Starting Methods of Synchronous Motor

     A device that converts electrical energy into mechanical energy running at synchronous speed is called Synchronous Motor. Its speed is constant irrespective of load. It is a doubly excited machine because its field winding is excited from a separate dc source. But the synchronous motor is not self-starting. The average synchronous motor torque is zero at the start. For a net average torque, it is necessary to rotate the rotor at a speed very near to synchronous speed. This is possible through various methods in practice.

     The different methods used to start a synchronous motor are :

Using Pony Motors : 

     By using the small pony motors like a small induction motor, we can start the synchronous motor. This small induction motor is coupled to the rotor of the synchronous motor. The function of this induction motor is to bring the rotor of the synchronous motor to the synchronous speed.

     Once the rotor attains the synchronous speed the pony motor is dis-coupled from the rotor. The synchronous motor continues to rotate at synchronous speed, by supplying d.c. excitation to the rotor through the slip-rings. One should remember that the motor used as the pony motor must have less number of poles than the synchronous motor used.

Using Small D.C. Machine :

     In the above method, we have a seen small induction motor to start the motor. Here we use d.c. motor instead of induction motor to bring the motor to synchronous motor.

     Once the d.c. motor brings the rotor of the synchronous motor to synchronous speed. The motor starts acting as the d.c. generator and starts giving excitation to the field winding of the synchronous motor.

Using Damper Winding :

     When a 3-phase supply is given to the synchronous motor it fails to start. In order to make it start copper bars circuited at both ends ( similar to the squirrel cage rotor of an induction motor ) are placed on the rotor, these bars or winding are known as ' Damper Winding '.

     Now when the supply is given the field winding setups a rotating magnetic field. Due to the damper winding used, the rotor starts rotating as an induction motor i.e., less than the synchronous speed at starting. Once d.c. excitation is given to the field winding and the motor is then pulled into synchronism.

Starting Methods of Synchronous Motor

     The damper winding is used to start the motor and hence can be used for starting purposes only. Because once the rotor rotates at synchronous speed the relative motion between the damper winding and rotating magnetic will be equal, and hence induced emf and current will be zero. The damper winding will be out of the circuit.

As a Slip Ring Induction Motor ( Synchronous Induction Motor ) : 

     In this method, an external rheostat is connected to the rotor through slip-rings. Here, ends of the damper winding are brought of the motor and connected either in star or delta. The rheostat is connected in series with the rotor. At starting high resistance is connected with the rotor to limited the current drawn by the motor. As the motor starts as a slip ring induction motor at starting, it draws large currents.

     When the motor picks up its speed, resistance is gradually cut off from the rotor circuit. As the speed reaches near to synchronous speed, d.c. excitation is given to the rotor and it is pulled into synchronism.

Starting Methods of Synchronous Motor

     The above figure shows the rheostat connected with the rotor circuit through slip-rings. From the figure as the dc supply is given current ' I ' flows through the positive terminal, then it divides as ' I/2 ' through each phase at star point.

     From these methods, damper winding is the most common method of starting a synchronous motor.

Reference - 

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