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The permanent magnet dc motor differs from the shunt machine in that

  • A.its flux is fixed and cannot be regulated
  • B.its armature carries no current in normal operation
  • C.it develops no torque at standstill
  • D.it needs no commutator or brushes at all

Answer: A

The magnets set the flux once and for all, so only armature voltage control is available and there is no field weakening region. In return the machine is compact, efficient and needs no field supply, which suits small servo and toy applications.

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