what is the difference between induction and synchronous
motor?explain in simple words?why 3 phase motor is given
preference over single phase motor.
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Answer Posted / m s kumar
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.
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