A thermal relay, also known as a thermal overload relay or thermal protection relay, is an electromechanical device used to protect electrical equipment, particularly electric motors, from damage due to excessive heat caused by overcurrent conditions. Thermal relays are often used in conjunction with contactors or motor starters to provide an additional layer of protection against motor overload.

Here's how a thermal relay typically works:

1. Bimetallic Strip: The key component of a thermal relay is a bimetallic strip. This strip is made of two different metals bonded together with different coefficients of thermal expansion. When heated, the two metals expand at different rates, causing the strip to bend.

2. Heater Element: The thermal relay is equipped with a heater element that is placed in the path of the current flowing to the motor. As the current increases, the heater element heats up.

3. Temperature Sensing: The heat generated by the current flowing through the heater element causes the bimetallic strip to heat up. As the temperature of the strip rises, it bends due to the differential expansion of the metals.

4. Contacts: The bending of the bimetallic strip actuates a set of contacts within the thermal relay. These contacts are typically connected in series with the control circuit of the motor or the coil of a contactor. When the bimetallic strip bends beyond a certain threshold, the contacts open, interrupting the circuit and de-energizing the motor.

5. Reset Mechanism: Thermal relays often include a reset mechanism that allows the contacts to close again once the temperature has dropped to a safe level. This reset may be automatic, manual, or remote depending on the design of the thermal relay.

The function of a thermal relay is to sense excessive heat caused by overcurrent conditions, such as when a motor is overloaded or stalled, and to disconnect the motor from the power source to prevent damage. By providing thermal protection, thermal relays help extend the lifespan of electrical equipment and improve overall system reliability.

Thermal relays are commonly used in various applications where electric motors are involved, including industrial machinery, pumps, fans, compressors, and HVAC systems. They provide an effective and relatively simple means of protecting motors from overheating and damage due to overloads.