A synchronous motor is a machine that rotates at a constant speed called synchronous speed and converts electrical power to mechanical power. Synchronous motors Starter winding fed with a three phase AC supply and field winding is excited using a DC power source hence this motor is also called a doubly excited machine. Now we are going to discuss starting methods of synchronous motor.
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The synchronous motor is not self-starting it requires some starting arrangements to make it self-starting.
Starting Methods of Synchronous Motor
- Starting with external prime mover
- Starting with Damper Winding
Synchronous Motor Starting With External Prime Mover
This Method uses an external motor to drive the synchronous motor at a synchronous speed then the motor is synchronised with the bus bar as the synchronous generator once the motor attains synchronous speed prime mover is disconnected and the rotor is excited using DC supply.
Hence rotor magnetic field pole is attracted to the unlike pole of the rotating magnetic field of the stator and magnetic locking happens between the stator and the rotor field. The rotor starts rotating at the speed of the stator magnetic field which is at synchronous speed.
starting motor is smaller in rating compared to the synchronous motor as the initial load is not connected to the synchronous motor before synchronizing. Hence prime-moving motor has to overcome the inertia of the synchronous motor at no load.
Synchronous Motor Starting with Damper Winding
This starting method of synchronous motor is widely used.
Damper winding consists of heavy copper bars inserted in slots of pole faces of the rotor and these bars are short-circuited at both ends of the rotor using end rings.
This arrangement forms a squirrel cage Winding. When three phase supply is fed to the stator.
Synchronous motor with Damper Winding arrangement will start as a 3-phase induction motor.
As the motor attains 85% to 97% of synchronous speed, DC excitation is applied to the field winding.
the rotor will then pull into step with the stator magnetic field and start rotating at synchronous speed.
