Temperature relay: circuit, principle of operation, purpose

Protection of electrical equipment from excessive load is provided by a temperature relay. Without it, conductors overheat, which leads to premature failure of insulation.

Operating principle

The function of the thermal relay is to disconnect the electrical circuit when the current flowing through it exceeds the rated current. The device consists of a thermal preheater, through which current flows, and a plate of bimetal, which deforms when heated and opens the contacts of the circuit. The higher the current, the faster the trigger occurs.

After opening the circuit, the thermoelement cools and returns to its original state.

Types of thermocouple operation schemes

The thermal relay operates according to two schemes:

• The switching contacts are forcibly closed;
• The circuit returns to its original state independently.

The first option relates to protective thermal relays (electromagnetic starters, circuit breakers, etc.). The second is used in the systems of temperature control of objects (refrigerator, iron, warm floor, etc.).

Bimetallic plate in the deflection acts on a group of contacts, which opens the electrical circuit. Due to the low response rate, the device does not extinguish the electric arc with the proper effect. On modern relays, devices are used that increase the rate of discontinuity.

Types of thermal relays

Thermal relays are selected based on the rated load of the motors with an excess of 20-30%. With this overload, triggering occurs after 20 minutes. Bimetallic plate bends slowly. In this connection, it acts on the contacts through the acceleration devices (jumping contact). There are the following types of thermal relays.

1. RTP - protect three-phase motors with currents on thermocouples up to 600 A and in direct current networks up to 150 A. The bimetallic plate is heated from the heater and the current passing through it. The pickup current is manually adjusted by first deforming the plate. The reset is made by the button, but there are modifications with self-return.
2. RTL - to protect three-phase asynchronous motors from prolonged overloads, with phase asymmetry, rotor wedging or heavy start-up. In such conditions electric drives of load-lifting mechanisms, pumps, ventilators, machine tools, etc. work. Relays are built in starters, and also make separate devices.
3. PTT - are included in the protection sets of three-phase asynchronous motors against prolonged overloads, unbalance phases, etc. They can be integrated into magnetic starters in alternating and direct current circuits.

Adjustment and adjustment

In the electrothermal relay, the setpoints should be periodically set. First, they are audited and the desired room temperature is ensured. At external inspection the condition of contacts, bimetallic plates, fastening elements and the mechanism is checked.

The setting for a thermal thermostat with adjustment is made up or down, where each scale division corresponds to a correction of 10 ° C. If the relay has a temperature compensation, no adjustment is required.

The adjustment is made to operate at a sixfold increase in I _nom. With a low inertia, the devices operate in the interval 0.5-4 s, and with a large one - from 4 to 25 s. Then the check is made with an increase in current up to 1.2 from I _nom. The relay should switch off the contacts after 20 minutes.

Simple temperature controllers

The temperature relay can be created on the basis of electronic circuits that are used to maintain a given temperature mode of a computer, a living room, an incubator, etc. A thermoregulator can be used for this, the circuit of which contains a sensor consisting of a measuring and supporting half-arm with a thermistor R ₂ and Resistors R ₁ , R ₃ , R ₄ .

When the temperature changes, the resistance value R ₂ changes. The discrepancy signal comes from the bridge to the input of the LM393 chip. It operates in the comparator mode, where from an analog signal at input 3 it jumps from a switched-off state to a working state. The signal from the output of the chip is amplified by the transistor Q ₁ , after which the fan starts. It cools the thermistor, after which the comparator turns off the fan. In this way, the temperature control is controlled by air cooling.

Sensor for floor thermostat

Under a similar principle, the system of the warm floor is controlled.

An alternating voltage of 230 V is applied to the input of the device, then it is converted in the transformerless power supply to a constant 15 V. The switching threshold is set by the divider R ₄ , R ₅ , R ₉ . When the floor is cold, the resistance of the thermistor R ₉ is 10 kΩ. The zener diode TL431 receives a signal above 2.5 V along the VD ₃ , R ₆ , HL ₂ , U ₁ chain. This is indicated by the diode HL ₂ . The triac VS ₁ turns on and the voltage is applied to the floor heating. When its temperature reaches a predetermined value, the resistance of the thermistor R ₉ (sensor) decreases so much that at the control input of the zener diode the signal value becomes less than 2.5 V. TL431 is closed and behind it is an optocoupler with a triac. As a result, the heater section is turned off. As soon as the floor starts to cool, the process repeats.

The minimum and maximum temperatures are set by resistors R ₄ and R ₅ . The switching threshold is adjusted after the R ₉ sensor is installed. It is located in the middle between the turns of the heater. The output power controlled by the temperature relay depends on the resistance R ₇ .

The open terminals of the sensor are closed with a shrink tube, and it is closed together with the cable with a screed or a layer of glue. Conclusions should be placed in a brass sleeve and filled with epoxy resin. Top of the floor is covered with tiles.

How to connect the thermostats to the heaters, you can see on the diagram, which is depicted on the body of many models. It can also be found in the description of the device.

The industrial thermal relay for the heater can be selected from the following types:

• Electromechanical - with manual adjustment of the switch;
• Digital - the control is performed by touch or tactile buttons, and the display shows the necessary information (current temperature and settings);
• Programmable - with the setting of the heater program for a specified period, as well as remotely controlled via a computer.

Conclusion

Many circuits are known, as a temperature relay is connected to the devices. Previously, they had to collect their own hands. Now on the market you can choose a thermostat, whose circuit optimally resembles a heater (electric boiler, warm floor, etc.). It is important to ensure the necessary functionality, reliability and safety in operation.