Indian Patents. 225243:A TRANSFORMER CORE

Title of Invention	A TRANSFORMER CORE
Abstract	The invention relates to a transformer core, comprising three legs and yoke parts connecting said legs, wherein the cross-section of said legs is regularly multi—edged with more than four edges. The core is made up of rings rolled from strips of constant width, wherein each of said rings make up part of two of said legs.

Full Text	FIELD OF INVENTION The present invention relates generally to transformer cores and especially to three-phase and one-phase cores comprising regularly multi-edged legs. BACKGROUND Three-phase transformer cores are usually made of transformer plates cut to E I shape for small transformers and to rectangular plates, which are laid edge to edge, in larger transformers. They have the drawback that the magnetic field has to pass via edges from plate to plate and that the magnetic field must go an unnecessarily long way and not always along a magnetic orientation. Designers of transformer cores have striven to obtain legs with an essentially circular cross- section because that gives the best efficiency of the final transformer. However, there is always a trade-off between efficiency and production requirements, leading to non-optimal transformer cores with non-circular legs. Strips cores for three-phase transformers have hitherto been difficult to manufacture. The efficiency of the core can be increased by cutting strips to variable width and winding rings, which are given a circular cross-section for single-phase transformers and semi-circular cross- section for three-phase transformers. This method results in a great deal of waste and the winding process is time consuming. US 4,557,039 (Manderson) discloses a method of manufacturing transformer cores using electrical steel strips having approximately a linear taper. By selecting a suitable taper, a hexagonal or higher order approximation of a circular cross section for the legs of the cores is produced. However, the tapered strips are difficult and time-consuming to produce and the design is not well adapted to large-scale production. In figs, la-c is shown a prior art three-phase transformer core according to Manderson, generally designated 10. The core has a general delta-shape, as is seen in the isometric views of fig. l,with three legs interconnected by yoke parts. In fig. la, a cross-sectional view of the core is shown before final assemble. The core comprises three identical ring-shaped parts 12, 13, and 14, the general shape of which appears from fig. 1. Each ring-shaped part fills up one half of two legs with hexagonal cross-sections, see fig. la, thus totaling the three legs of a three-phase transformer. The ring-shaped parts are initially wound from constant width strips to three identical rings 12a, 13a, 14a with rhombic cross-sections comorising two angles of 60 degrees and two angles of 120 degrees. These rings 12a-14a constitute the basic rings. The orientation of the strips also appears from figs, la and lb. Outside of the basic ring in each ring-shaped part there is an outer ring 12b,13b,14b of a regular triangular cross-section. The outer rings are wound from strips with constantly decreasing width. When the three ring-shaped parts 12-14 are put together, see fig. lb they form three hexagonal legs on which the transformer windings are wound. A drawback with this solution is that every size of transformer requires its own cutting of the strips. Also, the outer rings 12b-14b are made of strips with decreasing width, leading to waste and it also makes the transformer according to Manderson difficult to manufacture. Transformer cores are also described in the following documents: SE 163797, US 2,458,112, US 2,498,747, US 2,400,184 and US 2,544,871. However, the above mentioned problems are not overcome by the cores described in this documents. OBJECT OF THE INVENTION An object of the present invention is to provide a transformer core wherein the energy tosses are minimised. Another object is to provide a transformer core, which is easy to manufacture and avoids material waste. Another object is to provide a method of manufacturing a transformer that is well adapted for large-scale production. SUMMARY OF THE INVENTION The invention is based on the realization that a transformer core with one or more regularly multi-edged legs with more than four edges can be wound of strips of material with constant width. According to the invention there is provided a transformer core, comprising at least one leg and at least one yoke part, wherein the cross-section of said at least one leg is regularly multi- edged with more than four edges, characterised in that the core is made up of rings rolled from strips of constant width. Further preferred embodiments are defined in the dependent claims. BRIEF DESCRIPTION OF DRAWINGS The invention is now described, by way of example, with reference to the accompanying drawings, in which: fig.l is an isometric view of a prior art three-phase transformer core made of rings with rhombic and triangular cross-sections; fig. la and lb are transverse cross-sections of the core shown in fig.l before and after assembly, respectively; fig. 2 is an isometric view of a three-phase transformer according to the invention with legs with hexagonal cross-sections; figs. 2a and 2b are transverse cross-sections of the core shown in fig. 2 before and after assembly, respectively; figs.3a and 3b are transverse cross-sections of an alternative three-phase transformer core with legs with hexagonal cross-section before and after assembly, respectively; fig.3c is an isometric view of one of the frames of the transformer core shown in fig.3; fig.3d is an exploded view of the frame shown is fig.3c; fig.3c is a cut away isometric view of the frame shown in fig.3c; fig.3f is a cross-sectional view of the frame shown in fig.3c; fig.3g is a transverse cross sectional view of the three-phase transformer core as shown in fig.3 before assembly; fig.3h is a transverse cross-sectional view of the three-phase transformer core as shown in fig.3 after assembly; fig.3i is a front view of one of the rings forming part of the frame shown in fig.3c; fig.3j is a side view of the ring shown in fig.3i; Fig.3k is a cross-sectional view taken in the direction of arrows 3k-3k in fig.3i; Fig.31 is a front view of another of the rings forming part of the frame shown in fig.3c; Fig.3m is a side view of the ring shown in fig. 31; Fig.3n is a cross-sectional view taken in the direction of arrows 3n-3n in fig.3l; Fig.3o is a front view of another of the rings forming part of the frame shown in fig.3c; fig.3p is a side view of the ring shown in fig.3o; fig.3q is a cross-sectional view taken in the direction of the arrows 3q-3q in fig.30; Fig.4 is an isometric view of a three-phase transformer core with octagonal legs; Fig.4a is a transverse cross-section of the core shown in fig.4; Fig. 5 is a cross-section of a transformer leg with ten edges; Fig. 6 is a cross-section of a transformer leg with twelve edges; Figs.7-9 show an arrangement for influencing the leakage inductance and the harmonics in a three-phase transformer; Fig.10 is a transverse cross-section of a three-phase transformer core with specially shaped yoke parts for improving the magnetic flux; Fig.11 shows a three-phase transformer core with lined up legs; Figs. 12-14 show one-phase transformer cores according to the invention; and Figs. 15-17 show further improvements of the shape of the transformer core cross-section. DETAILED DESCRIPTION OF THE INVENTION Preferred embodiments of a three-phase transformer core according to the invention will now be described. Fig. 1 has already been discussed in connection with prior art and will not be explained further. In fig. 2 is shown a three-phase transformer core according to the invention, generally designated 20. In its general shape it is similar to the prior art transformer core shown in fig. 1 with a general delta-shape but is designed in an entirely different way. The core is made up of three ring-shaped parts 22,23,24 comprising several rings. These come in two widths, broad or narrow wherein the narrow rings are made up of strips of half the width of the broad rings. Also, they come in two heights, bw or high wherein the low rings have half the height of the high rings. Unless otherwise stated, these definitions will be used throughout this description. The strips are preferably made of transformer plate. Each of the ring-shaped parts 22-24 comprises a broad high basic ring 22a-24a, respectively, similar to those described with reference to fig. 1. Thus, these rings form in pairs four of the sides in the hexagonal legs. The remaining rhombs in the legs are built in different ways, see figs. 2a and 2b. In the first leg 25 in the background, the additional rhombic cross-section is composed of two rhomboids. The first one, designated 24b and belonging to ring-shaped part 24, is a broad bw ring. The second one, designated 22b and belonging to ring-shaped part 22, is a narrow high ring. In the second leg 26 to the right in fig.2, the additional rhombic cross-section is composed of one rhomboid and two rhombs. The rhomboid is filled by the narrow high ring 22b belonging to the ring-shaped part 22. The rhombs are filled by two narrow bw rings 23b, 23c belonging to the ring-shaped part 23. In the third leg 27 to the left in fig .2, the additional rhombic cross-section is also composed of one rhomboid and two rhombs. The rhomboid is filled by the broad bw ring 24b belonging to the ring-shaped part 24. The rhombs are filled by two narrow bw rings 23b, 23c belonging to the ring-shaped part 23. The reason that the ring-shaped part 23 comprises two bw narrow rings instead of one larger ring is that this larger ring can not be both narrow and high, as required in the left leg 27, and broad and low, as required in the right leg 26. Thus, instead two narrow low rings are used. All upper or lower yokes connecting the legs 25-27 have different shapes but all are built from one basic ring with a large rhombic cross-section plus one ring with a rhomboidal cross-section or two rings with a small rhombic cross-section. This gives all yokes the same total cross- section area. A second preferred embodiment is described with reference Figs. 3 and 3a-3q. This embodiment is for a three-phase transformer core having three frames 32-34 arranged in a triangular or delta pattern. (It should be noted that in a three-phase transformer core with hexagonal legs and yokes in a triangle, the innermost parts of the cross sections of the rings must be 60°. This is the definition of the orientation of the angle notations in this applications.) As explained below, the core has three legs, each of which extends through a transformer coil. The frames are composed of rings, each ring being a wound sheet of transformer plate sheet metal, the layers of each of which are offset, i.e. splayed or profiled. In cross-section, the legs form regular hexagons. The terms referred to earlier (broad, narrow, thick, thin, etc.) have the same meaning as they did earlier. Referring first to fig.3, a three-phase transformer core 30 is shown. Core 30 has frames 32,33,34. Each frame has vertically extending portions that run between opposite ends of a pair of opposed yokes. As explained below, the legs of each frame cooperate with legs of adjacent frame frames to form legs 35,36 and 37 of transformer core 30. Since each frame is identical to the other frames, only frame 32 will be discussed. Frame 32 has a broad, thick ring 32a, a narrow thin ring. 32b which goes partly over ring 32a, and a narrow thin ring 32c which goes partly over ring 32b. Ring 32 is shown alone in fig. 3c. Fig.3c is a rear view of frame 32a, viewed from inside core 30. It can be seen in fig.3c that broad, thick ring 32a is lying in one plane, that ring 32b has an edge lying on the forward edge of ring 32a as viewed on the left portion of fig. 3c, but is located so that the right edge of ring 32b is sitting near the rear edge of ring 32a. Ring 32c has its left hand surface flush against the left hand surface of ring 32b, and then is arranged so that it crossed over ring 32b and that its offset or splayed surface, labeled 301 is aligned with the offset or splayed surface 302 of ring 32b. The right hand edge of ring 32a is forward of the splayed portion of ring 32b. The splayed surface 303 of ring 32a angled in the opposite direction of surfaces 301 and 302. Frame 32 is shown the exploded form in fig.3d. It can again be seen that broad thick ring 32a is a wound ring, with its layers being skewed or inclined so that its innermost strip in the profile extends forwardly, out of the plane of the paper. Narrow thin ring 32c has its layers skewed inwardly, towards the plane of the paper, that is, opposite to the direction of the offset of ring 32a, as noted above, Ring 32b is narrow and thin, and the direction of the offset of its layers is the same as that of ring 32c. Fig. 3e shows a portion of frame 32 in cross- section. Ring 32a is at the innermost part of frame 32, ring 32b is close to the forward edge of ring 32a when viewed in the left hand corner of Fig. 3e, and then extends partly through ring 32a as can be seen from the direction of the windings of 32b in fig.3e. The latter is indicated by the angle u in fig. 3n.Narrow, thin ring 32c is rearward of ring 32b as shown in fig.3e, but is wound around and sits on ring 32b as can be seen in the right hand portion of frame 32. The cross-section shown in fig.3e on the left-hand portion shows a rhombus 32a and two rhomboids 32b and 32c. A cross-section showing all three frames 32,33, and 34 is shown in fig. 3a. This will be discussed, briefly, below. Fig. 3f is a top cross sectional view of frame 32 in fig.3c. It shows broad thick ring 32a, narrow thin ring 32b, and narrow thin ring 32c. The offsetting or splaying of the profiled rings 32a, b and c is clear from this view. Ring 32a is lower in the foreground and higher in the background. Rings 32b and 32c are higher in the foreground and lower in the background. The dimension "w" shows the width of ring 32a, and the width of rings 32b and 32c are "w/2". The thickness of ring 32a is shown as "t" and the thickness of each of rings 32b and 32c are "t/2". Fig. 3g is similar to fig.3a, and fig 3h is similar to fig 3b, and they will be described together. Transformer core 30 is composed of legs 35, 36 and 37. Frame 32 includes broad, thick ring 32a and narrow, thin rings 32b and 32c. Frame 33 has broad, thick ring 33a, and narrow thin rings, 33b and 33c. Frame 34 includes broad, thick ring 34a, and narrow rings 34b and 34c. Figs 3a and 3g shows transformer core 30 in exploded form, and core 30 is shown in assembled form in figs, 3b and 3h, with a cross-section through each of the legs depicted. The cross-section of leg 35 is composed of rhomb 32a, rhomb 32b, rhomb 32c, rhomb 34a, rhomb 34b and rhomb 34c. The cross section of leg 36 is made of rhomb 32a, rhomb 32b rhomb 32c, rhomb 33a, rhomb 33b and rhomb 33c. Finally, leg 37 has rhomb 33a, rhomb 33b, rhomb 33c, rhomb 34a, rhomb 34b and rhomb 34c. Each of the cross-section of legs 35, 36 and 37 is regular hexagons. The hexagon of leg 35 is defined by the bottom side and right edge of rhomb 32a, the right side edge of rhomb 32b and the right side edge of rhomb 32c (the last two edges are aligned to form one side of the hexagon), the edge of rhomb 32c and the aligned edge of 34c, the left edge of rhomb 34a and the lower edge of rhomb 34a. The hexagon of leg 36 is defined by the bottom side and right hand edge outside of rhomb 32a, the aligned right hand edges outside of rhombs 32c and 33c, the lower aligned edges of rhombs 33b and 33c, the lower edge of rhomb 33a and the left hand edge of rhomb 33a. Finally, the hexagon of leg 37 is defined by the upper right hand side and edge of rhomb 34a, the inner side of rhomb 33a, the bottom edge of rhomb 33a, the aligned bottom edges of rhombs 33b and 33c, the aligned left-hand of rhombs 34c and 34b, and the upper left hand edge of rhomb 34a. Turning next to figs. 3i-3k, ring 32a, which is identical to rings 33a and 34a, is shown in its front side and cut away views taken in the direction of arrows 3k-3k. Ring 32a includes opposite, parallel leg portions 305, 306, opposing yoke portions 307,308 and rounded corners 309-312. The offset or splaying is shown in the profile 303. Narrow thin ring 32b, which is identical to 33b and 33c, is shown in figs, 31-n. fig 31 shows a front view of 32b, fig. 3m shows a side view of 32b and fig 3n shows a cross-sectional view taken in the direction of 3n-3n. Ring 32b has opposing, parallel legs 314, 315 and opposing yokes 316, 317. Ring 32b has rounded corners 318-321. Ring 32b is turned as it passes partly through broad thick ring 32a, and this is shown by the angle m in fig. 3n. The offsetting or splaying of the wound layers of 32b is shown at 323. It should be observed that the offset of the profile shown in figs. 3l-3n is in the opposite direction from that of ring 32a shown in figs. 3i-3k. Front and side views of ring 32c (which is identical to rings 33c and 34c) are shown in figs.3o and 3p Fig 3q is taken in the direction of arrows 3q-3q in fig. 3o. Ring 32c has leg portions 325, 327, which are opposing and parallel to each other. Parallel yoke portions 328, 329 oppose each other Rounded corners 330-333 join the yokes and legs. The rings of transformer plate are offset or splayd are shown in figs. 30-3q at 335. It can be seen that the direction of the profile of offset portion 335 is to the right as viewed in fig. 3p, which is the same as the direction of the profile of ring 32b shown in fig. 3m. The turning is in the opposite direction reversed with respect to ring 326. The rhombic space outside of the basic rings described with reference to figs 2, 2a and 2b, could thus be filled in accordance with a couple of basic principles. The second embodiment was just described with reference to figs. 3 and 3a through 3q. In other words, the core, generally designated 30, has the same general shape as the first embodiment described above. However, in this embodiment the core comprises three identical frames 32-34, as explained above of which the rightmost one 32 will again be briefly described. The frames 32- 34 are similar to the part 23 described in connection with fig. 2. Referring to figs. 3a and 3g in the first leg 35, the leg part of frame 32 comprises the board thick ring 32a and two narrow thin or bw rings 32b, 32c wherein ring 32c is wound outside of ring 32b, and the leg part of frame 34 with broad thick rings 34a, and side-by-side narrow thin rings 34b and 34c. In the second leg 36, the leg part of frame 32 has the broad thick ring 32a, and the two narrow thin rings 32b, 32c placed one beside the others. Leg 37 has the leg part of frame 33 with broad thick ring 33a, narrow thin rings 33b, 33c, and leg part of frame 31 with broad thick ring 34a and narrow thin rings 34b, 34c. The two other frames 33,34 are identical to the first one 32. Thus, the production of the core can as a rule can be simplified, depending on the production volume, because all three frames 32-34 can be made from the same mould. A further possibility is to make broad thin rings and turn the leg parts 60°, forcing a corresponding bending of the yoke parts. The yoke parts then require more space and the bending is not so easy to effect. Making narrow thick rings and turning and bending as mentioned is also possible, but difficult. Additional variants, including those with smaller divisions, are also possible. A core with octagonal legs, generally designated 40, will now be described with reference to figs. 4 and 4a. In an octagonal cross-section, see e.g. the back leg 45, the sides turn 45 degrees, which means that they have a relative angle of 135 degrees to each other. Three rhombs, each with an angle of 45 degrees, thus get space in the innermost edges of the legs of the core. Outside of these rhombs, two squares are filled by rings with quadratic cross- sections. Finally. A rhomb fills the rest of the octagonal cross-section of the leg. From these six cross-subsections, three subsections compose the cross-section of a profiled ring going to the second leg 46. The remaining subsections compose the cross-section of a profiled ring going to the third leg 47. There is also profiled ring connecting the second and third legs 46, 47. The three profiled rings all contain two rings with equal leg parts. A first ring 42a,43a, 44a has rhombic cross-section and the yoke Darts bent 15 degrees. A second ring 42b, 43b, 44b outside of the first ring is quadratic and follows the form of the first ring 42a-44a. Using a solution from the embodiments with hexagonal legs descrtoed with reference to figs. 2 and 3, two outer rhombs compose the cross-section of an outer ring with the yoke parts bent 15 degrees. Alternatively, two inner rhombs compose an inner ring but bent 60 degrees. The next ring must now give an outer rhomb in one leg and an inner rhomb in the other leg and be bent 30 degrees. One type of profiled ring is to be preferred because it is difficult to bend a ring 60 degrees and one can not avoid a ring with both an outer rhomb and an inner rhomb. In part 42, the third ring 42c has a rhombic cross-section in the leg parts and is placed outermost in the back leg 45 but inside the right leg 46. These rhombs of the leg parts are obtained by displacing the outer strips of the ring to the right at the right leg 46 and to the left at the back leg 45. Furthermore, the legs are turned asymmetrically 30 degrees and the yoke parts are bent accordingly. The ring is given such circumference that it will lie outside of the other rings. The final result appears in figure 4. A 10-sided leg. Generally designated 50, will now be described with reference to fig. 5. The profiled rings contain all four rings with equal leg parts. A first ring 50a, a second ring 50b and a third ring 50c with rhombic cross-sections in their leg parts are attached to the 10-sided cross section. Thus they have the angles 36, 72 and 108 degrees and their yoke parts bent 24 degrees. A fourth ring 50d having a rhomboid cross-section with the angle 36 degrees lies mainly upon the first ring 50a. Its leg parts are turned out-wards 24 degrees, causing a 48 degrees bending of its yokes. The forth ring also causes the yoke parts of the third ring 50c to make a larger bow to give space. A fifth ring 50e has a rhombic cross-section in its leg parts with the angle 144 degrees when it lies outside of the third ring 50c, but the ring has a rhombic cross-section with the angle 72 degrees when it lies outside of the fourth ring 50d. The yokes are bent only 12 degrees. The arrows I the figure indicate that the cross-sections 50e belong to different profiled rings. There will also be a channel 51 suitable for cooling the legs. In an alternative embodiment, the channel is filled with a ring. This is an advantage when the rings co-operate by letting the magnetic filed go between them. The space can e.g. be disposed of in such a way that the upper part of the rings 50c obtains new rhombic cross-sections with the angle 72 degrees, causing the channels 52a and 52b to be formed. Further parts of ring 50c to the right can be pushed to ring 50e, which forms the spaces 53a and 53 b. It is possible to provide three-phase transformer cores with even more edges. Fig. 6 shows a 12-sided core, generally designated 60. The profiled rings are composed of four rings 60a-d with rhombic cross-sections with the angles 30, 60, 90, and 120 degrees, which are attached to the 12-sided cross-section and are turned 15 degrees. Inside of these rings there are two rings 60e, 60f with rhombic cross-sections with the angles 30 and 60 degrees, respectively, and turned outward 15 degrees. Attached to the fifth and sixth rings 60e, 60f there is space for a ring 60g with rhombic cross-section with the angle 30 degrees turned outward 45 degrees. Its other leg part is a rectangle outside of the sixth ring 601f and turned outward 15 degrees. Upon the ring 60d there is space for a ring 60h with a rhombic cross-section with the angle 150 degrees and the other leg part is a rectangle attached to ring 60d and outside ring 60f. The whole cross-section is then filled. Yoke parts are separated by giving some wider bows to give space for other yoke parts. The good properties of these transformer cores can be made even better for some transformer application, see fig. 7. The leakage, inductance can easily be increased by an additional core 29 of strips between the primary and secondary windings of the transformer. The strips are brought together at the top and bottom. The strips can be spread around the entire primary winding or be concentrated to one place, making the secondary winding eccentric. The non-linear magnetic properties of iron result in harmonics in the magnetic fields, voltages and currents. An additional leg placed in the center of the core will not get any magnetic field under perfectly symmetrical and distortion-free three-phase conditions. Common components in the phase voltages, like the third harmonics, will be influenced by a centre leg. Also a combination of strips between the windings and a centre leg is possible. In one embodiment, the centre leg is made of three rectangular poles 80 from strips given a height three times the width, laid on each other to a quadratic cross-section, see fig. 8. This is preferably triangular and a custom-made solution contains poles with a rhombic cross-section, of which three are put together to form a packet with the strip edges toward each other in a wave form, see fig. 9. Three packets are put together with small distances to form a leg with a............. cross-section approximating a triangle. The ends of the poles are bent outward to reach the yokes. To make the bends possible spacers between the poles are necessary. The spacers do not influence the magnetic properties because one pole from each packet 9ia-c; 92a-c; 93a-c is bent to each yoke. Also the strips are, at least on one side, parallel to the spacers. A rod, wound of strips in spiral form or as coils, is useful, especially if there are to be air gaps between the centre leg and the yokes. The spiral can be made wider at the ends to reduce the air gaps to the yokes. The flexibility of building cores like this good and is shown in fig. 10. The figure shows the core described in connection with fig. 4. A major part of the magnetic flux can pass from one profiled ring to another in the legs where they are touching each other. This enables the rotation of larger fluxes in the yoke triangle. With the present invention, it is also possible to provide a three-phase transformer core with lined up legs. This has the advantage that the transformer is narrower than with the delta shaped core. This type of transformer is ideal for placement on e.g. train wagons. Fig. 11a shows the transverse cross-section of a transformer with octagonal legs. All legs comprise four rhombs with an angle of 45 degrees and two squares. Rings running between adjacent legs are shown in the figure while those running between the outer legs are almost entirely hidden. In order to make transformer cores of this kind, the leg parts must be bendable and that the yoke parts can be bent and pass each other. There are several solutions, of which one is shown in the figure. The leg parts of the rings are bent outward and the yoke part inward or vice versa. The shape of the yoke parts is limited by the limited possibilities of plastic deformations but otherwise the yoke parts can have any shape. The principle shown in fig. 11 is to have sharp bends and straight yoke parts. The rings can also be placed on each other giving rounded bends in order to save material. The yokes between the left leg 115 and the center leg 116 are built up of a ring 112a with a rhombic cross-section in the leg part, a ring 112b with a square cross-section and both bent 22.5 degrees and a rhombic ring 112c turned 67.5 degrees in the leg parts. The rings 112a and 112b fit into the octahedrons close to the yoke side while the ring 112c fits into the opposing side. The yoke between the center leg 116 and the right leg 117 can only be placed in the center leg in the remaining positions: 114a-c. The cross-sections of the left and right legs 115, 117 are mirror images to the center leg 116 so that the rings running in the center leg are symmetric. The inner rings 114a, 114b have their closest positions in the right leg 117. However, the ring 114c with a square cross-section in the leg parts runs to the closest square-shaped position in the right leg. The reason behind that is that the ring 113a with a square cross- section between the outer legs is in an outer position on the yoke parts already present in order to reach the left leg. The turning of the yokes can be impossible to achieve. In an alternative embodiment, a heavily sloping fold is used instead. This is shown for the right 114c having the shortest yoke. The fold starts at one end of the yoke and ends at the other end, marked by 118a for the lower yoke and 118b for the upper yoke in fig. 11. Also, the yokes can be subdivided into several narrow rings. Also single-phase transformers will be more efficient if they are given polygonal cross-sections. Fig. 12 shows a transformer with an octagonal cross-section composed of rings with the same cross-sections as in the three-phase transformers but with the return loops going the closest way outside of the windings. The rings can be transposed and yet given an octagonal cross-section. A small reduction of the amount of plate can e.g. be obtained by looping up to the left of the ring looping rightmost in the figure. There must its cross-section be changed to a rhombic form close to rectangular form. A core with two legs can be made from the three-phase designs by bending the rings from one leg together to form only one more leg. A core is shown in fig. 13 with an octagonal cross-section in its legs. The turning of three leg-parts is 45 degrees and the bending is 90 degrees. A ring with a rectangular cross-section and the two rings outside of that ring are not deformed. Cores with hexagonal legs need only three rings made of strips with the same width. If that octagon edge where three rhomb edges meet, is put innermost in the core, the turnings will only be 22.5 degrees except for the rhomb in the middle, which must be turned 67.5 degree. Replacing this rhomb, is more realistic and is shown in fig. 14. A further improvement is made by letting the strips reach the circle, thus increasing the total cross-section. The segments outside of a polygonal leg can be filled by a thin rhombic ring of a strips with about half the width and the full height of the segment and wound to its total width. Folds in the strips along the middle of the rhomb as in fig. 15 two sides to one flat side giving a triangle, the sides of which are in contact with the core. With about 2/3 width and 8/9 height, a fold at the edge of the innermost strip makes a trapezoid cross-section as in fig. 16. The cross-section can also be rounded. By means of strips of constant width the leg parts can be given a cross-section shape closer to the shape of a circle, see figs. 17,17a and 17b. The right leg 172 in fig. 17 will be described as an example with reference to fig. 17a, wherein a transverse cross-section of that leg is shown. Innermost, there are rings 173 of e.g. 80% of full width and to a height of 9% of its width. There are three rings reaching a circumscribed circle, see fig. 17a. Four of the six segments have been filed with magnetic material and strips outside of the assembled core can fill the other segments. A ring 174 can be placed on the outer sides of the hexagons. Another embodiment is shown in fig. 17b, wherein the ring 174 has been replaced by broader strips in the other rings. Some of the advantages of the inventive transformer core have already been mentioned. Among the other advantages can be mentioned: lower no load losses, less weight, less volume, lower electrical leakage, a reduction of harmonics due to the symmetry of the phases of the three-phase transformer, easy maintenance etc. Preferred embodiments of a transformer core according the invention have been described. The person skilled in the art realizes that these can be varied with the scope of the claims. WE CLAIM 1. A transformer core, comprising three legs and yoke parts connecting said legs, wherein the cross-section of said legs is regularly multi-edged with more than four edges, characterized in that the core is made up of rings rolled from strips of constant width, wherein each of said rings make up part of two of said legs. 2. A transformer core as claimed in claim 1, wherein said legs have hexagonal cross-section. 3. A transformer core as claimed in claim 2, comprising nine rings. 4. A transformer core as claimed in claim 3, wherein said nine rings comprises three rings of a first width and a first height and six rings of a second width corresponding to half the first width and a second height corresponding to half the first height. 5. A transformer core as claimed in claim 4, comprising a first (32), a second (33) and a third (34) ring-shaped part, wherein each ring-shaped part comprises a first ring (32a, 33a, 34a) wound from strips of a first width to a first height, the cross-sections of said rings being rhombic with two angles of 60 degrees, a second ring (32b, 33b, 34b) wound from a strip of a second width essentially corresponding to half the first width, to a second height essentially corresponding to half the first height, said second ring having rhombic cross-section and being positioned on said first ring (32a, 33a, 34a), a third ring (32c, 33c, 34c) wound from a strip of the second width to the second height, said second ring having rhombic cross-section and being positioned in one position on said first ring (32a, 33a, 34a) adjacent to said second ring and in another position on said second ring, said first, second and third ring-shaped part being assembled whereby a three-phase transformer core with three legs with hexagonal cross- sections is formed. 6. A transformer core as claimed in claim 2, comprising seven rings. 7. A transformer core as claimed in claim 6 comprising: a first (22a), a second (23a) and a third (24a) ring wound from strips of a first width to a first height, the cross-sections of said rings being rhombic with two angles of 60 degrees, said first, second and third rings forming yoke parts together forming a triangle, a fourth ring (24b) wound from a strip of said first width to a second height essentially corresponding to half the first height, said fourth ring having rhomboidal cross-section and being positioned on said third ring (24a), a fifth ring (22b) wound from a strip of a second width essentially corresponding to half the first width, to said first height, said fifth ring having rhomboidal cross-section and being positioned on said first ring (22a), a sixth ring (23b) wound from a strip of the second width to said second height, said sixth ring having rhombic cross-section and being positioned on said second ring (23a), and a seventh ring (23c) wound from a strips of the second width to said second height, said seventh ring having rhombic cross-section and being positioned on said second ring (23a) and on said sixth ring (23b), whereby a three-phase transformer core with three legs with hexagonal cross-sections is formed. 8. A transformer core as claimed in claim 1, wherein said legs have octagonal cross-section. 9. A transformer core as claimed in claim 8, comprising a first, a second, and a third profile ring, each comprising three rings (42a, 42b, 42c) with two leg parts and two yoke parts, wherein a first ring (42a) having rhombic cross-section in its leg parts with an angle of 45 degrees and with the yoke parts bent 15 degrees in such 3 direction that the outer side faces of its leg parts are moved towards each other, a second ring (42b) having quadratic cross-sections in its leg parts and being positioned on said first ring, and a third ring (42C) having rhombic cross-sections in its leg parts, a first leg part having 45 degrees lying mainly on said first ring (42a) and a second leg part having 135 degrees lying on said second ring (42b). said first, second and third profile rings being assembled whereby a three- phase transformer core with three legs with octagonal cross-sections is formed. 10. A transformer core as claimed in claim 1, wherein said legs have a cross- section with ten edges. 11.A transformer core as claimed in claim 10, comprising a first, a second, and a third profile ring, each comprising five rings (50a-e) with two leg parts and two yoke parts, wherein a first ring (50a) having rhombic cross-sections in its leg parts with an angle of 36 degrees, a second ring (50b) having rhombic cross-sections in its leg parts with an angle of 72 degrees, a third ring (50c) having rhombic cross-sections in its leg parts with an angle of 108 degrees, a fourth ring (50d) having rhombic cross-sections in its leg parts with an angle of 36 degrees and lying mainly on the first ring (50a) and having its yoke parts turned outwards 24 degrees, and fifth ring (50e) having rhombic cross-sections in its leg parts with an angle of 144 degrees when it lies on the third ring (50c) but rhombic cross- section with an angle of 72 degrees when it lies outside the fourth ring (50d), and a channel (51) suitable for cooling the leg outside of the fifth ring (50e), said first, second and third profiles rings being assembled whereby a three-phase transformer core with three legs with ten-sided cross sections is formed. 12. A transformer core as claimed in claim 11, comprising cooling channels (52a, 52b, 53, 53b) caused by giving the outer part of the third ring (50c) a rhombic cross-section with an angle of 72 degrees and by displacing another outer leg part of the third ring toward the fifth ring (50e) when it goes within the complete leg. 13. A transformer core as claimed in claim 10, comprising multi-edged cross- sections, and profile rings comprising a first cluster of rings with rhombic cross-sections with different angles but in their leg parts turned the same angle and attached to the multi-edged cross-section. And a second cluster of rings with rhombic cross-section with different angles, but in their leg parts being turned at the same angle and attached to the first cluster and so on until innermost there arises space area for the rings, which in one of their leg parts is given a cross-section and turning differently from those in the other leg part. 14. A transformer core as claimed in claim 1, wherein all the rings have a rhombic cross-section with two angles of 60 degrees and two angles of 120 degrees. 15. A transformer core as claimed in claim 1, comprising an additional core (70) of strips between windings brought together at the top and the bottom of the core. 16. A transformer core as claimed in claim 1, comprising an additional core in the centre line of at least one strip pole, and if many, arranged three and three in a package (fig. 8 and 9), which poles are bent to each yoke. 17. A transformer core as claimed in claim 1, wherein segments between the cross-sections of the legs and circumscribed circle are partly filled by thin rings and/or slightly broader strips. The invention relates to a transformer core, comprising three legs and yoke parts connecting said legs, wherein the cross-section of said legs is regularly multi—edged with more than four edges. The core is made up of rings rolled from strips of constant width, wherein each of said rings make up part of two of said legs.

Full Text

FIELD OF INVENTION
The present invention relates generally to transformer cores and especially to three-phase
and one-phase cores comprising regularly multi-edged legs.
BACKGROUND
Three-phase transformer cores are usually made of transformer plates cut to E I shape for
small transformers and to rectangular plates, which are laid edge to edge, in larger
transformers. They have the drawback that the magnetic field has to pass via edges from
plate to plate and that the magnetic field must go an unnecessarily long way and not always
along a magnetic orientation.
Designers of transformer cores have striven to obtain legs with an essentially circular cross-
section because that gives the best efficiency of the final transformer. However, there is
always a trade-off between efficiency and production requirements, leading to non-optimal
transformer cores with non-circular legs.
Strips cores for three-phase transformers have hitherto been difficult to manufacture. The
efficiency of the core can be increased by cutting strips to variable width and winding rings,
which are given a circular cross-section for single-phase transformers and semi-circular cross-
section for three-phase transformers. This method results in a great deal of waste and the
winding process is time consuming.
US 4,557,039 (Manderson) discloses a method of manufacturing transformer cores using
electrical steel strips having approximately a linear taper. By selecting a suitable taper, a
hexagonal or higher order approximation of a circular cross section for the legs of the cores is
produced. However, the tapered strips are difficult and time-consuming to produce and the
design is not well adapted to large-scale production.
In figs, la-c is shown a prior art three-phase transformer core according to Manderson,
generally designated 10. The core has a general delta-shape, as is seen in the isometric views
of fig. l,with three legs interconnected by yoke parts. In fig. la, a cross-sectional view of the
core is shown before final assemble. The core comprises three identical ring-shaped parts 12,
13, and 14, the general shape of which appears from fig. 1. Each ring-shaped part fills up one
half of two legs with hexagonal cross-sections, see fig. la, thus totaling the three legs of a
three-phase transformer. The ring-shaped parts are initially wound from constant width strips
to three identical rings 12a, 13a, 14a with rhombic cross-sections comorising two angles of 60
degrees and two angles of 120 degrees. These rings 12a-14a constitute the basic rings. The
orientation of the strips also appears from figs, la and lb.
Outside of the basic ring in each ring-shaped part there is an outer ring 12b,13b,14b of a
regular triangular cross-section. The outer rings are wound from strips with constantly
decreasing width.
When the three ring-shaped parts 12-14 are put together, see fig. lb they form three
hexagonal legs on which the transformer windings are wound.
A drawback with this solution is that every size of transformer requires its own cutting of the
strips. Also, the outer rings 12b-14b are made of strips with decreasing width, leading to
waste and it also makes the transformer according to Manderson difficult to manufacture.
Transformer cores are also described in the following documents: SE 163797, US 2,458,112,
US 2,498,747, US 2,400,184 and US 2,544,871. However, the above mentioned problems are
not overcome by the cores described in this documents.
OBJECT OF THE INVENTION
An object of the present invention is to provide a transformer core wherein the energy tosses
are minimised.
Another object is to provide a transformer core, which is easy to manufacture and avoids
material waste.
Another object is to provide a method of manufacturing a transformer that is well adapted for
large-scale production.
SUMMARY OF THE INVENTION
The invention is based on the realization that a transformer core with one or more regularly
multi-edged legs with more than four edges can be wound of strips of material with constant
width.
According to the invention there is provided a transformer core, comprising at least one leg
and at least one yoke part, wherein the cross-section of said at least one leg is regularly multi-
edged with more than four edges, characterised in that the core is made up of rings rolled
from strips of constant width.
Further preferred embodiments are defined in the dependent claims.
BRIEF DESCRIPTION OF DRAWINGS
The invention is now described, by way of example, with reference to the accompanying
drawings, in which:
fig.l is an isometric view of a prior art three-phase transformer core made of rings with
rhombic and triangular cross-sections;
fig. la and lb are transverse cross-sections of the core shown in fig.l before and after
assembly, respectively;
fig. 2 is an isometric view of a three-phase transformer according to the invention with legs
with hexagonal cross-sections;
figs. 2a and 2b are transverse cross-sections of the core shown in fig. 2 before and after
assembly, respectively;
figs.3a and 3b are transverse cross-sections of an alternative three-phase transformer core
with legs with hexagonal cross-section before and after assembly, respectively;
fig.3c is an isometric view of one of the frames of the transformer core shown in fig.3;
fig.3d is an exploded view of the frame shown is fig.3c;
fig.3c is a cut away isometric view of the frame shown in fig.3c;
fig.3f is a cross-sectional view of the frame shown in fig.3c;
fig.3g is a transverse cross sectional view of the three-phase transformer core as shown in
fig.3 before assembly;
fig.3h is a transverse cross-sectional view of the three-phase transformer core as shown in
fig.3 after assembly;
fig.3i is a front view of one of the rings forming part of the frame shown in fig.3c;
fig.3j is a side view of the ring shown in fig.3i;
Fig.3k is a cross-sectional view taken in the direction of arrows 3k-3k in fig.3i;
Fig.31 is a front view of another of the rings forming part of the frame shown in fig.3c;
Fig.3m is a side view of the ring shown in fig. 31;
Fig.3n is a cross-sectional view taken in the direction of arrows 3n-3n in fig.3l;
Fig.3o is a front view of another of the rings forming part of the frame shown in fig.3c;
fig.3p is a side view of the ring shown in fig.3o;
fig.3q is a cross-sectional view taken in the direction of the arrows 3q-3q in fig.30;
Fig.4 is an isometric view of a three-phase transformer core with octagonal legs;
Fig.4a is a transverse cross-section of the core shown in fig.4;
Fig. 5 is a cross-section of a transformer leg with ten edges;
Fig. 6 is a cross-section of a transformer leg with twelve edges;
Figs.7-9 show an arrangement for influencing the leakage inductance and the harmonics in a
three-phase transformer;
Fig.10 is a transverse cross-section of a three-phase transformer core with specially shaped
yoke parts for improving the magnetic flux;
Fig.11 shows a three-phase transformer core with lined up legs;
Figs. 12-14 show one-phase transformer cores according to the invention; and
Figs. 15-17 show further improvements of the shape of the transformer core cross-section.
DETAILED DESCRIPTION OF THE INVENTION
Preferred embodiments of a three-phase transformer core according to the invention will now
be described.
Fig. 1 has already been discussed in connection with prior art and will not be explained
further.
In fig. 2 is shown a three-phase transformer core according to the invention, generally
designated 20. In its general shape it is similar to the prior art transformer core shown in fig. 1
with a general delta-shape but is designed in an entirely different way.
The core is made up of three ring-shaped parts 22,23,24 comprising several rings. These come
in two widths, broad or narrow wherein the narrow rings are made up of strips of half the
width of the broad rings. Also, they come in two heights, bw or high wherein the low rings
have half the height of the high rings. Unless otherwise stated, these definitions will be used
throughout this description. The strips are preferably made of transformer plate.
Each of the ring-shaped parts 22-24 comprises a broad high basic ring 22a-24a, respectively,
similar to those described with reference to fig. 1. Thus, these rings form in pairs four of the
sides in the hexagonal legs. The remaining rhombs in the legs are built in different ways, see
figs. 2a and 2b.
In the first leg 25 in the background, the additional rhombic cross-section is composed of two
rhomboids. The first one, designated 24b and belonging to ring-shaped part 24, is a broad bw
ring. The second one, designated 22b and belonging to ring-shaped part 22, is a narrow high
ring.
In the second leg 26 to the right in fig.2, the additional rhombic cross-section is composed of
one rhomboid and two rhombs. The rhomboid is filled by the narrow high ring 22b belonging
to the ring-shaped part 22. The rhombs are filled by two narrow bw rings 23b, 23c belonging
to the ring-shaped part 23.
In the third leg 27 to the left in fig .2, the additional rhombic cross-section is also composed of
one rhomboid and two rhombs. The rhomboid is filled by the broad bw ring 24b belonging to
the ring-shaped part 24. The rhombs are filled by two narrow bw rings 23b, 23c belonging to
the ring-shaped part 23. The reason that the ring-shaped part 23 comprises two bw narrow
rings instead of one larger ring is that this larger ring can not be both narrow and high,
as required in the left leg 27, and broad and low, as required in the right leg 26. Thus, instead
two narrow low rings are used.
All upper or lower yokes connecting the legs 25-27 have different shapes but all are built from
one basic ring with a large rhombic cross-section plus one ring with a rhomboidal cross-section
or two rings with a small rhombic cross-section. This gives all yokes the same total cross-
section area.
A second preferred embodiment is described with reference Figs. 3 and 3a-3q. This
embodiment is for a three-phase transformer core having three frames 32-34 arranged in a
triangular or delta pattern. (It should be noted that in a three-phase transformer core with
hexagonal legs and yokes in a triangle, the innermost parts of the cross sections of the rings
must be 60°. This is the definition of the orientation of the angle notations in this
applications.) As explained below, the core has three legs, each of which extends through a
transformer coil. The frames are composed of rings, each ring being a wound sheet of
transformer plate sheet metal, the layers of each of which are offset, i.e. splayed or profiled.
In cross-section, the legs form regular hexagons. The terms referred to earlier (broad, narrow,
thick, thin, etc.) have the same meaning as they did earlier.
Referring first to fig.3, a three-phase transformer core 30 is shown. Core 30 has frames
32,33,34. Each frame has vertically extending portions that run between opposite ends of a
pair of opposed yokes. As explained below, the legs of each frame cooperate with legs of
adjacent frame frames to form legs 35,36 and 37 of transformer core 30. Since each frame is
identical to the other frames, only frame 32 will be discussed.
Frame 32 has a broad, thick ring 32a, a narrow thin ring. 32b which goes partly over ring 32a,
and a narrow thin ring 32c which goes partly over ring 32b. Ring 32 is shown alone in fig. 3c.
Fig.3c is a rear view of frame 32a, viewed from inside core 30. It can be seen in fig.3c that
broad, thick ring 32a is lying in one plane, that ring 32b has an edge lying on the forward
edge of ring 32a as viewed on the left portion of fig. 3c, but is located so that the right edge
of ring 32b is sitting near the rear edge of ring 32a. Ring 32c has its left hand surface flush
against the left hand surface of ring 32b, and then is arranged so that it crossed over ring 32b
and that its offset or splayed surface, labeled 301 is aligned with the offset or splayed surface
302 of ring 32b. The right hand edge of ring 32a is forward of the splayed portion of ring 32b.
The splayed surface 303 of ring 32a angled in the opposite direction of surfaces 301 and 302.
Frame 32 is shown the exploded form in fig.3d. It can again be seen that broad thick ring 32a
is a wound ring, with its layers being skewed or inclined so that its innermost strip in the
profile extends forwardly, out of the plane of the paper. Narrow thin ring 32c has its layers
skewed inwardly, towards the plane of the paper, that is, opposite to the direction of the
offset of ring 32a, as noted above, Ring 32b is narrow and thin, and the direction of the offset
of its layers is the same as that of ring 32c. Fig. 3e shows a portion of frame 32 in cross-
section. Ring 32a is at the innermost part of frame 32, ring 32b is close to the forward edge of
ring 32a when viewed in the left hand corner of Fig. 3e, and then extends partly through ring
32a as can be seen from the direction of the windings of 32b in fig.3e. The latter is indicated
by the angle u in fig. 3n.Narrow, thin ring 32c is rearward of ring 32b as shown in fig.3e, but
is wound around and sits on ring 32b as can be seen in the right hand portion of frame 32.
The cross-section shown in fig.3e on the left-hand portion shows a rhombus 32a and two
rhomboids 32b and 32c. A cross-section showing all three frames 32,33, and 34 is shown in
fig. 3a. This will be discussed, briefly, below.
Fig. 3f is a top cross sectional view of frame 32 in fig.3c. It shows broad thick ring 32a, narrow
thin ring 32b, and narrow thin ring 32c. The offsetting or splaying of the profiled rings 32a, b
and c is clear from this view. Ring 32a is lower in the foreground and higher in the
background. Rings 32b and 32c are higher in the foreground and lower in the background.
The dimension "w" shows the width of ring 32a, and the width of rings 32b and 32c are "w/2".
The thickness of ring 32a is shown as "t" and the thickness of each of rings 32b and 32c are
"t/2".
Fig. 3g is similar to fig.3a, and fig 3h is similar to fig 3b, and they will be described together.
Transformer core 30 is composed of legs 35, 36 and 37. Frame 32 includes broad, thick ring
32a and narrow, thin rings 32b and 32c. Frame 33 has broad, thick ring 33a, and narrow thin
rings, 33b and 33c. Frame 34 includes broad, thick ring 34a, and narrow rings 34b and 34c.
Figs 3a and 3g shows transformer core 30 in exploded form, and core 30 is shown in
assembled form in figs, 3b and 3h, with a cross-section through each of the legs depicted. The
cross-section of leg 35 is composed of rhomb 32a, rhomb 32b, rhomb 32c, rhomb 34a, rhomb
34b and rhomb 34c. The cross section of leg 36 is made of rhomb 32a, rhomb 32b rhomb 32c,
rhomb 33a, rhomb 33b and rhomb 33c. Finally, leg 37 has rhomb 33a, rhomb 33b, rhomb 33c,
rhomb 34a, rhomb 34b and rhomb 34c. Each of the cross-section of legs 35, 36 and 37 is
regular hexagons. The hexagon of leg 35 is defined by the bottom side and right edge of
rhomb 32a, the right side edge of rhomb 32b and the right side edge of rhomb 32c (the last
two edges are aligned to form one side of the hexagon), the edge of rhomb 32c and the
aligned edge of 34c, the left edge of rhomb 34a and the lower edge of rhomb 34a. The
hexagon of leg 36 is defined by the bottom side and right hand edge outside of rhomb 32a,
the aligned right hand edges outside of rhombs 32c and 33c, the lower aligned edges of
rhombs 33b and 33c, the lower edge of rhomb 33a and the left hand edge of rhomb 33a.
Finally, the hexagon of leg 37 is defined by the upper right hand side and edge of rhomb 34a,
the inner side of rhomb 33a, the bottom edge of rhomb 33a, the aligned bottom edges of
rhombs 33b and 33c, the aligned left-hand of rhombs 34c and 34b, and the upper left hand
edge of rhomb 34a.
Turning next to figs. 3i-3k, ring 32a, which is identical to rings 33a and 34a, is shown in its
front side and cut away views taken in the direction of arrows 3k-3k. Ring 32a includes
opposite, parallel leg portions 305, 306, opposing yoke portions 307,308 and rounded corners
309-312. The offset or splaying is shown in the profile 303.
Narrow thin ring 32b, which is identical to 33b and 33c, is shown in figs, 31-n. fig 31 shows a
front view of 32b, fig. 3m shows a side view of 32b and fig 3n shows a cross-sectional view
taken in the direction of 3n-3n. Ring 32b has opposing, parallel legs 314, 315 and opposing
yokes 316, 317. Ring 32b has rounded corners 318-321. Ring 32b is turned as it passes partly
through broad thick ring 32a, and this is shown by the angle m in fig. 3n. The offsetting or
splaying of the wound layers of 32b is shown at 323. It should be observed that the offset of
the profile shown in figs. 3l-3n is in the opposite direction from that of ring 32a shown in figs.
3i-3k.
Front and side views of ring 32c (which is identical to rings 33c and 34c) are shown in figs.3o
and 3p Fig 3q is taken in the direction of arrows 3q-3q in fig. 3o. Ring 32c has leg portions
325, 327, which are opposing and parallel to each other. Parallel yoke portions 328, 329
oppose each other Rounded corners 330-333 join the yokes and legs. The rings of transformer
plate are offset or splayd are shown in figs. 30-3q at 335. It can be seen that the direction of
the profile of offset portion 335 is to the right as viewed in fig. 3p, which is the same as the
direction of the profile of ring 32b shown in fig. 3m. The turning is in the opposite direction
reversed with respect to ring 326.
The rhombic space outside of the basic rings described with reference to figs 2, 2a and 2b,
could thus be filled in accordance with a couple of basic principles. The second embodiment
was just described with reference to figs. 3 and 3a through 3q. In other words, the core,
generally designated 30, has the same general shape as the first embodiment described
above. However, in this embodiment the core comprises three identical frames 32-34, as
explained above of which the rightmost one 32 will again be briefly described. The frames 32-
34 are similar to the part 23 described in connection with fig. 2. Referring to figs. 3a and 3g in
the first leg 35, the leg part of frame 32 comprises the board thick ring 32a and two narrow
thin or bw rings 32b, 32c wherein ring 32c is wound outside of ring 32b, and the leg part of
frame 34 with broad thick rings 34a, and side-by-side narrow thin rings 34b and 34c. In the
second leg 36, the leg part of frame 32 has the broad thick ring 32a, and the two narrow thin
rings 32b, 32c placed one beside the others. Leg 37 has the leg part of frame 33 with broad
thick ring 33a, narrow thin rings 33b, 33c, and leg part of frame 31 with broad thick ring 34a
and narrow thin rings 34b, 34c.
The two other frames 33,34 are identical to the first one 32. Thus, the production of the core
can as a rule can be simplified, depending on the production volume, because all three frames
32-34 can be made from the same mould.
A further possibility is to make broad thin rings and turn the leg parts 60°, forcing a
corresponding bending of the yoke parts. The yoke parts then require more space and the
bending is not so easy to effect. Making narrow thick rings and turning and bending as
mentioned is also possible, but difficult. Additional variants, including those with smaller
divisions, are also possible.
A core with octagonal legs, generally designated 40, will now be described with reference to
figs. 4 and 4a. In an octagonal cross-section, see e.g. the back leg 45, the sides turn 45
degrees, which means that they have a relative angle of 135 degrees to each other. Three
rhombs, each with an angle of 45 degrees, thus get space in the innermost edges of the legs
of the core. Outside of these rhombs, two squares are filled by rings with quadratic cross-
sections. Finally. A rhomb fills the rest of the octagonal cross-section of the leg.
From these six cross-subsections, three subsections compose the cross-section of a profiled
ring going to the second leg 46. The remaining subsections compose the cross-section of a
profiled ring going to the third leg 47. There is also profiled ring connecting the second and
third legs 46, 47.
The three profiled rings all contain two rings with equal leg parts. A first ring 42a,43a, 44a has
rhombic cross-section and the yoke Darts bent 15 degrees. A second ring 42b, 43b, 44b
outside of the first ring is quadratic and follows the form of the first ring 42a-44a.
Using a solution from the embodiments with hexagonal legs descrtoed with reference to figs. 2
and 3, two outer rhombs compose the cross-section of an outer ring with the yoke parts bent
15 degrees. Alternatively, two inner rhombs compose an inner ring but bent 60 degrees. The
next ring must now give an outer rhomb in one leg and an inner rhomb in the other leg and
be bent 30 degrees. One type of profiled ring is to be preferred because it is difficult to bend
a ring 60 degrees and one can not avoid a ring with both an outer rhomb and an inner rhomb.
In part 42, the third ring 42c has a rhombic cross-section in the leg parts and is
placed outermost in the back leg 45 but inside the right leg 46. These rhombs of
the leg parts are obtained by displacing the outer strips of the ring to the right at
the right leg 46 and to the left at the back leg 45. Furthermore, the legs are
turned asymmetrically 30 degrees and the yoke parts are bent accordingly. The
ring is given such circumference that it will lie outside of the other rings.
The final result appears in figure 4.
A 10-sided leg. Generally designated 50, will now be described with reference to
fig. 5. The profiled rings contain all four rings with equal leg parts. A first ring
50a, a second ring 50b and a third ring 50c with rhombic cross-sections in their
leg parts are attached to the 10-sided cross section. Thus they have the angles
36, 72 and 108 degrees and their yoke parts bent 24 degrees. A fourth ring 50d
having a rhomboid cross-section with the angle 36 degrees lies mainly upon the
first ring 50a. Its leg parts are turned out-wards 24 degrees, causing a 48
degrees bending of its yokes. The forth ring also causes the yoke parts of the
third ring 50c to make a larger bow to give space. A fifth ring 50e has a rhombic
cross-section in its leg parts with the angle 144 degrees when it lies outside of
the third ring 50c, but the ring has a rhombic cross-section with the angle 72
degrees when it lies outside of the fourth ring 50d. The yokes are bent only 12
degrees. The arrows I the figure indicate that the cross-sections 50e belong to
different profiled rings. There will also be a channel 51 suitable for cooling the
legs. In an alternative embodiment, the channel is filled with a ring. This is an
advantage when the rings co-operate by letting the magnetic filed go between
them. The space can e.g. be disposed of in such a way that the upper part of the
rings 50c obtains new rhombic cross-sections with the angle 72 degrees, causing
the channels 52a and 52b to be formed. Further parts of ring 50c to the right can
be pushed to ring 50e, which forms the spaces 53a and 53 b.
It is possible to provide three-phase transformer cores with even more edges.
Fig. 6 shows a 12-sided core, generally designated 60. The profiled rings are
composed of four rings 60a-d with rhombic cross-sections with the angles 30,
60, 90, and 120 degrees, which are attached
to the 12-sided cross-section and are turned 15 degrees. Inside of these rings
there are two rings 60e, 60f with rhombic cross-sections with the angles 30 and
60 degrees, respectively, and turned outward 15 degrees. Attached to the fifth
and sixth rings 60e, 60f there is space for a ring 60g with rhombic cross-section
with the angle 30 degrees turned outward 45 degrees. Its other leg part is a
rectangle outside of the sixth ring 601f and turned outward 15 degrees. Upon
the ring 60d there is space for a ring 60h with a rhombic cross-section with the
angle 150 degrees and the other leg part is a rectangle attached to ring 60d and
outside ring 60f. The whole cross-section is then filled. Yoke parts are separated
by giving some wider bows to give space for other yoke parts.
The good properties of these transformer cores can be made even better for
some transformer application, see fig. 7. The leakage, inductance can easily be
increased by an additional core 29 of strips between the primary and secondary
windings of the transformer. The strips are brought together at the top and
bottom. The strips can be spread around the entire primary winding or be
concentrated to one place, making the secondary winding eccentric.
The non-linear magnetic properties of iron result in harmonics in the magnetic
fields, voltages and currents.
An additional leg placed in the center of the core will not get any magnetic field
under perfectly symmetrical and distortion-free three-phase conditions. Common
components in the phase voltages, like the third harmonics, will be influenced by
a centre leg.
Also a combination of strips between the windings and a centre leg is possible.
In one embodiment, the centre leg is made of three rectangular poles 80 from
strips given a height three times the width, laid on each other to a quadratic
cross-section, see fig. 8. This is preferably triangular and a custom-made solution
contains poles with a rhombic cross-section, of which three are put together to
form a packet with the strip edges toward each other in a wave form, see fig. 9.
Three packets are put together with small distances to form a leg with a.............
cross-section approximating a triangle. The ends of the poles are bent outward
to reach the yokes. To make the bends possible spacers between the poles are
necessary. The spacers do not influence the magnetic properties because one
pole from each packet 9ia-c; 92a-c; 93a-c is bent to each yoke. Also the strips
are, at least on one side, parallel to the spacers. A rod, wound of strips in spiral
form or as coils, is useful, especially if there are to be air gaps between the
centre leg and the yokes. The spiral can be made wider at the ends to reduce
the air gaps to the yokes.
The flexibility of building cores like this good and is shown in fig. 10. The figure
shows the core described in connection with fig. 4. A major part of the magnetic
flux can pass from one profiled ring to another in the legs where they are
touching each other. This enables the rotation of larger fluxes in the yoke
triangle.
With the present invention, it is also possible to provide a three-phase
transformer core with lined up legs. This has the advantage that the transformer
is narrower than with the delta shaped core. This type of transformer is ideal for
placement on e.g. train wagons.
Fig. 11a shows the transverse cross-section of a transformer with octagonal legs.
All legs comprise four rhombs with an angle of 45 degrees and two squares.
Rings running between adjacent legs are shown in the figure while those running
between the outer legs are almost entirely hidden.
In order to make transformer cores of this kind, the leg parts must be bendable
and that the yoke parts can be bent and pass each other. There are several
solutions, of which one is shown in the figure. The leg parts of the rings are bent
outward and the yoke part inward or vice versa. The shape of the yoke parts is
limited by the limited possibilities of plastic deformations but otherwise the yoke
parts can have any shape. The principle shown in fig. 11 is to have sharp bends
and straight yoke parts.
The rings can also be placed on each other giving rounded bends in order to
save material.
The yokes between the left leg 115 and the center leg 116 are built up of a ring
112a with a rhombic cross-section in the leg part, a ring 112b with a square
cross-section and both bent 22.5 degrees and a rhombic ring 112c turned 67.5
degrees in the leg parts. The rings 112a and 112b fit into the octahedrons close
to the yoke side while the ring 112c fits into the opposing side.
The yoke between the center leg 116 and the right leg 117 can only be placed in
the center leg in the remaining positions: 114a-c. The cross-sections of the left
and right legs 115, 117 are mirror images to the center leg 116 so that the rings
running in the center leg are symmetric. The inner rings 114a, 114b have their
closest positions in the right leg 117. However, the ring 114c with a square
cross-section in the leg parts runs to the closest square-shaped position in the
right leg. The reason behind that is that the ring 113a with a square cross-
section between the outer legs is in an outer position on the yoke parts already
present in order to reach the left leg.
The turning of the yokes can be impossible to achieve. In an alternative
embodiment, a heavily sloping fold is used instead. This is shown for the right
114c having the shortest yoke. The fold starts at one end of the yoke and ends
at the other end, marked by 118a for the lower yoke and 118b for the upper
yoke in fig. 11. Also, the yokes can be subdivided into several narrow rings.
Also single-phase transformers will be more efficient if they are given polygonal
cross-sections. Fig. 12 shows a transformer with an octagonal cross-section
composed of rings with the same cross-sections as in the three-phase
transformers but with the return loops going the closest way outside of the
windings. The rings can be transposed and yet given an octagonal cross-section.
A small reduction of the amount of plate can e.g. be obtained by looping up to
the left of the ring looping rightmost in the figure. There must its cross-section
be changed to a rhombic form close to rectangular form.
A core with two legs can be made from the three-phase designs by bending the
rings from one leg together to form only one more leg. A core is shown in fig. 13
with an octagonal
cross-section in its legs. The turning of three leg-parts is 45 degrees and the
bending is 90 degrees. A ring with a rectangular cross-section and the two rings
outside of that ring are not deformed. Cores with hexagonal legs need only three
rings made of strips with the same width.
If that octagon edge where three rhomb edges meet, is put innermost in the
core, the turnings will only be 22.5 degrees except for the rhomb in the middle,
which must be turned 67.5 degree. Replacing this rhomb, is more realistic and is
shown in fig. 14. A further improvement is made by letting the strips reach the
circle, thus increasing the total cross-section.
The segments outside of a polygonal leg can be filled by a thin rhombic ring of a
strips with about half the width and the full height of the segment and wound to
its total width. Folds in the strips along the middle of the rhomb as in fig. 15 two
sides to one flat side giving a triangle, the sides of which are in contact with the
core. With about 2/3 width and 8/9 height, a fold at the edge of the innermost
strip makes a trapezoid cross-section as in fig. 16. The cross-section can also be
rounded.
By means of strips of constant width the leg parts can be given a cross-section
shape closer to the shape of a circle, see figs. 17,17a and 17b. The right leg 172
in fig. 17 will be described as an example with reference to fig. 17a, wherein a
transverse cross-section of that leg is shown. Innermost, there are rings 173 of
e.g. 80% of full width and to a height of 9% of its width. There are three rings
reaching a circumscribed circle, see fig. 17a.
Four of the six segments have been filed with magnetic material and strips
outside of the assembled core can fill the other segments.
A ring 174 can be placed on the outer sides of the hexagons.
Another embodiment is shown in fig. 17b, wherein the ring 174 has been
replaced by broader strips in the other rings.
Some of the advantages of the inventive transformer core have already been
mentioned. Among the other advantages can be mentioned: lower no load
losses, less weight, less volume, lower electrical leakage, a reduction of
harmonics due to the symmetry of the phases of the three-phase transformer,
easy maintenance etc.
Preferred embodiments of a transformer core according the invention have been
described. The person skilled in the art realizes that these can be varied with the
scope of the claims.
WE CLAIM
1. A transformer core, comprising three legs and yoke parts connecting said
legs, wherein the cross-section of said legs is regularly multi-edged with
more than four edges, characterized in that the core is made up of rings
rolled from strips of constant width, wherein each of said rings make up
part of two of said legs.
2. A transformer core as claimed in claim 1, wherein said legs have
hexagonal cross-section.
3. A transformer core as claimed in claim 2, comprising nine rings.
4. A transformer core as claimed in claim 3, wherein said nine rings
comprises three rings of a first width and a first height and six rings of a
second width corresponding to half the first width and a second height
corresponding to half the first height.
5. A transformer core as claimed in claim 4, comprising a first (32), a second
(33) and a third (34) ring-shaped part, wherein each ring-shaped part
comprises a first ring (32a, 33a, 34a) wound from strips of a first width to
a first height, the cross-sections of said rings being rhombic with two
angles of 60 degrees, a second ring (32b, 33b, 34b) wound from a strip of
a second width essentially corresponding to half the first width, to a
second height essentially corresponding to half the first height, said
second ring having rhombic cross-section and being positioned on said
first ring (32a, 33a, 34a),
a third ring (32c, 33c, 34c) wound from a strip of the second width to the
second height, said second ring having rhombic cross-section and being
positioned in one position on said first ring (32a, 33a, 34a) adjacent to
said second ring and in another position on said second ring,
said first, second and third ring-shaped part being assembled whereby a
three-phase transformer core with three legs with hexagonal cross-
sections is formed.
6. A transformer core as claimed in claim 2, comprising seven rings.
7. A transformer core as claimed in claim 6 comprising:
a first (22a), a second (23a) and a third (24a) ring wound from strips of a
first width to a first height, the cross-sections of said rings being rhombic
with two angles of 60 degrees, said first, second and third rings forming
yoke parts together forming a triangle,
a fourth ring (24b) wound from a strip of said first width to a second
height essentially corresponding to half the first height, said fourth ring
having rhomboidal cross-section and being positioned on said third ring
(24a),
a fifth ring (22b) wound from a strip of a second width essentially
corresponding to half the first width, to said first height, said fifth ring
having rhomboidal cross-section and being positioned on said first ring
(22a),
a sixth ring (23b) wound from a strip of the second width to said second
height, said sixth ring having rhombic cross-section and being positioned
on said second ring (23a), and
a seventh ring (23c) wound from a strips of the second width to said
second height, said seventh ring having rhombic cross-section and being
positioned on said second ring (23a) and on said sixth ring (23b),
whereby a three-phase transformer core with three legs with hexagonal
cross-sections is formed.
8. A transformer core as claimed in claim 1, wherein said legs have
octagonal cross-section.
9. A transformer core as claimed in claim 8, comprising a first, a second, and
a third profile ring, each comprising three rings (42a, 42b, 42c) with two
leg parts and two yoke parts, wherein
a first ring (42a) having rhombic cross-section in its leg parts with an
angle of 45 degrees and with the yoke parts bent 15 degrees in such 3
direction that the outer side faces of its leg parts are moved towards each
other,
a second ring (42b) having quadratic cross-sections in its leg parts and
being positioned on said first ring, and
a third ring (42C) having rhombic cross-sections in its leg parts, a first leg
part having 45 degrees lying mainly on said first ring (42a) and a second
leg part having 135 degrees lying on said second ring (42b).
said first, second and third profile rings being assembled whereby a three-
phase transformer core with three legs with octagonal cross-sections is
formed.
10. A transformer core as claimed in claim 1, wherein said legs have a cross-
section with ten edges.
11.A transformer core as claimed in claim 10, comprising a first, a second,
and a third profile ring, each comprising five rings (50a-e) with two leg
parts and two yoke parts, wherein
a first ring (50a) having rhombic cross-sections in its leg parts with an
angle of 36 degrees,
a second ring (50b) having rhombic cross-sections in its leg parts with an
angle of 72 degrees,
a third ring (50c) having rhombic cross-sections in its leg parts with an
angle of 108 degrees,
a fourth ring (50d) having rhombic cross-sections in its leg parts with an
angle of 36 degrees and lying mainly on the first ring (50a) and having its
yoke parts turned outwards 24 degrees, and
fifth ring (50e) having rhombic cross-sections in its leg parts with an angle
of 144 degrees when it lies on the third ring (50c) but rhombic cross-
section with an angle of 72 degrees when it lies outside the fourth ring
(50d), and a channel (51) suitable for cooling the leg outside of the fifth
ring (50e),
said first, second and third profiles rings being assembled whereby a
three-phase transformer core with three legs with ten-sided cross sections
is formed.
12. A transformer core as claimed in claim 11, comprising cooling channels
(52a, 52b, 53, 53b) caused by giving the outer part of the third ring (50c)
a rhombic cross-section with an angle of 72 degrees and by displacing
another outer leg part of the third ring toward the fifth ring (50e) when it
goes within the complete leg.
13. A transformer core as claimed in claim 10, comprising multi-edged cross-
sections, and profile rings comprising a first cluster of rings with rhombic
cross-sections with different angles but in their leg parts turned the same
angle and attached to the multi-edged cross-section. And a second cluster
of rings with rhombic cross-section with different angles, but in their leg
parts being turned at the same angle and attached to the first cluster and
so on until innermost there arises space area for the rings, which in one of
their leg parts is given a cross-section and turning differently from those
in the other leg part.
14. A transformer core as claimed in claim 1, wherein all the rings have a
rhombic cross-section with two angles of 60 degrees and two angles of
120 degrees.
15. A transformer core as claimed in claim 1, comprising an additional core
(70) of strips between windings brought together at the top and the
bottom of the core.
16. A transformer core as claimed in claim 1, comprising an additional core in
the centre line of at least one strip pole, and if many, arranged three and
three in a package (fig. 8 and 9), which poles are bent to each yoke.
17. A transformer core as claimed in claim 1, wherein segments between the
cross-sections of the legs and circumscribed circle are partly filled by thin
rings and/or slightly broader strips.
The invention relates to a transformer core, comprising
three legs and yoke parts connecting said legs, wherein the
cross-section of said legs is regularly multi—edged with more
than four edges. The core is made up of rings rolled from strips
of constant width, wherein each of said rings make up part of two
of said legs.

Documents:

IN-PCT-2001-211-KOL-FORM 27.pdf

IN-PCT-2001-211-KOL-FORM-27-1.1.pdf

IN-PCT-2001-211-KOL-FORM-27.pdf

in-pct-2001-211-kol-granted-abstract.pdf

in-pct-2001-211-kol-granted-claims.pdf

in-pct-2001-211-kol-granted-correspondence.pdf

in-pct-2001-211-kol-granted-description (complete).pdf

in-pct-2001-211-kol-granted-drawings.pdf

in-pct-2001-211-kol-granted-examination report.pdf

in-pct-2001-211-kol-granted-form 1.pdf

in-pct-2001-211-kol-granted-form 18.pdf

in-pct-2001-211-kol-granted-form 2.pdf

in-pct-2001-211-kol-granted-form 26.pdf

in-pct-2001-211-kol-granted-form 3.pdf

in-pct-2001-211-kol-granted-form 5.pdf

in-pct-2001-211-kol-granted-reply to examination report.pdf

in-pct-2001-211-kol-granted-specification.pdf

in-pct-2001-211-kol-granted-translated copy of priority document.pdf

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Patent Number

225243

Indian Patent Application Number

IN/PCT/2001/211/KOL

PG Journal Number

45/2008

Publication Date

07-Nov-2008

Grant Date

05-Nov-2008

Date of Filing

22-Feb-2001

Name of Patentee

HOGLUND LENNART

Applicant Address

BLASTADSGATAN 126, S 589 23 LINKOPING

Inventors:

#	Inventor's Name	Inventor's Address
1	HOGLUND LENNART	BLASTADSGATAN 126, S 589 23 LINKOPING

PCT International Classification Number

H01F 27/25

PCT International Application Number

PCT/SE99/01518

PCT International Filing date

1999-09-02

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	09/146.501	1998-09-02	U.S.A.