Document evKOeOX4NyaBr7Kj6Lp4ZxNZ9

^-1--------------- *- ii.r Dec. 16, 1969 C. A. L1DDICOAT ELECTRICAL CAPACITORS Filed Nov. 21, 1968 3,484,664 ATTORNEY DSW 435967 STLCOPCB4089028 . . 3,484,664 Patented Dec. 16, 1969 12 3,484,664 the dielectric to withstand high voltage stresses per unit of ELECTRICAL CAPACITORS Charles A. Liddicoat, Wilmington, Del., assignor to Hercules incorporated, Wilmington, Dei., a corpo ration of Delaware Filed Nov. 21, 1968. Scr, No. 777,727 Int. Cl. HOlg 1/00 thickness, and decreases the capacitance to volume ratio so that in general the beneficial results are offset to a commercially significant extent. 6 Another method which has been proposed to improve impregnation of synthetic resin films involves, including U.S. Cl. 317--258 7 Claims either on the film surface or throughout its mass, a par ticulate inorganic dielectric material such as aluminum oxide. However, the use of aluminum oxide, as a dusting ABSTRACT OF THE DISCLOSURE 10 on the film, is subject to the disadvantage that it is diffi Liquid impregnated electrical capacitors having high corona starting voltage and capacitance contain as the dielectric spacer, two (or more) sheets of thermoplastic film, each sheet being separated from the other sheets by a single, very thin stratum of a conductive metal, such as aluminum deposited one of the sheets. cult to obtain and maintain a uniform distribution of the particles of the oxide on the film surface, and the incor poration of aluminum oxide into the resin prior to film _ formation is subject to the disadvantages that it is diffi cult to obtain uniform resinous dispersions and to ex trude the resin under conditions such as to obtain a satis factory film. Now', in accordance with this invention it has been The present invention relates to electrical capacitors found that capacitors with synthetic resin film dielectrics and, more particularly, to fully impregnated electric ca- u can be uniformly impregnated with the dielectric fluid pacitors having as the dielectric spacer a thermoplastic film. Capacitor dielectric spacers have in the past been made of kraft paper or other cellulosic materials and more recently of synthetic resin films, either alone or in conjunction with kraft paper sheets. Synthetic resin films in without the prior art disadvantages and additionally that the capacitors are more or less self-healing. Accordingly, the present invention relates to an elec n_ "i> trical capacitor comprising a pair~ of' cbiiducfive"'e'fec> Trodes separatecThy-aTHelecti ic spacer consisting essen tially of at least two sheets of non-porous thermoplastic general are superior to cellulosic sheet dielectrics in that they can withstand higher voltage stresses per unit of thickness, are less subject to flaws and defects, such as film, said sheets being in face to face relationship and having between adjacent faces a stratum of an electri cally conductive material about 10-5 to 10~s inch thick pin holes or conducting particles which reduce electrical on one of said adjacent faces and covering an area which strength, and have better power factor characteristics. In is at least coextensive with the area of the electrodes but view of these and other properties, capacitors with syn thetic resin film dielectrics can usually be made smaller than paper dielectric capacitors to obtain equivalent ca pacitance and voltage ratings. However, thin films of synthetic resins are subject to the drawback that they less than the area of the face, and a dielectric fluid im pregnating said spacer and filling any voids therein. The thermoplastic material which forms the solid die J lectric film of the capacitors of this invention can be any thermoplastic film-forming homopolymer or copolymer. tend to stick and to adhere tightly to one another, and to the electrode foils with which they are wound in the Many such polymers are known, and include for example the polyolefins such as the polymers of ethylene or the manufacture of capacitors. The resultant sticking (com- 4(j higher mono-alpha-olefins containing 3 or more carbon monly referred to as "blocking") of the synthetic resin film surfaces to adjacent surfaces of synthetic resin film or metal foil makes it difficult to impregnate such ca atoms as well as copolymers thereof with a different oiefin or a vinyl ester and the like; the polyesters such as poly (ethylene terephthalate); polystyrene and the vinyl pacitors after winding. Moreover, the sticking tendencies cause the finally rolled capacitor unit to have air voids polymers such as po!y(vinvl chloride), polytvinylidene chloride), vinylidene chloride-vinyl chloride copolymers, therein, and prevent ready flow of dielectric liquid to all vinyl chloride-acrylonitrie copoymers, and the like; the portions of the roll. As a result, electrical strength is polycarbonates; the polyamides; and the like. Particu markedly reduced, corona starting voltage is lowered and larly preferred because of the favorable mechanical and the capacitor units are subject to premature breakdown dielectric properties are the high molecular weight linear and shortened operational life. Additionally, the non- 5q polymers of ethylene and the stereo-regular crystalline porous nature of the synthetic resin films makes it difficult polymers of propylene, butene-1, pcntcnc-1, 3-mcthyl- to impregnate them adequately. Poor impregnation re butenc-1 and 4-methylpentene-l. sults in unoccupied pores ar.d voids in the interstices oc Films of these polymers can be formed for purposes of curring between the films and adjacent non-porous sur this invention by rolling, extruding, pressing, solvent 1 faces with resultant deterioration and premature break down. casting and melt casting. The films can also be stretched longitudinally and/or transversely' to impart uniaxial, One method which has been proposed to improve im biaxial or balanced biaxial orientation to the film. Pref pregnation of capacitors containing synthetic resin die lectrics is to use a porous material such as kraft paper erably. the film has a thickness of about 0.10 to 2.00 mils. The dielectric spacer, as previously stated, consists es in conjunction with the synthetic resin film. In these com- sentially of at least 2 sheets of thermoplastic film and positc or layered dielectric capacitors, the porous layer, adjacent sheets of the film arc separated from each oilier probably, due to a wicking action, provides better and by a very thin coating or stratum of an electrically con more uniform impregnation of the dielectric fluid. How ductive material supported on one of the adjacent sheets. ever, the use of a paper layer decreases the ability of The electrically conductive material is preferably a metal I DSW 435968 STLCOPCB4089029 - ; , 3.4S4.CG4 i; - 3 "- * 4 i | and suitably, aluminum, zinc, silver, gold, tin, lead, cad mium, etc. The stratum or coating can be applied to dried assembly is still under vacuum in a suitable evacu ated enclosure. Usually, sufficient impregnating liquid is f the film face by a number of means including cbcmical introduced to at least submerge the capacitor element in decomposition, gas decomposition, sputtering, etc., but the container. The pressure in the enclosure is then raised r> is preferably applied by vapor deposition in an atmospherc of reduced pressure (the so-called vacuum deposi to atmospheric pressure or greater and the assembly per mitted to stand for a number of hours for thorough pene i. tion technique). The exact thickness of the stratum or tration of the liquid impregnation. After impregnation, coating used will vary of. course, depending upon the the capacitor unit is sealed, as by applying a suitable particular conductive material. v quantity of solder to hold 14. In addition to the fore A thickness within the range of about 10~5 to 10~8 10 going processes, other techniques which generally utilize inch and preferably' about 10-6 to 10-7 inch ffiis been heat and/or pressure, can be utilized to aid in the im found to be best suited for obtaining the advantages of pregnating process. the invention since such thicknesses provide good wet Certain well known fluid impregnating liquids can be ting of the film and hence, thorough liquid impregnation, employed in the present invention. Examples of suitable maintain their integrity during normal capacitor construe- 15 impregnants are mineral oil, castor oil, cottonseed oil, sili tion, permit vaporization of the conductive material dur cone oil, polybutene and any of the various halogenated. ing any self-healing action without damage to the film, aromatic hydrocarbon compounds. Examples of the halo- but are sufficiently thin so that there is no current carry genated aromatic compounds a~re~ the; chlorinated diphen ing capacity. yls, the chlorinated diphcnvl ketones, pentachloronitrodi- The capacitors of the present invention are constructed 20 phenyl and its alkyl derivatives, chlorinated benzene and in the usual manner except for the solid dielectric mate benzene derivatives and the like. rial employed. Thus, as Is. shown in the accompanying By way of example, a group of capacitors similar in drawings, FIG. 1 is a perspective view of a partially un construction to sections of power factor correction ca coiled rolled capacitor assembly, FIG. 2 is a perspective pacitors was made, each containing a wound pair of alu view of a capacitor of the type of FIG. 1 after being 25 minum electrode foils 0.25 mil thick separated by two encased in a container and FIGS. 3 and 4 are fragmen sheets of 0.5 mil thick heat set, biaxial ly oriented poly tary cross-sectional views of typical dielectric spacers. propylene film, one sheet of which was coated with a With reference to the drawings, the capacitor assembly layer of aluminum, approximately 10~6 inch thick on the 1 of FIG. 1 can be made by convolutely winding elec surface adjacent to the other sheet. The aluminum coat trode strips 2 and 2' of metal foil such as aluminum, 30 ing was vacuum deposited on the film by transporting copper, tantalum, etc., with an interleaved dielectric the film across a heated aluminum vapor source. The spacer 3 comprising 2 sheets of non-porous thermoplas source was properly masked so that the marginal edges tic film 4 and 4' separated by the stratum of metal 5 de of the film were free of coating, and the coating widiti posited on film 4' (see FJG. 3). In FIG. 4 of the drawing, w'as 80% of the width of the film. The transport was ac the spacer comprises 3 sheets.of thermoplastic film 4, 4' 35 complished by means of a powered winding mechanism and 4" separated by metal strata 5' and S". Although the comprising two spool holders, associated supports and sheet of film which carries the conductive material is not jockey rollers. The coating was applied while the entire specifically shown by the drawings, it is understood that x'apor source, mask and transport mechanism were en either one of the faces of the adjacent sheets may be the closed in a chamber evacuated to 10-5 torr pressure at carrier, and that in the case when 2 or more sheets are 40 ambient temperature. A control croup of capacitors was employed, a single inner sheet thereof can be the carrier made of identical construction except that the polypro for 2 strata of the conductive material, i.e., a stratum on pylene film was not coated. Both groups were impreg each face. nated with chlorinated diphenyl dielectric liquid (Aroclor Electrical contact with the electrodes can be made by 1242) under the same conditions. The average corona laid-in tap straps 6 and 7 having their ends protruding 43 starting voltage of the control group of capacitors was from one end of the rolled capacitor assembly. The tap much lower than that of the group made with the alu straps can be welded to the electrodes, if desired, to ob minum coated polypropylene film. These tests indicate tain a better contact, and the lower part of the straps 6 that coating the surface of the film in accordance with the and 7 can be enlarged as at 8 to provide a larger contact invention improves the impregnability of thermoplastic di area with its cooperating electrode. 50 electric films as evidenced by the higher corona starting Prior to impregnating, the electrode-spacer assembly voltages. . 1 is usually placed in a container such as the metal con A second control group of capacitors of the same size tainer 10 shown in FIG. 2, and the cover 11 hermetically was also constructed in the manner of the example ex scaled to the container. . . cept that a paper-polypropvlcne film composite composed Although not illustrated, the unit 15 shown in FIG. 2 55 of two sheets of polyproylene film 0.5 mil thick sep further includes a dielectric liquid which occupies the re arated by a single sheet of kraft paper 0.5 mil thick wa: maining space in container 10 not occupied by the ca substituted for the two sheets of polypropylene film sep pacitor assembly and which also impregnates the dielec arated hv the aluminum coating. Tests conducted therecn tric spacer 3. _ showed that the capacitors of the example had a highei Before the cover is scaled on the container, the contact 60 capacitance per unit of size at essentially equivalent start tap straps 6 and 7 are respectively affixed to the terminals ing voltages than the control capacitors containing the 12 and 13 extending through and insulated from the paper-polypropylene film composite. cover. In order to allow withdrawal of moisture and air Tests conducted on a number of liquid impregnated from the assembly and the introduction of the impreg capacitors constructed as in the example except that films nating liquid, a small hole 14 is provided in the cover 11. 65 of linear polyethylene and polytethyler.c lerephlhabte) Before impregnation, the capacitor assemblies arc nor were substituted for the polypropylene film demonstrated mally vacuum dried to remove residual moisture. The that in all cases the life of the capacitor containing the drying temperature will vary depending on the length of dielectric spacer of the invention was considerably longer the drying cycle, but usually ranges from about 60 to than the life of the capacitor controls containing two about 150" C. with too low a temperature, the drying 70 sheets of film, neither of which was coaled. period is cxcesively long while too high a temperature What I claim and desire to protect by Letters Patent causes decomposition and shrinkage of the dielectric is: spacer. 1. An electrical capacitor comprising a pair of con ' The impregnating dielectric liquid is admitted to the ductive electrodes separated by a dielectric spacer con capacitor assembly through hole 14 preferably while the 75 sisting essentially of at least two sheets of nou-_porous DSW 435969 STLCOPCB4089030 3,484,664 56 thermoplastic film, said sheets being in face to face rela 5. The capacitor of claim 4 wherein the thermoplastic tionship and having between adjacent faces a stratum of is a polyolefin. an electrically conductive material about 10-5 to JO-* inch thick on one of said adjacent faces and covering an 6. The capacitor of claim 5 wherein the polyolefin is polypropylene. area which is at least coextensive with the area of the g 7. The capacitor of claim 6 wherein the film is biaxially electrodes but less than the area of the face, and a dielec oriented. tric fluid impregnating said spacer and filling any voids References Cited therein. UNITED STATES PATENTS 2. The capacitor of claim 1 wherein the conductive material is vacuum deposited on the thermoplastic film, to 3,340,446 9/1967 Cox 174--25 X 3. The capacitor of claim 2 wherein the conductive E. A. GOLDBERG, Primary Examiner material is a metal. 4. The capacitor of claim 3 wherein the metal is U.S. Cl. X.R. aluminum. 174--25; 317--260, 261 X DSW 435970 STLCOPCB4089031