A METHOD OF MAKING AN ELECTROMAGNETICAL APPARATUS

This invention relates to a method of making electromagnetic apparatus.

In particular the invention relates to a method of making an electromagnetic apparatus, including the steps of winding at least one electrical conductor into the form of a coil and shaping the conductor into leads at the coil ends, the conductor used for winding having at least one stack of substantially oblong cross-section insulated conductor strands and uncured resin within an outer insulation, stabilising the coil by a heat treatment which also cures the resin to bond together the insulated conductor strands therein, and then. either before or after assembling the stabilised coil with a magnetic flux carrier,further shaping the leads and separating the conductor strands at the end of each lead for electrical connection to a termination of the apparatus.

It is known that the coils of electromagnetic apparatus, for example transformers, exist within a leakage magnetic field created by current in the coils, that this magnetic field will induce eddy currents and stray losses in the conductors of the coils, and that to reduce this the conductors may be stranded. The insulated conductor strands may be continuously transposed along the or each electrical conductor, both to even out the lengths of the strands in the coil which pass through a magnetic field in the apparatus and to reduce current loss due to circulating currents. Coils can be subject to mechanical forces due to short circuit current. Stranded conductors may be strengthened, to reduce the possibility of distortion due to such mechanical forces, by bonding together the strands. An example of a coil having such a transposed stranded and bonded conductor is disclosed in GB-A-1463510.

A method of making an electromagnetic apparatus as specified in the penultimate paragraph is known in which uncured resin in the conductor used for winding the coil is provided as powdered epoxy resin individually coating each of the insulated conductor strands. However this has significant practical drawbacks in relation to the further shaping of the leads and the separating of the conductor strands which is required after the coil has been stabilised. It is desired that the coil should be rigid by virtue of the cured resin after the stabilising heat treatment, but if resin provided in this way is left in the leads and also cured therein by this heat treatment then the leads will be made rigid and incapable in this condition of being further shaped. Thus the cured resin has to be removed individually from each conductor strand in each lead while retaining the insulation on each strand. This removal has to be effected with cleaning chemicals or mechanical abrasion. For example on a large transformer winding having several stranded conductors, this removal of cured resin may require several days of manual work. The cleaned insulated conductor strands then have to be re-assembled to form the leads which are flexible and may be further shaped as required. The cleaned strands at the end of each lead may also then be separated, and have their insulation removed, for electrical connection to a termination by, for example, brazing. This process of dealing with leads made rigid by cured resin is particularly difficult if they are in the form of conductors with continuously transposed strands. An alternative is to disassemble each lead for removal of the uncured powdered epoxy resin before the coil stabilising heat treatment, but this is also time consuming and uncertain since some resin may be left in and cured and then have to be afterwards removed. GB-A-1463510 mentioned above has no disclosure or teaching concerning further shaping of the coil leads.

An object of the present invention is substantially to overcome the drawbacks just described.

According to the present invention there is provided a method of making an electromagnetic apparatus as defined in the second paragraph of this specification characterised in that the uncured resin is provided as impregnation in one or more carrier strips which closely contact the insulated conductor strands so that the coil stabilising heat treatment allows the resin to flow from the carrier strip (or strips) to the insulated conductor strands to bond them together, and in that before said heat treatment said carrier strip (or strips) is (or are) removed from a section of each lead where it is intended afterwards to effect said further shaping of that lead and said separating of the conductor strands, so that the conductor strands in those lead sections are not bonded together by said heat treatment thereby facilitating said further shaping and separating.

The basic idea of the invention is to provide uncured resin only as impregnation in one or more carrier strips. In this way, by selectively removing the impregnated carrier strip (or strips), the uncured resin may be easily and accurately completely removed from the coil leads where and only where required before the coil stabilising heat treatmnt. A stranded conductor for use in the method of the invention should also be more easily and cheaply provided with uncured resin than the above mentioned known conductor in which the individual strands are individually coated with powdered epoxy resin.

The carrier strips impregnated with uncured resin may be provided as individual coverings for each insulated conductor strand. Another possibility is that the carrier strip (or strips) impregnated with uncured resin may be provided as a covering around the stack of insulated conductor strands, or as coverings individually around each said stack, or as a single covering around all said stacks.

In a form of the method of the invention which is particularly useful where the conductor has a large number of strands, the conductor used for winding may have at least two said stacks of insulated conductor strands with a separator strip between the or each pair of stacks, the carrier strip (or strips) impregnated with uncured resin may comprise said separator strip(s), and, after the or each resin impregnated separator strip has been removed from said lead sections and before said heat treatment, the or each removed resin impregnated separator strip may be replaced along at least part of said lead sections by a separator strip which is not resin impregnated. The insulated conductor strands may be continuously transposed along the or each electrical conductor. In this form of the method of the invention using a resin impregnated separator strip (or strips) the conductor strands are not surrounded by cured resin after the coil stabilising heat treatment but are bonded together by the cured resin which has flowed to touch them along only one edge (or perhaps two opposite edges if there are more than two stacks) of their oblong cross-section. However the rigidity of the conductor in the coil achieved by this amount of bonding of the strands should be quite sufficient to withstand the mechanical forces to which it is subjected in operation of the apparatus.

In the method of the invention the insulated conductor strands may have substantially curved convex edges touching the carrier strip(s) impregnated with uncured resin, said heat treatment allowing the resin to flow between adjacently stacked said strands in the regions of those curved edges. This enhances the bonding of the strands compared with the case where they may have only flat edges.

In the method of the invention, the outer insulation of the conductor and/or the one or more carrier strips may comprise paper. In this case, in order to dry the paper without making it brittle, the coil stabilising heat treatment should be at a temperature above 100°C and below 140°C.

Examples of the invention will now be described with reference to the accompanying schematic drawings in which Figures 1 to 4 each show a corresponding cross-section of a conductor embodying the invention.

Referring now to Figure 1, an electrical conductor has two parallel, adjacent stacks of substantially oblong cross-section conductor strands 1. Two opposite edges 10 of each strand 1 are substantially curved convex. Normally the strands 1 will be initially formed with a retangular cross-section of which only the corners are then made radiussed. Four strands 1 are shown in each stack but there may be typically ten or more such strands in each stack. The strands 1 are normally of copper or aluminium and are individually insulated by an insulation film (not shown) which is normally enamel or the like. The strands 1 are within an outer insulation 2 which is normally paper wrapping or the like. A strip 3 is located between the two stacks in close contact with edges 10 of the conductor strands 1.

The strip 3, which is normally paper or the like, serves a double purpose in that it is a separator strip which keeps stable the configuration of the strands 1 within the conductor and in that it is a carrier strip impregnated with uncured resin, normally epoxy resin. The resin will be cured in a heat treatment step of making an electromagnetic apparatus, as described below, so as to bond together the strands 1. The insulated strands 1 are normally continuously transposed along the length of the conductor, in which case there will normally be an odd number of strands 1 in the conductor within the outer insulation 2. The transposition, not seen in Figure 1, may be in a clockwise or anti-clockwise direction as viewed in Figure 1. If clockwise, for example, then along the length of the conductor the strands 1 in the top stack will be continuously displaced from left to right, the right hand strand of the top stack will move down along the right hand side of the stacks to become the right hand strand of the bottom stack, the strands 1 in the bottom stack will be continuously displaced from right to left, and the left hand strand of the bottom stack will move up along the left hand side of the stacks to become the left hand strand of the top stack.

The electrical conductor described above with respect to Figure 1 is used in a method of making an electromagnetic apparatus, for example a transformer, as will be described below.

At least one stranded electrical conductor is wound into the form of a coil and the or each conductor is shaped into leads at the coil ends. The outer insulation 2 is temporarily removed from a section of each lead where it is intended later to further shape that lead and separate the conductor strands at the end of that lead. The resin impregnated strip 3 is then removed from that section of each lead, for example by cutting it away with scissors or the like. The removed resin impregnated strip 3 is then replaced, along at least the part of that lead section where it is intended later to further shape the lead, by a strip which is similar but not resin impregnated. The outer insulation 2 is then replaced on that part of each lead section.

The coil then undergoes a stabilising heat treatment, that is to say a heat treatment in which moisture is removed from the coil and the coil is shrunk to a required size. Where the insulation 2 and/or the strips 3 comprise paper then, in order to dry the paper without making it brittle, the coil stabilising heat treatment should be at a temperature above 100°C and below 140°C.

The coil stabilising heat treatment also cures the resin in the strip 3. During this curing process the resin is allowed to flow from the strip 3 to the adjacent edges 10 of the insulated conductor strands 1 to bond these strands 1 to the strip 3 and thereby together. Furthermore, the heat treatment may allow the resin to flow between the adjacently stacked strands 1 in the regions of their curved edges. The conductor strands 1 are not bonded together by this coil stabilising heat treatment in those lead sections where the resin impregnated strip 3 has been removed.

The stabilised coil is then assembled with a magnetic flux carrier, for example a metal core where the apparatus is a transformer. The unbonded lead sections are sufficiently flexible to be then further shaped so as to position them where required in respect of terminations of the apparatus. The conductor strands at the end of each lead are separated, more easily because they have not been bonded, stripped of their insulation, and individually electrically connected by, for example, brazing to a said termination of the apparatus. In a large power transformer, for example, one bus bar termination may have individually connected to it up to approximately 200 separated conductor strands derived from 6 or 7 conductor leads.

The further shaping of the leads and separating of the conductor strands may alternatively be a step in preparation of the stabilised coil before assembly with the magnetic flux carrier.

Referring now to Figure 2, an electrical conductor is shown which is similar to that shown in Figure 1 but in which, in addition to the separator strip 3 impregnated with uncured resin, two further strips 4 are provided parallel to the separator strip 3 and between the stacks of strands 1 and the outer insulation 2. The strips 4 are also carrier strips, normally of paper or the like, impregnated with uncured resin. Before the coil stabilising heat treatment the resin impregnated strips 4 are removed from the same lead sections from where the resin impregnated strip 3 is removed. The removed impregnated strips 4 may or may not be replaced in those lead sections by strips without resin impregnation. The coil stabilising heat treatment also cures the resin in the carrier strips 4 all owing the resin to flow from the strips 4 to and between the adjacent edges 11 of the insulated conductor strands to bond the strands 1 to the strips 4 and thereby provide extra bonded strength of the strands 1 in the conductor in the stabilised coil.

Referring now to Figure 3, an electrical conductor is shown which is modified with respect to that shown in Figure 1 primarily in that the carrier strip impregnated with uncured resin is provided as a covering 20 around and in close contact with both stacks of insulated conductor strands 1. The impregnated covering strip 20 will normally be a paper wrapping which is then overwrapped with the usual outer insulation paper wrapping 2. In this case the separator strip 3 may or may not be resin impregnated. Before the coil stabilising heat treatment the resin impregnated covering strip 20 is removed from the lead sections where appropriate and may be replaced by a strip without impregnation. If the separator strip 3 is resin impregnated this will also be removed from those lead sections and replaced by a separator strip which is not resin impregnated. The coil stabilising heat treatment cures the resin in the covering strip 20 allowing resin to flow from the strip 20 to and between the contacting edges 11 of the insulated conductor strands 1 and to the sides of the outer strand in each stack. This should provide sufficient bonded strength of the strands 1 in the stabilised coil. If the separator strip 3 is resin impregnated then extra bonded strength is provided.

Referring now to Figure 4, an electrical conductor is shown which is modified with respect to that shown in Figure 1 in that the separator strip 3 is not resin impregnated and in that carrier strips impregnated with uncured resin are provided as individual coverings 5, normally as paper wrapping, for each insulated conductor strand 1. Before the coil stabilising heat treatment these individual impregnated coverings 5 are removed from the lead sections where appropriate and may be replaced by coverings which are not impregnated with resin. The coil stabilising heat treatment cures the resin in the coverings 5 allowing the resin to flow to the closely contacting sides of the strands 1 thereby bonding the strands together in the stabilised coil.

The electrical conductors described above with respect to Figures 1 to 4 have two stacks of insulated conductor strands 1. These conductors may be modified to have more than two stacks of strands, although in practice there would not usually be more than three stacks. In this case, the conductors of Figures 1, 2 and 3 would require a resin impregnated separator strip 3 between each pair of stacks.

Continuous transposition of the insulated conductor strands 1 is normally provided along the length of the conductor having two or more stacks, but electrical conductors as described above with respect to Figures 1 to 4 may be provided without such transposition of the strands.

The electrical conductors described above with respect to Figures 2 to 4 may be modified to have only one stack of insulated conductor strands 1, simply by not having the separator strip 3. In this case the strands 1 may or may not be continuously transposed.

Electrical conductors and a method of making electromagnetic apparatus have been described above in relation to the electromagnetic apparatus being a transformer, in particular a large power transformer. Such conductors and such method may be used in relation to other types of transformers, or in relation to other electromagnetic apparatus such as electric motors or electric generators.