Abstract:
A rotating coupling allows a vacuum chamber in the rotor of a superconducting electric motor to be continually pumped out. The coupling consists of at least two concentric portions, one of which is allowed to rotate and the other of which is stationary. The coupling is located on the non-drive end of the rotor and is connected to a coolant supply and a vacuum pump. The coupling is smaller in diameter than the shaft of the rotor so that the shaft can be increased in diameter without having to increase the size of the vacuum seal.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under Prime Contract No. DE-FC36-93CH10580 awarded by the Department of Energy. The Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The field of the invention is superconducting electric motors, specifically those that require that the superconducting material of the rotor be cooled, requiring the use of a cryogenic coolant supply system and a vacuum chamber. 
     Superconducting motors provide increases in power and efficiency over motors of a conventional, non-superconducting design. However, the use of superconducting materials presents obstacles that increase the complexity of the motor. The most significant impediment to the use of superconducting materials is temperature. 
     The current state of the art in superconductor motor technology is the use of what are referred to as high temperature superconductors (HTS) in the rotor of an electric motor. Despite their nomenclature, high temperature superconductors require an operating temperature in the range of 30K to 70K. This requires the use of a coolant system to deliver a low temperature coolant, such as liquid neon or gaseous helium, to the superconducting material. It also requires that the superconducting material be enclosed in a vacuum chamber to provide thermal insulation. 
     The fact that the superconducting material is contained in the rotor, which must be allowed to rotate, poses a significant problem for the creation and maintenance of a vacuum chamber. One way to obtain a vacuum in the rotor is to manufacture it as a sealed vacuum chamber. This approach does not require that the rotor be connected to an external vacuum pump during operation. However, it does require that the welds and joints be of a very high quality. In addition, the composite materials commonly used in high temperature superconductors have inherently high outgassing rates that rapidly compromise the vacuum level. This requires that the motor be stopped and the rotor vacuum chamber be pumped out periodically to maintain a sufficient level of vacuum. 
     The second way to obtain a vacuum surrounding the superconducting material is to enclose the entire rotor (and sometimes the stator) in a stationary vacuum chamber. This allows that vacuum space to be constantly pumped by an external vacuum pump to maintain the requisite level of vacuum. The major disadvantage to this approach is that it requires rotating vacuum seals for the rotor shaft. The cost and complexity of rotating vacuum seals increases as the size of the shaft increases. Therefore, for very large motors, the use of rotating vacuum seals becomes prohibitively expensive. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention overcomes the cost and complexity associated with creating and maintaining a vacuum insulation about the superconducting rotor coils in electric motors with large rotor shafts by continually pumping out the vacuum space through a rotating vacuum seal that is smaller in diameter than the rotor shaft. By using seals that are much smaller than the size of the shaft support bearings, and that do not have to support high radial loads, seal life is improved, seal cost is reduced, and leakage is reduced. The vacuum chamber is attached to the rotor to rotate therewith. Because the diameter of the coupling is not dependent on the diameter of the rotor shaft, the shaft can be made as large as desired without incurring the cost and complexity of large vacuum couplings. 
     Specifically, then, the present invention provides a rotor for use with a superconducting electric motor. The rotor includes a rotor support shaft having an outer surface having a first diameter for receiving a support bearing and having an inner axial bore and a vacuum seal with an interface dividing stationary and rotating portion of the vacuum seal, the interface having a second diameter smaller than the first diameter. A superconducting rotor winding communicates with the rotor support shaft to rotate therewith and a vacuum jacket is attached to the rotor support shaft to surround the superconducting rotor winding thereby providing thermal insulation. The inner bore of the rotor support shaft communicates with an interior of the vacuum jacket and a non-rotating vacuum line communicates with the inner bore so as to provide a path of evacuation of the interior of the vacuum jacket through the inner bore into the vacuum line. The vacuum seal fits between the vacuum line and the inner bore with one of the stationary and rotating portions of the vacuum seal fitting against the vacuum line and one of the stationary and rotating portions of the vacuum seal fitting against the inner bore. 
     Thus it is one object of the invention to provide a means for continuously evacuating a running motor. The use of a vacuum seal with a smaller diameter than the motor shaft makes a continuous coupling between the rotor and an external vacuum pump more robust and less expensive. 
     The vacuum seal may fit against the inner surface of the inner bore and an inner periphery of the vacuum seal fits against an outer periphery of the vacuum line. 
     Thus it is another object of the invention to provide a coupling that fits unobtrusively within one motor shaft. 
     The inner bore may include a concentric partitioning tube having a central lumen leading to the superconducting rotor windings and the vacuum line may include an inner concentric cryogen supply line positioned so that when the vacuum line communicates with the inner bore, the cryogen supply line engages the central lumen of the partitioning tube and the vacuum line communicates with the space between the partitioning tube and the inner bore. 
     Thus it is another object of the invention to provide a continuous cryogen supply to a rotating rotor. 
     The cryogen supply tube overlaps with the partitioning tube to minimize conduction between the vacuum seal and the cryogen of the cryogen supply line. Both the vacuum line and the inner concentric cryogen supply line extend beyond the second seal and are joined at their edges to provide an extended thermal path between the cryogenic temperatures of the cryogen supply line and the second seal. 
     Thus it is another object of the invention to permit the use of vacuum seals that cannot function at cryogenic temperatures. 
     The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings that form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made to the claims herein for interpreting the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective side view of the rotor and shaft assembly of the present invention showing the position of the rotating coupling and its size relative to the size of the rotor and rotor shaft and support bearings; 
     FIG. 2 is a cross-sectional view along lines  2 — 2  of FIG. 1 of the rotor and shaft showing the concentric cryogen supply line and vacuum line interfitting with seals within a bore of one rotor shaft; and 
     FIG. 3 is a detailed view of FIG. 2 showing the dual cryogen and vacuum pathways provided by the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, the present invention provides a superconducting rotor  10  for an electric motor. The rotor  10  includes a generally cylindrical vacuum jacket  12  having closed bases  14  and  16 . Axial drive shaft  20  extends from base  16 , and axial support shaft  22  extends from base  14 . The shafts  22  and  20  are aligned with the central axis of the cylindrical vacuum jacket  12 . The drive shaft  20  may be solid for increased torque and flexibility in coupling. 
     Drive shaft  20  and support shaft  22  may be supported by conventional ball bearings  24  in a manner well known in the art and extend through the ball bearings  24  to provide outer end  26  of drive shaft  20 , that may be coupled to a machine receiving torque from the rotor  10  and to provide outer end  28 , of support shaft  22 , that may receive a combined cryogen/vacuum line  30 . Referring now to FIGS. 2 and 3, the combined cryogen/vacuum line  30  provides a cryogen pipe  32  concentrically located within a larger vacuum pipe  34 . Referring now principally to FIG. 2, the bearings  24  in turn may be supported by a housing  58  of a type well known in the art containing the armature and other features of the motor. 
     Continuing to refer to FIGS. 2 and 3, the support shaft  22  includes an axial bore  36  and fitted within the axial bore  36  at the end  28  is a ferrofluidic seal and bearing  38  abutting at its outer periphery the inner surface of the bore  36  and supporting at its inner periphery the outer surface of the combined cryogen/vacuum line  30 . As is well understood in the art, the ferrofluidic seal and bearing provides both a conventional radial ball bearing and by means of a ferrofluidic liquid, a vacuum seal between the sliding surfaces which define an interface between the moving a stationary portions of the ferrofluidic seal and bearing  38 . The interface is of a significantly smaller diameter than the diameter of the support shaft  22 . Such ferrofluidic seal and bearings are well known in the art and may be obtained from a number of commercial manufacturers including Ferrofluidics Corporation of New Hampshire. 
     After passage into the bore  36  and past the ferrofluidic seal and bearing  38 , the vacuum pipe  34  necks inward to a reduced diameter  40  to fit within a second ferrofluidic seal and bearing  42  having an inner periphery of smaller diameter than the inner periphery of ferrofluidic seal and bearing  38 . The second ferrofluidic seal and bearing  42  supports the outer surface of the necked portion of the vacuum pipe  34 . The outer periphery of the second ferrofluidic seal and bearing  42  fits within a spacer ring  44  spanning the distance between the outer periphery of the second ferrofluidic seal and bearing  42  and the inner surface of the bore  36  and forming part of a stationary portion of the second ferrofluidic seal and bearing  42 . The ring  44  is fixed to the support shaft  22  to rotate therewith. 
     Attached to the inner radial face of ring  44  (removed from the outer end  28  of the support shaft  22 ) is a radially outwardly flared lip of a partition tube  48  fitted coaxially within bore  36 . The partition tube  48  loosely surrounds the necked down portion of the vacuum pipe  34  and cryogen pipe  32  and extends through the vacuum jacket  12  into its inner volume. 
     Ring  44  includes a plurality of axial ports  46  aligning with an axial bore in the flared lip of partition tube  48  to provide communication between a space  49  defined within the ferrofluidic seal and bearing  38 , the ring  44 , the bore  36  of the support shaft  22  and outer surface of the vacuum pipe  34 , and a space  50  defined within the bore  36  of the support shaft  22  and the outer surface of the partition tube  48 . A port  53  cut in the outer surface of the vacuum pipe  34  provides a path  54  for drawing air from space  50 , through the ring  44  to space  49  and then into the vacuum pipe  34  which is connected externally to a vacuum pump (not shown). 
     Cryogen may pass along path  56  within the inner cryogen pipe  32  to a volume  52  inside the wall of the partition tube  48 . The vacuum pipe  34  and cryogen pipe  32  extend an arbitrary distance past the ferrofluidic seal and bearing  42  so as to provide a high thermal resistance between the cryogen and the ferrofluidic seal and bearing  42  and are joined together by stopper ring  51  which connects the outer surface of the inner cryogen pipe  32  to the inner surface of the vacuum pipe  34 . It will be understood that the cryogen pipe  32  will thus be more thermally isolated from the support shaft  22  as is connected by the ferrofluidic seal and bearing  42  and ring  44  by a relatively thin cross-section of an appropriately long thermal path. The loose fit between the vacuum pipe  34  within the partition tube  48  provides a gas passage from the end of vacuum pipe  34  and cryogen pipe  32  back to the ferrofluidic seal and bearing  42  but this is a relatively narrow cross section and dead-ended so there is little thermal conduction through gas trapped therein. 
     Referring now principally to FIG. 2, the support shaft  22  abuts the vacuum jacket  12  of the rotor  10  to sandwich a base of the vacuum jacket  12  between itself and a composite torque tube  60  axially aligned with the support shaft  22  inside the vacuum jacket  12 . The torque tube  60  provides a continuation of the support function of the support shaft  22 , however, with lower thermal conductivity provided both by material selection and its being hollow. The partition tube  48  extends from the bore  36  of the support shaft  22  into the torque tube  60  and then by means of a second outwardly flared lip expands radially to attach to the inner surface of the torque tube assembly  60 . An orifice  62  cut in the torque tube  60  to communication between space  50  and the interior of the vacuum jacket  12  so that the latter may be evacuated through vacuum pipe  34 . 
     The torque tube assembly  60  connects also to a coil support  64  which includes an internal cryogen distribution structure  66  allowing cryogen in volume  52  to pass through the cryogen distribution structure  66  to high temperature superconducting field windings  68  attached at the outer periphery of the support structure  64 . An AC flux shield  72  may be positioned outside of the high temperature superconductor windings  68  between the high temperature superconductor windings  68  and the armature  74 . 
     The cryogen introduced into volume  52  may thus communicate with an inner surface of the high temperature superconducting winding  68  without release to the general inner volume of the vacuum jacket  12  surrounding the high temperature superconducting windings  68 . In this manner, both vacuum and cryogen may be separately contained with the rotor  10 . 
     Axially, on the opposite side of the support structure  64  from the torque tube  60 , a similar torque tube  70  connects to the base  16  of the vacuum jacket  12  which is sandwiched between torque tube  70  and drive shaft  20  as described with respect to FIG.  1 . 
     Importantly, it will be noted that the size of the ferrofluidic seal and bearings  38  and  42  is substantially smaller than the size of the bearing  24  thus reducing the potential leakage area significantly decreasing the cost of the seals which also are not required to support any substantial radial loads which are handled by the bearing  24 . In this embodiment, vacuum vessel rotates with the shaft thus eliminating any further seal that would be required between the vacuum vessel and the shaft. 
     The rotor  10  thus formed may be surrounded by armature  74  of conventional design having standard conductors which are thus isolated from the high temperature superconductor windings  68  which are within the vacuum jacket  12 . An exciter of conventional design (not shown) may be fit either to the drive shaft  20  or to the support shaft  22 . 
     During operation, a vacuum pump is attached to the vacuum line and cryogen is inserted into the cryogen pipe  32  without the need for complex couplings and both lines are nonrotating. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but that modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments also be included as come within the scope of the following claims.