A basic thermal overload relay is a protective device used in electrical circuits to safeguard electric motors from overheating and potential damage caused by excessive current or prolonged operation. It operates based on the principle that excessive current flowing through the motor can generate excessive heat, which can be harmful to the motor's windings and insulation. The thermal overload relay helps prevent these issues by monitoring the motor's temperature and tripping the circuit if the temperature rises above a safe threshold.
Here's how a basic thermal overload relay protects electric motors:
Sensing Temperature: The thermal overload relay is equipped with a bimetallic strip or a heater element embedded in the motor's windings or near the motor's heat-sensitive parts. The bimetallic strip is composed of two different metals with different coefficients of thermal expansion, causing it to bend when subjected to temperature changes.
Calibration: The relay is pre-calibrated based on the motor's rated current and the allowable temperature rise for the specific motor. The temperature at which the relay should trip is set according to the manufacturer's specifications.
Current Monitoring: As the motor operates, it draws current from the power supply. The thermal overload relay is connected in series with the motor circuit. It continuously monitors the current passing through it.
Heating and Bimetallic Action: When the motor operates within normal conditions and the current is within the safe operating range, the bimetallic strip remains in its original position, and the relay stays closed.
Overload Detection: If the motor is subjected to an overload or a prolonged current surge, it will draw excessive current. The excess current causes an increase in the motor's temperature.
Bimetallic Trip: As the temperature rises beyond the pre-set threshold, the bimetallic strip inside the thermal overload relay heats up and bends due to the differential expansion of its two metal components. When the strip bends enough, it actuates a trip mechanism in the relay.
Circuit Interruption: When the trip mechanism is activated, the normally closed contacts of the relay open, breaking the electrical circuit to the motor. This stops the flow of current to the motor and prevents further heating.
Manual Reset (Optional): Some thermal overload relays come with a manual reset feature. After the relay trips and the motor cools down, the operator needs to reset the relay manually before the motor can be restarted.
Overall, the thermal overload relay serves as an essential protective device for electric motors, preventing damage caused by overheating due to overloads or prolonged operation. This protection is crucial in maintaining the motor's reliability and extending its operational life.