US2908880A - Magnetic core - Google Patents

Description

Oct. 13, 1959 Filed Aug. 8. 1955 A. G. STEINMAYER ET AL MAGNETIC CORE 3 Sheets-Sheet 1 BY ai@ United States Patent MAGNETIC CORE Alwin G. Steinmayer, Milwaukee, Wis., and James G. Everhart, Sherman, Tex., assignors to McGraw-Edison Company, a corporation of Delaware Application August 8, 1955, Serial No. 526,955

Claims. (Cl. 336-213) This invention relates to magnetic cores for stationary induction apparatus and in particular to multi-legged magnetic cores for electrical transformers.

It is an object of the invention to provide a new and improved multi-legged magnetic core for stationary induction apparatus.

It is a further object of the invention to provide a magnetic core of spirally wound magnetic strip material wherein the mean length of iron turn is appreciably less than in prior art cores of similar conguration.

Another object of the invention is to provide a magnetic core of concentrically nested, atwise curved, magnetic strip laminations which is appreciably lighter in weight than prior art cores of similar configuration.

A magnetic core constructed in accordance with the invention comprises a plurality of core sections in side by side relation across the width of the core. Each core section includes at least one closed loop of atwise curved, concentrically nested laminations, and the total cross sectional height of the laminations in the closed loops .and the window dimensions of the loops are so selected that the winding legs of the core formed jointly by the core sections are of cruciform cross section and the yokes of the core are of approximately semi-cruciform cross section, whereby the greatest amount of iron in the yokes is immediately adjacent the electrical winding and the mean length of iron turn is minimized.

The preferred embodiment of the `present invention is an improvement on the polyphase magnetic core which forms the subject matter of U.S. Patent No. 2,431,128 to Edwin A. Link entitled Three Phase Core and assigned to the present assignee. The Link core comprises two similar, contacting, closed magnetic core loops spirally wound of magnetic strip material and an outer core loop of magnetic strip material spirally wound in embracing relation about the contacting inner core loops. The

contiguous sides of the inner core loops constitute the able as a return path for the center winding phase flux.

However, the reluctance to ux travel in a direction perpendicular to the laminations is relatively high and very little of the center leg ilux transfers to the outer core loop.

loops, form the outer legs of the core. In such a magnetic core the magnetic flux from the inner loops, jointly forming the center phase winding leg, can transfer much more easily to the outer loops, as the flux will move parallel to the laminations.

For a better understanding of the principles and objects of the invention, reference should be had to the following description taken in conjunction with the accompanying drawing wherein: Fig. 1 is an elevation view of the core and coil assembly of a three phase induction apparatus embodying the invention; Fig. 2 is a sectional view taken on line 2-2 of Fig. 1; Fig. 3 is a sectional view taken on line 3-3 of Fig. 1; Fig. 4 is a plan view of the magnetic core of the apparatus of Fig. 1; Fig. 5 is an elevation view of an alternative embodiment of the invention; Fig. 6 is a sectional view taken on line 6 6 of Fig. 5; Fig. 7 is a sectional view taken on line 77 of Fig. 5; Fig. 8 is an elevation View of an alternative embodiment of the invention for a single phase, shell type transformer; Fig. 9 is a sectional view taken on line 9-9 of Fig. 8; and Fig. 10 is a sectional view taken on line 10-10 of Fig. 8,

All of the advantages of the present invention inhere in the preferred embodiment of the polyphase stationary' having a three legged, cruciform-in-cross section, mag- In accordance with the preferred embodiment of the Y invention, a cruciform-in-cross-section magnetic core is constructed so that the inner, wound strip loops which pass through the center phase winding and jointly from the center winding leg are in side-by-side relationship .with certain of the outer, wound strip loops which pass through the outer phase windings only and, with the inner netic core comprising a plurality of core sections 10, 11 and 12 in side-by-side relation across the width of the core, live such sections being shown in the drawing. Each core section 10, 11 and 12 can be separately constructed and is similar to the core of the aforementioned Link patent with the exception that only a single width of magnetic ribbon is utilized and consequently, the legs thereof are of rectangular cross section. Thus, the magnetic core of the preferred embodiment is subdivided, or sectioned, along parallel planes intersecting the three legs A, B and C of the core, said planes being vertical with the magnetic core disposed as shown in the drawing. Each core section 10, 11 and 12 comprises two inner contacting loops spirally wound of magnetic strip material and an outer loop spirally wound of magnetic strip material embracing the inner loops.V The core sections 10, 11 and 12 are so constructed that they jointly form magnetic core winding legs A, B and C of crucifornr cross section, and the yokes of the inner loops of certain core sections are in side-by-side relation to the yokes of the outer loops of core sections adjacent thereto to permit transfer of magnetic flux between the inner and outer loops parallel to the laminations.

In order to minimize eddy current losses in the finished transformer, suitable insulating means 16 such as single ply paper or a resinous varnish film is preferably pro vided between adjacent core sections 10, 11 and 12.

The core sections 10 at both outer faces of the magnetic core are identical, and each core section 10 comprises two inner, substantially rectangular, loops 20 and 21 having the longer sides thereof abutting and an outer substantially rectangular loop 22 embracing the inner loops 26 and 2l. Each of the loops 20, 21, and 22 is formed by winding a closed loop of magnetic ribbon, and the magnetic ribbon is of the same width d1 (see Fig. 4) in each of the loops 2i), 21, and 22. Any suitable magnetic strip material may be utilized in constructing the loops, although it is preferable to use cold rolled silicon steel having a direction of easiest magnetization lengthwise thereof. The inner core loops 20 and 21 are duplicates, and each is formed by. spirally windingmagnetic ribbon 24 of suitable width fiatwise on a mandrel to provide a closed convoluted loop of concentrically nested, flatwise curved, lamination layers. Although the Winding is preferably on a rectangular mandrel, the winding can be accomplished on a circular mandrel and zoos/,eso

tion. Each of the inner loops 20and 21 is woundto the same thickness t1 (see Fig. 3) of magnetic ribbon 24, and the inner and outer ends of the magnetic ribbon 24 may be brazed, tackfwelded" or otherwise secured in Va manner well known inthe art.- i

' AThe outer loop `2"2-may be spirally wound ofmagnetic ribbon '24; having the same -width'i1"as themagn'etic ribbon of the inner loops =20 and '21, ony ay rectangular mantlrel (notshown)4 and after-completion positioned so as toencircle the abutting linner loops A20 and 21, or the outer loop 22"may `be wound directly on the contacting inner loops 20 and 21. Preferably-the twoinner loops 20fand 2'1" are not removed from the mandrels on which they-y are wound, and the mandrels (not shown) are Iriountedion a common rotatable member to accomplish the'winding of Vthe outer loop in a manner similar to that disclosedinthe copending application Serial No. 502,374

of Robert P. Thompson and. Albert C. Wurdack, Jr., entitled; Magnetic Cjore and Method of Construction lhereof and having the saine assignee as therprese'nt invention. Preferably thel outer loop 22 is wound to approximately theV same thickness t1 of magnetic ribbon 24 'as each of the inner loops'2t1 and 21. If desired, the magneticstrip 2 4 from 0.116` or both'of the inner loops 2f() vandj21 may be, left unsevered afterA the completion thereof, andthe outerv loop 22,v may be spirally wound from an uncut length of magnetic. ribbon 24 continuous with the magnetic, ribbon of, oneor both of the inner loopsZ)l and 21 ina manner disclosed in patent applicaf `tion Serial No. 422,374toAnderson and Everhart entitled Polyphase Transformer and also vdisclosed` in the aforementioned ThompsonV and Wurdack application, both-of which'have the sarne assignee as the present invention. After the winding of the outer loop 22 is completed, vthe end ofthe magneticstrip material 24 may be tackwelded, brazed, or otherwise secured.

The core sections 11 Vadjacent to and disposed inward from` the core'sections. 10 are similar to the core sections 10, eXceptthat they are wound of wider magnetic ribbonand to a greater thickness. Each core section 11 c ornprises` two inner, substantiallyrect'angular loops25 and '26 having their longer'sides abutting and an outer, substantially rectangular loopk 27 embracing the' inner loops 25 and 26. Each of Vthel inner loops 25 and-'26 is formed by winding a closed loop o f magnetic ribbon ZShavinga width d which isgreater than thewidth d1 of the magnetic ribbonf24 forming 'the loops 20, 21 and 22 ofthe core section10. VThe winding of magneticl ribbon 28 is preferably on a `rectangular mandrel (not shown)y havingA thefsame height but beingY narrower than the mandrel on 'which the inner loops ,.20 and 21 are wound. As a consequence, the width w2.(see Fig. 4) of the ywindingpwindow of` theinner loops 25 and 26 is less than the width w1 of theI loops '20 and 21, but the height h (see Fig. 1) of the winding window is the same Y in the inner loops 25 and 26 as in the inner loops 20 and 21k of core section 10. The magnetic ribbon 28 is Wound to a greater thickness t2 (see Fig. 3) inthe inner loops 25 and 26 than the thickness t1 to which the magnetic strip material 24 is wound inthe inner loops 20 and 21 of core sections 10.

In a manner similar to that described for core section 10, the outer loop 27 may be spirally wound of magnetic strip material 28, having vthe same width d2 as the magnetic strip material forming the inner loops 25 and 26, 0n a rectangular mandrel (not shown) and after completion positioned so as to encircle the abutting inner loops 25 and 26, or the outer loop 27 maybe wound directly on the contacting innerloops 25 and 26. The outer loop 27 may be wound after the two inner loops 25 and26 are mounted on a common rotatable member as above described, and themagnetic ribbon 28` from which the outer 'loop 27 swound may be a free` length of uncut magnetic 4 strip material continuous with the magnetic strip material of one or both the inner loops 25 and 26. The magnetic ribbon 28 is wound to a thickness t2 approximately equal to the thickness to which the inner loops 25 and 26 are Wound, and the outer end thereof is brazed, tack welded or otherwise secured as described above.

The core section 12 is similar to the core sections 10 and 11 except that it is wound of still wider magnetic ribbon and to still greater thickness. Core section 12 comprises two inner, substantially rectangular loops 3Q and 31 having longer sides thereof abuttingfand anonte'r, substantially rectangular loop 32 embracing theinner loops 30 and 31. Each of the inner loops 30 and 31 is formed by spirally winding a closed loop of magnetic ribbon 34 having a width d3 (see Fig. 4) lwhich is greater than the width d2 of the magnetic ribbon 28 forming the loops 2,5, 26 and 2'7 of the core section 11. The winding is preferably on a rectangular mandrel (not shown) having the same height but beingnarrower than the mandrelon which the loops 25 and 26 are wound. As a consequence, the width w3 (see Fig. 4) of the winding window of thefinner loops 3i) and 31is less than the width `W2 of the inner loops 25 and 26, but the height h v(see Fig. l) of the winding window is the same as in the inner- loops 20, 21, 25 and 26 kof the sections 10 and 11. The magnetic ribbon 34Y is wound to a greater thickness tg (see Fig. 3) yin the inner loops Strand 31 than the thickness t2 to which the inner loops 25 and 26 are wound.

In a manner similar to that ldescribed for core sections 10 and '11, the outer loop 32 may be spirally wound of magnetic` strip material 34, having the same width d3 as the magnetic strip material forming the inner loops 30 and v31, and after completion positioned so as to encircle the abutting inner loops 30 and 31, or the outer loop 32 may be wound directly onV the contacting inner loops 30 and 31. The outer loop 3'2 may be wound after the two inner loops 30 and 31 are mounted on a common rotatable member as described above, and the magnetic ribbon 34 from which the outer loop 32 is wound may be a free length of uncut magnetic strip continuous with the magnetic strip ,material of one or both the inner loops 30 and 31. In forming the outer loop 32, the magnetic ribbon 34 is wound to a thickness r3 approximately equal to the thickness to which the inner loops 30 and 31a re wound, and the end thereof isbrazed, tack 'welded or otherwise secured as described above. A

The core ` sections 10, 111 and 12 may be separately strain relief annealed to remove all strains :due td the working of the magnetic ribbon during winding or during winding and subsequent shaping, depending on which process hereinabove described is followed. Preferably, however, the core sections 10, .111 and 12 are disposed in side-'by-siderelation as shown in Figs. l through 4, and clamped together, and the entire core is strain relief lannealed as a unit. Metal bands encircling the legs or yokes may be utilized to clamp the core sections together as Va unit during annealing. In the finishedjcore the ycore mountingframe (not shown) is preferably constructed to support and clamp the core sections 10,' 11 and 12 in the position shown in the drawing, and nonconducyting tapes may be utilized circumjacent they yokes to further bind the core sections 10, 1'1'and 12 together. l'

As shown in the drawing, the `thickness t1, t2 and t3 to whichV the inner and outer loops of the core sections 10, 11 and `12 are wound provide winding legs A, B and C which are` identical and of cruciform cross section and yokes which are of generally semicruciform cross section. The outer loops of the core sections are disposed in sideby-side relation to the inner loops of a core section` adjacent thereto to permit the, magnetic llux to pass over between innerrand vouter loops parallel yto thelaminations. As -best seenin Fig. 3 theryoke portion of outer loop?` 22 of each core section .10 is in side-by-side relation to the yoke portion of the inner loops 25 and 26 of a icor'eseetion y171. In a similar manner-, inner loops 30 and 3,1"o'f core section :12 are in side-by-side relation to outer loop 27 of both core sections 11. Inasmuch as the magnetic flux from the portion of the center phase winding leg formed by the inner loops 25 and 26, of core sections 11 can transfer more easily parallel to the lamnations to the outer loops 22 of core sections 10, and the magnetic lux from the portion of the center phase winding leg formed by the inner loops 30 and 31 of core section A12 can transfer more easily parallel to the laminations to the outer loops 27 of core sections 11, a greater proportion f the magnetic flux can return through the entire cross sectional area of the iron of the outer core legs than in the prior `art cores wherein the flux could only travel to the outer core loop through the relatively high reluctance paths perpendicular to the laminations and only a negligible percentage of llux Afrom the center leg transferred to the outer core loop.

Although the preferred method of construction has been described as including the separate construction of the core sections 10, 11 and 12, it will be appreciated that if a winding mandrel is provided having a stepped periphery identical to the winding window of the final core, the inner loops 20, 25 and 30 (or the inner loops 21, 26 and 31) can be wound simultaneously.

The separate phase conducting winding assemblies 40, 41 and 42 for connection to the different phases of a polyphase alternating current circuit are wound on the finished annealed core in any suitable manner. For example, the winding machine disclosed in the patent to Steinmayer et al. 2,305,999 can be utilized. After the conducting windings 40, 41 and 42 have been finished, suitable wedges may be driven into place to suitably hold the conducting winding assemblies firmly in position with reference to the core.

lt will be apparent that inasmuch as the greatest amount of iron in the yokes of the core is immediately adjacent to the winding window (in contrast to the core of the aforementioned Link patent wherein the yoke is of cruciform cross section and the greatest amount of iron in the yoke is disposed away from the winding window), the mean length of iron turn in the core of Figs. l through 4 is appreciably lessened in comparison to the core of the Link patent and consequently, that the weight of the core is reduced proportionally.

In an alternative embodiment of invention illustrated in Figs. through 7, all of the core sections are identical in height and the cross section of the yokes of the magnetic core is inverted from that of the embodiment of Figs. l through 4. The parts of the alternative embodiment are given the same reference numerals as the parts of the preferred embodiment to which they are similar with the addition of the prime designation. The core sections 110' are constructed in a manner similar to that of the core sections of the embodiment of Figs. 1 through 4, although for a given height of core the inner loops 20 and 21' are wound on a mandrel having a greater height than the mandrel used for Winding the inner loops 20 and 21. The inner loops 25 and 26' of core sections 11 are preferably wound on a mandrel which is shorter and narrower than the mandrel on which the inner loops 20 and 21 are Wound, and consequently, the width W2 of the window in the inner loops 25 and 26 is less than the width w1 of the window in the inner loops 20' and 21 andrfurther, the height h2 of the Window in the inner loops 25 and 26 is less than the height h1 in the inner loops 20 and 21.

The inner loops 30' and 31' of core section 12 are preferably Wound on a mandrel which is shorter and narrower than the mandrel on which the inner loops 25 and 26' are wound, and consequently, the width w3 of the window in the inner loops 30 and 31 is less than the `width wz of the Window in the inner loops 25 and 26 and further the height h3 of the window in the inner loops 30' and 31 is less than the height h2 in the inner loops 25 and 26.

^ core sections lie in a common plane.

The winding legs A', B' and C are all substantially identical and of cruciform cross section; The top surl faces of the core sections 16', 11" and 12' are in coni= mon plane, and similarly the bottom surfaces of these The yokes of inner loops 20 and 21 of each core section 10 are disposed in side-by-side relation to the yokes of the outer loop 27 of a core section 11', and the yokes of the inner loops 25 and 26 of the core sections 11 are disposed in sideby-side relation to the yokes of the outer loop 32 of the core section 12 to permit the magnetic iluX to transfer between inner and outer loops of the core sections parallel to the layers of magnetic strip material.

Although the illustrated embodiments have been shown and described as including two inner loops in each core portion, the invention is not so limited and comprehends any number of contacting inner loops in each core section. For example, in a four-legged magnetic core constructed in accordance with the invention, each core section comprises three inner loops in back-to-back relation in a common plane.

Although the core sections 10, 11 and 12 of the embodiments of Figs. l through 4 and Figs. 5 through 7 are illustrated as being constructed of magnetic ribbons 24, 28 and 34 of different widths, the invention is not so limited, and magnetic cores having Winding legs of cruciform cross section can be constructed in accordance with the invention wherein the same width magnetic ribbon is utilized in all the core sections. For example, it will be apparent that the equivalent of section 12 can be constructed by winding a plurality of narrower ribbons, each having the width of ribbon 24 (or of ribbon 28) to. thickness t3 in both the inner and outer loops to form a plurality of core sections each having a width of the.v ribbon 24 and disposing such sections in side by side re` lation to provide a total width equal to the width d3 of ribbon 34. In the magnetic core structure illustrated in Figs. 8 to 10, all of the core sections are formed of mag,- netic ribbon of the same width. The invention is particularly advantageous in polyphase magnetic cores of the types illustrated in the embodiments of Figs. l through 4 and Figs. 5 through 7, but single phase magnetic cores may also be constructed in accordance with the invention Which are lighter in weight and have a considerably smaller mean length of iron turn than prior art cores of similar configuration. It may be considered that two similar, closed, rectangular cores 39 and 40 comprise the single phase, shell type, three legged magnetic core struc ture 41 illustrated in Figs. 8 to l0. However, inasmuch as the two cores 39 and 40 are mechanically held as a unit in the finished apparatus and coact with a single electrical winding, the magnetic core structure 41 will be described as including a plurality of core sections 43, 44 and 45 disposed in side by side relation across the Width of the core, with each core section including two closed loops in back to back relation, one loop being in each core half 39 and 40. Thus, each core section 43, 44, and 45 is somewhat analogous to the core sections 10, 11 and 12 of the embodiments described hereinbefore in that it includes two closed, rectangular, wound strip loops in back to back relation, but no outer loop surrounds the back to back loops of core sections 43, 44 and 45. Thus, lluX transfer parallel to the laminations between inner to outer loops is not effected in the single phase magnetic core structure 41 of Figs. 8 to l0 as in the three phase cores of Figs. l to 4 and Figs. 5 to 7, but the features of decreased length of iron turn and consequent reduction in weight are inherent in the magnetic core structure 41 of Figs. 8 to l0.

As illustrated the single phase magnetic core structure 41 has a central leg 47 of cruciform cross section formed by the back to back sides of the core halves 39 and 40 and outer legs 49 and 50 of approximately semi-cruciform cross section. All of the closed loops of all of the sections 43, 44 and 45 are wound of the same width mag- 7 neticjribbon 51, but the winding isperformed on diierent mandrels (not shown) all of the. same height h4 buthavingdiierent widths W4, W5, and we. Each oftheloops 53and 54. of a core section 43 are wound from lmagnetic ribbon 51 on a mandrel (not shown) `of heighth., and width .W4 to a thickness t4 (See Fig. 10) andthe loops 53 and 54 are disposed in back to back relation to form acore section 43. Two core sections 43 areprovided in the ycore structure 41,- one at each outside edge thereof. Each of the loops 56 and 57 of a4 core Section 44 are wound from magnetic ribbon 51 on Va mandrel (not shown) having a height It., and a width W5, which is smaller than width W4, to a thickness t5 (see Fig. l0), which is greater than thickness t4, and the loops 56 and 57 are disposed in back toY back relation to form a core SCtOn 44. Two core sections 44 Iare provided inthe core structure 41, one being disposed inward from each core section 43. Each of the loopsV 59 and 60.of a core section 45 are wound from magnetic ribbon 51 on a mandrel having a height h4 and a width W6, Which is smaller than width W5, to a thickness t6 which is greater than the thickness t5, andthe loops .59 and 60 are disposed in back toback relation to form a core section-45.

Two core sections 45 are provided and are arranged in side to side relation inward from the core sections 44 and at the innermost portion of the core. As inthe preferred embodiment suitable insulating means 16 such as single ply paper or a resinous varnish lm may be provided between adjacent core sections 43, 44 and.45 to minimize eddy current losses in the finished transformer.

The back to back sides of the loops 53. and 54, the back to back sides of the loops 56 and 57," and the back to back sides of the loops 59 and 60 jointly `form the center leg 47 of the single phase shell type magnetic core structure 41, and insulating tape (not shown) may be wound around the abutting sides of all the loops 53, 54, 56, 57, 59, and 60 to hold the core sections as an integral unit. An electrical winding 62 shown in dash-dot lines may be wound in embracing relation about the center leg 47. One outer leg 50 of the single phase, shell type magnetic core structure 41 is formed jointly by the adjacent legs of the loops 54, 57 and 60; similarly the other outer leg 49 of core structure 41 is formed jointly by the adjacent legs of the loops 53, 56 and 59.

It will be -apparent that the yokes of the magnetic core structure 41 are of semi-cruciform cross section and consequently a greater amount of ironin theyokes is immediately adjacent to the electrical winding 62 than in a conventional wound core wherein the yokes are of cruciform cross section and that thus the magnetic core structure of Figs. 8 to 10 has a shorter meanlength of iron turn and smaller weight than a conventional cruciform-in-cross section wound core.

While we have shown and described particular embodiments of our invention, it will be `obvious to those skilled in the ait that Various changes and modifications may be made therein without departing from the invention, land therefore, it is intended in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent ofthe United States is:

1. A multi-legged magnetic core for stationary induction apparatus comprising, in combination, a plurality of core sections disposed in side-by-side relation, each core section at least including two similar, generally rectangular, closed inner loops of spirally and atwise wound magnetic strip material arranged back to back in the same plane and a generally rectangular, closed, outer loop of spirally and ilatwise wound magnetic strip material in embracing relation about said inner loops and said outer loop being in the plane of said inner loops, the magnetic strip material in each core section being in planes perpendicular to the plane of said innerl and outer loops and being of the same width in both the inner and outer loops of said corersection,` the abutting sides ofthe inner loops of all the core sections jointly formingV the central leg of the core, in each core section the outer'` which the inner and outer loops of the core-sections disposed at the outer portion of the core in avdirection perpendicular to said planes of the inner and outer loops` is wound being less than the thickness of magnetic strip,

material to which the inner and outer loops of the core sections disposed inward therefrom is wound, the winding legs formed jointly by the core sections being ofapproximate cruciform cross section andthe yokes join-v ing said winding legs -and formed jointly by said coil sections being of approximate semicruciform cross secL tion and the yoke portions of the inner loops of-certain of the core sections are in side-by-side relation to the yoke portions of the outer loop of a core section adjacent thereto.

2. Three phase electromagnetic induction apparatus comprising, in combination, three conductivewinding structures each adapted to be connected to `one ofthe phases of a three phase electrical system, said winding structures being side-by-side in a-'rowV and each'having a window for the reception of magnetic core material, andl a magnetic core having three winding legs, one of said legs passing through the window of each of said windingy structures, said core comprising a plurality of core sections disposed in side-by-side relation in a direction perpendicular to the median plane of said'core defined by the axes of said core legs, each core section'includinga first generally rectangular, closed, inner loopof radially nested, fiatwise curved, lamination layers passing through the window of one of the end winding structures and the window of the middle winding structure, aj secondy generally rectangular, closed, inner loop of radially|` nested, flatwise curved, lamination layers passing through the window of the middle winding structure and the window of the other end winding structures, said `first and second inner loops being back to back in acommonfplane parallel to said median plane, and-a generally rectangular closed outer loop of radially nested, flatwirse curved, lamination layers passing through'the windows of the two end winding structures and embracing the two inner loops, the laminations being of the same width in -the inner and outer loops of each core section, the height of magnetic strip material to which the inner and outer loopsofthe core sections disposed at the outer portions of the core in a direction perpendicular to said median plane iswound being less than the height of magnetic strip material to which the inner and outer loops of the core sections disposed inward therefrom is wound and the winding legs formed jointly by said core sections being of approximate cruciform cross section and the yokesformed jointly yby said core sections and connecting said winding legs being of approximate semicruciform cross sectiony and thejyokes of the inner loops of cetrain core sections beingin side@ by-side relation to the yokes of the outer. loop v ofacore section adjacent thereto, v Y Y f Y 3. Three phase electromagnetic induction apparatus comprising, Vin combination, Vthree `conductive each adapted to be connected to Vone. ofthe phases of a three phase Velectricahsystem, said ,windings beingside'- 4by-sid'e in a row and each having a win'dowffor the Vreception of magnetic core material, and a ymagnetic core having three winding legs, one` of. said legs; passing Vthrough the window of each of said windings, said'core comprising a plurality of core sections disposed in sideby-side relation, each core section including a first, generally rectangular, closed inner loop of radially nested, liatwise curved, magnetic strip laminations passing through the window of one of the end windings andthe window of the middle winding, Va second, generallyrectangular, closed, inner loop of radially nested, flatwse curved,

magnetic strip laminations passing through the window of the middle winding and the window of the other end winding, said first and second inner loops being back to back in a common plane, and a generally rectangular, closed outer loop of radially nested, flatwise curved, magnetic strip laminations passing through the windows of the two end windings and embracing the two inner loops, the magnetic strip laminations being of the same width in the inner and outer loops of each core section, the closed loops of the different core sections having diierent cross sectional heights of magnetic strip laminations, the arrangement being such that the legs formed jointly by the core sections are of cruciform cross section and the height lof the window in all of said iirst inner loops in a direction parallel to the core legs is the same and the height of the window of all of said second inner loops in a direction parallel to the core legs is the same and the yokes of the inner loops of certain core sections are in side-by-side relation to the outer loop of a core section adjacent thereto.

4. A three legged magnetic core for stationary induction apparatus comprising, in combination, a plurality of core sections disposed in side-by-side relation across the width of the core, each core section having portions in each of the legs of the core and each core section comprising two similar, generally rectangular, closed, inner loops of concentrically nested, flatwise curved, magnetic strip laminations, said inner loops being arranged back to back in the same plane, and a generally rectangular, closed, outer loop of concentrically nested, flatwise curved, magnetic strip laminations in embracing relation about said inner loops, the magnetic strip laminations being of the same width in both the inner and outer loops of each core section, the abutting sides of the inner loops of all of the core sections jointly forming the central leg of the core, in each core section the outer side of each inner loop and the side of the outer loop adjacent thereto forming a portion of an outer leg of the core, the height of the window in -the inner loops of all the core sections being the same in a direction parallel to the core legs, Ithe total cross sectional height of the magnetic strip laminations of the loops of the innermost core section being greater than that of the loops of the core sections disposed outward therefrom, the arrangement being such that the winding legs of said core formed jointly by the core sections are of cruciform cross sec- 10 tion and the yokes of said core connecting said winding legs and formed jointly by the core sections are of approximate semicruciform cross section and the yokes of the inner loops of certain of the core sections are in side-by-side relation to the yokes of the outer loop of a core section adjacent thereto.

5. A three legged magnetic core for stationary induction apparatus comprising, in combination, a plurality of core sections disposed in side-by-side relation across the width of the core, each core section having portions in each of the legs of the core and each core section comprising two similar, generally rectangular, closed, inner loops of concentrically nested, atwise curved, magnetic strip laminations, said inner loops being arranged back to back in the saine plane, and a generally rectangular, closed, outer loop of concentrically nested, atwise curved, magnetic strip laminations in embracing relation about said inner loops, the magnetic strip laminations being of the same width in both the inner and outer loops of each core section, the abutting sides of the inner loops of all of the core sections jointly forming the central leg of the core, in each core section the outer side of each inner loop and the side of the outer loop adjacent thereto forming a portion of an outer leg of the core and adjacent said portions jointly forming a winding leg of said core, the height of al1 the core sections being the same in a direction parallel to the core legs, the total cross sectional height of the magnetic strip laminations of the loops of the innermost core section being greater than that of the loops of the core sections disposed outward therefrom, the arrangement being such that the winding legs of said core formed jointly by the core sections are of cruciform cross section and the yokes of said core formed jointly by the core sections are of approximate semicruciform cross section and lthe yokes of the inner loops of certain of the core sections are in side-by-side relation with the yokes of the outer loop of a core section adjacent thereto.

References Cited in the file of this patent UNITED STATES PATENTS 2,431,128 Llllk NOV. 18, 1947 FOREIGN PATENTS 636,986 `Great Britain I May 10, 1950 660,965 Great Britain NOV. 14, 1951

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