Thermal relay working principle - Solutions - Huaqiang Electronic Network

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The main function of the Thermal Relay is to protect the electric motor or other electrical equipment, electrical line overload protection appliances.
During the operation of the motor, for example, during the operation of dragging the production machine, if the machine is not normal, or the circuit is abnormal and the motor is overloaded, the motor speed decreases and the current in the winding increases. To increase the winding temperature of the motor. If the overload current is not large and the overload time is short, 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, which will cause the motor winding to age, shorten the service life of the motor, and even burn the motor winding in severe cases. Therefore, this overload is unacceptable to the motor. The thermal relay is a protection device that uses the principle of the thermal effect of the current to cut off the motor circuit in the event of an overload that the motor cannot withstand, providing overload protection for the motor.
When using a thermal relay to protect the motor from overload, connect the thermal element to the stator winding of the motor, connect the normally closed contact of the thermal relay in series with the control circuit of the electromagnetic coil of the AC Contactor, and adjust the setting current adjustment knob to make the person The font lever is at an appropriate distance from the push rod. When the motor works normally, the current passing through the heat element is the rated current of the motor, the heat element heats up, and the bimetal is bent after being heated, so that the push rod just touches the herringbone lever, and cannot push the chevron lever. The normally closed contact is in a closed state, the AC contactor remains closed, and the motor operates normally.
If the motor is overloaded, the current will increase in the winding, which will cause the current in the thermal relay component to increase, so that the temperature of the bimetal will increase, and the degree of bending will increase. The lever of the glyph, the chevron of the chevron pushes the normally closed contact to open, so that the AC contactor coil circuit can be disconnected, so that the contactor releases and cuts off the power of the motor, and the motor is stopped to be protected.
The other effects of the thermal relay are as follows: the left arm of the herringbone lever is also made of bimetal. When the ambient temperature changes, the bimetal in the main circuit will have a certain deformation and twist. At this time, the chevron lever The left arm also has a twist and tear in the same direction, so that the interval between the herringbone lever and the push rod is basically unchanged, ensuring the accuracy of the thermal relay initiative. This effect is called temperature compensation.
The screw 8 is a normally closed contact reset method conditioning screw. When the screw orientation is to the left, after the motor is overloaded, the normally closed contact is disconnected. After the motor is parked, the thermal relay bimetal is cooled and reset. The moving contact of the normally closed contact is actively reset under the effect of the spring. At this moment, the thermal relay is in an active reset condition. When the screw is rotated counterclockwise to the right to a certain orientation, if the motor is overloaded at this time, the normally closed contact of the thermal relay is disconnected. Its moving contact will swing to the right side of a new balanced orientation. After the motor is powered off, the moving contact cannot be reset. It is necessary to press the reset button to reset the moving contact. At this moment, the thermal relay is in a manual reset condition. If the motor overload is faulty, in order to prevent the motor from starting again easily, the thermal relay should use the manual reset method. To adjust the thermal relay from the manual reset method to the active reset method, simply turn the reset conditioning screw clockwise into the proper orientation.