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Reactance voltage in a commutating coil is the emf induced by

  • A.the rotation of the coil through the main field
  • B.the resistance of the brush contact
  • C.the eddy currents in the armature core
  • D.the rapid reversal of current in that coil

Answer: D

Reversing tens or hundreds of amperes in a fraction of a millisecond induces a substantial emf of self-induction, which tries to keep the current flowing and produces sparking at the trailing brush edge. Interpoles generate an equal and opposite emf.

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