US2812505A - Magnetic core for stationary electrical induction apparatus - Google Patents

Description

Nov. 5, 1957 s. G. soMERvlLLE 2,812,505

MAGNETIC CORE FOR STATIONARY ELECTRICAL INDUCTION APPARATUS Filed May 27, 1952 3 Sheets-Sheet 2 Inventor: Gareth G. Somervle,

bg His Attorney- United rates Patent Ottico MAGNETIC CORE F OR STATIONARY ELECTRI- CAL INDUCTION APPARATUS Gareth G. Somerville, Pittsfield, Mass., assignor to General Electric Company, a corporation of New York Application May 27, 1952, Serial No. 290,280 9 Claims. (Cl. 336-5) This invention relates to magnetic cores for stationary electrical induction apparatus, and more particularly to three-phase, three-leg magnetic cores of the plate type lhaving longitudinally divided leg and yoke members.

size it is advantageous to assemble the core sections in such a manner as to allow the magnetic iiux to follow as much as possible the line of the grain of the ferromagnetic core laminations.

In the normal lapped core construction it can be seen that the magnetic flux must cut crosswise of the most favorable direction at the ends of the laminations in traversing from one core leg to the next. Due tothe crosswise ow of magnetic flux at the corners or" the core, relatively high losses occur at these points.

One construction which has been suggested to reduce the core loss at the corners of cores constructed of oriented magnetic material is to split the laminations of each core leg and yoke element into more than one lamination in each layer. Such a construction has been shown by Patent 2,467,823, Gordy, and 2,467,824, Granfield, both assigned to the same assignee as the present application.

Splitting the lamination layers has numerous advantages. Large cores having relatively Wide leg and yoke portions can be made from relatively narrow magnetic strips or punchings so that it is unnecessary to stock as wide a range of widths of magnetic material. Furthermore, the total area of overlap at the corners is less than with conventionally lapped corner joints of full width laminations, thus decreasing the effective area of crossgrain flux travel. Another advantage is that splitting of the laminations facilitates end sheeting of the cores, particularly ot the larger sizes, because it requires very much more force tcinsert a wide yoke lamination than it does to successively insert two narrow laminations between the ends of the leg laminations. A further advantage of split punchings is that it provides an improved iux path as compared with standard overlapped joints of cores in which the laminations are not spiit without requiring the additional cost and complexity of mitering the joints. This is particularly important when highly oriented magnetic material is used. As the core is split, it is more diilicut for magnetic flux to travel across the grain or crosswisc of the most favorable magnetic direction as the barrier produced by the split prevents such cross migration of the flux. Consequently, the tiux travels with the grain throughout a greater percentage of the mean length turn of the core. Also, the

2,812,505 Patented Nov. 5, 1957 narrower laminations reduce the eddy current losses in the core.

It is an object of this invention to provide a new and improved magnetic core structure of the split punching type.

It is a further object of this invention to provide a new and improved three-phase, three-leg magnetic core of the split punching type having an improved butt and lap joint arrangement.

It is a still further object of my invention to provide a magnetic core of the split punching type in which the split laminations are spaced from one another to provide a duct for passage of a cooling fluid.

In accordance ywith these objectives, this invention provides a split core arrangement for a three-phase, threeleg magnetic core structure in which the split punchings are so arranged as to form an outer core separated by an air gap from an inner core, with a connection between the outer and inner core being made at the center leg portion of the two respective core sections, and employing butt joints between mating laminations, with the yjoints of adjacent layers being overlapped by axially shifting adjacent lamination layers with respect to each other.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and use, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 represents a three-leg magnetic core constructed in accordance with my invention; Fig. 2 is an exploded view of several of the laminated layers of the core of Fig. l; Fig. 3 is a three-leg magnetic core having a modified type of interconnection between the inner and outer cores; Fig. 4 is an exploded View of several of the laminated layers of the core of Fig. 3; Fig. 5 is an elevation View, partly cut away, of a three-phase core in accordance with still another modification of my invention; while Fig. 6 is an exploded view of the core of Fig. 5.

Referring now to Fig. l, there is shown a magnetic core 1 comprising a plurality of layers of laminated magnetic material, with the laminations in each layer defining an outer core 2 and an inner core 3. The magnetic core 1 is provided with three leg members, the corresponding ends of the three leg members being connected together by yoke members. In the embodiment shown, each respective leg and yoke member comprises two laminations in each layer extending longitudinally of the respective leg or yoke element. Obviously more than two laminations could be used in each leg and yoke member. The left-hand leg of the three-phase core, with respect to the view shown in the drawing, comprises a longitudinally-extending lamination 4 in the outer core section and a longitudinally-extending lamination 5 in the inner core section. The right-hand leg of the core, with respect to the view shown in the drawing, comprises a longitudinally-extending lamination 6 in the outer core section and another longitudinally-extending lamination 7 in the inner core section. The center leg of the core for each layer comprises two longitudinally-extending laminations 8 and 9 lying in the inner core section and two insert members 1t) and 11 which respectively connect the upper ends of laminations 8 and 9 and the lower ends of laminations 8 and 9 to the outer core section 1, the respective insert members 10 and 11 extending to the outer edge of the respective yoke members for the upper and lower ends of the outer core section 1.

In accordance with my construction, the laminations of the yoke and leg members in both the inner and outer cores are respectively butt-jointed in their mating laminations, the butt joint arrangement being the same in each layer. As will be explained more fully hereinafter, I prefer to make all of the joints between the yoke and leg laminations lie in one direction, such as, for example, transversely of the longitudinal axis of the core, and then shift adjacent layers along the'longitudinal axis of the core inrorder to obtain the desired offset between joints for adjacent layers. Conversely, I could make the buttV joints between the yoke and' leg members extend Yparallel to the longitudinal axis of the core and then obtain the desired odset between joints for adjacent layers by shifting adjacent layers along the transverse axls of the core. Y

In accordance with my construction, the yoke portions of the core are butt jointed to the insert lamination mem,- bers which connect the inner and outer core sections by butt joints which extend parallel tothe longitudinal axis of the core, and in this case the desired offset between joints for adjacent layers is obtained by varying the width of the insert members joining the outer and inner'core sections. v 'Y Y VThe upper yoke portion of the outer core section comprises a lamination 12 which is bntted to the upper 'edge of leg lamination 4 and extends from the left-hand edge of the core to the upper left-hand edge of upper insert lamination 1t), to which it is butt jointed; and a lamination 13 extending from a butt joint with the right-hand edge of upper insert member 10 to the right-hand edge of the core, where it is butt jointed to the top edge of leg lamination 6. The yoke portion of the outer core section at the lower end of the core comprises a lamination 14 extending from the bottom left-hand edge of the core where it is butt jointed to the bottom edge of leg lamination 4 to the lower left-hand edge of lower insert member 11, to which it is butt jointed; and a lamination 15 extending from a butt joint with the lower right-hand edge of insert member 1l to the right-hand edge of the core where it is butt jointed to the bottom edge of leg lamination 6. The yoke portion of the inner core at the upper end of the core comprises a lamination 16 which is butt jointed to the upper edge of leg lamination 5 and which extends from the left-hand edge of lamination 5 to the lower portion of the left-hand edge of insert member lil, to which it is butt jointed; and a lamination 17 extending from a butt joint with the lower portion of the right-hand edge of insert memberlt) to the righthand edge of leg lamination 7, lamination 17 being butt jointed to the top edge of leg lamination 7.

The yoke portion at the lower end of the inner core comprises a lamination 18 butt jointed to the bottom edge of lamination 5 and extending from the lower lefthand edge of lamination 5 to the upper left-hand edge of insert member 11 to which it is butt jointed; and a lamination 19 extending from a butt joint with the upper right-hand edge of insert member 11 to the Vright-hand edge of leg lamination 7, lamination 19 being butt jointed to the bottom edge of leg lamination 7.

It will be noted that the inner and outer core sections are separated from one another around the entire periphery of the `core by an air gap, except where the insert members 10 and 11 join the inner and outer core sections.

Each of the respective layers comprising the magnetic core is similar to all the other layers except that the respective insert members for adjacent layers corresponding to insert members 10 and 11 are made of slightly different width in order to obtain an offsetting between adjacent layers of the joints between the insert members and the other portions of the core.

Referring to Fig. 2, which shows an exploded view, partially cut away, of three successive layers of the magnetic core, these respective layers having been designated by the numbers 2i), 21 and 22, it will be seen that the respective insert members 23, 24 and 25 are of different width fromV one another in order to obtainan oifsetting between'successive layers -of the joints between these respective insert members and the yoke laminations with which they are in abutting relation. The respective insert members 23, 24 and 25 are of varying width, the widths being so adjusted that the joints for the respective insert members are not superimposed upon one another. In the particular embodiment illustrated in Figs. l and 2, the insert members 23, 24 and 25 become successively wider, with'this sequence of steps then being repeated a pluralityY of times. However, it will be understood that my invention is not restricted to the use of any particular width sequence of insert members or to any particular number of steps in such a width sequence.

A further feature of my construction is the arrangement by which I obtain an oifsetting or overlapping of joints lying in adjacent layers other than the joints between the insert members and the yoke laminations. Since each layer of the core is similar to every other layer except for the difference in widths of the respective insert members, hereinbefore explained, it is desirable to provide an arrangement which will permit oisetting of the joints between the respective abutting leg and yoke laminations. As previously explained, the offsetting of the jointsof the insert members is obtained by varying the width of the insert members. The shifting of the corresponding leg and yoke lamiination joints for adjacent layers is accomplished by shift- ,ing each layer along an axis parallel to the longitudinal axes of the core legs so that each layer is offset with respect to the layers immediately adjacent it. This axial .offset arrangement of the layers with respect' to one another may be in a sequence of three steps, as shown in the embodiment of Fig. l, in which three'laye'rs are progressively offset from one another in the same direction, with the offset sequence being repeated a plurality of times. However, it will be understood that I do not intend to be restricted to any particular offset arrangement or to any particular number of steps in the offset sequence. Y

There is shown in Figs. 3 and 4 a magnetic core constructed of split laminations of the same general type as those shown in Figs. 1 and 2, but provided with a different arrangement for connecting the inner and outer core sections. Since the upper and lower halves of the magnetlc core structure of Figs. 3 and 4 are similar, only the upper half of the core structure is shown, and it will be understood that the lower half of the core is similar. The three-leg magnetic core 26 has an outer core section 27 and an inner core section 28. As will best be seen in the exploded view of Fig. 4, the magnetic core 26 of Fig. 3 is constructed of a sequence of layers 29, 30 and 31 which differ from one another only in the center leg portion of the core which connects the inner and outer core sections together. A Y

Referring to layer 29, it will be seen that the left-hand core leg, with respect to the View shown in the drawing, comprises a lamination 32 in the outer core section separated by an air gap from lamination 33 in the inner core section. The right-hand core leg, with respect to the view shown in the drawing, comprises a lamination 34 in the outer core section and a lamination 35 inthe inner core section. The center leg portion of layer 29 comprises a lamination 36 separated by an air gap from a lamination 37, each of these laminations extending to the upper edge of the core. Lamination 36 is located on the left-hand side of the center core leg, while lamination 37 is located on the right-hand side of the center core leg, with respect to the view shown in the drawing. The yoke portion of the core for layer 29 comprises a lamination 38 which is butt-jointed to the upper edge of leg lamination 32 in the outer core section and extends from the left-hand edge of the outer core section to the right-hand edge of leg lamination 34. In the inner core section a yoke lamination 39 is butt-jointed to the top edge of leg lamination 33 and extends from the left-hand edge of leg lamination 33 to the left-hand edge of center leg lamination 36; and yoke lamination 46 extends Vfrom the right-hand edge of center leg lamination 37 to the right-hand edge of leg lamina= tion 35, being butt-jointed to the top edge of leg lamination 35.

Layer 30 is the same as layer 29 as just described, except for the center leg portion of the core. Instead of having the center leg members extend to the outer edge of the core, as in the case of laminations 36 and 37 of layer 29, the center leg laminations stop short at the inner edge of the yoke laminations for the inner core section. In layer 3i) a short insert member 41 connects the outer core section to the center leg. Thus, insert member 41 extends from the inside edge of the yoke portion for the outer core section to the inner edge of the yoke portion of the inner core section, where it is in abutting relation with the top edge of the divided center leg. In layer 30, a Single yoke lamination 42 is preferably used in the yoke portion of the outer core section, lamination 42 extending from the left-hand edge of the outer core section entirely across the width of the core to the right-hand edge of the right-hand leg of the core.

The third laminated layer 31 is the same as layer 30 except that the insert member 43 which extends from the inner edge of the inner yoke section to the inner edge of the outer yoke section is of a different width than the corresponding insert member 41 of layer 30. Also, the overall width of insert member 43 is different than the overall width from the left edge of center leg lamination 36 to the right edge of center leg lamination 37 of layer 29. By thus staggering the widths of laminations 36 and 37 of layer 29, of insert member 41 of layer 30, and of insert member 43 of layer 31, the joints which these respective members make with adjacent yoke laminations in each of the respective layers are offset from one another in a manner which provides increased mechanical strength and improved magnetic properties in the joint region. In layer 31, a single yoke lamination 44 extends across the entire width of the core from the left-hand edge of the core to the right-hand edge of the core.

In order to obtain an offsetting of the transversely extending joints between yoke and leg lamination members, each lamination layer is shifted axially along the longitudinal axis of the leg members to obtain the desired offset relation. This offsetting of one layer with respect to another may be done in any desired manner. In the embodiment illustrated in Fig. 3 I have used a progressive three-step offset pattern in which each layer in the threestep sequence is progressively offset in the same direction, the sequence then being repeated a plurality of times.

There is shown in Figs. 5 and 6 a modified embodiment of my invention in which the inner and outer core sections are connected together at the center leg portion by the center leg laminations themselves, rather than by the use of insert members as shown in the embodiments of Figs. 1 4.

There is shown in Fig. 5 a magnetic core 45 having a left-hand core leg comprising laminations 46 and 47, a center core leg comprising laminations 48 and 49 and a right-hand core leg comprising laminations 50 and 51. A yoke lamination 52 extends across the entire width of the core from the left-hand edge of lamination 46 to the right-hand edge of lamination 51, lamination 52 being butt jointed to the top edge of laminations 46 and 51, respectively. Center leg laminations 48 and 49 extend to the bottom edge of yoke lamination 52. In the inner core section, the short yoke lamination 53 is butt jointed to the top edge of leg lamination 47 and extends from the lefthand edge of leg lamination 47 to the right-hand edge of leg lamination 48, to which it is butt jointed. A yoke lamination 54 extends from the right-hand edge of leg lamination 49 to the right-hand edge of leg lamination 5i), to the top edge of which it is butt jointed.

In the exploded view of Fig. 6, there are shown three successive layers 55, 56 and 57 of the core 45 of Fig. 5. Layer 55 is the uppermost layer shown and described in connection with Fig. 5. Layer 56 is similar to layer 55 except that the center leg laminations 58 and 59 of layer 56 are each respectively provided with notches in order to accommodate yoke laminations 60 and 61. By notching the leg laminations 58 and 59 as shown, the respective yoke laminations to which they are butt jointed are longer than the corresponding yoke laminations 53 and 54 of layer 55 and hence the joints between the yoke and leg laminations of layer 55 are offset from the corresponding joints for layer 56. In a similar manner, leg laminations 62 and 63 of layer 57 are notched to receive yoke laminations 64 and 65, respectively. The depth of the notches in leg laminations 62 and 63 of layer S7 are made deeper than the corresponding notches in laminations 58 and 59 of layer 56. Consequently, the joints between the yoke and center leg laminations of layer 57 are offset from the corresponding joints of layers 55 and 56.

While I have shown the use of a sequence of three successive layers 55, 56 and 57, in which the center leg laminations of the first layer in the sequence are not notched and the center leg laminations of the two succeeding layers are notched by successively greater amounts, it will be understood that my invention is not restricted to the three-step arrangement shown in Fig. 6 and that I may use a greater number of steps.

In order to obtain an offset between the transverselyextending joints lying in successive layers, I shift successive layers along the longitudinal axis of the core in the manner described in connection with the embodiments of Figs. -l-4. If the joints between the various leg and yoke members were to extend parallel to the longitudinal axis, then I would shift the successive layers along the transverse axis of the core to obtain the desired overlap between joints of successive layers.

It can be seen that I have provided a new and improved magnetic core of the ydouble core type in which each pair of mating leg and yoke laminations meet in a butt joint. I have provided an arrangement for offsetting the joints in successive layers in accordance with which successive layers are axially shifted with respect to one another, and have also provided the necessary offset at the center leg portion of the core by either using insert members of varying widths, or by using notched center leg laminations with successive layers having notches of varying depths.

Wherever I have shown the use of a single laminar layer in any particular arrangement, it will be understood that I may use a plurality of layers instead of using only one layer.

While there have been shown and described particular embodiments of my invention, it will be obvious to those skilled in the art that various changes and modications can be made therein without departing from the invention and, therefore, it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A three phase magnetic core of the plate type comprising a plurality of superposed layers of flat stacked magnetic material, said core comprising a center core leg member and two outer core leg members for the reception of electrical windings, each of said outer leg members being divided into at least two spaced apart stacks of magnetic laminations extending longitudinally of said respective core legs, a separate stack of yoke laminations joined to each end of each of the respective stacks of laminations of said outer core legs, said stacks of yoke laminations joining the respective opposite ends of said respective stacks of outer leg laminations to said center core leg, meeting leg and yoke laminations in said joined stacks of leg and yoke laminations being butt jointed to each other, said center core leg being connected at each of its ends to all of the stacks of yoke laminations at the respective ends of the core, the width of the portion of the laminations of said center core leg which are joined to the laminationsof said yoke members being diierent in successive-layers to provide a Vjoint overlap between successive layers at said'portion of said center leg, substantially all butt joints between the laminations of said outer core legs and said yoke members being parallel to each other, successive layers of said core'being shifted axially with respect to each other in a direction perpendicular to said butt joints between said yoke laminations andthe laminations of said outer core legs to provide an overlap between the successive layers at said joints.

2. A three phase magnetic core of the plate type comprising a plurality of superposed'layers of at stacked magnetic material, said core comprising a center core leg member and two outer core leg members for the reception of electrical windings, said leg members being joined together at their corresponding ends by yoke members, each of said leg and yoke members comprising in each of said layers at least two laminations extending longitudinally of the respective members and separated from each other by a duct extending for substantially the entire length of said respective members, the plurality of longitudinally separated laminations of each yoke and leg member in said plurality of layers defining at least two core Ysections separated from eachother by ducts extending lengthwise of said respective leg and yoke members, said respective core sections being connected to each other at the center leg member of said core to form closed magnetic paths between the outer core legs and said center-core leg, the laminationslying in said outer core legs being butt jointed to yoke members lying inthe same respective core sections as said laminations of said outer core legs, the width of the portion of said center leg which joins said core sections together being different in successive layers to provide a joint overlapbetweensuccessive layers at said portionof said center leg, substantially all `butt joints between yoke laminations and the laminations of said outer core legs being parallel toeach other, successive layers of said core being shifted with respect to each other in a direction perpendicular to said butt joints between said yoke laminations and the laminations 4of said outercore legsto provide an overlap between the successive layers at said butt joints.

3. A three phase magnetic coreof the plate type cornprising a plurality of superposed layers offlat stacked v`magnetic material, saidcore comprising a centerV core leg member and two outer core-leg members for the reception of electrical windings, each of said outer leg members being divided into atleast two spaced apart stacks of magneticlaminations extending longitudinally of saidrespective core legs, a separate stack of yoke laminations joined to each end of each of the respective stacks oflaminations of saidouter' core leg, said stacks of yoke laminations joining the respective opposite Yends of said respective stacks of outerleg laminationsrto s'aid center core leg, meeting legand yokerlami'nations in said joined stacks of center leg and outer leglaminations being butt jointed to Veach other, generally retangular shaped insert laminations in alignment with'said center c ore leg Vand having their longitudinal axes parallel to the longitudinal axis of said core, said insert members being positioned at opposite ends orr said center leg inthe respective laycrstof said core and magnetically joining together `t -e .yoke members at said center leg of .said core, insert members lying in successive layers Ahaving a different width transverse of said coreto provide an overlap between the joints of` successive layers,A substantiallywallbutt joints between the laminations of said outer core legs and said yoke members being parallel to each other, successive layers of said core being shifted axially .with v respect to each other in a direction perpendicular ,toY said lbutt joints between saidyoke laminations andthe laminationsV oflsaid cuter corele'gs toprovide van overlap betweenthe successive layersfand said joints'.

4Q A three phasel magnetic core fof the plate type comprising a plurality of superposed layers of flat stacked e magnetic material, said core comprising a center core leg member and two outer core leg members vforthe reception of electrical windings, said core legV members being joined together atl their corresponding ends Yby yoke members, each of'said leg and yoke Ymembers comprising in each of said-'layers atleast two laminations extending longitudinallyof the respective members and separated `from each other yby a duct extending for substantially the entirelength of said respective members, the plurality of longitudinally separated laminations of each yoke and leg member in said plurality of layers delining at least two core sections separated from each other by ducts extending lengthwise of said respective leg and yoke members, said respective core sections being connected to each other atthe center leg member of said core to form closed magnetic paths between the outer core legs and said center core leg, the Vlaminations lying Vin said outer core legs being butt jointedV to yoke laminations lying in the same respective core sections as said laminations lying in said outer core legs, said center leg member comprising at each respective end of said core relatively short magnetic insert members which are butt jointed to the laminations of Vsaid center leg member and also to the laminations of the yoke 'members at the respective ends of saidl coreto thereby connect said center leg member to said respective yoke members said insert members being substantially rectangular and having their longitudinal'axes parallel to the `longitudinal axis of said core, thewidths of insert members lying in successive layers being different to provide a joint overlap between successive layers, substantially all butt joints between yoke laminations and the laminations of said outer core` legs being parallel tvoeach other and to the same axis of rsaid core, successive layers of said core being shifted Vwith respect to each other in a direction perpendicular to tion of electrical windings, each of said core leg members comprising in each of said layers'an inner and an outer lamination lying in the same plane, said inner and outer laminations extending longitudinally of said respective leg members Yand beingv separated from each other -by a duct extending for substantially the entire length of said respective members, divided yoke members disposed at opposite ends of said core legs, each of said yoke members comprisingan outer and an inner yoke lamination, the respective outer laminations of said outer core legs being joined to each other at their'respective ends by said outer yoke laminations, the respective inner laminations ot"saidiouter vcore legs being respectively joined by' said inner yoke laminations to the respective one of said laminations of said center leg closest to said respective inner laminations of said outer core leg, said inner and outer yoke laminations of each ofsaid yoke members being spaced apartffrom each othertby a duct extending for substantially ,the Ventire length of said respective yoke laminatixnls,V said center leg laminations being Vjoined at the respective opposite ends of said core to said inner and outer yoke laminations, the portions of said center leg laminations extending between said inner and outer yoke membersat the'respective ends of said core having'notchesextendingtransversely of said center leg laminationsto receive said'inner yoke laminations, the center leg laminations lying' in'successive layershaving notches extending for different distances, substantially all joints between said outer leg members and said yoke Ymembers being butt joints extending parallel to Y leach other7 successive layers'of said core being axially joints between said yoke and said lammations of said outer leg in order to provide an overlapping between the joints of successive layers.

6: A three phase magnetic core of the plate type comprising a plurality of superposed layers of at stacked material, said core comprising a pair of outer core leg members joined at their ends by a pair of yoke members, each of said outer leg members and yoke members being divided into at least two spaced apart parallel stacks of magnetic laminations extending longitudinally of the respective members, at least the innermost of the stacks of said yoke members being centrally transversely divided and spaced apart, -a center core leg assembly extending parallel to said outer leg members and joining said spaced apart inner yoke stacks, the joints between said inner yoke stacks and said center leg assembly being parallel to the longitudinal axis of said core, the width of the portion of the laminations of said center leg assembly which are joined to the laminations of said inner stacks being different in successive layers to provide a joint overlap between successive layers at said portion of said center leg assembly, at least the central portion of 'said center leg assembly being split to provide a duct extending longitudinally of said center leg assembly, said center leg assembly joining all of the stacks of each of said yoke members, all of the joints between the laminations of said core being butt joints, substantially all of the joints between said yoke and outer leg members being parallel to each other, successive layers of said core being shifted axially with respect to each other in a direction perpendicular to the butt joints between said yoke and outer leg members to provide an overlap between the joints of successive layers between said yoke and outer leg members.

7. A three phase magnetic core of the plate type comprising a plurality of superposed layers of at stacked material, said core comprising a pair of outer core leg members joined at their ends by a pair of yoke members, each of said outer leg members and yoke members being divided into at least two spaced apart parallel stacks of magnetic laminations extending longitudinally of the respective members at least the innermost of the stacks of said yoke members being centrally transversely divided and spaced apart, a center core leg assembly extending parallel to said outer leg members and joining said spaced apart inner yoke stacks, the joints between said inner yoke stacks and said center leg assembly being parallel to the longitudinal axis of said core, the width of the portion of the laminations of said center leg assembly which are joined to the laminations of said inner stacks being different in successive layers to provide a joint overlap between successive layers at said portion of said center leg assembly, at least the central portion of said center leg assembly being split to provide a duct extending longitudinally of said center leg assembly, said center leg assembly joining all of the stacks of each of said yoke members, lall of the joints between the laminations of said core being butt joints longitudinal to one of the laminations at each respective joint and perpendicular to the other lamination at said respective joint, substantially all of the joints between said yoke and outer leg members being parallel to each other, successive layers of said core being shifted axially with respect to each other in a direction perpendicular to the butt joints vbetween said yoke and outer leg members to provide an overlap between the joints of successive layers between said yoke and outer leg members.

8. A three phase magnetic core of the plate type comprising a plurality of superposed layers of flat stacked material, said core comprising a pair of outer core leg members joined at their ends by a pair of yoke members, each of said outer leg members and yoke members being dicular to said butt divided into at least two spaced apart parallel stacks of magnetic laminations extending longitudinally of the respective members, all of the stacks of said yoke members being centrally transversely divided and spaced apart, a center core leg assembly extending parallel to said outer leg members and joining said spaced apart stacks, the joints between said yoke stacks and said center leg assembly being parallel to the longitudinal axis of said core, the width of the portion of the laminations of said center leg assembly which are joined to the laminations o'r said stacks being diterent in successive layers to provide a joint overlap between successive layers at said portion cf said center leg assembly, the central portion of said center leg assembly being split to provide a duct extending longitudinally of said center leg assembly, all of the joints between the laminations of said core being butt joints longitudinal to one of the laminations at each respective joint and perpendicular to the other lamination at said respective joint, substantially all of the joints between said yoke and outer leg members being parallel to each other, successive layers of said core being shifted axially with respect to each other in a direction perpendicular to the butt joints between said yoke and outer leg members to provide an overlap between the joints of successive layers between said yoke and outer leg members.

9. A three phase magnetic core of the plate type comprising a plurality of superposed layers of flat stacked material, said core comprising a pair of outer core leg members joined at their ends by a pair of yoke members, each of said outer leg members and yoke members being divided into at least two spaced apart parallel stacks of magnetic laminations extending longitudinally of the respective members, the innermost of the stacks of said yoke members being centrally transversely divided and spaced apart, a center core leg assembly extending parallel to said outer leg members and joining said spaced apart inner yoke stacks, the joints between said inner yoke stacks in said center leg assembly being parallel to the longitudinal axis of said core, the width of the portion of the laminations of said center leg assembly which are joined to the laminations of said inner stacks being different in successive layers to provide a joint overlap between successive layers at said portion of said center leg assembly, said center assembly being split to provide a duct extending longitudinally the entire length of said center leg assembly, said center leg assembly joining the outermost stack of said yoke laminations along a joint perpendicular to the longitudinal axis of said core, all of the joints between the laminations of said core being butt joints longitudinal to one of the laminations at each respective joint and perpendicular to the other laminations at said respective joints, substantially all of the joints between said yoke and outer leg members being parallel to each other, successive layers of said core being shifted axially with respect to each other in a direction perpendicular to the butt joints between said yoke `and outer leg members to provide an overlap between the joints of successive layers between said yoke and outer leg members.

References Cited in the le of this patent UNITED STATES PATENTS 2,467,823 Gordy Apr. 19, 1949 2,467,824 Grantield Apr. 19, 1949 2,698,924 Gordy Jan. 4, 1955 FOREIGN PATENTS 340,472 Germany Sept. 12, 1921 567,540 France Mar. 3, 1924 645,454 Germany May 27, 1937 493,739 Great Britain Oct. 13, 1938 551,732. Great Britain Mar. 8, 1943

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