The working principle of the thermal relay in the motor - News - Global IC Trade Starts Here.

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Thermal relays are essential protective devices used to safeguard electric motors and other electrical equipment from damage caused by overload conditions. These relays work based on the thermal effect of electric current, ensuring that the motor doesn't suffer from excessive heat buildup due to prolonged or severe overloads.
When a motor is in operation—whether it's driving machinery or performing other tasks—it may encounter abnormal situations such as mechanical jams or electrical faults. These issues can lead to an increase in current flowing through the motor windings, causing the motor speed to drop and the winding temperature to rise. If the overload is mild and short-lived, the motor can usually handle it without damage. However, if the overload persists for a long time or is significant in magnitude, the temperature of the windings can exceed safe limits, leading to insulation degradation, reduced lifespan, and even complete failure of the motor.
To prevent this, a thermal relay is employed. The thermal element of the relay is connected in series with the motor’s stator winding. Meanwhile, the normally closed contact of the thermal relay is integrated into the control circuit of the AC contactor’s electromagnetic coil. By adjusting the current setting knob, the bimetallic strip is positioned so that it remains just close to the push rod. Under normal operating conditions, the current passing through the thermal element matches the motor’s rated current. This causes the bimetallic strip to slightly bend, keeping the push rod in contact with the herringbone lever without triggering any action. As a result, the normally closed contact remains closed, allowing the AC contactor to stay engaged, and the motor continues to run smoothly.
In the event of an overload, the current through the thermal element increases, causing the bimetallic strip to heat up more significantly. This leads to greater bending, which pushes the herringbone lever. The movement of the lever then triggers the normally closed contact to open, cutting off the power supply to the contactor coil. This results in the contactor releasing, thereby disconnecting the main power supply to the motor and stopping it to prevent further damage. It’s important to note that thermal relays typically protect the motor by interrupting the control circuit rather than directly cutting the main power. This design ensures efficient and reliable protection against overloading, making thermal relays a vital component in industrial and commercial motor control systems.