Induction motors

[Jk1996]

Hi I’ve got the concept of Back emf and with a stalled motor not having any back emf to oppose the current the current drawn will remain high through the motor windings. But I’m trying to get my head around if we then have a high resistance for example on a contactor would this then increase the overall current in all three phases of the resistance is the same in all 3?

 

[electricpete]

During starting, the motor acts like constant impedance and a series resistance would reduce the current slightly (although the starting impedance is primarily inductive and also small magnitude, so you'd need a heckuva lot of resistance to make a dent).

During run the motor acts more like constant horsepower, inserting a small series resistance decreases the effective terminal voltage and generally increases the current drawn.

You can predict the results using the induction motor steady state equivalent circuit.


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(2B)+(2B)' ?