Starting Methods for Synchronous Motor

For explaining the principle of operation of a three-phase induction motor, consider a portion of the three-phase induction motor as shown in the figure.

Working of Three Phase Induction Motor

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 around the stator at synchronous speed (Ns).

$N_{s} = 120 \frac{f}{P}$

Where,

f = frequency

P = Number of Poles

(For more details about rotating magnetic field, read Production of rotating magnetic field).

This rotating field passes through the air gap and cuts the rotor conductors, which are stationary.

An EMF gets induced in every rotor conductor due to the relative speed between the rotating magnetic flux and the stationary rotor. 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, a 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 the 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 Lenz’s 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. This is how a three-phase induction motor starts running.

Slip in Induction Motor

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 the 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.