Sunday, 4 September 2016

Difference between Induction Motor and Transformer


As per the equivalent circuit representation, a three phase Induction Motor is generalized transformer but there are some differences between the Induction Motor and a Transformer, they are summarized below.

  • The basic and most important difference is that even though the equivalent circuit of Induction Motor and Transformer is same rotor of motor rotates where as secondary of transformer do not rotate.

  • Another difference is that Transformer is an alternating flux machine while Induction Motor is rotating flux machine. Rotating flux is only possible when three phase voltage or poly phase which is 120° apart in time is applied to a three phase winding or poly phase winding 120° apart in space then a three phase rotating magnetic flux is produced whose magnitude is constant but direction keeps changing. In transformer the flux produced is time alternating and not rotating because the three phase windings are not 120° apart in space rather they are wound on the limbs of Transformer.



  • There is no air gap between primary and secondary of Transformer where as there is an air gap between stator and rotor of Induction Motor. Air gap in Induction Motor is necessary so that Rotor may rotate. We always try to maintain as less air gap as mechanically possible. The magnetizing current required in motor is 25-40% of rated current of motor because of higher reluctance of air gap where as in transformer it is only 2 -5 % of rated primary current.
  • In an alternating flux machine frequency of induced EMF in primary and secondary side is same where as frequency of rotor EMF depends on slip. During starting when slip s = 1 the frequency of induced EMF in rotor and stator is same but after loading it is not same. Frequency of induced EMF in Rotor at a slip of s is sf.
  • Other difference is that Rotor core is mounted on a shaft set in bearings free to rotate in Induction Motor.


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