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Speed control of a dc motor above base speed is achieved by

  • A.inserting resistance in the armature circuit of the machine
  • B.reversing the connections to the field winding of the machine
  • C.weakening the field
  • D.raising the armature voltage above the rated value of the machine

Answer: C

Once the armature voltage has reached its ceiling, the only way to go faster is to reduce the flux, which raises the speed for the same back emf. Torque falls in proportion, so the region above base speed is one of constant power.

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