Thermal Overload Relay. Basic construction and function.

Typically use in a motor starters.
The thermal overload relay serves as a crucial protective device in electrical systems, primarily used to safeguard motors from overheating and potential damage due to excessive current or overload. It operates based on the principle of thermal sensing, monitoring the heat generated by the motor during operation.
When the motor draws excessive current with an ampere above the relay trip value, in an extended period, the thermal overload relay responds by tripping and disconnecting the power supply to the motor. This prevents the motor from overheating, reducing the risk of electrical damage and enhancing operational safety.
Thermal overload relays are mainly categorized based on their working principle. This 3 types of thermal relays are bimetallic, electronic, and melting alloy thermal overload relay.
in this presentation, we will discuss the bimetallic thermal overload relay. which is the most widely used thermal relay because of simplicity, both in terms of construction and operation.
A bimetallic strip is utilized as the main component of this relay. it contain two metals that are joined lengthwise. the two metals have different expansion rate when heated causing it to bend in one direction.
Note that the bimetallic strips in these types of thermal relay mechanisms are indirectly heated by coils. The coils are connected to the motor circuit, which enables them to match the temperature rise which is created by the amount of actual current being drawn by the motor.
Since bimetallic strips or strips are mechanically link in a tripping mechanism.
When the temperature reaches a certain level, the bimetallic element bends and activate the thermal overload internal switches to disconnect power to the equipment and signal a fault or alarm.