The thermal relay is an electrical device that protects the electric motor of any appliance from critical temperatures. Under high load conditions, an engine that drives any mechanism of an electric machine or electrical appliance consumes an increased amount of electricity. This energy can be many times higher than the norm for the engine. As a result of the overload process, the temperature begins to rise rapidly inside the electrical circuit. This, of course, may well lead to damage to this appliance. To prevent this from happening, special devices are additionally included in the electric circuits , designed to cut off the power supply in case of any emergency conditions (transients in electric networks, overloads, etc.). Such a safety device was called a thermal relay (sometimes you can find the name "thermal relay" in the literature). The main task of the thermal relay is to maintain the operating mode of the electrical appliance and its overall operational ability.
The thermal relay has a special bimetallic plate in its internal design. Under the influence of overloads and increased voltage in the electric network, such a plate bends (deforms), and in its normal state it has a fairly smooth surface. This bimetallic plate tightly closes the electrical contacts, and therefore the current can flow freely through the electric circuit.
With overvoltage and an increase in the value of the electric current in the circuit, the temperature begins to skyrocket. 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 rather slowly. If the contact is movable and directly connected with it, then a low deflection rate will not ensure the extinction of the arc arising when the circuit breaks. Therefore, the design of the thermal relay provides an accelerating device, the so-called "jumping contact". It follows that the choice of a thermal relay is based on its characteristic such as the dependence of the response time on the magnitude of the electric current.
Due to this gap, the machine will be stopped. After some time (usually half an hour - an hour), the plate cools down and returns to its previous state, which restores the operation of the circuit of the electric circuit. The device returns to operational state.
A thermal relay can be of several types. The relay TRP (for single-phase load), TRH (for two-phase load), thermal PTT relay (for long-term overload in a three-phase circuit) and thermal relay RTL (protection of electric motors from prolonged overloads) are widely used.