Abstract:
A switched reluctance machine having salient stator and rotor poles. Alternating ones of the stator poles having windings and the others having permanent magnets attached on their pole faces. The alternate stator pole windings are provided with polarities that are suitable for unidirectional and bidirectional current operation of the switched reluctance machine. The alternate poles with permanent magnets in the switched reluctance machines can have also concentric windings placed on them and excited with currents to further augment the flux linkages in the stator poles. The windings on the poles with permanent magnets can be excited from the same source as the windings on the poles without permanent magnets to enhance power output or provide power factor correction.

Description:
This application claims the priority benefit of U.S. provisional application 61/327,829 filed on Apr. 26, 2010, the entire content of which is incorporated here by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to increasing the torque and power output of a switched reluctance machine (SRM), having windings on alternate stator poles, by augmenting the stator flux with permanent magnets placed directly on the stator poles that have no stator windings. The invention also relates to an SRM having windings on all of its stator poles and permanent magnets only on the alternate stator poles, so as to increase the stator flux, torque, and power output. The invention further relates to unidirectional and bidirectional current operation of an SRM with permanent magnets on alternate stator poles having no windings on them. Still further, the invention relates to unidirectional and bidirectional current operations of an SRM having windings on all poles but permanent magnets only on alternate poles. 
     BACKGROUND OF THE RELATED ART 
     A switched reluctance machine (SRM) is well known in literature and its principle, theory of operation, and construction are described in R. Krishnan, “Switched Reluctance Motor Drives”, CRC Press, 2001. An SRM has windings on its stator poles and no windings or magnets on the rotor poles or rotor slots. SRMs are ideal for high torque, are highly fault-tolerant, are highly efficient, and have high thermal operating conditions. Recently, machines with SRM e-core structures (see Cheewoo Lee, R. Krishnan, and N. S. Lobo, “Novel Two-Phase Switched Reluctance Machine Using Common-Pole E-Core Structure: Concept, Analysis, and Experimental Verification,” IEEE Trans. Ind. Appl., vol. 45, no. 2, pp. 703-711, March-April 2009) and SRMs with unexcited common poles (see C. Lee and R. Krishnan, “New designs of a two-phase e-core switched reluctance machine by optimizing the magnetic structure for a specific application: Concept, design, and analysis,” IEEE Transactions on Industry Applications, vol. 45, no. 3, pp. 1804-1814, September-October 2009) have been introduced with high power density and high efficiency operation. In the SRM e-core structure and SRM with unexcited common poles, maximum efficiency has been extracted from an electromagnetic point of view. To increase torque density of an SRM further, a structural change in its stator or rotor cores or windings is not sufficient; other means must be used to obtain much higher power density and greater efficiency. One way to achieve higher power density is to focus on the excitation augmentation in a stator with permanent magnets (PMs). Such augmentation of the excitation must bestow the fundamental operational characteristics of the SRM so as to maintain the attractive features of: (1) dc excitation; (2) simplicity, with regard to the minimum number of power semiconductor devices employed to control current in a power electronic circuit; (3) absence of shoot-through faults; (4) high fault tolerance, (5) utilization of reluctance torque; and (6) high efficiency operation (see see R. Krishnan, “Switched reluctance motor drives”, CRC Press, 2001). 
     Many machine implementations have been in practice, such as one with PMs in a stator back iron (see R. Krishnan, “Permanent magnet synchronous and brushless dc motor drives”, CRC Press, 2009 and X. Luo, and T. A. Lipo, “Synchronous/permanent magnet hybrid AC machine,” IEEE Transactions on Energy Conversion, vol. 15, no. 2, pp. 203-210, 2000) to exploit the structural properties of an SRM. Placing PMs in a stator back iron creates operation that is not that of a switched reluctance machine drive system, but is that of a PM brushless direct current (dc) motor drive system, which causes the loss of the best operational features of SRMs while embracing only the standard features of a PM brushless dc motor (PMBDC) drive system. Different schemes for realizing an SRM with PMs in the stator are provided in detail in R. Krishnan, “Permanent magnet synchronous and brushless dc motor drives”, CRC Press, 2009, and briefly described here. Broadly, three kinds of PMs superimposed on SRM structures can be seen in the literature; they are: (1) PMs in a back iron of a stator, (2) PMs on stator pole faces, and (3) PMs embedded in the middle of stator poles. 
       FIG. 1  illustrates a machine  100  with PMs  101  and  102  disposed within a stator back iron  104 . PMs  101 ,  102  aid the flux arising from stator pole excitations. An SRM of this type is known in literature as a PM SRM or doubly-salient PM machine. The reversal of excitation currents in stator pole windings  103  will produce flux with polarity that is opposite to the flux from PMs  101 ,  102 . The opposite polarities of flux causes flux cancellation, due to opposing magneto-motive force. Flux produced by the stator current excitation passes through adjacent stator poles, resulting in greater flux leakage. The reluctance variation for the phase becomes smaller leading to smaller reluctance torque. Therefore, the main torque produced is primarily the synchronous torque of the machine. The doubly-salient PM machine behaves like a PM synchronous machine or, what is sometimes referred to in the literature, a PM brushless dc machine. PMs  101 ,  102  equivalently replace PMs in a rotor of a conventional PMBDC machine, with no apparent difference in performance. 
       FIG. 2  illustrates a single-phase machine  200  having two PMs disposed on the face of each stator pole.  FIG. 3  illustrates a three-phase machine  300  with PMs disposed on the pole faces. The structures of  FIGS. 2 and 3  support flux reversal in the back iron  205  of machines  200 ,  300  (see R. P. Deodhar, S. Andersson, L. Boldea et al., “Flux-reversal machine: A new brushless doubly-salient permanent-magnet machine,” IEEE Transactions on Industry Applications, vol. 33, no. 4, pp. 925-934, 1997). Machines  200 ,  300  are referred to as flux reversal machines. 
     In machines  200  and  300 , PMs  201  and  202  are installed on the face of each stator pole  203 . PMs  201 ,  202  are magnetized along the radial direction of machine  200 &#39;s and machine  300 &#39;s rotors  204 . Each stator pole  203  has two magnets  201 ,  202  that are half the width of the pole arc. Net flux in each stator pole  203  is the sum of the flux due to the windings and the flux of the PMs. Only half of the stator pole face is utilized by the flux when stator windings  206  are excited. The wide face of each stator pole, as illustrated in  FIGS. 2 and 3 , increases the difficulty of manually wrapping windings on the poles and automated winding insertion is expensive. 
       FIG. 4  illustrates a three-phase machine  400  having PMs embedded in the middle of each stator pole (see D. Ishak, Z. Q. Zhu, and D. Howe, “Comparative study of permanent magnet brushless motors with all teeth and alternative teeth windings,” IEE Conference Publication, pp. 834-839, 2004). PMs  401  are embedded in the center of poles  402  along the radial direction of rotor  403 . The magnetization of each PM  401  is along the circumferential direction of machine  400 . The performance of machine  400  is similar to that for flux reversal machines  200 ,  300 . Machine  400  is referred to as flux switching machine (FSM) in literature. The operation of an FSM is the same as that of a PM synchronous machine, and the stator windings are excited with three-phase alternating currents. FSMs have no reluctance torque and, thus, no relationship to SRM operation or characteristics. 
     In summary, doubly-salient PM machines, flux reversal machines, and FRMs: (i) are fundamentally alternating current (ac) machines, (ii) have PMs embedded in the stator instead of in the rotor, as is conventional, (iii) allow flux reversal in the stator poles to varying degrees, and (iv) have an SRM structure of salient stator and rotor poles, but function solely as PM synchronous or brushless dc machines, as they have very negligible reluctance torques. 
       FIG. 5  illustrates an SRM  500  having four excitation poles  501 ,  502 ,  503 , and  504 , two poles for each of two phases. Phase A employs excitation poles  501 ,  502 , and Phase B employs excitation poles  503 ,  504 . SRM  500  has windings  505  on each of excitation stator poles  501 ,  502 ,  503 ,  504 . Diametrically opposite excitation pole windings constitute a phase winding. SRM  500  has a common pole  506  sandwiched between each pair of adjacent excitation poles  501 ,  502 ,  503 ,  504 . Thus, SRM  500  has four common poles  506  and four excitation poles  501 - 504 . When phase A is excited, flux: (1) enters excitation pole  501 ; (2) is conveyed to back iron  507 ; (3) passes through each common pole  506  adjoining excitation pole  501 , (4) enters the air gap (i.e., the space existing between a rotor pole and its nearest stator or common pole at any instant) between the common pole conveying the flux and its nearest rotor pole  508 ; (5) enters the nearest rotor pole  508 ; (6) enters rotor back iron  509 ; (7) enters a rotor pole nearest excitation pole  501 ; (8) enters the air gap between excitation pole  501  and its nearest rotor pole; and (9) is conveyed back to excitation pole  501 . The main flux in excitation pole  501  splits between left- and right-side common poles  506  and likewise for excitation pole  502  of phase A. 
     The purpose of each common pole  506  is to carry the flux produced by the individual excitation pole nearest to the common pole and route it back to the excitation pole via the air gap, rotor pole, adjoining rotor back iron, rotor pole, and air gap. The same applies to phase B operation in the SRM. From the above-discussion, it may be inferred that the flux in the common poles does not reverse, regardless of which phase is conducting. The excitation poles experience no reversal of flux, as they are excited only with unidirectional current. Common poles  506  serve to carry the flux generated by excitation poles of both phases. 
     Flux generated in machine  500  is solely due to the excitation of phase windings  505  on excitation poles  501 - 504 . A challenge facing machine  500  is that the starting torque at an unaligned position of the rotor and stator poles is not very high. 
     SUMMARY OF THE INVENTION 
     Objects of the invention include: 
     1. achieving the fundamental behavior of a switched reluctance machine (SRM) in torque generation, i.e., the SRM&#39;s predominant torque production must be from variable reluctance of the machine; 
     2. achieving the fundamental structure of an SRM both in its stator and rotor shape and form, i.e., with double saliency retained; 
     3. no windings and no permanent magnets (PMs) on rotor poles of the SRM; 
     4. a stator with laminations having multiple poles, some of the stator poles carry windings (i.e., excitation poles) and the remainder having no windings (i.e., common poles); 
     5. PMs on stator poles and magnetized in the radial direction with parallel or radial orientation; 
     6. small thickness PMs on stator poles; 
     7. PMs of remnant flux density that retain magnetism and are made of ferrite, neodymium, samarium cobalt, or any other rare earth element; 
     8. PMs only on stator pole faces, either fully or partially covering the pole faces or on the sides of the poles along the stator pole height; 
     9. PMs only on stator poles that are not wound with coils for excitation; 
     10. unidirectional current excitation of SRM stator windings; 
     11. the magnetic flux in stator excitation poles and stator common poles remains in the same direction, with no flux reversal in the stator back iron and stator poles, when stator windings are excited with unidirectional current, magnetic flux; 
     12. common poles carry flux of PMs as well as part of flux generated by excitation poles, without any reversals or cancellations in the same direction when excitation poles of one phase or any other phase are excited; 
     13. high efficiency and high torque; 
     14. significant torque, at lower currents and lower load torques, due to PM excitation; 
     15. little effect from PMs at nominal operating load torque and higher torques, so as to operate like an SRM; and 
     16. highly-enhanced torque-per-unit-current at low and high torques. 
     A further object of the invention is to provide a path for excitation flux, when excitation current is reversed, so that stator pole flux due to current excitation and PM flux are not additive. The path for stator flux is through main excitation poles of the same phase. 
     Still further, an object of the invention is to have no leakage paths in a machine&#39;s back iron. Alternating current (ac) operation of the excitation windings is possible without PM flux opposing stator excitation pole flux and with only reluctance torque contributing to the dominant torque. The PM flux aids the stator excitation poles flux and at no time do they oppose each other. 
     These and other objects of the invention may be achieved, in full or part, by a stator having: (1) multiple excitation poles, each of which has an inductive winding disposed thereon; (2) a common pole that has a permanent magnet but no inductive winding disposed thereon; and (3) a stator back that electromagnetically interconnects the common pole and each excitation pole. 
     The above-mentioned and other objects of the invention may be achieved, in full or part, by a stator having: (1) multiple poles, each of which has an inductive winding disposed thereon and fewer than all of the poles have a permanent magnet disposed thereon; and (2) a stator back that electromagnetically interconnects the poles. 
     The above-mentioned and other objects of the invention may be achieved, in full or part, by an SRM having N excitation phases, N an integer greater than 0. The SRM has: (1) multiple stator poles associated with each of the N excitation phases, each stator pole having an inductive winding disposed thereon for receiving current of the associated excitation phase; (2) for each of the N excitation phases, a first permanent magnet disposed on each of fewer than all of the stator poles associated with the excitation phase; and (3) a stator back that electromagnetically interconnects all of the stator poles. 
     The above-mentioned and other objects of the invention may be achieved, in full or part, by a switched reluctance machine having: (1) a stator with: (a) multiple excitation poles, each of which has an inductive winding disposed thereon; (b) multiple common poles, each of which has a permanent magnet but no inductive winding disposed thereon; and (c) a stator back that electromagnetically interconnects the common pole and each excitation pole; and (2) a rotor that cooperates electromagnetically with the stator. The excitation and common poles are disposed around the stator with respect to one another such that when unidirectional currents: (a) are passed through the inductive windings to induce fluxes in the same radial direction from the rotor to the excitation poles or (b) are not passed through the inductive windings: (i) the fluxes flowing through the stator are unidirectional and never reverse direction, (ii) the fluxes flowing through the common poles are unidirectional and never reverse direction, and (iii) the induced fluxes and the fluxes generated by the permanent magnets are additive, without a negating effect on one another. 
     The above-mentioned and other objects of the invention may be achieved, in full or part, by an SRM having: (1) a stator with: (a) four excitation poles per phase, each of the excitation poles has an inductive winding disposed thereon, the windings for a pair of the excitation poles for each phase configured so as to produce fluxes through their respective excitation poles in a direction opposite to that produced by the windings for the other pair of the excitation poles of the phase, when a current is passed through the four windings of the phase; (b) common poles, each of which has a permanent magnet but no inductive winding disposed thereon, the number of common poles being equal to the number of excitation poles; and (c) a stator back that electromagnetically interconnects the common and excitation poles; and (2) a rotor that cooperates electromagnetically with the stator. The flux directions are determined with respect to a radial direction of the excitation poles toward the rotor. The excitation and common poles are disposed around the stator with respect to one another such that when the windings of any one phase are excited by passing an alternating current through these phase windings: (i) the fluxes generated by the excited windings and the fluxes generated by the permanent magnets are additive, without a negating effect on one another, for each direction the alternating current flows, and (ii) flux reversal through the common poles does not occur. 
     The above-mentioned and other objects of the invention may be achieved, in full or part, by a method of operating a switched reluctance machine (SRM), the SRM having: (a) a rotor, (b) a stator having multiple poles, each of which has an inductive winding disposed thereon and fewer than all of the poles have a permanent magnet disposed thereon; and (c) a stator back that electromagnetically interconnects the poles. The method including: (1) inducing flux within the SRM by passing current through one or more of the inductive windings that is not disposed on a stator pole having a permanent magnet thereon; and (2) inducing flux within the SRM by passing current through one or more of the inductive windings disposed on a stator pole having a permanent magnet thereon so as to correct a power factor applied to the SRM or enhance torque production of the SRM. 
     The above-mentioned and other objects of the invention may be achieved, in full or part, by a method of operating a switched reluctance machine (SRM) having N excitation phases, N an integer greater than 0, the SRM having: (a) multiple stator poles associated with each of the N excitation phases, each stator pole having an inductive winding disposed thereon for receiving current of the associated excitation phase; (b) for each of the N excitation phases, a first permanent magnet disposed on each of fewer than all of the stator poles associated with the excitation phase; (c) a stator back that electromagnetically interconnects all of the stator poles; and (d) multiple inductive windings disposed around the stator back, each of the stator back windings positioned between two of the poles associated with an excitation phase of the excitation phases, wherein for each of the stator back windings, one of the two poles has the first magnet disposed thereon and the other of the two poles does not have a first magnet disposed thereon. The method including: (1) exciting one phase of the SRM by passing current through the windings disposed on stator poles associated with the phase; and (2) exciting the stator back windings by passing current through them to correct a power factor of the SRM. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a related art machine with permanent magnets (PMs) disposed within a stator back iron; 
         FIG. 2  illustrates a related art single-phase machine having two PMs disposed on the face of each stator pole; 
         FIG. 3  illustrates a related art three-phase machine with PMs disposed on the pole faces; 
         FIG. 4  illustrates a related art three-phase machine having PMs embedded in the middle of each stator pole; 
         FIG. 5  illustrates a related art SRM having four excitation poles, two poles for each of two phases; 
         FIG. 6  illustrates a machine having PMs disposed on the faces of a stator&#39;s common poles; 
         FIG. 7  illustrates the machine of  FIG. 6  with stator pole winding polarities of the phases changed so that the flux produced by one of the excitation poles goes through the other excitation pole of the same phase; 
         FIG. 8  illustrates the closed flux path created by phase B excitation of the machine illustrated in  FIG. 7 ; 
         FIG. 9  illustrates a machine having small magnets along the axial lengths of common poles; 
         FIG. 10  illustrates a machine having a stator formed by two modular and separate segments; 
         FIG. 11  illustrates a modification of the machine illustrated in  FIG. 6  in which windings are disposed around each common pole; 
         FIGS. 12(   a ) and  12 ( b ) illustrate machines having modular and separate L-shaped stator segments; 
         FIGS. 13(   a ) and  13 ( b ) illustrate machines having I-shaped modular and separate stator segments; 
         FIG. 14  illustrates a related art machine having three stator poles per phase; 
         FIG. 15  illustrates a modification of the machine illustrated in  FIG. 14  in which PMs are disposed on two stator pole faces; 
         FIG. 16  illustrates a modification of the machine illustrated in  FIG. 14  in which PMs are disposed on four stator pole faces; and 
         FIG. 17  illustrates a modification of the machine illustrated in  FIG. 14  in which PMs are disposed internally within stator poles. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 6  illustrates a machine  600  having permanent magnets (PMs) disposed on the faces of a stator&#39;s common poles. A PM  601 ,  602 ,  603 ,  604  is mounted on the face of each of common poles  609  with south poles forming the base and north poles directly facing rotor poles  610  and the air gaps in between. Excitation poles  605 - 608  all generate and conduct flux away from the air gap existing between the rotor and stator poles, into the excitation poles, and then into stator back iron  611 . The polarities of current excitations (dot indicating current coming out and star indicating current going in the conductors) are shown in windings  612  around excitation stator poles  605 - 608 . The PM flux goes to the adjacent excitation poles of both phases under normal conditions, when excitation poles  605 - 608  are not energized. A dominant portion of the PM flux passes through the excited excitation pole. 
     For example, exciting phase-A winding on excitation stator poles  605 ,  607  will draw the flux from common pole PMs  601 - 604  to excitation poles  605 ,  607 . Thus, PM flux is harnessed to augment the flux generated by the excitation poles. The flux linkage of the stator excitation phases are enhanced with flux from the PMs. When the excited phase&#39;s flux linkages are enhanced as a function of stator excitation current for each rotor position, torque generated in the air gap increases. The increase in torque can be seen from fundamentals of electro-mechanics (see chapter 2 of R. Krishnan, “Switched reluctance motor drives”, CRC Press, 2001). 
     When machine  600 &#39;s stator windings  612  are unexcited, the flux in machine  600  is due to PMs  601 - 604 . The flux and flux linkage of the stator windings may be derived considering linear material characteristics. When a pair of diametrically opposite rotor poles are completely unaligned between stator poles, flux linkage is a minimum and will increase linearly with increasing excitation of the stator winding on the excitation pole. The flux linkages versus stator current characteristics will be a straight line having a small slope. The straight line flux linkage is very much similar to SRM characteristics except that SRM unaligned position flux linkages versus stator current characteristic will have a slightly smaller slope than machine  600 . As rotor poles  610  are moved from the completely unaligned position to complete alignment with stator poles  605 - 608 , the flux linkages versus stator current characteristics follow the shape of the magnetization (B-H) characteristics of machine  600 &#39;s steel laminations. 
     The flux linkage contribution of PMs  601 - 604  and stator phase winding excitation flux produce a larger area in the flux linkage characteristics as compared to an SRM without PMs. The difference in the area enclosed between: (1) the flux linkages versus stator current curve and (2) such characteristics at a position where the stator and rotor poles are completely out of alignment, identifies the work done for each operational cycle of a phase. The area enclosed between the flux linkages versus stator current curve is much larger than that of an SRM without PMs in the pole faces and explains the higher torque generating capability of the SRM with PMs in the pole faces. 
     Machine  600  produces cogging torque, which is a disadvantage, but cogging does not detract machine  600 &#39;s ability to generate higher torque than conventional PM machines. Torque generation is enhanced in machine  600  because the PM flux is governed by excitation poles  601 - 604 . By itself, the PM flux is divided between the excitation poles adjacent to the PM. PM flux focus is achieved by the excitation of a pole on either side of the common pole where the PM is embedded. Focusing of the PM flux also makes the flux flow unidirectionally, as both the PM and excitation fluxes have the same polarity and are in series. Absence of flux reversal reduces core losses in the stator laminations and contributes to an increase in efficiency. 
     The flux path of machine  600  is shortened by comparison to an SRM with four stator poles and two, six, eight, or ten rotor poles. In a four-pole, two-phase SRM, the entire stator back iron has flux flowing whereas in machine  600 , only half of stator back iron  611  has flux flowing. Only half the back iron carrying flux at any time amounts to no core losses in the other half of the stator back iron. Only half the stator and rotor back irons carry flux and, therefore, contribute to lower losses and higher efficiency compared to conventional SRMs where the entire stator and rotor back irons carry the flux. 
     So as to operate machine  600  with an alternating current (ac) power electronic converter, it is necessary only to change the stator pole winding polarities. The stator pole winding polarities of the phases must be changed so that the flux produced by one of the excitation poles goes through the other excitation pole of the same phase. 
       FIG. 7  illustrates the machine of  FIG. 6  with stator pole winding polarities of the phases changed so that the flux produced by one of the excitation poles goes through the other excitation pole of the same phase. Note that there is direct cancellation of flux in electric machines with PMs in the back iron, like those described in the Background of the Related Art section of this disclosure. The direct cancellation of flux forces the excitation pole&#39;s flux to be reduced and diverted via the remaining stator poles, thus diminishing the torque generation due to reluctance variation. The addition of flux achieved with machine  700  is a great advantage compared to conventional machines. 
       FIG. 8  illustrates the closed flux path created by phase B excitation of the machine illustrated in  FIG. 7 . The PM flux passes through to the top pole of phase B. The excitation pole flux for the bottom pole of phase B also closes its path through the top phase-B pole, through respective air gaps, rotor poles, rotor back iron, stator poles, and stator back iron. Flux from PM poles and excited stator poles leak to excitation poles of phase A. There is a slight crowding of flux in the top pole compared to the bottom pole of any excited phase, which may create an unbalanced normal (i.e., radial) force. The unbalanced radial force may be compensated for by having four excitation poles per phase, rather than two excitation poles per phase. 
     Increasing the number of rotor poles will also increase the number of common poles with PMs on them. If the number of excitation poles per phase and common poles with PMs is increased such that diametrically opposite excited poles each experience flux crowding, the net normal force can be made zero. With zero net normal force, stator acceleration and noise generation in the machine can be minimized. 
     The flux produced by the excitation poles of the phases add together and do not have to find another path as in the synchronous machines described in X. Luo, and T. A. Lipo, “Synchronous/permanent magnet hybrid AC machine,” IEEE Transactions on Energy Conversion, vol. 15, no. 2, pp. 203-210, 2000. The additive flux makes the magnetic circuit highly efficient. 
     Two PMs in the stator back iron are sufficient in the PM SRM described in X. Luo, and T. A. Lipo, “Synchronous/permanent magnet hybrid AC machine,” IEEE Transactions on Energy Conversion, vol. 15, no. 2, pp. 203-210, 2000, whereas machines  600 ,  700  require a minimum of four PMs. The number of PMs is the same as the number of excitation poles if a contiguous stator lamination is required. If the PMs have to be put in the back iron, instead of in the common stator poles, at least 8 PMs would be required. Thus, machines  600 ,  700  reduce the number of PMs relative to that of a conventional machine. Placing PMs in the back irons of machines  600 ,  700  would also result in no flux reversals in the common poles, stator back iron, and excitation poles. 
       FIG. 9  illustrates a machine  900  having small magnets along the axial lengths of common poles. Small magnets  901 ,  902  cover only a portion of the face of common poles  903 , so as to reduce the size of the PMs. Small magnets  901 ,  902  are easy to insert and are flush with the pole contour. Variations of magnet placements shown in machine  900  are possible in and around common poles  903 , such as in the faces of common poles  903  or along the radial part of each base of common poles  903 . Small magnets  901 ,  902  may cover the entire facial arc of common poles  903  or some portion thereof. 
       FIG. 10  illustrates a machine  1000  having a stator formed by two modular and separate segments. Each E-shaped modular segment  1001 ,  1002  has two excitation poles  1003 - 1006  and one common pole  1007 ,  1008 . Each modular segment  1001 ,  1002  has one excitation pole for phase A  1003 ,  1004  and one for phase B  1005 ,  1006 . By connecting the windings of respective phases in series or parallel from modular segments  1001 ,  1002 , phase A and B windings are realized. Common poles  1007 ,  1008  have PMs  1009 ,  1010  in their pole faces. The flux of PMs  1009 ,  1010  is steered to the phase poles by the excitation of the respective phase windings in the poles. The functioning of SRM  1000  is similar to the SRM with E-core described in Cheewoo Lee, R. Krishnan, and N. S. Lobo, “Novel Two-Phase Switched Reluctance Machine Using Common-Pole E-Core Structure: Concept, Analysis, and Experimental Verification,” IEEE Trans. Ind. Appl., vol. 45, no. 2, pp. 703-711, March-April 2009. Machine  1000  reduces the stator iron and packaging of a machine. 
       FIG. 11  illustrates a modification of the machine illustrated in  FIG. 6  in which windings are disposed around each common pole. Power factor correction (PFC) windings  1101  of machine  1100  may be excited by a switched, rectified ac current from an ac supply with the aid of a power electric circuit. PFC windings  1101  may be used in the stead of an inductor within a PFC circuit. Common poles  1101  carry the flux due to rectified currents in the PFC windings and thereby augment the excitation pole flux, thus generating increased torque. PFC windings  1101  serve to: (1) provide a physical inductor for PFC and (2) enhance torque generation with the flux created by the inductor windings on the common poles. Additionally, with machine  1100 : (1) no separate lamination core is required for winding inductor  1101 ; (2) no separate space for packaging inductor  1101  is required as is required for conventional power PFC circuits, with the result that the electronic package of the PFC circuit may be smaller; and (3) cooling of inductor  1101  is integrated with the cooling of the motor, thus replacing cooling management of two separate entities (i.e., motor and inductor) with only one, that of the motor. 
       FIGS. 12(   a ) and  12 ( b ) illustrate machines  1210  and  1220  having modular and separate L-shaped stator segments. Each of machines  1210 ,  1220  has L-shaped stator segments  1212 . PMs  1214  are disposed on the faces of common poles  1216  for each of machines  1210  and  1220 . Machine  1220  differs from machine  1210  in that the former has windings  1222  disposed on common poles  1216 . 
       FIGS. 13(   a ) and  13 ( b ) illustrate machines  1310  and  1320  having I-shaped modular and separate stator segments. Each of machines  1310 ,  1320  has I-shaped stator segments  1312 . PMs  1314  are disposed on the faces of common poles  1316  for each of machines  1310  and  1320 . Machine  1320  differs from machine  1310  in that the former has windings  1322  disposed on common poles  1316 . Windings  1322  on common poles  1316  are disposed to look differently from those on excitation poles  1324  of phases A and  13 . Windings  1322  around common poles  1316  may be used to enhance the excitation of common poles  1316  and as inductors for power factor correction. 
     Machines  1000 ,  1210 ,  1220 ,  1310 , and  1320  may be realized without continuity in stator lamination. Despite the discontinuity in the stator lamination, machines  1000 ,  1210 ,  1220 ,  1310 , and  1320  produce torque mainly contributed by reluctance variation. Therefore, these machines are controlled as a standard SRM drive system and any of the converter topologies for SRMs can be used for current control of the phase windings. 
     Machines  1000 ,  1210 ,  1220 ,  1310 , and  1320  do not need to have two stator modules, as one module is sufficient to actuate the rotor. With more than one stator module, failure of one or more modules but not all modules ensures fault tolerant operation, provided phase windings in the modules are separate from each other and controlled separately with power converters. 
     For machines  1220  and  1320 , windings  1222  and  1322  may be used both as an inductor for PFC and to augment the flux generated by the PMs and excitation poles. The additional flux generates higher torque, while providing PFC and drawing near sinusoidal current from an ac supply that delivers power for actuating the motor. Power stages involved in feeding the motor from the ac supply lines are: (1) ac to direct current (dc) rectification with control for power factor correction and (2) dc to controlled voltage/current to the motor phases. 
     The ac excitation of the phases of machines  1000 ,  1210 ,  1220 ,  1310 , and  1320  is very similar to that for machine  1100 , as described herein. H-bridge converters can be used for ac excitation of the windings, similar to the inverters used for the control of ac machines, with or without a split dc link. 
       FIG. 14  illustrates a related art machine  1400  having three stator poles per phase. Phase A employs three stator poles A 1 , A 2  and A 3 . When exciting stator poles A 1 , A 2 , and A 3  of phase A, half the flux from stator pole A 1  goes through the air gap between stator pole A 1  and a rotor pole R 1 , rotor pole R 1 , the back iron between rotor pole R 1  and a rotor pole R 2 , rotor pole R 2 , the air gap between rotor pole R 2  and stator pole A 2 , and the stator back iron between stator poles A 1  and A 2  before returning to stator pole A 1 . Likewise, the other half of the flux from stator pole A 1  goes through rotor pole R 1 , the back iron between rotor pole R 1  and a rotor pole R 3 , rotor pole R 3 , the air gap between rotor pole R 3  and stator pole A 3 , stator pole A 3 , and the stator back iron between stator poles A 1  and A 3  before returning to stator pole A 1 . 
     Stator pole A 1  has twice the number of winding turns (not illustrated) and twice the cross-sectional area across its pole face as do stator poles A 2  and A 3 ; the phase-B poles are likewise configured. The winding polarities can be in such a direction that no flux reversals occur in the back iron, as discussed in Krishnan Ramu and Nimal Savio Lobo, “Apparatus and method that prevent flux reversal in the stator back material of a two-phase SRM (TPSRM)”, U.S. Pat. No. 7,015,615 B2, Mar. 21, 2006. 
       FIG. 15  illustrates a modification of the machine illustrated in  FIG. 14  in which PMs are disposed on two stator pole faces. More specifically, machine  1500  has PMs  1502  disposed on the faces of stator poles A 1  and B 1 . Components  1504  and  1506  may be either permanent magnets, such as are illustrated in  FIG. 1 , or windings. Components  1504  and  1506  prevent flux reversals due to the flux generated by: (1) the permanent magnets or (2) exciting the stator back windings. Note that the polarities, north to south, of PMs  1502  are indicated by arrows in  FIG. 15 , with the head of the arrow indicating north and the tail of the arrow indicating south. 
       FIG. 16  illustrates a modification of the machine illustrated in  FIG. 14  in which PMs are disposed on four stator pole faces. More specifically, PMs  1602  are disposed on the faces of stator poles A 2 , A 3  and B 2 , B 3 . The structure of machine  1500 , in which PMs  1502  are disposed on stator poles A 1  and B 1 , is magnetically equivalent to the structure of machine  1600 , in which PMs  1602  are disposed on stator poles A 2 , A 3 , B 2 , and B 3 , as the flux encounters the same reluctance. Components  1604  and  1606  may be either permanent magnets, such as are illustrated in  FIG. 1 , or windings. Components  1604  and  1606  prevent flux reversals due to the flux generated by: (1) the permanent magnets or (2) exciting the stator back windings. 
     PMs  1602  do not require extensive bracing support to stay on the faces of poles A 2 , A 3 , B 2 , and B 3 . The placement of PMs  1602  is preferably near to the faces of stator poles A 2 , A 3 , B 2 , and B 3 , so as to minimize the leakage flux within PMs  1602 . 
       FIG. 17  illustrates a modification of the machine illustrated in  FIG. 14  in which PMs are disposed internally within stator poles A 1  and B 1 . As with the placement of PMs  1602  in  FIG. 16 , PMs  1702  in  FIG. 17  are preferably mounted within stator poles A 1  and B 1  near to the faces of the poles. A slot  1704  for mounting PM  1702  within each of stator poles A 1 , B 2  is big enough to embed PM  1702 , but PM  1702  is preferably wedged or glued so that it does not move within the slot. Components  1706  and  1708  may be either permanent magnets, such as are illustrated in  FIG. 1 , or windings. Components  1706  and  1708  prevent flux reversals due to the flux generated by: (1) the permanent magnets or (2) exciting the stator back windings. 
     Machines  1500 ,  1600 , and  1700  each have two phases and employ 6-stator and 3-rotor poles and proper winding polarities so as to prevent flux reversals in the stator back iron. The PMs illustrated in  FIGS. 15-17  may be arranged either on the larger poles or on the smaller poles for torque enhancement. This two-phase, 6-stator and 3-rotor pole arrangement is extendable to multiphase machines with greater than two phases. 
     Machines  1500 ,  1600 , and  1700  and multiphase machines do not require PM disposed on the common poles to achieve torque enhancement and operation that is free of flux reversal; instead, windings can be placed on the stator back iron between stator poles A 2  and B 2  and between stator poles A 3  and  133  of machines  1500 ,  1600 ,  1700  to produce electromagnetically equivalent PM flux. And windings disposed on the back iron may be used as inductors for power factor correction. 
     The foregoing has been a detailed description of possible embodiments of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention. Accordingly, it is intended that this specification and its disclosed embodiments be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.