Principle of operation and connection diagram of thermal relays

Forced cooling devices are used to protect household or industrial equipment with electric motors from overheating. If they are not enough, you can additionally connect a thermal relay. The device protects the network from overload, and the equipment from failure and costly repairs.

Reasons for using protective devices

Thermal relay RTN-7374

Network overload leads to an increase in the temperature of the power line, causing complex breakdowns and emergency situations. The heating device in this case creates conditions for turning off the electricity.

After a fault signal is received, it opens the circuit and the current surges do not affect the motor.

The thermal type relay is suitable for self-connection, is compact and simple in design.

Before using the device, you need to understand its design and operating features.

The principle of operation and design of the thermal relay

Regardless of the type, TP have the following device:

  • sensitive bimetallic plate - made of two metals with different coefficients of expansion;
  • heating coil;
  • mechanism of levers and springs;
  • contacts - normally open and permanently closed - are capable of changing their state when the protection is triggered.

The action of the device is based on the deflection of the plate towards the metal with a lower coefficient of thermal expansion at the moment of overload. A force is exerted on the contact system - the technique stops. After cooling, the plate returns to its original position.

Since the operation of the relay depends on the room temperature or additional heating, the design uses the principle of counterweight. For its implementation, an auxiliary temperature compensator plate with adjustment is used. It can bend in the opposite direction.

The current is checked with a special transformer. After processing the data and exceeding the rating above the setpoint, a pulse is sent to the shutdown mechanism. The external contactor opens, the load is blocked. The device is installed on a magnetic starter.

Main characteristics of the current relay

The main characteristic of the device is the dependence of the response time on the magnitude of the current flow. If there is a denomination, the maximum generation duration is infinite. An increase in indicators leads to a rupture of the insulation.

The optimal load on the motor should be at least 1.2-1.3 seconds under conditions of a 30% overload. If the indicator is higher, the winding or the entire motor heats up. The malfunction is eliminated only after a complete replacement of the equipment.

The relay voltage is selected in accordance with the network parameters - 220 or 380 V. Normal motor protection is possible only when choosing a relay that prevents phase imbalance.

The current rating can be viewed on the chassis bar.

Basic relay types

The compatibility of a relay device with a particular motor depends on its type. Manufacturers produce:

  • TRP. Single pole device with combi heating system that protects asynchronous motors. Suitable for a network with a direct current of no more than 440 V, insensitive to shocks.
  • RTL.Prevents motor malfunction in conditions of phase failure, current imbalance and overload, prolonged start, seizure. It is mounted on a DIN rail separately or together with a starter.
  • RTT. The main purpose of the devices is to prevent a prolonged start, overload, phase imbalance of asynchronous motors with squirrel-cage rotors.
  • TRN. Two-phase switch for control of starting and engine functionality. Suitable for AC mains, the contacts are manually returned to their original position.
  • RTI. Thermal RTI-relay has a minimum power consumption, compatible with circuit breakers or fuses. Installation is carried out on a special contactor.
  • Solid state. Compact devices without active nodes. The principle of their functionality is to check the operating and starting current, to determine the average temperature of the engine. Installed in hazardous areas.
  • RTK. A launcher that controls the temperature inside the equipment enclosure. It is used in circuits with relays as part of the automation equipment.

All devices only prevent emergencies, and do not protect the network from short circuits.

Connection, adjustment and designation of TP

It is necessary to install an electro-thermal relay with a magnetic starter, which connects and starts the engine. As an independent device, the device is placed on a DIN rail or mounting panel.

Device connection diagram

Wiring diagrams for starters with thermal types of relays depend on the type of device:

  • Serial connection with motor winding or starter coil to normally open contact (NC). The element works when connected to the stop key. The system is used when it is necessary to equip the engine with alarm protection. The relay is placed after the starting contactors, but in front of the motor, then the NC contact is connected.
  • Break of the starter zero by a normally closed contact. The circuit is convenient and practical - zero can be connected to the TP contact, a jumper is thrown from the second contact to the starter coil. At the moment the relay is triggered, a zero break occurs and the starter is de-energized.
  • Reversible scheme. The control circuit contains a normally closed and three power contacts. The electric motor is powered through the latter. When the protective mode is activated, the starter is de-energized and the motor stops.

Regardless of the scheme, to stop the equipment, press the "Stop" button.

Adjustment procedure


The device is configured at specialized stands with a low-power load transformer. The heating units are connected to its secondary mechanisms, and the voltage is controlled by an autotransformer. The current limit of the load is regulated by an ammeter connected through the secondary circuit.

The check is done like this:

  1. Turning the transformer handle to zero position with voltage applied. Then the load current is selected with the handle and the relay actuation time is checked from the moment the lamp goes out with a stopwatch. The norm is 140-150 seconds at a current of 1.5 A.
  2. Setting the current rating. Produced when the current rating of the heater does not match the rating of the motor. The adjustment limit is 0.75 - 1.25 of the heater rating.
  3. Setting the current setpoint.

For the last action, you will need to make a calculation:

  • determine the correction for the rated current without temperature compensation according to the formula ± E1 = (Inom-Io) / CIo. Io - current of zero setting, FROM - division price of the eccentric (C = 0.05 for open models and C = 0.055 - for closed ones);
  • calculate the correction taking into account the ambient temperature E2 = (t - 30) / 10where t - temperature;
  • calculate the total correction by adding the obtained values;
  • round the result up or down, translate the eccentric.

This method of adjustment is suitable for devices such as TRN and TRP with indicators similar to the average.

Manual adjustment

The thermal relay can be adjusted manually.The magnitude of the operating current can be set in the range from 20 to 30% of the nominal. The user will need to smoothly move the lever to change the bend of the bimetallic plate. The tripping current is also adjusted after replacing the thermal unit.

Modern switches are equipped with a test button for finding a breakdown without using the stand. Using the reset key, you can reset the settings in automatic or manual mode. An indicator is used to monitor the status of the device.

Marking of electrothermal relays

Decoding of TP marking

The designation of the device has the form of letters and numbers, depending on the manufacturer. Most often, marking is made on the basis of an abbreviated name, as well as:

  • parameters of the setting current - it is prescribed in brackets in numbers;
  • letter designation of the structure;
  • climatic performance in the form of a range;
  • current rating - numbers are used (1 - up to 25 A, 2 - up to 100 A, 3 - up to 510 A);
  • features of the device - compatibility with a DC and AC circuit, mono- and bistable devices, with acceleration or deceleration on / off, with or without winding.

All designations are registered in the relay passport.

Features of the choice of a thermal relay

The choice of a TR should begin with a study of the instructions. The technical document of the device contains the following information:

  • connection between load current and actuation period;
  • start state - cooling or overheating;
  • rated load of the electric motor - the optimal overload rate is 20-30%;
  • constant load time - from 5 to 10 minutes;
  • the period of continuous load - from 40 minutes to 1 hour;
  • dependence of plate heating on air temperature.

In case of deviation from the specified temperature, it is necessary to adjust the relay.

Thermal relay devices are characterized by high speed and large response range. They are easy to install yourself. To ensure timely engine shutdown in case of overload, the TR is adjusted on a special stand.
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