Theory of operation
When the motor is supplied with a.c. power supply, the stator poles get energised. This in turn attracts (opposite) the rotor poles, thus both the stator and rotor poles get magnetically interlocked. It is this interlock which makes the rotor to rotate at the same synchronous speed with the stator poles. The synchronous speed of rotation is given by the expression Ns=120f/P.
When the load on the motor is increased gradually, the rotor even though runs at same speed, tends to progressively fall back in phase by some angle, “β", called the Load Angle or the Coupling Angle. This Load angle is dependent on the amount of load that the motor is designed to handle. In other words, we can interpret as the torque developed by the motor depends on the load angle, “β".
The electrical working of a Synchronous Motor can be compared to the transmission of power by a mechanical shaft. In the figure are shown two pulleys, “A" & “B". Pulley “A" and the pulley “B" are assumed to be keyed on the same shaft. Pulley “A" transfers the power from the drive through the shaft, in turn making the pulley “B" to rotate, thus transferring power to the load.
The two pulleys which are keyed to the same shaft can be compared to the interlock between the stator & rotor poles.
If the load increases, the pulley “B" transfers the increase in load to the shaft, which is exhibited by the twisting of the shaft.
Thus the twist of the shaft can be compared to the rotor falling back in phase with the stator.
The twist angle can be compared to the load angle “β". Also when the load increases, the twisting force and the twist angle increases, thus the load angle “β" also increases.
If the load on the pulley “B" is increased to such an extent that it makes the shaft to twist and break, then the transmission of power through the shaft stops as the shaft is broken. This can be compared with the rotor going out of synchronism with the stator poles.
Thus Synchronous motors can run either at synchronous speed or they stop running.