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A brushless dc motor differs from a permanent magnet synchronous motor chiefly in its

  • A.inability to develop torque at standstill
  • B.back emf waveform and commutation scheme
  • C.use of a mechanical commutator and brushes for switching
  • D.requirement for a wound field on the rotor of the machine

Answer: B

A trapezoidal back emf commutated in six steps from Hall sensors is simple and cheap, while a sinusoidal machine with continuous position feedback gives smoother torque. Both are synchronous machines with magnet rotors.

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