What is the function of a thermal overload relay?

The function of a thermal overload relay is to open the circuit when excess heat is generated. This mechanism is crucial for protecting motors from overheating due to overcurrent conditions. When a motor draws more current than its rated capacity, typically because of a mechanical fault or overload, the thermal overload relay detects the increase in temperature that results from this excessive current.

This detection is achieved through the use of a bimetallic strip or a specific heat-sensitive element within the relay that bends or changes state as it heats up. Once the temperature reaches a predetermined threshold, the relay "trips," effectively disconnecting the motor from the power supply and preventing potential damage from overheating.

This protective function is vital for maintaining the longevity and reliability of motor systems. While closing electrical contacts during normal operation is an important function of relays in general, it does not specifically address the protective function of a thermal overload relay. Similarly, operating with a capacitive load and measuring thermal expansion of electrical parts do not pertain to the primary role of a thermal overload relay, which centers around safety and overheating prevention.