Protection of electrical equipment from excessive load provides a temperature relay. Without it, the conductors overheat, which leads to premature failure of the insulation.
Operation principle
The function of a thermal relay is to turn off the electrical circuit when the current flowing through it exceeds the rated current. The device consists of a thermal heater, through which an electric current passes, and a bimetal plate, which deforms when heated and opens the circuit contacts. The greater the current, the faster the operation.
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 refers to protective thermal relays (electromagnetic starters, circuit breakers, etc.). The second is used intemperature control systems for objects (refrigerator, iron, floor heating, etc.).
When deflected, the bimetallic plate acts on a group of contacts that open the electrical circuit. Due to the low response speed, the device does not extinguish the electric arc with the desired effect. Modern relays use devices that increase the speed of breaking the 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 the contacts through acceleration devices (jumping contact). There are the following types of thermal relays.
- RTP - protect three-phase motors with currents on thermocouples up to 600 A and in DC networks up to 150 A. The bimetallic plate heats up from the heater and the current passing through it. The tripping current is adjusted manually by the initial deformation of the plate. Return to the original state is made by the button, but there are modifications with self-return.
- RTL - to protect three-phase asynchronous motors from prolonged overloads, with phase asymmetry, rotor jamming 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 as separate devices.
- PTT - are included in the protection kits for three-phase asynchronous motors against continuous overloads, phase unbalance, etc. They can be built inin magnetic starters in AC and DC circuits.
Adjustment and tuning
In electrothermal relays, the operation settings should be periodically set. First, they are audited and the set temperature in the room is provided. During an external examination, the condition of the contacts, bimetallic plates, fasteners and mechanism is checked.
The setting for a thermal thermostat with adjustment is done in the direction of increasing or decreasing, where each division of the scale corresponds to an amendment of 10 ° С. If the relay is temperature compensated, no adjustment is required.
The setting is made to operate at a sixfold increase in Inom. With a small inertia, the devices operate in the range of 0.5-4 s, and with a large one - from 4 to 25 s. Then the check is done by increasing the current to 1, 2 from Inom. The relay should turn off the contacts after 20 minutes.
Simple thermostats
A temperature switch can be created on the basis of electronic circuits that are used to maintain a given temperature regime for a computer, living space, incubator, etc. For this, a thermostat can be used, the circuit of which contains a sensor consisting of a measuring and reference half-arm with thermistor R2 and resistors R1, R3, R4.
When the temperature changes, the resistance value R2 changes. The mismatch signal comes from the bridge to the input of the LM393 chip. It operates in the comparator mode, where from the analog signal at input 3 there is an abrupt transition from the off state to the working one. The signal from the output of the microcircuit is amplified by the transistor Q1, after which the fan starts. It cools the thermistor, after which the comparator turns off the fan. In this way, the temperature is controlled by air cooling.
Sensor for floor heating thermostat
The underfloor heating system is controlled in a similar way.
Alternating voltage 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 R4, R5 , R9. When the floor is cold, the resistance of the R9 thermistor is 10 kΩ. A signal above 2.5 V is supplied to the zener diode TL431 along the chain VD3, R6, HL2, U1. This is indicated by the diode HL2. Triac VS1 turns on and voltage is applied to floor heating. When its temperature reaches the set value, the resistance of the thermistor R9 (sensor) decreases so much that the signal value at the control input of the zener diode becomes less than 2.5 V. TL431 closes, followed by an opto-triac with triac. As a result, the heater section is switched off. Once the floor begins to cool, the process is repeated.
Minimum and maximum temperatures are set by resistors R4 and R5. The switching threshold is adjusted after installing the sensor R9. It is located in the middle between the coils of the heater. The output power controlled by the temperature relay depends on the resistance value R7.
Open sensor leads are closed with a heat shrink tube, and it, together with the cable, is covered with a tie or a layer of glue. The conclusions should be placed in a brass sleeve and filled with epoxy resin. From above, the floor is tiled.
How to connect a thermal relay to heaters can be seen in the diagram, which is depicted on the body of many models. It can also be found in the device description.
Industrial heater thermostat can be selected from the following types:
- electromechanical - with manual switch setting;
- digital - control is performed by touch or tactile buttons, and the display shows the necessary information (current temperature and settings);
- programmable - with setting the heater operation program for a given period, as well as remotely controlled via a computer.
Conclusion
There are many schemes known for how a temperature relay is connected to devices. Previously, they had to be collected by hand. Now on the market you can choose a thermostat, the circuit of which is optimally suited to the heater (electric boiler, underfloor heating, etc.). It is important to ensure the necessary functionality, reliability and safety in operation.
