a6- Rotary Machines

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*%CI,?p$$:.-'- ..--u - ."-.. ".*Brunel Technical College VOL 1SECT 1Department of Aeronautical Studies CHAP 7

CHAPTER 7ROTARY MA C H I N E S

Simple Generator.1. The simple d o c. gener ato r i s a device for converting mechanicalenergy in to e l ec t r ic a l energy.

I , , A ,briar, brle e ~ e & ~ u a ~ubyuc, c,dke~l L--~ bu- , .I - J 3 o--the load i s always i n one di re ct io n a simple switchin g arrangement, i s attached t o t he loop ends . This device i s c a l l e d t h e'commutator ' .The Simple Commutator.

I

*This co nsi sts of two halves of a copper rin g, each ha lf in sul at edfrom the o ther , and mounted on t h e same s h a f t as th e loop; eachsegment o f ' t h e commutator i s connected t o a loop end.1 e


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VOL 1SECT 1 -7CHAP 7

3 . A s shown i n th e ske tch th e change-over of the segments tak esplace when no EMJ? i s being induced i n the loop. A s the looprota tes . brush C i s i n contac t wi th the conductor pass ingthe North Pole, and bru sh D i s i n con tact with the conduotorpassing the south pole. When the loop has passed through 180'the oondition is still the same and the generated E51F a t t h ebrushes although o f a varying m pl i tud e i s always of the samepola r i ty .By increasing the number of loops and commutator segments a muchsmoozner UV&' f gr ea t er va lue can be obtained.

\ Multi-Bar Commutator.

This con sis ts of th e correct number of copper s t r i p s insu lat edfrom one anot her by mica; th e whole s t r u c t u r e i s then mounted andinsulated from the shaft .Brushes

5. These a re made of g rap hit e (carbon) which has a f a i r l y highres is tance . They ar e f a i r l y so f t and do not normally wear t hecommutator, but ad jus t t he ir contact su rface t o the shape of the


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VOL 1SECT 1c3-m7

u ~ m m ~ ca o r . Tne brushe-sSare kept i n con tac t with the commutatorby means of s pri ng s. The complete cr ad le and sp ri n g assembly i sknown a s th e brush ho lde r. This i s insulated from the case ofthe machine.Induced EMF.

6 . For a constant speed and f i e l d curre nt, th e induced emf i sconstant i s given by -- - . -- - - --E = K x 8 x n Volts - - _ n 7 - 1\ where K i s a cons tant

Z i s f lu x i n webersn i s revolut ions per sec .The armature conductor system has some resistance Ra. When th emachine i s supplying a load, Ra w i l l have a po ten t ia l d i f fere nceac ross i t a s cur ren t flows through th e load. Hence term inalv o l t s V w i l l f a l l .

and the val ue of armature cu rr en t can be determined byI a = E - V AmpsRaGenerator Efficiency

7. i ) Mechanical Eff ic iency = Total watts generated x 100%Mechanical Power Suppliedii) Elec t r i ca l Ef f i c i ency = Watts ava i l ab le i n ex te rna l c i r c u i t x 10@Total Watts Generatediii) Commercial Efficiency = Watt s i n ex te rna l c i r c u i t x 100%Mechanical Power SuppliedThus Mechanical Eff ic ienc y x E le ct r ic al Eff ic ienc y = CommercialEff ic iency.1 Horse Power = 746 Watts.

9

Output Rating,8 . This i s the output voltage which the generator w i l l genera te a tth e s ta te d speed and th e c urr en t which can be sa fe ly taken from th emachine without undue overheating, in t e rn a l v o l t s d rop o r sparking .

A t y p i c a l r a t i n g : ~ O O V , 50(XnA 2,000 rev/min.Regulation.

9. Generato rs ar e give n a maximum lo ad r a ti n g and should n o t beallowed to exceed the st at ed value. The f a l l i n terminal PDbetween no load and full-load i s known a s the r eg ul at io n of t hemachine. A well regulated machine w i l l show only a s l i g h t d ro p


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, L#VOL 1

SECT 1 ,-a-wp 7

VOLTAGE REGULATION = (No-load p.d . - Ful l - load pod) xNo Laad pode - - 4n.7.4The Simple Motor.

An el e c t r i c motor i s a devi ce f o r conve r ti ng e l ec t r i c a l ene rgyin to mechanical energy.When a current I s passed through the armature and the f ieldwinding i s exc i t ed a f o r ce i s s e t up which causes t h e armaturet o ro ta t e . The d i r e c t io n of ro ta t io n can be found by us ingLeft Hand Rule.Commutation.

11. In the case of a d.c. motor a commutator i s e s s e n t ia l i n o r de rt o r e v er se t h e d i r e c t i o n o f t h e c u r r e n t i n t h e a rm at ur e t oma i n t a i n t he r o t a t i on .Ef fi ci en cy of a Motor.

12. Efficiency = Mechan ical Power Output x 10@Elect r ical Power InputEf f i c i ency = E x l a x 100%

V x l a

The Horsepower developed by a motor = E x I a746


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VOL 1SECT 1CHAP 7

1 . This i s the turning fo rce which causes the conductor to r ot at e .The force i s propor t ional to the f lux and the current f lowingaround the loop.

TO- = B X I X N X A Nm - - - 4 n .6where B< = f l u x d e n s it y i n Wb m-2

I = curre nt i n ampsN = number of t ur ns of th e c o i l

, A = ar ea of loop i n m2Back e.m.f.

14. I n a motor th e conductors moving i n t he magnetic f i e l d musthave an induced e me f . produced i n them ( ~ a r a d a y ' Law). Thisinduced e.m.f. opposes th e motion cau sin g i t ( ~ e n z ' Law). Theva lue of t h e induced e.m.f. i s found i n e xa ct ly t he same waya s i n t h e g e n er at or .

INDUCEDe.m.f. = K x di x n V - - _ n 7.7where K = A Constant

iti = Flux i n webersn = Revolutions per secondA t a l l t imes th e back e.m.f. w i l l be i n oppos i tion to th e supplyvol tage . The armature cu rr en t I a w i l l depend upon the resistanceof t he armature and th e dif fer enc e between th e supply vol tag e V sand the back e.m.f. Vb,

I n a running machine the back e.m.f. w i l l be s l i g h t l y l e s s th anth e supply e.m.f. and th e cu rre nt w i l l be j u s t s u f f i c i e n t t o keepth e motor running a t a constant speed. If the load on the machineinc rea ses t h i s ten ds t o slow the machine down, th e induced backe.m.f. w i l l be reduced because th e ve lo ci ty has been reduced.Therefore Vb i s reduced, Ta i s increased and tends t o br ing t hemachine back t o normal speed.Pr ac ti ca l d.c. Motor Construction.

15. The construction i s s im i la r t o th a t of the doc . gene ra to r and i sswnmarised as follows :-a ) So ft i ro n yoke f i t t e d with shaped pole pieces, which areoften laminated.b ) The f i e l d windings may be s e r i e s o r shun t connected anda r e of copper wire. Heavy gauge f o r t he s e r i e s motor,f i n e gauge f o r t he shunt motor. The copper wire i t s e l f

i s sh el la c in su la te d and then wound with c ott on t ape.C ) The armature : Th is i s constructed from s o f t i r o n stampings

' .


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o r Laminations, which ar e assembled and bo lt ed to ge th eron the splined spindle. The armature has s l o t s i n i t t o t ak ethe armature coils . There ar e usua lly many co i l s and t h i sinvolves havlng many segments on the o o m u t a t o ~ . The multi-eegment oommutato~ onsists of a number of copper segmentsplaced together but insulated from one another by mica spaces.The ends of th e armature co il s ar e soldered t o the commutatorsegments.

pie ces . The brushes a r e us ua ll y made of carbon o r copper carbon,and ar e held i n posi ti on on the commutator by l i g h t sp rin gs on thebrush holder s. The brush hold ers co ns is t of metal tubes and canbe square , rec tangular or cyli ndr ical . The brushes make a s l i d i n gf i t within the brush holders. The brushes a r e concaved t o gi vea good conta ct a re a with t he commutator and connection t o t hebrush i s made by a copper fl e x or p i g ta il .Armature Reaction. \ 1 9 *n g a *

Fig. 7.6

In a machine the cu rr ent i n t he windings produces a f i e l d whichr e s u l t s i n a d i s t o r t i o n i n t he main f i e l d . In the motor thee l e c t r i c a l n e u t ra l a x i s lags on the geometr ic neu tra l ax i s i n thed i r ec t i on of r o ta t ion . Thus t o accomplish commutation witho utsparking a t the brushes then . the brushes must be retarded. I fin te rpo l es a re used t he i r po la r i t y must be th a t of the po lesimmediately behind i n th e di rec tio n of r ota tio n.In the ge nera to r the e le c t r i c a l ne u t ra l axis leads the geometricne u t ra l a x i s in the d i re c t ion of ro ta ti on and th e brushes must beadvanced.Field Excita t ion.

18. A s the method f o r cr eat i ng a f i e l d f or machines i s normallyelectromagnetic i t i s convenient i n most machines t o use t h e e.m.f.generated by the machine i t s e l f t o energ ise the f ie l d c o i l s .This i s possible sin ce the re sidu al magnetism i n the pole p ieces


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- a-ll;ows -a =al l e n 2 - L nerated wiien tile machine is --.111.l b U AJt:s ta r t ed and t h i s bu i lds up as t he f ie ld grows.The f i e l d wind ings may be i n s e r ies o r i n pa ra l le l wi th thearmature windings.Methods of Fie ld Ekc i ta t ion .

19. Separately excited machines.

LOAD

~a)SPEED

Where VB i s v o l t s drop across brushesIa Ra i s vol ts d rop ac ross armature

by*Fig. 7.7

Fig. 7.8In t h i s type of machine the terminal volt age i s f a i r l y c on st an t,because the f ie ld current i s independent of the vari,atio ns i n theload current . The only fa l l * n terminal voltage i s hue t o theres ist anc e of the armature and brushes. Variat ions of thegenerated e.m.f. may be accomplished by a va ri ab le s e r i e s r e s i s t o ri n the f i e l d c i r c u i t , t he re by l imi t i ng f i e ld c u r re n t and the re fo reflu x. The re gul at ion of a sep ar ate ly exc ite d machine i s good. Fora motor, field strength i s independent of load var ia t io ns so th atthe motor speed remains v i r tu al l y constan t fo r var ious loa ds.


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20. (a) Generator 9OADI n th i s case some of the armature current flows through the f i e l dwindings. The sm al le r t h e f i e l d c u r re nt the l a rg e r the c u r re nta va i l a b le to the ou ts ide c i rc u i t .Since the flux i s dependent on the ampere turns of the fieldwindings, they are made of many turns of f a i r l y h igh r e s i st a n c ewire. Varia t ion of output can again be achieved by t h e s e r i e sf i e l d r e s i s t a n c e RT.A shunt wound machine has a f a l l i n g c h a r a c t e r i s t i c due t o t h e f a c tthat the more current IL the load takes, the l e s s i s a v a i la b l e f o rt h e f i e l d .(b) Motor.

Here the applie d vol tag e i s a c ros s the f i e ld c o i l s and thearmature. Consequently i f the load increases the speed decreases,and th e back e.m.f. de creas es. This causes an increase i n armaturecurr ent and a corresponding increase i n torque so the spee dbuilds up again.


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'Yhe shun t wound motor i s v i r t u a l l y a constant speed machine and i t sloa d c ha ra c te r i s t i c i s steady. The speed does drop sl i gh t ly a thigh loads because of th e high increased IR drop i n t he armature.S er ies Wound Machines.

21. ( a ) Generator.

In t h i s case the f i e l d curr ent must be the same as the armaturecurrent and the load current . It fo llows the re fore , th a t i f t heload draws more cur rent the f i e l d cu rrent increase s l ikewise .The f i e l d windings i n t h i s case a re a few tu rn s of heavy gaugewire. Since f i e l d current increases with load, th ere must be aninc rease i n the generated e.m.f. t h i s re su l t s i n a r i s i n gc h a r a c t e r i s t i c .Regulation of the generated e.m.f. i s by means of th e pa r al le lres is tance Kf.

SPEED & F ig * 7 e 1 2TORQUE

LOADI n the s er ie s machine the f i e l d curr ent and t he armature curr ent ar eone and the same. I f the load on the machine i s increased i t w i l l


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b l V W U r n- 'I , he b a c ~ .m.f. w i l l be reduced, and the current throughthe armature and the f i e l d inc re as e s. This gives increased torquewhich makes the machine esp ec ia ll y usef ul f o r the purpose of e l e c t r i ct ac-ti n.Compound Wound Machines.

22. ( a ) Generator. Fig. 7.13

mer compoun_ded fel Compounded t

7

a

This i s a combination o f s e r i e s and shunt machines having bothse r ies and shunt f ie lds . It therefore oombines the loadcha ra cte ri s t i c of both types of machine and by su i tab le cho iceof each f i e ld a f a i r l y lev el graph can be obta ined.I f t he s e r i e s f i e l d i s connected s o t h a t i t s f i e ld a id s the s hun tf ie l d , the genera to r i s ca l led 'cumulatively' compound. I f these r ie s f i e l d opposes the shunt f i e l d , the genera tor i s c a l l e d'differentially ' compound.The voltage characteristic of the cumulative compound generatordepend on the ra t io of the turns i n the shunt and se r ie s f i e l d Iwindings. P E E D AND TORQUE


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I n a-sh-t motor t h e r e i s a time la g between the app lica tio n ofan increased load and the build ing up again of the speed. Thisi s an undes i rab le e f fe c t , espec ia l ly i f th e motor i s dr iv ing agenerator, because th e voltage output would drop durin g th e t imela g. For t h i s reason a machine i s produced having a serieswinding so that i t s f i e l d a i d s t h e s h un t f ie l d . A s soon as theloa d on th e motor i s increased the increasing current throughth e a rmature and the se r i es f i e l d w i l l produce a s t ro nge r se r i esf ie ld giving immediately higher torque. The result i s t h a t t hespeed remains s t emotor such as t h i s w i th a s e r i e s c o i l a i di n g t h e main f i e l d - i s\ c a l l e d a cumulative compound wound motor. I f t he s e r i e s c o i lwas wound t o oppose th e shun t f i e l d , i t would be known as ad if f e re n ti a l compound wound motor, bu t t h i s typ e i s ra re ly used .Losses i n R otary Machines.

. 23. These ar e of t he same nat ure f o r a l l ro ta ry machines i . e . motors,genera tors e tc .i ) Copper Losses. This i s due to th e ohmic re si st an ce of thearmature, and f i e l d windings and the I R drop across thebrushes.

A s armature and f i e l d c urr ent s flow through th es e windingsthey diss ipate power I ~ R . This i s wasteful power since i ti s taken from th e in put prime mover and not d el iv er ed t othe ou tput load c i rcu i t .I t i s minimised by keeping th e res is ta nc e of t he winding a ssmall as possible and t h i s i s done by using t h e be st butcheapest conducting mate rial fo r these windings e.g. copper.It i s al so minimised by making th i s wire a s th ic k a s possib le,the l im i t in g fac t or s here being the res u l t i ng bu lk of thefinished machine.

( i i ) I ron Losse s.( a ) H y st e r e si s L o s s T h es el os se s a r e d u e e s s e n t i a l l y t o t h efa c t t h a t t he i ro n am3tu re core and f i e ld po l e s a r e l oca t edwith in a rapid ly changing magnetic fi e ld . Thus they area lt e rn a te l y magnetised and demagnetised, th e magnetisingfo rc e encoun ters i n t he substance a s o r t of 'OPPOSITION' whichcauses the magnetising ef fe ct t o la g behind th e magnetisingforce . This lagging i s ca l l ed HYSTERESIS.The energy lo s s due to t h i s 'OPPOSITION' shows up a s he atproduced wi th in th e substance. To minimise th e hy st er es islo ss the armature core and f i e ld poles a re ge ner al ly made ofso f t i ron , SILICON s t ee l o r ce r ta in o ther a l loy s t h a t havea high permeabili ty.( b ) Eddy Curre nt Losses. Since the i r on armature core i s a l s o


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a conductor, and i s cu t t in g- l ine s of fo rce , an e.m.f.w i l l be induced r esu l t ing i n c i rc u la t ing curren ts whichw i l l cause heat. By Lenz' law th es e Eddy c ur re n ts w i l lc r e a t e a flwr opposing the foroe whloh created them and f o r t h i sreason they are wrdeslrab2e.They ar e minimised by con st ru cti ng t he cor e from many t hi nstampings which are insulated by paper or shellac. Thispreven ts c ur re nt s flowing through th e core and reduces theneax and. waste energy produced by eddy cu rr en ts . Thisarrangement of laminated cores i s al so used i n transformers ,

\ czhokes e tc .( c ) Fric t ional Losses . When a machine i s running, th ere ar evarious f r ic ti o na l lo sse s t o be overcome, each of which requi resa continuous expenditure of energy and results i n heating of ~7the rubbed parts. '3sThere i s f r i c t i o n l o s s i n the machine bearing, a t the surfaceof the commutator due t o th e rubbing of th e brushes , and i nth e armature core due to i t s fanning action (windage).When a generator o r motor m s t a f ix ed speed and genera tesa given vol tag e, th e mechanical l o s s (stray-power loss) i sconstant regard less of the electricaJ, output o r input of themachine, fo r speed and flux dens i ty are the only two factorstha t in f luence the stray-power l o s s .

ICORE Loss.

Remedies24. a ) Brushes Power l o s s a t th e brushes can be reduced byreducing :-

a ) Brush pressure on the commutator.b ) Total contact area of a l l the brushes .c ) Co-e ff ic ien t o f f r ic t ion .The type of brush used i s al so important e.g. brushesconsis t i ng large ly of pure graphite have a re la t i ve ly lowlos s , whereas harder graphite w i l l give an inc rease inbrush loss.


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( b ) Bearings. Sleeve bearings produce the gr ea te st lo ss , soba l l bea rings o r r o l l e r bea rings a r e p r e f e rr ed .(c) Windage. This i s due t o a i r r e s i s t ance and a i r f r i c t i o n a tth e su rf ac e of the armature. Where a fa n i s mounted on thesh af t f o r cooli ng purposes th e power absorbed by th e fa n mustbe added t o t he windage l os s.Loading Requirements of Pm- r s =d Motors.

25. Generators. Shunt Wound: This type should pr ef er ab ly be -s ta rt edwi thout any e lec t r ica l load , e s p e c i a l l y s i n c e it may f a i l t o e x ci t edue to lack of f i e ld cur ren t .Series Wound: In t h i s c as e t h e f i e l d i s an open c i rcu i t i f no loadi s applied and the machine cannot excite. It should therefore best ar te d on load.

26. Motors. Shunt Wound: This typ e should be s t a r t e d o ff loa d duet o t he f a c t t h a t t h e s t a r t i n g t or qu e w i l l be small ( to rque =kx I a x 8 ) as 8 w i l l be very small. The armatu re ha s -smalloppos i tion t o the cu r ren t s ince the re i s v ir tu al ly no back e .m.f .on s tar t ing.Series Wound: I n t h i s c as e t he s t a r t i n g to rq ue w i l l beexcept ional ly high s i nc e both Ia and 8 w i l l be very la rge . Themotor w i l l acc eler ate very quickly, so s er ie s machines can beused f o r t rac t ion purposes where t he s t a r t i ng load i s very heavy.Motor Generator.

27. This i s a device f o r converting one value of dc vol tag e t o anothervalue. It cons is t s o f a motor and a dynamo mounted on t h e sames h a f t . The do c. input turn s th e motor which ro ta te s th e dynamo t o givea d i f fe r en t d .c. ou tpu t. The above can be arra nged i n t hr eedifferent ways.1 ) Two sepa ra te machines with t h e i r s h a ft s coupled.2 ) ,Bu i l t in the same cas t ing wi th separa te a rmatures , f i e ldwindings and commutators f o r each machine.3 ) Using the same pole pieces and armature but having twodi ff er en t s e t s of windings on the armature, th e endsbrought out t o two commutators, one a t ei t h e r end. This

i s known a s a dyno-motor. The motor i s usua l ly shunt o rcompound wound i n ord er t o o bta in a ste ady c ha ra ct er is t i c.This makes th e generato r voltage independent of va ri at io nsof load or supply.Motor Alternator.

28. Thi s i s a s i m i l a r arrangement t o th e motor generator, except th a tthe genera tor i s replaced by an al ternator .


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The RotaryWansfoxmer.29 Fig. 7.15

FISLD

This i s a machine wit h one s e t of f i e l d windings and onearmature. The armature has two windings, one f o r re ce iv in gth e curr ent and turni ng th e armature, th e oth er winding thenbeing a generator and i t s output being then taken v i a acommutator a s shown. The motor windings may be shu nt o r s e r i e swound. Se ri es windings a re employed where quick s t a r t i n g i sdesi rab le . In th e case of the shunt wound type some type o fst ar t i ng res ist ance should be included.Rotary Converter. Fig. 7.16

The con str ucti on of t h i s type of machine i s r a t h e r s i m i l a r t o t h erot ary transformer but t o obta in an AC o utpu t s l i p r in g s a re u sedin s te ad of a commutator from th e outp ut t o th e machine. Themachine i s usually shunt wound and the effective output would be0.707 of t he a.c. peak value.Interference Suppression.

31. The most common form of interference i s that which i s caused bysparking. Sparking i s caused where co ntac ts a re co nti nu all y being -,opened and closed. The ef fe ct of sparking i s t o produce h igh


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VOL 1SECT 1CHAP 7

frequency currents i n the leads t o th e cont acts which w i l l causethe radiation of electro-magnetic waves from the leads (Radiowaves). These r e f . waves w i l l be picked up by receiving aerialsi n t h e v i c i n i t y and w i l l cause in terference. If t h i s i n t e r f e r e n c ei s bad normal re cep ti on may be impossible and fo r t h i s reasonspar ki ng must be reduced t o a minimum.Suppress ion Fil ters .

--22

These consist of a capacito r from the sparking contact t o ea rt hand a high frequency choke i n s er i es with th e supply. Thesechokes oppose any f luc tua t ions i n l i ne c u r ren t and the c a pa ci to r sof fe r easy pa ths t o e a r th fo r th e h igh f requency curren ts . As upp re s s ion f i l t e r i s shown for a genera tor bu t these f i l t e r s a r eal so used on the inpu t c i r cu i ts of motors. It may be nece ssa ry t oprovide more than one f i l t e r and fu rt he r sec ti ons can be addeda s shown i n t he diagram. These f i l t e r s a re only e f f ec t iv e whenconnected a s c lose as poss ible t o the spark source.A fu r the r p recaut ion i n avo iding in te r fe rence i s to s c re e n the l e a dsby using a cable such a s lead covered cable and eart hi ng t he ou tercovering. This forms an e l e c t r o s t a t i c s h i e l d .

I

The cast in g of the machine and au xi li ar y u n i t s should al so beearthed.


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I n many cases th e out put from a generator i s not smooth enough andi f it was used as th e h o t . supply fo r a rece iver it could oausea background hum (buzz). The output from the generator can bemade smooth using a smoothing f i l t e r across th e output leads . Thecapac i to r C w i l l charge t o the peak voltage. When th e ge ne ra to routpu t f a l l s the c a pa ci to r will p a r t i a l l y d i sc ha rg e i n t o t h ere s i s t ive loa d . The current taken by the load w i l l de te rmine howsmooth the output w i l l be, because t he more th e cu rr en t taken,the quicker the f a l l of the capa citor voltage .

34. B e t t er r e s u l t s can be obtained so th at the output from thegenerator i s smoother s t i l l .

In fig. 7;-20 C1 agai n charges t o the peak value and so does C2.5 offers opposit ion to the cur rent changes thus decreasing t he iramplitude. C2 o f f e r s an easy pa th fo r the remaining ripple when

, .c q a r e d w i t h the load. - 16 - ?. ?a,.-. z t,..-,. -z - '' .$?&

f l


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VOL 1SECT 1CHAP 7

Alternators .-- t \ -33 . \ a ) ~ r m a ~ ; u r e . 1s only rouna i n a l t e r na t or s o r smal l, power ra ti ng and no t g ener ally used.

(b) Rota t ing Fie ld . This machine has a s ta t ionary armaturewinding, o r s ta to r , and a rotor , or rota t in g f i e l d winding.

36. Stators of high or low speed machines are essentially the same.They co ns is t of lamin ated i r o n c ores, wit h *t he armat ure windings embeddedi n t h i s core . The core i s secured t o th e s t a to r frame. Thearmature windings a r e c ut by the ro ta t ing magnetic f i e l d . The voltagegenerated i n the armature as a r e s u l t of t h i s a c t i on i s appliedd i r e c t l y t o t h e lo ad .37. Advantages of Stator.

The fix ed connections of the s t a t o r to t he load means th at thesewindings ar e much more e as il y insul ated a t very high voltages, thani f s l i p r ing s were used.Rotors.

37. ( a ) Turbine Driven: These a r e high speed (1200 r . p m. or more)ro to rs which ar e cyli ndr ic al and small i n diameter, andhaving windings fir ml y embedded i n t he s l o t s provided. Thewindings are arranged to form 2 o r 4 d i s t i n c t f i e l d p o le s.(b) S a l i e n t - Pole: This type of r ot or i s used i n low speed(1200 r.p.m. o r l e s s ) rot or s and has a number of s ep ar at el ywound pole p iec es which ar e bol ted t o t he frame of th ero tor . The f i e l d windings are e i t he r connected i n ser ies ,o r i n s e r i e s g roups connected in pa ra l l e l . In e i t he r ca seth e ends of th e windings connect t o s l i p rin gs mountedon the shaft .Regardless of the type of ro to r f ie ld used, i t s windings areseparate l y excited, usually by a DC genera tor ca l led an exc i te r ,Altern ator Rating.

39. Every a l te rna tor ra t ing i s expressed i n VOLT-AMPERES o r K I LOVOLT-AMPERES (KVA) of the apparent power which the alternator cansupply. This means t o say th at the al te rn at or i s ra te d i n termsof the maximum c u r ren t which i t can safely carry, without damageto the in s u la t ion , as well a s in terms of the voltage output .


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VOL 1SECT 1CHAP 7

This simple diagram i l l u s t r a t e s a two pole s in gle phase a l t ern ato r.The rotor consists of two pol es of opposit e po la ri ty . A s the ro to rturns , i t s poles induce ac vol tages i n the s t a t o r windings.The two c o i l s of th e s ta to r winding ar e connected i n such a wayth a t the a c vo l t age s induced i n them are i n phase or series a iding.Two Phase.


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VOL 1 .,SECT 1CHAP 7

fTb.rn Dn l n ---- --42. The st a t o r con si st s of two si ng le phase windings completelysepara ted from one another, and phy sica lly spaced so th a t th e

ac voltage induced i n one i s ni ne ty degrees out of phase withthe vol tage induced i n the other . Spacing between the windingsi s such t h a t when one i s being cu t by maximum f l u x th e o the r i sbeing cu t by no f lu x a t a l l .

ro+ysr is i c ? e ~ t i ~ ~ lo t ha t used i~ a s i n g l e phasea1 ernator .

\

When th e r ot or p oles a re oppo site Phase A windings then maximumvoltage i s induced i n phase A and zero v o lt s i n phase B. A s t h erotor moves away from the A windings and approaches the B windingsth e voltag e induced i n phase A decreases from m a x i m u m value andth e vol tage induced in phase B inc re ase s from zero. With therotor poles opposite phase B windings volta ge induced i n phase Bi s maximum and th e vo lt ag e induced i n phase A has dropped t o zero.Three Phase,

fl)' Fig. 7.22\ \

The thre e phase alt er na to r has thre e s ing le phase windings sospaced th a t th e volt age s induced i n any one phase i s displaced by120' from th e o th er two. The three phases are independent of eachother . The voltage waveforms generated across each phase aredrawn i n Fig. 7.25 phase distance 120 from each other.8

A sim plif ied diagram of the.win dings i s shown, w i t h t h e r o t o romit ted for s implici ty.S ta r Connection. A

STAR POINT - IFig. 7.23

LINE VOLTAGE


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VOL 1SECT 1CHAP 7

'llhe po in t of connecti on i s cal le d the neut ra l Point , andthe vol tage from th i s point t o any one of th e l i n e leads w i l l be t h ephase voltage. The t o ta l vol tage o r l i n e vol tage across any twol i n e s i s t he vec to r sum of the individual phase voltages. Thel i n e v o lt ag e VL i s D . v ~r 1.73 t imes the phase voltage.Since the windings form only one path for the current betweenphases, th e l i n e and phase curren ts ar e equal; IL = Ip

Fig. 7.25

In the d e l t a connec tion the l in e vo l t ages a re equa l t o t he phasevol tage , but the l in e currents w i l l be equal t o the vec to r sum ofth e phase cur rents . Since th e phases a re 120 out of phase, th el i n e c u r r e n t s w i l l be 1.73 t imes th e phase cu rrent .Output Frequency.

46. The frequency of t h e a.c, generated by an al te rn at or depends on thespeed of ro ta ti on and on the number of p a i r s of r o t o r po les . -When a r ot or ha s ro ta te d through a su ff ic ie nt ly wide angle fo r twoopposi te poles (a north and a sou th) t o have passed one s ta to rwinding, the vo ltag e induced i n th e winding w i l l have passed througha complete cycle of 360'.

*

Frequency F = N x 2 Hz6 5 2= Rotor rpm x No. of Poles Hz

60 2For an e igh t po le a l t e rn a to r ro t a t ing a t 750 rpm = 750 x 8 = 50 Hz

60 2A.C. Motors.

47. An A.C. motor can be designed t o oper ate from a si ng le o r multi-phase a.c . supply, b ut i n e i t h er case i t operates on the sameprinc iple. The a.c , applied t o th e motor generates a r o t a t i n g


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*VOL 1SECT 1c l a p 7 '\

magnetic f i e l d cau-gthe ro to r of the m n t o r to k r r ? .Synchronous Motor.

48. This motor i s an a l t e rn a to r ope ra ted a s a motor i n which an a.c.i s appl ied t o the s t a t o r and a d.c. i s app li ed to the ro to r . Thesynchronous motor i s so cal led because i t s r o t o r i s synchronisedw i th the ro ta t ing f i e l d s e t up by the s t a t o r .S ta to r .

\ 49. The appl ica t ion of a t h ree phase supply t o th e s t a to r causes aro t a t i ng magnet ic f ie l d t o be s e t up round th e ro tor . Thep o l a r i t y of t h i s f i e l d w i l l be as shown i n the d iagram t h i s be ingdue to the current passing through the d e l ta connected s ta to r- windings.

3 PHASEAC INPUT Fig. 7.26

These windings A, AL - B, and C, C1 i n each phase a r e woundi n the same dire ct ion , but as these phases ca rry opposi te currents ,(e.g. B & C ) then opposite poles w i l l be produced t o giv e acombined s t a t o r f ie ld which re ac ts on the rot or .Rotor.

50. Because the rotor i s energised by d.c. i t behaves l i k e a barmagnet and the ref ore l i n e s up with t he magnetic f i e l d caused by th eappl ica t ion of three phase a .c . to the s ta to r . I f th e magneticf i e l d tu rns , t he ro to r w i l l t u rn w i th the f i e ld . If t h e r o t a t in gmagnetic field i s strong, i t w i l l exer t a s t ro ng turning force onthe rotor which w i l l be therefore able to turn a load as i t r o t a t e s .(se e Fig. 7.28)

51. The speed of ro ta ti on of th e magnetic f i e l d depends on thefrequency of the th re e phase a.c.- then since th e frequency i sfixed, synchronous motors ar e i n pra ct ic e s in gl e speed motors.They are used for loads which require constant speed from no loadr ig h t th rough t o the f u l l l oad cond i tion.


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VOL 1SECT 1CHAP 7

52. One of t h e disadvantag es of a synchronous motor i s t h a t it cannotbe s tarted from a s t a n d s t i l l by applying thr ee phase a.c . t o thes ta to r . The ins ta n t a .c. i s appl ied t o the s t a to r , a h igh speedro ta t ing f i e l d a ppe ars. Th is ro ta t ing f i e l d rus he s pa s t the ro to rpoles so quickly th at the rot or does not have a chance t o ge ts t a r t e d ; it i s repe l led f i r s t i n one d i r ec t i on and t he n i n t h eother. So a synchronous motor i n i t s pure form has no starting

3 ?++AS A. C. A P P L ~ C ~o STATOG.-23- Fig, 7.27


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*

VOL 1SECT 1CHAP 7

3 - usua l ly s t a r te d the refore wi th the he lp of a small inductionmotor, or with windings equivalent t o t h i s incorporated i n thesynchronous machine. When th e r o t o r has been brought ne ar t osynchronous speed by th e s t a r t i n g device, i t i s energised byconnecting I t t o a d .a , voltage source, The r o t o r t hen f a l l si n t o s t e p with the ro ta t ing f i e ld .Induction Motors.

r 12-0 curren ts a re induced i n the ro to r c i r cu i t by the ro t a t i ng -\ magnetic f i e l d i n the s t a t o r .

S ta to r .55. The s t a t o r cons tru ctio n of the inductio n motor, and of th esynchronous motor a r e almost id en ti ca l.

Rotor. .-.

SEW - CLOSED

Eh'D R INS

The rotor i s a laminated cylinder with s l ot s i n i t s surface .The "s qu ir re l cage rot or " i s made up of heavy copper barsconnected togethe r a t e it he r end by a metal ring made of copperor b rass. No ins u la t ion i s needed between the core and the barsbecause of the very low voltages generated i n the ro to r bars .The a i r gap between th e r oto r and the s t a t o r i s kept very smallso a s t o obtain maximum f i e l d str eng th .

57 . F'rinciple of Operation Fig. 7.29


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VOL 1SECT 1CHAP 7

/ W s t a - b i n g ~ a g r r e t i c d d ge11 d l V m i l nduces ane.m.f. i n the ro to r . The curren t i n the ro to r c i rc u i t causedby t h i s induced e.m.f. s e t s up a magnetic field. The two fieldsin te r a c t and c ause the ro to r t o tu rn .This ro ta t i ng f i e l d cu ts the ba rs of the r o t o r and induces acurr ent i n them and t h i s induced cur rent w i l l generate a magneticf i e l d around the conductors of t he ro to r, which w i l l t r y t o l i neup w it h t h e s t a t o r f i e l d . However, s inc e th e s t a t o r f i e l d i s- + - + . t - n - n n + . t n ~ rcllL O u a r ~ r g U L A r r ~ i t e i r ~ + n - m - m n n ~ + 4 n n - * - -. --t+LA 4 t ,u u ~ a L u A u u A A A A ~ UP W A U L -Lalways follow along behind it.

\

S l i p .58. It i s imposs ib le fo r the ro t o r o f an induction motor t o tur n a s

fas t as the ro ta t ing magnet ic f i e l d . I f t he speeds were thesame, no re l a t i v e motion would e x i s t between th e two and s o noinduced e.m.f. would re su l t i n th e ro to r.Without induced e.m.f, no tu rn ing fo rc e would be ex er te d on thero tor . The ro to r must ro ta te a t a speed lower than th a t of th ero t a t in g magnetic f i e l d i f re la t ive motion i s t o e x i s t betweenthe two.

Fig, 7.30

This percentage di ff er enc e between the speed of th e r ot at in gst a t o r f i e l d and the speed of the ro tor i s call ed "s l ip ". Thes mal ler the s l i p the c lo s e r the ro to r s pe ed w i l l approach the speedof t he s t a t o r f i e l d .

S l i p = Ss - R s x 100%SsSs = Synchronous speedRs = Rotor speed


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VOL 1SECT 1 ,\CHAP 7

e ro to r depends on th e torque requ irements of th eload. The bigger th e load, th e stro nge r the tur nin g forc e neededt o r o t a t e t h e r ot o r. But, the turning f orc e can only incre asei f the rotor induced e.m.f. increases; an8 t h i s e.m.f. om onlyincrease if the magnetic field outs through t he r o t or a t a f a s t e rr a t e . 30, to increase the re la t ive speed between th e f i e l d andt h e ro to r, th e r o to r must SLOW DOWN.

60. For heavier loads therefore the induction motor w i l l turn s lower-,loads. Actual ly only a s l i g h t changei n speed i s necessary to produce th e cu rre nt changes require d f o r

\ formal changes i n load . This i s because th e r ot or windings havesuch a very low resistance. Induc t ion motors a re there fore fo ra l l practical purpoges constant speed motors.

I Two Phase Ind uc tion Motors.

IAxts OF -4 % B M A ~ ~ ~ t ~ELD*

A two phase induction motor has i t s s t a t o r made up of two windingsp laced a t r ig h t ang les to each o ther around the s t a t or .I t i s arranged that the vol tages A - A1 and B - B1 a r e 90' outof phase. This means tha t the magnetic fi e ld s ar e al so go0 outof phase, and th es e on adding to ge th er a t every ins tan t dur ingtheir cycle produce a r e su l t a n t f i e l d which w i l l ro ta te onerevolut ion f o r each cycle of a.c.Fig. 7.32 shows two al te rn at in g magnetic f i e l d s which ar e 90' outof phase. By arranging thing s i n t h i s way a ro ta ti ng magnetic f i e l dhas been produced and t h i s w i l l pul l the rotor round with i t .Single Phase Motors.

62. A sin gl e phase induction motor has only one s t a t o r winding. Thiswinding generates a f i e l d which can be sa id t o a l te rn at e acro ssthe ax is of the s ing le winding rat he r than ro ta te .


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VOL 1SECT 1CX-IAp 7

Fig, 7.32LP PAQANT MOVL A L T ~ ~ N A - ~ ~a s b eOF-NO ~ T HOLE.

When the rotor i s s ta t i ona ry the expanding and collaps in g f i e l dinduces curren t i n the ro t o r which genera tes a ro t o r f ie ld . Thein te rac t ion of these f ie ld s exer ts a , f o r ce on the ro to r , whicht r i e s t o t ur n i t 180' from i t s pos i t ion . However, si nc e t h i sfo rc e i s exerted through the ' cent re of t he rotor, i t does nottur n, Thus the same st ar ti n g arrangements ar e necessary fo r asi ng le phase inducti on motor.

I f now, the rot o r i s turne d, the tu rning fo rc e i n th a t d i re c t i oni s aide d by the momentum of the r o to r , and a l s o by the inducedpo l ar i t y of t he r ot or , (a s shown) which can now be pul le d roundi n the d i re c t io n of ro ta t io n toward the south pole.The induced e.m.f, i n the r ot or has th e po l ar i t y shown becausei t i s the re la t i ve motion of t he r oto r which decides the dire ct i onof the induced e.m.f. and not the actual r ot at io n of th e rot ori t s e l f . So, the turnin g force on the rotor , due t o the inducedp o l a r i t y w i l l help the ro tor continue on i n the dir ect io n of i t soriginal movement.


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VOL 1SECT 1CHAP 7

-ovemenbs 01 t he f i e l d t a k es p la ce a t a f a s t e rr a t e than t he a ctu al movement, the ref ore the r e la t iv e movementof th e ro to r w i l l be i n the opposi te di re ct i on a s shown.

Fig.7.36 shows a simp lifi ed cap aci tor s t a r t motor. The statorco ns is ts of t he main winding and a star-t;ing winding which i sconnected i n pa ra ll el with t he main winding and spaced a tr igh t ang les to i t . The go0 e lec t r i ca l phase d i f f e rencebetween the two windings Is obtained by connecting th e a ux il i arywinding i n s er ie s wi th the capaoi tor and s t a r t i n g s w it ch .On s ta r t in g , the switch i s closed, p lac ing the capaci to r i nse r i es wi th the aux i l ia ry winding, The capaci tor i s of sucha va lue th a t the au xi l i a ry winding i s e f f e ct i v e ly a r e s i s t i v e ,capac it ive c i r cu i t i n which th e cur ren t l eads the l in e vo lt ageby approximately 45'. The main winding has enough inductancet o cause th e curren t t o la g the l i n e voltage by apprdximately 45'.

Brushless Generator

The two cu rre nt s a re the re fo re 90' out of phase and s o a r e t h emagnetic f i el d s they generate. The ef fe ct i s t ha t t he twowindings a c t as a two phase st a t o r and produce th e revo lvingf i e l d r equ ired t o s t a r t t he motor.

t

Brushes i n ro t ary machines of a i r cr af t f l y i ng a t low a l t i t u de sare genera l ly s uf f ic ien t ly lubr ic a ted by the moist a tmosphere.A t h ig he r a l t i t u d e s t h e a i r i s colde r and d r i e r and brush wearincrea ses s i gn if i ca nt ly, Brushless generators have been useds ince the ea r ly 1950 's t o avoid t h i s problem,Figure 7-36 shows a simple br us hl es s generator,A and A representthe s ta t i ona ry f i e ld winding of th e ex ci ter genera tor , B i s t h erotat ing armature, The ac output of the rotor i s r e c t i f i e d , i . e .converted t o dc, by a b r i d g e r e c t i f i e r C ( fo r d e t a i l s see Vol.1Sec 1Chap 9) . The dc output of the r e c t i f i e r i s f ed t of i e l d c o i l D which induses ac i n t h e s t a t o r c o i l s F and


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A part of t h i s ac output i s re c t i ied and fed to theftat ion ary f i el d windings A and A .67. , The rot atin g armature coil , br idge re ct if ie r , rota tin g f ie l d co iland permanent magnet E, a l l r o t at e on a hollow shaft. Coolinga i r i s forced through the hollow shaft. The permanent magnetavoids any need for flashing the field.

BRUSHLESS GENERATOR

BRUNEL TECHNICAL COLLEGE, BRISTOLGSB/PJR

a6- Rotary Machines

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