A basic thermal overload relay is a protective device used in electrical systems to safeguard electric motors from overheating due to excessive current draw or prolonged operation. It operates based on the principle that when an electric motor operates beyond its rated current or for an extended period, it generates excess heat that can damage the motor's insulation and winding, leading to failure.
Here's how a basic thermal overload relay works to protect electric motors:
Sensing Temperature: The thermal overload relay contains a bimetallic strip or a heater element connected to a temperature-sensitive device, such as a thermistor. As the electric motor operates, it generates heat, causing the temperature within the motor windings to rise. The temperature-sensitive device in the relay senses this rise in temperature.
Heating and Bimetallic Strip: The temperature-sensitive device heats up or causes the bimetallic strip to bend as the temperature increases. The bimetallic strip is made of two different metals with different coefficients of thermal expansion bonded together. As the strip heats up, the two metals expand at different rates, causing the strip to bend.
Contact Activation: The bending of the bimetallic strip or the state change of the temperature-sensitive device triggers the relay's contact mechanism. The relay's contacts are typically connected in series with the motor's power supply. When the contacts close, the motor circuit is completed, allowing current to flow to the motor.
Overload Protection: If the motor starts drawing excessive current or operates for an extended period, causing the temperature to rise beyond a safe threshold, the temperature-sensitive device will continue to heat up, leading to further bending of the bimetallic strip. Eventually, the strip will bend enough to mechanically open the relay's contacts.
Motor Shutdown: When the relay's contacts open, the motor circuit is interrupted, and the motor stops receiving power. This prevents the motor from further overheating and potentially damaging its components due to prolonged excessive current or extended operation.
Cooling and Reset: Once the motor is stopped, the excess heat can dissipate, allowing the temperature-sensitive device to cool down. As it cools, the bimetallic strip returns to its original position, and the contacts reset to their closed position.
Manual Reset: Some thermal overload relays require manual intervention to reset the contacts after the motor has cooled down and the cause of the overheating has been addressed. This manual reset ensures that the motor won't restart automatically without human intervention, preventing potential hazards if the initial problem hasn't been resolved.
In summary, a basic thermal overload relay is an essential safety device that protects electric motors by monitoring their operating temperature and interrupting the power supply if the temperature rises to a potentially damaging level due to excessive current or extended operation. This helps prevent motor damage and potential hazards in industrial and commercial applications.