Abstract:
A rotor is adapted to be used in a reluctance motor configured to generate a magnetic field around the rotor to form magnetic circuits passing through the rotor to produce a driving force corresponding to a torque generated by changes in magnetic reluctance in the magnetic circuits. The rotor includes a first salient pole group and a second salient pole group. The first salient pole group includes a plurality of first salient poles configured and arranged to be energized simultaneously with the first salient poles being spaced apart from each other in a circumferential direction of the rotor. The second salient pole group includes a plurality of second salient poles configured and arranged to be energized simultaneously with the second salient poles being spaced apart from each other in the circumferential direction of the rotor. The first salient pole group is magnetically insulated from the second salient pole group.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to Japanese Patent Application No. 2007-019609, filed on Jan. 30, 2007. The entire disclosure of Japanese Patent Application No. 2007-019609 is hereby incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a rotor for a reluctance motor and a reluctance motor equipped with the rotor. 
         [0004]    2. Background Information 
         [0005]    It has been proposed to use reluctance motors as drive sources for vehicles because the reluctance motors have a simple structure that does not require the use of permanent magnets and the reluctance motors are capable of rotating at high speeds. However, it is known that when a reluctance motor is operated at a high rotational speed or with a large torque output, the amount of torque ripple is large and the actual amount of torque obtained is smaller than a target amount of torque. Consequently, when a reluctance motor is operated at a high rotational speed or with a large torque output, it is necessary to increase or decrease the current of each phase at a high speed. However, since the speed at which the current is increased or decreased depends on the power source voltage of the device used to drive the reluctance motor, it is not possible to increase the speed at which the current is increased and decreased. 
         [0006]    Thus, Japanese Laid-Open Patent Publication No. 2001-178092 discloses a conventional reluctance motor that can alleviate the problem of low torque output. More specifically, the conventional reluctance motor disclosed in this publication is configured to perform a so-called electrical angle advancing control in which electrical energizing of the coil is started at an electrical angle that is earlier (advanced) than a prescribed energizing timing in order to compensate for a wider interval during which the current increases or decreases. 
         [0007]    In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved reluctance motor rotor. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure. 
       SUMMARY OF THE INVENTION 
       [0008]    With the electrical angle advancing control as described in the above-mentioned publication, since the electrical energizing is started at an angle where the inductance is small, the current rises quickly and the torque ripple is reduced. However, such a conventional reluctance motor will have the following problems. 
         [0009]      FIG. 4  is a simplified schematic cross sectional view of a comparison example of a switched reluctance motor having a U-phase, a V-phase, and a W-phase showing the effects of a magnetic flux occurring at any given phase. In  FIG. 4 , a rotor  2  includes a plate member (hereinafter called a “back yoke”)  2   a  having eight salient poles  2 P 1  to  2 P 8  connected integrally thereto. A stator  3  includes twelve salient poles  3 P 1  to  3 P 12 . When the rotor  2  is positioned as shown in  FIG. 4  and an electric current energizes coils (not shown) installed on each of the salient poles  3 P 1 ,  3 P 4 ,  3 P 7 , and  3 P 10  of the stator  3 , magnetic fluxes are generated in the directions indicated with the arrows D 1  and D 2  such that a magnetic circuit is formed in which the magnetic flux enters, for example, from the salient pole  2 P 1  of the rotor  2 , passes through the back yoke  2   a,  and returns to the stator  3  from the salient pole  2 P 3 . As a result, a torque is generated in the direction indicated by the arrow D T  and the rotor  2  can be rotated in the direction of the arrow D T . 
         [0010]    However, in reality, as exemplified with the salient pole  3 P 1  positioned at twelve o&#39;clock in  FIG. 4 , a flux leakage path develops in the salient pole  2 P 8  adjacent the salient pole  2 P 1  of the rotor  2  and magnetic flux flows in the direction of the arrow D 3  toward the salient pole  3 P 12 , which is an even-numbered salient pole adjacent to the salient pole  3 P 1 . Similarly, a flux leakage path develops in the other salient pole  2 P 2  adjacent the salient pole  2 P 1  of the rotor  2  and magnetic flux flows in the direction of the arrow D 4  toward the odd-numbered salient pole  3 P 3 . 
         [0011]    The flux leakage paths cause a negative torque to develop which opposes the torque acting in the direction of the arrow D T  and the size of the negative torque increases as the electrical angle is advanced. As a result, the torque output declines. This phenomenon becomes particularly marked when the reluctance motor is operated at a high rotational speed or with a large torque output. Accordingly, the problem of the torque output being low when the reluctance motor is operated at a high rotational speed or with a large torque output remains. 
         [0012]    The present invention was conceived based on recognition of the problems described above. One object of the present invention is to provide a reluctance motor rotor and a reluctance motor equipped with the same that are configured to suppress the development of flux leakage paths that cause a negative torque to occur, and thereby improving the torque output of the reluctance motor. 
         [0013]    In order to achieve the above object of the present invention, a rotor is adapted to be used in a reluctance motor configured to generate a magnetic field around the rotor to form magnetic circuits passing through the rotor to produce a driving force corresponding to a torque generated by changes in magnetic reluctance in the magnetic circuits. The rotor includes a first salient pole group and a second salient pole group. The first salient pole group includes a plurality of first salient poles configured and arranged to be energized simultaneously with the first salient poles being spaced apart from each other in a circumferential direction of the rotor. The second salient pole group includes a plurality of second salient poles configured and arranged to be energized simultaneously with the second salient poles being spaced apart from each other in the circumferential direction of the rotor. The first salient pole group is magnetically insulated from the second salient pole group. 
         [0014]    These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses preferred embodiments of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    Referring now to the attached drawings which form a part of this original disclosure: 
           [0016]      FIG. 1A  is a simplified schematic front view of a rotor in accordance with a first embodiment of the present invention; 
           [0017]      FIG. 1B  is a simplified schematic right side elevational view of the rotor illustrated in  FIG. 1A  in accordance with the first embodiment of the present invention; 
           [0018]      FIG. 1C  is a simplified schematic perspective view of the rotor illustrated in  FIGS. 1A and 1B  in accordance with the first embodiment of the present invention; 
           [0019]      FIG. 1D  is a simplified schematic front view of a reluctance motor that includes a stator and the rotor illustrated in  FIGS. 1A to 1C  in accordance with the first embodiment of the present invention; 
           [0020]      FIG. 2A  is a simplified schematic front view of a rotor in accordance with a second embodiment of the present invention; 
           [0021]      FIG. 2B  is a simplified schematic right side elevational view of the rotor illustrated in  FIG. 2A  in accordance with the second embodiment of the present invention; 
           [0022]      FIG. 2C  is a simplified schematic perspective view of the rotor illustrated in  FIGS. 2A and 2B  in accordance with the second embodiment of the present invention; 
           [0023]      FIG. 3A  is a simplified schematic front view of a rotor in accordance with a third embodiment of the present invention; 
           [0024]      FIG. 3B  is a simplified schematic cross sectional view of the rotor illustrated in  FIG. 3A  as taken along a section line  3 B- 3 B of  FIG. 3A  in accordance with the third embodiment of the present invention; and 
           [0025]      FIG. 4  is a simplified schematic cross sectional view of a comparison example of a switched reluctance motor having a U-phase, a V-phase, and a W-phase showing the effects of a magnetic flux occurring at any given phase. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    Selected embodiments of the present invention will now be explained with reference to the drawings. It will be apparent to those skilled in the art from this disclosure that the following descriptions of the embodiments of the present invention are provided for illustration only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. 
         [0027]    Referring initially to  FIGS. 1A to 1C , a reluctance motor rotor  10  (hereinafter simply called “rotor”) and a reluctance motor equipped with the rotor  10  are explained in accordance with a first embodiment.  FIGS. 1A to 1C  are a simplified schematic front view, a simplified schematic right side view, and a simplified schematic perspective view, respectively, of the rotor  10  in accordance with the first embodiment. 
         [0028]    As shown in  FIGS. 1A to 1C , the rotor  10  includes a back yoke  11  having a front plate member  11 F and a back plate member  11 B attached together. In the first embodiment, the front and back plate members  11 F and  11 B preferably have an identical shape and structure. Both of the front and back plate members  11 F and  11 B are arranged to be fixedly coupled coaxially to a rotational shaft of a reluctance motor RM along a rotational axis O shown in  FIGS. 1A to 1D . 
         [0029]    In the first embodiment, the front plate member  11 F of the back yoke  11  includes four salient poles  12 F connected to the front plate member  11 F along the outer periphery thereof with the salient poles  12 F being spaced apart from each other in a circumferential direction of the rotor  10 . Also, the back plate member  11 B of the back yoke  11  includes four salient poles  12 B connected to the back plate member  11 B along the outer periphery thereof with the salient poles  12 B being spaced apart from each other in the circumferential direction of the rotor  10 . 
         [0030]    As shown in  FIGS. 1A and 1C , each of the salient poles  12 F of the front plate member  11 F has a proximal portion  12   a  that connects to the front plate member  11 F and an outer wall  12   b  that extends from the proximal portion  12   a  along the direction of the rotational axis O toward the back plate member  11 B. Likewise, each of the salient poles  12 B of the back plate member  11 B has the proximal portion  12   a  that connects to the back plate member  11 B and the outer wall  12   b  that extends from the proximal portion  12   a  along the direction of the rotational axis O toward the front plate member  11 F. 
         [0031]    As shown in  FIG. 1A , the back yoke  11  of the rotor  10  is formed by assembling the front and back plate members  11 F and  11 B together such that the respective salient poles  12 F and  12 B are disposed alternately in the circumferential direction of the rotor  10 . Thus, the front and back plate members  11 F and  11 B are assembled together so that the salient poles  12 F and  12 B do not overlap each other as viewed in the direction of the rotational axis O. In the first embodiment, the front and back plate members  11 F and  11 B are connected integrally together with a connecting member  13  being interposed therebetween such that a gap  14  (empty, open space) is formed between the front and back plate members  11 F and  11 B as shown in  FIGS. 1B  and  1 C. Moreover, in the first embodiment, it is preferable to provide a member made of a material M having a high magnetic reluctance, particularly a non-magnetic material M, in the gap  14 . In other words, the front plate member  11 F and the back plate member  11 B are assembled together so that the salient poles  12 F are magnetically insulated from the salient poles  12 B. 
         [0032]    The front plate member  11 F of the back yoke  11  is arranged such that the four salient poles  12 F are connected to the front plate member  11 F to form a salient pole group (first salient pole group) that are energized simultaneously. Thus, a magnetic circuit system (first magnetic circuit system) is formed by the salient poles  12 F of the front plate member  11 F as indicated with the arrows D in  FIG. 1A . The back plate member  11 B is also arranged such that the four salient poles  12 B are connected to the back plate member  11 B to form a salient pole group (second salient pole group) that are energized simultaneously. Thus, a magnetic circuit system (second magnetic circuit system) is formed by the salient poles  12 B. Since the salient poles  12 F are connected to the front plate member  11 F and the salient poles  12 B are connected to the back plate member  11 B, which is magnetically insulated from the front plate member  11 F, a flux leakage path between, for example, one of the salient poles  12 F and an adjacent one of the salient poles  12 B as indicated with broken-line arrows D in  FIG. 1A  is interrupted. Therefore, a short circuit can be prevented from occurring in the rotor  10 . 
         [0033]    In other words, although the rotor  10  has a total of eight salient poles  12 F and  12 B, four of them (i.e., the salient poles  12 F) are connected to the front plate member  11 F to form a group of salient poles that are energized simultaneously, and the other four of them (i.e., the salient poles  12 B) are connected to the back plate member  11 B to form another group of salient poles that are energized simultaneously. Thus, two separate magnetic circuit systems are formed in the back yoke  11  of the rotor  10 . Since the gap  14  or the material M having a high magnetic reluctance (preferably a non-magnetic material) is provided between the front and back plate members  11 F and  11 B to magnetically insulate the front and back plate members  11 F and  11 B, the effects of magnetic flux leakage are greatly reduced. For example, when the salient poles  12 F are energized, the resulting magnetic flux has a much smaller effect on the salient poles  12 B than it would if the gap  14  or the material M was not provided between the front and back plate members  11 F and  11 B. As a result, the formation of flux leakage paths in the salient poles  12 B when the salient poles  12 F are energized can be greatly reduced. Similarly, when the salient poles  12 B are energized, the formation of flux leakage paths in the salient poles  12 F can be greatly reduced. 
         [0034]    Therefore, as shown in  FIG. 1D , when the rotor  10  according to the first embodiment is installed in the reluctance motor RM having a stator  103  with twelve stator salient poles  103 P 1  to  103 P 12  (which is similar to the stator  3  of the comparison example shown in  FIG. 4 ), the development of flux leakage paths that cause a negative torque to occur can be suppressed when the reluctance motor is operated at a high rotational speed or with a large torque output. As a result, the torque output of the reluctance motor (i.e., the torque decline that would otherwise result from the negative torque) can be improved in an effective manner. 
         [0035]    Additionally, the back yoke  11  of the rotor  10  includes the front and back plate members  11 F and  11 B each having a plurality of the salient poles  12 F and  12 B, respectively, and the front and back plate members  11  F and  11  B are assembled together such that the respective salient poles  12 F and  12 B are disposed alternately in the circumferential direction of the rotor  10  and two separate magnetic circuit systems are formed in the rotor  10 . As a result, identical members can be used as the front and back plate members  11 F and  11 B. Thus, the cost of manufacturing can be reduced because common (the same) members are used for both the front and back plate members  11 F and  11 B of the back yoke  11 . 
         [0036]    It is possible for each of the salient poles  12 F and  12 B to include only the proximal portion  12   a.  However, in the first embodiment, each of the salient poles  12 F and  12 B also preferably includes the outer wall  12   b  as illustrated in  FIGS. 1A to 1C . As a result, larger magnetic circuits can be formed in the back yoke  11  of the rotor  10 . 
         [0037]    When the back yoke  11  of the rotor  10  in accordance with the first embodiment includes the front and back plate members  11 F and  11 B with the gap  14  being formed therebetween, it is necessary to connect the front and back plate members  11 F and  11 B of the back yoke  11  together with the connecting member  13 . In such a case, it is preferable to take into account that a flux leakage path could develop through the connecting member  13 . Thus, the connecting member  13  is preferably made of the material M having a high magnetic reluctance (particularly a non-magnetic material) to connect the back yokes  11 F and  11 B together. Therefore, the formation of flux leakage paths that cause negative torque to develop can be suppressed even further. 
       Second Embodiment 
       [0038]    Referring now to  FIGS. 2A to 2C , a rotor  20  in accordance with a second embodiment will now be explained. In view of the similarity between the first and second embodiments, the parts of the second embodiment that are identical to the parts of the first embodiment will be given the same reference numerals as the parts of the first embodiment. Moreover, the descriptions of the parts of the second embodiment that are identical to the parts of the first embodiment may be omitted for the sake of brevity. 
         [0039]      FIGS. 2A to 2C  are a simplified schematic front view, a simplified schematic right side view, and a simplified schematic perspective view, respectively, of the rotor  20  in accordance with the second embodiment. 
         [0040]    In the second embodiment, a back yoke  22  of the rotor  20  has three plate members including a pair of outer plate members  22   a  (first and third plate members) and an inner plate member  22   b  (second plate member). The outer plate members  22   a  share a common shape (i.e., the outer plate members  22   a  are shaped the same). The inner plate member  22   b  is disposed between the outer plate members  22   a  as shown in  FIGS. 2B and 2C . 
         [0041]    More specifically, each of the outer plate members  22   a  has four proximal portions  23   a  connected to the outer plate member  22   a  to be circumferentially arranged around a rotational axis O with the proximal portions  23   a  being spaced apart from each other. An outer wall  23   b  extends from each of the proximal portions  23   a  along the direction of the rotational axis O. The outer plate members  22   a  are coupled together so that the proximal portions  23   a  of each of the outer plate members  22   a  are aligned as viewed in the direction of the rotational axis O, and the outer walls  23   b  of the outer plate members  22   a  are connected to form a plurality of salient poles  23  (in this example, four salient poles  23  are formed) as shown in  FIGS. 2A to 2C . 
         [0042]    The inner plate member  22   b  has four salient poles  24  that are circumferentially arranged along the outer periphery of the inner plate member  22   b  around the rotational axis O with the salient poles  24  being spaced apart from each other t. As shown in FIGS.  2 A and  2 C, each of the salient poles  24  has a proximal portion  24   a  that connects to the inner plate member  22   b  and an outer wall  24   b  that extends from the proximal portion  24   a  along the direction of the rotational axis O towards both of the outer plate members  22   a.    
         [0043]    In the rotor  20  of the second embodiment, the back yoke  22  is formed by connecting the two outer plate members  22   a  together with the inner plate member  22   b  arranged therebetween such that the salient poles  23  of the outer plate members  22   a  and the salient poles  24  of the inner plate member  22   b,  respectively, are disposed alternately in the circumferential direction of the rotor  20 . Thus, the salient poles  23  and  24  are arranged so as not to overlap each other as viewed in the direction of the rotational axis O. Each of the outer plate members  22   a  is connected to the inner plate member  22   b  with the connecting members  13  being interposed therebetween such that the gap  14  is formed between each of the outer plate members  22   a  and the inner plate member  22   b  as shown in  FIGS. 2B and 2C . Similarly to the first embodiment, it is preferable to provide a member made of the material M having a high magnetic reluctance, particularly a non-magnetic material M, in the gap  14 . Thus, the inner plate member  22   b  is magnetically insulated from each of the outer plate members  22   a  in the second embodiment. 
         [0044]    The outer plate members  22   a  of the back yoke  22  in the second embodiment are arranged such that the four salient poles  23  form a salient pole group (first salient pole group) that are energized simultaneously and form a magnetic circuit system (first magnetic circuit system), as indicated with the arrows D in  FIG. 2A . The inner plate member  22   b  is also arranged such that the four salient poles  24  form a salient pole group (second salient pole group) that are energized simultaneously and form a magnetic circuit system (second magnetic circuit system). Since the salient poles  23  and the salient poles  24  each connect to a different plate member (i.e., the outer plate members  22   a  or the inner plate member  22   b ), a flux leakage path between, for example, one of the salient poles  23  and an adjacent one of the salient poles  24  as shown in the broken-line arrows in  FIG. 2A  is interrupted. Thus, a short circuit is prevented from occurring in the rotor  20 . 
         [0045]    In other words, although the rotor  20  has a total of eight salient poles  23  and  24 , four of them (i.e., the salient poles  23 ) are connected to the outer plate members  22   a  to form a group of salient poles that are energized simultaneously, and the other four of them (i.e., the salient poles  24 ) are connected to the inner plate member  22   b  to form another group of salient poles that are energized simultaneously. Thus, two separate magnetic circuit systems are formed in the back yoke  22  of the rotor  20 . Since the gap  14  or the material M having a high magnetic reluctance (preferably a non-magnetic material) is provided between each of the outer plate members  22   a  and the inner plate member  22   b,  the effects of magnetic flux leakage are greatly reduced. For example, when the salient poles  23  are energized, the resulting magnetic flux has a much smaller effect on the salient poles  24  than it would if the gap  14  or the material M was not provided. As a result, the formation of flux leakage paths in the salient poles  24  when the salient poles  23  are energized can be greatly reduced. Similarly, when the salient poles  24  are energized, the formation of flux leakage paths in the salient poles  23  can be greatly reduced. 
         [0046]    Therefore, when the rotor  20  according to the second embodiment is used in a reluctance motor having the stator  103  in the similar manner in the first embodiment as shown in  FIG. 1D , the development of flux leakage paths that cause a negative torque to occur can be suppressed when the reluctance motor is operated at a high rotational speed or with a large torque output. As a result, the torque output of the reluctance motor (i.e., the torque decline that would otherwise result from the negative torque) can be improved in an effective manner. 
         [0047]    Moreover, the rotor  20  includes three plate members (i.e., the outer plate members  22   a  and the inner plate member  22   b ) to form a plurality of salient poles  23  and  24 . Two of the plate members (i.e., the outer plate members  22   a ) are connected together such that the proximal portions  12   a  and the outer walls  12   b  thereof are aligned with one another as viewed in the direction of the rotational axis O to form the salient poles  23 . The inner plate member  22   b  is disposed between the outer plate members  22   a  such that adjacent salient poles  23  and  24  of the outer plate members  22   a  and the inner plate member  22   b  are disposed alternately in the circumferential direction of the rotor  20 . With such an arrangement, the overall structural strength of the rotor  20  in accordance with the second embodiment can be further increased. However, it will be apparent to those skilled in the art from this disclosure that the basic idea of the rotor  20  in accordance with the second embodiment can be realized so long as the rotor  20  comprises three or more plate members each having a plurality of salient poles and the plate members are assembled together such that circumferentially adjacent salient poles of each of the plate members are disposed alternately in the circumferential direction of the rotor  20 . In such a case, even if the salient poles are provided in a cantilevered structure, the overall structural strength of the rotor can be increased because the lengths of the salient poles can be shortened. 
       Third Embodiment 
       [0048]    Referring now to  FIGS. 3A and 3B , a rotor  30  in accordance with a third embodiment will now be explained. In view of the similarity between the first and third embodiments, the parts of the third embodiment that are identical to the parts of the first embodiment will be given the same reference numerals as the parts of the first embodiment. Moreover, the descriptions of the parts of the third embodiment that are identical to the parts of the first embodiment may be omitted for the sake of brevity. The parts of the third embodiment that differ from the parts of the first embodiment will be indicated with a single prime (′). 
         [0049]      FIG. 3A  is a simplified schematic front view of the rotor  30  in accordance with the third embodiment of the present invention.  FIG. 3B  is a cross sectional view of the rotor  30  taken along a section line  3 B- 3 B of  FIG. 3A . The rotor  30  of the third embodiment differs from the rotor  10  of the first embodiment in that a back yoke  11 ′ of the rotor  30  in the third embodiment includes front and back plate members  11 F′ and  11 B′ that have substantially square shapes in the front view as shown in  FIG. 3A , which is made up of straight lines and curves. Similarly to the first embodiment, the front and back plate members  11 F′ and  11 B′ are preferably coupled together via the connecting member  13  with the gap  14  or the material M being interposed therebetween so that the front and back plate members  11 F′ and  11 B′ are magnetically insulated from each other. 
         [0050]    As shown in  FIG. 3A , the front and back plate members  11 F′ and  11 B′ include a plurality of salient poles  12 F′ and a plurality of salient poles  12 B′, respectively. The front and back plate members  11 F′ and  11 B′ are assembled together such that the salient poles  12 F′ and the salient poles  12 B′ are disposed alternately in the circumferential direction of the rotor  30  as shown in  FIG. 3A . Each of the salient poles  12 F′ and  12 B′ includes a proximal portion  12   a ′ and an outer wall portion  12   b ′ extending from the proximal portion  12   a′.    
         [0051]    Moreover, in the third embodiment, each of the proximal portions  12   a ′ of the salient poles  12 F′ is arranged to gradually widen in the circumferential direction of the rotor  30  as one moves closer to the rotational axis O as shown in  FIG. 3A . More specifically, as shown in the rightward salient pole  12 F′ in  FIG. 3A  as an example, both side surfaces fr of the proximal portion  12   a ′ are slanted generally along the circumferential direction of the rotor  30  so that a distance between the side surfaces fr becomes larger as one moves closer to the rotational axis O. 
         [0052]    As in the comparison example shown in  FIG. 4 , when the reluctance motor rotor has an even number of salient poles  2 P 1  to  2 P 8  that are divided into one group of evenly numbered salient poles  2 P 2 ,  2 P 4 ,  2 P 6 , and  2 P 8  and another group of oddly numbered salient poles  2 P 1 ,  2 P 3 ,  2 P 5 , and  2 P 7 , magnetic saturation tends to occur at the proximal portions of the salient poles  2 P 1  to  2 P 8 . An example of the proximal portion of one of the salient poles  2 P 1  to  2 P 8  is indicated with an area B in  FIG. 4 . 
         [0053]    However, in the rotor  30  of the third embodiment, this magnetic saturation can be prevented by making the external shape of each of the salient poles  12 F′ of the front plate members  11 F′ such that the proximal portion  12   a ′ of the salient poles  12 F′ gradually widens in the circumferential direction of the rotor  30 , thereby increasing the cross sectional area of the proximal portion  12   a ′ of each of the salient poles  12 F′. 
         [0054]    More specifically, in the rotor  30  of this invention, the proximal portion  12   a ′ of each of the salient poles  12 F′ has the two side surfaces fr configured such that the proximal portion  12   a ′ has the shape that gradually widens in a circumferential direction of the rotational axis O. Consequently, the cross section of the salient pole  12 F′ as taken along a plane perpendicular to the rotational axis O is larger at the proximal portion  12   a ′ and magnetic saturation can be prevented at the proximal portion  12   a′.    
         [0055]    Additionally, as shown in  FIG. 3B , each of the salient poles  12 B′ of the back plate member  11 B′ has an axially oriented surface fs so that the proximal portion  12   b ′ of the salient pole  12 B′ has a shape that widens gradually in the direction of the rotational axis O as one moves away from the rotational axis O. On the other hand, as shown in  FIG. 3B , the front plate member  11 F′ includes inner edge portions  11   e  provided in positions circumferentially between adjacent ones of the salient poles  12 F′ so as to face opposite the salient poles  12 B′ of the back plate member  11 B′. Each of the inner edge portions  11   e  is slanted so as to be parallel to the axially oriented face fs of the respective salient pole  12 B′ to form an escape with respect to the external shape of the respective salient pole  12 B′. 
         [0056]    In other words, in the rotor  30  of the third embodiment, the shape of each of the salient poles  12 B′ of the back plate member  11 B′ is arranged such that the proximal portion  12   a ′ of the salient pole  12 B′ widens gradually in the direction of the rotational axis O, and the shape of each of the inner edge portions  11   e  of the front plate member  11 F′ is arranged to form an escape with respect to the external shapes of the respective salient poles  12 B′ in order to prevent an interference between the front plate member  11 F′ and the back plate member  11 B′. As a result, the strength of the cantilevered structure of the salient poles  12 B′ can be improved. 
         [0057]    Although, in the rotor  30  of the third embodiment, the shapes of the salient poles  12 F′ of the front plate member  11 F′ are different from the shapes of the salient poles  12 B′ of the back plate member  11 B′, it is also acceptable to form the front and back plate members  11 F′ and  11 B′ to have the same shape as is done in the first embodiment. 
         [0058]    In the rotor  30  in accordance with the third embodiment, the salient poles  12 F′ and  12 B′ of the rotor  30  are connected into separate groups of salient poles (i.e., the salient poles  12 F′ and the salient poles  12 B′) that are energized simultaneously so as to form different magnetic circuit systems (first and second magnetic circuit systems) in the rotor  30 . Moreover, the gap  14  or the material M having a high magnetic reluctance is disposed between the magnetic circuit systems formed by the front plate member  11 F′ and the back plate member  11 B′. The material M is preferably a non-magnetic material. With the third embodiment, when a particular salient pole group (i.e., the salient poles  12 F′ or the salient poles  12 B′) is energized, the influence of the magnetic flux on the other salient pole group can be greatly reduced. As a result, the formation of leakage flux paths in the other salient pole group can be greatly reduced. 
         [0059]    Therefore, when the rotor  30  according to the third embodiment is used in a reluctance motor having the stator  103  in a similar manner as shown in  FIG. 1D , the development of flux leakage paths that cause a negative torque to occur can be suppressed when the reluctance motor is operated at a high rotational speed or with a large torque output. As a result, the torque output of the reluctance motor (i.e., the torque decline that would otherwise result from the negative torque) can be improved in an effective manner. 
         [0060]    Moreover, the reluctance motor provided with one of the rotors  10 ,  20  and  30  in accordance with the first to third embodiments is preferably provided with the stator  3  having the salient poles  3 P 1  to  3 P 12  as exemplified in  FIG. 4 . The rotors  10 ,  20  or  30  and the stator  3  are preferably arranged such that the ratio of the number of the salient poles of the rotor  10 ,  20  or  30  and the number of salient poles of the stator  3  is 2:3. Consequently, a reluctance motor can be obtained which has superior output efficiency and exhibits the excellent effects described above. 
         [0061]    In the first to third embodiments, each of the rotors  10 ,  20  and  30  is explained to have an even number of salient poles with the salient poles being separated into a group of evenly numbered salient poles and a group of oddly numbered salient poles that are disposed alternately in the circumferential direction of the rotor  10 ,  20  or  30 , each group of salient poles being arranged to be magnetically energized separately. However, it will be apparent to those skilled in the art from this disclosure that the number of salient poles and the method of exciting the salient poles can be varied from the illustrated embodiments. Furthermore, the various constituent features of the illustrated embodiments can be combined and interchanged as necessary in accordance with the particular objective and application at hand. 
       General Interpretation of Terms 
       [0062]    In understanding the scope of the present invention, the term “comprising” and its derivatives, as used herein, are intended to be open ended terms that specify the presence of the stated features, elements, components, groups, integers, and/or steps, but do not exclude the presence of other unstated features, elements, components, groups, integers and/or steps. The foregoing also applies to words having similar meanings such as the terms, “including”, “having” and their derivatives. Also, the terms “part,” “section,” “portion,” “member” or “element” when used in the singular can have the dual meaning of a single part or a plurality of parts. 
         [0063]    While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. For example, the size, shape, location or orientation of the various components can be changed as needed and/or desired. Components that are shown directly connected or contacting each other can have intermediate structures disposed between them. The functions of one element can be performed by two, and vice versa. The structures and functions of one embodiment can be adopted in another embodiment. It is not necessary for all advantages to be present in a particular embodiment at the same time. Every feature which is unique from the prior art, alone or in combination with other features, also should be considered a separate description of further inventions by the applicant, including the structural and/or functional concepts embodied by such feature(s). Thus, the foregoing descriptions of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.