Abstract:
The invention features an internally supported superconducting coil assembly. The invention includes several superconducting windings and at least one internal coil support member that forms a laminate stack alternating between an internal support member and a superconducting winding.

Description:
This invention arose in part out of research pursuant to Contract No. N00014-99-C-0296 awarded by the Office of Naval Research. 
    
    
     TECHNICAL FIELD 
     This invention relates to the construction and operation of superconducting rotating machines, and more particularly to superconductor winding construction for use in superconducting motors. 
     BACKGROUND 
     Superconducting air core, synchronous electric machines have been under development since the early 1960s. The use of superconducting windings in these machines has resulted in a significant increase in the magneto motive forces generated by the windings and increased flux densities in the machines. However, superconducting windings generate tremendous internal stresses that attempt to force the superconducting windings into circular shapes. Certain applications require the superconducting windings to be non-circular for various reasons and the internal stresses must be alleviated or supported. 
     SUMMARY 
     The invention features an internally supported superconducting coil assembly. The invention includes several superconducting windings and at least one internal coil support member that forms a laminate stack alternating between an internal support member and a superconducting winding. Embodiments of this aspect of the invention may include one or more of the following features. 
     The internal coil support members are especially advantageous when non-circular superconducting windings are utilized. In certain embodiments, a racetrack shaped superconducting winding is used. The racetrack shape is defined by two opposing arcuate end sections and two substantially straight side sections. The internal magnetic stresses generated by the superconducting winding attempts to force the superconducting winding to become round in shape. The internal coil support members help alleviate the internal stresses. The internal coil support members work better than external support members because the bending stresses are greatest near the center of the winding, away from any external supports. 
     In certain embodiments, the superconducting coil assembly laminate can be fixed to a rotor body for use in a rotating machine by passing a bolt through the laminate and into the rotor body. The bolt, or multiple bolts, will help unify the laminate into a unitary whole. The laminate may also be impregnated with epoxy to achieve a unitary whole. 
     The internal coil support members must have openings to allow electrical connection between adjacent superconducting windings that are separated by the internal coil support member. The internal coil support member is usually made of stainless steel, which further helps quench the magnetic forces. 
    
    
     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 . 
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     Referring to FIG. 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 . 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 150 N·m/Kg or more at 300 RPM or less. Furthermore, such motors are expected to provide a greatly improved gap shear stress characteristic in a range between 30 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 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 , as well as the heat generated by the conductor turns  24 . 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 an 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 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 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. 
     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  16  spaced from a stationary disk  14  (e.g., spaced 1-4 mm). Rotating disk  16  is formed of a high permeability laminated material (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 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, assigned to American Superconductor Corporation, assignee of the present invention, and is 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. The 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 Para-aramid/Epoxy straps. 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 within the coupling bands  66 , thereby facilitating a quicker build time for the rotor assembly  50 . 
     Referring to FIGS. 12 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  from the output shaft  82 . 
     Referring to FIGS. 12 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 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 (not shown) 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  impractical. 
     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 a mounting 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. 
     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.