A 3-phase 6 pole 400 V slip ring induction motor is running at a speed of 960 rpm. What is the additional resistance required in rotor circuit to reduce the speed by 10%, if rotor resistance is 0.2 ohm?
- A. 0.48 ohm
- B. 0.68 ohm
- C. 0.88 ohm
- D. 0.28 ohm
Answer: A. 0.48 ohm
Explanation: s1 = 0.04; N2 = 0.9×960 = 864 rpm gives s2 = 0.136. For the same torque R2/s is constant: (0.2 + Rext)/0.136 = 0.2/0.04, so Rext = 0.48 Ω.