Abstract:
A method of removing a stator coil ( 10 ) from a stator core ( 12 ) of an electrical generator, including vibrating an unbonded portion ( 50 ) of the coil ( 10 ) until a resin bond material ( 32 ) between a bonded portion of the coil ( 46 ) and a surface ( 34 ) of the core ( 12 ) fails due to high cycle fatigue to free the coil ( 10 ) from the core ( 12 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates generally to stator rewind procedures for electro-dynamic machines. More particularly, this invention relates to removing a stator coil from a stator core of an electrical generator when the coil has been resin bonded to the core. 
       BACKGROUND OF THE INVENTION 
       [0002]    In many modern generators the stator coil includes conductors separated and wrapped with a tape, e.g. a mica tape. This assembly is impregnated with a resin insulation that removes air, gas, and moisture, to provide a void-free insulation. In some generators the coil is impregnated first and then assembled to the core. In other generators the coil and core are assembled first, and the entire assembly is impregnated with the resin in a process to form a monolithic stator assembly. An example of this process is Global Vacuum Pressure Impregnation (GVPI). In a GVPI process the stator assembly is processed in an alternating vacuum and pressure environment that ensures uniform distribution of resin throughout the assembly. The resin is cured to form the monolithic stator assembly. Benefits of GVPI include improved structural strength and improved resistance to moisture and chemicals etc. 
         [0003]    When a stator rewind is necessary, where the stator coils must be removed and replaced, each coil must be removed from a slot within the stator. The coil is resin bonded into the slot, and the bond is necessarily strong. Conventional practice has been to engage the end windings of the coil and to pull them upward out of the slot with enough mechanical force to extract the coil. However, this leaves a lot of the resin still bonded to the surface of the slot, and a subsequent operation is necessary to remove the residual resin from the surface of the slot. This subsequent operation is labor intensive and time consuming. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    The invention is explained in the following description in view of the drawings that show: 
           [0005]      FIG. 1  is a schematic partial cross section of a stator coil in a stator core. 
           [0006]      FIG. 2  is a schematic partial cross section  2 - 2  of  FIG. 1 . 
           [0007]      FIG. 3  is a schematic partial cross section  3 - 3  of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0008]    The present inventor has recognized that the traditional method of coil removal by mechanical force actually tears the tape present on the windings, yet leaves the resin bond to the stator slot surface largely intact. The inventor further recognized this to be a result of a greater mechanical strength of the bond than the tape, which enables the bond to resist the mechanical extraction force until after the tape has yielded. The inventor further recognized that the resin bond is more rigid than the tape, and as a result is likely to have a lower resistance to high frequency mechanical fatigue than the tape. As a result the inventor has developed an innovative approach to removing the coil from the core that targets the weaker fatigue strength of the bond such that the bond breaks due to fatigue while the tape remains intact. Specifically, by applying high frequency vibrations to the coil, the bond is repeatedly stressed by the vibration forces until it breaks. However, because the tape is capable of withstanding these low displacement/high cycle forces better than the bond, it remains intact. When the process is complete, the coil is freed from the stator, the resin bond is destroyed, and only a reduced amount of the resin is left adhered to the slot. An advantage of this process is the reduced amount of time required for resin removal from the slot subsequent to the coil removal. 
         [0009]      FIG. 1  shows a schematic partial cross section of a stator coil  10  in a stator core  12 . The stator coil  10  includes an end winding  14  and a straight portion (bar)  16 . The bar  16  is disposed in a slot  18  present in and running a length L C  of the core  12 , and thus the original bond is characterized by an original bond length L BO  that may be equal to the slot length L C . Prior coil removal techniques involved securing a device to the end winding  14  and lifting the end winding  14  upward as indicated by arrow  20 . This upward movement pulled upward on the bar  16 , prying it from the slot  18 , by tearing the tape that forms the exterior of the coil  10 . 
         [0010]      FIG. 2  is a schematic partial cross sectional view as seen at section  2 - 2  of  FIG. 1  showing the stator coil  10  in the slot  18  of the core  12 . The coil  10  includes a conductor bundle  22  that includes individual conductors  24  separated by interposed tape  26 , and external tape  28  that surrounds the conductor bundle  22 . A resin bond  30  made of resin material  32  secures the external tape  28  to a surface  34  of the slot  18 . Under the old removal method, the external tape  28  would fail (tear) due to the applied mechanical force, while the resin bond material  32  would remain. This would necessitate the subsequent operations to remove the resin bond material  32  from the surface  34  of the slot  18 . 
         [0011]    The method disclosed herein includes vibrating the coil  10  in such a way that the resin bond material  32  reaches its fatigue limit before the external tape  28  reaches its fatigue limit. Due to the inherent characteristics of the materials, this can be accomplished by attaching a vibration inducing device  40  to the coil. Inducing vibration for a sufficient time causes the resin bond material  32  to reach its fatigue limit first, such that a crack forms in the bond  30  which begins to propagate along the bond  30 , thereby breaking the bond  30 . When the crack has propagated the entire original length L BO  of the bond the coil  10  is fully freed from the slot  18 . 
         [0012]      FIG. 3  is a schematic partial cross sectional view as seen at section  3 - 3  of  FIG. 2  showing the coil  10  and the core  12  after a vibration device  40  has induced and propagated a leading edge  42  of a crack partially along the bond  30  in a direction  44 . This propagation leaves a bonded portion  46  with intact resin bond material  48  as indicated by the hatch markings. The bonded portion is characterized by a bonded portion length L BP . This propagation also forms an unbonded portion  50  of unbonded portion length L UP  where failed resin bond material  52  is indicated by dots. As the crack progresses, the bonded portion length L BP  gradually decreases, and the unbonded portion length L UP  gradually increases. Failed resin bond material  52  may include some resin bond material  32  on the surface  34  of the slot and some resin bond material  32  on the external tape  28 . Some of the resin bond material  32  may become pulverized by relative movement between the unbonded portion  50  and the slot  18  during continued vibration. In this embodiment the vibration device  40  has induced motion in a direction indicated by arrows  54 . The vibration device  40  may be secured in any manner known to those in the art. In an exemplary embodiment the vibration device  40  is strapped to the unbonded portion  50 . 
         [0013]    So long as the yield strength of the external tape  28  is not exceeded by the vibration induced forces, nearly any frequency and amplitude of vibration may be selected. It is estimated that vibration amplitudes up to 0.100 inches (2.54 mm) may be permissible, however, this may vary in different applications. In an exemplary embodiment, a frequency and amplitude of vibration may be applied to the coil without change until the bond is broken. Alternately, the vibration device  40  may vary frequencies in any number of patterns. For example, the vibration device may be capable of delivering a range of frequencies, and the frequencies selected may vary within that range over time. It may vary in a step wise manner, where a first frequency is selected, and then a second etc. It may vary in a cyclic manner, such that the frequencies applied to the coil  10  sweep from lowest to highest etc. The range applied may or may not be adjustable. Any number of other loadings may be envisioned. Other scenarios include ramping the frequency, either from low to high or from high to low. Further, random frequencies may be employed. Any combination of the above is likewise possible. 
         [0014]    In addition to varying the frequency, the amplitude may be varied in a manner similar to how the frequency is varied. In particular, the amplitude of the vibrations may remain the same, may be stepped or ramped up or down, may cycle, and/or may be random. Further, instead of vibrations, an impulse loading may be employed. The frequency and the amplitude of the impulse may be varied just as they may be for the vibrations. In all instances the frequency and amplitude may be changed in unison with each other or independent of each other. 
         [0015]    The amount and frequency of the force imparted to the bond at the leading edge of the crack may be controlled to maximize a speed of the process. Fatigue failure is influenced both by a magnitude of applied force and a frequency of application. Increasing either the magnitude or frequency of applied force decreases the amount of time it takes for the fatigue failure to occur. However, the method requires that the magnitude of the force applied to the resin bond material  32  not exceed a mechanical yield strength of the external tape  28 . Consequently, in order to reduce the amount of time it takes to break the bond it may be desired in some embodiments to monitor the system to ensure a maximum acceptable force is applied at the maximum possible frequency to the leading edge  42  of the crack. 
         [0016]    The amount of force felt by the leading edge  42  of the bond  30  for every cycle of vibration depends on an amplitude of vibration of the unbonded portion  50 . In order to maximize an amplitude of vibration for a given vibratory input delivered by the vibration device  40 , in an exemplary embodiment a natural frequency may be considered. The vibrating assembly includes those parts subject to vibratory induced movement. This may include the unbonded portion  50  and the vibration device  40  if the vibration device  40  is attached to the unbonded portion  50 , since each influences the natural frequency at which the unbonded portion  50  will vibrate. A natural frequency of the vibrating assembly is related to both a mass and a length of the vibrating assembly. However, it can be seen that the unbonded portion  50  will change its unbonded portion length L UP  as the leading edge  42  of the crack moves along in direction  44 , which necessarily changes the mass and length of the vibrating system. This change in mass and length L UP  of the unbonded portion  50  will change the natural frequency of the vibrating system as the crack progresses. An increase in the mass and unbonded portion length L UP  will likely lower the natural frequency. Consequently, in an exemplary embodiment, the frequency of vibration imparted by the vibration device  40  may vary as a natural frequency of the vibrating assembly changes. This change in natural frequency may be sensed by sensors  56 , such as accelerometers, that may be used to monitor for motion. 
         [0017]    There may be a fundamental frequency and other resonant frequencies that are multiples of the fundamental frequency for the vibrating assembly. In an embodiment where more than one resonant frequency is present in the vibrating system, the highest resonant frequency the vibration device  40  can deliver may be selected. By doing this, the time it takes to reach fatigue failure is reduced because for any given time period more cycles are delivered at a higher frequency than at a lower frequency. 
         [0018]    It may be desired to remove a portion of the end winding  14  before operating the vibration device  40 . Trimming a portion of the unbonded portion  50  yields a trimmed unbonded portion  50  with a reduced mass, which increases the natural frequency of the trimmed vibrating system. Vibrating at or near this increased natural frequency in turn reduces the amount of time it takes to reach the fatigue failure at the leading edge  42 . In an exemplary embodiment, once the leading edge  42  of the crack has propagated far enough along the original length L BO  of the bond  30 , the vibration device  40  may be unsecured from its original location and moved in the direction  44  closer to the leading edge  42  of the crack, where it is secured into a subsequent location. In addition to this repositioning of the vibration device  40 , as described above, some (or some more) of the unbonded portion  50  may be trimmed and the trimmed vibrating assembly may continue to be vibrated. Trimming some (or some more) of the unbonded portion  50  increases the natural frequency of the trimmed vibrating assembly, which in turn decreases the amount of time before fatigue failure occurs at the leading edge  42  of the crack. This repositioning and trimming may be repeated as many times as is desired to propagate the crack the full original length L BO  of the bond  30 . Trimming as much of the unbonded portion as is possible will produce the highest natural frequencies, and hence the quickest time to fatigue failure. 
         [0019]    Vibrating an unbonded portion  50  of a coil  10  may produce complex waveforms. This is represented schematically by waveform  60  shown in  FIG. 3  below the coil  10 . The waveform  60  is flat in the bonded portion  46  because the bond  30  restricts motion in that region. However, in the unbonded portion  50  the waveform represents motion. In this figure the waveform  60  includes a wavelength L W  that is shorter than the length L UP  of the unbonded portion  50 . In such situations nodes  62  may be present where the amplitude of vibration is negligible. Where an amplitude of vibration is negligible, so is a force imparted to the resin bond material  32 . Should a node coincide with the leading edge  42  of a crack, negligible force would be imparted to the resin bond material  32 . Crack propagation would likely stall in this scenario, or at least slow down significantly. Consequently, dynamic modeling may be used to model the motion of the unbonded portion  50  to ensure that the leading edge  42  of the crack does not coincide with a node. Such dynamic modeling may employ sensors such as sensors  56  to monitor actual motion of the unbonded portion  50 . The sensors  56  may provide input to a control system used to control activation of the vibration device  40  such that the frequency/amplitude of the induced vibrations are actively controlled in response to changes in the dynamic properties of the system as the leading edge  42  of the crack progresses. 
         [0020]    In this exemplary embodiment only a single coil  10  is shown, and the vibration device  40  is shown as secured only to the single coil  10 . However, often there may be two coils in a given core slot  18 . The vibration device  40  could be secured to two different coils simultaneously, or a separate vibration device  40  could be attached to each coil. In such an exemplary embodiment, vibrations could be applied without regard for natural frequency. Alternately, the unbonded portions could be monitored and the frequency of vibration adjusted as the natural frequency of the system changes. 
         [0021]    The novel method of removing a stator coil from a stator core disclosed herein greatly improves stator rewind procedures. It simplifies removal of the coil by eliminating the need for mechanical systems large enough to forcefully extract the coil from the core. It reduces damage to the coil because the resin bond external to the core is broken, not the external tape that is part of the coil. It reduces the amount of resin left in the slot, and this reduces the effort and time required,for the subsequent final removal of any remaining resin from the core slot. Together these improvements yield a decrease in downtime and cost associated with a stator rewind. 
         [0022]    While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.