Abstract:
A stator winding bar comprises a plurality of strands that are transposed in the manner of a Roebel bar in an active part having a prescribed length, the active part being divided into several areas having different lengths arranged one after the other, in each area a transposition of the strands by a prescribed angle magnitude is provided, wherein the crimping distance within each area varies.

Description:
Priority is claimed to Swiss Patent Application No. CH 00505/05, filed on Mar. 23, 2005, the entire disclosure of which is incorporated by reference herein. 
   The present invention relates generally to the field of dynamoelectric machines and, in particular, to a stator winding bar having a plurality of strands that are transposed in the manner of a Roebel bar in an active part having a prescribed length 
   BACKGROUND 
   In the case of stator windings of dynamoelectric machines such as, for example, turbogenerators, stator winding bars are used that are placed into appropriate grooves in the stator and affixed there. In order to reduce eddy currents, the stator winding bars consist of a plurality of insulated strands that (as shown in  FIG. 5 ) are combined to form stacks  26 ,  27  arranged in parallel and that are then surrounded by an outer insulation. In order to reduce circulating current losses in the strands, the strands are transposed (Roebel transposition) according to a prescribed pattern (magnitude of the angle) within the active length (in the active part) of the stator winding bar. A stator winding bar formed in this manner is referred to as a Roebel bar. 
   A standard transposition of the kind known from U.S. Pat. No. 2,821,641 is shown schematically in  FIG. 1 . The stator winding bar  10  shown in  FIG. 1  comprises two stacks, each with ten strands  11 . In order to clearly depict the transposition, a strand—designated with the reference numeral  12 —is drawn with thicker lines. The stator winding bar  10  is supported with an active part  8  having a length L in a winding groove (not shown here) of the stator. The strands execute a transposition of 540° over the length L of the active part  8 , that is to say, each strand has completed a rotation of 540° around the longitudinal axis of the bar in this area. The stack change of the strands  11  needed for the transposition is made possible by appropriately crimping the strands. The transposition per unit of length of the stator winding bar can be larger or smaller. If the transposition per unit of length increases, the crimping distance k decreases ( FIG. 1 ), that is to say, the distance between the crimping sites of strands that are adjacent in the stack. The active part  8  is delineated on both sides by winding heads  9  in which the strands  11  are untwisted. 
   With the known stator winding bar  10  from  FIG. 1 , the active part  8  is divided into three areas A, B and C in each of which a transposition by 180° is provided, resulting in a transposition of 3×180°=540° over the entire length L of the active part  8 . The two outer areas A and C are the same length and they each extend over one-fourth of the length L of the active part  8 . The middle area B extends accordingly over half the length L/2 of the active part. 
   In the transposition shown in  FIG. 1 , due to the non-transposed bar parts in the winding head, unequal current distributions still nevertheless occur in the strands, since circular currents (also called circulating currents), which are caused by the magnetic field in the winding head, form in the loops formed by strands and the ears at the ends of the bars. Thus, U.S. Pat. No. 3,614,497 already proposed compensating for the effects of the non-transposed bar parts in the winding head by incorporating so-called voids (that is to say, non-transposed areas). The appertaining transposition diagram is shown in  FIG. 2 . 
   The stator winding bar  13  shown there, which again consists of two stacks, each with ten strands  14  and  15 , is once again divided over the length L of the active part  8  into the three areas A, B and C in each of which a transposition of 180° is provided. In this case, once again, the lengths of the areas A, B and C amount to L/4, L/2 and L/4. Unlike the solution shown in  FIG. 1 , however, a segment  16 ,  17 ,  18  is inserted in the middle of each area, no transposition occurring in said segments  16 ,  17 ,  18 . The crimping distance k is correspondingly shortened in the other segments of the areas A, B and C. 
   The introduction of the voids or non-transposed segments  16 ,  17  and  18  brings about a reduction in the undesired circulating current effects, but it does not totally eliminate them. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a transposed stator winding bar with which the circulating current losses in the bar can be greatly reduced in comparison to the solutions known from the state of the art. 
   The present invention provides a stator winding bar that includes a plurality of strands transposed in the manner of a Roebel bar in an active part having a prescribed length, the active part being divided into several areas having different lengths and arranged one after the other, in each of which areas a transposition of the strands by a prescribed angle magnitude is provided, characterized in that the crimping distance varies in each of the areas of the active part. 
   One embodiment of the invention is characterized in that a transposition totaling 540° is provided in the active part. The active part is divided into three areas, a transposition of 180° being present in each area. The middle area extends over half of the length of the active part, whereas the two other areas each extend over one-fourth of the length of the active part. Each area is divided into several segments arranged one after the other and the crimping distance is constant in each of the segments. 
   In particular, each area is divided into three segments arranged one after the other, with the crimping distance over the two outer segments of each area being equal to or smaller than the crimping distance over each middle segment. Within each area, the segments each have the same length, the segments of the middle area each being twice as long as the segments of the two outer areas. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
     The invention will be explained in greater detail below with reference to embodiments in conjunction with the drawing, which shows the following: 
       FIG. 1  a transposition diagram known from the state of the art with three areas of different lengths in which a uniform transposition of 180° is provided in each area; 
       FIG. 2  in a depiction comparable to  FIG. 1 , another transposition diagram known from the state of the art, in which voids without transposition have been inserted into each of the areas; 
       FIG. 3  in a depiction comparable to  FIG. 1 , a transposition diagram according to an embodiment of the invention; 
       FIG. 4  a comparative calculation of the current distribution in a stator winding bar with 80 strands according to the configuration shown in  FIG. 5  for the three transposition diagrams of  FIGS. 1 to 3 ; and 
       FIG. 5  the structure of the stator winding bar upon which the calculation of  FIG. 4  is based, with the applicable numbering of the strands. 
   

   DETAILED DESCRIPTION 
   The above-mentioned U.S. Pat. No. 3,614,497 proposes a transposition of 540° in the active part of the stator winding bars of rotary current windings, namely, with the additional insertion of so-called voids in order to compensate for the effects of the non-transposed parts of the bar in the winding head, and it is on this basis that the present invention achieves an improved transposition effect. 
   A feature of the invention is that the transposition of 540° in the active part is not effectuated through voids but rather through various tightly crimped segments that make it possible to reduce the so-called circulating current losses in comparison to the approach from the above-mentioned publication. Another advantage is that the stator winding bar does not have any non-transposed segments which, for production-related reasons, are undesired because of the divergent bar height (compared to segments of crimped strands). 
     FIG. 3  shows an embodiment of the transposition according to the invention that is comparable to  FIGS. 1 and 2 , in which here as well, a stator winding bar  19  has been taken as the basis that consists of two stacks, each with ten strands  20 ,  21 . Here, too, three areas A, B and C are present having the lengths L/4, L/2 and L/4 in each of which the transposition is 180°. Like with the standard 540° transposition ( FIG. 1 ), the two outer areas A and C having the length L/4 are structured identically and transposed in the same manner. The middle area B of the transposition according to the invention is structured analogously to the outer areas A and C but, in comparison to each of these areas A or C, extends over twice the length L/2. 
   When the stator winding bar is manufactured, the crimping sites arranged in the middle of the individual areas A, B and C are pulled apart. This creates larger crimping distances k in the corresponding segments  22   b ,  23   b  and  24   b  than in the standard 540° transposition of  FIG. 1 . Moreover, at the edges of the areas (segments  22   a,c ,  23   a,c  and  24   a,c ), the crimping distances are shorter than with the standard 540° transposition. Consequently, the areas A, B and C each have at least two different crimping distances k. Within the scope of the invention, however, more than two different crimping distances can be realized. Hence, a continuous change in the crimping distances can be achieved and the circulating current factor can be further minimized. However, this does not go as far as in the case of  FIG. 2 , where an entire area is not transposed at all. Since these different crimping distances look like the bellows of an accordion, this type of transposition could be called “540° accordion-transposition”. 
   As shown in  FIG. 3 , the segments  22   a,b,c ;  23   a,b,c ; and  24   a,b,c  each have the same length, and each segment  23   a,b,c  of the middle area B is twice as long as the corresponding segments  22   a,b,c  and  24   a,b,c  of the two outer areas A, C. 
     FIG. 4  shows results of the current distribution—calculated for a turbogenerator—in the individual strands (strand current in p.u. over the number of strands, that is to say, per value of the strand current without circulating current), which illustrate the above-mentioned effects. According to  FIG. 5 , the stator winding bars  25  upon which the calculations are based consist of two stacks  26 ,  27 , each with forty strands  28 , which have an insulated height of 2.2 mm. The calculation of the circulating current factor for the well-known standard transposition of 540° ( FIG. 1 , case  1 ) yields a value of 1.20 (circulating current losses relative to the purely ohmic losses). 
   A circulating current factor of 1.08 is obtained for  FIG. 2  (case  2 ), and a circulating current factor of 1.04 for the transposition according to  FIG. 3  (case  3 ) upon which the invention is based. 
   It should be taken into consideration that the stator winding bar transposition that needs to be used to attain the minimum circulating current factor cannot be specified precisely in advance since the increase in the crimping distances has to be carried out differently, depending on the machinery, and moreover, the minimum permissible crimping distances have to be taken into account.