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A universal motor is

  • A.a shaded pole machine with an auxiliary winding fitted
  • B.a series machine that runs on either alternating or direct current
  • C.an induction machine that will start on a single phase without help
  • D.a synchronous machine of fractional horsepower rating

Answer: B

Correct answer: a series machine that runs on either alternating or direct current

In a series machine the field and armature currents reverse together, so the torque keeps its direction whichever way the current flows - which is why the same machine works on alternating and direct supplies. Its very high speed and high torque per unit mass make it the machine of choice for hand tools and vacuum cleaners.

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