Bimetallic plate: device, principle of operation, practical application

Complex automation systems that perform the role of switching modes of operation of certain devices are built on the simplest elements. They have the property of changing one of their parameters (shape, volume, electrical conductivity, etc.) under the influence of one or several factors.

So, all modern heating elements are equipped with thermostats controlling 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 an elevated temperature is called a bimetallic plate. By name, you can guess that the composition of the plate has two metals. Each of them has its own value of the coefficient of thermal expansion. As a result, when one of these plates is heated, one component of it expands by a certain amount, and the other by another.

This leads to a bend, the shape of which depends on the difference in the 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 joint and can work as long as desired.

What components are used in bimetals

In order to connect metals together into a single bimetal, soldering, welding and riveting methods are used.

An example of a common bimetallic plate is the connection of brass and steel. Such a composite has a high thermal sensitivity.

There are analogues of bimetal from nonmetallic materials (glass, ceramics). They are designed to work in aggressive chemical environments where metal can not be used.

How does the bimetallic plate work?

The plate made of bimetal works as a part of various systems of thermal control and thermal control, or more precisely in the thermostat of many modifications. The simplest thermostat includes:

• Heat resistant body. It contains all the relay elements.
• Terminals - are used to connect an electrical circuit.
• Mechanical switches for contacts or contact groups. Close and open the electrical contacts, including or disconnecting the circuit.
• Dielectric rod or gasket. Transmits the mechanical action from the plate to the switch.
• Bimetallic plate. It is an element of reaction to the temperature change and creates pressure on the rod.
• Temperature sensor. Ordinary metal plate, directly connected to the control element. It has good thermal conductivity and transfers heat to the bimetal.

When the surface of the heater has a permissible temperature, the bimetallic plate is in a certain bent (flat) state, the electrical contacts are closed, the current flows in the heater circuit.

As the surface temperature rises, the bimetal begins to bask and gradually deforms, exerting pressure on the rod. At the same time, there comes a time 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 chain closes, and everything repeats again.

Often the relays are released with the ability to regulate the response of the temperature.

Bimetallic boiler plate

Natural gas heating systems are high-risk devices, and therefore include a variety of state monitoring sensors. So, the main element of safety is the thrust sensor. It determines the correct direction of the output of the 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 traction sensor is a bimetallic plate for the gas boiler. The principle of its 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 deformation of the plate and the operation of the gas valve.

Which devices use bimetal

The area of application of the bimetallic plate is unusually wide. Virtually all devices where temperature control is required are equipped with bimetal-based thermostats. This is due to the constructive simplicity and reliability of such relay systems. In the usual technology, thermostats stand:

• In domestic heating appliances: ovens, ironing systems, boilers, electric kettles, etc.
• Heating systems: electric convectors, gas and solid fuel boilers with electronics.
• In electropackets of automatic shutdown.
• In electronics in measuring instruments, as well as in pulse generators and time relays.
• In engines of thermal type.

In industrial equipment, 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. Necessity comes when:

• Bimetal has lost its properties or has changed, which does not correspond to the mode of operation of the device.
• The plate burned out (refers to thermal relays).
• If the fixing bolt is damaged or the ignitor burner fails (in gas boilers).
• When plate replacement is assumed to be routine maintenance activities.

In household appliances it is usually not changed. If the thermoregulation system fails, the replacement of the bimetallic plate takes place as a whole block, which go as spare parts to a specific model of the device. But often the reason for the failure of the thermostat is the ignition of the opening contacts, and not the bimetallic plate.