Thermal relay works heat is generated by the current flowing into the heating element, so that the bimetal has different coefficients of expansion is deformed when the deformation reaches a certain distance, the operation to push link, so that the control circuit is opened, so that the contact loss of power, disconnect the main circuit, overload protection of the motor. As the overload relay of the electric motor element, its small volume, simple structure and low cost it has been widely used in production.

Selection of thermal relay: thermal relay is mainly used to protect the motor overload, it is necessary to understand the situation of the motor, such as the working environment, the starting current, the nature of the load, the selection of the working system, allowing other overload.

1. The thermal relay should in principle safety second characteristic even coincide as close as possible motor overload characteristic, or characteristics of the motor under overload, while the motor overload and short instant start, should not be affected thermal relay (not action).

2. When the thermal relay system for protecting a long-term or long-term intermittent motor system, generally based on the rated current of the motor to choose. For example, setting value of the thermal relay may be equal to 0.95 to 1.05 times the rated current of the motor, relay or for heat setting the value of a current equal to the rated current of the motor, and then adjusted.

3. When the thermal relay for protecting a motor short-time work is repeated, only a range of adaptive thermal relay. If the lot number of operations within a short time, the belt speed should be used and the thermal relay current transformer saturation.

4. For reversing and off frequently special duty motor, should not be used as a thermal overload protection relay, while the use of motor winding embedded temperature relay or a thermistor for protection.