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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 the rated value. If the stator resistance and rotational losses are neglected, then the frequency of the impressed voltage required to obtain twice the rated torque at starting should be:

1. 10 Hz
2. 5 Hz
3. 4 Hz
4. 2 Hz

Correct answer: 4 Hz

Solution

For constant flux and negligible stator resistance, induction motor torque is proportional to slip frequency in the low-frequency region. Rated torque occurs at slip frequency \(s f = 0.04\times 50=2\) Hz. To get twice the rated torque at starting, the slip frequency must double to 4 Hz, so the impressed frequency is 4 Hz.

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