Principle of operation for Synchronous Motor

PRINCIPLE OF OPERATION FOR SYNCHRONOUS MOTOR

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 is produced will be resultant of all the three pulsating fields. It can be shown that the resultant field has a magnitude of 1.5ϕ_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 × ƒ/ 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 3.58 shows that the two fictitious stator poles marked N_S and S_S assumed to rotate clockwise at a synchronous speed N_S. The rotor poles (assumed to be only 2 in number), N_R and S_R are formed by the d.c excitation. When N_S and N_R together (and similarly S_S and S_R) like poles repel each other. Since N_S and S_S are moving in the clock direction, N_R and S_R tend to figure. Half a cycle later, the stator poles have moved, whereas the rotor poles have moved significantly. This situation is shown in figure N_S and 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 directions 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 (ie., The stator and rotor poles) get attracted and locked to one another. This is shown in figure 3.59.

Hence a synchronous motor, though not self starting, starts working as a motor if it is started up by some means.

It needs two separate supplies-one a d.c source for excitation of the rotor and other, a three-phase supply for the stator. Because of the interlocking between the stator and rotor poles, the motor runs only at one speed, the synchronous speed.