Thermal relay - design and principle of operation

A thermal relay is an electronic device designed to protect electric motors from current overloads. There are several most common types of thermal relays: TRN, PTT, TRP, RTL. Overloads greatly affect the durability of electrical power equipment. For any object there is a dependence of the magnitude of the current on the duration of its flow. This dependence characterizes the reliability and time of operation of a particular equipment. If a current flows that is greater than the rated current, then this creates additional aging of the insulation due to an increase in temperature.

thermal relay,

Thermal relay with bimetallic plate

The plate of such a thermal relay consists of two layers. One has a higher heating temperature (a large temperature coefficient), the other has a lower one. These two parts are bonded to each other in the places of contact by welding or by rolling in hot state.Thermal relay is based on the work of this plate. If it warms up, then it bends towards the material with the smallest coefficient of thermal expansion. The most common chromium-nickel steel or non-magnetic. A bimetallic plate in a device such as a thermal relay heats up due to the fact that, under the influence of the load current, heat is generated in the plate. Often, a special additional heater is made to enhance heating. This process is called combined heating. The plate is heated with the help of a heater, which is streamlined by the load current, and due to the heat from the current passing through the bimetal. Bending under the action of heat, the plate with its free end affects the relay contacts and opens it.

thermal relay

Thermal relay and its basic properties

As mentioned above, the main feature of the relay is the dependence of its response time on load currents (time-to-time characteristic). When checking these values, one should take into account the state from which the relay will operate: from cool or from overheated. When selecting all these parameters, it is necessary to clearly understand what functions this or that protective device will perform.

RTL thermal relay

Thermal relay and its selection parameters

Select this unit based on the rated current, load and voltage. It is also necessary to take into account the time-current characteristic. The heating of the plate is directly dependent on the ambient temperature, therefore, if the temperature is very different from the nominal one, it is necessary either to select another heating element or to conduct a continuous additional adjustment of the relay. The deflection of the plate itself is usually a slow process. Therefore, the plate acts on the contact system through a special accelerating device. Thermal relay RTL, designed to protect directly electric motors and generators, can provide protection against asymmetric current components and phase failure. Electrothermal and thermal RTL can be installed together with starters or separately.

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