A synchronous motor is a type of electrical machine that converts electrical energy into mechanical energy by rotating at a constant speed, known as the synchronous speed. This speed is determined by the frequency of the power supply and the number of poles of the motor.
One of the main characteristics of a synchronous motor is that it is not self-starting, meaning that it requires an external force to bring it up to the synchronous speed. This is because, at the start of the motor, the average torque on the rotor is zero.
To understand why the average torque on the rotor is zero at the start, consider a rotating magnetic field as equivalent to the physical rotation of two stator poles, N1 and S1, as shown in the figure. At any given instant, the stator poles are in line with the stator magnetic axis [A-B]. At this instant, the rotor poles are arbitrarily positioned as shown in the figure.
When a DC supply is applied to the stationary rotor, the unlike poles will try to attract each other. This will cause the rotor to be subjected to an instantaneous torque in the anticlockwise direction. However, due to the inertia of the rotor, it is unable to rotate in this direction. As the stator poles rotate at the speed of Ns rpm, the direction of the torque on the rotor changes. At the next instant, the stator poles are exactly reversed, but due to the inertia of the rotor, it is unable to rotate from its initial position.
At this point, the rotor is subjected to a torque in the clockwise direction, which tends to rotate the rotor in the direction of the rotating magnetic field. However, before this can happen, the stator poles change position again, reversing the direction of the torque on the rotor. This cycle continues, resulting in an average torque on the rotor of zero. As a result, the synchronous motor is unable to start itself and requires an external force to bring it up to the synchronous speed.