synchronous motor, Starting Torque, Types, Equivalent Circuit, Torque-speed characteristics

Introduction
Starting Torque
Types of Rotors
Power, Torque developed
Comparison with Induction motor
Starting Methods
Equivalent Circuit
Torque-speed characteristics
Changing the Load-Effects
Phasor diagrams-with Ref. Frame
Applications

1. Synchronous Motor
Introduction
StartingTorque
Types of Rotors
Power, Torque developed
Comparison with Induction motor
Starting Methods
Equivalent Circuit
Torque-speed characteristics
Changing theLoad-Effects
Phasordiagrams-with Ref. Frame
Applications

2. Introduction
Thesynchronousmotorrotatesatthesynchronous
speed i.e. the speed of theRMF.
Stator is similar in construction to that of an
induction motor, so same principle is appliedto
the synchronous motorrotor.
Fieldexcitationisprovidedontherotorbyeither
permanentorelectromagnets with number of
poles equal to the poles of the RMF caused by
stator

3. Introduction
Faradays Law of ElectromagneticInduction.
The rotor acting as a bar magnet will turn to line up with
the rotating magnet field. The rotor gets locked to theRMF
and rotates unlike induction motor at synchronous speed
under all loadcondition

4. Construction
Stator:
Stationary Armaturewinding
Stator core use a laminated construction Special
steel stamping and insulated from each other with
varnish or paper Reduce the eddy currentloss
Steel material Reduce hysteresisloss

5. Rotor
:Projected pole type as all the poles are projected out
from the surface of therotor.
Rotor have large diameter and small axiallength
The poles have damper winding.
The damper windings are used to reduce the
‘Hunting’

6. SMtypes
•Non salient-pole (NSP) synchronousmachine
•Salient-pole (SP) synchronousmachine
3

7. Rotor with non-salient and salientpoles
7

8. Pole numbers and synchronousspeed
Thestatorandtherotormayhavea
highernumber(alwaysaneven
integer)ofpoles.
The winding structure of a 2p pole
motor(pisthenumberofpole
pairs)issimilartotwo-pole
windingexceptthatsinusoidally
distributedwindingisrepeatedp
timesin360mechanicaldegrees
Fora2ppolemachine,eachcycle
of2πelectricalradiansofsupply
currentcausesthestatorfieldto
rotateby2/pmechanicalradians.
rev/sec
f
s
syn
N
p

electricalp
mechanical
20
a1
a1' a2'
a2
Average speed regulation =0

9. SynchronousMotor-General
A synchronous motor is electrically identical with an alternator or AC
generator.
A given alternator ( or synchronous machine) can be used as a motor,
when drivenelectrically.
Some characteristic features of a synchronous motor are asfollows:
1.Itrunseitheratsynchronousspeedornotatalli.e.whilerunningit
maintainsaconstantspeed.Theonlywaytochangeitsspeedistovary
thesupplyfrequency(becauseN
S=120f/P).
2.Itisnotinherentlyself-starting.Ithastoberunuptosynchronous
(ornearsynchronous)speedbysomemeans,beforeitcanbe
synchronized to thesupply.
3.Itiscapableofbeingoperatedunderawiderangeofpowerfactors,
bothlaggingandleading.Hence,itcanbeusedforpowercorrection
purposes,inadditiontosupplyingtorquetodriveloads.

10. StartingTorque
Itcannotbestartedfromastandstillbyapplying
ac to thestator.
WhenACsupplyis applied to the stator a high
speed RMF appears around the stator. This RMF
rushespasttherotorpolessoquicklythattherotor
is unable to getstarted.
Itisattractedfirstinonedirectionandtheninthe
other and hence no startingtorque.

11. Improvement of startingtorque
Itisstartedbyusingasquirrelcagewithinarotor
constructionandthereforestartsasaninduction
motor.
Atsynchronousspeedthesquirrelcagehasnopartto
play.

12. Different Torques of a SynchronousMotor
(a)StartingTorque:Itisthetorquedevelopedbythemotor
whenfullvoltageisappliedtoitsstator(armature)
winding.Itisalsosometimescalledbreakawaytorque.
(b)Running Torque: As its name indicates, it is the torque
developed by the motor under runningconditions.
Itisdetermined by the horse-powerandspeed of the
driven machine.
The peak horse power determines the maximum torque
that would be required by the drivenmachine.
Themotormusthaveabreakdownoramaximum
runningtorque greater than this value in order to avoid
stalling.

13. (c)Pull-in Torque: A synchronous motor is started as induction motor
till it runs 2 to 5% below the synchronousspeed.
Afterwards, excitation is switched on and the rotor pulls into step with
the synchronously rotating statorfield.
(d)Pull-outTorque:Themaximumtorquewhichthemotorcan
developwithoutpullingoutofsteporsynchronismiscalledthepull-
outtorque.
Motordevelopsmaximumtorquewhenitsrotorisretardedbyanangle
of90
o.
Anyfurtherincreaseinloadwillcausethemotortopulloutofstep
(orsynchronism)andstop.

14. Method ofStarting
Thereareseveralmethodstostartthesynchronousmotorsuchas:
(a)Auxiliary drive (induction motor or dcmotor),
(b)Induction start (using damper winding),etc
GeneralStartingProcedure:Therotor(whichisasyet
unexcited)isspeededuptosynchronousornearsynchronousspeedby
somearrangementandthenexcitedbytheDCsource.
Themomentthis(near)synchronouslyrotatingrotorisexcited,itis
magneticallylockedintopositionwiththestatori.e.therotorpolesare
engagedwiththestatorpolesandbothrunsynchronouslyinthesame
direction.
Itisbecauseofthisinter-lockingofstatorandrotorpolesthat
themotorhaseithertorunsynchronouslyornotatall.
However,itisimportanttounderstandthatthearrangement
betweenthestatorandrotorpolesisnotanabsolutelyrigidone.

15. Procedure for Starting a SynchronousMotor
Using the DamperWinding
Whilestartingamodernsynchronousmotorprovidedwith
damper windings, following procedure isadopted.
1.First,mainfieldwindingisshortcircuited.
2.Reducedvoltagewiththehelpofauto-transformersis
appliedacrossstatorterminals.Themotorstartsup.
3.Whenitreachesasteadystatespeed(asjudgebyitssound),a
weakDCexcitationisappliedbyremovingtheshort-
circuitonthemainfieldwinding.Ifexcitationissufficient,
thenthemachinewillbepulledintosynchronism.
4.Fullsupplyvoltageisappliedacrossstatorterminalsby
cuttingouttheauto-transformers.
5.Themotormaybeoperatedatanydesiredpowerfactorby
changingtheDCexcitation.

16. Advantages of SM overIM
1.SMcanbeusedforpowerfactorcorrection
2.SMsaremoreefficient(atunitypowerfactor)
3.ThefieldpolerotorofSMcanpermittheuseofwider
airgapthanthesquirrelcageIM.
4.SMcanoperatelagging,leadingandunitypowerfactor
5.SMmaylesscostlyforthesamehorsepower,speed
andvoltageratings.
6.Theygiveconstantspeedfromno-loadtofull-load.

17. Disadvantages of SM overIM
1.Theyrequiredcexcitationwhichmustbe
supplied from externalsource.
2.They have tendency tohunt.
3.They cannot be used for variable speed jobs
as speed adjustment cannot bedone.
4.Theycannotbestartedunderload.Their
staring torque iszero.
5.They may fall out of synchronism and stop
whenoverloaded.

18. Starting SynchronousMotors
Three basic approaches can be used to safely start a
synchronous motor:
Motor Starting by Reducing ElectricalFrequency
Motor Starting with an External PrimeMover
Motor Starting by Using AmortisseurWindings

19. The per phase power development in a synchronous machineis
as under:

21. Equivalent Circuit of a SynchronousMotor
Fig.36.9(a)showstheequivalent
circuitmodelforonearmaturephase
ofacylindricalrotorsynchronous
motor.
ItisseenfromFig.36.9(b)thatthe
phaseappliedvoltageVisthevectorsum
ofreversedbackemfi.e.–E
bandthe
impedancedropI
aZ
s.
In other words,V=(-E
b+I
aZ
s).
The angle between the phasor for V and E
b is called the load angle or power angle
of the synchronousmotor.

22. Power Developed by a SynchronousMotor
The armature resistance of a synchronous motor is negligible as compared to its
synchronousreactance.
Hence the equivalent circuit for the motor becomes as shown in Fig. 36.10(a).
From the phasor diagram of Fig. 36.10(b),it
is seenthatAB Eb sinIa XScos
EbV sin=VIaXS cos
Now, VI
acos= motor powerinput/phase
Pin 
EbV
sinperphase
XS XS
Pin 
3EbV
sinforthreephase
SinceStatorCulosseshavebeenneglected,P
inalso
representsthegrossmechanicalpower(P
m)
developed by themotor.
XS
Pm Pin 
3EbV
sin
The gross torque developed by the motor isT
g=9.55P
m/N
S
N-m-N
S inrpm.
Tg 9.55
Pm

9.553EbV
sin28.65
EbVsin
N m
NS NSXS NSXS

23. The Effect of Load Changes on
a SynchronousMotor
X
s
V
: fixed (from electricalcourse)
E
A :fixed(whenI
F fixedorusingpermanent magnets)
A
VE
P 3V
Icos 3
A
sin
(leadinglagging)
A AA
2
P(loadincreases) sinImoreheat(3IR)

jX
sI
A
P

24. The Effect of Field Current Changes on a
SynchronousMotor
jX
sI
AV
E
A R
AI
A
E
AjX
sI
A


25. Phasordiagram and the stator referenceframe
X
qI
qVsin, I
qVsin/X
q
E
fX
dI
dVcos,
PVcosI
qVsinI
d
d dq
V
2 X
dVE
f X
q

P sin
X 2XX

sin2
I cosI
q cos I
d sin



q-axis
d-axis
Id
V
jI
dX
d
Ef
I
jIX
qq
Iq
43

26. 51
Summary
Non-SPSMReluctance
SM
SPPMSP SM(Wound
Rotor)
inductanceL
q = L
d =L
sL
q <L
d L
q >L
dL
q <L
d
excitationEf Ef=0 Ef Ef
-δ|Tmax(deg)90 45 >90 <90

27. Synchronous MotorApplications
Synchronous motors find extensive application for
the following classes ofservice:
1.Power factorcorrection;
2.Constant speed, constant load drives
3.Voltageregulation.