Electrical inductive apparatus

Claim 1

Electrical inductive apparatus, comprising: an enclosure, an electrical winding in said enclosure adapted for connection to an electrical potential at power frequency, said electrical winding having areas of high electrical stress when connected to the electrical potential, dielectric means in said enclosure surrounding said electrical winding, said dielectric means having a predetermined dielectric constant which does not exceed about 2.2, an insulating structure for said electrical winding including solid insulating means, said insulating structure being disposed to add mechanical strength to at least certain of said areas of high electrical stress in said electrical winding, with said insulating structure having a compressive strength exceeding about 6000 psi and sufficient spring-back to retain winding tightness following short circuit mechanical stresses, said solid insulating means including an adhesive binder having a dielectric constant in the range of about 3 to 4 and a filler which includes microspheres having a dielectric constant of about 1. said adhesive binder and said filler being selected to provide a dielectric constant for said solid insulating means which is substantially less than the dielectric constant of said adhesive binder alone, the concentration of said filler in said insulating structure being in the range of 25 to 75% by weight, and wherein the diameter of said microspheres is 100 microns, or less, to provide the requisite compressive strength and spring-back characteristics without breakage of the microspheres, and a corona inception value compatible with the electrical operating potential.