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What is the working principle of thermal relays?
Apr 01, 2018

The thermal relay works by generating heat from the current that flows into the heat element, so that there are different expansion coefficient of the bimetal sheet deformation, when the deformation reaches a certain distance, it pushes the connecting rod action, so that the control circuit disconnect, so that the contactor lost power, the main circuit disconnected, to achieve overload protection of the motor. As the overload protection element of the motor, the relay is widely used in the production because of its small size, simple structure and low cost.

Role: Mainly used for overload protection of asynchronous motor, his working principle is overload current through the thermal components, so that bimetal plate heating bending to promote the action mechanism to drive the contact action, so that the motor control circuit disconnect to achieve motor power off parking, play an overload protection role. In view of the thermal bending of bimetal plates, heat transfer takes a long time, therefore, heat relays can not be used as short circuit protection, but only as overload protection of the overload protection of thermal relays.

Working principle:

Thermal relays are protective appliances for overload protection of electric motors or other electrical equipment and electrical circuits.

In the actual operation of the motor, such as drag production machinery in the process of work, if the mechanical appearance of abnormal or unusual circuit motor encountered overload, then the motor speed down, winding current will increase, so that the motor winding temperature rise. If the overload current is small and the time of overload 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, so that the motor winding aging, shorten the service life of the motor, and even burn the motor windings when it is serious. Therefore, this overload is the motor can not afford. Thermal relay is to use the thermal effect of the current principle, in the presence of the motor can not withstand overload, cut off the motor circuit for the motor to provide overload protection protection of electrical appliances.

When using a thermal relay to overload an electric motor, the stator windings of the heat element and the motor are connected in series, the constant close contact of the thermal relay is connected in the control circuit of the electromagnetic coil of the AC contactor, and the adjusting knob is adjusted to make the herringbone lever and the push rod to be separated by a proper distance. When the motor is working normally, the electric current of the electric motor is the rated current of the electromotor, the heat element is hot, the bimetal sheet is bent after heating, so that the push rod is in contact with the herringbone lever, and can not promote the herringbone lever. The constant close contact is in the closed state, the AC contactor keeps the suction and the motor is running normally.

If the motor is overloaded, the current in the winding increases, and the temperature of the bimetallic plate rises higher by increasing the current in the thermal relay element. The bending degree increases, pushes the herringbone to push the pole, the herringbone lever pushes the constant close contact, causes the contact to disconnect and disconnects the AC contactor coil circuit, causes the contactor to release, the power supply which cuts off the electric machine, The motor stops and is protected.

The other parts of the thermal relay function as follows: The left arm of the herringbone lever is also made of bimetallic sheets, when the ambient temperature changes, the main circuit of the bimetal sheet will produce a certain deformation and bending, then the herringbone dial the left arm will also occur in the same direction of deformation and bending, so that the herringbone lever and the distance between the putter basically remain unchanged, Ensure the accuracy of the thermal relay movement. The effect is called temperature compensation.

Screw 8 is the normal closed contact reset mode to adjust the screws. When the screw position is left, the motor is overloaded, the closed contact is disconnected, and after the motor stops, the thermal relay bimetal plate is cooled and reset. The dynamic contact of the constant closed contact is automatically reset under the action of the spring. At this time the thermal relay for the automatic reset state. When the screws are rotated counterclockwise to the right to a certain position, if the motor is overloaded, the constant close contact of the thermal relay is disconnected. The movable contact will be placed to a new equilibrium position on the right. When the motor is powered off, the moving contact cannot be reset. You must press the reset button and the moving contact can be reset. At this time the thermal relay is a manual reset state. If the motor overload is faulty, in order to avoid starting the motor again easily, the thermal relay should adopt manual reset mode. To transfer the thermal relay from the manual reset mode to the automatic reset mode, simply rotate the reset screw into the appropriate position.