Insulating materials are materials that are not electrically conductive under the allowable voltage, but are not absolutely non-conductive materials. Under certain external electric field strength, conductive, polarization, loss, breakdown, etc. may occur, and aging may occur in long-term use. . Its electrical resistivity is very high, usually in the range of 1010 to 1022 Ω·m. In the motor, for example, the insulating material around the conductor isolates the turns and isolates it from the grounded stator core to ensure safe operation of the motor.
A brief history of insulation development
Insulation material is the basis and guarantee for the development of electrical products. It plays a particularly important role in the development of electrical machinery and electrical industry. The development and progress of insulation materials depends on the development of polymer materials and directly restricts and influences the development and progress of electrical products. .
Insulating materials are the key to the advanced technology of electrical products, and also an important guarantee for the long-term safe and reliable operation of electrical products. Therefore, it is required to continuously develop new varieties of insulating materials, improve product performance and quality, and adapt to the continuous development of electrical products. After more than 50 years of development, China’s insulating materials industry has initially formed a relatively complete product, complete supporting facilities, and considerable production scale. And the industrial system of scientific research strength. Especially in the past 20 years, the variety of insulating materials has developed rapidly, the quality has been greatly improved, and the product level has reached a new height.
Before the 20th century, insulating materials were basically derived from natural materials or their products. The earliest electric motors used silk, cotton yarn and cotton cloth as insulation materials. In order to improve water resistance and the like, natural resin such as shellac is impregnated with vegetable oil or pitch.
At the beginning of the 20th century, due to the development of organic synthesis and polymer chemistry, humans made the first synthetic polymer, phenolic resin, which is also an important invention in the field of insulating materials. Once the phenolic resin was introduced, it was quickly widely used, and it was made into a phenolic resin-based impregnating varnish, plastic, impregnated fiber product and laminate. Then, urea formaldehyde resin, aniline formaldehyde resin, melamine formaldehyde resin, glycerin resin and the like appeared.
Since the 1930s, polyvinyl chloride, polyethylene, polytetrafluoroethylene, neoprene, polyvinyl acetal have been developed.
After the 1950s, industrial production of silicone resins, polyester films, unsaturated polyester resins, and epoxy resins, as well as industrial production of glass fiber and powder mica products, promoted the development of insulating materials. With the development of modern polymer chemistry and industry, the development of new insulating materials based on synthetic polymers has really begun. The polymers appearing in the previous paragraph were successively applied to insulating materials, and new types of insulating materials were rapidly developed, such as solventless paints for motor impregnation; thin film composite products as slot insulation for motors; powder mica products developed rapidly and used For large high-voltage generators; sulfur hexafluoride is available and is used in high-voltage electrical appliances.
In the 1970s, while the polymer industry was further developing into large-scale industrialization, the new F-class and H-class insulating material systems began to appear in the insulating materials industry, and polyimide, polyamideimide, and poly were successively developed. Heat-resistant insulating varnishes such as imides, polymaleimides, and polydiphenyl ethers, adhesives and films, and modified epoxy, unsaturated polyester, aramid fiber paper, and composite materials thereof New product. The heat resistance grade of electrical products has risen to B grade in large quantities, and new F-class and H-class insulation materials have been adopted in special motors such as metallurgy, cranes and locomotive motors.
After entering the 1980s , China carried out large-scale independent development of Class F and Class H insulation materials to improve performance, such as the appearance of modified diphenyl ether, modified bismaleimide, modified polyester Amine enameled wire lacquer, polyamideimide enamel coated wire lacquer, polyimide enamel coated wire lacquer, F grade, H grade glass fiber products, high performance polyimide film, F grade epoxy powder mica tape. Solventless impregnating resins and fast drying impregnating varnishes have developed rapidly. The use of less rubber powder mica tape and VPI (vacuum pressure impregnation) impregnating resin began.
Modern application of nanotechnology to develop nano-insulation materials. Nanotechnology can be used in many fields, including the field of insulating materials. The nano-scale (ranging from 1 to 100 nm) powder is uniformly dispersed in the polymer resin, and it is also possible to form or add nano-scale crystal grains or non-grain substances inside the polymer, and to form nano-scale micropores. Or bubbles. Due to the structural characteristics of nano-sized particles, the composite materials exhibit a series of unique and bizarre properties, making nanomaterials into a promising new material field. At present, China has carried out research in this area. For example, the preparation of polyimide/montmorillonite nanocomposite film by Sichuan University has been successful. The application of nanomaterials is bound to be a new and different performance that many traditional insulating materials cannot achieve, opening up the development prospects of new materials and new technologies.
Insulating material, also called dielectric, refers to a material that is not conductive or has a small conductive electrode under the action of a direct current voltage, and its resistivity is generally greater than 1010 Ω·m. The main function of the insulating material is to isolate the charged conductors of different potentials in the electrical equipment, so that the current can flow in a certain path, and can also be mechanically supported and fixed, as well as arc extinguishing, heat dissipation, energy storage, moisture proof, mildew proof. Or improve the potential distribution of the electric field and protect the conductor. Therefore, the insulating material is required to have as high an insulation resistance, heat resistance, moisture resistance as possible, and a certain mechanical strength is required.
Basic properties of insulating materials
Main performance indicators of insulating materials In order to prevent accidents caused by insulation damage of insulating materials, it is necessary to make the insulating materials meet the performance specifications stipulated by national standards. However, there are many performance indexes of insulating materials, and the characteristics of various insulating materials are also different. The main performance indexes of commonly used insulating materials are breakdown strength, heat resistance, insulation resistance and mechanical strength.
(1) Breakdown strength. Under the action of the electric field strength above a certain value, the insulating material will be damaged and lose its insulating properties. This phenomenon is called breakdown. The electric field strength when the insulating material is broken down is called the breakdown strength and the unit is: kV/mm.
(2) Heat resistance. When the temperature rises, the electrical resistance, breakdown strength, mechanical strength and the like of the insulating material are lowered. Therefore, it is required that the insulating material can work for a long time at a prescribed temperature and the insulation performance is ensured to be reliable. Insulation materials with different compositions have different heat resistance levels, and the heat resistance grades can be divided into seven grades of Y, A, E, B, F, H, and C, and the maximum limit operating temperature is specified for each grade of insulating material.
Grade Y: The limit temperature is 90 °C, such as wood, cotton yarn, paper fiber, acetate fiber, polyamide and other textiles and plastic insulation which is easy to thermally decompose and has low melting point.
Class A: The ultimate working temperature is 105 ° C, such as enameled wire, varnish, lacquer, oil paint and asphalt insulation.
Class E: The limit working temperature is 120 °C, such as glass cloth, oily resin paint, high-strength enameled wire, vinyl acetate heat-resistant enameled wire and other insulation.
Grade B: The ultimate working temperature is 130 ° C, such as polyester thin wax, mica, glass fiber, asbestos, polyester paint, polyester enameled wire and other insulation treated with corresponding resin.
Class F: Extreme working temperature is 155 ° C, such as mica bonded or impregnated with Class F insulating resin, coated mica, glass wool, asbestos, glass varnish and laminates based on the above materials, mica, powder products, chemistry Polyester and alkyd materials with good thermal stability, composite silicone organic polyester paint.
Class H: The ultimate working temperature is 180 °C, such as thick F-grade materials, mica, silicone mica products, silicon organic paint, silicone rubber polyimide composite glass cloth, composite film, polyimide paint, etc.
Class C: The limit operating temperature is greater than 180 °C. Refers to inorganic materials that do not use any organic binders and impregnating agents, such as quartz, asbestos, mica, glass, ceramics, and tetrafluoroethylene plastics.
(3) Insulation resistance. The resistance value exhibited by the insulating material is the insulation resistance. In the normal state, the insulation resistance is generally several tens of megaohms or more. The insulation resistance varies greatly depending on the temperature, thickness, and surface condition (moisture, dirt, etc.). Although the electrical resistivity of the insulating material is very high, it is under a certain voltage. There is always a small current passing through, and this current is called leakage current.
(4) Mechanical strength. According to the specific requirements of various insulating materials, the corresponding specified tensile strength, compression resistance, bending resistance, shear resistance, tear resistance, impact resistance and other strength indicators are collectively referred to as mechanical strength.
(5) Other characteristic indicators. Some insulating materials are presented in liquid form, such as various insulating varnishes. Their properties include viscosity, fixed content, acid value, drying time and gel time. Some insulating material properties also refer to permeability, oil resistance, elongation, shrinkage, solvent resistance, arc resistance, and the like.
Aging of insulating materials
Insulating materials will undergo physical phenomena such as polarization, conductance, medium heating, and breakdown under the action of an electric field. While being subjected to the electric field, they will also be affected by many factors such as mechanical and chemical factors. Long-term _T will appear aging. . Therefore, many failures of electrical products often occur in the insulating portion. The aging of the dielectric refers to the phenomenon that the electrical properties and mechanical properties of the dielectric gradually deteriorate over time during long-term operation. The main forms of aging are electrical aging, heat aging and environmental aging.
(1) Electrical aging. More common in high-voltage electrical appliances, the main reason is the partial discharge of the insulating material under high pressure.
(2) Heat aging. More common in low-voltage electrical appliances, the mechanism is under the action of temperature. The internal components of the insulating material are oxidized, cracked, and deteriorated, and undergo hydrolysis reaction with water to gradually lose the insulating properties.
(3) Environmental aging. Also known as atmospheric aging, it is due to ultraviolet ray, ozone, salt spray, acid and alkali and other factors caused by polluted chemical aging. among them. Ultraviolet light is the main factor. Ozone is produced by corona or partial discharge of electrical equipment. Once the insulating material has aged, its insulation performance is usually unrecoverable. The following methods are commonly used in engineering to prevent the aging of the insulating material.
(1) Add anti-aging agent in the process of making insulating materials.
(2) Outdoor insulation materials may be added with UV absorbers or separated by a barrier.
(3) The insulating material used in the hot and humid zone can be added with anti-fungal agents.
(4) Measures to strengthen local anti-corona and partial discharge of electrical equipment.