Patent Publication Number: US-2005121992-A1

Title: Counteracting magnetic field generator for undesired axial magnetic field component of a power generator stator and associated methods

Description:
FIELD OF THE INVENTION  
      The present invention relates to the field of power generation and, more particularly, to a power generator and related methods.  
     BACKGROUND OF THE INVENTION  
      A typical power generator includes a shaft and a rotor carried by the shaft. Surrounding the generator rotor is a generator stator. A turbine, such as a gas combustion turbine, a water-driven turbine, or steam-driven turbine rotates the shaft. The generator rotor is supplied DC power typically from an exciter also driven by the shaft. As the generator rotor is turned within the generator stator, electrical power is produced and is delivered to the utility power grid. Rotation of the rotor within the stator may create an undesired axial magnetic field component adjacent opposing ends of the stator. The undesired axial magnetic field component may cause eddy currents that would undesirably heat up the ends of the stator, unless addressed.  
      U.S. Pat. No. 6,608,419 to Shah et al. discloses a stepped away portion at each end of the stator. To combat the build-up of heat due to axial magnetic flux at each end of the stator core, the inner surface is stepped away from the rotor to increase the distance between the rotor and the stator core along the ends of the stator core. The increased distance reduces the axial magnetic flux on the ends of the stator core.  
      Multiple flux shunts are also disclosed in the Shah et al. patent. The flux shunts are positioned adjacent the stepped away portion of the prior art power generator to attract and redistribute the axial magnetic flux. The flux shunts attract and redistribute the axial magnetic flux by providing a low reluctance path for the undesired axial magnetic flux produced by rotation of the rotor.  
      U.S. Pat. No. 6,525,444 to Salem et al. discloses a laminate for reducing eddy currents and heating in a stator core to increase generator capacity. The metal lamination package comprises alternate layers of amorphous metal laminate and non-amorphous metal laminate.  
      Despite the above disclosed approaches to reduce an undesired axial magnetic field component in a power generator, it is still desirable to provide a more efficient and effective way to reduce, or counteract, the undesired axial magnetic field component at the ends of the stator.  
     SUMMARY OF THE INVENTION  
      In view of the foregoing background, it is therefore an object of the present invention to reduce an undesired axial magnetic field component created at the ends of a stator.  
      This and other objects, features, and advantages of the present invention are provided by a power generator comprising at least one counteracting magnetic field generator for generating a counteracting magnetic field to counteract an undesired axial magnetic field component. More specifically, the power generator may include a rotor, and a stator surrounding the rotor and having opposing ends. The stator may comprise a stator core and a plurality of windings carried by the stator core creating the undesired axial magnetic field component adjacent the opposing ends of the stator. The counteracting magnetic field generator may be associated with at least one end of the stator for generating a counteracting magnetic field for counteracting the undesired axial magnetic field component. Accordingly, by counteracting the undesired axial magnetic field component, eddy currents and associated heating may be reduced.  
      The counteracting magnetic field generator may comprise a first electrically conductive coil portion positioned for having an electrical current induced therein by the rotor, and a second electrically conductive coil portion positioned adjacent an end of the stator and connected to the first electrically conductive coil portion to receive the electrical current therefrom to generate the counteracting magnetic field. The stator may be spaced from the rotor to define a gap therebetween, and the first electrically conductive coil portion may be positioned in the gap in these embodiments. Alternately, or additionally, the stator core may include a recess therein receiving the first electrically conductive coil portion.  
      The windings may comprise end windings extending outwardly beyond respective ends of the stator core, and the second electrically conductive coil portion may be positioned adjacent at least one of the end windings. Relative positioning of the first and second electrically conductive coil portions may provide a desired phase offset for the counteracting magnetic field.  
      In other embodiments, the counteracting magnetic field generator may alternately comprise an electrically conductive coil portion adjacent an end of the stator, and a power source connected to the electrically conductive coil portion to generate the counteracting magnetic field. The power generator may further comprise at least one magnetic field sensor, and the power source may comprise a controller for controlling the counteracting magnetic field based upon the at least one magnetic field sensor. The controller may provide a desired phase offset for the counteracting magnetic field.  
      In addition to the counteracting magnetic field generator, the stator core may have at least one step at each end thereof, and/or a magnetic field shunt adjacent each end of the stator. This may further enhance the reduction of the undesired axial magnetic field component.  
      A method aspect of the present invention is for counteracting an undesired axial magnetic field component adjacent at least one end of a stator. The method may comprise generating a counteracting magnetic field adjacent the at least one end of the stator to counteract the undesired axial magnetic field component. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is schematic block diagram of a power generating apparatus including a counteracting magnetic field generator according to the present invention.  
       FIG. 2  is a schematic transverse cross-sectional view of an end portion of the stator of the power generator shown in  FIG. 1  illustrating an undesired axial magnetic field component, and the counteracting magnetic field produced by the counteracting magnetic field generator.  
       FIG. 3  is a schematic transverse cross-sectional view of the power generator shown in  FIG. 1 .  
       FIG. 4  is a schematic transverse cross-sectional view of the second embodiment of the counteracting magnetic field generator according to the present invention.  
       FIG. 5  is a schematic end view of stator end windings of a stator illustrating a second embodiment of the counteracting magnetic field generator according to the present invention.  
       FIGS. 6-10  are schematic plan views of an interior surface of the stator showing additional embodiments of electrically conductive coil portions of the counteracting magnetic field generator according to the present invention.  
       FIGS. 11-12  are schematic diagrams of two other embodiments of the electrically conductive coils of the counteracting magnetic field generator according to the present invention.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and prime notation is used to indicate similar elements in alternate embodiments.  
      Referring initially to  FIGS. 1-3 , a power generating apparatus  15  including a counteracting magnetic field generator  30  is now described. The power generating apparatus  15  illustratively includes a power generator  20  and an exciter  17  connected thereto. More particularly, the power generator  20  includes a generator rotor  22  and a generator stator  23  surrounding the generator rotor. The generator rotor  22  is mounted to a shaft  24  that is driven by a turbine  26 . The turbine  26  may be a steam turbine, gas turbine, or water turbine as will be appreciated by those skilled in the art. The counteracting magnetic field generator  30  is illustratively positioned adjacent both ends of the stator  23  ( FIG. 1 ) to generate a counteracting magnetic field  99 . The counteracting magnetic field  99  counteracts an undesired axial magnetic field component  97  ( FIG. 2 ) caused by windings of the stator  23  and windings of the rotor  22 . Those skilled in the art will appreciate that the counteracting magnetic field generator  30  may also be positioned adjacent only one end of the stator  23 .  
      The stator  23  illustratively has opposing ends, and comprises a stator core  21  having laminations. The stator  23  surrounds the rotor  22 , and an undesired axial magnetic field component  97  may be created adjacent the opposing ends of the stator. The counteracting magnetic field generator  30  is associated with at least one end of the stator  23  for generating a counteracting magnetic field  99  for counteracting the undesired axial magnetic field component  97 . More particularly, the counteracting magnetic field generator  30  may be positioned at the end of the stator core  21  between the laminations, as will be readily appreciated by those skilled in the art. Accordingly, by counteracting the undesired axial magnetic field component  97 , eddy currents and undesired heating are reduced.  
      The stator  23  is illustratively spaced from the rotor  22  to define a gap  29  therebetween, and illustratively comprises a plurality of windings  28 . The counteracting magnetic field generator  30  may illustratively be positioned in the gap  29 . More specifically, the counteracting magnetic field generator  30  illustratively includes first and second electrically conductive coil portions  32 ,  34 . The first electrically conductive coil portion  32  is positioned for having an electrical current induced therein by the rotor  22 , and the second electrically conductive coil portion  34  is positioned adjacent an end of the stator  23  and connected to the first electrically conductive coil portion to receive the electrical current therefrom to generate the counteracting magnetic field.  
      More particularly, the first electrically conductive coil portion  32  is positioned in the gap  29 . The stator core  21  may include a recess  27  therein receiving the first electrically conductive coil portion  32 . The stator core  21  may also have a stepped portion  19  at an end thereof, and may comprise a magnetic field shunt  18  adjacent an end of the stator core  21 . Of course, those skilled in the art will appreciate that the stepped portion  19  and the magnetic field shunt  18  may be positioned adjacent each end of the stator core  21 . The stepped portion  19  and the magnetic field shunt  18  advantageously reduce eddy currents and undesired heating of the power generator  20 .  
      Referring now additionally to  FIGS. 4-5 , a second embodiment of the power generator  20 ′ is now described. In the second embodiment of the power generator  20 ′, the plurality of windings  28 ′ carried by the stator core  21 ′ create an undesired axial magnetic field component adjacent the opposing ends of the stator  23 ′. The counteracting magnetic field generator  30 ′ may illustratively surround the windings  28 ′. The counteracting magnetic field generator  30 ′ illustratively includes a conductive coil portion  35 ′ adjacent an end of the stator  23 ′. The counteracting magnetic field generator  30 ′ also includes a power source  37 ′ connected to the electrically conductive coil portion  35 ′ to generate the counteracting magnetic field.  
      The power generator  20 ′ comprises a magnetic field sensor  40 ′, and the power source  37 ′ comprises a controller  42 ′ for controlling the counteracting magnetic field based upon the magnetic field sensor. The power source  37 ′ may provide a desired phase offset for the counteracting magnetic field. The other elements of this embodiment of the power generator  20 ′ are similar to those of the first embodiment of the power generator  20 , are labeled with prime notation, and require no further discussion herein.  
      Referring now additionally to  FIGS. 6-10 , embodiments of the counteracting magnetic field generator  30  are now described in greater detail. A first embodiment of the first and second electrically conductive coil portions are labeled as above, i.e.,  32 ,  34 . The alternate embodiments of the first and second electrically conductive coil portions  32 ,  34  are labeled as increasing in numbering by  100  for each alternate embodiment. Further, a graphical illustration is provided if  FIGS. 6-10  to note direction, in which Θ represents the peripheral direction, and Z represents the axial direction. Relative positioning of the first and second electrically conductive coil portions  32 ,  34  provide a desired phase offset for the counteracting magnetic field.  
      Referring more specifically to  FIG. 6 , for example, a plurality of first and second electrically conductive coil portions  32 ,  34  are illustrated. The plurality of first and second electrically conductive coil portions  32 ,  34  each have a polygonal shape, and are arranged in side-by-side relation along the interior surface of the stator  23 .  FIG. 7  illustrates another embodiment of the plurality of first and second electrically conductive coil portions  132 ,  134 . The plurality of first and second electrically conductive coil portions  132 ,  134  also have a polygonal shape, but are slightly more slanted than the plurality of first and second electrically conductive coil portions  32 ,  34  illustrated in  FIG. 6 . Further, the plurality of first and second electrically conductive coil portions  132 ,  134  illustrated in  FIG. 7  may be overlapped along the interior surface of the stator  123 .  
      In  FIG. 8 , a pair of first and second electrically conductive coil portions  232 ,  234  is illustrated, each having opposing polygonally shaped ends, and arranged in side-by-side relation along the interior surface of the stator  223 . In  FIG. 9 , the first and second electrically conductive coil portions  332 ,  334  are arranged in an end-to-end linear format along an inner surface of the stator  323 . In  FIG. 10 , the configuration of the pair of first and second electrically conductive coil portions  432 ,  434 , is similar to that of the pair of first and second electrically conductive coil portions  232 ,  234  illustrated in  FIG. 8 , and are positioned along a lower portion of the interior surface of the stator  423 .  
      Referring now additionally to  FIGS. 11-12 , further embodiments of the first and second electrically conductive coil portions  532 ,  534  and  632 ,  634  are now described in greater detail. In  FIG. 11 , the arrow  89  indicates the main stator core flux. Accordingly, the first and second electrically conductive coil portions  532 ,  534  are arranged in a loop configuration having polygonally shaped portions. In  FIG. 12 , an opposing main stator core flux  89  is illustrated. Accordingly, a cross-over point  91  joins the first and second electrically conductive coil portions  632 ,  634 . In  FIGS. 11 and 12 , a graphical illustration of direction is provided in which Θ represents the peripheral direction, Z represents the axial direction, and R represents the radial direction.  
      A method aspect of the present invention is for counteracting an undesired axial magnetic field component adjacent at least one end of a stator  23 . The method may comprise generating a counteracting magnetic field adjacent the at least one end of the stator  23  to counteract the undesired axial magnetic field component.  
      Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.