A 3-phase 400 V, 50 Hz, 4 pole slip ring induction motor is excited with rated voltage and rated frequency. A centre zero ammeter is connected in rotor circuit and it has been observed that it is completing 90 oscillations per minute. What is the speed of rotor field in rpm?
- A. 25 rpm
- B. 45 rpm
- C. 50 rpm
- D. 90 rpm
Answer: B. 45 rpm
Explanation: 90 oscillations/min = 1.5 Hz = sf, so s = 0.03; speed of the rotor field relative to the rotor = s×Ns = 0.03 × 1500 = 45 rpm.