Static induction electrical device

Abstract

This invention provides a static induction electrical device that improves the smoothness of magnetic flux distribution within an iron core. The static induction electrical device of this invention comprises a wound iron core and windings. The wound iron core is a stack of overlapping magnetic bodies, and has stepped joints at least on its inner circumference. The gap between the ends of the stepped joints gradually shortens towards the outer circumference. This provides a static induction electrical device with a simple structure that improves the smoothness of magnetic flux distribution within the iron core.

Description

Technical Field

[0001] This invention relates to static induction electrical equipment such as transformers and reactors. Background Technology

[0002] In static induction electrical equipment, such as the wound core of a transformer, the difference in magnetic circuit length between the inner and outer sides of the core material results in a tendency for high magnetic flux density on the inner side of the core, gradually decreasing towards the outer side. Furthermore, in wound cores, eddy current losses increase due to flux concentration caused by flux crossing at the ends of the core, leading to deterioration of magnetic properties. In particular, the magnetic flux density decreases on the outer periphery, resulting in high magnetic losses and further deterioration of magnetic properties. Therefore, there is a problem of the magnetic flux distribution on both the inner and outer periphery not being smoothed out.

[0003] However, most coiled iron cores use amorphous materials. Amorphous materials have better magnetic properties than silicon steel plates, but their thickness is only about 1 / 10 (about 0.025 mm) of silicon steel plates, making them very thin and hard, and difficult to process in transformer manufacturing.

[0004] Therefore, the manufacturing process of the coiled iron core of a transformer using amorphous materials involves stacking multiple sheets of amorphous materials into a layered structure, cutting them, stacking them again, and then winding them into the shape of an iron core.

[0005] As a joining structure between the ends of amorphous materials wound into a core shape, two joining structures are known: overlapping joining (overlapping joining) and stepped joining (top joining). Overlapping joining is a method in which the ends of the stacked amorphous materials are arranged to overlap. Stepped joining is a method in which the ends of the stacked amorphous materials are arranged to top each other without overlapping, but rather at a predetermined interval.

[0006] Then, as a technology for improving the magnetic flux distribution in the wound core and providing a transformer with improved core characteristics, a wound core is described, for example, in Japanese Patent Application Publication No. 2010-263233 (Patent Document 1). In Patent Document 1, in order to improve the magnetic flux distribution inside the core, focusing on the joint structure between the ends of the amorphous materials, a method is proposed to make the joint method an overlapping joint, and to smooth the magnetic flux distribution inside the core by adjusting the overlap length of the overlapping portion.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2010-263233 Summary of the Invention

[0010] The problem that the invention aims to solve

[0011] Patent Document 1 describes a structure that adjusts the magnetic resistance at the overlapping portion of the overlapping joint to smooth the magnetic flux distribution inside the iron core. However, the adjustment of the overlapping portion as shown in Patent Document 1 has its limitations. For example, if the overlapping portion is shortened, unless it has a margin of about 5 mm, the friction of the joint surface of the overlapping joint decreases, making it difficult to fix, and the deterioration of iron loss also increases.

[0012] Furthermore, the maximum length of the overlapping portion is determined by the total length of the lap joint and the number of parts into which the lap joint is divided, so the limit of the overlapping portion can be considered to be in the tens of millimeters. Therefore, it is difficult to achieve a sufficient gradient in the magnetic reluctance, and there is a possibility that the effect of smoothing the magnetic flux distribution inside the iron core is insufficient.

[0013] Furthermore, the longer the overlapping portion, the more the thickness increases from the inner circumference to the outer circumference of the coiled core. When the overlapping portion is formed on all joint surfaces, the thickness of the lap joint of the coiled core becomes twice that of the leg, leading to the increase in the size of the transformer and the amount of core, which in turn creates a secondary issue of increased cost.

[0014] The main objective of this invention is to provide a static induction electrical device that can further improve the smoothness of the magnetic flux distribution inside an iron core. Furthermore, while an example using amorphous materials was shown for the previously described wound iron core, this invention can also be applied using silicon steel sheets.

[0015] Technical solutions for solving the problem

[0016] One example of the main features of the present invention is a static induction electrical device having a wound core and windings, wherein the core is a stack of overlapping magnetic bodies, and at least on the inner circumferential side of the wound core has a stepped joint, wherein the gap between the ends of the stepped joint gradually shortens towards the outer circumferential side.

[0017] Furthermore, another feature of the present invention is a static induction electrical device having an overlapping joint on the outer periphery of the stepped joint, wherein the length of the overlap distance of the overlapping portion of the overlapping joint gradually increases as it moves toward the outer periphery.

[0018] Invention Effects

[0019] According to the present invention, a static induction electrical device with a simple structure and improved smoothness of magnetic flux distribution inside the iron core can be provided. Attached Figure Description

[0020] Figure 1This is an external view showing the external structure of the molded transformer to which the present invention is applied.

[0021] Figure 2 It means Figure 1 An external view of the coiled iron core.

[0022] Figure 3 This is an enlarged cross-sectional view of the section near the end of the coiled iron core according to the first embodiment of the present invention.

[0023] Figure 4 This is an explanation Figure 3 The diagram illustrates the relationship between the length of the overlapping sections and the changes in iron loss and the excitation current ratio.

[0024] Figure 5 This is an enlarged cross-sectional view of the section near the end of the coiled iron core according to the second embodiment of the present invention.

[0025] Figure 6 It represents the cross-section of the winding at the leg of the transformer, and is a cross-sectional diagram of the winding with a roughly rectangular cross-section.

[0026] Figure 7 It represents the cross-section of the winding at the leg of the transformer, and the cross-sectional view of the winding with an approximately square cross-section. Detailed Implementation

[0027] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the following embodiments, and various modifications and applications are also included within the scope of the technical concept of the present invention.

[0028]

Example 1

[0029] Next, the first embodiment of the present invention will be described. Figure 1 This illustrates the structure of the transformer to which the present invention is applied. Figure 2 express Figure 1 The appearance of the coiled iron core shown. Figure 3 express Figure 2 The enlarged cross-section of part P (overlap joint).

[0030] Figure 1 In this transformer, the main body 10 is generally composed of an iron core 11 and a winding (coil) 12 wound on the legs 11F of the iron core 11. The transformer main body 10 is of the inner iron type, and the iron core 11 is composed of legs 11F and a yoke 11Y connecting the two ends of the legs 11F. In a pair of iron cores 11, the legs 11F are closely fitted together, and the winding 12 is wound in the fitted portion. The winding 12 is composed of a primary winding 12P and a secondary winding 12S, with the primary winding 12P wound on the legs 11F and the secondary winding 12S wound around its outer periphery.

[0031] In addition, in the case of an oil-immersed transformer, the transformer body 10 is housed and fixed inside a container; in the case of a molded transformer, as... Figure 1 The diagram shows a structure in which winding 12 is molded with synthetic resin 13. The primary winding 12F and the secondary winding S are surrounded by synthetic resin 13 (molded), thereby ensuring electrical insulation and mechanical strength.

[0032] The core portion 11 is a structure in which plate-shaped amorphous material (in this embodiment, an iron-based amorphous alloy) or silicon steel plates are stacked on top of each other. On the yoke portion 11Y on the lower side of the core portion 11, an overlapping joint portion 14 is formed where the ends of the amorphous material or silicon steel plates are joined to each other. This overlapping joint portion 14 is generally mostly formed on the yoke portion 11Y on the lower side of the core portion 11.

[0033] Next, use Figure 2 and Figure 3 illustrate Figure 1 The main part of the overlapping joint 14 of the iron core 11. Figure 2 It means to Figure 1 The appearance of the core section 11 after it has been pulled out is shown.

[0034] Figure 2 In the middle, the iron core 11 is made of a laminated material formed by stacking thin strips of magnetic material (hereinafter referred to as "magnetic circuit forming strips") made of plate-shaped amorphous material or silicon steel plate, including relatively long legs 11F and relatively short upper yoke 11YU and lower yoke 11YB that integrally and continuously connect the two ends of a pair of legs 11F.

[0035] Here, in the lower yoke 11YB, a known overlapping joint 14 is formed, in which the ends of multiple magnetic circuit forming strips are magnetically joined together. Thus, the core portion 11 can form a closed magnetic circuit through this overlapping joint 14. In this embodiment, the overlapping joint 14 is a composite joint having a stepped joint and an overlapping joint. Then, a stepped joint is formed on the inner peripheral surface 11in side of the lower yoke 11YB, and an overlapping joint is formed from the outer peripheral side of the stepped joint to the outer peripheral surface 11out side of the lower yoke 11YB.

[0036] Figure 3 Indicates will Figure 2 The enlarged cross-section of the overlapping joint 14 of the P section. This cross-section represents the state of the section cut in the direction extending from the lower yoke 12YB of the core section 11 towards the leg section 11F.

[0037] Figure 3 In the middle, the overlapping joint 14 is formed on the inner peripheral surface 11in of the lower yoke 11YB (reference). Figure 2The stepped joint joint SL on one side, and the outer peripheral surface 11out of the lower yoke YB from the outer periphery of the stepped joint joint SL (reference) Figure 2 The overlapping joint OL consists of two layers.

[0038] As previously explained, the stepped joint SL is configured such that the ends of the magnetic circuit forming strips are not overlapped but are joined at predetermined intervals. The overlapping joint OL is configured such that the ends of the magnetic circuit forming strips overlap. The stepped joint SL is located on the inner circumference of the core 11, and the overlapping joint OL is located from the stepped joint SL towards the outer circumferential surface 11out of the core 11.

[0039] Figure 3 In the middle, viewed from the inner peripheral surface 11in of the lower yoke 11YB towards the outer peripheral side, the first magnetic circuit forming thin strip layer SL1, the second magnetic circuit forming thin strip layer SL2, the third magnetic circuit forming thin strip layer SL3, and the fourth magnetic circuit forming thin strip layer SL4 are stacked. These magnetic circuit forming thin strip layers SL1 to SL4 are stepped together, and each magnetic circuit forming thin strip layer SL1 to SL4 is abutted against each other at its own ends.

[0040] Then, the gap distance between opposite ends in the first magnetic circuit forming thin strip layer SL1 is set to "a", the gap distance between opposite ends in the second magnetic circuit forming thin strip layer SL2 is set to "b", the gap distance between opposite ends in the third magnetic circuit forming thin strip layer SL3 is set to "c", and the gap distance between opposite ends in the fourth magnetic circuit forming thin strip layer SL4 is set to "0", i.e., contact. The gap distances then have a relationship of "a>b>c>0". That is, the gap distance of the magnetic circuit forming thin strips that are closer to the inner circumferential surface (11 inches) gradually (including in stages) increases (in other words, it gradually (including in stages) decreases as it moves towards the outer circumferential side).

[0041] Furthermore, viewed from the stepped joint SL of the lower yoke 11YB towards the outer periphery, the fifth magnetic circuit forming thin strip layer OL1, the sixth magnetic circuit forming thin strip layer OL2, and the seventh magnetic circuit forming thin strip layer OL3 are stacked. These magnetic circuit forming thin strip layers OL1 to OL3 are overlapped and joined together, and the ends of each magnetic circuit forming thin strip layer OL1 to OL3 overlap each other.

[0042] Then, the overlapping distance of the overlapping portion at the end of the fifth magnetic path forming thin strip layer OL1 is set to "d", and the overlapping distance of the overlapping portion at the end of the magnetic path forming thin strip layer OL2 is set to "e". Then, each overlapping distance has a relationship of "d < e". That is, it is set that the overlapping distance of the magnetic path forming thin strip closer to the outer peripheral surface 11out is longer. In addition, the overlapping distance of the overlapping portion at the end of the magnetic path forming thin strip layer OL3 is not shown, but is set to be longer than the overlapping distance "e" of the magnetic path forming thin strip layer OL2.

[0043] Figure 3 In the figure, the magnetic flux moving in the magnetic path forming thin strip is shown by a dotted arrow. In the stepped joint portion SL, the magnetic flux moves across the two magnetic path forming thin strip layers that overlap each other twice (indicated by black dots). In addition, in the overlapping joint portion OL, the magnetic flux moves across the overlapping portion of its own magnetic path forming thin strip layer once (indicated by white dots). Such magnetic flux movement appears as a difference in magnetic resistance (the same applies to exciting current and iron loss). Then, the magnetic resistance in the iron core changes corresponding to the length of the portion (referred to as "crossing") across which the magnetic flux passes.

[0044] For example, when paying attention to the gap distance of the stepped joint portion SL, as described above, two magnetic flux crossings are required. The longer the gap distance is, for example, the distance (equivalent to "a") where the magnetic flux of the second magnetic path forming thin strip layer SL2 is concentrated adjacent to the portion of the gap distance "a" of the first magnetic path forming thin strip layer SL1 increases relative to the gap distance "b" of the second magnetic path forming thin strip layer SL2, so the magnetic resistance (including exciting current and iron loss) increases. The same applies to the gap distances "b" and "c".

[0045] In this way, in the stepped joint portion SL, from the inner peripheral side to the outer side of the iron core portion 11, as the gap distance gradually (including stepwise) shortens in the manner of "a > b > c", a characteristic of decreasing magnetic resistance can be obtained.

[0046] On the other hand, when paying attention to the overlapping portion of the overlapping joint portion OL, only one magnetic flux crossing is required, the magnetic resistance is smaller than that of the stepped joint portion SL, and the place where the magnetic flux is concentrated is the overlapping portion. Then, the overlapping distance "d" of the fifth magnetic path forming thin strip layer SL5 is shorter than the overlapping distance "e" of the sixth magnetic path forming thin strip layer OL2. Therefore, the concentration of magnetic flux in the overlapping portion of the fifth magnetic path forming thin strip layer OL1 is greater, and the magnetic resistance (including exciting current and iron loss) is greater. In this way, in the overlapping joint portion OL, from the inner peripheral side to the outer side of the iron core portion 11, as the overlapping distance gradually (including stepwise) lengthens in the manner of "d < e", a characteristic of decreasing magnetic resistance can be obtained.

[0047] According to this embodiment, in the overlapping joint 14 of the lower yoke 11YB of the core portion 11, a stepped joint joint SL with high magnetic reluctance is formed on the inner circumferential side of the lower yoke YB, and the gap distance between the ends of each magnetic circuit forming a thin strip layer gradually (including in stages) decreases towards the outer circumferential side. Furthermore, an overlapping joint OL with low magnetic reluctance is formed on its outer circumferential side, and the distance between the overlapping portions between the ends of each magnetic circuit forming a thin strip layer gradually (including in stages) increases towards the outer circumferential side. By adopting such a structure, the smoothness of the magnetic flux distribution from the inner circumferential side to the outer circumferential side in the core portion 11 can be improved.

[0048] Figure 4 This graph represents the gap distance in the stepped joint SL and the overlap distance in the overlapping joint OL on the horizontal axis, and the iron loss variation and excitation current ratio on the vertical axis. "+" (positive) represents the gap distance, and "-" (negative) represents the overlap distance. "0" indicates the contact state between the ends of the air section and the overlapping section. Additionally, for comparison, the area located at... Figure 3 The outermost sixth magnetic circuit forms a thin strip layer OL2 as a reference.

[0049] like Figure 4 As shown, for the gap distance of the stepped joint SL, represented by "a", "b", and "c", as the distance increases, the iron loss changes, the excitation current ratio increases, and the magnetic reluctance increases towards the inner circumference of the iron core 11. Furthermore, for the overlap distance of the overlapping joint OL, represented by "d" and "e", as the distance decreases, the iron loss changes, the excitation current ratio increases, and the magnetic reluctance increases towards the inner circumference of the iron core 11.

[0050] However, in overlapping and stepped connections, while the stepped connection side is described above as having greater magnetic reluctance and excitation current, this is not necessarily the case regarding iron losses. For example... Figure 4 As shown, in overlapping joints, as the overlap distance gradually decreases (represented by g), there is a phenomenon where the iron loss change Li (dashed white dot) increases sharply, resulting in greater losses than in stepped joints. Therefore, in this embodiment, the region with an overlap distance shorter than "g" is designated as a region not used in the overlapping joint OL (unused region).

[0051] Therefore, compared to forming the overlap portion 14 using only overlapping joints with low magnetic reluctance and excitation current, as in this embodiment, the magnetic reluctance is controlled by combining stepped joints and overlapping joints, thus enabling the formation of a core with lower losses. Furthermore, compared to forming only with overlapping joints, this embodiment offers a wider adjustment range, and the effect of suppressing the increase in core thickness can also be expected.

[0052] In the embodiments described above, there is little need for the adjustment portion formed by the stepped joint SL and the overlapping joint OL to extend from the inner peripheral surface 11in of the core portion 11 to the outer peripheral surface 11out of the core portion 11.

[0053] Generally, the magnetic flux density at a point approximately one-third of the distance from the inner circumferential surface to the outer circumferential surface of the overlap portion 14 is known to exhibit a value close to the average magnetic flux density of the entire core portion 11. Therefore, the adjustment portion, which is generally formed by the stepped joint portion SL and the overlapping joint portion OL as in this embodiment, is formed in the range from the inner circumferential surface 11in of the core portion 11 to one-third of the distance, and the distance between the ends of the overlap portion can be configured to be the same length in the outer circumferential side compared to this.

[0054] Thus, according to this embodiment, the wound core is a stack of overlapping magnetic bodies. A stepped joint is provided on the inner circumference of the wound core, and the gap between the ends of the stepped joint gradually (including in stages) decreases towards the outer circumference. Furthermore, an overlapping joint is provided on the outer circumference of the stepped joint, and the overlap distance of the overlapping portions of the overlapping joint increases towards the outer circumference. Therefore, a static induction electrical device with a simple structure and improved smoothness of magnetic flux distribution within the core can be provided.

[0055]

Example 2

[0056] Next, the second embodiment of the present invention will be described. Figure 5 The structure of the stepped joint SL in the second embodiment is shown. The stepped joint SL is formed by three blocks (SLset1, SLset2, SLset3), which differs from the first embodiment. Furthermore, the overlapping joint OL is omitted from the illustration.

[0057] Generally, magnetic circuits are formed in sets of 10 to 20 thin strips, as described in the first embodiment ( Figure 4 In order to make the difference in gap distance easy to understand, it is represented as the gap distance of the second magnetic circuit forming thin strip layer SL2 adjacent to the first magnetic circuit forming thin strip layer SL1, the third magnetic circuit forming thin strip layer SL3 adjacent to the second magnetic circuit forming thin strip layer SL2, etc., becoming shorter in sequence. However, in the actual iron core portion 11, it is preferable to form 5 to 10 layers with the same gap distance.

[0058] That is, when multiple magnetic circuits are stacked together as a thin strip (4 layers in this embodiment) as a group, the gap distance of the first step joint SLset1 of the 4 layers is set as "a", the gap distance of the second step joint SLset2 of the 4 layers on its outer periphery is set as "b", and the gap distance of the third step joint SLset3 of the 4 layers on its outer periphery is set as "c".

[0059] Here, the gap distances are set as in the first embodiment, from the inner periphery of the core portion 11 outwards, in the order "a>b>c". Therefore, as the gap distance gradually (including in stages) shortens towards the outer periphery, a characteristic of reduced magnetic reluctance can be obtained. Thus, as a practical structure, it is preferable to set the gap distances of the magnetic circuit forming strips with a predetermined number of layers (e.g., 5 to 10 layers) to the same distance and group them as one group, while different groups with gradually (including in stages) decreasing gap distances are grouped together on their outer sides.

[0060]

Example 3

[0061] Next, a third embodiment of the present invention will be described. The first and second embodiments relate to the overlap portion of the wound core, while the third embodiment relates to a transformer using the aforementioned wound core. Figure 6 and Figure 7 This indicates the relationship between the shape of the transformer windings and the shape of the core.

[0062] The cross-sectional shape of the core section 11 and the cross-sectional shape of the winding 12 are generally as follows: Figure 6 The diagram shows a rectangular shape. The iron core 11, which uses an amorphous material to form a magnetic circuit strip, is manufactured by winding a rectangular magnetic circuit strip. Therefore, the orientation of the main surface of the magnetic circuit strip is near-far from the paper, and the stacking direction is towards the paper. Figure 6 (The middle direction is up and down).

[0063] Therefore, increasing the thickness in the stacking direction increases the difference in magnetic circuit length between the innermost and outermost circumferences, resulting in a rectangular cross-sectional shape for the core 11 and winding 12, making it difficult to increase the stacking thickness. Consequently, the cross-sectional shape of the winding is as follows: Figure 6 As shown, the design features different lengths on the long side and the short side, which poses a risk of increased noise caused by the electromagnetic force of winding 12.

[0064] In contrast, to suppress noise caused by the electromagnetic force of winding 12, such as Figure 7The cross-sectional shapes of the core portion 11 and the winding 12 shown are preferably square. Then, by using a wound core with the overlapping joints described in the first and second embodiments, the magnetic circuit length can be adjusted. Therefore, even if the cross-sectional shapes of the core portion 11 and the winding 12 are square, the magnetic flux distribution within the core portion 11 can be smoothed, thus suppressing noise generation.

[0065] In this way, by adopting a transformer that combines the overlapping joints described in the first and second embodiments with a structure that makes the cross-sectional shape of the core 11 and the cross-sectional shape of the winding 12 square, the magnetic flux distribution can be smoothed and excessive excitation current can be suppressed, thus providing a transformer that suppresses losses and noise.

[0066] Furthermore, this invention can be implemented not only in transformers but also in other static induction electrical devices (e.g., reactors). Additionally, the above-described embodiments are provided for ease of understanding and are not intended to limit the scope to all described structures.

[0067] As described above, the present invention is a static induction electrical device having a coiled iron core and windings, characterized in that: the coiled iron core is a stack of overlapping magnetic bodies, and has a stepped joint at least on the inner circumferential side of the coiled iron core, wherein the gap between the ends of the stepped joint gradually shortens as it moves toward the outer circumferential side.

[0068] Therefore, it is possible to provide a static induction electrical device with a simple structure that improves the smoothness of magnetic flux distribution inside the iron core.

[0069] Furthermore, the present invention is not limited to the above-described embodiments, but includes various modifications. The above embodiments are described in detail for ease of understanding of the present invention and are not limited to having all the structures described. In addition, a part of the structure of one embodiment can be replaced with the structure of another embodiment, and the structure of another embodiment can be added to the structure of one embodiment. For the structures of each embodiment, other structures can be added, deleted, or replaced.

[0070] Explanation of reference numerals in the attached figures

[0071] 10…Transformer body, 11…Core, 11F…Leg, 11Y…Yoke, 11YU…Upper yoke, 11YB…Lower yoke, 11in…Inner circumferential surface, 11out…Outer circumferential surface, 12…Winding, 12P…Primary winding, 12S…Secondary winding, 13…Synthetic resin, 14…Overlap joint, SL…Stepped joint, OL…Overlapped joint.

Claims

1. A static induction electrical device having a wound iron core and windings, characterized in that: The wound core is formed by stacking thin strips of magnetic circuit, and has at least a stepped joint on the inner circumferential side of the wound core. The gap between the ends of the stepped joint gradually decreases towards the outer periphery of the coiled iron core. The outer peripheral side of the stepped joint has an overlapping joint. The length of the overlap distance of the overlapping portions of the overlapping joints gradually increases as one moves toward the outer periphery.

2. The static induction electrical equipment as described in claim 1, characterized in that: The stepped joint and the overlapping joint are formed on one side of the inner circumferential surface of the coiled iron core to a distance of about 1 / 3 of the distance between the inner circumferential surface and the outer circumferential surface.

3. The static induction electrical equipment as described in claim 1, characterized in that: A stepped joint assembly is formed by grouping multiple layers of magnetic circuit forming thin strips, the gap distance of the stepped joint assembly is set to be the same, and a plurality of stepped joint assemblies are formed from the inner peripheral side of the coiled iron core to the outer peripheral side, the gap distance of the plurality of stepped joint assemblies gradually shortens as they move toward the outer peripheral side.

4. The static induction electrical equipment as described in any one of claims 1 to 3, characterized in that: The cross-sectional shape of the coiled iron core on the inner side of the winding is square, and the cross-sectional shape of the winding is also square.

5. The static induction electrical equipment as described in any one of claims 1 to 3, characterized in that: The magnetic circuit forming strip is made of iron-based amorphous alloy or silicon steel sheet.

6. The static induction electrical equipment as described in claim 4, characterized in that: The magnetic circuit forming strip is made of iron-based amorphous alloy or silicon steel sheet.

7. A static induction electrical device comprising a wound core and a winding, wherein the wound core has a pair of longer legs opposite each other and a pair of shorter yokes connecting the ends of the pair of legs integrally, the winding being wound around the legs, the static induction electrical device being characterized in that: In the wound core, multiple magnetic circuits are stacked in overlapping thin strips, and the stacked magnetic circuits are magnetically joined by an overlapping joint formed in one of the yokes to form a closed magnetic circuit. The overlapping joint. The coiled iron core has a stepped joint formed on its inner circumferential side, and the gap between the ends of the stepped joint is formed to gradually shorten towards the outer circumferential side of the coiled iron core. The stepped joint also has an overlapping joint on the outer periphery, and the length of the overlapping distance of the overlapping portion of the overlapping joint increases in stages as it moves toward the outer periphery.

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