Patent GB-681860-A (GB)
Title: Improvements in or relating to the treatment of electrical windings
IPC: H01B3/44
Filed: 1949-12-14
Abstract: Members of electrical apparatus which include an electrical winding are treated with a completely reactive fluid material that polymerizes to a gel on being heated, by applying the fluid to the member to fill interstices in the winding while the member is at a temperature permitting such impregnation to take place, and passing an electric current through the winding to heat the same and the fluid material within and about it, to polymerize the material in and adjacent the winding to a non-flowable gel.  The windings may be heated to a temperature above that of the remainder of the member before immersion in the fluid material. Vacuum impregnation may be used.  After removal of the member, with its adherent coating of gel, from the liquid, it may be allowed to drain to remove unpolymerized composition, and then heated, e.g. by baking by infra-red lamps or in a gas, steam, or electrically-heated oven, to complete the polymerization of the adherent material.  Electrical members suitable for treatment according to the invention comprise coils such as hairpin, field, rotor, pancake, shell-type and bobbin-wound coils, stators, rotors, transformer coil and core assemblies, and electromagnetic coils such as solenoids, having windings composed of a conductor such as copper wire, which may be enamelled or covered with cotton, asbestos, glass fibre, paper, silk, or combinations thereof, while the winding or coil may be covered with an insulating material such as mica tape, glass, cotton or asbestos tape.  The conductors may be treated with a solvent-containing varnish prior to impregnation according to the invention.  The impregnant may comprise a single polymerizable compound such as diallyl phthalate, succinate, maleate, or adipate, allyl alcohol, methallyl acrylate, diallyl ether, allyl acrylate or crotonate, or mixtures thereof. Compositions comprising an unsaturated liquid monomer and an unsaturated resinous component, especially an alkyd resin, may also be used, particularly the reaction product of an ethylenic dicarboxylic acid or its anhydride, such as maleic, fumaric, monochloromaleic, itaconic or citraconic acid or maleic, itaconic or citraconic anhydride, or mixtures thereof, with a polyhydric alcohol such as glycol, glycerol, or pentaacrythritol or mixtures thereof.  A mixture of monostyrene and the reaction product of castor oil with maleic anhydride may also be used.  Saturated aliphatic or aryl dicarboxylic acids or anhydrides, e.g. succinic, adipic, sebacic or phthalic acid or phthalic anhydride may partially replace the unsaturated dicarboxylic acids or anhydrides, and in certain cases epoxides may replace the polyhydric alcohols.  The admixed liquid unsaturated polymerizable monomer may be monostyrene, alpha-methyl styrene, 2,4-dichlorostyrene, p-methyl styrene, vinyl acetate, methyl methacrylate, ethyl acrylate, diallyl phthalate, succinate or maleate, allyl alcohol, methallyl alcohol, acrylonitrile, methyl vinyl ketone, diallyl ether, vinylidene chloride, butyl methacrylate, allyl acrylate or crotonate, 1.3-chloroprene, or divinyl benzene, or mixtures thereof.  The composition may include a polymerization catalyst, suitable catalysts including peroxides, ozonides, per-halides and per-acids, e.g. benzoyl peroxide, tertiary butyl perbenzoate, tertiary butyl hydroperoxide, di-tertiary butyl peroxide, di-tertiary butyl di-perphthalate, 1-hydroxy cyclohexyl hydro-peroxide-1, and ascaridol, or the winding may be coated with the catalyst by dipping in a solution thereof in a volatile organic solvent which is evaporated, and the winding then immersed in the uncatalysed reactive composition and heated.  In examples, there are described, inter alia, impregnating compositions prepared by (1) dissolving the reaction product of castor oil and maleic anhydride and the reaction product of linseed oil, castor oil and maleic anhydride in monostyrene; (2) heating together tall oil, pentaerythritol and propylene glycol, then adding maleic anhydride and continuing the heating, finally mixing with monostyrene and a little tertiary butyl hydroperoxide; (3) dissolving the reaction product of diethylene glycol, maleic anhydride and tung oil, in methyl methacrylate.  Specification 588,392 is referred to.