Induction Motor as a Generalized Transformer & their Comparison


     The working principle of induction motor is similar to transformer i.e., the primary and secondary windings of a transformer are referred to as stator and rotor in case of induction motor. The transformer works on the principle of mutual induction i.e., the flux generated by the primary winding links with the secondary and induces a voltage in it.

     Similar to the transformer the flux produced by the induction motor stator links with the rotor. Since the rotor is short-circuited there will be a circulation of currents in the rotor, thereby transferring the energy by mutual induction. This is the main reason for an induction motor to be referred to as a transformer.


Similarities Between Induction Motor with that of a Transformer :

  • In an induction machine, the synchronously rotating air gap flux is due to the combined action of both stator and rotor MMF's. Similarly, the resultant mutual flux in a transformer is due to the combined action of primary and secondary MMF's.
  • The rotating airgap flux generates counter emf in the stator winding similar to the counter emf induced by the mutual flux in the primary winding of a transformer.
  • The short-circuit test performed on the high-voltage side of the transformer is similar to the blocked rotor test of induction motor where the rotor of the motor is blocked to rotate.
  • The stator and rotor windings of an induction machine possess resistances and leakage reactances just like the resistances and leakage reactances of the primary and secondary windings of a transformer.
  • As the transformer is loaded, the mmf of the secondary current reacts on the primary to draw more power from ac source. Similarly, with the increase in the shaft load, the rotor mmf reacts with the stator winding to extract more power from the ac source.
  • Both induction motor and transformer work on the principle of Faraday's laws of electromagnetic induction. Due to the above similarities, the induction machine is referred to as a generalized transformer.



     When a 3-phase induction motor is connected across the supply. The stator winding will set up a rotating magnetic field, and the speed of rotation depends upon supply frequency and the number of stator poles of the machine i.e., synchronous speed ( Ns = 120f/P ). This rotating magnetic field will induce an EMFs ( E1 & E2 ) in the stator and rotor.

Induction Motor as a Generalized Transformer

Where kW11 and kW2 are winding factors for the stator and rotor windings respectively. Therefore,

Induction Motor as a Generalized Transformer

     The above equation can be compared with the voltage equation of the transformer. But, the emf induced in the case of induction motor is due to stator distributed windings, whereas in the transformer the primary and secondary uses concentrated coils.

     Therefore, the characteristics of both the machines with and without load conditions are similar. The phasor diagram of the induction motor can be developed similar to that of a transformer. The below shows the phasor diagram of induction motor on-load.

Induction Motor as a Generalized Transformer


Difference Between Induction Motor and Transformer :

Induction Motor

Transformer

Induction as rotating parts in it.

Transformer as no rotating parts in it i.e., it is a stationary static device.

The voltage ratio of induction motor includes winding factors.

It requires no winding factors.

The per-phase values of emf induced in stator and rotor windings are given by,
E1 = √2π f1 Kω1 N1 Φ
E2 = √2π f2 Kω2 N2 Φ
Where, Φ is the average value of the rotating flux per pole.

The rms values of emf's induced in primary and secondary windings are given by,
E1 = √2π f1 N1 Φmax
E2 = √2π f2 N2 Φmax
Where, Φ is the maximum value of the core flux used.

The no-load current in induction motor varies between 30 to 50% of full-load current.

In transformer no-load current varies from 0 to 6% of full-load current.


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