Thermal relay for electric motor: principle of operation, device, how to choose

During the operation of power equipment, current overloads reduce the durability. Protection in such situations serves as a thermal relay for the electric motor, which turns off the power supply in the event of non-standard circumstances.

We propose to understand the design, principle of operation, types and nuances of connecting protective devices. In addition, we will describe what parameters and characteristics should be taken into account when choosing a thermal relay.

The content of the article:

  • The design of thermal relays
  • The principle of operation of the device
  • How to connect a thermal relay
  • Nuances when installing the device
  • Existing device types
  • How to choose a thermal relay
    • Basic device specifications
    • Choosing a device according to the rules
  • Conclusions and useful video on the topic

The design of thermal relays

Thermal relays of all kinds have a similar device. The most important element of any of them is the sensitive bimetallic plate.

The value of the operating current is influenced by the temperature indicators of the environment in which the relay operates. A rise in temperature reduces the response time.

To minimize this effect, device developers choose the temperature of the bimetal as high as possible. With the same purpose, some relays provide an additional compensation plate.

Heat Relay Design

The device consists of a case, a nichrome heater, a bimetallic plate, a latch, a screw, a lever, a moving contact and a return button (+)

If the design of the relay includes nichrome heaters, they are connected in parallel, serial or parallel-serial circuit with the plate.

The value of the current in the bimetal regulate with shunts. All parts are mounted in the housing. The bimetallic U-shaped element is fixed on the axis.

The cylindrical spring rests on one end of the plate. At the other end, it is based on a balanced insulation block. Performs turns around the axis and is a support for the contact bridge, equipped with silver contacts.

To coordinate the current setpoint, the bimetallic plate is connected to its mechanism with its left end. Adjustment occurs due to the influence on the primary deformation of the plate.

If the magnitude of the overload currents becomes equal to or greater than the setpoint, the insulation block is rotated by the plate. During its tipping occurs disconnecting the contact contact device.

Sectional thermal relay

Thermal relay TRT in the section. Here, the main elements are: housing (1), setting mechanism (2), button (3), axis (4), contacts silver (5), contact bridge (6), insulating block (7), spring (8), bimetallic plate (9), axis (10)

Automatically, the relay makes a return to its original position. The self-return process takes no more than 3 minutes from the moment the protection is turned on. Manual reset is also possible; for this, a special Reset key is provided.

When using it, the device takes its original position in 1 minute. To activate the button, it is turned counter-clockwise until it rises above the body. The installation current is usually indicated on the shield.

The principle of operation of the device

Performing a protective function, circuit breaker disconnects power supply chains. The thermal relay differs from it in that when the load is exceeded it simply gives out a control signal. With this protection, small currents are switched in one control circuit.

In the diagram in front of the thermostat is magnetic switch. When the circuits open in emergency order, there is no need to duplicate the work of the contactor. Consequently, the material is not consumed for the manufacture of power contact groups.

The most popular are devices equipped with bimetallic plates. The plate itself consists of two similar elements.

One of them has a significant temperature coefficient, and the other one is somewhat smaller. These two components fit snugly together.

Bimetallic plates

Since the components of the bimetallic plate are made of a pair of dissimilar metals having different expansion coefficients, the heat causes it to bend and interact with the contacts

Such a rigid bond is provided by welding or hot rolling. Due to the fact that the plate is fixed motionless, when heated, its bending towards the element with a lower temperature coefficient is observed. This principle is taken as the basis for creating thermal relays.

In their production, nickel-chromium steel and non-magnetic steel are used, which have a large value of the temperature coefficient. As a material with a small value of this parameter, use Invar - the connection of nickel with iron.

Circuit thermostat

According to this scheme operates a thermal relay. The loose end of the bimetallic plate during its deflection affects the contacts of the thermal switch (+)

A plate of bimetal heat the load currents. They often flow through a special heater. There is also a combined heating, in which, besides the heat given off by the heater, the bimetal also warms the current passing through it.

How to connect a thermal relay

A closed contact (normal connected), with which the thermal module is connected to a magnetic starter, is designated as NC or NC, which is decoded as normally closed. The letter combination of NO denotes a normally open contact.

In a simple scheme, it is used to give a signal indicating that the motor protection has been activated due to exceeding the threshold temperature.

When embedded into complex control circuits, it is capable of generating, in an emergency order, a signal for removing from the operating state of the conveyor.

Relay placement

The thermal relay is placed behind the contactors, but in front of the electric motor. Connecting the normal connectde contact to the “Stop” button on the control panel is carried out in a sequential scheme (+)

The designation of the terminals of the contactors dictates GOST: normally closed - 95-96, normally open - 97-98. The starter is connected to the first pair, the second is used for signaling circuits. Since the engine and the thermal relay must be protected from short-circuit, the circuit must contain a circuit breaker.

The device layout includes the “Test” and “Stop” or “Reset” buttons. Using the first one, the operability is checked, and the second one - the protection is manually disabled.

After turning on the protection, the motor is restarted using the rotary platoon switch. On the glass cover, the product is marked and sealed.

If we proceed from the type of connection, we can distinguish two large groups of thermal relays:

  • first group - devices mounted behind the magnetic starter and those that are connected using jumpers;
  • second group - devices mounted directly on the contactor starter.

In the latter case, when starting up, the main load falls on the contactor. Here the thermal module is equipped with copper contacts connected directly to the starter inputs.

Designation of relay elements on the diagram

Thermal relay circuit. The marks of control elements and conclusions are marked on it. Different models may have different designations (+)

To TR connect the wires from the engine. The relay itself in this scheme represents an intermediate node analyzing the current flowing in transit to the engine from the magnetic starter.

Nuances when installing the device

The speed of the thermal module can affect not only the current overload, but also the external temperature. Protection will work even in the absence of overloads.

It also happens that under the influence of forced ventilation the motor is subject to thermal overload, but the protection does not work.

To avoid such phenomena, you should follow the recommendations of experts:

  1. When selecting a relay, focus on the maximum allowed response temperature.
  2. Protection to be mounted in the same room with the protected object.
  3. For installation, choose places where there are no heat sources or ventilation devices.
  4. It is necessary to adjust the thermal module, focusing on the real ambient temperature.
  5. The best option is the presence in the design of the relay built-in thermal compensation.

An additional option of the thermal relay is protection in case of a phase failure or a fully mains supply. For three-phase motors, this moment is particularly relevant.

Thermal relay serial connection

The current in the thermal relay moves in series through its heating module and on to the engine. With the starter winding device connect additional contacts (+)

In case of malfunctions in one phase, the two others take on a larger current. As a result, overheating quickly occurs, and further off. With inefficient operation of the relay can fail and the engine and wiring.

Existing device types

The class of thermal relays includes several types: TRN, RTL, TRP, RTI, PTT. The use of each is due to the design features.

Two-phase current relay (TRN), used mainly for electrical protection of asynchronous motors with a short-circuited rotor. As a rule, they operate on mains with a nominal value of up to 500 V and a frequency of 50 Hz.

Equipped with a relay manual contact control mechanism. Dimensions of TRN make it possible to build them into complete devices of both closed and open type stations, coordinating the work of the drives. They do not perform the function of protection against short-circuit and they themselves need it.

Relay TRP have a vibration resistant, shockproof casing. Designed for the protection of three-phase asynchronous motors operating in conditions of high mechanical loads.

They are designed for a maximum current of 600 A and a maximum voltage of 500 V, and in circuits with a constant current - 440 V. Automatic equipment is insensitive to external temperature and works when the indicator exceeds 200 ° C.

RTL devices - three-phase, in addition to motor overload protection, prevent the rotor from jamming. They insure it against breakdowns in case of phase skew, with a prolonged start.

They work autonomously with KRL terminals and in modification with PML magnetic starter. Current working period - from 0.10 to 86 A.

Contactor and thermal relay

Contactor paired with thermal relay. When the device is triggered, a normally closed and normally open contact synchronously changes its position

PTT - the device protects asynchronous motors from current surge, phase skew, jamming and other abnormal situations. It is used both as an independent device and as a built-in PMA, PME actuators.

Product three-phase RTI It is endowed with the same functions as the previous one, but is used in the modification with the KTM and KMI starters.

How to choose a thermal relay

The engine needs a relay to protect when, for technological reasons, there is a potential threat of overloading it. The second case is the need to limit the start-up time in low voltage conditions.

These requirements are contained in the relevant instructions. In which a request is made to equip a protective product with a time delay. Implement all this with the help of thermal relays.

Basic device specifications

The basic data of the device protecting the engine are:

  1. The speed of contacts depending on the parameters of the current - time-current indicator.
  2. Operating current at which the TP is triggered.
  3. Limit current setting settings. In all devices manufactured by different manufacturers, this parameter differs slightly. Exceeding the nominal by 20% entails the operation of the device after 25 minutes.
  4. The rated current of the working bimetallic plate. This refers to the value, above which the relay does not turn off immediately.
  5. The current range in which the relay operates.

Information about the thermal relay can be obtained by decoding its label. The symbol for the type of performance may differ.

Thermal Switch Marking

Contactor paired with thermal relay. When the device is triggered, a normally closed and normally open contact synchronously changes its position (+)

Locations of domestic TP regulated by GOST 15150. Their work is influenced by such moments as the height of rise above sea level, vibration, shock, acceleration.

All these nuances of manufacturers reflect in the labeling of their products. Some of them additionally include information on work opportunities in the presence of hazardous substances and explosive gases.

Choosing a device according to the rules

Requirements for thermostat set forth in the instructions. It also stipulates that the defense should have a time delay. Implement all requests using special devices.

Time-current characteristics

Time-current characteristics of the TP and the protected engine. At short circuit currents, the heating elements of the relay become thermally unstable (+)

Analyzing the time characteristics of the TP, you need to take into account that the actuation can occur from a superheated or cold state.

Impeccable protection suggests that the curve depicting the optimum for trouble-free the functioning of the equipment the dependence of the current carrying time on the magnitude of the current for the relay and engine, different. The first must be lower than the second.

Relay Selection Table

The table shows the technical characteristics of the thermostat type RTL. According to it, you can choose a protective device with the necessary parameters for engine power (+)

The correct selection of the protective product is carried out on the basis of a parameter such as a working rated current. Its value is related to the rated load current of the motor.

Both international and domestic standards stipulate that the rated current of the motor is similar to the set value of the operation of the thermal relay.

This means that the inclusion in the operation of the device occurs when overloading from 20 to 30% or at Isr.x1.2 or 1.3 no later than 20 minutes.

On this basis, the choice should be made so that the failure current of the TP exceeds the rated current of the covered object by an average of 12%. The value of In is displayed in the passport of the device and on a plate attached to the case.

Based on it, both the TR and the starter corresponding to it are selected. The scale of the relay is calibrated in amperes and, as a rule, corresponds to the value of the current setpoint.

An example is the selection of a thermal relay for an asynchronous motor connected to a 380 V network with a power of 1.5 kW.

The working rated current for it is 2.8 A, which means that for a thermal relay the threshold current will be: 1.2 * 2.8 = 3.36 A. According to the table, the choice should be stopped on the RTL-1008, whose adjustment range is in the range from 2.4 to 4 A.

Protection operation

When a protection is triggered, first eliminate the root cause of the shutdown, and then return the “tweezer” to its original state using the return key

When the engine's passport data is unknown, the current is determined by using special devices - current-measuring clamp or multimeter with the appropriate option. Measurements are carried out on each of the phases.

It is important when choosing to pay attention to the voltage indicated on the device. If you plan to use the tandem TR-starter, you need to consider the number of contacts.

When the device is turned on in a three-phase network, a module is needed that has the function of protection for cases of conductor burnout or phase skew.

Conclusions and useful video on the topic

The scheme of effective motor protection:

Components of the thermal relay:

The principle of the interaction of various devices in different ways of connecting the thermal relay is the same. For better orientation in the diagrams, one should be able to “read” the device labeling. Ideally, all wiring should be carried out by a master who is authorized to work in high voltage conditions.

Is there anything to supplement, or have any questions on the choice and use of a thermal relay? You can leave comments on the publication, participate in discussions and share your own experience using devices. The form for communication is in the lower block.

Instructions for assembling a solid-state relay do it yourself

Instructions for assembling a solid-state relay do it yourselfRelays, Contactors, Sensors

Solid-state relay (TTR) is a device from a series of electronic components of non-mechanical action. The lack of mechanics opens up more opportunities for electronics lovers to make a solid-state ...

Read More
Electromagnetic relay: device, types, marking, connection and adjustment

Electromagnetic relay: device, types, marking, connection and adjustmentRelays, Contactors, Sensors

The conversion of electrical signals into the corresponding physical quantity - movement, force, sound, etc., is carried out with the help of drives. The drive should be classified as a transducer...

Read More
Pressure switch for compressor: device, labeling, connection and adjustment

Pressure switch for compressor: device, labeling, connection and adjustmentRelays, Contactors, Sensors

Using an air pneumatic relay allows you to automate the filling of the receiver of the compressor with compressed gas. The equipment operator with the pressostat does not need to monitor the proce...

Read More