The thermal relay is a protective device using the heat effect of current to disconnect the electric circuit. It is used as overload protection in circuit for motors in home electric appliances. According to the action modes, the thermal relays are divided into: 

1.Bimetallic strip The thermal bimetallic strip (Two kinds of solid metal with different expansion coefficient of are firmly welded together) is heated and bends to push one contact actuator to act. 

2. Fusible alloy type: The overload current gives out heat and reach a certain temperature, making the fusible alloy melt and act. 

3. The thermal relay is made by means of the characteristics and principles that the material permeability or resistance value changes with the temperature changing. Among these three kinds of thermal relays, the thermal relay of bimetallic strip is featured by simple structure, small volume and low cost, and at the same time, if proper thermal components are selected,  good inverse- time characteristic can be obtained. therefore, it has been widely applied, hereinafter this kind of relay is introduced. 

Working principles: The basic structures of the thermal relay are composed of thermal components, contact system, acting mechanism, resetting button, setting current device and temperature compensation components etc.. 

The thermal components include two parts, which are the main parts of the thermal relay, composed of bimetallic strip and resistance wire around the outside of the bimetallic strip. Bimetallic strip is welded with the two metal strips with different thermal expansion coefficient. During application, connect the resistance wire directly in series on the two phase circuit of the asynchronous motor. The normally closed contacts are connected on the branch of the contact coil of the motor control circuit. When the over current occurs in motor winding due to overload, and after a certain period of time, the heat generated by the heating elements make the bimetallic strip to bend, and to push the guide plate to move rightwards to a certain distance, then the guide plate pushes the temperature compensating plate and the lever to separate the dynamic contact and the static contact, so that the motor line contactor is deenergized and released to switch off the power supply and to be protected. After the power is switched off, the thermal relay begins to cool, and after a period of time the bimetallic strip will restore to its original form, then under the function of spring, the contacts automatically resets and close. 

This kind of thermal relay can be manually reset too. Then as long as screw the screw to a certain position to make the contact rotate beyond a certain angle, in this case, even if the bimetallic strip cools, the contacts cannot automatically reset, they must be manually reset. namely, press the reset button to make the contacts displace, which is necessary for the occasion preventing the motor from being started again on condition that the faults have not been cleared.