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The cooling time constant of a self-ventilated motor at standstill is

  • A.independent of whether the machine is running or standing still
  • B.longer than its heating time constant when running
  • C.shorter than its heating time constant
  • D.exactly equal to its heating time constant under all conditions

Answer: B

With the shaft-mounted fan stationary the machine sheds heat far more slowly, so the dissipation term falls and the time constant lengthens. That is why an intermittently loaded machine may not cool as much during its rest period as a simple calculation suggests.

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