US3043000A - Method of forming a conductive coil on a closed magnetic core - Google Patents

Description

July 10, 1962 F. A. HATFIELD 3,043,000

METHOD OF FORMING A CONDUCTIVE COIL ON A CLOSED MAGNETIC CORE 3 Sheets-Sheet 1 Filed April 24, 1958 INVENTOR. FREDERICK A. HATFIELD BY AT TORNEY July 10, 1962 F. A. HATFIELD 3,043,000

METHOD OF FORMING A CONDUCTIVE con ON A CLOSED MAGNETIC CORE Filed April 24, 1958 3 Sheets-Sheet 2 A. HATFIELD R m W W.

FREDERICK ATTORNEY July 10, 1962 F. A. HATFIELD 3,043,000

METHOD OF FORMING A CONDUCTIVE COIL ON A CLOSED MAGNETIC CORE Filed April 24, 1958 3 Sheets-Sheet 3 INVENTOR. FREDERICK Av HATFIELD ATTORN EY United States Patent 3,043,000 METHGD F FORMING A CONDUCTIVE COIL ON A CLOSED MAGNETIC CORE Frederick A. Hatfield, Zanesvilie, Ohio, assignor to McGraw-Edison Company, Milwaukee, Wis., a corporation of Delaware Filed Apr. 24, 1958, Ser. No. 730,664 4 Claims. (Cl. 29-15557) This invention relates to stationary induction apparatus having a closed magnetic core with an electrical coil wound thereon and to a method for constructing electrical windings on closed magnetic cores.

The present invention is an improvement of the methods of forming electrical windings on closed magnetic cores disclosed in US. Patents 2,305,999 to Steinmayer et al. and US. 2,334,131 to Schultz et al. which have the same assignee as the subject application and disclose the steps of revolubly supporting a tubular insulating shell circumjacent a straight winding leg and out of contact with the magnetic core and rotating the tubular insulating shell to wind wire inhelical layers onto the rotating shell. The copper space factor, ie, the ratio of net copper cross-sectional area to area of the window in the closed magnetic core, in an electrical transformer constructed in accordance with this method is relatively low. Ducts are provided in the electrical coil between helical layers of wire parallel to the longitudinal axis of the winding leg for the vertically upward circulation of cooling fluid, and the cooling ducts which occur within the core window are of little value in effecting cooling and take up space which, if filled with copper, would increase the copper space factor.

It is an object of the invention to provide an improved method of forming electrical coils on closed magnetic cores which results in a greater copper space factor than with prior art methods.

It is a further object of the invention to provide an electrical transformer, of the type wherein a closed mag netic core of continuous magnetic ribbon is encircled by an electrical winding, having a greater copper space factor than prior art transformers of this type.

Other objects and advantages of the invention will be apparent fromthe following description when taken in conjunction with the accompanying drawing wherein:

FIG. 1 is a view of an electrical transformer core and coil assembly, with a portion of the closed magnetic core broken away and in section, illustrating a step of a preferred method of practicing the invention;

FIG. 2 is .a View similar to FIG. 1 illustrating the subsequent method step of shifting inner helical layers of wire in a direction away from the core window until the portions of the inner helical layers within the core window are contiguous the winding leg of the core;

FIG. 3 is a view similar to FIGS. 1 and 2 showing the improved transformer construction of the invention and also illustrating'the method step of moving outer helical layers of wire relative to the inner helical layers until the portions of the outer helical layers within the core window are contiguous the inner helical layers;

FIG. 4 is a side elevation view of apparatus for rotatably supporting a tubular insulating shell circumjacent a straight winding leg of a magnetic core and out of contact with the core;

FIG. 5 is a view similar to FIG. 4 illustrating the step of rotatably supporting a second tubular insulating shell :ircumjacent and out of contact with the inner helical .ayers;

FIG. 6 is a detail view in perspective illustrating bearng means for revoluoly supporting the second tubular nsulating shell shown in FIG. 5;

FIG. 7 is a partial plan view of the apparatus of FIG. 4 with parts broken away and in section;

FIG. 8 is a partial plan view of the apparatus of FIG. 5 with parts broken away and in section; and,

FIG. 9 is a view similar to FIGS. 4 and 5 illustrating an alternative method in accordance with the invention wherein it is unnecessary to utilize two different sets of split gears to construct the electrical winding.

In accordance with the method of the invention, inner helical layers of conductor are wound around the straight winding leg of a closed magnetic core in radially spaced relation thereto, outer helical layers of conductor are wound around the inner helical layers in radially spaced relation thereto, and the inner and outer helical layers are shifted in a direction away from the core window until the portions of the inner helical layers within the core window are contiguous the winding leg and the portions of the outer helical layers within the core window are contiguous the inner helical layers, whereby a relatively high copper space factor within the core window is obtained in comparison to the constructions of the aforementionsd patents.

The invention will be described with reference to an electrical transformer core and coil assembly wherein the electrical coil has a low voltage winding adjacent a straight winding leg of a closed magnetic core and is separated by an insulating barrier from a high voltage winding radially outward therefrom. A preferred method of practicing the invention illustrated in FIGS. 4-8 will be described as a specific improvement over that disclosed in the aforementioned U.S. Patent 2,334,131 to Schultz et al. The magnetic core 10 of an electrical transformer embodying the invention is of the closed, wound strip type wherein continuous magnetic strip is wound layer upon a layer on an approximately rectangular mandrel and different widths of magnetic ribbon are utilized to provide winding legs 11 of approximately cruciform cross-section. The closed magnetic core 10 is clamped to a frame structure 12 by suitable clamping means 14 having a set screw 15 threaded therethrough and bearing against the mag netic core 10 in .a manner similar to that disclosed in US. Patent 2,334,131. Uprights 16 secured to the frame structure 12 carry bearing members 17 which removably interlock with the uprights 16 and are held in place by bolts 18. The bearing members 17 extend inwardly toward the clamping means 14 and are provided with semicylindrical inner flange portions 20 (see FIG. 7) surrounding a part of winding leg 11 and upon which split flange members 21 are revolubly supported. The flange members 21 are formed of two or more segments, two segments 22 and 23 being indicated in the drawing, detachably locked together by locking members 24 inset within recesses 25 provided in the flange members 21. The locking members 24 are provided with apertures at their opposite ends through which screws extend and are threaded into the segments 22 and 23 of the flange members 21, thus locking the segments 22 and 23 together circumjacent the winding leg' 11. The flange members 21 are spaced apart lengthwise of the winding leg 11 and .are provided with inwardly extending cylindrical flange portions 26 on which a tubular insulating shell 27 is formed.

Preferably, the flanged members 21 are provided with gear teeth which mesh with pinion gears 28 carried on a shaft 29 driven by suitable drive means. While the flange members 21 are being rotated, a sheet of insulating material, preferably coated with shellac, is wound on the annular flanges 26 until a tubular insulating shell 27 of requisite strength and thickness is formed. At least the end portion of the outermost layer of insulating material is shellacked so as to adhere to the adjacent layer of the multilayer tubular shell and prevent unwinding thereof. preferably the outer layer of the tubular insulating shell 2.7 is also of insulating sheet material and has its marginal edges folded back and forth to form a thickened marginal portion 3% (see P16. 7) approximately equal to the diameter of the wire to be utilized in the low voltage winding L. The end of the wire is then anchored, by means not shown, preferably to one of the flange members 21, and the shaft 29 is driven to rotate the flange members 21 and tubular insulating shell 27 and pull the wire into a first, inner, helical conductor layer 31 upon shell 27. Thereafter another layer 32 of insulating sheet material with thickened marginal edges 3t) is positioned on the finished wound conductor layer 31 of the conducting coil and, a second inner, helical wire layer 33 of the conducting coil is formed. This process is continued until the requisite number of inner helical layers of conductor 31, 33 etc. have been wound in place.

The above described apparatus and method steps do not constitute a novel part of the present invention and are disclosed in detail in the aforementioned US. Patent 2,334,131, and in accordance with the method of this patent the same steps are continued until a low voltage winding, a high voltage winding, and an insulating barrier therebetween are completed. After the conducting coil constructed in accordance with this patent has been completely wound, the bolts 18 are removed, the bearing members 17 are slid outwardly in opposite directions from within the annular flanges 26 on the flange members 21, and the flange members 21 are removed, thus leaving the finished coil in place on the permanently positioned tubular insulating shell 27. During the process of constructing the electrical coil, corrugated insulation or circumferentially spaced apart spacers parallel to the axis of rotation of the flanged gears 21 are positioned between helical layers of conductor to provide cooling ducts which permit vertically upward circulation of cooling fluid through the electrical coil, and the ducts which appear within the core window 34 are relatively ineffective in accomplishing cooling. Further, the cooling duets within the core window result in a relatively small copper space factor, i.e. the ratio of net copper cross-sectional area to the area of the window in the core, and it will be apparent that the size of the magnetic core and the transformer casing can be materially reduced if the copper space factor is increased.

In accordance with the method of the present invention, the above described steps are continued until sulficient inner helical wire layers 31, 33 etc. are wound to complete the low voltage winding L and sufficient layers of suitable insulating sheet material are wound over the inner helical wire layers of the low voltage winding L to form half b of the insulating barrier between the low voltage winding L and high voltage winding H to be constructed in a manner hereinafter described. It will be apparent that the invention is applicable to the construction of any type of electrical winding and that the phrase inner helical wire layers could equally well be those of a high voltage winding if it were desired to have the primary winding immediately adjacent the core and the low voltage winding radially outward therefrom.

After the inner coil I comprising the inner helical wire layers 31, 36 etc. of the low voltage winding L and half b of the insulating barrier are completed, the bolts 18, the bearing members 17, and the flange members 21 are removed. It will be apparent that after removal of these members, the tubular insulating shell 27 is spaced radially from the winding leg 11 by the thickness of the semicylindrical portions 20 of the bearing members 17 and the thickness of the cylindrical flange portions 26 of the split flanged gears 21. The inner coil 1, comprising tubular insulating shell 27, inner helical vw're layers 31, 33 etc. wound thereon and barrier half I), is then displaced in a direction away from the core window to the position shown in FIG. 2 wherein the portions of the shell 27 and inner helical wire layers 31, 33 etc. within the core window are contiguous the winding leg 11 and the axis of inner coil 1 is noncoincident with the longitudinal (1 axis of winding leg 11. A wedge 35 of wood or other insulating material may then be disposed between winding leg 11 and tubular insulating shell 27 to hold the inner helical wire layers 31, 33 etc. in such displaced position.

Second bearing members 37 illustrated in PEG, 6 are then secured to the uprights 16 by bolts 13 as shown in FIGS. 5 and 8. Each bearing member 37 is formed with a body portion 36a integrally joined by suitable means such as welding to a bearing portion 36b. The bearing portion 3611 has an indentation 33 of stepped configuration which conforms to the cruciform cross sections of winding leg 11 and an inwardly extending semicylindrical flange 39 partially surrounding the inner coil I.

The axis of the semicylindrical flange 39 of each second bearing member 37 is coincident with the axis of tubular insulating shell 27 and inner helical wire layers 31, 33 etc. but is offset from the longitudinal axis of the winding leg 11. The semicylindrical flanges 39 on the second bearing members 37 are adapted to support a second set of split flange gears 41, similar to but of greater inner diameter than the flange members 21, coaxial with the inner coil I. The split gears 41 are preferably formed of two segments 49 and 5t) detachably locked together and have inwardly extending cylindrical flanges 42 which surround the inner coil I. Suitable sheet insulating material, preferably shellac coated paper, is Wound on the cylindrical flanges 42 in suflicient layers to form a second tubular insulating shell 44 (see FIG. 8) of requisite strength and thickness similar to the first insulating shell 27. The second insulating shell 44- may comprise the radially outward half b of the insulating barrier between high and low voltage windings. The layers of sheet insulating material are wound by rotating shaft 29 carrying pinion gears 28 which mesh with the flange gears 41, and the outer layers of the tubular insulating shell preferably has its marginal edges folded back and forth to form a thickened marginal portion 30 approximately equal to the thickness of the conductor to be utilized in the primary winding H. The end of the wire is then anchored, preferably to one of the flange gears 41, by suitable means not shown, and the spit flange gears 41 are driven from shaft 29 to pull the wire into a first, outer, helical wire layer 46 around the second tubular insulating shell 44-. Thereafter another layer of insulating sheet material 47 with its thickened marginal edge 30 is positioned on the finished Wound outer helical wire layer 46 and a second outer helical wire layer 48 of the electrical coil is pulled onto the second tubular shell 44 on top of the first outer helical layer 46 by rotating the flange gears 41. The process is continued until the outer coil 0, comprising the insulating barrier half 12 and the requisite number of outer helical layers 46, 48 etc. of the primary winding H, have been wound in place. Thereafter the outer coil 0 is finished or covered in any suitable way. The particular manner in which the end leads or the intermediate leads are arranged forms no part of this invention and will not be discussed. After the desired number of outer helical wire layers 46, 48 etc. of the primary winding H are wound, the bolts 18 are removed and the second hearing members 37 are slid outwardly in opposite directions from within the cylindrical flange portions 42 of the split flange members 41. Thereafter the locking mem- Users 24 are removed and the segments 49 and 50 of the split flanged members 41 are removed.

After the bearing members 37 and split flange members 41 are removed, the outer coil 0 is radially spaced from the inner coil 1 by the radial thickness of the flange portion 39 of second bearing members 37 and the flange portion 42 of split gear members 41. Outer coil 0, comprising the second tubular insulating shell 44 and the outer helical wire layers 46, 48 etc., is displaced in a direction away from the core window to the position shown in F116. 3 wherein the portion of outer coil 0 within the core window is contiguous the inner coil I. Wedges 51 of wood or other suitable insulating material are positioned between the insulating barrier half b of the inner coil I and the second tubular insulating shell 44, i.e. the insulating barrier half b of the outer coil, to hold the outer helical wire layers 46, 48 etc. of the primary windingw H in this oifset position. It will be apparent that a relatively large, crescent-shaped cooling duct 52 (see FIG. 3) is thus provided between low voltage winding L and high voltage winding H for upward circulation of transformer oil, and that, inasmuch as no cooling ducts occur within core window 34, a high copper space factor is obtained in comparison to prior art transformers.

The phrases inner and outer are intended to refer to the helical Wire layers and coils which are Wound when flange members 21 and 41 respectively are utilized.

In place of bearing members 17 and 37, flange gears 21 can be rotatably supported directly on winding leg 11 and gears 41 on the outer periphery of inner coil 1 or both gears 21 and 41 can "be rotatably supported at a plurality of points about their periphery by pinion gears 65 as shown in FIG. 9.

FIG. 9 illustrates an alternative method of practicing the method of the invention wherein only one set of split flanged gears is required to construct the electrical winding, and this alternative method is described as a specific improvement over that disclosed in the aforementioned US. Patent 2,305,999. A split, or a multi-section, tubular insulating shell 60 is placed around the winding leg 11 or" the magnetic core 10. As shown in the drawing, the shell 65 is split as indicated at 71 and provided with removable end members 61. Preferably the end members 61 are split gears formed by two externally toothed seg ments 62 and 63 detachably joined together by clips 68 so as to encircle winding leg 11. The tubular insulating sleeve 60 is provided adjacent its ends with radially extending apertures which receive pins 64 carried by the gear segments 62 and 63 so that rotation of the end members 6-1 will also turn the tubular shell 60. The split gears 61 are rotatably supported at three points around their circumference by pinion gears 65 so that the tubular shell 65 is out of contact with the winding leg 11. Radially extending web portions 55 disposed laterally of the external teeth on pinion gears esprevent lateral movement of the end members 61 relative to the pinion gears 65, and it will be appreciated that web portions es may comprise circular discs ailixed to opposite sides of a conventional spur gear. At least one of the pinion gears 65 is rotatably driven by suitable drive means (not shown) to rotate the tubular insulating shell 60 and pull the wire conductor into inner helical layers 31, 33 etc. in a manner substantially identical to that disclosed in the aforementioned patent, and sufficient layers of shellac coated paper may be Wound over the inner conductor layers to build up the insulating barrier half 12.

At this stage of construction the toothed end members 51 are not removed as in the embodiment of FIGS. 48, but instead spacer supports 67 are placed radially outward from the inner coil I to rotatably support a second tubular .nsulating shell 44' in radially spaced relation to the inner :oil I. The spacer supports may assume different forms 1nd each may, for example, comprise a plurality of layers )f relatively narrow material having a suitably low co- :fiicient of friction spirally wound around the barrier half i adjacent one end thereof to build up a narrow annular :upport ring (not shown) for one end of the tubular hell. As illustrated, the spacer supports comprise elongated spacer members 67 extending'parallel to the axis if rotation of the shell 69 disposed at equiangular posiions around the outer periphery of the inner coil I to radilly space inner coil I from the outer coil 0. The spacer members 67 may be held in position by any suitable means uch as pressure sensitive tape (not shown) which permits emoval of the spacer members 67 after the outer helical vire layers 46, 48 etc. are wound. The second tubular isulating shell 44 placed around the spacer members 67 lay be of the split type or of the multi-section type if it is desired to remove the insulating shell after completion of the winding. As illustrated in the drawing, suitable shellac-coated insulating sheet material is wound in suflicient layers over the support spacers 67 to form a second tubular insulating shell 44 of requisite strength, and the end members 61 are rotatably driven through pinion gears 65 to pull the outer helical wire layers 46, 48 etc. upon the second tubular insulating shell 44. It will be appreciated that with this method of coil con struction, both the inner and outer helical wire layers are wound symmetrically about to the longitudinal axis of winding leg 11. After the desired number of outer helical layers 46, 48 etc. have been wound, the clips 68 are removed and the gear segments 62 and 63 withdrawn. If a split or a multi-section tubular insulating shell 44' were used, it may then be removed, but when the tubular insulating shell 44 is constructed from a plurality of layers of shellac-coated paper to form the insulating barrier half b between low and high voltage windings L and H, shell 44' may be left as a permanent part of the electrical coil. The spacer support rods 67 are then pulled out between the inner coil I and outer coil 0 to provide a radial space therebetween. The inner coil I, including inner helical wire layers 31, 33' etc. and

insulating barrier half b, is then displaced relative to winding leg 11 is a direction away from core window 34 to position the portion of the inner coil I within core window 34 adjacent winding leg 11, and the outer coil 0, including outer helical wire layers 46, 48 etc. and insulating barrier half b, is shifted relative to inner coil I in a direction away from core window 34 until the portion of outer coil 0 within core window 34' is contiguous the inner coil I, as shown in FIG. 3, whereby the copper space factor within the core window is appreciably increased in comparison to prior'art constructions.

It will be noted that in the manner of practicing the method of the invention illustrated in FIG. 9, no portion of the supporting members for the tubular insulating shell 60 is disposed between core 10- and shell 60 and further that the end members 6 1 are secured directly to the shell 60 and do not have cylindrical flanges positioned between core 11 and shell 60 as in the embodiment utilizing the apparatus of FIGS. 48. It will thus be readily apparent that in accordance with the method of practicing the invention illustrated in FIG. 9, the first tubular insulating shell may be spaced a smaller distance from the winding leg 11 and similarly the second tubular shell 44 may be radially spaced a smaller distance from the inner coil I than with the method illustrated in FIGS. 48.

In alternative form the spacer supports 67 may be hollow, compressible, elongated members extending parallel to the axis of rotation of the tubular insulating shell, which spacers 67 can be is shifted relative to the inner coil contiguous Within the core window.

It will be apparent that the pinion gears 65 provide means alternative to the bearing supports 17 and 37 for rotatably supporting the flanged members and that the pinion gears 65 may be utilized to rotatably support the split gears 21 and 41 of the FIGS. 48 embodiment. Preferably the shaft on which the upper one of the pinion gears 65 is mounted is movable vertically to facilitate removal of one set of gears and the installation of a second set of split gears having different diameter cylindrical flanges. The use of pinion gears 65 to support the split flanged members such as 21 and 41 permits the construction of the tubular insulating shells by spirally winding a plurality of layers of adhesive coated insulating paper.

In the embodiments of the method illustrated in the drawings, means are provided to support the split flanged members out of contact with the core and coil, i.e. in the embodimen illustrated in FIGS. 48 the flange members I to bring these parts crushed when the outer coil 0 21 and 4i are supported on bearing members 17 and 37, while in the alternative manner of practicing the invention illustrated in FIG. 9 the split gears 61 are supported at three points about the periphery thereof by pinion gears 65. However, the invention also comprehends rotatably supporting the split flanged members directly on the winding leg Ill or on a tube of paper encircling the winding leg on which the split gears having cylindrical flanges can revolve without injury to the magnetic material. When following this method of practicing the invention wherein the split gears are revolubly supported directly on the magnetic core, the inner insulating tube may be constructed by spirally winding a plurality of layers of adhesive coated insulating paper in the manner explained hereinbefore. Further, the split flanged gears such as 41 for constructing the outer coil 0 may be rotatably supported directly on the insulating barrier half b surrounding the inner coil I, and in this method of practicing the invention the inner coil 1 may be displaced relative to the winding leg either before the outer coil 0 is wound as in the FIGS. 48 embodiment, or subsequent thereto as in the embodiment illustrated in FIG. 9.

Although only two alternative methods and specific apparatus have been illustrated and described for practicing the method of the invention, many modifications and variations thereof will be apparent to those skilled in the art, and accordingly it is intended in the appended claims to cover all such modifications and variations which fall within the true spirit and scope of the invention.

It is claimed:

1. The method of winding a conductive coil on the straight winding leg of a closed magnetic core having a plurality of legs surrounding a window, comprising the steps of placing a pair of bearing members closely adjacent said winding leg in spaced relation lengthwise thereof, revolubly supporting cylindrical split flanges on said bearing members, supporting a first insulating tube on said flanges, rotating said cylindrical flanges to thereby rotate said first insulating tube and winding wire in inner helical layers on said rotating tube, removing said split flanges and said bearing members, displacing said inner helical layers to a position wherein the axis thereof is noncoincident with the longitudinal axis of said winding leg and the portion of said inner layers within said window is adjacent said winding leg, holding said inner layers in said displaced position, placing second bearing members closely adjacent said inner helical layers in concentric relation therewith and in spaced relation lengthwise thereof, supporting second cylindrical split flanges on said second bearing members, supporting a second insulating tube on said second flanges, rotating said second flanges to thereby rotate said second tube and winding wire in outer helical layers on said second insulating tube, removing said second cylindrical split flanges and said second bearing members, displacing said outer helical layers to a position wherein the axis thereof is noncoincident with the axes of said winding leg and said inner layers and the portion of said outer helical layers within said window is adjacent said inner layers, and holding said outer layers in said displaced position.

2. The method of winding a conductive coil on a straight winding leg of a closed magnetic core having a plurality of legs surrounding a winding window, comprising the steps of placing a pair of bearing members closely adjacent said winding leg in spaced relation lengthwise thereof, revolubly supporting first cylindrical flanges on said bearing members, supporting a first insulating tube on said first cylindrical flanges, rotating said first cylindrical flanges to thereby rotate said tube and winding wire in inner helical layers on said rotating tube, removing said first cylindrical flanges and said first bearing members,

displacing said first insulating tube and said inner helical with the longitudinal axis of said winding leg and the portion of said first insulating tube within said window is adjacent said winding leg, placing a pair of second bearing members closely adjacent and concentric with said inner helical layers in spaced relation lengthwise thereof, supporting second cylindrical flanges on said second bearing members, supporting a second insulating tube on said second cylindrical flanges, rotating said second cylindrical flanges to thereby rotate said second tube and winding wire in outer helical layers on said second insulating tube, removing said second cylindrical flanges and said second bearing members, and displacing said second insulating tube and said outer helical layers wound thereon relative to said inner helical layers to a position wherein the axis of said outer layers is noncoincident with the axis of said inner layers and the portion of said second insulating tube within said window is adjacent said inner layers, whereby a high copper space factor is obtained within said window, and holding said inner and outer layers in said displaced positions.

3. The method of forming a conductive coil on the straight winding leg of a closed magnetic core having a plurality of legs surrounding a window, comprising the steps of placing a first insulating tube and end flanges around said winding leg to provide a spool like structure, revolubly supporting and rotating said flanges to thereby rotate the tube and winding wire in inner helical layers on the rotating tube, removing said flanges, displacing said inner helical layers relative to said winding leg to a position wherein the portion of said inner layers within the window of said core are adjacent said winding leg and the axis of said inner helical layers is noncoincident with the longitudinal axis of said winding leg, placing a second insulating tube around said inner helical layers, revolubly supporting and rotating said second insulating tube and winding wire in outer helical layers on the rotating second insulating tube, and displacing said outer helical layers relative to said inner helical layers to a position where the portion of said outer layers'within said window is adjacent said inner layers and the axis of said outer layers is noncoincident with the axes of said winding leg and said inner layers, whereby a high copper space factor within said core window is obtained.

4. The method of forming a conductive coil on the straight winding leg of a closed magnetic core having a plurality of legs surrounding a window, comprising the steps of placing a first insulating tube and end flanges around said winding leg, revolubly supporting and rotating said flanges to thereby rotate said tube and winding wire in inner helical layers on said rotating tube, placing spacer supports radially outward from said inner helical layers, placing a second insulating tube around said inner helical layers and rotatably supporting said second insulating tube on said spacer supports, rotating said second insulating tube and winding wire in outer helical layers on said second insulating tube, removing said spacer supports, displacing said inner helical layers to a position wherein the axis thereof is noncoincideut with the axis of said winding leg and the portion of said inner layers within said core window is adjacent said winding leg, and displacing said o-uter helical layers to a position wherein the axis thereof is noncoincident with the axes of said winding leg and said inner helical layers and the portion of said outer helical layers within said winding window is adjacent said inner layers, whereby a high copper space factor is obtained within said core window.

References Cited in the file of this patent UNITED STATES PATENTS 829,780 Hall Aug. 28, 1966 (Uther references on following page) 9 UNITED STATES PATENTS Kent Nov. 20, 1923 Boyjajian June 10, 1941 Vienneau Dec. 22, 1942 Steinmayer et a1 Dec. 22, 1942 5

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