Protective appliances for overload protection of motors or other electrical equipment and electrical circuits - thermal relays

What is a thermal relay? What is its working principle? The thermal relay is a protective appliance used for overload protection of motors or other electrical equipment and electrical circuits. In actual operation of the motor, for example, when dragging production machinery to work, if there is an abnormality in the machinery or the circuit is abnormal and the motor encounters an overload, the motor speed will decrease and the current in the winding will increase, causing the motor winding temperature to rise. high.

If the overload current is not large and the overload time is short, and the motor winding does not exceed the allowable temperature rise, this overload is allowed. However, if the overload time is long and the overload current is large, the temperature rise of the motor winding will exceed the allowable value, causing the motor winding to age and shorten the service life of the motor. In severe cases, the motor winding may even be burned. Therefore, this kind of overload cannot be tolerated by the motor. Thermal relay is a protective appliance that uses the principle of thermal effect of current to cut off the motor circuit when an overload that the motor cannot bear occurs and provides overload protection for the motor.

The working principle of the thermal relay is that the current flowing into the thermal element generates heat, which causes the bimetallic sheets with different expansion coefficients to deform. When the deformation reaches a certain distance, the connecting rod is pushed to disconnect the control circuit, thereby causing the contactor to In case of power loss, the main circuit is disconnected to realize overload protection of the motor. As an overload protection component for motors, relays are widely used in production due to their small size, simple structure, and low cost.

Technical Parameters:

Rated voltage: The highest voltage value that the thermal relay can work normally, generally AC 220V, 380V, 600V.

Rated current: The rated current of the thermal relay mainly refers to the current passing through the thermal relay

Rated frequency: Generally speaking, its rated frequency is designed according to 45~62HZ.

Setting current range: The range of setting current is determined by its own characteristics. It describes that under certain current conditions, the action time of the thermal relay is inversely proportional to the square of the current.

It is mainly used for overload protection of asynchronous motors. Its working principle is that after the overload current passes through the thermal element, the bimetallic sheet is heated and bent to push the action mechanism to drive the contact action, thereby disconnecting the motor control circuit and realizing the motor shut-off when powered off. , plays the role of overload protection. In view of the fact that the heat transfer takes a long time during the thermal bending process of the bimetallic sheet, the thermal relay cannot be used for short-circuit protection, but can only be used for overload protection. The thermal relay can only be used for overload protection. The symbol is FR, and the circuit symbol is as shown on the right:

Thermal relay working principle

Thermal relay is an electrical protection component that uses the thermal effect of current to push the action mechanism to close or open the contacts. It is mainly used for motor overload protection, phase failure protection, current unbalance protection and other electrical equipment. Control during heating state.

The heating element made of resistance wire has a small resistance value. It is connected in series to the main circuit of the motor during operation. The bimetallic sheet surrounded by the resistance wire is made of two metal sheets with different linear expansion coefficients pressed together. Complete, the left end is fixed to the shell. When the current passing through the heating element exceeds its rated value and overheats, because the thermal expansion coefficient of the upper layer of the bimetallic sheet is small and the thermal expansion coefficient of the lower layer is large, the bimetallic sheet bends upward after being heated, causing the gusset plate to trip, and the gusset plate is The normally closed contact is opened under the tension of the spring.

The contacts are connected in series in the control circuit of the motor, so that the action coil of the contactor in the control circuit is de-energized, thereby cutting off the main circuit of the motor. The above is the working principle of the thermal relay, I hope it can help you.