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Commonly used thermal relay function

time:2022/1/30   source:华天电力  reading:1203 time

The thermal relay generates heat by the current flowing into the thermal element, which deforms the bimetallic sheets with different expansion coefficients. When the deformation reaches a certain distance, it pushes the connecting rod to act, so that the control circuit is disconnected, so that the contactor is de-energized. The main circuit is disconnected to realize the overload protection of the motor. As the overload protection element of the motor, the relay has been widely used in production due to its advantages of small size, simple structure and low cost.


Protection Relay Test Set.png

Commonly used thermal relay types are:


1. Fusible alloy type


 Using the overload current to generate heat, the fusible alloy will melt when it reaches a certain temperature value, and the relay will act.


2. Thermistor type


 A thermal relay made by utilizing the property that the resistance value of metal changes with temperature.


3, bimetallic sheet


 The bimetallic sheets with different expansion coefficients (such as manganese-nickel and copper sheets) are heated and bent to push the lever and make the contacts move.


Main technical parameters of thermal relay


Rated voltage: the highest voltage value that the thermal relay can work normally, generally AC 220V, 380V, 600V.


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


Rated frequency: Generally speaking, its rated frequency is designed according to 45~62HZ.


Setting current range: The setting current range is determined by its own characteristics. It describes that the action time of the thermal relay is proportional to the square of the current under a certain current condition.


The function of the thermal relay is: it is mainly used to protect the asynchronous motor from overload. Its working principle is that after the overload current passes through the thermal element, the bimetal sheet is heated and bent to push the action mechanism to drive the contact action, thereby breaking the motor control circuit. Open to realize the motor power off and stop, and play the role of overload protection. In view of the fact that the heat transfer takes a long time during the heating and bending of the bimetal, the thermal relay cannot be used as a short-circuit protection, but can only be used as an overload protection.

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