How to make a synchronous motor self-starting?

Assume that initially, the rotor of a synchronous motor is stationary. When a pair of rotating stator poles sweeps across the stationary rotor poles at synchronous speed, the stator poles will tend to rotate the rotor in one direction and then in the other direction. As a result, the rotor experiences a torque first in one direction and then in the other. However, the rotor has high inertia and the stator field slides by so fast that the rotor cannot follow it. As a result, the rotor cannot move and hence the resultant starting torque is zero, i.e., a synchronous motor cannot start by itself.

In order to make the synchronous motor self-starting, a damper winding (a squirrel cage type winding) is provided on the rotor. The damper winding is formed by embedding copper bars in the slots made in the pole faces of the salient poles of the rotor. The copper bars are short-circuited at the ends. The damper winding serves to start the synchronous motor.

Explanation

Initially, the stator winding of the motor is connected to a balanced 3-phase supply and the rotor winding or the main field winding is left open, i.e., unexcited as shown in the figure. It will produce a rotating magnetic field in the air gap which induces currents in the damper winding and the synchronous motor starts as an induction motor.

As the speed of the motor reaches near to the synchronous speed, at a right moment, the rotor field winding is excited with direct current. Now, the resulting rotor poles face the stator poles of opposite polarity and a strong magnetic force of attraction is set up between the rotor and stator poles. Consequently, the rotor poles magnetically locked with the stator poles.

Therefore, the rotor revolves at the same speed as the rotating magnetic field of the stator, i.e., at synchronous speed.

Under this condition, the rotor rotate at the same speed as the rotating magnetic field of the stator, thus the copper bars of the damper winding do not cut any flux and hence have no induced EMF and current in them. Thus, the squirrel cage portion of the rotor, i.e., the damper winding is removed from the operation of the motor.

Here, it is to be noted that due to the magnetic locking between the stator and the rotor poles, a synchronous motor can only run at synchronous speed. At a speed other than synchronous speed, this magnetic locking ceases and hence the average torque developed in the motor becomes zero. As a result, the synchronous motor comes to the rest with a severe disturbance on the supply line.

Point to Note

It is very important that the rotor field winding must be excited with a direct current at the right moment. If the DC excitation to the rotor winding is applied when the pole N_S faces the pole N_R, then the resulting magnetic repulsion will produce a violent mechanical sock. The motor will immediately slow down and the circuit breakers will trip.

In actual practice, the synchronous motor starters are designed to detect the precise moment when the field excitation should be applied.