A basic thermal overload relay is an essential protective device used to safeguard electric motors from damage caused by excessive heat. It operates based on the principle that when an electric motor is subjected to overcurrent or prolonged operation, it generates excessive heat, which can lead to insulation degradation, winding damage, and even motor failure.
The thermal overload relay works in conjunction with the motor's full load current and is designed to trip the motor circuit if it detects an abnormal increase in temperature, indicating an overload condition. Here's how it works:
Bimetallic elements: The thermal overload relay contains bimetallic elements, which consist of two different metals with different coefficients of thermal expansion bonded together. When subjected to heat, the bimetallic elements bend or deform at different rates, causing them to move.
Heater elements: These are connected in series with the motor circuit and are typically made of a material with a positive temperature coefficient, meaning their resistance increases with temperature.
Ambient temperature compensation: The relay may also have ambient temperature compensation to account for the external environmental conditions that could affect the motor's operating temperature.
Calibration: The thermal overload relay is calibrated to the motor's full load current and trip curve, which defines the relationship between the relay's response time and the level of current flowing through the motor.
Trip mechanism: When the motor is running within its normal operating conditions, the bimetallic elements and heater elements remain in a stable position. However, if the motor experiences an overload, the current flowing through the heater elements generates heat. This heat causes the bimetallic elements to bend, eventually reaching a critical point where they mechanically trip the relay.
Motor shutdown: When the thermal overload relay trips, it opens the motor circuit, cutting off power supply to the motor. This prevents further damage to the motor due to the overload.
Reset: After the relay has tripped and the motor cools down or the overload condition is addressed, the relay can be manually or automatically reset to allow the motor to restart.
Overall, the thermal overload relay acts as a safety device, preventing motors from operating under dangerous overload conditions and helping to protect the motor and associated equipment from damage or potential fire hazards. It is an important component in ensuring the safe and reliable operation of electric motors in various industrial and commercial applications.