Abstract:
A method of manufacturing a transformer coil encapsulated in casting resin utilizing a disposable casting mold where the disposable mold acts both as a winding mandrel and as an inner mold shell for resin encapsulation. The disposable casting mold may be oval shaped or circular shaped depending upon the desired shape of the coils to be encapsulated. The inner and outer mold shells are made from thin sheet metal and thus may be adjusted for the manufacture of a wide variety of sizes of oval shaped or circular shaped epoxy encapsulated coils.

Description:
RELATED APPLICATION 
     A related application is Ser. No. 09/405,017, filed Sep. 27, 1999, entitled “Method of Manufacturing a Transformer Coil With a Disposable Wrap and and Mold and Integrated Winding Mandrel” filed concurrently herewith by Thomas J. Lanoue, Wayne Lambert and Charles Sarver and assigned to the same assignee as the present application, the disclosure of which is incorporated herein by that reference thereto. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method of manufacturing transformer windings embedded in casting resin and more particularly to a method of manufacturing a transformer coil encapsulated in casting resin utilizing a disposable casting mold wherein the disposable mold acts both as a winding mandrel and as an inner mold shell for resin encapsulation. The invention is particularly suited to the manufacture of non-standard oval shaped or circular shaped epoxy encapsulated coils for dry type distribution transformers. 
     2. Description of the Prior Art 
     Currently the state of the art is precision made standard sized re-usable round shaped mandrels and molds in fixed size increments with costly tap inserts. Examples of prior art methods of encapsulating transformer coils are disclosed in U.S. Pat. Nos. 4,337,219, 4,540,536, 5,036,580 and 5,633,019. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a method of manufacturing a transformer coil encapsulated in casting resin utilizing a disposable casting mold wherein the disposable mold acts both as a winding mandrel and as an inner mold shell for resin encapsulation. It is a further object of the invention to provide a method of manufacturing a wide variety of oval or circular shaped resin encapsulated transformer coils by making the disposable inner and outer molds from low cost sheet metal. The invention process provides the advantages of flexible oval or circular dimensional sizes, flexible and incremental oval or circular shapes providing substantially reduced mandrel and molding costs, elimination of costly standard mold storage, elimination of the need for standard winding mandrels, and facilitates the opportunity to manufacture transformer coils in a round to oval configuration. 
     In accordance with the foregoing objects and advantages the present invention provides a method of manufacturing a transformer coil encapsulated in casting resin utilizing a disposable casting mold wherein the disposable mold acts both as a winding mandrel and as an inner mold shell for resin encapsulation. The method comprises the steps of forming an annular inner mold of predetermined shape around a winding mandrel of a coil winding machine to produce an integrated winding form/mandrel/inner mold for use in the winding process, placing the integrated winding/mandrel/inner mold in a winding machine, winding a coil around the inner mold while in the winding machine, removing the inner mold and coil wound thereon from the winding machine, forming an annular outer mold of predetermined shape over the finished coil to provide a manufactured coil and mold assembly, providing a seal for the manufactured coil and mold assembly to prevent casting resin leaks during the encapsulation process, filling the mold assembly with casting resin to encapsulate the coil, permitting the casting resin to harden on the coil, and removing the encapsulated coil from the casting mold assembly. 
     In accordance with another aspect of the invention the annular outer mold is provided with a mold dome. The method of the invention includes placing gasket material at the ends of the inner and outer molds and placing end plates against the material on the ends of the inner and outer molds to form a casting mold. The method further includes placing coil support plates at the top and each end of the mold dome of the outer mold to support the weight of the coil during encapsulation and curing processes, applying a predetermined compression force to the end plates ofthe casting mold and placing the casting mold containing the wound coil in a horizontal position in an evacuated chamber and pouring casting resin into an opening in the mold dome in the outer mold of the casting mold to encapsulate the coil. 
     In accordance with another aspect of the invention, the inner and outer molds are formed of sheet metal. 
     In accordance with another aspect of the invention, the predetermined shape of the inner and outer molds is oval shaped for the production of an oval coil. 
     In accordance with another aspect of the invention the predetermined shape of the inner and outer molds is circular shaped for the production of a round coil. 
     In accordance with another aspect of the invention at least one of the inner and outer molds is of one-piece construction. 
     In accordance with another aspect of the invention the inner mold includes inturned flanges at the radial ends of the sheet metal and removably secured together mechanically for ease of mold disassembly and the method includes the step of placing a gasket between the inturned flanges of the innermold before removably securing together the flanges and applying a liquid sealant to the outside of the mold gasket area. 
     In accordance with a further aspect of the invention, the method includes the step of applying a resin release agent to all exposed surfaces of the inner mold, applying a glass net to the exposed surfaces of the inner mold after the application of the resin release agent and prior to winding the coil onto the inner mold. 
     In accordance with a further aspect of the invention, the method includes the step of applying a glass net over the finished coil winding and applying a resin release agent to the inside of the outer mold prior to placing the outer mold over the coil. 
     In accordance with another aspect of the invention, the method includes the step of installing a plurality of compression rods to interconnect the end plates, and applying a torque to the compression rods to apply the predetermined compression force to the end plates of the casting mold. 
     In accordance with another aspect of the invention at least one of the inner and outer molds is of two-piece construction. 
    
    
     For a more detailed disclosure of the invention and for further objects and advantages thereof, reference is to be had to the following description taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a casting mold assembly useful in practicing the method of the present invention. 
     FIG. 2 is a perspective view of the mold assembly similar to FIG. 1 but with certain parts removed for purposes of clarity. 
     FIG. 3 is a sectional view taken along the lines  3 — 3  in FIG.  1 . 
     FIG. 4 is a top plan view of the mold assembly illustrated in FIG.  1 . 
     FIG. 5 is a perspective view of a casting mold assembly for manufacturing an oval transformer coil in accordance with the present invention. 
     FIG. 6 is a sectional view taken along the lines  6 — 6  in FIG.  5 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is particularly suited for manufacturing non-standard oval or round shaped epoxy encapsulated high voltage coils for dry type distribution transformers. The invention to be described herein includes a new process of winding a high voltage oval or round coil over a disposable mold which acts as both a winding mandrel and an inner mold shell for epoxy encapsulation. The invention also includes a simple technique of making the inner and outer molds using low cost sheet metal. The complete winding and molding processes can all be conducted on a standard winding machine for either aluminum or copper conductors wound as in a layer or disk type format. Referring to the drawings it will be seen that FIGS. 1-4 relate to a casting mold assembly for manufacturing a round or circular transformer coil in accordance with the present invention and FIGS. 5 and 6 relate to a casting mold assembly for manufacturing an oval shaped transformer coil in accordance with the present invention. 
     Referring to FIG. 1 there is illustrated a casting mold assembly  10  useful in practicing the method of the present invention. A mold assembly  10  includes an annular inner mold  12  of predetermined shape. As may be seen in FIG. 2 the inner mold  12  is of circular shape and preferably is formed from thin sheet metal so that it is disposable and inexpensive. In one form ofthe invention the inner mold  12  was formed from a mill standard mild steel of 0.048 thickness commonly referred to as 18 gauge material. For transformer coil KVA classes larger than 1500 KVA, a 14 gauge material is preferred. The inner mold  12  may be made of one piece or two piece construction. The two-piece construction is illustrated in FIGS. 1-3. The preferred fabrication method is roll forming, however, conventional press break multiple bending step processing is possible for the two-piece mold construction illustrated in FIGS. 1-3. The inner mold  12  has flanges  12   a  which may be secured mechanically by either bolting or clamping. A gasket material, such for example as 6.3 mm rubber (not shown) is inserted between the mold flange components  12   a  for sealing and spacing. The spacing generated by the gasket greatly enhances the ease of mold disassembly. Mechanical strength of both the round and oval coil design requires internal support longitudinally across the coil to maintain shape consistently. Typically there are three mechanical supports equally spaced inside the mold  12 , beginning at about 4 inches from the end of the mold. In the embodiment illustrated in FIGS. 1-4 for the round coil two inner mold support bars  18  have been illustrated. The ends of the inner mold support bars  18  are connected to spacer bars  20  by bolts  22 , FIG.  3 . 
     The annular outer mold  24 , FIGS. 2 and 3, is fabricated into either a round or oval shape and is constructed of 18 gauge or 1.22 mm thick cold rolled steel so as to be disposable. The outer mold  24  is of one-piece construction with an integrated dome and tap area  26 . The tap or dome area  26 , being an open exposed area of the coil and mold, makes it well suited for the epoxy filling position for the encapsulation process. This also serves as a reserve area for make up of any epoxy shrinkage. 
     The following is a detailed step by step manufacturing process for construction of a cast coil in accordance with the present invention. The sheet metal inner mold  12  is prepared by installing a rubber gasket to the flanges of the mold and securing it by bolting or clamping. A liquid silicone sealant preferably is applied to the outside of the mold gasket area and permitted to air dry for a predetermined period. It has been found that a minimum of four hours for air drying is suitable and this can take place during the winding process. The inner mold  12  is then installed onto an expanding mandrel of a standard winding machine (not shown) and expanded until secure. An epoxy release agent preferably is applied to all exposed surfaces of the inner mold  12  in accordance with recommendations of the epoxy manufacturer. Apre-pregnated glass netmaterial 28  is applied to the innermold  12  to ensure good surface coverage. It is preferable that the glass net be applied with the large strands in the radial direction of the coil. Sectional strips may be used on the outer surface of the coil to reduce material content. A coil winding  30  is then applied by the winding machine to the inner mold  12 . A pre-pregnated glass net  32  is then installed over the finished coil winding  30  to provide an outer layer of glass net which also is preferably applied radially. The outer mold  24  is then installed and secured over the finished coil  30 . This process may be performed while the inner mold  12  is on the mandrel or it may be performed at a remote assembly site. 
     After the coil  30  is removed from the winding equipment, the inner support members  18  are inserted. These support members  18  maintain the coil shape on large coils and remain inside the inner mold  12  during the entire encapsulation process. Prior to installing the outer mold  24 , an epoxy release agent is applied to the inside of the outer mold  24 . For best results continuous and consistent coverage is essential. Thereafter, the outer mold  24  is installed over the coil winding  30 . A gasket material  34  is placed over the inner and outer disposable mold ends as shown in FIGS. 1 and 4. The application of a small amount of suitable adhesive such as super glue to the mold and gasket mating surfaces will aid in securing the gasket  34  to the mold ends. A pair of end plates  36 ,  36  are set in place against the gasket and mold surfaces and temporarily secured in place while verifying the squareness and positioning ofthe inner and outer molds  12  and  24 . A plurality of compression rods  38  are installed to connect the end plates  36 . The compression rods  38  have threaded ends for receiving nut members  40 . At this point in the process the nut members  40  are not tightened to apply torque to the compression rods  38 . A plurality of coil support plates  42  are installed at the top and each end of the mold dome area  26  using small clamps  44 . This will support the weight of the coil  30  during the encapsulation and curing processes. Torque is now applied to the compression rods  38  by tightening the nut members  40 . This is preferably done by starting at one side of the end plates and moving in a crossing sequence until torque values in the order of 130 in lbs. to 140 in lbs. is established on all of the remaining compression rods  38 . Torque values preferably are established by 40 to 50 in. lb. increments. The length of the inner and outer molds  12  and  24  preferably are manufactured to close tolerances in the order of 0.4 mm. This is desirable for obtaining consistent gasket sealing to end plates. With the process described venting of the innermold area is desirable at the end plates at  36   a  for equalization of pressure during the casting process. 
     With the casting mold assembly  10  in a horizontal position it is placed in a vacuum chamber and a casting resin  48 , such for example as an epoxy resin, is introduced through the openings in the support plates  42  to fill the mold with the casting resin. Such a vacuum casting process is well known in the art. It is preferable that the mold assembly  10  be maintained horizontal or level during the gelling and curing process. After the curing process has been completed, the outer and inner molds are removed and the encapsulated coil is removed from the casting mold assembly. 
     While the present invention has been described in connection with a method of making a round winding configuration in FIGS. 1-4, it is equally applicable to oval winding configurations. The principle difference is in the shape of the inner and outer molds. An example of a mold assembly for oval shaped coils is illustrated in FIGS. 5 and 6. The casting mold assembly  50  shown in FIGS. 5 and 6 includes an oval shaped inner mold  52  and an oval shaped outer mold  54 . Both the inner and outer molds  52  and  54  are made of low cost sheet metal similar to the inner and outer molds  12  and  24  illustrated in FIGS. 1-4. A glass net material is applied over the inner mold  52  and a coil winding  56  is then applied by the winding machine to the inner mold  52 . Another layer of glass net is then installed over the finished coil winding  56  to provide an outer layer of glass net. The outer mold  54  is then installed and secured over the finished coil  56 . Mechanical strength of the oval coil design requires internal support longitudinally across the coil to maintain shape consistency. Typically there are three mechanical supports in the form of disks  58  equally spaced inside the mold, beginning about 4 inches from the end of the mold. The mechanical support work pieces may be fabricated on a punch machine to obtain the oval shape. The material thickness for the inner support may be 14 gauge (1.88 mm) or 11 gauge (3.17 mm). The other parts of the casting mold assembly  50  are similar to the parts of the casting mold assembly  10  and have been identified with similar reference characters with the addition of a prime. 
     From the foregoing it will be seen that the present invention includes a new process of winding either round or oval transformer coils over a disposable mold which acts as both a winding mandrel and as an inner mold shell for epoxy encapsulation. The invention also includes a simple technique of making the inner and outer molds using low cost sheet metal. The invention has many advantages over conventional fixed sized standard winding mandrels and casting molds. Two of the features or advantages are the use of low cost sheet metal to form a flexible sized oval or circular mandrel and the horizontal molding concept. The complete winding and molding processes can be all conducted on a standard winding machine for either aluminum or copper conductor wound in a layer or disk type format. The process of the present invention provides the advantages of flexible round or oval dimensional sizes, flexible and incremental round or oval shapes providing substantially reduced mandrel and molding costs, elimination of costly standard mold storage, elimination of the need for standard winding mandrels, controlled epoxy thickness for increased dielectric strength and reliability, improved manufacturer ability, reduction in epoxy cost, horizontal epoxy casting and facilitates the opportunity to manufacture transformer coils in a round or oval configuration. The present process using a disposable integrated winding mandrel represents a substantial improvement over the current state ofthe art which uses precision made standard sized re-usable round shaped mandrels and molds in fixed size increments. 
     While a preferred embodiment ofthe present invention has been described and illustrated, it is to be understood that further modifications thereof can be made without departing from the scope of the appended claims.