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Differences Between Induction Motor and Transformer

Comparison of Induction Motor With A Transformer

An induction motor may be considered to be a transformer with a rotating short circuited secondary. The stator winding corresponds to transformer primary and rotor winding to transformer secondary. However, the following differences between the two are worth noting:

(i) Unlike a transformer, the magnetic circuit of a 3-phase induction motor has an air gap. Therefore, the magnetizing current in a 3-phase induction motor is much larger than that of the transformer. For example, in an induction motor, it may be as high as 30-50 % of rated current whereas it is only 1-5% of rated current in a transformer.

(ii) In an induction motor, there is an air gap and the stator and rotor windings are distributed along the periphery of the air gap rather than concentrated on a core as in a transformer. Therefore, the leakage reactances of stator and rotor windings are quite large compared to that of a transformer.

(iii) In an induction motor, the inputs to the stator and rotor are electrical but the output from the rotor is mechanical. However, in a transformer, input as well as output is electrical.

(iv) The main difference between the induction motor and transformer lies in the fact that the rotor voltage and its frequency are both proportional to slip s. If f is the stator frequency, E2 is the per phase rotor e.m.f. at standstill and X2 is the standstill rotor reactance/phase, then at any slip s, these values are:

Rotor e.m.f./phase, 
E'2 = s E2Rotor reactance/phase,
X'2 = sX2Rotor frequency, f' = sf

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