3 Phase Induction motor

Working of a 3 phase Induction Motor

The induction motor works on the principle of Induction and hence the name Induction Motor. The rotating magnetic field produced by the stator cuts through the conductors of the rotor and hence induces voltage. The induced voltage circulates current in the windings of the rotor since the ends of the rotor are short circuited. Current carrying conductor experiences force when placed in a magnetic field. As a result the motor rotates.

What is important for an induction motor to works is a Rotating magnetic field. What is actually a rotating field?

I shall try to explain that through slides of images.

A 3-phase supply consist of same magnitude emf displaced by 120 degrees. Corresponding flux produced are also displaced by 120 degrees. Let us draw these wave forms and find there find their resultant sum for different angles. We find that the resultant of the 3 flux waveform always remain constant for any value of theta. When we plot this magnitude vs. theta graph in polar form we get a rotating magnetic field which is rotating with an angular frequency omega.

Consider the flux produced by the stator is rotating in clockwise direction. Since there is change of flux, there will be induction of voltage. The direction of the current flowing will be such that it opposes the cause of it that is the relative displacement of the flux and conductor. Hence the rotor will rotate in same direction. So 3 phase induction motor is a self-starting motor.


The rotor will always try to make the relative speed to 0 but will not be able to achieve that. As it reaches the Synchronous speed there will be no relative speed and hence no current and no torque. Hence 3 phase induction motor is also called asynchronous motor. However the average speed of the rotor remains almost constant.