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Speed control of a dc motor below base speed is normally achieved by

  • A.increasing the frequency of the supply feeding the machine
  • B.varying the armature voltage
  • C.weakening the field of the machine while holding the armature voltage fixed
  • D.inserting resistance in the field circuit of the machine

Answer: B

Armature voltage control gives constant torque capability down to standstill without wasting energy, since the flux and hence the torque per ampere are unchanged. Field weakening is reserved for speeds above the base value.

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