Insulation

Date: 2015-10-07; view: 510.

The most important insulation material is air. A variety of solid, liquid, and gaseous insulators are also used in electrical apparatus. In smaller transformers, generators, and electric motors, insulation on the wire coils consists of up to four thin layers of polymer varnish film. Film insulated magnet wire permits a manufacturer to obtain the maximum number of turns within the available space. Windings that use thicker conductors are often wrapped with supplemental fiberglass insulating tape. Windings may also be impregnated with insulating varnishes to prevent electrical corona and reduce magnetically induced wire vibration. Large power transformer windings are still mostly insulated with paper, wood, varnish, and mineral oil; although these materials have been used for more than 100 years, they still provide a good balance of economy and adequate performance.

The cable insulation usually consists of impregnated paper, synthetic polymers and compressed gases. Early cables mostly used impregnated paper insulation. Next stage was the introduction of oil-filled paper insulation. At present, polymers are widely used. The polymers most often used are polyvinyl chloride (PVC) for low-voltage cables, polyethylene (PE), cross-linked polyethylene (XLPE) and ethylene propylene rubber (EPR) for medium-voltage cables; and XLPE and EPR for high-voltage cables. Flexible insulating materials such as PVC are used to insulate the circuit and prevent human contact with a 'live' wire – one having voltage of 600 volts or less. Alternative materials are likely to become increasingly used due to EU safety and environmental legislation making PVC less economic.