Lecture notes. Materials used in electrical machines Classification and purpose of electrical materials

GENERAL INFORMATION ABOUT ELECTRIC TRAINS

Brief characteristics and main indicators of operating electric trains.

Electric trains are divided into mechanical parts, electrical equipment, and pneumatic equipment.

The mechanical part includes: the car body, bogies with wheel pairs and axle units, spring suspension, traction transmission, coupling devices and T.R.P.

Electrical equipment includes: current collectors, starting, control and braking equipment, traction motors, auxiliary machines, protection and control equipment for electric trains.

Pneumatic equipment: pneumatic devices and brake system devices, typhons, tanks, taps, etc.

REQUIREMENTS FOR ELECTRIC TRAINS.

Electric trains must provide high acceleration during startup (acceleration) and deceleration during braking. High speeds on hauls, passenger comfort.

Purpose and classification of electrical materials

For the production of electrical machines, apparatus and other equipment, special materials are used that have certain electrical or magnetic properties. Depending on this, electrical materials are divided into four groups: conductor, semiconductor, magnetic and electrical insulating.

Conductor materials are characterized by high specific conductivity and are used in electrical devices as conductors of electric current: windings and contacts in electrical machines, apparatus and instruments, wires and cables for the transmission and distribution of electrical energy.

Highly conductive materials: copper, aluminum and some alloys (brass, bronze, etc.)

High resistivity materials can be divided into three groups:

1) for precision measuring instruments and standard resistors (copper-manganese alloy - manganin)

2) for resistors and rheostats (constantan)

3) having a high operating temperature and intended for heating devices and load rheostats (alloys of nickel, chromium and iron - nichrome; chromium, aluminum and iron - fechral)

Carbon-graphite conductors are slightly inferior in conductivity to metals and their alloys and are used as conductive elements. Carbon conductor materials are based on graphite and carbon. Brushes used for electrical machines are divided into four main groups: carbon-graphite, graphite, electrographite and metal-graphite.

Semiconductor materials occupy an intermediate position in conductivity between conductors and dielectrics.

Magnetic materials are distinguished by their ability to enhance the magnetic field in which they are placed, i.e. have high magnetic conductivity. They are used for the manufacture of magnetic cores in electrical machines and transformers. Electrical devices mainly use iron, nickel, cobalt and their alloys.

LECTURE 10

ELECTRICAL MATERIALS. CLASSIFICATION

Electrical materials (for example, contact materials) are materials characterized by certain properties in relation to electric and magnetic fields and used in technology taking into account and thanks to these properties. Currently, the number of items of electrical materials used in radio, micro-, and nanoelectronics is several thousand. Moreover, the task of creating new materials with specified properties (optical, semiconductor, emissive, etc.) is becoming increasingly urgent.

The main areas of use of electrical materials are electrical power engineering, electrical engineering, and radio electronics.

Electric power industry is the production of energy and its supply to the consumer. These are power lines, transformer stations, and energy facilities.

Electrical engineering is everything that is associated with the conversion of electrical energy into other types of energy while simultaneously implementing technological processes:

electrothermal, - electric welding, - electrophysical, - electrochemical, etc.

Radio engineering is control systems for energy and electrical facilities, information transmission, processing, storage, etc.

Improvements in electrical technology have led to the creation of materials with new properties: higher strength, heat resistance, resistance to aggressive chemical reactions, and having high electrical insulating properties and low thermal conductivity.

Classification of electrical materials

Materials used in electronic technology are divided into electrical, structural and special purpose.

Based on their behavior in a magnetic field, electrical materials are divided into strongly magnetic (magnetic) and weakly magnetic. The former have found particularly wide application in technology due to their magnetic properties.

Based on their behavior in an electric field, materials are divided into conductor, semiconductor and dielectric.

Most electrical materials can be classified as weakly magnetic and practically non-magnetic. However, among magnetic materials one should distinguish between conductive, semi-conducting and practically non-conducting, which determines the frequency range of their application.

Conductor are materials whose main electrical properties are highly pronounced electrical conductivity. Their use in technology is mainly due to this property, which determines high specific electrical conductivity at normal temperature.

Semiconductor are materials that are intermediate in conductivity between conductor and dielectric materials and whose distinctive property is the strong dependence of specific conductivity on the concentration and type of impurities or various defects, as well as in most cases on external energy influences (temperature, illumination, etc.) .

Dielectric are materials whose main electrical property is the ability to polarize and in which the existence of an electrostatic field is possible. A real (technical) dielectric gets closer to the ideal one, the lower its specific conductivity and the less pronounced its slow polarization mechanisms associated with the dissipation of electrical energy and the release of heat.

When using dielectrics - one of the most extensive classes of electrical materials - the need to use both passive and active properties of these materials was quite clearly defined.

Active(controlled) dielectrics are ferroelectrics, piezoelectrics, pyroelectrics, electroluminophores, materials for emitters and shutters in laser technology, electrets, etc.

Conventionally, materials with electrical resistivity ρ are classified as conductors< 10 -5 Ом*м, а к диэлектрикам материалы, у которых ρ >10 8 Ohm*m. It should be noted that the resistivity of good conductors can be only 10 -8 Ohm m, and the best dielectrics can exceed 10 16 Ohm m. The resistivity of semiconductors, depending on the structure and composition of the materials, as well as on their operating conditions, can vary within
10 -5 -10 8 Ohm m. Metals are good conductors of electric current. Of the 105 chemical elements, only twenty-five are non-metals, and twelve elements can exhibit semiconductor properties. But in addition to elementary substances, there are thousands of chemical compounds, alloys or compositions with the properties of conductors, semiconductors or dielectrics. It is quite difficult to draw a clear boundary between the resistivity values of different classes of materials. For example, many semiconductors behave like insulators at low temperatures. At the same time, dielectrics can exhibit semiconductor properties when heated strongly. The qualitative difference is that for metals the conducting state is ground, and for semiconductors and dielectrics it is excited.

Electrical materials are a set of conductor, electrical insulating, magnetic and semiconductor materials designed to operate in electric and magnetic fields. This also includes basic electrical products: insulators, capacitors, wires and some semiconductor elements. Electrical materials occupy one of the main places in modern electrical engineering. Everyone knows that the reliability of electrical machines, apparatus and electrical installations mainly depends on the quality and correct selection of appropriate electrical materials. Analysis of accidents of electrical machines and devices shows that most of them occur due to the failure of electrical insulation, consisting of electrical insulating materials.

Magnetic materials are no less important for electrical engineering. Energy losses and dimensions of electrical machines and transformers are determined by the properties of magnetic materials. Semiconductor materials, or semiconductors, occupy a fairly significant place in electrical engineering. As a result of the development and study of this group of materials, various new devices have been created that make it possible to successfully solve some problems in electrical engineering.

With a rational choice of electrical insulating, magnetic and other materials, it is possible to create electrical equipment that is reliable in operation with small dimensions and weight. But to realize these qualities, knowledge of the properties of all groups of electrical materials is required.

Conductor materials

This group of materials includes metals and their alloys. Pure metals have low resistivity. The exception is mercury, which has a fairly high resistivity. The alloys also have high resistivity. Pure metals are used in the manufacture of winding and mounting wires, cables, etc. Conductor alloys in the form of wires and tapes are used in rheostats, potentiometers, additional resistances, etc.

In the subgroup of alloys with high resistivity, a group of heat-resistant conductor materials that are resistant to oxidation at high temperatures is distinguished. Heat-resistant, or heat-resistant, conductor alloys are used in electric heating devices and rheostats. In addition to low resistivity, pure metals have good ductility, i.e. they can be drawn into thin wire, into ribbons and rolled into foil less than 0.01 mm thick. Metal alloys have less ductility, but are more elastic and mechanically stable. A characteristic feature of all metallic conductor materials is their electronic conductivity. The resistivity of all metal conductors increases with increasing temperature, as well as as a result of mechanical processing, which causes permanent deformation in the metal.

Rolling or drawing is used when it is necessary to obtain conductor materials with increased mechanical strength, for example, in the manufacture of overhead line wires, trolley wires, etc. To return deformed metal conductors to their previous resistivity value, they are subjected to heat treatment - annealing without access to oxygen.

Electrical insulating materials

Electrical insulating materials, or dielectrics, are those materials that are used to provide insulation, i.e., they prevent the leakage of electric current between any conductive parts that are under different electrical potentials. Dielectrics have very high electrical resistance. Based on their chemical composition, dielectrics are divided into organic and inorganic. The main element in the molecules of all organic dielectrics is carbon. There is no carbon in inorganic dielectrics. Inorganic dielectrics (mica, ceramics, etc.) have the greatest heat resistance.

According to the production method, a distinction is made between natural (natural) and synthetic dielectrics. Synthetic dielectrics can be created with a given set of electrical and physicochemical properties, which is why they are widely used in electrical engineering.

Based on the structure of their molecules, dielectrics are divided into non-polar (neutral) and polar. Neutral dielectrics consist of electrically neutral atoms and molecules, which do not possess electrical properties before being exposed to an electric field. Neutral dielectrics are: polyethylene, fluoroplastic-4, etc. Among the neutral ones, ionic crystalline dielectrics (mica, quartz, etc.) are distinguished, in which each pair of ions constitutes an electrically neutral particle. Ions are located at the sites of the crystal lattice. Each ion is in vibrational thermal motion near the center of equilibrium - a node of the crystal lattice. Polar, or dipole, dielectrics consist of polar dipole molecules. The latter, due to the asymmetry of their structure, have an initial electric moment even before the influence of the electric field force on them. Polar dielectrics include bakelite, polyvinyl chloride, etc. Compared to neutral dielectrics, polar dielectrics have higher dielectric constants, as well as slightly increased conductivity.

According to their state of aggregation, dielectrics are gaseous, liquid and solid. The largest is the group of solid dielectrics. The electrical properties of electrical insulating materials are assessed using quantities called electrical characteristics. These include: volume resistivity, surface resistivity, dielectric constant, temperature coefficient of dielectric constant, dielectric loss tangent and dielectric strength of the material.

Specific volume resistivity is a value that makes it possible to estimate the electrical resistance of a material when direct current flows through it. The reciprocal of volume resistivity is called volume conductivity. Specific surface resistance is a value that allows one to estimate the electrical resistance of a material when direct current flows across its surface between the electrodes. The reciprocal of specific surface resistance is called specific surface conductivity.

The temperature coefficient of electrical resistivity is a value that determines the change in the resistivity of a material with a change in its temperature. With increasing temperature, the electrical resistance of all dielectrics decreases; therefore, their temperature coefficient of resistivity has a negative sign. Dielectric constant is a value that allows us to evaluate the ability of a material to create electrical capacitance. Relative dielectric constant is included in the value of absolute dielectric constant. The temperature coefficient of dielectric constant is a value that makes it possible to evaluate the nature of the change in dielectric constant, and therefore the insulation capacitance, with a change in temperature. Dielectric loss tangent is a value that determines power losses in a dielectric operating at alternating voltage.

Electric strength is a value that allows us to evaluate the ability of a dielectric to resist destruction by electric voltage. The mechanical strength of electrical insulating and other materials is assessed using the following characteristics: tensile strength of the material, tensile elongation, compressive strength of the material, static bending strength of the material, specific impact strength, splitting resistance.

The physicochemical characteristics of dielectrics include: acid number, viscosity, water absorption. The acid number is the number of milligrams of potassium hydroxide required to neutralize the free acids contained in 1 g of dielectric. The acid number is determined for liquid dielectrics, compounds and varnishes. This value allows us to estimate the amount of free acids in the dielectric, and therefore the degree of their effect on organic materials. The presence of free acids impairs the electrical insulating properties of dielectrics. Viscosity, or the coefficient of internal friction, makes it possible to evaluate the fluidity of electrical insulating liquids (oils, varnishes, etc.). Viscosity can be kinematic or conditional. Water absorption is the amount of water absorbed by a dielectric after it has been in distilled water for 24 hours at a temperature of 20° C and above. The amount of water absorption indicates the porosity of the material and the presence of water-soluble substances in it. As this indicator increases, the electrical insulating properties of dielectrics deteriorate.

The thermal characteristics of dielectrics include: melting point, softening point, drop point, vapor flash point, heat resistance of plastics, thermoelasticity (heat resistance) of varnishes, heat resistance, frost resistance, tropical resistance.

Film electrical insulating materials made from polymers are widely used in electrical engineering. These include films and tapes. Films are produced with a thickness of 5-250 microns, and tapes - 0.2-3.0 mm. High-polymer films and tapes are characterized by great flexibility, mechanical strength and good electrical insulating properties. Polystyrene films are produced with a thickness of 20-100 microns and a width of 8-250 mm. The thickness of polyethylene films is usually 30-200 microns, and the width is 230-1500 mm. Films from fluoroplastic-4 are made with a thickness of 5-40 microns and a width of 10-200 mm. Non-oriented and oriented films are also produced from this material. Oriented fluoroplastic films have the highest mechanical and electrical characteristics.

Polyethylene terephthalate (lavsan) films are produced with a thickness of 25-100 microns and a width of 50-650 mm. PVC films are made from vinyl plastic and plasticized polyvinyl chloride. Vinyl plastic films have greater mechanical strength, but less flexibility. Vinyl plastic films have a thickness of 100 microns or more, and plasticized polyvinyl chloride films have a thickness of 20-200 microns. Cellulose triacetate (triacetate) films are made unplasticized (rigid), blue-dyed, slightly plasticized (colorless) and plasticized (blue-dyed). The latter have significant flexibility. Triacetate films are produced in thicknesses of 25, 40 and 70 microns and a width of 500 mm. Film-electric cardboard is a flexible electrical insulating material consisting of insulating cardboard covered on one side with Mylar film. Film-electrocardboard on lavsan film has a thickness of 0.27 and 0.32 mm. It is produced in rolls 500 mm wide. Film-asbestos cardboard is a flexible electrical insulating material consisting of a Mylar film 50 microns thick, covered on both sides with asbestos paper 0.12 mm thick. Film-asbestos cardboard is produced in sheets 400 x 400 mm (not less) with a thickness of 0.3 mm.

Electrical insulating varnishes and enamels

Varnishes are solutions of film-forming substances: resins, bitumen, drying oils, cellulose ethers or compositions of these materials in organic solvents. During the drying process of the varnish, solvents evaporate from it, and physical and chemical processes occur in the varnish base, leading to the formation of a varnish film. According to their purpose, electrical insulating varnishes are divided into impregnating, coating and adhesive.

Impregnating varnishes are used to impregnate the windings of electrical machines and devices in order to secure their turns, increase the thermal conductivity of the windings and increase their moisture resistance. Coating varnishes make it possible to create protective moisture-resistant, oil-resistant and other coatings on the surface of windings or plastic and other insulating parts. Adhesive varnishes are intended for gluing mica sheets to each other or to paper and fabrics in order to obtain mica electrical insulating materials (micanite, mycalente, etc.).

Enamels are varnishes with pigments introduced into them - inorganic fillers (zinc oxide, titanium dioxide, red lead, etc.). Pigments are introduced to increase the hardness, mechanical strength, moisture resistance, blow resistance and other properties of enamel films. Enamels are classified as coating materials.

According to the drying method, varnishes and enamels are distinguished between hot (oven) and cold (air) drying. The former require high temperatures for their curing - from 80 to 200 ° C, while the latter dry at room temperature. Hot-drying varnishes and enamels, as a rule, have higher dielectric, mechanical and other properties. In order to improve the characteristics of air-drying varnishes and enamels, as well as to speed up curing, they are sometimes dried at elevated temperatures - from 40 to 80 ° C.

The main groups of varnishes have the following features. After drying, oil-based varnishes form flexible, elastic, yellow films that are resistant to moisture and heated mineral oil. In terms of heat resistance, the films of these varnishes belong to class A. In oil varnishes, scarce linseed and tung oils are used, so they are replaced by varnishes based on synthetic resins, which are more resistant to heat aging.

Oil-bitumen varnishes form flexible black films that are resistant to moisture, but easily dissolve in mineral oils (transformer and lubricating oils). In terms of heat resistance, these varnishes belong to class A (105° C). Glypthal and oil-glypthal varnishes and enamels have good adhesive ability to mica, papers, fabrics and plastics. The films of these varnishes have increased heat resistance (class B). They are resistant to heated mineral oil, but require hot drying at temperatures of 120-130 ° C. Pure glypthal varnishes based on unmodified glypthal resins form hard, inflexible films used in the production of solid mica insulation (hard micanites). After drying, oil-glyphthalic varnishes produce flexible, elastic, yellow films.

Silicone varnishes and enamels are characterized by high heat resistance and can work for a long time at 180-200 ° C, so they are used in combination with fiberglass and mica insulation. In addition, the films have high moisture resistance and resistance to electric sparks.

Varnishes and enamels based on polyvinyl chloride and perchlorovinyl resins are resistant to water, heated oils, acidic and alkaline chemicals, therefore they are used as coating varnishes and enamels to protect windings, as well as metal parts from corrosion. You should pay attention to the weak adhesion of polyvinyl chloride and perchlorovinyl varnishes and enamels to metals. The latter are first coated with a layer of primer, and then with varnish or enamel based on polyvinyl chloride resins. Drying of these varnishes and enamels is carried out at 20, as well as at 50-60 ° C. The disadvantages of this type of coating include their low operating temperature, amounting to 60-70 ° C.

Varnishes and enamels based on epoxy resins are characterized by high adhesive ability and slightly increased heat resistance (up to 130 ° C). Varnishes based on alkyd and phenolic resins (phenol alkyd varnishes) have good drying properties in thick layers and form elastic films that can work for a long time at temperatures of 120-130 ° C. The films of these varnishes are moisture and oil resistant.

Water-emulsion varnishes are stable emulsions of varnish bases in tap water. Varnish bases are made from synthetic resins, as well as from drying oils and their mixtures. Water-emulsion varnishes are fire and explosion-proof because they do not contain flammable organic solvents. Due to their low viscosity, such varnishes have good impregnation ability. They are used for impregnation of stationary and moving windings of electrical machines and devices operating for a long time at temperatures up to 105° C.

Electrical insulating compounds

Compounds are insulating compounds that are liquid at the time of use and then harden. The compounds do not contain solvents. According to their purpose, these compositions are divided into impregnating and filling. The first of them are used for impregnation of the windings of electrical machines and devices, the second - for filling cavities in cable couplings, as well as in electric machines and devices for the purpose of sealing.

Compounds can be thermoset (not softened after curing) and thermoplastic (softened upon subsequent heating). Thermosetting compounds include compounds based on epoxy, polyester and some other resins. Thermoplastics include compounds based on bitumen, waxy dielectrics and thermoplastic polymers (polystyrene, polyisobutylene, etc.). Impregnating and casting compounds based on bitumen in terms of heat resistance belong to class A (105° C), and some to class Y (up to 90° C). Epoxy and organosilicon compounds have the greatest heat resistance.

MBC compounds are made on the basis of methacrylic esters and are used as impregnation and potting compounds. After hardening at 70-100°C (and with special hardeners at 20°C) they are thermosetting substances that can be used in the temperature range from -55 to +105°C.

Unimpregnated fibrous electrical insulating materials

This group includes sheet and roll materials consisting of fibers of organic and inorganic origin. Fibrous materials of organic origin (paper, cardboard, fiber and fabric) are obtained from plant fibers of wood, cotton and natural silk. The normal moisture content of electrical insulating cardboard, paper and fiber ranges from 6 to 10%. Fibrous organic materials based on synthetic fibers (nylon) have a moisture content of 3 to 5%. Approximately the same humidity is observed in materials produced on the basis of inorganic fibers (asbestos, fiberglass). Characteristic features of inorganic fibrous materials are their non-flammability and high heat resistance (class C). These valuable properties are in most cases reduced when these materials are impregnated with varnishes.

Electrical insulating paper is usually made from wood pulp. Mica paper, used in the production of mica tapes, has the greatest porosity. Electric cardboard is made from wood cellulose or from a mixture of cotton fibers and wood (sulfate) cellulose fibers, taken in various ratios. Increasing the content of cotton fibers reduces the hygroscopicity and shrinkage of cardboard. Electric cardboard designed to work in air has a denser structure compared to cardboard designed to work in oil. Cardboard with a thickness of 0.1-0.8 mm is produced in rolls, and cardboard with a thickness of 1 mm and above is produced in sheets of various sizes.

Fiber is a monolithic material obtained by pressing sheets of paper, pre-treated with a heated solution of zinc chloride and washed in water. Fiber is amenable to all types of mechanical processing and molding after soaking its blanks in hot water.

Letheroid is a thin sheet and roll fiber used for the manufacture of various types of electrical insulating gaskets, washers and shaped products.

Asbestos papers, cardboards and tapes are made from chrysotile asbestos fibers, which have the greatest elasticity and the ability to twist into threads. All asbestos materials are resistant to alkalis, but are easily destroyed by acids.

Electrical insulating glass tapes and fabrics are made from glass threads obtained from alkali-free or low-alkali glasses. The advantage of glass fibers over plant and asbestos fibers is their smooth surface, which reduces the absorption of moisture from the air. The heat resistance of glass fabrics and tapes is higher than asbestos.

Electrical insulating varnished fabrics (varnished fabrics)

Varnished fabrics are flexible materials consisting of fabric impregnated with varnish or some kind of electrical insulating compound. The impregnating varnish or composition after hardening forms a flexible film, which provides good electrical insulating properties of the varnished fabric. Depending on the fabric base, varnished fabrics are divided into cotton, silk, nylon and glass (fiberglass).

Oil, oil-bitumen, escapon and organosilicon varnishes, as well as silicone enamels, solutions of silicone rubbers, etc. are used as impregnating compositions for varnished fabrics. Silk and nylon varnished fabrics have the greatest extensibility and flexibility. They can operate at temperatures no higher than 105° C (class A). All cotton varnished fabrics belong to the same heat resistance class.

The main areas of application of varnished fabrics are: electrical machines, apparatus and low voltage devices. Lacquered fabrics are used for flexible turn and groove insulation, as well as various electrical insulating gaskets.

Plastics

Plastics are solid materials that, at a certain stage of manufacturing, acquire plastic properties and in this state can be used to produce products of a given shape. These materials are composite substances consisting of a binder, fillers, dyes, plasticizers and other components. The starting materials for the production of plastic products are pressing powders and pressing materials. According to heat resistance, plastics are classified as thermosetting and thermoplastic.

Laminated electrical insulating plastics

Laminated plastics are materials consisting of alternating layers of sheet filler (paper or fabric) and a binder. The most important of the laminated electrical insulating plastics are getinax, textolite and fiberglass. They consist of sheet fillers arranged in layers, and bakelite, epoxy, organosilicon resins and their compositions are used as binders.

Special types of impregnated paper (in getinaks), cotton fabrics (in textolite) and alkali-free glass fabrics (in fiberglass) are used as fillers. The listed fillers are first impregnated with bakelite or silicone varnishes, dried and cut into sheets of a certain size. Prepared sheet fillers are collected into bags of a given thickness and subjected to hot pressing, during which individual sheets are firmly connected to each other using resins.

Getinax and textolite are resistant to mineral oils, therefore they are widely used in oil-filled electrical devices and transformers. The cheapest laminate material is wood laminate (delta wood). It is obtained by hot pressing of thin sheets of birch veneer, pre-impregnated with bakelite resins. Delta wood is used for the manufacture of power structural and electrical insulating parts operating in oil. To work outdoors, this material needs careful protection from moisture.

Asbestos textolite is a layered electrical insulating plastic obtained by hot pressing of sheets of asbestos fabric, pre-impregnated with bakelite resin. It is produced in the form of shaped products, as well as in the form of sheets and plates with a thickness of 6 to 60 mm. Asbogetinax is a laminated plastic produced by hot pressing of sheets of asbestos paper containing 20% kraft cellulose or asbestos paper without cellulose, impregnated with an epoxy-phenol-formaldehyde binder.

Of the considered layered electrical insulating materials, fiberglass laminates based on organosilicon and epoxy binders have the greatest heat resistance, the best electrical and mechanical characteristics, increased moisture resistance and resistance to fungal mold.

Wound electrical insulating products

Wound electrical insulating products are solid tubes and cylinders made by winding any fibrous materials pre-impregnated with a binder onto round metal rods. Special types of winding or impregnating papers, as well as cotton fabrics and fiberglass fabrics are used as fibrous materials. The binders are bakelite, epoxy, silicone and other resins.

The wound electrical insulating products, together with the metal rods on which they are wound, are dried at high temperature. In order to make the wound products hygroscopic, they are varnished. Each layer of varnish is dried in an oven. Solid textolite rods can also be classified as wound products, because they are also produced by winding blanks from textile filler impregnated with bakelite varnish. After this, the blanks are subjected to hot pressing in steel molds. Wound electrical insulating products are used in transformers with air and oil insulation, in air and oil switches, various electrical devices and electrical equipment components.

Mineral electrical insulating materials

Mineral electrical insulating materials include rocks: mica, marble, slate, soapstone and basalt. This group also includes materials made from Portland cement and asbestos (asbestos cement and asbestos plastic). This entire group of inorganic dielectrics is characterized by high resistance to electric arcs and has fairly high mechanical characteristics. Mineral dielectrics (except mica and basalt) can be machined, with the exception of thread cutting.

Electrical insulating products from marble, slate and soapstone are obtained in the form of boards for panels and electrical insulating bases for switches and low voltage switches. Exactly the same products from fused basalt can only be obtained by casting into molds. In order for basalt products to have the necessary mechanical and electrical characteristics, they are subjected to heat treatment in order to form a crystalline phase in the material.

Electrical insulating products made from asbestos cement and asbestos plastic are boards, bases, partitions and arc extinguishing chambers. To make this kind of product, a mixture consisting of Portland cement and asbestos fiber is used. Asbestos plastic products are produced by cold pressing from a mass to which 15% of a plastic substance (kaolin or molding clay) has been added. This achieves greater fluidity of the initial pressing mass, which makes it possible to obtain electrical insulating products of complex profile from asbestos plastic.

The main disadvantage of many mineral dielectrics (with the exception of mica) is the low level of their electrical characteristics, caused by the large number of pores present and the presence of iron oxides. This phenomenon allows the use of mineral dielectrics only in low voltage devices.

In most cases, all mineral dielectrics, except mica and basalt, are impregnated with paraffin, bitumen, styrene, bakelite resins, etc. before use. The greatest effect is achieved when impregnating already mechanically processed mineral dielectrics (panels, partitions, chambers, etc.).

Marble and products made from it do not tolerate sudden changes in temperature and will crack. Slate, basalt, soapstone, mica and asbestos cement are more resistant to sudden temperature changes.

Mica electrical insulating materials

These materials consist of mica sheets glued together using some kind of resin or adhesive varnish. Glued mica materials include micanites, micafolia and mycalentes. Glued mica materials are used mainly for insulating the windings of high-voltage electrical machines (generators, electric motors), as well as insulating low-voltage machines and machines operating in harsh conditions.

Micanites are hard or flexible sheet materials obtained by gluing sheets of plucked mica using shellac, glyphthalic, organosilicon and other resins or varnishes based on these resins.

The main types of micanites are collector, spacer, molding and flexible. Collector and spacer micanites belong to the group of solid micanites, which, after gluing mica, are pressed at high specific pressures and heat. These micanites have less thickness shrinkage and higher density. Molding and flexible micanite have a looser structure and lower density.

Collector micanite is a solid sheet material made from mica sheets glued together using shellac or glypthal resins or varnishes based on these resins. To ensure mechanical strength when working in the collectors of electrical machines, no more than 4% of adhesive is introduced into these micanites.

Spacer micanite is a solid sheet material made from sheets of plucked mica, glued together using shellac or glyphthalic resins or varnishes based on them. After gluing, the sheets of cushioning micanite are pressed. This material contains 75-95% mica and 25-5% adhesive.

Molding micanite is a solid sheet material made from sheets of plucked mica, glued together using shellac, glyphthalic or organosilicon resins or varnishes based on them. After gluing, the sheets of molding micanite are pressed at a temperature of 140-150° C.

Flexible micanite is a sheet material that is flexible at room temperature. It is made from sheets of plucked mica, glued with oil-bitumen, oil-glyphthalic or silicone varnish (without drier), forming flexible films.

Certain types of flexible micanite are covered with mica paper on both sides to increase mechanical strength. Flexible glass fiber is a sheet material that is flexible at room temperature. This is a type of flexible micanite, characterized by increased mechanical strength and increased resistance to heat. This material is made from sheets of plucked mica, glued together with silicone or oil-glyphthalic varnishes, forming flexible heat-resistant films. Sheets of flexible fiberglass are covered on both sides or one side with alkali-free fiberglass.

Mikafolia is a roll or sheet electrical insulating material, molded in a heated state. It consists of one or several, usually two or three, layers of mica sheets glued together and with a sheet of paper 0.05 mm thick, or with fiberglass, or with a fiberglass mesh. Shellac, glypthal, polyester or organosilicon are used as adhesive varnishes.

Mica tape is a rolled electrical insulating material that is flexible at room temperature. It consists of one layer of sheets of plucked mica, glued together and covered on one or both sides with thin mica paper, fiberglass or fiberglass mesh. Oil-bitumen, oil-glyphthalic, organosilicon and rubber solutions are used as adhesive varnishes.

Mikasilk is a rolled electrical insulating material, flexible at room temperature. Mikasilk is one of the varieties of mycalente, but with increased mechanical tensile strength. It consists of one layer of sheets of plucked mica, glued together and covered on one side with a cloth made of natural silk, and on the other with mica paper. Oil-glyphthalic or oil-bitumen varnishes were used as adhesive varnishes, forming flexible films.

Mikafat is a roll or sheet electrical insulating material that is flexible at room temperature. Mica cloth consists of several layers of plucked mica, glued together and covered on both sides with cotton fabric (percale) or mica paper on one side and fabric on the other.

Micalex is a mica plastic made by pressing from a mixture of powdered mica and glass. After pressing, the products are subjected to heat treatment (drying). Micalex is produced in the form of plates and rods, as well as in the form of electrical insulating products (panels, bases for switches, air capacitors, etc.). When pressing Micalex products, metal parts may be added to them. These products are amenable to all types of mechanical processing.

Mica electrical insulating materials

When developing natural mica and when manufacturing electrical insulating materials based on plucked mica, a large amount of waste remains. Their recycling makes it possible to obtain new electrical insulating materials - mica. This kind of material is made from mica paper, pre-treated with some kind of adhesive (resins, varnishes). Hard or flexible mica electrical insulating materials are obtained from mica paper by gluing with adhesive varnishes or resins and subsequent hot pressing. Adhesive resins can be introduced directly into the liquid mica mass - mica suspension. Among the most important mica materials, the following must be mentioned.

Sludinite collector is a solid sheet material, calibrated in thickness. It is obtained by hot pressing sheets of mica paper treated with shellac varnish. Collector mica is produced in sheets ranging in size from 215 x 400 mm to 400 x 600 mm.

Mica cushioning is a solid sheet material obtained by hot pressing sheets of mica paper impregnated with adhesive varnishes. Spacer mica is produced in sheets measuring 200 x 400 mm. Solid gaskets and washers are made from it for electrical machines and devices with normal and increased overheating.

Molding glass mica is a hard sheet material when cold and flexible when heated. It is obtained by gluing mica paper to fiberglass substrates. Molding heat-resistant glass mica is a solid sheet material molded in a heated state. It is made by gluing sheets of mica paper to fiberglass using heat-resistant silicone varnish. It is produced in sheets measuring 250 x 350 mm or more. This material has increased mechanical tensile strength.

Sludinite flexible is a sheet material that is flexible at room temperature. It is produced by gluing sheets of mica paper followed by hot pressing. Polyester or silicone varnish is used as a binder. Most types of flexible mica are covered with fiberglass on one or both sides. Flexible glass mica (heat resistant) is a sheet material that is flexible at room temperature. It is produced by gluing one or several sheets of mica paper to fiberglass or fiberglass mesh using organosilicon varnishes. After gluing, the material is hot pressed. It is covered with fiberglass on one or both sides to increase mechanical strength.

Sludinitofolium is a roll or sheet material, flexible when heated, obtained by gluing one or several sheets of mica paper with telephone paper 0.05 mm thick, used as a flexible substrate. The scope of application of this material is the same as that of micafolia based on plucked mica. Sludinitofolium is produced in rolls 320-400 mm wide.

Mica tape is a rolled heat-resistant material, flexible at room temperature, consisting of mica paper covered on one or both sides with fiberglass mesh or fiberglass. Mica tapes are produced mainly in rollers with a width of 15, 20, 23, 25, 30 and 35 mm, less often in rolls.

Glass-mica tape is a rolled, cold-flexible material consisting of mica paper, fiberglass mesh and mica paper, glued and impregnated with epoxy-polyester varnish. The surface of the tape is covered with a sticky layer of compound. It is produced in rollers with a width of 15, 20, 23, 30, 35 mm.

Glass mica-electrocardboard is a sheet material that is flexible at room temperature. It is obtained by gluing mica paper, electrical cardboard and fiberglass using varnish. Available in sheets measuring 500 x 650 mm.

Mica-plastic electrical insulating materials

All mica-plastic materials are produced by gluing and pressing sheets of mica-plastic paper. The latter is obtained from non-industrial mica waste as a result of mechanical crushing of particles by an elastic wave. Compared to mica-plastic materials, mica-plastic materials have greater mechanical strength, but are less homogeneous, since they consist of larger particles than mica-plastics. The most important mica-plastic electrical insulating materials are the following.

Collector mica plastic is a solid sheet material calibrated in thickness. It is obtained by hot pressing sheets of mica paper, pre-coated with a layer of adhesive. Available in sheets measuring 215 x 465 mm.

Mica cushioning is a solid sheet material made by hot pressing sheets of mica paper coated with a layer of binder. Available in sheets measuring 520 x 850 mm.

Molding mica is a pressed sheet material that is hard when cold and capable of being molded when heated. Available in sheets ranging in size from 200 x 400 mm to 520 x 820 mm.

Flexible mica is a pressed sheet material that is flexible at room temperature. Available in sheets ranging in size from 200 x 400 mm to 520 x 820 mm. Flexible glass mica plastic is a pressed sheet material, flexible at room temperature, consisting of several layers of mica paper, covered on one side with fiberglass, and on the other with fiberglass mesh or on both sides with fiberglass mesh. Available in sheets ranging in size from 250 x 500 mm to 500 x 850 mm.

Mica-plastic folium is a rolled or sheet material, flexible and moldable in a heated state, obtained by gluing several sheets of mica-plastic paper and pasted on one side with telephone paper or without it.

Mica plastic tape is a roll material flexible at room temperature, consisting of mica plastic paper covered with mica paper on both sides. This material is available in rollers with widths of 12, 15, 17, 24, 30 and 34 mm.

Heat-resistant glass mica plastic tape is a material flexible at room temperature, consisting of one layer of mica paper, covered on one or both sides with fiberglass or fiberglass mesh using silicone varnish. The material is produced in rollers with a width of 15, 20, 25, 30 and 35 mm.

Electroceramic materials and glasses

Electroceramic materials are artificial solids obtained as a result of heat treatment (firing) of initial ceramic masses consisting of various minerals (clay, talc, etc.) and other substances taken in a certain ratio. Various electroceramic products are obtained from ceramic masses: insulators, capacitors, etc.

During the high-temperature firing of these products, complex physical and chemical processes occur between the particles of the starting substances with the formation of new substances of a crystalline and glassy structure.

Electroceramic materials are divided into 3 groups: materials from which insulators are made (insulating ceramics), materials from which capacitors are made (capacitor ceramics), and ferroelectric ceramic materials, which have abnormally high values of dielectric constant and piezoelectric effect. The latter are used in radio engineering. All electroceramic materials are characterized by high heat resistance, weather resistance, resistance to electric sparks and arcs, and have good electrical insulating properties and fairly high mechanical strength.

Along with electroceramic materials, many types of insulators are made of glass. Low-alkali and alkali glasses are used for the production of insulators. Most types of high voltage insulators are made from tempered glass. Tempered glass insulators are superior in mechanical strength to porcelain insulators.

Magnetic materials

The quantities by which the magnetic properties of materials are assessed are called magnetic characteristics. These include: absolute magnetic permeability, relative magnetic permeability, temperature coefficient of magnetic permeability, maximum magnetic field energy, etc. All magnetic materials are divided into two main groups: soft magnetic and hard magnetic.

Magnetically soft materials are characterized by low hysteresis losses (magnetic hysteresis - a lag in the magnetization of a body from the external magnetizing field). They have relatively large magnetic permeability values, low coercive force and relatively high saturation induction. These materials are used for the manufacture of magnetic cores of transformers, electrical machines and devices, magnetic screens and other devices where magnetization with low energy losses is required.

Hard magnetic materials are characterized by large hysteresis losses, i.e., they have high coercive force and high residual induction. These materials, being magnetized, can retain the resulting magnetic energy for a long time, i.e., they become sources of a constant magnetic field. Hard magnetic materials are used to make permanent magnets.

According to their basis, magnetic materials are divided into metallic, nonmetallic and magnetodielectrics. Metallic magnetically soft materials include: pure (electrolytic) iron, sheet electrical steel, iron-Armco, permalloy (iron-nickel alloys), etc. Metallic magnetically hard materials include: alloy steels, special alloys based on iron and aluminum and nickel and alloying components (cobalt, silicon, etc.). Non-metallic magnetic materials include ferrites. These are materials obtained from a powdery mixture of oxides of certain metals and iron oxide. Pressed ferrite products (cores, rings, etc.) are fired at a temperature of 1300-1500° C. Ferrites are either magnetically soft or magnetically hard.

Magnetodielectrics are composite materials consisting of 70-80% powdered magnetic material and 30-20% organic high-polymer dielectric. Ferrites and magnetodielectrics differ from metal magnetic materials in having higher volume resistivity values, which sharply reduces eddy current losses. This allows these materials to be used in high-frequency technology. In addition, ferrites have stable magnetic characteristics over a wide frequency range.

Electrical sheet steel

Electrical steel is a soft magnetic material. To improve the magnetic characteristics, silicon is added to it, which increases the resistivity of the steel, which leads to a reduction in eddy current losses. This steel is produced in the form of sheets with a thickness of 0.1; 0.2; 0.35; 0.5; 1.0 mm, width from 240 to 1000 mm and length from 720 to 2000 mm.

Permalloy

These materials are iron-nickel alloys with a nickel content of 36 to 80%. To improve certain characteristics of permalloys, chromium, molybdenum, copper, etc. are added to their composition. Characteristic features of all permalloys are their easy magnetization in weak magnetic fields and increased values of electrical resistivity.

Permalloys are ductile alloys, easily rolled into sheets and strips with a thickness of up to 0.02 mm or less. Due to their increased resistivity and stability of magnetic characteristics, permalloys can be used up to frequencies of 200-500 kHz. Permalloys are very sensitive to deformation, which causes a deterioration in their original magnetic characteristics. Restoring the original level of magnetic characteristics of deformed permalloy parts is achieved by heat treating them according to a strictly developed regime.

Hard magnetic materials

Magnetically hard materials have large values of coercive force and high residual induction, and therefore, large values of magnetic energy. Hard magnetic materials include:

alloys hardened to martensite (steels alloyed with chromium, tungsten or cobalt);
iron-nickel-aluminum non-malleable alloys of dispersion hardening (alni, alnico, etc.);
malleable alloys based on iron, cobalt and vanadium (viccaloy) or based on iron, cobalt, molybdenum (komol);
alloys with very high coercivity based on noble metals (platinum - iron; silver - manganese - aluminum, etc.);
metal-ceramic non-malleable materials obtained by pressing powdered components followed by firing of pressed products (magnets);
magnetically hard ferrites;
metal-plastic non-malleable materials obtained from pressing powders consisting of particles of magnetically hard material and a binder (synthetic resin);
magnetoelastic materials (magnetoelasts), consisting of a powder of a magnetically hard material and an elastic binder (rubber, rubber).

The residual induction of metal-plastic and magnetoelastic magnets is 20-30% less compared to cast magnets made of the same hard magnetic materials (alni, alnico, etc.).

Ferrites

Ferrites are non-metallic magnetic materials made from a mixture of specially selected metal oxides with iron oxide. The name of ferrite is determined by the name of the divalent metal, the oxide of which is part of the ferrite. So, if the ferrite contains zinc oxide, then the ferrite is called zinc; if manganese oxide is added to the material - manganese.

Complex (mixed) ferrites are used in technology, having higher magnetic characteristics and greater resistivity compared to simple ferrites. Examples of complex ferrites are nickel-zinc, manganese-zinc, etc.

All ferrites are substances of a polycrystalline structure, obtained from metal oxides as a result of sintering powders of various oxides at temperatures of 1100-1300 ° C. Ferrites can only be processed with an abrasive tool. They are magnetic semiconductors. This allows them to be used in high-frequency magnetic fields, since their losses due to eddy currents are insignificant.

Semiconductor materials and products

Semiconductors include a large number of materials that differ from each other in internal structure, chemical composition and electrical properties. According to their chemical composition, crystalline semiconductor materials are divided into 4 groups:

materials consisting of atoms of one element: germanium, silicon, selenium, phosphorus, boron, indium, gallium, etc.;
materials consisting of metal oxides: cuprous oxide, zinc oxide, cadmium oxide, titanium dioxide, etc.;
materials based on compounds of atoms of the third and fifth groups of the Mendeleev system of elements, denoted by a general formula and called antimonides. This group includes compounds of antimony with indium, with gallium, etc., compounds of atoms of the second and sixth groups, as well as compounds of atoms of the fourth group;
semiconductor materials of organic origin, for example polycyclic aromatic compounds: anthracene, naphthalene, etc.

According to the crystal structure, semiconductor materials are divided into 2 groups: monocrystalline and polycrystalline semiconductors. The first group includes materials obtained in the form of large single crystals (single crystals). Among them are germanium and silicon, from which plates are cut for rectifiers and other semiconductor devices.

The second group of materials are semiconductors, consisting of many small crystals soldered to each other. Polycrystalline semiconductors are: selenium, silicon carbide, etc.

In terms of volumetric resistivity, semiconductors occupy an intermediate position between conductors and dielectrics. Some of them sharply reduce electrical resistance when exposed to high voltage. This phenomenon has found application in valve-type arresters to protect power lines. Other semiconductors dramatically decrease their resistance when exposed to light. This is used in photocells and photoresistors. A common property for semiconductors is that they have electron and hole conductivity.

Electrocarbon products (brushes for electric machines)

This type of product includes brushes for electrical machines, electrodes for arc furnaces, contact parts, etc. Electrocarbon products are made by pressing from the original powdery masses, followed by firing.

The initial powdery masses are made up of a mixture of carbonaceous materials (graphite, soot, coke, anthracite, etc.), binders and plasticizing substances (coal and synthetic tars, pitches, etc.). Some powders do not contain a binder.

Brushes for electrical machines are graphite, carbon-graphite, electrographite, metal-graphite. Graphite brushes are made from natural graphite without a binder (soft grades) and with the use of a binder (hard grades). Graphite brushes are soft and cause little noise during operation. Carbon-graphite brushes are made from graphite with the addition of other carbon materials (coke, soot), with the introduction of binders. The brushes obtained after heat treatment are coated with a thin layer of copper (in an electrolytic bath). Carbon-graphite brushes have increased mechanical strength, hardness and low wear during operation.

Electrographitized brushes are made from graphite and other carbon materials (coke, soot), with the introduction of binders. After the first firing, the brushes are subjected to graphitization, i.e., annealing at a temperature of 2500-2800 ° C. Electrographitized brushes have increased mechanical strength, resistance to shock load changes and are used at high peripheral speeds. Metal-graphite brushes are made from a mixture of graphite and copper powders. Some of them contain powders of lead, tin or silver. These brushes feature low resistivity values, tolerate high current densities, and have low transient voltage drops.

Conductor materials

Electrical insulating materials

Electrical insulating varnishes and enamels

Electrical insulating compounds

Unimpregnated fibrous electrical insulating materials

Letheroid is a thin sheet and roll fiber used for the manufacture of various types of electrical insulating gaskets, washers and shaped products.

Electrical insulating varnished fabrics (varnished fabrics)

Plastics

Laminated electrical insulating plastics

Wound electrical insulating products

Mineral electrical insulating materials

Marble and products made from it do not tolerate sudden changes in temperature and will crack. Slate, basalt, soapstone, mica and asbestos cement are more resistant to sudden temperature changes.

Mica electrical insulating materials

Micanites are hard or flexible sheet materials obtained by gluing sheets of plucked mica using shellac, glyphthalic, organosilicon and other resins or varnishes based on these resins.

Mica electrical insulating materials

Mica-plastic electrical insulating materials

Mica cushioning is a solid sheet material made by hot pressing sheets of mica paper coated with a layer of binder. Available in sheets measuring 520 x 850 mm.

Molding mica is a pressed sheet material that is hard when cold and capable of being molded when heated. Available in sheets ranging in size from 200 x 400 mm to 520 x 820 mm.

Electroceramic materials and glasses

Magnetic materials

Electrical sheet steel

Permalloy

Hard magnetic materials

Magnetically hard materials have large values of coercive force and high residual induction, and therefore, large values of magnetic energy. Hard magnetic materials include:

alloys hardened to martensite (steels alloyed with chromium, tungsten or cobalt);
iron-nickel-aluminum non-malleable alloys of dispersion hardening (alni, alnico, etc.);
malleable alloys based on iron, cobalt and vanadium (viccaloy) or based on iron, cobalt, molybdenum (komol);
alloys with very high coercivity based on noble metals (platinum - iron; silver - manganese - aluminum, etc.);
metal-ceramic non-malleable materials obtained by pressing powdered components followed by firing of pressed products (magnets);
magnetically hard ferrites;
metal-plastic non-malleable materials obtained from pressing powders consisting of particles of magnetically hard material and a binder (synthetic resin);
magnetoelastic materials (magnetoelasts), consisting of a powder of a magnetically hard material and an elastic binder (rubber, rubber).

The residual induction of metal-plastic and magnetoelastic magnets is 20-30% less compared to cast magnets made of the same hard magnetic materials (alni, alnico, etc.).

Ferrites

Semiconductor materials and products

materials consisting of atoms of one element: germanium, silicon, selenium, phosphorus, boron, indium, gallium, etc.;
materials consisting of metal oxides: cuprous oxide, zinc oxide, cadmium oxide, titanium dioxide, etc.;
materials based on compounds of atoms of the third and fifth groups of the Mendeleev system of elements, denoted by a general formula and called antimonides. This group includes compounds of antimony with indium, with gallium, etc., compounds of atoms of the second and sixth groups, as well as compounds of atoms of the fourth group;
semiconductor materials of organic origin, for example polycyclic aromatic compounds: anthracene, naphthalene, etc.

The second group of materials are semiconductors, consisting of many small crystals soldered to each other. Polycrystalline semiconductors are: selenium, silicon carbide, etc.

Electrocarbon products (brushes for electric machines)

The article provides information about the types of materials used in the manufacture of electric motors, generators and transformers. Brief technical characteristics of some of them are given.

Classification of electrical materials

Materials used in electrical machines are divided into three categories: structural, active and insulating.

Construction materials

are used for the manufacture of such parts and machine parts, the main purpose of which is the perception and transmission of mechanical loads (shafts, frames, bearing shields and risers, various fasteners, and so on). Cast iron, non-ferrous metals and their alloys, and plastics are used as structural materials in electrical machines. These materials are subject to requirements that are common in mechanical engineering.

Active materials

are divided into conductive and magnetic and are intended for the manufacture of active parts of the machine (windings and magnetic cores).
Insulating materials are used for electrical insulation of windings and other current-carrying parts, as well as for insulating sheets of electrical steel from each other in laminated magnetic cores. A separate group consists of materials from which electric brushes are made, used to drain current from the moving parts of electrical machines.

Below is a brief description of the active and insulating materials used in electrical machines.

Conductor materials

Due to its good electrical conductivity and relative cheapness in quality, electrical materials are widely used in electric machines, and recently also refined ones. The comparative properties of these materials are given in Table 1. In some cases, the windings of electrical machines are made of copper and aluminum alloys, the properties of which vary widely depending on their composition. Copper alloys are also used for the manufacture of auxiliary current-carrying parts (commutator plates, slip rings, bolts, etc.). In order to save non-ferrous metals or increase mechanical strength, such parts are sometimes also made of steel.

Table 1

Physical properties of copper and aluminum

Material	Variety	Density, g/cm 3	Resistivity at 20°C, Ohm×m	Temperature coefficient of resistance at ϑ °C, 1/°C	Linear expansion coefficient, 1/°C	Specific heat capacity, J/(kg×°C)	Specific thermal conductivity, W/(kg×°C)
Copper	Electrical annealed	8,9	(17.24÷17.54)×10 -9		1.68×10 -5	390	390
Aluminum	Refined	2,6-2,7	28.2×10 -9		2.3×10 -5	940	210

Temperature coefficient of resistance of copper at temperature ϑ °C

The dependence of copper resistance on temperature is used to determine the increase in the temperature of the winding of an electrical machine when it operates in a hot state ϑ g above the ambient temperature ϑ o. Based on relation (2) to calculate the temperature rise

Δϑ = ϑ g - ϑ o

you can get the formula

(3)

Where r g - winding resistance in a hot state; r x- winding resistance measured in a cold state, when the temperatures of the winding and the environment are the same; ϑ x- cold winding temperature; ϑ o - ambient temperature when the machine is operating, when resistance is measured r G.

Relations (1), (2) and (3) are also applicable for aluminum windings if 235 is replaced with 245.

Magnetic materials

For the manufacture of individual parts of the magnetic circuits of electrical machines, sheet electrical steel, sheet structural steel, sheet steel and cast iron are used. Due to its low magnetic properties, cast iron is used relatively rarely.

The most important class of magnetic materials consists of various grades of electrical steel sheets. To reduce losses on and into its composition, silicon is introduced. The presence of impurities of carbon, oxygen and nitrogen reduces the quality of electrical steel. The quality of electrical steel is greatly influenced by its manufacturing technology. Conventional electrical steel sheets are produced by hot rolling. In recent years, the use of cold-rolled grain-oriented steel, whose magnetic properties during magnetization reversal along the rolling direction are significantly higher than those of conventional steel, has been rapidly growing.

The range of electrical steel and the physical properties of individual grades of this steel are determined by GOST 21427.0-75.

Electrical machines mainly use electrical steel grades 1211, 1212, 1213, 1311, 1312, 1411, 1412, 1511, 1512, 3411, 3412, 3413, which correspond to the old designations of steel grades E11, E12, E13, E21, E22, E31 , E32, E41, E42, E310, E320, E330. The first digit indicates the class of steel by structural state and type of rolling: 1 - hot-rolled isotropic, 2 - cold-rolled isotropic, 3 - cold-rolled anisotropic with rib texture. The second number shows the silicon content. The third digit indicates the group according to the main standardized characteristic: 0 - specific losses at B= 1.7 T and f= 50 Hz (p 1.7/50), 1 - specific losses at B= 1.5 T and frequency f= 50 Hz (p 1.5/50), 2 - specific losses due to magnetic induction B= 1.0 T and frequency f= 400 Hz (p 1.0/400), 6 - magnetic induction in weak fields at 0.4 A/m ( B 0.4), and 7 - magnetic induction in average magnetic fields at a magnetic field strength of 10A/m ( B 10). The fourth digit is the serial number. The properties of electrical steel depending on the silicon content are given in Table 2

table 2

Dependence of the physical properties of electrical steel on silicon content

Properties	Second digit of steel grade
Properties	2	3	4	5

Density, g/cm 3
Specific resistance, Ohm×m
Temperature coefficient of resistance, 1/°C
Specific heat capacity, J/(kg×°C)

As the silicon content increases, the brittleness of steel increases. In this regard, the smaller the machine and, consequently, the smaller the size of the teeth and grooves into which the windings are placed, the more difficult it is to use steels with increased and high degrees of alloying. Therefore, for example, high-alloy steel is used mainly for the manufacture of transformers and very powerful generators.

In machines with current frequencies up to 100 Hz, electrical steel sheets with a thickness of 0.5 mm are usually used, and sometimes also, especially in transformers, steel with a thickness of 0.35 mm. At higher frequencies, thinner steel is used. The dimensions of electrical steel sheets are standardized, with sheet widths ranging from 240 to 1000 mm and lengths from 1500 to 2000 mm. Recently, the production of electrical steel in the form of strips wound on rolls has been expanding.

Rice. 1. Magnetization curves of ferromagnetic materials

1 - electrical steel 1121, 1311; 2 - electrical steel 1411, 1511; 3 - low-carbon cast steel, rolled steel and forgings for electrical machines; 4 - sheet steel 1-2 mm thick for poles; 5 - steel 10; 6 - steel 30; 7 - cold rolled electrical steel 3413; 8 - gray cast iron with content: C - 3.2%, Si 3.27%, Mn - 0.56%, P - 1.05%; I × A - scales along axes I and A; II × B - scales along axes II and B

Figure 1 shows various grades of steel and cast iron, and Table 3, according to GOST 21427.0-75, shows the values of specific losses p in the most common grades of electrical steel. The index of the letter p indicates the induction B in Tesla (numerator) and the magnetization reversal frequency f in Hertz (denominator), at which the loss values given in Table 3 are guaranteed. For grades 3411, 3412 and 3413, losses are given for the case of magnetization along the rolling direction.

Table 3

Specific losses in electrical steel

steel grade	Sheet thickness, mm	Specific losses, W/kg			steel grade	Sheet thickness, mm	Specific losses, W/kg
steel grade	Sheet thickness, mm	p 1.0/50	p 1.5/50	p 1.7/50	steel grade	Sheet thickness, mm	p 1.0/50	p 1.5/50	p 1.7/50
1211	0,5	3,3	7,7	-	1512	0,5	1,4	3,1	-
1212	0,5	3,1	7,2	-		0,35	1,2	2,8	-
1213	0,5	2,8	6,5	-	1513	0,5	1,25	2,9	-
1311	0,5	2,5	6,1	-		0,35	1,05	2,5	-
1312	0,5	2,2	5,3	-	3411	0,5	1,1	2,45	3,2
1411	0,5	2,0	4,4	-		0,35	0,8	1,75	2,5
1412	0,5	1,8	3,9	-	3412	0,5	0,95	2,1	2,8
1511	0,5	1,55	3,5	-		0,35	0,7	1,5	2,2
	0,35	1,35	3,0	-	3413	0,5	0,8	1,75	2,5
						0,35	0,6	1,3	1,9

Eddy current losses depend on the square of the induction, and hysteresis losses depend on the induction to a power close to two. Therefore, the total losses in steel, with sufficient accuracy for practical purposes, can be considered to depend on the square of the induction. Eddy current losses are proportional to the square of the frequency, and hysteresis losses are proportional to the first power of frequency. At a frequency of 50 Hz and a sheet thickness of 0.35 - 0.5 mm, losses due to hysteresis exceed losses due to eddy currents several times. The dependence of the total losses in steel on frequency is therefore closer to the first power of frequency. Therefore, specific losses for values B And f, different from those indicated in Table 3, can be calculated using the formulas:

(4)

where the value of B is substituted in teslas (T).

The specific loss values given in Table 3 correspond to the case when the sheets are isolated from each other.

For insulation, a special varnish or, very rarely, thin paper is used, and oxidation is also used.

During stamping, cold hardening of electrical steel sheets occurs. In addition, when assembling core packages, partial closure of the sheets occurs along their edges due to the appearance of burrs or burrs during stamping. This increases losses in steel by 1.5 - 4.0 times.

Due to the presence of insulation between the steel sheets, their waviness and heterogeneity in thickness, not the entire volume of the compressed core is filled with steel. The average filling factor of a bag with steel when insulated with varnish is k c= 0.93 with a sheet thickness of 0.5 mm and k c= 0.90 at 0.35 mm.

Insulation materials

The following requirements are imposed on electrical insulating materials used in electrical machines: high mechanical strength, heat resistance and thermal conductivity, as well as low hygroscopicity. It is important that the insulation be as thin as possible, since an increase in the thickness of the insulation impairs heat transfer and leads to a decrease in the fill factor of the groove with conductor material, which in turn causes a decrease in the rated power of the machine. In some cases, other requirements also arise, for example, resistance against various microorganisms in humid tropical climates, and so on. In practice, all these requirements can be satisfied to varying degrees.

Video 1. Insulating materials in electrical engineering of the 18th - 19th centuries.

Insulating materials can be solid, liquid or gaseous. The gases are usually air and hydrogen, which represent an ambient or cooling medium in relation to the machine and at the same time, in some cases, play the role of electrical insulation. Liquid oils are used mainly in transformer manufacturing in the form of a special type of mineral oil called transformer oil.

Solid insulating materials are of greatest importance in electrical engineering. They can be divided into the following groups: 1) natural organic fibrous materials - cotton paper, wood pulp-based materials and silk; 2) inorganic materials - mica, fiberglass, asbestos; 3) various synthetic materials in the form of resins, films, sheet material, and so on; 4) various enamels, varnishes and compounds based on natural and synthetic materials.
In recent years, organic fiber insulation materials have been increasingly replaced by synthetic materials.

Enamels are used for insulating wires and as outer insulation for windings. Varnishes are used for gluing layered insulation and for impregnating windings, as well as for applying a protective coating layer to the insulation. By impregnating the windings two or three times with varnishes, alternating with drying, the pores in the insulation are filled, which increases the thermal conductivity and electrical strength of the insulation, reduces its hygroscopicity and mechanically holds the insulation elements together.

Impregnation with compounds serves the same purpose as impregnation with varnishes. The only difference is that the compounds do not have volatile solvents, but are a very consistent mass, which, when heated, softens, liquefies and is capable of penetrating into the pores of the insulation under pressure. Due to the absence of solvents, the filling of pores during compounding is more dense.
The most important characteristic of insulating materials is their heat resistance, which decisively affects the reliability of operation and service life of electrical machines. According to heat resistance, used in electrical machines and devices, they are divided, according to GOST 8865-70, into seven classes with the following maximum permissible temperatures ϑ max:

The standards of previous years contain the old designations of some insulation classes: instead of Y, E, F, H, respectively, O, AB, BC, SV.

Class Y includes fibrous materials made of cotton paper, cellulose and silk that are not impregnated with liquid dielectrics or immersed in them, as well as a number of synthetic polymers (polyethylene, polystyrene, polyvinyl chloride, etc.). This insulation class is rarely used in electrical machines.

Class A includes fibrous materials made of cotton paper, cellulose and silk, impregnated or immersed in liquid electrical insulating materials, insulation of enamel wires based on oil and polyamide resole varnishes (nylon), polyamide films, butyl rubber and other materials, as well as impregnated wood and wood laminates. Impregnating substances for this class of insulation are transformer oil, oil and asphalt varnishes and other substances with appropriate heat resistance. This class includes various varnished fabrics, tapes, electrical cardboard, getinaks, textolite and other insulating products. Class A insulation is widely used for rotating electrical machines with power up to 100 kW and above, as well as in the transformer industry.

Class E includes insulation of enamel wires and electrical insulation based on polyvinyl acetal (viniflex, metalvin), polyurethane, epoxy, polyester (lavsan) resins and other synthetic materials with similar heat resistance. Insulation class E includes new synthetic materials, the use of which is rapidly expanding in low and medium power machines (up to 10 kW and above).

Class B combines insulating materials based on inorganic dielectrics (mica, asbestos, fiberglass) and adhesive, impregnating and coating varnishes and resins of increased heat resistance of organic origin, and the content of organic substances by weight should not exceed 50%. This includes, first of all, materials based on thin plucked mica (micalenta, micafolia, micanite), widely used in electrical engineering.

Recently, mica materials have also been used, which are based on a continuous mica ribbon of mica plates up to several millimeters in size and several microns thick.

Class B also includes various synthetic materials: polyester resins based on phthalic anhydride, polychlorotrifluoroethylene (fluoroplastic-3), some polyurethane resins, plastics with inorganic filler, etc.

Class F insulation includes materials based on mica, asbestos and fiberglass, but with the use of organic varnishes and resins modified with organosilicon (organosiloxane) and other resins with high heat resistance, or with the use of other synthetic resins of corresponding heat resistance (polyester resins based on ISO - and terephthalic acids, etc.). Insulation of this class must not contain cotton, cellulose or silk.

Class H includes insulation based on mica, fiberglass and asbestos in combination with organosilicon (organopolysiloxane), polyorganometallosilxane and other heat-resistant resins. Using such resins, micanites and mica, as well as steklomicanites, steklomicafolium, steklomicalents, steklosludinit, glass laminates and fiberglass laminates are produced.

Class H also includes insulation based on polytetrafluoroethylene (PTFE-4). Class H materials are used in electrical machines operating in very difficult conditions (mining and metallurgical industries, transport installations, etc.).

Class C insulation includes mica, quartz, fiberglass, glass, porcelain and other ceramic materials used without organic binders or with inorganic binders.

Under the influence of heat, vibration and other physical and chemical factors, the insulation ages, i.e., it gradually loses its mechanical strength and insulating properties. It has been experimentally established that the service life of class A and B insulation is reduced by half with an increase in temperature of every 8-10° above 100°C. Similarly, the service life of other classes of insulation also decreases with increasing temperature.

Electric brushes

are divided into two groups: 1) carbon-graphite, graphite and electrographite; 2) metalgraphite. To make brushes of the first group, carbon black, crushed natural graphite and anthracite with coal tar as a binder are used. Brush blanks are fired, the regime of which determines the structural form of the graphite in the product. At high firing temperatures, the carbon contained in soot and anthracite is converted into the form of graphite, as a result of which this firing process is called graphitization. Brushes of the second group also contain metals (copper, silver). The most common are brushes of the first group.

Table 4 shows the characteristics of a number of brands of brushes.

Table 4

Technical characteristics of electric brushes

Brush class	Brand	Nominal, A/cm 2	Maximum peripheral speed, m/s	Specific pressure, N/cm 2	Adapter for a pair of brushes, V	Friction coefficient	Characteristics for which the use of brushes is recommended
Carbon-graphite	UG4	7	12	2-2,5	1,6-2,6	0,25	Somewhat difficult
Graphite	G8	11	25	2-3	1,5-2,3	0,25	Normal
Electrographitized	EG4	12	40	1,5-2	1,6-2,4	0,20	Normal
	EG8	10	40	2-4	1,9-2,9	0,25	The most difficult
	EG12	10-11	40	2-3	2,5-3,5	0,25	Difficult
	EG84	9	45	2-3	2,5-3,5	0,25	The most difficult
Copper-graphite	MG2	20	20	1,8-2,3	0,3-0,7	0,20	The easiest