The basic difference between an induction motor(a.c) and a
synchronous AC motor is that in the latter a current is
supplied onto the rotor. This then creates a magnetic field
which, through magnetic interaction, links to the rotating
magnetic field in the stator which in turn causes the rotor
to turn. It is called synchronous because at steady state
the speed of the rotor is the same as the speed of the
rotating magnetic field in the stator.
By way of contrast, the induction motor does not have any
direct supply onto the rotor; instead, a secondary current
is induced in the rotor. To achieve this, stator windings
are arranged around the rotor so that when energised with a
polyphase supply they create a rotating magnetic field
pattern which sweeps past the rotor. This changing magnetic
field pattern can induce currents in the rotor conductors.
These currents interact with the rotating magnetic field
created by the stator and the rotor will turn.
1. Synchronous motor runs at synchronous speed but the
induction moror can't, if it does so then it will stop
because the rotor conductors will not cut the magnetic
field & induction principle will fails.
2. Synchrounous motor can improve power factor but
induction motor can't.
3. Synchrounous motor is costly as compared to induction
4. Synchrounous motor has two types excitations but in
induction motor has only one type excitation.
5. Synchronous motor can't run at other speed without
synchronous speed, if it does so then it will fall out of
synchronism & will stop.
6. There is requirement of any auxilary help for starting
the synchronous motor like pony motor,with the help of
manual rotation, by inserting the copper cage into its own
rotor(means starting as squirrel cage motor), but there is
no requirement of any help to start the induction motor.
induction motor is a self starting motor&it is single
but synchronous is a not a self starting machine&it is a
doubly excited machine.
in syncronous motor we connect rotor to dc supply due to
1. Synchronous motor is not self starting, but induction
motor is self starting.
2. In synchronous motor, a d.c. supply is required for rotor
excitation. But in induction motor, d.c. supply is not required.
3.By adjusting the d.c. field excitation of synchronous
motor, it can be made to operate from lagging to leading
p.f. But 3-phase induction motor always operates at low
4. Synchronous motor can be used for variable speed
operations because it runs at synchronous(or one) speed at
all loads. But induction motor can be used for variable
speed operations because it's speed vary with the loads.
5. Synchronous motor requires high cost as compared to
• Always turn at less than synchronous speed.
• Do not require control.
• Much cheaper to produce (true for squirrel cage
• Less efficient than synchronous machines.
• More suitable for explosive environments.
No maintenance (for squirrel cage machines). Synchronous
• Wound-rotor or permanent magnet to generate the
rotor magnetic field.
• Rotor magnetic field is stationary with respect
to the rotor.
• Always turn at synchronous speed.
• Require some form of control to operate.
• More expensive to produce than squirrel cage
• Not self-starting.
• More efficient than induction machines.
• Wound-rotor or squirrel cage to generate the
rotor magnetic field.
• Rotor magnetic field rotates with respect to the