It consists of a heating element, a bimetallic strip, contacts, and a transmission and adjustment mechanism. The heating element is a length of resistance wire with low resistance, connected in series within the main circuit of the motor being protected. The bimetallic strip is formed by bonding together two metal sheets with different coefficients of thermal expansion. In the strip shown in Figure 1, the lower layer has a higher coefficient of thermal expansion than the upper layer. When the motor is overloaded, the current flowing through the heating element exceeds the set current; the bimetallic strip heats up and bends upward, disengaging from the latching plate and causing the normally closed contact to open. Since the normally closed contact is connected to the motor's control circuit, its opening de-energizes the connected contactor coil; this causes the contactor's main contacts to open and cuts power to the motor's main circuit, thereby achieving overload protection.
After the thermal relay operates, the bimetallic strip cools down over a period of time, and the device can be reset by pressing the reset button.
