US3534311A

US3534311A - Transformer with magnetic shields

Info

Publication number: US3534311A
Application number: US814637A
Authority: US
Inventors: Clifford J Bell
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1969-04-09
Filing date: 1969-04-09
Publication date: 1970-10-13
Anticipated expiration: 1987-10-13
Also published as: FR2041149A1; BE748745A; FR2041149B1; ES377608A1

Description

Oct. 13, 1970 c, J, BELL 3,534,311

TRANSFORMER WITH MAGNETIC SHIELDS Filed April 9, 1969 4 Sheets-Sheet l FIG. I.

FIG. IA.

WITNESSES INVENTOR jz zna g m Clifford J. Bell ATTORNEY Oct. 13, 1970 c. J. BELL 3,534,311

TRANSFORMER WITH MAGNETIC SHIELDS Filed April 9, 1969 4 Sheets-Sheet 2 Oct. 13, 1970 1 E L 3,534,311

TRANSFORMER WITH MAGNETIC SHIELDS Filed April 9, 1969 4 Sheets-Sheet s Oct. 13, 1970 c. J. BELL 3,534,311

TRANSFORMER WITH MAGNETIC SHIELDS Filed April 9, 1969 v 4 Sheets-Sheet 4 /lll United States Patent O 3,534,311 TRANSFORMER WITH MAGNETIC SHIELDS Clifford J. Bell, Muncie, Ind., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Apr. 9, 1969, Ser. No. 814,637 Int. Cl. H01f 15/04 U.S. Cl. 33684 8 Claims ABSTRACT OF THE DISCLOSURE An electrical transformer of the shell-form type having a plurality of coils disposed in inductive relation with a magnetic core structure. The electrical transformer includes magnetic shielding members disposed to provide low reluctance magnetic circuits for the leakage flux of the coils. Certain of the magnetic shielding members are formed of a cast insulating resin filled with magnetic material. These cast magnetic shielding members are utilized adjacent the curved portions of the coil openings, and are cast to substantially the configuration of the opening it is to fill, to increase the volume between the coils and the magnetic core which is filled with magnetic material.

CROSS-REFERENCE TO RELATED APPLICATION Certain of the apparatus disclosed but not claimed in this application, is disclosed and claimed in co-pending application Ser. No. 814,633, filed Apr. 9, 1969, in the names of Cliiford J. Bell and Harold R. Moore, which application is assigned to the same assignee as the present application.

BACKGROUND OF THE INVENTION Field of the invention I The invention relates in general to electrical transformers, and more specifically to new and improved magnetic shielding means for transformers of the shell-form type.

Description of the prior art In electrical transformed of the shell-form type, the pancake coils are substantially oblong in configuration, having straight outer sides, with contiguous sides being joined by curved corners. Further each coil has an opening which has substantially straight sides in which the contiguous sides are also joined by curved corners. The pancake coils are assembled on a leg portion of a magnetic core structure, and, as is characteristic of the shell-form design, they extend outwardly from opposite sides of the magnetic core structure.

In order to provide the functions of supporting the pancake coils and preventing their movement due to short circuit stresses, clamping the laminations tightly together in the tongue or leg portion of the core which extends through the coil openings, and supporting the pancake coils in the proper assembled relation with the magnetic core structure, metallic coil support beams or members, and wedges are inserted through the openings in the coils at the top and bottom of the stack of core laminations, between the magnetic core and the end portions of the coil openings. Since these coil support members must support the weight of the pancake coils, they are made of metal, and in order to reduce the heating and losses in the support members, the more expensive non-magnetic metals, such as stainless steel, are resorted to, in order to provide the requisite strength without attracting leakage flux from the pancake coils. The leakage flux from the pancake coils is particularly high, however, at the locations of these oil supports, requiring that even non-magnetic coil support members be shielded by bundles of laminations of grain 3,534,311 Patented Oct. 13, 1970 oriented silicon steel. The bundles of laminations provide a low reluctance path for the leakage flux adjacent the metallic coil supports, and thus reduce heating and losses in the coil support members due to eddy currents. US. Pat. 2,370,045 discloses examples of these prior art support and magnetic shielding structures.

While the magnetic shunts or shielding members of the prior art reduce the heating in the coil support members, stray or leakage flux heating is still a problem in the larger transformer ratings, due to the increase in the density of the leakage flux and the increase in the size and thickness of the metallic parts. Leakage flux heating is especially severe at the ends of the openings or windows in the pancake coils, where the coil support members and their shielding means are disposed, and particularly at the curved corners of the coil openings, where the flux density is the highest and where effective shielding by prior art means has not been achieved. The metallic support beam, the edges of the magnetic core, and the edges of the adjacent magnetic shunt, overheat adjacent the curved corners of the coil opening. The support beam overheats because it is not shielded in this location, and the core and shield overheat because the flux does not strike the narrow edges of the core and shield laminations perpendicularly, as required for minimal heating. This overheating has been made evident by discoloration of the edges of the metallic support beams, and other adjacent metallic members, in transformers which have been taken out of service and disassembled. Overheating of metallic parts due to leakage flux is also evident due to the gassing of certain transformer units in service.

SUMMARY OF THE INVENTION Briefly, the present invention solves the problem of openings, such as the metallic support beam, metallic wedges, magnetic shields constructed of laminated steel, and the corners of the magnetic core, by constructing spe cial magnetic shield members which snugly fit the curved corners of the coil openings, and which fill the space between these curved corners and the adjacent metallic support and its shield members, and adjacent magnetic core. These magnetic shield members for the curved corners of the coil openings are cast to shape from an insulating resin system, which is filled with magnetic filler means, such as metallic laminations, mangetic metallic shot, or magnetic metallic powder, or any combination of these materials. When metallic laminations are used, they are aligned with their major surfaces substantially perpendicular to the curved surface of the coil opening, to provide a low reluctance path for the leakage flux. These cast, specially shaped magnetic shunts fill the curved corners of the coil openings with magnetic material and thus provide a low reluctance path for the leakage flux which protects the metallic support members, their magnetic shields, and the magnetic core, from overheating, and they support and uniformly compress the core laminations in areas which are unsupported in prior art arrangements.

BRIEF DESCRIPTION OF THE DRAWINGS Further advantages and uses of the invention will become more apparent when considered in view of the following detailed description and drawings, in which:

FIG. 1 is a fragmentary elevational view, in section, of an electrical transformer of the shell-form type, constructed according to the teachings of the prior art;

FIG. 1A is an enlarged fragmentary view of a portion of the transformer shown in FIG. 1;

FIG. 2 is a fragmentary view which illustrates how the prior art transformer of FIG. 1 would be modified according to a first embodiment of the invention;

FIG. 3 is a fragmentary view which illustrates how the prior art transformer of FIG. 1 would be modified according to another embodiment of the invention;

FIG. 4 is a perspective, partially cut away, of a transformer having certain magnetic shield members constructed according to the teachings of the invention; and

FIG. 5 is a perspective view which illustrates a coil support and magnetic shield member which may be used with the magnetic shield members of the invention.

' DESCRIPTION OF PREFERRED EMBODIMENTS Referring to the drawings, and FIG. 1 in particular, there is shown an elevational view, in section, of electrical inductive apparatus 10, such as a transformer or reactor, which may utilize the teachings of the invention. Specifically, inductive apparatus includes a magnetic core-winding assembly 12 disposed within a tank 14, which may be filled with a suitable insulating and cooling dielectric, such as oil. The electrical bushings for bringing electrical conductors through the wall of the tank 14 for connection to the windings therein, are not shown in order to simplify the drawing.

The magnetic core-winding assembly 12 is of the shellform construction, having a plurality of pancake coils, such as pancake coil 16, disposed in inductive relation with a leg portion 17 of the magnetic core structure 18. Magnetic core structure 18 has first and second similar assemblies or

sections

20 and 22. Each of the

magnetic core assemblies

20 and 22 are formed of a plurality of metallic, magnetic laminations 24, such as grain oriented silicon steel, which are stacked to form a rectangular structure having at least four interconnected stack sections arranged to define at least one window or opening through which the coil turns may pass. The two

assemblies

20 and 22 are disposed in side-by-side relation, with their adjacent stacked sections forming the winding leg portion 17, about which the pancake coils are assembled. Thus, in the customary construction of shell-form power transformers, the pancake coils extend outwardly from the top and bottom surfaces of the magnetic core assembly 18.

The pancake coils, such as pancake coil 16, are each constructed of a plurality of turns 26 of electrical co-nductor, such as copper or aluminum, with the turns being insulated and wound to provide a substantially rectangular configuration having rounded outer corners, and defining an opening 19 through which the winding leg portion 17 of magnetic core structure 18 may pass. The opening 19 in each of the pancake coils is also substantially rectangular in shape, having substantially straight sides joined by

curved corners

28, 30, 32 and 34. The pancake coils are disposed in side-by-side relation, with their openings in alignment, and they are interconnected to form the winding, or windings, of the electrical inductive apparatus or transformer 10. Transformer 10 may be single or polyphase, and it may be of the isolated winding, or of the autotransformer type.

In order to support the pancake coils, such as pancake coil 16, against movement in a direction parallel with the major surfaces of the pancake coils, such as may tend to occur during short circuit conditions due to differences in the locations of the electrical centers of the pancake coils, which produces a force component parallel with the major surfaces, in order to support the pancake coils and insulating barriers, such as insulating barrier 36, and in order to clamp the laminations 24 tightly together in the tongue or leg portion 17 of the magnetic core assembly 18, support means is disposed to extend through the coils, between the coils and leg portion 17, at the upper and lower spaces defined by the openings in the pancake coils and upper and lower surfaces of leg portion 17. For example, as shown in FIG. 1, metallic T-

beams

38 and 40, along with their magnetic shielding members illustrated generally as a plurality of

bundles

42 and 44 are provided at the upper and lower spaces, respectively. The

bundles

42 and 44 are formed of a plurality of stacked metallic laminations which are welded together, and which are then welded to the T-beams with the major surfaces of the laminations being perpendicular to the T-beams.

The metallic T-

beams

38 and 40 extend through the coils and are supported at their extreme ends outside the coils. Wedges, as required, are forced between the T-beams and pancake coils to compress the laminations of the leg portion 17 tightly between the upper and lower coil support members. The pancake coils and insulating barriers are also pressed together in a direction perpendicular to their major surfaces, between end frame structures attached to the tank walls, to hold the pancake coils against movement due to the component of force generated during short circuit conditions which tends to separate the coils in a direction normal to their faces.

The leakage flux from the pancake coils has a high density adjacent the coil support T-

beams

38 and 40, with the flux density being especially high adjacent the rounded corners of the pancake coil openings. In order to il1us trate the problem of shielding structures disposed adjacent the rounded corners of the coil openings, the portion of the transformer 10 shown in FIG. 1 which is within the circle A, is shown enlarged in FIG. 1A. As illustrated in FIG. 1A the leakage flux, illustrated by lines 46, intersects the rounded corner of the coil opening substantially perpendicular to the curved surface. The leakage flux 46 causes overheating of the T-beam 40 at its lateral edges, as it is difiicult to extend the bundles 44 of metallic laminations to the edges of the T-beam due to the curvature of the coil opening. Further, the edges of leg portion 17 and the edges of the bundle 44 of shielding laminations which are adjacent corner 32 overheat, as the leakage fiux in this area does not strike the thin edges of these laminations perpendicular thereto. Therefore, eddy currents are created in the end laminations of the shielding bundles and the end laminations of the leg portion 17, which cause excessive heating and losses in these areas.

The present invention solves the problem of overheating the metal disposed adjacent the curved corners of the coil opening, by providing especially shaped magnetic shunt members which closely conform to the curved openings. The specially shaped magnetic shunt members provide a low reluctance shunt path adjacent the edges of the T-beam, its magnetic shielding members, and magnetic core, to prevent the leakage flux from entering these members and causing heating and losses due to eddy currents.

A first embodiment of the invention is shown in FIG. 2, which is a fragmentary view illustrating how the transformer 10 shown in FIG. 1 would be modified according to this embodiment of the invention. Essentially, the fragmentary view shown in FIG. 2 is the same area encompassed by the circle A of FIG. 1. Like reference numerals in FIGS. 1 and 2 indicate like components. More specifically, FIG. 2 illustrates a special magnetic shielding member 50, which is formed of a cast resinous insulating material 51, such as an epoxy resin system, which is filled with magnetic material 52. The T-beam 40 shown in FIG. 1, is given the reference numeral 40', as it has a width which extends only to the point where the coil opening starts to curve at its corners. The solid magnetic shielding member 50 now provides the support required for the laminations 24 of the magnetic core leg portion 17 which was heretofore provided by the T-beam, and it additionally provides support for the laminations where they are unsupported by prior art structures. By reducing the width of the T-beam 40, it will extend approximately to the same location as the outer edge of shielding bundle 44, which allows the magnetic shielding member 50 to be constructed with a flat surface 53 which is disposed against the T-beam 40 and shielding bundle 44, a flat;

surface 55 which is perpendicular to the first flat surface 53, which is disposed against the bottom lamination of the leg portion 17, and a curved surface 57 which closely conforms to the curved surface 30 of the coil opening ing 19.

The resin system of which the magnetic shielding member 50 is formed may be any suitable resin which will maintain its strength at the elevated temperatures to which it will be subjected during normal usage in the transformer, and it should also be resistant to attack from the cooling fluid uilized in the transformer. In general, the resin should be a thermo-setting resin, with the epoxy resins being excellent because of their excellent physical strength at ambient and elevated temperatures, and their resistance to attack from transformer oils and other coolants utilized in transformers. Since suitable epoxy resin systems are well known in the art, it is not necessary to describe a specific epoxy resin system in detail.

In this embodiment of the invention, the magnetic shielding member 50 is filled with metallic magnetic particles 52, such as metallic shot and/or metallic powders. The metallic shot and powder should, in general, be selected to provide the maximum concentration of filler in the magnetic shunt member 50, in order to provide a low reluctance magnetic circuit. The filler particles may be a uniform size, or they may be graded to provide a predetermined range of sizes.

Suitable magnetic shunt members have been cast of epoxy resin filled with 200 mesh powdered iron, utilizing the ratio of to parts of resin to 80 to 90 parts of the powdered iron, by weight. They were formed in a mold which had sufficient length to allow the resulting cast magnetic shunt to be disposed through the entire length of the window of an electrical phase, and to intercept or butt against magnetic wall shields disposed on tank at either end. The mold shape closely conformed to the configuration of the opening into which the magnetic shunts were to be inserted, providing additional support for the tongue iron. Magnetic shield members constructed in this manner have been very effective in preventing heating of the steel parts separated from the flux source by the magnetic shield members, and the shunts themselves experienced very little heating, which illustrates that they form a low reluctance magnetic shunt circuit.

FIG. 3 is a fragmentary view which illustrates still another embodiment of the invention. FIG. 3 encompasses the same general area as the fragmentary view of transformer 1 shown in FIG. 2, and it illustrates how the transformer of FIG. 1 would be modified according to this embodiment of the invention. In this embodiment, instead of filling the specially formed shunt member of discrete metallic particles, such as metallic shot and/or powdered iron, the efficiency of the shunt is increased by embedding oriented bundles of metallic laminations within the shield member. More specifically, as shown in FIG. 3, a specially formed magnetic shunt member 60 is cast in a special mold, utilizing a resin system 61, in which is embedded a plurality of bundles 62, each formed of stacked laminations 64 which have been prebonded together to form coherent bundles of laminations. Each of the bundles 62 of laminations contain a relatively small number of laminations, in order to orient the laminations of each bundle substantially perpendicular to the curved surface of the corner of the opening in the pancake coils. For example, it has been found that bundles formed of one-half inch wide electrical steel laminations having a thickness of .012 inch, with the bundles having a build dimension of one-half inch, are very effective in providing a low reluctance magnetic shunt 60. The bundles of laminations may be formed by stacking a large plurality of laminations together and painting their edges with an epoxy resin, or other suitable resin system, and allowing the resin system to harden. Bundles having the desired number of laminations may then be cut using a sharp tool, from the bonded laminations, and they may then be placed in a mold which is filled with a resin system. The resin system should also be filled with discrete magnetic particles, such as powdered iron or shot 66, the specially formed cast magnetic shunt member 60, may be utilized with the conventional coil supporting structure shown in FIG. 2, or, as shown in FIG. 3, it may be used with the new coil support and magnetic shunt member 54, which will be hereinafter described, and which is claimed in the hereinbefore mentioned copending application. The cast magnetic shunt member 50 shown in FIG. 2 may also be used with the coil support and magnetic shunt member 54.

FIG. 4 is a perspective view of a transformer 70, which illustrates the placement of the specially formed magnetic shunt members, such as shown in FIGS. 2 and 3, adjacent the rounded or curved edges of the openings in the pancake coils, as well as illustrating a new and improved magnetic shielding structure which may be utilized with the specially formed corner shield members. Specifically, transformer 70 includes a magnetic corewinding assembly 72 disposed within a tank 84, which may be filled with a suitable insulating and cooling fluid, such as oil. The magnetic core-winding assembly 72 includes a plurality of pancake coils 74 disposed in inductive relation with a magnetic core structure 76. Magnetic core structure 76 is formed of first and second magnetic core sections 78 and 80 which are disposed in sideby-side relation to provide a leg portion 82 about which the pancake coils 74 are assembled.

The tank 84 is of the form-fit construction, having a lower portion 83 which closely conforms to the outer configuration of the lower extension of the pancake coils 74 from the magnetic core 76, and which includes a shelf 87 upon which laminations 77 are stacked in superposed relation to form the magnetic core sections 78 and 80. The laminations 77 of the magnetic core sections 78 and 80 are compressed tightly together against shelf 87 by end frame members, such as end frame members 86 and 88, which are welded to the inside of the upper portion 89 of tank 84. The close proximity of the lower tank walls 83 and end frame members 86 and 88 to the pan cake coils 74 provides a low reluctance path for the leakage flux from the coils, making it necessary to magnetically shield these components in order to prevent heating and losses therein.

In the prior art, the tank walls and end frame members are shielded with bundles of metallic laminations which are attached to the walls and end frame members, and which form magnetic circuits by butting the ends of the bundles of laminations together. However, the tolerances of the bundles, and the tolerances of the end frame members and tank make it difficult to obtain good butt joints between the ends of the bundles of laminations, with substantial gaps thus being introduced into the magnetic circuit. These gaps increase the reluctance of the circuits and reduce the efiiciency of the magnetic shunts.

In addition to shielding the tank walls and end frame members from the leakage magnetic flux from the plurality of pancake coils, in the prior art the coil support members are magnetically shielded, as hereinbefore described relative to FIG. 1. The bundles of metallic laminations are assembled by Welding across the edges of the laminations, and then the bundles of laminations themselves are welded to the metallic T-beam or support members. The welds across the edges of the magnetic steel laminations adversely affect their magnetic properties, which increases the reluctance of the magnetic circuits, and it would thus be desirable to be able to assemble the laminations into bundles and fix the location of the bundles, without resorting to welding. FIG. 4 illustrates new and improved magnetic shielding assemblies which perform the functions of supporting the pancake coils, and of providing low reluctance magnetic circuits between the leg portion of the magnetic coil structure and the pancake coils, and between the pancake coils and the tank and end frame members of the transformer.

More specifically, the new and improved magnetic shielding structures include lower and upper magnetic shielding assemblies 90 and 92, respectively. The lower magnetic shielding assembly 90 includes a plurality of bundles 94 of laminations 95, which extend in an insulating manner through the pancake coil openings, between the lower surface of the leg portion 82 and the lower end of the openings in the pancake coils 74. The separate metallic support member or T-beam is eliminated by constructing the bundles 94 to have the strength necessary to support the pancake coils 74. The bundles 94 of metallic, magnetic laminations are placed side-by-side across the bottom surface of the pancake coil openings, on the fiat portion of the coil opening, with the major planes of the laminations being perpendicular to the adjacent edges of the coil openings. The rounded or curved edges of the coil opening may be filled with specially formed magnetic shunt members 97 and 99, which aid in supporting the laminations of the leg portion of the magnetic core, as well as providing a low reluctance path for the leakage flux which is perpendicular to the curved edges of the coil openings. The specially formed magnetic shunt members 97 and 99 are formed of a cast resin system filled with magnetic metallic material, as hereinbefore described.

The bundles 94 of laminations 95, as well as the specially prepared magnetic shunt members 97 and 99, have their ends at one side of the coils 74 disposed adjacent the ends of a plurality of bundles 96 of laminations 101, which bundles are disposed perpendicular to the bundles 94, and which extend towards the bottom 85 of the tank 84. The major planes of the laminations 101 are perpendicular t the insulating barrier 103 disposed at the end of the stack of coils 74. The other ends of the bun dles 94, and the other ends of the special magnetic shunts 97 and 99 are disposed adjacent the ends of similarly disposed bundles of laminations (not shown) at the opposite side of the coils 74.

In order to connect the ends of

bundles

94 and 96 with a low reluctance joint, the laminations of

bundles

94 and 96 may be staggered to provide tongue-andgroove joints, which have a very low reluctance, even when the tongue-and-groove joints are not completely closed. Thus, a liberal tolerance may be utilized while constructing the bundles of laminations and the noncritical portions of the transformer, without substantially affecting the losses of the magnetic circuit.

The tank bottom 85 is shielded from the leakage flux from pancake coils 74 by a plurality of bundles 98 of laminations 105 which extend between the lower extremity of the pancake coils 74 and the tank bottom 85, and which are magnetically linked with the perpendicularly disposed bundles at each end of the stacked coil assembly, such as the bundles 96. The bundles 98 may be joined to the perpendicularly disposed bundles with tongue-and-groove joints, similar to the connection of the bundles 94 to the perpendicularly disposed bundles at each end thereof. Thus, there is a complete magnetic circuit or loop which encircles the lower portion of the pancake coils 74, which extends through the coil opening to shield the tongue iron from the leakage flux, it extends along the walls of the lower tank portion 83 to shield the adjacent walls from the leakage flux from the pancake coils, and it also extends across the bottom of the tank 84, between the bottom 85 and the pancake coils 74.

The lower shielding structure '90 also includes a second complete magnetic loop or circuit which encircles the lower portion of the pancake coils 74 perpendicular to the axis of the first magnetic loop or circuit. This magnetic loop protects the remaining side wall portions of the lower tank portion 83 from leakage flux from the bottom extension of the pancake coils 74 below the mag- 8 netic core structure 76. More specifically, the second magnetic loop portion of the lower magnetic shielding assembly includes a plurality of bundles 102 of laminations 107 which have their adjacent ends insulatingly butted against the outer lamination of the adjacent outer bundle of the bundles 96 of laminations. In like manner a pluarlity of bundles 106 are disposed perpendicularly to the bundles 96, with the adjacent ends of the bundles 106 being insulatingly butted against the outer lamination of the bundle at the other outer side of the bundles 96. The bundles are magnetically joined to a plurality of bundles104, such as by a tongue-and-groove joint, which bundles extend along a lower vertical transformer wall, parallel with the tank bottom 85, between the pancake coils and the adjacent transformer wall, to where their other ends are joined with a plurality of bundles disposed similar to the bundles 102. These bundles then have their ends butted against the bundles at the other end of the transformer which are similar in location to the bundles 96. In other words, bundles 102 and 104 are part of a substantially U-shaped structure 121 which is fitted about one end of the lower extension of the pancake coils 74 below the magnetic core 76, with the legs of the U- shaped structure 121 being butted against the vertically disposed bundles of laminations at opposite ends of the coils 74.

The bundles 106 are magnetically linked with a plurality of bundles 108, such as with tongue-and-groove joints, which extend along the adjacent lower vertical transformer wall to protect this wall from the leakage flux from the pancake coils 74. The bundles 108 join a plurality of bundles disposed similar to bundles 106,'except at the other end of the coils, such as with tongue-andgroove joints, which bundles extend perpendicularly against the bundles at the other end of the transformer which are disposed similar to bundles 96. Thus, bundles 106 and 108 are part of another U-shaped structure 123, which, along with the other U-shaped structure 121, and the vertically disposed bundles, from a complete magnetic circuit about the sides and ends of the lower extension of the pancake coils 74, which protect the bottom portion 83 of the transformer tank 84 from overheating due to stray leakage flux.

The upper shielding assembly 92 is similar to the lower shielding assembly 90, except the magnetic circuit which extends through the coil windows is not completed across the top of the coils, since it may interfere with leads and accessories disposed above the pancake coils. Further, it is unnecessary to shield the top of the pancake coils, as the top of the tank 84 is not immediately adjacent the top of the pancake coils. More specifically, the upper shielding assembly 92 includes a plurality of bundles shown generally at 112 which extend through the coil openings, between the upper ends of the openings in the pancake coils 74 and the upper surface of the magnetic core structure 76, with the adjacent ends of the bundles 112 at one end of the stacked coil assembly being joined to the ends of bundles which are disposed perpendicularly upward from the bundles 112, and with their remaining adjacent ends being joined to similarly disposed bundles at the opposite end of the stacked coils, to shield the end frames at opposite ends of the transformer. Two substantially U-shaped structures and 127, similar to the U-shaped structures 121 and 123, are disposed about opposite sides of the upper extensions of the pancake coils 74, with the ends of the

U-shaped structures

125 and 127 butting against the perpendicularly disposed bundles of laminations at opposite ends of the transformer, such as the bundles 110. The first U-shaped structure 125 includes a plurality of bundles 114 which are butted against the perpendicularly disposed bundles 110, a plurality of similarly disposed bundles at the opposite side of the transformer (not shown), and a plurality of bundles 137 which connect the ends of these bundles, such as with tongue-and-groove joints.

The second U-shaped structure 127 includes a plurality of bundles 11'5 disposed with their ends perpendicular to the plurality of bundles 110, a plurality of similarly disposed bundles at the other end of the transformer (not shown), and a plurality of bundles 116 which connect the ends of these bundles, such as with tongue-andgroove joints. Therefore, the upper shielding structure 92 provides a low reluctance path for the leakage flux within the coil openings, and also a low reluctance path for the leakage flux which protects the end frame members from the leakage flux of the pancake coils.

FIG. is a perspective view of

bundles

120 and 122 of laminations 125, which are constructed to enable the bundles to achieve the structural strength required to eliminate the T-beam or coil supports disposed in the upper and lower coil openings, between the coil openings and the tongue or leg portion of the magnetic core structure 76. As illustrated in FIG. 5, the

bundles

120 and 122 are formed such that they may be joined with tongue and groove joints, with either in-line or angular connections between the two bundles. Bundle 120' includes a plurality of

laminations

132, 128, and 124 of similar length, separated by a plurality of laminations 126 and 130 of a shorter length, in order to provide the tongueand-groove ends which will mate with the tongue-andgroove ends fo bundle 122. In this embodiment the loose laminations are wrapped tightly with a woven fabric tape 131, such as a cotton or glass tape, and the ends of the tape are temporarily secured while the entire tape area is coated and impregnated with a low viscosity resin 133, which is then cured. The low viscosity resin 133, which is preferably a thermosetting resin, penetrates the woven fibers of the fabric tape 131, and penetrates the stacked laminations for a short distance, which, upon curing, bonds the laminations into a coherent bundle having an integrity and strength greater than the bundles of the prior art which are welded across their edges to obtain the desired assembled relation. Instead of impregnating the tape and laminations after assembly, the material of which the laminations are formed, and the tape, may be pre-treated with resin, which is dried but not cured. Then, after the laminations are cut, stacked, and taped, the assembly is heated to flow the resin and cure it to a solid.

The adhesive or resin coating 133 may be any suitable resin which will maintain its strength at the elevated temperatures to which it will be subjected during normal usage of the transformer, and it should also be resistant to attack from oils and other dielectric fluids used within the transformer. Epoxy resins have been found to be excellent, because of their strength at elevated temperatures, and their resistance to attack from transformer oils and other chemicals. In addition to forming a solid high strength bundle of laminations, the impregnated and cured resin-tape structure provides the insulation required between the shield members and adjacent portions of the transformer.

While the bundles of laminations for shielding the end frames and tank of the transformer have been illustrated as being assembled and mounted on the pancake coils 74, it will also be evident that these magnetic shielding structures may be connected to the end frames and tank walls by suitable clips which are sized to hold the bundles, and which are welded to the end frames and tank walls.

In summary, there has been disclosed new and improved magnetic shielding members which are disposed at the curved corners of the coil openings to protect the coil support members, the coil support shielding means, and the adjacent leg portion of the magnetic core, from overheating due to the leakage flux entering the curved corners of the coil opening. The new and improved magnetic shunt members are formed to the desired shape from a cast resinous insulation system, which is filled to a high degree with magnetic material, such as laminations, shot, and/or powdered iron. In addition to effectively shielding an area which is unshielded in prior art structures, the new and improved shielding members provide support for the laminations of the leg portion of the magnetic core, where the laminations have been un supported in prior art structures.

Since numerous changes may be made in the above described 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.

I claim as my invention:

1. An electrical transformer comprising:

a magnetic core having a leg portion,

a plurality of electrical coils, each of said electrical coils having an opening defined by substantially straight sides joined by curved corners, said plurality of coils being positioned on said leg portion,

a plurality of magnetic shield members extending within said coils, between said coils and said leg portion, said magnetic shield members being spaced from one another, with each being adjacent a curved corner of the coil openings, said magnetic shield members being formed of a cast resinous insulating material filled with magnetic filler means, said magnetic shield members being shaped to closely conform to the adjacent leg portion and the curvature of the adjacent curved corner of the coil openings,

and support means extending within said coils, between said coils and said leg portion.

2. The electrical transformer of claim 1 wherein the opening in each coil has four sides and four curved corners, with a magnetic shield member being disposed adjacent each of the curved corners.

3. The electrical transformer of claim 2 wherein the support means includes first and second structures disposed between the leg portion and the coils, adjacent first and second predetermined opposite sides of the coil openings, respectively, and between the magnetic shield members disposed adjacent the curved corners contiguous to these predetermined opposite sides of the coil openings.

4. The electrical transformer of claim 1 wherein the support means includes a metallic body portion disposed adjacent the leg portion, and magnetic shielding means disposed between the metallic body portion and electrical coils.

5. The electrical transformer of claim 1 wherein the support means are bundles of metallic laminations, assembled and insulated to provide the support required for the coils, while also functioning as additional magnetic shields.

'6. The electrical transformer of claim 1 wherein the magnetic filler means of the cast resinous magnetic shield members includes finely divided iron particles.

7. The electrical transformer of claim 1 wherein the magnetic filler means of the cast resinous magnetic shield members includes a plurality of stacked metallic laminations arranged with their major planes substantially perpendicular to the curvature of the comers of the coil openings.

8. The electrical transformer of claim 7 wherein the magnetic filler means also includes finely divided iron particles.

References Cited UNITED STATES PATENTS 1,610,867 12/1926 Lennox 336-212 1,763,150 6/1930 Hebnew 336-219 2,370,045 2/1945 Keto 336-84 3,281,745 10/1966 Moore et al. 336-84 3,464,041 8/ 196 9 Waterman 336-84 XR THOMAS J. KOZMA, Primary Examiner US. Cl. X.R. 336-212, 215, 216