Thin Insulating Mica Sheet and Insulated Coil

The present invention relates to a thin insulating sheet and to a novel insulated coil. More particularly, it relates to a thin insulating sheet for a bare conductor insulation or a turn insulation of a coil of an electric instrument.

The insulating structure shown in the FIGURE has been known. The FIGURE is a sectional view of a coil. In the FIGURE, the reference numeral (1) designates a bare conductor; (2) designates a bare conductor insulating part; (3) an earth main insulating layer. A potential formed between the bare conductors (1) is maintained by the bare conductor insulating part or the turn insulating part (2) and a potential formed between the earth and the bare conductor is maintained by the earth main insulating layer (3).

As the bare conductor insulating part or the turn insulating part (2), a duplex glass winding, an enamel baked or enamel undercoat baked duplex glass winding, a film tape winding, and a mica tape winding have been employed. The duplex glass winding or the enamel baked or enamel undercoat baked duplex glass winding for the bare conductor insulating part or the turn insulating part has inferior corona discharge resistance in comparison with the mica tape winding.

The conventional mica tape winding as the bare conductor insulating part or the turn insulating part causes thick thickness of the tape to occupy a large space for the insulation and to give inferior space factor and to be uneconomical.

In order to overcome these disadvantages, it has been proposed to use an associated mica paper as an insulating material. The associated mica paper has superior heat resistance and electric insulating property such as corona discharge resistance and has been widely used. However, it has insufficient strength and the mica flake is easily removed to require special careful handling of the sheet since flaky mica is sheeted without a binder. When the associated mica paper is used as the insulating material, it has been indispensable to give certain structural strength by bonding the insulating substrates with a varnish or a binder.

It is an object of the present invention to overcome the disadvantages and to provide a thin insulating sheet for an insulated coil having excellent electric characteristics, heat resistance and impregnancy.

The foregoing and other objects of the present invention have been attained by providing a thin insulating sheet obtained by melt-bonding an associated mica sheet containing 1 to 9 wt.% of polymer fibril to a polymer film or a porous polymer sheet. The thin insulating sheet is used for the bare conductor insulation or the turn insulation.

The FIGURE is a sectional view of a conventional insulated coil.

In the present invention, a thin insulating sheet is produced by using an associated mica sheet made of flaky mica containing polymer fibril at a content of 1 to 9 wt.% and a polymer film or a porous polymer sheet by melt-bonding. An insulated coil having excellent electric characteristic heat resistance and impregnancy can be obtained by using the thin insulating sheet. The associated mica sheet can be produced in a thin thickness of 15-40 .mu.m which could not be given by the conventional associated mica sheet.

The polymer fibril (pulp) used in the present invention can be fibrous particles having many projections or ribbon type particles which form a paper by a wet paper sheeting machine. For example, a solution of an aromatic polymer is mechanically stirred in a precipitating medium. In the sheeting, using the pulp and the mica flake, it is possible to add an aromatic staple fiber (0.5-1.0 denier; filament fineness of 0.2-15.mu.; staple length of 1-15 mm) at a content of 1.5-4.0 wt.%.

The aromatic polymer can be aromatic polyamides having an aromatic main chain having units represented by the formula (1) or (2):

wherein Ar.sub.1, Ar.sub.2 and Ar.sub.3 are the same or different and respectively represent di-valent aromatic group. The typical aromatic polyamides include poly(m-phenyleneisophthalamide), poly(m-phenyleneterephthalamide); poly(p-phenyleneisophthalamide) and poly(p-phenyleneterephthalamide). The polymer can be a copolymer or can contain a non-aromatic component.

The resulting associated mica sheet is melt-bonded to a polymer film such as a heat resistant polymer film or a porous polymer sheet by heat-pressing whereby a thin insulating sheet for the coil insulation can be continuously produced without using a binder such as a varnish or an adhesive composition.

The heat resistant polymer film used for the melt-bond can be a polyamide film (trade name KAPTON manufactured by E. I. DuPont); a polyparabanic acid film (trade name TREDLON manufactured by Exxon Chemical Co.); a polyamide film (trade name A film manufactured by Nitto Denki Kogyo); and a polyester film (manufactured by Diafoil Co., Ltd.).

A content of the polymer fibril incorporated into the associated mica sheet is preferably in a range of 1-9 wt.%. When it is less than 1 wt.%, the strength of the associated mica sheet is too low and the mica flake is easily peeled off whereas when it is more than 9 wt.%, electric characteristics especially long charged life characteristic are adversely affected to be unfavorable.

The porous polymer sheet is preferably a porous aromatic polyamide paper and also can be produced by using fibril made of a polyester, phenol resin or polyacrylonitrile etc.

The present invention will be further illustrated by certain examples.

Each associated mica sheet containing 1-5 wt.% of aromatic polyamide fibril shown in Table 1 was melt-bonded to a polyamide film (KAPTON) (40-60 g/m.sup.2 or 25-45 g/m.sup.2) to produce each mica tape having a thickness of 35-65 .mu.m and a mica content of more than 50 wt.%.

Each coil was prepared by winding each mica tape on a bare conductor at predetermined turns and a main insulating layer was formed on it. A composition of an impregnation epoxy resin (Epon 828 manufactured by Shell Chemical Co.) and an acid anhydride curing agent (HN 2200 manufactured by Hitachi Kasei Kogyo with 0.5 PHR of benzyl dimethylamine) was immersed into each coil and cured to obtain each insulated coil having a bare conductor and turn insulation which had excellent heat resistance and electric characteristics. The characteristics of the resulting insulated coils are shown in Table 1.

An enamel undercoat duplex glass winding sheet (PEW/DGC: manufactured by Dainichi Nippon Densen) was used as a thin sheet and glass fiber was used instead of mica in a preparation of an insulated coil. The characteristics of the resulting insulated coils are shown in Table 1.

A polyester back mica tape (manufactured by Kobishi Denki) was used as film backing associated mica tape at a content of mica shown in Table 1 in the structure of the thin sheet to obtain an insulated coil. The characteristics of the resulting insulated coil are shown in Table 1.

In Table 1, as the dielectric breakdown strength of the coil, the characteristic after treating with the epoxy type resin is shown in the upper part and the characteristic in the non-impregnation is shown in parentheses, (), in the lower part.

Each associated mica sheet containing 1 to 9 wt.% of aromatic polyamide fibril shown in Table 2 was melt-bonded to each aromatic polyamide paper (40-60 g/m.sup.2 or 25-45 g/m.sup.2) to produce each mica tape having a thickness of 35-65 .mu.m and a mica content of more than 50 wt.%.

Each coil was prepared by winding each mica tape on a bare conductor at predetermined turns and a main insulating layer was formed on it.

In accordance with the process of Example 1, each insulated coil was produced and characteristics of the resulting insulated coil were tested. The results are shown in Table 2.

In the examples, 1 turn-half lap wound conductor and 2 turn-half lap wound conductor were employed for the tape winding. Thus, it is possible to employ an edge lap wound conductor or a simplex wound conductor depending upon a thickness of the insulating sheet, a dielectric breakdown strength and a size of a conductor. A number of turns of an insulated conductor can be also selected as desired. In the embodiment, a main insulating layer was formed around the insulated conductor. Thus, the present invention can be also applied for the insulated conductor to which a main insulation is not applied.

In accordance with the present invention, a thin mica tape having high strength obtained by melt-bonding an associated mica sheet containing polymer fibril to a heat resistant polymer sheet or a porous polymer sheet is wound on a conductor thereby providing an insulated coil which has excellent heat resistance and corona discharge resistance, and a high space factor and is economical. Thus, the insulated coil of the present invention has remarkably high value in an industrial field.