What is the primary purpose of thermal relays in motors?

The primary purpose of thermal relays in motors is to provide overload protection. These devices are designed to monitor the temperature within the motor circuit, responding to excessive current conditions that can lead to overheating. When the motor operates beyond its rated capacity for an extended period, the thermal relay detects the increased temperature, which is a sign of overload, and subsequently interrupts the power supply to prevent damage to the motor.

Thermal relays achieve this by using a bimetallic strip or thermistor that bends or changes resistance when heated, triggering the relay to open and break the circuit. This protection mechanism is crucial as it helps ensure the longevity and reliability of the motor by preventing failure due to overheating.

While overcurrent protection and voltage protection are also important in electrical systems, they are generally achieved through different types of protective devices, such as circuit breakers or fuses for overcurrent and voltage relays for voltage issues. Thermal protection can be seen as a subset of the broader category of overload protection, but it specifically refers to monitoring and responding to heat generated by excessive current, emphasizing the thermal aspect of overload conditions.