Abstract:
An electrical machine rotor includes a flux-conducting portion and a flux-inhibiting portion. The flux-conducting portion is conducive to conveying an electromagnetic flux and has a plurality of salient rotor poles and a portion of back material. The flux-inhibiting portion is less conducive to conveying an electromagnetic flux than the flux-conducting portion and is disposed entirely outside the boundaries of the rotor poles.

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 
       FIG. 1  illustrates a related-art switched reluctance machine (SRM)  100  having long rotor poles  102  and a back iron  104  that is narrow, but sufficient to carry flux for the SRM&#39;s intended use. The flux path within SRM  100  traverses through the entire heights of rotor poles  102 . The long rotor poles  102  of SRM  100  create two drawbacks: (i) the flux path through the long rotor poles is longer than necessary and (ii) the greater weight of the rotor laminations, due to the length of the rotor poles, results in high core losses. 
     SUMMARY OF THE INVENTION 
     The invention disclosed herein overcomes the high core loss existing in related-art switched reluctance machines (SRMs) due to long flux paths in the rotor. 
     These and other objects of the invention may be achieved, in whole or in part, by an electrical machine rotor that includes a flux-conducting portion and a flux-inhibiting portion. The flux-conducting portion is conducive to conveying an electromagnetic flux and has a plurality of salient rotor poles and a portion of back material. The flux-inhibiting portion is less conducive to conveying an electromagnetic flux than the flux-conducting portion and is disposed entirely outside the boundaries of the rotor poles. 
     Additionally, the objects of the invention may be achieved, in whole or in part, by an electrical machine rotor having a back material, a plurality of salient rotor poles radially extending from the perimeter of the back material, a flux-conducting portion, and a flux-inhibiting portion. The flux-conducting portion is conducive to conveying an electromagnetic flux and this portion includes the rotor poles and a first portion of the back material. The flux-inhibiting portion is less conducive to conveying an electromagnetic flux than the flux-conducting portion. The back material portion of the flux-conducting portion is disposed circumferentially around the rotor as an annular ring that forms the outer perimeter of the rotor between adjacent rotor poles. The first portion of the back material is capable of conveying as much electromagnetic flux away from each rotor pole, without saturating, as the rotor pole is capable of conveying to the first portion of the back material, without saturating. The flux-inhibiting potion is disposed within the back material at a lesser radial distance from the rotational axis of the rotor than the first portion of the back material. 
     Still further, the objects of the invention may be achieved, in whole or in part, by a system that identifies operational phases of an electrical machine. The system includes a rotor of the electrical machine; a first detector that detects the passage of a light beam through a first hole formed between opposing surfaces of the rotor; a second detector that detects the passage of a light beam through a second hole formed between the opposing surfaces of the rotor; and a phase identifying component that identifies the occurrence of a first operational phase of the machine based upon the detection of light passing through the first hole and identifies the occurrence of a second operational phase of the machine based upon the detection of light passing through the second hole. The first hole has a different radial distance from the rotational axis of the rotor than does the second hole. 
     Still further, the objects of the invention may be achieved, in whole or in part, by a method for identifying operational phases of an electrical machine. The method includes detecting the passage of a light beam through a first hole formed between opposing surfaces of a rotor of the electrical machine; identifying the occurrence of a first operational phase of the machine based upon the detection of light passing through the first hole; detecting the passage of a light beam through a second hole formed between the opposing surfaces of the rotor; and identifying the occurrence of a second operational phase of the machine based upon the detection of light passing through the second hole. The first hole has a different radial distance from the rotational axis of the rotor than does the second hole. 
    
    
     
       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 related-art switched reluctance machine (SRM) having long rotor poles and a narrow back iron; 
         FIG. 2  illustrates an SRM having a reduced flux path length for reducing core losses; 
         FIG. 3  illustrates the SRM of  FIG. 1  and the flux paths generated by an excitation pole A 1  when a phase A of the SRM is excited; 
         FIG. 4  illustrates the SRM of  FIG. 2  and the flux paths generated by an excitation pole A 1  when a phase A of the SRM is excited; 
         FIG. 5  illustrates the rotor of the SRM from  FIG. 2  in greater detail; 
         FIG. 6  illustrates an SRM having permanent magnets installed on the faces of common poles; and 
         FIG. 7  illustrates components of a system for detecting the position of an SRM rotor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  illustrates a switched reluctance machine (SRM)  200  having a reduced flux path length for reducing core losses. More specifically, the length of rotor poles  202  within SRM  200  are shorter than rotor poles  102  within related-art SRM  100 . 
       FIG. 3  illustrates SRM  100  of  FIG. 1  and the flux paths generated by an excitation pole A 1  when a phase A of SRM  100  is excited. Similarly,  FIG. 4  illustrates SRM  200  of  FIG. 2  and the flux paths generated by an excitation pole A 1  when a phase A of SRM  200  is excited. In each of SRMs  100  and  200 , the generated flux is conveyed through excitation pole A 1 , crosses the air gap between excitation pole A 1  and a rotor pole R 1 , and divides into two pathways within rotor pole R 1 . One flux pathway travels through the rotor back iron between rotor poles R 1  and R 2  and the other through the rotor back iron between rotor poles R 1  and R 3 . The flux traveling through the rotor back iron between rotor poles R 1  and R 2  is conveyed through rotor pole R 2 , across the air gap between rotor pole R 2  and a stator pole A 2 , and returns to excitation pole A 1  through stator back iron  106 . Similarly, the flux traveling through the rotor back iron between rotor poles R 1  and R 3  is conveyed through rotor pole R 3 , across the air gap between rotor pole R 3  and a stator pole A 3 , and returns to excitation pole A 1  through stator back iron  106 . 
     Although there is no significant difference in the stator flux paths for SRMs  100  and  200 , a significant difference exists in their rotor flux paths because the length of rotor pole  202  within SRM  200  is shorter than that of rotor pole  102  within SRM  100 . As may be determined by inspection of  FIG. 4 , flux path  410  passes through each of rotor poles R 1  and R 2  and flux path  412  passes through each of rotor poles R 1  and R 3 . Because each of rotor poles R 1 , R 2 , and R 3  within SRM  200  is shorter than its counterpart within SRM  100 , the total rotor flux path length for each of flux paths  410  and  412  is considerably reduced even though the flux path length through the rotor back iron of SRM  200  is greater than that of SRM  100 . Thus, assuming that the rotors of SRMs  100  and  200  are capable of conveying the same amount of flux using the same type of material, the rotor of SRM  200  can do so with less material than that of SRM  100 . The lesser amount of material reduces the weight and core losses of the rotor of SRM  200  with respect to that of SRM  100 . 
       FIG. 5  illustrates the rotor of SRM  200  in greater detail. Rotor  500  has shaped air slots  504 ,  506  in back iron  502  of the rotor material to inhibit the flow of flux through the portions of back iron  502  having, or obstructed by, air slots  504 ,  506 . Shaped air slots  504 ,  506  may have various shapes, such as the shape of air slot  504  and that of air slot  506 . Shaped air slots  504  form a first layer of air slots within rotor  500  and shaped air slots  506  form a second layer of air slots; the first layer of air slots being disposed closer to the rotational axis of rotor  500  than the second layer. Fewer or more layers may be used. Because air slots  504 ,  506  inhibit the flow of flux, the flux flowing between two rotor poles, such as R 1  and R 2  or R 1  and R 3 , is induced to pass through a shorter path of less resistance (i.e., reluctance), within back iron  508 , that is close to rotor poles  202 . The absence of material in shaped air slots  504 ,  506  decreases the weight and inertia of rotor  500 , which helps increase the acceleration of the machine and contributes to a faster speed-loop bandwidth of the machine. 
     The reduction in flux-path length between SRM  200  and SRM  100  is derived as follows. Rotor pole R 1  has height h rp , pole are w rp , outer radius r 1 , and rotor pole pitch θ rp . Because the flow of flux through rotor pole R 1  is divided in two between two paths  410 ,  412 , as the flux flows through rotor back iron  508 , the thickness of rotor back iron  508  needs only be half the arc of rotor pole R 1 . Thus, the mean length of the rotor flux path is: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     Assuming SRMs  100  and  200  have equal values of w rp , r 1 , and θ rp , the difference between their mean rotor flux-path lengths is:
 
 l   fc   −l   fn   =h   rpc [2−θ rp   ]+h   rpn [θ rp −2]  eq. (2)
 
     where l fc  is the mean flux-path length for SRM  100 , l fn  is the mean flux-path length for SRM  200 , h rpc  is the height of rotor pole  102 , and h rpn  is the height of rotor pole  202 .
 
Let:  h   rpc   &gt;h   rpn  
 
 h   rpc =(1 +k ) h   rpn   eq. (3)
 
     and the difference in mean length of the rotor flux paths is:
 
Δ l   f   =l   fc   −l   fn =(1 +k ) h   rpn {2−θ rp   }+h   rpn {θ rp −2}= kh   rpn [2−θ rp ]  eq. (4)
 
     θ rp &lt;1, typically ¼ or ⅓, and the difference in rotor pole heights between SRMs  100  and  200  is:
 
Δ h   rp   =h   rpc   −h   rpn   eq. (5)
 
Therefore, Δ l   f =[2−θ rp   ]Δh   rp   eq. (6)
 
Therefore, Δ l   f ≈2(Δ h   rp )}
 
     
       
         
           
             
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     where Δl f  is the change in the flux path length and is proportional to the amount of material that is not experiencing flux changes and flux. Because SRM  200  has a smaller flux path length, its core losses and excitation requirement are lower. 
     The ratio of SRM  200 &#39;s rotor flux-path reduction, calculated above, to the flux-path length within SRM  100  may be expressed through the equations:
 
 w   rp =( k   2 θ rp ) r   1  
 
0.3 &lt;k   2 &lt;0.55
 
     
       
         
           
             
               
                 
                   
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     Suppose, for example, that 
     r 1 =5 cm; k 2 =0.55 
     h rpn =1 cm 
     Δh rp =1.72 cm 
     θ rp =36°=0.63 rad 
     w rp =k 2 r 1 θ=1.72 cm 
     
       
         
           
             
               
                 Δ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   l 
                   f 
                 
               
               
                 l 
                 c 
               
             
             = 
             
               0.2594 
               . 
             
           
         
       
     
     This example indicates a 25.94% reduction in SRM  200 &#39;s mean rotor flux-path length, with a concomitant reduction of core loss in the rotor laminations. Core losses, for high-speed or high-efficiency machines, may be as much as 6 to 8% of the input power. 
       FIG. 6  illustrates a stator  600  having permanent magnets  602  installed on the faces of common poles  604 . Stator  600  may also have additional or alternative magnets (not shown) inserted into back iron  606 . Permanent magnets  602  have polarities ‘N’ for the north pole and ‘S’ for the south pole. Employing rotor  500  with stator  600 , so as to reduce core loss and weight of an SRM, does not affect the operation of such an SRM. 
     Air slots  504 ,  506  within rotor  500  can also be used for sensing the position of rotor  500 , without a physical rotary position sensor, such as an encoder.  FIG. 7  illustrates components of a system for detecting the position of an SRM rotor. A first light emitting diode (LED)  702  is disposed on one side of the rotor stack of rotor  500  so as to pass a beam of light  710  through a rotor air-slot hole  506  in the second air-slot layer. A second LED  704  is similarly disposed so as to pass a beam of light  712  through another air-slot hole  504  in the first air-slot layer. Alternatively, a single LED may pass light through air slots of both air-slot layers. A first light detector  706 , disposed on the other side of rotor  500 , detects light beam  710  as it passes through air slot  506 , and a second light detector  708 , similarly disposed, detects light beam  712  as it passes through air slot  504 . 
     Light detectors  706 ,  708  may each be a two state logic device, which sets one logic level when it detects light through the hole. When light does not pass through an air slot within the rotor lamination, due to the obstruction of the light beam by the rotor lamination, a zero signal is obtained. 
     For a two-phase SRM, first LED  702  is placed so that it corresponds to the active operation of one phase of the SRM, such as phase A, and second LED  704  is placed to correspond with the active operation of the other phase, such as phase B. Similarly, other ways of obtaining the active-operation signals can be easily derived, using the basic principle illustrated here, for use in controlling the operation of the SRM. The control signals may also be obtained with the help of an encoder or a pulse modulation based sensor. 
     The above-described rotor may be made of a ferromagnetic material, may have any number of rotor poles, and may be used within any SRM having any kind of stator and any number and configuration of stator poles. The rotor may have one or more layers of air slots that are placed below the rotor poles, within the back iron, and may be closer to the rotor shall than to the rotor poles. The air slots may have any shape or form and should have dimensions that leave enough rotor back iron for flux flow, without saturating the rotor laminations. The air slots should be configured so as not to affect the structural integrity of the rotor body. In one embodiment, the width of the rotor back iron existing between air slots  504 ,  506  and the lower extent of a rotor pole is at least half the rotor pole arc. The air slots may be of regular shapes, such as rectangles, trapezoids, ellipses, or circles and may be laid in many layers around the rotor shaft, with any number of air slots existing within a particular layer. One air slot may be disposed directly under each rotor pole so as to prevent flux from flowing readily beyond a particular distance below the rotor pole. 
     An LED and an infrared light sensor may be disposed with respect to the above-described rotor so as to generate signals for activating and commutating current in machine phases of an SRM. The air slots may be placed in layers such that the number of layers corresponds to the number of machine phases, so that absolute rotor positions corresponding to current control instants of machine phase windings may be determined. The light sources and sensors may be hung from the stator and attached to the stator pole sides. One light source may be used for all layers of air slots with multiple light sensors detecting the light passed by the flux barriers. The light sensors can be displaced from each other so that the phase shift between phases A and B can be obtained to energize the respective phases; this configuration for obtaining the phase shift has the benefit of requiring only one layer of flux barriers. Alternately, the physical phase shift between the air slots of two layers can be exploited, by having two light sensors and two light sources aligned with them, so that the phase shift between the two machine phases is obtained for energizing the respective phases. 
     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.