JP2005078934A - Insulation-covered electric conductor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an insulation-covered electric conductor suitable for forming a coil for a motor or a generator and tolerant of welding heat generated when a circuit is formed by providing a welding spot in a circuit in the coil. <P>SOLUTION: A multilayer insulation-covered electric conductor is provided as a covered metal conductor wherein a lower insulation cover layer that touches the uppermost layer among the multiple insulation cover layers is made of a polyamidimide resin, and the ratio between the number of amide groups and the number of imide groups in the polyamidimide resin is 45:55 to 1:99. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an insulation-coated electric conductor that can withstand welding heat when a circuit is formed by providing a welding point in a circuit in a coil, which is suitable for forming a coil such as a motor or a generator.

Conductors coated with electrical insulators are incorporated into various electrical equipment and are used in large quantities for coil applications. They are used particularly often in electrical equipment such as motors and generators. Of these, windings having a conductor cross-section other than a round shape have been conventionally used in which an insulating material such as glass or paper is horizontally wound around the covering material. These have been used for applications that require extremely high reliability, such as generator coils for power plants, transformer coils, and drive motors for vehicles.
In recent years, windings other than round shapes and generally rectangular shapes have come to be used for devices smaller than these devices. Even in these small devices, higher performance has been promoted in the shape of the coil, and the coil creation method is not a method of creating a coil by winding an electric wire, which was conventionally called a winding, around the circumference of the coil. A method has been adopted in which a coil is formed by joining coated conductors having a cross-sectional shape matching the shape. Using a winding with a conductor other than a round shape in this small device eliminates the gap between the coil core and magnetic field loss, resulting in improved performance. Further progress has been made by being able to reduce the size.
Making a conductor other than a circle makes it difficult to wind the wire directly long when forming a coiled circuit, so after forming the short conductor into the partial shape of the coil, the conductors are welded together As a result, a method of forming an entire circuit has been carried out. In order to form these coils, it is necessary to connect the conductors. An electric welding method such as fusing (electric welding is performed while applying pressure) or TIG welding has been adopted in a portion where soldering is conventionally performed to connect the conductors. This is because in soldering that has been used in the past, the lead contained in the solder has a large impact on the environment when the product is discarded, and the soldered part has low reliability against vibration of the equipment. Therefore, it is derived from the fact that a conductor (such as copper) that is currently used is required to be used as a connection material.

  Conventionally, various resins such as polyester have been used as coating materials used for round enamel wires. However, in fusing and TIG welding, the conductor is melted by directly applying heat to the conductors, and the conductors are connected to each other. Therefore, the insulation coating in the vicinity of the connection part is extremely high in temperature and is subject to great thermal deterioration. Become. For example, in order to connect copper to each other by ordinary welding, the temperature of copper needs to be higher than the melting point of copper, and for that purpose, the conductor temperature rises to about 1100 ° C. or higher. An increase in the conductor temperature causes thermal deterioration of the insulating coating in the vicinity, and the low molecular weight component in the coating material evaporates due to heat, causing the coating to swell (foam), and the electrical characteristics of the nearby coating material May decrease. Although it is well known that the heat of welding affects the coating, it is necessary to reduce this effect in order to improve the reliability of electrical equipment. When the conductor temperature of the welded portion is approximately 1100 ° C., the heat received by the coating that requires insulation performance is approximately 600 ° C. at a distance of 10 mm from the welded portion. For this reason, such swelling of the coating cannot be avoided with a conventionally used polyamideimide resin alone.

In order to avoid this problem, for example, a coated electrical conductor using a polyimide resin as the middle layer of the coating has been proposed (see Patent Document 1). This coated electric conductor has good instantaneous heat resistance, and does not cause voids or blisters in the coating even when processing conductors such as welding in the coil manufacturing process. It was to provide. However, with higher performance of equipment, there is a need for coated electrical conductors for coils with higher reliability.
JP 2002-109965 A

  The present invention provides a coated electrical conductor that can withstand welding heat when forming a circuit by providing a welding location in a circuit in the coil, which is suitable for forming a coil of a motor, a generator, or the like. It is for the purpose.

  The present inventors paid attention to the coating material of the coated electric conductor, and found a coating structure that can withstand the heat during welding and that does not cause abnormalities such as swelling of the coating. Regarding the coating composition that can withstand the heat during welding, the inventors focused on the fact that the heat during welding is instantaneously applied, and verified the heat resistance (instant heat resistance) of the coating. The instantaneous heat resistance of the coating is that it conducts from the conductor side during welding of the conductor, the coating material formed on the conductor side is most susceptible to thermal degradation, and the cracked gas generated from the coating material on the conductor side spreads over the entire coating. It was confirmed that voids and blisters (fine foaming) were generated. For this reason, in order to counteract the generation of voids, etc., we considered using a material that does not soften even when the coating is heated as a part of the coating, and a specific polyamideimide resin is applied to a specific layer of the multilayer insulating coating layer. It has been found that the object of the present invention can be achieved by use, and the present invention has been made based on this finding.

That is, the present invention
(1) A multilayer insulation coating electric conductor, wherein a lower insulation coating layer contacting the uppermost layer of the multilayer insulation coating layer is made of a polyamideimide resin, and the ratio of the number of amide groups to the number of imide groups of the polyamideimide resin is an amide group A coated metal conductor characterized by having an imide group of 45:55 to 1:99;
(2) Part or all of the imide groups in the polyamide-imide resin are derived from pyromellitic dianhydride, the coated metal conductor according to (1),
(3) The coated metal conductor according to (1) or (2), wherein the uppermost layer of the multilayer insulating coating layer is made of a polyamideimide resin,
(4) The coated metal conductor according to any one of (1) to (3), wherein the metal conductor is low-oxygen copper or oxygen-free copper having an oxygen content of 30 ppm or less,
(5) The coated metal conductor according to any one of (1) to (4), wherein the conductor has a cross section other than a circle, and
(6) The coated metal conductor according to any one of (1) to (5), wherein the coated metal conductor is a multilayer insulation coated electric conductor used in a coil constituting a circuit by providing a welding location in the coil Is to provide.
The reason why the effect of the present invention is exerted is not yet clear, but the polyimide resin has less decrease in elastic modulus even at room temperature than at room temperature. This is thought to be due to the small difference in elastic modulus.

The coated metal conductor of the present invention has good instantaneous heat resistance, and does not cause voids or blisters in the coating even for processing such as welding of a conductor that is heated at high temperature in a harsh coil manufacturing process, Since soundness is maintained, an insulated wire does not deteriorate. In addition, when the present coated metal conductor is used, it is possible to provide a highly reliable coil because the insulating coating hardly causes thermal deterioration even with respect to heat conducted from the conductor side. This has an excellent effect of enhancing the performance of the entire device using the coil and contributing to the reliability of the device.
In addition, when a polyamideimide resin having a modified imide ratio prepared by combining pyromellitic dianhydride is used, it is inexpensive and easy to use, and the production stability is good.
Further, when the uppermost layer is a polyamide-imide resin, the dielectric breakdown voltage is high and baking hardening is possible.
Further, when the metal conductor is low oxygen copper or oxygen free copper having an oxygen content of 30 ppm or less, the electrical resistance of the welded portion can be prevented from deteriorating and the strength of the welded portion can be maintained.
Further, the conductor having a desired cross-sectional shape can be used.

In the present invention, the ratio of the number of amide groups to the number of imide groups used to form the lower insulating coating layer in contact with the uppermost layer of the multilayer insulating coating layer is 45:55 to 1:99 in terms of amide group: imide group. Thus, the polyamide-imide resin in which the imide ratio is changed can be prepared by appropriately changing the monomer of the acid component during the synthesis of the polyamide-imide resin. Polyamideimide is generally prepared by an equimolar reaction of trimellitic anhydride (hereinafter referred to as TMA) and diphenylmethane-4,4′-diisocyanate (hereinafter referred to as MDI). It is possible to change the ratio of amide and imide by changing a part of this TMA to acid dianhydride. Examples of the acid dianhydride usable here include pyromellitic dianhydride (hereinafter referred to as PMDA), 3,3′4,4′-benzophenonetetracarboxylic dianhydride (hereinafter referred to as BTDA), 4 , 4′-oxydiphthalic dianhydride (hereinafter referred to as ODPA), 3,3′4,4′-biphenyltetracarboxylic dianhydride (hereinafter referred to as BPDA), and the isomers thereof. Other acid dianhydrides can also be used. In this case, for example, when a polyamideimide resin is prepared using TMA: 40 mol, PMDA: 10 mol, and MDI: 50 mol, the ratio of the number of amide groups: imide groups is 40:60 (molar ratio). Become.
The amide group: imide group is preferably 45:55 to 20:80, more preferably 45:55 to 30:70. When the number of imide groups per number of amide groups is too small, the heat softening temperature of the insulating film is lowered, and as a result, the heat resistant temperature is lowered. On the other hand, if the number of imide groups per amide group is too large, although it depends on the choice of monomer, the solubility in the varnish state deteriorates and the production of the electric wire becomes impossible.

Among these, an imide ratio-modified polyamideimide resin prepared by a combination of TMA and PMDA is the cheapest, easy to use, and has good production stability. However, other acid dianhydrides can also be used.
As a simple method, a commercially available polyamideimide resin varnish (for example, trade name HI406 manufactured by Hitachi Chemical Co., Ltd.) and a commercially available polyamic acid varnish (for example, product name # 3000 manufactured by Toray DuPont) are used. A polyamide-imide resin with a changed imide ratio can also be obtained by mixing. Generally, it is difficult to simply mix a polyamideimide varnish and a polyamic acid varnish, but the mixture is stabilized by heating to 60 to 80 ° C. while cooling and cooling to room temperature.
The specific method will be described in detail in the embodiments described later.

  Moreover, in this invention, the polyamide imide resin whose amide group: imide group is 45: 55-55: 45 can be used for layers other than the lower layer which contacts uppermost layer. As the polyamide-imide resin that can be used for these layers, commercially available products (for example, trade name HI406 manufactured by Hitachi Chemical Co., Ltd. (in terms of number, amide: imide is approximately 50:50)) can be used. . In addition, by a conventional method, for example, a product obtained by directly reacting a tricarboxylic acid anhydride and a diisocyanate in a polar solvent, or by reacting a diamine with a tricarboxylic acid anhydride in a polar solvent first, first an imide bond And then amidated with diisocyanates can be used. Polyamideimide resin has a lower thermal conductivity than other resins, has a high dielectric breakdown voltage, can be baked and cured, and is suitable as a resin for forming the uppermost layer.

In addition, as the resin forming the uppermost layer of the conductor coating of the present invention, a self-lubricating resin obtained by dispersing and mixing wax or a lubricant by a conventional method can be used. As the wax used for this, those usually used can be used without particular limitation, for example, synthetic waxes such as polyethylene wax, petroleum wax, paraffin wax, and natural waxes such as carnauba wax, cadilla wax, and rice wax. A wax etc. are mentioned. There is no restriction | limiting in particular also about a lubricant, For example, silicone, a silicone macromonomer, a fluororesin etc. can be used.
In the coated electric conductor of the present invention, the resin forming method for forming each resin layer of the coating is not particularly limited, and can be performed by various known methods.

  In addition, in the insulation-coated electric conductor, the lowermost layer of the insulation coating layer is a polyamide-imide resin, and the coated metal conductor is characterized in that an imide ratio-changing polyamide-imide resin is provided directly through another resin layer As a result, there is an effect that the insulating coating hardly causes thermal deterioration with respect to heat conducted from the conductor side when used in a motor or a transformer transformer.

  Conventionally used metal conductors can be used as the conductor, preferably low oxygen copper having an oxygen content of 30 ppm or less, more preferably low oxygen copper or oxygen-free copper having 20 ppm or less. If the oxygen content is low, when the conductor is melted with heat to weld it, there is no generation of voids due to oxygen contained in the welded portion, preventing the electrical resistance of the welded portion from deteriorating and the welded portion of the welded portion. Strength can be maintained. Further, the conductor having a desired cross-sectional shape can be used, but a conductor having a shape other than a circle is preferably used, and a rectangular shape is particularly preferable.

  The coated electric conductor of the present invention can be obtained, for example, by applying and baking a resin varnish corresponding to the coating layer to be formed on the conductor. The method of applying these resin varnishes on the conductor may be a conventional method. For example, a method using a varnish application die having a similar shape to the conductor shape, or if the conductor cross-sectional shape is a quadrangle, it is formed in a grid pattern. A dice called a “universal die” can be used. The conductors coated with these resin varnishes are baked in a baking furnace in the usual manner. The specific baking conditions depend on the shape of the furnace used, but in the case of a natural convection type vertical furnace of about 5 m, the passage time is set to 400 to 500 ° C. to 30 to 90 seconds. Can be achieved. In the present invention, the thickness of the lower layer contacting the uppermost layer is not particularly limited, but is preferably 4 to 35 μm, more preferably 5 to 18 μm. The thickness of the polyamideimide resin layer as a whole is preferably 10 to 40 μm, more preferably 10 to 35 μm. The total thickness of the coating film to be coated is about 15 to 130 μm, preferably 25 to 50 μm.

  The coated metal conductor of the present invention can be preferably used as a multilayer insulation coated electric conductor used in a coil constituting a circuit by providing a welding location in the coil.

Hereinafter, the present invention will be described in more detail based on examples.
[Creation 1 of polyamideimide varnish with changed imide ratio]
A synthesis device equipped with a heating / cooling device, a nitrogen introducing device, and a stirrer was prepared in a four-necked three-necked flask. Into it, 172.9 g (0.9 mol) of TMA, 21.8 g (0.1 mol) of PMDA, 250.1 g (1.0 mol) of MDI and 851.2 g of N-methyl-2-pyrrolidone as a solvent were added. The temperature was raised from room temperature to 140 ° C. over 2 hours with stirring in a nitrogen stream. Stirring was continued for 2 hours at 140 ° C. until carbon dioxide gas was vigorously generated from the flask and the solution in the flask became viscous. Thereafter, the mixture was cooled to room temperature to obtain 1216 g of a polyamideimide varnish composition 1 (resin concentration: 30% by weight).
[Creation 2 to 4 of polyamideimide varnish modified with imide ratio]
In the same manner as in Preparation 1, the components shown in Table 1 were added to obtain imide ratio-modified polyamideimide varnish compositions 2 to 4. The amount of each component used at that time and the amide: imide ratio of each composition are shown in Table 1 together with the results of the above composition 1.

[Example 1]
To a flat conductor (copper having an oxygen content of 15 ppm) with a chamfer radius r = 0.5 mm at four corners of 1.8 × 2.5 mm (thickness × width), in order from the lower layer, polyamideimide resin (PAI) (Hitachi Chemical) Co., Ltd., trade name HI406), varnish of composition 1 prepared in Preparation 1 of the above-mentioned imide ratio modified polyamideimide varnish, polyamideimide resin (PAI) (trade name HI406, manufactured by Hitachi Chemical Co., Ltd.) in this order. The total film thickness was 45 μm. Each film thickness is as shown in Table 2. When forming the film, a plurality of dies having a shape similar to the shape of the conductor were used, and baking was performed a plurality of times at 450 ° C. for about 15 seconds in a baking furnace having a furnace length of 8 m. The results of an evaluation test on this resin-coated conductor are shown in Table 5-1.

[Examples 2 and 3]
The resin and conductor used were the same as in Example 1, and the conditions for baking the resin coating were the same. However, each film thickness was changed as shown in Table 2. The results of an evaluation test on this resin-coated conductor are shown in Table 5-1.

[Comparative Example 1]
As shown in Table 2, polyamide imide resin (PAI) (manufactured by Hitachi Chemical Co., Ltd.) is applied to a flat conductor (copper of the same quality as in Example 1) of 1.8 × 2.5 mm and four corners of r = 0.5 mm. HI406) was formed, and the total film thickness was 45 μm. When forming the film, a plurality of dies having a shape similar to the shape of the conductor were used, and baking was performed a plurality of times in a baking furnace having a furnace length of 8 m at a baking temperature of 450 ° C. for approximately 15 seconds. The results of an evaluation test on this resin-coated conductor are shown in Table 5-1.

[Examples 4 to 6]
In the same manner as in Example 1, a rectangular conductor (copper having an oxygen content of 15 ppm) having a chamfer radius r = 0.5 mm at four corners of 1.8 × 2.5 mm (thickness × width) was applied from the lower layer to a polyamideimide resin ( PAI) (trade name HI406, manufactured by Hitachi Chemical Co., Ltd.), varnish of composition 2-4 prepared in preparations 2-4 of the imide ratio-modified polyamideimide varnish, polyamideimide resin (PAI) (product manufactured by Hitachi Chemical Co., Ltd.) Films were formed in the order of name HI406), and the total film thickness was 45 μm. Each film thickness is as shown in Table 3. When forming the film, a plurality of dies having a shape similar to the shape of the conductor were used, and baking was performed a plurality of times at 450 ° C. for about 15 seconds in a baking furnace having a furnace length of 8 m. The results of an evaluation test on this resin-coated conductor are shown in Table 5-2.

[Example 7]
A synthesis device equipped with a heating / cooling device, a nitrogen introducing device, and a stirrer was prepared in a four-necked three-necked flask. Among them, TMA 7.7 g (0.04 mol), ODPA (4,4′-oxydiphthalic dianhydride) 297.8 g (0.96 mol), MDI 250.1 g (1.0 mol), and as a solvent N-methyl-2-pyrrolidone (1109.7 g) was added, and the mixture was heated from room temperature to 140 ° C. over 2 hours with stirring in a nitrogen stream. Stirring was continued for 2 hours at 140 ° C. until carbon dioxide gas was vigorously generated from the flask and the solution in the flask became viscous. Thereafter, it was cooled to room temperature to obtain 1585 g of an imide ratio-modified polyamideimide varnish (ODPA resin concentration of 30% by weight). In this case, the amide: imide ratio is 2:98.
Using this imide-modified polyamide-imide varnish, a rectangular conductor (copper having an oxygen content of 20 ppm) having a corner of r = 0.8 mm at four corners of 2.0 × 3.0 mm, and a polyamide-imide resin from the lower layer as shown in Table 4. (PAI) (trade name HI406 manufactured by Hitachi Chemical Co., Ltd.), polyamideimide varnish (ODPA (2:98)) and polyamideimide resin (PAI) (trade name HI406 manufactured by Hitachi Chemical Co., Ltd.) described above. A film was formed in order, and the total film thickness was 50 μm. When forming the film, a plurality of dies having a shape similar to the shape of the conductor were used, and baking was performed a plurality of times in a baking furnace having a furnace length of 8 m at a baking temperature of 450 ° C. for about 20 seconds. The results of an evaluation test on this resin-coated conductor are shown in Table 5-3.

[Example 8]
A synthesis device equipped with a heating / cooling device, a nitrogen introducing device, and a stirrer was prepared in a four-necked three-necked flask. Among them, TMA 153.7 g (0.8 mol), BTDA (3,3 ′, 4,4′-benzophenone tetracarboxylic dianhydride) 64.4 g (0.2 mol), MDI 250.1 g (1. 0 mol) and 905.8 g of N-methyl-2-pyrrolidone as a solvent were added, and the temperature was raised from room temperature to 140 ° C. over 2 hours while stirring in a nitrogen stream. Stirring was continued for 2 hours at 140 ° C. until carbon dioxide gas was vigorously generated from the flask and the solution in the flask became viscous. Thereafter, the mixture was cooled to room temperature to obtain 1294 g of a polyamideimide varnish (BTDA: resin concentration of 30% by weight). In this case, the amide: imide ratio is 30:70.
A rectangular conductor of 2.0 × 3.0 mm and r = 0.8 mm at the four corners (copper of the same quality as in Example 7), as shown in Table 4, from the lower layer, polyamideimide resin (PAI) (Hitachi Chemical Co., Ltd.) Product name HI406), the above-mentioned imide ratio modified polyamideimide varnish (BTDA (30:70)), polyamideimide resin (PAI) (trade name HI406, manufactured by Hitachi Chemical Co., Ltd.) in this order, and the film thickness Was 130 μm. When forming the film, a plurality of dies having a shape similar to the shape of the conductor were used, and baking was performed a plurality of times in a baking furnace having a furnace length of 8 m at a baking temperature of 450 ° C. for about 20 seconds. The results of an evaluation test on this resin-coated conductor are shown in Table 5-3.

[Example 9]
A synthesis device equipped with a heating / cooling device, a nitrogen introducing device, and a stirrer was prepared in a four-necked three-necked flask. Among them, TMA 153.7 g (0.8 mol), BTDA (3,3 ′, 4,4′-benzophenone tetracarboxylic dianhydride) 64.4 g (0.2 mol), MDI 250.1 g (1. 0 mol) and 905.8 g of N-methyl-2-pyrrolidone as a solvent were added, and the temperature was raised from room temperature to 140 ° C. over 2 hours while stirring in a nitrogen stream. Stirring was continued for 2 hours at 140 ° C. until carbon dioxide gas was vigorously generated from the flask and the solution in the flask became viscous. Thereafter, the mixture was cooled to room temperature to obtain 1294 g of a polyamideimide varnish (BDTA: resin concentration 30% by weight) with a modified imide ratio. In this case, the amide: imide ratio is 30:70.
As shown in Table 4, from the lower layer to the above-mentioned imide ratio-modified polyamideimide varnish (BTDA (30: 30), with a rectangular conductor of 2.0 × 3.0 mm and r = 0.8 mm at the four corners (same copper as in Example 7). 70)) and polyamideimide resin (PAI) (trade name HI406 manufactured by Hitachi Chemical Co., Ltd.) in this order, and the film thickness was 130 μm. When forming the film, a plurality of dies having a shape similar to the shape of the conductor were used, and baking was performed a plurality of times in a baking furnace having a furnace length of 8 m at a baking temperature of 450 ° C. for about 20 seconds. The results of an evaluation test on this resin-coated conductor are shown in Table 5-3.

[Comparative Example 2]
A rectangular conductor of 2.0 × 3.0 mm and r = 0.8 mm at the four corners (copper of the same quality as in Example 7), as shown in Table 4, from the lower layer, polyamideimide resin (PAI) (Hitachi Chemical Co., Ltd.) Product name HI406), a coating film was formed in the order of the varnish of composition 1 prepared with the imide ratio-modified polyamideimide varnish 1, and the film thickness was 130 μm. When forming the film, a plurality of dies having a shape similar to the shape of the conductor were used, and baking was performed a plurality of times in a baking furnace having a furnace length of 8 m at a baking temperature of 450 ° C. for about 20 seconds. The results of an evaluation test on this resin-coated conductor are shown in Table 5-3.

Evaluation method Bending (edgewise bending)
Bending is performed at 180 ° in the direction of the edge surface of the coated conductor (edgewise bending). The bending radius was made equal to the dimension in the width direction of the conductor (1w bending). After this bending, a pinhole test specified in JIS C3003 was conducted to investigate the occurrence of pinholes. “Good” means that no film cracks are observed when bending is performed, and no pinholes are generated.

Instantaneous heat resistance (fusing)
The flat surface of the coated conductor was orthogonalized, the upper and lower portions of the intersecting portion were sandwiched between electrodes, and the roughness of the coating immediately adjacent to the welding was investigated when welding was performed under the current conditions described in the table. “Good” means no voids or burns.

Instantaneous heat resistance (electric welding)
The ends of the two coated conductors were peeled off by 5 mm, and the butted surfaces of those fixed in parallel so that the peeled surfaces were in contact with the edge surfaces were electrically welded. Conditions are according to the table. In this case, the roughness of the coating immediately adjacent to the weld surface was investigated. “Good” means no voids or burns.

Dielectric breakdown voltage It implemented using the metal foil method of JISC3003 description. The table shows the average value of n = 5. Moreover, it implemented also about the sample left still for 5 days in a 230 degreeC thermostat.

Claims (6)

  A multilayer insulation coating electric conductor, the lower insulation coating layer contacting the uppermost layer of the multilayer insulation coating layer is made of a polyamideimide resin, and the ratio of the number of amide groups to the number of imide groups of the polyamideimide resin is amide group: imide group And 45:55 to 1:99.   The coated metal conductor according to claim 1, wherein a part or all of the imide groups in the polyamide-imide resin are derived from pyromellitic dianhydride.   3. The coated metal conductor according to claim 1, wherein the uppermost layer of the multilayer insulating coating layer is made of a polyamideimide resin.   The coated metal conductor according to any one of claims 1 to 3, wherein the metal conductor is low-oxygen copper or oxygen-free copper having an oxygen content of 30 ppm or less.   5. The coated metal conductor according to claim 1, wherein a cross section of the conductor has a shape other than a circle.   The coated metal conductor according to any one of claims 1 to 5, wherein the coated metal conductor is a multilayer insulation coated electric conductor used for a coil constituting a circuit by providing a welded portion in the coil.