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Power Electronics - Engineering

Q1:

A 3 phase induction motor is controlled by voltage and frequency control such that the ratio V/f is constant. At 50 Hz, the speed is 1440 rpm. At 25 Hz, the speed is likely to be

A about 1350 rpm

B about 720 rpm

C about 360 rpm

D either (b) or (c)

ANS:B - about 720 rpm

In this case synchronous speed at 50 Hz is 1500 rpm. Therefore synchronous speed at 25 Hz is 750 rpm. Since slip is about 3 - 5%, speed is about 720 rpm.