How a synchronous motor starts?

Synchronous speed is referred as the speed of the rotating flux. So, as the name suggests a motor which rotates at synchronous speed is synchronous motor. Synchronous motors run at the same speed as that of the rotating flux. The speciality of a synchronous motor is that the starter is connected with 3 phase supply and the rotor is connected with dc supply to make it a permanent magnet.

Now when 3 phase supply is given to the motor there is a rotating flux generated, which rotates at the synchronous speed. Now the rotor which is a magnet which is not rotated automatically, or it can be said that the synchronous motor is not self starting. The reason behind that the rotating flux rotates at very high speed. So, the poles of the rotor can not get locked with the stator poles and the motor does not works. To start the operation of the synchronous motor the rotor is first rotated by an external prime mover or a separate motor. The main aim for doing this is to reduce the speed difference between the rotating flux and rotor poles. When the relative speed between these two almost reaches zero, the poles of the rotor gets attached or locked with the flux. In this way, the rotor starts to rotate at synchronous speed.

Another method of starting of synchronous motor is by using damper windings. In this case at first the supply (dc) to the rotor is switched off and the motor starts to rotate on the principle of squirrel cage induction motor. When the relative speed between the rotor and the rotating flux decreases the rotor is energized by dc supply and the rotor gets locked with flux and starts to rotate at synchronous speed.