Complex automation systems that play the role of switching the operating modes of various 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 temperature controllers that control the degree of surface heating. The basis of any thermostat is a bimetallic plate.
What is a bimetallic plate
An element with the property of deforming (bending) in one direction under the influence of elevated temperature is called a bimetallic plate. The name suggests that the plate contains two metals. Each of them has its own value of the coefficient of thermal expansion. As a result, when heating such a plate, one of its components expands by a certain amount, and the second by another.
This leads to bending, the shape of which depends on the difference in temperature coefficients. The strain rate is directly proportional to the temperature change. When cooling the plate, it acquires its original position. The plate is a monolithic connection and can work as long as you like.
What components are used in bimetals
In order to connect metals to each other in a single bimetal, methods of soldering, welding and riveting are used.
An example of a common bimetallic plate is the combination of brass and steel. Such a composite has a high thermal sensitivity.
There are analogues of bimetal from non-metallic materials (glass, ceramics). They are designed to work in aggressive chemical environments where metal cannot be used.
How a bimetallic plate works
The bimetal plate works as a part of various thermal control and thermal control systems, and more precisely, in the thermal relay of many modifications. The simplest thermal relay includes:
- Heat resistant housing. It contains all the elements of the relay.
- Terminals - are used to connect an electrical circuit.
- Mechanical switches of contacts or contact groups. Close and open electrical contacts, turning the circuit on or off.
- Dielectric rod or gasket. Transmits mechanical impact from the plate to the switch.
- Bimetal plate. It is an element of reaction to temperature changes and creates pressure on the stem.
- Temperature sensor. An ordinary metal plate directly connected to a control element. It has good thermal conductivity and transfers heat to bimetal.
When the surface of the heater has an acceptable temperature, the bimetallic plate is in a certain curved (even) state, while the electrical contacts are closed, current flows in the heater circuit.
With increasing surface temperature, the bimetal begins to heat up and gradually deforms, exerting pressure on the rod. In this case, the moment comes when the rod opens the contact of the mechanical switch, and the current in the heater circuit is interrupted. Then it cools, the plate cools, the circuit closes, and everything repeats again.
Often relays are released with the ability to control the operation according to the temperature.
Bimetal boiler plate
Natural gas heating systems are high-risk devices, therefore, they include various condition monitoring sensors. So, the main safety element is a traction 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 bimetal plate for a gas boiler. Its principle of operation is similar to 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 plate deformation and the gas valve to operate.
What devices use bimetal
The scope of the bimetallic plate is unusually wide. Almost all devices where temperature control is required are equipped with bimetal-based thermostats. This is due to the structural simplicity and reliability of such relay systems. In the usual technique, thermostats are:
- In household heating appliances: stoves, ironing systems, boilers, electric kettles, etc.
- Heating systems: electric convectors, gas and solid fuel boilers with electronics.
- In power accessories automatic shutdown.
- In electronics in measuring devices, as well as in pulse generators and time relays.
- In thermal type engines.
In industrial technology, bimetallic plates are installed in thermal relays designed to protect powerful electrical devices from temperature overloads: transformers, electric motors, pumps, etc.
When the plate is changed
All bimetallic plates have a long service life, but sometimes its replacement is inevitable. The need comes when:
- Bimetal has lost its properties or their change has occurred, which does not correspond to the operating mode of the device.
- The plate burned out (refers to the thermal relay).
- In case of violation of the fixing bolt or failure of the pilot burner (in gas boilers).
- When replacing a plate is expected to be scheduled maintenance activities.
In household appliances, they usually do not change it. If the thermoregulation system fails, then the bimetallic plate is replaced by a whole block, which go as spare parts for a specific model of the device. But often the cause of the failure of the thermostat is the burning of the opening contacts, and not the bimetallic plate.