Abstract:
Superconducting coils and methods of manufacture include a superconductor tape wound concentrically about and disposed along an axis of the coil to define an opening having a dimension which gradually decreases, in the direction along the axis, from a first end to a second end of the coil. Each turn of the superconductor tape has a broad surface maintained substantially parallel to the axis of the coil.

Description:
STATEMENT AS TO FEDERALLY SPONSORED RESEARCH  
       [0001] This invention was made with Government support under Prime Contract No. DE-FC36-93CH10580 awarded to Reliance Electric Co. by the Department of Energy. The Government has certain rights under this invention. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The invention relates to superconducting magnetic coils.  
           [0003]    Superconductors may be used to fabricate superconducting magnetic coils such as solenoids, multipole magnets, etc., in which the superconductor is wound into the shape of a coil. When the temperature of the coil is sufficiently low that the superconductor can exist in a superconducting state, the current carrying capacity as well as the magnitude of the magnetic field generated by the coil is significantly increased.  
           [0004]    Typical superconducting materials include niobium-titanium, niobium-tin, and also copper oxide ceramics such as members of the rare-earth-copper-oxide family (i.e., YBCO), the thallium-barium-calcium-copper-oxide family (i.e., TBCCO), the mercury-barium-calcium-copper-oxide family (i.e., HgBCCO), and the bismuth-strontium-calcium-copper-oxide family (with or without lead substitutes, i.e., (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O 10 , Bi 2 Sr 2 Ca 2 Cu 3 O 10  (BSCCO (2223)) , which perform particularly well because their superconductivity and corresponding high current density characteristics are achieved at relatively high temperatures (T c ≈108° K.)  
           [0005]    In fabricating certain superconducting magnetic coils, the superconductor may be formed in the shape of a thin tape which allows the conductor to be bent around the diameter of a core. For example, high temperature superconductor (HTS) is often fabricated as a thin tape in which multi-filament composite superconductor including individual superconducting filaments extends substantially the length of the multi-filament composite conductor and are surrounded by a matrix-forming material, which is typically silver or another noble metal. Although the matrix forming material conducts electricity, it is not superconducting. Together, the superconducting filaments and the matrix-forming material form the multi-filament composite conductor. In some applications, the superconducting filaments and the matrix-forming material are encased in an insulating layer (not shown).  
           [0006]    One approach for winding a magnetic coil with superconducting tape is known as pancake winding, in which the superconductor tape is wound one turn on top of a preceding turn thereby forming a plane of turns perpendicular to the axis of the coil. In applications where a series of pancake coils are to be used to form a coil, the pancake coils can be wound as double pancakes.  
           [0007]    In some applications, a superconducting magnetic coil assembly using pancake coils (whether single or double) may include several coils, coaxially disposed along the length of the coil assembly. The individual coils are interconnected using short lengths of superconducting wire or ribbon made from the superconducting materials of the type described above, for example, copper oxide ceramic.  
           [0008]    One example of this arrangement is described in U.S. Pat. No. 5,581,220, assigned to the assignee of the present invention and incorporated herein by reference, which describes a variable-profile (e.g., stair-stepped) superconducting magnetic coil with a varying radial cross section. Another example of a superconducting coil having a stacked arrangement of pancake coils is the “saddle-shaped” coil, which is oval-shaped or rectangular with rounded corners.  
           [0009]    The variable-profile and saddle-shaped coils are advantageously used in applications where the superconducting magnetic coil is required to conform to or be positioned within an annular region of an assembly, such as a rotating electric machine.  
         SUMMARY OF THE INVENTION  
         [0010]    The invention features a superconducting coil having a conical or tapered profile.  
           [0011]    In one aspect of the invention, the superconducting coil includes a superconductor tape wound concentrically about and disposed along an axis of the coil to provide a plurality of concentric turns defining an opening having a dimension which gradually decreases, in the direction along the axis, from a first end to a second end of the coil. Each turn of the superconductor tape has a broad surface maintained substantially parallel to the axis of the coil.  
           [0012]    The decreasing dimension opening defined by the winding configuration of the coil provides a coil having a tapered profile. The advantages of a tapered superconducting coil having this arrangement are numerous. For example, the tapered superconducting coil is well-suited for use in applications where the coil is to be positioned in annularly-shaped volumes, such as those commonly found in rotating electric machines (e.g., motors or generators.) In general, the tapered arrangement eliminates stepped profiles, common with other stacked arrangements. In particular, the tapered superconducting coil requires relatively fewer stacked individual coils to fill annularly-shaped volumes. This is in contrast to other superconducting coil assemblies, which require stacking of many more thin, individual coils to fill an annularly-shaped volume. Moreover, reducing the number of individual coils, in turn, reduces the number of electrical connections between the individual coils, thereby increasing the overall performance and reliability of a coil assembly using tapered coils.  
           [0013]    In addition, the superconductor tape of the present invention is wound with its broad surface maintained substantially parallel to the axis of the coil (as well as to adjacent turns.) This feature is particularly advantageous when the tape is formed of less flexible, brittle materials, such as ceramic-based high temperature superconducting materials.  
           [0014]    Furthermore, the tapered configuration provides better critical current (I c ) retention characteristics and allows for better coil grading.  
           [0015]    In another aspect of the invention, a superconducting coil assembly includes a plurality of superconducting coils in a stacked arrangement, each having the characteristics described above. Because the turns of superconductor tape for each coil has its broad surface maintained substantially parallel to the axis of the coil assembly, the individual coils are easily stacked without the need for spacers or wedges. In certain embodiments of this aspect of the invention, the coils are substantially identical, which is particularly advantageous in certain applications (e.g., rotating machines) where the coil assembly is to be placed within a predefined annularlyshaped volume.  
           [0016]    In certain stacked arrangements, a top and a bottom coil (e.g., pancakes) at each end of the stack (a first end coil and a second end coil, respectively) are preselected to have a higher critical current retention characteristic than critical current retention characteristic of the coils positioned between the top and bottom coils of the stack. Similarly, groups of coils at the top and bottom ends of the coil assembly may be preselected to have a higher critical current retention characteristic. Positioning top and bottom pancake coils with higher critical current retention characteristics in this manner can significantly lower the power loss of the total coil assembly. The top and bottom pancake coils can be preselected by their intrinsic properties or, alternatively, with small changes in the superconductor tape dimensions.  
           [0017]    Embodiments of these aspects of the invention may also include one or more of the following features.  
           [0018]    The superconductor tape is wound in a racetrack shape defining a pair of opposing arcuate end sections and a pair of opposing substantially straight side sections.  
           [0019]    The superconductor tape includes a multi-filament composite superconductor including individual superconducting filaments which extend the length of the multi-filament composite conductor and are surrounded by a matrix-forming material. The superconductor tape preferably includes an anisotropic high temperature superconductor, such as (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O. In alternative embodiments, the superconductor tape includes a copper oxide ceramic, such as those which are members of the rare-earth-copper-oxide family (i.e., YBCO.)  
           [0020]    In certain embodiments, the superconductor tape includes a pair of superconductor layers and at least one mechanical reinforcing layer. The pair of superconductor layers are disposed between a pair of mechanical reinforcing layers, each reinforcing layer including stainless steel.  
           [0021]    The superconducting coil is in the form of a pancake coil, such as a double pancake coil.  
           [0022]    In one embodiment, the superconductor tape is wound to provide a linearly tapered inner surface of the coil extending along the axis of the coil. In an alternative embodiment, the superconductor tape is wound to provide a curved inner surface of the coil extending along the axis of the coil. In this embodiment, the curved inner surface of the wound superconductor tape is cylindrically-shaped along the opposing substantially straight side sections and spherically-shaped along the opposing arcuate end sections.  
           [0023]    In another aspect of the invention, a method of providing a superconducting coil includes the following steps. A superconductor tape is wound about an axis of the coil to provide concentric turns defining an opening having an inner dimension with a broad surface of the superconductor tape maintained substantially parallel to the axis of the coil. The tape is wound so that the opening gradually decreases from a first end to a second end in the direction along the axis.  
           [0024]    Embodiments of this aspect of the invention includes one or more of the following features.  
           [0025]    The superconducting coil is wound using a mandrel having surfaces which define the taper. For example, the surfaces may be linearly tapered or curved (e.g., cylindrically-shaped or spherically-shaped.) In an alternative approach, a pair of heated plates are used to apply heat and pressure to mold the superconducting coil into a tapered profile.  
           [0026]    Other advantages and features will become apparent from the following description and the claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    [0027]FIG. 1 is a perspective, partially cutaway view of a superconducting coil in accordance with the invention.  
         [0028]    [0028]FIG. 2 is a cross-sectional side view of a portion of a superconductor tape for winding the superconducting coil of FIG. 1.  
         [0029]    [0029]FIG. 3 is an exploded view of a portion of the superconducting coil along line  3 - 3  of FIG. 1.  
         [0030]    [0030]FIG. 4 is a perspective view of a mandrel suitable for winding the superconducting coil of FIG. 1.  
         [0031]    [0031]FIG. 5A and 5B illustrate an alternative approach for forming the superconducting coil of FIG. 1.  
         [0032]    [0032]FIG. 6 is a perspective view of an alternative embodiment of a superconducting coil.  
         [0033]    [0033]FIG. 7 is an exploded view of a portion of the superconducting coil along line  7 - 7  of FIG. 6.  
         [0034]    [0034]FIG. 8 is a perspective view of a mandrel suitable for winding the superconducting coil of FIG. 6.  
         [0035]    [0035]FIG. 9 is a cross-sectional end view of a portion of a rotor including the superconducting coil of FIG. 6. 
     
    
     DETAILED DESCRIPTION  
       [0036]    Referring to FIG. 1, a tapered superconducting coil  10  includes a superconductor tape  12  wound generally in an elongated oval or racetrack shape. The “racetrack-shaped” superconducting coil  10  includes a pair of opposing and generally straight side sections  14   a  and a pair of opposing curved end sections  14   b , which together form a generally rectangularly-shaped coil with rounded corners. It is important to note that although coil  10  is “racetrack-shaped”, it does not have the shape or structure of the well-known racetrack coil. As will be described below in conjunction with one fabrication approach, coil  10  is wound about an axis  16  of the coil from a continuous length or series of lengths of superconductor tape, thereby forming a number of windings or turns  18  of the coil (see FIG. 3.) The turns, in combination, define an opening  19  which, as will be described in greater detail below, increases in size from the innermost turn to the outermost turn. This approach for winding the superconductor tape is often referred to as pancake winding, in which the superconductor tape is wound one turn on top of a preceding turn thereby forming a plane of turns perpendicular to axis  16  of coil  10 .  
         [0037]    Referring to FIG. 3, superconductor tape  12  includes broad sides  22  and narrow sides  24 . In one embodiment, superconductor tape includes a multi-filament composite superconductor layer  25  having individual superconducting filaments extending substantially the length of the multi-filament composite conductor and surrounded by a matrix-forming material, such as silver. The superconducting filaments and matrix-forming material together form the multi-filament composite conductor. In many applications, the superconducting filaments and the matrix-forming material are encased in an insulating layer (not shown).  
         [0038]    Referring to FIG. 2, in one embodiment, a pair of superconductor layers  25  are sandwiched between a pair of reinforcement members  26 , for example of stainless steel, which provide mechanical support to the superconductor layers  20 . One approach for fabricating a reinforced superconductor tape in this manner is described in U.S. patent application Ser. No. 08/701,375, assigned to American Superconductor Corporation, assignee of the present invention, and incorporated herein by reference.  
         [0039]    Referring to FIG. 3, each turn  18  of superconductor tape  12  is wound such that each turn is slightly offset, in the direction of axis  16 , from a preceding turn so that from the innermost turn  18   a  (FIG. 1) to the outermost turn  18   b , coil  10  is wound in tapered fashion along an imaginary line  28 . It is important to note that broad sides  22  of each turn  18  are parallel to each other and to axis  16 .  
         [0040]    Referring to FIG. 4, a mandrel  30  is formed here, for example, of aluminum and is used with a winding mechanism carrying spools of the superconductor layers and stainless reinforcing members (neither shown) to wind superconductor coil  10 . Mandrel  30  includes a central mount section  32  surrounded by opposing tapering side sections  34  and opposing tapering end sections  36 . Mandrel defines the shape and degree of taper for winding coil  10 . A tool (not shown) follows the surface of mandrel  30  and guides superconductor tape  12  in place on the mandrel.  
         [0041]    Referring to FIGS. 5A and 5B, an alternative approach for forming tapered superconducting coil  10  is shown. In this approach, superconducting coil  10  is formed first as a conventional pancake coil  10   a , without tapered edges; that is, each turn of coil  10   a  lies directly and entirely over the preceding turn. Kapton® film (a product of E. I. duPont de Nemours and Company, Wilmington, Del.) having a B-staged epoxy is wrapped “in-hand” with superconductor tape  12  during the winding process of the pancake coil. The epoxy has a tacky characteristic which helps hold the adjacent turns together. Referring to FIG. 5A, pancake coil  10   a  is then placed between a pair of heating plates  40  having tapered surfaces  42 , which define the shape and desired degree of taper for superconducting coil  10 . Referring to FIG. 5B, heating plates  40  are brought together (in the direction of arrow  43 , as shown in FIG. 5A) and heat and pressure are applied to pancake coil  10   a , thereby forming superconducting coil  10 . The heat and pressure are removed after a predetermined period of time, to allow the epoxy to cool so that the adjacent turns of superconducting coil  10  are bonded together.  
         [0042]    Referring to FIG. 6, an alternative embodiment of a superconducting coil  100  is shown wound with the same superconductor tape  12  described above for winding coil  10 . In this embodiment, however, superconductor tape  12  is wound so that regions of the tape along opposing side sections  104   a  are cylindrically tapered while regions of the tape along opposing end sections  104   b  are spherically tapered.  
         [0043]    Referring to FIG. 7, in particular, superconductor tape  12  is wound at both side sections  104   a  and end sections  104   b  in tapered fashion along an imaginary curved line  106 . Because side sections  104   a  are straight in the plane of the coil, tapering along these sections is based on a cylinder. At rounded end sections  104   b , on the other hand, the tapering is based on a quartered section of a sphere. Note that as was the case with the linearly tapered embodiment described above, the broad sides  22  of each turn of the superconductor tape in this case are still parallel to each other and to axis  16 .  
         [0044]    Referring to FIG. 8, a mandrel  130  for forming superconducting coil  100  is shown. Mandrel  130  is essentially the same as mandrel  30  described above except that the surfaces of opposing tapering side sections  134  and opposing tapering end sections  136  are rounded to define the curved shape of rounded side sections  104   a  and end sections  104   b.    
         [0045]    Superconducting coil  10  and superconducting coil  100  are both well-suited in applications where the coils are required to be positioned within angularly-shaped regions, such as those common in rotor assemblies of electric motors.  
         [0046]    With reference to FIG. 9, in one example, a rotor assembly  200  for a synchronous motor has a four-pole topology without its outer shield for enclosing the vacuum layer within the overall assembly. In this embodiment, rotor assembly  200  includes a torque tube  220  fabricated from a high-strength, ductile and non-magnetic material (e.g., stainless steel). The outer surface of the torque tube  220  supports four superconducting coil assemblies  230  (only two are shown,) each winding associated with a pole of the motor. A hollow core member  250  formed of a high permeability, high saturation flux density material (e.g., iron) is positioned within the inner volume of the torque tube to provide a low reluctance flux path for magnetic fields generated by coil assemblies  230 . Coil assemblies  230  are positioned within annular regions  240  defined by the outer surface of torque tube  220  and inner surface of pole caps  260 , which together with torque tube  220  define a cylinder.  
         [0047]    Because of their shape, annular regions  240  are not well-suited for receiving superconducting coil configurations, such as stacked pancake and racetrack coils. To fill annular regions  240  with these types of superconducting coils requires that the coil assemblies be constructed with individual coils (e.g., pancakes) which are staggered in stair-step fashion. In order to fill the annular region efficiently, a relatively large number of individual coils are required to be made thin, stacked, and then connected together. Connecting the individual coils in these arrangements is non-trivial and the torque tube is generally required to be machined with a corresponding stair-step surface to support the coil assemblies, adding cost and complexity to the manufacture of the motor.  
         [0048]    Because superconducting coil  10  and superconducting coils  100  are tapered, either linearly or in a curved manner, either coil assembly can conform within and fill annular regions  240 . Unlike, the stacked pancake arrangements described above, far fewer coils are needed to fill the space, thereby reducing the number of connections and increasing the reliability and performance of the coil assemblies. Thus, a more efficient, easy to assemble motor construction is provided. Moreover, tapered coils are advantageously positioned closer to the armature of the motor.  
         [0049]    Further, in many applications the annular regions may be formed such that the stacked tapered superconducting coils are substantially identical, which further reduces manufacturing costs. A stacked set of substantially identical tapered superconducting coils are simply connected and positioned within the annular region.  
         [0050]    Another important advantage of the conical or tapered superconducting coil is that, in a stacked arrangement, the configuration has the benefit of shielding inner ones of the stacked coils from fields perpendicular to the broad face of the superconductor tape. Thus, a series of the tapered superconducting coils can be stacked so that the those coils having better performance characteristics are placed on the top and bottom of the stack.  
         [0051]    Other embodiments are within the scope of the claims. For example, both single and stacked arrangements of tapered or conical superconducting coils have been described above. However, in certain applications it is desirable to wind the superconducting coil as a single pancake, rather than a number of thinner, stacked coils having the same width. The single pancake arrangement provides self-shielding and a higher critical current retention characteristic.