In a constant V/f control of induction motor, the ratio V/f is maintained constant from 0 to base frequency, where V is the voltage applied to the motor at fundamental frequency f.

Which of the following statements relating to low frequency operation of the motor is TRUE? (A) At low frequency, the stator flux increases from its rated value. (B) At low frequency, the stator flux decreases from its rated value. (C) At low frequency, the motor saturates. (D) At low frequency, the stator flux remains unchanged at its rated value.

Surbhi
15 जून
18065 दृश्य
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Your Answer

SA

Sadaqat
16 जुलाई

If you keep V/f constant , then field flux remains constant , hence the generated torque now depends only on rotor current , there is a relation of electromagnetic torque generated in motors ,Te = (3/2)*(P/2) *Fs*Fr *sin(delta) , by keeping Fs constant , you remove the coupling between flux and the armature current , hence Torque response will be instantaneous with change in rotor current and response will not get delayed ,with smooth control, so flux vs time should be a parallel line to time axis.
At low frequencies once you try to maintain V/f constant , stator applied voltage also reduces , and hence the stator impedance drop becomes significant , hence flux value reduces from rated . At high frequencies impedance drop is negligible compared to the applied voltage ,so we take V =E .
D. At low frequency, the stator flux remains unchanged at its rated value.