Abstract:
A unitary bimetallic shield ring for a superconducting generator rotor includes coaxial inner and outer cylindrical portions. The outer cylindrical portion comprises a first metallic material for conducting eddy currents to dissipate energy, and defines an interior face. The inner cylindrical portion comprises a second metallic material for providing structural support to the outer cylindrical portion. The inner cylindrical portion is continuously metallurgically joined with the interior face of the outer cylindrical portion. The shield is made by first forming a substantially continuous weld between the inner and outer layers, and then machining the shield.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to electromagnetic shielding for superconducting generator rotors.  
         [0002]     Superconducting generators include a cryogenically cooled rotor having rotor windings. The rotor is a rotating part that works together with a non-moving stator to produce electrical energy, which can be conditioned to the form of a high-voltage DC output. The superconducting rotor coils must be protected from magnetic flux variation originating in the stator. Such undesired magnetic flux may cause the generation of alternating electrical currents (i.e., eddy currents) in the rotor coils. Those alternating currents generate heat, and can thereby cause the rotor coils to cease to be superconductors.  
         [0003]     In order to protect rotor coils from noise, and occasionally to also provide dampening, a number of shields and damper shields have been disclosed. A damper shield disclosed by Sterrett, U.S. Pat. No. 4,039,870, is a two-layer assembly for providing shielding and mechanical dampening. The inner layer is a conductive copper alloy. The outer layer, forming the exterior of the shield, is a structural material. The two layers are welded together using explosive welding. The explosive welding process in conducted by placing the inner layer and outer layer inside one another, and placing an explosive charge inside the inner (copper alloy) layer and detonating it. However, this damper shield has a number of disadvantages. Eddy currents tend to form along exterior surfaces, and therefore an outer non-conductive structural layer lessens the ability of the damper shield to dissipate eddy currents at a distance from the rotor itself. In addition, placing explosive charges inside a cylinder is difficult, particularly where the bore of the cylinder (pre-detonation) is small.  
         [0004]     A damper shield disclosed by Cooper et al., U.S. Pat. No. 4,152,609, is a three-layer assembly. The inner and outer layers are non-conductive structural layers and the intermediate layer is a conductive layer. The respective layers are welded together. High-strength non-magnetic materials are specifically used for the outer layer because of mechanical forces concentrated there. The three layers are secured together by metallurgical bonding or mechanical keying. However, the damper shield presents difficulties with respect to the outer layer being non-conductive, which lessens eddy current dissipation capabilities at a distance from the rotor, and difficulties in manufacturing a three-layer assembly. Explosive welding, which can be used to form a metallurgical bond, is problematic where both the inner and outer layers are high-strength materials. Moreover, a mechanically-keyed assembly presents a risk of cracking and other damage during use.  
         [0005]     A shielding assembly is also disclosed by Khutoretsky et al., U.S. Pat. No. 4,820,945 for providing only shielding to a rotor. The assembly includes an inner cylinder of conductive material. An outer cylinder is formed of a structural material. A solid film lubricant is disposed between the inner and outer cylinders, and separates those cylinders such that they do not form a unitary, securely-joined structure. However, this shield presents difficulty with respect to poor thermal conduction to dissipate heat from the inner cylinder through the solid film lubricant, and also with respect to lessened eddy current dissipation at a distance from the rotor where the outer cylinder is non-conductive.  
         [0006]     The present invention provides an alternative electromagnetic shield for use with a superconducting generator or other dynamoelectric device.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     A unitary bimetallic shield ring for a superconducting generator rotor includes coaxial inner and outer cylindrical portions. The outer cylindrical portion comprises a first metallic material for conducting eddy currents to dissipate energy, and defines an interior face. The inner cylindrical portion comprises a second metallic material for providing structural support to the outer cylindrical portion. The inner cylindrical portion is continuously metallurgically joined with the interior face of the outer cylindrical portion.  
         [0008]     Further disclosed is a method of manufacturing a shield. The method includes providing a first cylindrical layer of a high-strength non-magnetic flux conducting metallic material, welding a second cylindrical layer of copper around the first cylindrical layer, and machining the second cylindrical layer to remove a portion of the second cylindrical layer. The weld is substantially continuous at an interface defined between the first cylindrical layer and the second cylindrical layer.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a longitudinal cross-sectional view of a portion of a superconducting generator system having a shield assembly according to the present invention.  
         [0010]      FIG. 2  is a lateral cross-sectional view of a portion of the superconducting generator system, taken along section A-A of  FIG. 1 , showing the shield assembly and a rotor. 
     
    
       [0011]     In  FIGS. 1 and 2 , only the shield assembly has been has been shown cross-hatched in section, and the other components of the superconducting generator system have been shown only schematically for clarity.  
       DETAILED DESCRIPTION  
       [0012]     The present invention provides an electromagnetic shield for a superconducting generator rotor, or similar device, to shield the rotor&#39;s windings from electromagnetic noise, giving rise to eddy currents. In particular high-order harmonics are of a concern, including the 5 th  and 7 th  harmonic pair, the 11 th  and 13 th  harmonic pair and the 17 th  and 19 th  harmonic pair. The 5 th  and 7 th  harmonic pair can be addressed using an active rectifier. The shield of the present invention can also be used to dissipate undesired electromagnetic noise to protect the superconducting coils from undesired heating. In one embodiment, the shield of the present invention can intercept about 100 watt losses due to the 11 th  and 13 th  harmonics and about 15 watt losses due to the 17 th  and 19 th  harmonics, which could otherwise cause the windings to develop local hot spots and cease to be superconductors.  
         [0013]     The shield according to the present invention is a cylindrical bimetallic assembly that includes an inner structural layer of a high-strength high electrical resistivity (i.e., nonmagnetic) metallic material and an outer electrically conductive metallic layer. The two layers are metallurgically bonded together to form a unitary shield structure. The outer layer conducts noise currents (i.e., alternating currents or eddy currents) to dissipate them, as heat, at a location spaced from the rotor windings.  
         [0014]      FIG. 1  is a longitudinal cross-sectional view of a portion of a superconducting generator system  10  having a shield assembly  12 . The system  10  includes a stator  14  and a cryogenically-cooled rotor  16 . The rotor  16  includes a shaft structure  18 , rotor windings  20 , and coolant pathways  22 . The coolant pathways allow a cryogenic coolant to enter the rotor  16  and travel through a complex path past the rotor windings  20  and eventually out of the rotor  16  through a central pathway  22 A. The stator  14  includes a stator body structure  24  and stator windings  26 . The rotor  16  and stator  14  are positioned about an axis of rotation  28  for the generator system  10 .  
         [0015]     The shield  12  is mounted around a portion of the rotor  16 , and is separated from the stator  14  by a small air gap or vacuum gap. The shield  12  is retained on the rotor  16  at both ends of the shield  12 . At one end, the shield  12  is placed at support notch  30  of the rotor  16 . The support notch  30  restrains longitudinal movement of the shield  12  with an interference fit. At its other end, the shield  12  is secured with a retention plate  32  that is secured to a support flange  34  of the rotor  16  by one or more screws  36 . Sealing elements  38  (e.g., OmniSeals®, available from Saint Grobain Performance Plastics, Garden Grove, Calif.) are provided to create a fluid seal between the shield  12  and the rotor  16  at both ends of the shield  12 .  
         [0016]      FIG. 2  is a lateral cross-sectional view of a portion of the generator system  10 , taken along section A-A of  FIG. 1 , showing the rotor  16  and the shield assembly  12 . As shown in  FIGS. 1 and 2 , the shield  12  is a unitary, bimetallic assembly that includes an inner layer  50  and an outer layer  52 , and has an elongate cylindrical shape. The inner layer  50  is a structural layer formed of a high-strength and high electrical resistivity metallic material, such as Inconel® 718 (a high-strength austenitic nickel-chromium-iron alloy) and MP35N® (available from Carpenter Technology Corp., Reading, Pa.). The outer layer  52  is formed of an electrically conductive metallic material having a relatively low resistivity, such as aluminum or copper. The copper can be oxygen-free copper or of a similar grade. In a preferred embodiment, the outer layer  52  is formed of copper, which has beneficial coefficient of expansion properties when used at temperatures where the rotor windings  20  can be superconducting.  
         [0017]     The inner and outer layers  50  and  52  are metallurgically joined, for example, using an explosive welding process, to produce a unitary bimetallic shield  12  where the inner and outer layers  50  and  52  are connected by a substantially continuous joint  54  along the interface of those layers. The outer layer  52  forms an exterior surface  56  of the shield  12 .  
         [0018]     In one embodiment, the shield  12  has the following nominal dimensions. The inner diameter of the of the inner layer  50  is 21.5265 centimeters (cm) (8.475 inches). The outer diameter of the inner layer  50  (and also the inner diameter of the outer layer  52 ) is 22.2885 cm (8.775 inches). The outer diameter of the outer layer  52  is 23.0505 cm (9.075 inches). The longitudinal length L of the shield  12  is 49.276 cm (19.4 inches). It should be recognized that these dimensions are exemplary and other dimensions are possible, as desired.  
         [0019]     In operation, the shield  12  reduces the risk that electromagnetic noise originating at the stator  14  will reach the rotor  16 . The electromagnetic noise is dissipated by the process of generating eddy currents in the outer (conductive) layer  52  of the shield  12 . Those eddy currents are dissipated as heat by the shield  12 , to reduce heating of the rotor  16 , and more particularly, to reduce heating of the superconducting rotor core  18  protected by the shield  12 .  
         [0020]     Also, when the shield  12  is installed on the rotor  16 , the high strength inner layer  50  of the shield  12  can provide compressive loading to the rotor  16 . This optional compressive loading permits the use of magnetic materials for the rotor core  18  that would otherwise not be acceptable at cryogenic temperatures (i.e., about 40° K or lower).  
         [0021]     The shield  12  can be manufactured as follows. A first cylinder corresponding to the inner (structural) layer  50  is provided. Then a second cylinder corresponding to the outer (conductive) layer  52  is positioned around the first cylinder. The second cylinder is slightly larger than the desired nominal finished dimensions of the outer layer  52  of the shield  12 . This permits the second cylinder to be more easily fitted over the first cylinder for fabrication. The first and second cylinders are then cleaned as desired. They are then positioned in an appropriate enclosure or pit for explosive welding, and supported for explosive welding. Explosive charges are placed around the second cylinder, relative to an exterior surface of the second cylinder (corresponding to the exterior surface  56  of the outer layer  52  of the shield  12 ). Explosive welding is conducted by detonating the charges to cause the material of the second cylinder to be metallurgically joined to the material of the first cylinder, and create the shield  12  with a substantially continuous connection between its inner layer  50  (corresponding to the first cylinder) and its outer layer  52  (corresponding to the second cylinder). Finally, the welded shield  12  is machined at its outer surface  56  and its inner surface  58 . Machining is performed to achieve desired nominal finished dimensions for the shield. The finished shield can then be mechanically installed on the rotor  16 .  
         [0022]     It should be recognized that the present invention provides numerous advantages. First, the location of the conductive material of the outer layer at the exterior surface of the shield allows electromagnetic noise to be dissipated at a location spaced from the superconducting windings of a superconducting generator rotor. Moreover, the bimetallic shield of the present invention has its conductive layer radially outside the structural layer, which may facilitate manufacturing.  
         [0023]     Although the present invention has been described with reference to several alternative embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For instance, dimensions of the shield can differ from the example given, as desired for particular application. In addition, a shield according to the present invention can be used with generators, induction motors, and other dynamoelectric systems.