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Residual Magnetism in an Induction Motor 4

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rockman7892

Electrical
Apr 7, 2008
1,161

I have had many discusses lately about when a motor was capable of regeneration. Obviously when the motor is driven past synchronous speed it generates a voltage but what about when the motor is shut off?

How long does it take the magnetic field in the motor to decay? Will there always be some sort of residual magnetism in the motor even after sitting for a while?
 
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Yes, I agree with David again from the simple equivalent circuit linked above that as s increases above 1 the current continues to increase ALTHOUGH the slope (increase in current per reverse speed) is relatively shallow since the locked rotor and backward-spinning-rotor impedance primarily inductive. If I get a chance I'll try it out with some typical motor parameters to see what is the increase in current from say 1.0 to 1.1

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At the risk of re-igniting old arguments, I'll step in here with a comment. The variable resistance in the "equivalent circuit" motor model acts as a substitute for the voltage drop of the speed-dependent generated EMF of the motor.

The generated EMF is a signed quantity and can "go negative". Obviously a physical resistance cannot be negative. But it is often simpler to analyze the system as if it were a (signed) resistance, so it is a convenient fiction.

Rockman -- your electrician was not wrong to talk about the regenerated voltage of the back-spinning motors being the problem. He was talking about the fundamental physics of the motor rather than the equivalent circuit.

Curt Wilson
Delta Tau Data Systems
 
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