EP4732313A1

EP4732313A1 - Iron cores for disassembly/transport transformers

Info

Publication number: EP4732313A1
Application number: EP24748630.1A
Authority: EP
Inventors: Naoyuki Kurita; Chie Kobayashi; Hiroaki Kojima; Kohei Yamaguchi
Original assignee: Hitachi Energy Ltd
Current assignee: Hitachi Energy Ltd
Priority date: 2023-07-26
Filing date: 2024-07-24
Publication date: 2026-04-29
Also published as: JP2025018338A; WO2025021884A1; CN121569355A; KR20260014043A

Abstract

The present invention provides an iron core for a disassembly/transport transformer that makes it possible to control a decline in the performance of the disassembly/transport transformer. The iron core for a disassembly/transport transformer used for a disassembly/transport transformer is a three-phase/five-leg iron core, wherein main legs, side legs, and a plurality of yokes connecting the upper and lower end parts of the main legs or the side legs are constituted by laminating a plurality of steel sheets, the lower yoke connecting the lower end part of the adjoining main legs as well as the lower yokes connecting each lower end part of a main leg and a side leg are each divided by two dividing parts, the steel sheets are laminated at the dividing part for a lap length, which is a given length, by shifting the position of a steel sheet at successive layers in the extending direction of the lower yoke, and the lap length at each of the dividing parts provided at the lower yoke that connects the adjoining main legs is different from the lap length at each of the dividing parts provided at each of the lower yokes that connects each lower end part of a main leg and a side leg.

Description

DESCRIPTION

Title of the Invention: IRON CORES FOR DISASSEMBLY/TRANSPORT TRANSFORMERS

FIELD OF THE INVENTION

[0001]

The present invention relates to an iron core of a disassembly /transport transformer to be used for a disassembly /transport transformer.

BACKGROUND OF THE INVENTION

[0002]

As one form of large-sized transformers with large capacity, disassembly /transport transformers have been known that are disassembled into a plurality of parts, which satisfy transport constraints, after the completion of production, assembly, testing, and inspections at factories, reassembled, tested, and inspected. [0003]

Iron cores for disassembly /transport transformers are constituted by laminating grain-oriented silicon steel sheets or the like in order to minimize reassembly work at installation locations.

Regarding iron cores for disassembly /transport transformers, a yoke iron core connecting a plurality of main leg iron cores to which winding wires are to be wound around might possibly be broken down into multiple parts. In such a case, the yoke iron core requires a plurality of joints.

[0004]

As such iron cores for disassembly /transport transformers, those described in Patent Document 1 have been known, for example. In the iron cores for disassembly /transport transformers described in Patent Document 1, a lower yoke iron core connecting three main legs of a three-phase/three-leg iron core, which was formed by laminating grain-oriented silicon steel sheets, is divided, a main leg iron core and a portion of the divided lower yoke iron core are assembled together and transported, and then, at the time of reassembly, yokes connecting the divided lower yoke iron core are constituted by laminating grain-oriented silicon steel sheets. In this case, the joining parts between the divided lower yoke iron core and connecting yokes are connected by a lap joint structure in which grain-oriented silicon steel sheets are laminated by shifting them at a given distance.

Patent Document 1 discloses setting ranges for the overlapping amount of grain-oriented silicon steel sheets (lap length) and the number of layers of grain-oriented silicon steel sheets (number of lap stages) in the abovementioned lap joint structure in order to control a decline in the performance of the disassembly /transport transformers.

[0005] While Patent Document 1 relates to disassembly /transport transformers using a three-phase/three-leg iron core, those described in Patent Document 2 have been known as a method for dividing and reassembling an iron core for three-phase/five-leg disassembly /transport transformers used for a transformer with greater capacity.

[0006]

Patent Document 2 discloses a method for dividing a lower yoke iron core, which connects three main legs of three-phase/five-leg iron core formed by laminating grain-oriented silicon steel sheets and also connects the main leg iron core with side legs on both sides, as well as a method for dividing the main leg iron core in the longitudinal direction.

Furthermore, in the method disclosed in Patent Document 2, a divided main leg iron core and a portion of a lower yoke iron core are integrally assembled before it is transported, and at the time of reassembling, yokes connecting the divided lower yoke iron cores are constituted by laminating grain- oriented silicon steel sheets to form an iron core for a disassembly /transport transformer.

Prior Art Document

Patent Document

[0007]

Patent Document 1: Japanese Laid-Open Patent Publication No. 2012-15210

Patent Document 2: Japanese Laid-Open Patent Publication No. 2010-272786

SUMMARY OF THE INVENTION

Problem to be solved by the invention

[0008]

Technologies disclosed in Patent Document 1 and Patent Document 2 relate to methods for disassembling and reassembling a three-phase/three-legs iron core and a three-phase/five-leg iron core, respectively, to make it possible to divide and transport a lower yoke iron core.

In the connecting part of the divided iron core, the problem is that iron loss caused by eddy current as well as excitation current increases because magnetic flux flowing inside the iron core is carried over to an adjoining grain-oriented silicon steel sheet, resulting in a decline in the performance of the disassembly /transport transformer.

[0009]

Patent Document 1 discloses setting ranges for the lap length/the number of lap stages of grain- oriented silicon steel sheets at the connecting part of the divided iron core in order to control a decline in the performance of the disassembly /transport transformer; however, since the structure of the connecting part of the divided iron core is identical, the effect of controlling a decline in the performance is limited. [0010]

To solve the abovementioned problems, the present invention discloses an iron core for a disassembly /transport transformer that can control a decline in the performance of the disassembly /transport transformer.

[0011]

Furthermore, the abovementioned purpose and other purposes of the present invention as well as novel features of the present invention will be revealed by the description of the present specification and attached drawings.

Means for Solving the Problem

[0012]

The iron core for a disassembly /transport transformer according to the present invention is an iron core for a disassembly /transport transformer used for a disassembly /transport transformer and is a three- phase/five-leg iron core comprising three paralleled main legs, two side legs arranged in parallel to those three main legs on the outer sides thereof, and a plurality of yokes connecting the upper and lower end parts of the adjoining main legs or side legs.

Moreover, in the iron core for a disassembly /transport transformer according to the present invention, three main legs, two side legs, and a plurality of yokes are constituted by each laminating a plurality of steel sheets, and, from among the plurality of yokes, the lower yoke connecting the lower end part of the adjoining main legs as well as the lower yokes connecting each lower end part of the main legs and the side legs are each divided by two dividing parts.

[0013]

Moreover, in the iron core for a disassembly /transport transformer according to the first present invention, the steel sheets are laminated at the dividing part for a lap length, which is a given length, by shifting the position of the facing steel sheet of the same layer at successive layers in the extending direction of the lower yoke. Moreover, in the iron core for a disassembly /transport transformer according to the first present invention, the constitution is such that the lap length at each of the dividing parts provided at the lower yoke that connects the lower end parts of adjoining main legs is different from the lap length at each of the dividing parts provided at each of the lower yokes that connects respective lower end parts of a main leg and a side leg.

[0014]

Moreover, the iron core for a disassembly /transport transformer according to the second present invention has a lap joint structure at each dividing part, wherein the steel sheets are laminated at the dividing part for a lap length, which is a given length, by shifting the position of the facing steel sheet of the same layer at successive layers in the extending direction of the lower yoke, and at the lap joint structure of each dividing part, the position of the facing steel sheet of the same layer is repeated with layers having the number of lap stages, which is a given number, as a repeating unit. Moreover, in the iron core for a disassembly /transport transformer according to the second present invention, the constitution is such that the number of lap stages at each of the dividing parts provided at the lower yoke that connects the adjoining main legs is different from the number of lap stages at each of the dividing parts provided at each of the lower yokes that connects respective lower end parts of a main leg and a side leg.

Effect of the Invention

[0015]

According to the constitution of an iron core for disassembly /transport transformer of the first present invention, the lap length at each dividing part is different between the lower yoke connecting the adjoining main legs of the three-phase/five-leg iron core and the lower yoke connecting a main leg and a side leg.

Since the lap length at each dividing part of the lower yoke is different depending on the position of the lower yoke, the difference of the magnetic flux density within the iron core at various positions can be adjusted by means of the lap length. This makes it possible to decrease iron loss that occurs at the iron core, so that the harmonic component of excitation current flowing through the winding wire wound around main legs can be decreased and excitation noise of the iron core can be lowered.

[0016]

According to the constitution of an iron core for disassembly /transport transformer of the second present invention, the number of lap stages at each dividing part is different between the lower yoke connecting the adjoining main legs of the three-phase/five-leg iron core and the lower yoke connecting a main leg and a side leg.

Since the number of lap stages at each dividing part of the lower yoke is different depending on the position of the lower yoke, the difference of the magnetic flux density within the iron core at various positions can be adjusted by means of the number of lap stages. This makes it possible to decrease iron loss that occurs at the iron core, so that the harmonic component of excitation current flowing through the winding wire wound around main legs can be decreased and excitation noise of the iron core can be lowered. [0017]

The problems, configuration, and effects other than those described above will be made clear by the explanation of the detailed description of the invention as shown below.

BRIEF DESCRIPTION OF THE DRAWINGS

[00018]

[FIG 1] A front view of a disassembly /transport transformer according to Example 1. [FIG 2] A bottom view of a disassembly /transport transformer according to Example 1.

[FIG 3] Enlarged cross-sectional views of A, B, and C parts in FIG. 1.

[FIG 4] A front view showing the definition of the size of each part for a three-phase/five-leg iron core used in a three-dimensional electromagnetic field analysis.

[FIG 5] A view of calculation results of magnetic flux density distributions at a central part of a three- phase/five-leg iron core, comparing Example 1 with a conventional constitution.

[FIG 6] The calculation results of the waveforms of excitation current in a three-phase/five-leg disassembly /transport transformer having a conventional joint structure.

[FIG 7] The calculation results of the waveforms of excitation current in the three-phase/five-leg disassembly /transport transformer according to Example 1.

[FIG 8] (A - C): views showing a method for producing the three-phase/five-leg disassembly /transport transformer according to Example 1.

[FIG 9] Enlarged cross-sectional views of A, B, and C parts in a three-phase/five-leg disassembly /transport transformer according to Example 2.

[FIG 10] Enlarged cross-sectional views of A, B, and C parts in a three-phase/five-leg disassembly /transport transformer according to Example 3.

[FIG 11] Enlarged cross-sectional views of A, B, and C parts in a three-phase/five-leg disassembly /transport transformer according to Example 4.

DETAILED DESCRIPTION OF THE INVENTION

[0019]

The following explains detailed description and examples with reference to sentences and drawings. However, the structures, materials, and other various constitutions described in specific terms here are not limited to those described here but can be combined and modified in an appropriate manner as long as the spirit of the present invention is not changed.

[0020]

The iron core for a disassembly /transport transformer according to the present invention is an iron core used for a disassembly /transport transformer.

The iron core for a disassembly /transport transformer according to the present invention is a three- phase/five-leg iron core constituted of three parallel main legs, two side legs arranged in parallel to the main legs on the outer sides of those three main legs, and a plurality of yokes connecting the upper and lower end parts of the adjoining main legs or side legs.

Moreover, the iron core for a disassembly /transport transformer according to the present invention is constituted by laminating a plurality of steel sheets for three main legs, two side legs, and multiple yokes of the abovementioned three-phase/five-leg iron core. Furthermore, the lower yoke of the multiple yokes connecting the lower end part of the adjoining main legs and the lower yoke connecting each lower end part of a main leg and a side leg are divided by two dividing parts.

[0021]

[0022]

[0023]

[0024]

Since the number of lap stages at each dividing part of the lower yoke is different depending on the position of the lower yoke, the difference of the magnetic flux density within the iron core at various positions can be adjusted by means of the number of lap stages. This makes it possible to decrease iron loss that occurs at the iron core, so that the harmonic component of excitation current flowing through the winding wire wound around main legs can be decreased and excitation noise of the iron core can be lowered. [0025]

In each of the abovementioned constitutions of an iron core for a disassembly /transport transformer according to the present invention, a wide variety of steel sheets used for disassembly /transport transformers including grain-oriented silicon steel sheets can be used as steel sheets constituting iron cores. [0026]

In the abovementioned iron core for a disassembly /transport transformer according to the first present invention, the constitution may be such that the average magnetic flux density within all main legs under rated excitation conditions can be made higher than the average magnetic flux density within all side legs, and the lap length at each dividing part provided at a lower yoke connecting the lower end part of the adjoining main legs can be made shorter than the lap length at each dividing part provided at a lower yoke connecting each lower end part of main legs and side legs.

In such a constitution, the lap length at the dividing part of the lower yoke relating to the main leg that has higher average magnetic flux density under rated excitation conditions is shorter, and since the magnetic resistance is larger when the lap length is shorter, the difference in magnetic flux density between the main leg and the side led can be lowered. This makes it possible to decrease iron loss that occurs at the iron core, so that the harmonic component of excitation current flowing through the winding wire wound around main legs can be decreased and excitation noise of the iron core can be lowered.

[0027]

In the abovementioned iron core for a disassembly /transport transformer according to the first present invention, the constitution may be such that the average magnetic flux density within all main legs under rated excitation conditions can be made lower than the average magnetic flux density within all side legs, and the lap length at each dividing part provided at a lower yoke connecting the lower end part of the adjoining main legs can be made longer than the lap length at each dividing part provided at a lower yoke connecting each lower end part of main legs and side legs.

In such a constitution, the lap length at the dividing part of the lower yoke relating to the main leg that has lower average magnetic flux density under rated excitation conditions is longer, and since the magnetic resistance is smaller when the lap length is longer, the difference in magnetic flux density between the main leg and the side led can be lowered. This makes it possible to decrease iron loss that occurs at the iron core, so that the harmonic component of excitation current flowing through the winding wire wound around main legs can be decreased and excitation noise of the iron core can be lowered.

[0028]

In the abovementioned iron core for a disassembly /transport transformer according to the first present invention: in a lap joint structure at each of the dividing parts of the lower yoke in which the position of the facing steel sheet of the same layer is shifted, the number of lap stages, which is the number of layers of the repeating unit for the position of the facing steel sheet of the same layer, is all made the same. [0029]

In the abovementioned iron core for a disassembly /transport transformer according to the first present invention: in a lap joint structure at each of the dividing parts of the lower yoke in which the position of the facing steel sheet of the same layer is shifted, the number of lap stages, which is the number of layers of the repeating unit for the position of the facing steel sheet of the same layer, can take any different value depending on the position of the lower yoke.

In such a constitution, the difference in magnetic flux density within an iron core depending on positions can be adjusted in a lap joint structure not only by the difference in the lap length but by the difference in the number of lap stages as well, so that the harmonic component of excitation current flowing through the winding wire wound around main legs can be decreased and excitation noise of the iron core can be lowered.

[0030]

In the abovementioned iron core for a disassembly /transport transformer according to the second present invention, the constitution may be such that the average magnetic flux density within all main legs under rated excitation conditions can be made higher than the average magnetic flux density within all side legs, and the number of lap stages at each dividing part provided at a lower yoke connecting the lower end part of the adjoining main legs can be made smaller than the lap length at each dividing part provided at a lower yoke connecting each lower end part of main legs and side legs.

In such a constitution, the number of lap stages at the dividing part of the lower yoke relating to the main leg that has higher average magnetic flux density under rated excitation conditions is smaller, and since the magnetic resistance is larger when the number of lap stages is smaller, the difference in magnetic flux density between the main leg and the side led can be lowered. This makes it possible to decrease iron loss that occurs at the iron core, so that the harmonic component of excitation current flowing through the winding wire wound around main legs can be decreased and excitation noise of the iron core can be lowered. [0031]

In the abovementioned iron core for a disassembly /transport transformer according to the second present invention, the constitution may be such that the average magnetic flux density within all main legs under rated excitation conditions can be made lower than the average magnetic flux density within all side legs, and the number lap stages at each dividing part provided at a lower yoke connecting the lower end part of the adjoining main legs can be made more than the lap length at each dividing part provided at a lower yoke connecting each lower end part of main legs and side legs.

In such a constitution, the number of lap stages at the dividing part of the lower yoke relating to the main leg that has lower average magnetic flux density under rated excitation conditions is larger, and since the magnetic resistance is smaller when the number of lap stages is larger, the difference in magnetic flux density between the main leg and the side led can be lowered. This makes it possible to decrease iron loss that occurs at the iron core, so that the harmonic component of excitation current flowing through the winding wire wound around main legs can be decreased and excitation noise of the iron core can be lowered. (Examples) [0032]

Next, specific examples will be described for disassembly /transport transformers using an iron core for a disassembly /transport transformer.

[0033]

(Example 1)

Next, the disassembly /transport transformer according to Example 1 is described with reference to FIG 1 through FIG 8.

FIG 1 is a front view of the disassembly /transport transformer according to Example 1, and FIG 2 is a bottom view of the disassembly /transport transformer according to Example 1.

The width direction, the height direction, and the depth direction when the disassembly /transport transformer is let stand are defined here as the x direction, the y direction, and the z direction, respectively. [0034]

The disassembly/transport transformer 10 shown in FIG 1 is constituted of providing a three- phase/five-leg iron core, wherein steel sheets are laminated in the z direction, and three main legs lu, Iv, and Iw for the three phases and two side legs 2 provided on the outer sides of those main legs are placed side by side thereby having the longitudinal direction in the y direction. [0035]

The main legs lu, Iv, and Iw as well as the main legs lu, Iv, and Iw and the side legs 2 are connected by an upper yoke iron core 2a on their upper end parts and by a lower yoke iron core 2b on their lower end parts.

The upper yoke iron core 2a consists of an upper yoke between main legs 3 connecting the adjoining main legs and an upper yoke between a main leg and a side leg 4.

The lower yoke iron core 2b consists of a lower yoke between main legs 5a, a lower connecting yoke between main legs 5b, a lower yoke between a main leg and a side leg 6a, and a lower connecting yoke between a main leg and a side leg 6b. More specifically, the lower yoke connecting the lower end part of the adjoining main legs are divided at two dividing parts, thereby making two lower yokes between main legs 5a and an intermediate lower connecting yoke between main legs 5b. Moreover, the lower yoke connecting the lower end part of the main legs and side legs are divided at two dividing parts, thereby making two lower yokes between a main leg and a side leg 6a and an intermediate lower connecting yoke between a main leg and a side leg 6b.

[0036]

Since steel sheets are laminated by changing their widths in the present example, the cross-sections of the main legs In, Iv, and Iw are approximately circular and the cross section of the side leg 2 is approximately elliptical as shown in FIG 2.

Moreover, the respective cross-sections of yokes 3, 4 constituting the upper yoke iron core 2a and yokes 5a, 5b, 6a, and 6b constituting the lower yoke iron core 2b are also approximately elliptical because they are laminated each other by changing the widths of steel sheets.

[0037]

Each yoke iron core 2a, 2b, the main legs lu, Iv, and Iw, and the side legs 2 are connected each other at connecting parts 7, wherein respective steel sheets are laminated each other with a predetermined lap length and a predetermined number of lap stages.

The lower yokes between main legs 5a and the connecting yokes 5b are connected each other at connecting parts 8a, wherein steel sheets are laminated each other with a predetermined lap length and a predetermined number of lap stages.

The lower yokes between a main leg and a side leg 6a and the connecting yokes 6b are connected each other at connecting parts 8b, wherein steel sheets are laminated each other with a predetermined lap length and a predetermined number of lap stages.

Here, the lap length refers to a laminating length of steel sheets when they are laminated each other in such a way that the position of the facing steel sheets of the same layer are laminated at successive layers at a predetermined length by shifting them in the extending direction of the lower yoke.

Furthermore, the number of lap stages refers to the number of layers as a repeating unit when the position of the facing steel sheets of the same layer are repeated with a predetermined number of layers as the repeating unit.

[0038]

As the steel sheet constituting the iron core for the three-phase/five-leg disassembly/transport transformer 10 of the present example, grain-oriented silicon steel sheets can be used, for example.

Nevertheless, the steel sheet constituting the iron core are not limited to the grain-oriented silicon steel sheet, but a wide variety of steel sheets employed for an iron core for a disassembly/transport transformer can be used.

[0039] Furthermore, three-phase winding wires 9u, 9v, and 9w are wound around the main legs lu, Iv, and

Iw.

The winding wires 9u, 9v, and 9w have low-voltage winding wires on their inner sides and high- voltage winding wires on their outer sides wound around in a laminating manner, and may have intermediatevoltage winding wires, tap winding wires, and the like wound around in the same manner depending on the application of transformers, thereby constituting the disassembly /transport transformer 10.

FIG 1 simplifies a plurality of those winding wires as one unit and shows the outermost outlines with broken lines.

[0040]

FIG 3 shows enlarged cross-sectional views of A, B, and C parts within the iron core for the three- phase/five-leg disassembly /transport transformer 10 as shown in FIG 1. The A part shows the connecting part connecting the lower yoke between main legs 5a with the connecting yoke 5b. The B part shows the connecting part 8b connecting the lower yoke between a main leg and a side leg 6a with the connecting yoke 6b. The C part shows the connecting part 7 connecting a main leg lu, Iv, Iw or a side leg 2 with a yoke iron core 2a, 2b.

[0041]

The present example constitutes each connecting part 8a, 8b, 7 with a lap joint structure as shown in FIG 3, wherein adjoining steel sheet layers 1 la and 1 lb are laminated each other by shifting the position of the facing steel sheet of the same layer for a given amount.

In FIG 3, the laminating amounts (lap lengths) of the steel sheets 1 la and 1 lb at the Apart, B part, and C part are defined as Lm, Ls, and Ln, respectively, and the laminating structure of the steel sheets 1 la and 1 lb are repeated in the vertical direction of FIG 3.

[0042]

In the lap joint structure of the connecting part 8a, 8b, 7 as shown in FIG 3, steel sheets are laminated each other in such a way that the position of the facing steel sheet of the same layer changes alternately for every layer. The lap joint structure laminated in this manner is referred to as a “one-stage alternate lamination” below.

In the one-stage alternate lamination lap joint structure, the position of the facing steel sheet of the same layer is repeated for two layers as a repeating unit, and therefore the number of lap stages, which is the repeating unit, is two (2-stage).

[0043]

Moreover, in each of the connecting parts 8a, 8b, and 7, the butt part of steel sheets of the same layer (Ila, 1 lb) is provided with a gap G for absorbing errors that occur at the time of producing an iron core.

However, any strict management of this gap G is practically unrealistic.

[0044] In the present example, the lap length Lm of the connecting part 8a (A part) located at the lower yoke connecting main legs is different from the lap length Ls of the connecting part 8b (B part) located at the lower yoke connecting a main leg and a side leg.

[0045]

Furthermore, it is preferable that the relative (longer or shorter) relationship of those lap lengths Lm and Ls should be determined based on the relative (higher or lower) relationship of average magnetic flux densities Bm and Bs under rated excitation conditions that occur in the main legs lu, Iv, Iw and the side leg 2 of the three-phase/five-leg iron core.

More specifically, if the lap lengths Lm and Ls are designed to be identical (Lm = Ls) as in the conventional manner, Lm should be made shorter than Ls when the average magnetic flux density Bm of the main leg is higher than the average magnetic flux density Bs of the side leg, and Lm should reversely be made longer than Ls when the average magnetic flux density Bm of the main leg is lower than the average magnetic flux density Bs of the side leg.

Among these constitutions, FIG 3 shows one in which the lap length Lm at the connecting part 8a (A part) is made shorter than the lap length Ls at the connecting part 8b (B part).

If the average magnetic flux density Bm of the main leg is lower than the average magnetic flux density Bs of the side leg, the lap length Lm at the connecting part 8a (A part) should preferably be made longer than the lap length Ls at the connecting part 8b (B part), contrary to FIG 3.

[0046]

Here, when the lap lengths are designed to be identical (Lm = Ls), the average magnetic flux densities Bm and Bs can be found by calculation based on the materials and sizes of each part of an iron core (main legs lu, Iv, Iw, side legs 2, yoke iron core 2a, 2b) and winding wires 9u, 9v, 9w, the amount of current flowing through winding wires 9u, 9v, 9w and what not.

[0047]

Furthermore, in FIG 3, the lap length Ln at the connecting part 7 (C part) is made equal to the lap length Ls at the connecting part 8b (B part).

Nevertheless, the lap length Ln at the connecting part 7 (C part) has no relevance to achieving the effect of the iron core according to the present invention and, therefore, is not restricted, that is, can be the length identical to any one of Lm and Ls or the length different from Lm, Ls.

[0048]

Next, the effect of applying the constitution of the present example to an three-phase/five-leg iron core having a divided lower yoke is described by a quantification method using the three-dimensional electromagnetic field analysis with reference to FIG 4 through FIG 7.

FIG 4 is a front view showing the definition of the size of each part for a three-phase/five-leg iron core used in a three-dimensional electromagnetic field analysis. A list of relative sizes of parts is shown in Table 1 using the diameter D of the main leg having a circular cross-section as a reference. The side leg has a elliptical cross-section having a short diameter Ds, the yoke has an elliptical cross-section having a short dimeter Dy, and the thickness in the depth direction of the iron core is D (identical to the diameter of the main leg).

[0049]

[Table 1]

[0050]

For the model of the three-sheet/five-leg iron core, the magnetization characteristics and iron loss characteristics of a grain-oriented silicon steel sheet 30ZH105 manufactured by Nippon Steel Corporation were defined in consideration of easy magnetization axis direction and laminating direction. Then, winding wires for the purpose of generating rated magnetic flux density were set based on an electromagnetic field analysis model and then a predetermined 50 Hz sinusoidal voltage was applied to the winding wires.

An equivalent magnetic resistance corresponding to a predetermined lap length was added to the connecting part between a main leg or a side leg and a yoke as well as to the connecting part of a lower divided yoke so as to reproduce the excitation impedance characteristics of the three-phase/five-leg iron core. [0051]

FIG 5 is a view showing magnetic flux densities along the central part (x axis) of the three- phase/five-leg iron core as shown in FIG 4, which was calculated based on the abovementioned conditions for the purpose of comparison. Broken lines 21 correspond to a conventional constitution, wherein calculation results are shown when all of the lap lengths Lm, Ls, and Ln at connecting parts in FIG 3 are 10mm. Solid lines 22 show calculation results of one embodiment of the present example, wherein the lap length Lm is 2mm and the lap lengths Ls and Ln are 10mm.

[0052] In FIG 5, according to the calculation results 21 for the conventional constitution, the magnetic flux density within V-phase main leg Iv is higher than the magnetic flux densities of U-phase main leg lu and W- phase main leg Iw, which shows the occurrence of biased magnetic flux density distributions among three- phase main legs. Furthermore, the magnetic flux densities within the side legs 2 on both ends are lower than the magnetic flux densities within the main legs by 5% or more, which shows biased magnetic flux densities at each part of the three-phase/five-leg iron core.

[0053]

On the other hand, according to the calculation results 22 for the constitution of the present example, it has been shown that the biased magnetic flux density distributions among three-phase main legs were lowered, the magnetic flux densities within the side legs 2 increased as compared with the calculation results 21 for the conventional constitution, and the biased magnetic flux densities were reduced for the entire iron core.

It is also shown that the calculated values of iron loss generated at the three-phase/five-leg iron core in the constitution of the present example were reduced by 0.5% to 1.0% as compared with the calculated values of iron loss generated at the three-phase/five-leg iron core in the conventional constitution.

[0054]

The following shows the comparison of calculated results for the waveforms of excitation current flowing through winding wires wound around the abovementioned three-phase/five-leg iron cores.

FIG 6 shows the calculated results of exemplified current waveforms in the three-phase/five-leg disassembly /transport transformer having a conventional joint structure, wherein all of the lap lengths Lm, Ls, and Ln at the connecting parts of the iron core as shown in FIG 3 were set to 10mm. In FIG 6, 3 lu, 3 Iv, and 31w show the waveforms of excitation current flowing through the winding wires of the U-phase, V- phase, and W-phase, respectively.

FIG 7 shows the calculated results of exemplified current waveforms in one embodiment of the three-phase/five-leg disassembly /transport transformer according to the present example, wherein Lm was set to 2mm, and Ls and Ln were set to 10mm. In FIG 7, 32u, 32v, and 32w show the waveforms of excitation current flowing through the winding wires of the U-phase, V-phase, and W-phase, respectively.

These waveforms of excitation current show the waveforms of excitation current flowing through high-voltage winding wires when the terminals of low- voltage winding wires were released, and a rated current 50 Hz sinusoidal voltage was applied between the terminals of high-voltage winding wires based on an electromagnetic field analysis model in which the low-voltage winding wires and high-voltage winding wires were wound around the main legs of a three-phase/five-leg iron core.

[0055]

The waveforms of excitation current 3 lu, 31v, and 31w of the three-phase/five-leg iron core corresponding to the conventional constitution as shown in FIG 6 indicate deformations in the sinusoidal waves, showing the superimposition of harmonic components that are caused by the nonlinear magnetization characteristics of an iron core.

On the other hand, in the waveforms of excitation current 32u, 32v, and 32w of the three-phase/five- leg iron core corresponding to the constitution of the present example as shown in FIG 7, the superimposition of harmonic components is under control as compared with those in FIG 6, producing waveforms similar to sinusoidal waveforms.

This effect allows us to expect excitation noise to be under control in the three-phase/five-leg iron having the constitution of the present example.

[0056]

The relative (higher or lower) relationship of the average magnetic flux densities Bm and Bs under the rated excitation conditions in the main legs lu, Iv, and Iw and the side legs 2 changes depending on the cross-sectional area of the horizontal plane, which is a plane perpendicular to the direction of magnetic flux, a distance between main legs, a distance between a main leg and a side leg, and what not.

In the constitution of the Example 1 as shown in FIGs 1 and 2, the main legs lu, Iv, and Iw have larger cross-sectional areas on the horizontal plane than the side legs 2 and, therefore, the average magnetic flux density tends to be larger in the main legs lu, Iv, and Iw than the side legs 2.

However, even when the main legs lu, Iv, and Iw have larger cross-sectional areas on the horizontal plane than the side legs 2, the average magnetic flux density of the main legs lu, Iv, and Iw could possibly be smaller than those of the side legs 2 depending on the other conditions such as a distance between main legs and a distance between a main leg and a side leg.

[0057]

The disassembly /transport transformer 10 of the present example can be produced by a method described below.

FIG 8A through FIG 8C show a method for producing the three-phase/five-leg disassembly /transport transformer of the present example.

[0058]

First of all, in FIG 8A, a T-shaped iron core part 40 formed by integrally assembling a main leg lu, Iv, Iw and a lower yoke between main legs 5a or a lower yoke between a man leg and a side leg 6a and an L- shaped iron core part 41 formed by integrally assembling a side leg 2 and a lower yoke between a man leg and a side leg 6a are provided on a bottom tank 12 side by side at a predetermined space by means of a crane or the like.

[0059]

Next, in FIG 8B, the lower connecting yoke between main legs 5b and the lower connecting yoke between a main leg and a side leg 6b are provided between T-shaped iron core parts and between a T-shaped iron core part 40 and a L-shaped iron core part 41 by laminating steel sheets, thereby connecting all of the iron core parts with lower yokes.

Furthermore, winding wires 9u, 9v, and 9w are inserted into the main legs In, Iv, and Iw by means of a crane or the like.

[0060]

Finally, in FIG 8C, the upper yoke 3 between main legs and the upper yoke 4 between a main leg and a side leg are provided by laminating steel sheets, thereby connecting all of the iron core parts with upper yokes.

Moreover, after furnishing wiring parts and the like between winding wires, an upper tank covering the three-phase/five-leg iron core in its entirety is installed to complete the production of the disassembly /transport transformer.

[0061]

The iron core for a disassembly /transport transformer of the present example is an iron core used for a disassembly /transport transformer 10.

The iron core for a disassembly /transport transformer of the present example is a three-phase/five- leg iron core constituted of three parallel main legs lu, Iv, and Iw, two side legs 2 arranged in parallel on the outer sides of the main legs lu, Iv, and Iw, a plurality of yokes 2a and 2b for connecting the upper and lower end parts of adjoining main legs lu, Iv, and Iw or side legs 2. Three main legs lu, Iv, and Iw, two side legs 2, and the plurality of yokes 2a and 2b are constituted by laminating a plurality of steel sheet layers.

Among the plurality of yokes 2a and 2b, the lower yokes connecting the lower end parts of adjoining main legs lu, Iv, and Iw and the lower yokes connecting the lower ends of main legs lu, Iv, and Iw and side legs 2 are divided into three by two dividing parts. At each dividing part, from among three divided lower yokes, two adjoining yokes (5a and 5b, 6a and 6b) are connected by laminating steel sheets to constitute the connecting part 8a, 8b.

At each dividing part, steel sheets 1 la and 1 lb are laminated at a predetermined lap length by shifting the position of the facing steel sheet Ila, 1 lb of the same layer at successive layers in the extending direction (x direction) of the lower yoke iron core 2b.

[0062]

According to the iron core for a disassembly /transport transformer of the present example, the constitution is particularly such that the lap length is made to be different at the connecting part, which is constituted at the dividing part of the lower yoke, between the lower yoke connecting adjoining main legs lu, Iv, and Iw and the lower yoke connecting a main leg with a side leg 2.

More specifically, the lap length Lm of the connecting part 8a (A part) connecting the lower yoke between main legs 5a with the connecting yoke 5b is different from the lap length Ls of the connecting part 8b (B part) connecting the lower yoke between a main leg and a side leg 6a with the connecting yoke 6b. Since the lap lengths Lm and Ln of the connecting part 8a and 8b are different at each dividing part of the lower yoke, the difference in the magnetic flux densities at various positions within an iron core can be adjusted by the lap lengths Lm and Ln. This makes it possible to decrease iron loss that occurs at the iron core, so that the harmonic component of excitation current flowing through the winding wire wound around main legs can be decreased and excitation noise of the iron core can be lowered.

[0063]

Moreover, when the average magnetic flux density Bm within the main legs lu, Iv, Iw is higher than the average magnetic flux density Bs within the side leg 2 under rated excitation conditions (Bm > Bs), the constitution can be made such that the lap length Lm at each dividing part provided in the lower yoke connecting the lower end part of adjoining main legs is shorter than the lap length Ls at each dividing part provided in the lower yoke connecting the lower end parts of a main leg and a side leg (Lm < Ls), as shown in FIG 3.

In such a constitution, the magnetic resistance increases as the lap length is shorter and, therefore, the difference in the magnetic flux density between a main leg and a side leg can be reduced. This makes it possible to decrease iron loss that occurs at the iron core, so that the harmonic component of excitation current flowing through the winding wire wound around main legs can be decreased and excitation noise of the iron core can be lowered.

[0064]

Moreover, when the average magnetic flux density Bm within the main legs lu, Iv, Iw is lower than the average magnetic flux density Bs within the side leg 2 under rated excitation conditions (Bm < Bs), the constitution can be made such that the lap length Lm at each dividing part provided in the lower yoke connecting the lower end part of adjoining man legs is longer than the lap length Ls at each dividing part provided in the lower yoke connecting the lower end parts of a main leg and a side leg (Lm > Ls), contrary to the one as shown in FIG 3.

In such a constitution, the magnetic resistance decreases as the lap length is longer and, therefore, the difference in the magnetic flux density between a main leg and a side leg can be lowered. This makes it possible to decrease iron loss that occurs at the iron core, so that the harmonic component of excitation current flowing through the winding wire wound around main legs can be decreased and excitation noise of the iron core can be lowered.

[0065]

(Example 2)

Next, the constitution of a disassembly /transport transformer according to Example 2 is described with reference to FIG 9. FIG 9 shows the enlarged cross-sectional views of the connecting parts, that is, A, B and C parts among steel sheets within the three-phase/five-leg iron core shown in FIG 1. The same reference numbers are used for the constitution identical to FIG 3 of Example 1 and any overlapping explanation will be omitted. [0066]

In the present example, as shown in FIG 9, connecting parts 8a, 8b, and 7 are constituted by a lap joint structure, wherein adjoining steel sheet layers 1 la, 1 lb, 11c are laminated by shifting the position of the facing steel sheet of the same layer for a given amount.

In FIG 9, steel sheets are particularly laminated in such a way that the position of the facing steel sheet of the same layer is repeated for three layers as a repeating unit in the lap joint structure of connecting parts 8a, 8b, and 7. The lap joint structure laminated in such a manner is referred to as a “three-stage step lap joint structure” below.

In the three-stage step lap joint structure, the position of the facing steel sheet of the same layer is repeated for three layers as a repeating unit and, therefore, the number of lap stages, which is the repeating unit, is three (3 -stage).

[0067]

The present example can also achieve the effect similar to that of Example 1 by making the lap length Lm of the connecting part 8a (A part) different from the lap length Ls of the connecting part 8b (B part) depending on the relative (higher or lower) relationship of the average magnetic flux densities Bm and Bs within the three-phase/five-leg iron core.

In FIG 9, from among such configurations, the lap length Lm at the connecting part 8a (A part) is made shorter than the lap length Ls at the connecting part 8b (B part).

In the present example as shown in FIG 9, the lap length Ln at the connecting part 7 (C part) is equal to the lap length Ls at the connecting part 8b (B part), in a manner similar to FIG 3 of Example 1. [0068] (Example 3)

Next, the constitution of a disassembly /transport transformer according to Example 3 is described with reference to FIG 10.

FIG 10 shows the enlarged cross-sectional views of the connecting parts, that is, A, B and C parts among steel sheets within the three-phase/five-leg iron core shown in FIG 1. The same reference numbers are used for the constitution identical to FIG 3 of Example 1 and FIG 9 of Example 2, and any overlapping explanation will be omitted.

[0069]

In the present example, as shown in FIG 10, connecting parts 8a, 8b, and 7 are constituted by a lap joint structure, wherein adjoining steel sheet layers Ila through 1 If are laminated by shifting the position of the facing steel sheet of the same layer for a given amount. In FIG 10, steel sheets are particularly laminated in such a way that the position of the facing steel sheet of the same layer is repeated for six layers as a repeating unit in the lap joint structure of connecting parts 8a, 8b, and 7. The lap joint structure laminated in such a manner is referred to as a “six-stage step lap joint structure” below.

In the six-stage step lap joint structure, the position of the facing steel sheet of the same layer is repeated for six layers as a repeating unit and, therefore, the number of lap stages, which is the repeating unit, is six (6-stage). [0070]

The present example can also achieve the effect similar to that of Example 1 and Example 2 by making the lap length Lm of the connecting part 8a (A part) different from the lap length Ls of the connecting part 8b (B part) depending on the relative (higher or lower) relationship of the average magnetic flux densities Bm and Bs within the three-phase/five-leg iron core.

In FIG 10, from among such constitutions, the lap length Lm at the connecting part 8a (A part) is made shorter than the lap length Ls at the connecting part 8b (B part).

In FIG 3 of Example 1 and FIG 3 of Example 2, the lap length Ln at the connecting part 7 (C part) is equal to the lap length Ls at the connecting part 8b (B part); however, in FIG 10 of the present example, the lap length Ln at the connecting part 7 (C part) is equal to the lap length Lm at the connecting part 8a (A part) [0071] (Example 4)

Next, the constitution of a disassembly /transport transformer according to Example 4 is described with reference to FIG 11.

FIG 11 shows the enlarged cross-sectional views of the connecting parts, that is, A, B and C parts among steel sheets within the three-phase/five-leg iron core shown in FIG 1. The same reference numbers are used for the constitution identical to FIG 3 of Example 1, FIG 9 of Example 2, and FIG 10 of Example 3, and any overlapping explanation will be omitted.

[0072]

In the present example, as shown in FIG 11, connecting parts 8a, 8b, and 7 are constituted by a lap joint structure, wherein adjoining steel sheet layers are laminated by shifting the position of the facing steel sheet of the same layer for a given mount. In this respect, the present example is similar to Example 1 through Example 3.

[0073]

However, in the present example, the number of lap stages, which is the number of layers of a repeating unit in a lap joint structure, is made different, instead of making the lap lengths Lm and Ls of connecting parts different at the connecting part 8a (A part) and the connecting part 8b (B part), as shown in FIG 11. The present example can also achieve the effect similar to that of Example 1 through Example 3 by making the number of lap stages at the connecting part 8a (A part) different from the number of lap stages at the connecting part 8b (B part) depending on the relative (higher or lower) relationship of the average magnetic flux densities Bm and Bs within the three-phase/five-leg iron core. [0074]

In FIG 11, from among such constitutions, the connecting part 8a (A part) has a one -stage alternate lamination in which the number of lap stages is two, and the connecting part 8b (B part) has a six-stage step lap joint structure in which the number of lap stages is six, i.e., the number of lap stages at the connecting part 8a (A part) is made smaller than the number of lap stages at the connecting part 8b (B part).

[0075]

Furthermore, the relationship of the number of lap stages is preferably be determined based on the relative (higher or lower) relationship of the average magnetic flux densities Bm and Bs under rated excitation conditions that occur in the main legs lu, Iv, and Iw and the side legs 2 of the three -phase/five leg iron core as shown in FIG 1.

More specifically, if the average magnetic flux density Bm of main legs is higher than the average magnetic flux density Bs of the side legs under rated excitation conditions (Bm > Bs) when the number of lap stages is designed identical conventionally, the number of lap stages at the connecting part 8a (A part) between main legs should be made smaller than the number of lap stages at the connecting part 8b (B part) between a main leg and a side leg.

In such a constitution, the magnetic resistance increases as the number of lap stages is smaller and, therefore, the difference in the magnetic flux density between a main leg and a side leg can be decreased. This makes it possible to decrease iron loss that occurs at the iron core, so that the harmonic component of excitation current flowing through the winding wire wound around main legs can be decreased and excitation noise of the iron core can be lowered.

[0076]

On the contrary, if the average magnetic flux density Bm of main legs is lower than the average magnetic flux density Bs of the side legs under rated excitation conditions (Bm < Bs) when the number of lap stages is designed identical as usual, the number of lap stages at the connecting part 8a (A part) between main legs should be made larger than the number of lap stages at the connecting part 8b (B part) between a main leg and a side leg.

In such a constitution, the magnetic resistance decreases as the number of lap stages is larger and, therefore, the difference in the magnetic flux density between a main leg and a side leg can be lowered. This makes it possible to decrease iron loss that occurs at the iron core, so that the harmonic component of excitation current flowing through the winding wire wound around main legs can be decreased and excitation noise of the iron core can be lowered. [0077]

In FIG 11, a combination of a one -stage alternate laminating in which the number of lap stages is two, and a six-stage step lap joint structure in which the number of lap stages is adopted for the number of lap stages at the connecting part 8a (A part) and the number of lap stages at the connecting part 8b (B part).

With regard to the constitution for different numbers of lap stages, other combinations are also possible. By way of example, a three-stage step lap structure in which the number of step stages is three can be used at either connecting part.

[0078]

(Modified Examples)

In each of the abovementioned examples, only one of the lap length and the number of lap stages is made different at the connecting part 8a (A part) and the connecting part 8b (B part); however, both of the lap length and the number of lap stages can possibly be made different.

Particularly, if the relative (longer or shorter) relationship of lap lengths and the relative (larger or smaller) relationship of the numbers of lap stages are determined by the relative (higher or lower) relationship of the average magnetic flux densities Bm and Bs of the main legs lu, Iv, Iw and side leg 2 under rated excitation conditions, the effect of decreasing the harmonic component of excitation current flowing through winding wires and the effect of decreasing the excitation noise of an iron core can further be enhanced.

[0079]

In the enlarged cross-sectional view as shown in FIGs 9 and 10, steel sheets are shifted in one direction from left to right vertically in the drawings with regard to the repeating unit of the lap joint structure.

However, it is possible to use a constitution in which steel sheets are shifted in one direction from right to left vertically in the drawing or a constitution in which the shifting direction is changed alternately such as the constitution of the B part through the D part in FIG 3 of Patent Document 1 in which steel sheets are shifted from left to right and then from left to right.

In the constitution of the B part through the D part in FIG 3 of Patent Document 1, the lap joint structure in which steel sheets are shifted is repeated for three layers in the same direction and, therefore, the number of lap stages, which is the number of layers of a repeating unit is three.

As compared with the constitution of the B part through the D part in FIG 4 of Patent Document 1 in which steel sheets are shifted in one direction from left to right, the constitution of the B part through the D part in FIG 3 of Patent Document 1 has the same number (three) of lap stages and the same electric resistance at connecting parts but is different in terms of the number of layers repeating the positions of steel sheets (period).

[0080] The present invention is not restricted by the abovementioned detailed description and examples but includes a wide variety of variations. For example, the abovementioned detailed description and examples are described in detail for the purpose of simply facilitating the understanding of the present invention but are not necessarily limiting the present invention to one provided with all the constitutions described above.

Description of the Reference Numbers

[0081] lu, Iv, Iw: Main leg

2: Side leg

2a: Upper yoke iron core

2b: Lower yoke iron core

3: Upper yoke between main legs

4: Upper yoke between a main leg and a side leg

5a: Lower yoke between main legs

5b: Lower connecting yoke between main legs

6a: Lower yoke between a main leg and a side leg

6b: Lower connecting yoke between a main leg and a side leg

7: Connecting part between a leg iron core and a yoke iron core

8a: Connecting part of a lower yoke between main legs

8b: Connecting part of a lower yoke between a main leg and a side leg

9u, 9v, 9w: Winding wire

10: Disassembly /transport transformer

Ila- Ilf: Grain-oriented silicon steel sheet

12: Bottom tank

12a: Upper tank

40: T-shaped iron core parts

41 : L-shaped iron core parts

Claims

1. An iron core for a disassembly /transport transformer used for a disassembly /transport transformer, the iron core being a three-phase/five-leg iron core comprising three paralleled main legs, two side legs arranged in parallel to those three main legs on the outer sides thereof, and a plurality of yokes connecting the upper and lower end parts of the adjoining main legs or side legs, wherein: the three main legs, the two side legs, and the plurality of yokes are constituted by each laminating a plurality of steel sheets, from among the plurality of yokes, the lower yoke connecting the lower end part of the adjoining main legs as well as the lower yokes connecting each lower end part of the main legs and the side legs are each divided by two dividing parts, the steel sheets are laminated at the dividing part for a lap length, which is a given length, by shifting the position of the facing steel sheet of the same layer at successive layers in the extending direction of the lower yoke, and the lap length at each of the dividing parts provided at the lower yoke that connects the lower end parts of the adjoining main legs is different from the lap length at each of the dividing parts provided at the lower yoke that connects respective lower end parts of the main leg and the side leg.

2. The iron core for a disassembly /transport transformer according to Claim 1, wherein the average magnetic flux density within all main legs under rated excitation conditions is higher than the average magnetic flux density within all side legs, and the lap length at each dividing part provided at a lower yoke connecting the lower end part of the adjoining main legs is shorter than the lap length at each dividing part provided at a lower yoke connecting each lower end part of main legs and side legs.

3. The iron core for a disassembly /transport transformer according to Claim 1, wherein the average magnetic flux density within all main legs under rated excitation conditions is lower than the average magnetic flux density within all side legs, and the lap length at each dividing part provided at a lower yoke connecting the lower end part of the adjoining main legs is longer than the lap length at each dividing part provided at a lower yoke connecting each lower end part of main legs and side legs.

4. The iron core for a disassembly /transport transformer according to Claim 1, wherein, in a lap joint structure at each of the dividing parts of the lower yoke in which the position of the facing steel sheet of the same layer is shifted, the number of lap stages, which is the number of layers of the repeating unit for the position of the facing steel sheet of the same layer, is all made the same.

5. The iron core for a disassembly /transport transformer according to Claim 1, wherein, in a lap joint structure at each of the dividing parts of the lower yoke in which the position of the facing steel sheet of the same layer is shifted, the number of lap stages, which is the number of layers of the repeating unit for the position of the facing steel sheet of the same layer, is made any different value depending on the position of the lower yoke.

6. An iron core for a disassembly /transport transformer used for a disassembly /transport transformer, the iron core being a three-phase/five-leg iron core comprising three paralleled main legs, two side legs arranged in parallel to those three main legs on the outer sides thereof, and a plurality of yokes connecting the upper and lower end parts of the adjoining main legs or side legs, wherein: the three main legs, the two side legs, and the plurality of yokes are constituted by each laminating a plurality of steel sheets, from among the plurality of yokes, the lower yoke connecting the lower end part of the adjoining main legs as well as the lower yokes connecting each lower end part of the main legs and the side legs are each divided by two dividing parts, the iron core has a lap joint structure at each dividing part, wherein the steel sheets are laminated at the dividing part for a lap length, which is a given length, by shifting the position of the facing steel sheet of the same layer at successive layers in the extending direction of the lower yoke, at the lap joint structure of each dividing part, the position of the facing steel sheet of the same layer is repeated with layers having the number of lap stages, which is a given number, as a repeating unit, and the number of lap stages at each of the dividing parts provided at the lower yoke that connects the adjoining main legs is different from the number of lap stages at each of the dividing parts provided at each of the lower yokes that connects respective lower end parts of the main leg and the side leg.

7. The iron core for a disassembly /transport transformer according to Claim 6, wherein the average magnetic flux density within all main legs under rated excitation conditions is higher than the average magnetic flux density within all side legs, and the number of lap stages at each dividing part provided at a lower yoke connecting the lower end part of the adjoining main legs is smaller than the number of lap stages at each dividing part provided at a lower yoke connecting each lower end part of main legs and side legs.

8. The iron core for a disassembly /transport transformer according to Claim 6, wherein the average magnetic flux density within all main legs under rated excitation conditions is lower than the average magnetic flux density within all side legs, and the number of lap stages at each dividing part provided at a lower yoke connecting the lower end part of the adjoining main legs is more than the number of lap stages at each dividing part provided at a lower yoke connecting each lower end part of main legs and side legs.