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 — C2R 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
C2R 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 C2R Loss in Rotary Converters
LIV. Armature C'2R 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 C2 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 practice1 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 bridge2 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.
0
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 metre1 ; but in subsequent calculations of commercial apparatus it is more convenient to have the results in terms of the wratts 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 10T
Watts per cubic inch at one cycle per second
Area x 16.4 = 4 TT x 107
Watts per pound at one cycle per second
Area x 16.4
' 4 TT x 107 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 found1 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 tester1 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 loss3 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 apparatus1 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 notes1 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 /
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ANALYSIS. |
||||||
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SILICON. I PHOSPHORUS |
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||||||||
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ANALYSIS
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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. 0 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 |
0 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 |
0 181 |
|
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 |
102 |
|
X. |
0.42 |
0.42 |
0.42 |
0.43 |
0.47 |
0.56 |
333 |
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,
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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 t2 x N2 x B2 x 10-10- in which
"W = watts per pound at 0 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 0 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 0 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 0 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 0 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 0 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-magnetic1 |
I1- |
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
l—J
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 |
0 |
0 |
3 |
2 |
25 |
2 |
0 |
0 |
0 10 |
|
6000 |
3 |
2 ° — |
3 |
50 |
0 |
0 2 |
1 |
15 |
2 |
0 |
0 |
0 10 |
|
|
6500 |
3 |
1 ! 2 |
1 |
35 |
0 |
0 2 |
0 |
10 |
1 0 |
0 |
0 5 |
||
|
7000 |
1 |
0 1 0 |
10 |
0 0 1 |
0 |
5 |
1 0 |
o |
0 5 |
||||
|
7500 |
0 |
0 110 |
5 |
0 |
0 0 |
0 |
0 |
1 |
0 |
0 |
0 5 |
||
|
8000 |
0 |
0 1 0 |
5 |
0 |
0 0 |
0 |
0 |
1 0 |
0 |
0 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 |
0 |
3 |
1 |
4 |
40 |
|
5500 |
1 |
2 |
5 |
1 |
45 |
0 |
0 |
1 0 |
5 |
0 |
3 |
0 |
2 |
25 |
|
|
6000 |
0 |
0 |
5 |
1 |
30 |
0 0 |
0 0 |
0 |
0 1 |
0 |
1 |
10 |
|||
|
6500 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 0 |
0 |
0 0 |
0 |
8 |
5 |
||
|
7000 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 0 |
0 |
0 |
0 |
|
|
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 0 |
2 |
35 |
|
|
5000 |
2 |
5 |
3 |
4 |
70 |
2 |
2 3 |
o |
45 |
1 |
0 0 |
0 |
5 |
||
|
5500 |
1 |
5 |
2 |
3 |
55 |
1 |
1 2 |
2 |
30 |
0 |
0 , 0 |
0 |
0 |
||
|
6000 |
1 |
3 |
1 |
2 |
35 |
1 |
1 1 |
0 |
15 |
||||||
|
6500 |
0 |
1 |
0 |
1 |
10 1000 |
5 |
|||||||||
|
7000 |
0 |
0 |
0 |
0 |
0 |
00 0 0 |
0 |
||||||||
|
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 0 K. |
Number of Samples O K. |
Number of Samples 0 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 |
0 |
0 |
2 |
4 |
30 |
|
6000 |
2 |
2 |
4 |
2 |
50 |
0 |
0 |
0 0 |
0 |
0 |
0 |
0 |
0 |
0 |
|
6500 |
0 |
2 |
2 |
1 |
25 |
0 |
0 |
0 0 |
0 |
0 |
0 |
0 |
0 |
0 |
|
7000 |
0 |
2 |
1 |
0 |
15 |
0 |
0 |
0 0 |
0 |
0 |
0 |
0 |
0 |
0 |
|
7500 |
0 |
1 |
0 |
0 |
5 |
0 |
0 |
0 0 |
0 |
0 |
0 |
0 |
0 |
0 |
|
8000 |
0 |
1 |
0 |
0 |
5 |
0 |
0 |
0 0 |
0 |
0 |
0 |
0 |
0 |
0 |
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 |
0 |
2 |
0 |
0 |
10 |
|
5500 |
3 |
4 |
2 |
3 |
60 |
0 |
0 |
0 |
2 |
10 |
0 |
0 |
0 |
0 |
0 |
|
6000 |
1 |
3 |
2 |
2 |
40 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
|
6500 |
1 |
2 |
0 |
1 |
20 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
|
7000 |
1 |
1 |
0 |
0 |
10 0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
|
|
7500 |
0 |
1 |
0 |
0 |
5 0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
|
|
8000 |
0 |
1 |
0 |
0 |
5 [ 0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
|
|
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 |
0 |
3 |
0 |
35 |
|
5000 |
4 |
3 |
4 |
3 |
70 |
3 |
1 |
2 |
3 |
45 |
1 |
0 |
1 |
0 |
10 |
|
5500 |
3 |
2 |
3 |
1 |
45 |
2 |
0 |
2 |
0 |
20 |
0 |
0 |
0 |
0 |
0 |
|
6000 |
1 |
1 |
1 |
1 |
20 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
|
6500 |
0 |
0 |
0 |
1 |
5 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
|
7000 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
|
7500 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
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 0 K. |
||||||||||||
|
2500 3000 3500 4000 4500 5000 |
5 5 4 3 2 0 |
5 5 4 2 1 0 |
5 5 4 3 2 0 |
5 5 4 3 1 0 |
Per Cent. 100 100 80 55 30 0 |
5 5 4 3 1 0 |
5 5 5 2 0 0 |
5 5 2 1 0 0 |
5 5 3 1 0 0 |
Per Cent. 100 100 70 35 5 0 |
5 5 4 0 0 0 |
5 5 4 1 0 0 |
5 4 2 0 0 0 |
p. • Cent. 5 100 5 100 5 75 0 5 0 : 0 0 s 0 |
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 |
0 |
0 |
0 |
5 0 |
0 |
0 |
1 |
5 |
|
1 |
2 |
0 |
2 |
25 |
0 |
0 |
0 |
0 |
0 0 |
0 |
0 |
0 |
0 |
|
0 |
0 |
0 |
0 |
0 0 |
0 |
0 |
0 |
0 i 0 |
0 |
0 |
0 |
0 |
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 |
0 |
j |
0 |
15 |
1 |
2 |
0 |
2 |
25 |
||
|
0 |
0 |
1 |
1 |
10 |
1 |
000 |
5 |
0 |
0 |
0 |
0 |
0 |
|||
|
0 |
0 |
0 0 |
0 |
0 |
0 |
0 0 |
0 |
0 |
0 |
0 |
0 |
0 |
|||
|
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
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 0 K. |
Number of Samples 0 K. |
Number of Samples 0 K. |
||||||||||||
|
2500 3000 3500 4000 4500 5000 |
5 5 5 4 0 0 |
5 5 4 0 0 0 |
5 5 5 1 0 0 |
Per Cent. 5 100 5 100 5 95 3 40 0 0 0 0 |
5 5 3 0 0 0 |
5 5 4 0 0 0 |
5 5 5 0 0 0 |
5 5 1 0 0 0 |
Per Cent. 100 100 65 0 0 0 |
5 5 2 1 0 0 |
5 5 4 0 0 0 |
5 5 2 0 0 0 |
5 5 0 0 0 o |
Per Cent. 100 100 40 5 0 0 |
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 |
0 |
1 |
2 |
1 |
20 |
3 |
1 |
1 |
0 |
25 |
0 |
1 |
1 1 |
15 |
|
|
4000 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 0 |
0 |
|
|
4500 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
|
5000 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
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 |
0 |
0 |
0 |
5 |
0 |
1 |
0 |
0 |
5 |
|
4000 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
|
4500 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
|
5000 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
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 winding1 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 0 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[uuiDle- 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 ICT8 :
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. |
|
|
0 |
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.naiisf^g.6oiafpitch
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