Abstract:
A generator including a frame ( 56 ); first and second frame rings ( 13 ) extending radially inwardly from an inside surface of the frame ( 56 ); a core ( 54 ) within the frame ( 56 ); key bars ( 100 ) spaced apart circumferentially and extending axially spanning a distance between the first and second frame rings ( 75 A and  75 B), the key bars coupled to the core; laminated spring bars ( 60 ) spaced apart circumferentially and extending axially to span a distance between the first and second frame rings, a first spring bar end ( 112 ) supported by the first frame ring ( 75 A), a second opposing spring bar end ( 111 ) supported by the second frame ring ( 75 B), each spring bar coupled to a key bar; wherein the laminated spring bar further includes spring bar subcomponents ( 60 A and  60 B) and couplers ( 98/99 ) for coupling the spring bar subcomponents.

Description:
FIELD OF THE INVENTION 
     The present invention relates to electric power generators and more specifically to an enhanced spring bar core support system, e.g., changing a resonant frequency of the core support system, for a generator. 
     BACKGROUND OF THE INVENTION 
     The generator stator core is the largest monobloc component in a turbine-generator set. The stator core comprises thousands of thin steel laminations stacked horizontally and clamped together to form a cylindrical stator core disposed within a generator frame. Each lamination defines a central opening and thus when stacked an axial opening extends through the core. The laminations are held together by a plurality of axial through-bolts that extend from end-to-end through the core. 
     A rotor is disposed within the central opening and rotated by a rotating turbine. Electrical current is supplied to rotor windings such that rotation generates electric current in stator windings. The stator current is supplied to a plurality of main and neutral electrical leads mounted to the generator frame then to electrical loads through a transmission and distribution system. 
     Steady-state and transient forces generated during normal operation and transient conditions impose substantial forces on the stator core. These forces can distort the core geometric shape, cause the laminations to vibrate, and damage the core, rotor and/or frame. Also, mechanical fatigue caused by these forces can lead to premature failure of generator components. 
     Certain of these forces (including especially steady state forces generated during normal generator operation) may excite resonant responses in the generator and in particular in the coupling components that attach the core to the frame. Once the resonant response begins, the magnitude of these forces may increase substantially. 
     To reduce the effects of the steady sate and transient forces, the generator frame is fixed to a stable support such as the floor of a power plant and the stator core is solidly affixed to the generator frame. According to the prior art different attachment techniques and corresponding attachment components are employed to affix the core to the frame. 
     Keybars are used in one attachment technique. These long, axially-disposed members are located along an outer circumference of the stator core, specifically within slots defined in the outer circumference. The radially inwardly facing surface of each keybar is held within the slot by a geometrically capturing interfacing shape (for example a dovetail shape). A radially outwardly facing surface of each keybar is attached to the stator frame using various intermediate hardware components. 
     One such intermediate attachment component comprises a resilient spring bar. Several spring bars are distributed circumferentially around an interior surface of the frame and each spring bar extends axially through the frame. A first surface of each spring bar is attached to radially inwardly facing generator frame ribs and an opposing second surface of each spring bar is attached to a key bar mounting block or plate. The key bar block or plate is attached to the keybar. 
     The end of each keybar (both the exciter end and the turbine end) comprises a threaded segment for receiving a threaded nut and mating washer. The nuts are tightened to provide a clamping force to the stator core. 
       FIG. 1  is a partial cutaway perspective view of a prior art electric generator  8  and a stator core  10  mounted within a generator frame that is not shown in  FIG. 1 .  FIG. 1  further illustrates a spring bar  15 ; a plurality of such spring bars  15  is distributed around a circumference of the core  10 . Each frame ring  13  comprises a circumferential component  13 A and a transverse component  13 B. 
     A first surface of each spring bar  15  is attached to a plurality of the transverse components  13 B by fasteners  19 . Each transverse component  13 B is welded to a circumferential component  13 A and each circumferential component  13 A is welded to an inside surface of the generator frame. 
     Each spring bar  15  extends an axial length of the core  10 . At a plurality of axially spaced-apart locations a second surface of each spring bar  15  is attached to a key bracket or key block  20  using fasteners  18 . Each key block  20  spans a width of a keybar  22  and a plurality of key blocks  20  are axially distributed along each keybar  22 . 
     The keybars  22  are fixedly captured within the core  10  by a geometrically capturing interface defined in an outer surface of the core  10 . The keybars  22  and the core grooves are shaped such that the keybars  22  are captured within the groove by the geometric capture feature, such as the illustrated dovetail shape. A fastener  7  is tightened to provide additional forces to secure the keybar  22  to the core  10 . Thus the core  10  is connected to the generator frame by serial coupling of the keybars  22  geometrically retained within core grooves, the key blocks  20  and the spring bars  15 . 
     Stator windings (also referred to as stator bars, but not illustrated) are disposed within winding slots  21 . Through-bolts extend axially through openings  23 . The through-bolts and mating nuts (neither illustrated in  FIG. 1 ) cooperate to exert inwardly-directed axial clamping forces on core end plates and laminations that comprise the core  10 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in the following description in view of the drawings that show: 
         FIG. 1  is a cutaway view of a prior art generator core within a frame. 
         FIG. 2  is a cutaway view of certain components for use in attaching the generator core to the frame according to the present invention. 
         FIG. 3  is a detailed illustration of certain components of  FIG. 2 . 
         FIG. 4  is a detailed illustration of certain components of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention relates to a system for attaching a generator core to a generator frame (i.e., a core-to-frame attachment system) that exhibits a natural frequency beyond a range of resonant frequencies that are excited during normal and transient generator operation. 
     It is known in the art that when a natural frequency of the core-to-frame attachment system is excited, the resulting forces can initiate cracks in the weld joints of this system. These forces can also cause the cracks to propagate from the point of origin, reducing the capability of this system to support the dead weight of the core and the torsional forces developed during operation. The cracks may also induce noise and vibrations within the generator. 
     As a result of the mechanical and electrical properties of generator components, generators in operation today have one resonant frequency at approximately twice the line frequency. For a 2-pole 3000 RPM generator generating electricity at a line frequency of 50 Hz one resonant frequency is at about 100 Hz and for 2-pole 3600 RPM generator producing electricity at a line frequency of 60 Hz one resonant frequency is about 120 Hz. For a 4-pole generator operating at 1500 RPM and a line frequency of 50 Hz the resonant frequency is also at about 100 Hz and for a 4-pole 1800 RPM generator operating at a 60 Hz line frequency the resonant frequency is also about 120 Hz. Note that both resonant frequencies are at twice the value of the line frequency. Thus these resonant frequencies are easily excited during operation of the generator. 
     The natural frequency of the core-to-frame attachment system of the present invention is higher than these resonant frequencies and other troublesome resonant frequencies produced during generator operation. 
     The invention is described for use with a generator core using spring bars to affix the core to the frame. However the principles of the invention can be applied to other dynamoelectric machines by employing similar core-to-frame attachment systems. 
       FIG. 2  is a partial cutaway view illustrating a core  54 , a generator frame  56  and three pairs of proximately-disposed U-plates  57 . A frame ring (not shown) is captured between each U-plate pair and welded to each of the U-plates  57  to attach the core  54  to the frame  56 .  FIG. 2  illustrates three pairs of the U-plates  57  each for mating with one of three frame rings. A generator for use with the present invention may have more or fewer than three frame rings depending on frame length and other generator parameters. Further, the frame rings may not be equally spaced along the generator axis. 
     Note that frame rings employed in the  FIG. 2  embodiment are unlike the frame rings  13  in  FIG. 1 . Instead of comprising a circumferential component  13 A and a transverse component  13 B as in  FIG. 1 , the frame rings for use in the  FIG. 2  embodiment comprise only the circumferential component that is captured between and welded to each pair of U-plates  57 . The frame rings are not shown in  FIG. 2 . 
     Continuing with  FIG. 2 , laminated spring bars  60  cooperate with key bars, key blocks and fasteners (none visible in  FIG. 2 ) to secure the core  54  to the generator frame  56 . These components are further illustrated in detail in  FIG. 3  and described below. 
     Stator winding ends  62  at the turbine end  63  and stator winding ends  64  at the exciter end  65  are illustrated in  FIG. 2 . 
       FIG. 3  illustrates additional details of the core-to-frame attachment components of the present invention. 
     An inwardly-facing surface of a key bracket or key block  70  is disposed in contact with a key bar  100 . In the illustrated embodiment the key block  70  comprises an inverted U-shaped member that contacts three surfaces of the key bar  100 . 
     In one embodiment the key bar  100  exhibits a dovetail shape that is geometrically captured within a corresponding axial groove in the core  54 , such as is illustrated in  FIG. 1 . 
     The following components extend radially outwardly from the key block  70 : a rectangular friction member or friction washer  74 , a laminated spring bar  60  further comprising spring bar components  60 A and  60 B, and a load bearing plate  80 . The spring bar components  60 A and  60 B are similarly shaped and in surface-to-surface contact. 
     These identified components are removably coupled by bolts  90 , mating washers  92  and load-indicating washers  93 . The bolts  90  extend through each of the components and are threadably received in threaded holes in the key bar  100 . The load indicator washers  93  comprise projections on at least one surface thereof that are deformed or crushed into a flat shape when a desired torque is applied to the bolts  90 . 
     In other embodiments more than the two illustrated bolts  90  and washers  92  and  93  may be used to couple these components. 
     Coupling the key block  70  to the spring bar  60  transmits the core forces (weight and forces developed during operation) from the core to the spring bar then to the frame rings and finally to the generator frame. 
     Top and bottom surfaces of the friction member  74  are formed to provide a desired frictional force (e.g., having a desired coefficient of friction when used between the laminated spring bar  60  and the key block  70 ) to maintain the torque applied by the bolts  90 . 
     In the illustrated embodiment of  FIGS. 3 and 4  the spring bar  60  comprises two laminated spring bar subcomponents  60 A and  60 B placed in aligned contact to form the spring bar  60 . Other embodiments may comprise more or fewer than two laminated spring bar subcomponents forming the spring bar  60 . However in certain applications it may be difficult to use a single spring bar in place of the two laminated spring bars  60 A and  60 B when these components are installed in-situ in an existing generator. 
     A spring bar span is generally defined as an axial distance between two consecutive frame rings  75 A and  75 B. The inventors have determined that shortening the span stiffens the spring bar and modifies the natural frequency of the core-to-frame attachment components. 
     A plurality of bolts  98  is disposed along a span to couple spring bar subcomponents  60 A and  60 B. The  FIG. 3  embodiment illustrates four bolts between the frame ring  75 A and the load bearing plate  80  and four bolts between the frame ring  75 B and the load bearing plate  80 . Other embodiments may have more or fewer than the illustrated number of bolts. 
     Each bolt  98  passes through the spring bar subcomponents  60 A and  60 B and threadably engages a mating nut hidden from view in  FIG. 3 . In one embodiment each of the bolts  98  comprises a shoulder bolt. The inventors have also determined that clamping the spring bar components modifies the natural frequency of the core-to-frame attachment components. 
     In addition to or in lieu of the bolts  98 , the spring bar subcomponents  60 A and  60 B are attached together by spot weld joints  99 . Preferably the spot weld joins  99  are formed by first forming one or more openings through the spring bar subcomponents  60 A and  60 B then forming a spot weld joint  99  within each opening. 
     Depending on a natural frequency of the core-to-frame attachment system, certain embodiments may use only the bolts  98 , other embodiments may use only the spot welds  99 , and still other embodiments may use both the bolts  98  and the spot welds  99 . 
     The coupled interface between the spring bar  60  and the key bar  70  affects the stiffness properties of the core-to-frame attachment system. For example, a longer key block shortens the span and thus provides additional stiffness. The distance between bolts  98  can also be modified to change the span length and the stiffness properties of the system. 
     An (inverted) T block  110  couples an end  111  of the laminated spring bar  60  to a proximate U-plate  57 . An opposing end  112  of the spring bar  60  similarly couples a T-block  110  to a proximate U-plate  57 . In the illustrated embodiment weld joints  122  are used as the coupling component. 
     The U-plates  57  are welded to respective frame rings  75 A and  75 B at weld joints  123 . Since the U-plate is welded to a frame ring, the U-plate is an integral part of the frame ring and the spring bar span may instead be defined as the distance between two consecutive U-plates. 
     As also illustrated in  FIG. 3 , the T blocks  110  are welded to the spring bar member  60 B at weld joints  114  and also coupled to the spring bar  60  by bolts  118  and mating nuts (the nuts not seen in  FIG. 3 ). 
     The T blocks  110  are not present in the prior art and the spring bar  60  is instead fixed to each U-plate. Thus the spring bar span is defined as the distance between consecutive U-plates. The present invention adds the T blocks  110  and by welding the T blocks to the U-plates shortens the span to a distance between the T blocks (or more precisely to the distance between the end points of the weld joints that couple the T blocks to the U-plate). 
     Although the key bars, spring bars and their associated components span an entire axial length of the core,  FIG. 3  illustrates a first span and the T block  110  of a second span. Thus according to one embodiment, each span comprises two T blocks  110  (one at each end of the span), one key block  70 , one rectangular friction member  74 , one load bearing plate  80  and other associated components disposed along the span. 
     With reference to  FIG. 3 , each spring bar span further defines two diamond-shaped openings  130 , with a variable number of these openings (two illustrated in  FIG. 3 ) on each side of the key block/load bearing plate assembly. 
     Generally, about fifteen spring bars and associated components are disposed circumferentially around the generator core  54 . 
     The coupling components of the present invention tune the natural frequency of the core support system to a frequency significantly different (a greater frequency in one embodiment) than the resonant frequencies of the generator. This is accomplished by controlling a length of the spans (which in turn controls a distance between the T blocks) and/or employing additional coupling subcomponents (e.g., the bolts  98  and/or the spot welds  99  for attaching the spring bar subcomponents  60 A and  60 B). For example, shortening the span and/or using additional coupling components to couple the spring bar subcomponents  60 A and  60 B increases the radial stiffness of the core-to frame attachment system. The weld joints  114  and  122  of  FIG. 3  are also important load-carrying members. 
     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.