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The commutation period of a dc machine is the time for which

  • A.the brush remains in contact with one segment only
  • B.a coil is short-circuited by a brush
  • C.a conductor passes beneath a main pole face
  • D.the armature completes one full revolution

Answer: B

Correct answer: a coil is short-circuited by a brush

The brush spans more than one segment, so for a brief interval it connects both ends of one coil together. During that interval the coil’s current must reverse completely, and the whole difficulty of commutation lies in getting it to do so before the short circuit is broken.

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