Abstract:
A conductor bar for a rotating electrical machine has an inner conductor with an essentially rectangular cross section sheathed in a straight section of the conductor bar by mica tape insulation.

Description:
[0001]    Priority is claimed to German Patent No. DE 10 2008 000 073.6, filed on Jan. 17, 2008, the entire disclosure of which is incorporated by reference herein. 
         [0002]    The present invention relates generally to the field of rotating electrical machines, and in particular to a conductor bar for a rotating electrical machine. 
       BACKGROUND  
       [0003]    In large rotating electrical machines, windings which are composed of individual conductor bars are used in the area of the stator and/or rotor. The conductor bars, which normally have a rectangular cross section, are inserted with a straight part into corresponding axial slots in the stator body or rotor body, and are connected to one another in an end winding at the ends which project out of the body (EP-A2-1 653 588). By way of example, the conductor bars are designed as illustrated in  FIG. 1 : the conductor bar  10  has an electrically conductive inner conductor  11  composed of copper, which generally comprises a multiplicity of conductor elements  12 . The inner conductor  11  is surrounded by insulation  14  whose first and innermost section is in the form of mica insulation  13 , in order to avoid discharges. The longitudinal edges  25  of the inner conductor play a major role in the breakdown resistance, and are characterized by an edge radius r i . 
         [0004]    The glass/mica insulation  13  as shown in  FIG. 2  is wound in the form of mica tapes  15  in a plurality of layers in a spiral onto the inner conductor  11  with an overlap OL of about 50% (by hand or by machine). The maximum tape width (B in  FIG. 6 ) is governed by the sharpest edge radius. Tape widths of 15 to 30 mm are normally used. 
         [0005]    Investigations on conductor bars which have broken down in long-term electrical tests show, in the majority of cases, that
       1. the breakdown occurs in the slot part (that is to say the straight part of the generator bar);   2. the breakdown occurs on the longitudinal edge ( 25  in  FIG. 1 );   3. the breakdown occurs in the first mica tape layer (that is to say directly on the inner conductor  11 ) on the edge of the tape ( 16  in  FIG. 3 ).       
 
         [0009]    Re point 2:the occurrence on the longitudinal edge  25  can be understood because this is where there is a considerable peak in the field strength; the field strength precisely above the copper is approximately: 
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         [0010]    In this case, U is the applied voltage, r i  is the radius of the inner conductor  11  and the thickness of the overall insulation  14 . The “natural” edge radius is given by the radius of the conductor elements  12  from which the inner conductor  11  is formed (this radius is normally 0.5 mm). 
         [0011]    The field peak effect can be ameliorated by rounding (for example by grinding) the inner conductor  11 . However, it is virtually impossible to achieve more than r i =2.5 mm since greater rounding cannot be achieved without grinding onto the conductor elements  12  (and thus constricting the current path). 
         [0012]    Re Point 3: The edge  16  of the first mica tape layer  15  represents a dielectric discontinuity (see the areas surrounded by dashed lines in  FIG. 3 ). This edge area is either filled with pure resin (with a different permeability to that of the mica tape  15 ) or is not filled at all (that is to say it is a cavity). As shown in  FIG. 4 , two mica tapes  17  and  18  are normally wound with a ¼ tape width offset and a half overlap OL, thus resulting in a discontinuity such as this every 6.25 mm, in the case of a tape width of 25 mm. 
       SUMMARY OF THE INVENTION  
       [0013]    An aspect of the invention is to provide a conductor bar in which the breakdown  probability at the edge is reduced. 
         [0014]    At least one first mica tape is wound around the inner conductor and adjacent parallel edges of the first mica tape, which is wound around the inner conductor, have an overlap which is substantially less than half the width of the first mica tape. The dielectric discontinuity at the edge of the conductor bar or of the inner conductor is reduced or overcome in that tape edges occur considerably less often in the first (or first and second) mica tape layer than in known conductor bars. 
         [0015]    One refinement of the invention is characterized in that the first mica tape is wound in a helical shape around the longitudinal axis of the conductor bar. The width of the first mica tape is in this case preferably greater than 50 mm and, in particular, is about 100 mm. 
         [0016]    Another refinement of the invention is distinguished in that the first mica tape is wound around the conductor bar with its longitudinal axis parallel to the longitudinal axis of the conductor bar. In this case, preferably, the first mica tape is wound with its longitudinal axis around the conductor bar such that the edges of the first mica tape lie in the side surfaces of the conductor bar. 
         [0017]    In this case, it is particularly advantageous if the width of the first mica tape is essentially the same as the external circumference of the inner conductor, as a result of which the parallel edges of the first mica tape form a butt joint running in the longitudinal direction. 
         [0018]    A further refinement of the invention is characterized in that the first mica tape is wound with a minute overlap such that the adjacent parallel edges of the first mica tape form a butt joint. 
         [0019]    In addition, the first mica tape on the lower face can be designed to be electrically poorly conductive. In particular, this could be achieved in that the first mica tape has a mica layer which is coated with graphite or carbon black on the lower face. 
         [0020]    However, it is also feasible that the first mica tape has a mica layer which is applied to an electrically conductive web layer, wherein the electrically conductive web layer may comprise, in particular, a carbon-fiber web or an electrically conductive polyester non-woven. 
         [0021]    In addition, a glass-fiber web can be applied to the mica layer. 
         [0022]    If the first mica tape is applied with the longitudinal axis parallel to the longitudinal axis of the conductor bar, it is advantageous for it to be fixed on the inner conductor by an adhesive substance. In this case, the adhesive substance is, in particular, an impregnation resin or a silicone elastomer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0023]    The invention will be explained in more detail in the following text with reference to exemplary embodiments and in conjunction with the drawings, in which: 
           [0024]      FIG. 1  shows the section through a conductor bar such as that on which the invention is based; 
           [0025]      FIG. 2  shows a conventional winding layout for the first mica tape with a half-width overlap; 
           [0026]      FIG. 3  shows the problem zones with the highly overlapping winding; 
           [0027]      FIG. 4  shows the problem zones with simultaneously winding with two offset tapes; 
           [0028]      FIG. 5  shows helical winding with the first mica tape with a small overlap, according to one exemplary embodiment of the invention; 
           [0029]      FIG. 6  shows helical winding without any overlap, and with a butt joint, according to another exemplary embodiment of the invention; 
           [0030]      FIG. 7  shows the winding with a butt joint, with the longitudinal direction of the tape parallel to the longitudinal direction of the conductor bar, according to a further exemplary embodiment of the invention; 
           [0031]      FIG. 8  shows a possible two-layer structure of the mica tape used for the invention; and 
           [0032]      FIG. 9  shows a possible three-layer structure for the mica tape used according to the invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0033]    In the first exemplary embodiment of the invention, a decrease is achieved in the breakdown probability on the longitudinal edge  25  of the inner conductor  11  by winding a mica tape of normal width in only one layer, with an overlap OL which is reduced in comparison to the conventional winding technique ( FIG. 5 ). This considerably reduces the frequency of the edges  16  per unit length of the conductor bar in the longitudinal direction. 
         [0034]    In the overlap limiting case of 0 ( FIG. 6 ), that is to say with the winding forming a butt joint  19 , the frequency of the edge is reduced by a factor of 4 in comparison to the prior art. In this preferred embodiment of the butt-jointed winding, the butt joint  19  in the first mica tape  17  is covered by a second, likewise butt-jointed, layer of a mica tape  18 . This type of winding should be restricted to the straight part of the conductor bar  10  (which is located in the winding slot) since it is not possible to wind with a butt joint in the curved area of the bar. In one particularly preferred embodiment, a tape with a considerably greater width B, in particular of more than 50 mm, for example 100 mm, should be used for this purpose. 
         [0035]    A second embodiment, by means of which it is possible to completely avoid any tape edges on the bar edge  25 , comprises the application as a first layer of a web of the length of the straight part and of the width of the copper bar circumference or inner-conductor circumference in the form of a butt joint ( FIG. 7 ). The butt joint  20 ,  21  of the mica tape  15  then runs in the axial direction of the conductor bar  10  in the area of the center of the narrow face or broad face of the bar, that is to say well away from the bar edge. In order to ensure that this mica tape remains on the bar, it must be adhesively bonded on, for example by impregnation resin or a silicone elastomer. In the latter case, an additional advantage is that the connection between the inner conductor  11  and the insulating sleeve (main insulation  14 ) is elastic. This allows shear stresses, produced by the difference in the thermal coefficients of expansion between copper and the main insulation  14 , to be converted to strain without this leading to tearing of the adhesive joint. 
         [0036]    A mica tape such as this can in one preferred embodiment be formed such that it is poorly conductive on its lower face. This can be achieved by means of a graphite or carbon-black coating. However, it is also feasible ( FIG. 8 ) for the mica (mica layer  22 ) to be coated onto an electrically conductive web layer  23 , in particular a carbon-fiber web or a conductive polyester non-woven. The mica tape  15  is then a two-layer material (for example C-fiber web/mica). 
         [0037]    The mica tape  15 ′ can, as shown in  FIG. 9 , also, however, be a three-layer material (for example C-fiber web/mica/glass-fiber web), in which a glass-fiber web  24  is also applied over the mica layer  22 . The conductive layer  23  is than used as complete internal corona-discharge protection, which is in close and permanent contact with the main insulation  14 . 
       LIST OF REFERENCE SYMBOLS  
       [0000]    
       
           10  Conductor bar 
           11  Inner conductor 
           12  Conductor element 
           13  Mica insulation 
           14  Main insulation 
           15 ,  15 ′ Mica tape 
           16  Edge 
           17 ,  18  Mica tape 
           19  Butt joint 
           20 , 21  Butt joint 
           22  Mica layer 
           23  Web layer (electrically conductive) 
           24  Glass-fiber web 
           25  Longitudinal edge 
         B Width (mica tape) 
         OL Overlap