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A mixed frequency test on a transformer or machine is used to

  • A.measure the efficiency at rated load directly
  • B.determine the insulation resistance of the windings
  • C.locate a short-circuited coil in the winding
  • D.produce rated temperature rise without rated load

Answer: D

Correct answer: produce rated temperature rise without rated load

The mixed frequency method superimposes two supplies of different frequency so that the machine draws currents producing the rated losses while delivering no useful output. The heating is then correct without a load to absorb full power, which is what makes the temperature rise of a very large machine testable at all.

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