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Flashover round a commutator is most likely when

  • A.the commutator surface carries a healthy patina
  • B.the machine suffers a sudden heavy overload
  • C.the machine is running unloaded at rated speed
  • D.the brushes have been newly bedded in

Answer: B

A surge of current distorts the flux badly and raises the voltage between segments, and an arc drawn at a brush can then track right round the commutator. Fast overcurrent protection and pole-face compensation both guard against it.

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