Thermal overload relays are widely used for overload protection of electric motors due to their simple structure, ease of installation and use. According to the magnitude and duration of the motor starting current, correctly configuring the connection mode of the thermal relay in the circuit is the key to properly selecting the setting current of the thermal relay.
Thermal overload relay current adjustment setting
When the motor is operated at rated current, no thermal relay protection is required. Even if the overload is 20% running, the design is considered to allow for a period of time and will not burn immediately. However, if the motor is blocked or the machine being dragged fails, the motor will be overloaded and burned; and the thermal relay can quickly cut off the circuit during this time to ensure the normal operation of the motor.
At present, when the thermal overload relay is used for a light-loaded starting motor, the starting current is generally about six times the rated current, and the starting time is not more than 5 seconds. At this time, the setting current of the thermal relay can be adjusted to be equal to the rated current of the motor. When the motor is overloaded by 20%, the thermal relay quickly cuts off the power supply in a specified time by directly connecting the main and auxiliary contacts in the circuit in series to protect the motor from overload. The thermal relay can escape six times the starting current within 5s without causing malfunction.
In the special occasions of heavy-duty relays (such as long-running and dragging impact loads), such as blowers and induced draft fans of gasifiers, since the starting time of these devices exceeds 5s, the thermal relay setting current should be adjusted to The rated current of the motor is 1.1~1.15 times. Sometimes, in order not to cause the thermal relay to malfunction during startup, the setting ratio of the thermal relay can only be increased. This makes the motor’s overload capacity exceed 20% of the design time, and the protection of the motor is not obvious. In order to ensure the reliability of the action in heavy load, the connection of the thermal overload relay in the circuit should adopt the circuit shown in Figure 1. Due to the action of TA, KA and KT, the circuit can also avoid the impact of the starting current when the starting time is long, and the setting value of the thermal relay can be adjusted to be equal to the rated current value. Malfunction.
The actual operation shows that the setting current of the thermal overload relay is reasonably selected, and the motor is correctly connected according to the use condition of the motor in different occasions.
Thermal relays protect the motor from overload in both light and heavy loads.
When using a thermal relay to protect the motor from overload, connect the thermal element to 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 the person The font lever is at an appropriate distance from the push rod. When the motor is working normally, the current passing through the heat element is the rated current of the motor, the heat element is heated, and the bimetal is bent after being heated, so that the push rod is just in contact with the herringbone lever, and the herringbone lever cannot be pushed. The normally closed contact is in a closed state, the AC contactor remains closed, and the motor operates normally.
If the motor is overloaded, the current in the winding increases, the current in the thermal relay element increases, the temperature of the bimetal rises higher, the bending degree increases, the herringbone lever is pushed, and the herringbone lever pushes the normally closed contact. The contact is disconnected and the AC contactor coil circuit is disconnected, the contactor is released, the power of the motor is cut off, and the motor is stopped to be protected.
The other parts of the thermal relay function as follows: the left arm of the chevron lever is also made of bimetal. When the ambient temperature changes, the bimetal in the main circuit will have a certain deformation and bending. At this time, the chevron lever The left arm also undergoes deformation and bending in the same direction, so that the distance between the chevron lever and the push rod remains substantially unchanged, and the accuracy of the thermal relay operation is ensured. This effect is called temperature compensation.
The screw 8 is a normally closed contact reset mode adjusting screw. When the screw position is to the left, after the motor is overloaded, the normally closed contact is disconnected, and after the motor stops, the thermal relay bimetal is cooled and reset. The moving contact of the normally closed contact is automatically reset by the action of the spring. At this time, the thermal relay is in an automatic reset state. When the screw is rotated counterclockwise to the right to a certain position, if the motor is overloaded at this time, the normally closed contact of the thermal relay is disconnected. Its moving contact will swing to the right side of a new equilibrium position. After the motor is powered off, the moving contact cannot be reset. The movable contact must be pressed after pressing the reset button. At this time, the thermal relay is in the manual reset state. If the motor overload is faulty, in order to avoid starting the motor again easily, the thermal relay should be manually reset. To adjust the thermal relay from manual reset to automatic reset, simply turn the reset adjustment screw clockwise into position.
Thermal overload relay parameter adjustment method
- The setting current of the thermal overload relay must be adjusted according to the rated current of the motor, and it is absolutely not allowed to bend the bimetal.
- Generally, the thermal overload relay should be placed in the manual reset position. If automatic reset is required, the reset screw can be rotated clockwise.
- If the thermal overload relay is activated after the motor is overloaded, the thermal overload relay must be reset after the heating element is cooled. The general reset time is 2 minutes for manual reset and 5 minutes for automatic reset.