TRANSFORMER CORE

Abstract: A transformer or inductor core of magnetic material has three legs (6, 8, 10) interconnected by yokes. The core comprises loops (12, 14, 16) of wires and/or strips of magnetic material, wherein each of the loops makes up part of two of the legs. Loops making up part of two different leg portions are interleaved in a common leg portion. This provides for mechanical stability and good magnetic properties for the core.

Description: TRANSFORMER CORE FIELD OF INVENTION The present invention relates generally to cores made of magnetic material and more particularly to transformer or inductor cores comprising legs and yokes interconnecting the legs. The invention also relates to transformers or inductors comprising such cores and a method of manufacturing such cores. BACKGROUND Transformer cores are constructed from ferromagnetic materials, such as iron. This material can be provided in the form of so-called transformer plate, which easily can be cut to strips or wires of constant width. These strips or wires can subsequently be put together to cores having one or more legs and interconnecting yokes . The Swedish patent publication SE 163 797 ( iegand) discloses a method of making transformer cores wherein three loops of magnetic material are put together to form a cage core with delta shaped yokes. However, the resulting core suffers from some disadvantages. Firstly, the mechanical stability of the core is poor, as the different loops tend to slide relatively each other. Secondly, the circular leg areas are not filled in an efficient way. At the present time, there are wires that have better mechanical and magnetic properties than rolled or milled strips. An example thereof is disclosed m the International patent publication WO 99/28919 (Asea Brown Boweπ), wherein a magnetic core assembly is made up of wires of magnetic material, however, the transition of magnetic flux between the loops via the legs is hampered by the air-gaps between the individual wires in the loops. These air-gaps can oe filled with mag- netic composite, for example, but tne efficiency is limited . SUMMARY OF THE INVENTION An object of the present invention is to provide a core of magnetic material wherein the problems of prior art are eliminated or at least mitigated. The invention is based on the realization that a transformer or inductor core can be made of loops or rings of strips or wires wherein the strips or wires from different rings are interleaved m the core legs. According to the present invention there is provided a core of magnetic material as defined m appended claim 1. There is also provided a three-phase transformer as defined in claim 14, a single-phase transformer as defined in claim 15 and an inductor as defined m claim 16. A method of manufacturing a core is defined in claim 17. With the core according to the present invention the above mentioned drawbacks of prior art are eliminated or at least mitigated. The magnetic flux path of the inventive core has been improved over prior art and the interleaving of the magnetic material also provides mechanical stability to the core. The dependent claims define further preferred emboαi- ments of the invention. BRIEF DESCRIPTION OF DRAWINGS The invention is now described, by way of example, with reference to the accompanying drawings, m which: FIG. 1 shows the general snape of a three-phase cage core ; fig. 2 is a cross-sectional view of the core shown ir. fig. 1 made from strips of magnetic material; fig. 2a is a detailed view of a funnel frame of magnetic material comprised m the core shown in fig. 2 ; fig. 2b is a side view of the funnel frame shown in fig. 2a; fig. 3 is a side view of part of the core shown m fig. 2; fig. 4 is a cross-sectional view of a single-phase transformer core derived from the three-phase core shown in figs. 2 and 3; fig. 5 is a plan view of a core made from wires of mag- netic material; fig. 6 is a cross-sectional view of the core shown i fig. 5; fig. 6a is a detailed view of a frame of magnetic mate ¬ rial comprised in the core of fig. 6; fig. 7 is a side view of a three-phase transformer with aligned legs; fig. 8 is a plan v--e of one layer of the core snown in fig. 7; fig. 9 is a plan viev of several interleaved _-a_ / ers of the core shown m f_-g. 7; fig. 9a is a detailed cross-sectional view of . leg of the core shown m f_g. 9; figs. 10 and 11 sho a side view and a top vie , respectively, of a frame used for manufacturir the inventive cage core; and fig. 12 is a side \ _-ew of an inductor core. DETAILED DESCRIPTIO N OF THE INVENTION In the following, a detailed description of preferred embodiments of a core according to the invention will be given. In the description, for purposes of exolana- tion and not limitation, specific details are set forth, such as particular ^ardware, applications, tech n iques etc. m order to provide a thorough understandi n g of the present invention, -owever, it will be apparent to one skilled in the art mat the present invention may be utilized m other e"Dθdιments that depart from tnese specific details. X other instances, detailed αescrip- tions of well-known methods, apparatuses, and circuits are omitted so as net to obscure the description of the present invention w_th unnecessary details. Also, al- though a specific number of strip or wire turns are shown in the figure, it will be appreciated that the numbers shown are to give a clear description of the in ¬ vention and that the actual number of turns and the dimensions will vary with the application, the wire or strip thickness etc. In fig. 1 there is shown a perspective view of the overall shape of an embodiment a core according to the invention. The core corroπses two generally delta shaped or triangular yokes, an upper yoke comprising portions 2a-c and a lower yoke comprising portions 4a-c, and three legs 6, 8, 10 interconnecting the corners of the yoke triangles. In the following cores of this general shape will be referred to as "cage cores" . The special cage core m this description has three legs that are wound at the same time of wires or strips of magnetic material, such as iron. Each leg has a cir- cular or essentially circular cross-section, as can be seen from figs. 2 and 6, for example. A first embodiment of a cage core according to the invention will now be described with reference to figs. 2, 2a, 2b, and 3. The core 1 is made of frames m the form of almost rectangular "funnels" wound from strips of transformer plate, for example. By using the term "funnel" the cross-section of the frames is accentuated. All strips have constant width, thus reducing costs and waste for the material and also facilitating the manufacturing of the core. An example of a funnel frame 12 taken from the core in fig. 2, marked therein by its cross-sections in legs 6 and 10, is shown m fig. 2a. It is appreciated that the funnel frame comprises a number of turns of strips 12a- e, m the present example five turns. Each turn is slightly offset from the neighoormg turns so as to create an essentially rhomboidal cross-section. When using strips of essentially rectangular cross-sections, the long sides of the turns become slightly jagged, but the small thickness of the strips results m a negligible jaggedness and that fact does not degrade the overall performance of the core. Tπe two cross-sections of a funnel frame are mutually converging rhomboids, see fig. 2a, thereby creating a "funnel". The angle α mdi- eating the slope of the rhomboids is 30 degrees, i.e., the two rhomboids have a mutual angle of 60 degrees. A side view of the frame 12 is shown m fig. 2b. The frame is made up of one strip wound to the desired number of turns. It can here be seen that each turn 12a-e has a smooth, generally rectangular shape with rounded corners. The rounded corners helps providing a satisfactory path for the magnetic field. Referring again to fig. 2 it is seen that a second funnel frame 14 having a slightly larger diameter than funnel frame 12 is wound between legs 6 and 8. In all other aspects, funnel frame 14 is identical to funnel frame 12. A third funnel frame 16 is wound between legs 8 and 10 and outside of funnel frames 12 and 14. The third funnel frame has the same general shape as the two first ones. However, to take into account the increasing width needed to fill up a circle, the third funnel frame comprises more turns than funnel frame 12 and 14. Thus, the three funnel frames 12, 14, 16 are mutually rotated 120 degrees, thereby forming a triangle. Subsequent funnel frames are wound outside of the first three ones in the same orαer, i.e., counter-clockwise when viewed m the figures. The widths of the subsequent funnel frames are adapted to make the resulting legs nearly circular in cross-section, as indicated c ^ the dashed circles around each leg 6, 8, and 10. In that way strips from one of the loops comprised of funnel frames positioned between two legs are interleaved with strips from another of the loops compriseo of funnel frames positioned between two legs, wherein one leg is common to the t..o loops. In order to avoid short-circuit within the core, eacr. funnel frame is isolated from adjacent funnel frames by means of strips of isolation. The final shape of the cage core made of strips also appears from fig. 3, which is a side view of the lower half of the core. Tne upper half (not shown) is a mirror image of the lower half. When making a transformer or inductor based on the cage core 1 shown m figs. 2 ana 3, windings are wound around the legs 6, 8, and 10. This results m a three- phase device with improved electrical and mechanical properties as compared with prior art cage cores. A derivative of the three-phase cage core shown in figs. 2 and 3 is the single-phase core 1' shown m fig. 4. This core is essentially identical to the three- phase core but with the funnel frames interconnecting legs 8 and 10 omitted. This single-phase core 1' com ¬ prises a first inner funnel frame 12' and a second funnel frame 14' wound outside of funnel frame 12' with a mutual angle of 60 degrees. This is identical to what s shown m fig. 2. However, instead of having a third funnel frame interconnecting legs 8' and 10', tne third funnel frame 16' is wound outside of and para_---el to funnel frame 12' between legs 6' and 10' . Subsequent funnel frames are wound alternately between _-egs 6', 8' and 6', 10' until a * - * essentially circular leg X has been built up. Whe 1 " completing a transformer cr inductor based on this s_ngle-phase core 1', windi n gs are wound around leg 6' . That legs 8' and 10' have nregu- lar cross-sectional shapes are thus of no rele * "ance. A second embodiment of a three-phase cage core according to the invention will now be described w_t- reference to figs. 5, 6, and 6a. In fig. 5, showing a plan view of a cage core 101, there is apparent ho a number of layers of wires are wound between three legs 106, 108, 110, thereby forming "funnels" similar tc those of the first embodiment. The wires have square cross- sectional shape, see fig. 6a, and to get smoct- layers in the legs and not saw tooth surfaces, the v__res m the yokes are bent towards the centerlme of ce core. In fig. 6 a first inner funnel frame 112 is s'o n positioned between legs 106 and 108. This funnel frame is also shown m detail m fig. 6a, wherein the perfect alignment of the wires constituting the funne. frame is apparent. In fig. 6a there are also shown the individual wires 112a having square cross-section. A second layer of v/ires 114 is wound outside cf funnel frame 112 and between legs 106 and 108. A third layer 116 is weund outside of the second funnel fra^e 114 m leg 108 and outside of the first funnel frame 112 in leg 110. Subsequent layers or funnel frames are wound outside of the first three funnel frames 112, 114, 116 and with varying widths so as to create essentially circular legs. Each interleaved layer of wires is isolated from t e adjacent layers so as to avoid short circuit with-.* *1 the core. As a rule, --^ a real core, the thickness of the wires is negligible compared with the thickness of the legs. For purpose of illustration, the layers of the cere shown m figs. 5 and 6 have been depicted as having air there betwee^, which causes adjacent layers to vary greatly in width. In practice, there would be many more layers of wires m the legs than shown m the figures. A third embodiment of a three-phase transformer core will now be described with reference to figs. 7-9, wherein the same inventive concept of interleaved layers of strips or wires is used. However, the core 201 shown lr figs. 7-9 is not a cage core but a core with three a.±gned legs, i.e., the three legs are positioned on a common straight line m a row. This configuration is advantageously used in a train waggon, for example, wnere a rectangular space is available for the core. The core comprises three legs 206, 208, 210, the end portions of which are interconnected by an upper yoke comprised of oortions 202a-c and a lower yoke comprised of portions 204a-c. The entire structure is made up of a number of rings of wires or strips of magnetic material. Each ring has two opposing essentially straight sides making up part of two legs and two opposing sides interconnecting the leg sides and thus making up part of the yokes. The basic configuration will rcrf be described with reference to figs. 7 and 8. A first ring 212 is positioned so as td make up part cf the left leg 206 and the center leg 208. A seccnd r_ng 214 is then put so as to overlap the first ring 212 _n tne area of the center leg 208. The first and the seccnd essentially planar rings 212, 214 are identical or almost identical to their shape, each having four straight sides interconnected by rounded smooth corners, giving a good path for the magnetic flux circulating m the core. Finally there is a third larger ring 216 positioned overlapping the first and seccd rings m the area of the left leg 206 and the right leg 210. The yoke sides of this larger ring 216 is curved and bent so as not to interfere with the first and second rings. The basic configuration shown m fig. 8 comprising three rings creates three legs and interconnecting yokes. In order to complete the core, further rings are added, see fig. 9. Although not necessary, the heights of the rings are adapted so as to create legs with close to elliptical cross-sectional shapes. Thus, each leg is comprised of interleaved rings alternately leading to one of the two other legs. Each of the rings 212, 214, 216 is made up of a number of turns of strips of magnetic material with constant width, see fig. 9a. Thus, the number of turns and the thickness of the strips will determine the height h of each ring while the width of f~e strips will determine the width w of the ring. In this embodiment the magnetic flux will go from one yoke to another via the sides of the rings of strips. An alternative to using strips is to use wires. In tne case wires are used the width of the rings shall be made so small that the transition of magnetic flux is easy obtained also from wires inside the ring. In the case of strips the approximation of cylindrical legs can be made by using narrow strips and by the interleaving of the rings yet get good transition of flux. A spool stand used when manufacturing a cage core according to the invention will now be described wit" reference to figs. 10 and 11. In fig. 10 there is shown a spool stand 30 for iron wires from the yoke side while in fig. 11 the same spool stand is snown from the side, wherein two grooves for legs are seen in the foreground. The spool stand is made of three parallel grodves 35a-c with dpenings outwards in respective directions shifted 120 degrees and convex portions 36 between the ends of the grooves. When manufacturing the core, rings or frames of iron wires are wound between two of the grooves 35a-c in a circular pattern, e.g., first between grooves 35c and 35a, then between grooves 35a and 35b, then between grooves 35b and 35c, then again between grooves 35c and 35a and so on, until circular or essentially circular legs have been built up. To assist shaping the legs, ribbons or laths (not shown) can be used. Supporting pipe halves 37 or divisible bearings 38 can also be used, see fig. 10. Alternatively, all or part of the core can be built from funnel-shaped, thin coils of wire that are manufactured separately by winding wire on stands with conical cross-section. In fig. 12 there is snown a side view of a cage core used as an inductor core. This core 301 is similar to the core described _th reference to figs. 2 and 3, having three legs 3C6, 308, 310 interconnected oy an upper and a lower yc - e . However, after manufacturing, each of the legs nas oeen severed m at least one place, resulting m two core halves, an upper and a lower half, separated by a gap of the distance "d". These gaps are filled with some suitable non-conductive material. This results in a core having a much larger magnetic resistance tnan the transformer core, giving it suitable properties for use as an inductor core. Also, the inherent mechanical stability of the core according to the invention makes this division of the core into two halves possible. Preferred embodiments of a core according to the invention have been described. The person skilled m the art realizes that these could be varied within the scope of the appended claims. Thus, m all the above-described embodiments, imperfections in the shape of the core could be corrected b_, filling with a magnetic composite material, for example. The filling might be necessary to maximize the effect of a transformer when the core is made of round wires m three separately wound rings. The overlapping wire layers allow the flux to pass between the yokes via the leg sections and thus improve the properties of the transformer. The funnel frames ma ιng up the inventive core have been shown with essentially rectangular shape with rounded corners. It will be realized that, due to the flexible nature of tne strip or the wire, the shape can deviate from what is shown m the drawings. Thus, the leg and/or yo e portions can be slightly curved or arched. Also, it has oeen stated that the cross-sections of the legs are circular or near circular. Tney can also devi- ate from the circular shape and be elliptical etc. In the wire core embodiment described with reference to figs. 5 and 6, the yokes are bent tov.ards the center- line of the core. However, when using wires the yokes can also have the shape shown in fig. 2, i.e., being straight between the legs. This is preferably used with wires of a cross-sectional shape different than square, such as circular, rhomboidal, or rhomoic. Also, the wires shown m fig. 6a have a square cross- sectional area. It will be appreciated that these wires also can have a rectangular cross-sectional area without altering tne general core shape shown m figs. 5 and 6. The wires used with the wire core have been described as iron wires. The wires can of course be of any suit- able magnetic material having the desired properties. They can also oe of an amorphous material. Although only the cage core configuration has been shown as an inductor core, see fig. 12, it is realized that the core with the legs m a row, see fig. 7-9, is equally suitaole for use as an inductor core. All cores described herein have a numoer of frames or rings of strips and/or wires. In the cage core embodi ¬ ments described with reference to figs. 1-6 the core is described as comprising a number of separate frames or rings of magnetic material. It will be appreciated that a single strip or wire can be used m several or even all frames, i.e., when one frame has been wound another is wound from the same strip or wire without cutting the first one or using a new one. This would allow for a continuous manufacturing process, possibly interrupted by a short period wherein the core is rotated 120 degrees to allow for winding of another of the three sides of the core.

Claims: CLAIMS 1. A core of magnetic material having a first (c; 6'; 106; 206; 306), a second (8; 8'; 108; 208; 308), and a third (10; 10'; 110; 210; 310) leg portion ana a first (2a-c; 102a-c; 202a-c) and a second (4a-c; 104a-c; 204a-c) yoke portion, said core comprising: - loops (12, 14, 16; 12', 14', 16'; 112, 114, 116; 213, 214, 216) of wires and/or strips of magnetic materia1 ; each of said loops making up part of two of said leg portions ; char ac te ri zed i n tha t - loops making up part of two different leg portions are interleaved in a common leg portion of said leg portions . 2. The core according to claim 1, wherein each of said loops comprises a plurality of layers of wires and/or strips of magnetic material. 3. The core according to claim 2, wherein each of said layers comprises a plurality of strips (12a-e), preferably strips of constant width, each of said strips being offset from adjacent strips. 4. The core according to claim 2, wherein each cf said layers comprises a plurality of wires (112a), preferably wires with a square cross-sectional area. 5. The core according to any of claims 1-4, wherein said leg portions (6, 8, 10; 106, 105, 110) are provided in a triangular pattern. 6. The core according to any of claims 1-4, wherein said leg portions (206, 208, 210) are provided in a row. 7. The core according to any of claims 1-c, wherein said magnetic material is an amorphous material . 8. The core according to any of claims l- ~ , wherein said leg portions have an essentially circular cross-sectional area. 9. The core according to any of claims 1-7, wherein said leg portions have an essentially elliptic cross-sectional area. 10. The core according to any of claims 1-9, wherein said leg portions comprise a magnetic composite material . 11. The core according to any of claims 1-10, wherein a single wire or strip is used in several loops . 12. The core according to any of claims 1-11, wherein all of said Idops (12', 14', 16') have one common leg portion (6' ) . 13. The core according to any of claims 1-12, wherein each of said legs (306, 308, 310) comprises a portion (322a-c) of non-conductive material. 14. A trree-phase transformer having a core of magnetic material with a first, a second, and a tnird leg interconnected by a first and a second yoke, and primary and secondary windings wound around said legs, characterized in that said core is a core (1, 101, 201) according to any of claims 1-11. 15. A single-phase transformer having a core of magnetic material with a first, a second, and a tnird leg interconnected by a first and a second yoke, and primary and seccndary windings wound around one of said legs (6'), wnerem said core (1') is a core according to claim 12. 16. A three-phase inductor having a core of magnetic material with a first, a second, and a third leg interconnected by a first and a second yoke, and windings wound around said legs, characterized in that said core is a core (301) according to claim 13. 17. A method of manufacturing a core of magnetic material having a first (6; 106; 206; 306), a second (8; 108; 208; 308), and a third (10; 110; 210; 31C) leg portion and a first (2a-c; 102a-c; 202a-c) and a second (4a-c; 104a-c; 204a-c) yoke portion, said method comprising the following steps: a) winding a first loop (12; 112; 212) of wires and/or strips of magnetic material, said first loop making up part of said first and second leg portions; b) winding a second loop (14; 114; 214) of wires and/or strips of magnetic material, said second loop making up part of said second and third leg portions; c) winding a third loop (16; 116; 216) of wires and/or strips of magnetic material, said third loop making up part of said tnird and first leg portions; d) repeating steps a)-c) until final cross-sectional shapes of said leg portions are obtained. 18, The method according to claim 17, wherein said loops are wound separately, preferably on stands with conical cross-section.

Inventors: HOEGLUND LENNART, HOEGLUND, LENNART
Applicants: HOEGLUND LENNART, HOEGLUND, LENNART
Application year: 2001
Main classification: H01F27
IPC codes: H01F3, H01F27, H01F30, H01F41