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Which of these best explains why dc motors dominated variable-speed work for a century?

  • A.They are more efficient than any ac machine
  • B.They require no maintenance of any kind
  • C.They can be built at larger ratings than ac machines
  • D.Torque follows one easily controlled current

Answer: D

One measurable current commands the torque and one voltage commands the speed, which is a control problem simple enough for the electronics of the time. Vector control brought the same simplicity to the cage machine and ended the monopoly.

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