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A three-phase synchronous motor connected to AC mains is running at full load and unity power factor. If its shaft load is reduced by half, with field current held constant, its new power factor will be

1. unity
2. leading
3. lagging
4. dependent on machine parameters

Correct answer: leading

Solution

For a synchronous motor, keeping field current constant while reducing shaft load reduces the torque angle. Since the motor was at unity power factor at full load, the same excitation becomes overexcited at the lower load, so the current becomes leading.

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