Abstract:
An electromagnetic machine stator has a common pole and a plurality of excitation poles. Each excitation pole has a coil wound around it for inducing a magnetic flux through the excitation pole. The common pole that does not have a coil wound around it for inducing a magnetic flux. A flux barrier, disposed within the common pole, inhibits the flow of flux from one part of the common pole across the flux barrier to another part of the common pole. The flux barrier is less conducive to the flow of flux than is the common pole.

Description:
This application claims priority to U.S. provisional application 61/409,638, filed on Nov. 3, 2010, the content of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE RELATED ART 
     High efficiency operation is an important requirement in many switched reluctance machine (SRM) applications. Efficiency is increased by enhancing torque generation for an excitation. Related-art SRMs have reached an upper limit in the efficiency achievable through their shapes and configurations, but have not achieved the high power-densities provided by synchronous machines with high-energy-density permanent magnets. 
       FIG. 1  illustrates an SRM having common poles that have no excitation windings. SRM  100  includes a stator  102 , a rotor  104 , and a rotor shaft  106  that rotates rotor  104  within stator  102 . Stator  102  has back iron  108  and salient excitation poles  110 ,  112  that each have a winding  114  through which an excitation current flows during an excitation phase associated with the excitation pole; excitation poles  110  are associated with phase A excitation, and excitation poles  112  are associated with phase B excitation. Stator  102  also has common poles  120  that have no excitation windings. Rotor  104  has back iron  116  and salient poles  118 ; rotor poles  118  may each be shaped (i.e., contoured) to provide a varying air gap as the rotor pole rotates past a stator pole or may be unshaped so as to provide a constant air gap with the stator pole as the rotor pole rotates past the stator pole. SRM  100  provides high power-density compared to an SRM having the same number of rotor poles and excitation poles, but no common poles. 
     Common poles  120  are disposed between excitation poles  110 ,  112  of different phases so as to prevent flux reversal within stator  102 . The pole arc of each common pole  120  equals one rotor pole pitch, which is the angular distance between two adjacent rotor poles; this common-pole arc enables the equivalent of one rotor pole to be fully under the common pole at all times. Although each of two rotor poles may be partially under a common pole at a particular moment, the combined area of the rotor pole faces under the common pole remains constant throughout the rotation of rotor  104  and this combined area is equal to the area of a single rotor pole face. 
     The variation of reluctance between a rotor pole and an excitation pole increases as the rotor pole moves toward the excitation pole. But the reluctance variation between a common pole and a rotor pole is small and almost insignificant compared to the reluctance variation experienced by an excitation pole as the rotor rotates. Thus, near-constant reluctance is presented to a common pole and negligible reluctance variation is contributed by common poles  120  to SRM  100 &#39;s overall reluctance variation. And because common poles  120  provide negligible reluctance variation, they do not appreciably contribute to torque generation; the machine torque comes almost entirely from the reluctance variation between the excited stator poles and their corresponding rotor poles. 
     Common poles  120 : (1) provide a path for the flow of return flux, (2) always carry unidirectional flux, and (3) cause a unidirectional flow of flux in stator back iron  108 , making the entire stator structure free of flux reversals. The absence of flux reversal minimizes core losses in SRM  100 , thus boosting the efficiency and, indirectly, the power density of SRM  100 . 
     SUMMARY OF THE INVENTION 
     The invention disclosed herein overcomes the negligible variation of reluctance within common poles of related-art switched reluctance machines (SRMs) and produces greater torque generation. These benefits are achieved without increasing the machine dimensions or winding turns. Thus, without increasing the weight and volume of steel laminations and winding copper within an SRM, the torque and output power may be increased. 
     The key to increasing torque generation and output power is to ensure that reluctance variation exists at all times for all overlapping surfaces of stator and rotor poles. This may be achieved by splitting, or creating air slots within, common poles of an SRM such that reluctance variation between the rotor poles and common poles is created as a rotor pole traverses under the common pole. 
     These and other objects of the invention may be achieved, in whole or in part, by an electromagnetic machine stator having a common pole and a plurality of excitation poles. Each of the excitation poles has a coil wound around it for inducing a magnetic flux through the excitation pole. The common pole does not have a coil wound around it for inducing a magnetic flux. A flux barrier, disposed within the common pole, inhibits the flow of flux from one part of the common pole across the flux barrier to another part of the common pole. The flux barrier is less conducive to the flow of flux than is the common pole. More simply, the flux barrier has greater reluctance than does the common pole. 
     Additionally, the objects of the invention may be achieved, in whole or in part, by an electromagnetic machine having a rotor and a stator. The rotor has a rotor pole, and the stator has: (1) an excitation pole with a coil wound around it for inducing a magnetic flux through the excitation pole and (2) a common pole without a coil wound around it for inducing a magnetic flux. A flux barrier, disposed within the common pole, inhibits the flow of flux from one part of the common pole across the flux barrier to another part of the common pole. The flux barrier is less conducive to the flow of flux than is the common pole. 
     Still further, the objects of the invention may be achieved, in whole or in part, by a segment of an electromagnetic machine stator. The stator segment has: (1) an excitation pole for conveying a magnetic flux when a coil wound around the excitation pole is excited by the flow of a current; (2) a leftmost portion of a first common pole; and (3) a rightmost portion of a second common pole. A back material interconnects the excitation pole with the leftmost portion of the first common pole and the rightmost portion of the second common pole, such that the leftmost portion of the first common pole is interconnected on one side of the excitation pole and the rightmost portion of the second common pole is interconnected on the other side of the excitation pole. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention are described in the following paragraphs of the specification and may be better understood when read in conjunction with the attached drawings, in which: 
         FIG. 1  illustrates a switched reluctance machine (SRM) having common poles that have no excitation windings; 
         FIG. 2  illustrates an SRM embodiment of the invention in which a single air slot exists within each common pole of the SRM; 
         FIG. 3  illustrates a common pole of the SRM illustrated in  FIG. 2  in greater detail; 
         FIG. 4  illustrates a second embodiment of the invention in which two air slots exist within each common pole of the SRM; 
         FIG. 5  illustrates a third embodiment of the invention in which multiple air slots exist within each common pole of the SRM; 
         FIG. 6  illustrates a fourth embodiment of the invention in which multiple air slots exist within each common pole of the SRM; 
         FIG. 7  illustrates a fifth embodiment of the invention in which two air slots exist within each common pole of the SRM; 
         FIG. 8  illustrates a sixth embodiment of the invention in which an SRM stator is composed of multiple isolated sections; and 
         FIG. 9  illustrates the torque ratio for an SRM with split common poles to one without split common poles as a factor of the ratio of the overlapping stator-to-rotor pole angle with respect to the rotor pole arc, for various values of the ratio between maximum and minimum air gaps, m. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  illustrates a switched reluctance machine (SRM) embodiment of the invention in which a single air slot exists within each common pole of the SRM. SRM  200  includes a stator  202 , a rotor  204 , and a rotor shaft  206  that rotates rotor  204  within stator  202 . Stator  202  has back iron  208  and salient excitation poles  210 ,  212  that each have a winding (not shown) through which an excitation current flows during an excitation phase associated with the excitation pole. Excitation poles  210  are associated with phase A excitation, and excitation poles  212  are associated with phase B excitation. Stator  202  also has common poles  220  that have no excitation windings. Rotor  204  has back iron  216  and salient poles  218 . 
       FIG. 3  illustrates a common pole of the SRM illustrated by  FIG. 2  in greater detail. Common pole  220  has an air slot  330  along a radial segment of the salient pole that bisects common pole  220  into mirror-image side parts  322 ,  324 . Air slot  330  has greater reluctance than the material of the common pole and, thus, inhibits the flow of flux between side parts  322 ,  324 . For example, as flux flows from a rotor pole  326  into side part  322  and subsequently into back iron  208 , air slot  330  inhibits this flux from passing into side part  324 , though this inhibition is not absolute. Similarly, flux flowing into side part  324  from a rotor pole  328  mostly flows through side part  324  and into back iron  208 , with little of the flux crossing air slot  330  so as to enter back iron  208  through side part  322 . Cylindrical hole  332  within air slot  330  serves as a bolt or rivet hole to secure together the laminations of stator  202 . Cylindrical hole  332  is disposed within back iron  208  and, thus, SRM  200  does not require space outside the stator laminations to mount the securing bolts or rivets. 
     As may be determined by inspection of  FIG. 3 , rotor pole  328  is nearly aligned with common pole  220  and has a maximum air gap,  342 , d max , with common pole  220  on the leftmost side of rotor pole  328  and a minimum air gap,  340 , d min , with common pole  220  on the rightmost side of rotor pole  328 . As explained below, the varying air gap between the rotor and stator poles improves the torque and power generation of SRM  200 . 
       FIG. 4  illustrates a second embodiment of the invention in which two air slots exist within each common pole of the SRM. Stator  402  has a common pole  420  with a longitudinal air slot  430 , along a radial segment of the pole, that inhibits the flow of flux between side parts  422 ,  424  in a manner similar to that provided by air slot  330  with respect to side parts  322 ,  324 . Cylindrical hole  432  serves both as an air slot to inhibit the flow of flux and as a bolt or rivet hole to secure together laminations of stator  402 . Because air slot  430  and cylindrical hole  432  do not form a continuous air slot that entirely bisects common pole  420  into two mirror image side parts, as do air slot  330  and cylindrical hole  332 , air slot  430  and cylindrical hole  432  do not provide as great an inhibition to the flow of flux between side parts  422 ,  424  as do air slot  330  and cylindrical hole  332  with respect to side parts  322 ,  324 . 
       FIG. 5  illustrates a third embodiment of the invention in which multiple air slots exist within each common pole of the SRM. Stator  502  has a common pole  520  with a longitudinal air slot  530  along a radial segment of the pole that inhibits the flow of flux between side parts  522 ,  524  in a manner similar to that provided by air slot  430  with respect to side parts  422 ,  424 . Two additional air slots  534  are disposed on opposite sides of air slot  530 , near the face of common pole  520 , such that one of air slots  534  is within side part  522  and the other is within side part  524 . Similarly, two additional air slots  536  are disposed on opposite sides of a cylindrical hole  532 , extending from back iron  508  into common pole  520 , such that one of air slots  536  extends into side part  522  and the other extends into side part  524 . Air slot  530  is a primary flux barrier due to its large size, and air slots  534 , 536  are auxiliary flux barriers due to their smaller sizes. Multiple flux barriers are more effective than a single barrier, but are more expensive to manufacture. Cylindrical hole  532  serves both as an air slot and as a bolt or rivet hole to secure together laminations of stator  502 . 
       FIG. 6  illustrates a fourth embodiment of the invention in which multiple air slots exist within each common pole of the SRM. Just as with stator  502  of  FIG. 5 , stator  602  has a primary barrier composed of an air slot  630 , a cylindrical hole  632 , and multiple auxiliary air slots  634 ,  636 . The larger, primary barrier more effectively bars the flow of flux than do the smaller, auxiliary barriers. Stator  602  differs from stator  502  in that longitudinal axes of auxiliary air slot barriers  634 ,  636  are disposed at a non-zero angle with respect to the longitudinal axis of primary air slot barrier  630 ; whereas the longitudinal axes of primary air slot barrier  530  and auxiliary air slot barriers  534 ,  536  are parallel. The inclined axes of auxiliary air slot barriers  634 ,  636 , with respect to the axis of primary air slot barrier  630  increases the effectiveness of the barriers to flux flow. 
       FIG. 7  illustrates a fifth embodiment of the invention in which two air slots exist within each common pole of the SRM. Stator  702  has a similar configuration to that of stator  402  illustrated by  FIG. 4 ; however, primary air slot  430  within stator  402  has a rectangular shape, whereas primary air slot  730  within stator  702  does not. Instead, air slot  730  has the shape of a column with a wider base at the foot of the column and a wider crown at the head of the column. 
       FIG. 8  illustrates a sixth embodiment of the invention in which an SRM stator is composed of multiple isolated sections. SRM  800  has a stator  802  composed of four isolated stator sections  802   a ,  802   b ,  802   c ,  802   d  that have no physical connection to one another through their respective back irons  808 . Each stator section  802   a ,  802   b ,  802   c ,  802   d  has an arcuate shape with a first side part  822  of a common pole  820  at one end of the arc and a second side part  824  of another common pole at the other end of the arc. For each stator section  802   a ,  802   b ,  802   c ,  802   d , an air slot  830  completely separates the first side part from the second side part of an adjacent stator section. Air slots  830  also completely separate back iron  808  of each stator section  802   a ,  802   b ,  802   c ,  802   d  from back iron  808  of an adjacent stator section. The adjoining first and second side parts of adjacent stator sections constitute a common pole  820 ; SRM has four such common poles  820 . Air slot  830  serves to inhibit the flow of flux between first side part  822  and second side part  824  by providing magnetic isolation between the two side parts. 
     The difference in performance, particularly in the torque, between an SRM having each of its common poles partially or fully separated into two parts and an SRM having no such separation can be derived based on a few assumptions. These assumptions are: 
     1. the magnetic equivalent circuit of the SRM is linear, though the magnetic equivalent circuit for the SRM can be modified to account for saturation; 
     2. the air gaps between the stator and rotor poles vary linearly up to an angle θrv; 
     3. θ is the overlap angle between the rotor pole and stator pole (excitation pole, common pole, or split part of the common pole); 
     4. m is the ratio of the maximum to minimum air gaps between the stator and rotor poles, when they are near alignment; 
     5. θrv is the rotor pole arc angle; 
     6. the air gap decreases from one end of the rotor (e.g., the leading edge of the rotor that comes first in close proximity to the stator pole as the rotor moves) to the other end of the rotor (e.g., the trailing edge of the rotor). 
     Based on the six assumptions identified above, the torque of the machines with split common poles and without split common poles can be derived from the equivalent magnetic circuit described in R. Krishnan, “Switched reluctance motor drives”, CRC Press, 2001, the content of which is incorporated herein by reference. From this derivation, the ratio of the torque (Tev) provided by a split common-pole SRM with 8 stator poles and 10 rotor poles and the torque (Te) provided by an SRM without split common poles may be determined and plotted against the ratio of the overlapping angle of the stator and rotor poles with respect to the rotor pole arc, θrv. 
       FIG. 9  illustrates the torque ratio for an SRM with split common poles to one without split common poles as a factor of the ratio of the overlapping stator-to-rotor pole angle with respect to the rotor pole are, for various values of the ratio between maximum and minimum air gaps, m. The following inferences can be drawn from the plots of  FIG. 9 : 
     1. Up to a value of 0.2 p.u. (per unit, i.e., the normalized value) of normalized overlap angle between the stator and rotor poles, the torque of an SRM having split common poles is lower than that of an SRM not having split common poles, by as much as 17%. The difference is reduced to zero as the normalized overlap angle approaches 0.2 p.u. 
     2. For normalized overlap angles greater than 0.2 p.u., the ratios of torque for an SRM with split common poles to that for an SRM without split common poles increase to peak values of 1.8 to 4.4 p.u. for values of ranging from 1 to 4. For normalized overlap angles greater than 0.2 p.u., phase current can be maintained throughout the phase, except at the tail end of the overlap where the phase current has to be turned off to prevent the machine from entering a negative-torque generating region. Preventing the machine from entering the negative-torque region prevents the highest torque from being harvested. The safest level up to which the phase current and, hence, torque production can be maintained is about 0.8 p.u. of overlap angle, in practice. But even for a lower overlap-angle-restricted operating point, the torque produced by a split common-pole SRM is about 1.6 to 2.4 times that of a machine without split common poles. 
     3. A unity ‘m’ value indicates that the air gap remains constant from the leading edge of the rotor pole face to the trailing edge. And even for a unity value of ‘m,’ an SRM having split common poles provides substantially greater torque, over a large range of normalized overlap angles of the stator and rotor poles, than does an SRM without split common poles. Thus, the use of split common poles is beneficial for increasing the torque produced by an SRM and increasing the power and power density of an SRM. 
     The best flux blocking capability, from one side or half of the common pole to the other side or half of the common pole, is achieved by entirely separating the sides/halves of the common poles, as illustrated in  FIG. 8 . But splitting the common poles entirety poses a problem in assembly. The use of air slots within the common poles provides substantial flux isolation across the common pole halves or sides, without having to assemble multiple separate parts. The above-described embodiments that do not have fully split or cut common poles do not provide perfect magnetic isolation of one side from the other side in the common pole, but do isolate them extremely well without destroying the integrity of a one-piece lamination having all the common and excitation poles. Perfect magnetic isolation, which is desirable for an SRM, is possible with completely cut common poles, but entails an expensive assembly process. 
     The above-described invention may be applied to an SRM having stator poles with excitation windings and common poles without excitation windings. Any number of stator and rotor poles may be used, and all forms of SRMs, including linear SRMs and transverse or axial flux-type SRMs, may be used. The common poles are split, such as with air gaps or air slots, into two parts so that flux sharing by the two halves of the common poles, split by the air gaps or air slots, is minimized. Other flux barriers within the common poles may also be used to inhibit the flow of flux between halves or separated parts of each common pole. 
     The common poles may be physically split all the way through with no connection between halves (i.e. the right and left parts of the common poles) of the common poles. In this instance, an integral stator segment may be created having an excitation pole, with its winding, flanked by right and left halves of two distinct common poles. Four such integral stator segments may be assembled to constitute a complete stator for an SRM. 
     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, drawings, 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.