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 acoustic noise, and is readily adaptable to existing hardware such as known controllers and the like.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally to a switched reluctance machine (SRM). The present invention relates more specifically to a SRM having a new relationship between the number of stator poles and rotor poles.  
         [0003]     2. Discussion of the Related Art  
         [0004]     A switched reluctance machine has salient poles on its stationary member stator and rotating member rotor. SRMs are advantageous in their operation because they do not require permanent magnets or field windings on the rotor. Because of simple and robust construction, fault tolerant capabilities and torque-speed characteristics, SRMs are very popular to provide auxiliary power in aircraft, automotive and vehicular systems, non-conventional energy sources, and other industrial machineries and equipments.  
         [0005]     SRMs are realized in a variety of forms. In particular, the constructions differ in the number of stator and rotor poles on the stationary and movable members, respectively, and in the number of independent circuits with which the controller is separately able to switch stator windings in and out of circuit. The stator poles have phase windings on them while the rotor poles have no windings. Each set of windings separately switched in and out of circuit by the controller constitutes one phase of the machine. The machine may have one or more phases.  
         [0006]     Torque produced by an SRM is function of the instantaneous phase inductance, and hence the instantaneous rotor position, and the stator coil currents. By nature of the machine characteristics and control strategy, maximum torque is produced when rotor &amp; stator poles are unaligned, while minimum torque is produced at the aligned position. This results in considerable torque ripple and is one of the factors limiting the application of SRMs. For example, it is known that humans can sense very low levels of torque perturbation. There is therefore 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  
       [0007]     The present invention provides a switched reluctance machine (SRM); whether operated as a motor, generator, or both; having a new relationship between the number of stator poles and rotor poles so as to provide a SRM with a minimal amount of torque ripple and acoustic noise while providing improved power density and torque production. Particularly, the present invention provides a SRM having a salient rotor 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 a  6 / 10 ,  8 / 14 , or  10 / 18  configuration.  
         [0008]     Although the invention will be principally described with reference to 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. Also, while the invention is described in relation to an exemplary form of rotary machines, it is equally applicable to other forms of rotary machines and to linear and inverted machines as well. Thus, although the present invention will be described with reference to preferred embodiments, modifications and rearrangements can be made and still be within the scope and spirit of the present invention. 
     
    
     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 6 stator poles and 4 rotor poles.  
         [0011]      FIG. 2  illustrates a 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  illustrates an SRM according to the present invention having 6 stator poles and 10 rotor poles.  
         [0014]      FIG. 5  illustrates flux lines in the SRM of  FIG. 4  at an aligned position.  
         [0015]      FIG. 6  illustrates a magnetic flux density in the known SRM of  FIG. 1 .  
         [0016]      FIG. 7  illustrates a magnetic flux density in the exemplary SRM of  FIG. 4 .  
         [0017]      FIG. 8  is a graph of rotor angle versus time for the exemplary embodiment of the present invention.  
         [0018]      FIG. 9  is a graph of torque produced versus time for the exemplary embodiment of the present invention.  
         [0019]      FIG. 10  is a graph of torque produced versus time for the known SRM of  FIG. 1 .  
         [0020]      FIG. 11  illustrates a SRM of the present invention with 8 stator poles and 14 rotor poles.  
         [0021]      FIG. 12  illustrates a SRM of the present invention with 10 stator poles and 18 rotor poles. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]     The exemplary embodiment will be set forth in the context of a rotary switched reluctance machine (SRM). It will be appreciated by the person having ordinary skill in the art that a SRM according to the present invention may be arranged in a variety of ways.  
         [0023]      FIG. 1  illustrates a known construction of a three-phase salient pole SRM  11 . The outer stator  13  has six poles  15 ,  17 ,  19 ,  21 ,  23 ,  25  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 central rotatable shaft  30 , has four rotor poles  29 ,  31 ,  33 ,  35 .  
         [0024]     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 to turn the rotor in a counter-clock-wise direction, or energize phase-C to turn the rotor in a clock-wise direction. Subsequent serial phase excitation would than result in continuous rotation of the rotor.  
         [0025]      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 .  
         [0026]      FIG. 4  shows the present invention in the form of a SRM  41  with a stator  42  having six poles in three phase-pairs  43 / 45 ,  47 / 49 ,  51 / 53 , i.e., the same number of stator poles as in the known SRM of  FIG. 1 , for direct comparison therewith, as further discussed below.  
         [0027]     The SRM  41  shown in  FIG. 4  is a three-phase machine with six stator poles. Each stator pole has a coil, collectively  55 , wound around it. Stator poles  43 / 45  with their associated coils represent phase A. Stator poles  47 / 49  and their coils represent phase B. Stator poles  51 / 53  and their coils represent phase C. Ten salient rotor poles, collectively  57 , are located on the rotor mechanism  59  which is concentric and affixed to a rotatable central shaft  61 , the rotor  59  and shaft  61  being within the concentric stator  42 .  
         [0028]     The electrical control circuit configuration  37  as shown in  FIG. 2  can be readily adapted for the present invention. From the aligned position as shown in  FIG. 4 , it will be appropriate to excite the coils of phase-B poles  47 / 49  or phase-C poles  51 / 53  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.  
         [0029]      FIG. 5  shows the flux lines for the present SRM  41  when phase-A is excited and rotor poles  65 ,  67  are aligned with stator poles  43 ,  45 . Similarity of flux lines for the present embodiment and that of the prior art demonstrate a symmetry of the structure in both the designs, indicate that present theories and knowledge concerning the physical behavior of known SRMs can be readily applied or adapted to operation of the present invention.  
         [0030]      FIG. 7  shows distribution of magnetic flux density for the present embodiment at the same instant. A comparison of magnetic flux density plots in  FIG. 6  for the known SRM of  FIG. 1  versus an SRM of  FIG. 7  according to the present invention shows the possibility of driving the present invention SRM into deep saturation more easily than a conventional SRM motor, thus further increasing the performance advantage of the present invention SRM.  
         [0031]     A computer simulation of the SRM  41  shown in  FIG. 4  was performed to generate the time graphs of  FIG. 8  and  FIG. 9  for rotor position and torque, respectively, produced by the SRM  41  ( FIG. 4 ). For this, the motor is assumed to be in a position of having two diametrically opposite rotor poles  57  aligned with the stator poles  43 ,  45  having the phase-A coils as shown in  FIG. 4 . From this instance on, the phase-B coils on stator poles  47 ,  49  are energized to cause the rotor  59  to align to a new position by turning counter-clockwise. Computer simulation with transient analysis capabilities is performed to generate accurate data for speed and torque.  FIG. 8  and  FIG. 9  graph the rotor angle and torque, respectively, of the present SRM  41  on a time scale. Note that the shaft  61  has to travel 12 degrees [{2*360/10-360/6}=12] from its zero position i.e., the initial position. Once the rotor  59  turns counter-clock-wise and rotor poles  57  are aligned with the phase-B coil stator poles  47 ,  49 , the rotor  57  will not move further without further excitation. It is thus apparent from the graph of  FIG. 9  that torque production decreases rapidly as the rotor approaches an aligned position and then finally drops to zero at the aligned position.  
         [0032]      FIG. 10  shows a comparable computer simulation for the conventional SRM  11  shown in  FIG. 1 , for a comparison and with the embodiment of  FIG. 4 . Note the high spike indicative of torque ripple in the conventional design versus the smoother torque curve of the present invention. As can be seen, the present invention SRM produced torque ramp-up smoothly at the beginning and then is nearly at the peak for a major portion of the rotor travel, and finally rapidly drops to zero at the aligned position. Torque smoothing can be aided further by pole shaping and current profiling to enhance the capabilities of this motor if necessary or desired.  
         [0033]     Thus, an SRM based on the present invention will show significant improvements in torque ripple and torque density, and in turn, significant improvements in efficiency and noise reduction. SRMs based on the present invention can be easily adapted to use control strategies, switching schemes, and circuit configurations utilized in the known art, thus making aspects of this invention very practical for present commercial implementation.  
         [0034]     As shown in  FIGS. 4, 11 , and  12  the present invention proposes a novel concept of a SRM having more rotor poles than stator poles in the following relationship:  
         [0035]     Number of Rotor Poles (R)=2 times the Number of stator poles (S) minus 2, or 
 
 R= 2 S− 2, where S&gt;4; 
 
 i.e., for any SRM having more than four stator poles. 
 
         [0036]     The present invention is illustrated in an embodiment of a rotary SRM of 6 stator poles and ten rotor poles as shown in  FIG. 4 . The R=2S−2 relationship is further illustrated in  FIG. 11  for an SRM  71  having 8 stator poles, collectively  73 , and 14 rotor poles, collectively  75 . The R=2S−2 relationship is further illustrated in  FIG. 12  for an SRM  81  having 10 stator poles, collectively  83 , and 18 rotor poles, collectively  85 .  
         [0037]     It will be apparent to the person of skill in the art that symmetries of stator and rotor are still preserved in these new designs and hence the method of operation applicable to prior art using the standard switching schemes and circuit topologies will be equally suitable for these new designs. Thus, any switching schemes, control strategies, and circuit configuration meant for a standard design of SRM can be used for these new designs too.  
         [0038]     The present invention thus gives 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 electromechanical energy conversion equipment.  
         [0039]     While certain exemplary embodiments have been put forth to illustrate the present invention, these embodiments are not to be taken as limiting to the spirit or scope of the present invention which is defined by the appended claims. Thus, although the present invention has been described with reference to preferred embodiment, modifications and rearrangements could be made and still the result would be within the scope and spirit of the invention.