Abstract:
A rotor core and winding assembly including: separable rotor core sections assembled to form the rotor core, where the core sections each have a substantially circular perimeter and are axially aligned when assembled as the core, and the winding assembly includes a pre-assembled superconducting field winding and a winding support, wherein the winding support extends between adjacent core sections in the assembled rotor core.

Description:
CROSS-RELATED APPLICATIONS 
     The application is a divisional of and claims priority to application Ser. No. 09/935,735 filed Aug. 24, 2001 now U.S. Pat. 6,795,720. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to a superconductive field coil winding in a synchronous rotating machine. More particularly, the present invention relates to a rotor core that supports a superconducting field winding assembly in a synchronous machine. 
     Synchronous electrical machines having rotor field coil windings include, but are not limited to, rotary generators, rotary motors, and linear motors. These machines generally comprise a stator and rotor that are electromagnetically coupled. The rotor may include a multi-pole rotor core and one or more field coil windings mounted on the rotor core. The rotor cores may include a magnetically-permeable solid material, such as an iron-core rotor. 
     Conventional copper windings are commonly used in the rotors of synchronous electrical machines. However, the electrical resistance of copper windings (although low by conventional measures) is sufficient to contribute to substantial heating of the rotor and to diminish the power efficiency of the machine. Recently, superconducting (SC) field coil windings have been developed for rotors. SC windings have effectively no resistance and are highly advantageous rotor coil windings. 
     Iron-core rotors saturate at an air-gap magnetic field strength of about 2 Tesla. Known superconductive rotors employ air-core designs, with no iron in the rotor, to achieve air-gap magnetic fields of 3 Tesla or higher. These high air-gap magnetic fields yield increased power densities of the electrical machine, and result in significant reduction in weight and size of the machine. Air-core superconductive rotors require large amounts of superconducting wire. The large amounts of SC wire add to the number of coils required, the complexity of the coil supports, and the cost of the SC coil windings and rotor. 
     High temperature SC rotor coil field windings are formed of superconducting materials that are brittle, and must be cooled to a temperature at or below a critical temperature, e.g., 27° K, to achieve and maintain superconductivity. The SC windings may be formed of a high temperature superconducting material, such as a BSCCO (Bi x Sr x Ca x Cu x O x ) based conductor. 
     High temperature superconducting (HTS) coil windings are sensitive to degradation from high bending and tensile strains. These coils must undergo substantial centrifugal forces that stress and strain the coil windings. Normal operation of electrical machines involves thousands of start up and shut down cycles over the course of several years that result in low cycle fatigue loading of the rotor. Furthermore, the HTS rotor coil windings should be capable of withstanding 25% over-speed operation during rotor balancing procedures at ambient temperature, and at occasional over-speed conditions at cryogenic temperatures during power generation operation. These over-speed conditions substantially increase the centrifugal force loading on the rotor coil windings over normal operating conditions. 
     SC coils used as the HTS rotor field winding of an electrical machine are subjected to stresses and strains during cool-down and normal operation. These coils are subjected to centrifugal loading, torque transmission, and transient fault conditions. To withstand the forces, stresses, strains and cyclical loading, the SC coils should be properly supported in the rotor by a coil support system. These coil support systems hold the SC coil(s) in the HTS rotor and secure the coils against the tremendous centrifugal forces due to the rotation of the rotor. Moreover, the coil support system protects the SC coils, and ensures that the coils do not prematurely crack, fatigue or otherwise break. 
     Developing coil support systems for HTS coil has been a difficult challenge in adapting SC coil windings to HTS rotors. Examples of coil support systems for HTS rotors that have previously been proposed are disclosed in U.S. Pat. Nos. 5,548,168; 5,532,663; 5,672,921; 5,777,420; 6,169,353, and 6,066,906. However, these coil support systems suffer various problems, such as being expensive, complex and requiring an excessive number of components. There is a long-felt need for a HTS rotor having a coil support system for a SC coil. The need also exists for a coil support system made with low cost and easy to fabricate components. 
     BRIEF SUMMARY OF THE INVENTION 
     A multi-piece rotor core for a superconducting synchronous machine has been developed. The rotor core includes passages transverse to the rotor axis. Through these passages extend coil support bars that are coupled to a superconducting coil winding. The coil winding extends around the rotor core, and is generally in a plane that includes the rotor axis. The rotor core has flat sides that are adjacent the long sides of the coil winding. 
     The rotor core is assembled from several rotor core sections. These sections are generally disk shaped and have a T-shaped cross-section. The rotor core sections have connection bosses to engage slots in adjacent rotor core sections. The core sections are assembled around a pre-formed superconducting winding and coil support. The assembly of rotor core sections form a solid core, except for the support bar passages that extend through the core axis. The core sections are held together by tie rods that extend through the assembly of sections. The rods are parallel to the rotor core axis and extend the length of the core. 
     Tension bars that extend between the sides of the rotor coil can provide support so that the coil will withstand the centrifugal forces of the rotor. To support opposite sides of the coil, the tension bars extend through rotor core. There is a desire to assembly the tension bar and coil winding before both are mounted on a rotor core. However, a solid rotor core will not allow for pre-assembly of the coil and tension members. Thus, there is a need for a rotor core and assembly technique that will allow an assembled coil and tension member to be mounted on a solid rotor core. 
     An assembly of rotor core sections permits the rotor core to be assembled around a coil winding assembly. The coil winding assembly may be assembled with the winding support to form a pre-formed coil winding assembly prior to the rotor core assembly. Pre-assembly of the field coil and winding support should reduce the rotor-coil production cycle, improve coil support quality, and reduce coil assembly variations. 
     The HTS rotor may be for a synchronous machine originally designed to include SC coils. Alternatively, the HTS rotor may replace a copper coil rotor in an existing electrical machine, such as in a conventional generator. The rotor and its SC coils are described here in the context of a generator, but the HTS coil rotor is also suitable for use in other synchronous machines. 
     In a first embodiment, the invention is a rotor in a synchronous machine, comprising: a superconducting field winding assembly having a coil winding and at least one winding support extending between opposite sides of the winding, and a rotor core formed of a plurality of rotor core sections, each of said core sections having a slot to receive said winding support. 
     In another embodiment, the invention is a rotor core and winding assembly comprising: separable rotor core sections assembled around the winding assembly to form said rotor core, where said core sections are axially aligned with said rotor core, and said winding assembly includes a pre-assembled a superconducting field winding and a center winding support. 
     Another embodiment of the invention is a method for assembling a rotor core around a superconducting field coil winding assembly comprising the steps of: fabricating said field coil winding assembly by assembling a field coil winding and a coil support prior to assembly of the rotor core, inserting a portion of each of a plurality of rotor core sections partially through said coil winding assembly, assembling the plurality of rotor core sections around said coil support, and securing the assembly of rotor core sections. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings in conjunction with the text of this specification describe an embodiment of the invention. 
         FIG. 1  is a schematic side elevational view of a synchronous electrical machine having a superconductive rotor and a stator. 
         FIG. 2  is a perspective view of an exemplary racetrack superconducting coil winding. 
         FIG. 3  is a cross-sectional view of an assembled rotor core with a coil winding. 
         FIG. 4  is a cross-sectional diagram of the assembled rotor core taken along line  4 — 4  in  FIG. 3 . 
         FIG. 5  is a perspective diagram of a rotor core end section. 
         FIG. 6  is a cross-sectional diagram of a rotor core section. 
         FIG. 7  is a cross section of the rotor core taken along line  7 – 7  of  FIG. 3 . 
         FIG. 8  is a cross-section of a coil winding, section of a tension bar and coil housing. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an exemplary synchronous generator machine  10  having a stator  12  and a rotor  14 . The rotor includes field winding coils that fit inside the cylindrical rotor cavity  16  of the stator. The rotor fits inside the rotor cavity of the stator. As the rotor turns within the stator, a magnetic field  18  (illustrated by dotted lines) generated by the rotor and rotor coils moves/rotates through the stator and creates an electrical current in the windings of the stator coils  19 . This current is output by the generator as electrical power. 
     The rotor  14  has a generally longitudinally-extending axis  20  and a generally solid, multi-piece rotor core  22 . The rotor core is an assembly of axially-aligned end core sections  44  and middle core sections  46 . The core  22  has high magnetic permeability, and is usually made of a ferromagnetic material, such as iron. In a low power density superconducting machine, the iron core of the rotor is used to reduce the magnetomotive force (MMF), and, thus, minimize the amount of superconducting (SC) coil wire needed for the coil winding. 
     The rotor  14  supports at least one longitudinally-extending, racetrack-shaped, high-temperature superconducting (HTS) field winding assembly  33  having an HTS winding (See  FIG. 2 ). The HTS field coil winding may be alternatively a saddle-shape or have some other shape that is suitable for a particular HTS rotor design. A rotor field assembly and coil support is disclosed here for a racetrack SC field winding. The rotor core assembly and coil support may be adapted for winding configurations other than a racetrack field winding mounted on a solid rotor core. 
     The rotor includes a pair of end shafts  24 ,  30  that are supported by bearings  25 . The end shafts may be coupled to external devices. For example, one of the end shafts  24  has a cryogen transfer coupling  26  to a source of cryogenic cooling fluid used to cool the SC field windings in the rotor. The cryogen transfer coupling  26  includes a stationary segment coupled to a source of cryogen cooling fluid and a rotating segment which provides cooling fluid to the HTS winding. This end  24  of the rotor may also include a collector  31  for electrically connecting to the rotating SC field winding. The opposite end shaft  30  of the rotor may be driven by a power turbine coupling  32 . 
       FIG. 2  shows an exemplary HTS racetrack field winding assembly  33  comprising a field coil winding  34  and a series of tension bars  35  (the coil support) extending between opposite sides of the winding. The winding assembly  33  is fabricated with the field winding  34  and tension bars  35  before the assembly  33  is inserted into the rotor core. The tension bars support the field coil windings with respect to the centrifugal forces that act on the windings as the rotor spins during operation. Accordingly, the tension bars are attached to the windings by a winding housing  36  (as shown in  FIG. 8 ). The housing and tension bars restrain the expansion of the field coil winding  34  that would otherwise occur with the tension bars  35 . 
     The SC field windings  34  of the rotor includes a high temperature superconducting (SC) winding  34 . Each SC winding includes a high temperature superconducting conductor, such as a BSCCO (Bi x Sr x Ca x Cu x O x ) conductor wires laminated in a solid epoxy impregnated winding composite. For example, a series of BSCCO 2223 wires may be laminated, bonded together and wound into a solid epoxy impregnated winding. 
     SC wire is brittle and easy to be damaged. The SC winding is typically layer wound SC tape that is epoxy impregnated. The SC tape is wrapped in a precision winding form to attain close dimensional tolerances. The tape is wound around in a helix to form the racetrack SC winding  34 . 
     The dimensions of the racetrack winding are dependent on the dimensions of the rotor core. Generally, each racetrack SC winding encircles the magnetic poles of the rotor core, and is parallel to the rotor axis. The field windings are continuous around the racetrack. The SC windings form a resistance free electrical current path around the rotor core and between the magnetic poles of the core. The winding has electrical contacts  41  that electrically connect the winding to the collector  31 . 
     Fluid passages  38  for cryogenic cooling fluid are included in the field winding  34 . These passages may extend around an outside edge of the SC winding  34 . The passageways provide cryogenic cooling fluid to the porous winding and remove heat from the winding. The cooling fluid maintains the low temperatures, e.g., 27° K., in the SC field winding needed to promote superconducting conditions, including the absence of electrical resistance in the winding. The cooling passages have an input and output fluid ports  39  at one end of the rotor core. These fluid (gas) ports  39  connect the cooling passages  38  on the SC winding to the cryogen transfer coupling  26 . 
     Each HTS racetrack field winding  34  has a pair of generally straight side portions  40  parallel to a rotor axis  20 , and a pair of end portions  42  that are perpendicular to the rotor axis. The side portions of the field coil winding are subjected to the greatest centrifugal stresses. Accordingly, the side portions are supported by the tension bars and housing. These bars and housing form a winding support system that counteract the centrifugal forces that act on the winding. 
       FIG. 3  is a schematic diagram of a multi-piece rotor core  22  with the winding assembly  33 , including the racetrack superconducting coil field winding  34  and tension bars  35 . The iron core is made of multiple core sections, which are generally several middle sections  44  and a pair of end sections  46 . Each of the core sections have a semi-rectangular shape (see  FIG. 7 ) with a pair of opposite flat sides  50  and a pair of opposite arc-shaped sides  52 . 
     When assembled, the flat sides  50  of the core sections are in alignment with each other, and similarly the arc-shaped sides are also in alignment. The middle core sections  44  have a generally “T” shape in cross sections, except for the two end sections (compare  FIGS. 5 and 6 ). The end sections  46  have a generally L-shaped cross section. 
     The sections of the rotor core are assembled around the winding assembly  33 . During assembly of the core, the narrow head  45  of each middle section slides between adjacent support bars  35  in the winding assembly. The narrow head of the end rotor core sections  46  slide between a tension bar  35  and an end  42  of the coil winding  34 . Each of the core sections has at least one tension rod slot  53  (middle sections  44  have a pair of opposite slots) which when mated with the slot in an opposite core forms an aperture  55  for a tension bar  35 . The assembly of rotor core sections permits integrating a fully assembled winding assembly  33  (which includes, for example, field winding  34  and tension bars  35 ) into the rotor core. 
     The core sections  44 ,  46  may be iron core forgings. The rotor core sections are assembled through rabbet joint fits for concentricity and alignment. Each core section has at least one boss  54  (middle sections have a pair of opposing bosses) that fit into a slot  56  on an adjacent core section. The boss-slot connection between the core sections aligns the core sections in the rotor core. Several tie-rods  58  extend laterally through rod holes  60  along the length of the rotor core. The tie rods have a nut or other faster at each end and hold the core sections together in compression. 
     A vacuum housing  64  may be formed over the field winding  34 , once the rotor core sections have been assembled around the winding assembly. A vacuum around the winding facilitates the superconducting characteristics of the winding. The vacuum housing provides a vacuum over the entire race-track shape of the coil winding. 
     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 all embodiments within the spirit of the appended claims.