Abstract:
An induction machine short circuit stop is interposed between a brace and a machine component, such as a generator building bolt, in order to limit component motion during a short circuit event. The stop is a block of resilient non-conductive material that is oriented proximal the machine component at a desired spaced gap. Gap dimensions may be modified by use of different height stop blocks and/or shims. The stop is affixed to the brace by straps, cordage or fasteners, such as threaded studs. The stop is suitable for retrofitting existing induction machines in the field or in refurbishing service centers.

Description:
BACKGROUND OF THE DISCLOSURE 
     1. Field of the Invention 
     The invention relates to short circuit stops for induction machines that restrict relative motion between a paired induction machine component and a brace. More particularly the invention relates to generator short circuits stops and their methods for installation, including replacement during generator retrofitting in service facilities or at field installation sites. 
     2. Description of the Prior Art 
     Induction machines, such as generators, have components capable of motion during machine operation. It is often desirable to limit motion range of components. For example, generator stators have a generally annular structure in which is captured a rotor. The generator stator annular structure has an end basket affixed to one end by building bolts having a stack of compressible end washers to allow a limited range of expansion and contraction. In a short circuit event it is desirable to restrain axial and radial movement of the building bolts. The building bolts are restrained by one or more braces that are interposed within the bolt&#39;s potential range of motion at a selective gap distance. A stop is interposed between the paired building bolt and brace. The stop may be selectively positioned to set a desired gap or spacing between itself and a paired component, such as a building bolt. Depending on the stop design, the gap may be altered or permanently set. 
     One type of existing generator stop is a threaded stop stud that is captured within mating female threads formed in the generator brace. The threaded stud is advanced into the brace so that it is selectively positioned proximal to the building bolt or other apparatus component whose motion is intended to be restrained by the stop. The threaded stop stud has a relatively thin diameter compared to its projection length out of the generator bracket, and thus is susceptible to bending during a generator short circuit event. Threaded stop stud replacement requires careful removal of the old stop stud, so as not to damage the corresponding female threads within the generator brace. Damaged female threads require repair before inserting a new threaded stop stud. The threaded stop stud dimensions vary among different generator models and, it may not be possible to ascertain with certainty needed replacement stud dimensions prior to generator disassembly. Thus additional generator down time may be incurred during the repair cycle while service technicians obtain replacement stud parts. 
     Another type of existing generator stop is an L-shaped bracket, one leg of which is permanently affixed to the generator brace, such as by welding. The other leg of the L-shaped bracket is oriented generally parallel to the brace and functions as the stop surface to restrain the building bolt or other apparatus component. Positioning the L-shaped bracket relative to the building bolt is accomplished by hand, and the bracket is clamped to the brace prior to the welding operation. Different generator configurations require different dimension brackets, and those dimensions may not necessarily be ascertained until the subject generator is removed from service and disassembled for inspection. This in turn requires ordering of L-bracket parts after initial disassembly, and possibly longer generator down time until the appropriate L-brackets are obtained. L-brackets are permanently attached to the generator brace, such as by welding procedures. Subsequent generator rebuilds and/or service may require time-consuming removal of the welded L-brackets, followed by re-welding of replacement brackets to the generator brace. 
     Thus, a need exists in the art for a short circuit stop for an induction machine, such as a generator, having robust construction that is not easily susceptible to damage in the event of a generator short circuit event. 
     Another need exists in the art for a short circuit stop capable of field installation without permanent attachment to induction machine components, such as generator braces, that can be selectively installed and uninstalled, preferably without modification of the underlying support components. 
     Another need exists in the art for a short circuit stop capable of meeting dimensional and configuration requirements of different induction machine designs, directly out of the box or by field fitting, so that the types of short circuit stop spare parts can be minimized in service inventory; or if not available in inventory, preferably quickly fabricated after initial repair disassembly and inspection so as to minimize repair down time. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the invention is to create a short circuit stop having robust construction that is not easily susceptible to damage in the event of a generator short circuit event. 
     Another object of the invention is to create a short circuit stop that capable of quick field installation, preferably without permanent modification of or attachment to induction machine components, such as generator braces, that can be selectively installed and uninstalled. 
     An additional object of the invention is to create a short circuit stop that is capable of being adapted to meet the dimensional and configuration requirements of different generator or other induction machine designs—directly out of the box or by field fitting—so that the types of short circuit stop spare parts can be minimized in service inventory; or if not available in inventory, preferably quickly fabricated after initial repair disassembly and inspection, so as to minimize repair down time. 
     These and other objects are achieved in accordance with the present invention by an induction machine short circuit stop that is interposed between a brace and a machine component, such as a generator building bolt, in order to limit component motion during a short circuit event. The stop is a block of material that is oriented proximal the machine component at a desired spaced gap. Gap dimensions may be modified by use of different height stop blocks and/or shims. The stop affixed to the brace by straps, cordage or fasteners, such as threaded studs. The stop is suitable for retrofitting existing induction machines in the field or in refurbishing service centers. 
     One aspect of the present invention features a stop apparatus for limiting relative movement between a paired induction machine component and a brace, including a stop block adapted for interposition between the paired components, with the stop block having a bottom face for abutment against one of the paired components, a top face in opposed spaced orientation relative to the other component, and defining a gap there between, and an engagement surface. A fastener is coupled to the engagement surface and the one of the paired components, for affixing the stop block thereto. 
     The present invention also features an induction machine apparatus having a paired induction machine component and brace capable of relative movement there between during an electrical short circuit event within the machine. A stop block is interposed between the paired components for limiting the relative movement between the paired components, with the stop block having a bottom face for abutment against one of the paired components, a top face in opposed spaced orientation relative to the other component and defining a gap there between, and an engagement surface. A fastener is coupled to the engagement surface and the one of the paired components, for affixing the stop block thereto. 
     Yet another feature of the present invention is a method for installing a short circuit stop block in an induction machine generator apparatus of the type having a building bolt and a generator brace that are capable of relative movement there between during an electrical short circuit event within the generator, so that the stop block limits such relative movement. The method comprises providing a stop block having a bottom face; a top face; and an engagement surface. The stop block is interposed between the building bolt and the generator brace by abutting the bottom surface against the generator brace. The stop block is affixed to the generator brace with a fastener coupled to the engagement surface and the generator brace. 
     The objects and features of the present invention may be applied jointly or severally in any combination or sub-combination by those skilled in the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows schematically a prior art induction machine generator stator end winding support structure that includes respective pairs of building bolt components and generator short circuit stops; 
         FIG. 2  is a side elevational view of a prior art generator stop; 
         FIG. 3  is a perspective view of another prior art generator stop; 
         FIG. 4  is a side elevational view of the generator stop of the present invention; 
         FIG. 5  is a perspective view of an alternate embodiment of the generator stop of the present invention; 
         FIG. 6  is a perspective view of an another alternate embodiment of the generator stop of the present invention; 
         FIG. 7  is a perspective view of an additional alternate embodiment of the generator stop of the present invention; 
         FIG. 8  is a perspective view of an yet another alternate embodiment of the generator stop of the present invention; and 
         FIG. 9  is a partial cross sectional elevational view of the generator stop of  FIG. 8 , taken along  9 - 9  thereof. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION 
     After considering the following description, those skilled in the art will clearly realize that the teachings of the present invention can be readily utilized in induction machine short circuit stops that restrict relative movement between a paired induction machine component and brace, such as between a generator building bolt and brace. 
     General System Overview 
       FIGS. 1-3  show an exemplary known induction machine generator  20  and known short circuit stops  40 ,  50 . Referring to  FIG. 1 , generator  20  has a generally annular-shaped stator core  22  within which is captured rotating shaft/rotor  24 . A generator end basket  26  is mounted on an axial end of the stator core  22  by a plurality of axially extending building bolts  28  that are oriented about the generator circumferential periphery. The building bolt  28  typically includes a stack of axially compressible washers that are captured by a spring clip and threaded nut that engages a mating threaded end of the male bolt structure, in order to accommodate axial expansion of the generator core structure during operation. 
     As previously described above, it is desirable to limit axial and radial movement of the building bolt  28 , such as during a short circuit event. As shown in  FIGS. 2 and 3 , a generator brace  30  is paired with a corresponding building bolt  28 . The generator brace  30  has respective bosses  32  that are oriented to restrain respective axial and radial movement of the building bolt  28 . Each boss  32  has a proximal face  34  that is oriented in opposed spaced relationship with the building bolt  28  as well as a distal face  36 . The respective bosses  32  also have threaded female bores  38  formed between the proximal  34  and distal  36  faces, for receipt of a threaded stop stud  40 . Each stop stud  40  has a proximal tip  42  in opposed spaced relationship with the building bolt  28 , the gap between which may be selectively varied by advancing or withdrawing the stop stud  40  distal tip  44  with an appropriate tool. 
       FIG. 3  shows a known alternative embodiment pair of short circuit stops  50  that restrain building bolt motion axially and radially. Each respective stop  50  constructed of L-shaped sheet metal material having a distal leg  52  affixed to the generator brace boss  32 , such as by spot or tack welds  56  and a proximal leg  54  oriented in opposed spaced relationship with the building bolt  28 . The gap between each respective stop proximal end  54  and its paired building bolt  28  is selectively and permanently set before affixing distal end  52  to its corresponding generator brace boss  32 . The short circuit stop  50  may be retrofitted to an existing generator boss  32  as a substitute for a prior threaded stop stud  40 : for example if replacement stop studs are not readily procured within a desired timeframe or if the threaded bore  38  is damaged when removing the existing stop stud. However, once a stop  50  is permanently affixed to a generator brace boss  32  it has to be removed by cutting or grinding operations in order to change the gap between the proximal end  54  and the building bolt, or if that stop is subsequently found to be damaged beyond repair in a future generator maintenance cycle. 
     Short Circuit Stop of the Present Invention 
     Referring to  FIGS. 4 and 5 , short circuit stops  60  of the present invention are interposed between the respective proximal faces  34  of the generator brace  30  and the paired building bolt  28 . The stop  60  is preferably constructed of a resilient non conductive material, such as an epoxy laminate, that may be molded in final form or shaped from a block of material. Other suitable materials include thermosetting or thermoplastic resins, as well as elastomers, that are suitable for the induction machine&#39;s operational parameters. As shown, the stop  60  preferably has a flat bottom face  62  that is abutted against a corresponding proximal face  34  of the generator brace boss  32  and a top face  64  that is in opposed, spaced orientation relative to the building bolt  28 . The gap between the stop block  60  and the building bolt  28  may be selectively adjusted by varying the block  60  thickness between the top  64  and bottom  62  faces and/or by selective addition of one or more spacers, such as shims  70 . The stop  60  top face  64  limits the building bolt  28  range of motion by direct compressible contact. Unlike the previously known stop studs  40  and stop brackets  50 , the stop  60  of the present invention is less likely to be permanently deformed or damaged during a short circuit event, due to its preferred resilient material construction, ability to distribute compressive loads throughout its structure and relatively large abutting surface contact area with the brace proximal face  34 . 
     The stop block  60  of  FIGS. 4 and 5  has an engagement surface  66  that is this embodiment is a necked, concave surface formed in the top face  64  and flanking lateral sides, so that the block structure has a generally dog-bone profile. As shown, an elongated flexible fastener, such as a non-conductive cordage, tape, cloth or strap  80  is affixed to stop block  60  by wrapping it about the engagement surface  66  and passing the respective ends  82 ,  84  through the generator brace bore  38  that previously retained a stop stud  40 . The flexible fastener strap  80  is tensioned about another stop block  60  or other rigid member that is oriented on the opposite side of the bore  38  on the distal face  36  of the boss  32 , and subsequently tied off or otherwise affixed to the boss. By passing the fastener strap  80  through the existing generator brace boss aperture  38  the block  60  is restrained against the boss proximal face  34  without the necessity of creating new fastener holes or attachment points as is required for installation of the prior art stop brackets  50 . Similarly, the boss aperture  38  threads need not be repaired in order to receive the strap fastener  80 , as might otherwise be required if a replacement stop stud  40  were utilized in lieu of the stop block of the present invention. Attachment of the stop block  60  of the present invention does not require permanent alteration of the generator brace  30  or its bosses  32 . The stop block  60  may be selectively removed from the generator by unfastening, or cutting of the fastener strap  80 . 
     While  FIG. 4  shows an exemplary fastener strap  80  having tied off ends  82 ,  84 , other types of fasteners may be utilized in the present invention. In  FIG. 5 , strap fastener  180  is a cable tie having engageable respective mating pawl  182  and ratchet  184  ends, that eliminate the need to tie free ends of the fastener. The pawl end  184  of cable tie fastener  180  is passed through the brace aperture  38  from the distal face  36  to the proximal face  34  of the boss  32 , over the stop block engagement surface  66  and retracted back through the aperture  38  to mate with the ratchet end  182 . In  FIG. 6 , Stop block  160  has a recessed engagement surface  166  and a pair of through bores  168  from the bottom surface  162  to the top surface  164  through which pass fastener strap  280 , that has respective mating hook and loop fastener ends  282 ,  284 . 
     Another stop block  260  embodiment is shown in  FIG. 7 , wherein the bottom surface  262  defines a threaded aperture  266  for mating receipt of threaded stud  140 . The stud  140  is advantageously captured within the generator brace threaded female bore  38 . Alternatively, the threaded stud  140  may have a smaller diameter than the threaded bore  38  and an alternative mating threaded engagement with a female threaded fastening element oriented outside the distal face  36  of the generator brace  30 . A screw with fastening head may be substituted for the stud  140 , in which case the screw distal end threads engage within the stop block engagement aperture  262 . One or more spacer shims  70  may be utilized to alter the gap between the stop block top face  264  and the corresponding paired building bolt  28 . 
     Another short circuit stop  360  embodiment is shown in  FIGS. 8 and 9 , that facilitates wrapping of the flexible fastening strap element  80  about the outer periphery of the stop block engagement surface  366  and the generator brace boss  32 . Stop  360  has a bottom surface  362  from which projects a male boss or stud  368  that is inserted into the generator boss aperture  38 , thereby limiting lateral shifting movement of the stop relative to the generator boss proximal face. 
     Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings. For example, the stop block of the present invention may be affixed to the building bolt rather than to the generator brace. While the stop block embodiments shown in the figures herein are affixed to the generator brace with a fastener engaging an engagement surface formed within the stop, the stop block may eliminate an engagement surface by adhesively affixing or otherwise bonding it to the generator brace or building bolt. Reinforced adhesive tape may be substituted as a fastening element. Alternatively, the fastening element may be integrally formed within the stop block. The stop block can be formed in non-rectangular shapes, including cylindrical, hemispherical or frustoconical profiles.