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The back emf of a dc motor is proportional to

  • A.the product of flux and speed
  • B.the armature current alone, irrespective of the speed of rotation
  • C.the armature resistance multiplied by the current flowing in it
  • D.the square of the speed at which the machine is running

Answer: A

Writing \( E = k\phi\omega \) makes the two dependences explicit, and it is the relation on which every method of speed control rests. Reducing the flux at fixed voltage therefore raises the speed, which is field weakening.

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