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.