Abstract:
Welded stator winding splice joints are provided that utilize gas tungsten arc welding to offer high performance and reliability. The application of welded stator winding joints can significantly raise operation reliability of rotary electric machines such as turbine generator and induction motors, simplify the manufacturing process, and reduce the cost and product cycle time.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to stator winding splice joints and, more particularly, to welded stator winding splice joints utilizing gas tungsten arc welding to provide high performance and reliability. 
     A rotary electric machine such as a turbine generator generally comprises a stator having an iron core and a coil winding, and a rotor rotatably supported within the stator and having a core and coil winding. The annular stator core has a plurality of axially extended core teeth to form coil slots therebetween at regular intervals circumferentially of the inner surface of the stator core. 
     The function of the stator winding is to provide a well defined path for electric current flow to and from an external system at a prescribed terminal voltage level and temperature level. The stator winding coils are made up of bundles of strands of insulated copper wires that are disposed in the slots of the stator core. Referring to FIG. 1, at the ends of each coil, the copper wires are brazed together to form bar leads  10 ,  12 . A pair of coils is connected by sandwiching the stator bar leads  10 ,  12  with two connection straps  14 ,  16  and brazing them together, as schematically shown at  18 , to provide an electric current path. There are several dozen such conventional stator winding splice joints contained in a rotary electric machine. The failure of one splice joint can result in the failure of the entire stator winding. Thus, each splice joint plays an important role in ensuring the normal operation of the rotary electric machine. The reliability of each splice joint has a direct and strong impact on the reliability of the whole machine. 
     As noted above, brazing is conventionally used to join the stator coils. During brazing, the base metal is not melted. Thus, the filler metal is chosen to melt at a lower temperature than the base metal. Although brazing is widely used, brazing has numerous disadvantages. For example, because good brazing depends on the capillary flow of the filler metal, surface cleanliness in brazing is much more critical than other joining processes, such as welding. Any contamination on the brazing surfaces may cause the ultimate failure of the splice joint. Further, the filler material in many brazed joints is considerably weaker than the joint base material. In addition, brazing produces waste disposal and hazards that are harmful to the environment. Furthermore, distortion can be experienced in brazing and the evaluation of strength of brazed joints is more complex than that of welded joints. In summary, brazing usually has a relatively low reliability. In spite of these shortcomings, the conventional design of the stator winding splice joints employs brazing as the joint method. 
     BRIEF SUMMARY OF THE INVENTION 
     Gas tungsten arc welding (GTAW) is a high-precision, high-quality, high reliability, low cost and simple joining process. GTAW produces the coalescence of metals by heating them with an arc between a non-consumable tungsten-electrode and the base metal. During the welding process, an inert gas such as helium sustains the arc and protects the molten metal from atmospheric contamination and oxidation. The advantages of GTAW include the fact that it produces superior quality welds that are generally free of defects; it is free of the spatter that occurs in brazing processes; it can be used with or without a filler metal; it can be used with a wide range of power supplies; it allows precise control of welding penetration depths; it can produce inexpensive autogenous welds at high speeds; and it can be used to weld almost all metals. 
     As noted above, conventional splice joints employ brazing as the joining method. Because of the strict cleaning requirements for the brazing surfaces, the brazing process is rather time consuming and costly. It therefore would be desirable to modify the stator winding splice joining process to increase the reliability of stator winding splice joints in rotary electric machines. It would also be desirable to simplify the manufacturing processes and reduce assembly cycle time. It would also be desirable to reduce labor material and reworking costs and to provide a technique for joining the winding that reduces the production of waste and hazards, in particular compared to the conventional brazing process. The invention addresses the aforementioned problems of the conventional brazing process and in particular is embodied in the use of a GTAW process to weld stator winding splice joints to increase manufacturing efficiency and maintain product quality stability. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These, as well as other objects and advantages of this invention, will be more completely understood and appreciated by careful study of the following more detailed description of the presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic perspective view of a conventional stator winding splice joint; 
     FIG. 2 is a schematic perspective view of a welded stator winding splice joint embodying the invention; 
     FIG. 3 is a cross-sectional view taken along line  3 — 3  of FIG. 2; 
     FIG. 4 is a cross-sectional view taken along line  4 — 4  of FIG. 2 
     FIG. 5 is a schematic perspective view illustrating an alternate embodiment of the invention; 
     FIG. 6 is a schematic perspective view illustrating a welded splice joint according to a further alternate of the invention; 
     FIG. 7 is a schematic cross-sectional view taken along line  7 — 7  of FIG. 6; 
     FIG. 8 is a schematic perspective view showing yet a further alternate embodiment of a welded stator winding splice joint embodying the invention; 
     FIG. 9 is a cross-sectional view taken along line  9 — 9  of FIG. 8; and 
     FIG. 10 is a schematic perspective view of a further alternate embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is advantageously and desirably embodied in at least one of two main weld configurations. One of the preferred weld configurations is a side weld and the second is a front weld. The side weld may be a vertical weld or a longitudinal weld. 
     A welded stator winding splice joint embodying the invention is schematically illustrated in FIG.  2 . FIG. 2 illustrates an embodiment of a side weld configuration wherein the welding is carried on the two sides of the connection strap  20  as vertical welds. More specifically, in this embodiment, a U-shaped strap  20  holds the top and bottom bar leads  22 ,  24  of the stator winding. A middle copper filler  26  is inserted between the two bar leads and a bottom copper filler  28  is placed under the bottom bar lead. As can be seen from FIGS. 2 and 4, the copper filler  26  width corresponds to the coil (bar lead) width and the length of the copper filler is the same as the corresponding dimension of the connection strap  20 . The copper fillers  26 ,  28  serve two primary functions. First, the copper fillers avoid the “melt-down” phenomena that usually occurs at weldment edges. Second, the copper fillers increase the effective flowing area of electric current to lower the joint electric resistance. A C-clamp schematically illustrated by arrows C, is applied on the strap top wall  30  and the bottom copper filler  28  to press the weldments tightly prior to welding. 
     The GTAW torch (not shown) used for welding the splice joint is preferably specially designed to be able to fit in the small space between adjacent splice joints. The number of weld seams  32  provided is determined by current density through the joint and the electric resistance of the joint. Usually, two or more seams are desirably provided on each strap side wall  34 ,  36  to ensure low splice joint resistance and low copper loss. To get a wide welding width across the interface between the strap  20  and bar leads  22 ,  24 , the GTAW polarity configuration is set as direct current electric position (DCEP). Where a thick strap is provided for high current density cases, welding slots  38 , as illustrated in FIG. 5, are desirably formed on-the strap side walls  34 ,  36  to help increase the penetration depth in the bar leads. In this embodiment, a filler metal, i.e., copper is needed to fill in the slots  38 . 
     A further alternative embodiment of the invention is illustrated in FIGS. 6 and 7. More particularly FIG. 6 is an exemplary embodiment of a front weld configuration wherein welding  40  is executed along the interfaces between the strap side walls  34 ,  36  and the bar leads  22 ,  24  at the coil front. Since the weldments are easy to access, a conventional torch can be directly employed in welding. In a high current density case, a combination of side welds  32  (FIG. 2) and front welds  40  (FIG. 6) will provide satisfactory welded joints. To obtain a deep penetration depth, the strap and the bar leads are prepared with some opening (not shown in FIG. 6) corresponding to the welding slots  38  illustrated in FIG.  5 . For the front weld configuration of FIG. 6, direct current electro-negative (DCEN) is selected. 
     A further alternate and perhaps the, simplest embodiment of a welded splice joint as an embodiment of the invention is shown in FIG.  8 . In this design, only one copper filler  46  is required to connect two bar leads  42 ,  44 . Welding  48  is performed along the interfaces of the filler  46  and the bar leads  42 ,  44 . Therefore, the alignment of the side surface of the bar leads has little impact on the welding quality. A lower joint electric resistance can be guaranteed by selecting a suitable welding length. The additional savings that are evidently achievable result from the elimination of the connection strap  20  used in the embodiments of FIGS. 2 and 6. 
     With reference to FIG. 10, to further simplify the welded splice joint design, the copper bar leads  52 ,  54  can be redesigned by changing their bending angles to allow them to contact each other. A single welding seam  58  is then made. In this manner, the middle copper filler  26 ,  46  can be eliminated. Therefore, a further cost reduction can be achieved in terms of resource and material savings. 
     As mentioned above, the welding machine is adapted to satisfy the particular requirements of the subject splice joint welding process. This can be done by improving a conventional welding machine to include automatic electro travel-speed control, high frequency starting capability, torch position/angle adjustment and 360° welding track rotating control to allow welding of any bar lead configuration. In order to enhance welding efficiency, multi-electrodes may be used to weld on the same side or two sides of the bar leads simultaneously. 
     As is evident to those skilled in this art, the aforedescribed welded stator winding splice joints may be adopted in all air-coil and hydrogen-coil generators with either forward flow or reverse flow ventilation. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.