Some materials used in electrical appliances and power supply circuits have dielectric properties, that is, they have a high resistance to current. This ability allows them not to pass current, and therefore they are used to create insulation for current-carrying parts. Electrical insulating materials are designed not only to separate current-carrying parts, but also to create protection against the dangerous effects of electric current. For example, the power cords of electrical appliances are covered with insulation.
Electrical insulating materials and their applications
Electrical insulating materials are widely used in industry, radio and instrument making, and the development of electrical networks. The normal operation of an electrical appliance or the safety of a power supply circuit depends largely onused dielectrics. Some parameters of a material intended for electrical insulation determine its quality and capabilities.
The use of insulating materials is subject to safety regulations. The integrity of the insulation is the key to safe work with electric current. It is very dangerous to use devices with damaged insulation. Even a slight electric current can have an effect on the human body.
Properties of dielectrics
Electrical insulating materials must have certain properties in order to perform their functions. The main difference between dielectrics and conductors is the large volume resistivity (109–1020 ohm cm). The electrical conductivity of conductors in comparison with dielectrics is 15 times greater. This is due to the fact that insulators by their nature have several times fewer free ions and electrons, which provide the current conductivity of the material. But when the material is heated, there are more of them, which contributes to an increase in electrical conductivity.
Distinguish between active and passive properties of dielectrics. For insulating materials, passive properties are most important. The dielectric constant of the material should be as low as possible. This allows the isolator not to introduce parasitic capacitances into the circuit. For the material that is used as the dielectric of a capacitor, the dielectric constant should, on the contrary, be as large as possible.
Insulation Options
To the main parameterselectrical insulation include electrical strength, electrical resistivity, relative permittivity, dielectric loss angle. When evaluating the electrical insulating properties of the material, the dependence of the listed characteristics on the magnitudes of the electric current and voltage is also taken into account.
Electrical insulating products and materials have a greater electrical strength in comparison with conductors and semiconductors. Also important for the dielectric is the stability of specific values during heating, voltage increase and other changes.
Classification of dielectric materials
Depending on the power of the current passing through the conductor, different types of insulation are used, which differ in their capabilities.
According to what parameters are electrical insulating materials divided? The classification of dielectrics is based on their state of aggregation (solid, liquid and gaseous) and origin (organic: natural and synthetic, inorganic: natural and artificial). The most common type of solid dielectric, which can be seen on the cords of household appliances or any other electrical appliance.
Solid and liquid dielectrics, in turn, are divided into subgroups. Solid dielectrics include varnished fabrics, laminates and various types of mica. Waxes, oils and liquefied gases are liquid electrical insulating materials. Special gaseous dielectrics are used much less frequently. This type also includesthe natural electrical insulator is air. Its use is due not only to the characteristics of air, which make it an excellent dielectric, but also to its economy. The use of air as insulation does not require additional material costs.
Solid Dielectrics
Solid electrical insulating materials are the widest class of dielectrics that are used in various fields. They have different chemical properties, and the dielectric constant ranges from 1 to 50,000.
Solid dielectrics are divided into non-polar, polar and ferroelectrics. Their main differences are in the mechanisms of polarization. This class of insulation has such properties as chemical resistance, tracking resistance, dendritic resistance. Chemical resistance is expressed in the ability to withstand the influence of various aggressive environments (acid, alkali, etc.). Tracing resistance determines the ability to withstand the effects of an electric arc, and dendritic resistance determines the formation of dendrites.
Solid dielectrics are used in various fields of energy. For example, ceramic electrical insulating materials are most commonly used as line and bushing insulators in substations. Paper, polymers, fiberglass are used as insulation for electrical appliances. For machines and devices, varnishes, cardboard, compound are most often used.
For use in various operating conditions, insulation is given some special properties by combining differentmaterials: heat resistance, moisture resistance, radiation resistance and frost resistance. Heat-resistant insulators are able to withstand temperatures up to 700 °C, these include glasses and materials based on them, organosilites and some polymers. Moisture-resistant and tropical-resistant material is fluoroplastic, which is non-hygroscopic and hydrophobic.
Radiation resistant insulation is used in devices with atomic elements. It includes inorganic films, some types of polymers, fiberglass and mica-based materials. Frost-resistant are insulations that do not lose their properties at temperatures up to -90 ° C. Special requirements are placed on insulation intended for devices operating in space or vacuum conditions. For these purposes, vacuum-tight materials are used, which include special ceramics.
Liquid dielectrics
Liquid electrical insulating materials are often used in electrical machines and apparatus. Oil plays the role of insulation in a transformer. Liquid dielectrics also include liquefied gases, unsaturated vaseline and paraffin oils, polyorganosiloxanes, distilled water (purified from s alts and impurities).
The main characteristics of liquid dielectrics are dielectric constant, electrical strength and electrical conductivity. Also, the electrical parameters of dielectrics largely depend on the degree of their purification. Solid impurities can increase the electrical conductivity of liquids due to the growth of free ions and electrons. Purification of liquids by distillation, ion exchange, etc. leads to an increase in the electrical strength of the material, thereby reducing its electrical conductivity.
Liquid dielectrics are divided into three groups:
- petroleum oils;
- vegetable oils;
- synthetic fluids.
The most commonly used oils are petroleum oils such as transformer, cable and capacitor oils. Synthetic fluids (organosilicon and organofluorine compounds) are also used in apparatus engineering. For example, organosilicon compounds are frost-resistant and hygroscopic, so they are used as an insulator in small transformers, but their cost is higher than the price of petroleum oils.
Vegetable oils are practically not used as insulating materials in electrical insulating technology. These include castor, linseed, hemp and tung oil. These materials are weakly polar dielectrics and are mainly used for impregnating paper capacitors and as a film-forming agent in electrical insulating varnishes, paints, and enamels.
Gas dielectrics
The most common gaseous dielectrics are air, nitrogen, hydrogen and SF6. Electrical insulating gases are divided into natural and artificial. Natural air is used as insulation between the current-carrying parts of power lines and electrical machines. As an insulator, air has disadvantages that make it impossible to use it in sealed devices. Due to the presence of a high concentration of oxygen, air is an oxidizing agent, and in inhomogeneous fields, a low electrical strength of air appears.
Power transformers and high-voltage cables use nitrogen as insulation. Hydrogen, in addition to being an electrically insulating material, is also forced cooling, which is why it is often used in electrical machines. In sealed installations, SF6 is most often used. Filling with SF6 gas makes the device explosion-proof. It is used in high-voltage circuit breakers due to its arc-extinguishing properties.
Organic dielectrics
Organic dielectric materials are divided into natural and synthetic. Natural organic dielectrics are currently used extremely rarely, as the production of synthetic ones is expanding more and more, thereby reducing their cost.
To natural organic dielectrics include cellulose, rubber, paraffin and vegetable oils (castor oil). Most of the synthetic organic dielectrics are various plastics and elastomers often used in electrical household appliances and other equipment.
Inorganic dielectrics
Inorganic dielectric materials are divided into natural and artificial. The most common of the natural materials is mica, which has chemical and thermal resistance. Phlogopite and muscovite are also used for electrical insulation.
To artificial inorganicdielectrics include glass and materials based on it, as well as porcelain and ceramics. Depending on the application, the artificial dielectric can be given special properties. For example, feldspar ceramics are used for bushings, which have a high dielectric loss tangent.
Fibrous electrical insulating materials
Fibrous materials are often used for insulation in electrical apparatus and machines. These include materials of plant origin (rubber, cellulose, fabrics), synthetic textiles (nylon, capron), as well as materials made of polystyrene, polyamide, etc.
Organic fibrous materials are highly hygroscopic, so they are rarely used without special impregnation.
Recently, instead of organic materials, synthetic fiber insulation has been used, which have a higher level of heat resistance. These include glass fiber and asbestos. Glass fiber is impregnated with various varnishes and resins to increase its hydrophobic properties. Asbestos fiber has low mechanical strength, so cotton fiber is often added to it.
