Abstract:
A dielectric fluid of refined soybean oil with additives of cycloaliphatic epoxide resin and butylated hydroxtolene covered in the winding of a metallized polypropylene film capacitor to prevent trapped air and moisture being absorbed in the winding.

Description:
BACKGROUND OF INVENTION  
         [0001]    This invention relates to dielectric fluids for electrical capacitors, particularly metallized polypropylene film capacitors.  
           [0002]    In the manufacture of metallized film capacitors, the selection of the dielectric fluid has a substantial influence on the electrical characteristics of the capacitor. The fluid is needed to prevent trapped air and moisture from absorbing into the winding of the metallized film and eroding away the metallized layer by electrical discharge.  
           [0003]    The dielectric fluid needs to have a low moisture content (less than 600 ppm), a dielectric constant similar to polypropylene which has a dielectric constant of 3, additives to scavenger acidic contaminates, and have low absorption by propropylene film.  
         SUMMARY OF THE INVENTION  
         [0004]    Accordingly, the present invention resides in a capacitor wherein a dielectric fluid consists essentially of refined soybean oil, and additives of cycloaliphatic epoxide resin and butylated hydroxytoluene. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    The present invention is illustratively shown and described in respect to the accompanying drawings in which:  
         [0006]    [0006]FIG. 1 is a perspective view of a convolutely wound metallized film capacitor body which is partially unwound to show the dielectric plastic film and metallized foil electrode structure;  
         [0007]    [0007]FIG. 2 is a perspective view of the capacitor body of FIG. 1 to which terminal leads are attached;  
         [0008]    [0008]FIG. 3 is a partial sectional view taken across lines  3 - 3  of FIG. 1; and  
         [0009]    [0009]FIG. 4 is a perspective view of the capacitor body of FIGS. 1 and 2 enclosed in a suitable housing. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0010]    Referring now to FIG. 1, there is illustrated a convolutely wound ac capacitor body  10 . The capacitor body  10  includes a pair of dielectric plastic film layers  11  and  12  and overlying metallized electrodes  13  and  14 , respectively providing a winding W. The dielectric plastic film layers  11  and  12  are metallized in such a manner so as to provide margins  15  and  16  which are free of metal. Such margins are useful in helping to prevent electrical short circuits between the metallized electrodes at the ends of the capacitor body  10 . In convolutely winding the capacitor body  10 , the metallized electrode  13  overlying the plastic film  11  is laterally offset with respect to the metallized electrode  14  overlying the plastic film  12  so that metallized electrode  13  and margin  16  appear at end  17  of the capacitor body so that metallized electrode  14  and margin  15  appear at end  18  of the capacitor body.  
         [0011]    As shown in FIG. 2, suitable terminal leads  19  and  20  are attached to the ends  17  and  18 , respectively, of the capacitor body  10  using any suitable attachment process. The attachment process must not result in electrically conductive material penetrating into the capacitor body  10  to cause the metallized electrode at the capacitor body edge to electrically short the other metallized electrode. An acceptable attachment process includes spraying the ends  17  and  18  of the capacitor body  10  with lead, a lead alloy, aluminum, an aluminum alloy, zinc, a zinc alloy, tin, a tin alloy, nickel, a nickel alloy and the like. The metal material at the ends of the capacitor body  10  are indicated by the reference numerals  21  and  22 . Lead strips  23  and  24  are suitably attached to the metal material as shown in FIG. 2. A suitable lead strip material is solder coated copper.  
         [0012]    [0012]FIG. 3 is a sectional view taken along the line  3 - 3  of FIG. 1 but limited to a small section of the capacitor body  10 . In FIG. 3, the plastic film layers  11  and  12  and their respective overlying metallized electrodes  13  and  14  are arranged in the same manner as shown in FIG. 1.  
         [0013]    Metallized electrode  13  is integrally joined to the plastic film  11  whereas metallized electrode  14  is integrally joined to the plastic film  12 . FIG. 3 shows a metallized electrode on but one side of the plastic film; however, the metallized electrode layers may be deposited on both sides of the plastic film. FIG. 3 shows a single side of the plastic film as being metallized. The application of the metallized electrodes  13  and  14  to their respective plastic film may be accomplished by any suitable process such as vapor, chemical or other known metal depostion processes.  
         [0014]    The dielectric plastic films or layers  11  and  12  should have high dielectric strength, low dissipation factor to prevent thermal runaway, broad temperature range capabilities, resistance to the dielectric fluid, and freedom from pinholes and conductive particles. The electric film layers  11  and  12  may be of any suitable homogenous polymer or plastic material such as polyester, poylethylene, polycarbonate, polystyrene, polysulfone, polyethylene terephthalate and, preferably, the polyoefins, particularly polypropylene.  
         [0015]    In FIG. 4, a capacitor body  10  is preferably sealed in a can or housing  40  to form a capacitor  41 . Capacitor  41  includes the metal housing or can  40  and terminals  42  and  43 . The capacitor body  10  is contained within the housing  40  and terminal lead  23  makes electrical contact with terminal  42  and the remaining terminal lead  24  makes electrical contact with terminal  43 . The housing is sealed through lid  45 .  
         [0016]    The dielectric fluid of the present invention consists essentially of refined soybean oil and additives of cycloaliphatic epoxide resin and butylated hydroxytoluene. The refined soybean is in amount of 98.9% by volume while the epoxide is in an amount of 1.0% by volume and the epoxide acts as a seavenger of acidic contaminants while the hydroxytoluene to acts as an antioxidant. The hydroxytoluene in amount of 0.1% by volume. The term “refined soybean oil” is meant to mean a fully refined 100% soybean salad oil which has been refined, bleached, and deordorized in a continuous and totally integrated process.  
         [0017]    The dielectric fluid is incorporated in the metallized film capacitor by dispensing the fluid into an open end of can  41  until the winding W is completely covered.