Commonly used insulating materials can be divided into inorganic insulating materials, organic insulating materials and mixed insulating materials according to their different chemical properties. The frequently used inorganic insulating materials are: mica, asbestos, marble, porcelain, glass, sulfur, mainly used for the winding insulation for motor and electrical equipment, baseplate of switch, insulators and etc.. Organic insulation materials include shellac, resin, rubber, cotton yarn, paper, linen, rayon, etc., which are mostly used in the manufacture of insulating varnish, the coating insulation of winding lead and so on. Mixed insulating materials are various molded ones processed by the above two kinds of materials, used as the base and housing of electrical appliances, and so on.
The role of the insulating material is to isolate different charged parts of the electrical equipment. Therefore, the insulation material should first have a higher insulation resistance and dielectric strength to avoid leakage, breakdown and other accidents. Second, there should be a better heat resistance to avoid deterioration caused by long-term overheating; In addition, excellent insulating materials should have good thermal conductivity, moisture and lightning resistance, higher mechanical strength and convenient processing. According to the above requirements, performance indicators of the commonly used insulating material are dielectric strength, tensile strength, specific gravity, expansion coefficient and etc..
Dielectric strength: the higher the voltage applied to the insulator, the greater the electric field force on the charge in the material, and the easier the ionization collides, resulting in breakdown of the insulator. The lowest voltage to break down the insulator is called breakdown voltage. To break down 1mm thick insulating material, the added kilovolt voltage is referred to as dielectric strength. As the insulation materials have a certain degree of dielectric strength, manufacturers have provided a certain allowable voltage for a variety of electrical equipment, all kinds of safety equipment (electrical pliers, test pencil, insulating gloves, insulating rods, etc.), various electrical materials, which is called the rated voltage. When using, the applied voltage shall not exceed the rated voltage, so as to avoid accidents.
Tensile strength: tensile strength refers to the tension that cross-sectional area of insulating material can withstand, for example, glass can withstand tension of 1400 Newtons cross-sectional area per square centimeter.
Insulation properties of insulating materials are closely related to the temperature. The higher the temperature, the worse the insulating properties. To ensure the dielectric strength, each insulating material has an appropriate maximum allowable working temperature, and below this temperature, it can be used for a long term safely, while beyond this temperature, it will quickly age. In accordance with the degree of heat resistance, the insulation material can be divided into A, E, B, F, H, C level. For example, the maximum allowable working temperature of class A insulation material is 105℃. Most of the insulation materials used for distribution transformers and motors are class A. Corresponding temperature for each level are as follows:
Class A: 105℃ , Class E: 120℃, Class B: 130℃, Class F: 155℃, Class H: 180℃, Class C: over 200℃.
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