US2300964A

US2300964A - Magnetic core structure

Info

Publication number: US2300964A
Application number: US376406A
Authority: US
Inventors: Henry V Putman
Original assignee: Westinghouse Electric and Manufacturing Co
Current assignee: CBS Corp
Priority date: 1941-01-29
Filing date: 1941-01-29
Publication date: 1942-11-03
Anticipated expiration: 1959-11-03

Description

Nov. 3, 1942. H v PUTMAN 2,300,964

MAGNETIC CORE STRUCTURE ATTO Nov. 3, 1942. H v, PUTMAN 2,300,964

MAGNETIC CORE STRUCTURE Filed Jan. 29, 1941 2 Sheets-Sheet 2 F1919. ffy/0. l 2, u n A "la a3-.94 32 ...r/33 N34 (-/77 mx ik, .1371* 4/38/.9

f l D l' 23 4Z 4Z 44 25 43 L/ 4 2/ 2/ fig: f1 9.' /Z 1 W-Zmt- V W j i l/ 7n 36-37 35 33 ffy' i /Cjgr /4 B G ntf G "700 fj, f 7m 66D/7ppanenA/af lbf/'b' 1600 5.6 Half, 1 lp /wgo nyu/0f "5" e/z waff; '500 lum ,40 Z/zolgoarmA/r/Ms vlfw /2000 IODOO [0000 sooo ma 4D D /20 [60 290 240 260 O M0 00 i200 /609 tm Z400 'fr ue Waffe /gaparen Neff: WITNESSES: INVENTOR Hen/y 1./ Puf/nan ATTORNEY Patented Nov. 3, 1942 MAGNETIC CORE STRUCTURE Henry V. Putman,

Sharon, Pas, assignorto Westinghouse Electric a Manufacturing Company,

a corporation of Pennsyl- East Pittsburgh, `Pa.,

vania Application January 29, 1941, Serial N0. 376,406

6Claims.

My invention relates to magnetic core structures for use in electrical induction apparatus such as transformers.

In electrical induction apparatus, such as distribution or power transformers, it has been the usual practice to employ a core structure of magnetic material formed of a stack of layers of thin sheets or laminations of magnetic material such, for example, as hot-rolled silicon steel. A portion of the magnetic structure is employed as a winding leg about which the copper circuit conducting windings of the transformer are wound. In order to keep the cost of the transformers as low as possible, it is desirable that the length of the mean turn of the current carrying conductor about the winding leg -be as small as practicably consistent with permissible losses and power performance of the transformer.

The smaller the cross-section of the magnetic material the greater will be the flux density necessary to develop a given electromotive force and the greater will be the exciting current flowing in the primary winding. The permissible exciting current for a given size of transformer has been a factor in limiting the permissible reduction in the cross-section of the magnetic material.

The exciting current is made up of two components or parts, namely, the magnetizing current that is necessary to force the lines of magnetic flux through the iron circuit of the core, and the iron loss current necessary to supply losses of energy in the iron core structure, such as those due to hysteresis and eddy current losses. The component of magnetizing current is controlled very largely by the permeability of the material used in the core structure, and the watts loss component of current is controlled by the iron loss characteristic of the material.

The magnetic iron or steel usually employed in transformer core structures is a high grade hotrolled silicon steel having a value of permeability and watts loss per cubic volume of substantially constant value, whether the lines of magnetizing flux pass through the steel in the direction of rolling, or pass in other directions up to a considerable departure from the direction of rolling. More expensive steels, such as Hipersil, have been developed having much higher permeability and much lower losses in the direction of rolling. In such material the permeability varies considerably as the direction of the lines of magnetizing force vary from the direction of rolling of the material and is the highest in the direction of rolling. The watts loss per cubic volume or per cubic weight of the material likewise varies markedly as the direction of magnetizing flux varies from the direction of rolling of the material and is lowest in the direction of rolling.

The permeability of the new steel in the direction of rolling is considerably higher at operating densities (say 13 to 15 kilogausses per square centimeter) than the permeability of commercial grades vof hot-rolled silicon steel at the same densities. Likewise the watts loss per unit volume or unit weight at operating densities when magnetized in the direction of rolling is lower than for commercial grades of hot-rolled silicon steel at the same density.

It will be seen, therefore, that if a transformer core is formed of sheet steel having a preferred or greater permeability and a preferred or lesser watts loss per unit volume when the flow of flux is through the steel in the direction of rolling than when it is through the material in other directions and the sheets are so arranged that the lines of magnetic flux pass through the steel in the direction of rolling, a higher permeability and lower watts loss will result, permitting the use of higher flux densities than are pennissible with the usual grade of silicon steel, and a reduction of the cross-section of the magnetic material otherwise required.

Magnetic cores have been built with a high grade steel having a preferred permeability and low watts loss in the direction of rolling in which the core structure is formed by winding a ribbon of the steel to form loops in which the lines of magnetic force travel in the direction of the ribbon. There are, however, certain applications in which it is desirable to form the core from stacks of sheets of laminated material, particularly in the larger sizes of core structures in which the continuous ribbon loops are not so readily adaptable.

It is an object of the invention to provide an electrical induction apparatus having a core structure made up of stacked sheets of steel having a preferred permeability or easiest magnetization in the direction of rolling so positioned that the lines of magnetization correspond substantially to the direction of rolling of the material.

It is a further object of my invention to provide a core structure of the above indicated character employing combination butt and lap joints adjacent the corners of a rectangular core structure in which the meeting edges are cut at an angle to the direction of the sheets to effect a high permeability of the core structure as a whole. and as low an iron loss as possible.

Other objects and advantages of the invention will be apparent from the following description of certain embodiments thereof, reference being had to the accompanying drawings, in which:

Figures l and 2 illustrate the manner in which magnetic flux passes through core members of different designs;

Figs. 3 to 7, inclusive, illustrate parts of the core structures of single phase transformer cores organized in accordance with the invention;

Figs. 8 to 12, inclusive, illustrate parts of a three-phase core structure; and

Figs. 13 and 14 show curves illustrating the comparative losses in two types of structures.

Fig. 1 illustrates a conventional arrangement of the laminatlons or sheet steel punchings commonly used in a core structure built from high grade hot-rolled silicon steel comprising in each layer of the core structure four punchings I, 2, I and l, arranged in the same plane, the punchings I and 3 being alike and the punchings 2 and I being alike. The several punchings are rectangular in shape, the punching I extending from Ia to la', the punching 2 extending from 2a to 2a', the punching 3 extending from 3a to la', and the punching 4 extending from 4a to la', so that one end of each of the four punchings is adjacent the side of its adjoining punching. In this form of construction, position of the four punchings will be shifted in the adjacent layer to provide an overlap between the punchings in the adjacent layers so that the punching I will extend from Ib to Ib', the punching 2 from 2b to 2b', the punching 3 from 3b to 3b', and the punching 4 from 4b to 4b', 4the punchings in the second described layer forming the same general pattern as those in the rst described layer but with the positions of the adjoining edges shifted from the full line locations Ia, 2a, 3a and la to the dotted line positions Ib', 2b', 3b and 4b.

In the structure shown in Fig. 1, the core legs 2 and l may be winding legs of the core structure about which the primary and secondary windings of the transformer are placed and the connecting portions I and 3 form the yokes connecting the winding legs to complete the magnetic circuit of the core structure. In this structure the lines of magnetic iiux pass lengthwise through the straight lportions of the legs and yoke, and flow about the corners of the structure something after the fashion indicated by the arrowed lines 8, so that near the ends of the punchings the flux must pass sidewise of the members across the grain of the steel. This construction is entirely satisfactory for cores ernploying conventional hot-rolled steel where the permeability is not much different at right-angles to the direction of rolling from the permeability in the direction of rolling, but experience has shown'that it is unsuited to the use of the new type steel having a much greater preferred permeability and lower watts loss when the lines of magnetic flux pass through the material in the direction of rolling than when these lines of flux pass through the material in other directions.

'For such new type steel, in order to make efcient use of the properties of the material, it is necessary to employ a construction in which the lines of flux follow the direction of rolling, or easiest magnetization, at all points. In Fig. 2 of the drawings it will be noted that the adjoining edges of the several sheets 1. 8, 9 and III comprising a layer of the core material are cut on a diagonal or at a angle to the direction of the sheets. In this core the flux will tend to follow the direction of easiest magnetization, as shown by the arrow lines II, although this may not be the shortest path around the corner. By following this path of easiest magnetization or least resistance, the lines of flux avoid the necessity of passing crosswlse to the direction of rolling near the ends of the sheets of each lamination.

The merit of the construction in which the adjoining edges of the laminations are cut at a diagonal to the direction of the sheet after the principle illustrated in Fig. 2, as compared to the conventional butt and lap joint illustrated in Fig. l, is readily shown by reference to the curves in Figs. 13 and 14. In Fig. 13, the values for true watts loss are plotted as abscissa for vary ing flux densities, plotted as ordinates and indicated as kilogausses per square centimeter. Curve I shows these values as obtained from tests on a core having anglar cut joints corresponding to the principle illustrated in Fig. 2, and curve II shows the values obtained from tests on a core having the butt and lap joint de sign shown in Fig. 1, the two cores being identical constructions, except for the different manner of making the joints. These cores were of the same identical material, that is, the new steel above referred to having the easiest magnetization in the direction of rolling, which is the direction of the sheets or laminations as as sembled in the core. It will be noted, for example, that, at a flux density of 15 kilogausses per square centimeter, the core loss lfor the angular cut joint construction shown in Fig. 2 is 156 watts, and for the construction shown in Fig. l the core loss is 212 Watts, so that the Fig. 2 construction has a watts loss that is TS1/2% of that for the Fig. 1 construction.

Fig. 14 shows two curves, III and IV, similar to Fig. 13, except that the abscissa represents apparent watts, including both the power and reactive losses in the core, or the exciting voltampere characteristic of the steel. The apparent watts is considerably larger than the true watts because the wattless component of exciting current is relatively large. The wattless component is also more sensitive to differences in flux density. In Fig. 14 the curve III represents the voltampere characteristic of the core having the diagonal or angular cut joints as in Fig. 2, and the curve IV shows the volt-ampere characteristic of a similar core except built with the lapped I-plate construction shown in Fig. l, the two curves being for the same two cores as are represented by the curves in Fig. 13. It will be noted from Fig. 14 that at a flux density of 15 kilogausses the apparent watts for the I-plate design of Fig. 1 is 2120 and the apparent watts for the Fig. 2 angular cut joint design is 660 or 31% oi' the apparent watts for the Fig. 1 construction.

This principle is made use of in the practical core structures shown in the following figures and the punchings are so arranged that the adjoining edges in different layers are so shifted as to form a combination butt and lap joint having a lower joint reluctance than if all the layers were built as in Fig. 2 in which the diagonal lines forming the junctions between the layers would be superimposed upon each other.

Referring to the core structure shown in Fig. 3 comprising four punchings I4, I5, I6 and Il in each layer of the core, the punchings I 4 and Il are alike and are shown in detail in Fig. 5, and

the punchings Il 1nd I1 are alike and are shown in detail in Fig. 4. Thearrangement oi' the four members oi' the core structure, that is, the two winding leg members made up from stacks of laminations I and I1, and the yoke members made up from stacks of laminations I4 and Il, form a window having a width W and a height L. The width oi' the laminations I4 and I9 is slightly greater than that of the laminations I5 and I1, the latter being indicated by the dimension N and the former by the dimension N-I-n. The inner edge of punchings I4 and I6, that is indicated in Fig. 5 as W-m, is likewise less than thewidth o! the window, the dimension m corresponding to the amount that the inner end of the diagonal edge of the punching shown in Fig. 5 is oil'set from the adjacent window corner as indicated in Fig. 3. It will also be noted that the inner edge oi' the punchings I5 and I1 is somewhat longer than the length of the window as indicated in Fig. 4 by the dimension L-I-n. Referring to the layer represented by full line positions of the punchings I4, Il, I6 and I1 in Fig. 3, it will be noted that the adjoining edges between laminations I8 and I1 extend i'rom a point I 9 that is offset from the outer corner of the core member by the dimension n to the point I9 corresponding to the corner of the window, and that at the opposite end of the punching I8 the adjoining edges between laminations I 5 and I6 extend from the point 2| at the outer corner of the core structure to a point 22 that is offset from the inner corner of the window by the dimension m. It will also be noted that the diagonally opposite corners are alike, that is to say, that in the upper right and lower left corner of the structure as shown in Fig. 3, the junction between the adjacent edges of the laminations shown in full lines extends outwardly from the corner of the window to a point oil'set from the outer comer of the structure, while in the lower right and upper lei't corners of the structure the adjoining edges of the laminations extend from the outer corner of the structure to a point oil'set from the inner corner of the window. It will also be noted that in the adjacent layer in which the adjoining edges between the laminations in the same layer are indicated in dotted lines, the locations of the adjoining edges are reversed so that at each corner where the junction of one layer starts outwardly from the corner of the window, the junction of the next layer starts inwardly from the outer corner of the structure. In effecting this reversal in the location of the adjoining edges between the laminations of a layer, the punchings I4, I6, as shown in Fig. 5, will be turned end for end so that the offset portion shown by the dimension n at the lower lei't of Fig. 5 will, in an adjacent layer of laminations, be at the lower right in Fig. 3 and will likewise alternate from the upper right to upper left position for the lamination I4. The punchings I 5 and I1 are alike at both ends and are shifted lengthwise in adjacent layers an amount indicated by the dimension n, so that for the punching I5, for example, in the first described layer represented in full lines in Fig. 3, this member will extend from points I9 and I9 at the upper end to the points 2| and 23 at the lower end, while in the second described layer of laminations indicated in dotted lines it will extend from the points 2I and 23 at the upper end to points I8 and I9 at the lower end. The successive layers of the lamination I1 vary in the same manner as for IB, the lamination I1 being in its lower position in the layer 75 3 in which the lamination I5 is in its higher position and vice versa.

Fig. 6 shows an arrangement of

laminations

24, 2B, 29 and 21 which is, in general, similar to that shown in Fig. 3 and in which punchings 2B and 21 correspond to that outlined in Fig. 4. and the punchings 24 and 29 correspond to that outlined in Fig. '1. The punching shown in Fig. 7 diilers from that shown in Fig. 5 in that the inner edges of the punchings 24 and 29 are of a length W corresponding to the full width of the window and extend at right angles for a dimension n so that the sloping portion p is the same at both ends of the punching corresponding to the dimension p of the diagonal edge of the punching shown in Fig. 4. With this arrangement of the upper and lower punchings which are used to build up the yoke portions of the core the small voids represented by the triangles I9, 22, 23 in Fig. 3 are done away with and the core structure presents a smooth inner surface about the comers of the window.

It will be noted that the constructions shown in Figs. 3 and 6 are such that although the acijacent edges of the sheets in the several punchings of adjacent layers or laminations of the structure are staggered so as to provide the butt and lap joints at the corners of the structure, the inner and outer edges of the layers oi punchings are in alignment, that is, the inner edges of the punchings forming the window of the structure line up with the corresponding edges of the adjacent layers of the magnetic material,` and the outer edges of the several laminations correspondingly align so as to make a compact structure having the minimum dimensions for the given amount of material used. This construction makes it possible to tap or block the core, that is, to apply pressure to the edges of the sheets in the stack in order to get the edges of all of the sheets lined up.

It will also be noted that the yoke members y above and below the windows as shown in Fig. 8 have a width of N-I-n, whereas the vertical or winding leg members of the core have a width N so that the yoke members are widened with respect to the winding leg members, that is, have a' greater cross-section than the winding leg members so that the flux density therein is less. This widening can be increased as desired by changing the angle of cut between the adjacent members of the sheets from that of as shown, without introducing any cross flux. The` flux will therefore follow the lines of easiest magnetization as shown by the arrowed lines II in Fig. 2.

It will also be noted that by cutting the adjoining edges of the sheets of magnetic material to provide the angular cut joints shown in Figs. 2, 3 and 6, the reluctance of the joint between adjacent sheets will be lowered because the length of the adjoining edges is increased by the ratio of the \/2 to 1 with respect to the right-angle cuts shown in Fig. 1. This increase in the length of the adjoining edges between adjacent members causes the flux density at the joint to be correspondingly reduced. This decrease in the reluctance oi the air gap in the diagonal butt and lapped joint shown in Figs. 3 and 6 greatly increases the efficiency of the core at the gap over that of a core having the conventional butt and lapped joints shown in Fig. i.

In Fig. 8 an arrangement of punchings is illustrated for building up a core having three legs such as might be used in a three-phase transformer. The central leg 32 is built up from punchings having the outline shown in Fig. 9 and the outer legs 33 and 3l from punchings having the outline shown in Fig. 10, while the

yoke members

35, 33, 3l and 33 are built up or punchings shaped as shown in Figs. il and 12. The

outer leg members

33 and 34 alternately change their position in different layers of the core structure in the same manner as the two outer leg punchings I6 and I1 in Fig. 3 extend- Ing from points 2l and 23 at the top to points Il and I3 at the bottom in one layer, as shown in full lines for the member 3|, and from the points I3 and I9 at the top to points 2I and 23 at the bottom as shown in dotted lines for the member 3l in Fig. 8. 'I'he outer edges of the member` 32, like the inner edges of the members 33 and 3l are longer than the windows in the core structure by the dimension n, the corner 42 on one edge being advanced by the dimension n lengthwise beyond the corner II on the opposite side. The slopes of the edges from the corners 3| and I2 to the points I3 at the ends of the members 32 are at an angle of 45 degrees to the direction o! the member 32. By placing the members 32 in the core so as to alternately vary the side that is uppermost, the end point 43 varies its position from point 43 to point M as shown in Fig. 8 and the points II and 42 shift sides. The point Il lcoincides with the corner of the window and the point I2 is a distance n vertically distant therefrom. Two diil'erent shaped punchings are required for the yoke pieces as shown in Fig. 11 and 12, the two members alternating from side to side.. For example, the member in Fig. 11 is shown in full lines at 31, and 36 in Fig. 8, and the member shown in Fig. 12 is shown inv iull lines at 35 and 38 in Fig. 8. In the adjacent layer positions, the relative positions of the members are reversed so that the member in Fig. 11 is at 35 and 3B as shown in dotted lines and the member in Fig. 12 is at 31 and 36 as shown in dotted lines. It will be noted that In Fig. 12 the punching on its inner side is the same length W as the width of the windows, and the edges adjoining adjacent punchings in the same layer extend outwardly from the inner corner oi the window to a point onset from the corner of the punching or core structure by the dimension n. I'he punching shown in Fig. 11 has a dimension on its inner edge of W-2m, so that when placed in position as shown in the full lines in Fig. 8 as yoke members 31 and 36, both inner corners are oIIset irom the corners ot the window by the dimension 1n, and the edge adjoining the adjacent outer leg punching in the same layer extends from its offset point to the outer corner of the core structure.

In the full outline of the sheets shown in Fig. 8, the outer leg member 33 is shown in its lower position extending from the points I8 and I9 at the top to points 2I and 23 at the bottom, while the leg 3l is in its upper position extending from points 2i and 23 at the top to points I9 and I8 at the bottom, and the center leg member 32 is in a position in which the corners 42 are at the upper right and lower left of the member, as viewed in Figs. 8 and 9, and the corners 4I are at the upper left and lower right as viewed in these figures. Also the punching shown in Fig. l1 is shown in the full line position in Fig. 8 as at the bottom of the left window and at the top of the right window, while the punching shown in Fig. 12 is shown at the top of the left window and at the bottom of the right window. In the layer of laminations having their adjoining edges shown in dotted lines, the positions ot the several punchings are shifted from the full line positions, the punching in the ieg 33 being shown in the upper position and the punching in the leg 34 being shown in its lower position, while in the middle leg 32 the position of the punching is reversed so that the end points 43, as shown in Fig. 9, occupy the positions Il as shown in Fig. 8, and the corners I2 are in the upper left and lower right corners of the punching as viewed in Fig. 8. Likewise the positions of the punchings shown in Figs. 11 and 12 are reversed, the punching of Fig. 11 being shown in dotted lines below the right-hand window and above the left-hand window, and the punching of Fig. 12 being shown in dotted lines below the left-hand window and above the right-hand window. This arrangement of the alternate layers oi' punchings used to build up the core structure provides an arrangement in which the several sheets or laminations employed are so positioned in the structure that the lines of magnetic ilux extend lengthwise of the members up to the adjoining edge ci' the next adjacent member in the same layer of punchings comprising the core, and the adjoining edges of the punchings in adjacent layers are ofl'set fromeach other to provide a diagonal overlap adjacent the corners oi' the windows in a manner similar to the arrangements shown in Figs. 3 and 4.

It will be apparent to those skilled in the art that many modiilcations in the detailed arrangements oi the parts of the structure may be made within the spirit of my invention and I do not wish to be limited otherwise than by the scope of the appended claims.

I claim as my invention:

l. In an electrical apparatus, a substantially rectangular magnetic core structure built up from layers of sheets of magnetic material formed of steel having preferred greater permeability and lower watts loss in the direction of rolling than in other directions and so positioned in the structure that the direction of magnetization is substantially coincident with the direction of rolling.I each layer comprising a plurality of sheets having adjoining edges adjacent the corners of the core structure and arranged about a substantially rectangular window, the junctions of all adjoining edges at the outer corners of the magnetic core extending along continuous straight lines at angles oblique to the direction oi' rolling so that substantially all iiux in each sheet passes between junctions at opposite ends of each sheet in a continuous straight path along the direction of rolling, the line of junction be tween said adjoining edges of the sheets of magnetic material of one layer at any corner of the window extending outwardly from the inner angle of the window. and the line of junction between the adjoining edges of any adjacent layer at the same corner extending outwardly :from a point offset from the inner angle of the window at an angle to provide an overlap of substantially constant width between the adjacent junction lines of said adjacent layers.

2. In an electrical apparatus, a substantially rectangular magnetic core structure built up from layers of sheets of magnetic material formed of steel having preferred magnetic properties in the direction of rolling and so positioned in the structure that the direction of magnetization is substantially coincident with the direction of rolling. each layer comprising a plurality of sheets having adjoining edges adjacent the corners of the core structure and arranged about a substantially rectangular window, the junctions of all adjoining edges at the outer corners of the magnetic core extending along continuous straight lines at angles oblique tothe direction of rolling so that substantially all flux in each sheet passes between junctions at opposite ends of each sheet in a continuous straight path along the direction of rolling, the line of junction between said adjoining edges of the sheets of one layer at a pair of alternate corners of the core extending from the inner angle of the window,

the line of junction between the adjoining edges of said layer at the other pair of alternate corners of the core extending from a point offset from the inner angle of the window, the sheets in any adjacent layer being so arranged that the points vfrom which the lines of junction extend are the reverse of those of the first-named layer, the junction lines inthe several adjacent layers at any one corner of the core extending in parallel directions to provide an overlap of substantially constant width between the junction lines of the adjacent layers.

3. In an' electrical apparatus, a magnetic core structure built up from layers of sheets of magnetic material formed of steel having preferred magnetic properties in the direction of rolling and so positioned in the structure that the direction of magnetization is substantially coincident with the direction of rolling, each layer comprising a plurality of sheets arranged to'form a substantially rectangular window, the junctions of all adjoining edges of any of the sheets in any layer extending along continuous straight lines at angles oblique to the direction of rolling so that substantially all flux in each sheet passes between junctions at opposite ends of each sheet in a continuous straight path along the direction of rolling, the sheets on two opposite sides of said window being of a diierent width than the sheets on the other two opposite sides ofthe window, the lines of junction between the adjoining edges of the sheets comprising one layer being arranged at two diagonally opposite corners of the window to extend from points olset from the outer corners of the structure along continuous straight lines to the inner corners of the windows, and the lines of junction between the adjoining edges of the sheets at the two remaining diagonally opposite corners in the same layer extending from the outer corners of the structure along continuous straight lines to points offset from the corners of the Window, the location of the lines of junction between all adjoining edges being reversed in adjacent layers between the two sets of diagonally opposite corners to provide an overlap of substantially constant width between the adjacent lines of junction of the adjacent layers.

4. In an electrical apparatus, a magnetic core structure comprising leg members and yoke members at angles of substantially 90 to each other to form a closed magnetic circuit about a rectangular window, the leg and yoke members being built up of layers of sheet members formed of steel having preferred magnetic properties in the direction of rolling and so positioned in the structure that the direction of magnetization is substantially coincident with the direction of rolling, each layer comprising a plurality of sheets having adjoining edges between leg and yoke members adjacent the corners of the corel the sheets in the several layers having their inner and outer lineal edges in alignment and the junctions of all adjoining edges between the sheets at substantially 45 to the lineal edges of the members so that substantially all flux in each sheet passes between junctions vat opposite ends of each sheet along a continuous straight path in the direction of rolling, the junctions of all the adjoining edges of said sheet members in a given layer being oiset from, and parallel to, the junctions of the adjoining edges of the sheet members in an adjacent layer at the corresponding corners of the core to provide an overlap of substantially constant width between the adjacent junction lines of the adjacent layers, said yoke members being of a dilerent width than the leg members, the members of the greater width having the ends adjoining adjacent members in the same layer offset dierently at the opposite ends thereof with respect to the corners of the structure.

5. In an electrical apparatus, a magnetic core structure built up from a plurality of layers of sheets of magnetic material formed of steel having preferredrmagnetic properties in the` direction of rolling and so positioned in the structure that the direction of magnetization is substantially coincident with the direction of rolling, said core structure comprising three winding leg members and yoke members connecting the ends of the leg members to form a substantially rectangular core having two substantially rectangular windows, the junctions of all adjoining edges between the sheets of the leg and yoke members in any layer at the outer corners of the core extending along continuous straight lines at oblique angles to the direction of the sheets so that substantially all iux in each sheet passes between junctions at opposite ends of each sheet in a continuous straight path along the direction of rolling, the line of junction between the adjoining edges of the sheets of one layer at a cor'' responding end of both windows extending outwardly from the inner angles of the windows and the line of junction between the adjoining edges of the sheets of the same layer at the other ends of the windows extending outwardly from points olset along the yoke from the inner angles of the windows, the lines vof junction between the adjoining edges of the sheets in different layers at any corner extending in parallel lines outwardly from the corner, the positions of the parallel lines being reversed in adjacent layers to provide an overlap of substantially constant width between the adjacent junction lilies of the adjacent layers.

6. In an electrical apparatus, a magnetic core structure built up from a plurality of layers of sheets of magnetic material formed of steel having preferred magnetic properties in the direction of rolling and so positioned in the structure that the direction of magnetization is substantially coincident with the direction of rolling, said core structure comprising three leg members and yoke members connecting the ends of the leg members to iorm a substantially rectangular core having two substantially rectangular windows, the junctions oi' all adjoining edges between the sheets forming the leg and yoke members in any layer at the outer corners of the core extending along continuous straight lines at oblique angles to the direction of the sheets so that substantially all iiux in each sheet passes between junctions at opposite ends of each sheet in a continuous straight path along the direction o1' rolling, the lines of junction between all the adtion o! the core having their inner edges o! a length less than the width of the window. the sheets in adjacent layers of the yoke portion having diierent lengths of the inner edges so that the junction lines in the several adjacent layers at any one corner of the core extend in parallel directions to provide an overlap of substantially constant width between the Junction lines oi the adjacent layers.

HENRY V. PU'IMAN.