A voltage source inverter is used to control the speed of a three-phase, 50 Hz squirrel cage induction motor. Its slip for rated torque is 4%. The flux is maintained at rated value. If the stator resistance and rotational losses are neglected, then the impressed voltage to obtain twice the rated torque at starting should be
- A. 10 Hz
- B. 5Hz
- C. 2 Hz
- D. 4 Hz
Answer: D. 4 Hz
Explanation: At rated condition, slip frequency fs = s × f = 0.04 × 50 = 2 Hz. With constant V/f ratio, torque is proportional to slip frequency. To develop twice rated torque at starting (where rotor speed = 0, so f_slip = f_supply), supply frequency must be 2 × 2 Hz = 4 Hz.