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The basic principle of thermal protector

Thermal relays are protective devices for overvoltage protection of motors or other electrical equipment and electrical circuits. In actual operation of the motor, for example, if an abnormal condition occurs in the machine or an abnormality in the line causes the motor to encounter a load during the operation of the towing production machinery, the motor speed will decrease and the working current in the winding will increase, causing the motor winding temperature to rise. If the load working current is small and the load time is short, the motor winding does not exceed the allowable temperature rise, and this type of load is acceptable. However, if the load time is long and the load working current is large, the temperature rise of the motor winding will exceed the specified value, causing damage to the motor winding and shortening the service life of the motor. In severe cases, the motor winding may even burn out. Therefore, this kind of load is unbearable for the motor. The thermal relay is a protective device that uses the thermoelectric effect principle of the working current to cut off the motor circuit when a load that the motor cannot bear appears, providing overvoltage protection for the motor.

The basic principle of thermal relay is that the working current flowing into the hot part generates heat energy, which causes the bimetallic strips with different expansion coefficients to deform. When the deformation reaches a certain distance, it promotes the operation of the connecting rod, interrupts the control circuit, and then causes the contactor to lose power, interrupts the main line, and completes the overvoltage protection of the motor. As an overvoltage protection component for motors, relays have been widely used in production due to their small size, simple structure, and low cost.

Technical parameters:

Rated voltage: The maximum voltage value at which the thermal relay can operate normally, usually AC220V，380V，600V。

Rated current: The rated current of the thermal relay mainly refers to the working current of the thermal relay.

Rated frequency: Generally speaking, the rated frequency is based on45~62HZdesign.

Rated working current range: The range of the set working current is determined by its own characteristics. It describes that under certain working current conditions, the operating time of the thermal relay is inversely proportional to the square of the working current. It is mainly used for overvoltage protection of asynchronous motors. Its basic principle is that after the load working current passes through the thermal component, the bimetallic strip is heated and bent to promote the operating mechanism to pull the contact to operate, thereby interrupting the motor control circuit to complete the power-off and stop of the motor, thus playing the role of overvoltage protection. Since the transfer of heat energy takes a long time during the heating and bending process of the bimetallic strip, the thermal relay cannot be used for short-circuit protection, but can only be used for overvoltage protection of overvoltage protection thermal relays. The symbol isFR, the circuit symbol is as shown on the right:

Thermal relay is an electrical protection component. It is a protection device that uses the thermoelectric effect of the working current to promote the operating mechanism to close or interrupt the contacts. It is mainly used for overvoltage protection, phase failure protection, working current unbalance protection of motors, and control of heating conditions of other electrical equipment.

The thermal component is made of resistance wire, and its resistance value is small. It is connected in series in the main circuit of the motor during operation. The bimetallic strip surrounded by the resistance wire is made of two metal strips with different linear expansion coefficients pressed together, and the left side is fixed to the casing. When the working current in the thermal component exceeds its rated value and overheats, the upper layer of the bimetallic strip has a small thermal expansion coefficient, while the lower layer has a large thermal expansion coefficient, which makes the bimetallic strip expand after being heated.CanUpper bend,therebyThe buckle plate trips, and the buckle plate disconnects the normally closed contact under the tension of the spring. The contact is connected in series with the control circuit of the motor.CauseThe contactor's operating coil in the control circuit is de-energized,middleThe mainwantCircuit.