How does the Induction motor work? (OR) Why does the Rotor rotate?
- When the 3 phase stator windings are fed by 3 phase supply, a magnetic flux of constant magnitude which is rotating at synchronous speed is set up.
- The flux passes through the air-gap and sweeps past the rotor surface thus it cuts the rotor conductors.
- Due to the relative speed between the rotating flux and the stationary rotor conductors, an emf is induced in the stationary rotor conductors as per the Faraday’s laws of electromagnetic induction.
- The frequency of the induced emf is the same as the supply frequency.
- Its magnitude is proportional to the relative velocity between the flux and the conductors.
- Its direction will be as per Fleming’s right hand rule.
- Since the rotor conductors form a closed circuit the rotor current is produced.
- This current’s direction will oppose the very cause producing it ( as per Len’s law)
- Here the cause is the relative velocity between the rotating flux of the stator and the stationary rotor conductors.
- Hence, in order to reduce the relative speed, the rotor begins to rotate in the same direction as that of the rotating magnetic flux and tries to catch up with the rotating flux. Thus the rotor of induction motor starts to rotate.
 What is the general working principle of Induction motor?
- The conversion of electrical power into mechanical power takes place in the rotating part of an electric motor.
- In DC motors the electrical power is conducted directly to the armature through brushes and commutator.
- Thus the DC motor can be called as conduction motor. But in case of AC motors, the rotor receives electric power, not by conduction but by induction.
- This is exactly in the same way as the secondary of two winding transformer receives its power from the primary.
- That is why such motors are known as induction motors.
- Thus an induction motor is also known as rotating transformer ( ie, one in which primary winding is stationary and the secondary is free to rotate)