Abstract
W. J. KIERNAN points out in a recent article (Bell Lab. Bee., 20, No. 12 ; August 1942) that until the recent development of synthetic resins and drying oils the impregnants applied to coils contained linseed or china-wood oils and natural resins, and that they 'set' by surface oxidation, leaving the interior in a semi-fluid state. Fatty acids were usually present in the unset portion, sometimes in sufficient strength to corrode copper. Varnishes made with synthetic resins of the phenol-aldehyde type polymerize on drying rather than oxidize, and produce a solid state throughout their mass. Corrosive tendencies are negligible or entirely absent because of their complete setting and low acid content. When thoroughly dried by baking at high temperatures, they are no longer thermoplastic. This allows coils to operate at higher temperatures without softening the impregnating material and permits substantial savings by reducing the size of the coils. The penetrating power of these synthetic resins and their high dielectric strength make them particularly valuable for high-voltage circuits where corona discharges might occur and, by oxidizing the nitrogen, produce nitric acid which breaks down the coil insulation. Retardation coils and power transformers of several types, designed for telephone apparatus, are now being impregnated on a production basis with synthetic resin varnishes. The results indicate advantageous possibilities for their wider use.
Article PDF
Rights and permissions
About this article
Cite this article
Impregnating Varnishes. Nature 150, 544–545 (1942). https://doi.org/10.1038/150544d0
Issue Date:
DOI: https://doi.org/10.1038/150544d0