Abstract:
A cryogenic rotary transfer coupling for a superconducting electromechanical rotating machine delivers supply flow from a cryogenic cooler to a rotor so as to cool the superconducting coils. The flow is then returned to the cooler and recirculated throughout the system. The structure includes a relative motion gap between stationary and rotating portions of the coupling. The gap is configured to greatly simplify the manufacture of the coupling while 1) being compatible with cooling systems having both cool return flow capability and warm return flow capability and 2) maintaining a high efficiency for the coupling.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electromechanical machines and, more particularly, relates to a system for maintaining superconducting coils of a rotor at a desired temperature. 
     2. Discussion of the Related Art 
     Electromechanical machines such as generators and motors include rotor and stator windings that create a magnetic field to rotate the rotor. Synchronous motors are well known in the art as comprising a rotor that rotates as a result of magnetic flux created between an armature winding and a stator winding. Synchronous motors having superconducting rotor coils are unique in that the coils operate without any resistance to electrical current. As a result, higher current densities may be achieved that are not possible in conventional conductors which, in turn, allows for stronger magnetic fields in electromechanical machines. These motors therefore have a notably higher efficiency than conventional motors. 
     Presently, low temperature superconducting coils are known to operate below approximately 10° K., and high temperature superconducting coils are known to operate above approximately 30° K. If the operating temperature rises significantly above the normal operating temperature for the superconductor, the coil will act as a conventional conductor, and electric losses will occur within the rotor. It is therefore important to design a coolant system that maintains the superconducting coil at its designed temperature. 
     In present high temperature superconducting devices, a cryogenic rotary transfer coupling delivers a coolant from a stationary cryogenic cooler to the rotor, thereby cooling the rotor coils, and returns warm coolant to the cooler. Because portions of the coupling rotate during operation, and other portions are stationary, a relative motion gap is formed between the rotating and stationary parts. This gap is a significant path for parasitic heat leakage into the coupling. Present relative motion gap arrangements can result in warmer flow than necessary returning to the cryogenic cooler, thereby decreasing the efficiency of the coupling. Additionally, the complex physical orientation of the annular gap adds cost and complexity to the rotor assembly during manufacturing, and requires tighter tolerances of the machined parts. 
     Additionally, in cooling systems that use a liquid helium supply flow and a gaseous helium return flow, a large temperature differential results between the two flows. As a result, significant conduction may occur between the outer walls of the supply tube and the outer walls of the return tube, thereby decreasing the efficiency of the overall cooling system. 
     The need has therefore arisen to provide a cooling system for a superconducting rotor that does not incorporate the difficulties in manufacturing and efficiency associated with prior art cooling systems. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is therefore a first object of the present invention to provide a cryogenic transfer coupling within a superconducting rotor having an annulus that permits the transfer coupling to be manufactured by a simple and cost-effective manufacturing process. 
     It is a second object of the invention to provide a cryogenic transfer coupling within a superconducting rotor having a relative motion gap that does not materially adversely affect the efficiency of the coupling. 
     It is a third object of the invention to manufacture a cooling system incorporating the above two objects that further permits the temperature of the return flow to be either slightly greater or significantly greater than the temperature of the supply flow. 
     In accordance with a first aspect of the invention, a cryogenic rotary transfer coupling is provided for delivering a cryogenic coolant, such as gaseous helium, from a cryogenic cooler to a supply flow path that extends axially through a rotor shaft, thereby permitting the coolant to enter the rotor and cool the superconductive coils. The coolant then returns to the cooler via a return flow path. Both the supply and the return flow paths have stationary parts connected to the cooler, and rotating parts extending into the rotor. A relative motion gap is therefore formed between the stationary and rotating parts of the coupling. Both stationary and rotating walls of the gap provide a solid conduction path from the ambient environment to the cold part of the coupling. Also the coolant filling the gap may contribute to heat leakage to the cold part via convection. In order to minimize this parasitic heat leakage, at least a portion of the gap is a narrow and long annulus bounded by two concentric thin wall tubes. The tube axis coincides with an axis of rotation, and both tubes are vacuum insulated. Depending on the coupling design, the relative motion gap may be continuous or it may comprise a plurality of segments. The coupling is designed such that every straight line extended from the rotor axis radially outwardly and perpendicularly to the axis will cross the relative motion gap no more than once. This design of the relative motion gap reduces the need for inserts and spacers that are necessary with couplings having other types of relative motion gaps. 
     In accordance with a second aspect of the invention, a plurality of vacuum cavities exists within the coupling, thereby eliminating heat transfer that would otherwise increase the temperature of the supply and return flows. Additionally, the temperature gradient within the gap is such that the temperature of the fluid flowing through the gap to be returned to the cooler is not increased significantly by the parasitic heat leak. The relatively cool return flow thereby reduces the energy needed to sufficiently cool the return flow and increasing the overall efficiency of the system. 
     In accordance with a third aspect of the invention, one embodiment is designed to accommodate cooling systems having a return flow of only a few degrees greater than the supply flow, thereby minimizing the concern for heat loss due to conduction between the return flow and the supply flow. In a second embodiment, the potential for conductive heat loss between the return flow and the supply flow is minimized, thereby accommodating cooling systems having return flow temperatures that are significantly greater than the supply flow temperature. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred exemplary embodiments of the invention are illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which: 
     FIG. 1 shows fragmentary side sectional elevation view of a rotor having a cryogenic transfer coupling constructed in accordance with a preferred embodiment of the invention; 
     FIG. 2 shows a fragmentary side sectional elevation view of a rotor having a cryogenic transfer coupling constructed in accordance with an alternate embodiment of the invention; 
     FIG. 3 shows a sectional end elevation view of the cryogenic transfer coupling of FIG. 1; 
     FIG. 4 shows a sectional end elevation view of a cryogenic transfer coupling of constructed in accordance with another alternate embodiment of the invention; and 
     FIG. 5 shows a sectional end elevation view of a cryogenic transfer coupling of constructed in accordance with another alternate embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Pursuant to a preferred embodiment of the invention, a rotor assembly is provided having superconducting coils disposed therein. The rotor assembly is connected to a cryogenic transfer coupling to maintain the superconducting coils at their operating temperature. Specifically, a cryogenic cooler is connected to a supply flow path that extends into the rotor shaft to cool the superconducting coils with a cryogenic fluid such as gaseous helium. A return flow path is connected to the supply flow path at one end and extends through the rotor shaft and is connected to the cryogenic cooler at a second opposite end. The cryogenic cooler cools the return flow to the desired temperature of the coils, and recirculates the coolant throughout the system. A caging wall encloses a rotating vacuum, is disposed radially inwardly of an outer wall of the rotor shaft, and defines the radially inward rotating boundary of the coupling. A stationary outer vacuum wall is disposed radially inwardly of the caging wall and surrounds another vacuum cavity between the return flow path and a relative motion gap formed between the rotating caging wall and stationary outer vacuum wall. The gap features the property that every straight line extended from the axis of rotation radially outwardly and perpendicularly to the axis crosses the relative motion gap only once. Additionally, the gap is constructed such that it does not reverse its direction of axial extension. Because of the relatively simple construction of the gap, the manufacturing process is more reliable and efficient, and is less costly than those of present cryogenic transfer couplings. 
     Referring to FIGS. 1 and 3, a cryogenic transfer coupling  10  delivers coolant from a cryogenic cooler  12  connected to a supply flow path  14  that extends into a rotor shaft  16  and is connected to a return flow path  20 . The cryogenic coolant is circulated throughout the cooling system, thereby cooling coils (not shown) of a rotor  18  in a known manner. The rotor shaft  16  is generally cylindrical and contains generally annular internal structures unless otherwise indicated that rotate and are generally symmetrical about an axis of rotation a—a. 
     In a high temperature superconducting rotating machine constructed in accordance with the preferred embodiment, the coolant comprises gaseous helium at a temperature of approximately 30° K. However, any fluid capable of cooling the coils inside the rotor  18  to their desired temperature could be used. Depending upon the chosen heat exchanger used to cool the superconducting coils, the return flow could be as little as only a few degrees above the temperature of the supply flow or could be significantly warmer. The embodiment of FIG. 1 is designed to accommodate return flow temperatures that are only slightly higher, for example by 10° K., than the supply flow temperature. 
     The supply flow path  14  comprises 1) a stationary portion  13  located axially upstream of a seal  34  to be described below, and 2) a rotating portion  15  located axially downstream of the seal. (The terms “axially downstream” and “upstream” are used throughout the disclosure to identify a direction of extension corresponding to the direction of the supply flow. Additionally, because the preferred embodiment comprises generally cylindrical and annular objects, directions of travel are referred to as being “radial” or “axial”. However, the preferred embodiment should not be limited to such structures, and the directions of travel and extension may also be properly interpreted as being lateral or longitudinal respectively in non-cylindrical, non-annular embodiments.) 
     Once the coolant has circulated through the rotor  18 , it returns through a return flow path  20  and is then transferred back to the cryogenic cooler  12  to be again cooled and recirculated through the cooling system. 
     The rotor shaft  16  rotates about roller bearings  22  during operation in a known manner. A wall  24  is disposed between an outer race of the bearing  22  and rotor shaft  16 , and wall  26  is disposed between a radially inward portion of the bearing  22  and a stationary outer wall  30  of a vacuum cavity  32 , thereby providing support for wall  30 . An annular wall  11 , defining the stationary portion  13  of the supply flow path  14 , extends axially downstream until turning radially outwardly and then axially upstream, and connecting with a stationary inner seat  36  for the seal  34 . Wall  30  extends axially downstream, crosses the return flow path  20 , and forms the inner seat  36 . At least one opening  72  exists in stationary wall  30  to permit the return flow to enter a stationary portion  42  of the return flow path  20 . Radially inwardly and axially upstream of inner seat  36  is an outer wall  38  of a vacuum cavity  40 . The vacuum cavity  40  prevents heat transfer due to convection between the return flow path  20  and the supply flow path  14 . 
     Wall  38  also forms an inner annular wall of the stationary portion  42  of return flow path  20 . Radially outwardly from wall  38  is an outer annular wall  44  of the stationary portion  42  of the return flow path  20  that also forms an inner wall of the vacuum cavity  32 . Walls  38  and  44  extend axially downstream and radially outwardly and are connected, and preferably welded, to wall  30 . 
     The rotor shaft  16  is connected to a rotating radially inwardly disposed wall  48  which is located axially downstream of bearings  22 , and which is connected to a rotating, axially extending caging wall  50 . The caging wall  50  extends downstream, crosses a rotating portion  54  of the return flow path  20  which extends radially outwardly and then axially downstream into the rotor, and forms a rotating outer seat  58  for the seal. At least one opening  74  exists in wall  50 , thereby permitting the return flow to enter the relative motion gap  28  from which the return flow enters the stationary portion  42  via the opening  72 . Outer annular wall  52  and inner annular wall  56  of the rotating portion  54  of the return flow path  20  extend axially upstream from the rotor until winding radially inwardly and are connected, and preferably welded, to caging wall  50 . The outer seat  58  extends axially downstream until turning radially inwardly and again axially downstream, thereby forming an outer tubular wall  60  of the rotating portion  15  of the supply flow path  14 . 
     The seal  34  acts as a barrier between the supply and return flows. While a labyrinth seal  34  is preferred, any non-contact or low-contact seal could be implemented. Rotor shaft  16 , wall  48 , caging wall  50 , outer seat  58  for the seal  34 , and outer wall  60  of the rotating portion  15  of the supply flow path  14  define a rotating vacuum cavity  62  that is further bound by an outer wall (not shown) of the rotor  18  as is known in the art. The rotating portion  54  of the return flow path  20  is disposed within vacuum cavity  62  and extends axially upstream and radially inwardly, merging with the gap  28 . Additionally, vacuum cavities  32  and  40  are further bound axially upstream by outer radial walls (not shown) between walls  30  and  44 , and between walls  38  and  11  respectively. In the preferred embodiment, walls  30  and  44  defining vacuum cavity  32 , and walls  38  and  11  defining vacuum cavity  40  are preferably made of stainless steel. However, any material that is capable of maintaining the vacuums may be used. 
     Because the return flow is only slightly warmer than the supply flow in FIG. 1, the efficiency of the system will not be materially adversely affected by the conduction to the supply flow path  14  from the return flow path  20  via the seal seats  36  and  58 , and by possible small bypass flow from the supply flow to the return flow through the seal  34 . The amount of heat loss due to conduction is a function of thickness and length of the return and supply tubes. Because the embodiment illustrated by FIG. 1 is designed to operate in systems in which the return flow is only slightly warmer than the supply flow, the length and thickness of the tubes are not critical. However, it is preferable to eliminate heat transfer that would otherwise increase the temperature of supply flow path  14  and return flow paths  20 . For instance, vacuum cavity  62  eliminates heat transfer due to convection from the warm rotor shaft  16  to the rotating portion  54  of the return flow path  20  as well as gap  28 , and additionally from the rotating portion of the return flow path to the rotating portion  15  of the supply flow path  14 . Vacuum cavity  40  eliminates heat transfer due to convection between the stationary portion  42  of the return flow paths  20  and the stationary portion  13  of the supply flow path  14 . Vacuum cavity  32  eliminates heat transfer due to convection between gap  28  and stationary portion  42  of the return flow path  20 . 
     The relative motion gap  28  is formed between the rotating caging wall  50  and stationary outer wall  30  of vacuum cavity  32 , and also extends radially outwardly between wall  48  and bearings  22 . As the return flow re-enters the coupling  10  from the rotor  18 , it will flow axially upstream within the rotating portion  54  of the return flow path  20 , and will then flow radially inwardly towards the stationary portion  42  of the return flow path. After passing through opening  74 , the return flow enters into the gap  28  during the radially inward path of travel, and then is forced through the opening  72  into the stationary portion  42  of the return flow path  20 . The coolant that is disposed within the gap  28  is sealed from the ambient environment by bearings  22  and associated seals (not shown), thereby preventing leakage into the ambient environment. 
     Because the bearings  22  are in contact with the ambient environment, they are typically going to be at room temperature. As a result, a large temperature gradient will exist (on the order of 300° K.) at the gap  28  as the helium will be warmest at a point adjacent the bearings  22  and will become cooler as the gap extends axially downstream towards the towards the radially extending portion of the return flow path  20 . Therefore, in order to decrease the heat leak into the coupling, the walls  30  and  50  are made thin and the gap  28  is made small. 
     As the gap  28  extends axially upstream from return flow path  20  and turns radially outwardly adjacent the bearings  22 , it does not again extend axially downstream (which would be the reverse direction of the preexisting axially upstream direction of extension). The gap  28  therefore changes direction by only approximately 90 degrees and does not reverse its direction of extension. Additionally, any straight line extending radially outwardly and perpendicularly from axis a—a will cross the gap  28  only once. As a result, the need for additional seals and inserts further directing the travel of the gas through the gap is advantageously eliminated. 
     While the return flow path  20  comprises annular stationary and rotating portions  42  and  54 , it could alternatively comprise a plurality of individual stationary and rotating tubes. In this case, the stationary return tubes would be disposed within a single vacuum cavity bound by wall  30 , inner seat  36 , and wall  11 . 
     An alternate embodiment of FIGS. 1 and 3 is shown in FIG. 4 in which reference numerals of like elements have been incremented by 100. Turning to FIG. 4, vacuum cavity  40  has been eliminated and replaced with a low conductivity plastic or composite wall  111  that is sufficiently thick to serve as a thermal barrier between the return flow path  120  and supply flow path  114 . Stainless steel vacuum jacket  144  forms the outer wall of the return flow path  120  and is surrounded radially outwardly by vacuum cavity  132 . Gap  128  is disposed radially outwardly from the vacuum cavity  132  and is bound by inner stainless steel wall  130 , and outer rotating caging wall  150 . 
     Another alternate embodiment of FIGS. 1 and 3 is illustrated in FIG. 5, in which reference numerals of like elements have been incremented by 200. Instead of the return flow path  120  being coaxial with the supply flow path  114  as in FIG. 4, supply flow path  214  and return flow path  220  are shown in a side-by-side orientation in FIG. 5 as being separated by a low conductivity plastic or composite wall  211  that has sufficient thickness to serve as a thermal barrier between the return and supply flow. A stainless steel metal jacket  244  surrounds wall  211  and forms the inner wall of vacuum cavity  232 . Stationary stainless steel outer wall  230  of vacuum cavity  232  also forms the inner wall of gap  228 , which is surrounded by rotating outer caging wall  250 . 
     A cryogenic rotary transfer coupling  310  in accordance with an alternate embodiment is shown in FIG. 2, in which reference numerals of like elements from FIG. 1 have been incremented by 300. Hence, cryogenic transfer coupling  310  comprises a supply flow path  314  connected to a cryogenic cooler that circulates a cryogenic coolant through a rotor shaft  316 , thereby cooling superconducting coils in the rotor, and returning the coolant to the cooler through return flow path  320 . Supply flow path  314  comprises a stationary portion  313  having an outer wall  311 , and a rotating portion  315  having an outer wall  360 . Wall  311  also defines a radial inner wall of a vacuum cavity  340 . Vacuum cavity  340  has a stationary radial outer wall  338  also serves as a radial inner wall for a stationary portion  342  of the return flow path  320  that is surrounded by wall  344 . Wall  344  also defines the inner wall of a vacuum cavity that is surrounded by a stationary outer wall  330 . Wall  330  extends axially downstream past the return flow path  320  and inner wall of the bearing  366  until ultimately connecting with wall  311 . At least one opening  372  exists in wall  330  at the point where it crosses the return flow path  320 . 
     The rotor shaft  316  contains a rotating caging wall  350  that is disposed between the shaft and bearing  322 , and on a rotating support  364  that is disposed between the shaft  316  and bearing  366 . The radially inward wall of bearing  322  rests on a tube  326  that surrounds a portion of wall  330  and that works in conjunction with caging wall  350  and the seal (not shown) associated with the bearing  322  to form a seal for the gap  328  formed between stationary wall  330  and the caging wall  350 . Caging wall  350  extends axially downstream between and beyond support  364  and bearing  366  until turning radially inwardly and again extending axially downstream to form the rotating outer wall of the rotating portion  315  of the supply flow path  314 . Caging wall  350  includes an opening  374  at the point where it crosses the return flow path  320 . After passing through opening  374 , the return flow enters into the gap  328  during the radially inward path of travel, and then is forced through the opening  372  into the stationary portion  342  of the return flow path  320 . 
     The rotating portion  354  of the return flow path  320  is preferably at least one tube  352  passing through an opening in rotating support  364  and radially inwardly bent and is connected and preferably welded to caging wall  350  at the opening  374 . A rotating vacuum cavity  362  is defined by the rotor shaft  316 , caging wall  350 , wall  360 , and an axially downstream radially oriented wall (not shown) of the rotor through which the rotating portions  352  and  315  of the return flow paths  320  and supply flow path  314  respectively extend. 
     Return flow path  320  is shown as comprising stationary annulus  342  and rotating tube  352 . Alternatively, as described in conjunction with the embodiment of FIG. 1, a plurality of return tubes could replace the annulus  342 . In this embodiment, a single stationary vacum cavity would be defined by walls  330  and  311 . 
     The embodiment of FIG. 2, as will be described below, is designed to accommodate heat exchangers in the rotor whose return flow is significantly warmer than the supply flow. However, it may also be used when the return flow is only slightly warmer than the supply. The arrangement in accordance with this embodiment seeks to minimize heat transfer between the return flow path  320  and the supply flow path  314  resulting from conduction as well as convection. Vacuum cavity  354  prevents heat transfer resulting from convection between the shaft  316  and the rotating portion  354  of the return flow path  320 , and from the rotating portion of the return flow path to the rotating portion  315  of the supply flow path  314  as well as gap  328 . Vacuum cavity  340  prevents heat transfer in the form of convection from the stationary portion  342  of the return flow path  320  to the stationary portion  313  of the supply flow path  314 , and also from the gap  328  in the stationary portion of the supply flow path. Vacuum cavity  332  prevents heat transfer in the form of convection from gap  328  to the stationary portion  342  of the return flow path  320 . Heat transfer in the form of conduction between the supply flow path  314  to the return flow path  320  is also minimized. Additionally, walls  350 ,  338 , and  330  are may be made of stainless steel with a low conducting plastic or composite material sandwiched inside to reduce conductive heat transfer along these walls. Additionally, the embodiment of FIG. 2 shows the return flow path  320  as being separated from the supply flow path  314  by a greater distance than in the embodiment of FIGS. 1 and 3. In addition, because the walls  350 ,  330 , and  338  are of minimal thickness, the embodiment of FIG. 2 is designed to accommodate cooling systems in which the temperature differential between the return flow and the supply is greater than in the embodiment of FIGS. 1 and 3. These features thereby increase the overall efficiency of the system. 
     Parasitic heat loss will occur in the gap  328  at the point where it is transferred from the stationary portion  313  to the rotating portion  315 . This supply flow is sealed from the return flow portion of the gap  328  by the seal  370  which is associated with the bearing  366 . As the return flow travels from the rotating portion  354  to the stationary portion  342 , it enters the gap  328  and then is forced into stationary portion  342  of the return flow path  320 . The return flow gap is sealed by walls  350 , bearing  322 , and wall  326  axially upstream, and axially downstream by associated bearing  366  and associated seal  370 . Because the temperature of bearings  322  and  366  will be approximately that of the outside environment, a temperature distribution will form within the return flow portion of the gap  328 , and will have its warmest portions adjacent the bearings and cooling as the gap extends from each bearing toward the return flow path  320 . Likewise, the supply flow portion of the gap  328  will be warmest adjacent bearing  366  and will cool as the gap extends towards the supply flow path  314 . 
     As in the embodiment illustrated in FIG. 1, the gap  328  is designed such that any straight line extending radially outwardly and perpendicularly from the axis of rotation b—b will cross the gap no more than once. Again, no reversal of the annulus formed by gap  328  in the axial direction is present in this embodiment, thereby simplifying the manufacturing of the coupling  310 . 
     Many changes and modifications may be made to the invention without departing from the spirit thereof. The scope of the changes will become apparent from the appended claims.