A synchronous motor is not inherently self-starting. Therefore, it requires some auxiliary means of starting. In order to start a synchronous motor, there are following two methods −
Starting with an external prime mover
Starting with damper windings
In this method of starting a synchronous motor, an external motor is used to drive the synchronous motor as shown in Figure-1.
The external motor brings the synchronous motor to synchronous speed and then the synchronous motor is synchronised with the AC supply as a synchronous generator. Then the prime mover (i.e., the external motor) is disconnected. Once synchronised, the synchronous machine will operate as a motor.
Now, the mechanical load can be connected to the shaft of the synchronous motor. Since the load is not connected to the synchronous motor before synchronising, thus the prime mover motor has to overcome only the inertia of the synchronous motor at no-load. Consequently, the rating of the prime mover or starting motor is much smaller than the rating of the synchronous motor.
At present, damper windings are most widely used method of starting a synchronous motor. A damper winding is made up of heavy copper bars inserted in slots cut into the pole faces of the rotor as shown in Figure-2. These copper bars are then short-circuited by end rings at both ends of the rotor.
Hence, this arrangement of copper bars and end rings form a squirrel cage winding.
When the armature of the synchronous motor is connected to a three-phase supply, the synchronous motor with damper winding will start as a 3-phase squirrel cage induction motor.
When the motor attains a speed nearer to the synchronous speed, the DC field excitation is applied to the rotor field windings. Then, the rotor will pull into step with the rotation magnetic field of the armature and hence the motor runs at synchronous speed.