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The backward field of a running single-phase machine produces

  • A.a large driving torque that assists the forward field
  • B.no torque and no loss of any kind while the machine runs
  • C.the whole of the useful torque the machine develops
  • D.a small braking torque and extra rotor loss

Answer: D

At a slip near two it is on the falling part of its characteristic and contributes little torque, but it still drives current in the rotor and heats it. That extra loss is one reason such machines are less efficient than three-phase ones.

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