Patent Publication Number: US-7589441-B2

Title: Circumferentially wound cooling tube structure for cooling a stator

Description:
RELATED APPLICATIONS 
   This application is a divisional of U.S. application Ser. No. 11/742,083, filed Apr. 30, 2007, which is a continuation of U.S. application Ser. No. 10/083,927, filed Feb. 27, 2002, which is a continuation-in-part (and claims the benefit of priority under 35 USC 120) of U.S. application Ser. No. 09/639,218, filed Aug. 15, 2000, which is a conversion of U.S. provisional application Ser. No. 60/149,129, filed Aug. 16, 1999. The disclosure of the prior application is considered part of (and is incorporated by reference in) the disclosure of this application. 

   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. 09/632,602, filed Aug. 4, 2000, entitled “Segmented Rotor Assembly For Superconducting Rotating Machines”; (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. 09/632,601, filed Aug. 4, 2000, entitled “Stator Support Assembly For Superconducting Rotating Machines”; (5) U.S. application Ser. No. 09/480,430, filed Jan. 11, 2000, entitled “Exciter and Electronic Regulator for Rotating Machinery”; (6) U.S. application Ser. No. 09/481,480, filed Jan. 11, 2000, entitled “Internal Support for Superconducting Wires”; (7) U.S. Ser. No. 09/480,396, filed Jan. 11, 2000, entitled “Cooling System for HTS Machines”; (8) U.S. application Ser. No. 09/415,626, filed Oct. 12, 1999, entitled “Superconducting Rotating Machine”; (9) U.S. Application No. 60/266,319, filed Jan. 11, 2000, entitled “HTS Superconducting Rotating Machine”; (10) U.S. Application No. 09/905,611, filed Jul. 13, 2001, entitled “Enhancement of Stator Leakage Inductance in Air-Core Machines”; (11) U.S. application Ser. No. 09/956,328, filed Sep. 19, 2001, entitled “Axially-Expandable EM Shield”; and (12) U.S. application Ser. No. 09/480,397, filed Jan. 11, 2000, entitled “Stator Construction For Superconducting Rotating Machines”. 
   TECHNICAL FIELD 
   This invention relates to rotating machines. 
   BACKGROUND 
   Superconducting air-core, synchronous electric machines have been under development since the early 1960&#39;s. The use of superconducting windings in these machines has resulted in a significant increase in the field electromotive forces generated by the windings and increased flux and power densities of the machines. 
   Early superconducting machines included field windings wound with low temperature superconductor (LTS) materials, such as NbZr or NbTi and later with Nb 3 Sn. The field windings were cooled with liquid helium from a stationary liquefier. The liquid helium was transferred into the rotor of the machine and then vaporized to use both the latent and sensible heat of the fluid to cool the windings. This approach proved to be viable for only very large synchronous machines. With the advent of high temperature superconductor (HTS) materials in the 1980&#39;s, the cooling requirements of these machines were greatly reduced and smaller superconducting machines were realizable. 
   In addition to the heat generated by the rotor assembly, the stator assembly also generates a considerable amount of heat that must be removed in order for the superconducting machine to operate efficiently. In conventional “non-superconducting” rotating machines, iron teeth are utilized between the individual stator coil assemblies, which act as heat sinks and remove the heat generated by the stator assembly. However, in superconducting machines, the flux density is so great between these stator coil assemblies that these iron teeth would immediately become saturated, resulting in Eddy current heating and operating inefficiency. 
   SUMMARY 
   According to an aspect of this invention, a stator assembly includes a plurality of stator coil assemblies and a stator coil support structure constructed of a non-magnetic, thermally-conductive material. The stator coil support structure includes an axial passage for receiving a rotor assembly, and a plurality of channels positioned radially about the axial passage. Each channel is configured to receive one or more of the stator coil assemblies. 
   One or more of the following features may also be included. Each stator coil assembly is surrounded by a ground plane assembly. The stator assembly further includes a magnetic annular assembly surrounding the stator coil support structure. The magnetic annular assembly includes a plurality of axial coolant passages. The stator assembly further includes a coolant circulation system for circulating a cooling liquid through the axial coolant passages. The non-magnetic, thermally conductive material is a sheet material which is laminated to form the stator coil support structure. The sheet material is an advanced thermal transfer adhesive. The sheet material is Grafoil. The stator assembly further includes an epoxy filler which fills any voids between the stator coil assemblies and the stator coil support structure. 
   According to a further aspect of this invention, a superconducting rotating machine includes a stator assembly having a plurality of stator coil assemblies, and a stator coil support structure constructed of a non-magnetic, thermally-conductive material. The stator coil support structure includes an axial passage for receiving a rotor assembly, and a plurality of channels positioned radially about the axial passage. Each channel is configured to receive one or more of the stator coil assemblies. A rotor assembly is configured to rotate within the stator assembly. The rotor assembly includes an axial shaft and at least one superconducting rotor winding assembly. 
   One or more of the following features may also be included. Each stator coil assembly is surrounded by a ground plane assembly. The stator assembly further includes a magnetic annular assembly surrounding the stator coil support structure. The magnetic annular assembly includes a plurality of axial coolant passages. The superconducting rotating machine further includes a coolant circulation system for circulating a cooling liquid through the axial coolant passages. The non-magnetic, thermally conductive material is a sheet material which is laminated to form the stator coil support structure. The sheet material is an advanced thermal transfer adhesive. The sheet material is Grafoil. The superconducting rotating machine further includes an epoxy filler which fills any voids between the stator coil assemblies and the stator coil support structure. The at least one superconducting rotor winding assembly is constructed using a high-temperature superconducting material. The high temperature superconducting material is chosen from the group consisting of: thallium-barium-calcium-copper-oxide; bismuth-strontium-calcium-copper-oxide; mercury-barium-calcium-copper-oxide; and yttrium-barium-copper-oxide. The superconducting rotating machine further includes a refrigeration system for cooling the at least one superconducting rotor winding assembly. 
   According to a further aspect of this invention, a method of manufacturing a stator coil support structure includes forming a non-magnetic, thermally conductive cylindrical structure and forming a plurality of axial channels radially about the non-magnetic, thermally conductive cylindrical structure. The method positions one or more stator coil assemblies in each of the channels. 
   One or more of the following features may also be included. Forming a non-magnetic, thermally conductive cylindrical structure includes laminating multiple layers of a non-magnetic, thermally conductive sheet material to form the non-magnetic, thermally conductive cylindrical structure. Forming a non-magnetic, thermally conductive cylindrical structure includes casting a non-magnetic, thermally conductive material to form the non-magnetic, thermally conductive cylindrical structure. A plurality of axial coolant passages are provided in the non-magnetic, thermally conductive cylindrical structure. An epoxy filler is deposited between the stator coil assemblies and the non-magnetic, thermally conductive cylindrical structure. 
   According to a further aspect of this invention, a method of manufacturing a stator coil support structure includes forming a non-magnetic, thermally conductive cylindrical structure and forming a plurality of axial slots radially about the non-magnetic, thermally conductive cylindrical structure. The method inserts into each axial slot a heat-sinking member, thus forming a channel between each pair of adjacent heating-sinking members. The method positions one or more of the stator coil assemblies in each of the channels. 
   One or more of the following features may also be included. Forming a non-magnetic, thermally conductive cylindrical structure includes laminating multiple layers of a non-magnetic, thermally conductive sheet material to form the non-magnetic, thermally conductive cylindrical structure. Forming a non-magnetic, thermally conductive cylindrical structure includes casting a non-magnetic, thermally conductive material to form the non-magnetic, thermally conductive cylindrical structure. A plurality of axial coolant passages are provided in the non-magnetic, thermally conductive cylindrical structure. An epoxy filler is deposited between the stator coil assemblies and the non-magnetic, thermally conductive cylindrical structure. 
   According to a further aspect of this invention, a stator assembly includes a plurality of stator coil assemblies, a magnetic annular assembly, and a plurality of non-magnetic, thermally-conductive heat sinking members positioned radially about the magnetic annular assembly. This forms a plurality of channels, each configured to receive one or more of the stator coil assemblies. 
   One or more of the following features may also be included. The magnetic annular assembly includes a plurality of axial coolant passages. A coolant circulation system circulates a cooling liquid through the axial coolant passages. The non-magnetic, thermally-conductive heat sinking members are constructed of a non-magnetic, thermally conductive sheet material. The sheet material is laminated to form the non-magnetic, thermally-conductive heat sinking members. The sheet material is a polymer-based adhesive or a graphite-based material. The stator assembly further includes an epoxy filler disposed between the stator coil assemblies and the non-magnetic, thermally-conductive heat sinking members. 
   According to a further aspect of this invention, a method of manufacturing a stator coil support structure includes forming a magnetic annular assembly and forming a plurality of non-magnetic, thermally-conductive heat sinking members. The heat-sinking members are positioned radially about the magnetic annular assembly. This forms a channel between each pair of adjacent heating-sinking members. One or more stator coil assembly are positioned in each of these channels. 
   One or more of the following features may also be included. Forming a plurality of non-magnetic, thermally conductive heat-sinking members includes laminating multiple layers of a non-magnetic, thermally conductive sheet material, or casting a non-magnetic, thermally conductive material, to form the non-magnetic, thermally conductive heat-sinking members. The method further includes providing a plurality of axial coolant passages in the magnetic annular assembly and depositing an epoxy filler between the stator coil assemblies and the non-magnetic, thermally conductive heat-sinking members. 
   One or more advantages can be provided from the above aspects of the invention. Stator coil assemblies can be positioned proximate thermally-conductive heat sinks. Accordingly, the stator coil assemblies and the stator itself can operate at lower temperatures. As these heat sinks are constructed of a non-magnetic material, heat sink flux saturation is eliminated. This elimination of flux saturation minimizes Eddy current stator heating which causes stator inefficiencies. Stator heating can further be reduced by incorporating a circulation system which circulates a cooling fluid through the stator coil support structure. 
   The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 

   
     DESCRIPTION OF DRAWINGS 
       FIG. 1  is a perspective view of a single layer, three-phase stator having coil windings; 
       FIG. 2  is an exploded perspective view of the stator of  FIG. 1  including external helical cooling tubes; 
       FIG. 3  is a cross-sectional schematic representation of the stator and the cooling tubes of  FIG. 2 ; 
       FIG. 3A  is a partial assembly of stator coils with cooling tubes; 
       FIG. 4  is side cross-sectional view of an alternative embodiment of a stator cooling system; 
       FIG. 5  is a end on cross-sectional view of the cooling system along the plan A-A of  FIG. 4 ; 
       FIG. 6  is an enlarged view about portion A of the cooling system of  FIG. 5 ; 
       FIG. 7  is a perspective view of an alternate embodiment of a stator cooling system for a coil winding; 
       FIG. 8  is a cross-sectional perspective view of the stator cooling system of  FIG. 7 ; 
       FIG. 9  is an end view of the stator cooling system of  FIG. 7 ; 
       FIG. 10  is a cross-sectional side view of a superconducting rotating machine; 
       FIG. 11  is a cross-sectional end view of the stator assembly of the superconducting rotating machine of  FIG. 10 ; 
       FIG. 12  is a flow chart of a method of manufacturing a stator coil support structure; 
       FIG. 13  is a flow chart of another method of manufacturing a stator coil support structure; 
       FIG. 14  is a flow chart of another method of manufacturing a stator coil support structure; 
       FIG. 15  is a flow chart of another method of manufacturing a stator coil support structure; 
       FIG. 16  is a cross-sectional end view of another embodiment of the stator assembly of the superconducting rotating machine of  FIG. 10 ; and 
       FIG. 17  is a flow chart of another method of manufacturing a stator coil support structure. 
   

   Like reference symbols in the various drawings indicate like elements. 
   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , a three-phase stator  1  includes multiple phase coil assemblies  8 - 13 , which are arranged into an inner layer of phase coil assemblies  11 ,  12 ,  13  and an outer layer of phase coil assemblies  8 ,  9 ,  10 . The outer layer coil assemblies  8 ,  9 ,  10  have end regions  8   a ,  9   a ,  10   a  which extend away from corresponding end regions of adjacent inner phase coils. Each phase coil assembly includes concentric coil windings  7  which are insulated from each other. Note that end regions  8   a ,  9   a ,  10   a  of the outer layer coil assembly  8 ,  9 ,  10  are exaggerated in  FIG. 1  and are not normally perpendicular to the stator central axis (see  FIG. 3 ). As will be described in greater detail below, the invention is directed to cooling systems which minimize exposure of coolant, here water to the high voltages within the stator coils, thereby allowing the use of fresh water, which contains ions. 
   Individual phase coil windings are made from any electrically conductive material, e.g., copper and aluminum. Typically, the phase coils are made from copper. Phase coil assemblies can be constructed using different methods. 
   In the embodiment shown in  FIG. 1 , for example, each phase coil assembly includes many concentric individually insulated coil windings. Typically, each phase coil assembly can include any number of concentric coil windings depending upon the stator motor design. Additionally, each concentric coil winding can include individually insulated coils assembled together to form the concentric coil winding. The individual coils can be insulated to withstand coil-to-coil voltage and assembled to form the concentric coil winding. Each concentric coil windings is then assembled to form a phase coil assembly, which is insulated to full phase-phase and phase-ground voltage levels. 
   In another method, a conductor is concentrically wound with adequate turn-to-turn insulation to form a phase coil assembly. Completed phase coil assemblies are insulated to full phase-phase and phase-ground voltage levels. In order to reduce eddy-current losses in these coils, it is generally desirable that any fully transposed Litz-type cable be employed. In certain applications, a Rutherford type conductor is employed. A Rutherford type conductor includes many smaller strands, which are fully transposed to decouple an AC field experienced by a conductor in any orientation. Rutherford conductors are also flexible making the task of coil fabrication easier. All phase coil assemblies are insulated to industry acceptable insulation classes (such as class H and F insulations), which normally dictate the highest temperature that the conductor could be operated at. Likewise, Rutherford type conductors are readily available from a number of vendors such as New England Electric Wire, Lisbon, N.H. 
   Referring to  FIG. 2 , a cooled stator system  100  includes a stator inner coil  14  received within a central bore  2  of the stator, an outer coil  17  wrapped about the outer surface of stator  1 , and end coils  101 ,  102  wrapped about ends  103 ,  104  of the stator. Outer coil  17  includes end portions  117 ,  119  which surround outer layers  105 ,  106  of phase coil assemblies  8 ,  9 ,  10  and a central portion  120  which surrounds a midsection  107  of the inner layer of all phase coil assemblies  8 - 13 . Each of inner coil  14 , outer coil  17 , and end coils  101 ,  102  is in fluid communication with inlets,  15 ,  18 ,  110 ,  112  and outlets  16 ,  19 ,  111 ,  113 , respectively. 
   As shown in  FIG. 3 , a cooled stator system  200  includes a phase coil  1  wrapped around a non-metallic bore tube  162  having an axis L. Cooling tubes  14 ,  101 ,  102  and  17  are applied to phase coil  1  and encased in core  160 . Core  160 , typically, is an iron core constructed from 0.02 inch thick iron laminations, e.g., those used by the motor industry. The laminations are cut in circular segments and assembled around the stator assembly  200 . Alternatively, core  160  is formed by winding an iron wire of high permeability. Core  160  is insulated by a varnish or oxide for eliminating eddy-current heating. Sufficient layers of this wire could be applied to produce a smooth cylindrical outer surface  170  shown in  FIG. 3 . 
   Cooled stator system  200  is inserted inside a motor housing. The entire assembly, including stator and motor housing, are impregnated with an epoxy to bond all components of the stator together to produce a monolithic structure. Inner coil  14  is supported within stator  1  by bore tube  162 . Inner coil  14 , outer coil  17 , and end coils  101 ,  102  are electrically insulated from stator  1  by an insulator  150 . Insulator  150  maintains coils  14 ,  17 ,  101 ,  102  at a ground potential permitting the use of fresh water, which contains ions. Insulator  150  is made from any insulating material that can withstand operating voltages and the heat generated by stator  1 . In general, insulator  150  has a thickness to withstand the operating voltage. The thickness of insulator  150  is determined by the dielectric strength (insulating properties) of the material. For example, the thickness of a high dielectric strength insulating material can be less than the thickness of a low dielectric strength insulating material. Typically, insulator  150  has a thickness between about 0.001 to 0.100 inches. Examples of insulative materials include, but are not limited to, epoxy, mica, and glass. 
   In operation, heat is transferred from the stator conductors through insulator  150  and into coils  14 ,  17 ,  101 ,  102 , which contain cooled fresh water. By having a higher fluid pressure at inlets  15 ,  18 ,  110 ,  112  than at outlets  16 ,  19 ,  111 ,  113  cold fluid is forced to flow through coils  14 ,  17 ,  101 ,  102 . Thus, heat transferred to the fresh water is removed from the cooled stator system. To improve cooling of stator  1 , inner coil  14  removes heat from the inside while the other outer coil  17 , and end coils  101 ,  102  remove heat from the outside.  FIG. 3A  shows phase coils  8 - 13  surrounded by cooled tube  17 . 
   Referring to  FIG. 4 , in another embodiment, a cooled stator system  200  includes a stator  1 , as shown in  FIG. 1 , encased by a thermally conducting material  24 . Thermally conducting materials  27  and  37  are formed by laminating a series of plates  21  around the midsection  107  of the stator  1 . The phase coils  8 ,  9 ,  10 ,  11 ,  12  and  13  are assembled around bore tube  162  such that they are contacting each other at the bore tube surface. However, coil sides are separated from each other at the outer surface of coil assembly  7 . This space is filled with wedge shape sections  37  (here, aluminum) of plates  21  as shown in  FIG. 6 . The aluminum wedge shape sections  37  help to remove heat from coil sides  7 . In certain applications, aluminum wedge shape sections  37  are manufactured in the form of laminations to reduce eddy-current losses. These laminations also have holes  25  which are used for passing fresh water for cooling. It is further possible to install these wedge shape sections on each phase coil ( 8  through  13 ), epoxy impregnate the phase coil assembly and test it electrically and thermally before incorporating it into the stator assembly. When all phase assemblies are assembled, stator coil assemblies with cooling wedge shape sections form the assembly  12  shown in  FIG. 5 . 
   Plates  21  are made from a thermally conducting material. Examples of thermally conducting materials include metals, e.g., copper, iron and aluminum, as well as flexible graphite materials, such as Grafoil®, a product of UCAR International Inc., Nashville, Tenn. Grafoil® advantageously has a thermal conductivity similar to that of copper while having an electrical resistivity characteristic approximately 100 times that of copper. Typically, the plates are formed from a non-magnetic material, e.g., copper or aluminum. Each plate  21  includes a body portion  320  and the wedge shaped section  37  which extends radially towards the central axis of the stator. Typically, plates  21  are aligned between ends  103 ,  104  of the stator such that a passage  25  from each plate forms an outer bore  29  for fitting a cooling tube. Outer bore  29  is parallel to the central axis (L) and provides a path for the flow of fresh water. Each plate  21  also can be insulated from adjacent plates to reduce eddy currents, which cause increased heating. 
     FIG. 5  shows a top view of plate  21  including the coils  306  of stator  1 , body  320 , and tooth portions  37 . Each plate can include passages  25 , equally spaced and radially positioned about the circumference of a stator midsection  107 . For example, each plate  21  can include a passage for each winding of the stator. As shown in greater detail in  FIG. 6 , coil  306  includes adjacent windings  6 ,  7  having inner ends  32  and outer ends  34 . Tooth portion  37  is wedged between adjacent windings so that the windings touch at inner ends  32 , i.e., on the bore side, and are spaced apart on outer ends  34 . Inner body  37  provides additional surface area for the transfer of heat between the windings and the coolant manifold. 
   Alternate embodiments may redirect fluid from one passage to another to form a serial fluid flow loop. For example in  FIG. 6 , passage  25  may be connected to passage  36  so that fluid from passage  25  goes through passage  36  before it leaves the cooling system. Other embodiments may cool warm water from the cooling system by running it through a heat exchanger before pumping it through the system again. Alternatively, water to the cooling system could come from a main water supply and could be discarded after use. 
   In still other embodiments, the stator winding assembly is cooled using a stator cooling system having a form similar to the stator winding itself. 
   Referring to  FIGS. 7-9 , for example, a stator winding  400  of the type similar to phase coil assemblies  11 ,  12 , and  13  of three phase stator  1  (see  FIG. 1 ) is shown independent from its neighboring phase coil assemblies. In this embodiment, a cooling system  410  includes a pair of cooling tubes  412 ,  414  concentrically wound about an axis  415  of stator winding  400  and positioned on opposing sides of stator winding  400 . Note that axis  415  is transverse to axis L of the embodiment of the cooled stator system  200  shown in  FIG. 3 . In particular, cooling tubes  412  are positioned to be in thermal contact with the inner surface and outer surface of stator winding  400 , respectively. 
   As was the case with the cooling tubes described above, cooling tubes  412 ,  414  are formed of a non-magnetic material, such as aluminum or stainless steel. In many applications, stainless steel is preferable because of its resistance to corrosion and low eddy current loss characteristics. 
   Unlike the embodiments described above in conjunction with  FIGS. 1-6 , cooling tubes  412 ,  414  are concentrically wound into a saddle-shaped, racetrack form, similar to that of stator winding  400 . As shown in  FIG. 8 , the cooling tubes are wound to conform to the generally curved surface of the stator winding and are wound in bifilar fashion. 
   By “bifilar”, it is meant that two lengths of each cooling tube are wound together, in parallel, one over the other (wound in-hand) so that each cooling tube  412 ,  414  has a helical arrangement with an inlet  416 ,  418  and outlet  420 ,  422  extending from the outer periphery of respective ones of the cooling tubes. Winding the cooling tubes using the bifilar approach advantageously allows the inlet and outlet to be positioned adjacent each other without requiring a length of the tube extending back over the wound cooling tube. Moreover, the cooling tubes themselves form a coil which links magnetic flux from the stator field winding, which it cools. The bifilar winding approach reduces voltage and circulating currents flowing through the cooling tube, thereby reducing eddy current losses. 
   In one approach for winding cooling tubes  412 ,  414  in a bifilar manner, a length of the cooling tube is folded upon itself at its midpoint to form a U-shaped bend  424  ( FIG. 7 ). The length of folded cooling tube is then concentrically wound outwardly, one turn over the other. 
   With respect to a multi-phase stator having multiple stator windings (e.g., the three-phase stator assembly of  FIG. 1 ), cooling tubes  412 ,  414  of cooling system  410  are individually potted to each of the stator windings thereby providing a separate and independently testable subsystem. 
   Referring to  FIG. 10 , a superconducting rotating machine  510  has a stator assembly  512  including stator coil assemblies  514   1-n . As is well known in the art, the specific number of stator coil assemblies  514   1-n  included within stator assembly  512  varies depending on various design criteria, such as whether the machine is a single phase or a polyphase machine. For example, in one 33,000 horsepower superconducting machine design, stator assembly  512  includes one hundred and eighty stator coil assemblies  514   1-n . These stator coil assemblies  514   1-n  are mounted on a stator coil support structure that is constructed of a non-magnetic, thermally-conductive material, thus minimizing Eddy current heating and the resulting stator inefficiencies. This will be discussed in greater detail below. 
   A rotor assembly  516  rotates within stator assembly  512 . As with stator assembly  512 , rotor assembly  516  includes rotor winding assemblies  518   1-n . In the same 33,000 horsepower superconducting machine design, rotor assembly  516  includes twelve rotor winding assemblies  518   1-n . These rotor winding assemblies, during operation, generate a magnetic flux that links rotor assembly  516  and stator assembly  512 . 
   During operation of superconducting rotating machine  510 , a supply voltage  520  is supplied to stator coil assemblies  514   1-n . By supplying supply voltage  520 , machine  510  is brought up to its operating speed, which is proportional to the frequency of supply voltage  520 . Accordingly, if the frequency of supply voltage  520  is held constant, machine  510  (i.e., rotor assembly  516 ) will rotate at a constant (or synchronous) speed. The torque generated by this now-rotating rotor assembly  516  is transferred to a load  521  (e.g., a propeller shaft of a ship, a conveyor belt on a production line, the drive wheels of a diesel locomotive, etc.). The rotor winding assemblies  518   1-n  are mounted on a support structure  517  which is connected to a first flange  519  that transfers the motor torque to a torque tube  522 . Torque tube  522  is connected to a second flange  523 , which is connected to an output shaft  524 . Flanges  519  and  523  may be incorporated into torque tube  522  or may be separate assemblies. 
   Output shaft  524  is supported by a pair of bearing plates  526 ,  528 , one at each end of rotor assembly  516 . The bearing plate  526  on the drive end  530  of superconducting rotating machine  510  contains a passage  532  through which output shaft  524  passes. Additionally, bearing plate  528  may also have a passage through which the output shaft  524  passes. Bearing plates  526 ,  528  position rotor assembly  516  at the proper position within stator assembly  512  so that rotor assembly  516  can freely rotate within stator assembly  512  while maintaining the proper gap “g” between rotor assembly  516  and stator assembly  512 . 
   During operation of superconducting rotating machine  510 , field energy  534  is applied to rotor winding assembly  518   1-n  through a slip ring/rotating disk assembly  535 . This signal can be in the form of a DC current. Rotor winding assemblies  518   1-n  require DC current to generate the magnetic field (and the magnetic flux) required to link the rotor assembly  516  and stator assembly  512 . Therefore, if field energy  534  is supplied in the form of an AC current, a rectifier/thyristor circuit (not shown) will be employed to convert the AC current into a DC current. 
   While stator coil assemblies  514   1-n  are non-superconducting copper coil assemblies, rotor winding assemblies  518   1-n  are superconducting assemblies incorporating either HTS (High Temperature Superconductor) or LTS (Low Temperature Superconductor) windings. Examples of LTS conductors are: niobium-zirconium; niobium-titanium; and niobium-tin. Examples of HTS conductors are: thallium-barium-calcium-copper-oxide; bismuth-strontium-calcium-copper-oxide; mercury-barium-calcium-copper-oxide; and yttrium-barium-copper-oxide. 
   As these superconducting conductors only achieve their superconducting characteristics when operating at low temperatures, superconducting machine  510  includes a refrigeration system  36 . Refrigeration system  36  is typically in the form of a cryogenic cooler that maintains the operating temperature of rotor winding assemblies  518   1-n  at an operating temperature sufficiently low to enable the conductors to exhibit their superconducting characteristics. 
   Rotor assembly  516  includes an asynchronous field filtering shield  538  positioned between stator assembly  512  and rotor assembly  516 . As rotor assembly  516  is typically cylindrical in shape, asynchronous field filtering shield  538  is also typically cylindrical in shape. Stator assembly  512  is typically powered by multiphase AC power or pulse-width modulated (PWM) power  520  at a frequency commensurate with the desired shaft speed. This, in turn, generates a rotating magnetic field that rotates about the axis of the cylindrically-shaped stator assembly  512 . As stated above, the frequency of the multiphase AC power  520  supplied to stator assembly  512  proportionally controls the rotational speed of superconducting machine  510 . Since AC or PWM signals naturally contain harmonics of their primary frequency (e.g., odd multiples of a 60 Hertz signal), it is desirable to shield the rotor winding assemblies  518   1-n  of rotor assembly  516  from these asynchronous fields. Accordingly, asynchronous field filtering shield  538 , which is fitted to rotor assembly  516 , electromagnetically shields rotor winding assemblies  518   1-n  from the asynchronous fields generated as a result of these harmonics present in three-phase AC power  520 . Asynchronous field filtering shield  538  is constructed of a non-magnetic material (e.g., copper, aluminum, etc.) and should be of a length sufficient to fully cover and shield rotor winding assemblies  518   1-n . In a preferred embodiment, asynchronous field filtering shield  538  is constructed of 6061T6 structural aluminum. The thickness of shield  538  varies inversely with respect to the frequency of the three-phase AC power  520  supplied to stator assembly  512 , which is typically in the range of 2-120 Hertz. Typically, the thickness of shield  538  varies from ½-3 inches depending on this supply frequency. 
   Shield  538  is connected to output shaft  524  via a pair of end plates  540 ,  542 . These end plates  540 ,  542  are rigidly connected to output shaft  524 . This rigid connection can be in the form of a weld or a mechanical fastener system (e.g., bolts, rivets, splines, keyways, etc.). 
   A vacuum chamber sleeve  543  surrounds the rotor winding assemblies  518   1-n . This vacuum chamber sleeve  543  is positioned between shield  538  and the rotor winding assemblies  518   1-n  and is connected on its distal ends to end plate  540 ,  542 . This connection can be in the form of a weld, a braze, or a mechanical fastener system (e.g., bolts, rivets, splines, keyways, etc.). Typically, vacuum chamber sleeve  543  is relatively thin (e.g., 3/16″) and is constructed of stainless steel. When vacuum chamber sleeve  543  is connected to the end plates, an air-tight chamber is formed which encloses the rotor winding assemblies  518   1-n . This air-tight chamber can then be evacuated, thus forming a vacuum within the chamber. This helps to insulate the rotor winding assemblies  518   1-n  (which are superconducting and kept cool) from output shaft  524  (which is warm). 
   As stated above, a gap “g” exists between stator assembly  512  and rotor assembly  516 . In order to reduce the size of superconducting rotating machine  510 , it is desirable to reduce the dimensions of this gap (or spacing) to a minimum allowable value. In the same 33,000 horsepower superconducting machine, this gap “g” has a value of just over one inch. Specifically, due to the maximization of the flux linkage, the efficiency of machine  510  is maximized when gap “g” is minimized. Unfortunately, when gap “g” is minimized, shield  538  gets very close to the windings of stator coil assembly  514   1-n . 
   During operation of superconducting rotating machine  510 , shield  538  will heat up as a result of eddy current heating caused by the presence of the asynchronous fields described above. As metals (especially aluminum) are known to expand when heated, it is important that rotor assembly  516  be capable of accommodating this expansion. This expansion can occur in two dimensions, both axially (i.e., along the direction of the output shaft  524 ) and radially (i.e., along the direction of the rotor assembly&#39;s radius). Accordingly, rotor assembly  516  typically includes a pair of interconnection assemblies  544 ,  546  for connecting shield  538  to end plates  540 ,  542 . These interconnections assemblies  544 ,  546  compensate for the thermal expansion of shield  538  by allowing for axial movement between shield  538  and end plates  540 ,  542  while restricting tangential movement. 
     FIG. 11  shows the details of a particular embodiment of stator assembly  512 . Referring to  FIGS. 10 and 11 , stator assembly  512  includes a stator coil support structure  600  for supporting and positioning the stator coil assemblies  514   1-n . In traditional, non-superconducting machines (e.g., induction motors), stator coil support structure  600  is constructed of a magnetic material (e.g., laminated sheet steel). By using a magnetic material, the non-superconducting machine will magnetically saturate the stator coil support structure, resulting in an increased flux density and, therefore, an increased flux linkage between the stator and rotor assemblies. 
   As is well known in the art, a superconducting rotating machine  510  achieves its desirable superconducting characteristics by maximizing efficiencies, minimizing heat build-up within the machine  510 , and maintaining the superconducting rotor winding assemblies  518   1-n  at an operating temperature sufficiently low to enable the conductors to exhibit their superconducting characteristics. The low resistivity of the superconducting rotor winding assemblies  518   1-n  allows for high levels of flux density and, therefore, high levels of flux linkage between the stator assembly  512  and the rotor assembly  516 . Accordingly, enhancing the flux density through the use of a magnetically-saturatable stator coil support structure is not required. 
   Further, it is undesirable to use a magnetically-saturatable stator coil support structure, as this results in the generation of hysteresis and eddy current losses, thus lowering the efficiency of the superconducting rotating machine  510  by heating the stator coil assembly. Accordingly, stator coil support structure  600  is constructed of a non-magnetic thermally-conductive material, such as: a polymer-based adhesive (e.g., Advanced Thermal Transfer Adhesive, available from the BTech Corporation, 120 Jones parkway, Brentwood, Tenn. 37027); or a graphite-based material (e.g., Grafoil, available from Union Carbide, 39 Old Ridgebury Road, Danbury, Conn. 06817). These materials have a favorable thermal transfer coefficient of at least 100 Watt/Meter Kelvin. Specifically, Advanced Thermal Transfer Adhesive has a thermal transfer coefficient of between 100 and 450 Watt/Meter Kelvin and Grafoil has a thermal transfer coefficient of between 140 and 375 Watt/Meter Kelvin. By comparison, glass epoxy material has a thermal transfer coefficient of ˜0.60 Watt/Meter Kelvin. 
   By using a non-magnetic, thermally-conductive material, stator heating resulting from the presence of eddy currents is eliminated. Further, any heat generated by stator coil assemblies  514   1-n  can be easily removed (this will be discussed below in greater detail). This non-magnetic thermally-conductive material can be in the form of a sheet material or a castable liquid. If the material is a castable liquid, it can be cast into the form of a cylindrical structure and then machined into it&#39;s final form. Alternatively, if the material is a sheet material, it can be laminated into the required shape. 
   Stator coil support structure  600  includes an axial passage  602  for receiving rotor assembly  516 . Channels  604   1-n  are positioned radially about the stator coil support structure  600 , thus forming teeth  605   1-n  that act as heat sinking members and absorb the thermal energy generated by the stator coil assemblies  514   1-n . These channels are designed and sized to each receive one or more of the stator coil assemblies  514   1-n . It is important to appreciate that while sixteen channels are shown, this is for illustrative purposes only and is not intended to be a limitation of the invention, as the specific number of channels  604   1-n  (and, therefore, stator coil assemblies  514   1-n ) utilized will vary depending on the design requirements of the superconducting rotating machine  510 . In the same 33,000 horsepower superconducting machine design, stator coil support structure  600  includes ninety channels  604   1-n , each of which includes two stator coil assemblies  514   1-n  (stacked on top of each other). Further, while channels  604   1-n  are shown being positioned about the inner perimeter of stator coil support structure  600 , this is for illustrative purposes only, as these channels  604   1-n  can be positioned about the outer perimeter of structure  600 . Each stator coil assembly  514   1-n  is surrounded by a ground plane  606 , such as a wire wound around the circumference of the stator coil assembly  514   1-n  and tied to ground. 
   Stator coil support structure  600  is typically surrounded by an outer annular assembly  501 . This assembly  601 , which is typically constructed of laminated sheet steel, is commonly referred to as the “back iron” and provides a flux return path for rotor assembly  516 . 
   Assembly  601  includes coolant passages  508   1-n  which are bored axially through assembly  101 . These coolant passages  608   1-n  allow a coolant (e.g., water, oil, air, or a suitable gas) to be circulated through assembly  601  by coolant circulation system  610 . As assembly  601  is in thermal contact with stator coil support structure  600 , due to conductive heat transfer, thermal energy is transferred from stator coil support structure  600  to assembly  601 . Convective heat transfer then transfers thermal energy to the coolant circulating through the coolant passages  608   1-n . 
   Since air is a relatively poor conductor of heat, an epoxy filler  612  (e.g., a low viscosity liquid resin) is utilized to fill any voids between stator coil assemblies  514   1-n  and channels  604   1-n . This epoxy filler  612  can be either drawn into these voids through the use of a vacuum or pushed into the voids using positive pressure. 
   Referring to  FIG. 12 , there is shown a method  620  of manufacturing a stator coil support structure. This method is utilized when the stator coil support structure is constructed of a sheet material and the “teeth” which separate the stator coil assemblies are an integral part of the stator coil support structure. 
   Method  620  includes laminating  622  multiple layers of a non-magnetic, thermally conductive sheet material to form a non-magnetic, thermally conductive cylindrical structure. Method  620  includes providing  624  a plurality of axial channels radially about the non-magnetic, thermally conductive cylindrical structure. Method  620  includes positioning  626  one or more of the stator coil assemblies in each of the channels. Method  620  further includes providing  628  axial coolant passages in the non-magnetic, thermally conductive cylindrical structure and depositing an epoxy filler  630  between the stator coil assemblies and the non-magnetic, thermally conductive cylindrical structure. 
   Referring to  FIG. 13 , there is shown a method  650  of manufacturing a stator coil support structure. This method is utilized when the stator coil support structure is constructed of a sheet material and the “teeth” which separate the stator coil assemblies are inserted into slots machined into the stator coil support structure. 
   Method  650  includes laminating  652  multiple layers of a non-magnetic, thermally conductive sheet material to form a non-magnetic, thermally conductive cylindrical structure, Method  650  includes forming  654  a plurality of axial slots radially about the non-magnetic, thermally conductive cylindrical structure and inserting  656  into each axial slot a heat-sinking member, thus forming a channel between each pair of heating-sinking members. Method  650  includes positioning  658  one or more of the stator coil assemblies in each of the channels. Method  650  further includes providing  660  a plurality of axial coolant passages in the non-magnetic, thermally conductive cylindrical structure and depositing  662  an epoxy filler between the stator coil assemblies and the non-magnetic, thermally conductive cylindrical structure. 
   Referring to  FIG. 14 , there is shown a method  700  of manufacturing a stator coil support structure. This method is utilized when the stator coil support structure is constructed of a cast material and the “teeth” which separate the stator coil assemblies are an integral part of the stator coil support structure. 
   Method  700  includes casting  702  a non-magnetic, thermally conductive material to form a non-magnetic, thermally conductive cylindrical structure. Method  700  includes forming  704  a plurality of axial channels radially about the non-magnetic, thermally conductive cylindrical structure, and positioning  706  one or more of the stator coil assemblies in each of the channels. Method  700  further includes providing  708  a plurality of axial coolant passages in the non-magnetic, thermally conductive cylindrical structure, and depositing  710  an epoxy filler between the stator coil assemblies and the non-magnetic, thermally conductive cylindrical structure. 
   Referring to  FIG. 15 , there is shown a method  750  of manufacturing a stator coil support structure. This method is utilized when the stator coil support structure is constructed of a cast material and the “teeth” which separate the stator coil assemblies are inserted into slots machined into the stator coil support structure. 
   Method  750  includes laminating  752  multiple layers of a non-magnetic, thermally conductive sheet material to form a non-magnetic, thermally conductive cylindrical and forming  754  a plurality of axial slots radially about the non-magnetic, thermally conductive cylindrical structure. Method  750  includes inserting  756  into each axial slot a heat-sinking member, thus forming a channel between each pair of heating-sinking plates. Method  750  includes positioning  758  one or more of the stator coil assemblies in each of the channels. Method  750  further includes providing  760  a plurality of axial coolant passages in the non-magnetic, thermally conductive cylindrical structure and depositing  762  an epoxy filler between the stator coil assemblies and the non-magnetic, thermally conductive cylindrical structure. 
   Now referring to  FIGS. 10 and 16 , it is shown that the stator coil support structure  100 ′ may actually consist of only non-magnetic thermally-conductive teeth  605   1-n ′. For example, an outer annular assembly  601 ′ (which is typically constructed of laminated sheet steel and is, therefore, magnetic) can be used to radially position non-magnetic thermally-conductive teeth  605   1-n ′ around the perimeter of assembly  601 ′ Therefore, the “back iron” (i.e., outer annular assembly  601 ) is used to support and position the stator coil support structure  601 , which actually consists of multiple non-magnetic thermally-conductive teeth  605   1-n ′. Since the teeth  605   1-n ′ between adjacent stator coils are constructed of a non-magnetic thermally conductive material, magnetic saturation is eliminated. Further, by placing these non-magnetic, thermally conductive teeth  605   1-n ′ in thermal contact with assembly  601 ′, thermal energy is easily transferred from teeth  605   1-n ′ to assembly  601 ′. 
   Referring to  FIG. 17 , there is shown a method  800  of manufacturing a stator coil support structure. A magnetic annular assembly is formed  802  and a plurality of non-magnetic, thermally-conductive heat sinking members are formed  804 . These heat-sinking members are positioned  806  radially about the magnetic annular assembly. This forms a channel between each pair of adjacent heating-sinking members. One or more of the stator coil assemblies are positioned  808  in each of the channels. 
   Forming  804  a plurality of non-magnetic, thermally conductive heat-sinking members includes laminating  810  multiple layers of a non-magnetic, thermally conductive sheet material, or casting  812  a non-magnetic, thermally conductive material, to form the non-magnetic, thermally conductive heat-sinking members. A plurality of axial coolant passages are provided  814  in the magnetic annular assembly and an epoxy filler is deposited  816  between the stator coil assemblies and the non-magnetic, thermally conductive heat-sinking members 
   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. Accordingly, other embodiments are within the scope of the following claims.