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

Synchronous Motor Starting Methods


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.

Damper Winding — Synchronous Motor Starting Methods

This is the most common synchronous motor starting method. 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.

synchronous motor starting methods

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.

Why Synchronous Motor is not Self Starting?

Assume the number of poles of stator P = 2, then the synchronous speed of the RMF will be = 3000 RPM.
So the stator poles N1 and S1 are rotating at a speed of 3000 RPM i.e. very rapidly in the clockwise direction.
Initially (at the start), the rotor is in an arbitrary position as shown in Figure.
We assume that the rotor is not being driven by any other prime mover.
The Figure shows the situation at an instant ‘A’. The rotor is in an arbitrary position. The stator poles S1 and N1 are rotating in clockwise direction.
Due to the force of repulsion between the like poles N1 – N2 and S1 – S2, the standstill rotor experiences a torque in the clockwise direction.

 why the synchronous motor is not self-starting

The rotor takes time to accelerate in the clockwise direction due to inertia. Before the rotor can accelerate in the clockwise direction, the stator poles change their polarity quickly.
At the instant ‘B’, which occurs a period half after instant ‘A’ the stator poles have acquired new polarity but the rotor is still in the previous position.
Now, due to the force of attraction between the unlike poles N1 – S2 and S1 – N2, the standstill rotor experiences a torque in the anticlockwise direction.

Thus, in the case of a synchronous motor, standstill rotor is subjected to a torque which is changing its direction at a very high speed, the rotor cannot respond to it due to its inertia and hence remains stationary.

This is why the synchronous motor is not self-starting.
Thanks for reading about synchronous motor starting methods.

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