|
(11) | EP 0 010 427 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
|
|
|
|
|||||||||||||||||||||||||||
| (54) | Transformer cores |
| (57) A method of manufacturing transformer cores of either the continuously wound or cut
wound type using electrical steel strip having approximately a linear taper. By selecting
a suitabletaper, a hexagonal (or higher order) approximation of a circular cross-section
for the legs of the cores is available. Two complementary core strips can be cut from
a single rectangular stock strip in a scrapiess manner. |
BACKGROUND OF THE INVENTION
(1) Field of the- Invention
[0001] This invention relates to transformer cores of the wound core type and is suitable for either single phase or three phase trans formers.
(2) Description of the prior art
[0002] There are a multitude of existing designs, which an be broadly catergorized as rectangular or cruciform. After examination of these existing types, it was considered that the cruciform types were philosophically superior, but the rectangular types embodied production advantages, including simplecity.
BRIEF SUMMARY OF THE INVENTION
[0005] It is a further preferred object to produce the cores without a large number of different widths of electrical steel strip being required.
[0006] A preferred feature of the present invention is the use of tapered electrical steel strip to produce a hexagonal or better approximation to circular cross-section for those portions of the core under the windings to enable the production of cores of near optimum geometry as a straight forward proandure
[0008] In the broad aspect, the present invention aosides in a method of manufacturing a transformer core from electrical steel strip including the step of:
cutting at least a portion of the strip with non- parallel sides.
[0010] In another aspect, the present invention resides in a transformer constructed using the method.
[0011] The invention, in its preferred simplest form, is the hexagonal form approximation, which can be achieved in either of two ways, both using a single width size of conventionally slit steel strip, which is then specially slit. For scrapless production of cores two identical cores can be slit from the normal parallel sided strip in such a manner that the tapered pieces are complementary to each others Two identical tapered strips can readily be cut from a suitable rectangular piece by cutting it at an appropriate angle.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE
DRAWINGS.
[0012] To enable the invention to be fully understood, a number of preferred embodiments will now be described with reference to the accompanying drawings, in which:
FIG. 1 is a front view of a single phase core of substantially hexagonal cross-section;
FIG. 1A is a cross-sectional view taken on line 1A-1A of FIG 1;
FIG. 1B shows the cutting plan of the strip for the core of FIG. 1;
FIG. 2 is a cross-sectional view of a modified form of the core of FIG. 1;
FIG. 2A shows the cutting plan of the strip for the sore of FIG. 2;
FIG. 3 is a front view of a single phase shell type core:
FIG. 3A is a cross-sectional view taken on line 3A - 3A of FIG. 3;
FIG. 4 is a front view of a three phase "delta" core;
FIG. 4A is a cross-sectional view taken on line 4A - 4A of FIG. 4;
FIG. 5 is a cross-sectional view of a three-phase star core;
FIG. 5A is a view of an assembled layer of the core of FIG. 5; and
FIG. 5B is a nlan view of the strips for the assembled layer of FIG. 5A.
DETAILED DESCRIPTION OF' THE PREFERRED
EMBODIMENTS
[0013] The production of the tapered electrical steel strip is implemented by a suitable slitting machine. Bocause the angle of taper is so small e.g. less than 1°, the axis of the slitting rollers is set perpendicular to the Strip of el ectrical steel, and the necessary taper is achieved by Forcing the rollers across the sheet. The need for precise control of the positioning of the slitting rollers meaus that the s] itting machine is best built with a single pair of rollers for the slitting operation. This means that it only has to accomodate the width of steel needed for the Largest core to be cut by the method. The details of the method are most easily illustrated by a description of the relevant parts of the machine. After dereeling, the strip is passed through a pair of plain rollers, comprising a driven roller to control the speed of the strip, and its idler. The strip then passes through two guides with tungsten carbide wear parts which control its lateral position, and then through a second pair of rollers similar to the first. The second idler is identical to the first, hut the other roller is machined to have a circumference which matches the number of pulses per revolution of the pulse generator (shaft encoder) that it drives. This unit thus measures the length along the strip as it is fed through, and the slitting roller assembly is immediately adjacent to it. The slitting rollers are mounted on a very rigid frame, and can be set so that they are preloaded to minimize deflection and with the desired amount of overlap. The roller frame is wider than the strip, since it must be able to mere back and forth across the strip. The frame is mounted on a machine bed and driven by a worm drive from a direct current motor geared down by a large amount because of the slow travel required. Included in this assembly is a second shaft encoder with its own small roller which enables the position of the slitting rollers to be known and controlled The conteol system for the slitting roller assembly is straightforward in that the motion of the slitting rollers sideways across the strip is direc ly proportional to the length of strip passing through. This can be implemented by ordinary iogie and servosystem components, but is better and simpler done by a microprocessor based computer which may control the rest of the machine. Thus the tapered strip required for the core designs can readily be produced.
[0014] Referring to FIGS. 1 , 1A and 1B, the core 10 which is continuously wound, has a substantially hexagonal cross-section as shown in FIG. 1A comprising a section 10A of increasing width, a central section 10B of maximum width, and a section 10C of decreasing width.
[0015] Referring to FIG. 1B, section 10A is wound from strip 11A which is cut from a rectangular stock strip 12, the strip 10A having a linear taper from a substantially zero width upto the width of the stock-strip 12.
[0016] Section 10B is wound from a strip 11B cut from a length of the stock strip 12 and has parallel sides.
[0017] Section 10C is wound from a strip 110 out from stock strip 12 and has a linear taper from the width of the stock strip 12 down to substantially zero width. As the strip 11A, 11C are of the same width, they can be cut from lengths of stock strip 12.
[0018] In the modified form of the core illustrated in FIGS. 2 and 2A, the core 20 has a section 20A of increasing width and a section 20B of decreasing width. The seetions both have a maximum width equal to the maximum width of the core and a minimun. width equal to one-half of the core, the sections 20A, 20B being wound from strips 21A, 21B respectively from rectangular stock strip 22 which has a width equal to 1.5 times the maximum width of the core 20. As the strips 21A, 20B are complementary, two transformer cores 20 can be cut from a single length of stock strip 22 without scrap. The single phase shell-type core 30 of FIGS. 3 and 3A has a pair of core frames 31A each with a cross-section which is substantially identical with an isosceles trapezium. When the two frames are placed back-to-back, the central leg is substantially hexagonal in cross-section (as shown in FIG. 3A). Each frame can be wound from a single strip of increasing width, such as strips 21A, 21B shown ifi-FIG. 2A. As these strips are complementary, the core 30 can be produced from a single piece of rectangular stock strip i.e. strip 22.
[0019] The three-phase delta core 40 shown in FIGS. 4 and 4A compaises three froames 41 each conjoined at their sides the other two frames, the legs of the core having a substantially hexagonal cross-section. Each frame 41 is continuously wound from a section 42 of fixed width ( equal to 0°5 times the diameter of the leg) cut from a length of rectanfuiar stock strip of that width, and then a section 43 of decreasing width. As the sections 43 of two of the frames can be complementary, they can be cut from rectangular stock strip having a width equal to 0.5 times the diameter of the core legs. The core 40 is assembled by placing the frames 41 in the configuration shown in FIGS. 4 and 4A and securing the frames together.
[0020] FIGS. 5, 5A and 5B show a three-phase "star" or "Y" core 50 with legs of substantially hexagonal cross-section. The core 50 comprises three frames 51A, 51B, 51C each of substantially C-shape in side view.
[0021] Each frame 51A, 51B, 51C is formed from a series of lamination lengths A,B,C respectively cut from a tapered strip 53.
[0022] The strip 53 is of increasing taper to form section 52A of each frame and of decreasing taper to form section 52B and may be cut as shown in FIG. 2A.
[0023] Lamination lengths A.B.C are cut to selected length of square-ended strip 53 with two cuts at 60° to the longitudinal axis of the strip 53. This cutting step for most size cores can ignore the taper on the layers which come from the taper, strip because of the small angle of taper.
[0024] Each layer is formed as shown in FIG. 5A where the angled cut ends of each lamination length are butted to the side of adjacent strip adjacent its free ends.
[0025] The method of assembling a star core is to lay together all the laminations so that one joint (e.g, the bottom joint) is assembled and the core has the appearance of three radial arms. The joint is clamped and the lamination lengths A,B, C are bent upward until they are perpendicular to the plane of the joint. The lengths are secured and then the top joint is sequentially folded together. The electromagnetic properties of the joint are best when each layer is rotated one third of a turn from the previous layer, that is when piece A of FIG. 5A is placed on each core leg in turn. This joint is applicable not only to cores with tapered strip, but to any core of this type, also known as Y (wye) cores. The section labelled E can be seen to protrude from the joint, but there is no advantage to be gained by removing it.
[0026] All of the new core designs, being of the wound core type, require annealing after they have been cut and formed to shape.
[0027] By using the tapered strip to form the cores, which can be continuously wound ( except for the star core 50 of FIGS. 5, 5A and 5B), the transformer designer can achieve great flexibility in desuign. By careful selection of the taper of the electrical strip, he can achieve almost any core cross-section which he may require and can almost achieve the theoretically optimum circular cross-section. For simplicity, the hexagonal cross-section as an approximation of the circular rrosssection is readily achievable.
[0028] The invention in addition to its application as a means of produc ing the hexagonal form approximation, can in a analogous fashon to the cruciform case, be used to produce octagonal of higher order even regular sided approximations to a circular cross-soction. However, for each pair of sides in excess of six an additional size of parallel strip is required to allow scrapless production of the core.
cutting at least a portion of the strip with a-parallel sides.
the portion of the strip is out with an approx- inkitoly linean lapel
the tapered core strips arc cut from a parallel side and tionof cloale ical steel strip, the tapered core strips being complemenlary to each other.
the parallel side portion has a width of approximately 1.5 times the width of the core section; and
the tapered core strips have a width of approximately 0.5 times the width of the core section.
cutting a fiist portion with a taper of increasing width; and
cutting a second por tion on with a taper of deereasing width, the length of the second portion being greater than the length of the first portio.
the electrical strip section has a width of tpproximately 1.5 times the width of the core section; anr
the minimmm widths of' the first portion and the second portion are approximately equal to 0.5 times the width of the core section.
cntting a third portion with substantially parallel sides to be interposed between the first portion and the second portion, the third portion having a width equal to the marimum width of the core section.
cutting a first portion for each frame with substantially parallel sidos. said first portion being of a width substantially equal to 0.5 times the diameter of the leg sections of the core; and
cutting a second portion for each frame with an increasing or decreasing taper.
cutting a first portion of increasing taper from a minimum width to a maximum width ;
cutting a second portion from a maximu width to a minimum width;
cutting cach portion into a plurality of iamination lengths, each group of three lanination lengths being separated from the adjaccnt group at each end by a cut at substantially 90° to its longitudinal axis, each said group forminga layer of the core, with the three lamination lengths in the group being sepatated form each other by cuta at subatantially 60° to the longitudinal axis;
butt joining the ends of two of the lamination lengths cut at substantially 60° to the sides of a third lamination length cut substantially 90° adjacent the ends of the third lamination length to form a lamination layer; and assembling the lamination layers to form the assembled core.