Abstract:
A flux probe ( 100 ) for measuring the rotor flux of an electric generator ( 10 ). The probe ( 100 ) is affixed to a probe carrier ( 102 ) and held within a stator coil slot ( 62 ) by affixing the carrier ( 102 ) to two opposing grooves ( 104 ) formed in sidewalls ( 81 ) of the slot ( 62 ). One or more stator coils ( 18 ) are also disposed in the slot ( 62 ) and retained therein by a wedge ( 74 ) and corresponding spring ( 72 ) that applies an radially outwardly directed force to the coils ( 18 ). In one embodiment the probe carrier ( 102 ) is affixed to a radially inwardly directed face ( 82 ) of the wedge ( 74 ).

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to magnetic flux probes for measuring the magnetic flux produced by an electric generator rotor, and more specifically to a flux probe adapted for installation without requiring removal of the rotor from the generator and to a method for installing the flux probe.  
       BACKGROUND OF THE INVENTION  
       [0002]     As shown in  FIG. 1 , a conventional electric generator  10  comprises a generally cylindrical rotor  12  carrying axial field or rotor windings (also referred to as rotor turns)  13  about the circumference thereof. Current flow through the field windings  13  generates a magnetic field within a stationary armature or stator  14 . One end  15  of the rotor  12  is drivingly coupled to a steam or gas-driven turbine (not shown in  FIG. 1 ) for supplying rotational energy to turn the rotor  12  and causing rotation of the magnetic field produced thereby. According to known dynamoelectric principles, the rotating magnetic field of the rotor  12  induces current flow in stator windings  18  (only one stator winding  18  is illustrated in  FIG. 1 ) of the stator  14 . An end  16  of the rotor  12  is coupled to an exciter (not shown) for supplying direct current to the rotor windings  13  to produce the aforementioned magnetic field.  
         [0003]     The stator  14  has a generally cylindrical shape and an annular traverse cross section defining a longitudinal bore extending there through for accepting the rotor  12 , which thereby extends substantially the longitudinal length of the stator  14 . The stator  14  comprises a core  17  further comprising a plurality of thin, generally annular high-permeability laminations disposed in side-by-side orientation. Each lamination comprises an insulative coating such that two adjacent laminations are electrically insulated to reduce eddy current flow within the core  17 . The alternating current generated in the stator windings  18  by action of the rotating magnetic field of the rotor  12  is conducted to main generator leads  19  for supplying the generated current to an external electrical load. The stator windings  18  are connected through end turns  20 . A gap  21  formed between the rotor  12  and the stator  14  typically measures between about one and two inches.  
         [0004]     The rotor  12  and the stator  14 , and other generator components not directly relevant to the present invention, are enclosed within a frame  23 .  
         [0005]      FIG. 2  illustrates rotor axial leads  54  through which the direct current generated by the exciter is coupled to the rotor windings  13 . A conventional two-pole rotor comprises eight to eighteen axial slots each carrying a plurality of mutually insulated conductive bars that are symmetrically oriented along the rotor circumference. For example, one rotor design comprises fourteen slots such that one rotor pole comprises seven slots. The conductive bars comprising a pole are interconnected by end turns, i.e., arcuate winding segments located at a rotor end for connecting the axially disposed conductive bars. The end turns are physically restrained by retaining rings  56 .  
         [0006]      FIG. 3  is a cross-sectional view of the stator  14 , illustrating a face  60  of one lamination of the stator core  17  and circumferential stator slots  62  defined by inwardly-directed circumferential teeth  64  extending axially along the stator  14 . The stator windings  18  are disposed within the slots  62  between two adjacent teeth  64 . Bolt/nut combinations  66 , or similar fasteners, extend axially through and secure the laminations to form the stator core  17 .  
         [0007]     As illustrated more clearly in the partial view of  FIG. 4 , the stator core  17  comprises a plurality of stacked laminations  67 . Two adjacent stator teeth  64  (only one is shown in  FIG. 4 ) formed in the laminations  67  define stator slots  62  for receiving the stator windings  18 . Typically, two stator windings  18  are disposed within each slot  62 . The two stator windings  18  are retained within the slot  62  by a shim  70 , a ripple spring  72  and a wedge  74  having opposed beveled edges  76  for engaging correspondingly shaped grooves  80  in sidewalls  81  of the teeth  64 . The ripple spring  72  is compressed between the wedge  74  and the shim  70  to apply a radially outwardly directed force retaining the two stator windings  18  in place within the slot  62 .  
         [0008]     It is known that a shorted rotor winding turn reduces the magnetic flux and modifies the power dissipation profile of the rotor windings. Such changes in power dissipation can produce non-uniform heating of the rotor windings, resulting in thermally induced rotor distortion and vibration that can damage the rotor and other generator components. The vibrations may also excite the natural resonant frequencies of generator components, including the pad on which the generator is mounted. Pad vibration can lead to severe damage to the generator.  
         [0009]     The primary cause of a short circuit is the breakdown of insulation separating the conductor bars or end turns of the rotor, by wearing of the rotor winding insulation or slot cell insulation that separates the rotor windings. The short circuits may occur only when the rotor is at rest or only when the rotor windings and end turns are subjected to centrifugal forces caused by rotation. The former type of short circuit can be detected by known static tests, while the latter can be detected only while the rotor is turning at or near its operational speed.  
         [0010]     The considerable economic value of a generator and the high cost of replacing generated power during a generator service outage encourages an owner to continue operating the generator so long as operation is safe and will not likely damage the generator. A single shorted rotor turns can generally be tolerated, although it may be necessary for the operator to add balance weights to compensate the vibration levels under different operating conditions. If the vibrations cannot be eliminated by rotor balancing it may be necessary to shut down the generator and remove the rotor to determine the location of each short and repair the affected rotor winding. Also, if the rotor windings develop several shorts the short circuit current can flow through the rotor body. The rotor body may be damaged by this current flow and will thus require a substantial repair effort.  
         [0011]     One technique for locating rotor short circuits uses a stator-mounted flux probe for measuring the rotor magnetic field flux, from which, using known techniques, the operator can determine the existence and location of rotor winding shorts. According to this prior art process, installation of the flux probe requires removal of the rotor to access the stator and mount the probe thereto. In one embodiment, the probe is bonded and pinned, using dowel pins for example, to a radially inwardly directed face  82  of the stator wedge  74  of  FIG. 4 . The rotor is then replaced and the generator returned to service. Once operational again, probe signals indicative of the rotor&#39;s magnetic field are analyzed to determine the location of the short circuit. Such a prior art probe installation process is an expensive and time consuming undertaking for the generator operator, requiring a shut down duration of several days. The cost of the shutdown, with respect to both the process for installing the probe and the cost for replacement power can be considerable. Once the location of the short is determined, the generator is again shutdown, the rotor removed and the necessary rotor repairs performed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The foregoing and other features of the invention will be apparent from the following more particular description of the invention, as illustrated in the accompanying drawings, in which like reference characters refer to the same parts throughout the different figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.  
         [0013]      FIG. 1  illustrates an axial cross-sectional view of an electric generator;  
         [0014]      FIG. 2  illustrates a perspective view of a rotor of the electric generator of  FIG. 1 ;  
         [0015]      FIG. 3  illustrates a radial cross section of a stator of the electric generator of  FIG. 1 ;  
         [0016]      FIG. 4  illustrates a partial view of the stator of  FIG. 3 ;  
         [0017]      FIGS. 5A and 5B  illustrate plan and elevation views of a first embodiment of a flux probe according to the present invention;  
         [0018]      FIGS. 6A and 6B  illustrate plan and elevation views of a second embodiment of a flux probe according to the present invention;  
         [0019]      FIGS. 7A and 7B  illustrate plan and elevation views of a third embodiment of a flux probe according to the present invention;  
         [0020]      FIGS. 8A and 8B  illustrate plan and elevation views of a fourth embodiment of a flux probe according to the present invention; and  
         [0021]      FIG. 9  illustrates a tool for inserting the flux probe according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     Before describing in detail the particular flux probe mounting apparatus and method in accordance with the present invention, it should be observed that the present invention resides primarily in a novel and non-obvious combination of hardware elements and method steps. Accordingly, these elements and steps have been represented by conventional elements and steps in the drawings, showing only those specific details that are pertinent to the present invention so as not to obscure the disclosure with details that will be readily apparent to those skilled in the art having the benefit of the description herein.  
         [0023]     According to the teachings of the present invention, as illustrated in the plan view of  FIG. 5A  and the elevation view of  FIG. 5B , a flux probe  100  is attached to a carrier  102  (by for example, pinning, bonding or fastening with any known fastening device) that is in turn mounted within opposing tooling wedge grooves  104  formed in sidewalls  81  of two adjacent teeth  64 . After insertion, the flux probe  100  is positioned at an appropriate axial location of the generator  10  to measure the rotor magnetic flux, from which the existence of a rotor short can be determined. Generally, the probe is positioned in a region of the rotor  12  between the end turns for detecting a short at any axial location of a rotor winding.  
         [0024]     Installation of the flux probe  100  according to the teachings of the present invention requires removal of only the generator end frame to provide access to the stator slot  62 . Thus removal of the rotor is avoided, offering the generator operator a substantial cost savings due to the need for only a relatively brief service outage.  
         [0025]     According to one embodiment of the present invention as illustrated in  FIG. 5A , the carrier  102  comprises three mating segments  102 A,  102 B and  102 C each having opposing-double-beveled edges  110  (see  FIG. 5B ) for mating with a corresponding tooling wedge groove  104 . As shown, the flux probe  100  is mounted on one of the three segments, for example the carrier segment  102 B, by any known technique, including bonding using an adhesive material. Two threaded studs  120  are disposed within axially directed oversized or slotted holes in the segments  102 A,  102 B and  102 C for threadable engagement with cooperating nuts  122 . After the carrier  102  and the attached flux probe  100  are properly inserted within the opposing tooling wedge grooves  104 , the studs  120  are tightened relative to the nuts  122 , urging the double-beveled edges  110  of the segments  102 A,  102 B, and  102 C against surfaces defining the grooves  104 . In another embodiment the stud  120  is threadably engaged with one or more of the wedge segments  102 A,  102 B and  102 C.  
         [0026]     One or more wires  124  carrying a signal representing the rotor magnetic flux extend from the probe  100  along the carrier segments  102 B and  102 C and then are directed external to the generator. The wires  124  are terminated in various known signal processing and monitoring devices for determining the existence of winding shorts in the rotor  12  according to known techniques.  
         [0027]     In another embodiment illustrated in  FIGS. 6A and 6B , a bladder  150  is installed below a carrier  152  to which the flux probe  100  is mounted. After probe installation, the bladder  150  is filled with a liquid epoxy resin material that cures and hardens to apply an upwardly-directed force to the carrier  152 , urging the carrier  152  against walls defining the tooling wedge grooves  104 , thereby securing the probe  100  in place.  
         [0028]     In yet another embodiment illustrated in  FIGS. 7A and 7B , the tooling wedge grooves are absent and thus a carrier  170  comprises substantially flat axial side surfaces  172 , which are urged against the sidewalls  81  as the stud/nut combinations  120 / 122  are tightened.  
         [0029]     In anther embodiment illustrated in  FIGS. 8A and 8B  a probe  179  is mated with a carrier  180 . In one embodiment, the mating is accomplished by inserting the probe  179  into an opening formed in the carrier  180  and bonding the mating surfaces with an appropriate bonding material. The carrier  180  is bonded to the tooling wedge grooves  104  and/or to an upper surface  182  of the wedge  74 . Any suitable epoxy or adhesive material can be used to affect the bond.  
         [0030]     A tool  190  illustrated in  FIG. 9  comprises a trigger  191  movably affixed to a handle  192  further connected to a hollow rod  194  that slidably encloses two fingers  196 . Application of a trigger force to the trigger  191  withdraws the two fingers  196  into the rod  194 , applying a grasping force to the probe  100 . Spring members  198  and  200  apply an inwardly directed bias force to the two fingers  196 . The gap between the stator  14  and the rotor  12  is generally in the range of one to two inches, suggesting use of a tool such as the tool  190  for positioning the flux probe, such as the probe  100  or  179 , in the tooling wedge groove  104  after removing the generator end frame. Installation also requires reaching beyond the end turns  20  of the stator  14 , illustrated in  FIG. 1 .  
         [0031]     Although the invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalent elements may be substituted for elements thereof without departing from the scope of the present invention. The scope of the present invention further includes any combination of the elements from the various embodiments set forth herein. In addition, modifications may be made to adapt the teachings of the present invention to a particular situation without departing from the invention&#39;s scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.