An MI cable is an example of a heat-resistant insulated wire. Such MI cable also known as mineral insulated cable which is formed by inserting a conductor into a heat-resistant alloy tube of a stainless steel alloy etc. charged with fine particles of a metal oxide such as magnesium oxide, or a glass braided tube insulated wire, said tube being made of textile glass fiber forming an insulating member, and the like. However, an 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 and its electrical and mechanical reliability leaves room for improvement 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 of insulated wire, 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 to such a degree that the heat-resistant organic material is not completely decomposed, wound and again heated to thermally completely 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 made oxidizing the surface of an aluminum conductor to form 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 an aluminum 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 of the wire turns caused by vibration or the like. Therefore, even if the aforementioned wire the surface of which is covered with a ceramic layer is employed for manufacturing a coil, a sufficient heat resistance cannot be attained when the coiled wire is fixed in the coil by 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 made 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 hold the wound wire in place by the oxide ceramics layer, thereby attaining a 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 voids 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 fill the voids between the inner turns of the wire forming the coil with the reacted solution of metal alkoxide, whereby cracks caused in the organic insulating layer remains effective in reducing the breakdown voltage so that it is impossible to attain a high insulation ability which must originally be provided by the organic insulating layer.