US3303448A

US3303448A - Joint structure for three-legged laminated core

Info

Publication number: US3303448A
Application number: US285711A
Authority: US
Inventors: Otis T Farry
Original assignee: Wagner Electric Corp
Current assignee: McGraw Edison Co
Priority date: 1963-06-05
Filing date: 1963-06-05
Publication date: 1967-02-07
Anticipated expiration: 1984-02-07

Description

Feb. 7, 1967 o. T. FARRY 3,303,448

JOINT STRUCTURE FOR THREE-LEGGED LAMINATED CORE Filed June 5, 1963 FIG- 3 INVENTOR OTIS T. FARRY United States Patent Other:

3,303,448 JOKNT STRUCTURE FGR THREE-LEGGED LAMINATED CORE Otis T. Parry, University City, Mo, assignor to Wagner Electric Corporation, St. Louis, Mo., a corporation of Delaware Fiied June 5, 1963, Ser. No. 285,711 4 Claims. (Cl. 336-212) This invention relates to magnetic core structures and more particularly to magnetic core structures of the flat stacked lamination type for stationary electrical induction apparatus.

Magnetic cores of the flat stacked type are usually formed by stacking a plurality of layers of assembled laminations cut or punched lengthwise from grain oriented magnetic strip material having a preferred magnetic direction lengthwise of the strip material. Such cores have been provided with mitered or bias joints between adjacent ends of adjoining laminations in each layer and with each such joint bridged or overlapped by a lamination in an adjacent layer. This construction reduces the core losses since the magnetic flux runs parallel to the preferred grain orientation of the strip material in substantially all parts of the core and prevents the possibility of high reluctance gaps being formed at the joints because each joint in each layer is overlapped by a lamination in an adjacent layer.

There were, however, certain undesirable features in the manufacture of cores of the above tpye having three or more legs. For example, in a three-legged, three-phase core, the middle leg laminations were usually provided at each end with an end edge which had two converging end edge portions extending at oblique angles to the longitudinal axis of the lamination and which joined each other at a point away from either side of the lamination. Usually, each adjoining yoke lamination at each end of the middle leg lamination had an end edge configuration complementary to an end edge portion of the middle leg so as to form mitered joints with the middle leg lamination. Since the end edges of these laminations in each layer were not complementary in configuration to the end edges of any of the other laminations of the layer, they could not be cut or punched from strip material without waste. It was necessary to use a special punching or cutting operation in forming these laminations; for example, in some cases, after each of the abovementioned laminations were partiaL ly formed, an end portion had to be clipped off to provide the second converging edge portion. Thus, not only was there considerable waste of magnetic material in producing the finished core structure, but special punches or cutting steps and additional handling operations were required, and these, of course, considerably added to the manufacturing costs.

It is an object of the present invention to provide a magnetic core structure of the flat stacked type having at least three leg members and a plurality of yoke members wherein the abovementioned undesirable features are obviated or over-come to a large degree.

Another object is to provide a magnetic core structure of the flat stacked type having at least three leg members and a plurality of yoke members which is efiicient in operation and especially economical to manufacture.

Another object is to provide a novel fiat stacked type of magnetic core structure having two or more core windows wherein a minimum number of differently shaped laminations is required.

Another object is to provide a novel three-phase magnetic core structure of the flat stacked type wherein all of the laminations of the structure can be cut or punched from a single strip of magnetic material without waste.

3,303,448 Patented Feb. 7, 1967 Another object is to provide a magnetic core structure of the flat stacked type having at least three parallel leg members which can be manufactured without any end clipping of laminations and therefore without waste pieces of magnetic material, otherwise attendant with such end clipping.

Still another object is to provide in a magnetic core of the fiat stacked type a novel T-joint or connection between a leg and a pair of yoke portions extending perpendicularly in opposite direction from the leg portion.

Other objects and advantages of the present invention will be apparent from the following detailed description and accompanying drawing.

Briefly, in accordance with one aspect of the present invention a magnetic core structure is provided wherein each layer of the structure includes at least three parallel leg laminations with the intermediate leg lamination forming with one yoke lamination a straight line butt joint perpendicular to the lengthwise dimension of the one yoke lamination, and with another yoke lamination, a joint having at least a major portion thereof at an oblique angle to the lengthwise dimension of the leg lamination, and with the joints in one layer overlapped by laminations in an adjacent layer.

In the drawing:

FIG. 1 is an exploded perspective view of a magnetic core structure in accordance with the present invention.

FIG. 2 is a fragmentary elevational view of one end of the core structure of FIG. 1, and

FIG. 3 is a plan view of magnetic strip material illustrating a manner in which the laminations of the core structure of FIG. 1 may be cut from a single strip of magnetic material without waste.

Referring now to FIG. 1, there is illustrated a threephase laminated magnetic core 10 which can be used in a suitable electric induction apparatus, such as a threephase transformer. Magnetic core 10 includes a plurality of superposed rectangular layers 12 of assembled laminations, the laminations being formed lengthwise from grain oriented magnetic strip material having a preferred magnetic direction lengthwise of the strip. Each layer includes three

parallel leg laminations

14, 15, and 16, and four

yoke laminations

18, 19, 20 and 21 assembled together to form two rectangular Windows. The layers are identical to each other but alternate layers of the core structure are laterally reversed or rotated about their longitudinal axes to provide overlapped joints, which will be further discussed hereinafter. Alternate layers, which are laterally reversed relative to the other layers, are indicated at 12a in FIG. 1.

In each layer, the laminations are so cut or punched that the butt joints between adjacent ends of adjacent leg and yoke laminations, except the joints between the intermediate or middle leg lamination 15 and yoke laminations l9 and 20, are generally mitered broken-line joints having a major portion at an oblique angle, preferably at 45 as shown, to the lengthwise dimension of the meeting laminations, and a minor portion perpendicular to the lengthwise dimension or longitudinal axis of one of the meeting laminations. The major portion of each butt joint at each of the outer corners of the layers runs parallel to a diagonal extending between the inside corner and the outside corner. The joints between the outer leg lamination 14 and the

yoke laminations

18 and 21 are offset on the leg side of their corner diagonals while the butt joints between the outer leg lamination 16 and

yoke laminations

19 and 20 are offset on the yoke side of their corner diagonals. By laterally reversing alternate layers, each of the outer corner butt joints in one layer is overlapped by a lamination in an adjacent layer in the core structure. Each of these corner butt joints thus includes a major joint or joint portion extending at an oblique angle to the lengthwise dimension of the meeting laminations to provide in adjacent layers generally mitered butt joints parallel to and offset equal distances on opposite sides of a corner diagonal.

The abovementioned joints at the corners of the core are provided by cutting the adjoining laminations which form the corners such that each has an end edge surface formed by a minor portion perpendicular to the length wise dimension of one of the meeting laminations and a major portion which is inclined or at an oblique angle to the lengthwise direction of the meeting laminations. Thus, in each layer, leg lamination 14 has a notch or minor end edge portion 24 cooperating with a straight or longitudinally extending side portion 25 of yoke lamination 18, and a major end edge portion 26 cooperating with a major end edge portion 27 of lamination 18 to form the butt joint between these meeting laminations. The

edge portions

26 and 27 of

laminations

14 and 18 are at an oblique angle to the lengthwise direction or longitudinal axis of their respective laminations. The lamination '18 has at one end a minor edge portion 23 perpendicular to the lengthwise direction of the lamination which forms with the outer side thereof an outside corner 29. The cooperating

edge portions

26 and 27 form a major portion of the butt joint which runs parallel to a diagonal between the outside corner 29 and an inside corner 30 which is offset on the leg side of the diagonal. Similarly, yoke lamination 21, which is identical to lamination 18, has a straight or longitudinally extending side edge portion 32 which cooperates with a notch or perpendicular minor edge portion 33 of the lamination 14, and a major oblique end edge portion 34 cooperating with a major oblique end edge portion 35 of lamination 14. Lamination 21 has a minor perpendicular edge portion 36 forming with the outer side of lamination 21 a corner 37 of the layer. The major portion of the joint formed by adjoining

oblique edge portions

34 and 35 runs parallel to a diagonal between the outside corner 37 and an inside corner 38 and is offset on the leg side thereof.

For convenience, an end edge surface of a lamination or a butt joint which is perpendicular to the lengthwise dimension or longitudinal axis of a lamination will be referred to herein-after merely as a perpendicular edge portion or perpendicular butt joint, and an edge portion of a lamination or butt joint which is at an oblique angle to the lengthwise dimension will be referred to hereinafter as an oblique edge portion or oblique butt joint. 7

The lamination 16 has a longitudinally extending side edge portion 40 cooperating with a notch or minor perpendicular edge portion 41 of yoke lamination 19, and a major oblique end edge portion 42 cooperating with a major oblique end edge portion 43 of lamination 19. Similarly, at the other end of the layer, an extending side edge portion 44 of lamination 16 cooperates with a notch or minor perpendicular edge portion 4-5 of lamination 2t), and a major oblique edge portion 46 of lamination 16 cooperates with a major oblique edge portion 47 of lamination 20. The lamination 16 has minor

perpendicular edge portions

48 and 49 at the opposite ends thereof which form with the outer side,

outside corners

50 and 51, respectively. The broken-line joint formed between leg lamination 16 and yoke lamination 19 has a major portion offset on the yoke side and parallel with a diagonal between an inside corner 52 and the outside corner 50. Similarly, lamination 16 and yoke lamination 20 form a broken-line joint having a major portion offset on the yoke side and parallel with a diagonal between an inside corner 53 and the outside corner 51. With this construction, the major portion of the butt joints in

adjacent layers

12 and 12a at each outer corner of core structure are parallel to and offset on opposite sides from a diagonal running between the inside and outside corners, as shown in FIG. 2. In this way, the flux runs parallel to the preferred magnetic direction around the corners of the core.

The middle leg laminations 15, and the yoke laminations form T-joints at opposite ends of the core structure. The middle leg lamination 15, at one end, has a notch or minor perpendicularly extending end edge portion 54 cooperating With a longitudinally extending side edge portion 55 of yoke lamination 18, and an oblique major edge portion 56 cooperating with an oblique major edge portion 57 to form a broken-line joint having a major portion at an oblique angle to the lengthwise dimension of these laminations which extend from the inside corner at 58 to the extremity or end tip 59 of the lamination 15. Similarly, at the opposite end of the layer, the laminat on 15 is provided with a perpendicular minor edge portion 60 cooperating with a longitudinally extending side por tion 61 of yoke lamination 21, and an oblique major end edge portion 62 cooperating with an oblique major portion as of lamination 21 to form a broken-line joint having a major portion at an oblique angle to the lengthwise dimension of these laminations. The joint between

lann nations

15 and 21 extends between the inside corner 64 and the opposite extremity or end tip 65 of lamination 15.

Yoke laminations

19 and 20 are identical to each other and form straight-line butt joints with the middle leg lamination 15 that are perpendicular to the lengthwise dimension of these yoke laminations. As seen in the drawing,

laminations

19 and 20 have straight end edge portions 68, 69, respectively, which are perpendicular to their lengthwise dimensions and which extend across the full width thereof. The

end edge portions

68 and 59 cooperate respectively with

side edge portions

70 and 71 of leg lamination 15 to form straight-line perpendicular joints at opposite ends of the layer. Edge 68 cooperates with edge portion 70 to form a joint extending between the inside corner at 72 and the end tip 59 of lamination 15, the joint being perpendicular to lamination 19. At the other end of the layer, end edge portion 69 cooperates with side edge portion 71 to form a joint extending between the inside corner at 73 and the end tip 65 of lamination 15 which is perpendicular to yoke lamination 20.

The end edge portions of

yoke laminations

19 and 20 also form small straight-1ine butt joints with

yoke laminations

18 and 21, respectively. A minor end edge portion 68a of end edge 68 cooperates with a minor perpendicular edge portion 74 of lamination 18 to form a joint perpendicular to yoke

laminations

18 and 19 which runs from the tip 59 of leg lamination 15 to the outer side of the layer or outer side edges of these yoke laminations. In like manner, a minor end edge portion 69a of end edge 69 of lamination 20 cooperates with a minor perpendicular edge portion 75 of yoke lamination 21 to form a joint perpendicular to the lengthwise direction of each of the

yoke laminations

20 and 21, and which runs from the other tip 65 of leg lamination 15 to the outer side edges of these yoke laminations.

As more readily seen in FIG. 2, the perpendicular joints between

laminations

15 and 19 in adjacent layers of the core are equally spaced on opposite sides of the longitudinal axis of the leg laminations 15, and the mitered or oblique joints between

laminations

15 and 18 in ad jacent layers are normal to each other and cross at the longitudinal axis of the leg laminations 15. Each joint between the leg lamination 15 and adjoining yoke laminations at each end of the layer is overlapped by a lamination in an adjacent layer. For example, yoke lamination 18 in each layer 12 overlaps the perpendicular butt joints between yoke lamination 19 and the leg lamination 15, and the minor perpendicular butt joint between the two

yoke laminations

18 and 19 in an adjacent layer 12a. Lamination 18 in each layer 12 also overlaps a portion of the oblique butt joint between lamination 18 and the lamination 15 in an adjacent layer 12a. Also, the middle leg lamination 15 in each layer 12 overlaps the other portion of the oblique butt joint between the lamination 18 and the lamination 15 in an adjacent layer 12a. This. construction provides a T-joint construction at each end of the core structure having overlapped joints wherein one of the joints, in. each layer is a mitered or oblique joint formed between the middle leg lamination and a yoke lamination. Since there is a mitered joint or a joint having at least a major portion thereof at an oblique angle to the longitudinal axes of the meeting laminations in each layer between a middle leg lamination and a yoke lamination and these joints are offset from one another in adjacent layers at each end of the structure, the core structure is provided with an efficient T-joint or connection between the yoke and middle leg portions of the structure.

Because each end edge of the middle leg lamination is complementary to the ends of other laminations of the layer, it can be formed from the same strip of magnetic material used in forming the other laminations without any waste resulting therefrom. Also, since yoke laminations 18 and 21 form joints with the end edges of the middle leg lamination in each layer and have end edges complementary to other laminations, they too can be cut or punched from the same strip of material without waste. Thus, it will be apparent that in each layer, each end of each lamination is complementary in configuration to an end of at least one other lamination and all of the laminations are of equal width so that all of the laminations in each layer of the core structure can be formed by cutting or punching laminations from a single strip of magnetic material without any waste, as is indicated in FIG. 3. In FIG. 3, the right-hand strip section represents a continuation of the left-hand strip section, the upper end of lamination 15 being formed simultaneously with the lower end of lamination 18. It will be apparent that there is no end clipping of laminations or waste pieces between adjacent laminations in the strip of material.

It will be noted from the illustrated embodiment that the middle leg lamination 15 is identical to the outer leg lamination 14, the

yoke laminations

18 and 21 are identical to each other and that laminations 19 and 2d are identical to each other. The yoke laminations 18 and 21 are also identical with the outer lag lamination 16 except in length. Also, one end of each of the yoke laminations 1 and is identical in configuration with an end configuration of

leg laminations

14 and 15. Since layers 12 are identical with layers 12a, except that layers 12a are laterally reversed, there are a minimum number of different kinds of laminations in the core structure and this simplifies the assembly of the core structure, and reduces the number of difierent kinds of punches and dies required to produce the laminations, as well as reducing the handling or labor involved in forming and assembling laminations.

Each end of each lamination in each layer, except one end of each of the

laminations

19 and 20, has a minor perpendicular edge portion in a plane perpendicular to one side thereof and a major oblique edge portion in a plane at an angle of 45 to a side or the longitudinal axis of the lamination, and the opposite ends of these laminations are normal to each other. Since all the laminations in the illustrated embodiment have like minor perpendicular and major oblique end edge portions and each lamination is symmetrically shaped since one end is a mirror image of the other, except the

laminations

19 and 20, the laminations may be first punched by employing two like punches with one reversed or by employing a single punch if the single punch or the strip material is reversed for alternate punching operations. The

laminations

19 and 20 may be first formed as an integral piece by the abovementioned punching operations and then subsequently punched or cut in half along a straight line perpendicular to its lengthwise dimension, if desired.

Since the

yoke laminations

19 and 20 have perpendicular end edge portions 68 and 69, respectively, which form butt joints with

side edge portions

70 and 71, respectively, there is no end clipping of these laminations and therefore no waste pieces in forming them, as was usually the case in the manufacture of other known cores containing generally mitered butt joints between the midb dle leg and yoke portions of a core structure having three or more legs.

From the foregoing, it is now apparent that a novel magnetic core structure meeting the objects set out hereinbefore is provided. It is to be understood that changes and modifications to the form of the invention set forth in the disclosure by way of illustration may be made by those skilled in the art without departing from the true spirit of the invention as defined in the claims which follow.

What I claim is:

1. A three-phase magnetic core structure having four outer sides comprising a plurality of stacked layers edgewise coincident in all four outer sides of the core structure, each of said layers including at least one sheet of assembled laminations, each sheet in each layer being identical and consisting of seven laminations comprising three parallel leg laminations including two outer leg laminations and a middle leg lamination, and four yoke laminations interconnecting said leg laminations to form two rectangular windows with joints between adjoining laminations, one of said outer leg laminations being identical in shape to said middle leg lamination and different in shape from the other of said outer leg laminations, two of said yoke laminations adjoining said one outer leg lamination being identical in shape to each other, the other two of said yoke laminations adjoining said other outer leg lamination being identical to each other and diiferent in shape from the first named two yoke laminations whereby there is a total of four differently shaped laminations in said core structure, the joints in each of said sheets between each of said outer leg laminations and said adjoining yoke laminations and between the middle leg lamination and the first named two of said yoke laminations each including a minor portion perpendicular and a major portion at an angle of 45 degrees to the longitudinal axis of one of the adjoining laminations forming the joint, the joints between the middle leg lamination and said other two of said yoke laminations being straight line joints parallel to the longitudinal axis of the middle leg lamination, said first named two yoke laminations also forming straight line joints with said other two yoke laminations, respectively, alternate layers of said core structure being reversed with respect to the other layers with each of the joints in each layer being bridged by a lamination in an adjacent layer.

2. A three-legged magnetic core structure having four outer sides comprising a plurality of stacked layers edgewise coincident in all four outer sides of the core structure, each of said layers including at least one sheet of assembled laminations formed lengthwise from magnetic strip material having a preferred magnetic direction lengthwise of the strip material, said sheet in each of said layers comprising three parallel leg laminations including two outer leg laminations and a middle leg lamination between said outer leg laminations; and four yoke laminations, two at each end of the sheet connecting the ends of said leg laminations together to form two rectangular windows with joints between adjoining laminations; each of the joints between said outer leg and yoke laminations at the corners of said core structure having a minor portion perpendicular to the longitudinal axis of one of the adjoining laminations and a major portion extending at an angle of 45 to the longitudinal axis of one of the adjoining laminations; the joints between said middle leg lamination and the two yoke laminations at each end of said sheet including a straight line joint extending parallel to the longitudinal axis of the middle leg lamination from a corner of one of said windows to the lengthwise extremity of said middle leg lamination, and a joint having a minor portion perpendicular and a major portion at an angle of 45 to the longitudinal axis of the middle leg lamination and extending from an inside corner of the other of said windows to said extremity of said middle leg lamination; the two yoke laminations at each end of said sheet adjoining each other to form a straight line joint therebetween extending parallel to the longitudinal axis of said middle leg lamination substantially from said extremity of said middle leg lamination to the adjacent outer end of said sheet; one of said outer leg laminations being identical in shape to said middle leg lamination and different in shape from the other outer leg lamination in said sheet; two of said yoke laminations in said sheet being identical to each other; the other two of said yoke laminations in said sheet being identical to each other; and all of the laminations in each of said layers being of equal width; alternate ones of said layers of said core structure being reversed relative to the other of said layers, each of said joints in each of said layers being bridged by a lamination in an adjacent layer.

3. A three-legged magnetic core structure having four outer sides comprising a plurality of stacked layers edgewise coincident in all four sides of the core structure, each of said layers including at least one sheet of assembled laminations formed lengthwise from magnetic strip material having a preferred magnetic direction lengthwise of the strip material, said sheet in each layer comprising three parallel leg laminations including two outer leg laminations and a middle leg lamination between said outer leg laminations; and four yoke laminations, two at each end of the sheet connecting the ends of said leg laminations together to form two rectangular windows with joints between adjoining laminations; all of said laminations being of equal width, the joints between said outer leg and yoke laminations at the corners of said core structure having a minor portion perpendicular to one of the adjoining laminations and a major portion extending at an oblique angle to one of the adjoining laminations; each of the joints between one of said outer leg laminations and the yoke laminations adjoining said one outer leg lamination being offset on the leg side from an imaginary straight line diagonal running from the outer to the inner core structure corner adjacent the joint, each of the joints between the other of said outer leg laminations and the yoke laminations adjoining said other outer leg lamination being oifset on the yoke side from an imaginary straight line diagonal running from the outer to the inner core structure corner adjacent the joint, the joints between said middle leg lamination and the two yoke laminations at each end of each sheet including a straight line joint extending parallel to the longitudinal axis of the middle leg lamination from a corner of one of said windows to the lengthwise extremity of said middle leg lamination, and a joint having a minor portion perpendicular and a major portion at an oblique angle to the longitudinal axis of the middle leg lamination and extending from an inside corner of the other of said windows to said extremity of said middle leg lamination; the two yoke laminations at each end of each sheet adjoining each other to form a straight line joint therebetween extending parallel to the longitudinal axis of said middle leg lamination substantially from said extremity of said middle leg lamination to the adjacent outer end of said sheet; in each said sheet said one outer leg lamination being identical in configuration to said middle leg lamination and different in configuration from said other leg lamination, said yoke laminations adjoining said one outer leg lamination being identical in configuration, and said yoke laminations adjoining said other outer leg lamination being identical in configuration; alternate ones of said layers of said core structure being laterally reversed relative to the other of said layers, each of said joints in each of said layers being bridged by a lamination.

in an adjacent layer.

4. The magnetic core structure according to claim 3 wherein said oblique angle is an angle of References Cited by the Examiner UNITED STATES PATENTS 2,407,625 9/1946 Brand 29-15561 X 3,212,042 10/1965 Twomey 336217 3,214,718 10/1965 Graham 336-217 FOREIGN PATENTS 561,545 4/1957 Italy. 975,473 12/ 1961 Germany.

LEWIS H. MYERS, Primary Examiner.

L. E. ASKIN, D. J. BADER, T. I. KOZMA,

Assistant Examiners.

Claims

1. A THREE-PHASE MAGNETIC CORE STRUCTURE HAVING FOUR OUTER SIDES COMPRISING A PLURALITY OF STACKED LAYERS EDGEWISE COINCIDENT IN ALL FOUR OUTER SIDES OF THE CORE STRUCTURE, EACH OF SAID LAYERS INCLUDING AT LEAST ONE SHEET OF ASSEMBLED LAMINATIONS, EACH SHEET IN EACH LAYER BEING IDENTICAL AND CONSISTING OF SEVEN LAMINATIONS COMPRISING THREE PARALLEL LEG LAMINATIONS INCLUDING TWO OUTER LEG LAMINATIONS AND A MIDDLE LEG LAMINATION, AND FOUR YOKE LAMINATIONS INTERCONNECTING SAID LEG LAMINATIONS TO FORM TWO RECTANGULAR WINDOWS WITH JOINTS BETWEEN ADJOINING LAMINATIONS, ONE OF SAID OUTER LEG LAMINATIONS BEING IDENTICAL IN SHAPE TO SAID MIDDLE LEG LAMINATION AND DIFFERENT IN SHAPE FROM THE OTHER OF SAID OUTER LEG LAMINATIONS, TWO OF SAID YOKE LAMINATIONS ADJOINING SAID ONE OUTER LEG LAMINATION BEING IDENTICAL IN SHAPE TO EACH OTHER, THE OTHER TWO OF SAID YOKE LAMINATIONS ADJOINING SAID OTHER OUTER LEG LAMINATION BEING IDENTICAL TO EACH OTHER AND DIFFERENT IN SHAPE FROM THE FIRST NAMED TWO YOKE LAMINATIONS WHEREBY THERE IS A TOTAL OF FOUR DIFFERENTLY SHAPED LAMINATIONS IN SAID CORE STRUCTURE, THE JOINTS IN EACH OF SAID SHEETS BETWEEN EACH OF SAID OUTER LEG LAMINATIONS AND SAID ADJOINING YOKE LAMINATIONS AND BETWEEN THE MIDDLE LEG LAMINATION AND THE FIRST NAMED TWO OF SAID YOKE LAMINATIONS EACH INCLUDING A MINOR PORTION PREPOENDICULAR AND A MAJOR PORTION AT AN ANGLE OF 45 DEGREES TO THE LONGITUDINAL AXIS OF ONE OF THE ADJOINING LAMINATIONS FORMING THE JOINT, THE JOINTS BETWEEN THE MIDDLE LEG LAMINATION AND SAID OTHER TWO OF SAID YOKE LAMINATIONS BEING STRAIGHT LINE JOINTS PARALLEL TO THE LONGITUDINAL AXIS OF THE MIDDLE LEGS LAMINATION, SAID FIRST YOKE LAMINATIONS LAMINATIONS ALSO FORMING STRAIGHT LINE JOINTS WITH SAID OTHER TWO YOKE LAMINATIONS, RESPECTIVELY, ALTERNATE LAYERS OF SAID CORE STRUCTURE BEING REVERSED WITH RESPECT TO THE OTHER LAYERS WITH EACH OF THE JOINTS IN EACH LAYER BEING BRIDGED BY A LAMINATION IN AN ADJACENT LAYER.