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Single-phase induction motors are built only in small ratings because

  • A.their performance and their supply loading both suffer at size
  • B.no rotor can be built large enough to suit a single-phase machine of that size
  • C.single-phase supplies cannot carry any appreciable current
  • D.their efficiency exceeds that of three-phase machines at size

Answer: A

A large single-phase load unbalances the supply and the machine's own efficiency and power factor are poor, so beyond a few kilowatts a three-phase machine is used instead. Domestic and small commercial duties are where they belong.

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