US3206835A

US3206835A - Method of assembling a 3-phase transformer core

Info

Publication number: US3206835A
Application number: US98369A
Authority: US
Inventors: Martin I Zwelling
Original assignee: McGraw Edison Co
Current assignee: McGraw Edison Co
Priority date: 1961-03-27
Filing date: 1961-03-27
Publication date: 1965-09-21
Anticipated expiration: 1982-09-21

Description

Sept. 21, 1965 M. 1. ZWELLING METHOD OF ASSEMBLING A 3-PHASE TRANSFORMER CORE Filed March 27, 1961 2 Sheets-Sheet 1 5 WW I W W M P 1965 M. I. ZWELLING 3,206,835

METHOD OF ASSEMBLING A 3-PHASE TRANSFORMER CORE Filed March 27, 1961 2 Sheets-Sheet 2 INVENTOR. flikn/v Z Zwezu/va United States Patent Delaware Filed Mar. 27, 1961, Ser. No. 98,369 6 Claims. (Cl. 29-155.57)

This invention relates to magnetic cores, more particularly to three phase cores for stationary induction apparatus, and to a method of making such cores.

Three phase magnetic cores for stationary induction apparatus may comprise three radial-laminated, U-shaped portions whose corresponding ends are joined by interleaved magnetic inserts of different shape having a most favorable magnetic direction. Use of magnetic inserts increases the number of joints in the magnetic circuit, and the greater the number of joints, the higher the reluctance. Moreover, care must be exercised in stacking the inserts so that the most favorable grain direction of the insert is aligned with the flux paths. Although, Y and T core structures have been produced having satisfactory values, numerous joint structures have been developed in attempts to lower the magnetic losses and exciting current, but such joint structures have not enabled the reduction in magnetic losses and exciting current to a value approaching the losses and exciting current of the magnetic material without joints therein. In some of these structures the joints between U-shaped laminations, or between U-shaped laminations and inserts, are approximately at right angles to the longitudinal axis of the laminations. The magnetic flux crosses such a joint by passing into an adjacent lamination or insert which bridges the joint and increases the magnetic flux density in the bridging part to a value nearly double the average density in the rest of the core, thereby causing the bridging part of the laminations to saturate prior to other portions of the core and increasing both loss and exciting current.

It is an object of the invention to provide a simple and economical three phase magnetic core which does not incorporate inserts and which has lowered exciting cur rent and losses approaching the values for the magnetic material without joints.

It is a further object of the invention to provide a three phase magnetic core having an improved joint construction wherein the magnetic flux density in the part of the laminations bridging the joints is not appreciably greater than the average flux density in the rest of the core, thus assuring minimum core loss and exciting current.

It is another object of the invention to provide a novel method of constructing three phase magnetic cores which prevents the portions of the bridging laminations opposite the joints from saturating prior to the rest of the core, thus insuring minimum losses and exciting current even when operating at high magnetic flux densities.

These and other objects and advantages of the invention will be better understood from the following detailed description when taken in conjunction with the accompanying drawing wherein:

FIG. 1 is a perspective view of a preferred embodiment of three phase magnetic core of the invention;

FIG. 2 is a partial view in perspective of the joint structure of the core of FIG. 1; and

FIGS. 3 and 4 illustrate steps of the preferred method of practising the invention.

Referring to FIGS. 1 and 2, a core and coil assembly is provided having three sets of phase windings A, B and C, shown in dotted lines, each set comprising primary and secondary windings for a particular phase. The preferred 3,206,835 Patented Sept. 21, 1965 embodiment of the invention is illustrated as comprising a T-core having three laminated, generally U-shaped

members

10, 11, and 12 with corresponding ends adjacent to and facing each other. The straight central portions 14 of the U-shaped

members

10, 11, and 12 constitute the three winding legs of the completed core and extend through windows in the phase windings A, B, and C, and the right angularly disposed, bent-over ends 15 of the three U-shaped members are joined together to form the core yokes. The

members

10, 11, and 12 are composed of flatwise bent, U-shaped laminations of magnetic strip material terminating in jointed relation in upper and lower

common yoke areas

16U and 16L of the completed core.

Members

10 and 11 are generally similar and respectively comprise U-shape laminations 17A and 17B each of which is approximately twice as wide as the U-shape laminations 18 of member 12. The U-shaped laminations 17 of

members

10 and 11 lie closely adjacent each other in the winding legs 14 but are spaced apart in the yokes 15. Member 12 has approximately twice as many U-shaped laminations 18 as the number of laminations 17 in each of the

members

10 and 11, and the laminations 18 in member 12 are nested closely adjacent each other so as to be in solid contact along both the winding legs 14 and the yokes 15.

Each lamination 18 of member 12 has a long, pointed, angularly disposed end 20 formed by a diagonal cut whose length is sufliciently greater than the /2 times the width of the magnetic strip material of lamination 18 to prevent saturation of the lamination bridging the butt joint formed by the diagonally cut end 20 prior to the rest of the core. The length of the diagonal cut 20 is preferably approximately two or more times the width of the magnetic strip material, e.g., a diagonal cut at an angle relative to the r longitudinal axis of the lamination 18 whose tangent is equal to approximately 0.5. The angularly disposed ends 20 reverse in successive layers of member 12 so that all angularly disposed ends 20 of odd-numbered

laminations

18A, 18C, 18E, etc. are similar and abut against the laminations 17A of member 10 and all angularly disposed ends 20 on even-numbered

laminations

18B, 18D, 18F, etc. extend in the same direction and abut against the ends of the laminations 17B in member 11. The ends 21 of the U-shaped laminations 17 in

members

10 and 11 are angularly disposed and are along diagonals at an angle relative to the longitudinal axis of the magnetic strip complementary to the angle at which the corresponding laminations 18 are cut, e.g., the ends 21 of the laminations 17A in member 10 may be disposed at an angle Whose tangent is equal to approximately 2 relative to the longitudinal axis of the magnetic strip material.

Each layer of the common yoke areas 16 comprises the end of a lamination 18 from member 12 and the end of a lamination 17 from one of the

members

10 or 11. The odd-numbered layers of the common yoke areas comprise an odd-numbered lamination 18 from member 12 and a lamination 17A from member 10; the even-numbered layers of the common yoke areas comprise an even-numbered lamination 18 from member 12 and a lamination 17B from member 11. The outer layer of each

common yoke area

16U and 16L comprises a lamination 18A from member 12 having a butt joint with a lamination 17A from member 10; the next radially inward layer comprises a lamination 18B from member 12 having a butt joint with a lamination 17B from member 11; the next radially inward layer comprises lamination from member 12 having a butt joint with a lamination 17A from member 10, etc.

The angularly disposed ends 20 of odd-numbered

laminations

18A, 18C, 18E, etc., and the butt joints formed with ends 21 of laminations 17A, are all disposed in the joints formed with the ends 21 of laminations 17B, are

in a plane perpendicular to the plane of the magnetic strip material.

7 The laminations 17 and 18 in the

members

10, 11, and 12 abut in joints having a length which is approximately two or more times the width of the laminations 18 in member 12, and each diagonal butt joint is disposed opposite a substantially uncut portion of an adjacent lamination. It is well known that the magnetic flux crosses the air gap of a butt joint between laminations by passing into the adjacent laminations which bridge the gap. Thus, to the 100 percent magnetic flux in the lamination bridging the gap is added the magnetic flux of the lamination having the butt joint, and the magnetic flux density in the bridging part of the laminations of conventional cores having right angle butt joints between the U-shaped core portions is nearly double the average flux density in the rest of the core, thus increasing core loss and exciting current of conventional cores markedly over the average value for the rest of the core.

The long diagonal butt joints formed between laminations 17 and 18 have a length approximately twice the width of the laminations 18 and are disposed opposite substantially uncut portions of adjacent laminations. The cross sectional area through the bridging lamination along the line of the long diagonal joint 20 is thus approximately 200 percent of the cross sectional area through a lamination 18 taken at right angles to the longitudinal axis thereof. Consequently the magnetic flux can transfer to the bridging lamination without increasing the magnetic flux density appreciably above the average density in the rest of the core, the bridging part of the adjacent lamination cannot saturate prior to other portions of the core even whenthe core is operated at high magnetic flux densities, and loss and exciting current are appreciably reduced in comparison to prior art construction.

The thickness of the stack of radially nested laminations 18 in member 12 preferably is approximately equal to the width of the laminations 17 in

members

10 and 11. With such relationship, the thickness of the stack of laminations 17 in the winding leg 14 of each

member

10 and 11 is approximately equal to the width of the laminations 18 in member 12 of the preferred embodiment. Consequently, rectangular cross sections through the winding legs 14 of the three

members

10, 11, and 12 are substantially identical, and the same size phase windings A, B, and C may be used in surrounding re lation to the winding legs 14 of the

U-shape members

10, 11, and 12.

In the preferred method of constructing the improved three phase magnetic core of the invention, first magnetic strip material of a width w is wound upon a rectangular mandrel (not shown) to a thickness t to form a first closed rectangular core having a pair of opposed winding leg sides 26 and a pair of opposed yoke sides 27 as shown in FIG. 3, and core 25 is annealed to permanently set the turns in rectangular shape. Second magnetic strip material having a width 2w, approximately twice as wide as the first magnetic material strip forming core 25, is wound upon a rectangular mandrel (not shown) having a pair of opposed sides of the same dimension as the rectangular mandrel on which core 25 was wound and to the same thickness t as core 25 was wound to form a second closed core 28 having a pair ofyoke sides 29 and a pair of winding leg sides 30 of approximately the same height as the winding leg sides 26 of core 25. Second core 28 is relief annealed to permanently set the turns thereof in rectangular configuration.

Long diagonal planar cuts are made through upper and

lower yoke sides

27U and 27L of the first core 25 and through upper and lower yoke sides 29U and 29L of second core 28 by suitable means such as a rotating abrasive disc. One half of first core 25 provides the U-shaped laminations 18 for member 12 of the three phase construction; one half of second core 28 provides the U- shaped laminations 17 for both member 10 and member 11 of the final three phase core.

The planes ABCD and AB'C'D of the cuts through the upper and

lower yoke sides

27U and 27L of the first core 25 are preferably perpendicular to the plane of the first magnetic strip material and along diagonals AB and AB' having a length approximately 2w or twice the width of the first magnetic strip. Preferably the cut is at an angle whose tangent is equal to approximately 0.5 relative to the longitudinal axis of the first strip, thus providing the long diagonal ends 20 on the laminations. The U-shaped laminations 18 in one half of first core 25, for example, the left half 25A of core 25 as shown in FIG. 3 after the two planar cuts ABCD and AB'C'D' are made form the member 12 of the completed core. Alternate laminations, for example, even-numbered

laminations

18B, 18D, 18F, etc., are reversed so that the long diagonal ends 20 cross in adjacent layers as shown in FIGS. 1 and 2.

Laminations 17A and 17B are formed by cutting upper and lower yoke sides 29U and 29L of second core 28 along planes HI] K and H'IJK' respectively in the manner illustrated in FIG. 4. The planes HI] K and HI'JK' of the planar cuts are each determined by two mutually perpendicular lines. One line HI for determining the plane of the cut HIJK through upper yoke side 29U is in the plane of the outer turn and along a diagonal at an angle relative to the longitudinal axis of the second magnetic strip complementary to the angle which the line AB makes with the longitudinal axis in the cut ABCD through core 25, and preferably line HI is at an angle whose tangent is equal to approximately 2 relative to the longitudinal axis of the second strip, .thus forming the angul-arly disposed ends 21 on the laminations. The planar cut HIJK is not made perpendicular to the plane of the magnetic material but rather is at an oblique angle thereto which will dispose the :angularly cut ends 21 of laminations 17A and 17B in planes perpendicular to the plane of the magnetic strip material when alternate laminations are removed and the laminations are displaced so as to be in solid contact along the winding leg. The second line I], which together with line HI defines the plane of the cut HIJK, is at right angles to the line HI, is in the plane defined by the edges of the wound second magnetic strip, and is at such an angle that the peripheral distance IL along the yoke side between the entrance of the cut at J at the inner turn and the entrance of the cut at I at the outer turn is approximately equal to the height of the stack of laminations 17A, or of 17B, in the member 18 or 11. Another way of stating this relation for the preferred embodiment is that IJ'L=tan 0.5 where the line Ll is perpendicular to the plane of the magnetic strip. The second line I] for determining the plane of the cut HIJ K is selected to dispose the cut ends 21 of laminations 17A and 17B of the

members

10 and 11 in planes perpendicular to the plane of the magnetic strip when the laminations 17A and the laminations 17B are displaced to a position wherein they lie closely adjacent each other in the winding leg 14 of the

members

10 and 11 in the final core.

The planar cut HIJ'K' through lower yoke side 29L of second core 28 is determined in a manner similar to that described for out HIJK through upper core side 29U.

One half of the second core 28, e.g., the left half 28A resulting from cutting core 28 in both yoke sides 29U and 29L as shown in FIG. 4, provides the laminations 17A and 1718 for the U-shaped members It] and 11 of the completed three phase core. Alternate laminations,

e.g., even-numbered laminations are removed from the core half 28A to provide the U-shaped laminations for one U-shaped member, for example, the laminations 17B of member 11. The remaining odd-numbered U-shaped laminations from core half 28A constitute the laminations 17A of member 10.

The

U-shaped members

10, 11 and 12 are disposed with corresponding ends facing and adjacent each other as shown in FIGS. 1 and T2, and the corresponding ends of the laminations 17A, 17B and 18 are then interfitted in jointed relation in the common upper and

lower yoke areas

16U and 16L to form the three phase core. The laminations 17A of member are nested closely adjacent each other along the leg 14 but are spaced apart along the yokes 15. The ends 21 of the laminations 17A would lie in a plane at an oblique angle to the plane of the magnetic strip material if equal spacings were provided between laminations 17A along both winding leg 14 and yokes 15, but the step of stacking laminations 17A closely adjacent each other along the winding leg 14 brings the ends 21 of laminations 17A into a plane perpendicular to the plane of the magnetic strip material so that they interfit with the long diagonal ends 20 of odd-numbered

laminations

18A, 180, 18E, etc. of member 12 in tight butt joints in the

common yoke areas

16U and 16L. In a similar manner the laminations 17B are spaced apart along the yokes but stacked closely adjacent each other along the winding leg 14, and the step of displacing laminations 17B until they are in solid contact along the winding leg 14 places the lamination ends 21 in a plane perpendicular to the plane of the magnetic strip so that they interfit with the long diagonal ends of the even-numbered

laminations

18B, 18D, 18F, etc. of member 12 in tight butt joints in the

common yoke areas

16U and 16L.

A second three phase magnetic core is then constructed from the right core halves 25R and 28R as shown in FIGS. 3 and 4. The first core right half 25R provides the laminations 18 of the member 12 of the second three phase core. Alternate laminations, e.g., even-numbered

laminations

18B, 18D, 18F, etc. are reversed in the right hand core half 25R to cross the long diagonal ends 20 in successive layers of U-shaped member 12.

Planar cuts MNJK and MN'J'K are made through yoke sides 29U and 29L of core half 28R in a manner substantially identical to cuts HI] K and H'IJK but oftset in the opposite direction along the periphery of the core. For example, line NJ is determined by making the peripheral distance LN between the entrance of cut MNJK in the inner and outer turns equal to the total thickness of the air spaces between laminations 17A, or between laminations 17B, in the yoke sides 15 in

member

10 or 11 of the final core but otfset to the right of the line L] which is perpendicular to the plane of the magnetic strip as seen in FIG. 4. In other Words LJN=tan 0.5 in the preferred embodiment. Alternate laminations, e.g., odd-numbered laminations 17A are removed from core half 28R after cuts MNJK and M'N'JK are made and are stacked together to form member 10, and the remaining even-numbered pieces are stacked together to form the member 11. The

members

10, 11, and 12 are then arranged with corresponding ends adjacent and facing each other, and the laminations 17A, 17B, and 18 are assembled in jointed relation in the

common yoke areas

16U and 16L to form a second three phase core similar to the embodiment of FIGS. 1 and 2.

Although the preferred embodiment has been illustrated and described as having diagonal butt joints whose length is approximately 2.23 times the width w of the magnetic strip material and as being constructed by a method including the steps of winding magnetic strip material to form a rectangular core and cutting through the rectangular core to provide the U-shaped laminations, it will be appreciated that the invention is not so limited and also comprehends three phase magnetic cores having shorter butt joints of a length sufiiciently greater than the /2 times lamination width w so that the portion of the bridging laminations does not saturate prior to the rest of the core and further encompasses methods of construction werein magnetic strip material is sheared to provide the desired laminations and bent to U-shape and annealed in such U-shaped configuration.

Although the invention has been illustrated and described with reference to a T-core, it will be appreciated that it also comprehends a Y-core wherein the ends of three U-shaped members abut in long diagonal joints having a length approximately twotimes greater than lamination width to prevent saturation of the bridging part of the laminations prior to other portions of the core. While only a single embodiment of the invention has been illustrated and described, many modifications and variations thereof will be apparent to those skilled in the art, and consequently it is intended to cover in the appended claims all such modifications and variations which fall within the true spirit and scope of the invention.

I claim:

1. The method of constructing a three phase magnetic core having three U-shaped portion of flatwise bent laminations, comprising the steps of winding first magnetic strip material to form a first closed rectangular core having a pair of opposed win-ding leg sides and a pair of opposed yoke sides, winding second magnetic strip material having approximately the same thickness and a width approximately twice that of said first magnetic material to approximately the same number of turns as said first magnetic material to form a second closed rectangular core having a pair of opposed yoke sides and a pair of opposed winding leg sides of approximately the said height as the winding leg sides of said first core, annealing said first and second cores to permanently set the turns thereof, cutting through both yokes of said first core along a plane perpendicular to the plane of the magnetic strip and along a diagonal having a length equal to approximately twice the width of the first strip tm-ateri-al, cutting through each yoke of the second core along a plane defined by two mutually perpendicular lines the first of which is in the plane of the outer turn of magnetic strip at an angle to the longitudinal axis complementary to the angle at which the first core is cut and the second of which is in the plane defined by the edges of the magnetic strip at an angle to the perpendicular to the plane of the magnetic strip equal to the angle at which the first core is cut and which will dispose the cut ends of the laminations in a plane :perpendicular to the plane of the magnetic strip when alternate laminations are removed and the remaining U-shaped laminations are in solid con-tact along the crosspiece thereof, reversing alternate laminations of one-half of said first core and stacking said alternate laminations so that the diagonal cut ends extend in opposite directions in successive layers to form the first U-shaped portion, removing alternate laminations from one-half of the second core and stacking the laminations so removed in solid contact along the cross-piece thereof to form the second U-shaped portion, stacking the remaining laminations from said one-half of said second core in solid contact along the cross-piece thereof to form a third U'shaped portion, and arranging said U-shaped portions with corresponding ends adjacent to and facing each other and with the laminations of said portions terminating in jointed relation in common yoke areas, abutting the diagonal cut ends of alternate laminations of said first portion with the cut ends of the laminations of the second portion in said common yoke areas, and abut-ting the corresponding diagonal cut ends of the remaining laminations of said first portion with the cut ends of the laminations of said third portion in aid common yoke areas.

2. The method of constructing a three phase magnetic core having three U-shaped portions of fiatwise bent laminations, comprising the steps of winding first magnetic material strip to form a first closed rectangular core, Winding second magnetic material strip having approximately the same thickness and a width approximately twice that of said first strip to approximately the same number of turns as said first magnetic trip material and into a second closed rectangular core having approximately the same height as said first rectangular core, annealing said first and second cores to permanently set the turn thereof, cutting through opposed sides of said first core in a plane along a diagonal having a length approximately twice the width of said first strip, cutting through each of one pair of opposed sides of said second core in a plane along a diagonal at an angle relative to the longitudinal axis of the second strip complementary to the angle at which said firs-t core was cut, reversing alternate laminations of one half of said first core and stacking said laminations so that the diagonal cut ends extend in opposite directions to form a first U-sha-ped portion of said three phase core, removing alternate laminations from one half of said second core to provide the laminations for a second U-shaped portion, the remaining laminations of said one half of said second core constituting a third U-shaped portion, arranging said U-shaped portions with corresponding ends adjacent to and facing each other and assembling the laminations of said U-shaped portions in jointed relation in common yoke areas, abutting the diagonal cut ends of alternate laminations of said first U-shaped portion with the cut ends of laminations of said second U-shaped portion in said common yoke areas, and abutting the diagonal cut ends of the remaining laminations of said first U-shaped portion with the cut ends of the laminations of said third U-shaped portion in said common yoke areas.

3. The method according to claim 2 including the step of stacking the laminations of the second and third U shaped portions in solid contact along the cross-piece thereof and wherein the cutting through the turns in each of said one pair of opposed sides of the second core is along a plane at an angle oblique to the plane of the magnetic strip defined by two mutually perpendicular lines one of which is in the plane of the outer turn of magnetic strip at an angle to the longitudinal axis complementary to the angle at which the first core is cut and the second is in the plane defined by the edges of the magnetic strip at an angle to the perpendicular to the plane of the magnetic strip equal to the angle at which the first core is cut and which will dispose the cut ends in planes perpendicular to the plane of the magnetic strip when alternate laminations are removed and the remaining U-shaped laminations are displaced to be in solid contact along the cross-piece thereof.

4. The method of constructing a three-phase magnetic core having three laminated, U-shaped portions with corresponding ends adjacent to and facing each other, comprising the steps of winding first magnetic strip material of width w to a thickness 2w to form a first closed rectangular core, winding econd magnetic strip of width 2w and approximately the same thickness as said first strip to a thickness approximately 2w to form a second closed rectangular core having approximately the same height as said first rectangular core, annealing aid first and second cores to permanently set the turns thereof, cutting through opposed sides of said first core along a diagonal having a length at least equal to 2w, cutting through opposed sides of said second core, reversing alternate laminations of one half of said first core so 70 that the diagonal cut ends thereof extend in opposite directions and stacking said laminations so reversed to form a first U-shaped portion of said core, removing alternate U-shape laminations of one half of said second core to provide laminations for a second U-sha-ped portion, the remaining laminations of said one half of said second core constituting a third U-shaped portion, arranging said U-shaped portions with corresponding ends adjacent to and facing each other and assembling the laminations of said U-shaped portions in join-ted relation in common yoke areas, abutting the diagonal cut ends of alternate laminations of said first portion with the cut ends of the laminations of said second portion in said common yoke areas, and abutting the diagonal cut ends of the remaining laminations of said first portion with the cut ends of the laminations of said third portion in said common yoke areas so that said laminations of said second and third portions are interleaved with the ends of the laminations of said first portion.

5. The method of constructing a three phase magnetic core comprising the steps of winding first magnetic material strip to form a first rectangular core, winding second magnetic material strip of approximately the same thickness as and substantially wider than said first strip to form a second rectangular core having approximately the same height as said first core, annealing said first and second cores to permanently set the turn thereof, cutting through the turns in opposed sides of said first core collectively along a diagonal having a length approximately twice the width of said first strip to form first U-shaped laminations, cutting through the turn in opposed sides of said second core collectively to form second U-shaped laminations, assembling first laminations in radially nested relation to form a first U-shaped bundle and reversing alternate first laminations so the diagonal cut ends cross in successive layers, assembling alternate second laminations to form a second U-shaped bundle,

assembling the remaining second laminations to form a third U-shape-d bundle, arranging said three U-shaped bundles with corresponding ends adjacent to and facing each other and assembling the corresponding ends of the laminations of said three U-shaped bundles in jointed relation in common yoke areas, abutting the diagonal cut ends of alternate laminations of said first bundle with the cut ends of laminations of said second bundle in said common yoke areas, and abutting the diagonal cut ends of the remaining laminations of said first bundle with the cut ends of laminations of said third bundle in said common yoke areas.

6. The method in accordance with claim 5 including the step of stacking the laminations of the second and third U-shaped bundles in solid contact along the crosspiece thereof and wherein the cutting through one of said cores is perpendicular to the :plane of the magnetic strip material and the cutting through the turn of the other core is at an oblique angle whose tangent relative to the perpendicular to the plane of the magnetic strip is equal to approximately 0.5.

References Cited by the Examiner UNITED STATES PATENTS 2,456,461 12/48 Dunn 29-155.61 2,579,578 12/51 Horstman et al 29'155.6l 2,840,889 7/58 Freegard 29l55.61 2,931,993 4/60 Dornbush 336-2l7 2,942,218 6/60 Attewell 336217 2,958,931 11/60 Hurt 29l55.61 2,964,836 12/60 Smith 29'155.57 2,968,087 1/61 Thompson 29155.57

WHITMORE A. WILTZ, Primary Examiner.

MILTON O. HIRSHFI'ELD, FRANK E. BAILEY,

JOHN F. CAMPBELL, Examiners.

Claims

5. THE METHOD OF CONSTRUCTING A THREE PHASE MAGNETICCORE COMPRISING THE STEPS OF WINDING FIRST MAGNETIC MATERIAL STRIP TO FORM A FIRST RECTANGULAR CORE, WINDING SECOND MAGNETIC MATERIAL STRIP OF APPORXIMATELY THE SAME THICKNESS AS AND SUBSTANTIALLY WIDER THAN SAID FIRST STRIP TO FORM A SECOND RECTANGULAR CORE HAVING APPROXIMATELY THE SAME HEIGHT AS SAID FIRST CORE, ANNEALING SAID FIRST AND SECOND CORES TO PERMANENTLY SET THE TURNS THEREOF, CUTTING THROUGH THE TURNS IN OPPOSED SIDES OF SAID FIRST CORE COLLECTIVELY ALONG A DIAGONAL HAVING A LENGTH APPROXIMATELY TWICE THE WIDTH OF SAID FIRST STRIP TO FORM FIRST U-SHAPED LAMINATIONS, CUTTING THROUGH THE TURNS IN OPPOSED SIDES OF SAID SECOND CORE COLLECTIVELY TO FORM SECOND U-SHAPED LAMINATIONS, ASSEMBLING FIRST LAMINATIONS IN RADIALLY NESTED RELATION TO FORM A FIRST U-SHAPED BUNDLE AND REVERSING ALTERNATE FIRST LAMINATIONS SO THE DIAGONAL CUT ENDS CROSS IN SUCCESSIVE LAYERS, ASSEMBLING ALTERNATE SECOND LAMINATIONS TO FORM A SECOND U-SHAPED BUNDLE, ASSEMBLING THE REMAINING SECOND LAMINATIONS TO FORM A THIRD U-SHAPED BUNDLE, ARRANGEING SAID THREE U-SHAPED BUNDLES WITH CORRESPONDING ENDS ADJACENT TO AND FACING EACH OTHER AND ASSEMBLING THE CORRESPONDING ENDS OF THE LAMINATIONS OF SAID THREE U-SHAPED BUNDLES IN JOINTED RELATION IN COMMON YOKE AREAS, ABUTTING THE DIAGONAL CUT ENDS OF ALTERNATE LAMINATIONS OF SAID FIRST BUNDLE WITH THE CUT ENDS OF LAMINTATIONS SAID SECOND BUNDLE IN SAID COMMON YOKE AREAS, AND ABUTTING THE DIAGONAL CUT ENDS OF THE REMAINING LAMINATIONS OF SAID FIRST BUNDLE WITH THE CUT ENDS OF LAMINATIONS OF SAID THIRD BUNDLE IN SAID COMMON YOKE AREAS.