Three phase induction motor works on the principle of electromagnetic induction. Due to the similarity in the working principle of transformer, it is also known as rotating transformer.
The three phase induction motors are the most widely used electric motors in industry. They run at essentially constant speed from no load to full load. However, the speed is frequency dependent and consequently these motors are not easily adapted to speed control. We usually prefer d.c. motors when large speed variations are required.
Principle of Operation
For explaining the principle of operation of a three phase induction motor, consider a portion of three phase induction motor as shown in the figure. The operation of the motor can be explained as under:
- When three phase stator winding of an induction motor is energized from a 3 phase supply, a rotating magnetic field is set up which rotates round the stator at synchronous speed (Ns = 120 f/P).
- The rotating field passes through the air gap and cuts the rotor conductors, which as yet, are stationary. Due to the relative speed between the rotating flux and the stationary rotor, e.m.f.s are induced in the rotor conductors. Since the rotor circuit is short-circuited, currents start flowing in the rotor conductors.
- The current-carrying rotor conductors are placed in the magnetic field produced by the stator. Consequently, mechanical force acts on the rotor conductors. The sum of the mechanical forces on all the rotor conductors produces a torque which tends to move the rotor in the same direction as the rotating field.
- The fact that rotor is urged to follow the stator field (i.e., rotor moves in the direction of stator field) can be explained by Lenz’s law. According to this law, the direction of rotor currents will be such that they tend to oppose the cause producing them.
- Now, the cause producing the rotor currents is the relative speed between the rotating field and the stationary rotor conductors. Hence to reduce this relative speed, the rotor starts running in the same direction as that of stator field and tries to catch it.
The rotor speed (N) of a three phase induction motor is always less than the stator field speed (Ns). This difference in speed depends upon load on the motor. The difference between the synchronous speed Ns of the rotating stator field and the actual rotor speed N is called slip.
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