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The demagnetising component of armature reaction arises because

  • A.the brushes are always set behind the magnetic neutral axis
  • B.the commutator reverses the armature current twice per revolution
  • C.saturation makes the strengthened pole tip gain less than the weakened one loses
  • D.the armature current flows in the opposite sense to the field current

Answer: C

One pole tip is strengthened and the other weakened by equal ampere-turns, but the iron of the strengthened tip is already near saturation, so the net flux falls. That is why the effect is small in a lightly saturated machine and appreciable in a heavily loaded one.

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