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what is the difference between induction and synchronous 
motor?explain in simple words?why 3 phase motor is given 
preference over single phase motor.
 Question Submitted By :: Electrical-Engineering
I also faced this Question!!     Answer Posted By  
# 1
Following are the differences between an induction and
synchronous motor:
1. An induction motor is singly excited motor (i.e.)power
supply is fed only to the stator. On the otherhand, a
synchronous motor is doubly excited (ie) stator must be
supplied from an ac source while the rotor is supplied from
a dc source.
2. An induction motor can only run at speed less than the
synchronous speed. While a synchronous motor can run only
at synchronous speed.
3. A 3phase induction motor is self starting while the
synchronous motor is not self starting (i.e.)it has to be
driven by another motor upto or near synchronous speed
before being pulled-up by the synchronously rotating flux
(magnetic locking).
4. In an induction motor Stator has field windings and the
rotor is the armature. In a Synchronous Motor, the field
winding is the Rotor while armature is the Stator

The relationship between frequency and speed is given by

f=pn/120 ; where f = frequency, n = speed,
p = no. of poles

For a 4 pole motor supplied from 3 phase 50Hz power supply,
the speed would be 1500rpm. This speed is called
synchronous speed. This means that the speed of induction
motor will always be less than 1500rpm whereas the
synchronous motor can run only at 1500rpm.

3phase machines are used for high power applications
usually of about 1kW whereas 1phase motors are used for
fractional horse power applications.

3phase induction motors are self starting whereas 1phase
motors are not self starting but can be made self starting
by temporarily started as 2phase motor having two windings
using capacitors for one phase.
Is This Answer Correct ?    41 Yes 1 No
M S Kumar
# 2
The main diffrence betwwen synchronous Moter & Induction
motor is.(On Designing Basis)..In the inductoin motor field
winding is on Stator & armature winding is on Rotor While
in Synchronous Motor the Field winding is on on Rotor &
Armature winding is on Stator
Is This Answer Correct ?    42 Yes 13 No
Tarkeshwar Mall
# 3
the induction motor run at lower than synchorous
speedsynchorous synchorous speed.
Is This Answer Correct ?    37 Yes 14 No
# 4
Synchronous motor : doubly excited (DC to the rotor , AC to the stator), not self starting , high PF (used as capacitor bank) , unity or leading PF , high efficiency (low inrush current) , Shorter operational life compared to the induction motor (brushes).

Induction motor : singly excited (AC to the stator), self starting , low PF , lagging PF , low efficiency (high inrush current) , longer operational life compared to the Synchronous motor (no brushes).
Is This Answer Correct ?    11 Yes 0 No
Mohammed Subhi Assaf
# 5
Synchronous motor working as a leading power factor and
induction motor working lagging power factor. synchronous
motor used for power factor improving.
Is This Answer Correct ?    12 Yes 2 No
Mukesh Kumar
# 6
the diffrence is comes at their speeds.
IM runs at iess than synchronus speed
sm runs at synch speed
Is This Answer Correct ?    13 Yes 4 No
# 7
IM is a self start motor while synchronous motor is not
self start.
Is This Answer Correct ?    9 Yes 1 No
# 8
induction motor has self starting torque. synchronous motor
has no self-starting torque therefor some external means be
used to start it.
Is This Answer Correct ?    7 Yes 1 No
Sharvan Kumar Saini
# 9
main difference is that why IM doesnt run at synchronous speed?
coz of the slip.
ns= 120F/P
slip=(ns-1)/ns at synchronous speed

at standstill condition slip (%)will be 1
so as the slip increases speed of the motor decreases.

slip in the case of IM at standstill condition is 1. so
obiviously IM wont run at synchronous speed.
Is This Answer Correct ?    6 Yes 1 No
# 10
synchronous motor is also for power factor
improvement.while induction motor not
Is This Answer Correct ?    5 Yes 0 No

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