Method for Application of a Tape of Insulating Material in the Longitudinal Direction of a Substantially Rectangular Electrical Conductor

1. Field of the Invention

The invention relates to the manufacture of insulated electrical conductors, and more particularly to method for application of insulating tape along the longitudinal direction of substantially rectangular cross-section electrical conductors.

2. Prior Art

U.S. Pat. Nos. 3,723,797, 3,777,198 and 3,775,628 describe rectangular electrical conductors on which tapes of corona-resistant insulating material and/or tape-formed polymer film have been applied by arranging the tape or film along the longitudinal direction of the conductor. The tape is then folded around the edges of the conductor and is fixed to the underlying material by means of a binder which has been applied to the tape prior to its application to the conductor. The insulating tape is folded around the edges of the conductor and fixed to the underlying material in the manner described above.

According to the invention it is possible to efficiently and continuously obtain conductors insulated with insulating tape, in which the tape insulation is practically free from blisters and has a well-defined, predeterminable geometry and location on the conductor.

It is an important feature of the present invention that the conductor is heated before the insulating tape is brought into contact therewith so that the conductor provides the required heating of the binder on the insulating tape. Thereby the binder is heated only on that part of the insulating tape which, at each moment of the application of the insulating tape, is in contact with the conductor. It is a further important feature of the invention that only part of the insulating tape is first fixed to the conductor before the other parts of the tape are fixed thereto. This enables the tape to have a definite location on the conductor and prevents sliding movements of the tape relative to the conductor, which movements are hard to avoid if the parts of the tape are fixed around the conductor simultaneously. The successive fixing of the parts of the tape also makes it possible to fold the tape around the conductor in a substantially blister-free manner. The heating is preferably performed by induction heating since such heating is easily carried out so quickly that no formation of blisters occurs in the layer of a varnish or a polymer film on the conductor. And also no harmful oxide formation occurs if the conductor lacks such a layer. According to one embodiment of the invention an additional heating of the conductor is performed after the insulating tape has been applied on one or more sides of the conductor. In this way the first heating can be limited so that the risk of the previously mentioned damage is avoided to an even greater extent.

As stated above, the insulating tape can be applied directly to the conductor or to a conductor which is already insulated, for example, with a varnish or a polymer film. The varnish can then be of such a type as is normally used for varnishing winding wire such as, for example, terephthalic acid alkyds, polyester imides, polyamide imides, polyimides, polyurethanes, silicones and epoxy resins. The polymer in the polymer film may, among other things, consist of a linear unbranched or branched polymer such as, for example, polyamide imide, polyimide, a polymer based on polyhydantoin, polyethylene glycol terephthalate, polycarbonate and polysulphon. According to the present invention, the polymer film can be applied in the longitudinal direction of the conductor and folded around the edges of the conductor. It is also possible to spin the film around the conductor. At least in the latter case another insulating tape is applied on top of the polymer film in the longitudinal direction of the conductor.

Other examples of insulating tapes which are suitable for application according to the present invention are among other things, besides the polymer film, different kinds of mica tapes, for example a tape built up of a mixture of small mica flakes (<5 mm.sup.2) and short fibers of a linear polymer, such as polyamide, of cellulose or glass, or a tape built up from small flakes of mica only. Also insulating tapes of paper, for example, can be used for insulating of conductors according to the present invention.

The binder, which is applied to the insulating tape before the tape is applied to the conductor, should be dry before coming into contact with the conductor. Examples of suitable binders are flexible polyurethane resins, epoxy resins modified with polyamides, polyester resins modified with isocyanates, epoxy resins modified with polyurethane resin and certain types of rubber.

The invention is described in greater detail by way of exemplary embodiments with reference to the accompanying Figures wherein:

FIG. 1 shows a diagrammatic side view of apparatus for carrying out the method according to the present invention;

FIGS. 2 to 6 are cross-sectional views of apparatus for sequentially applying tape to a conductor with the sectional views being taken along lines 2--2, 3--3, 4--4, 5--5 and 6--6, respectively, at the indicated stages of the apparatus of FIG. 1 in accordance with one embodiment of the invention; and

FIGS. 7, 8 and 9 illustrate alternative methods of applying insulating tape to an electrical conductor according to the present invention.

With reference to FIG. 1, a rectangular copper conductor 10, for example having a width of 8 mm and a thickness of 3 mm, which may be uninsulated or provided with an insulation such as in the form of a varnish layer of, for example, terephthalic acid alkyd or in the form of a film of, for example, polyamid imide, is conveyed from a storage roller 11 through an induction coil 12, where it is heated to a temperature of around 300.degree. C. An insulating tape 13, for example having a width of 20 mm and a thickness of 75 microns, and made from a mixture of equal parts by weight of small mica flakes and short fibers of an aromatic amide (e.g. NOMEX M from Du Pont, U.S.A.). On one side the tape 13 is coated with a thin layer of a binder consisting of, for example, a polyamide-modified epoxy resin (such as "AF-42" from Minnesota Mining and Manufacturing Company, U.S.A.). Tape 13 is conveyed towards the conductor from storage roller 14 with the binder side of the tape facing the conductor.

With reference to FIG. 2, the middle portion 13a of insulating tape 13 is brought into contact with a first conductor side 10a and is fixed to that conductor side in a device comprising a plane table 15 and rollers 16 which are held pressed against the upper side of the conductor by means not shown in the Figures. The pressure is effected, as is the case with the rollers described, infra, by spring forces acting on the shafts of the rollers. The rollers may advantageously be made of a plastic such as polytetrafluoro ethylene, possibly surrounded by a shell of metal. Securing of the middle portion 13a of tape 13 is obtained because the binder, which is dry from the beginning, melts at the middle tape portion 13a by the heat from the conductor, and then hardens. With continuing reference to FIG. 2 and additional reference to FIG. 3, each of tape portions 13b and 13c, extending on respective sides of conductor 10, is then folded around the edge of the conductor which is located between first conductor side 10a and adjacent conductor sides 10b and 10c, respectively, with a number of rollers 17, 17' (which are located on one side of the conductor and are shown in FIG. 1), and with rollers 18, 18' as described, infra. Rollers 17, 17' each have a cylindrical portion 17a located beneath middle portion 13a of tape 13, and a conical surface 17b for folding tape portion 13b. In the first of rollers 17 engaged by tape 13 and conductor 10, conical surface 17b forms a greater angle with side 10b than the corresponding conical surface of the other subsequently engaged roller 17'. On the other side of tape 13 and conductor 10 there are a pair of rollers (not shown) corresponding to rollers 17, 17' for folding part 13c of the tape, which rollers are displaced in position in the direction of movement of the conductor in relation to rollers 17, 17'. Also with regard to rollers 18, 18', 20, 20' and 21, 21' described, infra, only the rollers located on one side of the conductor are shown in FIG. 1. There are also rollers, corresponding to those shown, on the other side of the conductor and such rollers perform the same tasks as the shown rollers regarding the parts of insulating tape 13 which are located on the other side of conductor 10. The rollers on the other side of the apparatus have not been shown to avoid cluttering the drawing. Moreover, as such rollers perform identical functions to the shown rollers, there is no need to provide a description of their operation for purposes of practicing the invention.

With additional reference to FIG. 4, tape portions 13 bm and 13 cm, respectively, of tape portions 13b and 13c, respectively, located nearest the middle portion, are then fixed to the conductor by a number of rollers 18, 18' which also contribute to the folding of the tape portions 13bm and 13cm. Rollers 18, 18' each have a cylindrical part 18a supporting the middle portion 13a of tape 13 and conductor 10, and a part 18b with a surface parallel to the conductor and pressing against it as shown in FIG. 4. The fixation of tape portion 13bm to conductor 10 is achieved by the binder on part 13bm being heated by conductor 10. Rollers (not shown) corresponding to rollers 18, 18' on the other side of conductor 10, fold and fix part 13cm in a corresponding way. After passing through rollers 18, 18', conductor 10 and tape 13 pass through an induction coil 19 for additional heating of the conductor.

As shown in FIG. 5, each of tape portions 13d and 13e extending above conductor slides 10b and 10c, respectively, is thereafter folded around a respective edge of conductor 10 which is located between a side 10b and 10c onto conductor side 10d, opposite to the side 10a, by a number of rollers 20, 20' and 21, 21' as described, infra. Rollers 20, 20' each have a cylindrical part 20a engaging tape portion 13b on conductor side 10b and an upstanding part 20b, with a conical surface 20c confronting tape portion 13d in contacting relation therewith as shown in FIG. 5. In rollers 20, conical surface 20c forms a greater angle with conductor side 10d than the corresponding conical surface of rollers 20'. Rollers (not shown) on the other side of conductor 10 fold part 13e of the tape in a corresponding way and are staggered along the conductor with respect to rollers 20, 20'. Tape portions 13d and 13e are then additionally folded and fixed to conductor side 10d with rollers 21, 21'. Rollers 21, 21' each have a cylindrical part 21a in contacting relationship to tape portion 13b on conductor side 10b and a part 21b with a surface parallel to and confronting conductor side 10d to press tape portion 13d against conductor side 10d as shown in FIG. 6. The fixation of tape 13 is effected by the binder on tape portion 13d being heated by conductor 10. Rollers (not shown) on the other side of conductor 10 fold and fix part 13e in a corresponding way, and are staggered along the conductor with respect to rollers 21, 21'. A number of supports 29 are arranged to support conductor 10 in front of and between rollers 20 and 21. Supports 29 bear against the downwardly-directed pressure on conductor 10 exercised by rollers 20, 20' and 21, 21'. The conductor with the applied tape finally passes over plane table 22 and beneath a number of rollers 23 of the same type as the rollers 16 before it is wound up on wind-up device 24. It is also possible, of course, after fixing middle tape portion 13a, to first fold and fix parts 13b and 13d before parts 13c and 13e are folded and fixed. In such a case, all rollers on the other side of conductor 10 and corresponding to rollers 18, 18', 20, 20' and 21, 21' are positioned after rollers 21, 21' in the direction of transport of conductor 10 in the apparatus shown in FIG. 1.

FIG. 7 shows application of tape 25, which is narrower than tape 13, onto conductor 10 utilizing the invention. In this case, tape portion 25a is first fixed to conductor side 10a and then tape portions 25b and 25c are folded and fixed to conductor sides 10b and 10c. It is, of course, possible to apply a similar tape on conductor side 10d and the upper parts of conductor sides 10b and 10c.

FIG. 8 shows another way of applying two tapes 26 and 27, respectively, to conductor 10 in accordance with the invention. With regard to tape 26, tape portion 26a is first fixed to conductor side 10a and thereafter tape portions 26c and 26e are fixed in turn to conductor sides 10c and 10d. The tape 27 is fixed to conductor 10 simultaneously with, or after, tape 26 with tape portions 27a, 27b and 27d fixed in turn to conductor sides 10a, 10b and 10d, respectively.

FIG. 9 shows the application of a broader tape 28 than tape 13 onto conductor 10 utilizing the invention. In this case, tape portions 28a, 28bm, 28cm, and 28d can be applied in the manner previously described with respect to FIGS. 1 to 6. Tape portion 28e is applied after tape portion 28d, and its fixation to conductor 10 takes place by heat from the conductor upon contact therewith and with tape portion 28d, respectively, at the overlapping tape portion 28el.

The apparatus for moving the conductor through the tape applying apparatus does not form any part of the invention per se, and those skilled in the art will recognize that any well-known conveying techniques can be employed to transport the conductor and tape through the tape applying apparatus. In the embodiments described above, the tape and conductor are moved through the tape applying apparatus by rotation of the wind-up device 24.