Humidity resistance of insulating varnishes

Moisture is one of the most common aggressive environments for electrical equipment. Recently, manufacturers have become more concerned with the effects of humidity on the parts they produce. Many of the tests introduced in the past were based on systems type analysis. Under these conditions, the secondary insulation (impregnating varnish) and the primary insulation on the magnet wire are tested together. Work conducted at Schenectady Chemicals, Inc., indicates that conditions which bring about degradation of the secondary insulation may not be sufficient to degrade the primary insulation. Thus, secondary insulation having poor moisture resistance could appear to be quite satisfactory when tested over a good, water resistance primary coating. This study presents a method for evaluating the secondary insulation alone, eliminating the predominant effect of the primary insulation.