Thermal relay is a maintenance appliance used for overload maintenance of electric motors or other electrical equipment and electrical lines.
In the actual operation of the motor, if the machinery is abnormal or the circuit is abnormal, the motor will be overloaded, the speed of the motor will decrease, the current in the winding will increase, and the winding temperature of the motor will increase. If the overload current is not large and the overload time is short, the motor winding does not exceed the promised temperature rise, and the overload is promised. However, if the overload time is long and the overload current is large, the temperature rise of the motor winding will exceed the promised value, which will make the motor winding age, shorten the service life of the motor, and even incinerate the motor winding in serious cases. So, this overload is unacceptable for the motor. Thermal relay is a maintenance appliance that uses the thermal effect principle of electric current to intercept the motor circuit when the motor is unacceptable, and provides overload maintenance for the motor.
When using a thermal relay to carry out overload maintenance on the motor, the thermal element is connected in series with the stator winding of the motor, the normally closed contact of the thermal relay is connected in series in the control circuit of the electromagnetic coil of the communication toucher, and the setting current adjustment knob is adjusted so that the herringbone lever and the push rod are at an appropriate distance. When the motor is operating normally, the current through the thermal element is the rated current of the motor, the thermal element is heated, and the bimetal sheet is bent after being heated, so that the push rod just touches the herringbone lever, and can not advance the herringbone lever. The normally closed contact is in a closed state, the communication toucher insists on sucking, and the motor runs normally.
If the motor is overloaded, the current in the winding increases, the temperature of the bimetal sheet rises higher through the increase of the current in the thermal relay element, and the degree of tortuosity increases, and the herringbone lever is promoted, and the herringbone lever is promoted to the normally closed contact, so that the contact is disconnected and the communication touch device coil circuit is disconnected, so that the touch device releases and intercepts the power supply of the motor, and the motor is parked and maintained.
The effect of other parts of the thermal relay is as follows: the left arm of the herringbone lever is also made of bimetal sheet, when the ambient temperature changes, the bimetal sheet in the main circuit will undergo a certain deformation and twist, and the left arm of the herringbone lever will also occur in the same direction of deformation and bending, so that the distance between the herringbone lever and the push rod is basically unchanged, and the accuracy of the thermal relay action is ensured. This effect is called the temperature compensation effect.
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, and after the motor is parked, the thermal relay bimetal sheet cools and resets. The moving contacts of the normally closed contacts will actively reset under the effect of the tension spring. At this moment, the thermal relay is in active reset condition. When the screw is rotated counterclockwise to the right to a certain position, if the motor is overloaded, the normally closed contact of the thermal relay is disconnected. Its moving contacts will swing to a new equilibrium position on the right. After the motor is powered off and parked, the moving contact cannot be reset. The reset button must be pressed and then the contact can be reset. At this moment, the thermal relay is in a manual reset condition. If the motor is overloaded, in order to prevent the motor from being easily started again, the thermal relay should be manually reset. To adjust the thermal relay from the manual reset method to the active reset method, simply screw the reset conditioning screw clockwise into the appropriate position.
Thermal relay function
It is mainly used for overload maintenance of asynchronous motors, and his operating principle is that after the overload current passes through the thermal element, the bimetal sheet is heated and twisted to propulsion the action mechanism to drive the contact action, so as to disconnect the motor control circuit to complete the motor power-off parking, and play the effect of overload maintenance. In view of the fact that the heat transfer of the bimetal sheet needs to be long in the process of heating and twisting, therefore, the thermal relay cannot be used for short-circuit maintenance, but only for overload maintenance.
The phase-off maintenance function of the thermal relay is provided by a differential expansion mechanism composed of internal and external push rods. When the motor is operating normally, the current through the thermal relay heating element is normal, and the two push rods on the surface and inside are moved forward to the appropriate position. When the power supply is disconnected and the phase is missing, the phase current is zero, the bimetal sheet of the phase is cooled and reset, so that the inner push rod moves to the right, and the bimetal sheet of the other two phases increases the degree of tortuosity due to the increase of current, so that the outer push rod moves more to the left, because of the differential expansion effect, the normally closed contact is pushed into the normally closed contact to be disconnected in a short time after the phase failure fault is presented, so that the communication touch device is released, and the motor is powered off and parked and maintained.
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