Abstract :
Thermosetting varnishes based on oil-modified phenolic resins appear to have certain advantages over the conventional oil varnishes for the treatment of coils which operate under oil. These anticipated advantages are associated with their heat-hardening characteristics and the absence of metallic driers. Oxidizing oil varnishes dry and harden by oxidation; hence, when applied as a thin film they will harden satisfactorily. However, if they are applied thickly only the surface hardens, and the varnish underneath remains fluid indefinitely. These varnishes also contain ingredients which may adversely affect the insulating oil. Laboratory tests were therefore made to compare the behaviour in oil of small coils (a) untreated, (b) treated with thermosetting varnishes and (c) treated with conventional oil varnishes. The criterion adopted in these comparisons was the ultimate effect of the specimen coil on the sludging and general properties of the mineral oil in which it had been immersed. Other factors likely to influence the effect of the coils, such as the conductor, conductor covering, the presence of air and susceptibility of the oil to deterioration, were also taken into consideration. The tests were made at temperatures between 75? and 80? C and the normal period of heating was about 10 weeks. The results of the experiments indicate that thermosetting varnishes are superior to oxidizing oil varnishes for use under oil. Their greater oil resistance eliminates direct contamination of the oil, and the permanency of the varnish coating effectively suppresses the potential catalytic activity of the copper in causing oxidation of the oil. There are also indications that the thermosetting varnish inhibits the oiloxidation process. On the other hand, there were equally definite indications that, unless adequately dried, conventional oil varnishes were liable to accelerate the formation of sludge and the development of acidity in the oil. Nevertheless, it was evident that t- reatment of a coil in one of the more oil-resisting grades of such oxidizing varnish was preferable to leaving the coil in an unvarnished condition. The results of the subsidiary tests confirm the effectiveness of a coating of tin or enamel on the copper as a barrier to the catalytic activity of the metal. They also demonstrate the marked influence of the inherent characteristics of an oil upon the manner and progress of its subsequent deterioration and of the predominating effect of oxidation in the oil-deteriorative process.
