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3-phase induction motor speed control

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crutschow

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thank you all for the overwhelming responses;) this has somehow benefited me greatly

I have a question. I read that the higher the resistance of the rotor, the lower the speed. of the rotor. Is this due to the fact that as the resistance gets higher, the current in the rotor is lower and thus results in a lower speed due to the electromagnetic induction?

Some sites say that a higher resistance results in a higher torque and a higher torque results in a lower speed. Why is that so?
In an induction motor the rotor current is generated by induction and the value is determined by the slip frequency. (The slip frequency is determined by the difference between the field rotation rate and the rotor speed. For example, if a 2-pole, 60Hz induction motor it operating at 3500rpm, then the slip frequency is (3600-3500) / 60 = 1.6Hz.) The rotor conductors cutting the magnetic field flux lines at the slip rate generate a rotor voltage. The voltage is proportional to the slip frequency. This voltage divided by the rotor resistance determines the rotor current. And the amount of rotor current required is determined by the torque load on the motor, more torque requires more rotor current.

From the above you can see that a higher rotor resistance will require a higher slip frequency (lower speed) to generate a given rotor current. Motors with a higher rotor resistance will thus run at a lower speed and change speed more with load.

Don't know of any reason that a higher rotor resistance would result in higher torque. Normally the opposite is true. Sometimes extra resistance in temporarily inserted in the rotor circuit of large induction motors to reduce the start-up surge current and consequent rotor heating. The rotor current can be very large at start-up otherwise since the slip frequency is equal to the line frequency at zero rpm. But adding resistance does not increase torque, it only reduces start-up current.
 
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