Unit 7_Synchronous_Motor asyncronous and syncronous.pdf

A synchronous motor is one in which the rotor normally rotates at the same speed as the revolving field in the machine. The stator is similar to that of an induction machine consisting of a cylindrical iron frame with windings, usually three-phase, located in slots around the inner periphery. The difference is in the rotor, which normally contains an insulated winding connected through slip rings or other means to a source of direct current (see figure).
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The principle of operation of a synchronous motor can be understood by considering the stator windings to be connected to a three-phase alternating-current supply. The effect of the stator current is to establish a magnetic field rotating at 120 f/p revolutions per minute for a frequency of f hertz and for p poles. A direct current in a p-pole field winding on the rotor will also produce a magnetic field rotating at rotor speed. If the rotor speed is made equal to that of the stator field and there is no load torque, these two magnetic fields will tend to align with each other. As mechanical load is applied, the rotor slips back a number of degrees with respect to the rotating field of the stator, developing torque and continuing to be drawn around by this rotating field. The angle between the fields increases as load torque is increased. The maximum available torque is achieved when the angle by which the rotor field lags the stator field is 90°. Application of more load torque will stall the motor.

One advantage of the synchronous motor is that the magnetic field of the machine can be produced by the direct current in the field winding, so that the stator windings need to provide only a power component of current in phase with the applied stator voltage—i.e., the motor can operate at unity power factor. This condition minimizes the losses and heating in the stator windings.

The power factor of the stator electrical input can be directly controlled by adjustment of the field current. If the field current is increased beyond the value required to provide the magnetic field, the stator current changes to include a component to compensate for this overmagnetization. The result will be a total stator current that leads the stator voltage in phase, thus providing to the power system reactive volt-amperes needed to magnetize other apparatuses connected to the system such as transformers and induction motors. Operation of a large synchronous motor at such a leading power factor may be an effective way of improving the overall power factor of the electrical loads in a manufacturing plant to avoid additional electric supply rates that may otherwise be charged for low power-factor loads.

Three-phase synchronous motors find their major application in industrial situations where there is a large, reasonably steady mechanical load, usually in excess of 300 kilowatts, and where th

1. Unit 7
SYNCHRONOUS MOTOR
PART-II
Electrical Machines-II 1
Dr. D. A. Tamboli

2. Electrical Machines-II 2
▪Introduction
▪Construction
▪Principleofoperation,
▪Methodsofstarting,
▪Equivalentcircuit
▪Performanceandvectordiagramwithdifferent
excitations
▪VandinvertedVcurves
▪Hunting-itscausesandremedies
Content

3. 3
Synchronous Motor-General
▪Itmayberecalledthatad.c.generatorcanberunasad.c.motor.
▪Similarlyasynchronousmotoriselectricallyidenticalwithan
alternatorora.c.generator.
▪Infact,agivensynchronousmachinemaybeused,atleast
theoretically,asanalternator,whendrivenmechanicallyorasa
motor,whendrivenelectrically,byconnectingitsarmature
windingtoa3-phasesupply.Itisthencalledasynchronousmotor.
▪Asthenameimplies,asynchronousmotorrunsatsynchronous
speed(Ns=120f/P)i.e.,insynchronismwiththerevolvingfield
producedbythe3-phasesupply.
▪Thespeedofrotationis,therefore,tiedtothefrequencyofthe
source.Sincethefrequencyisfixed,themotorspeedstays
constantirrespectiveoftheloadorvoltageof3-phasesupply.
▪However,synchronousmotorsarenotusedsomuchbecausethey
runatconstantspeed(i.e.,synchronousspeed)butbecausethey
possessotheruniqueelectricalproperties.

4. 4
Some characteristic features of a synchronous motor:
1.Itrunseitheratsynchronousspeedornotatalli.e.whilerunningit
maintainsaconstantspeed.Theonlywaytochangeitsspeedisto
varythesupplyfrequency(becauseNs=120f/P).
2.Itisnotinherentlyself-startingandanauxiliarymeanshastobe
usedforstartingit.Ithastoberunuptosynchronous(ornear
synchronous)speedbysomemeans,beforeitcanbesynchronized
tothesupply.
3.Itiscapableofbeingoperatedunderawiderangeofpower
factors,bothlaggingandleading.Hence,itcanbeusedforpower
correctionpurposes,inadditiontosupplyingtorquetodriveloads.
4.Synchronousmotorsaregenerallyofthesalientpoletype.

5. 5
Construction
▪Likeanalternator,asynchronousmotorhasthefollowingtwoparts:
▪Astatorwhichhouses3-phasearmaturewindingintheslotsofthe
statorcoreandreceivespowerfroma3-phasesupply.
▪Arotorthathasasetofsalientpolesexcitedbydirectcurrenttoform
alternateNandSpoles.Theexcitingcoilsareconnectedinseriesto
twoslipringsanddirectcurrentisfedintothewindingfroman
externalexcitermountedontherotorshaft.Rotorholdsthefield
winding.Therotorcanbeofsalient-poletypeorcylindricaltype.
▪Synchronousmotorislikelytohuntandso
damperwindingsarealsoprovidedinthe
rotorpoles.
▪Thestatoriswoundforthesamenumberof
polesastherotorpoles.
▪Animportantdrawbackofasynchronous
motoristhatitisnotself-startingand
auxiliarymeanshavetobeusedforstartingit.
▪Synchronousspeed,Ns=120f/P

6. 6
Advantages
1.Forlargepower(50kw-2500kw)andlowspeedapplications
synchronousmotorarecheaperandsmallsizecomparedto
equivalentI.M
2.Constantspeed,andchangeinspeedcanbeachievedbychangein
frequency.
3.SMcanbeusedforpowerfactorcorrectioninadditionto
supplyingtorquetodriveloads.
4.SMcanoperatelagging,leadingandunitypowerfactorbutIM
canoperateonlylaggingpowerfactor.
5.Synchronousmotorscanbeconstructedwithwiderairgapsthan
inductionmotorswhichmakesthesemotorsmechanicallymore
stable
6.Itusuallyrunsathigherefficiencies,especiallyinlow-speedunity
powerfactor.

7. 7
Disadvantages:
1.Synchronousmotorsrequiresdcexcitationwhichmustbesuppliedfrom
externalsources
2.Synchronousmotorsareinherentlynotselfstartingmotorsandneeds
somearrangementforitsstartingandsynchronizing
3.ThecostperkWoutputisgenerallyhigherthanthatofinductionmotors
4.Thesemotorscannotbeusedforvariablespeedapplicationsasthereisno
possibilityofspeedadjustmentunlesstheincomingsupplyfrequencyis
adjusted(VariableFrequencyDrives)
5.Theycannotbestartedonloadbecauseitsstartingtorqueiszero
(Tst/TFL=0orTFL/Tst=ꝏ).
6.Thesemotorshavetendencytohunt,soadditionaldamperwindingis
necessary.
7.Collectorringsandbrushesarerequiredresultinginincreasein
maintenance
8.Synchronousmotorscannotbeusefulforapplicationsrequiringfrequent
startingorhighstartingtorquesrequiredbecausewhenloadingonthe
synchronousmotorincreasesbeyonditscapability,thesynchronism
betweenrotorandstatorrotatingmagneticfieldislostandmotorcomesto
halt.

8. 8
Synchronous motors find extensive application for the following classes of
service:
1. Power factor correction;
2. Constant speed, constant load drives; and
3. Voltage regulation.
Applications
1.Large power and low head pumps
2.Becauseofthehigherefficiencypossiblewithsynchronousmotors,they
canbeusedforloadswhereconstantspeedisrequired.Typical
applicationsofahigh-speedsynchronousmotor(above500rpm)are
exhaustfans,blowers,centrifugalpumpsandcompressors.
3.Steel mills and rolling mills
4.Sincesynchronousmotorscanbebuiltforspeedsaslowas120RPM.
Theyarewell-suitedfordirectconnectiontoreciprocatingcompressors.
5.Crushesandblowers
6.Synchronouscondenserinpowerhousesandsub-stationsinparallelto
thebusbarstoimprovepowerfactor.
7.Itisusedtocontrolthevoltageattheendoftransmissionlinebyvarying
itsexcitation.

9. 9
BASIS OF
DIFFERENCE
SYNCHRONOUS MOTOR INDUCTION MOTOR
Type of ExcitationA synchronous motor is a doubly excited
machine.
An induction motor is a single excited machine.
Supply SystemIts armature winding is energized from an AC
source and its field winding from a DC source.
Its stator winding is energized from an AC
source.
Speed It always runs at synchronous speed. The speed
is independent of load.
If the load increased the speed of the IM
decreases & always less than the syn. speed.
Speed controlNot possible Possible but difficult
Load
characteristics
Motor is sensitive to sudden load variations and
hunting results
Phenomenon of hunting absent
Maintenance More less
Starting It is not self starting. It has to be run up to
synchronous speed by any means before it can
be synchronized to AC supply.
Induction motor has self starting torque.
Operation A synchronous motor can be operated with
lagging and leading power by changing its
excitation.
An induction motor operates only at a lagging
power factor.
Usage It can be used for power factor correction in
addition to supplying torque to drive
mechanical loads.
An induction motor is used for driving
mechanical loads only.
Efficiency It is more efficient than an induction motor of
the same output and voltage rating.
Its efficiency is lesser than that of the
synchronous motor of the same output and the
voltage rating.
Cost Costlier than an induction motor of the same
output and voltage rating
Cheaper than the synchronous motor of the
same output and voltage rating.

10. 10
▪Thestatoriswoundforthesimilarnumberofpolesasthatofrotor,and
fedwiththreephaseACsupply.The3phaseACsupplyproducesa
rotatingmagneticfieldinstator.
▪TherotorwindingisfedwithDCsupplywhichmagnetizestherotor.
Principle of operation of Synchronous motor
▪Consideratwopolesynchronousmachineasshown
infigurebelow.Now,thestatorpolesarerevolving
withsynchronousspeed(letssayclockwise).Ifthe
rotorpositionissuchthat,Npoleoftherotorisnear
theNpoleofthestator,thenthepolesofthestator
androtorwillrepeleachother,andthetorque
producedwillbeanticlockwise.
▪Thestatorpolesarerotatingwithsynchronousspeed,
andtheyrotatearoundveryfastandinterchangetheir
position.Butatthisverysoon,rotorcannotrotate
withthesameangle(Nless,polemore,dueto
inertia,diameterhighduetolargersize),andthe
nextpositionwillbelikelythesecondschematic
whererotorpoletrytolockwithstatorpole

11. 11
▪Inthiscase,polesofthestatorwillattractthepolesofrotor,andthe
torqueproducedwillbeclockwise.
▪Hence,therotorwillundergotoarapidlyreversingtorque,andthemotor
willnotstart.
▪But,iftherotorisrotateduptothesynchronousspeedofthestatorby
meansofanexternalforce(inthedirectionofrevolvingfieldofthe
stator),andtherotorfieldisexcitednearthesynchronousspeed,thepoles
ofstatorwillkeepattractingtheoppositepolesoftherotor(astherotoris
also,now,rotatingwithitandthepositionofthepoleswillbesimilar
throughoutthecycle).Now,therotorwillundergounidirectionaltorque.
Theoppositepolesofthestatorandrotorwillgetlockedwitheachother,
andtherotorwillrotateatthesynchronousspeed.

12. 12
▪ThismotorsaresomewhatsimilartoIM,thereforethisphenomenoncan
beexplainedinanotherwayas
▪Toproduceunidirectionaltorque,relativespeedbetweenstatorandrotor
mmf=0.IncaseofIM,thespeedofstatorfluxisNsandinitiallywhen
therotorisstationary,relativespeedbetweenthestatorandrotoris(Ns-
N)(ButatstandstillN=0,fr=f).Nowwhenrotorpicksupthespeed,fr=sf
andN=Ns(1-s).Thereforespeedofrotorinspaceorw.r.t.stator=sNs+
N=Ns,sounidirectionaltorqueisproduced.
▪Butincaseofsynchronousmotor,statormmf(flux)speedisNs,Butthe
rotorspeediszeroandduetosalientpolesexcitedfromdcsupplyare
fixed,noemfisinducedinrotorlikeIM.Sotherelativespeedbetween
statorandrotorisnotequaltozero,sounidirectionaltorqueisnot
produced.
▪Toproducetheunidirectionaltorque,therelativespeedbetweenstator
androtormmfshouldbenearlyequaltozero,sosynchronousmotorare
notselfstarting.

13. Electrical Machines-II 13
▪Introduction
▪StartingmethodsofSynchronousmotor
I.Auxiliary motor starting
II.By using a damper winding
III.Starting with synchronous induction motor
Content

14. 14
Some characteristic features of a synchronous motor:
1.Itrunseitheratsynchronousspeedornotatalli.e.whilerunningit
maintainsaconstantspeed.Theonlywaytochangeitsspeedisto
varythesupplyfrequency(becauseNs=120f/P).
2.Itisnotinherentlyself-startingandanauxiliarymeanshastobe
usedforstartingit.Ithastoberunuptosynchronous(ornear
synchronous)speedbysomemeans,beforeitcanbesynchronized
tothesupply.
3.Itiscapableofbeingoperatedunderawiderangeofpower
factors,bothlaggingandleading.Hence,itcanbeusedforpower
correctionpurposes,inadditiontosupplyingtorquetodriveloads.
4.Synchronousmotorsaregenerallyofthesalientpoletype.

15. 15
Starting methods of Synchronous motor
The different methods that are generally followed to start
the synchronous motor are
I.Auxiliary motor starting (By using small DC motor or a
pony motor or induction motor) -starting under no load
or light load conditions
II.By using a damper winding -starting under no load or
light load conditions
III.Starting with synchronous induction motor –started
under loaded condition-practical case

16. 16
I] Starting with auxiliary motor starting (By using small DC
motor or a pony motor or induction motor)
▪Initially3phasesupplyisgiventostator,itproducesarotatingmagnetic
fieldthatrotateswithsynchronousspeed.
▪Inthismethod,therotorofthesynchronousmotorisbroughttoits
synchronousspeedwiththehelpofanexternalDCmotororinduction
motor.Thisexternalmotoriscalledtheponymotor.
▪Atthattime,theDCexcitationofthefieldsystemisremoved.Ifthedc
excitationisgivenatstarting,itproducedrotationalemfduetorelative
motionbetweenstatorandrotoraddwithDCemf(Excitation).Sothis
highvoltagemaydamagetheinsulation.
▪ToavoidthisDCexcitationisdisconnectedwhileconnectingto
auxiliarymotororsometimesitmaybeconnectedtoahighresistance
(5-6timesoffieldresistance)whichproducesatorqueandmotor
accelerateswithhigherspeed.
▪Oncetherotorattainsthesynchronousspeed,thed.c.excitationtothe
rotorisswitchedonandresistanceisremoved.Oncethesynchronismis
establishedponymotorisdecoupled.Themotorthencontinuestorotate
assynchronousmotor.

17. 17
II]Startingbyusingadamperwinding
▪Inasynchronousmotor,inadditiontothenormalfieldwinding,the
additionalwindinghavingofcopperbarsplacedintheslotsinthepole
faces.
▪Thebarsareshortcircuitedwiththehelpofendrings.Suchanadditional
windingontherotoriscalleddamperwinding.Thiswindingasshort
circuited,actsasasquirrelcagerotorwindingofaninductionmotor.
▪Now,whenathree-phasesupplyisgiventothestatorofasynchronous
motor,itwillstartasathree-phaseinductionmotoratsubsyn.speed.
▪ThenDCsupplyisgiventothefieldwinding.Ataparticularinstantmotor
getspulledintosynchronismandstartsrotatingatasyn.speed.
▪Asrotorrotatesatsynchronousspeed,therelativemotionbetweendamper
windingandtherotatingmagneticfieldiszero.Hencewhenmotoris
runningassynchronousmotor,therecannotbeanyinducedEMFinthe
damperwinding.Sodamperwindingisactiveonlyatstart,torunthemotor
asaninductionmotoratstart&afterwardsitisoutofthecircuit.
▪Asdamperwindingisshortcircuitedandmotorgetsstartedasinduction
motor,itdrawshighcurrentatstartsoinductionmotorstarterslikestar-
delta,autotransformeretc.usedtostartthesynchronousmotorasan
inductionmotor.

18. 18
III] Starting with synchronous induction motor as a Slip Ring IM
▪Theabovemethodofstartingsynchronousmotorasasquirrelcageinduction
motordoesnotprovidehighstartingtorque.Sotoachievethis,insteadof
shortingthedamperwinding,itisdesignedtoaformathreephasestaror
deltaconnectedwindingsimilartoslipringIM.
▪Thethreeendsofthiswindingarebroughtoutthroughsliprings.Anexternal
rheostatthencanbeintroducedinserieswiththerotorcircuit.Sowhenstator
isexcited,themotorstartsasaslipringinductionmotorandduetoresistance
addedintherotorprovideshighstartingtorque.Theresistanceisthen
graduallycutoff,asmotorgathersspeed.Whenmotorattainsspeednear
synchronous.
▪DCexcitationisprovidedtotherotor,thenmotorsgetspulledinto
synchronismandstartsrotatingatsynchronousspeed.Thedamperwindingis
shortedbyshortingthesliprings.
▪Theinitialresistanceaddedinthe
rotornotonlyprovideshigh
startingtorquebutalsolimits
highinrushofstartingcurrent.
Henceitactsasamotor
resistancestarter.

19. Electrical Machines-II 19
▪Equivalentcircuit
▪PowerFlowwithinaSynchronousMotor
▪PowerstagesinaSynchronousMotor
▪Motoroperationonnoload&load
▪PhasorDiagramsWithDifferentExcitations
▪EffectofincreasedloadwithConstantFieldExcitation
▪EffectofChangingFieldExcitationatConstantLoad
Content

20. 20
Equivalent Circuit of a Synchronous Motor
Equivalentcircuitmodelforone
armaturephaseofacylindrical
rotorsynchronousmotor.
It is seen from phasor diagram that the
phase applied voltage Vis the vector
sum of reversed back emf i.e. –Eb and
the impedance drop IaZs.
In other words, V=(-Eb+IaZs).
The angle αor δbetween the phasor for Vand Ebis called the load
angle, torque angle or power angle of the synchronous motor.

21. 21
▪Unliketheinductionmotor,thesynchronousmotorisconnectedtotwo
electricalsystems;ad.c.sourceattherotorterminalsandana.c.system
atthestatorterminals.
▪Undernormalconditionsofsynchronousmotoroperation,novoltageis
inducedintherotorbythestatorfieldbecausetherotorwindingis
rotatingatthesamespeedasthestatorfield.
▪Onlytheimpresseddirectcurrentispresentintherotorwindingand
ohmicresistanceofthiswindingistheonlyoppositiontoit.
▪Inthestatorwinding,twoeffectsaretobeconsidered,theeffectofstator
fieldonthestatorwindingandtheeffectoftherotorfieldcuttingthe
statorconductorsatsynchronousspeed.
▪Theeffectofstatorfieldonthestator(orarmature)conductorsis
accountedforbyincludinganinductivereactanceandeffectofarmature
reactionreactanceinthearmaturewinding.Thisiscalledsynchronous
reactanceXs.
▪AresistanceRamustbeconsideredtobeinserieswiththisreactanceto
accountforthecopperlossesinthestatororarmaturewindingasshown
inFig.
▪Thisresistancecombineswithsynchronousreactanceandgivesthe
synchronousimpedanceofthemachine.

22. 22
▪Thesecondeffectisthatavoltageisgeneratedinthestatorwindingby
thesynchronously-revolvingfieldoftherotor.
▪Thisgeneratede.m.f.Ebisknownasbacke.m.f.andopposesthestator
voltageV.
▪ThemagnitudeofEbdependsuponrotorspeedandrotorfluxфper
pole.
▪Sincerotorspeedisconstant;thevalueofEbdependsupontherotor
fluxperpolei.e.excitingrotorcurrentIf.
▪This equivalent circuit helps considerably in understanding the
operation of a synchronous motor.
▪A synchronous motor is said to be normally excited if the field
excitation is such that Eb = V.
▪If the field excitation is such that Eb < V, the motor is said to be under-
excited.
▪The motor is said to be over-excited if the field excitation is such that
Eb > V. As we shall see, for both normal and under excitation, the
motor has lagging power factor. However, for over-excitation, the
motor has leading power factor.
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23. 23
Power Flow within a Synchronous Motor
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acosɸperphase,
Totalinputforastar-connected3-
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Themechanicalpowerdeveloped,
somewouldgotomeetironand
fractionandexcitationlosses.
▪Hence,thepoweravailableat
theshaftwouldbelessthanthe
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24. 24
Power stages in a Synchronous Motor

25. 25
Power developed by a Synchronous Motor

26. 26
MotoronLoad
▪Ind.c.motorsandinductionmotors,anadditionofloadcausesthemotor
speedtodecrease.Thedecreaseinspeedreducesthecountere.m.f.
enoughsothatadditionalcurrentisdrawnfromthesourcetocarrythe
increasedloadatareducedspeed.Thisactioncannottakeplaceina
synchronousmotorbecauseitrunsataconstantspeed(i.e.,synchronous
speed)atallloads.
▪Whathappenswhenweapplymechanicalloadtoasynchronousmotor?
▪Therotorpolesfallslightlybehindthestatorpoleswhilecontinuingto
runat299r.p.msynchronousspeed.Theangulardisplacementbetween
statorandrotorpoles(calledtorqueangleα)causesthephaseofback
e.m.f.Ebtochangew.r.t.supplyvoltageV.
▪Thisincreasesthenete.m.f.Er
inthestatorwinding.
Consequently,statorcurrent
Ia(=Er/Zs)increasestocarry
theload.

27. 27
Thefollowingpointsmaybenotedinsynchronousmotoroperation:
1)Asynchronousmotorrunsatsynchronousspeedatallloads.Itmeets
theincreasedloadnotbyadecreaseinspeedbutbytherelativeshift
betweenstatorandrotorpolesi.e.,bytheadjustmentoftorqueangle
α.
2)Iftheloadonthemotorincreases,thetorqueangleaalsoincreases
(i.e.,rotorpoleslagbehindthestatorpolesbyagreaterangle)butthe
motorcontinuestorunatsynchronousspeed.Theincreaseintorque
angleαcausesagreaterphaseshiftofbacke.m.f.E
bw.r.t.supply
voltageV.ThisincreasesthenetvoltageE
rinthestatorwinding.
Consequently,armaturecurrentI
a(=E
r/Z
s)increasestomeettheload
demand.
3)Iftheloadonthemotordecreases,thetorqueangleaalsodecreases.
ThiscausesasmallerphaseshiftofE
bw.r.t.V.Consequently,thenet
voltageE
rinthestatorwindingdecreasesandsodoesthearmature
currentI
a(=E
r/Z
s).

28. 28
Motor on Load With Constant Excitation
▪Inasynchronousmachine,abackemfE
b(aslikeDCmotor)issetupin
thearmature(stator)bytherotorfluxwhichopposestheappliedvoltage
V.
▪Thisbackemfdependsonrotorexcitationonly(andnotonspeedasin
DCmotors).
▪Thenetvoltageinarmature(stator)isthevectordifference(not
arithmetical,asinDCmotors)ofVandEb.
▪Armaturecurrentisobtainedbydividingthisvectordifferenceof
voltagesbyarmatureimpedance(notresistanceasinDCmachines).
▪Fig.showstheconditionwhenthemotor(properlysynchronizedtothe
supply)isrunningonno-loadandhasnolossesandishavingfixed
excitationwhichmakesE
b=V.
▪ItisseenthatvectordifferenceofE
bandVis
zeroandsoisthearmaturecurrent.
▪Motorintakeiszero,asthereisneitherload
norlossestobemetbyit.
▪Inotherwords,themotorjustfloats.

29. 29
▪Ifmotorisonno-load,butithaslosses,thenthevectorforEbfalls
back(vectorsarerotatinganti-clockwise)byacertainangleαorδ,
sothataresultantvoltageE
RandhencecurrentI
aisbroughtinto
existence,whichsupplieslosses.
▪Theangleθ(knownasinternalangle)bywhichIalagsbehindE
R
isgivenbytan??????=??????
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0
.
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▪MotorinputP=VI
acosɸper
phase.Thusatnoload,the
motortakesasmallpowerVI
a
cosɸ/phasefromthesupplyto
meettheno-loadlosseswhileit
continuestorunatsynchronous
speed.

30. 30
▪If,now,themotorisloaded,thenitsrotorwillfurtherfallbackin
phasebyagreatervalueofangleαorδ-calledtheloadangleor
couplingangleortorqueangle.
▪ThiscausesEb(itsmagnitudeisconstantasexcitationisfixed)tolag
behindVbyagreaterangle
▪TheresultantvoltageERisincreasedandmotordrawsanincreased
armaturecurrent,thoughataslightlydecreasedpowerfactor.
▪MotorinputP=VI
acosɸperphase.
▪Mechanicalpowerdeveloped
bymotor/phase
▪??????
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wherecosα−ɸ=cosine of
angle between E
band I
a

31. 31
PhasorDiagramsWithDifferentExcitations
▪Fig.(i)-thephasordiagramfornormalexcitation(Eb=V),
▪Fig.(ii)showsthephasordiagramforunder-excitation.Inbothcases,the
motorhaslaggingpowerfactor.
▪Fig.(iii)showswhenfieldexcitationisadjustedforunityp.f.operation.Under
thiscondition,theresultantvoltageErand,Iaareminimum.
▪Whenthemotorisoverexcited,ithasleadingpowerfactorfromFig.(iv).
1)Foragivenload,thepowerfactorisgovernedbythefieldexcitation;aweak
fieldproducesthelaggingarmaturecurrentandastrongfieldproducesa
leadingarmaturecurrent.
2)Thearmaturecurrent(Ia)isminimumatunityp.fandincreasesasthep.f.
becomeslesseitherleadingorlagging.

32. 32
EffectofincreasedloadwithconstantfieldExcitation
▪Wewillthestudytheeffectofincreasedloadwithconstantexcitationmay
bei)normalexcitationEb=Vii)underexcitationEb<V&iii)Eb>V.
▪Whatevermaybetheexcitation,itwouldbekeptconstant.AsRais
negligibleascomparedtoXssothatphaseanglebetweenEr&Ia=900
(i)Normalexcitationwithmotorisrunningatlightload
▪Duetolightload-torque/loadangleα
1issmall,Er1issmall,henceIa1is
smallandɸ1issmallsothatcosɸ1islarge
▪Supposeloadonmotorincreases-formeetingthisextraload,motormust
developextratorquebydrawingextraarmaturecurrent.
▪UnlikeDCmotor,asyn.motorcan’tincreaseitsIabydecreaseinspeed
andbackemfbecausebothareconstantinitscase.

33. 33
What actually happens is as under:
1.Rotor falls back in phase i.e. load angle increases to α
2as shown in Fig. 2
2.The resultant voltage in armature is increased considerably to new value
E
R2.
3.As a result, Ia1 shifts (increases) to I
a2, thereby increasing the torque
developed by the motor. ɸ1 increase to ɸ
2, so that power factor decreases
from cos ɸ
1to the new value cos ɸ
2.
4.Since increase in Iais much greater than the slight decrease in power
factor, the torque developed by the motor is increased (on the whole) to a
newvaluesufficientto
meettheextraloadputon
themotor.
5.Itwillbeseenthat
essentiallyitisby
increasingitsI
athatthe
motorisabletocarrythe
extraloadputonit.

34. 34
(ii)Underexcitation
▪Asshowninphasordia.,withasmallloadandhence,smalltorqueangle
α
1,Ia1lagsbehindVbyalargephaseangleɸ1whichmeanspoorpower
factor.
▪Amuchlargerarmaturecurrentmustflowfordevelopingthesamepower
becauseofpoorpowerfactor.
▪Asloadincreases,ER1increasestoER2,consequentlyIa1increasesto
Ia2andpowerfactorangledecreasesfromɸ1toɸ2.
▪DuetoincreasebothinIaand
powerfactor,powergenerated
bythearmatureincreasesto
meettheincreasedload.
▪Asseen,inthiscase,change
inpowerfactorismorethan
thechangeinIa.

35. 35
(iii)Overexcitation
▪Whenrunningonlightload,α
1issmall,butIa1iscomparativelylarger
andleadsVbyalargerangleɸ1.
▪Liketheunderexcitedmotor,asmotorloadisapplied,thepowerfactor
improvesandapproachesunity.
▪Thearmaturecurrentalsoincreasestherebyproducingthenecessary
armaturepowertomeettheincreasedload.
▪Inthiscase,powerfactor
angleɸdecreases(orpower
factorincreases)atafaster
ratethanthearmaturecurrent
therebyproducingthe
necessaryincreasedpowerto
meettheincreasedload
appliedtothemotor.

36. 36
Themainpointsregardingtheabovethreecasescanbesummarized
asunder:
1.Asloadonthemotorincreases,Iaincreasesregardlessof
excitation.
2.Forunder-andover-excitedmotors,powerfactortendsto
approachunitywithincreaseinload.
3.Bothwithunder-andover-excitation,changeinpowerfactoris
greaterthanIawithincreaseinload.
4.Withnormalexcitation,whenloadisincreasedchangeinIais
greaterthaninpowerfactorwhichtendstobecomeincreasingly
lagging.

37. 37
EffectofChangingFieldExcitationatConstantLoad
▪Inad.c.motor,thearmaturecurrentIaisdeterminedbythe
differencebetweenVandEbdividingbythearmatureresistance
Ra.
▪Similarly,inasynchronousmotor,thestatorcurrent(Ia)is
determinedbydividingthedifferencebetweenvoltage-phasor
resultant(Er)betweenVandEbbythesynchronousimpedanceZs.
▪Oneofthemostimportantfeaturesofasynchronousmotoristhat
bychangingthefieldexcitation,itcanbemadetooperatefrom
laggingtoleadingpowerfactor.
▪Considerasynchronousmotorhavingafixedsupplyvoltageand
drivingaconstantmechanicalload.Sincethemechanicalloadas
wellasthespeedisconstant,thepowerinputtothemotor(=3VIa
cosɸ)isalsoconstant.
▪Thismeansthatthein-phasecomponentIacosɸdrawnfromthe
supplywillremainconstant.Ifthefieldexcitationischanged,back
e.m.fEbalsochanges.Thisresultsinthechangeofphaseposition
ofIaw.r.t.Vandhencethepowerfactorcosɸofthemotorchanges.

38. 38
a)ExcitationDecreased
▪AsshowninFig.(b),supposeduetodecreaseinexcitation,backe.m.f.is
reducedtoE
b1atthesameloadangleα
1.TheresultantvoltageE
R1causesa
laggingarmaturecurrentIa1toflow.EventhoughIa1islargerthanI
ain
magnitudeitisincapableofproducingnecessarypowerVI
aforcarrying
theconstantloadbecauseI
a1cosɸ1componentislessthanI
asothatVI
a1
cosɸ1<VIa.
▪Hence,itbecomesnecessaryfor
loadangletoincreasefromα1to
α2.Itincreasesbacke.m.f.
fromE
b1toE
b2which,inturn,
increasesresultantvoltagefromE
R1
toE
R2.Consequently,armature
currentincreasestoI
a2whosein-
phasecomponentproducesenough
power(VI
a2cosɸ
2)tomeetthe
constantloadonthemotor.

39. 39
(b)ExcitationIncreased
▪TheeffectofincreasingfieldexcitationisshowninFig.(c)where
increasedE
b1isshownattheoriginalloadangleα
1.Theresultant
voltageE
R1causesaleadingcurrentI
a1whosein-phasecomponentis
largerthanIa.Hence,armaturedevelopsmorepowerthantheloadonthe
motor.Accordingly,loadangledecreasesfromα
1toα
2whichdecreases
resultantvoltagefromE
R1toE
R2.
▪Consequently,armaturecurrent
decreasesfromI
a1toI
a2whosein-
phasecomponentI
a2cosɸ
2=I
a.In
thatcase,armaturedevelopspower
sufficienttocarrytheconstantload
onthemotor.
▪Hence,wefindthatvariationsin
theexcitationofasynchronous
motorrunningwithagivenload
producevariationsinitsloadangle
only.

40. 40
EffectofExcitationonArmatureCurrentandPowerFactor
(i)Underexcitation
▪Themotorissaidtobeunder-excitedifthefieldexcitationissuchthatEb
<V.Undersuchconditions,thecurrentIalagsbehindVsothatmotor
powerfactorislaggingasshowninFig.(i).
▪SinceEb<V,thenetvoltageErisdecreasedandturnsclockwise.Asangle
θ(=90°)betweenErandIaisconstant,therefore,phasorIaalsoturns
clockwisei.e.,currentIalagsbehindthesupplyvoltage.Consequently,the
motorhasalaggingpowerfactor.
▪ThemagnitudeofIisincreasedbutitspowerfactorisdecreased.WhenIf
isreduced,themotorpullouttorqueisalsoreducedinproportion.

41. 41
(ii)Normalexcitation
▪ThemotorissaidtobenormallyexcitedifthefieldexcitationissuchthatEb
=V.
▪Notethattheeffectofincreasingexcitation(i.e.,increasingEb)istoturnthe
phasorErandhenceIaintheanti-clockwisedirectioni.e.,Iaphasorhascome
closertophasorV.
▪Therefore,p.f.increasesthoughstilllagging.Sinceinputpower(=3VIa
cosɸ)isunchanged,thestatorcurrentIamustdecreasewithincreaseinp.f.
▪SupposethefieldexcitationisincreaseduntilthecurrentIaisinphasewith
theappliedvoltageV,makingthep.f.ofthesynchronousmotorunity
Fig.(iii).Foragivenload,atunityp.f.theresultantErand,therefore,Iaare
minimum.

42. 42
(iii) Over excitation
▪ThemotorissaidtobeoverexcitedifthefieldexcitationissuchthatEb>
V.
▪Under-suchconditions,currentIaleadsVandthemotorpowerfactoris
leadingasshowninFig.6.10(iv)).NotethatErandhenceIafurtherturn
anti-clockwisefromthenormalexcitationposition.Consequently,Ialeads
V.
▪WhenIisinphasewithV,thecurrentdrawnbythemotorisminimum

43. 43
Fromtheabovediscussion,itisconcludedthat
1.Ifthesynchronousmotorisunder-excited,ithasalaggingpower
factor.
2.Astheexcitationisincreased,thepowerfactorimprovestillit
becomesunityatnormalexcitation.Undersuchconditions,the
currentdrawnfromthesupplyisminimum.
3.Iftheexcitationisfurtherincreased(i.e.,overexcitation),the
motorpowerfactorbecomesleading.
4.Themagnitudeofarmaturecurrentvarieswithexcitation.The
armaturecurrent(Ia)isminimumatunityp.fandincreasesasthe
powerfactorbecomespoor,eitherleadingorlagging.
5.FortheForthesameinput,armaturecurrentvariesoverawide
rangeandsocausesthepowerfactoralsotovaryaccordingly.

44. Different Torques of a Synchronous Motor
Varioustorquesassociatedwithasynchronousmotorareas
follows:
1.Startingtorque,
2.Runningtorque,
3.Pull-intorque,and
4.Pull-outtorque.
StartingTorque:
▪Thisisthetorquedevelopedbythesynchronousmotoratstartwhen
ratedvoltageisappliedtothestator.
▪Itisalsocalledbreakawaytorque.Itisnecessarytoovercomefriction
andinertia
RunningTorque:
▪Itisthetorquedevelopedbythesynchronousmotorunderrunning
conditions.
▪Itisdecidedbythehorsepowerandspeed.Thepeakhorsepower
determinesmaximumtorquethatwouldberequiredbythedriven
machine.Themotormusthaveabreakdownoramaximumrunning
torquegreaterthanthisvalueinordertoavoidstalling.

45. Different Torques of a Synchronous Motor
PullinTorque:
▪InitiallySynchronousmotorisstartedasinductionmotortillitruns2to
5%belowthesynchronousspeed.Afterwards,excitationisswitchedon
andtherotorpullsintostepwiththesynchronouslyrotatingstatorfield.
▪Theamountoftorquedevelopedbythemotoratthetimeofpullinginto
synchronousspeediscalledPULLinTorque
PulloutTorque:
▪Themaximumtorquewhichthemotorcandevelopwithoutpullingout
ofsteporsynchronismiscalledthepullouttorque.
▪WhentheSynchronousmotorisloaded,therotorfallsbackwithrespect
tostator(i.e.synchronously-revolvingstatormagneticfieldthoughit
keepsrunningsynchronously)byananglecalledloadangleδorα.
▪Asδorαincreasesmagneticlockingbetweenstatorandrotordecreases.
▪Motordevelopsmaximumtorquewhenitsrotorisretardedbyanangle
of90
o
.Anyfurtherincreaseinloadwillcausethemotortopulloutof
step(orsynchronism)andstop.

46. V & Inverted V Curves
▪Whenthefieldcurrentissufficientenoughtoproducetheairgapflux,as
demandedbytheconstantsupplyvoltageV,thenthemagnetizingcurrent
orlaggingreactiveVArequiredfromacsourceiszeroandthemotor
operateatunitypowerfactor.Thefieldcurrent,whichcausesthisunity
powerfactoriscallednormalfieldcurrent.
▪Ifthefieldcurrentisnotsufficientenoughtoproducetherequiredairgap
fluxasdemandedbyV,additionalmagnetizingcurrentorlaggingreactive
VAisdrawnfromtheACsource.Thismagnetizingcurrentproducesthe
deficientflux(constantflux-fluxsetupbydcsupplyrotorwinding).
Henceinthiscasethemotorissaidtooperateunderlaggingpowerfactor
andtheissaidtobeunderexcited.
▪Ifthefieldcurrentismorethanthenormalfieldcurrent,motorissaidto
beoverexcited.Thisexcessfieldcurrentproducesexcessflux(fluxsetup
byDCsupplyrotorwinding–resultantairgapflux)whichmustbe
neutralizedbythearmaturewindingandcausesleadingcurrent.This
propertyofthemotorrendersitextremelyusefulforphaseadvancing
(andsopowerfactorcorrecting)purposes.
46

47. 47
▪HencethearmaturewindingdrawsleadingreactiveVAor
demagnetizingcurrentleadingvoltagebyalmost90
o
fromtheAC
source.Henceinthiscasethemotoroperateunderleadingpower
factor.Thiswholeconceptofexcitationandpowerfactorof
synchronousmotorcanbesummedupinthefollowinggraph.Thisis
calledVcurveofsynchronousmotor.

48. 48
The variations of p.f. with excitation are shown in Fig. The curve for p.f. looks
like inverted ‘V’ curve. It would be noted that minimum armature current
corresponds to unity power factor.

49. 49
V-Curves and Inverted V-Curves
▪Itisclearfromabovediscussionthatifexcitationis
variedfromverylow(underexcitation)toveryhigh
(overexcitation)value,thencurrentIadecreases,
becomesminimumatunityp.f.andthenagain
increases.Butinitiallaggingcurrentbecomesunity
andthenbecomesleadinginnature.
▪Excitationcanbeincreasedbyincreasingthefield
currentpassingthroughthefieldwindingof
synchronousmotor.Ifgraphofarmaturecurrent
drawnbythemotor(Ia)againstfieldcurrent(I
f
)is
plotted,thenitsshapelookslikeanenglishalphabet
V.Ifsuchgraphsareobtainedatvariousload
conditionswegetfamilyofcurves,alllookinglikeV.
SuchcurvesarecalledV-curvesofsynchronous
motor.
▪Asagainstthis,ifthepowerfactor(cosф)isplotted
againstfieldcurrent(If),thentheshapeofthegraph
lookslikeaninvertedV.Suchcurvesobtainedby
plottingp.f.againstIf,atvariousloadconditionsare
calledInvertedV-curvesofsynchronousmotor.

50. 50
▪Inordertodrawthesecurvesexperimentally,themotorisrun
fromconstantvoltageandconstantfrequencybusbar.
▪Powerinputtothemotoriskeptconstantatadefinitevalue.
▪Thefieldcurrentisincreasedinsmallstepsandcorresponding
armaturecurrentsarenoted
▪Thuswegetonevcurveforparticularconstantmotorinput.
▪Similarcurvescanbedrawnbykeepingmotorinputconstantat
differentvalues

51. Hunting: Its Cause & Remedies
51
▪Unloadedsynchronousmachinehaszerodegreeloadangle.Onincreasing
theshaftload,graduallyloadanglewillincrease.
▪LetusconsiderthatsuddenlyloadP
1isappliedtounloadedmachineshaft,
somachinewillslowdownmomentarily.
▪Alsoloadangle(δorα)increasesfromzerodegreeandbecomesδ
1.
▪Duringthefirstswingelectricalpowerdevelopedisequaltomechanical
loadP
1.Equilibriumisnotestablishedsorotorswingsfurther.Loadangle
exceedsδ
1andbecomesδ
2.
▪Nowelectricalpowergeneratedisgreaterthanthepreviousone.Rotor
attainssynchronousspeed.Butitdoesnotstayinsynchronousspeedandit
willcontinuetoincreasebeyondsynchronousspeed.
▪Asaresultofrotoraccelerationabovesynchronousspeedtheloadangle
decreases.Soonceagainnoequilibriumisattained.Thusrotorswingsor
oscillatesaboutnewequilibriumposition.Thisphenomenonisknownas
huntingorphaseswinging.Huntingoccursnotonlyinsynchronous
motorsbutalsoinsynchronousgeneratorsuponabruptchangeinload.

52. 52
❖CausesofHunting
1.Suddenchangeinload.
2.Suddenchangeinfieldcurrent.
3.Aloadcontainingharmonictorque.
4.Faultinsupplysystem
❖EffectsofHunting
1.Itmayleadtolossofsynchronism.
2.Producesmechanicalstressesintherotorshaft.
3.Increasesmachinelossesandcausetemperaturerise.
4.Causegreatersurgesincurrentandpowerflow.
5.Itincreasespossibilityofresonance

53. Remedies of Hunting
▪UseofDamperWinding:Itconsistsoflowelectrical
resistancecopper/aluminumbrushembeddedinslotsofpolefaces
insalientpolemachine.Damperwindingdampsouthuntingby
producingtorqueoppositetoslipofrotor.Themagnitudeofdamping
torqueisproportionaltotheslipspeed.
▪UseofFlywheels:Theprimemoverisprovidedwithalargeand
heavyflywheel.Thisincreasestheinertiaofprimemoverandhelps
inmaintainingtherotorspeedconstant.
▪Designingsynchronousmachinewithsuitablesynchronizingpower
coefficients.
53

54. Thank you