The effective and durable operation of electrical machines and installations directly depends on the state of insulation, for the device of which electrical materials are used. They are characterized by a set of specific properties when placed in an electromagnetic field, and are installed in devices taking into account these indicators.
The classification of electrical materials allows you to divide into separate groups of electrical insulating, semiconductor, conductive and magnetic materials, which are complemented by the main products: capacitors, wires, insulators and finished semiconductor elements.
Materials work both in separate magnetic or electric fields with certain properties, and are exposed to several radiations simultaneously. Magnetic materials are conventionally divided into magnets and weakly magnetic substances. In electrical engineering, highly magnetic materials are most widely used.
Material science
Material is called a substance characterized by a chemical composition different from other objects, properties and structure of molecules and atoms. The substance is in one of four states: gaseous, solid, plasma or liquid. Electrical and structural materials perform various functions in the installation.
Conductor materials transmit electron flux, dielectric components provide insulation. The use of resistive elements converts electrical energy into heat, structural materials retain the shape of the product, for example, the case. Electrical and structural materials necessarily perform not one, but several related functions, for example, a dielectric experiences loads in the electrical installation, which brings it closer to structural materials.
Electrotechnical material science is a science that deals with the determination of properties, the study of the behavior of a substance under the influence of electricity, heat, frost, a magnetic field, etc. Science studies the specific characteristics necessary for creating electrical machines, devices, and plants.
Conductors
These include electrical materials, the main indicator of which is the pronounced conductivity of the electric current. This is because electrons are constantly present in the mass of matter, weakly bonded to the nucleus and being free charge carriers. They move from the orbit of one molecule to another and create a current. The main conductive materials are copper, aluminum.
Conductors include elements that have a specific electrical resistance ฯ <10 -5 , while an excellent conductor is a material with an indicator of 10 -8 Ohm * m. All metals conduct current well, out of 105 elements of the table only 25 are not metals, and from this heterogeneous group of 12 materials conduct electric current and are considered semiconductors.
The physics of electrotechnical materials allows their use as conductors in a gaseous and liquid state. As liquid metal with a normal temperature, only mercury is used, for which it is a natural state. The remaining metals are used as liquid conductors only in a heated state. Conducting fluids, such as electrolyte , are also used for conductors . The important properties of conductors that make it possible to distinguish them by the degree of electrical conductivity are considered to be the characteristics of thermal conductivity and the ability to thermal generation.
Dielectric materials
Unlike conductors, the mass of dielectrics contains a small number of elongated free electrons. The main property of a substance is its ability to obtain polarity under the influence of an electric field. This phenomenon is explained by the fact that under the influence of electricity the bound charges move towards the acting forces. The displacement distance is the greater, the higher the electric field strength.
Insulating electrical materials the closer to ideal, the lower the conductivity, and the less pronounced the degree of polarization, which allows us to judge the dissipation and release of thermal energy. The conductivity of the dielectric is based on the action of an insignificant amount of free dipoles, shifting towards the field. After polarization, the dielectric forms a substance with different polarity, that is, two different signs of charges are formed 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 laser emitters.
The main electrical materials - dielectrics with passive properties, are used as insulation materials and conventional type capacitors. They are able to separate two sections of the electrical circuit from one another and prevent the flow of electric charges. With their help, insulation of live parts is carried out so that electrical energy does not go into the ground or on the housing.
Dielectric Separation
Dielectric materials are divided into organic and inorganic materials, depending on the chemical composition. Inorganic dielectrics do not contain carbon, while organic forms have carbon as the main element. Inorganic substances, such as ceramics, mica, have a high degree of heating.
Electrotechnical materials according to the production method are divided into natural and artificial dielectrics. Widespread use of synthetic materials is based on the fact that manufacturing allows you to give the material the desired properties.
According to the structure of molecules and the molecular lattice, dielectrics are divided into polar and nonpolar. The latter are also called neutral. The difference is that atoms and molecules, before the action of an electric current on them, have or not have an electric charge. The neutral group includes fluoroplastic, polyethylene, mica, quartz, etc. Polar dielectrics are composed of molecules with a positive or negative charge, such as polyvinyl chloride, bakelite.
Dielectric Properties
As dielectrics are divided into gaseous, liquid and solid. The most commonly used solid electrical materials. Their properties and application are evaluated using indicators and characteristics:
- volume resistivity;
- the dielectric constant;
- surface resistivity;
- thermal permeability coefficient;
- dielectric losses expressed by the tangent of the angle;
- material strength under the influence of electricity.
Volume resistivity depends on the ability of the material to resist the passage of constant current through it. The inverse of resistivity is called volumetric conductivity.
Surface resistivity is determined by the ability of the material to resist direct current flowing along its surface. Surface conductivity is the inverse of the previous figure.
The coefficient of thermal permeability reflects the degree of change in resistivity after increasing the temperature of the substance. Typically, with increasing temperature, the resistance decreases, therefore, the coefficient value becomes negative.
The dielectric constant determines the use of electrical materials in accordance with the ability of the material to create electrical capacity. The relative permeability index of a dielectric is included in the concept of absolute permeability. The change in insulation capacity is shown by the previous indicator of the coefficient of thermal permeability, which simultaneously shows an increase or decrease in capacity with a change in temperature.
The dielectric loss tangent reflects the degree of power loss of the circuit relative to the dielectric material exposed to AC electric current.
Electrical materials are characterized by an indicator of electrical strength, which determines the possibility of destruction of a substance under the action of voltage. When identifying mechanical strength, there are a number of tests to establish an indicator of the compressive strength, tensile, bending, torsion, upon impact and splitting.
Physical and chemical characteristics of dielectrics
Dielectrics contain a certain number of released acids. The amount of caustic potassium in milligrams needed to get rid of impurities in 1 g of the substance is called the acid number. Acids destroy organic materials, have a negative effect on the insulating properties.
The characteristic of electrical materials is supplemented by 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 in the water at a given temperature. This characteristic indicates the porosity of the material, an increase in the indicator worsens the insulating properties.
Magnetic materials
Magnetic properties assessment indicators are called magnetic characteristics:
- magnetic absolute permeability;
- magnetic relative permeability;
- thermal magnetic 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 magnitude of the magnetization of the body lags the current magnetic field. They are more permeable to magnetic waves, have a small coercive force and increased induction saturation. They are used in the construction of transformers, electromagnetic machines and mechanisms, magnetic screens and other devices where magnetization with small energy omissions is needed. These include pure electrolyte iron, iron - armco, permalloy, electrical steel 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 store stored energy. They have a large coercive force and a large capacity of residual induction. Elements with such characteristics are used for the manufacture of stationary magnets. Representatives of the elements are iron-based alloys, aluminum, nickel, cobalt, silicon components.
Magnetodielectrics
These are mixed materials containing 75-80% magnetic powder in the composition, the remainder of the mass is filled with an organic high-polymer dielectric. Ferrites and magnetodielectrics have increased values โโof volume resistivity, small eddy current losses, which allows their use in high-frequency technology. Ferrites have a stable performance at different frequency fields.
Field of use of ferromagnets
They are used most effectively to create the core of transformer coils. The use of the material allows you to significantly increase the magnetic field of the transformer, while not changing the current strength reading. Such ferrite inserts can save electricity during the operation of the device. Electrotechnical materials and equipment, after switching off the external magnetic action, retain magnetic indicators and maintain the field in the neighboring space.
Elementary currents do not pass after turning off the magnet, thus creating a standard permanent magnet that works effectively in headphones, telephones, measuring instruments, compasses, sound recorders. Permanent magnets that do not conduct electricity are very popular in use. Get them by combining iron oxides with various other oxides. Magnetic iron belongs to ferrites.
Semiconductor materials
These are elements that have a conductivity value that lies in the interval of this indicator for conductors and dielectrics. The conductivity of these materials directly depends on the manifestation of impurities in the mass, external directions of exposure and internal defects.
The characteristic of electrical materials of the semiconductor group indicates a significant difference between the elements from each other in terms of their structural lattice, composition, and properties. Depending on the specified parameters, the materials are divided into 4 types:
- Elements containing atoms of one type: silicon, phosphorus, boron, selenium, indium, germanium, gallium, etc.
- Materials containing metallic oxides - copper, cadmium oxide, zinc oxide, etc.
- Materials combined in the antimonid group.
- 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 makes it possible to separate them into non-magnetic and weakly magnetic ones. Among the magnetic components, semiconductors, conductors and non-conductive elements are distinguished. A clear distribution is difficult to accomplish, as many materials behave differently under changing conditions. For example, the operation of some semiconductors at low temperatures can be compared with the action of insulators. The same dielectrics work 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 sheet fiberglass components, fiberglass, mixtures of electrically conductive and refractory metals. The use of equivalent mixtures allows you to identify the strengths of the material and apply them as intended. Sometimes a combination of composite components leads to the creation of a completely new element with other properties.
Film materials
Films and tapes, as electrotechnical materials, have won a large field of application in electrical engineering. Their properties differ from other dielectrics in flexibility, sufficient mechanical strength and excellent insulating characteristics. The thickness of the products varies depending on the material:
- films are made with a thickness of 6-255 microns, tapes produce 0.2-3.1 mm;
- polystyrene products in the form of tapes and films produce a thickness of 20-110 microns;
- polyethylene tapes are made with a thickness of 35-200 microns, a width of 250 to 1500 mm;
- PTFE films are made with a thickness of 5 to 40 microns, a width of 10-210 mm.
The classification of electrotechnical materials from the film allows us to distinguish two types: oriented and non-oriented films. The first material is used most often.
Varnishes and enamels for electrical insulation
Solutions of substances that form a film during solidification are modern electrical materials. This group includes bitumen, drying oils, resins, cellulosic ethers or compounds and combinations of these components. The transformation of the viscous component into an insulator occurs after evaporation from the mass of the applied solvent, and the formation of a dense film. According to the method of applying the film is divided into adhesive, impregnating and coating.
Impregnating varnishes are used for windings of electrical installations in order to increase the coefficient of thermal conductivity and resistance to moisture. Coating varnishes create an upper protective coating against moisture, frost, oil for the surface of the windings, plastic, insulation. Adhesive components are able to glue mica plates with other materials.
Compounds for electrical insulation
These materials appear to be a liquid solution at the time of use, followed by solidification and hardening.Substances are characterized in that they do not contain solvents. Compounds also belong to the group of "electrical materials". Their types are filling and impregnating. The first type is used to fill the cavities in the cable joints, and the second group is used to impregnate the motor windings.
Compounds are made thermoplastic, they soften after rising temperatures, and thermosetting, stably preserving the form of hardening.
Fibrous, non-impregnated electrical insulation materials
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Thin fiber is produced in sheets and rolled into a roll for transportation. It is used as a material for the manufacture of insulation strips, shaped dielectrics, washers. Asbestos-impregnated paper and asbestos board are made from chrysolite asbestos, splitting it into fibers. Asbestos is resistant to alkaline conditions, but is destroyed in acidic conditions.
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. Materials for the installations are used with selected characteristics, which makes it possible to produce new functional equipment with improved performance.