Working principle of thermal relay and selection precautions

Thermal relaysare a crucial member of the relay family, frequently used in production and holding significant importance.

Working Principle of Thermal Relays

The heating element in a thermal relay, which generates heat, should be connected in series with the motor circuit. This allows the thermal relay to directly detect motor overload currents. The sensing element of a thermal relay is typically a bimetallic strip. A bimetallic strip is a composite of two metal sheets with different linear expansion coefficients, mechanically pressed together. The layer with the larger expansion coefficient is called the active layer, while the layer with the smaller expansion coefficient is called the passive layer. When heated, the bimetallic strip expands linearly. Due to the different linear expansion coefficients of the two metal layers and their close contact, the bimetallic strip bends toward the passive layer. The mechanical force generated by this bending causes the contacts to operate.

Dissecting a Thermal Relay

A thermal relay consists of a heating element, a bimetallic strip, contacts, and a transmission and adjustment mechanism. The heating element is a low-resistance resistor wire connected in series with the main circuit of the protected motor. The bimetallic strip is formed by pressing two metal sheets with different thermal expansion coefficients together. When the motor is overloaded, the current flowing through the heating element exceeds the set current, causing the bimetallic strip to bend upward due to heat, separating from the plate and opening the normally closed contact. Since the normally closed contact is connected to the motor's control circuit, its opening de-energizes the connected contactor coil, thereby opening the contactor's main contacts and de-energizing the motor's main circuit, thus providing overload protection.

Function of a Thermal Relay

It is primarily used to provide overload protection for asynchronous motors. Its operating principle is that when the overload current passes through the thermal element, the bimetallic strip heats and bends, pushing the actuator and actuating the contacts, thereby disconnecting the motor's control circuit and shutting down the motor, thus providing overload protection. Because the heat transfer from the bimetallic strip during the bending process takes a long time, thermal relays cannot be used for short-circuit protection; they can only be used as overload protection for overload protection thermal relays.

Purpose of thermal relay

Thermal relays are used primarily for circuit overload protection.

 Their operating principle is that when an overload current passes through a thermal element, the bimetallic strip heats and bends, pushing the actuator and actuating the contacts, thereby disconnecting the circuit and stopping the load, thus providing overload protection. Because the heat transfer from the bimetallic strip during its bending process takes a long time, thermal relays cannot be used for short-circuit protection, but only for overload protection.

Precautions for selecting thermal relays