Thermal relay is mainly used to overload protect the asynchronous motor. Its working principle is that after the overload current passes through the thermal element, the double metal sheet is bent to push the action mechanism to drive the contact action, so as to disconnect the motor control circuit and realize the motor shutdown, and play the role of overload protection.Given the long time of heat transfer required during thermal bending of bimmetal plate, the thermal relay cannot be used as short circuit protection, but only as overload protection of overload protection thermal relay.
When the thermal relay is used to overload protect the motor, connect the thermal element and the stator winding of the motor in series, the normally closed contact of the thermal relay is connected in the control circuit of the electromagnetic coil of the AC contactor, and the setting current adjustment knob is adjusted to make the herringbone lever an appropriate distance from the push rod.When the motor works normally, the current through the thermal element is the rated current of the motor. When the thermal element warms up, the double metal sheet is bent after heating, so that the push rod just contacts with the herringbone lever, but cannot push the herringbone rod.The normally closed contacts are closed, the AC contactor remains engaged, and the motor operates normally.
If the motor overload situation, the current in the winding increases, through the current in the thermal relay element bimetallic temperature rises higher, bending degree, promote the herringbone lever, herringbone lever push often close the contact, make the contact disconnected and disconnected the ac contactor coil circuit, make the contactor release, cut off the motor power, the motor stop and protected.
Other parts of the thermal relay is as follows: herringbone lever left arm is made of bimetallic, when the ambient temperature changes, the main circuit will produce certain deformation bending, then the left arm in the same direction, so that the distance between herringbone lever and the push rod remains unchanged, ensure the accuracy of the thermal relay action.This action is called the temperature compensation action.
Screw 8 is a adjusting screw with normally closed contact reset.When the screw position is on the left, after the motor overload, the often closed contact is disconnected, after the motor stops, the hot relay bimetallic sheet cooling reset.The moving contacts of the normally closed contacts will automatically reset under the action of the spring.At this point, the thermal relay is automatically reset state.When the screw is rotated counterclockwise to a certain position, if the motor is overloaded, the normally closed contact of the thermal relay is disconnected.The moving contacts will reach a new balance position on the right.The moving contact cannot be reset after the motor is powered off.The reset button must be pressed before the contact is reset.At this point, the thermal relay is in a manually reset state.If the motor overload is faulty, in order to avoid easily starting the motor again, the thermal relay should adopt the manual reset mode.To adjust the thermal relay from the manual reset mode to the automatic reset mode, simply screw the reset adjustment screw clockwise to the proper position.
Post time: Mar-28-2022
