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slip ring induction motor

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#1 kunalsarkar


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Posted 23 February 2006 - 02:28 PM

We have found that during regenerative braking (lowering motion in hoists) if we increase the rotor resistance lowering speed increases. How can we explain that?

#2 marke


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Posted 23 February 2006 - 06:22 PM

Hello kunalsarkar

Welcome to the forum.

If you increase the rotor resistance, you move the slip at which the maximum torque occurs. If the motor is driving the load, the motor will slow down to the point where the motor torque equals the load torque.
If the motor is being driven by the load, the motor will speed up to the point where the motor torque equals the load torque.

If you wish to slow down the rate of descent, this is achieved by reversing the motor and adding resistance in the rotor. i.e. the motor is actually trying to lift the load, but the slip of maximum torque is shifted above 100%.

There used to be commercial slip ring controllers that were designed to do this with a combination of rotor and stator control. The stator voltage was controlled with SCRs to give a continuous control over the torque generated by the motor and control the decceleration speed.
The problem with pure rotor resistance control is that this does not determine the speed, rather it alters the motor torque and the speed is determined by the intersect of the motor torque and load torque curves.

Best regards,

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