# Full text of "Electric generators"

## See other formats

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 Electric Generators. V ^ i . \ \ V g | h j "* b 9 c s o S V I ly g ^ f - ,\ \ *"-L ^ ~, * i & * j j A r" S 3J \ 1 4 \\ j - ^ * 5 i )n v Si , - 1 3 j , ft -- 5 i . ! % * i N x. \ ^ \ 1 gij t) \ 5 : g s -s u 5 v - i a ^ * i \ 1 i A 3 V b V \ \ aj ^ \ \ i - , eo 5 \ v ca i y UJ tj ^! ^ I S V<; i \ V ^^ a ^ ! 1 t ? s s \ i -\ J 1 c- \ g| N > t J" 1 1 y r - ^ 1 V R v - x^- 1 j S ^ * s >^ i^i f \ 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 ps r VV. | J SPOOL fST 1 1 1 refifi 6 tspeeol of Armature /S80\ft grMm Iviip zratbr e oF Air- 18 C 1 rise c F err iptrc Lure measured b ( I es st ance ^~ ^ ^ X \ C > A / ^ y ,f ,v V -/ I / f 2_ / [11S7C) " i! 3 Time 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. o i-Jr-pS n "O $S.IJ4| ^SoSg 1 ^ 0.0- 3= ~ : S ^!s.^ "it-ti ktfil , ^ ^aau. - u U 3. HI 3Unj.VU3dlH3J. JO 3SIU JO 3SIV SE. W tf p H < tf W PM s w H K O Q W W tf oo (J < tf W HH HH tf I I tf W tf fe o 1 5 CO i ! X O Q O 0- 5 Co !"_ II ^QOQOC^t* 113 ^^ 1 * i O s TOO j PERPH. RAD. SURFACE a" . WATTS PER SO.. IN. AT 20 C.. ULTIMATE RISE IN C BY RES. - RISE C PER /io WATT PER SQ..IN. AT 20 i L 3 u 1 \ \ I \\ Ul 1 \ il u c Fig.^05. toP4~i5-97-500. RISE OF TEMPERATURE OF FIELD SPOC BV RESI \ I j! j : c i I i UNAR> ASURED VINPIN \ si ?i ERATUREAS ME RES IN FIELD ^ X s oi^l 5 It i \ ^ >FTEMP AM PI N V fc X, ill W 5 ( f x \ ^ 1S5 ill u ^ QQ Amps in Spool Res of spool atWC ohms. Watts dissipated at 20 C Perph. Rad. surface " Watts per sy.inch at WC Ultimate rise in C by ffes. * * 5i ] - 1^ ** 8"|-_ JjJ MP4-35-975-SOO RISE OF TEMPERATURE OF Fl ELD SPOOLS ARMATURE STATIONARY. .> 1 fe I Rise of Temperature as measured ( Amperes in Fieldwindinq -15 T .c I 1C c C a o o i *o E c ^ cL ^ E ^ k. ^ i \ V, \ 4 tQ \ g \ i \ \ 1 u. \ \ \ cu 1 V L l * 2 *s ^ | 2 u CL L o p **** \ ST. f O 7J V3H /V/ S ^Wf M ui SM s^ . \ k \ 6 10 _L un ~2 a 2 OF TEMPERATURE OF FIELD RUNNING ATA PERIPHERAL SP IATURE AS MEASURED BV RES HS bj a *7 2 | -!si ;* o I." i o [WATTS DISSIPATED AT 20 C -61 -.108.. 168- PERPH. RAD. SURFACE n 31Z..S1Z.312 WATTX PEH XO IN.AT 2O C. -796 >347 iS4 . 2&15. .40 SL.7.&S ATZO-G-J5-Z.-M-...13A I 5 i ^ t i i 2 1 j I OJ. O3SV V3M n JLTIMATE RISE. IN C BY RES ?AS *C PERtfo WATT PER SO. IN. : | ! | A t> g 5 ^ w u *p 4 3 a 4 J TEMPEF AMPS. IN SPOOL.... IRES. OF SPOOL AT z ; . i 1 \f>i i | i . i . f X ^ "*-^ f33U93O Nl 3tmj.VU3dlN3J. JO SSI a Pn H O ft 03 ^ < P3 H W P^ M P3 W PH fe O H O fc w 1=) A fe ^ Nl OS X3d JJ.VM*U3d3. NIHIN3J. 3SIU Fig.108. MP 4 35-975-500. RISE OF TEMPERATURE OF FIELD SPOOLS. ARMATURE RUNNING ATA PERIPHERAL SPEED OF3500FT PER WIN. TEMPERATURE AS MEASURED BY THERMOMETER. UNCORDED SPOOL. CORDED SPOOL. o ujd -10 " 0) IO Q. LQ 5 10 C K ^ o N CO o UJ J In s ffi 891 ffOl 19 S-99l"SOF "l.9- ~ O.on J.V O3J.VdlSSia SJ.J.VM SOI- 801 - -SOI SOt.--90CffOf- SINHO O.OZ J.V -lOOdSJO S1V VK-L si, -SZ-L---I SL- -loads NI SUWK 1 WATTS PER SQ.IN.AT 20 C ...-1 99. .;3SZ..--5S -19Z.. :340. .. .-S30 ULTIMATE RISE IN C BY THERM 13-5 -ZO-3L. 35.1 11 18 .30.-7 1 1 > -s Pi V F z -^n S- i ^ K Ul CL u -^ ^ ^3\ 1* |9 \ > 4sn <vsz- oja jsfvy RISE OF TEMPERATURE OF FIELD SPOOLS UNNING ATA PERIPHERAL SPEED OF48OO Temperature as measured ty Thermometer A Outside of Uncorded Spool. B ,, ., Corded C Absolute temperature of Air. Uncorded spool. Corded Sf 1 n* 1010 Siva ll Ul 1 SPEED OF ARMATURE CONSTAN A~UNGORD ED SPO OL. B" CORDED SPOOL. PERATU I [ \\ J 5 T \, ""^l 1 \ , Of MM HBSOLU r- s = 1 di tfff FSWtf Res.of spool at 20 C ohms. Watts dissipated at 20 C Perpn. Had. suffice D ~ Watts per sq.in. atZO C Ultimate nse in "C by Therm. Rise ~Cper\ nattpersq. in. atZO C - u s - > 1 } ( i PERPH. c 8 i i b i i] \ n 5^ te. I K _j 1 ^ ^ \ V 8 2 - o O. Nl 3Unj.\/U3dH3JL JO 39IH 3. Nr3HnJ.VH3dW31 JO 3SIH Watts Dissipated in Conductors. 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. GO SO v" 2 UJ ft K 3 9 S gJ UJ a *,<7 %1 P ISE OFT NNING A Temt MP4-35 EMPERA TAPER If teroture as 75 MB 61 312 196 v 53-5 3 1Z -975-50 CURE OF >HERALS neasured by 1 125 0. FIELD SF PEED OF Resist?* >OOLS iP^$ ARM.RU 48OOFT.F >ER MIN. ^ ^ Amps, in i Res ofspc Watts dist Perph Rac Walts per Ultimate i P.ise Cper pool ol atZO"Cohms. ipatedat20C 1 surface o sq.in.at2O~C -isem CbyRes \,watt.persqinat20 C id of Arm coristantat 48( / 108 108 08 1685 n 312 / 82 6&15 t 12-7 / / / r 1^-H- / tfrtonp- / Absal utecemp-a Air ~**~~- ^^ 1 \ r \ o <>-< -/ ^p* ^^ r* 3 1& kj 8 i / a ; * / | s. T TIME IN HOURS 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 in it 30 to sa re = : : Fuf.126. to 34 W HO eo w HO aa xteeot ran KM tot uoc IIM not aoo ittt <M KM imatt H r<* vt H f rv 01 irr 1C rr >^ ( c/^ fY 1 ? it t =M /a it- a tl yt t e - - ,-" 1 ^ 3 ~ 1 ^ ^ ^ ^\ ,-*" ,x. X * ^ Fig.ffl. ( X x s s ^ / ^ ^ X ^ ^ / x t X / / , ^ s /* ^ */ x s g / /, // // y A // A ^ 2 ,- J& sas c) iso ao . jio jt Unctirrcctea Density - Ki 0, /SO (7* W 0///7C5 />e^ iS^ //7 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. /?/ -3 129 3 Z <f 1 0. 1- a \ ct 2 z \ <n t? 5 (0 [ \. \ \ V, \ \ \ \ V s \ \ \ 3^Od O73 HOV3 HOUJ JUniVWUV F i \ ie \ i A \\ z 5 CL * j \ a 10 o ^\ P4 ^ Q \ W) i^j ^\ e V 2 \v cs 0. ^ 5 1 \ S in \\ IH | \ \ g* f V ^ 1 z 3 \ ^ \ < IT j ^ % 3 1- \ ^ U1 \ V o \ i s \ \ \ HOYS MOUJ suruvHUV ONuajLN? xmj 130 Electric Generators. (O O tt i o -J c. TJOd O13IJ H3V3 NOVJ SJW/7V03JT .- \ \ iu g \ s z. X (0 AT NO LOAD. \ \ SATURATION C \ - \ \ - \ \ \ s \ \ \ \ \ \ \ \ \ \ *9 C^ ^ nod cnau HOYS ONIUSJLNS xrnj Typical Magnetic Circuits. 131 <i -1- 1 \ \ o \ \ d \ O [ \ a \ \ - Q i \ UJ \ \ i J \ s & 5 O \ X 2; a. \ N u: h- v \ - g \ UJ > V ^ U| Q s \ v ^ t UJ \ > s y % \ \ \ . ^ \ s \ s 3 \ J , x \ t^ X \ X x X, H3V3 HOUJ JOOdSOJJIJ HOV3 NOVJ 3VfUVNUV 132 Electric Generators. \ JE 0. SATURATION CURVE a: (\i \ AT NO LOAD. \ \ \\ \ \ \ \ \ \ \ \ ( \ \ s V. \ l 31Od 013IJ H3V3 HOUJ ZU/UVNUV 3NIU3J.N3 MU S3NI1V03H \ ^ k. I Q <r SEVERAL STANDARD MACHINES. \ SATURATION CURVE AT NO IX) AVERAGE OF TESTS MADE ON \ | \ \ \ ^ \ V 1 \ \ \ \ *v OOdS 013IJ HOV3 HOtfJ 3ttnJ.VNVV ONIU3JLN3 Xmj S3NnV03H 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 ARYI OF A I TUI NG A RMA ?N & IR Gt TURI PER PS -LAIN N. LENGTH J TURNS PER 17- 8" \ \ \ Rg.166. , \ \ \ s^ \ . V ^ PERIN.LE ) C ^ V *^ --^ , a ULTIMATE VALUE OF 56-6 IN AIR "\ N^ ^ P.TURN& < X "^v^^^ =; - ~- . ,, , 38-2 ,, --^ - . ^~ . - . 00 BE I* OJ A EAH Lf/Vi THO FAIf GAP INJ AN NCI- o * 8 73 ;e zo no. 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. __ -. "!. **. -*, -H \ \ ^^ --. >**, \ ^ "X \ si S s ^^ V., \ > S. \ S v> s s s s, s A s s, M .CoiU m/ va m f #L f>t 7n ti wirub ^ "V, 3 coiLs in/ cnv slot/ & on^e/ -in/ cudJcu^&i ^ *ll )f-. v. >^ Ceils i-n> cudjcucent slots. i , -^ 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 j;,c too 3SO 30C ISO wo , 5(7 100 . Un, 16 7. ^* . .- -^ X ^ / X / i to LL ad Saturation Curve Depthafrqap Yie at 7. rp.r / / / f / / t / / / 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 :: ( ei cT ,v& A TT 1 \; "1 : <: @) "^ ~ @ a C + eX J ^ %jr,^ *! j< ---in- 9K * K^7-,(d) i o,-\^ 2 c 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 fsooo Compounding Curve For SOO VolU Rd188 SIX POLE - 2 K.W. SOO VOLT, " GENERATOR FOR I35 R.RM Cur/es oTPoCential distributional noload&atfrftlotdSOBw!:: between brushes inboth cases. Speed-ISO r.pjn.inoc raced speexJi. I 040 4^ -4Mf | ^- - - J ; ,,. -/ _ i / , 5 l 1 / \ 9 ~ / \ ? K $ / / ^ V | /* > ". *^ V >- if . L5 JP J j i C l |( 5 r | (j >! 7; > ] u ,T i t> M 41 I ^^L * "^r-rt- 5 " i . < . : B - I o \ / \ 2 \ . T : 1 Pe , tt fC .- - ion > c. 2 nc CJF * OT, r dKMX r^jS 1 F SIX POLE, ZOO K.W. SOO VOLT GENERATOR FOR 135 R.PM. 3.200 Core Loss. 1 / zeoo 1.400 1.200 3.000 ^1.600 cc / / / / / / / / 600 eoo tee ZOO / / / / / / .- X SO 100 ISO 200 250 300 350 400 450 600 SJSQ fcf (HSC.M) VOLTS 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. o fl *&** \ O * * 5 I ^ ^ *-*)- , ~p 1 ^ a Q, Hria ( o * * o 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. f i ,-K I .A I 4, r -J=d ----i-pd n?-|-- 7^~^i> n3n;sJ> 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 f L s X 1 5 v a - ^ k X 3 7 ~^ >[ S \ V in ec \ S o ^ J fj i * rr* t S " ,-j n 5 LT1 19 a uj <^ S j^ r a! ? .v . z 1: ir u ^ H \ \ \ _i p ^ \ gt X . ol u a 1 \. B ^ ^ ~ ^^ ^ i ! i i : C j i i s S I 10 1 1 i i 8 3 s 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 ^S^^IP III < 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 _l_L-L4-J i-U T-4 4-4-- -M- ! 1_U_4_ L_iV -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 / *Cf M / toe //.I / too *i 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. { X f OL 1 2. O OK Yi WO VOLT G 3SERVED CORE L T 32O R .P U1ERA1 . M OK / / / / f / V / ^ x / a ,S ~S _ - 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 3-0 M f V) I-S hi j S 6 1 3 AUD WATTS PCH POUND IH AltMATUaS. COfiE > ,x -"" X K x ," / ^ ,-" ^ J " . x* x<r j 8 x x< - *> ^ "? 1X *~ y ^ ^ 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. 02 tf o H O (punoj .lad SWAY 0001 \ = MX j -H (M CO 09 * M M B3 3 IO ^ CO 04 CO a M 3 -J * IO _; 00 s s CO CO ** IO g .0001 ao 10 00 :- <" * U9 <l C9 CO s 9 ^< CO 9 -:\ 1 Q i- 2 Q H 9 CQ ^v 9 5 IO p s CO S O5 * H rH H oi -i ffl a = s?oig Avotaq .tysudQ J sauipiiH So O CO 00 t- id S g co i- CO 00 3 CD 8 ~ S 00 ao t 3 CO eo CO CO fw - CO i CO g -* IO ^i S ^ d i- s = puooag jad sap.CQ 8 8 = 5 S r- 5 Q 3 F, s 5 8 5 g Q i-. 5 CO 8 o 5 5 8 >n us sd CD co ed t~ l> t- i> 00 00 00 r S 01 1C id S3 S To g punoj .iad < SSO r I 3JOQ JO SUttAY OS t^ t- < s @ 00 S g 1 t. ft S 3 fe 8 8 -f i s s B 8 00 * 99 -i eo o * ^ H - M "- 1 oi H CO OJ ^ SUOl^TJU raiTn jo ;UPAV pnX .8 ^S o 00 n i - M E CO I ^ i S i IO I O 9 CH 8 7t I CD ^*i < 00 g r e CM i to Q 55 IO 1 10 IO ^ 00 IO O5 ^t s IO o M ~ eo" * rt ^ rt rt M *~ qi83X jo iqSiaAY XJS o CD >9 i CO 99 Q i- r s H IO ^4* to 1 g CO i IO i N *4 s CD 1C e O 9 C-4 ? IO 8 g s;o[g A\o[aq suoiquuiuiBq jo ^uSpAY *s D | , r - ^ r Q " 10 -! oo CO CO o l - * i i C C9 CN O s i CO | -^ ^J i Q e 4i ^i 1 S -1 I jc ** s (M ** S^BAY u ! sso-i 8JOQ paAJasqo pnox O CO^ vrT 1 oT -3 K n T Q S c4 e -1 IO* s r^ ^* Q g of O IO o-i" o ep e> -r 10" H-" CO o co" O C9 of c IO_ oT g 01 of H 00 e IO i of to Cl 1- of g * oJ sauiiojoi u; s5O[g Avopq A ^isuaQ J^ 00 s ~r 03 9 00 co t - 1- CO 3 19 o> ?: oo t~ 00 J> 41 o> CO I- n CO CO t- i> CO 90 ad ^i IO c CO *# 63 I - CO 00 83Ul( -OHM u; moax ?oo>i (jo; XIISUBQ !)najday ^1 m s g S3 CO ?, 04 c l CO 9 99 CO to 1 | g Qi M O] CO 41 49 CO CO CO * CO 2 S 310,1 aad xn\j{ 5;rasireax 05 paums -sy qiaax J Jaqiun^ (M s CO g -M -* Oc r s >~ 30 C ^ CO - r oo I - CO 810J jad ajrutmuy Sui -aa^ug xn^q sauiittfiaj^ 03 a y? S ^< CO CO ^ o oo c s lH I-l IO J- fH o> IO o (N o * CO CD to t^ CO 10 CO w -c t CO ^ " O> Ol CO DO H > t t-I sS 10 fj d CO to 8!)Oig jo aaqumjj i 3 |H i o ~] -1 ct p ?! i -Ti i * 01 1 - CN O ^ CH 7 g i 01 c i CD S ( i R fH cfi -I IO IP5!J 05 ;uy atoj jo OI^H sc S4 ^ l> Cl s :~ J - CD *~ 8 a 8 o * CO I - 00 S j; i 00 B 1C i - ?j IO CQ g 00 o WOQ jo mPIAV 9 Mioajjg ^C! CO a g O o | M 00 CO 06 s CD f- 1C a .H CO to 00 OJ 00 os So 5S 30 .5 co <* t <N 8 9! t -i c -M 41 -M O o ^ * s ~. so r- CO) a-ioo jo qiPIAV ssoao gS s 1C s s &J iq M IO IO c5 I "5 M s CD |Q -i ! - IO IO 10 to o 00 <M fH s JH ti TH il ^ H (\i) ?<>IS A\opq suoi) Buiuii3 r [ jo qjdaQ C<-~ IO 1- -1 CD S 2 IO t^ 8 IO 1 - M to 01 M f ^" -f CO S exi <4> CO 10 01 00 CO 1- 09 CO * 8 "* 00 00 t 00 - 00 e 00 d CO t- 00 IO ^* CD < a ^J 10 oi co IO CO ( ,3) aottf may IB qioox jo iflpjAY -s| :: >a s -H to t- * t- co a ~ ^i 90 i i s i 1! <* g I- ..~ 5 GO *y to 2 i t~ ^i 19 8 g 3J 1 (-3) ?OOH IT; qioox jo qapiAV a J3-S i 00 ^s CN 95 i 9 1 t- 9 ^p IO "* V i ; M i s ^1 i CO as 9O 41 CO |Q -_; -r ?. : . ^! cfi CN o w op i 1 s " ( a) ^oiguuyjoinpiAY c .a 10 s (O CD H ^J4 oo I 99 3 9 s M * to 3 ^i o E >-: tf) N <* s -* -* M *4< * i CO CO (0) "}<>lg ajn^uuy jo q}da<i 00 00 cS I- 00 IO i 1 in i t- to i oo to f2 i IO j^ S i 00 V LO g CO "S 01 S ff S CO o 55 CO (g) -suon Tjuiuwq aatnuittiy jo ja^auiBiQ fciuajui CO ""S eJ ^i IO 3 co S 2 IO S Z S to c^ 68.1875 1C d M 3 to J2 2 "* v 1C M M 35 OS * s 9 o o5 * CO 3 CO ^rfl i s 8 10 t-I to to td 10 t-; 1^ (y) a.in jTJui.iy jo aaiauiBiQ i^oiaixg c^ "g "} Cl g 5 U} GN g S 1C S o> to M 3 c> 2 IO CO co V ff 8 10 i o CO o -1 id ^i M o> -. IO CO oi o 9 ^< 3 % 8 10 CO UO^OOHSUOQ jo a^Q J on i / 1 00 ? 1 00 s ffi 1> 1 | 1 1 09 | 1 i 1 1 1 I OP 9 r g i X 00 1 h- C5 00 a^nuijV jad snoi^niOAaa paadg 8 >9 00 s o 5 & S S s e a g 3 -i 8 S Q ^ i 1 O n Si i Q CN s 1 saioj jo aaqumx 50 m 00 co 00 CO CO CO e fH ce 00 00 I-l CO O H CO CO CO CO 01 IH CO CO O O w H O 02 CO 02 O O o p w t> P5 W 02 O Q 02 fc o i i 02 s I-l Q M i i X 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 JFuj246. " %.26o -Kf -y-jZT Jr/ * -^"V I v| \k. I-T- . /5S , I I rgN 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 %.^.r^"o/%" t p f **g 1 Torque. Curve for 33 Wheels Ratio or Gearing 1J8 / x X / X . / / , / / / / / , / 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. ** 3 Q. tt.40 Fiq.266 \S F GEARED RAILWAY MOTOR RATED DRAW BAH PULL. OF 80 OL BS 4T S*> TZD OF ll-t-HILES PER HOUR Horse. Poiror C urve ; ^ r / / / / ^ / x / / / / S x X ^ ? 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 . r/ *x ft I SSTT Zi_ , 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 a ** 000 t [S a 5 soo 5 Q 700 00 1 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. 1 B I 31 f- 32 1 1 80 1 Z8 M M tz Q CO i i ^ * i N K 10 6 t e j 11 / \ / \ \ / \ v \ V / X ^s ^ -^ / too / too / 100 / *. U " zu 30 40 SO CO 1 (*Vi6.t) Amperes Input. 00 90 100 fa 10 20 3D 10 JO 60 TO 90 SO 10 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 / / ^ AMPERES it__i_l__i / . t / , / / / / X 10 / *" / /" / X 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 30 80 70 SO SO JO to 10 < BO to 70 0) hi 2 so s X JO w 10 7 ^= ~~s -. *> - ~ . ^ \ / \ \ \ \ _J \ \ 1 \^ s. \, ^, ^-. - ^ , -, 1 i - , ~ . J_ 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. } jo H" X ^ : T D t ; i- - : - * - -- T -& 5 ^ t 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 - --la t- - I \_ x^ \ > i -j 1 h~ : s a ~j ^ 4^ 3b .~~" ^ i . *L i X i it JL ,/> J j ji_ k r ( K^; 3 \ " IL. ; o r 4 if , d) V J 1 8? ? 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* > O^lff \j :a -\ (Ti 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 f Ht Or -S .* .- - r -fioi- , 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 , - " ^- > -,.- ;.-" get .-..; CM tM ISC ;.<.- .;, e , . -^ ^ ^ s_ / / / / / i- / ^ / X / ^ / f / / /" / 2 A*\ PCXt -s f It LO row 60 SO VO ISO MO ICO 180 20O ZX 200 260 28O SOO Pig. 32,1. tvvu i i M ; ] || I ll i - , - DIRECT CONNEC i | Mil 1 :TEb RAILWAY MOTOR! -+CURVEOFSPEEDANDL tRAwmR piiili AT SOOVOL ij ! / 4:2: I 7 ?t x "jWC VI a ^A S Xz S 1 1? 1 **BW ^ ,y s X X s5 < ffltll y? ^j f I<H ^* Sppr $ :: :::: f ~*~"" i ~ < -Si^ " woe r ! "*~ ~~~ 3 Z i / / I/ <O yyj . . { j_ jMpeffs i H.-HJ- 1 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 <>; 06 0!> r: 02 Oi FCg.343 OF I MOORE S RESULTS FOR RELATION BETWEEN RESISTAW CARBON BRUSH CONTACT & BRUSH P ;E RF-Rfi LIRE. \ \ Peripheral Speed Commutator- 14OOit.pcrmin. \ \ \ ^ X, % "^ --^ v X *~ - ^Amp S.per 5^. in x X " v - \ ^ ^-^ -- ---. ~~~ " " . -~ ~. ^-~. -~--. . _ .^ -32 2 Am pi pers j. in. . ~ , ~ 64-fi " " " 2 -4 -6 S 10 12. 14 i-e 18 2-O Z-Z 3-A 2-6 Z-8 SO 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 <u iu 03 a i ? 02 MOO RES RESULTS FOR .RELATION BETWEEN RESISTANCE ___ OF CARBON BRUSH CONTACTANO O 4OO SOO 120O 1GOO 2000 Z400 2800 &OO &28Pi SPEED IN FEET PER MINUTE. Pt 16 RAYMONDS R OF CARBON BRU OF COMMUTATOR A ESUL HES J1PV ;sur TSFO AT V/ ARID s-?-^ R CONTACT RESISTANCE p kRIOUS PERIPHERAL SPEEDS us CURRENT DENSITIES. vs E rush Pre ( ifes. perSq. It 1 X\ \^ X s ^^ ^ * ^ ^ ^ - I W0(7 rtper M.n.t eriph tra/J />eec "^ & WO - 2O 3O HO 50 60 f*SMi CURRCHT DENSITY ATCONTACT IH AMPERES PEP SQ.Ifl 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 ; "PZ7 Q2t k WW OH fig 346 . | \ I \ I \ i /L i i4 as oa an .. .-. ARNOLD S Vi^ cuavEs OF COMTACT RESISTANCES oM CARBON I COPPER BRUSHES. > n Peripheral speed Commutator -IZOOpermin Diameter of Commutator = /" Commutator composed of 4&copptr segman Brush oressun = l-7/os per sa./n. & 1 % ft t" to UK U f. tut <>H ! ffl 4 (so*.) 30 w eg fir ffo 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 b-C= 1-600 (5031 B) b-c-o C-0=S67 CU-b-1 600 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. 9 99 591 OS CO M II a C R Loss in Armature Conductors of Rotary Converters. w o <0 -- .w > 9w ^ A ^/ v / v / v/ x r^^i 3X X A A * X ) 289 2 P Rotary Converters. V(luiiO) OOOOOOOOOOOOOOOOOOOO oooooooooooooooooooo CD" CD" CO" CO" CM CD (M CD CM CD CM CD CM CO CM CO CM CO <M CO co co co tn cS SK cocococoocooeoocoocoocoocoocooco COOOOQO^OX>kOOOOCOlOCX)lOOOlOGOiOCCiOCO + + + +I + I + I + I + I 1 1 1 1 1 1 1 . M^ OOS -T- ^ U9J cocococo co co co co co co co co cocococo co co co co co co co co i IT IT IT 1 O r- 1 O i 1 O i 1 O r 1 O i 1 O i 1 O i 1 O i 1 + + + + + + + + 1 1 1 1 -U CO go 3 O UO^OBJ J9MOJ SuiJ9pISUOQ OiClOO O O IO IO IO O 1O id + + + + + + + + 1 1 1 1 snonurjnoQ oooooooooooooooooooo 10101010101010*010101010101010*0*0010*0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 + oooooooooooooooooooo B (^u9ajnQ) CM" CM" <M" CM" CM" co" CM" i i o CO ,5 pH CO *, CO O QO qu e^jns9 jj CM CM CM CM CM CM CM CM IO CM IO CM IO CM IO Ol 1^> C>1 IO CM 1^ 998 ~^ 1U8J -JTIQ Sui^ Bajg^jy CD CD CD CD CD CD CD CD . *. CD CD >-*. CD CD CO CD + + + + + + + + + + + + 1 1 40 CO -H SO 14 i-, 6 JO^OB^ JiaAYOjJ UlI8ptSUOQ ui } BtiJ9(}jy i-O iO IO 1O 1O 1O IO ^O iO iO 1O ^O ^O id CD CD CD co co co co" CD . . CD . . CD . . CD _, co ._. CD . . CD CDCDCDCOCDCDCOCDCDCOCD^ ^CD^ t> ^CD + + + + + + + + + + + + 1 1 ^tre.unQ OOOOOOOOOOOOOOOOOOOO snonuyiuoQ 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 + ; .(((U8jjnQ) (M O CM O CM CM CM CM CM CM CM CM O CM O <M O CM O O CM IO CM id CM CM CM CM CM CM CM (M 1C CM IO <M IO CM IO 1O c-f CM" CM" CM" CM" CM" co" i i CD ^U9JJHQ IO 1O 1O IO IO ^O IO IO IO IO XO IO XO iO CD O CD O> CD CO CD CD CD CD CD CD <O CD *O CD O CO O CO S luuipasoy i ( Current in ui VEUJ9 r ny CD r ^ CD . . CD CD co CD co CD CD co , . co , . CD . . CD . . CD COtDCDCDCOCDCOCDCDCDCDCD^ Z3 ^CD snonui juo ") OOOOOOOOOOOOOOOOOOOO 1 1 i I 1 ! 1 1 > 1 1 1 1 1 1 1 1 1 1 1 - .lo^onpuof) I-H !M CO -<i ifi CD t OOCiO" iCMCO^kOCOl COC5O 1 \M I . . r r CTJ A :Z C R Loss in Armature Conductors of Rotary Converters. 291 O o o OS 05 os o o OS o 05 o os o o os os o o o o os 10 o o O os 10 o o o o OS IO o OS O O IO O o o IO IO O IO IO IO IO o o 10 os o o os os o OS co co CO CO co CO co co CO Ol CO Ol CO Ol CO 01 eo CO 01 co 01 eo co 01 co co 01 co co o O CO CO 00 00 1 1 eo 00 1 co OO 1 CO OO 1 co 00 1 eo eo CO OO 1 1 eo O OO IO 1 + eo o OO IO 1 + eo O CO IO 1 + CO OO 1 O CO IO 00 I-H + + O CO IO 00 O CO IO CO O CO IO 00 + + O CO IO 00 1 4- co eo CO 00 CO OO o -^ O G o <u y CO - eo co co eo l-H l-H 1 1 CO eo I-H 1 eo CO I-H 1 co CO I-H 1 CO eo 1 co eo CO CO I-H l-H 1 1 CO eo 1-H O 1 eo eo i-H O 1 CO CO 1-1 O 1 co eo T-H 1 co CO O l-H CO co O I-H co co O i-H CO CO O . i CO co I-H co eo CO CO "CO eo I-H ^H <U || CO i IO IO cd co co co 1 1 IO CO 1 IO co 1 co CO 1 CO* CO 1 co co co co 1 1 IO CO" CO 1 IO cd CO 1 IO 1 cd co 1 IO 10 < CO IO CO CO 10 IO ^ co IO IO cd co CO CO co" CO op O O IO IO o IO o IO 10 o 10 o o IO IO IO IO IO IO IO IO IO O IO O IO IO o o IO IO IO IO + + o o IO IO 1 1 o o IO IO 1 1 o IO 1 o o o o o o o o o o o o o o o o o o o o iO t* of IO of b- **" b- b- b- b- b- b- b- *"" of lO^ b- id O l-H IO b- IO b- of IO b- of IO of o co" IO Ol o IO 1 co" Ol 1 co" Ol 1 o" Ol 1 co co Ol O> 1 1 co co 1 1 cd co Ol Ol ! 1 O O 1 + CO 1 O CO IO Ol 4- 1 O CO IO Ol + 1 O co" IO Ol l-H + 4- O CO 10 01 1 1 + + O CO lO 01 1 + O co" IO Ol 1 + o IO 1 co oo" s- O Q> ,=< PH b- i~~\ co b 1 b- 1 t^ CO b- 1 co b- 1 b- b- cd co 1 1 b- b- co" co b- b- 1 1 b- b- co co b- b- 1 1 t^ g o 1 b- 1 b- 1 b- _ CO ^""^ b- 1 o g b- b- b- o ^o o> IO o 1 o IO CO 1 IO CO co 1 CO co 1 IO IO co cd co co 1 1 IO IO cd cd co co 1 1 IO IO co co co co 1 1 10 <=> 1 IO co CO 1 IO ^ CO 1 1O *" co 1 IO co 1O o <> co oi IO f *o c - > co w ] o o IO IO O IO 4- o IO + o o o IO O IO IO IO IO o o 10 10 o o IO IO o IO o o IO IO o o IO IO o o IO IO o o IO O + + o o IO IO 1 1 o o IO O 1 1 IO 1 o 01 O b- IO Ol of o o IO of b~ Ol o Ol b- 01 o 01 O t- O Ol of Ol Ol b- b- Ol Ol b- b- Ol Ol b- b- Ol Ol b- Ol Ol b- b- 01 Ol o 01 O b- (^^ C"-l O b- O 01 O b- 1O Ol O(" O 01 O b- 1O Ol of o 01 b- IO Ol of o o IO of o _^ ** ~3 g5 S C4_l G O o O 10 ,- IO 10 IO IO IO IO IO IO 1O IO O 1O IO IO IO IO <=> ^ ^ OS II 43 O co o co o CO co co co CO CO co co CO co co O CO O co O CO O CO O CO o 1 1 + 1 + 1 + 1 1 1 1 ! 1 1 1 1 1 4- 1 + 1 + 1 1 4- 1 + 1 O w S-, Q IO hi M 10 IO 10 IO IO IO IO IO IO IO IO IO IO 10 IO IO 10 IO -t- 3 * 10 W o g 1 co co 1 1 ^-s CO co 1 co co co co 1 1 co co co co 1 1 co co co co 1 1 CO CO 1 co co co co 1 1 co < ~> to 1 ^ CO 1 1 * CO co J " - 2 I O IO o 10 + o IO + o IO o IO o o IO IO o o IO IO O IO IO IO o IO o o IO IO o o IO IO o o IO IO IO lO 4- + o o IO IO 1 1 o o IO IO 1 1 o IO 1 X _ o 00 m I-H Ol 01 Ol CO Ol 01 IO Ol CO Ol b- 00 Ol Ol OS O 01 co I-H Ol eo co CO "* co co IO eo CO b- eo eo 00 CS eo co rH Ol CO ^ IO CO b- 00 292 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" <u ^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 A f^d ^_ o ^TTBijnsay; + + + + + + + + + + + + + 1 + 1 + 1 + 1 I ll a ti)^ c 3 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 OlOlO OlOXOlOlOlOlOiOOiOlOlOlOVOlOlOlO 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 cS ^5 ^<d <*H O O CO ^aaaanQ 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 . t>D si>,$ ^^ o o 998 -r ^uaj JUQ Bup BUja^y ^co^oo-^oo^cso-^oo^co^oo^co-*^^-* ^OO^OO^OO-*OO^OO^OO^HOO^OO^^-*^t< + + + + + + + + + + + + + + + + + 1 + 1 13 JO^OB^ .laAvoj coxocovoeoiocoiocokocoiocoiocoioeoo scoco b C 6 Sauapisuoo ^ou q.u9janQ Sui^^aja^iy OOtOOOtDOOtOCOtOOOtOOOtOOOtOOOtDOOOOCO r O cOt^cOt^cot^cOt^cot^eoi>.cot^cO-t^cococOcO + + + + + + + + + + + + + + + + + + + 1 ^uajjiiQ snonui^uoQ oooooooooooooooooooo OiOiOOOlOiOiOOiniOlOiOkOkOiOlOlOiOS 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 PH ^uajauQ ju B^nsa jj i lOOi lOOi-HtOi (tOi itO^^tOi i tO i itOi itOr itO 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 <u h c 3 O ^uoaang 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! JL! a) .^t S- A . IO IO CO CO CO CO CO O CM O <M O lO CO CM CO IO CO <M CO o IO co" (M co o CO IO <M IO CO CO O CN CO o IO i co iO CO IO iO CO CO 10 co co 10 co 10 co CO o co CO o I-H 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 4 1 4 1 4 1 4 1 4 4 4 10 10 co co co co eo I-H ^H I-H 1 1 ! 1 1 CO* 1 CO 1 IO CO 1 CO 1 IO CO 1 CO I-H 1 IO cd 1 IO CO CO I-H 1 1 eo l-H 1 10 1 IO co 4 CO* 1 co IO IO co co 1 4 10 CO* 1 CO 4 IO co 1 IO CO CO iO l-H IO CO t CO IO I-H IO CO lO CO kO CO IO CO IO co IO co IO CO IO co CO co CO CO CO CO CO CO CO IO CO IO CO GO CO GO* CO 1 + 1 1 1 00 1 co 1 CO 1 CO 1 CO \ eo 1 so 1 co oo 1 1 CO 1 00 co 1 00 co 4 00 co 1 00 CO CO OO CO CO 1 4 GO CO 1 00 eo 4 00 CO 1 no CO co 00 co co o o o o o IO IO IO IO lO 1 1 1 + + o IO IO IO o IO o 1O o IO 1O IO o IO IO o o IO IO IO o o IO o IO o IO 10 IO IO 1 10 o^ 10 10 c> CO O CO CO IO os -* 00 I-H CO IO "* eo IO -* I-H eo IO ^ I-H eo IO CO -* I-H IO -* I-H CO o I-H o CO o I-H 10 O eo O OS 00 10 co o o Oi 00 IO CO o o OS 00 IO eo o o 00 o OS 00 . 43 o c -* o> s II 10 (3 _w r-4 O> <N Ol OS <M C5 <M os <M OS <M os (M OS (M OS I-H OS I-H a l-H os I-H 1 1 IO "* IO lO OO OS CO I 1 1 4- 1 IO 00 CO IO CO CO IO CO CO IO CO IO CO 00 co 10 1 CO CO 1 10 4 CO CO 1 3 1 os IO os I OS OS 1 4- 1 + 1 1 1 1 1 1 1 1 1 1 1 1 1 GO GO 1 **< 1 CO 00 1 1 4 1 4 1 4 1 ^ 1 eo co co co 10 co IO CO iO CO 10 CO IO CO iO CO IO eo IO CO CO CO CO CO CO CO CO CO CO CO CO CO CO CO CO CO CO t^- + 1 + 1 1 CO 1 CO 1 GO 1 CO 1 00 1 o 1 CO 1 CO 00 1 1 CO 1 CO CO 1 CO 1 GO CO 1 co 1 CO CO CO CO 1 4 CO CO 1 00 CO 4 00 CO 1 00 CO 4 GO co 1 CO CO 00 CO 1 o o o o o IO IO IO IO IO 1 1 1 4 4 o o o o o o o o o o o IO o IO 4 o o o o IO IO o iO 4 IO 4 IO IO o IO o IO 1 + + + + + CO O CO CO O o o CO o co CO O CO o co o o r--. CO o o t- o CO O co o o CO o co o GO o OO > "S o g t^- Q Vs 03 c -< t-i r*j c IQ t- CO t- t~ CO ^ CO ^ 10 t^ 10 t. 10 t- 10 ^ 10 ^ 10 t- 10 ^ 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 I-H co 01 I-H 4 CO CN 1 I-H CO l-H OJ 4 1 I-H 4 CO <M 1 I-H GO CO 1-1 00 CO GO OO 1 CO CO CO CO CO CO CO CO 10 CO IO CO IO CO IO CO IO co IO 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 CO 00 t-- co 1 1 CO t 1 CO CO 1 co 1 GO CO 1 co 1 CO CO CO t- I 1 CO co 1 co 1 00 co 1 CC co 4 00 co 1 OO co 00 CO 1 o o o o o 1 I 1 4 4 o o o o 4 4- o lO o 10 + o IO 4 IO IO o lO o IO o IO o IO iO 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 ^ ^ / / / / / g / / s / 1 / 1 ? -} / _,* s Ov / _s s ^ 1 / / ^ \s^ 1 / ^ ^ ^ > / / ^ S <o It 7 ^ *s 1 ^ ^S" 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 *$,&&gt; 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>. to | V. 1 I? \ u " 5 / \ 2 , / \ 1 / \ * / *S K" 1 "\ ,^_ , - , - . , 3 4 ? 6 T 8 9 Amperes Field Excitation. My 415. m LO of 7,7 AD PHASl SOOEw. Re>fhjoi4 c M -C-J HARACTERISTH Cycles. 500Vol Notary Coma -.s. (S 1 U i Power Fcuctffr in> terms of Amperes per Collector Ring. i I I \ \ \ \ \ ^ ^ ~, ~^, Power Factor 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. t Three PhMeBotnry Converter Current, Potential; of 1 5 Volte between ringg. ISO X ;.?<? \< I x */ 1 IKX - H 2 $&n gy ^ 1 ( 2 88 > too ; BOO 1 \ fW> \ / \ / TOO > \ rf>^ X, *r* > ,, g J.X- fiflfl s X, ,cJ > > \ **1L T//*/. 5J ,W \ WO \ - v / V r. / \< / \ / \ s / \ / s , / s t r 000 1000 ISOO 2000 Z00 3OOO SbOO WOO tSOO tswa A mpere Turns per Fi eld Spool. 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 Power FcLCCorin* terms of Ampere Tarns per KeW/ Spool/. Ctnscant Alternating CujrenjbPoteiWuaJU of 15~Vdlts between/ rings rrn nu _ tot \v i -grr ^ -4 & 3*1 B^ ,-- in at CT ^ f& l^ ft irtf ^ ^ Tf* 3 ^ \ <^ ^ J Jii ,b 4 tf / A I >\ / \ / \ / ^ \ / / \ / \ to> / s. s JOx flt Lj^- X. ^ J> *-^ H , r^~ K>0 1000 16OO ZOOO ZSOO 3000 3500 4OOO 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. fowe rfgjC jUlt/J: toi^.ui) terms ofJ( wnrnaja^ug Curr mjienw frer Cottdctttr J&jtq \ I, Consti eiw Pptaihial/ of 75Volts~b&M 7 / I l 1* ^ -^ woo 800 eoo too zoo ^ ^ 35 ss < . ^ X ^ <> 4 ^ 1 a j . *J 32 -:r ^ - a \ t - . i p \ 1 \ \ M \ x . "^ ^ ^ . 1 . ^ j ^ T \ Power Factor 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 Three* PhMS&Rotfiry Gorwer DietriJmJtJuorv ofre^vJjtant TnagTietjo -motive* farce- over armature* surface . Cer. Trmc the* vGur / \ / \ / \ y / \ / \ \ / \ / \ / \ \ / -15 -15 -15 -15 -15 15 +15 +15 +15 +1;5 +1-5 O -15 -15 -1-5 -J S -1-6 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. 5 ,.42 <j. iure^rrwi6e ruoisiry ujrwcrver. IKstnJbuUtww of di& resultants armcuture / \ V ma* or jncto moliM nature sort force- av er th& / / S N f s \ 1 \ / \ \ / \ / \ y \ \. , / S v <1 ~M m "A H W\ W\ V W\ i IcTM 1 m "M s ,1 ! w fcj 4 v H A \ 74 1 ^ .- i - 1 / . "fcf |@j 11 -861 -867 867 867 -8O1 851 -67 - Sfi? -867 -iS67 --.S67 -567 -S67 O O -867 -67 - CT -*G7 000 -67 -aS7 -i867 -667 -667 -iS7 -67 -VS67 --Sffl -8ffJ -Bffl * -113-113 -867 -8C7 -7 -SCT-S67 -SCT -/73 "113 -1K -8E7 -567 -867 -67 -67 - -"7 * 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/ 2cr> Z iO 220 200 IffO 160 120 wo CO 20 X ^ X X X / ^ X ^ ^ ^ ^ ^ _ . *** Jmp&vIurneperFielcL Spoob-O ]/ olts on/ Conuruubat hree Pliose* Tl&tofe, ur = 550 7 J "3 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 |i i! 5SS, 50 US n" CO Mni aiii! 51^1 99HS 3939* M*M lilil i & 4> V Pound. h- 00 CO 1005 -fr-l ifl X) 00 -H 00 ^^ ^^^C^WCO fiO^OOO C5 3 O f ,C ?: Hill IIHI Pounds per * d ~=$ ? ?e HI! OOOSO 9-ll^OJo r-I CS vir^^i-^ , _J * *" *"" ..... OrHOrM OT-ll-iO^ -H^4 iilll IlN! w ri i i IN o 3S jio iSSiS S2gg S^S&^ S^ o^;^ i>^c!S 5 S 5 S a SSSSS? SSSsl -3S?5c5 222^ -- S3 1/5 I | li- FH -^ O CQ O O d | ^P rH rH i-H W <N CO -^ i CO 30 O 00 O i-^ ^ O^i W t^ iO IN. ^ SS6H 11SI1 Q^ o si S o 30 m *C Oi ^ O OO w) pH > -^fl COOOOC-1 ^ ^OCS3 i-HrH^lCOCO ^COt~--OSC^ lOa^rHOS CiOJGOCiO XOff^t^O US^O J _; i-i r-i oo* co ^ to t^ as fri m* o i ^ o* o co o" * oo i- rH W N -^ iCOOOOtM Scr^c-io ^3 ^? 12 M 5 H H P W o S ^ j O i Iil3 "rH r-lrHC^WCO * t?J t C5 - *.-5l^O OOQ Ottt-i fe O PH ^ O H I o o -* (M O CO O Sco go 3 _OO r- 2^^S ^S^2$ =^corH<N wni-co rHi-HCiwco -^lotocoo CO . I> : ~;^ 1 * oo^ao^ci ^cocscox ocot^-o ~* r- rH C-I G^l CO ^iCCOOOO OOt-^-Hr^^I ^irsOSOQC^ rH rHrHC^C-JCO "*"iC?O30rH O ** CO W ^ 5 ^ O iO O PH OH M K ic nil rHrHrHG^l^ CoWi<i>t^-CS W IO O ih rH OOCOOrH OOJOM^CO 00^*r*r ^ pq a H M T2 S ^rf .^=00 aS8a g o ss -g tf tq 5 PH ** O % rr" > O If S o ih i^ 55 o 0000 rHrHrHS-lC-J CO^iCt C3 rH-**GOOOCS IN* ^> Ob *V CQ OOrHGOt^O& OOiOOOO rHrHrH5>JOl CO-fCOI^-Ca T-) us 2 ^ o ic 3! s i~ i_- PH *r PH 8 r I ii 8 .... o.oooo ^.. o.oooo .gjgjjja o.oooo Seoeoco^ co^o^^S M K. O K 1 1 5 1-3 H (_} a, o ^3 5 ai8 O CO O I-^^HCOCOO gj^oeor-i ^prHOT-io o<oc-ic:S ^P^^^ 7 1 c- ^ c 1 5 5 III 111 PQ ^ H d d ISIS COC lC l MrH i-HrHi-Ht-HrH r^O- OO OOOOO OOOCO H B a o d ff2| SUBS IliSg lilll SSSli |SS1 o a Q 1 S2ggf IK 2 j 2 d CQ 82 ig C 1 ! l?J 7-1 ?1 i rH rH r-t r- rH O O O O OCOOO O C O O OOOOO illli Hill i || IP- 368 Appendix. H M ^ O o P3 8 J s H s I I << H a p3 o H 2 S a PH PQ PH r M 3 ^^ i ~ rH -* CO * CD O COOS 1^ 00 CM CM CO 1~ o^gissj I-H m co co co CO 1^ OS rH CO cs i~ m m * m >*x m x *< OS * l^ * CO CM i-H i-H O O 2 SB CD in -* co SiiSs 883 co co i-- o in CO CM CM i-H IN O t^ 1C CO rH rH CO CM rH I-H I-H ^ . I d CD 00 CM 00 CO rH OS 00 00 O CM rH rH * OS X CDO1> CM (N B.B HJ 3 rH rH CM CM CO, T* 1C CD X O CM m X CM t^ CD X CO CM CM CO * CD X X X l^ O l^ OS CO X l^ i 1 i I CM CM (M CM X CM ilill lllslll co p t rHCM Ei^d lal CO CO COX OS X OCS CM 1-H OS I-H CO CM CO CD OS t~O * X CO O X rH CO * CO i-H i-H O O m cp X CM CO * fM C-l O O X t>- 5" (2 00 i 8000 -* O 1-H , m i i o CM OS CD CO CM rH rH t~ r~ t~ o * OSOrH -*05 rH CO CO 1^ * CM i-H 1C CO rH Sd os m x co CO t~ O CO Ot CO CM X COrH in co x i-u^ * m s IP g-s-d ||8 IIISI X CO CO CM CO OS rH CD i-H 1^ O CM in rH mi~ CD o co I-H x CD m x in CM o x x m o co co OS J> CO TX CO i-H * X CO CO CO CM i-H r-i i-H CSt-CO^KNrH CM rH rH rH rH kj tUX! il o5 ||- - - o.xoo rH CM CO -* in CD t~ X CR O rH i-H i-H i-H CM rH CM CO ^f in CM CM CM CM CM CO t^XCS O CM CM CM CM CO rH CM CO * 1C CD CO CO CO CO CO CO d ills CO C5 -HH O (M iC CD O ** 00 rH rH C<] O3 W liiil m * m co x * OS rH 1C OS CO CO CD X CM CM O COt~ rH CO ** OS CM CM rH CM * 1C 1C 8 8 rH rH rH CM rt S " aSS ,0=0X0 t~ x * x in co COCOl-HI^^ -* rH i-H CM CM 1C X d ills Issil rH CD i-H in * l-H OS t-Xl-H co co * m i^ OS X CM CO t~ o x co CM m m ^ ic ic I-H COrH -*05O d 3 1-H I-H I-HOJ co co in t^ o CM CO O CD CM rH rH CM CM CO Ci O in 1C CS co m CD i^ x X X O CM CO rH CM L" O CO i 1 O i-H rH CM CM 1C X d oo ;o co * CO CO OS CM 00 CO MI 1~ i 1 * i-H rH rH CS CM CO -^ 1C CD CO rH CO * -M X CO l^ CO t l^ O O O x m o x x t~CO CS) X O rH O CO I-H * 1 -s S g rHrHi-HCM CM CO 1C COOS rH rH rH CM CO CO * CO i^ X o xx m ocs CM ** 00 * X * S. o (N 01 10 o Il il * X CD 1^ rH ! CO CD OS CO i-H CD O r-l * CO CM 1^ -* * CM CD l~ OS CM CD i-H * CM CD * CO CO CO CD CO i-H OS CM OS rH O 1C t- O CM CD 1 o rH rH CN CM CO * CD OS O t~ CO X i-HrHrHCM CM -* CO 1>- CO t~ CO * 1C CDt~ CM X in X CO CD rH CO t^ CM -t CS 1-H rH i-H CM * CO d 1C 1C f- 00 oooo m x , co * rH C<1 in CO rH I-H I-H rH rH CM Ic CM oo K t^ CM co eo * m SSsss in x I-H i>- m * o co x * i-H CM CD * OS O 1CX CO OS V D 8 i-H rH OI CM CO * 1C X o co co I-H m rH rH i-H CM CM CM O CM rH rH CO *** C CD l> SXCM rH * in (N CO rH i-H * d ^i OS CO 00 1*- OS CO OS CO O rH * CO OS CM CM CO * in CO GOO CO 00 t^ 1C OS CO O <N XOS * X CO CM * XO CD * X CO * O d o CM CM CO 1C J>> i 1 rH i-H (N CM CO * in CD CD xo m co m CD O CJO.Q s 5 I oooo rH CM CO TX in o.x.o rH -M CO **< O CO 1^ X O5 O rH CM CO -* irj CM CM CM CM CM COt~ XOS O CM CM CM CM CO rH (N CO * 1C CO CO COCO CO CO CO IJJjT ot^ CM CM CO O CO -* i-l OS O CO CM * X 1^ CD 1C * CO o o o o o <N ic i-i r- ^ <N (M r-l i-H oooo CO O5 rH 1>- CO TO O -^ O CM CM -* OS OS CO CO CO X CO OS ill Hill 1C CD CO 1C CO CD X co o X C: c] "" d d * m o oo i- * o m rH O t^- 1C CO CO CO CM CM CM i-J os oo co m (N rH i-l r-l rH CO G-J O O5 30 rH t-Hr-HOO Sf- co in ^ oooo 00000 :::: H "s" d g d _00 CO CM * CM CM CM CM 1C tM t- O CO i-H O5 i^- CO ^* G<) rH i-H i-H r~ * W O5 O CO N CCrH 005 X rH rH rH O O 1 O COCO O 00 CD * (N rH O g X . . .I-H o -o . Q -S d a) . .rHOSCO rHgS^ gCMgc^ lissl m m o o o .a i 1 * m o o m CM oo * TJI ^* CO CO * OS X O CM CM CM CM CO OiO X -^ S2S33 OOO 00000 1C X OS CM 1C IC -* * CO oooo CM X in IN S o o o o o 000 = X CD -* CO C-l oo ooo slllll LJ 9 C 000 rH CM M T* US co t- wo; o rH CM CO * 1C Sr^rnS ! rH <M CO * in o i-, x os o rH <M CO JP irt SO SJ go Appendix. 3G9 hi HUH am * 53H3 gSSH 39533 W VM ."25ii I|i? - rH |t jo i-^ PH I-H >1 7J j,j J3 ?3 ^< O ;c i-^ an r-J -.1 .X ? .* .* * * " SrHrnS sssss s|5g fjsjjslij i|*| Illf f ^H ^H i-H r-t OJ C 1 ! 7-1 W JO "5 l - ri . ~1 S* i l" .- ~ *, >O 1^ 35 71 * 1- O i- i- l 71 rH r-l rH 71 71 SO - 5 *i i SrgO 111 O O 25Z.-SOO gooffigi poo 7i ^S riS215 ^M rl 1 ^ ".~. .* 3 . ?qqq_ S2SSS UPlll lla si 3323" Ssas JO iO 35 - 5 S S 5: S 3 g * 8 S g s : ; i i 1 sllli lll jjll ^Ts -^ iilil iliil ll ii s^ss ^i^Z ^gH IS-S3 1Q9!H o sj II O_ O_O_O^OOO rHOICO^lO COt~.0035O rHT-lCOtO OJ^-OOOJO i-H~)-o-M. , BS8i;3W cor~ ss S2 T ~r -r-rX 1 illllsS SSilii HSII 33323 co^SSm a, Se ,o *-* 00 < S ^^OC^O ^ r. i - c 5 c:^33r:^ c^i 71 73 o in i^- 30 < re i i-H t-3 pH IPH r- * T I i 1 >J M CO^iCCOO CCt -CCtC-l C 1 l~ : 7 ix ^* t ^ O ^1 o ii ID O 00 i c; jC o "M "M re r; ^-1 oo o ooi^T^oO 1 "* ^ gggliSS SSfjSS 35ggff 2385 2S2?3g -,^. .0^.,,. ttinnili mil rH rH 71 5J n ~f 10 x 35 - 71 : 7! to 71 ffl I- X C: SO in 30 rH rH rHi-HM7ICO COTincOX CS rH O5 O 00 .J O I 1 S g 1111112 Bsllli SSl lg IS53? SIS = 5S5 x^,,xx =,ir = xx ^ JO?I"* Oi OOt^*MOC ^"JS*I 717iS5o Stt- -S? O-iSso 1 * rH rH 71 *1 - a5 S rHrnrH. 0,^ aSSSa gjg.^ 3|gS| O Q O K O c p < fe e o H as < w g & o 1 2 -t t- o ^ x c-i o >-: o o i~ I C 71 t t^ O T O SO SO X 71 SO rH -f m 1 O C O ;rH i-HrHrH3-17-] COSO^inCO OCO^l^-rH l>-inXC5O 3571SOr-IX ^**in35SOi.O i-i x ic -t -t i ~* -f u- -~ i~~ o c o o ^ isof sst- ^22*!Hs EJ35t! l 9: oxsooio 35sot^sox somsoo eooo- i-H rH rH 0-1 CM CO SO in CO I- X C 71 -r I- 7! r. /: SO -f ?. 71 ?. 7 T- : sliiisi ISs: OO-TCM O US O^t^^X ^^X)?7^;-t ^^L-l^t^. o _ SO M in t~ X ~ -r X M C i-l 3; SO SO 7-1 X ^ C & !3 l~ i i-< rH i-< oi so * * ~ o i i s: so e- in so i-< T r-l rt i-l 71 M M * O O SO -< ri 1.0 so o 9 r- 9 m >Q * r~ y I- 33 OS j- SO 5 = 2 >-~. S r. SO s r-lr-1 ^H.l. . >T -*Tl~ O._,^O = O rHCMCO^lO CO1^0035O rH<MSO^O CCr^XOO rHCMCO^riO Ot-3035O rH71SO-fin COt^X3SO rH7J^-jiiA tCt^X* riS in ^ X = ^ rHrHrHrHrH rHi-HrHrH71 (MCMCM717-1 717171*3 SOSOSOCOCO SOSOSOSO- --W-*- 9 -JE 9 1 = ? r-! P2 i ^, A r-i rn 00 l_: ~ ~ so so OSO--S03: O-r:ccr: 7_ . i - qqq ciqqq S8888 25255 55555 55555 88888 55555 55555 5555 ^ S S 8 S S S ? ? L" v " " i. * 1/5 O mo oo o I-H so i.T, o so 7-1 x S3 -H (M c n x i^ * o -i> so ci r^ c ^ x x i* i~ 1- l-^i -r-T SOr7717J71 7)71 ,-if-t,-i,-ir-i T . : : OO o o5ooo o c o -r o => o s = o o .= .= o c = 555 55 iX f ocooioiw e-i^o^ao co-*^ . -J 71 O ^- ^O SO rH 35 I- tO -^ 71 rH C < B oo oco Soooo 3Sc58 5ii5* " * I 5 5 i 370 Appendix. m O P fc OD ^ 02 O w PM O H O O r I OD w PH cq o <u <D S * S -H -M PHJ- U) ^ i "I 02 _rt o o -2 g 3 c QJ rH O] Ci) f^ r*! [> Q .2 & Mr^ O ^2g S O.Q CS -M g . P-l ^ C nj S rv] s -s PH S5 o g^ "- 1 .B ^ Ss IT S 3 ^ 2.2 rH O) 03 cS Mg 1 |S grS S ZL rH ^ S O ? rH Q rH X S - o OH CD spunoj qoui CO CO COCO (M(N GM<M ^COCO S.IIAVIQ oyioadg C5 OS OSO3 00 OOCC OOOO COOJCS od oo odod t^ t^z- t-^t-^ co oood qoui a-itjnbg jad epunoj q^Sua.ng . i ui33j^ wan oyioadg ; ; ; ; ; ; ; : g 2 S Sac -wot WH o o o o o o o JO ,)s-U.l.I.ill| - !>U3Q -18J O O OO O * rH i-HrHOOCOtOCNCO !}U9Q Jo gd o IB euiqo "Bid ui too 1 puis Suoi 00 OO OSO OS OO OSO r-iiOOCDt^t t--Tt<CD 1U8Q jodQ !,B qOUI oiqno Jad siuqo-o-ioijv CO OO Oi Ci iO ** tOCO J>-OSOSl>- ^ I ^rHm^< (oqno ?U3Q aad siuqo -OJOijv) ^"90 ^ad iv aou^^sisa^j oyiosdg CD i-^ oso" id co idt^ ostrji-H-*oioioioo rt< ^1 ^4 iQ CD CO COCD COt^-OOSrHrH<N^irH spunoj qouj I Is Is 1 ii g g M AIIAVIIQ oifioadg ^1CD?O CSOS t^i^OOOO^O OS OOS oi <N N ei IN t-^ o t^ os oo od oo as 06 qoui 3-nmbg 3 [ i B u a x aquuniifi OOO : : : : : : : : : : : : St^" o" O O U139HI T^H oijioadg <M <M CN O rH O .... O O Saa -duioj Suma K 1 : : :f :| : : : : 1 | JO 8ST38.1QUT *^U9r) J3fJ CO IOCS rH CO 5 OS "* OS <* CO CM coco oo w ooo i^S 1 " 1 ; ; ; ** J !juao -Sad TB suiqo ;d "I TOO pu eguol 1i3 T 3J !AV JO30UE!)sisa^i ^ O rfi -*t^OOCir3 OOCOCOOO (N^O F H] f H r H r H r HC^<Nl--" -iCC COCD *CMrJl O rH IN t> rH rH otqnQ jad suiqo-OJOi]^ O ift ^j r- liO^-^COrHCOCD rJiCOTjiiOIr- lO 0013 OOr-lrH(MCOCOOSCSrHCvlC5CDOOO O COcD*tJ i-H lO (aqnQ !>uaQ asd suiqo ojoiw) "1U3Q -^ad (j^ aou G jsisa^T ouioadg i-H CO rH CO i-H rH ^ rH r3 . ** .> .cu .0 .x-s .*^v. . o .0 .0 c d .co& ^ cs <JC L-*^ Qc - *i*-^Os Ci o o> O) C_Ofc tXrC rG pC <irH COM* ^^ ^50 w 5 fi . i 5 ^ rn ^ g^q os * ^ ttg -.S - ft $ ".5 S-.S * ^ s * "** "^ -^ L, 13 c ^< ^.s -^fi-^cS-t-jc-^S^r-i . oo" ^C c - C &7c * 53 <<:<;<<:-<^-<-<o3SDooao .0 Sw Appendix. 00 00 CO CO il IO to CC CO CO CO CO CO CO CO Cl C-4 - .-..-, oooo 0606 CM CM CM CM CM OS os m *o ci cs d o coco QO^H^^ Si^ 1 ^ eo" r>T co" rH SO OS O ** CM <M CM 1 o o .... oo ^ o o rH i-4 r-l CO <M i-H . * * .t- , 2 5 * o r* us S--- SSS 8 S 10 CO i-< CM OO O3 CO CO 7 1 7 I ~- N I to CO OOO5 IO Ci CO 00 00 00 CM CO * r-i o* CO 00 S 2 s 3 S CM to oo ao t^. O 1^ 1 CO r-* CO CO CO CO IO * CO * s o CO O O5 t^ 00 rH O S to ^ t- C& * O CM O * CO CO CO OS rH t-l S cfSS cs CM co r- co co co * 00 00 CM <N CB IO <N i-l CM CO s co oio 00 -*<N* CM OS t~ CO O CM ffl O * to t~ Ol CO IO CO CO CO * * O CO 5 S CJ CO -H i-J r- co co tO 00 tOQ CM d * s CO ^ * CO i 1 CM CM aJ ! ! J> be S - . B : . -S -o => -S? ! 1) . cj . . O- .O -O ! C~ . O Cl. i r; c C fe "". S g cj cs , a) 2J, c S e e *d "d - "> ^ w ^^ E Q> 3 tC Q ^_^ o S -^ 2 O - S .> c ^ 3 p R C C *">2 ^ ^ C" " ^ C B ft^ S^c^-y-Cv,: (BC Cc S^ "S^OJ f?| C Sft 9 P ".9 .9 ejiJ-l -^ " c - 5 I "cS"E5;| -S ^ ..S..CT> g *? * 2, ^r^ cS 2"ft = ^ B S -^ r)2 l - fc "5^S,- = . die si!ilgcl|l|!i!l! : g-3g-s.- : c : a^ 2-"^ i< &51-S^ 1, rt ^3^-|-g s , * > <B" ^ /*Pax*8a SNgSgN .2 S & 5 5 s fill li^i Mi^i *& a . ..o I S t~ "Po *QSS iS = =~-oSo S2 E t | 3 1 s 1 "s 1 1 E | Q 1 1| "" J J J "I l * o ^ * * ^ lllltl^lElsg 3 S ^S- s b| > o c gc-gccccccs^c-g.c-gft S* 2 " S S*ca) 1:! fe!uOa> c c5 c OooogoO -^~2- ti c S = ^ o^ S^^sls^^^Q^ ^ S_2 &J2 c._g ^x* 3 5 cs^ioi o 8 aMsBgMjsMs of _2 E S " "* f~ "" ^ a e S S S S S S S S E S Cu S fc ft< Jcbn bo bo ao cc in S M HHHH * NN t-H <O m 10 CM C*3 O O <N O O CO CM CM CO <N to g S CM SJS^.^. S S CO CO CO CO CM CM X * it oo oo - c cs *- ^ *~ *""SS r1 t^ ^1 CO to I~ CM O O CO CO" r-H CO CJ ^ 00 iO O I I |2 2 J 3SJ3 |_ 00 t^ ^** <$ ao o "^ o oo c^i co 10 co co ^ CM * n -v ?j ^ tH rH CM CM 3 CO CM 00 C5> 5 -^J iri oo * OO iO * ^ ^> co ci d * a C g ft & 3 H.E .s O _ ; C e ^co E * 5-._-^ s -c 5 ^ * S % S "^ & p. OJ C3 I ** "-^ ^ -a) >O & * < g ^a .5 , a .So S. l l : h & " o co E ^&," . : I- : " lfll"i- : * fc CD C jy ^ _ .^ **C ^dO al^l ss c =s .<c;;- =^.X2C;; E 8 1rlJ-- - SS ?<NS I 5 .*s S I.- : & 3 M : Z~,ati O x" O ft . ft ft< >To fcTo *-HH !;-H- - O) X3 "^ ^ ^ *** ftj ft . ft . * o o c 61 fell! S gsltll 1 ^! 8 - ^ -|iWI 8 J5l j. C OCD ! . -?;- orf o o .o . -, * x r X^^! 3 ^ -O) "^ _T^" B * C C CC ; .-ftc fta, = r a 22r i SQ!2 5 c c S2~5-2 ! u g "_ * "5 "o u - o oo o o ii S^ = .- O 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>&) ^ ^ ^ ^ / ^ / / ) 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 UNIVERSITY OF CALIFORNIA LIBRARY