Abstract:
A method of repairing a crack in at least one side of a dovetail portion of a generator rotor coil slot wall, the dovetail portion having received at least two axially adjacent steel wedges and including at least a radial entry surface, an inwardly tapered surface and an intermediate radial surface, the method comprising a) machining a groove at least partly along the inwardly tapered surface to remove damaged material from the coil slot wall; and b) replacing the at least two axially adjacent steel wedges.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to generator rotors and specifically, to the repair of generator rotors that have experienced fretting damage and/or cracks in the rotor teeth located at the butt joints of the axially aligned rotor wedges. 
   Conventional dynamoelectric machines, such as generators used with gas and steam turbines, employ forged rotors of magnetic material into which radial slots are machined for receiving the conductive turns of field windings which are interconnected such as to produce a desired magnetic flux pattern. Typically, included in such conventional rotor slots are creepage blocks at both the top and bottom ends of the slot as well as coil slot wedges for resisting the radially outward forces exerted on the windings when the rotor is operational. 
   The slot wedges, which are generally dovetail shaped, are used to maintain the copper coils in place while the rotor is spinning at, for example, 3600 revolutions per minute. In the prior art such coil slot wedges were normally 6 to 12 inches long with a number of such wedges being required for each coil slot, particularly in the longer rotors with high electrical ratings. In an effort to decrease the number of parts that are required for assembly as well as increasing the overall speed of such assembly, full length wedges have been used in certain applications. For other applications, tolerances preclude the use of full length wedges and, in these cases, two or more wedges are used in each rotor slot. Cracks have been found, however, in the radial slot walls of several generators, at the butt joint between adjacent rotor wedges, apparently due to fretting damage found at the ends of steel wedges. If allowed to remain in the rotor, these cracks can grow and potentially cause a catastrophic failure of the rotor. 
   In the past, repairs to cracked generator rotor teeth have involved removing the damaged rotor tooth material. These repairs, however, have not done anything additional to eliminate the future occurrence of additional fretting fatigue damage. 
   It is also known to use aluminum rotor wedges in certain generator rotors, but these have not previously been recognized in the art as useful for the elimination of fretting fatigue. 
   One prior solution to the fretting fatigue problem was to machine away the damaged tooth material and to replace the multiple original short steel rotor pole wedges with a single, full-length aluminum wedge. As already mentioned above, however, there are many generator rotors that cannot make use of full length wedges and require the continued use of plural shorter wedges with original dimensions. 
   BRIEF DESCRIPTION OF THE INVENTION 
   This invention provides a process for repairing a generator rotor which has fretting fatigue damage in the rotor teeth at the butt joints of steel rotor wedges. The repair process eliminates existing cracks by machining away any damaged material to create a shape which minimally reduces the low and high cycle fatigue life of the rotor. 
   In one embodiment, the slot side wall is machined along a vertical entry surface and a tapered radially outward dovetail surface to remove the damaged material. The machined areas are concave in shape in a manner that only minimally reduces the low and high cycle fatigue life of the rotor. 
   In another embodiment, the machining process is continued into the intermediate vertical (or radial surface of the dovetail, and, in this area, extended axially in opposite directions such that the machined area takes on an inverted T-shape. 
   Future fretting damage is eliminated not only by removing the damaged material, but also by replacing the original steel wedges with similarly sized aluminum wedges. This solution is advantageous in that it does not require large scale machining or a high speed rotor balance operation, and is therefore suitable for implementation on site. When the wedges are replaced, the butt joint between adjacent wedges may be aligned with, or offset from the repaired area. 
   Accordingly, in one aspect, the present invention relates to a method of repairing a crack in at least one side of a dovetail portion of a generator rotor coil slot wall, the dovetail portion having received at least two axially adjacent steel wedges and including at least a radial entry surface, an inwardly tapered surface and an intermediate radial surface, the method comprising: a) machining a groove at least partly along the inwardly tapered surface to remove damaged material from the coil slot wall; and b) replacing the at least two axially adjacent steel wedges. 
   In another aspect, the invention relates to method of repairing a crack in at least one side of a dovetail portion of a generator rotor coil slot wall, the dovetail portion adapted to receive at least two axially adjacent steel wedges and including at least a radial entry surface, an inwardly tapered surface and an intermediate radial surface, the method comprising a) machining a first concave groove in the radial entry surface and only partly along the inwardly tapered surface to remove damaged material from the coil slot wall; and b) replacing the at least two axially adjacent steel wedges with aluminum wedges, such that the groove is centered on a butt joint between two axially adjacent replacement wedges. 
   In still another aspect, the invention relates to a method of repairing a crack in at least one side of a dovetail portion of a generator rotor coil slot wall, the dovetail portion adapted to receive at least two axially adjacent steel wedges and including at least a radial entry surface, an inwardly tapered surface and an intermediate radial surface, the method comprising a) machining a first groove in the radial entry surface and along the inwardly tapered surface; machining a second groove in the intermediate radial surface in an orientation transverse to the first groove; and b) replacing the two axially adjacent steel wedges with aluminum wedges, such that said groove is centered on a butt joint between two axially adjacent replacement wedges. 
   The invention will now be described in detail in connection with the drawings identified below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side elevation of a conventional generator rotor including radially oriented coil slots; 
       FIG. 2  is a partial cross section of the rotor body taken along the lines  2 — 2  of  FIG. 1 ; 
       FIG. 3  is a cross section through a typical coil slot and contents thereof; 
       FIG. 4  is a cross section of a rotor slot with damaged areas machined away in accordance with a first exemplary embodiment of the invention; 
       FIG. 5  is an enlarged detail taken from  FIG. 4 ; 
       FIG. 6  is a partial perspective view of the rotor slot, showing damaged areas machined away from opposite sides of the slot wall; 
       FIG. 7  is a plan view of the rotor slot shown in  FIG. 6 , with adjacent wedges shown in phantom; 
       FIG. 8  is a side elevation of one of the rotor slot side walls shown in  FIGS. 5 and 6 ; 
       FIG. 9  is a partial perspective of a radial coil slot and wedge, showing damaged material machined away in accordance with a second exemplary embodiment of the invention; 
       FIG. 10  is a partial cross section of the radial coil slot shown in  FIG. 9 ; and 
       FIG. 11  is a partial side elevation of the coil slot shown in FIG.  10 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2  illustrate a typical rotor  10  for a dynamoelectric machine wherein the rotor includes conventional elements such as rotor end shaft portions  12 ,  14  and couplings  16 ,  18  for connection with a turbine or gear reduction unit. Of particular significance here are the axially oriented coil slots  20  arranged circumferentially about the mid-section of the rotor, that are used for holding the copper field windings or coils. 
   With further reference to  FIG. 3 , the coil slots  20  are each radially directed and typically contain, in a radially outward sequence, insulated copper coils  22 , a creepage block  24 , and a plurality of axially aligned slot wedges  26 . The wedges have a generally dovetail shape in cross section, and are located and arranged so as to maintain the copper coils  22  and creepage block  24  in place while the rotor is spinning. The slot wedges  26  may also contain ventilation holes  28  (one shown) which are in general alignment with ventilation channels  30  (one shown) which pass through the copper conductors or coils, as well as through the creepage block  24 . 
   Fretting fatigue damage has occurred in the coil slots  20 , specifically in the slot walls, at the butt joints of the axially adjacent steel rotor wedges  26  (a butt joint  30  between a pair of adjacent wedges  32 ,  34  is shown in phantom in FIG.  7 ). Fretting occurs whenever a junction between components is subjected to a cyclic load that leads to small tangential displacement over part of the interface. A two dimensional (radial-axial) finite element fretting model was developed to model the mechanics of contact between the rotor slot  20 , and particularly the dovetail-shaped portion  36  of the slot wall  38  (see  FIGS. 4 and 5 ) and the wedge  26 , where fretting initiated cracks were observed in the rotor. The model provided a qualitative understanding of the fretting mechanism and provided a basis for preliminary assessment of various wedge redesign options. 
   The objective of the analysis was to understand the fretting behavior in terms of the fretting damage parameter (FDP) as the machine is being ramped up to Full Speed (FS) at ω=3600 rpm. In order to investigate the fretting behavior as the machine undergoes start-up or shut-down, a similar analysis was performed at ω=569 and 805 rpm. 
   The effect of the wedge material was also considered in the analysis. Steel (E=206.85 GPa) as well as aluminum wedges (E=70.329 GPa) are currently being used. The rotor material was a high strength steel, and a friction coefficient of μ=0.47 was used for steel-aluminum contact. Large differences were observed in the stick-slip behavior for aluminum and steel wedges at 360 rpm and 569 rpm. Steel wedges have a larger sliding region and higher magnitudes of slip when compared to aluminum wedges. Steel wedges also show a significantly higher fretting damage at the same rotor speed. 
   The highlights of the findings of the analyses are summarized:
         (1) Fretting damage tendency is highest at locations near the wedge ends rather than the center of the wedge. This correlates with the field experience that fretting cracks and damaged material have been observed at the butt joints between wedges.       

   (2) Fretting damage tendency is much higher for steel wedges than aluminum wedges. This correlates with the field experience that fretting cracks have been observed only in coil slots next to the pole where steel wedges are used. All other coil slots (that nominally see the same mechanical loading as the coil slots next to the pole) use aluminum wedges where no fretting cracks have been observed. 
   (3) Fretting damage accumulation occurs during start-up and shutdown as opposed to Full Speed Operation. This correlates with the field observation that fretting cracks have been observed in machines with large number of start/stops or peaking units (e.g. 2 units that demonstrated Fretting Damage had 900 start, 1600 starts). 
   The present invention has two significant aspects in terms of repair of fretting cracks in rotor teeth. First, the damaged material is ground out of the rotor and a local stress field is altered near the repaired ends. With reference to  FIGS. 4 and 5 , the machined area is in the wedge-receiving dovetail portion  36  of the slot wall  38 . As already noted, this is the area of the slot wall that is susceptible to fretting cracks, and particularly, at the interface of two abutting steel wedges. Initially, a suitably shaped and sized machining bit (e.g., a 0.625 dia. bit) is used to machine the vertical slot entry surface  40  and thereafter, most of the radially outer tapered dovetail surface  42 , best seen in  FIGS. 5 and 8 . The machining work creates a first concave groove  43 , including groove portions  44  and  46 . Note that a small area  48  ( FIG. 8 ) of the tapered surface  42  remains in its original state, and, as such, the groove portion  46  does not extend into the radial intermediate surface  47 . Once the material is machined to form groove portions  44  and  46 , the sharp edges along the boundaries of both portions are radiused. Machining in this manner only minimally reduces the low and high cycle fatigue life of the rotor. Where appropriate (i.e., depending on the crack location and extent of the crack), the repair may be effected by forming the first groove portion  46  only. When the wedges are replaced, the butt joint between adjacent wedges  32 ,  34  may be centered on the machined regions (i.e., on the radial centerline through the groove  43 , as shown in FIG.  7 . Alternatively, the wedge butt joint may be axially offset from the repaired area. 
   Once the machine repair is completed, the steel wedges  32 ,  34  are replaced, preferably with aluminum wedges of the same size, located in the same positions relative to the repaired areas. This has been shown to minimize the potential for future fretting damage in the rotor. However, the replacement wedges may be steel, particularly if, for example, aluminum wedges are not available. 
     FIGS. 9-11  illustrate a variation in the above described repair procedure. Here, the coil slot wall  50  is machined on the vertical (or radial) entry surface  52 , the upper or radially outer tapered surface  54  of the dovetail configuration, and into the vertical or radial intermediate surface  56  of the dovetail. Specifically, the tool is utilized to machine a narrow, concave groove  57  including groove portion  58  in the radial surface  52 , and radially aligned groove portion  60  in the tapered surface  54 , overlapping into the transition zone between surfaces  54 ,  56  at  62 . A second tool (e.g., a 1.25 in dia. disk) is then used to machine a second groove  64  transverse to the groove  57 , i.e., in the axial direction of the slot. Thus, the machined grooves take the shape of an inverted “T,” with first groove  57 , including groove portions  58 ,  60  serving as the stem and second groove  64  serving as the cross member of the “T”. It is also noted that the centerline through grooves  57 ,  60  may, again, be aligned with the joint between the wedges, or axially offset therefrom. Note end surface  59  of the wedge  61  in FIG.  9 . Further in this regard, in the exemplary embodiment, the groove portions  58 ,  60  of the first groove  57  have equal widths of about 0.37 inch while second groove  64  has a width less than half that, i.e., 0.12 inch. From either side edge of groove portion  60 , the second groove  64  projects about 0.25 inch. Here again, after machining, all surface edges defining the grooves  57  and  64  are radiused to reduce stress. As in the first described embodiment, the groove portion  58  in the radial surface  52  may be eliminated where appropriate. 
   It should also be noted that the drawings illustrate repaired areas on opposite sides of the slot. If a crack appears on only one side of the slot, the repair procedure may be confined to that side. However, the opposite side may also be machined as a preventative measure. 
   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.