Motor thermal protection is the control of the protection of electrical components, its working principle is to use the thermal effect of current to promote the action mechanism, so that the control circuit of the AC contactor coil loss of power and broken, so as to cut off the series in the main circuit of the circuit power supply. It is mainly used for overload protection of motors. Some thermal relays also have the functions of phase break protection and current imbalance protection. Mainly used for motor overload protection, some motor thermal protection also has phase break protection, current imbalance protection function. There are two thermal components, which are the main part of the thermal protection of the motor. They are composed of a bimetallic sheet and a resistance wire around the bimetallic sheet. Bimetallic plates are welded by two metal plates with different expansion coefficients. In normal operation, the three groups of resistance wires are directly connected in series on the three-phase circuit of the motor load. The normally closed contact is connected to the contactor coil branch of the motor control circuit.

The reasonable selection and correct use of motor thermal protection directly affect the safe operation of electrical equipment, so the following problems should be emphasized during installation

(1) When installing wiring of the motor thermal protection , the dirt on the contact surface should be removed to avoid the circuit obstruction or excessive contact resistance affecting the working characteristics of the motor thermal protection.

(2) The connection wire of the main loop of the motor thermal protection should not be too thin to avoid the heating of the connection terminal and the wire affecting the normal work of the motor thermal protection.

(3) The motor thermal protection is installed with other electrical appliances and should be installed under the output of other electrical appliances to avoid the influence of other action characteristics by the heat of other electrical appliances.