Three-Phase Synchronous Motor – Construction and Working Principle

A 3-phase synchronous motor is a 3-phase synchronous machine which is operated as a motor i.e. converts electrical energy input into mechanical energy output.

A synchronous motor has a unique feature that is it runs at a constant speed equal to the synchronous speed at all load provided that the load on the motor does not exceed the limiting value. If the load on the motor exceeds the limiting value, then the motor will come to rest and the average torque developed by the motor becomes zero. Because of this, a synchronous motor is not inherently self-starting.

A synchronous motor is a doubly-excited machine. Its stator winding or armature winding is connected to the AC supply while the rotor winding or field winding is excited by a DC source.

Construction of Three-Phase Synchronous Motor

A synchronous motor has the following two parts (refer Figure-1) −


The stator is the stationary part of the machine and is built up of sheet steel laminations having slots on its inner periphery. A three-phase winding is placed in these slots which is called armature winding and receives power from a 3-phase supply.


The rotor of the synchronous motor has set of salient poles carrying a field winding which is supplied with direct current through two slip-rings by a separate DC source to form alternate N and S poles. The DC source is generally a small DC shunt generator mounted on the shaft of the motor.

Note – The stator of a synchronous motor is wound for the same number of poles as the rotor poles.

Working Principle of Synchronous Motor

Consider a 3-phase, 2-pole synchronous motor having two rotor poles NR and SR as shown in Figure-2. The stator is also being wound for two poles NS and SS. A three-phase AC supply is connected to the stator winding and a DC voltage is applied to the rotor field winding.

The stator winding produces a rotating magnetic field which revolves around the stator at synchronous speed. The DC voltage applied to the rotor sets up a two-pole field which is stationary so long as the rotor is not running. Hence, under this condition, there exists a pair of revolving stator poles (NS-SS)and a pair of stationary rotor poles (NR-SR).

Now, suppose at any instant, the stator poles are at positions as shown in Figure-2. From Figure-2, it is clear that poles NS and Nrepel each other and so do the poles SS and SR. Hence, the rotor experiences a torque in the anticlockwise direction

After a period of half-cycle of the AC supply, the polarities of the stator poles are reversed but the polarities of the rotor poles remain the same as shown in Figure-3. Under this condition, the poles SS and NR attract each other and so do the poles NS and SR. Due to this, the rotor tends to move in the clockwise direction.

Since the stator poles change their polarities rapidly, they tend to pull the rotor first in one direction and then after a period of half cycle in the other direction. But the rotor has high inertia, consequently, the rotor does not move and we say that the starting torque is zero. In other words, a synchronous motor is not self starting.

Updated on: 30-Oct-2021

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