Following are the differences between an induction and
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
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.
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
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).