Abstract:
A magnesium methylate coating useful for insulating electromechanical hardware that comprises a composition of magnesium methylate, methyl alcohol, silica and talc.

Description:
TECHNICAL FIELD 
     The present invention relates to insulative coating compositions for insulating stacked laminations used in electromechanical hardware, such as rotors and stators in electrical motors and generators, and a method of making rotors and stators for use in electric motors and generators using these coatings. 
     BACKGROUND ART 
     Electrical components such as rotors and stators for motors and generators are typically manufactured from a plurality of thin steel laminations which are stacked together to form the rotor or stator. Treating each lamination with an insulative coating reduces the eddy current losses inherent in motors and generators. Some chemical compounds that have been commonly used for such insulative coatings include those made from aluminum chromate and containing hexavalent chromium. These are environmentally unsafe and hazardous to personnel involved in manufacturing the components. Accordingly, workplace exposure to hexavalent chromium is strictly limited by regulation and further regulations concerning air and water levels of chromium compounds have been issued or are expected to be issued. 
     Solutions of magnesium methylate in methanol are known to be effective in forming thin insulative coatings. See, for example, Suchoff, U.S. Pat. No. 2,796,364. Magnesium methylate is believed to be free of the health and safety risks associated with hexavalent chromium compounds. Applying a solution of magnesium methylate in an organic solvent to a metal surface and heating the surface to evaporate the solvent produces a thin coating of magnesium oxide on the surface of the metal. A thicker coating would result in improved electrical insulation between the laminations, and in a corresponding reduction in eddy current losses. There is a need for thicker, more effective magnesium methylate coatings, which would provide improved reduction in eddy current losses in motor and generator rotors and stators, compared to prior art coatings. There is also a need to avoid the health and safety risks associated with other coatings, such as hexavalent chromium compounds. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a coating composition includes magnesium methylate, methyl alcohol, silica and talc. The coating can also include a wetting agent, such as a nonionic, phosphate surfactant. The composition may include methyl alcohol, and magnesium methylate in a concentration up to ten percent by weight. The composition may also include silica in a concentration up to 15% by weight, and talc in a concentration up to 25% by weight. It is also advantageous to add a small amount of wetting agent, in order to ease the preparation of the coating, in an amount not exceeding 1% by weight. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a depiction of a stator lamination of the present invention. 
     FIG. 2 is a partial cross-section of a coated lamination of the present invention. 
     FIG. 3 is a partial cross section of a stack of laminations of the present invention, stacked into a rotor stack. 
    
    
     DESCRIPTION OF THE INVENTION 
     Samples were made of the several coatings used in this invention. Test strips of steel were coated with the coatings and tested for surface electrical resistance. The coatings used were several percentages of magnesium methylate in methyl alcohol and magnesium methylate in methyl alcohol with talc and silica. We also tested rings coated with magnesium hydroxide, steam oxide, and a chromate coating, such as the chromate coatings of the prior art. Magnesium hydroxide, milk of magnesia, readily forms a coating on samples. “Steam oxide” is a coating formed by reacting steel laminations with live steam inside an autoclave. Results of surface resistance testing are shown in Table 1, and of course the higher the resistance, the better the coating. 
     
       
         
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 2% Magnesium 
                   
                   
                 5% Mag 
               
               
                   
                 Bare 
                   
                   
                 Steam 
                 Methylate 
                   
                   
                 Methylate 
               
               
                 Coating 
                 steel 
                 Chromate 
                 Mg(OH) 2   
                 Oxide 
                 in MeOH 
                 4% 
                 8% 
                 3% silica, 6% talc 
               
               
                   
               
             
             
               
                 Surface 
                 0 ohms 
                 10-30 
                 1000 
                 925 
                 350 
                 540 
                 880 
                 1000 
               
               
                 Resistance 
               
               
                   
               
               
                 Testing done with .014″  thick Hiperco 50 rings.  
               
             
          
         
       
     
     In addition, more samples were prepared using these coatings on Hiperco 50 steel. Rings were stamped for core loss tests and subsequently coated with the coatings mentioned. These rings are 1.5″ o.d. and 1.25″ i.d., and are stacked about ⅜″ high, for a core loss test per ASTM A697. This test measured the AC core losses in a given stack. AC core loss depends on the steel used, the lamination thickness, and on the quality of surface insulation separating one lamination from the next. In core loss tests, unlike the surface resistance tests, the lower the loss, the better the material used. Results are shown in Table 2. 
     
       
         
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Core Loss in Watts per lb. for 15 or 20 Kilogauss 
               
             
          
           
               
                   
                   
                   
                   
                   
                 5% Magnesium 
               
               
                   
                   
                 10% Mg Methylate 
                 Magnesium 
                 Steam 
                 Methylate in MeOH, 
               
               
                 Coating 
                 Chromate 
                 in methyl alcohol 
                 Hydroxide 
                 Oxide 
                 with 3% silica and 6% talc 
               
               
                   
               
               
                 AC15 
                 20.9 
                 21.2 
                 21.6 
                 21.2 
                 21.1 
               
               
                 AC20 
                 37.4 
                 36.9 
                 37.5 
                 37.0 
                 36.5 
               
               
                   
               
               
                 Testing done with .014″  thick Hiperco 50 rings.  
               
             
          
         
       
     
     In one embodiment of the invention, the coating may be prepared in steps. A solution is prepared by dissolving magnesium methylate in methyl alcohol. This solution may be thickened by adding silica in a concentration of up to 15% by weight, or by adding talc in a concentration up to 25% by weight. The solution may be stirred to encourage uniformity, and a wetting agent may be added to aid in the dispersal. In one embodiment of our coating, the composition can include about 4 to 6% magnesium methylate by weight, about 2 to 4% silica by weight, and about 5% to 7% talc by weight, with zero to 1% by weight wetting agent, and the remainder methyl alcohol. In one embodiment of our invention, a final concentration of 6% talc, 5% magnesium methylate, and 3% silica, with 0.4% Victawet 12 surfactant, in a solution of methyl alcohol, was demonstrated to produce an effective coating. Victawet 12 is a surfactant produced by Akzo Nobel, USA, Chicago, Ill. 
     A stack of laminations may be prepared using these coatings in a method according to another aspect of our invention. Such a stack may comprise a rotor or a stator, such as a rotor or stator useful in a motor or generator. FIG. 1 is a depiction of a stator lamination of the type which may be coated according to the present invention. Part  10  is such a lamination. FIG. 2 is a partial cross section of a lamination coated according to the present invention. Lamination  10  includes the steel lamination  12  and is coated on the bottom and top sides with coating  14  and  16 . These laminations  12  may be stacked into a stack  18 , such as depicted in FIG.  3 . The laminations may be joined together as by welding, adhering, or impaling onto a rotor shaft. 
     According to our invention, the method includes the steps of preparing the coating, coating the laminations, drying the laminations, and stacking the laminations together. In one aspect of our invention, the method includes the steps of preparing a coating of magnesium methylate and applying it to a plurality of laminations. The laminations are then dried at a temperature sufficient to drive off the methyl alcohol solvent. A temperature of 200 to 450° F. is sufficient for this purpose. The laminations are then stacked together to form a rotor or stator. The laminations are desirably made of a high-performance material in order to achieve the lowest core loss or eddy current loss possible in motors or generators made according to this invention. Accordingly, laminations are preferably made from alloys of cobalt and iron, such as the Hiperco series of electrical steels from Carpenter Technology Corp. of Reading, Pa., or the Vacodur steels from Vacuumschmelze GmbH, Hanau, Germany. 
     Additional embodiments of our invention will include a method of using these steels in combination with the coating of the present invention, in order to make motors and generators which have low eddy current losses and low core losses. Thus, in one embodiment of our invention, a solution of zero to ten percent by weight magnesium methylate in methyl alcohol is prepared, and applied to laminations made of Hiperco 50. The coating is applied to the laminations by dipping, spraying, brushing, roller coating or by automatic coating machines. The coated laminations are dried at a temperature from 200 to 400° F. The laminations are then stacked together to form a rotor or stator, and joined by applying adhesive, or by welding together, or by pressing together, as on a shaft. In other embodiments of our invention, the coatings described are prepared and applied to laminations. In one embodiment, a coating is prepared from about 4 to 6% magnesium methylate by weight, about 2 to 4% silica by weight, and about 5% to 7% talc by weight, with zero to 1% by weight wetting agent, and the remainder methyl alcohol. This coating is applied to laminations, especially laminations made from Hiperco 50 steel. The coating is applied to the laminations by dipping, spraying, brushing, roller coating or by automatic coating machines. The laminations are then dried, at a temperature sufficient to drive off the solvent, preferably at a temperature from 200 to 400° F. 
     In another embodiment of our invention, a coating is prepared with 6% talc, 5% magnesium methylate, 3% silica, and 0.4% Victawet 12 surfactant, in a solution of methyl alcohol. The coating is applied to steel laminations, especially laminations made from Hiperco 50. The laminations are allowed to air dry for a brief period, such that when oven drying commences, the coating is not disturbed by the removal of the solvent. The laminations are then dried at a temperature of 200 to 400° F. The laminations are then removed from the oven, stacked, and formed into rotors or stators as desired. 
     Alternatively, instead of laminations, a coil of steel may be coated, and dried as described. Thus, in another embodiment of this invention, a solution of the desired coating is prepared, and placed into a machine designed to coat coils of steel. The steel is then run through the bath of solution or coating. Desirably, the coated steel coil is then dried and re-rolled. The steel may then be processed, such as by running through a punch press, in which laminations are stamped out for subsequent assembly into rotors and stators. In one embodiment of our invention, a solution is prepared of magnesium methylate in methyl alcohol, and is placed into a bath or tank of a coil-coating machine. A coil of Hiperco 50 steel is placed onto the coil-coating machine. The coil is coated with the solution and is then dried, at a temperature of 200 to 400° F. The coil is re-reeled and is then or subsequently punched into laminations useful for rotors or stators. In another embodiment of our invention, a coating is prepared from about 4 to 6% magnesium methylate by weight, about 2 to 4% silica by weight, and about 5% to 7% talc by weight, with zero to 1% by weight wetting agent, and the remainder methyl alcohol, and is placed into a bath or tank of a coil-coating machine. A coil of Hiperco 50 steel is placed onto the coil-coating machine. The coil is coated with the solution and is then dried, at a temperature of 200 to 400° F. The coil is re-reeled and is then or subsequently punched into laminations useful for rotors or stators. In another embodiment of our invention, a coating is prepared from about 5% magnesium methylate by weight, about 3% silica by weight, and about 6% talc by weight, with 0.4% by weight wetting agent, and the remainder methyl alcohol, and is placed into a bath or tank of a coil-coating machine. A coil of Hiperco 50 steel is placed onto the coil-coating machine. The coil is coated with the solution and is then dried, at a temperature of 200 to 400° F. The coil is re-reeled and is then or subsequently punched into laminations useful for rotors or stators. 
     The foregoing description of preferred embodiments of the invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The description was selected to best explain the principles of the invention and practical applications of these principles to enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention not be limited by the specification, but be defined by the claims set forth below.