I. Working principle of thermal relay
Thermal relays are protective electrical devices used for overload protection of motors or other electrical equipment and circuits.
In actual operation, such as when driving production machinery, if an abnormal situation occurs in the machinery or an abnormal circuit causes the motor to encounter an overload, the motor speed will decrease, the current in the windings will increase, and the temperature of the motor windings will rise. If the overload current is small and the overload time is short, the motor windings will not exceed the allowable temperature rise, and this overload is permissible. However, if the overload time is long and the overload current is large, the temperature rise of the motor windings will exceed the allowable value, causing the motor windings to age, shortening the motor's service life, and in severe cases, even causing the motor windings to burn out. Therefore, this type of overload is unacceptable for the motor. A thermal relay utilizes the principle of the thermal effect of current to disconnect the motor circuit when an overload occurs that the motor cannot withstand, providing overload protection for the motor.
When using a thermal relay for overload protection of a motor, the thermal element is connected in series with the motor's stator winding, and the normally closed contact of the thermal relay is connected in series with the control circuit of the AC contactor's electromagnetic coil. The current setting knob is adjusted so that the V-shaped lever and the push rod are at an appropriate distance apart. When the motor is operating normally, the current through the thermal element is the motor's rated current. The thermal element heats up, and the bimetallic strip bends, causing the push rod to just contact the V-shaped lever without pushing it further. The normally closed contact is in the closed state, the AC contactor remains engaged, and the motor operates normally.
If the motor is overloaded, the current in the winding increases. This increased current in the thermal relay element causes the bimetallic strip to heat up further, bending more. This pushes the V-shaped lever, which in turn pushes the normally closed contact, causing it to open and disconnect the AC contactor coil circuit. This releases the contactor, cuts off the power to the motor, and stops the motor, thus providing protection.
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 strip. When the ambient temperature changes, the bimetallic strip in the main circuit will deform and bend to a certain extent. At this time, the 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 push rod remains basically unchanged, ensuring the accuracy of the thermal relay's operation. This function is called temperature compensation.
Screw 8 is the normally closed contact reset adjustment screw. When the screw is positioned to the left, after a motor overload, the normally closed contact opens, and after the motor stops, the bimetallic strip of the thermal relay cools and resets. The moving contact of the normally closed contact automatically resets under the action of the spring. At this time, the thermal relay is in automatic reset mode. 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 opens. Its moving contact will swing to a new equilibrium position on the right. After the motor stops due to power failure, the moving contact cannot reset. The moving contact can only reset after the reset button is pressed. At this time, the thermal relay is in manual reset mode. If the motor overload is faulty, the thermal relay should be manually reset to avoid easily restarting the motor. To switch the thermal relay from manual reset mode to automatic reset mode, simply turn the reset adjustment screw clockwise to the appropriate position.
II. Thermal relay function
It is mainly used for overload protection of asynchronous motors. Its working principle is that when an overload current passes through a heating element, it heats and bends a bimetallic strip, which in turn pushes an actuating mechanism to actuate the contacts, thereby disconnecting the motor control circuit and stopping the motor, thus providing overload protection. Because the heat transfer during the bending process of the bimetallic strip takes a relatively long time, thermal relays cannot be used for short-circuit protection, but only for overload protection.
The phase loss protection function of the thermal relay is provided by a differential amplification mechanism composed of inner and outer push rods. When the motor is working normally, the current through the thermal element of the thermal relay is normal, and both the inner and outer push rods move forward to the appropriate positions. When a phase loss occurs due to a power outage, the current in that phase is zero, the bimetallic strip of that phase cools and resets, causing the inner push rod to move to the right. The bimetallic strips of the other two phases bend more due to the increased current, causing the outer push rod to move further to the left. Due to the differential amplification effect, the normally closed contact is pushed to open within a very short time after the phase loss fault occurs, causing the AC contactor to release, and the motor to stop and be protected.
