Electrical Apparatus magazine, August 2002 Insulating 'Varnish'--What it is, What it Does

Most windings of electrical apparatus are impregnated with an insulating liquid subsequently heat-cured to form a solid. It serves to bond conductors rigidly together; to promote heat dissipation; and to exclude moisture or other contaminants. Originally, varnishes were used--mixtures of an organic resin with a solvent and a catalyst. While in contact with air, and particularly during oven curing, such a liquid gives off solvent vapors. The result is air pollution and the creation of undesirable voids within the finished winding.

The past 30 years have therefore brought general usage of so-called "solventless" or "100% solids" impregnating resins in which the liquid component is chemically reactive, becoming part of the cured solid rather than passing off as vapor. Development efforts continue to still further reduce any emissions prior to complete cure.

Other changes in resin composition (and impregnation processes as well) are being made in the effort to reduce processing time; increase tank stability of the material; improve the extent to which winding voids are filled; and offer greater resistance to moisture or chemical attack. Of particular importance has been the development of resin additives to increase the liquid's thixotropy, rendering the same material capable of exhibiting a low viscosity suitable for either vacuum-pressure impregnation (VPI) or a higher viscosity for random-wound low voltage coils.

A more recent development arose from the increased use of pulse width modulated (PWM) inverters to supply motors with variable frequency power. In many such applications, high-frequency overvoltages have caused premature motor insulation failures. Magnet wire coatings were quickly improved to achieve higher turn-to-turn dielectric capability. However, the impregnating resin used has also been found to have an important influence on the corona inception voltage or CIV in such windings, leading to changes in the chemistry of that material as well.

The service industry needs to be aware of such developments, which may require changes in the repair process. New resins may not be fully compatible with existing insulation systems or rewind methods.

Industry news...

Featured Technical Article
August 2002

On this page is a summary of the Electrical Apparatus August 2002 featured technical article, by Richard L. Nailen, P.E. , "Insulating 'Varnish'--What it is, What it Does" Isolierender 'Lack' - was ist das, was tut er? ... Le 'vernis' isolant - ce que c'est et ce qu'il fait ... 'Barniz' Aislante - Lo Que Es, Lo Que Hace..

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