Why is it necessary to supply rated voltage at rated frequency?
Answers
If an induction motor is being supplied with a voltage less than rated value, at rated frequency, the magnitude of magnetizing/useful air gap flux decreases, as flux=V/f. So, the magnitude of net air gap vector will decrease. Torque=air gap flux*rotor flux*sin(delta). So, the effective torque decreases & so it runs with less speeds. We know that if rotor speed reduces, slip increases & hence motor should accelerate. But as voltage supply to stator is of less magnitude, the length of rotating stator flux vector also reduces, but the synchronous speed is same (proportional to frequency). So the rotor will try to accelerate (due to high slip), but due to lesser magnitudes of both the fluxes, it cant & so our motor runs at speeds less than the case when rated supply is given to it. Actually this speeding up of rotor enhances more in the desired operating region only.
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because the frequency is somewhat the number of vibrations to be passed off in one seconds so to get max resultant the voltage value must be fixed❗