Synchronous motors run at synchronous speed. The synchronous speed of a motor depends on the supply frequency and the number of poles in the motor.
Synchronous speed is given by
Where, f = supply frequency and p = number of poles.
We can change the synchronous speed of the motor by changing the supply frequency and the number of poles. But the motor would always run with this speed for a given supply frequency and the number of poles.
Synchronous motors have lots of advantages but being not self-starting unlike 3 phase induction motors, is a major disadvantage. In synchronous motors, the stator has 3 phase windings and is excited by 3 phase supply whereas the rotor is excited by DC supply. The 3 phase windings provide rotating flux whereas the DC supply provides constant flux.
The torque produced on the rotor is a pulsating one and not uni-directional. Considering the frequency to be 50 Hz, from the above relation we can see that the 3 phase rotating flux rotates about 3000 revolutions in 1 min or 50 revolutions in 1 sec. At a particular instant rotor and stator poles might be of the same polarity (N-N or S-S) causing a repulsive force on the rotor and the very next second it will be N-S causing attractive force. But due to the inertia of the rotor, it is unable to rotate in any direction due to attractive or repulsive force and remain in standstill condition. Due to this, the motor cannot start on its own. The rotor of the synchronous motor has to be brought to synchronous speed by using external means.
Below are the techniques used for starting a synchronous motor:
Starting a Synchronous Motor Using an Induction Motor
We need to bring the rotor of the synchronous motor to synchronous speed before we switch on the motor. For that reason, we directly couple a small induction motor (pony motor) with the synchronous motor. Note here, that the number of poles of the induction motor should be less than the synchronous motor else it will never be able to achieve the synchronous speed of the synchronous motor. This is because an induction motor always has a speed less than the synchronous speed and for it to become equal to the synchronous speed of the synchronous motor, its own speed has to be increased. After the rotor of the synchronous motor is brought to the synchronous speed, we switch on the DC supply to the rotor. After that, we simply de-couple the induction motor from the synchronous motor shaft.
Starting a Synchronous Motor Using a DC Machine
It is similar to above method with a slight difference between the two. A DC machine is coupled to the synchronous motor. The DC machine works like a DC motor initially and brings the synchronous motor to synchronous speed. Once it achieves the synchronous speed, the DC machine works like a DC generator and supplies DC to the rotor of the synchronous motor. This method offers easy starting and better efficiency than the earlier method.
Starting a Synchronous Motor Using Damper Windings
In this method, the motor is first started as an induction motor and then starts running as a synchronous motor after achieving synchronous speed. For this, damper windings are used. Damper windings are additional windings consisting of copper bars placed in the slots in the pole faces. The ends of the copper bars are short-circuited. These windings behave as the rotor of an induction motor. When 3 phase power is supplied to the motor, the motor starts running as an induction motor at a speed below synchronous speed. After some time DC supply is given to the rotor. The motor gets pulled into synchronism after some instant and starts running as a synchronous motor. When the motor reaches synchronous speed, there is no induced emf in the damper windings anymore and hence they don’t have any effect now on the working of the motor. This is the most commonly used technique for starting synchronous motors.
Starting a Synchronous Motor Using Slip Ring Induction Motor
Here we connect one external rheostat in series with the rotor. The motor is first started as a slip ring induction motor. The resistance is gradually cut-off as the motor gains speed. When it achieves near synchronous speed, DC excitation is given to the rotor, and it is pulled into synchronism. Then it starts rotating as a synchronous motor.