What Is a Thermal Overload Relay?

A thermal overload relay is a small electromechanical device that protects motors from overheating. These relays help to control the electrical current that goes to the motor to prevent it from overheating. If the motor pulls too much electricity for an extended period of time, the relay may flip and cut the power to the motor to prevent damage.

The thermal overload relay has three bimetal pole, or strip, relays that are equipped with bimetal tripping elements. The current for the motor runs through these three elements. The tripping elements and poles are heated directly by the electrical current or indirectly by the current by passing through the insulated heating winding that forms part of the protective housing around the pole, or strip. When too much electrical current passes through the pole, it causes it to overheat. As the pole overheats, it begins to bend. This causes the tripping elements to engage, releasing the relay and causing the contacts to switch positions, shutting off the electrical supply to the motor. Because they are designed to help avoid overheating and causing damage to the motor, the temperature readings given by the thermal overload relay might be hotter than the motor actually is. The relay works to shut off the excess electrical current before it reaches the motor which results in the temperature discrepancy.