Abstract:
A new stator core cooling concept is provided that can be implemented with a conventional core structure having cooling ducts interspersed at intervals along the axial length of the stator core. To enhance cooling of the stator coils, the cooling ducts are radially closed off so that cooling air is forced to flow circumferentially, preferably through small holes, channels, or slots, to impinge directly on the armature bar surface. Accordingly, rather than flowing cooling gas through the cooling ducts and immediately discharging it into the annular gap between the rotor and stator, cooled gas flows radially down through the cooling duct and then must flow circumferentially whereupon it impinges directly on the armature bar surface itself.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to gas cooled generator stators and, more particularly, to a structure and method for impingement cooling of generator stator coils. 
     During the process of producing electricity, power generators also create heat that must be dissipated away from the generator. Generators are typically gas-cooled by ventilated cooling systems that circulate cooling gasesthrough ducts in the rotor and stator. 
     By way of example, FIG. 1 shows a cross-section of one-half of a generator  10 , (see axial center line  12  and longitudinal center-line  14 ) havinga reverse flow ventilated cooling system. In this example, a portion  16  of the flow of cooling gases is directed to the rotor  18 . The cooling gases are drawn through ventilation ducts  20  in the rotor by centrifugal forces created by the spinning rotor. As the gases flow through the rotor, heat in the rotor is transferred to the gases. Thus, heated gas exits the rotor ducts  20  at the surface of the rotor  18  and enters an air gap  22  between the rotor  18  and stator  24 . Fans  26  mounted at the ends of the rotor  18  (only one of which is shown in FIG. 1) draw these heated gases through the annular gap  22  and direct the same via an external duct  28  to a heat exchanger  30  for cooling the gas. 
     The stator  24  is cooled by ventilation flow path(s) that are separate from the flow paths in the rotor  18 . Gas  32  cooled by the heat exchanger  30  enters a plenum chamber  34  surrounding the stator  24 . Cooling gas tends to flow in greater volume and velocity through stator ducts near the ends of the stator because the end sections of the stator are closest to the exhaust fans  26 . This potential imbalance in the flow of cooling gas through the stator is preferably compensated for by varying the spacing and cross-sectional area of stator cooling ducts along the length of the stator to optimize the distribution of cooling gases through the stator and minimize the necessary pressure head needed for the cooling gases. The cooled gas flows to the stator outer circumferential surface  38  and into the cooling ducts  40  defined between the packets  42  of stator core laminations. 
     Referring to FIG. 2, the armature bars  44 ,  46  are secured in the stator coil slot  48  with filler  50 , top-ripple spring(s)  52 , and stator wedge  53  to restrain the bars radially, and with side-ripple spring(s)  54  to increase friction between the bars  44 ,  46  and the side walls of the slot  48 . Referring to the lower armature bar  44  for convenience, the heat that is generated in the copper strands  56  of the armature bar  44  is thermally conducted along a heat flow path  58  from the strands  56 , through a layer of insulation  60  to the side walls of the slot  48 . 
     As noted above, cooling ducts  40  are incorporated into the stacks of laminations defining the stator core  24 . Referring to FIG. 3, space blocks  62 ,  64  are provided axially between adjacent stator core packets  42  for defining the axial dimensions of the ducts  40  and for directing the cooling air flow radially through the stator  24 . As the gas flows radially inwardly through the stator  24 , heat from the stator coils  44 ,  46  is transferred to the gas. In conventional systems, the cooling gas ducts  40  are open ended so that the cooling gas flows radially directly into the annular gap  22  between the rotor  18  and stator  24  and then flows axially along that gap under the influence of the fans  26  for return to the heat exchanger  30 . 
     As is apparent from the foregoing, the current-carrying copper conductors  56  of the typical stator coil/armature bar  44  are indirectly cooled. That is, in systems of the type described above, the coolant does not directly contact the current-carrying copper conductors  56  of the armature bar  44 , nor indeed most of the bar  44 . Instead, there is a thermal conduction path  58  from the armature bar  44  to the walls of the stator slot(s)  48 . Thereafter the heat must be conducted through the lamination packet  42  to the adjacent cooling duct(s)  40 . 
     This thermal conduction path  58 , however, includes regions of imperfect contact between, e.g., the armature bar  44  and the side walls of the slot  48 . The imperfect contact is inherent in the assembly of the multiple components. For example, because of the nature of the armature bar  44 , it is not perfectly flat. Moreover, because of the assembly tolerance of the laminations that define the packets  42 , the stack of laminations from which the core  24  is made may not align perfectly, so the slots  48  are not perfectly straight. More specifically, laminations may be slightly rotated clockwise or counterclockwise relative to a next adjacent lamination. Dead-air spaces are formed when individual adjacent laminations are slightly offset from each other in the peripheral direction. As a result, there are voids in the thermal conduction path, referred to as lamination stagger. Voids and imperfect contact of the type described above cause increased thermal resistance between the bar, which is the source of heat, and the cooling duct which is where the heat is taken away. High thermal resistance results in a higher operating temperature of the armature bar, which limits the output performance of the generator, since there is an imposed limit on the bar operating temperature. 
     In addition to the aforementioned imperfect thermal conduction paths in/along the stator slot, there is a further thermal resistance conduction path through the stack of stator core laminations. More specifically, once the heat is conducted through the thermal-contact resistance in the slot, heat needs to flow peripherally, radially and/or axially through the stack of iron laminations eventually to the cooling duct surface. The axial conduction path in particular presents a high resistance to heat flow since there is thermal contact resistance and often an enamel layer in between individual laminations in the stack. 
     BRIEF SUMMARY OF THE INVENTION 
     In view of the problems and inefficiencies observed with the traditional methods of heat removal, a more effective heat removal path would be highly desirable. The end result of a more effective heat removal path would be a lower operating temperature for the stator armature bar. In that regard, the power output for a generator is limited by how hot the armature bar can get. Indeed, the stator RTD temperature is monitored by the customer and will limit the output of the machine. If the stator bar can be cooled more effectively, the machine can produce more output for the same size unit. This in turn results in an increase in power density which realizes a cost reduction because more power can be made onto the grid with a smaller size machine. 
     The invention is embodied in a new stator core cooling concept that can be implemented with the basic core structure described above, having cooling ducts interspersed at intervals along the axial length of the stator core. More particularly, the concept of the invention is embodied in a stator core in which cooling ducts are radially closed off. Thus, cooling air is forced to flow circumferentially preferably through small holes, channels, or slots to impinge directly on the armature bar surface. Thus, rather than flowing cooling gas through the cooling ducts and immediately discharging it, cooling gas flows radially through the cooling duct and then must flow circumferentially whereupon it impinges directly on the armature bar surface itself. 
     In a preferred embodiment of the invention, once the cooling gas impinges on the armature bar surface, it is directed to flow axially, along at least a portion of the length of the armature bar. Advantageously, the axial flow extends into the lamination packet. The cooling gas flow may then be directed, for example, into the gap between the stator and rotor and through aperture(s) in the stator wedge. 
     Accordingly, the invention is embodied in an electromagnetic generator that comprises a stator structure concentrically disposed to a rotor structure, the stator structure including stator core packets annularly disposed around the rotor and radial stator cooling ducts defined between the stator core packets, with each of the stator cooling ducts extending radially between the outer stator surface and the inner stator surface proximate the rotor, the stator structure further defining a plurality of axially extending stator slots and tooth portions circumferentially between the slots, and a baffle structure having part circumferential portions for intercepting radial cooling air flow through the cooling ducts and redirecting the flow in a circumferential direction. 
     The invention is further embodied in a method of impingement cooling a stator coil that comprises providing a stator structure having a radially outer surface and a radially inner surface, and including stator core packets and radial stator cooling ducts defined between the stator core packets, each of the stator cooling ducts extending radially between the outer stator surface and the inner stator surface, the stator structure further defining a plurality of axially extending stator slots and tooth portions circumferentially between the slots; flowing cooling air radially inwardly through the cooling ducts; intercepting the radial cooling air flow in a vicinity of a tooth portion of the stator structure; and directing the intercepted cooling air circumferentially onto at least one stator coil disposed in an adjacent slot of the stator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These, as well as other objects and advantages of this invention, will be more completely understood and appreciated by careful study of the following more detailed description of the presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic cross-sectional diagram of a generator showing a reverse flow cooling system; 
     FIG. 2 is a broken away, cross-sectional view of a conventional stator slot illustrating the conventional thermal-conduction path from armature bar to stator slot to lamination; 
     FIG. 3 is a broken away, cross-sectional view taken along line A—A in FIG. 1 showing a conventional cooling duct; 
     FIG. 4 is a broken away, cross-sectional view taken along line A—A in FIG. 1 showing a structure for providing impingement cooling in accordance with the present invention; and 
     FIG. 5 is a view similar to FIG. 4 but taken along line  5 — 5  in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is embodied in a cooling system wherein rather than depending solely upon the conduction of armature bar heat laterally from the armature bar through the insulation wall, side ripple spring(s), if present, to the cooling duct, and heat conduction through the stator slot and laminations to the cooling duct surface, the cooling gas is directed onto and along the armature bar surface, preferably through cooling jet or nozzle openings. Since the flow is directed onto the armature bar, the system of the invention utilizes impingement cooling. The heat transfer co-efficient associated with directly impinging jets is several times higher than normal through-flow duct coefficients, so that the armature bar realizes much higher heat transfer. After the cooling jets impinge on the sides of the bar, the gas flow continues through axial cooling channels incorporated in the stator slot so that the cooling gas flows along the sides of the bar and continues to cool it. The flow must ultimately be discharged from the stator slot into the annular gap between the rotor and stator. In the presently preferred embodiment, the heated cooling gas discharges through the stator wedge structure into the annular gap between the rotor and the stator. The discharge hole in the stator wedge is oriented such that exit gas flow impinges a second time, on the side of the stator tooth tip surface, which is a second source of significant heating. Once the cooling air discharges through the stator wedge onto the tooth surface, the flow exits through the annular gap to the cooling fan mounted on the end of the rotor, for exhausting hot gas from the interior of the machine. 
     With reference to FIGS. 4-5, the cooling flow path in accordance with the invention from where it enters the stator core, to where the cooling work is primarily effected, to where it exits, will now be described with greater specificity. For convenience, the components of the inventive system that correspond to those of the conventional systems are identified by the same or similar reference numerals incremented by  100 . 
     The cooling gas enters from the radially outer surface of the stator core (not shown in FIG. 4, but illustrated at  38  in FIG.  1 ). That cooling gas enters the ducts  140  defined circumferentially by pairs of space blocks  162 . As discussed above, the cooling gas duct  140  is formed axially between axially adjacent packets  142  of laminations that the space blocks  162  separate and the gas flows through the cooling gas duct, radially inwardly of the stator core  124 . As the cooling gas flows through this duct, some heat is removed as the gas passes through the stator core yoke  125  since the stator core yoke  125  is an active electromagnetic component where heat is generated. The cooling flow continues radially down into the tooth portion  127  of the duct where it continues to remove heat since the stator core teeth  127  are also active electromagnetic components. The cooling flow now approaches a serpentine space block  166  which redirects the cooling flow to realize the improved cooling effect of the present invention. 
     The serpentine spacer  166 , as illustrated in particular in FIG. 4, traverses the periphery of the stator slot  148  and bridges the radially inner periphery  149  of the cooling air duct  140  between adjacent stator slots  148 , thereby to preclude radial flow of cooling air into the gap  122  between the rotor (not shown in FIG. 4) and stator  124 . More specifically, the spacer  166  includes circumferential portions  168 ,  169  and radial portions  170 ,  171 . The circumferential portions  168  intercept radial cooling air flow and direct it circumferentially, toward a respective adjacent stator slot  148 . 
     As illustrated in FIG. 4, typically two armature bars, lower armature bar  144  and upper armature bar  146  are provided inside each stator slot  148 . The cooling gas flows from the tooth portion  127  through apertures hereinafter referred to as impingement holes  172 , defined in the radial portions  170  of the serpentine spacer  166 . The cooling gas passes through the impingement holes as cooling jets that impinge on the surface of the upper and/or lower armature bars  144 ,  146 , directly cooling those bars, which are the primary sources of the stator heat. In an exemplary embodiment, the impingement holes are small diameter holes that form nozzles in the serpentine spacer. However, rather than drilled holes, milled slots may be provided for manufacturing simplicity, since adjacent laminations  142  of the stator core form the closure for such nozzles, thereby forming a closed passage for the impingement jet passages. The slots or holes could also be formed in a molded non-metallic piece integrated with a serpentine partition. In the illustrated embodiment, the impingement holes  172  are shown in the radial portion  170  of the serpentine spacer  166  on only one side of the armature bars  144 ,  146 , however, holes may be provided in either or both radial portions  170 ,  171  bordering the armature bars  144 ,  146 . Also, the cooling jets may be disposed to direct cooling air onto either or both of the lower and upper armature bars  144 ,  146 , and may be disposed to direct cooling air onto the radial end of bar  144  through circumferential portions  169  as shown in FIG.  4 . Where holes  172  are provided on both radial portions of each slot, a space block similar to space block  64  (FIG. 3) may be provided between adjacent slots  148  to divide and direct cooling gas to the holes  172  of each radial portion  170 ,  171 . 
     The cooling gas that flows through the impingement holes  172  impinges on and directly cools the lower and/or upper bars  144 ,  146 , as noted above. In the presently preferred embodiment, after the gas jets impinge on the lower and/or upper bars, the cooling air flow progresses axially through axial flow path(s)  174 , for example, along at least a portion of the length of the armature bar(s)  144 ,  146 . To provide for cooling gas impingement and define axial flow path(s), a plurality of projections  176  project generally circumferentially from the radial portion(s)  170  of the serpentine spacer  166  to define a gap between the radial portion  170  and the armature bar(s)  144 ,  146 . The projections may be in the form of projecting elongated ribs and/or discreet dimples for defining axial flow path(s)  174  and/or radial flow path(s) for the cooling air. 
     As noted above, the cooling flow preferably progresses axially along at least a portion of the length of the stator coils. To provide for such axial flow in the illustrated embodiment, the impingement holes  172  are defined in the cooling duct  140  (as illustrated in FIG. 4) and axial downstream therefrom, e.g., within the lamination packet  142 , discharge hole(s)  178  is/are defined in the stator wedge  153 , as shown in FIG.  5 . Since core loss, that is magnetic hysteresis loss, is generated in the tooth tip  129 , it is advantageously cooled in the illustrated embodiment. Accordingly, the discharge hole(s)  178  may be oriented in such a way that the cooling gas now impinges on the sides of the tip  129  of the stator tooth  127 . The stator tooth tip is thus the last portion of the stator core to be washed by cooling gases. 
     Once the exiting cooling gas impinges on the tooth tip, it is discharged into the air gap  122  which is the annular gap between the stator radially inner surface  180  and the rotor outer diameter surface (shown in FIG.  1 ). The cooling gas discharging into the air gap travels axially to the exit of the cooling circuit, typically by means of the cooling fan mounted on the end of the rotor that is exhausting hot gas from the interior of the machine. 
     As is evident from the foregoing, along the axial length of the stator core there will be a multiplicity of exiting jets with respect to air flow through the cooling ducts that have cooled small increments of the axial length of the stator coils/stator core. Thus, there is a repeating pattern provided with the cooling system described hereinabove composed of radial inflow through the cooling ducts, impingement jets on the armature bar surfaces, axial flow through channels along side of the bars, discharge through hole(s) in the stator wedge, preferably oriented to impinge on the tooth tip, and exiting into the air gap. It is preferred that cooling packets of this type be relatively short in the axial direction since a greater number of short packets results in minimum gas temperature rise through each packet and significant surface area exposed to cooling. Since typically in machines of the type described hereinabove, the cooling patterns are symmetrical about the longitudinal center line, half the total cooling flow will discharge from each end of the machine. 
     In the illustrated embodiment, the serpentine space blocks are provided at the radially inner end of otherwise generally conventional cooling air ducts, thereby to direct the radial flow circumferentially as impingement jets onto the stator coils. In the embodiment illustrated in FIG. 4, the space block that conventionally extended into the tooth is omitted. However, that space block may be retained and either terminate with the circumferentially adjacent space blocks  162  or when, for example, impingement holes are defined on both radial portions  170 ,  171  of the serpentine spacer, can extend into the tooth  127  as in the conventional structure, to divide cooling flow between those radial portions  170 ,  171  of the serpentine spacer and thus between adjacent slots  148 . To facilitate cooling flow along the requisite portion of the axial length of the stator core packets, side ripple springs or other fillers may be provided at suitable axial and/or radial portions of the stator slots for defining axial cooling flow paths between the bar(s) and the slot wall(s). 
     While the invention has been described with reference to a cooling system having radial inflow sections, the invention could be applied to a system having radial outflow sections. In that regard, to be adapted to radial outflow sections, rather than providing part circumferential portions  168  at the radially innermost end of the tooth, corresponding part circumferential portions would be provided at the top end of the respective stator tooth, generally circumerentially adjacent part circumferential portions  169 , with walls corresponding to radial portions  170  and  171  projecting radially inwardly therefrom. With such an assembly, the cooling air would flow radially outwardly, be redirected to flow circumferentially through impingement holes onto the armature bars, and then flow axially along the armature bars to exit into the next axially downstream duct, e.g. through perforations in the circumferential wall portion  169 . 
     Thus, while the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.