Why Induction Motor is called Rotating Transformer?

Induction motor as a rotating transformer: The rotor of the squirrel cage  induction motor is short-circuited by end rings. When three phase supply fed to stator of three phase induction motor, rotating magnetic flux is produced and the flux gets linked to the rotor conductor. The rotor conductors are short circuited by end rings and the current starts flowing in the rotor conductor. Due to an interaction of the main flux and the rotor current torque is produced.

why induction motor is called rotating transformer?

In a transformer, AC supply is fed to the primary and the flux produced travels through the magnetic core and gets linked to the secondary of the transformer. The air gap is absent in the transformer whereas the induction motor has air gap. The nature of the flux produced in the transformer is pulsating type whereas the flux produced in the stator of induction motor is of rotating type.

The principle of working of  induction motor and transformer is based on the Faraday’s and Lenz’s law of electromagnetic induction. If we compare the transformer with slip ring induction motor, the voltage induced in the rotor across slip rings  under slip rings open condition is according to the turns ratio of the induction motor. The voltage induced across the slip ring  is analogous to the secondary terminal voltage of the transformer.

The only difference between a transformer and the induction motor is that the secondary winding of induction motor or rotor part of induction motor is free to move or rotate while in a transformer the secondary and primary winding are firmly fixed in a position.

The stator winding of induction motor is placed 120 degree electric apart for production of rotating magnetic field, whereas the winding of transformer  are usually arranged concentrically around the core limb. The voltage induced in the secondary of the  transformer is governed by the turns ratio of the transformer and, the voltage induced in the rotor winding is governed by the turns ratio of stator to rotor and the relative speed between synchronous speed and the actual speed of the motor.

The transformer does not have the air gap. but the induction motor has the air gap for its operation. Without air gap, the motor rotation is not possible, therefore, air gap is kept in the motor.

That is why Induction Motor is called Rotating Transformer.

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