Abstract:
A wound field synchronous machine includes direct oil cooling along a conductive sleeve with elongated semi-arcuate shaped channels which alternate with damper bar channels containing tie-rod supports structures. With a reduction in sleeve thickness permitted by the direct cooling, the overall weight of the machine is reduced.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a wound field synchronous machine, and more particularly to a coolant system for a reduced weight electric generator system. 
   Vehicles, such as aircraft, often utilize an electric generator system to provide electric power. The generators convert mechanical energy from rotation of the engine into electrical energy for the vehicle. 
   The use of conductive sleeves in wound field synchronous machines is commonplace. In some aerospace applications where weight is a critical feature there may be a desire to also use the sleeve to provide additional stiffness to the rotor structure. However, electrical losses may be present in the conductive sleeve due to the changing magnetic field. These losses may cause the temperature of the sleeve to increase to undesirable levels in particular locations related to the changing magnetic fields. A decrease in sleeve thickness provides a weight reduction, but the reduced thickness is a tradeoff in which the temperature increase is exacerbated and the rotor assembly stiffness is reduced. 
   Accordingly, it is desirable to provide a reduced weight electric generator system that operates at a reduced temperature yet increases rotor assembly stiffness. 
   SUMMARY OF THE INVENTION 
   A wound field synchronous machine according to the present invention includes direct oil cooling through a rotor core adjacent a conductive sleeve with elongated semi-arcuate shaped channels which alternate with damper bar channels that contain tie-rod supports structures. The directly cooled conductive sleeve operates at a lower temperature than that of conventional construction. The direct oil cooling minimize localized hot spots and permits the sleeve to be designed for a lower operating temperature. Since the yield strength of the sleeve increases as the max temperature decreases, the lower operating temperature permits the sleeve to be manufactured to a thinner construction yet still maintains the desired yield strength. The thinner sleeve decreases the electro-magnetic losses from eddy currents which subsequently further lowers the sleeve operating temperature such that the sleeve may be manufactured to a still thinner thickness so that an optimal tradeoff of weight and stiffness is achieved such that the overall weight of the machine is thereby reduced. 
   The present invention therefore provides a reduced weight electric generator system that operates at a reduced temperature yet increases the rotor assembly stiffness. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows: 
       FIG. 1  is a schematic view of an exemplary generator system use with the present invention; 
       FIG. 2  is a schematic view of a coolant oil system; 
       FIG. 3A  is a perspective view of a generator system according to the present invention; 
       FIG. 3B  is a sectional view taken along the line  3 B— 3 B; 
       FIG. 3C  is a sectional view taken along line  3 C— 3 C; 
       FIG. 3D  is a perspective view of the rotor assembly; 
       FIG. 3E  is a perspective view of the rotor laminations and windings; 
       FIG. 3F  is a sectional view of the rotor taken along the axis of rotation transverse to the winding bundle 
       FIG. 3G  is a sectional view of the rotor taken through the axis of rotation 
       FIG. 3H  is an expanded view of  FIG. 3F  illustrating a winding bridge; 
       FIG. 3I  is a perspective view of the rotor laminations without the winding bundle 
       FIG. 3J  is an exploded view of the rotor end turn supports and the laminations; 
       FIG. 3K  is a sectional view taken along the line  3 G— 3 G in  FIG. 3B ; and 
       FIG. 3L  is a schematic view of coolant oil flow through the rotor assembly. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  illustrates a general schematic block view of a brushless electric generator system  10  that includes a rotor assembly  12  driven about an axis of rotation A by a prime mover such as a gas turbine engine E. It should be understood that although the system is described in terms of a synchronous generator, it may also be utilized as a synchronous motor such as in an aircraft starter generator system. It should also be understood that although a particular component arrangement is disclosed in the illustrated embodiment, other arrangements will benefit from the instant invention. 
   The generator system  10  preferably includes an integral step-up gearbox G, a lubrication system L (also illustrated schematically in  FIG. 2 ), and a PM generator (PMG) for Generator Control Unit (GCU) power. A Main Generator (MG) is preferably a 170/170 kVA two-pole wound field synchronous machine (WFSM) operating nominally at 24000 rpm. Rotor field current is supplied from the output of a rotating rectifier assembly (RRA)  30  which is powered by an Exciter (a smaller WFSM having the field winding on the stator). The GCU provides electrical current to the Exciter field converted from the PMG. Cooling is provided by a coolant oil flow through the common rotor shaft  16 . The RRA  30  is in the coolant oil flow path such that a multiple of shaft orifices  44  (illustrated schematically) provide a coolant spray to the RRA, Exciter and MG end-turns. It should be understood that the RRA  30  may be located around and/or within the shaft  16 . 
   The system  10  includes a rotor assembly  12  which supports a series of permanent magnets  18 . A stator  20  provided with windings  22  surrounds the magnets  18  and defines the PMG. Electrical current generated in the winding  22  during rotation of the rotor shaft  16  is provided via suitable conductors and an electronic controller (not shown), to windings  24  in an exciter stator  26 . Axially aligned with the exciter stator  26  and carried by the rotor shaft  16  are windings  28  in which alternating current is induced during rotation of the rotor shaft  16 . 
   The alternating current induced in the windings  28  is communicated to the RRA  30  where it is rectified to direct current; usually from three phase alternating current. Direct current from the RRA  30  is then fed to a main field winding  32  located in alignment with a main generator stator assembly  34  of the MG. The main stator assembly  34  includes windings  36  in which an alternating current is induced and which, by suitable conductors (not shown), may be connected to suitable loads. 
   To further increase system capacity, the rotor shaft  16  is provided with an oil inlet  40  and an oil outlet  42  to provide a conduit for coolant oil to flow therethrough and be sprayed into the rotor assembly  12  through spray orifices  44 . The coolant oil operates to liquid cool the rotor assembly  12  as well as for lubrication purposes ( FIG. 2 ). 
   Referring to  FIG. 3A , the rotor assembly  12  includes a rotor retaining sleeve  46  which surrounds a main rotor core  47  having a multiple of rotor laminations  48  and a main rotor winding bundle  50  ( FIG. 3B ). The laminations  48  are stacked generally in the plane normal to the axis of rotation A. It should be understood that a solid non-laminated rotor core will also be usable with the present invention. The main rotor winding bundle  50  preferably have five layers of 34-33-34-33-34 turns respectively of #14AWG for a total of 168 turns. 
   Referring to  FIG. 3C , the rotor retaining sleeve  46  stiffens the rotor core  47  during high speed rotation of the rotor assembly  12 . To significantly decrease the machine weight through primarily electromagnetic (E/M) design changes along the main centerline, the main rotor shaft  16  is preferably manufactured of a Titanium alloy. The shaft weight reduction permits a thinner rotor retaining sleeve  46  and a related decrease in the stator ID and OD. This permits a lighter rotor assembly  12  and consequently a thinner and lighter rotor retaining sleeve  46  which permits a smaller E/M gap. Transient performance improves, especially since the electromagnetic gap decrease allowed by the thinner sleeve yields a more power dense, but higher reactance machine. The stator core may then be designed for the constraints dictated by the temperature rise of the windings. The constraints for the number of winding layers and maximum wire size permit determination of the final rotor field winding (and lamination configuration). Since the yield strength of the rotor retaining sleeve  46  increases as the max temperature decreases, the rotor retaining sleeve  46  may be thinner. The thinner sleeve decreases the electromagnetic losses which lowers the rotor retaining sleeve  46  operating temperature and permits a still further decrease in the rotor retaining sleeve  46  thickness until a preferred thickness is obtained. 
   The rotor retaining sleeve  46  is preferably manufactured of a single material such as non-magnetic stainless, or titanium or a multi-layered sleeve such as a layer of copper, aluminum or silver explosion bonded to a non-magnetic stainless steel or titanium. The outer layer principally for conductivity, inner layer principally for strength. Alternatively, the rotor retaining sleeve  46  may be manufactured of a non-metallic composite material. 
   Referring to  FIG. 3D , a multiple of sleeve cooling channels  52  and a multiple of damper bar channels  54  are defined along an edge of the laminations  48  (also illustrated in  FIGS. 3F–3H ). One or more of the damper bar channels  54  may contain a tie-rod  56  ( FIG. 3H ) mounted therein. The damper bars form part of the damper winding or damper circuit. This is a principally conductive circuit located principally in quadrature with the main field winding. It is similar to a “squirrel cage” rotor conductor circuit found in induction machines. The damper bars (typ. hard copper) are mechanically and electrically connected (typ. brazed or welded) to endplates (typ. copper strips joining damper bars of one pole to those of another) on either end of the core. The purpose of the damper circuit is to reduce or eliminate speed oscillation. Any magnetic field impressed upon the rotor that is rotating asynchmously with the rotor will produce a current in the damper bars and thus a torque which tends to bring the rotor in synchronism with the dominant rotating magnetic field. Preferably, the tie-rods  56  span approximately 76 degrees per pole face about axis of rotation A. Used here, the primary purpose is to mechanically stiffen the rotor—the rotor is weakest in the axis of the damper circuit, so it is convenient to remove a few damper bars and use the now empty slots (in the rotor, where the damper bars typically are) for “tie-rods.” 
   The multiple of sleeve cooling channels  52  are interspersed between damper bar channels  54  to extract heat from the sleeve  46  ( FIG. 3C ). The multiple of sleeve cooling channels  52  are in fluid communication with the lubricant system L through an end-turn support  58 A ( FIGS. 3I and 3J ). 
   A relatively thin spacer  60  such as a ribbon shaped spacer is weaved between layers of the main rotor windings  50  at either end to allow coolant oil to penetrate (and exit) the winding bundle  50  ( FIGS. 3B and 3J ). That is, the spacer  60  provides some separation between the winding bundle layers such that the coolant oil may flow therebetween. 
   Referring to  FIG. 3J , the end-turn support  58  defines coolant oil channels  62 A– 62 B. The wire bundle coolant channels  62 A ( FIG. 3B ) provide a dedicated flow path into the winding bundle  50  ( FIG. 3K ). The coolant channels  62 B wrap the wiring bundle  50  to provide a coolant oil path to a cooling opening  62 C which feeds the sleeve cooling channels  52  adjacent the sleeve  46  ( FIGS. 3C and 3L ). That is, the channels  62 A provide oil coolant flow into the winding bundle  50  and coolant channels  62 B provide flow generally around the winding bundle and into the channels  52 . Notably, centrifugal force drives the coolant oil outward to the sleeve cooling channels  52 . 
   The dedicated coolant path provided by the sleeve cooling channels  62  and the separation of the winding layers facilitate cooling and minimize localized hot spots formed in the rotor retaining sleeve  46  which allows the rotor retaining sleeve  46  to be designed to lower temperatures than that of conventional construction. 
   The directly cooled conductive sleeve operates at a lower temperature than that of conventional construction. The direct oil cooling minimize localized hot spots and permits the sleeve to be designed for a lower operating temperature. Since the yield strength of the sleeve increases as the max temperature decreases, the lower operating temperature permits the sleeve to be manufactured to a thinner construction yet still maintains the desired yield strength. The thinner sleeve decreases the electromagnetic losses from eddy currents which subsequently further lowers the sleeve operating temperature such that the sleeve may be manufactured to a still thinner thickness so that an optimal tradeoff of weight and stiffness is achieved such that the overall weight of the machine is thereby reduced. 
   Referring to  FIG. 3H , coolant oil flow enters the shaft  16  from the transfer tube  65  ( FIG. 3K ). The orifices  44  ( FIGS. 1 and 2 ) divert some of the coolant oil to provide dedicated spray cooling to the RRA  30  exciter stage, main stator windings, gear mesh, and bearings. The primary rotor cooling path, however, is through the RRA  30  and into the main field  32  and sleeve  46 . 
   The coolant oil flow from the shaft  16  impinges directly on the end region of the main rotor winding bundle  50  which is supported by the end support  58 . The end region is supported in the saddle shaped region  64  ( FIG. 3J ). The coolant channel  62 A permits the spacer  60  to be wound between winding layers as well as provide a coolant oil flow path. The coolant channels  62 B permit coolant oil to pass around and beneath the main rotor winding bundle  50  and flow into the sleeve cooling channels  52 . 
   Coolant oil flows through the sleeve cooling channels  52  and returns to the interior of the shaft  16  through coolant openings  68  in the end support  58 B. The coolant oil in the winding bundle returns to the rotor shaft  16  through the coolant channels  62 B in the opposite end support  58 B. A portion of the return flow may alternatively or additionally exit the shaft  16  through stator spray orifices  70  (illustrated schematically) to provide cooling spray to other components ( FIG. 1 ). 
   Alternatively, to increase the stiffness of the rotor, tie rods are placed in the in slots from which alternating damper bars are removed as the span of the damper slots tends to be relatively more significant than the actual number. The tie rods may be threaded along one end segment and have a head on the other. The tie-rods are then passed through, for example, the shaft window overhang and though the empty damper bar slots. 
   An alternative method for sleeve cooling utilizes the damper bar channels as the oil coolant channels rather than the dedicated channels disclosed above. 
   It should be understood that relative positional terms such as “forward,” “aft,” “upper,” “lower,” “above,” “below,” and the like are with reference to the normal operational attitude of the vehicle and should not be considered otherwise limiting. 
   It should be understood that although a particular component arrangement is disclosed in the illustrated embodiment, other arrangements will benefit from the instant invention. 
   Although particular step sequences are shown, described, and claimed, it should be understood that steps may be performed in any order, separated or combined unless otherwise indicated and will still benefit from the present invention. 
   The foregoing description is exemplary rather than defined by the limitations within. Many modifications and variations of the present invention are possible in light of the above teachings. The preferred embodiments of this invention have been disclosed, however, one of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For that reason the following claims should be studied to determine the true scope and content of this invention.