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GIFT OF 
MICHAEL REESE 




ELECTRIC GENERATORS. 



BY 



HORACE FIELD PARSHALL 



HENRY METCALFE HOBART. 




LONDON : 
OFFICES OF "ENGINEERING," 35 AND 36, BEDFORD STREET, STRAND, W.C. 

NEW YORK: 
JOHN WILEY AND SONS, 43, EAST NINETEENTH STREET. 

1900. 
[All rights reserved.] 



x*. 







85388 




[Fro?n a photograph by Elliott and Fry 

DR. JOHN HOPKINSON, F.R.S. 



THIS BOOK IS DEDICATED, BY PERMISSION, 



THE LATE DR. JOHN HOPKINSON, F.R.S., 



THE FOUNDER OF THE 



SCIENCE OF DYNAMO DESIGN." 



TABLE OF CONTENTS. 



PAET I. 

PAGE 

MATERIALS ... ... ... ... ... 1 

TESTING OF MATERIALS ... ... ... ... ... 1 

Conductirity Tests Permeability Tests Ring Method Other Per 
meability Testing Methods Methods not Requiring Ballistic Galvanometer 
Determination of Hysteresis Loss Conversion of Units Hysteresis Losses 
in Alternating and Rotating Fields Methods of Measuring Hysteresis Loss 
without Ballistic Galvanometer Hysteresis Testers. 

PROPERTIES OP MATERIALS ... ... 14 

The Magnetisation of Iron and Steel Cast Iron Malleable Cast Iron 
Cast Steel Mitis Iron Nickel .Steel Forgings. 

ENERGY LOSSES IN SHEET IRON ... ... 28 

Annealing of Sheet Iron Deterioration of Sheet Iron Effect of Pressure 
Hysteresis Loss Eddy Current Losses Estimation of Armature Core 
Losses. 

INSULATING MATERIALS ... ... ... ... ... 38 

Effect of Temperature upon Insulation Resistance Description of 
Insulation Testing Methods for Factories Description of Transformer for 
making Insulation Tests Method of Test Methods of Insulating Coils. 

ARMATURE WINDINGS ... ... ... GO 

Continuous - Current Armature Windings Ring Windings Drum 
Windings Multiple-Circuit Windings Two-Circuit Windings Formula for 
Two-Circuit Windings Single Windings Multiple Windings Windings for 
Rotary Converters Alternating - Current Armature Windings Induction 
Motor Windings. 

FORMULAE FOR ELECTROMOTIVE FORCE ... 78 

Continuous-Current Dynamos Alternating-Current Dynamos Curve of 
E.M.F. Assumed to be a Sine Wave Values of K for Various Waves of 
E.M.F. and of Magnetic Flux Distribution in Gap Rotary Converters Three- 
Phase Rotary Converters Polyphase Machines Electromotive Force and Flux 
in Transformers. 



viii Table of Contents. 

PAGE 

THERMAL LIMIT OF OUTPUT ... ... ... 90 

Magnets Armatures Internal and Surface Temperature of Coils Heat 
Losses C 2 R due to Useful Currents in the Conductors Foucault Currents 
Hysteresis Loss in Cores Heating and Efficiency of Railway Motors, Arc 
Dynamos, Constant Potential Dynamos Commutator Heating Friction 
Loss. 
DESIGN OF THE MAGNETIC CIRCUIT ... ... ... ... ... 115 

Leakage Coefficient Armature Core Reluctance Air Gap Reluctance 
Reluctance of Complete Magnetic Circuit Estimation of Gap Reluctance 
Reluctance of Core Projections Calculation for Magnetic Circuit of Dynamo 
Field Winding Formula Application to Calculation of a Spool Winding for 
a Shunt- Wound Dynamo Typical Magnetic Circuits Magnetic Circuit of 
the Transformer Magnetic Circuit of the Induction Motor Examples. 
CONSTANT POTENTIAL, CONTINUOUS-CURRENT DYNAMOS ... ... ... 143 

Armature Reaction Application of Fundamental Considerations to the 
Proportioning of Dynamos Influence of Armature Reaction in Two Extreme 
Cases- Conditions essential to Sparkless Commutation Determination of the 
Number of Poles for a Given Output Multiple-Circuit Windings Two-Circuit 
Windings Multiple Windings Two-Circuit Coil Windings Voltage per 
Commutator Segment as Related to Inductance Inductance Constants 
Practical Definition of Inductance Description of Experimental Tests of 
Inductance Illustrations of the Calculation of the Reactance Voltage. 

DESCRIPTION OF MODERN CONSTANT POTENTIAL, COMMUTATING DYNAMOS ... 179 

1,500-Kilowatt, GOO- Volt, Railway Generator ... ... ... 179 

200- Kilowatt, 500-Volt, Railway Generator ... ... 190 

300-Kilo watt, 125-Volt, Lighting Generator ... ... 201 

250-Kilowatt, 550-Volt, Power Generator ... ... 215 

CORE LOSSES IN MULTIPOLAR COMMUTATING MACHINES ... ... ... 228 

ELECTRIC TRACTION MOTORS ... ... ... ... ... 232 

Description of a 24 Horse- Power Geared Motor for a Rated Draw-Bar Pull 

of 800 Ib. at a Speed of 11.4 Miles per Hour ... ... ... 233 

Description of a 27 Horse-Power Geared Railway Motor for a Rated Output 

of 27 Horse-Power, at an Armature Speed of 640 Revolutions per Minute ... 242 
Description of a 117 Horse-Power, Gearless Locomotive Motor for a Rated 

Draw-Bar Pull of 1,840 Ib., at 23.8 Miles per Hour, on 42-in. Wheels ... 256 

COMMUTATORS AND BRUSH GEAR ... ... ... ... ... 268 

Contact Resistance of Brushes Brushes of Various Materials, Copper, 
Carbon, Graphite. 

PART II. 

ROTARY CONVERTERS ... ... ... ... 283 

C 2 R Loss in Armature Conductors of Rotary Converters Single-Phase 

Rotary Converters Windings for Rotary Converters Three-Phase Rotaries 

Six-Phase Rotaries Interconnection of Static Transformers and Rotary 
Converters Four-Phase Rotary Converters Twelve-Phase Rotary Converters. 
Design for a Six-Phase, 400-Kilowatt, 25-Cycle, 600- Volt, Rotary Converter ... 311 



Table of Contents. ix 

PAGE 

Tabulated Calculations and Specifications for a 900 - Kilowatt, Three - Phase, 

Kotary Converter ... ... ... ... ... 329 

The Starting of Rotary Converters ... ... ... ... 340 

Synchronising Rotary Converters ... ... ... ... 345 

Methods of Adjusting Voltage Ratio in Rotary Converter Systems ... ... 346 

Running Conditions for Rotary Converters ... ... ... ... 351 

Predetermination of Phase Characteristic Curves of Rotary Converters 351 

" Surging " Effect ... ... ... ... 362 

Compound- Wound Rotary ... ... ... ... ... 363 

Series- Wound Rotary ... ... ... ... ... 365 

Rotary Without Field Excitation ... ... ... 365 

APPENDIX ... ... ... ... ... ... 367 

Tables of Properties of Copper Wire of Various Gauges Curve for Sheet 
Iron at High Densities Curve of Properties of Various Metallic Materials. 

INDEX 373 



ERRATA. 

Page 1, line 9. For " in the metallic " read "in the magnetic." 

Page 201, tenth line from bottom. For "Figs. 190 to 193 " read "Figs. 207 to 210." 

Page 230. For " Table LXIX." read " Table XLIX." 

Page 255. For the page heading, "27 Horse-Power Geared Railway Motor," read 
"117 Horse-Power Railway Motor." 

Page 296. For the title of Fig. 372, for "Two-Circuit Winding" read "Six-Circuit 
Winding." 



LIST OF ILLUSTRATIONS. 



PIG. 

1 Permeability Bridge ... ... ... ... ... 6 

2 Permeability Bridge ... ... ... ... ... 8 

3 Cyclic Curve of Sheet Iron ... ... ... ... 9 

4 Sample for Hysteresis Tester ... ... ... ... 11 

5 Hysteresis Tester ... ... ... ... ... 12 

6 Hysteresis Tester ... ... ... ... ... 13 

7 Hysteresis Tester ... ... ... ... ... 15 

8 to 11 Magnetic Curves for Cast Iron ... ... ... ... 19 

12 Magnetic Curves for Malleable Iron ... ... ... 21 

13 Mixtures of Steel and Cast Iron ... ... ... ... 21 

14 and 15 Magnetic Curves for Cast Steel ... ... 21 

16 to 19 Magnetic Curves for Cast Steel ... ... ... ... 23 

20 Magnetic Curves for Mitis Iron ... ... ... ... 26 

21 Magnetic Curves for Nickel Steel ... ... ... ... 26 

22 Magnetic Curves for Wrought-Iron Forgings ... ... ... 26 

23 Magnetic Curves for Steel and Wrought Iron ... ... ... 26 

24 Magnetic Curves for Forgings and Steel Castings ... ... 28 

25 Effect of Temperature of Annealing on Hysteresis Loss in Sheet Iron ... 30 

26 " Ageing " Curves for Basic, Open-Hearth Steel ... 30 

27 " Ageing " Curves for Acid, Open-Hearth Steel ... ... 30 

28 " Ageing " Curves for Sheet Iron ... ... ... ... 30 

29 to 32 " Ageing " Curves for Sheet Iron ... ... ... ... 32 

33 and 34 Effect of Pressure upon Hysteresis Loss in Sheet Iron ... ... 33 

35 Curves for Hysteresis Loss in Sheet Iron ... ... 34 

36 Curves for Eddy-Current Loss in Sheet Iron ... ... ... 34 

37 Characteristic Insulation Resistance Curve for Cloth ... ... 43 

38 and 39 Transformer for Insulation Tests ... 43 

40 to 44 Apparatus for Insulation Tests ... 44 

45 Circuit Connections for Insulation Tests ... ... ... 48 

46 to 51 Insulation Curves for " Mica-Canvas " 48 and 49 

52 to 57 Insulation Curves for "Mica Long-cloth " ... 50 and 51 

58 to 63 Insulation Curves for " Shellac d Paper" 54 

64 to 69 Insulation Curves for "Red Paper" ... ... 56 

70 Gramme Ring Winding with Lateral Commutator ... ... 61 

71 Multiple-Circuit, Drum Winding ... ... ... 64 

72 Six Circuit, Double Winding ... ... ... ... 65 



xii List of Illustrations. 

FIG. PAGE 

73 Two-Circuit, Single Winding 67 

74 Two-Circuit, Double Winding 

75 Two-Circuit Winding for Three-Phase Rotary Converter ... 71 

76 Six-Circuit Winding for Three-Phase Rotary Converter ... 72 

77 Urn-Coil Single-Phase Winding ... 74 

78 Uni-Coil Single-Phase Winding with Parallel Slots 74 

79 Multi-Coil Single-Phase Winding ... 74 

80 Y-Connected, Three-Phase Winding ... 74 

81 A-Connected, Three-Phase Winding ... 74 

82 Three-Phase, Non-Overlapping, Fractional Pitch Winding, with 14 Field 

Poles and 21 Armature Coils ... ... 74 

83 Three-Phase Armature, 10 Poles and 12 Coils ... 76 

84 Quarter-Phase Armature, 10 Poles and 8 Sets of Coils 76 
85 to 87 Induction Motor Windings ... 76 

88 Types of Winding ... 84 

89 Rotary Converter Characteristic Curves ... ... 86 

90 and 91 Form Factor Curves ... ... ... 87 

92 to 96 Thermal Tests of a Field Spool ... ... ... 94 

97 and 98 Thermal Tests of a Field Spool ... ... 94 and 95 

99 to 112 Thermal Tests of Influence of Peripheral Speed on Temperature Rise 96 to 101 

113 Armature Slot of a Large Alternator ... ... ... 105 

114 to 116 Curves Relating to Core Loss in Railway Motor Armature 106 and 108 

117 Curves of Rate of Generation of Heat in Copper by Resistance ... 110 

118 Curve of Insulation Resistance of a Transformer at Various Tem 

peratures ... ... ... ... HO 

119 to 124 Leakage Factor Diagrams of Dynamos ... ... ... 120 

125 Diagram for Illustrating Reluctance of Core Projections ... ... 123 

126 Sheet Iron Curves for High Densities ... ... ... 126 

127 Tooth Density Correction Curves ... ... ... ... 126 

128 to 137 Typical Magnetic Circuits and their Saturation Curves ... 129 to 134 

138 Magnetic Circuit of a Transformer ... ... ... ... 137 

139 Curve for Calculating Hysteresis Loss in Transformer Cores ... 136 

140 Curves for Calculating Eddy-Current Loss in Transformer Cores ... 136 

141 Magnetic Circuit of Induction Motor ... ... ... 137 

142 and 143 Curves of Distribution of Resultant Magnetomotive Force in Induction 

Motors ... ... 138 and 139 

144 to 146 Diagrams of Distorting and Demagnetising Effects of Armature Current 147 
147 Curves of Gap Distribution of Magnetic Flux with Various Leads of 

Brushes ... ... ... ... ... 148 

148 to 160 Diagrams and Curves of Armature Inductance... ... 161 to 174 

161 Diagram for Illustrating Reactance Calculations ... ... 175 

162 to 166 Drawings of 1,500 Kilowatt Railway Generator ... 181 to 185 
167 and 168 Saturation and Compounding Curves of 1,500 Kilowatt Railway 

Generator ... ... ... ... ... 188 

169 to 183 Drawings of 200-Kilowatt Railway Generator ... 191 to 196 

184 to 188 Results of Tests of 200-Kilowatt Railway Generator ... 202 

189 to 206 Drawings of 300-Kilowatt Lighting Generator ... ... 204 to 213 

207 to 210 Curves of Results of Tests of 300-Kilowatt Lighting Generator ... 213 



List of 



Xlll 



FIG. PAGE 

211 to 233 Drawings of 250-Kilowatt Electric Generator ... ... 216 to 226 

234 to 236 Characteristic Curves of 250-Kilowatt Electric Generator ... 227 and 228 

237 and 238 Diagram and Curve for Calculating Core Losses in Multipolar Corn- 
mutating Machines ... ... ... ... 229 

239 to 254 Drawings of 24 Horse-Power Geared Railway Motor 234 to 240 

255 to 258 Characteristic Curves of 24 Horse- Power Geared Railway Motor ... 240 

259 to 277 Drawings of 27 Horse-Power Geared Railway Motor ... 242 to 250 

278 to 283 Characteristic Curves of 27 Horse-Power Geared Railway Motor 250 and 251 

284 to 319 Drawings of 117 Horse-Power Gearless Railway Motor ... 253 to 264 

320 to 323 Characteristic Curves of 117 Horse-Power Gearless Railway Motor ... 265 

324 to 331 Commutators for Traction Motors ... 268 and 269 

332 to 340 Commutators for Traction Generators ... ... 269 and 270 

341 Diagram of Arrangements for Measuring Contact Resistance of Brushes 271 

342 to 346 Curves of Properties of Commutator Brushes ... ... 271 to 274 

347 to 352 Brush Holders for Radial Carbon Brushes for Traction Motors 275 and 276 

353 and 354 Carbon Brush Holder for Small Launch Motor... ... ... 276 

355 to 358 Carbon Brush Holders for Generators ... ... 276 and 278 

359 Holder for a Copper Gauze Brush ... ... ... ... 278 

360 and 361 Bay liss Reactance Brush Holder ... ... ... 279 

362 and 363 Brush Holder Constructed of Stamped Parts ... ... 279 

364 and 365 Holder for Carbon Brushes ... ... 279 

366 Sine Curves of Instantaneous Current Values in Three Phases of a 

Rotary Converter ... ... ... ... ... 286 

367 Diagrams of Instantaneous Current Values in Line and Windings of a 

Rotary Converter ... ... ... ... ... 287 

368 and 369 Developed Diagrams of Rotary Converter Winding ... 288 and 289 

370 Two-Circuit Single Winding for Single-Phase Rotary ... ... 295 

371 Two-Circuit Singly Re-Entrant Triple Winding for Single-Phase Rotary 296 

372 Six-Circuit Single Winding ... ... ... ... 296 

373 Six-Circuit Single Winding for Three-Phase Rotary ... ... 297 

374 Two-Circuit Single Winding for Three-Phase Rotary ... ... 298 

375 Two-Circuit Singly Re-Entrant Triple Winding for Three-Phase Rotary 299 

376 Six-Circuit Single Winding for Six-Phase Rotary ... ... 300 

377 Two-Circuit Single. Winding for Six-Phase Rotary ... ... 301 

378 Two-Circuit Singly Re-Entrant Triple Winding for Six-Phase Rotary ... 302 

379 Diagrammatic Comparison of Six-Phase and Three-Phase Windings ... 303 

380 Inter-Connection of Static Transformers and Rotary Converter . . . 304 
381 and 382 " Double-Delta " Connection and " Diametrical " Connection ... 305 

383 Six-Phase Switchboard ... ... ... 307 

384 Six-Circuit Single Winding for Four-Phase Rotary ... ... 308 

385 Two-Circuit Single Winding for Four-Phase Rotary ... 309 

386 Two-Circuit Triple Winding for Four-Phase Rotary ... ... 310 

387 Diagrammatical Representation of Conditions in Four-Phase Rotary 

Converter Winding ... ... ... ... 310 

388 and 389 Connection Diagrams for Twelve-Phase Rotary Converter ... ... 311 

390 to 393 Drawings of Six-Phase 400-Kilowatt Rotary ... ... 313 to 315 

394 and 395 Curves of Six-Phase 400-Kilowatt Rotary ... 316 

396 to 398 



Drawings of Three-Phase 900-Kilowatt Rotary 



331 and 332 



XIV 



List of Illustrations. 



FIG. PAGE 

399 to 402 Characteristic Curves of Three-Phase 900-Kilowatt Rotary ... 333 
403 Diagram of Connections for Starting Rotary Converter by Compensator 

Method ... ... 341 

404 and 405 Methods of Synchronising Rotary Converters ... ... ... 343 

406 to 408 Three-Pole, 2,000 Ampere, 330-Volt Switch for Rotary Converters 

344 and 345 

409 Diagram of Connections for Using Induction Regulators for Controlling 

the Voltage Ratio in Rotary Converters ... ... ... 347 

410 Diagram of Connections for Controlling the Voltage Ratio in Rotary 

Converter System by an Auxiliary Booster ... ... 348 

411 Diagram of Connections for Controlling the Voltage Ratio on a Portion 

of a Rotary Converter System by an Auxiliary Booster ... 349 

412 Combined Rotary Converter and Series Booster ... .. 350 

413 Combined Rotary Converter and Auxiliary Synchronous Motor for 

Giving Adjustable Voltage Ratio ... ... ... 350 

414 to 418 Phase Characteristic Curves of Rotary Converters ... 354 to 357 

419 and 420 Distribution of Resultant Armature Magnetomotive Force over the 

Armature Surface of a Rotary Converter ... 358 and 359 

421 Curves of a Series- Wound Rotary ... ... ... 363 

422 Curves of a Rotary without Field Excitation ... ... 364 

423 Curve for Sheet Iron at High Densities 372 



LIST OF TABLES. 



TABLE PAGE 

I. Data of Ten First-Quality Samples of Cast Steel ... ... ... 22 

II. Data of Ten Second-Quality Samples of Cast Steel ... ... 24 

III. Data of Twelve Samples of Mitis Iron ... ... ... 24 

IV. Analyses of Samples of Sheet Iron and Steel ... ... ... 27 

V. Results of Tests on " Ageing " of Iron ... ... ... 31 

VI. Properties of Iron and Steel, with Special Reference to Specific Resistance 36 

VII. Preece s Tests of Annealed Iron Wire ... ... ... 36 

VIII. Influence of Carbon on Specific Resistance of Steel ... 37 

IX. Influence of Silicon on Specific Resistance of Steel ... 37 

X. Influence of Manganese on Specific Resistance of Steel ... 38 

XI. Puncturing Voltage of Composite White Mica ... ... 38 

XII. Insulation Tests on Sheets of Leatheroid ... ... 39 

XIII. Summary of Qualities of Insulating Materials ... ... ... 42 

XIV. Insulation Tests on " Mica Canvas " ... ... ... ... 47 

XV. Insulation Tests on "Mica Long-Cloth" ... 52 

XVI. Insulation Tests on Shellac d Paper ... ... ... ... 53 

XVII. Insulation Tests on Red Paper ... ... ... ... 55 

XVIII. Subdivision of Windings for Rotary Converters ... ... ... 70 

XIX. Drum Winding Constants ... ... ... 80 

XX. Correction Factors for Voltage of Distributed Windings ... ... 81 

XXI. Values for K in E.M.F. Calculations for Multi-Coil Windings 82 

XXII. Values for K in E.M.F. Calculations for Multi-Coil Windings, with 

Various Pole Arcs ... ... ... ... ... 83 

XXIII. Values for K in E.M.F. Calculations for Windings with Various Per 

centages Spread 

XXIV. Values for Voltage Ratio for Single and Quarter-Phase Rotary Converters 85 
XXV. Values for Voltage Ratio for Three-Phase Rotary Converters ... 85 

XXVI. Values of Number of Turns in Series between Collector Rings in Rotary 

Converters ... ... ... 87 

XXVII. Values for Form Factor ... 88 

XXVIII. Values for Form Factor ... 89 

XXIX. Temperature Correction Coefficients for Copper ... ... ... 102 

XXX. Current Densities in Copper and Corresponding Specific Rates of 

Generation of Heat in Watts per Pound ... ... ... 108 

XXXI. Magnetic Flux Densities in Sheet Iron, and Corresponding Specific Rates 

of Generation of Heat in Watts per Pound ... ... ... 109 

XXXII. Current Densities in Various Types of Apparatus ... ... 109 

C 



xvi List of Tables. 

TABLE 

XXXIII. Calculation of Reluctance of Core Projections 

XXXIV. Calculation of Reluctance of Core Projections 
XXXV. Calculation of Reluctance of Core Projections 

XXXVI. Test of Armature Reaction 

XXXVII. Inductance Tests 

XXXVIII. Inductance Tests 

XXXIX. Inductance Tests 

XL. Inductance Tests 

XLI. Inductance Tests 

XLII. Inductance Tests 

XLIII. Inductance Tests 

XLIV. Inductance Tests 

XLV. Inductance Tests 

XLVI. Inductance Tests 

XL VII. Inductance Tests 

XLVIII. Inductance Tests 

XLIX. Core Loss Results 

L. Tests on Graphite and Carbon Brushes 

LI. Output of Rotary Converters 

LII. Output of Rotary Converters 

LI II. Armature C 2 R Loss in Rotary Converters 

LIV. Armature C 2 R Loss in Rotary Converters 

LV. Armature C-R Loss in Rotary Converters 



PAGE 

125 
125 
125 
149 
160 
162 
162 
162 
163 
164 
165 
167 
167 
168 
168 
171 
230 
280 
284 
285 
290 
292 
294 



APPENDIX. 

LVI. Table of Properties of Copper Wire B. and S. Gauge 

LVII. Table of Properties of Copper Wire S. W. G. Gauge 

LVIII. Table of Properties of Copper Wire B. W. G. Gauge 

LIX. Physical and Electrical Properties of Various Metals and Alloys 



367 
368 
369 
370 



PREFACE. 



present volume is an amplification of the notes of a series of 
lectures, delivered first by Mr. Parshall and continued by Mr. 
Hobart, at the Massachusetts Institute of Technology, some six years ago. 
The original notes met with so cordial an appreciation from Lord Kelvin, 
the late Dr. John Hopkinson and others, that the authors determined 
to follow out a suggestion made, and publish a book on the design of 
Electric Generators. The work of revising the original notes gradually led 
to the bringing together of an amount of material several times larger than 
was at first intended, and a comprehensive treatment of the subject 
prevented reducing this amount. In this form the work appeared as a series 
of articles in " ENGINEERING," during the years 1898 and 1899. The 
interest taken in the series, together with the fact that the experience 
of the Authors, covering as it does the period during which most of the 
modern types of machines have been developed, justifies the publication 
of the treatise, despite the present large number of books on the theory 
of commutating machines. 

In dealing with the practice of designing, three sub-divisions can be 
finally made : 

The first may be taken as relating to the design of the magnetic 
circuit. The classical papers of Doctors John and Edward Hopkinson 
have dealt with this subject so completely that there remains but little to 
be written ; and this relates chiefly to the nature and properties of the 
different qualities of iron and steel which may be used in the construction 
of the magnetic circuit. 

The second sub-division considers the phenomena of commutation and 
the study of dimensions, with a view to securing the greatest output 



xviii Preface. 

without diminishing the efficiency. The theory of commutation has 
become better understood since electrical engineers began to deal with 
alternatina- currents and to understand the effects of self-induction. How- 

o 

ever, owing to the number of variables affecting the final results, data 
obtained in practice must be the basis for the preparation of new designs. 
In this work will be found a statement of such results, and numerical 
values experimentally obtained from representative commutating machines. 
One familiar with the theory of commutation can, with comparative 
certainty, from the values and dimensions given, design machines with 
satisfactory commutating properties. 

The third sub-division relates to what we have termed the " Thermal 
Limit of Output," that is, the maximum output with safe heating. It can 
be fairly said that while the theory of all the losses in a commutating 
dynamo are understood, yet, with the exception of the C 2 R losses, it is 
still a matter of practical experience to determine what relation the actual 
losses bear to what may be termed the predicted losses. It is invariably 
found that the iron losses are in excess of those which may be predicted 
from the tests made upon the material before construction. The hysteresis 
loss in the armature core is generally found to be greater, owing to the 
mechanical processes to which the material in the core has to be sub 
jected during the process of construction. Owing, probably, in a large 
measure to a species of side magnetisation, the eddy-current loss is found 
to be greater than is indicated by calculations based upon the assumption 
of a distribution of magnetic lines parallel to the plane of the laminations. 
If the armature conductors are solid, the losses therein by foucault 
currents may often be considerable, even in projection type armatures, 
especially when the projections are run at high densities. Under load 
losses, not including friction, there have to be considered the foucault 
current loss in the conductors due to distortion, and the increased 
loss in the armature projections from hysteresis and eddy currents likewise 
due thereto. There is also the loss brought about by the reversal of the 
current in the armature coil under commutation. It is apparent, therefore, 
considering that each of these variables is dependent upon the form of 



Preface. xix 

design, the material used, and the processes of construction, that only an 
approximate estimate as to the total loss can be made from the theoretical 
consideration of the constants. We believe, therefore, that these con 
siderations will justify the length with which we have dealt with the 
thermal limit of output. 

The various other sections give information which we have found 
indispensable in designing work. The General Electric Company of 
America, and the Union Elektricitiits-Gesellschaft of Berlin, have kindly 
placed at our disposal the results of a large amount of technical experience, 
which have formed a very substantial addition to the results of our own 
work. We have endeavoured to show our appreciation of this liberal 
and, unfortunately rare, policy, by setting forth the conclusions at which 
it has enabled us to arrive, in a manner which we hope will render the 
work a thoroughly useful contribution to technical progress in dynamo 
design. Apart from the papers of the Hopkinsons, the treatise on 
Dynamo Electric Machinery by Dr. Sylvanus Thompson, has had the 
greatest influence in disseminating thorough knowledge of the theory of 
the dynamo. It was, in fact, after considering the contents of these 
works that we decided to prepare our treatise on the present lines ; with 
the aim to supply, however imperfectly, a work which shall assist in 
applying to practice the principles already clearly enunciated in these 
treatises. 

We acknowledge with pleasure the valuable assistance and suggestions 
which we have received from many friends in the preparation of the 
work. 



PART I. 

ELECTRIC GENERATORS. 




ELECTRIC GENERATORS. 



MATERIALS. 

A CONSIDERABLE variety of materials enters into the construction 
of dynamo electric apparatus, and it is essential that the grades 
used shall conform to rather exacting requirements, both as regards electric 
and magnetic conductivity as well as with respect to their mechanical 
properties. 

TESTING OF MATERIALS. 

The metallic compounds employed in the metallic and conducting 
circuits must be of definite chemical composition. The effect of slight 
differences in the chemical composition is often considerable ; for instance, 
the addition of 3 per cent, of aluminium reduces the conductivity of copper 
in the ratio of 100 to 18. 1 Again, the magnetic permeability of steel 
containing 12 per cent, of manganese is scarcely greater than unity. 

The mechanical treatment during various stages of the production also 
in many cases exerts a preponderating influence upon the final result. 
Thus, sheet iron frequently has over twice as great a hysteresis loss when 
unannealed as it has after annealing from a high temperature. Cast copper 
having almost the same chemical analysis as drawn copper, has only 50 per 
cent, conductivity. Pressure exerts a great influence upon the magnetic 
properties of sheet iron. 2 Sheet iron of certain compositions, when 
subjected for a few weeks, even to such a moderate temperature as 60 deg. 
Cent., becomes several times as poor for magnetic purposes as before 
subjection to this temperature. 3 

It thus becomes desirable to subject to chemical, physical, and electro 
magnetic tests samples from every lot of material intended for use in the 

1 Electrician, July 3rd, 1896. Dewar and Fleming. 2 See page 33, and Figs. 33 and 34. 
3 See pages 30 to 32, and Figs. 26 to 32. 

B 



2 Electric Generators. 

construction of dynamo-electric apparatus. This being the case, the 
importance of practical shop methods, in order that such tests may be 
quickly and accurately made, becomes apparent. 

CONDUCTIVITY TESTS. 

The methods used in conductivity tests are those described in text 
books devoted to the subject. 1 It will suffice to call attention to the recent 
investigations of Professors Dewar and Fleming, 2 the results of which show 
that materials in a state of great purity have considerably higher conduc 
tivity than was attributed to them as the results of Matthiessen s experi 
ments. Manufactured copper wire is now often obtained with a conductivity 
exceeding Matthiessen s standard for pure copper. 

Copper wire, drawn to small diameters, is apt to be of inferior conduc 
tivity, due to the admixture of impurities to lessen the difficulties of 
manufacture. It consequently becomes especially desirable to test its 
conductivity in order to guard against too low a value. 

The electrical conductivity of German silver and other high resistance 
alloys varies to such an extent that tests on each lot are imperative, if 
anything like accurate results are required. 3 

PERMEABILITY TESTS. 

Considerable care and judgment are necessary in testing the magnetic 
properties of materials, even with the most recent improvements in 
apparatus and methods. Nevertheless, the extreme variability in the 
magnetic properties, resulting from slight variations in chemical composition 
and physical treatment, render such tests indispensable in order to obtain 
uniformly good quality in the material employed. Various methods have 
been proposed with a view to simplifying permeability tests, but the most 
accurate method, although also the most laborious, is that in which the 
sample is in the form of an annular ring uniformly wound with primary and 
secondary coils, the former permitting of the application of any desired 



1 Among the more useful books on the subject of electrical measurements are Professor 
S. W. Holman s Physical Laboratory Notes (Massachusetts Institute of Technology), and 
Professor Fleming s Electrical Laboratory Notes and Forms. 

2 Electrician, July 3rd, 1896. 

3 A Table of the properties of various conducting materials is given later in this volume. 




Permeability Tests. 3 

magnetomotive force, and the latter being for the purpose of determining, 
by means of the swing of the needle of a ballistic galvanometer, the 
corresponding magnetic flux induced in the sample. 

DESCRIPTION OF TEST OF IRON SAMPLE BY RING METHOD WITH 
BALLISTIC GALVANOMETER. 

The calibrating coil consisted of a solenoid, 80 centimetres long, 
uniformly wound with an exciting coil of 800 turns. Therefore, there 
were 10 turns per centimetre of length. The mean cross-section of 
exciting coil was 18.0 square centimetres. The exploring coil con 
sisted of 100 turns midway along the solenoid. Reversing a current 
of 2.00 amperes in the exciting coil gave a deflection of 35.5 deg. on the 
scale of the ballistic galvanometer when there was 150 ohms resistance 
in the entire secondary circuit, consisting of 12.0 ohms in the ballistic 
galvanometer coils, 5.0 ohms in the exploring coil, and 133 ohms in external 
resistance. 

H = 47rnC ; 1=10.0; C = 2.00; 
10 / I 

.: H=!l x 10.0 x 2.00 = 25.1, 
10 

i.e., 2.00 amperes in the exciting coil set up 25.1 lines in each square 
centimetre at the middle section of the solenoid; therefore 18.0 x 25.1 
= 452 total C G S. lines. But these were linked with the 100 turns of 
the exploring coil, and therefore were equivalent to 45,200 lines linked with 
the circuit. Reversing 45,200 lines was equivalent in its effect upon the 
ballistic galvanometer to creating 90,400 lines, which latter number, con 
sequently, corresponds to a deflection of 35.5 deg. on the ballistic 
galvanometer with 150 ohms in circuit. Defining K, the constant of the 
ballistic galvanometer, to be the lines per degree deflection with 100 ohms 
in circuit, we obtain 

90400 

K= Q , , T -^ ?r = 1690 lines. 
35.5 x 1.50 

The cast-steel sample consisted of an annular ring of 1.10 square 
centimetres cross-section, and of 30 centimetres mean circumference, and 
it was wound \vith an exciting coil of 450 turns, and with an exploring coil 
of 50 turns. With 2.00 amperes exciting current, 



4 Electric Generators. 

Reversing 2.00 amperes in the exciting coil gave a deflection^ of 
40 deg. with 2,400 ohms total resistance of secondary circuit. Then with 
100 ohms instead of 2,400 ohms, with one turn in the exploring coil instead 
of 50 turns, and simply creating the flux instead of reversing it, there 
would have been obtained a deflection of 



2400 1 .. 1 x 40 = 9.60 deg.; 



x x 



100 50 

consequently the flux reversed in the sample was 

9.60 x 1,690 = 16,200 lines. 

And as the cross-section of the ring was 1.10 square centimetres, the 

density was 

16,200 -f 1.10 = 14,700 lines per square centimetre. 

Therefore the result of this observation was 

H = 37.7; B = 14,700; p = 390. 

But in practice 1 this should be reduced to ampere turns per inch of length, 
and lines per square inch ; 

Ampere-turns per inch of length = 2 H = 75.4. 

Density in lines per square inch = 6.45 x 14,700 = 95,000 

This would generally be written 95.0 kilolines. Similarly, fluxes of 
still greater magnitude are generally expressed in megalines. For instance, 

12.7 megalines = 12,700,000 COS lines. 

1 Although mixed systems of units are admittedly inferior to the metric system, present 
shop practice requires their use. It is, therefore, necessary to readily convert the absolute 
B H curves into others expressed in terms of the units employed in practice. In absolute 
measure, iron saturation curves are plotted, in which the ordinates B represent the density in 
terms of the number of C G S lines per square centimetre, the abscissae denoting the magneto 
motive force H. B/H equals p, the permeability. In the curves used in practice the 
ordinates should equal the number of lines per square inch. They are, therefore, equal to 
6.45 B. The abscissae should equal the number of ampere-turns per inch of length. Letting 
turns = n, and amperes = C, we have 

H = "" , I being expressed in centimetres. 

1 \J L 

I Q TT 

. . Ampere-turns per centimetre of length = , 



Ampere-turns per inch of 2 - 5 ^ x 1Q 



; 

4 7T 

Ampere-turns per inch of length = 2.02 H. 
Therefore ampere-turns per inch of length are approximately equal to 2 H. 



Permeability Tests. 5 

OTHER PERMEABILITY TESTING METHODS. 

The bar and yoke method, devised by Dr. Hopkinson, permits of the use 
of a rod-shaped sample, this being more convenient than an annular ring, 
in that the latter requires that each sample be separately wound, whereas 
in the rod and yoke method the same magnetising and exploring coils 
may be used for all samples. However, the ring method is more absolute, 
and affords much less chance for error than is the case with other methods, 
where the sources of error must either be reduced to negligible proportions, 
which is seldom practicable, or corrected for. Descriptions of the Hop 
kinson apparatus are to be found in text-books on electro-magnetism, 1 
and the calculation of the results would be along lines closely similar to 
those of the example already given for the case of an annular ring sample. 

METHODS OF MEASURING PERMEABILITY NOT REQUIRING BALLISTIC 

GALVANOMETER. 

There have been a number or arrangements devised for the purpose 
of making permeability measurements without the use of the ballistic 
galvanometer, and of doing away with the generally considerable trouble 
attending its use, as well as simplifying the calculations. 

Those in which the piece to be tested is compared to a standard 
of known permeability have proved to be the most successful. The 
Eickemeyer bridge 2 is a well-known example, but it is rather untrust 
worthy, particularly when there is a great difference between the standard 
and the test-piece/ 

A method of accomplishing this, which has been used extensively 
with very good results, has been devised by Mr. Frank Holden. It is 
described by him in an article entitled " A Method of Determining 
Induction and Hysteresis Curves " in the Electrical World for December 
15th, 1894. The principle has been embodied in a commercial apparatus 
constructed by Mr. Holden in 1895, 3 and also in a similar instrument 
exhibited by Professor Ewing before the Royal Society in 1896. 4 

1 Also J. Hopkinson, Phil. Trans., page 455, 1885. 

2 Electrical Engineer, New York, March 25th, 1891. 

3 "An Apparatus for Determining Induction and Hysteresis Curves," Electrical World, 
June 27th, 1896. 

4 "The Magnetic Testing of Iron and Steel," Proc. Inst. Civil Engineers, May, 1896. 



6 Electric Generators. 

Holden s method consists essentially of an arrangement in which two 
bars are wound uniformly over equal lengths, and joined at their ends 
by two blocks of soft iron into which they fit. The rods are parallel, 
and about as close together as the windings permit. In practice it has 
been found most convenient to use rods of about .25 in. in diameter, and 
about 7 in. long. Over the middle portion of this arrangement is placed 
a magnetometer, not necessarily a very sensitive one, with its needle 
tending to lie at right angles to the length of the two bars, the influence 
of the bars tending to set it at right angles to this position. Means are 




FIG. 1. 



provided for reversing simultaneously, and for measuring, each of the 
magnetising currents, which pass in such directions that the north end 
of one rod and the south end of the other are in the same terminal block. 
It is evident that whenever the magnetometer shows no effect from the 
bars, the fluxes in them must be equal, for if not equal there would be 
a leakage from one terminal block to the other through the air, 
and this would affect the magnetometer. This balanced condition is 
brought about by varying the current in one or both of the bars, and 
reversing between each variation to get rid of the effects of residual 
magnetism. 

For each bar 



H = 



10* 



Permeability Tests. 7 

where 

n = number of turns. 
C = Current in amperes. 
I = distance between blocks in centimetres. 

As the same magnetising coils may always be used, and as the blocks 
may be arranged at a fixed distance apart, 



o 

and 

H = KG. 

The B H curve of the standard must have been previously deter 
mined, and when the above-described balance has been produced and 
the magnetomotive force of the standard calculated, the value of B is 
at once found by reference to the characteristics of the standard. If 
the two bars are of the same . cross-section, this gives directly the B in 
the test-piece, and H is calculated as described. The method furnishes 
a means of making very accurate comparisons, and the whole test is 
quickly done, and the chances of error are minimised by the simplicity 
of the process. The magnetometer for use with bars of the size 
described need not be more delicate than a good pocket compass. 
Although two pieces of quite opposite extremes of permeability may 
be thus compared, yet it takes less care in manipulating, if two 
standards are at hand, one of cast-iron and one of wrought iron or cast 
steel, and the standard of quality most like that of the test-piece should 
be used. 

Sheet iron may be tested in the same way, if it is cut in strips 
about .5 in. wide and 7 in. long. This will require the use of specially- 
shaped blocks, capable of making good contact with the end of the bundle 
of strips which may be about .25 in. thick. In general the cross-sections 
of the test-piece and standard in this case will not be equal, but this 
is easily accounted for, since the induction values are inversely as the 
cross-sections when the total fluxes are equal. In Figs. 1 and 2 are 
shown both the Holden and the Ewing permeability bridges. 



Electric Generators. 






e 



Hysteresis Tests. 9 

DETERMINATION OF HYSTERESIS Loss. 

The step-by-step method of determining the hysteresis loss, by carrying 
a sample through a complete cycle, has been used for some years past, and 
is employed to a great extent at the present time. Such a test is made 
with a ring-shaped sample, and consists in varying by steps the magneto 
motive force of the primary coil, and noting by the deflection of a ballistic 
galvanometer the corresponding changes in the flux. From the results a 
complete cycle curve, such as is shown in Fig. 3, may be plotted. If this 
curve is plotted with ordinates equal to B (C G S lines per square centi 




metre), and with abscissae equal to H, (- j ), its area divided by 4 * 

(conveniently determined by means of a planimeter), will be equal to the 
hysteresis loss of one complete cycle, expressed in ergs per cubic centi 
metre 1 ; but in subsequent calculations of commercial apparatus it is more 
convenient to have the results in terms of the w r atts per pound of material 
per cycle per second. The relation between the two expressions may be 
derived as follows : 



CONVERSION OF UNITS. 
Ergs per cubic centimetre per cycle 

Area complete cyclic curve 

4 7T 

1 Fleming, Alternate Current Transformer, second edition, page 62. 



10 Electric Generators. 

Watts per cubic centimetre at one cycle per second 

Area 

= 4 7T X 10 T 

Watts per cubic inch at one cycle per second 

Area x 16.4 
= 4 TT x 10 7 

Watts per pound at one cycle per second 

Area x 16.4 



4 TT x 10 7 x .282 
(One cubic inch of sheet iron weighing .282 Ib.) 

.-. Watts per pound at one cycle per second = .0000058 x ergs per cubic centimetre per 
cycle. 

HYSTERESIS LOSSES IN ALTERNATING AND ROTATING FIELDS. 

Hysteresis loss in iron may be produced in two ways : one when the 
magnetising force acting upon the iron, and consequently the magnetisation, 
passes through a zero value in changing from positive to negative, and the 
other when the magnetising force, and consequently the magnetisation, 
remains constant in value, but varies in direction. The former condition 
holds in the core of a transformer, and the latter in certain other types of 
apparatus. The resultant hystereris loss in the two cases cannot be 
assumed to be necessarily the same. Bailey has found 1 that the rotating 
field produces for low inductions a hysteresis loss greater than that of the 
alternating field, but that at an induction of about 100 kilolines per square 
inch, the hysteresis loss reaches a sharply defined maximum, and rapidly 
diminishes on further magnetisation, until, at an induction of about 130 
kilolines per square inch, it becomes very small with every indication of 
disappearing altogether. This result has been verified by other experi 
menters, and it is quite in accord with the molecular theory of magnetism, 
from which, in fact, it was predicted. In the case of the alternating field, 
when the magnetism is pressed beyond a certain limit, the hysteresis loss 
becomes, and remains, constant in value, but does not decrease as in the 

1 See paper on " The Hysteresis of Iron in a Rotating Magnetic Field," read before the 
Royal Society, June 4th, 1896. See also an article in the Electrician of October 2nd, 1896, 
on " Magnetic Hysteresis in a Rotating Field," by R. Beattie and R. 0. Clinker. Also 
Electrician, August 31st, 1894, F. G. Bailey. Also Wied. Ann., No. 9, 1898, Niethammer. 



Hysteresis Tests. 



11 



case of the rotating magnetisation. Hence, as far as hysteresis loss is con 
cerned, it might sometimes be advantageous to work with as high an 
induction in certain types of electro-dynamic apparatus as possible, if it can 
be pressed above that point where the hysteresis loss commences to decrease ; 
but in the case of transformers little advantage would be derived from high 
density on the score of hysteresis loss, as the density, except at very low 
cycles, cannot be economically carried up to that value at which the 
hysteresis loss is said to become constant. 




FIG. 4. 



METHODS OF MEASURING HYSTERESIS Loss WITHOUT THE BALLISTIC 

GALVANOMETER. 

To avoid the great labour and expenditure of time involved in 
hysteresis tests by the step-by-step method with the ballistic galvano 
meter, there have been many attempts made to arrive at the result in a 
more direct manner. The only type of apparatus that seems to have 
attained commercial success measures the energy employed either in 
rotating the test-piece in a magnetic field, or in rotating the magnetic field 
in which the test-piece is placed. 

The Holden hysteresis tester 1 is the earliest of these instruments, and 

1 "Some Work on Magnetic Hysteresis," Electrical World, June 15th, 1895. 



12 



Electric Generators. 



appears to be the most satisfactory. It measures the loss in sheet-iron 
rings when placed between the poles of a rotating magnet, and enables the 
loss 3 to be thoroughly analysed. The sheet-iron rings are just such as 
would be used in the ordinary ballistic galvanometer test (Fig. 4, page 11). 
The rings are held concentric with a vertical pivoted shaft, around 
which revolves co-axially an electro-magnet which magnetises the rings. 
The sample rings are built up into a cylindrical pile about in. high. 




FIG. 5. 

Surrounding but not touching the sample to be tested is a coil of insulated 
wire, the terminals of which lead to a commutator revolving with the 
magnet. The alternating electromotive force of the coil is thus rectified, 
and measured by a Weston voltmeter. Knowing the cross-section of the 
sample, the number of turns in the coil, the angular velocity of the magnet, 
and the constants of the voltmeter, the induction corresponding to a certain 
deflection of the voltmeter, can be calculated in an obvious manner. 1 

1 For electromotive force calculations, see another page in this volume. 



Hysteresis Tests. 



13 



The force tending to rotate the rings is opposed by means of a helical 
spring surrounding the shaft and attached to it at one end. The other end 
is fixed to a torsion head, with a pointer moving over a scale. The loss per 
cycle is proportional to the deflection required to bring the rings to their 
zero position, and is readily calculated from the constant of the spring. 

By varying the angular velocity of the magnet, a few observations give 
data by which the effect of eddy currents may be allowed for, and the 
residual hysteresis loss determined ; or, by running at a low speed, the 
eddy current loss becomes so small as to be practically negligible, and 
readings taken under these conditions are, for all commercial purposes, the 
only ones necessary. A test sample with wire coil is shown in Fig. 4, 
whilst the complete apparatus may be seen in Fig. 5, page 12. 

A modification (Fig. 6) of this instrument does away with the adjust- 




FIG. 6. 



ment of the magnetising current and the separate determination of the 
induction for different tests. In this case the electro-magnet is modified 
into two of much greater length, and of a cross-section of about one-third 
that of the sample lot of rings. The air gap is made as small as 
practicable, so that there is very little leakage. A very high magneto 
motive force is applied to the electro-magnets, so that the flux in them 
changes only very slightly with considerable corresponding variation in the 
current. With any such variation from the average as is likely to occur in 
the rings on account of varying permeability, the total flux through them 
will be nearly constant, with the magnetisation furnished in this manner. 
The sample rotates in opposition to a spiral spring, and the angle of rotation 
is proportional to the hysteresis loss. In general a correction has to be 
applied for volume and cross-section, as the rings do not, owing to varia 
tions in the thickness of the sheets, make piles of the same height. The 



14 Electric Generators. 

magnets are rotated slowly by giving them an impulse by hand, and the 
reading is made when a steady deflection is obtained. 



EWING HYSTERESIS TESTER. 

In Professor Ewing s apparatus 1 the test sample is made up of about 
seven pieces of sheet iron f in. wide and 3 in. long. These are rotated 
between the poles of a permanent magnet mounted on knife-edges. The 
magnet carries a pointer which moves over a scale. Two standards of 
known hysteresis properties are used for reference. The deflections corres 
ponding to these samples are plotted as a function of their hysteresis losses, 
and a line joining the two points thus found is referred to in the subsequent 
tests, this line showing the relation existing between deflections and 
hysteresis loss. The deflections are practically the same, with a great 
variation in the thickness of the pile of test-pieces, so that no correction 
has to be made for such variation. It has, among other advantages, that 
of using easily prepared samples. The apparatus is shown in Fig. 7. 



PROPERTIES OF MATERIALS. 

The magnetic properties of iron and steel depend upon the physical 
structure ; as a primary indication of which, and as a specific basis for the 
description of the material, chemical analysis forms an essential part of 
tests. The physical structure and the magnetic properties are affected to a 
greater or less degree according to the chemical composition ; by annealing, 
tempering, continued heating, and mechanical strains by tension or com 
pression. The rate of cooling also influences the magnetic properties of the 
material ; the permeability of cast iron, for instance, is diminished if the 
cooling has been too rapid, but it may be restored by annealing, the only 
noticeable change being that the size of the flakes of graphite is increased. 
The permeability of high carbon steels may also be increased by annealing 
and diminished by tempering, and that of wrought iron or steel is diminished 
by mechanical strain ; the loss of permeability resulting from mechanical 
strain, may, however, be restored by annealing. 

The effect on the magnetic properties, of the different elements entering 
into the composition of iron and steel, varies according to the percentage of 



1 Electrician, April 26th, 1895. 




Composition of Iron and Steel. 



15 



other elements present. The presence of an element which, alone, would 
be objectionable may not be so when a number of others are also present ; 
for instance, manganese in ordinary amounts is not objectionable in iron and 
steel, as the influence it exerts is of the same nature as that of carbon, but 




FIG. 7. 



greatly less in degree. Some elements modify the influence of others, 
while some, although themselves objectionable, act as an antidote for 
more harmful impurities : as for instance, in cast iron, silicon tends to 
oft-set the injurious influence of sulphur. The relative amounts and the 

1 Electrician, April 26th, 1895. 



16 Electric Generators. 

sum of the various elements vary slightly, according to the slight 
variations in the process of manufacture. On account of the more or 
less unequal diffusion of the elements, a single analysis may not indicate 
the average quality, and may not, in extreme cases, fairly represent the 
quality of the sample used in the magnetic test. It is necessary, therefore, 
to make a great number of tests and analyses before arriving at an 
approximate result as to the effect of any one element. The conclusions 
here set forth, as to the effect of various elements, when acting with the 
other elements generally present, are the result of studying the analyses 
and magnetic values when the amounts of all but one of the principal 
elements remained constant. The results so obtained were compared 
with tests in which the elements that had remained constant in the first 
test varied in proportion. 

It will be seen that this method is only approximate, since variations 
of the amount of any element may modify the interactions between the 
other elements. The statements herein set forth have been compared 
with a great number of tests, and have been found correct within the 
limits between which materials can be economically produced in practice. 

In general, the purer the iron or steel, the more important is the 
uniformity of the process and treatment, and the more difficult it is to 
predict the magnetic properties from the chemical analysis. It is sig 
nificant to note that, beginning with the most impure cast iron, and 
passing through the several grades of cast iron, steel and wrought iron, 
the magnetic properties accord principally with the amounts of carbon 
present, and in a lesser degree with the proportions of silicon, phos 
phorus, sulphur, manganese, and other less usual ingredients, and that 
an excess of any one, or of the sum of all the ingredients, has a noticeable 
effect on the magnetic properties. Carbon, on account of the influence 
it exerts on the melting point, may be regarded as the controlling element, 
as it determines the general processes ; hence also the percentage of 
other elements present in the purer grades of iron. However, its influence 
may sometimes be secondary to that of other impurities ; as, for instance, 
in sheet iron, where a considerable percentage of carbon has been found 
to permit of extremely low initial hysteresis loss, and to exert an influence 
tending to maintain the loss at a low value during subjection to pro 
longed heating. 

The properties of iron and steel require separate examination as to 
magnetic permeability and magnetic hysteresis. The permeability is of 



Properties of Cast Iron. 17 

the greatest importance in parts in which there is small change in the 
magnetisation ; hence such parts may be of any desired dimension, and 
may then be either cast, rolled, or forged. On account of the electrical 
losses by local currents when the magnetism is reversed in solid masses 
of metals, parts subjected to varying magnetic flux have to be finely 
laminated. Thicknesses of between .014 in. and .036 in. are generally 
found most useful for plates, which must be of good iron to withstand 
the rolling process. Some impurities affect the hysteresis more than 
the permeability. Hysteresis tends towards a minimum, and the per 
meability towards a maximum, as the percentage of elements, other than 
iron, diminishes. 

In the case of comparatively pure iron or steel, alloyed with nickel, 
it is found, however, that the permeability is increased beyond that which 
would be inferred from the other elements present. The purest iron 
has been found to have the highest permeability, yet the iron in which 
the hysteresis loss has been found smallest is not remarkable for its purity, 
and there was no known cause why the hysteresis was reduced to such 
a noticeable extent. The treatment of the iron, both during and subse 
quent to its manufacture, exerts a great influence upon the final result. 

THE MAGNETISATION OF IRON AND STEEL. 

Cast Iron. Cast iron is used for magnetic purposes on account of the 
greater facility with which it may be made into castings of complex form. 
Considering the relative costs and magnetic properties of cast iron and steel, 
as shown in the accompanying curves, it is evident that cast iron is, other 
things being equal, more costly for a given magnetic result than cast steel. 
The great progress in the manufacture of steel castings has rendered the 
use of cast iron exceptional in the construction of well-designed electrical 
machines. 

The cast iron used for magnetic purposes contains, to some extent, all 
those elements which crude iron brings with it from the ore and from the 
fluxes and fuels used in its reduction. Of these elements, carbon has the 
greatest effect on the magnetic permeability. The amount of carbon 
present is necessarily high, on account of the materials used, the process 
employed, and its influence in determining the melting point. In cast iron 
of good magnetic quality, the amount of carbon varies between 3 per cent. 
and 4.5 per cent.; between 0.2 per cent., and 0.8 per cent, being in a com- 

D 



18 Electric Generators. 

bined state, 1 and the remainder in an uncombined or graphitic state. 
Combined carbon is the most objectionable ingredient, and should be 
restricted to as small an amount as possible. Cast irons having less than 
0.3 per cent, of combined carbon are generally found to be of high magnetic 
permeability. Fig. 8 shows curves and analyses of three different grades 
of cast iron. The effect of different proportions of combined carbon may be 
ascertained by comparison of the results with the accompanying analyses. 
In Fig. 9 is given the result of the test of a sample carried up to very high 
saturation. It is useful for obtaining values corresponding to high 
magnetisation, but as shown by the analysis and also by the curve, it is a 
sample of rather poor cast iron, the result being especially bad at low 
magnetisation values. The cast iron generally used for magnetic purposes 
would be between curves B and C of Fig. 8. 

Graphite may vary between 2 per cent, and 3 per cent, without 
exerting any very marked effect upon the permeability of cast iron. It is 
generally found that when the percentage of graphite approximates to the 
lower limit, there is an increase in the amount of combined carbon and a 
corresponding decrease of permeability. A certain percentage of carbon is 
necessary, and it is desirable that as much of it as possible should be in the 
graphitic state. Sulphur is generally present, but only to a limited extent. 
An excess of sulphur is an indication of excessive combined carbon, and 
inferior magnetic quality. Silicon in excess annuls the influence of sulphur, 
and does not seem to be objectionable until its amount is greater than 
2 per cent., its effect being to make a casting homogeneous, and to lessen 
the amount of combined carbon. The amount of silicon generally varies 
between 2.5 per cent, in small castings, and 1.8 in large castings. Phos 
phorus in excess denotes an inferior magnetic quality of iron. Although in 
itself it may be harmless, an excess of phosphorus is accompanied by an 
excess of combined carbon, and it should be restricted to 0.7 per cent, or 
0.8 per cent. Manganese, in the proportions generally found, has but 
little effect ; its influence becomes more marked in irons that are low in 
carbon. 

Figs. 10 and 11 show further data relating to irons shown in Fig. 8, 
grades A and C respectively. 

Malleable Cast Iron. When cast iron is decarbonised, as in the 
process for making it malleable, in which a portion of the graphite is 



1 Arnold, "Influence of Carbon on Iron," Proc. Inst. C.E., vol. cxxiii., page 156. 



Magnetisation Curves of Iron. 



to K 

s. 2 W iuj 

Ll U 3 i<0 

31& S 




20 Electric Generators. 

eliminated, there is a marked increase in the permeability. This is due, 
however, to the change in the physical structure of the iron which accom 
panies the decarbonisation, as unmalleable cast iron, of chemical analysis 
identical with that of malleable iron, has but a fraction of the permeability. 
In Fig. 12 are shown the magnetic properties of malleable cast iron ; 
Fig. 1 3 illustrates the magnetic properties of mixtures of steel and pig iron. 

Cast Steel. The term " cast steel," as used in this place, is intended to 
refer to recarbonised irons, and not to the processes of manufacture where 
there has been no recarbonisation, as in irons made by the steel process. 
Cast steel used for magnetic purposes has been generally made by the open- 
hearth or Siemens-Martin process, the principal reason being that this 
process has been more frequently used for the manufacture of small cast 
ings. The Bessemer process could, perhaps, be used to greater advantage 
in the manufacture of small castings than the open-hearth process, since, on 
account of the considerable time elapsing between the pouring of the first 
and last castings, there is frequently by the open-hearth process a change 
of temperature in the molten steel, and likewise a noticeable difference in 
the magnetic quality. In the Bessemer process the metal can be main 
tained at the most suitable temperature, and the composition is more easily 
regulated. 

Cast steel is distinguished by the very small amount of carbon present 
which is in the combined state, there being generally no graphite, as in the 
case of cast iron, the exception being when castings are subjected to great 
strains, in which case the combined carbon changes to graphite. It may be 
approximately stated that good cast steel, from a magnetic standpoint, 
should not have greater percentages of impurities than the following : 

Per Cent. 
Combined carbon ... ... ... ... ... ... ... 0.25 

Phosphorus ... ... ... 0.08 

Silicon 0.20 

Manganese ... 0.50 

Sulphur 0.05 

In practice, carbon is the most objectionable impurity, and may be 
with advantage restricted to smaller amounts than 0.25 per cent. The 
results of a great number of tests and analyses show that the decrease in 
the permeability is proportioned to the amount of carbon in the steel, other 
conditions remaining equal ; that is, that the other elements are present in 
the same proportion, and that the temperature of the molten steel is 



Magnetisation Curves of Iron and Steel. 



21 




HONlOS U3d S3NHOTIM 



22 



Electric Generators. 



increased according to the degree of purity. Cast steel at too low a 
temperature considering the state of purity, shows a lower permeability 
than would be inferred from the analysis. Manganese in amounts less than 
0.5 per cent, has but little effect upon the magnetic properties of ordinary 
steel. In large proportions, however, it deprives steel of nearly all its 
magnetic properties, a 12 per cent, mixture scarcely having a greater 
permeability than air. Silicon, at the magnetic densities economical in 
practice, is less objectionable than carbon, and at low magnetisation 
increases the permeability up to 4 or 5 per cent. ; x but at higher densities it 
diminishes the permeability to a noticeable extent. The objection to 
silicon is that when unequally diffused it facilitates the formation of blow 
holes and, like manganese, has a hardening effect, rendering the steel 
difficult to tool in machining. Phosphorus and sulphur, in the amounts 
specified, are not objectionable ; but in excess they generally render the 
steel of inferior magnetic quality. 

In Tables I. and II. are given the analyses and magnetic proper 
ties of what may be termed good and poor steel respectively. In Fig. 14, 
curves A and B represent the average values corresponding to these two 
sets of tests. 

The extent to which the percentage of phosphorus affects the result, 
may be seen from the curves of Fig. 15. The curves of Fig. 16 show the 
deleterious effect of combined carbon upon the magnetic properties. The 
magnetic properties of steel are further illustrated in Figs. 17, 18, and 19. 

TABLE I. DATA OF TEN FIRST QUALITY SAMPLES OF CAST STEEL. 





Kilolines per Square Inch. 


Ampere- Turns per 




Inch of Length. 


1. 


2. 


3. 


4. 


5. 


6. 


7. 


8. 


9. 


10. 


Average. 


30 


78.6 


77.5 


78.0 


83.2 


84.0 


79.4 


84.5 


78.0 


81.4 


84.0 


80.9 


50 


91.0 


87.7 


89.6 


93.0 


94.2 


89.6 


93.5 


88.5 


91.5 


93.5 


91.2 


100 


102 


98.6 


100 


102 


107 


100 


104 


99.4 


102 


103 


101.8 


150 


107 


104 


107 


106 


113 


106 


110 


105 


108 


107 


107.3 



A nalysis. 



Carbon ... 


.240 


.267 


.294 


.180 


.290 


.250 


200 


.230 


.170 


.180 


.230 


Phosphorus 


.071 


.052 


.074 


.047 


.037 


.093 


.047 


.100 


.089 


.047 


.057 


Silicon ... 


.200 


.236 


202 


.120 


.036 


.230 


.173 


.160 


.150 


.120 


.195 


Manganese 


.480 


.707 


.655 


.323 


.550 


.410 


.530 


.450 


.390 


.323 


.482 


Sulphur ... 


.040 


.060 


.050 


.050 


.050 


.030 


.030 


.040 


.020 


.050 


.042 



1 See Electrical World, December 10th, 1898, page 619. 



Magnetisation Curves of Cast Steel. 



23 




H3NI 6S H3d S3Nno^l)l 



24 Electric Generators. 

TABLE II. DATA OP TEN SECOND QUALITY SAMPLES OP CAST STEEL. 



Ampere -Turns per 
Inch of Length. 


Kilolines per Square Inch. 


1. 


2. 


3. 


4. 


5. 


6. 


7. 


8. 


9. 


10. 


Average. 


30 


68.3 


68.3 


69.0 


58.0 


60.0 


64.5 


67.0 


64.5 


60.0 


73.0 


65.3 


50 


82.0 


82.0 


84.5 


72.2 


74.8 


78.0 


80.5 


80.0 


76.0 


87.0 


79.7 


100 


96.0 


94.1 


97.5 


87.0 


89.6 


92.2 


92.9 


94.8 


91.0 


101 


93.6 


150 


102 


100 


102 


92.8 


96.0 


98.7 


98.7 


101 


96.5 


106 


99.4 



Analysis. 



Carbon ... 


.250 


.280 


.195 


.333 


.337 


.366 


.409 


.318 


.702 


.380 


.357 


Phosphorus 


.087 


.076 


.028 


.059 


.045 


.151 


.063 


.107 


.084 


.066 


.077 


Silicon ... 


.210 


.210 


.683 


.292 


.302 


.476 


.444 


.203 


.409 


.550 


.378 


Manganese 


.790 


.720 


.815 


.681 


.642 


.617 


.640 


1.636 


.088 


.790 


.742 


Sulphur... 


.020 


.030 


.040 


.060 


.070 


.010 


.010 


.030 


.050 


.030 


.038 



Mitis Iron. In Table III. are given analyses and magnetic properties 
of aluminium steel, frequently referred to as " mitis iron." The action 

TABLE III.- DATA OF TWELVE SAMPLES OP MITIS IRON. 



Ampere-Turns per 



Kilolines per Square Inch. 



Inch or Length. 


1. 


2. 


3. 


4. 


5. 


6. 


7. 


8. 


9. 


10. 


11. 


12. 


Aver 
age. 


30 
50 
100 
150 


81.3 

87.6 
95.5 
100 


93.5 
100 
109 
114 


93.5 
101 
108 
113 


82.0 
93.5 
104 
109 


89.6 
96.8 
105 
110 


91.5 
101 
108 
112 


90.3 
98.6 
106 
110 


69.6 
81.6 
92.0 
98.0 


64.5 

76.7 
89.5 
95.5 


83.1 
92.2 
102 

108 


82.0 
92.2 
103 
108 


76.0 

86.5 
96.5 
101 


83.1 
92.3 
101.5 
106.5 



Analysis. 



Carbon ... 


.065 


.105 


.106 


.125 


.136 


.212 


.214 


.216 


.235 


.241 


.242 


.260 


.180 


Phosphorus 


.083 


.093 


.112 


.166 


.053 


.056 


.052 


.128 


.065 


.093 


.094 


.120 


.093 


Silicon ... 


.073 


.045 


.050 


.046 


.111 


.126 


.111 


.083 


.122 


.072 


.099 


.020 


.080 


Manganese 


.112 


.108 


.099 


.120 


.191 


.405 


.401 


.167 


.107 


.248 


.253 


.140 


.196 


Sulphur ... 


.150 


.050 


.050 


.050 


.030 


.040 


.040 


.010 


.030 


,030 


.030 


.030 


.045 


Aluminium 


.079 


* 


.059 


.183 


.008 


.273 


* 


.152 


.055 


.120 


.119 


.080 


.113 



Not determined. 



of aluminium in steel is, like that of silicon, sulphur, or phosphorus, 
of a softening nature. It seems to act more powerfully than silicon, the 
castings having a somewhat greater degree of purity and a higher 
magnetic quality than steel castings made by processes of equal refinement. 
It will be seen from the analyses that the aluminium is present in amounts 
ranging from 0.05 per cent, to 0.2 per cent., and that this permits of making 



Magnetic Properties of Iron and Steel. 25 

good castings with about one-half as much silicon and manganese as in 
ordinary cast steel. The amount of carbon, also, is generally somewhat 
less. An inspection of these tests and analyses of mitis iron shows that 
they do not furnish a clear indication as to the effect of the various impurities. 
It will be noticed, however, that in those of poor magnetic qualities there is 
generally an excess of impurities, this excess denoting a lack of homogeneity 
and a greater degree of hardness than in those of good quality. 

Mitis iron is, magnetically, a little better than ordinary steel up 
to a density of 100 kilolines, but at high densities it is somewhat 
inferior. The magnetic result obtained from mitis iron up to a density of 
100 kilolines is practically identical with that obtained from wrought-iron 
forgings. 

A curve representing the average of the twelve samples of Table III., 
is given in Fig. 20. 

Xickel Steel. Some of the alloys of steel with nickel possess remark 
able magnetic properties. 1 A 5 per cent, mixture of nickel with steel, 
shows a greater permeability than can be accounted for by the analysis of 
the properties of the components. The magnetic properties of nickel alloys 
are shown in Fig. 2 1. 2 

Forgings. Forgings of wrought iron are, in practice, found to be 
of uniform quality and of high magnetic permeability. In curves A and B 
of Fig. 22 are shown the magnetic properties of wrought iron, nearly 
pure, and as generally obtained, respectively. The former is made by 
the steel process at the Elswick Works of Messrs. Sir W. G. Armstrong 
and Co., Limited, but owing to its excessively high melting point, 
it is only manufactured for exceptional purposes. Curve D illustrates an 
inferior grade of wrought iron, its low permeability being attributable 
to the excess of phosphorus and sulphur. Curve C shows the properties 
of a forging of Swedish iron, in the analysis of which it is somewhat 
remarkable to find a small percentage of graphite. 

For the wTought-iron forgings and for the sheet iron and sheet steel 
generally used, curve B should preferably be taken as a basis for calcula 
tions, although the composition of the sheets will not be that given 



1 For information as to the remarkable conditions controlling the magnetic properties of 
the alloys of nickel and iron, see Dr. J. Hopkinson, Proc. Royal Soc., vol. xlvii., page 23 ; 
and vol. xlviii., page 1. 

Various investigations have shown that the permeability of steel is greatly lessened by 
the presence of chromium and tungsten. 

E 



26 



Electric Generators. 




HONI 6S U3d S3NHOTIU 




6S H3d S3NHOTIH 




S3NHOTDI 



Magnetic Properties of Iron and Steel. 



27 



by the analysis. The composition of some samples of sheet iron and 
sheet steel, the results of tests of which are set forth on pages 30 to 32, 
is given in Table IV. Such material however is subject to large 
variations in magnetic properties, due much more to treatment than to 
composition. 

TABLE IV. ANALYSIS OP SAMPLES. 



Brand. 


Silicon. 


Phosphorus. 


Manganese. 


Sulphur. 


Carbon. 


I. 


.019 


Not determined 


.490 


Not determined 


.120 


IT. 


.007 


Not determined 


.420 


Not determined 


.062 


III. 


.009 


.083 


.510 


.026 


.056 


IV. 


.003 


Not determined 


.570 


Not determined 


.044 


V. 


trace 


.029 


.020 


trace 


.050 


VI. 


.005 


.059 


.500 


.048 


.040 


VII. 












VIII. 


.003 


.018 


.490 


.014 


.052 


IX. 












X. 













In comparing wrought-iron forgings with unforged steel castings, 
Professor Ewing notes 1 that the former excel in permeability at low 
densities, and the latter at high densities. This he illustrates by the 
curves reproduced in Fig. 23, in which are given results for Swedish 
wrought iron and for a favourable example of unforged dynamo steel 
by an English maker. He states that annealed Lowmoor iron would 
almost coincide with the curves for Swedish iron. 

Professor Ewing further states that there is little to choose between 
the best specimens of unforged steel castings and the best specimens 
of forged ingot metal. The five curves of Fig. 24 relate to results of 
his own tests, regarding samples of commercial iron and steel. Of these 
curves, A refers to a sample of Lowmoor bar, forged into a ring, annealed 
and turned ; B to a steel forging furnished by Mr. K. Jenkins as a 
sample of forged ingot metal for dynamo magnets ; C to an unforged 
steel casting for dynamo magnets made by Messrs. Edgar Allen and Co. 
by a special pneumatic process ; D to an unforged steel casting for 
dynamo magnets made by Messrs. Samuel Osborne and Co. by the Siemens 
process ; E to an unforged steel casting for dynamo magnets made by 
Messrs. Friedrich Krupp, of Essen. 2 



1 Proc. lust. Civil Engineers, May 19th, 1896. 

2 Proc. Inst. of Civil Engineers, May 19th, 1896. 



28 



Electric Generators. 



ENERGY LOSSES IN SHEET IRON. 

The energy loss in sheet iron in an alternating or rotating magnetic 
field consists of two distinct quantities, the first being that by hysteresis or 
inter-molecular magnetic friction, and the second that by eddy currents. 
The loss by hysteresis is proportional to the frequency of the reversal of 
the magnetism, but is entirely independent of the thickness of the iron, and 
increases with the magnetisation. There is no exact law of the increase of 
the hysteresis with the magnetisation, but within the limits of magnetisa 
tion obtaining in practice, and those in which such material can be pro 
duced to give uniform results, the energy loss by hysteresis may be taken 




A LOW MOOR BAR, FORCED INTO RING, ANNEALED fc TURNED. 
B - STEEL FOROINO FOR DYNAMO MAGNETS. 
C- - CASTING - - .. UNFORCED 

D - UNFORCED STEEL CASTINGS (SIEMENS PROCESS) 

(KRUPP) 



GO GO 100 TZO 14O 160 1SO 200 

AMPERE TURNS PER INCH OF LENGTH. 



to increase approximately with the 1.6 power of the magnetisation, as was 
first pointed out by Mr. C. P. Steinmetz. 1 

Professor Ewing and Miss Klaassen, 2 however, from a large number of 
tests, found the 1.48 power to be better representative at the densities 
generally met in transformers. Other extensive tests point to the 1.5 
power as the average. 3 

The hysteresis loss is independent of the temperature at ordinary 
working temperatures, but from 200 deg. Cent, upward the loss decreases 
as the temperature increases, until at 700 deg. Cent, it has fallen to as low 
as from 10 per cent, to 20 per cent, of its initial value. Obviously this 



1 Elec. Eng., New York, vol. x., page 677. 

2 Electrician, April 13th, 1894. 
8 Elec. World, June 15th, 1895. 



Energy Losses in Sheet Iron. 29 

decrease at very high temperatures is of no commercial importance at the 
present time. 1 

The magnitude of the hysteresis loss is somewhat dependent upon the 
chemical composition of the iron, but to a far greater degree upon the 
physical processes to which the iron is subjected. 

Annealing of Sheet Iron. The temperature at which sheet iron is 
annealed has a preponderating influence upon the nature of the results 
obtained. Extended experiments concerning the relation of hysteresis loss 
to temperature of annealing, show that the higher the temperature the 
lower the hysteresis loss up to about 950 deg. Cent. 2 Beyond this 
temperature deleterious actions take place ; the surfaces of the sheets 
become scaled, and the sheets stick together badly. A slight sticking 
together is desirable, as it insures the iron having been brought to the 
desired high temperature, and the sheets are easily separated ; but soon 
after passing this temperature (950 deg. Cent.), the danger of injuring the 
iron becomes great. 

Curves A and B of Fig. 25 show the improvement effected in two 
different grades of iron, by annealing from high temperatures. 3 

Deterioration of Sheet Iron. It has been found that the hysteresis 
loss in iron increases by continued heating. 4 No satisfactory explanation 
of the cause of this deterioration has yet been given. Its amount depends 
upon the composition of the iron, and upon the temperature from which it 
has been annealed. The best grades of charcoal iron, giving an exceed 
ingly low initial loss, are particularly subject to deterioration through so- 



1 Tech. Quarterly, July, 1895; also Elek. Zeit., April 5th, 1894; also Phil. Mag., Septem 
ber, 1897 ; also in a very complete and valuable paper by D. K. Morris, Ph.D., " On the Mag 
netic Properties and Electrical Resistance of Iron as dependent upon Temperature," read 
before the Physical Society, on May 14th, 1897, are described a series of tests of hysteresis, 
permeability, and resistance, over a wide range of temperatures. 

2 This temperature depends somewhat upon the composition of the iron, being higher the 
more pure the iron. 

3 In this and much of the following work on hysteresis and on the properties of insulating 
materials, the authors are indebted to Mr. Jesse Coates, of Lynn, Mass., and to Messrs. 11. 
C. Clinker and C. C. Wharton, of London, for valuable assistance in the carrying out of tests. 

4 " On Slow Changes in the Magnetic Permeability of Iron," by William M. Mordey, 
Proceedings of the Royal Society, January 17th, 1895 ; also Electrician, December 7th, 1894, 
to January llth, 1895. A recent very valuable contribution to this subject has been made 
by Mr. S. R. Roget, in a paper entitled "Effects of Prolonged Heating on the Magnetic 
Properties of Iron," read before the Royal Society, May 12th, 1898. It contains some very 
complete experimental data. 



Electric Generators. 



called " ageing." Iron annealed from a high temperature, although more 
subject to loss by " ageing," generally remains superior to the same grade of 
iron annealed from a lower temperature. This was the case in the tests 
corresponding to Figs. 26 and 27, but there are many exceptions. 

Table V. shows the results of " ageing " tests at 60 deg. Cent, 
on several different brands of iron. It will be noticed that in the 
case of those brands subject to increase of hysteresis by " ageing," the 
percentage rise of the annealed sample is invariably greater than that of the 




FLg.27. ACID 
ncLKriouarteiiHG of IRON TO / 







at esi> c. ^_ 












35 . 










I 














ANALYSIS. 










SILICON. 

I PHOSPHORUS 


> ... .-.Of 

..*; 

-01 


a 






[ CARSON... . 



E 










zoo too eoo BOO 1000 tzoo 1*00 tea 



0- + i -S t.l 



JFt9^J6.BASIC OPEN-HEARTH STEEL 



E! 







Sampll 


^T~ 






















s 


-J^A 


















eis^L 


J2iL 


sL2ifc 


t 














^ 




















ANALYSIS. 








SILICON :OO8 
PHOSPHORUS... -.1 OO 
MANGANESE -SIS. 
SULPHUR rO3S 
CARBON :O8I 


























z 


oo too eoo too looo izot> HOO ifo 


TO A TiHrimTum or so-c 




ANALYSIS 



SILICON 0031 

PHOSPHORUS.. . oas 

MANGANESE gl 

SULPHUR- 026 

CARBON. 056 



unannealed sample, and that often the annealed sample ultimately becomes 
worse than the unannealed samples. 

Brands III., V., and VI., are the same irons whose " ageing " records are 
plotted in Figs. 28, 31, and 29 respectively. 

From these investigations it appears that iron can be obtained which 
will not deteriorate at 60 deg. Cent., but that some irons deteriorate rapidly 
even at this temperature ; and that at a temperature of 90 deg. Cent, even 
the more stable brands of iron deteriorate gradually. Consequently, so far 
as relates to avoidance of deterioration through " ageing," apparatus, even 
when constructed with selected irons, should not be allowed to reach a 
temperature much above 60 deg. Cent. 



" Ageing " of Sheet Iron. 



31 



TABLE V. RESULTS OP TESTS ON AGEING OP IRON. 

(From Tests by R. C. Clinker, London, 1896-7.) 

Temperature of ageing = 60 deg. Cent., except where otherwise stated. 
The chemical analyses of these samples are given in Table IV., on page 27. 



Brand of Iron. 


Hysteresis Loss in Watts per pound at 100 Cycles per Second, 
and 24,000 Lines per Square Inch. 


Increase in 1000 Hours. 




j 

!3 

a 

h- 1 


After Ageing for 


200 400 
Hours. Hours. 


600 
Hours. 


800 
Hours. 


1000 
Hours. 


I. 

Unannealed 
Annealed 


1.00 
0.41 


1.00 
0.43 


1.00 
0.43 


1.00 
0.43 


1.00 
0.43 


1.00 
0.43 


per cent. 


5 


II. 

Unannealed 
Annealed 


0.46 
0.39 


0.46 
0.39 


0.46 
0.40 


0.46 
0.41 


0.46 
0.42 


0.46 
0.43 



10 


III. 

Unannealed 
Annealed 


0.38 
0.33 


0.38 
0.33 


0.38 
0.33 


0.38 
0.33 


0.38 
0.37 


0.38 
0.39 




18 1 


IV. 

Unannealed 
Annealed 


0.86 
0.42 


0.90 
0.50 


0.94 

0.58 


0.97 
0.66 


1.01 
0.74 


1.04 
0.83 


21 
98 


V. 

Unannealed 
Annealed 


0.35 
0.36 


0.40 
0.40 


0.43 
.45 


0.45 
0.50 


0.47 
0.53 


0.49 
0.55 


40 
53 


VI. 

Unannealed 
Annealed 


0.65 
0.39 


0.71 
0.41 


0.83 
0.49 


1.00 
0.62 


1.09 
0.78 


1.19 
0.90 


83 
130 


VII. 

Unannealed 
Annealed 


0.80 
0.43 


0.82 
0.44 


0.82 
0.45 


0.82 
0.45 


0.82 
0.45 


0.82 
0.45 


3 
6 


VIII. 

Unannealed 
Annealed 


0.36 
0.31 


0.36 
0.32 


0.36 
0.34 


0.36 
0.35 


0.37 
0.35 


0.37 
0.35 


3 
13 


IX. 


0.58 


0.58 


0.58 


0.58 


0.60 


0.64 


10 2 


X. 


0.42 


0.42 


0.42 


0.43 


0.47 


0.56 


33 3 



1 Temperature raised to 90 deg. after 600 hours. 

2 Temperature raised to 90 deg. after 650 hours. 

3 Temperature raised to 90 deg. after 670 hours. 



32 Electric Generators. 

An examination of the results indicates that a rather impure iron gives 
the most stable result. It is believed that by annealing from a sufficiently 
high temperature, such impure iron may be made to have as low an initial 
hysteresis loss as can be obtained with the purest iron. The lower melting 
point of impure iron, however, imposes a limit ; for such iron cannot, in 
order to anneal it, be brought to so high a temperature as pure iron, 





4-7 

4 

o-e 
o-s 

0-3 
O.Z 








^ 


^ 


, 


-o- 













^o- 





/ 


^ 












ANALYSIS. 


f 

TEMP? C 


FACEIN 


,-60 C 




CARBOH O-OS 
SILICON- .TRACE 

MANGANESE - -.Q-OZ 
SULPHUR .TRACE 
PHOSPHORUS- 0-029 









































ss 



Kef 


3Z 
















^^t 


\<2 










/ 


i 




leaL~ 




- -O 






^ 














TE 


MP* OF 


ACCING 


SO C 




V 





























MIT../ 1 * 



2500 3000 



o xsa soo iso 1000 aso iroo vso 

u HOURS 

Saix. 



because the surface softens and the plates stick together at comparatively 
low temperatures. 

The curves of Figs. 30, 31, and 32 represent the results of interesting 
^ageing " tests. In Fig. 30 the effect of a higher temperature upon the 
annealed sample is clearly shown. 

Effect of Pressure. Pressure and all mechanical strains are injurious 
even when of no great magnitude, as they decrease the permeability and 
increase the hysteretic loss. Even after release from pressure, the iron only 
partly regains its former good qualities. In the curves of Fig. 33 is shown 



Properties of Shwf Iron. 



33 



the effect of applying pressure to two different grades of iron, the measure 
ments having been made after the removal of the pressure. 

Another interesting case is that shown in the curves A, B, and C, 
of Fig. 34. These show the results of tests upon a certain sample of sheet 
iron, as it was received from the makers, after it had been annealed, and 



0-9 



EFFECT OF PRES8UREUPON THE 
HYSTERESIS-LOSS OFSHEET- 
JRON SUBSEQUENT TO THE 
REMOVAL O 



SILICON .QC9 

PHOSPHORUS __;090 

MANGANESE, -4-74 

SULPHUR... _.__ -O4-O 
CARBON.. _-O72 



4OOO 8OOO 1200 16OO ZOOO 24OO 
PRESSURE IN LBS. PER SQ. INCH. 



CARBON :O4 

PHOSPHORUS ....-II 7 

MANGANESE -368 

SILICON .-202 

SULPHUR . .. -I 




4 6 8 -JO 12 14 16 

AMPERE TURNS PER INCH OF LENGTH. 



after being subjected to a pressure of 40,000 Ib. per square inch, respectively. 
It will be seen that the annealing in this case materially increased the per 
meability, but that subjecting the sample to pressure diminished the per 
meability below its original value. 

The value of the hysteresis losses while the iron is still under pressure 
is probably much greater. Mr. Mordey refers to a case in which a pressure 



34 



Electric Generators. 



of 1,500 Ib. per square inch was accompanied by an increase of 21 per cent, 
in the core loss. Upon removing the pressure, the core loss fell to its original 
value. 1 Re-annealing restores iron which has been injured by pressure, to 
its original condition. 

This matter of injury by pressure, particularly so far as relates to the 
increase while the iron remains under pressure, is one of considerable im 
portance, and in assembling armature and transformer sheets, no more tem 
porary or permanent pressure should be used than is essential to good 
mechanical construction. 

Hysteresis Loss. The curves of Fig. 35 give values for the hysteresis 
losses that can be obtained in actual practice. Curve B is for sheet steel 




These results should be increased I0?.for transfi 

via, less than 50 Ibs of Iron & S/. For those. ith From 50 U 
100 Ibf. Also niqktr losses with Iron not pruper/y annea/eol 
tWe C hascrWHein ok tL ~ g 



~H- test 



iple., 



Orthodox. Values forfcldv Current Lost in Sheet 
F 10 Microhm, ier 



bnprtur \ 

Actual losses inTransFormcrs.diie bo tdoly Curre 
SQ7.txlt>Ofi in excefS of thtbc values 




such as should be used for transformer construction, and all iron used in 
transformer work should be required to comply with these values. For 
transformer work, iron of .014 in. thickness is generally used. 

For armature iron there is no occasion for such exacting requirements, 
and curve A is representative of the armature iron generally used. Iron 
for armatures is usually .025 in. to .036 in. in thickness. Curve C gives 
the best result yet secured by Professor Ewing. It was from a strip of 
transformer plate .013 in. thick, rolled from Swedish iron. 2 Its analysis was : 

Per Cent. 

Carbon .02 

Silicon .032 

Manganese ... ... ... ... ... ... ... ... trace only. 

Phosphorus .020 

Sulphur ... ... .003 

Iron (by difference) ... ... ... ... ... ... 99.925 

1 "On Slow Changes in the Magnetic Permeability of Iron," by William M. Mordey, 
Proceedings of the Royal Society, January 17th, 1895. 

2 Proceedings q/ the Institution of Civil Engineers, May 19th, 1896. 



Properties of Sheet Iron. 35 

This iron ages very rapidly. The iron of Fig. 28 is only 6 per cent, 
worse initially when annealed, and at 60 deg. Cent, it does not deteriorate. 
Its analysis has already been given. 



EDDY CURRENT LOSSES. 

In sheet iron the eddy current losses should theoretically conform to 

the formula : l 

W = 1.50 x t 2 x N 2 x B 2 x 10- 10 - 
in which 

"W = watts per pound at deg. Cent. 

t = thickness in inches. 
N = periodicity in cycles per second. 
B = density in lines per square inch. 

The loss decreases .5 per cent, per degree Centigrade increase of 
temperature. The formula holds for iron, whose specific resistance is 
10 microhms per centimetre cube, at deg. Cent., and which has a weight 
of .282 Ib. per cubic inch. These are representative values for the grades 
used, except that in sheet steel the specific resistance is apt to be consider 
ably higher. 

Curves giving values for various thicknesses of iron are shown in 
Fig. 36. 

Owing possibly to the uneven distribution of the flux, particularly at 
the joints, the observed eddy current losses are, in transformer iron, from 
50 to 100 per cent, in excess of these values, even when the sheets are 
insulated with Japan varnish or otherwise. 

Estimation of Armature Core Losses. With regard to the use of 
curve A in the estimation of armature core losses, the values obtained from 
curve A may for practical purposes be considered to represent the 
hysteresis component of the total loss. To allow for other components of 
the total core loss, the values obtained from curve A should be multiplied 
by from 1.3 to 2.5, according to the likelihood of additional losses. Briefly, 
this large allowance for eddy current losses in armature iron is rendered 
necessary owing to the effect of machine work, such as turning down, 
filing, &c., these processes being destructive to the isolation of the plates 
from each other. 

1 For thicknesses greater than .025 in., magnetic screening greatly modifies the result. 
Regarding this, see Professor J. J. Thomson, London, Electrician, April 8th, 1892. Professor 
Ewing, London, Electrician, April 15th, 1892. 



36 



Electric Generators. 



The curves in Fig. 36 are chiefly useful for transformer work, and are 
of little use in armature calculation, as they refer only to the eddy current 
Tosses due to eddy currents set up in the individual isolated sheets, and 
in armatures this often constitutes but a small part of the total loss. 

The irons used for magnetic purposes have approximately the resis 
tance and density constants given in Table VI. ; in which are also given, 
for comparison, the corresponding values for very pure iron and for com 
mercial copper : 

TABLE YI. 





Spt cific Resis 










tance at deg. 


Increase in 


Specific 


Pounds per 




per Centimetre 


deg. Cent. 


Gravity. 


Cubic Inch. 




Cube. 












per cent. 






Cast i ron ... 


100 


.1 


7.20 


.260 


Cast steel 


20 


A 


7.80 


.282 


Wrought iron and very mild steel 


10 


.5 


7.80 


.282 


Nearly pure iron ... 


9 


.6 








Commercial copper 


1.6 


.388 


8.90 


.322 



Mr. W. H. Preece gives the Table, reproduced below, of values 
(Munroe and Jameson Pocket-book), which shows in a striking manner the 
dependence of the specific resistance of iron upon the chemical composition. 

TABLE VII. PREECE S TESTS OF ANNEALED IRON WIRE. 



Number of Sample 


1. 


2. 


3. 


4. 


O. 


6. 


7. 


8. 


Carbon 


0.09 


0.10 


0.15 


0.10 


0.10 


0.15 


0.44 


0.62 


Silicon 


trace 


trace 


0.018 


trace 


0.09 


0.018 


0.028 


0.06 


Sulphur ... 




0.022 


0.019 


0.035 


0.03 


0.092 


0.126 


0.074 


Phosphorus 


o!bi2 


0.045 


0.058 


0.034 


0.218 


o .077 


0.103 


0.051 


Manganese 


0.06 


0.03 


0.234 


0.324 


0.234 


0.72 


1.296 


1.584 


Copper 


trace 


trace 


trace 


trace 


0.015 


trace 


ti ace 


trace 


Iron 


99.69 


99.70 


99.44 


99.60 


99.11 


98.74 


98. :0 


97 41 


Ohm mile at 60 deg. Fahr. 


4546 


4502 


4820 


5308 


5974 


6163 


7468 


/ f i J. 

8033 


Specific resistance (microhms per 


















cubic centimetre at deg. Cent.) 
Specific resistance in microhms per 


9.65 


9.60 


10.2 


11.3 


12.7 


13.1 


15.9 


17.1 


cubic inch at deg. Cent. 


3.80 


3.78 


4.02 


4.45 


5.00 


5.15 


6.25 


6.75 


Resistance wire 1 ft. long and 


















.001 in. in diameter at deg. Cent. 


57.9 


57.5 


61.2 


67.7 


76.2 


78.5 


955 


103.0 



No. 1. Swedish charcoal iron, very soft and pure, 
i) 2. ,, ,, good for P. 0. speci- 

tication. 

3. ,, ,, not suited for P. 0. 

specification. 



No. 4. Swedish Siemens-Martin steel 0.10 carbon. 

,, 5. Best puddled iron. 

,, 6. Bessemer steel, special soft quality. 
7. ,, ,, hard quality. 

,, 8. Best cast steel. 



Specific Conductivity of Iron and Steel. 37 

Although prepared in connection with telegraph ami telephone work, it is 
of much significance to transformer builders, and points to the desirability 
of using as impure iron as can, by annealing, have its hysteresis loss 
reduced to a low value, since the higher specific resistance will proportion 
ately decrease the eddy current loss. Such comparatively impure iron will 
also be nearly free from deterioration through prolonged heating. Of 
course its lower melting point renders it somewhat troublesome, owing to 
the plates tending to stick together when heated to a sufficiently high tem 
perature to secure good results from annealing. Transformer builders in 
this country have generally used iron of some such quality as that of 
sample No. 1, and have been much troubled by " ageing." Most trans 
formers in America have been built from material whose chemical compo 
sition is more like Samples 4, 5 and 6, and the transformers have been very 
free from " ageing." At least .4 per cent, of manganese should be present, 
owing to its property of raising the specific resistance. 

Reference should here be made to a paper by M. H. Le Chatelier, read 
before FAcademie des Sciences, June 13th, 1898, in which is given very 
useful data regarding the influence of varying percentages of carbon, silicon, 
manganese, nickel, and other elements, upon the electrical resistance of 
steels. The results relating to the influence of varying percentages of 
of carbon, silicon, and manganese are of especial importance, and are con 
sequently reproduced in the following Tables : 

TABLE VIII. INFLUENCE OF CARBON. 

Speci6c Resistance in Microhms Composition. 

per Centimetre Cube. C. Mn. Si. 

10 O.OG ... 0.13 .. 0.05 

12.5 ... 0.20 0.15 0.08 

14 0.49 ... 0.24 ... 0.05 

16 0.84 0.24 0.13 

18 1.21 0.21 0.11 

18.4 1.40 ... 0.14 0.09 

19 1.61 ... 0.13 0.08 

TABLE IX. INFLUENCE OF SILICON. 

Resistance in Microhms per Composition. 

Centimetre Cube. C. Si. 

12.5 0.2 ... 0.1 

38.5 0.2 ... 2.6 

15.8 0.8 0.1 

26.5 ... 0.8 ... 0.7 

33.5 ... 0.8 ... 1.3 

17.8 ... 1.0 0.1 

25.5 ... 1.0 ... 0.6 

32.0 1.0 1.1 



ietre Cube. 


C. 


17.8 


0.9 


22 


0.9 


24.5 


1.2 


40 


1.2 


66 magnetic 
80 non-magnetic 1 


I 1 - 



38 Electric Generators. 

TABLE X. INFLUENCE OF MANGANESE. 

Resistance in Microhms per Composition. 

Mn. Si. 

0.24 ... 0.1 

0.95 ... 0.1 

0.83 ... 0.2 

1.8 ... 0.9 

13. 0.3 



INSULATING MATERIALS. 

The insulating materials used in dynamo construction vary greatly, 
according to the method of use and the conditions to be withstood. 
The insulation in one part of a dynamo may be subjected to high electrical 
pressures at moderate temperatures ; in another part to high temperatures 
and moderate electrical pressures ; in still another part to severe mecha 
nical strains. No one material in any marked degree possesses all the 
qualities required. 

Mica, either composite or solid, has been very largely used on 
account of its extremely high insulating qualities, its property of with 
standing high temperatures without deterioration, and its freedom from 
the absorption of moisture. In the construction of commutators mica 
is invaluable. The use of mica, however, is restricted, on account of 
its lack of flexibility. 

Moulded mica, i.e., mica made of numerous small pieces cemented 
together, and formed while hot, has been used to insulate armature coils 
as well as commutators. Its use, however, has not been entirely satis 
factory, on account of its brittleness. 

Composite sheets of mica, alternating with sheets of paper specially 
prepared so as to be moisture proof, have been found highly suitable 
for the insulation of armature and field-magnet coils. The following 
Table shows roughly the electrical properties of composite sheets of 

white mica : 

TABLE XI. 

Thickness. Puncturing Voltage. 

0.005 ... ... 3,600 to 5,860 

0-007 ... ... ... 7,800 10,800 

0.009 ... ... ... 8,800 11,400 

0-011 ... ... 11,600 14,600 

1 In another paper by the same author are set forth results showing the influence of 
tempering upon the electric resistance of steel. Comptes Rendus de I Academie des Sciences, 
June 20th, 1898. 



Properties of Insulating Materials. 



39 



The other materials that have been found more or less satisfactory, 
according to method of preparation and use, are linen soaked with linseed 
oil and dried ; shellaced linen, which is a better insulator than oiled 
linen, but liable to be irregular in quality and brittle ; oiled bond- 
paper, which is fairly satisfactory when baked ; " press board," which 
shows very good qualities, and has been -used with satisfaction to insulate 
field-magnet coils. 

Where linseed oil is to be employed, the material should be 
thoroughly dried before applying the oil. 

Red and white vulcanised fibres are made by chemically treating 
paper fibre. They have been used as insulators with varying success, 
the main objection to them being their decidedly poor mechanical qualities, 
so far as warping and shrinking are concerned. This is due to their 
readiness to absorb moisture from the air. Baking improves the insu 
lating qualities, but renders the substance brittle. Whenever it is 
necessary to use this material, it should be thoroughly painted to render 
it waterproof. The insulating quality varies according to the thickness, 
but good vulcanised fibre should withstand 10,000 volts in thicknesses 
varying from ^ in. to 1 in., this puncturing voltage not increasing with 
the thickness, owing to the increased difficulty of thoroughly drying 
the inner part of the thick sheets. 

Sheet leatheroid possesses substantially the same qualities, and 
is made according to the same processes as vulcanised fibre. A thickness 
in this material of ^ in. should safely withstand 5,000 volts, and should 
have a tensile strength of 5,000 Ib. per square inch. 

TABLE XIT.- TESTS ox SHEETS OF LEATHEROID. 





Insulation Strength. 


Thickness. 


Total Volts. 


Volts pt-r Mil. 


in. 






BT 


5,000 


320 


^3" 


8,000 


256 


3 

7T 


12,000 


256 


T? 


15,000 


240 


4 


15,000 


120 


3 
TT 


6,000 


32 


i 


6,000 


24 



With such materials as vulcanised fibre and sheet leatheroid, 
increase in thickness is not necessarily accompanied by increased 



40 Electric Generators. 

insulation resistance, owing to the difficulty of obtaining uniformity 
throughout the thickness of the sheet. This is well shown in the tests 

O 

of leatheroid sheets of various thicknesses, given in the preceding Table. 

Hard rubber in various forms is sometimes useful, owing to its 
high insulating qualities. Its use is restricted, however, from the fact 
that at 70 deg. Cent, it becomes quite flexible, and at 80 deg. Cent, it 
softens. 

Hard rubber should stand 500 volts per mil. thickness. Sheets 
and bars of hard rubber should stand bending to a radius of 50 times 
their thickness, and tubes to a radius of 25 diameters. 

Slate is used for the insulation of the terminals of dynamos, &c. 
Ordinarily good slate will, when baked, withstand about 5000 volts per 
inch in thickness. 

The chief objection to slate is its hygroscopic quality, and it requires 
to be kept thoroughly dry ; otherwise, even at very moderate voltages, 
considerable leakage will take place. Where practicable, it is desirable 
to boil it in paraffin until it is thoroughly impregnated. 

Slate is, moreover, often permeated with metallic veins, and in 
such cases is quite useless as an insulator. Even in such cases its 
mechanical and fireproof properties make it useful for switchboard and 
terminal-board work, when re-eriforced by ebonite bushings. 

Marble has the same faults as slate, though to a less extent. 

Kiln-dried maple and other woods are frequently used, and will 
stand from 10,000 to 20,000 volts per inch in thickness. 

The varnishes used for electrical purposes should, in addition to other 
insulating qualities, withstand baking and not be subject to the action 
of oils. Of the varnishes commonly used, shellac is one of the most 
useful. There are a number of varnishes on the market, such as Insullac, 
P and B paint, Sterling Varnish, Armalac, &c. 

One of the special insulating materials readily obtainable that has 
been found to be of considerable value is that known as " vulcabeston," 
which will withstand as high as 315 deg. Cent, with apparently no 
deterioration. This material is a compound of asbestos and rubber, 
the greater proportion being asbestos. \ 7 ulcabeston, ordinarily good, 
will withstand 10,000 volts per J in. of thickness. 

As results of tests, the following approximate values may be 
taken : 

Red press-board, .03 in. thick, should stand 10,000 volts. It should 



Properties of Insulating Materials. 41 

bend to a radius of five times its thickness, and should have a tensile 
strength along the grain of GOOD Ib. per square inch. 

Red rope paper, .01 in. thick, having a tensile strength along the 
grain of 50 Ib. per inch of width, should stand 1000 volts. 

Manilla paper, .003 in. thick, and having a tensile strength along 
the grain of 200 Ib. per inch of width, should stand 400 volts. 

TESTS ON OILED FABRICS. 

Oiled cambric .007 in. thick stood from 2500 to 4500 volts. 

cotton .003 6300 7000 

paper .004 ,, 3400 4800 

.010 5000 volts. 

A number of composite insulations are in use, consisting generally 
of split mica strips pasted with shellac on to sheets of some other 
material. The principal ones are : 

1. Insulation consisting of two sheets of .005 in. thick red paper, with 
one thickness of mica between them, the whole being shellaced together 
into a compound insulation .015 in. thick. This stands on the average 
3,400 volts. 

2. Combined mica and bond-paper of a thickness of .009 in. had a 
breaking strength of from 2,000 to 3,000 volts. 

3. Composition of mica and canvas. Mica strips are pasted together 
with shellac on to a sheet of canvas, and covered with another sheet of 
canvas shellaced on. The mica pieces are split to be of approximately the 
same thickness about .002 in. and lapped over each other for half their 
width, and about -g- in. beyond, so as to insure a double thickness of mica at 
every point. Each row of strips is lapped over the preceding row about 
i in. 

The sheets thus prepared are hung up and baked for 24 hours before 
use. The total thickness should be taken at about .048 in., using canvas 
.013 in. This will stand about 3,000 RM.S. volts. 

4. Composition of mica and longcloth, made up with shellac in the 
same manner as preceding material. 

5. White cartridge paper shellaced on both sides, and baked for 12 
hours at 60 deg. Cent. The total thickness is .012 in., and it will stand 
about 1,500 volts per layer. 

It will doubtless have been observed that the quantitative results 
quoted for various materials are not at all consistent. This is probably in 



42 



Electric Generators. 



part due to the different conditions of test, such as whether tested by con 
tinuous or alternating current ; and if by alternating current the form 
factor and periodicity would effect the results, and it should have been 
stated whether maximum or effective (E.M.S.) voltage was referred to. 
Continuous application of the voltage will, furthermore, often effect a 
breakdown in samples which resist the strain for a short interval. It is 
also of especial importance that the material should have been thoroughly 
dried prior to testing; though on the other hand, if this is accomplished 
by baking, as would generally be the case, the temperature to which it is 
subjected may permanently affect the material It thus appears that to be 
thoroughly valuable, every detail regarding the accompanying conditions 
and the method of test should be stated in connection with the results. 

The importance of these points has only gradually come to be 
appreciated, and the preceding results are given for what they are worth. 
It is true that some tests have been made which are more useful and 
instructive, and various materials are being investigated exhaustively as 
rapidly as practicable. Such tests are necessarily elaborate and expensive 
and tedious to carry out, but it is believed that no simple method will give 
a good working knowledge of the insulating properties of the material. 

TABLE XIII. SUMMARY OF QUALITY OF INSULATING MATERIALS. 





Electrical. 


Thermal. 


Mechanical. 


Hygroscopic. 




Mica 


Excellent 

j) 
Very poor 
Good 
Fair 
Good 


Excellent 
Good 


Excellent 
Poor 
Good 

)> 
Excellent 

Good 
Fair 

;> 


Poor 
Good 

>> 
> 
>> 
Poor 

5) 

Fail- 
Poor 


Excellent 
Fair 
Poor 

Good 

>> 
Poor 
Fair 
Poor 


Hard rubber 
Slate 


Marble ... ... 


Vulcabeston 
Asbestos 


Vulcanised fibre ... 
Oiled linen 
Shellaced linen 



EFFECT OF TEMPERATURE UPON INSULATION RESISTANCE. 

The resistance of insulating materials decreases very rapidly as the 
temperature increases, except in so far as the high temperature acts to 
expel moisture. Governed by these considerations, it appears that the 
apparatus should, so far as relates to its insulation, be run at a sufficiently 
high temperature to thoroughly free its insulation from moisture. The 



Testing Insulating Materials. 43 

great extent of these changes in insulation resistance is very well shown in 
the accompanying curve (Fig. 37) taken from an investigation by Messrs. 
Sever, Monell and Perry. 1 It shows for the case of a sample of plain 
cotton duck, the improvement in insulation due to the expulsion of 
moisture on increasing the temperature, and also the subsequent 
deterioration of the insulation at higher temperatures. 



DESCRIPTION OF INSULATION TESTING METHODS FOR FACTORIES. 

The subject of testing insulating materials can be approached in two 
ways, having regard either to the insulation resistance or to the disruptive 




strength. Messrs. Sever, Monell and Perry, in the tests already alluded 
to, measured the former, but for practical purposes the latter is often 
preferable. 

Various methods of testing insulating materials have been devised 
from time to time ; but after many experiments on different lines the 
following has been evolved, and has been found very suitable for investi 
gations in factory work. The apparatus required consists of: 

1. A special step-up transformer for obtaining the high potential 
from the ordinary alternating current low potential circuits. The design 
of this transformer is illustrated in Figs. 38 and 39, which are fully 
dimensioned. 



1 " Effect of Temperature on Insulating Materials," American Institute of Electrical 
Engineers, May 20th, 1896. 



44 



Electric Generators. 



2. A water rheostat for regulating the current in the primary of the 
transformer. This consists of a glass jar, containing two copper plates 
immersed in water, the position of the upper one being adjustable. 

3. A Kelvin electrostatic voltmeter, of the vertical pattern , for 
measuring the effective voltage on the secondary of the transformer. 

4. A testing board for holding the sample to be tested. This, as 
shown in Figs. 40 to 43, consists of two brass discs ^ in. thick and 1^ in. in 
diameter, the inside edges of which are rounded off to prevent an excess of 
intensity at these points. These are pressed together against the sample 
by two brass strips, which also serve to apply the voltage to the 



lJ 





discs. The pressure between the discs is just enough to hold the sample 
firmly. 

5. An oven for keeping the sample at the required temperature. It 
consists (as shown in Fig. 44) of a wooden box containing a tin case. There 
should be an inch clearance between the two, which should be tightly filled 
with asbestos packing all round, except at the front where the doors are. 
The tin case is divided horizontally by a shelf, which supports the testing 
board, while beneath is an incandescent lamp for heating the oven. Holes 
are drilled at the back to admit the high potential leads and lamp leads, 
and there is a hole in the top to admit a thermometer. 

Adjustment of the temperature is made by having a resistance in series 
with the lamp, the amount of which can be adjusted till enough heat is 
generated to keep the temperature at the required value. 



Transformer for Insulation Testing. 45 



DESCRIPTION OF STEP-UP TRANSFORMER. 

Core. The core is of the single magnetic circuit type, and is built up 
of iron punchings lj in. by 7f in., and Ij in. by 4|- in., for sides and ends 
respectively, and .014 in. thick. Every other plate is japanned, and the 
total depth of punchings is 3^ in., giving with an allowance of 10 per cent. 
for lost space, a net depth of iron of 2.92 in., and a net sectional area of 
3.65 square inches. With an impressed E.M.F. of form factor = 1.25, the 
density is 36.4 kilolines per square inch. 

The primary and secondary coils are wound on opposite sides of the 
core on the longer legs. 

Primary Coils. The primary consists of two coils form-wound, and 
these were slipped into place side by side. The conductor is No. 13 
S.W.G. bare = .092 in. in diameter. Over the double cotton covering it 
measures .103 in., the cross-section of copper being .0066 square inch. 
Each coil consists of 75 turns in three layers, giving a total of 150 primary 
turns. 

Secondary Coils. The secondary is wound in six sections on a wooden 
reel, with flanges to separate the sections, as shown in Figs. 38 and 39. 
The conductor is No. 33 S.W.G. bare, .010 in. in diameter. Over the 
double silk covering it measures .014 in., the cross-section of copper being 
.000079 square inch. Each coil consists of 1,600 turns, giving a total of 
9,600 secondary turns. 

Insulation. The primary coils are wrapped with a layer of rolled tape 
(white webbing) 1 in. by .018 in. half lapped and shellaced before being put 
on the core ; they are slipped over a layer of " mica -canvas " on the leg. 
The secondary coils are wound direct on the wooden reel, which is 
shellaced ; they are covered outside with two or three layers of black tape 
(1 in. by .009 in.), shellaced. 

Advantage of this Type for Insulation Tests. By having the primary 
and secondary on different legs, the advantage is gained that, even on short 
circuit, no great flow of current occurs, because of the magnetic leakage. 

Connection Boards. The transformer is mounted on a teak board, on 
which are also placed the secondary connection posts, as shown in Fig. 45. 
The primary leads are brought to another teak board, which is for con 
venience mounted on the top of the transformer. This board is fitted with 
fuses. 



46 Electric Generators. 

A number of samples may be tested simultaneously by connecting the 
testing boards in parallel, as shown in the diagram of connections given in 
Fkr. 45. A is a single-pole switch in the main secondary circuit, and 

O ^ * 

B, B, B are single-pole switches in the five branches. 

The method of test is as follows : A number of samples 4 in. square 
are cut from the material to be tested, and are well shuffled together. Five 
samples are taken at random, placed between the clips of the testing boards 
within the ovens, and brought to the temperature at which the test is 
to be made. They should be left at this temperature for half an hour 
before test. 

The apparatus may, of course, be modified to suit special requirements; 
but, as described, it has been used and found suitable for investigations 
on the disruptive voltage of various materials. 

As an example of such an investigation, we give one in Table XIV. 
that was made to determine the effect of different durations of strain and 
different temperatures on the disruptive strength of a composite insu 
lation known as mica-canvas. 

Two hundred samples, measuring 4 in. by 4 in., were cut and well 
shuffled together, in order to eliminate variations of different sheets. 
Before test, all samples were baked for at least 24 hours at 60 deg. Cent. 

METHOD OF TEST. 

Five samples were placed between the clips of the testing boards, 
and the voltage on the secondary adjusted by the water rheostat to 
2,000 volts, as indicated by a static voltmeter. Switch A was open 
and switches B, B, B closed (Fig 45). Switch A was now closed for 
five seconds, and if no sample broke down the voltage was raised to 3,000, 
and Switch A again closed for five seconds. This application of the voltage 
is practically only momentary, as the capacity current of the samples 
brings down the voltage slightly because of magnetic leakage in the 
transformer, five seconds not being a long enough interval to admit of re 
adjusting the pressure to the desired value. 

When any sample broke down, as indicated by the voltmeter needle 
dropping back to zero, it was disconnected from the circuit by its 
switch, B ; it being easy to determine which sample had broken do\vn 
by lifting switches B, B, B, one by one, till one of them drew out 
an arc. 



Insulation Tests. 



47 



The remainirg samples were then subjected to the next higher 
voltage, and so on until all five samples had broken down. 

TABLE XIV. INSULATION TESTS ; MICA-CANVAS. 
Temperature 25 deg. Cent. 



Effective 
Voltage 
Impretss d 


Duration 5 Seconds. 


Duration 10 Minutes. 


Duratioa 30 Minutes. 


Number of Samples Unpierced. 


Number of Samples Unpierced. 


Number of Samples Unpierced. 


2000 


5 


5 5 


5 


percent. 
100 


5 


5 


5 


5 


percent. 
100 


5 


5 


5 


percent. 
5 100 


3000 


5 


5 5 


5 


100 


5 


5 


5 


5 


100 


5 


5 


5 


5 100 


4000 


5 


5 4 


5 


95- 


5 


5 


5 


5 


100 


5 


3 


3 


3 70 


4500 


5 


5 . 4 


5 


95 4 


2 


5 


5 


80 


5 


2 


9 


3 60 


5000 


4 


5 4 


5 


90 


1 


1 


3 


3 


40 


4 


1 


} 


1 35 


5500 


4 


4 3 


5 


80 








3 


2 


25 


2 








10 


6000 


3 


2 




3 


50 





2 


1 


15 


2 








10 


6500 


3 


1 ! 2 


1 


35 





2 





10 


1 





5 


7000 


1 


1 


10 


1 





5 


1 


o 


5 


7500 





110 


5 














1 








5 


8000 





1 


5 














1 





5 



Temperature 60 deg. Cent. 



2000 


5 


5 


5 


5 


100 


5 5 


5 


5 


100 


5 


5 


5 


5 


100 


3000 


5 


5 


5 


5 


100 


5 5 


5 


5 


100 


5 


5 


5 


5 


100 


4000 


5 


3 


5 


4 


85 


4 2 


2 


5 


65 


1 


4 


2 


4 


55 


4500 


5 


3 


5 


3 


80 


1 2 


2 


3 


40 


1 


3 


2 


4 


50 


5000 


3 


2 


5 


o 


60 


1 


1 


o 


2 


30 





3 


1 


4 


40 


5500 


1 


2 


5 


1 


45 








1 


5 





3 





2 


25 


6000 








5 


1 


30 











1 





1 


10 


6500 



































8 


5 


7000 












































7500 
































8000 
































Temperature 100 deg. Cent. 


2000 


5 


5 


5 


5 


100 5 


5 


5 5 


100 


5 


5 5 


5 


100 


3000 


5 


5 


5 


5 


100 5 4 


5 


5 


100 


555 


5 


100 


4000 


4 


5 


5 


4 


90 


4 


4 


5 


5 


90 


250 


4 


60 


4500 


4 


5 


4 


4 


85 


3 


3 


3 


3 


60 


1 


3 


2 


35 


5000 


2 


5 


3 


4 


70 


2 


2 3 


o 


45 


1 








5 


5500 


1 


5 


2 


3 


55 


1 


1 2 


2 


30 





, 








6000 


1 


3 


1 


2 


35 


1 


1 1 





15 












6500 





1 





1 


10 1000 


5 












7000 

















00 













7500 































A series of four tests, as above, were taken, making a total of 
twenty samples tested under the same conditions. 



48 



Electric Generators. 



A set of twenty samples was tested with the impressed voltage kept 
constant for ten minutes, and another set, in which it was kept constant 
for thirty minutes. 

A complete series of tests was made under the above three con 
ditions at three different temperatures 25 deg. Cent., 60 deg. Cent., 
and 100 deg. Cent. The samples were left in ovens for at least half 




MICA CANVAS 



IICA CANVAS 









\ 


\ 








Fi 


gM. 




\ 










TSfftt 

unmet 


rune ei 

T DVKi 


c 

TIOHS 




^\ 


\ 


^ 














v\ 


\ 














> 


-:-. 


^_ 


**~ <tj -iooo 2000 3000 woo seoo eooo noo at 
trrec-TiYt VCLTAGZ /r/ BtssfO 




an hour, at approximately the right temperature, before being tested. 
The temperature during test did not vary more than 10 per cent. 

The results of these tests are given in the Table above, and they 
are plotted as curves in Figs. 46 to 51, the effective (R.M.S.) voltage 
impressed as abscissae, and the percentage of samples not broken down 
at that voltage as ordinates. In Figs. 46, 47, and 48 curves are 
plotted for same temperatures and different durations, while in Figs. 49, 



Insulation Tests of Materials. 



50, and 51 they are plotted for different temperatures for the same 
duration. 

As the form of the electromotive force wave would affect the results, 
and as it was impracticable to keep account of the same, the current 
being supplied by Thomson-Houston and Brush alternators running 
in parallel and at various loads, the effects were eliminated as much as 
possible by making tests on different sets of samples on different days. 

It is evident from the results obtained that 3000 R.M.S. volts 



MICA CANVAS 



MICA CANVAS 




*t/j moo aooo 3000 woo sow eooo 7000 BOOO 

tFFECTW VOiTAGC. IHHIES&CD 



5 
^ 



3 



FCg.5C. 



MICA CANVAS 







PERCENTAGE Of SAMPLES UNPIERCED 

it * ft n 6 

<s S S o 5 


MICA CANVAS 








\ 










Fu 


T.S7. 




v 

\\ 


\ 








DURA 
UFKt 


1C ft 30 K 
CHT TZ* 


INUTZS 
fVMTUK 


V 


\X 


















\\ 














\ 
\ 


V 








^ 



mrtiesfD 



is the limit of safe-working voltage of this material under all conditions 
tried. 

It would also appear from curves in Figs. 46, 47, and 48, that 
with the momentary application of the voltage, the material does not 
have time to get so strained as for a longer duration of the applied 
voltage, and that between the ten-minute and thirty-minute durations the 
difference is not so marked. 

From curves in Figs. 49, 50, and 51, it seems that in the case of 
this material the temperature does not have much effect on the disrup 
tive voltage, although at 60 deg. and 100 deg. the shellac becomes 
softened, and the sample may be bent back on itself without cracking. 

H 



50 



Electric Generators. 



HO ?3~ldlHVS JO 33VJ.N33U3d 




XO 



MO SJIdWVS JO 39VJLN33U3d 



Insulation Tests of Materials. 



51 



A corresponding set of tests was made on material called " mica long- 
cloth," which differed from the "mica-canvas" only in the nature of the cloth 
upon which the mica was mounted. The "long-cloth" is an inexpensive 
grade of linen serving merely as a structure upon which to build the mica. 

The mode of manufacture is the same as that of " mica-canvas," except 




ZOOO 3OCO 4OOO f>OOO 6000 

EFFECTIVE VOLTAGE IMPRESSED. 



DURATION 3O MINUTES 
DIFFERENT TEMPERATURES 




WOO 300O 4OOO MOO 6JOO 

EFFECTIVE VOLTAGE IMPRESSED. 



that the sheets of "long-cloth" are first impregnated with shellac and then 
dried. The mica is then put on in the same manner as with the " mica- 
canvas." The " long-cloth" is .0052 in. thick, and the mica varies from 
.001 in. to .009 in., but averages .002 in. The total thickness of the "mica 
long-cloth" completed, averages .025 in. This includes two sheets of 
" mica long-cloth," with interposed mica, the mica having everywhere at 



52 



Electric Generators. 



least a double thickness. When made up, the sheets were placed for three 
or four hours in an oven at 60 deg. Cent. The sheets were then cut up 
into samples measuring 4 in. by 4 in., and were again baked for twenty- 
four hours before testing. 

TABLE XV. MICA LONG-CLOTH. 

Temperature, 25 deg. Cent. 



Effective Voltage 


Duration 5 Seconds. 


Duration 10 Minutes. 


Duration 30 Minutes. 


Impressed. 


Number of Samples K. 


Number of Samples O K. 


Number of Samples K. 












Per 

Cent. 








Per 
Cent. 










Per 
Cent. 


2000 


5 


5 


5 


5 


100 


5 


5 


5 5 


100 


5 


5 


5 


5 


100 


3000 


5 


5 


5 


5 


100 


5 


5 


5 5 


100 


5 


5 


5 


5 


100 


4000 


5 


5 


5 


5 


100 


4 


4 


5 5 


90 


5 


5 


4 


5 


95 


4500 


4 


5 


5 


5 


95 


4 


3 


3 5 


75 


4 


5 


3 


5 


85 


5000 


4 


5 


5 


4 


90 


3 


2 


1 2 


40 


2 


1 


3 


4 


50 


5500 


3 


2 


5 


3 


65 


2 


1 


1 ! 1 


25 








2 


4 


30 


6000 


2 


2 


4 


2 


50 





























6500 





2 


2 


1 


25 





























7000 





2 


1 





15 





























7500 





1 








5 





























8000 





1 








5 






























Temperature, 60 deg. Cent. 



2000 


5 


5 


5 


5 


100 


5 


5 


5 5 


wo* 


5 


5 


5 


5 


100 


3000 


5 


5 


5 


5 


100 


5 


5 


5 5 


100 


5 


5 


5 


5 


100 


4000 


5 


5 


5 


5 


100 


5 


5 


5 5 


100 


4 


5 


5 


5 


95 


4500 


5 


5 


5 


5 


100 


3 


3 


1 5 


60 


2 


2 


1 


2 


35 


5000 


4 


4 


3 


5 


80 


1 


2 


1 


3 


35 





2 








10 


5500 


3 


4 


2 


3 


60 











2 


10 

















6000 


1 


3 


2 


2 


40 
































6500 


1 


2 





1 


20 
































7000 


1 


1 








10 





























7500 





1 








5 





























8000 





1 








5 [ 





























Temperature, 100 deg. Cent. 


2000 


5 


5 


5 


5 


100 


5 


5 


5 


5 


100 


5 


5 


5 


5 


100 


3000 


5 


5 


5 


5 


100 


5 


5 


5 


5 


100 


5 


5 


5 


5 


100 


4000 


5 


4 


5 


5 


95 


D 


5 


4 


5 


95 


5 


3 


3 


3 


70 


4500 


5 


4 


5 


5 


95 


4 


4 


2 


5 


75 


4 





3 





35 


5000 


4 


3 


4 


3 


70 


3 


1 


2 


3 


45 


1 





1 





10 


5500 


3 


2 


3 


1 


45 


2 





2 





20 

















6000 


1 


1 


1 


1 


20 
































6500 











1 


5 
































7000 















































7500 
















































The results which are given in the Table and plotted as curves, show 
much the same character as those for " mica-canvas," the limit of safe 
working being about 3,000 R.M.S. volts as before. The results as plotted 



Insulation Tests of Materials. 



53 



in the curves support the former conclusion, that with five seconds duration 
of the application of the voltage, the material is not so much strained as 
by longer applications. As before, also, the temperature does not appear to 
affect the disruptive voltage. 

These tests show the material to be quite as good electrically as " mica- 
canvas," nothing being gained by the extra thickness of the latter. The 
" mica-canvas " and the " mica long-cloth" had the same thickness of mica, 
but the canvas is so much thicker than the " long-cloth " as to make the 
total thickness of the "mica-canvas" .048 in., as against a thickness of 
only .025 in. for the "mica long-cloth." The insulation strength is 
evidently due solely to the mica. 

TABLE XVI. SHELLAC D PAPER (Two Sheets). 
Temperature, 25 deg. Cent. 



Effective Voltage 
Impressed. 


Duration, 5 Seconds. 


Duration, 10 Minutes. 


Duration, 30 Minutes. 


Number of Samples O K. 


Number of Samples O K. 


Number of Samples K. 


2500 
3000 
3500 
4000 
4500 
5000 


5 

5 
4 
3 

2 




5 
5 

4 

2 

1 




5 
5 
4 
3 
2 




5 
5 
4 
3 
1 



Per 
Cent. 

100 
100 
80 
55 
30 



5 

5 
4 
3 
1 




5 
5 
5 
2 




5 
5 
2 
1 




5 
5 
3 
1 




Per 

Cent. 

100 
100 
70 
35 
5 



5 
5 
4 






5 
5 
4 
1 




5 
4 

2 






p. 

Cent. 
5 100 
5 100 

5 75 
5 

: 
s 



Temperature, 60 deg. Cent. 



2500 
3000 
3500 
4000 
4500 
5000 



2500 
3000 
3500 
4000 
4500 
5000 



5 


5 


5 


5 


100 i 5 


5 


5 


5 


100 5 


5 


5 


5 ilOO 


4 


5 


4 


5 


90 


5 


3 


5 


5 


90 4 


4 


4 


5 


85 


4 


4 


3 


4 


75 


2 


3 


3 


3 


55 2 


2 


3 


2 


45 


2 


3 


3 


3 


55 


1 











5 








1 


5 


1 


2 





2 


25 





















































i 















Temperature, 100 deg. Cent. 





5 


5 


5 5 


100 


5 


555 


100 


5 


5 


5 


5 


100 




3 


3 


4 4 


70 


2 


2 1 2 


35 


1 


3 


2 


2 


40 




2 1 


3 


2 


40 


2 





j 





15 


1 


2 





2 


25 










1 


1 


10 


1 


000 


5 










































































































In the following set of tests the same method of procedure was 
employed, the material in this case being so-called " Shellac d Paper," 
which consists of cartridge paper about .010 in. thick, pasted with shellac 
on both sides and then thoroughly baked. The average thickness when 
finished is about .012 in. This material is often used as insulation between 
layers of the windings of transformers, in thicknesses of from one to three 



54 



Electric Generators. 




SSIdlNVS JO 39VJ.N33U3d 



Insulation Tests of Materials, 



55 



sheets, according to the voltage per layer. It was found convenient to 
test two sheets of the material together, in order to bring the disruptive 
voltage within the range of the voltmeter. The use of two thicknesses also 
tended to produce more uniform results. As will be seen, the duration of 
the application of the voltage, and the temperature up to 100 deg. Cent., 
exert a slight but definite influence upon the results. But at 100 deg. 
Cent, the shellac becomes quite soft. 

The tests show that this material withstands a little over 1000 R.M.S. 
volts per single sheet, although in employing it for construction, a factor of 
safety of two or three should be allowed under good conditions, and a still 
higher factor for the case of abrupt bends and other unfavourable conditions. 

Further tests showed the disruptive strength of this material to be 
proportional to the number of sheets. 

Curves and Tables are given below of the results obtained in similar 
tests on a material known as " Red Paper." It is .0058 in. thick, and is of 
a fibrous nature, and mechanically strong ; hence especially useful in 
conjunction with mica, to strengthen the latter. 

TABLE XVII. RED PAPER (Four Sheets). 
Temperature, 25 deg. Cent. 



Effective Voltage 
Impressed. 


Duration 5 Seconds. 


Duration 10 Minutes. 


Duration 30 Minutes. 


Number of Samples K. 


Number of Samples K. 


Number of Samples K. 


2500 
3000 
3500 
4000 
4500 
5000 


5 
5 
5 
4 




5 
5 
4 





5 
5 
5 
1 




Per 
Cent. 

5 100 
5 100 
5 95 
3 40 




5 
5 
3 





5 

5 
4 





5 
5 

5 





5 
5 
1 






Per 
Cent. 

100 
100 
65 





5 
5 
2 

1 




5 
5 
4 





5 
5 
2 






5 
5 




o 


Per 
Cent. 

100 
100 

40 
5 





Temperature, 60 deg. Cent. 



2500 


5 


5 


5 


5 


100 


5 


5 


5 


5 


100 


5 


5 


5 


5 


100 


3000 


5 


5 


5 


4 


95 


5 


5 


5. 


5 


100 


4 


2 


2 


5 


65 


3500 





1 


2 


1 


20 


3 


1 


1 





25 





1 


1 1 


15 


4000 












































4500 















































5000 
















































Temperature, 100 deg. Cent. 



2500 


5 


5 


5 


5 


100 


5 


5 


5 


5 


100 


5 


5 


5 


5 


100 


3000 


5 


5 


5 


5 


100 


3 


2 


2 


3 


50 i 3 


3 


2 


1 


45 


3500 


2 


3 


2 


3 


50 


1 











5 





1 








5 


4000 















































4500 















































5000 
















































56 



Electric Generators. 




Insulation Tests of M.iteriftJs. 57 

The method of test was the same as that employed in the case of the 
preceding set of tests on " Shellac d Paper ; " and for the reasons set forth 
in those tests, it was found in this case convenient to test four sheets of 
the material together. 

An examination of the curves and Tables will show that the limit of 
safe working is 2,500 R.M.S. volts for four sheets, or 625 volts for a single 
sheet, other tests having been made which showed the breakdown pressure 
to be proportional to the number of sheets. 

It also appears from the curves, that " Red Paper " has a more uniform 
insulation strength than the materials previously tested. As in the case of 
" Shellac d Paper," it showed weakening of the insulation at a temperature 
of 100 deg. Cent. 

From tests such as the four sets just described, very definite conclu 
sions may be drawn. For instance, if it were desired to use " mica-canvas " 
as the chief constituent of the main insulation of a 2,000 volt transformer, 
which should withstand an 8,000 volt breakdown test, between primary and 
secondary, for one half hour, three layers of this composite insulation would 
be sufficient and would probably be inserted ; though the chances would be 
in favour of its withstanding a 10,000 or 12,000 volt test if due attention 
is given to guarding against surface leakage, bending and cracking and 
bruising of insulation, and other such matters. A comparison with the 
tests on "mica long-cloth," would, however, show that a given insulation 
strength could be obtained with a much thinner layer. 

There are on the market patented composite materials giving still 
better results. But they are expensive, and hence it is often impracticable 
to use them. 

In designing electrical machinery, similar tests of all insulating 
material to be used should be at hand, together with details of their 
mechanical, thermal, and other properties, and reasonable factors of safety 
should be taken. 

Armature coils are often insulated by serving them with linen or 
cotton tape wound on with half-lap. A customary thickness of tape is 
.007 in., and the coil is taped with a half over-lap, so that the total 
thickness of the insulation is .014 in. The coils are then dipped in some 
approved insulating varnish, and baked in an oven at a temperature of 
about 90 deg. cent. These operations of taping, dipping, and drying, are 
repeated a number of times, until the required amount of insulation is 
obtained. It has been found in practice that a coil treated in this manner, 

i 



58 Electric Generators. 

and with but three layers of .007-in. tape (wound with half over-lap), dipped 
in varnish twice after the first taping, once after the second, and twice after 
the third, i.e., five total dippings, and thoroughly baked at 90 deg. cent, 
after each dipping in varnish, withstands a high potential test of 
5,000 R.M.S. volts, which is considered sufficient for machines for not over 
600 volts. Armature coils insulated in the above manner are generally 
placed in armature slots lined with an oil-treated cardboard of about .012 in. 
in thickness ; but this contributes but little to the insulation strength, 
serving rather to protect the thin skin of varnish from abrasion when 
forcing the coil into the armature slot. In this treatment of the coils, 
great care must be taken to see that the taping be not more than one half 
over-lap, and that the varnish does not become too thick through evapora 
tion of the solvent. All coils should be thoroughly dried and warmed 
before dipping, as the varnish will then penetrate farther into them. The 
slot parts of coils are dipped in hot paraffin and the slots lined with oil- or 
varnish-treated cardboard, to prevent abrasion of the insulations. The 
greatest of care should be used in selecting insulating varnishes and com 
pounds, as many of them have proved in practice to be worthless ; a 
vegetable acid forming in the drying process, which corrodes the copper 
through the formation of acetates or formates of copper which in time lead 
to short-circuits in the coil. Some excellent preparations have their 
effectiveness impaired by unskilful handling. If, for instance, the first coat 
of the compound is not thoroughly dried, the residual moisture corrodes 
the copper and rots the insulations. By far the best method of drying is 
by the vacuum hot oven. By this method, the coils steam and sweat, and 
all moisture is sucked out. A vacuum oven, moreover, requires a much 
lower temperature, consequently less steam, and very much less time. 
Such an oven is almost a necessity where field spools have deep metal 
flanges, for in the ordinary oven, in such cases, the moisture simply cooks 
and steams, but does not come out. Cases have occurred where spools 
have been kept in an ordinary drying oven for ten days at a temperature of 
90 deg. cent., and then the spools had to be further dried with a heavy 
current to sweat the moisture out. Field spools may be treated with tape 
and varnished in the same manner as armature coils, thus doing away with 
the needless metal flanges, and also saving space. 

As further instances of taping and varnishing, may be cited the 
cases of some coils treated with the same kind of tape and varnish as 
already described. In one case, a half over-lapped covering of .007-in. 



Method* of Insulating Coih. 59 

tape, giving a total thickness of .014 in., had seven successive dippings and 
bakings, resulting in a total thickness of tape and varnish of .035 in. 
Coils thus insulated withstood 6,000 R.M.S. volts. An insulation suitable 
for withstanding 15,000 R.M.S. volts consists in taping four times with 
half over-lap, and giving each taping three coats of varnish, making in all, 
eight layers of .007-in. tape, and 12 layers of varnish. The total thickness 
of insulation was then about .09 in. The quality of the tape, the thick 
ness of the varnish, and the care in applying and drying the varnish, play 
an important part. 

One disadvantage of this method of insulating armature coils by 
taping and impregnating with varnish and baking, consists in the brittleness 
of the covering ; and a coil thus treated should preferably be warmed 
before pressing it into place on the armature. 

Other methods of treating coils, such as dipping the slot part of the 
coil in shellac and then pressing it in a steam-heated press form, thus 
baking the slot part hard and stiff , have the advantage of rendering the 
coils less liable to damage in being assembled on the armature, and also 
make the coils more uniform in thickness. Coils thus pressed are sub 
sequently taped and dipped in the way already described. 

Coils may be treated in a vacuum, to a compound of tar and linseed oil, 
until they become completely impregnated. They are then forced into 
shape under high pressure. Coils thus prepared cannot be used in 
rotating armatures, as the centrifugal force tends to throw the com 
pound out. 



60 Electric Generators, 



ARMATURE WINDINGS. 

CONTINUOUS-CURRENT ARMATURE WINDINGS. 

In the design of dynamo machines a primary consideration is with 
respect to the armature windings. Many types have been, and are, at 
present employed, but the large continuous-current generators now most 
extensively used for power and lighting purposes, as well as in the numerous 
other processes where electrical energy is being commercially utilised on a 
large scale, are constructed with some one of a comparatively small number 
of types of winding. Although the many other types may be more or less 
useful in particular cases, it will not be necessary for our present purpose to 
treat the less-used types. 

The windings generally used may be sub-divided into two chief classes 
one, in which the conductors are arranged on the external surface of a 
cylinder, so that each turn includes, as a maximum, the total magnetic flux 
from each pole, termed drum windings ; the other, in which the conductors 
are arranged on and threaded through the interior of a cylinder, so that 
each turn includes as a maximum only one-half of the flux from each magnet 
pole ; this is known as the Gramme, or ring winding. 

One of the chief advantages of the Gramme winding is that the volt 
age between adjacent coils is only a small fraction of the total voltage, 
while in drum-wound armatures the voltage between adjacent armature coils 
is periodically equal to the total voltage generated by the armature. On 
account of this feature, Gramme windings are largely used in the armatures 
of arc-light dynamos, in which case the amount of space required for 
insulation would become excessive for drum windings. There is also the 
practical advantage that Gramme windings can be arranged so that each 
coil is independently replaceable. 

Gramme-ring windings have been used with considerable success in 
large lighting generators, the advantage in this case being that the armature 
conductors are so designed that the radial ends of each turn at one side of 
the armature are used as a commutator ; and with a given number of con 
ductors on the external surface of the cylinder, the number of the commu 
tator bars is twice as great as in the drum-wound armature an important 



Continuous- Current Armature Windings. 



01 



feature in the generation of large currents. Having one commutator 
segment per turn, the choice of a sufficient number of turns keeps the 
voltage per commutator segment within desirably low limits. The use of 
a large number of turns in such cases, while permitting the voltage per 
commutator segment to be low, would entail high armature reaction, mani 
fested by excessive demagnetisation and distortion, if the number of poles 
should be too small ; but by the choice of a sufficiently large number of 
poles, the current per armature turn may be reduced to any desired extent. 
While it is necessary to limit the armature strength in this way, the cost 




of the machine is at the same time increased, so that commercial consider 
ations impose a restriction. 

Fig. 70 is an outline drawing of the armature and field of a 12-pole 
400-kilowatt Gramme-ring lighting generator, of the type just described. 
Machines of this type have been extensively used in large central stations 
in America, and it is one of the most successful types that have ever been 
built. 

In small machines where, instead of two-face conductors, there is often 
a coil of several turns between adjacent commutator segments, the Gramme 
ring is, on the score of mechanical convenience, inferior to the drum wind 
ing ; since, in the case of the latter, the coils may be wound upon a form, 
and assembled afterwards upon the armature core. This is only made 



62 Electric Generators. 

practicable in the case of a Gramme ring, by temporarily removing a 
segment of the laminated core. This plan has obvious disadvantages. 

These two practical classes of windings, Gramme ring and drum, may 
be subdivided, according to the method of interconnecting the conductors, 
into "two-circuit" and "multiple-circuit" 1 windings. In the two-circuit 
windings, independently of the number of poles, there are but two circuits 
through the armature from the negative to the positive brushes ; in the 
multiple circuit windings, there are as many circuits through the armature 
as there are poles. 

Making comparison of these two sub-classes, it may be stated that 
in the two-circuit windings the number of conductors is, for the same 
voltage, only 2/N times the number that would be required with a 
multiple-circuit winding, N being the number of poles ; hence a saving is 
effected in the labour of winding and in the space required for insulation. 
This last economy is frequently of great importance in small generators, 
either lessening the diameter of the armature or the depth of the air gap, 
and thereby considerably lessening the cost of material. 

It has been stated that Gramme-ring armatures have the advantage 
that only a small fraction of the total voltage exists between adjacent coils. 
This is only true when the Gramme armature either has a multiple-circuit 
winding, or a certain particular type of two-circuit winding, known as 
the Andrews winding, i.e. the long-connection type of two-circuit 
Gramme-ring winding. This reservation having been made for the sake 
of accuracy, it is sufficient to state that multiple-circuit Gramme-ring 
windings are the only ones now used to any extent in machines of any 
considerable capacity ; and, as already stated, these possess the advantage 
referred to, of having only a small fraction of the total voltage between 
adjacent coils. 

DRUM WINDINGS. 

In the case of drum windings, it is obvious that all the connections 
from bar to bar must be made upon the rear and front ends exclusively ; it 
not being practicable, as in the case of Gramme-ring windings, to bring 
connections through inside from back to front. From this it follows that 
the face conductors forming the two sides of any one coil must be situated 
in fields of opposite polarity ; so that the electromotive forces generated in 

1 This term applies to single armature windings. 



Drum Windings. 63 

the conductors composing the turns, by their passage through their 
respective fields, shall act in the same direction around the turns or coils. 

Bipolar windings are, in some cases, used in machines of as much as 
100 or even 200 kilowatts output; but it is now generally found desirable 
to employ multipolar generators even for comparatively small outputs. 
The chief reasons for this will be explained hereafter, in the section relating 
to the electro-magnetic limit of output. 

Drum windings, like Gramme-ring windings, may be either multiple- 
circuit or two-circuit, requiring in the latter case, for a given voltage, only 
2/N times as many conductors as in the former, and having the advantages 
inherent to this property. Owing to the relative peripheral position of 
successively connected conductors (in adjacent fields), two-circuit drum 
windings are analogous to the short-connection type, rather than to the 
long-connection type of two-circuit Gramme-ring windings. The multiple- 
circuit drum windings are quite analogous to the multiple-circuit Gramme- 
ring windings, the multiple-circuit drum possessing, however, the 
undesirable feature of full armature potential between neighbouring 
conductors ; whereas one of the most valuable properties of the multiple- 
circuit Gramme-ring winding is that there is but a very small fraction of 
the total armature potential between adjacent conductors. 

In Fig. 71 is given the diagram of a multiple-circuit drum winding. 
It is arranged according to a diagramatic plan which has proved convenient 
for the study of drum windings. The radial lines represent the face 
conductors. The connecting lines at the inside represent the end connections 
at the commutator end, and those on the outside the end connections at the 
other end. The brushes are drawn inside the commutator for convenience. 
The arrowheads show the direction of the current through the armature, 
those without arrowheads (in other diagrams) being, at the position shown, 
short-circuited at the brushes. By tracing through the winding from the 
negative to the positive brushes, it will be found that the six paths through 
the armature are along the conductors and in the order given in the six 

following lines : 

7 58 9 60 11 2 13 4 15 6 

56 5 54 3 52 1 50 59 48 57 

27 18 29 20 31 22 33 24 35 26 

16 25 14 23 12 21 10 19 8 17 

47 38 49 40 51 42 53 44 55 46 

36 45 34 43 32 41 30 39 28 37 

In making the connections, each conductor at the front end is 

o 

connected to the eleventh ahead of it ; and at the back to the ninth behind 



64 



Electric Generators. 



it. In other words, the front end pitch is 11, and the back end pitch is 
- 9. In practically applying such a diagram, the conductors would 
generally be arranged with either one, two, or four conductors in each slot. 
Suppose there were two conductors per slot, one above the other ; then 
the odd- numbered conductors could be considered to represent the upper 
conductors, the lower ones being represented by conductors with even 
numbers. In order that the end connections may be of the ordinary 



Fig.71. 




double-spiral arrangement or its equivalent, the best mechanical result will 
be secured by always connecting an upper to a lower conductor ; hence the 
necessity of the pitches being chosen odd. 

The small sketch at the top of Fig. 7 1 shows the actual location of the 
conductors on a section of the armature. There might, of course, have 
been only one conductor per slot ; or, when desirable, there could be more 
than two. The grouping of the conductors in the diagram in pairs is 
intended to indicate an arrangement with two conductors per slot. But 
in subsequent diagrams it will be more convenient to arrange the face 
conductors equi-distantly. 




Multiple- Circuit Windings. 



65 



The following is a summary of the conditions governing multiple- 
circuit single windings, such as that shown in Fio-. 71 ; 

a. There may be any even number of conductors, except that in iron 
clad windings the number of conductors must also be a multiple of the 
number of slots. 

b. The front and back pitches must both be odd, and must differ by 2 ; 
therefore the average pitch is even. 

c. The average pitch y should not be very different from c/n when c = 
number of conductors, and n = number of poles. For chord windings, y 



Fig. 72. 




SIX-CIRCUIT, DOUBLE WINDING. 



should be smaller than c/n by as great an amount as other conditions will 
permit, or as may be deemed desirable. 

Multiple-circuit windings may also be multiple- wound, instead of being 
single-wound, as in the above instance. We refer to a method in which 
two or more single windings may be superposed upon the same armature, 
each furnishing but a part of the total current of the machine. The rules 
governing such windings are somewhat elaborate, and it is not necessary at 
present to go fully into the matter. In Fig. 72 is shown a six-circuit 
double winding. Each of the two windings is a multiple-circuit winding, 
with six circuits through the armature, so that the arrangement results in 

K 



66 Electric Generators. 

only one-twelfth of the sixty conductors being in series between negative 
and positive brushes ; each of the conductors, consequently, carrying one- 
twelfth of the total current. This particular winding is of the doubly 
re-entrant variety. That is to say, if one starts at conductor 1, and traces 
through the conducting system, conductor 1 will be re-entered when only 
half of the conductors have been traced through. The other half of the 
conductors form an entirely separate conducting system, except in so far as 
they are put into conducting relation by the brushes. If fifty-eight con 
ductors are chosen, instead of sixty, the winding becomes singly re-entrant, 
i.e., the whole winding 1 has to be traced through before the original con- 

* o o o 

ductor is again reached. 

A singly re-entrant double winding is symbolically denoted thus (p\ 
and a doubly re-entrant double winding by O. There is no limit for such 
arrangements. Thus we may have 

Sextuply re-entrant, sextuple windings, O O O O O O 

Triply re-entrant, sextuple windings, 
Doubly re-entrant, sextuple windings, 
Singly re-entrant, sextuple windings, 

by suitable choice of total conductors and pitch. In practice, multiple 
windings beyond double, or at most triple, would seldom be used. Such 
windings are applicable to cases where large currents are to be collected at 
the commutator. Thus, in the case of a triple winding, the brushes should 
be made of sufficient width to bear at once on at least four segments, and 
one-third of the current passing from the brush will be collected at each of 
three points of the bearing surface of the brush, such division of the current 
tending to facilitate its sparkless collection. A double winding has twice 
as many commutator segments as the equivalent single winding. Another 
property is that the bridging of two adjacent commutator segments by 
copper or carbon dust does not short-circuit any part of the armature 
winding, and an arc is much less likely to be established on the commutator 
from any cause. 

Two-CmcuiT DRUM WINDINGS. 

Two-circuit drum windings are distinguished by the fact that the pitch 
is always forward, instead of being alternately forward and backward, as in 
the multiple-circuit windings. 




Two Circuit Windings. 



67 



The sequence of connections leads the winding from a certain bar 
opposite one pole-piece to a bar similarly situated opposite the next pole- 
piece, and so on, so that as many bars as pole-pieces are passed through 
before another bar in the original field is reached. 

A two-circuit single winding in a six-pole field is shown in Fig. 73. 
Two-circuit windings have but two paths through the armature, independ 
ently of the number of poles. Only two sets of brushes are needed, 
no matter how many poles there may be, so far as collection of the current 



Fig. 73 







TWO CIRCUIT, SINGLE WINDING. 



is concerned ; but in order to prevent the commutator being too expensive, 
it is customary in large machines to use as many sets of brushes as there 
are pole-pieces. Where more than two sets of brushes must be used, that 
is, in machines of large current output, the advantages possible from equal 
currents in the two circuits have been overbalanced by the increased spark 
ing, due to unequal division of the current between the different sets of 
brushes of the same sign. 

An examination of the diagrams will show that in the two-circuit 
windings, the drop in the armature, likewise the armature reaction, is 
independent of any manner in which the current may be subdivided among 



08 Electric Generators. 

the different sets of brushes, but depends only upon the sum of the currents 
at all the sets of brushes at the same sign. There are in the two-circuit 
windings no features that tend to cause the current to subdivide equally 
between the different sets of brushes of the same sign ; and in consequence, 
if there is any difference in contact resistance between the different sets of 
brushes, or if the brushes are not set with the proper lead with respect to 
each other, there will be an unequal division of the current. 

When there are as many sets of brushes as poles, the density at each 
pole must be the same ; otherwise the position of the different sets of 
brushes must be shifted with respect to each other to correspond to the 
different intensities, the same as in the multiple -circuit windings. 

In practice it has been found difficult to prevent the shifting of the 
current from one set of brushes to another. The possible excess of current 
at any one set of brushes increases with the number of sets ; likewise the 
possibility of excessive sparking. For this reason the statement has been 
sometimes made that the disadvantages of the two-circuit windings increase 
in proportion to the number of poles. 

From the above it may be concluded that any change of the armature 
with respect to the poles will, in the case of two-circuit windings, be 
accompanied by shifting of the current between the different sets of 
brushes ; therefore, to maintain a proper subdivision of the current, the 
armature must be maintained in one position with respect to the poles, and 
with exactness, since there is no counter action in the armature to prevent 
the unequal division of the current. 

But in the case of multiple-circuit windings, it will be noted that 
the drop in any circuit, likewise the armature reaction on the field in 
which the current is generated, tend to prevent an excessive flow of 
current from the corresponding set of brushes. On account of these 
features (together with the consideration that when there are as many 
brushes as poles the two-circuit armatures require the same nicety of 
adjustment with respect to the poles as the multiple-circuit windings), the 
latter are generally preferable, even when the additional cost is taken into 
consideration. 

In the section upon " The Electro-magnetic Limit of Output," it will 
be shown that the limitations imposed by the use of practicable electro 
magnetic constants restrict the application of two-circuit windings to 
machines of relatively small output. 

Two-circuit windings may be multiple as well as single-wound. Thus 



Two Circuit Windings. 



69 



in Fig. 74 we have a two-circuit, doubly re-entrant, double winding. An 
illustration of the convenience of a double winding, in a case where either 
one of two voltages could be obtained without changing the number of face 
conductors, may be given by that of a six-pole machine with 104 armature 
conductors. The winding may be connected as a two-circuit single winding 
by making the pitch 17 at each end, or as a two-circuit doubly re-entrant 
double winding, by making the pitch 17 at one end and 19 at the other. 




TWO CIRCUIT, DOUBLE WINDING 



The second would be suitable for the same watt output as the first, but at 
one-half the voltage and twice the current. 



FORMULA FOR Two-CmcuiT WINDINGS. 
The general formula for two-circuit windings is : 

C = n y _ 2m. 



where 



C = number of face conductors. 

n = number of poles. 

y = average pitch. 

m = number of windings. 



70 Electric Generators. 

The m windings will consist of a number of independently re-entrant 
windings, equal to the greatest common factor of y and m. Therefore, 
where it is desired that the m windings shall combine to form one re-entrant 
system, it will be necessary that the greatest common factor of y and m be 
made equal to 1. 

Also, when y is an even integer the pitch must be taken alternately, as 
(y l) and (y+ 1), instead of being taken equal to y. 

Thus, in the case of the two-circuit single windings we have 

C = n y 2 

and in double windings (m being equal to 2) we have 

C = n y + 4. 

As a consequence of these and other laws controlling the whole subject 
of windings, many curious and important relations are found to exist 
between the number of conductors, poles, slots, pitches, &c., and with 
regard to re-entrancy and other properties. 1 



WINDINGS FOR ROTARY CONVERTERS. 

As far as relates to their windings, rotary converters consist of con 
tinuous-current machines in which, at certain points of the winding, con 
nections are made to collector rings, alternating currents being received or 
delivered at these points. 

The number of sections into which such windings should be sub 
divided are given in the following Table : 

TABLE XVIII. 

Two-Circuit Multi. -Circuit 

Single Single 

Winding. Winding. 

Sections per Pair 
Sections. Poles. 

Single-phase rotary converter ... ... 2 2 

Three-phase rotary converter ... ... 3 3 

Quarter-phase rotary converter ... ... 4 4 

Six-phase rotary converter ... ... 6 6 

For multiple windings, the above figures apply to the number of 

1 y - 3 and y + 3, etc., also give re-entrant systems, but the great difference between the 
pitches at the two ends would make their use very undesirable except in special cases ; thus, for 
instance, it would be permissible with a very large number of conductors per pole. 



Rotary Converter Windings. 



71 



sections per winding : thus, a three-phase converter with a two-circuit 
double winding would have 3x2 = 6 sections per pair of poles. In the 
case of the three-phase rotary converter winding shown in Fig. 75, 
which is a two-circuit single winding, connection should be made from a 
conductor to one of the collector rings, and the winding should be traced 
through until one-third of the total face conductors have been traversed. 
From this point, connection should be made to another collector ring. 
Tracing through another third, leads to the point from which connection 




THREE-PHASE ROTARY CONVERTER, TWO-CIRCUIT SINGLE WINDING. 

should be made to the remaining collector ring, between which and the first 
collector ring the remaining third of the total number of conductors would 
be found to lie. It is desirable to select a number of conductors, half of 
which is a multiple of three, thus giving an equal number of pairs of con 
ductors in each branch. Where a multiple-circuit winding is used, the 
number of conductors per pair of poles should be twice a multiple of three. 
A multiple-circuit three-phase rotary converter winding is given in Fig. 76. 
Further information regarding the properties of rotary converters, and the 
resultant distribution of current in their windings, is reserved for the 
section on "Rotary Converters." 



72 



Electric Generators, 



ALTERNATING CURRENT WINDINGS. 

In general, any of the continuous-current armature windings may be 
employed for alternating current work, but the special considerations 
leading to the use of alternating currents generally make it necessary to 
abandon the styles of winding best suited to continuous- current work, and 
to use windings specially adapted to the conditions of alternating current 
practice. 

Attention should be called to the fact that all the re-entrant (or closed 
circuit) continuous-current windings must necessarily be two-circuit or 




fliree Pftate Rotary Converter SixCircu/c Winding 

multiple-circuit windings, while alternating current armatures may, and 
generally do, from practical considerations, have one-circuit windings, i.e., 
one circuit per phase. From this it follows that any continuous-current 
winding may be used for alternating current work, but an alternating 
current winding cannot generally be used for continuous-current work. In 
other words, the windings of alternating current armatures are essentially 
non-re-entrant (or open circuit) windings, with the exception of the ring- 
connected polyphase windings, which are re-entrant (or closed circuit) 
windings. These latter are, therefore, the only windings which are 
applicable to alternating-continuous-current commutating machines, 



Alternator Winding*. 73 

Usually for single-phase alternators, one slot or coil per pole-piece is 
used (ns represented in Figs. 77 and 78), and this permits of the most 
effective disposition of the armature conductors as regards generation of 
electromotive force. If more slots or coils are used (as in Fig. 79), or, in 
the case of face windings, 1 if the conductors are more evenly distributed 
over the face of the armature, the electromotive forces generated in the 
various conductors are in different phases, and the total electromotive force 
is less than the algebraic sum of the effective electromotive forces induced 
in each conductor. 

But, on the other hand, the subdivision of the conductors in several 
slots or angular positions per pole, or, in the case of face windings, their 
more uniform distribution over the peripheral surface, decreases the 
inductance of the winding, with its attendant disadvantages. It also 
utilises more completely the available space, and tends to bring about a 
better distribution of the necessary heating of core and conductors. There 
fore, in cases where the voltage and the corresponding necessary insulation 
permit, the conductors are sometimes spread out to a greater or less extent 
from the elementary groups necessary in cases where very high potentials 
are used. Windings in which such a subdivision is adopted, are said to 
have a multi-coil construction (Fig. 79), as distinguished from the form in 
which the conductors are assembled in one group per pole-piece (Figs. 77 
and 78), which latter are called unicoil windings. 

In most multiphase windings, multi-coil construction involves only very 
slight sacrifice of electromotive force for a given total length of armature 
conductor, and in good designs is generally adopted to as great an extent 
as proper space allowance for insulation will permit. 

It is desirable to emphasise the following points regarding the relative 
merits of unicoil and multi-coil construction. With a given number of 
conductors arranged in a multi-coil winding, the electromotive force at the 
terminals will be less at no load than would be the case if they had been 
arranged in a unicoil winding ; and the discrepancy will be greater in 
proportion to the number of coils into which the conductors per pole-piece 
are subdivided, assuming that the spacing of the groups of conductors is 
uniform over the entire periphery. 

But when the machine is loaded, the current in the armature causes 
reactions which play an important part in determining as will be shown 

1 Otherwise often designated "smooth core windings," as opposed to "slot windings." 

L 



74 



Electric Generators. 



later the voltage at the generator terminals ; and this may only be 
maintained constant as the load comes on, by increasing the field excitation, 
often by a very considerable amount. Now, with a given number of 
armature conductors, carrying a given current, these reactions are greatest 
when the armature conductors are concentrated in one group per pole-piece 



Fig.80. 



Uni Coil Single phase Winding n/tft 
parallel slots 






Thre.e Phase non overlapping 
Fractional pitch minding 
i4 Field Poies ZlArmatvrecoi/s 



(Figs. 77 and 78) ; that is, when the unicoil construction is adopted ; and 
they decrease to a certain degree in proportion as the conductors are 
subdivided into small groups distributed over the entire armature surface, 
that is, they decrease when the multi-coil construction (Fig. 79) is used. 
Consequently, there may be little or no gain in voltage at full load by the 



Induction Motor Windings. 75 

use of a unicoil winding over that which would have been obtained with 
a multi-coil winding of an equal number total of turns, although at no load 
the difference would be considerable. This matter will be found treated 
from another standpoint in the section on " Formulae for Electromotive 
Force." 

Multi-coil design (Fig. 79) also results in a much more equitable 
distribution of the conductors ; and, in the case of iron-clad construction, 
permits of coils of small depth and width, which cannot fail to be much 
more readily maintained at a low temperature for a given cross-section of 
conductor ; or, if desirable to take advantage of this point in another way, 
it should be practicable to use a somewhat smaller cross-section of 
conductor for a given temperature limit. A final advantage of multi-coil 
construction is that it results in a more uniform reluctance of the magnetic 
circuit for all positions of the armature ; as a consequence of which, 
hysteresis and eddy current losses are more readily avoided in such designs. 
A thorough discussion of this matter is given in the section relating to 
the design of the magnetic circuit. 

The unicoil winding of Fig. 77 may often with great advantage be 
modified in the way shown in Fig. 78, where the sides of the tooth are 
parallel, enabling the form-wound coil to be readily slipped into place. The 
sides of the slots are notched for the reception of wedges, which serve to 
retain the coil in place. Parallel-sided slots become more essential the 
less the number of poles. For very large numbers of poles, radial slots are 
practically as good. 

Fig. 80 shows a Y-connected unicoil three-phase winding; Fig. 81 
differs from it only in having the windings of the three-phases A connected. 

Fig. 82 gives a portion of a three-phase winding, with fourteen field 
poles and twenty-one armature coils (three coils per two-pole pieces). 
This is a representative of a type of windings known as fractional pitch 
windings, the relative merits of which will be discussed in the section on 
the design of polyphase generators. The diagrams in Figs 83 and 84 give 
two more examples of fractional pitch polyphase windings. 1 

INDUCTION MOTOR WINDINGS. 

The windings of induction motors are not essentially different from 
many already described. In order to keep the inductance low, the 

1 See also British Patent Specification No. 30,264, 1897. 



Electric (j-emrators 




Induction Motor Windings. 77 

windings both for the rotor and stator are generally distributed in as 
many coils as there can be found room for on the surface, instead of being 
concentrated in a few large coils of many turns each. This becomes of 
especial importance in motors of large capacity ; in smaller motors the 
windings may consist of comparatively fe\v coils. This is the case in 
Fig. 85, where the stator winding of a 7^- horse-power four-pole three- 
phase motor is divided up into two slots per pole-piece per phase. The 
rotor, whose winding is generally made up of few conductors, each of large 
cross-section, is often most conveniently arranged with but one conductor 
per slot, as shown in Fig. 85. The connection diagrams of these stator 
and rotor windings are given in Fig. 86. Fig. 87 gives a useful type of 
winding for either the stator or the rotor of induction motors, the con 
ductors, represented by radial lines, being, in the case of the stator, 
generally replaced by coils. 

The matter of induction motor windings will be more completely 
considered in the section devoted to the design of induction motors. 



78 Electric Generators. 



FORMULA FOR ELECTROMOTIVE FORCE. 

In this section, the dynamo will be considered with reference to the 
electromotive force to be generated in the armature. 

CONTINUOUS-CURRENT DYNAMOS. 

The most convenient formula for obtaining the voltage of continuous- 
current dynamos is : 

V - 4.00 TNM 10- 8 
in which 

V = the voltage generated in the armature. 
T = the number of turns in series between the brushes. 
N = the number of magnetic cycles per second. 

M = the magnetic flux (number of C G S lines) included or excluded by each 
of the T turns in a magnetic cycle. 

V, the voltage, is approximately constant during any period considered, 
and is the integral of all the voltages successively set up in the different 
armature coils according to their position in the magnetic field ; and since 
in this case, only average voltages are considered, the resultant voltage is 
independent of any manner in which the magnetic flux may vary through 
the coils. Therefore we may say that for continuous-current dynamos, the 
voltage is unaffected by the shape of the magnetic curve, i.e., by the 
distribution of the magnetic flux. 

It will be found that the relative magnitudes of T, N, and M may 
(for a given voltage) vary within wide limits, their individual magnitudes 
being controlled by considerations of heating, electro-magnetic reactions, 
and specific cost and weight. 

This formula, if correctly interpreted, is applicable whether the 
armature be a ring, a drum, or a disc ; likewise for two-circuit and 
multiple-circuit windings, and whether the winding be single, double, 
triple, &c. 



E.M.F. in Continuous-Current Dynamos. 79 

To insure, for all cases, a correct interpretation of the formula, it will 
be desirable to consider these terms more in detail : 

T = turns in series between brushes, 

= total turns on armature divided by number of paths through armature from 

negative to positive brushes. 

For a Gramme-ring armature, total turns = number of face conductors. 
For a drum armature, total turns = ^ number of face conductors. 

With a given number of total turns, the turns in series between 
brushes depend upon the style of winding, thus : 
For two-circuit winding, 

If single, two paths, independently of the number of poles. 
If double, four paths, independently of the number of poles. 
If triple, six paths, independently of the number of poles, <fec. 

For multiple-circuit winding, 

If single, as many paths as poles. 

If double, twice as many paths as poles. 

If triple, three times as many paths as poles, &c. 

N = the number of magnetic cycles per second 

R.P.M. x number of pairs of poles 
60 

It has been customary to confine the use of this term (cycles per 
second) to alternating current work, but it is desirable to use it also 
with continuous currents, because much depends upon it. Thus N, the 
periodicity, determines or limits the core loss and density, tooth density, 
eddy current loss, and the armature inductance, and, therefore also affects 
the sparking at the commutator. It is, of course, also necessarily a 
leading consideration in the design of rotary converters. 

Although in practice, dynamo speeds are expressed in revolutions per 
minute, the periodicity N is generally expressed in cycles per second. 

M = flux linked successively with each of the T turns. 
In the case of the 

Gramme-ring machine, M = \ flux from one pole-piece into armature. 
Drum machine, M = total flux from one pole-pieoe into armature. 

(M is not the flux generated in one pole-piece, but that which, after 
deducting leakage, finally not only crosses the air-gap, but passes to the 
roots of the teeth, thus linking itself with the armature turns.) 



80 



Electric Generators. 



Armature cores are very often built up as rings for the sake of 
ventilation, and to avoid the use of unnecessary material ; but they may 
be, and usually are, wound as drums, and should not be confounded with 
Gramme-wound rings. 

The accompanying Table of drum-winding constants affords a 
convenient means of applying the rules relating to drum windings. 

TABLE XIX. DRUM-WINDING CONSTANTS. 






Class of Winding. 




Number of Poles. 


4. 


6. 8. 


10. 


12. 


14. 16. 


f 


TW.,U^l f Sin le 


1.667 


1.667 


1.667 


1.667 


1.667 


1.667 


1.667 


Volts per lOOconductors ^rcml \ Double 


.833 


.833 


.833 


.833 


.833 


.833 


.833 


per 100 revolutions per 1 [ Triple 


.556 


.556 


.556 


,556 


.556 


.556 


.556 


minute and flux equal 


Two ( Slllgle 


3.33 


5.00 


6.67 


8.33 


10.00 


11.67 


13.33 


to one megaline 


iwo :, \ Double 


1.667 


2.50 


3.33 


4.17 


5.00 


5.83 


6.67 


* 


( Triple 


1 111 


1.667 


2.22 


2.78 


3.33 


3.89 


4.44 


Average volts bet ween (, iy[ u uiDle- f Dingle 
commutator segments, -4. \ Double 


.1333 

.0668 


.200 
.100 


.267 
.1333 


.333 
.1667 


.400 
.200 


.467 
.233 


.533 
.267 


i T ! Circuit 1 m i 

per megaline and per J (, Iriple 


.0445 


.0667 


.0888 


.1111 


.1333 


.1555 


.1778 


100 revolutions perl 


m f Single 


.267 


.600 


1.068 


1.668 


2.40 


3.27 


4.27 


minute (independentof ,","+. ^ Double 


.1333 


.300 


.534 


.834 


1.200 


1.635 


2.14 


number of conductors) ( 


I Triple 


.0888 


.200 


.356 


.556 


.800 


1.09 


1.42 



ALTERNATING CURRENT DYNAMOS. 

For alternating current dynamos it is often convenient to assume that 
the curve of electromotive force is a sine wave. This is frequently not the 
case ; and, as will presently be seen, it is practicable and often necessary to 
consider the actual conditions of practice instead of assuming the wave of 
electromotive force to be a sine curve. 



CURVE OF ELECTROMOTIVE FORCE ASSUMED TO BE A SINE WAVE. 
The formula for the effective no-load voltage at the collector ring is : 

V = 4.44 T N M 10- 8 , 

this being the square root of the mean square value of the sine wave of 
electromotive force whose maximum value is : 

V = 6.28 T N M 10- 8 . 

In order that these formulae may be used, the electromotive force wave 
must be a sine curve, i.e., the magnetic flux must be so distributed as to 



E.M.F. in Alternating Current Dynamo*. 81 

give this result. The manner of distribution of the magnetic flux in the 
gap, necessary to attain this result, is a function of the distribution of the 
winding over the armature surface. 

T = number of turns in series between brushes. 

N number of magnetic cycles per second. 

M = number of C G S lines simultaneously linked with the T turns. 

The flux will be simultaneously linked with the T turns only in the 
case of unicoil windings, i.e., windings in which the conductors are so grouped 
that they are all similarly situated in respect to the magnetic flux ; in other 
words, they are all in the same phase. 1 

The effective voltage at no load, generated by a given number of turns, 
will be a maximum when that is the case ; and if the voltage for such a 
case be represented by unity, then the same number of conductors arranged 
in " two-coil," "three-coil," &c., windings will, with the same values for T, 
N, M, generate (at no load) voltages of the relative values, .707, .667, &c. ; 
until, when we come to a winding in which the conductors are distributed 

7 O 

over the entire surface, as in ordinary continuous-current dynamos, the 
relative value of the alternating current voltage at no load, as compared 
with that of the same number of turns arranged in a unicoil winding will 



be .637 (which = 2 ). 

\ IT/ 



Tabulating these results we have : 

TABLE XX. 

Correction Factor for Voltage 

of Distributed Winding. 
Unicoil winding ... V = 1.000 

Two-coil winding ... V .707 x unicoil winding. 

Three-coil winding ... V = .GC7 x ,, ., 

Four-coil winding ... V = .654 x ,, ., 

Many-coil winding ... V = .637 x ,, ,, 

The terms uni-, two-, three-coil, &c., in the above Table indicate 
whether the conductors are arranged in one, two, three, &c., equally-spaced 
groups per pole-piece. The conditions are equivalent to the component 
electromotive forces generated in each group ; beii g in one, two, three, &c., 
different phases, irrespective of the number of resultant windings into which 
they are combined. 

1 Fig. 88, on page 84, will be of assistance in understanding the nomenclature employed 
in designating these windings. 

M 



82 Electric Generators. 

The values given in the Table may be easily deduced by simple vector 
diagrams. 

Instead of using such " correction factors," the following values may 
be substituted for K in the formula V = K T N M ICT 8 : 

TABLE XXI. 





Values for K in Formula. 




For Effective Voltage. 


For Maximum Voltage. 


Unicoil winding 
Two-coil ,, 
Three-coil 
Four-coil 
Many-coil 


4.44 
3.13 
2.96 
2.90 
2.83 


6.28 
4.44 
4.19 
4.11 
4.00 



(In all the preceding cases, as they apply only to sine wave curves, the 
maximum value will be 1.414 times the effective value.) 



VALUES OF K FOR VARIOUS WAVES OF ELECTROMOTIVE FORCE AND 
OF MAGNETIC FLUX DISTRIBUTION IN GAP. 

The relative widths and arrangement of pole arc and armature coil 
exert a great influence upon the magnitude of the effective (and maximum) 
voltage for given values of T, N, M, because of the different shapes of the 
waves of gap distribution and induced electromotive force. This is shown 
by the following Tables, where are given the values of K in the formula : 

V = KTNM10- 8 , 

it being assumed that the magnetic flux M emanates uniformly from the 
pole face, and traverses the gap along lines normal to the pole face. This 
assumption being usually far from the facts, the following results must be 
considered more in the light of exhibiting the tendency of various relative 
widths of pole face and the various arrangements of armature coil, rather 
than as giving the actual results which would be observed in practice. The 
results are, nevertheless, of much practical value, provided it is clearly kept 
in mind that they will be modified to the extent by which the flux spreads 
out in crossing the gap from pole face to armature face. 

The following Table applies to cases where the various components of 
the total winding are distributed equi-distantly over the armature, 



E.M.F. in Alternating Current Dynamos. 

TABLE XXII. VALUES FOR K. 
In the Formula V = K T N M 10" 8 , where V = Effective Voltage. 



83 





Pole Arc (expressed in per Cent, of Pitch). 


Winding. 






10. 


20. 


30. 


40. 


50. 


60. 


70. 


80. 


90. 


100. 


Unicoil i 12.6 


8.96 


7.28 


6.32 


5.66 


5.17 


4.78 


4.46 


4.21 


4.00 


Two-coil ... 


8.96 


6.32 


5.17 


4.21 


4.00 


3.64 


3.40 


3.12 


3.00 


2.83 


Three-coil 


7.30 


5.15 


4.21 


3.84 


3.55 


3.35 


3.08 


2.90 


2.76 


2.55 


Four coil ... 6.32 


4.44 


4.00 


3.72 


3.45 


3.24 


3.02 


2.83 


2.63 


2.45 


Many-coil ... 3.93 3.79 


3.63 


3.44 


3.27 


3.08 


2.88 


2.70 


2.52 


2.32 



When the coils are gathered in groups of a greater or less width, the 
values of K should be taken from Table XXIII. given below. 

A better understanding of the nomenclature employed in these 
two Tables will be obtained by an examination of the diagrams in 
Fig. 88. 

Probably the method used in obtaining these values (simple graphical 
plotting) is substantially that used by Kapp in 1889. The six values he 
gives check the corresponding ones in Tables XXII. and XXIII. 

TABLE XXIII. VALUES OF K. 
In the Formula V = K T N M 1Q- 8 , where V = Effective Voltage. 



Spread of Armature 
Coil in per Cent. 



Pole Arc (expressed in per Cent, of Pitch). 



of Pitch. 


10. 


20. 


30. 


40. 50. 


60. 


70. 


80. 


90. 


100. 





12.60 


8.96 


7.28 


6.32 


5.66 


5.17 


4.78 


4.46 


4.21 


4.00 


10 


9.80 


8.20 


6.85 


6.00 


5.50 


5.05 


4.74 


4.42 


4.15 


3.88 


20 


8.20 


7.40 


6.55 


5.75 


5.25 


4.90 


4.60 


4.35 


4.05 


3.75 


30 


7.10 


6.55 


6.00 


5.45 


5.05 


4.75 


4.45 


4.20 


3.90 


3.60 


40 


6.20 


5.80 


5.45 


5.15 


4.85 


4.55 


4.30 


4.00 


3.72 


3.43 


50 


5.60 


5.32 


5.10 


4.85 


4.60 


4.35 


4.10 


3.85 


3.60 


3.27 


60 


5.08 


4.90 


4.71 


4.55 


4.39 


4.15 


3.95 


3.68 


3.40 


3.10 


70 


4.72 


4.60 


4.44 


4.30 


4.18 


3.95 


3.75 


3.45 


3.20 


2.90 


80 


4.44 


4.30 


4.15 


4.00 


3.85 


3.66 


3.50 


3.25 


3.00 


2.75 


90 


4.18 


4.00 


3.90 


3.75 


3.60 


3.40 


3.20 


3.00 


2.78 


2.55 


100 


3.93 


3.79 


3.63 


3.44 3.27 


3.08 


2.88 


2.70 


2.52 


2.32 



It thus appears that by merely varying the spread of the pole arc and 
the armature coil, there may be obtained for given values of T, N, and M, 
values of the effective electromotive force, varying from a little more than 
half the corresponding value for a sine wave, up to several times that 
value (in fact, with an infinitely small spread of pole arc, provided the flux 
could be maintained, an infinitely large value of K would be obtained). 
The maximum value increases at the same time, in a still greater proportion. 



84 Electric Generators. 

ROTARY CONVERTERS. 

In rotary converters we have an ordinary distributed continuous- 
current winding, supplying continuous-current voltage at the commutator, 
and alternating-current voltage at the collector rings. The same wii ding, 
therefore, serves both for continuous-current voltage and for alternating 
voltage. 

Suppose that such a distributed winding, with given values of T, N, 
and M, generates a continuous-current voltage V at the commutator. 
Imagine superposed on the same armature a winding, with the same 
number of turns T in series, but with these turns concentrated in a unicoil 
winding. For the same speed and flux, and assuming a sine wave curve of 



vr^s OF ->f.:ja. Hs 




P&an-3Q%oF pitch 



?ok arc JU%of pitch 
Four coil winding 

I I I I I I 

I ! FJearc 50%ef pitch 

vmnnni uuuuvu mnmmr- - s . naiis f^ g . 6 oi a f pitch 

Polearc-W-ofpitch 
Spread ofn dg IOC? of pitch 

In the above- diayraais,th6 Slotted type of armature is 
represented The application aflhe iltMtrationt 13 the case of 

i cans armature merely rtj"irsi Uiat the conductors be supposed 
to begroupeot on the surface cftfis armatvrrinL * : stnr- rslatt re 



electromotive force, this imaginary superposed winding would supply 
1.11 V, ( = -y ] effective volts to the collector rings. But, re-arranging 

this same number of turns in a " many-coil " (distributed) winding, would, 
for the same speed and flux, reduce the collector ring voltage to 

.637 x 1.11 x V = .707 x V. 

Therefore, in a distributed winding, with T turns in series, there will be 
obtained a continuous-current voltage V, and an alternating-current voltage 
.707 V, on the assumption of a sine wave curve of electromotive force. 

But often the electromotive force curve is not a sine wave, and the 
value of the voltage becomes a function of the pole arc. Thus, examining 
the case of a single or quarter-phase rotary converter by the aid of 
the Tables for K, the results given below are obtained. 



E.M.F. it L Rotary Converters. 85 

TABLE XXI\ r . SINGLE AND QUARTER-PHASE ROTARY CONVERTERS. 



Spread of Pole Arc 


Kin V = KT NM 10- 


Kfor 


Ratio of Alternating 
Voltage between Collpctor 


in for 
per Cent, of Pitch. Collector- Ring Voltagp. 


Continuous-Current 
Voltage. 


Rings to Contii.uous- 
Current Voltage at 








Commutator. 


10 


3.93 


4.00 


.982 


20 


3.79 


4.00 


.947 


30 


3.63 


4.00 


.908 


40 


3.44 


4.00 


.800 


50 


3.27 


4.00 


.816 


60 


3.08 


4.00 


.770 


70 


2.88 


4.00 


.720 


80 


2.70 


4.00 


.675 


90 


2.52 


4.00 


.630 


100 


2.32 


4.00 


.580 



THREE-PHASE ROTARY CONVERTERS. 

An examination of three-phase rotary converters will show that the 
conductors belonging to the three phases have relative positions on the 
armature periphery, which may be represented thus : 

222221111111111333333333322222222221111111111333333333322222 
333333333322222222221111111111333333333322222222221111111111 

Consequently, it appears that the coils of one phase have a spread 
equal to 66.7 per cent, of the pitch. Observing also that each three- 
phase alternating branch has two-thirds as many turns in series between 
collector rings as has each branch, considered with reference to the commu 
tator brushes, we obtain the following Table of values : 

7 O 

TABLE XXV. THREE-PHASE ROTARY CONVERTERS. 









Ratio of Alternating 


Spread of Pole Ai c 


Kin V = KTNM 10~ 8 


Kfor 


Voltage between Collector 


in 


for 


Continuous-Current 


Rings to Continuous- 


per Cent, of Pitch. 


Collector-Ring Voltage. 


Voltage. 


Current Voltage at 
Commutator. 


10 


4.89 


4.00 


.815 


20 


4.70 


4.00 


.785 


30 


4.53 


4.00 


.755 


40 


4.39 


4.00 


.732 


50 


4.25 


4.00 


.710 


60 


4.02 


4.00 


.670 


70 


3.82 


4.00 


.636 


80 


3.52 


4.00 


.585 


90 


3.26 


4.00 


.544 


100 


2.96 


4.00 


.495 



86 



Electric Generators. 



The last column, giving the ratio of alternating-current voltage 
between collector rings, to continuous-current voltage at commutator, is the 
one of chief interest. This ratio varies from .495, when the pole arc is 
equal to the pitch, up to .815 with a 10 per cent, pole arc. 

These results only apply to rotary converters when independently 
driven, unloaded, from some mechanical source, or when driven unloaded 
as a continuous-current motor. That is to say, the electromotive forces 
referred to are counter-electromotive forces. When driven synchronously, 
the ratio of the terminal voltages may be made to vary through a very 
wide range by varying the conditions of lag and lead of the current in 



Three, phase, evnAnafna cen&zrter, Z5 Cycles pa- fecand. 

Jteiatuzv betmrxn, alttmeutuiy rolts & mnptrw the- generator & ccrurrrter 

ftjdd, taxAJtatijQfis bexsva so cirj/(itei o*f aimgrj to Kcire- 5fternusicU4 rofos 

at CGntuiu0u4 current, commutator. jfl 



Tctrard, left fto/irf- To*ard right hand:- 
Strona csnrertfr exaCat^ iStraqp generator 



-_jf Converter fielct zero. 
AUpcintt (e Outrighthare, 
neaature ccmrertiJ-fvelaL 




Mtfr" -*?;,-uj volts beiuetn, fnUrcfyr rings ,_ 

JSO 37(J JW 390 tOO 110 <KO 



the armature. In Fig. 89 is given a curve showing through what a 
very extended range this ratio may be varied, according to the conditions 
of load and excitation. 



TABLE XXVI. 



Proportion that T is of Turns on Arm. 



converter. 


2-Circuit Winding. 


Multiple- Circuit Winding. 


Single-phase rotary ... 
Quarter-phase rotary ... 
Three-phase rotary 


* 

\ 
* 


1 


2 x number of pairs of poles 
1 


2 x number of pairs of poles 
1 


3 x number of pairs of poles 



E.M.F. in Polyphase Apparatus. 



87 



In rotary converters, Table XXVI. will be of assistance in 
determining the value of T (number of turns in series between collector 
rings). 

Polyphase Machines. In considering polyphase machines in general, 
it may be said that the most convenient way of considering the relations 
between V, T, N, and M, is to make the calculations for one phase. Thus 
in the case of a three-phase machine, one would calculate the volts per 




phase, by placing in the formula the turns in series per phase, for T. Then 
if the winding is "delta" connected, this will give also the volts between 
collector rings (since there is only the winding of one phase lying between 
each pair of collector rings). If, on the other hand, the winding is Y 
connected, the volts between collector rings will be >/ 3, (1.732) times the 
volts per phase. Thus the calculation should be carried out with reference 
to one phase, the results of interconnecting the windings of the different 
phases being subsequently considered, 



Electric Generators. 



ELECTROMOTIVE FORCE AND FLUX IN TRANSFORMERS. 

In the case of transformers, the relation between voltage and flux is 
dependent upon the wave form of the applied electromotive force, and 
determinations of these quantities involve the use of the term "form factor," 
proposed by Fleming. 1 He defines the form factor as the ratio of the 
square root of the mean of the squares of the equi-spaced ordinates of a 
curve, to the true mean value of the equi-spaced ordinates. The mean 
square value he denotes by the letters R.M.S. (root mean square), and the 
mean value by the letters T.M. (true mean). 



Form factor = 



R.M.S. 
T.M. 



= / 



In the case of a rectangular wave, the R.M.S. value, the T.M. value 
and the maximum value are equal, and the form factor becomes equal to 1. 
In this case the form factor has the minimum value. 

Peaked waves have high form factors. Denoting the form factor byy, 
the relation between voltage, turns, periodicity, and flux may be expressed 
by the equation 

V = 4.00/T NM 10- 8 . 

The extent of the dependence of the form factor upon the proportions 
and winding of the generator may be obtained from the two following 
Tables, the first of which applies to equidistantly distributed windings, and 
the second to windings in which the face conductors are gathered in groups 
more or less spread over the surface of the armature, these groups 
alternating with unwound spaces. 

TABLE XXVII. VALUES FOR FORM FACTOR (/). 



Winding. 



Pole Arc (Expressed in Per Cent, of Pitch). 





10 


20 


30 


40 


50 


60 


70 


SO 90 


100 


Uni-coil... 


3.33 


2.24 


1.82 


1 58 


1 41 


1.29 


1.19 


1.12 


1.06 


1.00 


Two-coil... 


2.24 


1.58 


1.29 


1.12 


1.00 


1.10 


1.18 


1.26 


1.34 


1.41 


Three-coil 


1.82 


1.29 


1.06 


1.08 


1 15 


1.21 


1.22 


1.19 


1.17 


1.15 


Four-coil 


1.57 


1.12 


1.07 


1.13 


1.16 


1.14 


1.11 


1.12 


1.17 


1.22 


Many-coil 


1.02 


1.04 


1.06 


1.08 


1.09 


1.11 


1.12 


1.14 


1.15 


1.15 



Alternate Current Transformers, vol. i., second edition, page 583* 



E.M.F. and Flux in Transformers. 
TABLE XXVT1I. VALUES FOR FORM FACTOR (/). 



89 



Spread f Arm-uure 
Coil in per Cent, of 
Pitch. 


Pole Arc (Expressed in Per Cent, of Pitch.) 


10 


20 30 


40 


50 


60 


70 


80 


JO 


100 





3.33 


2.24 


1.82 


1.58 


1.41 


1.29 


1.19 


1.12 


1.06 


1.00 


10 


2.61 


2.05 


1.73 


1.53 


1.37 


1.26 


1.17 


1.11 


1.05 


1.02 


20 


2.05 


1.83 


1.59 


1.48 


1.31 


1.23 


1.13 


1.08 


1.04 


1.04 


30 


1.73 


1.59 


1.50 


1.40 


1.25 


1.19 


1.12 


1.07 


1.06 


1.06 


40 


1.53 


1.48 


1.40 


1.30 


1.21 


1.16 


1.12 


1.09 


1.08 


1.08 


50 


1.37 


1.31 


1.25 


1.21 


1.17 


1.13 


1.12 


1.09 


1.09 


1.09 


60 


1.26 


1.23 


1.19 


1.16 


1.13 


1.13 


1.12 


1.11 


1.11 


1.11 


70 


1.17 


1.13 


1.12 


1.12 


1.12 


1.12 


1.12 


1.12 


1.12 


1.12 


80 


1.11 


1.08 


1.07 


1.09 


1.09 


1.11 


1.12 


1.13 


1.14 


1.14 


90 


1.05 


1.04 


1.06 


1.08 


1.09 


1.11 


1.12 


1.14 


1.15 


1.15 


100 


1.02 


1.04 


1.06 


1.08 


1.09 


1.11 


1.12 


1.14 


1.15 


1.15 



From the formula V= 4.00 fT N M 10 s , it appears that for a given 
effective voltage V, the flux M may be low in proportion as the form factor 
f is high. This is a distinct advantage in the case of transformers, since 
their core loss is dependent upon the density of the flux circulating in their 
iron cores. If a given voltage can be obtained with a small flux, the trans 
former can be operated at a higher all-day efficiency. Commercial 
generators of different types differ often by 25 per cent, and more, as 
regards the form factor of their electromotive force waves. The pre 
determination of the form factor thus becomes a matter of considerable 
interest in the design of alternating-current generators. 

While, however, peaked waves insure low core losses for transformers 
on the circuits, they have the disadvantage that the maximum electro 
motive force is more in excess of the effective electromotive force than for 
the less peaked waves. It is, therefore, generally undesirable to so 
proportion a generator as to obtain an excessively peaked wave. 

The curves of Figs. 90 and 91, page 87, correspond to values given in 
the Tables, and show the extent of the variations obtainable. 



.N 



90 Electric Generators. 



THERMAL LIMIT OF OUTPUT. 

Viewed from a thermal standpoint, the maximum output of an electric 
machine is determined by the maximum increase of temperature con 
sistent with good working. The limiting increase of temperature may be 
determined with respect to durability of the insulating materials used, the 
efficiency, and the regulation. The increase of temperature is commonly 
expressed by the ratio of the heat generated in watts, to the radiating 
surface in square inches, i.e., watts per square inch radiating surface. The 
increase of temperature of any surface above the atmosphere, and therefore, 
also, the permissible expenditure of energy per square inch radiating 
surface, varies according to the nature of the surface, its speed, location, 
&c. For static surfaces, such as the surfaces of field magnets, the increase 
of temperature may be taken to be about 80 deg. Cent, per watt per 
square inch, as measured by a thermometer placed against the cylindrical 
surface. For cylindrical surfaces of the same nature, but rotated with 
a peripheral speed of about 3,000 ft. per minute, the increase of temperature 
per watt per square inch may be taken to be between 30 deg. Cent, and 
40 deg. Cent. The increase of temperature per watt per square inch 
increases as the surface speed is diminished. Thus for smooth-core 
armatures the increase of temperature is about 25 per cent, greater at a 
peripheral velocity of 2,000 ft. than at a peripheral velocity of 3,000 ft. per 
minute. For ventilated armatures of ordinary design, i.e., armatures with 
interstices, the increase of temperature is between 15 deg. Cent, and 
20 deg. Cent, per watt per square inch for a peripheral speed of 3,000 ft. 
per minute, and between 10 deg. Cent, and 12 deg. Cent, for a peripheral 
speed of 6,500 ft. per minute. 1 The increase of temperature per watt per 
square inch varies somewhat with the temperature of the surface, but 
remains fairly constant for the temperatures used in practice. 

In transformers submerged in oil in iron cases, the rise in 
temperature, as measured by the increased resistance of the windings, 
is about 35 deg. Cent, per -^ watt per square inch of radiating surface of 

1 The increase of temperature, as determined from resistance measurements, will generally 
le from 50 per cent, to 100 per cent, in excess of these values. This is clearly shown in the 
various tests described in the following pages. 



General Considerations Relating to Temperature Rise. 01 

the iron case, at the end of ten hours run. Before this time has elapsed, 
small transformers will already have reached their maximum temperature, 
but transformers of 25 kilowatts capacity and larger may continue increasing 
in temperature for a much longer period. However, transformers are 
seldom called upon to carry their full load for a longer period than 
10 hours. The same transformers, without oil, will have 30 per cent, 
greater rise. 

Large transformers are generally artificially cooled by forced circu 
lation of oil, air, or water, the latter being circulated in pipes coiled about 
the transformers ; and sometimes in the low potential coils of very large 
transformers, the conductors are made tubular, the cooling medium being 
forced through them. With artificially-cooled transformers, by using 
sufficient power for forcing the circulation, the rise of temperature may be 
kept down to almost any value desired. But, of course, the power applied 
to this purpose lowers the efficiency of the equipment. 

Although constants such as those given above are very useful for 
obtaining a general idea of the amount of the increase of temperature, they 
should be used with discretion, and it should be well understood that the 
rise of temperature is greatly modified by various circumstances, such as : 

Field-magnet coils depth of winding ; accessibility of air to surface of 
spools ; force with which air is driven against spool surfaces ; shape and 
extent of magnet cores on which coils are located ; season, latitude, nature 
of location, i.e., whether near boiler-room or in some un ventilated corner, 
or in a large well-ventilated station, or under a car, &c. 

Armature windings and cores similar variable factors, particularly 
method and degree of ventilation ; shape and details of spider ; centrifugal 
force with which air is urged through ventilating ducts ; degree of freedom 
from throttling in ducts ; number of ducts ; freedom of escape of air from 
periphery ; and peripheral speed. Thus it will be readily understood 
that the values for rise of temperature per watt per square inch have to be 
determined from a number of conditions. 

Small machines quickly reach the maximum temperature ; large 
machines continue to rise in temperature for many hours. Hence the length 
of a heat run should be decided upon with reference to the nature of the 
apparatus and the use to which it is to be put. The heat should be 
distributed in proportion to the thermal emissivity of each part, with due 
regard to the permissible rise of temperature. Heating is of positive 
advantage, in so far as it is limited to temperatures that will keep the 



92 Electric Generators. 

insulation thoroughly dry, and thus tend to preserve it. But it is 
disadvantageous as regards preservation of insulation, in so far as it 
overheats and deteriorates it. The permissible temperature is thus 
dependent upon the nature of the insulation. In railway motors, the field 
conductors are insulated with an asbestos covering, as the location of the 
motors does not permit of their being sufficiently large to run cool under 
heavy loads. 

MAGNETS. 

The radiating surface of magnets of ordinary design, i.e., those in which 
the diameter of the magnet coil approximately equals the length, is 
ordinarily taken to be the cylindrical surface ; no account being taken of the 
ends, which in general are not very efficient for the radiation of heat ; when, 
however, the magnets are very short, and the surface of the ends large, 
they should be considered. 

ARMATURES. 

Radiating surface of armatures in general, is taken to be the surface of 
those parts in which heat is generated, that are directly exposed to the air. 
Due allowance should be made for the different linear velocities of different 
portions of the armature windings. Thus in the ordinary Siemens type of 
armature the radiation per square inch, or thermal eniissivity, at the ends, 
averages only about two-thirds that at the cylindrical surface, the difference 
being due to the difference in surface speed. In the case of armatures 
of very large diameter, the thermal emissivity at the ends becomes 
approximately equal to that of the cylindrical portion when the armatures 
are not very long. When the armatures have a length approaching half 
the diameter of the armature, the thermal emissivity at the ends may 
considerably exceed that midway between the ends of the armatures, unless 
special means for ventilating are resorted to. 

In the " barrel " type of winding, now largely used, the end 
connections are approximately in the same cylindrical surface as the 
peripheral conductors, being supported upon a cylindrical extension from 
the spider. Here the entire armature winding revolves at the same 
peripheral speed, and is in the best position as regards ventilation. 

The radiation of heat from an armature is not affected greatly by 
varying the surface of the pole-pieces, within the limits attained in ordinary 



Estimation of Temperature Rise. 93 

practice. If, however, the magnets are rectangular in section, and placed 
closely together, the radiation of heat from the armature may be 
considerably restricted. Further, unless the magnets are so placed with 
respect to each other that the heat of each is carried off independently of 
that of the others, special means for ventilating will have to be resorted to, 
and the values given above will not hold. Such constructions as the last 
two mentioned are not recommended for general practice. 

EXAMPLE OF ESTIMATION OF TEMPERATURE RISE. 

Diameter of a certain ironclad armature ... ... = 35 in. 

Length, overwinding ... ... ... ... = 25 

Speed ... ... = 360 revs, per min. 

Internal diameter ... ... ... ... ... = 18 in. 

35 x TT x 25 ... ... = 2750. sq. in. 

18 x TT x 25 = 1420. 

x (252-182) x 2 ... 470. 



Total radiation surface ... ... = 4640. ,, 

35 

Peripheral speed = IT x - x 360. = 3300. ft. per min. 

If well ventilated by internal ducts, it should be very safe to take 
22 deg. Cent, rise of temperature per watt per square inch. 

Watt*. 

Core loss 5000 

Armature C 2 R 2600 



Total loss 7600 

7600 

= 1.64 watts per sq. in. 



4640 
. . 1.64 x 22 = 36 deg. Cent, rise of temperature at end of 10 hours run at full load. 

INTERNAL AND SURFACE TEMPERATURE OF COILS. 

The importance of determining the internal temperature of coils, by 
resistance measurements, instead of relying upon the indications of a 
thermometer placed upon the surface, is well shown by the results of the 
following test. An experimental field-magnet coil was wound up with 
2,646 total turns of No. 21 B.W.G., the winding consisting in 38 layers, 
from every pair of which, separate leads were brought out, to enable the 



94 



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Experimental Heat Tests. 



95 



temperature of all parts of the coil to be determined by resistance 
measurements. 

Two distinct tests were made, one with the armature at rest, and the 
other with the armature running at a peripheral speed of 2,000 ft. per 
minute. Each test lasted two hours, the current through the coil being 
maintained constant at one ampere throughout both tests. Every ten 
minutes a reading was taken on a voltmeter across each pair of layers, thus 
giving a record of the change in resistance as the test progressed. A 
dimensional sketch of the coil, pole-piece, and armature is given in Fig. 92, 
and the results of the tests are plotted in the curves of Figs. 93, 94, 95, 
and 96. 

For the armature at rest (Fig. 93) shows the ultimate rise of 



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temperature in the different layers plotted against the positions of those 
layers ; and Fig. 94 shows the rise of temperature in the innermost 
layers, the middle layers, and the outside layers, plotted against time. 
The curves show well that without the aid of the circulation of air set 
up by the rotation of the armature, the metal of the field -magnet core is 
as effective in carrying away the heat, as is the air which bathes the 
surface of the spool. For the armature running at a peripheral speed of 
2,000 revolutions per minute, the results are plotted in the curves of 
Figs. 95 and 96. The latter figure shows that with the circulation 
of air set up by the rotation of the armature, the outside of the 
coil is maintained much cooler than is the inner surface adjoining the 
field-magnet core. But the most significant conclusion to be drawn 
from the tests is that shown by Figs. 93 and 95, namely, that the 
temperature of the interior layer of a coil may considerably exceed the 



96 



Electric Generators. 



temperature corresponding to the average rise of resistance of the total 
winding. 

In Figs. 97 and 98 are given respectively a sketch of the field-magnet 
and spool of a machine, and the result of a heat test taken upon it, in 
which the average temperature of the field spools was determined from 
time to time, by means of resistance measurements of the field winding. 

The influence of the peripheral speed of the armature upon the 
constants for determining the temperature increase of field spools, as well 



Fig. 99 




as the effect of covering the wire with a final serving of protecting cord, 
are clearly shown by the results of the following test made upon the 
field spools of a continuous-current generator of 35 kilowatts rated output. 
The tests were made with a wide range of field excitation, and the 
temperatures were determined both by thermometric and resistance 
measurements. The results afford a check upon the more general values 
given on page 90 for predetermining the temperature rise of spools. 

In Fig. 99 is given a dimensional sketch of the machine, and in 
Figs. 100 to 111 are given curves of results of the various heat runs. 
The curves of Fig. 112 summarise the average results obtained. 



Influence of Peripheral Speed on Temperature Rise. 97 

Out of the four field spools, two only were under observation, ie., 
the top two. On one of these two spools the cording and insulation was 
taken oft", and the winding exposed directly to the air ; the remaining 
spools remained corded. For the purpose of measuring the outside 
temperature of the spools, thermometers were placed, for the one spool 
on the outside of the winding, and for the other spool on the outside of 
the cording ; the third temperature measurement was determined from the 
resistance increase of the four spools in series. Thus, three temperature 
measurements were made : 

1st. On the outside of the uncorded spool, by thermometer. 

2nd. ,, ,, corded ,, ,, 

3rd. Increase of temperature of the four spools by resistance. 

The four spools were connected in series, the amperes input 
being kept constant, and the volts drop across the four spools noted. 

In the first case, the armature remained stationary, and results were 
obtained with .5, .75 and 1 ampere. These results are set forth in the 
curves of Figs. 100 to 105. 

The armature was then revolved at a peripheral speed of 2000 ft. 
per minute, and temperature rises observed at .75, 1 and 1.25 amperes. 
In this case, a different procedure was adopted. On the temperature 
reaching a constant value with .75 ampere, the test was carried on, the 
amperes being raised to 1, and again, after reaching a constant value, 
to 1.25 amperes. At this point the temperature reached a value above 
which it was not advisable to go. Results of this test are set forth in 
the curves of Figs. 10G and 107. 

Two further tests were carried out on similar lines, at peripheral 
speeds of 3,500 ft. and 4,800 ft. per minute, results of which are set forth 
in the curves of Fi^s. 108 to 111. 

O 

From the curves of Fig. 112, in which the average results of all these 
tests are summarised, it will be noted that a considerable increase of 
speed above 2,000 ft. per minute does not, for this machine, reduce 
the temperature rise to any very great extent. 

On each of the curves a table is given, setting forth the working 
data, and the constants derived from the tests. It will be noted that 
the results are figured from the assumption that the watts dissipated 
remain constant, whereas in reality they vary as the temperature alters ; 
but as this variation would complicate the calculations, these are based 
on the resistance at 20 deg. Cent., namely, 108 ohms per spool. 

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RISE OF TEMPERATURE OF FIELD SPOOLS. 
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TEMPERATURE AS MEASURED BY THERMOMETER. 

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101 



The peripheral radiating surfaces of the two spools differ, owing 
to the cording having been removed in the one case ; therefore, in figuring 
on the thermometer measurements of the corded and uncorded spools, 
their respective radiating surfaces are used ; but in the case of the 
measurements of temperature rise by resistance, a mean peripheral 
radiating surface is taken. 

It should furthermore be noted that the higher the peripheral speed 
of the armature, the less is the difference between the temperature rise 
observed from thermometric readings on the surfaces of the corded and 
the uncorded spools. 



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The armature had two ventilating ducts, each one half- inch wide, 
through which air was thrown out centrifugally, after entering through 
the open end of the armature spider. 

HEAT LOSSES C 2 R DUE TO USEFUL CURRENTS IN THE CONDUCTORS. 

Heat generated, due to the current and resistance, is calculated 
directly from these two factors. The resistances should be taken to 
correspond to the temperature the conductors attain in practice. To 
determine this temperature, resistance measurements are much more 
reliable than thermometric measurements. For standard sizes of wire, 
the resistance is most conveniently determined by ascertaining from tables, 



102 Electric Generators. 

the ohms per 1000 ft. of the size of wire in question. Then the length 
of wire in the magnet spool or armature, as the case may be, should be 
computed from the number of turns and the mean length of one turn. 
The total resistance can then be obtained. 

The Appendix contains Tables of this description, which give 
the properties of commercial copper wire for three standard gauges, 
namely, B. and S. (American); S.W.G. (Board of Trade); and B.W.G. 
(Birmingham Wire Gauge). They have been arranged with especial 
reference to convenience in designing- electrical apparatus, but they do 
not differ greatly from the Tables arranged for exterior wiring and 
other purposes. They serve as a basis for thermal calculations, and 
are also useful in the calculation of spool windings, as considered in 
the section on the design of the magnetic circuit. 

Example. A certain transformer has/ in the primary, 1200 turns 
of No. 7 B. and S. Mean length of one turn = 28 in. = 2.33 ft. 
Total length -2. 33 x 1200 == 2800ft. No. 7 B. and S. has (see Table 
in Appendix), at 20 deg. Cent., .497 ohms per 1000 ft. Therefore the 
primary resistance at 20 deg. Cent. = 2.8 x .497 = 1.40 ohms. Suppose 
full load current =13 amperes. Then the primary C 2 .R = 169 x 1.40 
= 237 watts. 

Specific resistance of commercial copper at deg. Cent. 

= .00000160 ohms per cubic centimetre. 
= .00000063 ohms per cubic inch. 

i.e., between opposite faces of a cubical unit. The above constants are 
of use when other than standard sizes of wire are employed. In con 
nection with them it should be kept in mind that the resistance of copper 
changes about .39 per cent, per deg. Cent. Where more convenient, 
and where greater accuracy is desired, use may be made of the following 
factors by which the resistance at deg. Cent, should be multiplied in 
order to obtain the resistance at the temperature employed : 

TABLE XXIX. 

Deg. Cent. 

... ... 1.000 

20 1.080 

40 1.160 

60 ... ... 1.250 

80 ... 1.337 

100 ... 1.422 



Foucault Currents. 103 

Example. An armature has a conductor .GO in. by .30 in. = .180 
square inches in cross-section. It has an eight-circuit double winding. 
Total turns = 800. Mean length of one turn = 60 in. Turns in series 

between brushes = - =50. Therefore, length of windino- between 

8x2 

positive and negative brushes = 50 x 60 = 3000 in. Cross-section = 
8x2x .18 = 2. 88 square inches. Therefore resistance at deg. 

n 3000 x .00000063 

Cent. = = .OOOGSo ohms. Suppose the full - load 

Z.oo 

current of 4000 amperes heats the armature conductors to 60 deg. Cent. 
Then the armature C 2 R at 60 deg. Cent. = 4000 2 x .000655 x 1.25 
= 13,100 watts. 

The Tables of properties of commercial copper wire is supplemented 
by a Table in the Appendix, giving the physical and electrical 
properties of various metals and alloys. This Table, used in connection 
with the others, permits of readily determining resistances, weights, 
dimensions, &c., of various conducting materials. 



FOUCAULT CURRENTS. 

In addition to the C 2 R losses in the conductors, there are losses due 
to parasitic currents, often termed eddy, or foucault currents, when solid 
conductors, if stationary, are exposed to the influence of varying induction 
from magnetic fields ; and whenever they are moved through constant 
magnetic fields, except in cases where the solid conductors are shielded from 
these magnetic influences. 

In armatures with smooth-core construction, the conductors are not 
screened from the magnetic field, consequently there may be considerable 
loss in the conductors, from foucault currents. This loss has been found to 
vary greatly, according to the distribution and density of magnetism in the 
air-gap, and cannot be accurately predetermined. 

In practice this loss is kept as small as possible ; in the case of bar 
windings, by laminating the bars and insulating them from each other ; or 
in the case of wire windings, by using conductors xV"^ 11 - or ^ ess m diameter, 
and twisting these into a cable. The amount by which the foucault current 
loss can be lessened in this last method is forcibly illustrated by the 
following example : The winding of a certain armature consisted of four 



104 Electric Generators. 

wires in parallel, each 0.165 in. in diameter. These conductors were 
replaced by 19 strands of cable having the same cross-section of copper, and 
the total loss of the armature was diminished by one-third. 

In iron-clad dynamos, the conductors are more or less protected from 
eddy currents by being embedded in. slots. This exemption from such 
losses depends upon the extent to which the teeth overhang, and upon the 
density in the teeth ; very high density throwing part of the lines through 
the slots, instead of permitting them all to be transmitted along the teeth. 
Even where the tooth density is low, stranded conductors must sometimes 
be used in iron-clad armatures. As an instance, may be cited the case of 
an alternating current armature with a slot of the proportions shown in 
Fig. 113. Here solid conductors of the proportions shown were at first used, 
but the cross-flux set up by the armature current was perpendicular to the 
plane of the conductors, and excessive heating resulted from the eddy 
currents set up in the solid conductors. Stranded conductors should be 
used in such a case. 

Stranded conductors are open to the objections of increased first cost, 
and of having from 15 per cent, to 20 per cent, higher resistance for given 
outside dimensions. This increased resistance is not entirely due to the 
lesser total cross-section of the component conductors, but also partly to 
their increased length, caused by the twist given them in originally making 
up the conductor. The stranded conductor, constructed, in the first place, 
with a circular cross-section, is pressed to the required rectangular section, 
in a press operated by hydraulic pressure. No precautions, such as 
oxidising, or otherwise coating the surface of the component wires, are 
necessary. The mere contact resistance suffices to break up the cross 
currents. 

Closely related to the losses just described, are the eddy current losses 
in all solid metal parts subjected to inductive influences. This occurs chiefly 
in pole-faces ; but if the proportions of the armature are such that, 
in passing the pole-pieces, the reluctance of the magnetic circuit is much 
varied, eddy currents will be found throughout all solid parts of the 
entire magnetic circuit. Consequently, in such cases, not only the 
pole-pieces, but the entire magnetic yoke, should be laminated. Such 
a construction has been used in alternators, with the result that, especially 
in the case of uni-slot armatures, a very marked improvement has been 
made in efficiency and in heating. 

In continuous-current machines, the surface of the armature is broken 



Foucault Currents in Non- Laminated Parts of Dynamos. 105 

up by a large number of small slots, and the disturbance is mainly local, the 
reluctance of the magnetic circuit, as a whole, remaining unchano-ed. 
Nevertheless, in such cases, the loss in the neighbourhood of the pole-face 
may be large, and will be found to depend chiefly upon the depth of the air- 
gap as related to the width of the slot opening. Instances have occurred 
in small machines, where increasing the depth of the air-gap from J- in. to 
J in., has greatly modified the magnitude of such pole-face losses, 
Straight-sided armature slots give, of course, much greater losses in the 
pole-face than slots with overhanging projections, while if the slots are 
completely closed over, the loss is practically eliminated. 

Pole-faces frequently consist of a laminated structure, cast in, or 
sometimes bolted on, to the upper portion of the magnet core. Another 




type of construction consists in laminating the entire magnet core and casting 
it into the solid yoke. 

In the neighbourhood of conductors and coils which are the seat of 
high magneto-motive forces, solid supports, shields, and the like, should be 
avoided, unless of high resistance, non-magnetic material, such as man 
ganese steel. For this reason spool flanges could also well be made of 
manganese steel. 

Eddy-current losses in the sheets of armature cores are dependent 
upon the square of the density of the flux, the square of the periodicity, 
and the square of the thickness of the sheets. Also upon the care with 
which the laminations are insulated from each other. It is, therefore, 
important to avoid milling and filing in slots, as this tends to destroy the 
insulation, and makes a more or less continuous conductor parallel to the 
copper conductors. Consequently, the eddy-current loss is quite largely 



106 



Electric Generators. 



dependent upon the relative magnitudes of flux, number of turns, and 
length of armature parallel to the shaft, as upon these quantities depends 
the volts per unit of length tending to set up parasitic currents in the 
armature core. Owing to the less amount of machine work, smooth-core 
armatures are much more apt to be free from parasitic currents in the 
core. The more such losses from eddy currents are anticipated from 
the nature of the design, the greater should be the safety factor applied 
to the value of the core loss as derived from the curves of Figs. 35 and 36 
(see page 34). 

Armature punchings should, when possible, be assembled without any 
milling or riling. Cases are on record where the milling of armature slots 




has increased the core loss to three times its original value, the metal 
removed by milling being merely a thin layer from the sides of the slot. 
Even light filing increases the core loss considerably. Most of the increase, 
in both these cases, is due to the burring of the edges making a more or 
less continuous conductor, although there is also a slight increase due to 
injuring the quality of the iron by mechanical shock. 

In a modern railway motor, this matter was studied by testing the 
core loss at various stages of the process of manufacture. The curves 
of Fig. 114 represent the average results from tests of two armatures. 

Curve 1 was taken after assembling the punchings. 
11 2 ,, teeth were wedged straight. 

11 3 ,, slots were slightly filed. 

4 winding. 



Hysteresis Loss in Armature Cores. 107 

The difference between curves 3 and 4 gives the eddy-current loss in 
the conductors. The particular shape of the curves possesses no especial 
significance in connection with the object of the investigation, and is 
merely due to the armature having been driven at the various speeds 
corresponding to the conditions of practice for the corresponding values of 
the current. 



HYSTERESIS Loss IN CORES. 

The hysteresis loss in armature cores may be estimated directly from 
curve A of Fig. 35 (page 34), which represents the magnetic grade of iron 
generally used in armature construction. However, the temperature of 
annealing, and the subsequent treatment of the iron, materially influence 
the result. 

In Fig. 115 (page 108) are given three curves of total core losses of 
three railway motor armatures. 

Curve 1. Iron annealed after punching. 
Curve 2. Iron annealed before punching. 
Curve 3. Iron not annealed. 

Nevertheless, it is very likely that in the case of a railway motor 
armature, the rough conditions of service soon largely destroy any 
temporary gain from annealing subsequent to punching. 

In Fig. 1 1 6 the total core loss in the armature with unannealed 
iron has been analysed, and the hysteresis and eddy current components 
are shown in curves Nos. 2 and 3, the resultant loss being given in 
curve No. 1. 

The question of core loss is not of vital importance in armatures, 
being of chief interest from the thermal standpoint. But with trans 
formers it is of the utmost importance, as it is the controlling factor in 
determining the all-day efficiency. Special consideration will be given 
hereafter to the matter of core loss in transformers. At this point it 
will be sufficient to state that iron of at least as good quality as that 
shown in Curve B of Fig. 35, should be specified and secured. Even 
with sheets carefully japanned, or separated by paper, the eddy-current 
loss in transformers will be from once and a half to twice the theoretical 
value given in the curves of Fig. 3G. This may, perhaps, be explained 
by supposing the flux not to follow the plane of the sheet, but to 
sometimes follow a slightly transverse path, thus having a component in 



108 



Electric Generators. 



a direction very favourable for the setting up of eddy currents in the 
plane of the sheets. In Figs. 139 and 140, on page 136, will be found 
curves especially arranged for convenience in determining transformer core 
losses. 

In addition to considering the subject of heating from the standpoint 
of degrees rise of temperature per watt per square inch of radiating 
surface, it is useful in certain cases to consider it on the basis of rate 
of generation of heat, expressed in watts per pound of material. Similarly 
to the manner in which the curves of Figs. 35 and 36 give the rate of 
generation of heat in iron by hysteresis and eddy currents, there are 
given in Fig. 117 curves showing the rate of generation of heat in copper, 



Railway motor core tots 




Piq.116. Rai/noy Armature. Uaarrnealeet Iron 



,,. 
(W76) 




Amperes 



due to ohmic resistance. One s conception of the relative magnitudes 
of these quantities in copper and iron is rendered more definite by a study 
of the values given in Tables XXX. and XXXI. : 



TABLE XXX. COPPER. 



Current Density in 



Rate of Generation of Heat by Ohmic Resistance. Watts per Pound. 



Amperes per 
Square Inch. 


Deg. 
Cent. 


20 Deg. 
Cent. 


40 Deg. 
Cent. 


60 Deg. 
Cent. 


80 Deg. 

Cent. 


100 Deg. 

Cent, 


500 


.50 


.54 


.58 


.62 


.67 


.71 


1000 


2.00 


2.15 


2.33 


2.48 


2.68 


2.84 


1500 


4.40 


4.74 


5.1 


5.5 


5.9 


6.2 


2000 


7.9 


8.4 


9.1 


9.8 


10.6 


11.2 


2500 


12.3 


13.3 


14.3 


15.3 


16.5 


17.5 


3000 


17.7 


19.0 


20.6 


22.8 


23.7 


25.0 



Specific Rate of Generation of Heat. 
TABLE XXXI. SHEET IRON. 



109 



Flux Density 
(Kilolines per 
Square Inch). 


Rate of Generation of Heat by Hysteretic Resistance (and by Ohmic Resistance to 
the Extent to which Eddy Currents are Present). 


25 Cycles. 


60 Cycles. 100 Cycles. 


125 Cycles. 


20 
40 
60 
80 
100 
120 
140 


.10 
.27 
.56 
.92 
1.4 
2.0 
2.8 


.25 
.75 
1.5 
2.5 
3.8 
5.4 
7.7 


.44 
1.3 
2.8 
4.8 
7.3 
10.5 
15 


.59 
1.85 
4.0 
6.7 
10.5 
15 
22 



Table XXXI. should also be used in calculating iron losses at high 

o o 

densities, as it extends beyond the range of the curves of Figs. 35 and 36. 

Smooth-core armatures can be run at higher current densities than 
iron-clad armatures, owing to the better opportunity for cooling. Like 
wise with iron-clad armatures, those with a few large coils have to be 
designed with lower current densities than those in which the winding 
is subdivided into many smaller coils. 

In Table XXXII. are given some rough figures for the current 
densities used in various cases : 

TABLE XXXII. 



Small high-speed armatures 

Large ,, 

Small low-speed armatures 

Large 

Transformers with forced circulation of oil or air 

Large transformers immersed in oil or air 

Small 



Amperes per 
Square Inch. 
2500 to 3500 


1500 


2500 


1500 


2000 


1100 


1600 


800 


1500 


500 


900 


500 


1100 



In the case of small transformers the current density could be very 
much higher without causing excessive temperature rise, but such trans 
formers would have poor regulation. On the other hand, large trans 
formers, when properly designed, have better regulation than is necessary, 
the current density being limited from thermal considerations. Although 
many large transformers are so poorly designed that a few hours run at 
full load heats them up to above 100 deg. Cent., this is bad practice, as 
it causes deterioration both of insulation and of iron. 1 A rise of not more 
than 60 deg. Cent, should be aimed at, even with large transformers. 

1 See pages 29 to 32 for discussion of deterioration of iron at high temperatures. 



110 



Electric Generators. 



The curve of Fig. 118 shows that even a rise of 60 cleg. Cent, reduces 
the insulation resistance of a transformer to a small percentage of its 
resistance when cold. In other words, insulating substances have a very 
large negative temperature coefficient. In this case, where the insulating 
material was a composition of mica and cloth, the transformer being 
immersed in oil with which the insulation was thoroughly impregnated, 
the average temperature coefficient between 20 deg. Cent, and 80 deg. 

































s* 











^^ 


1 
























^ 


<* 








^ 


i-^^ 


^^ 




jl 






















^ 








^ 


^ 


















0. 


r 






^ 


^ 
















1 














X 




in? 

" 


3 






















1 










/ 


^ 




^ 


































/ 




^ 


^ 


































/ 


/ 


x 


n a t 


.1) 


fo 


Tera 


i/<)/7 of Heat u 


id 


ypt 


r 










5 




/ 


/ 












iy 


Hi 


3IS 


tar 


ce 
















1 


/, 


/ 






































^ / 


/ 








































w 










































j? 










































y_ 
























































Watts per 


po^ 


ma 





















2600 
2100 
1200 

eooo 
isoo 

1600 
HOO 

1100 

woo 

BOO 
600 
WO 
200 



133TI 



I 2 3 5 6 7 8 3 10 II IS 13 II IS If 17 IS 13 20 SI & 13 



rkANSFORHEH INSULATION V^sf 
Ae Voltane used For testing 
ranges From 20 to 100 Volts 
f Temperature measured by Res 
"f Tranf former Coila 



Temperature in C 



Cent, was .8, that is, the insulation resistance increased 80 per cent, per 
deg. Cent, decrease of temperature. But the ability of this insulating 
material to withstand the disruptive effects of very high potentials is 
practically unimpaired. Consequently, it is important to distinguish 
carefully between the ability to withstand the application of high voltages 
and the insulation resistance, as measured in megohms. The insulation 
resistance in megohms returns to its original high value when the 
transformer is again cold. 



Heating and Efficiency of Electric Machines for Various Purposes. Ill 

RAILWAY MOTORS. 

The necessity in this class of apparatus of having high efficiency at 
light loads (which is the condition under which railway motors operate the 
greater part of the time), requires that they shall be designed with an 
efficiency curve which quickly reaches its maximum, and falls off very much 
at larger loads. As a consequence, a good railway motor cannot be 
operated for long periods at its full rated drawbar pull, without reaching an 
excessive and dangerous temperature. The need for compactness also 
requires running at high temperature under the condition of long-sustained 
full load. In the section relating to the design of railway motors, this 
matter is more fully considered. 

ARC DYNAMOS. 

Arc dynamos are designed to maintain constant current, partly, and 
sometimes almost entirely, by inherent self-regulation. This requires a 
large number of turns both on field and armature, and in order to obtain 
reasonable efficiency, the conductors have to be run at very low-current 
densities. As a consequence, a properly designed arc dynamo will 
run much cooler than would be at all necessary from the thermal stand 
point. Such a machine must be, of course, large and expensive for its 
output. 

In apparent contradiction to the above statement stands the fact that 
almost all arc machines at present in operation run very warm. But this 
is because almost all arc machines as now in use have such low efficiencies, 
particularly at anything less than full load, as to render it extremely 
wasteful to continue them in service. By throwing them all out and 
installing well-designed apparatus, the saving in maintenance would quickly 
cover the expenses incurred by the change. 

CONSTANT POTENTIAL DYNAMOS. 

In constant potential dynamos it should be the aim to have the 
electromagnetic and thermal limits coincide. Forty or fifty degrees 
Centigrade rise in temperature during continuous running is generally 
considered entirely satisfactory, although the requirements for Admiralty 
and other Government work are usually more rigid. In constant-potential 
machines the efficiency is so high (especially when compared with the engine 



112 Electric Generators. 

efficiency) when the temperature limit is satisfactory, that the efficiency 
should seldom be a determining factor. Proper thermal and electro 
magnetic constants should be the limiting considerations. 

In dynamos it is customary to quote the efficiency at the tem 
perature reached by the machine at the end of several (generally ten) 
hours run ; but in the case of transformers, it is generally quoted at 
20 deg. Cent. Nothing except prevailing practice justifies these con 
tradictory methods. 



COMMUTATOR HEATING. 
The heating of the commutator arises from three causes the 

o 

mechanical friction of the brushes, the C 2 R due to the useful current 
flowing across the contact resistances, and the heating due to the waste 
currents caused by short-circuiting of adjacent segments, and by sparking. 
Copper brushes may, under good conditions, be run up to a density of 
200 amperes per square inch of contact surface, and even higher in small 
machines. Carbon brushes should preferably not be run above 40 amperes 
per square inch of contact surface, except in small machines, where, with 
good conditions, much higher densities may be used. The pressure need 
seldom exceed 2 Ib. per square inch of brush-bearing surface, and a 
pressure of 20 oz. per square inch corresponds to good practice. In the 
case of railway motors this has to be considerably increased, because of the 
excessive jarring to which the brushes are subjected. 

At a peripheral speed of commutator of 2,500 ft. per minute, which 
corresponds to good practice, the rise of temperature of the commutator 
will seldom exceed 20 deg. Cent, per watt per square inch of peripheral 
radiating surface for unventilated commutators ; and with special 
ventilating arrangements depending upon centrifugal flow of air, this 
figure may be considerably improved upon. The total rise of tempera 
ture should preferably not exceed 50 deg. Cent, for continuous running 
at full load. 

The contact resistance offered by carbon brushes at a pressure of 
20 oz. per square inch of bearing surface, and at ordinary current 
densities and peripheral speeds, may be taken at .03 ohms per square 
inch of contact surface. That is, if there are, for instance, four positive 
and four negative brushes, each with 1.25 square inches of bearing 



Estimation of Temperature Rise of Commutator. 1 1 3 

surface, the resistance of the positive brushes will be - = .006 ohms 

4x 1.25 

and this will also be the resistance at the negative brushes ; consequently, 
the total contact resistance will be .012 ohms from positive to negative 
brushes. 

The contact resistance of copper brushes need not exceed .003 ohms, 
per square inch of contact surface, and with good conditions will be less. 

In estimating the friction loss, the coefficient of friction at the standard 
pressure, and with the commutator and brushes in good condition may be 
taken equal to .3. 

To illustrate the application of these constants in estimating the 
heating of a commutator, the case may be taken of a six-pole 120-kilowatt 
generator with a 30 in. diameter commutator, whose length, parallel to 
shaft, is 8 in., and which is furnished at each of its six neutral points with 
a set of four carbon brushes, each having a bearing surface of 1.5 in. 
x .75 in. = 1.13 square inches. Consequently, there being twelve posi 
tive and twelve negative brushes, the total cross-section of contact for 
the current is 12 x 1.13 = 13.5 square inches. 

The capacity of the machine is 480 amperes at 250 volts ; conse 
quently, the current density is 36 amperes per square inch. Taking the 
contact resistance at .03 ohms per square inch, the total contact resistance 

.03 
amounts to 1 Q x 2 = .0045 ohms from positive to negative ter- 

A. . XN -L J. O 

minals. Therefore the C 2 E loss is 480 2 x .0045 = 1050 watts. Pres 
sure is adjusted to about 1^ Ib. per square inch. Total pressure 1.25 x 
13.5 x 2 = 34 Ib. Speed = 300 revolutions per minute. Peripheral 
speed = 2360 ft. per minute. Therefore, foot-pounds per minute = 
2360 x 34 x .3 - 24,000 foot pounds = .73 horse-power = 545 watts. 

Watts. 
C 2 R = 1050 

Friction ... 545 

Allow for stray losses ... ... ... ... ... ... 100 

Total commutator loss ... ... = 1695 

Radiating surface = 8 x 30 x TT = 760 sq. in. 
Watts per sq. in. = 1695 -r 760 = 2.2. 

Figuring the rise at 20 deg. Cent, per watt per square inch, there is 
obtained : 

Total rise temperature = 2.2 x 20 = 44 deg. Cent. 

Q 



114 Electric Generators. 

Careful tests fail to show any considerable decrease in resistance 
of contact on increasing the brush pressure beyond 20 oz. per square 
inch, nor does it change very greatly for different speeds and current 
densities ; at least not enough to be worth taking into account in the 
necessarily rough approximate calculations. It will, of course, be under 
stood that when brushes or commutator are in poor condition, friction, 
C 2 R and stray losses, are certain to greatly increase. 

FRICTION Loss. 

The loss through windage and bearing friction necessarily is very 
dependent upon the nature of the design and the method of driving. 
When the armature is directly driven from the engine shaft, and is not 
provided with an outboard bearing, the loss has to be shared by both 
engine and dynamo. With belt-driven dynamos a third bearing beyond 
the pulley is sometimes necessary. The loss due to belt friction is not 
properly ascribable to the dynamo. If the armature and spider are 
furnished with internal fans and flues, or other ventilating arrangements, 
the advantage in cooling thereby gained necessarily involves increased 
friction loss. In a line of high-speed alternators thus designed, the friction 
loss ranged from one per cent, in the large sizes up to three per cent, in 
the small sizes, the range being from 400 kilowatts to 60 kilowatts 
capacity, and the machines being belt-driven, the belt losses, however, 
not being included. The speeds were from 360 revolutions per minute for 
the 400 kilowatt, up to 1500 revolutions per minute for the 60 kilowatts. 

Some similar continuous- current belt-driven generators, for rather 
lower speeds, had friction losses ranging from .8 per cent, in the 500 kilo 
watt sizes up to 2 per cent., or rather less, in the 500 kilowatt sizes. 

Large direct-coupled slow-speed generators will have considerably 
less than 1 per cent, friction loss, and such machines for 1000 kilowatts 
and over should have friction losses well within J- per cent. 



Design of the Magnetic Circuit. 115 



DESIGN OF THE MAGNETIC CIRCUIT. 

la practice, the solution of magnetic problems is generally largely 
empirical, on account of the very great difficulty in calculating the 
magnetic leakage, as well as in determining the precise path which will 
be followed by the magnetic flux in those parts of the magnetic circuit 
which are composed of non-magnetic material, such as in dynamos and 
motors the air gap between the pole-face and the armature surface. 
In closed circuit transformers no such difficulties arise, and the deter 
mination of the reluctance of the magnetic circuit becomes comparatively 
simple. 

Analogies between electric and magnetic circuits are misleading, 
since a magnetic circuit of iron located in air is similar to an electric 
circuit of high conductivity immersed in an electric circuit of low conduc 
tivity, the stream flow being proportional to the relative conductance 
of the two circuits. Moreover, in magnetic circuits the resistance varies 
with the flux in a manner dependent upon the form and materials of the 
magnetic circuit. 

For the purpose of calculation it is assumed that the magnetic flux 
distributes itself according to the reluctance of the several paths between 
any two points. The difference of magnetic potential between two points 
is equal to the sum of the several reluctances between these points, 
multiplied by the flux density along the line over which the reluctances 
are taken. The permeability of air being unity, and that of iron being 
a function of the flux density, it follows that a proportion of leakage 
flux, or flux external to the core of an electro-magnet, increases with the 
flux density in the core, and with the magnetic force. Practically, the 
function of a magnetic circuit is to deliver from a primary or magnetising 
member a definite magnetic flux to a secondary member. Thus, in the 
case of a dynamo or alternator, the function of the field magnets or primary 
member is to deliver a certain flux to the armature ; in the case of a 
transformer, that of passing through the secondary coils a certain magnetic 
flux. The secondary member reacts upon the primary member, and affects 
the effective magnetic flux according to the amount of current generated 



116 Electric Generators. 

in tho secondary member. Tnis reaction acts to change the magnetic flux 
in the secondary member in two ways, first by reducing the resultant 
effective magneto-motive force acting on the magnetic circuit ; and, 
secondly, by affecting the magnetic leakage by altering the differences 
of magnetic potential and distribution of magnetic forces around the mag 
netic circuit. 

In the case of a generator with brushes set with a forward lead, the 
reaction is such as to demagnetise the field magnets and increase the 
leakage. 

In the case of a motor with brushes set with a forward lead, the 
reaction is such as to increase the flux through the armature by added 
magneto-motive force and diminished leakage. 

In the case of an alternating-current generator, the reaction is such 
as to diminish the flux with lagging armature current, or with leading 
current to increase the flux. 

In the case of a transformer with lagging current, the effect is to 
diminish the effect of the primary current, and with leading current to 
increase this effect. 

As stated above, however, the leakage in general is affected according 
to the magneto-motive force between any two points. The effective 
flux in any magnetic circuit is equal to the resultant magneto- motive 
force divided by the reluctance of the magnetic circuit. Obviously, then, 
in the design of a magnetic circuit the effects of these reactions have to 
be carefully calculated. In the design of the field-magnet circuit of 
dynamos and alternators, the influence of the armature reaction on the 
effective magneto-motive force may be taken into consideration in the 
calculations by assuming a certain definite maximum armature reaction. 
These armature reactions will be discussed subsequently. Obviously, 
the flux density and magnetising force may in all cases vary very widely 
for a given total flux. Therefore, fulfilling equivalent conditions as to 
efficiency and heating, there is no fixed ratio between the amount of 
copper and iron required to produce a certain magnetic flux. The design 
ing of a magnetic circuit may then be said to be a question of produc 
ing in the secondary member a given effective magnetic flux, and with 
a given amount of energy expended in the primary magnetic coils, and 
with a minimum cost of material and labour ; and the most economical 
result is arrived at by means of a series of trial calculations. The energy 
wasted in the field magnets should not, in the case of continuous-current 



Armature Magnetomotive Force. 117 

machinery, generally exceed 1 or 1^ per cent, of the rated output, the 
permissible values being dependent mainly upon the size and speed . 
In all cases there is, of course, the condition that the magnetising coils 
shall be so proportioned as not to heat beyond a safe limit. 

In the case of transformers the condition becomes different. There 
is a constant loss of energy in the magnetic circuit, due to hysteresis. The 
amount of energy consumed in the magnetising coils at no load is 
negligible. At full load it is a considerable fraction of the total loss. 
Transformers are seldom worked at full load for any length of time, 
consequently the open circuit losses should be made consistent with the 
mean load of the transformer. The general design of the magnetic circuit 
of an alternating-current transformer may then be said to consist, for a 
given stated output, in securing a satisfactory " all day " efficiency and 
satisfactory thermal conditions for a minimum cost of material and labour, 
both the iron and copper losses being considered. 

In the case of continuous-current dynamos, the armature reaction as a 
factor in determining the design of the field magnets, is of greater impor 
tance now than heretofore. Thorough ventilation of the armature has so 
reduced the heating, that from this standpoint the output of dynamos has 
been greatly increased. The general introduction of carbon brushes, and a 
more thorough knowledge of the actions in commutation, has greatly 
increased the output for good operation from the standpoint of sparking. 
Thus the magnetomotive force of the armature has naturally become a 
much greater factor of the magnetomotive force of the field magnets. 
Taking the magnetomotive force of the armature as the line integral 
through the armature from brush to brush, there are numerous examples 
of very good comnmtating dynamos in which the magnetomotive force of 
the armature at full load is equal to that of the field magnets. In several 
large dynamos designed by Mr. H. F. Parshall, which have now been in 
use for so long a time that there is no question as to satisfactory operation, 
the magnetomotive force of the armature at full load was 50 per cent, 
greater than the magnetomotive force of the field magnets ; and the number 
of turns required in the series coils to maintain constant potential was 
approximately equal to that in the shunt coils to give the initial magnetisa 
tion. It is found in practice that the component of the armature magneto 
motive force opposing the field magnets, i.e., the demagnetising component, 
is from 18 to 30 per cent, of the total armature magnetomotive force. 
This corresponds to a lead of the brushes of from 9 to 15 per cent, of the 



118 Electric Generators. 

total angular distance between successive neutral points, i.e., to an angular 
lead of from 16 deg. to 27 deg., the angular span of two magnetic fields 
(north and south) being taken as 360 deg. 

The armature reaction, therefore, in modern practice greatly increases 
the amount of material required in the field-magnet coils and in the field- 
magnetic circuit, by increasing the economical length of the magnetic core 
and coils, which in turn tends to increase the magnetic leakage, and there 
fore to require greater cross-section of magnetic circuit. As yet, however, 
practice has not been sufficiently developed to reach the limit beyond which 
the total cost of the dynamo is increased, by increasing the armature 
reaction. The field magnet may, therefore, be considered, in general 
practice, a subservient member. The limit, of course, to the armature 
reaction is frequently reached in the case of such compound dynamos as are 
required to give an approximately constant potential over the whole 
working range. 

In the case of alternators, the thermal limit of output has been 
increased by ventilation, as in commutating machines. By the introduction 
of a general system of air passages, shorter armatures have become possible, 
consequently natural ventilation of the armature has been vastly increased. 

The tendency in recent practice has been to limit the output of 
alternators from the standpoint of inherent regulation, and the thermal 
limit of output has been generally determined to conform with the 
conditions laid down as to regulation and inductance. Alternators 
designed to work over inductive lines for power purposes are very 
frequently designed with one-half the armature reaction that would be 
used in the case of lighting machines. 

A full discussion of the armature reaction of alternators will be 
given in a later section. It may be stated here, that in uni-slot 
single-phase alternators, the value of the reluctance of the magnetic 
circuit becomes very dependent upon the position of the armature 
slot with respect to the pole-face ; hence the reluctance undergoes 
a periodic variation of n cycles per revolution of the armature, n 
being the number of field-poles. The variation is generally of so 
great an amplitude as to make it important to construct the entire 
magnetic circuit of laminated iron, otherwise the field frame becomes the 
seat of a very substantial loss of energy through eddy currents. Although 
this loss is less serious in multi-slot single-phase alternators and in poly 
phase alternators, it should be carefully considered ; and it will often be 



Coefficient of Magnetic Leakage. 119 

found desirable in such machines to adopt a laminated construction of the 
entire field frame. Even in continuous-current machines, the loss may 
sometimes be considerable, being of greater value, the fewer the slots per 
pole-piece, the wider the slot openings and the shorter the air gap. But 
in continuous-current machines, there are almost always enough slots to 
insure the restriction of the magnetic pulsations to the vicinity of the pole- 
face, and hence it is often the practice to laminate the pole-faces only. 
But in all alternators, even with multi-slot armatures, present practice 
requires that the magnet cores, at least, shall be laminated for the entire 
length. The pulsations of the flux throughout the magnetic circuit, due to 
periodic variations in the reluctance, reach their greatest extent in the 
inductor type of alternator, and constitute one of the objections to most 
varieties of this type of alternator. 



LEAKAGE COEFFICIENT. 

The coefficient by which the flux which reaches the armature and 
becomes linked with the armature turns must be multiplied in order to 
derive the total flux generated by the field coils, is known as the " leakage 
coefficient," and in most cases is considerably greater than unity. It is 
evident that the " leakage coefficient " should increase with the load, 
since the armature ampere turns serve to raise the magnetic potential 
between the surfaces of the adjacent pole-faces, and tend to increase the 
component of flux leaking between adjacent pole tips and over the surface 
of the armature teeth above the level of the armature conductors. The 
annexed diagrams give the values of the leakage coefficients as determined 
from actual measurements for several cases. It will be noted that in 
Fig. 122 are given results both with and without current in the armature. 
(See Figs. 119 to 124.) 



ARMATURE CORE RELUCTANCE. 

The reluctance of the armature core proper is generally fixed by 
thermal conditions, which are dependent upon the density and periodicity 
at which the core is run, the reluctance being chosen as high as is consistent 
with the permissible core loss. 



120 



Electric Generators. 







Reluctance of Magnetic Circuit. 121 

AIR GAP RELUCTANCE. 

The reluctance between the armature core and the faces of the pole- 
pieces is determined by the space required by the armature conductors and 
the necessary mechanical clearance between the armature surface and the 
pole-faces. 1 

RELUCTANCE OF COMPLETE MAGNETIC CIRCUIT. 

The reluctance for a given length of magnetic circuit should be such 
that the combined cost of magnetic iron and magnetising copper is a 
minimum. The length of the magnetic circuit should be such that, with 
what may be termed the most economical densities, the cost of the copper 
and iron is a minimum. By magnetising copper is meant that amount of 
copper required by the magnetising coils to give, under fixed thermal 
conditions, that magnetomotive force that will maintain the proper flux 

1 In discussing the sparking limit of output of a smooth-core armature, it has been 
frequently asserted that the sparking limit of a generator is a function of the depth of the 
air gap. But the inductance of the armature coils when under commutation is not appreciably 
diminished by increasing the depth of the air gap, except in machines where the brushes have 
to be set forward into the near neighbourhood of the pole-tip, which is not necessary in 
well-designed generators. Therefore, the depth of the air gap has no relation to the magnetic 
sparking output, except in so far as it may alter the distribution of magnetism in the gap. 
Beyond a certain limit, increasing the depth of the air gap acts deleteriously on the sparking 
limit, since the distribution of the magnetic flux in the gap becomes such that the permissible 
angular range of commutation is very small. In the case of toothed armatures (which are 
now common practice), the air gap in good practice is made as small as is consistent with 
mechanical safety. The density in the projections is carried to a very high value, it being 
generally recognised that the greater the magnetic density at the pole-face, the greater 
armature reaction is possible without sparking. To satisfy this condition alone, a high 
density in the projections becomes necessary. It has, however, been pointed out that, with 
the projection normally worked out, magnetic distortion in the air gap may be made greatly 
less than in the case of a well-designed smooth-core armature. In the smooth-core machine 
the distortion in the gap is proportional to the armature reaction; whereas in the case of 
highly magnetised projections the distortion is greatly less than proportional to the armature 
reaction. Considered with relation to the inductance of the armature coils, it appears that 
the inductance of the coils becomes smaller and smaller as the magnetic reluctance in the 
circuit surrounding the coils becomes increased. All of these conditions may be included 
broadly by saying that for a given output there is a certain limiting minimum reluctance 
in the air gap, having regard both to distortion and self-induction. As will be shown later, 
however, sparkless commutation has to be considered not only in its relation to the inductance 
of the armature coils and to the strength of the reversing field, but also in respect to the 
nature of the collecting brushes. Generally speaking, visible sparking, or that external 
to the brushes, is least injurious to the commutator. 

R 



122 Electric Generators. 

through the armature at full load. The densities should be taken to 
correspond with the full voltage generated by the armature. The propor 
tions of the magnets should be taken to correspond with the magneto 
motive force required at full load. 

For a given density the magnet coils should be of a certain length ; 
if too long, the cost of the iron will be excessive ; if too short, the cost of 
the copper will be excessive, since the radiating surface of the coil will be 
too restricted. The depth of the magnet coil must, in practice, be 
restricted ; otherwise, the temperature of the inner layers will become 
excessive. 1 



ESTIMATION OF GAP RELUCTANCE. 

The magnetomotive force (expressed in ampere turns) expended in 
maintaining a flux of D lines per square inch, across an air gap of 
length L (expressed in inches) is.313 x D x L. The proof of this is as 
follows : 

D lines per sq. in. = - lines per square centimetre. 

6.45 

B= 



6.45 

For air 

H = B. 

H= D . 

6.45 



4 it nC 
But H = 1fl 7 , / being length expressed in centimetres, and n C being 

ampere turns (number of turns x current). 



n 1 u / 

n C = . _ x H x . 

4 TT 

10 D 

= x x 2.54 L. 

4 TT 6.45 

= .313 x D x L. 



1 The increase of temperature of the magnet coils should be determined by the increase 
in their resistance. Placing the thermometer on the external surface, unless the winding 
is very shallow, is not a satisfactory indication as to whether or not the inner layers may 
not be so hot as to increase the resistance of the coil so much that its magnetomotive force 
at a given voltage is greatly diminished. 



Reluctance of Core Projections. 123 

RELUCTANCE OF CORE PROJECTIONS. 

The armature projections between the conductors are generally 
magnetised well towards saturation, so that the determination of the 
magnetic force required for a given flux across this part of the magnetic 
circuit is of importance. The following method will be found useful : 

The magnetic flux divides between two paths : 

1. The iron projections. 

2. The slots containing the conductors, and the spaces between the 
laminations. 

The proportion of the flux flowing along each path is proportional to 
its magnetic conductance. There are several considerations which make 
the cross-section of the iron path small compared with that of the other 
paths. 

1. In practice the width of the tooth is generally from 50 to 80 per 
cent, of the width of the slot. 

2. The slot is broader in a direction parallel to the shaft than the iron 
portion of the lamination, because of the 25 per cent, of the length of the 
armature frequently taken up by insulation between laminations, and by 
ventilating ducts. 

3. This 25 per cent, of insulation and ducts, itself offers a path, which 
in the following calculation it will be convenient to add to the slot, 
denoting the total as the air path. 

It thus appears that although the iron path is of higher permeability, 
the air path has sufficiently greater cross-section, so that it takes a con 
siderable portion of the flux ; and it will be readily understood that the 
resultant reluctance of the paths in multiple being considerably less, and 
the density of the flux being decreased at a point where the permeability 
increases rapidly with decreasing density, the magnetomotive force 
necessary for a given flux may be greatly less than that required to send 
the entire flux through the projections. 

Let a = width of tooth. 
b = slot. (See Fig. 125.) 
,, k = breadth between armature heads, of iron part Ju, < 6 

of lamination. 
a k = cross-section of iron in one tooth. 



- = cross-section of slot (because 25 per cent, of the breadth of the 
7 *} 

armature is taken up by ventilating ducts and insulation 

between laminations, and the breadth of the slot exceeds that 
of the iron in the tooth by that amount). 



1 "24 Electric Generators. 

If in any particular design this proportion varies from 25 per cent., 
new calculations may be made, if the magnitude of the variation is sufficient 
to warrant it. Moreover, there is 25 per cent, of ventilating ducts and 
insulation in the breadth of the tooth itself The cross-section of this will 

be .25 _- = .33 a k. It will be convenient to add this to the slots, and 
./ o 

denote the total as the air path. 

Cross^section of air path = - - + .33 a k = 1.34 b k 4- .33 a k. 

.75 

This air rath, thereioie, takes in all jaths except the iron lamination. 

Let I = depth of tooth and slot. 

X = lines to be transmitted by the combined tooth and slot, and 
u = permeability of iron in tooth, at true density. 

Let the X lines so divide that there shall be 

n in iron path, and X - n in air path. 
= density in iron path. 



at 

and 

X - n 



1.34 b k ~ .03 a k 



density in air path. 



Conductivity of iron path = " ; 

f, , . . 1.34 b k + .33 a k 
Conductivity of air path = . 

Xow. the fluxes n and X n in iron and air will be directly propor 
tional to the respective conductivities : 

aku 



X - n 1.34 b k + .33 n k 1.34 b + .33 a 

~~i~ 

1.34 b n ~ .33 a = atX i n ; 
n (1.34 b + .33a + a ) = a a >* : 
X 1.34 b + .33a + a m 



Let B = true density in iron, and B 1 = density calculated on the 
assumption that the iron transmits the entire flux. Therefore, the ratio of 

X (the total lines) to n (those in iron*, i.e. , will equal the ratio of B 1 



Reluctance of Core Projections. 



125 



(the density figured on the assumption that all the lines are in iron), to B 
(the actual density in iron). 

B 1 = N = 1.34 b + .33a + a ft 
B n a ft 

In Table XXXIII. are calculated some values of TY for different 



B 



values of 



a 



1. - = 1 



TABLE XXXIII. 
(i.e., width tooth = width slot) 



f - 

B 



2. - .75 ( 
o 

3. - = .50 ( 
b 



. _ 
B 



2.12 + 



B 1 _ 3.00 -1- 

T>~ ~~ ~ 

-D 



The next step in this process requires reference to the iron curves of 
Fig. 126. From these curves Table XXXIV. is derived : 

TABLE XXXIV. 



Corrected 
Iron Densities. 


Densities Figured on Assumption that Iron Transmits 
Entire Flux. 


B. 


(;- ) (;-) 


(=.) 


17,000 
18,000 
19,000 
20,000 
21,000 
22,000 
23,000 


133 17,200 
92 18,400 
56 19,500 
33 21,000 
23 22,500 
17 24,200 
13 26,000 


17,300 
18,500 
19,800 
21,300 
23,000 
24,700 
26,800 


17,400 
18,600 
20,000 
21,800 
23,700 
26,000 
28,300 


TABLE XXXV. DENSITIES IN INCHES. 


Corrected Iron 
Densities. 


Densities Figured on Assumption that Iron Transmits 
Entire Flux. 


Kilolines per Square 
Inch. 


1 = L 1 = 75 


-..*, 


110 111 


112 


113 


116 


119 


120 


121 


123 


127 


128 


129 


129 


136 


138 


141 


136 


145 


149 


153 


142 


156 


160 


lh> 


149 


168 


173 


183 



126 



Electric Generators. 



In the curves of Fig. 127, the values of the densities in the 
Tables have been transposed into kilolines density per square inch, and are 
thus available for use in dynamo calculations, where the process simply 
consists in figuring the iron density as if the iron transmitted the entire 
flux, and obtaining from the curves a corrected value for use in figuring the 
magnetomotive force. The number of teeth to be taken as transmitting 
the flux has to be determined by judgment, and is influenced by the 
length of the gap. Generally, increasing by one, the number lying 



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directly under the pole-face gives good results for machines with very 
small air gaps, while two or three extra teeth should be added for 
larger gaps. 



CALCULATION FOR MAGNETIC CIRCUIT OF DYNAMO. 

The following example of a very simple case may be of interest, 
as giving some idea of the general method of handling such problems : 

A certain ironclad dynamo has an air-gap density of 40 kilo- 
lines (per square inch), the density in the magnet core is 90 kilolines, 
and in the magnet yoke 80 kilolines. The frame is of cast steel. 
The tooth density is 110 kilolines, and the armature density is 
50 kilolines. 



Length of gap 

magnet core (as related to the magnetic circuit) 

yoke (corresponding to one spool) 

,, tooth 

,, armature (corresponding to one spool) ... 



in. 
.25 
10 
6 

1.5 
4 



Field Winding Calculation. 127 

Required number of ampere-turns per spool at no load : 

Ampere-turns for gap = .313 x 40,000 x .25 3130 

Ampere-turns for magnet core (from curve A of Fig. 14, page 21) 

-- 47 x 10 470 

Ampere-turns for yoke = 29 x G ... ... ... 170 

Ampere-turns for teeth (from curve B of Fig. 22) == 150 x 1.5= 230 

Ampere-turns for armature core = 6x4 20 



Total 4020 

Therefore ampere-turns per pole-piece at no load = 4020. 

It thus appears that, for practical purposes, it is much more direct to 
proceed as in the above example, than to go through a laborious calcula 
tion of the total reluctance of the magnetic circuit, incidentally bringing in 
the permeability and other factors, as described in many text-books. 



FIELD WINDING FORMULA. 

In making field winding calculations, the following formula is of great 
service. 

Lb. =- 

watts 

in which 

Lb. = Pounds of copper per spool. 

Ampere-feet = Ampere-turns x mean length of one turn, expressed in feet. 
Watts = watts consumed in the spool at 20 deg. Cent. 

This formula is derived as follows : 

Resistance between opposite faces of a cubic inch of commercial copper at 20 deg. Cent. 
= .00000068 ohms. 

If length in inches = L, and cross-section in square inches = S, then 

T , .00000068 L 

-l\i 

.00000068 L 2 

h Li = . 

Let / = mean length of one turn in inches. 

t = number of turns. 
It = L. 

.00000068 /- f 2 

O Ju = - 

= .00000068 C 2 F P 

e- H 



128 Electric Generators. 

Git 

= ampere-feet (ampere-turns x mean length of one turn in feet). 

Git = 12 x ampere-feet. 
G- P? = 144 (ampere-feet) 2 . 
C 2 R = watts. 

/ampere-feet 
.68 x!44 x ( ^ 00 

b Li 

watts 

/ ampere-feet 

.32 x .68 x 144 x / ^^ 

Lb. = .32 S L = 



watts 



/ampere feet \ 

Lb. . ( 1000- ) 

watts 



APPLICATION TO CALCULATION OF A SPOOL WINDING FOR A SHUNT- 
WOUND DYNAMO. 

Thus, suppose the case of a machine for which it had been determined 
that 5,000 ampere-turns per spool would be required. Assume that the 
mean length of one turn is 4.0 ft. Then 



/ampere-feetV /5000 x 4\ 2 _ 

V 1000 / \ 1000 ) 



The radiating surface of the spool may be supposed to have been 
600 square inches. After due consideration of the opportunities for 
ventilation, it may be assumed to have been decided to permit .40 watts 
per square inch of radiating surface at 20 deg. Cent, (it, of course 
increasing to a higher value as the machine warms up). 



watts = 600 x .40 = 240 per spool. 

x 4 
~240~ 



31 x 400 
Ib. copper per spool = ~-j7j =52 Ib. 



This illustrates the application of the formula, but it will be of interest to 
proceed further and determine the winding to be used. 

A six-pole machine will be taken, designed for separate excitation 
from a 250 volt exciter. In order to have room for adjustment, as well as 
to allow for probable lack of agreement between the calculated and 
actual values, it is desirable to have but 220 volts at the winding terminals 
under normal conditions of operation. This is 220/6 = 36.7 volts per 
spool. 



Typical Magnetic Circuits. 
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130 



Electric Generators. 



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Electric Generators. 






















































































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Field Winding Calculation. 



133 



The conditions as regards ventilation indicate a rise of 30 deg. Cent, 
in the temperature of the spool winding under the conditions of operation. 
Then the watts per spool are : 

1.17 x 240 = 280 watts at 50 deg. Cent. 

280 

Amperes = - _ = 7.6 
36.7 

rp , 5000 

lurns per spool = = 655 



7.6 
TYPICAL MAGNETIC CIRCUITS. 



Rg.186. 




depth of armature -4 



NES FLUX IN ARMATURE 
PER FIELD POLE. 



1tiO 
16O 
14O 
12O 

4O 
























X 
























x 


X 
























X 


X 
























x 
























x 


x 


?* FACTOR ASSUMED -1.20 
SPEED -/27OR.P.M. 












/ 


x 
























X 
























x 


x 
























x 





























46 



10 H 1Z 13 14 



AMPERE TURNS PER POLE PIECE +/0 3 



And as the mean length of one turn is 4.0 ft., the total length of 
winding is : 



655. x 4 = 2620 ft, 
Pounds per 1000 ft. = =19.8 



From the Table of properties of commercial copper wire, it will be 



134 



Electric Generators. 



found that No. 12 B. and S. has 19.8 Ib. per 1,000 ft., and is, therefore, the 
proper size. Generally, the desired value for the pounds per 1,000 ft. does 
not come out very nearly like that of any standard size of wire. In such a 
case, the winding may be made up of two different sizes of wire, one 
smaller and the other larger than the desired size. Generally, however it 
is sufficiently exact to take the nearest standard size of wire. 

Suppose the space inside the spool flanges to have been 10 in. long, 
then, after insulating, 9^ in. would probably be available for winding. 
From the Table of properties of commercial copper wire it will be found 




t A . 4 & X-J A II essential parts 
substanba/fy to scale 





























S 


















. 


























/ 










E 












1 









































































? 


















Cnojs depth arm 


Is 






















^ 






















lam I0 /i 


i - 




















, 
























3 ducts each % wide 


- 


















/ 












































/ 






























sSi 














/ 






St 


/; 


r 


/ 


iv 


S 


(. / 


. ! 
























/ 










.1 


r 


D 


S 


IJ 
















S S 










/ 










































, 















































\ 






















































































/ 














































^ _ 






An 


l 


?r 


" t 


u 


n 


t 


t 


i 


i 


i 


^ 


i C 

















< ><.;/ 1000 2000 3000 4000 SOOO 



that double cotton-covered No. 12 B. and S. has a diameter of .091 in. 
Therefore it should have 9.5/.091. = 105 turns per layer. Plan to take only 
100 turns per layer, so as to have a margin. 

Number of layers = 655/100 = 6.6 layers. 

Therefore, winding will consist of 6.G layers of 100 turns each, of 
D.C.C. No. 12 B. and S., and will require 220 volts at its terminals when 
warm, it carrying 7.6 amperes. 

Calculations relating to the compounding coils of machines will be 
given later, after the theory of armature reaction has been developed. 

It is now proposed to give experimentally determined no-load satura 
tion curves for several different types of machines, together with sufficient 




Magnetic Circuit of the Transformer. 135 

of the leading dimensions of the machines to enable the results to be 
profitably studied and compared. 

In the case of Fig. 128, two machines were tested. Same fields, but 
one armature having slots as shown at A and B, and the other as shown at 
C, D, and E. The armature coils used in the tests were those in slots A 
and C respectively. For figuring the flux in the case of A, the " form 
factor " was taken as 1.25. For C, the " form factor" was taken as 1.11. 
In the case of a winding at B, the results would probably have corre 
sponded to an appreciably different " form factor" from that used for A. 
In the tests the coils contained in the slots B were not employed. 

The saturation curves A and C exhibit the results and show the total 
reluctance of the magnetic circuit to be substantially the same for the two 
cases. In Figs. 129 to 137, inclusive, nine other examples are given, the 
necessary data accompanying the figures. 

MAGNETIC CIRCUIT OF THE TRANSFORMER. 

The calculation of the magnetic circuit in the case of transformers 
cannot, of course, be at all completely dealt with until the whole matter of 
transformer design is taken up in a later section. But the following 
example will give a general idea of the considerations involved, and 
will illustrate the use of B-H and hysteresis and eddy current curves : 
Ten-kilowatt Transformer. The magnetic circuit is shown in the 
accompanying sketch (Fig. 138). Primary voltage == 2,000 volts. 
Secondary voltage =100 volts. Primary turns = 2,340, periodicity 80 
cycles per second. E = 4 F.T.N.M. x 10~ 8 . Assume that the trans 
former is to be used on a circuit having a sine wave of electromotive 
force. The "form factor" of a sine wave rs 1.11 ; hence 

F - 1.11 

2000 = 4 x 1.11 x 2340 80 x M x 1Q- 8 
M = 240,000 lines = .24 megalines. 

Effective cross-section of magnetic circuit = 3.13 x 3.13 x .90 1 = 8.8 
square inches. 

Density = 27.3 kilolines per square inch. 

First calculate magnetising component of leakage current. From 
curve B of Fig. 22 (page 26), we find that at a density of 27.3 kilo- 

1 Ninety per cent, of the total depth of laminations in iron, the remaining 10 per cent, 
being japan varnish or paper for insulating the laminations from each other. 



136 



Electric Generators. 



lines, there is required about three ampere-turns of magnetomotive force 
per inch length of magnetic circuit. 

Mean length of magnetic circuit = 59.5 in. 
.-. Require magnetomotive force of 59.5 x 3 = 179 ampere turns. 



There are 2,340 turns. 



179 



.. Require a maximum current of = .077 amperes. 



R.M.S. current = 



.077 



2340 
= .054 amperes. 



Density in Kilolines per SiJ,. Inch. 

M * 0, Q> 5 C 

o o 5 o o o 


Curve For calculating Hysteresis loss 
in TransFormer cores. 


Take dotted curve For Transformers 
oF less Urtan 3 K.W. capacity. 


^ 








^ 


^^*"^ 






^^ 








^^ 








^ 










/ 











1234 

Watts per pound at 100 Cycles per Sec. 
Fig14(j. 




Loss decreases-Si* per degree 
Centegraote, increase oF 
temperature. 
Thickness pF plate* OI4 . 



i 4S1SR -20 -4-0 -60 -80 1-00 1-20 1-40 1-60 t-BO Z-00 

Watts per pound at 20 Centigrade. 

Next estimate core loss component of leakage current. Weight of 
sheet iron = 59.5 x 8.8 x .282 = 148 Ib. At 80 cycles and 27.3 kilo- 
lines, Fig. 139 shows that there will be a hysteresis loss of .6 x .8 = .48 
watts per pound. 

T7 U 4. 2 > 000 

Volts per turn per square inch 01 iron cross-section = - 

2,340 x 8.8 

= .097. From Fig. 140 the eddy current loss is found to be .21 watts 
per pound. 

Consequently hysteresis and eddy current loss will be .48 -I- .21 = .69 
watts per pound. Total iron loss = 148 x .69 = 102 watts. Core loss 
component of leakage current = 102 -=- 2,000 = .051 JR.M.S. amperes. 



Magnetic Circuit of the Induction Motor. 



13; 



Resultant leakage current = /y/.054 2 + .05 1 2 = .074 amperes. Full 

i A 10,000 

load current = 



2,000 



= 5.0 amperes. 



Consequently resultant leakage current = 1.4 per cent, of full-load 
current. Core loss = 1.02 per cent, of full-load rated output. 

Example. Find core loss and leakage current for the same trans 
former with the same winding when running on a 2,200-volt 60 cycles 
circuit. 



MAGNETIC CIRCUIT OF THE INDUCTION MOTOR. 

In Fig. 141 is represented the magnetic structure of a six-pole three- 
phase induction motor. The primary winding is located in the external 



JKgMI. 



Fuj.138. 




, MAGNETIC CIRCUIT Of INDUCTION MOTOR. 

6 Poles, stater (primary], has 64 slots, delta connected 
dphase minding, with 108 turns tn series per phase, for 110 volts at 60 Cycles. 

stator, which has 54 slots. There are 12 conductors per slot, consequently 
12 x 54 = 648 total face conductors, 324 turns, and 108 turns in series 
per phase. The motor is for 100 volts, and 60 cycles, and its primary 
windings are A connected. When run from a sine wave circuit, we have 

o 

110 = 4 x 1.11 x 108 x 60 x M x 10~ s 
M = .38 megalines. 

Before proceeding to the calculations directly concerned in the de 
termination of the magnetising current for the magnetic circuit of this 
induction motor, it will be necessary to study the relations between 
magnetomotive force and flux distribution in this type of magnetic circuit 
and winding. 

In Fig 142, a portion of the gap face of the primary is developed 
along a straight line, and the slots occupied by the three windings are 



138 



Electric Generators. 



lettered A, B, and C. The relative magnitudes of the currents in the 
three windings at the instant under consideration are given numerically 
immediately under the letters, and the relative directions of these currents 
are indicated in the customary manner by points and crosses. The instant 
chosen is that at which current in phase A is at its maximum, denoted 
by 1, the currents in B and C then having the value .5. 

The curve plotted immediately above this diagram shows the 
distribution of magnetic flux in the gap, at this instant, on the assumption 
that the gap density is at each point directly proportional to the sum total 
of the magnetomotive forces at that point. Thus the magnetic line which, 
in closing upon itself, may be conceived to cross the gap at the points 



Di&trib it/an 



JNDtJCTIQN 



of Res tit an 




M and N, is linked with the maximum ampere turns. Taking the 
instantaneous current in conductors of phase A as 1, and in phases B and 
C as .5, and for the monent considering there to be but one conductor per 
slot, the total linkage of ampere turns with the line m n is 3 x 1 + G x.5 
= 6, and the maximum ordinate is plotted at this point with the value 6. 

In the same way the other ordinates are plotted. From this curve it 
appears that the resultant of the magnetomotive forces of the three phases 
at the points M and N is two times the maximum magnetomotive force of 
one phase alone. This is a general property of such a three-phase winding. 

Moreover, an analysis of the curve shows the maximum ordinate to be 
1.6 times as great as the average ordinate. But this is only in this 
particular case. With different numbers of slots per pole-piece, this value 
would vary, and, owing partly to the increased reluctance in the high 



Magnetic Circuit of the Induction Motor. 



139 



density teeth, the curve would tend to be smoothed out and become less 
peaked. Consequently, the distribution of the flux density should be taken 
to h;ive a sinusoidal form. Practical calculations of the magnetising 
current agree best with observed results when the maximum value of the 
air-gap density over the pole-face is taken equal to >/2 times the average 
value. 

The above considerations are sufficient, as they enable us to determine 
the maximum values of magnetomotive force and flux, and it is from such 
values that the maximum magnetising current is derived. But it will be of 
interest to refer also to Fig. 143, in which are represented the conditions 
one-twelfth of a complete cycle (30 deg.) later, when the current in phase B 

INDUCTION MOTOR. 
JEuj.143. Distribution of Resulbanb Magnebcmobive Force*. 




has become zero, the current in phases A and C having become .867. 
Figs. 142 and 143 represent the limiting values between which the 
resultant magnetomotive force fluctuates as the magnetic field proceeds in 
its rotatory course about the magnetic structure. Various experimenters 
have shown this small variation in intensity to be, in practice, practically 
eliminated. An examination of the diagrams of Figs. 142 and 143 
shows that the maximum ordinates are 5.2 and 6 respectively, which 
corresponds to the theoretical ratio of 

^: 1 = 1:1.16. 



From Fig. 141 the following cross-sections of the magnet circuit per 
pole-piece at different positions are obtained : 



140 Electric Generators. 

Sq. In. 

A. Cross-section air gap per pole-piece at face of stator, i.e., surface 

area of exposed iron of projections ... 21 

B. Ditto for rotor face... ... 21 

C. Cross-section at narrowest part of projections in stator ... 10 

D. Cross-section at narrowest part of projections in rotor ... ... 8 

E. Cross-section in laminations back of slots in stator ... ... 10 

F. Cross section in laminations back of slots in rotor ... ... 8 



FLUX DENSITY. 

Average. Maximum. 

A. ... ... ... 18 kilolines ... 25 kilolines 

B. 18 , ... 25 

C. ... 38 54 

D. 48 68 

E. 38 

F. 48 

The depth of the air gap is -g-% in. (.047 in.), and the ampere-turns for 
the air gap amount to 

.313 x 25.000 x .047 = 370. 

For the iron, should allow about 8 ampere-turns per inch of length 
of the magnetic circuit, which, through the high density teeth, is about 
9 in. 

Ampere-turns for iron = 8 x 9 = 72 
Total ampere-turns per pole-piece = 370 + 72 = 442. 

Magnetomotive force of the three phases is equal to two times the 
maximum ampere-turns per pole-piece per phase. There are 18 turns per 
pole-piece per phase, therefore, letting C = R. M. S. amperes per phase, 
we have 

1.41 x C x 18 x 2 = 442. 

442 

C = T .11 = 8.7 amperes = magnetising current per phase. 

1.41 x 18 x 2 

w*)R 

Taking the core loss at 300 watts, the friction at 150 volts, and the 
C 2 R loss running light, at 50 watts, gives a total power, running light, of 
500 watts, or 167 watts per phase. Energy component of leakage current 

167 
per phase = = 1.5 amperes. 



Examples. 141 



Resultant leakage current per phase = A/ 8.7 2 + 1.5 2 = 9 amperes. 
Ditto per line leading to motor = 9 x />/3 = 15.6 amperes. 
Letting power factor, running light, equal P, we have 



P x 9 x 110 = 168 
P - .17. 



EXAMPLES. 

The following examples relate to matters treated of in the foregoing 
sections : 

1. A three-phase generator has 24 poles, 36 slots, 20 conductors 
per slot, Y connection. Volts between collector rings at no load 
and 500 revolutions per minute = 3500. What is the flux from 
each pole-piece into the armature, assuming the curve of electro 
motive force to be a sine wave ? (For type of winding, see Fig. 82, 
page 74.) 

2. A continuous-current dynamo has a two-circuit single winding (drum). 
Its output is 100 kilowatts at 550 volts. The current density in the 
armature conductors is 1200 amperes per square inch. It has 668 face 
conductors. Mean length of one armature turn is 75 in. 

What is the cross-section of the armature conductors ? 

What is the resistance of the armature from positive to negative 
brushes at 60 deg. Cent. ? 

The dynamo has six poles. If the speed is 200 revolutions per 
minute, what is the magnetic flux entering the armature from each 
pole-piece ? 

3. A six-pole continuous-current generator with a two-circuit, single 
winding, gives 600 volts with a certain field excitation and speed. There 
are 560 face conductors, arranged two per slot in 280 slots. If this 
winding is tapped oft at two points, equi-distant with reference to the 
winding, what would be the alternating current voltage at two collector 
rings connected to these points ? 

Assume the pole arc to be 60 per cent, of the polar pitch. 
4. 100-kilowatt dynamo, 250 volts, 4 poles ; 500 revolutions per 
minute ; armature wound with a two-circuit, triple-winding ; 402 face 

conductors arranged in 201 slots. Therefore - = 201 total turns. 

2 o 



142 Electric Generators. 

500 x 2 
= 33.5 turns in series between brushes. = 16.7 cycles per 

second. 

250 = 4 x 33.5 x 16.7 x 10~ 8 . 
.-. M = 11.2 megalines. Take leakage factor = 1.20. 

Flux in magnet cores = 11.2 x 1.20 = 13.5 megs. Magnet cores 
of cast steel, and run at density of 95 kilolines per square inch, therefore 

,. 13,500,000 i r- i.- 

cross-section = - =142 square inches. Circular cross-section. 

Diameter 13.5 in. 

Length armature core parallel to shaft = 16 in., of which 12 in. is 
solid iron, the remainder being occupied by venlilating ducts and the space 
lost by the japanning of the iron sheets. Diameter armature = 30 in. 
Length air gap = J in. Length magnet cores = 12 in. Length 
magnetic circuit in yoke = about 24 in. per pole-piece. Yoke of cast iron 
and run at density of 35 kilolines. Tooth density =120 kilolines. Core 
density = 70 kilolines. Therefore, depth of iron under teeth = 

11,200,000 , ., . 

0.7 in. Length magnetic circuit in armature = 



2 x 70,000 x 12 

10 in. per pole-piece. Pole arc measured along the arc = 17.5 in. Cross- 
section of pole-face = 16 in. x 17.5 in. = 280 square inches. 

11,200.000 

Pole-face density = - - = 40 kilolines. 

2bO 

Ampere-turns per pole-piece for yoke... = 24 x 60 = 1400 

Ampere-turns per pole-piece for mag 
netic core 12 x 50 600 

Ampere-turns per pole-piece for teeth... == 1.5 x 350 525 

Ampere-turns per pole-piece for arma 
ture core ... ... ... 10 x 12 = 120 

Ampere-turns per pole piece for air gap = .25 x 40,000 x .313 = 3130 

Total ampere-turns per pole-piece at no load and 250 volts = 5775 



Constant Potential, Continuous- Current Dynamos. 143 



CONSTANT POTENTIAL, CONTINUOUS-CURRENT 

DYNAMOS. 

THE problems peculiar to the design of the continuous-current dynamo 
are those relating to commutation. The design of the magnetic circuit, 

o o o 

and considerations relating to the thermal limit of output, to efficiency 
and to regulation, although matters of importance in obtaining a satis 
factory result, are nevertheless secondary to the question of commutation ; 
and they will consequently be considered incidentally to the treatment of 
the design from the commutating standpoint. 

Under the general class of constant potential dynamos are included 
not only dynamos designed to maintain constant potential at their termi 
nals for all values of the current output, but also those designed to 
maintain constant potential at some distant point or points, in which 
latter case the voltage at the generator terminals must increase with the 
current output, to compensate for the loss of potential in the transmission 
system. 

In the commutating dynamo, great improvement has been made in 
the last few years in the matter of sparkless collection of the commutated 
current ; in consequence of which, the commutator undergoes very little 
deterioration; and it is customary to require the dynamo to deliver, without 
harmful sparking, any load up to, and considerably in excess of, its rated 
output, with constant position of the brushes. This has been made neces 
sary by the conditions of service under which many of these machines must 
operate ; and the performance of such machines is in marked contrast to 
that of the dynamos of but a few years ago, in which the necessity of 
shifting the brushes forward in proportion to the load was looked upon as 
a matter of course. The change has been brought about by the better 
understanding of the occurrences during commutation, and to the gradual 
acquisition of data from which satisfactory constants have been deduced. 
One of the most important factors has been the very general introduction 
of high- resistance brushes, the use of copper brushes now generally being 
resorted to only for special purposes. 



144 Electric Generators. 

Radial bearing carbon brushes are now used very extensively, and 
although they were at first considered to be applicable only to high poten 
tial machines, where the quantity of current to be collected would not 
require too large and expensive a commutator, their use has been extended 
to low-voltage machines of fairly large output, the advantages being con 
sidered to justify the increased cost of the commutator. Various types of 
brushes have been developed, intermediate in resistance between carbon and 
copper, and different grades of carbon brushes, from high-resistance grades 
with fine grain for high potential machines, to grades of coarser grain and 
lower resistance for low potential machines. A corresponding develop 
ment has been taking place in the design of brush-holding devices. In the 
construction of the commutator, care is now taken to insulate the segments 
by mica, which shall wear at as near as possible the same rate as the copper 
segments ; and the construction of the commutator has now reached a stage 
where uneven bars and other sources of trouble of earlier days now no 
longer give concern. Of less importance, owing to the greatly increased 
durability of the modern commutator, are the modes of construction 
whereby sectors of the commutator may be renewed without disturbance 
to the remainder of the commutator. This is a method much employed 
in large commutators. Amongst the examples of modern dynamos which 
follow the discussion of matters of design, will be found illustrations of 
various types of commutator construction. 

The advance thus briefly summed up, in the mechanical design and in 
the careful choice of material for brushes, brush holders, and commutators, 
has been in no small measure responsible for the improvement in corn- 
mutating dynamos, and, when accompanied by correct electro-magnetic pro 
portions, has enabled manufacturers to dispense with the many ingenious 
but complicated windings and devices arranged to modify sparking by 
making use of various electro-magnetic principles requiring auxiliary 
windings, subsidiary poles, and other additions. Some of these non- 
sparking devices accomplish their purpose very effectively ; but, notwith 
standing the care and ingenuity displayed in their application, it does 
not appear likely that it will be commercially profitable to resort 
to them, since the careful application of ordinary methods appears to 
have already brought the constant potential commutating dynamo to that 
stage of development where the thermal limit of output of armature 
and field is reached below that output where harmful sparking occurs. 
Further improvement rendering it permissible to use more highly 



Armature Reaction. 145 

conducting brushes without encountering sparking, would of course result 
in a saving in the cost of the commutator, and from some source or other 
such improvement may appear. But as the saving can apparently only be 
effected at the commutator, it will not be sufficient in amount not to be 
more than offset by the increased cost of resorting to any of the auxiliary 
windings and devices yet proposed. 



ARMATURE REACTION. 

The study of the problems relating to sparking resolves itself down 
principally to the study of the reaction of the armature, which will now 
be considered and illustrated with relation to its influence upon the propor 
tioning of commutating dynamos, the choice of windings, and, finally, 
by descriptions of some modern dynamos. 

When discussing the formula) for electromotive force and the design 
of the magnetic circuit, it w T as pointed out that considerations relating 
to armature reaction make it necessary to modify the conclusions 
arrived at when these phenomena are left out of consideration. The 
formula for the electromotive force E = K T N M 10~ 8 , has already 
been given. Additional conditions are, however, imposed by the necessity 
of giving T, the turns, and M, the flux, such relative values as to 
fulfil the conditions necessary to obtain sparkless collection of the 
current, and satisfactory regulation of the voltage, with varying load. 

The requirements for commutating or reversing the current in the 
coil that is to be transferred from one side of the brush to the 
other, consist in so placing the brushes that when the coil reaches the 
position of short-circuit under the brushes, it shall have just arrived in 
a magnetic field of the direction and intensity necessary to reverse the 
current it has just been carrying, and to build up the reversed current 
to a strength equal to that of the current in the circuit of which it is 
about to become a part. In such a case, there will be no spark when 
the coil passes out from the position of short circuit under the brush. 
Now it is plain that, as the current delivered from the machine is 
increased, it will require a stronger field to reverse in the coil this 
stronger current. But, unfortunately, the presence of this stronger 
current in the turns on the armature, so magnetises the armature as to 
distort the magnetic field into a position in advance of the position of 
the brushes, and also to weaken the magnetic flux. The brushes must 



146 Electric Generators. 

therefore be shifted still further, whereupon the demagnetising effect of the 
armature is again intensified. Finally, a current output will be reached 
at which sparkless collection of the current will be impossible at any 
position, there being nowhere by the time the brushes are moved 
to it any place with sufficient strength of field to reverse and build up 
to an equal negative value the strong armature current, during the time 
the coil is passing under the brush. 

These distorting and demagnetising effects of the armature current 
are made quite plain by the diagrams given in Figs. 144, 145 and 146, 
in which the winding is divided into demagnetising and distorting belts 
of conductors. 

In Fig. 144 the brushes are in the neutral zone, and the current 
is distributed in the two sets of conductors, so as to tend to set up 
a flux at right angles to that which, the armature carrying no current, 
would be set up by the field. The resultant flux will be distorted 
toward the forward pole tip, considered with reference to the direction 
of rotation. Therefore, at this position of the brushes, the electro 
magnetic effect of the armature is purely distortional. Similarly, if, as 
in Fig. 145, the brushes were moved forward through 90 deg. until 
they occupied positions opposite the middle of the pole faces, and if in 
this position, current were sent through the brushes into the armature, 
(the armature with this position of the brushes being incapable of 
generating current), the electromagnetic effect of the armature would 
be purely demagnetising, there being no component tending to distort 
the field ; and in any intermediate position of the brushes, such, for 
instance, as that shown in Fig. 146, the electromagnetic effect of the 
armature current may be resolved into two components, one demagnetising, 
and due to the ampere turns lying in the zone defined by two lines (a a) 
drawn perpendicularly to the direction of the magnetomotive force of the 
impressed field, and passing through the forward position of the two 
brushes, and the other component due to the ampere turns lying outside 
of the zone, and purely distortional in its tendency. Fig. 146, of course, 
represents roughly the conditions occurring in actual practice, Figs. 144 and 
145 being the limiting cases, shown for explanatory purposes. 

In this connection, the results will be of interest of a test of 
armature reaction under certain conditions. A small four-pole iron-clad 
generator of 17-kilowatt capacity, at 250 volts, with a four-circuit 
single- winding, was tested with regard to the distribution of the magnetic 



Experimental Investigations of Armature Reaction. 



147 



flux in the gap. For this purpose the gap was divided up into a 
number of sections, from each of which successively an exploring coil 
was withdrawn. The coil was in circuit with a resistance box, and 
with the moveable coil of a Weston voltmeter. From the deflections 
and the total resistances of the circuit, the intensity of the flux at 
different portions of the gap was determined. These determinations 
were made with the armature at rest. As shown on the curves of 
Fig. 147, readings were taken, first with the field excited, but with 
no current in the armature, (curve A), and then with full-load current 



Fig.144, 




Fig.145. 





in the armature, and for various positions of the brushes. With the 
brushes at the neutral point (curve B), the distortion is at a maximum, but 
there is no demagnetisation. It would have been expected that the 
distortional crowding of the lines would have so increased the maximum 
density as to slightly diminish the total flux at the excitation used, this 
excitation being maintained at a constant value throughout the test. The 
integration of curves A and B, however, gives equal areas, consequently 
there was in this case no diminution of the total flux. 

But when the brushes are shifted over to the middle of the pole face 
(curve E), the demagnetisation becomes very marked, as may be seen, 



148 



Electric Generators. 



not only by the shape of the curve, but by its total area which is 
proportional to the total flux, but there is no longer any distortion. 
This last curve (curve E), representing the flux distribution corresponding 
to the position of the brushes at the middle of the pole face, should have 
been symmetrical, its lack of symmetry possibly being due to variation 
in the depth of the gap. 

Dr. Hopldnson 1 has made experiments upon the distribution 




of the magnetic flux in the air gap of two Siemens Brothers bipolar 
dynamos, the results of which correspond very closely with his 
calculations with reference to the influence of armature reaction. A 
similar analysis of the curves of Fig. 147 also confirms the theory of 
armature reaction. The machine experimented upon had a four-circuit 

1 " Original Papers on Dynamo Machinery and Allied Subjects." By John Hopkinson. 
Whittaker and Co., London, 1893. 



Experimental Investigations of Armature Reaction. 149 

drum- winding, with 79 coils of six turns each, in 79 slots in the 

79x6 
periphery. There were, therefore, = 119 turns per pole piece 

on the armature. The armature current being 71.5 amperes, there were 
71.5-1-4 = 18 amperes per turn; consequently, lly x 18 = 2140 ampere 
turns per pole piece on the armature. The area of the curves, which 
are proportional to the flux entering the armature, are as follows : 

A. 49 square centimetres =100 per cent. 

B. 49 = 100 

C. 36 ,, = 74 ,, 

D 27 55 

*- - 1 D j> )) 

E. 20 = 41 

For curves A. and B. the demagnetising component is zero, there 
being, however, in the case of B, maximum distortion, which would 
have been expected to so increase the maximum gap density as to 
cut down the total flux due to the 3,000 field ampere turns per pole 
piece. This was not, however, the case. 

In curves C, D, and E, the demagnetising component of the armature 
strength rose to x 2,140 = 710 at C, f x 2,140 == 1,420 at D, and 
to the full strength of 2,140 ampere turns at E. These results can be 
tabulated as follows : 

TABLE XXXVI. 



1 


2 


3 


4 


5 


(i 


i 


Designa 
tion of 
Curve. 


Percentage that 
Flux Entering 
Armature is of 
Total Flux at no 
Load. Determined 
from Area of 
Curves of Fig. 147. 


Field Ampere 
Turns, 
Maintained 
Constant 
throughout 
the Tests. 


Armature 
Ampere 
Turns, 
Maintained 
Constant 
throughout 
the Tests. 


Demagnetising 
Component of 
Armature Ampere 
Turns Determined 
from Position of 
Brushes. See Dia 
grams of Figs. 144, 
145, and 146. 


Resultant 
Ampere 
Turns, Deter 
mined from 
Columns 3 
and ">. 


Percentage that 
Resultant Am 
pere Turns are 
of no Load 
Ampere Turns, 
Determined 
from Column 6. 


A 


100 


3000 








3000 


100 


B 


100 


3000 


2140 





3000 


100 


C 


74 


3000 


2140 


710 


2290 


76 


D 


55 


3000 


2140 


1420 


1580 


53 


E 


41 


3000 


2140 


2140 


860 


29 



The large percentage of flux in curve E (41 per cent.), as compared 
with the small percentage of resultant ampere turns (29 per cent.), 
is explained by the fact that with the brush at the middle of the pole face, 



150 Electric Generators. 

as was the case in curve E, many of the armature turns are so situated 
in space as not to be linked with the entire flux, and consequently 
cannot be so effective in demagnetisation. In other words, the armature 
turns are uniformly distributed, instead of being concentrated in a coil 
placed so as to fully oppose the field coils. The extent of this non- 
effectiveness is proportional to the pole arc, but with the positions of 
the brushes which would occur in practice, the demagnetising component 
of the armature ampere turns would be fully effective. 

It will be observed that for curves A, B, C and D, the proportion 
of flux to resultant ampere turns is very close. 



APPLICATION OF THESE CONSIDERATIONS TO THE PROPORTIONING OF 

DYNAMOS. 

If it were not for these effects, due to the electromagnetic reaction 
of the armature, the proportioning of dynamos would resolve itself 
into a determination of those values of T and M in the formula E = 
KTNM x 10~ 8 , which would, with a minimum cost of material, give 
the desired current and voltage ; suitable cross-section of copper and 
iron being chosen, to secure immunity from excessive heating. Thus 
suppose the problem should arise, of the best design for a 500-volt 
100-kilowatt generator, to run at 600 revolutions per minute. The current 
output is 200 amperes. Let us try a two-pole drum winding with 10 face 
conductors. Then T == 5 ; N = 10 ; 500 = 4 x 5 x 10 x M x lO" 8 , M = 
250,000,000 lines. The armature iron could not properly be run at 
more than 100,000 lines per square inch. Therefore, the cross-section 
of the armature = 2,500 square inches at least. It thus appears that 
the armature would have to be 50 in. in diameter and 50 in. long, or 
else some other equally extreme dimensions. The field turns would be 
of great length, and as the air gap density would be very high, there 
would be need for very many field ampere turns. Without carrying 
the calculations any farther, it is apparent that, as regards cost of materials 
alone, the machine would be poorly designed. 

On the other hand, suppose the armature had 2000 face conductors. 
Then T = 1000 ; 500 = 4 x 1000 x 10 x M x 10~ 8 , . . M = 1,250,000 
lines. Necessary cross-section = 12.5 square inches as far as regards 
transmitting the flux. Therefore, the magnet cores would be 4 in. in 
diameter. But to have on the armature 2000 face conductors, each 



Influence of Armature Reaction. 1 5 1 

carrying 100 amperes, would require a very large armature, probably 
as large a diameter as was necessary in the former case ; but then it was 
a question of carrying a large magnetic flux, which determined the size 
of the armature. In this case we should have a very large weight of 
armature copper, but otherwise the material would not cost much, if we 
look no further into the matter of field copper than relates to that 
necessary to obtain the required flux at no load. But, nevertheless, on the 
score of material alone, some intermediate number of conductors would 
be found to give a more economical result. 



INFLUENCE OF ARMATURE REACTION IN THESE TWO EXTREME CASES. 

In the first case, that of the armature with only five turns, there 

5 x 100 
would have been but ~ =250 ampere turns per pole-piece on the 

armature, which, as far as armature reaction effects are concerned, would 
be entirely negligible ; but, as relates to the collection of the current, there 

would be = 200 average volts between commutator segments, and 

Zi. 

this would have corresponded to such a high inductance per coil as 
to have rendered quite impossible the reversal of 100 amperes, 20 times 
per second, with any ordinary arrangement of commutator and brushes. 
In the other case (that of the machine with 1000 armature turns), there 
would have been one volt per turn, a value which, with the methods of 
construction generally employed, would correspond to a very low inductance 

indeed ; but there would have been on the armature - = 50,000 

t 

ampere turns per pole-piece, which would completely overpower the field 
excitation, and the design would be entirely out of the question. 

We find, therefore, that while in the first case the armature reaction 
is small, the inductance per commutator segment is excessive. In the 
second case the inductance per commutator segment is small ; the armature 
is altogether too strong. With but two poles, some intermediate value 
would have to be sought for both quantities ; probably something like 100 
turns would give a fairly good result. 



152 Electric Generators. 

CONDITIONS ESSENTIAL TO SPARKLESS COMMUTATION. 

As a consequence of armature reaction and inductance, it becomes not 
only desirable but necessary to limit the armature strength to such an 
amount (at full load current) as shall not too greatly interfere with the 
distribution and amount of the magnetic flux set up by the magnet spools. 
It is furthermore necessary to make each armature coil between adjacent 
commutator segments of so low inductance as to permit of the complete 
reversal of the current by means of the residual flux in the commutating 
field. The location and amount of this residual flux is determined by the 
strength of the armature, and the position of the brushes and the reluctance 
of the gap. To best understand the method of fulfilling these conditions, 
attention should be given to the following illustrations, which lead up to a 
very definite method for assigning the most desirable electromagnetic 
proportions to constant potential dynamos, particularly with reference to 
the determination of the proper number of poles. , 



DETERMINATION OF THE NUMBER OF POLES FOR A GIVEN OUTPUT. 

Suppose we want a 50-kilowatt 400-volt bipolar generator. We 
conclude to limit the armature strength to 3,000 ampere turns per pole- 
piece, and the volts per commutator segment to 16 volts (a very high limit). 

Amperes output = = 125 amperes. Therefore, each conductor 

125 3,000 

carries _ 62.5 amperes. Turns per pole-piece = r9 _ = 48, i.e., 96 

Z ()_.. 

total turns. ^- =25 commutator segments between brushes, or 50 total 

commutator segments. Therefore - - = about two turns per coil (i.e., per 

o u 

commutator segment). 

In the 100 kilowatt machine for the same voltage, to retain the same 
strength of armature, and the same volts per commutator segment, we 
must have only one turn per coil. 

For these values of armature strength and volts per commutator 
segment we have now reached the limiting output, and the problem arises : 
What shall be done in the case of a machine of twice the size, in this case 
200 kilowatts, if the type of winding remains the same ? We cannot have 
less than one turn per commutator segment, so we find that in a bipolar 



Determination of Number of Poles for Multiple Circuit Windings. 153 

machine it will be necessary to either double the armature strength, in 
which case we can retain the low voltage per commutator segment, or we 
can double the voltage per commutator segment, and keep the armature 
strength of the same low value used in the previous cases ; or we can 
compromise by raising both limits to a less extent. This latter plan is that 
which would be adopted to retain the bipolar design. But the result 
would be unsatisfactory as regards sparking, and even though it could be 
made passable at this output, the same question would arise with the next 
larger size. But by the use of a multipolar design, the difficulty is entirely 
overcome. Suppose we let our 200-kilowatt 400-volt machine, have four 
poles. Then there will be four paths through the armature, each carrying 

a quarter of the total current. Amperes output =- - = 500 amperes. 

Therefore amperes per conductor = - - = 125. The turns per pole-piece 

- = 24. We have, also, 24 commutator segments per pole-piece, 

giving _ -= 16.6 volts per commutator segment. 
24 

A machine can consequently be made to operate entirely satisfactorily, 
as regards sparking, by designing it with a proper number of poles. 



MULTIPLE CIRCUIT WINDINGS. 

With multiple-circuit windings, the armature strength and the volts 
per bar may be reduced to any desired extent by sufficiently increasing the 
number of poles. Thus, suppose that in a certain case the conditions given 
are that the armature strength of a 500-kilowatt 600-volt generator shall 
be 4,000 ampere-turns per pole-piece, and that there may be 15 volts per 
commutator segment. Then the number of poles would be determined 
as follows : 

Commutator segments per pole-piece -- 40. 

1 

Therefore 40 turns per pole piece. 

= 100 amperes per armature branch. 

Full load current 5< ~^ - = 833 amperes. 

000 

833 
Therefore we want = 8 poles. 

X 



154 Electric Generators. 

But suppose it were considered advisable that this generator should 

have only 3000 ampere-turns per pole-piece on the armature, and that it 

should have but 8 volts per commutator segment, then turns per pole-piece 

600 



= /a. 



A j 3000 

Amperes per armature conductor = - _ -= 40 

75 

QOO 

Therefore number of poles = =20. 



Two-CmcuiT WINDINGS. 

But in the case of two-circuit windings, these values cannot be 
adjusted by changing the number of poles, for the reason that the current 
divides into two paths through the armature, independently of the number 
of poles, instead of dividing into as many paths as there are poles. 

Suppose, for example, that it were desired to use a two-circuit winding 
in a 500-kilowatt, 600-volt generator, and to have 15 volts per commutator 
segment. Then : 

Number of segments per pole-piece = = 40. 

i o 

T? m A 500,000 

Full load amperes = = 833. 

Amperes per turn = - =417. 

Therefore, ampere-turns per pole-piece on armature = 40 x 417 
= 16,700. 

This would be impracticable. To reduce this to 6000 ampere-turns, the 
turns have to be reduced, and consequently the commutator segments, to 

x 40 = 14 per pole-piece. There would then be = 43 volts 



16,700 14 

per commutator segment, which, with ordinary construction, would corres 
pond to so high a reactance voltage in the short-circuited coil (in a machine 
of this output) as not to be permissible. Moderate values can only be 
obtained by interpolating commutator segments in accordance with some 
well-known method, or by the use of double, triple, or other multiple 
windings. Such methods generally give unsatisfactory results, and two- 
circuit windings are seldom used for machines of large output. When 
they are used, in such cases, exceptional care has to be taken to counteract 



Limitations of Two-Circuit Windings. 155 

their objectionable features by the choice of very conservative values for 
other constants. 



MULTIPLE WINDINGS. 

But the use of multiple windings (such, for instance, as the double 
winding of Fig. 74), permits of employing two-circuit windings. 

Thus, suppose in the case of the design of a 350-kilowatt, 250-volt 
generator, it appears desirable, when considered with reference to cost of 
material, or for some other reason, to use 14 poles; and that, fuithermore, 
a two-circuit multiple winding is to be used. The question arises, how 
many windings shall be employed, in order to have only 9 volts per 
commutator segment, and to permit not over 5,000 ampere-turns per 
pole-piece on the armature ? 

- == 28 commutator segments per pole-piece. 
/ 

Therefore, 28 turns per pole-piece. 

m, f 500 

.Therefore, = IbO amperes per turn. 

28 

350,000 
Amperes output = SHTT" = 1*00 amperes, 

1400 



180 



= 7.1 



Therefore there must be eight paths through the armature from the 
positive to the negative brushes. Consequently, a two-circuit quadruple 
winding is required. 

It may, however, be well to again emphasise the fact that poor results 
generally follow from the adoption of such windings, except in cases where 
a width of commutator can be afforded which permits of dispensing with all 
but two sets of brushes. 1 By adopting such a width of commutator, one of 
the savings effected by the use of multipolar designs is lost. By careful 
designing, two-circuit double and sometimes two-circuit triple windings 
have given good results. 

1 If only two sets of brushes are retained, the short-circuited set of conductors no longer 
consists of the two corresponding to one turn, but now includes as many in series as there 
are poles. A high reactance voltage is consequently present in this short-circuited set. The 
presence of the full number of sets of brushes, if correctly adjusted, should reduce this, but 
cannot in practice be relied upon to do so. 



156 Electric Generators. 

TWO-CIRCUIT "CoiL" WINDINGS. 

But two-circuit single windings can be very properly applied to 
machines of such small capacity, that, when good constants are chosen, 
they work out to have one or more turns per segment. It follows 
that, within certain ranges, any desired values of armature strength 
and volts per commutator segment may be obtained ; not, however, by 
a suitable choice of poles, but by the use of a suitable number of turns 
between commutator segments. Suppose, for instance, a 10-kilowatt 
100-volt generator, with an armature strength of 2,000 ampere turns 
per pole-piece, and with 5 volts per commutator segment. 

Then 

o 100 

Segments per pole-piece = = 20 

D 

10,000 
Full load current = = 100 amperes. 

Amperes per conductor = _ - = 50. 

2000 

Turns per pole-piece = 40. 
50 

Therefore, = two turns per commutator segment. 
20 

If 3,000 ampere-turns had been permissible, we should have used 

3,000 

27)7)0" x ^ = 3 turns per commutator segment. 

Finally, it may be stated that two-circuit armatures are built multi- 
polar mainly from considerations of cost, and should not be used for 
large outputs except in special cases. 

Aside from the reasons dependent strictly upon the magnetic limit 
of output, it may be said that two-circuit windings are unsatisfactory 
whenever the output is so large as to require the use of more than two sets 
of brushes (in order to keep the cost of the commutator within reasonable 
limits), because of the two-circuit windings lacking the property of 
compelling the equal subdivision of the current among all the sets of 
brushes used. Selective commutation occurs, one set of brushes 
carrying for a time a large part of the total current ; this set of brushes 
becoming heated. This trouble is greater the greater the number of 
sets of brushes, and the practicability of two-circuit windings may be 
said to be inverselv as the number of poles. If, however, in multiple 



Voltage per Commutator Segment, as Related to Inductance 157 

circuit windings the part of the winding opposite any one pole-piece 
should tend to take more than its share of the current, the increased 
armature reaction and CR drop tends to restore equilibrium, this 
property constituting a great advantage. 



VOLTAGE PER COMMUTATOR SEGMENT AS RELATED TO INDUCTANCE. 

As already stated, the average voltage between commutator segments, 
although it can be relied upon to give good results, if care is used in 
special cases, is not a true criterion of the inductance of a coil. For, 
in different types, this expression may have the same value for coils of 
different inductances. 

Thus, if the design is for an armature in which the conductors are 
located in holes beneath the surface, the inductance will be very high, and 
it would be necessary to limit the average voltage per commutator 
segment to a very low value. If the slots are open, the inductance 
will be somewhat lower, and in a smooth core construction with the 
winding on the surface, the inductance is very low. In this latter case, 
a much higher value for the average volts per commutator segment 
could be used. 

The possible value also varies according to whether carbon or 
copper brushes are used. Carbon 1 brushes may be much less correctly 
set and still have sparkless commutation, due to the high resistance ot 
the brush limiting extreme variation of current in the short-circuited 
coil, as well as because the brushes are not so subject to injury 
through this cause, as would be the case with copper brushes ; 
consequently, the average volts per commutator segment may be permitted 
to be three or four times as great as with copper brushes, without 
endangering the durability either of the brushes or of the commutator ; and 
on account of this, it is found desirable to increase the density in the 

1 There has lately been a tendency amongst some designers to attribute still other 
properties to high-resistance brushes, and even to maintain that they play an important part, 
not only in limiting the short-circuit current, but in accelerating the building up of the 
reversed current. However, one would feel inclined to hold that the main element in the 
commutating, i.e., stopping and reversing of the current, is attributable to the influence of the 
residual commutating field ; and that while the carbon brush aids in promptly arresting the 
original current, it is perhaps of still more importance in virtue of its possessing a certain 
inertness in combination with the copper commutator segments which renders the sparking 



158 Electric Generators. 

air gap to coiTespond with this higher inductance between commutator 
segments. 

We have now shown that although the preliminary design for a 
commutating machine may be arrived at from the maximum permissible 
armature reaction and the number of commutator segments per pole 
necessary for good commutation, the average voltage between the 
commutator segments is not the ultimate expression as regards com 
mutation. The ultimate expression must be in terms of the inductance 
of the coil or coils included between a pair of commutator bars. 

In general, commutation occurs when a coil is in a feebly magnetised 
field, so that the inductance can be approximately calculated from the 
magnetomotive force of the coils, and the reluctance of the magnetic 
circuit around which the coils act. The frequency of reversal is determined 
from the thickness of the brush and the commutator speed. 

The commutated current consists of two components : one a wattless 
magnetising component, and the other an energy current, due firstly to 
the dissipation of energy by C 2 R loss in the coil, and secondly to 
eddy currents generated internally in the copper conductors, and in the 
surrounding mass of metal. 

It follows from this that there is a loss increasing with the load in 
commutating machines due to the commutation of the currents. There 



much less destructive than between copper brushes and copper segments. It has the property 
of burnishing the commutator, giving it a lustrous refractory surface. 

The following bibliography comprises the most recent contributions to the discussion of 
the subject of sparking in commutating dynamos : 

Weymouth ; " Drum Armatures and Commutators." 

Reid; "Sparking; Its Cause and Effects;" Am. Inst. Elec. Engrs.; December 15th, 
1897. Also The Electrician, February llth, 1898. 

Thomas; "Sparking in Dynamos." The Electrician, February 18th, 1898. 

Girault : " Sur la Commutation dans les Dynamos a Courant Continue. " Bull, de la 
Soc. Int. des Electr., May, 1898, vol. xv., page 183. 

Dick ; " Ueber die Ursachen der Funkenbildung an Kollektor und Biirsten bei Gleich- 
strom-dynamos." Elek. Zeit., December 1st, 1898, vol. xix., page 802. 

Fischer-Hitmen ; " Ueber die Funkenbildung an Gleichstrom-maschinen." Elek. Zeit., 
December 22nd and 29th, 1898, vol. xix., pages 850 and 867. 

Arnold; "Die Kontactwiderstand von Kohlen und Kupferbiirsten und die Tempera 
ture rhohung eines Kollektors." Elek. Zeit., January 5th, 1899, vol. xx., page 5. 

Kapp ; "Die Funkengrenze bei Gleichstrom-maschinen." Elek. Zeit., January 5th, 1899, 
vol. xx., page 32. 

Arnold and Mie ; " Ueber den Kurzschluss der Spulen und die Kom mutation des 
Stromes eines Gleichstromankers. Elek. Zeit., February 2nd, 1899. vol. xx., page 97. 



Inductance Constants. 159 

are also other load losses in commutating machines, brought about by 
the distortion and the increasing magnetisation in the iron, so that the 
hysteresis and eddy current losses increase from no load to full load, as 
also the eddy current losses in the armature conductors themselves l It 
has been generally assumed on the part of designers that these losses in 
the armatures of commutating dynamos do not increase with the load. 
This, however, is incorrect. The increase does exist, and is in general of 
the same nature as the increase in these losses in alternators, due to the 
load, although they may be restricted to a greater extent by proper 
designing. The effect of the induced eddy currents on commutation is 
often appreciable, since the frequency of commutation is generally from 
200 to 700 cycles per second. For this reason, calculations on inductance 
in reference to commutation have to be considered with reference to the 
particular construction of the armature core. Constants as to inductance 
are, therefore, best determined by actual measurements. In practice, a 
good average expression is, that one ampere turn will give a field of 
20 C.G.S. lines per inch of length of armature core. 

It is convenient to assume this as as a basis upon which to work 
out a design. As the design developes, the figures should be corrected 
according to the dimensions selected. This is the most satisfactory 
method, and several tests will be described, the results of which have a 
direct bearing upon the value of the constant. By a study of these 
results one may determine the most desirable proportions to give to the 
armature slot in order to bring the inductance down to, or even below, 
the value of 20 C.G.S. lines per ampere turn and per inch of length of 
armature lamination. In cases where it is impracticable to use such slot 
proportions as shall give the minimum value, the tests afford an indication 
of the value to be used. It is, of course, very desirable that such 
experiments should be independently carried out on the particular line 
of commutating dynamo with which the individual designer is concerned. 
In this connection, that is, in relation to inductance in commutating 
dynamos, interest attaches, not to the inductance of the armature winding 
as a whole, as in the case of alternating-current dynamos, 2 but to the 

1 See Fig. 114, on page 106, for experimental confirmation of this statement. 

2 Rotary converters contain the elements of both these types, and in their subsequent 
treatment it will appear that while the coil undergoing commutation should have the least 
practicable inductance, the inductance of the coils in series between collector rings must have 
a suitable value for reasons entirely other than those related to commutation. 



160 



Electric Generators. 



inductance of those components of the winding which simultaneously 
undergo commutation at the brushes. In well-designed dynamos of this 
type such coils will, at the time of commutation, be located in the space 
between pole-tips, practically at the position of minimum inductance. 
The measurement of this inductance was the object of the tests now to 
be described. 

PRACTICAL DEFINITION OF INDUCTANCE. 

A coil has an inductance of one henry when it is situated in a 
medium of such permeability, and is so dimensioned, that a current of 
one ampere sets up a magnetic flux of such a magnitude that the product 
of the number of lines linked with the coil, by the number of turns in 
the coil is equal to 100,000,000. If the coil has but one turn, then its 
inductance, expressed in henrys, becomes 10~ 8 times the number of lines 
linked with the turn when one ampere is passing through it. If the 
coil has T turns, then not only is the magnetomotive force T times as 
great (except in so far as saturation sets in), but this flux is linked with 
T turns ; hence the product of flux and turns, i.e., the total linkage, the 
inductance of the coil, is proportional to the square of the number of 
turns in the coil. 



DESCRIPTION OF EXPERIMENTAL TESTS OF INDUCTANCE. 

First Experiment. In Fig. 148 is shown a sketch of a commutating 
dynamo with a projection type of armature with a four- circuit single 
winding. The inductance of several groups of coils was measured with a 
2 5 -cycle alternating current, and the results, together with the steps of the 
calculation, are set forth in the following Tables. 

TABLE XXXVII. MINIMUM INDUCTANCE. 

Conductors in position of minimum inductance are in the commutating zone, i.e., midway 

between pole corners. 



Number of 


Amperes 




Impe 


Resist 


React 


Induct 


C.G.S. Lines per 


Turns 
Under 


in 

these 


Volts. 


dance 
in 


ance 
in 


ance 
in 


ance 
in 


Ampere Turn and per 
Inch of Length of 


Test. 


Turns. 




Ohms. 


Ohms. 


Ohms. 


Henrys. 


Lamination. 


4 


75 


.594 


.00790 


.00692 


.00388 


.0000247 


15.0 


5 


65 


.728 


.0120 


.00865 


.00708 


.0000450 


18.0 


6 


68 


.944 


.0139 


.0104 


.00930 


.0000592 


16.5 



Experimental Tests of Inductance. 



1(51 



The air gap of this machine was afterwards shortened from its original 
depth of about .188 in. to about .1 in., and the inductance in the position of 
maximum inductance was again measured. In the position of minimum 
inductance, the values are unaffected by the depth of the air gap. 



Rg.148. 

Gap /#75 






slOCS r ft? 

Conductors per slot 6 

Turns per. coil 3 

lengthof Armature -6-f 



Ho of slots- 110 
No of Poles - 4 
Conductors per slot - 
Ho of commutator i 



Turn* per slot -3 
No of commutator 



No.of Polea. 
No of Slot* 

Conductors per slot - 12 

Turns per slot - 6 

No of Commutator segments 159 

Length of krmaburt, t-T 



Eg.160. 




(SOS-IK] 



N9 of Slot* == 166 

Gross length/ ofATrnntare^ lamirvcutions 11-25. 



Second Experiment. A commutating dynamo, illustrated in Fig. 149, 
has a four-circuit single winding consisting of 75 coils of three turns each, 
arranged in 75 slots. Tests with 25-cycle alternating current were made 
on the inductance of from one to five adjacent coils, and the results are set 
forth in Table XL. 



162 



Electric Generators. 



TABLE XXXVIII. MAXIMUM INDUCTANCE. 
Conductors in position of maximum inductance are under the middle of the pole faces. 



Number 


Amperes 




Impe 


Resist 


React 


Induct 


C.G.S. Lines per 


of Turns 


in 


Volts. 


dance 


ance 


ance 


ance 


Ampere Turn and per 


Under 


these 




in 


in 


in 


in 


Inch of Length of 


Test. 


Turns. 




Ohms. 


Ohms. 


Ohms. 


Henrys. 


Lamination. 


2 


73 


.391 


.00535 


.00346 


.00407 


.0000260 


65.0 


3 


71 


.730 


.0103 


.00529 


.00890 


.0000567 


63.0 


4 


reo) 

J23f 


rl.095) 
{ .378) 


.0174 


.00692 


.0159 


.000102 


63.5 


5 


22 


.594 


.0270 


.00865 


.0256 


.000163 


65.0 


6 


22 


.770 


.0350 


.0104 


.0333 


.000212 


59.0 



TABLE XXXIX. CONDUCTORS IN POSITION OF MAXIMUM INDUCTANCE WITH SHORTENED 


AIR GAP. 


Number 


Amperes Impe- 


Resist 


React 


Induct 


C. G. S. Lines per 


of Turns 


in 


Volts. 


dance 


ance 


ance 


ance 


Ampere Turn and per 


Under 


these 




in 


in 


in 


in 


Iiich of Length of 


Test. 


Tests. 




Ohms. 


Ohms. 


Ohms. 


Henrys. 


Lamination. 


1 


80.5 


.189 


.00235 


.00173 


.00138 


.00000876 


87.6 


2 

2 


40.0 
78.0 


.230 
.472 


.00575 
.00605 


.00346 I 
.00346 } 


.00452 


.0000288 


72.0 


3 


20.5 


.256 


.0125 


.00519 ) 








3 


39.0 


.500 


.0128 


.00519 \ 


.0116 


.0000735 


81.5 


3 


76.5 


1.02 


.0133 


.00519 ) 








4 
4 


20.5 
38.0 


.432 

.850 


.0210 
.0224 


.00692 \ 
.00692 ] 


.0202 


.000129 


80.5 


5 


19.5 .640 


.0328 


.00865 


.0314 


.000200 


80.0 


6 


19.7 


.915 


.0465 


.0104 


.0452 


.000288 


80.0 



Hence shortening the air gap has increased the inductance in the position of maximum inductance 
by about 27 per cent. 

TABLE XL. POSITION OF MINIMUM INDUCTANCE. 



Number 
of Coils 
Under 
Test. 


Number 
of Turns 
Under 

Test. 


Amperes. 


Volts. 


Impe 
dance 
in 
Ohms. 


Resist 
ance 
in 
Ohms. 


React 
ance 
in 
Ohms. 


Induct 
ance 
in 
Henrys. 


C.G.S. Lines per 
Ampere Turn 
and per Inch of 
Length of 
Lamination. 


3 


9 


63 


2.25 


.0357 


.0309 


.0173 


.000110 


15.5 


4 


12 


58 


3.00 


.0518 


.0412 


.0308 


-.000197 


15.6 


5 


15 


52 


3.70 


.0710 .0515 


.0482 


.000307 


15.6 



Position of Maximum Inductance. 



1 


3 


61 


.75 


.0123 


.0103 


.00655 


.000042 


53 


2 


6 


58 


1.95 


.0339 


.0206 


.0268 


.000171 


54 


3 


9 


52 


3.45 


.0668 


.0309 


.0590 


.000376 


53 


4 


12 


21 


2.30 


.111 


.0412 


.103 


.000655 


52 


5 


15 


20 


3.30 


.165 


.0515 


.156 


.00099 


50 



Attention should again be drawn to the fact that it is the minimum inductance, which corresponds to 
the inductance in the position of commutation, which is of chief interest in the present section. 




Experimental Tests of Inductance. 



163 



Tables XXXVIII. and XXXIX., and the last half of Table XL., 
relating to the position of maximum inductance, are useful for a correct 
understanding of the relation of the proportions of the magnetic circuit 
of the armature coil to the resulting inductance, but are not directly 
applicable to the conditions obtaining during commutation. 

Third Experiment. Tests were made with GO-cycle alternating current 
upon the inductance of a six-pole commutating generator, the armature 
of which had 166 slots with a six-circuit single- winding of 166 complete 
coils, each of two turns. Fig. 150 gives the dimensions. The results 
are set forth in Table XLI. 

TABLE XLI. POSITION OF MINIMUM INDUCTANCE. 





















C.G.S. Lines per 


Number 
of Coils 
Under 

Test. 


Number 
of Turns 
Under 

Test. 


Am 
peres. 


Volts. 


Impe 
dance 
in Ohms. 


Mean 
Impe 
dance. 


Resist 
ance in 
Ohms. 


React 
ance in 
Ohms. 


Induct 
ance 
in Henrys. 


Ampere Turn 
and per Inch 
Length of 
Armature 




















Lamination. 


1 

1 


2 
2 


98.5 
126.5 


.46 
.585 


.00467 
.00463 


.00465 


.0015 


.00439 


.0000117 


26.0 


2 


4 


85.0 


1.42 


.0167 












2 


4 


95.7 


1.62 


.0169 


.0168 


0030 


.0165 


.0000440 


24.5 


2 


4 


105. 


1.79 


.0169 












3 


6 


65.3 


2.24 


.0343 












3 


6 


75.0 


2.60 


.0346 


.0345 


.0045 


.0342 


.000091 


21.8 


3 


6 


87.0 


3.00 


.0345 












4 


8 


65.5 


3.74 


.0571 












4 


8 


76.0 


4.36 


.0573 


.0573 


.0060 


.0570 


.000152 


21.1 


4 


8 


87.0 


5.00 


.0575 













Position of Maximum Inductance. 



1 


2 


89.8 


.71 


.0078 












1 


2 


95.2 


.77 


.0081 


.0080 


.0015 


.0078 


.0000208 


4( 


1 


2 


111.8 


.91 


.0081 












2 


4 


71.0 


2.24 


.0316 












2 


4 


78.0 


2.42 


.0310 


.0312 


.0030 


.0310 


.000082 


4 


2 


4 


84.2 


2.60 


.0309 












3 


6 


72.3 


4.68 


.0648 












3 


6 


83.7 


5.38 


.0643 


.0644 


.0045 


.064 


.000170 


45 


3 


6 


89.3 


5.74 


.0643 












4 


8 


66.6 


7.14 


.1072 












4 


8 


77.0 


8.32 


.1062 


.1052 


.0060 


.105 


.000279 


3* 


4 


8 


86.3 


8.9 


.1031 













164 



Electric Generators. 



Fourth Experiment. This relates to the carcass of a 30 horse-power 
railway armature, the leading dimensions of which are indicated in Fig. 151. 
Only four coils, of three turns each, were in position in four adjacent 
armature slots. The armature was out of its field frame, which was 
equivalent to its being in the position of minimum inductance. The testing 
current was supplied at a frequency of 100 cycles per second. Gross length 
of armature lamination = 8.5 in. The results obtained are set forth in the 
following Tables : 



TABLE XLTI. POSITION OF MINIMUM INDUCTANCE. 



















C.G.S. Lines per 


Number 
of Coils 
Under 

Test. 


Number 
of Turns 
in these 
Coils. 


Amperes Volts at 
in these Ter- 
Turns. minals. 


Impe 
dance 
in Ohms. 


Resist 
ance 
in Ohms. 


React 
ance 
in Ohms. 


Induct 
ance 
in Henrys. 


Ampere Turn 
and per Inch 
Gross Length 
of Armature 
















Lamination. 


1 


3 


55.5 1.11 


.0200 


.0085 


.0181 


.0000286 


37.4 


1 


3 


47.0 .94 


.0200 


.0085 


.0181 


.0000286 


37.4 


1 


3 


34.0 .68 


.0201 


.0085 


.0182 


.0000287 


37.5 


1 


3 


31.5 


.62 


.0195 


.0085 


.0176 


.0000278 


37.7 


2 


6 


51.9 


2.78 


.0536 


.017 


.0507 


.000080 


26.2 


2 


6 


42.5 


2.27 


.0536 


.017 


0507 


.000080 


26.2 


2 


6 


36.3 


1.97 


.0542 


.017 


.0513 


.000081 


26.5 


2 


G 


31.4 


1.71 


.0545 


.017 


.0517 


.000082 


26.7- 


3 


9 


23.7 


2.27 


.0960 


.026 


.0924 


.000147 


21.4 


3 


9 


18.9 


1.84 


.0974 


.026 


.0937 


.000149 


21.6 


3 


9 


16.9 


1.62 


.0959 


.026 


.0921 


.000146 


21.2 


3 


9 


15.8 


1.50 


.0947 


.026 


.0910 


.000145 


21.1 


4 


12 


19.8 


2.91 


.147 


.034 


.143 


.000227 


18.5 


4 


12 


15.9 


2.51 


.158 


.034 


.154 


.000245 


20.0 


4 


12 


14.4 


2.15 


.149 


.034 


.145 


.000230 


18.8 


4 


12 


12.4 


1.88 


.152 


.034 


.148 


.000235 


19.2 



Mean of the four observations for three turns 
> six ,, 

> ,, nine ,, 

,, twelve,, 



37.5 
26.4 
21.3 
19.1 



Fifth Experiment. Fig. 152 gives a sketch showing the leading 
dimensions of the dynamo experimented upon. The armature was in place 
in the cast-steel frame. Testing current had a periodicity of 100 cycles per 
second. The gross length of the armature lamination = 8.7 in. The 
results are given in Table XLIII. 



Experimental Tests of Inductance. 
TABLE XLIII. POSITION OP MINIMUM INDUCTANCE. 



165 



Number 
of Coils 
Under 
Test. 


Number 
of Turns 
in these 
Coils. 


Amperes 
in these 
Turns. 


Volts at 
Ter 
minals. 


Impe 
dance 
in Ohms. 


Resist 
ance 
in Ohms. 


React 
ance 
in Ohms. 


Induct 
ance 
in Henrys. 


C.G.S. Lines per 
Ampere Turn 
and per Inch 
Gross Length 
of Armature 
















Lamination. 


1 


3 


39.0 


.838 


.0215 


.0065 


.0205 


.0000330 


42.2 


1 


3 


43.5 


.941 


.0216 


.0065 


.0206 


.0000332 


42.4 


1 


3 


46.0 


.992 


.0216 


.0065 


.0206 


.0000332 


42.4 


2 


G 


20.0 


1.18 


.0590 


.0130 


.0584 


.0000924 


29.5 


2 


6 


21.5 


1.24 


.0577 


.0130 


.0562 


.0000895 


28.6 


2 


6 


24.0 


1.39 


.0580 


.0130 


.0565 


.0000900 


28.8 


2 


G 


25.0 


1.45 


.0581 


.0130 


.0565 


.0000900 


28.8 


3 


9 


14.9 


1.84 


.124 


.0195 


.122 


.000194 


27.6 


3 


9 


16.9 


2.05 


.122 


.0195 


.120 


.000191 


27.2 


3 


9 


18.9 


2.29 


.122 


.0195 


.120 


.000191 


27.2 


3 


9 


20.9 


2.52 


.121 


.0195 


.119 


.000190 


26.9 


4 


12 


13.4 


2.46 


.184 


.026 


.182 


.000290 


23.2 


4 


12 


14.8 


2.74 


.185 


.026 


.183 


.000291 


23.3 


4 


12 


15.8 


3.01 


.190 


.026 


.188 


.000299 


23.9 


4 


12 


18.3 


3.44 


.188 


.026 


.186 


.000296 


23.7 


Mean of the observations with thre 
six 
,, nin 
twe 


e turns ... 


42.3 
28.9 
27.2 
23.5 




3 ,, 


Ive .. 



Sixth Experiment. This experiment was made in respect to the 
inductance of an armature of a 25 horse-power tramway motor. 
The following data applies to this armature : 

Diameter of armature ... ... ... ... ... ... 16 in. 

Number of slots ... ... ... ... ... ... 105 

coils ... ... ... ... ... ... ... 105 

Turns per coil ... ... ... ... ... ... 4 

Conductors per slot ... ... ... ... ... .. 12 

Gross length of armature laminations ... ... ... ... Sin. 

The inductance tests were made with a current of a periodicity 
of 100 cycles per second. 

Inductance measurements were made upon one, two, three, and 
four coils in series, and under the condition of minimum inductance, 
which was considered to correspond with the armature in air, and then 
with air gaps of various lengths arranged by a special pole-piece of 
laminated iron of the dimensions shown in Fig. 153, which shows the 
pole-piece in place, with pieces of leatheroid between it and the armature. 
Owing to this pole-piece being of the same radius as the armature, on 



16G 



Electric. Generators. 



inserting the leatheroids a gap was obtained which was larger at the 
inner edge of the pole-piece than at the outer (see Fig. 153), so that in 
the calculations and curves a mean gap is given. 




C.G.S. LINES PER AMPERE TURNS PER INCH LENGTH 

.OFARMATURE FOR VARIOUS TURNS IN SERIES , 

4 TURNS PER COIL. GROSS LENGTH OF ARM. LAM . .S 




16 



In Tables XLIV. to XLVII. inclusive, and in the curves of 
Figs. 154 and 155, are given the results of these tests. 



Experimental Tests of Inductance. 167 

TABLE XLIV. ONE COIL OF FOUR TURNS PER COIL. RESISTANCE = 0.014 OHMS. 



Amperes. 


Volts. 


Imped 
ance. 


Reactance. 


Cycles 
per 
Second. 


Induct 
ance in 
Henrys. 


C.G.S. Lines 
per Ampere 
Turn and per 
Inch Length 
of Armature. 


Mean. 


Mean Air 
Gap. 


















111. 


23.75 


1.08 


.0455 


.0433 


97 


.0000710 


55.5 






23 


1.07 


.0466 


.0444 


97 


.0000728 


57.0 


56.6 


CO 


20.2 


.945 


.0468 


.0466 


97 


.0000732 


57.2 






23.5 


1.325 


.0562 


.0549 


99 


.0000884 


69.0 






22 


1.268 


.0576 


.0558 


99 


.0000897 


70.0 


69.8 


23 

VT 


19.75 


1.120 


.0568 


.0551 


99 


.0000887 


69.3 






20 


1.385 


.0693 


.0678 


99 


.000109 


85.2 






225 


1.56 


.0694 


.0679 


99 


.000109 


85.2 


85.5 


1 1 

64 


24 


1.675 


.0698 


.0684 


99 


.000110 


86.0 






245 


2.18 


.0891 


.0880 


99 


.000141 


110.0 






20 


1.725 


.0863 


.0852 


99 


.000137 


107.0 


108.2 


3 
32 


22 


1.91 


.0868 


.0857 


99 


.000138 


107.8 






22 


2.53 


.1151 


.1141 


99 


.000189 


143.6 






20 


2.29 


.1145 


.1137 


99 


.000183 


143.0 


142.5 


a 

Vf 


18 


2.03 


.1128 


.1119 


99 


.000180 


141.0 







TABLE XLV. Two COILS OF FOUR TURNS PER COIL. RESISTANCE = 0.033 OHMS. 



Amperes. 


Volts. 


Impedance. 


Reactance. 


Cycles per 
Second. 


Inductance in 
Henrys. 


C. G. S. Lines 
per Ampere 
Turn and per 
Inch Length of 
Armature. 


Mean. 


Mean 
Air 
Gap. 












in. 


21 


2.64 .1256 


.1212 


99 


.000195 


38.1 






19 


2.42 


.1274 


.1230 


99 


.000198 


38.7 


38.2 


CO 


17.5 


2.18 


.1245 


.1202 


99 


.000193 


37.8 






17 


2.85 


.1676 


.1645 


100 


.000262 


51.3 






15.5 


2.61 .1680 


.1646 


100 


.000262 


51.3 


51.0 


If 


13 


2.15 .1655 .1620 


100 


.000258 


50.4 






13 


2.81 .216 


.213 


100 


.000340 


G6.4 






15 
16.5 


3.20 
3.55 


.213 

.215 


.210 

.212 


100 .000334 
100 .000338 


65.3 
GG.l 


65.9 


1 1 

04 


12.5 


3.48 


.278 


.276 


100 .000440 


86.0 






11 
10 


3.03 
2.77 


.275 .273 
.277 .275 


100 

100 


.000435 
.000438 


85.0 85.6 
85.8 


3 

3TT 


10 


3.59 


.359 .358 


99 


.000576 


112.5 


9 


3.20 


.356 


.355 


99 


.000572 


111.7 


111.6 


3 


8 


2.82 


.353 


.352 


99 


.000567 


110.7 







168 Electric Generators. 

TABLE XLVI. THREE COILS OF FOUR TURNS PER COIL. RESISTANCE = .0473 OHMS. 



Amperes. 


Volts. 


Impedance. 


Reactance. 


Cycles per 
Second. 


Inductance in 
Henrys. 


C.G.S. Lines 
per Ampere 
Turn and per 
Inch Length of 
Armature. 


Mean. 


Mean 
Air 
Gap. 


















in. 


15 


3.68 


.245 


.240 


99 


.000386 


33.5 






13.5 


3.35 


.248 


.243 


99 


.000391 


33.9 


33.7 


CO 


12 


2.96 


.246 


.241 


99 


.000388 


33.7 






10 


3.47 


.347 


.344 


98 


.000558 


48.5 






9 

8 


2.98 
2.45 


.331 
.306 


.328 
.303 


98 
98 


.000533 
.000492 


46.3 
42.7 


45.8 


2 3 
4 


17 


7.8 


.458 


.452 


98 


.000737 


63.8 






15 


6.75 


.450 


.447 


98 


.000726 


63.0 


63.2 


11 


14 


6.3 


.450 


.447 


98 


.000726 


63.0 






13 


7.84 


.603 


.601 


98 


.000976 


84.6 






12 
10 


7.08 
5.32 


.590 
.532 


.588 
.530 


98 
98 


.000958 
.000863 


83.3 

74.7 


80.8 


3 
32" 


18 


14.6 


.812 


.811 


98 


.001317 


114.2 






16 


12.5 


.782 


.781 


98 


.001270 


110.1 


111.1 


3 


15 


11.6 


.774 


.773 


98 


.001255 


109.0 







TABLE XLVII. FOUR COILS OF FOUR TURNS PER COIL. RESISTANCE = .0637 OHMS. 















C.G.S. Lines 






Amperes. 


Volts. 


Impedance. 


Reactance. 


Cycles per 
Second. 


Inductance in 
Henrys. 


per Ampere 
Turn and per 
Inch Length of 


Mean. 


Mean 
Air 
Gap. 














Armature. 




















in. 


19 


7.42 


.390 


.385 100 


.000613 


29.9 




17 


6.47 


.380 


.375 


100 


.000598 


29.3 


29.5 


CO 


14 


5.32 


.380 


.375 


100 


.000598 


29.3 






15 


8.23 


,544 


.539 


100 


.000872 


42.6 






13 


7.06 


.543 .538 


100 


000871 


42.6 


41.5 


2 3 
75~4 


11 


5.48 


.500 


.495 


100 


.000802 


39.2 






10 


7.58 


.758 


.755 


100 


.00120 


58.7 






9 


6.64 


.738 


.735 


100 


.00117 


57.3 


56.1 


1 1 

6"T 


8 


5.40 


.675 


.672 


100 


.00107 


52.3 






17 


19.04 


1.12 


1.117 


100 


.00178 


87.0 






15 


16.25 


1.082 


1.079 


100 


.00172 


84.2 


84.8 


3 

3"2" 


13 


13.75 


1.057 


1.054 


100 


.00170 


83.2 






17 


24.0 


1.411 


1.410 


100 


.00225 


110 






15.5 


21.3 


1.375 


1.374 


100 


.00219 


107 


107.5 


3 


14 


19.0 


1.356 


1.355 


100 


.00216 


105.5 




04 



The curves in Figs. 154 and 155 are plotted from the above results. 



Experimental Data of Inductance. 



1G9 



No results are given for the position of zero air gap, since great 
inaccuracy was introduced by the pole-piece not making a uniform 
magnetic contact each time it was replaced. 

Seventh Experiment. The armature of a 20 horse-power railway 
motor characterised by an especially small number of slots (twenty-nine) 
was measured as to inductance, and it is interesting to note that despite 
the concentration of many turns in each slot, the inductance as expressed 
in terms of the number of C.Gr.S. lines per ampere turn and per inch 



NGTH OFARM.LAMIN9 

S 5 

SC.Cs 




^ 






C.G.&. 

OF Af 
COIL. 


LINE 
MAT 

GRO! 


B PEI 

URE F 

iS LE 


I AM 
OR V 
NCTH 


PER 
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OF A 


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RMA 


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TURI 


PER 
PS 

-LAIN 


N. LENGTH 
J TURNS PER 
17- 8" 




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Rg.166. 




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PERIN.LE 

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ULTIMATE VALUE OF 56-6 IN AIR 








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length of armature lamination, is but very little greater than in machines 
with many slots and but few conductors per slot. 

The principal dimensions of the armature are given below, and in 



Fig. 156. 



Diameter of armature 
Number of slots ... 
,, coils ... 

Turns per coil 
Conductors per slot 
Gross length of armature laminations 
Length of air gap average 



11 in. 

29 

87 
G 

36 

9 in. 
wV in. 



The values for the position of minimum inductance were taken with 
the armature out of its frame ; i.e. , in air. 



170 



Electric Generators. 



SECTIONAL VIEW 

OF 
RAILWAY MOTO 




(50S7.E) 




(5087. F) 



ARRANGEMENT OF COILS IN SLOTS 



















Mh. 


























































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-. 


"!. 


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s 






























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va 


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f>t 


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3 coiLs in/ cnv slot/ 


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Ceils i-n> cudjcucent slots. 






















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, 


-^ 












































































B 



































































































































































































































































































































































































































































































































































































no 



u 

,; 70 



(soar o) 



IB 



TURNS IN SERIES 



Experimental Tests of Inductance. 



171 



For the position of maximum inductance, the armature was in its 
frame with the coils under test directly under the pole face. The pole face 
was built of laminations. 

Fig. 157 shows the arrangement of the coils in the slots, and also serves 
as a key to the combinations of coils taken. Taking slot 1, it was found 
that the inductance of coils A, B, and C were practically the same. 

The results are plotted in Fig. 158. In the curve marked A, the 
turns are situated in one and the same slot except for the last point 
(i.e., twenty-four turns), in which case, eighteen turns were in one slot 
and six turns in the adjacent one. In curve B, the turns were situated 
six in each slot, (i.e., one coil per slot), the slots being adjacent. 

The observations are given below in tabulated form. 

TABLE XLVIII. 













C.G.S. Lines uer 


Amperes. Impedance. 


Mean 
Impedance. 


Reactance. 


Cycles pel- 
Second. 


Inductance in Ampere Turn and 
Henrys. Per Inch Length 












of Armature. 


1 













15 

17 
19 



One Coil of 6 Turns. Position of Minimum Inductance. 

Slot 1, Coil B. Resistance = .0230 ohms. 
.0793 



.0782 
.0784 



.0786 



.0752 



97 



.0001237 



Two Coils of 6 Turns per Coil. Position of Minimum Inductance. 
Slot 1, Coils B and C. Resistance = .048 ohms. 



8 


.299 


10 


.290 


11 


.291 


Slot 1, 


10 


.204 


13 


.199 


15 


.195 



.293 



.289 



97 



.000476 



36.7 



Slot 1, Coil B. Slot 2, Coil B. Resistance - .049 ohms. 



.199 



.195 



96 



.000322 



9 
11 
13 



13 
15 
17 



13 
15 
17 



Three Coils of 6 Turns per Coil. Position of Minimum Inductance. 

Slot 1, Coils A, B, and C. Resistance = .0738 ohms. 
5.78 ! .643 



6.68 
7.7 



.607 
.593 



.614 



.609 



97 



.0010 



34.3 



Slot 1, Coils A and B. Slot 2, Coil B. Resistance = .0722 ohms. 



5.26 
6.52 
7.23 



.404 
.407 
.426 



.412 



.405 



96 



.000673 



23.1 



Slot 1, Coil B. Slot 2, Coil B. Slot 3, Coil B. Resistance = .0722 ohms. 



4.4 

5.08 
5.72 



.338 
.339 
.336 



.338 



.330 



96 



.000548 



18.1 



172 



Electric Generators. 
TABLE XLVIIT. Continued. 

















C. G. S. Lines 


Amperes. 


Volts. 


Impedance. 


Mean 
Impedance. 


Reactance. 


Cycles per 
Second. 


Inductance in 
Henrys. 


per Ampere 
Turn and per 
Inch Length 
of Armature. 



Four Coils of 6 Turns per Coil. Position of Minimum Inductance. 
Slot 1, Coils A, B, and C. Slot 2, Coil B. Resistance = .0976 ohms. 



13 
15 
17 



9.5 
10.5 



10.17 
11.5 

13.08 



.782 
.767 
.769 



.772 



.765 



96 



.001272 



Slot 1, Coil A and B. Slot 2, Coils A and B. Resistance = .098 ohms. 



6.02 
6.97 
7.62 



.752 
.732 
.746 



.743 
.74 



.736 



96 



.001223 



24.6 



23.6 



Slot 1, Coils A and B. Slot 2, Coil B. Slot 3, Coil B. Resistance - .0984 ohms. 



8.5 
10 

12 



5.45 
6.27 
7.30 



.642 
.627 
.608 



.626 



.620 



97 



.001020 



19.7 



Slot 1, Coil B. Slot 2, Coil B. Slot 3, Coil B. Slot 4, Coil B. Resistance = .0984 ohms. 



10 
13 
15 



15 
13 
10 



10 
9 



10 
11 
12 



15 
14 
13 



9 

10 
11 



5.25 
6.65 

7.47 



.525 
.512 

.498 



.511 



.501 



97 



.000824 



15.9 



One Coil of 6 Turns. Position of Maximum Inductance. 
Slot 1, Coil B. Resistance = .0232 ohms. 



2.16 
1.89 
1.42 



.144 
.145 
.142 



.144 



101 



.000224 



69.2 



Two Coils of 6 Turns per Coil. Position of Maximum Inductance. 

Slot 1, Coils B and C. Resistance = .0469 ohms. 
5.6 .56 

4.94 .55 .553 .551 100 .000877 

4.4 .55 



Slot 1, Coil B. Slot 2, Coil B. Resistance = .0479 ohms. 



4.35 
4.81 
5.32 



.435 
.437 
.443 



.438 



.436 



101 



.000687 



67.7 



53.0 



Three Coils of 6 Turns per Coil. Position of Maximum Inductance. 
Slot 1, Coils A, B, and C. Resistance = .0735 ohms. 



19.2 

18 

16.6 



1.28 
1.28 
1.28 



1.28 



1.28 



102 



.0020 



Slot 1, Coils A and B. Slot 2, Coil B. Resistance = .0748 ohms. 



9.6 
10.7 
11.85 



1.07 
1.07 
1.08 



1.07 



1.07 



101 



.00169 



68.9 



58.3 



Experimental Tests of Inductance. 

TABLE XLVIII. Continued, 



173 

















C. G. S. Lines 


Amperes. 


Volts. 


Impedance. 


Mean 
Impedance. 


Reactance. 


Cycles per 
Second. 


Inductance in 
Henrys. 


per Ampere 
Turn and per 
Inch Length 
















of Armature. 



Slot 1, Coil B. Slot 2, Coil B. Slot 3, Coil B. Resistance = .0739 ohms. 



11 

12 
13 



12 
13 

H 



12 
13 
15 



9.2 

10 
10.85 



.837 
.834 
.835 



.835 



.830 



97 



.00136 



46.8 



Four Coils of 6 Turns per Coil. Position of Maximum Inductance. 
Slot 1, Coils A, B, and C. Slot 2, Coil B. Resistance = .0984 ohms. 



23.3 
25.3 
27.3 



1.94 
1.95 
1.95 



1.94 



1.94 



103 



.0030 



59.2 



Slot 1, Coils A and B. Slot 2, Colls A and B. Resistance = .0992 ohms. 



22.4 

24 

27.6 



1.87 
1.85 
1.84 



1.85 



1.85 



101 



.00292 



57.6 



Slot 1, Coils A and B. Slot 2, Coil B. Slot 3, Coil B. Resistance = .101 ohms. 



13 
15 
17 



20.7 
23.6 
26.5 



1.59 
1.57 
1.56 



1.57 



1.57 



101 



.00247 



48.7 



Slot 1, Coil B. Slot 2, Coil B. Slot 3, Coil B. Slot 4, Coil B. Resistance = .0986 ohms. 



15 


19.6 


1.31 












16 


20.9 


1.31 


1.31 


1.31 


101 


.00206 


40.6 


17 


22.2 


1.31 













Eighth Experiment. These measurements related to an armature of an 
alternating current dynamo. The considerable number of slots, however, 
make the results instructive from the standpoint of commutating 
machines. First, the coils A A and B B of Fig. 159 were connected 
in series, and the inductance was measured at a periodicity of 30 cycles 
in the position of minimum and maximum inductance, the position 
shown in Fig. 159 being, of course, the position of maximum inductance. 

The values deduced from the observations were : 



Position of minimum inductance 



maximum inductance 



20. C.G.S. lines per ampere turn 
and per inch gross length of 
armature lamination. 



Then the turns in four adjacent slots were connected in series, 
and then, as shown in Fig. 160, inductance was measured in the positions 



174 



Electric Generators. 



of minimum and maximum inductance. The following results were 



obtained :- 



Position of minimum inductance 



maximum inductance 



13. C.G.S. lines per ampere turn 
and per inch gross length of 
armature lamination. 

1 

* v 11 11 11 



net, cores 
parallel to shaft 
IX, . 




JDepth, 
of Gap 31 



CciL AA ? W> txtrne in/ Series 
CdL BB = 12 tarrte vuSert&s 
Gross depth/ ~LcaninMULone 12> " 
Net/ depth 6-6" 



Fy.160. 




A study of these tests indicates that in projection armatures, it is 
practicable to so proportion the slots and conductors as to obtain as small a 
flux as 20 C.G.S. lines per ampere turn and per inch of gross length of 
armature lamination for the coils in the position of minimum inductance. 
When the conditions conform approximately to any particular case 
regarding which more definite experimental data is available, this more 
exact data should of course be employed. 

The experimental data in the possession of other designers relating to 
the types with which they are accustomed to deal, may lead them to the 



Calculation of Reactance Voltage. 



175 



use of numerical values for this constant other than those indicated by the 
preceding tests ; but it will be at once admitted that the chief value of such 
data lies more in the relative results obtained for various machines, than in 
the absolute results. The method of applying the constant must hold 
equally for all types, but doubtless the most suitable value to take for the 
constant will vary to some extent according to the degree of divergence 
between the types. 



ILLUSTRATIONS OF THE CALCULATION OP THE REACTANCE VOLTAGE. 

The determination of the inductance having so important a bearing 
upon the design, the method will be explained by working out several 
cases ; and when in the following sections several complete working designs 



The, 
posit 



HnATURE CONDUCTORS 
onductors fnarjft/otS. are/ tyb the 
sho>w. short circuited at A B 



Fig.161 




are described, the value of the inductance as related to the general 
performance of the machine will be considered. All the following cases 
relate to drum windings : 

Case I. In a four-pole continuous-current dynamo for 200 kilowatts 
output at 550 volts and a speed of 750 revolutions per minute, the armature 
is built with a four-circuit single-winding, arranged in 120 slots, with four 
conductors per slot. The commutator has a diameter of 20 in., and has 
240 segments. 

The brushes are .75 in. thick. The segments are .26 in. wide; 
consequently as there is one complete turn per segment, three complete 
turns is the maximum number undergoing short circuit at one brush at 
any instant. 

Considering a group of adjoining conductors in the slots occupying the 
commutating zone between two pole tips, six of these conductors, occupying 
one and one-half slots will be short-circuited, three at one set of brushes 



176 Electric Generators. 

and three at another, as shown diagrammatically in Fig. 101. Now the 

full-load current of this machine is- - = 364 amperes, the current per 

550 

OCA 

circuit being - - = 91 amperes. Consequently, while any one coil is short- 
circuited under the brush, the current of 91 amperes in one direction 
must be reduced to zero, and there must be built up in it a current of 
91 amperes in the other direction by the time it emerges from the 
position of short circuit under the brush, to join the other side of the 
circuit. This change is at times occurring simultaneously in a group of 
six adjacent conductors. 

A coil has an inductance of one henry when it is situated in a medium 
of such permeability, and is so dimensioned that a current of one ampere 
sets up a magnetic flux of such magnitude that the product of the number 
of lines linked with the coil by the number of turns in the coil is equal to 
100,000,000. If the coil has but one turn, then its inductance becomes 
10~ 8 times the number of lines linked by the turn when one ampere is 
passing through it. In the case under consideration, the coil is of one turn, 
but the varying flux linked with it, and hence the voltage induced in it is 
proportional not only to the rate of change of its own current, but to the 
rate of change of the currents in the adjacent turns simultaneously under 
going commutation at different sets of brushes, and at different points of 
the surface of the same brushes. In this case five other turns are 
concerned in determining this varying flux, hence the voltage induced 
will be six times as great as if the coil had alone been undergoing 
commutation at the moment. It will not be the square of six times 
as great, since it is the voltage in the one turn that it is required to 
determine. 

Had the six turns in series belonged to the one coil undergoing 
commutation, then the induced voltage would have been the square 
of six times as great as for a one-turn coil. 

Gross length of lamination = 10 in. 

Flux set up in one turn, per ampere in that turn and per inch of length of armature 
lamination = 20 C.G.S. lines. 

Hence flux of self-inductance = 10 x 20 = 200 lines. 

Self-inductance = 200 x 10~ 8 = .0000020 Henrys. 

Mutual inductance of one turn with relation to the six turns simultaneously undergoing 
commutation = 6 x .0000020 = .000012 Henrys. 

Circumference of commutator = 20 x TT = 62.8 in. 



Calculation of Reactance Voltage. 177 

Revolutions per second = 750 -f- GO = 12.5 

Peripheral speed of commutator == 62.8 x 12.5 = 785 in. per second. 

Thickness of radial carbon brush = .75 in. 

i O 

Current is completely reversed in ^- = .00095 seconds, which is the time of comple 
tion of a half-cycle. Consequently, the reversal occurs at an average rate of 



2 x .00095 
= 530 cycles per second. 

We are now prepared to obtain the reactance of the turn, and shall, 
for want of a better, make the in this case very unwarranted assump 
tion of a sine wave rate of variation : 

Reactance = 2 x IT x 530 x .000012 = .040 ohms. 
Reactance voltage = 91 x .040 = 3.6 volts. 

This is the voltage estimated to be induced in the turn during 1 

o o 

the process of commutation. In each of the other five turns independently 
undergoing commutation under other sets of brushes, and under other 
parts of the bearing surface of the same set of brushes, there is also 
an induced voltage of 3.5 volts. 

In this design, the factors most concerned in the process of commuta 
tion are the following : 

Reactance voltage of short-circuited coil ... ... ... 3.6 volts 

Inductance per commutator segment ... ... ... ... .000012 henrys 

Armature ampere turns per pole-piece ... ... ... ... 5500 ampere turns 

Current per armature circuit ... ... ... ... ... 91 amperes 

Average voltage per commutator segment ... ... ... 9.2 volts 

Case II. A six-pole continuous-current dynamo has a rated output 
of 200 kilowatts at 600 revolutions per minute and 500 volts. 

The armature has a six-circuit winding, arranged in 126 slots, 
with eight conductors per slot. The commutator has 252 segments. 
There are two turns in series per segment. The diameter of the commuta 
tor is 20 in. and the width of a segment is .24 in. The thickness of 

O 

the radial bearing carbon brushes is .63 in., consequently the maximum 
number of coils short-circuited at any time at one set of brushes is 
three. Hence 3x2x2=12 conductors grouped together in the 
neutral zone between two pole tips, and occupying one and one-half 
slots, are simultaneously undergoing commutation, that is, six conductors 
at one set of brushes and the other six at the next set. 

Gross length of lamination 9 in. 

2 A 



178 Electric Generators. 

Flux set up in 12 turns by 1 ampere in those turns, and with 
9 in. length of armature lamination = 12 x 20 x 9 = 2160 C.G.S. lines. 
Mutual inductance of one coil (two turns) with relation to the six coils 
simultaneously undergoing commutation = 2160 x 10~ 8 x 2 = .0000432 
henry s. 

Circumference of commutator = 62.8 in. 

Revolutions per second = 600 4- 60 = 10. 

Peripheral speed commutator = 62.8 x 10 = 628 in. per second. 

Thickness of radial bearing carbon brush = .63 in. 

f* o 

Current completely reversed in -| - = .0010 seconds. 

62o 

Average rate of reversal = 500 cycles per second. 

Reactance = 2 x TT x 500 x .0000432 = .136 ohms. 

. ., 200,000 

Amperes per armature circuit = - = bo. 7 amperes. 

Reactance voltage = 66.7 x .136 9.1 volts. 

(This, of course, is an undesirably high figure, and would only be 
permissible in connection with especially good constants in other respects.) 

Reactance voltage of short-circuited coil ... ... ... 9. 1 volts 

Inductance per commutator segment ... ... ... ... .000043 henrys 

Armature ampere turns per pole-piece ... ... ... .. 5600 ampere turns 

Current per armature circuit ... ... ... ... ... 67 amperes 

Average voltage per commutator segment ... ... ... 12 volts 



Case III. A 10-pole lightning generator has a rated output of 300 
kilowatts at 125 volts and 100 revolutions per minute. It has a 10-circuit, 
single-winding, arranged, four conductors per slot, in 180 slots. The 
commutator has 360 segments, one segment per turn. Diameter of 
commutator is 52 in., and the width of a segment is .45 in. 

The thickness of the radial bearing carbon brushes is 1 in., and the 
maximum number of coils short-circuited at any time at one set of brushes 
is three. Hence six conductors, grouped together at the neutral zone 
between any two pole tips, are concerned simultaneously in the 
commutating process. 

Gross length of lamination = 17.6 in. 

Flux set up in six turns by one ampere in each of them, and with 
17.6 in. length of armature lamination = 6 x 20 x 17.6 = 2,110 C.G.S. lines. 



Fifteen- Hundred Kilowatt Railway Generator. 179 

Mutual inductance of one coil of one turn, with relation to the six 
oils simultaneously undergoing commutation = 2,110 x 10~ 8 x 1 = .0000211 
henrys. 

Circumference of commutator = 52 x TT = 1G4 in. 
Revolutions per second = 100 -f- GO = 1.67 revolutions. 
Peripheral speed commutator = 1G4 x 1.67 == 274 in. per second. 
Thickness of radial bearing carbon brush = 1 in. 

Current completely reversed in = .00365 seconds. 

274 

Average rate of reversal = = 137 cycles per second 
Reactance - 2 x TT x 137 x .0000211 = .018 ohms. 
Rated full load current output = = 2400 amperes. 

1 ^jD 

2400 
Current per armature conductor = 240 amperes. 

Reactance voltage = 240 x .018 = 4.3 volts. 

Reactance voltage of short-circuited coil ... ... ... 4. 3 volts 

Inductance per commutator segment ... ... ... .000021 henrys 

Armature ampere turns per pole-piece... ... ... ... 8600 ampere turns 

Current per armature circuit ... ... ... ... ... 240 amperes 

Average voltage per commutator segment ... ... ... 3.5 volts 



MODERN CONSTANT POTENTIAL COMMUTATING DYNAMOS. 

Direct- Connected, 12-Pole, 1,500-Kilowatt, 600-Volt Railway Generator. 
Speed = 75 Revolutions per Minute. This machine is remarkable in that, 
at the time it was designed no commutating dynamo of more than a 
fraction of its capacity had been constructed. Owing to the great weight 
of the various parts, and the short time in which the machine had to 
be constructed, it was assembled and tested for the first time at the 
Columbian Exposition. 

It was found that the machine complied with the specification in 
all particulars as to heating, and that sparking did not occur between the 
limits of no load and 50 per cent, overload. Mention is made of this, since 
this was the first of the modern traction generators developed in the 
United States ; and the constants of this machine, which were novel at 
that time, have since become common in the best practice in designing. 
Perhaps the most remarkable feature of this machine is the range of load 
at which sparkless commutation occurs, and the great magnetic strength 
of the armature as compared with that of the field-magnets. This result 



180 Electric Generators. 

was accomplished, first, by comparatively low inductance of the armature 
coils ; secondly, high magnetisation in the armature projections, which 
to some extent keeps down distortion of the magnetic field ; and, thirdly, 
by the over-compounding of the machines to suit railway practice : that 
is, no load volts of 550 and full load volts of 600. The increase 
of magnetisation corresponding to this increase of voltage is a condition 
favourable to sparkless commutation ; and it will be noted from the 
particulars given of the machine, that the magnetising force of the series 
coil at full load is approximately equal to that of the shunt coil at 
no load. 

Drawings are given, Figs. 162 to 166, showing the construction, 
and in Figs. 167 and 168 are given saturation and compounding curves for 
this machine. The following specification sets forth the constants of the 
machine and the steps in the calculations. 

SPECIFICATION OP 12-PoLE, 1,500-KiLowATT, 600-VoLT GENERATOR, FOR SPEED OF 

75 REVOLUTIONS PER MINUTE. 

Number of poles ... ... ... ... ... ... ... 12 

Kilowatts ........................ 1500 

Revolutions per minute ... ... ... ... ... ... 75 

Frequency in cycles per second ... ... ... ... ... 7.5 

Terminal volts, no load ... ... ... .. ... ... 550 

full load ...... ... GOO 

Amperes, full load ... ... ... ... ... ... 2500 

DIMENSIONS. 

Armature : 



Diameter over all ... ... ... ... ... ... 126 in. 

Length over conductors ... ... ... ... ... ... 48^ ,, 

Diameter at bottom of slots 12 If , 

4 " 

Internal diameter of core 103^ , 

4 

Length of core over all ... ... ... ... ... ... 33f 

Effective length, magnetic iron ... ... ... ... ... 26.8 

Pitch at surface ... ... ... ... ... ... ... 33 in. 

Insulation between sheets ... ... ... ... ... 10 per cent. 

Thickness of sheets ... ... ... ... ... ... .014 in. 

Depth of slot ..................... 2i 

Width of slot at root ii 

16 " 

surface ... ...... ... }j 

Number of slots ... ... ... ... ... ... ... 345 

Minimum width of tooth ... ... ... ... ... .412 in. 

Width of tooth at armature face ... ... ... ... .763 

,, conductor ... .. ... ... ... ... 7_ 

Depth of 



Fifteen- Hundred Kilowatt Railway Generator. 



181 




182 Electric Generators. 

Number of ventilating ducts ... ... ... ... ... 8 

Width of each ventilating duct ...... \ in. 

Effective length of core -f total length ... ... ... .795 

Magnet Core : 

Length of pole face ... ... ... ... ... ... 33^ in. 

Length of pole arc ... ... ... ... ... ... 24^ ,, 

Pole arc -=- pitch ... ... ... ... ... ... ... .73 

Thickness of pole-piece at edge of core... ... ... ... 1 T 9 ^ in. 

Radial length of magnet core ... ... ... ... ... 18 

Width of magnet core ... ... ... ... ... ... 14 

Thickness of magnet core ... ... .. ... ... 30 ,, 

Diameter of bore of field ... ... ... ... ... 126| 

Depth of air gap ... ... ... ... ... ... ... yV 

Spool : 

Length over flanges ... ... ... ... ... ... 17|- in. 

of winding space ... ... ... ... ... ... 16^ ,, 

Depth 3| 

Yoke: 

Outside diameter ... ... ... ... ... ... ... 190| in. and 180| in. 

Inside ,, ... ... ... ... ... ... ... 168 in. 

Thickness, body ... ... ... ... ... ... ... 6^ ,, 

Length along armature ... ... ... ... ... ... 36 ,, 

Commutator : 

Diameter ... ... ... ... ... ... ... 86 J ,, 

Number of segments ... ... ... ... ... ... 696 

,, ,, per slot ... ... ... ... ... 2 

Width of segment at commutator face ... ... ... ... .342 in. 

root .313 

Depth of segment ... ... ... ... ... ... 3 ,, 

Thickness of mica insulation ... ... ... ... ... .05 ,, 

Available length of surface of segment ... ... ... ... 1 9|- 

Cross-section of commutator leads ... ... ... ... . 1 30 squai-e inches 

Brushes : 

Number of sets ... ... ... ... ... ... ... 12 

Number in one set ... ... ... ... ... ... 6 

Width 2.5 

Thickness... ... ... ... ... ... ... ... .75 

Area of contact of one brush ... ... ... ... ... 1.875 

Type of brush ... ... ... ... ... ... ... Radial carbon 

MATERIALS. 

Armature core ... ... ... ... ... ... ... Sheet iron 

spider Cast iron 

Conductors ... ... ... ... ... ... ... Copper 



Fifteen-Hundred Kilowatt Railway Generator. 183 




184 Electric Generators. 

Commutator segments ... ... ... ... ... ... Copper 

,, leads ... ... ... ... ... ... German silver 

Spicier ... ... ... ... ... ... ... ... Cast iron 

Pole piece ... ... ... ... ... ... ... ... Cast steel 

Yoke 

Magnet core ... ... ... ... ... ... ... ,, 

Brushes ... ... ... ... ... ... ... ... Carbon 

TECHNICAL DATA. 

Armature, no load voltage ... ... ... ... ... 550 

Number of face conductors ... ... ... ... ... 1 392 

Conductors per slot ... ... ... ... ... ... 4 

Number of circuits ... ... ... ... ... ... 12 

Style of winding ... ... ... ... ... ... ... Single 

Gramme ring or drum ... ... ... ... ... ... Drum 

Type construction of winding ... ... ... ... ... E volute end 

connections 

Mean length one armature turn ... ... ... ... ... 1 76 in. 

Total armature turns ... ... ... ... ... ... 696 

Turns in series between brushes.. . ... ... ... ... 58 

Length between brushes ... ... ... ... ... ... 10,200 in. 

Cross-section, one armature conductor ... ... ... ... .161 

Ohms per cubic inch at 20 deg. cent 00000068 ohms. 

Resistance between brushes at 20 deg. Cent. ... ... ... .043 ,, 

60 .050 

Volts drop in armature at 60 deg. Cent. ... ... ... 10.3 

,, brush contact ... ... ... ... ... 2.5 

,, series winding ... ... ... ... ... 1.9 

Terminal voltage, full load ... ... ... ... ... 600 

Total internal voltage, full load... ... ... ... ... 620 

Amperes per square inch in armature winding ... ... 1290 

,, ,, commutator connections ... ... 3200 

Commutation : 

Average voltage between commutator segments ... ... 10.3 

Armature turns per pole... ... ... ... ... ... 58 

Amperes per turn ... ... ... ... ... ... 208 

Armature ampere turns per pole ... ... ... ... 12,100 

Segments lead of brushes ... ... ... ... ... 6^ 

Percentage lead of brushes ... ... ... ... ... 10.8 

,, demagnetizing ampere turns ... ... ... 21.6 

,, distorting ampere turns ... ... ... ... 78.4 

Demagnetizing ampere turns per pole ... ... ... ... 2610 

Distorting ,, ... ... 9490 

Frequency of commutation (cycles per second) ... ... 227 

Number of coils simultaneously short-circuited per brush ... 2 

Turns per coil ... ... ... ... ... ... ... 1 

Number of conductors per group simultaneously undergoing 

commutation 4 



Fifteen-Hundred Kilowatt Railway Generator. 185 




2 B 



186 Electric Generators. 

Flux per ampere turn per inch length armature lamination ... 20 (assumed). 

Flux linked with four turns = 36.7 x 20 x 4 2700 

Inductance in one turn constituting one coil, in henrys = 

1 x 2700 x 10- .000027 

Reactance short-circuited turn ... ... ... ... ... .0385 ohms 

Reactance voltage = .0385 x 208 8.0 volts. 

In operating these machines, the brushes are set at a constant lead 
of 6|- segments for all loads, and the output may temporarily exceed the 
full load rated output by 50 per cent. 

MAGNETIC DATA. 

Coefficient of magnetic leakage ... ... ... ... ... 1.15 

Megalines entering armature per pole-piece at no load and 

550 volts 31.6 

Megalines entering armature per pole-piece at full load and 

620 inter, volts 35.6 

Armature : 

Section ... ... ... ... ... ... ... ... 241 square inches 

Length (magnetic) ... ... ... ... ... ... 1 9 in. 

Density at no load ... ... ... ... ... ... 66 kilols, 

at full load ... ... ... ... ... ... 74 

Ampere turns per inch length no load ... ... ... ... 15 

full load 18 

Ampere turns, no load ... ... ... ... ... ... 290 

full load 340 

Teeth : 

Transmitting flux from one pole-piece ... ... ... ... 24 

Section at roots ... ... ... ... ... 264 square inches 

Length ... 2.125 in. 

Apparent density at no load ... ... ... ... ... 120 kilols. 

full load ... 135 ,, 

Corrected density at no load ... ... ... ... ... 116 ,, 

full load ... 126 

Ampere turns per inch length, no load... ... ... ... 1800 

full load ... ... 1400 

Ampere turns no load ... ... ... ... ... ... 1700 

full load ... 3000 

Gap : 

Section at pole face ... ... ... ... ... ... 820 square inches 

Length gap .43 in. 

Density at pole face, no load ... ... ... ... ... 39 kilols. 

, , , full load 44 

Ampere turns, no load ... ... ... ... ... 5300 

full load ... 6000 



Fifteen- Hundred Kilowatt Railway Generator. 187 

Magnet Core : 

Section ... ...... 420 square inches 

Length (magnetic) ... ... ... ... ... ... 20 in. 

Density, no load ... 87 kilols. 

full load 98 

Ampere turns per inch length, no load... ... ... ... 67 

full load 160 

Ampere turns, no load ... ... ... ... ... ... 1350 

full load 3200 

Magnet Yoke : 

Section ... ... ... ... ... ... ... ... 225 square inches 

Length per pole ... ... ... ... ... 27 in. 

Density, no load ... 81 kilols. 

full load 91 

Ampere turns per inch length, no load ... 49 

full load 110 

Ampere turns, no load ... ... ... ... 1320 

full load 3000 

AMPERE TURXS PER SPOOL. 

No Load and No Load and 
550 Volts. 620 Internal Volts. 

Armature core 290 340 

teeth ... 1700 3000 

Air gap 5300 6000 

Magnet core ... ... 1350 3200 

Yoke 1320 3000 



9960 15,540 

Demagnetising ampere turns per pole-piece at full load ... 2600 

Allowance for increase in density through distortion ... ... 1000 

Total ampere turns at full load of 2500 amperes and 600 

terminal volts ... ... ... ... ... ... 19,140 

If the field rheostat is so adjusted that the shunt winding shall supply 
the 9,960 ampere turns necessary for the 550 volts at no load, then, when 
the terminal voltage has risen to 600 volts at full load, the shunt winding 

will be supplying - x 9,960 = 10,840 ampere turns. The series winding 

OU 

must, at full load, supply the remaining excitation, i.e., 19,140 -- 10,840 = 
8,300 ampere turns. The armature has 1,392 face conductors, hence the 
armature strength expressed in ampere turns per pole-piece is, at full load 
current of 2,500 amperes (208 amperes per circuit) : 

1392 
l x 208 = 12,100 ampere turns per pole-piece, on armature. 



188 



Electric Generators. 



CALCULATION OF SPOOL WINDINGS. 
Shunt : 

Mean length of one shunt turn ... 

Ampere turns per shunt spool at full load 

Ampere feet 

Radiating surface one shunt spool 

Permit .36 watts per square inch at 20 deg. Cent. 

Then shunt watts per spool at 20 deg. Cent. ... 

And ,, ,, 60 

Pounds copper per coil = - 650 Ib. 

405 



8.5 ft. 

10,840 

92,000 

1130 square inches 

405 

468 



A margin of 16.6 per cent, in the shunt rheostat when coils are hot 
leaves 83 per cent, of the available 600 volts, or 500 volts, at the terminals 



f . H 

600 
SSt 

too 

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too 

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wo 

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twoo lepoo 



of field spools. This is equivalent to 432 volts, or 36 volts per spool, 
when spools have a temperature of 20 deg. Cent. 



Hence require - = 11.3 amperes in shunt coils. 
36 



Turns per shunt spool 



10,800 
11.3 



960 

8150 ft. 
79.8 



Length of 960 turns 

Pounds per 1 000 feet 

No. 6 B. and S. gauge weighs 79.5 Ib. per 1000 feet. 

Bare diameter = .162 in. D.C.C.D. = .174 inch. 

Cross section = .0206 square inch. 

Current density = 546 amperes per square inch. 

Length of the portion of winding space available for shunt 

coil 9.0 inches. 
Depth of winding, 3.9 inches. 



Series Winding. The series winding is required to supply 8,300 
ampere turns at full load. With 4.5 turns per spool^ the full load current 



Fifteen-Hundred Kilowatt Railway Generator. 189 

will give 2,500 x 4.5 = 11,250 ampere turns. Consequently, 650 amperes 
must be diverted through the diverter rheostat, leaving 1,850 amperes 
in the series winding, giving 8,300 ampere turns. 

The 4.5 turns consist of ten bands in parallel, each 7 in. wide by 
T \y in. thick. 

Cross-section conductors... ... ... ... ... ... 4.375 square inches 

Current density ... ... ... ... ... ... ...424 amperes per sq. in. 

Resistance of 12 spools at 20 deg. Cent. ... ... ... .000855 ohms. 

Series C 2 R at 20 deg. Cent, per spool . . . 244 watts 

60 282 

Weight series copper per spool ... ... ... ... ... 650 Ib. 

ESTIMATED CORE Loss. 

Total weight armature laminations ... ... ... ... 26,000 Ib. 

Cycles per second ... ... ... ... 7.5 

Kilolines density in core ... ... ... 74. 

Cycles x Density 

1000 

Corresponding watt core loss per pound ... ... .9 

Total estimated core loss ... ... ... ... ... 23,400 watts 

THKRMAL CALCULATIONS. 

Armature : 

C 2 R loss at 60 deg. Cent. ... 25,850 watts 

Core loss (estimated value) ... ... ... ... ... 23,400 ,, 

Total armature loss ... ... ... ... ... ... 49,250 

Peripheral radiating surface armature ... ... ... ... 19, 100 square inches 

Watts per square inch radiating surface armature ... ... 2.6 watts 

Peripheral speed armature, feet per minute ... ... ... 2480 

Rise in temperature at 15 deg. Cent., rise per watt per square 

inch ... ... ... ... ... ... 39 deg. Cent. 

Spool : 

Total C 2 R loss at 60 deg. Cent., per spool ... ... 750 watts 

Peripheral radiating surface one spool ... ... ... 2080 square inches 

Watts per square inch of radiating surface, warm ... ... .41 watts 

At 80 deg. Cent, rise per watt per square inch, rise in 

temperature of field spool is ... ... ... ... 33 deg. Cent. 

Commutator : 

Area bearing surface all positive brushes ... ... ... 67.5 square inches 

Amperes per square inch of brush bearing surface ... ... 37 amperes 

Ohms per square inch bearing surface of carbon brushes ... .03 ohm 

Brush resistance, positive + negative ... ... ... ... .00089 ohm 

Volts drop at brush contacts ... ... ... ... ... 2.22 volts 

C 2 R at brush contacts ... 5550 watts 

Brush pressure ... ... ... ... ... ... 1.25 Ib. 



190 Electric Generators. 

Coefficient of friction ... ... ... ... ... ... .3 

Peripheral speed of commutator in feet per minute ... ... 1700 

Brush friction 1040 watts 

Stray power lost in commutator ... ... ... ... 750 ,, 

Total commutator loss ... ... ... ... ... ... 7340 

Radiating surface commutator ... ... ... ... ... 5400 square inches 

Watts per square inch of radiating surface ... ... ... 1.36 watts 

Rise in temperature at 20 deg. Cent, rise per watt per square 

inch ... ... ... ... ... ... ... ... 27 deg. Cent. 

EFFICIENCY CALCULATIONS. 

Watts. 

Output at full load ... 1 , 500,000 

Core loss (estimated) ... 23,400 

C- R armature at 60 deg. Cent. ... ... ... ... 25,850 

Commutator and brush loss ... ... ... ... ... 5,550 

Shunt spools C 2 R at 60 deg. Cent. ... ... ... 5,650 

,, rheostat ,, ,, ... ... ... ... 1,130 

Series spools - C 2 R at 60 deg. Cent. 3,380 

rheostat 1,190 



Total input 1,566,150 

Commercial efficiency at full load and 60 deg. Cent. = 95.7 per cent. 

WEIGHTS (POUNDS). 
Armature : 

Magnetic core 

Teeth 

Copper 

Commutator, segments ... 

Twelve magnet cores and pole-pieces 

Yoke ... ... 

Twelve shunt coils 

,, series coils 

Total spool copper 




6-PoLE 200-KlLOWATT RAILWAY GENERATOR. 

Figs. 169 to 183 relate to a six pole railway generator for an output 
of 200 kilowatts (500 volts and 400 amperes) at a speed of 135 revolutions 
per minute. The constants of this machine are set forth in the following 
specification, which also exhibits the steps in the calculation : 

Number of poles ... ... ... ... ... ... ... 6 

Kilowatts 200 

Revolutions per minute ... ... ... ... ... ... 135 

Frequency in cycles per second ... ... ... ... ... 6.75 

Terminal volts 500 

Amperes ... ... ... ... ... ... ... ... 400 



Two-Hundred Kilowatt Railway Generator. 



191 




192 Electric Generators. 

DIMENSIONS. 
Armature : 

Diameter over all ... ... ... ... ... .. 59^ in. 

Length over conductors ... ... ... ... ... 36|- ,, 

Diameter at bottom of slots ... ... ... ... ... 56 

Internal diameter of core ... ... ... ... ... 38| ,, 

Length of core over all ... ... ... ... ... 14^,, 

Effective length, magnetic iron ... ... ... ... ... 9.9 ,, 

Pitch at surface ... ... ... ... ... ... ... 31.1 ,, 

Insulation between sheets ... ... ... ... ... 10 per cent. 

Thickness of sheets .025 in. 

Depth of slot ... ... ... ... ... ... If 5, 

Width of slot at root ... -416,, 

,, at surface ... ... ... ... ... .416 

Number of slots ... ... ... ... ... ... 220 

Minimum width of tooth... ... ... ... ... ... .384 in. 

Width of tooth at armature face ... ... ... ... .429 ,, 

,, conductor ... ... ... ... ... ... .057 

Depth of conductor ... ... ... ... ... ... .658 ,, 

Number of ventilating ducts ... ... ... ... ... 5 

Width of each ventilating duct ... ... ... ... ... T 7 T in. and f in. 

Efficient length of core -=- total length... ... ... ... .70 

Magnet Core : 

Length of pole face ... ... ... ... ... ... 13. in. 

Length of pole arc ... ... ... ... ... ... 23.1 ,, 

Pole arc 4- pitch ... ... ... ... ... ... ... .74 

Thickness of pole-piece at edge of core ... ... ... ... 1 T 9 ^- in. 

Radial length magnet core ... ... ... ... ... 15-J- 

Diameter of magnet core ... ... ... ... ... 14^,, 

Bore of field (diameter) ... ... ... ... ... ... 59.9 ,, 

Depth of air gap ... ... ... ... ... ... ... .33 ,, 

Spool : 

Length over flanges ... ... ... ... ... ... 15| ,, 

Length of winding space... ... ... ... ... ... 14^,, 

Depth of winding space ... ... ... ... ... ... 2J ,, 

Yoke : 

Outside diameter ... ... ... ... ... ... ...112}, in. and 106.\ in. 

Inside diameter ... ... ... ... ... ... ... 96-i in. 

Thickness ... ... ... ... ... ... 8 in. and 5 in. 

Length along armature ... ... ... ... ... 17 J in. 

Commutator : 

Diameter ... ... ... ... ... ... ... ... 39 ,, 

Number of segments ... ... ... ... ... ... 440 

,, segments per slot ... ... ... ... ... 2 

Width of segment at commutator face ... .. ... ... .240 in. 

segment at root .210 ,, 



Two-Hundred Kilowatt Railway Generator. 



193 

















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194 



Electric Generators. 



DIMENSIONS continued. 



Thickness of mica insulation 
Available length of surface of segment. 
Cross-section commutator leads... 



.04 in. 



.01 square inch 



Brushes : 

Number of sets ... 

In one set 

Length (radial) ... 

Width ... ... - 

Thickness ... .., ... ... ... ... ... ... J ,, 

Area of contact (one brush) ... ... ... ... ... 1.00 square inch 

Type of brush ... ... ... ... ... ... ... Radial carbon 



6 

3 

2 in. 

o 

* j> 




MATERIALS. 



Armature core 

spider ... 

Conductors 

Commutator segments 
leads 

spider 

Pole-piece ... 

Yoke 

Magnet core 

Brushes 



Sheet Steel 

Cast iron 

Copper 

)i 

Rheotan 
Cast-iron 
Cast steel 



Carbon 






aJ: ct.J. 



PUT 



=11 




196 



Electric Generators. 



TECHNICAL DATA. 



Armature, no load voltage 

Number face conductors... 

Conductors per slot 

Number circuits ... 

Style winding 

Gramme ring or drum 

Type construction of winding 

Mean length, one armature turn 

Total armature turns 

Turns in series between brushes 

Length between brushes ... 



500 

1760 

8 

G 

Single 
Drum 

Barrel-wound 

107 in. 

880 

147 

15,700 in. 



Cross-section, one armature conductor ... ... ... ... .0375 square inches 



Kg 188. 




Ohms per cubic inch at 20 deg. Cent. ... 
Resistance between brushes at 20 deg. Cent. . . . 

,, ,, uU ,, ,, 

Volts drop in armature at 60 deg. Cent. 

,, in brushes and contacts 
Total internal voltage, full load 
Amperes per square inch in armature winding... 

,, ,, commutator connection 

Commutation : 

Average voltage between commutator segments 

Armature turns per pole 

Amperes per turn... 

Armature ampere turns per pole 

Segments lead of brushes 

Percentage lead of brushes 

,, demagnetising ampere turns 

,, distorting ampere turns 

Demagnetising ampere turns per pole ... 
Distorting ampere turns per pole 



.00000068 
.048 ohms 
.055 
22 volts 

3 

525 

1780 

6670 



6.8 

147 

66.7 

9800 

7 

9.6 
19.2 
80.8 
1880 
7920 



Two-Hundred Kilowatt Railway Generator. 197 

Frequency of commutation (cycles per second)... ... ... 275. 

Number of coils simultaneously short-circuited per brush ... 3 

Turns per coil 

Number of conductors per group simultaneously undergoing 

commutation... ... ... ... ... ... ... 12 

Flux per ampere turn per inch length armature lamination ... 20 (assumed) 
,, linked with 12 turns with one ampere in those turns = 

14.25 x 20 x 12 3420 lines 

Inductance of two turns constituting one coil = 2 x 3420 x 

10- 8 .000068 henrys 

Reactance short-circuited coil ... ... ... ... .118 ohms 

,, voltage short-circuited coil ... ... ... 7.85 volts 

The amount and distribution of the magnetomotive force may be 
roughly estimated as follows : 

Megalines entering armature per pole-piece, no load ... ... 12.6 

full load 13.3 

Coefficient of magnetic leakage ... ... ... ... ... 1.15 

Megalines in magnet frame, per pole-piece, no load ... ... 14.5 

,, ,, full load ... ... 15.3 

Armature : 

Section ... ... ... ... ... ... ... ... 174 square inches 

Length, magnetic... ... 15 in. 

Density, no load ... ... 72 kilolines 

full load... ... 76 

Ampere turns per inch length, no load ... ... ... 22 

full load... 26 

,, no load ... ... ... ... ... 330 

full load ... 390 
Teeth : 

Transmitting flux from one pole piece ... ... ... ... 29 

Section at roots ... 110 square inches 

Length ... 1.6 in. 

Apparent density, no load ... ... ... ... ... 115 kilolines 

full load ... ... 121 

Corrected density, no load .. ... ... ... ... 113 ,, 

full load 118 

Ampere turns per inch length, no load ... ... ... ... 350 

full load 500 

,, no load 560 

,, full load 800 

Gap : 

Section at pole face ... 300 square inches 

Length gap .33 in. 

Density at pole face, no load ... ... ... ... 42 kilolines 

,, full load 45 ,, 

Ampere turns, no load ... ... ... ... ... ... 4500 

full load . 4800 



198 Electric Generators. 

Magnet Core : 

Section ... ... ... ... ... ... ... ... 159 square inches 

Length (magnetic) ... ... ... ... ... ... 16.4 in. 

Density, no load ... ... ... ... ... ... ... 91 kilolines 

full load 96 

Ampere turns per inch length, no load... ... ... ... 80 

,, full load... 100 

no load 1320 

full load .. ... 1640 

Magnet Yoke : 

Section ... ... ... ... ... ... ... 220 square inches 

Length per pole ... ... ... ... ... ... ... 27 

Density, no load ... ... ... ... ... ... ... 66 kilolines 

,, full load ... ... ... ... ... ... 70 ,, 

Ampere turns per inch length, no load... ... ... ... 34 

full load 40 

,, no load 920 

full load 1080 

AMPERE TURNS PER SPOOL. 

No Load and 525 Internal 
No Load and Volts, Corresponding 

500 Volts. to a Full Load Terminal 

Voltage of 500. 

Armature core ... ... ... 330 390 

teeth 560 800 

Gap 4500 4800 

Magnet core 1320 1640 

yoke ... ... 920 1080 



7630 8710 

Demagnetising ampere turns per pole, at full load ... 1880 

Allowance for increase in density through distortion ... 400 



Total ampere turns at full load and 500 terminal volts 10,990 

CALCULATION OF SPOOL WINDINGS. 
Shunt : 

Mean length one shunt turn = 50 in. = 4.16 ft. 
Ampere turns per spool = 7630. 

feet = 7630 x 4.25 = 31,800. 

Radiating surface one field spool = 870 square inches. 
Permit .35 watts per square inch at 20 deg. Cent. 
.. .35 x 870 = 305 watts per spool. 

Shunt watts per spool - x 305 = 212 watts. 

1 \/j t/ t)\s 



copper per spool = 

pere feet\ 2 

ug 



/ampere feet\ 2 

V 1000 ) 31 x 1010 



watts 212 



Two-Hundred Kilowatt Railway Generator. 199 

Of the 500 volts available for excitation, should plan to make use 
of 90 per cent., or 450 volts at 60 deg. Cent., or 390 volts at 20 de\ 

Cent. This is - =65 volts per spool at 20 deg. Cent. Hence 

212 + 65 = 3.25 amperes 

^* C* Q ( \ 

Consequently turns per shunt spool = (- = 2350 turns 

o.Zo 

Length of 2350 turns = 2350 x 4.16 = 9800ft. 

Pounds per 1000 ft. = 15.2. No. 13 B. and S. has 15.7 Ib. per 1000 ft., and has a 

diameter of .072 in. bare, and .082 in. double cotton covered. 
This should be wound in 14 layers of 168 turns each. Cross-section No. 13 = 

.00407 square inch. 
Hence current density in shunt winding = 800 amperes per square inch. 



Series Winding. This must supply 10,990 7630 = 3360 ampere 
turns at full load of 400 amperes, of which 70 amperes should be carried 
through a diverting shunt, leaving 330 amperes for the series coils. 
Hence there must be 10 turns per spool. 

Mean length series turn 53 in. 

Total length ten turns = 530 in. 

Series C 2 R. per spool = 93 watts per spool. 

Hence resistance per spool = 93-7- 330 2 = .00085 ohms. 

Copper cross-section = .425 square inch. 

Series winding per spool may consist of two coils of flat strip copper 7 in. wide and 

.06 in. thick, wound five turns per coil. Weight series copper one spool = 

70 Ib. 
Current density in series winding = 770 amperes per square inch. 



THERMAL CALCULATIONS. 
Armature : 

C 2 R loss at 60 deg. Cent. 8800 watts. 

Core loss (observed value) 2760 watts. 

Total armature loss 11,560 watts. 

Observed increased temperature by increased resistance of armature winding 

63 deg. Cent. 

Peripheral radiating surface armature = 6800 square inches. 
Watts per square inch armature radiating surface = 1.70. 
Increased temperature per watt per square inch armature radiating surface 

37 deg. Cent., as determined from resistance measurements. 
Peripheral speed armature (feet per minute) = 2030. 
Increased temperature of armature by thermometer = 30 deg. Cent. 
Ditto, per square inch peripheral radiating surface = 17.7 deg. Cent. 



200 Electric Generators. 

Spool : 

Total C 2 R loss at 60 deg. Cent., per spool, = 353 watts. 

Observed increased temperature by increased resistance of winding = 45 deg. Cent. 

Peripheral radiating surface, one spool 870 square inches. 

Watts per square inch spool radiating surface = .405. 

Increased temperature per watt per square inch spool radiating surface = 111 deg. 
Cent., as determined from resistance measurements. 

By thermometer the observed increase in temperature of spool was only 1 6 deg. Cent. 
Commutator : 

Area of all positive brushes ... ... ... ... ... 9.0 square inch. 

Amperes per square inch, brush-bearing surface ... ... 44.5 

Ohms per square inch bearing surface, carbon brushes . . . .03 

Brush resistance, positive + negative ... ... ... ... .0067 ohms 

Volts drop at brush contacts ... ... ... ... ... 2.7 

C 2 R as brush contacts (watts) ... ... ... ... ... 1070 

Brush pressure, pounds per square inch ... ... ... 1.25 

Total brush pressure, pounds ... ... ... ... ... 22.5 

Coefficient of friction ... ... ... ... ... ... .3 

Peripheral speed commutator, feet per minute... ... ... 1330 

Brush friction, watts ... ... ... ... ... ... 270 

Stray power lost in commutator, watts ... ... ... ... 200 

Total commutator loss, watts ... ... ... ... ... 1540 

Radiating surface, square inches ... ... . . ... 800 

Watts per square inch radiating surface ... ... 1.92 

Observed rise temperature ... ... ... ... ... 36 deg. Cent. 

Increased temperature per watt per square inch radiating 

surface ... ... ... . . ... ... ... 19 deg. Cent. 

With further reference to the temperature measurements, the machine 
on which the increase of temperature was observed, had been run at full 
load for nine hours, and had probably about reached its maximum 
temperature. The spool windings were equivalent to, but not identical 
with, those described in this specification. In all other respects, the 
construction was substantially that described. 

EFFICIENCY CALCULATIONS. 

Watts. 

Output at full load ... ... 200,000 

Core loss 2,760 

Commutator and brush loss ... ... ... ... ... ... 1,540 

Armature C 2 R loss at 60 deg. Cent. ... ... ... ... 8,800 

Shunt spools - C 2 R loss at 60 deg. Cent 1,470 

rheostat - C 2 R loss at 60 deg. Cent 180 

Series spools - O 2 R loss at 60 deg. Cent. ... ... ... 640 

rheostat (diverter) C 2 R loss at 60 deg. Cent 130 

Total output 215,520 



Two-Hundred Kilowatt Railway Generator. 201 

WEIGHTS (Pouxos). 
Armature : 

Core magnetic ... ... ... ... ... ... ... 3, GOO 

Teeth... 400 

Spider 1,000 

Copper ... ... ... ... ... ... ... ... 1,150 

Commutator : 

Segments ... ... ... ... ... ... ... ... 450 

Complete without shaft 12,000 

Frame : 

Six pole-pieces ... ... ... ... ... ... ... 750 

Six magnet cores ... ... ... ... ... ... ... 4,100 

Yoke ... ... 11,000 

Field Windings : 

Six shunt coils ... ... ... ... .. ... ... 890 

Six series coils ... ... ... ... ... ... ... 420 

Total spool copper ... ... ... ... ... ... ... 1,310 

Other parts 3,800 

Machine complete with base plate ... ... ... ... ... 33,000 

The results of tests of this machine are given in the curves of Figs. 
184 to 188, relating respectively to saturation, compounding, core loss, 
efficiency, and gap distribution. 



10-POLE 300-KlLOWATT LIGHTING GENERATOR, 

A ten-pole lighting generator, designed by Mr. A. H. Moore, and built 
in 1897 by the Union Elektricitats-Gesellschaft, of Berlin, is illustrated in 
Figs. 189 to 206. Its rated output is 300 kilowatts at 125 volts and 
2,400 amperes, and at a speed of 100 revolutions per minute. In 
Figs. 190 to 193 are given curves of this machine derived from the 
results of tests and covering the subjects of saturation, core loss, com 
pounding, and efficiency. The most interesting feature of this design is 
that carbon brushes are used, notwithstanding the low tension and heavy 
current. 

In this instance the commutator is crowded considerably, and, as will 
be seen in the following specification, the temperature rise at the 
commutator was largely in excess of that at other parts of the machine. 
Mr. Moore has modified the design in this respect by lengthening the 
commutator segments about 25 per cent. 

2 D 



202 



Electric Generators. 



Fy.184. 



5SO 

500 
4SO 
400 
350 
300 



160 
100 



Saturation Curve. 



Rg.1S5. 



n,ooo 

10.000 
9000 

e.000 
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Compounding Curve For SOO VolU 



Rd188 SIX POLE - 2 K.W. SOO VOLT, 
" GENERATOR FOR I35 R.RM 


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SIX POLE, ZOO K.W. SOO VOLT 
GENERATOR FOR 135 R.PM. 



3.200 


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SIX POLt,200K.W. SOO VOLT 
OENERATOR FORI35RRM. 

Efficiency and Losses 
at 500 Volts. 




cluttmy ?l>c<.v ;**.; 



Three- Hundred Kilowatt Lighting Generator. 203 

The calculations are arranged below in the form of a specification : 

Number of poles ... ... ... ... ... ... ... 10 

Kilowatts 300 

Revolutions per minute ... ... ... ... ... ... 100 

Frequency in cycles per second ... ... ... ... ... 8.33 

Terminal volts, no load ... ... ... ... ... ... 110 

full load 125 

Amperes, full load ... ... ... ... ... ... 2400 



DIMENSIONS. 
Armature : 

Diameter over all ... ... ... ... ... ... 65i in. 

Length over conductors ... ... ... ... ... ... 33| ,, 

Diameter at bottom of slots ... ... ... ... ... 61f ,, 

Internal diameter of core ... ... ... ... ... 50| ,, 

Length of core over all ... ... ... ... ... ... 17| ,, 

Effective length, magnetic iron ... ... ... ... ... 12.7 ,, 

Pitch at surface ... ... ... ... ... ... ... 20.5 

Per cent, insulation between sheets ... ... ... ... 10 

Thickness of sheets . . ... ... ... ... ... .025 in. 

Depth of slot ... ... ... ... ... ... ... If ,, 

Width of slot at root ... ... ... ... ... .59 ,, 

,, ,, surface ... ... ... ... ... ... .59 ,, 

Number of slots ... ... ... ... ... ... ... 180 

Minimum width of tooth ... ... ... ... ... .478 in. 

Width of tooth at armature face ... ... ... ... .539 ,, 

,, conductor ... ... ... ... ... ... .197 ,, 

Depth of conductor ... ... ... ... ... ... .650 ,, 

Number of ventilating ducts ... ... ... ... ... 7 

Width of each ventilating duct ... ... 4 in. 

O A 

Effective length of core -h total length ... ... ... .72 

Magnet Core : 

Length of pole-face ... ... ... ... ... ... 1 6 in. 

Length of pole arc (average) ... ... ... ... ... 13.3 ,, 

Pole arc H- pitch ... ... ... ... ... .65 

Thickness of pole-piece at edge of core ... ... ... 1J in. 

Radial length of magnet core ... ... ... 12i 

Diameter of magnet core... ... ... ... 13 ,, 

Bore of field (diameter) 65j| 

Depth of air gap ... ... ... ... -3 ,, 

Spool : 

Length over flanges ... ... ... ... 12f in. 

Length of winding space ... ... ... Hf 

Depth of winding space... ... ... ... %$ 



Tl tree- Hundred Kilowatt Lighting Generator. 205 

Yoke : 

Outside diameter ... Ill in. and 105 in. 

Inside diameter ... ... ... ... ... 97 in. 

Thickness... ... ... ... ... ... ... ... 7 in. and 4 in. 

Length along armature ... ... ... ... ... ... 1 6 in. 

Commutator : 

Diameter ... ... ... ... ... ... ... 52 ,, 

Number of segments ... ... ... ... ... ... 360 

,, per slot 2 

"Width of segment at commutator face ... ... ... ... .425 in. 

root .372 

Thickness of mica insulation ... ... ... ... ... .03 ,, 

Total depth of segment ... ... ... ... ... ... 3.0 ,, 

Approximate useful depth of segment ... ... ... 1.5 ,, 

Maximum length of segment ... ... ... ... ... 12| ,, 

Available length surface of segment ... ... 11| ,, 

Cross-section commutator leads... ... ... ... ... .059 square inch 

Brushes : 

Number of sets ... ... ... ... ... ... ... 10 

Number in one set ... ... ... ... ... ... 8 

Width 1.25 in. 

Thickness ... ... ... ... ... ... ... 1 ,, 

Area of contact of one brush ... ... ... ... ... 1.25 square inches 

Type of brush ... ... ... ... ... ... ... Radial carbon 

MATERIALS. 

Armature core ... ... ... ... ... ... ... Sheet steel 

,, spider... ... ... Cast iron 

conductors ... ... ... ... ... ... Copper 

Commutator segments ... ... ... .. ... ,, 

,, leads ... ... ... ... ... ... Rheotan 

,, spider ... ... ... ... ... ... Cast iron 

Pole-pieces ... ... ... ... ... ... Cast steel 

Yoke 

Magnet cores ... ... ... ... ... ... ... ,, 

Brushes ... ... ... ... ... ... ... ... Carbon 

TECHNICAL DATA. 

Armature, no load voltage ... ... ... ... ... 110 

Number of face conductors ... ... ... ... ... 720 

Conductors per slot ... ... ... ... ... ... 4 

Number of circuits ... ... ... ... ... ... 10 

Style of winding ... ... ... ... ... ... ... Single 

Gramme ring or drum ... ... ... ... ... ... Drum 

Type construction of winding ... ... ... ... ... Barrel-wound 

Mean length one armature turn... ... ... ... ... 88.5 in. 

Total armature turns 360 



206 



Electric Generators. 



Fig. 191. 




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Fig. 184. 




Three-Hundred Kilowatt Lighting Generator. 




207 



Turns in series between brushes... ... ... ... ... 36 

Length between brushes ... ... ... ... ... 3190 in. 

Cross-section one armature conductor ... ... .128 square inch 

Ohms per cubic inch at 20 deg. Cent. ... ... ... .00000068 ohms 

Resistance between brushes at 20 deg. Cent ... .00171 

60 deg. Cent. ... .00198 

Volts drop in armature at 60 deg. Cent. ... ... ... 4.75 

,, ,, brushes and contacts and series winding ... 3.25 

Terminal voltage, full load ... ... ... ... ... 125 

Total internal voltage, full load ... ... ... ... 133 

Amperes per square inch in armature winding... ... ... 1880 

,, ,, commutator connections ... ... 4000 

Commutation : 

Average voltage between commutator segments ... 3.5 

Armature turns per polo... ... ... ... ... ... 36 

Amperes per turn ... ... ... ... ... ... 240 

Armature ampere turns per pole-piece ... ... ... ... 8650 

Segments lead of brushes ... ... ... ... ... 3 

Percentage lead of brushes ... ... ... ... ... 8.3 

,, demagnetising ampere turns ... ... ... 16.6 

,, distorting ampere turns ... ... ... ... 84.4 

Demagnetising ampere turns per pole ... ... ... ... 1450 

Distorting 7200 

Frequency of commutation (cycles per second)... ... ... 138 

Number of coils simultaneously short-circuited per brush ... 3 

Turns per coil ... ... ... ... ... ... ... 1 

Number of conductors per group simultaneously undergoing 

commutation ... ... ... ... ... ... ... 6 

Flux per ampere turn per inch length armature lamination ... 20 

Flux linked with six turns with 240 amperes in those turns = 

17.6 x 20 x 6 2110 lines 

Inductance in one turn constituting one coil, in henrys = 1 x 

2110 x 10- 8 ... .0000211 henrys 

Reactance short-circuited turn ... ... ... ... .0183 ohms 

voltage = .0183 x 240 4.4 volts 



MAGNETOMOTIVE FORCE CALCULATIONS. 

Megalines entering armature, per pole-piece, at no load 
,, ,, at full load 

Coefficient of magnetic leakage ... 
Megalines in magnet frame, per pole-piece, at no load 

full load 



9.17 
11.1 
1.15 
1.05 
1.28 



Armature : 

Section ... ... ... ... ... ... ... ... 143 square inches 

Length (magnetic) ... ... ... ... ... ... 10 in. 




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Three- Hundred Kilowatt Lighting Generator. 209 

Density at 110 load ... ... 63.5 kilols. 

full load ... 77.5 

Ampere turns per inch length, no load ... ... 14 

,, full load 23 

Ampere turns, no load ... ... ... ... ... 140 

full load ... 230 

Teeth : 

Transmitting flux from one pole-piece ... ... ... 14 

Section at roots ... ... ... ... ... ... ... 8.5 square inches 

Length ... ... ... ... ... ... ... ... 1.75 in. 

Apparent density at no load ... ... ... ... ... 108 kilols. 

full load 130 

Corrected density at no load ... ... ... ... ... 106 ,, 

full load ... ... 125 

Ampere turns per inch length, no load ... ... ... ... 100 

full load ... 750 

Ampere turns, no load ... ... ... ... 180 

full load ... 1310 

Gap: 

Section at pole-face ... 213 square inches 

Length ... ... ... ... ... ... ... ... .3 in. 

Density at pole-face, no load ... ... ... ... ... 42,800 

full load ... 52,000 

Ampere turns, no loud ... ... ... ... ... ... 4,050 

Ampere turns, full load ... ... ... ... ... ... 4,900 

Magnet Core : 

Section ... ... ... ... ... ... ... ... 132 square inches 

Length (magnetic) ... ... ... ... ... ... 13.5 in. 

Density, no load ... ... ... ... ... ... ... 79.0 kilols. 

full load... 96.5 

Ampere turns per inch length, no load ... ... 48 

full load 93 

no load ... ... ... ... ... ... 650 

full load ... 1250 

Magnet Yoke: 

Section ... ... ... ... ... ... ... ... 156 square inches 

Length per pole ... ... ... ... ... ... ... 15 in. 

Density no load ... 67.0 kilols. 

full load... ... 82.0 

Ampere turns per inch length, no load ... 

full load 58 

no load 480 

full load ... 870 

2 E 



210 Electric Generator*. 

AMPERE TURNS PER SPOOL. 



Armature cor6 ... 


No Load 
and 
110 Volts. 

140 


No Load and 133 
Internal Volts, corres 
ponding to 125 
Terminal volts at 
Full Load. 

230 


Armature teeth 


180 


1310 


Grap 


4050 


4900 


JMa^net COre 


650 


1250 


voke 


480 


870 









5500 

Demagnetising ampere turns per pole-piece, at full load 
Allowance for increase in density through distortion . . . 



Total ampere turns at full load and 125 terminal volts = 10,560 

If the rheostat in the shunt circuit is adjusted to give 5,500 ampere 
turns at 110 volts, then, when the terminal voltage is 125, the shunt 

125 
excitation will amount to - - x 5,500 = 6,250 ampere turns. 10, 5GO 0,250 

= 4,310 ampere turns must be supplied by the series winding. 

CALCULATION OF SPOOL WINDINGS. 
Shunt : 

Mean length of one shunt turn = 51 in. = 4.25 ft. 
Ampere turns per shunt spool at full load = 6250. 

feet = 26,600. 

Radiating surface one field spool = 730 square inches. 
Permit .36 watts per square inch at 20 deg. Cent. 
.-. 263 total watts per spool. This is divided up into 84 watts in series winding and 

177 iu shunt. 

Shunt watts per spool at 60 deg. Cent. = 204. 
Q-, /Ampere feet\ 2 

31 x ( Tooo ) 

Pounds 

watts 

.-. Shunt copper per spool = ^^f 125 Ib. 

Plan to have 90 per cent, of the available 125 volts, or 113 volts, at the terminals 
of the field spools when hot, the remainder being consumed in field rheostat. 
This is 98 volts at 20 deg. Cent, or 9.8 volts per spool. 

177 

Hence require - = 18.1 amperes per spool, 
y * o 

Turns per shunt spool = - = 345. 

18.1 

Length of 345 turns = 1470 ft. 
Pounds per 1000 ft. = 85. 



Three-Hundred Kilowatt Lighting Generator 



211 



Fig. 201. 




212 Electric Generators. 

No. 8 B.W.G. has 82.4 Ib. per 1000 ft. 

Bare diameter = .165 in. D.C.C.D. = .177 in. 

Cross-section .0214 square inches. Current density = 845 amperes per square 

inch. 
Length of the portion of winding space available for shunt winding = 6f in. 

Winding consists of 10 layers of 35 turns each, of No. 8 B.W.G. 

Series Winding. The series winding is required to supply 10,560 
G250 = 4,310 ampere turns at full load. 

With two turns per spool, the full load current will give 2400 x 2 = 
4800 ampere turns. Consequently, 250 amperes must be diverted through 
the diverter rheostat, leaving 2,150 amperes in the series winding, giving 
4,300 ampere turns. 

The two turns consist of flat strips wound on edge spirally, as shown 
in Figs. 196 and 197. The conductor is made up of 44 strips 1.10 in. by 
.079 in., making up a total cross-section of 3.8 square inches : 

Current density = 630 amperes per square inch. 

Mean length of turn = 51 in. 

Resistance of ten spools at 20 deg. Cent. = .000183 ohms. 

Series C 2 R = 2150- x .000183 = 840 watts. 

Ditto per spool = 84 watts. 

At 60 deg. Cent. = 97 watts. 

Weight series copper = 1250 Ib. 

THERMAL CALCULATIONS. 
Armature : 

C 2 R loss at 60 deg. Cent. ... ... 11,400 watts 

Core loss (observed value) ... ... ... ... ... 4.150 

Total armature loss ... ... ... ... 15550 

Observed increased temperature by increased resistance of 

armature winding ... ... ... ... ... ... 64 deg. Cent. 

Peripheral radiating surface armature ... ... ... ... 7,000 square inches 

Watts per square inch radiating surface armature ... ... 2.22 

Increased temperature per watt per square inch armature 

radiating surface ... ... ... ... ... ... 29 deg. 

Peripheral speed armature, feet per minute ... ... ... 1720 

Increased temperature of armature by thermometer ... ... 29 deg. Cent. 

Ditto, per square inch peripheral radiating surface ... ... 13 ,, 

Spool : 

Total C 2 R loss at 60 deg. Cent, per spool 301 watts 

Observed increased temperature by increased resistance of 

winding 64 deg. Cent. 

Peripheral radiating surface of one spool 730 square inches. 

Watts per square inch of spool radiating surface .41 



Tliree- Hundred Kilowatt Lighting Generator. 



213 
























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214 Electric Generators. 

Increased temperature per watt per square inch of spool 

radiating surface ... ... ... ... ... ... 156 deg. Cent. 

By thermometer the increase in temperature of spool was ... 46 ,, 

Ditto, per square inch radiating surface ... ... ... 112 ,, 

Commutator : 

Area of all positive brushes (bearing surface) ... ... ... 50 square inches 

Amperes per square inch of brush bearing surface ... 48 amperes 

Ohms per square inch bearing surface of carbon brushes . . . .03 ohms 

Brush resistance, positive + negative ... ... .00120 ohms 

Volts drop at brush contacts ... ... ... ... ... 2.9 volts 

C 2 B at brush contacts ... ... ... ... ... ... 6900 watts 

Brush pressure, pounds per square inch ... ... ... 1.25 Ib. 

Ditto, total ... 125. 

Coefficient of friction ... ... ... ... ... ... .3 

Peripheral speed of commutator in feet per minute ... ... 1365 

Brush friction ... ... ... ... ... ... ... 1160 watts 

Allowance for stray power lost in commutator ... ... 500 ,, 

Total commutator loss ... ... ... ... ... ... 8560 ,, 

Radiating surface commutator ... ... ... ... ... 1920 square inches 

Watts per square inch of radiating surface ... ... ... 4.45 

Observed rise in temperature ... ... ... ... ... 80.5 deg. Cent. 

Increase in temperature per watt per square inch of radiating 

surface ... .. ... ... ... 18 deg. Cent. 

These temperature observations were made on the machine after it 
had been run on full load for eight hours. As readings were made only at 
the end of the test, it cannot be stated that the machine was not still 
increasing in temperature. 

EFFICIENCY CALCULATIONS. 

Watts. 

Output at full load 300,000 

Core loss ... ... ... ... ... ... ... ... 4,150 

Commutator and brush loss ... ... ... ... ... 8,560 

Armature C 2 R loss at 60 deg. Cent. ... ... 11,400 

Shunt spools -C 2 R loss at 60 deg. Cent 2,040 

,, rheostat ... ... ... ... ... ... ... 230 

Series spools ... ... ... ... ... ... ... 970 

,, rheostat (diverter) C 2 R loss at 60 deg. Cent. ... ... 100 



Total input ... 327,450 

Commercial efficiency at full load and 60 deg. Cent. = 91.6 per cent. 

WEIGHTS (POUNDS). 

A rmature : Ik 

Magnetic core ... ... ... ... ... ... ... 3, 500 

Teeth ... 560 

Spider and flanges ... ... ... ... ... ... 7,000 

Copper 1,310 




Six-Pole 2,5Q-Kilowatt Electric Generator. 215 

Comimitator : 

Segments ... 

Spider and press rings 

Complete armature and commutator without shaft 

Frame : 

Ten pole pieces ... ... ... ... .. ... ... 1,000 

,, magnet cores ... ... ... ... ... ... 5,000 

Yoke 8,500 

Ten-shunt coils ... ... ... ... ... ... ... 1,250 

Ten-series 1,250 

Total spool copper ... ... ... ... ... ... 2,500 

Other parts 3,000 

Machine complete ... ... ... ... ... ... 34,500 

In Figs. 207 and 208, page 213, are given the results of tests of 
saturation and core loss. 

Points A and B of Fig. 209 are experimental values. The curves of 
Fig. 209 show approximately the ampere turns that would be required for 
various outputs, if the terminal voltage increased in a straight line from 
110 volts at no load, up to 125 volts at full load. This would not auto 
matically increase in a straight line, but the deviation was not tested. 
Curves of losses and efficiencies are given in Fig. 210. 



SlX-PoLE 250-KlLOWATT ELECTRIC GENERATOR. 

The following is one of the latest designs: In Figs. 211 to 224 are 
given diagrammatical sketches, setting forth the electromagnetic dimen 
sions to which the ultimate designs should correspond. Figs. 225 to 233 
show some interesting details of construction of frame, spider, commutator, 
brush holders, bearing, &c., suggested among other alternative schemes, 
in the mechanical development of the generator. 

SPECIFICATION. 

Number of poles ... ... ... ... ... C 

Kilowatts 250 

Revolutions per minute ... ... ... ... ... 320 

Frequency in cycles per second ... ... ... ... ... 16 

Terminal volts, full load ... ... ... ... ... ... 550 

,, ,, no load ... ... ... ... ... ... 500 

Amperes ... ... ... . ... ... ... 455 



216 



Electric Generators. 





Six-Pole 250 } - Kilowatt Electric Generator. 



217 



Armature : 

Diameter over all... 
Length over conductors . . . 
Diameter at bottom of slots 
Internal diameter of core 



Kg.%18 



DIMENSIONS. 



46 in. 

32.3 

43.4 
30 



Fig. 2,14. 




Fig. ZK. 



, 11 torn* of -012 StexL Wire ,14- turos of -058 Phosphor Bronze TPEne 





Length of core over all ... 
Effective length, magnetic iron 
Pitch at surface ... 
Insulation between sheets 
Thickness of sheets 



12.3 in. 
9.9 

24 

w * 

10 per cent. 
.014 in. 

2 F 



218 



Electric Generators. 



Depth of slot 
Width of slot at root 

,, ,, surface . 

Number of slots .., 
Minimum width of tooth 



1.28 in. 

.582 

.582 

150 

327 
** j) 



ttg.%18 




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III 


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Width of tooth at armature face 
Width of conductor 
Depth of conductor 
Number of ventilating ducts 
Width of each ventilating duct . . 



Efficient length of core -=- total length . . . 



.379 in. 
.10 
.45 

3 

.44 in. 
.80 



Six-Pole 250-Kiloivatt Electric Generator. 219 

Magnet core, length of pole face ... ... ... ... 12.3 

Length of pole arc ... ... ... ... ... 17 in. 

Pole arc -f- pitch ... ... ... ... ... ... ... .70 

Thickness of pole-piece at edge of core ... ... ... ... .50 

Radial length, magnet core ... ... ... ... ... 10.5 

Diameter of magnet core ... ... ... ... ... 12.3 

Bore of field (diameter) ... ... ... ... ... ... 46f in. 

Depth of air gap ... ... ... ... ... ... ... yV > 

Spool : 

Length over flanges ... ... ... ... ... ... 10.5 in. 

,, of winding space... ... ... ... ... ... 9.3 ,, 

Depth ... 2.75,, 

Yoke : 

Outside diameter ... 81.1 in. 

Inside diameter ... ... ... ... ... 72.1 ,, 

Thickness ... ... ... 4.5 ,, 

Length along armature ...... 15 ,, 

Commutator : 

Diameter ... ... ... ... ... ... ... ... 37.4 in. 

Number of segmenta ... ... ... ... ... ... GOO 

,, ,, per slot ... ... ... ... ... 4 

Width of segment at commutator face ... ... ... .167 in. 

Thickness of mica insulation ... ... ... ... .030 ,, 

Available length surface of segment ... ... ... 9.06 ,, 

Cross-section commutator leads ... ... ... ... ... .03 square inch 

Brushes : 

Number of sets ... ... ... . . 6 

Number in one set ... ... ... ... 4 

Width of brush ... 1.75 in. 

Thickness of brush .625 ,, 

Area of contact one brush . 1.09 square inches 

Type of brush . . . Carbon 



MATERIALS. 

Armature core . . . Sheet iron 

Spider ... ... ... Cast iron 

Conductors . . . Copper 

Commutator segments ...... ... ,, 

,, leads ... ... ... ... ... ... ,, 

,, spider Cast iron 

Pole-pieces ... ... ... ... ... ... ... Cast steel 

Yoke ... ... ... ... ... ... ... ,, 

Magnet cores ... ... ... ... ... ,, 

Brushes Carbon 



220 



Electric Generators. 



TECHNICAL DATA. 



Armature : 

No load voltage ... 
Number face conductors. 
Conductors per slot 



500 
1200 

8 





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T-4 4-4-- -M- 




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-UU-JH-L 



Number of circuits 
Style winding 
Gramme ring, or drum 



6 

Multiple 
Drum 



Six-Pole 250-fCilowatt Electric Generator. 



221 



Type of construction of winding Barrel-wound 

Mean length, one armature turn 84.5 in. 

Total armature turns . . . GOO 

Turns in series between brushes. . . 100 

Length between brushes ... ... ... ... 8450 in. 

Cross-section one armature conductor ... ... .045 square inch 

Ohms per cubic inch at 20 deg. Cent. ... .00000068 

Resistances between brushes at 20 deg. Cent. ... ... .0213 ohms 

60 .0245 

Volts drop in armature at 60 deg. Cent. 11.3 

,, brushes and contacts ... ... ... 2.1 

Total internal voltage, full load... ... ... ... 564 

Amperes per square inch in armature winding ... 1700 

,, ,, commutator connections ... 2500 

Commutation : 

Average voltage between commutator segments ... 5.5 

Armature turns per pole... ... ... 100 

Amperes per turn ......... 76 

Armature ampere turns per pole ... ... ... ... 7600 

Segments lead of brushes ... ... 8 

Percentage ,, ... ... 8 per cent. 

,, demagnetising ampere turn ... 16 

,, distorting ,, ,, ... ... 84 

Demagnetising ampere turns per pole ... ... ... ... 1220 

Distorting ., 6380 

Frequency of commutation, cycles per second ... ... ... 500 

Number of coils simultaneously short-circuited per brush ... 4 

Turns per coil ... ... ... ... ... ... 1 

Number of conductors per group simultaneously undergoing 

commutation... ... ... ... ... ... ... 8 

Flux per ampere turn per inch length armature lamination ... 20 

Flux linked with eight turns with one ampere in these turns 1970 lines 

Inductance of one turn in henrys = 1 x 1970 x 10" 8 ... ... .0000197 

Reactance short-circuited coil ... ... ... ... ... .062 ohms 

,. voltage short-circuited coil .. 4.7 volts 



MAGNETO-MOTIVE FORCE CALCULATIONS. 

Megaliiies entering armature, per pole piece, no load ... 7.80 

full load 8.80 

Coefficient of magnetic leakage ... ... ... ... 1.15 

Megalines in magnet frame, per pole piece, at no load 8.97 

full load 10.1 

Armature : 

Section ... ... ...132 square inch 

Length, magnetic ... ... ... ... 13.0 



222 Electric Generators. 

Density, no load ... ... ... ... 59 kilolines. 

,, full load ... ... ... ... ... 66 ,, 

Ampere turns per inch length, no load ... ... 11 

,, ,, ,, full load ... 13 

,, no load ... 140 

full load ... 179 

Teeth ; 

Transmitting tiux from one pole-piece ... ... 20 

Section at roots ... ... ... ... ... ... 65 

Length ... 1.28 

Apparent density, no load 132 kilolines 

full load 148 

Corrected ,, no load ... ... ... ... ... 124 ,, 

full load 134 

Ampere turns per inch length, no load . . . 700 

full load 1250 

no load 890 

full load ... 1600 

Cap : 

Section at pole-face ... ... 210 square inch 

Length gap ... ... ... ... ... .31 in. 

Density at pole-face, no load ... ... 37.2 kilolines 

full load ... 42 

Ampere turns, no load ... .. ... ... 3640 

full load ... 4150 

Magnet Core : 

Section ... ... ... ... ... ... ... ... 119 square inch. 

Length (magnetic) ... ... ... ... 12.75 in. 

Density, no load ... ... ... 76 kilolines 

full load... 85 

Ampere turns per inch length, no load... 35 

,, ,, ,, full load 46 

,, no load ... .. ... 450 

full load . 590 

Magnetic Yoke : 

Section ... ... ... ... ... ... ... 140 square inches 

Length per pole ... ... ... ... ... ... ... 1 8 in. 

Density, no load ... ... ... ... ... ... ... 64 kilolines 

full load ... ... ... 72 

Ampere turns per inch length, no load... ... ... ... 25 

full load 32 

., no load ... ... ... ... ... ... 450 

full load 570 



Six-Pole 250-KUowatt Electric Generator. 



223 




Fi0.Z. 




224 Electric Generator*. 

AMPKRE TURNS PER SPOOL. 



Armature core 


No] 
No Load and po 
500 Volts. at 

140 


^oad and 564 Volts, Corres 
nding to Internal Voltage 
Full Load, when Terminal 
Voltage is 550. 

170 
1600 
4150 
590 

570 


, teeth 


890 


Gap 


3640 


Magnet core 


450 


voke 


450 






5570 

Demagnetising ampere turns per pole, at full load 
Allowance for increase in density through distortion . . . 
Total ampere turns at full load and 550 terminal volts 


7080 

1220 

700 
8920 



If the rheostat in the shunt circuit is adjusted to give 5570 ampere 
turns at 500 volts, then when the terminal voltage is 550 the shunt 

550 
excitation will amount to - x 5570 = 6130 ampere turns. 

500 

8900 6130 = 2270 ampere turns, must be supplied by the series 
winding. 

CALCULATION OF SPOOL WINDING. 
Shunt : 

Mean length of one shunt turn ... ... ... ... = 48.5 in 4.05 ft 

Ampere turns per shunt spool at full load ... ... ... 6,130 

Ampere feet ..................... 24,800 

Total radiating surface of one field spool ... ... ... 530 square inches 

Proportion available for shunt = x 530 ... 365 

8900 

Permit .40 watts per square inch at ... ... ... ... 20 deg. Cent. 

. . 365 x .40 = 146 watts per shunt spool at ... ... ... 20 ,, 

And 168 watts per shunt spool at ... ... ... ... 60 ,, 



31 x 
Shunt copper per spool = 31 x 615 = 131 Ib. fLb. = 



. . 

1 46 watts J 

Plan to have 80 per cent, of the available 550 volts, i.e., 440 volts, 
at the terminals of the field spools when hot, the remainder being con 
sumed in the field rheostat. This is 382 volts at 20 deg. Cent., or 63.5 

volts per spool. Hence require - = 2.3 amperes per spool. 

DO. i) 



Six-Pole 2,50-Kilowatt Electric Generator. 225 

/ I O/"\ . 

Turns per shunt spool - ... ... 2600 

Length of 2660 turns 10,800ft. 

Pounds per 1000 ft 12.1 

No. 14 B. and S. has 12.4 Ib. per 1000 ft. 

Bare diameter ... ... ... ... ... ... ... .0641 in. 

D.C.C. diameter .075 

Cross-section ... ... ... ... ... ... ... .00323 square inch 

Amperes per square inch ... ... ... ... ... 710 

Length of the portion of winding space available for shunt winding, 6.5 in. 
Winding consists of 33 layers of 81 turns each, of No. 14 B. and S. 



SERIES WINDING. 

The series winding is required to supply 2770 ampere turns at full 
load of 455 amperes. 

Planning to divert 25 per cent, through a rheostat in parallel with 
the series winding, we find we have .75 x 455 = 342 amperes avail 
able for the series excitation ; hence each series coil should consist of 

2770 
= 8 turns. 

342 

Mean length of series turn ... ... ... ... ... 48.5 in. 

Total length of eight turns 388,, 

Radiating surface available for series spool ... ... ... 165 square inches 

Permit .40 watt per square inch in series winding at 20 deg. Cent. 
Watts lost per series spool at 20 deg. Cent. = .40 x 165 = 66. 

/ / 

Hence resistance per spool at 20 deg. Cent. = - = .00057 ohms. 

342- 

Copper cross-section = .46 square inch. 

Series winding per spool may consist of eight turns made up of four strips of sheet 

copper 2.3 in. x .050 in. 

Weight of series copper in one spool = 58 Ib. 
Current density series winding = 740. 

THERMAL CALCULATIONS. 
Armature : 

C 2 R loss at 60 deg. Cent. ... 5050 watts 

Core loss ... 4000 

Total armature loss ... ... ... ... ... ... 9050 ,, 

Peripheral radiating surface of armature ... ... ... 4700 square inches 

Watts per square inch radiating surface ... ... ... 1.93 

Peripheral speed armature feet per minute ... ... ... 3850 

Assumed increase of temperature per watt per square inch in 

radiating surface as measured by increased resistance 25 deg. Cent. 

Hence estimated total increase temperature of armature = 48 ,, 

2 G 



226 



Electric Generators. 



Commutator : 

Area of all positive brushes 

Amperes per square inch brush-bearing surface 

Ohms per square inch bearing surface carbon brushes 

Brush resistance, positive and negative 

Volts drop at brush contacts 

C 3 R at brush contacts 

Brush pressure, assumed 1.25 Ib. per square inch 



13.1 square inch 
35 amperes 

.03 ohm. 

.0046 

2.1 volts 

950 watts 

32.8 Ib. 




Coefficient friction ... ... ... ... ... ... .3 

Peripheral speed of commutator, feet per minute ... ... 3130 

Brush friction ... ... ... ... ... ... ... 700 watts 

Allowance for stray power lost in commutator ... ... 150 ,, 

Total commutator loss ... ... ... ... ... ... 1800 ,, 

Radiating surface in square inches ... ... ... ... 1100 

Watts per square inch radiating surface of commutator ... 1.64 
Increase of temperature per watt per square inch radiating 

surface ... ... ... ... ... ... ... 20 deg. Cent. 

Total estimated increase of temperature of commutator ... 33 ,, 



Six-Pole 2,50-Kilowatt Electric Generator. 



227 



EFFICIENCY CALCULATION. 



Output, full load ... 

Core loss ... 

Commutator and brush losses 

Armature C 2 R at 60 deg. Cent 

Shunt spools C 2 R at 60 deg. Cent. 

,, rheostat at 60 deg. Cent 

Series spools C-R at 60 deg. Cent. 

,, rheostat at 60 deg. Cent. ... 
Friction in boaiings, and windage 



Commercial eltieiency at full load and 60 cleg. Cent. ... 

1 If, 2, itl 8lx POLt250K.W. 550 VOLT GENERATOR 
^7 *" J ^- 320 R.P M. 



Watts. 

250,000 

4,000 

1,800 

5,050 

1,000 

250 

460 

150 

2,000 

264,710 
94.4 per cent. 



MO 

.1,TC 






j 


ATI 


RATION 


cut 


vt 
















/ 






























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M 








/ 














toe 

//.I 


























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too 


































































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c -iooo 4000 eooo sooo 

HIS AHPfne TURNS PCR SPOOL. 


M 



r !X POLEZSOK.W SSOVOLT GtNtKATun 
i*j.%3Q.K 320R.PM. 

EFFICIENCY A LOSSES. 



Fig.l 



SIX POLE 2SOKW. 550 VOLT GENERATOR 

320 R P.M. 
COMP JVNOING CURVES 



30CO 
8000 

fCCC 
SOOO 
WOO 

saoo 

2000 
1000 




3OO 4 CO 



WEIGHTS. 



Armature : 

Magnetic core 
Teeth 
Spider 
Shafting . . . 
End flanges 
Copper 



Lb. 

2,100 
210 
860 

1,700 
750 
730 



228 



Electric Generators. 



Commutator : 

Segments ... 

Spider 

Rings 

Other parts of armature and commutator 

Armature complete, including commutator and shaft... 

Field : 

Six pole-pieces and magnet core ... 

Magnet yoke 

Six shunt coils 

Six series coils 

Total spool copper 

Brush gear 

Bedplate and bearings ... 

Machine complete 



G80 
530 
260 
180 
8,000 

2,400 

5,000 
790 
350 

1,140 
300 

2,600 
20,000 



Fig. 235. 






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In Figs. 234, 234A, and 234B are given saturation, compounding, and 
efficiency curves in accordance with estimated values. This machine 
has recently been completed. Figs. 235 and 236 show the results of 
saturation and core loss tests. They agree very well with the pre 
determined values of the above specification. As shown in Fig. 235, the 
excitation required at no load and 500 volts was, by observation, 5400 
ampere turns, as against the predetermined value of 5570 ampere turns 
given in the calculation on page 224. 



CORE LOSSES IN MULTIPOLAR COMMUTATING MACHINES. 

In determining the core losses of electric generators, it is frequently 
convenient to resort to empirical devices, as a check upon more theoretical 
methods, owing to the conditions in practice affecting the results. As 



Core Losses in Multipolar Commutating Machines. 



229 



already explained in an earlier section of this series, the machine-work 
upon the armature, the periodic variations in the magnetic reluctance, with 
resulting eddy current and hysteric losses in the magnet frame, and the 
eddy currents in the armature conductors, supports, shields, &c., all tend to 
introduce uncertain factors. 



FIG. 237. 




FIG. 238. 



CURVE EXHIBITING 7K5 RF.!. . .TIC"; BETWEEN 

CYCLSS Pft SECOVO/C XILOLINCS DENSITY BELOW SLOTS+IOOO 



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WATTS Pea POUKO 



In the Table on page 230 are set forth the dimensions and the 
observed core losses of twenty-three large multipolar commutating 
machines, in the design of which there was a wide range of periodicies and 
magnetic densities. The results set forth in this Table are useful in 
drawing practical conclusions as to the probable core losses of new designs. 
Although in these designs the rate of dissipation of energy in the teeth is 
high, the small percentage which the mass in the teeth bears to the total 



Electric Generators. 



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Core Tkmex m Multipolar Commutating Machines. 231 

mass of the core of the armature, makes it practicable, as shown by the 
results given in the Table, to draw conclusions from a comparison of the 
watts per pound of total laminations as related to the periodicity and to the 
density below slots. But this would not be found to be the case, except 
when tooth densities are chosen, lying within the limits generally adopted, 
since the higher the density in the projections, the more considerable is the 
loss due to eddy currents in the embedded copper conductors, in con 
sequence of the stray field crossing them. Another factor affecting the 
value of the core loss in commutating dynamos, is the influence of the 
conditions during commutation of coils, in relation to which the frequency 
of commutation has an important bearing. 

The curve given in Fig. 238 is plotted from the tabulated results, and 
will be found useful for this type of machine. 

Suppose, for example, we wish to predetermine the core loss of a 
uiultipolar generator having, say, eight poles and running at 240 revolutions 
per minute. From previous calculations we find it requires 7000 Ib. weight 
of total laminations, including teeth and core body, allowing a full load 
working density of 76 kilolines per square inch cross-section area of the 
core body. Now, eight poles at 240 revolutions per minute would be 
sixteen cycles per second. 

Cycles x density in kilolines 16x76_, 0;) 

~~ "~~ 



~~ 1000 "1000~~ 

According to curve, Fig. 199, we obtain 2.1 watts per pound,- and as 
there is 7,000 Ib., the total core loss will be 2.1 x 7,000 = 14,700 watts. 

For the range of periodicity and flux density covered by the above 
tabulated machines, an average value of 1.7 is obtained for K. Hence the 
following approximate rule is derived : 

Watts per Ib. = 1.7 x cycles per second x kilolines density. 



232 Electric Generators. 



ELECTRIC TRACTION MOTORS. 

Motors for electric traction must, from the nature of their work, be 
designed to be reversible, and to have the brushes set in a fixed position at 
a point midway between pole ends. Since the brushes cannot be shifted, 
the magnetic field cannot be utilised to reverse the current in the short- 
circuited coil ; in fact, whatever impressed magnetic flux is passing 
through the coil while it is short-circuited under the brush, is in such a 
direction as to tend to maintain the current in its original direction, instead 
of assisting to reverse it. The commutation may be termed brush commu 
tation, and the commutating element is in the resistance of the brushes. 
For satisfactory commutation, traction motors are designed with very high 
magnetisation at full load. Much higher densities are practicable, as 
regards the heating limit, than in machines running at constant loads, since 
the average current intake by a traction motor is not ordinarily above 
one-fourth of its rated capacity, so that in average work the magnetisation 
of the air gap and armature core is not very different from that in machines 
designed for constant load. At rated capacity, however, the magnetisation 
in the projections and armature core is frequently 50 per cent, higher than 
in machines designed for constant load, and at rated load the heat 
generated per square inch of radiating surface is generally more than double 
that of machines for constant load. 

Because of the unfavourable commutating conditions, the armature 

O 

reaction of railway motors and the reactance voltage of the short-circuited 
coil, should be comparatively small at rated capacity. This is the more 
important on account of the desirability of lessening the diameter of the 
armature, so as to shorten the magnetic circuit and diminish the weight of 
the motor. Material progress has been made in this direction by putting 
three or even four, coils in one slot, where in former practice but one, 
corresponding to one commutator bar, was placed in one slot. This is a 
condition which would be adverse to satisfactory commutation with 
reasonable heating, in large generators for constant load ; but in the case 




24 Horse-Power Geared Railway Motor. 233 

of railway motors, on account of the lesser number of projections and 
consequent less room occupied for insulation, the cross-section of the pro 
jections has been increased so that a higher magnetisation in the gap is 
permissible, under which condition sparking is diminished at heavy loads. 
A material advance has been made in efficiency at average loads, and in 
sparking, by greatly increasing the magnetisation of the armature core 
proper. 

It may be fairly said that all efforts to improve commutation have 
been, first, to increase magnetisation, so that distortion is diminished ; and 
secondly, to diminish the inductance of the armature coils by employing 
open and wider slots. Machines have been constructed of 300 and 400 
horse-power capacity, capable of being reversed in either direction without 
much sparking. That the commutation is never so perfect as in the case of 
machines where the reversing field can be utilised, is shown by the gradual 
roughening of the commutator, which requires more attention than in the 
case of generators or other non-reversible machines. The remarkable 
progress that has been made in the design of this class of machinery 
will be apparent by comparing the drawings and constants of well- 
known types of machines, with those of machines constructed but a few 
years ago. 



DESCRIPTION OF A GEARED RAILWAY MOTOR FOR A BATED DRAWBAR PULL 
OF 800 LB. AT A SPEED OF 11.4 MILES PER HOUR. 

This motor has been in extensive use for some years, hence it does not 
represent the latest developments, except in so far as modifications have 
been introduced from time to time. The fundamental design, however, is 
not in accordance with the best examples of recent practice. On account 
of its established reputation for reliability, it is still, however, built in large 
numbers. Its constants are set forth below, in specification form, and in 
Figs. 239 to 254, pages 234, 236, and 240, are given drawings of the 
motor. 

SPECIFICATION. 

Number of poles ... ... ... ... . . ... ... 4 

Rated drawbar pull 800 Ib. 

Under standard conditions at this rating, the field windings are 

2 H 



234 



Electric Generators. 






24 Horse-Power Geared Railway Motor. 



235 



connected in parallel with an external shunt which diverts from the field 
winding, 30 per cent, of the total current. 

Revolutions of armature per minute at this rating ... ... 555 

Number of teeth on armature pinion ... ... ... ... 14 

axle gear ... 67 

Ratio of gear reduction ... ... ... ... ... ... 4.78 

Revolutions of axle per minute ... ... ... ... 116 

Speed of car in feet per minute on 33-in. wheels ... ... 1000 

miles per hour ,, ... ... 11.4 

Foot-pounds per minute, output for above drawbar pull and 

speed 800,000 

Horse-power output for above drawbar pull and speed ... 24.2 

Kilowatts output for above drawbar pull and speed ... ... 18.1 

Efficiency of above rating, motor warm ... ... ... 79.5 per cent. 

Corresponding kilowatts input ... ... ... ... 22.8 

., amperes ,, ... ... ... ... 45.5 

Terminal voltage ... ... ... ... .. ... ... 500 

Frequency in cycles per second at rated conditions ... ... 18.5 

DIMENSIONS. 
Armature : 

Diameter over all ... ... ... ... ... ... 16 in. 

,, at bottom of slots ... ... ... ... ... 13.2,, 

Internal diameter of core ... ... ... ... ... 4i 

Length of core over all ... ... ... ... ... ... 8 ,, 

Effective length, magnetic iron ... ... ... ... ... 7.2,, 

Pitch at armature surface ... ... ... ... ... 12.6,, 

Japan insulation between laminations ... ... ... ... 10 per cent. 

Thickness of laminations ... ... ... ... ... ... . 025 in. 

Depth of slot ... 1.40 

Width of slot at root, die punch ... ... ... ... .240 ,, 

,, ,, surface, die punch ... ... ... ... .240 ,, 

Number of slots ... ... ... ... ... ... ... 105 

Minimum width of tooth ... ... ... ... ... .164 in. 

Width of tooth at armature face ... ... ... ... .239 ,, 

Size of armature conductor, B. and S. gauge ... .. ... No. 9 

Bare diameter of armature conductor ... ... ... ... ,114 in. 

Cross-section ... ... ... ... ... ... ... .0102 square inch 

Magnet Core: 

Length of pole face ... ... ... ... ... ... 8 in. 

arc 8.25 

Pole arc -4- pitch ... ... ... ... ... ... ... .655 ,, 

Length of magnet core ... ... ... ... ... ... 8 in. 

Width 7.75 

Diameter of bore of field... ... ... ... ... ... 16 ;i 9 ^ 

Length of gap clearance above armature ... ... ... ^ ,, 

below 3 



236 



Electric Generators. 



Commutator : 

Diameter ... 
Number of segments 



per slot 



8J in. 
105 
1 



Fig 246 



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" 





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t 



Width of segment at commutator face 

root 

Thickness of mica insulation 
Available length of surface of segment 



.214 in. 

.128 
.04 




24 Horse-Power Geared Railway Motor. 237 

Brushes : 

Number of sets ... ... ... ... ... ... ... 2 

,, brushes in one set ... ... ... ... ... 1 

Length, radial ... ... ... ... ... ... ... 2f in. 

Width ... 24- 

4 

Thickness ... ... ... ... ... ... ... ... .5 

Area of contact of one brush ... ... ... ... ... 1.125 square inches 

Type of brush radial carbon 

TECHNICAL DATA. 

Terminal voltage ... ... ... ... ... ... ... 500 

Number of face conductors ... ... ... ... ... 840 

Conductors per slot ... ... ... ... ... ... 8 

,, coil 4 

Number of circuits ... ... ... ... ... ... 2 

Style of winding ... ... ... ... ... ... ... Single 

Gramme ring or drum ... ... ... ... ... ... Drum 

Type of construction of winding... ... ... ... ... Formed coil winding. 

Number of coils ... ... ... ... ... ... ... 105 

Mean length of one armature turn ... ... ... ... 43 in. 

Total armature turns ... ... ... ... ... ... 420 

Turns in series between brushes ... ... ... ... ... 210 

Length between brushes ... ... ... ... ... ... 9000 in. 

Cross-section of one armature conductor ... ... ... .0102 square inch 

Ohms per cubic inch at 20 deg. cent. ... ... ... ... .00000068 ohms. 

Resistance between brushes at 20 deg. Cent. ... ... ... .305 ,, 

,, ,, 95 ,, ... ... ... .o94 ,, 

Volts of drop in armature at 95 ... ... ... 18 

Mean length of one field turn ... ... ... ... ... 4G.5 in. 

Field conductor, B. and S. gauge ... ... ... ... No. 6 

Bare diameter ... ... ... ... ... ... ... .162 in. 

Cross-section of field conductor ... ... ... ... ... .0205 square inch 

Turns per field spool ... ... ... ... ... ... 203 

Number of field spools ... .. ... ... ... ... 2 

Total field turns in series ... ... ... ... ... 406 

,, length of spool copper ... ... ... ... ... 18.800 in. 

resistance of spool winding at 20 deg. Cent. ... ... .625 ohm. 

Q<i 81 

>j j) 

Thirty per cent, of the main current of 45.5 amperes is 
diverted from the field winding by a suitable shunt 

resistance, hence current in field winding is ... ... 32 amperes 

Volts drop in field winding at 95 deg. Cent 26 volts 

Resistance brush contacts (positive plus negative) ... ... .055 ohm 

Volts drop in brush contacts ... ... ... ... ... 2.5 volts 

armature, field, and brushes ... ... 46.5 ,, 

Counter electromotive force of motor ... ... 453.5 ,, 

Amperes per square inch in armature winding... 

field 1560 



238 Electric Generators. 

Commutation : 

Average voltage between commutator segments ... ... 18 

Armature turns per pole... ... 105 

Amperes per turn 

Armature ampere turns per pole ... ... ... ... 2400 

Frequency of commutation (cycles per second) ... ... 250 

Number of coils simultaneously short-circuited per brush ... 3 

Turns per coil ... ... ... ... ... ... ... 4 

Number of conductors per group simultaneously undergoing 

commutation ... ... ... ... ... ... 24 

Flux per ampere turn per inch length of armature lamination 20 
Flux linked with 24 turns with one ampere in those turns 

= 20 x 8 x 24 = 3840 

Inductance of four turns = 4 x 3480 x 10~ 8 - 000154 henrys 

But in a two-circuit winding with four poles and only two sets of 
brushes, there are two such four-turn coils in series, being commutated 
under one brush, and their inductance is = 2 x .000154 = .000308 henrys. 

Reactance of these two short-circuited coils ... ... ... .484 ohm 

Amperes in short-circuited coils ... ... ... ... 22.8 

Reactance voltage of short-circuited coils ... ... ... 11 volts 

MAGNETOMOTIVE FORCE. 

Megalines entering armature, per pole-piece ... ... ... 2.92 

Coefficient of magnetic leakage ... ... ... ... ... 1.25 

Megalines per field-pole ... ... ... ... ... ... 3.65 

Armature : 

Section ... ... ... ... ... ... ... ... 62.8 square inches 

Density 46.5 kilols. 

Length (magnetic path) ... ... ... ... ... ... 4 in. 

Ampere turns per inch of length ... ... ... ... 8 

for armature core ... ... ... ... 30 

Teeth : 

Transmitting flux from one pole-piece ... ... ... ... 19 

Section at roots ... ... ... ... ... ... ... 22.5 square inches 

Length ... ... ... ... ... ... ... ... 1.4 in. 

Apparent density at root tooth ... ... ... ... ... 130 kilols. 

Corrected ,, ... ... ... ... ... 125 ,, 

Ampere turns per inch of length ... ... ... ... 700 

for teeth ... 980 

Gap : 

Section at pole face ... ... ... ... ... ... 66 square inches 

Length, average of top and bottom ... ... ... ... .14 in. 

Density at pole face ... ... ... ... ... ... 44 kilols. 

Ampere turns for gap ... ... ... ... ... ... 1920 



24 Horse-Power Geared Railway Motor. 



239 



Cast-Steel Portion of Circuit : 

Average cross-section ... ... ... ... ... ... 52 square inches 

Length, magnetic ... ... ... ... ... ... 9 in. 

Average density ... ... ... ... ... 70 kilols. 

Ampere turns per inch of length ... ... 35 

,, for cast-steel frame, per pole-piece ... ... 320 

Only two of the four poles carry exciting windings ; hence of the 203 
turns on one spool, only 101.5 are to be taken as corresponding to one 
pole-piece. Thirty per cent, of the main current being diverted from the 

fields, the field exciting current is 32 amperes, and field ampere turns per 

pole-piece are 32 x 101.5 = 3250 ampere turns. These are probably 
distributed somewhat as follows : 

Ampere turns for armature core ... ... ... ... 30 

teeth 980 

gap ... 1920 

frame... ... ... ... ... ... 320 

Total ampere turns per pole-piece ... 3250 

THERMAL CONSTANTS. 
Armature : 

Resistance between brushes at 95 deg. Cent. ... ... ... .394 ohm 

Amperes input at rated capacity ... ... ... ... 45.5 amperes 

Armature C 2 R loss at 95 deg. Cent. ... ... ... ... 815 watts 

Total weight of armature laminations, including teeth ... 314 Ib. 

,, observed core loss (only apparently core loss) ... ... 800 watts 

Watts per pound in armature laminations ... ... ... 2.55 ,, 

Total of armature losses ... ... ... ... ... ... 1615 

Length of armature (over conductors) ... ... ... ... 12 in. 

Peripheral radiating surface of armature ... ... ... 600 square inches 

\Vattspersquareinchperipheralradiatingsurface ... 2.7 watts 

Field Spools : 

Total resistanceof the two field spools at 95 deg. Cent. ... .81 ohm 

Amperes in spool winding ... ... ... ... ... 32 amperes 

Spool C?R loss at 95 deg. Cent. 830 watts 

Commutator : 

Area of bearing surface of positive brush ... ... ... 1.13 square inches 

Amperes per square inch of brush-bearing surface ... 40 amperes 

Ohms per square inch of bearing surface of carbon brushes ... .03 ohm 

Brush resistance, positive + negative ... ... ... ... .053 ,, 

Volts drop at brush contacts ... ... 2.4 volts 

C 2 R at brush contacts 1 1 watts 

Brush pressure per square inch ... ... ... ... ... 2 Ib. 

Total brush pressure ... ... ... ... ... ... 4.5 ,, 










SCARED RAILWAY MOTOR 








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SOO tOO 600 SOO IOOO 1SOO 1100 1600 ISOO 

(1SI6C) DRAW BAR PULL 



GEARED RAILWAY MOTOR. 

Ftg.2ffl- FOR RATED DRAW SAB PULL CF SCO i 
AT SPEED CF H-4-MILS.S PER HOUR. 

Speed Curve For 33 Wheels & 
Gearing Kg trio of 4-76. 



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Fiq.266 

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GEARED RAILWAY MOTOR 
RATED DRAW BAH PULL. OF 80 








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? fi 12 16 20 29 29 32 36 90 

(we o) HORSE -rones* OUTPUT 



Fig.^S. 



100 
30 
60 



GEARED RAILWAY MOTOR. 



Curve of Commercial Efficiency. 



12 16 20 24 28 32 36 40 

Miles per Hour. 




10 20 jo +0 

<*" ef > Amperes I 



24 Horse-Poiver Geared Railway Motor. 241 

Coefficient of friction ... ... ... ... ... .3 

Peripheral speed of commutator, feet per minute ... ... 1240ft. 

Brush friction 36 watts 

Stray power lost in commutator (allowance) ... ... ... 50 

Total commutator loss ... ... ... ... ... ... 198 

Peripheral radiating surface 100 square inches 

Watts per square inch radiating surface of commutator ... 2 watts 

EFFICIENCY CALCULATIONS. 

Watts. 

Output at rated capacity "... ... ... ... ... 18,100 

Core loss ... ... ... ... ... ... 800 

Commutator and brush loss ... ... ... ... ... 198 

Armature C- R loss at 95 deg. Cent 815 

Field spool C 2 R ... 830 

Gearing friction ... ... ... ... ... ... ... 2 000 



Total input 22,743 

Commercial efficiency at rated capacity and 95 deg. Cent. = 79.5 per cent. 1 

WEIGHTS. 

b. 

Armature core (magnetic) ... ... ... ... ... 250 

teeth 67 

copper... ... 60 

Commutator bars ... ... ... ... ... ... 45 

Armature complete ... ... ... ... ... ... 635 

Magnet pole ... ... ... ... ... . . ... 520 

Spool copper ... ... ... ... ... ... ... 129 

Machine complete ... ... ... ... ... ... 1525 

111 Figs. 255, 256, 257, and 258 are given respectively curves of draw 
bar pull, output, speed, and efficiency for this motor. 

In many of the more modern street-railway motors, the design has 
followed lines differing in many respects from those of the motor just 
described. Thus several armature coils are arranged in one slot, largely 
reducing the number of slots, and the pole- faces are laminated, since other 
wise these few wide slots would set up too great an eddy current loss in 
the pole-face. It has been found preferable to have one field spool per 
pole-piece, instead of having two salient and two consequent poles. The 
armature diameter has been largely reduced, and sparking is minimised by 
running not only the teeth, but also the core, up to extremely high 
magnetic density ; nevertheless, owing to the greatly reduced mass of the 

1 Tri this result, the loss in the diverting shunt to the field spool winding is not 
allowed for. 

2 I 



~y v /r 

ff* 



242 



Electric Generators. 



armature iron, the core loss is small. A motor designed on these lines, and 
of not very different capacity from the one just described, will next be 
described. 



GEARED RAILWAY MOTOR FOR A RATED OUTPUT OF 27 HORSE-POWER AT 
AN ARMATURE SPEED OF 640 REVOLUTIONS PER MINUTE. 

The rating of this motor is in accordance with the now generally 
accepted standard practice of limiting the temperature rise of field and 



g 



g.ZGZ Centre, Section, of Pole-piece 




armature to 75 deg. Cent., as measured by thermometer after a full-load 
run of one hour s duration. The motor is illustrated in Figs. 259 to 277 
inclusive. 

Applying this same standard permissible temperature rise to runs of 
different durations, the following Table gives the corresponding ratings at 
500 terminal volts : 



27 Horse-Power Geared Railway Motor. 



243 



Length of Run. Hours. 



Amperes. 
75 
51 
39.5 
32.5 
23.5 
17 
14.5 
14 



Horse-Power. 
38.2 
27 
21.3 
17.5 
12.5 

8.6 

6.9 

6.6 



Uia 266 




The following specification is prepared on the basis of the rating of 
27 horse-power for one hour s continuous operation at full load. In tram 
way service, of course, the motor is on the average called upon to develop 
but a small percentage of its full capacity ; and hence such a motor, when 
continuously in service under normal conditions, runs much cooler than the 
above-quoted temperatures. 



244 Electric Generators. 

SPECIFICATION. 

Number of poles ... ... ... ... ... ... ... 4 

Rated horse-power output ... ... ... ... ... 27 

kilowatts 20.2 

Efficiency at above rating and at 95 deg Cent ... ... 79 per cent. 

The efficiency is a little higher at lighter loads, and is at its 
maximum at about two-thirds full-rated load, so that it is high through 
out the entire range of working, that is, from quarter load to heavy 
overloads. (See efficiency curve in Fig. 282.) 

Kilowatts input at rated load ... ... ... ... ... 25.6 

Terminal voltage ... ... ... ... ... ... ... 500 

Corresponding amperes input ... ... ... ... ... 51 

,, revolutions per minute of armature ... ... G40 

Number of teeth on armature pinion ... ... ... .... 14 

,, axle gear 67 

Ratio of gear reduction ... ... ... ... ... ... 4.78 

Revolutions of axle per minute ... ... ... ... ... 134 

Speed of car in feet per minute, on 33-in. wheels ... ... 1160 

miles hour ... ... 13.1 

Output in foot-pounds per minute, at normal rating ... ... 890,000 

Pounds drawbar pull, at normal rating... ... ... ... 770 

Frequency at rated conditions in cycles per second ... ... 21.4 



DIMENSIONS. 
Armature : 

Diameter over all... ... ... ... ... ... .. 11 in. 

,, at bottom of slots ... ... ... ... ... 8.42 ,, 

Internal diameter of useful magnetic portion of core ... ... 6.17 ,, 

Length of core over all ... ... ... ... ... ... 9 ,, 

Number of ventilating ducts, each in. wide ... ... ... 3 

Effective length of magnetic iron ... ... ... ... 7.42 in. 

Pitch at armature surface ... ... ... ... ... 8.65 ,, 

Japan insulation between laminations ... ... ... ... 10 per cent. 

Thickness of laminations... ... ... ... ... ... .025 in. 

Depth of slot 1.29 

Width of slot at root i| ^ 

,, at surface ... ... ... ... ... ... Jjj ,, 

Number of slots ... ... ... ... ... ... ... 29 

Minimum width of tooth... ... ... ... ... ... .445 in. 

Width of tooth at armature face ... ... ... ... .724 ,, 

Size of armature conductor, B. and S. gauge ... ... ... No. 10 

Bare diameter of armature conductors ... ... ... ... .102 in. 

Cross-section 0081 square inches 



246 Electric Generators. 

Magnet Core : 

Length of pole face ... ... ... 9 in. 

arc 6.1 

Pole arc -7- pitch... ... ... ... ... ... ... .69 

Length of magnet core ... ... ... ... ... ... 8|- in. 

Width 4 

Diameter of bore of field ... ... ... ... ... ... 11 3 9 2 ,, 

Length of gap clearance above armature ... ... ... |- ,, 

,, ,, below ,, ... ... ... 3 2 ,, 

Commutator : 

Diameter ... ... ... ... ... ... ... ... 8 in. 

Number of segments ... ... ... ... ... ... 87 

segments per slot ... ... ... ... ... 3 

Width of segment at commutator face ... .. ... ... .243 in. 

segment at root ... ... ... ... ... .108 ,, 

Thickness of mica insulation ... ... ... ... ... .050 ,, 

Available length of surface of segment ... ... ... ... 2|- 

Brushes : 

Number of sets ... ... ... ... ... ... ... 2 

,, in one set ... ... ... ... ... ... 2 

Length, radial ... ... ... ... ... ... ... 21 in. 

Width ... 1 

Thickness ... ... ... ... ... ... ....... \ ,, 

Area of contact of one brush ... ... ... ... ... 625 square inches 

Type of brush ... ... ... ... ... ... ... Radial carbon 

MATERIALS. 

Armature core ... ... ... ... ... ... ... Sheet steel 

Magnet frame ... ... ... ... ... ... ... Cast ,, 

Pole faces... ... ... ... ... ... ... Sheet ,, 

Brushes ... ... ... ... ... ... ... ... Carbon 

TECHNICAL DATA. 

Terminal voltage ... ... ... ... .... ... ... 500 

Number of face conductors ... ... ... ... ... 696 

Conductors per slot ... ... ... ... ... ... 24 

,, coil ... ... ... ... ... ... 4 

Number of circuits ... ... ... ... ... ... 2 

Style of winding ... ... ... ... ... ... ... Single 

Gramme ring or drum ... ... ... ... ... ... Drum 

Type construction of winding ... ... ... ... ... Formed coil winding 

Number of coils ... ... ... ... ... ... ... 87 

Mean length of one armature turn ... ... ... ... 38.5 in. 

Total armature turns ... ... ... ... ... ... 348 

Turns in series between brushes ... ... ... ... 174 

Length between brushes... ... ... ... ... ... 6700 in. 

Cross-section of one armature conductor .. ... ... .0081 square inch 



27 Horse-Power Geared Railway Motor. 



247 



to .dotted, line reproseTifa outline of IrundaAwrvJ ig 2J2SecAJ). 




Section, 





f 



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Shows shape* of" pifce before being 



248 



Electric Generators. 




27 Horse-Power Geared Railway Motor. 



249 



Ohms per cubic inch at 20 deg. Cent .00000068 

Resistance between brushes at 20 deg. Cent. ... ... ... .28 ohm 

j) .00 ,, 

Volts drop in armature at 95 deg. Cent. ... ... ... 18.3 volts 

Mean length of one field turn ... ... ... ... ... 36 in. 

Size of field conductor, B. and S. gauge ... ... ... No. 5 

Bare diameter ... ... ... ... ... ... ... .182 in. 

Cross-section of field conductor ... ... ... ... ... .026 square inch 

Turns per field spool ... ... ... ... ... ... 156.5 

Number of field spools ... ... ... ... ... ... 4 

Total field turns in series ... ... ... ... ... 626 

length of spool copper ... ... ... ... ... 22,000 in. 

,, resistance spool winding at 20 deg. Cent. ... ... .59 ohm 

>t )> yo . < o ,, 

Volts drop in field winding at 95 deg. Cent. ... ... ... 38.6 volts 

Resistance brush contacts (positive + negative) ... ... .048 ohm 

Volts drop in brush contacts ... ... ... ... ... 2.4 volts 

,, ,, armature, field, and brushes ... ... ... 59.3 ,, 

Counter electromotive force of motor ... ... ... ... 441 

Amperes per square inch in armature winding ... ... 3130 

field 10 9 

,, ,, ,, ,, . . . ... ... i;7~v 

Commutation : 

Average voltage between commutator segments ... ... 21 

Armature turns per pole ... ... ... ... ... 87 

Amperes per turn ... ... ... ... ... ... 25.5 

Armature ampere turns per pole ... ... ... ... 2200 

Frequency of commutation, cycles per second ... ... ... 270 

Number of coils simultaneously short-circuited, per brush ... 2 

Turns per coil ... ... ... ... ... ... ... 4 

Number of conductors per group, simultaneously undergoing 

commutation ... ... ... ... ... ... 16 

Flux per ampere turn per inch-length of armature lamination 20 lines 
,, linked with 16 turns with 1 ampere in those turns, 

= 20 x 9 x 16 2880 

Inductance of four turns = 4 x 2880 x 10~ 8 ... ... ... .000115 henrys 

In a four-pole, two-circuit winding, and with only two sets of 

brushes, there are two such four-turn coils in series, being 

commutated under the brush, and their inductance is ... .000230 henrys 

Reactance of these two short-circuited coils ... ... ... .39 ohm 

Amperes in short-circuited coils ... ... ... ... 25.5 amperes 

Reactance voltage of short-circuited coils ... ... ... 9.9 volts 

Magnetomotive Force Estimations : 

Megalines entering armature, per pole piece ... ... 2.96 

Coefficient of magnetic leakage ... ... ... ... 1.25 

Megalines per field pole ... ... ... ... 3.70 

Armature : 

Section ... ... ... ... ... 16.7 square inches 

Density ... ... 177 kilols. 

2 K 



250 



Electric Generators. 




Fiq. 
*s 



6CAACO RAILWAV MOTOR. 

218 AN ARMATURE SPEED or e^o n. 



ISO 
160 

no 
ISO 

1500 
1400 
1300 
1200 
1100 

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or<j/e Curve, For 33 Wheels an A 
6**r Ratio of 4-78. 


GEARED RAILWAV MOTOR. 
FOR A RATED OUTPUT OF 2^ H.P. AT 
AH ARMATURE SPEfD OF 4-0 /?.flM. 

g 279 Speed Curve for 33 Wheels 
Geoir Ratio of 4-78. 




















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1C.I.) **IPRES in PUT 



27 Horse-Poiver Geared Railway Motor. 



251 



But, as is evident from the drawing of Fig. 2GO, many lines will 
flow through the inner parts of the punchings, and also, to a certain 
extent, through the shaft, and a corrected density may be taken of, 
say, 130 kilolines. 



GEARED RAILWAY MOTOR 



700 

to 
to 


Fin. 280 

t7 ARHATUKC. SPCEOOfSaO HP*. 

Horse Power Curve 






















































































































































































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GEARED RAILWAY MOTOR 



Core Loss Curve. 



422 



.%. 



IS ZO IS go 83 1O IS 

if OUTPUT 



GEARED RAILWAV MOTOR. 
FOR A RATED OUTPUT OF 27 H. P. AT 
N ARMATURE SPEED OF 640 R. P.M. 

Curve of Commercial Efficiency. 



20 JO 40 SO 60 70 SO .90 100 

I Amperes in Field. 



GEARED RAILWAY MOTOR 
FOR A RATED OUTPUT OF 27 H P AT AH 
K ^Rft ARMATURE. SPIED OF 610H PM 

^ Thermal Characteristic Curve, 

assuming a rise in Armature&-Fieldof7S c 



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10 ib so 40 so so i 

n. i Amperes Input 

Length (magnetic) 
Ampere turns per incl 
for arm 


90 90 100 


1 Z 3 
TIME IN HOI 


9 a 

rS 

3 in. 
900 
2700 


i of length 
ature core 







Teeth : 



Transmitting flux from one pole-piece 
Section at root of six teeth 
Length 



6 

20 square inches 
1.29 in. 



252 Electric Generators. 

Apparent density in root tooth ... ... ... ... ... 148 

Corrected ,, ,, ... ... ... ... ... 138 

Ampere turns per inch of length ... ... ... ... 1 300 

for teeth 1680 

Gap : 

Section at pole-face ... ... ... ... ... ... 55 square inches 

But owing to the special method of constructing the pole-face 
(see Figs. 262 and 263), whereby the entire surface is not 
equally effective, a corrected section at pole-face should 
be taken, equal to, say ... ... ... ... ... 45 square inches 

Mean length of air gap ... ... ... ... ... ... .14 in. 

Pole-face density (from corrected section) ... ... ... 66 kilols. 

Ampere turns for gap ... ... ... ... ... ... 2900 

Cast Steel Portion of Circuit : 

Average cross-section ... ... ... ... ... ... 39 square inches 

Length (magnetic) ... ... ... ... ... ... 7.5 in. 

Average density ... ... ... ... ... ... ... 96 kilols. 

Ampere turns per inch of length ... ... ... ... 90 

for cast-steel frame per pole piece ... ... 670 

Each spool carries 156.6 turns, and in this motor full field is always 
used, i.e., no portion of the main current is diverted through an auxiliary 
shunt. Hence 

Ampere turns per field spool at full rated load are equal to 156.5 x 51 = 

7950 ampere turns. 

This magnetomotive force of 7,950 ampere turns can be considered to 
be distributed somewhat in the following manner : 

Ampere Turns. 
Armature core ... ... ... ... ... ... ... 2700 

Teeth 1680 

Gap ... ... 2900 

Steel Frame 670 



Total magnetomotive force per pole piece ... ... 7950 

It is not intended to convey the impression that any high degree of 
accuracy is obtainable, in these magnetomotive force estimations in railway 
motors ; but working from the observed results, and from the known 
dimensions of the apparatus, and the assumed properties of the material 
employed, some rough idea of the distribution of the magnetomotive force 
is obtained. 




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254 



Electric Generators. 



THERMAL CONSTANTS. 



Armature : 



Resistance between brushes at 95 deg. Cent. ... 
Amperes input at rated capacity 
Armature C 2 R loss at 95 deg. Cent. 
Total weight of armature laminations including teeth 
observed core loss (only apparently core loss) . 
Watts per Ib. in armature laminations ... 
Total of armature losses ... 
Length of armature, over conductors ... 
Peripheral radiating surface of armature 
Watts per square inch peripheral radiating surface . 

Field Spools : 

Total resistance, all field spools at 95 deg. Cent. 
Current in spool winding 
Spool C 2 R loss at 95 deg. Cent. 



.36 ohm 
51 amperes 
925 watts 

120 Ib. 
1120 watts 

9.3 
2045 
13.5 in. 

465 square inches 
4.4 watts 



.76 ohm 
51 amperes 
2000 watts 



Commutator : 

Area of bearing surface of positive brushes ... ... ... 1.25 square inches 

Amperes per square inch of brush-bearing surface ... ... 40.5 amperes 

Ohms per square inch of bearing surface of carbon brushes . . . .03 ohm 

Brush resistance, positive + negative ... ... ... .. .048 ,, 

Volts drop at brush contacts ... ... ... ... ... 2.4 volts 

C 2 R at brush contacts (watts) ... ... ... ... ... 122 watts 

Brush pressure, pounds per square inch ... ... ... 2 Ib. 

Total brush pressure ... ... ... ... ... ... 5 ,, 

Coefficient of friction ... ... ... ... ... ... .3 

Peripheral speed of commutator (feet per minute) ... ... 1850ft. 

Brush friction ... ... ... ... ... ... ... 46 watts 

Allowance for stray power lost in commutator ... ... 50 ,, 

Total commutator loss ... ... ... ... ... ... 216 ,, 

Peripheral radiation surface ... ... ... ... ... 95 square inches 

Watts per square inch peripheral radiating surface of com 
mutator ... ... ... ... ... ... ... 2.3 watts 

EFFICIENCY ESTIMATIONS. 

Watts. 

Output at rated capacity ... ... ... ... ... 20,200 

Core loss 1,120 

Commutator and brush loss ... ... ... ... ... 218 

Armature C 2 R loss at 95 deg. Cent. ... ... ... ... 925 

Field 2,000 

Gearing friction ... ... ... ... ... ... ... 1,200 



Total input 



25,663 



Commercial efficiency at rated capacity and 95 deg. Cent. = 79 per cent. 



27 Horse-Poiver Geared Railway Motor. 255 

9 

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256 Electric Generators. 

WEIGHTS. 

Armature laminations 

complete (with pinion 
Motor complete (without axle gear and gear case) 



Ib. 

- 120 
= 357 
= 1460 



In Figs. 278 to 283 are given, respectively, curves of D.P.B., speed, 
output, core loss, efficiency, and thermal characteristics. 



DIRECT-CONNECTED RAILWAY MOTOR. 

This motor gives an output of 117 horse-power at a speed of 
23.8 miles per hour on 42-in. wheels. It contributes 1,840 Ib. to the 
drawbar pull of the 3 5 -ton locomotive, for the equipment of which, four 
such motors are employed. Consequently the total draw-bar pull of this 
locomotive at the above speed is 7,350 Ib., but the motor is capable of 
exerting a torque far in excess of this figure ; in fact, up to the limit of 
the tractive effort possible for a locomotive of this weight, before slipping 
takes place. Drawings for this motor are given in Figs. 284 to 319, and 
its constants are set forth in the following tabularly-arranged calculation : 

Number of poles ... ... ... ... ... ... ... 4 

Drawbar pull at 23.8 miles per hour ... ... ... ... 1840 Ib. 

Corresponding speed (miles per hour) ... ... ... ... 23.8 miles 

Speed in feet per minute... ... ... ... ... ... 2100ft. 

Diameter of driving wheels ... ... ... ... ... 42 in. 

Armature revolutions per minute ... ... ... ... 190 

Output in foot-pounds per minute for above drawbar pull and 

speed 3,800,000 

Ditto in horse-power ... ... ... ... ... ... 117 

kilowatts 87.5 

Corresponding kilowatts input ... ... ... ... ... 95.8 

Terminal voltage ... ... ... ... ... ... ... 500 volts 

Current input ... ... ... ... ... ... ... 192 amperes 

Frequency in cycles per second ... ... ... ... ... G. 3 5 cycles 

DIMENSIONS. 
Armature : 

Diameter over all ... ... ... ... ... ... 22| in. 

Length over conductors ... ... ... ... ... ... 45 j ,, 

Diameter at bottom of slots ... ... ... ... ... 19.04 ,, 

Internal diameter of core ... ... ... ... ... 9| ,, 

Length of core over all ... ... ... ... ... ... 28 

Effective length, magnetic iron ... ... ... ... ... 25.2 

Pitch at armature surface 17.7 , 



117 Horse-Power Railway Motor, 



257 




2 L 



258 Electric Generators. 

Japan insulation between laminations ... ... ... ... 10 per cent. 

Thickness of laminations ... ... ... ... ... .025 in. 

Depth of slot ... ... 1.73 

Width at root .52 ,, 

,, ,, surface ... ... ... ... ... ... .52 ,, 

Number of slots ... ... ... ... ... ... ... 61 

Minimum width of tooth ... ... ... ... ... .463 in. 

Width of tooth at armature face ... ... ... ... .635 ,, 

,, conductor ... ... ... ... ... ... .10 ,, 

Depth .60 

Apparent cross-section of armature conductor... ... ... .060 square inches 

This is a pressed stranded conductor, made up of 49 strands 
of No. 19 B. and S. gauge. The cross-section of a No. 
19 gauge wire is .0101 square inch, hence the cross- 
section of the 49 strands is 49 x .0101 ... ... ... .0495 square inch 

Fig.285 

^O.SZT MA* 
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1 lit 




But allowance must also be made for the increased resistance 
due to the increased length of the individual strands 
when twisted in the process of forming. Hence the 
equivalent cross-section of solid copper should be esti 
mated at ... ... ... ... ... ... ... .046 square inches 

This was the experimentally-determined value in this case, and is 
fairly representative of stranded conductors of about these dimensions. 

Magnet Core : 

Length of pole-face ... ... ... ... ... ... 28 in. 

arc ... ... ... ... ... ... 13.2 

Pole arc -4- pitch... ... ... ... ... ... ... 73 per cent. 

Length of magnet core ... ... ... ... ... ... 28 in. 

Width ... 9| 

Diameter of bore of field ... ... ... ... ... ... 23 T ^ 

Length of gap clearance above armature ... ... ... yV 

below ... ... 

Commutator : 

Diameter ... ... ... ... ... ... ... ... 19 

Number of segments ... ... ... ... ... ... 183 

,, ,, per slot 



117 Horse-Poiver Railway Motor. 259 

Width of segment at commutator face ... ... ... ... .286 in. 

root .200 

Thickness of mica insulation ... ... ... ... ... .04 

Available length of surface of segment... ... ... ... 8 ,, 

Brushes : 

Number of sets ... ... ... ... ... ... 2 

in one set ... ... ... ... ... ... 4 

Length (radial) ... ... ... ... ... ... ... 2i in. 

Width ... ... ... if 

Thickness ... ... ... ... ... ... ... ... TT 

Area of contact of one brush ... ... ... ... ... 1.2 square inch 

Type of brush Radial carbon 

MATERIALS. 

Armature core Sheet Steel 

spider No. 3 metal 

flanges Cast iron 

,, conductors Pressed stranded 

copper 

Commutator segments ... ... ... ... ... ... Copper 

spider ... ... ... ... ... ... Malleable cast iron 

Pole-pieces ... ... Sheet steel 

Yoke and magnet cores ... ... ... ... ... ... Cast ,, 

Brushes ... ... ... ... ... ... ... ... Carbon 

TECHNICAL DATA. 

Terminal voltage ... ... ... ... ... ... ... 500 volts 

Number of face conductors ... ... ... ... ... 366 

Conductors per slot ... ... ... ... ... ... 6 

Number of circuits ... ... ... ... ... ... 2 

Style winding ... ... ... ... ... ... ... Single 

Gramme ring or drum ... ... ... ... ... ... Drum 

Type construction of winding ... ... ... ... ... Barrel wound 

Mean length of one armature turn ... ... ... ... 103 in. 

Total armature turns ... ... ... ... ... ... 183 

Turns in series between brushes ... ... ... ... 91 

Length between brushes ... ... ... ... ... ... 9400 in. 

Virtual cross-section of one armature conductor ... ... .046 square inch 

Ohms per cubic inch at 20 deg. Cent ... .00000068 

Resistance between brushes at 20 deg. Cent. ... ... ... .070 ohms 

70 .084 

Volts drop in armature at 70 deg. Cent. ... ... ... 16 volts 

Mean length of one field turn ... ... ... ... ... 95 in. 

The winding on the small spools consists of fifteen turns, whose 

section is made up of two strips of .050 in. by .875 in., in multiple with 



260 



Electric Generators. 




117* Horse- Power Railway Motor. 



261 





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117 Horse-Power Railway Motor. 263 

two of .060 in. by .875 in. Insulation between turns consists of a 
thickness of .010 in. of asbestos. 

Cross-section of field conductor on small spools ... ... .193 square inch 

The winding on the large spools consists of seventy-six turns, whose 
section is made up of a strip of .050 in. by 2j in., in multiple with one of 
.060 in. by 2 in. 

Cross-section of field conductor on large spools ... ... .234 square inch 

Total turns on all four spools all are in series ... ... 182 

Resistance of two small spools at 70 deg. Cent. ... ... .012 ohm 

lar g e i, .047 

Total spool resistance at 70 deg. Cent ... ... .059 ,, 

Volts of drop in field ... ... ... ... ... ... 11 volts 

Resistance of brush contacts (positive + negative) .. ... .012 ohm 

Volts of drop in brush contacts... ... ... ... 2 volts 

,, ,, armature, field, and brushes ... ... ... 29 ,, 

Counter electromotive force of motor ... ... ... ... 471 ,, 

Amperes per square inch in armature winding ... ... 2100 

,, ,, winding of small spools ... ... 1000 

,, large ... 820 

Commutation : 

Average voltage between commutator segments ... ... 10.7 

Armature turns per pole... ... ... ... ... ... 46 

Amperes per turn ... ... ... ... ... ... 91 

Armature ampere turns per pole ... ... ... ... 4200 

Frequency of commutation, cycles per second ... ... ... 138 

Number of coils simultaneously short-circuited per brush ... 3 

Turns per coil ... ... ... ... ... ... ... 1 

Number of conductors per group simultaneously undergoing 

commutation... ... ... ... ... ... ... 6 

Flux per ampere turn per inch of length of armature lamina 
tions 20 

Flux linked with six turns with one ampere in those turns ... 3360 

Inductance of one turn ... ... ... ... ... ... .0000336 henrys 

The armature having a two-circuit winding with four poles 
and only two sets of brushes, there are two such turns in 
series, being commutated under the brush, and their 

inductance is ... ... ... ... ... ... .OOOOG7 henrys 

Reactance of short-circuited turns ... ... ... ... .058 ohm 

Amperes in ,, ,, ......... 91 

Reactance voltage of short-circuited turns ... ... ... 5.3 volts 

MAGNETO-MOTIVE FORCE ESTIMATIONS. 

Megalines entering armature, per pole piece ... ... ... 20.6 

Coefficient of magnetic leakage taken at ... ... ... 1.15 

Megalines in magnet frame, per pole-piece 



264 



Electric Generators. 




117 Horse-Power Railway Motor 



265 



Armature : 
Section 
Density 

Length, magnetic 
Ampere turns per inch of length 
for armature core 



240 square inch 

86 kilolines 

6 in. 

40 

240 



DIRECT CONNECTED RAILWAY MOTOR. 
Ticy .320^- SATURATION CURVE 

When dnven on open circuit at 190 r p. m, field 

















































































































































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:TEb RAILWAY MOTOR! 






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DIRECT CONNECTED RAILWAY MOTOR. 
Fl<J-322_ COK LOS s CURVES. 

I. Core Loss from analysis of efficiency curve 
//Core Loss when driven at speeds corresponding to 
those of Curve Land with corresponding field ex- 
utations butw/th no current in the armature. 
30OO{ dfd with brushes raised 



2800 
2600 
2400 
2200 
2000 
JSOO 
1600 
1WO 

noo 
looo 

SOO 

600 

ooo 

200 



120 100 iso iea zoo zta &o za> zao 300 



DIRECT CONNECTED RAILWAY MOTOR. 
CURVE OF COMMERCIAL EFFICIENCY. - 



>x oo no GO igo an 




Jeeth : 

Transmitting flux from one pole-piece ... ... ... ... 13 

Section at roots ... ... ... ... ... ... ... 152 square inches 

Length ... ... ... ... ... ... ... ... 1.73 in. 

Apparent density at root tooth ... ... 13" kilolines 

Corrected ... ... ... ... ... 127 

Ampere turns per inch of length ... ... ... ... 1000 

for teeth ... 1730 

2 M 



26 G Electric Generators. 

Gap : 

Section at pole-face ... ... ... ... ... ... 370 square inches 

Length gap, average of top and bottom ... ... ... .28 in. 

Density at pole-face ... ... ... ... ... ... 56 kilolines 

Ampere turns for gap ... ... ... ... ... ... 5000 

Cast-Steel Portion of Circuit : 

Average cross-section ... ... ... ... ... ... 240 square inches 

Length, magnetic... ... ... ... ... ... ... 17 in. 

Average density ... ... ... ... ... ... ... 102 kilolines 

Ampere turns per inch of length ... ... ... ... 105 

Ampere turns for cast-steel frame (per pole-piece) ... ... 1780 

In the following Table is given the estimated subdivision of the 
magnetomotive force observed among the different portions of the magnetic 
circuit : 

Ampere Turns. 
Armature core ... ... ... ... ... ... ... 240 

teeth 1730 

Gap 5000 

Cast-steel frame ... ... ... ... ... ... ... 1780 

Total ampere turns per field spool ... ... ... ... 8750 

The field excitation is furnished by two small spools on the top and 
bottom poles, and two large spools on the other two poles. There being 
fifteen turns per small spool, and seventy-six per large spool, the average 

1 ^ _L *7 (\ 

excitation per spool at full rated load is x 192 = 8,750 ampere 

_j 

turns. 

THERMAL CONSTANTS. 
Armature : 

Resistance between brushes at 70 deg. Cent. ... ... ... .084 ohm 

Amperes input at rated capacity ... ... ... ... 192 amperes 

Armature C 2 R loss at 70 deg. Cent. ... ... ... ... 3100 watts 

Total weight of armature laminations, including teeth ... 1900 Ib. 

Watts per pound in arniature laminations ... ... ... 1.15 watts 

Total core loss (apparently core-loss) ... ... ... ... 2200 ,, 

,, of armature losses ... ... ... ... ... ... 5300 ,, 

Peripheral radiating surface of armature ... ... ... 3250 square inches 

Watts per square inch peripheral radiating surface ... ... 1.63 watts 

Field Spools : 

Total resistance of four field spools at 70 deg. Cent. ... ... .059 ohms 

Spool C 2 R loss at 70 deg. Ctnt.... 2200 watts 



117 Horse-Power Railway Motor. 



267 



Commutator : 

Area of bearing surface of all positive brushes ... ... 4.8 square inches 

Amperes per square inch of brush-bearing surface ... 40 amperes 

Ohms per square inch of bearing surface for carbon brushes ... .03 ohm 

Brush resistance, positive + negative ... ... ... ... .0125 

Yolts drop at brush contacts ... ... ... ... ... 2.4 volts 

C 2 R at brush contacts ... ... ... ... ... ... 460 watts 

Brush pressure, pounds per square inch 2 Ib. 

Total brush pressure ... ... ... ... ... ... 19.2 ,, 

Coefficient of friction ... ... ... ... ... ... .3 

Peripheral speed commutator, feet per minute ... ... 915 

Brush friction ... ... ... ... ... ... ... 120 watts 

Allowance for stray power lost in commutator ... ... 150 ,, 

Total commutator loss ... ... ... ... ... 730 ,, 

Radiating surface 510 square inches 

Watts per square inch of radiating surface 1.43 watts 

EFFICIENCY ESTIMATIONS. Watts. 

Output at rated capacity ... ... ... ... 87,500 

Core loss 2,200 

Commutator and brush loss ... ... ... ... ... 730 

Armature C 2 R loss at 70 deg. Cent 3,100 

Field spool C 2 R loss at 70 deg. Cent 2,200 

Total input 95,730 

Commercial efficiency at rated capacity and 70 deg. Cent. = 91.3 per cent. 

WEIGHTS. Lb. 

Weight of armature laminations ... ... ... ... 1,900 

Total weight of armature copper ... ... ... ... 270 

,, with commutator ... ... ... 3,000 

Total weight of spool copper ... ... ... ... ... 1,300 

,, frame with field coils ... ... ... 9,000 

Total weight of motor ... 1 2,000 

Insulation resistance, measured on 500 volts circuit, was, for the 

average of several motors, 2 megohms from frame to windings of 
armature and field, at 20 deg. Cent., and 30,000 ohms at 70 deg. Cent. 

The results of experimental tests of efficiency, saturation, speed, 
torque, and core loss, are given in Figs. 320 to 323. 



268 



Electric Generators. 




Commutators and Brush Gear. 



269 




270 



Electric Generators, 




FCg.337. 





Fig. 33 9 



Fig 340. 



Commutators and Brush Gear. 



271 



COMMUTATORS AND BRUSH GEAR. 

A number of illustrations of various types of commutators are given 
in Figs. 324 to 340. Figs. 324 to 331 illustrate designs widely employed 
in traction motors, that of Figs. 330 and 331 being used on a 100 horse 
power direct-connected motor, the three former in smaller, geared motors. 




Moore* Im/e&iigajiuonj of ifie Relation* between* 
Rfeiftartcf. of Ccu-boh, brush Contorts and, dtrrent 
Derufity irv Amperes per effuare, Inch, of CantauA Surface, 

Arrangement of Apparatus 
Jieeietance. mea&uredj from, AtoB. 



01 



06 



Of, 



04 



OS 



02 



O1 



I MOORE S RESULTS FOR 

RELATION BETWEEN RESISTANCE 

BRUSH CONTACTS AN D CURRENT DENSITY IN 

AMPERES PER SO.IN. OF CONTACT SURFACE. 



:B OF CARBON 




ZO SO 40 

AMPERES PER SO IN. 



Figs. 332 to 334 give some early designs of Mr. Parshall s, which 
have been much used with general success in many later machines, 
especially traction generators. Other useful modifications and alternative 
designs are shown in Figs. 335 to 340, the last one being employed in a 
1,600-kilowatt generator. 



272 



Electric Generators. 



Commutator segments should preferably be drawn, although good 
results have also been attained with drop-forged segments ; cast segments 
have been generally unsatisfactory. It is not on the score of its superior 
conductivity that wrought-copper segments are necessary, since the loss 
due to the resistance itself is negligible, but it is of primary importance 
that the material shall possess the greatest possible uniformity throughout, 
and freedom from any sort of flaw or inequality. Any such that may 
develop during the life of the segments will render the commutator 
unequal to further thoroughly satisfactory service until turned down or 



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otherwise remedied, as the effect of uneven wear, once started, is cumu 
lative. For similar reasons great care must be exercised in the selection 
of the mica for the insulation between segments ; it should preferably be 
just soft enough to wear at the same rate as the copper, but should in 
no event wear away more slowly, as under such conditions the commutator 
will not continue to present a suitably smooth surface to the brush. 

The writers have found the method of predetermining the com 
mutator losses and heating, set forth briefly on page 112, to give very 
good results, and to amply cover practical determinations. But an 
intelligent handling of the subject of the relations existing between 
commutator speeds, brush pressure, and contact resistance, is facilitated 



Contact Resistance of Brushes. 



273 



by a study of the results of tests that have been made, showing the 
dependence of these values upon various conditions. 

The most complete and careful tests on carbon brushes at present 



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available, appear to be those conducted by Mr. A. H. Moore, in 1898, 
and the results are graphically represented in Figs. 341 to 344. In 
Fig. 341 is given a sketch showing the disposition and nature of the 
parts. A rotating cylinder, A, of 6.8 in. diameter, of cast copper, took the 

2 N 



Electric Generators. 



place of a commutator, and this introduced an element of doubt as to 
whether a segmental structure of hard-drawn copper segments and mica 
would have given the same results. But inasmuch as the constants 
derived from these tests agree with those which have been found to lead 
to correct predictions of the performance of new commutators, it may 
be safely concluded that this point of dissimilarity was of no special 
consequence. In all other respects the tests seem especially good. The 
set of tests also includes values for the resistances of the brush holders, 
but with good designs of brush holders the resistance should be negligible ; 



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CARBON I COPPER BRUSHES. > 

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AMPERES PER SO IN. 



hence it has been deemed advisable not to divert attention from the 
important results relating to contact resistance, by the addition of these 
less useful observed values. 

Mr. E. B. Raymond has, in America, conducted tests on this same 
subject. Some of the results for carbon brushes are shown in the curves 
of Fig. 345, and it will be observed that, for all practical purposes, his 
results, like Mr. Moore s, lead to the general working constants given on 
page 112. 

Dr. E. Arnold, in the Elektrotechnische Zeitschrift, of January 5th, 
1899, page 5, described investigations on both copper and carbon brushes, 



Brush Gear. 



275 




276 



Electric Generators. 




Brush Gear. 277 

from which have been derived the curves set forth in Fig. 346, showing 
the relative values for the contact resistances in the two cases. Dr. Arnold 
also points out that while the coefficient of friction for carbon brushes 
on copper commutators is in the neighbourhood of .3, he has found .2 
to be a more suitable value for copper-gauze brushes. But in the 
absence of thorough tests in support of this, the writers would be 
inclined to continue using a coefficient of .3 for both carbon and copper 
brushes. 

Of course, all values relating to this whole matter of commutator 
losses must necessarily be, in practice, but little better than very roughly 
approximate, as they are so dependent upon the material, quality, and 
adjustment of the brushes, and the condition of their surfaces, as also upon 
the construction, condition, and material of the commutator and brush 
holders, and fully as important as anything else upon the electromagnetic 
properties of the design of the dynamo. 

A collection of designs of brush holders for generators and railway 
motors, are given in Figs. 347 to 365, the first six (Figs. 347 to 352) 
being for use with radial carbon brushes on traction motors, where the 
direction of running is frequently reversed. In Figs. 353 and 354 is 
shown a brush holder which has been used on a 3 horse-power launch 
motor, for reversible running, with carbon brushes. Figs. 355 to 358 
illustrate useful types for generators with carbon brushes, and in Fig. 359 
is shown a holder designed for a copper-gauze brush. 

The Bayliss reaction brush holder, shown in Figs. 360 and 361, is one 
of the latest and most successful developments in brush-holder design. 
Another design, where the holder is constructed largely, of stamped 
parts, is given in Figs. 362 and 363. The holder shown in Figs. 364 
and 365 is essentially a modification of the design represented in 
Fig. 357. 

Of carbon brushes, a wide range of grades have been used, ranging 
from the soft, amorphous, graphite brushes, up to hard, rather crystalline, 
carbon brushes. The latter have the lower specific resistance, 1 a lower 
contact resistance, and a lower coefficient of friction on copper com 
mutators, and are for most cases much to be preferred. Tests made by 

1 Some types of graphite brushes have a lower specific resistance than some types of 
carbon brushes. A great deal depends upon the composition and upon the methods of 
manufacture. By varying these, a wide range of specific resistances may be obtained, both 
for carbon and for graphite brushes. 



278 



Electric Generators. 




Brush Gear 



279 




280 Electric Generators. 

Mr. Raymond, show the extent of these differences between graphite and 
carbon brushes of two representative grades. 

TABLE L. RAYMOND S TESTS ox GRAPHITE AND CARBON BRUSHES. 

Amperes per Square Inch of Ohms per Square Inch of 

Brush-bearing Surface. Brush-bearing Surface. 

Graphite. Carbon. 

10 .075 .048 

L O .045 .035 

30 .033 .026 

40 .027 .022 

50 .022 .019 

60 .019 .017 

70 .017 

80 .015 

The above results were obtained at peripheral speeds in the neighbour 
hood of 2,000 ft. per minute, and with brush pressures of about 1.3 Ib. per 
square inch. 

While the coefficient of friction for carbon brushes is about .3, 
Mr. Raymond obtained the value of .47 for these graphite brushes. 

The specific resistance of a good grade of carbon brush is 2,500 
microhms per cubic inch, i.e., about 4,000 times the resistance of copper. 

Another objection to graphite brushes, at any rate on higher potential 
commutators, say 500 volts, is that they are liable to have their contact 
surface gradually pitted out to a greater extent than occurs with the 
hard-grained, coarser carbon brushes. Nevertheless, the matter of 
obtaining the best commutating conditions for each particular case, still 
remains partly experimental, and graphite brushes have, in certain instances, 
been found helpful, although the commutator surface requires more con 
stant attention to be kept clean and bright ; indeed, with soft graphite 
brushes it is almost impossible to obtain such a hard, glazed commutator 
surface, as with coarser, harder carbon brushes. 

There are very many more varieties of brushes, made of all sorts of 
materials, and giving many intermediate grades of resistances, lying 
between the limits of carbon and copper. It is not worth while to attempt 
to classify and describe these varieties of brushes ; their relative merits are 
dependent partly upon the choice of materials, but still more upon the 
methods of constructing the brush from these materials. Scarcely any 
one type of brush and grade of resistance, is suitable for any considerable 
range of variety of dynamo-electric machine. 



PART II. 



ROTARY CONVERTERS. 



2 o 



ROTARY CONVERTERS. 



A ROTARY converter is, structurally, in many respects similar to 
a continuous-current generator, the chief outward difference consist 
ing in the addition of a number of collector rings, and in the commutator 
being very much larger, in comparison with the dimensions of the rest of 
the machine, than in an ordinary continuous-current dynamo. Under 
the usual conditions of running, the armature is driven, as in a plain 
synchronous motor, by alternating current supplied to the collector rings 
from an external source. Superposed upon this motor current in the 
armature winding, is the generator current, which is delivered from the 
commutator to the external circuit, as continuous current. Occasionally 
rotary converters are used for just the opposite purpose, namely to convert 
continuous into alternating current. With this latter arrangement, how 
ever, some sort of centrifugal cut-off governor should always be used, as 
the reactions on the field strength occasioned by sudden changes in the 
alternating current load, may so weaken the field as to cause dangerous 
increase of speed. But in by far the greater number of cases, the appa 
ratus is employed for transforming from alternating to continuous current. 
The most interesting property of a rotary converter, is the overlapping 
of the motor and generator currents in the armature conductors ; in virtue 
of which, not only may the conductors be of very small cross section for 
a given output, from the thermal standpoint, but, the armature reactions 
also being neutralised, large numbers of conductors may be employed on 
the armature, which permits of a very small flux per pole piece, and a 
correspondingly small cross section of magnetic circuit. But the commu 
tator must be as large as for a continuous-current generator of the same 
output, hence a consistently designed rotary converter should be charac 
terised by a relatively large commutator, and small magnetic system. This 
is best achieved by an armature of fairly large diameter and small axial 
length; and this, furthermore, gives room for the many, though small, 
armature conductors, and for the many poles required for obtaining reason- 



284 



Rotary Converters. 



able speeds at economical periodicities. The mechanical limit imposed by 
centrifugal force, becomes an important factor in the design of the armature 
and commutator of a rotary converter, as compared with continuous-current 
generators. 

In some installations, a good deal has been heard of "surging" troubles 
in operating rotary converters. These were largely due to insufficiently 
uniform angular velocity of the engine driving the Central Station gene 
rators, whose power was ultimately used to operate the rotary converters. 
This lack of uniformity in angular velocity, had the effect of causing cumu 
lative oscillations in the rotary converters, in their efforts to keep perfectly 
in synchronism with the direct-driven generators throughout a revolution. 
This caused especial difficulty when it was attempted to operate several 
rotary converters at different points in parallel. The true solution for 
these difficulties is to have engines of such design as to give uniform 
angular velocity. In describing the proper lines on which to design rotary 
converters, it will be assumed that this condition, as regards the generating 
set, has been complied with ; otherwise it is necessary to employ auxiliary 
devices to counteract such causes, and there results a serious loss in 
economy, through the dissipation of energy in steadying devices. 

TABLE LI. OUTPUT IN TERMS OF OUTPUT OF CONTINUOUS-CURRENT GENERATOR FOR EQUAL 
C 2 R Loss IN ARMATURE CONDUCTORS FOR UNITY POWER FACTOR AND ON THE 
ASSUMPTION OF A CONVERSION EFFICIENCY OF 100 PER CENT. 



Type of 
Rotary Converter. 


Number of 
Collector Rings. 


Uniform Distribution of 
Magnetic Flux over 
Pole-Face Spanning Entire 
Polar Pitch. 


Uniform Distribution of 
Magnetic Flux over 
Surface of Pole-Faces 
Spanning 67 Per Cent, of 
Entire Polar Pitch. 


Single phase 


2 


.85 


.88 


Three 


3 


1.34 


1.38 


Four ,, 


4 


1.64 


1.67 


Six 


6 


1.96 


1.98 


Twelve ,, 


12 


2.24 


2.26 



The extent to which the motor and generator currents neutralise one 

o 

another, and permit of small armature conductors to carry the residual 
current, varies with the number of phases. Table LI. gives the output 
of a rotary converter for a given C 2 li loss in the armature conductors, 




C R Loss in Armature Conductors of Rotary Converters. 



285 



in terms of the output of the same armature when used as a continuous- 
current generator, this latter being taken at 1.00. 

Table LII. shows the extent to which the preceding values have to be 
modified for power factors other than unity. 

TABLE LII. OUTPUT IN TERMS OF OUTPUT OF CONTINUOUS-CURRENT GENERATOR FOR 
EQUAL C 2 R Loss IN ARMATURE CONDUCTORS FOR 100 PER CENT. EFFICIENCY, AND 
FOR UNIFORM GAP DISTRIBUTION OF MAGNETIC FLUX OVER A POLE-FACE SPANNING 
67 PER CENT. OF THE POLAR PITCH. 



Type of 
Rotary Converter. 


Number of 
Collector Rings. 


Power Factor of 


1.00. 


0.90. 


0.80. 


Single phase 


2 


.88 


.81 


.73 


Three ,, 


3 


1.38 


1.28 


1.17 


Four ,, 


4 


1.67 


1.60 


1.44 


Six 


6 


1.98 


1.92 


1.77 


Twelve ,, 


12 


2.26 


2.20 


2.05 



The writers have investigated by graphical and other methods the 
subject of the C~ H loss in the armature of a three-phase rotary converter, 
in comparison with the C 3 R loss for the same load delivered from the 
commutator when the machine is used in the ordinary way as a mechani 
cally driven continuous-current dynamo. Not only are the results of 
considerable value, but a study of the graphical method of investigation 
pursued leads to an understanding of many interesting features of the 
rotary converter. 

As a basis for the analysis, Figs. 366, 367, 368, and 369 were prepared. 
In Fig. 366 are given sine curves of instantaneous current values in the three 
sections of the armature winding (as it would be if the alternating currents 
alone were present), and also the corresponding curves of resultant current 
in the three lines leading to the collector rings. The first three curves are 
lettered a, b, and c, and a current clockwise directed about the delta is 
indicated as positive. The line currents are derived by Kirchhoff s law that 
the sum of the currents from the common junction of several conductors 
must always equal zero. Outwardly directed currents are considered 
positive. These curves of resultant line current are designated in Fig. 366 as 
a-b, b-c, and c-a. Thirteen ordinates, lettered from A to M, divide one com- 



286 



Rotary Converters. 



plete cycle up into 30 deg. sections. In Fig. 367 are given diagrams of line and 
winding currents from each of the ordinates from A to F. The remainder, 
i.e., from G to M, would merely be a repetition of these. An examination 
shows that these six diagrams, so far as relates to current magnitudes, are 
of two kinds, of which A and B are the types. In A, the three current 
values in the windings, are respectively 0, .867 and .867, whilst these 
become in B, .5, .5 and 1.00. Hence it is sufficient for practical purposes 
to study the current distribution in the armature conductors, corresponding 




to positions A and B, and to then calculate the average C 2 R loss for these 
two positions. For this purpose, developed diagrams have been mapped 
out in Figs. 368 and 369, for the winding of a rotary converter, from whose 
commutator 100 amperes at 100 volts are to be delivered from each pair 
(positive and negative), of brushes. The number of poles is immaterial. 
The armature has a multiple-circuit single winding, and it may be assumed 
that there are two conductors per slot, though this assumption is not 
necessary. It was thought best to take a fairly large number of conductors, 
and to take into account, just as it comes, the disturbing influence of the 
brushes, which somewhat modifies the final result. Of course, this 



C*R Loss .in Armature Conductors of Rotary Converters. 287 

disturbing influence would vary with the width of the brushes. Com 
paratively narrow brushes are shown, and this will tend to off-set the 
number of conductors being considerably less than would be taken in 
practice for this voltage. 

The assumption is made that the rotary converter is of 100 per cent, 
efficiency, only calling for an input equal to the output. To supply 
100 amperes to the commutator brushes calls for 50 amperes per conductor, 
so far as the continuous-current end is concerned. This is shown in 




Orb 1-6OO 



b-C t-7SZ 




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(5031 B) 




b-c-o 



C-0=S67 




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b O-t-bOO 



direction and magnitude by arrowheads and figures at the lower ends of 
the vertical lines representing face conductors. 

100 volts and 100 amperes give 10,000 watts per pair of poles. 
Therefore, input per phase = 3330 watts. Volts between collector rings 
= volts per winding = 100 x .615 = 61.5 volts. 1 Amperes per winding 

3330 

TTT-Z = 54 amperes (effective). In this analysis, which considers 

O 1 . 

1 The Estimation of the Electro-Motive Force in Rocary Converters, Tables of Values 
of the Ratio of the Alternating Voltage between Collector Rings to the Continuous-Current 
Voltage at the Commutator, and the Estimation of the Effect of the Pole Face Spread upon 
these Values ; have already been given on pages 84, 85, and 86, in the section on Formulae 
for Electro-Motive Force. 



Rotary Converters. 



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Rotary Converters. 





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C R Loss in Armature Conductors of Rotary Converters. 291 



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of 


IO b- 

of 


IO 

of 








































o co" 

IO Ol 


o 

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1 


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Ol 

1 


co" 

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1 


o" 

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1 


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1 1 


co co 

1 1 


cd co 

Ol Ol 

! 1 


O O 

1 + 


CO 

1 


O CO 
IO Ol 

4- 1 


O CO 
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+ 1 


O co" 
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l-H 

+ 4- 


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10 01 

1 1 

+ + 


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1 + 


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1 + 


o 

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co 

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,=< PH 


b- 

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1 





b- 

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co 
b- 

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co" co 
b- b- 

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1 1 


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1 


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10 

<=> 
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10 10 


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+ + 


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1 1 




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1 




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of 


o 
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of 


b~ 

Ol 




o 


Ol 
b- 
01 


o 01 

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of 


Ol Ol 
b- b- 
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b- b- 


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b- b- 
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b- 


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b- b- 
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o 01 

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(^^ C"-l 
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<=> ^ ^ 

OS II 43 


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co co 


CO 


co co 


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o 




1 1 


+ 


1 


+ 


1 


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1 1 


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1 1 


1 


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4- 1 


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+ 1 


1 4- 


1 + 


1 


O w S-, 

Q IO hi M 


10 




IO 




10 


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IO 


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10 


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1 





co 

co 

1 





1 


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1 


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1 1 


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IO 


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4- + 


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292 



Rotary Converters. 





-(^113.1.1113) 


oooooooooooooooooooo 
oooooooooooooooooooo 
t^ co t^ co t^ ?r> t^ > t^ to t^ <x> t^ o i- o t^ o i>- o 

o" o" o" cT o" o" co" to" crT to" 








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^tiajjnQ 


1C 10 1C >O IO 1O lOlOiOiOiOlOiOlO 

to co to co to co to co to co to eo to to to to to to cd to 
C<IO<MOC^O<NOC 5 IO<NOC i IC<IC^<MdC^C<IC^ 


oj 

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f^d 

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^TTBijnsay; 


+ + + + + + + + + + + + + 1 + 1 + 1 + 1 




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P 


009 -f- <* 
-ati3 gai^muoHv 


10 m io 10 10 10 lomioioioioioio 
tocoto coto cotoeoto eotoectototototdtcStoto 

t^lOt~>Ot~lOt^lOt^IOt^iOt^t^l^l>.t^l>-l>.l>. 

1 1 I-H ! I I 1 I 1 I 1 

+ + + + + + + + + + + + + 1 + 1 + 1 + 1 




S 


JO^OIBJ J3AVOJ 


ooiccoiocoiocoiocoiocoiocoeocoeocoeococo 


1 

3 
o 


Sauepisuoo 
jon ^u^j.n\3 
Sni^ BUJa^iv 


OOtOOOtDOOtOOOtOOOtOCOtOOOOOOOCOOOOOGOGO 

co t w t co .t co t eo fr eo t co eo eo co eo eo eo erj 

+ + + + + + + + + + + + + 1 + 1 + 1 + 1 




^uaaanQ 
sncmui^uoQ 


oooooooooooooooooooo 

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1 1 1 1 1 1 1 1 1 I 1 ! 1 1 1 1 1 1 1 1 




5 (^u9jjno) 


lOOiOOiOOiOOiOOiOOiOOiOOOOxOO 

coioco>ocoioooiocoocoiocoiocoioeoocoo 
^^^-TH^-^-^ITHOSOI 

i 1 i-H i 1 i i i I i i i I i I OO CO 


o 




OC^CiC-TCiC-TCSC-lOC^asCMCSfMOSC-TCl^-lOir i 


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ims^psa^j 


IOOOIOOOIOOOIOOOIOOOOCOIOOOIOCOO^IO-* 

cococococococoooaso 

I + I + I+ + +I + I + I + I + I 1 1 1 


1" 




i 1 (M i I (M i I (M i 1 (M t I <M i iC^li iCTi iCTi ii li li I 


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JUQ Bup BUja^y 


^co^oo-^oo^cso-^oo^co^oo^co-*^^-* 

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+ + + + + + + + + + + + + + + + + 1 + 1 


13 


JO^OB^ .laAvoj 


coxocovoeoiocoiocokocoiocoiocoioeoo scoco 


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+ + + + + + + + + + + + + + + + + + + 1 




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snonui^uoQ 


oooooooooooooooooooo 

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1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 III 




-(^uajjno) 


t^O*Ot~Ot-Ol^Ot~Ol~Ot^Ob-Ot^O 

coocoocoocoocoocoocoocoocooeoo 

i 1 CO i (OOr It- i 1 t i 1 t>- i 1 t- i 1 t>- i ItI It-i It-- 

t^ t^ 






t^eOt^COt^Ob-lOt^lO^-iCt^lOI>-iOl>.lOl^lO 




1 

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i I OO i 1 OO I-H C^l i I <M i I CM i I C-4 r 1 (M i t C-l i 1 (M i ( !M 

II + II + I+ +1 + 1 + 1+ +1 + 


2 




cococococoxoeoiocomcoiocoiocoiocoioeoio 


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c 

3 

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npi3UJ8 ; [Y 


COCOOOCXJCOtOCOtOOOtOOOtOOOtOOOtDCOtOCOtO 

cococococot cot~cot^cotcot^cotcot>cofe 

+ \ + ! + + + + + + + + + + + + + + + + 




^ugjanQ 
snonupuoQ 


oooooooooooooooooooo 

lOlOlOiO OlOlOldlOiO OlOlOlOlOlOIOOlOIO 

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 III 




jo^onp 


i ((MCO^lOtOt-OOOOt l(MeO"*lOtOt GOO5O 









H! 

s 
^ 



C-R Loss in Armature Conductors 

oooooooooooooooooo 
oooooooooooooooooo 

O O O^ t~ CO^ t CO t CO l^ CO t^ CO t CO l^ O l^ 

i-T co" -<" o" o" o" o" o" o" co" 


of 

o 



o 
co" 


Rotary 

000 

o o o 

I- O t- 

co" 


ClfM^s. 
rH N 

Converters. 293 

o o o o o o 

O O O O O 
^^ t^ ^^ <^2 ^^ co 

co" co" I-H" CO" O" 














































? 

^ o 

l-H ^ 

10 a! 

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a) .^t 

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CO CO CO CO CO 
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lO 

CO 
CM 


CO 



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<M 


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o 


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co" 

(M 


co 
o 


CO 
IO 
<M 


IO 

CO CO 
O CN 


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o 


IO 


i 


co 


iO 
CO 


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CO CO 


10 

co 


co 


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co 


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co 


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o 


co 


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o 

I-H 


1 1 1 1 1 


1 


1 


1 


1 


1 


1 


1 


1 1 


1 


1 


4 


1 


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1 4 


1 


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1 


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10 10 

co co co co eo 

I-H ^H I-H 

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1 


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1 


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t 


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I-H 


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CO 


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CO 


IO 


co 


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co 


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co 


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co 


CO 


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CO 


CO 


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1 + 1 1 1 


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1 


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1 


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1 


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1 


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1 


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co 


00 
co 


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o o o o o 

IO IO IO IO lO 

1 1 1 + + 



o 




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CO O CO CO IO 

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CO 


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10 O 
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00 


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co 


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. 43 

o c 

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s 

II 10 

(3 _w 


r-4 O> <N 





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1 1 


IO "* IO lO OO 

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IO 


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CO 


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CO 

CO 


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co 


10 

1 


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1 


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os 


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1 














































4- 1 + 1 1 


1 


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GO 

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co 


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CO CO CO CO t^- 

+ 1 + 1 1 


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co 
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1 1 1 4 4 


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c -< 

t-i r*j 
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t- CO t- t~ CO 


^ 


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t^ 


10 


t. 


10 t- 


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^ 


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10 


t- 10 


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IO 


t^ 


co 


t^ 


CO 


CO 


I-H CO I-H 1C CO 
^H CO I-H I-H 00 

1 1 1 4 4 


I-H 


CC 
CO 


I-H 
r-H 


co 

(M 


- 


CO 


I-H 


CO I-H 
<M l-H 


CO 
(M 

1 


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co 

01 


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4 


CO 
CN 

1 


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l-H OJ 

4 1 


I-H 

4 


CO 
<M 

1 


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CO 


1-1 


00 
CO 


GO 
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1 


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CO 


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10 


CO 


IO 


CO 


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IO 


CO 


IO 


co 


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co 10 


co 


IO 


co 


co 


co 


co 


CO 


GO CO 00 GO 00 

co co co co co 
4141 + 


CO 

co 


00 

co 


CO 

co 


CO 

l^ 


. CO 

eo 


CO 

l^ 


CO 

eo 


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t-- co 

1 1 


CO 

t 

1 


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CO 

1 


co 

1 


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1 


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1 


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I 1 


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co 

1 


co 

1 


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co 

1 


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co 
4 


00 

co 
1 


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co 


00 
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1 


o o o o o 
1 I 1 4 4 








o 


o 





o 


o 







4 




4- 


o 

lO 


o 

10 

+ 


o 

IO 

4 



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o 

lO 


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IO 


o 

IO 


o 

IO 







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o 

10 

1 


I-H d CO "^ IO 
d Ol O-l C^l d 


<M 


d 


<M 


<M 


co 


CO 


CO 


co eo 


eo 


CO 


CO 


00 
eo 


CO 


O I-H 


<M 


co 


-* 


iO 


co 


t~ 


GO 



294 Rotary Converters. 

instantaneous values, a sine wave current curve has been assumed, working 
from the maximum value of 54 x ^2 = 76.5 amperes. 

When the current is in phase with the electromotive force, the 
distribution of things for positions A and B respectively, is as shown 
in the diagrams of Figs. 368 and 369. There are 48 conductors, correspond 
ing to two poles, and these are numbered from 1 to 48. Any 48 successive 
conductors will give the same result. The values and arrowheads at the 
upper part of the lines representing the face conductors, give the 
instantaneous values and directions of the currents corresponding to 
the instantaneous conditions. The figures and arrowheads at the middle 
of these lines give the instantaneous values and directions of the resultant 
currents. These results are also given in Tables LIU. and LIV., where a 
current from bottom to top is regarded as positive, and from top to bottom, 
as negative. There are also given values for lagging currents, the results 
from which show a rapid rise in C 2 R loss. 

These results are summed up in Table LV., the figures given being 
the average for positions A and B : 

TABLE LV. PER CENT. THAT ARMATURE C 2 R Loss is OP THAT OF SAME ARMATURE IN A 
CONTINUOUS-CURRENT GENERATOR FOR THE SAME OUTPUT, ASSUMING 100 PER CENT. 
CONVERSION EFFICIENCY. 

Power Factor. Per Cent. 

1.00 58 

.87 85 

.50 ... ... 375 

oo 

Some indefiniteness is introduced by the exact position and width of 
the brushes under the condition of power factor of unity, the results for 
this value being higher, in proportion as the number of conductors per 
pole is low. But for the other values of the power factor, this indefiniteness 
does not appear. It will be noted that, just before reaching the position 
of short-circuit under the brush, the current is often the sum of the 
alternating and continuous currents. 

Throwing the results into the above form, brings out forcibly the fact 
that it is only for comparatively high-power factors that the residual C 2 R 
loss is so greatly decreased. 



Windings for Single-Phase Rotaries. 



295 



SINGLE-PHASE ROTARY CONVERTERS. 

The winding is connected up to the commutator segments, exactly as 
for an ordinary continuous-current dynamo. For the alternating-current 
connections the winding is tapped, for a two-circuit winding, at some one 
point, to one collector ring. Then after tracing through one-half of the 
armature conductors, a tap is carried to the other collector ring. This case 



Fig 370 




WINDING FOR A SINGLE-PHASE ROTARY CONVERTER. Two-CiucuiT SINGLE WINDING WITH 

64 CONDUCTORS, Six POLES, PITCH 11. 



of a two-circuit single winding, connected up as a single-phase rotary 
converter, is illustrated in the winding diagram of Fig. 370, which relates 
to a six-pole armature with 64 conductors. 

In Fig. 371 is given a diagram for a six-pole single-phase rotary 
converter, w r ith a t\vo-circuit singly re-entrant triple winding. This 
winding has 72 conductors. Single-phase rotary converters, with two- 




WINDING FOR A SINGLE-PHASE ROTARY CONVERTER. TWO-CIRCUIT SINGLY RE-ENTRANT 
TRIPLE WINDING WITH 72 CONDUCTORS, Six POLES, PITCH 11. 




WINDING FOR A SINGLE-PHASE ROTARY CONVERTER. TWO-CIRCUIT SINGLE WINDING WITH 
72 CONDUCTORS, Six POLES, FRONT PITCH 13, BACK PITCH 11. 



Advantages of Polyphase over Single-Phase Rotaries. 297 

circuit multiple windings, have two taps per winding, hence the two-circuit 
triple winding of Fig. 371 has 2x3-6 equi-distant taps. 

In Fig. 372 a six-circuit single winding, also with 72 conductors, is 
connected up as a single-phase rotary converter. For such a winding 



Fig. 31 3. 




"\ \ *^ N,- "* \ 



"X x*v - 

S ""N^ r ^-_-^ 



r^tL /y^X x x N - *--\ \ > 

/ / /"/-y^../^^ -^^- ^ \,^ \ \ X -V * 1 

/ f / sTs^ r^ n : I I 1 / > s!0\\ \ \ * 




WINDING FOR A THREE-PHASE ROTARY CONVERTER. Six-CmcuiT SINGLE WINDING WITH 
108 CONDUCTORS, Six POLES, FRONT PITCH 19, BACK PITCH 17. 



there are two taps per pair of poles, hence six taps in all, the winding- 
being divided up into six equal sections of 12 conductors each. 

In single-phase rotary converters, the overlapping of the commutator 
and collector-ring currents is so much less complete than for multiphase, 
as shown already on pages 284, 285, Tables LI. and LIL, as to render their 

2 Q 



298 



Rotary Converters. 



use very uneconomical, because of the reduced output in a given machine. 
There is the further disadvantage that a single- phase rotary cannot be 
run up to synchronism from the alternating-current side. In general, the 
operation of single-phase rotary converters is distinctly unsatisfactory, and 



, / \ 

4 / v x ^ \ 

^^^ / --../ "v/\ .*- 




WINDING FOR A THREE-PHASE ROTARY CONVERTER. TWO-CIRCUIT SINGLE WINDING WITH 
90 CONDUCTORS, EIGHT POLES, PITCH 11. 

they are rarely used except for small capacities. An examination of the 
windings shows that, due to the distribution of the conductors over the 
entire peripheral surface, the turns in series between collector rings are 
never simultaneously linked with the entire magnetic flux ; in fact, such 
a winding used as a pure alternating current single-phase generator, gives 



Advantages of Polyphase over Single-Phase Rotaries. 299 

but 71 per cent, as great a voltage at the collector rings as the same 
machine used as a continuous-current dynamo would give at the 
commutator. 1 The ratio of the outputs, under such conditions, is for 
equal loads in the armature conductors, 71:100. It will be seen in the 
following that this is largely avoided when the winding is subdivided for 




WINDING FOR A THREE-PHASE ROTARY CONVERTER. TWO-CIRCUIT SINGLY RE-ENTRANT 
TRIPLE WINDING WITH 108 CONDUCTORS, Six POLES, PITCH 17. 

polyphase connections, and the relative advantages of these different 
polyphase systems is largely dependent upon the extent to which they are 
free from this objection. 

1 A discussion of the ratio of commutator and collector-ring voltages in rotary converters 
has already been given on pages 84 to 86, in the section relating to Formulae for Electro 
motive Force. 



300 



Rotary Converters. 
THREE-PHASE ROTARY CONVERTERS. 



The earlier rotaries were generally operated as three -phasers, the 
output for a given C 2 R loss in the armature winding being 38 per cent, 
greater than for the same armature as used in a continuous-current 



Fig. 31 6 




WINDING FOR A SIX-PHASE ROTARY CONVERTER. Six-CmcuiT SINGLE WINDING WITH 108 
CONDUCTORS, Six POLES, PITCH, FRONT 19, BACK 17. 

generator. To-day, however, most rotaries are being arranged to be 
operated either as four or six-phasers, with the still further advantages 
of 67 per cent, and 98 per cent, increased output respectively, for a given 
heating in the armature conductors. These are the values given in 
Table LI. 



Three- Phase Rotaries. 



301 



For three-phase rotary converters, there are three sections per pair 
of poles in multiple-circuit single windings, and three sections per pair 
of poles per winding in multiple-circuit multiple windings. There are 
three sections per winding, regardless of the number of pairs of poles 



/\/\/\ 





^ \ x> y> / 
V \/ V 

5034 H V 



WINDING FOR A Six PHASE ROTARY CONVERTER. TWO-CIRCUIT SINGLE WINDING WITH 90 

CONDUCTORS, EIGHT POLES, PITCH 11. 

in two-circuit windings. Thus, a six-pole machine, with a six-circuit 
triple \vinding,would have !j x 3 = 9 sections. At equal ninths through 
the winding from beginning to end, leads would be carried to collector 
rings, three leads to each of the three collector rings. But if 
the armature had had a two-circuit double winding, there would have 



302 



Rotary Converters. 



been but three sections per winding, regardless of the number of poles ; 
hence, for this two-circuit double winding there would be 2 x 3 = 6 
sections, and six leads to the three collector rings. In Figs. 373, 374 and 
375 are given diagrams of three-phase rotary converter windings, from a 




WINDING FOR A SIX-PHASE ROTARY CONVERTER. Two CIRCUIT SINGLY RE-ENTRANT TRIPLE 
WINDING WITH 108 CONDUCTORS, Six POLES, PITCH 17. 

study of which familiarity with the inherent characteristics of such windings 
may be obtained. The most distinctive characteristic is the overlapping 
distribution of the conductors of the three phases, in consequence of 
which any one portion of the periphery of the armature carries conductors 
belonging to two phases. At one portion, the conductors will belong 
alternately to phases 1 and 2, then to 2 and 3, and then to 3 and 1, then 



Six-Phase Rotaries. 



303 



again to 1 and 2, the repetition occurring once per pair of poles. As a 
consequence of this property, the conductors of any one phase are 
distributed over two-thirds of the entire periphery, and when the width 
of the magnetic flux exceeds one-third of the polar pitch and it is 
generally, when spreading is considered, at least three-quarters of the 
polar pitch all the turns of one phase will not be simultaneously linked 
with the entire flux, and the consequence is a lower alternating-current 
voltage per phase than if simultaneous linkage of all the turns of one 
phase with the entire flux occurred. Hence, for a given heating, the output 
is limited, although already, because of more effective linkage of turns 
and flux, 56 per cent, higher than for single-phase rotaries. 



SIX-PHASE ROTARY CONVERTER. 

This disadvantage is mainly overcome in the so-called six-phase rotary 
converter, in which as will appear later the conductors of any one 



THREE PHASE 




phase are distributed over only one-third of the entire periphery, as a 
result of which an almost simultaneous linkage of all the turns of one 
phase, with the entire magnetic flux, is obtained. The resultant output 
of such a machine, for a given heating of the armature conductors, 
increases, as stated in Table LI. on page 284, in the ratio of 1.38 to 1.98, 
i.e., by 44 per cent, beyond that of an ordinary three-phase machine. As 
a matter of fact, this so-called six-phase is only a special case of three- 
phase arrangement. This distinction will be subsequently made clear. 

Figs. 376, 377, and 378 are the same winding diagrams as for Figs. 373, 
374, and 375 (pages 297, 298, and 299), but with the connections made for 
so-called " six-phase," with six collector rings. This requires in each case 
subdividing the winding up into just twice as many sections as for the 
case of three-phase windings. A study of these windings will show that 



304 



Rotary Converters. 



with these connections with six sections (where before there were three), 
the first and fourth, second and fifth, and third and sixth, taken in pairs, 
give a distribution of the conductors, suitable for a three-phase winding, 
each of the above pairs constituting a phase. Furthermore, each portion 
of the periphery is now occupied exclusively by conductors belonging to 
one phase, i.e., the first and fourth groups, the second and fifth, or the 
third and sixth, and in this way is distinguished from the previously 
described three-phase windings in which the phases overlapped. 

This distinction will be made more clear by a study of the diagrams 



o-iven in Fie. 371). 

o o 




\ HIGH TENSION 
( BUSBARS 



HIGH TENSION SWITCHES 



fig. 380. 



LOW TENSION SWITCHES 



\TO THE COLLECTOR 
I RINGS 



INTERCONNECTION OF STATIC TRANSFORMERS AND ROTARY CONVERTERS. 

For three-phase rotary converters, the transformers should preferably 
be connected in " delta," as this permits the system to be operated with 
two transformers in case the third has to be cut out of circuit temporarily 
for repairs. 

A satisfactory method of connection is given in Fig. 380. 

For six-phase rotary converters, either of two arrangements will be 
satisfactory. One may be denoted as the " double delta " connection, and 
the other as the " diametrical " connection. Let the winding be represented 
by a circle (Fig. 381), and let the six equidistant points on the circumference 
represent collector rings, then the secondaries of the transformers may be 
connected up to the collector rings in a " double delta/ as in the first 
diagram, or across diametrical pairs of points as in the second diagram. 
In the first case it is necessary that each of the three transformers have 



Six- Pli ase Rotaries. 



305 



two independent secondary coils, as A and A 1 , B and B l , C and C 1 , whereas 
in the second case there is need for but one secondary coil per transformer. 
The two diagrams (Fig. 382) make this clear. 

In the first case, the ratio of collector ring to commutator voltage is 
the same as for a three-phase rotary converter, it simply consisting of two 
"delta" systems. In the second case, the ratio is the same as for a 
single-phase rotary converter, it being analogous to three such systems. 



Fig 381 





"DOUBLE- BELT*. CONNECTION. 



"DIAMETRICAL CONNECTION. 



DOUBLE-DELTA CONNECTION 



DIAMETRICAL CONNECTION 



m 




mm mtw 



TO 6 COLLECTOR RINGS. 



Style of Connection for .Six- Phase 
Rotary Converter. 

Double-delta connection 
Diametrical 



TO 6 COLLECTOR RINGS. 



TABLE LVI. 



Ratio of Collector Ring 

Voltage to 
Commutator Voltage. 

.612 

.707 



The latter the "diametrical " connection, is, on the whole, to be 
preferred. The higher voltage at the collector rings, permits of carrying 
lighter cables about the station in wiring up from the static transformers 
to the rotary converter. It also only requires two secondary leads to be 
brought out per transformer and it simplifies the switching arrangements. 

2 R 



306 Rotary Converters. 

A switchboard connection suitable for a plant with four, six-phase 
rotary converters is given in Fig. 383, where it is arranged that the 
synchronising shall be done on the high-tension side of the transformer. 
This method of synchronising avoids the necessity of six-bladed, heavy 
current, low-tension switches. The switches A and B are more for the 
purpose of connectors ; the line circuits are intended to be made and 
broken by the high-tension, quick-break switches C. Another feature of 
the arrangement shown, is that it brings the entire alternating-current 
system to the left of the line L, and the entire continuous-current system 
to the right of the line L, thus keeping them entirely separate. The 
particular scheme shown, has two independent sets of high-tension feeders 
coming to the two feeder panels shown. 

In conclusion, it may be said that six-phase rotary converters have, 
in practice, been found to run stably, and have been free from surging and 
flashing. The six collector rings can hardly be said to constitute any 
serious disadvantage, and there is the already explained gain of 44 per 
cent, in output from the standpoint of the heating of the armature 
conductors. This latter is, of course, an important advantage ; but it 
must be kept in mind that this gain does not apply to the commutator, 
which must be for a given output just as large for a six-phase rotary 
as for a three-phaser. 



FOUR- PHASE ROTARY CONVERTERS. 

In Fig. 384 is given a six-circuit single winding connected up as a 
four-phase rotary converter. Here we subdivide the winding into four 
sections per pair of poles hence in this case 4 x = 12 total sections, 
and four collector ring s. 

o 

A two-circuit single winding connected up for a four-phase rotary 
converter, is shown in Fig. 385. It is subdivided into four sections; the 
rule for two-circuit windings used as four-phase rotary converters, being 
that they shall have four sections per winding, independent of the number 
of poles. Hence, in the two-circuit triple winding shown in Fig. 386, the 
winding is subdivided into 4 x 3 = 12 sections. All these four-phase 
windings are characterised by the winding per phase having a spread of 
50 per cent, of the polar pitch. Sections 1 and 3, as also 2 and 4, are 
really in the same phase, in this sense such rotary converters are sometimes 



Switchboard for Six-Phase Rotaries. 

JfflOaHOJ.1 



307 




I 



308 



Rotary Converters. 



called two-phase, also occasionally quarter-phase. The distribution is also 
well shown in Fig. 387. 

There are also in four-phase, as in six-phase, alternative methods of 



Fig.384 




WINDING FOR A FOUR-PHASE ROTARY CONVERTER. SIX-CIRCUIT SINGLE WINDING, WITH 
96 CONDUCTORS, Six POLES, PITCH 17 AND 15. 

connecting from secondary transformer terminals to collector rings. The 
diametrical connection is to be preferred, and for the same reasons as in 
the case of six-phase. 



Twelve-Phase Rotary Converter. 



TWELVE-PHASE ROTARY CONVERTERS. 



309 



Another interesting combination of apparatus permits of obtaining 
the advantages of a 12-phase rotary converter with only two static 
transformers. Each transformer has one primary and four equal secondary 




WINDING FOR A FOUR-PHASE ROTARY CONVERTER. T\VO-CIRCUIT SINGLE WINDING, AVITII 
80 CONDUCTORS, Six POLES, PITCH 13. 

coils. The primaries are excited from two circuits in quadrature with each 
other, and there are twelve tappings into the armature per pair of poles in 
a multiple-circuit winding, and twelve tappings per winding, independently 
of the number of poles in two-circuit windings. The diagram, Fig. 388, 



310 



Rotary Converters. 



sets forth the underlying idea as applied to a bi-polar armature, the circle 
representing the winding, tapped at the points 1 to 12. Transformers I. 




WINDING FOR A FOUR-PHASE ROTARY CONVERTER. Two CIRCUIT TRIPLE-WINDING, WITH 
96 CONDUCTORS, Six POLES, PITCH 17. 

and II. have their primaries connected to circuits in quadrature with each 
other. 

The 60 deg. chords represent the transformer secondaries 11-9, 3-5, 



fig 387 



FOUR PHASE. 



n 



a s s BJ 





12-2, and 8-6, while the two diameters represent the series-connected 
pairs of secondaries 1-7 and 10-4. Obviously the whole idea is based on 



Twelve-Phase Rotary Converter. 



;;i I 



two inscribed hexagons, the one standing at an angle of 90 deg. from the 
other. The four equally-wound secondary coils conform to the equality 
requirement between sides and radii. 

By letting the transformer primaries have different windings, the 
well-known method of changing from three to quarter-phase permits of 
retaining the greater economy and other advantages of three-phase 



JFig.388 







pain A av n 





TO 8 COLLECTOR 
KINGS 



CONT 
UHRNT 
fCUIT 




I 


ROTARY 




1. 





.0 


I 


E 


C 





i; 


i 


:. 


SS 








CONVERTER 






























AK"/.TL K. 


























J 




LEADS TO 6 COLLECTOR ff/NGS 



transmission, and these further advantages of only two transformers per 
rotary, and greatly increased output per rotary. This system is sufficiently 
indicated in diagram, Fig. 389. 

DESIGN OF A SIX-PHASE 400-KiLOWATT, 25-CvcLE, 600-VoLT ROTARY 

CONVERTER. 

The first question to decide is the number of poles. The periodicity 
being given, the speed will be inversely as the number of poles. High 
speed, and hence as few poles as are consistent with good constants, will 
generally lead to the best results for a given amount of material. 



312 Rotary Converters. 

In considering the design of continuous-current generators, it was 
shown that the minimum permissible number of poles is determined by 
the limiting armature interference expressed in armature ampere turns per 
pole-piece, and by the reactance voltage per commutator segment, for 
which, in the very first steps of the design, the average voltage per 
commutator segment is taken. But in polyphase rotary converters, the 
superposed motor and generator currents leave a very small resultant 
current in the armature conductors, and in six-phase rotary converters 
this is so small that armature interference would not be a limiting 
consideration ; in fact, as many turns per pole-piece will be used on the 
armature as other considerations, first among which is that of permissible 
peripheral speed, shall determine. As the motor and generator currents 
cancel each other to a very considerable extent, the conductors have only 
to be of relatively small cross-section in order to carry the resultant 
current ; nevertheless, by the time each conductor is separately insulated, 
no extraordinarily large number can be arranged on a given periphery, 
and hence no excessive armature interference can result. With insuffi 
ciently uniform angular velocity per revolution of the generator supplying 
the rotary converter, this assertion could not safely be made. In such a 
case, the pulsations of the motor component of the rotary converter current, 
caused by the inability of the rotary converter to keep in perfect step with 
the generator, and by the consequent oscillatory motion superposed upon 
its uniform rate of revolution, greatly decrease the extent to which the 
motor and generator components neutralise one another, and hence results 
a large and oscillatory armature interference. But where a satisfactory 
generating set is provided, armature interference in the rotary converter 
is not a limiting consideration. 

The reactance voltage of the coil under commutation, must be made 
as low as possible, as one has, in rotary converters, a kind of " forced " 
commutation," that is, one does not make use of a magnetic field to 
reverse the current in the short-circuited coil. The brushes remain at 
the neutral point for all loads, since any alteration in their position 
from the neutral point would interfere with the proper superposition 
of the collector ring and commutator currents. Moreover, the collector 
ring current must continue independently of the commutation going 
on in the generator component of the resultant current. The process 
is complicated, and for practical purposes it appears desirable to estimate 
a nominal reactance voltage based upon that which would be set up in 



Six-Phase, Four-Hundred Kilowatt Rotary Converter. 313 

the short-circuited turns by the reversal of the continuous-current 
component. 

The diameter of the armature is chosen as large as is consistent with 
retaining the armature conductors in place, using a reasonable amount of 
binding wire, figured with a conservative factor of safety. Upon this 
armature is generally placed as large a number of conductors as current 




and magnetic flux densities permit. For some ratings, however, a 
sufficiently low reactance voltage may be obtained without approaching 
extremes, either of armature diameter or of number of armature conductors. 
Another limitation often met with in rotary converter design, is that of 
width of commutator segment at the commutator face. It is not desirable, 
on machines of several hundred kilowatts output, that the commutator 
segments should be much less than J in. in width. For a given diameter 
and number of poles, this at once restricts the number of commutator 
segments, and, on the basis of one turn per commutator segment, also 

2 s 



314 



Rotary Converters. 



restricts the number of armature turns. For large rotary converters, 
two turns per segment would almost always lead to an undesirably high 
reactance voltage of the coil being commutated. 

The speed, expressed in revolutions per minute, is, in rotary 
converters, generally two or three times as high as for good continuous- 
current generators of the same output, and with an equal number of 




poles. Hence the frequency of commutation is also very high, often from 
600 to 1000 complete cycles per second. Consequently the inductance of 
the short-circuited coil must be correspondingly low, in order not to lead to 
high reactance voltage. 

Rotary converters have been built with two commutators, to escape 
the limitations referred to, of high peripheral speed, and narrow com 
mutator segments. This method is rather unsatisfactory, since the chief 
gain would be in connecting the two commutators in series ; but by so 



Six-Phase, Four-Hundred Kilowatt Rotary Converter. 315 

doing, the entire current output has to pass through both, and the 
commutator losses are thereby doubled, while the cost of each commutator 
is so slightly reduced below that of one, as to render the construction 
expensive. A parallel connection of the two commutators at once sacrifices 
the chief gain, there only remaining the advantage of commutating but 
half the current at each set of brushes; but this will not permit of very 
great reduction of the number of segments. Moreover, there is the 
further difficulty that unequal contact resistance at the brushes would 
bring about an unequal division of the load between the two windings. 

In smaller rotary converters, it sometimes becomes practicable to 
employ multiple windings (i.e., double, or occasionally even triple). In 
such cases, the tendency to increase the frequency of commutation must 





FULL SIZE DETAIL OF SLOT. 



not be overlooked. If, for instance, one uses a double winding, the 
calculation of the time during which one armature coil is short-circuited, 
must be made with due regard to the fact that the two terminals of this 

o 

coil are connected, not to adjacent but to alternative segments, and the 
intervening segment is, so far as time of short circuit is concerned, to be 
considered as a wide insulating gap. Hence, for a given width of brush, 
the time of short circuit is considerably reduced ; but as the number ot 
paths through the armature from the positive to the negative brushes has 
been doubled, the current to be reversed is half what it would be for the 
equivalent single winding. No general conclusions, however, should be 
drawn, and the reactance voltage must be estimated for each particular 
case, from the inductance of the coil, the frequency of its reversal under 
the brush, and the current to be reversed. 



316 



Rotary Converters. 



In a similar manner, if one were comparing the relative advantages 
of, say, four and six poles, one should keep distinctly in mind that while 
the final effect on the frequency of reversal may not be great (because of 
the inverse change in speed), the inductance per turn (largely dependent 
upon the length of the armature), may be quite different, and that the 
current to be reversed, is, in the case of the larger number ol poles, less 
than in the machine with few poles. It is much safer to make rather 
complete comparative calculations, as the probability of overlooking the 



SATURATION CURVE 

400K.W. 25 CYCLES. 600 VOLTS. 

JKg.<394 Rotary Converter. 



TOO 
C50 
600 
550 
BOO 
160 
WO 
350 
3OO 
Z50 
ZOO 
ISO 
WO 
SO 






















^ 


^ 




















/ 


/ 






















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/ 
















g 








/ 


/ 
















s 








/ 


















1 






/ 




















1 

? 




-} 


/ 
















_,* 


s 


Ov 




/ 














_s 


s 






^ 

1 


/ 


/ 












^ 


\s^ 






1 




/ 










^ 


^ 










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> 


/ 


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^ 


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<o 
It 


7 




^ 


*s 


















1 


^ 


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J 


% 

T 




50 600 700 &?<V 9000 
AMPERE TURNS. 



!< 30 



EFFICIENCY AND LOSSES. 

4-OOK.W. 25 CYCLES. 600VOLTS. 
Rotary Converter. 



iearinq & ail BrusfiTnccion + Windage 




E" 
i 

O e 



100 2OO 300 WO 500 600 TOO 8OO 
(it7f> AMPERES OUTPUT 



effect of a certain change, on all the constants involved, is very 
considerable. 

As a general rule, it is preferable to arrange the conductors in many 
slots, thus having but few per slot. It is also necessary to keep as small 
as possible, the width of slot opening, and it should not be much, if any, 
greater than the radial depth of the air gap. This is important, because 
laminated pole-faces should not be used where there is the least possibility 
of " surging," due to inconstant angular velocity per revolution of the 
generating set. Where, with laminated pole-pieces this " surging " is 
present to any extent, it will be diminished, and sometimes prevented, if 
solid pole-faces of good conductivity, such as wrought-iron forgings of 



Six-PJiase, Four-Hundred Kilowatt Rotary Converter. 317 

good quality, are used. The tendency of the superposed oscillations of 
the armature, and the consequently varying magnetic field, is to set up 
induced currents in this pole-face, which react, and in turn tend to check 
these oscillations. This may be accomplished with minimum loss of energy, 
by suitably arranged copper circuits ; but under favourable conditions, the 
surging will be of small extent, and may be made negligible with but little 
dissipation of energy in the wrought-iron pole-faces. The magnet cores 
may be of cast steel, but this has not so high specific conductivity as the 
best wrought iron, which latter should be employed for the pole-faces. 
The prevention of the surging will also be more complete, the shorter the 
air gap, but the high speeds of rotary converters generally render very 
small clearances undesirable. 

Given the output, periodicity, and the voltage, trial calculations made 
with the foregoing various considerations in mind, lead one very definitely 
to the choice of a certain number of poles and the corresponding speed, 
best combining good constants in operation with economy in material. At 
most, the choice will lie between two successive numbers of pairs of poles, 
in which case both designs should be thoroughly worked out, and the 
constants and cost compared. 

For a six-phase rotary converter for 400 kilowatts output at 25 cycles, 
and 600 volts at commutator, the following design is worked out. The 
number of poles is eight, and the speed is 375 revolutions per minute. 
A good design with six poles and 500 revolutions per minute could have 
been obtained, and excellent practice in the application of these principles 
would be found in working out a corresponding specification for such a 
machine, and then making a comparison of the costs of material. 

The eight-pole design is illustrated in Figs. 390 to 393, inclusive, and 
in Figs. 394 and 395 are given the estimated saturation and efficiency 
curves. 

TABULATED CALCULATION AND SPECIFICATION FOR A 400-KiLOWATT SIX- 
PHASE ROTARY CONVERTER. 

DESCRIPTION. 

Q 

Number of poles ... 

Kilowatt output . . . 

Speed, revolutions per minute ... 

Terminal volts, full load ... 

Amperes ... 

Frequency (cycles per second) ... 



318 Rotary Converters. 

DIMENSIONS. 

Armature : 

Diameter over all ... ... ... ... ... ... ... 58 in 

Length over conductors ... ... ... ... ... ... 29 ,, 

Diameter of core at periphery ... ... ... ... ... 58 ,, 

,, ,, bottom of slots ... ... ... ... 55 ! 

,, ,, laminations ... ... ... 40 ,, 

Length of core over laminations ... ... ... ... 9^ ,, 

Number of ventilating ducts ... ... ... ... ... 4 

Width of each ventilating duct ... ... ... ... ... f in. 

Effective length, magnetic iron ... ... ... ... 7.2,, 

,, of core -r total length... ... ... ... -76,, 

Length round periphery ... ... ... ... ... ... 183,, 

Pitch at surface 22.8 

Insulation between sheets ... ... ... ... ... 10 per cent. 

Thickness of sheets ... ... ... ... ... ... .014 in. 

Depth of slot 1.25 

Width of slot at root .28 

,, at surface... ... ... ... ... ... .28 

Number of slots ... ... ... ... ... ... ... 300 

Gross radial depth of lamination ... ... ... ... 9 in. 

Radial depth below teeth ... ... ... ... ... 7.75 ,, 

Width of teeth at root .303,, 

,, armature face ... ... ... ... .330,, 

Size of conductor 05 in. x .45 in. 

Magnet core, length of pole-piece ... ... . . ... 9.5 in. along shaft. 

Length of pole-arc ... ... ... ... ... ... 14 in. 

Thickness of pole-piece at edge ... ... ... ... ... If 

Pole-piece to consist of soft wrought-iron forging, so as to have 
maximum specific conductivity. 

Pole-arc -r pitch ... 61 per cent. 

Length of core, radial ... ... ... ... ... ... 14 in. 

Diameter of magnet core ... ... ... ... ... 12 ,, 

Bore of field 58J 

Clearance ... ... ... ... ... ... ... ^ ,, 

Spool : 

Length ... 1 4 in. 

,, of shunt winding space ... ... ... ... ... !!>> 

of series ,, 2f ,, 

Depth of shunt ,, ... ... ... ... 2 ,, 

,, of series ,, ... ... ... ... ... 2 ,, 

,, of winding space ... ... ... ... ... ... 2 

Yoke : 

Outside diameter 104 in. and 95 J in. 

Inside ... ... ... ... ... ... ... 88 in. 

Thickness 3| 

Length along armature ... ... ... ... ... ... 20 ,, 



Six-Phase, Four-Hundred Kilowatt Rotary Converts. 31<J 

Commutator : 

Diameter ... 52 5 in 

Number of segments ... QQQ 

per slot ... ... ... 2 

Width of segments at surface ... .23 in. 

at root ... ... ... 21 

Total depth of segments ... 

,, length of segment ... \\ 

Available length of segment ... 9 

Width of insulation between segments ... ... .045 

Collector : 

Diameter ... ... ... ... 15 in. 

Number of rings ... ... 6 

Width of ring ... ... ... ... 2 in. 

,, between rings ......... 1 

Length over all ... .. ... ... ... 22 , 

JJrushes : Continuous Alternating 

Current. Current. 

Number of sets ... ... ... ... ... 8 6 

. in one set ... ... ... ... 4 3 

Radial length of brush ... ... .. ... 2J in. 

Width of brush 11,, 1 i n . 

Thickness of brush .63 1 

Dimensions of bearing surface, one brush ... 1.5 in. x .75 in. 1 in. x 1 in. 

Area of contact, one brush ... ... ... 1.13 square inches 1 square inch. 

Type of brush ... ... Radial carbon Copper. 

Insulation : 

On core in slots Oil-treated cardboard about 

.012 in thick. 
Of conductor ... ... ... ... ... Varnished linen tape. 

ELECTRICAL. 
Armature : 

Terminal volts full load ... ... ... ... ... ... 600 

Total internal volts ... ... ... 614 

Number of circuits 

Style of winding ... ... ... ... Multiple circuit drum. 

Times re-entrant ... ... ... ... ... ... ... 1 

Total parallel paths through armature ... 

Conductors in series between brushes ... ... ... 150 

Type construction of winding ... ... ... ... Bar 

Number of face conductors ... ... ... ... 1200 

slots ... 300 

conductors per slot ... 

Arrangement of conductors in slot 
Number in parallel making up one conductor ... 



320 Rotary Converters. 

Mean length of one armature turn ... ... ... ... 78 in. 

Total number of turns ... ... ... ... ... ... 600 

Turns in series between brushes ... ... ... ... 75 

Length of conductor between brushes ... ... ... ... 5850 in. 

Cross-section, one conductor ... ... .. ... ... .0225 square inch 

,, eight conductors in parallel ... ... ... .18 ,, 

Ohms per inch cube at 20 deg. Cent. ... .00000068 

Per cent, increase in resistance 20 deg. Cent, to 60 deg. Cent. 16 

Resistance between brushes, 20 deg. Cent. ... ... ... .022 ohm. 

Resistance between brushes, 60 deg. Cent. ... ... ... .0256 

It has already been seen that in six-phase rotaries 1.96 times the 
output may be taken from the commutator for the same C 2 R loss in the 
armature conductors, as in a continuous-current generator with the same 
winding. Hence, for a given load, the resultant current in the armature 
conductors is a little over half that delivered from the commutator. In 
the present machine, the full load output is 667 amperes. Allowing for 
efficiency, and not quite unity power factor, we may take the current in 
the armature conductors at 667 x .55 = 370 amperes. 

C R drop in armature at 60 deg. Cent. ... ... ... 9.5 volts 

,, series coils ... ... ... ... ... ... 1 ,, 

,, brush contact surface ... ... ... ... 2.2 ,, 

,, not allowed for in above ... ... ... ... 1.3 in cables and 

connections 

Amperes per square inch, conductor ... ... ... ... 2050 figured on 

resultant current 
,, ,, brush-bearing surface ... ... 37 figured on current 

output from 
commutator 

,, shunt windings ... ... ... 980 

,, ,, series windings ... ... 1000 

All but the armature current density and drop results are derived later 
in the specification, but are brought together here for reference. 

SPACE FACTOR. 

In transformers, it is the aim to secure as high a ratio as possible of 
the total section of copper to the space in which it is wound, for a given 
specified insulation resistance. The same ratio, termed " space factor," is 
of service in proportioning the conductors and insulation to the armature 
slots. 

Sectional area of slot = 1.25 x .28 = .35 square inches. 
Sectional area of copper in slot = 4 x .0225 = .09 square inches. 
"Space factor" = .09 -4- .35 = .26 



Six-Phase, Four-Hundred Kilowatt Rotary Converter. 321 

i.e., 26 per cent, of the space is occupied by copper, and 74 per cent, by the 
necessary insulation. 

Commutation : 

Average volts between commutator segments ... ... 8 

Armature turns per pole ... 75 

Resultant current per conductor = = 46 amperes. 

8 

Resultant armature strength per pole = 46 x 75 = 3450 ampere turns. 

As the brushes remain at the mechanical neutral point, these exert 
only a distorting tendency, and do not have any demagnetising effect so 
long as the power factor of the alternating- current component is unity. 
It is also to be noted that, while the resultant armature current is 
46, amperes, the 3450 corresponding ampere turns are by no means fully 
effective as magnetomotive force, being positive and negative in successive 
groups sometimes even in successive turns opposite one pole-piece. (See 
Figs. 368 and 369, pages 288 and 289.) 

DETERMINATION OF REACTANCE VOLTAGE OF COIL UNDEK COMMUTATION. 

Diameter of commutator ... ... ... ... 52.5 in. 

Circumference of commutator ... ... ... ... 165 ,, 

Revolutions per second ... ... ... ... ... ... 6.25 

Peripheral speed, inches per second ... ... ... 1030 

Width of brush surface, across segments ... ... .75 in. 

Time of one complete reversal ... ... ... ... .00073 sees. 

Frequency of commutation, cycles per second ... ... 685 

Coils short-circuited together per brush 

Turns per coil ... ... ... ... ... ... ... 1 

Turns short-circuited together per brush 

Conductors per group commutated together ... ... 6 

Flux per ampere turn per inch gross length armature 

lamination ... ... ... ... ... ... ... 20 

Flux through six turns earring one ampere 

Inductance one coil of one turn ... ... .0000114 henrys 

Reactance of one coil of one turn ... ... ... .049 ohm 

Current in one coil (continuous-current component) ... 83.5 amperes 

Reactance voltage, one coil ... ... ... 4.1 volts 



PROPORTIONING THE BINDING WIRE. 

This is an important consideration in machines which must run at the 
high speeds customary with rotary converters. Cases might easily occur 
where an otherwise good machine might be designed ; but on calculating 

2 T 



322 Rotary Converters. 

the binding wire, it would be found to require a larger portion of the 
total peripheral surface than could properly be devoted to it. 

Length of conductor between brushes ... ... ... ... = 5850 in. 

Cross-section of conductor between brushes ... ... ... = .18 square inch 

Weight of armature copper = 5850 x .18 x .32 = 340 Ib. 

Every pound of material at the periphery is subject to a centrifugal 
force of .0000142 D N 2 pounds, where 

D = diameter in inches. 

N = revolutions per minute. 

Hence, in this case, to a force of 

.0000142 x 58 x 375 2 = 115 Ib. 

The iron laminations are dovetailed into the spider, so the binding 
wire need only be proportioned to retain the weight of the copper wire in 
place. 

Total centrifugal force - 340 x 115 = 39,000 Ib. 

Force per square inch of armature surface = - =7.4 Ib. 

29 x 58 x TT 

Total projected area = 29 x 58 = 1680 square inches. 

Total stress on binding wire = 1680 x 7.4 = 12,500 Ib., or 6250 Ib. per side. 

Using phosphor-bronze binding wire, and estimating on the basis of 
a tensile strength of 100,000 Ib. per square inch, with a factor of safety 

of 10, we require 

6250 x 10 



100,000 



= .63 square inch. 



Taking No. 12 Stubbs wire gauge with a diameter of .109 in., and 
cross-section of .00933 square inch, 72 of these would be required. These 
should be arranged in nine bands of eight turns each. Three of these 
bands should be over the laminated body of the armature, and three over 
each set of end connections. (See Fig. 392 on page 315.) 

MAGNETIC CIRCUIT CALCULATIONS. 

Megalines from one pole at full load and 600 terminal volts 

(614 internal volts) 8.20 

Coefficient of magnetic leakage ... ... ... ... ... 1.15 

Megalines in one pole at full load ... ... ... ... 9.5 



Six-Phase, Four-Hundred Kilowatt Rotary Converter. 



323 



Armature : 

Core section = 7.75 x 7.2 x 2 . 
Length, magnetic 
Density (kilolines) 
Ampere turns per inch 
Ampere turns 



Teeth : 



Number transmitting ilux per pole piece 
Section at face 



= 112 square inches 

7 in. 

73 

20 

140 



64 square inches 
i) roots ... ... ... 

Mean section ... " 

Length 

... ... 1 O^i " 

Apparent density (kilolines) ... 7 V*" 

Width of tooth " a " (mean) 

clnf (t 7, "^ 

" S10t ...... ... . 90 

Ratio "a" 4- "6" 

Corrected density 

Ampere turns per inch ... ," 

Ampere turns 

... 1 .) 1 

Gap : 

Section at pole face 

T ...loo square inches 

Length, one side ... . . 

.2o in. 
Density at pole face (kilolines) ......... 61 

Ampere turns (.313 x 61,000 x .25) ... 4800 

Magnet Core : 

Section -, -, .> 

_ llo square inches 

Length ... , . . 

14 in. 
Density (kilolines) ............ g , 

Ampere turns per inch ......... KQ 

Ampere turns ... ... ... ^-QQ 

Yoke : 

Section-2x 62 ... ... 124 square inches 

Length (per pole) ......... 17 in 

Density (kilolines) 

Ampere turns per inch ... ... ... g^Q 

SUMMAUY OF AMPERE TURNS. 

Armature core ......... 140 

,, teeth ... ............ 1370 

Ga P ......... 4800 

Magnet core ... ... ... ... ... 700 

Yoke ............ 640 



Total per spool 



7650 



324 Rotary Converters. 

SPOOL WINDINGS. 
Shunt : 

Mean length, one turn ... ... ... ... ... ... 3.66 ft. 

Ampere turns per shunt spool, full load ... 7,650 

Ampere feet 28,000 

Radiating surface, one field spool ... ... ... ... 700 square inches 

Watts per square inch to be allowed at 20 deg. Cent.... ... .40 

Watts per spool at 20 deg. Cent. 280 

,, ,, shunt winding at 20 deg. Cent. ... ... 220 

series ,, 60 

,, ,, shunt winding at 60 ,, ... ... 255 

Shunt copper per spool ... ... ... ... ... ... 110 

Volts at terminals of spool at 20 deg. Cent. ... ... ... 56 

Amperes per shunt spool ... ... ... ... ... 3.92 

Turns 1950 

Total length of shunt conductor ... ... ... .. 7150ft. 

Resistance per spool at 20 deg. Cent. ... ... ... ... 14.4 ohms 

Pounds per 1000 ft 15.4 Ib. 

Size of conductor ... ... ... ... ... ... No. 15 S.W.G. 

Dimensions bare ... ... ... ... ... ... ... .072 in. in diam. 

Dimensions double cotton covered ... ... ... ... .082 ,, 

Cross-section 00407 square inches 

Current density, amperes per square inch ... ... ... 980 

Available winding space ... ... ... ... ... 10 in. 

Number of layers ... ... ... ... ... ... 17 

Turns per layer ... ... ... ... ... ... ... 115 

Rotary converters do not run so well with much lag or lead, and the 
superposition of the motor and generator currents is far less perfect ; but 
it is often found convenient to use a series coil of some 25 per cent, of the 
strength of the shunt coil, and to have, on the side of the machine, a 
switch, which, when completely open, sends all the main current, except 
a very small percentage, through the series winding, the small balance 
passing through a diverter rheostat. In the next position, about half of 
the current is diverted through the rheostat, the series coil being much 
weaker, and in the final position, the series coil is completely short- 
circuited, all the current being diverted from it. This enables the series 
winding to be employed to the extent found desirable, considered with 
relation to the high-tension transmission line, as well as to the low-tension 
continuous-current system, on which latter system, it is desirable to have 
the terminal voltage increase with the load. 

By adjusting the shunt excitation so that the current lags slightly at 
no load, and by having sufficient series excitation, the total field strength 
increases as the load comes on, and thus controls the phase of the motor 



Six-Phase, Four- Hundred Kilowatt Rotary Converter. 325 

current. At some intermediate load the motor current will be exactly 

in phase with the electromotive force, and at higher loads will slightly 
lead, thus also maintaining rather higher commutator voltage. 

Series : 

Ampere turns, full load ... ... ... ... ... ... 2000 

Full load amperes ... ... ... ... ... ... 6G7 

Amperes diverted ... ... ... ... ... 167 

,, in series spool ... ... ... ... ... ... 500 

Turns per spool ... ... ... ... ... ... ... 4 

Size of conductor used ... ... ... ... ... ... 2 in. by .05 in. 

Number in parallel ... ... ... ... ... ... 5 

Total cross-section ... ... ... ... ... .5 sq. in. 

Current density, amperes per square inch ... ... 1000 

Mean length of one turn ... ... ... ... ... 3.66ft. 

Total length, all turns on eight spools ... ... ... 1400 in. 

Resistance of eight spools at 20 deg. Cent. ... .0019 ohm 

Series C 2 R watts, total at 20 deg. Cent. ... 475 

,, per spool at 20 deg. Cent. ... ... 60 

CO ... JO 

Weight of series copper ... ... ... 225 Ib. 



CALCULATIONS OF LOSSES AND HEATING. 
Armature : 

Resistance between brushes ... ... ... ... ... .0256 ohm at 

60 deg. C. 

C-R loss at 60 deg. Cent. . . 3500 watts figured from 

resultant current 

Frequency, cycles per second = C = ... 25 

Weight of armature teeth ... ... 245 Ib. 

core 2310 

Total weight armature laminations = 2555 
Apparent flux density in teeth (kilolines) 

Flux density in core (kilolines) = D = 73 

C.D. -j- 1000 = ... 1-83 

K = ... 1-65 

5^-5l = watts core loss per Ib. = ... ... 3.02 

1000 

Total core loss = 3.02 x 2555 = 7,700 watts 

,, armature loss = 11,20( ,, 

Armature diameter ... 58 in. 

length ... ... 34 

Peripheral radiating surface . . . 5300 square inches 

,, speed, feet per minute ... ... 5700 

Watts per square inch in radiating surface 

Assumed rise of temperature per watt per square inch by 

thermometer, after 10 hours run 20 deg. Cent. 



326 Rotary Converters. 

Total rise estimated 011 above basis ... ... ... ... 42 

Assumed rise of temperature per watt per square inch by 

resistance, after 10 hours run ... ... ... ... 30 ,, 

Total rise estimated on above basis ... ... ... ... 63 ,, 

It will be observed that the total weight of iron in armature, i.e., 
2555 Ib., is multiplied by the "watts core loss per pound" to obtain total 
core loss. This includes loss in teeth, as the curve (see Fig. 238, page 229) 

from which the constant was taken, is so proportioned as to allow for core 
and tooth losses for this type of construction and range of magnetic 
densities. 

COMMUTATOR LOSSES AND HEATING. 

Area of all positive brushes ... ... ... ... ... 18 square inches 

Amperes per square inch contact surface ... ... ... 37 

Ohms per square inch contact surface, assumed ... ... .03 

Brush resistance, positive and negative ... ... ... .0033 

Volts drop at brush contacts ... ... ... ... ... 2.2 

C 2 R loss 1500 watts 

Brush pressure ... ... ... ... ... ... ... 1.25 Ib. per sq. in. 

total ... 45 Ib. 

Coefficient of friction ... ... ... ... ... ... .3 

Peripheral speed ... ... ... ... ... ... ... 5150 ft. per min. 

Brush friction 70,000 ft.-lb. per min. 

,, .. ... ... ... ... ... ... 1600 watts 

Stray watts lost in commutator, assumed ... ... ... 400 

Total watts lost in commutator ... ... ... ... ... 3500 

Diameter of commutator ... ... ... ... ... 52.5 in. 

Length ... 9 

Radiating surface ... ... ... ... ... ... 1500 square inches 

Watts per square inch radiating surface ... ... ... 2.3 

Assumed rise of temperature per watt per square inch after 

10 hours run ... ... ... ... ... ... 15 deg. Cent. 

Total rise estimated on above basis ... ... ... ... 35 

COLLECTOR LOSSES AND HEATING. 

Total contact area of all brushes ... .. ... ... 18 square inches 

Amperes per square inch contact surface ... ... 110 

Ohms per square inch contact (assumed) ... ... ... .003 

Total resistance of brushes per ring ... ... ... ... .001 

Volts drop at brush contacts ... ... ... ... ... .34 

C 2 R loss at brush contacts per ring ... ... ... ... 110 watts 

in six rings 660 

Brush pressure, pounds per square inch ... ... ... 1.0 

,, total pounds ... ... ... ... ... 18 

Coefficient of friction .3 



Six-Phase, Four-Hundred Kilowatt Rotary Converter. 327 

Peripheral speed, feet per minute ... ... 1470 

Brush friction, foot-pounds per minute... 8000 

,, ,, watts lost ... ... ... 180 

Total watts lost in collector ... ... ... ... 840 

Diameter collector ... ... ... ... 15 in. 

Effective length of radiating surface ... ... 12 

Radiating surface 570 square inches 

Watts per square inch radiating surface ... ... ... 1.5 

Assumed rise of temperature per watt per square inch after 

10 hours run ... ... ... ... ... 20 dog. Cent. 

Total rise estimated on above basis 30 



SPOOL LOSSES AND HEATING. 
Spool : 

C 2 R loss at 60 deg. Cent, per shunt coil ... ... 255 watts 

,, ,, per series coil ... ... ... 70 ,, 

Total watts lost per spool ... ... .. 325 ,, 

Length of winding space, total .. ... ... ... ... 14 in. 

Circumference of spool ... ... ... ... ... ... 50 ,, 

Peripheral radiating surface per spool ... ... ... ... 700 square inches 

Watts per square inch radiating surface ... ... ... .465 

Assumed rise of temperature per watt per square inch by 

thermometer, after 10 hours run ... ... ... 80 deg. Cent. 

Total rise estimated on above basis ... ... ... ... 37 ,, 

Assumed rise of temperature per watt per square inch by 

resistance, after 10 hours run ... ... ... ... 120 ,, 

Total rise estimated on above basis 56 ,, 



EFFICIENCY. 

Output, full-load watts ... 400,000 

Core loss ... 7,700 

Armature C 2 R loss at 60 deg. Cent. ... 3,500 

Commutator losses 3,500 

Collector losses ... 
Shunt spools losses 2,040 

rheostat losses 
Series spools losses 

,, diverter losses 
Friction, bearings and windage ... 
Input, total 
Commercial efficiency, full load ... 95 per cent. 

MATERIALS. 

Armature core ... Sheet steel 

spider ... 

conductors Copper 



328 Rotary Converter*. 

Commutator segments ... ... ... ... ... ... Copper 

,, leads ... ... ... ... ... ... Rheotan 

,, spider ... ... ... ... ... ... Cast iron 

Pole-piece ... ... ... ... ... ... ... ... Wrought-iron forging 

Yoke ... ... ... ... ... ... ... ... Oast steel 

Magnet core ... ... ... ... ... ... ... ,, 

Brushes ... ... ... ... ... ... ... ... Carbon and copper 

Brush-holder ... ... ... ... ... Brass 

,, yoke ... ... ... ... ... ... Gun-metal 

Binding wire ... ... ... ... ... ... ... Phosphor bronze 

Insulation, commutator ... ... ... ... ... ... Mica 

armature ... ... ... ... ... ... Varnished linen tape 



WEIGHTS. 

Armature : Lh. 

Laminations ... .. ... ... ... ... ... 2,550 

Copper ... ... 340 

Spider ... 1,550 

Shaft -. 1,230 

Flanges 700 

Commutator : 

Segments 1,000 

Mica 80 

Spider ... ... 1,000 

Press rings ... ... ... ... ... ... ... 200 

Other parts 300 

Collector, complete ... ... ... ... ... ... 700 

Armature, commutator, collector, and shaft complete... ... 9,650 

Magnet : 

Cores 3,550 

Pole-pieces ... ... ... ... ... ... ... 400 

Yoke 5,000 

Field : 

Shunt coils ... 880 

Series ,, ... ... ... ... ... ... ... 225 

Total copper ... 1,105 

Spools complete ... ... ... ... ... ... ... 1.800 

Bedplate, bearings, &c. ... 6,300 

Brush rigging ... ... ... ... ... ... ... 450 

Other parts ... 1,000 



Complete weight rotary converter ... 30,360 



Three-Phase, Nine-Hundred Kilowatt, Rotary Converter. 329 

TABULATED CALCULATIONS AND SPECIFICATIONS FOR A 900-KiLowATT THREE- 
PHASE ROTARY CONVERTER. 

The machine is illustrated in Figs. 39G, 397 and 398 ; and curves of 
its performance are given in Figs. 399 to 402. 

DESCRIPTION. 

Number of poles ... ... ... ... ... 12 

Kilowatt output ... ... ... ... ... ... ... 900 

Speed, revolutions per minute ... ... ... ... ... 250 

Terminal volts, full load ... ... ... ... ... ... 500 

,, no load ... ... ... ... ... 500 

Amperes, output ... ... ... ... ... ... ... 1800 

Frequency, cycles per second ... ... 25 

DIMENSIONS. 
Armature : 

Diameter over all ... ... ... ... ... ... ... 84 in. 

Length over conductors ... ... ... ... ... ... 27 ,, 

Diameter of core at periphery ... ... ... ... ... 84 ,, 

,, ,, bottom of slots ... ... ... ... 81 i ,, 

,, ,, ,, laminations ... ... ... 62 ,, 

Length of core over laminations ... ... 12.5,, 

Number of ventilating ducts 

Width, each ... \ in. 

Effective length, magnetic iron ... ... ... ... 9.9 ,, 

,, of core -r total length ... ... ... .79 

Length round periphery ... ... ... ... 2G4 in. 

Pitch at surface .... . . . ...... - >, 

Insulation between sheets 10 per cent. 

Thickness of sheets .016 in. 

Depth of slot ... 1-25 

Width of slot at root -44 

,, ,, surface ... ... -44 ,, 

Number of slots ... 

Gross radial depth of laminations H " 

Radial depth below teeth 9 -75 in. 

AVidth of tooth at root... 

,, ,, armature face -* n 

Size of conductor ... - 1 - 5 in - b J - 400 in - 

Magnet Core : 

Length of pole-piece along shaft 

pole-arc, average 

Pole-piece and core consists of sheet-iron punchings .04 in. 
thick, japanned on one side, and built up to a depth of 

2 v 



330 



Rotary Converters. 



12 in. The edges of pole-face are chamfered back 3 in. 

by -^ in., and a copper bridge 14 in. by ^ in., extending 

1-| in. under pole tips, is inserted between poles to 

prevent " surging." 

Pole arc -f- pitch... ... ... ... ... ... ... .722 

Length of core radial ... ... ... ... ... ... 9}| in. 

Size of magnet core (laminations) ... ... ... ... 12 in. by 12 in. 

Bore of field ... 84 1 in. 

Clearance (magnetic gap) ... ... ... ... ... TIT > 

Spool ; 

Length ... ... ... ... ... ... ... ... 8 T 7 ^ in. 

,, of shunt-winding space ... ... ... ... 4.9 ,, 

,, ,, series-winding space... ... ... ... ... 3.5 ,, 

Depth of winding space ... ... ... ... ... 2| ,, 

Yoke : 

Outside diameter 123 in. & 114 in. 

Inside diameter ... ... ... ... ... ... ... 1 05 in. 

Thickness ... ... ... ... ... ... ... 4| ,, 

Length along armature ... ... ... ... ... ... 22 ,, 

Beyond the 22-in. length along armature, projects ori one side 

a ring 1| in. wide, which is grooved to receive the brush 

rocking gear. 

Commutator : 

Diameter... ... ... ... ... "... ... ... 54 in. 

Number of segments ... ... ... ... ... ... 576 

,, ,. per slot 

Width of at surface .24 

,, ,, at root ... ... ... ... ... .215 

Total depth of segment ... ... ... ... ... ... 2 in. 

,, length of segment ... ... ... ... ... 17 J ,, 

Available length of segment ... ... ..." ... ... 14 ,, 

Width of insulation between segments ... ... ... .05 

Collector : 

Diameter ... ... ... ... ... ... ... ... 24 in. 

Number of rings... ... ... ... ... ... .... 3 

Width of each ring ... ... ... ... ... ... 3| in. 

,, between rings ... ... ... ... ... 1| ,, 

Length over all ... ... ... ... ... ... ... 18 in. 

Continuous Alternating 

Current. Current. 

Number of sets ... ... ... ... 12 

Number in one set ... ... ... 8 8 

Radial length of brush ... ... ... 2 in. 

Width of brush ... I],, 1J in. 

Thickness of brush ... ... ... f G ,, 

Dimensions of bearing surface (one brush) 1.25 in. by .87 in. 1.25 n. by 1.1 in. 

Area of contact (one brush) ... ... 1.08 square inch 1.35 square inch 

Type of brush ... ... ... ... Radial carbon Copper 



Three-Phase, Xine-Hundred Kilowatt, Rotary Converter. 331 




332 



Rotary Converters. 



TECHNICAL DATA. ELECTRICAL. 
Armature : 

Terminal volts, full load ... ... ... 

Total internal volts 

Number of circuits 

Style of winding ... 

Times re-entrant ... 

Total parallel paths through armature ... 

Conductors in series between brushes ... 

Type construction of winding ... 



500 

513 

12 

Multiple-circuit drum 

1 

12 

96 

Bar 




3) 


; 


I 

Oj 


\ 


o. 


J) 















- 




















- 














_y~^- 


^ 






Number of face conductors 

,, slots ... 

,, conductors per slot ... 

Arrangement of conductors in slot 
Number in parallel making up one conductor . . . 
Mean length of one armature turn 
Total number of turns ... 
Turns in series between brushes 
Length of conductor between brushes ... 
Cross-section, one conductor 

,, 12 conductors in parallel 

Ohms per inch cube at 20 deg. Cent. ... 

Per cent, increase in resistance 20 deg. Cent, to 60 deg. Cent. 
Resistance between brushes 20 deg. Cent. 

60 



1152 
288 
4 
2 by 2 

1 

78 in. 

576 

48 

3744 in. 
.05 square inch 
-60 

.00000068 

16 per cent. 

.00425 

.00493 



Three-Phase, Nine-Hundred Kilowatt, Rotary Converter. 333 

Assuming the current in three-phase rotary converter armature to 
be about three-fourths of that for continuous-current generator of same 



Observed/ Curves 

of 

SATURATION 

ow 000 Kw. 25 Cycles 500 Volts 
Three, Phase Rotary Converter. 



Rg390 



x! 



Observed; 
CORE LOSS. 

on, 900 Kw. 25 fycles 500 Volt* 
Three. Phase Rotary Converter. 



*.= 



. 1000 ZOOO 30004000 5000 6000 7000 8000 3000 10000 IWn 

FueloL Ampere turns per Spool. 




(5l42 S c f *$,&> i* 00 f 50 " *> iOO MO fOO 

Voits Armature. 



Observed 

PHASE CHARACTERISTIC 
on, 900 Kw. 25 Cycles 600 Volts 
Three, Phase Ratary Converter. 




A 



fitr 



^&fad^ 



7v/// Art7, 



EFFICIENCY & LOSSES. 



aw 900 Kw. 25 Cycles 500 Volt* 
Three, Phase Rotary CoMerter . 



3 4 

Amperes ui Field 



200 W> 600 800 1000 IWI MOO I60H 1800 ZOtl 2200 TMO 

(tutij Ampere Output,. 



output, and a power factor of not quite unity, we may take current in 
armature conductor as 1,800 x .8 = 1,440 amperes. 



334 Rotary Converters. 

CR drop in armature at 60 cleg. Cent. ... ... ... 7.1 volts 

,, series coils ... ... ... ... ... ... 1.6 ,, 

at brush contact surfaces ... ... ... ... 2.1 

,, not allowed for in above ... ... ... .. 1 5 volts for cables and 

connections ; figured 
on component cur 
rents 
Amperes per square inch conductor (armature) ... ... 2400 

,, ,, ,, brush-bearing surface ... ... 34.5 

,, shunt windings ... ... ... 970 

,, ,, ,, series windings ... ... ... 970 

Space Factor : 

Sectional area of slot = 1.25 x .44 = .55 square inch. 

,, ,, copper in slot = 4 x .125 x .4 = .2 square inch. 

"Space factor" = .2 -f- .55 = .364, or 36.4 per cent, of total space is occupied by 
copper, leaving 63.6 per cent, for the necessary insulation. 

Commutation : 

Volts between segments, average ... ... ... ... 10.4 

Armature turns per pole ... ... ... ... ... 48 

Resultant current per conductor = =120 amperes. 

Resultant armature strength = 120 x 48 = 5800 armature 
ampere turns per pole. 



DETERMINATION OP REACTANCE VOLTAGE OP COIL UNDER COMMUTATION. 

Diameter of commutator ... ... ... ... ... 54 in. 

Circumference of commutator ... ... ... ... ... 170 ,, 

Revolutions per second ... ... ... ... ... ... 4.2 

Peripheral speed, inches per second ... ... ... ... 708 

Width of brush surface across segments .. ... ... .87 in. 

Time of one complete reversal, seconds ... ... ... .00123 

Frequency of commutation, cycles per second ... ... ,.. 407 

Coils, short-circuited together per brush ... ... ... 3 

Turns per coil ... ... ... ... ... ... ... 1 

Turns short-circuited together per brush ... .... .. 3 

Conductors per group commutated together ... ... ... 6 

Flux per ampere turn per inch gross length armature lamina 
tion 20 

Flux through six turns carrying one ampere ... ... ... 1500 

Inductance one coil of one turn... ... ... ... ... .000015 henrys 

Reactance of one coil of one turn ... ... ... ... .039 ohms 

Current in one coil, amperes ... ... ... ... ... 150 

(continuous-current 

component) 

Reactance voltage, one coil ... ... ... ... ... 5.8 volts 



Three-Phase, Nine- Hundred Kilowatt, Rotary Converter. 335 

BINDING WIRE. 
Length of conductor between brushes ... ... 3774 in. 

Cross -section of conductor between brushes ... ... .0 square inch 

Weight of armature copper ... ... ... ... ... 3744 x .0 x .32 

= 721 II.. 
Centrifugal force... ... = .0000142 D N- Ib. 

Therefore, .0000142 x 84 x 250 == 74.7 Ib. exerted as centrifugal 
force by every pound of copper conductor on armature, and as there are 
721 Ib. weight of copper conductors, the total centrifugal force = 721 x 74.7 
= 54,000 Ib. 

Part of the centrifugal force is resisted by strips of hard wood driven 
into dovetail grooves running parallel to the length of the shaft at the 
tops of the slots, while the end projections and connections are held in 
place by 84 strands of No. 11 B. and S. phosphor-bronze wire arranged 
over both ends, in bands of six strands each, seven of these bands being 
employed for each end. 

MAGNETIC CIRCUIT CALCULATIONS. 
Megalines from one pole at full load and 500 terminal volts 

(512.5 internal volts) ... 10.4 

Assumed coefficient of magnetic leakage ... 1.20 

Megalines in one pole at full load ... ... 12.5 

The magnetic reluctance and the observed total number of ampere 
turns per field spool required, were probably distributed approximately 
as follows : 

Armature : 

Core section ... ... ... 9.9 x 9.75 x . 

= 194 square inches 

Length of magnetic circuit 1 i n - 

Density (kilolines) 
Ampere turns per inch . . . 
Ampere turns 
Teeth : 

Number transmitting flux per pole-piece 

Section at face ... 76 square inches 

roots ... 
Mean section ......... 

Length ... 1-25 in. 

i "}J. 
Apparent density (kilolines) 

Width of tooth (mean) "a" 

slot " b " 
Ratioofa-^-6 ... 

1 OQ 

Corrected density (kilolines) 

1 1 fiO 

Ampere turns per inch . 

1460 
Ampere turns 



336 Rotary Converters. 

Gap : 

Section at pole-face ... ... ... ... ... ... 190 

Length .1875 

Density at pole-face (kilolines) ... ... ... ... ... 54.5 

Ampere turns - .313 x 54,200 x .1875 = 3200. 

Magnet Core : 

Section (effective) ... ... ... ... ... ... 135 square inches 

Length 9j| in. 

Density (kilolines) ... ... ... ... ... ... 95 

Ampere turns per inch ... ... ... ... ... ... 53 

Ampere turns ... ... ... . . ... ... ... 530 

Yoke : 

Section magnetic 2 x 136 = 272 square inches. 

Length per pole ... ... ... ... ... ... 14.5 in. 

Density (kilolines) ... ... ... ... ... ... 48 

Ampere turns per inch ... ... ... ... ... ... 29 

Ampere turns ... ... ... ... ... ... ... 430 

SUMMARY OP AMPERE TURNS. 

Armature core ... ... ... ... ... ... ... 180 

teeth ... ... 1460 

Gap ... 3200 

Magnet core ... ... ... ... ... ... ... 530 

Yoke ... . ... ... 430 

5800 
SPOOL WINDINGS. 

Ampere turns per shunt spool, full load ... ... ... 5800 

Watts per spool at 60 cleg. Cent. 405 

,, shunt winding at 20 deg. Cent 200 

series ,, 143 

shunt at 60 deg. Cent 240 

Shunt copper per spool ... ... ... ... ... ... 110 Ib. 

Volts at terminals of spool at 20 deg. Cent. ... ... ... 36 

Amperes per shunt spool ... ... ... ... ... 6.3 

Resistance at 20 deg. Cent, per spool, ohms ... ... ... 5.7 

Turns per shunt spool ... ... ... ... ... ... 912 

Total length of shunt conductor ... ... ... ... 4400ft. 

Pounds per 1000 ft 24.9 

Size of conductor ...No. 11 B. and S. gauge. 

Dimensions bare ... ... ... ... ... ... ... .0907 in. in diameter 

double cotton covered ... ... ... ... .101 ,, ,, 

Cross-section ... ... ... ... ... ... ... .00647 square inch 

Current density, amperes per square inch ... ... ... 970 

Available winding space ... ... ... ... ... 4 in. 

Number of layers 

Turns per layer ... ... ... ... ... ... 40 



Three-Phase, Nine-Hundred Kilowatt, Rotary Converter. .337 



Series : 

Ampere turns, full load ... ... 3030 

Full-load amperes ... ... ... 1800 

Amperes diverted ... 350 

,, in series spools ... ... ... ... ... 1450 

Turns per spool ... ... ... ... 2. , 

Size of conductor used ... ... ... ... ... ... 2.5 in. by .075 in. 

Number in parallel ... ... ... ... ... 8 

Total cross section ... ... ... 1.5 square inch 

Current density, amperes per square inch ... ... ... 970 

Mean length of one turn ... ... ... ... 4.83ft. 

Total length, all turns on 12 spools ... ... 150 ft. = 1800 in. 

Resistance of 12 spools at 20 cleg. Cent. ... ... .000816 ohm 

Series C 2 R watts, total at 20 deg. Cent. 1718 

,, ,, per spool ... ... ... ... 143 

at 60 deg. Cent. ... 165 

Total weight of series copper, pound ... ... ... 864 

CALCULATION OF LOSSES AND HEATING. 
Armature : 

Resistance between brushes, ohms ...00493 at 60 deg. Cent. 

C-R loss at 60 deg. Cent, 9700 

Frequency, cycles per sec. C - 
Weight of armature teeth ... 500 lb. 

core 6500 

Total weight of laminations ... 70( 

Flux density in teeth, kilolines . . . 

core = D = 
C.D. + 1000 

Observed core loss per pound, watts 
^ _ watts core loss per pound _ 2.05 

~7o7D7 -r ioooy~ 

Total core loss ... 

on FJ.-.A 
,, armature losses 

o | 

Armature diameter 

,, length 
Peripheral radiating surface 

speed, feet per minute 
Watts per square inch radiating surface 



COMMUTATOR LOKSKS AND HEATING. 

Commutator : 

Area of all positive brushes 

Amperes per square inch contact surface 

nu~ assumed 

Ulims ,, n 

Brush resistance, positive and negative 
Drop at brush contacts ... 
C-R loss at brush contacts 



51 square inches 

35 

.03 

.00116 ohm 

2.1 volts 
3700 watts 

2 x 



338 Rotary Converters. 

Brush pressure, pounds per square inch ... ... ... 1.15 

total 117 Ib 

Coefficient of friction ... ... ... ... ... ... .3 

Peripheral speed, feet per minute ... ... ... ... 3550 

Brush friction, foot-pounds per minute ... ... ... 124,000 

watts 2800 

Stray watts lost in commutator, assumed ... ... ... 600 

Total 7100 

Diameter of commutator ... ... ... ... ... 54 in. 

Available length of commutator ... ... ... ... 14 ,, 

Radiating surface ... ... ... ... ... ... 2400 square inches 

Watts per square inch of radiating surface ... ... ... 2.9 

Assumed rise of temperature per watt per square inch, after 

10 hours run ... ... ... ... ... ... 15 deg. Cent. 

Total rise estimated on above basis ... ... ... ... 43 ,, 

COLLECTOR LOSSES AND HEATING. 

Total contact area of all brushes ... ... ... ... 33.5 square inches 

Amperes per square inch of contact surface ... ... ... 150 

Ohms per square inch of contact (assumed) ... ... ... .003 

Total resistance of brushes per ring ... ... ... ... .00027 

Volts drop at brush contacts ... ... ... ... ... .48 

C~R loss at brush contacts per ring ... ... ... ... 850 

,, ,, ,, in three rings ... ... ... 1700 

Brush pressure, pounds per square inch ... ... ... 1.6 

,, ,, total pounds ... ... ... ... ... 54 

Coefficient of friction ... ... ... ... ... ... .3 

Peripheral speed, feet per minute ... ... ... ... 1,580 

Brush friction, pounds per minute ... ... ... ... 25,500 

,, ,, watts lost ... ... ... ... ... 600 

Total watts lost in collector 2, 300 

Diameter collector ... ... ... ... ... ... 24 in. 

Effective length radiating surface ;.. ... ... ... 11 ,, 

Total radiating surface ... ... ... ... ... ... 820 square inches 

Watts per square inch radiating surface ... ... ... 2.8 

Assumed rise of temperature per watt per square inch, after 

10 hours run ... ... ... ... ... ... 15 deg. Cent. 

Total rise estimated on above basis ... ... ... ... 42 ,, 

Field Spool Losses : 

Spool C-R loss at 60 deg. Cent, per shunt coil ... ... 240 

C 2 R loss at 60 deg. Cent, per series coil ... ... ... 165 

Total loss per spool, watts ... ... ... ... ... 405 

,, in 12 spools, watts ... ... ... ... ... 4850 

EFFICIENCY. 

Full load, watts output 900,000 

Core loss 19,850 



Three-Phase, Nine-Hundred Kilowatt, Rotary Converter. 339 

Commutator losses ... ... ... ... 7100 

Collector losses ... ... ... ... 2300 

Armature C 2 R loss at 60 deg. Cent 9,700 

Shunt spools C 2 R loss at GO cleg. Cent. ... 2 900 

Shunt rheostat C-R loss at 60 deg. Cent 300 

Series spools C 2 R loss at GO cleg. Cent. ... 1,700 

Series diverter C 2 R loss at GO deg. Cent. ... ... 500 

Friction, bearings, and windage ... ... ... 5 100 



Total input 949,450 

Commercial Efficiency : 

Full load ... 95 per cent. 

Materials : 

Armature core ... ... ... ... ... ... ... Sheet steel 

,, spider... ... Cast iron 

,, conductors ... ... ... ... ... ... Copper 

Commutator segments ... ... ... ... ... ... 

,, leads ... ... ... ... ... ... Stranded copper 

,, spider ... ... ... ... ... ... Cast iron 

Pole-piece... ... ... ... ... ... ... ... Laminated sheet iron 

Yoke ... ... ... ... ... ... ... ... Cast steel 

Magnet core ... ... ... ... ... ... ... Laminated sheet iron 

Brushes ... ... ... ... ... ... ... Carbon 

Brush-holder ... ... Brass 

,, yoke ... ... ... ... ... ... Gun-metal 

Binding wire ... ... Phosphor-bronze 

Insulation, commutator ... ... ... Mica 

WEIGHTS. 
Armature : 

Laminations ... ... ... ... 7,000 

Copper 

Spider ... ... 3,000 

Shaft 3,000 

Flanges ... 800 

Commutator : 

Segments -> 100 

Mica 
Spider 
Press rings 
Sundry other parts 

Collector rings, complete ... 1,0/0 

Armature, commutator, collector, and shaft complete 

Magnet : 

Yoke 13 000 

Poles 6 < 0(X) 



340 Rotary Converters. 

Field : 

Shunt coils, copper ... ......... ... ... ... 1,320 

Series ,, ,, ... ... ... ... ... ... 860 

Total copper ... ... ... ... ... ... ... 2,180 

Spools complete, including flanges and all insulation ... ... .5,600 

Bedplate, bearings, Ac. ... ... ... ... ... ... 18,000 

Brush gear ... 1,200 

Sundry other parts ... ... 2,200 



Total weight of rotary converter ... ... 66,000 

THE STARTING OF ROTARY CONVERTERS. 

The starting and synchronising of rotary converters may be accom 
plished in any one of several ways. The simplest, at first sight, is to 
throw the alternating- current terminals of the rotary converter directly on 
the alternating-current mains ; but this, although often practicable, has 
several disadvantages. By this method, the current rush at the moment 
of starting is generally in excess of the full-load current input to the rotary 
converter, and as it lags in phase by a large angle, it causes a serious 
drop of line voltage, and affects the normal line conditions, to the serious 
detriment of other apparatus on the line. This large current gradually 
decreases as the rotary converter s speed increases. The action of the 
rotary converter, in starting, is analogous to that of an induction motor. 
The rotating magnetic field set up by the currents entering the armature 
windings induces but very ineffectively secondary currents in the pole- 
faces, and the mutual action between these secondary currents and the 
rotating field imparts torque to the armature, which revolves with 
constantly accelerating speed, up to synchronism. Then the circuit of the 
rotary converter field spools is closed, and adjusted to bring the current 
into phase. But when the armature is first starting, the field spools 
are interlinked with an alternating magnetic flux, generated by the current 
in the armature windings, and, in normally-proportioned field spools, with 
several hundreds or thousands of turns per spool, a dangerously high 
secondary voltage is generated in these spools. Hence they must be 
insulated better than field spools ordinarily are, not only between layers, 
but between adjacent turns ; and wire with double or triple cotton covering 
should be used. However, the most frequently-occurring breakdown due 
to this cause, is from winding to frame, and hence extra insulation should 
be used between these parts. 



Methods of Starting Rotary Converters. 

The terminals of the different field spools should be connected up to 
a suitable switch, arranged so that the field winding may be conveniently 
broken up into several sections ; otherwise, if a thousand volts or so are 
induced in each spool, the strain on the insulation between the ends of 
these spools in series, and frame is severe. 

At starting, this switch must always be open, and must not be closed 
until the armature has run up to synchronous speed, which is observed by 
the line current falling to a much smaller value. This special switch is 
then closed, and afterwards the main field switch, whereupon a still further 
decrease in the line current occurs, due to improved phase relations, and 
the process of synchronising is completed. 

By means of a compensator, this heavy current on the line at starting 



Fig 403 



I 



THREE COMPENSATORS 
OR ONE THREE PHASE 
COM PENS ATOR> 




may be dispensed with. The connections for a three-phase rotary with 
compensator, are as shown in the diagram of Fig. 403. 

At the instant of starting, the collector rings are connected to the 
three lowest contacts, hence receive but a small fraction of the line voltage, 
and would receive several times the line current; i.e., if the taps into 
the compensator winding are, say, one-fifth of the way from common 
connection to line, then the rotary converter has one-fifth the line voltage 
and five times the line current. As the converter runs up in speed, the 
terminals are moved along until, at synchronism, the collector is directly 

on the line. 

Another difficulty encountered when the rotary converter is started 
from the alternating-current end, is the indeterminate polarity at the 
commutator, when the rotary is made to furnish its own excitation. 
Unless some independent source of continuous current is available at the 
rotary converter sab-station, the rotary is dependent for its excitation upon 



342 Rotary Converters. 

the polarity that its commutator happens to have at the instant of 
attaining synchronism. If there are two rotary converters at the 
sub-station, and the first comes up with the wrong polarity, then it 
may be allowed to run so, temporarily, till the second one is synchronised. 
The second one can be given either polarity desired, by using the first 
as an independent source of continuous current. Then from the second 
one, the polarity of the first may be reversed into the correct direction, 
and the second rotary converter shut down. Obviously, however, this 
indeterminafceness of the initial polarity constitutes a further inconvenience 
and objection to starting rotary converters by throwing them directly on 
to the alternating-current line. But in the case of large capacity, slow- 
speed rotary converters, consequently machines with heavy armatures, it 
has been found practicable to control the polarity of the first machine when 
it is started up from the alternating current side. One must stand ready 
by the field switch as the machine approaches synchronism, when the 
pointer of the continuous-current voltmeter will commence to vibrate 
rapidly about the zero mark with short swings. These will finally be 
followed by a couple of fairly slow, indecisive, long swings, in opposite 
directions from the zero mark. Near the maximum point of whichever 
of these swings is in the direction of the desired polarity, the field switch 
should be closed, and the machine will excite itself, provided the field 
terminals are correctly positive and negative. Otherwise which might 
happen on the first run, or after alterations the field terminals will require 
to be reversed. 

The required line current is greatly reduced by starting generator 
and rotary converter up simultaneously. The latter is then, from the 
instant of starting, always in synchronism with its generator, and the 
conditions of running are arrived at with a minimum strain to the system. 
But the conditions of routine operation rarely render this plan practicable. 

A method sometimes used, is to have a small induction motor direct 
coupled to the shaft of the rotary converter for the purpose of starting 
the latter with small line currents. This, however, is an extra expense, 
and results in an unsightly combination set. 

Where there are several rotary converters in a sub-station, a much 
better way is that described in a recent British patent specification, in 
which the station is provided with a small auxiliary set consisting of an 
induction motor direct coupled to a continuous-current dynamo, the latter 
being only of sufficient capacity to run the rotary converters one at a time 



Metlnxk of 



Rota ) // Converter*. 







343 



344 



Rotary Converters. 



up to synchronous speed as continuous-current motors. When this 
speed is arrived at, and synchronism attained, between the alternating- 
current collector rings and the line, the switch between them is 
closed, and the rotary converter runs on from the alternating-current 
supply. 

In many cases, a continuous-current system derives its supply partly 
from continuous-current generators and partly from rotary converters. In 




such cases, the rotary converter is simply started up as a motor from the 
continuous-current line, and then synchronised. 

On the Continent it is very customary to operate storage batteries 
in the sub-stations, in parallel with the rotary converters, the batteries 
being charged by the rotaries during times of light load, and helping out 
the rotaries with heavy loads. They are known as buffer batteries," 
and are of considerable assistance in maintaining uniform voltage and 
more uniform load on the generating plant. Moreover, they render the 
sub-station independent of the rest of the system for starting up the rotary 
converters. 



Methods of Synchronising Rotary Converters. 



345 



SYNCHRONISING ROTARY CONVERTERS. 

One has the choice of synchronising the rotary converter either by 
a switch between the collector rings and the low potential side of the 
step-down transformers, or of considering the step-down transformers and 
the rotary converter to constitute one system, transforming from low- 
voltage continuous current to high-voltage alternating current, and 
synchronising by a switch placed between the high-tension terminals of 
the transformers and the high-tension transmission line. This latter plan 



fig 40 




Pig 408. 



o o 



o 
o 



o 



is, perhaps, generally the best ; as for the former plan, one requires a 
switch for rather heavy currents at a potential of often from 300 to 
400 volts ; and such a switch, to be safely opened, is of much more 
expensive construction than a high-tension switch for the smaller current. 
Moreover, for six-phase rotaries, the low-tension switch should preferably 
have six blades, as against three for the high-tension switch. It is much 
simpler in six-phase rotary converters to have an arrangement which 
obviates opening the connections between the low-tension terminals of the 
transformers and the collector ring terminals, although in such cases some 

2 Y 



346 Rotary Converters. 

type of connectors should be provided which may be readily removed when 
the circuits are not alive, for purposes of testing. 

The arrangement shown in Fig. 404 represents a plan for synchronising 
and switching, on the high-tension circuits, and adapted to six-phase 
rotaries. 

Fig. 405 shows diagrammatically a plan for a three-phase system where 
the switching is done on the low-tension circuits. The quick-break switch 
used, which is necessarily of rather elaborate construction, is illustrated in 
Figs. 406, 407, and 408. This switch was designed by Mr. Samuelson. 
The switch is designed for the breaks to occur on the back of the board, 
thus protecting the operator. 

VOLTAGE RATIO IN ROTARY CONVERTER SYSTEMS. 

As already shown, there is a tolerably definite ratio between the 
alternating-current voltage at the collector rings and the continuous- 
current voltage at the commutator. This lack of flexibility is, to a certain 
degree, a source of inconvenience ; hence, methods whereby it may be 
avoided possess interest. A rotary converter with adjustable commutator 
voltage, is desirable for the same purposes as an over-compounded 
generator, and also for charging storage batteries. 

If the generators, transmission line, transformers, and rotary con 
verters possess sufficient inductance, the commutator voltage may be 
varied within certain limits by variations of the field excitation of 
converter or generator, or both. By weakening the generator excitation 
or strengthening the rotary excitation, the line current may be made to 
lead, and a leading current through an inductive circuit causes an increased 
voltage at the distant end of the line. Hence, by suitable adjustment of 
the excitation, the voltage at the collector rings of the rotary, and con 
sequently also its commutator voltage, may be increased. Strengthening 
the generator field or weakening the converter field, or both, causes the 
current to lag, and results in a decreased commutator voltage. These 
effects may be intensified by placing inductance coils in series in the 
circuits. 

Another method of controlling the commutator voltage is by 
equipping the step-down transformers with switches whereby the number 
of turns in primary or secondary, and hence the ratio of transformation, 
may be adjusted. A much better method consists in employing an 



Methods of Adjusting Voltage Ratio in Rotary Converter Systems. 347 

induction regulator between the transformer secondary terminals and the 
rotary converter. This consists in a structure like an induction motor. 
Series windings are put on the one element, say the stator, and potential 



HIGH TENSIOH ALTEfNATING BUS BAPS 




windings on the rotor. The rotor may be progressively advanced through 
a certain angle, and at each angular position will raise or 
voltage at the collector rings by a certain amount, by virtue of 1 
action of the series and potential coils. The connections 
diagraniinatically in Fig. 409. 



348 



Rotary Converters. 



A small auxiliary rotary converter, having a voltage equal to the 
amount by which it is desired to increase or decrease the commutator 
voltage of the main rotary, and with a current capacity equal to that of 
the main rotary, may be employed with its commutator in series with that 
of the main rotary. The auxiliary rotary should have field coils capable 
of exerting a great range of excitation. Its collector should be supplied 
from a special transformer or transformers, with the primary and secondary 
coils considerably separated, so as to permit of much magnetic leakage 
between them. This gives large inductance to the small branch circuit 



J J 






J I 


J 


J 


11 11 1 


1 


1 11 



CONTINUOUS CURBfNT 




1 


..T. 


THREE PHASE LINE 

7, 






JLL 


--] 


__ 






\ 


:: 


1 




1 












IT 




-I 


f U 





AUXILIARY BOOSTER 



MAIN ROTARY 



leading to the auxiliary rotary, and by regulation of its field excitation, 
a very wide range of voltage at its commutator is secured. It has the 
great advantage over inductance in the main circuit that it gives a wide 
range of voltage variation for the combined set, consisting of main and 
auxiliary rotary, without working at low-power factors. This is obviously 
the case, since the main rotary may be adjusted to work at a power 
factor of unity, while it is only the relatively small amount of energy 
consumed by the small capacity auxiliary rotary, which is supplied at a 
low power factor. The effect on the power factor of the main system, 
caused by the power factor of the small rotary, may be completely 



Methods of Adjusting Voltage Ratio in Rotary Converter Systems. 34 J 




350 



Rotary Converters. 



neutralised, and the resultant power factor restored to unity by the simple 
method of running the large main rotary with a slight over or under 
excitation, and hence with a power factor slightly lower than unity, to 
compensate for the lagging or leading current, as the case may be, con 
sumed by the small auxiliary rotary converter. The scheme is illustrated 
diagrammatically in Fig. 410. 

A similar piece of apparatus has been used for the express purpose 
of charging storage batteries from a 500-volt line. With maximum 
excitation, it supplied 200 volts more, giving the 700 volts required by the 
battery toward completion of the charge. This rotary converter had a 
shunt winding, and also a negative series coil, and when finally adjusted 
it had the interesting property of automatically charging the battery from 
a minimum potential in the neighbourhood of 530 volts at the commence- 




seaies BOOSTER 



ment of the charge, up to about 700 volts when fully charged. Moreover, 
the current, amounting to some 40 amperes at the commencement, gradually 
fell off to about 30 amperes when the battery was fully charged. That is, 
when the battery charge is low, and this rotary converter is thrown on in 
series with the 500-volt line, it automatically regulates its own excitation 
so that, while giving 30 volts and 40 amperes at first, it finished up with 
200 volts and 30 amperes. Its shunt coils are excited from its own 
commutator ; hence at gradually increasing voltage. 

Its series winding is connected to act in opposition to the shunt 
winding. This negative series winding was at first put on to protect the 
rotary from the effect of sudden variations of voltage on this 500-volt 
circuit. Thus, if the line voltage suddenly rose to 520 volts, the addition 
of the rotary voltage would have sent a much heavier current into the 
battery ; a negative series winding tended to equalise the resultant voltage 
in spite of line variations, and proved to contribute very markedly to the 



MefJwds of Adjusting Voltage Ratio in Rotary Converter Systems. 351 

automatic regulation of current and voltage to the varying requirements 
during the process of charging the storage battery. 

In Fig. 411 is given a diagram of its connections. 

An alternative scheme to that of a small auxiliary rotary converter, 
and, perhaps, on the whole, the best arrangement of all, consists in the 
addition of a small continuous-current machine on an extension of the 
shaft of the main rotary converter. If its fields are excited in series with 
the load, and its commutator connected in series with that of the main 
rotary converter, the combined set may be adjusted to over-compound to 
any desired extent. Fig. 412 gives a diagram of this scheme. 

A great disadvantage of both these last schemes is that the com 
mutator of the auxiliary machine carrying the main current must have 
substantially as great a radiating surface as the main commutator, and 
hence is expensive. The commutator losses are also doubled. 

Still another interesting arrangement for giving an adjustable ratio 
of conversion of voltage, is that illustrated in Fig. 413, wherein a small 
synchronous motor is directly connected on the shaft of the rotary, which 
requires no collector rings ; those of the synchronous motor serving for 
the set. The synchronous motor has a separate field system, by varying 
the excitation of which, the percentage of the voltage consumed in the 
synchronous motor, is varied, and consequently also the total ratio of 
conversion. This scheme avoids the losses in an extra commutator, and 
is a very flexible method. 

RUNNING CONDITIONS FOR ROTARY CONVERTERS. 

The conditions relating to starting rotary converters have been con 
sidered on pages 340 to 344. After being finally brought to synchronous 
speed, there remain various adjustments requisite to secure the most 
efficient performance, and to adapt them to best fulfil the special 
requirements. 

Phase Characteristic. The term " phase characteristic" is generally 
applied to a curve plotted with field excitation (preferably expressed in 
ampere-turns per field spool), for abscissae, and with amperes input per 
collector ring, as ordinates. Such a curve has been given for no load in 
Fig. 400, on page 333, and from an examination of it, one learns that, 
at normal voltage between collector rings (310 volts in the machine 
in question), and a field excitation of 0.4 amperes (5800 ampere- 



352 Rotary Converters. 

turns per pole), there was required only about 80 amperes per phase 
to run the rotary converter unloaded. This is the condition of 
minimum current input ; with weaker field excitation the current 
lags, and with stronger it leads, in both cases increasing rapidly in 
amount with the varying field excitation. The curve shows that with 
no field excitation, the current per phase increases to about 2100 amperes, 
and it also reaches approximately this same value with twice the normal 
field excitation. 

If the current is in phase at the point of minimum current input, then 
the volt-amperes will be equal to the sum of the no-load losses. 



NO-LOAD LOSSES. 

Watts. 
Core and stray losses at normal voltage ... ... ... ... = 20,000 

Friction and collector C 2 R losses... ... ... ... ... = 8,000 

Shunt field self excitation = G.4 x 500 ... ... = 3,200 



Total no-load losses =31,200 

Watts per phase ... ... ... ... ... ... ... = 10,400 

310 
"Y" voltage =,- = 180 volts. 

Current per phase (i.e., entering each collector 

10,400 

ring) = ? ... ... ... ... ... = 58 amperes. 

180 

Hence we have an unaccounted-for balance of 80 58 = 22 amperes. 



This is due partly to a difference in the wave forms of the generator 
and the rotary, but chiefly to so-called "surging" effects, and will be a varying 
value, depending upon the motive power driving the generating alternator, 
and upon the methods employed to limit the effect. It will be considered 
in a subsequent paragraph. 

Neglecting the "surging" effect, for a given field excitation, the power 
factor of the incoming current may be estimated. Thus the curve of Fig. 
400 shows that with the excitation of 3.2 amperes (half the normal excita 
tion) there is an incoming current of 1000 amperes per phase. One 

thousand amperes entering a collecting ring corresponds to j= = 580 
amperes in the armature conductor. 



/ re-determination of Phase Characteristic Curve*. 353 

Resistance of armature between commutator brushes has been 
given as .005 ohm at 60 cleg. Cent. = R. (See page 332.) 

Then the resistance of one branch (i.e., one side of the A) will be 1 33 
R = .0067 ohm. 1 

In each branch there will be a C 2 R loss of 580 2 x .0007 = 2250 watts, 
and therefore a total armature C 2 R of 3 x 2250 = G750 watts. The field 
excitation with regulating rheostat losses will be one-half its former value, 
i.e., 1650 watts. The core loss and friction remain substantially as before, 
but the collector C 2 R loss is increased by 500 watts. 

SUMMARY. 

Watts. 
Armature C 2 R ... ... ... 6750 

Field self-excitation ......... 1 (550 

Core and stray losses ......... 20 000 

Friction and collector C Z R losses ... 8,500 



Total of losses 36,900 

Total per phase ... 12,300 

Volt-amperes input phase = 580 x 310 = 180,000. 

\ f) 3 

Hence power factor = - = .068. 
180 



1 Proof that, if R = armature resistance between commutator brushes, then 1.33 R 
= resistance of one side of the A. 

Take the case of the present rotary. It has 12 poles, and a multiple-circuit single 
winding. Therefore, there are 12 paths through the armature from the positive to the 
negative brushes. There are 576 total turns on the armature. Hence, each of the 12 paths 
has 48 turns. R = the resistance of the 12 paths in parallel. . . 12 R = resistance of one 
path of 48 turns. But between two collector rings, the 576 total turns are divided into three 
groups of 192 turns each. One side of the A is made up of one such group arranged in six 

192 
parallel paths of - - = 32 turns each ; 32 turns in series will have a resistance of 

x 12 R = 8 R, 

48 

8 R 
and six paths in parallel will have a resistance of = 1.33 R, and this equals the 

resistance of one side of the A. Q.E.D. 

Any difficulties in understanding this subdivision of the winding into groups and parallel 
paths may be removed by a study of the winding diagram for the multiple-circuit single 
winding shown in Fig. 373, on page 297. Analogous investigations of two-circuit single 
windings, and of multiple windings of both the two-circuit and multiple-circuit type, will 
yield the same result, i.e., that the resistance of one side of the A is equal to 1.33 R, 
for three-phase rotaries. For an examination of these latter cases, one may make use of 
the winding diagrams of Figs. 374 and 375, on pages 298 and 299. 

2 z 



Rotary Converters. 

Similar Ccalculations for other values of the field excitation, give data 
for plotting other phase characteristic curves for no load, that is, for no 

/VO LOAD PHASE CHARACTERISTICS. 
&UJ.414. O f dOOEw. 25 Cycle* dOOVoUs 

Three Phcuse RotMjy Converter. 
Power factor uv terms ofFLelci ExcitaJtL0n>. 



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output from the commutator. Thus in Fig. 414 the power factor is plotted 
in the terms of the field excitation ; and in Fig. 415 in terms of the amperes 
input per collector ring. These curves have all corresponded to no load, 



Prc-detr.rmination of />J,, Mfi CJ^mcterixtic CWiw. 355 

but other phase characteristic curves may be obtained for various 
conditions of load. 

In Fig. 416 are given phase characteristic curves at no load, half load 
and full load for a 125-kilowatt rotary converter. It will be observed 
that the phase characteristic curves with load possess the same general 
features as the curve for no load, though less accentuated. 



PHASE CHARACTERISTICS 



fig 416. 



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In Fig. 417 these curves are transformed into three others in which the 
power factors are plotted in terms of field excitation ; and in Fig. 418 the 
power factors are plotted in terms of amperes input per collector ring. 

Figs. 414, 416, and 417 show the importance, especially with light 
loads, of careful adjustment of the excitation. The power factor falls off very 
rapidly indeed with variations of the field excitation from the normal value. 
However, with load, the variations are comparatively moderate, and field 
regulation can then advantageously be employed as a means of phase 



356 



Rotary Converters. 



control ; and through the intermediation of line and armature inductances, 
sometimes aided by auxiliary inductances employed for the express purpose, 
a considerable working range of voltage, at the commutator of the rotary 
converter, may be obtained. 

This brief description of the phase characteristic curves permits of 
now explaining, in a rough, practical way, what causes the current to lag 
or lead with varying field excitation, and also what controls and determines 



PHASE CHARACTERISTICS. 



of 
Fig417 Three Phase- Rotary Converter 

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Ampere turns per Field Spool. 



the extent by which it shall lag or lead. Suppose a generator, say by 
hand regulation of the field excitation, is made to furnish 310 volts, under 
all conditions of load and phase, to the collector rings of a rotary converter. 
(Assuming the rotary converter to be of very small capacity relatively to 
that of the generator, these variations will not materially affect the gene 
rator voltage, which will remain approximately constant.) 

It has been shown that there will be substantially 500 volts at the 
commutator when there are 310 volts between collector rings. This is 
fairly independent of the field excitation. But figuring from the 310 volts 



Pre-determination of Phase Characteristic Curves. 



357 



at the collector rings, or the 500 volts at the commutator, the result 
arrived at is that there is a magnetic flux M per pole-piece, linked with 
the armature winding turns. When the field excitation is such as to afford 
the requisite magnetomotive force for impelling this flux M against the 
reluctance of the magnetic circuit, there will be no current in the armature, 
or, rather, only the small amount necessary to supply the power represent 
ed by the no-load losses. But if the field excitation is weakened, say, to 
one-half, then, since there is still the same terminal voltage, it follows that 
there must also be the same flux M impelled through the same magnetic 



PHASE CHARACTERISTICS. 
of125R*. 35 Cycle 115Volt,. 
Three Phase Rotary Converter. 









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circuit. The remaining part of the required magnetomotive force has, 
therefore, to be sought for elsewhere. It is, in fact, furnished by a lagging 
armature current which then flows into the collector rings. This com 
ponent does no work, hence it is 90 deg. out of phase. The resultant 
current is composed of the energy component which overcomes the losses, 
and this wattless current. Thus in the analysis on page 352 of the phase 
characteristic curve of Fig. 400, it was found that reducing the field 
excitation from 6.4 amperes, (corresponding to unity power factor), to 
3.2 amperes, increased the input from 80 amperes per collector ring to 
1,000 amperes per ring. The magnetising component of this 1,000 amperes 
- 80 2 , and hence scarcely differed for 1,000 amperes. There 



was 



358 



Rotary Converters. 



are, therefore. - . = 580 amperes per side of the " delta," or - = 97 
V3 6 

amperes per armature conductor. This, assuming a sine wave of incoming 
current, is 97 x \/2 = 138 maximum amperes. A current of 6.4 amperes 
in the field corresponded to a magnetomotive force of 5,800 ampere-turns. 
This, with 3.2 amperes, was reduced to 2,900 ampere-turns, the remaining 
2,900 ampere-turns per pole-piece being supplied by the lagging current in 
the armature winding. The 12-pole armature has 576 total turns, or 48 per 



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15 +15 +15 +15 +1;5 +1-5 



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current/ vahLee ~w the- vifper candvucJUirs. 
* ,, <r ,, ,, lower ,, 

aw a KJResviJLtant/ Current/ vaJjuues per pear of Caruhmtjors. 

pole-piece; but these 48 turns per pole-piece belong to three different 
phases, hence there are 16 turns per pole-piece per phase. The maximum 
ampere-turns per phase are 

16 x 138 = 2,200 ampere turns. 

In Figs. 419 and 420 are shown, diagrammatically, the arrangement 
of the conductors of the different phases in the armature slots of a three- 
phase rotary, and directly above, the corresponding curve of magneto 
motive force due to the currents in the armature conductors. Fig. 419 
represents the instant when these relative current values in the phases 
A, B, and C are, respectively, 1, .5, and .5. In Fig. 420 these have become 
.867, 0, and .867. Hence it is in Fig. 419, that one phase reaches the 
maximum value 1 , and as there are six conductors per pole-piece per phase, 



P re-determination of Phase Characteristic Curves. 



359 



its maximum magnetomotive force may be represented by G. But 
although, in Fig. 419, the corresponding maximum value of the magneto 
motive force of the three phases is ( J, it becomes 10.4, one-twelfth of a cycle 
later, at the instant represented by Fig. 420. Hence, in a three-phase rotary 
converter winding, the maximum magnetomotive force exerted by the 

armature conductors of all the phases is, per pole-piece,- - = 1.73 times 
as great as the maximum magnetomotive force per pole-piece per phase. 



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* InstanbaneouA current/ vabuues uv C/te upper conductors 



,,.,, a 0. ResvJjicwb current vabuues per pair of conductors. 

Now, for the case under consideration (the 900-kilowatt rotary), the 
value of 2,200 ampere-turns per pole-piece was found for the maximum 
magnetomotive force per phase. Therefore, the maximum resultant 
armature reaction for the three phases would be 

1.73 x 2,200 = 3,800 ampere-turns per pole-piece. 

But it is only in opposition to the nux at the very centre of the pole-face, 
that the armature magnetomotive force would exert this strength. 
Approaching both sides, it shades off towards zero, as may be seen from the 



360 Rotary Converters. 

curves of magnetomotive force distribution of Figs. 419 and 420, whereas 
the field spool against which it reacts, is linked with the entire pole-piece. 
In practice, these magnetomotive force curves would be smoothed out into 
something like sine curves. Hence we may take the average magneto- 

O C AA 

motive force exerted over the whole pole-face as about - , = 2,700 

A/ 2 

ampere-turns. This corresponds fairly well with the 2,900 ampere-turns 
by which the field excitation was reduced. 

At first sight, it would appear that this checks well enough for all 
practical purposes, but an analysis of the curves of many other rotary 
converters resulted in almost always finding that 10 to 25 per cent, less 
magnetomotive force on the armature, suffices to replace the field excita 
tion ; which leads to the conclusion that it is the location of this magneto 
motive force in the armature conductors themselves which enables it, with 
from 10 to 25 per cent, less magnitude, to replace the in this respect less 
effectively situated magnetomotive force in the field spools, the flux set up 
from which latter, suffers diminution, by magnetic leakage, on the way 
to the armature. 

The difference between three-phase and six-phase windings, as regards 
the manner of distribution of the conductors of the different phases over the 
armature surface, has already been pointed out on page 303, and is 
illustrated diagrammatically in Fig. 379. Bearing in mind the difference 
there explained, it should be further noted that the so-called six-phase 
winding gives a distribution of its armature magnetomotive force in 
accordance with the diagrams for the magnetomotive force in induction 
motors, which were shown and explained on pages 137 to 140. It 
is there shown that the three phases of such a winding, exert a 
resultant magnetomotive force, whose maximum value is equal to two 
times the maximum value of the magnetomotive force per phase. But 
by Figs. 419 and 420, on pages 358 and 359 ante, it has been 
shown that in the winding of the ordinary three-phase rotary converter 
(when the windings of the different phases overlap), this maximum value 
is only 173 times the magnetomotive force per phase. A six-phaser will, 

therefore, give equally effective response to field variations, with but 3 

2.00 

or 87 per cent, as great an incoming current, as will a three-phase rotary con 
verter. This is a distinct advantage, even for the shunt- wound and for the 
compound-wound rotary, but it is still more important in the case of the 



Pre-determination of Phase Characteristic Curves. 361 

series rotary, and for the rotary without field excitation (which will shortly 
be discussed), since the chief objections to these latter types relate to the 
large incoming current due to absence of control of field excitation, except 
by means of armature reactions. 

The choice of as many turns per pole-piece on the armature, as good 
constants, in other respects, will permit, is, of course, conducive in all 
types of rotaries to the best result, from the standpoint of securing the 
required magnetomotive force from the armature with as little idle current 
as possible. 

By similar methods the magnetomotive force relations may be 
analysed from the phase characteristics with load. Under these con 
ditions, i.e., with current delivered from the commutator, there are further 
considerations : The demagnetising influence of the commutated current 
may be neglected, as the brushes remain at the neutral point, and even the 
distorting influence upon the magnetic distribution may be considered 
to be substantially offset by the overlapping energy component of the in 
coming alternating current. The main difference appearing in the analysis 
of the phase characteristic with load, is that the energy component, except 
with great weakening or strengthening of the normal field, will be a very 
appreciable component of the total resultant incoming alternating current. 
Thus, in Fig. 416 (page 355 ante], the upper curve represents the phase 
characteristic with full load output of 1100 amperes at 115 volts from 
the commutator. At normal field of 2750 ampere-turns, the amperes 
input per collector ring are 1030. Reducing the field excitation to 
zero, increases this incoming current to 1290 amperes. The output is 
125,000 watts. 

The internal losses under these conditions of full-load output and zero 
field excitation, are approximately as follow 

Watts. 

Total armature C 2 R loss 

Bearing and all brush friction 2 ,70( 

Core loss 

Brush C 2 R losses 3 > 500 



Total internal loss ... 13,900 

Watts output... ... - 125 > 000 



Total watts input ... . 138,900 

Total watts input per phase . , . 



8 A 



362 Rotary Converters. 

Voltage per phase ... ... ... ... ... ... 75 volts. 

Energy component of current per phase in armature ... 616 amperes. 

Observed current input per collector ring ... ... ... 1,290 ,, 

,, ,, in armature winding ... ... ... 745 

Magnetising component = ^745- 616 2 = ... ... 406 

The armature has a six-circuit single winding with 180 total turns; therefore, 

10 turns per pole-piece per phase. 

Magnetising current per turn = =135 amperes. 

o 

Maximum magnetomotive force per phase = ^2 x 135 x 10 = 1,900 ampere-turns. 
Hence maximum of resultant magnetomotive force of armature per pole-piece 
= 1.73 x 1,900 = 3,300 ampere-turns. 

3,300 
Average value over pole-face = ,- = 2,300 ampere-turns. 

These serve to set up the same magnetic flux through the armature 
winding, for which 2,750 ampere-turns per field spool were required. The 
latter, however, were less favourably situated, there being much magnetic 
leakage to be deducted from the initial flux set up. 

" Surging " Effect. Reference has been made to the " surging " effect 
in rotary converters as being chiefly responsible for the discrepancy 
between the observed current input, when the field is adjusted for minimum 
input, and the energy-current input. This additional current is of the 
nature of an interchanging current amongst the generators and rotary 
converters. When, in the first place, the source of power driving the 
generator has not a constant angular effort, the flywheel may not be 
sufficiently large to make the angular velocity uniform throughout the 
revolution. 

The rotary converter, to remain strictly in synchronism, must respond 
perfectly to those changes in angular velocity. Of course, it cannot do 
so perfectly, so the result is that at one instant it lags behind by a more 
or less small fraction of an alternation, (distance from mid-pole-face 
position), and takes more current ; then it accelerates more rapidly, gains 
on the generator, and swinging too far forward, on account of its 
momentum, acts for the instant as a generator, returning current to the 
source of its supply. This is the nature of the superposed current above 
referred to. 

According to the degree of unevenness of the angular speed of the 
generator, and to the absolute and relative inertia of the moving parts 
of the generators and rotary converters, this superposed swinging motion 
may be more or less great, and may, either between generators and rotary, 






Compound-Wound Rotary Converters. 363 

or between rotaries, develop into sympathetic swings of considerable 
magnitude, leading, in some cases, to falling out of phase, but more often 
to serious and rather destructive sparking at the commutator, due to the 
pulsations. As already pointed out, these troubles may be remedied in 
practice by employing copper coils or plates specially located between 



SERIES ROTARY 

Th Phase,. 25r] 100 Rv output,. 




80 100 KV 140 

Continuous Current Amperes oucpuc from Commutator. 



pole-pieces ; or more easily, but less economically and effectively, by using 
wrought-iron pole-pieces of the highest practicable conductivity, with small 
clearance between pole-face and armature. 

Compound- Wound Rotary. The purpose of the compounding coil 
(series winding) has already been set forth (see page 324), and it merely 
remains to state that in practice it has been found to distinctly diminish 
the tendency to stability when the "surging" effect is present to any 



364 



Rotary Converters. 



extent. Nevertheless, it is an aid to automatic phase regulation, being, 
of course, more especially valuable where quick changes of load are 
constantly occurring, as in the operation of tramways. For gradually 
varying load, pure shunt excitation with hand regulation is more 
satisfactory, unless the generator is driven with an extremely uniform 



angular motion. 



$10,422 ROTARY CONVERTER 

** Without/ Field/ 



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200 

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160 

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SO 04) 



w/ Continuous Current Amperes output from Commutator. 

The current delivered from the commutator of a rotary converter is 
never very uniform ; it has always a superposed alternating-current com 
ponent, which may be readily demonstrated by sending such a commutated 
current through a reactance coil of sufficient inductance, when there may 
be observed across the terminals of the coil (by an alternating-current 
voltmeter) a difference of potential many times in excess of the CE, drop. 1 
Although this is best observed by means of the drop across it, such a 



1 See Jour. Inst. Elec. Engrs., vol. xxvii., page 710, 1898. 



Series Converters ami Converters without Field Excitation. 365 

reactance coil tends to eliminate these variations, and they are much less 
than when no inductance is in circuit. A compound winding will, to a 
certain degree, have this same effect ; and while the difficulties attending 
its use are probably partly due to this effect, it should, at the same time, 
in some measure tend to make the commutated current more free from 
superposed variations. The series winding is cut out when starting up 
from the continuous-current side, and this is conveniently accomplished by 
a double-throw switch, which in one position connects the junction of the 
series winding and the negative brushes to the starting rheostat, and in 
the other position connects this point with the equalising bar. 

Series Rotary. The shunt winding may be dispensed with altogether 
in a rotary converter, the excitation being supplied by the series winding 
alone. The conditions, however, are not satisfactory, as the excitation is 
controlled entirely by the load current ; and from what we have learned 
by a study of phase characteristics, such wide variation of excitation cannot 
be made to give an economical power factor for any extended range of 
load. Curves taken upon a 550-volt, 100-kilowatt rotary, operated in this 
manner, are given in Fig. 421. 

Rotary without Field Excitation. A rotary with no field winding 
supplies its excitation by virtue of the magnetising effect of the lagging 
currents flowing through its armature, and which enter from the collector 
rino-s. In Fig. 422 is given a curve of the alternating-current in terms of 
the continuous-current output for the above-mentioned 100-kilowatt rotary 
when operated with no field excitation. In this case, the excitation of the 
generator was raised from 5,500 ampere-turns per spool, when no amperes 
were delivered from the commutator of the rotary converter, up to 
7,000 ampere-turns per spool at full load amperes delivered from the 
commutator of the rotary converter. This served to maintain the com 
mutator potential of the rotary, constant at 550 volts, throughout the 
whole range of load. This increased excitation of the generator was 
necessary,as it also was of only 100-kilowatt capacity, and the large 
demagnetising magnetomotive force of the lagging armature current acting 
against its own impressed field, required to be overcome by the increase 
of field excitation from 5,500 to 7,000 ampere-turns per spool. Such 
rotaries without field windings have, however, actually been employed 

commercially. 

The advantage of having, for rotaries of this type, a very 
armature, even to the sacrifice of the most favourable values for other 



366 Rotary Converters. 

constants, will now be clearly seen. The armature winding will thereby 
be enabled to supply the required magnetomotive force with less excessive 
magnetising currents from the source of supply. The use of six collector 
rings (so-called six-phase), has in this respect an advantage of 14 per 
cent., for a given armature and winding, over the ordinary method with 
three rings. 



Appendix. 



367 







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3G9 



hi 


HUH am * 53H3 gSSH 39533 W VM 


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sliiisi ISs: 



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o o5ooo o c o -r o => o s = o o .= .= o c = 555 55 



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B oo oco Soooo 3Sc58 5ii5* " * I 5 5 i 









370 



Appendix. 



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<<:<;<<:-<^-<-<o3SDooao .0 Sw 



Appendix. 



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372 



Appendix. 



Fig. 126, on page 126, gave a saturation curve for sheet iron at 
high densities, but for the purposes of that section investigation of the 
reluctance of core projections the curve was plotted in C.G.S. units. 



SATURATION CURVE 
for Sheet Iron, at Jiicfh, densities. 



j 

9 



5. 
i 

s> 

i 

3 
3 no 



i 

(5<U>&) 














^ 


^ 


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) 2OO 4OO GOO 8OO WOO 1200 1<WO 1600 iSiK 
Amoere. turns ner Inch nf 1 print h 



FIG. 423. 



As a sheet iron curve for high densities is constantly required for reference, 
it has been re-plotted in Fig. 423, in the system of units employed through 
out the other sections of the work. 



( 373 ) 



INDEX. 



AGEING of iron, 29 

Air-gap reluctance, 121 

Alloys, table of physical and electrical properties, 

370 

Alternating current machine windings, 71 
Aluminium steel, magnetic properties of, 24 
Analyses, chemical 

Cast iron, 19 

Cast steel, 20, 22, 24 

Mitis iron, 24 

Wrought iron, 27 
Annealing sheet iron, effect of, 29 
Armatures 

Magnetomotive force of, 116 

Radiating surface of, 92 
Armature coils, method of insulating, 57 
Armature core losses, 35 
Armature core reluctance, 119 
Armature, reaction of 

Alternators, 118 

Continuous-current dynamos, 117 

Continuous-current, constant potential dynamos, 

145 
Armature windings 

For alternating-current machines, 71 

For continuous-current machines, 60 

Gramme ring, 62 

Drum, two-circuit, 66 

Drum-multiple circuit, 62, (58 

For induction motors, 75 

For rotary converters, 70 

Symbols for, 66 



BINDING-WIRE for rotary converters, 321 

Bond-paper, oiled, insulating properties of, 39 

Brushes, carbon 
Use of, 144 

Contact resistance and friction loss, 273 
Comparative tests of carbon and graphite, 277 

Brush-gear, 271 



CAMBRIC, oiled, insulating properties of, 41 
Cartridge-paper, insulating properties of, 41 
Chemical analysis of 

Cast iron, 19 

Cast steel, 20, 22, 24 

Mitis iron, 24 

Wrought iron, 27 
Coils, internal and surface temperature of, 9. 5 

Methods of insulating, 57 
Commutation, essential conditions, 152 
Commutators, heating of, 112 
Conductivity tests, 2 
Conductors, watts dissipated in, 101 

Foucault currents in, 103 
Conversion of magnetic units, 4 

Of hysteresis loss units, 9 
Converters. See Rotary Converters. 
Copper wire, properties of, 367, 368, 369 
Core losses 

Estimation of, 35 

In commutating machines, 229 
Correction factor for voltage of distributed winding, 

81 

Cotton, oiled, insulating properties of, 41 
Curves, hysteresis, 30, 32, 33, 34 

Permeability, 19, 21, 23, 26 and 126 

Saturation, for high densities, 372 

Tooth density correction, 126 

DETERIORATION of iron, 29 

Drum windings 
Two-circuit, 66 
Multiple circuit, 62, 68 

Dynamos 

Continuous-current constant potential, 143 
Influence of armature reaction, 151 
Proportioning of, 150 

1,500 K.W. railway generator, description, 179 
200 K.W. railway generator, description, 190 
30(3 K.W. lighting generator, description, 201 
250 K.W. railway generator, description, 215 



374 



Index. 



EDDY current losses 

In conductors, 103 

In pole faces, 105 

In sheet iron, 35, 105 
Efficiency of 

Arc dynamos, 111 

Constant potential dynamos, 111 

Railway motors, 111 
Electromotive force 

In alternating-current dynamos, 80 

In continuous-current dynamos, 78 

In polyphase apparatus, 87 

In rotary converters, 84 

In transformers, 88 

FIBRE, vulcanised, insulating properties of, 39 
Field winding 

A calculation for shunt dynamo, 128 

Formula for, 127 

Method of insulating, 58 
Flux in transformers, 88 
Forgings, magnetic properties of, 25 
Form factor, 88 
Formula for 

Eddy current loss, 35 

Electromotive force, 78 

Field winding, 127 

Magnetomotive force, 3 

Reluctance, 124 

Two-circuit windings, 69 
Foucault currents. See Eddy currents. 
Friction loss, 114 

GENERATION of heat, specific rate, 109 
Generators. See Dynamos, 
Gramme ring windings, 62 

HEAT, specific rate of generation of, 109 
Heat losses. See Losses. 
Heating of 

Arc dynamos, 111 

Commutators, 112 

Constant potential dynamos, 111 

Railway motors, 111 
Hysteresis 

Curves of, in actual practice, 34 

Determination of (general), 9 

Effect of pressure on, 32 

In alternating and rotating fields, 10 

In cores, 107 

Method of measurement without ballistic gal 
vanometer, 11 

Testers, 11 and 14 

Variation with magnetisation, 28 



INDUCTANCE 

Constants, 159 

Experimental tests, 160 

Practical definition of, 160 
Induction motor windings, 75 
Insulating coils, methods of, 57 
Insulation resistance, effect of temperature on, 42 
Insulation testing methods for factories, 43 
Iron, cast 

Effect of chemical composition on, 16 

Magnetic properties of (general), 14 

Specific resistance, 36 
Iron, malleable cast 

Effect of chemical composition on, 18 
Iron, Mitis, magnetic properties of, 24 
Iron, Nickel, 25 
Iron, sheet 

Ageing of, 29 

Eddy current losses, 35 

Magnetic properties of, 25 

Temperature of annealing, 29 
Iron, Swedish 

Analysis of, 34 

Magnetic properties, 25 
Iron, wrought 

Analysis of, 27 

Magnetic properties of, 25 

Specific resistance of, 36 



LEAKAGE coefficient, 119 

Leatheroid, insulating properties of, 39 

Limit of output, thermal, 90 

Linen, oiled, insulating properties of, 39 

Linen, shellaced, insulating properties of, 39 

Losses C 2 R, 101 

C 2 R in rotary converters, 285 

Eddy current, 35, 103 

Friction, 114 

Hysteresis, 9, 28, 32, 107 



MAGNETS, radiating surface of, 92 
Magnet winding 

A calculation for shunt dynamo, 128 

Formula for, 127 
Magnetic circuit 

A calculation for, 126 

Design of, 115 

Of the induction motor, 137 

Of transformers, 117, 13o 

Reluctance of, 121 

Typical forms of, 129 
Magnetisation of iron and steel, 17 



Index. 



375 



Magnetomotive force 

Of armatures, 117 

Of rotary converter armatures, 358 
Malleable cast iron 

Effect of chemical composition on, 18 
Manilla paper, insulating properties of, 41 
Marble, insulating properties of, 40 
Materials, insulating - 

Effect of temperature on, 42 

Method of testing, 43 

Properties of, 39 

Materials, Magnetic, properties of, 14 
Metals table of physical and electrical properties, 

370 

Mica, insulating properties of, 38 
Mica-canvas, insulating properties of, 47 
Mica longcloth, insulating properties of, 50 
Mitis iron, magnetic properties and analysis of. 24 
Motors, railway, 233 

Description of geared 24 horse-power motor, 233 

Description of geared 27 horse-power motor, 242 

Description of direct-connected 117 horse-power 
motor, 256 

NICKEL iron, magnetic properties of, 25 
Nickel steel, magnetic properties of, 25 

OILED bond paper, insulating properties of, 39 
Oiled cambric, insulating properties of, 41 
Oiled cotton, insulating properties of, 41 
Oiled linen, insulating properties of, 39 
Oiled paper, insulating properties of, 41 
Output, thermal limit of, 90 
Oven, vacuum, for drying coils, 58 

PAPER, Insulating properties of different makes, 41 
Permeability, curves, 19, 21, 23, 26, 126, 372 

Bridges, 6, 8 

Effect of pressure on, 32 

Tests with ballistic galvanometer, 3, 

Tests without ballistic galvanometer, 5 
Phase characteristics of rotary converters, 351, 
Poles, determination of number for given output, 

152 

Press-board, insulating properties of, 39 
Pressure, effect of, on permeability and hysteresis, 

32 
Proportioning of dynamos, 150 

RAILWAY generators. See Dynamos. 
Railway motors. See Motors. 
Reactance voltage, calculation of, 175 



^* 
C 

V 

5 



Reluctance of 

Air-gap, 121 

Armature core, 119, 123 

Complete magnetic circuit, 121 

Core projections, 123 
Resistance, insulation 

Effect of temperature on, 42 
Resistance, specific, of iron and steel, 36 
Ring windings, 62 

Rope-paper, red, insulating properties of, 41 
Rotary converters, general, 283 

Advantages of polyphase over single phase, 297 

Compound-wound, 363 

C-R loss in armatures of, 285 

Four-phase, 306 
nterconnection with static transformer, 304 

Magnetomotive force of armatures, 358 

Output with different number of phases, 284 

Phase characteristics of, 351 

Running conditions for, 351 

Series-wound, 365 

Single phase, 295 

Six phase, 303 

Six phase, 400 K.W., description of, 311 

Starting of, 340 

Surging effect of, 284, 352, 362 

Synchronising of, 345 

Three phase, 300 

Three phase, 900 K.W., description of, 329 

Twelve phase, 309 

Winding of, 75 

Without field excitation, 365 
Rotary converter systems 

Adjusting voltage ratio in, 347 

Proportioning binding wire, 321 

Space factor, 320 
Rubber, hard, insulating properties of, 40 

SATURATION curve for high densities, 372 
Sheet iron 

Ageing of, 29 

Eddy current losses in, 35 

Magnetic properties of, 25 

Temperature of annealing, 29 
Sheet steel 

Ageing of, 29 

Eddy current losses in, 35 

Magnetic properties of, 25 

Temperature of annealing, 29 
Shellaced linen, insulating properties of, 39 
Shellaced paper, insulating properties of, 41 
Slate, insulating properties of, 40 
Space-factor in rotary converters, 320 
Specific resistance of iron and steel, 36 



376 



Index. 



Steel, aluminum, magnetic properties of, 24 
Steel, cast 

Effect of chemical composition, 20 

Magnetic properties of, 14 

Specific resistance of, 36 
Steel, nickel, magnetic properties of, 25 
Steel, sheet- - 

Ageing of, 29 

Eddy current losses in, 35 

Magnetic properties, 25 

Temperature of annealing, 29 
Surging effect in rotary converters, 284, 352, 362 
Swedish iron 

Analysis of, 34 

Magnetic properties of, 25 

Switch for synchronising rotary converters, 345 
Switchboard for six- phase rotary converters, 307 
Symbols, winding, 66 
Synchronising of rotary converters, 345 

TEMPERATURE 

Effect on disruptive voltage, 49, 53 

Effect on insulation resistance, 42 

Of annealing sheet iron, 29 
Temperature rise in dynamos, 90 

Estimation of, 93 

Of commutators, 112 

Influence of peripheral speed, 97 
Temperature rise in transformers, 109 
Tests, ageing, 31 

Conductivity, 2 

Heat, 95 

Hysteresis, 11, 13, 14 

Inductance, 160 

Insulation, 46 

Permeability, 2, 5 
Thermal limit of output, 90 
Tooth-density correction curves, 126 
Traction generators. See Dynamos. 



Traction motors. See Motors. 
Transformers 

Electromotive force and flux in, 88 

For insulation testing, 45 

Interconnection with rotary converters, 304 

UNITS, conversion of hysteresis loss units, 4 
Conversion of magnetic units, 9 

VACUUM oven for drying coils, 58 
Voltage of 

Alternating-current dynamos, 80 

Continuous-current dynamos, 78 

Polyphase apparatus, 87 

Rotary converters, 84 

Transformers, 88 
Voltage per commutator segment as related to 

inductance, 157 

Vulcabeston, insulating properties of, 40 
Vulcanised fibre, insulating properties of, 39 

WINDAGE loss, 114 
Windings, armature 

For alternating-current machines, 71 

For continuous-current machines, 60 

Gramme ring, 62 

Drum, two-circuit, 66 

Drum, multiple-circuit, 62, 68 

For induction motors, 75 

For rotary converters, 70 

Symbols for, 66 
Windings, field 

A calculation for shunt dynamo, 128 

Formula, 127 

Wood, insulating properties of, 40 
Wrought iron 

Analysis of, 27 

Magnetic properties of, 25 

Specific resistance of, 36 




1 KIMX.D AT T1JE BEDFORD PRESS, 20 & 21, BEDKORDBVRY, STRAKD, LONDON, W.f. 



01945 






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