A basic thermal overload relay is an electrical device used to protect electric motors from overheating and potential damage. It works by monitoring the motor's operating temperature and interrupting the motor circuit if the temperature exceeds a certain predetermined threshold. Here's how it functions:
Sensing Temperature: The thermal overload relay contains a bimetallic strip or a heating element connected to a temperature sensor. When current flows through the motor's circuit, the bimetallic strip or heating element heats up. The rate of heating is proportional to the current passing through the motor.
Bimetallic Strip Operation: A bimetallic strip consists of two different metal strips bonded together. Each metal strip has a different coefficient of thermal expansion. As the temperature increases, the two metals expand at different rates, causing the bimetallic strip to bend or deform.
Contact Operation: The deformation of the bimetallic strip or heating element triggers a mechanical action, which, in turn, causes the relay's contacts to open. The contacts act as a switch that interrupts the electrical circuit supplying power to the motor.
Circuit Interruption: Once the contacts open, the power supply to the motor is cut off, and the motor stops running. This prevents further overheating and potential damage to the motor.
Cooling and Reset: With the power supply interrupted, the motor cools down. When the motor's temperature decreases below the predetermined threshold, the bimetallic strip or heating element returns to its original state, causing the contacts to close again. At this point, the motor can be restarted, and it will operate normally.
It's important to note that thermal overload relays are designed to protect motors from sustained overloads and excessive current draw, such as during motor stalls or when the motor is operating under heavy loads for an extended period. They are not designed to protect against short-circuit conditions or electrical faults, for which other protective devices like fuses or circuit breakers are used.
In more advanced applications, electronic motor protection relays can provide additional features like adjustable trip settings, phase loss protection, and communication capabilities, offering enhanced protection and monitoring capabilities for electric motors.