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A two-speed pole-changing machine is efficient at both speeds because

  • A.the supply frequency is changed to suit each speed
  • B.the slip stays small at each of them
  • C.the losses are dissipated outside the machine
  • D.the flux is reduced at the lower speed

Answer: B

Each connection has its own synchronous speed and the machine runs a few per cent below it, so the rotor loss is small in both cases. That is quite unlike voltage or resistance control, which obtain low speed by increasing the slip deliberately.

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