Discussion
The V/f drive has displaced most other induction motor speed control methods because it is
Answer: C
Correct answer: efficient over the whole speed range
The older methods all threw away the energy they used to reduce the speed - rotor resistance dissipated the slip power as heat, and stator voltage control ran the machine at high slip to do it. The V/f drive changes the synchronous speed itself, so the machine runs at a few per cent slip whatever the speed and the efficiency is preserved. Above base speed it cannot hold rated torque, since the flux must be weakened once the voltage ceiling is reached.
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