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INTERNATIONAL LIBRARY OF TECHNOLOGY
A SERIES OF TEXTBOOKS FOR 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
Coi»7rlstat. 1906, by Intb&hational Tkztbook Compant.
Entered at Stationers' Hall. London.
Deslrn of Alteniatinff>Current Apparatus: Copyright. 1906, by iKTBUfATioifAi. Textbook Company. Entered at Stationers' Hall, London.
Electric Transmission: Copyriffbt, 1906. by iNTBRNATioirAL Textbook Company. Entered at Stationers' Hall. London.
Line Corjtraction: Copyriffbt. 1906. by Intern ation a i. Textbook Company. Entered at Stationers' Hall, London.
Switchboards and Switchboard Apidlaoces: Copyright. 1906. by Internationai. Textbook Company. Entered at Stationers' Hall. London.
Power Transformation and Measurement: Copyright. 1906, by Internationai. Textbook Company. Entered at Stationers' Hall, London.
All rlirhts reserved.
Printed in the United States.
//ISB
UURR PRINTING HOUSE,
FRANKFORT AND JACOB STREETS,
NEW YORK. 113
, t » ^«^'
1
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 complete in itself, and no textbooks are required other than those pre- pared 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 con- genial 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.
• • •
ni
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, illustrations 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
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
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 /' 7? 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
Armattu-e 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
... Ill
iv CONTENTS
Design of Alternating-Current Appa- ratus— Cmiiinued Section Page
Density in Air Gap 20 48
Design 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
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 aiid 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 2?
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
vi CONTENTS
Dbsign 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 66
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
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
Switchboards 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
T^ransformers 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
viii CONTENTS
Power Transformation and Mkasurkment
Continued Section Page
m
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 86
DESIGN OF ALTERNATING CURRENT APPARATUS
■ (PART 1)
ALTERXATOBS
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.
2m 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 n<»tice of copyright, see page immediately following the title page. 46—2
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.
LIMITATION 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 reiirulatioiu 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 fuU^
§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 excited 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-ewtipg circuit- However, while armature reaction
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.
HKATIXG OV ALTERNATOR ARMATTJRB8
7. The final temperature that an armature attains when carrying its normal load depends not only on the actual amount of energy wasted in 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,
§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 continuouisly, 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.
6 DESIGN OF ALTERNATING § aO
RELATION BETWEEN I'B L088 AND OinTUT
10. The I' R loss 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 tlierefore 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
Ciinw »hawlpn If latfon A«(«eM» armatun t*a laf* output ofalttrttalor,
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- sect ion. 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
§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 /* R 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 I* R loss, and consequently the no-load rise in tem- perature may be considerable.
HTSTERESIS liOSS
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.
8 DESIGN OF ALTERNATING §20
ExAUPLB. — The armature core of an alternator having 12 poles and running at a speed of SIHI revolutions per minute is worked at a maxi- mum magnetic density of 20.tKM) lines |)er square inch. If the volume of the core is 2.000 cubic inches, how many watts will be wasted in hysteresis 7
Xagnmlte aatulty B llintt per a» <»«A) FiO. a
Solution. — If the machine runs at SOO rev. per min. and has 12 poles, the frequency of the magnetic cycles in the armature core must be V X Vo""' <"' ^ cycles per siecond.
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
.22 X 2.000 X 60 iVu = ——-icni — = ^^^ watU. Ans.
§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.
KDDY-CUBBENT liOSS
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
10 DESIGN OP 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.
BADIATENG 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.tJ to 2.S watts per square inch of cylindrical
§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 7^7^ loss, so that we will, in general, be safe in assuming that an allowance of from .8 to 1.4 watt I"" 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.
ARMATITRK 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
12
DESIGN OF 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 iV, 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 wijl 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 sho^n 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
Pig. 4
§20
CURRENT APPARATUS
13
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.
PIO. 5
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. G; that is, the current
Fio. e
..J
U 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 tidvantage of in designing armature windings, namely, the crowding together of the lines to one side of the pole piece.
§20 CURRENT APPARATUS 16
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.
ARMATUBB SBIiF-rNTDUCTION
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 field.s.
32. 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 gurrent is passed through the coil, a magnetic field is set up
16 DESIGN OF ALTERNATING g 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
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 mure coils placed in separate slots, thus reducing the number of conductors per slot. For exam- ple, suppose an armature has G 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 C X .OS = 13 henry.
§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 (a), 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 (^), so that the air gap ab \% made much shorter. Also, the slots being deep and narrow compared with (cl)^ 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 teeth are not used as ii^uch in alternators as in direct-current machines, and the tendency of making the slots narrower and deeper is, on the whole,
18 DESIGN OF 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 low 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.
SJ4. 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 (tf), 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.
36, On account of the number of variable quantities 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
§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 _ .
"lo^""^
where ^ 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,
J 6,400 X 40 ^^^^^ ,
L = * = .00266 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 2 tt « A, where n is the frequency. The voltage necessary to overcome the reactance is ''l-nnL /, where / is the current in the armature.
* •*
Electric Generators," Parshall and Hobart.
20 . DESIGN OP ALTERNATING §26
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.
36. 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 AliTERNATOR
ARMATURES
27. 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 thd
§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 16,000.
28. The frequency of a great many modern machines IS lower than that formerly used, 60 or 25 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 3,500 to 5,500 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 WINDENGS
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 wHh 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
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; (b) 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.
SINGLE-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
where T = number of turns connected in series on the
armature ; ^ = total magnetic flux from one pole; n = frequency;
^ = 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.
§20
CURRENT APPARATUS
23
SINGLE-PHASE DISTRIBUTED WINDINGS
32. 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
4.44 ^ 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
Pig. 8
distribute these slots equally around the armature. We will put half as many conductors as before in each slot, so that
24
DESIGN OP 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
A
Pio. 9
^ 4.44 ^ r«
(1)
since there are \ the total turns T active in each set. The resultant E. M. F. Ot: will therefore be
£ = liM,^^XiX*/a = iilS^X.707
10'
10'
(2)
That is, the E. M. F, that is obtained at tw load from a twO'Coil single-phase winding is .101 times that which would have been obtained with the same total number of turns grouped into a uni-coil ivinding. 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 kuid, it may give as
§20
CURRENT APPARATUS
26
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
b
"> —
E^±i±±^^jae7
■^d
10'
PlO. 10
consist of — turns, and the three E. M. F.'s Oa^ O b^ and Oc
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
(3)
and the resiiltant E. M. F. O d. Fig. 10, will be
_ 4.44 * Tn , 4.44 * T>i ^^,„ ...
£ = ---j(,.- -X i = - — jQ, X.6G7 (4)
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
E = iil^ X .636 (5)
34. The more the winding is spread out, the greater the number of crossings of the coils at the ends of the armature, making such windings difficult to insulate for high voltages. Such windings, therefore, have the disadvantage of being nlore expensive to construct and insulate, in addition to giving a lower E. M. F. at no load for a given length of active conductor. They have the advantage of giving better regulation or small drop in voltage when loaded, and also give a smooth E. M. F. curve. Also, the heating is more uniformly distributed than when a concentrated winding is used. For single-phase armatures in general, we may then write the E. M. F. equation as follows:
^ 4.44 4^ Tn , ,^.
E = jQi X k (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 = ] ; for a two-coil winding, ^ = .707; for a three-coil winding, /(• = .007; for a uniformly distributed winding, yt = .030.
§ 20 CURRENT APPARATUS 27
POLYPHASE ARMATURE WINDINGS
35. Concentrated, or uni-coil, polyphase windings have aheady 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°/
o
Let the total number of tufns 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
At
sets will be
n — n ^ ^'^^ ^ ^^
The.se two E M. F*s will be related as shown in Fig. 11, and the resultant E. M. F. will be
^ 4.44 ^ Tn , ^ ,^o
£ = 77;^ X i X 2 cos 15°
10
4 44 0 7^ ;/ = — lJ--X.965 (7)
Hence, tAe voltage venerated per phase by a two-coil three- phase winding is .965 times that which would be generated by a single-coil winding. In other words, the splitting up of the winding has resulted in a voltage reduction of but
28 DESIGN OP ALTERNATING §20
3^ 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
windings we may then summarize the following: The E. M. F. generated per phase in a polyphase armature is given by the expression
c- 4.44 ^ Tn , ,Q,
£ = 10^ X k (8)
where T = number of turns connected in series per pAase ; 0 = 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, >(' = 1 ; for a two-phase winding uniformly distrib- uted, k = .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, I' = .965.
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 difTerent phases together.
"-^.
FlO. 18
§20 CURRENT APPARATUS 29
ARRAJf GEMSNT 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 b' is then connected to r', 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 tlie 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 1\ from there we pass to the corresponding slot under the next pole, that is, slot 7,
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 ;/. From n a connection is made to ;;/, and from /// 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 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.
§20 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
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, 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.
42, Fig. 15 shows an- other style of disk and slot in common use. This disk is provided with 10 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.
§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 hold the conductors down in place, giving a construc- tion very similar to that commonly employed for direct-current armatures.
44. Stationary arma- fio. i6
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
Fig. 17
supporting iron framework A. As the core is built up, the joints between the different segments are staggered, or the
34 DESIGN OF ALTERNATING §20
segments are overlappeil, 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 oi 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 //
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.
46. 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 p^g- w
assembling disks to make up a core, it is usual to place a heavy sheet of paper about every ^ inch or J inch of core, in order to make sure that the path for eddy currents will be effectually broken up.
ARMATURE 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,
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. 30 shows two views of a spider and core suitable for disks of moderate size punched in one piece. The spider proper consists of a
hub ft provided with four radial arms ^ 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 </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.
Pig, -iO shows the construction used with armatures hav- ing a ?n)n)I number of heavy armature coils. In such cases
DESIGN OF ALTERNATING
§2
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. 23 shows a spider suitable for a ''■"■ ^ large armature built up with
segments like those shown in Fig. 19. This style of spider
Pic. xa ts common for machines with large diameter of armature
§ao
CURRENT APPARATUS
31
running at low speeds. The rim r of the spider is made nun-continuous, in order to avoid strains in casting as much as possible.
47. When the ariiiature 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
•s
E
m
Pig. 28
stationary armature frame for a machine of large diameter. The frame casting is usually made in two pieces A and B, the lower half being provided with projections a, a, by which the spider is bolted to ihe bed or foundation. The seg- mental core stampings (/, d are held in place by the dovetail grooves f, c. These segments are clamped between the end rings e^ e by means of the bolts /. The end rings e are shown made up in segments on account of their large diameter.
88 DESIGN OF ALTERNATING §20
ARMATURE C'ONI>UCrrOR8
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. Y A section of a conductor made
; If up of two bare wires in mul ^"''^ tiple is shown at (^i), and four
W (b) (e) (d) bare wires at (d), the con-
^'°- ^ ductors being in each case cov-
ered by the cotton wrapping i. 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 ^^y 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 {(/) shows a cross-section of an armature-winding bar. The dimension // is usually considerably greater than by 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.
SAO
CURRENT APPARATtJS
FORMS OF ARMATURE COILS AND BARS
61. 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 large 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 --.i.. «.
are shown in Fig. 35 (a) and {6). The straight portion cc and rf</ 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 («), while in others the ends shown at {d) are used.
fi!£. 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 * is- bent down into a different plane ^'o- * 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. 13, 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
^^*s*a^^^'
,1111111111^11111
40 DESIGN OP ALTERNATING §20
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 flanges, especially if the armature revolves. The side a a lies in a lower plane than bb^ 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^ 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. b form the connections to the other bar. Fig. 29 shows one element or turn of such a winding. The part c 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 "a" ^
point a^ the element being ^*®- *
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.
8 80 CURRENT APPARATUS 41
Fig. 3"i shows a portion of the bar winding on the station- ary armature of one of the large 5,nW-kilowatt al- ternators of the Manhattan Eleva- ted Railway, New York. In this case there are three bars in each slot, the bars being first ^'°- ">
insulated separately and then bound together. The figure shows the arrangement of the end connections in two
different planes, so that they can pass each other with a good clearance. This armature has a distributed winding
with 4 slots or 12 conductors per pole per phase. The armature is wound for three phases and delivers current at 11,000 volts.
DESIGN OF ALTERNATING
ARMATURK IXSULATION (C01L8)
56. Alternator armatures are generally called on to generate much higher voltages than are common with cimtiiiuous-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.
67. 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 sectron of a coil consisting of 33 turns. Between each layer of wire is a layer of
§20 CURRENT APPARATUS 43
insulation / 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.
ARMATUIIE INSULATION (SLOTS)
58, 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
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 c, consisting of several turns of copper wire or ribbon, is wound in place after the slot insulation has
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- 1 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
§30 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 et, 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, pw*. w
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 no. tt holding the bars in place. A wooden strip is
usually inserted between the band wires and bars in order
to protect the winding.
62. The present practice in armature construction, espe- cially for high pressures, is to place the insulation 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.
46 DESIGN OF ALTERNATING §20
63* In using two-layer windings, care should be taken
b 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 {/, in order to make sure of good insulation between the coils and core. The spider flanges should also be thor- oughly insulated with paper and mica c Fio. 88 wherever there is any possibility of the
current jumping from the coils to the spider.
MAGNETIC DENSITIES
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
§iM) 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 twice 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 ARMATURE CORE
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 rf, 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 rototion, 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.
48 DESIGN OF ALTERNATING §20
BSNSmr IK 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.
DESIGN OF ALTERNATING- CURRENT APPARATUS
(PART 2)
DESIGN OF 100-KELOWATT SINGLE- PHASE ALTERNATOR
1. 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 = /:, voltage at full load = 2,200 = E\ (5) allowable rise in temperature, 40° C. ; (6) general type of machine, belt-driven, revolving arma- ture, stationary field.
§ 21
For Dotico of copyright, ••• p«ff« imm«di«t«ly following Uio tlUo pftgt« 45—6
2 DESIGN OF ALTERNATING §21
2. 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
^^ P 600
/ = 12
and the machine must be provided with twelve poles to give the required frequency at a speed of 600 R. 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.
§21 CURRENT APPARATUS 3
DIMENSIONS OF CONDUCTOR ANT) CORE
4. The current output at full load will be
^_ watts __ kilowatts X 1,000 .^.
■" fuTr-foadTolFage "" T' ^^
100X1,000 = 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.
6. The cross-section of the conductor that is used on the atmature 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.
4 DESIGN OF ALTERNATING §21
Then,
A = Im (2)
In this case / = 46.4 and m = 550. Therefore, the cross- section of the conductor will be
46.4 X 650 = 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 j^ 26,180 circular mils. Two bare No. 9 wires 'J 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 .^.
^- rTrmT^"^^ ^"^^
6,000 X 12 oi Q • u
= --^-r.7. = 31.8 mches
600 X ^
We will therefore adopt 31| 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
§21 CURRENT APPARATUS 5
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 1.96 per cent, of the output. The approximate I* R 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 siich 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 PR loss. This means that we should have
about —-2 square inch of cylindrical surface for each watt
I^ R loss. This would call for a surface of 1,960 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 13.05 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 annature overheating.
10, We have now determined the a[)|)r()xiniate dimen- sions of the armature core, and are in a position to calculate
6 DESIGN OF ALTERNATING §21
the magnetic flux ^ 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 4> 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
W
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. E generated at no load are now known, the number of turns T necessary to generate the voltage E 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 <P Tn
£ =
10'
or T = -j-r- :^ (6)
4.44 X ^ X « ^ '
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
^ 2,000X100,000,000 4.44 X 2,328,000 X 60
13. From the above, it is seen that we must place as nearly 322 turns on the armature as possible. There are
§ai CURRENT APPARATUS 7
twelve slots, or six coils; hence, there would be H*
= 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 66 turns per coil and 66 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 .343 + 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^i 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 akercd in any way to
8 DESIGN OF 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 ^, so that the voltage generated by the 336 conductors
will be 2,000 volts. The final length of armature may be
obtained as follows:
E V 10' We have <P = , ^^ ^\ (6)
4.44 X / w ^ '
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
<P
2,235,000 ^^ . . , ,„.
= ^A Lc^ = 55- 8 square mches (7)
air-gap density 40,000
and the length of the pole and armature core parallel to the shaft will be
f^^ = ^ = 13.42 inches (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 13jV inches as the final value for the length of the core parallel to the shaft (see 4, 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.
§21
CURRENT ArPARATUS
9
Fig. 3 shows a cross-section of the core, and Fig. 4 shows a portion of the armature iVenUUiUHg duet
lying between two pole * \ *• ^^t'
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 number flowing from the pole
Pig. 8
Fig. 4
piece. Hence, the flux through the armature core is \ ^. The area of cross-section of iron required will then be
A.^ \
l^
B.
(9)
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
. , 2,235,000 ^„ ^^ . .
^c = t X * * = 37.26 square mches
This is the area of cross-section of iron, and it is equal to
the radial depth of the core under the slots {ad, 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 13j\ 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 1^ inches,
leaving 13^\ — 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 ' ' = 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 2|i inches; hence, the total radial depth of the disk is 2|| + 3^'^ = 5} inches, and the inside diameter is 31| — 2 X 5| = 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 ARIVIATURE LOSSES
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 /^ K loss.
§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
-xs
I
-H
--:
Pig. 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
i^ot) R = }^^ = .cm oi.m
We will take the resistance as .7 ohm, in order to make some allowance for the resistance of the connections between the coils
U DESIGN OF ALTERNATING §21
19, The full-load current is 45.4 amperes; hence, the PR loss al full load wi'i 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 P R loss exceed- ing the allowable amount. The outer cylindrical surface of the armature as obtained from the final dimensions is ?r x 31 J X 13y\ = 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.
80, 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 |?r (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 IH 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
„, 5,020 X .42 X 60
^" = 100 = ^'^^^
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 /* 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 w.ould be somewhat less than this on account of the bearing friction, brush friction, e^c.
§21 CURRENT APPARATUS
ARMATUKi: WINDING FOR TWO-PHASE ALTEENATOB
S3. 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 3,000 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 90° 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
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.
83. 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 numtier 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 Pia. s
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
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 ^ so that E will remain the same. The slot may then be arranged as shown in Fig. 6. Allowing 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 single- phase armature, we will allow only I 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 /'i^ loss in this 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 wire 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
Slots.
Fig. 7
§21 CURRENT APPARATUS 15
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 WINDEN^G FOR THREE-PHASE
AliTERNATOR
36, 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 thtf 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 = |/3 EI
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,
J 100,000 or / = — — -_ = 26.2 amperes
2,200 4/3
26, If the line current at full load is 26.2 amperes, the full-load current in the armature conductors must also be
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.
37. The line voltage at no load is to be 2,000; conse- quently, the voltage generated in each phase will be *
= 1,154 volts, because the armature is Y connected. We
have
r^ 4.44 ^ Tn ,
where E is the voltage at no load generated in each phase. In this case, the constant ^ is 1, because we are using a con- centrated winding, there being only one slot for each pole and phase. 7" is the number of turns in each phase. The magnetic fiux ^ will be considered the same as before, because the dimensions of the pole pieces and armature have not been altered. We then have
y, ^ Ex 10"
4.44 X ^ X n
^ 1,154X10" ^_. ,
^^ ^ = 4.44x2,235,0()0-^r6Q = ^^^ '"^"^' ""^^'^^
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
I 31 CURRENT APPARATUS 17
armature will therefore be provided with 18 coils, each con- sisting of 33 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.
38. 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 ^'"-^
+ a X .04 + 2 X .06 = .79 inch.' The depth of the slot will be 8 X .14 + 2 X .04 + 2 X OC = 1.33 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
nensions shown in Fig. 8.
le other dimensions of the
I disk remain the same as for
j those previously calculated.
39, The /' J? 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 /' /i 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
18 DESIGN OF ALTERNATING §21
altered. There might possibly be a slight increase in the length, owing 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
— ^-=.628 ohm
The current in each phase at full load is 26.2 amperes. Hence the PR 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 this 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 — i=r.
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 ha,ve aU been made on the supposition that
§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.
COMPLETED ARMATURES
33. 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 ic: fare the ventilating ducts for allowing a circulation of air through the core. The cast-brass shields s
are supported from the armature spider, and are used to protect the projecting ends of the calls. The armature is shown complete with the collector rings rand the rectifier f. 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
20 DESIGN OF ALTERNATING § 21
on the spider flanges and are held down by the bands a. The disks are carried by the spider i and are clamped up Tjy the end plates e. The copper bars ti, efare the connections between the winding and the collector rings. It will be
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 FIELD 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. 1"^), provided with a number of poles i/projectiiLg 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 6 is very often cast with the base of the machine, . especially in machine-s of moderate size. In larger machines
§21
CURRENT APPARATUS
21
the lower half is cast separately and provided with projec- tions c^ Cy by means of which it is bolted to the bed. The halves are held together by means of the bolts e. Some
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.
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 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
PlO. 18
82 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
\_ c,"- ^ a ^ Qygj. 30 QOQ ^)p 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-tron pole pieces have come largely into use, and are employed on nearly all modern 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 c, 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 ic is generally small compared with the length of the armature, except perhaps on large slow-speed machines.
§21
CURRENT APPARATUS
23
(a)
36. 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 ^'^- ^^
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 (^) 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 revolvlngr 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
24
DESIGN OP ALTERNATING
§21
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 b, 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
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.
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 b, held together by bolts/,
§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.
FIBLB-MAGXET COFLS
38. Fleld-magrnet colls may be wound on spools con- structed similar to those used for the field coils for continuous-
j'-
.." )' ? II
■ —■ >i
— «*•
3e=
"--•U »•-
ji.
J I.
PIG. 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^ 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 provided, as at a. Fig. 20.
Pig. 20
36 DESIGN OF ALTERNATING 1 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- Held alternators are wound with fiat copper strip bent on
&
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
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
§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 FFEU) 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 by 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.
28 DESIGN OF ALTERNATING §21
DESIGN OF FFEIiD
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 cen 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 machine perform well electrically.
§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 31f -f | = 32J inches. The poles cover 50 per cent, of the armature, and the length of the arc will be
w X bore of poles X .5
number of poles
(10)
or
JT X 32.125 X .5 , ^ . ,
arc = — = 4.2 mches
12
The distance between the sides of the pole will be about 44 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, 13tV inches.
Pig. 24
46. 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 J 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 g inch, as shown in Fig. 24, for
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 32J + 1^> + J = 48J inches.
MAGNETIC' FLUX THROUGH POLE PIECES AJn> TOKE
46. The magrnetic flux that passes through the arma- ture from one pole piece is 0. 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 0 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
P __ flux through core /ii\
■^ cross-section ^ *
3,120,000 ^. ,^^,. . ,
= jj jr.-f- = 50,400 hnes per square mch
It will be noticed that this density is well below that point at which wrought iron begins to saturate, so that
§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,
^' 3,129,000 • -2- = -^-y = 1,564,500
and the required cross-section of the yoke will be
A = flux through yoke _ i ^' /^ox
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 as the allowable value of B„, thus giving for the required cross-section
A = h^ = 52.1 square inches, 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 ^...^Tzr^^^^^mmm. 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 oil 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..
Pig. 26
32
DESIGN OF ALTERNATING
§21
CALCULATION OF FIELD AMPERE-TUBN8
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-c-d-e-/^
Fig. ae
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
§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 cd-^-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 .376 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 atid Dy?iamo Design, Part 2, we fine 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.
52, 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
4^—7
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.
63« 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.
64. 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 + 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.
CALCULATION OF SKl'ARATKLY 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
§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 ^j 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^y 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 ^ represent the E. M. F. generated by the c.xiter, and e the drop in the rheostat. The pressure applied to one pair of coils will then be
t
2
where/ = number of poles;
2 ^E-e) or — ^^ —
P The current in the field will be
2(/:-r)
E. M. F. p ,^o\
%•=:—.- — = ^- (U)
resistance A
where R is the resistance of a pair of spools,
36 DESIGN OF ALTERNATING §21
But the hot resistance R oi a, pair of spools may be expressed as follows :
ye=Z«><_Z (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
. ^ 2 jE- e) m
/ X /» X r ^ '
and fft = — a / L- ^ —
_ pXl^XtT - 2{E-e)
(16)
The values ©f the quantities T and i 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.
§21 CURRENT APPARATUS 87
57. 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
. , ., 12X41X6,700 ,.^^, circular mils = ^^r^- — = 14,022
The nearest size to this is No. 9 B. & S. having a cross- section of 13,090 circular mils. We 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 WW = 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,
J. 1 . (T
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 126 mils, and if the coil is wound in eight layers, the depth of winding will be
38 DESIGN OP 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
I 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-
1 pleted layer, since the
difference is so small.
The length of winding
space occupied by the
coil will be 30 x .136
= 3. 78 inches, or, say,
SI 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 3J 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.
COMPOirVD, OR SERIES-FIELD, '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 over com pounded. 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.
§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.
63, 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
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 f jj Jf x 6,700 = 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 2,500 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 'H^ = 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
§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 ^ + ^ + ^ = '((^\ 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 7yV + 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 8 J 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 49f 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.
42 DESIGN OF ALTERNATING §21
liOSS IN FIEIiD COIJLS
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
jp 12 X 240 X 41 . , , .
A, = TTTuin ~ ^ 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 , ,^ u
The PR 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
§21 CURRENT APPARATUS 43
alternators of good design. For a 100-kilowatt machine the
Jt«IaMo» btltt«»n /itld i'r lost and output of mtttrmtittr.
Pig. «8
average loss is about 1.7 per cent., which is slightly lower than that for the machine just calculated.
MECHANICAL CON8TEUCTION
FIEU) FRAME AND BED
70. Fig. 39 shows the field ftn,me, 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 with 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
§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 | inch or | inch up to 1^- 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 sit a a to receive the rockef-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.
COLLECTOR 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 riniors in combination with a i-ectifter. 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
46
DESIGN OF ALTERNATING
§^1
§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 ^, 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 A, 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.
48 DESIGN OF ALTERNATING §21
72. 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 800 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.
§21
CURRENT APPARATUS
49
o
45—8
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 IJ 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 c, while mica insulation is provided between the segments a and b, 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 ^, 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 AJ^D BRUSH HOLDERS
74. Copper bruslies 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 Fig. 82 brushes for each col-
lector ring, though this is hardly as essential as with
§21 CURRENT APPARATUS • 61
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 S. 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.
BRUSH-HOLDER STtTDS
75. Brusli-liolder studs follow the same general design as those used for continuous-current 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
52
DESIGN OF ALTERNATING
§21
washer h. The stud is insulated from the rocker-arm by a heavy hard-rubber bushing / and washers b. The bushing / is let into the washers b^ as shown, in order to break up the path by which the current tends to
Pig. 88
jump from the stud to the supporting casting. The sharp corners of the casting should also be removed, as shown at ;//. 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
Fig. 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 meiins of the cap c and the cap bolts d. Connection is
§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 .^ tf' 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. F'g- »
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 rectifier 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 C(f. 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^.
77. Fig. 30 shows a rocker-arm suitable for the single- phase machine designed. The arms a, b are 150° apart, and
54
DESIGN OF ALTERNATING
§21
carry the rectifier studs, the arms r, d for the collector-ring studs being carried on the same rocker. The hub e 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. 86
V-i*^
together by bolts g^ g. 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 according 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
§21
CURRENT APPARATUS
55
100-kilowatt machine is shown in Fig. 37. This is designed for a pulley journal, 13 in. X 4 in., and a collector end journal, 10 in. x 3| in. The keyway a is for the armature spider key. The central portion of the shaft where the spider fits on is usually made a little large, so that the spider may be forced into place. The keyway for the pulley is shown at b. All internal corners of the shaft should be
Fig. 87
rounded, as shown at r, r, and oil grooves //, 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.
PULLEYS
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
50
DESIGN OF ALTERNATING
§21
diameter of pulley =
12 5'
TT X R. P. M.
(17)
where 5' = belt speed in feet per minute.
Applying this to the lOO-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 600
We will make the diameter of the pulley 28 J 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
^
17
ff
—J
PlO. 38
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. Assumine^ that a double thick belt is used, we may determine the width of belt necessary by means of the fol- lowing formula;
width of belt
W .7X^ (18)
where W = output of generator in watts.
§21 CURRENT APPARATUS 57
Applying this to the 100-kilowatt machine, we get
•j.i- rt_ 1. IV 100,000 ^^ ^ . ,
width of belt = .7 X , * ^ = 15.5 inches
4,o00
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.
CONISTBCTIONS
80r 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 b^ 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 b^ 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 r, 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
68
DESIGN OF ALTERNATING
§21
shown in Fig. 40. In this case the terminal T is connected to one end b of the primary ab oi 2^ 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 cd oi this transformer connects directly to the two sides of the rectifier, which, in turn, connects to the series-field by
-»^AAAAAA/V*
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
§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 fiiild.
FlO. 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 c^ 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
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 full 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. Tl"ys 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.
82. 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
§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
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 iiy means of small brass connectors similar to those shown in Fig. 43. Three of the commoner forms are here shown. They all consist of two brass plates e^f 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
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
'I III
' ' ' ' '
iT
Tl
<h)
FIG. 48
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.
Pig. 43
86. Fig. 43 shows an assembled com pound- wound machine with stationary field and revolving armature, such
1 81 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 l> 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.
DESIGN OF ALTERNATING CURRENT APPARATUS
(PART 3)
TKANSFORMEKS
!• It has been shown that a certain amount of loss always occurs in a transformer so long as its primary ir> 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
§ 22
For notice of copyright, sec page immediately following the title page. 45—9
8 DESIGN OF ALTERNATING §23
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.
3. 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
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
gas 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 I* 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
BUatt^n NfHMB */ftcUitrv anA owtpMt o/ (mw/vrawra.
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 r«acbed-
DESIGN OF ALTERNATING §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 care should be taken in the selection of the iron for transformer cores. It should be borne in mind that the hysteresis loss goes on continuously, whether the trans- former is loaded or not, and that everything possible should be done to keep this loss small by using only the best qual- ity of core iron. The stampings should be about 13 or 14 mils thick for l*45-cycle transformers, but may be slightly thicker than this for transformers of low frequency. The oxide on the iron, with the addition of a paper sheet at intervals along the core, is usually sufficient to insulate the sheets from each other. Some makers coat the plates with an insulating varnish or japan and do not depend on the oxide film.
HEATING OF TRANSFORMERS
4. Since the efficiency of transformers' is generally high, the energy lost in them is small, and in transformers of ordinary size there is generally enough radiating surface to get rid of the heat generated. Transformers up to 5()-kilowatt capacity can usually be made with sufficient ventilation to get rid of the heat generated, but for larger sizes it is often necessary to use special cooling arrange- ments. Air blasts are frequently used to carry the heat away from the core and windings of large transformers. Sometimes the core and windings are immersed in oil kept cool by water circulating in pipes. Transformers of smaller size are usually designed so that the case may be filled with oil. This helps to give the windings good insulation, and keeps down the temperature by conducting the heat from the windings and core to the outside casing. The student
§22 CURRENT APPARATUS 6
should bear in mind that while these special devices are in many cases necessary to get rid of the heat, it does not follow by any means that the transformer is inefficient; on the contrary, the efficiency is usually very high, and these devices are necessary only because the transformer of itself does not present enough radiating surface to get rid of the heat. No definite rules can be given as regards the number of watts that can be radiated per square inch of core or case surface that will apply to all types of transformers. This radiation constant varies widely for transformers of different sizes and forms, but unless the efficiency is very low, the dimensions of transformers under 40 or 50 kilowatts are usually such that they can get rid of the heat generated without undue rise in temperature.
MAGNETIC DENSITY IN CORE
6. Transformer cores are worked at low magnetic densi- ties in order to keep down the core losses and magnetizing current. The hysteresis loss is proportional to the frequency, and the eddy-current loss to the square of the frequency; hence, for an allowable amount of core loss it follows that higher magnetic densities can be used with low-frequency than with high-frequency transformers. For 60-cycle trans- formers, the maximum value of the magnetic density may be taken from 28,000 to 32,000 lines per square inch. For 125-cycle transformers, the density may be from 19,000 to 21,000 lines per square inch. The densities in individual cases may vary from the above, but the average values used are generally within the limits given.
6. The allowance of copper per ampere in the primary and secondary coils should be large, in order to keep down the copper loss and prevent overheating. The coils are usually heavy, and it is also important to have a liberal cross-section of copper, in order to prevent overheating. The cross-section per ampere should be about the same both for primary and secondar-y coils. When the core type is
6 DESIGN OF ALTERNATING §22
used, there is usually room for a liberal cross-section of copper, but in the shell type the winding space is more restricted, and the coils cannot be made very large without considerably increasing the bulk of the iron core. The number of circular mils allowed per ampere varies greatly in transformers of different makes and sizes. In general, the allowance should not be less than 1,000 or 1,200 circular mils per ampere, and in many of the later types of trans- formers the allowance may be as high as 2,000, or over.
ABRANGEMENT OF COIL£l ANI> CORE
7. The arrangrement of colls and core has already been described for two of the common types. The core type can be usually arranged so that it can be taken apart and the coils slipped off in case repairs are necessary, while the shell construction usually requires the removal of each plate before the coils can be reached. Transformers have been made with the core built in sections, as shown in Fig. 3. In this case the upper part a is built up separately, and forms a cover that can be removed from the main part of the core when it is desired to get at the coils. This construction is, however, objectionable, because it introduces small air gaps into the magnetic circuit at b^ b, thereby increasing the mag- netic reluctance. In designing transformer cores, every effort should be made to have the magnetic circuit continuous. Fig. 4 shows an arrangement of coils and core suitable for a transformer of large size. The stampings a and b are cut as shown, the joints being at c, d^ and e. As the core is piled up, these joints are staggered, as shown by the dotted lines, thus making the iron path for the lines practically continuous and doing away largely with the bad effects of the joints. The primary and secondary coils are wound in a number of sections, each consisting of a flat coil, these sections being sandwiched, as shown, in order to reduce the magnetic leakage between them. Splitting up the coils in this way also makes it easier to insulate the transformer for
833
CURRENT APPARATUS
high voltages, because it cuts down the voltage across any one of the coils. The coils are usually separated from each
other by a built-up sheet of mica, or other material having high insulating properties. Large cores are frequently
DESIGN OF ALTERNATING
i ZZ
provided with ventilating ducts between the laminations, as shown at/". The laminations are liekl apart l)y brass cast- ings, and the channels so formed allow air to circulate through the core, the whole construction being similar to that used for ventilated armature cores. Fig. 5 shows another arrangement of coils and core that also makes use of thin flat coils. In this case the stampings a and ^ sur- round one side of the coil only, a separate set of stampings
being used to form the magnetic circuit around the other side. This is the construction used by the Westinghouse Company for several of their larger transformers. The projecting ends of the coils c are frequently spread out like a fan, so as to allow air to circulate freely between them.
WINIHNO AND IXSUI.ATION OF COILS
8. Since transformer coils are usually of simple shape, they can generally be iatlie-wound and thoroughly insulated. High insulation is of great importance in transformers, and every precaution should be taken to see that the primary and secondary coils are not only well insulated from the core, but also from one another. Fig. (i shows the shape of a primary coil commonly used for shell transformers. The coils mnst withstand a high impressed line E. M. F., and the voltage between layers may therefore be considerable
§22
CURRENT APPARATUS
9
Insulation /should be placed between each layer; this may be composed of. oiled linen tape or other good insulating material. The outside of the coil is heavily taped and after- wards treated with insulating varnish and baked. Addi- tional insulation in the form of mica and paper, or in some cases oiled hard-wood pieces, is placed between the coils and the core. The insulation between primary and secondary should be specially good. Some makers allow a clear air space between the coils, in addition to the insulation on
d, 1 1 1 1 I 1 1 1 1 1 1 1 1 1 1 1 1 1 1
PlO. 6
the coils themselves. If connection should be established between the primary and secondary, and there should hap- pen to be a ground on the primary mains, a difference of potential would exist between the secondary service wires and the ground that would be equal to the primary voltage. Such a difference of potential between the service wires and the ground would be very dangerous to life; hence, the importance of thorough insulation between the primary and secondary.
9. The conductor used for the primary winding usually consists of copper wire, except in large transformers, where copper strip may be used to advantage. For the secondary, a conductor of large cross-section is usually required, because the secondary voltage is generally low and the cur- rent correspondingly large. For transformers of moderate output, the secondary conductor can generally be made of a number of wires in multiple. In most large transformers, the secondary conductor is made up of copper strip. Fig. 7 shows a flat secondary coil made up in this way. Such a
10
DESIGN OF ALTERNATING
§2-^
coil would be suitable for the transformer shown in Fig. 5. The details of construction and method of calculating the
r
FlO. 7
different parts will be best understood by working out an example. We will therefore take up the design of a trans- former of the core type such as would be suitable for lighting work.
DESIGN OF 8-KIIiOWATT TRANSFORMER
10. In starting out to design a transformer, the follow- ing quantities are either known or assumed : Useful second- ary output in kilowatts (K. W.) ; primary voltage {£p); secondary voltage (E^) ; frequency of system on which the transformer is to be operated («). For ordinary lighting transformers, Ep is in the neighborhood of 1,000 or 2,000 volts; ii,, 50 or 100 volts; and «, 60 or 125 cycles per second.
11. We will take for an example an 8-kilowatt trans- former of the core type to be designed for 2,000 volts pri- mary and 50 or 100 volts secondary, the secondary being wound in two coils, which may be connected in parallel for 50 volts or in series for 100 volts. The frequency will be taken as 60. A good transformer of this output should have a full-load efficiency of 90.8 or 96.9 percent.; conse- quently, in designing it we should aim to keep the losses
§22 CURRENT APPARATUS 11
down to such an amount that the efficiency will be, say, 96.8 per cent. We have
^ . watts output
efficiency = ; — ~ —
' watts input
Hence, for an output of 8,000 watts, the input will be
The total loss at full load, therefore, should not exceed 264 watts. This total loss is made up of three parts, namely, the losses due to the resistance of the coils, hyster- esis, and eddy currents. The /' R loss and the core losses should be about equally divided; that is, the copper loss should be about equal to the sum of the hysteresis and eddy- current losses. If the transformer is used only a short time during each day, it might be well to allow the /' R loss to be a little larger than the core losses, but the above relation holds approximately correct for well-designed trans- formers. In the present case, we will aim at making the copper loss, say, 140 watts, and the core loss 124 watts. This division of the losses should give a satisfactory trans- former for lighting work.
DETBRMIXATION OF CORE VOLUME
12. Since the transformer is to operate on a 60-cycle system, we will take 30,000 lines per square inch as a fair value for the maximum magnetic density in the core. At this frequency arid density, there will be a definite amount of loss per cubic inch of iron in the core, depending on the quality of the iron used. We will assume that the curve A^ Vig, 2, Part 1, represents the quality of the iron in this respect. From this curve, we find that the loss per cubic inch per 100 cycles at a density of 30,000 is about .25 watt. The loss at 60 cycles will therefore be yVo X .25 = .15 watt.
The total core loss is to be 124 watts. This is the loss due to hysteresis and eddy currents combined. The eddy- current loss should be quite small if the core is properly
12
DESIGN OF ALTERNATING
§22
laminated : hence, we will take the hysteresis loss alone as 110 watts, and allow 14 watts for the loss due to eddy currents. If the loss per cubic inch is .15 watt, then the
volume of iron in the core will be i—— = 733 cubic inches.
. 15
h 8,
.5 a-
1
i-a-
l^a
t
J
BIMEXSIONS OF COKE
13. The volume of iron in the core has now been deter- mined, and it remains to proportion the core itself. Fig. 8
shows the style of core used for this type of transformer, and in proportioning it due regard must be had to the windings that are to be placed on the cores r, c. We will make the core square in cross- section, with the corners chamfered slightly, as shown in the figure. If the cross-section a x a is made very small, the cores will be long and thin, the mag- netic flux 4> will be small, and the coils will have to be provided with a large number of turns to generate the required E. M. F. Long cores also give rise to a long magnetic circuit, thus increasing the magnetizing current. On the other hand, if the cores are made very short, the wire will have to be piled up deep, in order to get it into the winding space, and the yokes across the ends will have to be made longer. Deep windings also mean a long length of wire for a given number of turns, resulting in a large amount of copper. The best proportions to be given to the core are therefore largely a matter of experience. For pre- liminary dimensions, we will use the proportions indicated in Fig. 8, all the dimensions being here expressed in terms of the thickness of the cores. We will make the height of the core = 7 ^. The volume of the core will then be
i
T 1
n
Fio. 8
V = (2 X 3.5a + 2 X 5 a) a'
(1)
§22
CURRENT APPARATUS
13
«• being the area of cross-section and 6 a the distance between the yokes. This
gives
17 a* = V = 733 cubic inches
733
a =
•/733 Vl7
3f
This makes a just about 3J inches. This is the value of the thickness of the core if it were solid iron. Part of the cross-section is, how- ever, taken up by insulation between the plates, and the corners are cut off slightly, so we will make the core 3| inches square. The other dimensions follow from this, so we will take the dimensions given in Fig. 9 as a basis for further working out the design. The distance between the inside edges of the cores will be
i«*
T
a.
I I I
^
ei'^i
I I
I I
J..
J
~I
$
PlO. 9
5'
Tir
inches, and the
space between the yokes available for the windings will be 18^ inches.
DIMENSIONS OF CONDUCTORS
14. We will wind the secondary coil next the cores, and place the primary over it. The secondary current at full load will be
secondary watts __ 8,000 secondary volls ~ 100
= 80 amperes
(2)
The secondary coil will be wound in two sections, one section being placed on each core. Each section will have a sufficient number of turns to generate 50 volts, and the
14 DESIGN OF ALTERNATING §22
conductor will be capable of carrying 80 amperes. If an output of 100 volts and SO amperes is required, the coils may be connected in series and their E. M. F/s added. If an output of 160 amperes at 50 volts is desired, the coils may be connected in parallel. In either case, the full-load current in the conductor will be 80 Amperes. In this type of transformer, a large cross-section is usually allowed per ampere, because there is plenty of room for the coils, and the number of turns is usually large. We will therefore allow 2,000 circular mils per ampere to obtain the approxi- mate size of the conductor. We then have
Circular mils cross-section of secondary conductor = 80 X 2,000 = 160,000 circular mils.
Six No. 6 B. & S. wires in parallel will give 6 X 26,250 = 157,500 circular mils. We will make up the secondary
conductor, as shown in Fig. 10, using six bare
wires in multiple and covering the whole with
•^^1^^ S a cotton insulation having a double thickness
IUQ^J r of 20 mils. The bare diameter of the wire is
1:344^ .162 inch; hence, the width of the conductor
FIG. 10 over all will be 2 X .162 + .02 = .344 inch.
The height of the conductor will be 3 X .162 +".02
= .506 inch.
16. The watts supplied to the primary at full load are 8 264. Hence, the approximate primary current will be
primary watts 8,264 ..„, .„.
^^ " u- = :r7^7^ = *^1^'^ amperes (3)
primary volts 2,000 '^ ^ '
The primary current at full load will be very nearly in phase with the E. M. F. ; or, in other words, the power factor will be very nearly 1. The magnetizing current should be quite small, so that the primary current at full load will be but slightly larger than the above amount. We will call the full-load primary current 4.25 amperes, in order to allow a little for the magnetizing current. Allow- ing the same cross-section per ampere in the primary as in the secondary, we get
§22 CURRENT APPARATUS 15
Circular mils of primary conductor = 4.25 X 2,000 = 8,500 circular mils.
A No. 11 B. & S. wire has a cross-section of 8,234 circular mils, which is nearly the number required. The diameter of this wire over the insulation may be taken as .101 inch.
CALCULATION OF PRIMARY AND SECONDARY TURN8
16. The primary coil has to be provided with a sufficient number of turns to generate a counter E. M. F. equal and opposite to that of the mains. The impressed E. M. F. is equal and opposite to the resultant of the E. M. F. generated by the primary and the E. M. F. necessary to overcome the resistance of the primary. The drop through the primary at no load due to the ohmic resistance is so
0
^mall that it may be neglected in comparison with the E. M. F. that is generated by the primary coil, so that we may take the E. M. F. so generated as equal numerically to the impressed E. M. F. The number of turns required to set up this E. M. F. will depend on the magnetic flux 0 that threads through the turns. The maximum magnetic flux through the coil will be
* = B max. X A (4)
where B max. is the maximum value that the magnetic den- sity reaches during a cycle, and A is the area of cross-section of iron in the core on which the coil is wound.
In this case, B max. is 30,000 lines per square inch, and the area of cross-section of the iron is 3^ X 3^ = 12.25 square inches; hence,
4> = 30,000 X 12.25 = 367,500 lines
Taking the E. M. F. generated in the primary as the equal and opposite of the line voltage, we may write
4.44(^7;// .-
16 DESIGN OP ALTERNATING §22
where ^ = maximum value of the magnetic flux through
the core; Tj, = number of turns on primary coil; ;/ = frequency (cycles per second) ; Ej, = impressed primary voltage. Applying this to the present example, we have
o A/w. ^i^X 367,500 X T; X 60 2,000 = -^^
or T; = -m-iT'..^ ^T^KTTT^ = ^.^-^^^ J^early
2,000 X 10" 4.44 X 367,500 X 60
We will therefore provide the primary coil with, say, 2,040 turns, and place 1,020 on each of the cores, as this number will give an even number of turns on each coil. Dropping two turns would not appreciably affect the work- ing of the transformer, as the magnetic density would have to be increased but very slightly to make up for the dif- ference.
17. The number of secondary turns T, will be
T, X J (6)
where £, is the secondary voltage, since the turns must be in the same ratio as the voltages generated. This will give for the present case
The total number of secondary turns will therefore be 102, or there will be 51 turns on each coil, using the conductor shown in Fig. 10.
AIIRANGKMEXT OF PUIMAUY AXD SK<ONI>ARY COILS
18. The coils will be arranged on the core as shown in the section through the coils and core, Fig. 11. The coils are here shown circular in cross-section; very often they are approximately rectangular in shape, the secondary being
%2% CURRENT APPARATUS 17
wound directly on the core and the wooden pieces a, a omitted. In the larger sizes of transformer of this type, cylindrical coils are commonly used. The secondary will be wound next the core, in order to make the length of the heavy second- ary conductor as short as possible. The coil may be held firmly in posi- tion by oiled hard- wood blocks a placed between it
, , . , PlO. 11
and the iron core b.
The diameter of the coils could be made somewhat less by chamfering the corners more than shown, but this would decrease the cross-section of iron, so that very little would be gained in the end. Both coils are heavily insulated with linen tape, and provision is made for a clear space of -^ inch between the primary and secondary. The length of the cores between the yokes is 18^ inches (see Fig. 9). Each second- ary coil contains 51 turns The breadth of each turn is .344 inch, so that 51 turns will take up a length along the core of 51 X .344 = 17.5 inches. The secondary coil can therefore be made up of one layer of 51 turns of the con- ductor shown in Fig. 10. This arrangement will allow about -^ inch clearance at each end between the secondary winding and the yoke, in addition to the taping. The arrangement of this winding will be readily understood by referring to the section of the coils shown in Fig. 13. The mean diameter of the secondary coil will be 5| inches and the mean length of a turn 17.28 inches,
19. The primary coil is placed over the secondary, as shown in Fig. 11. The space of -^ inch is allowed to insure
18 DESIGN OF ALTERNATING §22
good insulation between the coils ; sometimes a mica insulating
shield is placed between the coils. In case the transformer is
immersed in oil, the film of oil between the coils forms an
insulating layer that is not easily broken down. We will
make the primary coil slightly shorter than the secondary,
and adopt a clear winding space, say, 17J inches in length.
This will remove the high-tension primary windings a little
farther from the yokes and avoid danger of breakdown.
The diameter of the primary conductor over the insulation
is .101 inch; hence, in a layer 17 J inches long we can place
17 25
' = 170 turns, nearly. We must place 1,020 turns on
each coil, so that we can arrange the winding by using six layers of 170 turns per layer. The two primary coils are connected in series across 2,000-volt mains; hence, the pressure across each coil will be 1,000 volts, and there will be 166 volts generated in each layer. The pressure tending to break down the insulation between the beginning of the first layer and the end of the second will therefore be the maximum value corresponding to an effective pressure of 333 volts. It is necessary, therefore, to insulate each layer from the one next to it, and particular care should be taken at the ends of the coil, where a breakdown between layers is most liable to occur. We will allow 20 mils for insulating tape between each layer and ^ inch all around for the outer taping on the coil. This will make the thickness of the primary coil 6 X .101 + 5 X .020 + ^ = .831 inch.
The mean diameter of the primary coil will be about 7| inches, and the mean length of a primary turn 23. 17 inches.
20. All the essential dimensions of the transformer have now been determined. With the primary winding calcu- lated above, the outside diameter of the primary coil will be about 8| inches. The distance from center to center of cores is 5^\-\-'Sl = 9^\ inches, so that there would be a space between the coils of }| inch, and the design is suitable as far as the accommodation of the windings goes.
§22 CURRENT APPARATUS 19
EFFICIENCY
181, In designing the transformer, we aimed at securing a certain efficiency, and so proportioned the core that the hysteresis loss should not exceed 110 watts. The design has been worked out, and it is found that the windings obtained can be accommodated on this core. It now remains to be seen whether the copper loss in these coils is within the allowable amount. If the copper loss is exces- sive, we must remodel the design of the coils so as to bring it to nearly the allowable amount. In order to calculate *•' copper losses in the primary and secondary, we musi determine their resistance.
, 22. In calculating the resistance of the coils, we will take the resistance of a mil foot of copper as 12 ohms, as it is the hot resistance that we must consider. Since there are 51 turns on each secondary coil, and the length of each turn is 17.28 inches, the resistance of each coil will be
o total length in inches 51 X 17.28 ,^„_ .
K^ = -. -. \ TV— = — .. t>r^ t>.w. — = .OOoo onm
' area m circular mils 157,506
and the resistance of the two coils in series will be .0112 ohm. The loss in the secondary at full load will therefore be
I.^R. = (80)' X .0112 = 71.68, say, 72 watts (7)
23. Each primary coil has 1,020 turns, and the length of each turn is 23.17 inches. The resistance of each pri- mary coil will then be
^ 1,020X23.17 o Qry u Rp = gT^^^ = ^-87 ^^"^s
and the resistance of the two coils in series will be 5.74 ohms. The primary /' R loss will therefore be
//ypp = (4.25)' X 5.74 = 103.7 watts (8)
20 DESIGN OP ALTERNATING §22
The total /*/? loss in the coils as designed is about 176 watts instead of the 140 watts allowed. The difference, however, is not great enough to make a very large differ- ence in the efficiency. It will be noticed that the less in the primary coils is rather high, since the loss should be about equally divided between the primary and secondary. This can be remedied to some extent by lowering the pri- mary resistance, i. e., by using a larger wire for theprijnary winding. We will have room enough to do this because we found that there would be a clearance of \^ inch between the coils. Suppose we try a No. 10 wire for the primary and see if this will give a more satisfactory result. The insulated diameter of this wire will be about .112 inch. The number of turns that can be placed in one layer will be
17 25
' = 154. We will therefore use six complete layers with
154 turns each, and one additional layer with 96 turns. The coil at the part where it is wound seven layers deep will have a thickness of 7 X .112 -f 6 X .020 + ^ = 1.029 inches. This will increase the mean diameter slightly and make the mean length of a turn about 23.3 inches. The cross- section of the wire will now be 10,380 circular mils, so that the resistance of each primary coil will be
^ 1,020 X 23.3 o OG K ^-=--10,380 - =^-2»ohms
and the resistance of the two coils will be 4.58 ohms. The loss in the primary at full load will then be
/p'7?p = (4.25)'' X 4.58 = 83 watts, nearly
This makes the total /' R loss 72 + 83 = 155 watts, instead of 176. This change in the primary winding makes the loss in the primary and secondary more nearly equal, and brings the total loss down nearly to the required amount. We will therefore adopt this winding in place of the one previously calculated. The outside diameter of the primary coils will now be a little over 8^ inches, so that there will still be a clearance of about J inch between them
%n
CURRENT APPARATUS
21
when the transformer is assembled. The total loss at full load will be 110 + 155 + U = 279 watts. The full-load efficiency will then be |JfJ = .9GG3, or about .17 per cent, lower than was assumed when starting out to design the transformer.
EFFICIENCY CUBVE
24. The curve showing the relation between the effi- ciency and output can be readily plotted when the efficiencies at different loads are known. We will therefore calculate the efficiency for one-eighth, one-fourth, one-half, three- fourths, and full load, also for one-fourth overload. In order to do this, we will assume that the core loss remains constant. For example, at one-fourth load the useful out- put is 2,000 watts, and the secondary current 20 amperes.
TABIiE I
|
-TJ |
>s |
>» (fl |
|
•S3 |
5^ - |
etc £ "32 « |
|
o*^ |
C 4-> OS |
5 fc o. |
|
Fra Full |
O 3 > 03 |
8 S 6 |
|
i |
I,O0O |
lO |
|
i |
2,000 |
20 |
|
i |
4,ooo |
40 |
|
* |
6,ooo |
60 |
|
Full load |
8,ooo |
80 |
|
J overload |
I0,000 |
ICO |
c
0)
u d
a
« M
§3
^ So-
.60 1. 12
2.16 3.20 4-25 5- 30
|
s 3 |
lary Loss |
dary Loss |
• s |
Input rox.) |
|
£ |
1*= |
h |
~ a a 0. |
|
|
0 u |
P^2^ |
C«N |
0 |
P |
|
124 |
1.65 |
I.I2 |
126.8 |
1,127 |
|
124 |
5.72 |
4.48 |
134.2 |
2,134 |
|
124 |
21.39 |
17.92 |
163.3 |
4,163 |
|
124 |
46.9 |
40.32 |
211. 2 |
6,211 |
|
124 |
83.00 |
72.00 |
279.0 |
8,279 |
|
124 |
128.65 |
112.00 |
365.0 |
10,365 |
C at
H flu
88.73
93- 72 96.08 96.60 96.63 96.48
The primary current will be that corresponding to the sec- ondary current of 20 amperes (or 1 ampere, since the ratio of transformation is 1 to 20) plus the current necessary to set up the magnetization and supply the losses. The primary current at one-fourth load may be taken as approximately 1.12 amperes, since the amount of current required to supply the losses will be very small at this load. The
n DESIGN OF ALTERNATING §22
primary /' R loss will be (1.13)' X 458 — 5.72 watts. The secondary /^ R loss will be (SO)' X .0113 = +.48 watts. The core loss is 134 watts; hence, the total loss will be 134.3 watts. The input will then be 3, 134 watts approxi- mately, and the output 2,000, giving an efficiency at this load of 93.73 per cent. The calculations and results for the other loads are given in Table I.
. 35. These values of the load and efficiency give the curve shown in Fig. 13. The student should compare this
Output (R fraettmu t^ full toad. Xf/ltientjf eirrr fur IrnHtformfr detlffned.
curve with that shown in Fig. 1. The scale used for the efficiency in Fig. 12 is larger than that in Fig. 1, in order to show the variation of efficiency more clearly, but it will be noticed that the curves have the same general character- istics. The variation in efficiency in this case is not more than 3 per cent, from one-fourth load to 25 per cent, over- load. It will be seen from the table that the efficiency begins to drop off when the transformer is overloaded, owing to the rapid increase of the /' A' losses.
§22 CURRENT APPARATUS 23
AL.L-DAY EFFICIENCY
!36« The efficiency that actually determines the cost of operating the transformer is the all-day efficiency, or the ratio of the watts useful output per day to the watts sup- plied during the day. This will depend on the length of time during the day that the transformer is doing useful work. For example, suppose the transformer were worked during the 24 hours an amount equivalent to full load for 6 hours, and that it remained idle an amount of time equiva- lent to 18 hours. The core losses would go on for the whole 24 hours, because the pressure is maintained across the lines, whether the transformer is working or not. The watt- hours wasted in the form of core losses in 1 day would therefore be 124 x 24 = 2,976. The copper losses during 1 day would be equivalent to the sum of the primary and secondary full-load copper losses for 6 hours. Hence, the watt-hours energy wasted in PR losses per day will be 155 X 6 = 930. The useful energy delivered during the day is equivalent to full load for 6 hours, or 8,000 X 6 = 48,000 watt-hours. The energy that must be supplied during the day is 48,000 + 2,976 + 930 = 51,906 watt-hours, and the all-day efficiency under these conditions is ^f J^ = .925, nearly. This means that of all the energy delivered to the transformer during 24 hours, 92.5 per cent, is con- verted into useful energy and the remainder wasted. If the transformer were loaded for a longer period during the day, the useful work done would be greater and the /' R loss would also be greater. The core loss would remain the same as before, so that the all-day efficiency would depend on the relation between the copper and iron losses. For example, suppose the transformer were fully loaded for 10 hours instead of 6. The useful work would be 80,000 watt- hours and the energy wasted in copper losses 1,550 watt- hours. The core loss would be 2,970, as before, and the total energy supplied would be 84,520, giving an all-day effi- ciency of about 94.6 per cent. The condition of load for which any given transformer will give its maximum all-day
24 DESIGN OP ALTERNATING §22
efficiency depends, therefore, on the relation between the copper and iron losses. It also follows that if the trans- former is to be loaded for only a short period during the day, the iron losses should be small if the all-day efficiency is to be high.
MAGNETIZING CURRENT
S57. The current that the primary of a transformer takes from the line when its secondary is an open circuit is usually spoken of as the mafirnetlzlng: current, although, strictly speaking, it is the resultant of the magnetizing current proper and the current that represents the energy necessary to supply the core losses. It is important that this no-load current should be small, because if a large number of trans- formers are connected to the line, the sum of all the mag- netizing currents required by the separate transformers may represent a considerable current to be supplied from the station. This means that the alternator may be delivering a fairly large current when no useful work is being done. It is true that this current may not represent very much power, because it is considerably out of phase with the E. M. F., but it loads up the lines and alterna^.or, and thus limits their useful current-carrying capacity. The no-load current is made up of two components, one of which is the magnetizing current, or the current that sets up the ampere- turns necessary to drive the flux around the core. The other component represents that current which is neces- sary to supply the core losses, and is in phase with the impressed E. M. F. The core loss in this case is 124 watts; hence, this component of the no-load current will be j^^^ = .062 ampere.
28. The component of the no-load current that repre- sents the current necessary to set up the magnetic flux may be obtained as follows : For a magnetic density of 30,000 lines per square inch, we will require about 5.5 ampere-turns per inch length of the circuit for a good quality of transformer
§22 CURRENT APPARATUS 26
iron. The mean path for the magnetic flux is shown by the dotted line. Fig. 9; the length of this circuit is about GO inches. The ampere-turns required to set up the flux will then be 60 X 5.5 = 330. The number of primary turns surrounding the circuit is 2,040. We then have magnetizing current X 2,040 = 330, or current = .162 ampere.
The no-load current is therefore made up of the two com- ponents .062 and .162 at right angles to each other, and its value is /„, = 1^.062' + .162* = .17 ampere.
This is the current that the transformer will take from the line when it is operating under no load. This does not mean, however, that it is consuming . 17 X 2,000, or 340 watts, because the no-load current is always considerably out of phase with the E. M. F., and, as a matter of fact, the trans- former consumes only sufficient power to make up for the core losses and the slight loss in the primary due to the no-load current. At no load, the power factor may be con- siderably less than 1, but as the load is increased, the cur- rent and E. M. F. shift into phase until the power factor at full load is very nearly unity.
BEGXTLATION
29. As the secondary voltage will fall off as the load is applied, it is important that this falling off should be slight. In well-designed transformers the falling off in secondary voltage may vary from 1 to 2.5 or 3 per cent., depending on the output. This drop is due to magnetic leakage and the resistance of the primary and secondary coils. In the type of transformer designed, the falling off due to mag- netic leakage will be quite small, because the coils are wound one over the other, making the path between the coils, through which leakage is set up, long and of small cross-section. The leakage drop would not likely amount to more than .2 or .3 volt. The drop in the secondary coils at full load will be current X resistance = 80 X .0112 = .89 volt. The drop in the primary at full load due to
|
i |
^ |
JUil |
1 |
||
|
h — |
^df |
^n. .' |
a| |
||
|
■" — |
jl z |
||||
|
f-^ — ■ |
--W| !'- -. '!, ' |
||||
|
t |
■ J| |
1=^ |
§22 CURRENT APPARATUS • 27
the primary resistance will be 4.25 x 4.58 = 19.46 volts.
This drop of 19.46 volts in the primary will cause a corre-
19 46 sponding drop of - - = .97 volt, in the secondary. The
total drop due to leakage and resistance combined will therefore be under 2 volts, or 2 per cent, of the output, which is close enough regulation for all practical purposes.
CONSTRUCTION
30. The construction and arrangement of the core and coils will be understood by referring to Fig. 13. This shows an elevation of the assembled transformer, with a longitudinal section of the coils showing the windings and insulation. The core is built up out of thin iron strips, which are interleaved at the corners, so as to practically do away with joints in the magnetic circuit. The plates are shown held in position by clamps ^, drawn up by bolts b. The terminals of the coils should be very heavily insulated, and may be run to a connection board within the transformer, or taken directly out through the case. Transformers in sizes up to 20 or 30 kilowatts are usually placed in an iron case arranged for mounting on poles. These cases should be weather-proof, and made as light as possible con- sistent with the necessary strength. They are generally designed with a view to being filled with oil. Fig. 14 shows a case suitable for the transformer designed. This is made of cast iron about ^^ or \ inch thick. The case a is provided with a cover ^, which is bolted on by means of the bolts d. The overlapping flange and gasket c serve to make the cover water-tight. The transformer, which is shown by the dotted outline, is held in place by setscrews. The pri- mary terminals are brought out through the bushings ^, and four bushings y are provided on the front of the case for the secondary terminals. The bushings should be of heavy hard rubber or porcelain, and so constructed that they will prevent leakage of current from the lines to the case.
28
DESIGN OP ALTERNATING
§22
These outlets should, of course, be directed downwards, so that the wires may be looped into them, thus preventing water from getting into the case. Lugs g, g should be pro- vided on the back of the case for attaching suspension hooks. Fuses are usually provided between the primary and the
line, but these are generally mounted outside the trans- former case in separate fuse boxes of special construction. Secondary fuses are not provided at the transformer, the fuses in connection with the secondary service wires being depended on to protect the secondary circuit. For large indoor transformers, only sufficient covering is used to
§22
CURRENT APPARATUS
29
protect the coils, a regular case being unnecessary, as well as interfering with the ventilation.
31. The transformer that has been worked out is one that would be used on an ordinary lighting circuit. The method of designing a step-up transformer would be essen- tially the same, except that extra precautions would be taken to insure very high insulation, and a larger allowance of winding space would be necessary. The design of a shell transformer may be also carried out in about the same way. The core proportions shown in Fig. 15 may be taken as a
Pig. 15
starting point. All dimensions are referred to the width of the tongue a^ which carries the lines through the coils. The length of the core may be from 3 to 7 times a. The height of the winding space is usually from 2 to 3 times ^, and the breadth from .7 to .8 times a. The thickness of the outer part of the shell around the coils is necessarily one-half of a, because this part of the core carries one-half the flux pass- ing through the coils. In this type of transformer the allowance of copper will usually be somewhat less than in the core type, because the winding space is more restricted.
30 DESIGN OF ALTERNATING §22
INDUCTION MOTORS
32. In many respects the action of an Induction motor
resembles that of a transformer, and, consequently, parts of its design can be carried out by methods similar to those used in designing transformers. The primary of the induc- tion motor, that is, the part into which currents are led from the line, corresponds to the primary coil of the trans- former, while the secondary, or the part in which the cur- rents are induced, corresponds to the secondary. This relation holds, whether the primary or secondary is the revolving part; but in all that follows we will consider the primary as being fixed and the secondary as revolving. In such an arrangement, the fixed primary is commonly spoken of as the field, or stator, and the secondary as the arma- ture, or i-otor. The essential difference between an induc- tion motor and a transformer is that in the latter case the secondary core and windings are fixed as regards the primary, and the E. M. F. generated in the secondary is made use of to supply useful electrical energy to an outside circuit; while in the former case the secondary core and windings revolve with regard to the primary, and the mechanical torque action between the primary and secondary is made use of to deliver mechanical energy. The currents generated in the secondary are not led int^o an outside circuit, but flow within the secondary itself, in order that they may react on the field produced by the primary and so cause the armature or secondary to exert the required effort at the pulley. A transformer supplied with a constant primary pressure will furnish a nearly constant secondary pressure independently of the load ; an induction motor when supplied with a con- stant primary pressure will run at nearly constant speed independently of the load.
§22 CURRENT APPARATUS 31
lilMITATION OF OUTPUT
33. The output of induction motors, like that of alternators and transformers, is limited principally by the heating effect due to the various losses that occur in the motor when it is loaded. The principal loss is that due to the resistance of the primary and secondary conductors, although the hysteresis and eddy-current losses may also be considerable if the motor is not properly designed. If an induction motor is considerably overloaded, the armature currents react excessively on the field, causing excessive magnetic leakage along the air gap, and greatly lessening the torque between the field and armature. If the overload is sufficiently great, the torque will be reduced to such an extent that the motor will stop. Usually, however, an induction motor may be loaded for short periods beyond its full-load capacity without danger of overheating or stopping.
PRIMARY CORE I.OS3ES, MAGNETIC DENSITIES, ETC.
34. The losses in the primary are made up of the core loss due to hysteresis and eddy currents, and the copper loss due to the resistance of the primary winding. The frequency of the changes in the magnetism of the primary is the same as the frequency of the current magnetizing it; hence, the lower the frequency at which the motor is operated, the higher is the allowable value of the magnetic density in the primary core. The core densities used for such motors should be about the same as those used for transformers operating at the same frequency. The curve, Fig. 16, shows the relation between the maximum value of the density and the frequency, based on values given by Kolben. The densities are low, and lie between 40,000 and 20,000 lines per square inch throughout the range of fre- quencies commonly met with in practice. This curve gives the density in the core proper; the density in the teeth of the primary and secondary may be double these values
32 DESIGN OF ALTERNATING 1 3»
without making the hysteresis loss very large, the volume of the teeth being small. Motors are also commonly built in which the mag- netic density will be found less than that given by the 3 curve, but the val-
i" ues shown should
( not, as a rule, be
J exceeded. Induc-
5 tion motors, like
i alternators, are
j generally built
iwith several poles, so that the mag- netic flux is sub- S d i V i d e d . The
« required cross-sec-
A tion of iron in the
yoke is therefore small, and a low »««««■*.«<«« A, /-*.,«,■. if«« magnetic density may be used with- o u t making the machine very heavy. The eddy-current loss in the primary, like that in transformer cores, can be kept down to a very small amount if thin disks are used. The thickness of stampings used is about the same as for alternator arma- ture cores, namely, from .013 inch to .018 inch.
SK<:ON'IIAUY <,(»UB MWSK8, MAGNETIC KKNSITIES, ETC.
35. The core lo.sses in the secondary are usually quite small. This is due to the fact that the frequency of the reversals of magnetism in the secondary is low. If the armature were standing still, the slip between primary and secondary would be 100 per cent., and the frequency of the magnetic cycles in the secondary would be the same as in
§22 CURRENT APPARATUS 33
the primary. When, however, the motor is running under normal conditions, the slip may not be more than from 2 to 5 per cent. The frequency of the magnetic cycles in the armature will therefore be only from 2 to 5 per cent, of the frequency in the field, and the core losses will be cor- respondingly small.
INDUCTION-MOTOR WINDINGS
PBIMARY WIKBING
36. The winding on the primary must be so designed that it will generate a counter E. M. F. equal and opposite to that of the mains, neglecting the small drop due to the resistance of the coils. It is therefore determined in a man- ner similar to that used for the calculation of the primary winding for a transformer. In some of the earlier forms of induction motors the coils were wound on salient poles, but in modern machines they are placed in slots in the same way as windings for alternator armatures. Most induction motors are of the two-phase or three-phase type, and the field winding of such machines is carried out in the same way as the winding for the armature of a two-phase or three-phase alternator. The primary winding may be concentrated or distributed, the latter arrangement being most generally used for machines operating at moderate pressures. We may write for induction-motor windings
£= jQi — x>& (9)
as explained in connection with alternator windings. In this formula
/f = E. M. F. generated by or impressed on each phase;
T = number of turns connected in series per phase;
0 = maximum total magnetic flux from one pole;
;/ = frequency (cycles per second) ;
>fe = a constant depending on the style of winding used.
45—11
34 DESIGN OF ALTERNATING §22
For a concentrated winding, that is, one with one group of conductors per pole per phase, k = I, For a uni- formly distributed two-phase winding, ^ = .90. For a uniformly distributed three-phase winding, k = .95. If the winding is only partially distributed, the value of k will lie between the values just given and 1. It will be noticed that for a given value of the flux, frequency, and number of volts applied, the number of turns required for a dis- tributed winding is but slightly more than that required for a concentrated winding, the difference being about 10 per cent, for a two-phase motor and 5 per cent, for a three- phase. The distributed windings are preferred, because with them there is less magnetic leakage between the primary and secondary; this decreases the inductance and improves the power factor of the motor. Generally, the primary slots occupy about one-half the circumference of the primary core, as this arrangement allows a fair amount of space for the windings without forcing the density in the teeth too high.
37. The cross-section of the conductor used for the pri- mary winding is determined by the full-load current that the motor takes in each phase. The relation of this current to the full-load current taken from the mains will, of course, depend on the way in which the different phases are con- nected up. The primary is usually stationary, and cannot therefore radiate its heat as readily as if it were revolving. For this reason, the current density should be kept as low as possible without making the space occupied by the windings too large. Induction-motor fields usually i)resent quite a large radiating surface, and are, moreover, generally sup- plied with air ducts, through which a draft is caused by the armature. If it were not for this, the allowance per ampere would have to be considerably more. The circular mils allowed per ampere varies greatly in different makes of machines. In some it may be as low as 500 or 600, and in others it may be 1,100 or 1,200. Much depends on the way in which the machine is ventilated, but it is always best to
§22
CURRENT APPARATUS
35
make the allowance as large as possible without interfering with the design in other respects.
38. The primary winding may be made up of bars or coils, depending on the voltage at which the machine is to operate, coils being used on most machines of moderate size. These are arranged in the same way as has already been described for two-phase and three-phase armatures, and what has been said as regards the insulation, etc. of such armatures applies also to induction-motor primaries. The primary winding is very often arranged in two layers, form- wound coils being used.
SECOND ABT WIia>ING
39. The number of conductors used for the secondary Tvlndlngr is largely a matter of choice. The motor may be built with any ratio of transformation, that is, with any ratio of primary to secondary conductors, and work well. It is desirable, however, to make the resistance of the secondary low, and to get as large a cross-section of copper as possible into the slots. For this reason, it is usual to pro- vide the secondary with only one or two bars to each slot, the space taken up by insulation being thus reduced to a minimum. The bars are generally rectangular in section, though in some machines
round bars have been /. v
used.
40. The secondary conductors are in many cases grouped into a reg- ular two-phase or three- phase bar winding. It is necessary to use a wound secondary of this kind when it is desired to insert resistance in series with the sec- ondary, either for the purpose of securing a good starting torque or regulating the speed. When this is done, the
FIO. 17
36 DESIGN OF ALTERNATING §22
winding is connected up according to the Y method, and the three terminals brought to collector rings, as shown in Fig. 17. The three phases /,, /„ and/, are thus connected to the three resistances r,, r„ and r,, as shown. When the motor is being started, the phases are connected to the points rt, b, and r, and the resistance is gradually cut out as the motor runs up to speed.
41. When it is not desired to insert resistance in the secondary circuit, a plain squirrel-cage winding may be used. There is in this case only one bar in each slot, all of them being connected by copper short-circuiting rings at each end of the armature. The squirrel-cage construction gives a durable and efficient armature, because the winding is extremely simple, and the end connections between the bars are of very low resistance. Since the voltage generated in an induction-motor secondary is very low, the insulation between the bars and core need not be heavy, as the danger of burn-outs is almost nil and short circuits do not count for much, because the bars are short-circuited by the end connecting rings. Usually, the number of slots in the secondary is different from the number in the primary, though this is not absolutely necessary. The use of a dif- ferent number of slots tends to avoid any dead points at starting, and prevents the motor from acting merely as a static transformer with a short-circuited secondary.
POWER FACTOR
42. It is important that the power factor of an induc- tion motor be high, otherwise it will take an excessive amount of current for a given amount of power delivered, on account of the angle of lag between the current and E. M. F. In order that the power factor may be high when the motor is loaded, the magnetic leakage and consequent inductance must be kept low. This may be done by using a small air gap, subdivided windings, and slots that are partially opened at the top.
§22 CURRENT APPARATUS 37
liENGTH OF AIR GAP
43. The current necessary to set up the magnetic flux through the field will be largely dependent on the leng^tli of air gap between the primary and secondary, because this constitutes by far the greater part of the reluctance of the magnetic circuit. In a transformer it is not necessary to have any air gap in the magnetic circuit; hence, the magnetizing current can be made quite small. In an induc- tion motor, however, an air gap is unavoidable, and all that can be done is to reduce this to the smallest possible amount. The air gap is therefore made as small as the necessary mechanical clearance will permit. For very small motors the single air gap may not be more than yf^ inch. For larger machines it must be greater than this, on account of the difficulty of centering large armatures exactly, and to prevent the armature touching the field in case the bearings should wear slightly.
GENERAL DATA
44. The following figures, given by M. A. C. Eborall,* will serve as a guide for the Values of some of the various items entering into the design of induction motors. These apply for the most part to motors designed for the ordinary frequencies of 50 to 60 cycles. These must be taken as a general guide only, and individual machines might show values differing considerably in some particulars from these and yet give good results.
46. Perlplieral Speeds. — From 4,000 to 7,000 feet per minute. The speed of large motors is usually somewhat higher than that of the smaller machines.
46. Kumber of Poles. — Two to 7 J horsepower, 4 poles; 10 to 30 horsepower, 6 poles; 40 to 100 horsepower, 8 poles.
47. Pull-Iioad Efficiency. — Table II gives ordinary values for the full-load efficiency.
* London Electrician^ 1900.
38
DESKiN OF ALTERNATING
§22
TABL.K II
|
Brake Horsepower |
2 |
5 |
lO |
25 |
50 |
|
Polyphase motors Single- phase motors |
•75 •72 |
•79 •75 |
■85 .80 |
•87 •83 |
.90 •85 |
48. Full-IiOad Povrer Factor. — Table III gives ordi- nary values for the full-load power factor.
|
TABIiE |
III |
||||
|
Brake Horsepower |
2 |
5 |
10 |
25 |
50 |
|
Polvphase motors |
.78 •72 |
.80 •75 |
•85 .80 |
•87 •83 |
.88 |
|
Single-phase motors |
■85 |
49. TiengtYi of Air Gap. — The following values (Table IV) give the minimum length of air gap that it is safe to use for mechanical reasons. In some machines larger air gaps than these are employed. The lengths given refer to a single gap only.
TABLE IV
|
Rotor Diameter |
Air-Gap Length Inch |
|
Between k inches and 8 inches |
rV 1 |
|
Between 9 inches and 12 inches Between i k inches and 20 inches |
|
|
Between 24. inches and '?2 inches |
|
|
Between ±0 inches and 60 inches |
|
|
^ |
60. Density of MaKnetisin in Htator Teetli. — Table V gives values for the density in the stator teeth.
§22
CURRENT APPARATUS
39
TABIuE V
Horsepower
2 to 7.5 10 to 30 40 to 100. Above 100,
Density in Lines Per Square Inch
65,000 70,000 80,000 85,000
The density in the air gap should not exceed 30,000 lines per square inch, and is usually considerably lower than this.
61. Density of Magrnetlsm In Rotor Teeth. — Table VI gives values for the density in the rotor teeth.
TABI.E VI
|
Horsepower |
Density in Lines Per Square Inch |
|
2 to 7 . S |
80,000 |
|
10 to ^0 |
85,000 90,000 100,000 |
|
40 to 100 |
|
|
Above 100. . . ! |
|
62. Current Densities per Square Inch. — With low and medium pressure semi-enclosed motors, the amperes per square inch cross-section of stator or field conductor will be between 1,500 and 1,100, according to size. This corre- sponds approximately to 850 to 1,150 circular mils per ampere. With high-tension motors, somewhat smaller values must be taken on account of the space occupied by the insulation.
63. Volume of Current In Stator and Rotor. — The
ampere-conductors, i. e., the product of the current and
40
DESIGN OF ALTERNATING
§22
conductors, for each inch periphery should have values about as shown in Table VII.
fii
rABIilC VII
Horsepower
2 to 7.5
10 to 30
40 to 100.
100 to 150
Above 200,
Ampere-Conduc- tors Per Inch of Periphery
250
330
430
570 600
64. Slip at Full lioad. — Table VIII gives approximate values of the slip at full load in per cent, of synchronous
speed.
TABIiE Vm
|
Horsepower |
Slip Per Cent. |
|
2 to K |
7 S . 4 3 |
|
7I to li*. |
|
|
20 to 40 |
|
|
KO to 100 |
|
|
J wv^ *«r«r..... ..■■..... |
DESIGN OF lO-HORSEPOWER MOTOR
65, In order to illustrate the design of a simple induc- tion motor, we will take an example and make the calcula- tions required for the windings and core. Many of the mechanical details are similar to those that have already been described for alternators, so that they need not be taken up in detail; those parts that differ materially will be described as the design is worked out. We will take for an example a 10-horsepower three-phase motor with stationary primary and revolving secondary. The primary will be
§22 CURRENT APPARATUS 41
provided with a distributed winding placed in slots, the secondary being provided with a squirrel-cage winding. We will suppose that the following quantities are given: Output at pulley, 10 horsepower; line voltage, 220 volts; frequency, 60 cycles per second ; power factor at full load, . 85 ; commercial efficiency at full load, 85 per cent.
rtTLL-IiOAD CimRENT IN PRIMARY
66. The output is to be 10 horsepower, or 10 X 746 = 7,460 watts = IV. The actual power to be delivered to
the motor at full load will therefore be ' ., = 8,776 watts
.oo
= W.
The true watts delivered to the motor at full load are equal to the product of the volts and amperes into the power factor cos 0, where 0 is the angle of lag between the cur- rent and E. M. F. We then have
true watts .^^.
apparent watts = — (1")
cos ^ = .85 in this case; hence, we have
apparent watts = ^^^ = 10,324 = W" For a three-phase motor we have
where E is the voltage between the lines, and / the current in each line ; hence,
10,324 = 220X /X i/3
j= 10.324 ,^
220 X l/3
The full-load current in the line will therefore be
27.1 amperes, and the current in each phase will also
be 27.1 amperes if we adopt a Y winding for the primary.
If we used a A winding, the current in each phase would
27 1 be — ~ = 15.7 amperes, nearly.
^3
42 DESIGN OF ALTERNATING g 22
SIZE OF PRIMARY COXDUCTOR
57. Since the current in each phase is comparatively small, we will use the Y method of connection for the pri- mary winding. The current in the primary conductor will therefore be 27.1 amperes. We will .provide 850 circular mils per ampere as a fair allowance of copper for the primary. We then have 27.1 X 850 = 23,035 circular mils.
A No. 6 B. & S. has a cross-section of 26,251 circular mils, and three No. 11 wires in multiple give, a cross- section of 24,702 circular mils. Two No. 9 wires in parallel will give 20,188 circular mils, so that any of these arrange- ments would give the requisite cross-section. When it comes to arranging the dimensions of the slot', a decision can be made as to which arrangement can be used to best advantage.
PERIPHERAIi SPEED AND DIAMETER OF ARMATURE
68. If the speed of rotation and the frequency are fixed, the number of poles for which the field must be wound is at once determined ; or, if the number of poles and frequency be fixed, the speed of rotation at no load at once follows, because at no load the speed of the armature is almost exactly equal to that of the revolving field, the slip being very small. If we wind the field so as to ha.ve six poles, the
speed at no load will be very nearly s = -. — -^— ? — -. —
'^ ^ '' number of poles
2 X 60 = — - — = 20 revolutions per second, or 1,200 revolutions
per minute. If the field were wound for eight poles, the speed would be 900 revolutions per minute. As this motor is not very large, 1,200 revolutions per minute will be a fair speed for it. If we used the eight-pole arrangement, we would obtain a lower speed, but the motor would be larger and more expensive; we will therefore adopt the six-pole 1,200-revolution arrangement.
§22 CURRENT APPARATUS 43
69. Induction motors are run at moderately high periph- eral speeds, usually between 4,000 and 7,000 feet per min- ute, the larger motors having the higher peripheral speed. For a motor of the size under consideration, 4,500 feet per minute will be a fair value. The outside diameter of the armature will therefore be
, peripheral speed X 12 4,500 X 12 ^ . o«>. • u
«o = - — T^ T^ 1^/ = -^tt^^t;; = 14.324 mches
" R. P. M. X JT 1,200 X JT
We will therefore adopt 14| inches as the outside diameter of the armature. The circumference of the armature will be about 45.16 inches. The inside diameter of the field will be equal to the outside diameter of the armature plus the air gap required for mechanical clearance. For an armature of this diameter ^y inch on each side should be sufficient, so that the inside diameter of the field will be 14|-+-^ X Vff = I'^rV inches. The inside circumference of the field will be about 45.35 inches.
PRIMARY WINDING
60. We will use a primary winding: that is subdivided. If the winding is subdivided to a large extent, a large num- ber of slots will be required to accommodate it. It is usually sufficient, however, for motors ranging from 10 to 100 horsepower, to use from two to four coils per pole per phase, and for the present case we will take three coils per pole per phase as a trial arrangement. The winding will be arranged in two layers; hence, there will be as many slots as coils. The number of slots will therefore be 3 X G X 3 = 54.
61. Before fixing upon the size of the slots, it will be necessary to determine the number of conductors. We will design the primary so as to make the ampere-conductors per inch of periphery as nearly 300 as possible, as this should give good results for a motor of this size. The circumfer- ence of the stator is 14 j\ x 3.1416 = 45.35 inches; hence,
44
DESIGN OF ALTERNATING
§22
the total ampere-conductors will be 45.35 X 300 = 13,605. The current in each conductor is 27.1 amperes; hence,
13,605
number of conductors should be
27.1
= 502, approxi-
mately. There are 54 slots, and as the winding is to be arranged in two layers, there must be an even number of conductors per slot, so that the nearest number will be 10. This will make the nearest total number of conductors 540.
62, In order to obtain a slot that will be fairly deep compared with its width, we will use three No. 11 wires in
^ a4r gap
Short eirctUHwg ring
Fig. 18
multiple, with a cotton wrapping on each wire. The diam- eter of the wire over the insulation will be .101 inch, and
§22 CURRENT APPARATUS 45
allowing 65 mils on each side for slot insulation, taping, and
clearance, the width of the slot will be 3 X .101 + 2 X .065
= .433 inch. The space taken up by the 54 slots will
be 54 X .433 = 23.38 inches, thus leaving 45.35 - 23.38
= 21.97 inches for the teeth. Each tooth will therefore
21 97 be ' = .407 inch wide at the circumference. This is 54
not much less than the width of the slot, and will give ample cross-section of iron to carry the flux, because the density in the tooth will not be much more than twice that in the air gap, and as the latter will not be more than 20,000 to 25,000 lines per square inch, there will be no dan- ger of the teeth becoming saturated.
The slot must have sufficient depth to accommodate 10 wires in addition to the slot insulation, the dividing insulation between the upper and lower layers of coils, and the wedge or fiber strip used to hold the coils in place. We will allow -^j inch for the middle insulation, and ^^ inch for the holding in strip. The total depth of the slot will then be 10 X .101 + 2 X .065-f ^^ + ^4 = 1-390, or, say, m inches, in order to allow a small amount for clearance. The dimensions of the slot and the arrangement of the ten turns of three-wire conductor are shown in Fig. 18, the coils being held in place by wooden or fiber strips slipped into notches in the teeth.
MAGNETIC FLUX IN POLES
63. By the magrnetic flux ^ is meant the total max- imum number of lines that flow from one pole piece. The pole pieces of an induction motor are not sharply defined like those of an alternator field, but gradually merge from one into the other.
Fig. 19 will help to convey an idea as to the way in which the flux is distributed around the face of an induction-motor field. The inner circle represents the face of the field, which for the present will be considered as unbroken by slots. If a current is sent through the windings, six poles
46
DESIGN OF ALTERNATING
§22
will be formed, as shown, and these poles will be continually shifting around the ring. We will consider the instant
when the centers of the poles are at the points marked A^, 5. The magnetic den- sity is greatest op- posite the center of the pole, and may be represented by the arrow a b directed ou t w-ards from a south pole, or a* b' directed inwards from a north pole. As we move away from a pole the field intensity d i - m i n i s h e s until it becomes zero at the point midway between the poles,, and begins to increase again in the opposite direction. This vari- ation in the magnetic density at the various points of the pole face is represented approximately by a sine curve, and if the line a b represents the maximum value of the density,
the average value of the density will be ab x -y since the
2 average value = maximum value x -. Hence, if B repre-
2 sents the maximum value of the density, B X - will be the
TZ
average density. The total flux 4> is equal to the area of the pole face multiplied by the average value of the density; or
2
^ = arc ef x length of field parallel to shaft X B X -
Pig. 19
n
Arc ef =
n X diameter of field number of poles
t ^ :r X diameter of field _ , .u r ^ u » 2
hence, ^ = r > n X length of field X B X -;
^ number of poles if
§22 CURRENT APPARATUS 47
or we may write, for the length of field parallel to the shaft,
where ^ = flux from one pole; / = number of poles; df, = inside diameter of field ; B = magnetic density in the air gap (maximum).
Hence, from th^ formula we can obtain the length of the field parallel to the shaft when we know the value of ^ and have decided on the air-gap density to be used. The other quantities in the equation are already known. We can obtain the value of the flux from the formula
E- 4.44 ^ Tn , ^= 10' ^^
We will take k -=• .95, as the winding is nearly uniformly
distributed. There are eighteen coils in each phase, with
5 turns each, so that the number of turns T in series per
phase is 90. The voltage generated in each phase will be,
220 neglecting the resistance drop, — = = 127 volts, because the
armature is Y connected. We then have
4.44 X <P X 90 X 60 X .95
127 =
10*
127 X 10" °'" * = 4.44 X 90 X 60 X. 95 = SS^'^OOl'^^^. approximately
64. The magnetic density in the air gap should not be forced too high, or a large magnetizing current will be required to set up the flux. From 20,000 to 30,000 lines per square inch may be taken as fair values for the air-gap den- sity. The density at the top of the teeth would of course be more than this. We will take 20,000 lines per square inch in this case. Applying formula 11, we have for the length of the core parallel to the shaft, the field diameter being 14j\ = ^^^ ^^^h»
48 DESIGN OF ALTERNATING §22
. _ 557,500 X 6 X 16 _
' - 2 X 231 X 20,000 " ^"^^ '""^''^^
The length of the iron part parallel to the shaft should therefore be, say, 5ff inches, in order that the air-gap den- sity shall not exceed 20,000 lines per square inch. The length of core over all will be somewhat greater than this, owing to the space taken up by insulation between the disks and by the air ducts if the latter are used. We will allow I inch for an air duct in the center of the core, and \ inch for the space taken up by the insulation, thus making the spread of the laminations over all 6|^ inches.
65. All the dimensions of the primary have now been determined except the depth of the iron under the slots, that is, the dimension d^^ Fig. 18. This must be made such that there shall be a sufficient cross-section of iron to keep the magnetic density down to the proper amount. Referring to the curve, Fig. 16, we find that a fair value for the magnetic density in the iron of a 6P-cycle motor is about 30,000 lines per square inch. The magnetic leakage in such a motor is small, and we may take the flux in the field as practically the same as that in the air gap. The flux through a cross-section of the yoke under the slots will be \ 0, because the flux from one pole will divide, one half flowing in one direction and the other half in the other direction. The area of cross-section of iron in the yoke will therefore be
10
>4 — ^
which gives
278,750 30,000
"^v = on Arm = ^-^^ square inches
The actual length of iron parallel to the shaft is 5}f inches; hence, the depth of iron under the slots must be
^c = F^T^S ~ ^'^ inches, nearly
§22
CURRENT APPARATUS
49
y-jT-\^ar-
V --- ^=
-* i "ft
..^.--
f: -
l-^-J
K-
as
J««^
6
45—12
50 DESIGN OF ALTERNATING §22
We will therefore make the dimension rf^, Fig. 18, If inches. The inside diameter is 14 1?^ inches, and the depth of the slots 1^ inches, so that the outside diameter of the stampings for the primary will be 14yV + 2xli| + ^Xl| = 20^ inches.
The complete dimensions of the primary are shown by (a), Fig. 20. A section through one of the primary slots is given at {b), showing the air duct d and a section of the laminations. The primary laminations are provided with a keyway k for holding the stampings in place and bringing the slots into line. There will be 54 slots of the dimensions shown in Fig. 18, equally spaced around the inner periphery.
SECONDARY WINDING
66, The design of the secondary follows largely from that of the primary. The outside diameter is already known, and the length of the secondary core over all par- allel to the shaft will be the same as the length of the primary, 6|^ inches. We will provide the secondary with a squirrel-cage winding, although a secondary with a regular three-phase Y winding might be used if it were desired to insert resistance when starting. It is advisable, though not absolutely necessary, to use a number of slots for the secondary different from that used in the primary, as it tends to prevent dead points at starting. We will there- fore try 60 slots for the secondary winding, and see if this number gives a satisfactory design in regard to the size of the slots and bars.
ROTOR CONDUCTORS AND CORE
67. The magnetizing action of the currents in the secondary of an induction motor is, at each instant, equal and opposite to the magnetizing action of the currents in the primary, as is the case in an ordinary transformer. The total volume of current in the secondary may then, for purposes of calculation, be taken equal to that in the pri- ip^ry. In this case we have a total of 540 stator conductor^
§22 CURRENT APPARATUS 51
carrying a current of 27.1 amperes. Hence, the total volume of current is 540 X 27.1 = 14,634 ampere-con- ductors. If, therefore, we use 60 bars on the armature, the current in each bar will be approximately ^^V^ = 243.9 amperes. The voltage that must be generated in the secondary at full load in order to set up this current in the bars will depend on the resistance of the bars, the higher the resistance, the greater being the necessary E. M. F. and the greater the slip. It is desirable, therefore, in order to secure close speed regulation and high efficiency, to make the resistance of the bars as low as practicable. The core losses in the secondary are very small on account of the low frequency of the magnetism in the secondary, so that as far as heating is concerned, we might allow a large /'i? loss in the conductors; an allowance as low as 300 or 400 circular mils per ampere would not likely give rise to any undue heating. We will, however, allow 500 circular mils per ampere, as this larger cross-section will tend toward better speed regulation and higher efficiency. The cross-section of the secondary bars will then be 243.9 X 500 = 121,950 circular mils = .096 square inch, nearly. The usual practice is to make the secondary slots for squirrel- cage armatures rather broad and shallow, as shown in Fig. 18. This brings the conductors near the surface of the rotating member, and also allows the bars to be placed in the best position for connecting to the end short-circuit- ing rings. The distance between centers of the secondary
slots will be .,' = .753 inch, or a little over | inch. A
60
bar y\- inch by -^ inch has a cross-section of very nearly .096 square inch; a bar of these dimensions will be placed in the slot as shown in Fig. 18. A bar of this size will have a cross-section of approximately 121,800 circular mils, allow- ing a little for rounding the corners. The width of the bar is ^ inch = .438 inch; hence, there is .753 — .438= .315 inch left for the tooth and the insulation. This will allow the teeth to be made ^^ inch projected width at the circumference and still le^ye sufficient space for insulation. Since the
52. DESIGN OF ALTERNATING §22
voltage generated in the secondary is very low, a light slot insulation is all that is necessary. In this case there will be room enough for .017 inch insulation around the bar. The secondary slots are made nearly closed at the top, as shown in Fig. 18, and the bars are pushed in from the end.
68, The bars are connected up into closed circuits by means of the short-circuiting rings r, Fig. 18, one at each end of the armature, the bars being bolted to the copper rings by means of the flat-headed countersunk bolts s. In order to secure good contact, the projecting ends of the bars should be milled to conform with the surface of the ring. The lower the resistance of the end rings, the better, but as the path of the current through these rings is short, there is little advantage gained by putting a large amount of copper into them. We will make the thickness of the rings the same as that of the bars, i. e., ^\ inch, and will make the rings ^ inch wide, in order to secure a good con- tact between them and the bars.
69. The complete dimensions of the stator and rotor have now been determined with the exception of the inner diameter of the rotor disks. The flux through the rotor will be practically the same as that in the stator. The rotor might be worked at a higher magnetic density than the stator without serious loss, because of the low secondary frequency. However, we will use the same density in both, so that the depth of iron under the secondary or rotor slots will be 1| inches. The total depth of the slots is f inch, so that the inner diameter of the rotor is 14| — 2 (| + 1|) = 10| inches.
HEAT LOSSES
70. The principal dimensions have now been deter- mined, and it remains to be seen whether the motor will deliver its rated output without overheating. In order to do this, we will make an approximate estimate of the
§22 CURRENT APPARATUS 53
n R losses. The /" A' loss in the secondary may be deter- mined approximately as follows: The cross-section of each armature bar as finally adopted will be about 121,800 circu- lar mils. The bars should project a short distance out of the slots, so we will call the length of each bar about 8^ inches. The hot resistance of each bar will then be
R = lggg^^'" '""!^"^ = -1^1,-, = .000069 ohm Circular mils 121,800
The total /* R loss in the armature will be (243.9)' X .000069 X 60 = 246 watts. There will also be a certain amount of loss in the short-circuiting rings and at the joints, but the total /^ R loss will probably not exceed 300 watts. The outside cylindrical surface of the armature is 45.16 X 6.687 = 302 square inches, nearly, which gives a surface of over 1 square inch per watt /' R loss. The core losses in the secondary will be very small, so that the secondary will carry its load without any danger of over- heating.
?!• In order to estimate the /' R loss in the primary at full load, we must first determine the length of a primary turn. There are in all 54 coils and 54 slots, the coils being arranged in two layers. There are six poles, so that if one side of a coil lies in the top of slot No. i, the other side will lie in the bottom of slot No. 10, as shown in the winding diagram, Fig. 22. The coil will then span over -^^ of the circumference of the field, as shown in Fig. 21. This figure represents two coils of the field winding' in place, the inner face of the field being developed out flat. When the coils are in place, the ends a, a and b, b will project out past the core, forming a cylindrical winding. The ends of the coils are arranged on such a slant that they will fit in as shown without crowding. From this layout of the coils, the length of an average turn can be obtained, and in the present case it is found to be about 36 inches. There are 18 coils in series per phase and 5 turns per coil, making a total of 90 turns. The cross-section of the conductor is 3 X 8,234
54
DESIGN OF ALTERNATING
§22
= 24, 702 circular mils, since there are three No. 11 wires in parallel. The resistance per phase will therefore be
R =
90 X 36
24,702
= .131 ohm, nearly
The PR loss per phase will then be (27.1)* X .131 = 96.2 watts, and the total PR loss in the field will be 96.2 X 3 = 288.6, say, 290 watts. The exposed cylindrical
Pig. tl
surface of the field core alone is 20^ X 3.1416 X 6fJ = 430.7 square inches. The surface exposed by the pro- jecting windings will be approximately 200 square inches, so that there is an effective radiating surface of 630. 7 square inches for getting rid of the heat developed in the primary, without counting the radiating surface that would be pro- vided, to a certain extent, by the frame of the machine in
FlO. 23
§22 CURRENT APPARATUS 56
contact with the field. The radiating surface as a whole, therefore, should be sufficient to get rid of the losses with- out an undue rise in temperature, especially as the hyster- esis loss in the primary core would not be as large as the I* R losses, the density being low and the volume of iron comparatively small.
PIEIiI> WINDING AND CONNECTIONS
72. Fig. 22 shows the arrangement of the primary or field winding, one phase being drawn in complete. The groups of conductors for the other two phases are indicated by the light and dotted lines, the connections between them being made in the same way as those for the phase drawn in. The rules governing the connecting up of such a winding have already been explained in connection with polyphase- alternator armatures. Each of the heavy outlined figures represents a field coil of 5 turns; the lighter lines (two to each coil) projecting from the inner point of the coils rep- resent the terminals of the coils. There are 64 slots, or 9 slots corresponding to each pole; hence, the E. M. F.'s in all the conductors in the 9 slots under any one pole will be in the same direction, as shown by the arrowheads. For example, the E. M. F.'s in the conductors in slots 7, 8, 9, 10^ 11, 12, 13, H, 16 will all be in one direction, say directed from the front to the back, while those in slots 16, 17, 18, 19, 20, 21, 22, 23, and 2Jt. will have their E. M. F.'s in the opposite direction, corresponding to a pole of opposite polar- ity. The 18 coils shown belonging to one phase must all be connected in series, so that the E. M. F. 's in the conductors in the different slots belonging to this phase will be summed up. Suppose we start with the terminal 1\ ; we will pass five times around the coil, bridging from slot U6 to slot /, in agreement with the arrowheads, and come out at /; we will connect / to /', and go five times around the next coil, finally coming to x and completing the connections of that group of coils. We then pass on to the next group, con- necting X \,id y (so as to agree with the arrows), and so on
56 DESIGN OP ALTERNATING §22
around the field until the whole 18 coils are connected in series, finally coming to 7",. We will connect T^ to the common connection of the Y winding, 1\ being then one of the terminals of the motor that is connected to the line. The other two phases are connected up in exactly the same way, the connections between the terminals of the different phases and the common junction being made according to the rules already given. This winding could also be con- nected up A, the only difference being in the connections of the phase terminals with each other and with the terminals of the machine.
MECHAKICAI. CONSTRUCTION
ARMATURE
73. The armature core is built up in almost exactly the same way as cores for alternator or continuous-current armatures, the disks being mounted on a spider and clamped together by means of end flanges drawn up and held in place by capscrews or bolts. If a wound secondary is used, it is customary to provide the spider with projecting flanges for supporting the winding, as already explained for alternator armatures with distributed windings. Where the squirrel- cage construction is used, no supports are necessary, the bars and short-circuiting ring being stiff enough to hold themselves in place.
SHAFTS
74. Shafts for induction motors are usually made excep- tionally heavy, considering the power that they must trans- mit. They should, in general, be heavier than the shafts used for alternators of corresponding speed and output. The air gap in induction motors is so small that a very stiff shaft is required, the slightest bending of the shaft being sufficient to either let the armature touch the field or bring very heavy magnetic pulls on the shaft, due to the shorten- ing of the air gap on one side. The shafts for these motors are
((,
<
^
JcL_
y Tpv^^^^:^^^;^g^
W/'i^'/mU y
III I '^ll/r II I „,„„.
»
$22 CURRENT APPARATUS 57
shorter than those required for alternators and continuous- current machines, because no room need generally be allowed for collector rings. Fig. 23 shows the induction motor that has been worked out. This will give an idea as to the style of 'shaft used for such machines.
FIELD FRAME, BEDPLATE, ETC.
75, The arrangement of the parts of an induction motor of this size will be understood by referring to Fig. 23. In this case the field frame forms the main supporting casting of the machine, being provided with feet as shown. It serves the double purpose of supporting the field stampings and forming a base for the machine. In some of the larger sizes of induction motors, the field frame is bolted to a separate bed in the same manner as shown for the field of the alternator. For machines of moderate size, the con- struction shown in Fig. 23 answers quite well, and is cheaper than that which makes use of a separate bed. The self-oiling bearings are carried by the two end plates //, //, which are bolted to the field frame, as shown, and carry the bearings g^ g and the shaft /, with pulley /. These end- bearing supports also serve to protect the field coils c. The conductors in the field slot are shown at d, d^ and ^ is a section of the field laminations. The armature laminations a are supported by the spider c and held by the cap bolts and end flange, as shown. The armature bar is shown pro- jecting from the slot, the ends being bolted to the short- circuiting rings. The field frame k is provided with a number of ribs r, which are bored out to fit the outer cir- cumference of the stampings.' A number of openings o are cored in the frame to allow ventilation. The terminals of the field winding are led through the cored openings/,/) to the terminals ;/, which are mounted on the slate terminal board ;;/, from which the connections to the line are made. It will be seen that, on the whole, the construction of such a motor is very simple, there being no brushes, brush holders,
58
DESIGN OF ALTERNATING
§22
collector rings, etc. Fig. 24 shows a perspective view of an induction motor of the same general type as the one worked
Pig. 94
out. The main mechanical features of Fig. 24 will be understood by referring to Fig. 23, so that further comment is unnecessary.
76. Two-phase and single-phase induction motors are designed in the same way as three-phase machines, the only essential difference being in the arrangement of the wind- ings. The calculation of two-phase armature windings has already been described, and the calculations for a two-phase induction-motor field are made in the same way.
ELECTRIC TRANSMISSION
INTRODUCTORY
1. Electric transmission may be defined as the trans- ferring of power from one point to another by means of electricity. The power so transmitted may be used for any of the numerous applications to which electricity is now adapted, such as operating motors, lights, electrolytic plants, etc. The distance over which the power is transmitted may vary from a few feet, as in factories, to many miles, as in some of the modem long-distance transmission plants.
2. A power-transmission system consists of three essen- tial parts: (a) The station containing the necessary dyna- mos and prime movers for generating the electricity; (d) the line for carrying the current to the distant point; and {c) the various receiving devices by means of which the power is utilized.
3. Electric transmission may be carried out by using direct current, alternating current, or a combination of the two. Generally speaking, in cases where the transmission is short, direct current is used, though alternating current is now also largely used for short-distance transmission, as, for example, in driving factories. When the distance is long, it is necessary to use alternating current. In cases where the distance is long and where alternating current is not well adapted to the operation of the receiving devices, the current transmitted over the line is alternating, but it is changed to direct current at the distant end and there dis- tributed, thus forming a combination of the two systems. The special applications of electric transmission to railway
For mUice of co^righi^ see ^age immeduUely lolUmring the iiiU Pag*
128
2 ELECTRIC TRANSMISSION §23
and lighting work will be taken up later in connection with those branches of the subject; for the present, the object is only to bring out important points relating to the sub- ject of electric-power transmission generally.
Power transmission is extensively used in connection with water powers that would in many cases be of little use on account of their being located away from railways or com- mercial centers. It is also coming into extensive use in factories to replace long lines of shafting and numerous belts, which are wasteful of power. Its most important use, however, is in connection with the operation of electric rail- ways, where the power is transmitted from the central sta- tion to the cars scattered over the line. The style of apparatus used will depend altogether on the special kind of work that the plant is to do, and the type best adapted for a given service will be described when the ■ different transmis- sion systems are treated later. Power stations will be taken up by themselves; the present Section will be confined to the methods and appliances used for carrying out electrical transmission.
POWER TRANSMISSION BY DIRECT
CURRENT
4. Up to within a comparatively recent date, electric transmission for power purposes was carried out by means of the direct current, alternating current being used when the power was required for lighting purposes only. Later, however, alternating-current motors and rotary converters came into use, and at the present time, large transmission systems use alternating current for both light and power.
6. Dynamos and Motors Used. — Direct-current d3ma- mos may be of either the constant-current or the constant- potential type. Practically all the current is distributed at constant potential and in America compound-wound dynamos are generally used. The motors used in connection with such constant-potential systems are generally of the shunt or compound type.
§23 ELECTRIC TRANSMISSION 3
6. simple Poiver-TransmlBsloii System. — About the simplest possible example of electric- power transmission is that shown in Fig. 1. Here a compound- wound dynamo A is driven by means of an engine not shown, and sends current
through the motor B \
by means of the lines { M, M. The dynamo is driven at constant speed and its series- winding is adjusted so that the pressure at the terminals of the dynamo rises slightly as the current in- creases, due to the , . ; increase of the load on the motor. This slight rise in voltage is to make up for the loss in pressure in the line, as will be ex- plained later. The pressure at the motor remains nearly con- stant, no matter what ' "'^'^ load the motor may be carrying, but the current supplied in- creases as the load is increased. When
both lights and motors are operated, such a system will probably use a pressure of 110 or 220 volts at the receiving
4 ELECTRIC TRANSMISSION §23
end of the circuit; if used for power alone, a pressure of 250 or 500 volts will be employed. It should be mentioned that when the receiving end of a circuit is spoken of, the end distant from the station is meant, because this is the end where the various devices, such as lamps, motors, etc., receive their current.
7. liost Power and Line Drop. — In order that a transmission plant may be efficient, the generating apparatus, line, and motors must be efficient. Dynamos and motors of good make are generally satisfactory as regards efficiency, and the question is. How efficient can the line be made? In answer to this, it might be said that the loss of power in the line could be made as small as we please if expense were no consideration. All conductors, no matter how large, offer some resistance to the current and there is bound to be some loss in power. By making the conductor very large we can make the loss small, because the resistance will be low, but a point is soon reached where it pays better to allow a cer- tain amount of power to be lost rather than to further increase the size of the conductor. The pressure necessary to force the current over the line is spoken of, in power- transmission work, as the drop in the line, because this pressure is represented by a falling off in voltage between the dynamo and the distant end of the line.
8. If R is the resistance of the line and / the current flowing, the drop is, from Ohm*s law, e = I R. The power, in watts, lost in the line is / R X / = PR. The power lost, due to the resistance encountered by the current, reap- pears in the form of heat. The power generated by the dynamo is equal to the product of the pressure generated by the dynamo and the current flowing; or, if E^ repre- sents the dynamo pressure, then
watts generated = IVi = ExI (1)
The power delivered at the end of the line is equal to the product of the pressure at the end of the line multiplied by
§23 ELECTRIC TRANSMISSION 5
the current, or, if E^ represents the pressure at the distant, or receiving, end, then
watts delivered = IV, = E^I (2)
It should be particularly noted at this point that the cur- rent / is the same in all parts of the circuit. Thus, in Fig. 1 the same current flows through the motor that flows through the dynamo, tmless there is a leakage at some point between the lines, and this would not be the case if the lines were properly insulated. What does occur is a drop or loss in pressure between the station and the receiving end, but there is practically no loss in current except, perhaps, in a few cases where the line pressure is exceedingly high or the insulation unusually bad. This point is mentioned here because the mistaken idea that there is a loss of current in the line is a common one.
9. We have already seen that the number of watts lost in the line is given by the equation W = P R.
The lost power must also be equal to the difference between the power supplied and the power delivered, or W= Wr" W,, =E,/-E,/, =/{E,^E,).
El — E, represents the loss of pressure, or the drop, and it is at once seen that the greater the drop, the greater the loss in power. For example, a 5-per-cent. drop in voltage is equivalent to a 5-per-cent. loss of power in the line.
10. In order to transmit power, we must be willing, then, to put up with a certain amount of loss, or what is equivalent, with a certain amount of drop in the line. The amount of drop can be made anything we please, depending on the amount of copper we are willing to put into the line. The percentage of drop allowed is seldom lower than 5 per cent, and not often over 15 per cent, except on very long trans- mission lines; 10 per cent, is a fair average. In cases where the distribution is local, as, for example, in house wiring, the allowable drop from the point where the current enters the building to the farthest point on the system may be as low
6 * ELECTRIC TRANSMISSION §23
as 1 or 2 per cent. If the drop is excessive, the pressure at the end of the line is apt to fluctuate greatly with changes of load and thus render the service bad. In a few special cases there may be conditions that warrant the use of an excess- ive drop, but in general the above values are the ones com- monly met with.
11. When the loss, or drop, in a circuit is given as a percentage, this percentage may refer either to the voltage at the station end of the line, or the voltage at the receiving end. For example, suppose we take the case where the per- centage loss refers to the voltage at the station end, and let
/fi = voltage at dynamo;
R^ = voltage at end of line;
% = percentage loss (expressed as a number, not as a
decimal); e = actual number of volts drop in the line.
Then. E, = -^^f^^^ (3)
And '^m~f'%-^ <*^
Example. — The voltage at the end of a lighting circuit is to be 110 and the allowable drop is to be 3 per cent, of the dynamo voltage. {a) Wha: will be the dynamo voltage? (d) What will be the actual drop, in volts, in the circuit?
Solution. — (a) We have Ei = —.m — ^ "^ 113.4. Ans. (d) The drop e = -^^^3- - HO = 3.4 volts. Ans.
12. It is frequently more convenient to express the loss as a percentage of the power delivered at the end of the line. For example, if the voltage at the end of the line were 110, and the loss were to be an amount equivalent to 3 per cent, of the power delivered, instead of 3 per cent, of the power generated, it would mean that the allowable drop was 3 per cent, of 110, or 3.3 volts, instead of 3.4 volts. Railway generators are commonly spoken of as being adjusted for
§23 ELECTRIC TRANSMISSION 7
10 per cent, loss when they are wound so as to generate 500 volts at no load and 550 volts at full load; i. e., 50 volts, or 10 per cent, of 500, is allowed as drop in the line, 500 being: the voltage at the end of the line. In expressing the loss as a percentage, then, it should be distinctly understood as to whether this percentage refers to the power generated or the power delivered, otherwise there is liable to be con- fusion. The best way is to express the drop directly in volts and then there can be no doubt as to what is meant. In what follows, we will, when expressing the loss as a per- centage, refer to the power delivered unless it is otherwise specified, as this method is now very generally followed.
lilNE CAIiCUIiATIONS
13. Calculations for Tipvo-Wire System. — We are now
in a position to look into the method of determining the size of wire necessary for a given case. First consider the simple transmission system, shown in Fig. 1. The problem of determining the size of a line wire usually comes up about as follows: Given a certain amount of power to be transmitted over a given distance with a given amount of loss; also, given the required terminal voltage; determine the size of line wire required. The whole problem of deter- mining the size of line wire simply amounts to estimating the size of wire to give such a resistance that the drop will not exceed the specified amount. All the formulas for this purpose are based on Ohm's law, and are simply this law arranged in a more" convenient form to use. There have been a large number of these formulas devised, each for its own special line of work, and the one that is derived below is given because it is as generally applicable as any.
14. In the first place, if the watts or horsepower to be delivered and the voltage at the end of the line are given, we can at once determine the current, because
/ = f (6)
46—13
8 ELECTRIC TRANSMISSION §23
in which IV, is the power delivered. Furthermore, the drop e in the line is known or specified, and since
e = /R (6)
or /? = y, the resistance /? of the line is easily determined.
15. Referring to Fig. 1, it is seen that the totallength L of line through which the current flows is twice the distance from the dynamo to the end of the line. It has already been shown that the resistance of a wire is directly proportional to its length and inversely proportional to the area of its
cross-section, or/? = — -—, where A'is a constant that depends
A
on the units used for expressing the length L and the area of cross-section A. In practice, it is generally most con- venient to have the length L expressed in feet and the area A in circular mils. When these units are used, the quantity A' is the resistance of 1 mil-foot of wire; i. e., the resistance of 1 foot of wire nsW inch in diameter. If the area of cross- section of the wire were only 1 circular mil, it is evident that the resistance of L feet of it would be /CL, and if the area
of the wire were A circular mils, its resistance would be .
A
The resistance of 1 mil-foot of copper wire, such as is used
for line work, may be taken as 10.8 ohms. This resistance
will, of coiu"se, vary with the temperature and also with the
quality of the wire used, but the above value is close enough
for ordinary line calculations. The following formula may
then be used for calculating the resistance of any line:
y? = ^^^^ (7)
A
where /? = resistance in ohms;
L = length of line in feet (total length, both ways); A = area of cross-section in circular mils.
16. What is usually desired is the area of the wire required for the transmission, not the resistance, and by combining formulas 6 and 7 this can be obtained.
§23 ELECTRIC TRANSMISSION 9
|
We have ' |
e = IR, |
|
ft but |
r, 10.8 L A • |
|
hence, |
10.8 L I '~ A ' |
|
or |
^ _ 10.8 L I |
(8)
e
Expressing this formula in words, the required area of cross-section in circular mils
_ 10.8 X length of line in feet X current in amperes
drop in volts
This rule for determining the size of wire for a given transmission may be written as follows:
Bale. — Take the continued product of 10, 8 y the total length of the line in feety and the airrent in amperes; divide by the drop in voltSy and the result will be the area of cross-section in circular mils.
17. It will be noticed that the size of wire has been determined by making it of such dimensions that the drop will not exceed the allowable amount. In other words, the drop has been made the determining factor and no attention has been paid to the current-carrying capacity of the wire. li the distance were very short and the drop allowed were large, the size of the wire as given by the formula might be such that it would not carry the current without greatly overheating. This is an important consideration where wires are run indoors, because the distances are then short and the rise in temperature of the wire needs to be carefully considered, owing to the fire risk. This point will be taken up in connection with interior wiring. For line work such as we are now considering, the distances are usually so long that the size of wire as determined by the allowable drop is nearly always much larger than would be necessary to carry the current without overheating.
10
ELECTRIC TRANSMISSION
§23
18. The formula just given is also often written in the form
A = 2L-6.^ (9)
where D is the distance (one way) from the station to the center where the power is delivered. Evidently, D is only one-half the length of wire through which the current flows; i. e., L — ^D\ hence thej^^onstant 21.6 is used instead of 10.8.
19. Formulas 8 and 9 may be applied to a large number of cases if care is taken to see that the proper values are substituted. The length L or distance D must always be expressed in feet. The use of the formulas will be illus- trated in connection with the following examples. Table I, giving the area in circular mils of the various sizes of wire according to the Brown & Sharpe gauge, is here inserted for convenient reference in connection with the examples.
TABIiE I
SECTIONAL AREA OF B. & S. WIRES
|
No. |
Cross- Section |
|
B.&S. |
Circular Mils |
|
0000 |
2ii,6oo |
|
000 |
167,805 |
|
00 |
133.079 |
|
0 |
105,535 |
|
I |
83,694 |
|
2 |
66,373 |
|
3 |
52,634 |
|
4 |
41,742 |
|
5 |
33,102 |
|
6 |
26,251 |
|
7 |
20,816 |
|
8 |
16,509 |
|
9 |
13,094 |
|
10 |
10,381 |
No. B.&S.
II 12
13
14
15 16
17 18
19
20 21 22
23
24
|
Cross- Section |
No. |
Cross-Section |
|
Circular Mils |
B.&S. |
Circular Mils, |
|
8.234 |
25 |
320 |
|
6,530 |
26 |
254 |
|
5,178 |
27 |
202 |
|
4,107 |
28 |
160 |
|
3,257 |
29 |
127 |
|
2,583 |
30 |
lOI |
|
2,048 |
31 |
79.7 |
|
1,624 |
32 |
63.2 |
|
1,288 |
33 |
50.1 |
|
1,022 |
34 |
39.7 |
|
810 |
35 |
31.5 |
|
642 |
36 |
25.0 |
|
509 |
37 |
19.8 |
|
404 |
38 |
15.7 |
§23
ELECTRIC TRANSMISSION
11
Example 1. — In Fig. 1 the pressure at the receiving end of the line is to be 500 volts, and 40 kilowatts is to be transmitted with a drop of 50 volts. The distance from the station to the end of the Ime is 3 miles. Calculate the cross-section of wire necessary and g^ve the nearest size B. & S. that will answer.
Solution.— 40 K. W. = 40,000 watts; hence, current = HW = 80 amperes. The distance from the station to the end of the line is 3 mi., but the current has to flow to the end and back again, so that the length of line L through which the current flows is 6 mi., or 31,680 ft. Applying formula 8,
A = - ---
10.8X31,680X80
50
= 547,430 circular mils, nearly. Ans.
This is considerably larger than any of the B. & S. sizes, so that a stranded cable would be used.
Pio. 2
Example 2. — It is desired to transmit 20 horsepower over a line i mile long with a drop of 10 per cent, of the voltage at the receiving end. The voltage at the end of the line is to be 110. Find the size of wire required.
Solution. — 20 horsepower = 20 X 746 watts; hence,
20X746
current =
110
=T 135.6 amperes
The drop is to be 10 per cent, of the voltage at the receiving end; hence, drop e = ^ — = 11 volts. The length Z, is 1 mi., since the distance from the station to the end is i mi., and applying formula 8,
A = -
10.8 X 5,280 X 135.6
11
— = 702,950 circular mils, nearly. Ans.
This also would call for a large cable.
Example 3. — Fig. 2 shows a simple transmission system as used in connection with a street railway. The feeder a c runs out from the station and taps into the trolley wire x^y at the point c. The pressure
12 ELECTRIC TRANSMISSION §23
between the trolley and track at the point r is to be 500 volts, and the drop in the feeder is to be 10 per cent, of the voltage at the car when a current of 60 amperes is being supplied. The current returns through the track, and we will suppose in this case that the resist- ance of the return circuit is negligible. Required the cross-section of the feeder ac.
Solution. — In this case the drop takes place altogether in the
wire a Cy because the resistance of the return circuit through the rails is
taken as zero; hence, the length L used in the formula will be } mi.,
or 3,960 ft., and not twice this distance, as in the previous examples.
500 X 10 The drop in voltage will be ^ = — r^ — = 50, and since the current
is 60 amperes, we have
A = - — '- 1^ = 51,322 circular mils. Ans.
By referring to the wire table, it will be found that this is nearly a No. 3 B. & S.
20. In making^ line calculations, it seldom happens that the calculated value will agree exactly with any of the sizes given in the wire table. It is usual in such cases to take the next larger size, unless the smaller size should be con- siderably nearer the calculated value. Generally, the load operated on a line always tends to increase, because busi- ness increases, and it is better to have the line a little large, even if it entails a slightly greater cost when the line is erected.
21. Formula 8 may also be used for determining the drop that will occur on a given line with a given current. In this case the formula is written,
volts drop = ^ = — '—- — (10)
A
Example. — Power is transmitted over a No. 3 B. & S. line for a distance of 4,000 feet. What will be the drop in the line when a cur- rent of 30 amperes is flowing?
Solution. — The length of wire through which the current flows is 2 X 4,000 = 8,000 ft. The cross-section of a No. 3 B. & S. wire is 52,634 circular mils; hence,
. _ 10.8X8,000X30 .^„ .
volts drop = -^ ,.^, = 49.2. Acs.
oJ,oo4
§23 ELECTRIC TRANSMISSION 13
bxampl.es for practice
1. A dynamo delivers current to a motor situated 850 yards distant. The current taken by the motor at full load is 30 amperes, and the pressure at the motor is to be 220 volts. The drop in the line is to be 8 per cent, of the voltage at the receiving end. Required: (a) the drop in volts ; (d) the size of the wire in circular mils and also the nearest
size B. & S. A / ^^^ ^^-^ ^^^^
\{*) 93,886 cir. mils.; use No. 0 wire
2. A current of 40 amperes is transmitted from a station to a point 1 mile distant through a No. 0 B. & S. wire: (a) What will be the drop, in volts, in the wire? {d) How many watts will be wasted in the wire? . I {a) 43.2
^^^' \(d) 1,728
USE OF HIGH PRESSURE
22« By referring to the first two examples in Art. 19,
it will be noticed that the wire called for is very large,
although the amount of power transmitted is not very
great nor the distance long. Suppose a fixed number of
watts IVn to be transmitted with a given voltage B, at the end
of the line; then, the current that must flow through the
IV line is -a^. We have seen that the loss in the line is B,
/" R; i. e., if the current be doubled the loss becomes four times as great. If, then, the E. M. F. be doubled, we will be able to transmit the same amount of power with one-half the current, and hence with one-quarter the loss. Or, putting it the other way, and supposing that the loss is to be a fixed amount, we can, by doubling the pressure and thereby halving the current, use a wire of four times the resistance. For example, suppose we have to transmit 20 kilowatts at a terminal pressure of 500 volts and that the loss in the line is to be limited to 2 kilowatts. The current would be /= H?i^^ = 40 amperes, and 7*7? = 2,000 watts, otAO*R = 2,000; hence, ^ = Uu^ = 1.25 ohms. Now, sup- pose that a terminal pressure of 1,000 volts instead of 500 is used and that the same amount of power is transmitted with the same number of watts loss as before. The current will now be 7 = YoVo^ = 20 amperes, and 7* 7? = 2,000 watts, as
14
ELECTRIC TRANSMISSION
§23
before. We will then have 20'^ = 2,000; R = ^Vo^ = 5 ohms.
In other words, for the same amount of loss and for the same amount of power delivered^ the allowable resistance of the line can be made foUr times as great if the pressure is doubled. Since the lenfifth is supposed to be the same in both cases, this
'teidfVttastaf.
means that doubling the pressure makes the amount of cop- per required just one-fourth as great. If the pressure were increased threefold, the amount of copper required would be one-ninth as great, other things being equal. This may be stated as follows: For the same amottnt of power delivered and for the same loss in Power ^ the amottnt of copper required for transmission over a given distance varies inversely as the square of the voltage.
§23 ELECTRIC TRANSMISSION 15
23. Edison Three- Wire System. — From the preceding it is seen that an increase in the voltage results in a large decrease in the amount of copper required. Incandescent lighting was first carried out at a pressure of 110 volts, but this pressure rendered the use of large conductors necessary, and systems were therefore brought out that would permit the use of a higher pressure. In street-railway work, a pressure of about 500 volts soon became the standard, because this appeared to be the limit to which the voltage could be pushed for this class of work without danger to life.
The Edison three- wire system allows current to be supplied at 110 volts, although the transmission itself is really carried out at 220 volts, and therefore results in a large saving in copper over the 110- volt system. The three- wire system is shown in Fig. 3. Two compound dynamos A and B are connected in series across the two lines d e and h k. Each dynamo generates 110 volts, so that the pressure between the two outside wires is 220 volts, because the two machines are connected in series. A third wire, called the neuiraly is connected to the point / between the machines, so that between the lines de and fg there is a pressure of 110 volts, and between fg and hk3, pressure of 110 volts also.
24. In order to illustrate the action of such a system, suppose there are six 32-candlepower lamps on one side and four on the other, each lamp taking, say, 1 ampere. A current of 4 amperes will flow from the positive side of B through the line h k and through the lamps to the neutral wire. At the same time, a current of 6 amperes will tend to flow out from the positive pole of A over the line f g through the left-hand set of lamps and back through e </, as shown by the arrows. In the neutral wire there is a current of 6 amperes tending to flow in one direction and a current of 4 amperes tending to flow in the other direc- tion, the result being that the actual current is the differ- ence between the two, or 2 amperes, as shown by the full arrow; or, looking at it in another way, there is 4 amperes Sowing directly across from h k to de and 2 amperes flowing
16
ELECTRIC TRANSMISSION
§23
from A through the neutral wire /^ and back through ed to A, thus making 6 amperes in the line e d. If the cur- rents taken by the two sides were exactly balanced, no current would flow in the neutral wire and there would be practically a 220-volt, two-wire transmission. In any case, the current in the neutral wire is the difference between the currents in the two sides, and its direction will depend on which side is the more heavily loaded.
25. A three-wire system should always be installed so that the load on the two sides will be as nearly balanced as possible. The simplest way to estimate the size of the con- ductors is to first calculate the size of the outside wires,
/Mi'U.'
e^eyb/fs.
u
■ t f
MofOK
wovo/rs.
Pio.4
JOlamps 3ZaR
treating it as if it were a 220-volt, two-wire system. When motors are operated on the three-wire system, they are usually wound for 220 volts and connected across the outside lines. The following example will illustrate the method of calcu- lating the wires for a three-wire transmission:
Example. — Two dynamos deliver power over a distance of 1 mile to sixty 32-candlepower lamps, thirty lamps on each side of the circuit, as shown in Pig. 4. A motor that requires a current of 40 amperes is also connected across the outside wires. Each lamp requires a current of 1 ampere, and the pressure at the lamps is to t)e 110 volts. Calculate the size of wire required for the two outside conductors if the drop in pressure is not to exceed 10 per cent, of the voltage at the end where the power is delivered.
Solution. — The first thing to determine is the current. Thirty lamps are connected on each side and these lamps are connected in
§23 ELECTRIC TRANSMISSION 17
multiple, each taking 1 ampere. The current in the outside lines due to the lamps is, therefore, 30 amperes. The motor is connected directly across the outside lines; hence, the current due to the motor is 40 amperes, and the total current in the outside lines is 70 amperes. The pressure across the outside wires must be 220 volts at the end of the line, because the pressure at the lamps is to be 110. The drop in the outside wires is, therefore, 220 X .10 = 22 volts. The length of the outside wires is 2 mi., or 10,560 ft. Applying formula 8,
. , ., 10.8 X 10,560 X 70 -..^o Qon a circular mils = ^ = 362,880. Ans.
This would require a stranded cable.
26. The neutral wire is often made one-half the cross- section of the outside wires, though practice differs in this respect. It is seldom, however, made less than one-half, and in a number of cases it is made equal in cross-section. Of course, if the load could be kept very nearly balanced at all times, a small neutral wire would be sufficient, but it is impossible to keep the load balanced, and hence it is usual to put in a neutral of at least one-half the cross-section of the outside wires. In the above example, a No. 000 wire would probably be large enough for the neutral. For dis- tributing mains, where there is much liability to unbalan- cing, the neutral is made equal in size to the outside wires. In some special cases, three-wire systems are arranged so that they can be changed to a two-wire system by connecting the two outside wires together to form one side of the circuit, the neutral wire constituting the other. If this is done, the neutral would have to carry double the current in the outside wires and would be made twice as large as the outside wires.
27. Since the outside wires are only i the size required for the same power delivered by means of the two-wire, 110- volt system with the same percentage of loss, it follows that, even if the neutral wire be made as large as the out- side wires, the total amount of copper required is only 4 + i, or I of that required for the two-wire, 110- volt system. The amount of copper in the neutral wire is only i that required for the two-wire system, because it has i the cross- section and its total length is 2 that for the two-wire system.
18 ELECTRIC TRANSMISSION §23
28. From the preceding it is seen that the three-wire system of distribution effects a considerable saving in copper, owing to the use of a higher pressure. Three-wire systems operating 220-volt lamps with 440 volts across the outside wires have been introduced with considerable success, thus making a still further reduction in copper. The tendency has naturally been to use as high pressure as possible, but there are grave difficulties in the way of transmitting cur- rent at high pressure by means of direct current. These difficulties may be classed under the heads (a) difficulty of generating direct current at high E. M. F.; and (b) difficulty of utilizing direct current at high pressure after it has been generated.
29. Machines for the generation of direct current must be provided with a commutator, and this part of a well- designed machine gives comparatively little trouble if the pressure generated does not exceed 700 or 800 volts; beyond this point, it becomes a difficult matter to make a machine that will operate without sparking. Moreover, in direct- current dynamos, the armature winding has to be divided into a large nimiber of sections or coils, and the numerous crossings of these coils make it exceedingly difficult to insulate such armatures for high pressures.
30. Even if it were possible to generate high-pressure direct current, it would be difficult to utilize it at the other end on account of the danger to life. About 500 to 600 volts is as high as it has been found safe to operate street railways, the consideration of safety setting this limit on the pressure used. Moreover, it is just as difficult to build motors for high-pressure direct current as it is dynamos, and for most purposes the high-pressure current would have to be reduced to low pressure before it could be utilized with safety at the distant end of the line. This transformation could be effected by using a high-voltage motor to drive a low-voltage dynamo. In some cases, these two machines might be combined into one having an armature provided with two windings and two commutators, this armature being arranged so as to revolve
§23
ELECTRIC TRANSMISSION
19
in a common field mag^net. The high-tension current from the line is led into one winding throug^h one commutator and drives the machine as a motor. The second set of winding^s connected to the other commutator cuts across the field and sets up the secondary E. M. F., thus applying ciu*- rent to the low-pressure lines. A machine of this kind is known as a dynamotor. It is thus seen that the trans- formation of direct current from high pressure to low pressure involves the use of what is essentially a high-pres- sure, direct-current motor — a piece of machinery that is liable to give more or less trouble for the reasons already stated.
SPECIAL. THREE-WIRE SYSTEMS
31, The ordinary three-wire system requires two dyna- mos, and a number of special systems have been devised whereby a three-wire system may be operated from a single machine. Some of these systems will be found described in
290 V
UOW*
h
PlO. 5
connection with Electric Lighting. Perhaps the most common method, outside of the regular system using two' machines, is the use of a single large dynamo connected across the outside wires and a balancing set consisting of a pair of small
20 ELECTRIC TRANSMISSION §23
machines connected in series across the lines to take care of the unbalanced portion of the load, the neutral wire being connected between ^the machines, as described in Electric Lighting.
32, Dobrowolsky Three-Wire System. — Fig. 5 shows a method invented by Dobrowolsky for running a three-wire system from a single dynamo. A A is sn ordinary direct- current armature connected to its commutator in the usual manner. Two diametrically opposite points of the winding are connected to the rings r, r', and from these connection is made to the terminals of a choke coil. The coils c^ d have an equal number of turns, and as they are wound on the laminated iron core e^ they have a high inductance. The pressure applied to the terminals of r, d is alternating, because connection is made to the armature winding through slip rings r, r*. Since the E. M. F. applied to c^d is alternating, the coils will not short-circuit the armature because of the counter induced E. M. F. Also, since c and d have an equal number of turns, the point ^will always be at a potential midway between that of the two terminals attached to the collector rings, and if the neutral wire / is attached to the junction of c and d^ the pressiu"e between / and either outside wire will be one-half that between the outside wires. If the system becomes unbalanced, a direct current flows through /, but the choke coil offers no opposition other than the slight ohmic resistance of c and dy because this current is steady and cannot therefore set up a counter E. M. F. Also, if a direct current flows into the coils through /, it divides, half flowing through c and half through d ^ and since the two parts of the direct current circulate around the core in oppo- site directions, the magnetizing effect of the direct current is zero, and it does not therefore interfere with the choking effect that the coils exert on the alternating current.
33« Fig. 6 shows how this system has been applied by the Westinghouse Company. In order to get a more uniform action, the winding is tapped at four points, as in Fig. 6 (^i), and these points connected to four collector rings in exactly
ELECTRIC TRANSMISSION
21
the same way as for a quarter-phase rotary converter, the commutator and brushes being here omitted. The four nags A„B„A„B„ Fig. 6 (6), are connected to the choke coils Ci, C, and the mid-points x of each coil, or rather pair of coils, are connected to the neutral wire a. If the choke coils could be mounted in the armature and revolved with
it, the connections would be equivalent to those shown in Pig. 6 (c), and but one collector ring would be required to connect the neutral wire with the neutral point O. In some cases three pairs of choke coils are used connected to six equally spaced points in a manner similar to that shown in Fig. 6 (o), each point connecting to a collector ring. The
ELECTRIC TRANSMISSION
diagrams are here shown for two-pole machines; for maid- pohir machines there would be a connection to each ring for each pair of poles.
34. Fijr. 7 shows a method of operating a three-wire, direct-current system from two-phase, alternating-current mains. An arrangemeDt of this Icind is useful where the greater part of the output of a plant is utilized as alternating current, but where it is desired to use part of it for operating direct-current motors on the three-wire system or supply an existing three-wire, direct-current system from an altematiag-
cnirent station. A and B are two transformers with their primaries con- nected to the two phases and their secondaries connected in series and feeding a two-phase, A 77~ »~^^ three-wire rotary con-
verter. The mid-point ' Cof the two secondaries
is connected to the neutral wire N. It is evident that point C is always at a potential half way between that ■ of the outside wires, or in other words the pressure between C and D OT C and E is always half that between E and D, and the pressure between N and For N and G is half that between F and G, which is the condition required for a three- wire system.
35. lUroct-Current Convcpter. — Referring again to Pig. ft, it will be seen that instead of driving the armature .-/ by means of a belt and thereby operating a three-wire system from a single dynamo, the armature may be driven
§23 ELECTRIC TRANSMISSION 23
by means of current supplied from an outside source. When operated in this way the machine acts as a direct- current converter, and by means of it direct current can be transformed to another direct current at half the voltage, or the current supplied can be delivered as another at twice the origfinal voltage. For example, in Fig. 6, current at 220 volts can be supplied at the brushes and a current of twice the amount delivered at 110 volts. Or, if current is supplied at 110 volts to one pair of the three terminal wires, it will be converted to a current of one-half the volume at 220 volts. Direct-current converters have been used in some cases where it is desired to operate 250-volt motors from a 600-volt power circuit. These machines have so far been used but little for this class of work, motor dynamos or dynamotors having been used instead.
POWEK TRANSMISSION BY ALTER- NATING CURRENT
36. The difficulties encountered in the generation and utilization of high-tension direct current led engineers to adopt alternating current for places where the power had to be transmitted over considerable distances. At first, alter- nating current was used for lighting work only, because the single-phase alternators first introduced were not capable of readily operating motors, although they were quite satisfac- tory for the operation of incandescent lamps. With the introduction of polyphase alternators along with the induc- tion motor, the use of alternating current for power purposes became very common, and plants using line pressures as high as 60,000 volts are in regular operation.
37. Alternating current is well adapted for high-pressure work, because not only can it easily be generated, but what is even of greater importance, it can be readily transformed from one pressure to another. The winding of an alter- nator armature is very simple, no commutator is necessary, and the problem of generating high pressures becomes a
45— u
24 ELECTRIC TRANSMISSION §23
comparatively easy one. In some cases, the current is gener- ated at a low pressure and raised by step-up transformers for transmission over the line. At the distant end it is easily lowered, by means of step-down transformers, to any pressure required for the work to which it is to be put.
8INGIiE-PHAS£ TRANSMISSION
38. The simplest scheme for alternating-current trans- mission is that which uses a single-phase dynamo; i. e., a machine that generates a single alternating current. In Fig. 8, A represents a simple alternator generating current at a high pressure. This current is transmitted over the line to the distant end, where it is sent through the pri- mary of transformer B, which lowers the pressure to an amount suitable for distribution to the lamps /. The syn- chronous motor ^f is operated directly from the line, because it can be wound for a high voltage. If, however, this high pressure about the motor should for any reason be objection- able, step-down transformers could be used. As already mentioned, such systems are installed for lighting work almost exclusively. At first a pressure of 1,100 volts at the alternator, or about 1,000 at the end of the line, was commonly used. Later, pressures of 2,200 and 2,000 volts became the ordinary practice. In cases where the distance was very long, step-up transformers were used, as shown in Fig. 9. Here the current from the alternator A is first sent into the primary of the transformer Z", which raises the voltage to any required amount, with, of course, a corresponding reduction in current. At the other end, the transformer 7^ steps down the high line pressure to whatever pressure is suitable for local distribution.
39. The single-phase system has been used in the past to a limited extent for the operation of synchronous motors. The ordinary single-phase synchronous motor will not start up even if it is not loaded and this is a great drawback to its use. The single-phase system is therefore seldom installed where power is to be transmitted for the operation
26 ELECTRIC TRANSMISSION §23
of alternating-current motors of large size. The motor M shown in Fig. 8 is the same in construction as an alter- nator, but it would have to be provided with some arrange- ment for bringing it up to speed. It is possible that in the future single-phase, alternating-current motors may be so improved that this system will be used much more largely for power purposes than it is now. Experiments have already been made in the operation of electric railways by means of single-phase motors constructed similar to series direct-current motors, but having laminated fields. The results obtained have been so satisfactory that a large increase in the use of single-phase current for power purposes may be expected, though at present the single-phase series motor has not been used to any great extent in regular com- mercial work.
TWO-PHASE POWER TRANSMISSION
40. The great advantage of the two-phase system over the single-phase is that it allows the operation of rotary- field induction motors and two-phase synchronous motors. Fig. 10 shows a two-phase system. In this case, we have taken the simplest arrangement, where the alternator feeds directly into the line without the use of step-up transformers. If, however, the distance is very long, step-up and step-down transformers could be connected in each phase, in a manner similar to that shown in Fig. 9. A is the alternator supply- ing the two currents differing in phase by 90° to the four line wires. By B are two transformers supplying lights. One is connected on phase No. 1 and the other on phase No. 2, so as not to unbalance the load on the alternator. C, C are two large transformers supplying alternating current at 389 volts to the rotary transformer Z?, which changes it to direct current at 550 volts suitable for operating the street-railway system E. F, F are two transformers supplying a two-phase induction motor G, H shows a two-phase synchronous motor. This is the same in construction as the generator A, and it is not necessary to use transformers with it, as it can be con- structed for the same voltage as the generator.
28 ELECTRIC TRANSMISSION §23
THREE-PHASE POWER TRANSMISSION
41. In the three-phase system, if the load on all three phases is kept nearly balanced, as it usually is in practice, only three wires are needed. For the same amount of power, line loss, and distance of transmission, the three-phase sys- tem requires only three-fourths the amount of copper called for by the single-phase or two-phase systems. For this reason, it is often used for the transmission itself, even if the power is generated by means of two-phase alternators. By a special arrangement of transformers, described later, two currents differing in phase by 90° can be transformed into three differing in phase by 120°. Fig. 11 is similar to Fig. 10, except that it is arranged for a three-phase transmission. There is little choice between the two-phase and three- phase systems so far as actual operation is concerned, the chief point in favor of the three-phase system being the saving in line wire.
42. In many large transmission systems, it is customary to generate the power in one large central station and distribute it at high pressure to a number of substations located at the various distributing centers. At these sub- stations the current is transformed down and passed through rotary converters, if direct current is necessary, and dis- tributed to the various devices to be operated. This is commonly done in connection with both lighting and street- railway work. If alternating current alone is used, the volt- age is merely stepped down by means of large transformers.
At present, the three-phase system is the one most largely used for power transmission purposes. When the power is used for railway operation, the alternating current is changed into direct current, because heretofore alternating-current motors have not proved as satisfactory as direct-current motors for railway operation, hoisting, or other variable speed work. However, recent developments in the line of the single-phase series motor with laminated field seem to indicate that motors of this or similar type can be built so as
30 ELECTRIC TRANSMISSION §23
to have sufi&ciently largfe output and at the same time run without sparking. These motors have properties much the same as series-wound, direct-current motors. They give a good starting torque and are well adapted to variable speed. A great deal of experimenting is at present being done with them, and it is probable that the single-phase system will, in the future, be a strong competitor of the two-phase and three-phase systems for railway work.
lilNB CAIiCUIiATIONS FOR ALTERNATING
CURRENT
43. The factors that determine the size of line wire for a direct-current transmission apply also, in a general way, to alternating-current systems. The resistance of the line causes a drop in pressure between the station and the dis- tant end, and the line must be proportioned so that this drop will not be excessive. If the load to be carried is practically non-inductive, and if the distances are not long, the same rules that have already been given for direct-current circuits may be applied with sufficient accuracy to alternating-current lines. If, however, the lines are long, say more than 2 or 3 miles, there are other effects that must be taken into account. It must be remembered that the current is continually changing, and this introduces effects not met with in continuous-current circuits where the current flows steadily in one direction. The size of wire required will depend not only on the amount of the load, but also on the kind of load, i. e., on whether it consists wholly of motors or lights, or a combination of the two. In direct-current circuits, it makes no difference, so far as the drop in the line is concerned, how far the wires are strung apart on the poles, but in an alternating-current circuit this may have an appreciable effect.
The effects of self-induction and capacity on alternating- current transmission lines have already been given in con- nection with the subject of alternating currents. On all but very long transmission lines the effects of capacity are not serious, but the inductance of the line may have quite a large
§23 ELECTRIC TRANSMISSION 31
influence on the line drop. The relation between the line drop, terminal E. M. F., and generator E. M. F. has been shown by means of an E. M. F. diagram, and by laying out such a diagram, the size of wire for any particular case could be obtained. For ordinary line calculations, however, it is convenient to use formulas that may be easily applied, and that will give results accurate enough for most practical purposes.
FORMUIiAS FOR IjINE CAIiCUIiATIONS
44. Esttmatlon of Cross-Section of Ijlnes. — In a
direct-current transmission line a certain drop in voltage is equivalent to a corresponding loss in power. With alter- nating current, the percentage drop in pressure may be quite different from the percentage loss in power. In case alter- nating current is used, the drop in voltage will very likely be more than the corresponding loss in power, because of the self-induction of the line. Just what the drop will be, corresponding to a given loss in power, depends on the size of the wire, distance apart on the poles, etc. The exact calculation of line wires for alternating current i^ a compli- cated matter, but in nearly all the cases that arise in prac- tice they can be estimated with sufficient accuracy by means of comparatively simple formulas. The following formulas, originated by Mr. E. J. Berg, will be found convenient for estimating alternating-current lines. The different quantities entering into the calculations are as follows:
D = distance in feet over which power is transmitted (this distance is to be taken one way only, i. e., it is the single distance);
W% = total watts delivered at the end of the line (this number must express the actual watts delivered, not the apparent watts);
P = percentage of power lost in line (it should be noted that this percentage is that of the power delivered, not the power generated; also, it is the percentage power lost, not the percentage drop in voltage);
32 ELECTRIC TRANSMISSION §23
E^ — voltag^e required at the receiving end of the line, i. e., the voltage at the end where the power is deliv- ered; / = a constant having the following values: 2,400 for a single-phase system operating lights only; 3,000 for a single-phase system operating motors and lights; 3,380 for a single-phase system operating motors only; 1,200 for a three-wire, three-phase and four- wire, two-phase
system, all lights; 1,600 for a three- wire, three-phase and four-wire, two-phase
system, motors and lights; 1,690 for a three-wire, three-phase and four-wire, two-phase system, all motors.
The cross-section of the wire required for any given case may then be calculated from the following formula:
circular mils = *- (11)
Example. — 300 horsepower is- to be transmitted by means of the three-phase system over a distance of 5 miles with a loss of 10 per cent, of the power delivered. The pressure at the end of the line is to be 4,000 volts and the power is to be used altogether for operating motors. Calculate the size of line wire required.
Solution.— In this case the distance D is 5,280 X 5 = 26.400 ft. The watts delivered wiU be 300 X 746 = 223,800. P = 10 and E^ = 4,000. The constant / for this case will be 1,690; hence, we have from formula
. , ., 26,400X223,800X1,690 ^„ .^-
circular mils = iq x 4,00b~X lyOOO " = ^^'^^^
or about a No. 2 B. & S. Ans.
45. Estimation of Current in liines. — The current in the line wires of an ordinary direct-current line is easily obtained by dividing the watts delivered by the voltage at the end of the line. The current in the case of alternating- current systems can be calculated by using a similar formula and multiplying by a constant, to allow for the circumstances under which the current is used, as follows:
current in line = — '— (12)
§23 ELECTRIC TRANSMISSION 33
where IV^ = watts delivered;
£^ = voltage at the receiving end of the line; T = constant referred to above.
Values of Constant T
Single-phase system, all lights 1.052
Single-phase system, motors and lights 1.176
Single-phase system, all motors 1.250
Two-phase, four-wire system, all lights 526
Two-phase, four-wire system, motors and lights .588
Two-phase, four- wire system, all motors 625
Three-phase system, all lights 607
Three-phase system, motors and lights 679
Three-phase system, all motors 725
Example 1. — 100 kilowatts is delivered by means of the two- phase, four-wire system to a mixed load of motors and lights. The pressure at the receiving end of the line is 2,000 volts. Calculate the current in each line wire.
Solution.— 100 K. W. = 100,000 watts. For this case the con- stant T will be .588; hence,
100,000 X .588 ^ . .
current = ^ ^r^ = 29.4 amperes. Ans.
Example 2. — 200 kilowatts is transmitted by means of the three- phase system, the voltage between lines at the receiving end being 4,000 volts. The load consists wholly of motors; calculate the current in each line.
Solution.— 200 K. W. = 200,000 watts. For this case the value
of T will be .725; hence,
200.000 X .725 «^ _ .
current = . ,^^^ = 36.25 amperes. Ans.
4,000
46. Estimation of Drop. — The volts drop in the line
p £^
for a continuous-current system would be ~;^7r» when P is
the percentage of delivered power lost and E, is the voltage at the receiving end of the line. This formula can be made to give the approximate drop in an alternating-current line by multiplying it by a constant that takes into account the conditions under which the line is operated, as follows:
volts drop in line = — — ^ (13)
34
ELECTRIC TRANSMISSION
§23
The value of M depends on the frequency, the power factor of the load, and the size of the line wire; its value, under various conditions, is g^iven in the following table:
TABLE n
|
1 • |
m S3 O ■ n < 211, 6oo 167,805 I33»079 105.535 83.694 66,373 52,634 41,742 33.102 26,251 20,816 16,509 |
Values of M |
r |
||||||
|
30 Cycles |
60 Cycles |
12s Cycles |
|||||||
|
CO PQ s o d |
0 1 |
4^ % 0 |
a 0 la |
0 « bi |
40 "^ •0 8 2 0 '•1* 0 |
a 0 S 0 «i* 0 ;3 |
0 *•> if |
1 u 0 3.06 2.62 2.25 1.96 1.74 1-54 1.38 1.26 1. 16 1.08 1. 01 1.00 |
0 « 0 0 |
|
OOOO OOO oo o I 2 3 4 5 6 7 8 |
1.26 1.20 1. 15 I.IO 1.06 1.03 1.02 1.00 1.00 1.00 1.00 1.00 |
1.27 I. 17 1.08 I.OO 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 |
1.24 I. 14 1.05 I.OO 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 |
1.64 1.49 1.39 1.30 1.23 1. 16 I. II 1.07 1.04 1.02 1.00 1.00 |
1.85 1.63 1.46 1.32 1. 21. I. II 1.04 1.00 1.00 1.00 1.00 1.00 |
1.85 1.62 1.42 1.28 1. 16 1.06 1.00 1.00 I.OO 1.00 1.00 1.00 |
2.44 2.15 1.92 1.73 1.57 1.44 1.35 1.26 1. 19 1. 14 1.09 1.06 |
3.M 2.67 2.29 1.99 1.73 1.53 1.38 1.22 I. II 1.03 1.00 1.00 |
Example. — 600 kilowatts is to be transmitted a distance of 6 miles by means of the three-phase 60-cycle system. The loss in the line is to be limited to 10 per cent, of the power delivered, and the pressure at the receiving end of the line is to be 6,000 volts. The current is to be supplied to a mixed load of motors and lights. Calculate: (a) the size of the line wire; {b) the current in each line; (r) the volts drop in the line; and (^) the pressure generated by the dynamos at full load.
Solution.— (j) 600 K. W. = 600,000 watts. 6 mi. « 6 X 5,280 = 31,680 ft. Using formula 11, we have, since /for this case is 1,500,
, .- 31,680X600,000X1,500 «« «««
circular mils = ^^ - ^^ x e.OOO" = ^^'^
A No. 1 B. & S. wire would therefore be used. Ans.
§23 ELECTRIC TRANSMISSION 35
(d) In order to obtain the current in each line we use formula 12, and for this case, the value of T will be .670; hence,
^ 600,000 X. 679 a^? a ^ a
current = ^ ^^^^ = 67.9 anlperes. Ans.
OjlMJU
{c) In order to calculate the volts drop in the line, we use formula 13. For a No. 1 wire and a frequency of 60 cycles on a combined lamp and motor load, the value of the constant ^is found to l>e 1.21 by referring to the table; hence,
,^ , 10X6,000X1.21 .-^ . volts drop = :r7rT = 726. Ans.
(d) Since the drop in the line is 726 volts, the pressure at the dynamo must be 6,000 -h 726 = 6,726 volts when the full-load current is being delivered. Ans.
Note.— In the above example, the drop In the line woold have been only 600 volts if continuoas current were used.
EXAMPLES FOR PRACTICE
1. 250 horsepower is to be supplied to 60-cycle induction motors by means of the two-phase, four- wire system over a line 3 miles long. The pressure at the distant end of the line is to be 4,000 volts and the loss in the line is to be limited to 8 per cent, of the power delivered. Cal- culate: {a) the size of the wire required; {d) the current in each line
wire; (c) the drop in the line.
Ans.^
(a) 39,000 cir. mils, nearly; about No. 4 B. & S. 29.14 amperes 320 volts
liJ!
2. A three-phase alternator delivers 400 horsepower to a mixed load of motors and lights. The pressure at the distant end of the line is 3,000 volts. Calculate the current in each line. Ans. 67.54 amperes
3. 5,000 incandescent lamps are supplied with current from a single- phase alternator, having a frequency of 125, over a distance of 3 miles. The loss in the line is to be limited to 10 per cent, of the power deliv- ered, and the pressure at the end of the line is to be 3,000 volts. Allow 60 watts for each lamp supplied and calculate: (a) the size of the line wire; (d) the current in the line; {c) the volts drop in the line; (d) the
{a) 126,720 cir. mils, or about
No.OOB. &S. {d) 105.2 amperes (r) 576 volts (d) 3,576 volts
voltage at the generator.
Ans.
36 ELECTRIC TRANSMISSION §23
THE SELECTION OP A SYSTEM
47. From the foregoing: it is seen that the engineer has a large number of systems to choose from when installing a given plant, and the selection of a system for any given case is a matter that requires careful consideration. We will, therefore, endeavor to sum up the principal advantages and disadvantages of the different systems as an aid in determin- ing the system to be used in any given case.
The selection of a system, so far as its bearing on the location of the station is concerned, is comparatively unim- portant in ordinary street-railway work, as the 500-volt, direct-current system is the standard American practice, due allowance being made for distance. But in the case of lighting and power distribution over large districts, and for long-distance railway work, the problems require careful analysis.
DIRECT-CURRENT SYSTEMS
48. If lighting and motive power are required, the first points to be considered are the characteristics of the town and nature of the business to be expected. In compactly built, thickly settled places, where a good site for a station can be had within a mile from the most distant lights or motors, there is no better or cheaper system, either in first cost, economy, or convenience of operation than the direct- current system, and whether it should be two- or three-wire, circumstances will determine. Where distances exceed 1 mile, boosters can be used advantageously, or the double-bus system of high and low potential. These last two arrange- ments are described more in detail later. In the follow- ing we will state the potential on the system of distribution, and due allowance must be made for drop in E. M. F. between generators and the point where the energy is utilized.
§23 ELECTRIC TRANSMISSION 37
49. The tTvo-Tvlre, 220-voIt system is in successful operation, and the 220-volt incandescent lamp is perfected for use on a commercial basis. There can be no question of the great advantage of a 220-volt, two-wire system over the three-wire system in simplicity and reduced cost of copper. It must be recognized, however, that greater care is required in insulating and installing all interior fittings that require more or less handling.
50. Three-Wire, 220- Volt System. — The advantages of the three-wire, 220-volt, direct-current system are many, among which may be mentioned the following; some of these also apply to the 220-volt, two-wire system.
1. Low potentials in dynamos, station apparatus, and street lines, and consequent perfect safety to the dynamo attendants, linemen, and the public.
2. Greatly lessened leakage, and therefore reduced risk from fire.
3. Convenience, cheapness, and ease of connection to the wiring on the consumers* premises.
4. The reading at the station, of pressure returned from extreme feeder ends by means of pressure wires, as described later, indicates quite accurately the pressure at the consumers* premises.
5. As the dynamos are run in parallel on the system in conjunction with station methods of regulation and control, it is possible to tie the mains and feeders together wherever convenient, thus insuring by equalization a more uniform pressure, no matter to what extent the electrical center or heavy load in the district may shift during the 24 hours. By enabling the lightly loaded lines to supplement those that are heavily loaded, this system of intermeshing conductors equalizes the potential and gives the best results from a given weight of copper.
6. The use of direct current makes possible the employ- ment of storage batteries as an adjunct to the central station, thus lessening the hours during which it may be necessary to operate a considerable portion of the steam plant, minimizing
38 ELECTRIC TRANSMISSION §23
the labor accotint, and enabling one to run the boilers, engfines, and dynamos at a higher efficiency during the period they are in operation, and to shut them down as soon as the load is low enough to justify throwing all or a portion of it on the storage battery. Moreover, in case of a sudden or heavy demand for extra current, such as may be occasioned by bad weather or sudden thunder storm, the battery is always on hand, ready to be thrown on instantly to supple- ment the dynamos, whereas it requires some time to start an idle engine and throw in its dynamos.
7. Electrolytic and electroplating work can be done with the direct current, but is impossible with alternating currents, except at considerable expense and complication for rotary converters or other transforming devices.
8. The measurement of power, calculation of conductors, and arrangement of circuits are simpler than in the alter- nating system, on account of the absence of induction and consequent lag effects.
9. Simple and efficient motors are readily installed and operated, and form a considerable source of income.
10. The broad establishment of the business, the vast amotmt already served by the three-wire system, and its standardized methods largely influence its adoption.
But the three-wire system has manifest disadvantages, the most prominent of which are as follows:
1. The two sides of the system must be kept at nearly equal loads, as want of balance occasions a difference in potential between the positive and negative sides, and conse- quently a difference in the brilliancy of the lights.
2. If overhead lines are used for large currents, they are cumbersome, costly, and extremely liable to disaster from high winds or lightning.
3. It is impossible to cover a very large extent of territory at 260 volts potential without great expense for copper.
4. A ground on any part of the wiring, no matter how trifling in itself, may be a fault on the whole system, and if not promptly eliminated may give rise to a bad short circuit.
§23 ELECTRIC TRANSMISSION 39
5. Switchboard and other connections are complicated because of the use of three wires and the operation of the dynamos in pairs connected two in series.
51. Tliree-Wlre, 600-Volt System. — ^A larger extent of territory can be served by the use of the three-wire, 250-500-volt, direct-current system because it has greater capabilities of expansion, with less investment in copper for lightly loaded or scattered territory, as well as requiring less copper in heavily loaded business districts. The advantage stated for the 500-volt, three-wire system with the same cur- rent distribution and the same station location, is that it will cover, at the same cost of copper, a territory four times as large as with a 250-volt, three-wire systen;. Increased risks are encountered as mentioned, as regards insulation within buildings and for underground distributing systems because of higher potential, but these conditions are successfully met by the employment of standard appliances. The important point is that the ignorant consumer shall be fully protected when current is supplied him at potentials bordering on the danger line.
ALTERNATING -CURRENT STSTEM8
52. The alternating-current system has great value in the special field of transmission for long-distance and house-to-house supply in scattered territories, and is excel- lent and comparatively economical as a temporary expedient for developing business in a new territory. Before alter- nating current can be used in compact territory in com- bination with, or to replace, direct current, the following improvements are necessary:
1. A type of motor must be developed that will meet all commercial requirements, which can be used successfully for all classes of business without causing disturbance of the fixed potential of the system.
2. A universal system of supply that does not require transformers or anything except a meter to be located on the premises of the customer,
45—15
40 ELECTRIC TRANSMISSION §23
*
3. Some type of apparatus that will replace the storage battery as used in connection with direct current.
Alternating current cannot be used in connection with storage batteries, except through the employment of a rotary converter or motor generator for charging the battery. The use of such converting apparatus will be justified when the amount of current supplied and compensation received is sufficiently large to overbalance the extra cost for special equipment and the losses incurred for conversion of energy.
The direct-current motor can be better applied for general power work, and in some respects is superior to the alter- nating-current motor in its electrical operation. The dis- turbing effects on the system are less, when starting and stopping large motors. The initial cost of direct-current motors and their few necessary auxiliaries is much less than that of alternating-current motors. Alternating-cur- rent induction motors, on the other hand, have the advan- tage over direct-current of not requiring a commutator and brushes. Direct current is best adapted for elevator work.
With direct current at least 80 per cent, of the manu- factured power can be accounted for through the meters on a good system, whereas with the alternating-current system, from 50 per cent, to 60 per cent, only of the power can be accounted for; the rest is lost in transformers and special devices.
The comparative usefulness of the two systems for com- mercial distribution is illustrated in Chicago, where with a maximum output of 25,000 kilowatts, 20.4 per cent, is for 60-cycle distribution covering a territory of 58 square miles, and 79.5 per cent, is for direct-current distribution over a territory of 10 square miles.
The concensus of expert opinion is that the alternating- current system has not attained the requisite degree of per-
«
fection for general distribution, in compact territory, though for long-distance work it is indispensable. In compact terri- tory it cannot be used with storage batteries; the motor cannot be used for general power purposes. It is therefore
§23 ELECTRIC TRANSMISSION 41
evident that there is not yet any single ideal system that can be universally applied to serve all local conditions; special requirements, the environment of the station, and relative commercial importance of the various classes of service must be taken into account in determining what is most desirable for each given locality.
53. The problem for a combination system may, for example, be solved as follows:
For incandescent lighting and motive power in the business and near-by residential districts, the three-wire, direct-current system, 220 volts.
For incandescent lighting and some classes of motive power in scattered and long-distance territory, the alter- nating-current system, 2,300 volts primary; 110 to 220 volts secondary.
For arc lighting in streets, the enclosed series-arcs on the alternating-current system.
If the bulk of the power is transmitted over a long dis- tance, or supplied to a widely scattered area, the two-phase or three-phase systems would be installed; that is, only one kind of current would be furnished from the station, and if direct current were essential for any special purpose, it would be transformed at the consumers* premises by means of a rotary converter.
In general, it is well to avoid too great a variety of apparatus in a station, because it necessitates several sets of duplicate machines. Considerations of economy are fre- quently sacrificed in order to make the generating units in a given station uniform as to size and output.
FREQUENCY
54. The choice of a proper frequency in alternating- current systems is important. The early single-phase plants were designed for from 125 to 150 cycles, and some poly- phase machines have been built for these frequencies. The high inductive effects, troubles in parallel operation, and the
42 ELECTRIC TRANSMISSION §23
difficulty of obtaining low speeds have caused such higfh frequencies to be abandoned in favor of 60 cycles or less. In polyphase plants, therefore, 60, 40, and 25 cycles have come to be the standard frequencies. The choice of frequency should be governed by a careful consideration of the apparatus to which the plant is to furnish power.
If the alternating current is to be used for lighting pur- poses only, a high frequency affords the advantage of low first cost, and such a system might be even single phase. However, the demand for electric power is now so great that a low-frequency polyphase system is nearly always used in modem alternating-current installations. The cost of transformers, per kilowatt, diminishes as the frequency increases and this is one of the reasons why high frequency was used in the early installations when belt-driven, high- speed alternators were used almost exclusively. With the introduction of slow-speed, direct-driven machines, low frequencies became desirable, and the increasing use of induction motors, synchronous motors, and rotary con- verters also led to the introduction of lower frequencies. A frequency of 60 cycles is suitable for incandescent light- ing, arc lighting, and some motive power. When the current is used nearly altogether for power purposes, it is better to use lower frequency; 60 cycles will only be found satisfactory with synchronous motors, rotary converters, and similar apparatus when the speed regulation of the motive power is very good, because of the hunting or periodic surgings in speed that are liable to occur. A frequency of 40 cycles permits current for both lighting and power purposes to be supplied to advantage. It is within the limit of reasonable safety for operating rotary converters and is the lowest limit for satisfactory working of incan- descent and arc lights; 40-cycle equipments are not in general use and should only be adopted after analyzing all anticipated or existing conditions and finding that 60 cycles cannot be used with reasonable safety. A frequency of 25 cycles is very commonly used where the current is supplied wholly for power purposes.
§23 ELECTRIC TRANSMISSION 43
COST OF CONDUCTORS
55. In order to determine the best potential for a power transmission, it is necessary to consider carefully the cost of the transmission circuit. The weight of the electric con- ductor decreases as the square of the potential employed, and increases as the square of the distance. Dividing the potential by the distance gives a convenient figure, which can be used for all potentials and distances. The curves on the diagram, Fig. 12, given by the General Electric Company, furnish a ready means of obtaining the amount of copper required for a given power transmission. The figures on the curves indicate volts per mile; i. e., potential of line at generator divided by distance in miles. The weight of copper, potential, and line loss are in terms of the power delivered at the end of the line, and not of generated power. The curves are correct only for three-phase current with 100 per cent, power factor. Two-phase, single-phase, or continu- ous-current transmission requires one-third more copper. Five per cent, has been allowed for sag and waste in weights of copper given.
Example. — If copper is worth 15 cents per pound, what will the cost of copper be for a line (three-phase) to transmit 1,000 kilowatts at 10,000 volts over a line 10 miles long, with a loss of 5 per cent, of the delivered power?
Solution.— -Since 1,000 K. W. at 10,000 volts is to be delivered
1- in • 1 --Li. c ^1 1. 10,000 volts
over a line 10 mi. long with 5 per cent, loss, we have — tt^ — r —
^ ' '^ 10 mi.
= 1,000 volts per mi. Looking on the 1,000-volt curve, we find 5 per
cent, loss corresponds to 57 lb. of copper per kilowatt delivered. 1,000
K. W. X 57 = 57,000. If copper costs 15c. a pound the cost will be
57,000 X $0.15 = $8,550. Ans.
44
ELECTRIC TRANSMISSION
§23
5
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§23 ELECTRIC TRANSMISSION 45
COMBINED OPERATION OF DIRECT-
CURRENT DYNAMOS
OPERATION OF DYNAMOS IN SERIES
56. Dynamos are not very often run in series. Perhaps the most common case is where they are run in pairs of two in series on the three- wire system. Whenever dynamos are connected in series, their pressures are added in the same way as the voltage of two or more cells connected in series, but the current output is not increased. Series-wound dyna- mos are sometimes run in series, especially when used for arc lighting. In this case, the connections are very simple; the positive pole of one machine is connected with the negative pole of the other, so that the pressures of the two machines are added together instead of opposing each other. Gen- erally speaking, series-wound, shunt-wound, or compound- wound machines may be run in series with very little difficulty; in the case of the last named type, the compound coils' must of course be connected in series in the line. In most cases, however, the demand is for a large current out- put rather than for a high voltage; hence, plain series running is not common, except, perhaps, on arc-light circuits.
OPERATION OF DIRECT-CURRENT DYNAMOS IN
PARALLEL.
57. Dynamos, both direct and alternating, are much more frequently operated in parallel than in series. In Fig. 18 each machine generates the same voltage, and the pressure between the lines is the same as if a single machine were used; i. e., the pressure between the lines is not increased by adding machines in parallel, but the cur- rent delivered to the line is increased because the line current is the sum of the currents delivered by each of the machines.
46 ELECTRIC TRANSMISSION §23
Each machine is connected through its main switch M, M' to the heavy conductors C, D, like terminals of each machine being connected to the same bar. Each machine, when so connected, delivers current to the main bue-bars C, D and thence to the line.
It is not as easy a matter to operate machines in parallel as in series. It is evident that the voltage of each of the machines must be kept at the proper amount if the com- bination is to operate satisfactorily; for, suppose the E. M, F. of B should fall below that of A, then A would send oirrent through B and run it as a motor, and B would thus be
taking current from A instead of helping it feed into the line. There are a number of things that must be taken into account when machines are run in parallel that do not have to be considered when they are run separately. Compound- wound machines are run in parallel more than any other type in this country, though shunt machines are frequently run in this way also. Series machines are seldom run in parallel, for reasons to be given later. We will, however, first consider the series machine briefly, because the com- pound-wound machine is a combination of the series- and shunt- wound machines.
§23
ELECTRIC TRANSMISSION
47
8BRIES DYNAMOS IN PARAL.UCL
58. Suppose two series dynamos are in parallel, as shown in Fig, 14, and assume that they are delivering current to a load of some kind and that each machine supplies, say, one- half of the current. Now, it the E. M. F. of one of the machines A drops slightly, due to a slight variation in speed or any other cause, the amount of current delivered by A will decrease, and thus decrease the field excitation, because the current through the field coil is the same as the current delivered* by A. This lowering of the field excitation of A will still further cut down its E. M. F. and matters will go from bad to worse until, in a very short time, A will be driven as a motor, unless the belt on the heavily loaded machine should slip and thus bring down its voltage. The trouble is
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made still worse by the fact that the extra load thrown on B will raise its E. M. F., because the field of B will be strengthened. Moreover, when A is rtm as a motor, its direction of rotation will be reversed; and this may result in considerable damage. It is thus seen that two series machines connected in parallel, as shown in Fig. 14, will be very unstable in their action, and it is not practicable to so operate them.
59. Equalizing: Connection. — The unstable condition just referred to can be remedied by using an equalizing con- nection, or equalizer, as it is commonly called.' This is shown in Fig. 15, where the wire c d is the equalizer. It is a wire of low resistance connecting the points c and d
48
ELECTRIC TRANSMISSION
§23
where the series-coils are attached to the brushes; e and / are the regular terminals of the machine. Now suppose that the machine B delivers a greater current than A\ part of this current will flow to the -h line through the coil df^ but part of it will also take the path d~c-e through the field coil ^^ of
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machine A, The result is that part of the current delivered by B helps to keep up the field excitation of A, thus bringing up its voltage and equalizing the load between the machines. \i A delivers the greater part of the load, due to a drop in the voltage of B^ then part of the current flows through the path c-d-f and strengthens the field of B,
SHUNT DYNAMOS IN PARALLEIi
60. Shunt dynamos will operate very well in parallel. They have two properties that make their parallel operation a comparatively easy matter. In the first place, they are capable of exciting their own field no matter whether they are delivering current to the main circuit or not. In the second place, their voltage drops slightly with an increase in the load, and this tends to make their parallel operation stable. Suppose two shunt machines are arranged as shown in Fig. 16; A and B are the armatures, 5, S' the shunt field windings, and r, r' the adjustable field rheostats. Z, L' are switches in the field circuit and M^ M^ main switches con- necting the machines to the line. Suppose that machine A is in operation, as indicated by the closed position of switches L and M, To throw machine B in parallel, it is run up to speed and the switch L' closed; B will at once
§23
ELECTRIC TRANSMISSION
49
begin to pick up its field and run up to voltag:e. If the two machines are generating the same voltage and if their polarity is the same, as it should be, a voltmeter connected to blocks i, 2 will give no deflection, because the tendency of the machine A to send current through the voltmeter will be opposed by B, This state of affairs can be brought about by adjusting the rheostat r' until the voltmeter indi- cates that the voltages of the machines are equal, after which the switch M' may be closed and the field excitation of B again adjusted _
until the proper share — r*** i-PI H^^--^
of the load is carried. d a w\
In practice, it is gen- erally found better to have the voltage of B about 1 or 2 per cent, higher than that of A when the machine is thrown in. Very often, when shunt machines are arranged for parallel operation, the field is connected across the bus -bars instead of the armature of each machine. When this is the case, the field connection is made as indicated by the dotted lines ry^ r^ y'y instead of being connected as shown by the full lines rx, r^ x'. The effect of this is that the switch M must be closed before A will pick up, assuming that B is not in operation. If A is running and B is to be thrown in, then the switch U is closed and ^'s field is at once excited from the mains, so that B comes up to voltage almost immediately; after the voltage has been adjusted, switch M' may be thrown in as before.
Pio.16
61, We will suppose that the two shunt machines, Fig. 16, are running properly in multiple and will now see whether
50 ELECTRIC TRANSMISSION §23
their operation will be stable or not. It has already been seen that the shunt dynamo lowers its voltag:e as the current output increases. Now suppose that the voltage of A should drop slightly on account of a drop in speed or from any other cause. The tendency will be to throw the bulk of the load on B^ with the result that B'^ voltage will also drop on account of the above-mentioned property. The dropping of ^'s voltage will relieve it of part of its load and will make it divide with A, It is thus seen that there is an automatic tendency for the load to equalize. Again, suppose that the load on the line is suddenly increased, and that machine B takes more than its share of the current; the large current delivered by B will cause its E. M. F. to drop to more nearly that of A^ and the load will thus be equalized. If the voltage of one machine should for any reason become so low that the other machine runs it as a motor, no harm is liable to result, because the direction of rotation of the machine as a motor will be the same as when driven by the engine as a dynamo. As far as parallel running goes, the shunt dynamo is satisfactory, but it has been replaced by the compound machine, because the latter will maintain the line voltage with an increase of load; whereas, with shunt machines, the line voltage will fall off, unless the switch- board attendant cuts out some field resistance.
COMPOUND MACHINES IN PARALT^EIi
62, Since the compound machine is a combination of the series and shunt machines, one would naturally infer that the arrangement for parallel running would be a com- bination of the two preceding ones. Fig. 17 shows the connections in their simplest possible form; machines A and B are of equal size and the equalizer E runs directly between them; c and / are the + terminals of the machines, while c d and / e represent the leads, or cables, running to the switchboard; g h and k I are the negative leads running to the negative bus-bar h L There would, in practice, be a main switch in each of these negative leads, but as they
§23
ELECTRIC TRANSMISSION
61
are not essential for the present purpose they have been omitted. As shown by the full lines in Fig. 17, the shunt windings of the machines are connected in what is known as sliort shunt; i. e., the shunt field is connected across the brushes. Sometimes the shunt field is connected in lonsT shunt across the terminals of the machine or across the bus-bars. It makes very little difference as to the performance of the machine which connection is used.
Fig. 17
Most compound machines are provided with low-resistance shunts 5", S across their series-coils in order that the degree of compounding may be adjusted. These shunts should be adjusted so that the machines, when running separately, will give the same degree of compounding, which means, in the present case, that when each machine is delivering the same current, the voltage generated will be the same, because we are now assuming that A and B are of equal
52
ELECTRIC TRANSMISSION
§23
size. Another condition that must be fulfilled is that the resistance between the points a and d must be the same as between b and e. Since we are, for the present, assuming^ that the machines are of the same size and make, the resist- ance of their series-coils a c and b / will be almost exactly the same. The resistances of the switchboard leads c d and / e must, therefore, be equal; the resistance of the equalizer E should be as low as possible, and it should never be more than the leads c d or { e.
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63. We will now examine the action of the machines under a varying load. In the first place, if the resistance between ad \% equal to that between be and the machines are delivering: equal currents, then the drop through ad will equal the drop through b e and points a and b will be at the same potential. Since current can only flow between points at different potentials, there will be no current in E under such circumstances. Suppose, however, that A delivers a greater current than B\ then the drop in ad will exceed that in ^ ^ and current will flow through the path a-E-b-f-Af^-e and thus build up the voltage of machine B and equalize the load. If B delivers more current than A, the drop in ^ ^
§23 ELECTRIC TRANSMISSION 63
exceeds that in a it and current flows through the path b-E-a-c-M-d, builds up the voltage of A, and makes A take its share of the load.
64. In Fig. 17 the equalizer E is shown as connectine the positive brushes. This is usually the case in practice, though it would work just as well if both a and b were nega- tive brushes and c i the negative terminals of the machines. It is only necessary to see that the equalizer connects those brushes to which the series-coils are attached, and also to see that the brushes are of the same polarity on each of the
machines. In some cases, the equalizer wire is run directly between the machines as shown, but often a third wire is run from points a and b to the switchboard and there con- nected to an equalizer bar, as shown in Pig. 18. This represents a very common arrangement, triple-pole switches being used; the two outside blades for the + and — leads and the middle blade for the equalizer. There is a differ- ence of opinion as to whether it is better to run the equalizer to the switchboard or run it directly between the machines, as in Fig. 17, The most recent practice tends toward running it directly and placing the equalizer switch near the machine.
54
ELECTRIC TRANSMISSION
§23
This undoubtedly makes the connections shorter and thus leads to better reg:ulation. In such cases, the equalizer switch is usually mounted on a pedestal near the machine, as shown in Fig. 19.
65. In some railway plants, especially in those where large g^enerators are used, the main switch that is on the same side of the machine as the equalizer is placed on the stand near the machine alongside the equalizer switch. These two
O90rLm»_^^
switches are at practically the same potential, and there is no objection to placing them near each other. In case this is done, one of the bus-bars is placed under the floor near the machines and connected directly to the main switch. This shortens the connections considerably and makes the equal- ization of the load closer. It also has the advantage of simplifying the switchboard connections and avoiding crowding on the generator switchboard panels. Fig. 20
§23 ELECTRIC TRANSMISSION 55
shows the arrangement referred to. For lighting switch- boards or for small railway boards, both terminals of each machine are nm to the switchboard. In Fig. 20 the main connections only have been shown, the shunt coils of the machines and all minor connections being . omitted. The switches a and b are the equalizer and main + switches, respectively, the equalizer switch being connected to the brush to which the series-field c is attached. The + lead from b connects to the + bus-bar under the floor. Note that these leads should all be of the same length in order to secure close equalization. In the case of machines 1 and 2 the leads are doubled back as shown at d in order to make them of the same length as those running from the more distant machines. The general method of starting up, say, machine 1 and throwing it in parallel with others is as follows: See that all switches on the generator panel of the machine are open, and get the dynamo up to speed. Then close the equalizer switch a and the + switch b. Also, close the field switch on the generator panel. Some of the current furnished by the other machines will flow through the series-coils r, because the series-coil of machine 1 is in parallel with the other series-coils. This current in the series-coils will cause the machine to pick up rapidly, and since the shunt circuit is also closed, the machine soon comes up to full voltage. The voltage is then adjusted by means of the rheostat until it is equal to or a little higher than that of the other machines, and the negative switch e is then closed, thus placing the machine in parallel with the others. This method of pro- cedure applies to the case where the +, — , and equalizer switches are independent of each other, as is usually the case in modem installations. When triple-pole switches are . used, as in Fig. 18, all three must of course be closed together after the machine has been allowed to pick up its field and has had its voltage adjusted. After the machine has been thrown in parallel, its load is adjusted by varying the field excitation. In case the machine is provided with a circuit-breaker, as is nearly always the case on modern switchboards, the circuit-breaker should be closed before the
45— a 6
56 ELECTRIC TRANSMISSION §23
main switch. If any rush of current then occurs when the main switch is closed, the circuit-breaker is free to act and disconnect the machine.
66. Main and Equalizer Cables. — In connecting the machines to the switchboard, cables of ample capacity should be used. For most cases it will be sufficient to allow from 1,200 to 1,500 circular mils per ampere. For very large currents it is advisable to use two or three cables in parallel rather than a single large cable, as better radiating facilities are thereby provided. The equalizer should be of the same size as the main cables. In some cases an allowance as low as 1,000 circular mils per ampere is made for these main cables, but the better practice is in favor of a more liberal cross-section.
67. So far, in all that has been said, the machines were supposed to be alike in size and general design. Under such circumstances, there is generally no great difficulty in getting compound machines to operate properly in parallel. Trouble is often experienced, however, when it comes to operating machines of different construction and size. Some field mag- nets will respond to changes in field excitation much more quickly than others, and other differences in design may have considerable effect on the performance of the machines when they are run in parallel. With two machines of different size, the problem is to get the load to divide between them in pro- portion to their size. For example, suppose a large machine A is connected in parallel with a smaller machine B, as shown in Fig 21. Each is supposed to be adjusted so that it gives the same degree of compounding when operated by itself. Also, when each machine is delivering its proper share of the load, the drop between a b must equal the drop between c d. For example, if / is the full-load current of A, R the resist- ance between a and by P the full-load current of B, and R' the resistance between c and d^ then I R must equal P R^. Now, the resistance of the series-coils cannot very well be altered in order to bring about the required condition of affairs, so that the only remedy is to insert resistance of
§23
ELECTRIC TRANSMISSION
57
some kind in the leads eb or fd until the above drops become equal. This resistance will, of course, be very small and may be made up of a short piece of heavy German-silver strip or even an extra amount of cable in one of the leads. In the figure, it is indicated at x^ though it may be necessary to insert it in the main lead of machine B, The resistance must be inserted in series with the machine giving the least drop between the points mentioned above. Many times the attempt is made to bring about the adjustment by changing
Fig. 21
the shunts 5, y, but such attempts are useless, because just as soon as the machines are put in parallel, j and s' are also in parallel and are practically equivalent to one large shunt across the fields of both machines. The consequence is that any change in the shunts affects both machines. The adjust- ment must, therefore, be made in the main lead between the series-coil and the bus-bar, and any resistance so inserted must have the same carrying capacity as the series-coils. A change in the shunt across the series-coils will change the
58 ELECTRIC TRANSMISSION §23
compounding: of the machines as a whole, but it will not better their condition as regards the correct division of the load.
68. Compound Machines in Parallel Witli Bhunt Machines. — It is not practicable to run a compound machine in parallel with a shunt machine. If, for any reason, the com- pound machine takes a little more than its share of the load, the strengthening of its series-coils makes it still further over- load itself, with the result that the field rheostat of the shunt machine calls for constant attention. The only way to run this combination satisfactorily is either to cut out the series- coils of the compound machine, thereby making both plain shunt machines, or else provide the shunt machine with compound coils.
COMBINED RUNNING OF ALTERNATORS
AliTERNATORS IN SBRIES
69. Alternators cannot be run in series unless their arma- tures are rigidly connected by being mounted on the same shaft, so that the E. M. F.*s generated by the two machines will always preserve exactly the same relation with regard to each other. If the machines are driven separately, the E. M. F.'s may aid each other at one instant and oppose each other the next, thus making their operation unstable. There is, in any event, little occasion for operating alternators in series; the object of series operation is usually to obtain a high voltage, and this can readily be generated in a single alternator, or, if the alternator does not furnish a sufficiently high voltage, the pressure can easily be raised by means of transformers.
ALTERNATORS IN PARAIil^EIi
70. Alternators can be operated in parallel, although they are, as a rule, more troublesome than direct-current machines. This is especially the case if they are very dif- ferent in size and design. For example, alternators with the old-style, smooth-core armatures are hard to run in parallel
§23
ELECTRIC TRANSMISSION
59
with modem machines having toothed armatures. In fact, in many of the older lighting stations special precautions were taken at the switchboard to see that two alternators should never be thrown in parallel.
71. Alternators are operated in parallel in much the same way as direct-current machines, so far as connections are concerned; i. e., they are usually connected to bus-bars through the intervening main switches. If the alternators are compound wound, equalizing connections should be used;
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Pio. 22
but very many are operated with a separately excited field only and no equalizing connection is necessary, the whole scheme of connection corresponding more nearly to the running of shunt-wound machines in parallel.
Suppose two single -phase alternators A and B are con- nected in parallel. In order that the machines may operate properly and each take its proper share of the load, it is, of course, necessary to have their voltages equal or nearly so. There is another important condition that must also be fulfilled; the machines must be in synclironlstn. This
«) ELECTRIC TRANSMISSION §23
means that both machines must run at exactly the same frequency, for if this were not the case, they would g^et out of step. Before two alternators are thrown in parallel, equality of frequency is the most important condition to be fulfilled. A slig^ht difference in phase will cause an exchange of current between the machines, but they will pull each other into phase if the frequencies are equal.
72, Bynchronf zfnK. — The state of sjrnchronism may be ascertained by means of syncliroiilzliis lamps connected as shown in Fig. 22. T, 'P are two small transformers having their primary coils connected to the alternators, as shown. It should be noted that similar terminals i, i' are connected to similar sides of the machines. The secondaries are connected in series through a pair of lamps /, / and a plug switch m. If the machines are exactly in phase, termi- nals S and S' will have the same polarity at the same instant and the polarities of i and 4' will also be alike. But since like terminals are connected together, the two secondary voltages will just neutralize each other, as indicated by the arrows, and the lamps will not glow. If the machines were directly opposite in phase, the lamps would light up to full candlepower. It is evident that by reversing the connections of one of the transformers the state of sjrnchronism will be indicated by the lamps being bright. When machine B is started and the plug inserted at ;», the lamps rapidly fluctuate in brightness; but as ^ comes more nearly in S3rnchronism the fluctuations become much slower. When they have become as slow as one in 2 or 3 seconds, the main switch M' is thrown in at the middle of one of the beats when the lamps are dark. In some cases, the connections are so made that the lamps are bright when synchronism is attained. Whether the state of synchronism will be indicated by light or dark lamps depends simply on whether the transformer secondaries are connected so as to assist or to oppose each other.
73. Synchronizlnj? Two-Pliase and Three-phase MachlncH. — Fig. 22 shows the synchronizing arrangement for a single-phase machine. For a two-phase or three-phase
§23
ELECTRIC TRANSMISSION
61
machine the same arrangement may be used, but care must be taken to make sure that the transformers 7", T* are con- nected to corresponding phases on each of the machines. This may be determined by using two pairs of transformers; i. e., one regular pair, as in Fig. 22, and a temporary pair on one of the other phases. For example, on a two-phase machine an arrangement similar to that shown in Fig. 22 should be made for each of the phases, and when the con- nections are right, each set of phase lamps will light or
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become dark, as the case may be, at the same instant, show- ing that both phases are ready for parallel operation. After it is known that the connections are all right, the temporary pair of transformers may be removed and only one pair used, as in Fig. 22.
74. Fig. 23 shows a common scheme of connections used for synchronizing with lamps. In this case the connections are shown for three machines, each machine being provided
62 feLECTRIC TRANSMISSION §23
with its plug receptacle p. One small transformer / is con- nected across the bus-bars, and the other V can be connected to any one of the machines by inserting the plug in its receptacle. For example, suppose the main switch of machine No. 1 is closed, as indicated by the dotted lines, and that it is desired to operate machine No. 2 in parallel with No. 1. Machine No. 2 would be brought up to speed and the plug inserted at receptacle 2, thus connecting V to the machine. With the connections as shown, synchronism is indicated when the lamps bum to full brightness, hence the generator switch of machine No. 2 would be thrown in when the lamps are at the middle of a beat and at full brightness. The same arrangement could be used for synchronizing with dark lamps, the only change being that the synchronizing plug would be cross-connected, thus making the transformers oppose each other. Should the alternators generate a low voltage, as is sometimes the case when they are used in connection with step-up transformers or for low-voltage work, it is not necessary to use transformers /, /'. All that is necessary in such cases is to connect the terminals of the synchronizing circuit direct to the machines or bus-bars and insert a suf- ficient number of lamps in series to stand the maximum voltage applied to them. Another plan in low- voltage work is to use autotransformers that step down the voltage to an amount suitable for the lamps.
75. Use of Voltmeter for Synchronizing:. — As
explained above, lamps have been used very largely in the past for indicating synchronism, but they are not entirely satisfactory for this purpose. Lamps do not indicate the point of synchronism as closely as desirable, especially when large generating units are involved, and they do not give any accurate idea as to how much the machine being synchronized is out of phase or whether it is coming into or going out of phase. If a large machine is connected to the bus-bars when out of phase, even by a slight amount, a heavy cross-current will flow, and this frequently results in burned switch contacts, to say nothing of possible worse
§23
ELECTRIC TRANSMISSION
63
results. A number of schemes have been adopted for indicating the point of synchronism more exactly than is possible with lamps. Fig. 24 shows an arrangement of con- nections by which the machine voltmeters are used. If a voltmeter is connected in the same way as synchronizing lamps, the pressure applied to it at synchronism will be either zero or double the ordinary pressure, depending on how the transformers are connected. This would make
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the point of synchronism, as indicated by the instrument, come either at the zero end of the scale where considerable changes in voltage might make very little change in the reading, or at the maximum point of the swing where a considerable change in phase difference is necessary to cause an appreciable change in the resultant voltage. A scheme for three-phase systems, devised by Mr. J. E. Wood- bridge, and shown in Fig. 24, overcomes these objections by
64 ELECTRIC TRANSMISSION §23
making the voltage applied to the voltmeter at synchronism the resultant of two E. M. F.'s differing in phase by 60° instead of two that are in phase or 180° out of phase, as is ordinarily the case. The two transformer secondaries, connected in series through the voltmeter by means of the synchronizing plug, are attached to two different phases of the three-phase system in such a way that their E. M. F.*s differ in phase by 60°. Thus the resultant E. M. F. applied to the voltmeter is, when the machines are in phase, equal to the normal E. M. F., thus bringing the pointer some- where near the mid-point of the scale. The rate of change of the resultant E. M. F. due to changes of phase relation is also high with this connection, thus giving a more accu- rate indication of the exact instant at which the machines are in phase.
In Fig. 24 the connections are shown for a pair of high- pressure alternators, and two potential transformers /, / are provided for each machine. The junction of the two trans- former secondaries is grounded, as shown; this not only simplifies the connections by making the ground serve as one synchronizing bus, but, what is of more importance, it precludes the existence of a high pressure between the switchboard instruments and the ground in case the insula- tion between primary and secondary should break down. By using suitable plugs in the receptacles a, b, the voltmeter can be used either to indicate the voltage of the machine, or for synchronizing purposes; lamps are also provided, as shown, to indicate synchronism along with the voltmeter. The plug for the machine that is already in operation connects points 1 and 4, as shown at a, and the plug for the machine being synchronized connects points 2', 4', S\ as shown at b. This connects voltmeter d in series (by way of the ground connections) with coils e and h, and the lamps in series with coils e and^. The E. M. F.*s of e and h differ in phase by 120°, but the coils are connected in opposition so that one E. M. F. is reversed with respect to the other and the two E. M. F.'s which combine to act on the voltmeter differ in phase by 60°, as previously mentioned*
§23 ELECTRIC TRANSMISSION 65
The E. M. P.'s of e and g are in phase so that the volt- meter will indicate normal voltage, and the lamps /' will be dark at synchronism. When the voltmeter is to be used in the regular way to indicate the machine voltage, a plug is inserted that connects the upper contacts 1', 2', thus connecting the voltmeter across the transformer and indicating the voltage between the outside wires.
76. Iilncoln Synchronizer. — Voltmeters and other devices are used in many ways to indicate synchronism, and it is impossible to here treat all the different methods. Also, a number of synchronism ^
indicators, or synchrono- scopes, have been brought out; Fig. 25 shows one of these devised by Mr. Paul M. Lincoln. The terminals of the potential transformers are connected to the binding posts a a, bb, and when the incoming machine is in syn- chronism, the hand h remains stationary in the vertical position. If the machine that is being brought into syn- ^^
chronism is running too fast,
the hand revolves slowly to the right; if running too slow, it moves to the left. The following description of the principle of operation of this instrument is that given by Mr, Lincoln.
Suppose a stationary coil F has suspended within it a coil A, free to move about an axis in the planes of both coils and including a diameter of each. If an alternating current be passed through both coils, A will take up a position with its plane parallel to F. If, now, the currents in A and Fhe: reversed with respect to each other, coll A will take up a posi- tion 180° from its former position. Reversal of the relative directions of currents in A and F is equivalent to changing their phase relation by ISO", and therefore this change of
66 ELECTRIC TRANSMISSION §23
180° in phase relation is followed by a corresponding change of 180° in their mechanical relation. Suppose, now, that instead of reversing the relative direction of currents in A and 7% the change in phase relation between them be made gradually and without disturbing the current strength in either coil. It is evident that when the phase diflEerence between A and F reaches 90°, the force between A and F will become zero, and a movable system, of which A may be made a part, is in condition to take up any position demanded by any other force. Let a second member of this movable system consist of coil B^ which may be fastened rigidly to coil A, with its plane 90° from that of coil A, and with the axis of A passing through a diameter of B. Further, suppose a current to circulate through B, whose* difference in phase relative to that in A is always 90°. It is evident under these conditions that when the difference in phase between A and Fis 90°, the movable system will take up a position such that B is parallel to /% because the force between A and Fis zero, and the force between ^and F\s2l maximtun; similarly, when the difference in phase between B and F is 90°,^ will be parallel to F\ that is, beginning with a phase difference between A and F oi 0°, a phase change of 90° will be followed by a mechanical change in the movable system of 90°, and each successive change of 90° in phase will be followed by a corresponding mechanical change of 90°, For intermediate phase relations, it can be proved that under certain conditions the position of equilibrium assumed by the movable element will exactly represent the phase relations; that is, with proper design, the mechanical angle between the plane of F and that of A, and also between the plane of /^and that of B, is always equal to the phase angle between the current flowing in F and the currents in A and B, respectively.
77. Fig. 26 shows the general arrangement of the instru- ment. As seen from the figure, the construction is similar to that of a small motor. The field A A is built up of iron laminations, and is wound with coils 7% Fthsit are connected
§23
ELECTRIC TRANSMISSION
67
in series and joined to the secondary of the potential trans- former whose primary is connected to the bus-bars. The armature core ^ is of the drum type, and is wound with two coils C and D that are approximately at right ang^les to each other. These coils are connected in series, and their junc- tion X is connected to the middle ring 2 of three collector rings mounted on the shaft. The other two terminals are con- nected to rings 1 and 5. The middle ring, through its brush, connects directly to one terminal of the~ potential transformer of the machine to be synchronized. Ring 3 connects to a choke coil or inductance L] ring 1 con- nects to one terminal of a non- inductive resistance R. The remaining terminals of R and L are joined to y and connect to the other terminal of the poten- tial transformer. The induct- ance L and resistance R are adjusted so that the currents in the coils C and D differ in phase by very nearly 90°. The cur- rent in the coils /% F will lag nearly 90° behind the E. M. F. E, because of the high inductance of the field coils; consequently, the magnetism set up by the field will be 90° behind the E. M. F. E. When the current in coil D is in phase with the field magnetism, D will swing around until it assumes the vertical position where its plane is at right angles to that of the field. The current in D is 90° behind E\ because of the inductance L\ hence, at synchronism the current in D is in phase with the field magnetism, and the pointer assumes the vertical position.. The current
Fio. 26
68 ELECTRIC TRANSMISSION §23
in C is in phase with E', and hence differs in phase from the field current by 90°; hence, at synchronism no torque is exerted on coil C if the frequencies of E and E' are equal. But if E and E^ differ in phase by 90°, then the current in Z> is at right angles to the field and the current in C is in phase with the field magnetism; consequently, coil C assumes the vertical position, and the hand swings around through 90°. For a phase difference of less than 90° the pointer assumes an intermediate position. If the machines do not have equal frequencies, i. e., if the machine being synchronized is running too fast or too slow, the phase differ- ence between the field on one hand and C and D on the other is constantly changing, and, therefore, the pointer will revolve at a speed depending on the difference in speed of the alternators. From the direction of rotation, the attend- ant can tell at once whether the machine being synchronized requires speeding up or slowing down. The synchronizers made by the General Electric and Westinghouse companies operate on the above principle, and are now generally used instead of lamps or voltmeters.
78. The foregoing will give a general idea as to some of the methods in common use for indicating synchronism. As before stated, there are a great many possible arrange- ments and modifications of the connections, but the prin- ciples involved are much the same in all of them. Some devices have been proposed to make the action of syn- chronizing automatic; that is, to close the main switch automatically when the point of synchronism is reached instead of leaving the time of closing to the judgment of the operator. The object is to prevent the machines from being thrown together at the wrong time, and although a number of such automatic devices have been patented, they have not as yet come into general use. One arrangement for closing the switch is that patented by Mr. Lincoln in con- nection with the synchronizer just described. An electrical contact is arranged so that a circuit will be established when the pointer is anywhere within an arc, such as a b^
§23 ELECTRIC TRANSMISSION 69
Fig. 26. This arc represents the amount of phase difference that is allowable and yet have the machines g:o tog^ether without making^ a disturbance. The current through this electric contact operates a switch or relay that in turn closes the main switch. It is necessary that the relay shall only operate when the pointer is revolving at a very low speed; or, in other words, when contact exists for a considerable time. This is accomplished by providing the relay with a dashpot that prevents it from closing unless the current through its magnet is maintained for an appreciable length of time. If this were not done, the machines would be thrown together when their frequencies were unequal, because the hand in its revolution would make contact with the arc and close the circuit. It is only when the hand is moving very slowly that the switch should be operated.
FEATURES CONNECTED WITH PARALLEL OPERATION
79. When two alternators are running in parallel, each will hold the other in step and they will each rtm at such a speed as to give the same frequency; if the alternators have the same number of poles, their speeds will be exactly the same. When direct-current generators are operated in parallel, they do not necessarily run at the same speed and the load carried by each machine can be varied by changing the field excitation. When the load is increased, the engine speed drops a little and the governor admits more steam to the cylinders, thus increasing the power supplied. In the case of alternators, the machines are compelled to run at the same speed, and each alternator will deliver power in pro- portion to the power supplied to it from its prime mover. Changing the field excitation will not change the power delivered; the only effect of changing the field strength will be to set up local currents between the machines. The field strength should be adjusted so that, for a given total current delivered, the current delivered by each machine will be a minimum; or, so that the sum of the currents as indicated by the machine ammeters will equal the total current as nearly as possible.
70 ELECTRIC TRANSMISSION §23
The problem, then, of making a proper division of the load is more difficult in the case of alternators than direct- current machines. The alternators are compelled to run at the same speed just as if they were actually geared to a com- mon shaft, and any decrease in the speed of one must be accompanied by a corresponding decrease of speed in the other. Now, the governors of steam engines and water- wheels are designed so that a certain small decrease in speed is necessary, with increase of load, to make them operate* For example, suppose a steam engine is carrying a light load and running at a certain speed. If the load is increased, the speed must drop a slight amount before the governor can operate to admit steam sufficient to carry the load, and the engine continues to run at a slightly lower speed on the heavy load than it did on the light load. There is therefore a certain engine speed for each load.
Now, suppose that two alternators are running in parallel and that each is supplying half the amount of power taken by the system. If the external load is increased, the amount of power supplied to each alternator must also increase, and, if the load on the machines is to be kept equal, each engine must increase its power output by an equal amount. We have just seen that to increase the pow^r output the engine speed must drop slightly, and as the alternators must always run in synchronism, it follows that both engines must, for a given increase in load, drop their speeds an equal amount. In other words, to secure equal division of load the engines must per- form in exactly the same way as regards change in speed with change in load. If one drops its speed more than the other, it takes the load and the other machine may even be driven as a synchronous motor. The question, then, of proper division of load is one that relates more to the engines than to the alter- nators, and in choosing engines for this kind of work every effort should be made to have them alike as regards their change in speed with change in load. The engines may run at exactly the same speed for a given load, but if their speeds do not drop by the same amount with increase in load, the out- put will not divide properly between the machines.
§23 ELECTRIC TRANSMISSION 71
When machines are belt-driven, great care must be taken to see that the pulleys are exactly the correct dimensions to give the speeds required for operating in synchronism; because, if this is not the case, there will be considerable belt slippage, and there will also be considerable cross- current between the two machines.
80, Hunting of Alternators. — When alternators are coupled directly to slow-moving steam engines, difficulty is frequently encountered in connection with their parallel operation. This is specially the case when the alternators deliver a current of high frequency. The machines surge, or hunt, that is, the speed may fluctuate during each revolution, thus causing large periodic cross-currents to flow between the machines and seriously affecting the voltage of the system. This surging may become so bad as to cause the machines to fall out of synchronism and render parallel operation impossible. If rotary converters or synchronous motors are operated from the alternators, surgings are also set up in them and the voltage fluctuation and sparking caused thereby may be so serious as to make satisfactory operation very difficult to accomplish.
The cause of these surgings has been found in many cases to be due to periodic variations in the speed of the engine, and various methods have been tried to suppress them. The turning effort exerted on the crankpin of a steam engine is not uniform at all parts of the stroke, the pressure at the various points depending on the steam distribution in the cylinder or cylinders, on the position of the crankpin, angularity of the connecting-rod, etc. The result is, that while the speed of the engine may remain practically con- stant so far as the number of revolutions per minute is concerned, there will be momentary variations in speed during each revolution. It takes but a small momentary variation in angular velocity to throw the machines con- siderably out of phase, especially if the alternator has a large number of poles. For example, if a direct-connected alter- nator has 60 poles, the angular distance between centers of
45—17
72 ELECTRIC TRANSMISSION §23
poles will be 6°, and this corresponds to a phase difference of 180°, The periodic variation in the angular velocity of the revolving field or armature sets up corresponding varia- tions in phase difference and results in periodic surges of ' current between the machines. This trouble has been inves- tigated quite fully by Mr. W. L. R. Emmett*, who found that the energy necessary to maintain these current oscil- lations was in a number of cases supplied from the steam cylinders of the engines, and that it could be largely pre- vented by fixing the governor so that it would not respond to these sudden varia- tions and admit the steam necessary to maintain them. The governor must, how- ever, be capable of re- sponding to changes in the regular load on the machine, other- wise enough power would not be fur- nished to the alter- nator to enable it to carry its share of the load. In order to fix the governor so that Pio.str it would respond to'
gradual changes in the load, but not to momentary oscillations, it was pro- vided with a dashpot similar to that shown in Fig. 27. This dashpot was designed by Messrs. H. W. Buck and Harte Cook. It consists of a cylinder A in which a piston B moves; two by-passes b, f are provided, and at the end of each is placed a valve c or c" ordinarily held closed by springs d, d'. Each valve is provided with a small by-pass e, ^, and the whole cylinder, including the ports, is filled with
§23 ELECTRIC TRANSMISSION 73
heavy oil. Unless valves Cyd are raised, the only passag^e for the oil, to allow movement of the piston, is through the small ports, and the piston is therefore practically locked. A sudden fluctuation in the governor will not move c or ^, but a steady pressure on the piston, due to a prolonged raising or lowering of the speed, will move them, and the oscillations of the governor and steam in the cylinders are thereby damped out, thus suppressing the hunting action of the alternators.
81. In order to prevent hunting effects, engine builders have endeavored to secure tmiform angular velocity of their engfines. In some cases this is accomplished by the use of very heavy flywheels, but it is a question whether heavy fly- wheels are on the whole advisable. Some authorities claim that the momentum of heavy flywheels tends to maintain the oscillations, and that it is better to use fairly light flywheels and design the engine so that the turning effort on the shaft will be nearly tmiform. By using two or more engines coupled to the same shaft with their cranks at the proper angle to each other, this result can be attained quite closely. This is readily accomplished by cross-compound engines, either horizontal or vertical, and both tjrpes are largely used for driving alternators. In the case of the large alternators of the Manhattan Elevated Railway^ New York, each alternator is driven by four engines, two of which are vertical and two horizontal. There is a crankpin at each end of the shaft, and to it is connected one vertical and one horizontal engine. The cranks are displaced 135° and since the four cylinders give eight impulses during each revolution, the turning moment is so uniform that no flywheel other than the revolving field of the alternator is necessary.
82. Use of Damping Devices. — ^Another method that has been used to prevent hunting is to provide special wind- ings or conductors on the alternator field, so that the currents set up in them will oppose any shifting action and thus retard the oscillations. This device has been used much more on E^uropean alternators than on those built in America?
74 ELECTRIC TRANSMISSION §23
Fig. 28 (a) shows the method of arranging a damper (French amortisseur) of this kind, due to Hutin and Leblanc. A is the laminated pole piece of a revolving field alternator and is provided with the usual exciting coil B. Near the surface of the pole piece are a number of slots in which copper bars c are placed. These bars are connected together at each end of the pole by means of copper straps, thus forming the bars into a number of closed circuits similar to the squirrel-cage armature of an induction motor. As long as the magnetic flux passing from the pole face into the armature remains stationary with respect to the pole face, no currents are set up in the bars. If, however, there is any momentary shift- ing of the field, heavy currents are set up in the bars, and
PiO.28
these currents dampen the motion, thus smoothing out any tendency toward fluctuation. Fig. 28 (b) shows a field con- struction used by the Westinghouse Company that has some- what the same effect. Copper bridges A are placed between the poles; these serve to hold the coils in place and dampen hunting effects.
83. Hunting sometimes occurs even when the alternators are driven by prime movers, such as steam or water tur- bines, that give an absolutely uniform angular velocity. In this case the effect is due to certain relations between the properties of the electric circuit, such as its self-induction, capacity, etc., and the momentum of the moving masses of the machinery. The result is a cumulative pendulum effect that may be overcome by changing some of the above properties
§23 ELECTRIC TRANSMISSION 76
of the circuit or by damping the alternator, synchronous motors, rotary converters, or other devices on the system. For example, a change in field excitation will frequently overcome the difficulty. Fig. 29 shows another arrange- ment used for preventing htmting of rotary converters and
alternators. The pole piece ^___
is provided with a slot * in ^
the.center, in which is placed \
a heavy copper bar. The pole is also encircled by a heavy conductor forming two local circuits, in which heavy currents are set up if there is any shifting of the Held. Rotary converters are also frequently provided with copper bridges between the poles, about as shown in Fig.28 (i), to dampen the hunting. Fig. 30 shows an anti-hunting device used on General Electric converters. fio. 2»
The copper casting a, b, e, I bridges across the pole tips and is held in place by a bolt passing through ab. By draw- ing up this bolt, edges e / are forced apart against the pole tips. The sides cd lie in slots provided in the pole faces.
84. Generally speaking, the practice in America is to obtain engines that will give a nearly uniform angular velocity, though damping devices are also used. Damping devices add to the cost and also slightly lower the efficiency of the machines to which they are applied. Engine builders will now guarantee engines not to give a departure from uniform motion during a revolution that will cause more than 2i° to 3° of phase displacement of the E, M. F. furnished by each of the alternators or a total maximum phase displace- ment of 5° to 6°. If the displacement does not exceed this amount, the operation should be satisfactory. In America
76 ELECTRIC TRANSMISSION §23
damping devices are more commonly used on rotary con- verters than on alternators.
When steam-driven alternators are being synchronized, it is necessary to have some convenient means of controlling the engine speed from the switchboard. One way of doing this is to have a small reversible electric motor attached to the governor and arranged so that it can vary the tension on
^ a spring attached to
the governor weights or vary the position of a weight on a lever arm attached to the governor. This motor is readily ^°*^ started, stopped, or
reversed from the switchboard, so that the attendant has the speed of the engine under control and can make the slight variations in speed necessary to secure equality of frequency. Also, this device allows the point of cut-off to be varied when the engine is in regular operation, thus regulating the amount of power supplied to the alternator. As explained above, the current delivered by each alternator when running in synchronism depends on the amount of power supplied to the alternator, so that by adjusting the governor, the output of each machine, as shown by its indicating wattmeter on the switchboard, can be regulated.
85. Compound -Wound Alternators in Parallel.
Most of the large alternators now installed are of the revolving field type and are not generally provided with a compound field winding. For large units it is found that a carefully designed machine gives sufficiently close voltage regulation with a plain, separately excited winding, so that the extra complication of compound field excitation is not warranted. Where a compound winding is used on the fields, it is necessary to provide an equalizing connection somewhat similar to that used for a direct-current machine. Fig. 31 shows the connections necessary for running two
ELECTRIC TRANSMISSION
78 ELECTRIC TRANSMISSION §23
compound-wound, three-phase alternators in parallel, the connections for the separately excited field being omitted in order to simplify the diagram. The terminals of the series- field winding on each machine connect through switches Ay A to the equalizing wires by h. An adjustable resistance r is connected across each field, so that the effect of the series-coils can be varied to suit the character of the load on the machines. With the synchronizing connections shown in the figure, the lamps will be bright at synchronism, though the lamps could be made dark by simply changing the cross- connections used with the plug on the machine being syn- chronized. In this case an ammeter is used in one phase only, and is all that is necessary to indicate the current, provided the load is of such a nature that it is not liable to become unbalanced. In many cases it is customary to use an ammeter in each line, so that the current in all three phases will be indicated.
LINE CONSTRUCTION
INTRODUCTION
1. lilne construction may be considered conveniently tinder two heads: (a) overhead construction; (.b) underground construction,
t'or nearly all work in towns and small cities or for cross- country work, the lines are supported on poles. In cities, the current is now usually distributed, at least so far as the central part of the cities is concerned, by means of wires or cables run in underground tubes or ducts. This method is, of course, much more expensive than the overhead method; but the large increase in the number of wires used for different electrical purposes has rendered underground dis- tribution in cities almost absolutely necessary.
LINE CONDUCTORS
2. The line wire is, in the vast majority of cases, of copper. Aluminum is now coming into use for this purpose, and in the future it may replace copper for some lines of work. Iron or steel is seldom used for a line conductor, because its resistance is too high. There is one case, however, in which it is largely used as a return conductor, and that is in con- nection with electric railways, where the current is led back to the power house through the rails.
COPPER CONBUCTORS
3. Bare and Insulated Wires. — Line conductors are usually in the form of copper 'wire of round cross-section whenever the conductor is of moderate size. For conductors
For notice of coPyrt£ht, see Page immediately following the title Page
LINE CONSTRUCTION
§24
of large cross-section, stranded cables are used, made up of a number of strands of small wire "twisted together. This con- struction makes the conductor flexible and easy to handle. When these wires or cables are strung in the air, they are usually insulated by a covering that consists of two or three
braids o£ cotton, soaked in a weather-proof compound com- posed largely of pitch or asphalt. For underground work, the conductor is first insulated with rubber, or paper soaked in
compound, and the whole covered with a lead sheath to keep out moisture. Fig, 1 shows a stranded cable for underground work provided with an insulating layer of paper and a lead
sheath. Fig. 2 shows an ordinary triple-braid weather-proof overhead line wire, and Fig. 3 a weather-proof overhead cable. When the pressure used on tbe line is very high, say 10,000 volts or more, bare wires are generally used, because the ordinary weather-proof insulation is of little or no
§24 LINE CONSTRUCTION 3
protection against such pressures and only gives a false appearance of security. The practice for such lines is, there- fore, to use bare wire and to insulate it thoroughly by means of specially designed insulators.
WIRB GAUGES
4. Various standards or ^^ire gauges have been adopted by different manufacturers, but the safest and best way is to express the diameter of a wire in milsy or thousandths of an inch, and its area of cross-section in circu- lar mils. The American, or Brown & Sharpe, gauge is used almost exclusively in America in connection with electrical work, but it is always well to give the diameter of the wire as well as its gauge number, so as to avoid any possibility of mistake. When wires or cables larger than the regular B. & S, sizes are specified, their cross-section is given in circular mils. Explanations regarding the B. & S. gauge and the expression of area in circular mils, etc. have already been given, so it will not be necessary to repeat them here. As we shall have occasion to refer to the B. & S. wire table frequently, Table I is repeated here for convenience. This gives the dimensions, weight, etc. of bare copper wire according to the B. & S. gauge for both annealed and hard- drawn wire; most wires and cables are of annealed copper. The use of hard-drawn copper is confined principally to trolley wire for street railways and telephone and telegraph line wires.
5. Table II gives the approximate weights of weather- proof line wire, such as is used for ordinary outside lines.
6. Table III gives the approximate dimensions of strandard insulated weather-proof cables for overhead work. Such cables are always designated by their area of cross- section in circular mils, and not by gauge number. In fact, any conductor larger than No. 0000 is usually desig- nated by its area in circular mils. Cables such as those given in Table III are extensively used for street-railway feeders or for any other purpose requiring a large conductor.
i S 2 3 IS SS* S,^S
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6
LINE CONSTRUCTION
§24
ALUMINUM CONDUCTORS
7. Mention has already been made of the fact that aliiminum is beings used for electrical conductors, because this metal can now be sold at a figure low enoug^h to compete with copper. Its conductivity is only about 60 per cent, that of copper, so that for a conductor of the same resistance a larg^er cross-section is required. Aluminum is, however, so much lighter than copper that the larger cross- section can be used and still compete with the latter metal, although the cost per pound of the aluminum is considerably
TABIiE n APPROXIMATE WEIGHTS OF WEATHER-PROOF WIRE
(American Electrical IVorks)
TRIPLE-BRAIDEt) INSULATION
|
Size |
Feet per Pound |
Pounds per 1,000 Feet |
Pounds per Mile |
Carrying Capac- ity, Amperes, National Board Fire Underwriters |
|
0000 |
1.34 |
742 |
3,920 |
312 |
|
000 |
1.64 |
609 |
3,215 |
262 |
|
00 |
2.05 |
487 |
2,570 |
220 |
|
0 |
2.59 |
386 |
2,040 |
185 |
|
I |
3.25 |
308 |
1,625 |
156 |
|
2 |
4.10 |
244 |
1,289 |
131 |
|
3 |
5.15 |
194 |
1,025 |
IIO |
|
4 |
6.26 |
160 |
845 |
92 |
|
5 |
7.46 |
134 |
710 |
77 |
|
6 |
9.00 |
III |
585 |
65 |
|
8 |
13.00 |
73 |
385 |
46 |
|
10 |
20.00 |
50 |
265 |
32 |
|
12 |
29.00 |
35 |
182 |
23 |
|
M |
38.00 |
26 |
137 |
16 |
|
i6 |
48.00 |
21 |
113 |
8 |
|
i8 |
67.00 |
15 |
81 |
5 |
§24
LINE CONSTRUCTION
TABL.E 11— (Continued) Double-Braided Insulation
|
Size • |
Feet per Pound |
Pounds per 1,000 Feet |
Pounds per Mile |
Carrying Capac- ity, Amperes, National Board Fire Underwriters |
|
0000 |
1.40 |
711 |
3,754 |
312 |
|
ooo |
1.75 |
570 |
3,010 |
262 |
|
00 |
2.29 |
436 |
2,300 |
220 |
|
0 |
2.81 |
355 |
1.875 |
185 |
|
I |
3.56 |
281 |
1,482 |
156 |
|
2 |
4-49 |
223 |
1,175 |
131 |
|
3 |
5.45 |
184 |
969 |
no |
|
4 |
6.82 |
147 |
774 |
92 |
|
5 |
9.10 |
no |
580 |
77 |
|
6 |
10.35 |
97 |
510 |
65 |
|
8 |
15.52 |
64 |
340 |
46 |
|
10 |
22.00 |
45 |
237 |
32 |
|
12 |
40.00 |
25 |
132 |
23 |
|
M |
56.00 |
18 |
95 |
16 |
|
i6 |
76.00 |
13 |
69 |
8 |
|
i8 |
100.00 |
10 |
53 |
5 |
higher. Line -construction work is somewhat more diffi- cult with aluminum than with copper; joints are more difficult to make and there is greater liability of the spans breaking. Table IV jg^ives the properties of aluminum wire of the grades made by the Pittsburg Reduction Company and Table V gives the resistance. The values in these tables are taken from a pamphlet issued by the above company. A comparison of some of the properties of aluminum and copper is given in Table VI.
8
LINE CONSTRUCTION
§24
TABliE in
STANDARD WEATHER-PROOF FEED-WIRE
{Roeblt'nj^'s)
|
Circular Mils |
Outside Diameters Inches |
Weights Pounds |
Approximate Leng^th on Reels Feet |
g Capacity, lal Board iderwriters |
|
|
1,000 Feet |
Mile |
Carrying Natioi Fire Ui |
|||
|
1,000,000 |
li |
3,550 |
18,744 |
800 |
1,000 |
|
900,000 |
lit |
3,215 |
16,975 |
800 |
920 |
|
800,000 |
iH |
2,880 |
15,206 |
850 |
840 |
|
750,000 |
itV |
2,713 |
14,325 |
850 |
|
|
700,000 |
lA |
2,545 |
13.438 |
900 |
760 |
|
650,000 |
li |
2,378 |
12,556 |
900 |
|
|
600,000 |
i-sV |
2,210 |
11,668 |
1,000 |
680 |
|
550,000 |
11% |
2,043 |
10,787 |
1,200 |
|
|
500,000 |
li |
1,875 |
9,900 |
1,320 |
590 |
|
450,000 |
i-h |
1,703 |
8,992 |
1,400 |
|
|
400,000 |
iiV |
1,530 |
8,078 |
1,450 |
500 |
|
350,000 |
I |
1,358 |
7,170 |
1,500 |
|
|
300,000 |
IS. 1 6 |
1,185 |
6,257 |
1,600 |
400 |
|
250,000 |
fl |
1,012 |
5,343 |
1,600 |
§24
LINE CONSTRUCTION
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10
LINE CONSTRUCTION
§24
TABIilS V
TABLB OF BBSI8TANCS8 OF PURE ALUMnOTM WIRE*
|
u |
Resistance at ?s° P. |
|||
|
Am. Ga' B. AS. |
R |
Ohms |
Feet |
Ohms |
|
Ohms |
per |
Ohm |
per |
|
|
ixxio Feet |
Mile |
Pound |
||
|
oooo |
.08177 |
.43172 |
12,229.8 |
.00042714 |
|
ooo |
.10310 |
.54440 |
9,699.00 |
.00067022 |
|
oo |
.13001 |
.68645 |
7,692.00 |
.0010812 |
|
o |
.16385 |
.86515 |
6,245.40 |
.0016739 |
|
I |
.20672 |
1. 09150 |
4,637.35 |
.0027272 |
|
2 |
.26077 |
1.37637 |
3,836.22 |
.0043441 |
|
3 |
.32872 |
1.73570 |
3,036.12 |
.0069057 |
|
4 |
.41448 |
2.18850 |
2,412.60 |
.010977 |
|
5 |
.52268 |
2.75970 |
1,913.22 |
.017456 |
|
6 |
.65910 |
3.48020 |
1,517.22 |
.027758 |
|
7 |
.83118 |
4.38850 |
1,203.12 |
.044138 |
|
8 |
I.Of)8o2 |
5.53550 |
964.180 |
.070179 |
|
9 |
I. 32135 |
6.97670 |
756.780 |
.11156 |
|
lO |
1.66667 |
8.8(xxx> |
600.000 |
.17467 |
|
II |
2.I0I20 |
11.0947 |
475-908 |
.28211 |
|
12 |
2.64970 |
13.9900 |
377.412 |
.44856 |
|
13 |
3.34120 |
17.6420 |
299.298 |
.71478 |
|
14 |
4.31800 |
22.8000 |
231.582 |
I. 1623 |
|
15 |
5.I9I7O |
27.4620 |
192.612 |
1.7600 |
|
i6 |
6.69850 |
35.3<^>8o |
149.286 |
2.8667 |
|
17 |
8.44720 |
44.6020 |
118.380 |
4.5588 |
|
i8 |
10.6518 |
56.2420 |
93.8820 |
7.2490 |
|
19 |
13.8148 |
72.9420 |
72.3840 |
12.192 |
|
20 |
16.9380 |
89.4300 |
59.0406 |
18.328 |
|
21 |
21.3580 |
112.767 |
46.8222 |
29.142 |
|
22 |
26.9200 |
142.138 |
37.1466 |
46.316 |
|
23 |
33.9620 |
179.320 |
29.4522 |
73.686 |
|
24 |
42.8250 |
226.120 • |
23.3508 |
117.17 |
|
25 |
54.0000 |
285.120 |
18.5184 |
186.28 |
|
26 |
68.1130 |
359.650 |
14.6814 |
296.32 |
|
27 |
85.8650 |
453.370 |
11.6460 |
485.56 |
|
28 |
108.277 |
571.700 |
9.2358 |
749.02 |
|
29 |
136.535 |
720.900 |
7.3242 |
1,191.0 |
|
30 |
172.170 |
908.980 |
5.8087 |
1,893.9 |
|
31 |
212.120 |
1,119.98 |
4.7144 |
2,941.5 |
|
32 |
273.970 |
1,445-45 |
3-6528 |
4,788.9 |
|
33 |
345.130 |
1,822.30 |
2.8974 |
7,610.7 |
|
34 |
435.380 |
2,298.80 |
2.2969 |
12,109. |
|
35 |
548.920 |
2,898.20 |
1. 8218 |
19,251. |
|
36 |
692.070 |
3,654.20 |
1.4449 |
30,600. |
|
37 |
872.930 |
4,609.20 |
1. 1456 |
48,661. |
|
38 |
1,100.62 |
5,8lT.20 |
.c>oS6 |
76,658. |
|
39 |
1,387-47 |
7,325-80 |
' .7207 |
121,881. |
|
40 |
; 1,749.50 1 |
9,236^50 |
.5716 |
193,835. |
*Calculated on the basis of Matthiessen's standard.
§24
LINE CONSTRUCTION
11
TABIiE VI COMPARISON OF PROPERTIES OF COPPER AND ALiUMINITM
Conductivity (for equal sizes) , . .
Wei eh t (for equal sizes)
Weight (for equal leng^th and re- sistance)
Price, aluminum 29c.; copper i6c. (bare line wire)
Price (equal resistance and length, bare line wire)
Temperature coefficient, degree F.
Resistance of mil-foot (20° C.) . .
Specific gravity
Breaking strength (equal sizes) . .
Tensile strength (pounds per square inch, hard drawn)
Coefficient of expansion, degree F.
Alntnintim
Copper
.54 to .63 .33
.48
1.81
.868 .002138
18.73 2.5 to 2.68
I
40,000 .0000231
I I
.002155
10.05
8.89 to 8.93
I
60,000 .0000093
IRON WIRE
8. Iron Tvlre is used largely for telegraph and telephone work, but it is seldom employed in connection with electric transmission because of its high resistance. The approxi- mate value of the resistance per mile of a good quality of iron wire may be determined by the formula
r> 360,000
(1)
where d = diameter of wire in mils.
9. For steel ivlre, which is often used in place of iron wire, this formula becomes approximately
^ ^ 470,000 (2)
The various grades of iron wire on the market are termed **Extra Best Best/* **Best Best/* and **Best*'; the resistances of the diflEerent grades are shown in Table VII.
12
LINE CONSTRUCTION
§24
TABIiB Vn DIMENSIONS AND RESISTANCE OF IRON WIRE
|
6 • • 0) |
Diameter in Mils » <f |
•-5 oil |
Weiarht Pounds |
Breakinsr StrenflTth Pounds |
Resistance per at 68° P. |
MQe |
|||
|
1 a |
1. 000 Feet |
I MUe |
Iron |
Steel |
B* B. B. |
B. B. |
Steel |
||
|
o |
340 |
115,600 |
304.0 |
1,607 |
4,821 |
9,079 |
2.93 |
3.42 |
4.05 |
|
I |
300 |
90,000 |
237.0 |
1,251 |
3,753 |
7,068 |
3.76 |
4.40 |
5.20 |
|
2 |
284 |
80,656 |
212.0 |
1,121 |
3,363 |
6,335 |
4.19 |
4.91 |
5.80 |
|
3 |
259 |
67,081 |
177.0 |
932 |
2,796 |
5,268 |
5.04 |
5.90 |
6.97 |
|
4 |
238 |
^,644 |
149.0 |
787 |
2,361 |
4,449 |
5.97 |
6.99 |
8.26 |
|
5 |
220 |
48,400 |
127.0 |
673 |
2,019 |
3,801 |
4.99 |
8.18 |
9.66 |
|
6 |
203 |
41,209 |
109.0 |
573 |
1,719 |
3,237 |
8.21 |
9.60 |
11.35 |
|
7 |
180 |
32,400 |
85.0 |
450 |
1,350 |
2,545 |
10.44 |
12.21 |
14.43 |
|
8 |
165 |
27,225 |
72.0 |
378 |
1,134 |
2,138 |
12.42 |
14.53 |
17.18 |
|
9 |
148 |
21,904 |
58.0 |
305 |
915 |
1,720 |
1544 |
18.06 |
21.35 |
|
lO |
134 |
17,956 |
47.0 |
250 |
750 |
1,410 |
'18.83 |
22.04 |
26.04 |
|
II |
120 |
14,400 |
38.0 |
200 |
600 |
1,131 |
23.48 |
27.48 |
32.47 |
|
12 |
109 |
11,881 |
31.0 |
165 |
495 |
933 |
28.46 |
33- 30 |
3936 |
|
13 |
95 |
9,025 |
24.0 |
125 |
375 |
709 |
37.47 |
43.85 |
51.82 |
|
M |
83 |
6,889 |
18.0 |
96 |
288 |
541 |
29.08 |
57-44 |
67.88 |
|
15 |
72 |
5,184 |
13.7 |
72 |
216 |
407 |
65.23 |
76.33 |
90.21 |
|
i6 |
65 |
4,225 |
II. I |
59 |
177 |
332 |
80.03 |
93.66 |
110.70 |
|
17 |
58 |
3,364 |
8.9 |
47 |
141 |
264 |
100.50 |
120.40 |
13900 |
|
I8 |
49 |
2,401 |
6.3 |
33 |
99 |
189 |
140.80 |
164.80 |
194.80 |
GERMAN-SILVER WIRE
10. German-silver wire is used principally in resist- ance boxes or electrical instruments where a high resistance is required. The resistance of this wire varies greatly according to the materials sind methods of manufacture used. It is an alloy of copper, nickel, and zinc, and has a resistance anywhere from 18 to 28 times that of copper. Its resistance changes only to a small extent with changes in temperature, a feature of value in connection with rheostats and resistance boxes.
§24
LINE CONSTRUCTION
13
Table VIII gives some of the properties of German-silver wire containing 18 or 30 per cent, of nickel.
TABIiE vin
GERMAN-SILVER WIRE
{Roebline^s)
|
Resistance per 1,000 Feet |
Maximum Cur- rent Carrying Capacity in |
||
|
Number |
International Ohms |
||
|
B.&S. Gauge |
Amperes |
||
|
i8-Per-Cent. Wire • |
30-Per-Cent. Wire |
iS-Per-Ceat. Wire |
|
|
6 |
7.20 |
11.21 |
|
|
7 |
9.12 |
14.18 |
|
|
8 |
11.54 |
17.95 |
|
|
9 |
14.55 |
22.63 |
|
|
10 |
18.18 |
28.28 |
8.5 |
|
II |
22.84 |
35.53 |
5.4 |
|
12 |
28.81 |
44.82 |
4.6 |
|
13 |
36.48 |
56.75 |
3.8 |
|
M |
46.17 |
71.82 |
3.2 |
|
15 |
58.21 |
90.55 |
2.7 |
|
i6 |
72.72 |
113. 12 |
2.3 |
|
17 |
93.40 |
145.29 |
1.9 |
|
i8 |
118.20 |
183.87 |
1.65 |
|
19 |
145.94 |
227.02 |
1. 21 |
|
20 |
184.68 |
287.28 |
.99 |
|
21 |
232.92 |
362.32 |
.88 |
|
22 |
295.38 |
459.48 |
.66 |
|
23 |
370.26 |
575.96 |
.55 |
|
24 |
468.18 |
728.28 |
.488 |
|
25 |
590.22 |
918.12 |
.434 |
|
26 |
748.08 |
1, 1 63.68 |
.385 |
|
27 |
937.98 |
1,459.08 |
.343 |
|
28 |
1,191.24 |
1,853.04 |
|
|
29 |
1,481.22 |
2,304.12 |
|
|
30 |
1,891.8 |
2,942.8 |
|
|
31 |
2,388.6 |
3,715.6 |
|
|
32 |
2,955.6 |
4,597.6 |
|
|
33 |
3,751.2 |
5,835.2 |
|
|
34 |
4,764.6 |
7,411.6 |
|
|
35 |
6,031.8 |
9,382.8 |
|
|
36 |
7,565.4 |
11,768.4 |
14 LINE CONSTRUCTION §24
OVERHEAD CONSTRUCTION
POXiEB
11. Selection of Poles. — The poles used to the great- est extent in this country are of the following; kinds of wood: Norway pine, chestnut, cypress, and white cedar. The averag^e lives of these, under average conditions, are placed by g^ood authority at the following: values: Norway pine, 6 years; chestnut, 15 years; cypress, 12 years, white cedar, 10 years. Cedar poles are undoubtedly used to the greatest extent. Considering: their strength, they are light in weight, and, by some authorities, are considered the most durable, when set in the ground, of any American wood suitable for pole purposes. In some of the Western States, Califor- nia redwood is used for poles.
12. Sizes of Poles. — The best lines in this country use no poles having tops less than 22 inches in circumference. If the poles tai>er at the usual rate, the specification that a pole shall have a top 22 inches in circumference, or approxi- mately 7 inches in diameter, is usually sufficient, for the diameter at the butt will then be approximately correct, no matter what may be the length of the pole. When a pole line has to carry but a few small wires, it is not necessary to have them as large as 7 inches at the top, and poles with a 6-inch top will answer every purpose. For long-distance transmis- sion work, only the most substantial line construction is allowable, because every precaution must be taken to make the service continuous. Long transmission lines usually have to carry heavy wires, and moreover they are often in very exposed localities; for this class of work, therefore, specially heavy poles are used. The length of poles used in any given case is fixed by several considerations. It will
§24
LINE CONSTRUCTION
15
depend to some extent on the number of cross-arms to be accommodated, but more frequently the length is determined by the location of the pole. In any given transmission line it is necessary to use a number of different pole lengths and select the poles so that the tops will be graded, thus avoiding ups and downs in the wire as much as possible. A poorly graded line requires a greater length of wire than a well graded one, and this is objectionable not only on account of the extra cost of the wire, but also because of the larger line loss due to the larger resistance. Table IX shows the size of poles used on the Bay Counties high-tension transmission
TABIiB IX
DIMENSIONS OP POL.E8
|
Height |
Diameter of Top |
Diameter of Butt |
Depth in Ground |
|
Feet |
Inches |
Inches |
Feet |
|
25 |
8 |
12 |
5 |
|
40 |
9 |
14 |
6 |
|
45 |
10 |
15 |
6i |
|
50 |
II |
16 |
7i |
|
60 |
12 |
18 |
8 |
line in California*. Where angles occur in the line, the poles are set 1 foot deeper than shown by the figures in the last column of the table.
13. Spacing: of Poles. — Practice varies as to the spa- cing of poles. Of course, the number and sizes of the wires to be carried are the most important considerations in deter- mining this point, but the climatic conditions, especially with regard to heavy wind and sleet storms, should also be considered. In general, it may be said that the best lines carrying a moderate number of wires use 40 poles to the mile, while for exceptionally heavy lines, the use of 52 poles to the mile, or 1 pole every 100 feet, is not uncommon practice.
•Journal of Electricity, Power, and Gas, Vol. XI, No. 8.
16 LINE CONSTRUCTION §24
As a general rule, which it is safe to follow in the majority of cases, 35 or 40 poles to the mile should be used. For city work, the poles should be set on an average not farther apart than 125 feet.
CROBS-AItHS
14. The cross-arms should be made of well-seasoned, straight- grained Norway pine, yellow pine, or creosoted white pine. Cross-arms are made in standard sizes, the
length of the arm depending on the number of pins it is intended to hold. The standard cross-arm is 3i inches by 4j inches, and varies in length usually from 3 to 8 feet. They are usually bored for li-inch pins and provided with holes for two a-inch bolts. The arms are generally braced by flat iron braces, about li inches wide by i to ^ inch thick. These braces are shown in Fig. 4, which gives a view of an ordinary pole top provided with two 4-pin cross-arms. This pole top represents the style of construction suitable for fairly light work, such as is used for local light and
§24
LINE CONSTRUCTION
17
power distribution. For long transmission lines, heavier cross-arms are used. For example, those used by the Standard Company, of California, on a line designed to handle current at 60,000 volts, are di inches by 5f inches, and the holes for the pins are 42 inches apart, this wide distance between the wires being necessary on account of the high voltage. The older Niagara line used cross-arms 4 inches by 6 inches, and the later line 5 inches by 6 inches.
15. Fig. 5 shows the pole top used on the first Niagara transmission line. It was designed to accommodate twelve
Pig. 5
transmission wires, the insulators being placed side by side on the cross-arms as shown in the left-hand half of the figure. It was found that this arrangement did not work well because it was an easy matter to start short circuits between the wires, and the arc thus started traveled along the line wires until the power was shut off. By adopting the triangular arrangement shown at the right, the distance between the wires was doubled and all three wires made equidistant from each other. The apex of the triangle formed by the wires was placed downwards, as this arrangement
Id Line construction §24
makes it more difficult to lodge sticks or wires across the circuit than if the single wire is placed on the top arm with the other two beneath it, though the latter arrangement is used quite often. The Niagara line is designed to oper- ate at 20,000 volts. The supports a, a at each end of the cross-arms were intended to hold barb wire that was grounded at regular intervals in order, to conduct off light- ning discharges. The barb wire was also intended to act to a certain extent as a guard wire to prevent articles from falling on the line. It was found, however, that sleet and snow caused these guard wires to break and fall across the linesi
thus giving rise to so much trouble that they were finally removed. Barb wire is nevertheless used successfully in con- nection with a number of transmission plants, and affords an efficient protection against lightning, but it is necessary to use wire that is heavy enough to stand the strains put on it. Ordinary light barb wire as used for fences is not heavy enough for work where it has only one support in, say, every 100 feet, as is the case on a pole line. Another method that is some- times used for arranging two three-phase circuits is to use three cross-arms with two wires on each cross-arm, the pins being so placed that the wires come at the corners of a regular hexagon.
§24 LINE CONSTRUCTION
PINS
16. One style of pin by which insulators are mounted on cross-arms is shown in Fig. 6. This shows the ordinary pin used for light lines; pins used for heavy long-distance lines are considerably larger and stronger. They may be made of locust, chestnut, or oak (the woods being preferred in the order named), and are turned wilh a coarse thread on the end on which the insulator is to be secured; the shank A'is li inches in diameter.
1^-- — I
T
Pro. 8 PiD. 9
The pin should be secured in the hole by driving a nail through the arm and the shank. This renders it difficult to extract the shank of the pin in case a new one is required; but, on the other hand, it prevents the pin pulling out, which sometimes occurs when this precaution is not taken. For heavy lines, pins having an iron bolt passing through them are sometimes used. Fig. 7 shows a pin of this kind, designed by F. Locke, with a heavy insulator for carrying a cable in the groove a.
20 LINE CONSTRUCTION §24
In the case of high-tension, long-distance lines, exception- ally strong pins should be used. These are made of wood, because with high pressures any metal is objectionable near the insulator. Fig. 8 shows the style of pin used by the Standard Company previously referred to. These pins are made of blue gum wood {Eucalyptus), specially treated with linseed oil to prevent them from absorbing moisture. This pin is also shown in Fig. 14 in connection with the insulator that it supports. Fig. 9 (a) and {b) shows two styles of pin used on the Niagara transmission lines; (b) is the old- style pin, which was found to be too weak; (u) shows the heavier pin used on the later line. Note that in {a) the hole for the pin does not pass completely through the cross-arm. About 1 inch of wood is left at the bottom, as this is found to greatly strengthen the cross-arm.
17. Insalators in this country are usually made of glass, while in Europe porcelain is more commonly used. Porcelain, when new, is a better insulator than glass; but it is more costly, and under the action of cold the glazed surface becomes cracked. When this happens, the moisture soaks into the interior structure, and its insulating quality is greatly impaired. Tests recently made have shown that when newly put up, the insulation resistance of porcelain insulators is from 4 to 8 times better than glass, but that, along railroads and in cities, smoke forms a thin film on each material, so that at the end of a few months their insulating properties are nearly alike. On country roads, away from railroad
§24 LINE CONSTRUCTION 21
tracks, the porcelain insulators maintain a higher insulation than the glass during rain storms, but in fine weather it is not so high. Porcelain has an advantage over glass in that it is not so brittle, and therefore is less hkely to break when subjected to mechanical shocks. It does not condense and retain on its surface a thin film of moisture so readily as glass, i, 6,, it is less hygroscopic. On the other hand, glass insulators are not subject to such an extent as porcelain to the formation of cocoons and cobwebs under them, the transparency of the glass serving to allow sufficient light to pass through the insulator to render it an undesirable abode for spiders and worms. As cocoons, cobwebs, etc. serve to lower the insulation of the line to a great extent, this is an advantage that, in this country, it is not well to overlook.
18. Types of Insulators. — For ordinary work with moderate pressures, glass insulators are used. The style of insulator will depend to some extent on the size of wire to be supported. Wires smaller than No. 6 or 8 B. & S. are seldom used for power transmission lines; hence, the glass insulators, as a rule, must be heavier than the kind used for telegraph or telephone work. Fig. 10 shows an insulator, knownas the D, G. (deep groove) , that is well adapted for ordinary lines. This insulator is so called to distinguish it from those with smaller grooves, such as are used for telephone or telegraph work. It is provided with two petticoats, or flanges, a,^ over which leakage must take place before the current can leak from the wire to the pin. The use of anumberof petticoats increases the leakage distance and provides a high insulation; insula- tors used on high-tension lines are provided with several
LINE CONSTRUCTION
§24
petticoats. When heavy cables are used, it is customary to carry them on especially heavy insulators and to tie down the cable on top of the insulator instead of tying it to the side. Fig. 7 shows a common type of such insulator; the cable rests in the groove a and is held in place by a tie- wire twisted around the cable and passing under the ears at b, c. Good quality glass insulators, such as those just described, may be used for any lines where the potential is not over 2,000 or 3,000 volts; for higher pressures, it is necessary to use a larger insulator giving a higher degree of insulation. Fig. 11 shows a Locke insulator of glass that is suitable for any pressure up to 5,000 volts. This insulator is 4^ inches in diameter, and, it will be noted, is provided with three petticoats, thus giv- ing a long leakage dis- tance from the wire to the pin. Fig. 12 shows a still larger insulator; this one is suitable for pressures up to 25,000 volts and is 6j inches in diameter. For high pressures, pior- celain insulators have been largely used; as yet there does not seem to be any settled opinion as to just which is the better, glass or porcelain, for this kind of work, and on some lines using very high pressures the insulators are made partly of porcelain and partly of glass. Fig. 13 shows a type of porcelain insulator used for one of the Niagara-Buffalo trans- mission lines. These insulators are elliptical, or helmet, shaped and have an eave, or ridge, a on each side, the pbject of which is to run off the water to the end of the
§24 LINE CONSTRUCTION 23
insulator, where it will drop clear of the cross-arm. Fig;, 9 (a) shows a section of the later type of insulator used on the Niagara lines, and Fig. 14 shows a style that is used on high- tension lines in California that operate at pressures as high as 40,000 to 60,000 volts; in fact, lines equipped with these insulators have been operated experimentally at 80,000 volts. This insulator is made in two parts, the upper part being of porcelain and the lower of glass. The parts are cemented together by a mixture of sulphur and sharp sand, and the upper part is made of porcelain because moisture does not cling to it as readily as to glass. Glass offers- a greater resistance to puncture than porcelain, so that by combining the two materials a very efficient insulator is ob- tained, and the cost is also reduced materially. The lower part of the pin is covered by a por- celain sleeve that pro- tects the pin from any arc that might tend to strike from the eave of the insulator, and it also . protects the pin from the weather. The upper fio. u,
part of the insulator is
provided with a ridge around the edge and a projecting lip at one side, so that rain falling on the insulator drips clear of the cross-arm. These insulators are subjected to a test pressure of 120,000 volts for a period of 5 minutes in order to detect any defective insulators before they are put up oo the line.
TYING, SPLICING. ETC.
19. TylnK. — Fig. l.") shows the method of tying that is commonly used for small insulators. The tie-wire a is from 12 to 16 inches in length and should be insulated to the SErnie extent as the wire to be tied. The line wire is laid in
24 LINE CONSTRUCTION §24
the groove of the insulator, after which the two ends of the tie-wire, which have been passed half way around the insulator, are wrapped tightly around the wire. Some linemen pre- fer to wrap one end of the tie-wire over and the other end under the line wire. Fig. 16 shows a method of tying used where the wire lies on top of the insulator as with the Niagara type. Fig. 17 shows the method of tying to the insulator shown in Fig. 14. In this case a No. 4 aluminum tie-wire is used to tie the aluminum cable.
20. BpliclnK. — The American wire joint shown in Pig. 18 is generally used for splicing solid wires. The wires are placed side by side and each end wound around the other. All joints should be soldered. The rules of the National Board of Fire Under- writers require that all line joints shall be mechanically and electrically perfect before being soldered; i. e., solder should not be depended on to make the joints strong mechan- ically or efficient as an elec- trical conductor. In other words, soldering should always be done simply as a safegfuard against any diminution in the electrical conductivity of the joint. Large copper cables are
joined either by weaving the strands together and soldering, or by using a copper sleeve into which the ends of the cable are fastened.
Aluminum wires and cables are very often joined by means of a mechanical coupling, as aluminum is not easily
§24 LINE CONSTRUCTION 25
soldered. Fig. 19 shows an aluminum mechanical joint used on a number of California lines. The cable passes throueh the sleeves a, a', which are provided with right- and left- banded threads, so that they can be drawn tightly together by the threaded sleeve 6. The ends of the cable are first sawed off square, and after they have been passed through the sleeves, about 1 inch of each cable strand is bent back on itself, and the bunch so formed is forced into the conical part
of the sleeve. A small tapered aluminum plug is then driven into the center, thus wedging the strands firmly, after which the ends are securely screwed together. Another method of using this joint is to turn back on itself about li inches of the core wire of the cable, and after the strands have been forced into place and the joint screwed up tight, the space between the wires is filled. with solder. In this case the tumed-back wire takes the place of the aluminum wedge and spreads out the cable so that it is impossible for it to pull
through after the joint is filled with solder. Either method makes a very strong joint of which the resistance is less than a corresponding length of the cable. Aluminum wires are fre- quently joined by using a long aluminum sleeve or tube having an elliptical cross-section. This sleeve fits the wires snugly when they are slid into it side by side, and after they are in place they are twisted together. This is a good method for splicing solid wires; for stranded cables a sleeve joint is to be preferred.
26 LINE CONSTRUCTION §24
21, Strin^in^ Aluminum Wire. — Owing to the pecu- liar physical properties of aluminum wire, special care has to be taken in stringing it; otherwise, breaks in the line will be frequent. Slight impurities in aluminum wire affect both its mechanical and electrical properties to a marked extent. Its coefficient of expansion with increase in tem- perature is high, and if the stress on the wire is as high as 14,000 to 17,000 pounds per square inch, the wire stretches and takes a permanent set. In stringing the wire, it is therefore important to allow sufficient sag, in accordance with the temperature, so that when the wire contracts it will not be unduly strained. Neglect to do this has resulted in numerous breaks in some of the line wires that have been erected. An aluminum line in warm weather looks as if it had too much sag, but the contraction is so large with decrease in temperature that this slack is very largely taken up in cold weather. Table X, given by the Pittsburg Reduction Company, shows the deflection at the center of the span that should be allowed for various spans together with the tension under which the wire should be put up.
In this table X = deflection in inches at center of span; 5" = factor by which weight of wire per foot is multiplied to obtain tension.
Example. — Suppose a No. 4 aluminum wire is strung on poles 150 feet apart; what sag should be allowed at the center, if the temperature at the time the wire is strung is 30^ F.?
Solution. — Opposite the span 150, and under the column for 30°, we find that the deflection X should be 24 in. The weight of No. 4
200 9
aluminum wire per mile is 200.9 lb., or the weight per foot is _ '
= .038 lb. Hence, the tension will be A' X .038 = 1,390 X .038 = 52.8 lb. Ans.
22. In stringing the wire it is customary to pull up a number of spans at a time. The deflection is measured by hanging a target on the wire close to the insulator at each end of the span. One form of target consists of an iron strip with cross-marks of different colors corresponding to different deflections. This strip is hung from the wire by
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28 LINE CONSTRUCTION §24
means of a hook, and when the lowest point of wire comes in line with the point corresponding to the deflection called for by the temperature at which the wire is strmig, the line is tied to the insulator. The correct deflection is easily
Pxo. ao
determined by the lineman sighting from one target to the other while the wire is being pulled up (see Fig. 20). Each line foreman is provided with a thermometer and table of deflections. These refined methods are not necessary in connection with the stringing of copper wire, and if the cost of copper and aluminum were equal, copper would doubtless s. ^ .y be used on account of its
X X X
(a)
— superior mechanical qual- ities. However, in many "Ny S/ \/ cases quite a large saving
can be effected on long
"X "X X lines by using aluminum,
^^ and this accounts for its
use in connection with this
■^v— A< X/ kind of work. Aluminum
(e) has not as yet been used
FiQ. a to any great extent for
•underground work. The greater cross-section for a given conductivity is here a decided objection, because it would for a given current capacity make the cables considerably larger than those using copper, and this in turn would call for a larger amount of insulating material. With bare overhead lines these objections have little or no weight.
23. Transposition of Transmission liines. — When a number of alternating-current transmission lines are run side by side, the alternating magnetic field set up by the currents in one line may set up £. M. F.'s in the other lines,
§24 LINE CONSTRUCTION 29
thus causiag unbalancing of the voltage and affecting the line drop. This disturbing action can be avoided by traits- posUig or spiraling the wires so that the effect produced on one section of the line will be exactly counterbalanced by that produced in another. The most perfect example of spiraling is found in a cable where the conductors that make up the circuit are twisted together and the lines make a complete spiral every few inches. Such a cable has practically no inductive effect on a neigh- boring cable. Of course, in overhead transmission work, transpositions are not made very numerous because they make the wires harder to trace up in case of trouble and may, on high-pressiu:e work, , tend to promote crosses. In fact, some lines that work satisfactorily are not transposed at all. The Niagara lines are trans- posed in six sections be- tween Niagara Falls and Buffalo, about 23 miles. Practice seems to differ greatly with regard to the frequency with which high-pressure lines should pio.ii
be spiraled. In some cases
they are not spiraled at all; in other cases they are spiraled every 2 or 3 miles. Telephone lines, if strung on the same poles with transmission lines should be transposed every fourth or fifth pole, otherwise the telephones may be so noisy as to render conversation very difficult. Fig. 21 (a) shows
30
LINE CONSTRUCTION
§24
the transposition of a single-phase line; (d) a two-phase line, and (c) a three-phase line. Fig. 22 shows a transposition on a high-tension, three-phase line, each wire being shifted around one pin, or one-third of a turn. Where transpositions
J^jtf^jt? 36 4S 44 4a sa sa eo
ThouMtnda cf fd/Ai.
Pio. 23
are made in this way, it is advisable to place the pins on the cross-arms of each pole a little farther apart than the standard distance, so that the lines will not come too close together where they pass each other at the center of the span.
§24 LINE CONSTRUCTION 31
24. lieaka^e on Hi|?li-Tension liines. — On a high- tension line there is always some loss due to leakage, although if the lines be well separated and carefully insu- lated, this loss may be kept within reasonable limits. The leakage takes place between the wires either directly through the intervening air or over the insulators. When the pressure is raised to a high amount, a brush discharge takes place between the wires and the loss due to this dis- charge may be considerable, if the wires are not well separated. The curves in Fig. 23 show the results of some tests made by Mr. R. D. Mershon* to determine the relation between the loss, the pressure, and the distance between wires. These tests were made on a line about 2i miles in length. It is seen that there is a certain pressure, for each distance between wires, beyond which the loss increases very rapidly and that the nearer the wires are together, the lower the pressure at which the curves begin to rise rapidly. The loss by leakage at the insulators, of course, depends to a considerable extent on the design of the insulator, and also on its condition, i. e., whether wet or dry. It is difficult, therefore, to state very definitely what this loss is, but a nimiber of measurements show that it is in the neighbor- hood of 2 watts per insulator for lines operated at 25,000 volts, and does not exceed 4 watts with a pressure as high as 44,000 volts.
^Transactions American Institute of Electrical Engineers, Vol. XV.
S2 LINE CONSTRUCTION §24
UNDERGROUND CONSTRUCTION
25. In cities, it is necessary to place the wires under- gjound, especially in the business districts. The best way to do this is to provide a regular tunnel, or subway y in which the various wires, or cables, can be placed and which will be large enough to allow a man to walk through for inspection or repair. This method is, however, very expensive and can only be used in a few very large cities. Another method is to use conduits through which to run the cables. These con- duits usually consist of tubes of some kind that are buried in the ground and thus provide ducts into which the cables may be drawn. The ducts terminate in manholes usually placed at street intersections, by which access may be had to the cables and from which they may be drawn into or out of the ducts. A third method and one that has been largely used in cities for distributing current for lighting purposes, is to bury tubes containing insulated conductors in the ground. In this system the conductors cannot be withdrawn, as in the conduit system, and there is a separate tube for each set of conductors. The Edison tube system belongs to this variety, and a very large amount of lighting and power distribution on the three-wire, low-pressure system has been carried out by using underground conductors of this kind.
CONDUITS
26, A large variety of conduits are in use, and it has not been definitely settled as yet just which type is the best; but the following will serve to give an idea as to some of the more common forms that have stood the test of actual work and are in extended use.
27. Creosoted-Wood Conduit. — A form of conduit that was at one time largely used is composed of sections
§24
LINE CONSTRUCTION
33
of wooden tubing, the fiber of the wood being impregnated with creosote, in order to prevent its decay. This form of conduit is commonly known as pump-log: conduit. A section of this conduit is shown in Fig. 24; the ends are doweled in order to preserve the proper alinement in joining. These sections are usually 8 feet in length, and have circular holes through their centers from li to 3 inches in diameter, according to the size of cable to be drawn in. The external cross-section is square and 4i inches on the side, in the case of a tube having a 3-inch internal diameter. Such a conduit as this, if properly impregnated with creosote, will probably have a life of from 15 to 20 years, and perhaps much longer, this point being one concerning which there is considerablie argument and which, probably, time alone will decide. In
Pio.a4
Pio.25
some cases, difficulty has been experienced with creosoted- wood conduits on account of the creosote attacking the lead covering of the cables.
28. Cement-Iilned Pipe Conduit. — This conduit is made by the National Conduit and Cable Company. The sections shown in Fig. 25 are usually 8 feet long and are
34 LINE CONSTRUCTION §24
made as follows: A tube is made of thin wrought iron, No. 26 B, W. G., .018 inch thick, and securely held by rivets 2 inches apart. The tube is then lined with a wall of Rosendale cement 4 inch thick, the inner sur- face of which is polished while dryine, so as to form a perfectly smooth tube. This tubing comes in three sizes, each having a length of 8 feet and internal diameters of 2, 2j, and 3 inches, the latter being the standard size. Each end is provided with a cast- iron beveled socket joint, by the use of which perfect alinement may be obtained by merely but- ting the ends together. These beveled socket joints also allow of slight bends being made in the line of conduit as it is being laid.
29. Tltrifled-Clay or
Terra-Cotta Couduit. — A form of conduit that is probably used
in good construction work to a greater extent than any
other is made of vitrified clay. This material has the
advantage of being abso- lutely proof against all
chemical action, and unless
destroyed by mechanical
means will last for ages.
Besides this, its insulating
properties are high and
it is comparatively cheap
and easily laid.
Clay, or terra -co tta con- , . , . Fio. 27
duits arc m.itle m two gen- eral forms — multiple duct and single duct. Of the former type the most common is the 4-duct, two sections of which
§24 LINE CONSTRUCTION 35
are shown in cross-section in Fig. 26. Tbey are also made with 2, 3, 4, 6, and 9 ducts.
30. The form of clay conduits now most commonly used is the single duct shown in Fig. 27; this is usually made in 18-inch lengths, has an internal diameter of from 3 to Ai inches, and is 4f inches square outside. This duct has a
-'"''' r --- ^--1 ■ -I lb
great advantage over the multiple-duct sections in the greater ease of handling and also in the fact that it is much less liable to become warped or crooked in the process of burning during its manufacture than the larger and more complicated forms. Like the cement-lined pipe, it is laid on a bed of concrete.
cemented together with mortar, and enclosed on all sides and on top by concrete. In laying, a wooden mandrel, such as is shown in Fig. 28, 3 inches in diameter and about 30 inches in length, is used. At one end is provided an eye a, which
36 LINE CONSTRUCTION §24
may be engaged by a hook, in order to draw it through the conduit, while at the other end is secured a rubber gasket b having a diameter slightly larger than that of the interior of the duct. One of these mandrels is placed in each duct when the work of laying is begun. As the work progresses, the mandrel is drawn along through the duct by the workmen.
by means of an iron hook at the end of a rod about 3 feet long, the method of doing this being shown in Fig, 29. By this means, the formation of shoulders on the inner walls of the ducts at the joints is prevented, and any dirt that may have dropped into the duct is also removed. The cylindrical part of the mandrel insures good alioement of
§24 LINE CONSTRUCTION 37
the ducts, thus securing a perfect tube from manhole to manhole.
31. Fig. 29 illustrates the method of laying this con- duit, and shows how the joints should be broken in the various layers so as to insure a maximum lateral strength to the structure. All conduits should be laid to such grades that there will be no low points or traps in the conduit that will not drain into the manholes.
Figs. 30 and 31 show two arrangements of conduit used for distributing power from the Niagara Falls power station.* These are made of clay ducts laid in cement and covered, as shown, with concrete. The arrangement shown in Fig. 30 was used whenever the sewers were low enough to admit of good drainage, because it allowed a more convenient arrange- ment of cables in the manholes than the grouping shown in Fig. 31. Drainage was provided by the drain tiles a, a
*L. B. Stillwell, Transactions American Institute of Electrical Engineers, Vol. XVWI.
38 LINE CONSTRUCTION §24
surroiinded by loose gravel. These conduits are arranged so that there is never more than one duct between any duct and the ground, the object being to facilitate the dissipation of heat generated in the cables.
32. Bltumlnlzed-Flber Conduit. — Another kind of conduit that has recently been introduced is made of fibrous material treated with bituminous compound in such a way as to make a hard, dense tube. This conduit is light, strong, impervious to moisture, and has high insulating properties. Joints are made by fitting the lengths together in the same way as the pump-log conduit. Before placing a length in position, the end is dipped in hot pitch, or similar compound, so that when the end is pushed in, a water-tight joint is formed. The ordinary size of this conduit is 3 inches inside diameter and it is made in 7-foot lengths. The wall of the tube is about f inch thick. The conduit is usually laid in concrete, as described for the clay conduit, but owing to the nature of the joints it is not necessary to use mandrels if ordinary care is taken.
MANHOIiES
33, Manlxoles form a very important part in cable sys- tems and require careful designing to properly adapt them to the particular conditions to be met. They are usually placed about 400 feet apart, and, if possible, at the inter- section of streets. They should be located with a view to making the line of conduit between them as nearly straight as possible. The size of the manhole will depend on the number of ducts that are to be led to it, as well as the num- ber of men that will be required to work in it at one time. Manholes 6 feet square and from 5 to 6 feet high will usually be required for large systems, while for smaller systems, or the outlying portions of large ones, they may be made as small as 4 feet in length, in the direction of the conduit, 3 feet wide and 3 or 4 feet high.
Manholes may be constructed of either concrete or hard- burned brick laid in Portland-cement mortar. The foundation
§24 LINE CONSTRUCTION 39
should consist of a layer of concrete at least 6 inches thick. The walU, if of brick, should be laid in cement mortar, and should, also, be thoroug;hly plastered on the outside with the same mortar. They should never be less than 8 inches thick, and should be made double this thickness where large manholes are constructed in busy streets. As the brickwork is laid up, the supports for the iron brackets that hold the cables around the sides should be built in. The roof should
be of either arched brick, concrete, or structural iron, sup- porting some form of cast-iron manhole cover, of which there are several types on the market.
34. Fie. 32 shows a cross-section of a ventilated man- hole well suited for ordinary power-distribution work. It has been found better, on the whole, to provide manholes with ventilated covers and good sewer connections than to close them up tight, as was formerly done. If they are tightly sealed, gases are liable to accumulate and cause explosions. In Fig. 32 the manhole is provided with twQ
]
40 LINE CONSTRUCTION §24
sewer connections, so that in case the bottom one gets clogged up, the water will be able to flow through the side connection instead of backing up into the ducts. Both con- nections are provided with traps to keep out the sewer gas, and the bottom connection is equipped with a backwater valve to keep water from backing into the manhole. A removable cover is provided at the backwater valve, so that any dirt that accumulates can be cleaned out.
The roof of the manhole is made by laying 3'' X 3" I beams across the top and filling between them with brick, the whole being covered with a layer of cement. The man- hole cover may be either round or rectangular, the round type being preferred. Fig. 33 {a) and (d) shows two sectional views of the style of manhole used with the conduit shown in Figs. 30 and 31. The roof of this manhole is made of concrete arches supported by the side wall and by two I beams, as shown; a, a, a are the ducts of the main conduit, and dy b the ducts of the conduit through which the branch lines are taken. The cables pass around the side of the manhole, and are held in place on the racks R, R. The manhole is provided with a sewer connection at Sy and the drains that run alongside the conduit also attach to the sewer connection, as shown.
35. Fig. 34 {a) shows an elliptical manhole made of con- crete. This shape of manhole is becoming popular because it allows the cables to be easily bent to lie against the sides of the manhole. The rectangular comers of a square manhole are practically waste space, because the cables cannot be forced into these corners, or if the attempt is made to force them in, they are almost sure to be damaged. The elliptical form therefore utilizes the material to the best advantage. The main features of the construction are shown by the figure, so that little explanation is necessary. The main part a is of concrete, molded in a suitable form, and in this case the conduit b is of the 9-duct multiple type. The 2'' X 4'' tim- bers c are built into the concrete to form a base for the cable brackets. This manhole is comparatively small, so
(3) Fio. 84
§24 LINE CONSTRUCTION 43
that the holder d for the cast-iron cover e, forms the roof. This manhole, like nearly all those now constructed, is of the ventilated type. In case manholes are situated above the level of the sewer, the water that accumulates in them is usually removed by means of a water siphon. Fig. 34 (b) shows the cast-iron roof and cover.
36. After all work on the conduit and manholes has been completed, the cables are drawn into the ducts. In order to do this, it is necessary to have a wire or rope pass- ing through the duct; this is introduced by the process called roddinf^, which consists in pushing a number of jointed rods into a duct from one manhole until the first rod reaches the other manhole. The rods are joined together by screw connections or bayonet joints, as they are pushed in. When the chain of rods reaches between the two manholes, a rope or wire is attached to one end and pulled through, the rods being disjointed one by one as they reach the second manhole.
The introduction of the wire into the duct may often be greatly facilitated by using, instead of the rods, a steel wire about 4 inch in diameter and provided with a ball about 1 inch in diameter at its end. This wire may be pushed through a smooth duct without trouble for distances up to 500 feet. If an obstruction is found during the rodding that cannot be removed by means of the rods or by water, the distance to the obstruction can readily be measured on the withdrawal of the rod. The conduit should then be opened, the difficulty removed, and the structure repaired. This difficulty, how- ever, should never be met when proper care is taken in laying the conduit.
37. Dravrln^ In. — The process of dra-wlng in the
cable is illustrated in Fig. 35. The cable reel should be mounted on horses, so as to be free to revolve in such a manner that the cable will unwind from its top. The end of the rope leading through the duct should then be attached to the cable by grips made specially for the purpose or by binding it with iron wire for a distance of 18 inches or 2 feet
44 LINE CONSTRUCTION §24
from the end. Fig. 35 (d) shows a section of a cable grip of iron pipe made to fit the cable snugly. It is fastened to the cable, as shown, by common wood screws, and the piece ti- to which the drawing-in rope is fastened is screwed into the end of the iron pipe. Another form of cable grip is shown
Pio. U
in Fig. 36. Whenever a hole is made in the end of the cable for fastening the drawing-in rope, the end should be cut off when the cable has been drawn in, the moisture driven Out, and the end sealed if a joint is not to be made at once. The other end of the rope is passed over the grooved rollers, arranged on heavy planks mounted in the distant manhole, as shown, and is secured to a capstan or some form of windlass, by which a slow and steady pull may be exerted.
A man should be stationed in the manhole at which the cable enters to properly guide the cable into the duct, to prevent it from being kinked or unduly strained. It is well to use a special funnel-shaped guide, made of wood or lead, at the entrance of the duct, in order to further insure the fable against injury by the corners of the duct. This guide
§24 LINE CONSTRUCTION 45
is shown in Fig. 35 {a). It is sawed longitudinally into two sections, as shown in the left part of Fig. 35 (a), where the cable is to continue on through a manhole and where it would therefore be impossible to remove the cylindrical protector were it not sawed in two. Fig. 37 shows another arrangement for drawing in cables. In this case the windlass is arranged vertically in the manhole itself.
DISTRIBUTION FROM MANHOLES
38, Cables. — The construction of the cables themselves depends on the kind of service to which they are to be put. Two kinds of insulation are available — rubber and paper. With good rubber insulation, a small puncture in the lead sheath may not impair the insulation for some time, because the rubber is, to a large extent, proof against moisture. On the other hand, paper insulation will be damaged if the lead sheath becomes punctured so as to admit moisture. Paper insulation is, however, cheaper than rubber, and if the cables are carefully installed will give excellent service. Fig. 38 shows a paper-insulated cable designed for 6,600- volt, three-phase transmission. The three conductors are insulated with paper wrapping to a thickness of i inch. These three strands are then twisted together and covered with a wrapping of paper tV inch thick, over which the i-inch lead covering is forced. The paper is treated with insulating compound and the space between the strands, shown black in the figure, is filled with jute treated with insulating compound.
39. Underground cables have been regularly operated in America at a pressure of 25,000 volts. These cables were made for the St. Croix Power Company, and both paper- insulated and rubber-insulated cables were installed, the construction of the cables being similar to that shown in Fig. 38. The paper insulation on each conductor is W inch thick, and the outside paper jacket is liV inch thick. In the rubber cable, the insulation on each con- ductor is A inch thick, and the jacket surrounding the
46 LINE CONSTRUCTION §24
conductors is W inch tliick. The sheath is of lead with 3 per cent, of tin added.
40. Junction Boxes. — In underground electric-power distribution, it is important to have the various parts of the sjrstem so arranged that they can be disconnected, if neces- sary, because faults are liable to develop, and if the various sections can be readily disconnected, it makes the location of the defective portion very much easier to find; also, when
the defective part is located, it can easily be cut out without interfering with the operation of the remainder of the system. Again, at a manhole or other distribution center, where a □umber of distribution cables are connected to the main
§24 LINE CONSTRUCTION 4?
feeders running; to the power station, it is necessary to insert fuses, so that any branch will at once be cut o£E from the main cables in case of an overload, short circuit, or other defect giving rise to a rush of current. On low-pressure net- works, the distribution cables are attached to the main cables, or feeders, by means of junction boxes, which are provided with suitable fuse terminals. Junction boxes are made in a
great many different styles, but they are usually in the form of cast-iron boxes, containing suitable fuse-contact terminals and arranged so that they can be fastened to the side walls or roof of the manhole. These boxes must of course be water-tight.
41. Pig. 39 shows a typical junction box designed for fastening to the side walls or roof; it is known as a four-way box, because it accommodates four positive and four negative branch cables; it is designed for use on low-pressure, three- wire work. A and B are the positive and negative bars, which are made of copper and are well insulated from each other. These bars are connected to the cable terminals through copper fuses /, so that in case a short circuit occurs on a line, the fuses will blow and thus prevent damage. The short neutral bar shown in the bottom of the box attaches directly to the cables, because it is not usually con- sidered necessary or even desirable to place a fuse in the neutral. The small wires p, p are pressure wires that run back to the station and there connect to the voltmeter, so that the voltage at the center of distribution, represented by the
LINE CONSTRUCTION
§24
junction box, may be determined at any time. These pres- sure wires are protected by fuses placed in the small fuse receptacles b, b, b. Each pressure wire connects to one side of a cut-out b and the other sides connect to the +> — .
and neutral bars. The cables pass into the box througb water-tight rubber gaskets, and the box is closed by a water-tight cover.
Fig. 40 shows a recent type of junction box made by the General Electric Company. This differs considerably from those of the ordinary type, as it is designed to be placed in the roof of the manhole and access gained to it from the street. In many manholes there is very little room for placing junction boxes on the side walls without interfer- ing with tHe cables, and moreover manholes are sometimes filled with gas or water so that it is a difficult matter to get at the boxes to replace fuses or disconnect defective cables. Fig. 40 (a) is an exterior view of the box and {b) shows it
§24
LINE CONSTRUCTION
49
located in a manhole. All cables enter through the bottom, the lead sheath being joined to a nozzle bjr means of a wiped joint and the nozzle secured against the box by means of a union, as shown, thus making a joint that is gas- and water-tight, yet easily connected or disconnected. Fig, 40 (c) shows the arrangement of the fuses. The main cables connect, through fuses, to the castings a, 6, c and the branch cables are connected to these through fuses d, e, etc. The box
is intended for a three-wire system and 1, 2, 3 are small blocks to which the pressure wires are connected. In Fig. 40 (d), the location of the }unction box /, with reference to the manhole opening g, is shown. The junction box is made water-tight by means of the inner cover h, which is screwed down against a gasket. After the box is installed, a small hole is made close to the inner cover and opening into the manhole; this prevents any great accumulation of water
50 LINE CONSTRUCTION §24
between the inner and outer covers, so that there is little tendency for the gasket to leak. The junction box is covered by a loose cover k similar to that used for the manhole. If desired, the lower part of the box can be filled with oil, similar to that used in transformers; this is advisable with paper-insulated cables, as the oil will prevent moisture from working its way into the insulation.
42. Sepvlee Boxes. — When the conduit system of dis- tribution is used, and where customers have to be supplied, small handhoUs are provided wherever distributing points may be necessary. These are much smaller and shallower than manholes and only run down as far as the conduit. In these handholes a service box is placed. Fig, 41 shows
one style of service box with its cover removed. A,B, and C are the main cables that run straight through the box without being cut. D, E are the three-wire branch-service cables, or tubes, for supplying current to the buildings. These are attached to the main cables by means of suitable clamps, and after the cover is bolted in position the box is filled with insulating compound. Fig. 42 shows another style of service box for use on the three-wire system. In this four-way box the main cables are fastened to terminals instead of passing straight through. Fig. 43 shows a handhole with its service box arranged for delivering current to overhead conductors. The main feeders, running from manhole to manhole, are placed in the lower tiers of conduits, and the service mains
§24 LINE CONSTRUCTION 51
that run bacic from the manholes are run in the upper row, so that they will be accessible for the connection of : boxes.
43. Joining Cables. — For low-pressure work, cables are usually joined in the manholes by means of coupling boxes or junction boxes. Sometimes, however, joints must be made without the use of these boxes, in which cases the job must be very carefully done.
First, the soldered
end of the cable is cut off and the cable care- fully examined for moisture. If a little moisture be present and there is still more than enough room for the joint, it is allow- able to cut ofiE another short length. If indi- cations of moisture are still present, heat should be applied to the
Pio. 42
lead covering, starting from a distance and proceeding along the cable to the end. Thus, the moisture is driven out at the cut. When the use of torches is not allowed on account of gas in the manholes, hot insulating compound, such as boiling paraffin, may be poured over the cable. This process is known as boiling out. To ascertain whether moisture is present, the piece last cut off is stripped of its lead covering and plunged into hot insulating compound. If bubbles rise, moisture is still present.
44. High-Tenslon Cable Jolut. — Fig. 44 shows a typical hlKli-teuslon cable Joint. After all moisture has been driven out, the lead sheath is cut off for a suitable dis- tance from each end and the cable insulation is also cut back as indicated. A piece of lead pipe A of considerably larger diameter than the cable and a little longer than
52 LINE CONSTRUCTION §24
the total length of sheath stripped od is then slipped back on the cable. A copper sleeve (d) connects the abutting ends of the cable, and is sweated in place with solder worked in through the slot in the top of the sleeve. The sleeve is then covered with tape until it is brought up to a level with the cable insulation and a paper insulating sleevec that has previously been slipped back over the cable insula-
Pra. «
tion is placed over the joint and held there by a wrapping of string. The lead sleeve is now slipped into place and the ends hammered down around the cable sheath as indicated, and then soldered to the sheath with a plumber's wiped joint. These joints should be very carefully made so that there will be no opportunity for moisture to work into the cable and thus cause a breakdown. Two V-shaped openings are made in the top of the sleeve by cutting the lead and turning it back, as shown in (f); through one of these hot insulating compound is poured until the joint is filled. One
§24 LINE CONSTRUCTION . 53
of the openings allows the air to pass out while the compound is poured in at the other. In joining high-tension cables, the greatest care must be talien to have the joint perfect in every
particular. A slight defect may lead to a serious breakdown after the cable has been in use a short time.
EDISON UNDEKGKOUND-TUBE SYSTEM 45. The Edlsou underin'ound-tube system differs from the conduits previously described in that the con- ductors are placed in iron tubes that are buried in the ground.
The conductors are, therefore, '*■'"
not removable. This arrange- ment has been used extensively for three-wire 110-220 volt dis- tnbution in the larger cities. The conductors themselves are "'**
usually in the shape of round copper rods; the main tubes are designed for use on the three-wire system and are, therefore, provided with three rods, as shown in the section in Fig. 45. Each rod is wound with an open spiral of rope that serves to keep the rods separated in case the insulating material in the tubes should become soft. After
54
LINE CONSTRUCTION
§24
the rods have been provided with the rope spiral, they are bound together by means of a wrapping of rope and inserted in the iron pipe, the rods projecting for a short distance at each end. The whole tube is then filled with an
insulating compound that becomes hard when cold. The tubes are made in 20-foot lengths and are laid in the ground about 30 inches below the surface of the pavement. They
are joined together by means of the coupling boxes shown in Fig. 46 {a) and (d). Fig. 46 (a) shows the lower half of the box only, with the main tubes entering each end. The conductors are connected together by means of short, flexible, copper cables r, c, r, provided with lugs ^, d, that fit over the rods and are sol- dered in place. A cover d similar to the lower half e is then placed in position and the two securely bolted together by means of flange bolts, as shown in (d). After this has been done, melted compound is poured through an opening in the upper casting and the joint is complete. Fig. 47 shows two styles of connectors used for connecting the ends of the rods; (a) is a stranded
Pio.47
§24 LINE CONSTRUCTION 65
copper cable with temiiDals, and {6) is a laminated cop- per connector. Fig. 48 indicates a length of pipe with a coupling.
46. Where branches are talcen o£E the mains, T coupling boxes are used, as indicated in Fig. 49. This box, also, is filled with insulating compound that soon becomes bard and prevents the flexible comiections from coming in contact with one another. At the centers of distribution (usually a
street intersection) junction boxes are provided; these cor- respond to the manholes of the conduit system. The main supply wires, or feeders, run from the station to these junc- tion boxes, whence the mains are run to the various districts
66 LINE CONSTRUCTION §24
where light or power is supplied. Fig. 50 shows one of these junctioD boxes. The tubes enter at the lower part of the cast-iron box, and the mains are connected to the feeders through fuses that bridge over between the rings shown at the top. These fuses must be proportioned according to the size of the conductor in the tube to which they are
connected. The allowable carrying capacities of underground tubes and cables have been made the subject of a large number of tests by the manufacturers, who furnish tables giving the limit to which their cables or tubes may be loaded with safety. The junction box shown in Fig. 50 is made water-tight by clamping down the cover by means of the
§24
LINE CONSTRUCTION
57
studs d, b, and the whole is then covered with a cast-iron plate resting in the groove c and coming flush with the street surface.
47. The underground tubes and fittings are rather expensive, but they are comparatively cheap to install, as all that is necessary is to dig a shallow trench and lay the tubes in the ground. This system has the disadvantage that if any trouble occurs it is somewhat awkward to get at
TABIiB XI
CABBTING CAPACITY OF UNDEB- OBOIINl) TUBES
|
Size of Each Conductor Circular Mils |
Maximum Current in Each of Two Conductors |
|
4 1 ,ooo |
100 |
|
8o,ooo |
200 |
|
100,000 |
235 |
|
120,000 |
260 |
|
150,000 |
295 |
|
200,000 |
350 |
|
250,000 |
400 |
|
300,000 |
450 |
|
350,000 |
495 |
|
400,000 |
540 |
|
450,000 |
580 |
|
500,000 |
620 |
it, as the conductors cannot be pulled out as in a conduit system. When trouble occurs, the usual method of pro- cedure is to dig a hole at one of the couplings and separate the ends. By making a few breaks in this way at different points, the section in which the ground or short circuit is present can soon be located and the defective length of tube removed. Another and quicker method of locating grounds will be described later,
45—21
58 LINE CONSTRUCTION §24
48. The Edison tube system is not now used as largely as it once was for the main distributing lines or feeders. The present practice is to carry the main conductors from the station to the various distributing points in ducts, so that they may be drawn out if necessary. The tube sys- tem is, however, well adapted for the distributing mains, and is largely used for this purpose, because it allows ser- vice connections to be made easily and cheaply. Table XI gives the cross-section of the rods used in the standard tubes that are now used for distributing mains. Each tube has three conductors of the same size, and the table shows the allowable current when two of the conductors are loaded. If the system is balanced, the third wire will carry but a small current.
TESTS
49. In testing lines or apparatus, it is frequently neces- sary to make rough tests that will show whether or not circuits are continuous, broken, crossed, grounded, or properly insulated. These tests do not require accurate measurements; they are. merely made for the purpose of determining the existence of a faulty condition.
50. Magrneto Testing Set. — The most common, and probably, all things considered, the most useful, form of testing instrument for rough testing is that consisting of a magneto generator and bell mounted compactly in a box provided with a strap for convenience in carrying.
TESTING lilNES FOR FAIJIiTS
51. Faults on a line may be of two kinds: the line may be entirely broken, or it may be unbroken but in contact with some other conductor or with the ground. The former fault is termed a break; the latter a cross^ or ground. A break may be of such a nature as to leave the ends of the conductor entirely insulated, or the wire may fall so as to form a cross or ground. A cross or ground may be of such
§24 LINE CONSTRUCTION 59
low resistance as to form a short circuit or it may possess higfh resistance, thus forming what is called a leak. There are a number of different methods used for locating faults, and as those most suitable depend to a considerable extent on the kind of work for which the lines are used, most of the points relating to testing will be left until the different sub- jects with which they are connected are considered.
62. Continuity Tests, — In testing wires for continuity, the terminals of the magneto set should be connected to the terminals of the wire and the generator operated. A ringing of the bell will usually indicate, that the circuit is continuous. This is a sure test on short lines, but should be used with caution on long lines and with cables, because it may be that the electrostatic capacity of the line wires themselves will be sufficient to allow enough current to flow through the ringer to operate it, even though the line, or lines, be open at some distant point.
63. Testing for Crosses or Grounds. — In testing a line for crosses or grounds, one terminal of the magneto should be connected to the line under test, both ends of which are insulated from the ground and from other conductors. The other terminal of the magneto set should be connected successively with the earth and with any other conductors between which and the wire under test a cross is suspected. A ringing of the bell will, under these conditions, indicate that a cross exists between the wire under test and the ground or the other wires, as the case may be, and the strength with which the bell rings, and also the* pull of the generator in turning, will indicate, in some measure, the extent of this cross.
64. Here, however, as in the case of continuity tests, the ringing of the bell is not a sure indication that a cross exists if the line under test is a very long one. The insula- tion may be perfect and yet permit a sufficient current to pass to and from the line through the bell to cause it to ring, these currents, of course, being due to the static
60 LINE CONSTRUCTION §24
capacity of the line itself. In testing: very long lines or comparatively short lines of cable, the magneto set must be used with caution and intelligence on account of the capacity effects referred to. For short circuits in local test- ing, however, the results may be relied on as being accurate. Magneto testing sets are commonly wound in such manner that the generator will ring its own bell through a resistance of about 25,000 ohms. They may, however, be arranged to ring only through 10,000 ohms, or where espe- cially desired, through from 50,000 to 75,000 ohms. The first figure mentioned — 25,000 ohms — is probably the one best adapted for all-round testing work.
CURRENT DETECTOR GALVANOMETER
65. In order to test for grounds, crosses, or open circuits on long lines or on cables, without the liability to error that is likely to arise in testing with a magneto set, a cheap form of galvanometer for detecting currents, called a detector galvanometer, may be used. In testing for grounds or crosses, the galvanometer should be connected in series with several cells of battery and one terminal of the circuit applied to the wire under test, it being carefully insulated at both ends from the earth and from other wires, while the other terminal of the galvanometer and batteries should be connected successively to the ground and to adjoining wires. A sudden deflection of the galvanometer needle will take place whenever the circuit is first closed, this being due to the rush of current into the wire that is necessary to charge it. If the insulation is good, the needle of the gal- vanometer will soon return to zero; but if a leak exists from a line to the ground or the other wire with which it is being tested, the galvanometer needle will remain per- manently deflected.
In testing for continuity, the distant end of the line should be grounded or connected with another wire that is known to be good, and the galvanometer and battery applied, either between the wire under test and the ground or the wire
§24
LINE CONSTRUCTION
61
under test and the good wire. In this case, a permanent deflection of the galvanometer needle will denote that the wire is continuous, while if the needle returns to zero it is an indication of a broken wire.
5€f. Test for Insulation Resistance. — One thing that it is important to know about lines is the state of their insu- lation. In order to determine this, measurements of the insulation resistance between the line and ground must be made, and if this resistance is found to be dangerously low, the trouble should at once be looked up and remedied. One of the most convenient methods for measuring insulation resistance is by means of a good high-resistance voltmeter. The voltmeter is much easier to handle than a reflecting
Pzo. 61
galvanometer, and if the resistance of the voltmeter is known, insulation resistance measurements may be made with very little trouble. Suppose in Fig. 51 we wish to measure the insulation resistance of the line A A. The voltmeter is first connected across the lines at Fin the usual manner and the voltage of the dynamo D obtained. Call this reading V, After taking the reading F, the voltmeter is connected between the line B B and the ground, as shown at F,, and a reading F, obtained. In this case the current passes through the insulation from / to E^ through the ground to E^ and thence through Vx to /. It is evident that if the insulation resistance of the line A A is very high, very little current will flow through the voltmeter,
62
LINE CONSTRUCTION
§24
and a small deflection will be the result. If the resistance r of the voltmeter is known, then the insulation resistance of the line will be
r,_ ( V- r.) r
^- — V. —
(3)
ExAMPLB. — The insulation resistance of an electric-light main was tested by means of a Weston voltmeter having a resistance of 18.000 ohms. When connected across the lines, the voltmeter gave a reading of 110 volts. When one line was connected to g^round through the voltmeter, the reading was only 4 volts. What was the insulation resistance of the other line?
Solution. — We have by formula 3,
^ ^ (110 - 4) 18.000 ^ 106X18^ ^ ^^ ^^^^ ^ 4 4
Note.— The insulation resistance of lines is nsnally expressed in megohms. 1 mesrohm beinsr equal to 1.000.000 ohms. The resistance of the line in this case would therefore be .477 mesrohm.
TESTS FOR GROUNDS OR CROSSES
67. Varley lioop Test. — One of the most common methods for locating a ground or cross is by means of the Varley loop test. In Fig. 62, G is a sensitive galvanom- eter connected across the arms of a Wheatstone bridge in the ordinary manner; A B and A Care the ratio arms and CD
Pig. 52
the rheostat or balance arm of the bridge; D E is the faulty line, B E ^ good line, and 7^ is the location of the fault. The two lines should be connected together at E and the ends of the loop BED, so formed, connected across the terminals of the bridge as the unknown resistance. Call y the resistance
§24
LINE CONSTRUCTION
63
of the loop from B to F and x the resistance from D to F. With the battery connected between A and Z>, as in the ordinary method of using the Wheatstone bridge, balance the bridge. This will give, by working out the unknown resistance in the usual manner, a resistance i? equal to the sum of the resistances of the two wires forming the loop; that iSy I^ = y + X. Or, the resistance jR of the whole loop may be calctdated, if the length and size of the line wire are known.
«
Fig. 63
Now disconnect the battery from D and connect it to the groimd, as shown in Fig. 53. Then balance the bridge again, and the resistance x may be obtained by means of the follow- ing formula:
X =
(4)
m + n
in which m, «, and/ are the values of the resistances in the arms A By A C, and C D. After obtaining the resistance x from D to the fault F along the line D Ehy means of for- mula 4, the distance (in feet or miles) from the testing end/? to the fault F may be obtained by dividing this resistance x by the resistance of a unit length (a foot or a mile, as the case may be) of the line wire D E, The result obtained by this test is independent of the resistance at the fault between the line and the ground.
ExAMPLB. — A ground occurred on a conductor of a cable 10,000 feet long composed of three No. 10 wires. One good wire was used to
64
LINE CONSTRUCTION
§24
complete the loop. On testing with one end of the battery grounded as in Pig. 53, the bridge was balanced with the following resistances: Hf = 10 ohms, n = 1,000 ohms,/ = 1,642 ohms. Where was the ground, the resistance per 1,000 feet of the conductor being .9972 ohm?
Solution. — The length of the loop formed by joining the two wires of the cable at the distant end will be 20,000 ft.; hence, ^ = 20 X .9972
1,000 X 19.944 - 10 X 1.642
= 19.944, and x =
1,000 +10
distance of the fault from the testing station must be
3.4891
» 3.4391. Hence, the
.9972
X 1,000 s 3,498.9 ft. Acs.
Pio. 54
58* liOcatinfi: a Partial Ground Witlioat an Avail- able Good Wire. — ^The following method for locating a partial ground or leak is rather unreliable in practice, because the resistance of the partial ground may change
between the two measurements, and so give a more or less incorrect result. However, it is about the only way where there is no available good wire and where the tests must be made from one end only. The normal resistance of the
§24
LINE CONSTRUCTION
65
line must be known from some previous measurement, unless it can be calculated from the leng^th and size of the wire. Let this resistance be a; then measure the resistance of the line B B\ with the distant end B' gfrounded as shown in Fig. 54, and call this c. Also measure the resistance with the distant end open, as in Fig:. 55, and call this d ohms. Then the resistance x to the partial ground from the test- ing station is given by the following formula:
x^c- <(b-c)(a-c) (5)
Pio. 56
By dividing x by the resistance per unit length of the wire, known from some previous measurements or by a cal- culation from its size, length, and a table of resistances for the kind of wire under consideration, the distance to the grounded point may be obtained.
-90-
Pig. 67 ^
59. To liocate a Cross by the Varley Liobp Method.
First insulate the distant ends of the two crossed wires. Then connect as shown in Fig. 56 and measure the resist- ance from D io B through the cross F. Let the resistance of the cross be 2 ohms and the resistance found by balancing the bridge be R ohms. Then,
R ^ X ^-y + z (1)
66 LINE CONSTRUCTION §24
Now ground either wire, say D E^ anywhere beyond the cross, and connect as shown in Fig. 57. When the bridge is again balanced, we have
^ = ^ (2)
n p + X
From equations (1) and (2), x = ^-^-^J!.
tn -\- n
This is the same as formula 4. By dividing x by the
resistance of the wire DE per unit length, we have the
distance from D to the fault along the wire DE.
LOCATING GROUNDS AND CROSSES ON CONDUCTORS OF
LOW RESISTANCE
60. The above tests, in which the location of a ground or cross is determined by means of resistance measure- ments, are capable of giving the location quite closely, provided the wire is fairly small, say less than No. 8 or 10 B. & S. When the wire is large, as it nearly always is in connection with power-transmission systems, bridge methods do not give the location close enough, because it is evident that a small resistance corresponds to a long length of conductor. The location of faults on these large conductors is of special importance in connection with underground distributing systems, and the bridge methods cannot usually be applied on account of the low resistance of the conductors. When a cross occurs between the con- ductors of an underground cable, it nearly always results in a ground also, because the consequent short circuit fuses the cable, thus making connection between the core and the sheath. One way of locating faults on underground cables is by the cut-and-try process already mentioned. A manhole is opened at a point near the middle of the line, and the cable is cut. Each half is then tested and the half on which the fault exists is then cut out at its middle point, and so on until the fault is located between two manholes. This method is slow and expensive, especially where high-tension cables are used, because the making of joints in such cables is a slow and costly operation.
§24 LINE CONSTRUCTION 67
61, Another method of locating faults is to run a heavy current through the cable so as to bum the insulation at the fault, and thus fill the duct and manhole with smoke. On opening the manholes the presence of the smoke indicates the location of the fault. This method, while more rapid and less expensive than the cut-and-try method, has the disadvantages that the burning of a cable, especially if near a manhole, is liable to injure other cables, and also that the burning is liable to ignite accumulated gases and thereby cause a subway explosion.
62. Fig. .58 shows, diagrammatically, a method recom- mended by Mr. Henry G. Stott,* which is particularly useful for locating faults on underground cables of large size. A A
(D-D jg o
D D ¥ ^ P
I
Pig. 58
is the cable running through a series of manholes Ei, E,y etc. A ground has developed say at G\ and this ground has to be located. C is a small direct-current dynamo; an arc light machine answers very well, because it maintains a fairly constant current, irrespective of the resistance of the circuit. B is a, current reverser, which is revolved by means of a small motor. Brushes /, ^, which press on the rings d, Uy are connected to the terminals of C, and the contact arcs c, d are connected to the conductor and ground by means of brushes h, k. The rings a and b are connected to arcs c and d^ so that as the contacts revolve, the current flowing through the cable to the fault G' and back to G is periodically reversed.
•Transactions American Institute of Electrical Engineers, Vol. XVIII.
68
LINE CONSTRUCTION
§24
S
The speed of the mo- tor is such that the current is reversed once in about every 10 seconds. The fault is located by first opening: a manhole about the middle of the line, say at i?„ and laying: a compass D on the cable. The direct current, which need not be greater than 8 or 10 amperes, will cause the needle to swing: first to one side and then to the other every 10 sec-
. onds. If the needle £ swing:s in this way at -£*„ it shows that the fault is beyond E^\ hence, by this test, one-half of the cable is eliminated. The man- hole is then closed and another test made at say E^, At E^ no reversals of the com- pass will be obtained, because the current does not flow in the cable beyond the fault. The fault is therefore located be- tween E^ and E^\ by opening: a few inter- mediate manholes the
§24 LINE CONSTRUCTION 69
defective part is soon located between E^ and E^^ and this section of cable can be removed and the fault remedied. It will be noticed that, with this method, the cable is not cut and the time required to make the test in each manhole is very short, so that the trouble is quickly located, and there are no joints to be made afterwards save those actually needed to replace the defective part of the cable.
In case the cable system carries alternating current and has no permanent ground attached to it, this device may be used for locating a fault even while the alternating current is on the system. The testing device is simply connected to the feeder network as shown, but in series between it and the network is placed a reactance coil, for example, the primary of a transformer, the circuit being opened at e and the coil connected in series as shown at /. This avoids damage to the dynamo C by preventing a rush of current from the alternating-current generators in case another ground should occur on the other side of the system while the test was being made, thus producing a short circuit. Before applying the test it is a good plan to break down the insulation resistance of the fault by applying a high potential, between the conductor and ground, for a few seconds.
Fig. 59 shows the style of reverser used in applying this test. An induction motor M drives the shaft s by means of a worm-gear. The two-part commutator revolves in oil so as to give a quick reversal of the current.
SWITCHBOARDS AND SWITCHBOARD
APPLIANCES
SWITCHBOARD APPLIANCES
SWITCHES
1. Introduction. — The methods available for the transmission of electrical energy have been described in a general way, and it will now be necessary to examine more closely the various devices that are used for the control of the output of the generating plant. In order that a trans- mission system shall be under control, and also that the amount of the output, the condition of the lines, etc. shall be known, it is necessary to have various controlling and pro- tective devices in the station. These are usually grouped together at one central point on the switchboard^ and consist of switches, fuses, circuit-breakers, ammeters, voltmeters, ground detectors, lightning arresters, power factor indicators, wattmeters, and other auxiliary devices.
2« Probably the most important appliances on the switch- board are the switches, which are used for connecting or disconnecting circuits or dynamos from the rest of the sys- tem. Switches must be carefully selected with a view to the work that they have to perform. They must have ample carrying capacity and be capable of breaking the full-load current of the dynamo or circuit, without destructive burning or arcing. The style of switch used for any installation will depend on the voltage and current to be handled. For
For notice of copyright, see page immediately following the title page.
?25
a SWITCHBOARDS AND §2S
convenience, we will consider switches as divided into two classes: low-tension, for handling pressures up to about 1 ,000 volts, and high-tension, for pressures above this amount.
LOW -TENS ION SWITCHES
3. For pressures up to 1,000 volts, plain knife switches are generally used, though this style of switch with a broad separation of the blades and contacts has been used on pressures as high as 2,500 volts. For work of the latter class, how- ever, it is preferable to use a switch of the quick-break variety, and even for pressures of 500 volts, quick-break knife switches are commonly used. Fig. 1 shows
PlO. I VtQ. 3
a typical two-pole knife switch designed for front connec- tions and provided with fuses. Fig. 2 shows a similar switch without fuses and intended for mounting on a switchboard. When the switch is opened, connection is broken between the two clips at each side; thus opening both sides of the circuit. Knife switches should be substantially constructed and should have a contact surface at the clips of at least
25
SWITCHBOARD APPLIANCES
1 square inch for every 60 to 100 amperes, the allowable cur- rent density being greater in small switches than in large ones. Bolted contacts will carry 200 amperes per square inch, and laminated contacts, such as are described later on in connection with circuit-breakers, will carry from 300 to
TABIiB I CURRENT DENSITIES FOR COPPER STUDS
|
Diameter of |
Current Density |
Diameter of |
Current Density |
|
Stud Inches |
Amperes per Square Inch |
Stud Inches |
Amperes per Square Inch |
|
i |
1,200 |
li |
950 |
|
i |
1,150 |
li |
850 |
|
1 |
1,100 |
2 |
800 |
|
li |
1,000 |
3 |
700 |
500 amperes per square inch. For copper studs the current densities, shown in Table I, should not be exceeded if the temperature rise is to be limited to about 20° C.
For the same temperature rise the current density must be smaller in large studs than in small ones, because in the large studs the heat is not so readily radiated.
Fig. 3
4. The blades should be made of good conducting mate- rial, preferably of drawn copper, and the clips should be stiff enough to give a good, firm contact. For pure copper, the blades should have a cross-sectional area of about 1 square inch
45—22
SWITCHBOARDS AND
§26
per 1,000 amperes. Fig. 3, together with Table II, shows the dimensions, in inches, of General Electric knife switches.
Knife switches should always be mounted with the handles up; this is in accordance with a rule of the Fire Underwriters,
TABIiE II DIMENSIONS OF KNIFE SWITCHES
|
Capacity |
Dimensions Common to All |
Slncle-Pole |
Double- Pole |
Triple- Pole |
Pour- Pole |
|||||||
|
Amp. |
Volts |
A |
B il |
S |
6ft |
G i |
i |
C i |
D |
^ |
D |
D |
|
25 |
125 |
4ft |
4f |
4i |
4l |
|||||||
|
50 |
125 |
it |
a |
iH |
6ft |
i |
1 |
* |
4tt |
4* |
4* |
5 |
|
lOO |
125 |
2 |
2i |
2 |
6i |
i |
1 |
1 |
5f |
64 |
64 |
6t |
|
25 |
250 |
2i |
2} |
iH |
6ft |
i |
t |
f |
sft |
sf |
5f |
sft |
|
50 |
250 |
2* |
2} |
iH |
6ft |
f |
t |
t |
5« |
Si |
5* |
6ft |
|
100 |
250 |
2} |
3 |
2 |
6i |
i |
i |
} |
64 |
7l |
7i |
7A |
|
200 |
125-250 |
3i |
3l |
21^ |
6tt |
i |
I |
i |
7i |
84 |
84 |
84 |
|
300 |
125-250 |
at |
3* |
3i |
7} |
i |
if |
I |
9 |
94 |
94 |
9* |
|
500 |
125-250 |
4J |
4i |
4t |
8i |
i |
2i |
it |
iii |
"J |
II* |
134 |
|
800 |
125-250 |
5t |
5 |
5 |
9i |
I |
2f |
2 |
i2i |
I3» |
I3f |
144 |
|
1,200 |
125-250 |
5i |
5i |
4i |
10 |
li |
2i |
li |
12I |
134 |
I3i |
|
|
1,500 |
125-250 |
5t |
6 |
4J |
lol |
li |
2} |
2} |
13 |
I3i |
i3f |
which requires switches to be so placed that when open they will not tend to fall closed of their own accord.
5. Fig. 4 shows a style of quick-break switch that has proved very successful and is suitable for pressures as high as 2,000 to 2,500 volts if the current is not large. It has
been very widely used on direct- current railway switchboards. The switch blade, of drawn copper, is ^^'^ made in halves
A, B^ which are connected by two springs c, one on each side of the blade. When the handle is pulled out, the half A leaves the clip E and thus stretches the springs. When the bottom blade flies out, it leaves clip E very quickly, thus drawing out the arc and breaking it almost instantaneously.
§26 ' SWITCHBOARD APPLIANCES
HIGH-TENSION SWITCHES
6. In long-distance transmission plants using^ alternating current, the pressures are very high, and in some cases also the volume of current is large. A switch to interrupt a heavy current at high pressure has to be carefully designed, and a great many types have been brought out. These may be divided into three general classes:* (1) Those in which the arc is interrupted in the open air; (2) those in which the arc is interrupted in a confined space; (3) those in which the arc is broken under oil.
These switches may be arranged for hand operation or they may be designed to operate automatically in case the current exceeds the allowable limit. If used in the latter way, they are generally called circuit-breakers to distinguish them from the non-automatic type. In many cases it is necessary to have high-tension switches arranged so that they may be operated from a distant point, because it is not practicable or even desirable to have high-pressure switches of large capacity mounted on or near the operating board.
SWITCHES BREAKING ARC IN OPEN AIR
7. In this type of switch the arc is simply pulled out until it is broken. Fig. 5 shows a modification of the switch shown in Fig. 4. This switch will handle a moderate cur- rent at pressures up to 5,000 or 6,000 volts, but where the volume of current is large, it is better to use a switch belonging to class (3).
The switch (Fig. 5) is constructed so as to give a long, quick break, and is mounted on grooved insulators i, 2, 5, 4. This insulating material passes through the panel, so that in no place does the metal switch stud come in contact with the marble. This is a necessary precaution in cases where very high pressures are handled, because the marble cannot be depended on to give good enough insulation. Blade A has
^Classification given by E. W. Rice, Jr. Transactions American Institute Electrical Engineers, Vol. XVIII.
6
SWITCHBOARDS AND
§25
a hole in the end instead of a handle, and the switch is pulled open by means of a hook in the end of a handle about 3 feet long, thus allowing the attendant to stand back some distance and avoid the danger of being burned by the arc. To avoid arcing from one switch to the next, marble barriers C are mounted at right angles to the main part of the board.
For handling very high pressures, such as 20,000 volts and upwards, air-break switches have been used to quite a large extent. In these switches, the movable contact is generally
mounted on one end of a long arm, so that when the arm is thrown out, a break of several feet is made in the circuit.
8. Stanley Plug Switch.
Fig. 6 shows a type of air-break switch made by the Stanley Elec- tric Manufacturing Company, and used on pressures as high as 30,000 volts, at which pressure it is capable of handling a current of 25 amperes. A long wooden handle a is provided with a ter- minal b on its upper end, and this terminal is connected to a plug c by means of a flexible cable d. When plug c is inserted, it makes contact with a terminal sunk well below the surface of the marble, where it cannot be touched acci- dentally. Also, it is locked in position, so that the circuit cannot be accidentally opened at this point. The terminals e and / are mounted on ribbed porcelain insulators, and are made in the form of tapered points, as shown, so that the tip b may be slid over them. Hard-rubber guides arranged below the porcelain insulators engage with the projection cast on b, so that the handle a must be pulled straight down for a short distance when the switch is being opened, thus preventing terminals ^, / from being bent.
Pxo. 5
§ 25 SWITCHBOARD APPLIANCES 1
When the handle has been pushed up into place, it is
held by clamps £-, k. The switch shown in Fig. 6 is
of the double-throw type, i. e.,
terminal c can be connected to
either ^ or /; a marble barrier k
is placed between the terminals
to prevent arcing across. When
the switch is opened, the handle
is pulled down until the contact
is separated from the taper plug,
and it is then swung back over the
operator's shoulder and moved
away from the board until the
arc is ruptured. The tapered
terminals and the terminal on
the handle are provided with
zinc tips, as it has been found
that the arcing does not roughen
up the zinc to the same extent
as copper. One advantage of
this type of switch is that the
live terminals are at the top of
the board out of reach of the
operator. By unlocking plug e,
the handle with its cable may V\o. e
be removed entirely if it is desired to clear the board.
SWITCHES BKEAKINO THE ARC IN A CONFINED SPACE
9. Westlnghonse PlunKer Switch. — Fig. 7 shows a Westingbouse switch where the arc is broken in a confined space. The terminals are mounted at each end of a porcelain cylinder, A copper rod or plunger passing through these contacts or bushings completes the circuit, and when the plunger is withdrawn, the arc is formed in the confined space between the bushings. A small outlet is provided in the side of the tube, and when the arc is formed, the blast caused by the sudden expansion of the air in the confined space, together with the cooling action of the porcelain walls,
SWITCHBOARDS AND
§25
extin^ishes the arc. If the pressure to be handled is very high, a Qumber of these cylinders are connected in series, thus producing a long break. The .cylinders 1, 2, 3, etc. and plungers I', 2', 3' are mounted on the back of the board and are operated by a lever on the front. In the figure the switch is shown thrown out, but when the plunger is in, bushings a and 6, c and rf are connected to- gether, and the path of the current is a-b-e-d-e to ^°- ' line. When the plunger is
withdrawn, the arc is broken between a and b, e and d.
Stanley Slide ewitch and Circuit-Breaker.
Fig. 8 shows a Stanley slide s\7ltch provided with ;
S25
SWITCHBOARD APPLIANCES
automatic attachment that will open the switch whenever the current exceeds the amount for which the circuit-breaking device is adjusted. The attachment consists of a solenoid a through which the main current flows. When the current exceeds the allowable amount, the solenoid releases a catch and a spring throws the switch out. If it is not desired to use the switch as a circuit-breaker, the automatic device can be cut out. The switch terminals are mounted in the insu-
lating blocks b, y, of which there are two for each pole; in this case there are six terminals, the switch being three-pole. For each pole there is a cross-piece <r provided with blades d, d' that make contact with the terminals when they are forced in by swinging the handle (/ up. The motion of (f is transmitted to the cross-pieces c by means of a rack and pinion, and when the switch is opened the blades are withdrawn, from the
10 SWITCHBOARDS AND §25
clips; as soon as they leave the insulating: pieces, a shutter arrangement closes the opening, thus preventing the arc from following the blades. Switches of this type are made in a number of different sizes and are capable of handling as high as 60 amperes at 3,300 volts. The present practice, however, is to use oil switches for most high-pressure work.
11. Stanley Stab Switch. — Fig, 9 shows a simple form of high-tension switch that is capable of handling a current of 10 amperes at pressures as high as 7,000 volts. When the rod a is inserted, contact is made between the bushings d, c mounted in a thick fiber insulating tube. When a is with- drawn, the marble ball d drops from the cavity e in which it is held by the rod, and takes the position shown, thus effectually smothering the arc. The vent / provides an exit for the heated air. Switches of this type are particularly adapted for high-pressure, series-arc lighting circuits or series-incandescent lighting circuits.
SWITCHES BREAKING ARC T7NDER OII«
12. It has been found that circuits carrying large cur- rents at high pressure can be successfully broken by sepa- rating the terminals under oil, and oil-break switches have come much into use within the last few years. Circuits in which there is more or less inductance, producing a lagging current, require more effective switching devices than those in which there is no inductance, because the induced E. M. F. always tends to prolong the arcing when the switch is opened. Oil switches have proved very efficient on circuits of this kind. As soon as the switch terminals are separated under •oil, the oil fills the gap and arcing is effectually suppressed with a comparatively short separation of the terminals. It was at first thought that the very sudden break caused by a switch of this kind might give rise to severe strains on the insulation of the system, but this has not proved to be the case, and oil switches are now very largely used, both in central stations and also in connection with motors or other apparatus using alternating current. There are many different
SWITCHBOARD APPLIANCES
reliable makes of oil switches, but for purposes of illustration we will select a few examples of the General Electric type.
13. General Electric OH Switches.— Fig. 10 (a) and (*) shows a switch desigaed for mounting on the front of the switchboard or for individual use with motors or other appa- ratus. The same style of switch is made for mounting behind the switchboard with the operating handle on the front of the board; (if) shows the switch with the oil tank re- moved. In this case a triple- pole, single- throw switch is illus- trated, though the same type is made in single-pole, double- pole, and four-pole, and for either single-throw ' or double-throw. The terminals a, a, a are mounted in the porce- lain insulators 6, b, b. The contacts c are hinged as shown, and are connected together by a wooden cross- piece e connected to the operating handle. The other contacts d (^>
make a firm wiping ^"'- '"
contact with c when the switch is closed. The wires leading to and from the switch are attached to the terminals a, a, a
12 SWITCHBOARDS AND §25
so that they do not pass through the oil tank, and there is, therefore, no chance for oil leakage if the tank is not filled too full. This type of switch is recommended for use with all inductive appliances, such as induction motors, that operate at 250 volts or higher. It is not intended for circuits operating at pressures higher than 3,500 volts or in cases where the load exceeds 850 to 1,200 kilowatts, three-phase, under emergency conditions; i. e., under a short circuit or very heavy overload.
14. Fig. 11 (a), (d)f and (c) shows another General Electric switch of larger capacity. This is made single-, double-, triple-, and four-pole, and for single-throw only. The load that it can rupture under emergency conditions must not exceed 3,600 kilowatts, and the pressure 6,600 volts. For potentials exceeding 5,000 volts, the switch is not mounted on the back of the switchboard, as shown in Fig. 11, but is placed in a fireproof compartment entirely detached from the board. The operating handle on the board is con- nected with the switch by means of a series of levers. By this arrangement, the danger of fire at the switchboard is minimized and the operating devices can be entirely separated from the high-tension parts of the switch. Fig. 12 shows the general arrangement referred to, though, of course, the actual arrangement of the levers would depend on the relative location of the operating board and switch. These switches are arranged for simple hand control, or they can be provided with an attachment that will open them automat- ically in case the load becomes excessive, thus combining the feature of a switch with that of an automatic circuit-breaker. Fig. 11 (a) shows the operating handle provided with the automatic attachment; {d) shows the arrangement for hand control; (c) shows the construction of the switch proper with the oil tank removed. The terminals are held in the porce- lain insulators d, b, b, which are ribbed in order to inter- pose a large leakage surface between the terminals and the Eramework of the switch. When the operating handle is pushed in, the metal cross-pieces r, r, c are raised by the
SWITCHBOARD APPLIANCES
14
SWITCHBOARDS AND
§25
system of levers and brought into contact with the fingers df d, d, thus completing the circuit. Each cross-piece c is attached to a wooden rod e, and these rods are attached to a common crosshead that is moved up or down by the levers controlled by the operating handle. When the oil tank is in
flflfl
WOoatn/toa'
y,*Y,^//,f/^//,j'jy^^^//M^A/^^,/JM^J/M.
place, the contacts c and d are completely submerged in oil.
15. The automatic trip- ping mechanism used when the switch is mounted on the board is shown in (a). It consists of two solenoids/,/, which, when the current be- comes excessive, draw up their cores, which strike the lever g, g. This releases the link h that connects the operating handle with the switch and allows the switch j terminals to separate. The link h slides out through the operating handle, but the handle itself remains in. The projecting link, there- fore, acts as an indicator and shows that the switch has opened automatically. When the switch is opened by hand, the button k on top of the operating handle must first be pressed down.
16. Fig. 13 shows the connections for the tripping coils when the tripping mechanism is placed at the switch as in Fig. 11. The windings of the coils, Fig. 13, are connected to the secondaries of two current transformers, the primaries of which are in series with the mains, as shown. If the current in the mains becomes excessive, the current in the secondaries and tripping coils increases in like proportion, and if the current exceeds the value for which the armatures
m
Pio. 12
§25
SWITCHBOARD APPLIANCES
15
of the coils are adjusted, the switch is opened by the opera- tion of either one or both of the coils.
When the switch is not mounted on the board, the tripping coil is operated through an overload relay or auxiliary pair of magnets, as shown in Fig. 14. In Fig. 12, the tripping coil is located at a, and consists of a single coil, the arma- ture of which moves the light wooden rod b and allows the switch to open promptly whenever there is an overload. In Fig. 14, a is the tripping coil and^.c the coils of the over- load relay situated on the switchboard or at any other con- venient point. Under normal conditions the contacts d, e of
the relay short-circuit the tripping coil, but in ca-ie the current becomes excessive, either one or both of the coils draw up their cores and raise contacts d, e, thus making the current from the series-transformers take the path through the trip- ping coil a and opening the switch.
17, Oil Switch of I-arRe Capacity. — Fig. 15 shows two views of a General Electric oil switch of large capacity for use in central stations handling large alternating currents at high pressure. The switch is arranged for control from a distant point, the movements being effected by means of an
16
SWITCHBOARDS AND
§25
electric motor. These switches have also been built for operation by compressed air, and the Westinghouse Com- pany make a somewhat similar switch operated by solenoids. The casing of the switch shown in Fig. 15 is made of brick, and is provided with a removable iron door. The casing is divided into three compartments, one for each phase, and since they are separated by brick partitions, a bum- out, if it should occur, cannot spread to other parts. These switches are designed with a view to using the smallest possible amount of oil, because where there are a large number placed in a plant, the presence of a large quantity of oil in the switches would introduce a serious
To Line
Cur/ettt Transformer
OilJmkh
Oiter LoMi Rttaff9
Trip Coil
Fig. 14
fire risk. In each compartment is a pair of brass cylinders «, a with a contact sleeve at the bottom of each. These cans or cylinders are lined with insulating material, are filled with oil, and are provided with porcelain insulating sleeves b at the top through which slide copper rods c. The two rods are connected together by the cross-piece d^ so that when the rods are pushed down into the contact sleeves in the bottom of the cylinders, the two cylinders are electrically connected, the current passing from one cylinder to the other by way of rods Cy c and cross-piece d. The cross-pieces d are attached to a crosshead e by means of wooden rods /, and the motion of the crosshead is controlled by means of the motor g.
§25
SWITCHBOARD APPLIANCES
17
The motor is thrown into gear with a worm that operates a worm-wheel in the casing: A, whenever the solenoid k is excited from the switchboard. On the worm-gear shaft is a crank / which together with a link m forms a togglejoint. When the switch is out, as shown in the figure, spring n is compressed and the switch tends to close, but is prevented
Pig. 16
from doing so because the toggle / m is on center. As soon as the motor is started from the switchboard, the crank / is moved off center and the crosshead e is at once forced down. The crank / is driven from the worm-gear by means of a ratchet, so that as soon as the toggle is moved off center^
18 SWITCHBOARDS AND §25
the crank is carried around throngh nearly a half revolution independently of the movement of the motor. As soon as the crank stops, the ratchet at once takes hold and the crank is turned through the remainder of the half revolution until the toggle is again on center. The switch is now completely closed, and the motor is stopped automatically by means of a rotating switch moved by the worm-gear shaft. When the switch is closed, spring o is compressed and springs p are stretched. The switch is opened by starting the motor from the switchboard, as before, thus throwing the toggle off ranter again and allowing the springs to throw up the cross- head. In the opening operation, the springs / assist spring o, so that the opening is quicker than the closing, the time
required being about 1 second. For switches that have to handle large currents, the rods c, c are provided with auxiliary bell-shaped contacts g, g, which, when moved down to the dotted position, make contact with the upper part of the cylinders, thus relieving the rods of the current. When the switch moves up, these contacts leave the cylinder before the contact is broken inside the cylinder, so that no arcing takes place at the auxiliary contacts. The cylinders are mounted on ribbed porcelain insulators r,r, and are arranged so that they can be easily removed from these supports. The switch shown in Fig. II* has a range of movement of 17 inches and is capable of handling 300 to 800 amperes at 12,000 volts.
§25 SWITCHBOARD APPLIANCES 19
18. Stanley Oil Switches. — Figs. 16 and 17 show two types of Stanley oil switch. The switch shown in Fig. 16 is of the double-pole, double-throw type with the oil tanks mounted side by side. Fig. 17 shows a three-pole, single- throw switch with the tanks mounted one behind the other, so that the switch can be mounted on a narrow panel. The oil tanks a, b, c are of cast iron with an enamel lining, and are mounted under the marble slab d to which the fixed switch terminals e are attached. The slab d is supported by iron castings, and the switch arms / are operated by means of the levers, as indicated, thus throwing the blades g into or out of contact with the iixed clips. The terminals t are protected by wooden boxes, and the operating handle k is thoroughly
iusulated from the working parts of the switch by the wooden arm /. The tanks are arranged so that they can be easily refilled. There are two breaks in each leg of the circuit; in Fig. 17, for example, there are two fixed clips <f in each tank, and the two corresponding blades g are connected together.
BUS-BARS 19. Bus-bars should have a cross-section of at least 1 square inch per 1,000 amperes and should be arranged .so that the heat generated in them can he readily radiated. They should be substantially mounted and carefully insu- lated, particularly in cases where a high pressure is used. The bars are usually of flat rectangular cross-section; and if
20
SWITCHBOARDS AND
§25
large current-carrying capacity is required, a number of thiD bars are built up with air spaces between to allow ventila- tion. Thus, a bar made up of four bars i inch thick with a }-inch air space between each bar would be much better than a solid bar 1 inch thick.. Heavy solid bars should not on any account be used with alternating current. Where bars are made up of a number of thin bars with air spaces between, joints are readily made by interleaving the bars and bolting through, thus giving a large contact area. Round bars and copper tubes are occasionally used for bus-bars but they are not as desirable as flat bars except,
perhaps, for high-tension boards, where the current to be handled is small and where it is desirable to have the bars covered with insulating material.
Fig. 18 shows a simple method of mounting bus-bars for small low-pressure switchboards. Fig. 19 shows a method that has been largely used on 500-volt railway switchboards.
20. CuiTyingf Capacity otBiis-Bars. — Bus-bars should be of liberal cross-section, otherwise the loss in them may be considerable. For aliiiiiinnm bars, a density of from 500 to 600 amperes per square inch is allowable. Cast copper is much inferior to rolled or drawn copper as a conductor.
§25
SWITCHBOARD APPLIANCES
21
and the density in cast bars, studs, or fittings should not exceed 500 amperes per square inch. Brass can carry from 100 to 350 amperes per square inch, depending on the amount of copper in its make-up.
21. Mountliig for Hlg^h-Tenslon Bus-Bars. — When bus-bars have to handle a large current at high pressure, it is very important that they be mounted so that there is practically no possibility of a short circuit taking place between them. A short circuit on such bars might cause a great deal of damage, particularly if a number of machines happened to be feeding into the bars at the time. It has
^W
become customary, therefore, in large stations supplying cur- rent at high pressure, to mount the bars on fireproof supports and separate them by fireproof partitions so that each bar shall be in a compartment by itself. Fig. 20 shows the method of mounting 6,600-volt bus-bars in a large station in New York city. The bus-bar a is made up of four rolled copper bars 3 inches wide by i inch thick, and is bolted to a stud i> that is covered with an insulating tube c The bar, with its connecting stud, is supported on a firebrick slab rf, this slab being built into the brickwork e /. Thorough insulation is provided by the grooved porcelain insulato'-s f,^.
SWITCHBOARDS AND
and connections are made to the bar by means of the cable terminals h, k and plate k. Firebrick or soapstone slabs
projecting at right angles to the wall barriers between adjacent bars.
/ are used as
VOI/rMKTEB CONNECTIONS
22. It is customary, on switchboards, to make one volt- meter answer for several machines or circuits by providing
& W w
suitable voltmeter plugs or a voltmeter switch by means of which the instrument can be connected to the circuit or
§25 SWITCHBOARD APPLIANCES 23
machine on which a voltage reading is desired. Figs. 21 and 22 show a common plugging arrangement. A pair of voltmeter bus-wires a, b are con- nected to the voltmeter V, Fig. 21, and taps connect from a, b to the plug receptacles 1,1'. The dif- ferent dynamos are connected to 2, 2' and when a voltmeter read- ing is desired on, say, machine A, Fio.m a plug. Pig. 22, is inserted into the receptacle, thus con> necting 1,2 and l',2'.
23. Pressure Wires. — In many cases, particularly on systems supplying current for lighting purposes, it is necessary to know the pressure at the point where the current is utilized rather than at the station. In some cases, especially on low-pressure, three-wire systems, pressure ■wires a, b are run back to the station, as shown in Fig. 23.
f^3aare H^ivj.
U^
^ay^
The current required to operate the voltmeter is so small that there is practically no drop in the pressure wires; they can, therefore, be of small cross-section (usually No. 8 or No. 10 when. strung on poles); insulated iron wire is some- times used for the purpose.
24. CompeiisatlDK Voltmeter. — In order to avoid the use of pressure wires, compensating voltmeters, or compensators, are sometimes used with alternating-current circuits. The comiiensator is a device used in connection with the voltmeter to decrease the voltmeter reading as the load
24
SWITCHBOARDS AND
§25
increases, by an amount proportional to the drop in the line. The attendant then increases the field excitation of the alternator and brings the pressure up to such an amount that the voltage at the distributing point is correct.
Fig. 24 shows the connections for one of the earlier types' of Westinghouse compensating voltmeter. It consists of a series-transformer with both primary and secondary coils wound in sections. The primary is in series with the main circuit, and the secondary connects to a small auxiliary coil c on the voltmeter in such a manner that the current in c opposes the action of the current in the regular voltmeter coil d that
Cfi/rffe/r^afffr
ciir3
PlO. 24
is fed from the small potential transformer Zl When the voltage at the distributing end of the line is at its correct value, the hand of the voltmeter indicates the standard voltage. When the load increases, the current through the primary of the compensator also increases; this, in turn, increases the current in the secondary and the auxiliary coil. The hand on the voltmeter, therefore, goes back, and the pressure must be raised to bring it back to the standard point. By plugging in at different points on the primary and by set- ting at different points on the secondary, the compensator may be adjusted for operation on almost any of the circuits
§25 SWITCHBOARD APPLIANCES 25
ordinarily met. After it is once set to suit the particular line on which it is to work, it requires no further attention.
25. The MersbLon Compensator. — The compensator just described answers very well for lines that have little self-induction and that supply a non-inductive load. Where, however, the load is inductive, as, for example, a load of motors or of motors arid lamps, the reactance of the line may have a very gfreat influence on the drop in voltage, and the compensator must compensate not only for the ohmic drop in the line, but also for the drop due to the reactance. The Merslion compensator, brought out by the Westinghouse Company, is designed to accomplish this.
26. The principle of this compensator is briefly as fol- lows: The E. M. F. supplied at the end of the line is always equal to the resultant difference between the E. M. F. gen- erated and the E. M. F.*s necessary to overcome the resistance and reactance. If, then, three E. M. F.'s are set up at the station that are proportional to the above three E. M. F.'s and bear the same phase relation with regard to one another, and if these E. M. F.'s are combined in the same way as the line E. M. F.*s, it is evident that their resultant will make the voltmeter indicate the E. M. F. at the end of the line. For example, take the simple case shown in Fig. 25 (a). A is an alternator supplying current to the line. T' is an ordinary potential transformer, the secondary of which gives a voltage proportional to the generator voltage and in step with it. If the voltmeter V were connected directly to 7^', it would evidently indicate the station voltage, but what is wanted is an indication of the voltage at the far end of the line, and in order to get this, the voltage of V must be reduced by an amount equal to the sum of the drops caused by the reactance and resistance. An adjustable reactance a and an adjustable resistance b are therefore inserted in the circuit. The drop through b will be pro- portional to and in phase with the resistance drop, and the voltage across a will be proportional to and in phase with the inductive drop. From the way in which the connections are
26
SWITCHBOARDS AND
§25
made, it is easily seen that the voltage acting on Fis a com- bination of the voltages of 7^', a, and b. The drop across a and b will increase as the current in the line increases;
Ufte
•7
7ra/t^rmer.
WJRyvwWV
(•>
U/te
Thfno/brmer
-#• To Lca€f
Une
Curre/rf Transformer
Compensator
(h)
Fig. 25
hence, the voltmeter reading will decrease (because the connections are made so that the pressures across a and b cut down the E. M. F. applied to V), The voltmeter will,
§25 SWITCHBOARD APPLIANCES 27
therefore, indicate the true pressure at the end of the line because both the ohmic and inductive drops are accounted for. Fig. 25 (a) is intended to illustrate the principle only; the actual connections are more nearly as indicated in Fig. 25 id). Here A is the alternator, as before, and T' the potential transformer. 7" is a small current transformer, the primary of which is connected in series with the line and the secondary to the compensator proper, which consists of three parts, a, ^, and D. The E. M. F. generated in the secondary of T' is proportional to and in step with the gen- erator E. M. F. The current in the secondary of T is pro- portional to the load; a is a non-inductive resistance and d is a reactance coil wound on an iron core. These coils are connected in series, and the current supplied from the sec- ondary of T' passes through them. The E. M. F. across a is therefore in step with and proportional to the resistance drop in the line; while that across d is in step with and pro- portional to the back E. M. F. due to the reactance of the line. Z> is a small transformer in shunt with a; its secondary E. M. F. is in step with and proportional to the E. M. F. across a; b is also* provided with a secondary that gives an E. M. F. in step with and proportional to the E. M. F. across b. All these devices, i. e., a, b, and Z>, are in one piece of apparatus, and terminals from the secondaries of D and b are brought out to two multipoint switches, so that the number of turns in each may be adjusted to suit different lines. For three- phase circuits, a and b are supplied from two series-trans- formers whose primaries are connected in series with two of the lines and whose secondaries are in parallel. The volt- meter compensator made by the General Electric Company operates on practically the same principle.
FUSES AND CIRCUIT-BREAKERS
27. Either fuses or circuii-breakers may be used to pro- tect the generators or circuits from an excessive flow of current, due either to a short circuit or overload. Fuses are not used as much as they once were, as it has been found that circuit-breakers are more reliable. The
28 SWITCHBOARDS AND §25
circuit-breaker may be a separate device, or the main switch may be provided with an automatic tripping device, as already described.
FUSES ,
28* A fuse consists of a strip or wire of fusible metal inserted in the circuit, and so proportioned that it will melt and open the circuit if the current for any reason becomes excessive. Fuses are often made of a mixture of lead and bismuth, though copper and aluminum are also used. Aluminum is used very largely for high-tension fuses.
For low-tension switchboards, plain open fuses may be used; but for high-tension work, it is necessary to have them arranged so that the arc formed when they blow will not hold over. Moreover, it is necessary to have high-tension fuses arranged so that they can be renewed without danger to the switchboard attendant.
29. Fig. 26 (a) shows a type of fuse block used by the General Electric Company on alternating-current switch- boards; id) shows the shape of the aluminum fuse used in the block. The fuse holder is made in two parts, the lower part A being of porcelain and the upper part B of lignum vitae. The lower part is provided with blades c that fit between the clips ^, </', in the same way as the blades of a knife switch. These clips lie in slots in the marble board F and are connected to the line and dynamo by means of terminals ^ and k. By adopting this arrangement, the whole block may be detached from the board by simply pulling it straight out, thus pulling the blades out of the clips. The fuse is shown at /, and is clamped by means of the screws m^ n, A vent hole p is provided in the lignum- vitae cover, and the rush of air through this vent, together with the confined space, results in the suppression of the arc. This fuse block is suitable for currents up to 150 amperes at 2,500 volts. For higher pressures fuse blocks are used in which the fuse is pulled wide apart as soon as it blows, thus breaking the arc.
The use of the fuses on low-tension lighting switchboards
§25 SWITCHBOARD APPLIANCES 29
is not as commoa as it once was, their place being taken by the automatic circuit-breaker. Fuses are, however, used considerably on alternating-current boards and also for pro- tecting individual circuits on low-tension, direct-current boards. They are not as convenient or reliable as circuit- breakers, because it takes time to replace them when they blow, and only too often they are replaced with a heavier fuse or even a copper wire, which is of scarcely any use as a pro- tection. Again, fuses of the same size do not always blow at
T^X_
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(i>>
the same current, as much depends on the nature of the fuse- block terminals. If the clamps are not screwed up tightly, local heating will result; and the fuse will blow with a smaller current than it should. Also, it has been found that a fuse of a given cross-section and material will carry a heavier current when the distance between the terminals is short than when it is long, on account of the conducting away of the heat by the terminals.
30. Fig. 27 shows a type of high-tension enclosed fuse made by the Stanley Electric Company. The fuse is held in the holder a, which can be pulled out of the clips b when a
30 SWITCHBOARDS AND §25
fuse is to be renewed. Suitable blades are provided at each end to engage with clips b. The clips and connecting studs are thoroughly insulated by the porcelain insulators c, c, which prevent leakage of current to the supporting panel d. The fuse k passes through a fiber tube e and is held at each end by screws /; tube e is enclosed in the hard-rubber tube / of large diameter. At each end of the fuse is a cavity in which
is placed a carbon ball g, and when the fuse blows the balls are forced up against the openings leading to the ter- minals, thus cutting off the arc. These fuses can handle a current of 50 amperes at 20,000 volts. There is a small hinged lid k on top that is thrown up when the fuse blows, and thus acts as an indicator to show which fuse has blown.
§25 SWITCHBOARD APPLIANCES 81
The high-tension fuse used by the Westinghouse Company consists of two long hinged wooden arms that are held together by the fuse against the action of a spring. As soon as the fuse melts, the arms separate, thus placing a break of several feet in the circuit and rupturing the arc.
CIBCUIT-B
31, Some circuit-breakers have already been described in connection with high-tension switches. The circuit- breaker is essentially an automatic switch that opens the circuit whenever the current exceeds the allowable limit. It is therefore intended more as an automatic safety device than as a switch for regularly opening or closing the circuit.
■^
Circuit -breakers are made in great variety, handling cur- rents from a few amperes up to several thousand, and are con- structed for both alternating current and direct current. In nearly every case they consist of a switch of some kind that is held closed against the action of a spring. The main cur- rent passes through an electromagnet or solenoid, and when the current for which the breaker is set is exceeded, this magnet attracts an armature or core and operates a trip, thijs allowing a switch to fly out. In some cases the breaker
32 SWITCHBOARDS AND §25
opens both sides of the line, though often they are sinele- pole and open one side only. We will illustrate here a few examples to show their general method of operation.
32. Geueral Electric Circuit-Breakers. — Pigs. 28 and 29 show a type of General Electric circuit- breaker desif^ed for 125- or 250-volt circuits. One of the principal features of this circuit-breaker ts the main contact used. It consists of a U-shaped laminated contact a which is pressed firmly against the con- tacts b, b by means of a togglejoint, when handle h is forced down. Each main contact is provided with a pair of light auxiliary con- tacts m, m that can be easily renewed. These wipers press against the carbon blocks fi, p, and when the breaker flies out, the arc is Anally broken between the carbon blocks and the wipers. Laminated contacts are not liable to stick and they make a very good contact because of the firm pressure and the slight wiping action caused by the closing of the breaker. The tripping coil 5 attracts the arma* ture -4 when the current becomes excessive and trips the breaker, which is promptly opened by the spring /. The current for which the breaker is set may be adjusted by means of the screw v and the breaker may be tripped by hand at any time by pulling down on the knob w. The breaker shown in Fig. 28 is a double-pole; Fig, 29 shows a similar breaker of the single-pole type.
§25
SWITCHBOARD APPLIANCES
33
33. General £lectric M K Circuit-Breaker. — ^This breaker, Fig. 30, has been very widely used for 500-volt, direct-current, railway switchboards and is here shown as an example of the class of circuit-breakers in which a magnetic field is used to extinguish the arc. In Fig. 30, ^ is a heavy tripping coil through which the main current passes. The
Pig. ao
current enters the coil through the stud A; from the coil it passes to a connection on the back of the heavy copper con- tact block C When the breaker is closed ready for service, as shown in the figure, the main current passes from C to the laminated contact Z>, D and out to the line through the heavy block £, which has a terminal like A in the rear.
34
SWITCHBOARDS AND
§25
When the breaker is closed, the hinged iron armature F is held up by a spring G, the tension of which depends on the adjustment of a thumbscrew J. Attached to plate F is 3. trigger H, that has a shoulder against which a projection on the main handle yoke K bears. To set the breaker, the main handle L is pulled down hard; this forces Z>, D up against blocks C and E^ and also causes the projection on K to engage trigger H, which holds the circuit-breaking parts in place. In setting the switch, spring if/ is extended. When the breaker trips, solenoid B draws down armature F, and with it trigger H, which liberates the switch yoke and allows
Pzo. n
the strong spring M to pull down Z>, Z7, and hence open the circuit at C and E, In order to prevent burning of the main contacts, a shunt path is provided, as indicated by the circuit T-S-R-P-O-P-R-S-U, Fig. 31. 5, 5 are two magnetizing coils that set up a strong magnetic field between the auxiliary contacts P, P, When the breaker is closed, the contact piece O is pushed up between contacts P, P which are pressed firmly against O by springs (2. Q- Wh^n the breaker trips, contact D, D leaves C, ^ a littlf
§25 SWITCHBOARD APPLIANCES 35
before O leaves P, P, so that for a short interval the main current takes the path through the auxiliary contacts and blow-out coils S, S. A strong magnetic field is thus set up and when the circuit is finally broken at the auxiliary contacts, the arc is instantly blown up through an opening in the top of the breaker. Whatever burning action there may be is thus transferred to the auxiliary contacts, which are easily renewed or repaired.
34. Cutter Circuit-Breaker. — Fig. 32 shows the Cutter (l. T. E.) laminated -type circa It- breaker. The main contact a is lam- inated and is pressed against the contact surfaces by means of the handle working through a toggle joint ate. The tripping coil is shown at d and when the current ex- ceeds the amount for which the breaker is set the core inside d is suddenly drawn up, thus striking a trig- ger and allowing the breaker to fly out. The position of the core in d can be changed by adjusting screw e, thereby vary- ing the current at which the breaker trips. Auxiliary carbon contacts b, b do riot open until after the main contact so that the burning action is confined to the carbon contact surfaces. The Westinghouse circuit-breakers arc very similar in general appearance and operation to the type shown in Fig. 32, the main difference being in the arrangement of the tripping coll. 45—24
36
SWITCHBOARDS AND
§25
GROUND DBTECTORS
35. Ground detectors are used to determine whether or not a line or conductor, that should normally be insulated, is in contact with the ground or any conductor leading to the ground. A voltmeter makes a very good ground
(a)
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V 8
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I
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detector, because it not only indicates whether a ground is present, but by its deflection it shows whether the path of the current to ground is one of high resistance or low resistance.
§25
SWITCHBOARD APPLIANCES
37
In order to indicate grounds, the voltmeter may be con- nected as shown in Pig. 33 (a). If the line a should be grounded, as indicated by the dotted line, and the switch blade placed on point I, no deflection would result. If, however, the blade is moved to point 2, current will pass from line a through the ground on the line to the voltmeter to point 2, and thence to the line d, thus completing the circuit. When a deflection is obtained on point 2, it shows that line a is grounded; and when obtained on point I, it shows that line d is grounded. If the ground is of high resistance, the deflection will be comparatively small; if of low resistance, the deflection will be large. In Fig. 33 (a), the current will flow through the volt- meter in the opposite direc- tion on point 2 from what it will on point I; hence, the voltmeter must have its zero point in the center of the scale, so that it can read either way. Voltmeters, however, have their zero point at the left-hand end of the scale, and it is convenient to have a switch that will allow the ordinary voltmeter to be used either as a voltmeter or ground detector. Fig. 33 (^) shows an arrangement for doing this. When the switch is in the position 1-1\ the voltmeter F is connected directly across the line and gives the voltage on the system; when in the position 3-3\ the voltmeter indicates any grounds, such as C, that may be present on line d. When 5 occupies the position 2-2\ V indicates grounds on line a, as at C.
Pxo.84
36, Another very common arrangement for detecting grounds is shown in Fig. 34, where two lamps c, d are con- nected in series across the lines. The voltage for which these lamps are designed is equal to that of the dynamo, so that when the two are connected in series, they will burn dull red. At the point between the lamps, a connection is
38
SWITCHBOARDS AND
§25
made to ground through a switch or a push button /. If con- tact is made at / and there is no ground on either line, the brilliancy of the lamps will not be altered. If there is a ground on by as indicated at G', lamp d will go out when switch / is closed, and c will burn brightly. This lamp detector is simple, and while it serves as an indicator of grounds, it is not as satisfactory as the voltmeter detector, as it does not give accurate indications as to the resistance of the fault.
37. Fig. 35 shows a lamp ground detector suitable for a three-wire, low-tension system. Three lamps A, /„ /, are connected in series across one side of the system, and
a ground connection is made at x through key K, When all three lines are clear of grounds, the lamps will bum at a dull red, they will all be equal in brightness, and their color will not change when key K is pressed. If line C be- comes grounded at C, then, when A^ is pressed, /, and /, will go out, and /, will come up to full candlepower. If a ground occurs at G" on line B, lamp /, will go out and /,, /, will brighten up, but will not come up to full candle- power because two of them will be in series between B and C If there is a ground at G'" on line Ay all the lamps will come up to full candlepower, because they will all get the full voltage, /, being across A B and /», /, in series across A C.
• 38. The ground detectors just described apply more par- ticularly to low-tension, direct-current installations, but similar arrangements may be adapted to high-tension, alternating- current systems by using potential transformers. Fig. 36 shows one method used by the Westinghouse Company on their alternating-current switchboards. The regular
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Pig. 35
§25
SWITCHBOARD APPLIANCES
39
voltmeter F, with which the switchboard is equipped, is here used also as a ground detector. P is a. plug switch by means of which points 1 and 2 or 1 and 3 may be connected together. Under ordinary conditions, the plug is in 1 and 2, thus con- necting the primary of the potential transformer across the line, and I/' serves as an ordinary voltmeter. 5" is a key that connects one side of the line to ground through the trans- former primary. If there happens to be a ground on the side dj as shown at G\ the voltmeter will give a reading when ►S is pressed. By placing the plug in points 1 and 5, side a may be tested. When the key 5* is not pressed, the lever 5 is against contact 4y so that V is connected as an ordinary voltmeter.
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39. Blectpostatlc Ground Detectors. — Ground detectors operating on the electrostatic principle are much used on high-pressure, alternating-current switchboards. They have the advantage that they require no current for their operation and may be left connected to the circuit all the time, thus indicating a ground as soon as it occurs. They also give an indication without its being necessary to make an actual connection between the line and ground, as is the case with all the detectors previously described. Fig. 37 illustrates the principle of a Stanley electrostatic ground detector, which is especially adapted to high-pressure, alter- nating-current lines because the instrument is not in actual connection with either of the lines. The fixed vanes 1 and 4,
40
SWITCHBOARDS AND
§25
2 and 3 are connected together in pairs, as shown. The movable vane Fis connected to the ground and is held in the central position shown in the figure by means of small spiral springs S. The pairs of fixed plates are not connected direct to the lines, but are attached to plates a, a' of two small condensers which consist simply of two brass plates, mounted in hard rubber but separated from each other.
Q
Fio.VI
Plates d, V are connected to the lines. When no grounds are present, 1 and 4, 2 and 3 become oppositely charged by reason of charges induced on plates a, a' by plates ^, ^. The forces acting on the vane Fare therefore equal and opposite. Now, suppose that line B becomes grounded at (7'^ This is equivalent to connecting vane V to line B\ V takes up a charge similar to 2 and 3\ hence, it is repelled by 2 and 3
§26 SWITCHBOARD APPLIANCES 41
and is attracted by 1 and 4, thus giving: a deflection. If A becomes grounded, a deflection in the opposite direction is obtained. Instruments of this kind can, of course, only be used in places where the pres- sure is fairly high, as the electrostatic forces pro- duced by charges due to low pressures would not be large enough to oper- ate an instrument unless it were made much too delicate to be of prac- tical use in a light or power station. In most electrostatic detectors, the lines are connected *■
directly to the fixed sectors I, 2, 3, 4 and the-con> densers C, C are omitted.
. 40. Fig. 38 shows an electrostatic ground detector made by the Wagner Elec- tric Company. The fixed quadrants are shown at a, a, and the movable vane at d, b. The quadrants are connected to the line wires, and the vane is connected to ground. The vane is held nor- mally in its central position by means of a spring, and the pointer is deflected ^**- *• whenever a ground
occurs on either line. The [Manciple of action is the
42
SWITCHBOARDS AND
§25
same as that of the electrostatic ground detector just
described.
41. Figs. 39 and 40 show a General Electric, three- phase, electrostatic ground detector. It is practically three single-phase detectors combined in one instrument. When no ground exists, the three needles point toward the center. When a ground occurs on one of the lines, the two adjacent needles are de- flected toward the segments to which the grounded line is connected. Should a ground occur on two lines, the needle between the segments con- _ negted to the grounded lines — will be deflected toward the ~ one having the lower resist- ance ground and the two remaining needles will be
n
l/se
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Pio. 40
deflected toward the grounded segments.
POTENTIAIi REGUIiATORS
42. Where a number of feeders are supplied from a single dynamo or set of bus-bars, it is often necessary to provide means for raising or lowering the pressure on these feeders independently of each other. When alternating current is used, the pressure on the feeders can be easily adjusted by using potential pcgrnlators. These appliances, while not usually placed on alternating-current switchboards, are so closely con- nected therewith that they are here described. . There are many types of regulators but they all take the form of a special type of transformer with the primary connected across the mains and the secondary in series with one of the mains.
25
SWITCHBOARD APPLIANCES
43
43. Use of Transformer to Raise Voltag^e. — An
ordinary transformer connected as in Fig. 41 can be used to raise or lower the primary voltage by an amount equal to the secondary voltage of the transformer. When the double- throw switch is in the position indicated by the dotted lines, the primary is across the mains and the secondary in series with the lower main, thus adding 100 volts in this case or subtracting 100 volts if the connections be such that the secondary E. M. F. opposes the line E. M. F. When the switch is thrown to the right, the boosting trans- former is cut out.
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i
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1
Pio. 41
44. 8tllli»vell Begrulator. — Fig. 42 shows the connec- tions for a Stlllwell regulator. It operates in the same way as the transformer in Fig. 41 but the secondary 5" is provided with a number of taps connected to a switch i^so that the amount by which the voltage is raised or lowered can be adjusted. The primary P is connected to a reversing switch b so that the secondary E. M. F. can be made either to aid or oppose the primary E. M. F., thus using the regu- lator either to raise or lower the line pressure. The contact arm N is made in two parts, connected through a small reactance coil r, the object being to prevent momentary short-circuiting of the transformer sections during the instant the arm bridges over adjacent contact segments. By
44 SWITCHBOARDS AND §26
following out the connections, it will be seen that the sec- ondary is in series with the main circuit and the primary across the circuit, as in Fig. 41.
45. C B Regulator. — The C R reirulator, made by the General Electric Company, operates in a manner very
similar to the Stillwell regulator. Fig. 43 shows the general appearance of the regulator, and Fig. 44 the connections. The reversing switch operates automatically and is placed in the secondary circuit, and not in the primary as in the Stillwell
§25 SWITCHBOARD APPLIANCES 45
reeulator. In Fig. 44 the reversing switch is indicated at the lower part of the figure, and consists of an arm that is moved by the arm of the main switch so as to con- nect a with either c or b. The windings consist of a primary and secondary, the former connected across the circuit, and the latter divided into a number of steps, in series with the circuit. When the reversing switch and the main switch arm are in the posi- tions shown in Fig. 44, the main current flows through the whole of the secondary winding, and the maximum increase in voltage is ob- tained. As the dial switch arm is turned, the sections of the secondary are succes- sively cut out as contact is ^^- **
made at d, e, f, etc.; when the arm reaches g, the whole of the secondary winding is cut out, and the voltage sup-
plied to the feeder is the same as that furnished by the gen- erator. When the arm is started on a second right-handed
\
46 SWITCHBOARDS AND §25
revolution, the reversing switch is shifted automatically, so that point a is connected with dy and as the move- ment of the dial switch is continued to the right, the sections of the secondary are successively cut in, and the current now flows through them in the reverse direction to what it did before. The second revolution, therefore, lowers the feeder pressure below that of the generator; when the second revolution has been completed, the switch is auto- matically stopped. The dial switch is made so that when the handle is turned, springs are first compressed and the blade then unlocked by a cam so that it flies from one con- tact to the next almost instantly. The switch blade is slightly narrower than the distance between the contacts, so that there is no short-circuiting of the transformer sections.
■
46. A number of regulators are in use in which the volt- age in the secondary is varied by changing the position of the secondary with regard to the primary, instead of cutting turns in or out. By having the secondary coil movable, it can be arranged so that the amount of magnetic flux passing through it can be varied, thus varying the amount of the pressure added or subtracted. In other regulators, both the primary and secondary coils are fixed, and a movable core arranged so that the magnetic flux passing through the secondary can be made to vary.
§25 SWITCHBOARD APPLIANCES 47
PROTECTION FROM LIGHTNING AND
STATIC CHARGES
47. There are sources of danger to electrical equipments that may arise outside the station and that may cause great loss unless ample provision is made for protection. Among these are danger from lightning, danger from static charges, or other effects commonly referred to as static, and danger from short circuits caused by either of the former. Damage from lightning occurs on systems having overhead lines, but static charges and the damage resulting therefrom can occur on systems having either overhead or underground lines.
PROTECTION FROM lilGHTNING
48. Damage from lightning is due to an excessive differ- ence of potential that may exist between the atmosphere and the earth, and as overhead electrical conductors offer a path of comparatively low resistance, the atmospheric electricity will seek such path to the earth, unless prevented by suitable methods of lightning protection. Any properly designed piece of apparatus should have sufficient insulation to withstand a potential considerably higher than that nor- mally imposed on it, and to produce a ground, a lightning discharge must cause an excessive rise in the potential of the circuit. It frequently happens that the weakest point of insu- lation is at the switchboard or generator, and in the absence of sufficient protection, great damage will result at the station.
49. Overhead lines are always liable to accumulate a certain charge of static electricity even if they are not actually struck by lightning. Long transmission lines should be well protected against lightning, as they frequently nm through exposed and mountainous country. If these high- pressure discharges travel along the line and get into the
48
SWITCHBOARDS AND
§25
dynamos at the power station, they are almost sure to puncture the insulation of the machines and cause a bum-out. To guard against this, lightning arresters should be provided.
50. Simple liigrlitning: Arrester. — The term llgrlit- ning: arrester does not correctly express the use of these devices, because they do not arrest the discharge coming in over the line; they merely divert the charge by providing a path that the lightning will take to ground in preference to passing into the dynamo and making a path for itself to the ground by puncturing the insulation of the machine.
A lightning discharge is generally oscillatory in character, hence it will not pass through zn inductive path if an
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JIXD C ] C
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Pig. 46
alternative non-inductive path is provided for it. The object of a lightning arrester is to furnish a non-inductive path to ground and at the same time make provision for. suppressing the arcing that usually follows a discharge. Fig. 45 shows a line equipped with lightning arresters of the simplest possible form. The plates 7, 2 are connected to the lines and are separated by small gaps gy g from plates 5, 3 which are connected to the ground. The gap in the arrester should be more easily jumped across by the discharge than the weakest insulation on the dynamo; otherwise, the discharge may jump through the insulation to the ground instead of jumping across the air gap. The air gap must, of course, be long enough so that the pressure generated by the dynamq
§25 SWITCHBOARD APPLIANCES 49
itself will not be able to jump across it. For pressures up to 500 volts, a gap of iV inch should be suflScient.
51. Reactance, or Cboke, Colls. — In order to force the discharge to pass through the arrester, choke colls, react- ance colls, or kicking colls, as they are variousljr called, are inserted between the arrester and the device to be protected. Such coils consist of a few tiUTis of wire or
copper strip connected in the circuit as shown at A, A in Fig. 45. The discharge, in prefer- ence to overcoming the inductance of these coils, will jump the air gaps and pass ofi to ground. Fig. 46 shows a typical reactance coil of small size suitable for low-tension work. Fig. 47 shows a Westingbouse choke coil made of flat copper ribbon and mounted on a heavy glass insulator. This coil is for use on a high-tension circuit; hence, thorough insulation from the ground is necessary. '
52. Suppression of Arcing. — The
simple arrangement of air gaps shown in in « —
Fig. 45 would not be suitable for electric- light and power circuits for the following reason: If a dis- charge comes in over both the lines at once, as is quite likely to happen, because the lines usually run side by side, an arc will be formed across both the gaps, and current from the dynamo will follow the arc. This will practically short- circuit the dynamo, and such a large current will flow that the plates or contact points of the arrester will be destroyed. It is necessary, then, to have in addition to the air gap some means for suppressing or blowing out the arc as soon as it is formed. It is also necessary that as soon as the discharge has passed, the arrester will be in condition for the next discharge. Generally speaking, the arc from a direct -current machine is not as easily extinguished as that from an alternator; probably because every time the current passes through its zero value it loses some of its ability to hold the arc. In some cases, the arc is broken by being drawn out
50 SWITCHBOARDS AND §25
until it can be no longer maintained; in others, the air gap is so placed thai it will be surrounded by a magnetic field, so that when the arc is formed it is forced across the field and stretched out until it is broken. Another method is to make the arc occur in a confined space so that it will be smothered out. Still another method is to make the cylinder or plates between which the arc jumps of a so-called non- arcing metal, the vapor of which offers a high resistance to the discharge. Some arresters will work on either direct or alter- nating current; but, generally speaking, the arrester has to be selected with reference to the voltage of the circuit on which it is to be used and also with refer- ence to the kind of current.
53. Ground Connections for Ll^litnlnK Arresters. Arresters will be of little or no use if good ground connections are not provided for them. The following methods of making ground connections are recom- mended by the Westinghouse Company: A ground connec- tion for a line or pole lightning ^"^■*' arrester is shown in Fig. 48.
A galvanized -iron pipe is driven well into the ground and the top of it surrounded by coke, which retains moisture; the wire is run down the pole and connected to the top of the pipe as indicated. The wire is sometimes incased in galvanized -iron pipe for about 6 feet from the base of the pole and if this is done, it is well to solder the ground wire to the pipe at a. The following method of making the groimd connections at the station is recommended: A hole 6 feet square is dug 5 or 6 feet deep in a location as near the arresters as possible,
§25 SWITCHBOARD APPLIANCES 61
preferably directly under them. The bottom of this hole is then covered with charcoal or coke (crushed to about pea size) to a depth of al>out 2 feet. On top of this is laid a tinned, . copper sheet, about 5 feet by 5 feet, with the ground wire {about No. 0 B. & S.) soldered completely across it. The plate is then cov- ered with a 2-foot layer of coke or charcoal and the remainder of the hole filled with earth, running water being used to settle it. This will give a good ground, if made in good, rich soil; it will not give a good ground in rock, sand, or gravel. Sometimes grounds are made by put- ting the ground plate in a running stream. This, however, does not give as good a ground as is com- monly supposed, because running water is not a par- ticularly good conductor and the beds of streams very often consist of rock. When lightning arresters
are installed, all wires leading to and from them should be as straight as possible. Bends act more or less like a choke coil and tend to keep the discharge from passing oft by way of the arrester.
ARRKSTBRS FOR DIRECT CURRENT
54. Garton Arrester. — Fig. 49 illustrates the Garton arrester. The discharge points are of carbon, shown at // and/ These are about a'-j inch apart, and the lower one is connected to ground; / is a coil of wire wound on the tube.c.
45— 2B
63 SWITCHBOARDS AND §25
closed at the top; ^ is a small core o( iron attached to the rod d, which in turn connects, by means of a small flexible cable, to one end of a resistance b. The other end of the coil connects to the other end of the resistance, to which the line also connects. The resistance b is made up of a stick of graphite, which, having practically no induct- ance, offers little or no opposition to the discbarge and is used to limit the rush of current that follows the dis- charge. The discharge comes in over the line to a, passes through b to the rod d, thence to the carbon point A, and jumps the air gap to the ground. The discharge is followed by current from the dynamo, and. since the coil is in shunt with the resistance, part of the current jvill flow through the coil, thus drawing up the core e and breaking the arc between e and k. The fact that the arc also takes place in the enclosed tube tends to put '^'*- ** it out. As soon as the discharge has
passed, the core drops back and the arrester is ready for the next discharge. This arrester can be used on either direct- or alternating-current circuits.
55. Westinghonse Arrester. — Fig. 50 shows a West- Inichouse arrester used on direct-current circuits. It has no movable parts, and the arc is extinguished by smothering it in a confined space. Two terminals b, 6 are mounted on a lignum-vitse block and are separated by a space somewhat less than i inch- This space is crossed by a number of charred grooves, so that although the resistance in ohms between the terminals is very high, the lightning will readily leap across the space. The block A is covered by a second block, not shown in the figure, that excludes the air and conflnes the arc to the small space between the terminals.
SWITCHBOARD APPLIANCES
When the arc tends to follow the discharge, the small space is soon filled with a metal- lic vapor that will not support combustion. It should be noted that this arrester is intended for use on direct-current cir- cuits only, where the pres- sure does not exceed 600 or 700 volts.
56. General Elec- tric Arrester. — In the General Electric ar- resters for direct cur- rent, the arc is blown out by making it occur in a mag:netic field provided by an electromagnet. Fig. 51 shows a direct-current arrester with the cover removed; the case and cover are made of porcelain. The
Pio. w
part (6) holds the blow-out coil c with its polar projections A, /i;
54
SWITCHBOARDS AND
§25
r is a graphite resistance for limiting: the current. The electrodes are mounted in the cover and are held by clips ky k*\ the air gap a is about .025 inch in length. When the cover is in place, clips ^', k! make contact with the tongues ky k, and give the scheme of connections shown in Fig. 52. Here a represents the air gap, shown also at a, Fig. 51 (a), x y is the blow-out coil, r r' the graphite resistance. The ground connection is made to the lower end / of the resistance, and the line is connected to the upper electrode. The terminals of the blow-out coil
To Line
- resisfonu»
Pig. 62
connect to z and /, so that the coil is in parallel with a por- tion of the resistance. When a discharge comes in over the line, it jumps the air gap and passes to the ground through the resistance, and when the current follows the discharge, part of it passes through the blow-out coil. When the cover is placed in position, the air gap a falls between the pole pieces h^ h, and the arc is blown out through an open- ing in the cover. A portion of the resistance r' is in series with the coil and spark gap, and thus limits the amount of current that tends to follow the discharge. The ordinary type of this arrester is suitable for any direct-current circuit using pressures of 850 volts or less.
ARRESTERS FOR AI.TERNATING CURRENT
57. West Inpfh oil se Arrester for Alternating^ Cur- rent.— Fig. 53 shows a type of arrester that has been largely used by the Westinghouse Company on alternating-current
§25
SWITCHBOARD APPLIANCES
65
circuits. It is known as the Wurts non-arcing arrester, and consists of a number of milled cylinders a, a separated from each other by small air gaps. The end cylinders are con- nected to the lines and the middle cylinder to the ground. With this arrangement, a single arrester does for both sides of the line; where, however, the line pressure is high, a separate arrester is used for each side; and for very high pressures, such as are used on long-distance lines, a number of arresters are connected in series. When a discbarge comes in over the line, It jumps the gaps between the
cylinders and passes to the ground. It is claimed that the arc does not hold over, because the gases formed by the volatilization of the metal will not support an arc. The cylinders are made of what is known as non-arcing metal. Others claim that the suppression of the arc is due to the cooling effect of the cylinders and the alternating nature of the current. These arresters should be examined from time to time and the cylinders rotated slightly so that they will present fresh surfaces to each other.
56 SWITCHBOARDS AND §25
58. General Electric Arrester for Alternating Current. — Fig- 54 shows an arrester used by the General Electric Company for alternating-current circuits. It is somewhat similar to the Wurts arrester, except that fewer spark gaps are used and a non-inductive resistance r is inserted in the circuit in order to limit the current following the discharge. The spark gaps a, a are between the heavy metal cylinders b, b, b, the middle one of which is connected to ground in the double-pole arrester shown. This arrester, like the previous one, is not suitable for use on direct-current circuits.
The arresters just described have been shown as arranged for indoor use in the station. They may, how- ever, be used on the line, in which case they should be mounted in a weather-proof box made of iron or wood. The connections to and from the arresters should be made with wire not less than No. 4 B, & S.
59. Westlnghouse Ar r e ster for Hlffh-Tenslon LlncB. — When lightning arresters are used on high- tension lines, they usually consist of a number of air gaps connected in series between the line and the ground, the total length of air gap being so proportioned that the normal voltage of the system, even if one line becomes grounded, will not cause a current to jump across the gaps; the gaps are generally used in connection with a resistance that will prevent a rush of current after a discharge, A choke coil is also used to choke back the electrostatic wave passing along the line, and make it take the path to ground. Fig. 55 shows one of the air-gap units used with Westinghouse high-tension lightning arresters. It consists of seven knurled cylinders a, a, sepa- rated by six a'^-inch air gaps, and made of non-arcing metal.
§25
SWITCHBOARD APPLIANCES
57
The cylinders are arranged so that they can be revolved in the porcelain holders b, b in case the parts facingf each other should be burned by the discharge.
Fig. 56 shows the connections of a Westing^lioase lovr- equivalcnt arrester as arranged for a 6,000-volt circuit. The line to be protected is connected at point A. Two sets of gaps B and C are connected in series and to the ground through a series-resistance iff ^ The gaps C are shunted by a resistance R and are known as shunted gaps; gaps B are called series-gaps. When the potential at A rises to an abnormal amount due to a lightning discharge or other cause, a discharge leaps across the series-gaps B, If the discharge is heavy, it will meet with a large amount of opposition in the resist- ance Ry and will pass over gaps C and resistance R' to ground. The current that tends to follow the discharge and that is maintained by the dynamo will take the path
Pio.55
lb Un€
7 T~ t
Cioui
Pio.56
through R instead of passing across gaps C, so that the effect of the shunted resistance is to withdraw the arc from gaps C and at the same time cut down the volume of current so that the series-gaps can suppress the arc. By using this arrangement a smaller number of gaps at B is needed than
68 SWITCHBOARDS AND §25
would otherwise be necessary. The series-resistance R' is used to limit the initial flow of current and prevent burning of the cylinders B.
Fig. 57 shows the arrangement of one of these arresters with its choke coil. The spark gaps are at a, a, while the
Fro. 57
resistances are mounted in suitable holders *, b. The arrester shown in Fig. 57 is for 8,500 volts. For arresters of higher voltage than this, the series-resistance is not mounted on the same panel with the other parts, but is placed separately on suitable columns that provide thorough insulation.
60. In the selection of lightning arresters the following points should be kept in mind:
1. "The width and number of spark gaps should not be so great as to require the potential of the lightning charge to
§25 SWITCHBOARD APPLIANCES 59
be as high or higher than the potential necessary to rupture the insulation of the system.
2. On account of its nature, a lightning arrester is evidently exposed to severe potential strains; consequently, all live parts must be well insulated. On arresters for low voltages it is not a difficult matter to secure proper insulation, as the construction of the arrester itself affords protection. On high-tension arresters, however, proper insulation is a more difficult matter.
3. The general design and construction of the arresters, together with the necessary adjuncts, should be such as to with- stand very heavy lightning discharges without destruction.
4. As current is apt to follow the slightest discharge, it is necessary that the arrester should be designed to break the arc quickly without permitting an excessive flow of current.
5. Line terminals should not be exposed in arresters in such a manner as to permit of the accumulation of dust, dirt, bugs, cobwebs, etc., which may facilitate the formation of short circuits and resulting arcs across terminals.
6. Arresters should be designed to handle heavy dis- charges of atmospheric electricity without permitting the same to follow the circuit and puncture the insulation of the station apparatus.
♦
61. The importance of adequate protection becomes greater with the increased extension of the system, for the reason that the larger systems encounter different atmos- pheric conditions by extending over greater areas, and the possibility of trouble increases, also the amount of possible damage resulting from breakdowns. Thunder storms that may occtu: miles distant might be unknown at the station except for the snapping of the arresters or some sudden discharge.
The object should be to select the best method of pro- tecting the system and then to apply a sufficient number of lightning arresters judiciously located in suitable positions on the system to prevent absolutely any disruptive discharges from entering the station and damaging the apparatus.
60 SWITCHBOARDS AND §25
Special sets of arresters should be connected immediately outside of the station. On account of the extreme sudden- ness of the surges caused in the line by lightning discharges and other static disturbances, the gaps of the arrester, and ground connection also, must be able to discharge electricity very freely, in fact more rapidly than it appears on the line; otherwise, a dangerous rise of potential on the line will not be prevented.
INSTALLATION OF ARRESTERS
62. Before arresters are installed, the characteristics of the surrounding territory should be carefully studied, and if possible, statistics obtained regarding the frequency and severity of atmospheric electrical disturbances. The informa- tion obtained may be somewhat of a guide as to the amount of protection necessary.
63. liocation of Arresters. — As regards the location of lightning arresters, electric systems may be divided into two groups:
1. Systems in which the individual pieces of apparatus, such as transformers, motors, arc lights, etc., are many in number and widely scattered. In these cases lightning arresters should be located at a number of points for the purpose of protecting the whole line; they should be more numerous on the parts of the line particularly exposed, and fewer in number on the parts that are natturally protected, especially those parts shielded by tall buildings or numerous trees. Special efforts should be made to protect the station by connecting sets of arresters on each line and causing the discharge to pass to ground before it enters the station. No definite statement can be made as to the number of arresters needed per mile, as the requirements will vary widely according to atmospheric disturbances in the locality.
2. Systems in which the apparatus is located at a few definite points, as on a high-tension transmission line. In such cases the arresters should, in general, be located to protect especially those points where apparatus is situated;
§25 SWITCHBOARD APPLIANCES 61
that is, should be placed with the object of protecting the apparatus rather than the line as a whole. Where circuits are part underground and part overhead, sets of arresters should be connected at the points of entrance to and exit from the underground system.
When determining the safest method of mounting and insulating the arresters, it should be estimated that all parts of the arrester except the grounded end of the series-resist- ance may be momentarily at line potential during the dis- charge; therefore, the necessity of extra insulation becomes self-evident.
Two high-tension arresters attached to different line wires should not be placed side by side without either a barrier or a considerable space between them. It is prefer- able to place them on different poles.
PROTECTION BY CONTINUOUS DISCHARGE
64. For overhead systems, excellent protection has been secured by the placing of barbed wires on the pole lines above the lines used for distribution; the barbed points serve to collect the electricity, and the barbed wires should be thoroughly grounded, at least as frequently as every three or four poles. An easy method of doing this is to put a copper plate under the base of the pole, having the ground- wire connection soldered on the plate and stapled along the surface from the base of the pole to the top, where it is con- nected to the barbed wire. The effect of this sort of protec- tion is to discharge the atmospheric electricity silently and continuously, and this method under severe test has proved successful over large areas, with systems reaching from 30 to 50 miles or more from the station.
Fig. 68 shows the principle of the Westinghouse tank arrester, a type that has been much used on street-railway circuits where one side of the system is grounded. The arrester is connected to the series of choke coils S by closing plug switches A^, /f, /f. The arrester consists of tanks T, Ty T containing carbon electrodes ^ , ^, c\ the line is attached at L
62
SWITCHBOARDS AND
§25
and the other end of the choke coil goes to the dynamo or line bus-bar. A circulation of running water is maintained through the tanks and there is thus a continuous non-induc- tive path of high resistance to ground for any charges that may accumulate on the line. The water has such a high
ToHkfdtMft
Pig. 58
I ToDiVt/l,
resistance that the leakage of dynamo current to ground is not large. There is some leakage, however, and this type of arrester is only connected to the system during thunder storms, but while connected it affords very efficient protection.
PROTECTION FROM STATIC CHARGES
65. static Effects on High-Tenalon Systems. — It
has been found on systems where high pressure is used that under certain circumstances, parts of the system may be subjected to pressures very much higher than the normal. These effects, for want of a better name, are spoken of as being due to **static.'* They may be caused by any sudden change in the E. M. F. of the system, as, for example, when a dead circuit is suddenly connected to live bus-bars, when a transformer is switched on to a circuit, when a circuit is suddenly cut off from the bus-bars, etc. These effects are not due so much to the static charges themselves, but to the fact that when a device is switched on to a live circuit,* a current wave at once tends to pass through the device, and if this wave meets with opposition, pressures much higher than the ordinary pressure of the system may be set up. This is somewhat analogous to the case where a current of
§25 SWITCHBOARD APPLIANCES 63
water is flowing rapidly through a pipe. There will be a certain pressure on the walls of the pipe due to the head of water, and this pressure will be practically constant. If, however, the flow of water be stopped by suddenly closing a valve in the pipe, the pressure will for an instant rise to a very high amount, producing the well-known water-hammer effect. These sudden rises in pressure on high-tension cir- cuits may result in puncturing the insulation of transformer coils, armature coils, cable insulation, or other parts exposed to the high pressure. Take the case where a transformer is suddenly connected to a source of high E. M. F. The wind- ings tend to become charged instantly, but owing to the self-induction of the coil the current wave that tends to enter is choked back and a pressure may be set up between the various layers of the winding that is very much higher than the normal, thus tending to cause a breakdown. To overcome these bad effects, a choke coil may be inserted in series with the device to be protected. This coil chokes back or flattens out the wave, and allows the pressure applied to the device to rise gradually. The choke coil must be heavily insulated, and large enough to flatten out the wave so that the latter will not injuriously affect the device to be protected. This means that the coil must be large, and it is difficult to insert a large choke coil in the circuit without causing a considerable waste of energy and drop in voltage. Another method of protection is to use a choke coil in combination with a spark gap that will break down whenever the pressure rises above a predetermined amount. This arrangement is practically the same as a lightning arrester, and a number of large plants have their lines fully equipped with lightning arresters even though the distributing lines are entirely underground and hence safe from lightning discharges. The lightning arresters are in such cases installed to protect the cables against abnormal pressures caused by the so-called static effects.
&&• Static Interrupter. — In some cases, especially on high-tension lines operating at pressures higher than 16,000
64 SWITCHBOARDS AND §25
or 18,000 volts, a device known as a static Interrupter is installed to protect large transformers and other apparatus from the high pressures mentioned above. Fig. 69 shows the essential parts of the device as made by the Westing- house Company; one line only is shown in the figure, but it is necessary, of course, to place one of the interrupters in each line. -^ is a choke coil and B the primary coil of
tt a transformer or the OOOOObO] winding of other appa-
Lme
0000000
* ratus to be protected;
C is a condenser con- nected between A and B\ the other terminal of C is connected to ground through a ^ ^^^'"^ fuse D. If the primary
Fio. 69 coil D were suddenly
switched on to a live line without the interposition of A or C, a very high potential would at once be developed at point E^ because the current wave could not penetrate the layers of the winding instantly. The coil A retards the wave, and further- more the condenser C having a large capacity compared with the coil By takes up a considerable portion of the charge, thus reducing the potential of E for the time being and allowing the charge to progress well through the coil before the pressure at E rises to the full amount. In other words, the condenser C acts in much the same manner as an air chamber used on a water pipe to prevent the shock due to a water hammer. By using the condenser in conjunction with the choke coil, a much smaller coil is sufficient than if the coil were used alone, and it can thus be designed so that it will not insert an objectionable amount of resistance or inductance in the circuit. In practice, the coil A and con- denser C are mounted together in a case filled with oil, so that the interrupter has about the same appearance as an ordinary oil-insulated transformer. The interrupters are connected directly to the apparatus to be protected so as to practically form part of the apparatus, because they must be so situatecj
§25 SWITCHBOARD APPLIANCES 65
that they will come between the device to be protected and the source of static disturbance, as, for example, a hig^h- tension switch.
Overhead systems will naturally be equipped with light- ning arresters and these will serve to a considerable extent as protection against static discharges. Underground systems carrying current at high potential are liable to accumulation of static charges that may cause a rupture of the cable insu- lation. Assuming that alternating current of high potential is transmitted through an underground system, it will be found that there is a static charge developed in the cable covering or, under some conditions, in the conduit ducts. Certain types of conduit have been found to develop con- denser capacity under these conditions. A 6-foot section of 3-inch, creosoted, pump-log conduit was tested for capacity with an insulated wire drawn through it and connected in circuit with a high-potential current. In the darkness, a faint blue light could be distinguished on the interior surface of the duct. When circuits are quickly opened, the cable tends to set up violent oscillations of the system, and the resultant static potential is liable to rupture, at its weakest point, the insulation of the cable. Static charges are also liable to accumulate on generators and switchboard appara- tus. Electrostatic ground detectors should be used to show the appearance of any static charge on the line, and on which particular conductor it may be located.
FIEIiD RHEOSTATS
67. Field rheostats are inserted in the field circuits of the generators in order that the voltage may be adjusted by varying the field strength. The rheostat must therefore be able to carry the field current continuously without over- heating. The resistance of the rheostat will depend on the resistance of the field winding with which it is used, and the range of voltage variation desired. Very often the rheostat has a maximum resistance about equal to that of the field, though in many cases it is not necessary to have as much a$
66 SWITCHBOARDS AND §26
this. Field rheostats are made in a great variety of styles and sizes suited to various classes of machines. They are also constructed for various methods' of mounting, but all consist of a suitable resistance connected to a multipoint switch of some kind so that the amount of resistance in the field circuit can be varied. Small or medium-sized rheostats are generally mounted on the rear of the switchboard and operated from the front by a hand wheel. For large rheostats the resistance can be separate from the board with leads
running to the switch located on the back of the board, or the switch can be mounted with the resistance and be operated from the switchboard by means of chain and sprocket wheels, or from a pedestal, with a hand wheel, placed in front of the board. Either of the latter methods are preferable to run- ning leads from the resistance to the board, because quite a number of wires are required and there is danger of some becoming broken. In very large stations, the rheostats are
§25
SWITCHBOARD APPLIANCES
67
often bulky and must be placed quite a distance from the switchboard; in such cases the rheostat switch is moved by means of a small motor controlled from the switchboard.
68. Fig. 60 shows a General Electric field rlieostat of a type much used for 5(X)-volt railway switchboards. The rheostat is mounted on the back of the board and operated by the hand wheel IV in front. The resistance wire or strip is wound on asbestos tubes that are afterwards flattened and clamped between pieces of sheet iron covered with asbestos, the iron strips serving to conduct the heat from the wire. In rheostats of large capacity, the resistance is in the form of cast grids. Fig. 61 shows the connections for the
rheostat, Fig. 60. A .^in^/to/ a>s^>^^^^
small resistance c is connected to the contact rings df b and contacts a, a'. When the arm is in a position where a, a' are on adjacent con- tact points, resistance c, which is equal in amount to the resistance be- tween the rheostat con- tacts, is in parallel with the resistance between the contacts. Thus, by using resistance r, the change in resistance due to a move- ment of the arm from contact to contact is one-half what it would be if no auxiliary resistance were used. The varia- tions in field strength are, therefore, as gradual as in an ordinary rheostat using twice the number of contacts.
69. Field Switches. — Field switches are used to open the field circuits of dynamos and they are, therefore, of com- paratively small current-carrying capacity. Field windings, particularly those of large alternators or high-voltage, direct- current machines, have a high inductance, and if the circuit is suddenly opened very high E. M. F.'s may be induced,
Pig. 61
45— 2(j
68
SWITCHBOARDS AND
§25
/"/e/cf J^//C/f.
I
t
Lamp
sufficient in many cases to break down the field insolation. It is therefore necessary, with such machines, to arrange the field switch so that when the field circuit is broken, a path is at the same time established through a 4ischarge resist- ance. This allows the induced E. M. F. to set up a current through the local circuit thus provided, and strain on the windings is avoided. Fig. 62 shows a common arrangement of field switch and discharge resistance as used for 500-volt street-railway generators. The tongue / is wide enough to
bridge over the gap between the con- tact segments a, a' of the switch 5*, which is shown in the position that it occupies when the generator is in operation. The current then passes through the field rheostat r and the switch S, as indicated by the arrow- heads. When the switch is moved to the position indicated by the dotted line, connection between the field and the negative side of the. armature is broken, but before the break takes place, tongue / comes into contact with a', so that the shunt field, the rheostat r, discharge resistance r', and pilot lamp / all form a closed cir- cuit. The shunt field is thus able to discharge through this closed circuit. When the machine is being started, the tongue / is placed in its mid- position, so that current can flow through r' and / as well as through the shunt field and rheostat r. As the machine builds up, the pilot lamp becomes brighter, thus giving the attendant an indication as to whether the machine is '^picking up*' properly or not. After the machine has come up to voltage, the switch is moved to the position shown in the figure and the pilot lamp is cut out. On some boards, five or six lamps in series are used in place of the resistance r' and the single lamp /. Another type of field
Atmontn
Pio. 02
§25 SWITCHBOARD APPLIANCES 69
switch with Se Id-discharge resistance, as used in the exciting circuit of alternators, is shown in Pig. 73.
70. Recording Wattmeters. — Well-equipped switch- boards are generally provided with one or more recording vfattmeters, to record the output, in kilowatt-hours, of each machine or of the station as a whole. Readings of the total output are very valuable in making tests on the effi- ciency of the station and in keeping track of the cost per
kilowatt-hour. Sometimes it may be desirable to know the output of individual machines, but usually a knowledge of the total output is sufficient and a single total output record- ing meter is installed, as shown at 11, Fig. 65.
Fig. 63 shows a Tliomisnii reeordliiK wattmeter for use on direct-current switchboards. These meters have to carry large currents, hence their construction differs some- what from the ordinary Thomson meter, though the principle
^
70 SWITCHBOARDS AND §25
of •operation is the same. The series-coils of the ordinary meter are here replaced by the heavy copper bar a, through which the current passes, connection being made on the back of the board to the lugs b, b. Above and below this bar are the two small armatures r, c, which are con- nected, in series with a resistance, across the line, so that the current in them is proportional to the voltage. Cur- rent is led into the armatures through a small silver commutator d^ as in the ordinary recording meter, and the reacjing is registered on a dial e in the usual way. The damping magnets used to control the speed are con- tained in the case /. The main current flowing through the crosspiece a sets up a field around the crosspiece, and this field acts on the two armatures r, c. This instrument is constructed so that outside magnetic fields have little or no influence on it. In some of the older styles of meters, the magnetic field surrounding the heavy conductors on the back of the board affected the meter. In this meter any stray field affects both the armatures f, ^, which are so connected that an outside field tends to turn them in opposite direc- tions, and the disturbing effect is thus neutralized. The field set up by the instrument itself is in opposite directions on the upper and lower sides of a, so that these two fields propel the armatures in the same direction. For alternating- current boards, total-output recording meters of the induction type are used.
§25 SWITCHBOARD APPLIANCES 71
SWITCHBOARDS
71. The s^vltcliboard is a necessary part of every plant. Its object is to group together at some convenient and accessible point the apparatus for controlling and distributing the current, and the safety devices for properly protecting the lines and machines. Scarcely any two switchboards are alike in every particular; their layout and the type of apparatus used on them depend on the character of the system used, the number and size of dynamos, the number of circuits supplied, etc.
72. General Construction. — Switchboards were for-
•
merly made of wood and consisted simply of a built-up board or wall sufficiently large to accommodate the instruments. This construction was objectionable on account of the fire risk, and the only type of wooden board now allowed by the Fire Underwriters consists of a skeleton frame of well- seasoned hardwood filled and varnished to prevent absorption of moisture. A skeleton board of this kind is cheap and is suitable for those places whefe the expense of a slate or marble board is not warranted. Modem boards are nearly always made of slate, marble, soapstone, or brick tile. Slate is usually satisfactory for low-tension work, but it should be avoided on high-tension boards, because it is liable to contain metallic veins. A good quality of marble is the material generally used for modem boards. The slabs making the boards may vary from i inch to 2 or 2i inches in thickness, depending on their size. Most central-station slate or marble boards are made 2 inches thick with a bevel arotmd the edge of i or f inch. They are supported by bolting to angle irons /, /, Fig. 64, and are stood out from the wall by means' of braces 6^6, Station boards built up as shown in Fig. 64 are usually about 90 inches high. It has become customary to build up boards in panels, each panel carrying
72 SWITCHBOARDS AND §25
the apparatus necessary for a generator or one or more feeders. Those carrying the instruments for the generators . are known as generator panels; those carrying the instru- ments for the feeders, as feeder panels. This system allows the board to be easily extended as the plant grows in size, as panels can be added to those already in use. The extra panels are attached as indicated by the dotted lines in
Fig. 64, the panels being held together by means of bolts passing through holes h in the angle irons. For high-pressure boards using over 3,000 volts.'the marble should be polished on both sides in order to secure better insulation. Also, if oil switches are mounted on the back of the board, the mar- ble should be coated with varnish or similar substance to prevent absorption of oil.
§25
SWITCHBOARD APPLIANCES
73
DIRECT- CURRENT SWITCHBOARDS
73. Railway Sivltcli board. — Fig. 65 shows a typical direct-ciirrent switchboard as arranged for street-railway operation on the ordinary 500- volt rail-return system. The board consists of three generator panels Ay A, Ay one total-
output panel By and five feeder panels C, C, etc. One of the generator panels is left blank to provide for a future gener- ator. Each generator panel is equipped with + and — main switches i, i, voltmeter plug 2, field switch 5, pilot-lamp
74 SWITCHBOARDS AND §25
receptacle ^, field rheostat (operated by handle 5), machine ammeter 6, and machine circuit-breaker 7. The total-output panel carries a voltmeter 9 that can be connected to either machine by means of the voltmeter plug, a total-output ammeter 10 that indicates the combined current output of the generators; recording wattmeter 11 records the total output in kilowatt-hours. Each feeder panel is equipped with a single-pole feeder switch 12y a feeder ammeter 13, and a feeder circuit-breaker 14, Since on a ground-return railway system the current returns through the rails, which are connected to the negative bus-bar, the feeders are connected to the positive bus-bar only, hence single-pole feeder switches are used.
Fig. 66 shows the connections for the board. Two feeder panels only are shown and the instruments and switches are numbered to correspond with Fig. 65. If lightning-arrester reactance coils are used on the switchboard, they will be inserted as indicated on the left-hand feeder panel. The equalizer switches are mounted on pedestals near the gener- ators and the equalizer connections are not brought to the switchboard. When the voltmeter plug is inserted in either receptacle, terminals a and f, 6 and d are connected, thus placing the voltmeter across either machine; the voltmeter connections are made at the lower terminals of the main switch, or **back'* of the switch, so that voltmeter readings can be taken before a machine is thrown in parallel by closing the switch.
74. Lil^litins or Power Switcliboard. — Fig. 67 shows connections for a simple two-wire board suitable for two generators and three two-wire feeders. Three bus-bars are provided, the equalizer bar being mounted on the board. Each generator panel has a machine ammeter a connected across ammeter shunt 5, circuit-breaker by voltmeter plug r, main switches d, field rheostat e, and pilot lamps ky A. As this board is intended for low pressure, 110 to 250 volts, field switches and field-discharge resistances are not pro- vided. A total-output ammeter Af is connected between the
§25 SWITCHBOARD APPLIANCES
Pia.17
76 SWITCHBOARDS AND §25
generator and feeder panels to indicate the combined current output of the generators; voltmeter V indicates the voltage of either machine. Each feeder panel is equipped with a feeder circuit-breaker ^ and feeder switch /. The lamps k^ I may be connected either across the bus-bars, as shown for /, or to the feeders, as at k. In the latter case the lamp will go out when the circuit-breaker of the corresponding feeder trips, and the lamp thus serves as a circuit-breaker telltale. If a lamp ground detector were used on the board, it would be connected as shown by the dotted outline at D.
In large stations there are, of course, a large number of generator and feeder panels on the switchboard. This increases the size of the board, but each generator or feeder added merely repeats the connections of the other panels and no new features are involved.
AliTERNATING-CURRENT SWITCHBOARDS
75. The arrangement of ordinary alternatingr-current
boards is, in many respects, similar to that of direct-current boards. They are usually built up in panels in the same way as the boards previously described. Owing to the fact that alternators are generally separately excited, the switchboard contains some extra apparatus connected with the exciter that is not found on direct-current boards. The wiring and connections will also depend on whether single-phase or polyphase alternators are used.
76. Siii|?le-Pliase Generator Panel. — Fig. 68 (a) and {b) gives front and rear views of a typical alternating-current panel for one single-phase generator. Such a board would be used where only one single-phase machine is operated on a single line, and represents about the simplest possible arrangement. This panel is equipped as follows: Main switch a, electrostatic ground detector ^, voltmeters, ammeter d^ voltmeter switch e, field switch /, generator rheostat gy exciter rheostat ^, main fuses ^, and potential transformer/. The main switch a is of the quick-break type and is provided with the marble banier / between the blades to prevent arcing
§25 SWITCHBOARD APPLIANCES 77
across. The switch / is used to disconnect the field of the alternator from the exciter and is provided with auxiliary car- bon contacts to prevent burning at the blades. The rheostat^ is mounted on the back of the board and is operated by a hand wheel in front. This rheostat is connected in series with the field of the alternator, so that the field current may
be adjusted. The rheostat h is in the shunt field of the exciter and serves to regulate the exciter voltage. Sometimes the rheostat g is not used, the field current of the alternator being increased or decreased by raising or lowering the exciter volt- age by means of the rheostat k. It is best, however, to have the rheostat f also, especially if two or more alternators are
78
SWITCHBOARDS AND
§25
excited by the same exciter, because it then allows the field current of each alternator to be adjusted independently of the others. The voltmeter c is connected to the machine through the potential transformer /, and a small voltmeter switch e
Affenrafor f/e/d
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Fig. 69
is somtimes placed in circuit so that the instrument may be cut out of circuit when not needed. The main fuses k are of the enclosed type. No synchronizing device is needed on this board, as it is intended for a single machine only.
§25 SWITCHBOARD APPLIANCES 79
77. The rear view of the board will give a good idea as to the way in which the wiring is arranged. Heavy rubber- covered wire should be used for this work, and especial care should be taken to see that everything is thoroughly insu- lated and neatly done. The leads from the alternator connect to terminals 1 and 2, and the line connects to ter- minals 3 and 4. The potential transformer t used to lower the pressure for the voltmeter, is mounted on an iron frame- work at the base of the board, and when the lightning arresters are placed on the board, they are usually mounted on a similar framework rather than on the back of the board itself. This makes them stand out so that they do not crowd the wiring on the back. Fig. 69 shows the general scheme of connections on a board similar to that shown in Fig. 68.
78. Switchboards for Parallel Running?. — When alternators are operated in parallel, it is necessary to pro- vide bus-bars and have the different machines arranged so that they may feed into them. Fig. 70 shows connections for two three-phase machines arranged for parallel running, as used by the Westinghouse Company. Main fuses are here provided between the alternator and main switch, and these may or may not be placed on the switchboard itself. The field excitation is carried out in the same way described in connection with Figs. 68 and 69, about the only difference being that field plugs r, c' are used instead of field switches. Three ammeters are provided for each generator, one in each leg of the three-phase system. In many cases, how- ever, two ammeters only are used, as shown on the feeder circuit. T and T^ are the potential transformers that furnish current to the voltmeters V, V^ and also to the synchronizing lamps /, /'. The voltmeter is also made to serve as a ground detector by using the plug switches R, R^ and ground keys k, k'. The synchronizing lamps are connected to the transformers by inserting plugs /,>>^
79. Usually when a number of alternators are oper- ated in parallel, it is advisable to have their exciters arranged so that they may be operated in parallel also. If
SWITCHBOARDS AND
one exciter breaks down, the others may then supply the alternator that would ordinarily be supplied by the disabled
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machine. Again, in large plants, it is quite customary to supply all the alternators with their field current from one
§26 SWITCHBOARD APPLIANCES 81
or two larg^e exciters that feed into a pair of exciter bus- bars, from which the several alternators are supplied.
80. General Arrangement of Hlgli - Pressure Siprltcliboards. — In low-pressure work, the switchboard consists of a group of slate or marble panels on which the switches, bus-bars, instruments, and all devices necessary* for the control of the station output are placed. Such crowd- ing of the parts is dangerous on a high-pressure board, and the tendency in large stations is to separate the high-pressure switches and bus-bars so that a short circuit on one part will not spread to others and result in a serious interruption of the service. The switchboard panels in this case carry only the instruments and small switches necessary for controlling the main switches that are usually operated either by compressed air, electric motors, or electromagnets. No parts carrying high pressure are exposed on the surface of the board, thus insuring safety to the attendant; a switchboard arranged on this plan occupies a large amount of space. Fig. 71 shows a cross-section of the switchboard in the Waterside station of the New York Edison Company. This board controls the output of 16 generators, each having a capacity of 4,500 kilo- watts at 6,600 volts. The board is a good example of a number that have been installed in modern stations deliver- ing a large output at high pressure, and brings out the method of separating the various parts. The main cables from the generator first pass through the generator oil switch A, and from there they lead to the two selector oil switches B. The object of these switches is to allow the generator to be con- nected to either of the sets of bus-bars C, D, There are, therefore, two oil switches in series between any generator and the bus-bars into which it is feeding, so that if one switch fails to operate at any time, the generator can be cut off by means of the other. From the bus-bars, the current passes to a non-automatic oil switch ^, and then through an automatic oil switch /% from whence it passes out on the feeder G, R'^ and F' are a similar pair of switches for another feeder. //, //' are knife-blade switches that allow
§25 SWITCHBOARD APPLIANCES 83
any feeder to be connected to either pair of bus-bars. These switches are never opened while the current is on; other knife-blade switches K^ K' allow switches B to be discon- nected from the bus-bars. The potential transformers used for supplying: current to the voltmeter, wattmeters, or other instruments are shown at Z, and the current transformers are shown at M, It will be noted that all the transformers, bus-bars, knife switches, and working parts of the oil switches are separated from each other by brick partitions, and the various parts are so widely separated that there is little danger of fire communicating from one to the other.
The instruments connected with the control of the feeders are mounted in the upper gallery at Ny there being a panel for each feeder. On these panels are mounted the feeder ammeters, indicating wattmeter, power factor indicator, pilot switches for controlling the feeder oil switches, and all other devices connected with the control and measurement of the outgoing current.
81. The apparatus for the control of each generator is mounted on a pedestal at 6>, there being a pedestal for each generator. This pedestal has mounted on it the rheostat dial switch for adjusting the field excitation of the alternator, the resistance controlled by this switch being mounted at P in the gallery below. In addition to this, each pedestal is provided with a field switch for cutting off the exciting current, a switch for controlling the engine speed when synchronizing, synchronizing plug, and pilot switches for controlling the main generator switches A and the selector switches B, The ammeters, voltmeters, and other instru- ments connected with the generators are mounted at ^ on a small panel immediately above the generator pedestal. By mounting the generator controlling apparatus on separate pedestals instead of side by side on panels, the connections are kept separated to better advantage, and the devices are also separated, so that there is less danger of throwing the wrong switches.
The current for exciting the fields of the generators is
46—27
84 SWITCHBOARDS AND §25
supplied from motor-generator sets 5, each consisting: of an alternating-current motor coupled to a direct-current gen- erator. The apparatus for starting and controlling each of these sets is mounted on a pedestal T^ and the instruments connected therewith are mounted on panels u directly above the pedestal. F is a low-pressure, direct-current switchboard from which the exciter current is supplied.
From the above it will be seen that a high-pressure switch- board for a large station involves a wide variety of apparatus and occupies a large amount of space. The switchboard used in the large station of the Manhattan Elevated Railway, New York, is similar in its general design and handles current at 11,000 volts. In this station the operating board is equipped with small strips of brass that represent the main bus-bars, and the handles of the switches are so arranged that when moved, they apparently close or open the diagrammatic circuit on the controlling board. Signal lamps are also arranged to show whether a switch is on or off, the whole object being to arrange the controlling board so that the attendant will see just what connections exist between generators and bus-bars, and also what the result will be if certain switches are oper- ated. The object in arranging the controlling board in this diagrammatic fashion is to lessen the danger of confusion when connections have to be rapidly changed — a feature of special importance where large generating units are involved.
82. Fig. 72 shows a switchboard installation for a high- tension station of comparatively small output. This view shows the arrangement of one of the feeder panels. The lever /, for operating the feeder switch, is placed on the panel p that rests on the floor of the lower switch- board gallery. The levers operate the oil switches A^ A by means of the rods and bell-crank levers, shown in the figure. One of these rods b is of wood, so that the operating handle is effectually insulated from the switch. The bus-bars B are provided in duplicate and consist of copper rods well insu- lated with oiled tape. They pass through hard-rubber insulators that are supported by fiber pieces attached to the
§25
SWITCHBOARD APPLIANCES
85
angle-iron framework. Each feeder is provided with a cur- rent transformer /, none of the indicating instruments being connected directly to the high-tension lines. Each feeder is also provided with high-tension enclosed fuses C,
83. Fig. 73 shows the general scheme of connections for two of the generators and one of the feeders. This layout may be taken as an example where the generator supplies current at high pressure to the lines without the intervention
Pig. 72
of step-Up transformers. Each generator is provided with an ammeter Z>, supplied from a current transformer /, and a voltmeter supplied from a potential transformer /,. A second ammeter C is also connected in the field exciting circuit, so that the field current may be read at all times. The current transformer supplies the current coils of the indicating wattmeter A and the recording wattmeter E, A indicates the watts delivered by the alternator, and E
86 SWITCHBOARDS AND §25
records the watt-hours or kilowatt-hours. The indicating wattmeter indicates the load on each machine, so that the attendant can see at a glance whether or not each machine is taking its share of the load and can adjust the governor on the engine or waterwheel accordingly. The switch^ is for connecting the alternator field to the exciter bus-bars, and it is provided with two long clips between which a resistance h is connected, so that when the switch is opened this resist- ance is connected across the field terminals, thus taking up the discharge from the field and avoiding the danger of puncturing the field insulation. The construction of this switch is indicated in the small detail sketch (a). The long clips are formed so that when the switch is completely closed, the blades connect the lower and upper clips, but do not make contact with the middle clips. The synchronizing plugs are shown at ^, e\ and /, / are the synchronizing lamps. Each feeder running out from the station is provided with an oil switch, fuses, and two feeder ammeters. Sometimes three ammeters are used on the outgoing lines, as an ammeter on each line is often of service in indicating the condition of the line and also in showing whether the load is balanced or not In some cases the fuses, are replaced by automatic circuit- breakers, while in others the switch is provided with an automatic tripping device, so that the switch will open the circuit in case there is an overload or short circuit on the line. Current transformers K are connected in the bus-bars between the alternators and the feeders in order to supply total output ammeters.
84. Example of Double-Current Generator Instal- lation.— Fig. 74 shows a simplified diagram of connections for two double-current generators feeding into a three-wire, direct-current system for supplying near-by points and fur- nishing alternating current, through step-up transformers, to high-tension feeders running to outlying points. All auxiliary apparatus, such as ammeters, voltmeters, etc., is omitted in order to bring out the main connections more prominently. The method of operation shown in Fig. 74 is used by the
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88 SWITCHBOARDS AND §25
Chicago Edison Company. Two double-current generators are direct driven from a single steam engine and direct cur- rent at about 125 volts is supplied from the commutators and three-phase alternating current at from 75 to 80 volts from the collector rings 1, 2, 3. The commutators are connected in series and are attached to the neutral bus-bar. The shunt fields of the generators are arranged for excitation from the direct-current bus-bars, and the + and — brushes of the pair of generators are connected to the + and — bus-bars of the three-wire system. In order to permit independent control of the alternating voltage, potential regulators are inserted, as shown. These regulators are of the induction type described later in connection with the use of rotary con- verters. After passing through a low-tension switch, the alternating current is led to the primaries of three step-up transformers A,B,C that raise the pressure from 80 volts to 4,500 volts. Each transformer is provided with two primary coils that are connected to two corresponding phases of the generators, as indicated by the numbers on the terminals of the primary coils. Each primary is provided with low-tension fuses. The two secondaries of each transformer are con- nected in parallel, and the three groups are A connected to the high-tension bus-bars. The alternating-current sides of the two double-current generators are therefore connected in parallel through the step-up transformers and feed into com- mon high-tension bus-bars from which alternating current at high pressure is supplied to feeders running to distant centers of distribution. It is thus seen that by using double- current machines, a variety of service can be supplied from .a single generating outfit and the generators kept loaded to best advantage.
85. The foregoing will give the student a general idea as to the arrangement of switchboards and the apparatus used in connection with them. The variety of apparatus used in switchboard work is so great that it is impossible to treat all types. Many stations have now become so large that it has been found necessary to make the switchboard
§25 SWITCHBOARD APPLIANCES " 89
proper simply a place for grouping the small auxiliary devices needed to operate the main devices. It is now common to find field rheostats, field switches, main switches, etc. operated electrically or pneumatically from a distant point, and this method of operation has naturally introduced a large number of new switchboard appliances. Generally speaking, the tendency is to carry on this remote control by means of electricity rather than compressed air, as the electric current has proved just as reliable and is easier to apply. In some cases small electric motors are used for operating switches, rheostats, or other devices, especially where a rotary motion is required. In other cases a solenoid or electromagnet is simpler and more easily applied.
POWER TRANSFORMATION AND MEASUREMENT
TRANSFORMERS AND TRANSFORMER CONNECTIONS
1. Transformers vary somewhat as to their construc- tion, but al] have the three essential parts, i. e., the primary and secondary coils or £7oups of coils and the iron core that
serves to carry the magnetic flux through the coils. Their construction also depends to some extent on whether they are to be used outdoors or indoors. Fig. 1 shows a typical transformer for outdoor use mounted on a pole in the
For Jiolia ol eotyixM. m fage immtdialtly loJIotniiii Ihe lilU fax*
126
2 POWER TRANSFORMATION §26
usual manner. Where transformers are large, say above 25 or 30 kilowatts capacity, it is not advisable to mount them on poles if it is possible to avoid it.
2. Primary Fuses. — Transformers are operated on constant-potential circuits almost exclusively; hence, if a short circuit occurs on either primary or secondary, there will be a heavy rush of current, which will do damage unless the transformer is instantly disconnected from the circuit. This is accomplished by inserting fuses in the primary between the transformer and the line. The fuses also protect the trans- former against over- loads. Fuses should be placed in each side of the primary, as indi- cated at b, b. Fig. 1, and should be so mounted as to be easily replaced by the lineman. Primary fuse blocks are made so that the fuse holder may be entirely discon- pj^ nected from the primary
mains when the fuse is being renewed; in other words, the fuse block serves the purpose of a switch as well as a fuse holder. In some cases the blocks are double-pole, but when the primary pressure is high, it is better to use two single-pole fuse blocks. Double- pole blocks are not recommended for transformers of greater capacity than 2,o00 watts.
Fig, 2 (n) shows a General Electric double-pole primary switch and fuse block, with one fuse holder {b) removed for replacing a fuse. The fuse lies in a deep slot e in the porce- lain holder (h), and is fastened to the clips d,d. When the holder is in place, the clips engage with the terminals /, /, thus completing the connection to the transformer primary.
826 AND MEASUREMENT 3
When a fuse is to be renewed, the porcelain base is pulled out and the liuetnan can replace the fuse without danger.
Fig. 3 shows a single-pole block made by the Stanley Company. In this case, the lid of the iron box is placed at the bottom and the fuse holder A is pulled out, thus breaking connection with the terminals /, /. The fuse ^ runs through
a block of wood A, thus confining the arc and preventing it from arcing and burning the terminals /, /.
Where large transformers are operated in substations, automatic switches or circuit-breakers are used instead of fuses to disconnect the transformer from the line in case of a short circuit or overload.
POWER TRANSFORMATION
§26
Fig. 4
TRANSFORMERS ON SINGIiE-PHASE CIRCUITS
3. Transformers In Parallel. — Transformers may be connected in parallel so as to feed a sing^le circuit, as shown in Fig. 4, but care must be taken when. making the connec- tions. Suppose that the two transformers are of the same type, so that they will both be wound alike. The primary ter- minals Px and Pn must be connected to one of the mains, and P^ and A to the other main; the secondary terminals a and c will then have the same polarity at the same instant, which is the result desired. It will be noticed that, from the way in which the secondaries are connected, they oppose each other, and that little or no current will flow until the outside circuit is connected. In practice, it will be found that a current will flow between the trans- formers, but it will not be large. Suppose, how- ever, that the secondary terminals are connected as shown in Fig. 5; the coils are now in series so that the E. M. F.'s act together to set up a cur- rent through the coils, thus resulting in a short circuit. In connect- ing up the secondaries, before making the final connections it is always well to make sure that the proper secondary terminals are being connected together. This can be found out by connecting two of them
Fio. 6
26
AND MEASUREMENT
together and then connecting^ the other two through a piece of small fuse wire or fine copper wire. If the fuse blows, it shows that the connections should be reversed. It is often more convenient to reverse the primary terminals than the secondary, especially if the latter have been joined up per- manently. Reversing the primary has, of course, the same effect as reversing the secondary, and it is usually easier to carry but, because the primary connections are lighter and easier to handle.
4. Generally speaking, it is not advisable to operate several transformers in parallel, or banked^ as it is some- times termed. This is especially true if the transformers are
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small and scattered, as on many lighting systems, although it was occasionally done some years ago, when transformers were not made in large sizes. Suppose that a number of transformers are operating in parallel, as shown in Fig. 6. If they do not all have the same voltage regulation, the load may divide unequally between them and one or more of them take more than its share. The result is that the fuses of the heavily loaded transformer blow, and a heavier load is thrown on the remaining transformers, thus blowing their fuses. Of course, if the transformers are all of the same size and of similar design, such trouble is not very likely to happen; but it is better, if possible, to have each trans- former supply its own share of the load, and if more capacity
6
POWER TRANSFORMATION
§26
IS needed, to use one large transformer rather than a number of small ones.
5. Transformers are very often wound with their. pri- maries and secondaries in two sections, so that they can be connected in series for high voltage and in parallel for low voltage. For example, in Fig. 7 the transformer is wound with two primary coils /*, Px, each designed for 1,000 volts and two secondary coils each wound for 50 volts. By con- necting the coils Py Px in series, the transformer may be operated on 2,000-volt mains, and if the secondaries are also connected in series, it will supply current to 100-volt second- ary mains. If the two primaries P^ P^ are connected in
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parallel, as shown in Fig. 8, they may be operated on 1,000- volt mains, and if the secondaries are connected in series, they will supply current at 100 volts. If desired, the secondaries could be connected in parallel to supply cur- rent at 50 volts, but the 50-volt secondary circuit has practically gone out of use. A pressure of 50 volts was, at one time, used for incandescent lamps operated from trans- formers, but has given place to 100 to 110 volts, because the latter pressure requires less copper and it is now possible to obtain 100- to 110- volt lamps that operate fully as satis- factory as those made for 50 volts. Transformers are now
§26
AND MEASUREMENT
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frequently wound so that they can be connected for either 104 or 208 volts on the secondary.
6« In many places, plants that were origfinally installed to operate at 1,000 volts primary pressure have been chang^ed to 2,000 volts, in order to allow a larger load to be carried without increasingf the size of the line wires. In such cases it has been common practice to connect old 1,000-volt trans- formers in pairs, as shown in Fig. 9.
7. Transformers on the Three-Wire System. — ^The general tendency is to use a few large transformers for supplying a given district rather than a number of small ones. Small trans- ......i.i.^.ii_....^— ^^— —
formers are wasteful 2000 \ibff f^/mcrry kia/ns.
of power, and though ' each in itself may not represent a very large loss, yet when a large number are connected the total amount of energy that might be saved during a year by using a few large transformers may be surprisingly large. Of course, in most cases where the customers are scattered it is impossible to avoid using a number of small transformers, but in business districts it is generally easy to use a few large transformers of high efficiency. These are frequently connected in pairs so as to feed into three-wire secondary mains m, m, m, as shown in Fig. 10. The primaries are connected directly across the line in parallel, and the second- aries are connected in series with the neutral wire connected between them at the point o. Care must be taken in con- necting the secondaries to see that the terminals a, b are of opposite sign. If they are correctly connected, a pair of lamps /» / connected in series across the outside lines
Pio.9
8
POWER TRANSFORMATION
§26
should bum at full brigfhtness. If they are wrongly con- nected, the lamps will not light at all, showing that terminals Uj b are of the same polarity and that f , d are also the same, the secondaries being connected so that the two outside mains are of the same polarity with a common return wire in the middle. If two transformers are of the same style and make, the terminals of corresponding polarity will usually be brought out of the case in the same way. For example, in Fig. 4, terminals a^ c would be of the same polarity at the same instant. It is always best, however, to test out the connections before connecting permanently, and this is
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especially necessary in case two transformers of difEerent make or type are being dealt with.
8. Core-Type Transformers on Three- Wire System.
When ordinary transformers of the core type are used to supply current to a three-wire secondary system, as shown in Fig. 11 {a)y the voltage on the two sides of the circuit may become greatly unbalanced if the load is not equally divided. For example, in Fig. 11 (a) take the extreme case where the side a is not loaded at all. Secondary coil s will have no current and will therefore set up no counter mag- netization, whereas coil ^ will have a current due to the load on side b. Thus the magnetic flux in the two sides of the
§26 AND MEASUREMENT 9
core becomes unequal, as roughly indicated by the dotted lines, and the secondary E. M. P. is considerably higher on the side a than on the loaded side b. In order to overcome this difficulty, the General Electric Company wind the sec- ondary in a number of sections i,i,j,j, Fig, 11 (b), and cross-connect these coils as indicated. The result is that no matter how unbalanced the load may be, the magnetizing effect of the secondary is the same on both cores and the voltage remains practically the same on both sides.
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TRANSFOBMEBS ON TWO-PHASE C1HCUIT8
9. As most two-phase circuits are operated with four wires, such a system is practically equivalent to two single- phase circuits. If it is necessary to connect two trans- formers in parallel, as shown at (a). Fig. 12, their primaries must be connected to the same phase. If they are connected to different phases, as indicated by the dotted lines running to phase 1, a local current will flow through the secondary coils, because the secondary currents will not be in phase and there will be intervals when the E. M. F. of one will be greater than that of the other. The secondaries may, however, be connected in series, as shown at {b), forming a
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§26 AND MEASUREMENT 11
kind of three-wire system. If the voltage of each secondary is E^ the voltage between the two outside wires will be J5*X 1.414. This is because the E. M. F.'s in the two coils are not m phase. This method of connecting transformers, however, is not to be recommended, as the voltages on the two sides of the three-wire system are apt to become unbalanced. If a three-wire system is desired, it is better to use the connections shown at (^), where both primaries are connected to the same phase. The E. M. F.'s in the two secondary coils are, in this case, in phase with each other and the pressure across the outside wires is twice that of one secondary coil.
10. In connecting transformers to a two-phase system, the aim should be to get the load on the two phases as nearly balanced as possible. Of course, where motors are operated, both phases are used, and, hence, there is not much danger of an unequal division of load. When lamps are connected, one transformer or set of transformers at one point on the circuit can usually be balanced against another group at some other point, so that the load as a whole will be equally divided. Fig. 13 shows different methods of connecting transformers on a two-phase system, using three line wires. In this case the central wire acts as a common return, and the voltage between the outside wires is 1.414 times that of each phase. The same remarks apply here as in the previous case, and the three-wire arrangement shown at {h) is not as generally satisfactory as that shown at (r). In both cases the primary pressure is shown as 2,000 volts, and transformers with a ratio of 20 to 1 are taken for the sake of illustration.
TRANSFORMERS ON TOREE-PHASE CIRCUITS
11. Until recently it has been customary in America to use three single-phase transformers for transforming from one pressure to another on three-phase circuits; the three transformers may be connected up either Y or A. With the A arrangement, the power supply will not be entirely
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§26 AND MEASUREMENT 13
crippled even if one of the transformers should become damaged; also transformers wound for standard line volt- ages can be used. In some cases, however, the primaries are connected across the lines according to the Y scheme, as shown at {a)^ Fig. 14, and since there are two primary coils in series between any pair of mains, the pressure on any one primary coil is less than that between the mains. When the primaries are Y connected, the secondaries are usually Y con- nected also, as shown at (a). Sometimes, however, the primaries are Y connected and the secondaries A, as shown at {d). If transformers having a ratio of 20 to 1 were con- nected in this way, the secondary pressure would not be the
100 primary pressure divided by 20, i. e., 100 volts; but t~«^, or
57.7 volts. In order to get 100 volts secondary with this scheme of connections, the transformers would have to be
wound with a ratio of ^ -^o to 1, i. e., 11.55 to 1, approxi- mately. Fig. 14 (c) shows transformers with both primaries and secondaries A connected. The arrangements shown at {a) and {c) are the ones commonly used for three-phase work, as scheme (b) either calls for special windings on the transformers or else gives rise to odd secondary volt- ages. If the primaries are to be A connected, each primary coil must be wound for the full-line voltage. If the pri- maries are Y connected, each primary coil is wound for the line pressure divided by 1.732. It is possible to use only two transformers on- a three-phase system, as shown in Fig. 14 (d) , but this arrangement is not, on the whole, as desir- able as the A connections, because if one breaks down the service is crippled. It is equivalent to the delta arrange- ment with one side left out. The connections shown in (c) are used more largely than any of the others.
12. Phase-Chan^ ngr Transformers. — By combining two E. M. F.'s that differ in phase by 90°, an E. M. F. of any desired amount and phase relation to the original E. M. F.'s can be obtained. For example, in Fig. 15 (a), suppose it is desired to produce an E. M. F. ^ of the amount represented
14
POWER TRANSFORMATION
§26
by the line oc and having: the phase relation of oc. This £. M. F. can be reg^arded as made up of the two com- ponents oh and^A at right angles to each other; hence, if two E. M. F.*s Ex and E^^ having the values represented by the lines oh and oa^ and differing in phase by 90°, are com- bined, the result will be the required E. M. F. E, In Fig. 15(*) , A and B are the primaries of two transformers connected to a two-phase system. The E. M. F.'s Ex and E^ induced in their secondaries will therefore differ in phase by 90° and Ex and E^ can be made any desired value by suitably
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loojooj
PiO. 16
proportioning the windings. If the two secondaries are con- nected in series, the E. M. F. between the lines will be the geometric sum of Ex and i?„ as shown in (a). For example, in (^) , in passing from line 1 to line 2 we go through each coil in the same direction; that is, we pass from aX.o b and from c to din the direction indicated by the arrows. We will call this the positive direction. In {d), in passing from a\.o h we go through the coil ab\n the positive direction, but, with the connections of the second coil reversed, as shown, we pass through cd from d to c against the arrow. The line oa (c) is therefore reversed with regard to its position in (a) and
§26
AND MEASUREMENT
IS
the E. M. F. E between lines 1 and 2 is now denoted by the line oc, which is the same in amount as in (a)y but has a different phase relation. Fig. 15, therefore, shows a method of obtaining a single phase current of any desired amount or phase relation, from two currents differing in phase by 90^.
13. Scott Two-Phase, Tliree-Phase Transformer.
One of the most common examples of phase transformation is the changing of two-phase currents to three-phase, or vice versa, by means of the arrangement devised by Mr. C. F. Scott. In Fig. 16 {a)y A and B are the primary coils of two transformers connected to a two-phase system. The second- ary oiAyi. e., the coil acy is provided with a winding such
A
\smmmj
B UQOQQQQQ,
Z^vfiwm
W
■8'(nrd55Bo&s««ed
Fio. 16
that its voltage E will be the required voltage of the three-
phase system. The secondary of B has -^ or .87 times as
many turns as the coil a r, so that the voltage generated in it is .87 E. One end of coil de is connected to the middle point b of coil a c, as shown. With this arrangement of wind- ings and connections, three currents differing in phase by 120° will be delivered to lines i, 2, 3 when the primaries are supplied with two currents differing in phase by 90°. The same connections are shown in a simplified form in (b)^ the three-phase lines being attached to points 1, 2, and 3, The E. M. F. between 1 and 2 is that generated in the secondary
16 POWER TRANSFORMATION §26
ac. The E. M. F. between 2 and S is the E. M. F. generated \VL he combined with the E. M. F. generated in be. The E. M. F. between 3 and 1 is that in ^^ combined with that in ba. It must be remembered that the E. M. F. in be is at right angles to the E. M. F/s in a ^ and be. Coming back to (tf) and noting that the positive direction through the coils is marked by the arrows we can lay off the line oe in {e) to represent the E. M. F. between lines 1 and 2. The E. M. F. between points a and b is marked a — ^ in {e) and is represented by one-half of oe. Also, the E. M. F. between b and e would be represented by ^ — r. The — sign does not here signify subtraction; it simply denotes that the E. M. F. referred to is taken between the points b and r. The E. M. F. between lines 2 and 8 is found by adding, geometrically, the E. M. F. af — ^ to ^ — ^. In passing from line 2 to 5 we pass from ^ to ^ against the arrow, or in other words the E. M. F. ^ — ^ is the equal and opposite of ^ — ^ and is represented by <? / in (r ) equal to one-half of oe^ but drawn to the left of o. Coil de is passed through in the positive direction so that the E. M. F. ^/ — ^ will be repre- sented by the line oh above the horizontal, and the E. M. F. between lines 2 and 3 will be the resultant of ^/ and oh^ or oi. The E. M. F. between lines 3 and 1 is e — d combined with b — a. The E. M. F. between e and d is the equal and opposite of that between d and e\ hence, it is represented by ok, which is equal and opposite to oh. The E. M.'F. b — a is equal to and in the same direction as r — ^; hence, it is represented by o /, and the resultant of ^ / and ^ ^ is og, which is the pressure between lines 3 and 1, The three secondary- line pressures represented by the lines oe, of, and og, are therefore of equal amount and differ from one another in phase by 120°, as is required for a three-phase system.
For long transmission lines, it is more economical to use the three-phase than the two-phase system; hence, where power is generated by two-phase alternators and stepped up for transmission over long distances, as, for example, at Niagara, the current is often transformed from two-phase to three-phase as just explained.
§26
AND MEASUREMENT
17
14« Capacity of Transformers on Two- and Three- Phase Systems. — ^When transformers are connected on a two-phase system each transformer must be of capacity suffi- cient to carry half the load. If the three-pha^e system using three transformers is used, each transformer must be capable of carrying one-third the load. When the transformers are used to operate induction motors, a safe plan to follow is to install 1 kilowatt of transformer capacity for every horsepower delivered by the motor. Thus, a 20-horsepower, two-phase, induction motor will require two 10-kilowatt transformers; a 30-horsepower, three-phase motor will require three 10- kilowatt transformers; and so on. Table I, issued by the General Electric Company, shows the size and number of transformers suitable for 60-cycle, three-phase induction motors.
TABIiK I
CAPACITT OF TRANSFORMERS FOR
THREE-PHASE INDUCTION
MOTORS
|
Capacity |
of Transformers |
||
|
Horsepower |
I |
Cilo^ ers |
watts |
|
of Motor |
Two Transforms |
Three Transformers |
|
|
I |
.6 |
.6 |
|
|
2 |
1.5 |
I.O |
|
|
3 |
2.0 |
1.5 |
|
|
5 7i |
3.0 4.0 • |
2.0 3.0 |
|
|
10 |
5.0 |
4.0 |
|
|
15 |
7.5 |
5.0 |
|
|
20 |
10.0 |
7.5 |
|
|
30 |
150 |
lO.O |
|
|
so |
25.0 |
15.0 |
|
|
75 |
25.0 |
18 POWER TRANSFORMATION §26
SUBSTATION EQUIPMENT
15. General Features. — The high-tension alternating current, for large transmission systems, is usually delivered to a number of substations rather than to scattered groups of transformers, and it is therefore necessary to study the equipment of these substations. In some cases the power is delivered from the substation in the shape of alternating current; in others, it is transformed to direct current and delivered to the various receiving devices, such as lamps, motors, etc. Part of the output may be delivered as direct current and part as alternating, either at the same frequency as the current generated in the main station or at a different frequency. It is thus seen that the character of the equip- ment in a substation may vary greatly, and will depend on the character of the service. If the power is used for oper- ating a street railway where direct current at a pressure of 500 to 600 volts is required, the substation must be equipped with rotary converters for changing the alternating current to direct. Also, since the alternating current is transmitted at high pressure, it is necessary to provide transformers to step-down the incoming line voltage to an amoimt such that the converters will give the required direct-current voltage. The current can also be transformed from alter- nating to direct by using 'motor-generator sets, i. e., sets consisting of an alternating-current motor connected to one or more direct-current generators. Motor generators are more expensive than rotary converters of equal output, and are not quite so efficient; hence, the latter, especially in America, are much more generally used. For some classes of work, motor generators have advantages, and their operation on fairly high frequencies, over 60 cycles, is more satisfactory than that of rotary converters. They are used considerably on 60-cycle systems where the direct current is used for lighting work which requires close
§26 AND MEASUREMENT 19
voltage regulation. In a motor-generator set the two sides of the system are entirely separated, and disturbances on one side are not so liable to affect the other as with rotary converters. It is often practicable to wind the motor to take the high-tension line current without the inter- vention of step-down transformers, but even allowing for this the motor generator is not as economical, either as regards first cost or efficiency of operation,, as the rotary converter. By using frequencies from 40 to 25 cycles per second, little difficulty is found in operating rotary con- verters; and at theses frequencies they are largely used for the conversion of alternating current to direct current, or vice versa.
16. In some cases the output of a substation is delivered wholly as alternating current, and the substation contains simply the static transformers needed for raising or lower- ing the pressure, together with the switchboard appliances used to control the incoming and outgoing current. In sub- stations where the output is in direct current supplied to lighting or railway systems, it is common practice to provide a storage battery in order to equalize the load, the battery being charged during intervals of light load and discharged when the heavy load comes on. The use of a number of substations supplied from one large central station results in a comparatively constant load on the central station, especially when storage batteries are used in those substations that are situated in densely populated districts and are called on for a very heavy output at certain hours during the day. One of the chief advantages in supplying the power from a large central station is the uniformity of load obtained throughout the day, thus allowing the generating units to be worked at their best efficiency.
The equipment of a substation may be conveniently con- sidered under three heads, namely: (a) Apparatus for Con- trolling the Incoming Current; (d) Apparatus for Transform- ing the Current; (c) Apparatus for Controlling the Outgoing Current.
20 POWER TRANSFORMATION §26
APPARATUS FOR CONTROIiLING THE INCOMING
CURRENT
17. The apparatus for controlling the incoming current is generally grouped on a regular high-tension switchboard, and is separated, at least so far as the high-tension parts are concerned, from the devices controlling the outgoing current. If lightning arresters are used, they are placed at a point near where the wires enter the building; very often they are placed in a separate building. The arrangement of the con- trolling devices, of course, differs in different stations, but the incoming lines should first pass through a circuit-breaker or main switch so that all current may be cut off from the station. In many cases oil switches are used, and are so arranged that they may be either opened by hand or auto- matically whenever the current exceeds the allowable amount. Arranged in this way, the switches fulfil the requirements of both a circuit-breaker protecting the apparatus in case of overload, and a main switch that can be opened by hand when desired. Switches of the air-break type and those in which the arc is broken in a confined air space are also made to operate automatically in case of overload; all of these types are in common use for protecting the incoming lines.
18. Tlme-Lilnilt Relay. — In most substations, espe- cially in those where rotary converters are operated, it is not desirable to have the circuit opened every time there is a momentary overload, because it allows the converters to fall out of synchronism and it takes some time to get things imder way again. Besides, momentary overloads will not, as a rule, damage anything, while a long continued overload or short circuit will. For these reasons it is advisable to equip the circuit-breakers, or automatic switches, on the incoming lines with a tlme-llmlt relay, which controls the current in the tripping coils and will not allow the circuit to be opened until a certain interval of time has elapsed after the occurrence of the short circuit or overload. If the overload should pass off during this interval, the relay goes
AND MEASUREMENT
21
back automatically to its initial position, and the circuit is not opened. If, however, the overload should continue beyond the limit for which the relay is set, contact is made and the tripping coils energized, thus opening the circuit.
Time-limit relays have been made in a variety of forms. Fig. 17 shows one type intended for two-phase or three- phase circuits and used on a number of the Niagara lines. The coils a, a are connected to the secondaries of current transformers whose primaries are in series with the main lines. If the current in either phase exceeds the allowable amount, either one or both of the armatures i>, b are pulled down, thus releasing the clockwork mech- anism c. If the short circuit or overload is not removed within the time limit for which the relay is set, say 3 to 5 sec- onds, the clockwork makes a contact that allows current to flow through the tripping coil of the circuit- breaker and thus opens the circuit. If the overload or short '''°' "
circuit should disappear during the time limit, the armatures b, b rise, thus preventing the clockwork from making contact. By equipping the various circuit-breakers on a system with this attachment, it is possible to set them so that in case a short circuit or overload occurs on a certain section, the circuit-breaker nearest that section will go out before those nearer the station. In other words, the breakers near the station are set so as to hold on for a longer interval than the more distant ones, thus preventing a shut-down of the machinery due to some fault on a distant part of the system. The time that must elapse before the relay makes contact
22
POWER TRANSFORMATION
§26
can be adjusted by varying the angle made by the vanes </,
Fig. 17. Fig. 18 shows the connections for one type of high-
T9im€. tension circuit-breaker
operated by a time -limit relay. Current is supplied to the coils of the relay by the secondaries of the cur- rent transformers Ay A'. The incoming lines are attached to studs a, a of the circuit-breakers, and the main current crosses over to studs b^ b by way of the laminated contacts Cy c, which are forced up against the studs when the breaker is set. Each pair of contact studs a ^ is shimted by a long enclosed fuse mounted in holders so that it can be quickly re- placed by a new one in case it blows. When the breaker opens, thus with- drawing c from a and ^, the main current flows momen- tarily through the fuse and the circuit is, therefore, finally opened by the fuse, which is capable of taking care of the arc. If the cur- rent becomes excessive and holds on beyond the time limit for which the relay is set, contact d touches €, thus allowing
the cells / to send a current through the tripping coils of
the breaker.
Fig. 18
§26 AND MEASUREMENT 23
19. Westluffhouse TIme-JLimIt Heloy. — Fig. 19 shows a relay made by the Westinghouse Company, In this case the time-limit feature is regulated by means of a dashpot. A solenoid a is connected to the secondary of the current transformer, and the movable core i rests on a lever c pivoted at d. To the end of £ is attached the piston rod e, which carries the piston of the dashpot /. The lever c, counter- balanced by the weight ^ , is normally held in the position shown in the figure, by the weight of core 6 resting on it. The arm A, also pivoted at d, carries the contact springs k, i and its position can be adjusted, up or down, by an adjusting screw on the
cover of the instrument. Lever c carries a contact piece m that connects k, I if lever c rises far enough. When the cur- rent in a exceeds the allowable amount, core 6 is lifted, thus allow- ing the counterweight £- to raise lever c. The movement of c is con- trolled by the dashpot / and the time during which.the overload may exist before the circuit ^"^ '*
is opened is determined by the position of arm k. When lever c has moved high enough to make contact between k and /, the circuit-breaker is tripped and the main circuit opened. Should the overload pass off before the time limit is reached, 6 drops back and lever c is forced down before it has had time to make contact between k and /.
20. Reverse-Current Relay. — In a large distributing
system where a number of substations are connected to the main station, and to each other, by a network of cables, it is necessary to provide some means for preventing current
24
POWER TRANSFORMATION
§26
from flowing back toward a defective part and thereby main- taining a short circuit. This point will be understood more clearly by referring to Fig. 20, where A is the main station from which current is supplied to the substation B, Usually a number of cables in parallel are run between the main station and the substations in order to allow the use of cables of reasonable dimensions, and also to provide for uninter- rupted service in case one or more cables should break down. Suppose that c and d represent two three-wire cables, supply- ing the substation B with three-phase current. When both are in use, the ends at the substation and at the main station are connected to common bus-bars. Suppose that a short circuit occurs at / on cable c. The rush of current through
Pig. 20
the fault will, of course, open the circuit-breaker on cable c at the main station, but since d and c are connected together by the substation bus-bars, there is nothing to prevent a heavy current from flowing out over d and back through c to the fault /, thereby causing the circuit-breakers of cable d to open and completely shut off the power from the substation. In order to prevent this, reverse-current relays are installed at the end of the feeders, and their duty is to trip the circuit-breakers the instant the flow of energy through any of the cables reverses. Of course, where a substation is supplied by a single set of feeders and fur- nishes current to a secondary system which is not capable
AND MEASUREMENT
of feeding current back to the line, reverse cairent relays are not needed.
Fig, 21 shows an arrangement of reverse-current relays used on the Niagara system, and also in a number of other installations. A, A are the circuit-breakers, and B, B the reverse-current relays. These relays are similar in construction to small direct-current motors having laminated fields. The field windings are excited by current from the secondaries of two potential transformers i, t', and the armatures are sup- plied with current from the current transformers c, c'. The armatures are not allowed to turn, since their motion is limited by an arm playing between two slops as shown. When the current is flowing in its normal direction from the cables to the bus-bars, the arm of the relay bears against the lower stop, which is not connected electrically to any other part. If, however, the flow of energy is from the bus- bars to the cables, the flow of current at each instant
in the armature is reversed with respect to that in the fields. and the armature at once swings around in the opposite direction until the arm touches the upper stop, thus closing the battery circuit and tripping the circuit -breaker.
26 POWER TRANSFORMATION §26
This feeding-back action can also occur, if reverse-current circuit-breakers are not used, where a substation supplied through even a single set of feed-wires runs rotary con- verters which, on their direct-current side, are in parallel with storage batteries. If a short circuit occurs on the cable and it is cut off from the main generating station, the converters can still operate with direct -current fur- nished by the battery. They thus run inverted, taking the direct current from the batteries, converting it into alter- nating current, and feeding back to the line through the transformers. The current thus fed back to the fault in the cable will be very large, and may cause injury to the apparatus if means are not taken to prevent it by means of reverse-current circuit-breakers.
APPARATUS FOR TRANSFORMING THE CURRENT
21, If the ctirrent supplied from the substation to the consumers is utilized as alternating current, the substation is equipped with step-down transformers that supply alter- nating current directly to the secondary network. If the ciu-rent is utilized as direct current, it is necessary to install rotary converters or motor generators in addition to the step-down transformers.
22, Substation Transformers. — Transformers used in substations do not differ materially from ordinary trans- formers except as regards their size and the methods used to seciu"e cool running. They are usually of very large output as compared with those used for ordinary local lighting and power distribution. Their efficiency is very high, but on account of the comparatively small radiating surface that they present to the air, it is neces- sary to provide special means for getting rid of the heat, either by means of an air blast or by water that circulates through a coil of pipe placed in the upper part of the transformer case. With the latter method, the transformer case is filled with oil, and as the heated oil rises to the
§26 AND MEASUREMENT 27
upper part of the case it is there cooled by the water in the pipes, and descends to the lower part, thus keeping up a continuous oil circulation that carries the heat away from the coils and core.
Fig. 22 shows a Westinghouse 2,250-kilowatt substation transformer; (a) shows the coils and core assembled before being placed in the case. The core laminations a, a are built with openings ^, d at intervals so that the oil can circulate through the core and conduct the heat from the internal parts. The primary and secondary coils are each wound in several sections in the form of large flat coils, which are then sandwiched together, making a construc- tion that reduces magnetic leakage, and at the same time cuts down the voltage generated in any section of the winding. The ends of the coils project beyond the lamina- tions at the top and bottom as shown at c, and the terminals of the coils lead to a .terminal board mounted on top. The transformer is placed in a cylindrical tank made of riveted boiler plate. Fig. 22 (^), and is completely submerged in oil. Four coils of pipe placed in the upper part of the tank are connected in parallel by pipes a, a attached to common inlets and outlets. Each coil is provided with a valve, so that in case it becomes defective, it can readily be cut out without disturbing the flow of water through the others. This transformer, being of very large output, has an efficiency of 98.63 per cent, at full load, 98.2 per cent, at half load, 97.2 per cent, at quarter load, and 98.5 per cent, at one-half overload.
Fig. 23 shows a sectional view of an air-blast transformer of the General Electric type. The construction of the coils Ay A and core Z?, B is such that air spaces are left between the parts, and the transformer is mounted over an air chamber in which about i ounce air pressure is main- tained by motor-driven fans. The air passes through the openings in the core, between the coils, and out at the top and sides; suitable dampers are provided by means of which the flow can be regulated. This makes an efficient and cleanly method of cooling large transformers.
POWER TRANSFORMATION §26
§26 AND MEASUREMENT 29
Fig. 24 shows a group of nine air-blast transformers of 150 kilowatts each. A motor-driven fan is mounted at each end of the chamber and either fan has sufficient capacity to keep the transformers cool, thus providing a reserve blowing outfit in case one breaks down. The power required to operate the fans does not usually exceed one-tenth of 1 per cent, of the transformer output.
23. Polyphase Transformers. — In Europe, two-phase and three-phase transformers have been quite commonly used, and three-phase substation transformers are now manufactured in America. By using polyphase transformers, a saving in material is effected, thus reducing the cost per kilowatt. Also, a considerable saving in space is gained because a polyphase transformer, of given output, takes up less room than an equivalent output in single-phase transformers. This is i an important considera- tion in stations located in large cities. On the piu. Z3
other hand, the use of single-phase transformers is some- what safer, because if a breakdown occurs it is liable to damage but one of the transformers.
Fig. 2r5 shows the general arrangement of a three-phase core-type transformer. The primary and secondary coils, which are wound on the cores /I, B, C, may be connected Y or A. The magnetic flux in the core follows the same changes as the currents. E[tch cure acts alternately as the return path for the fiux in the other two cores, just as each
30 POWER TRANSFORMATION §26
line wire acts alternately as the common return for the other two in a three-phase line. The iron in the core is thus worked
to better advantag'e than when three separate single-phase transformers are employed. A two-phase transformer can be made by winding coils on cores A and C and leaving core B without coils; B will then act as the return path for the fluxes set up by the coils on A and C. Since these two fluxes will differ in phase by 90°, the re- sultant tlux in B will be ^'°^ VS times the flux in A
or C; hence, for a two-phase transformer, the central core B will have a cross-section \2 times that of ^ or CT instead of being equal as shown for the three-phase transformer.
AND MEASUREMENT
ROTARY CONVERTERS
24. The main features of rotary converters were described in connection with alternating-current apparatus. The types generally used are the two-phase or quarter-phase, three-phase, and six-phase; in America, the three-phase converter is used more largely than either of the others. Each converter is provided With its transformer or set of transformers in case it is necessary to step-down the line voltage. In some stations, notably in railway power plants, the alternating current is generated at low pressure when the
Higfi Tirtsiort 8u3-bora I [
I I II
greater part of the power is used near the station. In such plants, the near-by portions of the system are supplied with direct current from rotary converters placed in the main station and supplied with current directly from the alternators without the intervention of step-down transformers. If a very large percentage of the power was used as direct current for near-by points it would probably be cheaper to install double -current generators and dispense with the converters. In the majority of cases, however, where
32 POWER TRANSFORMATION §26
converters are used it is necessary to use transformers to supply a suitable voltage.
25. Connections for Six-Phase Rotary Converters.
It has been shown that the output of a rotary converter is increased by increasing the number of phases, and six- phase converters are used to a considerable extent, especially where the machines are of large output. Six phases are easily obtained from three by providing each of the three transformers with two secondary coils, as shown in Fig. 26. Coils i, d, and 5 are connected A, as also are 2, 4y and 6, one group being reversed as regards the other, thus giving the double-delta arrangement indicated in Fig. 27. The collector rings are attached to the points a, d, c^ etc., thus supplying the converter with six currents differing in phase
by 60°. The use of six phases introduces some additional complication in the connec- tions between the transformer secondaries and the converter, and also requires six collector rings, but this extra complication is more than offset by the increased output of the converters. Sometimes switches are inserted between the transformer secondaries and the converter, but more often the switching is done on the primary side because the secondary current is usually large and the switching devices correspondingly heavy.
26. Voltage Regulation of Rotary Converters.
Usually it is necessary to arrange converters so that their direct-current voltage can be increased with increase of load so as to keep the voltage constant at distant points on the system. It was pointed out in connection with the theory of rotary converters, that the voltage of the direct-current side could be raised or lowered within certain limits by changing the field excitation of the converter. The change in field excitation with increase in load is usually obtained by providing the machine with a compound field winding similar to that on a compound-wound, direct-current dynamo. If the load were not of a suddenly fluctuating character, the
§26
AND MEASUREMENT
33
necessary field regulation could be obtained by adjusting the rheostat in the shunt-field circuit, and a series-field winding would not be needed.
In order to admit of voltage regulation by varying the field strength of the converter, it is necessary to have a cer- tain amount of reactance on the alternating-current side; this can be provided by inserting reactance coils between the transformers and the collector rings, as shown in Fig. 28. Ay B, and C are the
T
?
T
/tf^Tem
T
T
Smrfeh.
step-down transform- ers, and Z^ is a lami- nated core on which the three reactance coils are wound.
Another method of regulating the volt- age of a converter is to provide the trans- former secondaries with a number of taps connected to a multi- point switch, thus allowing the number of secondary turns to be varied. This method does not ad- mit of as gradual a variation in voltage as some others, but it is simple and well adapted to cases where regulation is desired.
A third method of regulation is to insert a potential reg- ulator between the transformer secondaries and the collector rings. These regulators are made in a variety of forms, but they are nearly always some special type of transformer; the general features of this method of regulation will be under- stood by referring to Fig. 29. The secondary coils s, s, s of
To a c.
BuaBan
PiO.28
a considerable range in voltage
34
POWER TRANSFORMATION
§26
the regfulator are connected in series with the leads running between the transformers and the converter; the primaries P^PiP ai'e connected across the three phases as shown. Since the secondary coils are in series with the mains, it is evident that their E. M. F.'s will be added to or subtracted from those of the main transformers. If provision is made for varying the value of the E. M. F.'s generated in s^ Sy s, or for changing their phase relation with respect to the E. M. F.'s of the main transformers, the E. M. F.*s applied to the con- verter can be raised or lowered by an amount equal to the pressure generated in s. In some regulators, the eflEective
E. M. F. of the series-coils is varied by cutting turns in or out, as, for example, in the Stillwell regulator. Provision is also made for reversing the E. M. F. of the coil with respect to the circuit, so that the main E. M. F. can be raised or lowered. Another scheme is to arrange the magnetic circuit or the secondary coil so that by moving the coil or a portion of the core, the amount of magnetic flux passing through the secondary can be varied, thus changing the value of the induced E. M. F.
p
lOQOOOQQOQQd.
riflQQfiQQQQQOari
JMOQPOQOQPO.
Pig. 29
27. Fig. 30 (a) shows the general appearance of a three- phase induction potential regulator made by the General Electric Company and intended for regulating the voltage of a rotary converter. The stationary part of this regulator. Fig. 80 id), consists of a laminated structure a, a with inwardly projecting teeth exactly similar to the field of an induction motor. This is provided with distributed bar windings d, by which are connected in series with the mains
§26 AND MEASUREMENT 36
running to the converter. The primary consists of a lamina- ted core r, c similar to the armature core of an induction motor; this is mounted on a vertical shaft s so that the core can be turned throug^h a limited range by means of the hand wheel h, which operates a worm engaging with a segmental gear attached to s. The primary is provided with three wind- ings distributed in the slots, and connected across the phases as described in connection with Fig. 29. In this type of regu- lator, the field set up induces an E. M. F. of constant amount in each secondary winding. The adjustment of the amount of * 'boost'* is effected by varying the phase relation of the sec- ondary E. M. F. to that in the primary. For example, if the secondary induced E. M. F. and the primary E. M. F. are in phase, i. e., with the north and south poles of the primary and secondary windings facing each other, the maximum amount of increase in voltage will be obtained. With the secondary E. M. F. exactly opposite in phase to the primary, the E. M. F. will be lowered by an amount equal to the induced E. M. F. For intermediate positions of the primary, intermediate phase relations are obtained and the E. M. F. will be raised or lowered by an amount corresponding to the value of the com- ponent of the secondary E. M. F. that is in phase with or in opposition to the line E. M. F. With a regulator wound for four poles, a movement of 90° will give the total range of voltage, and as the movement is not large the current can be conducted into the primary by means of flexible cables. These regulators are also arranged for operation by means of a small motor, thus allowing them to be placed at some distance from the switchboard.
28. Methods of Starting Rotary Converters. — In
cases where direct current is available, rotary converters are usually started by driving them up to synchronism as direct-current motors. In many substations, storage batteries furnish a source of direct current that is available at all times for starting purposes. Of course, when one converter has been started it can be used as a source of direct current for starting others. In some cases, where a storage battery is
POWER TRANSFORMATION §26
• not available, direct
current is obtained from a small motor- generator set coil- ststins: of ao induc- tion nwtor coupled to a direct-curre dynamo. One advan- taE:e in starting from the direct-curren side is that the direct current furnished by the converter is al- ways of the same polarity, that is, the positive terminal, say, is always posi- Pi^ap tive; whereas, when
the converter is brougbt up to speed by allowing alter- nating current to flow through the arma- ture,the terminal may be positive at one time, and the next time the converter is started it may show a negative polarity.
When starting from the alternating -cur- rent side, the field is unexcited and when the current is first thrown on. the volt- meter connected to the direct-current Fm.)o side will show no
§26
AND MEASUREMENT
37
deflection because the E. M. F. between the direct-current terminals is then rapidly alternating, and, hence, will not effect a voltmeter of the Weston direct current or similar type except perhaps to cause a trembling of the needle. As the converter comes up to speed, the frequency on the direct-cur- rent side becomes slower and the voltmeter needle begins to vibrate, its rate of vibration becoming slower as the converter gets more nearly into synchronism. At exact synchronism, the E. M. F. on the direct-current side is steady; whence, the voltmeter reading becomes steady. The field should be excited just before synchronism is attained, and the polarity of the direct -current terminals will depend on which side of the zero the voltmeter pointer happens to be when the field is excited. If the ex- citing switch is closed with the pointer on the wrong side, the polarity will be Rufwan^ wrong.
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Another objection ^*^'^- to starting with alter- nating current is that when the current first flows through the armature it sets up an alternating flux through the field coils that may induce extremely high E. M. F.'s in them. Since the field coils are usually connected in series, the total E. M. F. generated may be so high as to endanger the insulation of the coils. When this method of starting is used, it is customary to install a special switch for disconnecting the field coils from each other while the converter is being started. Just before synchronism is attained, the coils are connected in the usual way and supplied with exciting current. It is not usually advisable to apply the full alternating-current voltage to the collector rings until the machine has come up
38 POWER TRANSFORMATION §26
to speed, because the full voltage will give rise to an objec- tionable rush of current. To cut down the voltage at start- ing, a starting compensator similar to that used in connection with induction motors is suitable, but a simpler arrangement is to bring out taps from the transformer 'secondaries and connect these to a double-throw switch so that in one position of the switch the converter receives half the secondary voltage, while in the running position the full voltage is applied. Pig. 31 shows this arrangement.
One considerable advantage in starting from the alternating- current side is that the converter does not have to be synchro- 1. nized; it is brought
I into synchronism by
V,^^ the alternating cur-
^IJ I ,.! [ rent. This is an im-
portant considera- tion when a machine ^must be started in -^a hurry. Starting from the alternating- current side does not give rise to undue 1 1 disturbances if the
II frequency of the
no.tt converter is fairly
low, say 25 cycles per second. On many switchboards connections are pro- vided so that the converters may be started with either direct or alternating current.
When the converter is started from the direct-current side, it is necessary to insert a resistance in the armature circuit. Fig. 32 shows a type of starting rheostat used for this pur- pose. On account of the unequal lengths of the switch clips, the three sections of the resistance are successively short- circuited as the switch is closed. As the converter starts up as an unloaded direct-current motor, it comes up to speed quite rapidly and a simple switch giving four or five resistance steps is suiScient.
§26
AND MEASUREMENT
39
Where direct current is not available, the converter may be started by means of a small induction motor having its armature mounted on an extension of the shaft. This method is used by the Westinghouse Company. It involves the use of a small auxiliary motor on each converter, and if the station contained many machines it might be cheaper and more satisfactory to install a small motor generator set and start from the direct-current side.
29.^ Synclironiziii^ Rotary Converters. — Rotary con- verters and synchronous motors are synchronized with the
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line E. M. F. in the same way as an alternator is synchro- nized with the bus-bar E. M. F. Lamps, voltmeters, or synchrono scopes may be used to indicate the point of syn- chronism. Fig. 33 shows a Lincoln synchronizer used to indicate when either of two rotary converters is in syn- chronism. In this case the converters are fed directly from low-pressure bus-bars and potential transformers are not needed in connection with the synchronizer. When the pressure is more than 400 or 500 volts, potential trans- formers should be used. Synchronizing lamps are also pro- vided, enough lamps being connected in series to stand
40 POWER TRANSFORMATION §26
the voltage. If converter No. 2 were to be synchro- nized, plugs would be inserted at a, ^, and e^ thus con- necting the upper terminals of the synchronizer to the bus- bars and the lower terminals to the corresponding phase of the converter. When the synchronizer is used on pres- sures somewhat above those for which it is made, it is necessary to insert resistances as shown at r^ and r,. In new installations, synchronoscopes are now used in pre- ference to lamps.
APPARATUS FOR CONTROLIjING THE OUTGOING
CURRENT
30. The apparatus for the control of the outgoing current is generally grouped on a switchboard by itself. In most cases the current is delivered at comparatively low pressure; hence, the devices used on the switchboard for the outgoing current differ materially from those on the incoming lines. Generally, the delivered current is used for electric lighting and power, or street-railway pur- poses, and the switchboard appliances used are the same as if the power were supplied from an ordinary station. Rotary converters are operated in parallel and connected up on the direct-current side in exactly the same way as direct-current machines. If they are compound wound an equalizing connection must be used.
LOCATION AND GENERAIj ARRANGEMENT OF
SUBSTATIONS
31. One of the greatest advantages of the distribution of power by means of substations is that the substations may be placed at or near the centers where the heaviest demand for current exists. They do not have to be located with reference to coal or water supply, and the price of real estate becomes a comparatively small item, because substa- tions have a very large output compared with the ground space they occupy. They can also be placed in locations
§26 AND MEASUREMENT 41
where a power plant would not be permitted on account of the smoke and dirt caused thereby. Substations can, for these reasons, be placed near the center of the load, and thus efiEect a great saving in the amount of copper required for feeders.
32. Fig. 34 shows the interior of a trpkal substation, one of the substations in Buffalo, N. Y., supplied with power from the Niagara power plant. All the machinery and controlling devices are here placed in one room, and a
single attendant is all that is needed. It is a fireproof building provided with a hand-operated overhead traveling crane for handling the machinery during installation, or in case repairs are necessary. The step-down transformers A, A are ranged along one side, and the three rotary con- verters B, B, B along the other. Each converter is of 400 kilowatts capacity and is supplied by a group of three 150-kilowatt transformers, the secondaries of which are con- nected to the converter; air-blast reactance coils, placed
42 POWER TRANSFORMATION §26
behind the transformers, are inserted between the trans- formers and the converter in order to permit voltag^e regnla- tion by variation in field strength. The converters are six- pole machines supplied with 25-cycle current, and run at a speed of 600 revolutions per minute.
The incoming current at 10,000 volts enters in the base- ment by means of a lead-covered cable and passes through the hand-operated oil switch C, which is provided for cutting off all power from the station in case of emergency or for any other reason. From C, the current passes through the high-tension circuit-breakers located on the switchboard D, and provided with time-limit relays. After passing through the circuit-breakers, the current goes to the high-tension bus-bars E and from there to the three high-tension oil switches F mounted in a brickwork casing. In the figure, one of the iron covers is removed showing the three cells of one switch. Each switch controls the current in the primaries of a group of three transformers supplying a rotary converter. The potential transformers for supplying current to the voltmeters and synchronizing lamps are. shown at jf , ^ on top of the oil switches. The switchboard for con- trolling both the incoming and outgoing currents is shown at H immediately below the gallery containing the high- tension switches and circuit-breakers. The portion of the switchboard that contains the instruments for the alternating current is at the right-hand end at A'; three panels are provided, one for each converter and group of three trans- formers. The switch handles for operating switches F are mounted on these panels and are thoroughly insulated, by insulating joints, from the switches themselves. The ammeters are supplied from current transformers, so that none of the appliances on the switchboard with which the operator might come in contact are exposed to the high pressure; all the high-pressure devices are confined to the upper gallery.
From the high-tension switches /% the current passes to the primary coils of the transformers and the induced cur- rent in the secondaries passes tQ the collector rings of the
§26
AND MEASUREMENT
43
6
44 POWER TRANSFORMATION §26
converters. The direct current passes to the panels I, 2, 3, each of which is provided with a direct-current ammeter and circuit-breaker in addition to the main switches. The out- going feeders are connected to the feeder panels 4, 5, ff, etc., each of which is provided with an ammeter, circuit-breaker, and main switch. Panel 9 carries an ammeter that measures the combined output of the converters, a voltmeter for measuring the direct-current voltage, and a recording watt- meter for registering the output of the substation. The voltmeter can be connected to any converter by means of plug connections on each converter panel. The subbase of each converter panel carries a single-pole switch for the field, and a double-pole transfer switch for connecting which- ever converter is to be started to the starting switch on the subbase of panel 9, Each converter is provided with an iron-clad magnet m mounted on the end of the bearing casing. A current is sent through this magnet at regular intervals, thus making the* shaft oscillate back and forth and keeping the brushes from wearing ridges in the com- mutator. Mechanical devices that have the advantage of not requiring any current for their operation have also been designed for maintaining an oscillation of the shaft.
Fig. 35 shows the arrangement of a typical substation for an electric railway. The arrangement of the trans- formers, rotary converters, etc. is clearly shown, so that further comment is unnecessary.
CONNECTIONS FOR SUBSTATIONS
33. The connections used for the various appliances in a substation vary considerably in different installations, so that it is impossible to give any scheme that is generally applicable. For example, those for a substation supplying a street-railway system will differ from those for one supplying current for lighting purposes. In order to give an illustra- tion of connections a few typical examples of substations for supplying direct current will be selected. In the first case the substation is to be supplied with current over one or
»
§26 AND MEASUREMENT 45
both of a duplicate set of hig:h-tension transmission lines. Two compound-wound rotary converters are used, which are to be arranged for parallel operation. The converters are to be started by means of direct current supplied by either one of the machines, it being assumed that one converter is always in operation. In case both were shut down for any cause, they could be started from the power station by starting up the alternator and bringing the converters and alternator up to speed together. Fig. 36 shows a scheme of connections that might be used for such a sub- station. It must be understood, however, that the connec- tions in individual cases might differ considerably from those shown, and yet give practically the same results. The differences would not lie so much in the main connec- tions as in those of the auxiliary parts, such as the various instruments, synchronizing devices, etc.
34, Path of Main Current. — The wiring, as a whole, can be divided into two sections; that between the con- verter 8 and the incoming lines 1, 2, and that between the converter and the outgoing feeders 20, 20. In the first section the current is alternating, while in the second it is direct. The main current enters on either one or both of the three-phase lines 7, 2, and passes to the high-tension bus-bars 3, 3. High-tension switches 1^ 2' are provided to cut off all current from the station. From the bus-bars 3, 3, the high-tension current passes to the converters through the switches 4, 4'. We will confine our attention from this point to one converter, as the connections of each are exactly alike. After reaching switch 4, the current passes through the high-tension fuses 5 to the primary coils of the step-down transformers 6, The switch 4 is frequently provided with an automatic tripping device that will open the circuit in case of overload, in which case the fuses 5 are not needed. In other cases a non-automatic switch is used at 4, and automatic circuit-breakers instead of fuses at 5\ the transformers 6 step-down the line voltage to an amount suitable for conversion. For example, in this case the
46 POWER TRANSFORMATION §26
converters will supply a voltage of about 550 for street-rail- way purposes, and the voltage supplied by the secondaries of 6 will, for a three-phase converter, be 550 X .612 = 337 volts, approximately. From 6 the low-pressure alternating current passes through the reactance coils 7, which are inserted to allow voltage regulation; in case potential regu- lators are used instead of reactance coils, they are inserted at this point. From 7, the current passes to the collector rings of the converter H and is transformed to direct current at 550 volts. The direct current passes through the main switches lU ^i' to the direct-current bus-bars 14. Since this substation supplies an ordinary street railway operating with an overhead trolley or third rail, the negative bus-bar is con- nected to the track and ground, while the positive connects to the outgoing feeders, which in turn are attached to the trolley wire'^or third rail, as the case may be.
35* Connections for Synchronizing:. — Each of the incoming lines is provided with a potential transformer i' or /", and each converter is also provided with a high- tension transformer, such as t"' connected between the switch and the transformer primaries. In series with the secondaries of each transformer is a synchronizing lamp /i, /„ etc. Suppose that current is being supplied over line 1 and that converter H is to be synchronized. The converter is started, switch 4 being open, by supplying it with direct current. It generates an alternating current that is stepped- up by transformers 6 and supplies the primary of f with an alternating E. M. F. By inserting plugs at a and c the secondaries at f and /"' are connected in series with each other and with lamps / and /,. If one plug c is cross-con- nected, as indicated by the dotted lines, the lamps will .be bright at synchronism. The synchronizing arrangement is essentially the same as that described in connection with the operation of alternators in parallel.
36. Voltmeter Connections. — In order to obtain a reading of the voltage on either incoming line, a voltmeter V is provided. By means of a voltmeter plug, connecting the
§26 AND MEASUREMENT 47
upper and lower terminals of either of the receptacles ^, /, the voltmeter can be made to indicate the voltage on either line. The voltage of the high-tension side of either converter can be measured by means of the voltmeter F', which is connected to the voltmeter receptacles g, h. The voltage of the direct-current side of the converters is indi- cated by the voltmeters O, (7 connected to the voltmeter receptacles p, f/. The voltage of a converter can thus be compared with the voltage of the line or direct-current bus-bars to which it is to be connected.
37. Ammeter Conuectloiis. — Each converter is pro- vided with an ammeter / connected to the secondary of a current transformer inserted between the switch 4 and the transformer primaries. In some cases an ammeter is inserted in each line wire, especially in large installations, though this is not absolutely necessary. In some cases, also, ammeters are placed on the incoming lines, series-trans- formers, of course, being used so as to thoroughly insulate the instruments from the high-tension line. The direct-current side of each converter is provided with an ammeter 21 con- nected across a shunt 12. Ammeter C indicates the total direct current, since its shunt is connected in series with the main bus-bar between the converters and the feeders. The feeders are provided with feeder ammeters i, V connected across the shunts 19 y 19^.
38. Circuit-Breakers. — In this case the incoming lines are not equipped with automatic circuit-breakers, though, if the substation formed part of a large network, circuit- breakers would likely be inserted at kk^y and these would be equipped with reverse-current and time-limit attachments. On the direct-current side each converter is provided with a circuit-breaker i.?, 13' connected between the converter and the direct-current bus-bars. Each feeder is also provided with a circuit-breaker, as indicated at 18y 18',
39. Equalizer Connection. — The positive brushes of the converters are connected by means of an equalizer cable in which the equalizer switch 15 is inserted. Note that
48 POWER TRANSFORMATION §26
the equalizer connects the two brushes to which the series- field winding^s are attached.
40. Shunt-Field Connections. — One end of the shunt field connects to the + brush, and the other to one terminal of the field rheostat R, The other rheostat terminal con- nects to the blade of the field switch m. When switch m is moved to the right, thus cutting the current off from the shunt field, the pilot lamp /«, resistance r, and rheostat R are connected across the field terminals, thus allowing the induced E. M. F., caused by the interruption of the field current, to discharge through this closed circuit. Switch m is in the position shown in the figure when the converter is in operation. Switch n allows the shunt field to be excited either from the direct-current bus-bars or from the con- verter itself. When it is partly closed, the blade makes contact with the long clip and the field is excited from the bus-bars; when fully closed, the field is connected across the brushes.
41, Method of Startln^^. — Suppose converter 8' is in operation supplying current to the direct-current bus-bars, and that 8 is to be started and thrown in parallel with 8', Switches 4, 11, 11', 15, n, and m are supposed to be open. Close the equalizer switch 15) place field switch m in the position shown in the figure, and close switch n until the blade makes contact with the long clip. The shunt field will then be excited by current from 8', because one end of the field is connected through R, nt, and n to the negative bus- bar, and the other end is connected to the positive side of 8' through the equalizer. Close switch 11, thus allowing cur- rent to flow through the series-coils 9, The field is now fully excited and the converter can be started as a direct- current motor by allowing current to flow through its arma- ture. This is done by throwing the switch s to the upper position and gradually closing the starting switch S, The speed of 8 can be adjusted by moving the field rheostat R, and when the point of synchronism is attained, as indicated by the synchronizing lamps, switch 4 is closed. After 5 has
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§26 AND MEASUREMENT 49
been closed and the resistance cut out, switch 11^ should be closed And switches S, s opened; also, n should be fully closed, thus connecting the shunt field across the terminals of the converter and allowing the field to remain excited even if switches 11, 11\ and 15 are open. The transfer switch s is provided so that the starting rheostat S can be connected to either converter.
This method of starting from the direct-current side is sometimes modified as follows: The converter is speeded up as before and the field rheostat is adjusted so that the machine runs somewhat above synchronism. Then switches 11, 11\ and n are opened, thus cutting oflE the direct current and opening the field circuit. The converter is then running above synchronism under its own momentum, but is genera- ting no E. M. F. Switch 4 is then closed and the converter is brought .into synchronism by the alternating current, and as it is already running at nearly synchronous speed the amount of current required is not nearly as great as if the converter were started from rest by allowing alternating current to flow through the armature. The field circuit is then closed, the direct-current voltage adjusted, and the con- verter thrown in parallel on the direct-current side in the usual manner. This method of starting is sometimes advantageous when the load on the direct-current bus-bars is of a very fluctuating nature. The variations in voltage may under such circumstances make it difficult to syn- chronize with the lamps in the ordinary way.
In case the converters are started by means of an auxiliary induction motor mounted on the shaft, switches S, s are omitted and the necessary connections for the starting motor are provided instead.
42, Fig. 37 shows connections for a substation contain- ing two rotary converters supplying current to a two-wire lighting or power system. The connections are, on the whole, very similar to those just described but differ from them in minor details. The switchboard is divided into two parts — the alternating-current board at the right and the
50 POWER TRANSFORMATION §26
direct-current board at the left. The alternating-current board consists of two panels, each of which is equipped with a main switch, which may be located some distance from the panel but yet be operated therefrom; a voltmeter, ammeter, power-factor indicator, overload relays, synchroni- zing lamps, synchronizing plug, and potential, and current transformers. Each direct-current panel is equipped with two single-pole main switches, field rheostat, machine ammeter, circuit-breaker, voltmeter plug, and starting switch for starting from the direct-current side. Each feeder panel, of which one is shown in the figure, is equipped with a double-pole feeder switch, feeder ammeter, circuit-breaker, and lightning arrester. In addition to the instruments on the generator and feeder panels, a total output ammeter and a total output recording wattmeter are connected between the converters and feeders so as to measure the combined output of the machines. Also, two voltmeters arfe provided — one to indicate the bus-bar voltage and the other to indi- cate the voltage of the direct-current side of either converter. These instruments, together with the total output meters, are often mounted on a panel by themselves.
It will be noted in Fig. 37 that the connections are such that the converters can be started from either side. Each machine is provided with a double-throw starting switch on the alternating-current side by means of which the converter is supplied with a reduced voltage at starting. The primaries of the transformers are provided with a number of taps to adapt them to different line voltages, and reactance coils are inserted between the secondaries and the collector rings. The main switch is provided with an automatic tripping attachment that is operated by the overload relays. The synchronizing connections are such that either the synchro- nizing lamps or voltmeter may be used. Each converter is equipped with a power-factor indicator, which shows whether the current taken from the bus-bars is lagging or leading. The operation of this type of power-factor indi- cator will be explained later after polyphase meters have been taken up.
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§26 AND MEASUREMENT 61
43, The method of starting from the direct-current side is briefly as follows: On the alternating-current side, the starting switch is thrown to the lower position and the main oil switch is open. The field break-up switch and the equal- izer switch at the machine are also closed. The break-up switch is used only when the converter is started from the alternating-current side. The + main switch, the circuit- breaker, and the single-pole starting switch are then closed, first making sure that the starring rheostat switch is open. Closing the + main switch and the equalizer switch places the series-coils in parallel with the series-coils of the converter that is already in operation and also connects one end of the shunt-field winding to the + side of the system. As soon as the starting rheostat switch is placed on the first point, cur- rent flows through the armature and shunt field. The con- verter then starts as a direct-current motor and comes up to speed as the starting rheostat switch is pushed in. After this switch has been fully closed, the — main switch is closed and the rheostat switch opened. The converter is now synchronized by varying the field strength, and when the lamps or voltmeter indicate synchronism the oil switch is closed.
When a converter is started from the alternating-current side, the switches on the direct-current side are open and the field break-up switch is also open. The double-throw start- ing switch is thrown to the upper position and the main oil switch closed. When the machine has attained speed, the starting switch is thrown over to the full-voltage position. The field is then excited and, after making sure that the polarity of the direct-current side is correct as indicated by the direct-current voltmeter, the converter is thrown in par- allel on the direct-current bus-bars.
44. Fig. 38 is a diagram of connections similar to Fig. 37 except that, since the direct current is delivered to a railway system, the arrangement of the apparatus on the direct- current side is different. The connections on the alternating- current side are shown for one converter only; they are the
52 POWER TRANSFORMATION §26
same as in Fig. 37. The negative bus-bar is placed near the machines instead of on the direct-current switchboard, and the negative main switch is placed alongside the equalizer switch, the converters being equalized on the negative side. The negative bus-bar is connected directly to the rail or return circuit, so that the direct-current panels are single-pole and the connections thereby simplified. The arrangement shown in Fig. 38 is used by the General Electric Company and the direct-current ammeters are of the Thomson astatic type, in which the magnetic field is supplied by electromag- nets excited from the bus-bars. Each ammeter has a pair of wires to supply the exciting current in addition to the usual pair connecting to the ammeter shunt. The series-field of each converter is provided with a shunt to regulate the amount of compounding; this shunt can be cut out.by means of the switch shown in the figure. This is necessary when starting from the alternating-current side; otherwise, the alternating E. M. F. induced in the series-coils would set up a large current through the shunt. These diagrams give a general idea of the connections iised for substations, but it must be remembered that they admit of considerable varia- tion and must be adapted to the requirements of each particu- lar case. It is not possible therefore to lay down any general scheme that is applicable to all cases.
§26 AND MEASUREMENT 53
MEASUREMENT OP POWER ON POLY-
PHASE CIRCUITS
INSTRUMENTS USED FOR POWER MEASUREMENT
45. Reference has already been made, in connection with alternating currents, to the measurement of power on alter- nating-current circuits. The measurements there described related to simple single-phase circuits; the influence of the power factor on the actual power delivered was pointed out, and the use of the wattmeter was explained. As the appli- cations of polyphase currents to power transmission have now been described, it will be advisable to consider the methods available for measuring the power supplied to two- phase and three-phase systems.
46. On account of the fact that the power factor of alternating-current circuits, either single-phase or polyphase, is seldom 100 per cent, or unity, power measurements are sel- dom made with ammeters and voltmeters as in direct-current work. The three ammeter and three voltmeter methods are inconvenient, liable to considerable error, and are never used if wattmeters are available. Good portable wattmeters are now obtainable at a price but little greater than that of ammeters or voltmeters. The wattmeter does not indicate the product of the volts and amperes, but the product, volts X amperes X cos <^, where cos <l> is the power factor.
In making practical power measurements we may wish to obtain simply a reading of the total watts supplied at any given time or we may wish to obtain the total work done, in watt-hours or kilowatt-hours, during a certain period of time. In the first case, indicating wattmeters would be used to make the measurements, while in the second it would be necessary to use recording wattmeters, or watt-hour meters, as they should more properly be called.
54 POWER TRANSFORMATION §26
INDICATING WATTMETERS
47, The indicating: vratt meters used for power meas- urement on polyphase circuits are in nowise different from those already described for use on sing^le-phase circuits. Many reliable makes of portable wattmeters are now avail- able and these are used for commercial measurements. The number of wattmeters required for a given test depends on the conditions under which the test is made.. In some cases one wattmeter is sufficient; in others, two are necessary, as will be shown. In connection with poljrphase measure- ments, it is well to bear in mind the fact that if the differ- ence in phase between the currents in the two coils of a Siemens type of wattmeter becomes more than 90°, the twisting action on the movable coil reverses, and, hence the deflection reverses. In ordinary single-phase circuits this condition does not arise, but it is possible in certain cases to have a greater phase difference than 90° on three-phase circuits, and the negative deflection referred to above must be taken into account.
RECORDING WATTMETERS
48. The Tliomson recording Tvattmeter has been described; it operates on either direct or alternating current and can be used for measurements on polyphase or single- phase circuits. Meters of the induction type, having no commutator, are simpler in construction than the commu- tator meter, and have rapidly come into favor. They, of course, have the disadvantage that they cannot be Used on direct current, whereas the Thomson meter can be used on either direct or alternating, a considerable advantage where a company supplies both kinds of current. Also, induction meters must be used on circuits having the fre- quency for which they are adjusted; if used on circuits of other frequency their indications will be incorrect.
49. Induction wattmeters are made in many different forms, but they all operate on about the same principle.
AND MEASUREMENT
55
They are essentially small induction motors desifned to operate with single-phase or polyphase current. Figs. 39 and 40 illustrate the operation of this class of recording meter, though it will he understood that it is possible to have a different arrangement of the parts and yet have the meter operate equally well. In Fig. 39. a is a coil of fine wire wound on the laminated iron core 6; c, c are coils of a few turns wound on a core d, which is entirely sepa- rate from b. An alumi- num disk e is mounted on the shaft / so that the outer part of the disk revolves past the ends of the cores on which the coils are wound. Fig. 40 shows a section of the coils and core taken along the line fg. Coils c, c are connected in series with each other and with the circuit so that all the current sup- plied passes through them. The potential coil a is connected across the circuit so that the current in it is propor- tional to the voltage; c and a therefore corre- m.. ».
spond to the current and potential coils of an ordinary watt- meter. The magnetism set up in core b will be proportional to the voltage, and that set up in core d will be proportional to the current. Coil a has a high inductance and an additional inductance is usually connected in series with it; in any event,
--i\-
56
POWER TRANSFORMATION
§26
the meter is so designed that the current in coil a will lag approximately 90° behind the E. M. F., thus making the magnetism in b lag 90° behind the E. M. F. The current in coils c, c is, of course, in phase with the current supplied to the circuit in which the meter is connected. The alter- nating magnetic field set up, say, by coil a induces eddy cur- rents in the disk, which spread out somewhat as indicated by the dotted lines o, Fig. 39 (b). These currents are reacted on by the field that emanates from the poles of core d and the disk is made to rotate.
In order that the meter shall give an accurate indication of the work done in the circuit, the driving torque on the disk must be proportional to EI cos <^, where cos <^ is the power factor of the circuit. Let us first consider the case where the
power factor is 1, i. e., where the line current and line E. M. F. are in phase. The current in a is at right angles to the line E. M. F. and the induced eddy cur- rents in the disk are at right angles to the mag- netic flux, because these currents depend on the rate of change of the flux, and the flux is changing most rapidly when the magnetizing current is passing through zero. The magnetism in ^ is in phase with the current; hence, for the power factor of 1, the currents in the disk are in phase with the magnetism set up by the series- coils; consequently, the driving torque is a maximum for the given values of the line current and E. M. F. Suppose that we have the same current and E. M. F. but that the power factor is less than 1. The line current will lag behind the E. M. F., the magnetism in ^will not reach its maximum at the same instant as the currents in the disk, and the driving torque will be reduced, thus making the meter run slower. A magnetic brake is provided by making the disk revolve between the poles of permanent magnets in the same manner as in the TbQmson meter. This makes the speed at
Pig. 40
826 AND MEASUREMENT 67
all times proportional to the driving; torque. If it were possible for the circuit to have a power factor of zero, i. e., if the line current lagged 90° behind the E. M. F,, the torque action on the disk would be zero, because the induced cur- rents would be at right angles to the magnetism in d. In other words, when the currents in the disk were a maximum there would be no field for them to react on, and when the field magnetism was at its maximum there would be no currents in the disk. The meter would not therefore record any power even though current would be flowing in coils a,c. This is
as it should be, because with zero power factor, the watts supplied would be zero no matter what the values of the cur- rent and E. M. F. might be. The induction meter can there- fore be made to record the number of true watts expended in a circuit no matter what value the power factor may have-
50. Fort Wayne Induction Wattmeter. — Fig. 41 shows a Fort Wayne single-phase induction wattmeter. D is the armature, which, in this meter, takes the form of an inverted aluminum cup. E is the damping magnet that exerts a drag on the armature and makes its speed pro- portional to the driving torque. The current and potential
46— .11
58
POWER TRANSFORMATION
§26
coils are at the back of the armature; a is one current coil, and the other coil occupies a similar position on the opposite side of the armature. The speed of the meter can be adjusted by shifting the magnet E up or down, thus vary- ing the amount of the armature embraced by the pole pieces of the permanent magnet.
51. Stanley Induction Wattmeter. — In the Stanley recording wattmeter the armature is an aluminum disk acted on by current and potential coils in much the same manner as previously described. The most interesting feature of this
meter is the method of suspending the disk. Instead of rest- ing on a pivot, as in most meters, the disk a, Fig. 42, is sus- pended magnetically. It is mounted on a small, hollow, steel shaft b through which passes, a fine steel wire c stretched taut by means of the screw d and spring e. The P»«« shaft b has in it two
small brass bushings, one at each end, that bear against the wire and keep the disk from tipping side wise, otherwise the disk has no support. A permanent magnet / is provided with pole pieces gy h shaped as shown; ^^ is a brass plug. From the way in which the pole pieces are shaped the lines of force passing across the gap at / hold the shaft in a central position between the poles so that the shaft and disk are magnetically suspended and revolve with very little friction. The reduc- tion in the friction makes the meter more accurate, particu- larly on light loads, and there is no pivot to be damaged by shock or vibration. The recording dial is operated by gears driven from the shaft by the teeth shown at w,
§26 AND MEASUREMENT 69
MEASUREMENT OF POWER ON TWO-PHASB
CIRCUITS
52. In making power measurements on polyphase cir- cuits, the methods used will depend, to some extent, on whether the load on the system is balanced or not. The load in such a system is said to be balanced when the current in each of the phases is alike, and the power factor of the load on each phase also alike. In other words, the loads on the different phases of a balanced system are alike in every particular; under such circumstances it would be accurate enougfh to simply measure the power delivered to one phase and multiply the result by the number of phases. Unfortu- nately, an exact balance is seldom realized in practice,
Current Coil
P/JOse/
Pressure Coil
P/?a5e^.
Pio. 48
although induction motors, synchronous motors, and rotary converters in themselves constitute a nearly balanced load, because they take current from the different phases in practically equal amounts. When a mixed load of lights and motors is operated, it is almost impossible to obtain an exact balance.
53. TNvo-Phase, Four- Wire System, — Fig. 43 shows the usual method of connecting wattmeters for measuring power on a two-phase, four-wire system. Each phase is provided with a wattmeter, there being a current coil in each phase; the pressure coils are connected across the phases. In series with the pressure coil there would be a resistance, $1$ ip all wattmeters of the electrodjrnamometer type; thi^
60 POWER TRANSFORMATION §26
resistance is not shown in the accompanying: fig^ures, and the fine-wire coil can be taken to represent the complete potential circuit of the wattmeter including: the usual pro- tective resistance.
Fig. 43 shows two distinct circuits containing wattmeters. It is evident that the sum of the two readings will give the total power supplied to the motor or other devices to which the lines are connected. Also, the sum of the readings will give the power supplied whether the load is unbalanced or not, because each wattmeter measures the actual number pf watts supplied to the phase in which it is connected. Fig. 44 shows the two wattmeters used on a two-phase system with a common return. Recording wattmeters of
/yjase/
PAisaP
VS^S^
\smssmy—\
SJ
.QMftQj
Pxo.44
the induction type are made, in which two sets of series- coils and two potential coils act on a common armature, thus practically combining two single-phase meters into a single meter, so that only one instrument is required to measure the energy no matter what the power factor may be or how unbalanced the current in the two phases.
54. Induction Wattmeter for Unbalanced Poly- I)hase Circuits. — Fig. 45 shows a General Electric poly- phase meter of the induction type for measuring energy supplied to unbalanced two-phase, three-phase, or monocyclic circuits. It operates on exactly the same principle as the single-phase induction wattmeter and is essentially two sets of single-phase meter coils acting on a common disk armature a. The two potential coils b, b are shown above the disk; they are connected in series with the reactance
■ §26 AND MEASUREMENT 61
coils c, c. There are four current coils, two of which are shown at d, d. A pair of current coils is situated under each potential coil and current is supplied to the front pair by means of the conducting strips e, e. The ends of the series-coils connect to terminals /; /, g, g, to which the mains are connected; h is one of the two magnets that retard the disk. Each set of coils b, d, d constitutes a single-phase induction meter, and as both these act on the same disk a, it follows that the resultant effort that turns
the disk is a combination of the efforts exerted by the two sets of coils, and the record given by the meter is, therefore, a true indication of the watts supplied. In Fig. 45, one set of series-coils d, d would be connected in series with phase 1, and the other set in series with phase 2. The potential coils b, b would be connected across the two phases. In a three-phase circuit the two sets of series-coils would be connected in series with the two outside wires, and the potential coils would be connected between the outside wires and the middle wire.
62 POWER TRANSFORMATION §26
66« Use of a Single Wattmeter on Ti?vo-Phase Circuit. — Figs. 46 and 47 show two methods of measuring: the power on a two-phase circuit with a single wattmeter: these can be used in case the load* is balanced. In Fig. 46, the current coil is connected in the commor. return wire, and a reading is first taken with the poten- tial coil connected across phase 2, as shown by the full line. The connection a is then transferred to a\ thus connecting the potential coil across the other phase. The
Phcfse/
1
^OWfl*^! -^ TaL^wf
\ — ssmi
Pig. 46
sum of the two readings gives the total power supplied no matter what the power factor of the load may be. In Fig. 47, the potential coil is connected across the outside wires, while the current coil is connected in the middle wire. The reading of the wattmeter gives the total number of watts because, if the system is balanced, the resultant cur- rent will differ in phase from the resultant E. M. F. by the same amount that the current in each phase differs from
P/kise/
^TJOT^
P/tase^
pJMAilft^
-^To laaa
Fig. 47
the E. M. F. of each phase. The resultant E. M. F., i. e.,
the E. M. F. E' between the outside wires, is V2 E^ where E is the E. M. F. of each phase. The resultant current /', i. e., the current in the middle wire, is V2 /, where / is the current in each phase. The reading of the wattmeter is E' P cos <^, where <^ is the angle of lag. The watts supplied to phase 1 are *E I cos <^ and the same to phase 2, so that the total
watts supplied are 2 E I cos <^. Now E^ ^ ^ E and /'
= aS /; hence, E^ P cos<^ = V2 ifi" V2/cos* = 2i5'/cos<^.
§26
AND MEASUREMENT
63
That is, a single wattmeter connected as shown in Fig, 47 indicates the total number of watts supplied provided the load is balanced. These methods of using a single watt- meter are convenient, but it is always best to use the two wattmeters if they can be obtained, because one cannot always be certain that the load is balanced.
MEASUREMENT OF POWER ON THREE-PHASE
CIRCUITS
56. Power may be measured on a three-phase circuit by using one, two, or three wattmeters. Two-wattmeter measurements are the most common, as the use of a single wattmeter requires either that the load be exactly balanced, or that the connections be transferred from one phase to another and the load kept constant during the change.
"wsm^
Mb
T
Ms
I
Fig. 48
67. Use of Three Wattmeters. — Let ABC, Fig. 48, represent the three windings of a Y-connected three-phase alternator. In a balanced system, ^i, ^„ and e^ being equal, the line E. M. F.'s E^, E^, E^ are also equal, and are equal
to the E. M. F. in one winding multiplied by Vs. The cur- rent in each line will be the same as the current in the winding to which it is connected, and in a balanced system the three currents will be equal. Three wattmeters with their current coils A' B' O connected in the lines and their potential coils a b c connected across the corresponding winding, will measure the power delivered no matter whether the load be balanced or unbalanced, inductive or non-inductive.
64
POWER TRANSFORMATION
§26
It is evident from the way in which .the wattmeters are connected that the potential applied to the pressure coil is equal to that generated in the winding with which the cur- rent coil is in series. Hence, the reading of wattmeter A' will be ex /\ cos <^, where <\> is the angle of lag between the current and E. M. F. The other two meters will give the power developed in phases B, C, and the sum of the three readings gives the total power. If the load were exactly balanced it would be necessary to use but one watt- meter and multiply its reading by 3 to obtain the total power. In case ABC represented the windings of an induc- tion motor, synchronous motor, transformers, or in fact a load of any kind, this method of measuring the power could
-^otJco^
E9
T
Et
.MO.QQ.QQ.
Fig. 49
be applied, though, as shown later, it is possible to measure an unbalanced three-phase load with two meters, and the three-meter method is therefore little used.
In most cases it is not possible to get at the neutral point x. Fig. 48, to connect the potential coils. In such cases an artificial neutral point may be obtained, as shown in Fig. 49, by connecting three non-inductive resistances x, y^ z across the three phases, and attaching their neutral point x^ to the potential coils. These resistances might be made up of wire wound non-inductively, or of incandescent lamps. The sum of the three wattmeter readings would then give the total power supplied as before.
§26
AND MEASUREMENT
65
58. Use of Single Wattmeter With V Resistance. If the load were balanced, it would be sufScient in Fig. 49 to use but one wattmeter and multiply its reading by 3.
""I_/irini?r
> . -r ~~
Fig, 50 shows the connections for a single wattmeter used in this way. The resistances r„ r, correspond to resistances X, s. Resistance r, is the usual protective resistance in series with the movable wattmetercoil. Fig.Sl shows a Thomson re- cording wattmeter with V resistance; a is the starting coil of the watt- meter intended to com- pensate for the friction and to secure more ac- curate readings on light loads. By comparing I Figs. 50 and 51 it will ■ be seen that the connec- tions are identical, the recording meter being connected in exactly
the same way as the
indicating instrument. Fig. 52 shows the con- nections of a recording
meter on a three-phase balanced circuit where the pressure is over 500 volts; the potential circuit is here supplied through the small step-down transformers t, t. For very
66
POWER TRANSFORMATION
§26
high-pressure circuits, the current coils would be connected
to the secondaries of current transformers in- stead of directly in the circuit. Fig. 53 shows the connections of a Wagner indicating watt- meter for measuring the power on a balanced three-phase circuit. The stationary current coils A, A are connected in series with the sec- ondary of a current transformer C instead of being placed directly in the circuit. The movable potential coil B is supplied with cur- rent from the small transformers Z?, D. A Y resistance is used, the two branches being in the separate cage £;
the protective resistance P is used to limit the current in
the potential coil.
Pzo. 52
59. Use of Two Wattmeters on Three-Phase Cir- cuits.— The most common method of measuring the power supplied to a three-phase circuit is by means of two watt- meters connected as shown in Fig. 64. The current coils A^ B are connected in two of the lines, and the potential coils between these two lines and the third line. If the power factor of the load is over .5, i. e., if the angle of lag is less than 60°, the sum of the two wattmeter readings gives the power supplied. If the power factor is less than .5, i. e., if the angle of lag is greater than 60°, the diflEerence of the readings gives the power.
§26
AND MEASUREMENT
67
Since the coil A, Fig, 54, is connected in one line and the potential coil a between the outside and middle lines, it is evident that even on a non-inductive load the current in A is not in phase with the current in a. On a non-inductive load
0-4--0
o-t-o
Pig. S3
the current in A will differ in phase from the E. M. F. between 1 and 2 by 30°, and the current in B will differ in phase from the E. M. F. between 2 and 3 by 30°. In Fig. 55, suppose that X represents the neutral point of the system, and that the lines x-l\ x-2\ x-3' represent the three voltages, differing
ToUfte
7b Load
b
B
i 3
Fio. 54
in phase by 120°. Then the voltage between lines 1 and 2 is equal to I'-x plus 2'-x, and is found by producing x-2' backwards and finding the resultant x-4. This resultant is 30° behind the voltage x-1^. The voltage between lines
68
POWER TRANSFORMATION
§26
2 and 3 is x-S' found by producinsT x-^' backwards and combining with x-2'. Since the wattmeters are connected symmetrically, as in Fig. 54, we must consider the E. M. P. acting on coil b as the E. M. F. taken in the direction 3—2 or 5'-2' instead of 2'-5', since we have taken the other E. M. F. in the direction 1-2 or 1^-2'. The E. M. F. act- ing on coil b will therefore be represented by r-6' equal and opposite to x-S', Now, when the load is non-inductive, the current in coils A and B is in phase with the E. M. F.'s x-i' and x-3', so that the E. M. F. acting on coil a is 30^ behind the current in a, and the E. M. F. acting on b is 30° ahead of the current in B.
fi^o. 65
If the load is inductive, the currents in the coils A, B instead of coinciding in phase with x-1' and x-S' will lag by an angle <^, cos <\> being equal to the power factor of the load. The current will then be represented by the lines ^-/» and AT-Za lagging <^ degrees behind x-l^ and x-3^. Lines x-4 and x-6^ represent the E. M. F.'s applied to coils a, b so that with an inductive load the phase difference between the currents in A and a is 30° — <^, and between the currents in B and b it is 30° + <^. If we represent the pressures x-l\ x-2' , xS', etc. by e, the pressure x-4 or the line pressure
will be V3 e. The watts indicated by A will be V3 e A cos
§26 AND MEASUREMENT 69
(30° - <^), and the watts indicated by B, VSe /, cos (30° + *), and the sum of these two readings gives the power.*
60. It is now easily seen why a power factor of less than .5 will give a negative reading on one of the wattmeters. If the lag is 60°, the current in B differs in phase from that in ^ by 30 + <^ = 90°; no effort is exerted on the swing- ing coil of the wattmeter and no deflection is given. If the lag becomes greater than 60° a torque is exerted in the reverse direction on the movable coil, and a negative deflection is obtained. For power factors greater than .6, both wattmeters will give positive readings, but their readings will not be alike and both positive unless ^ becomes zero, i. e., unless the power factor is 100 per cent, or unity. If the angle of lag becomes 90°, both wattmeters will read alike, but one will be positive and the other nega- tive, so that their -sum will be zero. This is as it should be, because when the lag is 90° the current flowing in the circuit is wattless and no power is expended. The conditions under which the test is made will nearly always indicate whether or not a negative reading is to be expected. If there is any doubt on the matter, connect the meters to a load of lamps and after all connections have been made so that both meters read properly, take off the lamps and connect the load under test. If one of the meters gives a reverse reading it shows that the reading is negative and that the difference in the two readings must be taken to give the number of watts sup- plied. Fig. 56 shows the connections of a Wagner indicating
*That the sum of these two readings gives the power is easily shown for the case of a balanced circuit where A = A. We have, power
= W = %§£ A cos (30** - 0) -h V8 £ A cos (30° -h0). From trigo- nometry we know that cos (30° -f- 9) = cos 30° cos ^ — sin 30° sin 9, and cos (30° — ^) = cos 30° cos ^ -h sin 30° sin 9. Substituting these values for cos (30° -f- 9) and cos (30° — 9) , we have
JT = 2 V3 e A cos 30° cos ^,
but cos 30° = ' ; hence, W^ = 3 £ A cos 0, but e A cos ^ is the power
in one phase, and 3 e A cos <p is the total power, so that the sum of the two wattmeter readings g^ves the total power supplied to the circuit.
70
POWER TRANSFORMATION
§26
wattmeter for measuring the watts on a three-phase circuit with balanced or unbalanced loads and with any power factor. It consists essentially of two wattmeters; AA^A' A* are the two sets of current coils and B, B' the two movable potential coils mounted on the same shaft. The torque due to the two wattmeters is thus added or subtracted, as the case may be, and the pointer attached to the shaft indicates , the actual number of watts expended. The current coils are supplied from current transformers, and each of the movable coils has a non-inductive resistance in series with it. This
Pig. 56
wattmeter is also suitable for measurements on an unbalanced two-phase system.
The recording wattmeter, shown in Fig. 45, is used largely for measurements on three-phase circuits. Since the two wattmeter elements act on a common armature, if one of them gives a negative turning effort, the net turning effect on the disk is reduced and the record on the dial is due to the difference of the effects of the two wattmeters. The instrument, therefore, gives an accurate record, no matter \srhat the power factor may be.
§26 AND MEASUREMENT 71
61. Measurement of Poiwrer Factor. — The fact that the ratio of the two wattmeter readings, Fig. 64, varies with the power factor of the load affords a method of determin- ing the power factor from the wattmeter readings.* Of course if ammeter and voltmeter readings are available the power factor can be calculated, since it is equal to
— -— the true number of watts being obtained from
apparent watts
the wattmeter readings and the apparent watts from the voltmeter and ammeter readings. For a three-phase circuit
the apparent watts would be Vs £" /. When two wattmeters are used, as shown in Fig. 54, the power factor of a three- phase circuit can be determined from the ratio of the read- ings alone, and ammeter and voltmeter readings are not necessary. The ratio of the readings is
V3 e / cos (30° + ^) ^ cos(30° + <^);
V3 e I cos (30° - <^) " cos (30° - <^) cos (30° +<^ ) _ cos 30° cos <^ - sin 30° sin <^ cos (30° -<^ ) "" cos 30° cos <^ + sin 30° sin ^
Vs
but cos 30° = -~, and sin 30° = i; hence,
It
??sj 30° J- ^ ) ^ 2 ^^ ' ^ ^ V3 cos » - sin »
cos (30° - <^ ) V3 ^ . i • A V3 cos <^ -H sin *
--- cos 9 + ¥ sm 9
XT -x * 1 4.U • V3 COS <^ — sin <^ J V Now if we take the expression — .^ , and sub-
V3 cos ^ + sin ^ stitute different values for <^, we will get the ratio of the wattmeter readings corresponding to those values. For example, if <^ = 60° we have ratio of wattmeter readings
>/3 X 2 - - -
^ ^ =0. An angle of lag of 60° cor-
V3xi + -| ^ responds to a power factor of .5. For an angle of lag of *E. J. Berg, Electrical \Vorld and Engineer, Vol. XXXIX,
72
POWER TRANSFORMATION
§26
30^, power factor = cos 30® = .866, we have ratio of reading^s
>3 X ^ - i \3 X^^ + i
1 2
By thus taking^ different values of the power factor we can plot a curve, Fig. 57, showing the relation between the ratio of the wattmeter readings and the power factor of the load.
Example. — The power supplied to a three-phase induction motor is measured by means of two wattmeters connected as shown in Fig. 54.
|
ffff |
|||||||||||||||||||||
|
•1 |
^ |
||||||||||||||||||||
|
^ |
^ |
||||||||||||||||||||
|
7f |
/ |
/ |
|||||||||||||||||||
|
ff |
,/^ |
X |
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|
m. |
/ |
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|
^ |
^ |
1 |
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|
y |
X |
90 |
5 |
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»• |
k ^ |
% |
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^ |
4f |
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9 |
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1 |
\ .1 |
» A |
» A |
r .4 |
1 ,i |
V A |
1 .4 |
» ^ |
» .i |
I i |
► .J |
I .i |
» .J |
1 .4 |
1 .4 |
1 .4 |
1 .1 |
r .4 |
» .1 |
» J |
1 |
-Ratio of Reading s- (negative)
«-♦-*■
-Ratio of Readings- (poj/tiv^
Fig. 57
The reading of A is 2,000 watts, and that oi^B 6,000 watts. What is the power factor of the motor corresponding to this load, and what is the total power supplied to the motor?
2 000 Solution.— The ratio of the two readings is -'.r^ = .333 and is
b,U0U
positive, because both readings are positive. Hence, referring to
Fig. 57, we take the ratio .333 on the right of the center line, and find
that the power factor corresponding to this ratio is about .74. The
total power supplied to the motor will be 2,000 -f 6,000 = 8,000 watts.
62. Po-wer-Factop Indicators. — The power-factor indi- cator made by the General Electric Company and used on
§26 AND MEASUREMENT 73
three-phase circuits is based on the foregoing principle. It consists of a fixed current coil, connected in series with the middle line, within which two potential coils are mounted on a vertical shaft. These coils are connected between the middle and outside lines. The resultant effort tending to deflect the shaft will evidently vary with the power factor, because the phase relation of the currents in the movable coils to the current in the fixed coil will change with the power factor and the instrument can be calibrated so that the pointer attached to the movable coils will indicate the power factor.
Another type of power-factor indicator that is commonly used is the same in construction as an indicating wattmeter.
except that the potential coil is connected in series with an inductance so that the current in it is 90° behind the current in the main coils when the power factor is 1. The result is that with a power factor of 1 there is no deflection of the pointer because there is no torque action between the two coils. With a power factor less than I, lagging current, the pointer swings in one direction and with a power factor greater than 1, leading current, the pointer swings in the other direction. Fig. 58 shows the front of a Wagner power- factor indicator operating on this principle.
63. Heaaurement of Power With One Wattmeter. The power supplied to a balanced three-phase load may be tneasured with a single wattmeter, as shown in Fig. 59, by
74
POWER TRANSFORMATION
§26
first taking a reading with the potential coil connected at c and then quickly transferring the connection to c*. The sum of the two readings will give the power if the power factor is over .5; if less than .5, the difference in the readings would
C^!
To Line
*-«
jSi^^
Pxo. S9
be taken. It is necessary, however, to use two wattmeters unless the load can be kept constant while the connections are being changed or in case the load is not balanced.
A.
4
JG
JT
I
/Wr
T^azw
O
\\mmmm
W \Ti
2fs^
Trvn^otmer^
kp b7
L
i>9/lMr»
Pig. 00
64. IPo^wev Measurement on Monocyclic Circuit.
Fig. 60 shows the connections for a Thomson recording wattmeter measuring energy supplied to a motor operated on the monocyclic system. The meter has two coils A^ B^
§26 AND MEASUREMENT 75
which are connected in series with two of the lead wires run- ning to the motor. As shown in the figure, the coils are in series with the leads C, D. If it is found that the speed of the meter diminishes when the load on the motor increases, field coil A should tie connected in series with the main E instead of C.
INSTALLATION OF RECORDING WATTMETERS 65. Location. — Recording wattmeters should be located so that they can be easily reached either for the purpose of taking readings or inspecting them. They are too often placed in out-of-the-way places where they are very difficult to get at. They should not be placed in a position where they will be subjected to vibration as, for example, near a door that is continually being opened and shut. The loca- tion should be such that the meter will not be exposed to dampness or chemical fumes of any kind.
CONNECTIONS FOR METERS
66. The method of connecting meters to the circuit varies with the size and make of the meter. It is impossible to
give here all the different connections and, moreover, it ts not necessary or desirable to do so, as the makers send
76 POWER TRANSFORMATION §2
§26
AND MEASUREMENT
77
out instructions with the meters, and these instructions are liable to chang^e with changes in the construction of the meters. Therefore, only a few of the most common connec- tions used on direct-current or single-phase alternating- current circuits will be described.
67. Connections for Tbomson Becordlng^ Watt- meter.— Fig. 61 shows the method of connecting a Thomson recording wattmeter of small capacity on a two-wire circuit.
To Looef
To TranafOi
(a)
^>000(^
To Transforfrttr
■^^s?
To Load-
Pio. 64
When the meter is of large capacity, only one side of the circuit is run through it and a small potential wire is run in from the other side, so as to put the armature across the circuit. This method of connection is shown in Fig. 62. Fig. 63 shows a meter connected to a three-wire circuit.
68. Connections for Stanley Induction Wattmeter.
Fig. 64 (a) and (d) shows the methods of connecting a Stanley wattmeter. The black terminal B on the meter must always connect to the transformer or other source of
78
POWER TRANSFORMATION
§26
E. M. F. The white terminal W connects to the load. It is necessary to have these connections correct or the meter will not rotate in the proper direction. The potential wire p connects from the meter to the wire that does not enter the meter.
The connections for induction wattmeters are much the same no matter what the make may be, the current coil or coils being connected in series with the circuit, and the potential coil across the circuit. What differences there may be are due to the manner in which the leads are brought
Potenfkrt SfuA
Current SfidiOs
TbLkie
ToLoQtl
Fig. 65
out of the meter case. In most cases current transformers are used in connection with meters on high-tension lines, the current coils being connected in series with the second- ary of a current transformer instead of in series with the main circuit. On high-potential circuits, the potential coils are supplied from potential transformers that step-down the voltage applied to the coils. Of course, when current and potential transformers are used in connection with a meter, the instrument is always calibrated so that it will take account of the current or voltage transformations and
§26
AND MEASUREMENT
19
indicate the number of watts in the main circuit. Fig. 65 shows the connections for a General Electric induction watt- meter of the polyphase type used on switchboards. In this Case, potential transformers /, /' are used to step-down the voltage and current transformers /, /' to transform the cur- rent. The connections shown are such as would be used on a three-phase circuit or a three-wire, two-phase circuit with the common return wire in the middle.
TESTING AND ADJUSTING RECORDING
WATTMETERS
69. Recording wattmeters should be checked up occa- sionally to see if they record correctly. If a rough test only is required the meter may be loaded with a specified number of lamps of which the power consumption per lamp is known; if a more accurate test is desired, the recording
Pio. 66
meter is usually checked by comparing it with a standard indicating wattmeter.
70. Checking a Thomson Recording: Wattmeter.
Figs. 66 and 67 show connections for checking a two-wire Thomson meter. Either set of connections may be used. The meter is set to work on a load of lamps, or other con- venient resistance, the standard direct-reading wattmeter
80
POWER TRANSFORMATION
§26
being connected as shown. A chalk mark is made on the meter disk, so that the revolutions may be easily counted, and the revolutions are taken for 40 to 60 seconds, the observer using a stop-watch. Another observer reads the standard instrument, and the load is kept as nearly constant as possible throughout the test. The meter watts may then be calculated from the following formula:
Meter watts = ^^^^^ ^
T
(1)
where R = number of revolutions in T seconds;
T = time in seconds of R revolutions;
K = constant of meter. The constant K used in formula 1 was, in the older types of meter, marked on the dial and was a number by which the dial reading had to be multiplied to give the true reading
Pig. 67
of the meter. In recent types of Thomson meter, the ge&rs in the recording train are arranged so that the dial reads directly and no constant is marked on it except in meters of large capacity. In recent meters the constant K used in formula 1 will be found marked on the revolving disk.
The actual watts are obtained from the standard meter; hence, the percentage by which the meter is correct is found by dividing the number of watts given by formula 1 by the number of watts given by the standard meter.
§26
AND MEASUREMENT
81
Example. — The disk of a 10-ampere, 100-volt Thomson meter makes 10 revolutions in 60 seconds. The average number of standard watts as indicated by the standard meter is 303. Find the percentage error of the recording meter. The constant of the meter is i.
Solution. — From formula 1, we have
Meter watts »
3,600 X 10 X i 60
= 300
tH = .99, or 99%. Ans.
The meter is, therefore, 1 per cent, too slow, and the damping mag- nets should be shifted in a little so that the retarding action on the disks will not be so great.
71. If a standard wattmeter is not available for testing: purposes, separate ammeters and voltmeters may be used for direct-current work, but they are not as convenient.
JJSUL
^9^
Pio.es
In Figs. 66 and 67 it will be noticed that the energy con- sumed by the potential circfuit of either meter is not measured by the other; that is, the current in the armature of the Thomson meter does not pass through the fields of the standard meter, neither does the current in the shunt of the standard pass through the field colls of the Thomson meter.
To test a Thomson meter used on a three-wire circuit (110-220 volts), the connections may be made as shown in Figf. 68. The potential circuits are wound for 110 volts. The field coils can, therefore, be connected in series, and the standard meter connected as shown. In formula 1 , however,
S2
POWER TRANSFORMATION
§26
K should be taken as one-half the constant marked on the dial or disk. Aside from this, the meter can be tested in the same manner as a two-wire meter.
72. Checking: a Stanley Wattnoteter. — Fig. 69 shows the connections for checking a Stanley wattmeter and the connections for testing any two-wire induction wattmeter
Trtm^ermtr
Pig. 09
would be very similar. A is the recording wattmeter and B the standard instrument. With the Stanley meter the watts are given by the following formula:
Meter watts =
(2)
where only R = number of revolutions in T seconds;
T = time in seconds for R revolutions; K = 2i constant marked on the meter case. This formula applies also to the Fort Wayne induction meters, the values of K being given for different sizes of meters, in a table fiunished by the manufacturers.
READING RECORDING WATTMETERS
73. The dials of most wattmeters record either watt-hours or kilowatt-hours. In some cases, as with the earlier types of Thomson meter, the reading taken from the meter dials must be multiplied by a constant in order to obtain the watt-hours. This constant is usually marked on the dial. However, the general practice now is to make the dials of meters direct reading except in the case of meters of large capacity. If no constant is marked on the dial it can be assumed that the meter is direct reading.
/oaaao /c^oco
f€^OOO^O0
n
4000
*^O0O^OOO
m
f,OQC
IV
/.OOQOOO too,ooo
/,ooo
/o,ooo
/,ooo,ooo /oo,ooo /c,coo
t/dooo^ooq,
VJ
/,OCQ
f^OOO^OOO
iooo
Pio. 70
84 POWER TRANSFORMATION §26
74. Beading Tlxomson Meter. — The Thomson meter has five dials. The lowest reading pointer is the one to the extreme right (facing the meter); it is marked 1,000, which means that one complete revolution of the hand indicates 1,000 watt-hours, and that each division therefore represents 100 watt-hours. The next one to the left is 10,000 to a revolution, or 1,000 for a division, and so on. Fig. 70 shows six different readings, by studying which the student should be able to take readings from any meter.
Beginning at the left, number the pointers 1, 2, 3, 4, and 5. Then, in /, Fig. 70, pointer 5 is on 2 and is read 200. Pointer 4 is two-tenths of the way between 8 and 9 and is read 8,000. Pointer 3 is read 10,000. Pointer 2 has not gone through its first division; likewise pointer 1. The state- ment is then 18,200.
The statement of // is 5,718,900 (not 5,719,900, as it fre- quently would be read) . Pointer 4 should not be read 9 until pointer 5 has completed its revolution and is again at 0,
The statement of ///is 99,800 (not 109,800), because the 100,000 mark will not be reached until pointer 5 has passed from 8 to 0, when 4 and 3 will be at (?, pointer 2 at i, and pointer 1 just past the zero mark.
The statement of /F is 9,990,800. Pointer 1 is slightly misplaced. Otherwise, the reasons given above will apply to this statement.
The statement of V is 8,619,900. Pointer 2 is misplaced. It should be two-tenths of the way between 6 and 7 instead of nearly over ^, as shown.
The statement of V/ is 834,200. Pointer 4 is misplaced. It should be two-tenths to the right of 4 instead of to the left of 5, These misplaced hands are frequently met with in practice and are generally caused by a knock in removing the cover, or, perhaps, they are a little eccentric.
Rule. — To ascertain the number of watt-hours thai has been used by a consuyner from one date to anothery subtract the earlier statefnent from the latter aytd ynultipiy by the constant of the meter ^ if one is marked on the dial. In case no constant is
§26 AND MEASUREMENT 85
marked on the meter, the cansiant is 1, and Ike readings are taken as given by the dial.
Example. — An electric company supplies power to operate a motor (or ODe of its customers. The rale charged is 5 cenls per kilowatt- hour. The reading of the meter on January 30 is 8.819.900, and on February 28, it is 9,990,800. The constant of the meter is 2. What should be the amount of the bill for the month?
Solution.— The number of watt-hours supplied between Jan. 30 and Feb. 28 » (9,990,800-8,619,900) X2 = 2,741,800.
2,741,800 watt-hours = 2,741.8 K. W.-hours, which at 5 cents per K. W.-hoor would unount to 2,741.8 X .05 - $137.09. Ans.
SPECIAli METERS
75. The Trro-Rate Meter. — Most electric-light stations
have their period of heaviest load for a few hours only in
the evening. During the daytime the plant is lightly loaded,
and a large i)art of the machinery is standing idle. Id order
to obtain a day load and thus work the plant to best advan- tage, some companies supply power during the daytime at specially low rates in order to induce customers to use electric motors. For measuring the power supplied to such
86 POWER TRANSFORMATION §26
customers, two-rate meters are sometimes used. A two-rate meter is one that records the power during certain hours of the day at a rate different from that at other hours. One of the earlier types made by the General Electric Company was a regular Thomson recording: meter provided with two dials and recording trains, which were arranged so that a self-winding clock would throw either one or the other into gear with the meter shaft at the proper time. The energy recorded on the two dials was then charged for at different rates.
In the later type of General Electric two-rate meter an ordinary Thomson meter A^ Fig. 71, with a single dial is used. Connected to the potential circuit of ^ is a self- winding clock mechanism contained in the case B. The case also contains a resistance, which, during certain hours, is inserted in series with the armature of the wattmeter, thus making the meter run at a reduced speed during those hours. The two-rate attachment, therefore, makes the meter run slow during certain hours, which is equivalent to charging for the power at a low rate during those hours.
76. Maximum -Demand Meter. — The maximum amount of current that the various customers consume determines in large measure the capacity of the station equip- ment. Some customers might use large currents for short intervals only, but the plant must be capable of handling these large currents; in some cases, therefore, the maximum demand for current is taken into account in making up the bill; for example, all current over a certain amount is charged for at a higher rate. One style of instrument used for indi- cating the maximum current used above a certain amount is the Wright maximum-demand meter^ shown in Fig. 72. It consists of a U-shaped tube, hidden partly by the scale in the figure, which has bulbs A, B on either end; a branch tube C is attached near B and carried down over the scale. The lower end of tube C is closed. The current flows from D to E through the resistance strip F coiled around bulb A, The tube is partially filled with liquid, which remains in it as long
§26 AND MEASUREMENT 87
as the current does not exceed a certain amount. If, how- ever, the current exceeds the allowable amount, the expan- sion of the air in A due to the heating of strip F will force liquid into the tube C. Any increase in the current will force
over more liquid, and from the height of the column of liquid in tube C the charge can be estimated. The U-shaped tube is mounted on an arm that can be swung up after the reading has been taken, thus emptying tube C into the U-shaped tube.
INDEX
NoTB.— All Items In this index refer first to the section and then to the pagre of the section. Thus. "Bus-bars 25 19" qieans that bus-bars will be found on paffe 19 of section 25.
A Sec. Page
Adjusting and testing recordinsr
wattmeters 26 79
Air sap. Density In 20 48
•' " Length of 22 88
and bore of
poles .... 21 28 of Induction motor.
Length of 22 87
All-day efficiency of transformer . 22 28 Alternatinsr-current apparatus. De-
sisrnof 20 1
" current apparatus, De- sign of 21 1
'* current apparatus. De- sign of 22 1
** current, Arresters for 25 54 ** current. Line calcu- lations for 23 80
" current. Power trans- mission by 23 23
" current switchboards . 25 76 ** current systems ... 28 39 current systems. Fre- quency In 23 41
Alternator, armature. Heating of . 20 4 " Armature winding for
three-phase 21 15
*' Armature winding for
two-phase 21 18
** armatures, PeHpheral
speed of 20 20
Design of 100-kilowatt single-phase .... 21 1 Alternators, Combined running of 23 58
Design of 20 1
** Electrical c o n n e c -
tions for 21 57
Hunting of 23 71
in parallel 23 58
" parallel. Com- pound-wound . . 23 76 '* series 28 58
••
Sec. Page Aluminum, Comparison of prop- erties of copper and 24 11
*' copductors 24 6
** line wire. Resistance,
tensile strength, and
weight of 24 9
" wire, Deflections and
tensions for . 21 27 '* " Resistances of
pure 24 10
" Stringing. ... 24 26
Ammeter connections 26 47
Apparatus and line tests 24 58
" for controlling incom- ing current ... 26 20 •• controlling out- going current . . 26 40 " " transforming cur- rent 26 26
Arcing, Suppression of 25 49
Area of B. & S. wires. Sectional . 23 10
Armature conductors 20 38
core 22 56
" Density in .... 20 47
" Design of 21 4
disks 20 31
** Inductance, Calculation
of 20 18
Insulation 20 42
** losses. Calculation of . 21 10 " Mechanical construc- tion of 22 56
or rotor 22 30
Radiating surface of . . 20 10
reaction 20 11
self-induction 20 15
slots. Insulation of ... 20 ^3
spiders 20 34
teeth. Density in .... 20 46
windings 20 21
" winding for three-phase
alternator 21 15
IX
INDEX
5^.
Armature wiadlns: for two - phase
alternator 21
** windings. Polyphase . . 20
Armatures. Completed 21
Heatinsr of alternator 20 " Peripheral speed and
diameter of 22
Peripheral speed of
alternator 20
Arrangement of windings 20
Arrester for alternating current.
General Electric . 25 ** " altematins: current,
Westinfifhouse . . 25 '* " hisrh - tension lines,
Westinsrhouse . . 25
Garton 25
General Electric .... 25 Simple lifirhtninf .... 25 *• Westinshouse .... 26 ** Westin^house low- equivalent 25
Arresters for alternating current 25
" direct current ... 25 " Ground connections for
liirhtnins: 25
Installation of 25
** Location of 25
B
Bare and insulated wires 2i
" copper wire. Dimensions.
wetfirhts. etc. of 24
Bed, frame, and field. Construction
of 21
Bedplate and field frame of induc- tion motors 22
Bituminized-fiber conduit 24
Bore of poles and lensrtb of airsrap 21
Box. Pour-way 24
Boxes. Junction .24
Service .24
Brush-holder studs 21
" holders and brushes .... 21
Bus-bars 25
" *' Carrying: capacity of . . 25 " •• Mounting for high-
tension 25
C
Cable. Drawins: in 24
joint. Hisrh-tension 24
Cables. Di.stribution of, for man- holes 24
Joinins: 24
Main and equaliser .... 23
Paze
13
27
19
4
42
20 29
66
54
66 51 58
48 62
57 64 61
50 60 60
43
67 38 28 47 46 50 51 60 19 20
21
43 61
45 M 66
Sec. Page Calculation of armature induct- ance 20 18
" " armature losses . 21 10 ** ** primary and sec- ondary turns . 22 15 * " separately-ex- cited windinsr . 21 34 Calculations for alternating: cur- rent. Line 23 30
'* for two-wire system 23 7
** Formulas for line . . 23 31
Line 23 7
Capacity of transformers for three- phase induction motors 26 17 " of transformers on two- and three-phase sys- tems 26 17
** of undereronnd tubes,
Carrying: 24 57
Carrylna: capacity of bus-bars . . 26 20 *' " ** undenrround
tubes ... 24 57
Cement-lined pipe conduit 24 33
Checkins: a Stanley wattmeter . . 26 82 of Thomson recording:
wattmeter 26 79
Choke, or reactance, coils 25 49
Circuit-breaker and slide switch,
Stanley 25 8
Cutter 25 35
Cutter laminated- type 25 85
General Electric
MK 25 83
** breakers 25 5
25 31
26 47
" ** and fuses .... 25 27
General Electric 25 82 Circuits, Induction wattmeter for
unbalanced polyphase 26 60 ** Measurement of power
on three-phase 26 63
** Measurement of power
on two-phase 26 50
** Transformers on sinffle-
phase 26 4
•* Transformers on three- phase 26 11
** Transformers on two-
phase 26 9
" Use of two wattmeters
on three-phase 26 66
Coils nnd core. Arrangfcment of . 22 6 Arrapsrement of primary and secondary 22 16
INDEX
zi
Sec.
CoHs. Pield-magnet 21
Insalatlon of armature ... 20
" field 21
•• Kicking 25
** Loss in field 21
** Reactance, or choke .... 25 ** Winding and insulation of . 22 Collector rinsfs and rectifier .... 21 Combined operation of direct-cur- rent dynamos .... 23 " running of alternators . 23
Compensating voltmeter 25
Compensator, Mershon 25
Completed armatures 21
Compound machines in parallel . 28 '* machines in parallel
with shunt machines 23 ** or series-field, windinsr 21
" wound alternators in
parallel 23
Conductor and core. Dimensions of 21 ** Size of primary .... 22
Conductors, Aluminum 24
and core. Rotor ... 22
Armature 20
Copper 24
Cost of 28
" Dimensions of ... . 22
Line 24
** of low resistance. Lo-
cating: grounds and
crosses on 24
Conduit, Bituminised-fiber .... 24 '* Cement-lined pipe .... 21
** Creosoted-wood 24
Pump-loflT 24
Vitrified -clay or terra- cotta 24
Conduits 24
Connection. Equaliser 26
Equalisins: 28
Connections. Ammeter ...... 26
Field windins: and . . 22 '* for alternators. Elec- trical 21
for meters 26
for six-phase rotary
converters .... 26 for Stanley induction
wattmeter 26
for substations ... 26 for synchronizinflT . . 26 for Thomson record- inflT wattmeter ... 26
Shunt-field 26
Voltmeter 25
25 42 27 49 42 49 8 45
45
68 23 25 19 50
58 38
76
8 42
6
50 88
1 48 18
1
66 88 83 82
»•
••
84 82 47 47 47 55
67 75
82
77 44 46
77 48 22
Sec.
Connections. Voltmeter 26
Construction. Line 24
*' of armature. Me- chanical ... 22 " " collector rinjers
and rectifier . 21
'* switchboards . 25
Overhead 24
line ... 24
of shafts 22
** " transformers . . 22
** Undersrround .... 24
line. . 24
Continuity tests 24
Converter, Direct-current 23
Converters, Connections for six- phase rotary .... 26 Methods of starting
rotary 26
Rotary 26
** Synchronizing rotary 26
** Voltagfe resfulation of
rotary 26
Copper and aluminum. Compari- son of properties of . . 24
" conductors 24
loss 22
" studs. Current densities of 25
wire 24
** *' Dimensions, weisrhts,
etc. of bare 24
Core and coils. Arran^ment of . . 22 *' ** conductor. Dimensions
of 21
Rotor .... 22
** Armature 22
** Construction and arrangfe-
ment of transformer ... 22
** Density in armature 20
** Desifim of armature 21
" Dimensions of 22
** losses 20
" " and masrnetic densities 22
" Maflmetic density in 22
** tjrpe transformers on three- wire system 26
** volume. Determination of . . 22
Cores. Transformer 22
Cost of conductors 23
Creosoted-wood conduit 24
Cross-arms 24
" section of lines. Estimation
of 23
Crosses and grounds on conduct- ors of low resistance. Loca- ting 24
Fiaze 46
1
56
45 71
1 14 66 27
1 82 59
32
85 81 89
32
11 1 1 3 1
4
6
8 50 56
27 47
4 12
7 31
5
8 11
4
43 82 16
31
66
«•
zn
INDEX
Sec,
Crosses or grounds, Testinff for . 24
Tests for . . 24
CRrearulator 25
Current densities for copper studs 25 densities per square Inch
in induction motors . . 22
" detector f^alvanometer . . 24
Half of main 26
in lines. Estimation of . . 28
•* primary. Full-load ... 22 " stator and rotor of in- duction motors,
Volume of 22
Magnetic 22
" Power transmission by
direct : . . 23
Curve. Efficiency 22
Cutter circuit-breaker 25
" laminated- type circuit- breaker 68
D
Damptns: devices, Use of 28
Deflections and tensions for alumi- num wire 24
Densities. Masmetic 20
Density in air zap 20
" armature core 20
teeth 20
•• core. Masmetic 22
** of magnetism in rotor
teeth . 22 •• " •' in stator
teeth . 22 Desism of altematins: - c u r r e n t
apparatus 20
" " alternating -current
apparatus 21
'* *' alternatins: -current
apparatus 22
" armature core .... 21
*• field 21
" 8-kiIowatt transformer 22
" ** lO-horsepower motor . 22 " lOO- kilowatt sinsrlc-
phase alternator . . 21
•* magmets 21
Detectors, Electrostatic srround . 25
Ground 25
Determination of core volume . . 22 Diameter and speed of armature.
Peripheral 22
Dimensions and resistance of Iron
wire 24
** of conductors .... 22
** ** conductor and core 21
Page
59
62
44
8
89 60 46 82 41
89 24
2 21 85
25
78
27 46 48 47 46 5
39
89
1 4
28 10 40
1 20 89 36 11
42
12
13
8
iyec.
Dimensions of core 22
" knife switches ... 25
•• poles 24
** weifrbts, etc. of bare
copper wire .... 24
Direct-current. Arresters for ... 25
converter 28
dynamos 23
" *' Power transmis- sion by 23
switchboards ... 23
Systems 23
Disks. Armature 20
Distribution of cables for man- holes 24
Dobrowolsky three-wire system . 23 Double-current generator installa- tion 25
Drawing in cable 24
Drop, Estimation of 28
Dynamos and motors for direct- current power trans- mission 23
** Direct-current 28
** in parallel. Series ... 28
Shunt ... 23
*' *' series. Operation of . 28
£
Eddy-current loss 20
Edison three-wire system .... 23
*' underground-tube system . 21
Efficiency curve 22
Pull-load 22
*' of transformer 22
All-day. 22
Electric transmission 23
Electrical connections for alterna- tors 21
Electrostatic ground detectors . . 25
Equaliser cables. Main and .... 23
" connection 26
Equalizing connection 23
Equipment, Substation ..... 26
Estimation of cross-section of lines 23
" " current in Ikies ... 23
" drop 28
F
Faults. Testing lines for 24
Peed-wire. S tandard weather-proof 24
Feeder panels 25
Field coils. Insulation of 21
" Loss in 21
*• Design of 21
frame and bed, Construction
of 21
Page 12
4 15
4
51 22 45
2 73 36 31
45 20
86 43 S3
2 45 47 48 45
9
15 53 21 87 19 23 1
57 89 56
47 47 18 31 82 88
68 8 72 27 42 28
43
INDEX
Sec.
^ield frame and bedplate of Induc- tion motors 22
maffnet coils 21
** maffnets. Design of 21
•' or stator 22
" rheostat, General Electric . . 25
" rheostats . . " 2S
•• switches 25
" wiudinff and connections . . 22
Fields. Revolvins: 21
Flax in poles. Maarnetic 22
Formulas for determinins: resist- ance of wire 24
for line calculations . . 23
Port Wayne Induction wattmeter . 26
Four-way box 24
** wire system. Two-phase . . 26
Frame, field, and bed. Construe*
tionof 21
Frequency in altematingr-current
systems 23
Full-load current in primary ... 22
'* •• efficiency 22
•• ** power factor 22
•• •• Slip at 22
Puses 25
** and cfircult-breakers .... 25
•* Primary 26
G
Galvanometer. Current-detector . 24
Garton arrester 25
Gausres. Wire 24
General Electric arrester 25
Electric arrester for alter- nating current 25
** Electric circuit-breakers . 25 Electric field rheostat . . 25 " Electric oil switches ... 25 Electric MK circuit- breaker 25
Generator installation, Example
of double-current . . 25
panel. Sinsrle-phase . . 25
" panels 25
German-silver wire 24
Ground connections for llgrhtnlns:
arresters 25
detectors 25
Electrostatic. . 25 ** and crosses on conductors of low resistance. Lo-
catlns: 24
** or crosses. Testins: for . . 24 Tests for ... 24
|
Xlll |
|
|
Sic, |
Pa^e |
|
26 |
45 |
|
22 |
52 |
|
20 |
4 |
|
22 |
4 |
|
23 |
13 |
|
25 |
81 |
|
25 |
21 |
|
24 |
51 |
|
24 |
31 |
|
25 |
56 |
|
25 |
5 |
|
25 |
62 |
|
23 |
71 |
|
20 |
7 |
|
26 |
54 |
|
26 |
72 |
Half of main current
57 Heat losses
25 HeatinflT of alternator armatures .
20 " " transformers
30 Hiffh pressure. Use of
67 '* " switchboards . . .
65 ** tension bus-bars. Mountins: 67 for
66 " " cable joint
23 ** ** lines. Leakage on .
46 Westlngrhouse
arrester for 11 " •• switches
31 *' " systems. Static
67 effect on
47 Huntins: of alternators
59 Hysteresis loss
43 I
Indicating wattmeters 26
41 Indicators. Power-factor 26
41 Inductance, Calculation of arma-
87 ture 20 18
38 Induction motor. Length of air srap
40 of 22 37
28 •• " windlnsrs .... 22 88
27 ** motors 22 30
2 ** motors. Capacity of
transformers for three- phase 26 17
motors. Current den- sities per square inch
in 22 39
motors. Field frame and
bedplate for 22 57
motors. General data on 22 87 motors. Limitation o f
output of 22 81
motors. Number of poles
of 22 37
motors. Peripheral
speeds of 22 87
motors. Power factor of 22 86 motors. Primary wind- ins: of 22 38
motors. Secondary
windlns: of 22 35
motors. Volume of cur- rent in stator and
rotor 22 89
wattmeter. Connections
for Stanley 26 77
wattmeter for unbal- anced polyphase cir- cuits 26 60
wattmeter, Fort Wayne 26 57
|
60 |
|
|
51 |
|
|
8 |
•• |
|
58 |
|
|
«■ |
|
|
56 |
it |
|
82 |
|
|
67 • |
*( |
|
11 |
|
|
•i |
|
|
88 |
|
|
•• |
|
|
86 |
• |
|
76 . |
|
|
72 |
(* |
|
12 |
|
|
i* |
|
|
50 |
|
|
36 |
|
|
39 |
•• |
|
•« |
|
|
66 |
|
|
59 |
|
|
62 |
•t |
XIV
INDEX
Sec. Page
Induction wattmeter. Stanley ... 26 58
wattmeters .26 54
Iron losses 22 1
^ire 24 11
/>^Ioss 22 1
loss and output. Relation
between 20 6
Installation of arresters 25 60
recordinsr watt- meters 26 76
Insulated wires. Bare and 24 1
Insulating armature slots 20 43
Insulation and windinsr of coils . . 22 8
of armature coils ... 20 42
field coils 21 27
resistance. Tests f or . . 24 61
Insulators 24 20
Types of 24 21
Interrupter. Static 25 68
J
Joining cables 24 51
Joint. Hiffh-tenslon cable 24 61
Junction boxes 24 46
K
Klckinsr coils 25 49
Knife switches. Dimensions of . . 25 4
L.
Laminated-type circuit- breaker,
Cutter 25 85
Lamps. Synchronizin&r 28 60
Leakacre on higrb-tension lines ... 24 81
Length of air gap 22 88
Lifhtinfir or power switchboard . . 25 74 Li&rhtnins: arresters. Ground con- nections for ..'... 25 50 arrester. Simple .... 25 48 Protection from .... 25 47
Limitation of output 20 2
of output of induction
motors 22 81
Lincoln synchronizer 28 65
Line and apparatus tests 24 58
" calculations 28 7
for alternatinsr
current .... 28 80
Formulas for . . 28 81
conductors 24 1
** construction 24 1
Overhead .... 24 14
Underground . . 24 32
drop. Lost power and .... 23 4
protection by continuous dis-
charsre 25 61
Sec. Line protection from static charges 25 wire. Resistance, tensile strencrth. and weisrht of
aluminum 24
Lines. Estimation of cross-section
of 23
cucrentin . . 28 Leakage on hisrh-tension . . 24 Tcstincr. for faults 24
Transportationfof transmis- sion 24
Locating a cross by the Varley
loop method 24
a partial srround without
an available srood wire 24 grounds and crosses on conductors of low re- sistance 24
Location of arresters 25
Long shunt 28
Loop test. Varley 24
Loss, Copper 22
Eddy-current 20
Hysteresis 20
;; /"^ .' ." 22
in field coils 21
^ Losses. Calculation of armature . 21
Core 20
]' Heat 22
Iron 22
Lost power and line drop 28
Low-equivalent arrester. Westing- house 25
tension switches 25
M
Machines in parallel. Compound . 28 Magnetic current *a
densities 20
*• •• J f *
and primary
core losses . 22
and secondary
core losses . 22
density in core 22
flux in poles 22
through pole pieces
and yoke 21
Magnetism in rotor teeth. Density
of 22
" stator teeth. Density
of 22
Magneto testing set 24
Magnets. Design of field 21
Main and equalizer cables .... 23
Main current. Half of . r 26
Manholes 24
Pore «2
9
81 82 31 58
28
Gb
64
86
60 51 62
1 9 7 1 42 10 / 52 1 4
67
2
50 24 46
81
82
5
45
80
39
89 58 20 56 45 88
INDEX
XV
Sec, Page
Maximum -demand meter 26 86
Measurement and transformation
of power 26 1
of power factor . . 26 71 " of power on poly-
phase circuits . . 26 53 " of power on three-
phase circuits . . 26 63 '• of power on two-
phase circuits . . 26 69 " of power with one
wattmeter .... 26 78
Mechanical construction 21 43
Mershon compensator 25 25
Meter, Maximum-demand .... 26 86
Readinsr Thomson 26 84
Two-rate 26 85
Meters, Connections for 26 75
Special 26 85
MK circuit-breaker. General Elec- tric 25 83
Motor. Desifim of 10-horsepower . 22 40 " IienflTth of air crap of induc- tion 22 87
Motors and dynamos for power transmission by direct- current 23 2
Capacity of transformers for three-phase induction 26 17 " Current densities per
square inch in induction 22 89 ** Field frame and bedplate
for induction 22 57
*' General data on induction 22 37
Induction 22 30
" Limitation of output of in- duction 22 81
** Number of poles of induc- tion 22 87
" Peripheral speeds of in- duction 22 37
*' Power factor of induction 22 86 *' Primary windini; of induc- tion 22 83
'* Secondary windinfif of in- duction 22 35
'* Volume of current in stator
and rotor of Induction . 22 39 Mountinsr for bifh-tension bus-bars 25 %\
O
Oil switch of larsre capacity .... 25 15
" switches. General Electric . . 25 11
Stanley 25 19
Operation of direct-current dyna- mos, Combined 23 45
Sec. Page Operation of direct-current dyna- mos in parallel . . 28 45 dynamos in series . 23 45
Output. Limitation of 20 2
" of induction motors. Limi- tation of 22 81
Overhead construction 24 1
line construction .... 24 14
P
Panel, Sinffle-phase srenerator . . 25 76
Panels, Feeder 25 72
Generator 25 72
Parallel operation. Features con- nected with 23 69
Peripheral speed and diameter of
armature ... 22 42 of alternator
armatures . . 20 20 speeds of induction
motors 22 87
Phase-chanffingr transformers . . 26 13
Pins 24 19
Pipe conduit, Cement-lined .... 24 33
PlufiT switch, Stanley 25 6
Plunger switch, Westincrhouse . . 25 7
Poles 24 14
" Bore of, and lensrth of air
gap 21 28
" Dimensions of 24 15
" Magnetic flux in 22 45
'* of induction motors, Number
of 22 37
*• Selection of 24 14
" Siaes of 24 14
*• Spacing of 24 15
Polyphase armature windingrs . . 20 27 " circuits. Measurement
of power on 26 53
transformers 26 29
Potential regulators 25 42
Power factor. Pull-load 22 88
indicators 26 72
" " of induction motors 22 86 " measurement. Instruments
used for 26 58
" measurement on mono- cyclic circuit 26 74
" or lighting switchboard . . 25 74 " on three-phase circuits.
Measurement of 26 63
transformation and
measurement 26 1
" transmission by alternating
current 23 23
XVI
INDEX
Sec, Page Power transmission by direct cur- rent .... 23 . 2 system. Sim-
pie 28 8
Three-phase. 28 28
Two-phase . 23 26
Pressure wires 24 47
25 23
Primary and secondary coils ... 22 16
• turns. . . 22 15
conductor 22 42
" core losses and magnetic
densities 22 81
PuIMoad current in ... 22 41
fuses 26 2
windinsr 22 88
22 48
of induction
motors .... 22 88 Protection from lishtninsr and
static charges 25 47
Pulleys 21 65
Pump-loff conduit 24 88
R
Radiating surface of armature . . 20 10
Railway switchboard 25 78
Rcaotaoce. or choke-coils 25 49
Reaction. Armature 20 11
Reading recording wattmeters . . 26 82
Thomson meter 26 84
Recording wattmeter. Checking of
Thomson 26 79
" wattmeter. Connections
for Thomson .... 26 77
" wattmeter, Thomson . 25 69
•• . 26 64
wattmeters 25 69
26 54
Installation
of .... 26 75
Reading . . 26 82
Testing and
adjusting 26 79
Rectifier and collector rings ... 21 45
Regulation of transformers .... 22 25
" ** rotary converters.
Voltage 26 32
Voltage 20 2
Regulator, CR 25 44
Stillwell a*) 43
Regulators. Potential 25 42
Relation between I^ R loss and
output 20 6
Relay, Reverse-current % 23
Sec.
Relay. Time-limit 26
Westinghonse time-limit . 26 Resistance and dimensions of iron
wire 24
" of wire. Formulas for
determining 24
Test for insulation . . 24
Reverse-cnrrent relay 26
Revolving fields 21
Rheostat. General Blectrlc field . . 25
Rheostats. Field 25
Rotary converters 26
Connections for
six-phase . . 26 Methods of
starting ... 26 Synchronising . 26 Voltage regula- tion of .... 26
Rotor or armature 22
** conductors and core .... 22 ** teeth. Density of magnetism
in 22
Rule for determining size of wire
for a given transmission .... 28
8
Scott two-phase, three-phase trans- former 26
Secondary coils. Arrangement of
primary and .... 22 ** core losses and mag- netic densities . . * .22
winding 22
" winding of induction
motors 22
Sectional area of B. A S. wires . . 23
Selection of a system 28
Self-induction, Armature 20
Separately-excited winding. Calcu- lation of 21
Series dynamos in parallel .... 28
" field, or compound, winding 21
" Operation of dynamos in . 23
Service boxes 24
Shafts 21
Construction of 22
Short shunt 28
Shunt dynamos in parallel .... 28
" field connections 26
" Long 28
" machines. Compound ma- chines in parallel with . . 23
•• Short 23
Simple lightning arrester 25
" power transmission system 23
20
12
11
61 2S
67 65
SI
82
85 89
82 80 60
89
9
15 16
82
60
35 10 86 15
84 47 88 45 50 54 66 51 48 48 51
58
51
48
8
INDEX
xvii
Sfc. Page Sincrle-phase alternator. Desism of
100- kilowatt .... 21 1 circuits. Transform- ers on 26 4
" " concentrated wind-
insr 20 22
'* " generator panel ... 25 76 transmission .... 23 24 '* wattmeter on two-phase cir- cuit 26 62
wattmeter with resistance . 26 65 Six-phase rotary converters, Con- nections for 26 82
Size of primary conductor .... 22 42 ** wire for a ffiven transmis- sion 23 9
Sizes of poles 24 14
Slide switch and circuit-breaker.
Stanley 25 8
Slip at full load. Table of 22 40
Slots. Insulation of armature ... 20 48
SpacinfiT of poles ' . . 24 15
Speed and diameter of armature.
Peripheral 22 42
of alternator armatures.
Peripheral 20 20
Spiders. Armature 20 84
Splicing and tyinar 24 28
Stab switch. Stanley 25 10
Stanley induction wattmeter ... 26 68 induction wattmeter. Con- nections for 26 77
" oil switches 25 19
'* plus: switch 25 6
*' slide switch and circuit- breaker 25 8
stab switch 25 10
wattmeter, Checkincr a . . 26 82 Starting rotary converters.
Methods of 26 35
Static charares. Line protection
from 25 62
" effect on hiflrh-tension sys- tems 25 62
interrupter 25 68
Stator and rotor of induction
motors 22 39
or field ,. . 22 30
teeth. Density of maffnet-
Ism in 22 39
Steel wire. Resistance of 24 11
Stillwell reirulator 25 43
Strinsrinfr aluminum wire 24 26
Studs. Brush-holder 21 51
Substation equipment 26 18
*' transformers 26 26
5^. Page
Substations. Connections for ... 26 44 " Location and ireneral
arransrement of . . 26 40
Surface of armature. Radiating . . 20 10
Switch of lar^re capacity. Oil ... 25 15
Stanley plug 25 6
stab 25 10
Wcstinsfhouse plunder ... 25 7 Switchboard and switchboard ap- pliances 25 1
** appliances 25 1
Power or liffhtinsr . . 25 74
Railway 25 78
Switchboards 25 71
Altematinsr- current 25 76
" Direct-current ... 25 78
for parallel runnin&r 25 79 '* General arransre- ment of hiarh-pres-
sure 25 81
" General construc- tion of 25 71
Switches 25 1
** breaking arc in a confined
space . . 25 7
'• in open air 25 6
" under on. . 25 10
Dimensions of knife . . . 25 4
Field 25 67
General Electric oil ... 25 11
HlfiTh-tension 25 6
'* Low-tension 25 2
Stanley oil 25 19
Synchronism 23 59
Synchronizer. Lincoln 28 65
Synchronizing 23 60
Connections for . . 26 46
lamps 23 60
" rotary converters . 26 89 ** two-phase and three-phase ma- chines 23 60
" Use of voltmeter for 23 62
System, Dobrowolsky three- wire . 28 20
Edison three-wire .... 28 15
Selection of a 28 86
Systems, Altematinsr-current ... 23 89
Direct-current 23 36
Special three-wire .... 28 19
T
Table of approximate weifirhts of
weather-proof wire . . 24 6 " ** capacity of transformers for three-phase induc- tion motors 26 17
ZVIU
INDEX
Sec. Page Table of carryhif capacity of
• nndersrround tubes . . 24 67 ** " compariKon of properties of copper and alumi- num 24 11
•* current -densities for
copper studs 26 8
*' deflections and tensions
for aluminum wire . . 24 27 " " density of magnetism in
rotor teeth 22 89
" " density of magnetism in
stator teeth 22 89
*' " dimensions and resist- ance of iron wire ... 24 12 ** *' dimensions of knife
switches 26 4
** *' dimensions of poles ... 24 16 '* *' dimensions, weisrhts, etc.
of bare copper wire . . 24 4 " " full-load efficiency ... 22 88 " *' •* power factor . . 22 86 " German-silver wire ... 24 IS " " length of air gap .... 22 38 " ** resistance of pure alumi- num wire 24 10
'* ** resistance, tensile strength, and weight of aluminum line wire . . 24 9 ** sectional area of B. A S.
wires 23 10
" " slip at fullload 22 40
'* " standard weather-proof
feed-wire 24 8
" valves of coefficient ilf . 28 84 ** " volume o f current I n stator and rotor of induction motors ... 22 40 Teeth, Density in armature .... 20 46 Tensions and deflections for alumi- num wire 24 27
Terra-cotta or vitrified-clay con- duit 24 34
Test for grounds or crosses ... 24 62 " " insulation resistance ... 24 61
" Varley loop 24 62
Testing and adjusting recording
wattmeters 26 79
" for crosses or grounds . . 24 69
lines for faults 24 58
" set, Magneto 24 58
Tests. Continuity 24 59
Line and apparatus .... 24 58
Thomson meter. Reading 26 24
recording wattmeter . . 25 69 " . . 26 64
5^.
Thomson recording wattmeter.
Checking of 26
recording wattmeter. Connections for ... 26 Three-phase alternator. Armature
winding for .... 21 '* ** circuits. Measure- ment of power on 26 circuits. Trans- formers on ... . 26 circuits. Use of two wattmeters on . . 26 " *' power transmission . 23 " wire system, Co re -type
transformers on . . 26
'* system. Dobrowolsky 28
" system. Edison ... 23
'* " 220-volt system .... 23
** 550-volt system .... 23
" " system .Transformers
on 26
" systems. Special ... 23
Time-limit relay 26
•• Westinghouse . . 26 Transformation and measurement
of power 26
Transformer, All-day efficiency of 22 " core. Construction
and arrangement
of 22
cores 22
Design of 8-kilowatt 22 " Efficiency of .... 22
'* Scott two-phase.
three-phase ... 26
Transformers 22
'* and transformer
connections ... 26 ** Construction of . . 22
" for three-phase in-
duction motors. Capacity of ... 26
Heating of 22
in parallel 26
*' on single-phase cir- cuits 26
" on three-phase cir- cuits 26
" on three-wire
system 26
" on two- and three- phase systems. Capacity of ... 26 " on two-phase cir- cuits 26
Phase-changing . . 26
Fiaj^e
79
77
15
63
11
66 28
8 20 15 87 89
7 19 20 28
1 28
27 4
10 19
15 1
1 27
17
4 4
4
11
17
9 18
INDEX
Sec. Faze
Transformers. Polyphase 26 29
Resrulation of ... 22 26
Substation 26 26
" Use of, to raise volt- age ,- ' ^ ' ^
Transforming current. Apparatus
for 26 26
Transmission by altematins: cur- rent. Power ... 23 23 " by direct current.
Power 23 2
Electric 23 1
lines. Transmission
of 24 28
Sinsrle-phase .... 23 24 system. Simple
power 23 8
Three-phase power 23 28 Two-phase power . 23 26 Transportation o f transmission
lines 24 28
Tubes, Carrying capacity of under-
irround 24 57
Turns. Calculation of primary and
secondary 22 15
Two-phase alternators, Armatiire
windlni; for 21 13
" and three - phase systems, Capacity of transformers
on 26 17
" phase-circuit. Use of a sinsrle
wattmeter on a ... 26 62 '* *' circuits. Measurement
of power on 26 69
'* " circuits. Transformers
on 26 9
" " four-wire system ... 26 69 power transmission . . 23 26 *' " three - phase trans-
former, Scott .... 26 15
** rate meter 26 85
" wattmeters on three-phase
circuits. Use of 26 66
" wire system. Calculations for 28 7
" 220-volt system 23 87
Tyincr and spliclnir 24 28
U
Unbalanced polyphase circuits.
Induction wattmeter for 26 60
Undersrround construction .... 24 1 line construction . . 24 82 " tube system. Edison 24 58 ** tubes. Carry ins: ca- pacity of 24 67
V Sec, Page
Values of coefficient M 23 84
Varley loop method. Locating a
cross by 24 65
" test 24 62
Vitrified-clay, or terra-cotta, con- duit 24 84
Voltage reirulaiion 20 2
" " of rotary con- verters ... 26 32
Voltmeter, Compensatinsr 25 23
connections 25 22
26 46
" for synchronizing. Use
of 28 62
Volume of current in stator and
rotor of induction motors .... 22 89
W
Wattmeter, Checking a Stanley . . 26 82 Checking of Thomson
recording 26 79
** Connections for Stan-
ley induction .... 26 77 " Connections for
Thomson recording 26 77
Port Wayne induction 26 57
Stanley induction ... 26 58
" on two-phase circuit . 26 62
** Thomson recording . 25 69
" . 28 54
" with resistance. Use of
single 26 65
Wattmeters, Example of use of
three 26 68
'* for unbalanced poly- phase circuits. In- duction 26 60
Indicating 26 54
Induction 26 54
'* Installation of re- cording 25 76
" on three-phase cir- cuits 26 66
" Reading recording . 26 82
Recording 25 69
26 54
Testing and adjust- ing recording ... 26 79 Weather-proof feed-wire. Standard 24 8 ** ** wire. Approximate
weights of .... 24 6 Weights, dimensions, etc. of bare
copper wire 24 4
Westinghouse arrester 25 52
" arrester for altera
nating current . . 25 54
XX
INDEX
Westinffhouse arrester for hifh tension lines . . *' low -equivalent ar
rester .... " plunsrer switch . " time-limit relay Windinsr and connections. Field " insulation of coils Calculation of separately- excited 21
Compound or series-field 21 for three-phase alter- nator. Armature .... 21 for two-phase alternator.
Armature 21
of Induction motors. Pri- mary 22
of induction motors.
Secondary 22
Primary 22
Secondary 22
Sinfifle-phase concen- trated 20
Windings. Armature 20
" Arranflreracnt of .... 20
** Induction-motor .... 22
*' Polyphase armature . . 20
|
Sec, |
Pa£e |
|
25 |
56 |
|
25 |
57 |
|
25 |
7 |
|
26 |
28 |
|
22 |
55 |
|
22 |
8 |
84
15
18
33
85 43
50
22 21 29 83 27
Sec. Pki£e Wire. Approximate weights of
weather-proof 24 6
" Copper 24 1
" Deflection and tensions for
aluminum 24 27
*' Dimensions and resistance
of iron ... 24 12 ** *■ weififhts. etc. of
bare copper . 24 4
" for a fiiven transmission . . 23 9 ** Formulas for determinincr
resistance of 24 II
" ffauires 24 8
German-silver 24 12
" Iron 24 11
Resistance, tensile strensrth. and weight of aluminum
line 24 9
'* Resistances of pure alumi- num 24 10
" Steel 24 11
StrinsrinR aluminum .... 24 26
Table of German-silver . . 24 13
Wires. Bare and insulated .... 24 1
Pressure 24 47
25 23
Sectional area of B. & S. .23 10