How bimetal strips are actuated as overload element in motors

  • When the load current through the heating element increases from full load current, the temperature of the heating element continues to increase.
  • The bi-metal element that is placed adjacent to the heating element will heat up through heat transfer.
  • Due to the differential elongation of both the elements/strips, the bimetal element bends from a vertical position and touches the fixed contact.
  • The trip relay is thus activated and trips the alternator or motor.
  • If the motor is restarted immediately after a trip event, the bimetal element will continue to heat up and bend from the transferred heat.
  • The tripping time taken by the bimetal element will be lesser than the first time since the bimetal has already retained warmth.
  • This is called the "memory feature" and is not available in current overload relays.

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