What is a thermal relay and what is it for?

Construction

Let's start by telling you what the thermal protection relay consists of. The basis of the work of the RT is the phenomenon described by the physical law of Joule-Lenz:

The amount of heat allocated to the section of the electrical circuit is proportional to the square of the current and resistance of this section.

This phenomenon is successfully used in a thermal release. A short section of the chain, which acts as a heat radiator, is wound up by a spiral on the insulator. All the current passing through the electric machine passes through this section. Directly near the spiral is a bimetallic plate, which when heated, bends and acts on the contact group. The plate consists of two dissimilar metals having different expansion coefficients when heated, combined into one element.

The photo below shows the section of the operating device. Through the conductors passes three phases of power to the electric motor. The heating coil is located on top of the bimetallic plate to reduce false triggering from external influences. The plates abut the movable bar, which pushes the release mechanism. Above is the spring regulator of the current setting, for fine-tuning the operation limits, and two groups of contacts( open NO and closed NC).

How

works As you see the thermal relay, now we go further and tell you how this device works. As we said earlier, RT protects the engine from prolonged overload.

Each motor has a rating plate with the nominal operating current. There are mechanisms in the operation of which the working current can be exceeded, both during start-up and in the work process. With prolonged exposure to such overloads, windings overheat, insulation failure, and failure of the engine itself occur.

This thermal protection relay is designed to act on the control circuit, by deactivating the circuit, opening the contacts, or giving a warning signal to the duty personnel closing the contacts. The device is installed after the start contactor in the power circuit in front of the motor in order to monitor the current passing.

Parameter setting is made upwards from the rated current of the motor, by 10-20%, according to the passport data. Turning off the machine does not happen immediately, but after a certain time. It all depends on the ambient temperature and the overload current, and can vary from 5 to 20 minutes. Incorrectly selected parameter will lead to false triggering or ignoring overload and equipment failure.

Graphical designation of the device in the diagram according to GOST:

For more details on how the thermal relay is arranged and how it works, you can by viewing this video:

Purpose

I would like to say immediately thatthere are different types and types of thermal relays and accordingly the scope of each classification is its own. In brief, let's talk about the appointment of the main types of devices.

RTL - three-phase, designed to protect the motor from overload, phase distortion, delayed starting or jamming of the rotor. The contactors of the PML are attached to the contacts or as an independent device with KRL terminals.

PTT - for three phases, designed to protect short-circuited motors from overcurrent currents, phase distortion, jamming of the rotor of the engine, delayed start of the mechanism. Can be fixed to PMA and PME starters, as well as independently installed on the panel.

RTI - protect the electric motor from overload, phase asymmetry, long start-up and jamming of the machine. A three-phase thermal relay is attached to the KMT and KMI series starters.

TRN - two-phase relay, controls the operating and starting mode, has only a manual return of contacts, the operation of the device depends little on the ambient temperature.

Solid state three-phase relay, do not have moving parts, do not depend on the environment, they are used in explosive areas. It monitors the load current, acceleration, phase failure, jamming of the mechanism.

RTK - temperature control is carried out by a probe located in the body of the electrical installation. It is a thermal relay, and controls only one parameter.

RTE is an alloy melting relay, the conductive conductor is made of a metal alloy, melts at a certain temperature and mechanically breaks the chain. This thermal relay is integrated directly into the monitored device.

As can be seen from our article, there is a great variety of control over the condition of electrical installations, differing in type and appearance, but equally fulfilling the protection of electrical equipment. This and all that I wanted to tell you about the device, the principle of operation and the appointment of thermal relays. We hope that the information was useful and interesting for you!

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