Method of manufacturing insulated coil

Abstract: A method of manufacturing an insulated coil by winding a wire (4), which is formed by covering the outer peripheral surface of a conductor (1) with a mineral insulating layer (6), into a coil comprises the steps of applying (3) a precursor solution (7) of an oxide insulating material onto the surface of the wire covered with the mineral insulating layer (6) for winding the wire into a coil.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of manufacturing a coil which is insulated by a mineral material. Description of the Background Art Examples of a heat-resistant insulated wire are an MI cable (mineral insulated cable) which is formed by inserting a conductor in a heat-resistant alloy tube of a stainless steel alloy etc. charged with fine particles of a metal oxide such as magnesium oxide, a glass braided tube insulated wire employing textile glass fiber for an insulating member, and the like. However, the MI cable is unsuitable for winding into a coil since the density of its conductor cannot be increased. On the other hand, the glass braided tube insulated wire is inferior in heat resistance in addition to electrical and mechanical reliability since its internal layer may contain an organic material, and the density of its conductor cannot be increased. Thus, the glass braided tube insulated wire is also unsuitable for winding into a coil. In a well-known method of manufacturing a coil, a mixture prepared by mixing and dispersing ceramic particles into a heat-resistant organic material is applied onto the outer surface of a conductor, dried or entirely heat treated in such a degree that the heat-resistant organic material is not completely decomposed, wound and again heated to thermally decompose the heat-resistant organic material contained in the wound wire, thereby fixing the ceramic particles around the conductor. Also known is an alumite wire which is prepared by oxidizing the surface of an aluminum conductor as a thin ceramic wire which is flexible to some extent, and it is also possible to manufacture a heat-resistant insulated coil by winding such a conductor into a coil. However, a wound coil is generally fixed with impregnation of an organic material such as enamel, in order to prevent dislocation caused by vibration or the like. Therefore, even if the aforementioned wire whose surface is covered with a ceramic layer is employed for manufacturing a coil, sufficient heat resistance cannot be attained when the coiled wire is fixed through an organic material. In order to solve such a problem, Japanese Patent Laying-Open Gazette No. 63-237404 discloses a method of dipping a coil which is prepared by winding a wire in a solution of reacted metal alkoxide for applying the solution onto the surface of the coil and then converting the material forming the solution layer into oxide ceramics by heating. According to this method, it is possible to fix the wound wire by the oxide ceramics layer, thereby attaining superior heat resistance as compared with the conventional method of employing an organic material. In such a method, however, it is difficult to fill up void portions between inner turns of the wire forming the coil with the solution of metal alkoxide. When a wire formed by covering the surface of a conductor with a mineral insulating layer is wound into a coil, bending stress is applied to the mineral insulating layer to crack the same. In the method disclosed in the above prior art, it is impossible to impregnate the void portions between the inner turns of the wire forming the coil with the reacted solution of metal alkoxide, and cracks caused in the organic insulating layer remain intact in such portions. Thus, the breakdown voltage is so reduced that it is impossible to attain high insulability, which must originally be provided by the organic insulating layer. SUMMARY OF THE INVENTION An object of the present invention is to provide a method of manufacturing an insulated coil, which can provide high insulability to a wound coil. The present invention is directed to a method of manufacturing an insulated coil by winding a wire, which is formed by covering the outer peripheral surface of a conductor with a mineral insulating layer, into a coil. The inventive method comprises the steps of applying a precursor solution of an oxide insulating material onto the surface of the wire which is covered with the oxide solution in an intermediate stage of winding the wire into a coil, winding the wire, and thereafter drying the precursor solution of the oxide insulating material applied onto the surface of the wire. The precursor solution of the oxide insulating material employed in the present invention is preferably prepared by hydrolyzing and polycondensing metal alkoxide or metal carboxylate containing at least one description of a metal selected from the group of Si, Al, Zr, Ti and Mg. In the present invention, the wire formed by covering the outer peripheral surface of a conductor with a mineral insulating layer may be prepared from the following material, for example: (1) an alumite wire formed by anodically oxidizing the surface of an aluminum conductor; (2) a wire formed by applying silicon resin which is converted to ceramics by heating or a material prepared by mixing ceramic particles into the said silicon resin, or a wire obtained by heating the resin layer to convert fully or partially the same to ceramics; or (3) a wire formed by applying a ceramic precursor solution which is prepared by hydrolyzing and polycondensing a raw material of metal alkoxide or metal carboxylate onto the surface of a conductor, or a wire obtained by converting fully or partially the material forming the solution layer to ceramics by heating. Although such a wire is relatively flexible, a large number of cracks are caused in the film when the wire is wound beyond the limit of toughness of ceramics since the insulating film material is formed of ceramics. Such cracks lead to a breakdown in energization, as hereinabove described. The present invention is adapted to form a ceramic insulating layer in order to prevent such cracks by filling up the cracks with ceramics. According to the present invention, the thickness of the mineral insulating layer is preferably not more than half the diameter of the conductor. If the thickness exceeds this value, the mineral insulating layer may be significantly damaged in winding to cause difficulty in recovery through application of the precursor solution of the oxide insulating material, while it is impossible to increase the density of the conductor for serving as winding. When the coil is applied to a vacuum use or the like, the precursor solution of the oxide insulating material which is applied onto the surface of the wire is preferably converted to ceramics by heat treatment. Such conversion is adapted to reduce the possibility of gas evolution, thereby suitably applying the coil to a vacuum use. However, it is not requisite to convert the applied precursor solution to ceramics. If only a small number of cracks are caused in the mineral insulating layer and can be filled up with a small amount of the precursor solution, the solution may simply be dried. In this case, it is also possible to convert the solution to ceramics by heat which is generated during employment. According to the inventive method, it is possible to fill up void portions and cracks, which may be caused in the mineral insulating layer covering the wire surface by winding, with the precursor solution of the oxide insulating material. Thus, it is possible to prevent reduction in insulability caused by cracks of the mineral insulating layer. According to the present invention, it is also possible to fix a wound coil tightly because void portions between inner turns of the wire are filled up with the precursor solution. The aforementioned metal alkoxide or metal carboxylate may be prepared as a solution having relatively low viscosity. Thus, it is possible to apply the solution onto the surface of a wire which is covered with a mineral insulating layer and fill up fine cracks caused in the mineral insulating layer, thereby improving insulability. According to the inventive method, the precursor solution of the oxide insulating material is applied onto the surface of the wire in an intermediate stage of winding the wire into a coil. Thus, void portions between inner turns of the wire for forming the coil are also filled up with the precursor solution of the oxide insulating material so that cracks caused in the mineral insulating layer which is formed on the surface of the wire can be filled up with the precursor solution also in these portions. Thus, reduction of insulability is prevented and a high breakdown voltage is attained. These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic block diagram showing a method of manufacturing a coil according to the present invention; and Fig. 2 is a sectional view showing a wire which is wound into a coil according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Fig. 1, a wire 1, which is formed by covering the outer peripheral surface of a conductor with a mineral insulating layer, is passed through a felt mesh board 2. A precursor solution is dripped on the felt mesh board 2 from a precursor solution supply tube 3. Thus, the felt mesh board 2 is impregnated with the precursor solution. This precursor solution is applied onto the surface of the wire 1, which is passed through the felt mesh board 2. Such a wire 4 coated with the precursor solution is wound on a core 5. Thus, the precursor solution is applied onto the surface of the wire 1, and the wire 4 coated with the precursor solution is wound into a coil. Referring to Fig. 2, a precursor solution filling layer 7 is defined between turns of a wire 1 which is wound on a core 5. As shown in Fig. 2, the precursor solution filling layer 7 is also defined on the surfaces of the turns of the wire 1 for forming the inner part of a coil, whereby cracks that may be caused in a mineral insulating layer 6 are filled up with the precursor solution filling layer 7. Thus, the coil manufactured according to the inventive method exhibits a high breakdown voltage. After winding, the precursor solution filling layer 7 is dried up. If necessary, the precursor solution filling layer 7 may be converted to ceramics by heat treatment at a higher temperature. Example 1 An aluminum wire of 1 mm in wire diameter was covered with an oxide film of about 20 µm in thickness, to prepare an alumite wire, which exhibited a breakdown voltage of about 300 V. This alumite wire was wound on a bobbin, while a solution of tetrabutoxysilane, which is alkoxide of Si, was applied onto the surface of the alumite wire. The solution of tetrabutoxysilane was prepared by heating/mixing an alcohol solution, to which water and a catalyst were added, at 80°C. The alumite wire was wound on a bobbin of 100 mm in diameter with application of the solution of tetrabutyoxysilane, and then heated with the bobbin at 300°C for one hour. The wound alumite wire exhibited a breakdown voltage of at least 300 V before and after heating. No reduction of the breakdown voltage was recognized even if the coil of the alumite wire was heated to 400°C for 10 hours. For the purpose of comparison, an alumite wire similar to the above was wound on the same bobbin of 100 mm in bobbin diameter, with no application of the solution of tetrabutoxysilane. In this case, the breakdown voltage of the alumite wire was reduced to about 200 V, and partially to less than 100 V. As obvious from the result of such comparison, the coil manufactured according to the present invention exhibits a high breakdown voltage also when the same is wound. Example 2 6 g of 2-ethyl-hexanoic zirconate [Zr[OC(O)CH(C₂H₅)C₆H₁₂]₄] and 2 g of 2-ethyl-hexanoic aluminate [Al[OC(O)CH(C₂H₅)C₆H₁₂]₃] were dissolved in 100 ml of dibutyl ether. Thus, a Zr/Al mixed solution was prepared. The Zr/Al mixed solution was applied onto a copper conductor of 0.5 mm in diameter which was plated with a nickel layer of about 10 µm in thickness, and mineralized by heat treatment performed in such a degree that substantially no organic component was left. The wire whose outer peripheral surface was thus covered with a mineral insulating layer was wound on a bobbin with application of the aforementioned Zr/Al mixed solution. This bobbin was 50 mm in bobbin diameter. After the winding, the coil was heat treated in the atmosphere at 400°C for two hours. The as-formed coil exhibited a breakdown voltage of 500 V. Example 3 A nickel-plated copper wire of 1 mm in wire diameter was vapor-decreased with perchloroethylene. Concentrated nitric acid of 1.2 N was added to a solution prepared by mixing 3 mole percent of tetraethyl orthosilicate, 35 mole percent of water and 62 mole percent of ethanol by 3/100 mol with respect to tetraethyl orthosilicate, and this mixture was heated/stirred at 70°C for two hours, to prepare a coating solution. This coating solution was applied onto the surface of the vapor-degreased nickel-plated copper wire, which was then heat treated to produce a wire covered with silicon oxide. 5 m mol of n-butoxy zirconium, 15 m mol of n-butoxy aluminum, 45 m mol of ethanol amine and 100 ml of diethyleneglycolmonomethylether were mixed to prepare a solution A. On the other hand, 80 m mol of n-butyl silicate, 100 m mol of water, 1.6 m mol of nitric acid and 100 ml of diethylene glycol monomethyl ether were mixed, heated/stirred at 80°C for five hours, and then stood for cooling to the room temperature, thereby preparing a solution B. The solution A was gradually dripped into the solution B. In such dripping, it is necessary to cool the solution B with ice. After the dripping was completed, the mixed solution was stirred in a constant humidity/constant temperature bath of 30°C in temperature and 50 % in humidity for 10 hours, thereby preparing a coating solution. This coating solution was applied onto the surface of the aforementioned wire having a mineral insulating layer, which was wound on a bobbin of 30 mm in bobbin diameter. After such winding, the coil was heat treated in the atmosphere at 200°C for four hours to convert partially to ceramics. The as-formed coil exhibited a breakdown voltage of 800 V. As hereinabove described, any coil manufactured according to the present invention exhibits a high breakdown voltage, which cannot be attained by the prior art. In the aforementioned embodiment, the precursor solution of the oxide insulating material is applied onto the wire which is not yet wound on the bobbin. In a modification of the present invention, the precursor solution may alternatively be applied onto a wire which is wound on a bobbin. In this case, the precursor solution is successively applied to the surfaces of the turns of the wire which is wound on the bobbin. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being limited only by the terms of the appended claims.

Claims: A method of manufacturing an insulated coil by winding a wire (1), formed by covering the outer peripheral surface of a conductor with a mineral insulating layer (6), into a coil, said method comprising the steps of: applying a precursor solution of an oxide insulating material onto the surface of said wire (1) covered with said mineral insulating layer (6) in an intermediate stage of winding the same into a coil; and drying said precursor solution of said oxide insulating material applied onto the surface of said wire (1) after winding the same. A method in accordance with claim 1, wherein said drying step comprises a step of converting said precursor solution of said oxide insulating material to ceramics by heat treatment. A method in accordance with claim 1, wherein said precursor solution of said oxide insulating material is prepared from alkoxide of at least one description of a metal selected from the group of Si, Al, Zr, Ti and Mg. A method in accordance with claim 1, wherein said precursor solution of said oxide insulating material is prepared from carboxylate of at least one description of a metal selected from the group of Si, Al, Zr, Ti and Mg. A method in accordance with claim 1, wherein the thickness of said mineral insulating layer (6) is not more than half the diameter of said conductor.

Inventors: INAZAWA SHINJI C O OSAKA, SAWADA KAZUO C O OSAKA, YAMADA KOUICH C O OSAKA, INAZAWA, SHINJI, C/O OSAKA, SAWADA, KAZUO, C/O OSAKA, YAMADA, KOUICH, C/O OSAKA
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1990
Main classification: H01B7
IPC codes: H01B13, H01B3, H01F41, H01B7