A basic thermal overload relay is an essential protective device used to safeguard electric motors from damage caused by overheating. It is designed to monitor the motor's operating temperature and interrupt the power supply when the temperature exceeds a predetermined threshold. The primary function of a thermal overload relay is to prevent the motor from drawing excessive current for extended periods, which can lead to overheating and potential motor failure.
Here's how a basic thermal overload relay protects electric motors:
Bimetallic Elements: The core component of a thermal overload relay is one or more bimetallic elements. These elements consist of two different metals with different coefficients of thermal expansion bonded together. When subjected to a rise in temperature, the metals expand at different rates, causing the bimetallic strip to bend or deform.
Motor Current Monitoring: The thermal overload relay is connected in series with the motor's power supply, and it continuously monitors the current flowing through the motor. Under normal operating conditions, the current level remains within a safe range, and the bimetallic elements do not reach their trip point.
Temperature Sensing: The bimetallic elements are in contact with a heater element, which simulates the motor's temperature rise. The heater element heats up due to the current flowing through it, and in turn, it transfers heat to the bimetallic elements.
Time-Delayed Response: The thermal overload relay is designed with a time-delay feature to allow for temporary current spikes during motor startup. This ensures that the relay does not trip unnecessarily during short-duration high-current events.
Trip Point Setting: The thermal overload relay has an adjustable setting for the trip point or the temperature at which it activates. The setting is typically based on the motor's thermal withstand capabilities and the ambient conditions in which the motor operates.
Trip and Disconnect: If the motor current remains above the safe threshold for an extended period or if there is a significant current spike, the heater element will heat the bimetallic elements to a critical point. When this happens, the bimetallic elements deform rapidly, causing the relay's contacts to open, breaking the circuit and disconnecting the motor from the power supply.
Reset and Restart: After the thermal overload relay trips and disconnects the motor, it enters a latched state. To restart the motor, the cause of the overheating (e.g., overload or excessive ambient temperature) must be addressed, and the relay must be manually reset. This safety feature prevents the motor from restarting automatically and potentially being damaged further.
In summary, a basic thermal overload relay protects electric motors by continuously monitoring motor current and simulating the motor's temperature rise through bimetallic elements. If the motor exceeds the predetermined safe operating temperature, the relay interrupts the power supply, preventing damage to the motor due to overheating.