Principle of Operation for Synchronous Motors

PRINCIPLE OF OPERATION

When a sinusoidal (single-phase) voltage is applied to a winding, the magnetic field produced by the resultant current flow will also be sinusoidally varying with respect to time. This means that the field is pulsating. Now when a three-phase voltage is applied to a three phase winding. The flux produced will be the resultant of all the three pulsating fields.

It can be shown that the resultant field has a magnitude of 1.5 φ_m where φ_m is the maximum value of the flux due to a single phase current: Further it can also be shown that the direction of the field changes continuously. i.e., the field is rotating in space at a speed given by:

N_S = 120 f / P

where ƒ is the frequency of supply and P is the number of poles. This speed is called the synchronous speed.

Hence it is to be remembered that when a three-phase supply is given to a three- phase winding a magnetic field of constant magnitude but rotating at a constant speed N_S is produced.

Figure 3.54 shows the two fictitious stator poles marked N_S and S_S assumed to rotate clock wise at a synchronous speed N_S. The rotor poles marked N_S and S_S assumed to rotate clockwise at synchronous speed N_S. The rotor poles (assumed to be only two in number) N_R and S_R and formed by the d.c excitation. When N_S and N_R are together (and similarly S_S and S_R) like poles repel each other. Since N_S and S_S are moving in the clockwise direction, N_R and S_R all moving in the clockwise direction, N_R and S_R tend to figure 4.46(a). Half a cycle later, the stator poles have moved, whereas the rotor poles have moved significantly. This situation is shown in Figure 3.54(b) N_S and S_R and similarly S_S and N_R get attracted and the rotor tries to rotate in the clockwise direction. This implies that the rotor experiences torque in different direction every half a cycle. As a result, the rotor is at standstill due to its large inertia. This explains why a synchronous motor has no starting torque and cannot start by itself.

However, if the rotor is now rotated separately by a prime mover in the same direction as the synchronously rotating stator field, and at a speed near N_S then it is possible that at some instant of time, N_S and S_R and similarly S_S and N_R (i.e., the stator and rotor poles) get attracted and locked to one another. This is shown in Figure3.55 (a) and (b).