Specifically the present invention is directed to light-weight, self-cooling compact transgun transformers which are integrally mounted to welding guns, forming the transgun assembly. Such transformers are characterised by windings that are closely coupled together both electrically and thermally, whereby the heat generated in both the primary and secondary windings is carried away by the fluid, flowing through the secondary winding. As they are used mainly in car production lines there is an additional requirement that they have a low external temperature too, which would not harm an operator entering in contact with them.

The primary winding for resistance welding transformers is preferably a set of multi-turn coils, made from rectangular profiled wire. with generally planar side portions. They are wound with each turn of the conductor stacked on top of the previous turn to form a coil with the exposed sides of the conductors tying in two substantially planar portions at each side of the coil.

The secondary winding is formed preferably from a square-shaped hollow member. Each secondary turn is preferably the same size as a primary coil.

Such planar primary and secondary windings are interleaved in order to achieve optimum magnetic coupling, minimum leakage fields and low thermal resistance between the sections of the primary and secondary windings. They are electrically insulated from each other and from the core in a manner known in the art. This winding structure is then positioned around the center leg of a suitable E-shaped core or double C-core to make the transformer.

In one of the well known designs the primary sections is formed from rectangular enamelled wires (Fig. 1). The problem with these wires is that it isn't economically feasible to make broad and thin enamelled rectangular wires, which dictate the number of sections of the primary winding in real designs to be preferably 6 or 8. As a result some sort of insulation between those sections is needed, occupying space which could be used for windings with larger cross section, hence lower resistance.

Other designs are based on uninsulated copper foil, which is wound together with an insulating strip when manufacturing the primary coils. The two most common types of strip coils are the full width foil type winding (Fig. 2) and the narrower spiral type strip coil (Fig. 3). The full width foil type occupies the complete width of the transformer winding and, because of its width, is made of very thin material. At the same time as the adjacent turns must be insulated between themselves a significant amount of insulation space relative to the conductor space is required, i. e. a low space factor results. Another drawback of this design is the poor heat transfer between the distant turns of the primary and the tubular secondary windings.

A set of spiral type strip coils may also be utilised whereby such windings have a strip conductor shape which is narrower and thicker than thin foil material.

In this case the thickness of the copper strip becomes an order of magnitude larger than that of the insulating strip, while the number of insulations between the primary sections is reduced to 1 or 0 (Fig. 3 and Fig. 7a). In modern transformers the thickness of the flat conductor is between 0,5 and 2 mm, while the insulating strip is around 0,1 mm. This achieves a much better space factor than the above mentioned designs while maintaining good thermal coupling between the primary and secondary windings.

An important drawback of this design, however, consists in the difficulty to guarantee the necessary insulation between adjacent turns under all circumstances. Because of this the flat conductor strip is wound together with an insulation strip extending beyond both edges of the conductive strip ("i"on Fig. 4).

As a result the effect of reduced number of insulations is cancelled, as the thickness"e"of the insulation between a coil and its adjacent secondary, as well as the thickness"2e"of the insulation between adjacent primary coils are disproportionally large : typically 1-3 mm. An additional effect of this thick insulation is the reduced thermal conductivity between the primary and secondary windings, leading to higher temperatures and larger copper losses in the transformer.

Another drawback of this design is that the different widths of the copper and insulating strips don't allow manufacture of well ordered coils (i. e. with successive turns stacked exactly on top of each other), because the winding jig has to be wide enough to accommodate the wider insulating strip. This allows the copper strip to deviate to some extent ("t"on Fig. 4) from its ideal position in the middle of the insulating strip. The result is that the effective width of the coil becomes larger than the width of the copper strip itself, hence a thicker insulation and worse thermal coupling between the primary and secondary windings.

While striving to improve the thermal coupling between the primary and secondary sections to achieve low external temperature it is advantageous to have the worse possible thermal conductivity between the primary coils-having the highest temperature as a result of the indirect cooling through the secondary- and the magnetic core. This would reduce the external temperature of the transformers, making their handling more comfortable.

For various reasons the welding transformers are usually encapsulated with epoxy resin, and the heat transfer between neighbouring parts depends primarily on the thickness of the material-i. e. epoxy resin-between them. The current art knows only one way to lower the heat transfer : leaving more space for epoxy or other insulating materials where necessary, thus arriving again at a poor space factor.

Therefore, it is desirable, and it is the object of this invention, to create a winding structure having a suitably shaped strip type conductor, maximising the space factor and allowing areas with selectively increased or decreased heat transfer between the primary winding and the surrounding members.

The objects of the invention are accomplished by winding the primary coils from a flat conductor having an insulating strip and a suitably shaped conductive strip, both strips with essentially equal width, these primary coils being then tightly assembled with the secondary winding and appropriate insulation into a winding structure, and positioned in inductive relationship with a magnetic core to form a transformer. Before assembly the primary coils, which are in contact with the magnetic core, are hermetically sealed with suitable rubber (e. g. butyl rubber) to prevent epoxy resin entering the spaces in contact with the surrounding core and protected from outside with an appropriate insulation (e. g. polyimid foil, polyfol etc.), preventing the epoxy resin from entering this space when sealing the transformer. To allow the epoxy resin to enter the tight space between the planar sides of the primary coils and the adjacent secondary turns on the surface of the latter suitable channels are engraved, through which the epoxy can penetrate the space between the insulation and the secondary winding.

The objects of the invention and the advantages which may be attained by its use will become more apparent upon reading the following detailed description of the invention taken in conjunction with the drawings, wherein the reference numbers identify corresponding parts and areas.

Fig. 1 shows the standard cross section of enamelled rectangular wires.

Fig. 2 is a section of one possible design of the winding structure with full width foil primary winding.

Fig. 3 shows one leg of the winding structure when 4 primary coils are interleaved with 2 secondary turns.

Fig. 4 shows a primary coil according to the current art.

Fig. 5 shows symmetric and asymmetric shapes of the flat conductor according to the present invention.

Fig. 6 shows primary coils made from flat conductors according to the present invention.

Fig. 7 is a section of one leg of the complete winding structure when 3 primary coils are interleaved with 2 secondary turns. It shows the channels on the secondary winding and the sealing of the outside surfaces of the primary coils.

According to the principles of the present invention primary coils are wound from a flat conductor having an insulating strip 1 on one side and a flat copper strip 2, whose narrow edges are trapezoidally shaped (Fig. 6). Both the insulating and the conductive strips have essentially equal width. Depending on the quality of the winding jig the successive coil turns will lie exactly on top of each other (Fig. 6a) or have some deviation (Fig. 6b). In either case the insulation between adjacent turns is guaranteed due to the air gap between the edges, even in cases where the insulating strip is somehow misplaced. As the welding transformers are for primary voltages between 220 and 600 V, the voltage between adjacent turns is less than 10 V, so even if there is no additional insulation between them-e. g. epoxy resin-an air gap of 0,1 mm is sufficient guarantee for the proper function of the transformer. At the same time the flat side of the trapezoid has a surface equalling between 60 and 90% of the flat conductor thickness.

When the primary coils are assembled together with the secondary winding (Fig. 7) the absence of protruding insulating strips allows the planar side surfaces of the primary turns 2 to be in almost immediate thermal contact with the adjacent surface of the secondary 5 (Fig. 7b), thus maximising the thermal conductivity between them. In fact the only thermal barrier between the primary coil and the secondary turn is the inevitable electric insulation 9, which is typically around 0,15 mm, or 5 to 20 times less than in existing designs. In order to preserve the insulation 9 from mechanical damage during assembly and operation it is preferable that the edges of the trapezoids are rounded. In reality the distance between the primary coil and the surface of the secondary should be maintained somehow larger to allow the epoxy resin penetrate these volumes 13, replacing the air.

Further improvement in this respect can be achieved if small channels 14 are engraved on the surface of the secondary 5, through which the epoxy can flow into said tight volumes (Fig. 7a & b).

It may be desirable to keep the air in selected zones though, as is the case with the external surface of the winding structure 12, where it is in close contact with the surrounding magnetic core (Fig. 7a & c). This can easily be achieved if the corresponding coil surface is covered with a suitable insulating material-e. g. polyimide foil-which is then glued to the coil on its edges with appropriate sealant 11. The air remaining in such volumes of the transformer acts as a thermal insulator, decreasing the heat transfer between the hot primary coil and the magnetic core. This results in lower temperatures at the surface of the welding transformer, which is a very desirable feature. To further improve this effect a more pronounced trapezoidal form 4 on this side of the flat conductor can be formed, thus minimising the direct thermally conductive contact to the neighbouring members by replacing it with more air. The narrowness of the trapezoid's crest is limited only by mechanical considerations : a too edge-shaped crest can bend or even detach during winding and short-circuit primary turns. A 0, 2-0,4 mm flat (in reality slightly rounded) crest (Fig. 5b) is considered to be a reasonable trade-off between mechanical strength and poor thermal conductivity.

As mentioned above the air gap between successive turns is a sufficient insulation at the envisaged voltages, but to prevent small loose metal particles. which are almost inevitable during actual manufacturing, from migration within the space 12 possibly short-circuiting adjacent primary turns, it is preferred that this surface of the primary coil be fixed with some sort of insulating enamel.

The invention so disclosed yields several positive effects, namely : a) A better space factor of the winding structure, allowing : 1. More copper in the same area (and more power within the same volume), 2. Reduced core window in exchange for more magnetic core, achieving lower strength of the magnetic field and lower magnetising current within the same overall dimensions, or 3. Reduced overall dimensions of the transformer while preserving its power. b) Significantly improved thermal conductivity between the windings, thus reducing their temperatures and copper losses. c) Improved thermal insulation between the hottest primary windings and the magnetic core, thus achieving lower temperatures at the transformer surface.

All this allows substantially higher production rate, due to the lower secondary impedance, lower temperatures of the windings (and losses therein) and better thermal KVA rating of the transgun transformer.

The foregoing is a complete description of a preferred embodiment of the present invention. Various changes and modifications may be made without departing from its spirit and scope as it is intended that all of the matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative rather than limiting.