Starting Methods of Synchronous Motor

A Synchronous motor does not automatically start on its own. As a result, it needs some Additional starting tools. There are two Starting Methods of Synchronous Motor :

• Starting with an External prime mover
• Starting with damper Windings

Starting with an External Prime Mover

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 speeds up the Synchronous motor, which is then Synchronised with the AC supply to function as a synchronous generator. The External Motor, or prime mover, is then disconnected. The synchronous machine will function as a motor once it has been Synchronised.

The Synchronous motor’s shaft can now be connected to the Mechanical load. The prime mover motor only needs to Overcome the synchronous motor’s inertia at no load because the load is not connected to it prior to synchronisation. As a result, the rating of the starting or prime mover motor is significantly lower than the rating of the synchronous motor.

Synchronous Motor Starting with Damper Windings

Currently, the most popular method of starting a synchronous motor is with damper windings. As seen in Figure 2, a damper winding is constructed from thick copper bars that are inserted into rotor pole face slots. End rings at both ends of the rotor then short-circuit these copper bars.

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.

The rotor field windings receive the DC field excitation. When the motor reaches a speed that is closer to the synchronous speed. The motor will then run at synchronous speed as the rotor will then pull into step with the armature’s rotating magnetic field.