This is a digital copy of a book that was preserved for generations on library shelves before it was carefully scanned by Google as part of a project to make the world's books discoverable online.
It has survived long enough for the copyright to expire and the book to enter the public domain. A public domain book is one that was never subject to copyright or whose legal copyright term has expired. Whether a book is in the public domain may vary country to country. Public domain books are our gateways to the past, representing a wealth of history, culture and knowledge that's often difficult to discover.
Marks, notations and other marginalia present in the original volume will appear in this file - a reminder of this book's long journey from the publisher to a library and finally to you.
Usage guidelines
Google is proud to partner with libraries to digitize public domain materials and make them widely accessible. Public domain books belong to the public and we are merely their custodians. Nevertheless, this work is expensive, so in order to keep providing this resource, we have taken steps to prevent abuse by commercial parties, including placing technical restrictions on automated querying.
We also ask that you:
+ Make non-commercial use of the files We designed Google Book Search for use by individuals, and we request that you use these files for personal, non-commercial purposes.
+ Refrain from automated querying Do not send automated queries of any sort to Google's system: If you are conducting research on machine translation, optical character recognition or other areas where access to a large amount of text is helpful, please contact us. We encourage the use of public domain materials for these purposes and may be able to help.
+ Maintain attribution The Google "watermark" you see on each file is essential for informing people about this project and helping them find additional materials through Google Book Search. Please do not remove it.
+ Keep it legal Whatever your use, remember that you are responsible for ensuring that what you are doing is legal. Do not assume that just because we believe a book is in the public domain for users in the United States, that the work is also in the public domain for users in other countries. Whether a book is still in copyright varies from country to country, and we can't offer guidance on whether any specific use of any specific book is allowed. Please do not assume that a book's appearance in Google Book Search means it can be used in any manner anywhere in the world. Copyright infringement liability can be quite severe.
About Google Book Search
Google's mission is to organize the world's information and to make it universally accessible and useful. Google Book Search helps readers discover the world's books while helping authors and publishers reach new audiences. You can search through the full text of this book on the web
at http : //books . google . com/|
Digitized by VjOOQIC
21 5 N. RANDALL A, ■:
by Google
Digitized by VjOOQIC
Digitized by VjOOQIC
Digitized by VjOOQIC
Digitized by VjOOQIC
Digitized by VjOOQIC
Digitized by VjOOQIC
INTERNATIONAL LIBRARY OF TECHNOLOGY
A SERIES OF TEXTBOOKS F6R PERSONS ENGAGED IN THE ENGINEERING
PROFESSIONS AND TRADES OR FOR THOSE WHO DESIRE
INFORMATION CONCERNING THEM. FULLY ILLUSTRATED
AND CONTAINING NUMEROUS PRACTICAL
EXAMPLES AND THEIR SOLUTIONS
DESIGN OF ALTERNATING-CURRENT
APPARATUS
ELECTRIC TRANSMISSION
LINE CONSTRUCTION
SWITCHBOARDS AND SWITCHBOARD
APPLIANCES
POWER TRANSFORMATION AND
MEASUREMENT
SCRANTON:
INTERNATIONAL TEXTBOOK COMPANY
13B
Digitized by VjOOQIC
Copyrififht, 1906. by International Textbook Company.
Entered at Stationers' Hall, London.
Desigm of Alternatkifir-CaiTent Apparatus: Copyright, 1905, by International
Textbook Company. Entered at Stationers' Hall, London. Electric Transmission: Copyright. 1905. by International Textbook Company.
Entered at Stationers' Hall, London. Line Corjtriliction: Copyrigrht, 1906, by International Textbook Company.
Entered at Stationers' Hall, London. Switchboards and Switchboard Appliances: Copin^srht. 1905, by International
Textbook Company. Entered at Stationers' Hall, London. Power Transformation and Measurement: Copyright. 1905. by International
Textbook Company. Entered at Stationers' Hall, London.
All rights reserved.
Printed in the United States.
//18b
burr printing house,
frankfort and jacob streets,
NEW YORK. 219
Digitized by VjOOQIC
104378
MAR 3 0 1S07
SB
\1>
PREFACE
The International Library of Technology is the outgrowth of a large and increasing demand that has arisen for the Reference Libraries of the International Correspondence Schools on the part of those who are not students of the Schools. As the volumes composing this Library are all printed from the same plates used in printing the Reference Libraries above mentioned, a few words are necessary regarding the scope and purpose of the instruction imparted to the students of — and the class of students taught by — these Schools, in order to afford a clear understanding of their salient and unique features.
The only requirement for admission to any of the courses offered by the International Correspondence Schools, is that the applicant shall be able to read the English language and to write it sufficiently well to make his written answers to the questions asked him intelligible. Each course is com- plete in itself, and no textbooks are required other than those prepared by the Schools for the particular course selected. The students themselves are from every class, trade, and profession and from every country; they are, almost without exception, busily engaged in some vocation, and can spare but little time for study, and that usually outside of their regular working hours. The information desired is such as can be immediately applied in practice, so that the student may be enabled to exchange his present vocation for a more congenial one, or to rise to a higher level in the one he now pursues. Furthermore, he wishes to obtain a good working knowledge of the subjects treated in the shortest time and in the most direct manner possible.
iii
Digitized by VjOOQIC
iv PREFACE
In meeting these requirements, we have produced a set of books that in many respects, and particularly in the general plan followed, are absolutely unique. In the majority of subjects treated the knowledge of mathematics required is limited to the simplest principles of arithmetic and mensu- ration, and in no case is any greater knowledge of mathe- matics needed than the simplest elementary principles of algebra, geometry, and trigonometry, with a thorough, practical acquaintance with the use of the logarithmic table. To effect this result, derivations of rules and formulas are omitted, but thorough and complete instructions are given regarding how, when, and under what circumstances any particular rule, formula, or. process should be applied ; and whenever possible one or more examples, such as would be likely to arise in actual practice — together with their solu- tions— are given to illustrate and explain its application.
In preparing these textbooks, it has been our constant endeavor to view the matter from the student's standpoint, and to try and anticipate everything that would cause him trouble. The utmost pains have been taken to avoid and correct any and all ambiguous expressions — both those due to faulty rhetoric and those due to insufficiency of statement or explanation. As the best way to make a statement, explanation, or description clear is to give a picture or a diagram in connection with it, illustraticms have been used almost without limit. The illustrations have in all cases been adapted to the requirements of the text, and projec- tions and sections or outline, partially shaded, or full-shaded perspectives have been used, according to which will best produce the desired results. Half-tones have been used rather sparingly, except in those cases where the general effect is desired rather than the actual details.
It is obvious that books prepared along the lines men- tioned must not only be clear and concise beyond anything heretofore attempted, but they must also possess unequaled value for reference purposes. They not only give the maxi- mum of information in a minimum space, but this infor- mation is so ingeniously arranged and correlated, and the
Digitized by VjOOQIC
PREFACE V
indexes are so full and complete, that it can at once be made available to the reader. The numerous examples and explanatory remarks, together with the absence of long demonstrations and abstruse mathematical calculations, are of great assistance in helping one to select the proper for- mula, method, or process and in teaching him how and when it should be used.
The first portion of this volume contains an exceptionally distinct and intelligible treatise on the complex problems relating to the design of alternating-current apparatus. The correct proportions and relative location of the different parts of the machines are clearly set forth and illustrated by numerous figures showing the details of the construction. The design of alternators, motors, and transformers is fully discussed. The various systems of transmitting electrical energy, and the methods used in calculating the size of wires, and installing the wires for overhead and underground trans- mission systems, are described in great detail, and complete wire data tables are furnished. The treatment of switchboards in this volume is very complete and is superior to anything yet published. The recent styles of oil switches, circuit-breakers, measuring instruments, etc. are fully explained and illustrated, and their location indicated on the switchboard diagrams. Under the heading Power Transformation and Measurement, a very clear treatise is given of the installation of transform- ers and substations and the methods of power measurements.
The method of numbering the pages, cuts, articles, etc. is such that each subject or part, when the subject is divided into two or more parts, is complete in itself; hence, in order to make the index intelligible, it was necessary to give each subject or part a number. This number is placed at the top of each page, on the headline, opposite the page number; and to distinguish it from the page number it is preceded by the printer's section mark (§). Consequently, a reference such as § 16, page 26, will be readily found by looking along the inside edges of the headlines until § 16 is found, and then through § 16 until page 26 is found.
International Textbook Company
I3-B
Digitized by VjOOQIC
Digitized by VjOOQIC
CONTENTS
Design of Alternating-Current Appa- ratus Section Page
Alternators 20 1
Limitation of Output 20 2
Heating of Alternator Armatures .... 20 4
Relation Between/*^ Loss and Output . 20 6
Core Losses 20 7
Hysteresis Loss 20 7
Eddy-Current Loss 20 9
Radiating Surface of Armature 20 10
Armature Reaction 20 11
Armature Self-induction 20 15
Peripheral Speed of Alternator Arma- tures 20 20
Armature Windings 20 21
Single-Phase Concentrated Winding ... 20 22
Single-Phase Distributed Windings ... 20 23
Polyphase Armature Windings 20 27
Arrangement of Windings 20 29
Construction of Armatures 20 31
Armature Disks 20 31
Armature Spiders 20 34
Armature Conductors 20 38
Forms of Armature Coils and Bars ... 20 39
Armature Insulation (Coils) ...... 20 42
Armature Insulation (Slots) 20 43
Magnetic Densities 20 46
Density in Armature Teeth 20 46
Density in Armature Core '^ '^ . 20 47
iii
Digitized by VjOOQIC
iv CONTENTS
Dbsign of Alternating-Current Appa- ratus— Continued Section Pagi
Density in Air Gap 20 48
Desigfn of 100-Kilowatt Single-Phase
Alternator 21 1
Dimensions of Conductor and Core ... 21 3
Design of Armature Core 21 4
Calculation of Armature Losses 21 10
Armature Winding for Two-Phase Alter- nator 21 13
Armature Winding for Three-Phase Alter- nator 21 15
Completed Armatures 21 19
Design of Field Magnets 21 20
Revolving Fields 21 23
Field-Magnet Coils 21 25
Insulation of Field Coils 21 27
Design of Field 21 28
Bore of Poles and Length of Air Gap . . 21 28 Magnetic Flux Through Pole Pieces and
Yoke ... 21 30
Calculation of Field Ampere-Turns ... 21 32
Calculation of Separately Excited Winding 21 34
Compound, or Series-Field, Winding . . 21 38
Loss in Field Coils 21 42
Mechanical Construction 21 43
Field Frame and Bed 21 43
Collector Rings and Rectifier 21 45
Brushes and Brush Holders 21 50
Brush-Holder Studs 21 51
Shafts 21 54
Pulleys 21 55
Connections 21 57
Transformers 22 1
Transformer Cores 22 4
Heating of Transformers 22 4
Magnetic Density in Core 22 5
Arrangement of Coils and Core .... 22 6
Digitized by VjOOQIC
CONTENTS V
Design of Alternating-Current Appa- ratus— Continued Section Page Winding and Insulation of Coils .... 22 8 Design of 8-Kilowatt Transformer ... 22 10
Determination of Core Volume 22 11
Dimensions of Core 22 12
Dimensions of Conductors 22 13
Calculation of Primary and Secondary
Turns 22 15
Arrangement of Primary and Secondary
Coils 22 16
Efficiency 22 19
Efficiency Curve 22 21
All-Day Efficiency 22 23
Magnetizing Current 22 24
Regulation 22 25
Construction 22 27
Induction Motors 22 30
Limitation of Output 22 31
Primary Core Losses, Magnetic Densities,
Etc 22 31
Secondary Core Losses, Magnetic Den- sities, Etc 22 32
Induction-Motor Windings 22 33
Primary Winding 23 33
Secondary Winding 22 35
Power Factor 22 36
Length of Air Gap .22 37
General Data .22 37
Design of 10-Horsepower Motor .... 22 40
Full-Load Current in Primary 22 41
Size of Primary Conductor 22 42
Peripheral Speed and Diameter of Arma- ture 22 42
Primary Winding 22 43
Magnetic Flux in Poles 22 45
Secondary Winding 22 50
Rotary Conductors and Core 22 50
Digitized by VjOOQIC
vi CONTENTS
Design of Alternating-Current Appa- ratus— Continued Section Page
Heat Losses 22 52
Field Winding and Connections 22 55
Mechanical Construction 22 56
Armature 22 56
Shafts 22 56
Field Frames and Bedplate 22 57
Electric Transmission
Introductory 23 1
Power Transmission by Direct Current . 23 2
Line Calculations 23 7
Power Transmission by Alternating Cur- rent 23 23
Single-Phase Transmission 23 24
Two-Phase Power Transmission 23 26
Three-Phase Power Transmission .... 23 28
Line Calculations for Alternating Current 23 30
Formulas for Line Calculations 23 31
Selection of a System 23 36
Direct-Current Systems 23 36
Alternating-Current Systems 23 39
Cost of Conductors 23 43
Combined Operation of Direct-Current
Dynamos 23 45
Operation of Dynamos in Series .... 23 45 Operation of Direct-Current Dynamos in
Parallel 23 45
Combined Running of Alternators .... 23 58
Alternators in Series 23 58
Alternators in Parallel 23 58
Line Construction
Introduction 24 1
Line Conductors 24 1
Overhead Construction 24 14
Cross-Arms 24 16
Pins 24 19
Digitized by VjOOQIC
CONTENTS vii
LiNB Construction — Continued Section Page
Tying, Splicing, Etc 24 23
Underground Construction 24 32
Conduits 24 33
Manholes 24 38
Edison Underground Tube System ... 24 53
Tests 24 58
Testing Lines for Faults 24 58
Switchboard^ and Switchboard Appli- ances
Switchboard Appliances 25 1
Switches . 25 1
Bus-Bars 25 19
Fuses and Circuit-Breakers 25 27
Ground Detectors 25 36
Potential Regulators . 25 42
Protection From Lightning and Static
Charges 25 47
Field Rheostats 25 65
Switchboards 25 71
Direct-Current Switchboards 25 73
Alternating-Current Switchboards .... 25 76
Power Transformation and Measurement Transformers and Transformer Connec- tions 26 1
Transformers on Single-Phase Circuits . 26 4
Transformers on Two-Phase Circuits . . 26 9
Transformers on Three-Phase Circuits . . 26 11
Substation Equipment 26 18
Apparatus for Controlling the Incoming
Current 26 20
Apparatus for Transforming the Current . 26 26 Apparatus for Controlling the Outgoing
Current 26 40
Location and General Arrangement of
Substations 26 40
Connections for Substations 26 44
Digitized by VjOOQIC
viii CONTENTS
Power Transformation and Measurement
Continued Section Pagt Measurement of Power on Polyphase Cir- cuits 26 53
Instruments Used for Power Measurement 26 53
Indicating Wattmeters 26 54
Recording Wattmeters 26 54
Measurement of Power on Two-Phase Cir- cuits 26 59
Measurement of Power on Three-Phase
Circuits 26 63
Installation of Recording Wattmeters . . 26 75 Testing and Adjusting Recording Watt- meters .26 79
Reading Recording Wattmeters 26 82
Special Meters 26 85
Digitized by VjOOQIC
DESIGN OF ALTERNATING- CURRENT APPARATUS
(PART 1)
ALTERNATORS
1. The design of alternators is in many respects similar to that of multipolar continuous-current machines, many of the parts being very similar. For example, the method of calculating the field ampere-turns, and the design of the field in general, is much the same in these two classes of machines. A great many of the mechanical details are also similar, and much of what has already been given as applying to continuous-current machines applies also to alternators.
3. Some of the calculations connected with the design of alternators are, however, not so easily made as for direct - current machines, and the production of a good design depends largely on the skill and previous experience of the designer. For example, there is a large variety of arma- ture windings to select from, and the designer has to decide which winding is best adapted for the work that the alter- nator has to do. Such calculations as the estimation of armature inductance, armature reaction, etc. are difficult to make without having had previous experience with machines of the same type as that being designed. The quantities are, in general, easily determined after the machine has
§20
For notice of copyri^jht, see page immediately following the title page.
45—2
Digitized by VjOOQIC
2 DESIGN OF ALTERNATING § 20
been built, but their previous calculation is difficult. For this reason the design of alternators is, on the whole, more empirical than that of continuous-current machines. There is also a greater choice as to the mechanical arrangement of the different parts, since either the field or armature may be the revolving member.
lilMITATION OF OUTPUT
3. The output of an alternator, like that of a direct- current machine, may be limited by the heating of the arma- ture. This heating is due to two causes, namely, the /' R loss in the armature conductors, and the core loss due to the hysteresis and eddy-current losses in the mass of iron con- stituting the armature core. Both these losses appear in the form of heat, and cause the armature as a whole to rise in temperature. Since the maximum temperature at which an armature can be run with safety is limited by the tempera- ture to which the insulating material may be subjected con- tinuously without injury, it follows that this heating effect is an important factor, limiting the output of the machine.
4. The output may in some cases be limited by self- induction and armature reaction. If the inductance of the armature is very high, a considerable part of the E. M. F. generated may be used to force the current through the armature itself, thus reducing at the terminals of the machine the E. M. F. available for use in the external cir- cuit. In other words, if an alternator having an armature with high self-inductance is run with a constant field excita- tion, the voltage between the collector rings will fall off as the load is applied. Most alternators have to be built under a certain guarantee as to voltage regulation. By the volt- age regulation is meant the percentage that the voltage rises when the full load is thrown off an alternator. That is, suppose an alternator, when carrying full load, generates 2,000 volts, and when the load is thrown off the voltage rises to 2,100, the field excitation and speed remaining the same. The increase is 100 volts, or 5 per cent, of the full-
Digitized by VjOOQIC
§20 CURRENT APPARATUS 3
load voltage, and the regulation would be 5 per cent ; the per- centage always refers to full-load voltage, because full load is taken as the normal operating condition of the machine.
6. In most of large slow-speed alternators of the revolv- ing field type the ventilation is so good that the full-load current can be delivered with a rise in temperature well within the safe working limit. If, however, these machines are not carefully designed they may not give the voltage regulation required. The voltage may drop more than the allowable amount when full load is applied because of the armature reaction and self-induction. In such cases, there- fore, the output that the machine can deliver without exceed- ing the specified limit of voltage regulation may be limited by the armature reaction and self-induction, and not by heating. For certain classes of work close regulation is very important, and in many cases the regulation becomes a more important factor in the design, so far as limitation of output is concerned, than heating.
As pointed out later, the regulation depends a great deal on the character of the load that the machine carries. ' The regulation might be very good on a non-inductive load and so poor on an inductive load that the machine could not be made to maintain its voltage even with the fields ej^cited to the fullest extent. A statement of the regulation should always include a statement of the character of the load for which the regulation is given, i. e. whether non-inductive or inductive, and, if the latter, the power factor.
6. In high-speed alternators, such as those driven by belts or by steam turbines, the armature presents compar- atively small surface for the dissipation of heat, and unless special means are provided for ventilation, the heating effect will be an important factor in determining the allowable output. In direct-current machines, sparking at the com- mutator often limits the output, but obviously this does not apply to alternators, because no commutator is used, except in some cases as an auxiliary part in connection with the field-exciting circuit. However, while armature reaction
Digitized by VjOOQIC
4 DESIGN OF ALTERNATING §20
cannot cause sparking in an alternator it has a decided influence on the voltage regulation, and its effects must be carefully considered.
HEATING OP AXTERNATOU ARMATUBB8
7. The final temperature that an armature attains when carrying its normal load depends not only on the actual amount of energy wasted iji the armature, and that appears in the form of heat, but also on the readiness with which the armature can get rid of this heat to the surrounding air. The armature will always keep on increasing in tem- perature until it reaches a point where it radiates the heat to the air as fast as it is generated. The rise in temper- ature necessary to accomplish this will evidently depend largely on the construction of the armature. A well-venti- lated armature will get rid of more heat per degree rise than a poorly ventilated one ; hence, every effort should be made, in designing an armature, to arrange it so that the air can circulate freely around the core and conductors. This is best done by mounting the armature disks on an open spider, and providing air ducts through the iron core, which allow a circulation of air when the machine is run- ning. By adopting this construction, makers have been able to reduce the size of armature for a given output com- pared with the size required for the same output when the older style, with surface windings and unventilated core, was used. The heat loss due to hysteresis and eddy currents in the core is about the same, whether the machine is loaded or not. Suppose an alternator to be run on open circuit with its field fully excited. There will be no loss in the armature- conductors, because the machine is furnishing no current. The mass of iron in the core is, however, revolv- ing through a magnetic field, and there will consequently be a hysteresis loss in the iron, and eddy currents will be set up in the armature disks. These will cause the arma- ture to heat up until the rise in temperature is sufficient to radiate these core losses. When the machine is loaded,
Digitized by VjOOQIC
§20 CURRENT APPARATUS 5
we have, in addition to the above, the heat loss in the con- ductors due to the current that is now flowing. The result is that the armature increases further in temperature until it reaches a final temperature that allows the armature to get rid of all the heat generated in it. If the armature is over- loaded, the PR loss becomes excessive, and a point is soon reached where it becomes unsafe to load the machine further.
8. What was said regarding the safe heating limit of the insulating materials used in the construction of con- tinuous-current armatures applies also to armatures for alternators. There is no good reason why an alternator armature should be worked at a higher temperature than that of a direct-current machine, although in many alterna- tors, especially some of the older styles, the limit is much higher. In modern machines, however, the rise of temper- ature is very little, if any, higher than in continuous-current machines of corresponding output and speed. The final temperature when running fully loaded should not exceed 40° to 50** C. above that of the surrounding air.
9. The total temperature that the armature attains ' when fully loaded depends on the temperature of the sur- rounding air. It is not safe to count on less than 20° C. for the average temperature of the surrounding air, because the air in dynamo rooms in summer often goes far above this. A fair rise in temperature may therefore be taken as from 70° to 80° F., or from 40° to 50° C. These are the ordinary values used in rating machines, and if an alter- nator will deliver its full load continuously, with a rise in temperature not exceeding the above, it should be per- fectly safe, as far as danger from overheating goes. The rise in temperature of the field coils is generally not quite as high as that of the armature, but it must be remembered that while the outside layers of the coils may be compara- tively cool, the inner turns may be quite hot, and it is the greatest temperature that any part of the coils attains that must be taken into account.
Digitized by VjOOQIC
6 DESIGN OF ALTERNATING §20
REIiATION BETWEEN I' R liOSS AND OUTPUT
10. The /*R'\oss in an armature at full load usually bears a certain ratio to the output of the machine. An alternator with an excessive I* R loss in the armature con- ductors would have a low efficiency. It is therefore impor- tant that the armature be so designed that the heat loss in the winding shall not exceed a certain proportionate amount of the total output. This loss can be decreased by decreas- ing the resistance of the armature winding. The resistance can be decreased by either shortening the length of wire on
s
Curve shotting relation between artnature I^R loeuJt outjyut of alternator,
PlO. 1
the armature or by increasing its cross-section. A certain length of active conductor is necessary for the generation of the E. M. F. ; hence, to keep down the /^R loss, we must use an armature conductor of large cross-section. The size of conductor, if increased too much, calls for a large armature for its accommodation, and the machine is thus rendered bulky and expensive. All that can be done, there- fore, is to design the armature winding so that the heat loss will be as small as is consistent with economy of construc- tion. Older types of alternators had a large armature
Digitized by VjOOQIC
§20 CURRENT APPARATUS 7
/* R loss, but the curve drawn in Fig. 1 may be taken as giving the average loss for ordinary alternators. The abscissas of this curve give the output in kilowatts, and the ordinates, the PR loss in per cent, of the output. It will be understood that the loss in individual machines might vary somewhat from the values shown, but the curve shows the average relation for machines where the /* R armature loss is not excessive. It will be noticed that this loss is a much larger percentage for small machines than for large ones. For machines over 100 K. W., the percentage loss does not decrease much with increased output.
CORE liOSSES
11. The core losses have already been mentioned as one of the causes producing heat in the armature. These losses are present also in continuous-current armatures, but their effects are usually much less than in alternators. In some alternators the core losses are nearly if not quite as great as the PR loss, and consequently the no-load rise in tem- perature may be considerable.
HYSTERESIS LOSS
12. The nature of this loss has already been explained in connection with the design of direct-current machines and the method of calculating it pointed out, so that it will not be necessary to dwell further on it here. The curves shown in Fig. 2 will be found useful for calculating the hysteresis loss in alternating-current apparatus. Curve A shows the relation between the maximum magnetic density and the watts lost per cubic inch per 100 cycles for a good quality of soft transformer iron. Curve B shows the loss for ordinary armature iron of good quality. In order to obtain the total hysteresis loss for a given mass of iron, multiply the value given by the curve corresponding to the maximum density at which the iron is worked, by the volume in cubic inches and the frequency and divide the result by 100.
Digitized by VjOOQIC
8 DESIGN OF ALTERNATING §20
Example. — The armature core of an alternator having 12 poles and running at a speed of 600 revolutions per minute is worked at a maxi- mum magnetic density of 20,000 lines per square inch. If the volume of the core is 2,t)00 cubic inches, how many watts will be wasted in hysteresis ? •
Magnetic d&mtity B {lines per bq. inch) FlO. 2
Solution. — If the machine runs at 600 rev. per min. and has 12 poles, the frequency of the magnetic cycles in the armature core must be V X Vo"' ^r 60 cycles per second.
By referring to curve //, Fig. 2, we find the loss per cubic inch per 100 cycles corresponding to a density of 20,000 to be about .22 watt. Hence, the total loss will be
... .22x2.000x60 __. ^^ . Wu = ^.^. = 264 watts. Ans.
Digitized by VjOOQIC
§20 CURRENT APPARATUS 9
13« The hysteresis loss, other things being equal, increases directly with the frequency. It is on this account that this loss is usually greater in alternator armatures than in those used for direct-current machines, because the fre- quency of the former is usually much higher than that of the latter. Special care should therefore be taken in the selection of core iron for all kinds of alternating-current apparatus. It will also be noticed that the hysteresis loss, being proportional to the 1.6th power of the magnetic density, will increase quite rapidly as the density is increased. It follows, therefore, that the core densities used should be low, otherwise the hysteresis loss may become excessive. It is usual to employ lower core densities in alternating-cur- rent machines than in continuous-current machines, because the frequency is usually fixed by the conditions under which the machine has to work, and a low density is therefore necessary to keep down the hysteresis loss.
BDDY-CURREXT IX>SS
14. The other core loss mentioned above, namely that due to eddy currents, is not usually very large, provided proper care is taken in building up the armature core. This loss is due to local currents circulating in the armature disks, and the eddy-current loss is really an /' R loss caused by the resistance offered to these currents by the iron con- stituting the core. If the core is thoroughly laminated, the paths in which these currents flow are so split up that the currents are confined to the individual armature disks. This keeps down the volume of the eddy currents, and if the disks are well insulated and made of thin iron, the eddy- current loss may be made very small. Anything that makes electrical connection between the disks may largely increase this loss. For example, filing out the slots, or burring over the disks, or passing uninsulated clamping bolts through the core may result in an increased loss. It
Digitized by VjOOQIC
10 DESIGN OF ALTERNATING §20
is well, therefore, to avoid filing or milling the slots unless it is absolutely necessary to render them smooth enough to receive the insulating troughs and armature conductors. The eddy-current loss is proportional to the square of the frequency, other things being equal; hence it is usually greater in alternators than in direct-current machines. If proper precautions are taken in building up the core, the eddy-current loss should be small compared with the /* R and hysteresis losses. It is difficult to calculate this loss beforehand, on account of the large variations caused in it by defects in the insulation of the core disks from each other.
RADIATING SURFACE OF ARMATURE
16. The armature has to present sufficient radiating surface to get rid of the heat dissipated without a rise in temperature exceeding, say, 40° or 50° C. This means that the size of the armature will, for a given output and given amount of loss, depend on the ease with which it can radiate the heat. The number of watts that an armature can radiate per square inch of surface per degree rise in tem- perature varies greatly with the style and construction of the armature and the peripheral speed at which the arma- ture is run, so that it is not possible to give any values for this radiation constant that will be applicable to all styles of armatures. A well- ventilated iron-clad alternator arma- ture should be able to radiate from .04 to .06 watt per square inch of cylindrical surface (circumference of iron core X length parallel to shaft) per degree rise. These values are for machines running at peripheral speeds of from 4,000 to 5,000 feet per minute; if the peripheral speed were higher, the watts radiated per square inch per degree rise would be correspondingly increased. This means, then, assuming 40° C. to be the allowable rise, that a well- ventilated armature of the above type should be capable of radiating from l.G to 2.8 watts per square inch of cylindrical
Digitized by VjOOQIC
§20 CURRENT APPARATUS 11
surface. In well-designed alternators, the sum of the hys- teresis and eddy-current losses will not, as a rule, be greater than the /*AMoss, so that we will, in general, be safe in assuming that an allowance of from .8 to 1.4 watt /' R loss for each square inch of surface will give an armature of sufficient radiating surface to keep the total rise in tempera- ture due to all the losses from exceeding 40° C. This will give a preliminary value for the surface of the armature on which to base subsequent calculations, bearing in mind that the dimensions so obtained are not necessarily final, and may be modified as the design is worked out further, pro- vided always that the armature is made of such dimensions that it will be able to get rid of the heat generated. Machines have been built in which the surface per watt is less than that given above, but it will usually be found that such machines run very hot when fully loaded unless their peripheral speed is very high or their ventilation exception- ally good. Alternator armatures of the iron-clad type can usually be constructed so as to secure good ventilation, especially if they are of fairly large diameter, so there should be no difficulty in radiating the amount of heat just given. The watts per square inch as given are referred to the outside cylindrical surface; of course, the ends of the core, and to a certain extent the inside also, help to radiate the heat, but it is more convenient for purposes of calcula- tion to refer the watts radiated per square inch to the out- side core surface rather than to the surface of the armature as a whole.
ARMATURE REACTION
16. Armature reaction, in connection with alternators, has already been mentioned in a general way, and it now remains to be seen just how it affects the action of a machine when loaded. The matter of armature reaction plays an important part in the design of continuous-current machines, as has already been shown in the section on the design of
Digitized by VjOOQIC
12
DESIGN OP ALTERNATING
20
such dynamos. If the armature of a continuous-current machine is capable of overpowering the field, bad sparking will result at the commutator. This, however, cannot occur in the case of an alternator, and the only bad effect that the reaction can have is to cause a weakening and distortion of the field, with a consequent reduction of the voltage gener- ated in the armature.
PlO. 8
17. Let N^ Fig. 3, represent one of the north poles of an alternator, surrounded by its magnetizing coil a. The
lines of force will flow into the armature from the pole piece, as indicated by the lines and arrowheads. We will consider the instant when the coil c c' has^ its opening directly under the pole, or when the center of the tooth b is opposite the center of the pole piece. If there is no self-induction pres- ent, the current flowing through the armature will be in phase with the E. M. F. generated ; consequently, at the position shown in the figure, the current in the coil will be zero, because the coil is cutting no lines of force, and the E. M. F. generated is consequently zero. It follows, then, that under this particular set of conditions the armature coil has no disturbing effect on the lines of force set up by the field. The direction of rotation is indi- cated by the arrow, and a moment later the bundle of conductors in the slot c is under the center of the pole, as shown in Fig. 4. The current in the conductors will now be at its maximum value, be- cause the E. M. F. generated is at its maximum. The current will be flowing down through the plane of the paper, and the bundle of conductors lying in
PIO. 4
Digitized by VjOOQIC
20
CURRENT APPARATUS
13
PIO. 6
the slot will tend to set up lines of force around themselves, as shown by the dotted lines, and in the direction shown by the arrowheads. It will be noticed that this field set up by the conductors tends to strengthen the right-hand side of the pole and weaken the left-hand side by a like amount. The resultant effect is therefore to crowd the field forwards in the direction of rotation, making it denser at the right-hand side, as shown in Fig. 5. It is therefore seen that in this respect the effect of armature reaction is similar to the effect observed in direct-current machines; but in an alter- nator with coils, as shown in the figures, the effect on the field is not steady, but varies as the teeth move past the poles. The student should note that in this case the armature and load are assumed to have no self-induction, and also that the armature reaction tends only to change the distribution of the field and not to weaken it.
18. Armatures always have more or less self-induction, especially if they are provided with heavily wound coils sunk
in slots. The effect of this self-induction is, of course, to cause the current in the armature to lag behind the E. M. F. It is necessary, then, to see how this lagging of the current affects the reaction of the armature on the field. In this case the current in the coil does not die out at the same instant as the E. M. F., but persists in flowing after the E. M. F. has become zero. The cur- rent, instead of being zero when the tooth is under the pole, will then be flowing as shown in Fig. 6 ; that is, the current
PlO. 0
Digitized by VjOOQIC
14 DESIGN OF ALTERNATING §20
in the conductors in slot c persists in flowing, as 'shown in Fig. 5, after the conductors have moved out from under the pole piece. This current flowing in the armature coil will set up lines of force through the coil in the direction shown by the dotted arrows, i. e., directly opposed to the original field. The armature reaction, therefore, not only tends to distort the field, but also tends to weaken it when there is a lagging of the armature current due to self-induction in the armature or external load. This reaction of the arma- ture on the field would of course cause a falling off in the voltage of the machine if the field magnets were not strengthened to counterbalance its effects. It is instructive to note here that if it were practicable to have a condenser in connection with the armature, the current could be made to lead the E. M. F., and the armature reaction would then tend to magnetize the field instead of demagnetize it.
19. It is seen from the above that in alternator arma- tures in which there is an appreciable amount of self-induc- tion present, we have two effects similar to those produced by the cross ampere-turns and back ampere-turns of a continuous-current armature, the former tending to distort the field, and the latter acting directly against it and tend- ing to weaken it. The bad effects of this reaction can be reduced, as in the case of direct-current machines, by length- ening the air gap. The actual amount of distortion or demagnetization is not easily calculated, as it evidently changes with the changes in the current, and also depends on the armature inductance, which is itself difficult to esti- mate without data from machines of the same type. The distribution of the field can be determined after the machine has once been built, and unless the air gap is very short, the distortion is not sufficient to badly affect the working of the machine.
20. One effect of armature reaction is sometimes taken advantage of in designing armature windings, namely, the crowding together of the lines to one side of the pole piece.
Digitized by VjOOQIC
§20 CURRENT APPARATUS 15
This practically makes the effective width of the pole face less and allows the use of coils on the armature with an opening somewhat less than the width of the pole face, without danger of the E. M. F.'s in the different turns of the coil opposing each other.
ARMATURE SELF-INDtJCTIOX
21. It has just been shown that self-induction is indi- rectly responsible for the demagnetization of the field, which in turn produces a falling off in voltage. Self- induction also calls for a considerable E. M. F. to force the current through the armature, and this causes a still further diminution in the E. M. F. obtained at the ter- minals. This drop in voltage has already been explained in the section on Alternators, A machine with high armature self-induction will not maintain a constant terminal pres- sure unless the field is strengthened as the load is applied, and such machines therefore require heavily compounded fields.
22. In general, armatures wound with a few heavy coils bedded in slots have a high self-induction, because the coils are able to set up a large number of lines around themselves when a current flows through the armature. Machines with this style of armature winding usually give an E. M. F. curve that is more or less peaked and irregular. Such windings are easily applied to the armature, and being of very simple construction, they necessitate very few crossings of the coils at the ends where the coils project from the slots. They are, therefore, easy to insulate for high volt- ages, and are extensively used on alternators for operating incandescent lights.
23. The inductance depends on the way in which the coils are arranged in the slots. Fig. 7 (a) shows a cross- section of a slot containing a heavy coil of 40 turns. When current is passed through the coil, a magnetic field is set up
Digitized by VjOOQIC
16 DESIGN OF ALTERNATING §20
that encircles the coil as indicated by the dotted lines. The self-induced E. M. F. will depend on the strength of this field and on the number of turns with which the field is linked. The strength of field depends on the current, the number of turns, and the reluctance of the magnetic path surrounding the turns. If the reluctance is a constant quantity, it is evident that the self-induced E. M. F. for a given current will increase as the square of the number of
(m)
/^M^/»/K'Z:
Fig. 7
turns per coil or conductors per slot. Such being the case, the inductance could be decreased by splitting up the single coil into two or more coils placed in separate slots, thus reducing the number of conductors per slot. For exam- ple, suppose an armature has 6 coils of 40 turns each, and that the inductance of each coil is .02 henry. The coils are supposed to be connected in series, so that the total inductance of the armature will be G X .02 = .12 henry.
Digitized by VjOOQIC
§20 CURRENT APPARATUS 17
Suppose, now, the winding is split up into 12 coils of 20 turns each, the shape and arrangement of the coils being kept the same. We will then have the same total number of turns as before, but will have half as many turns per coil or half as many conductors per slot. The inductance of each coil will therefore be one-fourth of what it was before, because the inductance will decrease as the square of the number of turns per coil. The inductance per coil will then be ^ X 02 = .005 henry, and the total inductance will be .005 X 12 = .06 henry, or one-half of what it was in the former case. In order, then, to decrease the inductance of an armature, the number of turns per coil must be decreased, or, what amounts to the same thing, the number of conductors per slot must be decreased.
In the preceding example, it has been assumed that the reluctance of the path around the coil is the same for the heavy coil as for the light coil. This, however, is not the case in practice, and the reduction of inductance by subdividing the winding is not as great as the theoretical example just given would indicate. In Fig. 7 (^), it will be noticed that the greater part of the reluctance of the magnetic path occurs at the air gaps around the top of the slots, as indicated at a b. With a wide shallow slot, the reluctance of the path c d between the sides of the slot is also larger. When the coil is split up, it is necessary to use narrower slots and teeth, as shown at (b), so that the air gap ab \^ made much shorter. Also, the slots being deep and narrow compared with (^), the reluctance between the sides of the slot itself is less. The result is that the decrease in the number of conductors per slot may be offset to a consider- able extent by the decreased reluctance, so that the product of the flux times turns may not be reduced to nearly so great an extent as the decrease in the turns per coil would lead one to expect. With the narrower slots in (^), the higher tooth density tends to keep up the reluctance of the magnetic path, but saturated teefh are not used as much in alternators as in direct-current machines, and the tendency of making the slots narrower and deeper is, on the whole,
45—3
Digitized by VjOOQIC
18 DESIGN OP ALTERNATING §20
to reduce the reluctance of the path for the magnetic flux that is responsible for the setting up of the induced E. M. F. While, therefore, the splitting up of the winding does not reduce the inductance in proportion to the square of the number of turns per coil, yet it does reduce it considerably, and for machines where iow armature inductance and close voltage regulation are desired, the winding is usually split up in the manner described. This subdivision of the wind- ing will be described more fully later.
JS4. Calculation of Armature Inductance. — Since the inductance of the armature coils depends on the reluctance of the magnetic path around the coils, it is evident that it ' will be influenced not only by the size and shape of the slots, but also by the position of the armature with regard to the field, and also by the length of the air gap between armature and field. For example, in Fig. 7 {a), when the bundle of conductors is under the poles, as shown, the inductance is a maximum because the iron pole face helps to carry the flux around the conductors. If the air gap were very short, it is evident that the reluctance of the path for the induced flux would be much less with the slot under the poles than when between the poles, because in the latter case the path between the tops of the teeth would be wholly through air. It is evident that with a long air gap there would be little difference in the inductance under the poles and between the poles. The inductance is there- fore not constant, but varies with the position of the slots with regard to the pole pieces. It is also evident that the number of lines set up through a coil will be proportional to the length of the laminated core, i. e., the length parallel to the shaft, so that for an equal number of turns, short arma- atures have a lower inductance than long ones.
25. On account of the number of variable qua'ntities that enter into the calculation of the inductance, it is not possible to lay down any rule that will apply to all sizes of slot, air gap, length of core, etc. Inductance calculations
Digitized by VjOOQIC
§20 CURRENT APPARATUS 19
are based on data obtained from tests of machines of similar type to the one being designed. Parshall * gives a number of tests made to determine the inductance of various arma- tures and shows that the field set up around a coil varies from 13 to 140 or 150 lines per ampere-turn per inch length of armature core. The latter high values are for armatures with a very short air gap and with the conductors under the poles in the position of maximum inductance. For fairly wide slots, and with the conductors in the position of minimum inductance between the poles, the value is from 15 to 20 lines per ampere-turn per inch length of core. For example, suppose an armature coil had 40 turns and that we take 20 lines per ampere-turn per inch length of core as a fair value for the field set up around the coil. Also, suppose that the armature core is 8 inches long. The flux through the coil will then be 20 X 8 X 40 = 6,400 lines for a current of 1 ampere. We have
<PX T _ -
where (P is the flux corresponding to a current of 1 ampere, T the number of turns, and L the inductance in henrys. Then, in this case,
,. 6,400 X 40 ^^^^^ ,
L = ' 3 — = .00256 henry
The probable value of the flux can usually be calculated from data obtained from tests on similar machines, and data of this kind is absolutely necessary if accurate estimates of inductance are to be made. The preceding example will, however, give the student an idea as to the elements on which the value of the inductance depends. If the induct- ance L is known, the armature reactance is easily obtained from the expression ^-nn L, where n is the frequency. The voltage necessary to overcome the reactance is 2nnLI^ where / is the current in the armature.
**• Electric Generators," Parshall and Hobart
Digitized by VjOOQIC
20 DESIGN OP ALTERNATING §20
Alternators provided with armatures of low inductance give a much better E. M. F. regulation than those having high inductance, because the reaction on the field is not only less, but much less of the E. M. F. generated is used up in driving the current through the armature. In other words, such machines, if provided with a constant field excitation, will show only a moderate falling off in terminal voltage from no load to full load. On this account, it is quite common to find such machines built without any compound or series-winding on the fields, all the regulation necessary being accomplished by varying the current sup- plied to the field coils by the exciter. Such alternators give a smooth E. M. F. curve that approximates closely to the sine form, and alternators of this type are being used exten- sively for power-transmission purposes.
26. An excessive amount of armature inductance, and consequent damagnetizing armature reaction, has been used to make alternators regulate for constant current. In such machines the armature inductance is made very high, and a small air gap is used between the armature and field. If the current delivered by such- a machine tends to increase by virtue of a lowering of the external resistance, the arma- ture reaction on the field increases and the field is weak- ened. This cuts down the voltage generated, so that the voltage adjusts itself to changes in the load, and the cur- rent remains constant.
PERIPHERAIi SPEED OF AliTERNATOB AR^IATURES
37. Alternators have been built to run at peripheral speeds much higher than those used for continuous-current machines. This was the case in many of the older types of lighting machines running at a high frequency. Since the frequencies employed were high, the revolutions per minute of the armature also had to be high in order to avoid using a very large number of poles. This high speed of rotation usually resulted in high peripheral speeds also, because the
Digitized by VjOOQIC
§20 CURRENT APPARATUS 21
armature could not be made very small in diameter. Such machines often ran at peripheral speeds as high as 7,000 or 8,000 feet per minute. Modern revolving-field machines for direct connection to waterwheels often run 7,000 or 8,000 feet per minute, and steam turbine alternators from 12,000 to 15,000.
5S8. The frequency of a great many modern machines is lower than that formerly used, 60 or 26 cycles per second being standard values. The lowering of the frequency was accompanied by a lowering of the peripheral speed, and the peripheral speeds of revolving armature alternators compare favorably with those of multipolar direct-current machines of the same output. Peripheral speeds for belt- driven 60-cycle alternators may be taken from about 3,600 to 6,600 feet per minute. The peripheral speed of some of the larger direct-connected alternators may be even lower than this, just as the peripheral speed of multipolar direct- current generators is usually lower than that of belt-driven machines. Alternators of the inductor or revolving field construction can be run at higher peripheral speeds than those with a revolving armature on account of the mechani- cal construction of the revolving field or inductor being more substantial than that of a revolving armature.
ARMATURE WINDINGS
29« The foregoing articles have dealt with different subjects relating to the behavior of armatures. We will now take up those subjects that deal more particularly with their design. Some of the most important points in the design of an armature are the selection of the type of winding to be used for a given case, the method of connect- ing it up, and the means used for applying the winding to the armature. Alternator windings have already been dealt with to some extent in the section on Alternators^ but the following articles are intended to bring out some points of difference between concentrated and distributed windings
Digitized by VjOOQIC
22 DESIGN OF ALTERNATING §20
that are necessary for the designing of armatures for alter- nators and fields for induction motors.
30. Alternator windings may be divided into two gen- eral classes, namely: (a) uni-coil or concentrated wind- ings; {d) multi-coil or distributed windings. These may further be subdivided into (1) uni-coil single-phase wind- ings; (2) multi-coil single-phase windings; (3) uni-coil poly- phase windings; (4) multi-coil polyphase windings.
The uni-coil windings for single-phase, two-phase, and three-phase machines have been treated in the section on Alternators, We will presently examine single-phase multi- coil, or distributed windings, to see how the spreading out of the winding affects the voltage generated by the armature.
SINGL.B-PHASE CONCENTRATED WINDING
31. A single-phase concentrated winding has only one slot or bunch of conductors under each pole ; consequently, the conductors are practically all active at the same instant, and the maximum E. M. F. is obtained with a given length of active armature conductor. This E. M. F. is given by the formula
^ 4.44 ^ Tn '
^ = 10- -
where T = number of turns connected in series on the armature ; ^ = total magnetic flux from one pole; n = frequency;
i? = E. M. F. generated in armature, or E. M. F. obtained between the collector rings at no- load. Such windings have therefore the advantage of giving a high E. M. F. for a given length of conductor, but they have the disadvantage that they give rise to high armature self-induction and consequent falling off in terminal voltage when the machine is loaded. Also, the heating of the coils is likely to be greater than if they were spread out.
Digitized by VjOOQIC
20
CURRENT APPARATUS
23
SINGLE-PHASB DISTRIBUTED WINDINGS
3!8« It has been shown that the self-induction can be reduced by splitting up the coils and distributing them over the armature. Such distribution is, however, always accom- panied by a lowering of the E. M. F. generated, even though the total number of turns be kept the same. Sup- pose, for example, we have a single-phase armature with T turns, connected in series and arranged with only one slot or bunch of conductors under each pole. The E. M. F. generated will then be
i? =
4.44 <P Tn 10-
Suppose, now, we spread the winding out so that there will be two sets of conductors or two slots for each pole, and
Pio. 8
distribute these slots equally around the armature. We will put half as many conductors as before in each slot, so that
Digitized by VjOOQIC
24
DESIGN OF ALTERNATING
§20
the total number of conductors and turns will remain the same as before. This will give us a winding similar to that shown in Fig. 8. This shows an eight-pole single-phase winding with two slots per pole piece. By examining the figure, it is evident that with such an arrangement the con- ductors in slot b are, at the in- stant when they are directly between the poles, generating zero E. M. F., while those in a are generating the maxi- mum E. M. F. The E. M. F. that will be obtained between the collector rings will be the sum of the two, as shown in Fig. 9. Oa represents the E. M. F. generated in one set of conductors, while O b repre- sents the E. M. F. generated in the other. These two E. M. F. 's will be equal, and will be given by the expression
- £44±Tn^j
PiO. 0
^^LU^^^ (1)
since there are ^ the total turns T active in each set. resultant E. M.¥,Oc will therefore be
The
^ 4.44 ^Tn , ^ 4.44 ^ Tn „^„ ,^.
E = —. X i X V2 = j^, X .707 (2)
10"
10"
That is, the E, M, F. that is obtained at no load from a twO'Coil single-phase winding is ,707 times that which would have been obtained with the same total number ofturtis grouped into a uni'Coil winding. By spreading out the winding in this way, the no-load voltage has, for the same number of active conductors, been reduced about 30 per cent. ; the inductance of the armature has, however, been reduced considerably ; so that, although we may not get an armature that will give as high a voltage at no load, it may give as
Digitized by VjOOQIC
20
CURRENT APPARATUS
25
high a terminal voltage when loaded, and a machine pro- vided with such a winding would hold its voltage more nearly constant throughout its range of load.
33. The subdivision of the winding might be carried still further, and three slots for each pole piece used. The E. M. F.*s in the three sets of conductors would then be related as shown in Fig. 10. Each of the groups would
K
{
PlO. 10
consist of — turns, and the three E. M. F.*s O a^ O b^ and Oc
o
would be displaced 60"^ from each other, instead of 90°, as shown in Fig. 9, because there are three groups of conduc- tors per pole, and the distance from center to center of the pole pieces corresponds to 180°. The E. M. F. generated in each set will be
and the resultant E. M. F. O d. Fig. 10, will be
(3)
^ = iifAZ:^X| = i:MAZ:^X.667
10"
10"
(4)
Digitized by VjOOQIC
^
26 DESIGN OF ALTERNATING §20
The effect of spreading out the coils into a three-coil winding is, therefore, to reduce the no-load terminal E. M. F. still further, and at the same time to reduce the self-induc- tion. It will be noticed that the difference in the voltages given by a two-coil and by a three-coil winding is not nearly so great as that between the voltages of the two-coil and single-coil windings. If the winding is spread out still more, the E. M. F. generated is reduced by very little, and if the subdivision is carried out so that the winding becomes uni- formly distributed over the whole surface of the armature, the formula becomes
r^ 4.44 ^ Tn ^^^ ,-.
£= —, X.636 (5)
34 'S^^ more the winding is spread out, the greater the number of c*^<^ssings of the coils at the ends of the armature, making such wi^^^^^^s difficult to insulate for high voltages. Such windings, ^ ^''efore, have the disadvantage of being more expensive?^ -struct and insulate, in addition to
giving a lower l£ >, ' ^^ ^^^d for a given length of
active conductor. L r ' : '^^ advantage of giving better
regulation or small aa ,c t;.^ --r ''''" loaded, and also
give a smooth E. M. F. curve. v^,^;.^heating is more uniformly distributed than when a concentrated T'^iding is used. For single-phase armatures in general, we may tn^^. write the E. M. F. equation as follows:
£ = il^X>t (6)
where T = total number of turns connected in series on the armature ; ^ = total flux from one pole ; n = frequency;
k = constant depending on the style of winding used. For a single-coil or concentrated winding, k = 1; for a two-coil winding, y^ = . 707 ; for a three-coil winding, * = . 667 ; for a uniformly distributed winding, k = .636.
Digitized by VjOOQIC
20 CURRENT APPARATUS 27
POLYPHASE ARMATURE WINDINGS
35. Concentrated, or uni-coil, polyphase windings have already been described in the section on Alternators. The two- and three-phase windings there described consist of one group of conductors, or one slot for each pole and each phase. Polyphase windings can, however, be distributed in a manner similar to that just given for single-phase wind- ings, and such distributed windings are in common use for induction motors, polyphase alternators, and polyphase syn- chronous motors. The distribution of such windings is accompanied by a lowering of the terminal E. M. F., as in the case of single-phase windings, though this decrease in the E. M. F. is not nearly so great. Suppose, for example, we have a three-phase winding with two groups of conduct- ors per pole per phase. We will have then six groups of conductors for each pole, and as the distance from center to center of poles is equivalent to 180°, the E. M. F.'s in the two
180° groups of each phase will differ in phase by - ^ -, or 30°.
6
Let the total number of turns per phase be T. Then, the
number of turns in each of the two sets constituting each
T phase will be — , and the E. M. F. generated in each of the 2
sets will be
^ c- 4.44 4>r« ,
£.^E.= 10- >< ^
These two E M. F's will be related as shown in Fig. 11, and the resultant E. M. F. will be
r^ 4.44 ^ Tn , ^ ^^o E = —, X i X 2 cos 15°
= ^^^X.965 (7)
Hence, the voltage generated per phase by a two-coil three- phase winding is . 965 times that zuhich ivonld be generated by a single-coil zvindiftg. In other words, the splitting up of the winding has resulted in a voltage reduction of but
Digitized by VjOOQIC
28 DESIGN OP ALTERNATING §20
3J per cent. If a three-coil winding were used, the E. M. F. would be reduced still further, and if a uniformly distrib- uted winding covering the whole surface of the armature were employed, the constant would become .95. If a uni- formly distributed winding is used on a two-phase machine, the value of the constant becomes .90. For polyphase
PIO. 11
windings we may then summarize the following: The E. M. F. generated per phase in a polyphase armature is given by the expression
j^ 4.44 ^Tn , ,Q.
E = Yo" ^ ^ ^^^
where T = number of turns connected in stries per phase ; ^ = flux from one pole ; n = frequency;
k = constant depending on the arrangement of the winding.
For a winding with one group of conductors per pole per phase, k = 1; for a two-phase winding uniformly distrib- uted, ^= .90; for a three-phase winding uniformly dis- tributed, k = .95; for a three-phase winding with two groups of conductors per pole per phase, k = .905.
The student will notice particularly that formula 8 gives the voltage per phase, not the voltage between the collector rings or terminals of the machine. This latter voltage will evidently depend on the method adopted for connecting the different phases together.
Digitized by VjOOQIC
Digitized by VjOOQIC
Digitized by VjOOQIC
\QiiS
Digitized by VjOOQ IC
Digitized by VjOOQIC
Digitized by VjOOQIC
Digitized by VjOOQIC
20 CURRENT APPARATUS ' 29
ARRANGEMENT OF WINDINGS
36. The method of arranging these distributed windings will be understood by referring to Figs. 12 and 13. Fig. 12 shows a six-pole two-phase coil-wound armature with two slots per pole per phase. The coils are shown by the heaVy outlines, the winding being in two layers, so that there are as many coils as slots. Only one phase is drawn in complete, so as not to confuse the drawing. Take the coil A. One side e of this coil lies in the top of a slot, and the other side / lies in the bottom of the corresponding slot under the next pole. The light lines a, a' represent the terminals of the coil A, and the light connections show the connections between the coils constituting one phase. Starting from collector ring i, we pass from a around coil A and come to a'; a' is joined to ^, so that the current passes around coil B in agreement with the arrows; the terminal t' is then connected to c\ so as to pass through coil C in the direction of the arrows. This process is repeated until the twelve coils constituting the phase are all connected in series and the remaining terminal / is brought to collector ring 2, The other phase, of which the active conductors are indi- cated by the light lines, is connected up in exactly the same way and its terminals brought to the collector rings 3 and 4' This gives a completed two-phase winding that consists of two coils for each pole and each phase, all the coils in each phase being connected in series and each phase connected to its pair of collector rings.
37, Fig. 13 represents a three-phase bar-wound arma- ture with two slots for each pole and each phase. The armature is wound for eight poles, so that there are 32 bars or conductors connected up in series in each phase. One phase is shown connected up, the conductors belonging to the other two phases being indicated by the dotted and dot-and- dash lines. Starting from the collector ring r„ we connect to the bottom conductor in slot /; from there we pass to the corresponding slot under the next pole, that is, slot 7,
Digitized by VjOOQIC
30 ' DESIGN OF ALTERNATING §20
and connect to the top conductor in that slot. In this way we pass twice around the armature, connecting up the bars in accordance with the arrows, coming finally to the point n. From n a connection is made to m^ and from m we pass twice around the armature again in the opposite direction, and come finally to the point j, which is connected to the common junction >& if a Y winding is employed. This con- nects all the conductors belonging to this phase in series. The bars constituting the other two phases are connected in a similar way, and the three phases connected up in the Y or A combination, according to the rules that have been given in the section on Alternators. A three-phase alternator X provided with a winding like that shown in Fig. 13 would be suitable for a machine designed to deliver a large current output at a low voltage. In such a case, the number of armature conductors required would be com- paratively small, and bars could be used to advantage. A similar scheme of connection could be used for a coil-wound armature, except that each element of the winding would consist of a number of convolutions instead of the single turn, as shown in Fig. 13.
38. By referring to Figs. 12 and 13, it will be noticed that in such two-layer windings the top conductors are always connected across the front and back of the arma- ture to bottom conductors ; that is, a conductor in the top of one slot is not connected to the top conductor in the corresponding slot under the next pole. This is done to make the arrangement of the end connections such that they do not interfere with each other as already explained in connection with direct-current dynamos. The two-layer type of winding is on this account extensively used, and its application will be taken up further in connection with induction-motor design.
Digitized by VjOOQIC
CURRENT APPARATUS 31
CONSTRUCTION OF ARMATURES
39. On the whole, the mechanical construction of alter- nator armatures is very similar to that employed for arma- tures for multipolar direct-current machines. There are differences in the electrical features, arising from the differ- ent type of winding usually employed and the absence of commutator connections. The construction of many of the armatures is simpler than that necessary for continuous- current machines, on account of the smaller number of coils used in making up the armature winding.
ARMATURE DISKS
40. Most of the armature disks used are adapted for armatures of the drum type. Such disks or disk segments are stamped from well-annealed mild steel. It is essential that whatever material is used, the hysteresis factor should be low, especially if the armature is to be run at a high fre- quency. It is almost the universal practice at present to use toothed cores, although smooth-core armatures were quite common in some of the older types of alternators. Core iron should be from .014 in. to .018 in., or from 14 mils to 18 mils, thick. Iron thicker than this is frequently used in direct-current machines, but it is not safe to use iron much thicker in alternator-armature cores on account of the danger of increasing the eddy-current loss. Some makers depend on the oxide on the disks for the insulation to pre- vent eddy currents, while other makers give the disks a coat of japan before they are assembled to form the core.
41. The variety of disks used for alternator armatures is large. Some are designed for stationary armatures of large diameter, while others are for rotating armatures of comparatively small diameter. The different styles of slots used are also numerous. Fig. 14 represents a common style
Digitized by VjOOQIC
32 DESIGN OF ALTERNATING §20
of disk used for lighting alternators. This disk is provided with as many teeth and slots as there are poles on the alter- nator. Each tooth is provided with the projections ^, a^
which hold the coils in place and obviate the ne- cessity of band wires. A keyway k is provided by which the disks are keyed to the spider supporting them. It is well to notice,
in passing, that core disks for alternators are usually quite shallow, the depth of iron d under the slots being small compared with that usually found in direct-cur- ^o. 14 rent armatures, making the
disks appear more like rings. This is accounted for by the fact that in an alternator the total flux that the armature conductors cut in one revolution is divided up among a large number of poles; consequently, the flux from any one pole is comparatively small. The flux through the core under the teeth is one-half the flux from the pole piece; the cross-section of iron necessary to carry it is therefore small, and a large depth of core is unneces- sary to obtain the required cross-section.
43. Fig. 15 shows an- other style of disk and slot in common use. This disk is provided with 16 slots, and would be suitable for ^'°- ^^
an eight-pole two-phase winding. The same style of disk with 24 slots would answer for the three-phase winding.
Digitized by VjOOQIC
§20 CURRENT APPARATUS 33
The disk shown in Fig. 15 is provided with slots that have dovetailed grooves near the circumference. After the coil is placed in position, a wooden wedge is fitted into these grooves, thus holding the coil firmly in place and doing away with the necessity of band wires.
43. When the armature is wound with bars, straight slots are frequently used. Fig. 16 shows such a disk pro- vided with 48 equally spaced slots. A disk of this kind would be suitable for an armature core for the wind- ing shown in Fig. 13. It would be necessary in this case to use band wires to I hold the conductors down in place, giving a construc- tion very similar to that commonly employed for direct-current armatures.
44. Stationary arma- fig. le »
tures for large machines are placed externally to the revolv- ing field, and the coils are placed in slots around the inner periphery. Since such armature cores are generally of large diameter, the armature disks have to be punched out in sections, as shown at c in Fig. 17. These sections are pro- vided with dovetail projections b that fit into slots in the
Pig. 17
supporting iron framework A. As the core is built up, the joints between the different segments are staggered, or the
45—4
Digitized by VjOOQIC
84 DESIGN OP ALTERNATING §20
segments are overlapped, so as to form a core that provides a magnetic circuit practically as good as if the disks were punched in one piece. The use of the dovetail projecting lugs avoids the use of bolts passing through the disks to hold the latter in place. Unless bolts are insulated, they are
liable to give rise to eddy cur- rents by short-circuiting the disks. Some makers, how- ever, use disks as shown in Fig. 18, provided with holes h for the clamping bolts. The slots used for such stationary armatures must of course be provided with grooves of some kind to receive holding-in strips or wedges, as it is not pos- sible to use band wires in such a case.
45. Revolving armatures are also frequently made of such large diameter that it is not practicable to punch the disks in one piece. In such cases, again, the disks are made in segments, and are held in place either by bolts passing through them or by dovetail projections fitting into grooves in an extension of the arma- ture spider arm. This con- struction will be understood by referring to Fig. 19. In F'g- ^»
assembling disks to make up a core, it is usual to place a heavy sheet of paper about every \ inch or \ inch of core, in order to make sure that the path for eddy currents will be effectually broken up.
AKMATURE SPIDERS
46, Disks for revolving armatures are usually supported on spiders similar to those used for direct-current multipolar armatures. These spiders are made of cast iron or steel,
Digitized by VjOOQIC
20
CURRENT APPARATUS
35
and are necessarily strongly constructed. They should be so made as to clamp the disks firmly in place, and be amply strong to bear any unusual twisting action they may have to withstand due to an accidental short circuit. Fig. 20 shows two views of a spider and core suitable for disks of moderate size punched in one piece. The spider proper consists of a
JJ9 Slats
r^}
PtO. 20
hub a provided with four radial arms d that fit the inner diameter of the disk. The hub is bored out so that it fits very tightly on the shaft, and a key is provided to avoid any chance of turning. The core disks d are clamped firmly in place by two heavy cast-iron end plates c, c that are pressed up and held by the bolts e. These bolts pass under the disks, so that there is no danger of their giving rise to eddy currents. The key / prevents the disks from turning on the spider and insures the alinement of disks, which is necessary to make the teeth form smooth slots when the core is assembled.
Fig. 20 shows the construction used with armatures hav- ing a small number of heavy armature coils. In such cases
Digitized by VjOOQIC
36
DESIGN OF ALTERNATING
20
the coils are stiff and the ends project out past the end of
the core without being supported. ~ In case a distributed winding is used, the coils are numerous, and being small, they are frequently not stiff enough to support them- selves; hence, the clamping rings of the spider are in such cases ' provided with flanges, as shown in Fig. 21. The end connections of the coils lie on the flat cylin- drical surfaces a^ a^ and are tightly bound down in place by means of band wires. Fig. 22 shows a spider suitable for a ^"°- *^ large armature built up with
segments like those shown in Fig. 19. This style of spider
C— Iru
Fig.
IS common for machines with large diametei of armature
Digitized by VjOOQIC
20
CURRENT APPARATUS
37
running at low speeds. The rim r of the spider is made non-continuous, in order to avoid strains in casting as much as possible.
47. When the armature is the stationary part of the machine, a stationary frame of some kind must be used to support the stampings. This consists usually of a rigid cast-iron framework provided with end plates, between which the armature disks are clamped. The construction will be understood by referring to Fig. 23, which shows a
ni
Pig. 88
stationary armature frame for a machine of large diameter. The frame casting is usually made in two pieces A and /?, the lower half being provided with projections a^ a, by which the spider is bolted to the bed or foundation. The seg- mental core stampings d, d are held in place by the dovetail grooves c^ c. These segments are clamped between the end rings €^ e by means of the bolts /. The end rings e are shown made up in segments on account of their large diameter.
Digitized by VjOOQIC
38 DESIGN OF ALTERNATING 8 20
AJEtMATURE CONDUCTORS
48. The style of conductor used on the armature will depend to a great extent on the current that it is to carry and the space in which it is to be placed. High-voltage machines of moderate output are usually wound with double or triple cotton-covered magnet wire. Frequently two or more wires are used in multiple in order to secure the requi- site cross-section. This gives a more flexible conductor than a single large wire, which would be difficult to wind.
49. It is often advantageous to use bare wire in making up such conductors and cover the combination of wires with
insulation, as shown in Fig. 24. A section of a conductor made up of two bare wires in mul- tiple is shown at (a), and four bare wires at (*), the con- ductors being in each case cov- ered by the cotton wrapping t. This construction not only saves space, but the insulation also serves to hold the wires in place. Conductors of special shape are used on some machines. For example, square wire and copper ribbon are often employed. Fig. 24 (c) shows a section of a copper ribbon conductor with its cotton insulation. Such ribbons are usually from ^^ inch to ^ inch thick, and should be made with rounded edges, to prevent danger of cutting through the insulation.
60. Copper bars are largely used for armatures designed to deliver large currents. Fig. 24 (d) shows a cross-section of an armature-winding bar. The dimension // is usually considerably greater than b, in order to adapt the bar to an armature slot that is deep and narrow. These bars are rolled to any required dimensions, the corners being slightly rounded, as shown, to prevent cutting of the insulation.
Digitized by VjOOQIC
20
CURRENT APPARATUS
89
: VI 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 r
^"^""IIIIIIINMIIIIIIII
(a)
illlMIIIIIIIIIIIIJIIlll,lll
IINIIIIIIIIIIIIIIl
(b)
Fig. 25
FORMS OF ARMATURE COIL.S AND BARS
51. The simplest form of coil for alternator armatures is that used on ordinary single-phase machines with uni-coil windings. The coils usu- ally consist of a fairly lar^e number of turns, and are wound on forms, so that the finished coil is of such shape that it fits snugly into place in the slots. Such coils are heavily taped to insulate them thoroughly and make them hold their shape. Coils of this type are shown in Fig. 25 (a) and (b). The straight portion cc and dd lies in the slots, the end parts projecting out over the ends of the armature core. In some cases the ends are curved as at {a)y while in others the ends shown at (d) are used.
53. In many polyphase windings it is necessary to shape these heavy coils so that they may cross each other at the ends of the armature. This is accomplished by shaping one of the coils as shown in Fig. 26. The end of the coil d is bent down into a different plane P'^- ^ from that of a, so that the coils
cross each other without touching, and insure good insulation.
53. When coils are used for a distributed winding like that shown in Fig. 12, they are generally shaped like the coil shown in Fig. 27, which is the same as those used on barrel-wound direct-current armatures. This is a form- wound taped coil, consisting usually of a comparatively
Digitized by VjOOQIC
40
DESIGN OF ALTERNATING
20
FlO. 28
small number of turns. The straight portions a a and bb lie in the slots, while the end portions project beyond the core and are usually supported by fianges, especially if the armature revolves. The side a a lies in a lower plane than b b^ so that the upper and lower end connections do not interfere with each other. The terminals /, / of the coil are usually brought out at the points shown. At the points r, c the coil is so formed as to bring the end connec- tions d^d into a plane above ^, a^ and thus bring the side b b in the top of the slot. Sometimes the terminals are brought out at the corners a, b, if this brings them in a position more convenient for connection to the other coils.
54. Bar windings are frequently made in two layers. Fig. 28 shows a form of bar suitable for a winding such as
that shown in Fig. 13. The straight part a a lies in the slot, and the end portions ^, b form the connections to the other bar. Fig. 29 shows one element or turn of such a winding. The part \ c lies in the top of the slot, and the two bars making up the element are soldered together at the point d. Fig. 30 shows a similar element for a wave bar winding, except that there
is no soldered joint at the "o'^ t>
point ^, the element being ^'o- ^
composed of one continuous copper bar first bent into the long U form shown in Fig. 31, and then spread out to form the winding element shown in Fig. 30. Bars of the style just described are used also for some styles of induction- motor armatures. The portion of the bar forming the end connection has to be taped in order to insulate it from its neighbors. The part in the slot is frequently taped also, though in some cases the insulation from the core is pro- vided wholly by the insulating trough.
Digitized by VjOOQIC
j20
CURRENT APPARATUS
41
Fig. 32 shows a portion of the bar winding on the station- ary armature of ^^^^^^-^thh^^^
one of the large ifT^^"^ ^^^''^^Ife^^i^^^ _-*-*!& 5,()0()-kilowatt al- JF ^^^^^5i||j^j^(!C>^^
ternators of the ^^^^^^„^
Manhattan Eleva- ^"^^^^^
ted Railway, New ^^^^
York. In this case m
there are three #
bars in each slot, ^
FlO. 80
the bars being first
insulated separately and then bound together. The figure
shows the arrangement of the end connections in two
Q.
FIO. 81
different planes, so that they can pass each other with a good clearance. This armature has a distributed winding
PIO. 88
with 4 slots or 12 conductors per pole per phase. The armature is Wound for three phases and delivers current at 11.000 volts.
Digitized by VjOOQIC
42 DESIGN OF ALTERNATING 8 20
ARMATURE INSTXATION (COILS)
55* Alternator armatures are generally called on to generate much higher voltages than are common with continuous-current machines. The pressures generated by ordinary lighting alternators are usually in the neigh- borhood of 1,000 or 2,000 volts. Power-transmission alternators with stationary armatures have been built to generate as high as 10,000 or 12,000 volts. These are the values of the pressures generated in effective volts, and when it is remembered that the maximum value of the pres- sure to which the insulation is subjected is considerably greater than the effective value, it will be seen that the insulation of these armatures must be carefully carried out to insure against breakdowns. The insulation should be capable of standing a pressure at least three or four times as great as that at which it is ordinarily worked.
56. For very high-voltage machines it is best to use the type with stationary armature, as it is easier to insulate a stationary armature thoroughly. The allowable space for insulation on a stationary armature is usually greater than on a revolving one, and, moreover, the insulation is more likely to remain intact. A revolving armature also necessi- tates collector rings, brush-holder studs, etc., which have to be insulated for high pressures; whereas with the station- ary armature only three terminals are required, which are comparatively easy to insulate.
57. When the coils each contain a large number of turns, the voltage gen- erated per coil will be large; conse- quently, it is not only necessary to insulate the outside of the coil thor- oughly, but each layer must also be insulated from its neighbor. Fig. 33 shows a section of a coil consisting of 32 turns. Between each layer of wire is a layer of
Digitized by VjOOQIC
§20 CURRENT APPARATUS 43
insulation i turned up at the ends, so as to thoroughly insulate the individual layers. The whole coil is covered with a heavy wrapping of insulating tape /, and in Addi- tion is baked to drive out 'all moisture and treated with insulating varnish. The thickness of tape will depend on the voltage of the machine. Linen tape of good quality, treated with linseed oil, forms about the best material for this purpose, as it has high insulating properties and does not deteriorate with a moderate amount of heating Such tape is usually about .007 to .010 inch (7 to 10 mils) thick, and is wound on half lapped. Where extra high insulation is required, the tape may be interleaved with sheet mica. Coils for distributed windings do not usu- ally contain a large enough number of turns to require insulation between the separate layers. They may be taped and treated with the same materials as the heavier coils, but the outside taping is usually not so heavy. With such windings, the material lining the slot is depended on largely for the requisite insulation.
ARMATURE TNSUL.ATIOX (SLOTS)
68. The taping on the coils is not always depended on alone for the insulation. The slots are often lined with insulating material that is not likely to be damaged by putting the coils in place. Slot insulation is usually made up in the form of troughs or tubes composed of alternate layers of pressboard and mica. The mica is depended on mainly for the insulation, the pressboard being used as a bonding material to hold the mica in place. These tubes may be either made up separately or formed in place in the slots. The mica is usually stuck on the pressboard with shellac or other insulating varnish, which becomes dry when hard and makes the trough hold its shape. Fig. 34 shows the slot insulation for an armature made up of disks similar to those shown in Fig. 13. The hardwood strip a is first
Digitized by VjOOQIC
44
DESIGN OF ALTERNATING
§20
laid in the bottom of the slot, and the paper and mica trough b formed in place before the bonding varnish becomes dry. The coil r, consisting of several turns of copper wire or ribbon, is wound in place after the slot insulation has
Pig. 84
become dry, and a wooden wedge d, pushed in from the end of the armature, holds the winding firmly in place. An insulating piece e is also placed between the wedge and the winding.
59. Fig. 35 shows an- other form of slot insu- lation; / is the taping on the coil and i the paper and mica insulating trough. The top of the trough is left projecting up straight until the coil is placed in the slot, after which it is bent over as shown, protecting the coil from any injury while the wedge a is being forced ^^^' * into place. These wedges
should be cut so that the grain of the wood lies across
Digitized by VjOOQIC
§20 CURRENT APPARATUS 45
the slot, otherwise there is danger of their becoming loose due to shrinkage.
60. Fig. 36 shows the arrangement of slot insulation for a coil-wound two-layer armature. The in- _ sultating trough i runs around the slot and
laps over the top of the coil as before. In addition to this, the upper and lower groups of conductors are separated by the insulating strip a, which must be sufficiently thick to stand the total voltage generated. This arrangement also makes use of the wedge ^^^^ construction for holding the coils in place. fxo. 8«
61. Fig. 37 shows the insulation for a two-layer bar-
■ — ■ wound armature with straight slots. This style of slot would be suitable for the bar winding shown in Fig. 13. In such cases the bars have to be placed in the slots from the top, the bent ends preventing their being pushed in from the end. This necessitates the use of straight slots and band wires for riG. vi holding the bars in place. A wooden strip is
usually inserted between the band wires and bars in order
to protect the winding.
63. The present practice in armature construction, espe- cially for high pressures, is to place the itisulation on the coil rather than in the slot. The coils after being wound are first thoroughly baked and then placed in hot insulating compound under pressure, so that the insulating varnish is forced into the coil. The coil is then taped with several layers of oiled linen, each layer being treated with varnish and baked before the next is applied. This gives a dense hard insulation that offers a high resistance to puncture and is more homogeneous than the ordinary slot insulation. The only insulation used in the slot itself is a thin layer of leatheroid or fiber to prevent abrasion of the coil while it is being forced into position.
Digitized by VjOOQIC
46
DESIGN OF ALTERNATING
20
63.
In using two-layer windings, care should be taken to have the top and bottom layers very thoroughly insulated from each other. The insulating troughs a, Fig. 38, should project a short distance beyond the core d, in order to make sure of good
r-^rrr^^ insulation between the coils and core. '''-' The spider flanges should also be thor- oughly insulated with paper and mica c
wherever there is any possibility of the
PIO. 88
current jumping from the coils to the spider
MAGNETIC DBNSriTBS
DENSITY IN ARMATURE TEETH
64. Where armatures are wound with a few heavy coils, the teeth between the coils are large, in some cases nearly as wide as the pole faces. In such armatures the magnetic density in the teeth will not be much higher than that in the air gap. When a distributed winding is used, the sur- face of the armature is split up more by the slots, and the area of cross-section of iron in the teeth is reduced. This gives rise to a higher magnetic density in the teeth than in the air gap.
65. It was pointed out, in connection with the design of continuous-current machines, that in such machines it was desirable to have the magnetic density in the teeth high, because highly saturated teeth prevent the armature from reacting strongly on the field and thus aid in suppressing sparking. In the case of alternators, however, high densi- ties in the teeth are avoided, because the effects of arma- ture reaction are not nearly so serious in these machines, and the high density might prove detrimental by causing excessive hysteresis and eddy-current losses. In general, therefore, in alternator design, the magnetic density in the
Digitized by VjOOQIC
§20 CURRENT APPARATUS 47
core teeth is kept as low as possible. The density, however, cannot be made very low, as this would mean large teeth and a correspondingly large armature. Where distributed windings are used, it will generally be found that the width of the slot and width of tooth are made about equal, thus reducing the effective iron surface of the armature to about one-half and making the magnetic density in the teeth about cwice that in the air gap. It will be remembered that both the hysteresis loss and eddy-current loss increase very rapidly with the density ; consequently, it is easily seen that if the density in the teeth is very high, the amount of loss in them may be considerable, on account of the high fre- quency at which alternators usually run. It also follows that, for the same amount of loss, it would be allowable to use a higher magnetic density with a low-frequency alter- nator than with one running at a high frequency.
DENSITY IN ARMATTTIE COBE
66. The density in the armature core proper, that is, the portion of the core below the armature slots, should also be low, in order to keep down the core losses. This density can be made almost as low as we please by decreas- ing the inside diameter of the core, thus making the depth ^, Fig. 14, large, and increasing the cross-section of iron through which the lines have to flow. If, however, the inside diameter were made very small, the core would be heavy, and since the hysteresis loss is proportional to the volume of iron, very little would be gained by decreasing the density beyond a certain amount. Armature cores for alternators are usually worked at densities varying from 25,000 to 35,000 lines per square inch, the allowable density being higher in low-frequency machines than in those run- ning at high frequencies. Where armatures are run at very high speeds of rotation, the density may be allowed to run a little higher than the above values, in order to make the core as light as possible, provided the frequency is not too high.
Digitized by VjOOQIC
48 DESIGN OF ALTERNATING
DENSITY IN AIR GAP
67. The allowable density in the air gap will depend, to a certain extent, on the material of which the pole pieces are made. If cast-iron pole pieces are used, the density must be kept fairly low, otherwise there will be danger of the cast iron becoming saturated. It is best, therefore, to make the air-gap density in such machines in the neighbor- hood of 30,000 lines per square inch. If the pole pieces are made of wrought iron, as they nearly always are in modern machines, the density may be as high as 40,000 or 60,000 lines per square inch. The density could be even higher than this without danger of saturating the wrought iron, but if the air-gap density is carried too high, a very large mag- netomotive force must be supplied by the field coils in order to set up the flux. For these reasons the average air-gap density should usually be somewhere near the values given above.
Digitized by VjOOQIC
DESIGN OF ALTERNATING- CURRENT APPARATUS
(PART 2)
DESIGN OF lOO-KILOWATT SINGLE- PHASE ALTEBNATOB
!• The general considerations governing the design and construction of alternator armatures having been given, we will now apply these to the special case of the design of an armature for a single-phase alternator, in order to illustrate the calculation of the different dimensions. As a starting- point, we will assume that the following quantities are known, and in this particular case are as given below, the design being worked out from these quantities. The student will understand, however, that most of the formulas are per- fectly general, and that these special values are only taken to illustrate ,a typical case in order to make the design clearer. The following quantities are in general known or assumed: (1) Output at full load ; (2) frequency; (3) speed; (4) voltage at no load, voltage at full load; (5) allowable safe rise in temperature; (6) general type of machine.
For the case under consideration we will take the follow- ing: (1) Output at full load, 100 kilowatts; (2) frequency, 60 cycles per second; (3) speed, 600 revolutions per minute; (4) voltage at no load = 2,000 = E^ voltage at full load = 2,200 = E\ (5) allowable rise in temperature, 40° C. ; (G) general type of machine, belt-driven, revolving arma- ture, stationary field.
§21
For notice of copyright, see page immediately following the title page.
45—5
Digitized by VjOOQIC
2 DESIGN OF ALTERNATING §21
8. It will be noted that the armature is to deliver 2,000 volts on open circuit and 2,200 volts when the machine is fully loaded. This is done so that the voltage at the dis- tant end of the line may remain practically the same from no load to full load. This increase in voltage is accomplished by strengthening the field by means of the series-coils, so that, so far as the voltage generated by the armature is concerned we design it to generate 2,000 volts, and leave the increase of 200 volts to be brought about by the action of the field.
3* Since the speed and frequency are fixed, the number of poles is also fixed by the relation
where s = revolutions per second;
/ = number of poles; n = frequency.
We then have
^" 2 ^ 60 / = 12
and the machine must be provided with twelve poles to give the required frequency at a speed of 600 {l. P. M. We might have used a speed of 900 R. P. M. and eight poles, the frequency being the same in either case. It is better, however, to use the lower speed (600 R. P. M.) for a machine of this capacity, so we will adopt the twelve pole 600 R. P. M. design. The field will be external to the armature, and will be provided with twelve equally spaced poles projecting radially inwards. We will also follow the usual practice and make the distance between the poles equal to the width of the pole face, or, in other words, make the width of pole face equal to one-half the pitch. The pole pieces will, therefore, cover one-half the surface of the armature.
Digitized by VjOOQIC
§21 CURRENT APPARATUS 3
DIMENSIONS OF CONDUCTOR AND COBE
4. The current output at full load will be
, _ watts _ kilowatts X 1,000 .^.
- full-load voltage " T' ^^
100 X 1,000 = 2,200 =^^'^^^P^^^
The machine must therefore be capable of delivering a current of at least 45.4 amperes continuously without the temperature rise above the surrounding air exceeding 40° C.
5. The cross-section of the conductor that is used on the armature is determined by the current that it must carry, and this in turn depends on the way in which the different armature coils are connected up. Since the armature under consideration must generate a high voltage, we will use an open-circuit winding and connect all the armature coils in series. The current flowing through the armature con- ductor at full load will then be the same as the full-load current output of the machine, that is, 45.4 amperes. The student should compare this with the calculations determin- ing the size of wire used on a continuous-current armature. It will be seen that in this latter case the current in the armature conductor was less than the total current output of the machine depending on the number of paths in the winding. In some of the older types of alternators, the armature conductors were worked at a high current density, in some cases less than 300 circular mils per ampere being allowed. For machines of good design, the number of cir- cular mils per ampere usually lie between 500 and 700. For a trial value, take 550 circular mils per ampere in order to determine the approximate necessary cross-section of the conductor.
Let
A = area of cross-section of conductor in circular mils;
/ = current in conductor;
m = circular mils per ampere.
Digitized by VjOOQIC
4 DESIGN OF ALTERNATING §21
Then,
A = Im (2)
In this case / = 45.4 and m = 550. Therefore, the cross- section of the conductor will be
45.4 X 550 = 24,970 circular mils
A No. 6 B. & S. wire would give 26,250 circular mils, which is quite near to the cross-section required, or two No. 9 wires in parallel would give a cross-section of 26,180 circular mils. Two bare. No. 9 wires 18 covered with a double wrapping of cotton should be used, because the two wires in mul- tiple will give a more flexible and easily wound conductor. The double thickness of this cover- ing will be about 15 mils. The diameter of No. 9 wire is .114 inch; hence, the width of the conductor over all will be .243 inch and the thickness .129 inch. Fig. 1 shows a cross- section of the conductor, illustrating the arrangement of the insulation.
DESIGN OF ARMATURE CORE
6. The diameter of the armature is determined by the speed of rotation and the allowable safe value of the periph- eral speed. A safe peripheral speed for a belt-driven machine of this type may be taken at about 5,000 feet per minute. Hence, the diameter of armature in inches equals
, _ peripheral speed x 12 .^.
"^^ - RTP. M. X ^ ^"^^
5,000 X 12 o, Q . u = -^r:7T - = 31.8 mches 600 X ^
We will therefore adopt 31 J inches = 31.75 as the outside diameter of the armature core.
7. The length of the armature core parallel to the shaft, or the spread of the laminations, must be large enough
Digitized by VjOOQIC
§21 CURRENT APl>ARATUS 6
to enable the armature to present sufficient radiating sur- face to get rid of the heat generated. In other words, the armature must be large enough to do the work required of it without overheating. The core losses and /'-^loss of the machine under consideration cannot be determined exactly until the dimensions of the armature have been determined. The curve shown in Fig. 1, Part 1, gives the relation between the output and /' R loss for machines of good design, and it is seen that for a machine of 100-kilowatt capacity, the P R loss should be about 1.95 per cent, of the output. The approximate PR loss may then be taken as 100,000 X .0195 = 1,950 watts.
8. This armature is of rather large diameter and runs at a fairly high peripheral speed. Good ventilation should easily be obtained by constructing the spider to allow free access of air and by providing the core with ventilating ducts. With such an armature there should be no difficulty in radiating about 2.8 watts for each square inch of core surface with a rise in temperature of 40° C. The core losses are apt to be quite large ; hence, to be on the safe side, we will allow half this radiation capacity for the core losses and half for the /' R loss. This means that we should have
about — square inch of cylindrical surface for each watt
I* R loss. This would call for a surface of 1,950 X .7 = 1,365.0 square inches.
9. The outside circumference of the armature is 31.75 X^ = 100 inches, nearly ; hence, the approximate length of arma- ture core parallel to the shaft should be about 13.65 inches. As a basis for further calculation, we will adopt a trial length of core of say 14 inches. It may be found necessary to modify this dimension slightly, as the design is worked out further, but it should not be made much less than this, or there will be danger of the armature overheating.
10. We have now determined the approximate dimen- sions of the armature core, and are in a position to calculate
Digitized by VjOOQIC
6 DESIGN OF ALTERNATING § 21
the magnetic flux 0 after we have decided on the density to be used in the air gap. This machine will be provided with wrought-iron pole pieces; hence, we may take 40,000 lines per square inch as a fair value for the magnetic density in the air gap. The total magnetic flux ^ from one pole will be the area covered by the pole multiplied by the mag- netic density. The poles cover one-half the circumference ; hence, the length of arc on the armature covered by each pole will be
number of poles
3.14 X 31.75 X .5 12
= 4.16 inches
The length of the pole face is the same as the length of the armature core, i. e., 14 inches; hence, the area of the pole face is 14 X 4.16 = 58.2 square inches.
The total flux from each pole will therefore be 58.2 X 40,000 = 2,328,000 lines.
11. Since the flux^, the frequency », and the E. M. F. £ generated at no load are now known, the number of turns T necessary to generate the voltage £ can be calcu- lated. This armature will be provided with six coils or twelve slots, that is, one slot for each pole; consequently^ all the conductors may be considered active at once, and we may use the formula
4.44 * Tn
£ =
10-
^=4.44x<Px;i ^*>
The voltage to be generated at no load is 2,000, the fre- quency is 60, and the flux 0 is 2,328,000; hence, we have
^ ^ 2,000 X 100,000,000 _ 4.44 X 2,328,000 X 60 ""
18. From the above, it is seen that we must place as nearly 322 turns on the armature as possible. There are
Digitized by VjOOQIC
§21 CURRENT APPARATUS 7
twelve slots, or six coils; hence, there would be ^p
= 53.6 turns per coil and 53.6 conductors in each slot.
This number would not be practicable,
since we should arrange the coils so that
they will wind up into a number of layers
without any fractions of turns. We must
therefore arrange the coils to give the
required number of turns as nearly as
possible, and then modify the length of
the turns, so that the voltage generated
will not be altered. Suppose we arrange
the coil and slot as shown in Fig. 2,
using 8 turns of the twin conductor in
each layer, and having 7 layers per coil. ^^®' '
This will give 56 turns per coil and 56 conductors per slot.
13. The dimensions of the slot may now be determined from the known number of conductors that are to be placed in it, and the necessary space that must be allowed for insu- lation. We will allow .06 inch or 60 mils all around for the paper and mica tube that composes the slot insulation, and .04 inch or 40 mils for lapping around the coil. In addition to this, we will allow for six layers of insulation, 10 mils thick, between the layers of the coil. This will make the necessary width of the slot 7 X .129 + 6 X .01 + 2 X .04 + 2 X .06 = 1.163 inches. The necessary depth of slot will be 8 X .243 + 2 X .04 + 2 X .06 = 2.144 inches.
In order to be sure that the coil will slip into the slot with- out having to be forced, and also to compensate for any slight roughness, we will adopt the dimensions shown in Fig. 2, namely, 1^ inches wide by 2^ inches deep. We will make the wooden wedge ^ inch thick, and the opening at the circumference the same width as the slot, in order to allow the coil to be slipped easily into place.
14. In order to obtain an even number of turns per coil, the total number of turns has been increased from 322, as first calculated, to 336. It follows, therefore, that if the dimensions of the armature are not altered in any way to
Digitized by VjOOQIC
8 DESIGN OP ALTERNATING §21
compensate for this increase in the number of conductors, the machine would give more than 2,000 volts when run at a speed of 600 revolutions per minute. In order, therefore, to keep the voltage generated the same, each conductor must be shortened a small amount, so that the poles and armature core will also be shortened. This will reduce the flux 0, so that the voltage generated by the 336 conductors will be 2,000 volts. The final length of armature may be obtained as follows:
We have * = , 7, ' (6)
4.44 X Tn ^ ^
and in this case
. 2,000 X 100,000,000 _ ^,_. _^^ ,
* = 4.44 X 336 X 60 ^ ^'^^^'^^^' ^^^^^^
That is, in order to keep the voltage the same, the flux is reduced from 2,328,000 to 2,235,000. The area per pole will then be
—' A T~ = —^TTT^rTTT- = ^^-^ square inches , (7)
air-gap density 40,000 ^ ^ '
and the length of the pole and armature core parallel to the
shaft will be
area 55.8 ,« 40 • u /q\
— -, = -J-—; = 13.42 mches (8)
polar arc 4.16 ^ '
It will thus be noticed that the armature core is shortened slightly, thus shortening up each conductor and making the length of active wire the same with the 336 conductors as it would have been if 322 had been used. We will therefore take 13^^ inches as the final value For the length of the core parallel to the shaft (see /„, Fig. 3).
16. All the essential dimensions of the armature core have now been determined except the diameter of the hole in the disks. This inner diameter of the core is determined by the cross-section of iron that must be pro- vided to carry the magnetic flux through the armature core from one pole to the next, and this cross-section in turn depends on the density at which the core is worked.
Digitized by VjOOQIC
21
CURRENT APPARATUS
Fig. 3 shows a cross-section of the core, and Fig. 4 shows a portion of the armature
lying between two pole pieces. In order to deter- mine the inside diameter, we must first obtain the distance d^, or the depth of the iron below the bot- tom of the slots. The lines of force flow from the north to the south pole, as shown in the figure, and it will be seen that the number of lines flowing through the portion a b under a slot is one-half the total
j9n%ila/U/ng duet U 13jj^
5
K''^'-'--- '"■•■■■'<
mm
%
I
PIO. 8
number flowing from the pole
Fig. 4
piece. Hence, the flux through the armature core is \ ^. The area of cross-section of iron required will then be
_4*
A =
B.
(9)
Digitized by VjOOQIC
10 DESIGN OF ALTERNATING §21
where B^ is the magnetic density at which the core is worked. We will take the value of B<, as 30,000 lines per square inch. This will make
^0 = i X gQ J^^ = 37.25 square inches
This is the area of cross-section of iron, and it is equal to
the radial depth of the core under the slots {ab^ Fig. 4)
multiplied by that length of core parallel to the shaft which
is actually occupied by iron. The over-all length of the
core parallel to the shaft is 13^ inches, but part of this
is taken up by the varnish, or other insulation, between the
disks, as well as the portion taken up by the air ducts.
In the present case, we will provide the armature with
three air ducts, each | inch wide, as shown in Fig. 3, the
disks being spaced apart this distance by suitable ribbed
brass castings, or by a special spacing disk. These three
ducts will therefore occupy a linear distance of \\ inches,
leaving 13^V — 1^, or 12^ inches to be occupied by the
iron and insulation on the disks. We will take 11^ inches
as the actual length of iron, the disks being insulated by
having a thin coating of japan placed on every other disk.
37 25 The required radial depth will then be -pp-=- = 3.23 inches.
11. 0
We will therefore make the depth of iron 3^ inches. (See
Figs. 3 and 4.) The total depth of the slot is 2fi^ inches;
hence, the total radial depth of the disk is 2|J + 3^V
= 5| inches, and the inside diameter is 31^ — 2 X 5J
= 20 inches. The dimensions of the disk are, therefore, as
shown in Fig. 4. There are twelve slots of the dimensions
shown in Fig. 2, these slots being spaced equally 30° apart.
CAIiCUIiATION OF ARMATURE L.OSSES
16. The dimensions of the armature having been deter- mined, it is now necessary to calculate the losses to see if the armature will deliver the required output without the losses exceeding the allowable amount. We will first calcu- late the /• R loss.
Digitized by VjOOQIC
§21
CURRENT APPARATUS
11
17. The resistance of the armature can be determined quite closely, since the length of wire on it can be estimated and the cross-section is already known. The length of wire can be obtained by laying out one of the coils to scale and measuring up the mean length of a turn. The coil must bridge over the distance from the center of a north pole to that of a south pole, and the ends of the coil must be rounded out so as to clear the armature core. The coil will be
Fig. 5
shaped as shown in Fig. 5. The straight portion of the coil will be made 15 inches long, in order to allow the coil to project about | inch from the slots at each end before it begins to turn. The mean turn, shown dotted, is the turn through the center of the coil. Its length is readily deter- mined from the drawing; in this case it is about 54 inches. The total length of conductor on the armature will there- fore be 54 X 336 = 18,144 inches, or 1,512 feet.
18. The hot resistance of any known length of a con- ductor may be found as follows:
D _ length of wire in inches '~ area in circular mils
Applying this to the armature just worked out, we find
We will take the resistance as .7 ohm, in order to make some allowance for the resistance of the connections between the coils.
Digitized by VjOOQIC
12 DESIGN OF ALTERNATING § ^1
19. The full-load current is 45.4 amperes; hence, the PR loss at full load will be (45.4)' X .7 = 1,442 watts. This shows that the PR loss is well under the limit of 1,950 watts and that the armature would be capable of deliver- ing a little over 45.4 amperes without the PR loss exceed- ing the allowable amount. The outer cylindrical surface of the armature as obtained from the final dimensions is tt x 31} X 13 f'^ = 1,343 square inches, nearly, which allows a little over .9 square inch per watt PR loss, which should be an ample allowance for an armature of this type.
20. The hysteresis loss may be calculated when the volume of iron, magnetic quality of the iron, and fre- quency are known. The area of the end of the core is ^TT (31.75' — 20') = 477.3 square inches, nearly.
The area of each slot is about 3.4 square inches, and the total area taken out by the slots 40.8 square inches, leaving 436.5 square inches as the area of the disks. The actual length of iron parallel to the shaft is 11 J inches; hence, the volume of iron in the core is 436.5 X 11.5 = 5,020 cubic inches.
The magnetic density in the core is 30,000 lines per square inch. Referring to curve B^ Fig. 2, Part 1, we find that for a density of 30,000 the loss per cubic inch per 100 cycles is .42 watt. Hence, the hysteresis loss in watts is
21. The eddy-current loss is not easily obtained, but the combined core losses in this case would likely be fully as great as, if not greater than, the P R loss of 1,442 watts. If the combined losses were, say, 3,000 watts, the electrical efficiency at full load would probably be in the neighborhood of 94 or 95 per cent., as there would be about 2 per cent, loss in the field and various connections. The commercial efficiency would be somewhat less than this on account of I lie bearing friction, brush friction, etc.
Digitized by VjOOQIC
21 CURRENT APPARATUS 18
ARMATURE WIXDrNG FOR TWO-PHASE ALTERNATOR
22. The armature just worked out has been designed to deliver a single current at 2,000 volts pressure. Suppose it were desired to provide this armature, or rather an arma- ture of the same general dimensions, with a winding that would deliver two currents at 2,000 volts pressure, and differ- ing in phase by 90°. We could use two windings, each con- sisting of six coils connected in series, the two sets being displaced GO*' from each other with regard to the poles. The total output, as before, is to be 100 kilowatts; hence, the output per phase will be 50 kilowatts, and the current in
t- i_ . r 11 1 J -11 t. 50 X 1,000 ^. „
each phase at full load will be — :ri^ — = 2^- 7 amperes.
The current in the armature conductor is, therefore, one- half of that in the single-phase machine, and, using the same current density, we may make the conductor of a single No. 9 wire instead of two in multiple.
23. The voltage generated in each phase is to be 2,000. The total magnetic flux is the same, since the size of the pole pieces and armature is not
altered; hence, the number of con- ductors in each phase must be 336. Each coil on the two-phase armature will therefore consist of 56 turns of No. 9 B. & S. wire, provided we can arrange this number satisfactorily in the slot. If we use 7 layers with 8 turns per layer, we will have a slot of the same width as before, but only a little over half as deep. This will result in a slot that is not very deep fio. 6
compared with its width, whereas it is generally better to have the slot considerably greater in depth than in width. It will give a much better proportioned slot if we use only 5 layers, and place 11 turns in each layer, or 55 turns per
Digitized by VjOOQIC
14
DESIGN OF ALTERNATING
§21
coil instead of 56. This will lower the voltage slightly, but will leave the dimensions of the core the same, and com- pensate for this slight decrease by strengthening the field a small amount. In other words, we will compensate for the decrease in the number of turns by increasing <P so that E will remain the same. The slot may then be arranged as shown in Fig. 6. AJlowing the same amount for insulation as before, the width of the slot will be equal to 5 X .129 + 4 X .01 + 2 X .04 + 2 X .06 = .885 inch. The depth of the slot will be 11 X .129 + 2 X .04 + 2 X .06 = 1.619 inches.
We will therefore make the slot ^ inch wide and 1| inches deep. As this coil is lighter than the one used for the singlp- phase armature, we will allow only \ inch for the wooden wedge, and make the upper part of the slot as shown in Fig. 6. We will leave the inner diameter of the disk the same, the cross-section of iron being slightly greater than before, on account of the smaller depth of the slots. The disk for this two-phase armature will then be of the dimen- sions shown in Fig. 7. In this case the disk is provided with 24 slots of the dimen- sions shown in Fig. 6, there being 12 slots for each phase.
24. The PR loss in this I armature would be practi- ' cally the same as that in the single-phase . armature pre- viously calculated. The re- sistance of each phase will be about double the resistance of the single-phase armature, because in each phase there is about the same length of wire as before, but this wif-e has only one-half the cross-section of that used for the single- phase machine. We may, therefore, take the resistance per phase as 2 X .7 or 1.4 ohms. The /'A' loss per phase will be (22.7)' X 1.4 = 721 watts, and the total loss in the two
Fig. 7
Digitized by VjOOQIC
§21 CURRENT APPARATUS 16
phases will be 1,442 watts, as before. The radiating sur- face has not been altered in any way, so that the two-phase armature should deliver its output without overheating. The core losses will also be about the same, because the volume of the core and the magnetic density have not been altered materially.
ARMATURE WiNDrNG FOR THREE-PHASE ALTERNATOR
26« Suppose it were desired to wind the above arma- ture so that it would deliver 100 kilowatts to a system by means of three currents differing in phase by 120°. It would be necessary to supply the armature in this case with three sets of coils displaced from one another 120° with regard to the poles. Each set would consist of six coils connected in series, the three groups being connected together according to either the Y or A method and the terminals led to the collector rings. In this case it will be supposed that the Y method of connection is used, because the current in each phase is small and the line voltage high. By adopting the Y method, the voltage to be generated per phase is reduced, thus calling for a smaller number of turns per coil than would be required if the armature were A con- nected. The total output, as before, is to be 100 kilowatts, and the line pressure at full load, 2,200 volts. We have, for a three-phase machine,
watts output = ^Z E I
where / is the full-load line current, and E the voltage between the lines at full load. For the present case, we have, therefore, 100,000 = 4/3/2,200,
, 100,000
or / = '- — -. = 26.2 amperes
2,200 4/3
86. If the line current at full load is 26.2 amperes, the full-load current in the armature conductors must also be
Digitized by VjOOQIC
16 DESIGN OF ALTERNATING §21
26.2 amperes, because, in a Y-connected armature, the cur- rent in each phase is the same as the line current. We will allow 550 circular mils per ampere, as before, to get an approximate estimate of the area of cross-section of con- ductor required. This gives 550 X 26.2 = 14,410 circular mils.
No. 9 wire has a cross-section of 13,090 circular mils, while No. 8 has a cross-section of 16,510 circular mils. We will use the No. 8 wire, since it is on the large side, and will thus tend to make the /* R loss less. The diameter of this wire when covered with a double wrapping of cotton will be about .14 inch.
27. The line voltage at no load is to be 2,000; conse- quently, the voltage generated in each phase will be '
y3
= 1,154 volts, because the armature is Y connected. We
have
4.44 (PT;.
^ - 10" ^ ^
where E is the voltage at no load generated in each phase. In this case, the constant k is 1, because we are using a con- centrated winding, there being only one slot for each pole and phase. T is the number of turns in each phase. The magnetic flux ^ will be considered the same as before, because the dimensions of the pole pieces and armature have not been altered. We then have
4.44 X ^ X n
^ 1,154X10" ^^. ,
^^ ^ = 4.44x2,235,00-51^50 = ''^ ^^^^^' ^^^^^^
These 194 turns are to be split up into the six coils con- stituting one phase. We can use 32 turns per coil, and thus have 192 turns in each phase instead of 194. This slight decrease in the number of turns could be compensated for by increasing the field strength slightly. The three-phase
Digitized by VjOOQIC
§21
CURRENT APPARATUS
17
armature will therefore be provided with 18 coils, each con- sisting of 32 turns of No. 8 wire. These coils are to be divided into three sets of six coils, each of the three sets being con- nected up Y.
Pig. 8
)^^saattnm
28. The arrangement of the slot
that would probably be best adapted
to this number of turns would be four
layers with eight turns per layer, as
shown in Fig. 8. We will allow the
same thickness of insulation as in
the previous examples, thus making
the width of the slot 4 X .14 + 3 X .01
+ 2 X .04 + 2 X .06 = .79 inch. The depth of the slot will
be 8 X .14 + 2 X .04 + 2 X .06 = 1.32 inches.
We will therefore adopt the dimensions |^ inch by
1| inches as the width and depth, and make the wedge | inch
thick, as in the last case. Fig. 9 shows the dimensions of the disk for this machine. It is provided with 36 slots, equally spaced and of the dimensions shown in Fig. 8. The other dimensions of the disk remain the same as for those previously calculated.
29. The /" R loss for this armature should not differ greatly from the loss calcu- lated for the other two. We can easily make an approxi- mate estimate of the /' R loss in such a three-phase armature as follows: The mean length of a turn will be very nearly the same as that obtained for the single-phase machine, because the angular distance that the coils span remains the same and the length of the armature core has not been
45—6
Fig. 9
Digitized by VjOOQIC
18 DESIGN OF ALTERNATING §21
altered. There might possibly be a slight increase in the length, Qwing to the shape that must be given to the ends of some of the coils in order to allow them to pass each other at the ends of the armature, but it will be sufficiently accu- rate to take the length of a turn the same as before, namely, 54 inches, for the present purpose. The total length of con- ductor in each phase will be 54 X 192 = 10,368 inches. The hot resistance of each phase will therefore be
Te^sio^-^^^^^"^
The current in each phase at full load is 26.2 amperes. Hence the /^ R loss in each phase will be (26.2)' X .628 = 431 watts, approximately. We will take the loss in each phase at, say, 500 watts, in order to allow for the loss due to the resistance of the connections. The total loss in the armature would therefore be 1,500 watts, or about the same as for the other armatures. The radiating surface is the same as in the other two cases, so that tiiis armature should deliver 100 kilowatts within the specified temperature limit. The core losses, as before, would remain nearly the same, since the volume of iron has not been changed appreciably. The coils of the two-phase and three-phase armatures would, if anything, run cooler than those of the single-phase machine, because the coils are lighter and the heating effect is distributed among a larger number of coils.
30. The three-phase armature might have been designed for a A winding, in which case each phase would be provided with a sufficient number of turns to generate 2,000 volts.
26 2
The current in the conductor would, however, be only —7—,
or 15.1 amperes; so that, while the number of turns must be increased, the cross-section of the conductor may be decreased in the same ratio, and the size of armature slot will be about the same in either case.
31. The above calculations for single-, two-, and three- phase armatures have all been made on the supposition that
Digitized by VjOOQIC
§21 CURRENT APPARATUS 19
unicoil, or concentrated, windings were used. The method of designing the armature when distributed windings are used is, in general, the same, with the exception that the formula giving the relation between the E. M. F., flux, and turns must be modified to suit the style of armature wind- ing used. The effect of using distributed windings has already been pointed out, and calculations relating to such windings will be given in connection with induction-motor design.
COMPIiETED ARMATURES
32. Fig. 10 shows a finished armature with collector rings. This armature has a concentrated winding, as indi- cated by the small number of large slots around its circum- ference. The wooden wedges for holding the coils in place are shown at w: c are the ventilating ducts for allowing a circulation of air through the core. The cast-brass shields J
PlO. 10
are supported from the armature spider, and are used to protect the projecting ends of the coils. The armature is shown complete with the collector rings r and the rectifier t. Fig. 11 shows a large three-phase armature with a distributed winding. It will be noticed that this armature has a large number of narrow slots and is similar in appearance to a continuous-current armature, except for the absence of the commutator and its connections. The ends of the bars rest
Digitized by VjOOQIC
20 DESIGN OF ALTERNATING - § 21
on the spider flanges and are held down by the bands a. The disks are carried by the spider b and are clamped up by the end plates c. The copper bars d^ d are the connections between the winding and the collector rings. It will be
Fig. n
noticed that this armature is not provided with a rectifier, because this style of armature is of such low inductance that the machine can be made to regulate closely enough without the use of a set of series-coils on the field.
DESIGN OF FIEL.D MAGNETS
33. Stationary field magnets for alternators are gen- erally constructed in about the same way as those for multi- polar continuous-current machines, the mam difference being the large number of poles with which an alternator field is usually provided. The design almost universally adopted for stationary fields consists of a circular yoke a^ usually of cast iron (see Fig. 12), provided with a number of poles d projecting radially inwards toward the armature. The field is usually made in halves, so that the upper part a may be removed to give access to the armature. The lower half b is very often cast with the base of the machine, especially in machines of moderate size. In larger machines
Digitized by VjOOQIC
§21
CURRENT APPARATUS
21
the lower half is cast separately and provided with projec- tions c, c, by means of which it is bolted to the bed. The halves are held together by means of the bolts e. Some
G
^
IRI
D
PIO. 12
makers build fields of this description, which are divided on the vertical diameter, allowing the halves to be separated sidewise in order to get at the armature. In some small machines the yoke is made in one piece, and the machine is so arranged that the armature may be drawn out endwise.
e
34, The pole pieces used with these stationary fields are usually straight; that is, they are not provided with pole shoes or polar projections of any kind. Pole shoes are not necessary, because the length of the polar arc is generally small. Some of the older types of machines were provided with cast-iron pole pieces cast with the yoke, but most modern machines pig. is
have wrought-iron pole pieces built up out of plates and cast welded into the yoke. Fig. 13 shows a form of cast-iron pole piece that was used on some of the older machines. This is a straight pole piece b cast with the yoke a. In order to prevent
Digitized by VjOOQIC
22 DESIGN OF ALTERNATING §21
eddy currents being set up in the pole pieces by the changes of magnetism in the pole face due to the coarse teeth and slots of the armature sweeping past it, the surface of the pole is broken up by a number of thin U-shaped pieces of sheet iron c cast into the pole. This limits the paths in which the eddy currents flow, and thus cuts down the heat- ing of the poles due to them. Cast-iron poles cannot be
^ — -^^^^^ worked at a magnetic density much
U— ?'7\ ^ —^ ^^^^ 30,000 or 35,000 lines per square
•^ inch, and there is always more or less
loss in the polar surface due to eddy currents. In order, therefore, to do away with this eddy-current loss and to permit the use of a higher magnetic density, laminated wrought-iron pole pieces have come largely into use, and are employed on nearly all n»odern alternators. Fig. 14 shows a common ^'° ^^ form of this type of pole. The pole is
built up of soft iron stampings b, which are clamped together between the end plates d, d by means of the bolts r, c. This built-up pole piece is cast into the yoke a. The plates used for these poles are usually from ^ inch to \ inch in thickness. If the bolt at the inner end of the pole piece is very near the end of the pole, it should be lightly insulated by a paper tube; otherwise it may, by short-circuiting the plates, allow eddy currents to flow. The length of these pole pieces parallel to the shaft is made equal to the correspond- ing length of the armature core. The breadth of the pole w is determined by the polar arc that the pole must span. It will be noticed that the cross-section of these pole pieces is, in general, rectangular, or nearly so, and the field coils are therefore nearly rectangular. Circular field coils and field cores, which are so common with direct-current machines, are seldom met with on alternators, because the width of the pole lu is generally small compared with the length of the armature, except perhaps on large slow-speed machines.
Digitized by VjOOQIC
§21
CURRENT APPARATUS
23
(a)
35. The yoke a b, Fig. 12, is nearly always made of cast iron. The magnetic flux through the yoke of an alternator is usually small, and as the yoke must have con- siderable cross.-section to make it strong enough, mechanically in any event, there is no object in using cast steel to make the cross-section ^*^- ^5
small, as is frequently done in the case of direct-current machines. Usually, the yoke is worked at a low density in order to give sufficient cross-section to make it strong enough mechanically. The shape of the cross-section is largely a matter of design, so long as the requisite area of iron is provided. Fig. 15 {a) shows a plain rectangular section with rounded corners; {b) shows a section that is frequently used, the well-rounded corners and the elliptical back giving the yoke a more graceful appearance than the plain rectangular section. Fig. 15 {c) shows a section that is commonly used. In this case the yoke is provided with flanges that make it stiff and that also give the yoke a solid appearance, although the cross-section of metal in it may be quite small (see Fig. 12). Fig. 15 {d) shows a flanged construction with the flanges moved in from the edge of the yoke. The breadth of the yoke is usually some- what greater than the length of the pole pieces parallel to the shaft, so that the yoke will partially cover the ends of the field coils.
REVOLVING FIELDS
36. A number of different constructions are used for revolvliifir fields, depending on the methods adopted for furnishing the field excitation. A common type is that in which the radial pole pieces are bolted to a cast-steel rim, each pole piece being provided with an exciting coil, as in the case of the stationary field just described. Fig. 16
Digitized by VjOOQIC
24
DESIGN OF ALTERNATING
§5J1
shows a pole piece and coil for this type of field. The pole a is built up out of sheet-iron plates and secured by the stud d to the rim by which is carried on the spokes of the field spider. Stud d screws into the bar c that passes through openings in the stampings, and the projections on the pole
PlO. 17
serve to hold the coil in place. In some cases the poles are made straight and the coil held in place by projecting lugs on the end clamping plates. Fig. 17 shows a similar pole piece, the plates in this case being dovetailed into the field ring and held firmly in place by a key e driven in at one side.
Fig, 18
37. Revolving fields have been built so as to require only one exciting coil for all the poles. A field of this type is shown in Fig. 18. The exciting coil c is circular. The field casting is in two parts a and /;, held together by boltsy,
Digitized by VjOOQIC
§21
CURRENT APPARATUS
25
and each casting has a crown of six poles, as shown. When current is sent through the coil, lines of force thread through it; all the projections d attached to one side being, say, north poles, and all those attached to the other side, south poles. This construction gives rise to large magnetic leakage, and is now seldom used.
FIELD-MAGNKT COIL.8
38. Field-magnet coils may be wound on spools con- structed similar to those used for the field coils for continuous-
FlG. 19
current machines. These spools are made so as to slip over the pole pieces, and are usually held in place by pins pro- jecting from the pole or by cap bolts screwed through lugs projecting from the end flanges of the spool. Fig. 19 shows an end elevation and a cross-sectional view of a spool of the style commonly used. The shell b is made of heavy sheet iron, and is flanged up at the ends, so that it may be riveted or soldered to the brass end flanges ^, a. These flanges are usually recessed and provided with ribs to make them stiff and at the same time secure lightness. The ends of the spool are rounded out as shown, so as to give clearance for the heads of the bolts that clamp the pole pieces together. In designing field coils and spools, care must be taken to see that the depth of winding is not made such that the coils will interfere with each other when they are placed on the poles, and sufficient clearance must be j)rovided, as at a. Fig. 20.
Pig. 90
Digitized by VjOOQIC
26 DESIGN OF ALTERNATING §21
39. Field coils are usually wound with double cotton- covered magnet wire, though in some large machines copper strip is used. The field spools of most modern revolving- field alternators are wound with flat copper strip bent on
PlO. 21
edge, as shown in Fig. 21, when (a) represents one of the laminated pole pieces, with its end insulations. A coil partly pulled apart is shown at (^). Insulation is placed between the layers of strip, and the outer edge of the strip
Fig. 22 Pig. 23
is left bare. A coil wound in this way is very solid and substantial, and the heat is readily radiated because the exposed strip conducts the heat to the air from the inner part of the coil. When field coils are provided with two
Digitized by VjOOQIC
§21 CURRENT APPARATUS 27
sets of windings (separately excited and series), the coils may be arranged on the spool, one on top of the other, as shown in Fig. 22, or side by side, as in Fig. 23. The con- struction shown in Fig. 23 is the better, because it admits of higher insulation and allows one coil to be repaired, in case of breakdown, without disturbing the other. On many modern machines the field coils are wound on forms and held in shape by taping so that it is not necessary to use spools.
INSULATION OF FIKLX) COILS
40. In many cases the fields are excited by coils that are provided with only one winding excited from a separate continuous-current machine. The exciter voltage in such cases is usually low, and it is unnecessary to take any unusual precautions in insulating the spools, as the maxi- mum pressure tending to break down the insulation would not likely exceed 100 or 200 volts. Such' spools may there- fore be insulated in the same way as those for ordinary con- tinuous-current machines.
41. Where the spools are provided with two windings, the series-winding is, in many cases, in direct connection with the armature, thus carrying the high potential to the field coils and subjecting the insulation to a large stress. Such windings must be thoroughly insulated, not only from one another, but also from the spools. Figs. 22 and 23 show the methods of insulating these coils. The shell is covered with several layers a of paper and mica interleaved, the insulation between the coils in Fig. 22 being also of the same material. The end insulations b, b and insulation d between the coils, Fig. 23, are made either of heavy collars of paper and mica, or of hardwood veneer treated with oil or other insulating material. Every precaution should be taken to make the insulation of these spools high, as they are liable to be subjected to just as high a voltage as the armature windings.
Digitized by VjOOQIC
28 DESIGN OF ALTERNATING §21
DESIGN OF FIEIiD
42. We will illustrate the method of obtaining the field dimensions by working out the design of a field suitable for the single-phase armature previously calculated. This field will be of the radial pole type shown in Fig. 12, the pole pieces being of wrought iron, as shown in Fig. 14.
BORE OF POLES AND LENGTH OF AIR GAP
43. Before proceeding with the design of the field, we must decide on the length of air gap to be used. It was shown, in connection with continuous-current machines, that for any given armature it was necessary to have a cer- tain length of air gap; otherwise, the armature would react on the field so as to cause sparking when the machine was loaded. It has also been shown that the general effect of the armature reaction in an alternator is to weaken the field. If we wish an alternator to give good regulation, we can cut down the effect of the armature on the field by using a large air gap, and on this account it is quite common to find alter- nators provided with an air gap that is much larger than is necessary for mechanical clearance. A short gap would have the advantage of requiring only a small amount of magnetizing power on the field to set up a given flux; but, on the other hand, it would allow the armature to react strongly, the actual length of air gap used not being deter- mined from considerations of the sparking limit, as it is in the case of direct-current machines. For belt-driven machines up to 250 or 300 kilowatts, | inch to | inch may be taken as fair values for the length of the double air gap. If the gap is made very large, of course a large amount of exciting power is required, so that it does not pay to increase the length of the gap much beyond the values given above. For large direct-connected machines, the gap necessary for mechanical clearance will usually be found sufficient to make the machnie perform well electrically.
Digitized by VjOOQIC
§21 CURRENT APPARATUS 29
44:. For the machine under consideration, we may, therefore, make the double air gap | inch and the bore of the pole pieces 31 J + | = 32 J inches. The poles cover 50 per cent, of the armature, and the length of the arc will be
n X bore of poles X .5
number of poles
(10)
ttX 32.125 X. 5 ,„. , or arc = — = 4.2 mches
I/O
The distance between the sides of the pole will be about 4-J inches, as shown in Fig. 24. The length of the pole piece parallel to the shaft will be the same as the length of the armature core, 13^ inches.
45. All dimensions of the pole pieces are now known except their radial depth /, Fig. 24. The pole piece must be made long enough to accommodate the winding without making it too deep. ^^°- ^
Short pole pieces result in a yoke of small diameter and a correspondingly light machine. On the other hand, the spool winding must usually be deep when short spools are used. The depth of winding may not only be limited by the space between the poles, but deep windings are objec- tionable on account of their liability to overheat and the larger amount of copper required for them. If, however, the cores are made longer than is necessary, the winding is made unnecessarily shallow and the yoke of large diam- eter, thus making the machine heavy and the magnetic circuit long. In machines of the type under consideration, the length of the pole piece is usually from 1| to 2^ times as long as it is wide. For a trial value, we will therefore take 8 inches as the length /. This can later be increased or decreased slightly to suit the windings, if found neces- sary. We will also allow | inch, as shown in Fig. 24, for
Digitized by VjOOQIC
30 DESIGN OF ALTERNATING §21
the thickness of the flat part on the inside of the yoke against which the coils rest. This will make the inside diameter of the yoke 32| + 16 + f = 48| inches.
MAGNETIC FLUX THROUGH POUE PIECES AND YOKE
46. The magrnetlc flux that passes through the arma- ture from one pole piece is ^. A certain number of the lines leak across from one pole piece to the other without passing through the armature; hence, in order to get ^ lines in the armature, we must have ^' lines in the pole piece, where ^' is equal to ^ multiplied by the coefficient of leakage. The coefficient of leakage is generally somewhat greater for alternators than for direct-current machines, because the poles are usually fairly close together and expose quite a large surface from which leakage may take place. The larger the air gap compared with the leakage path between the poles, the greater will be the amount of leakage, since the lines always flow by the path offering the least resistance. The coefficient of leakage also varies with the size of the machine, being smaller for large machines than for small ones, and may have values ranging from 2 to 1.3 or less in very large machines. We will take the coefficient of leakage for the machine under consideration as 1.4.
47. The useful flux ^ from one pole is in the present case 2,235,000 lines. The flux through each pole piece will therefore be <!>' = 2,235,000 X 1.4 = 3,129,000.
The magnetic density in the field cores will be
- flux through core /-i-fx
Of = : (H)
■^ cross-section ^ '
3,120,000 ^^, ^^^ ,. . ^
= Y\ ToT" ~ 56,400 Imes per square mch
It will be noticed that this density is well below that point at which wrought iron begins to saturate, so that
Digitized by VjOOQIC
§21 CURRENT APPARATUS 31
the sectional area of the pole pieces as determined by the polar arc is ample for carrying the magnetic flux.
48. The magnetic flux through the yoke is one-half that through the pole piece, because the lines divide, one half flowing in one direction and the other half in the other direction. The number of lines flowing through the cross- section of the yoke is, therefore,
<P' 3,129,000
Y = -^-i = 1»564,500
and the required cross-section of the yoke will be
. __ flux through yoke __ i ^' /i«>\
"" Allowable density in yoke ~ B^ ^ ^
where B^ is the magnetic density at which the yoke is worked. The yoke density is usually low, as already explained, the yoke being made of cast iron. We will take 30,000 lines per square inch a§ the allowable value of Bj,, thus giving for the required cross-section
. 1,564,500 ^,, . , ,
^ ^ qTwwT" ~ ^^--^ square mches, nearly
We will make the yoke 17 inches wide, so as to allow it to project over the pole pieces at each end. If we made the yoke rectangular in sec- tion, as shown by the dotted outline. Fig. 25, the thickness would be about 3^ inches to give the requisite cross-sec- tion. Instead of using the rectangular shape, we will increase the thickness at the center to 4 inches and round off the yoke as shown, so as to keep the area about the same. This will give a heavier- looking yoke, and one that will present a better appearance generally than that with a rectangular section.
Fig. 25
Digitized by VjOOQIC
32
DESIGN OF ALTERNATING
§21
CALCULATION OF FIELD AMPERE-TURXS
49. Since the dimensions of the field frame, armature, and air gap are now known, and the magnetic densities in these different parts are also known, the ampere-turns required to set up the magnetic flux can be calculated. In order to do this, it is best to consider one of the simple magnetic circuits shown by the dotted line a-b-^-d-e-f.
Fig. 26
Fig. 26. This path is made up of a portion of the yoke, two pole pieces, the double air gap, and the portion of the armature core shown. The dotted line represents the length of the average path through which the lines flow, and the ampere-turns supplied by the separately excited
Digitized by VjOOQIC
§21 CURRENT APPARATUS 33
coils on the two poles must be sufficient to set up the mag- netic flux around this path. We may, for convenience in making calculations, split up the ampere-turns required for the whole circuit into the following parts:
1. Ampere-turns required for the double air gap c d-\-ef.
2. Ampere-turns required for the circuit through the two pole pieces be -\- af.
3. Ampere-turns required for the path through the yoke a b,
4. Ampere-turns required for the path through the arma- ture d e,
60. The effective area of cross-section of the air gap through which the lines ^ flow will be taken as about equal to the area of the pole face. The lines will fringe to some extent at the edges of the pole, thus actually increasing the effective area slightly. The area is, however, cut down somewhat by the air ducts in the core, so that this will tend to counterbalance any increase in area due to fringing. We will therefore assume that the density is as taken at the out- set, namely, 40,000 lines per square inch. The permeability of air is 1, and the total length of air gap is | inch; hence, ampere-turns required for double air gap = H X /x .313 = 40,000 X .375 X .313 = 4,700, nearly.
51. The magnetic density in the pole pieces has already been determined and found to be 56,400 lines per square inch. The length of path through the two pole pieces is 2 X 8 = 16 inches. By referring to the magnetization curves. Dynamos and Dynamo Design^ Part 2, we find that it requires about 11 ampere-turns per inch of length to set up a density of 56,400 lines per square inch through wrought iron. Hence, ampere-turns required for field cores = 11 X 16 = 176.
53. The yoke has been made of such cross-section that the density in it is 30,000 lines per square inch. The length of the path ab through the yoke can be scaled from the 45—7
Digitized by VjOOQIC
34 DESIGN OF ALTERNATING §21
drawing, and in this case is about 14| inches. For a den- sity of 30,000 lines per square inch, the ampere-turns required per inch of length for cast iron are about 50. Hence, ampere-turns required for yoke = 50 X 14^ = 725.
53. The armature has been made of such cross-section that the density in the core is about 30,000 lines per square inch. The length of the path through the core can be obtained from the drawing; in this case it is about 12 inches. The ampere-turns required per inch of length for wrought iron at this density will be about 8. Hence, ampere-turns required for armature core = 8 X 12 = 96.
54. The total ampere-turns that must be supplied by one pair of the separately excited field coils will be the sum of the ampere-turns required for the different parts of the magnetic circuit; hence, total ampere-turns = 4,700+176 -h 725 + 96 = 5,697, say 5,700.
The student will note that because the magnetic densities in the iron parts of the circuit are low, and also because the lengths of the different paths are short, the ampere-turns required for the iron part of the circuit are small compared with those required for the air gap, which has a high mag- netic reluctance. The ampere-turns required for the arma- ture core might in many cases be neglected without serious error. It follows from this that if it is found necessary later to lengthen or shorten the pole pieces slightly, in order to accommodate the winding, the corresponding resulting change in the ampere-turns will not be appreciable.
CALCULATJON OF SEPARATELY EXCITED WINDING
65. Having determined the ampere-turns to be supplied by each pair of separately excited coils, the next step is to design a winding for these coils that will supply the required number of ampere-turns. The size of wire can readily be determined when the mean length of a turn and
Digitized by VjOOQIC
§21 CURRENT APPARATUS 35
the voltage across the coils are known. In order to get at a value for the mean length of a turn, we must adopt a trial value for the depth of the winding. Suppose we make the spool flanges 1^ inches deep, as this will give a spool of dimensions well suited to the field shown in Fig. 26, allow- ing plenty of clearance space between the coils when they are slipped over the poles. The clearance between the shell and field core will be, say, ^ inch all around, and we will allow ^ inch on each side for the thickness of the shell and insulation. The series and separately excited coils will be arranged side by side, as shown in Fig. 23. We will have a clear depth of winding of 1 inch, allowing for clearance and insulation as above. The shape of the spool will be as shown in Fig. 19, and the mean length of a turn can readily be measured off the drawing. In this case the mean length of a turn will be about 41 inches, or 3j^ feet.
56. The separately excited coils are connected in series, so that the voltage across any pair of coils will be the volt- age across all the coils divided by the number of pairs of poles on the machine. The voltage applied to the separately excited field is equal to the voltage generated by the exciter less whatever drop there may be in the regulating rheostat. Let E represent the E. M. F. generated by the e/.citer, and e the drop in the rheostat. The pressure applied to one pair of coils will then be
I 2 where/ = number of poles;
The current in the field will be where R is the resistance of a pair of spools.
Digitized by VjOOQIC
36 DESIGN OF ALTERNATING §21
But the hot resistance R of a, pair of spools may be expressed as follows :
R = :^??L>^ (14)
where /« = mean length of a turn in inches;
T = number of turns on a pair of spools; m = circular mils cross-section of field wire.
Substituting in formula 13 the value of R as given by formula 14, we get
. ^ li^JZfl^ (15)
(16)
The values of the quantities T and / are not known sepa- rately, but their product is known, since it is the ampere- turns supplied by one pair of spools. Hence, we may write
circular mils cross-section of separately excited field wire
_ number of poles X mean length of a turn in inches X ampere-turns ~ 2 (voltage of exciter — drop in field rheostat)
Or, the cross-section in circular mils of the wire necessary for the separately excited winding of an alternator is found by taking the product of the number of poles, the mean length of a turn in inches, and the ampere-turns supplied by one pair of spools, and dividing by twice the voltage of the exciter less the drop through the field rheostat.
The size of wire could be worked out equally well by con- sidering the ampere-turns supplied by all the coils instead of a single pair, and taking the total voltage instead of the voltage across a pair of spools. It is best, however, to make the calculations with reference to a pair of spools in order to avoid confusion, because the ampere-turns were calcu- lated for a pair of spools.
Digitized by VjOOQIC
§21 CURRENT APPARATUS 37
67. The exciter voltage E is commonly 110 volts, though other voltages are sometimes used with large machines. The use of 110 volts is common, because it permits the use of an ordinary 110-volt incandescent dynamo as an exciter. We will assume that the field for which we are making cal- culations is supplied from a 110-volt exciter, and that the normal drop in the rheostat is 10 volts. This will make the pressure across the twelve field coils 100 volts total. We then have
. , .. 12X41X5,700 ,^^^^
circular mils = — — — '- = 14,022
/vOO
The nearest size to this is No. 9 B. & S. having a cross- section of 13,090 circular mils. \Ve will therefore adopt this size of wire for the separately excited field, the slight differ- ence in cross-section being compensated for by cutting out a little of the rheostat resistance.
58. The current density in the field should be consider- ably lower than ir the armature, because the field windings are deeper and the heat is not so easily dissipated. The current in the separately excited winding is about the same, no matter what load the alternator is carrying, and in this respect is not like the current in the series-coils, which varies with the load. For these reasons, it is not safe to allow much less than 1,000 or 1,200 circular mils per ampere in the separately excited winding, and in cases where the wind- ing is very deep a larger allowance than this may be required. In the present case we will take 1,100 circular mils per ampere as a fair value, thus limiting the current to VtVo^ = 11.9 amperes.
59. With a field current of 11.9 amperes, the number of
turns required per pair of spools will be ' = 478 turns,
nearly. Each coil should then have 239 turns of No. 9 B. & S. double cotton-covered wire. The diameter of this wire over the insulation will be about 120 mils, and if the coil is wound in eight layers, the depth of winding will be
Digitized by VjOOQIC
38 DESIGN OF ALTERNATING §21
1.008 inches, so that an eight-layer winding will fit the 1-inch winding space on the spool. If we use thirty turns
to a layer, we will have 240 turns per spool. This is an increase of one turn over the number actually required, but it will be better to use this winding than to have an uncom- pleted layer, since the difference is so small. The length of -winding space occupied by the coil will be 30 X .126 = 3.78 inches, or, say, 3| inches, so as to be ^^®- ^ sure of enough room.
The separately excited coil will therefore be wound with eight layers of No. 9 wire with thirty turns per layer, the winding space occupied being 3| inches long and 1 inch deep. The use of 240 turns per spool, instead of 239 turns, will not affect the current appreciably. The upper coil 5, Fig. 27, shows the arrangement of this coil on the spool.
COMPOUND, OR SERIES-FIELiD, WINDING
60. The compound winding must provide a sufficient number of ampere-turns to compensate for the falling off in voltage at the terminals due to the resistance of the arma- ture and the combined effects of armature inductance and armature reaction. The compound winding must also pro- vide the ampere-turns necessary for any increase in terminal voltage in cases where the machine is to be overcompounded. The calculation of the compound winding depends to a large extent on data obtained from machines of a similar type. Its determination for a machine of new type is always -more or less experimental.
Digitized by VjOOQIC
§21 CURRENT APPARATUS 39
61. The current that is led through the series-winding is first rectified, as explained in former articles, and as the current increases in proportion to the load, the field is strengthened proportionally, provided the. magnetic circuit is not saturated. This is usually the case with alternators, so that we may assume that any change in the field current is accompanied by a corresponding change in the field strength. It is not usual to send the whole of the current around the series-fields; part of it is shunted through a German-silver resistance', by varying which the amount of compounding can be varied. This allows a considerable adjustment of the series-coils, so that their effect on the performance of the machine can be varied through a wide range^ without changing the series-winding in any way. Sometimes the whole current is not rectified, a portion of it being shunted around by means of a resistance connected to the two sides of the rectifier. In this case the shunt must revolve with the armature, and is usually mounted on the armature spider. Revolving shunts are generally used on machines of any considerable size, as they avoid the difficulty of commutating a large current. Compound coils are only necessary on the fields of machines that have high armature inductance or resistance, or on machines that must give a considerable rise in voltage from no load to full load. Other types of machines can be made to give suffi- ciently good regulation by the use of separately excited coils only. Most of the alternators of large output installed in modern power plants are plain separately excited machines.
• S2. The drop due to the resistance of the armature is easily calculated when the armature resistance is known, as it is equal to the product of the armature resistance and the full-load current. In this case, therefore, the armature drop will be 45.4 X .7 = 31.78 volts.
63. The machine is to supply 2,000 volts at no load and 2,200 volts at full load; the compound winding must there- fore strengthen up the field sufficiently to generate this
Digitized by Google i
40 DESIGN OF ALTERNATING §21
200 additional volts, as well as the 31.78 volts required to overcome the resistance of the armature. If there were no armature inductance or armature reaction, the total volts that would be generated at full load would be about 2,232. The ampere-turns supplied by two separately excited coils (i. e., 5,700) are sufficient to generate 2,000 volts; hence, if the above conditions were attained, the ampere-turns on the field at full load would have to be ||ff X 5,700 = 6,361, and the ampere-turns that would be supplied by the series- coils would be 6,361 — 5,700 = 661, or about 331 on each spool. For a machine, of this kind, however, this would represent only a very small part of the series ampere-turns that would actually be required, because, in the first place, • the field is weakened by the reaction of the armature, and, secondly, a large E. M. F. has to be generated to force the current through the armature against its induct- ance. In machines of this type the compound ampere-turns may be as much as two-thirds or more of the ampere-turns supplied by the separately excited coils. In the present case, therefore, we will design each spool so that it will be capable of supplying about 2,600 ampere-turns. If this should prove to be somewhat more than is actually required, it can easily be cut down by allowing more current to flow through the shunt.
64. We will assume that 70 per cent, of the current at full load flows through the series-coils, the remaining 30 per cent, flowing through either the revolving or stationary shunts. This will make the current in the series-coils 45.4 X .70 = 31.78, say 32 amperes, nearly. The number of turns required for each series-coil will then be ^jp = 78.4 turns.
65. The current density in the series-coils should be about the same as that in the separately excited windings. If we allow 1,100 circular mils per ampere, as before, we get a cross-section of 32 X 1,100 = 35,200 circular mils. Two No. 8 wires in parallel give 33,020, while two No. 7
Digitized by VjOOQIC
§21 CURRENT APPARATUS 41
wires give 41,640. We will adopt the conductor made up of two No. 8 wires, because the current in the series-coils is not apt to be continuously at 32 amperes, and we can there- fore afford to use a cross-section that is a little on the small side. The outside diameter of No. 8 wire with cotton insu, lation is about .140 inch; hence, in a winding space 1 inch deep we can place seven layers. If we use 11 turns per layer, we will have 77 turns per coil, and can compensate for the slight decrease in the calculated number of turns (78.4) by changing the shunt a little, so as to cause a correspond- ingly larger amount of current to flow through the coils. Each turn consisting of two wires in parallel will occupy a length along the winding space of .280 inch, and 11 turns will take up a space of .280 x 11 = 3.080 inches, say 3^ inches. We will allow ^ inch at each end and between the coils for the hard-wood insulating collars, thus making the total axial length taken up by the windings and insulation 3| + 3| -|- ^ = 7iV inches. The brass flanges on the spools will be about { inch thick, so that the total space taken up on the pole piece will be 7^^ + i = 8^ inches. The radial length of the pole piece as originally assumed was 8 inches; it will therefore be necessary to lengthen out the poles a little, in order to accommodate the spool, and * increase the diameter of the yoke correspondingly. It is best to have the pole project beyond the spool flange a little, as it keeps the flanges away from the armature and makes it easier to fasten the spools in place. We will therefore make each pole piece 8J inches long instead of 8 inches. Fig. 27 shows a section of the spool with both windings in place. The pole piece is indicated by the dotted outline. This change in the length of the pole piece will make the inside diameter of the yoke 49| inches, and the outside diameter 57| inches, as shown in Fig. 26, where the final dimensions are encircled by rings. The spools are held in place on the poles by pins (not shown in the figure), which are fixed in the pole pieces so as to prevent the coils slipping down on to the armature.
Digitized by VjOOQIC
42 DESIGN OF ALTERNATING §21
liOSS IN FIEIiD COLLS
66. The loss In the field coils should be determined, in order to see if sufficient radiating surface is provided to dissipate the heat. The resistance of the twelve separately excited coils will be
^ _ 12 X 240 X 41 ^ , . ^ ,
• 13 09Q ~ ^ ohms, approximately
since there are 240 turns on each spool.
The i^ R loss in the separately excited coils will therefore be (11.9)'' X 9 = 1,274 watts.
67. The resistance of the twelve series-coils is
^ 12 X 77 X 41 , _ , ^^=— 33:020— =^-^^^^"^'
The I* R loss in the series-coils will therefore be (32)* X 1.15 = 1,178 watts, nearly.
68. The total loss in the field will be 2,452 watts, or about 2.4 per cent, of the output. This is the maximum loss when the machine is working at its full output. The average field loss would probably not be over 2 per cent, of the output, as the loss in the series-coils would not be as high as -1,178 watts all the time. The loss per coil will be ^f|^ = 204 watts. The surface of each coil (not counting the ends) is about 350 square inches. This area is obtained by multiplying the perimeter of the coil as obtained from the drawing by the length of the coil along the pole piece. This area gives an allowance of 1.7 square inches of surface per watt, which is sufficient to insure a rise in temperature not exceeding 40° C. As far as heating goes, the design of the winding is therefore satisfactory.
69. The curve shown in Fig. 28 gives the relation between the average field PR loss and the output for
Digitized by VjOOQIC
§21 CURRENT APPARATUS 43
alternators of good design. For a 100-kilowatt machine the
9
3
S€laiion between field I*B io§§ and output of tUiematwr^
Fig. 28
average loss is about 1.7 per cent., which is slightly lower than that for the machine just calculated.
MECHAiaCAX. CONSTRUCTION
FIELD FRAME ANO BED
70. Fig. 29 shows the field frame, with bed and bear- ings, for the machine designed, and will serve to illustrate the general method of construction used for machines of this type. In this case, the field is shown as a separate casting bolted to the base, but, as mentioned before, many machines are constructed wMth the lower half of the field cast with the base. Where the machine is of large size, it becomes difficult to cast the field and bed together, and the construction shown is usually adopted in such cases. The field is usually set down into the bed, as this lowers the center of gravity and tends to make the
Digitized by VjOOQIC
Digitized by VjOOQIC
§ 21 CURRENT APPARATUS 45
machine run steadier. The distance between the centers of bearings is determined by the over-all length of the armature and the space taken up by the collector rings. The bed itself is almost exactly similar to the beds used for multipolar continuous-current machines; it is made hollow and provided with ribs to insure stiffness. The thickness of metal in the bed will vary from about J inch or | inch up to IJ inches or 1^ inches for machines varying in size from about 50 to 500 kilowatts. Self-oiling bearings of the ring type are used almost exclusively. The bearing pedestals, as shown in Fig. 29, are cast with the base, though in many large machines it is common practice to cast them separately and bolt them to the bed. The bearing cap and pedestal is grooved at a a to receive the rocker-arm, which carries the rectifier brushes. Some makers place the recti- fier and collector rings outside the bearing and bring the connecting wires through the shaft ; in such cases the out- side end of the bearing cap and pedestal must be grooved to receive the rocker-arm. Machines of the type shown are usually arranged so that they can be mounted on rails in the same manner as continuous-current machines.
COLLECrOU RINGS AND RECTIFIER
71. One of the distinguishing features of an alternator is the arrangement by which the current is collected. The commutator of the continuous-current machine, which is usually made up of a large number of parts, is replaced, in a simple alternator, by two or more plain collector rings. In case the alternator is compound-wound, the commutator is replaced by two or more collector rinp^s in combination with a rectifier. Although there are, in general, a small number of parts connected with a collector as compared with a commutator, the mechanical construction of the col- lector must be carefully carried out, because it is often necessary, where revolving armatures are used, to secure high insulation. Fig. 30 shows a Construction that may be
Digitized by VjOOQIC
46
DESIGN OF ALTERNATING
^n
Digitized by VjOOQIC
§21 CURRENT APPARATUS 47
used for simple collector rings. Such a pair of rings would be suitable for a single-phase alternator with a separately excited field winding only. The same construction could be used for separately excited two-phase or three-phase machines, the only difference being in the number of rings employed. The rings r, r are made of cast copper, which must be free from blowholes or imperfections tending to cause uneven wear. . These rings are usually made heavier than is necessary for collecting and carrying the current, in order to make them strong mechanically and to allow for wear. Fig. 30 shows the construction used for rings that are subjected to a pressure of about 2,000 volts. The rings are cast with a hub ^, which supports the rings by means of the spokes c. The insulation d between the disks is usually made of either red fiber or hard rubber, the latter being preferable, especially for high potentials. These insulating disks should be at least \ inch thick, in order to keep them from breaking easily, and they should also project some distance above the surface of the rings, in order to avoid any danger of the current arcing over from one ring to the other. The insulating washers and collector rings are assembled on a shell e, made either of cast iron or brass, the latter being preferable for collectors of small size. This shell is thoroughly insulated with several layers of mica, and the assembled collector is clamped firmly in place by means of the nut /and washer^. When the col- lector is of large diameter, it is usually clamped up by means of bolts instead of the nut/" The connections to the rings are made by two copper studs //, which pass through the back of the shell and connect to each of the rings by being screwed into one of the spokes, as shown. These studs are heavily insulated throughout their length by tubes made of mica or hard rubber. After the ter- minals of the armature winding have been attached to the studs, all exposed parts should be heavily taped to avoid any danger of arcing from one terminal to the other. Where the studs pass through the back of the shell, they are insulated by thick hard-rubber bushings k.
Digitized by VjOOQIC
48 DESIGN OF ALTERNATING §21
73, The dimensions of the rings are determined quite as much by mechanical considerations as by the current that they are to collect. The surface of the rings should be wide enough to present sufficient collecting surface, and they should be thick enough to allow for a reasonable amount of wear. Such rings should collect at least 200 amperes per square inch of brush contact surface. This assumes that copper brushes are used, which is often the case with alternators. The freedom of carbon brushes from cutting and their better performance generally have resulted in their being used largely on alternators, though, of course, their advantages as regards the suppression of sparking do not have the force here that they do with direct-current machines. Carbon brushes require about three times as much contact surface, for a given current, as copper brushes, and this large collecting area is usually obtained by using a number of brushes distributed around the circumference of each ring, instead of increasing the width of the ring itself. The rings should not be made of too large diameter, or the rubbing velocity between the brush and ring will be high, thus tending to cause uneven wear and cutting. On the other hand, if the rings are made of very small diameter, they must be made wide to present sufficient collecting surface, thus necessitating the use of wide brushes. If a large collecting surface is required, it is best to use a ring of moderately large diam- eter, and use several brushes on each ring. From 1,500 to 2,500 feet per minute are fair values for the peripheral speed of collector rings for belt-driven machines. The rings shown in Figs. 30 and 31 are 10 inches in diameter.
On large revolving-field alternators, the collector rings are usually made of cast iron instead of copper. This is much cheaper, and it is found that carbon brushes bearing on cast-iron rings give excellent results, the iron ring taking on a good polish. On these large machines, the collector rings are usually made in halves, suitably fastened together, so that the rings may be put in place or removed without disturbing any of the heavy parts of the alternator.
Digitized by VjOOQIC
§21
CURRENT APPARATUS
49
46—8
Digitized by VjOOQIC
50
DESIGN OF ALTERNATING
§21
73, For compound-wound machines, it is necessary to have a rectifier in addition to the collector rings. The rings and rectifier are usually built up together, though some makers mount them on the shaft separately. Fig. 31 shows a combined pair of collector rings and rectifier suit- able for the single-phase machine designed. The rings are made 10 inches in diameter and 1^ inches wide, the con- struction used being the same as that already described. The rectifier is made up of two castings, each having six sections, those belonging to one casting being marked a, and those belonging to the other, b. These two castings are separated by the mica collar r, while mica insulation is provided between the segments a and /;, as in a regular con- tinuous-current commutator. One set of segments connects to one of the collector rings through the hubs, as shown at d. The other rectifier casting is connected to the stud e^ which is, in turn, connected to one terminal of the armature winding. The other stud is connected to the remaining collector ring. The details of construction will be under- stood by referring to the drawing, as they are almost identical with those described in connection with Fig. 30.
BRUSHES AND BRUSH HOLDERS
74, Copper brushes are generally used on the smaller sizes of alternators, and copper leaf or wire brushes similar
to those used for di- rect-current machines are employed on many machines, though carbon brushes are now largely used on account of their superior wearing qualities. It is best to have at least two brushes for each col- essential as with
Fig. 82
lector ring, though this is hardly as
Digitized by VjOOQIC
§21 , CURRENT APPARATUS 51
direct-current machines, because collector-ring brushes do not need as much attention while the machine is running as those used with commutators; for this reason, a large num- ber of machines are built with only one brush for each collector ring. Two or more brushes should, however, be used for each terminal of the rectifier, because these brushes are liable to need more or less adjustment while the machine is running. The holders used should be so designed that the copper brush will press on the rings at an angle of about 45°. Any good form of copper brush holder used on continuous-current* machines will answer equally well for an alternator. Such a holder should be arranged so that the brushes may be lifted from the com- mutator and held off, and the pressure of the brush on the ring should be* easily varied. The pressure of the brush on the ring may be provided by making the brush itself act as a spring, or the holder may be provided with a spring, the tension of which is adjustable. Fig. 32 shows a simple type of holder that has been used considerably on alternators. The brush is made long enough between the holder h and the ring r to render it flexible and allow it to follow any unevenness of the surface. The pressure on the ring can be varied by changing the position of the holder on the stud by means of the clamp 5. One advantage of this style of holder is that the current has no loose contact surfaces to pass through between the brush to the brush-holder stud. The carbon brush holders used on alternators are similar to those used on direct-current machines and require no special description.
BRTTSH-HOLBER STUDS
76, Brush-holder studs follow the same general design as those used for ccmtinuous-curi ent machines, special care being taken to have them very well insulated. Fig. 33 shows a common type of stud and the method used for insulating it. The brass stud a is circular in cross-section and is provided with a shoulder g that clamps against a
Digitized by VjOOQIC
62
DESIGN OF ALTERNATING
§21
washer //. The stud is- insulated from the rocker-arm by a heavy hard-rubber bushing / and washers b. The bushing / is let into the washers ^, as shown, in order to break up the path by which the current tends to
Fig. 38
jump from the stud to the supporting casting. The sharp corners of the casting should also be removed, as shown at m. The cable terminal d is clamped between the washer c and the nut e. Fig. 34 shows another method that is sometimes used for mounting and insulating brush- holder studs. A hard-rubber tube a fits tightly over the
FlO. 34
stud b and completely covers it except at the points where the brush holders and cable connections are placed. The brush-holder stud is clamped to the rocker-arm, as shown, by means of the cap c and the cap bolts d. Connection is
Digitized by VjOOQIC
§21 CURRENT APPARATUS 53
made to the cable at the end of the stud. This construc- tion gives very good insulation between the stud and the rocker, because the insulation is unbroken and no path is open for the current to jump across unless it punctures the tube itself.
76. The studs that carry the rectifier brush holders should be mounted on a rocker-arm, so that they may be adjusted, with reference to the field, in the same manner as the brushes of a direct-current ma- chine. The studs for the collector- ring brushes may be carried on the same rocker-arm, or may be mounted on a stationary stand bolted to the bed of the machine. The collector-ring brushes do not need to occupy any definite position relative to the field; hence, it is not necessary that they should be mounted on the rocker-arm, though this is very often done for the sake of convenience and cheapness of construction. The angular distance between the arms of the rocker carrying the rectifie.r studs will depend on the number of poles on the machine. Sup- pose Fig. 35 represents the rectifier for the twelve-pole machine worked out. All the light sections belong to one casting and the dark ones to the other. The angular dis- tance from center to center of segments is 30°. When one set of brushes is on a light segment, the other set must be on a dark segment; hence, the brushes might occupy the position cd'. This, however, would bring the brushes too close together, and we will place the rocker-arms so as to make them as far apart as possible, and still have them conveniently located. We will therefore place the rocker- arms carrying these brush-holder studs 150° apart, thus bringing the brushes into the position c d,
77, Fig. 36 shows a rocker-arm suitable for the single- phase machine designed. The arms a^ b are 150° apart, and
Digitized by VjOOQIC
54
DESIGN OF ALTERNATING
§21
carry the rectifier studs, the arms c, 3 for the collector-ring studs being carried on the same rocker. The hub^ is bored to fit the groove in the bearing cap, and the rocker is made in halves, as shown, so as to be easily removable, and held
Pig. 80
kFJ
together by bolts ;f,^. The lug /is tapped out to receive a handle, which serves both to shift the rocker and clamp it in any desired position by screwing it down against the seat on which the rocker moves.
SHAFTS
78. Shafts for alternators are designed a<:cording to the same rules as those for direct-current machines. These shafts are usually made larger than the size called for by the power to be transmitted. Stiffness is an essential fea- ture of all armature shafts, and in order to secure this, they are made quite large, considering the actual amount of power that they must transmit. This is necessary, because the shaft must not only support the weight of the armature, but it may also be called on to stand heavy magnetic pulls if the field is not evenly balanced. A shaft suitable for the
Digitized by VjOOQIC
§21
CURRENT APPARATUS
66
100-kilowatt machine is shown in Fig. 37. for a pulley journal, 13 in. X 4 in., and journal, 10 in. x S^ in. The keyway a is spider key. The central portion of the spider fits on is usually made a little large, may be forced into place. The keyway shown at d. All internal corners of the
This is designed
a collector end for the armature
shaft where the so that the spider for the pulley is
shaft should be
Fig. 87
rounded, as shown at c^ c, and oil grooves d, d should be provided to prevent the oil from working its way out of the boxes by creeping along the shaft. In many cases the exciter is driven from a pulley mounted on an extension of the armature shaft. The shaft must then be furnished with a keyway on the extension for the exciter pulley, as shown by the dotted lines.
PULI.EY8
79, Ordinary cast-iron pulleys are usually employed. Broad-faced pulleys are usually provided with two sets of arms, and the pulleys, on the whole, are constructed some- what heavier than those used for general transmission work. Large pulleys should be made in halves, and strongly bolted together both at the hub and rim. The diameter of the pulley is determined by the linear speed at which it is allowable to run the belt. . A fair average value for this belt speed may be taken from 4,000 to 5,000 feet per minute for machines varying in size from 50 to 500 kilo- watts. It is not advisable to run the belt at a speed much higher than 5,500 feet per minute, as the grip between the belt and pulley becomes less with higher speeds. The diam- eter of the pulley in inches is then given by the expression
Digitized by VjOOQIC
56
DESIGN OF ALTERNATING
§21
12 5'
diameter of pulley = ^^ ^^ ,, (17)
^ ^ TT X R. P. M. ^ '
where 5' = belt speed in feet per minute.
Applying this to the 100-kilowatt machine, and taking 4,500 feet per minute as a fair value for the belt speed, we get
diameter of pulley = -— - — ^.-- = 28.6 inches ^ ^ 3.14 X GOO
We will make the diameter of the pulley 28J inches, as shown in Fig. 38. The face of the pulley must be slightly wider than the belt necessary to transmit the given amount of power at the required belt speed. The belt must be of
PlO. 88
such width that the strain on it per unit width will not be more than the belt can safely carry. The amount of power that can be transmitted per unit width of belt depends on the quality and thickness of the belt as well as on the belt speed. Assuming that a double thick belt is used, we may determine the width of belt necessary by means of the fol- lowing formula;
W
width of belt = .7 X
(18)
where W = output of generator in watts.
Digitized by VjOOQIC
§21 CURRENT APPARATUS 57
Applying this to the 100-kilowatt machine, we get
'ji.u r u 1. w 100,000 , ^ ^ . ,
width of belt = .7 X ^ * ^ = 15.5 inches 4,500
We will allow | inch on each side of the belt, thus making the face of the pulley 17 inches wide. Fig. 38 shows a pulley 28^ in. x 17 in. suitable for this machine. The pulley is provided with one set of arms only, as the face is not very wide. Setscrews are provided to prevent the pulley work- ing endwise on the shaft.
COITN'ECTIONS
80. The electrical connections for alternators have already been shown diagrammatically; it is now necessary to see how these are carried out on the machine. We will first consider the connections suitable for a single-phase compound-wound machine of the type designed. Fig. 39 represents the connections of such a machine. T^and T' are the two terminals of the armature winding, one of which is connected to one collector ring by means of the stud a. The other terminal T' is connected to one side of the recti- fier by the stud d, the other side of the rectifier being con- nected to the remaining collector ring. If a revolving shunt is used across the rectifier, it is necessary to have another connection stud, shown by the dotted line. The revolving shunt is then connected between this stud and d, thus placing the shunt across the rectifier and allowing a certain portion of the total current to flow by without being rectified. The line wires lead from the two collector rings, and the rectifier brushes are connected to the series-field by means of the connection boards c, c. The connections between the series-field, armature, rectifier, and collector rings shown in Fig. 39 are those that are used on the General Electric Company's machines of this type. The Westinghouse Company uses a different arrangement for supplying the rectified current to the series-coils, which is
Digitized by VjOOQIC
58
DESIGN OF ALTERNATING
§21
shown in Fig. 40. In this case the terminal T is connected to one end b of the primary a b oi a, small transformer. The other end of this primary connects to the collector ring, as shown, so that all the current flowing through the armature passes through this coil. The secondary ^^ of this transformer connects directly to the two sides of ^he rectifier, which, in turn, connects to the series-field by
-•VNAAAAAA^
Pig. 89
means of the brushes. The other collector ring is con- nected directly to the winding, as shown. In this case it is seen at once that there is no electrical connection between the armature and the series-coils, the latter being supplied by an induced current from the secondary c d. This trans- former, which is usually quite small, must, of course, revolve with the armature, and in some of the smaller
Digitized by VjOOQIC
§21
CURRENT APPARATUS
59
machines the spokes of the spider form the core of the transformer. The use of this transformer renders the insu- lation of the series-coils easier, because it separates the armature connections entirely from the field.
Fio. 40
81, -The connections for the field coils vary little in different makes of machines, so we will take those shown in Fig. 39 as a typical case. The windings of the field coils are connected up so as to make the poles alternately N and S. Care must be taken that the series-coils are not connected in such a way as to oppose the separately excited coils instead of aiding them. The terminals of the separately excited coils are led directly to the connection boards r, c. The terminals of the series-coils are also led to the same boards, and from there connected to the rectifier brush- holder studs by means of flexible cables. The stationary
Digitized by VjOOQIC
60 DESIGN OF ALTERNATING §21
shunt d IS connected to the same terminals on the connec- tion boards as the series-field. This shunt may be attached to the machine or placed on the switchboard ; it is usually made up of German-silver wire or ribbon of such size that it will not overheat with the maximum current it may be called on to carry. The connections and winding of the separately excited coils are generally the same, no matter what the current output or voltage of the machine may be. The series-connections may, however, be varied somewhat in machines with different current outputs. When the cur- rent output is large, the series-coils are sometimes grouped in two sets connected in parallel, thus reducing the cur- rent in the field conductor and allowing the use of smaller and more easily wound wire. For example, the 100-kilo- watt machine designed had a full-load current output of 45.4 amperes at 2,200 volts; if the same machine were built for 1,100 volts, the current output would be 90.8 amperes at fuU load. In the first case the series-field was designed to carry 32 amperes ; in the second case it would have to carry 64 amperes. Generally, we would wish to get the same num- ber of ampere-turns on each pole in either case ; so, instead of winding the coils with half as many turns of wire, large enough to carry double the current, we can connect the six upper coils in series and connect them in parallel with the six lower coils, which are also connected in series. This will keep the current in the coils the same, although the line current is doubled. This is often done in practice, as it allows the coils that were designed for a machine of certain voltage to be used for a machine of half that voltage without changing the coil winding in any way.
83, The line connections are usually made directly to the collector-ring studs when the machine is provided with a revolving armature. When the armature is stationary, the armature terminals are simply run to a connection board, to which the lines are attached. Fig. 41 shows a simple form of connection board, suitable for the connec- tions shown in Fig. 39. The base a should have high
Digitized by VjOOQIC
§21
CURRENT APPARATUS
61
insulating properties, and is preferably made of porcelain, or hardwood treated with oil. Slate is not a good material for this purpose, because it is liable to contain metallic veins. Cable terminals c are provided for the connections, and these are held in place by screws d passing through
PIO. 41
from the back of the base. These screws are well counter, sunk, and the holes filled in with insulating compound, in order to obviate any danger of the connections becoming grounded on the frame of the machine. The nuts e clamp the terminals firmly in place against the brass blocks b,
83. Connections between the individual field coils are usually made by means of small brass connectors similar to those shown in Fig. 42. Three of the commoner forms are here shown. They all consist of two brass plates ^, / pro- vided with grooves to receive the ends of the coils, and clamped together by screws, as shown. The ends of the coils usually consist of heavily insulated wire brought out from the winding. In some cases where the coils are wound with copper strip, connection between the coils is made by simply clamping the ends of the strip together between brass washers.
84. Special reference has not been made to the design of fields for two- and three-phase machines, because there is very little difference between such fields and the one
Digitized by VjOOQIC
62
DESIGN OF ALTERNATING
§21
worked out for the single-phase machine. The only differ- ence might be a slight change in the series-winding and the
jCX
o-
rrr3
■o
(a)
O
■o
(b)
o
^M
o
re J
Pig. 42
connections to the rectifier. The winding of the separately excited coils would be the same, because the exciter voltage would not be changed, and all three fields were assumed to furnish the same magnetic flux.
FlO. 48
. 85. Fig. 43 shows an assembled compound-wound machine with stationary field and revolving armature, such
Digitized by VjOOQIC
§21 CURRENT APPARATUS 63
as we have worked out. The lower half of the yoke is in this case cast with the bed, and the yoke itself is provided with flanges. The col- lector-ring brushes are here shown mounted on a stand a, and the rec- tifier brushes are car- ried on a rocker b mounted on the inside end of the bearing. The* ar rangement of cables, connection boards, etc., will be readily
seen by referring to ^
the figure. Fig. 44 ""^^^ '^"
shows a large alter- ^®' ^
nator designed to run at low speed. This machine is pro- vided' with a stationary armature and revolving field, the collector rings shown on the shaft being used to convey the exciting current into the field coils.
Digitized by VjOOQIC
Digitized by VjOOQIC
DESIGN OF ALTERNATING- CURRENT APPARATUS
(PART 8)
tra:n^sformers
1, It has been shown that a certain amount of loss always occurs in a transformer so long as its primary it; connected to a source of E. M. F. ; this loss may be divided, for convenience, in two parts, namely, iron losses and cop- per losses. The iron losses are those that occur in the iron core of the transformer, and are due to hysteresis and eddy currents. They are practically constant for all loads, because they are dependent on the magnetic density in the core, and this changes but little from no load to full load. The I*R loss, or copper loss, in the coils increases with the load. The combined effect of these losses is to heat up the coils and core, so that the amount of power that a transformer is capable of delivering is limited by the heat- ing effect. The transformer could therefore be loaded until the coils reached the maximum temperature that the insu- lation on the wire could stand without injury; any further increase in load would result in the transformer being eventually burned out. Aside from the danger of over- heating, a transformer should not be worked much beyond Its rated load, because of the falling off in efficiency. If the load is forced too high, the P R loss becomes excessive, and
For notice of copyright, see page immediately following the title page. 4^—9
Digitized by VjOOQIC
2 DESIGN OF ALTERNATING §22
the transformer works uneconomically, even if it does not happen to overheat.
Overloading a transformer also causes a falling off in the secondary voltage, which is very objectionable if the trans- former is used for lighting work.
2. A transformer should be so designed that it will do the work of transforming the current with the least possible cost. This means that the efficiency must not only be high at full load, but that it should also be high throughout a
T^rantfurmer effiei&ncy cwrve.
FlO. 1
wide range of load. Fig. 1 shows the efficiency curve for a transformer of good design. It will be noticed that the efficiency increases very rapidly at first, being as high as 60 per cent, with only one-sixteenth of the full load on the secondary. The efficiency varies but slightly between one- fourth load and full load, and when the transformer is over- loaded, the efficiency begins to fall off. A transformer is seldom worked at its full capacity all the time; hence, it is important to have a good efficiency through a wide range of load, as shown by the curve. The efficiency can be made
Digitized by VjOOQIC
§22 CURRENT APPARATUS 3
high by employing anything that will keep down the losses; but for a transformer of given size, the efficiency cannot be increased beyond a certain point without greatly increasing the weight and cost. For example, the /" R loss might be made very small by using a large cross-section of copper, but this would necessitate a large winding space, thus increasing the bulk of the transformer and making the core heavy. Increasing the efficiency beyond a certain point is attained only by a large increase in cost, and a transformer may, in general, be said to be well designed when it gives the highest all-day efficiency consistent with an economical distribution of iron and copper. The curve. Fig. 2, shows the relation between output and full-load efficiency that should be attainable in good transformers. The efficiency
IMatUm beivnen effieUney and mUfut of trang/ormerB.
FIG 2
increases rapidly with the output for transformers of small size, but changes slowly after outputs of 4 or 5 kilowatts are reached. Some very large transformers have an effi- ciency as high as 98 per cent., or slightly over, but it is only in transformers of large size that such a high efficiency is reached.
Digitized by VjOOQIC
DESIGN OF ALTERNATING 8 22
TRANSFORMER CORES
3. Transformer cores have been made in a large num- ber of different shapes, but the two most generally used types are the core and shell varieties. Good transformers may be designed using either the core or shell construction, and large numbers of both styles are in common use. Great