Abstract:
A switched reluctance machine (SRM) having a rotor and stator pole numerical relationship of S number of stator poles and R number of rotor poles, where R=2S−2, when S is greater than 4; provides improved power density, torque production, torque ripple, and is readily adaptable to existing hardware such as known controllers and the like.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to a switched reluctance machine. The present invention relates more specifically to switched reluctance machines having a rotor pole and a stator pole numerical relationship of R=2S−2, where S is a number of stator poles, with S&gt;4, and R is a number of rotor poles. 
       BACKGROUND OF THE INVENTION 
       [0002]    A switched reluctance machine (SRM) is a type of synchronous machine which can operate as a motor or a generator. Though there are no major differences in construction, SRM operates as a generator, when used to convert mechanical energy into electrical energy, or as a motor, when used to convert electrical energy into mechanical energy, and often one SRM will operate in both modes in a cycle. Hence, herein after, we shall use the term “machine” instead of motor and/or generator to include both of these operating modes. 
         [0003]    SRMs typically include a stator having a plurality of salient stator poles and a rotor having a plurality of salient poles. During operation of this configuration, each of the stator poles are successively excited to generate a magnetic attraction force between the stator poles and corresponding rotor poles to rotate the rotor. 
         [0004]    In general, the SRMs are simple machines with a robust construction and a number of advantages including fault tolerant capabilities, extended constant power torque-speed characteristics, and the absence of windings or permanent magnets on the rotor and high peak torque-to-inertia ratios make them well suited for high-speed applications. SRMs find application in aerospace, high speed applications, and consumer appliances, such as washing machines and electric bicycles. Additionally, SRMs are considered as strong contenders for auxiliary power application in vehicular systems, non-conventional energy sources, and other industrial machineries and equipments. 
         [0005]    However, despite the advantages, known SRMs have had limited commercial success because of a number of limitations, including high levels of torque ripple, acoustic noise, vibration, and relatively low torque density. These limitations can be partly attributed to their salient pole structure and control strategy. Therefore, there is a desire in the art to minimize the problem of torque ripple, increase torque production, and otherwise improve the operation of SRMs. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides new configurations of switched reluctance machines (SRM) having an improved relationship between the number of stator poles and rotor poles so as to provide a SRM with a minimal amount of torque ripple while providing increased power density and torque production. Particularly, the present invention provides SRM configurations having a rotor pole and stator pole numerical relationship of S number of stator poles, where S&gt;4, and R number of rotor poles, which can be expressed as R=2S−2, such as a S/R pole count in 6/10, 8/14, or 10/18 configurations. 
         [0007]    The SRM of this invention can be designed as a rotary, a linear, an axial or an external rotor type of machine, with three or more phases. The SRM of this invention does not mandate any unusual requirements on the power electronics and control techniques and is readily and easily adaptable to existing and contemporary control strategies, switching schemes, and circuit configurations developed for conventional SRMs, thus making it very practical for present commercial implementation and adoption. Further, known methods for improving the performance of conventional SRMs including pole shaping, current profiling, short flux excitation, sensorless algorithms, minimal flux reversing operations, can be extended to the SRMs of this invention to derive similar performance enhancements. 
         [0008]    The SRM of this invention can offer several advantages over known SRMs including: high efficiency with lower copper loss; improved thermal performance; lower torque ripple; higher torque density; and lower costs for mass production. It is expected that these performance advantages will boost the acceptance level of the SRMs and successfully fulfill the promises of SRMs being potential candidates for many applications. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The objects and features of this invention will be better understood from the following detailed description taken in conjunction with the drawings wherein: 
           [0010]      FIG. 1  illustrates a known SRM with six stator poles and four rotor poles; 
           [0011]      FIG. 2  is a schematic of a typical control circuit configuration for an SRM; 
           [0012]      FIG. 3  illustrates flux lines in the known SRM of  FIG. 1  at an aligned position; 
           [0013]      FIG. 4  is a perspective view of an SRM according to one embodiment of the invention with an axial configuration having six stator poles and ten rotor poles; 
           [0014]      FIG. 5  is a perspective view of a stator for the embodiment of  FIG. 4 ; 
           [0015]      FIG. 6  is a perspective view of a stator with an alternative coil position for an embodiment of an axial SRM; 
           [0016]      FIG. 7  is a perspective view of a rotor for the embodiment of  FIG. 4 ; and 
           [0017]      FIG. 8  is a SRM according to one embodiment of the invention with an external rotor configuration and having six stator poles and ten rotor poles. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    Although the invention will be principally described with reference to embodiments of a SRM having six stator poles and ten rotor poles, machines of other sizes and having other than three phases or six stator poles may be designed in accordance with the invention. 
         [0019]      FIG. 1  illustrates a known construction of a three-phase salient pole SRM  11 . The known SRM  11  includes a stator  13  with six stator poles  15 ,  17 ,  19 ,  21 ,  23 , each having a coil, collectively  27 , wound around each stator pole. The coils on diametrically opposite stator pole pairs i.e.  15 / 17 ,  19 / 21 , and  23 / 25  are connected in series or in parallel to form a phase of the machine. In general, the number of poles in a stator is double the number of phases. Hence, the machine shown in  FIG. 1  is a three-phase machine (Phases A, B and C) with six stator poles  15 / 17 ,  19 / 21 , and  23 / 25 , respectively. The rotor  28 , affixed to a central rotatable shaft  30 , has four rotor poles  29 ,  31 ,  33 ,  35 . 
         [0020]    To operate the SRM  11  as a motor, each phase is normally connected to an electrical energy source through semiconductor devices.  FIG. 2  illustrates one such circuit configuration  37 . Current flow can be diverted to the different Phases A, B, C, by rotor position-based control of the switches S 1  through S 6 . Clock-wise sequencing of phase excitation would produce counter-clock-wise rotation of the shaft and vice versa. Usually a phase is kept energized until any two of the rotor poles align themselves with those stator poles having energized coils. This position is referred to as a minimum reluctance position because reluctance to the flux path is at its least between opposite stator poles when the coils on those stator poles experience current flow. The next phase would then be energized once the rotor poles are aligned with corresponding stator poles, e.g.,  15 / 29  and  17 / 33  as shown for the position in  FIG. 1 . In the shown position, it is appropriate to energize phase-B, stator poles  19 / 21 , to turn the rotor in a counter-clock-wise direction, or energize phase-C, stator poles  23 / 25 , to turn the rotor in a clock-wise direction. Subsequent serial phase excitation would than result in continuous rotation of the rotor. 
         [0021]      FIG. 3  shows a distribution of flux lines, collectively  39 , when phase-A is energized and rotor poles  29 ,  33  are aligned to corresponding stator poles  15 ,  17 , respectively. At this minimum reluctance position, the SRM  11  will produce the least torque and hence it is no longer efficient to continue exciting phase-A. Exciting phase-B will cause the rotor to align itself with stator poles having coils connected to Phase B poles  19 ,  21  to offer a minimum reluctance path to the flux lines established by current in the Phase B coils and hence rotor  28  will turn counter-clockwise to the next aligned position with the Phase B poles  19 ,  21 . 
         [0022]    U.S. Pat. No. 7,230,360, issued on 12 Jun. 2007, herein incorporated by reference, described an SRM having a rotor pole and stator pole numerical relationship of S number of stator poles, where S&gt;4, and R number of rotor poles, which can be expressed as R=2S−2. This SRM showed significant improvements in torque ripple, torque density, efficiency and noise reduction over conventional SRMs. 
         [0023]      FIG. 4  shows a perspective view of an SRM  51  according to one embodiment of this invention. The SRM  51  has an axial configuration including a stator  53  positioned between a pair of rotors  55  which rotate about an axis. In this embodiment, the stator  53  and the rotors  55  are manufactured from stacked layers of laminated silicon steel sheets which provide low core losses, however, any magnetic material could be used. The SRM of this axial configuration is modular or stackable and can include any number of stators  53  and rotors  55  necessary to achieve a desired torque output or any other design consideration. In another embodiment, the SRM can include a single stator and a single rotor. 
         [0024]      FIG. 5  shows a perspective view of the stator  53  of  FIG. 4 . The stator  53  has a disk-like shape with a first stator surface  57  and a second stator surface  59 . The first stator surface  57  and the opposing second stator surface  59  are generally parallel to each other and each include a plurality of stator poles  61 ,  62 ,  63 ,  64 ,  65 ,  66  evenly distributed about a circumference of the stator  53 . The stator poles  61 ,  62 ,  63 ,  64 ,  65 ,  66  project outward, e.g., generally perpendicular, from the corresponding one of the first stator surface  57  or the second stator surface  59 . In this embodiment, each stator surface  57 ,  59  includes six stator poles in three-phase pairs  61 / 62 ,  63 / 64 ,  65 / 66 . Each stator pole  61 ,  62 ,  63 ,  64 ,  65 ,  66  has a coil, collectively  67 , wound around it. Each of the coils  67  is made of a magnetic wire, preferably copper, wrapped around a respective stator pole. Stator poles  61 / 62  with their associated coils represent phase A. Stator poles  63 / 64  and their coils represent phase B. Stator poles  65 / 66  and their coils represent phase C. In operation, the six stator poles on the opposing sides of the stator  53  operate in synch with each other. 
         [0025]      FIG. 6  shows the stator  53  of  FIG. 4  with an alternative coil arrangement. In this embodiment, each of a plurality of coils  69  are wound around a portion of the stator  53  and adjacent to a corresponding one of the stator poles  61 ,  62 ,  63 ,  64 ,  65 ,  66 . In this alternative arrangement, a single winding of coils can be used to energize a pair of stator poles, one on the first stator surface  57  and one on the second stator surface  59 . 
         [0026]      FIG. 7  shows the rotor  55  of  FIG. 4 . In this embodiment, the rotor  55  has a disk-like shape with a first rotor surface  71  and a second rotor surface  73 . The first rotor surface  71  and the second rotor surface  73  are positioned on opposite sides of the disk-like shape and are generally parallel to each other. In  FIG. 7 , the rotor  55  includes a plurality of rotor poles  75  evenly distributed about a circumference of the rotor  55  and which project generally perpendicular from the first rotor surface  71 . In an alternative embodiment, the rotor  55  can include a second set of rotor poles which project generally perpendicular from the second rotor surface  73 . 
         [0027]    The electrical control circuit configuration  37  as shown in  FIG. 2  can be readily adapted for the present invention. From the aligned position of phase A, it will be appropriate to excite the coils of phase-B poles  63 / 64  or phase-C poles  65 / 66  for counter-clock-wise or clock-wise rotation. This will cause the rotor poles to align themselves to the corresponding stator poles to offer a least reluctance path. 
         [0028]    In the embodiment of  FIG. 4 , the SRM  51  has six stator poles  61 ,  62 ,  63 ,  64 ,  65 ,  66  and ten rotor poles  75 . However, the number of stator poles and the number of rotor poles can be any number that is defined by the formula: number of rotor poles (R)=(2 times the number of stator poles (S)) minus 2, or R=2S−2, where S&gt;4, such as a S/R pole count in a 6/10, 8/14, or 10/18 configuration. 
         [0029]      FIG. 8  illustrates another embodiment of the present invention in the form of an SRM  81  with an inverted configuration. In this embodiment, the SRM  81  has an external rotor  83  which is concentric with an internal stator  85 . In this embodiment, the external rotor  83  and the internal stator  85  are manufactured from stacked layers of laminated silicon steel sheets which provide low core losses, however, any magnetic material could be used. The SRM  81  is a three-phase machine with six stator poles in three phase-pairs  91 / 92 ,  93 / 94 ,  95 / 96 . Each stator pole  91 ,  92 ,  93 ,  94 ,  95 ,  96  has a coil, collectively  97 , wound around it. Each of the coils  97  is made of a magnetic wire, preferably copper, wrapped around a respective stator pole. Stator poles  91 / 92  with their associated coils  97  represent phase A. Stator poles  93 / 94  and their associated coils  97  represent phase B. Stator poles  95 / 96  and their associated coils  97  represent phase C. Ten salient rotor poles, collectively  87 , are located on the external rotor  83 . 
         [0030]    The electrical control circuit configuration  37  as shown in  FIG. 2  can also be readily adapted for the present invention. From the aligned position as shown in  FIG. 8 , it will be appropriate to excite the coils of phase-B poles  93 / 94  or phase-C poles  95 / 96  for counter-clock-wise or clock-wise rotation, respectively. This will cause the rotor poles to align themselves to the corresponding stator poles to offer a least reluctance path. 
         [0031]    In the embodiment of  FIG. 8 , the SRM  81  comprises six stator poles  91 ,  92 ,  93 ,  94 ,  95 ,  96  and ten rotor poles  87 . However, the number of stator poles and the number of rotor poles can be any number that is defined by the formula: number of rotor poles (R)=(2 times the number of stator poles (S)) minus 2, or R=2S−2, where S&gt;4, such as a S/R pole count in a 6/10, 8/14, or 10/18 configuration. 
         [0032]    The SRM configurations of this invention are not limited to any particular switching schemes, control strategies, or circuit configuration thus making aspects of this invention very practical for present commercial implementation. For example, the methods of operation discussed above for current SRMs, such as standard switching schemes and circuit topologies, will be equally suitable for the SRM configurations of this invention. 
         [0033]    The SRMs of the present invention give machine designers an additional degree of freedom to realize better efficiency, reduced noise and torque ripple, desirable torque-speed profiles, higher power density, and superior torque characteristics. These performance advantages can help boost the acceptance level of the SRMs and successfully fulfill the promises of SRMs being potential candidates for electro-mechanical energy conversion equipment. 
         [0034]    It will be appreciated that details of the foregoing embodiments, given for purposes of illustration, are not to be construed as limiting the scope of this invention. Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention, which is defined in the following claims and all equivalents thereto. Further, it is recognized that many embodiments may be conceived that do not achieve all of the advantages of some embodiments, particularly of the preferred embodiments, yet the absence of a particular advantage shall not be construed to necessarily mean that such an embodiment is outside the scope of the present invention.