Thermal overload relays are key electrical devices for motor drag. The principle is to use the physical properties of the bimetal sheet to deform due to overheating, which can be regarded as a normally open and normally closed power switch, and is connected to the control circuit driven by the motor. For some reason, the position of the thermal relay becomes open, the control circuit motor main contactor coil loses power and the main contact separates and the motor terminates. Overheat protection for the motor.
Select the rated voltage and setting value of the thermal relay, and the thermal relay can be selected according to the rated voltage of the motor. .95～1.05 times or the middle value is equivalent to the rated voltage of the motor. Adjust the operating current of the thermal relay setting value to the rated voltage value of the motor.
一. The basic principle and efficacy of thermal relay
1. Since the bimetal plate must transfer heat for a long time when it is thermally bent, the thermal relay cannot be used for short circuit maintenance, but can only be used as overvoltage protection for the thermal relay. fr.
2. The thermal relay is made of heating element. Bimetallic sheet. Contact point, transmission system and adjustment structure. The heating element is a low-resistance heating wire connected to the main circuit that maintains the motor. The bimetallic sheet is rolled from two copper sheets with different coefficients of linear expansion. The bimetal plate shown below has a larger coefficient of linear expansion and a smaller top. When the motor is loaded, according to the current of the heating element exceeding the full current, the bimetal plate bends when heated to break free of the splicing, thus cutting off the normally closed point. Since the normally closed point is connected to the control circuit of the motor, the conference encourages the connected contactor coil to turn off the power, then the main contact switch of the AC contactor is disconnected, and the main circuit of the motor turns off the power to achieve overvoltage protection.
3. After the thermal relay poses, the bimetal can be calibrated by pressing and holding the reset switch for a period of cooling.
4. Many specifications of thermal relays also have phase failure protection. Through the phase failure of the internal protection thermal relay. The differential protection composed of the outer swing rod is given by the enlarged organization.
5. If the motor is loaded, the winding current expands, according to the current expansion in the thermal relay element, the bimetallic ambient temperature increases, and the deformation level increases, promoting the herringbone lever, the herringbone lever promotes the normally open and normally closed circuit breaker, disconnecting Communication contactor coil power circuit, release the AC contactor, cut off the power switch of the motor, and stop the motor to maintain it.
6. When using the thermal relay to realize the motor overvoltage protection, connect the thermal component in series with the motor stator winding, connect the normally open and normally closed circuit breaker of the thermal relay in series with the control circuit of the magnetic coil of the DC contactor, and adjust the fixed current adjustment knob to make The distance between the herringbone lever and the pendulum lever is moderate. When the motor is running normally, according to the current of the hot part is the rated voltage of the motor, the hot part is very hot, and the bimetallic plate bends after being heated, so that the pendulum rod collides with the herringbone lever, which cannot promote the herringbone lever. The normally open and normally closed circuit breaker is closed, the DC contactor remains closed, and the motor runs normally.
7. The functions of other parts of the thermal relay are as follows: the upper left arm of the herringbone lever is also made of bimetallic sheet. When the temperature changes greatly, the bimetal in the main circuit will cause certain deformation and bending. At this time, the upper left arm of the herringbone lever will also deform and bend in the same direction, so that the distance between the herringbone lever and the pendulum rod is basically the same to ensure the accuracy of the thermal relay posture. This effect is called temperature compensation.
8. It has a calibration method for adjusting the screw. When the screw part is close to the left, the normally closed circuit breaker is often opened after the motor is loaded. After the motor is stopped, the bimetal cooling of the thermal relay is calibrated. The moving contacts of normally open and normally closed circuit breakers can be automatically calibrated under the action of springs. At this point, the thermal relay is automatically calibrated. When the screw is turned counterclockwise to the right and adjusted to a certain part, if the motor is loaded at this time, the normally open and normally closed circuit breaker of the thermal relay will be disconnected. Its dynamic breaker will be placed in a new balance position on the right. The dynamic circuit breaker cannot be calibrated after the motor has been powered off. Before calibration, the reset switch must be pressed. At this point, the thermal relay is manually calibrated. If the motor load is a common fault, in order to prevent the motor from starting easily again, the thermal relay should choose the manual calibration method. If the thermal relay is adjusted from the manual calibration method to the automatic calibration method, just turn the adjustment screw clockwise to the moderate part.
Post time: Jul-08-2022