Thermal relay is the protective electric device for overload of motor or other electrical equipment or electrical circuit . It is a kind of protective device using the principle of heating effect of the current to cut off the motor when the motor can't bear the overload in order to provide overload protection for the motor. During actual operating of the motor, for example, when the production machine driven by the motor is working, if it  is abnormal or the circuit is abnormal, the motor will be overloaded, then the motor rotating speed will be reduced and the current in the winding will be increased, making the temperature of motor winding rise. If the not overload current is not big and the overload time is shorter, the winding of the motor does not exceed the temperature rise, this overload is permitted. But if the overload time is longer and the overload current is big, the temperature rise of motor winding  will exceed the allowable values, thus making the motor winding age and shortening the service life of the motor, and in severe case even making the motor winding burn. So, this overload cannot be born by the motor. The thermal relay is a kind of protective device using the principle of heating effect of the current to cut off the motor when the motor can't bear the overload in order to provide overload protection for the motor. When the thermal relay is used for overload protection of motor, connect the thermal element  with the stator winding of the motor in series and connect the normally closed contacts of thermal relay with the control circuit of electromagnetic coil of AC contactor in series, and adjust the adjusting knob of the setting current, in order to make the herringbone shifter lever and the push rod be separated by a proper distance. When the motor is working normally, the current through the thermal element is the rated current of the motor. The thermal element heats and the bimetallic strip is heated and bends, making the push rod just contact with the herringbone shifter lever  and not push the herringbone shifter lever. The normally closed contacts are in the closed state, the AC contactor keeps closing and the motor is normal operating. If the motor overload occurs, the current in the winding will be increased and the current through the thermal relay component will be increased too, so that the bimetallic strip temperature will rise higher, and the bending degree of bimetallic strip will be increased, thus pushing the herringbone shifter lever, which pushes the normally- closed contacts to make the contact disconnect and to disconnect the AC contactor coil circuit, thus making the contactor release and cutting off the power supply of the motor, at last the motor is shut down and protected. Functions of Other Part of Thermal Relay Are As Follows: The left arm of the herringbone shifter lever is made of bimetallic strip. When the environmental temperature changes, the bimetallic strip of the main circuit will bend and deform at a certain degree, at this moment the left arm of the herringbone shifter lever will deform and bend in the same direction, thus making the gap between the herringbone shifter lever and the push rod basically keep unchanged, so that the accuracy of the thermal relay action is ensured. This effect is called temperature compensation function. Screw 8 is the adjustment screw for reset way of the normally closed contacts. When the screw position keeps left and the motor is overloaded, the normally closed contact disconnects and the motor is stopped, the bimetallic strip of thermal relay is cooled and reset. Under the action of the spring, the moving contact of normally closed contact will automatically reset. At this moment the thermal relay is in automatic reset state. Turn the screw counterclockwise to the right to a certain position, if the motor is overloaded, the normally closed contact of thermal relay will disconnect. The dynamic contact will swing to a new equilibrium position at right side. After the motor is shut down due to power -off, the moving contact cannot reset. The moving contact cannot reset only after the reset button of the moving contact has been pressed. At this moment, the thermal relay is in manual reset state. If the motor overload is caused by failure, in order to avoid easily starting the motor again, the thermal relay shall be manually reset. If the thermal relay is adjusted from manual reset mode  to automatic reset mode, it is only required to  clockwise screw the adjusting screw for resetting to the appropriate position. The rated current of the thermal relay shall be greater than the motor rated current. Then according to the rated current, the model of thermal relay is selected.