Abstract:
A superconducting PM machine has a stator, a rotor and a stationary excitation source without the need of a ferromagnetic frame which is cryogenically cooled for operation in the superconducting state. PM material is placed between poles on the rotor to prevent leakage or diffusion of secondary flux before reaching the main air gap, or to divert PM flux where it is desired to weaken flux in the main air gap. The PM material provides hop-along capability for the machine in the event of a fault condition.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    The benefit of priority is claimed herein based on U.S. Provisional Application No. 60/413,248 filed Sep. 24, 2002. This application is also a continuation-in-part of Hsu, U.S. application Ser. No. 09/872,048, filed Jun. 1, 2001, and entitled “Method and Machine for High Strength Undiffused Brushless Operation.” 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH  
       [0002] This invention was made with Government support under Contract No. DE-AC05-000R22725 awarded to UT-Battelle, LLC, by the U.S. Department of Energy. The Government has certain rights in this invention. 
     
    
     
       TECHNICAL FIELD  
         [0003]    The field of the invention is electrical machines including motors and generators and the application of superconducting technologies to such machines.  
         BACKGROUND ART  
         [0004]    The need for a high power, small volume, high efficiency, low cost, reliable electric machine is well understood. The applications include, but are not limited to, ship propulsion, industrial drives, power generation and transportation. The development of high temperature superconducting (HTS) wires offers a bright future for superconducting devices. In this context, “high temperature” means operation above a temperature of about 40 degrees Kelvin, which is still well below zero degrees Fahrenheit. A well known superconducting cooling assembly is referred to as a cryostat.  
           [0005]    Dombrovski et al., U.S. Pat. No. 6,313,556, issued Nov. 6, 2001, discloses an AC synchronous motor with a stator, a rotor and a rotor winding of HTS wires which are cooled by a cryostat. The cryostat includes a refrigeration system that is connected through conduits in an extension of the rotor shaft into a vacuum chamber in the interior of the rotor. A cryogenic transfer coupling is provided to allow that portion of the superconducting structure which is inside the rotor to rotate with the rotor.  
           [0006]    The reliability of a rotating cryostat is lower than that of a stationary cryostat for a comparable cost of manufacture. In the rotating system, the superconducting coils not only have to bear the motor torque but also have to be thermally isolated. This places a stringent requirement on the mechanical design which translates into a complicated design for the cryostat, with lower reliability and increased cost. Furthermore, when the machines of the prior art experience a failure in the superconducting system and lose the excitation flux, the motor cannot be rotated. The so-called “hop-along capability,” which is the ability to rotate the rotor of a motor even under failure conditions, is practically zero.  
           [0007]    Recently, a new type of machine referred to as a high strength undiffused brushless machine has been disclosed in Hsu U.S. application Ser. No. 09/872,048, filed Jun. 1, 2001. In this machine, additional excitation is provided by a stationary excitation winding which is positioned next to the rotor so as to induce a rotor-side flux in the rotor. Permanent magnet (PM) material is positioned in between the poles of the rotor to control the flux diffusion of the secondary flux produced by the additional excitation. The flux provided by the stationary excitation winding is thus available to increase or decrease a resultant flux in the main air gap. This invention is applicable to both axial gap and radial gap machines.  
           [0008]    In known superconducting machines there is also a problem that back iron requirements are large in order to isolate the alternating flux. This adds to the weight of a high temperature superconducting motor. The overall wound core length that includes the core length and the length of the two winding end turns of a known superconducting machine may be quite long. Even when helical coils are used instead of the conventional armature coils, the overall core length is still quite long for a machine with lower numbers of poles in comparison with a PM machine that is capable for a machine with higher number of poles.  
           [0009]    There are also limitations to increasing the number of poles in the prior art machines. Unlike a PM motor, the number of poles of a non-PM superconducting motors cannot be increased to a number as high as that of a PM motor. This is because the armature flux of a PM motor is guided by the iron but the armature flux of a non-PM superconducting motor flows freely in the air.  
           [0010]    The present invention has been made to overcome the limitations of the prior art.  
         SUMMARY OF THE INVENTION  
         [0011]    Prior superconducting motors and generators have used a rotating cryostat. The present invention uses a stationary cryogenic assembly together with permanent magnets (PM) to provide a new type of superconducting motors and generators. The problems associated with the conventional superconducting machines and conventional PM machines, such as reliability, hop-along capability, mechanical stress, cost, thermal isolation, and manufacturability problems, are significantly overcome through this new technology.  
           [0012]    In the machine of the present invention, torque does not act on the superconducting coils. The present invention can use either high temperature or low temperature superconducting wires.  
           [0013]    Unlike the prior superconducting machines, the superconducting machine of the present invention has a hop-along capability as a result of the presence of the PM material.  
           [0014]    Unlike the prior superconducting machines, the superconducting machine of the present invention can be built with a high number of poles and with less length of superconducting wire than prior high temperature superconducting machines.  
           [0015]    The machine of the present invention can be an axial-gap machine, a radial-gap machine or a radial-gap inverted machine. The inverted machine has an armature disposed in a central chamber in the rotor, which is separated from the armature by the main air gap. A stationary cryogenic assembly encircles the rotor and is spaced from the rotor by a secondary air gap.  
           [0016]    The machine of the present invention can be built as a motor or a generator.  
           [0017]    The machine of the present invention can use a multiple DC excitation coil arrangement.  
           [0018]    The superconducting machine of the present invention can be built as either a disc-shaped or a barrel-shaped machine.  
           [0019]    Unlike a conventional PM machine, the main air gap flux of this new machine can be weakened through control of the secondary excitation.  
           [0020]    Unlike a conventional PM machine, the main air gap flux density of this new machine can be enhanced through control of the secondary excitation.  
           [0021]    Unlike a conventional PM machine, the manufacturability of this new machine can be ensured by a predetermined excitation or by a magnetic short-circuiting approach.  
           [0022]    The flux produced by the superconducting coils in the machine of the present invention is returned through the air, without requirements for back iron.  
           [0023]    These and other objects and advantages of the invention will be apparent from the description that follows and from the drawings which illustrate embodiments of the invention, and which are incorporated herein by reference. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 is a longitudinal section view of a prior art machine;  
         [0025]    [0025]FIG. 2 is a longitudinal section view of an axial gap machine of the present invention;  
         [0026]    [0026]FIGS. 3 and 4 are front and back perspective views of a rotor in the machine of FIG. 2;  
         [0027]    [0027]FIG. 5 is longitudinal section view of a first radial gap machine of the present invention;  
         [0028]    [0028]FIGS. 6 a - 6   d  are detail views of part of the rotor assembly in the machine of FIG. 5;  
         [0029]    [0029]FIG. 7 is a side elevational view of the rotor assembly in the machine of FIG. 5 with a part in section;  
         [0030]    [0030]FIG. 8 is a longitudinal section view of the radial gap machine of FIG. 5 with the addition of boundary superconducting coils;  
         [0031]    [0031]FIG. 9 is a longitudinal section view of a radial gap machine of the present invention with one additional rotor pole section;  
         [0032]    [0032]FIGS. 10 a - 10   d  are detail views of part of the rotor assembly in the machine of FIG. 9;  
         [0033]    [0033]FIG. 11 is a longitudinal section view of a radial gap machine of the present invention with three additional rotor pole sections;  
         [0034]    [0034]FIGS. 12 a - 12   d  are detail views of part of the rotor assembly in the machine of FIG. 11; and  
         [0035]    [0035]FIGS. 13 a - 13   e  are detail views of rotor pole configurations for the machines of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0036]    [0036]FIG. 1 illustrates a superconducting machine of the prior art, as disclosed in Dombrovski et al., U.S. Pat. No. 6,313,556, issued Nov. 6, 2001.  
         [0037]    As seen in FIG. 1, a superconducting motor  10  of the prior art includes a rotor  12  supported on a rotor shaft  11 , a cryogenic refrigeration system  13  for supplying a cooling fluid to the rotor  12 , and a stator  14  which produces a rotating electromagnetic field which causes the rotor  12  to rotate, a water cooling system  15  for the stator  14  and a power source  16  for the stator  14 .  
         [0038]    The cryogenic refrigeration system  13  is connected through conduits  17 ,  18  to a rotating cryogenic transfer coupling  19 . Refrigerant fluid is transmitted through conduit  17  to the interior of the rotor  12 . The refrigerant may comprise any suitable cryogenic fluid such as gaseous helium, liquid nitrogen, liquid neon, or liquid oxygen. For the purpose of this example, the refrigerant will be assumed to be gaseous helium. The rotor  12  rotates with the rotor shaft  11  and includes a rotor winding  20 , a coil support structure  21  which supports the rotor winding  20 , and a vacuum jacket  22  surrounding the coil support structure  21  to thermally insulate them. The coil support structure  21  is positioned axially centrally of the rotor  12  and has an outer radial portion  22  which is hollow so as to form a cavity  23  that houses the rotor winding  20 . The coil support structure  21  is in intimate thermal contact with cryogenic fluid supplied by the supply conduit  17  and, therefore, is sufficiently cooled by the cryogenic fluid to provide effective conductive heat transfer between the rotor winding  20  and the cryogenic fluid, thereby rendering the rotor winding  20  superconductive.  
         [0039]    In this machine, the stator  14  was also cooled by circulating a liquid coolant such as water through the stator  14  in a closed loop via supply and return conduits  25  and  26  extending between the water cooler  15  and the stator  14 .  
         [0040]    In contrast to the above-described prior art, the superconducting machine of the present invention is a further development of the machine first disclosed in Hsu, U.S. application Ser. No. 09/872,048, filed Jun. 1, 2001, and entitled “Method and Machine for High Strength Undiffused Brushless Operation.” The disclosure there is hereby incorporated by reference. To summarize, this machine has three major portions: 1) a wound armature core, 2) a stationary DC field excitation, and 3) a rotor situated between them. The machine can be configured either as an axial-gap machine or a radial-gap machine. The rotor acts as a flux inverter that changes the stationary DC flux to a multiple-pole flux rotating with the rotor. There is no torque produced between the stationary DC field excitation and the rotor, because the flux remains constant when the rotor is turning. The torque production of the rotor on the side facing the armature is the same as that of a synchronous machine or a brushless DC machine.  
         [0041]    As an advantage over this non-superconducting PM machine, the field excitation portion of the superconducting machine of the present invention does not require a back iron component as the flux return path for the flux produced by the superconducting coils. This flux can go through the air. This not only makes the machine lighter but also eliminates a saturable component. Consequently, the fact that the air-gap flux density can be increased in turns raises the power density of the machine.  
         [0042]    Like the PM non-superconducting motor, PM material in the present invention is positioned in the spaces between the poles of the rotor, to control diffusion or leakage of the flux between the rotor poles. Flux from the DC field excitation is guided through the multiple rotor poles to the main air gap, unless the polarity of the DC current in the excitation winding is reversed so as to allow diversion of the PM fluxand weaken flux in the main air gap. The PM material can also produce flux to ensure the hop-along capability.  
         [0043]    The following examples for the axial-gap and the radial-gap versions illustrate the present invention. FIG. 2 shows an axial-gap version of the invention. FIG. 2 is a sectional view of a disc-shaped motor  30  having a motor housing  31  with two portions  32  and  33 . A stator  34  has a number of teeth  35   b  and a number of stationary armature windings  35   a  wound around the teeth  35   b  to form stator poles. A disc-shaped rotor  36  is mounted on a shaft  37  having a front end supported in bearing  38  in a central opening in housing portion  32  and a rear end supported in bearing  39  in a central opening in housing portion  33 . Disc-shaped cover  40  closes the opening in the housing portion  32  and an annular cover piece  41  encloses the bearing cavity in housing portion  33 .  
         [0044]    The rotor  36  is seen best in FIGS. 3 and 4, in which rotor poles  42 ,  43  are formed by teeth on two respective portions  44 ,  45  that are joined by brazings  46  of nonmagnetic material. Permanent magnet (PM) material  47  is placed in the spaces  48  formed between the teeth  42 ,  43  to block leakage of flux for enhancing the main air-gap flux or to prevent the PM flux go to the main air-gap for field weakening depending on polarity of the secondary field excitation.  
         [0045]    Returning to FIG. 2, the side of the rotor seen in FIG. 3 faces the stator  34  across a main air gap  50 . Behind the rotor  36 , four superconducting coils  49   a - 49   d  are positioned for secondary excitation of the rotor  36  across a secondary air gap  51 . The superconducting coil(s)  49   a - 49   d  are arranged in a stationary cryogenic enclosure  52 . The wall of this enclosure  52  is avacuum jacketfor thermal isolation. The inside of this enclosure receives circulating refrigerant fluid from a source (not shown). The superconducting disc coils  49   a - 49   d  are positioned and arranged as shown in FIG. 2 to produce a two-pole (N and S) ring flux pattern. Instead of back iron, the superconducting coils  49   a - 49   d  and enclosure  52  are enclosed by a portion of the housing  33  having either ferromagnetic or non-ferromagnetic properties. Consequently, magnetic saturation of the housing is not a limiting factor for producing the high air-gap flux density in the rotor poles. The number of poles in this machine can be high for reducing the overall dimensions of the machine.  
         [0046]    A disc-shaped flux shield  53  is positioned between the superconducting coil enclosure  52  and the secondary air gap  51 . The purpose of this flux shield  53 , which is known in the art, is to magnetically short circuit any alternating flux which is not in synchronism with the rotation of the rotor  36 , this flux being induced largely by the stator windings  35   a.    
         [0047]    A feature of this motor  30  is provided when the superconducting field excitation portion  49   a - 49   d  experiences a fault condition and cannot function. In this case the PM material  47  between the rotor poles  42 ,  43  produces air-gap flux in the main air gap  50 . The motor  30  still can “hop along” at a reduced speed.  
         [0048]    [0048]FIG. 5 shows a radial-gap superconducting high strength undiffused PM machine  60   a . The technological principle of a radial-gap machine  60   a  is similar to that of the axial gap superconducting machine  30 . This motor  60   a  is shown with a motor housing  61   a , a stator core  62   a , and stator or armature windings  63   a . A barrier  86   a  is provided to form an optional cooling chamber for the stator  62   a ,  63   a . Cooling fluid would be circulated through the chamber from an external source (not shown).  
         [0049]    A rotor  64   a  of generally cylindrical shape is supported by shaft portions  65   a ,  66   a  for rotation in insulated bearings  67   a ,  68   a . Inside the rotor shaft portion  65   a  is a second pair of insulated bearings  69   a , which allow the rotor  64   a  to rotate around a stationary chamber  70   a  whose wall is avacuum jacket for containing a cryogenic fluid  71   a  and superconducting coils  72   a ,  73   a . The chamber  70   a  is encircled by a flux shield  74   a  to shield the coils  72   a ,  73   a  from alternating flux, induced primarily by stator windings  63   a . The coils  72   a ,  73   a  are mounted on a common support of non-magnetic material  75   a , which provides separation between the fluxes from the respective coils  72   a ,  73   a . Wire connections  90   a  for the coils  72   a ,  73   a  and a conduit  76   a  extend through a hollow cylindrical tube  77   a  for the refrigerant fluid leads into the chamber  70   a . Each coil  72   a ,  73   a  provides a pole (N or S) from inside the rotor  64   a.    
         [0050]    As seen in FIGS. 6 a - 6   d , the rotor  64   a  is an assembly that is formed by joining two complementary pieces  78   a ,  79   a  of ferromagnetic material (steel) using non-magnetic brazings  80   a  (FIG. 5) so as not to magnetically short circuit the two pieces  78   a ,  79   a . As seen in FIGS. 6 b  and  6   d , each rotor section  78   a ,  79   a  provides twelve spaced apart poles  81   a ,  82   a  of one respective polarity (N or S) around its circumference. The number of poles of each polarity can be different from the “twelve” used for this example. As further seen in FIG. 7, when the rotor sections  78   a ,  79   a  are assembled, this produces an alternating and complementary arrangement of north (N) poles  81   a  and south (S) poles  82   a , with PM material  83   a  positioned in spaces between the poles  81   a ,  82   a.    
         [0051]    Referring again to FIG. 5, the wound stator core  62   a , the stationary DC field excitation coils  72   a ,  73   a , and the rotor  64   a  situated between them are the three major components in a radial-gap superconducting machine. A cryogenic cooling chamber  70   a , AC flux shield  74   a , and superconducting disc coils  72   a ,  73   a  form the DC flux excitation assembly. The north and south pole fluxes produced by the superconducting disc coils go into the two pieces  78   a ,  79   a  of the rotor  64   a  facing the excitation. The rotor is a flux inverter that changes the stationary DC flux to a multiple-pole flux rotating with the rotor. There is no torque produced between the stationary DC field excitation and the rotor, because the flux remains constant when the rotor is turning. The torque production of the rotor  64   a  on the side facing the stator  62   a ,  63   a  is the same as that of a synchronous machine or a brushless DC machine.  
         [0052]    [0052]FIG. 8 shows a radial gap version of the machine similar to FIG. 5 with one modification. In FIG. 8, parts similar to the parts in FIG. 5 have the same number except for a “b” suffix. The modification incorporates two additional boundary superconducting coils  84   b ,  85   b , which are provided to provide a stronger pair of N and S poles in the rotor  64   b . The machine has a motor housing  61   b , a stator core  62   b , and stator or armature windings  63   b  corresponding to the parts for the previous embodiment. A rotor  64   b  of generally cylindrical shape is supported by shaft portions  65   b ,  66   b  for rotation in insulated bearings  67   b ,  68   b . Inside the rotor shaft portion  65   b  is a second pair of insulated bearings  69   b , which allow the rotor  64   b  to rotate around a stationary chamber  70   b  whose wall is a vacuum jacket for thermal isolation. The stationary chamber  70   b  contains a cryogenic fluid  71   b  and superconducting coils  72   b ,  73   b ,  84   b , and  85   b . The chamber  70   b  is encircled by a flux shield  74   b  to shield the coils  72   b ,  73   b ,  84   b , and  85   b  from alternating flux, induced primarily by stator windings  63   b . The coils  72   b ,  73   b  are mounted on a common support of non-magnetic material  75   b , which provides separation between the fluxes from the respective coils  72   b ,  73   b . In addition, two boundary superconducting coils  84   b ,  85   b , are provided to provide a stronger pair of N and S poles in the rotor  64   b . Wire connections  90   b  for the coils  72   b ,  73   b ,  84   b ,  85   b  and a conduit  76   b  for the refrigerant fluid extends through a hollow cylindrical tube  77   b  leading into the chamber  70   b.    
         [0053]    [0053]FIG. 9 shows a radial gap machine of the present invention with an additional rotor pole section. This provides for sets of three (N-S-N) pole sections on the inner surface of the rotor. This embodiment demonstrates the technology for the machine with a relatively long rotor  64   c  with a smaller diameter by providing additional pole sections inside the rotor  64   c . The inner periphery of the rotor is divided into a center cylinder  88  and two side cylinders  78   c ,  79   c . The axial length of a taper portion of the rotor poles  82   c  (FIG. 10 c ) is about half of those shown in FIGS. 7 and 8. Consequently, the radial thickness of the rotor can be made thinner.  
         [0054]    In FIG. 9, parts similar to the parts in FIGS. 5 and 8 have the same number except for a “c” suffix. This includes a motor housing  61   c , a stator core  62   c , and stator or armature windings  63   c  corresponding to the parts illustrated in the previous figures. A rotor  64   c  of generally cylindrical shape is supported by shaft portions  65   c ,  66   c  for rotation in insulated bearings  67   c ,  68   c . Inside the rotor shaft portion  65   c  are the second insulated bearings  69   c , which allows the rotor  64   c  to rotate around a stationary chamber  70   c  whose wall is a vacuum jacket for thermal isolation. The stationary chamber  70   c  contains a cryogenic fluid  71   c  and superconducting coils  72   c ,  73   c ,  84   c , and  85   c . The chamber  70   c  is encircled by a flux shield  74   c  to shield the coils  72   c ,  73   c ,  84   c , and  85   c  from alternating flux, induced primarily by stator windings  63   c . The coils  72   c ,  73   c  are spaced apart in this embodiment to provide an additional pole section inside the rotor. The additional two boundary superconducting coils  84   c ,  85   c , are provided to provide a stronger pair of N and S poles in the rotor  64   c . Wire connections  90   c  for the coils  72   c ,  73   c ,  84   c ,  85   c  and a conduit  76   c  for the refrigerant fluid extends through a hollow cylindrical tube  77   c  leading into the chamber  70   c.    
         [0055]    In this embodiment, there is an extra rotor pole section  88  of the rotor  64   c  that fits between end pieces  78   c ,  79   c . As seen in FIGS. 10 a  and  10   b , the middle section  88  provides twelve radially extending and spaced apart south (S) poles  88   c . This section fits between two end sections  78   c ,  79   c , one of which is shown in FIGS. 10 c  and  10   d , and both of which having twelve radially extending and spaced apart south (N) poles  82   c . The north (N) poles are offset by an angle from the south (S) so that the poles will fit together in a complementary fashion similarly to the depiction in FIG. 7, with the south (S) poles  88   c  being twice as long as, and extending alongside, the north (N) poles from each of the respective end sections  78   a ,  79   a.    
         [0056]    [0056]FIG. 11 shows an embodiment that is designed to have an even longer rotor  64   d  and with a relatively smaller diameter, using an arrangement of six spaced apart superconducting coils  72   d ,  73   d ,  84   d ,  85   d ,  86   d  and  87   d  disposed in a stationary cryogenic chamber  70   d  surrounded by an AC flux shield  74   d . These coils provide five stationary poles inside the rotor  64   d . The rotor  64   d  is formed by end pieces  78   d ,  79   d  and several intermediate pieces  88 ,  89 ,  90 , which provides sets of five alternating pole sections (N-S-N-S-N) on the inner surface of the rotor  64   d . The rotor  64   d  is formed by cylindrical end pieces  78   d ,  79   d  and cylindrical intermediate pieces  88 ,  89 ,  90  for providing the alternating poles on the outer surface of the rotor  64   d.    
         [0057]    [0057]FIGS. 12 a  and  12   b  show the details of one of the intermediate rotor sections  88 ,  90  providing twelve radially extending and circumferentially spaced south (S) poles  90   d . FIGS. 12 c  and  12   d  show the details of one of the intermediate rotor sections  89  providing twelve radially extending and circumferentially spaced north (N) poles  89   d . In addition, end rotor sections  78   d ,  79   d  would provide twelve radially extending and circumferentially spaced north (N) poles. When the sections  78   d ,  79   d ,  88 ,  89 ,  90  are joined by non-magnetic joints, the south poles and north poles of the intermediate sections  88 ,  89  and  90  overlap each other and the shorter poles of the end sections  78   d ,  79   d , which are similar to those sections in previous embodiments to provide a rotor assembly  64   d  with 5×12 N-S rotor poles.  
         [0058]    [0058]FIGS. 13 a - 13   e  shows examples of different pole shapes of a radial-gap superconducting rotor. The rotor poles  81 ,  82  can be either straight or skewed with respect to the stator teeth. The pole shape when seen from the side can be rectangular for poles  81   e ,  82   e  as seen in FIG. 13 b  or trapezoidal for poles  81   f ,  82   f  as seen in FIG. 13 c . The poles can be flared at the pole face to provide a pole face of enlarged area facing the main air gap  50  as seen for rectangular poles  81   g ,  82   g  in FIG. 13 d  and as seen for trapezoidal poles  81   h ,  82   h  in FIG. 13 e . Similar poles shapes can be employed with the axial gap machine of FIG. 2.  
         [0059]    This has been a description of several preferred embodiments of the invention. It will be apparent that various modifications and details can be varied without departing from the scope and spirit of the invention, and these are intended to come within the scope of the following claims.