The working principle of thermal relay

Thermal relays are protective electrical appliances used for overload protection of motors or other electrical equipment and electrical circuits.
In the actual operation of the motor, such as driving the production machinery to work, if the machine is abnormal or the circuit is abnormal and the motor encounters an overload, the motor speed will decrease, the current in the winding will increase, and the winding temperature of the motor will be increased. Elevated. 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 windings will exceed the allowable value, which will age the motor windings, shorten the service life of the motor, and even burn the motor windings in severe cases. Therefore, this kind of overload cannot be tolerated by the motor. The thermal relay is a protective electrical appliance that uses the thermal effect principle of the current to cut off the motor circuit when an overload that the motor cannot bear, and to provide the motor with overload protection.
When using a thermal relay to protect the motor against overload, connect the thermal element in series with 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 people A proper distance between the glyph lever and the push lever. When the motor is working normally, the current passing through the heating element is the rated current of the motor. The heating element generates heat, and the bimetallic sheet bends after being heated, so that the push rod just touches the herringbone lever, but cannot push the herringbone lever. The normally closed contact is in the closed state, the AC contactor remains closed, and the motor runs normally.
If the motor is overloaded, the current in the winding will increase, and the temperature of the bimetal will rise higher through the increase of the current in the thermal relay element, the degree of bending will increase, and the herringbone lever will be pushed, and the herringbone lever will push the normally closed contact. , The contactor is disconnected and the AC contactor coil circuit is disconnected, the contactor is released, the power supply of the motor is cut off, and the motor is stopped and protected.
The functions of other parts 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 deform and bend to a certain extent, then the herringbone lever The left arm will also deform and bend in the same direction, so that the distance between the herringbone lever and the push rod remains basically the same, ensuring the accuracy of the thermal relay action. This effect is called temperature compensation.
Screw 8 is the adjustment screw of the normally closed contact reset mode. When the screw position is to the left, after the motor is overloaded, the normally closed contact is disconnected. After the motor stops, the thermal relay bimetallic plate cools and resets. The moving contact of the normally closed contact will automatically reset under the action of the spring. At this time, the thermal relay is in an automatic reset state. When the screw is turned counterclockwise to the right and adjusted to a certain position, if the motor is overloaded at this time, the normally closed contact of the thermal relay is disconnected. The moving contact will swing to a new equilibrium position on the right. After the motor is powered off and stopped, the moving contact cannot be reset. You must press the reset button to move the contact to reset. At this time, the thermal relay is in manual reset state. If the motor overload is faulty, in order to avoid starting the motor easily again, the thermal relay should be manually reset. To adjust the thermal relay from manual reset mode to automatic reset mode, just turn the reset adjustment screw clockwise to the proper position.