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A dc motor is preferred to an induction motor where

  • A.the highest possible efficiency at large rating is required
  • B.the environment is dusty or hazardous
  • C.the machine must run for years without maintenance
  • D.fine speed control was needed before converters existed

Answer: D

Simple armature voltage and field control gave a wide, smooth speed range long before the variable-frequency drive existed. Every other consideration favours the cage machine, which is why the dc motor has largely been displaced.

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