Thermal relay: operating principles, types, wiring diagram + regulation and labeling

Durability and reliability in the operation of any installations with electric motor depends on various factors. However, to a large extent on the service life of motor current overload influence. To warn them connect the thermal relay, protecting the main working body electric machines.

We'll show you how to pick up the device, predicting the maturing of emergencies exceeding the maximum allowable current indicators. In the present article we described the principle of action, given the types and their characteristics. Give advice on the connection and correct setting.

The content of the article:

  • Why protective devices?
  • Design and operation TP
  • The basic characteristics of the current relay
  • Types of thermal relay
  • Connection, adjustment and marking
    • Wiring Devices
    • Subtleties adjustment relay elements
    • Markings for electric relays
  • Conclusions and useful videos on the topic

Why protective devices?

Even if the drive is intelligently designed and used without violating the basic rules of operation, there is always the risk of malfunctions.

For emergency operating modes include single phase and multi-phase faults, thermal overload electrical jamming of the rotor and the destruction of the bearing unit, the open phase.

Functioning in a mode higher loads, the electric motor consumes a huge amount of power. And at regular indicators exceeds the nominal voltage equipment is heated intensively.

As a result, quickly wear out the insulation, leading to a significant reduction in the service life of electromechanical installations. To eliminate such a situation, in the electric current path is connected the thermal relay. Their main function - to ensure normal operation of consumers.

They shut off the motor with a certain time delay, and in some cases - immediately to prevent the destruction of or damage to the insulation of individual power plant components.

thermal protection device

Current relay permanently protects the electric motor by phase discontinuity and technological accelerations and braking the rotor. These are the main reasons due to which there are emergency modes

In order to prevent lowering of insulation resistance employ protective devices, but if tasked to prevent a breach of the cooling, connect special devices built-in thermal protection.

Design and operation TP

Structurally standard electrothermal relay is a small device which consists of a sensitive bimetallic plate, the heating coil, lever-spring system and electric contacts.

Bimetallic plate is made from two dissimilar metals, usually chromium-nickel and invar steel, firmly joined together during the welding process. A metal has a large temperature coefficient of expansion than the other, so they are heated at different rates.

When overcurrent unfixed portion to flex plate material with a lower coefficient of thermal expansion. This exerts a force on a protection device contacts and activates the system electrical disconnection when overheated.

In most models of mechanical thermal relay has two contact groups. One pair - normally open, and the other - closed permanently. When the safety device is activated, the contacts change state. The first lead and the second becomes open.

electronic relay

The electronic TP involve special sensors and sensitive probes that respond to the current increase. The microprocessor such protective devices are programmed parameters that define the situation when it is necessary to disconnect the power supply

Detects a current transformer integrated, whereupon electronics processes the received data. If the current value at this point in time is greater than the set point, the momentum instantly transmitted directly to the switch.

Opening the external contactor, relay electronic mechanism locks the load. itself thermal relay for motor mounted on the contactor.

The bimetallic plate may be heated directly - by the action of the peak load current to the metal strip or indirectly, by a separate fuser. Often these principles are combined in one unit of thermal protection. When combined heating device has a better performance.

bimetallic strip

After cooling plate returns to its original state. Switching contacts are closed automatically or you want to force to bring them to a closed state

The basic characteristics of the current relay

The main characteristic of the thermal protection switch is pronounced dependence of the response of the current flowing through it - the larger the value, the faster it will work. This indicates a certain inertia of the relay element.

The directional movement of the carrier particles via any electrical charge, circulation pump and electric boiler generates heat. At rated current of its permissible length tends to infinity.

And at values ​​exceeding the nominal parameters, equipment raises the temperature, which leads to premature deterioration of the insulation.

Turn off motor

Open circuit instantly blocks the further growth of temperature characteristics. This makes it possible to prevent overheating of the engine and prevent the emergency shutdown of the electrical installation

Rated load of the motor - the key factor that determines the choice of instrument. Indicator in the range of 1.2-1.3 indicates successful activation in overcurrent of 30% over the time interval of 1200 seconds.

The duration of overload may adversely affect the electrical condition - only for short periods of motor winding is heated for 5-10 minutes, which has a small mass. A heated during prolonged whole engine, with serious breakdowns. Or even may need to replace burned-out equipment with new.

In order to maximize protect the object against overload, should be specifically used thermal relay under it, while operation which will correspond to the maximum permissible value of a specific overload motor.

In practice, to collect Voltage monitoring relay for each engine type is inappropriate. Employ one relay element to protect the motors of various design. At the same time guarantee reliable protection in the full operating range limited by the minimum and maximum load, it is impossible.

The dependence of the load current

Increasing the current indicators do not immediately lead to a dangerous state of emergency equipment. Before the rotor and the stator is heated up to the temperature limit, it will take some time

So there is no urgency to the protective device to respond to each, even a slight increase in current. The relay must switch off the electric motor only in cases where there is a danger of rapid deterioration of the insulation layer.

Types of thermal relay

There are several types of relays for protection of electric motors against phase failure and current overload. They differ in design features, the type used by the MP and the use of different engines.

TRP. Single-pole switching device with a combined heating system. Designed to protect three-phase asynchronous motors from overcurrent. TRP is used in electric DC basic voltage under normal operation no more than 440 V. Characterized by resistance to vibration and shock.

RTL. Protects the motor in the following cases:

  • in case when one of the three phases;
  • asymmetry and overload currents;
  • Long starts;
  • jamming actuator.

They can be fitted with terminals CRL separately from or magnetic actuators mounted directly on PML. Mounted on standard rails type, protection class - IP20.

PTT. Protect the three-phase asynchronous machines with short-circuit rotor of the tightened starting mechanism, prolonged overload and asymmetry, that is, phase unbalance.

Relay type PTT

PTT can be used as component parts in a variety of motor control circuits, as well as for integration into starters PMA series

TRN. Two-phase switches, which control the electrical motor starting and operation. Virtually independent of the temperature of the environment, have only manual reset system contacts in the initial state. They can be used in DC networks.

RTI. The electrical switching apparatus with a constant, albeit low power consumption. Mounted on the contactor KMI series. Working together with the fuses /breakers.

Solid-current relay. Are small electronic devices into three phases, in which the design has no moving parts.

They operate on the principle of calculating the average values ​​of the engine temperature, for carrying out permanent monitoring of the working and starting current. Different immune to changes in the environment, and therefore is used in hazardous areas.

RTC. Starting switches to control the temperature in the casing of electrical equipment. Used in automation applications where thermal relays act as accessories.

Thermal overload relays

To ensure reliable operation of the electrical relay element must possess qualities such as sensitivity and speed, and selectivity

It is important to remember that no one type of the above-discussed devices is not suitable for circuit protection against short circuits.

thermal protection device only prevents accidental modes that arise in the abnormal operation mechanism or overload.

Electrical equipment may burn out before the relay action. For comprehensive protection they need to be complemented by fuses or circuit breakers compact modular design.

Connection, adjustment and marking

Overload switching device, unlike the electric machine does not break the power circuit itself, but only sends a signal to temporarily disable the object when the emergency mode. Normally energized contact he works as a button "stop" contactor and connected in series.

Wiring Devices

The design of the relay does not need to repeat everything a function of the contacts with the successful activation, since it connects directly to the MP. This design allows to save material for the power contacts. It is much easier to connect the control circuit a small current, turning off the three phases once with great.

In many circuits connect the thermal relay to the object used permanently closed contact. It was successively connected to the button "stop" control panel and designated NS - NC or NC - normal connected.

Open contact with such a circuit may be used to initialize the thermal trip protection. Scheme connecting electric motors, which are connected in thermal protection relays may vary significantly depending on the presence of additional devices or technical features.

The circuit relay connections

In a simple scheme the standard TR is connected to the output of the low-voltage starter for electric motors. Auxiliary contact device compulsorily connected in series with the actuator coil

This will provide protection against electrical overloads. In the case of an unacceptable value exceeding the limit current relay element opens the circuit, momentarily disabling MT and the motor from the power supply.

Connection installation and thermal relays tend to produce together with the magnetic actuator designed for switching and start the electric drive. However, there are species that are mounted on a DIN-rail or a special panel.

Subtleties adjustment relay elements

One of the main requirements for the motor protector is a clear effect devices when an emergency operation of the motor operation. It is important to pick it up and adjust the settings as false positives are absolutely unacceptable.

Connecting current relay

Electrothermal relay that is best suited to a particular type of engine on all technical parameters can provide protection against overload on each phase, to prevent protracted installation start, to prevent accidents from jamming rotor

Among the advantages of using current protection elements should also note the rather high speed and a wide range of operation, ease of installation. To ensure the timely disabling the motor in case of overload, thermal overload relays must be configured on a special platform / stand.

In this case, inaccuracy is eliminated because of the natural variation of uneven nominal currents in NEs. To test the protective device on the stand is used the method of fictitious loads.

After a thermocouple was passed electric current low voltage to simulate actual heat load. After that, the timer accurately determine the exact response time.

When configuring the basic settings should strive to such indicators:

  • at 1.5 times the current device must turn off the motor after 150 s;
  • at 5... 6-fold current it must disconnect the motor 10 from.

If the response time is incorrect, the relay element must be adjusted by a control screw.

relay element

To work correctly, be sure to set the unit to the largest permissible electric motor current and temperature of the air

This is done in cases when the nominal motor current and NEs are different, and if the ambient temperature is lower than the nominal (+40 ºC) more than 10 degrees centigrade.

electrothermal tripping current switch decreases with increasing temperature around the object in question, as the heating of the bimetallic strip is dependent on this parameter. When significant differences need to further adjust the TP or find a more suitable thermocouple.

Sharp fluctuations in temperature characteristics strongly influence the performance of the current relay. It is therefore very important to choose NE, ability to perform the basic functions based on actual values.

Guide TP connection

TR is recommended to place in the same room with the protected electrical installation. They must not be installed close to heat generators, heating furnaces and other heat sources

To relay with temperature compensation, these restrictions do not apply. Setting current protective device can be adjusted 0,75-1,25h rated current fuser. The setting is performed in stages.

First calculate a correction E1 without temperature compensation:

E1= (IMr.-Ine) / C × Ine,


  • IMr. - nominal load current of the motor,
  • Ine - the nominal operating current of the heating element in the relay,
  • c - the scale interval, that is, the eccentric (c = 0,055 for protected starters, c = 0,05 for the public).

The next step - determining the amendments E2 at ambient temperature:

E2= (Ta-30)/10,

where ta (Ambient temperature) - ambient temperature in degrees Celsius.

The last step - finding a total correction:

E = E1+ E2.

The total correction E can be labeled "+" or "-". If the result is a fractional value, it is required to be rounded up to the whole at lower / higher modulus side, depending on the nature of the current load.

To set up a relay, the cam is transferred to the total value of the resulting amendments. High temperature reduces the dependence of the work of the protective device against external indicators.

Configuring the current relay

Thermal overload relays permits manual fine adjustment of the device operating current value within ± 25% of the rated current of the electromechanical installation

Adjustment of these parameters by a special lever, movement of which changes the initial bending of the bimetallic plate. Setting the operating current in a wider range by replacing the thermocouple.

In modern switching overload protection devices have a test button that allows you to check the serviceability of the device without a special stand. There is also a button to reset all settings. They can be reset automatically or manually. In addition, the product comes complete with an indicator of the current state of the appliance.

Markings for electric relays

Protective devices are chosen depending on the power of the electric motor. The main part of the key features is hidden in the legend.

Marking thermal relay

Here is the marking of thermal relay KEAZ plant. It is important when choosing to pay attention to the nominal current value of the model, so that it was sufficient

Focus should be on individual points:

  1. Setting range current values ​​(in parentheses) from different manufacturers differ minimally.
  2. Letter symbols of a particular type of performance may vary.
  3. Climatic performance often supplied as a range. For example, UHL3O4 need to read as follows: UHL3O4.

Today you can buy a variety of variations of the device: relays for AC and DC, monostable and bistable, devices with deceleration when on / off, thermal protection relays accelerating elements TR without retaining coil with a single winding or several.

These settings are not always displayed in the marking device, but must be specified in the data sheet electrical products.

With the device, and marking species electromagnetic relay acquaint next articleWith which we encourage you to read.

Conclusions and useful videos on the topic

Design and operation of the current relay for the effective protection of the motor unit on the example of the PTT 32P:

Proper protection against overloads and phase failure - the key to long-term trouble-free operation of the electric motor. Video on how to relay element responds in the event of abnormal operation of the mechanism:

How to connect the thermal protection device to the MP, schematic diagrams electrothermal relays:

Relay thermal overload protection - a mandatory element of any functional electric control system. It responds to the current flowing to the motor, and is activated when the temperature reaches the electromechanical setting limit values. This makes it possible to maximize the life of the environmentally friendly electric motors.

Please write comments in the box located below. Tell us how you select and adjust the thermal relay for its own electric motor. Share useful information, ask questions, share photos on the topic of the article.

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