# [Induction Motor] how to understand this concept?

Discussion in 'Homework Help' started by kougou, Nov 15, 2013.

1. ### kougou Thread Starter New Member

Oct 20, 2012
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0
Hi all

I am trying to understand the concept of 3 phase wound rotor induction motor.
I understand why when you feed in 3 phase current to the stators that are separate spatially 120 degree will get you a rotating magnetic field.

However, I am having much trouble in understanding the picture below; the representation of the rotor as the "3 phase transformer secondary".

I know the rotor is only like squirel cage, so graphically we could represent it as a bunch of current loop wrapping on the inner circle. However, this figure presents it as the "3 phase transformer secondary winding" confuse me.

How am I suppose to interpret this?

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2. ### MaxHeadRoom Expert

Jul 18, 2013
10,836
2,509
The drawing is not strictly correct, the NPQ are actually shorted turns, Hence the high current at power on, the resultant current in these fields are attracted to the rotating field and will eventually almost reach the same rate as the rotating field the difference is called the slip frequency, 4-7 cycles, off load.
If the coils were open as shown, the result would be a stationary rotor and very little current in the stator.
This slip is necessary and why a Induction motor cannot be synchronous without other action.
The motor does represent a 3ph transformer with shorted turn secondaries at switch-on.
Max.

Last edited: Nov 15, 2013