Thermal relays are actively used to protect electric motors from overloads.
Although quite a few types of these devices have been created, their field of application is almost the same.
Content
- Design features
- Main types
- Principle of operation
- Connection diagrams
- Selection recommendations
When choosing a thermal relay for an electric motor, it is necessary to know the design features of the device, as well as the principle of its operation.
Novice electricians, in addition, will have to figure out the connection diagrams of the device.
Design features
The device and principle of operation of a thermal relay (TR) is based on the Joule-Lenz law - allocated to section of the electrical circuit, the amount of heat is proportional to the resistance of this section and the square of the force current. This physical phenomenon is actively used today in thermal disconnectors. A small section of the electrical circuit, acting as a radiator, is wound on the insulator in a spiral.
The current passing through the electrical equipment also flows in this area. Next to the spiral there is a plate made of a bimetallic alloy. Upon reaching a certain temperature
it bends and acts on a group of contacts.The peculiarity of the plate is that it is made of two metals with different coefficients of thermal expansion, which make up one element.
The design of the device is shown in the figure.
Three phases of the electric motor power supply are connected to the conductors. The heating winding is located above the bimetallic plate, which reduces the number of false alarms of the device. The plates rest against a movable structural element, which acts on the disconnector mechanism. In the upper part of the device there are two groups of contacts (closed NC and open NO), as well as a spring-type current load regulator.
Main types
Since there are quite a few types of electric thermal relays, it is worth getting to know them. They differ in their field of application and even have their own classification. Among the main types of TR are:
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RTL is a three-phase device that protects the electric motor from phase imbalance, rotor jamming, as well as prolonged start-up. A relay of this type can be mounted on the contacts of a PML-type starter or work independently with the KRL terminal block. - RTT - the device is designed to operate in a three-phase power network and perform functions similar to RTL. The device can be used independently when mounted on a panel or installed on starters of the PME and PMA types.
- RTI is a three-phase relay necessary to protect motors from phase asymmetry, jamming and long starting. It can be mounted on two types of starters - KMI or KMT.
- TRN is a solid-state device designed for use in two-phase power grids. It allows you to control the start-up and operation of the electric motor. The device is equipped with a manual mechanism for returning the contacts to the initial position. Particular attention should be paid to the fact that the temperature of the external environment has practically no effect on the operation of the relay.
- RTK - a probe located in the body of the electrical equipment is used to control the temperature. This relay is thermal, it is able to control only one parameter.
- RTE is an alloy melting device. Its main conductor is made of a specific metal, which melts when it reaches a specific temperature. As a result, the electrical circuit is disconnected.
Thus, despite the existing differences, all these devices are designed to solve one problem - the protection of electrical equipment.
Principle of operation
Having become familiar with the design and types of devices, it is necessary to understand the principle of operation of a thermal relay. The manufacturer installs a technical data plate on each electric motor. One of the most important among them is the indicator of the nominal working electric current. Today, there are many units in use, during start-up or operation of which this value can be significantly exceeded.
If overloads are observed for a long time period, then the coils may overheat, the insulation layer will break down and the motor will fail. Protective TPs are capable of affecting the control circuit by opening contacts or giving a warning signal to service personnel. The devices are mounted in the power circuit in front of the engine in order to be able to control the indicator of the current passing through the unit.
During the setting of the protective device, the parameters are set to the larger side of the nominal passport value by an amount from 10 to 20%. The question of setting up the relay must be approached responsibly, since the disconnection of the circuit during overload does not occur instantly. This may take 5-20 minutes depending on various factors.
Connection diagrams
Most often, when connecting TP to magnetic starters, a group of normally closed contacts is used. In this case, they are connected in series to the "Stop" button. If such a scheme is used, then the normally open contacts can be used in the alarm system when the device is triggered. In more complex automated systems, this group of contacts is often used to activate emergency protocols for stopping service conveyor chains.
The TP connection can be done independently, but first you need to understand the design features of the device and the principle of its operation. Regardless of the type of device used and the number of terminals of the magnetic starter, there should be no difficulties with the introduction of the TR into the circuit.
The device is mounted in front of the electric motor, and its normally closed contacts must be connected in series with the equipment stop button.
Selection recommendations
When choosing a device, you must focus on the area of its use, as well as the available functionality. Problems with finding the right protective device almost never arise. At this time, special attention should be paid to the following points:
- Single-phase TRs with automatic reset return to their original state after a certain period of time. If the electric motor is still overloaded at this moment, the device will work again.
- Relays with an ambient temperature compensation system are capable of operating over a wide temperature range.
- Some models of devices have the ability to monitor the state of the phases. They will work not only when the motor overheats, but also if a phase failure was detected, their reversal or imbalance.
- There are TRs that can operate when the electrical equipment is underload. This situation is possible, for example, when the pump starts running dry.
The cost of the relay is in a wide price range. When choosing a device, you need to carefully study its technical characteristics. In the passport, you can also find recommendations for connecting the TR. However, this process is not difficult, and problems are extremely rare.
