Overload protection of electrical equipment is provided by a temperature relay. Without it, the conductors overheat, which leads to premature failure of the insulation.
Operating principle
The function of the thermal relay is to turn off the circuit when the current flowing through it exceeds the nominal. The device consists of a thermal heater, through which electric current flows, and a plate of bimetal, which deforms when heated and opens the contacts of the circuit. The greater the current, the faster the response.
After opening the circuit, the thermocouple cools down and returns to its original state.
Types of thermocouple operation schemes
The thermal relay operates in two ways:
- switching contacts are forcibly closed back;
- the circuit returns to its original state on its own.
The first option relates to protective thermal relays (electromagnetic starters, circuit breakers, etc.). The second is used in temperature control systems of objects (refrigerator, iron, underfloor heating, etc.).
A bimetallic plate during deflection acts on a group of contacts with which an electric circuit is opened. Due to the low speed of operation, the device does not extinguish the electric arc with the proper effect. On modern relays, devices are used that increase the speed of open circuit.
Types of Thermal Relays
Thermal relays are selected according to the rated load of the motors, exceeding it by 20-30%. With such an overload, operation occurs after 20 minutes. The bimetallic plate bends slowly. In this regard, it acts on contacts through acceleration devices (jumping contact). The following types of thermal relays are distinguished.
- RTP - protect three-phase motors with currents on thermocouples up to 600 A and in DC networks up to 150 A. The bimetal plate is heated by the heater and the current passing through it. The trip current is manually adjusted by initial deformation of the plate. Returning to the initial state is done by the button, but there are modifications with self-resetting.
- RTL - to protect three-phase asynchronous motors from prolonged overloads, with phase asymmetry, jamming of the rotor or during heavy starting. In such conditions, electric drives of hoisting mechanisms, pumps, fans, machine tools, etc. work. Relays are built into starters, and also made into separate devices.
- RTT - are included in the sets of protection for three-phase asynchronous motors from prolonged overloads, phase unbalance, etc. They can be built into magnetic starters in AC and DC circuits.
Adjustment and adjustment
In electrothermal relays, the operation settings should be periodically set. First, they are revised and a predetermined room temperature is provided. During an external examination, the condition of contacts, bimetallic plates, fasteners and mechanism is checked.
Installation for thermal thermal relay with adjustment is made in the direction of increase or decrease, where each division of the scale corresponds to a correction of 10 ° C. If the relay has temperature compensation, no adjustment is required.
The setting is made to operate with a six-fold increase in I nom. With low inertia, devices operate in the range of 0.5-4 s, and with large inertia, from 4 to 25 s. Then the check is done with increasing current up to 1.2 from I nom. The relay should disconnect the contacts after 20 minutes.
Simple thermostats
A temperature relay can be created on the basis of electronic circuits that are used to maintain the specified temperature conditions of a computer, a living room, an incubator, etc. For this, a temperature controller can be used, 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 value of resistance R 2 changes. The mismatch signal comes from the bridge to the input of the LM393 chip. It works in the comparator mode, where from the analog signal at input 3 it jumps from the off state to the working one. The signal from the output of the microcircuit 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 regime is controlled by air cooling.
Sensor for thermostat for underfloor heating
By a similar principle, the underfloor heating system is controlled.
An alternating voltage of 230 V is supplied to the input of the device, then it is converted in a 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 R 9 thermistor is 10 kOhm. A signal higher than 2.5 V is supplied to the zener diode TL431 through the VD 3 , R 6 , HL 2 , U 1 chain. This is indicated by the HL 2 diode. 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 closes, followed by an opto-simistor with a triac. As a result, the heater section is turned off. As soon as the floor begins 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 installing the R 9 sensor. It is located in the middle between the turns of the heater. The output power controlled by the temperature relay depends on the resistance value R 7 .
The open terminals of the sensor are closed with a heat shrink tube, and together with the cable they are closed with a screed or a layer of glue. The findings should be placed in a brass sleeve and coated with epoxy. The floor is tiled.
How to connect a thermal relay to heaters can be seen in the diagram, which is depicted on the case of many models. It can also be found in the device description.
The industrial thermal relay for the heater can be selected from the following types:
- electromechanical - with manual adjustment of the switch;
- digital - control is carried out by touch or tactile buttons, and the necessary information is displayed on the display (current temperature and settings);
- programmable - with setting the heater program for a given period, as well as remotely controlled via a computer.
Conclusion
Many schemes are known for connecting a temperature relay to devices. Previously, they had to be assembled with their own hands. Now on the market you can choose a thermostat, the scheme of which is optimally similar to a heater (electric boiler, heated floor, etc.). It is important to provide the necessary functionality, reliability and safety at work.