Starting Methods of Synchronous Motors

Various methods of starting of synchronous motors are described as follows:

When dc supply and dc compound motor are available, the synchronous motor is coupled and started by means of a dc compound motor.
The speed of dc motor is adjusted by the speed regulator.
The synchronous motor is then excited and synchronized with AC supply mains.
At the moment of synchronizing, the synchronous motor is switched on with the AC mains and either the dc motor is disconnected from the dc supply mains or the field of the dc machine is strengthened until it begins to function as a generator.
Now the synchronous machine is operating as a motor, from ac supply mains and dc machine acts as load on it.
The synchronous motor can also be started by the exciter mounted on an overhung synchronous motor bracket and shaft extension.
Here again, an available dc source operates the exciter as a motor during the starting period then after the synchronous machine is’ brought up to speed and synchronised, the exciter assumes its normal function.

A small direct coupled induction motor called the pony motor, may be used for starting the synchronous motor unless the motor is required to start against full load torque.
The induction motor frequently has two poles less than the synchronous motor and so is capable of raising the speed of the latter to synchronous speed.
Before switching on the AC supply to the synchronous’ motor, it must be synchronised with the bus bars.
After normal operation is established, the pony motor is sometimes decoupled from the synchronous motor.
This method is not very satisfactory and not suited to industrial needs. Modern machines are usually of the self starting type and are arranged to start as induction motors.

The synchronous motor is made self starting by providing a special winding on the rotor poles, known as damper winding or squirrel cage winding.
The damper winding consists of short circuited copper bars embedded in the face of the field poles.
AC supply given to the stator produces a rotating magnetic field which causes the rotor to rotate, therefore in the beginning synchronous motor provided with damper winding starts as a squirrel cage induction motor.
The exciter moves along the rotor-when the motor attains about 95% of synchronous speed, the rotor winding is connected to exciter terminals and the rotor is magnetically locked by the rotating field of the stator and the motor runs as a synchronous motors.