Abstract:
A method of servicing, or repairing a crack in, a dovetail portion of a dynamoelectric machine&#39;s rotor coil slot wall. The coil slot wall includes a radial entry surface connected to an inwardly tapered surface. The inwardly tapered surface is connected to an intermediate radial surface. A machining step machines a first groove in the radial entry surface and inwardly tapered surface. The first groove extends linearly to the intermediate radial surface. The first groove is configured so that a smooth and linear surface is created along a length of the first groove. A second machining step machines a second groove in the intermediate radial surface in an orientation substantially perpendicular to the first groove. A portion of the second groove contacts a portion of the first groove. At least one of the first groove and the second groove are configured to remove damaged material from the coil slot wall.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    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. 
         [0002]    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. 
         [0003]    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. 
         [0004]    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 
       [0005]    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 substantially 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. 
         [0006]    In one embodiment, the slot side wall is machined along both a vertical entry surface and a tapered radially outward dovetail surface to form a single groove that removes 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. 
         [0007]    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. 
         [0008]    Future fretting damage is substantially 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. 
         [0009]    Accordingly, in one aspect of the present invention a method is provided for servicing, or repairing a crack in, at least one side of a dovetail portion of a dynamoelectric machine&#39;s rotor coil slot wall. The rotor coil slot wall includes at least a radial entry surface connected to an inwardly tapered surface, and the inwardly tapered surface is connected to an intermediate radial surface. The method includes a first machining step that machines a first groove in the radial entry surface and the inwardly tapered surface. The first groove extends linearly to the intermediate radial surface, and the first groove is configured so that a smooth and linear surface is created along a length of the first groove. A second machining step machines a second groove in the intermediate radial surface in an orientation substantially perpendicular to the first groove. At least a portion of the second groove contacts a portion of the first groove. At least one of the first groove and the second groove are configured to remove damaged material from the coil slot wall. 
         [0010]    In another aspect of the present invention a method is provided of servicing, or repairing a crack in, at least one side of a dovetail portion of a rotor coil slot wall in a generator. The rotor coil slot wall includes a radial entry surface connected to an inwardly tapered surface, and the inwardly tapered surface is connected to an intermediate radial surface. The radial entry surface and the intermediate radial surface are substantially parallel to each other. The method includes a first machining step that machines a first groove in the radial entry surface and the inwardly tapered surface. The first groove extends linearly from an outer rotor surface to the intermediate radial surface, and the first groove is configured so that a smooth and linear surface is created along a length of the first groove. A second machining step machines a second groove in the intermediate radial surface in an orientation substantially perpendicular to the first groove and substantially axially with respect to the generator. At least a portion of the second groove contacts a portion of the first groove. At least one of the first groove and the second groove are configured to remove damaged material from the coil slot wall or to act as a preventive measure against damage to the coil slot wall. 
         [0011]    The invention will now be described in detail in connection with the drawings identified below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a side elevation of a conventional generator rotor including radially oriented coil slots; 
           [0013]      FIG. 2  is a partial cross section of the rotor body taken along the lines  2 - 2  of  FIG. 1 ; 
           [0014]      FIG. 3  is a cross section through a typical coil slot and contents thereof; 
           [0015]      FIG. 4  is a cross section of a rotor slot with damaged areas machined away, according to an aspect of the present invention; 
           [0016]      FIG. 5  is an enlarged detail taken from  FIG. 4 , according to an aspect of the present invention; 
           [0017]      FIG. 6  is a perspective view of the rotor slot, showing damaged areas machined away from the slot wall, according to an aspect of the present invention; 
           [0018]      FIG. 7  is a plan view of the rotor slot shown in  FIG. 6 , with adjacent wedges shown in phantom, according to an aspect of the present invention; and 
           [0019]      FIG. 8  is a side view of the rotor slot, showing the first and second grooves machined into the slot wall, according to an aspect of the present invention, 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]      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. 
         [0021]    With farther 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 . 
         [0022]    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  70  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 wail  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. 
         [0023]    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 wail  38 , As already noted, this is the area of the slot wail that is susceptible to fretting cracks, and particularly, at the interface of two abutting steel wedges. The outer rotor surface  50  forms an outer most portion of the rotor. The coil slots  20  include a radial entry surface  51 , an inwardly tapered surface  52  and an intermediate radial surface  53 . An opposing side of slot  20  will have the same surfaces, but in an opposing configuration. The radial entry surface  51  extends radially downward until it connects to the inwardly tapered surface  52 . The inwardly tapered surface angles over and is connected to the intermediate radial surface  53 . 
         [0024]    A repair method according to the present invention may use a suitably shaped and sized machining bit (e.g., a 0.625″ diameter bit) to machine a first groove  54  in both the radial entry surface  51  and the inwardly tapered surface  52 . The first groove  54  extends linearly to the intermediate radial surface  53 . The first groove  54  is also configured to prevent the formation of edge discontinuities along the surface of the first groove between the radial entry surface  51  and the inwardly tapered surface  52 . The mid-region of the first groove removes material in likely crack locations. Post-machining inspections will be facilitated, especially in likely crack locations, by the linear (i.e., in a straight line) surface of the first groove. If the first groove had a non-linear configuration (e.g.. formed by two lines/surfaces at an angle to each other) then post-machining inspections could be more difficult due to this edge discontinuity between the two lines/surfaces. By removing (or preventing) the edge discontinuities, post-machining eddy current inspections (e.g., inspections for cracks) are facilitated by the continuous smooth and linear surface along the first groove. The first groove  54  may be oriented to extend in a direction of about 20 degrees from the radial entry surface  51 . Alternatively, the first groove  54  may be oriented to extend in a direction of about 70 degrees from the outer rotor surface  50 . These angles are examples only and the specific angle chosen may vary from the examples given as desired in the specific application. An important feature is that the first groove extends substantially linearly from the outer rotor surface  50 /radial entry surface  51  to the intermediate radial surface  53 . This linear configuration prevents formation of an edge discontinuity on the groove surface between the radial entry and inwardly tapered portions of coil slot  20 . As examples only, the first groove  54  may have a maximum depth of about 0.195 inches and a width of about 0.37 inches or more. The maximum depth is chosen as this geometry removes cracks more readily, that is the deepest part of the profile (where most material is removed) is at the most likely location of possible cracks. However, it is to be understood that the specific values chosen for the depth and width will vary in specific applications and for the specific machine. 
         [0025]    A second groove  55  may be machined in the intermediate radial surface  53 . The second groove  53  is oriented in a substantially perpendicular direction with respect to the first groove  54 . For example, the second groove  55  may be machined in an axial direction with respect to rotor  10 . The second groove  55  may also be configured to contact a portion, of the first groove  54 , as shown in  FIG. 4 . The first groove  54  and/or the second groove  55  are configured to remove damaged material (e.g., a crack) from the coil slot wail, and both grooves  54 ,  55  may act as a preventative measure against future damage to the coil slot wall. The machining work may create a first concave groove  54  and a second concave groove  55 . Once the material is machined to form the grooved portions, the sharp edges along the boundaries of both portions may be 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  54  only. When the wedges are replaced, the butt joint between adjacent wedges may be centered on the machined regions (i.e., on the radial centerline through the groove  54 . Alternatively, the wedge butt joint may be axially offset from the repaired area. 
         [0026]      FIG. 6  is a perspective view of the rotor slot, showing damaged areas machined away from the slot wall (as shown In cross-section by  FIG. 5 ). The first groove  54  extends from the outer rotor surface  50  and/or radial entry surface  51  in a linear manner until it reaches the intermediate radial surface  53 . The first groove  54  may be configured to be wider than the second groove  55 . For example, if the height (measured radially) of the second groove  55  is about 0.35 inches, then the width (measured axially) of the first groove  54  may be about 0.70 inches. As can be seen the second groove extends in a axial direction along the intermediate radial surface. The axial length of the second groove may be about 1.0 inches to about 2.0 inches, or any suitable length as desired in the specific application. Both the first groove and the second groove are configured to contact each other and all groove edges may be radiused to minimize or eliminate stress points. 
         [0027]      FIG. 7  is a plan view of the rotor slot shown in  FIG. 6 , with adjacent wedges shown in phantom. A butt joint  70  is shown between two axially adjacent rotor wedges  32 ,  34 . The first groove  54  is centered on and located across from the butt joint  70 , as this is the region most likely to experience fretting damage. Thus, the machined first and second grooves  54 ,  55  take the shape of an inverted “T,” with first groove  54  serving as the stem and second groove  55  serving as the cross member of the “T”. It is also noted that the centerline through grooves  54 ,  55  may, again, be aligned with the butt joint between the wedges, or axially offset therefrom. 
         [0028]      FIG. 8  is a side view of the rotor slot, showing the first and second grooves machined into the slot wall, according to an aspect of the present invention. This view would be seen from inside the coil slot  20  looking towards the slot wall  38 . The first groove  54  forms a smooth surface and a linearly extending groove that facilitates post-machining inspections at the locations along the groove where cracks are most likely to have been removed. 
         [0029]    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. 
         [0030]    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. 
         [0031]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.