A 6 pole, 3 phase alternator running at 1000 rpm supplies power to an 8 pole, 3 phase, induction motor which has a rotor current of frequency 2 Hz. What is the speed at which the motor operates?
- A. 1000 rpm
- B. 960 rpm
- C. 750 rpm
- D. 720 rpm
Answer: D. 720 rpm
Explanation: Alternator generated frequency f = 6 × 1000 / 120 = 50 Hz. Motor synchronous speed Ns = 120 × 50 / 8 = 750 rpm. Slip s = 2/50 = 0.04; motor operating speed N = 750(1 − 0.04) = 720 rpm.