A thermal relay is a protective device used to safeguard electric motors, electrical equipment, and circuits against overload.
During actual operation-such as when a motor is driving production machinery-abnormalities in the machinery or the circuit can cause the motor to experience an overload. This results in a drop in motor speed and an increase in winding current, causing the temperature of the motor windings to rise. If the overload current is low and the duration is short-meaning the winding temperature does not exceed the permissible limit-such an overload is acceptable. However, if the overload persists for a long time or the current is excessive, the winding temperature will exceed the permissible limit; this accelerates winding aging and shortens the motor's service life, or in severe cases, causes the windings to burn out. Consequently, the motor cannot withstand this type of overload. Thermal relays operate on the principle of the thermal effect of electric current; they cut off the motor circuit when an intolerable overload occurs, thereby providing overload protection.
When using a thermal relay for motor overload protection, the heating element is connected in series with the motor's stator windings, and the relay's normally closed (NC) contact is connected in series within the control circuit of the AC contactor's electromagnetic coil. Additionally, the setting current knob is adjusted to establish an appropriate gap between the chevron-shaped lever and the push rod. During normal operation, the current flowing through the heating element equals the motor's rated current. The heating element generates heat, causing the bimetallic strip to bend until the push rod just touches the chevron-shaped lever without actually moving it. The NC contact remains closed, the AC contactor stays engaged, and the motor continues to run normally.
If the motor becomes overloaded, the winding current increases. The increased current flowing through the thermal relay element causes the bimetallic strip to heat up further and bend more sharply. This movement pushes the chevron-shaped lever, which in turn actuates the NC contact, causing it to open. This breaks the circuit to the AC contactor coil, causing the contactor to release and cut off the motor's power supply; the motor stops, thereby ensuring its protection. The functions of other components of the thermal relay are as follows: The left arm of the V-shaped lever is also constructed from a bimetallic strip. When the ambient temperature changes, the bimetallic strip in the main circuit undergoes a certain degree of deformation (bending); simultaneously, the left arm of the V-shaped lever bends in the same direction. This ensures that the distance between the V-shaped lever and the push rod remains essentially constant, thereby guaranteeing the operational accuracy of the thermal relay. This function is known as temperature compensation.
Screw 8 serves to adjust the reset mode of the normally closed (NC) contact. When the screw is positioned to the left, the NC contact opens upon motor overload; once the motor stops, the bimetallic strip cools and returns to its original shape, allowing the moving contact to automatically reset under spring tension-placing the relay in "automatic reset" mode. If the screw is turned counter-clockwise to the right, the NC contact opens during an overload, but the moving contact shifts to a new equilibrium position on the right side. After the motor stops, the moving contact does not reset automatically; the reset button must be pressed to return it to its original position-placing the relay in "manual reset" mode. Manual reset is preferable when the overload is caused by a fault, as it prevents the motor from being easily restarted. To switch the relay from manual to automatic reset mode, simply turn the adjustment screw clockwise to the appropriate position.
