Working Principle and Structure of a Thermal Relay

The thermal relay is mainly used for overload protection of motors, and its working principle is based on the thermal effect of current.  

1. Core Components The thermal relay mainly consists of the following parts:  

(1) Thermal element: Usually composed of temperature-sensitive components such as bimetallic strips or thermistors.  

(2) Contact system: Normally closed contacts (NC) are connected in series in the control circuit and open in case of overload.  

(3) Operating mechanism: Converts the deformation of the bimetallic strip into contact operation.  

(4) Reset button: Used to manually reset the relay.  

(5) Current setting device: Used to adjust the operating current of the thermal relay.  

(6) Temperature compensation element: Compensates for environmental temperature changes.  

2. Working Process The working process of the thermal relay is as follows:  

(1) Normal operation:  

• The motor current is within the rated value.  

• The heat generated by the thermal element slightly bends the bimetallic strip, but not enough to activate the contacts.  

(2) Overload state:  

• The motor current exceeds the rated value.  

• The thermal element generates more heat, causing the bimetallic strip to bend further.  

• When the bending reaches a certain degree, it drives the linkage mechanism to operate, disconnecting the control circuit.  

• The motor powers off, achieving overload protection.  

(3) Phase loss state:  

• When one phase is disconnected,  

• The bimetallic strip of that phase cools down, driving the differential mechanism to disconnect the relay.  

3. Equivalent Circuit Model The equivalent circuit model of the thermal relay mainly considers the thermal element and the contact part:  

(1) Thermal element part: Connected in series with the main circuit, equivalent to a resistive element whose resistance changes with temperature.  

(2) Contact part: Normally closed contacts connected in series with the control circuit, equivalent to a temperature-controlled switch.