Method of manufacturing a transformer coil with a disposable mandrel and mold

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.

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.

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 12a
 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 12a 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 netmaterial28 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
 36a 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.