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Induction Motor


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(1) How do we analysis a motor performance under load conditions?

(2) Can i say that if for a no-load speed control by varying supply frequency, the volatge will be reduced or increase according to the frequency.....Normally, what's the percentage of changes? Let say i got a motor at speed 50hz and voltage at 220Vspeed=1415 rpm. Then when i reduced the frequency to 10hz....the voltage becomes 72.6v and speed is 299rpm....How do i know the pecentage of drop?

Pls advise.Thanks.

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Hello Shermaine


re 2)

The induction motor requires flux in the iron to operate. If the flux is too high, there will be excessive iron loss and the motor will over heat. If the flux is too low, the motor will not develop torque. We can control the flux by adjusting the magnetising current. If you look at the equivielnt circuit of an induction motor, you will see that there is a shunt inductor that represents the magnetising current. Being an inductor, the current becomes both voltage and frequency dependent. z=jwL As we reduce the frequency, we need to reduce the voltage by the same proportion in order to prevent the flux becoming too high. We refer to this as a constant v/f ratio.

When we get down to a very low frequency, the resistance of the winding becomes the dominant current limiting component and so we apply a voltage boost to compensate for this.


Best regards,

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