Thermal relay - the device and the principle of operation

The thermal relay is an electrical device that protects the electric motor of any electrical appliance from the critical temperature values. Under high load conditions, an engine that drives any machinery of an electrical machine or electrical appliance consumes an increased amount of electricity. This energy can many times exceed the prescribed rate for the engine. As a result of the process of congestion, the temperature inside the electrical circuit begins to increase rapidly. This, of course, may well lead to the breakdown of this electrical appliance. To prevent this, electrical circuits additionally include special devices designed to stop the supply of electricity in any emergency (transients in electrical networks, overloads, etc.). Such a protective device was called a thermal relay (sometimes it can be found in the literature as a "thermal relay"). The main task of the thermal relay is to maintain the operating mode of the electrical appliance and its general operational capacity.

The thermal relay has in its internal design a special bimetallic plate. Under the influence of overloads and increased voltage in the electrical network, such a plate bends (deformed), and in its normal state it has a fairly flat surface. This bimetallic plate tightly closes the electrical contacts, and therefore the current can flow freely through the electrical circuit.

With overvoltage and an increase in the value of the electric current in the circuit, the temperature begins to increase rapidly. This contributes to the heating of the main element of the thermal relay - a two-layer metal plate. The latter begins to bend and breaks the flow of electricity, since the thermal relay is designed to cut off the load and voltage when the electric network is overloaded.

However, the bimetallic plate bends slowly enough. If the contact is mobile and directly connected to it, then a low deflection rate will not ensure the extinction of the arc that arises when the circuit breaks. Therefore, in the design of the thermal relay an accelerating device is provided, the so-called "jumping contact". From this it follows that the choice of a thermal relay is based on such a characteristic of it as the dependence of the response time on the magnitude of the electric current.

In view of this rupture, the machine will be stopped. After a while (usually half an hour - hour), the plate cools down and returns to its previous state, thereby restoring the circuitry of the electrical circuit. The device returns to the working state again.

A thermal relay can be of several types. The relay TRP (for single-phase load), TRN (for two-phase load), thermal RTT relay (for long-time overload in three-phase circuit) and RTL thermal relay (protection of motors from continuous overloads) are widely used.