US3436704A

US3436704A - Electrical transformer construction

Info

Publication number: US3436704A
Application number: US586116A
Authority: US
Inventors: August I Keto; Anthony J Palumbo
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1966-10-12
Filing date: 1966-10-12
Publication date: 1969-04-01
Anticipated expiration: 1986-04-01

Description

April 1, 1969 KETQ ETAL ELECTRICAL TRANSFORMER CONSTRUCTION Filed Oct. 12, 1966 N i J 0 m M m m V00 NH K A .L M 0 hwm i WM A Q WITNESSES 0%. 4;? m4.

April 1, 1969 A. l. KETO ET 3ELECTRICAL TRANSFORMER CONSTRUCTION Filed oct. 12. "less United States Patent 3 436 704 ELECTRICAL TRANS FOI QMER CONSTRUCTION August I. Keto, Sharpsville, and Anthony J. Palumbo,

Sharon, Pa., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Oct. 12, 1966, Ser. No. 586,116 Int. Cl. H01f /14, 27/02 US. Cl. 33670 8 Claims This invention relates in general to the construction of electrical transformers, and more specifically to new and improved transformer apparatus of the type which utilizes at least one partially cast coil, and to new and improved methods of constructing the same.

The availability of improved coated metallic foil or strip conductor, and improved casting resins, has made the encapsulation of electrical windings attractive for electrical transformers. Since foil wound windings have a single conductor turn per layer, the voltage stress between adjacent turns is very low, permitting the use of thin coatings of electrical insulation, on at least one side of the foil, to insulate the turn-to-turn voltage stress. Layer insulation, which is inserted between the radial layers of turns on wire wound coils, is completely eliminated. Foil wound windings also have a distinct advantage over wire wound windings from the standpoint of potting or encapsulating the winding. Since the voltage between layers of Wire wound coils may be very high, the encapsulating material must impregnate the winding and supplant all of the air in the voids between the layers.

The viscous nature of encapsulating materials makes it extremely diflicult to completely impregnate wire wound windings, with the air in the voids ionizing under high layer-to-layer voltage stresses, producing corona which degrades the surrounding insulation and generates radio interference. The low layer-to-layer stresses in foil wound windings makes it unnecessary to impregnate the windings, as any air trapped between turns will not be subjected to Stresses high enough to cause ionization.

Encpsulating foil wound windings in casting resin, however, is not without its problems. Care must be taken in the design of the electrical apparatus to provide liberal clearances, and to avoid thin section casting with its inherent air entrapment problems Also, the structure of the apparatus, and methods of providing the structure, have all been very costly, with high tooling costs accompanying high manufacturing costs, or the resulting structures are not sound electrically, or a combination of these disadvantages. For example, one method proposed for the manufacture of electrical transformers is a step-by-step casting procedure, wherein the inner low voltage coil is wound and cast, degreased and shot blasted, the high voltage coil is wound over the cast inner low voltage coil, and this assembly is cast, and the outer low voltage coil is wound about the cast high voltage coil. The outer low voltage coil is not cast. This method requires expensive, special shape molds for each casting step, and for each transformer rating, resulting in low production unless a large number of expensive molds are provided. Also, extreme care must be taken to prevent any voids in the high-low insulation, which is the cast material disposed between the low voltage coils and the high voltage coils. Any voids in this area will cause corona and eventual failure of the transformer. Also, with this method, the outer low voltage coil is mechanically protected only by the thin coating of insulation on the foil, with the very real possibility of damage to the outer low voltage coil requiring extreme care in the handling, storage, assembly with the magnetic core, and tanking, which increases manufacturing costs and rejects.

Another method proposed eliminates the step-by-step casting procedure by disposing spacers between the windings and then encapsulating. Test results, however, have shown excessive corona and inability to withstand test voltages. Manufacturing tolerances on the coils cause voids between the spacers and the coils which are not filled with the potting resin. These voids produce the corona. This type of winding is also slow and costly to manufacture, due to the extreme care required in spacing the coils, and the encapsulating step required to produce a structure which is corona free.

Therefore, it would be desirable to provide new and improved transformer apparatus, and methods of assembly, which eliminate the necessity of having a large number of special shape molds in order to obtain high production, which provides void-free high-low insulation between the low and high voltage coils, which relaxes the dimensional tolerances on the windings without producing voids in which corona may form, which strengthens the winding structure without using an excessive amount of casting material, and which protects the outside low voltage winding against mechanical damage.

Accordingly, it is an object of the invention to provide a new and improved transformer structure which utilizes at least one partially cast coil or winding.

Another object of the invention is to provide a new and improved transformer structure which utilizes cast resin to encapsulate at least a portion of one of its windings, and to strengthen the structure against short circuit stresses, without encapsulating the whole winding structure.

A further object of the invention is to provide a new and improved transformer structure which utilizes new and improved solid insulating means between the high and low voltage windings, with the thickness of the insulating means providing the necessary high-low insulation without regard to and without detriment from any air spaces between the solid insulating means and adjacent winding structures.

Another object of the invention is to provide a new and improved method for constructing electrical transformers in which one of its coils is at least partially encapsulated with cast solid insulating means.

Still another object of the invention is to provide a new and improved method for constructing transformers in which at least a portion of the insulation on at least one of its coils is provided by cast solid insulation, and which does not require the use of specially shaped molds.

Briefly, the above cited objects are accomplished by providing pre-formed high-low insulation members. Electrically conductive coatings on opposite sides of the highlow insulating members are electrically connected to the immediately adjacent winding, providing a substantially zero potential gradient between the windings and the insulating member. The ends of the high voltage winding are cast in solid insulation which completes the insulation for the high voltage winding and strengthens the structure against short circuit stresses. Circumferentially spaced apart ends of the high-low insulating structures provide ducts through the winding structure which allow cast solid insulating means to be introduced at one end of the structure, and use the ducts to flow to the opposite end, thus encapsulating both ends of the high voltage winding in one casting operation. The ducts are also filled with the cast solid material, which precludes the generation of corona therein.

The method of manufacture, in one of the embodiments of the invention, eliminates costly specially shaped molds for casting the ends of the high voltage windings in solid insulation. In this embodiment of the invention the ends of the low voltage windings project past the high voltage winding at both ends of the structure, which automatically provide cavities or molds for the cast solid insulation, except for means for sealing the bottom portion of the assembly until the resinous material is at least precured.

Further objects and advantages of the invention will become apparent from the following detailed description, taken in connection with the accompanying drawings, in which:

FIGURE 1 is an elevational view, partially in section, of the inner low voltage winding, whose construction is the first step in the method taught by the invention,

FIG. 2 is an elevational view, partially in section, which illustrates the placement of the first pre-formed high-low insulating structure relative to the low voltage winding assembly shown in FIG. 1,

FIG. 3 is an elevational view, partially in section which illustrates the position of the high voltage winding assembly relative to the assembly shown in FIG. 2,

FIG. 4 is an elevational view, partially in section. which illustrates the placement of the second pro-formed highiow insulating structure relative to the assembly shown in FIG. 3,

FIG. 5 is an elevational View, partially in section, which illustrates the disposition of the outer low voltage winding relative to the assembly shown in FIG. 4,

FIG. 6 is a cross-sectional view of the assembly shown in FIG. 5, taken along the line V1VI,

FIG. 7 is an elevational view, partially in section, which illustrates the assembly shown in FIG. 5 after casting,

FIG. 8 is an elevational view, partially in section, which illustrates the assembly shown in FIG. 7 with an outer protective coating of electrical insulating material,

FIG. 9 is a pictorial view which illustrates more clearly the two pre-formed high-low insulating structures and their relationship relative to one another, and

FIG. 10 is a schematic diagram which illustrates the electrical connections from the high and low voltage windings to the electrically conductive coatings on the preformed high-low insulating structures.

Referring now to the drawings, and FIG. 1 in particular, there is shown an elevational view, partially in section, of a coil or winding 10 which may be used as the inner low voltage winding of a transformer constructed according to the teachings of the invention. Winding 10 is formed of an electrically conductive metallic strip, sheet or foil conductor having a predetermined width dimension, such as copper or aluminum, and having a predetermined number of conductor turns 12, shown in the magnified insert 11, which are continuously wound upon a mandrel, or insulating tube and mandrel, to provide a predetermined inner opening 14 sized to receive the leg portion of magnetic core means (not shown). The edges of the strip material form first and second coaxially disposed end surfaces 13 and 15, on the substantially tubular winding structure 10. As is the characteristic of foil wound coils, each layer of the coil includes but one turn, with the turns being separated from one another by insulatin g means 16, shown in the magnified insert 11. Insulating means 16, shown in the magnified insert 11. Ining enamel, disposed on one or both of its major sides, such as an epoxy base enamel, or it may be a thin film of electrical insulation, such as Mylar, which is wound with the foil conductor to interleave the conductor turns 12. Electrical insulating means 16 may be extremely thin, which contributes to a high space factor for foil wound coils, as the layer-to-layer voltages and the turn-to-turn voltages are one and the same, and are very low relative to the high layer-to-layer voltage stresses found in wire wound coils.

After the step of winding the inner low voltage coil 10, a pre-formed insulating structure 20, having inner and outer

major surfaces

21 and 23, respectively, is disposed about the inner low voltage winding 10, as shown in FIG. 2. Insulating structure forms the first highlow insulating means, and may be formed in any suitable manner. For example, preformed insulating structure 20 may be formed of a butyl rubber, or a resin, such as an epoxy, and it may be flexible or a cast solid.

If flexible, it is simply wrapped about the inner low voltage coil 10. If a cast solid, it is telescoped over the inner low voltage winding or coil 10. The pre-for-rned solid insulating structure 20 has many advantages. For example, when forming the high-low insulation by casting in place between the high and low voltage windings, there are many factors which cannot be accurately controlled which contribute to non-uniformity in the cast structure from unit to unit, with air voids being formed in some or all of the units which lowers the corona inception voltage. By using pre-formed high-low insulation, its manufacture may be carefully and closely controlled to provide a void-free structure specifically formulated for the particular high-low voltage stress to be encountered, without compromising its electrical characteristics in order to obtain other characteristics necessary when casting the high-low insulation between the high and low voltage windings.

The pre-formed high-low insulation structure 20 also has many other advantages, both from cost and functional viewpoints.

If pre-formed insulation structure 20 is flexible, its length is predetermined to provide a gap or opening 22 between its ends when wound about low voltage winding 10. Or, if insulating structure 20 is cast, the wall section is not continuous, with the insulating structure 20 having first and second circumferentially spaced ends which provide a gap 22. The ends may be tong-ued or ribbed, as shown more clearly in FIGS. 6 and 9, for purposes which will be hereinafter explained.

Insulating structure 20 also has electrically

conductive coatings

26 and 28 disposed on the inner and outer

major surfaces

21 and 23, respectively.

Coatings

21 and 23 are electrically isolated from one another, and may be disposed such that they remain a predetermined distance from the edges of the structure, to increase the creep distance between the coatings. Thus, the first function of the gap or opening 22 in the wall section of insulating structure 20 is to prevent the electrically

conductive coatings

26 and 28 from forming a short circuit about the low voltage winding 10 and winding leg portion of the magnetic core. The second function of the opening 22, which will be fully described hereinafter, is to provide a duct for liquid casting resin in the casting step of the assembly of the apparatus.

The electrically

conductive coatings

26 and 28 may be in the form of excellent electrical conductors, such as copper or aluminum, or they may be semiconductive, i.e., having a voltage dependent resistivity, such as coatings containing particulated silicon carbide, or they may be partially conductive, such as coatings containing particulated carbon.

The inner electrically conductive coating 26 is electrically connected to the outer conductive turn of low voltage coil 10 by electrical connection 29. Therefore, the electrically conductive coating 26 is at substantially the same potential as the outer conductor turn of winding 10. Thus, it is not essential that the insulating structure 20 and coating 26 be disposed tightly against winding structure 10. Any air or void between coating 26 and winding 20 will be at substantially zero electrical stress or potential gradient. The high-low insulating structure 20 may thus be cast and the winding structure 10 wound to relaxed tolerances, which lowers manufacturing costs, as any space between the insulating structure and winding will not be subjected to a corona inducing stress. As will hereinafter be explained, all of the high-low stress will be applied to the high-low insulating structure 20. Coating 26 also provides a smooth, rounded, equipotential surface which prevents the concentration of voltage gradients about the edges of the winding and other sharp surfaces.

The next step in the method of forming a transformer according to the teachings of the invention is shown in FIG. 3. FIG. 3 illustrates the placement of the high voltage winding structure 30 relative to low voltage Winding 10 and the high-low insulating structure 20. High voltage winding structure 30 has a predetermined number of conductor turns and has a predetermined longitudinal dimension in the direction of the width of the electrically conductive strip, which in this embodiment of the invention is preferably less than the width dimension of the strip of which. the low voltage Winding is wound.

High voltage winding 30 may be formed by continuously winding an electrically conductive strip or foil upon the high-low insulating structure 20, until providing the required number of conductor turns. High voltage winding 30 may be formed of electrically conductive strip and the turns insulated, similar to the low voltage winding 10, hereinbefore described, with the edges of the strip forming first and second coaxially disposed spaced end surfaces 35 and 37.

High voltage winding 30 has its inner conductor turn electrically connected to the outer electrically conductive coating 28 via conductor 32, which reduces the electrical stress between the inner turn of winding 30 and coating 28 to substantially zero, and applies the total electrical stress between windings 20 and 30' to the insulating structure 20.

In some instances, high voltage winding 30 may be formed of two or more axially spaced, electrically connected sections, with each section being formed of electrically conductive strip. In this event, coating 28 will also be formed to have the same number of axially spaced sections as the high voltage winding, with the coatings being isolated electircally from one another, and which are electrically connected to the outer turn of its adjacent high voltage section. In this event, the axially spaced high voltage winding section, in this embodiment of the invention, :would still have a combined width which is less than the width of the low voltage winding 10.

The next step in the transformer assembly is shown in FIG. 4, and comprises the placement of a second highlow insulating structure 40 about high voltage winding 30. Insulating structure 40 is similar in construction to the high-low insulating structure 20, hereinbefore described, except for having an inner opening sized to encompass the outer dimension of high voltage winding 30 or, if flexible, a length capable of encircling high voltage winding 30, except for a gap or opening 42 similar to the gap 22 provided in insulating structure 20. Insulating structure 40 has inner and outer

major surfaces

44 and 46, respectively, and similar to insulating structure 20, has an electrically conductive coating 47 disposed on its inner surface 44, and an electrically conductive coating 49 disposed on its outer surface 46. Electrically

conductive coatings

47 and 49 may be formed as hereinbefore described relative to

coatings

26 and 28. Insulating structure 40 may be wrapped about high voltage winding 30, if flexible, or teleseoped thereover, if it is a cast solid. The inner electrically conductive coating 49 is electrically connected to the outer turn of high voltage winding 30, via electrical conductor 48, which reduces to substantially zero the potential gradient between coating 47 and winding 30, applying the electrical stress to the high-low insulating structure 40, and providing a smooth, equipotential surface which aids in reducing concentrations of electrical stress.

The next step in the assembly and construction of a transformer according to the teachings of the invention is shown in FIG. 5. This step involves the coaxial placement of the outer low voltage winding or coil 50 relative to the assembly shown in FIG. 4, which thus forms the complete winding assembly 60. The outer low voltage winding 50 may be continuously wound about the highlow insulating structure 40, and is formed of conductive strip or foil, similar to the inner low voltage winding 10, having a predetermined width and a predetermined number of continuous conductor turns separated by suitable insulating means. The outer low voltage winding 50, like the inner low voltage winding 10, preferably extends past the high voltage winding 30 at both ends thereof, for a predetermined distance. The edges of the conductive strip form first and second coaxially disposed, spaced end surfaces 55 and 57.

In order to form an area of substantially zero potential gradient between the outer low voltage winding 50 and insulating structure 40, and confine the electrical stress between windings 50' and 30 to insulating structure 40, which is specially prepared for this function, the inner turn of outer low voltage winding 50 is electrically connected to the outer conductive coating 47 on insulating structure 40 via electrical conductor 52.

Winding structure 60 is now physically ready for the casting operation. However, since foil or strip wound coils undergo considerable growth during the casting and curing cycles, it is preferable to include means for consolidating the windings and stabilizing their dimensions before casting. This consolidating and stabilizing step may be accomplshed by including a thin coating of a suitable adhesive disposed on at least one of the major surfaces of the conductive foil of which the

windings

10, 30 and 50 are wound. This consolidation and stabilizing of foil wound coils is described in detail in co-pending application Serial No. 506,350, filed Nov. 4, 1965, now abandoned, which is assigned to the same assignee as the present application. If the coils include such consolidating means, such as an epoxy adhesive, the next step in the method of constructing the transformer would be to heat the winding assembly 60 to fiow, set, and cure the adhesrve.

FIG. 6 is a cross-sectional view of the winding assembly 60 shown in FIG. 5, taken along the line VIVI, which more clearly illustrates the ducts formed in winding structures 60 by

openings

22 and 44 provided by the circumferentially spaced ends of insulating

structures

20 and 40, respectively. These ducts or

openings

22 and 42 are used to provide cast solid insulation at both ends of the winding structure '60 with a single step casting operation. In other words, by orienting winding structure 60 as shown in FIG. 5, the liquid casting resin may be introduced at the top of the structure, in the natural depression or cavity formed by the portions of the low voltage windings 10 and 50 which extend past the high voltage winding 30, and this casting resin will flow downwardly through

openings

22 and 42 to fill the cavities formed at the bottom of the winding structure 60 by the portions of low voltage windings 10 and 50 which extend past high voltage winding 30. Thus, in the casting step, costly, specially shaped molds are not required, as in this embodiment of the invention the construction of winding assembly '60 automatically forms the major portion of the mold for containing cast resinous insulation, which will insulate the ends of the high voltage winding 30 from ground and from the inner and outer low voltage windings .10 and 30. It is only necessary to place the winding structure in a very simple mold which will seal the bottom of winding assembly 60 during the casting step. If the windings are not formed to automatically provide cavities for containing the cast solid insulation to insulate the ends of high voltage winding 30, winding assembly 60 may be placed in a mold which will provide sufiicient space at each end of the winding assembly for the cast solid insulation.

After placing winding structure 60 into a suitable mold, the liquid cast resinous material is introduced into the top of the mold, the cast resinous insulation is directed to the bottom of the mold through openings or

ducts

22 and 42, to fill the space about the lower end of high voltage winding 30. In order to provide ample duct space without an excessive gap between the spaced ends of the insulating structures, the ends may be tongued, as il lustrated in FIG. 6. In other words, the wall thickness of insulating

members

20 and 40 may be reduced for a predetermined distance adjacent their ends, to increase the cross-sectional area of the ducts and facilitate resin flow through the ducts. After filling the bottom space in the mold,

openings

22 and 42 fill with the cast resin, and then the space about the upper end of high voltage winding 30 is filled with the cast resin. Winding assembly 60', after the cast resin is introduced, is shown in the elevational view, partially in section, of FIG. 7. FIG. 7 illus trates how the cast resin completes the insulating requirements of the high voltage windings 30 at the bottom of the structure 60 by cast solid insulating means 62, and at the top of structure 60 by cast solid insulating means 64. The cast solid insulation means 62 and 64 adheres to the pre-formed insulating

structures

20 and 40, to high voltage coil structure 30, and to the ends of low voltage windings and 50, to form a sealed, solid insulating system in which the solid insulation is stressed in puncture, and not in creep.

After the cast insulating means is introduced into the winding structure 60, the cast resin is cured by a suitable heating cycle. For example, a suitable heating cycle using an epoxy resin system, would be 2-4 hours at a predetermined temperature to set the resin system, after which the mold may be removed and reused, and 8 hours at a predetermined temperature to cure the resin system.

Many resin systems may be used for forming the cast solid insulating

means

62 and 64, which should preferably be thermosetting, although thermoplastic resin systems may also be used if their softening temperature is above the operating temperature range of the completed transformer.

In general, the resin system should be rigid, have a low coeflicient of thermal expansion tailored to closely match that of the conductive foil of which the high voltage winding 30 is formed, have excellent crack resistant characteristics, and a high thermal conductivity. An epoxy system found to be excellent is described in detail in co-pending application Serial No. 456,038, filed May 6, 1965, which is assigned to the same assignee as the present application.

Winding assembly 60 is now complete, except for mechanically protecting the exposed surfaces of low voltage windings 10 and 50 during the various steps in the assembly and handling of the transformer. This mechanical protection is provided, as shown in FIG. 8, by applying a thin coating of electrical insulation 70 about the whole winding assembly 60. Coating 70 may be applied by spraying, fluidized bed coating, or in any other suitable manner, and may be any suitable electrical insulating material, such as an epoxy system, or system containing butadiene-styrene, or silicone rubber.

Winding assembly 60 is thus completed, and as shown in FIG. 8, may be assembled with a suitable magnetic core assembly 80, which is shown in dotted outline, to form a core-winding assembly 90 which may be disposed in a suitable tank filled to a predetermined level [with a fluid insulating and cooling dielectric (not shown), such as oil.

Thus, a transformer 100 is formed, which utilizes cast insulation in a way which derives all of its benefits, without its accompanying drawbacks. This beneficial arrangement is obtained by using two pre-formed insulating

tructures

20 and 40, shown pictorially in FIG. 9 to more clearly illustrate their adjacent, spaced, concentric placement, and

openings

22 and 42 which interrupt the

conductive coatings

26, 28, 47 and 49, preventing a short circuit about the windings and core, and which also serves to enable the casting operation to be formed in one step. FIG. 9 also illustrates the construction of insulating

structures

20 and 40* more clearly, illustrating the inner and outer

major sides

21 and 23, respectively, of insulting structure 20 and how the structure forms the opening or gap 22 by first and second circumferentially spaced ends 91 and 92, respectively, which are disposed in spaced relation. This view also clearly illustrates the inner and outer

major surfaces

44 and 46, respectively, of insulating structure 40 and how opening 42 is formed by first and second circumferentially spaced ends 93 and 94-. The

pre-formed insulating

structures

20 and 40 may be specially prepared to :withstand the specific high voltage stresses encountered in a specific application, and it may be prepared without the deleterious voids which may occur when casting thin high-low insulation sections in place. Thus, thin section casting, with its inherent thin section air entrapment problems, is completely eliminated, the pre-formed insulating structure will perform the required function better, and the thickness of the insulating

structures

20 and 40 may be selected for the particular electrical stresses of the specific transformer, without resorting to costly tooling and mold changes. Thus, changeover from one type of transformer rating to another may be quickly and efficiently made.

The use of pre-formed insulating structures, along with their conductive coatings, enables all of the electrical stress between the windings to be applied to the insulating

structures

20 and 40, and makes it unnecessary to impregnate any space between the pre-formed insulating structures and the adjacet windings. Since the turns of each of the foil windings adjacent to a conductive coating is electrically connected thereto, which is schematically illustrated in FIG. 10, any space between the windings and electrically conductive coatings has substatially zero potential gradient, which precludes the formation of corona. FIG. 10 also illustrates terminals 99 and 101 on low voltage winding 10, terminals 97 and 98 on high voltage winding 30, and terminals and 96 on low voltage winding 50, which have been omitted on the other views of these windings for purposes of simplicity.

The smooth curved electrically conductive coatings form equipotential surfaces which prevent concentrations of electrical stress and shield the sharp edges of high voltage winding 30.

Further, the disclosed construction lends itself to low cost progressive winding of the transformer, whereby each transformer winding is wound about the prior assembled section, and the pre-formed insulating structures, with the discotninuous wall sections, allow the casting to be performed in one step. The disclosed construction requires a minimum in the way of special tooling and molds, enabling quick changes to be made in the dimesions and ratings of transformers with minimum cost and changeover time.

Since numerous changes may be made in the abovedescribed apparatus and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

We claim as our invention: 1. Electrical inductive apparatus comprising: first, second and third winding structures disposed in spaced, adjacent, concentric relation, respectively;

said first, second and third winding structures each having a predetermined number of conductor turns formed of electrically conductive strip material having predetermined width dimensions;

said first, second, and third winding structures each having first and second coaxially disposed end surfaces formed by the edges of said electrically conductive strip material;

first and second solid insulating means disposed between said first and second winding structures, and between said space second and third winding structures, respectively; said first and second solid insulating means having first and second coaxially disposed end portions, first and second circumferentially spaced end portions, which form ducts extending between said first and second coaxially spaced end portions, and inner and outer major surfaces which have an electrically conductive means disposed thereon; means electrically connecting the electrically conductive means on said inner and outer major surfaces to the winding structure immediately adjacent thereto; and

cast solid insulating means disposed about the first and second end surfaces of at least said second winding structure, and in the ducts formed by the circumferentially spaced end portions of said first and second solid insulating means.

2. The electrical inductive apparatus of claim 1, including means for mechanically protecting the exposed surfaces of said first and third winding structures, said means being in the form of a coating of solid electrical insulation.

3. The electrical inductive apparatus of claim 1, wherein the width dimensions of the electrically conductive strip of which said first and third winding structures are formed, exceed the width dimension of the electrically conductive strip material of which said second winding structure is formed, providing a winding assembly having a cavity at each end thereof defined by the first and second edge surfaces of said second winding structure and the portions of said first and third winding structures which extend past the first and second edge surfaces of said second winding structures, said cavities containing said cast solid insulating means.

4. The electrical inductive apparatus of claim 3, including electrical insulating coating means disposed over the outer surfaces of the winding assembly formed by said first, second and third Winding structures.

5. A method of constructing an electrical winding assembly, comprising the step of: Winding a strip of electrically conductive material having a predetermined width dimension, to form a first low voltage winding; providing a first pre-formed solid insulation structure having first and second major surfaces, each having an electrically conductive coating thereon, and first and second end portions; disposing said first pre-formed solid insulating structure around said first low-voltage winding, with its first and second end portions being circumferentially spaced apart; electrically connecting the electrically conductive coating disposed adjacent to said first low voltage Winding, to said first low voltage winding; winding a strip of electrically conductive material having a predetermined width dimension on said first pre-formed solid insulating structure to form a high voltage winding having first and second coaxially spaced end surfaces, and also complete a first duct between the first and second circumferentially spaced ends of said first pre-formed solid insulating structure; electrically connecting the remaining electrically conductive coating on said first pre-formed solid insulating structure to said high voltage winding; providing a second preformed solid insulating structure having first and second major surfaces, each having an electrically conductive coating thereon, and first and second end portions; disposing said second pre-formed solid insulation structure around said high voltage winding, with its first and second ends being circumferentially spaced apart; electrically connecting the electrically conductive coating disposed adjacent to said high voltage winding, to said high voltage winding; winding a strip of electrically conductive material having a predetermined width on said second preformed solid insulating structure, to form a second low voltage winding and also complete a second duct between the first and second circumferentially spaced ends of said second pre-formed solid insulating structure; electrically connecting the remaining electrically conductive coating on said second pre-formed solid insulating structure to said second low voltage Winding; casting a resinous insulating material about the first and second coaxially disposed ends of said high voltage winding, by introducing said resinous material at one end of said high voltage winding and using said first and second ducts to direct said resinous material to the other end; and curing said resinous material to form cast solid insulation which insulates the ends of said high voltage winding from said low voltage windings and ground, and which fills said first and second ducts to prevent the formation of corona therein.

6. The method of constructing the electrical winding assembly of claim 5, including the step of coating the exposed portions of said first and second low voltage windings with electrical insulating means, for mechanical protection.

7. The method of constructing the electrical winding assembly of cliam- 5 wherein the strip width dimension of which said first and second low voltage windings are wound exceeds the strip width dimension of which said high voltage winding is wound, which provides a cavity at each end of said high voltage winding for receiving said cast resinous insulation.

8. The method of constructing the electrical winding assembly of claim 5 wherein the strip of which said first and second low voltage, and said high voltage windings are formed has a coating of adhesive on at least one side thereof, and including the step of heating said first and second low voltage and high voltage windings prior to the step of casting the resinous insulating material, to consolidate said windings and stabilize their dimensions.

References Cited UNITED STATES PATENTS 1,837,245 12/1931 Wheeler 336-XR 2,998,583 8/ 1961 Worcester 336-206 XR 3,084,299 4/1963 Lord 336-70 3,265,998 8/1966 Park 33670 LEWIS H. MYERS, Primary Examiner. THOMAS J. KOZMA, Assistant Examiner.

U.S. Cl. X.R. 29-605; 33684, 96, 232

Claims

1. ELECTRICAL INDUCTIVE APPARATUS COMPRISING: FIRST, AND SECOND AND THIRD WINDING STRUCTURES DISPOSED IN SPACED, ADJACENT, CONCENTRIC RELATION, RESPECTIVELY; SAID FIRST, AND SECOND AND THIRD WINDING STRUCTURES EACH HAVING A PREDETERMINED NUMBER OF CONDUCTOR TURNS FORMED OF ELECTRICALLY CONDUCTIVE STRIP MATERIAL HAVING PREDETERMINED WIDTH DIMENSIONS; SAID FIRST, SECOND, AND THIRD WINDING STRUCTURES EACH HAVING FIRST AND SECOND COAXIALLY DISPOSED END SURFACES FORMED BY THE EDGES OF SAID ELECTRICALLY CONDUCTIVE STRIP MATERIAL; FIRST AND SECOND SOLID INSULATING MEANS DISPOSED BETWEEN SAID FIRST AND SECOND WINDINGS STRUCTURES, AND BETWEEN SAID SPACE SECOND AND THIRD WINDING STRUCTURES, RESEPECTIVELY; SAID FIRST AND SECOND SOLID INSULATING MEANS HAVING FIRST AND SECOND COAXIALLY DISPOSED END PORTIONS, FIRST AND SECOND CIRCUMFERENTIALLY SPACED END PORTIONS, WHICH FORM DUCTS EXTENDING BETWEEN SAID FIRST AND SECOND COAXIALLY SPACED END PORTIONS, AND INNER AND OUTER MAJOR SURFACES WHICH HAVE AN ELECTRICALLY CONDUCTIVE MEANS DISPOSED THEREON; MEANS ELECTRICALLY CONNECTING THE ELECTRICALLY CONDUCTIVE MEANS ON SAID INNER AND OUTER MAJOR SURFACES TO THE WINDING STRUCTURE IMMEDIATELY ADJACENT THERETO; AND CAST SOLID INSULATING MEANS DISPOSED ABOUT THE FIRST AND SECOND END SURFACES OF AT LEAST SAID SECOND WINDING STRUCTURE, AND IN THE DUCTS FORMED BY THE CIRCUMFERENTIALLY SPACED END PORTIONS OF SAID FIRST AND SECOND SOLID INSULATING MEANS.