Abstract:
An electrical generator ( 80 ) utilizes supplemental blowers or fans ( 140 ) to move additional cooling fluid through coolant flow paths ( 116, 124 ) in the generator, thus reducing the internal temperature of the generator. The supplemental blower comprises a blower ( 160 ) for supplementing cooling fluid flow through a discharge coolant flow path ( 155 ) and/or a supplemental blower ( 176, 178 ) for supplementing cooling fluid flow through an inlet coolant flow path ( 156 ). Either supplemental blower has the effect of lowering the peak internal generator temperature ( 134 ).

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to an apparatus for removing heat from electrical generators, and more specifically to the use of external blowers or fans to improve heat removal from electrical generators. 
     BACKGROUND OF THE INVENTION 
     As shown in  FIG. 1 , conventionally an electric generator  10  comprises a rotor  12  carrying axial field or rotor windings  13  producing a magnetic flux field that rotates within a stationary armature or stator  14 . One end  15  of the rotor  12  is drivingly coupled to a steam or gas driven turbine (not shown in  FIG. 1 ) for providing rotational energy to turn the rotor  12 . The opposing end  16  is coupled to an exciter (not shown) for providing the direct current carried by the rotor windings  13 . 
     The stator  14  comprises a core  17  including a plurality of thin, high-permeability circumferentially slotted laminations placed in a side-by-side orientation and insulated from each other to reduce eddy current losses. Stator coils  18  (see  FIG. 3 ) are wound in the inwardly directed slots of the stator core  17 . Alternating current is generated in the stator windings  18  by the action of the rotating magnetic filed emanating from the rotor windings  13 . The current is carried to the main leads  19  for connection to an external electrical load. 
     The rotor  12  and the stator  14  are enclosed within a frame  20 . Each rotor end comprises a bearing journal for mating with bearings  30  attached to the frame  20 . The rotor  12  further carries a blower  32  for forcing cooling fluid through the generator elements as described further below. The cooling fluid is retained within the generator  10  by seals  34  located where the rotor ends penetrate the frame  20 . The generator  10  further comprises coolers  36  through which the cooling fluid passes to release the heat absorbed from the generator components, after which the cooling fluid is recirculated through the generator elements. 
       FIG. 2  further illustrates the elements of the rotor  12 , including ventilation slots  50  on opposing ends of the rotor  12  and ventilation ports  52  located near the rotor center. As will be described further below, cooling fluid passes through the ventilation slots  50  and the ventilation ports  52  for cooling the rotor  12 . The direct current generated by the exciter is coupled to the rotor windings through axial leads  54 . The coil ends are held in place by retaining rings  56 . 
       FIG. 3  is a cross-sectional view of the stator  14 , illustrating the various components described herein, including a face  60  of one lamination of the stator core  17  and the inwardly directed slots  62  carrying the stator coils  18 . A somewhat distorted perspective view of the center region of stator  14  is provided to illustrate stator coils  18  and slots  62  extending along the axial length of stator  14 . 
     Generator cooling is required to remove the heat energy produced by electrical losses resulting from the large currents flowing through the generator conductors, including the direct current flowing through the rotor windings  13 , and the alternating current induced in the stator coils  18 . Mechanical losses such as windage caused by the spinning rotor  12  and friction at the bearings  30  are also heat sources. 
     As generator electrical output ratings increase, additional heat is generated within the generator and must be removed through the use of more effective cooling systems. Generally, as the heat removal requirements increase, the basic premise of the cooling system operation progresses from air cooling, to hydrogen cooling, to hydrogen inner cooling, and finally, to cooling the stator with flowing water. Certain of these cooling techniques can also be used in combination, and there are multiple variations for each cooling system. Each cooling system type is described briefly below. 
     Air-cooled generators can be configured as either open or closed. Open air-cooled generators use outside air. The air passes through the generator components only once, after which it is exhausted back outside the generator. Closed air-cooled cooling systems include a heat exchanger, also referred to as a cooler, for cooling the heated airflow after it has passed through the generator. The cooled air is then recirculated back through the generator. Cold water is pumped through tubes of the heat exchanger over which the hot air passes, transferring heat from the air to the water. 
     Although air can be used as the cooling fluid, hydrogen is preferred as it possess excellent thermodynamic and heat transport properties, is lighter than air, and is 10 to 20 times more efficient as a cooling medium than air. One important negative aspect of hydrogen cooling is the explosive mixture formed by hydrogen and air over a wide range of hydrogen concentrations. Therefore, the seals  34  are provided at the boundaries of the generator frame  20  to prevent hydrogen leakage. 
     Hydrogen cooled generators are subdivided into two groups, conventional-cooled and inner-cooled. In the conventional system, the hydrogen flow removes excess heat energy from the rotor  12  and stator  14  by circulating hydrogen around and through coolant paths within and proximate the generator components, including especially the rotor  12  and the stator  14 . The blower  32  creates high and low pressure zones within the generator, establishing hydrogen gas flow paths from high-pressure zones to low-pressure zones to remove heat from the generator components. 
     To provide rotor cooling, the hydrogen is directed through channels (not shown) in the hollow rotor windings, entering at the ventilation slots  50  at both rotor ends and exiting into a rotor/stator gap  64  (see  FIG. 1 ) via the ventilation vents  52  in a mid-region of the rotor  12 . The flow continues in the gap  64  as the hydrogen flows back to hydrogen coolers  36 . 
     Cooling hydrogen is also directed through the stator core  16  in both the radial and the axial directions. In radially cooled generators, the blower  32  causes hydrogen to flow axially along the outside of the stator  14  then radially inwardly toward the gap  64  (due to the lower pressure within the gap  64 ) through openings or vents between certain of the stator core laminations. The hydrogen in the gap  64  is directed back to the coolers  36  where the absorbed heat is removed, and the hydrogen is then recirculated back through the generator  10 . In an axially cooled stator, the hydrogen flow is directed from the coolers  36  axially along the outside of the stator  14  to the opposite end of the stator  14  then through axial ducts in the stator core  16  back toward the coolers  36 . 
     As mentioned, the heated hydrogen flow exiting from the stator  14  and rotor  12  is directed by the blower  32  through the hydrogen coolers  36  mounted at the turbine end of the generator frame  20 . Within the coolers  36  the hydrogen is cooled as it passes over water filled tubes. The cooler hydrogen flow is recirculated to continue the heat removal process. 
     A hydrogen inner-cooled system includes cooling ducts in the form of hollow passages in the stator coils  18 , in addition to the axial or radial stator cooling ducts in the stator core  16  as described above. As the hydrogen passes through the cooling ducts, heat is absorbed from the conductors of the stator coils  18 . 
     In a water cooling system the rotor  12  and stator core  16  are cooled with hydrogen as described above, while the stator coils  18  are cooled by pumping water through hollow conductors forming the stator coils  18 . The water is cooled by outboard heat exchangers and recirculates through the stator coils  18 . 
     BRIEF SUMMARY OF THE INVENTION 
     An electric generator includes supplemental cooling devices to lower the temperature of the generator elements and in particular to lower hot spot temperatures within the stator core. The generator includes the conventional stator core carrying a plurality of stator coils. The rotating rotor windings are responsive to the externally applied current for generating current in the stator coils by magnetic induction. To cool the generator components, cooling fluid (typically air or hydrogen) is forced into the generator by primary blowers affixed to opposing ends of the rotating rotor. The blowers cause the cooling fluid to traverse one or more coolant flow paths proximate to one or more of the stator core, the plurality of stator coils, the rotor shaft and the plurality of rotor windings. A coolant outlet discharges the cooling fluid from the generator, or the cooling fluid is cooled, by passing through a cooling unit, and then recirculated through the generator. Supplemental discharge and inlet blowers proximate discharge and inlet zones within the stator core provide additional cooling fluid to lower the internal temperature of the generator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features of the invention will be apparent from the following more particular description of the invention, as illustrated in the accompanying drawings, in which like reference characters refer to the same parts throughout the different figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
         FIG. 1  is a cross-section view of a prior art electric generator; 
         FIG. 2  is a pictorial illustration of a rotor of the electric generator of  FIG. 1 ; 
         FIG. 3  is a pictorial cross-section illustration of a stator of the electric generator of  FIG. 1 ; and 
         FIGS. 4 through 10  are schematic illustrations of generator cooling systems constructed according to the teachings of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Before describing in detail the particular external zone ventilation system for electric generators in accordance with the present invention, it should be observed that the present invention resides primarily in a novel combination of hardware elements related thereto. Accordingly, the hardware elements have been represented by conventional elements in the drawings, showing only those specific details that are pertinent to the present invention, so as not to obscure the disclosure with structural details that will be readily apparent to those skilled in the art having the benefit of the description herein. 
       FIG. 4  illustrates, in schematic form, a generator  80  to which the teachings of the present invention can be applied. The generator  80 , referred to as an air-cooled generator, is cooled by filtered air drawn into the generator interior through filters and inlet silencers. According to the prior art, the air is exhausted from the generator  80  through exhaust silencers conventionally located at the top of the generator, or through coolers where the air is cooled for recirculation through the generator. 
     The generator  80  comprises a rotor  82 , drivingly coupled to a turbine (not shown) at an end  84 , and at an end  86  coupled to an exciter (not shown) for supplying direct current to the rotor windings. As described above, the rotating magnetic field creates a current flow in stator windings located within a stator core  87 . 
     Blowers  88 A and  88 B, mounted for rotation on a rotor shaft  90 , comprise a plurality of blades for circulating cooling air through the generator  80  as the rotor shaft  90  rotates. In one embodiment, the blowers  88 A and  88 B draw cool air into the generator  80  via inlet flow paths  92 . In this embodiment, the cooling air is cooled below ambient by air-to-water coolers, for example, and drawn into the generator  80  via the inlet flow paths  92 . In another embodiment, the heated air discharged from the generator  80  is circulated through coolers and recirculated back into the generator  80 . 
     Rotation of the blowers  88 A and  88 B creates a plurality of pressure zones within the generator  80 , which cause cooling air flow, as indicated by the arrowheads in FIG.  4 . In one coolant flow path, cooling airflows from the blower  88 B into a rotor end slot  104 , through the hollow rotor windings, exiting the rotor  82  at rotor vents  106 . In another coolant flow path airflows from the blower  88 B axially through a rotor/stator gap  114  then radially outward through radial core vents  116  between laminations of the stator core  87 . After passing through the radial core vents  116 , the heated cooling fluid is collected by a duct  117  and discharged from the generator  80  via a discharge port  118 . In one embodiment the heated air is exhausted to the atmosphere. In another embodiment the heated air passes through a cooler for recirculation back into the generator  80  via the inlet flow paths  92 . 
     In a coolant flow path  120  the cooling air flows axially through a duct  123  and radially inward through an inlet vent  124  in the stator core  87 . Once in the rotor/stator gap  114 , the cooling air from the inlet vent  124  flows radially outward through the radial core vents  116  as indicated by the arrowheads in FIG.  4 . Note the duct  123 , and the other duct elements described below are incorporated within or external to the generator housing, i.e., the ducts are outside the electrically active elements of the generator  80 . 
     The blower  88 A creates similar coolant flow paths on the opposing end of the generator  80 . In one coolant flow path, cooling air flows from the blower  88 A into a rotor end slot  126 , through the hollow rotor windings, exiting the rotor  82  at the rotor vents  106 . In another coolant flow path air flows from the blower  88 A axially through the rotor/stator gap  114  then radially outward through the radial core vents  116  between laminations of the stator core  87 . In a coolant flow path  128  the cooling air flows axially within a duct  129  and radially inward through the inlet vent  124  in the stator core  87 . Once in the rotor/stator gap  114 , the cooling air flows radially outward through the radial core vents  116 . 
     Similar cooling flow paths are created in a left segment  130  of the stator core  87 . These cooling flow paths are not shown in FIG.  4 . Also, it is recognized by those skilled in the art that the various cooling flow paths illustrated in  FIG. 4  are not discrete and independent as might be suggested from the use of arrowheads to represent the cooling paths. There is considerable mixing and converging of the various cooling paths depending on the operational parameters and design of the generator  80 . Thus the arrowheads are intended only to generally indicate the cooling flow paths. 
     In the description of the coolant flow paths presented above, the hottest stator areas, represented generally by reference characters  134 , tend to be in a region between the inlet vent  124  and the opposing ends of the stator core  87 . The coolant flowing in these regions has absorbed heat during traversal of the coolant paths  120  and  128 , during flow through the inlet vent  124  and through the rotor/stator gap  114 . Thus the coolant has a reduced capacity to substantially cool the stator core  87  in the areas  134 . Since the generator current capacity is typically limited by the temperature of the hottest stator regions, it is desirable to lower the temperature of the areas  134 , thus allowing an increase in the generator capacity. 
     According to the teachings of the present invention, the hot spot temperatures can be reduced by the placement of a supplemental external cooling blower  136  in the region of the inlet vent  124 . A similar supplemental external cooling blower is located in the left segment  130  of the stator core  87 . In one embodiment, the external cooling blower  136  comprises blades caused to rotate by the action of a motor to which electricity is supplied for imparting the rotational energy. External connotes that the blower  136  (and the other blowers described as “external” herein) is mounted within the ventilation ducts and is powered from a separate energy source, as compared with the blowers  88 A and  88 B, which are powered by rotation of the rotor shaft  90 . The blower  136  can be provided as original equipment or added to the generator  80  after it has been placed in service if a capacity increase is required to supply the expected loads. 
     The blower  136  provides additional coolant flow through the inlet vent  124 , which in turn lowers the temperature in the hottest regions  134  and allows an increase in the generator capacity. In one embodiment, the blower  136  supplies ambient cooling air or air cooled below ambient by a heat exchanger arrangement not shown in  FIG. 4 , to the inlet vent  124 . In another embodiment, the blower  136  is not externally vented, but instead draws additional air into the inlet vent  124  from the coolant flow paths  120  and  128  to increase the cooling capacity of the coolant flow. Further, rather than a single blower  136  as shown in  FIG. 4 , one blower (not shown in  FIG. 4 ) can be positioned in each of the ducts  123  and  129  to provide additional coolant flow through the inlet vent  124 . These blowers can be externally vented and responsive to ambient or cooled air, or can draw additional air into the inlet vent  124  from the coolant paths  120  and  128 . 
     In one embodiment, calculations showed that use of the blower  136  increased the generator capacity by about 5% to 15%. Note that the blower  136  is not required for normal operation of the generator  80  (thus it is referred to as a supplemental blower) and therefore can be placed in service only when additional generating capacity is required. 
     In another embodiment as shown in  FIG. 5 , a supplemental external motor-driven blower or fan  138  is positioned in the duct  117  that receives discharged coolant from the radial core vents  116 . By increasing the coolant flow through the radial core vents  116 , the temperature in the regions of the stator core  87  proximate the radial core vents  116  is reduced. 
     In another embodiment as shown in  FIG. 6 , the inlet flow paths  92  are supplemented with an external inlet blower  140  that operates in series with the blowers  88 A and  88 B, boosting the inlet air pressure to supply additional cooling air for the generator  80 . By supplying additional coolant flow to the rotor  80  and the stator core  87 , hot spot temperatures can be reduced and the operating range of the generator  80  increased. 
     The embodiments illustrated in  FIGS. 4 ,  5  and  6  can be used in various combinations or singularly to reduce the temperature in various regions of the generator  80 . 
     The generator cooling arrangement illustrated in  FIGS. 4 ,  5  and  6  is referred to as three-zone cooling, i.e., two outlet or discharge zones through the radial core vents  116  on opposing sides of the stator core  87  and an inlet zone through the inlet vent  124 . A five-zone cooling arrangement is illustrated by a generator  150  of  FIG. 7  (and FIG.  8 ). In this cooling arrangement the stator core  87  comprises radial core discharge vents  154  and  155  carrying coolant in a radially outward flow through ducts  162 ,  168  and  170  as shown. Radial core inlet vents  156  carry coolant radially inward to the rotor/stator gap  114  from which the coolant is directed axially along the rotor  82  and radially outward through the radial core discharge vents  154  and  155 . 
     The cooling fluid discharged through the radial core discharge vent  155  tends to be the hottest of the cooling paths in the generator  150  because it has absorbed heat from the rotor  82 , the rotor/stator gap  114  and the stator core  87 . Also, due to the many divergent cooling paths within the generator  150 , the coolant flow volume through the center is lower. As a result, a hot spot tends to develop in the area of the radial core discharge vent  155 . Since the generator temperature, and especially the hot spot temperature, limits the allowable generator output due to the potential for overheating and possible component damage, it is desired to reduce the hot spot temperatures. 
     According to the teachings of the present invention, a supplemental external motor-driven fan or blower  160  is positioned in the duct  162  that receives cooling fluid from the radial core discharge vent  155  and adjacent radial core discharge vents  154 , to draw more cooling air through this discharge region. As a result, the temperature in this region declines. Note that the blower  160  is not powered by rotation of the rotor shaft  90  and thus is referred to as a supplemental external blower. Typically, such a blower would be powered by a motor. Ducts  168  and  170  also discharge heated air from the generator  150 . 
     The discharge from the blower  160  can be externally vented, or cooled using a heat exchanger, not shown in  FIG. 7 , for recirculation back through the generator  150 . However, the blower  160  is not required for normal operation of the generator  150  and can therefore be used only when extra generating capacity is required. 
     In another embodiment, supplemental external motor-driven fans or blowers  164  and  166  can be positioned in either or both of the ducts  168  and  170 . The radial core discharge vents at opposing ends of the stator core  87  that feed the ducts  168  and  170  tend to be the coolest zones of the generator  150  and thus the blowers  164  and  166  may not be required to lower the internal temperature. The embodiments with the blower  160  and the blowers  164  and  166  can be combined as required to lower the internal temperature of the generator  150 . 
     In yet another embodiment, see  FIG. 8 , ducts  172  and  174  providing cooling air to the radial core inlet vents  156 , include supplemental external motor-driven fans or blowers  176  and  178 , respectively. The blowers  176  and  178  can provide additional external ambient cooling air or external air cooled below ambient. Alternatively, the blowers  176  and  178  can accelerate the flow of cooling air through the radial core inlet vents  156 . 
     In generators with multiple discharge and inlet zones, i.e., more than the three and five zones exemplified herein, according to the teachings of the present invention supplemental blowers and fans can be employed with one or more of the discharge zones and with one or more of the inlet zones to lower the generator hot spot temperatures. 
     Application of the teachings of the present invention to a generator with a single discharge zone, commonly referred to as a single-zoned machine, is illustrated in  FIG. 9. A  generator  200  comprises radial core discharge vents  202  and a center radial core discharge vent  204 . A supplemental external motor-driven fan or blower  206  is positioned within a duct  208  for drawing additional cooling air through the center radial core discharge vent  204 . In one embodiment seals  210  are positioned to segregate the coolant flow from the center radial core discharge vent  204  (typically the hottest cooling fluid) from the radial core discharge vents  202 . These two coolant flows would be separately vented. This embodiment is especially applicable to hydrogen-cooled generators. 
     The teachings of the present invention can also be applied to reverse-ventilated machines, that is, where the cooling fluid is drawn from the machine ends rather than blown into the ends. Thus the coolant flows in a reverse-ventilated machine are reversed from the flows illustrated in the  FIGS. 4 through 9 . One example of a three-zone reverse ventilated generator  220  is illustrated in FIG.  10 . Rotor-shaft mounted blowers  222 A and  222 B direct cooling fluid through ducts  224  and  226  to a cooler  228 , where the coolant is cooled and directed through radial core inlet vents  230  in the stator core  87  to the rotor/stator gap  114 . From there, a portion of the cooling fluid is directed to a radial core discharge vent  231 . Another portion is directed to the blowers  222 A and  222 B and drawn back through the ducts  224  and  226 . According to the teachings of the present invention, an external motor-driven fan or blower  236  is positioned within a duct  238  for receiving cooling fluid from the radial core discharge vent  231 . Thus the blower  236  accelerates the flow of cooling fluid through duct  238  and reduces the stator core temperature in the region of the radial core discharge vent  231 . A similar coolant flow arrangement and a supplemental external motor-driven fan or blower is positioned within the left segment  130  of the generator  220 . 
     While the invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalent elements may be substituted for elements thereof without departing from the scope of the present invention. The scope of the present invention further includes any combination of the elements from the various embodiments set forth herein. In addition, modifications may be made to adapt a particular situation to the teachings of the present invention without departing from its essential scope thereof. For example, generators with any number of inlet and discharge zones can be accommodated by appropriate modifications to the teachings of the present invention. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.