Method of manufacturing wound transformer core

Abstract

In a single phase transformer core and transformer, thin-strip metal is wound into multiple rings of different widths and arranged to define a ring-like structure having a stepped, substantially circular cross-section without any cuts or gaps in the magnetic path, or the core is wound from a tapered strip that is configured to define a substantially circular cross-section when wound, while in a three phase transformer core and transformer two inner frames, each made of one or more wound rings are arranged side-by-side and an outer frame of one or more rings is wound around the two inner frames, the core being covered with epoxy prior to winding coils on it.

Claims

1. A method of making a thin-strip metal transformer core for a transformer, comprising winding one or more rings, each ring formed by winding multiple turns of continuous thin-strip metal, wherein the thin-strip metal is controlled so that the turns lie one on top of the other with the center line of the strip for each turn aligned in a plane; configuring the one or more rings to define two or more straight core legs having a substantially circular cross-section, and freezing the core by applying an epoxy or other shell to the outer surfaces of the core before applying any transformer coil windings.

2. The method of claim 1, wherein each ring is wound from parallel sided thin-strip-metal, and rings of different strip widths are wound on top of each other to define the substantially circular cross section for the core legs.

3. The method of claim 2, wherein the core is configured to have two straight core legs, which are connected at each end by a yoke.

4. The method of claim 1, the method further comprising winding a first set of two or more rings of different strip widths on top of each other to define a first frame; winding a second set of two or more rings of different strip widths on top of each other to define a second frame; arranging the first and second frames next to each other to define a first core leg of a transformer core between them, and winding a third set of two or more rings of different strip widths on top of first and second frames, to define a second and a third core leg located on either side of the first core leg.

5. The method of claim 1, wherein the freezing comprises applying an epoxy to the core at one or more stages as the rings are being wound, or continuously as the rings are being wound, or once all of the rings have been wound.

6. The method of claim 1, wherein at least some of the rings are wound from strips of thin-strip metal, the sides of which are non-parallel for at least part of the length of the strip.

7. The method of claim 6, wherein the strip for each ring includes a first end defining a starting end and a second end defining a terminating end, and wherein one or more of the rings has a non-parallel sided, tapered portion at the starting end or the terminating end or at both the starting and terminating ends.

8. The method of claim 7, the method further comprising winding a first ring from a thin-strip metal having a non-parallel sided, tapered portion at the starting end, to define a first frame; winding a second ring from a thin-strip metal having a non-parallel sided, tapered portion at the starting end, to define a second frame; arranging the first and second frames next to each other to define a first core leg of a transformer core between them, and winding a third ring from a thin-strip metal having a non-parallel sided, tapered portion at the terminating end on top of first and second frames, to define a second and a third core leg located on either side of the first core leg.

9. The method of claim 7, wherein the non-parallel sided, tapered portions are non-linear tapered portions.

10. A method of making a transformer, comprising winding a transformer core, which includes winding one or more rings, each ring formed by winding multiple turns of continuous thin-strip metal, wherein the thin-strip metal is controlled so that the turns of the strip lie one on top of the other with the center line of the strip for each turn aligned in a plane, and configuring the one or more rings to define two or more straight core legs having a substantially circular cross-section, the method further comprising freezing the core by applying an epoxy or other shell to the outer surfaces of the core, and winding transformer coil windings onto at least some of the legs after the freezing step.

11. The method of claim 10, wherein the freezing comprises applying an epoxy to the core at one or more stages as the rings are being wound, or continuously as the rings are being wound, or once all of the rings have been wound.

12. The method of claim 10, wherein each ring is wound from parallel sided thin-strip metal, and rings of different strip widths are wound on top of each other to define the substantially circular cross section for the core legs.

13. The method of claim 12, wherein the core is configured to have two straight core legs, which are connected at each end by a yoke.

14. The method of claim 10, wherein the winding of the core includes, winding a first set of two or more rings of different strip widths on top of each other to define a first frame; winding a second set of two or more rings of different strip widths on top of each other to define a second frame; arranging the first and second frames next to each other to define a first core leg of a transformer core between them, and winding a third set of two or more rings of different strip widths on top of first and second frames, to define a second and a third core leg located on either side of the first core leg.

15. The method of claim 10, wherein at least some of the rings are wound from strips of thin-strip metal, the sides of which are non-parallel for at least part of the length of the strip.

16. The method of claim 15, wherein the strip for each ring includes a first end defining a starting end and a second end defining a terminating end, and wherein one or more of the rings has a non-parallel sided, tapered portion at the starting end or the terminating end or at both the starting and terminating ends.

17. The method of claim 16, wherein the winding of the core includes, winding a first ring from a thin-strip metal having a non-parallel sided, tapered portion at the starting end, to define a first frame; winding a second ring from a thin-strip metal having a non-parallel sided, tapered portion at the starting end, to define a second frame; arranging the first and second frames next to each other to define a first core leg of a transformer core between them, and winding a third ring from a thin-strip metal having a non-parallel sided, tapered portion at the terminating end, on top of first and second frames, to define a second and a third core leg located on either side of the first core leg.

18. The method of claim 16, wherein the non-parallel sided, tapered portions are non-linear tapered portions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a front view of one embodiment of a wound transformer core of the invention,

(2) FIG. 2 shows a cross-sectional view through a leg of the core of FIG. 1,

(3) FIG. 3 shows a cross-sectional view through a leg of another embodiment of a wound core of the disclosure,

(4) FIG. 4 shows a front view of one embodiment of a single phase transformer core of the invention showing the use of winding tubes to wind coils on the legs of the core,

(5) FIG. 5 is a flow chart describing the winding process of the disclosure,

(6) FIG. 6 shows one embodiment of a single tapered strip of the disclosure (Haihong type taper),

(7) FIG. 7 shows another embodiment of a single tapered strip of the disclosure (tapered both left and right sides of the strip but at only one end),

(8) FIG. 8 shows one embodiment of a double tapered strip of the disclosure (both first and second ends tapered the same way),

(9) FIG. 9 shows another embodiment of a double tapered strip of the disclosure (both first and second ends tapered the same way with tapers extending to the middle of the strip),

(10) FIG. 10 shows another embodiment of a double tapered strip of the disclosure (taper at first end being different to taper at the second end)

(11) FIG. 11 shows a cross-sectional view through a leg of a wound core of the disclosure made using a strip such as that illustrated in FIG. 8,

(12) FIG. 12 shows a cross-sectional view through a leg of a wound core of the disclosure made using a strip such as that illustrated in FIG. 9,

(13) FIG. 13 shows cross-sectional view through a leg of a wound core of the disclosure made using a strip such as that illustrated in FIG. 10,

(14) FIG. 14 shows a yet another embodiment of a double tapered strip of the disclosure,

(15) FIG. 15 shows a cross-sectional view through a leg of a wound core of the disclosure made using a strip such as that illustrated in FIG. 14,

(16) FIG. 16, shows one embodiment of a core winder of the invention in which a laser cutter is mounted on the winder,

(17) FIG. 17 shows a strip slit to define two strips with tapered sides,

(18) FIG. 18 shows an exaggerated cross section through a ring formed from a straight sided strip with double taper starting with the narrow end of the strip,

(19) FIG. 19 shows an exaggerated cross section through a ring formed from a straight sided strip with double taper, starting with the wide end of the strip,

(20) FIG. 20 shows a cross-section through another embodiment of a core leg of the present disclosure,

(21) FIG. 21 shows a strip (not to scale) for forming a core as shown in FIG. 20,

(22) FIG. 22 shows a front view of one embodiment of a single phase core of the application,

(23) FIGS. 23-24 show front views of two embodiments of a three phase core of the application,

(24) FIG. 25 shows a top view of a strip of thin-strip metal for use in one embodiment of a core of the application, and

(25) FIG. 26 shows a cross-section through a frame of the core embodiment of FIG. 24.

DETAILED DESCRIPTION OF THE INVENTION

(26) The present invention provides a wound transformer core that is produced as a new ring-like wound core configuration that allows coils subsequently to be wound on the legs of the core without cutting the wound layers of the core.

(27) FIG. 1 shows a top view of one embodiment of a wound transformer core 100 of the invention. The ring-like structure of the core 100 is made up of multiple rings 102 of magnetic steel, e.g., amorphous metal strip or nano-grain steel material or silicon steel, of different widths (not shown in FIG. 1) wound on top of one another. Insofar as the thickness of the magnetic steel is less than 2.5 mil, e.g., amorphous metal or nano-grain steel, the strip material will be referred to herein in as thin-strip metal.

(28) The different widths of the rings 102 are best illustrated in the cross-sectional views of FIGS. 2 and 3. The ring-like structure defining the core 200 is made up of multiple rings, each of which is wound from one or more strips of magnetic steel. The different width rings define a cross-sectionally stepped structure within circumscribed circle 204 as shown in FIG. 2. The first or inner ring 210 and last or outer ring 220 in this embodiment are made from one or more strips of a first width, which is narrower than the strips of the other rings. The second and second last rings 212, 222 are made from strips of a second width that is wider than the first width. In this embodiment, a middle ring 230 is included between the second ring 212 and second last ring 222, the middle ring 230 being formed from strips of a third width that is wider than the second width, and has twice as many layers as either of the first or second rings. The widths of the rings are arranged to define a completed ring-like structure with a cross-section that has a stepped configuration but is substantially circular as depicted by the circle 204. It will be appreciated that while the above embodiment made use of 5 rings, more or fewer rings could be used and the number of layers per ring may vary in order to achieve a substantially circular cross-section. Thus, for example, the embodiment shown in FIG. 2 makes use of rings that have related step heights. In particular, in this embodiment, the number of layers which defines the thickness of the ring (build of the ring) is the same for each ring except the middle ring, which has twice the build. Thus the change in strip width of the rings gets more pronounced toward the inside and outside of the ring-like structure.

(29) In the embodiment of FIG. 3, the number of rings has been increased and the change in width from one ring to the next has been kept constant. In order to achieve a substantially circular cross-section for the ring-like structure the step height is reduced from one ring to the next as one moves from the middle ring 300 to the outer and inner rings. In other words the build for the first and last rings 310, 350 is less than that for the second and second-to-last rings 312, 352, which in turn is less than the build for the third ring 314 and third-to-last ring 354, which is less than that of the fourth ring 316 and fourth-to-last ring 356.

(30) It will therefore be appreciated that depending on the approach taken in winding a substantially round cross-sectional core, the number of layers per ring may vary from one ring to the next or remain constant. Similarly the change in ring width (as defined by the strip width) from one ring to the next may be a constant change or may become more pronounced toward the inner and outer rings.

(31) It will also be appreciated that practical considerations may require a less than optimum fill factor. The number of strip widths that are included determines how fully the circumscribed circle can be filled. Therefore by necessity, reducing the number of strip widths reduces the fill factor that can be achieved. Also, for a defined number of strip widths an optimum fill factor may be achieved if both the strip widths and the build may be selected. On the other hand, this optimum fill factor may not necessarily be achieved if the design of the core is limited to pre-defined strip widths based on strip widths available to the manufacturer in its inventory.

(32) The rings in the above embodiments were wound in a race-track configuration by winding onto an oblong former to define a core with two substantially straight, parallel legs joined at their ends by curved yokes. In another embodiment the rings were wound in a circular configuration and subsequently deformed to define two substantially parallel legs. Tests have, however, shown that greater winding speed is achieved and less stress is introduced into the core if the rings are initially wound in circular fashion to define a circular ring-like structure that is subsequently deformed to define two substantially straight, parallel legs joined at their ends by curved (e.g., semi-circular) yokes.

(33) By forming a core with two substantially straight, parallel legs, split winding tubes can be placed around the legs and attached together for winding the primary and secondary transformer coils onto the legs without the need to cut the core in order to place or land the coils on the legs. This is shown in FIG. 4 in which two winding tube halves 400 are placed around each substantially straight core leg 410, 412 and connected together, e.g., by means of tape to define a winding tube 402 as shown around the leg 410. In this embodiment the two tube halves are secured to each other by means of a thermally graded tape wrapped around the tube 402. One or more gears or sprockets 420 that are split into two halves 422, 424 are secured to the winding tubes for purposes of rotating the tubes by means of a motor that engages the sprocket or gear with a chain or complementary gear. In this embodiment the sprocket or gear halves 422, 424 are provided with tabs 426 having threaded holes in them for receiving bolts to connect the sprocket or gear halves to each other. In order to transfer the energy from the sprocket or gear 420 to the tube 402 the gears engage the tube in a non-slip relationship, e.g., by providing the sprocket or gear with radially inwardly extending tabs for engaging complementary notches or holes in the tube.

(34) By avoiding the need to cut the core material into sections in order to unlace the core and land the coils on the core, a substantial benefit is achieved over prior art wound distributed gap cores or stacked butt-lap and step-lap cores, which comprise cut sections of core material that overlap each other to define a complete flux path. The overlapping arrangement of sections is adopted in distributed gap, butt lap and step-lap cores to allow the core to be opened up or unlaced in order to land the coils on the core. Thereafter the core layers have to be put together again or re-laced. The problem with this prior art approach is that the core material provides the path (roadway) along which the flux travels. At every cut and gap the flux must make a detour. In butt lap cores the problem is particularly acute since the gap may be large, which makes it much more difficult for the flux to find an alternative path, thus leading to fringing, etc. Techniques such as step-lap cutting shorten the detour since the detour may be only one laminate thickness away, however, the flux still has to make a detour, which leads to losses. Every cut and gap increases the reluctance of the core and adds to losses and sound. Prior art wound core configurations such as distributed gap cores retain this problem by winding the magnetic material, e.g., amorphous metal in sections onto a former, thereby allowing the core subsequently to be opened up or unlaced in order to receive the core. In the case of annealed amorphous metal this process is particularly problematic due to the brittleness of the material after annealing or domain setting. The unlacing and re-lacing invariably results in a substantial amount of flaking of the amorphous metal, which increases the risk of pieces of amorphous metal later causing electrical shorting problems.

(35) Thus by adopting a cross-sectional core shape that allows a winding tube to rotate on the leg of the core while avoiding large air gaps between the core and the coils, the present invention allows the coils to be wound on the leg. This avoids having to create gaps in the magnetic field path and avoids unlacing and re-lacing of core layers in order to land the coils on the core legs.

(36) As depicted by the flow diagram of FIG. 5, in one embodiment, before applying the first layer of the coil windings, the surfaces of the core are coated with an epoxy (step 502), which in this embodiment is a two-part epoxy (Huntsman 5865 A & B) that cures at room temperature. This has the benefit of strengthening the core. The epoxy also provides a smoother outer surface to the core, thereby making it easier for the winding tube to rotate on the core leg. In this embodiment the resin or epoxy coating is further strengthened by adding fiberglass chop into the resin or epoxy. Phenolic balloons are added to the resin to provide sound dampening as depicted by step 500. Once the epoxy has cured, the winding tube is rotatably attached to the core (step 504). In one embodiment, a low voltage winding is then wound (step 508), onto each of the legs. The windings may include a polymeric or other insulation e.g., DuPont Nomex™. One or more layers of insulating material are then applied over the low voltage winding (step 514), before winding the high voltage winding on top of the low voltage winding. As is known in the art, a low and high voltage winding can be applied to each leg or the high voltage winding can be wound on one leg and the low voltage winding on the other leg. Typically, spacers or ducts are included as part of the coil design process and are included into the windings for cooling purposes. Thus spacers of the pre-designated thickness are axially arranged around the circumference of the winding between some or all of the winding layers (step 510). Spacers are provided in this embodiment between the low voltage and the high voltage windings (step 520).

(37) If the high voltage winding is to be disc wound the process begins by winding discs with insulated rectangular wire (step 524). As insulator for the wire, DuPont Nomex™ is used in one embodiment. In another embodiment a wire insulated with a polymer film that is compatible with the resins used between the layers of windings is used. Axially arranged duct spacers are again provided between the layers of the windings depending on the design requirements of the transformer (step 530).

(38) When the high voltage winding is complete a thin coating of compatible polymer or a varnish is applied to the windings by spraying the polymer onto the windings using nozzles or by dipping the entire transformer in a polymer or varnish.

(39) In the above embodiment, the coils were implemented using aluminum or copper wire having a round cross section or a rectangular cross section and covered with Nomex. The invention could instead be implemented using foil material and insulators. For instance foil conductors in the form of sheets could be wound onto the core legs using a winding tube. In one embodiment the foil is wound in two side-by-side sections that are then connected beginning to end so that the direction of the winding is the same for both sections (either both clockwise or both anti-clockwise). The invention is not limited to only one or two sections of foil. More than two sections can for instance be implemented. This also has the advantage that each section is narrower allowing resin to penetrate more easily between the coils.

(40) The core and resultant transformer, whether used as a single phase transformer or used as a set of three single phase cores wired to define a three phase transformer, produces numerous advantages. The process produces an amorphous core with minimum stress in the core, which does not require multiple cuts or post annealing manipulation in order to land the coils. The simple ring-like configuration of each single phase core does not require physical interconnection of core material with other single phase cores and does not require cutting or splicing together of layers of core material, thereby allowing each core structure to be wound separately and very quickly. Thus a core can be produced in much less time than any type of distributed-gap wound core. The configuration and process entirely eliminates damage to the core material due to cuts and gaps. Flaking and chips are virtually eliminated since there is no need to unlace the layers of core material in order to land the coils on the core, or to re-lace the core material. Testing has shown core losses using amorphous metal to be 6-10% lower than comparable Evans or five-legged amorphous cores. The cores produced may be operated at induction levels higher than amorphous Distributed Gap cores. By providing a core configuration in which the cross-sectional area of the yokes and legs are the same, the core can be operated at higher induction levels than three-phase configurations having different cross-sectional areas in parts of the core, such as Stadium cores provided by Haihong in China, and Hexaformer cores, both of which result in reduced core material in the yokes compared to the legs of the core. Audible sound levels have also been found to be lower at the higher induction levels than is the case with amorphous distributed gap cores.

(41) The single-phase transformers produced by this method are suitable for applications in which any other single-phase transformers are utilized, either as stand-alone single-phase transformers or wired together with other single-phase transformers to provide three-phase transformers. Cores using this invention may readily be produced ranging from 15 kVA through 3.3 MVA.

(42) The present application also includes the forming of a wound transformer core from one or more strips of magnetic steel material, wherein at least some of the strips have tapered sides. For instance, a single strip can be used having a first and a second end with a double-sided taper toward each end, which may or may not have a central un-tapered portion, as discussed in greater detail below.

(43) As discussed further below, and as described in co-filed application to the same applicant, entitled “Laser Slitting of Magnetic Steel” the taper may be cut with a laser.

(44) As is discussed in co-filed application entitled “Laser Slitting of Magnetic Steel”, the use of a laser to slit or cut magnetic steel can be applied to the manufacture of existing tapered ribbons such as the Haihong (or Stadium) configuration core, which to the best of the applicant's understanding makes use of a single tapered ribbon in forming its three-phase wound core. The Haihong strip of magnetic core material 600 appears to be limited to a single taper along one side 602 that extends the full length of the strip of magnetic core material from a first end 610 to a second end 612 as illustrated in FIG. 6. It will be appreciated that the depictions of the strip shapes given above and the other embodiments discussed below are not to scale. Actual strips will be significantly longer and the taper cut more gradually, and in many instances slightly curved.

(45) FIG. 7 shows another embodiment of a single tapered strip 700 of the invention. In this embodiment the taper is applied to both the left side 702 and the right sides 704 of the strip and again extends along the entire length of the strip from the first end 710 to the second end 712. It will be appreciated that when this strip is wound starting with the narrow end, the wound ring will gradually get wider. In order to provide a substantially circular cross-section to the core, a second ring has to be wound on top of the first ring that tapers the opposite way, i.e. starts wide and ends narrow to define, for example a hexagonal cross-section. In another embodiment, a central ring wound from parallel-sided strip material can be formed between the inner and outer rings to defined, for example, a core with an octagonal cross-section.

(46) FIG. 8 shows one embodiment of a double tapered strip 800 of the present disclosure in which both the first end 810 and the second end 812 are narrowed relative to a central portion 830 of the strip. In other words the strip 800 defines two opposite tapers. In this embodiment both the one side 802, e.g., the left side, and the other side 804, e.g., the right side of the strip are equally tapered and the tapers at the two ends do not extend all the way to the middle. In other words the strip 800 defines a central portion 830 with parallel sides.

(47) FIG. 9 shows another embodiment of a double tapered strip 900 of the invention that is similar to that of FIG. 8. However in this embodiment the taper at the first end 910 and the taper at the second end 912 extend to the middle of the strip. As in the strip of FIG. 8, the strip 900 is tapered identically along both the left and right sides, 902, 904.

(48) FIG. 10 shows yet another embodiment of a double tapered strip 1000 of the present disclosure, in which the taper at the first end 1010 is different to the taper at the second end 1012.

(49) In the above embodiments the tapers were depicted as straight-sided or linear tapers, for purposes of simplicity. In practice, however the taper will be curved in order to take account of the increasing path length as the ring's diameter increases. This will be discussed in greater detail below with reference to FIGS. 18-21.

(50) The effect of these different double tapered strips (when path length is taken into account and the taper, correctly curved) is shown in FIGS. 11 to 13. It will be appreciated that the impact of the taper will become more pronounced as the ring diameter increases, thus requiring a concave or convex taper, as will be discussed in further detail below with respect to FIGS. 18 and 19.

(51) FIG. 11 shows a simplified cross-sectional view through a leg of a wound core of the invention made using a strip similar to that illustrated in FIG. 8 (but with the appropriate curvature to the taper). Since the central portion 830 of strip 800 is not tapered the resultant ring has a cross section that is substantially rectangular with truncated corners 1100. By correctly choosing the curvature of the taper and length of each portion of the strip, an octagonal cross-section can be achieved.

(52) FIG. 12 shows a cross-sectional view through a leg of a wound core of the invention made using a strip similar to that illustrated in FIG. 9 (but with the appropriate curvature to the taper). In the strip 900 the tapers at the two ends 910, 912 extended to the middle of the strip thereby defining a wound ring with a 6-sided cross-section, which may be arranged to be hexagonal if the curvature of the taper is calculated correctly.

(53) FIG. 13 shows a cross-sectional view through a leg of a wound core of the invention made using a strip similar to that illustrated in FIG. 10 (but with the appropriate curvature to the taper). In this embodiment the taper at the first end 1010 was different to that at the second end 1012, not only to take account of the greater path length toward the outer portion of the ring, but also to achieve a different cross-sectional profile to the core ring, resulting in a wound ring with a non-uniform or substantially egg-shaped cross-section.

(54) In the embodiments discussed above the tapers were depicted as linear tapers for purposes of simplicity, but for the purpose of achieving multi-sided cross-sections. The present disclosure also allows for non-linear tapers such as the strip 1400 shown in FIG. 14, which provides a curved taper at both the left and right sides 1402, 1404 of the first end 1410 and the second end 1412 that is sufficient to achieve a cross-section with curved outer surface. The result is that when the strip 1400 is wound into a core ring the cross-section of the ring of this embodiment is substantially circular as shown in FIG. 15, which shows a cross section of the resultant core ring formed from winding the strip of FIG. 14.

(55) FIG. 16, shows one embodiment of a core winder 1600 of the invention in which one or more laser cutters, e.g., a continuous 20 nm wavelength laser, is mounted on the winder. In this embodiment two pay-out reels 1602, 1604 are used to wind two ribbons or strips 1630, 1632 onto a single former or winding head 1610, which in this embodiment is depicted as a race-track former 1610. It will be appreciated that the two ribbons or strips 1630, 1632 are wound simultaneously one on top of the other onto the same former 1610 to produce a single common race-track-shaped ring. Instead only one of the pay-out reels 1602 or 1603 could be loaded onto the winder to wind only one strip onto the former 1610. Since a single ring is formed in both cases, the use of two strips wound simultaneously on top of one another onto a common former, also referred to herein as a 2-ply or multi-ply arrangement, will be considered herein as forming a ring from a single or continuous strip or ribbon since multiple wound strips are wound simultaneously as one. The laser cutter in this 2-ply embodiment is implemented as 4 laser units 1620 for cutting both the left and right sides of each strip 1630, 1632 as the two strips are wound from the two pay-out reels onto the former 1610. In this embodiment the cut-off portions along the left and right sides of the strips 1630, 1632 are discarded or collected as scrap.

(56) In another embodiment, where a substantial amount of material would be discarded, the cut off portion can be wound onto one or two separate take-up reels. For instance, in one embodiment a strip 1700 such as that shown in FIG. 17 could be slit diagonally as shown, to end up with two tapered sections 1702, 1704. It will be appreciated that although each section is tapered along one side only, the two sections 1702, 1704 can be considered as double sided tapered strips by defining the central axes as 1712, 1714, respectively. The one strip 1702 can be wound onto a first take-up reel to form the inner ring of a core since it gradually increases in width. The second strip 1704 will be starting with the wide end and therefore define a gradually narrowing ring. This can subsequently be re-wound onto a new or third take-up reel to define a ring that is gradually increasing and thus usable as an inner ring for another core.

(57) As mentioned above with respect to FIGS. 11-13, the cross-sections shown in FIGS. 11-13 are simplified depictions and do not accurately reflect the cross-section when taking into account the changing diameter of the ring as it is wound. Considering again the embodiment of FIG. 17, if strip 1702 were wound to define the inner ring of a core, then taking into account the ever increasing path length of each successive layer due to the increasing build on the take-up reel, the cross-section of the first ring would have concave sides and look substantially as depicted in FIG. 18. Similarly, if the strip 1704 were wound onto a second take-up reel, it would have a cross-section with convex sides, substantially as shown in FIG. 19. A further complication that arises is if this strip 1704 were then re-wound onto a third take-up reel and thereafter wound on top of the inner ring formed by strip 1702. The increased path length of this outer ring (since it is formed onto the inner ring, which itself has a certain build) coupled with the ever increasing path length with each layer built on top of the previous layer will result not only in convex side walls to the outer ring but also in fewer layers and a smaller build for the outer ring. In practice, therefore the taper of the strips would have to be adjusted for the inner and outer rings and the outer ring would have to be made from a longer strip than the inner ring if, for instance a hexagonal or octagonal cross-section is to be achieved.

(58) One embodiment of the present disclosure involves forming a substantially square cross-sectional core with rounded corners, as shown in FIG. 20, which can be achieved by trimming both sides 2102, 2104 towards each end 2100, 2106 of the strip at a taper to form a leading tapered portion 2112 at a first or starting end 2100 of the strip as shown in exaggerated fashion in FIG. 21. The leading end 2100 has the sides 2102, 2104 slit at a curve for the portion 2112, to define convex surfaces (For ease of depiction, the curved sides of portion 2112 are simply shown as straight lines in FIG. 21. In practice, the curved sides will depend on the desired cross sectional shape and the cross sectional area of the core, since for larger cores the curved corners of the cross section will have a larger radius and will be longer, thus affecting a longer portion 2112 of the strip). For the trailing end 2106, the tapered portion 2110 is tapered over a longer distance than the tapered portion 2112, since the path length of each turn will be getting longer toward the trailing end 2106 due to the ever increasing build of the core (Again, for ease of reference the tapered sides of portion 2110 are simply shown as straight tapers but in practice may be curved to define convex sides to provide a core cross section that is substantially square 2308, 2350, 2352 with rounded corners as depicted in FIG. 20).

(59) In one embodiment of the present application, a single phase transformer core is formed using an amorphous metal or other thin-strip metal that has tapered ends as discussed with respect to FIG. 21 to provide a core as depicted in FIG. 22, with a leg cross section through legs 2200, 2202 similar to that shown in FIG. 20. Unlike the embodiments of FIGS. 2 and 3, the entire single phase core in this embodiment can be wound from a single strip of thin-strip metal

(60) Since the present invention deals with thin-strip material, which has little structural integrity on its own, an exo-skeleton has to be formed that will support the core. In one embodiment, an epoxy is applied to the core windings, e.g., by spraying, as depicted by reference numeral 2204. The epoxy can be a UV-cured (ultra-violet cured) resin and can be applied during the winding process, continuously or at various stages of the winding. Instead the epoxy can be applied once the core winding is complete. The epoxy coating over the core covers and strengthens the core as a precursor to winding any coils onto the core to form a transformer. In the present invention, coils are wound onto the core legs using winding tubes that are rotatably affixed to the core legs and then rotated in order to wind the coils onto the core tubes, as discussed above with respect to FIG. 4.

(61) In contrast, in the case of prior art cores such as wound digital gap cores, coils are wound separately and then placed onto the core legs by opening up the core (unlacing the core). In other words the core is made up of strip sections, each forming one turn of the wound core, rather than a continuously wound strip of multiple turns. This lacing and unlacing process damages the core and adds unlacing and re-lacing steps. Also, since part of the core cannot be protected by an epoxy or other protective layer until the coils have been placed on the legs, a second epoxy application step has to be introduced once the core layers are re-laced, in order to cover the rest of the core with epoxy.

(62) In the present invention, the core is wound from one or more continuous strips of thin-strip metal, each strip being wound up to form a ring having multiple turns, for example, several hundred turns of amorphous metal or nano-grain steel. In the case of a cruciform configuration, such as the one shown in FIG. 2, the core is made up of multiple rings of different strip width wound on top of each other to form the staggered cruciform cross-section. The center lines of the strips, depicted for example by line 1420 in the FIG. 14 embodiment, or by lines 1712 and 1714 in the embodiment of FIG. 17, are aligned in a plane one on top of each other as the strip is wound up into a ring. Insofar as the core is made up of multiple rings, the center lines of the strips in the various rings are also aligned with each other.

(63) In other embodiments of the present application, a three phase transformer core is formed, either using multiple parallel-sided strip widths as depicted in FIG. 23 or using strip widths with tapered ends as shown in FIG. 24.

(64) In the embodiment of FIG. 23, two inner frames 2302, 2304 are formed. Each frame 2302, 2304 is made up of multiple rings (in this case 3 rings). Frame 2302 is wound from three parallel sided strip widths to form three rings 2310, 2312, 2314 of ever increasing width. Similarly, frame 2304 is wound from three parallel sided strip widths to form three rings 2320, 2322, 2324 over increasing width. The two frames 2302, 2304 are then mounted side by side on a core winding machine to serve as take-up for a third, outer frame 2306. When two such frames 2302, 2304 are placed next to each other they define a central leg 2308 having a cross-section similar to that shown in FIG. 2, which for purposes of this application will be referred to as a substantially round cross-section.

(65) The frame 2306 is also made of three strip widths, which are of the same width as the strip widths used in the rings of frames 2302, 2304. However, in the case of frame 2306 the first ring 2330 to be wound onto frames 2302, 2304 has the widest strip width (same width as that used for rings 2314, 2324). The next two rings wound onto the first ring 2330 become progressively narrower. In other words, ring 2332 has the same strip width as rings 2312, 2322, while ring 2334 has the same width as rings 2310, 2320.

(66) The effect of forming frame 2306 around frames 2302, 2304 is to provide substantially straight parallel legs 2350, 2352 on either side of central leg 2308, each of the legs having a substantially circular cross-section similar to that depicted in FIG. 20. Like leg 2208, the legs 2350, 2352 have a cross section similar to that shown in FIG. 2.

(67) Again the core is frozen using one or more epoxy coatings to cover the core before the coils are wound onto the three legs 2308, 2350, 2352.

(68) In the embodiment of FIG. 24, two inner frames 2402, 2404 are each formed from a strip similar to the strip 2500 shown in FIG. 25, having a tapered portion 2502 at the first or starting end 2504. In this embodiment the taper is a non-linear taper, in which the sides of the strip converge in non-parallel, non-linear fashion. (In another embodiment, a strip with a linear taper can be used instead). The rest of the strip has parallel longitudinal sides 2510, 2512. The effect of winding a strip 2500 onto a former having two parallel sides is to form a frame cross-section similar to that depicted in FIG. 26. When two such frames 2402, 2404 are placed next to each other they define a central leg 2410 having a square cross-section with rounded corners, similar to that shown in FIG. 20, which for purposes of this application will be referred to as a substantially round cross-section.

(69) The two frames 2402, 2404 are mounted side by side on a core winding machine to serve as take-up for a third, outer frame 2406. The outer frame 2406 is wound from a thin-strip metal that has a tapered portion at one end similar to strip 2500 shown in FIG. 25. However, in this case the parallel portion is chosen as the starting end, while the tapered portion 2502 is at the terminating end. Also, since the path length of each turn of the third (outer) frame 2406 is much longer than that of the first and second frames 2402, 2404, the strip for frame 2406 will be much longer and the tapered portion will also be longer than the tapered portions of the strips used for frames 2402, 2404. The effect of forming frame 2406 around frames 2402, 2403 is to provide substantially straight parallel legs 2450, 2452 on either side of central leg 2410. Like leg 2410, the legs 2450, 2452 have a cross section similar to that shown in FIG. 20.

(70) Again the core is frozen using one or more epoxy coatings to cover the core before the coils are wound onto the three legs 2410, 2450, 2452.

(71) While the invention has been described with respect to specific embodiments it will be appreciated that the invention is not so limited but includes other implementations as defined by the scope of the claims and as may be readily determined by someone familiar with the art.