Electrotechnical materials, their properties and application

Effective and durable work of electrical machines and installations directly depends on the state of insulation, for the device of which electrotechnical materials are used. They are characterized by a set of certain properties when placed in the conditions of an electromagnetic field, and installed in devices with these indicators.

Classification of electrical materials can be divided into separate groups of electrical insulating, semiconductor, conductor and magnetic materials, which are supplemented by the main products: capacitors, wires, insulators and finished semiconductor elements.

The materials work in separate magnetic or electric fields with certain properties, and are exposed to several radiation simultaneously. Magnetic materials are conditionally divided into magnets and weakly magnetic substances. In electrical engineering, the most widely used are strongly magnetic materials.

Materials Science

A material is a substance characterized by a different chemical composition, properties, and structure of molecules and atoms. The substance is in one of four states: gaseous, solid, plasma or liquid. Electrotechnical and structural materials perform a variety of functions in the installation.

Conducting materials transfer the flow of electrons, dielectric components provide insulation. The use of resistive elements converts electrical energy into thermal, structural materials retain the shape of the product, for example, the body. Electrotechnical and structural materials necessarily carry out not one, but several accompanying functions, for example, dielectric in the work of an electrical installation undergoes loads, which brings it closer to structural materials.

Electrotechnical materials science is a science engaged in determining properties, studying the behavior of matter under the influence of electricity, heat, frost, magnetic field, etc. Science studies the specific characteristics necessary for the creation of electrical machines, devices and installations.

Conductors

These include electrotechnical materials, the main indicator of which is the pronounced conductivity of an electric current. This is because the mass of matter is constantly present electrons, weakly bound to the core and being free charge carriers. They move from the orbit of one molecule to another and create a current. The main conductor materials are copper and aluminum.

Conductors include elements that have a specific electrical resistance ρ <10 ^-5 , while an excellent conductor is a material with an index of 10 ^-8 Ω * m. All metals conduct a good current, out of 105 elements of the table only 25 are not metals, and from this heterogeneous group of 12 materials conduct an electric current and are considered semiconductors.

The physics of electrotechnical materials allows their use as conductors in gaseous and liquid state. As a liquid metal with a normal temperature, only mercury is used, for which this is a natural state. The remaining metals are used as liquid conductors only in the preheated state. Conductors are also used for conducting conductive liquids, for example electrolyte. Important properties of conductors that allow them to be distinguished by the degree of electrical conductivity are the characteristics of thermal conductivity and the ability to thermal generation.

Dielectric materials

Unlike conductors, the mass of dielectrics contains a small number of free electrons of elongated shape. The main property of a substance is its ability to receive polarity under the action of an electric field. This phenomenon is explained by the fact that under the action of electricity, the associated charges move towards the acting forces. The displacement distance is greater the higher the electric field strength.

The insulating electrotechnical materials are closer to the ideal, the lower the specific conductivity index, and the less pronounced the degree of polarization, which allows one to judge the dispersion and release of thermal energy. The conductivity of a dielectric is based on the action of a small number of free dipoles shifting toward the action of the field. After the polarization, the dielectric forms a substance with different polarities, that is, two different signs of charges form on the surface.

The use of dielectrics is most extensive in electrical engineering, since the active and passive characteristics of the element are used.

Active materials, with manageable properties, include:

• Pyroelectrics;
• Electroluminophores;
• Piezoelectrics;
• Ferroelectrics;
• Electrets;
• Materials for emitters in the laser.

Basic electrotechnical materials - dielectrics with passive properties, are used as insulating materials and capacitors of the usual type. They are able to separate two sections of the electrical circuit from each other and prevent the flow of electrical charges. With their help, insulation of live parts is carried out so that electrical energy does not escape into the ground or the housing.

Separation of dielectrics

The organic and inorganic materials are divided dielectrics, depending on the chemical composition. Inorganic dielectrics do not contain carbon, whereas organic forms have a basic element of carbon. Inorganic substances, such as ceramics, mica, have a high degree of heating.

Electrotechnical materials by the method of production are divided into natural and artificial dielectrics. A wide application of synthetic materials is based on the fact that the manufacture allows you to give the material the desired properties.

According to the structure of molecules and molecular lattice, dielectrics are divided into polar and non-polar. The latter are also called neutral. The difference lies in the fact that before the onset of action of an electric current or atoms, the atoms and molecules have or do not have an electric charge. The neutral group includes fluoroplastic, polyethylene, mica, quartz, etc. Polar dielectrics consist of molecules with a positive or negative charge, for example, polyvinyl chloride, bakelite.

Properties of dielectrics

The state of dielectrics is divided into gaseous, liquid and solid. The most common are hard electrotechnical materials. Their properties and applications are assessed using indicators and characteristics:

• Volume resistivity;
• the dielectric constant;
• Surface resistivity;
• Coefficient of thermal permeability;
• Dielectric losses, expressed by the tangent of the angle;
• Strength of the material under the action of electricity.

The volume specific resistivity depends on the ability of the material to resist the flow of a constant current through it. The index, the reciprocal of the resistivity, is called the volume specific conductivity.

The surface resistivity is determined by the ability of the material to resist a constant current flowing along its surface. Surface conductivity is the reciprocal of the previous indicator.

The coefficient of thermal permeability reflects the degree of change in the resistivity after raising the temperature of the substance. Usually, as the temperature increases, the resistance decreases, hence, the value of the coefficient becomes negative.

Dielectric permeability determines the use of electrical materials in accordance with the ability of the material to create electrical capacity. The index of the relative permittivity of a dielectric is included in the concept of absolute permeability. The change in the insulation capacity is indicated by the previous parameter of the thermal permeability coefficient, which simultaneously shows an increase or decrease in capacity with a change in the temperature regime.

The loss factor of the dielectric reflects the degree of power loss of the circuit relative to the dielectric material subjected to the action of an electric alternating current.

Electrotechnical materials are characterized by an index of electrical strength, which determines the possibility of destruction of a substance under the influence of stress. When there is a mechanical strength, there are a number of tests to establish the index of compressive strength, tension, bending, torsion, impact and splitting.

Physical and chemical indices of dielectrics

In dielectrics, a certain number of released acids are contained. The amount of caustic potassium in milligrams needed to get rid of impurities in 1 g of substance is called the acid number. Acids destroy organic materials, have a negative effect on insulation properties.

The characteristic of electrotechnical materials is supplemented with a coefficient of viscosity or friction showing the degree of fluidity of the substance. Viscosity is divided into conditional and kinematic.

The degree of water absorption is determined depending on the mass of water absorbed by the test-size element after a day of being in water at a given temperature. This characteristic indicates the porosity of the material, an increase in the index worsens the insulation properties.

Magnetic materials

The indicators of the evaluation of magnetic properties are called magnetic characteristics:

• Magnetic absolute permeability;
• Magnetic relative permeability;
• Thermal permeability coefficient;
• Energy of the maximum magnetic field.

Magnetic materials are divided into hard and soft. Soft elements are characterized by small losses when the magnetization of the body lags behind the effective magnetic field. They are more permeable to magnetic waves, have a small coercive force and an increased induction saturation. They are used in the arrangement of transformers, electromagnetic machines and mechanisms, magnetic screens and other devices where magnetization with small energy gaps is necessary. These include pure electrolyte iron, iron - armco, permalloy, electrotechnical steel in sheets, nickel-iron alloys.

Solid materials are characterized by significant losses when the degree of magnetization lags behind the external magnetic field. Having received magnetic impulses once, such electrotechnical materials and products are magnetized, and for a long time they save the accumulated energy. They have a large coercive force and a large capacity of residual induction. Elements with such characteristics are used for manufacturing stationary magnets. Representatives of the elements are alloys on an iron basis, aluminum, nickel, cobalt, silicon components.

Magnetodielectrics

These are mixed materials, 75-80% containing in the magnetic powder, the remainder of the mass is filled with an organic high-polymer dielectric. The ferrites and magnetodielectrics have higher values of volume resistivity, small vortex current losses, which makes it possible to use them in high-frequency technology. Ferrites have a stable index at various frequency fields.

Scope of use of ferromagnets

They are used most effectively to create cores of transformer coils. The application of the material allows to increase the magnetic field of the transformer much, without changing the current intensity. These inserts of ferrites save energy consumption during operation of the device. Electrotechnical materials and equipment, after switching off the external magnetic effect, retain the magnetic indices, and maintains the field in the neighboring space.

Elementary currents do not pass after turning off the magnet, thus creating a standard permanent magnet that effectively works in headphones, telephones, measuring devices, compasses, sound recorders. Very popular in use are permanent magnets that do not conduct electricity. They are obtained by combining iron oxides with other various oxides. Magnetic iron ore refers to ferrites.

Semiconductor Materials

These are the elements that have the value of the conductivity, located in the gap of this index for conductors and dielectrics. Conductivity of these materials directly depends on the manifestation of impurities in the mass, external directions of action and internal defects.

The characteristic of electrotechnical materials of a group of semiconductors speaks about essential distinction of elements from each other on a structural lattice, structure, properties. Depending on the specified parameters, the materials are divided into 4 types:

1. Elements containing atoms of one kind: silicon, phosphorus, boron, selenium, indium, germanium, gallium, etc.
2. Materials containing in the composition of metallic oxides - copper, cadmium oxide, zinc, etc.
3. Materials, grouped in the antimonide group.
4. Organic materials - naphthalene, anthracene, etc.

Depending on the crystal lattice, semiconductors are divided into polycrystalline materials and single-crystal elements. The characteristic of electrotechnical materials allows them to be divided into non-magnetic and weakly magnetic. Among the magnetic components distinguish semiconductors, conductors and non-conductive elements. Clear allocation is difficult to perform, as many materials behave differently in changing conditions. For example, the operation of certain semiconductors at low temperatures can be compared with the action of insulators. The same dielectrics act as semiconductors when heated.

Composite materials

Materials that are subdivided not by function, but by composition, are called composite materials, these are also electrotechnical materials. Their properties and application are due to a combination of materials used in the manufacture. An example is the sheet glass-fiber components, fiberglass, mixtures of conductive and refractory metals. The use of equivalent mixtures allows us to identify the strengths of the material and apply them to their intended use. Sometimes the combination of composite components leads to the creation of an absolutely new element with other properties.

Film materials

A large area of application in electrical engineering has been won by films and tapes, like electrotechnical materials. Their properties differ from other dielectrics by their flexibility, sufficient mechanical strength and excellent insulation characteristics. The thickness of the products varies depending on the material:

• The films are made 6-255 μm thick, the tapes are produced 0.2-3.1 mm;
• Polystyrene products in the form of tapes and films produce a thickness of 20-110 μm;
• Polyethylene tapes make a thickness of 35-200 microns, a width of 250 to 1500 mm;
• Fluoroplastic films are made of a thickness of 5 to 40 μm, the width is 10-210 mm.

The classification of electrotechnical materials from the film makes it possible to distinguish two types: oriented and undirected films. The first material is used most often.

Varnishes and enamels for electrical insulation

Solutions of substances forming when the film solidifies, are modern electrotechnical materials. This group includes bitumens, drying oils, resins, cellulose ethers or compounds and combinations of these components. The transformation of a viscous component into an insulator occurs after evaporation from the mass of the applied solvent, and the formation of a dense film. By the method of application, the films are divided into adhesive, impregnating and coating.

Impregnating varnishes are used for winding electrical installations in order to increase the thermal conductivity and moisture resistance. Covering varnishes create an upper protective coating against moisture, frost, oil for the surface of windings, plastic, insulation. Adhesive components are able to glue lamina mica with other materials.

Compounds for electrical insulation

These materials are represented by a liquid solution at the time of use, followed by solidification and solidification. Substances are characterized by the fact that the composition does not contain solvents. Compounds also belong to the group "electrotechnical materials". Their types are filling and impregnating. The first type is used to fill the cavities in the cable clutches, and the second group is used to impregnate the motor windings.

Compounds produce thermoplastic, they soften after raising the temperatures, and thermosetting, stably retaining the form of solidification.

Fibrous impregnated electrical insulating materials

For the production of such materials, organic fibers and artificially created constituents are used. Natural vegetable fibers of natural silk, flax, wood are converted into materials of organic origin (fiber, fabric, cardboard). Humidity of such insulators varies in the range of 6-10%.

Organic materials from synthetics (kapron) contain moisture only from 3 to 5%, the same saturation with moisture and in inorganic fibers (glass fibers). Inorganic materials are distinguished by their inability to ignite with significant heating. If the materials are impregnated with enamels or varnishes, the flammability rises. The supply of electrical materials is made to the enterprise for the manufacture of electrical machines and devices.

The lethroid

Fine fiber is produced in sheets and rolled into a roll for transportation. It is used as a material for the manufacture of insulation pads, shaped dielectrics, washers. Paper with asbestos impregnation and asbestos cardboard is made from chrysolite asbestos, splitting it into fibers. Asbestos has resistance to alkaline medium, but is destroyed in acid.

In conclusion, it should be noted that with the use of modern materials for the insulation of electrical appliances, their service life has significantly increased. For the hulls of plants, materials with the selected characteristics are used, which makes it possible to produce new functional equipment with improved performance.