O RI GINA L PA PE RS

COMMERCI AL DYNA MO

DES I GN

W I LLIA M L WAT E RS M E E H . , . . , . .

En inccr in ( Mar : o Hi /x S eed M ac/tinn Westin nause Elec i g y f g p y, g tr c 65

u r r /zi E er N t o l lec Man actu in Co . rme / C c n ine a i na E f g fi y f g ,

o l rin /r u r C . and Co nsu tin En ineer Wa o se Ai , g g , g

Brake and Canadian Westing/ww e Co t .

F I R S T E D I T I O N

NEW O Y RK.

$OHN W ILEY 8c SONS

LONDON: C HA PMA N 8c H A LL, 1.1m m 1911 COPYRIGHT 19 11

BY W I I W A T ERS LL A M L. n t kno w led e but i no ra ncc tlla t bc m o g , g , g C HA RLES DARWIN

cfzm ma n a debto r to Ii i: ro u io n y p fe ,

; b a l mend to e he p and orna ment tbcrcunto .

Fu ncns BA CON

3 4 36 52

I NT R ODUC T I ON

— Commercial Engi neering The nin eteenth century was nota s i ble for the achievement of the eng neer , and there is little doubt that the men responsible for this pioneer w o rk were engineers in i the broades t sense of the word . They were eng neers in their ability to manipulate the forces and materials of nature ; but they were also far- sighted and level-headed men of aflairs in their ap l preciation of the economic va ue of their work , and in the man

a em en t . g of their projects These men , by their genius and i s h enterpr se , and by their hard common ense and solid ac ieve i ment , forced the world to recogn ze the engineer as destined to play a leading part in the future . Public attention became

an d focussed on the engineer , numbers of men were attracted

r to the profession , either through natural inclination or th ough hope of profit . But as was to be expected , a large percentage an d of these men were imperfectly educated , possessed only a

an specialized talent in certain directions , without y of the broad minded comprehensive spirit of the pioneers . This change soon reacted on public opinion , and the engineer lost his high stand ing ; so that the public and the world of commerce came to re $ ” - gard him as a specialized crank , a species of high grade. artisan ,

w as who though useful enough in his place , devoid of any ability to take a sane and comprehensive view of a situation , or to man

an age undertaking in which he might be interested . The com m ercial u world , though distr sting the ability of an engineer to hi s manage enterprises , came to realize more and more their iii iv INTRODUCTION

f . o economic value and financial possibilities The result this was , that the enterprises which the engineering world proposed , gradually came to be exploited by business men , who had to

o f n a greater degree the confidence the fi ancial powers . These

to men , who attempted manage such undertakings , were usually

o f n o t altogether ignorant the work they took up , and it is sur f E prising that this development produced indi ferent results . n

in eers ff o ff o o f g , being e ectively cut fr m the commercial side their work , became narrower ; while the increasing specialization left the commercial men in still greater ignorance o fthe products they were handling , and their management became more and more wasteful . The inefficiency o fthis arrangement has gradually be come evident to all , and it is now generally recognized that the executive and commercial head of any large engineering enter prise must po s ess some engineering knowledge . It is also realized

fo r that , given an opportunity a broader education , an engineer can be readily trained to become an efficient and high- grade com m ercial en executive , while it is almost impossible to instil an gineerin g knowledge into o n e whose training has been restricted f to . o are the commercial world As a result this , engineers being given opportunities to obtain a broader commercial education , and it is recognized in high financial quarters that in the future t hey will have to depend o n the engineering profession for high grade operating men in all engineering enterprises . Already we o f the n find engineers at the head a number of large concer s , and it is probable that the time is n o t far distant when it will be the exception to find the operation o f an engineering under taking in the hands o fany other than an engineer . —I Mr $ o hn I Be s . t . gg was my good fortune to work under f $ . o n e o Mr . ohn I Beggs , one of the first and the most prominent o f the commercial engineering executives in this country , at a time when I was so interested in purely engineering problems

that I was in danger o f drifting away from broader interests . Fo r twenty-five years in active touch with the electrical engineer

as o r ing industry , a manufacturer , as an organizer and operator

o f public service corporations , obtaining his engineering and INTRODUCTION

r commercial knowledge th ough hard personal experience , Mr . Beggs was pre-eminently fitted to guide and encourage the i youthful engineer . Continually mprovin g and rendering com m ercially practicable much of the apparatus used in connection

s with electric lighting or railway intere ts , and doing this with

o f n n s the purpose obtai i g re ults , rather than of being accorded

o r n credit public recog ition , he was one of the first to realize that the engineer was destined to become the dominant i G i hi factor in large eng neering corporations . iv ng s young

' engineers a free hand in all branches of commercial and

i n n neces engineer ng work , only guidi g and checki g them where sary , he developed organizations and men in a way , which , while

r b it made the men his grateful and enthusiastic admire s , esta lished his reputation as one of the foremos t organizers in the engineering indu—stry , and left his mark on the methods and . s apparatus of to day In inscribing the e few papers to Mr . I i $ . I n ohn Beggs , am o ly tak ng an opportunity of express in g my indebtedness , and my appreciation of the education it w as to be in touch with him . I have entitled the papers $ n u Commercial Design , because engi eering questions of p rely academic interest are made subservient to those broader s n is ues , where desig and operation must be considered in their relation to the commercial success and future of the undertak ing . It is in this feature of my work that I have to ao

in e c fiu n e . knowledge the of Mr Beggs , and to state that I have in a great measure to thank him for any small succes s I may have had .

Histor o Pa e s — y f p r These papers , which have been written

f r d s ix at di ferent pe io s during the past years , are the result of fifteen years ’ experience as a designing and manufacturing engineer in some of the most important electrical manufacturing E concerns in urope and America . They cover various subjects

o f relating to the design electrical power machinery , in which I f happened to have been interested at di ferent times , and they are reproduced here in the hope that they may be more accessible , to any whom they may interest , than they would be in the pro vi INTRODUCTION

ceedings of the various institutions . The student will find there is no lack of treatises on dynamo design by capable men , and this book makes no pretense to be a comprehensive work . It is f merely o fered as a supplement to such treatises , as a series of articles , which written by one who is actively engaged in prae tical manufacturing , may cover a few points of special interest

s to the tudent or designing engineer , that could not be satis facto rily covered in a more general work .

Decembe r, 19 10 . C ONT E NT S

PAG E

DOUB LE CURRENT GENE RATORS IN THE IR RELATION To DOUB LE CURRE NT SUPPLY

PREDETERMI NATI ON o r SPARR ING IN D IREC T CURRENT MACHINES

ROT ARY CON $ ERTERS AND MOTOR-GENERATORS

SHUNT AND COMPOUND ROT AR Y CON$ ERTERS FOR RAI LWAY WORK — T HE NON SYN CHRONOUS OR IN DUC TION GENERATOR I N POWER STATI ON

MO DERN DE $ E LOPMENTS IN SIN GLE PHAS E GENERATORS

PERF ORMANC E SPE CI FI CATIONS AND RATI NGS

INPUT-OUTPUT EFFI CIEN CY T ES TS

DIREC T CURRENT T URBO GENERAT ORS

COMMERCIAL ALT ERNAT OR DESIGN

THE design of alternators has been treated times w ithout

l i . a. number , but usual y the commercial element in the design , ,

e . the relation of factory cost to selling price , has be n ignored An engineer has been defined as a man $ who can do for ” one dollar what any fool can do for two , and as this definition in applies connection with machine design , the man who can des ign the cheaper machine to satisfy a given specification is

the better designin g engineer . S peaking generally , there is no type of alternator that will a h compare with the intern l revolving field construction , in w ich

- each po le carries a separate field coil of edge o n copper strap . The revolving armature is cheaper for high frequencies and low

a (50 voltages , the inductor type is good for sm ll cycle high

speed machines , while the disc alternator with no iron in the

armature is an excellent machine for high frequencies . But d these , though sufficiently satisfactory in their own limite field ,

do not compare with the revolving field type for general work . — F undamental Types of Co ns tructio n The revolving field

L. 2 E . alternator took its present form about 189 when Mr . C . Brown designed some alternators which were practically modern

1 . $ . machines ; while in 893 Mr . S de Ferranti installed some

Mr. v i M hin I n nd m . n n L. B w B B r as c u de rs ta fro C E . ro that the ro w o e enfab rik de se rve to s o me e x te nt the cre dit fo r the des ign o fthe Po rt s mo uth Mr - f l e n o s o . Fe n w as the s to use e d e o u co e r s o r a t r at r , th ugh rra ti fir t g pp trap

fie ld magne ts . DESIGN

FIG 1 . . ALTERNATOR DESIGN

FIG . 2 . 4 COMMERCIAL ALTERNATOR DESIGN

21 0 KW. P E f alternators at ortsmouth , ngland , which were o f similar design to those o Mr . Brown . Before that date alter n ato rs o f o f this type were a clumsy amateur design , and their performance was , generally speaking , very poor . Strangely

o ut enough , these two engineers , after bringing a high grade design, apparently abandoned further development , and their machines to- day are almost identical with their machines o ften years ago . It has taken the different manufacturing concerns a long time to recognize the superiority of the Brown and Ferranti

e fo r typ standard work , and it is practically only during the

r o r last th ee four years that this type has been generally adopted .

co n struc The result is that , except for a few minor details , the tion of these alternators has been improved very little since they were first introduced ; while the excellence , from a commer f f ’ cial o o . point of view , the electrical design Mr Brown s early machines seems hardly recognized even yet by some engineers ; and we have alternators o n the market to-day which are fo r a given performance decidedly more expensive than those which he designed ten years ago . The armature frame in the Brown type of machine was only a skeleton cast iron frame fo r clamping the laminations to gether , and was provided with large ventilating holes ; while the ends of the armature coils stood out from the laminations , quite

- free and exposed to the full windage of the magnet wheel . The f numerous holes gave excellent cooling e fect , but they reduced

f o f the strength and sti fness the frame , so that the armature

- had to be stiffened by a series o ftie rods o r struts . This con

o f struction , which saves material at the expense labor , has

G S o n become standard with erman and wiss firms , though account of its unsightly appearance it has never found favor 1 f Fi . in this country . This t ype o alternator , shown in g , has retained practically its original form up to the present time and developments that have taken place have been mostly in the

Ferranti type . American and English engineers have followed the Ferranti ALTERNATOR DESIGN 6 COMMERCIAL ALTERNATOR DESIGN

Fi . 2 type shown in g , and made the armature frame stiff enough to stand without bracing . The trouble with this Construction was originally the poor ventilation of the armature . Ventilating spaces were either not used , or if they were , there was no proper circulation of air through them . The end connections on the armature winding and the ends o f the coils were packed tight

- together , or were closed in by cover plates permitting no ventila tion . Thus , the armature winding was usually the hottest part of the machine ; so that even allowing temperature rises of 4 ° 5 C. , these machines could not be rated as they should , solely on account of the poor ventilation . When American and E nglish engineers took up the revolving field type of alternator, F the badly ventilated erranti type was adopted , and the great importance of ventilation was not recognized , so that the de velo pm ent of alternator design in America and England has been comparatively slow . T he improvement in ventilation which has recently taken place in this type o falternator is really the greatest forward step that has been made , and it has given the designer immense help in increasing the outp ut of machines . 4 i F . F . 3 shows an old , badly ventilated armature , while igs g —— F . and 8 show a more up to date well ventilated machine . In igs 4 and 8 it will be seen that where the ends of the armature coils

s ne cros o another they are separated by an air space , and that t the end covers are provided with ventila ing holes , to allow a r circulation of air around the coils ; thus the armatu e winding ,

f o f instead o being the hottest part the machine , becomes the coolest . The armature core is well provided with vent spaces both at the centre and at the ends , and the air passing through

is these vents is free to escape at the back o fthe core . Th type of armature coil has the additional advantages that if lightning

o r gets into the machine a coil is burnt out , the damage is con

o n e n o t fined to coil , so that we do have half a dozen burnt out l Al i as usual y happens . so , all the coils on the armature are al ke

and made on the same former , so that the question of spare f ff coils is simplified . The dif erence in the cooling e ect between

8 COMMERCIAL ALTERNATOR DESIGN

f n o t o n these dif erent designs may seem to be much paper , but ° it results in the difference between a temperature rise o f 45 f and o n e o o n actual test . It means that we need only take into c o nsideration efficiency and regulation in designing a machine , knowing well that if these are satisfactory we can

o f o n - guarantee a temperature rise even low speed machines . When designing any machine we have the choice o ftaking a

a o f large di meter and making the machine short , or taking 3.

FIG . 5.

n f small diameter a d making the machine long . The di ference in

6 . the cooling effect between these two is obvious from Figs . 5 and

Fi . 5 In g the machine is small in diameter and long , the poles are

close together and the winding crowded , while all the heat from the field- coils has to be dissipated from the small exposed surface f Fi . at the ends o the coils . In g 6 the alternator is large in

u o f field diameter and short , and practically the whole s rface the f o r . coil is available cooling In addition , the field coils being e a f rise , giv s e will e sily o fset in addition The difference in

6 FI G . .

a construction w ith armature and mag 3 and 5 and one w ith armature and 4 F . 6 magnets as shown in igs and is so obvious , that it is quite

surprising to find the poorly ventilated type still on the market . The only inference to be drawn is that the firms building them

n have o t given the subj ect due thought .

S ec alio n - I . n a r p s the electric l part of the design , the fi st thing to be decided is the specification to which the machin e 10 COMMERCIAL ALTERNATOR DESIGN

firm is to be built . The with which I am connected has adopted as standard : ° A temperature rise of 35 C . on a continuous full load run ; ° 50 - A temperature rise of 50 C . on per cent over load for two hours ;

' A o f5 7 - regulation to per cent on non inductive load , accord ing to the size of the machine ; And gives a guarantee that all machines will without damage give continuously 25 per cent current over-load at zero power factor . Detail Desi n —G g iven the specifications , we have next to decide what diameter we shall make the machine What magnetic densities to take in the iron$ What current density in the conductors $ What percentage of the pole pitch shall the $ $ pole face be What air gap Of course , these questions can

ff- f only be answered o hand as the results o experience . But generally speaking we can , after a few trials , decide on the best design . We have only to consider the efficiency , the regulation , and the cost ; as with a good design the temperature need not be considered . The efficiency of a machine w ithin ordinary limits practically depends on the magnetic densities in the iron and the current densities in the copper . The higher the densities , the cheaper and the less efficient the machine . The copper loss in the armature is 1 2 usually between and per cent . Apart from the efficiency this 5 7 is decided by the regulation , because if we allow only to per 1 cent voltage drop on P F , we cannot well have more than

2 f lo w - per cent o this as C R drop . This means that in speed machines with a large number of poles , the current density in the - few armature is very low , while in high speed machines with poles the current density can be much higher . In practice it 12 varies from 00 to 3000 amperes per square inch . The iron

no t densities do vary much in standard machines , as the most economical densities are very fairly constant and independent of the speed ; and any attempt to obtain higher efficiencies by de r creased i on densities will increase the cost rapidly . COMME RCIAL ALTERNATOR DESIGN 1 1

The best ratio of po le face to pole pitch is largely a matter of F ffi 70 E M. . co e opinion . If it is large , say per cent , then the cient (the Kapp coefficient) is reduced , and the total flux of the machine increased and hence the magnets made heavier . On the other hand , with a wide pole face we have more teeth to - m carry the flux , and for a given tooth density the achine is

- shorter . But the armature core plates are correspondingly l deeper , so that the on y saving is a slight decrease in the length f of mean turn o the armature winding . The larger the per centage of pole face to pole pitch the greater the magnetic leak to i age , and a certa n extent the less the synchronizing power of the alternato r . So that there is little to be gained by much a variation of this ratio , and it seems dvantageous to keep it T o w 6 0 6 5 . , say between and per cent

The air gap is decided by the regulation of the machine . An immen se amount has been written on various theoretical metho ds

i a of calculat ng the regulation of alternators , but broadly spe king the regulatio n depe nds on the ratio of the ampere-turns on the

- - armature to the ampere turns for the air gap . In an alternator the armature conductors are cut by magnetic lines due to the

- i . e . armature current , , the armature self induction flux ; and by

n - magnetic li es due to the magnet current . But the self induction of the armature varies w ith the magnitude of the current in u i the armat re and magnets , and with the r relative position ; while the useful flux due to the magnets varies with the current in the armature and field , on account of the varying perm e i o f ab lity the iron and the magnetic leakage . So it is obvious the conditions to be taken into account are so complicated that

e o w it becom s quite impossible to treat them the retically , ithout a so m king many assumptions that the results , even when n obtained , can ot be directly applied . What a designer has to do is to work theoretically through a few simple special cas es himself and the results will give him an idea on what lines to work . Then by means of experimenting on a number of ma chines he can develop an empirical method for calculating the regulation . Afterwards as he gets more and more experience 12 COMMERCIAL ALTERNATOR DESIGN

n with alternators , he introduces further refi ements , and taking the regulation curves obtained by empirical methods he corrects them a little by eye . ’ o f Speaking generally from a designer s point view , an alter nator should be calculated fo r a certain regulation o n a lo w

- F F r 0 . o i power factor load , say P , if the mach ne is satis

- f r factory on low power factors , it will be satisfactory o non

n o t . hi inductive loads , while the converse is true Other t ngs

- being equal , the larger the air gap the better the regulation

- o n the low power factors . But the leakage coefficient o f the

18 1m o rtan t machine an p factor , and this increases with the air

i o f gap . Th s coefficient is , course , taken into account in drawing the n o load saturation curve ; but we have to remember that o n full load o flo w power- factor the leakage coefficient is much

o n o f increased (the leakage is often doubled) , account the additional ampere- turns required o n the magnets to overcome the demagnetizing ampere turns on the armature . So that if f the leakage coe ficient is already high , and if the density in the magnet iron is also high , we run a considerable risk of saturating the magnet circuit so that we cannot obtain the rated voltage o n

f l - loads o a o w power factor . It was this trouble that caused — inductor machines to become obsolete for low power factor loads , as they are particularly sensitive to leakage and are always

. S n o w orked at high densities peaking generally , if the load leakage coefficient o fa machine is higher than and if the

ih . density in the magnets is greater than lines per sq . , the designer has to be very careful o r he will be in difficulties . The regulation on non- inductive loads is not affected by the length o fthe air-gap to the same extent as the regulation o n lo w

— S o fo r power factors . machines which are only intended lighting o r rotary converter work usually can be economically designed with a smaller air- gap and higher densities than machines fo r motor work . In Europe practically every alternator sold has to operate

fo r F F motors , so that the regulation either P or for P

0 has to be guaranteed . In America , on account of the COMMERCIAL ALTERNATOR DESIGN 13

indt ctio n patent situation , i motors are used only to a limited

o f e extent , and as a result this it has b come standard practice

h o n n o n - to sell mac ines a regulation guarantee for inductive ,

f r . rather than o inductive loads This is very unsatisfactory . Almost every load that an alternator has to carry is to a certain

e . o extent inductive , g. , arc lamps , transformers on light l ads ,

o o rectifiers , induction m tors , and synchr nous motors , unless the

o f excitation is carefully adjusted . And as the regulati n o an alternator on P F is usually about twice as bad as o n

1 o o . a u w o P F , it is obvi us that a m re s tisfactory g arantee uld

c o be to give regulation o n indu ctive lo ads . Pra tically the nly exception to this is the cas e of an alternato r fo r use exclusively

w o o for running ro tary co nvert ers . And even ith a c mp und

o c t o - o wound r tary converter and an indu ive line , the p wer fact r is usually lo w and the c urrent lagging fo r light lo ads ; w hile if

o to o the t the r taries have be started fr m alternating curren side , a generator with po o r regulatio n o n lo w power-facto rs is very

o not iceable and may give tr uble .

It is extremely diffi cult. to meas ure the regulatio n fo r P F I o n c w t o o t o w o n any ma hine i h g d regula i n , hile a large machine

c o s s T he c n o be s it is practi all y imp ible . result a nl y figured a

f w b t o are. s o the di ference etween large quantities , and there many disturbing features that the result when o btained is no t

. e as to a very accurate On the other hand , it is quite y me sure the regulatio n o n a very lo w po wer- factor by loading on to a

o c o o o o o n e sec nd machine running as a syn hr n us m t r , the first operating as a generato r and varying the excitatio n of the mo tor and generato r till full l o ad current is flowing at full -lo ad

- c w voltage . The po wer fa t o r in such a test ill be very low and

c t o . can , with sufficient accura y , be aken zer Alternato rs can be des igne d s o as to satisfy a clo se regulatio n specification fo r n o n-inductive lo ads and yet be almo st wo rthless fo r o fl w - s carrying loads o power factor . S o that a such machines can be made cheaper than if they were required to give a

o n reasonable regulation inductive loads , there is a temptation fo r manufacturers to take advantage o fthe fact that the regula 14 COMMERCIAL ALTERNATOR DESIGN

o n 1 to o n f tion is only guaranteed P F , install e o these cheaper machines . It is probably this fact which is responsible fo r the number o f alternators having poor regulation o n lo w

- power factors , that have been installed in this country . It

’ would certainly be an advantage from the customer s point o f

o f view , and probably in the end from that the manufacturer

fo r f l also , if the regulation were guaranteed a load o o w

- power factor . This would make it necessary from the co m m ercial standpoint to alter somewhat the lines o n which modern

o f alternators are designed , but the cost the machines would

n o t . necessarily be much increased A modern alternator gives ,

7 o n P F 1 22 n say , per cent regulation , and per cent o P F When operating with a normal power- factor o f

o f 17 n o about and a regulation about per cent , it does t help the station engineer to know that if he had a n o n - induct ive load he would have good regulation . Such a machine could

re— o n f r s o to 6 be designed somewhat di fe ent lines , as have per

o n P F 1 12 o n P F cent regulation , and per cent and about o n e per cent lower efficiency without increasing the cost appreciably . Such a machine would be much mo re satis factory fo r general work and could probably be sold fo r co n s id erably more than the machine designed only fo r wo rk o n — n o n inductive loads .

o f Other things being equal , the regulation an alternator is

to better the more saturated the magnet circuit , and this applies

- low power factor loads as well as to high . It can be considered

o r simply as an experimental fact , the explanation can be accepted that the voltage drop in an alternator is partly due to

o f f the reaction the armature ampere turns , and that the e fect o fa definite percentage change in the ampere- turns is less when the magnets are saturated than when they are n o t . Obviously

r o f r to the pa t the magnetic ci cuit saturate is the magnet core , as the less its cross section , the less its perimeter and the less the f weight o fthe magnet copper . In the type o magnet shown in i Fi . 7 g , it is mpossible to saturate the pole core , and the large amount o firon and copper necessary always makes this design

1 6 COMMERCIAL ALTERNATOR DESIGN

o f a the radiating surface , increasing the depth the m gnet wind ing and hence increasing the length of mean turn o fthe magnet coil somewhat at the same time . It also slightly decreases

- fo r i the ampere turns the magnetic c rcuit . If we use a large

o f pole pitch , giving plenty space between the poles , together

, with a short armature and high peripheral speed , we can easily avoid the increase in temperature due to decrease in radiating f surface . S o that the limiting factor in reducing the length o

F I G . . 7 the pole core becomes the additional weight o fcopper due to the

o f o n increased length mean turn the magnet winding , caused f by the extra depth o the winding . With good design we can usually reduce the length o f pole core to about o n e inch fo r every 1500 ampere turns required o n full load ; so that o ur leakage coefficient will generally n o t exceed which is n o t excessive . T o show the effect o fthese various points o n the design o fa machine , let us take a definite example . COMMERCIAL ALTERNATOR DESIGN 17

W 1 2 22 7 00 . 750 K. . 6 0 00 Output cycles , poles , volt

E 95 lectrical efficiency at full load , % Regulation 7% fo r P F 1 16 % for P F 25% P F 0 °

d P F 1 30 C. Temperature rise on full loa , Armature , °

20 . Magnets , C This low magnet temperature being adopted so that the temperature will not become excessive with the increased los ses which will result when operating on loads of

- low po wer factor . A is a machine which has a pole pitch and diameter large

us e u . enough to round poles , and has sat rated pole cores It r has a strong a mature an d strong magnets .

a B has a sm ller pole pitch , so that it is a longer machine , and has un saturated fields . It has a weak armature and field , and the magnet winding is crowded .

Inte rnal diame te r o farmature . f Le ngth o armature co re . Po le pitch Air gap

l . ee r . Pe riphe ra s . f t pe min Slo ts per po e T urns pe r co il M e t co re sectio n

In no o n in e t co e e r s . in . ti r , p q Re gulatio n P P F . 8 P F 0 ’ Lo sse s magne t C R . . 2 Armature C R . . Iro n lo s s Effi cie ncy T e mpe rature rise o farmature

magne ts . We ight magne t co ppe r po les

whee l . armature co ppe r laminatio ns

frame . f Co s t o ab o ve mate rial . . 18 C OMMERCIAL ALTERNATOR DESIGN

S o on the principal items that enter into the cost of material , 25 the saving is about per cent , due to the use of strong armature and magnets , large pole pitch , and short saturated magnet cores . Probably the saving o fthe cost of the complete machine would 2 2 be about 0 or 5 per cent . The designs o fthese two machines are a little exaggerated , but they show very well the saving in cost that can be made . C is the same machine as A but designed with a weaker

so to o n lo w armature , as have better regulation , especially

- power factors , at the expense of a lower efficiency . The cost is about the same . The chief points fo r a cheap design fo r an alternator with good voltage regulation on inductive loads are strong , saturated

- magnets , a reasonably large pole pitch , and as large an air gap as can be used without excessive leakage . In machines of small output with a large number of poles , it is impossible to get a really economical design . The diameter is decided by the number o f poles and little is gained by making the machine less than

5 6 n o t or inches long , so the cost is reduced very much with the G output . enerally speaking , if the output of the machine is 1 KW 0 . . less than per pole , the design is unnecessarily expensive , while machines in which the length of the armature is about

equal to the pole pitch are usually the most economical . It is

fo r 6 0 fo r this reason that cycle alternators small outputs , and 120 or 133 cycle alternators o f all outputs are usually made belt -driven ; the saving in cost by doing this often being 50 E 50 ‘ per cent . In continental urope , where cycles is the usual

- practice , belt driven alternators have never met with much favor ; the universal custom being to direct- connect the alterna

- tor to a low speed engine . The result of this has been that the fly-wheel type o f alternator has practically become standard

for this work , and the poles of the alternator are simply bolted

f - to the rim o the fly wheel . This type allows considerable saving in cost in small 50 or 6 0 cycle machines and possesses so many other advantages that it is being introduced into this

country . COMMERCIAL ALTERNATOR DESIGN 1 9

‘ — S tea m T urbine Diiven A lt ernators Alternators fo r direct connection to steam turbines have lately come into prominen ce ;

h in e o the c ief consideration these machines b ing , of c urse , the high speed at which they operate . In order to prevent the

o f o length becoming excessive , the diameter a turbo alternat r

o f o is made as large as possible , and peripheral speeds fr m

c c to feet per minute are adopted . The me hani al

o f co n stresses in the metallic parts the magnets are high , but a

- s ervative factor . o f safety can be. maintained if high grade materials are employed . The greatest difficulty consists in arranging the mechanical stres ses o n the insulatio n of the ro t o r in such a way that the insulatio n is no t damaged . If this is no t o o f o o done , and if the winding and c nstituent parts the r t r are n o t firmly fixed so that relative mo tio n canno t take place

c ro under the influence of the entrifugal forces , continual t uble f will result due to changing o balance .

The electrical design o fa turbo alternato r is mu ch the sa me.

' - o f c c . e as that a belt driven ma hine , ex ept that the speed b ing very high the efficiency is go od ; s o that the magnetic and cu rrent densities in the armature are limited o nly by magnetic saturatio n and the difficulty o fdissipating the heat in the extremely lo ng armatures used o n these machines . The po le pit ch and the

o n . o f ampere turns the armature being large , the magnets are

o necessity very strong and the air gap large , so that the questi n

o of magnetic leakage becomes imp rtant . In turbo alternat o rs

’ lo w - m th just as in speed machines , saturated magnets armature

o and magnets as str ng as can conveniently be adopted , result in economical designs ; but as the mechanical co nditio ns are so

o o much more severe , g od mechanical design has a more imp rtant

o n Is lo - c influence the cost than the case in a s w speed ma hine . — Genera l Co mpariso n When co m m encmg to wo rk o ut a machine an experienced designer can usually estimate the mest ‘ ' economical diameter to adopt ; and he knows from e x pe x iiin ce approximately the number o f ampere-turns he can take per inch periphery o n the armature fo r an alternator o fgiven po le pitch and type . This decides the number o f turns on the 20 COMMERCIAL ALTERNATOR DE SIGN

— FIG . 8 . r e W f 2 W . o 75 K. 6 00 3 s e 6 0 A matur inding , Pha , c l e Ge e r o Cy n at r . COMMERCIAL ALTERNATOR DESIGN 2]

- armature and the ampere turns o n the magnets . He then

completes the first rough design , and working out the perform

r l ance cu ves , he can usua ly see very quickly in what way to modify it so as to obtain the best design possible under the

circumstances . The speed and frequency are the chief factors

o f In deciding the design a machine , but the voltage , the con

itio ns o r d of operati n , the equipment of the facto y in which it is i t o be bu lt , the facilities for obtaining castings and for shipping

w f the completed machine , all are points hich a fect the design

bo ns idered and have to be by the practical designer , since the prime object in a commercial design is rather to make profits for the manufacturing company than to produce the most perfect

‘ i to r mach ne . The points which have be taken into conside ation are so numerous and varied that it is impos sible to give general

. o rules fo r practical design . All that can be d ne is to give general directions and then it is a question of ability and experience

n u til the engineer can produce the best results .

f r . N o . E . L s . eglecting a moment the de igns of Mr C Brown , the greatest change in the des ign of alternators in the last ten years is the impro ved ventilation and the increas ed magnet

1893 o air- strength . In we were w rking with gap densities of to and magnets with to ampere-turns

d l —d a - per pole on full loa , whi e to y we have air gap densities of to and magnets giving anything up to

- e r o d - ampere turns p pole on full load , for r inary belt driven or

cn in e t e r a - g yp alte nators , and up to double this on ste m turbine

driven machin es . The change has been made so gradually that ff it has been hardly noticeable , but the e ect can be seen if I give

des i ned the dimensions on two machines g and tested , one in 1894 1903 and the other in , this latter machine being shown

in Fig. 8 . Both the designs are typical of the condition of the

alternator design at those dates . 22 COMMERCIAL ALTERNATOR DESIGN

W. 3 s e 70 K. Pha O 275 K-W 3 se utput K S P Pha W. e h . i50 . ingl i M P. . M S e e . P 6 00 R . . p d 6 00 R . Cycle s 6 0 6 0 T o f e s ff T ype magn t Lau e n ype . S tandard Re vo lving T Fie ld ype . ” ” Inte rnal diame te r armature 37 38 Le ngth o farmature laminatio ns 10 5 10 Armature ampe re -turns 1380 1050 A mpe re-turns o n magne ts 4700 7500 ll l o e f c e nc er ce n 9 0 94 Fu ad fi i y , p t Re gulatio n P F 1 1 1 % P F 8 n o s 14 . Wo uld o t give v lt % Full lo ad te mpe rature rise o far 2 ° mature 3 C . 6 80 T o a l e o fco e r lb s . t w ight pp , T o tal we ight o fmachine 1 1000 T o tal co s t o fmate rial $490

(T w o b e aring machine and 15C . co ppe r)

The output has been increased about four times and we have hi as a much better mac ne regards performance , while the cost is about the same . The older machine having such a low out

a put needs large amount of unnecessary material , to reduce the losses so as to give a reasonable efficiency . While in the larger output machine we can afford a considerably higher loss without reducing the efficiency , so that the weight of mate rial is little more ; and the better mechanical design has made the cost of material in the two machines about the same . Speak ing generally , the whole result has been accomplished by using stronger magnets and higher densities throughout , which are possible on account of the improved ventilation . Alternating current design has rather stagnated o f late on

o f account of the limited competition , and most the recent developments have come from the other side of the water . But

o f o f I think it has been shown that , from the point view dollars and cents , it is certainly worth while to spend time and ability somewhat lavishly in designing alternators .

Q4 DOUBLE —CURRENT GENERAT ORS

load , having its maximum during the day , and the lighting load its maximum in the evening , the result is that the alternating current sets are idle during the day and the direct current sets are idle in the evening , so that we have only about half the plant fi in use at o ne time . It is obvious that a saving in rst cost and in operating expense will be made if the tw o systems are tied

s o o ut o n e together in some way, that they can help another at times o f heavy load . This can be done by having both an a — lternator and a direct current generator coupled to one engine , o r v — o r by ha ing double current generators , by tying the supply

- circuits together by rotary converters and motor generator sets . A modification o f the latter method which has recently be come popular consists in installing alternating current gen erato rs only , and providing rotary converters to transform a portion o fthe power to direct current . $ From the point o f View of the station engineer the double current generator should be the best solution . The efficiency is higher than when rotary converters o r motor generator sets

to are used , and it ought be considerably cheaper than either o fthe other methods . The objection to it is that the voltage o n the alternating current side o fthe generator bears a definite

to o n - so o ne ratio that the direct current side , that cannot be varied without the other ; while the variation o fthe load on one f side a fects to a slight extent the voltage o n the other . The

as o f relative importance of these objections h , course , to be F ’ decided in each individual cas e . rom the manufacturer s point o fview the objection to double- current generators is that they are special machines , and usually require new designs and

r special patterns o tools .

- Of course , we can take any direct current generator , provide

to it with collector rings , and use it supply alternating current ; but the difficulty is usually that the frequency is unsuitable .

r bii ilt fo r An alte nator can be any commercial frequency , speed ,

f fo r r - or voltage , without any serious di ficulty ; but a di ect current generator , given the speed , voltage , and output , the question of commutation and cost decide withinnarrow limits the number DOUBLE—CURRENT GENERATORS 25

an d . a of poles , hence indirectly the frequency If it is necess ry to change this number of poles cons iderably in order to obtain i e aspecial frequency , there is often d fficulty with the d sign , which res ults in in creased cos t . So if so me latitude can be allowed in - selecting the frequency and speed for double current generators ,

o it is advisable to cho se them so that , if possible , the generator does not differ very much from some standard direct-current machine .

the The following table , gives the frequency of alternating current that could be obtained from standard direct- current generators . The number of poles and the speeds of the machines are N E taken from those of the ational lectric Company , but there is little difference in these respects between the machines o f be the various manufacturers , so that they can regarded as applying approximately to all standard makes of generators .

S 'ru u T l ' a n m s - - G $ B I $ . E N I N E DR I EN . ELT DR E N v Dm z x .

W t in . K. . Ra g

o 0 250 t 2 50 . o o . 50 l . 500 o . 50 0 l 5 0 o . 2 o t V t V lt V t V lt V l V l .

Considering as standard frequencies for double- current 25 40 an d 6 0 generators , , , it is evident that some of these stand ard direct-current machines could be very conveniently used as

- w double current generators ith only a slight change in speed . A W 2 K. . 50 standard , volt engine type machine would make 25 - a good cycle double current generator , the only chan ges nec

- essary being to provide collector rings , and to increase the air gap and put more copper on the field magnets to make the regulation satisfactory when operating as an alternatin g-current — 26 DOUBLE CURRE NT GENERAT ORS

n o t generator . These changes would increase the cost of the 2 0 . generator more than per cent On the other hand , if we took i W. 5 500 K. 00 a , volt engine type mach ne , very radical changes — would be required to make this into a 25 cycle double current n generator , as it would be necessary to i crease the number of

10 24 o r 30 i l poles from to , wh ch wou d practically double the

i . K. W cost o fthe mach ne But if we make this 500 . machine

— d - - belt driven instead of irect connected to a slow speed engine , it is evident from the table of frequencies that a standard 2 machine would be satisfactory for 5 cycles . Twenty-five or forty cycle machines are not in any way — difficult to build at the most it is a question of special designs and patterns . But with 6 0 cycle generators we begin to have difficulties with the commutator on account of the high periph

- . S 6 00 eral speed ixty cycle , volt , double current generators and rotary converters can undoubtedly be built , but at the present date they are not such reliable . machin es as those for lower frequencies , and there is no brush gear now on the market which is quite satisfactory fo r the peripheral speeds necessary

6 0 6 00 . in a cycle , volt machine The higher the speed o f a standard direct- current machine fo r a given output , the higher the frequency ; thus we would expect that the higher the frequency of a double- current genera tor the higher the speed at which it should operate ; and it appears from the table that the most satisfactory 6 0 cycle double-current generators will be those driven by steam

fo r turbines . Direct current generators direct connection to steam turbines and suitable fo r operation under American co n ditio ns , are not at present on the market , but in all probability they will be shortly ; and it appears probable that this type o f machine will solve the problem of 6 0 cycle double—current generators . G 25 - enerally speaking then , cycle double current generators , — if of large size , can be direct connected to a steam engine , while K - W. . 500 . for smaller units than , they are better belt driven Forty cycle machines should always be belt-driven if the cost DOUBLE - CURRENT GENERAT ORS 27

i fo r 6 0 — is to be reasonable , wh le cycle double current generators apparently the only reasonable solution is to have steam turbine - driven sets . Of course , double current generators can be made f r 6 0 l 6 00 o any frequency or voltage up to cyc e , volts , and at i l any speed des red , it is on y a question of cost ; but to obtain a

n reasonable price or delivery , and to have a u it which will at some time in the future have a second- hand value better than

a the scrap , it would be advis ble to consider above table of frequencies and outputs when laying out a station for double

rr current supply with do uble cu ent generators .

D 19 1 NOTE w . 0 [ , ]

In the past three years great improvements have been made in the cons truction o fcommutato rs an d brush gear for operation at high speeds ; this work having been done in connection with development of 6 0 cycle rotaries and direct-current turbo as generators , which should be reliable in operation as the cor 2 - i responding 5 cycle and slow spe ed units . The result of th s work has been to revolutionize the methods of cons tructing high speed commutators and brush gears , so that at the present time

e s commutators are being built to operate p rfectly , with rea on

at ui 6 0 c clc able attention , the speeds req red by v rotaries and

- t . e direc current turbo generators This being the case , the stat b 6 0 6 00 ment above , in regard to the relia ility of cycle volt double-current generators and rotary converters should be modified accordingly .

PREDETERMINATION OF S PARKING I N DIRECT CURRENT MACHINES

I G art SPEAK N generally , dynamo design did not become an until after the old two pole smooth- core Siemens and Edison

The machines came into exte nsive use for electric lighting . original design of these machines was more or less guesswork ; but

h had e after a few mac ines been made to op rate satisfactorily , the l h engineer was able to lay out a complete ine of mac ines , design

his r ing them partly by enginee ing intuition , and partly by some

e i as empirical rul s , wh ch he decided on he built successive

s . me h ni machines . The armatures were de igned more from a c a t n d cal than from an electrical standpoint , their leng h bei g limite by the stiffness of the shaft rather than by qu estions of com mutation ; while they were unventilated , and in consequence the output was limited by hea tin g . The armatures being of the o - - smo th core type , the self induction of the armature coils w as . so usually small , even with the length of armatures in general use , that it was unnecessary to consider it in connection with the

. I tendency of the machine to spark t was , however , generally recognized that if the magnets were too weak the machine was

- liable to spark , so the length of the air gap was usually deter ri mined by some empi cal rule obtained by experiment . When slotted armatures were first adopted extens ively they were de m - m signed along the same lines as s ooth core ar atures . They were so badly ventilated that the output was limited by heating to abo ut one-half that of a modern armature ; but in spite of 29 30 SPARKING IN DIRECT CURRENT MACHINES this it was found necessary to use carbon brushes to obtain good

T o ff commutation . economize in tools several di erent lengths o f o n armatures were frequently built the same diameter , while to economize space , the armatures were often built smaller in E diameter and longer than they otherwise would be . xperience with these different forms of armature made it very evident that a long armature had a greater tendency to spark than a short o n e ; and this became especially noticeable as the ventilating was improved and the output correspondingly increased on f account o the cooler operation . Previous to this a great deal had been written on the theory

rac of commutation in dynamos , but had been ignored by the p tical designers , who had more faith in experimental results .

But this bad behavior of long armatures as regards sparking , called attention to the theoretical work , and designers began to consider whether o r not the self-induction of the armature coils

n o t did , after all , decide the amount of current the machine would commutate without sparking . In the first attempts to

- take into account the self induction of the commutated coil , the self-induction o f a one- turn coil was considered as being simply proportional to the length of the armature core ; that is , the shape or size of the slot , the number of coils per slot , and

- t he . self induction of the end connections , were all neglected T his gave a very simple formula fo r the self-induction : z L n l . Where l length of armature

n number of turns per coil , And the self- induction o fcommutation (the reactance

o f voltage as it was called) , which is an estimate the difficulty of commutating the current , was given by 2 E I n i f i being the current per coil and f the frequency of commuta tion . This formula gave satisfactory results when applied to ma

chines designed along the same general lines . The allowable value o fthe reactance voltage could be obtained from experiment

32 SPARKI NG IN DIRE CT CURRENT MACHINES

to o n e induction of a wide slot compared a narrow , and it was soon acknowledged that the shape o f the slot should be con sidered - o f in calculating the self induction the commutated coil . When designing an armature fo r small self- induction it would be natural to make it large in diameter and short in length ; that

- . i to is , with a large pole pitch But in carrying th s an extreme

n o t it was found that it did give the good results expected . It was suggested that this result was due to the fact that the self

o f induction the end connections had been neglected , and that in

— o f armatures with large pole pitch and short length core , the self- induction o fthe end connections was comparable with that of the conductor embedded in the slots . In the light o fthese experiences it is evident that the design of a direct - current machine in regard to sparking is a compromise

n between a number o fconflicting conditions . It is o t possible to obtain a formula which will give a strict measure o fthe com mutating qualities o fall machines ; but by taking into considera tion the more important conditions which affect the sparking , it is possible to obtain o n e which will give fairly accurate results when applied to machines similarly designed , and which will give some idea o fthe tendency to spark when applied to machines f ff o . S widely di erent design uch a formula , when it has been

to f applied numerous machines of di ferent types , so that the

fo r allowable values the sparking constant have been determined ,

r can be taken as a fai working formula , and can be placed in the same category as empirical formul ae fo r determining the regu

f rm l ar n lation o falternators . Such o u m e o t intended to reduce

to - designing mere slide rule work , but are intended simply to give an idea as to the experimental results to be expected from an individual design .

As outlined above , the most important conditions to be taken into consideration are the self- induction electromotive force o f the commutated coil , and the inequalities introduced by the conditions o f commutation .

- Electro mo tive F o rce Due to S elf I n duction . This is given by — the formula : E Self induction of one coil $ number o fcoils SPARKING IN DIRECT CURRENT MACHINES 33 commutated in series ; x current in coil $ frequency of co mm u

The self-induction of one coil (self- induction of one con ductor embedded in the slot self- induction of one end conn ec tion) $ (number of turns per

- The self induction of one conductor embedded in the slot lk . Where I is the length of the core and k is a constant depending on the dimensions of the slot . By determining the se lf- induction of a large number of slots

fin d s k f we that thi constant can , with su ficient accuracy , be taken as a function of the ratio r , where

W dth of slot .

Depth of slot .

can n T k A curve be plotted connecti g and , determined ex perimentally from tests on a number of armatures ; such a c urve is shown in Fig. 9 . 3 The se lf-induction of the end connections can be taken as ’ s length of end connections x constant c . And a the length of m end connection is approxi ately proportional to the pole pitch ,

e - the s lf induction of two end connections can , with sufficient

r 0 . accuracy , be w itten p 2 - n” l Hence the self induction of one coil ( k p c) . l N u The number of coi s commutated in series , , is , of co rse ,

- one in a parallel or lap wound armature , and equal to the number

- of pairs o fpoles in a series or wave wound armat ure . The current per coil i in a two-circuit series or wave- wound m - ar ature is equal to one half the total current in the machine , while in a parallel or lap- wound armature it equals the total current divided by the number o fpoles . The se lf~inductio n pressure of the commutated coil is then given by

the Where f is frequency of commutation , and is equal to number of commutator bars $ speed in rev . per min . 3 34 SPARKING IN DIRECT CURRENT MACHINES

The width of the brus h is neglected in calculating the fre

uen c o f q y commutation , since it is found by experiment that within the ordinary limits o fpractice the thickness o fthe brush

o n o f i has little effect the operation a mach ne , unless the current density is excessive . The probable explanation of this is that a thicker brush gives more time for commutation to take place ; but it also means that more coils are commutated at the same

9 . FIG .

ff . time , thus introducing the e ect of mutual induction These two effects apparently counterbalance each other to a great

extent . — I nequalities Due to Co nditio ns of Co mmutatio n These are due to the use of few slots ; more than one coil per slot ; and to

dead coils . If there is only one coil per slot the use of few slots does not SPARKING IN DIRECT CURRENT MACHINES 35

ff u l m is in itself a ect commutation , n ess the nu ber of slots ex tre mely small; fo r though the slot may move through an ap

r ciable i l e t p e arc wh le the coi is b ing commu ated , the conditions are exactly the same for every coil when it is commutated .

n So there is no tendency to inequality in the conditio s , and if it is possible to set the brus hes so that one coil can be co m m utat l ed satisfactorily , then commutation wi l be satisfactory for all

sa the coils . But if the number of slots is extremely small , y less than six per hole , then the coil will move in such a widely varying magnetic field , and will come so close to the strong field under - u the pole tip while it is being commutated , that the local c rrents under the brush are liable to produce marking of the commuta

- to r bars even if the brus hes apparently do not spark . Of course this is only important in very low voltage machines and it is unnecessary to take it into account in any constant which is to be a criterion of the tendency to spark . It is sufficient to say

- - that the number of commutator segments in the polar gap ,

are - that is , the between the two pole shoes , must never be less than two and should generally be three or more .

With more. than one coil per slot inequalities are introduced : due to the difference in the value of the se lf- induction of the various coils ; and due to their commutation under different conditions . The self-induction of all the armature coils will be the same v when there are only two coils per slot , as it is ob ious that the configuration o fthe conductors and neighboring iron is the same for both coils . But when there are three or more coils per slo t

- w l as the self induction of the various coils i l vary , they occupy different relative positions in regard to the iro n ; the self-induc tion of the center coil being less than the self-induction of the outer coil . Investigating conditions at the point of commuta

n - tion in a modern generator by mea s of a pilot brush , it is found that commutation usually takes place at a po int where there is practically no resultant magnetic field ; that is , at a po int where the magnetic field of the armature just counterbalances

. In the average field due to the magnets other words , there is 3 6 SPARKING IN DIRE CT CURRENT MACHINES resistance commutation ; the armature current is commutated r by the varying resistance of the , rather than by a eversing $brush to due passing through a magnetic field . This being the case it is only necessary to consider the self-induction

; 1 —S n r r Be a E n d o f FIG 0 . ho wing Po s iti o o fA matu e at ginning nd a r Co mmut tio n Pe i o d .

- are those coils which have the greatest self induction . If these commutated satisfactorily by means o f the varying resistance o f -in the brushes , then the coils which have a smaller self H th duc tion will also be satisfactorily commutated . ence e variation in the self- induction o f the coils need n o t be co n SPARKING IN DIRECT CURRENT MACHINES 37

s ider d u c s e , and in the form la all that it is ne essary to con ider is t he self-induction of those coils which have the greates t self

The chief inequality introduced into the commutation by the adoption o fmore than o ne coil per slot is due to the various

S S o o s o o f Fm . r T w o s r l o r a e l l . A mature with o C il pe t h wing P iti n A m tur

r whe n the T w o Co ils a e be ing Co mmuta te d .

coils in the slot being commutated w hen they are in different

. Fi . 11 w magnetic fields This is evident from g , hich shows the po sition of the armature when the first and the last coil in the slot are being commutated . If the brushes are set so that the magnetic field is right for the first coil it will be incorrect for the 38 SPARKI NG IN DIRECT CURRENT MACHINES

last one , and vice versa . So whenever the machine is loaded to its limit the commutating conditions may be so bad fo r some of

o f - the coils , that in time some the commutator bars will become pitted and the well - known regularly recurring marking o f the

- commutator bars will develop . The question is how to take this inequality into account in the sparking formula . To do this , we make the assumptions that the magnetic field varies uniformly from the neutral point

- to the pole tip , and that in order to obtain perfect commutation

12 FIG . . it is necessary to move the brushes from a position on the neutral

- point at no load , to a position half way between the neutral - b point and the pole tip at full load . Calling the distance e

e— 2 d tween the neutral point and the pol tip , and assuming the brushes fixed on the line 0 P half-way between the pole- tip and the neutral point , then if any coil is commutated when it is $ ” a P at Q distant from , the machine will only commutate —a perfectly a current corresponding to 1 o f full load . d

13. $ust what this assumption means can be seen from Fig.

40 SPARKING IN DIRECT CURRENT MACHINES

to n slots corresponding variation from no load to full load , he ce an equality o f slot - pitch gives an inequality factor

s o that the sparking constant should be multiplied by the inequality factor

Curves can. very easily be plotted fo r different numbers o f

o to l sl ts per pole and coils per slot , in order facilitate the ca cu f o . Fi 1 . lation this inequality factor Such curves are shown in g. 5 The assumptions o n which this calculation is based are to a great extent rational , and though we cannot pretend that the

— r ur T n S l o s e r o le T re e o ls r S l o A mat e with we ty t p P and h C i pe t .

calculation has a rigid basis , yet it is probably as correct as the other sparking calculations , and used with discretion it gives

. fairly, reliable results The inequality introduced by the use o fa dead coil o n the armature is similar to that due to several coils per slot . The dead coil produces a break in the uniformity o f the winding ; and if the position o fthe brush is correct fo r commutation o f the coil immediately o n o n e side o fthe dead coil then it will be o n e o u t o fthe fo r segment correct position , the coil immediately

n f o the other side o the dead coil . The inequality introduced

fo r can be calculated , and allowed , in exactly the same way as 4 1

S o k s in position to the dead coil . ma ing the same a sumptions

n -S before ; if there are commutator egments per pole , and if

S l o ts per Pole

FI G . 15 .

75 - face cent of pole pitch , then the inequality 8 2 Thus if there are 0 segments per pole , a n 40 dead coil produces an equality equal to per cent of the load , and the inequality should be introduced into the sparking co n stant by the factor A curve can readily be plotted between 4 2 SPARKING IN DIRECT CURRENT MACHINES the inequality factor and the number o fcommutator segments

o . F i . 1 per p le Such a curve is shown in g 7 . — Ge neral F o rmu la Co mbining all the various factors which affect sparking we get as o ur complete formula fo r a sparking constant

P being the inequality fact o r resulting fro m a number o f

l6 — D . e s f e e FIG . Armatur with e ad Co il S ho wing Po s iti o n s o Armatur wh n th T w o s x D a r u a e Co il Ne t t o e d Co il a e b e ing Co mm t t e d .

o to coils per slot , and Q the c rresponding factor due the presence f o a dead coil .

n o t This formula is put forward as being scientifically exact , but as an empirical formula which has gradually been built up SPARKING IN DIRECT CURRENT MACHINES 43

o f if e as the result experience , d ferent terms having b en added to the formula from time to time when it was found necessary to take different. conditions int o acc o unt . As the

w e formula stands it gives good results , when know the value o fC which can be allowed fo r the particular design o f machine considered . The relative values o fC that have been fo und allowable in different cases are so mewhat as follows

2- po le

4- tw o —c c pole , series ir uit winding 6 — pole ,

4—o lc w o p , multiple und

fi—o p le , gradually increas ing to

24- o w o p le , multiple und

o f o s o These relative values , c ur e , nly apply when the machines

- in o n . each class are designed with similar c stants That is , they

o sh uld have approximately the same densities in the teeth , and appro ximately the same ratio o fampere turns per pole on the armature (armature reac tio n) to the ampere- turns required fo r If the teeth and air gap . these vary much it is difficult to get

T he r - co nsistent results . b ush gear and the current density in the brushes also play an impo rtant part in the sparking . If the

- o r the co in brush gear is weak mechanically , if mmutator is

o to w bad c ndition , sparking is sure take place ; while ith the

o w s average carb n brush , burning ill u ually take place when the current density reaches 50 amperes per sq . in . The shape

- o fthe po le tips has some effect o n the operatio n of the machine .

to o But as long as they are not close together , and as long as they are shaped s o that the commutation field varies

l . gradua ly , the exact shape need. give us no concern The dens ity in the armature core (behind the teeth) has also some effect o n the allowable sparking constant ; and if the core is highly saturated a higher constant can be used than if it is unsaturated . 4 4 SPARKING I N DIRECT CURRENT MACHINE S

Assuming that all these conditions are u nifo rm and s atis fac

o f tory , the variation in the allowable value C , found in actual n o t practice , shows that the formula does take into account all f s o o the conditions that a fect the sparking , the f rmula must be

— - m a o r S e e s er o le c . 1 7 . o FIG . C mut t gm nt p P pit h

used with considerable discretion . It cannot be claimed that

it is in any way accurate , but it can be considered an empirical

o f w working fo rmula , capable giving good results hen carefully

used ; and as such it is put forward . ROTARY CONVERTERS AND MOTOR-GENERATORS

AT the present time the alternatin g c urrent motor in a

- W 100 K. . o r motor generator set of capacity larger , is usually a synchronous moto r ; an induction motor is seldom used for h n this purpose . The reasons for this are , t at the laggi g current taken by an induction motor renders it undesirable at the end

in of a long l e , that from an operating standpoint the mechanical construction o f an induction motor makes it less reliable than a synchronous motor , and that the cost of an induction motor has be en heretofore appreciably higher than that of the co rresponding synchronous motor . The usual objections to — the synchronous motor that it has —a low starting to rque and that it requires external excitation do not apply to the

o f in - case a synchronous motor used a motor generator set , as a high starting torque is unnecessary and as there is always some way of exciting the motor whether it is coupled to a direct current or to an alternating current generator . It has thus become practically standard practice to use synchrono us motor generator sets in all sizes except where the output is too small for a stan dard synchronous motor . This being the case it is only necessary to cons ider synchronous motor- generator sets in comparison with rotary converters . Motor-generators and rotary converters can be dis cussed from

n two points of view ; that of the operating engi eer , or that of the f designer and manufacturer . As the operating point o view is probably most familiar to engineers , that will be considered first . 45 — 46 ROTARIES AND MOTOR GENERATORS — Co s t and F lo or S pace The main points that concern the engineer when installing transforming machinery are the first f cost o the machinery , its efficiency , and its reliability and

flexibility of operation . Incidentally , the floor space occupied , and sundry other things have to be taken into consideration .

o f - o r o r The cost a motor generator rotary converter , rather the price at which it is sold by the manufacturer , depends upon the output and the speed , and incidentally upon the competition

among the firms that are trying to secure the business . The choice of the speed fo r either machine being usually left to the manufacturer, is as high as is consistent with good mechanical and electrical design . The following table gives speeds in

R . P . M . which may be regarded as more or less standard for such

o f ff . machines di erent output , frequencies , and voltages

25 CYCLE S

- M T R N RAT R S . R TA RT RS O O GE E O O RY CON $ E E . K W . .

250 o s . 6 00 o s . 2 50 o s 6 00 o s . V lt V lt V lt . V lt

6 0 CYCLE S

- M T N AT S . R TA Y C $ T O OR GE ER OR O R ON ER ER S .

s $ . 2 0 o l . 6 00 o s o s 0 o . 5 V t lt 2 50 V lt . 6 0 V lts

In comparing the cost of motor-generators and rotaries it may be assumed that it will always be necessary to use trans formers with the latter in order to get the comparatively low

— 48 ROT ARIES AND MOT OR GENERAT ORS

volts without transformers . The efficiencies are the combined efficiencies of the sets ; rotaries and transformers in the one case , and motors and generators in the other . In the case of rotaries ,

flo o r- under the head of cost and space , the first figure refers to the rotary and the second to the transformers .

K 2 5 T 2 Y E S 1500 . w 7 O S 5 V L , C CL

Ro ar Co r e r Mo o r- e e ra o r t y nve t . t G n t .

318000 6 300 $24300 c 5 E fficie n y lo ad 93 . 1 93 . 5

240 125 34 5 2 l . f . f F o o r s pace sq t 3 0 s q . t .

The above are sale prices f . o . b . factory and do not include All freight o r erection charges . necessary rheostats and shunts

fo r are included , but no induction regulators the rotaries . In all three cases it will be n o ticed that the rotary converter f and transformers are the more efficient , the di ference in efficiency being about 3 per cent at full load and about 6 per cent at half f load . The value of this di ference in efficiency has to be decided

- in each case by the cost of producing the extra kilowatt hours . In a water-power plant the efficiency is an unimportant feature ; im in a steam plant , where the cost of fuel is high , it is quite portant . The floor space taken up by a rotary converter and its transformers is about 25 per cent greater than that taken up by

tw — - o f a o bearing motor generator set . The floor space is only importance in the case of a sub- station in a city where real estate is valuable , and in such cases the transformers could be placed in a gallery over the rotaries if desired . As the rotaries them selves only take up about tw o - thirds the floor-space of a motor generator set , the advantage would , with this arrangement , be with them . — Operating Characteristics The relative desirability of rotary converters and motor- generators from the operating point of — ROTARIE S AND MOT OR GENERAT ORS 49 view depends upon the question of their reliability and flexibility

- of operation . As regards flexibility , the motor generator is of

- course by far the better . With motor generators the power factor of the motor may be adjusted to unity , or if desired a leading current may be introduced into the line without affecting l the operation of the motor , whi e the voltage on the direct current side may be adjusted within wide limits either by us e N of the shunt rheostat , or by compounding . either of these adjustments can be applied conveniently to a rotary . In a rotary converter the ratio of the voltage on the direct current side to that on the alternating current side is practically

n ff constant ; that is , any drop or rise of voltage on the li e a ects proportionally the direct current voltage of the rotary . In attempting to regulate the power factor of the rotary or to in tro duce lea ding or lagging currents into the line by varying the

field strength , we are liable to alter the alternating current ff i voltage at the end of the line , and hence to a ect the d rect

' cm re nt voltage of the rotary . Variation of the shunt current within wide limits is also objectionable , because it often has a tendency to produce hunting . Theoretically a rotary converter can be compounded or

- o r - over compounded , rather the line can be over compounded , producin g the same effect on the direct c urrent terminal voltage

- as if the rotary itself was over compounded . This may be done.

- by introducing an artificial self induction into the line , and producing by means of a series winding on the c o nverter fields a leading current approximately proportional to the lo ad on the ro tary . This leading current will then raise the voltage of the

- line because of the self induction present . This compounding is .

s o however , at be t a rough method and can be used nly on sys m te s in which exactness of voltage is unimportant , as it is some what difficult to adjust the self- induction and the series winding i ff to give the requ red e ect . With this method the power factor of the rotary and that of the system are also varied within wide limits as the load varies and a change of voltage affects all other r machine y on the line . So that the cases are limited in which — 50 ROTARIES AND MOTOR GENERATORS this method can be used to regulate the direct-current voltage o n a rotary . A method o fvoltage control w ith rotary converters is used o n some o f the Edison systems . An induction regulator is inserted in the alternating current circuit between the trans m for er and the rotary , and is usually controlled from a switch board by means o f a pilot motor . Such a regulator increases

o f 20 the cost the apparatus about per cent , decreases the total

1 o n e- efficiency about per cent , adds about third to the floor space required by the transformers , and introduces additional complications into the system . Usually , therefore , the necessity fo r employing an induction regulator is a strong argument against the use o frotary converters in the particular installation considered . A rotary converter is more liable to hunt and to flash over on short circuits , and is a somewhat more complicated piece of

- . s n apparatus than a motor generator set On the other hand , a y chronons motor wound fo r a pressure of over volts is n o t so reliable as a transformer wound fo r the same voltage . Generally

o f $ o f speaking , from the point iew reliability of operation , there is little choice between 25 cycle rotaries and 25 cycle

- 6 0 advan motor generators , although in a cycle installation the f - tage is decidedly in favor o the motor generator . There is no doubt that satisfactory 6 0 cycle rotaries can be made up to

6 0 o f25 0 volts , but their design is more difficult than that cycle rotaries so that it is natural they should require more attention , — than motor generato r sets . Motor- generators have another advantage over rotary con

v erters n o t . r in that they are so liable to hunt Of cou se , hunt ing can be prevented , but not usually without introducing some

o n corresponding disadvantage . Dampers may be placed the

o f ef pole faces , but with the disadvantage causing some loss in ficien cy o r extreme uniformity of engine speed may be obtained at the expense o fa heavy fly- wheel ; while the small line drop that is usually found necessary fo r the reliable parallel operation o frotary converters requires considerable expense fo r condue ROTARIE S AND MOTOR- GENERAT ORS 51

r tors . Synch onous motors are not so liable to hunt as rotary

‘ converters , and the conditions that are good enough to insure the satisfactory parallel operation of alternators are usually all that are required to prevent hunting in synchronous motors . ’ F - rom the operating engineer s standpoint , a motor generator is preferable to a rotary converter in almost every respect , except as to efficiency and cost ; and even as to cost a motor generator is the cheaper for low voltages and large outputs .

Consequently , when comparatively cheap medium size units are wanted , and close voltage regulation is unimportant , rotary

. r converters are used But when large units are desi ed , and

r the voltage regulation is impo tant , as in incandescent lighting ,

- motor generators are employed . This applies to both 6 0 and 25 cycles .

So n far , rotary converters have only been co sidered for tran sforming from alternating current to direct c urrent . In

r regard to inverted rotary converte s ; that is , rotaries for trans m u for ing from direct current to alternating c rrent , almost the

. I a same remarks apply n ddition , however , inverted rotaries are subject to another disadvantage : that the power factor of the load on the alternating current side affects the magnetic

re flux , and in consequence the speed and frequency of the tary . A heavy inductive load on the alternating current side

. h tends to make the rotary run away T is , of course , can be i prevented by an automatic speed lim t device ; or , to a cer tain extent , by separately exciting the rotary from an under saturated exciter which it drives mechanically , or by making the armature o fthe rotary very weak in comparison w i th its field

. i magnet But these devices are makesh fts , and none of them can keep the speed absolutely constant under these conditions . And when the speed varies it also causes variation in the speed u of all induction and synchrono s motors , driven from the rotary , i which is highly objectionable . Th s , combined with its other faults , makes an inverted rotary usually less desirable than a

- motor generator . — 52 ROT ARIES AND - MOTOR GENERATORS

n o Ro tar o nverte s and Mo to r-Gen er — Desig f y C r ators . Generally speaking , and within reasonable limits , the higher the speed of

o f a machine the less is its cost . so that it is to the interest the TE manufacturer to run at as high a speed as possible . (See NO

fo r p . The permissible speed any alternator is limited

only by mechanical considerations , while the maximum speed at which a direct-current generator o r a rotary converter of a given output can be run is limited by the operating charac

teristics in regard to sparking . Given the approximate speed at

- o f which a direct current machine will run , the number poles

which it should have , is fixed within narrow limits by questions f f o sparking and economy o design . As the number of poles

and speed determine the frequency , it is easily seen how the

o f fo r o f choice speed which a rotary given output , frequency ,

and voltage may be built is limited .

KW 25 6 b e . 00 Suppose a . , cycle , volt rotary is to designed : to insure the cheapest machine the number o fpoles

must be as few as possible so that the speed can be high . There

to are amperes to commutate ; this , a great extent , deter t o f . o u mines the number poles On laying the design , it is

8 o r 10 12 found that poles will suffice , but that poles will be

L 2 . . more conservative . et 50 rev . per min be decided upon

- 21 in . Assuming a pole pitch of , we obtain an armature diameter

f 24 tw o f o 80 in . o , slots per pole , coils per slot , length armature

core in . The slots are comparatively narrow , only in .

- s o e . wide , that solid pol faces or copper dampers can be used 1 This makes a very good machine . Fig. 8 shows such a

machine .

- W. K. 6 00 If a , volt motor generator is to be designed , the most suitable number o fpoles o n the direct current generator

o n can be decided , and the speed may be made as high as is

consistent with good commutation . The speed may be as high f 300 . . o as rev per min , but , as in the case the rotary , a more conservative machine would result if the speed were kept down

2 . 50 . to rev per min Twelve poles is a suitable number , and as a somewhat better sparking constant is required than in a — ROT ARIES AND MOT OR GENERAT ORS 53

i w . rotary , the armature is bu lt ith a slightly larger diameter

16 o An o f86 . w armature in diameter , ith sl ts per pole and three

w o f n o t m coils per slot , is satisfactory . The idth slot is li ited , as

- to laminated pole faces are be used , so that a wide slot can be adopted with its consequent reduction in the self- induction of the commutated coil . This gives an armature length of

i . n . , and also makes a very good machine

1 . K. W. 6 o 8 . 00 l 25 c le o r o e e r FIG V t , Cy R ta y C nv rt .

This is o ne instance in w hich w e find that the rotary co n

- w verter and motor generator ill operate at the same speed .

o f In this case , the cost the rotary and transformers will be less

- than the corresponding motor generator set .

o n K. W 2 2 . 75 5 Suppose , the other hand , a , volt , cycle to machine is be designed . It is found that the minimum number of poles fo r a rotary o fthis output and voltage is about 54 ROT ARIE S AND MOTOR - GE NERAT ORS

2 2 0 . 1 0 50 . . This gives a pole rotary at rev per min , with an

1 . 1 24 r 30 3 . o n e armatu e in diameter , in long , slots per pole , and

o c il per slot . The c o rresponding generator fo r the motor- generato r set 25 1 0 . may be run at rev per min . by making it with 8 poles . s : 1 14 10 . . o o Thi gives armature in diameter , in l ng , sl ts

tw o f . o per pole , and coils per slot In this case the cost the mo to r- generator will be less than that o f the corresponding

o o n o f i r tary and transformers , account h gher speed at which

o - o - the mot r generat r can be run . Such a motor generator is

F 1 . shown in ig. 9 ’ o Speaking generally fr m the designer s standpoint , there is little to choo se between the difficulty o f designing a rotary converter and that o f designing a direct current generator o f

o f the same output , speed , and voltage . The design a rotary is subject to more limitations than that o f a direct current f generato r . The number o c o mmutato r segments per pair o f

o f poles must be divisible by the number phases , and the number o f o f slots per . pair poles should preferably be also divisible by f the same number . Also the relative dimensions o the slot and air gap are limited by the fact that eddy currents must be

- o r avoided in the solid pole faces , copper dampers , which are

o to . usually empl yed prevent hunting On the other hand , the absence o farmature reaction in a rotary converter is a consider able po int in its favo r as regards tendency to sparking . Investigating the conditi o ns o f c o mmutation in a direct

o f current generator by means a pilot brush , when the machine is o perating at full rated output with brushes set in the normal

position , it is generally found that resistance commutation is

. o taking place ; that is , the brushes are advanced just far en ugh fo r the armature cross magnetization field to neutralize the direct field due to the magnets at the p o int o f commutation .

o n As the load the machine is increased , the increased armature reaction causes the resultant field at the point o fc o mmutation to bec o me o fthe oppo site sign to that which wo uld be required fo r to perfect commutation , thus tending make the brushes

— 56 ROTARIES AND MOT OR GE NERAT ORS

n t same machine as a generator . The sparking does o appear to increase s o rapidly in a rotary converter when the load is f raised . The result o this is that the sparking constant in a rotary is usually permitted to be about 25 per cent higher than

o f in a generator , and in consequence this a rotary converter

fo r may be designed a lower peripheral speed , and with a longer armature core than the corresponding generator .

n - o There being o resultant . armature reaction in a r tary converter , some manufacturers design such machines with a high armature - reacti o n and lo w volts per commutator bar ; that is , with a strong armature and weak field . This design requires less material in the construction and tends to lower the

n cost o fthe machine . But this saving is o t s o pronounced as o n e might think at first sight , as the labor cost is increased quite materially when the number o fcoils o n the armature and the f o . number commutator bars are increased In addition , a strong armature is n o t conducive to good o peration in a rotary converter ; it tends to make the brushes flash badly o r even to

t o r flash over when star ing from the alternating current side , when hunting , and it also reduces the synchronizing power of

. o f$ the machine Considering the question from all points iew , it is usually found most satisfactory to design rotary converters with about the same armature reactio n and volts per bar as the corresponding direct current machines . — Ro tary Co nverter A rmature Win dingfi The copper loss in

' the armature o fa polyphase rotary co n verter is usually consider

in the n ably less than correspondi g direct current generator , so that such rotaries are often designed w ith a much smaller

- cross section o f c o pper than would be used in a generator .

This is bad practice . The copper loss in a rotary converter

n o t armature is equally distributed , the loss in the bars nearest the collector leads being usually much greater than in those f midway between the leads . And although the di ference in temperature at the end o fa temperature run cannot generally f be detected by a thermometer , the di ference is very appreciable

s : in the case o fsudden overload at a lo w power factor . And case — ROT ARIES AND MOTOR GENERAT ORS 57 are on record where the armature bars connected to the leads In a large rotary have bee n fused before the other bars got

F r dangerously hot . o heavy railway work the section of the copper in a rotary armature ought to be at least equal to that

n . in the correspondi g generator As regards heating , six phase rotary converters have an advantage over two or three phase , but as they result in extra complications in the cables and sw itchboard they are seldom employed except in large units . In any case a six phase rotary may have to operate

r m th ee phase at so e time , so it should be designed simply as a three phase machin e with three extra collector rings . This m e being the case , the re arks before made in regard to the s ction of the armature copper and heating of conductors also apply to six phase mac hines . One advantage sometimes claimed for the six phase rotary is that having a greater number of equipote ntial connections a is on the armature than either a two or three ph se , there a greater tendency towards equality in distribution of current between the various sets of brushes on the commutator . This would be true if the collector rings were the only equipotential connections on the armature . But in addition to the collector r a rings , the a mature winding of the modern high speed rot ry is usually provided with an equipotential connection for every 6 0 25 second slot , while in cycle or large cycle rotaries where the commutating conditions are more severe , one connection to every

. So re slot is frequently employed it is obvious that , in this spect at least , the six phase rotary as usually constructed pre sents no advantage over the two or three phase . Undoubtedly the larger the number of phases in the armature winding of a rotary converter , the more accurately and uniformly do the alternating and direct currents in the conductors neutralize one another , and in consequence , the less the pulsation of the armature reaction and the less the variation in the commutating conditions . But with the modern well propo rtioned rotary

i s converter , th s variation and pulsation is not a seriou matter , so that it should not influence the choice of the number of phases — 58 ROTARIES AND MOTOR GENERATORS any more than the necessity for additional equipotential con n ectio ns should . An important feature in the design of the armature of a rotary converter is often overlooked , and that is equality or balancing o fthe phases . If the windings o fthe different phases

’ on a rotary armature are not all exactly equal , and placed on the armature in an exactly similar and symmetrical position with regard to one another, then the phases will be unbalanced , with the result that the load will not be balanced among the phases , and that there will be a greater tendency to hunting . If the three ammeters in the three phases of such a rotary are

a watched , the load can be seen changing from one phase to n other . This is the reason why rotary converters with series wound armatures are usually unsatisfactory . It is often im

. 6 r possible to balance the phases A pole , th ee phase rotary 224 having a series wound armature with coils , must have the — — rings connected to coils 1 26 51 . An 8 pole three phase rotary 520 having a multiple wound armature with coils , must have 1—44— its rings connected to coils 86 . The phases of both these armatures are unbalanced . To have the phases perfectly balanced the number of commutator bars per pair of poles

o f should not only be divisible by the number phases , but the number of slots per pair of poles should also be divisible by the

f r i ff same number . The reason o th s is that to have the di erent phase windings all symmetrically and x similarly placed on the armature , all the coils that are connected to the phase leads must be in the same relative positions in their slots .

n in tar on verters an d S nchro no us M t r Hu t g of Ro y C y o o s . Rotary converters are more liable to hunt than synchronous f . G o motors enerally speaking , conditions operation and design that will enable two alternators to operate satisfactorily in

o n e parallel without hunting , will also enable of them to operate satisfacto rilv as a synchronous motor when driven by the other machine under the same conditions . A single rotary driven from an alternating current system , will not operate under given conditions quite so well as regards hunting, as a motor — ROTARIES AND MOTOR GENERATORS 59

r generator . The armature reaction of a rotary converte is considerably higher in proportion than that of a synchronous i . n motor Therefore , its sy chron zing power is less and the fact that the direct current side is so in timately conn ected to the li l n e cu ari . alternati g current side , makes it p y sensitive to hunting But the main difficulties with rotaries in regard to hunting are experienced when two or more rotaries are runnin g in parallel o n n the same alternating current system , and feedi g into the same direct current system . These difficulties are especially marked when the rotari es are running in parallel upon the same

n - alter ating current and the same direct current bus bars . Under similar conditions motor-generators are no more sen sitive to hunting than under the conditions of sin gly operated

' ffi r . the di culties units But with rotaries are often so se ious , that operatin g engineers have found it necessary to insist that manufacturers provide dampin g or an ti-hunting devices for all

n d machines intended to be co necte in this way , and also to install artificial choke coils between the collector rings and the

- alternating current bus bars , in order to limit the interchange of current . Artificial damping devices of various types and forms have

s use been tried , but the only one in exten ive at the present time A is a heavy grid of copper embedded in the pole face . nother construction now in use for accomplishing the same result ,

so h r consists in solid pole faces s aped , and with the armatu e slots and air gap so proportioned , that eddy currents due to

as the teeth are avoided . As far can be seen from practical s n operation , the e two methods of preventing hunti g seem ff ri equally e ective . They both enable rota es to be run in t parallel satisfac orily , as long as the variation in speed of the engines and the pressure drop in the feeders is not excessive . As regards the relative effects of a copper grid damper and a solid pole face upon the efficiency under working conditions , it is difficult to speak with any degree of accuracy as so much depends on the uniformity of speed of the engines . But it is

s - probable there is a constant los in the copper grid damper , — 6 0 ROTARIES AND MOTOR GENERAT ORS

when , as is often the case , it is used with large armature teeth and small air gaps . Solid pole faces require a larger air gap o r i narrower slot , wh ch in turn demands more copper on the 6 0 magnets , this being especially the case in cycle converters .

o f$ On the other hand , from a mechanical point iew it is a nuis ance to have to attach auxiliary copper grids or dampers to

o f any pole face , while the cost the dampers themselves is not

. G insignificant enerally speaking , then , there is little choice

- between solid pole faces and auxiliary dampers , so it would

o f - perhaps be best to advise the use solid pole faces in all cases , as they are simpler and more mechanical . — Mechanical Design Though rotary converters have been

i o f used for qu te a number years , yet their detail design seems to have received less care than has been given to generator

- design . A rotary in a sub station carrying a street railway o r interurban load is usually subjected to rough treatment , and consequently should be of robust design . All parts should be as accessible as possible in order to facilitate repairing . Two features in the design o frotaries that are often faulty

ear ' an d are the alternating current collector g leads , and the starting resistance used for starting from the direct current side . The alternating current end of a rotary is often designed sim ilarl - y to the old revolving armature alternators ; that is , the leads are strap- copper soldered to the armature conductor and

- attached to the armature end plate wi th a few cleats . While

- are the collector rings mounted solid upon the shaft , separated b and insulated by fiber or wood discs and ushings , the leads being embedded in this insulation where they pass through o r

Fi . 20 n . are co nected to the rings This construction is shown in g , and it is hardly necessary to state that it is unsatisfactory for S heavy railway work . olid copper leads , unless very carefully are and solidly cleated to the armature end plates , liable t to break due to vibration , while soldered joints are liable to mel

- under overloads . Collector rings mounted solid on the shaft are liable to break down due to warping or cracking of the to insulation , and to get hot on overloads due poor cooling ROT ARIES AND MOT OR - G E NERAT ORS 6 1

n e facilities . Also whe the rings are insulat d by wooden discs projecting between the rings they cannot be easily turned o ff

o r when they become cut grooved by the metal brushes . The most substantial c o nstruction is probabl y to use cable for the leads and to connect them to the w inding by special lugs riveted

- and silver soldered to the armature conductors . The rings should be carried on arms projecting from a spider and should be freely

— 20 R o ar o n e e r r e o lle c o - s FIG . . t y C v rt A matur and C t r Ring .

o fo r o o s c pen to the air c ling , and be ea ily a cessible for turning ff o whenever they become gro o ved o r uneven from wear . In fact they should be designed exactly like the collector gear o fan up

- - fie ld o to date revolving alternato r . When this c nstruction is adopted a temperature rise o fover 15 degrees is rarely attained

- o n o o . o 2 . n rmal l ad Such collect r rings are shown in Fig. 1 — 6 2 ROTARIES AND MOT OR GE NE RATORS

Regarding a starting resistance fo r starting a rotary or a

— fo r motor generator from the direct current side , it is often gotten that such work is much more severe than starting a o r o - to to motor . A rotary a mot r generator has be run up speed a nd t hen synchronized , and when synchronizing the speed has

2 — o lle c o r- s o n o r o r 1 . A . FIG . C C t Ring R ta y C nve te r .

o n t o be exact . If the voltage the direct current system is vary

to ing , it often requires several minutes synchronize , and if it is varying suddenly , the speed can only be adjusted by means o fthe starting resistance because shunt control is n o t quick — enough . Starting resistances fo r rotaries o r motor generators s hould be designed s o that the last steps can be kept in circuit

— ROT ARIES AND MOT OR GE NE RAT ORS can only be decided after e verv feature o fthe situation has been duly considered . Broadly speaking , however , the tendency

— - to day is toward motor generator sets in lighting systems and

o n rotary converters traction systems . This seems to be per haps the mo st rational conclusion .

- s c n Motor generators and rotary converter , all things o s idered to , are more difficult design and build than the ordinary

- - standard engine type generator . They are high speed machines and usually operate in sub- stations where the conditions o f

and o n o t f operation the supervisi n are o the best . The question o f their reliability in continuous operation should therefore receive the most careful c o nsideration from the designer and manufacturer ; and as suggestions and criticisms from operating engineers are o f the utmost value they should always be

to o r welcomed and investigated , in order determine whether n o t they contain features valuable enough to warrant the modification o fstandard designs .

NOTE

In the past tw o years the standard speed fo r rotary co n verters - o a and motor generat rs has appreciably incre sed , this being due partly to improvements in mechanical design ; partly to the development o f c o mmutating poles fo r direct current generators and t o a certain extent fo r ro tary converters ; and partly to o ur increased knowledge o fp o wer system phenomena

to o f o and the education power stati n engineers , which has per m itted closer commercial designing . The present commutating pole units require careful adjustment , but , after adjustment

to o are much less sensitive operating c nditions , and require less attention and maintenance .

in creas m o f As stated above , g the speed any unit tends to decrease the cost , though this is true only within certain limits . This is exemplified by the direct current generators which are n o w built fo r operation at the very high speed required fo r

to o n direct connection steam turbines , and whose cost is , account o f the mechanical difficulties o fconstruction due to the — ROTARIES AND MOTOR GENERAT ORS

l

high speed , appreciably greater than that of the corresponding medium speed unit . Given the approximate speed at which a

co m m utat direct current generator is to operate , and adopting a ing pole construction , the number of poles which the machine

o f should have is fixed within narrow limits by questions cost , f ll . w e care u v efficiency , and . mechanical design If design a

DC . o A . O. unit of given rating , whether , or (with c mmutating

o f f the poles) , for a number dif erent speeds , and figure efficiency

ST ANDARD ROT ARY CONVE RT ER AND MOT OR GENE RAT OR S PEE DS (19 10)

25 C YC LE S

- $ s O Y ON $ . MO TOR G E N E R A T OR . R TAR C ERTER

K. W .

0 s 6 00 $ o t s . 25 o t . 2 s 6 00 o s . 50 Vo lt . V lt V l l

6 0 C YC LES

o f in and cost manufacture for each design , we find that with creased speed the efficiency improves and the cost decreases till

a o n a cert in speed , dependent the rating , is reached ; and that u if the speed is still f rther increased , the efficiency is reduced and the cost increased . There is a similar relationship between

o f the number poles , and the efficiency and cost . The smaller

enerall the number of poles , the lower the frequency and , g v speaking , the larger the flux per pole , and in consequence the

. w heavier the magnetic circuit On the other hand , hen the number of poles is excessive , the frequency is high and the 5 6 6 ROT ARIE S AND MOT OR -GE NE RAT ORS diameter o f the armature becomes very large ; while the cost o f the armature and field winding , and in a direct current ma

o o f s chine , the c st commutator and brush gear , also increase

o f rapidly . The most economical and efficient number poles

K 0 2 e e . 2 . 6 0 l r r . l 3 . W. o 5 c e R o o FIG , V t , Cy ta y C nv t r

to o fo r ' an o f ad pt y machine given speed and rating , whether

A . o r C . . D C , can be readily determined by a few trial designs , and any change in this number usually results in increased

cost and decreased efficiency . D Adopting a modern commutating pole design fo r the C . — RIES AND MOTOR GENERATORS 6 7

re nerato r set of given rating , we can select the n ber o fpoles and speed without any restrictions mentioned above . But with a rotary converter

the o f fixed , so that number poles must vary

Ie speed . This restriction is more serious the

Ic y, as we are dealing with a smaller number of is often easier to develop an economical design o o rat r than it is for a rotary converter . The ents in commutating pole designs fo r direct have allow ed an appreciable increase in speed

' o r o sets compared with r tary converters , so but o time the comparis n in cost , efficiency , and vecn tw o t v es o f these p units is , if anything ,

- 0 the motor generator than it w as in 1905.

[Pres ented beore the A f neers Ma 28 1906 ngi , y , ]

SHUNT AND COMPOUND WOUND ROTARY CONVERT ERS FOR RAILWAY WORK

T HE ques tion of shunt or compound excitation for rotary convert ers has been discussed so much that it is of interest to compare the relative merits of the two . As the S hunt

m an d s winding is obviously the si plest , cheapest , mo t convenient

l to way of exciting a rotary converter , it wou d be well begin by considerin g why a compound winding is ever used . In a direct current circuit it is often an advantage to have a system which is to a great extent self-regulating as regards s voltage . This is especially the ca e where the load changes

r e - t f equ ntly , and in such cases a compound wound genera or is

w fi ld- used . A series inding on . the generator e coils tends to make the voltage at the generato r terminals rise as the load h i comes on the mac ine , and th s rising characteristic is employed to counteract the increasing voltage drop in the feeders and mains due to in creas ing load . By changing the series winding on the generator the rising tendency of the terminal voltage can be varied to almost any extent . — Compo unding ofEate ries I n a rotary converter the ratio of the voltages at the terminals on the two sides of the machine is approximately constant , and independent of the load or the magnitude of the excitation ; that is , when transforming from alternating current to direct current , the direct current ter minal voltage bears an almost constant ratio to the alternating l current terminal voltage under all conditions . So the on y 6 9 70 SHUNT AND COMPOUND WOUND ROT ARIES way of varying the direct current terminal voltage is to vary the alternating current voltage supplied to the machine . If the direct current voltage is to rise as the load comes on the u rotary , then the alternating c rrent voltage which is supplied must be made to vary in the same way . Assuming that appro x i mately constant voltage is supplied at the generator end of

the alternating current feeders , and that the circuit between the generator and the rotary converter contains sufficient self induction , then the voltage at the rotary converter end of the alternating current line can be raised or lowered by intro du in c g a leading or a lagging current into the system . A leading or lagging current can be introduced into the system by over o r under exciting the rotary . S o by putting a series winding on the magnets of the rotary the excitation will be in

s a ro x i crea ed as the load comes on , and a leading current pp mately proportional to the load introduced in the alternat ing current system . This will tend to raise the alternating current voltage supplied to the rotary and , in consequence , the f terminal voltage at the direct current side . By means o this r system , a rota y converter can be compounded in a manner similar to that employed for compounding a direct current

o r generator , . rather the line can be compounded so as to cause the voltage at the direct current terminals of all the rotaries on the line to increase as the load comes on . Here then is a system that gives autOm atie control of the voltage as the load varies . Such a system is obviously ex trem el nf t y useful and convenient . U or unately it presents a — e . . a number of disadvantages in practice , g series winding is required on the rotary magnets ; it is practically always necessary to insert artificial self- induction in the alternating current line , so as to bring its reactance up to the required value ; and extra switchboard arrangements are required . This results

in ffi . increased complications and cost , and a loss of e ciency A compound- wound rotary costs about 5 per cent more than a - 5 shunt wound , and reactance coils usually cost about per cent

h eflicienc of the cost of the rotary , while t e y of the system

72 SHUNT AND COMPOUND WOUND ROTARIES

instead of over or under- compounded the result is more satis

o f factory , as the natural tendency the solid poles or dampers

c . is to hold the magnetism and voltage onstant Unfortunately ,

- however , small sub stations are usually supplied from com paratively small power stations ; and with a varying load on the rotaries one of the main effects noticed in the sub-station n is the fluctuation of the speed of the e gines , and consequently i t r in the speed of the rotar es and the al e nating current voltage . The effect of this variation in speed Often completely masks ll a results from the compound winding .

In addition , this method of compoundingis often a nuisance . There must be careful adjustment o f the $series winding and — o f the reactance coils at the sub- stations before obtaining the

ff r desired e ect . Then if the rotaries a e changed to another a station , or if the line conditions are changed$ the djustment

' has to be made again . If other co mpo undiw o und : rotaries are - tr to installed in the same sub station , there is ouble adjust E them so that they divide the load properly at all loads . qual iz er r connections are used , but the results are ra ely altogether ‘ f th i . o tar e satisfactory The dif erent characteristics of er s, and the variation in the brush- contact resistance Or temperature of

- .If the machines all tend to upset the adjustment . Shunt wound ar fi rotaries are not dividing the load properly , v ying the eld rheostat will quickly adjust the load . And in any case shunt - h e wound rotaries , like shunt wound generators , av , a much greater natural tendency to divide the load properly than have - i d n un compound wound machines . With this automat c e i po d

- varies ~ arid ing , the power factor of the rotary system , often

- i . o n varies widely , with the load The power factor cann t be t erfered o f with , without disturbing the complete system regula

- - tion . A shunt wound rotary tends to keep the power factor the same at all loads ; and in any cas e the power-factor can be adjusted by means of the field rheostat without in any way f upsetting the regulation . The result o all the complication and disadvantages of a compound - wound rotary with reactance c 1 oils is , that , often after the system has been in operation for SHUNT AND COMPOUND WOUND ROTARIE S 73

- - some time , the series magnet coils and the reactance coils are cut out and the rotary operated as a straight shunt machin e . It is then more under the control of the operator and less liable to give trouble . — Mo st S atisfactory S ystem Probably the best system for gen

- eral work is to a have shunt wound rotary , standard transform - . S ers , and no reactance coils omewhat over excite the rotary so as - o to keep the power fact r a leading one at all times , and then a m d le ve the machines to take care of the selves , only a justing the excitation in cas e they fail to divide the load properly . In the case of a large system the feeder drop is small and the fluctuations in load are unimportant , so that the voltage is fairly constant at all times . The only work then for the sub-station attendant is to cut in or out an extra rotary as the load requires it , and to see that each machine in circuit carries approximately its proper propo rtion of the load . In the case of interurban systems operating several small

- sub stations , the voltage will vary with the load on account of the feeder drop . The direct current voltage will be high on

w Fo r light loads and ill fall on full load . a feeder line with

- sub stations at various points this is an idea l condition . When the load is light on any sub—station it will mean that most of the load on the system is being carried by other stations . The voltage will then be high at the lightly loaded stations and lower at the more heavily loaded ones , so that the lightly loaded l stations will tend to help out the heavily loaded ones , resu ting — in ideal conditions a tendency to distribute the load propor

i n l t o ate y be tween the stations at all times . The rotaries should be excited so as to obtain either unity power factor or a leading one at all loads , and then the feeder drop will automatically take care of the distribution of load , and there will be tendency always to divide the load proportionately between all the sub stations . Thus with S hunt-wound rotaries instead of compound-wound

- e machines with reactance coils , there r sults a cheaper , more efficient , and less complicated outfit , one less liable to give trouble 74 SHUNT AND COMPOUND WOUND ROTARIES and one which will give better results both in large and in small stations . It is hardly to be wondered at then that there is at the present time a tendency to make rotaries shunt-wound ; and it is probable the time is not far distant when the compound wound rotary will be considered as a type to be used only to - s e meet special requirements , the shunt wound rotary being cepted as standard for all railway work . 4 F ebrua ry 1 ,

THE NON-SYNCHRONOUS OR INDUCTION GENERATOR IN CENTRAL STATION AND OTHER WORK

THAT an induction motor could act as a generator and return power to the line when driven at a speed above that of synchro

n a . nism , has been k own for many ye rs It has , however , always s s been regarded as more or less of a cientific curio ity , and except S s in the case of the wiss three phase mountain railway , where the motors are sometimes allowed to run as generators to brake the t train on des cending heavy grades , the induction genera or has had but few commercial applications . The fact that the characteristics of this generator are such that it must receive is a lagging current from the system , the magnitude of which for a given machine definite ly decided by the slip of the generator s i c above synchroni m , comb ned with the fa t that when connected n a to a circuit it has no defi ite volt ge or frequency of its own , i make it lack the flexib lity of the synchronous generator . In 1895 it was proposed to operate an induction generator in con an s junction with unloaded synchronou motor , the generator to supply the watt component and the motor the wattless com ponent of the current in the system . But though this suggestion

n caused the induction generator to become a practical machi e , it is easily understood that , on account of the lesser flexibility

a when comp red with the synchronous generator , it has not appealed to the central station engineer as a desirable addition to his equipment . The ques tion as to the advisability of adopting the induction 75 — 76 T HE NON SYNCHRONOUS GENERATOR

fo r b generator power station work , was decided adversely y engineers at the time when the steam engine and the water

u - - t rbine were the only practical prime movers . But to day we

. have to deal with the steam turbine and the gas engine , and the introduction o f these two new types o f prime- mover has al together changed the situation in regard to the use o f this

fo r generator power station work . It often happens in such cases that the real meaning and possibilities o fthe introduction of types of machinery with such fundamentally new characteris tics as the steam turbine and gas engine , are not recognized until they are accidentally forced upon us . This has been the case in the present instance ; the question o fthe use o f the induction generator with steam turbines was not seriously considered until it was brought up indirectly . In 1904 the Baltimore Copper Smelting and Rolling Company 2 K. W . 00 was installing a , volt , direct current generator for electrolytic work . It was desirable to have the good steam economy on variable loads , the small floor space and reduced maintenance of the steam turbine , and at the same time to gen K 2 W. ra 2 r u . . 00 e te 00 volt di ect c rrent A , ampere , volt , direct current turbine generator was not c onsidered practical , and an induction generator together with a rotary converter was suggested as an alternative . It was

K. W h o f ui . decided to adopt t is type eq pment , and a ,

- M 4 R . P. 1 0 30 . six phase , volt , cycle , generator together

- 1 . . . . 5 R P KW. 0 with a , six phase , M rotary was installed to supply the required amperes direct current at 200 volts .

fo r w as A direct current exciter the rotary provided , so that the direct current voltage could be varied from 100 to 230 volts with out any danger o finstability . The exciter is compound wound to give constant voltage on the direct current side of the rotary, and the power- factor gradually rises from about 25 per cent at

96 . no load , to about per cent at full load When starting up

the set , the rotary is run up to speed from an auxiliary source

f r . o direct cu rent , and the generator by its turbine They are

then thrown together , the generator driving the rotary as a — T HE NON SYNCHRONOUS GENE RAT OR 77

z synchronous motor , and the rotary supplying the magneti ing u current of the generato r . The governor of the t rbine decides the frequency of the set , and the slip of the rotary behind the n 1 generator is proportional to the load , bei g about per cent at i n full load . Th s equipment has been ru ning now for about three years an d operates perfectly . The generator is similar - u — in arrangement to the old open type t rbo generator , and in cons equence is rather noisy , but with the modern enclosed t type of genera or, the air circulation could be better arranged , resulting in a quieter running machi is This installation given in detail , as it is one instance where the adoption of this apparently inflexible type of generator l i s x resu ted in an installation , which comb ne the fle ibility of the l i standard direct or a ternat ng current generator, with the heavy i overload capacity of the rotary converter , and the reliab lity and robustness of cons truction of the induction generator . N0 other

‘ electrical equipment which could have been installed would l have given the good resu ts that were obtained . And though the existing conditions rather forced the choice of the equip in h s ment t is case , the result so thoroughly met expectation , that engineers began to consider whether the development of

fime mo vers had the last few years in regard to p , not changed the induction generator from a scientific curiosity to a machine offering great advantages for certain conditions of power station work .

i o he I nduc tion — General Characterist cs ft Generato r . As most engineers are more familiar with the characteristics and per fo rman ce of the induction motor than they are with those of

m S the co mercial induction generator , it will be well to how how the characteristics of the two machines are allied . An induction i motor of given characteristics , carry ng a certain definite load , h runs at a speed fixed relatively to that of sync ronism , and it takes a current the magnitude and phase of which are definite . The torque exerted by the motor is proportional to the product

- of the current induced in the short circuited secondary , and the

magnetic flux . The electromotive force and the current induced — 78 T HE NON SYNCHRONOUS GENERATOR

in the secondary, and hence the torque , are proportional to the rate at which the secondary conductors cut the primary magnetic

o f o r field , that is , to the slip the rotor above below the speed of synchronism . As the speed of the motor gradually rises to

o f that synchronism , the current in the , secondary , and the torque gradually fall , till at the speed of synchronism they both

o f become zero . If the speed the machine still continues to increase , the secondary conductors cut the primary flux in the reverse direction , so that the induced electromotive force , the secondary current , and the torque become negative ; that is , the machine requires mechanical power to drive it above the speed of i synchronism . The mach ne now returns electrical power to the

. n circuit , and has become a generator When run ing as a motor , the current is never in phase with the impressed voltage , for two reasons : (1) the motor requires a certain wattless magnet 2 izing current ; and ( ) the motor windings , both primary and

- secondary , have a certain amount of self induction . This makes

curr ent ( 5 the lag behind the impressed voltage by an angle 7 ,

o f depending on the characteristics the machine , and on the load it is carrying . When the machine runs above synchronism as m a generator , it still takes from the circuit the wattless agnet

- izing current , and the circuits still possess their self induction .

The result of this is , that as the watt component has reversed ° m u s 180 in direction , the pri ary c rrent lags les than behind the voltage impressed by the circuit on the generator, hence the current leads the electromotive force supplied by the generator as to the circuit . Thus we have a fundamental characteristic o f - fo r the non synchronous generator , that a given load , it runs

o f ni at a certain definite speed above that synchro sm , and that , (a) it supplies a watt current which represents the power de r b livered by the generator to the ci cuit , and ( ) it takes a wattless hi magnetizing current from the system , the magnitude of w ch depends on the voltage , and on the watt component of the H f current . ence this type o generator cannot supply a lagging current to the outside circuit , and can only deliver power to a circuit which is able to provide the lagging magnetizing current

— 80 T HE NON SYNCHRONOUS GENERATOR

unit , while the remainder of the load is taken by this latter machine . When additional load comes on the station , it comes

first on the synchronous generator , and then , as this machine S lows down and allows the slip of the induction generator to r i increase , part of the load is transfe red to th s latter machine .

o f In any case , the voltage regulation of the system is that the synchronous unit , and the voltage of the circuit under any condition of load is decided by the excitation of this machine . If the induction generator is operating in parallel with a synchronous motor or rotary converter , the same remarks apply .

The voltage regulation is that of the synchronous machine , and the voltage of the circuit is decided by the magnitude of its

. a excitation When the energy load on the two machines incre ses , the additional load comes first on the synchronous machine , energy being supplied to the system from the momentum o fits rotating part ; and then as the machine slows down a little , the

r load is transferred to the induction generator , the synch onous machine supplying only any increase in the wattless component o f u the external load , together with the additional lagging c rrent required by the generator when carrying the increased energy load . The synchronous machine supplies at all times all the lagging wattless current in the circuit , and the governor of the prime- mover driving the induction generator decides the tre queney of the circuit , the synchronous machine slipping behind the generator an amount sufficient to allow this latter machine to supply all the power required by the circuit . — No n-synchro no us Generato rs in Po w er S tatio n Wo rk Oh vio usly the disadvantage of this type of generator for power station work is , that it cannot supply a lagging wattless current , and that it requires an additional lagging wattless current to

- excite it . The power factor of the current supplied by such a generator is a direct measure of the amount of wattless current required to excite it under that particular load . The power factor whi ch can be obtained in designing any induction genera o u S tor , depends the ize , speed , voltage , and frequency of the T HE NON~ SYNCHRONOUS GENERAT OR 1

i all n . Lo w e machi e speed , h gh voltage , and high fr quency , tend

- to low er the power factor which can be obtained . Table I gives the characte ristics of steam turbine and gas

W. engine driven induction generators of from K. to

T ABLE I ST EA M T URB I NE DRI $ E N 25 CYCLE I N DUCT I O N T YPE GE N E RA T O RS

D LOA .

- E fficie ncy. Po we r facto r .

9 7 6 9 7 7 9 7 5 9 7 0 9 7 5 9 7 0 9 5 0 9 5 9 96 5 98 2 9 8 2 9 7 9 9 7 9 9 7 3 96 4 96 5 96 9 96 0 9 8 3 98 2 98 0 98 O 9 7 7 9 7 4 96 5 97 0 96 5 98 5 98 4 98 2 98 2 98 0 9 7 5 96 8 9 7 3 9 7 1

GO CYCLE

9 7 6 9 7 7 9 7 5 96 4 96 9 96 4 1300 0 9 4 0 9 5 0 94 5 220 0 9 8 2 9 8 2 9 7 9 9 7 2 96 8 96 2 1300 0 9 4 5 9 5 3 9 4 8 2200 98 3 98 2 98 0 96 0 9 7 2 9 7 0 13000 94 5 9 5 6 9 5 5 2200 98 5 98 4 9 8 2 9 7 6 9 7 6 9 7 1 9 5 3 9 5 6 9 5 5

GAS E NGI NE D RI $ E N 25 CYCLE I N DUCT I O N T YPE GE N E RAT ORS

7 2 9 4 0 94 2 9 2 0 8 9 3 90 9 88 7 7 6 9 5 5 9 5 1 9 4 0 9 2 4 9 3 2 9 0 7 7 7 9 5 7 9 5 2 9 4 2 9 2 6 93 4 9 1 0

6 O CYCLE

W. K. 25 6 . for or volts , and for or 0 cycles The a t ble shows that on high speed volt generators , the power er factor rises as high as p cent , and that we can obtain on 6 — 2 T HE NON SYNCHRONOUS GENERATOR

such machines a power-factor which averages 97 per cent to 98 - - per cent , from one half to one and one quarter load ; while the

— 10 o f ul no load magnetizing current , is less than per cent the f l hi o f . load current the machine T s being the case , the fact that u these generators require a wattless c rrent to excite . them

r ceases to be a serious objection , and it would appea that the

o f only important limitation this type of machine is , that it ur cannot supply lagging wattless c rent to the outside circuit . - 6 0 The low speed , cycle machines are relatively poor as regards

- r o f power factor and exciting cu rent , so that the use induction generators would not be advocated under these conditions , unless their other characteristics made them particularly ad vantageo us for the special conditions under which they were to

advan be used . It will also be seen from Table I that another tage of this type of generator is the extremely high efficiency at

- all loads that is obtained in high speed machines . As a result of this , these generators have a low temperature rise at normal

r . rated load , and have a la ge overload capacity The normal ratings o fthe individual machines given in the table were chosen so that their characteristics would be best at from one-half to one - All and one quarter rated load . the machines given can gen

o n e- crate from two and half to five times their rated output , and as far as general characteristics are concerned could be rated 50

in per cent higher . It was stated above that the slip of the duction generator relatively to the synchronous unit must in

crease from no load to full load , in order that the former may

carry its due share of the load . S o if it is required that the load

always be automatically divided between the two machines ,

e- we must adjust the governor of the synchronous prim mover, so that its drop in speed from no load to full load is equal to the drop in speed of the non-synchronous prime-mover plus the

- full load slip of the non synchronous generator . This means that the speed and frequency of the synchronous generator must

change with the load , but we see from the slip given in the table

ni that this change is u mportant . The slip varies from per

- cent to per cent in high speed machines , and from per T HE NON—SYNCHRONOUS GENERAT OR 83

- m i cent to per cent in low speed ach nes , and this is about as close

rim m v r as the governors can be made to regulate in any p e o e . The greatest commercial field for the induction generator is undo ubtedlyin connection with steam turbine driven genera o f b i tors . This type generator is more suita le both mechan cally and electrically for high speed work than any other type of elec trical machine . The squirrel cage secondary with heavy copper bars , each bar held in a separate closed slot , and practically l requiring no insu ation , is an ideal construction mechanically , and is one which can be operated at very high temperature n without damage . Compari g this with the rotating magnets f of the standard synchronous turbo alternator , the dif erence is

w very great . The magnet inding of a synchronous turbo alter nator cons ists of a number of turns of thin strap separated by insulation ; while the windings often reach high temperat ures

- due to over load at low power factors , and are in addition subj ect to heavy centrifugal stresses and a potential difference of 125 h volts to ground . We can se e t at such a construction does not compare favorably w ith that of the squirrel cage rotating sec o ndary of an induction generator ; and as a breakdown on the field of a synchronous turbo generator usually puts the machine

advan out of commission for a couple of weeks , the commercial

-s tages possessed by the non ynchronous generator are obvious . The Simplicity in construction and insulation of the rotating s parts , the ea e with which the centrifugal stresses necessarily be present can taken care of , and the absence of a complicated

winding or brush gear , obviously tend to reduce the cost of the induction generator compared with that of the standard syn hr c o no us . generator The actual cost of any machine , as dis tin uished g from the sale price , depends on the performance e i sp cification to wh ch it is designed , and on how closely it

is rated , but it can readily be seen that the induction generator offers facilities for cheaper design and manufacture h which are not presented by the sync ronous generator . n th I u n — The Ex citatio of e nd ctio Gene rator . A synchronous

generator requires direct current excitation , while an induction — 84 T HE NON SYNCHRONOUS GENERATOR

r rre gene ator requires alternating cu nt for its excitation . The induction generator is excited by a lagging current taken usually hi h from a synchronous mac ne , and as t is synchronous unit h requires direct current excitation to produce t is lagging current , it can be said that indirectly the induction generator does require n a direct current exciter . But o account of the small air gap

o f of this type generator , it requires much less excitation than a synchronous machine . The actual capacity of the exciters required by a power station consisting of induction generators ,

- depends on the power factor of the load on the system . The

— - capacity will usually vary from one quarter to one half o fthat which would be required for the corresponding synchronous generators ; though if the power station feeds a cable system

re with high electrostatic capacity , this will supply part of the quired lagging excited current , and reduce the size of the required exciter . The charging current of the New York Interborough 1 i 05 . e . system is given as about amperes at volts , , about E . i K V . A . and that of the New York d son system is about 4 4 5 . . . F i . e 0 0 . amperes at volts , , about K V A rom Table I it is evident that the capacity charging current of the New York Edison system is sufficient to supply full load magnetizing K 2 W. 5 and wattless current required by a . , volt ,

- cycle turbine driven non synchronous generator, when running — on a non inductive external load . In the same way the capacity charging current of the Interborough system would be sufficient 2 K. W. 5 to supply the wattless current of a , volt , cycle turbine driven generator . If we had a cable system , such I as the nterborough , distributing at volts , it would have 190 a charging current of amperes at volts , which would W be sufficient to supply the wattless component for K. .

25 r — in volt , cycle tu bine driven non synchronous genera f tors . So we can see that the electrostatic capacity o a large cable system will play an important part , when the introduction of such generators into some of the large New York power stations is considered . In a system consisting o f induction generators supplying T HE NON—SYNCHRONOUS GENERATOR 85

power to rotary converters , it is unnecessary to have any ex citers o r syn chronous units in the power station . The first d rotary started up , must be brought up to spee from the i direct current side and then thrown on the generator circu t , i n when it w ll excite the latter , the voltage bei g decided by the excitation of the rotary . We can see from Table I that the power-factor of an induction generator can be made to remain ll - - practica y constant from one half to one and one quarter load , so that the amount of wattless current taken by the generator throughout its normal working range will be practically pro H portional to the watt component of the current . ence , assum ing we can neglect the capacity charging current of the cable system ; if we have a number of rotaries operating on the circuit ; we can adj ust the shunt excitation so as to give the correct volt i age at no load , wh le the series excitation can be adjusted to obtain any desired voltage characteristic as the load comes on o n r the system , the rotaries comp undi g the generato s by their series winding . This compo unding of the generators as the load c sub - ff s omes on any station , a ect of course all the other sub stations fed from the generators ; so if we are not regulating for c onstant voltage , it may be advisable in some cases to introduce

- d as artificial self induction into the rotary fee er circuits , so to

u s r over compound these circ it rather than the gene ators , and

s - avoid disturbing the voltage on other unloaded ub stations . r In a la ge system , the capacity current of the cables cannot be l neglected , so the wattless current to be supp ied by the rotaries

r . S will not be di ectly proportional to the load uch a system , however , usually requires constant voltage at the direct current

van terminals , and when such is the case it may be found ad tageo us to install compound-wound rotaries with automatic volt age regulators to control the shunt excitation . The voltage regulators could then be made to keep the generator voltage l constant , whi e the series winding would serve to compound each individual feeder in order to compensate for the voltage drop in that feeder . In such a system , all regulators controlling the voltage on a given group of generators would be tied together , 86 T HE NON—SYNCHRONOUS GENERATOR

o ne so that any regulator could not act before , or act against the others , and make the shunt excitation converters different for the individual rotaries . s - A suming the self induction is negligible , then if there is no

- electrostatic capacity in the system , the power factor of the rotaries is practically the same as that of the generators ; but if there is capacity which helps to supply the lagging current , then the power-factor of the rotaries will be higher than that of

. the generators Taking once more the Interborough system , K . W and assuming there are . of volt turbine

- an d ‘ driven non synchronous generators in the power station ,

W - K. . th of rotary converters in the sub station , then e capacity current supplies 13 per cent of the wattless current taken by the generator on full load , and the full load power 98 factor for the rotaries will be per cent . Such a power station of induction generators , having no direct current exciters and exciting circuits , is much simpler as regards cables and switch board connections than a S imilar station with synchronous

. n eces generators , and is much simpler to operate There is no sity for synchronizing the generators ; they are run up to speed and thrown on the line in series with a choke coil (to limit the

S - rush of current) ; the choke coil being then hort circuited , the generators are automatically excited from the rotaries and take r - are care of themselves . The governors of the p ime movers m e te rs t controlled by pilot from the swi chboard , and the load can be distributed at will among the different generators without any adjustment of the excitation to keep the power-factor con stant , as would be necessary with synchronous generators . il This results in an ideally simple station , as there are no aux iary circuits , and the switchboard is practically limited to the main generator and feeder switches and instruments . — S hort Circuits and Reso nance During the last few years we have heard a great deal regarding resonance and high power surges in large installations with cable distributing systems of high electrostatic capacity . These phenomena can be investi

in gated mathematically detail , if we choose to make a number

— 8 T HE NON SYNCHRONOUS GENERATOR it tends rather to eliminate disturbances from the line than to

- originate them . A S hort circuit on a system results in the voltage falling to zero , consequently any induction generator o n

the circuit losing its excitation becomes dead , and does not tend

- to supply either power , current , or voltage to the short circuit . F urther , the voltage wave form of an induction generator is

fo r virtually a sine wave all loads , and has no tendency at all to hi introduce higher harmonics w ch might produce resonance . If the synchronous machines supplying the wattless curr en t in the

circuit have a badly distorted wave form , the magnetizing cur

o f l rent the induction generator wil also be distorted , but there will be a strong tendency to damp out all harmonics in the volt age wave form of the system ; and we can say that , generally speaking , the induction generator acts as a strong damper to remove all harmonics in the voltage wave form of the system , introduced by distortion of the wave form of any synchronous machines . This distortion in a synchronous machine , being due to the armature reaction of the watt component of the current ,

re rather than the wattless , we see that the best conditions as gards freedom from distortion and harmonics are obtained by l i the use of a rotary converter or un oaded synchronous mach ne , rather than a loaded synchronous generator o r motor to supply the wattless current required to excite an induction generator .

24 S Fig. hows approximately the relation of the watt compo

‘ nent to the wattless component o f the current supplied by a

f r l W. o K. r induction generator , the cu ve being its norma rated voltage o f volts ; fo r a different voltage the values o f S current , both watt and wattless , hould be multiplied by the ratio of the new voltage to volts . We can see from this , that the magnitude o fthe watt current bears a definite relation to

o f that the wattless , and that the watt current , and consequently

o n the load the machine , cannot change without the wattless cur the . F curve rent also changing urther , for each point on the , . slip of the induction generator ahead o fthe synchronous machine

has a certain definite value . This shows that when a short circuit comes on a system consisting of an induction generator T HE NON—SYNCHRONOUS GENERAT OR 89

er - l and synchronous gen ator or motor , the short circuit wi l come

u n . on the synchrono s machi e If the voltage drops to zero , the induction generator will be dead ; but if the S hort-circuit is not a severe enough to reduce the volt ge of the system to zero , then it may still supply current to the circuit . The amount which it supplies will depend on the way in which the excitation of the synchronous machines is changed by any automatic voltage regulators . But a change in le ad which can be taken care of by l - voltage regu ators hardly comes under the class of short circuits ,

Wa ss urr cur ve A and s li curv B le c e ) e . tt nt ( . p ( ) 24 FI G . .

s ff s s and as the e latter e ects are the only seriou one , we w ill con sider them alone . We see from the above that the induction generator takes no part in the sudden surge of current which occurs on a short h a circuit , so that t is surge cannot be gre ter than that which is i supplied by the synchronous mach nes in circuit . This sudden s surge which take place when any synchronous machine , whether r - generator , motor , or rotary , is sho t circuited is equal to :

Electromotive force of syn chronous machine Total impedance in circuit

After the current has flowed for an appreciable interval , so that the magnetism of the synchronous machine has had time to — 90 T HE NON SYNCHRONOUS GENERATOR

r change , the armatu e reaction cuts down the electromotive force generated , and the current falls to the value commonly

- known as the short circuit value , this being the value of the

- u . K. W current on a continuous short circ it The . , volt Manhattan generators will give a continuous short-circuit r ur current about th ee times full load c rent , but the instantaneous value of the current on a sudden short-circuit is about seven times

- - this ; that is , about twenty one times normal full load current . If these generators are supplying rotary converters these

- r rotaries will also supply power to the short ci cuit . Operating

K. h W. as generators , the Man attan converters give about full-load current on a continuous S hort-circuit on the alternating

- o f o current side , with the self induction the transformers and rea tive coils in circuit ; and the instantaneous value on sudden short:

$ S - circuit is about three times this value . o a short circuit on a system consisting of the Manhattan generators supplying power

S - to rotary converters will give , on sudden hort circuit , a rush of current equal to twenty-four times the total full-load current of the generators in circuit ; while after a short period the value of - u - the short circuit c rrent will fall to about one sixth of this value . Assuming that we had induction instead of Synchronous genera

in - tors the power station , the short circuit current would be limited to that from the rotaries , and the sudden surge would

- be equal to three times full load current . The voltage of the ‘ the ro taries system would fall to zero , and would supply a t S - gradually decreasing curren to the hort circuit , until their rotational energy was expended , and they had come to rest .

We see then that with induction generators in the power station , the magnitude of the sudden surge on a short-circuit would be reduced to one-eighth of that which would take place with the present synchronous generators . As a result of this the voltage l - rise wou d be only one eighth , and the power of the surge

- n would be one sixty fourth . These figures do o t need any comment .

There is one point , however , that must be considered when operating non-synchronous generators o n a system containing

— 9 2 T HE NON SYNCHRONOUS GENERATOR

n o t neutral and outer terminals , but in the electromotive force between the outer terminals . They therefore appear in a three

r- o r r - phase , fou wire system , in a th ee phase system with ground ed neutral . The other harmonics appear , no matter what the connections are . Though the presence of these harmonics may not cause harm in any individual case , it is always possible di that there may , under certain con tions of circuit and load , be harmonics of a frequency sufficiently close to that o fresonance to cause serious rise of potential . And if generators o fdifferent ff characteristics operate together , or if there are loads of di erent f magnitude and phase , or di ferent excitation on the synchronous

- generators or motors , cross currents are liable to be produced hi between the machines , t s being especially the case when running with a grounded neutral . Rotary converters are very much better than any other class of synchronous machines as regards distortion of wave-form when operating with unity

- power factor , as they have practically no armature reaction ; so that the electromotive force wave generated under such con ditio ns is approximately a sine . The above remarks on syn chro no us n generators and motors , then , o ly apply to rotary converters to a limited extent . Induction generators have no distortion of field due to armature reaction , and as long as the iron in the magnetic circuit

ur - is not sat ated , the electromotive force wave form of these

f r generators is virtually a sine wave o all conditions of load . In consequence of this there can be no cross currents between

i f e generators of th s type , due to dif erence in wav form , and F no tendency to produce resonance in the circuit . urthermore , an induction generator acts as the strongest possible damper in a

i e c rcuit ; and if there is any surge , unbalancing of phases , dist r

- o r tion of wave form hunting present , the induction generator will tend to damp it out , and to restore the original condition of steady sine wave operation . If we then have a system con sisting of induction generators supplying power to rotary con verters as , it would be nearly perfect as possible in its freedom t from surges and resonance . The rotary conver er would give T HE NON- SYNCHRONOUS GENERATOR 93

ni u in the mi mum distortion of any synchrono s mach e , and the induction generator would tend to damp o ut any disturbance that occurred on the line . Such a system would certam be much superior to any one containing synchronous generators

m e te rs and synchronous , in regard to liability to disturbances

n o f from resona ce or surges , and in all probability the engineers such a system operatin g with grounded neutral would not be made aware that such phenomena existed . I have endeavored to show that the induction generator is very much superior to the synchronous from almost every point o of view , for the purpose of supplying p wer to motor generators or rotary converters through an underground cable system ; and that rotary converters are les s liable to introduce lin e disturb i ances than syn chronous motors . In some cases it m ght be considered advisable to install both synchronous and induction generators , or the station might be one in which the units first installed were synchronous while the later extens ions were induction generators . In such cas es it is readily seen that the

s as advantage outlined above , are obtained to a degree which depends on the ratio of the capacity in induction u nits to those r in synchronous . It should be emembered that the induction generator and rotary converter give the bes t combination to e u insure fre dom from line dist rbances , and that the synchronous generator and synchronous motor give the worst ; combinations of the two systems give results interm ediate between these two . — S mall Pow er S tatio ns Above we have considered in detail large systems which can easily provide the necessary lagging exciting current for the induction generator , either from the charging current of a cable system or from synchronous me te rs l or rotary converters in the system . With sma ler stations which supply power direct to motor and lighting circuits , the conditions are not so favorable to the induction generator , as

- this type is primarily one for high power factor loads , and is at a distinct disadvantage in a station where the load is of low

- - power factor . The advantages of the non synchronous genera — 94 T HE NON SYNCHRONOUS GENERATOR to r as outlined above are however , so great , that each particular case S hould be considered in detail to determine whether its

- adoption is advisable . Usually the low power factor load on such a station consists mainly o fm e te rs operating only during the daytime , whereas the heavy peak load is the lighting load at

r night . We can therefore install synch onous generators sufficient

- to carry the low power factor day motor load , and induction

n generators to assist in carrying the lighting load at night . Co sidering a 6 0 cycle station having a day load o f

- 70 o f with a power factor of per cent , and a night load with a power-factor o f 95 per cent ; and assume r fu ther that we have , exclusive of spares , two K . V . A . e synchronous turbo g nerators to carry the day load , and two

‘

K. W. induction generators to assist in carrying the night load . This night load consists of watt K . V . A . and wattless while the two induction generators 4 require in addition 6 0 K . V . A . to excite them . We shall then have the two synchronous units carrying a load o f

- - . . 2 K V . A at 5 per cent power factor and the non syn chronons machines carrying a load of K. W. at power f factor o per cent . It is nearly always more economical to supply wattless cur rent in a power station from unloaded high speed synchronous

m . otors , than from the main synchronous generators This is m ore especially the case with steam turbine or very slow-speed

’ u such m achin es nits , as cannot be economically designed with the good regulation and the margin on the field magnets necessary

- i o f to handle satisfactorily a low power factor load . A mach ne either o f these types to carry satisfactorily a certain kilowatt load at power- factor 70 per cent will be abo ut double the weight

o f. a unit to carry the same kilowatt rating at unity power factor . In the station considered in the preceding paragraph , we assumed that the synchronous generators supplied the wattless current required fo r the induction generators and outside circuit .

r It would be better , however , to make the synch onous generators

K . V . A . units with poor regulation , and install also a

9 6 T HE NON- SYNCHRONOUS GENERATOR

o f engine driven alternator , because the irregularity speed in a gas engine is usually such that the dampers have to perform e fl - heavy work in accelerating and r tarding the y wheels . We can readily see that there can be easily an increased loss of three

n to five per cent from these two causes , which would o t be de te cted except in a gas consumption test , when running in parallel f . o i with other units Instead synchronous un ts , we can install

- induction generators , and have high speed synchronous motors

i r runn ng light , to provide the necessary lagging cu rent for the hi outside circuit and for exciting the generators . In t s cas e any

r o n change in load comes fi st the synchronous motors , causing a change in their speed , and as a result a transference of the

AS f change in load to the generator . the voltage o the generator

o f would be decided by the excitation the synchronous motors , the voltage regulation o fthe station is that o fthe motors ; so that for constant potential it may be advisable in some cases to control the motor excitation by an automatic voltage regulator . The size of the direct current exciter necessary for the synchron ous motors would depend on the power-factor of the load on the station , and would be greater, the greater the lagging current

. G required by the external circuit enerally speaking , the size

- — of the exciter required would be from o n e quarter to one half of that necessary fo r the corresponding synchronous generators . The probable arrangements would be a direct connected exciter

hr on each sync onous motor , which could also be used as a start ing motor ; and one gas engine driven exciter fo r starting up the

r fi st motor .

o n 25 Taking as the load such a volt , cycle power

- . 0 . . 7 station , K V A at per cent power factor , we would M ‘ 75 R . P. . have four synchronous generators , each

K . . . W. 75 W . 12 K. R P 5 . requiring a exciter ; or four , M 500 induction generators , together with four

W. M 6 0 K. P. . R . synchronous motors with direct coupled starting E motor exciters . ach synchronous motor would supply the

o n e K . V . A . exciting current required by induction generator ,

fo r together with K . V . A . wattless current the external — T HE NON SYNCHRONOUS GENERAT OR 9 7

e circuit . The relative efficienci s on the load of 70 per cent power factor are as follows :

I d uc io e e rat o r S nc hro no us n t n G n a nd S n chro no us

ne ra to r.

o to r .

These efficiencies given for the synchronous generator do not take into account the additional losses due to the increased th fl - o r friction of e larger y wheel , and the losses in the dampers solid po le faces which occur with the gas engine driven syn chro no us generato r . These additional losses will reduce the efficiency of the synchronous generator below that of the induc tion generator set . The cost of the electrical equipment would

ff as - not be very di erent in the two cases , and the larger fly wheel ,

h r shaft , and bearings , required for the sync ronous generato

n as r the would i cre e the cost conside ably , it is probable that in duction generator equipment would be somewhat cheaper .

I as t must be remembered that this is an extreme c e , because the low power-factor of 70 per cent taken for the outside load is very s much again t the induction generator , and that if the power l l t factor were higher , the resu t wou d be more favorable to tha type . It might be supposed that unless the gas engine was provided

fl - e l e with a very heavy y whe l , there wou d be the same troubl with hunting of the induction generator and synchronous motor that we would have with the syn chronous generator . But such is not the case . There will undoubtedly be cross currents be i tween the machines , the magnitude of wh ch will depend on the variation in the speed of the gas engine , but it .will be practically

o f h s imp ssible to break them out of step . The worst ef ect of t i interchange of current between the machines is the heatin g and losses in the armature conductors , and the pulsation in voltage

o f due to the interchange of wattless current . This pulsation 9 8 T HE NON—SYNCHRONOUS GENERATOR voltage is diminished by dampers o n the pole- faces o fthe syn

r ch onons motor , but it will usually be perceptible when only o ne H generator is running . owever , as these gas engine driven units will be used mainly for power work in mills , the slight pulsation of voltage will be unimportant . — It is by n o means settled that extremely heavy fly wheels and powerful dampers are a practical and advisable solution o fthe parallel running difficulties with gas engine driven synchronous

So n generators . though induction generators are o t put forward as the only solution of the gas engine driven alternator question , they probably offer the most practical solution , and the one which will recommend itself most highly to the conservative power station engineer and manufacturer . — S team T urbine Driven Direct Current Units The other special application o fthe non-synchronous generator above re — hi ferred to w ch is, to operate with a rotary converter to supply direct current—is to meet the special case in which a steam turbine is desired as a prime-mover in the production of direct

ni current by large units . Mi mum overhead clearance , small u floor space , poor fo ndations , objection to the vibration of i reciprocating engines , h gh steam economy required over a wide range of loads , reduced maintenance and supervision , might render the adoption o f a steam turbine necessary ; and as the direct current turbo generator has as yet\ hardly established its position as a conservative and reliable machine , at any rate in

o f large units , it would be necessary to use some type alternating current generator in combination with a motor-generator or E i rotary converter . Two years ago the Milwaukee lectric Ra l L K W. way and ight Co . wished to install an auxiliary . , 300 volt direct current steam-driven generating equipment in the basement of their new Public Service Building . As the head 12 room was limited to about feet , and as the vibration would be objectionable , reciprocating engines were out of the question .

K. W. They installed two horizontal steam turb—ine driven synchronous alternators , and two synchronous motor generator

en sets . This would have been an ideal case for an induction g

100 T HE NON- SYNCHRONOUS GENERATOR

i des rable addition to a power station . It is a question whether the steam turbine driven induction generator and rotary co n verter are not superior to the engine driven direct current gen

S crator in almost every case , and this combination hould always be carefully considered when any new direct—current station is laid out , or any extensions are added to existing plants . I have endeavored to S how that the o n e great disadvantage of the induction generator—its inability to carry a lagging — wattless current load S hould not always prevent its successful adoption ; and that the important advantages it possesses in its

ni cbnstru ctio n excellent mecha cal , high efficiency , good charac teristics r - balanc in rega d to short circuits and resonance , strong

o r ing and damping action , absence of rotating windings collector

- rings , absence of direct current excitation and exciting circuits ,

fo r ease of parallel running , facility control of load by governor ,

S o f and general implicity and flexibility operation , render it in hi many cases the most advisable mac ne to adopt . This type of generator suffers from the fact that it was judged and condemned f in the early days o electric power stations . At that time there was no real field for this generator ; but the introduction o fthe steam turbine and the gas engine , with the modern development r of la ge power stations , have so fundamentally altered condi tions , that the induction generator is no longer an interesting

o f ro misin t es fo r curiosity , but one the most p g yp power station equipment . At the present time this machine can be considered as having been placed on a demonstrated commercial basis ; advan and while it may have limitations , it possesses so many tages , and its sphere of usefulness is so extensive , that it must be acknowledged to offer greater future possibilities than almost any other type of power station equipment .

NOTE

The following data on the steam consumption obtained from tests at the Baltimore Copper Smelting and Rolling Co . plant

o f referred to earlier in the paper , are interest in comparing the cies of the two types of direct current generating equip The first column gives the actual steam consumption of W K. 00 . steam turbine driven induction generator and n 14 0 . converter , the turbine bei g run with lb steam pres 1 ° 8 in . vacuum and 35 superheat . The second column he corresponding figures on a Corliss engine and direct ted e generator , the steam consumption b ing based on a

I . H. P. m consumption of lb . per at 80 per cent of full

INDS OF STEAM PER KILOWATT HOUR AT DIRECT CURRENT T ERMINALS

S tea m T urbine Co rlis s E ngine

u E ui e . Eq ipme nt . q pm nt

1 04 SINGLE PHASE GENERATORS

dangerous heating and low efficiency . The mechani cal stresses o n r the ends of the armature coils , due to the cu rent , result in vibration or distortion of the windings , and often in damage to the insulation or complete destruction of the coils ; these s tresses being particularly serious in S ingle phase railway genera

o u o f r tors , account the sudden va iations in load and frequent

- AS short circuits to which these machines are subjected . the

f o n r e fect of the mechanical stresses the armatu e coils , and the

a ro x i losses due to the pulsation of armature reaction , increase pp mately proportionally to the square of the pole-pitch in gen e rato rs ff of standard design , it is easily seen why such e ects which were negligible in the old single phase alternators of small pole pitch have become quite important in the modern steam f turbine driven generator . The seriousness o these difficulties w i hen first met with was so great , that even with n the last two years responsible engineers have stated it was impossible to

- build satisfactory low frequency , high speed , single phase gen e rato rs o flarge capacity ; and it is only by careful study and e xperimenting that the modern machine of this type has been developed . — Lo sses Due to Puls atio n ofA rmature Reactio n I n a poly phase generator the armature current produces a magnetic flux which rotates synchronously with the field magnet . This mag netic flux may result in increased losses in the laminated arma t ure core , but being of practically constant magnitude , causes f r very little loss in the iron o the magnetic ci cuit . On the other

a S hand , the rmature current in a ingle phase generator produces a pulsating magnetic flux which is stationary in space , and it is e asily seen that this flux will cause hysteresis and eddy-current l f osses throughout the whole magnetic circuit . The exact ef ect o f the armature reaction flux on the rotating magnets depends , o f course , on the relative phase of the armature current and

e - o f lectromotive force ; that is , on the power factor the load

c - arried by the generator . When the power factor is unity and

rr a the armature cu ent is in ph se with the electromotive force , the armature reaction flux is a cross-magnetization ; when the SINGLE PHASE GENERAT ORS 105

° power-factor is zero and the armature current is 90 out of phas e with the electromotive force , the armature reaction flux is a I hi n demagnetization . n the special case in w ch the rotati g field f magnet is cylindrical , without projecting poles , the ef ect of the armature reaction flux on the magnets is more nearly indepen d

- ent of the power factor of the armature current . But in any l n o s case this flux is a pu sati g one , and there are imp rtant lo ses in the field magn ets , due to their rotation through this cross magnetizing or demagnetizing flux . An estimate of the com bin ed loss es in the armature and field magnets due to the pulsat

a . ing armature re ction , can be obtained in a number of ways We can measure the increase of the power required to rotate the field magnets due to normal R . M . S . c urrent in the armature

: coils , with

1 r . . Direct cu rent in the armature winding 2 . Alternating current of synchronous frequency in the arma ture .

- r d . 3. Armatu e short circuited and field excite Or with the magnets stationary we can 4 . Send normal frequency alternating current through the

arm ature and measure the losses by a wattmeter . These methods must all be regarded as convenient tests which are found by experience to give some indication of the 4 magnitude of the losses . Method has the additional advan tage that we can vary the relative position of the armature re

flux - u s action and the pole faces , and th s inve tigate the variation of the losses in a single phase generator with the power-factor of

the lo ad . The only exact methods of measuring the losses are As - 1 . unknown losses in a motor generator (Hopkinson)

method efficiency test . 2 F . rom a comparison of the temperature rises obtained on l fu l load with those obtained with known losses . f d Un ortunately , both of these tests are iffit to make

a and ccurately , especially on a large machine , probably in practice they do not give results which are any more accurate 106 S INGLE PHASE GENERATORS

than the other more approximate methods . So at the present time we have to acknowledge that though we know a great deal about the relative values of the losses under various conditions , o our kn wledge of their absolute values is limited . — Po le-F ace Dampers Losses caused by a pulsating flux in the magnetic circuit are due to

1 H . . ysteresis 2 E . ddy currents . An d the relative magnitudes of the two depend on the amount of solid metal in the path of the flux . If the whole magnetic circuit is laminated , then the losses are practically all due to hysteresis . On the other hand , if we have solid cast steel poles there will be eddy currents in these poles which will partly choke back the pulsation o fthe flux so that the hysteresis loss will

- be reduced . But in this case there will be eddy current losses i in addition to the hysteresis , and the extent to wh ch the total loss is changed will depend on the proportions and design of the magnetic circuit . If we place a heavy copper damper in the

- path of the pulsating flux , it will provide a low resistance path for the eddy currents , so that the pulsating flux and consequent i hysteresis loss will be reduced practically to zero , wh le on account of the low resistance of the damper circuit the eddy loss will not be appreciable . The extent to which the losses and the pulsation of the flux vary according to the presence o feddy currents , can be determined for any particular design , by varying

n o r the thick ess of the lamination , or by changing to solid poles the addition of dampers . It is usually found that the losses are greatest with heavy laminations or solid poles ; that they are less for thin laminations , and practically zero when heavy low resistan ce d ampers are used either with solid or laminated poles .

2 o f Fig. 5 shows the pulsation the armature reaction flux in 2 — W. 500 K. 0 a , single phase , pole generator , as determined by

- - means of search coils wound o n the pole faces . 0 shows the 29 B S e fo r No . pulsation laminated poles , gauge ; hows the sam machine with solid poles ; and A the same solid pole-faces covered

A a e o f with a gin . copper plate . The magnitud the pulsations in

108 SINGLE PHASE GENERATORS system acts in regard to this reverse rotating flux in the same way as the short- circuited secondary of an induction motor or transformer ; a current is induced in the damper which produces a field neutralizing the rotating flux . The eddy and hysteresis loss in the iron o fthe magnetic circuit which would be caused

nl by this rotating flux is thus eliminated , and the o y loss is that

r due to the current ci culating in the damper . If we make the conductors forming the squirrel cage of sufficiently low resistance , this damper loss becomes negligible , with the result that the entire loss due to the pulsating armature reaction of the single phase generator is practically eliminated . To S how how serious this matter o flosses becomes in high speed , two pole , single phase machines without dampers , the following table is given , showing the loss and full load tempera r ture rise on three tu bo generators at full load , both with and without dampers :

T WO O E 2 E E E AT S ME 5 Y E SI NGLE H S G N R ORS . P L , C CL , P A A CURRENT PE R ARMAT URE CONDUCT OR ONE AND T HREE H SE UNDER ALL O DIT IO S AND ALL OSSES I N PE R P A , C N N , L CENT OF S INGLE PHASE RAT ING

AS S I GL PHAS . T HREE PH E . N E E

h e s . h e s . Witho ut Dampe rs . Wit o ut Damp r Wit Damp r

K S o l 7 50 W. id 1000 S o lid 1000 Laminate d 0 2

r S It will be seen that in these th ee machines , operating ingle

phase , there is due to the pulsating flux an average loss of per 2 ° 1 5 C. cent and an average temperature rise of , without dampers ; with dampers the average loss is per cent and the ° F nl temperature rise 30 C . igures are given o y on compara tively small machines on account o fthe difficulty of measuring S INGLE PHASE GE NERAT ORS 109

o n losses o n large machines . But t ests larger generators up to

W the to K. . capacity show that improvement due heavy copper dampers is even mo re striking in large machines than it is in small . So far as experience goes at the present time , it may be said that the use o fsuch dampers is the c o mplete so lution o fthe difficulties due. to pulsating armature reac tio n met with

lo w tw o o o . in large , frequency , p le , single phase generat rs — Ill echanicalS tres ses o n Armature Calla That it w as necessary to mechanically brace the end connections of the armature coils o na direct current machine subjected to sudden loads and

- S hort circuits has been know n for many years . But until

— FI G . 26 . o ie ld M n fo r K W 2 l cle S in le e . . o e R tary F ag t , P , Cy , g se Ge ne o Pha rat r . quite recently additional suppo rts for alternator armature coils

fo r as the co n were seldom provided . The reason this was that tin uo us short - circuit current o f an alternator is o nly about tw o o r o w as n o t three times n rmal , it considered that the mechanical stresses o n the ends o f the small po le pitch co ils generally in

o c use were sufficiently great t ause trouble . Only during the last few years has it been demo nstrated by experience that coil

o n o supports large p le pitch alternators are not only advisable ,

o f - but necessary , and that on account the numerous short circuits 1 1 0 SINGLE PHASE GE NE RAT ORS they are particularly necessary o n S ingle phase machines operat ing o n traction circuits .

- When an alternator is suddenly short circuited , the first rush o fcurrent is limited only by the self- inducti o n and resistance in c o f o f lo w - ircuit ; and in the case an alternator self induction , this first rush o fcurrent o n sudden short- circuit may be 20 to 30 times no rmal full load current . As the mechanical stress o n the

o f o f end the armature coils varies with the square the current , the o n 400 to 90 stress the armature coils will be 0 times normal . K 2 2 .W. 5 A , pole , cycle , single phase generator has a

f 1 f - o 00 in . o pole pitch about , and the length the end connection at o n e o f o n e 18 end armature coil will be about 0 in . The mechanical stress o n the end connections at o n e end o fo n e arma — ture coil o f this machine o n a sudden short circuit is appro x i mately 5 tons ; and usually o n lo w - frequency high speed genera

o f o n tors large . capacity , the mechanical stresses the end connections at o ne end o f o n e armature coil in the case o fa sud

- 2 1 o den S hort circuit are from to 0 tons . When it is c nsidered

r to that this esults in a sudden mechanical shock the winding , we

o f can realize the strength the coil supports required , and can understand the disastrous results sometimes Obtained o n short circuits , when such supports are omitted . It is obvious from these stresses that coil supports must be o fmetal and o fheavy cross

o f section . The difficulty suitably insulating metal coil sup

l to ports has caused numerous other materia s be used , but

o f though supports wood , porcelain , and similar insulating materials have been tried , it is easily understood that they have

Fi . proved unsatisfactory o n machines o f large pole pitch . g 27 shows a form o f coil support and bracing which has

o f proved very satisfactory fo r such machines . It consists a

w o f bronze strap hich clamps the coils , by means wood insulating blocks and insulated bronze bolts , to malleable iron brackets , which also serve to support the copper strap connectors between

f o fa the various groups o coils . The support and its method p plication are evident from the illustration ; it is placed in position after the machine is wound and is removable in a few

SINGLE PHASE GENERAT ORS

n o t minutes at any time . It is suggested that this is the only satisfactory type o f coil support that can be used ; it is merely given as a type which has proved successful in actual operation KW o n to . . machines up capacity , and which has appar ently solved the difficulties due to mechanical stresses o n the end co n n e cio n s Of large pole pitch generators liable to sudden

r - variations in load o frequent short circuits .

tw o lo w The main difficulties met with in large frequency , high speed , single phase generators , that have been described above , can at the present time be regarded as having been suc

sf ll o f o n ces u y overcome . The use heavy copper dampers the

- to pole faces , and heavy bronze coil supports applied the ends o f to the armature coils , in such a way as take directly the

o n - n o w mechanical stresses which develop short circuits , has L made such generators a practical success . ike every other

o f - new type electrical machinery , the large turbine driven ,

o f single phase generator has had its period development , but at the present time it may be said that such generators fo r 15 2 KW 5 o f to . . and cycle , in units capacity , can be ,

- O o n and are , built with the same success as that btained slow speed polyphase generators . [Pres e nted befo re the Natidna l Electric Light s e A so ciatio n , $ u n 3,

PERFORMANC E SPECIFICATIONS AND RATINGS

n — o f Umformily Of Rati g. The question uniformity in the rating of generat o rs and mot o rs is in a de cidedlv unsatisfacto ry

t as o e ve r c o ndition at the present ime , alm st v manufacturer and

o o r o r every purchaser rates his generat rs m to s , and specifies their

o s performance in a different way . The result ft hi is that when an operator decides a new generat o r is required he specifies as

as c fo r nearly possible a unit he onsiders suitable the work , buys

the c w one that manufa turer estimates ill fill his specifications ,

to i w o o and then proceeds to test , f nd hat utput he can btain from

s it under the conditions that exi t in his statio n . T he operating e ngineer and the manufacturer are both to blame fo r this state o faffairs ; the manufacturer because he o ften designs and builds

. o f o o his machine to take care s me arbitrary the retical , rather

o o f o the o o n than practical , c nditions operati n ; and perating gineer because he does no t more clo sely study the operating

o f conditions his machinery , and insist that the manufacturer

r o supply generators o mot rs suitable to his requirements . So long as the mechanical constru ctio n o fa machine is satisfactory a nd o ut ih such that repairs can be easily carried , no further

uiries s h q are usually made by the purcha er , provided t e name plate carries the nominal rating required . The tendency is to pay altogether to o much attention to the figures stamped on

- w r the name plate hen buying a generator o motor . The result is that almost every engineer knows examples o funits ’w ith the

o r f same nominal rating , built either by the same by di ferent 8 1 13 1 14 SPE CIFICAT IONS AND RAT INGS

o f manufacturers , some which under certain operating conditions

o f 50 are capable carrying per cent more load than others . The

fo r o f to remedy this , course , is have some uniform and rational

o f to method rating adopted by all manufacturers , and then specify and select machines suitable fo r the work they have to perform .

r ur n P er—F ct r — m T empe at e a d 0 w a o . The tw o ost important points in which specification and performance guarantees are unsatisfactory seem to be

1 o n . The temperature rise alternating and direct current generators o r motors ; and

2 - o f fo r . The power factor the load which guarantees are made o n alternating current generators .

o f co n The present system temperature guarantees , which sists in stating the maximum temperature rise in any part o f

f o f the machine , under numerous di ferent conditions load and f r o . T he varying periods , is both unsatisfactory and irrational temperature limit o f output in any generator o r motor is n o t m t decided by the te perature rise , but by the absolu e temperature

n f o f the insulation . At o e o the specified loads the armature iron may be the hottest part , at another the field coils , while at a third it may be the armature winding o r commutator . As each o fthese parts has a different limiting temperature to which it can be subjected without damage , it is o bvious that such a sys tem o funiform guarantees is misleading . The operating engin eer is n o t practically interested in knowing that the temperature

o f o n o ut rise his generator , as measured by a thermometer the

o f 35 o n fo r 24 45 side the winding , is degrees full load hours ,

o n 25 fo r 24 55 o n degrees per cent overload hours , and degrees f 50 per cent overload o r o n e hour . What he must know is

e the maximum load he can safely carry continuously , and in som cases the overload he can safely carry fo r tw o o r three hours .

o n o n This load depends , obviously , the room temperature and the limiting temperature to which the insulation can be sub f j ected without damage . The most rational method o tempera

to ture rating is , then , specify the maximum continuous rating

1 1 6 SPE CIFICAT IONS AND RAT INGS

- o f power factor , and p ssibly a higher temperature rise specified o r the field coils . Any method o f giving alternato rs a different rating fo r every operating power-factor wo uld probably be to o complicated

fo r . practical work It has , therefore , been proposed to continue to give all machines a nominal rating in kilovolt- amperes at 100

— to per cent power factor , and in addition give the maximum load which they will safely carry at various lower operating power

l w - . o fo r factors This maximum load at power factors , is decided

o f fo r some machines by the question holding up voltage , and

o f s o fo r others by the heating the field coils , that cases in which temperature is the limit the maximum load should 2 r e . 5 be referred to a definite room temperatu e , g. , degrees

o f centigrade . This method rating alternators gives fo r the purpose o fcomparison a nominal rating at 100 per cent power

as factor , and in addition gives the purchaser exact information to the operative characteristics o fthe proposed unit un der any

o f particular condition load . If the operating engineer knows

- to the power factor at which the machine will be required operate , he S hould have no difficultyj n deciding as to the suitability Of

f r the unit o his requirements . The question o fpower- factor is equally important in rotary

. G converters and synchronous motors enerally speaking , the

- to 100 fo r power factor should always be adjusted per cent , unless some special reason , definitely specified and understood at the time the machine was purchased . A case recently came to the writer ’s knowledge in which a system was operating its rotaries

90 - ro at per cent power factor , and when the manufacturer p

o m o f tested behalf his generators and rotaries , the operating engineer stated that he considered 90 per cent a $ mighty good

- P fo r fo r power factor . ossibly it would be induction motors , but ‘ r ro tary converters it is a mighty bad o n e . Synch onous motors

to - o f are often used correct the power factor the line , but when they are installed with that intention the maximum kilovolt

- amperes at the required power factor should be specified , exactly as in the case o fan alternating current generator . A synchron SPECIFICATIONS AND RATINGS 1 17

100 e r - ous motor designed to operate at p cent power factor , is just as unsuitable for operating at a low power-factor as an alternator would be in a similar case . —N imitin Comtitions . L g eglecting the question of efficiency , the limit for operating conditions should , in all cases , be decided m by the resultant injury to the machine . The li it of tempera m l ture rise is decided by the da age to the insu ation , or to the

i . mechanical construction , of the part of the mach ne considered Some insulation will not stand continuously a temperature higher than 90 degrees centigrade without deterioration ; other insulation will stan d 300 degrees safely . The temperature limit on a commutator or collector-rings is usually decided by the

in s shr kage of in ulation or unequal expansion of the materials , o i l causing l ss of mechan cal ba ance , or loss of accuracy on the i wearing surface . The lim t of the allowable sparking on a t - e s commuta or or collector rings is the resulting temp rature ri e , a t - s the dam ge to the surface of the commutator or collec or ring , f and the disintegration of the brushes . All thes e e fects mus t be considered in relation to the duration of the specified load ; and

c s as in such cases it is difficult for the pur ha er to decide , without l m an actua test , the amount of da age that will result from a to a certain condition of operation , he must , a gre t extent , de pe nd upon the guaran tees o fthe manufacturer ; which guaran

l e c tees , however , wil be subsequ ntly checked by the a tual opera tion of the machine in service . — Tes ting and S pecifications Testing to determine in what degree a unit meets the specified detailed performance guarantees is always a very difficult question . It is almost impossible to get an accurate direct test of the voltage regulation of any l ff alternator , as the result is measured on y as the di erence of two high readings , and a variation in any one of the conditions ff . l of test a ects the result Un ess made in a laboratory , away from masses of iron which would affect the accuracy of the in struments - - , an input output efficiency test of a motor generator can not be made with a greater accuracy than 2 or 3 per cent on account of the impossibility of obtaining accurate instrument 1 18 SPE CIFICATIONS AND RATINGS

f . o the readings under practical conditions In both these cases , direct method of test has to be abandoned in favor o fan indirect method , which enables more accurate results to be obtained .

are i u Temperature tests very d fficult to carry out acc rately , and unless careful precautions are taken by experienced observers , it is often impossible to be sure of results to 5 degrees . Generally speaking , the customer will more profitably spend his time in

o f investigating the conditions operation , and in specifying a hi mac ne suitable to operate under these conditions , than in

r r making tests to determine regulation , temperatu e rise , and othe similar characteristics which will be of only doubtful accuracy and

r value when made . If a generator o motor is specified suitable

fo r for the work , then the most satisfactory and convincing test the machine is the manner in which it performs its work ; and by means of suitable performance specification this should be made

n something defi ite , and something to which the customer can

. S hold the manufacturer If this were done , we hould have fewer disputes on the question o f whether or n o t a machine a e s tisfies its contract guarantees , we should have fewer unsuitabl

S units installed , and I think we hould have better operating con ditio ns on most power systems .

— 120 INPUT OUTPUT EFFICIENCY TESTS

f r continually changing and rarely the same o any two tests . In

o bser addition to the inaccuracies of the instruments , errors in l vation also affect the resu ts , and it requires extremely careful work o n the part o faccurate and experienced observers to repeat

. o f i readings consistently to one per cent As a result th s , it is very doubtful whether the readings taken under practical operat o r - ing conditions , in the factory in the sub station , can be o h

tain ed . with a greater average accuracy than two per cent And , w hen it is considered that this inaccuracy o ftwo per cent may

n occur in both the input and output readi gs , resulting in a o f possible error four per cent in the combined efficiency , we can see that such a test is valueless to check guarantees . An additional source of inaccuracy in a commercial input output sub-station efficiency test is that due to the power-factor o f any synchronous alternating current machine tested , being 100 other than per cent , and that due to the increased loss caused by inaccurate setting and poor condition of the brushes in a direct current machine . There are a number of such conditions

f ar which af ect the efficiency as tested by this method , which e apt to be overlooked , or which are controlled with difficulty under practical conditions . When we contrast the above Objections with those which r can be raised against the sepa ate loss method , we realize that the extra complication o fthis method is justified by the more accurate results obtained . In the separate loss method each individual loss is measured separately , and a possible error of two o r r th ee per cent in the measurement of the losses , will usually not make an error o fmore than per cent in the combined effi

i n E c e c . y ach loss can be measured under favorable conditions ,

r o under the particular conditions which are being considered , and when the test is completed there can be no suspicion that the accuracy o f the results has been influenced by unknown factors . Separate measurement o f the individual losses also

S ho w hows at once the losses are distributed , and allows the operating characteristics o f the machine under various con ditio ns f t o load o be defined more accurately . INPUT—OUTPUT EFFICIE NCY TE STS 121

The input-output method of tes ting rotary converters and motor generators has long been cons idered convenient and n sufficient by operati g engineers , on account of the ease with

a . which re dings can be obtained But , from what has been said l above , it wi l be realized that the additional time and expense of measuring the efficiency by the separate loss method is well d j ustified by the more accurate results , and by the ad itional information obtained in regard to the characteristics and opera

- tion of the machine . The input output method can be used as a rough check in cases where accuracy is not of importance , but in all cases where an accurate efficiency test is required there seems to be little doubt that the separate loss test is the only one which can be relied upon .

124 DIRE CT CURRENT T URBO GE NERATORS

ur 3. An alternating c rent synchronous or induction generator

to r direct connected the steam turbine , and supplying cu rent to a rotary converter which transforms the alternating to direct current .

The rotary converter method has , up to the present time , proved to be the most conservative arrangement , and prob

r ably will continue s o fo large units . The unipolar generator has been used in a few special cases , but both it and the combined rotary and alternating current generator , are at the present time being gradually displaced by the direct- driven direct current turbo generator fo r all small capacity units . — Histo rical. Direct current turbo generators have been built

E f r in urope o the past fifteen years , but their design and opera tion has not until recently been sufficiently satisfactory fo r them to be considered under American conditions . The early direct current turbo generators were built with smooth core armature

. R and copper brushes , so that the operation was poor ecently , E however , several uropean manufacturers have built direct

K. W current turbo generators in sizes up to . and l amperes which have proved much more satisfactory . Al the machines above referred to operated with metallic brushes , and naturally suffered from the handicap of excessive maintenance cost . But about two years ago , when the direct current turbo m ainten generator was extensively adopted , the question of ance became so serious, that operating engineers took matters into their own hands and insisted o n replacing the metallic

- brushes by high grade carbon o r graphite brushes . In a number of instances these European direct current turbo generators , which were originally built and shipped from the factory to operate with metallic brushes , have , because of the difficulty of keeping this brush gear in running condition , been

C . modified after installation , so as to operate with arbon brushes

$ The result of this has been that European manufacturers are now adapting their machines where possible to operate with carbon r brushes . American manufacturers realized early that the di ect current turbo generator would never be a satisfactory commercia l DIRECT CURRENT TURBO GENERATORS 125

b machine until it could be built with car on brushes . But it is

n o ly within the last five years , that the skill of designers and manufacturers has been equal to constructing direct current generators to operate satisfactorily w ith carbo n brushes at speeds materially higher than those o fthe standard belt-driven

w o generator . By careful design ith auxiliary commutating p les and by accurate shop-work it has been po s sible to build motor generators fo r doub c the speed which formerly considered

2 . 2 8 5 K W. 1 5 0 . l rb P. M. D . . T o o R . C Ge e , V t , u n rato r .

w o f f possible , hile the development direct current generators o r coupling to steam turbines has proceeded underthe same con ditio ns . It can n o w be considered that the direct current turbo generator has been developed , suitable for satisfactory operation with carbon brushes under American conditions . They

10 to 300 125 are being built in sizes from kilowatts at volts , 50 500 250 and from to kilowatts at volts , while designers are h working on still larger units . Alt ough 6 00 volt generators do i not seem to be in a great demand for this type of un t , a number 1 26 DIRECT CURRENT TURBO GENERAT ORS

o o fo r have been in satisfact ry peration some time , notably a

ftw 5 K o 00 . W K. W o . unit (consisting . generators coupled to o n e i steam turbine) , operating at which was n N R 1 9 7 . stalled by the orth Shore ailway Company , California in 0 — , Design s The following is a list o fappro ximately standard s peeds which have been found most suitable fo r these generators :

o s V lt .

250

All o fwhich machines can be satisfactorily built to operate with

- commutating poles when carefully designed . At o n e time eu gin e ers considered that a commutating pole o falmost any design

v fo r was a uni ersal remedy all commutating troubles , but ex perien ce with direct current turbo generators and other high speed machines , has shown that this is very far from being true . The commutating pole must be proportioned as carefully as the other parts o fa machine ; and it was the neglect o f this fact which caused the failure and abandonment o fthis device when

first used many years ago . The tw o factors w hich limit the design o fdirect current turbo

o o f generators are the commutation , and the collecti n large currents at high speeds . The commutating difficulties can be e satisfactorily overcom in the generators given in the above list ,

u fo r if a properly designed interpole construction is sed , though generators o fmo re special o r more extreme ratings a complete system o fdistributed compensating winding in the po le- faces is

f r usually necessary . The limiting speed o which it is possible to build large direct current turbo generators is decided by the

a maximum commut tor peripheral speed , which can be conserva

1 28 DIRE CT C URRE NT T URBO GE NERAT ORS

S o f peripheral peed feet per minute . We have

to o amperes c mmutate , which will require at least f our, and

ix o preferably S p les . The minimum allowable distance between brushes o n the commutator fo r a machine o fthis size and type

a 7 o r 8 S o 1 is bout inches , while preferably it h uld be 0 o r 12

o - inches . The m re conservative figure would result in a 14 inch

- . f r R P . . o o diameter commutator at M a four p le machine ,

- - 2 o . . f r o r 1 R P . o a inch commutat r at M a six pole machine , 1 though adopting 7 5, inches distance between brushes we could

- — operate the six po le machine at using a 14 inch diameter commutator . But adopting the more c o nservative

o - w e co m figure , and a f ur pole machine at must

o w mutate amperes per p le , which ill require approximately — 1 ” twenty S ix 4 x brushes . Thus we will require a commutator

14 56 o r tw o inches in diameter and approximately inches long , 14 2 commutators each inches in diameter and 8 inches long . This example shows the difficulties in o perating large direct current turbo generators at high speeds when a conservative design is followed ; and explains also w hy generat o rs Of extreme

to o o r s o fi rating , in regard v ltage current , become dif cult t o build . — C o mmu tato r an d Brus h- Genn On account o fthe careful de

S o f o sign and accurate hopwork required , the question c llecting large currents at high speed with carbon brushes is the most difficult pro blem in connection with the design and manufacture F Of direct current turbo generato rs . lexible metallic brushes will operate whether the commutator runs true o r n o t ; while

lo w fo r o f having contact resistance , they are suitable collection large currents ; and this explains why they were ado pted univer T f sally o u the early European machines . he di ficulty in operat ing w ith this type o f brush is due to the fact that it is almost

to r impossible enti ely eliminate sparking , unless carbon trailing tips are used ; while it is necessary to keep the brushes carefully trimmed if the operation is to be at all reliable . If the brushes a re n o t o f trimmed frequently , the trailing edge a copper gauze DIRECT CURRENT T URBO GENERAT ORS 129

o r wire brush becomes ragged , and when the brush is in such condition a short - circuit o r sudden violent change in load is liable to make the machine flash over . A new type of copper leaf-graphite brush has been used recently in Europe w i th better

the a o results , but operation cannot be considered s tisfact ry , and

- the cost is high . Carbon trailing tips have been used with metallic brushes , but this results in a complicated and sensitive

- w fi to brush gear , hich is almost as dif cult manufacture and keep in o perative conditio n as a brush-gear using entirely carbo n

o fo r o brushes . The only reas n the adoption of a carb n trailing tip and copper brush c o mbination is that it makes possible the use o fa smaller commutator than wo uld be ne cessary w ith all

- c arbon brush gear . In addition to the excessive attention re

— f r 2 125 l) . rm 00 l . 30 e o o . C . FIG A atur V t , T u bo n o r Ge e rat r .

- so quired , the life of metallic brush gear of all types is very short that the cost of maintenance usually beco mes prohibitive ; and this is the main reason why engineers consider that the o nly satisfactory solution of the commutat o r pro blem on a direct

o e current turbo generator is the use of carb n or graphit brushes .

t o f s The bet er the quality these bru hes , the more satisfactory the o s are operation , but the greater the difficulty of btaining p f brushes o r renewals . It is an open question w hether it is better c ommercially to design these machines to operate with ordinary

o r good quality graphitic carbon brushes , with some special high

. no o w grade imported brush There is question , h ever , but that the direct current turbo generato r sho uld have only carbon o r graphite brushes if it is to give satisfactory commercial service under American conditions . 9 130 DIRE CT CURRE NT T URBO GENE RAT ORS

The question o f go od operation with carbon brushes is a

o n e mechanical , and requires a commutator which runs ab so

l o lute y true under all c nditions and at all times . Commutators to carry a large current at high speed are always relatively small in diameter and long . The diameter is fixed by the revolutions per minute , and the maximum peripheral speed which can be

o f satisfactorily operated with the particular grade carbon used , and with the degree o faccuracy obtainable in the commutator manufacture . The peripheral speed usually adopted at the

m to present ti e in America is from feet per minute , although peripheral speeds 40 per cent higher than this have been used by European manufacturers with a special grade o f f . o brush With the diameter fixed , the length the commutator is decided by the questions o ftemperature rise and correct spac~ f ing o fthe requisite number o fbrushes . When the length o the commutator is greater than about 30 inches it becomes usually advisable to build tw o commutato rs o fhalf the length instead

Of o n e o ffull length . These tw o commutators can be arranged

o n e o f o r o n e either at each end the armature , both in tandem at end o fthe armature ; in the latter the bars o fthe tw o commuta

I s tors being connected by suitable lugs . A difficulty which ex

erien ced o r tw o o s p with any long commutator , with commutat r in o f o o f tandem , is the lack unif rmity in the distribution current

n h r h- f o eac b us . between the di ferent brushes \ arm With two

o n e o f diffi commutators , at either end the armature , we have culty in distributing the current equally between the tw o co m m utato rs ; this difficulty being especially marked if a S ingle

armature winding instead o ftw o independent windings is used . The difficulty in obtaining uniform distribution o f current is

o f o f about the same with each these three types commutators , and it can be avoided only by selection o f a suitable type o f

— o f brush holder , with good quality brushes uniform quality ,

and a suitable arrangement o fgenerator leads . The standard construction adopted fo r direct current turbo

- generators is a cylindrical commutator o fthe S hrink ring type . Radial commutators have been used to a certain extent in

132 DIRE CT CURRENT T URBO GENERAT ORS as with this construction it is difficult to keep the mechanical f stresses within reasonable limits , and the advantage o the shrink- ring construction is that the stresses can be directly

to calculated and arrangements made take care o f them . The S hrink- rings should be o fhigh grade steel o f sufficiently heavy

so to section , that the stresses due centrifugal action become ff moderate . They must also be sti enough to retain their circular form and to prevent any local distortion o f the commutator .

P to n ractice varies in regard u dercutting the mica segments .

When soft graphite brushes are used , undercutting the mica seg

fo r ments is essential good running , but with hard brushes it is n - o t . Probably the best results are obtained o n these high speed commutators when graphite brushes and undercut mica are used .

o ut The undercut grooves should , however , be cleaned occasion

o o f ally t prevent the accumulation carbon dust and dirt . — Mechanical Co ns tructio n The mechanical construction o f

o f the armature is great importance , since it is essential that the balance o fthe armature should n o t change after the machine is

n hin n put in operation . This necessitates that the pu c gs do o t

n o r o n become loose move the shaft , and that the armature wind ing does n o t move under the action o f centrifugal forces. The

un chin s o n o n e p g are usually either pressed the shaft at a time , o r o n o ut an o n built up a mandril bored , d shrunk the shaft , no intermediate spider being used o n account o fthe small diameter . Opinion varies as to whether the armature coils are better held

o r in position by wedges by wire bands , but the most satisfactory a o f o n rrangement seems to be the use bands the small , and

o n wedges o n large armatures . The end connections the arma ture are probably better held in position by steel wire bands .

fo r Bronze rings have been used this purpose , but there is danger that they may become loose and change the balance o f the

to . armature , as it is very difficult fix them securely The ques tion o finsulation o fthe armature winding is extremely impor

to tant , as the armature winding is exposed carbon and copper

o f o n dust from the commutator , and the collection dirt such a DIRECT CURRENT TURBO GENERAT ORS 133 high speed armature is very much greater than on the corre i spo ndi ng low speed . On account of th s it is necessary to be extremely careful in insulating all bare metal on the armature , s o that there will be no danger of flashing over dirty surfaces to ground ; while the insulation on the armature coils must be care

s fully baked and pres ed , so that there will be no shrinkage and consequent movement of the coils . The whole question of satis factory armature and commutator construction lies in working ut u o h o the numerous details in design and manufact re , so as to

mechani tain an armature and commutator , satisfactory both c ally and electrically at the time it is built , and so thoroughly seasoned before put in operation that it will not change appre ciably with time . The question of vibration is one of the most serious difficulties

r r to be considered in these machines . It is difficult to p ede te mine the critical speed of a direct current turbo generator armature; but it is very important that this critical speed of the u generator , when coupled to the steam t rbine , shall not be close to the normal running speed . This usually requires that the armature must be designed w ith the maximum possible diameter t i of shaf , and it is generally necessary to sacrif ce the advantage of low commutator peripheral speed to enable a sufficiently stiff shaft to be used . The ques tion of permanency of balance is equally important , and this requires that there be no relative movement of the component parts of the armature with time , and also that the shaft neither spring nor deflect under the in fluence of the temperature variations obtained . Direct current turbo generators as built a few years ago would operate perfectly

n on test , when first built , but after running six months mecha ical vibration and deterioration of commutator were frequently so great that they could no longer be considered commercial . One of the most satisfactory constructions for small units is a two bearing set , the turbine wheel being. overhung and the two bearings self-aligning ; as this construction obviates any trouble

. i due to lack of alignment With larger un ts , however , it is no longer suitable on account of the axial space required by the tur 134 DIRECT CURRENT T URBO GENERATORS

o r bine , and a three four bearing set with a coupling , preferably

be o m o ne es . a rigid , necessary Such sets again require careful alignment and careful fitting of the coupling and bearings ; other - l wise there will be trouble with vibration . Oil ring ubrication is f e fective in the smaller sizes , but forced flow lubrication is usually

K. 50 W. required in capacities above , if the temperature of the bearing is to be kept within reasonable limits and operation to be reliable . Foreign practice is usually to completely enclose the arma

ir ture , except the commutator , and to supply cooling a from a hi special duct . T s is hardly considered good American practice on account of the difficulty of access , and usually on small machines a semi- enclosed construction with natural cooling is

. a r adopted On large machines , however , as the noise is pp e ciabl - y more than in corresponding low speed units , it may ultimately be found advisable to adopt a more enclosed construction . — Present S ituatio n At the present time the direct current turbo generator can hardly be considered as commercially suit 2 W. able fo r the American market above 500 K. at 50 volt ; and

S n ecesso r the probability is that in larger izes it will be y, for the present , to use an alternating current turbo generator and rotary i converter as a substitute , though th s substitute may be only

W K. W . temporary in the 750 K. . and possibly the . sizes in E it Considerably larger sizes are at present use in urope , but should be remembered that operating conditions there , are by

o f no means as severe as they are here . A typical example European direct current turbo generator installation which was recently inspected by the writer on a large steamship , exemplifies w this latter point . It consisted of four units operating ith me tallic brushes ; the normal load being sufficien t only to fully load i two machines , and the load being changed around from one un t Th to another . egenerators after operating six days in this way ’ were subject to three o r four days overhauling while the boat

w was in port , hich overhauling consisted in replacing the brushes

- with a newly trimmed set , and in carefully sand papering the