Abstract: In order to offer the epoxy resin composition allowing much lower viscosity and higher heat resistance (glass transition temperature) of cured product to coexist and lowering hygroscopic property, and the insulation coil and one-piece impregnation type rotating machine with improved insulation performance using the composition, the epoxy resin composition made of a mixture of 1 to 5 by weight of sulfonium salt cationic polymerization initiator and 3 to 15 by weight of reactive diluent with 100 by weight of epoxy resin is provided. The insulation coil that is an insulation coil made of plain conductor wire formed into a specified shape and wound with insulation material, impregnated with the epoxy resin composition and then cured is provided.

Abstract: An insulation system for winding of electric rotating machines having thermal resistance of class H or more and excellent adhesiveness between a conductor and an insulating layer and resistance to thermal stress can be produced by wrapping a plurality of layers of an insulating tape or sheet having thermal resistance of class H or more around a winding conductor, impregnating the thus treated winding conductor with a solventless varnish consisting essentially of 1 equivalent of an epoxy compound containing at least two vicinal epoxy groups, more than one equivalent of an organic polyisocyanate, and then heating the inpregnated winding conductor at a temperature above 60.degree. C. for a time sufficient to form and cure a resin containing as recurring units at least two isocyanurate rings directly bonded to one another through the residue of the polyisocyanate and two oxazolidone rings directly bonded to each other through the residue of the polyepoxide in the presence of 0.