Abstract:
A dynamoelectric machine includes a stator having teeth fabricated from a non-magnetic material and containing at least one embedded conductor. The teeth are unitary with a back portion that is mounted to a stator back iron. Permeance variations induced by a stator winding mounted on the non-magnetic stator teeth are low which facilitates a reduction of motor noise. Specifically, since the non-magnetic teeth reduce production of permeance variations, changes in air gap forces between the rotor and the stator are decreased.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/199,424, filed Apr. 25, 2000. 
     
    
     
       BACKGROUND OF INVENTION  
         [0002]    This invention relates generally to synchronous machines and, more particularly, to synchronous machines with High Temperature Superconducting rotors.  
           [0003]    A synchronous machine typically includes a motor housing, a stator including a plurality of armature windings mounted between a plurality of stator teeth, and a rotor assembly. The rotor assembly includes a rotor core and a rotor shaft extending through the rotor core. The rotor core can be either a salient pole or a cylindrical configuration, and includes a plurality of field windings mounted thereon. The motor housing includes at least one endshield and houses at least a portion of the rotor assembly. Synchronous machines also typically include at least one bearing sized to receive and support the rotor shaft, and at least one inner bearing cap separated from the bearing. Typically, the bearing is positioned between an endshield and an inner bearing cap and facilitates rotation of the rotor shaft when the armature windings are energized.  
           [0004]    Recent technological advances have allowed synchronous machines to utilize HTS (high temperature superconducting) ceramic field windings in lieu of conventional copper windings. The HTS windings typically are fabricated from bismuth-2223 ((Bi,Pb)2Sr2Ca2Cu3O10) and are loaded with significantly larger currents than conventional copper windings can sustain. Therefore, machines with HTS windings can generate more powerful magnetic fields in a given volume of space compared to machines with conventional windings. Currently, a cryogenically cooled superconducting machine utilizing a toothless stator winding is able to match the power output of an equally rated conventional machine with as little as one-third the size and weight of the conventional machine.  
           [0005]    In a known machine with HTS windings, there are permeance variations in the stator due to the use of conventional slotted magnetic metal cores that generate varying forces in the air gap. The varying air gap forces can produce noise by exciting the machine&#39;s structure and the torque is limited by the need to provide space for the teeth structure. However, there are many applications where motor noise is undesirable and small size is important, such as, for example, in a submarine.  
           [0006]    Accordingly, it would be desirable to facilitate a reduction in permeance variations and increase torque (power) density in a stator of a synchronous machine with HTS field windings.  
         SUMMARY OF INVENTION  
         [0007]    The present invention is, in one aspect, a machine in which a stator is fabricated such that the teeth of the stator are of a non-magnetic material. Since the teeth are non-magnetic, the teeth do not contribute to generation of noise due to variations in magnetic fields, as do the magnetic teeth in known stators. Specifically, the non-magnetic teeth facilitate a reduction of permeance variations induced by a plurality of stator windings mounted on the stator teeth, thereby lessening variations in the air gap forces between a rotor and the stator. Additionally, utilizing non-magnetic teeth allows for the use of additional windings embedded in the non-magnetic teeth to increase current density and torque. Accordingly, noise caused by the variations in air gap forces is reduced and torque is increased.  
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0008]    [0008]FIG. 1 is a perspective view of a portion of a known magnetic metal stator.  
         [0009]    [0009]FIG. 2 is a perspective view of one embodiment of a stator magnetic metal yoke with non-magnetic teeth.  
         [0010]    [0010]FIG. 3 is a perspective view of an alternative embodiment of a stator magnetic metal yoke with non-magnetic teeth.  
         [0011]    [0011]FIG. 4 is a cross-sectional view of an alternative embodiment of a stator having non-magnetic teeth.  
         [0012]    [0012]FIG. 5 is a cross-sectional view of the stator shown in FIG. 4 during fabrication.  
         [0013]    [0013]FIG. 6 is a cross-sectional view of a stator having a plurality of winding embedded non-magnetic teeth.  
         [0014]    [0014]FIG. 7 is a cross-sectional view of a stator during fabrication.  
         [0015]    [0015]FIG. 8 is a cross-sectional view of the stator shown in FIG. 7.  
         [0016]    [0016]FIG. 9 is a cross sectional view of a synchronous machine including the stator shown in FIG. 2. 
     
    
     DETAILED DESCRIPTION  
       [0017]    [0017]FIG. 1 is a perspective view of a portion of a known magnetic metal stator  10  including a plurality of teeth  12  and a yoke or back iron  14 . A plurality of armature windings (not shown) are placed in a plurality of slots  16  defined by teeth  12 . Teeth  12  are metal and are fabricated on a plurality of laminations that are stacked together to form stator section  10 . Teeth  12  are unitary with back iron  14 .  
         [0018]    [0018]FIG. 2 is a perspective view of one embodiment of a stator  20  including a plurality of non-magnetic teeth  22  and a stator magnetic back iron  24  including a plurality of tooth slots  26 . In an exemplary embodiment, non-magnetic teeth  22  are fabricated from a glass laminate. In an alternative exemplary embodiment non-magnetic teeth  22  are fabricated from a non-magnetic fiber, such as, for example, a carbon fiber. It is contemplated that the benefits of reduced permeance variations in an air gap accrue to all stators having a plurality of non-magnetic teeth fabricated from any non-magnetic material. Non-magnetic teeth  22  are connected individually to back iron  24  by inserting a back section  28  of each tooth  22  into a respective tooth slot  26 . In an exemplary embodiment, each back section  28  includes a semi-cylindrical groove  30 , and back iron  24  includes at least one semi-cylindrical groove  32  positioned such that fully seating a particular tooth  22  into a respective tooth slot  26  aligns semi-circular groove  30  with semi-circular groove  32  to form a substantially cylindrical opening  34  such that insertion of a rod (not shown) into opening  34  keys each tooth  22  to a respective tooth slot  26 . In an alternative embodiment, each back section  28  includes a slot (not shown), and back iron  24  includes at least one slot (not shown) positioned such that fully seating a particular tooth  22  into a respective tooth slot  26  aligns the back iron slot with the tooth slot such that insertion of a rectangular piece of material keys each tooth  22  to a respective tooth slot  26 . It is contemplated that the benefits of non-magnetic stator teeth accrue to all stators having non-magnetic teeth keyed thereon using any method of keying, such as, for example, dovetail keying and spline keying. In an alternative embodiment, each back section  28  is attached to a respective tooth slot  26  utilizing conventional adhesives. In a further alternative embodiment, each back section  28  is keyed and adhesively bonded to each respective tooth slot  26 . Teeth  22  are spaced to define a plurality of slots  36  that can accommodate a plurality of armature windings (not shown). Since teeth  22  are non-magnetic, utilizing stator  20  in a machine with a HTS rotor results in a low noise signature due to the reduced permeance variations that non-magnetic teeth  22  provide in comparison to a machine utilizing stator  10 . However, only half of an inner periphery  38  of stator  20  is used for torque production.  
         [0019]    [0019]FIG. 3 is a perspective view of an alternative embodiment of a stator  40  including a back iron  42  and a non-magnetic tooth section  44  including a plurality of teeth  46  defining a plurality of slots  48  that can accommodate a plurality of armature windings (not shown). Teeth  46  are fabricated unitarily with a back portion  50  of tooth section  44 . Back portion  50  is substantially circular and includes at least one key  52  extending therefrom to key back portion  50  to back iron  42  utilizing a key receiving portion  54  of back iron  42 .  
         [0020]    In an exemplary embodiment, back portion  50  is keyed and adhesively bonded to back iron  42 . In an alternative embodiment, back portion  50  includes a plurality of keys extending therefrom. In a further alternative embodiment, back iron  42  includes at least one key (not shown) and back portion  50  includes at least one key receiver portion (not shown). Since teeth  46  are non-magnetic, utilizing stator  40  in a machine with a HTS rotor results in a low noise signature due to the reduced permeance variations that non-magnetic teeth  46  provide in comparison to a machine utilizing stator  10  (shown in FIG. 1).  
         [0021]    [0021]FIG. 4 is a cross-sectional view of an alternative embodiment of a stator  60  including a plurality of non-magnetic teeth  62 . Stator  60  has a substantially circular outer surface  64  and an inner surface  66  including a plurality of arcuate sections  68  interspersed with a plurality of key receiving sections  70 . Each tooth  62  includes a retaining key  72  extending radially outward. In one embodiment, teeth  62  are molded with unitary keys  72  and are inserted in a machine along an axial direction by sliding each key  72  into a respective key receiving section  70 . In an alternative embodiment, teeth  62  and keys  72  are machined utilizing conventional machine tools. Since teeth  62  are non-magnetic, utilizing stator  60  in a machine with a HTS rotor results in a low noise signature due to the reduced permeance variations that non-magnetic teeth  62  provide in comparison to a machine utilizing stator  10  (shown in FIG. 1).  
         [0022]    [0022]FIG. 5 is a cross-sectional view of stator  60  (shown in FIG. 4) during fabrication according to one embodiment. A plurality of spacers  74  are removably mounted to inner surface  66  at each arcuate section  68  forming a plurality of radially extending cavities  76 . Each cavity  76  is filled with filling material and a resin that is cured to produce rigid non-magnetic teeth  62  as shown in FIG. 4. Key receiving sections  70  are filled with the tooth material thereby forming and installing retaining key  72  unitary with teeth  62  in receiver sections  70  in a single operation.  
         [0023]    In an exemplary embodiment, teeth  62  include a plurality of embedded conductors forming a plurality of first armature windings. Additionally, a plurality of second armature windings are wound around teeth  62  allowing for an increased effective current density and, hence, increased torque over stators without embedded windings as explained in more detail below. Each cavity  76  is wound with conductors to form a first set of armature windings. In an exemplary embodiment, other filler material, such as, for example, but not limited to, glass fibers and polymers are added to cavities  76  either before or after forming the first windings. The filler material is selected to affect the strength, rigidity, and/or thermal conduction properties of teeth  62 . After the windings and the filler material are positioned in cavities  76  a resin is added and allowed to cure forming non-magnetic teeth  62  containing a first set of armature windings (not shown in FIG. 5). After the resin has cured, spacers  74  are removed, and teeth  62  are wound with a second set of armature windings (not shown in FIG. 5). Accordingly, in one embodiment, all of an inner periphery  78  of stator  60  is used to produce torque. The number of windings is increased in a stator including a first set of windings embedded in a plurality of non-magnetic teeth and a second set of windings are wound around the non-magnetic teeth resulting in a higher mean winding current density and higher torque. Although retaining key  72  is shown in the context of a dovetail key, it is contemplated that any method of keying can be utilized to obtain the benefits of winding embedded non-magnetic teeth.  
         [0024]    [0024]FIG. 6 is a cross-sectional view of a stator  90  including a plurality of winding embedded non-magnetic teeth  92  and a plurality of second windings  94  wound around non-magnetic teeth  92 . Stator  90  is substantially similar to stator  60  shown in FIGS. 4 and 5, and components that are identical to components in stator  60  are identified in FIG. 6 using the same reference numerals used in FIGS. 4 and 5. Stator  90  has a substantially circular outer surface  64  and an inner surface  66  including a plurality of arcuate sections  68  interspersed with a plurality of key receiving sections  70 . Each tooth  92  includes a key  72  extending radially outward and a plurality of conductors  96  forming a plurality of first windings  98 . After teeth  92  are fabricated as explained above with resin and filler, removable spacers  74  are removed, second windings  94  are wound around teeth  92 , and stator  90  is utilized in a machine to provide a higher current density than a machine without winding embedded non-magnetic teeth.  
         [0025]    The higher current density allows for a machine with more torque than a machine without winding embedded teeth. Alternatively, the higher winding current density allows for a machine of significantly less size than a machine without winding embedded teeth. It is contemplated that the benefits of winding embedded non-magnetic teeth accrue to all types of electric machines including, for example, but not limited to, all synchronous machines, all non-synchronous machines, and direct current (DC) machines having stator windings. Additionally, the benefits accrue to inside-out or doubly-wound machines, i.e., machines with a stator at least partially mounted within a rotor bore.  
         [0026]    [0026]FIG. 7 is cross-sectional view of a stator  110  during fabrication, stator  110  is substantially similar to stator  60  shown in FIGS. 4 and 5, and components that are identical to components in stator  60  are identified in FIG. 7 using the same reference numerals used in FIGS. 4 and 5. Stator  110  includes a substantially circular outer surface  64  and an inner surface  66  including a plurality of key receiving sections  70  interspersed with a plurality of second key retaining sections  112 . A plurality of spacers  114  are removably mounted to inner surface  66  at each second key retaining section  112  forming a plurality of radially extending cavities  76 . In an exemplary embodiment, spacers  114  are keyed to inner surface  66 . Each cavity  76  is filled with a resin and cured to produce non-magnetic teeth, such as non-magnetic teeth  62  as shown in FIG. 4 or conductor embedded non-magnetic teeth  92  as shown in FIG. 6. Spacers  114  may be driven out axially to remove spacers  114  from inner surface  66 . In an alternative embodiment, spacers  114  are fabricated from a frangible material and are destroyed thereby removing spacers  114  from inner surface  66 . Key receiving sections  70  are filled with the tooth material thereby forming and installing retaining key  72  unitary with teeth  62  in receiver sections  70  in a single operation.  
         [0027]    [0027]FIG. 8 is a cross-sectional view of stator  110  (shown in FIG. 7) after fabrication. Stator  110  includes a plurality of first winding embedded non-magnetic teeth  92  interspersed with a plurality of second winding embedded non-magnetic teeth  120 . Stator  110  has a substantially circular outer surface  64  and an inner surface  122  including a plurality of second winding key receiver sections  112  interspersed with a plurality of key receiving sections  70 . Each first winding embedded non-magnetic tooth  92  includes a key  72  extending radially outward and a plurality of conductors  96  forming a plurality of first windings  98 . After teeth  92  are fabricated, as explained above with resin and/or filler, removable spacers  114  are removed, second windings  94  are wound around teeth  92 , and additional resin and filler is used to fabricate second winding non-magnetic teeth  120  as explained above regarding the fabrication of first winding non-magnetic teeth  92 . Because inner surface  122  includes second winding key receiver sections  112 , fabricating second winding non-magnetic teeth  120 , forms and installs a plurality of second winding retaining keys  126  unitary with teeth  120  in a single operation. Accordingly, a substantially continuous inner tooth surface  128  is provided. Because second windings  94  are embedded in second non-magnetic teeth  120  that are keyed to stator  110 , second winding retaining keys  126  support at least a portion of the torque produced by second windings  94  and, hence, first winding non-magnetic teeth  92  do not fully support the torque produced by second windings  94  providing for a structurally sound and longer lasting winding support system, while also providing the benefits of a higher current density and lower noise.  
         [0028]    [0028]FIG. 9 is a cross sectional view of a synchronous machine  140  including stator  20  (shown in FIG. 2) including a bore  142  therethrough and back iron  24 . Machine  140  further includes a housing  144  supporting a plurality of bearings  146 . A rotor shaft  148  is rotatably positioned within bearings  146  and extends through bore  142 . A field windings support member  150  is mounted on shaft  148  and supports a plurality of HTS windings  152 . A rotor jacket  154  surrounds windings  152 . Rotor jacket  154  is in flow communication with a vacuum pump  156  that maintains a pressure inside jacket  154  substantially lower than atmospheric pressure. A cryogenic cooler  158  and an exciter  160  are coupled to a first end  162  of shaft  148 . Back iron  24  is mounted to housing  144 . A plurality of armature windings  164  are mounted between non-magnetic teeth  22  that are separated from windings  152  by an air gap  166 .  
         [0029]    During operation of machine  140 , cryogenic cooler  158  provides sufficient cooling to windings  152  allowing windings  152  to conduct as superconductors when energized. Superconducting windings  152  produce strong magnetic fields in gap  166 . The fields extend into armature windings  164  and teeth  22 , and are strongest in gap  166 . However, since teeth  22  are non-magnetic, permeance variations are reduced over a machine with stator  10 . Since permeance variations can cause noise, reducing permeance variations reduces noise. Although, an exemplary embodiment is described in the context of a synchronous machine, it is contemplated that the benefits of the invention accrue to a wide variety of rotary and linear electrical machines including, for example, but not limited to, reluctance machines, squirrel cage machines, direct current machines, and permanent magnet machines.  
         [0030]    Utilizing a stator with non-magnetic teeth in a machine with a HTS rotor results in a low noise signature due to the reduced permeance variations that the non-magnetic teeth provide in comparison to a machine with magnetic teeth. In an exemplary embodiment, non-magnetic teeth  22  are winding embedded non-magnetic teeth resulting in a quiet and more powerful machine than an approximately equal sized machine without a stator including winding embedded non-magnetic teeth. Accordingly, an efficient and low noise machine is provided.  
         [0031]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.