The rotation of synchronous motor happens when opposite poles of rotating stator field and stationary rotor field magnetically lock with each other. In other words, it is due to the magnetic overlocking of rotor and stator field poles that synchronous motor runs at synchronous speed. However this magnetic overlocking is not happen at the start due to which synchronous motor is not self-starting. Let me explain why?
Magnetic Overlocking in Synchronous Motor:
When we give three phase AC supply to the stator, rotating magnetic field is produced in the stator which rotates at synchronous speed. DC supply is also given to the rotor coils which magnetizes the rotor and creates north and south poles alternatively. Since rotor is excited with dc supply, hence field poles of the rotor are fixed (rotor magnetic poles are not rotating). Once rotor becomes a magnet, it tries to get locked with the rotating stator magnetic poles, but it will fail to catch up due to its high inertia.
At the starting of synchronous motor ↗ , rotor could have in any position. Rotor poles are fixed since they are excited from constant DC supply. Let us consider one instant point when the stator poles (N1,S1) is at vertical position along A-B point as shown in the fig (A). At that instant, rotor flux produces fixed poles (N2,S2) are at arbitrary position, fig (A).
During this particular instant of time, stator field is moving in the clockwise direction. By analyzing the fig (A), we can say that rotor N pole will face the stator N pole and rotor S pole will face stator S pole.
Since like pole repels each other, hence force is exerted on the rotor in anticlockwise direction due to which it starts to move in that direction. It should be noted that stator poles are rotating very fast (at synchronous speed). Due to high inertia, rotor hardly rotates in anticlockwise direction and at the same instant stator poles change their positions. After half of the time period, stator poles are exactly reversed but rotor is unable to rotate during this time period due to its high inertia. This can be seen in the fig (B).
By analyzing the fig (B), it can be seen that poles are now opposite. In that scenario, rotor north pole will attract towards stator S pole and rotor south pole will attract towards stator north pole. Hence direction of force on the rotor is now become clockwise fig(B). However, rotor is again unable to attain such high speed to catch the stator rotating magnetic field due to its high inertia. Since rotor cannot move from its particular position, stator poles again change their position and the direction of torque become reversed again. In simple words, we can say that magnetic overlocking between the stator and rotor poles cannot take place due to which machine fails to start.
From the above discussion, we can say that whatever be the starting position of rotor, synchronous motor is not self-starting.
Synchronous Motor starting Methods :
In order to make synchronous motor self-starting, we need external prime mover to get the rotor in motion. The rotor is made to rotate by an external prime mover at such specific speed (near to synchronous speed) so that the rotor poles would easily interchange their pole position along with the rotating stator poles. In that case, rotor poles gets magnetically locked with stator field and machine starts to runs at synchronous speed.
In order to make synchronous motor self-starting, damper winding (also known as squirrel cage winding) is provided on the rotor of synchronous motor. Damper winding consists of straight copper bars evenly spaced in the pole faces of salient pole rotor↗. The bars are connected with end rings which short circuited all the bars at each end to form a cage.
When three phase supply is given to the stator, rotating stator field cut the rotor bars and induces current in the damper cage windings. Once the current is induced in the squirrel cage bars, the motor will start as three phase Induction motor ↗ .
When motor reaches to such speed sufficient for synchronizing, rotor field winding is excited with DC supply. Hence current will flow in the coils of rotor and magnetize the rotor. The magnetize rotor now magnetically locks with the rotating stator magnetic field and machine starts to run at synchronous speed (i.e. rotor rotate at same speed as the stator field).
Moreover, squirrel cage bars of the rotor are also rotating at same speed as that of the stator field. Once synchronous speed is achieved, magnetic flux no more cut the squirrel cage rotor bars (since there is no relative speed between the stator field and rotor bars) which will result in zero induced current in the damper windings.
One more thing to be noted that once the opposite rotor and stator field poles magnetically locked with each other at synchronous speed, force of attraction between them is enough to keep the rotation of motor. Hence machine start working as synchronous motor. In this way, damper windings helps to make the synchronous motor as self-starting motor by providing the starting torque.