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Speed control of a drive by reducing the supply voltage is thermally poor because

  • A.the machine runs at large slip and heats its rotor
  • B.the machine draws no current at reduced voltage
  • C.the losses of the machine fall faster than its output
  • D.the machine's cooling improves as the speed is reduced

Answer: A

Low speed is obtained by increasing the slip, and the rotor loss is the slip times the air gap power, so the rotor takes a large share of the input. Frequency control obtains the same speed with the slip left small, which is the whole difference.

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