Power Electronics - Section 3 Question & Answers

Power Electronics - Section 3 Question & Answers 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) - ANSWER: B 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. A 3 phase induction motor is fed by a 3 phase ac regulator to change the stator voltage. The variation in speed will be more if A. load torque is constant B. load torque increases with increase in speed C. load torque decreases with increase in speed D. either (a) or (b) - ANSWER: B If load torque is constant, the change in speed is very small. A 3 phase semiconverter when supplying a certain load has a rectification efficiency of 0.6 and transformer utilisation factor of 0.35. When a 3 phase full converter is supplying the same load A. rectification efficiency will be more than 0.6 and TUF will be more than 0.35 B. rectification efficiency will be more than 0.6 but TUF will be less than 0.35 C. rectification efficiency will be less than 0.6 and TUF will be less than 0.35 D. rectification efficiency will be less than 0.6 but TUF will be more than 0.35 - ANSWER: A A high frequency series inverter consists of