A basic thermal overload relay is a protective device used to safeguard electric motors from overheating, which can occur due to excessive current draw or prolonged operation. It operates by monitoring the temperature of the motor and interrupting the power supply if the temperature exceeds a certain threshold.
Here's how a basic thermal overload relay works to protect electric motors:
Temperature Sensing: The thermal overload relay contains a bi-metallic strip or other temperature-sensitive element that is in physical contact with the motor windings. As the motor operates, it generates heat, and this heat is transferred to the temperature-sensitive element.
Bi-Metallic Strip Operation: The bi-metallic strip is composed of two different metals with distinct thermal expansion rates bonded together. As the temperature rises, the metals expand at different rates, causing the strip to bend or warp. This bending action is used to activate the protective mechanism.
Contacts Activation: The bending of the bi-metallic strip due to heat causes a mechanical action that triggers the opening or closing of electrical contacts within the thermal overload relay. When the strip bends sufficiently, it operates a mechanism that opens the normally closed (NC) contacts. These contacts are typically in series with the motor's power supply.
Circuit Interruption: When the normally closed contacts open, the power supply to the motor is interrupted. This prevents further current from flowing through the motor windings. As a result, the motor stops running, and the heat generation is halted.
Cooling and Resetting: Once the motor stops, it begins to cool down. As the temperature drops, the bi-metallic strip also cools and returns to its original shape. When it contracts sufficiently, it resets the mechanism, causing the contacts to close again. The motor can be restarted once the contacts are closed.
Adjustability: The thermal overload relay usually allows for adjustment of the temperature threshold at which it operates. This threshold is set based on the motor's rated current and the desired level of protection. Different motor sizes and applications may require different temperature settings to ensure optimal protection without causing unnecessary interruptions.
In summary, a basic thermal overload relay protects electric motors by monitoring the motor's temperature and interrupting the power supply if the temperature rises to a dangerous level. This prevents the motor from overheating and potentially damaging itself or other connected equipment.