Abstract:
A switched reluctance motor has a stator with a first set of poles directed toward levitating a rotor horizontally within the stator. A disc shaped portion of a hybrid rotor is affected by the change in flux relative to the current provided at these levitation poles. A processor senses the position of the rotor and changes the flux to move the rotor toward center of the stator. A second set of poles of the stator are utilized to impart torque upon a second portion of the rotor. These second set of poles are driven in a traditional switched reluctance manner by the processor.

Description:
ORIGIN OF THE INVENTION 
     This invention was made by an employee of the United States Government together with government support under contract awarded by the National Aeronautics and Space Administration and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon of thereof. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to switched reluctance motors, and more particularly to a bearingless switched reluctance motor. 
     2. Prior Art 
     Switched reluctance motors are typically constructed of a stator having an even number of poles, usually four or more, and a rotor having an even number of poles, also usually two or more. The poles on the rotor are typically oriented in a protruding manner crosswisely around the rotating shaft in an outward manner. The poles on the stator typically protrude inwardly with a concentrated winding thereabout in the form of a coil. The coils, which are wound on each of the pairs of opposing stator pole portions, are connected in series with each other whereby a magnetic flux is generated between each pair of opposing stator pole portions when current is supplied to the coils which are wound thereon. 
     In switched reluctance motors, the protruding poles of the stator attract the protruding poles on the outer peripheral surface of the rotor to generate torque and as a result, the rotor rotates. As a pole of the rotor approaches a pole of the stator, the supply current to the poles is changed, typically by means of switching elements in response to the rotational position of the rotor whereby rotatory torque is produced. 
     It is recognized that switched reluctance motors have advantages over reluctance, induction and permanent magnet motors due to their reliability, durability, low cost and possible operation in adverse environments including high temperatures, intense temperature variations or high rotational speeds. However, due to rotor eccentricity of rotors due to mechanical flaws during machining, conventional switched reluctance rotors suffer from vibration caused by large magnetic attraction forces on the rotor in the radial direction. Furthermore, traditional bearings are subject to wear and require lubrication. 
     Several patents have been obtained for improvements to switched reluctance motors including at least U.S. Pat. Nos. 5,909,071, 5,969,454, 5,880,549, 5,917,263, and 5,945,761. Few except the &gt;549 Patent, if any, are directed to bearingless switched reluctance motors. Nevertheless at least these efforts to reduce the effects of vibration and noise in a switched reluctance motor have been made. 
     Bearingless switched reluctance machines, which are believed to have been achieved only in development laboratories so far, employ magnetic bearings instead of traditional bearings and are thus referred to as bearingless since there is no mechanical bearing between the rotor and the stator during operation. Of course, a mechanical bearing is usually provided should the magnetic bearing fail. By magnetically suspending the rotor relative to the stator, further efforts to suppress vibration may be employed. 
     U.S. Pat. No. 5,880,549 discusses a prior art switched reluctance motor construction. The rotor rotates relative to the stator while being levitated by magnetic forces. In addition to the &gt;549 Patent, a number of papers have been authored by Akira Chiba, Masahiko Hanazawa, Ken Shimada, Tadashi Fukao, and Azizur Rahinan regarding bearingless switched reluctance machines. These individuals have studied the effects of magnetic saturation on a traditional bearingless switched reluctance motor, the mathematical formulations of forces affecting a traditional bearingless switched reluctance motor, and the addition of a feed forward compensator to adjust for the locus of magnetic centers. The studies of these individuals appear to center primarily around the use of a main four pole winding to rotate the rotor while utilizing a two pole winding to apply radial force to the winding with all of the stator poles having both windings thereon. 
     Accordingly, a need exists for an improved bearingless switched reluctance motor. 
     SUMMARY OF THE INVENTION 
     Consequently, it is a primary object of the present invention to provide a bearingless switched reluctance machine having higher load carrying capacity, higher stiffness and/or greater vibration suppression capacity. 
     It is a further object of the present invention to provide a rotor for use with a switched reluctance motor having higher load carrying capacity, higher stiffness and/or greater vibration suppression capacity. 
     Another object of the present invention is to utilize a single set of windings of a switched reluctance motor wherein a plurality of the poles are dedicated to levitation of the rotor while a separate plurality of poles are dedicated to rotating the rotor. 
     Accordingly, the present invention provides a bearingless switched reluctance motor having a stator with a plurality of pairs of poles and a hybrid rotor having a plurality of pole pairs in a laminated bundle on a first portion, and a stack of circular laminations forming a circular disc on a second portion. In the illustrated embodiment, an eight pole stator is utilized with a rotor having a first portion with six poles. Of course, other strator/rotor combinations could be utilized as well. A second portion is a disc member. Levitation may be produced and vibration may be suppressed by utilizing feed back and feed forward commands in the control software. The stator has four poles which are exclusively utilized to levitate or maintain the horizontal alignment of the rotor within the stator. The disc portions of the rotor is affected by the levitation poles of the stator. The other four poles are utilized in an opposing pair fashion to apply torque to rotate the rotor about the stator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings in which: 
     FIG. 1 is a sectional view of an 8/6 switched reluctance motor illustrating a preferred embodiment of the present invention; 
     FIG. 2 is a diagramic representation of magnetic flux through the levitation poles shown in FIG. 1; and 
     FIG. 3 is a side, exploded elevational view of the rotor used in the motor of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the FIG. 1, a sectional view of a motor  10  comprised of stator  20  and a hybrid rotor  40 . While a motor  10  is discussed and illustrated, similar principles could be applied to a generator. Stator  20  is comprised of a stack of plate laminations  22  that are typically formed of a ferromagnetic material such as laminated silicon steel which is stamped out to salient pole form Laminations  22  are stacked face-to-face and suitably adhered to one another by means known in the art. Other materials and construction techniques for stator design may also be utilized. Stator  20  includes a plurality of like, inwardly extending stator poles  24  having inwardly facing stator pole faces  26 , which are preferably concave. In the embodiment shown, the stator  20  has eight (8) stator poles, designated  24   a ,  24   b ,  24   C ,  24   d ,  24   e ,  24   f ,  24   g  and  24   h . It is most preferred to have a number stator poles which are multiples of four, such as eight, or a twelve, the preferred embodiment of eight is illustrated. However, with other controllers, six, ten or other numbers of stator poles could also be utilized. A gap  28  is defined between adjacent stator poles  24 . Stator pole faces  26  define a central bore  12  for receiving rotor  40  and may be concave in shape. The central bore  12  is substantially filled between the stator poles  24  with hybrid rotor  40  with a clearance space between the rotor  40  and stator poles  24 . 
     The stator  20  is preferably equipped with a single set of windings, or coils. The main windings  32   a ,  32   b ,  32   c ,  32   d ,  32   e ,  32   f ,  32   g  and  32   h  are preferably divided into two sets of uses. First, four windings are utilized exclusively for levitation of the rotor  40  within the stator  20 . In the preferred embodiment  32   a ,  32   c ,  32   e , and  32   g  are not utilized to impart rotation to the rotor, but are instead, utilized to produce fluxes to support the rotor  40  within the bore  12  within the stator  20 . This leaves the remaining four poles,  32   b ,  32   d ,  32   f , and  32   h  available to drive the stator as will be explained in further detail below. This separation of motoring and bearing functions allows the controller to be very simple, hence economical. Enhanced performance, at the expense of greater complexity can be achieved by a more complex controller that would produce both motoring and bearing forces in each of the two sets of four coils. 
     The hybrid rotor  40  is illustrated as having a first portion  42  resembling a six pole scalloped rotor having poles  44   a ,  44   b ,  44   c ,  44   d ,  44   e  and  44   f  and a second portion  43  resembling a circular disc. Both of the first and second rotor portions  42 , 43  are preferably stacked laminations, however, other constructions could also be utilized. When utilized with an eight pole stator, only two pairs of opposing poles  44  of the first portion  42  and stator poles  24  may be aligned at a moment in time due to the geometric compatibility of the two configurations. In FIG. 1, stator pairs  24   c ,  24   g  are aligned respectively with rotor pairs  44   b ,  44   e . A similar geometrical relationship exists with a 12/8 stator pole to rotor pole configuration. This is believed to be relevant to the operation of this design. 
     In order to rotate the hybrid rotor  40  relative to the stator  20 , current is provided to the stator windings  32  in a two pole manner to draw a pair of rotor poles  44  toward the pair of stator poles  24 . In the eight stator pole configuration illustrated, since four poles are dedicated to levitation, four poles, or two pairs of two are available to drive the rotor  40 . In the illustrated embodiment, the drive pole pairs would be  24   b ,  24   f  along with  24   d , 24   h.    
     As current is provided through stator windings  32   b , 32   f  in stator pole pair  24   b , 24   f , a flux is created which draws the first portion  42  of hybrid rotor  40 , if in the position illustrated, specifically rotor pole pair  44   a , 44   d , toward alignment with stator pole pair  24   b , 24   f  which will rotate the hybrid rotor  40  clockwise. As the poles near alignment with one another, the current is turned off. Meanwhile, rotor pole pair  44   b , 44   e  has moved closer to stator pole pair  24   d , 24   h  which may then be energized with windings  32   d , 32   h  to pull rotor pole pair  44   b , 44   e  toward alignment with the stator pole pair  24   d , 24   h . When alignment is near, or not past alignment, the current is turned off to windings  32   d , 32   h  to prevent reverse torque on the rotor  40 . Now rotor pole pair  44   f , 44   c  will be near stator pole pair  24   b , 24   f  and the windings  32   b , 32   f  may be energized again. In this manner, the two stator pole pairs may drive the three rotor pole pairs. Current is typically provided in a square, or trapezoidal wave form or otherwise, to the stator drive windings  32   b ,  32   d ,  32   f  and  32   h  to drive the first portion  42  about the stator  20 . 
     While the four poles  24   b ,  24   d ,  24   f  and  24   h  are driving the rotor and thus rotate a shaft  66 , the other four poles  24   a ,  24   c ,  24   e , and  24   g  are maintaining the rotor  40  in a desired position within the bore  12  within the stator  20 . These are referred to as levitation poles. This is done in a manner commonly used in conventional magnetic bearings. In the illustrated embodiment, the stator poles  24   a ,  24   e  are utilized as North poles and stator poles  24   c ,  24   g  are utilized as South poles. The hybrid rotor  40  includes a circular laminate stack as a second portion  43  which is affected by the flux generated between the North and South poles. FIG. 2 is a schematic of the North and South pole arrangement of the levitation poles  24   a ,  24   c ,  24   e  and  24   g.    
     Position sensors  50 ,  52 , which can be any of several types, shown in FIG. 1, sense the x,y position of the third portion  62  of the rotor  40  and provide the data to a processor  70 . The sensors  50 , 52  are illustrated at right angles to one another. Fortunately, the speed of modern processors allows computations to occur on the order of microseconds which allows for correcting signals to be provided to adjust the flux distribution through the North and South poles to return a rotor  40  toward its centered position relative to the stator  20 . 
     In FIG. 2, the second portion  43  has drifted toward the three o&#39;clock position. In operation, the sensors  50 , 52  would sense this change in position, the processor  70  shown in FIG. 1 would interpret the data provided by the sensors  50 , 52  and a correcting signal would be provided to the left South pole. Specifically, in this instance, the flux in pole  24   g  would be increased while the flux in pole  24   c  is decreased to pull the second portion  43  toward the center of the bore  12 , or the second portion  43  illustrated in phantom. It is also preferable to maintain a bias flux through the North South poles as illustrated in FIG.  2  and then utilize a control current, as needed, to modify the bias flux to move the second portion  43 , and thus the rotor  40  back toward center of the bore  12 . Additionally, a closed loop between the sensor, to the processor, to the stator poles may be utilized. While the above description has been directed to an 8/6 stator/rotor configuration, similar logic could be applied to a 12/8 stator/rotor configuration and others. 
     FIG. 3 shows a side perspective and exploded view of a hybrid rotor  40 . A first portion  42  is illustrated in contact with second portion  43 . While the two portions  42 , 43  could be insulated from one another, they are in contact with one another in the preferred embodiment. Connectors may extend through bores in the portions  42 , 43  to locate the portions  42 , 43  between plate  60  and base  62 . Plate  60  would be adjacent to portion  42  but has been shown in an exploded view to better illustrate the rotor portion  42 . Sleeve  64  connected to plate  60  allows the portions  42 , 43  to be located relative to shaft  66  such as with set screw  68 . While the set screw illustrated in FIG. 3 would tend to affect the balance of the rotor  40 , other, more symmetric designs could be employed to allow higher speeds without significant vibration. The circumference of the second disc portion  43  is illustrated as extending radially at least beyond a portion of the spaces between adjacent rotor poles and preferably substantially as far as the rotor poles  44 . 
     In the illustrated embodiment, speeds of over 5000 rpm were realized. With a more balanced mounting system about the shaft  66 , speeds of over 10,000 rpm are believed to be easily attained. It can also be seen that variation in the thickness of the first and second portions  42 , 43  could be utilized for specific tasks: a thicker first portion  42  will enable a greater torque for a given levitation stiffness and vice versa. 
     Referring back to FIG. 1, a processor  70  is illustrated receiving signals from sensors  50 ,  52  at inputs  72  which detect movement of the rotor  40  from the center of the bore  12 . Upon sensing a movement the bias current sent from outputs  74  is adjusted with a correcting current to at least one specific output  74  to attempt to pull the rotor  40  back toward center. Meanwhile outputs  76  are provided with current in a Aswitched@ manner to provide torque, and thus rotate rotor  40  about the stator  20 . A feed forward compensator may be utilized as well in the processor  70 . Of course, more sophisticated processors  70  could be utilized having more inputs and outputs, but the basic operation of the preferred embodiment has been found to operate satisfactorily with this construction. 
     Numerous alternations of the structure herein disclosed will suggest themselves to those skilled in the art. However, it is to be understood that the present disclosure relates to the preferred embodiment of the invention which is for purposes of illustration only and not to be construed as a limitation of the invention. All such modifications which do not depart from the spirit of the invention are intended to be included within the scope of the appended claims.