Patent Publication Number: US-6911759-B2

Title: Stator coil assembly for superconducting rotating machines

Description:
RELATED APPLICATION 
   This application is a continuation application and claims the benefit of priority under 35 USC §120 of U.S. application Ser. No. 09/632,412, filed Aug. 4, 2000, now abandoned. The disclosure of the prior application is considered part of and is incorporated by reference in the disclosure of this application. 
   INCORPORATION BY REFERENCE 
   The following applications are hereby incorporated by referenced into the subject application as if set forth herein in full: (1) U.S. application Ser. No. 09/632,599, filed Aug. 4, 2000, entitled “Superconducting Synchronous Machine Field Winding Protection”; (2) U.S. application Ser. No. 09/632,776, filed Aug. 4, 2000, entitled “HTS Superconducting Rotating Machine”; (3) U.S. application Ser. No. 09/632,600, filed Aug. 4, 2000, entitled “Exciter For Superconducting Rotating Machinery”; (4) U.S. application Ser. No. 09/632,602, filed Aug. 4, 2000, entitled “Segmented Rotor Assembly For Superconducting Rotating Machines”; and (5) U.S. application Ser. No. 09/632,601, filed Aug. 4, 2000, entitled “Stator Support Assembly For Superconducting Rotating Machines”. 

   TECHNICAL FIELD 
   This invention relates to the construction and operation of stators for use in superconducting motors. 
   BACKGROUND 
   Typical electric machines (e.g. motors and generators) include a rotor assembly that rotates relative to a surrounding stator assembly. The stator assembly typically is wound from copper, which is ductile and has a relatively high electrical conductivity. The copper wire or cable used to wind the stator typically includes an outer insulation layers so that when the copper conductor is wound over itself, individual turns are electrically isolated from each other. As the level of current flowing through the stator increase so does the amount of copper. 
   SUMMARY 
   The invention features a stator for use in a rotating machine having an in-land wound construction. 
   In a general aspect of the invention the stator includes a first electrical conductor; and a second conductor wound, in-hand, over the first conductor and along a longitudinal axis of the stator. The second conductor is electrically isolated from the first conductor along the length of the first and second conductors. 
   Another aspect of the invention features a method for constructing a stator for use in a rotating machine. The method includes winding, in hand, and along the longitudinal axis, a first electrical conductor over a second conductor, the second conductor electrically isolated from the first conductor along the length of the first and second conductors. Providing a stator having this construction has numerous advantages. For example, the in-hand winding construction allows multiple conductors to be combined to increase the overall current handling capability of the stator while substantially maintaining the “packing factor” (i.e., ratio of current-carrying conductor to overall conductor). The packing factor is substantially maintained because the amount of turn-to-turn insulation winding between typical conductors is reduced. 
   Embodiments of these aspects of the invention may include one or more of the following features. The first conductor has an end electrically connected to an end of the second conductor. The first conductor is wound over the second conductor along the axis in a first direction to form a first layer of the stator and, at an end region of the stator, the first conductor is wound over the second conductor along the axis in a second direction, opposite the first direction to form a second layer of the stator. In essence, this arrangement provides a wound in-hand layer construction. 
   In one embodiment, at the end region, the position of the first conductor and the second conductor are transposed. With this arrangement voltages induced in the circuits formed by the conductors are identical to a first order and any circulating currents between the circuits are minimized. Thus, overall looses in the stator are reduced. 
   These and other features and advantages of the invention will be apparent from the following description of a presently preferred embodiment and the claims. 

   
     DESCRIPTION OF DRAWINGS 
       FIG. 1  is a cross-sectional perspective view of a superconducting motor in accordance with the invention. 
       FIG. 2  is a generic cross-sectional view of the superconducting motor of FIG.  1 . 
       FIG. 3  is a perspective view of a stator assembly of the superconducting motor of FIG.  1 . 
       FIG. 4  is a perspective view of a single phase of stator coils of the stator assembly of FIG.  3 . 
       FIG. 5  is a perspective view of a single phase of stator coils mounted on the support tube of the stator assembly of FIG.  3 . 
       FIG. 6  is a cross-sectional perspective view of a stator coil section of the stator assembly of FIG.  3 . 
       FIG. 6A  is a schematic of two stator coils and an associated cooling loop. 
       FIG. 7  is a cross-sectional perspective view of a rotor assembly of the superconducting motor of FIG.  1 . 
       FIG. 8  is a cross-sectional perspective view of an output shaft and vacuum chamber of the rotor assembly of FIG.  7 . 
       FIG. 9  is a perspective view of rotor coils mounted on a rotor body of the rotor assembly of FIG.  7 . 
       FIG. 10  is a cross-sectional view of the rotor coil stack with internal support members of the rotor coils of FIG.  9 . 
       FIG. 11  is a perspective view of an axial buckle of the rotor assembly of FIG.  7 . 
       FIG. 12A  is a perspective view of a tangential buckle of the rotor assembly of FIG.  7 . 
       FIG. 12B  is a perspective view of the tangential buckle of  FIG. 12  mounted with a spring. 
       FIG. 13A  is a cross-sectional perspective view of the tangential buckles mounted within the rotor assembly of FIG.  7 . 
       FIG. 13B  is a cross-sectional perspective view of the axial buckles mounted within the rotor assembly of FIG.  7 . 
       FIG. 14  is a perspective view of a cryogenic cooling system and mounting flange of the superconducting motor of FIG.  1 . 
       FIG. 15  is a block diagram of a cryogenic cooling system of the superconducting motor of FIG.  1 . 
       FIG. 16  is a cross-sectional view of a single layer conductor used for winding a stator coil. 
       FIG. 17  is a cross-sectional side view of a double pancake coil wound two-in-hand. 
       FIG. 18  is a cross-sectional side view of a double pancake coil wound three-in-hand. 
   

   Like reference symbols in the various drawings indicate like elements. 
   DETAILED DESCRIPTION 
   Referring to  FIGS. 1 and 2 , a superconducting synchronous motor  10  includes a rotor assembly  50  cooled by a cryogenic cooling system  100 , here a Gifford McMahon (GM) cooling system, and surrounded by a stator assembly  20 . Both the stator assembly  20  and the rotor assembly  50  are mounted in a housing  12  to protect the components and any users of the superconducting motor  10  and to transmit motor torques to the mounting foundation. As will be described in greater detail below, each of these components and assemblies have features which contribute toward both increasing the overall performance, as well as reducing the overall size of motor  10 . In particular, superconducting synchronous motor  10  can be shown to produce torque densities as high as 75 N·m/Kg or more at 500 RPM: or less. Furthermore, such motors are expected to provide a greatly improved gap shear stress characteristic in a range between 15 psi and 100 psi. 
   Referring to FIGS.  1  and  3 - 5 , the stator assembly  20  includes, in this embodiment, one hundred eight stator coils  22  wound around a support tube  34 , and arranged in a multi-phase configuration, here a 9-phase configuration. The twelve stator coils  22  per phase provide a 12-pole arrangement. A back iron  36  is constructed by wrapping magnetic wire around the stator coils  22 . The stator coils  22  are wound into a diamond pattern, with one or more stator coil  22  diamond representing a single pole. The stator coils  22  are arranged around the support tube  34  by overlapping sides of adjoining stator coils  22  in the same phase. 
   Referring to  FIG. 6 , cooling conduits  30  are positioned to be in thermal contact with each stator coil  22  to facilitate cooling of the stator assembly  20 . Each cooling conduit  30  is constructed from a thin walled, high electrical resistivity alloy for minimizing eddy current heating. Each coolant passage of the cooling conduit  30  is distinct and electrically isolated from the adjacent coolant passage. Because the cooling conduits  30  are generally constructed from an electrically conductive material, an electrically insulating tape  28  is wrapped about the stator coil  22  to electrically insulate the stator coil  22  from surrounding components that are at ground potential, particularly the cooling conduits  30 . In particular, the electrically insulating tape  28  maintains the cooling conduits  30  at ground potential, thereby permitting the use of fresh water, which contains ions. The electrically insulating tape  28  is made from a material having a thickness that can withstand operating voltages of the conductor turns  24 , and can transport the heat generated by the conductor turns  24  to the cooling conduits  30 . The thickness of the electrically insulating tape  28  is determined by the dielectric strength (insulating properties) of the material and operating voltage, typically between about 0.001 to 0.100 inches. Examples of materials for the electrically insulating tape  28  include, but are not limited to, epoxy, mica, and glass tapes. 
   In this embodiment, the stator coils  22  are formed of an array of multiple conductor turns  24 . Each conductor turn  24  is electrically isolated from all adjacent turn by insulation  26 . Insulation  26  may be formed of the same material as electrically insulating tape  28 , but has a reduced thickness (e.g., 0.001 to 0.030 inches). 
   Referring to  FIGS. 6 and 6A , cooling conduits  30  are mounted adjacent to and in contact with the electrically insulating tape  28  surrounding each stator coil  22 . Each cooling conduit  30  has a number of passages extending therethrough for receiving a coolant from a fresh water external source  200 . With reference to  FIG. 3 , each cooling conduit  30  has an opening (not shown) at the end regions of each stator coil  22 . Therefore, one hundred eight openings are in fluid communication with a manifold assembly (not shown) to allow fluid into each cooling conduit  30  from the external source  200 . On the other side of the stator coils  22 , one hundred eight openings are in fluid communication with a return  202 . In one embodiment, the manifolds are end caps (not shown) circumferentially mounted to the front and back edge of the stator assembly  20 . 
   A porous copper thermally conductive member  32 , which has low electrical conductivity, is disposed about the stator coil  22  and cooling conduits  30  to facilitate cooling of the entire stator coil  22 . In other embodiments, this could be constructed from a wire or an insulated braid disposed about the stator coil  22 . Absent the thermally conductive member  32 , the stator coil  22  would only be cooled at the contact point between the cooling conduit  30  and the electrically insulating tape  28 . Because of this contact point cooling, a thermal gradient would be induced through the electrically insulating material  28 . This thermal gradient creates thermal stresses between the cooling conduit  30  and the electrically insulating tape  28 , which can cause premature failure in the stator assembly  20  due to electrical breakdown at this interface. Additionally, with high power density embodiments, the cooling conduit  30  cannot be mounted on a wide side of the stator coil  22  due to the required high packing densities. To minimize the peak temperature, the thermally conductive member  32  is positioned around the stator coil  22  and the cooling conduit  30  to allow heat transfer from the sides of the stator coil  22  that are not in direct contact with the cooling conduit  30 . 
   In certain embodiments, cooling of the stator assembly  20  is further enhanced by varying the thickness of the electrically insulating material  28 . The electrically insulating material  28  isolating the conductor turns  24  in each diamond-shaped stator coil  22  from the grounded thermally conductive member  32  experiences varying dielectric stress dependent on the electrical location of the coil within a given phase of the stator assembly  20  with stator coils  22  connected in series. The two stator coils  22  at the end of the phase are connected directly to line voltage and their electrically insulating material  28  experiences maximum dielectric stress between conductor turn  24  and the thermally conducting member  32 . The coils electrically located midway between the ends of the phase are exposed to approximately half the dielectric stress due to the voltage drops in the stator coils  22  between the end and middle of the phase. The thickness of the electrically insulating material  28  is varied in uniform steps directly proportional to the voltage variation. In one embodiment, the minimum thickness of the electrically insulating material  28  thickness is calculated by the relationship T ins *(0.5+(1/N)), where T ins  represents the maximum thickness of the electrically insulating material  28  at coils connected to the line voltage and N represents the even number of stator coils  22  in each phase. The electrically insulating material  28  thickness will proportionally vary in uniform steps between the maximum thickness, T ins , and the minimum thickness. Varying the thickness of the electrically insulating material  28  will help facilitate cooling, since thicker electrically insulating material  28  will not be used where it is not needed. 
   In another embodiment, the stator coils  22  in each phase may be arranged and connected in pairs in a two layer winding with stator coils  22  having the thinnest and thickest electrically insulating material  28  being paired. Stator coils  22  with the next thinnest and next thickest electrically insulating material  28  are then paired, this process being continued until the final two middle stator coils  22  are paired. 
   In certain other embodiments, the benefits of varying the thickness of the electrically insulating material  28  can be enhanced by varying the cross sectional area of each of the two stator coils  22  in the above described pairs of stator coils  22 . The cross sectional area of the conducting turns  24  in the stator coil  22  with thin electrically insulating material can be decreased as higher power can be dissipated due to the decreased thermal resistance of the thin electrically insulating material  28 . This makes room in the same coil pair to decrease the power dissipation in the remaining coil with thick electrically insulating material  28  by increasing the cross sectional area of its conducting turns  24 . Typically winding temperature rise is reduced by  30  percent compared with the result of using conventional art with uniform insulation thickness and uniform wire cross sectional areas. Increased resistance to voltage breakdown between the conducting turns  24  and the adjacent thermally conductive member  32  can be obtained compared with conventional art by increasing the thickness of electrically insulating material  28  on each of the coils in the above coil pairs for the same higher temperature as obtained with conventional art. 
   In another embodiment, a flexible graphite tape is used to substitute for the porous copper thermally conductive member  32 . The flexible graphite tape is wrapped around the stator coil  22  and cooling conduits  30  to facilitate cooling of the entire stator coil  22 . As was the case with the thermally conductive member  32  described above in conjunction with  FIG. 6 , the flexible graphite tape serves to dissipate heat from the stator coil  22  in a similar way as copper while having more desirable electrical characteristics. Flexible graphite tape is available from UCAR Carbon Company, Inc., PO Box 94637, Cleveland, Ohio 44101, and sold under the trade name Grafoil®. Grafoil® was originally developed for high temperature gasketing applications and is described in U.S. Pat. No. 3,404,061. The thermal conductivity and electrical resistivity of such flexible graphite tapes are a function of its density. For example, one type of Grafoil tape has the following characteristics: 
   
     
       
         
             
             
             
           
             
                 
             
             
               Grafoil Density 
               Electricity Resistivity 
               Implied Thermal 
             
             
               (lbs/ft 3)   
               (μΩm) 
               Conductivity (W/m° C.) 
             
             
                 
             
           
          
             
               70 
               6.7 
               140 
             
             
                 
                 
               (measured) 
             
             
               80 
               4.0 
               230 
             
             
               90 
               2.5 
               375 
             
             
                 
             
          
         
       
     
   
   From the above data, we can see that a Grafoil® tape having a density characteristic of 90 lbs/ft 3  has a thermal conductivity characteristic of 375 W/m° C., comparable to copper (400 W/m° C.). 
   The advantages of using flexible graphite are numerous. Flexible graphite has a thermal conductivity characteristic comparable to copper, but has an electrical resistivity characteristic  100  times larger than copper at room temperature. Because of its high electrical resistivity, flexible graphite does not generate significant eddy-current heating in an AC field. In addition, flexible graphite has only 1% of the loss of a copper material of identical thickness in the same magnetic field. This loss is sufficiently low that it does not interrupt the electric current flowing through the flexible graphite. Other valuable characteristics of flexible graphite include being highly sealable over extended periods of time, permanently resilient, non-hardening and non-aging. 
   The flexible graphite tape is generally required to be applied under pressure. There are several approaches for bonding the flexible graphite tape to the stator coil  22 . In one approach, after the flexible graphite tape is wrapped around the outer surface of the stator coil  22  and cooling conduits  30 , a layer of glass tape (e.g. Kevlar®) is applied over the flexible graphite tape with high tension. The assembly is then epoxy impregnated. In another approach, the flexible graphite tape is first bonded to a glass tape with a thickness of 1-2 mil before wrapping. 
   Referring to  FIG. 7 , the rotor assembly  50  includes a rotor body  58 , onto which the superconducting rotor coils  52  are fixed, mounted onto an output shaft  82  by an array of tangential buckles  70  and axial buckles  60 . As will be explained in detail below, the tangential buckles  70  and the axial buckles  60  transfer the torque and forces produced by the rotor coils  52  to the output shaft  82 , while also thermally isolating the cryogenically cooled rotor body  58  from the output shaft  82 . The tangential buckles  70  and axial buckles  60  are mounted between rotor body ribs  59  and output shaft plates  84 , as will be described in detail below. Vacuum chamber walls  86  are integrally mounted to the output shaft  82 , enclosing the rotor assembly  50  and acting as a cryostat. As will be described in detail below, a closed cryogenic cooling loop  118  (Shown in  FIG. 2 ) is used to conduct heat from the rotor coils  52  to the cryocooler  104  where the heat can be dissipated. In particular embodiments, vacuum chamber  86  includes an outer cylindrical wall that, for reasons discussed below, serves as an electromagnetic shield  88 . 
   Referring to  FIGS. 7 and 8 , the output shaft  82  includes multiple plates  84  extending radially outward from the output shaft  82  surface. The multiple plates  84  include a first set of circumferentially extending plates  84 A positioned around the output shaft  82  and a second set of longitudinally extending plates  84 B positioned along the output shaft  82 . Walls of the plates  84  form generally rectangular pockets, here thirty in number, around the surface of the output shaft  82  into which the tangential buckles  70  and axial buckles  60  mount. The plates  84  also include radial slots. Specifically, longitudinal plates  84 B include radial slots  85 B in every rectangular pocket wall around the output shaft  82  formed by the longitudinal plates  84 B for mounting the tangential buckles  70 . Similarly, the circumferential plates  84 A define radial slots  85 A in every other rectangular pocket wall around the output shaft  82  formed by the circumferential plates  84 A for mounting the axial buckles  60 . However, the present embodiment only utilizes three axial buckles displaced within the rectangular pockets in the middle of the rectangular pocket array. That is, no radial slots  85 A are found on the outer circumferential plates  84 A. 
   Referring again to  FIG. 2 , as discussed above, a vacuum chamber  86  is integrally mounted to the output shaft  82  and encloses the rotor assembly  50 . The vacuum chamber  86  also encloses the circumferential plates  84 A and longitudinal plates  84 B, and is sized to allow the rotor body  58  and rotor coils  52  to be mounted to the output shaft  82 . The output shaft  82  extends beyond the vacuum chamber  86  and the plates  84  at both ends. On one end, the output shaft  82  extends to connect to an external load that the motor  10  will drive. At the other end, the output shaft  82  connects to a rotating half of a brushless exciter  16 . 
   The brushless exciter, shown in  FIG. 2 , includes a rotating disk  6  spaced from a stationary disk  14  (e.g., spaced 1-4 mm). Rotating disk  16  is formed of a high permeability powder core or laminated material core (e.g., iron) and includes a pair of concentric grooves within which a pair of coil windings is disposed. Stationary disk  14  is similarly formed of a high permeability material and includes a pair of concentric grooves within which a pair of coil windings is disposed. In essence, this arrangement provides a transformer having a primary, which rotates relative to a secondary of the transformer (or vice versa). An important feature of this particular arrangement is that the flux linkage generated by stationary disk  14  and rotating disk  16  when stationary is the same as when the rotating disk rotates. This feature advantageously allows superconducting rotor coils  52  to be charged prior to rotating disk  16  rotating (i.e., before motor  10  operates). The structure and operation of the brushless exciter is described in U.S. patent application Ser. No. 09/480,430, entitled “Exciter and Electronic Regulator for Superconducting Rotating Machinery,” filed on Jan. 11, 2000, assigned to American Superconductor Corporation, assignee of the present invention, and incorporated herein by reference. 
   The rotor assembly includes an electromagnetic shield  88  wrapped around the vacuum chamber  86 , formed preferably from a non-magnetic material (e.g., aluminum, copper). In embodiments in which vacuum chamber  86  is formed of a different material, such as stainless steel, electromagnetic shield  88  can be mechanically located around the outer wall of the vacuum chamber  86 . Electromagnetic shield  88  also acts as an induction structure (i.e., supports induction Currents) and is, therefore, multi-purposed. Specifically, electromagnetic shield  88  intercepts AC magnetic fields from the stator before they impact the superconducting windings  26  of the rotor assembly  12 . Further, because electromagnetic shield  60  acts as an induction structure, it can be used to operate the synchronous superconducting motor  10  at start-up in an induction mode. The electromagnetic shield  88  allows the superconducting motor  10  to operate as an induction motor for start up or in a continuous mode as a backup mode in case of a catastrophic failure of the cryogenic systems. This mode of operating a synchronous motor is described in U.S. patent application Ser. No. 09/371,692, which is assigned to American Superconductor Corporation, assignee of the present invention, and incorporated herein by reference. 
   Referring to  FIG. 9 , the rotor assembly  50  further includes superconducting rotor coils  52  mounted to a stainless steel rotor body  58  for support. The rotor body  58  also carries the closed cryogenic cooling loop  118  that cools the rotor coils  52 . The rotor body  58  is tubular with an inner surface  90  and an outer surface  92 . The outer surface  92  may be generally cylindrical in shape, or may have flats machined to accept the rotor coils  52 . The machined flats may, for example, give the outer surface  92  a general pentagonal, hexagonal or heptagonal shape. In the present invention, twelve flats have been machined to accept twelve flat rotor coils  52 . 
   The rotor body  58  includes rotor body ribs  59  to mount the tangential buckles  70  and axial buckles  60 , which interface with the output shaft  82 . The rotor body ribs  59  are circumferentially fixed on the inner surface  90  and extend radially inward from the inner surface  90  of the rotor body  58 . 
   In this embodiment, the superconductor in the rotor coils  52  is a high temperature copper oxide ceramic superconducting material, such as Bi 2 Sr 2 Ca 2 Cu 3 O x  or (BiPb) 2 , commonly designated BSCCO 2223 or BSCCO (2.1)223. Other high temperature superconductors including YBCO (or superconductors where a rare earth element is substituted for the yttrium), TBCCO (i.e., thallium-barium-calcium-copper-oxide family), and HgBCCO (i.e., mercury-barium-calcium-copper-oxide family) are also within the scope of the invention. Rotor coils  52  may be formed with pancake coils either single or double layers. In certain embodiments, double pancake coils with the two coils of a pair being wound from the same continuous length of superconducting tape may be used. In this case, a pancake coil may include a diameter smaller than its associated pancake coil of the double pancake. An approach for using this approach is described in U.S. Pat. No. 5,581,220, which is assigned to American Superconductor, the assignee of the present invention, and incorporated herein by reference. Preferred embodiments are based on the magnetic and thermal properties of high temperature superconducting composites, preferably including superconducting ceramic oxides and most preferably those of the copper oxide family. Tile structure and operation of the superconducting windings is described in U.S. patent application Ser. No. 09/415,626, entitled “Superconducting Rotating Machine,” filed on Oct. 12, 1999, assigned to American Superconductor Corporation, assignee of the present invention, and incorporated herein by reference. 
   Referring to  FIG. 10 , the rotor coils  52 , as described above, are fabricated with an internal support  54  to help stabilize the structure because the racetrack configuration produces tremendous bending stresses that attempt to push the superconducting coil assembly apart. To overcome this limitation, the rotor coils  52  are fabricated in a laminated configuration with internal coil supports  54 , alternating between superconducting windings  126  and internal support  54 . External supports, such as the inner spacer  140  and the outer spacer  142 , do not sufficiently alleviate the internal stresses associated with non-circular and non-linear configurations, such as the racetrack configuration. The addition of internal coil supports  54  combined with the inner spacer  140  and outer spacer  142  gives mechanical strength to the rotor coil  52  and reduces the internal strains in the superconducting coils  126 . The internal strains are reduced by using the internal coil supports  54  partly because the peak strains are located at the inside diameter of the superconducting coils  126 , far removed from any external support structures that could be employed. 
   In the present embodiment, the internal coil support  54  is 40-mil thick stainless steel. However, it can be appreciated that various thicknesses and materials (such as copper or fiberglass composites) would work for their intended purposes, as various embodiments would require different thicknesses to optimize performance. In certain embodiments, a thermally conductive coating can be applied to the internal coil support  54  to provide better heat conductivity to cryogenic cooling tubes  118  located within the rotor body  58 . For example, the internal coil support can be coated with copper. 
   A fastener can be used to tie the internal coil supports  54  together. For example, the layers can be mechanically fastened together by passing a bolt, or multiple bolts, through the internal coil supports  54  at a point within the annular opening  136  created by the superconductor windings  126  and fixing the assembly and top cap  144  to the rotor body  58 . 
   The bolts tie the internal coil supports  54  together into a unitary whole, resulting in even greater mechanical strength. The rotor coils  52  can also be epoxied together, with or without fasteners, to further fix the lamination together. 
   The internal coil support member  54  will also have various openings (not shown) to facilitate electrical connections between adjacent superconductor windings. Each superconducting coil assembly in the rotor coils  52  has to be electrically connected. Since the internal support members  54  are placed between each rotor coil  52 , an opening must be provided to allow the electrical connection between each rotor coil  52 . 
   Referring to  FIGS. 11 and 13B , the axial buckles  60  are assembled in the rotor assembly  50  to prevent axial movement between the rotor body  58  and the output shaft  82 . The axial buckles  60  also thermally isolate the cryogenically cooled rotor body  58  from the output shaft  82  by using a thermally isolating coupling band  66  between the coupling members  62  and  64 . 
   A generally U-shaped coupling member  62  is mounted to the rotor body  58  by sliding the open end over the rotor body rib  59 . The rotor body rib  59  constrains the U-shaped coupling member  62  in the axial direction. Two smaller coupling members  64  are mounted in opposing radial slots  85 A in the circumferential output shaft plates  84 A by a narrow shoulder  65  on one face of the smaller coupling members  64 . The narrow shoulder  65  slides into the radial slot  85 A while the rest of the smaller coupling member  64  is wider than the radial slot  85 A, thereby preventing the smaller coupling member  64  from moving beyond the slot  85 A. The two smaller coupling members  64  are mechanically coupled to the U-shaped coupling member  62  by thermally isolating coupling bands  66 . The thermally isolating coupling bands  66  are formed as straps made of reinforcing epoxy (e.g., Para-aramid epoxy). By using thermally isolating coupling bands  66 , the output shaft  82  and the rotor body  58  are thermally isolated from each other since the coupling bands  66  are the only direct connection between the U-shaped coupling member  62  and the smaller coupling members  64 . This thermal isolation helps prevent the output shaft  82  from acting as a heat sink. 
   The coupling bands  66  wrap around spherical ball end couplings  69  mounted in the U-shaped coupling member  62  and the smaller coupling members  64 . The spherical ball end coupling  69  in one of the smaller coupling members is a cam  68 , which is used to preload the coupling bands  66 . Surrounding the cylindrical pins  72  and cam  68  are spherical ball ends  69 . The spherical ball end couplings  69  hold the coupling band  66  and provide misalignment take-up. The spherical ball end couplings  69  maintain even loading to the coupling band  66 . The coupling bands  66  are preloaded by turning the cam  68  to vary the tension. The coupling bands  66  are 180° apart, which allows one cam to tension both coupling bands  66  at the same time and put both coupling bands  66  in uniaxial tension. This configuration also constrains the rotor body  58  and output shaft  82  in both axial directions. The adjustability of the cam  68  allows each axial buckle  60  to be quickly preloaded by adjusting to any manufacturing tolerance differentiation for example within the coupling bands  66 , thereby facilitating a quicker build time for the rotor assembly  50 . 
   Referring to  FIGS. 12A and 13A , the tangential buckles  70  are assembled in the rotor assembly  50  to transfer the rotational forces between the rotor body  58  and the output shaft  82 . The tangential buckles  70  also thermally isolate the cryogenically cooled rotor body  58  from the output shaft  82  by using a thermally isolating coupling band  66  between the coupling members  72  and  74 . 
   An X-shaped coupling member  74  is mounted to the output shaft  82  by two recessed slide mounting areas  78  located on opposing legs of the X-shaped coupling member  74 . These recessed slide mount areas  78  are positioned such that the X-shape coupling member  74  mounts parallel to the axis of the output shaft  82 . The recessed slide mounting areas  78  slide down into the radial slot  85 B in the longitudinal plates  84 B, which constrain the X-shaped coupling  74  in the circumferential and axial directions. Two spherical ball end coupling  69  are mounted between the rotor body ribs  59  by pressing a cylindrical pin  72  through the rotor body ribs  59  and a spherical ball end coupling  69 . The spherical ball end couplings  69  are mechanically coupled to the X-shaped coupling member  74  by thermally isolating coupling bands  66 . As discussed above, the thermally isolating coupling bands are Para-aramid/Epoxy straps, which thermally isolate the rotor body  58  form the output shaft  82 . 
   Referring to  FIGS. 12A and 12B , the coupling bands  66  wrap around spherical ball end couplings  69  mounted in the X-shaped coupling member  74 , in the two legs not defining the recessed slide mounting area  78 , and around the spherical ball end coupling  69  mounted in the rotor body ribs  59 . The coupling bands  66  are mounted 180° apart, which allows both coupling bands to be in uniaxial tension. The X-shaped coupling member  74  defines an opening  80  therethrough sized to accept a spring  96 , which preloads both bands in uniaxial tension. The opening  80  is defined so as to be perpendicular to the axis of the output shaft  82  when the X-shaped coupling member  74  is mounted to the output shaft  82 , allowing the spring  96  to push the X-shaped coupling member  74  radially outward. The spring  96  allows the tangential buckle  70  to be preloaded by compressing the spring  96 . The spring  96  also allows for some compliance when the tangential buckle  70  is assembled within the rotor assembly  50 . The compressed spring  96  allows each tangential buckle  70  to be quickly preloaded by adjusting to any manufacturing tolerance differentiation for example within the coupling bands  66 , thereby facilitating a quicker build time for the rotor assembly  50 . The preload feature also facilitates loading the coupling bands  66  in pure tension. By loading the coupling bands  66  in pure tension, the assembly can transmit an extremely large torque between the rotor body  58  and the output shaft  82 . 
   The longitudinal output shaft plates  84 B are sized within axial slots ( FIG. 13A ) in the rotor body  58  such that they will bottom out during a high fault loading situation, thereby preventing the coupling bands  66  from breaking. If a sudden shock load is applied to the motor  10 , metal-to-metal contact will occur. The advantage to designing such a shock system is that the coupling bands  66  do not have to be sized for fault and shock loads, which would make the coupling bands  66  less practical. 
   Referring to  FIGS. 2 ,  14  and  15 , a cryogenic cooling system  100  is used to maintain a cryogenic fluid at cryogenic temperatures and move the cryogenic fluid to and from a cryogenic cooling loop  118  located adjacent and in thermal communication with the rotor coils  52 . The cryogenic fluid is moved through the cryogenic cooling loop  118  by a cryogenically adaptable fan  114 . This system helps maintain the rotor coils  52  at cryogenic temperatures, because the superconducting rotor coils  52  have to be maintained at cryogenic temperatures (i.e., below −79° C.) to operate properly and efficiently. The cryogenic cooling system  100  includes multiple cryogenically cooled surfaces  102 , here Gifford-McMahon cold heads, mounted in cryocooler assemblies  104 , a mounting flange  106  and a cryogenically adaptable fan  114 . The cryogenic cooling system  100  utilizes a closed loop system for efficiency and ease of maintenance. 
   The advantage of more than one cryogenically cooled surface  102  is efficiency and ease of maintenance. First, more than one cryogenically cooled surface  102  in series will allow each cryogenically cooled surface  102  to work less to lower the temperature of the cryogenic fluid. Also, if one cryogenically cooled surfaces  102  malfunctions, the redundancy in the system will be able to overcome the loss. Further, if one cryogenically cooled surface  102  does malfunction, the malfunctioning cryogenically cooled surface  102  can be isolated from the system by proper valving, and maintenance performed without shutting down the system or introducing contaminants into the system. 
   The cryocooler assembly  104  mounts to the outside of the superconducting motor  10  via amounting flange  106  fixed to the housing  12 . The fixed cryocooler assembly  104  is in fluidic communication with a cryogenic cooling loop  118 . In an embodiment with a rotating thermal load, such as the rotor coils  52 , the cryocooler assembly  104  interfaces with the rotating cryogenic cooling loop  118  by interfacing with a rotary seal  108 , here a ferrofluidic rotary seal. The rotary seal  108  allows the cryocooler assembly  104  to remain fixed while the cryogenic cooling loop  118  rotates with the rotor assembly  50 . The cryocooler assembly  104  is maintained stationary, rather than rotating, due to undesirable high gravity heat transfer seen internal to the cryocooler assembly  104  if it were to rotate. The cryogenic cooling loop  118  is in thermal communication with the rotor coils  52 , maintaining the rotor coils  52  at a cryogenic temperature. 
   The cryocooler assembly  104  is open to the vacuum chamber  86  of the rotor assembly  50 . Keeping the internal area of the cryocooler assembly  104  at vacuum helps to isolate the portion of the cryogenic cooling loop  118  that is located within the cryocooler assembly  104  from outside temperatures. The vacuum isolation further helps improve the efficiency of the cryogenically cooled surfaces  102 . 
   The cryogenic fluid, helium in this embodiment, is introduced into the system from a cryogenic fluid source  116 . The cryogenic cooling system is a closed system, but cryogenic fluid will have to be added periodically should any leaks develop. Other cryogenic fluids, such as hydrogen, neon or oxygen, may also be used. 
   The cryogenic fluid must be moved from the cryocooler  104  to the portion of the cryogenic cooling loop  118  located within the rotor body  58 . A cryogenically adaptable fan  114  is employed to physically move the cryogenic fluid. The advantage of a fan is that a fan does not require a heat exchanger to warm the fluid to the temperature of an ambient compressor, is inexpensive and is relatively small. In comparison, a prior art room temperature compressor in conjunction with a heat exchanger is more expensive and is much larger. Further details of the operation of the cryogenic cooling system  100  can be found in U.S. patent application Ser. No. 09/480,396, entitled “Cooling System for HTS Machines,” filed on Jan. 11, 2000, assigned to American Superconductor Corporation, assignee of the present invention, and incorporated herein by reference. 
   As discussed above in conjunction with FIGS.  1  and  3 - 5 , each stator coil  22  is wound into a diamond pattern. In many rotating machine applications, particularly those having air cores, the conductor used to wind the stator coils are Rutherford-type conductors. The Rutherford-type conductor generally includes a number of strands, each of which is transposed so that, over a given length (i.e., a full transposition pitch), the strand occupies the cross-sectional position of every other strand in the conductor. Each strand includes an outer insulative coating, such as enamel. With this arrangement, eddy current heating caused by radial and tangential magnetic fields are significantly reduced. Thus, Rutherford-type conductors are well-suited for use in constructing stator windings. 
   However, in constructing a Rutherford-type conductor, mechanical fabrication constraints limit the realizable geometry of the conductor. For example, for Rutherford-type cables formed with a substantially rectangular geometry, mechanical fabrication constraints generally limit the aspect ratio of the conductor (width:height) of the conductor to be about 10:1. In certain applications, such as high current applications, it may be necessary to wind more than one Rutherford-type conductor in-hand. For example, winding a coil two-in hand means that two conductors are wound one over the other, thereby increasing the overall cross-sectional area of conductor. Thus, winding a pair of conductors, each having a 10:1 aspect ratio, would form a conductor with roughly a 5:1 aspect ratio. 
   Referring to  FIG. 16  each single layer of a conductor  400  includes an outer insulation layer  402 , for example, a 1-2 mil thick layer of Kapton®, a product of E.I. dupont de Nemours and Company, Wilmington, Del. This outer insulation layer is commonly referred to as the “turn-to-turn” insulation. When winding multiple layers in-hand, the interface between the broad faces of the pair of conductors is occupied by two layers of turn-to-turn insulation. Thus, the ratio of the cross-sectional area of the current-carrying conductor to the overall cross-sectional area of the conductor, referred to as the “packing factor” decreases. Of course, as more single layer conductors are wound in-hand, the smaller the packing factor and the larger the turn-to-turn insulation penalty. 
   Referring to  FIG. 17 , stator coil  410  includes two pancakes  412 ,  414 , each wound two-in hand. That is, each pancake  412 ,  414  includes a first conductor  416  and a second conductor  418  wound over the other. It is important to note that the relative positions of the first conductor  416  and second conductor  418  are reversed in pancakes  412 ,  414 . In other words, as shown in  FIG. 17 , first conductor  416  is above second conductor  418  in pancake  412 , while in pancake  414 , first conductor  416  is below second conductor  418 . The transposition of the first conductor and the second conductor takes place at a base  420  of the coil. One approach for manufacturing the double pancake stator coil  410  is first wind out an appropriate length of first and second conductors  416 ,  418 . Pancake  412  is then wound from the base  420  to the outside diameter so that the ends of conductors  416 ,  418  are accessible at the outer diameter. Pancake  414 , in similar fashion, is then wound from the base to the top of the coil. First and second conductors are electrically isolated from each other using a relatively thin layer of insulation  419  (e.g., 1-2 mil mylar tape) or a layer of Formvar, but are electrically connected at an end region of the diamond-shaped stator coil  22 . Ground wall insulation  422  is then applied over the pancakes  412 ,  414 . With this arrangement, voltage induced in the circuits formed by first and second conductors  416 ,  418  are identical to a first order and any circulating currents between the circuits are minimized, thus reducing overall losses of the coil. 
   Referring to  FIG. 18 , in another embodiment, a stator coil  430  include two pancakes  432 ,  434 , each wound three-ill hand. As was the case above, each pancake  432 ,  434  includes a first conductor  436 , a second conductor  438 , and a third conductor  440  wound over each other. In this three-in hand winding approach, first pancake  432  is formed so that second conductor  438  is sandwiched between the other conductors, with first conductor  436  above the second conductor  438  and third conductor  440  below of the second conductor. Second pancake,  434 , however, is wound such that first conductor  436  is below second conductor  438  and third conductor  440  is above the second conductor. The transposition of the first conductor and the third conductor takes place at a base  441  of the coil. All three conductors,  436 ,  438 ,  440  are electrically isolated from each other using insulation, but are electrically connected at the end regions of the coil and ground wall insulation  442  is then applied over the pancakes  432 ,  434 . 
   Diamond-shaped stator coils for use with motor  10  can be formed from either of the two-in hand or three-in hand double pancake coils described above. One approach is to secure end regions of the coils and then the longer sides of the coil are grasped and rolled or twisted under, thereby forming the hairpin end regions of the stator coils. In a variation of this approach, the stator coil can be bent into a saddle form, such as those described in U.S. Ser. No. 09/415,626, filed Oct. 12, 1999, and entitled SUPERCONDUCTING ROTATING MACHINES, which is incorporated herein by reference. 
   A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the components described could be adapted to produce other superconducting rotating machines, such as a superconducting generator. Accordingly, other embodiments are within the scope of the following claims.