Complex automation systems that play the role of switching the operating modes of certain devices are built on the simplest elements. They tend to change any of their parameters (shape, volume, electrical conductivity, etc.) under the influence of one or more factors.
So, all modern heating elements are equipped with thermostats that control the degree of surface heating. The basis of any thermostat is a bimetallic plate.
What is a bimetallic plate
An element that has the property of deforming (bending) in one direction under the influence of elevated temperature is called a bimetallic plate. By the name, you can guess that the plate contains two metals. Each of them has its own value of the coefficient of thermal expansion. As a result, when such a plate is heated, one component of it expands by a certain amount, and the second by another.
This results in a bend whose shape dependsfrom the difference in temperature coefficients. The rate of deformation is directly proportional to the change in temperature. When the plate is cooled, it acquires its original position. The plate is a monolithic connection and can work indefinitely.
What components are used in bimetals
In order to connect metals together into a single bimetal, soldering, welding and riveting are used.
An example of a common bimetallic plate is a combination of brass and steel. This composite has high thermal sensitivity.
There are analogues of bimetal from non-metallic materials (glass, ceramics). They are designed to work in harsh chemical environments where metal cannot be used.
How a bimetal strip works
A bimetal plate works as part of various thermal control and thermal control systems, or rather, in a thermal relay of many modifications. The simplest thermostat includes:
- Heat resistant housing. It contains all the elements of the relay.
- Terminals - used to connect the electrical circuit.
- Mechanical switches of contacts or contact groups. Make and break electrical contacts, turning the circuit on or off.
- Dielectric rod or gasket. Transmits mechanical action from plate to switch.
- Bimetallic plate. It is an element of response to temperature changes and creates pressure on the stem.
- Temperature sensor. Ordinary metalplate directly connected to the control element. It has good thermal conductivity and transfers heat to the bimetal.
When the surface of the heater has an acceptable temperature, the bimetallic plate is in a certain curved (even) state, the electrical contacts are closed, and current flows in the heater circuit.
When the surface temperature rises, the bimetal begins to heat up and gradually deforms, putting pressure on the rod. In this case, there comes a moment when the rod opens the contact of the mechanical switch, and the current in the heater circuit is interrupted. Then it cools down, the plate cools, the circuit closes, and everything repeats again.
Relays are often produced with the ability to control the response by temperature.
Bi-metal boiler plate
Natural gas heating systems are high-risk devices, so they include various condition monitoring sensors. So, the main element of safety is a thrust sensor. It determines the correct direction of the exit of combustion products, that is, from the combustion chamber towards the chimney. This prevents carbon monoxide from entering the room and poisoning people.
The main component of the draft sensor is a bimetallic plate for a gas boiler. Its principle of operation is similar to that of any bimetal, and the dimensions and parameters of the material are calculated in such a way that exceeding the temperature of 75 degrees in the channel leads to deformation of the plate and actuation of the gas valve.
What devices use bimetal
The scope of the bimetallic plate is extremely wide. Almost all devices where temperature control is required are equipped with bimetal thermostats. This is due to the constructive simplicity and reliability of such relay systems. In our usual technique, thermostats are:
- In household heating appliances: ovens, ironing systems, boilers, electric kettles, etc.
- Heating systems: electric convectors, gas and solid fuel boilers with electronics.
- In the automatic switch-off boxes.
- In electronics in measuring instruments, as well as in pulse generators and time relays.
- In thermal engines.
In industrial technology, bimetallic plates are installed in thermal relays designed to protect powerful electrical devices from thermal overloads: transformers, electric motors, pumps, etc.
When changing the plate
All bimetal strips have a long service life, but sometimes replacement is inevitable. The need comes when:
- The bimetal has lost its properties or has changed, which does not correspond to the operating mode of the device.
- The plate is burned out (applies to thermal relays).
- When the fixing bolt is broken or the igniter burner fails (in gas boilers).
- When insert replacementexpected scheduled maintenance activities.
In household appliances, it is usually not changed. If the thermoregulation system fails, then the replacement of the bimetallic plate occurs as a whole block, which come as spare parts for a specific model of the device. But often the reason for the failure of the thermostat is the burning of the NC contacts, and not the bimetallic plate.
