Why Induction Motor draws more No Load Current and High Starting Current?


Induction Motor is also called as a rotating transformer because it also has two windings i.e. primary winding and secondary winding. The secondary winding of the induction motor is the rotating part. The no-load current and starting current of induction motor is different from the transformer. In this article, you will know why Induction Motor draws more No Load Current and High Starting Current.



Why Induction Motor draws more No Load Current?


(1) In an Induction motor, the no-load current comprises two parts, one is magnetizing component and another is loss component. 


Induction Motor equivalent Circuit


As the air gap between the stator field winding and rotor winding is large, the more magnetizing current is needed to create flux which will cut the rotor conductor. But in the case of a transformer, there is a core arrangement which helps to conduct the flux from primary winding to secondary winding so less amount of flux is enough to create emf in the secondary winding.
So the main point is, more flux production is needed in induction motor than a transformer that is why induction motor draws more No Load current than a transformer.



(2) A transformer in No Load means the secondary winding is open, but an induction motor in no-load means no external mechanical load is connected to the shaft but the secondary winding may be open or short. So in no-load condition, as the secondary winding of the transformer is open so no current flowing through it, so the transformer draws only magnetizing current.


Transformer equivalent Circuit


But the secondary winding of the induction motor always shorted(e.g. squirrel cage induction motor) in no load or full load current. So you may understand, in no-load condition, the induction motor does not draw the only magnetizing current it also draws some extra current for which it is rotating.

So it is an important reason why Induction Motor draws more No Load Current than a transformer.


Why Induction Motor draws High Starting Current?


At the starting time, the rotor of the induction motor is stationary and we also know that the secondary winding is short-circuited. So when we just ON the power supply emf will induce in the secondary winding due to the flux of primary winding. So a high current will flow through the secondary winding. The current in rotor winding will create flux which opposes the flux of primary winding. So it is clear that at the starting time a high current will flow through the secondary or rotor winding which forces the primary winding to draw high current from the power supply.

During the running condition, the relative speed between rotor and stator rotating filed is decreased so the flux cut also decreases, therefore, the emf in secondary winding will decrease and current also decrease. So in running condition induction motor draws less current compared to starting time.



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Why Induction Motor draws more No Load Current and High Starting Current? Why Induction Motor draws more No Load Current and High Starting Current? Reviewed by Author on 21:23 Rating: 5
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