Methods of Starting of Synchronous Motor

Since a synchronous motor is inherently not self-starting, the following methods of starting of synchronous motor are adopted generally:
 

By Means of Auxiliary Motor

 
In this method, a small induction motor known as pony motor is used to bring the motor speed to synchronous speed. After the speed has reached to its synchronous value, DC field is switched on, motor gets synchronized and then supply to the pony motor is disconnected and it acts as a load on the main motor. In some systems, pony motor is mechanically decoupled.
 

By Using a Small DC Machine

 
Many times a large size synchronous motors are provided with a coupled DC generator which acts as an exciter. Its DC output is used to supply the field winding of the synchronous motor.  At the start, this DC exciter is fed with DC supply to run it as a motor.
 
This motor brings the speed of the motor to synchronous speed. The field of the synchronous motor is then excited and motor gets synchronized.
 

By Using the Damper Winding

 
This is a most common method of starting of the synchronous motor. In this method, the motor is started as squirrel cage induction motor by a providing a special winding on the rotor poles called damper winding.
 

damper winding in synchronous motor

 
The damper winding is an additional winding included on the rotor side. The construction of damper winding is shown in the figure. This damper winding consists of a number of copper bars embedded into the holes provided on the outer periphery of the rotor and then short-circuiting these bars by brazing them to end rings.  This construction is similar to that of the squirrel cage rotor of an induction motor.
 
When the synchronous motor is connected to three phase supply, the motor starts as a squirrel cage induction motor. As soon as it attains about 95% of synchronous speed, the rotor winding is connected to DC mains and the rotor field is locked with stator field and the motor runs as a synchronous motor.
 
When the rotor is rotating at synchronous speed, the relative velocity between the RMF (rotating magnetic field of stator) and the rotor is zero. Hence the induced EMF and current in damper winding are zero. Hence damper winding is ineffective when the rotor is rotating at synchronous speed but we can use the damper winding for starting the motor. Thus damper winding allows the synchronous motor to be started as a squirrel cage motor and makes it self-starting. Now, we can use the induction motor starters to start the synchronous motor.

 

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