Conductive and insulating materials

All kinds of electrical equipment not only use conductive materials to conduct current, but also use insulating materials to isolate the conductive parts, limit the current flow in a certain path, and protect personal safety to avoid electric shock. Therefore, conductive materials and insulating materials are always used in combination with each other. Silver is the best conductive material, but it cannot be widely used due to its high price. Silver is often used on the contacts of electrical appliances to reduce the contact resistance at the contacts. Copper is the most commonly used conductive material. Copper has a high electrical conductivity, but if it contains a small amount of impurities, its electrical conductivity will drop significantly. The copper wires of the cables and motor coils are made of electrolytic copper with high conductivity, and its copper content is 99.99%. The conductivity of aluminum is similar to that of copper, but lighter than copper. Most of the current transmission lines use steel core aluminum wire or aluminum alloy wire. Certain achievements have been made in 'substituting aluminum for copper' in the electrical and electrical industries. The resistance value of manganin and constantan is little affected by temperature, so it is especially suitable for making standard resistance wires. Nickel-chromium alloy is suitable for making electric resistance wire, because its maximum working temperature can reach 1,100℃. There are many types of insulating materials, and only a few types and their general uses are briefly introduced below. (1) Gas insulation material Air is a kind of insulating material. At present, most of the overhead wires use bare wires, and air is the insulation between overhead wires. (2) Liquid insulating material Transformer oil is the most commonly used liquid insulating material. It has good insulating properties and is usually used as an insulator in equipment such as transformers, oil switches, and varistors. Transformer oil must be very pure. If it contains a small amount of water or impurities, its insulation performance will be greatly reduced. Transformer oil is easy to volatilize and burn, so do not let sparks close to electrical appliances containing transformer oil. (3) Asphalt, which is a solidifying insulating material, is brittle at low temperatures, and softens and melts when the temperature rises. Its hygroscopicity is extremely small, and it can be protected from moisture by coating it on the surface of the coil. Insulating varnish-usually used to make enameled wire. (4) Solid insulating materials Commonly used solid insulating materials include the following types: rubber, plastic-usually used as insulators for wires. Cardboard-usually used to make the skeleton of the coil. Cotton yarn and silk-can be used to make yarn covered thread and silk covered thread. Black tape-usually used to wrap wire connectors. Mica-usually used as the insulator of electric heaters and the medium in capacitors. Ceramics-usually used as insulators for high-voltage transmission lines, as an insulator between bare wires and the earth. As mentioned earlier, insulating materials are not absolutely non-conductive. If the voltage applied to the insulating material is too high, the insulating material will experience a breakdown of the insulating material due to too much electric field strength. In order to prevent the insulation material in the electrical equipment from breaking down due to excessive electric field strength, and to prevent the wire from overheating due to the passing of too much current, so as to ensure that the load can work reliably and economically, it is allowed There are certain regulations on the voltage value applied to each electrical device. Usually this specified voltage value is called the rated voltage of the electrical equipment. Before using any electrical appliance, you must first see whether the rated voltage of the electrical appliance is the same as the power supply voltage. If the power supply voltage is higher than the rated voltage of the electrical appliance, it must not be connected, otherwise, the appliance will burn out; If the voltage is lower than the rated voltage of the electrical appliance, it should not be connected, otherwise, the electrical appliance will not work normally.