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Thermal protection built into a single-phase motor operates on

  • A.the frequency of the supply feeding the machine
  • B.the temperature of the winding, opening the circuit
  • C.the speed of the rotor, opening the auxiliary circuit
  • D.the current in the starting capacitor alone

Answer: B

Correct answer: the temperature of the winding, opening the circuit

A thermal protector is buried in or against the winding and opens when the winding gets too hot, from whatever cause - a stalled rotor, an overload, blocked ventilation or a failed capacitor. Sensing temperature rather than current means it protects against all of them, and it usually resets itself once the machine has cooled.

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