Working principle of thermal relay
Thermal relays are protective appliances used for overload protection of motors or other electrical equipment and electrical circuits.
In the actual operation of the motor, such as the production machinery is dragged to work, if the machinery is abnormal or the circuit is abnormal, the motor encounters an overload, the motor speed decreases, the current in the winding will increase, and the motor winding temperature Rise. If the overload current is not large and the overload time 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, aging the motor winding, shortening the service life of the motor, and even burning the motor winding in severe cases. Therefore, this type of overload cannot be sustained by the motor. Thermal relay is the use of the thermal effect principle of current to cut off the motor circuit when there is an overload that the motor cannot withstand, and provide a protective appliance for the motor to protect it from overload.
When using a thermal relay to protect the motor from overload, connect the thermal element and the stator winding of the motor in series, connect the normally closed contact of the thermal relay in series to the control circuit of the electromagnetic coil of the AC contactor, and adjust the setting current adjustment knob to make people The zigzag lever and the push rod are at an appropriate distance. When the motor is working normally, the current through the heat element is the rated current of the motor. The heat element heats up, and the bimetal is bent after being heated, so that the push rod is just in contact with the herringbone lever, but cannot push the herringbone lever. The normally closed contact is in the closed state, the AC contactor remains closed, and the motor runs normally.
If the motor is overloaded, the current in the winding increases, and the current in the thermal relay element increases the temperature of the bimetallic plate higher, and the degree of bending increases, pushing the herringbone lever, and the herringbone lever pushes the normally closed contact , The contact is opened and the coil circuit of the AC contactor is opened, the contactor is released and the power of the motor is cut off, and the motor is stopped and protected.
The functions of the other parts of the thermal relay are as follows: the left arm of the herringbone lever is also made of bimetallic sheet. When the ambient temperature changes, the bimetallic sheet in the main circuit will produce a certain deformation and bending, then the herringbone lever The left arm will also be deformed and bent in the same direction, so that the distance between the herringbone lever and the push rod is basically kept unchanged, ensuring the accuracy of the thermal relay action. This effect is called temperature compensation.
The screw 8 is an adjustment screw for a normally closed contact reset method. When the screw position is left, after the motor is overloaded, the normally closed contact is disconnected, and after the motor is stopped, the bimetal of the thermal relay is cooled and reset. The moving contact of the normally closed contact will automatically reset under the action of the spring. At this time, the thermal relay is automatically reset. 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 opened. The moving contact will swing to a new balanced position on the right. After the motor is powered off and stopped, the moving contact cannot be reset. You must press the reset button before moving the contacts to reset. At this time, the thermal relay is in 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 mode to automatic reset mode, simply turn the reset adjustment screw clockwise to the appropriate position.
How to choose thermal relay? The following will answer you one by one
(1) Type selection
In general, two-phase thermal relays can be used, but when the three-phase voltage balance is poor, the working environment is poor or unattended motors, three-phase thermal relays should be used. For delta-connected motors, thermal relays with phase-off protection devices should be used.
(2) Rated current selection of thermal relay
The rated current of the thermal relay should be greater than the rated current of the motor. Then select the type of thermal relay according to the rated current.
(3) Selection and setting of thermal element rated current
The rated current of the heating element should be slightly larger than the rated current of the motor. When the starting current of the motor is 6 times the rated current and the starting time does not exceed 5S, the setting current of the heatless parts is adjusted to be equal to the rated current of the motor; when the starting time of the motor is long, impact load is dragged or parking is not allowed At this time, the setting current of the heating element is adjusted to 1.1-1.15 times of the rated current of the motor.
Thermal relay is a kind of protection relay made by the principle of current thermal effect, which is widely used for overload protection of motor. How to choose and use thermal relays reasonably is an old topic, but at present, many units still have motor accidents due to unreasonable selection and use of thermal relays. Therefore, the author reminds first-time contacts that, in addition to observing general regulations, the following points should be noted for the selection and use of thermal relays:
1. The relationship between the specifications of the relay and the installation position
For different star / delta control positions, the specifications of the selected thermal relay are also different. As shown in Figure 1, the current setting value of the selected thermal relay is equal to the rated current value of the protected motor. In Figure 2, the setting value of the selected thermal relay current should be 1 / Ö3 of the rated current of the protected motor.
2. Reset form
Thermal relays generally have two reset forms: manual reset and automatic reset. The conversion of these two reset forms can be completed by the adjustment of the reset screw. When the thermal relay leaves the factory, the manufacturer generally sets it to the automatic reset state. In use, the thermal relay should be set to the manual reset state or the automatic reset state according to the specific conditions of the control circuit. Under normal circumstances, the principle that the protected motor should not restart automatically even if the thermal relay is automatically reset after the thermal relay protection action should be followed, otherwise the thermal relay should be set to the manual reset state. This is to prevent the motor from repeatedly damaging the equipment after repeated restarts before the failure is eliminated. For example: the control circuit of manual start and manual stop controlled by button, the thermal relay can be set to automatic reset; the automatic start circuit controlled by automatic component should set the thermal relay to manual reset.
3. Selection of thermal relays with different rated current levels but the same thermal element adjustment range
For example, in the thermal relays of the JR16 series, in the rated current of 20A and 60A, the setting value of the thermal element has an adjustment range of 14A ~ 16A. At this time, the ambient temperature used by the thermal relay and the environment of the protected motor should be checked. temperature. When the ambient temperature used by the thermal relay is higher than the ambient temperature of the protected motor by more than 15 ℃, the thermal relay with a larger rated current level should be used; when the ambient temperature of the thermal relay is lower than the ambient temperature of the protected motor by 15 ℃ When using, the thermal relay with rated current smaller than one should be used. In addition, the load of the motor and the adjustment range that the thermal relay may require should also be considered.
4. It is used to adjust the setting current when overload protection of repeated short-time working motor
Only after multiple tests and adjustments at the site can reliable protection be achieved. The method is to first adjust the setting current of the thermal relay to be slightly smaller than the rated current of the motor. If it is found to operate frequently during operation, then gradually increase the setting value of the thermal relay until the operation requirements are met.
5. Selection of connecting wires
The connection wire of the thermal relay is too thick or too thin will also affect the normal operation of the thermal relay, because the thickness of the connection wire makes the heat dissipation different, which will affect the current thermal effect of the thermal relay. The selection of connecting wires for thermal relays of various specifications can be in accordance with the manufacturer's instructions or consult the electrician's manual.
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