Anode, Drocess for anodizing, anodized wire and electric device comprising such anodized wire TECHNICAL FIELD The invention relates to an anode for batch anodizing of a length of metallic conductor wire. More specifically, the invention relates to an anode for generating an electrically insulating oxide layer on the surface of a wire length of electric conductor wire of metal by anodizing, wherein the anode is immersed into an electrolyte. The invention also relates to a process for batch anodizing of a wire length of metallic conductor wire, in which such an anode is utilized, an insulated conductor wire with an electrically insulating surface layer generated by batch anodizing according to the invented anodizing process, and an electric device compri- sino one or more such insulated conductor wires which have been provided with an electrically insulating surface layer by batch anodizing.

BACKGROUND ART An electric device such as a cable, a transformer, a gene- rator, a motor, etc., comprises at least one current- carrying and voltage-carrying metallic body, hereinafter in this application referred to as a conductor. Certain high- voltage electric devices include conductors which comprise a large number of conductor wires of a more slender dimension, referred to in this application as strands. The strands are composed, for example laid, into parts of strands, windings or coils in generators and other electric machines. The voltage difference between the laid strands is low but the laid strands are still often insulated relative to one another.

According to the prior art, single-wire conductor wires which, as strands, are laid into coarser conductors, included in high-voltage devices, are insulated preferably

by organic insulating materials, such as thermoplastic resins, waxes or varnishes. However, organic insulating materials have a poor capacity to withstand influence in severe environments. For example, they have a low tempera- ture resistance and they must often be applied in relatively thick layers. The organic insulations also give rise to drawbacks and costs when recycling/recovering consumed or rejected conductor material since they have to be taken care of or destructed separately from the metallic conductor wire. For use in severe environments such as at high temperatures, under vacuum, in chemically aggressive environments or in environments involving a high fire hazard, it is known to use inorganic insulating materials such as insulating materials based on glass fibre or mica, which are usually wound onto the conductor wire. Further, it is known to insulate aluminium wire or aluminium-coated copper wire with a surface layer of aluminium oxide by ano- dic oxidation, anodizing. To obtain an oxide layer with a suitable structure and adhesion for the use described above, a treatment cycle is required which cannot be realized by means of a continuous electrochemical process but requires a batch process. A problem in this connection is to arrange a wire of a wire length sufficient for a coil in an electric high-voltage device. The wire length in a coil usually amounts to between a few two hundred metres and a few kilometres. The wire must in its entirety be connected to the positive pole of a current source in such a way that the potential difference is minimized over the long wire length.

The wire, which preferably has a diameter of 1 to 5 mm, must usually be supported and kept in such a way that essentially the whole surface may be anodized.

It is an object of the invention to suggest an anode which comprises a holder suitable for anodizing, in one batch, a conductor wire with a wire length of 100 metres to 10 kilo- metres.

It is also an object of the invention to suggest an anodi- zing process for batch anodizing of a conductor wire, an anodized conductor wire manufactured by means of this process, as well as an electric device comprising one or more such anodized wires.

SUMMARY OF THE INVENTION The above is achieved by means of an anode which is adapted for batch anodizing of a wire length, comprises the wire to be anodized and a holder for supporting and holding the conductor wire, during the anodizing, in such a way that essentially the whole surface of the conductor wire can be anodized, whereby, according to the invention, the wire is spirally wound onto the holder and the holder comprises at least three guide cams with guide slots in which the spirally wound wire is fixed. Preferably, the guide slots are formed such that the contact surfaces between the holder and the wire are minimized essentially into point contacts.

The guide slots of the guide cams hold the spiralled conductor wire such that all the turns are separated by a space. By this space between each turn and by the contact surfaces between holder and wire having been minimized, it is ensured that essentially the whole envelope surface of the conductor wire is anodized and that a homogeneous oxide layer is generated. The wire and the holder are immersed into an electrolyte during the anodizing, and the wire is connected at both its ends to the positive pole of a current source. The wire comprises, at least in an outer layer, aluminium or copper which during the anodizing is oxidized into an oxide layer with good adhesion, a suitable structure and a thickness of less than 15 pn, preferably less than 10 Am. This ensures the workability without the oxide layer being damaged or flaked and a sufficient electrical insula- tion for the limited potential difference between two strands included in the same winding or coil in a high- voltage electric machine. In certain embodiments, the wire is a solid wire of some of these metals or of an alloy based

on aluminium or copper, and in other embodiments a wire with a core of a first metal or alloy coated with a layer com- prising aluminium or copper; the core may, of course, com- prise aluminium or copper. That is to say, a core of alu- minium may be coated with a copper layer, a core of copper with an aluminium layer or a core of copper or aluminium be coated with an alloy comprising copper and aluminium, res- pectively, where the content of the coating differs from the content of the core. Preferably, during anodizing of a batch of metal wire, a conductor wire with a wire length of 100 metres to 10 kilometres is wound onto the holder. The inven- tion has proved to be exceedingly suitable for anodizing conductor wire with a diameter of 0.1 to 6 mm, preferably 1 to 5 mm.

The guide cams are preferably made of an electrically con- ducting material such as aluminium, copper or titanium.

Under certain conditions, it is also suitable to connect both holder and wire ends to the positive pole of the current source.

In a preferred embodiment, the holder comprises a central shaft and three bars, oriented parallel to the shaft and fixed to the central shaft by means of radially oriented arms. The bars are arranged with guide slots or with guide cams fixed to the bars.

An anode as described above is preferably suitable for use during a batch anodizing where the electrochemical, thermal or other parameters are varied during the process cycle, or during a batch anodizing which goes on for such a long time that a process where the wire is continuously drawn through the anodizing bath is not suitable.

Depending on the electrochemical conditions prevailing in the electrolytic bath, either the whole holder is made of an electrically conducting material and connected as an anode to the positive pole of the current source, or only the

guide cams fixed to the bars are made of an electrically conducting material and connected as an anode to the posi- tive pole of the current source. In the latter case, the guide cams are electrically insulated from the supporting parts of the holder. The electrolyte may contain other metal ions or anions, which during the oxidation are included in the oxide layer and act in a stabilizing way thereon or improve the adhesion to the underlying metal or in certain cases change the surface properties of the oxidized conduc- tor wire, which surface properties facilitate water rejec- tion, the application of polymer-based surface layers out- side oxide layers, etc. The contents of such additives to the electrolyte may be controlled during the anodizing.

Alternatively, other electrochemical process parameters, which influence the inclusion of such compounds in the oxide layer, may be controlled.

Through the anode and the batch anodizing, described above, it is possible to anodize conductor wires of copper or aluminium or conductor wires coated with any of these metals, such as aluminium-coated copper wires with an electrically insulating oxide layer, which exhibits an adhesion and structure such that they may be treated and formed into coils or windings in a high-voltage electric machine. The adhesion and the structure ensure that the electrical insulation may be achieved with a thin oxide layer, which makes them suitable to use as strands in coils and windings for electric high-voltage devices. It is particularly advantageous to use the anode and the anodizing process mentioned above for anodizing strands which are coordinated into an electric conductor which is wholly or partially surrounded by a polymer-based insulation and arranged to be included in a winding of a high-voltage elec- tric machine.

A conductor wire according to the invention, as well as a high-voltage electric machine comprising such a conductor wire, has an improved poor capacity of resisting the

influence of severe environments. For example, they have a high temperature resistance and may be applied in relatively thin layers. Nor do they give rise to any drawbacks or costs during recycling/recovery of consumed or rejected conductor material since the insulation may be taken care of and destructed together with the metallic conductor wire.

In the following, the invention will be explained in greater detail and be exemplified by means of a preferred embodiment with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS The anode in the figure comprises a holder in the form of a central shaft 1 and three bars 2, 2a, 2b, 2c, arranged parallel to the shaft and fixed to the central shaft by means of radially oriented arms 3a, 3b, 3c. On the bars 2, 2a, 2b, 2c, guide cams 4, 4a, 4b, 4c are fixed. The guide cams 4, 4a, 4b, 4c are of aluminium and are shown in more detail in Figure 2. The guide cams 4, 4a, 4b, 4c may be electrically insulated from the supporting parts 2, 2a, 2b, 2c, 3a, 3b, 3c of the holder. The wire 5 which is to be anodized is spirally wound onto the holder and each turn is fixed in the guide slots of the guide cams 4a, 4b, 4c. At both ends the wire 5 is electrically connected to the positive pole of a current source (not shown). During anodizing, the whole holder with the wound-on wire 5 is immersed into an electrolyte, an electrolytic bath. In the electrolytic bath, the electrochemical, thermal or other parameters are controlled during the process cycle. In certain cases, the process cycle during the batch anodizing takes such a long time that, even if the process parameters are kept essentially constant during the whole anodizing, a process where the wire is continuously drawn through the anodizing bath is not suitable. Depending on the electro- chemical conditions prevailing in the electrolytic bath, either the whole holder is made of an electrically conduc- ting material and connected as an anode to the positive pole

of the current source, or only the guide cams 4a, 4b, 4c, fixed to the bars, are made of an electrically conducting material and connected as an anode to the positive pole of the current source. In the latter case, the guide cams 4a, 4b, 4c are electrically insulated from the supporting parts of the holder. The electrolyte may contain other metal ions or anions, which, during the oxidation, are included in the oxide layer and act in a stabilizing manner thereon, or improve the adhesion to the underlying metal or, in certain cases, change the surface properties of the oxidized con- ductor wire, which properties facilitate water rejection, the application of polymer-based surface layers outside oxide layers, etc. The contents of such additives to the electrolyte may be controlled during the anodizing. Alter- natively, other electrochemical process parameters, which influence the inclusion of such compounds in the oxide layer, may be controlled. By means of an anode according to Figures 1 and 2, the batch anodizing may be carried out and a thin oxide layer, less than 10 Fm, with good adhesion to the underlying metal and a suitable structure be generated on conductor wires of copper or aluminium or conductor wires coated with any of these metals such as a aluminium-coated copper wires with an electrically insulating oxide layer.

Since the oxide layer exhibits a good adhesion and a suitable structure, the conductor wires may be treated and formed into coils or windings in a high-voltage machine without the electrical insulating capacity being reduced by cracks or flaking. This fact, and the small thickness of the oxide layer, contribute to the conductor wires according to the present invention being very suitable to use as strands in coils and windings for electric high-voltage devices. It is especially advantageous to use the anode and the anodi- zing process, described in the foregoing, for anodizing strands which are coordinated into an electric conductor which is wholly or partially surrounded by a polymer-based insulation and is adapted to form part of a winding of a high-voltage electric machine. A conductor wire according to the invention, as well as a high-voltage electric machine comprising such a conductor wire, have an improved poor ability to resist the influence of severe environments. For example, they have a high temperature resistance and may be applied in relatively thin layers. Nor does this give rise to any drawbacks or costs during recycling/recovery of consumed or rejected conductor material since the insulation may be taken care of and be destructed together with the metallic conductor wire