A thermal overload relay is an overload relay that utilizes the principle of current heating, using an electrical thermistor bimetallic strip as its sensing element. The electrical thermistor bimetallic strip is made by heating and rolling two alloys with significantly different coefficients of linear expansion. When heated, the bimetallic strip bends from the high-expansion layer (active layer) to the low-expansion layer (passive layer). When the current is too high (exceeding the set value), the element actuates due to heat, thereby cutting off the power supply to the controlled circuit and the protected equipment through its normally closed contact.
I. Function of Thermal Overload Relay
1. Protecting the Motor: The main function of a thermal overload relay is to protect the motor from overload damage. When an motor experiences an overload, its winding temperature rises. The thermal overload relay detects the temperature change of the windings to determine if the motor is overloaded. If an overload occurs, the thermal overload relay will promptly cut off the power supply to prevent damage to the motor.
2. Power Line Protection: In addition to protecting the motor, the thermal overload relay can also protect the power line. When the motor is overloaded, its current increases, which may cause the power line to overheat or melt. The thermal overload relay detects changes in current to determine if the power line is overloaded. If an overload is detected, the thermal overload relay will promptly cut off the power supply to prevent damage to the power line.
3. Improve power system safety: Thermal overload relays can effectively prevent damage to motors and power lines, thereby improving the safety of the power system. By using thermal overload relays, the failure rate of the power system can be reduced, minimizing economic losses and social impact caused by failures.
II. Working principle of thermal overload relay
The working principle of a thermal overload relay is mainly to determine whether the motor is overloaded by detecting changes in the temperature and current of the motor windings. Specifically, a thermal overload relay mainly includes the following components: a heating element, a bimetallic strip, an electromagnetic coil, and contacts.
1. Heating Element: The heating element is the core component of a thermal overload relay, and its main function is to generate heat. Heating elements are typically made of materials such as resistance wire or heating elements. When current passes through the heating element, heat is generated. The amount of heat generated by the heating element is directly proportional to the magnitude of the current; therefore, the magnitude of the current can be indirectly detected by detecting the heat generated by the heating element.
2. Bimetallic Strip: The bimetallic strip is another important component of a thermal overload relay, its main function being to sense temperature changes. A bimetallic strip is typically made of two metal strips of different materials bonded together, one of which has a higher coefficient of thermal expansion. When heat generated by the heating element is transferred to the bimetallic strip, it bends. Due to the different coefficients of thermal expansion of the two metals, the degree of bending of the bimetallic strip is proportional to the temperature change. Therefore, the degree of bending of the bimetallic strip can be detected indirectly to detect temperature changes.
3. Electromagnetic Coil: The electromagnetic coil is the control component of the thermal overload relay. Its main function is to drive the contacts to actuate via electromagnetic force. When the bimetallic strip bends, one end of it will contact the electromagnetic coil. When the bimetallic strip bends to a certain extent, one end will contact the electromagnetic coil, energizing it. Once energized, the electromagnetic coil generates electromagnetic force, driving the contacts to actuate.
4. Contacts: The contacts are the actuating components of the thermal overload relay, and their main function is to cut off the power supply. When the electromagnetic coil is energized, it generates electromagnetic force, driving the contacts to actuate. After the contacts actuate, the power supply to the motor is cut off, thus protecting the motor.
In summary, thermal overload relays are commonly used motor protection devices. Their main function is to promptly cut off the power supply when a motor experiences an overload, preventing damage to the motor. The working principle of a thermal overload relay is primarily based on detecting changes in the temperature and current of the motor windings to determine if an overload has occurred. By using thermal overload relays, motors and power lines can be effectively protected, improving the safety of the power system.
