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Synchronous motor-General:

A synchronous motor is electrically identical with an alternator or a.c. generator. In fact, a given synchronous machine may be used, at least theoretically, as an alternator, when driven machinanically or as a motor, when driven electrically, just as in the case of d.c. machines. Most synchronous motors are rated between 150 kW and 15 MW and run at speeds ranging from 150 to 1800 r.p.m.

Some characteristic features of a synchronous motor are worth nothing.

1. It runs either at synchronous speed or not at all i.e while running it maintains a constant speed. The only way to change its speed is to vary the supply frequency.
2. It is not inherently self-starting. It has to be run upto synchronous (or near syncronous) speed by some means, before it can be synchronized to be supply.
3. It is capable of being operated under a wide range of power factors, both lagging and leading. Hence, it can be used for power correction purposes, in addition to supplying torque to drive loads.

Principle of operation:

As shown in Art. 34.7, when a 3- winding is fed by a 3-supply, then a magnetic flux of constant magnitude but rotating at synchronous speed, is produced. Consider a two-pole stator of Fig. 38.2, in which are shown two stator poles (marked N and S) rotating at synchronous speed, say, in clockwise direction. With the rotor position as shown, suppose the stator poles
Stator poles are at that instant situated at points A and B. The two similar poles, N (of rotor) and N (of stator) as well as Sand S will repel each other, with the result that the rotor tends to rotate in the anticlockwise direction.

But half a period later, stator poles, having rotated around, interchange their positions i.e. N_s is at point B and S_s at point A. Under these conditions, N attracts S and Ss attracts N. Hence, rotor tends to rotate clockwise (which is just the reverse of the first direction). Hence, we find that due to continuous and rapid rotation of stator poles, the rotor is subjected to a torque which is rapidly reversing i.e., in quick succession, the rotor is subjected to torque which tends to move it first in one direction and then in the opposite direction. Owing to its large inertia, the rotor cannot instantaneously respond to such quickly-reversing torque, with the result that it remains stationary.

Now, consider the condition shown in Fig. 38.3 (a). The stator and rotor poles are attracting each 1oth. Suppose that the rotor is not stationary, but is rotating clockwise, with such a speed that it turns through one pole pitch by the time the stator poles interchange their positions, as shown in the figure. Here, again the stator and rotor poles attract each other/. It means that if the rotor poles also shift their positions along with the stator poles, then they will continuouly experience a unidirectional torque clockwise  torque, as shown in the figure.

Synchronous machine-Lecture Video:

Summary of this lecture: Synchronous motor and Synchronous generator