Patent Publication Number: US-2022216751-A1

Title: Rotor for rotating electric machine

Description:
This application claims priority from Japanese Patent Application No. 2021-001635 filed on Jan. 7, 2021, the disclosure of which is hereby incorporated by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a rotor that is to be provided in a rotating electric machine, and particularly relates to techniques for suitably determining a position of a void portion provided in a rotor core of the rotor. 
     BACKGROUND OF THE INVENTION 
     There is known a rotor which is to be provided in a rotating electric machine, and which includes a cylindrical-shaped rotor core provided with a pair of flux barriers and a void portion. The pair of flux barriers have respective magnet receiving portions in which a pair of magnets are provided. The pair of flux barriers are disposed on respective opposite sides, in a circumferential direction of the rotor core, of a q-axis extending in a radial direction of the rotor core, symmetrically with each other with respect to the q-axis. The void portion is disposed on the q-axis along which a stator magnetic flux is caused to pass. A rotor provided in a rotating electric machine disclosed in JP-6042976-B2 is an example of such a rotor. In the rotor in this Japanese Patent Publication, a pair of magnets disposed on respective opposite sides of the q-axis are arranged to form an inverted-V shape, so that a distance between the pair of magnets and a magnetic resistance therebetween are increased in a direction away from an outer peripheral side of a rotor core of the rotor toward an inner peripheral side of the rotor core. Therefore, it is regarded that almost no magnetic field line passes on the inner peripheral side than an arc connecting between intermediate portions of the pair of magnets, so that the void portion is located on the inner peripheral side than the arc (see FIG. 2 of JP-6042976-B2). 
     SUMMARY OF THE INVENTION 
     However, in the above-described rotor in the above-identified Japanese Patent Publication, the void portion is on a straight line connecting between inner-peripheral-side end portions of the pair of magnets between which magnetic field lines is caused to pass, so that the passage of a magnet magnetic flux is impeded or obstructed by the void portion whereby a motor torque of the rotating electric machine could be reduced due to reduction of an amount of the magnetic flux, and a motor efficiency of the rotating electric machine could be reduced due to increase of a core loss caused by increase of a density of the magnetic flux. If the void portion is located in a position far from the magnets, there could be no risk of obstruction to the passage of the magnet magnetic flux. However, the location of the void portion in such a position far from the magnets reduces performance for cooling the magnets, and accordingly reduces the motor torque due to demagnetization caused by excessive heating of the magnets. 
     The present invention was made in view of the background art described above. It is therefore an object of the present invention to make it possible to dispose the void portion in a position close as much as possible to the magnets while suppressing obstruction to passage of the magnet magnetic flux. 
     The object indicated above is achieved according to the following aspects of the present invention. 
     According to a first aspect of the invention, there is provided a rotor that is to be provided in a rotating electric machine, the rotor comprising: a cylindrical-shaped rotor core provided with a plurality of flux barriers including a pair of flux barriers that are disposed on respective opposite sides, in a circumferential direction of the rotor core, of a q-axis extending in a radial direction of the rotor core, wherein a pair of magnets are provided in the pair of flux barriers, such that a distance between the pair of magnets on a perpendicular plane perpendicular to an axis of the rotor core is increased in a direction away from an outer peripheral side of the rotor core toward an inner peripheral side of the rotor core, and such that each of the pair of magnets has a rectangular shape in a cross section parallel to the perpendicular plane, wherein a void portion, which is provided in the rotor core, is disposed on the q-axis, and is located on the inner peripheral side than an imaginary line connecting between inner-peripheral-side corners of the pair of magnets on the perpendicular plane, each of the inner-peripheral-side corners being one of corners of a corresponding one of the pair of magnets which is located on the inner peripheral side than the others of the corners of the corresponding one of the pair of magnets, wherein a first width is defined by a shortest distance between the pair of flux barriers on the perpendicular plane, wherein a second width is defined by a distance from the void portion to an intersection point at which the imaginary line intersects with a shortest line segment whose length corresponds to a shortest distance between one of the pair of magnets and the void portion on the perpendicular plane, and wherein the second width is smaller than the first width. It is noted that the above-describe feature that “the void portion is located on the inner peripheral side than an imaginary line” may be paraphrased as “the void portion is located on an inner side of an imaginary line in the radial direction”, and the above-described feature that “one of corners of a corresponding one of the pair of magnets is located on the inner peripheral side than the others of the corners of the corresponding one of the pair of magnets” may be paraphrased as “one of corners of a corresponding one of the pair of magnets is located on an inner side of the others of the corners of the corresponding one of the pair of magnets in the radial direction”. 
     According to a preferred arrangement of the first aspect of the invention, the pair of flux barriers are disposed symmetrically with each other with respect to the q-axis, and include respective magnet receiving portions in which the pair of magnets are received to be disposed symmetrically with each other with respect to the q-axis and arranged to have respective attitudes that form an inverted-V shape that opens toward the inner peripheral side of the rotor core, wherein the imaginary line is a straight line or a curved line convexed toward the inner peripheral side of the rotor core, and defines a boundary of a magnet magnetic flux of the pair of magnets, wherein the first width is a q-axis salient pole width through which a stator magnetic flux is allowed to pass along the q-axis, and wherein the second width is a q-axis magnetic-flux passage width through which the stator magnetic flux is allowed to pass in a region between the one of the pair of magnets and the void portion. 
     According to a second aspect of the invention, in the rotor according to the first aspect of the invention, wherein a pair of second magnets, in addition to the pair of magnets as a pair of first magnets, are provided in the pair of flux barriers, symmetrically with each other with respect to the q-axis, such that the pair of second magnets are located on the outer peripheral side than the pair of first magnets, wherein each of the pair of second magnets has a rectangular shape in a cross section parallel to the perpendicular plane, such that a distance between the pair of second magnets on the perpendicular plane perpendicular to the axis of the rotor core is increased in the direction away from the outer peripheral side of the rotor core toward the inner peripheral side of the rotor core, and such that the pair of second magnets each having the rectangular shape in the cross section have respective sides whose extensions intersect with each other, and wherein the imaginary line is an arc having a center of curvature that lies at an intersection point of the extensions of the sides of the pair of second magnets and passing through the inner-peripheral-side corners of the pair of first magnets. 
     According to a third aspect of the invention, in the rotor according to the first or second aspect of the invention, the imaginary line is a straight line connecting between the inner-peripheral-side corners of the pair of magnets. 
     According to a fourth aspect of the invention, in the rotor according to any one of the first through third aspects of the invention, the plurality of flux barriers provided in the rotor core form a plurality of V-shaped arrangement patterns in each of which at least two of the plurality of flux barriers are arranged to have a V shape or U shape that opens toward the outer peripheral side of the rotor core, such that the plurality of V-shaped arrangement patterns are arranged at a constant angular interval around the axis, wherein the q-axis is an intermediate line between each adjacent two of the plurality of V-shaped arrangement patterns, which are adjacent to each other in the circumferential direction of the rotor core, and wherein the pair of flux barriers are two of the plurality of flux barriers, such that the two of the plurality of flux barriers are disposed on the respective opposite sides of the q-axis in the circumferential direction, and such that the two of the plurality of flux barriers are closer to the q-axis, than the others of the plurality of flux barriers, in the circumferential direction. 
     According to a fifth aspect of the invention, in the rotor according to any one of the first through fourth aspects of the invention, the second width is not smaller than a half of the first width. 
     In the above-described rotor that is to be provided in the rotating electric machine, the void portion is located on the inner peripheral side than the imaginary line connecting between the inner-peripheral-side corners of the pair of magnets that are disposed on the respective opposite sides of the q-axis, so that it is possible to suppress the void portion from obstructing passage of the magnet magnetic flux and accordingly to suppress reductions of a motor torque and a motor efficiency of the rotating electric machine due to provision of the void portion. Further, the second width is smaller than the first width, wherein the first width is defined by the shortest distance between the pair of flux barriers that are disposed on the respective opposite sides of the q-axis, and wherein the second width is defined by the distance from the void portion to the intersection point at which the imaginary line intersects with the shortest line segment corresponding to the shortest distance between the one of the magnets and the void portion. Thus, the void portion is provided in a position close as much as possible to the pair of magnets while assuring an cross sectional area through which the magnet magnetic flux and the stator magnetic flux are allowed to be pass, so that it is possible to appropriately obtain performance for cooling the pair of magnets owing to the void portion, and accordingly to suppress reduction of a motor torque of the rotating electric machine which could be caused by demagnetization of the magnets due to excessive heating of the magnets. Since the stator magnetic flux having passed through the q-axis salient pole width Wq 1  is divided by the void portion into two directions, it is possible to assure the cross sectional area through which the stator magnetic flux is to pass, although the second width is smaller than the first width. It is noted that the first width may be referred to as an q-axis salient pole width through which the stator magnetic flux is allowed to pass along the q-axis, and that the second width may be referred to as q-axis magnetic-flux passage width through which the stator magnetic flux is allowed to pass between the one of the magnets and the void portion. 
     In the rotor according to the second aspect of the invention, the imaginary line is the arc having the center of curvature that lies at the intersection point of the extensions of the sides of the pair of second magnets (that are disposed on the outer peripheral side than the pair of first magnets) and passing through the inner-peripheral-side corners of the pair of first magnets. Thus, the imaginary line is close to a boundary line defining an actual boundary of the magnet magnetic flux of the pair of first magnets, so that it is possible to appropriately obtain the effect of the first aspect of the invention. 
     In the rotor according to the third aspect of the invention, the imaginary line is the straight line connecting between the inner-peripheral-side corners of the pair of magnets, so that the void portion is located on the inner peripheral side than the straight line as the imaginary line. It is possible to appropriately obtain the effect of the first aspect of the invention, since a degree of obstruction to the magnet magnetic flux can be made smaller than in the arrangement disclosed in the above-identified Japanese Patent Publication (JP-6042976-B2) in which the void portion is located on the straight line connecting between the inner-peripheral-side corners. The rotor according to this third aspect of the invention corresponds to an arrangement of the second aspect of the invention in which the intersection point of the extensions of the sides of the pair of second magnets is located in a position that is substantially infinitely far, for example, from the inner-peripheral-side corners of the pair of first magnets. 
     In the rotor according to the fourth aspect of the invention, the plurality of flux barriers form the plurality of V-shaped arrangement patterns in each of which at least two of the plurality of flux barriers are arranged to have the V shape or U shape, such that the plurality of V-shaped arrangement patterns are arranged around the axis, wherein the q-axis is the intermediate line between each adjacent two of the plurality of V-shaped arrangement patterns, which are adjacent to each other in the circumferential direction of the rotor core, and wherein the pair of flux barriers are two of the plurality of flux barriers, such that the two of the plurality of flux barriers are disposed on the respective opposite sides of the q-axis in the circumferential direction, and such that the two of the plurality of flux barriers are closer to the q-axis, than the others of the plurality of flux barriers, in the circumferential direction. The pair of flux barriers are arranged to have respective attitudes that cooperate with each other to form an inverted-V shape. The present invention is advantageously applied to these arrangements. 
     In the rotor according to the fifth aspect of the invention, the second width is smaller than the first width and is not smaller than the half of the first width, so that it is possible to appropriately obtain performance for cooling the pair of magnets owing to the void portion that is disposed in a position close to the pair of magnets, and also to appropriately assure the cross sectional area through which the magnet magnetic flux and the stator magnetic flux are allowed to be pass. Since the stator magnetic flux having passed through the first width is divided by the void portion into two directions, it is possible to assure the cross sectional area through which the stator magnetic flux is to pass, owing to the arrangement in which the second width is smaller than the first width and is not smaller than the half of the first width. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view for explaining a rotating electric machine including a rotor that is constructed according to an embodiment of the present invention; 
         FIG. 2  is a view showing in enlargement a part of a cross section of the rotor taken in a plane perpendicular to an axis O of the rotor shown in  FIG. 1 , wherein the part corresponds to a quarter circle about the axis O; 
         FIG. 3  is a view showing in further enlargement a portion of the rotor of  FIG. 2 , wherein the portion is in the vicinity of a q-axis of the rotor; and 
         FIG. 4  is a view corresponding to the view of  FIG. 3  and showing another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The rotating electric machine is referred also to as a rotating machine, and may be an electric motor, an electric generator, or a motor generator that can be used selectively as the electric motor or the electric generator. The rotating electric machine is, for example, a permanent-magnet synchronous motor. Although being constituted preferably by a rare-earth element magnet, each of the magnets may be constituted by any of other permanent magnets. The plurality of flux barriers provided in the rotor core form the plurality of V-shaped arrangement patterns in each of which at least two of the plurality of flux barriers are arranged to have a V shape or U shape that opens toward the outer peripheral side of the rotor core, such that the plurality of V-shaped arrangement patterns are arranged at the constant angular interval around the axis. The q-axis is the intermediate line between each adjacent two of the plurality of V-shaped arrangement patterns, which are adjacent to each other in the circumferential direction of the rotor core. The pair of flux barriers are two of the plurality of flux barriers, such that the two of the plurality of flux barriers are disposed on the respective opposite sides of the q-axis in the circumferential direction, and such that the two of the plurality of flux barriers are closer to the q-axis, than the others of the plurality of flux barriers, in the circumferential direction. The pair of flux barriers are arranged to have respective attitudes that form an inverted-V shape such that a distance between the pair of magnets on the perpendicular plane perpendicular to the axis of the rotor core is increased in the direction away from the outer peripheral side of the rotor core the toward the inner peripheral side of the rotor core. The arrangement patterns of the plurality of flux barriers may be modified as needed. In each of the flux barriers, a corresponding one or at least two of the magnets is provided, for example, such that a void or free space is left in the each of the flux barriers. However, the corresponding one or at least two of the magnets may be embedded in the entirety of the each of the flux barriers. Further, it is possible to employ an arrangement in which each of the flux barriers is divided into a plurality of divided flux barriers such that the first and second magnets are provided in respective ones of the divided flux barriers. 
     Although being preferably a through-hole through which a cooling fluid is to be caused to pass, as needed, the void portion may be a lightening hole or recess, too. The imaginary line, which defines the boundary of the magnet magnetic flux of the pair of first magnets, is preferably the straight line connecting between the inner-peripheral-side corners of the pair of first magnets on the perpendicular plane, or the curved line or arc having the center of curvature that lies at the intersection point of the extensions of the sides of the pair of second magnets and passing through the inner-peripheral-side corners of the pair of first magnets. However, the imaginary line may be modified as needed. Where the imaginary line is constituted by the arc, the center of curvature of the arc does not necessarily have to lie at the intersection point of the extensions of the sides of the pair of second magnets but may be lie at an intersection point of extensions of sides of the pair of first magnets or lie on the outer peripheral side than the intersection point of extensions of sides of the pair of first magnets. An angle defined between the extensions of the sides of the pair of second magnets is smaller than an angle defined between the extensions of the sides of the pair of first magnets, but may be substantially equal to or larger than the angle defined between the extensions of the sides of the pair of first magnets. The provision of the pair of second magnets is not essential but may be omitted. It is preferable that the q-axis magnetic-flux passage width as the second width is smaller than the q-axis salient pole width as the first width, and is not smaller than the half of the q-axis salient pole width. However, the q-axis magnetic-flux passage width may be smaller than the half of the q-axis salient pole width. Each of the pair of first magnets and pair of second magnets, which has the rectangular shape in the cross section parallel to the perpendicular plane perpendicular to the axis of the rotor core, may be constituted, for example, by a plurality of magnet pieces which are arranged to be in close contact with each other and which cooperate with each other to form the rectangular shape in the above-described cross section. 
     EMBODIMENTS 
     Hereinafter, preferred embodiments of the invention will be described in detail with reference to the accompanying drawings. The figures of the drawings are simplified or deformed as needed, and each portion is not necessarily precisely depicted in terms of dimension ratio, shape, etc. 
       FIG. 1  is a schematic cross-sectional view for explaining a rotating electric machine  10  including a rotor  12  according to an embodiment of the present invention, taken in a plane containing an axis O of the rotating electric machine  10 .  FIG. 2  is a view showing in enlargement a part of a cross section of the rotor  12  taken in a plane perpendicular to the axis O, wherein the part corresponds to a quarter circle about the axis O (in an angular range of 90°). The rotating electric machine  10  is a permanent-magnet embedded type synchronous motor serving as a motor generator that can be used selectively as an electric motor or a generator. The rotating electric machine  10  is advantageously used as a drive power source of a battery electric vehicle such as a hybrid electric vehicle, for example. The rotating electric machine  10  includes the above-described rotor  12  and a stator  14  that are coaxial with the axis O. In the following description of the present embodiment, the axis O of the rotating electric machine  10  is also used to refer to an axis of the rotor  12 , an axis of the stator  14  and an axis of a rotor shaft  20 . The stator  14  includes a cylindrical tubular-shaped stator core  16  disposed on an outer peripheral side of the rotor  12 , and a plurality of stator coils  18  wound on the stator core  16 . The stator core  16  is constituted by a multiplicity of annular-shaped steel plates that are laminated in an axial direction, i.e., a direction parallel to the axis O, with each of the steel plates being in an attitude perpendicular to the axis O, and is fixed in a casing (not shown) through fixing means such as press-fitting and screw bolts. 
     The rotor  12  includes the above-described rotor shaft  20 , a cylindrical tubular-shaped rotor core  22  attached onto an outer circumferential surface of the rotor shaft  20 , and a multiplicity of magnets  24  embedded in the rotor core  22 . The rotor core  22  is constituted by a multiplicity of annular-shaped steel plates that are laminated in the axial direction, i.e., the direction parallel to the axis O, with each of the steel plates being in an attitude perpendicular to the axis O. A pair of end plates  30 ,  30  are provided on axially opposite sides of the rotor core  22 , and are fixed to the rotor shaft  22 . The rotor shaft  20  is provided with a flange portion  32 , so that the rotor core  22  is sandwiched or clamped between the flange portion  32  and a nut  34  that is in thread engagement with the rotor shaft  20 , whereby the rotor core  22  is fixed onto the rotor shaft  20 . Each of the magnets  24  is constituted by a rare-earth element magnet, and is covered with an insulation film, as needed. 
     As shown in  FIG. 2 , the rotor core  22  is provided with four kinds of magnet receiving holes in the form of flux barriers  40   a ,  40   b ,  40   c ,  40   d  (hereinafter referred simply to as flux barriers  40  unless they are to be distinguished from one another). Each of the flux barriers  40  is formed through the rotor core  22  in the axial direction, and includes a magnet receiving portion or portions in which the magnet or magnets  24  are received. The magnet or magnets  24  received in the magnet receiving portion or portions of each of the flux barriers  40  are fixed by adhesive or the like so as to have a certain attitude. As shown in  FIG. 2 , the magnets  24  are categorized into six kinds of magnets A, a, B, b, C, c. In the following description, each of the magnets  24  will be referred to as magnet A, a, B, b, C or c as needed. Each of the magnets A, a, B, b, C, c has a rectangular shape in its cross section parallel to a perpendicular plane that is perpendicular to the axis O, as shown in  FIG. 2 , and has a rectangular prism shape whose length is substantially equal to an axial length of the rotor core  22 . Each of the magnets A is fixed in a corresponding one of the flux barriers  40   a . Each of the magnets a is fixed in a corresponding one of the flux barriers  40   b . Each of the magnets B and each of the magnets b are fixed in a corresponding one of the flux barriers  40   c . Each of the magnets C and each of the magnets c are fixed in a corresponding one of the flux barriers  40   d . Each flux barrier  40   c  has a bent portion in a lengthwise intermediate portion in it cross section, and the corresponding magnet B and the corresponding magnet b are fixed on respective opposite sides of the bent portion. Similarly, each flux barrier  40   d  has a bent portion in a lengthwise intermediate portion in it cross section, and the corresponding magnet C and the corresponding magnet c are fixed on respective opposite sides of the bent portion. Further, each flux barrier  40  has voids or free spaces left on respective opposite sides of the corresponding magnet  24  (that is fixedly received in the each flux barrier  40 ), and the voids or free spaces can serve as passages through which a cooling fluid is to pass to cool the corresponding magnet  24 . 
     The four kinds of flux barriers  40   a ,  40   b ,  40   c ,  40   d  form a plurality of V-shaped arrangement patterns Pv (eight V-shaped arrangement patterns Pv in the present embodiment) in each of which the flux barriers  40   a ,  40   b ,  40   c ,  40   d  are arranged to have a V shape or U shape that opens toward the outer peripheral side of the rotor core  22 , such that the plurality of V-shaped arrangement patterns Pv are arranged at a constant angular interval around the axis O of the rotor core  22 . Each of the V-shaped arrangement patterns Pv has a shape symmetrical with respect to a d-axis that passes through the axis O. Each flux barrier  40   a  in which the corresponding magnet A is fixedly received and the corresponding flux barrier  40   b  in which the corresponding magnet a is fixedly received are symmetrical with each other with respect to the d-axis. Each flux barrier  40   c  in which the corresponding magnets B, b are fixedly received and the corresponding flux barrier  40   d  in which the corresponding magnets C, c are fixedly received are symmetrical with each other with respect to the d-axis. That is, in the present embodiment, each of the V-shaped arrangement patterns Pv has a double structure in which a pair of flux barriers  40   a ,  40   b  is provided on the outer peripheral side of another pair of flux barriers  40   c ,  40   d . It is noted the provision of the pair of flux barriers  40   a ,  40   b  is not essential so that it can be regraded that each of the V-shaped arrangement patterns Pv (having the V shape or U shape that opens toward the outer peripheral side of the rotor core  22 ) can be constituted by only the flux barriers  40   c  provided with the magnets B, b and the flux barriers  40   d  provided with the magnets C, c. 
     In the rotor  12  described above, an intermediate line between each adjacent two of the plurality of V-shaped arrangement patterns Pv, which are adjacent to each other in a circumferential direction of the rotor  12 , is a q-axis, so that a stator magnetic flux (magnetic flux of rotating magnetic field produced by the stator  14 ) is caused to pass through the rotor  12  along the q-axis when the rotor  12  is used as a component of the rotating electric machine  10 . On respective opposite sides of the q-axis in the circumferential direction, a pair of the flux barriers  40   c ,  40   d  are located symmetrically with each other with respect to the q-axis, such that the flux barriers  40   c ,  40   d  are closer to the q-axis, than the others flux barriers  40 , in the circumferential direction, and such that the pair of the flux barriers  40   c ,  40   d  cooperate with each other to form an inverted V shape that opens toward the inner peripheral side of the rotor core  22 . Further, the magnets B, b and the magnets C, c, which are fixedly received in the pair of the flux barriers  40   c ,  40   d , are located symmetrically on the opposite sides of the q-axis in the circumferential direction, and have respective long sides of the rectangular-shaped cross sections, wherein the long sides cooperate to form an inverted V shape that opens toward the inner peripheral side of the rotor core  22 . The magnets b, c, which are located on the inner peripheral side than the magnets B, C, constitute “pair of magnets (first magnets)′ recited in the appended claims, and will be referred also to as first magnets b, c in the following description where appropriate. The magnets B, C, which are located on the outer peripheral side than the magnets b, c, constitute “pair of second magnets′ recited in the appended claims, and will be referred also to as second magnets B, C in the following description where appropriate. An angle of a vertex S 1  of the inverted V shape of the first magnets b, c is larger than an angle of a vertex S 2  of the inverted V shape of the second magnets B, C, as shown in  FIG. 3 . Namely, the angle defined between extensions of the long sides of the first magnets b, c is larger than the angle defined between extensions of the long sides of the second magnets B, C. The angle of the vertex S 1  is not smaller than 90°, and is about 120° in the present embodiment. In  FIG. 2 , solid line arrows fs represent examples of magnetic field lines of the stator magnetic flux passing along the q-axis. A shortest distance between the flux barriers  40   c ,  40   d  disposed on the respective opposite sides of the q-axis on the above-described perpendicular plane corresponds to a first width, i.e., a q-axis salient pole width Wq 1  through which the stator magnetic flux is allowed to pass along the q-axis. Although a direction of the magnetic field lines fs of the stator magnetic flux is downward in the examples shown in  FIG. 2 , the direction of the magnetic field lines fs may be upward in  FIG. 2 . 
     The rotor core  22  further has cooling holes  42  that are formed through the rotor core  22  in the axial direction. Each of the cooling holes  42  is disposed on the q-axis, and is located on the inner peripheral side than the flux barriers  40   c ,  40   d . The cooling holes  42 , through which the cooling fluid is to pass, are provided for cooling the magnets  24 , particularly, the first magnets b, c that are located on the inner peripheral side than the other magnets A, a, B, C. Each of the cooling hole  42  has a shape which is symmetrical with respect to the q-axis in the circumferential direction, and which is curved to be convexed toward the outer peripheral side. The cooling holes  42  are located on the inner peripheral side than the first magnets b, c that are located on the inner peripheral side than the other magnets A, a, B, C. Each of the cooling holes  42  corresponds to “void portion” recited in the appended claims. 
     The first magnets b, c are opposite to each other in terms of N-S polarity, so that the magnet magnetic flux is produced between mutually opposed surfaces of the respective first magnets b, c. Similarly, the second magnets B, C are opposite to each other in terms of N-S polarity, so that the magnet magnetic flux is produced between mutually opposed surfaces of the respective first magnets second magnets B, C. In  FIG. 2 , broken line arrows fr 1 -fr 4  represent examples of magnetic field lines of the magnet magnetic flux produced between the first magnets b, c and between the second magnets B, C. The magnet magnetic flux passes between the first magnets b, c in a region defined between the magnetic field line fr 1  and the magnetic field line fr 2 , wherein the magnetic field line fr 1  connects between inner-peripheral side end portions of the long sides of the respective first magnets b, c that are opposed to each other with the q-axis being located therebetween, and wherein the magnetic field line fr 2  connects between outer-peripheral-side end portions of the long sides of the respective first magnets b, c (see  FIG. 2 ). Similarly, the magnet magnetic flux passes between the second magnets B, C in a region defined between the magnetic field line fr 3  and the magnetic field line fr 4 , wherein the magnetic field line fr 3  connects between inner-peripheral-side end portions of the long sides of the respective second magnets B, C that are opposed to each other with the q-axis being located therebetween, and wherein the magnetic field line fr 4  connects between outer-peripheral-side end portions of the long sides of the respective second magnets B, C (see  FIG. 2 ). Although a direction of the magnetic field lines fr 1 -fr 4  of the magnet magnetic flux is leftward the examples shown in  FIG. 2 , the direction of the magnetic field lines fr 1 -fr 4  may be rightward in  FIG. 2 . A path of each of the magnetic field lines fr 1 -fr 4  is likely to vary depending on attitudes and magnetic charges of corresponding ones of the magnets b, c, B, C and also a magnetic permeability of the rotor core  22 . 
       FIG. 3  is a view showing in further enlargement a portion of the rotor  12 , wherein the portion is in the vicinity of the cooling hole  42  and the pair of flux barriers  40   c ,  40   d  that are provided on the respective opposite sides of the q-axis.  FIG. 3  shows an imaginary line Lm that is an arc having a center of curvature that lies at the vertex S 2  of the inverted V shape of the pair of second magnets B, C, i.e., the intersection point of the extensions of the sides of the pair of second magnets B, C (that are located on the outer peripheral side than the first pair of first magnets b, c), and passing through inner-peripheral-side corners bi, ci of the pair of first magnets b, c on the above-described perpendicular plane, wherein each of the inner-peripheral-side corners bi, ci is one of corners of a corresponding one of the first magnets b, c which is located on the inner peripheral side than the others of the corners of the corresponding one of the first magnets b, c. The imaginary line Lm defines a boundary of the magnet magnetic flux produced between the first magnets b, c. The cooling hole  42  is located on the inner peripheral side than the imaginary line Lm, so that it is possible to suppress the cooling hole  42  from obstructing passage of the magnet magnetic flux and accordingly to suppress reductions of a motor torque and a motor efficiency of the rotating electric machine  10  due to provision of the cooling hole  42 . The imaginary line Lm is close to the magnetic field lines fr 1  (shown in  FIG. 2 ). With the cooling hole  42  being located on the inner peripheral side than the imaginary line Lm, the obstruction by the rotor core  22  to the passage of the magnet magnetic flux can be appropriately suppressed. The imaginary line Lm is changed depending on a position of the vertex S 2 , namely, the attitudes of the respective second magnets B, C, so that the imaginary line Lm could be located on the outer peripheral side than the magnetic field lines fr 1  whereby the passage of the magnet magnetic flux could be somewhat obstructed by the cooling hole  42 . However, since the imaginary line Lm is located on the inner peripheral side than at least a straight line connecting between the inner-peripheral-side corners bi, ci of the pair of first magnets b, c, it is possible make a degree of obstruction to the magnet magnetic flux smaller than in the arrangement disclosed in the above-identified Japanese Patent Publication (JP-6042976-B2) in which the cooling hole  42  is disposed on the straight line connecting between the inner-peripheral-side corners bi, ci. 
     In  FIG. 3 , reference sign “Ls” denotes a shortest line segment whose length corresponds to a shortest distance between the cooling hole  42  and the magnet b that is one of the first magnets b, c, on the above-described perpendicular plane. In the present embodiment, each of the long sides of the rectangular cross section of the first magnet b intersects with the q-axis with its inclination of about 60° with respect to the q-axis, and the cooling hole  42  is opposed to one (i.e., an inner-peripheral-side one) of the long sides in a direction perpendicular to the long sides, so that the shortest line segment Ls is defined between the one of the long sides and the cooling hole  42 , as is apparent from  FIG. 3 . The shortest line segment Ls intersects with the above-described imaginary line Lm at an intersection point P. The shortest distance Wb, which corresponds to a length of the shortest line segment Ls, can be expressed by expression (1) given below, where “Wd” represents a d-axis magnetic-flux passage width that is a distance from the intersection point P to the first magnet b, and “Wq 2 ” represents a q-axis magnetic-flux passage width that is a distance from the intersection point P to the cooling hole  42 . The d-axis magnetic-flux passage width Wd is a width through which mainly the magnet magnetic flux is allowed to pass between the first magnets b, c. The q-axis magnetic-flux passage width Wq 2  is a width through which mainly the stator magnetic flux is allowed to pass in a region between the first magnet b and the cooling hole  42 . Thus, the shortest line segment Ls intersects with the imaginary line Lm at the intersection P, and the q-axis magnetic-flux passage width Wq 2  is present between the intersection point P and the cooling hole  42 , so that the stator magnetic flux can appropriately pass through the region between the first magnet b and the cooling hole  42 . 
         Wb=Wd+Wq 2  (1)
 
     The cooling hole  42  is located such that the q-axis magnetic-flux passage width Wq 2  is smaller than the above-described q-axis salient pole width Wq 1 . In the present embodiment, the q-axis magnetic-flux passage width Wq 2  is smaller than the q-axis salient pole width Wq 1 , and is not smaller than a half of the q-axis salient pole width Wq 1 , as indicated in expression (2) given below. That is, with the q-axis magnetic-flux passage width Wq 2  being smaller than the q-axis salient pole width Wq 1 , the cooling hole  42  is avoided from being far from the first magnet b more than necessary whereby the reduction of the cooling performance is suppressed, and the cooling hole  42  is located in a position close as much as possible to the first magnet b whereby the cooling performance for cooling the first magnet b can be appropriately obtained. Further, since the q-axis magnetic-flux passage width Wq 2  is not smaller than the half of the q-axis salient pole width Wq 1 , a cross sectional area through which the magnet magnetic flux and the stator magnetic flux are allowed to be pass in a region between the first magnet b and the cooling hole  42  can be appropriately assured. Owing to this arrangement, it is possible to avoid the cooling hole  42  from obstructing passages of the magnet magnetic flux and the stator magnetic flux, and to accordingly appropriately suppress reduction of performances of the rotating electric machine  10  such as reduction of the motor torque due to reduction of an amount of the magnetic flux and reduction of the motor efficiency due to increase of a core loss caused by increase of a density of the magnetic flux. Since the stator magnetic flux having passed through the q-axis salient pole width Wq 1  is divided by the cooling hole  42  into two directions, it is possible to appropriately assure the cross sectional area through which the stator magnetic flux is allowed to pass, owing to the arrangement in which the q-axis magnetic-flux passage width Wq 2  is not smaller than the half of the q-axis salient pole width Wq 1 . The position and the shape of the cooling hole  42  are determined such that the following expression (2) is satisfied. 
       (1/2) Wq 1≤ Wq 2≤ Wq 1  (2)
 
     The magnetic field lines fs of the stator magnetic flux is divided, by presence of the cooling hole  42 , into opposite sides of the cooling hole  42 , as shown in  FIG. 2 , and the divided magnetic field lines fs are caused to pass through a region between the cooling hole  42  and the first magnet b and through a region between the cooling hole  42  and the second magnet c, as shown in  FIG. 2 . In this instance, since the first magnets b, c and the cooling hole  42  are disposed symmetrically with respect to the q-axis, so that a relationship between the first magnet c and the cooling hole  42  is substantially the same as a relationship between the first magnet b and the cooling hole  42  whereby substantially the same effect can be obtained between the first magnet c and the cooling hole  42 , as between the first magnet b and the cooling hole  42 . 
     As described above, in the rotor  12  of the rotating electric machine  10  according to the present embodiment, the cooling hole  42  is located on the inner peripheral side than the imaginary line Lm that is the arc having the center of curvature that lies at the vertex S 2 , i.e., the intersection point of the extensions of the sides of the pair of second magnets B, C and passing through the inner-peripheral-side corners bi, ci of the pair of first magnets b, c, so that it is possible to suppress the cooling hole  42  from obstructing passage of the magnet magnetic flux and accordingly to suppress reductions of the motor torque and the motor efficiency of the rotating electric machine  10  due to provision of the cooling hole  42 . Further, the q-axis magnetic-flux passage width Wq 2  is smaller than the q-axis salient pole width Wq 1 , wherein the q-axis salient pole width Wq 1  is defined by the shortest distance between the pair of flux barriers  40   c ,  40   d  that are disposed on the respective opposite sides of the q-axis, and wherein the q-axis magnetic-flux passage width Wq 2  is defined by the distance from the cooling hole  42  to the intersection point at which the imaginary line Lm intersects with the shortest line segment Ls corresponding to the shortest distance Ls between each one of the first magnets b, c and the cooling hole  42 . Thus, the cooling hole  42  is provided in a position close as much as possible to the pair of first magnets b, c while assuring an cross sectional area through which the magnet magnetic flux and the stator magnetic flux are allowed to be pass, so that it is possible to appropriately obtain performance for cooling the pair of first magnets b, c owing to the cooling hole  42 , and accordingly to suppress reduction of the motor torque of the rotating electric machine  10  which could be caused by demagnetization of the magnets due to excessive heating of the magnets. Since the stator magnetic flux having passed through the q-axis salient pole width Wq 1  is divided by the cooling hole  42  into two directions, it is possible to assure the cross sectional area through which the stator magnetic flux is to pass, although the q-axis magnetic-flux passage width Wq 2  is smaller than the q-axis salient pole width Wq 1 . 
     Further, in the present embodiment, the imaginary line Lm is the arc having the center of curvature that lies at the vertex S 2 , i.e., the intersection point of the extensions of the sides of the pair of second magnets B, C (that are disposed on the outer peripheral side than the pair of first magnets b, c) and passing through the inner-peripheral-side corners bi, ci of the pair of first magnets b, c. Thus, the imaginary line Lm is close to the magnetic field lines fr 1 , i.e., a boundary line defining an actual boundary of the magnet magnetic flux of the pair of first magnets b, c. Owing to this arrangement, it is possible to suppress the cooling hole  42  from obstructing passage of the magnet magnetic flux and also to appropriately obtain performance for cooling the pair of first magnets b, c owing to the cooling hole  42  that is disposed in a position close as much as possible to the pair of first magnets b, c, while assuring the cross sectional area through which the magnet magnetic flux and the stator magnetic flux are allowed to be pass. 
     Further, the plurality of flux barriers  40  form the plurality of V-shaped arrangement patterns Pv in each of which at least two of the plurality of flux barriers  40  are arranged to have the V shape or U shape, such that the plurality of V-shaped arrangement patterns Pv are arranged around the axis O of the rotor core  22 , wherein the q-axis is the intermediate line between each adjacent two of the plurality of V-shaped arrangement patterns Pv, which are adjacent to each other in the circumferential direction, and wherein the pair of flux barriers  40   c ,  40   d  are two of the plurality of flux barriers  40 , such that the two of the plurality of flux barriers  40  are disposed on the respective opposite sides of the q-axis in the circumferential direction, and such that the two of the plurality of flux barriers  40  are closer to the q-axis, than the others of the plurality of flux barriers  40 , in the circumferential direction. The pair of flux barriers  40   c ,  40   d  are arranged to have respective attitudes that cooperate with each other to form the inverted-V shape. The present invention is advantageously applied in the present embodiment. 
     Further, the q-axis magnetic-flux passage width Wq 2  is smaller than the q-axis salient pole width Wq 1  and is not smaller than the half of the q-axis salient pole width Wq 1 , so that it is possible to appropriately assure performance for cooling the pair of first magnets b, c, owing to the cooling hole  42  that is disposed in a position close to the pair of first magnets b, c, and also to appropriately assure the cross sectional area through which the magnet magnetic flux and the stator magnetic flux are allowed to be pass. Since the stator magnetic flux having passed through the q-axis salient pole width Wq 1  is divided by the cooling hole  42  into two directions, it is possible to assure the cross sectional area through which the stator magnetic flux is allowed to pass, owing to the arrangement in which the q-axis magnetic-flux passage width Wq 2  is smaller than the q-axis salient pole width Wq 1  and is not smaller than the half of the q-axis salient pole width Wq 1 . 
     In the above-described embodiment, the imaginary line Lm is the arc having the center of curvature that lies at the vertex S 2 , i.e., the intersection point of the extensions of the sides of the pair of second magnets B, C (that are disposed on the outer peripheral side than the pair of first magnets b, c) and passing through the inner-peripheral-side corners bi, ci of the pair of first magnets b, c. However, the imaginary line Lm may be also a straight line connecting between the inner-peripheral-side corners bi, ci of the pair of first magnets b, c that are disposed on the inner peripheral side than the other magnets A, a, B, C, as in another embodiment of the present invention shown in shown in  FIG. 4 . Owing to the arrangement in which the cooling hole  42  is located on the inner peripheral side than the straight line as the imaginary line Lm, it is possible to suppress the cooling hole  42  from obstructing passage of the magnet magnetic flux and accordingly to suppress reductions of the motor torque and the motor efficiency of the rotating electric machine  10  due to provision of the cooling hole  42 . Since the magnetic field line fr 1  passes on the inner peripheral side than the straight line as the imaginary line Lm, the passage of the magnetic field line fr 1  could be obstructed by the cooling hole  42 . However, a degree of obstruction to the magnet magnetic flux can be made smaller than in the arrangement disclosed in the above-identified Japanese Patent Publication (JP-6042976-B2) in which the cooling hole  42  is disposed on the straight line connecting between the inner-peripheral-side corners bi, ci of the pair of first magnets b, c. 
     Further, in the embodiment shown in  FIG. 4 , the position and the shape of the cooling hole  42  are determined such that the q-axis magnetic-flux passage width Wq 2  is smaller than the q-axis salient pole width Wq 1 , wherein the q-axis magnetic-flux passage width Wq 2  is defined by a distance from the cooling hole  42  to the intersection point P at which the imaginary line Lm that is the straight line intersects with the shortest line segment Ls whose length corresponds to the shortest distance between each one of the magnets b, c and the cooling hole  42  on the perpendicular plane. Owing to this arrangement, the cooling hole  42  is provided in a position close as much as possible to the pair of first magnets b, c while assuring the cross sectional area through which the magnet magnetic flux and the stator magnetic flux are allowed to be pass, so that it is possible to appropriately obtain performance for cooling the pair of first magnets b, c owing to the cooling hole  42 , and accordingly to suppress reduction of the motor torque of the rotating electric machine  10  which could be caused by demagnetization of the magnets due to excessive heating of the magnets. 
     Further, the position and the shape of the cooling hole  42  are determined such that the above-described expression (2) is satisfied, specifically, such that the q-axis magnetic-flux passage width Wq 2  is smaller than the q-axis salient pole width Wq 1  and is not smaller than the half of the q-axis salient pole width Wq 1 , so that it is possible to appropriately obtain performance for cooling the first magnets b, c owing to the cooling hole  42  that is disposed in a position close to the first magnets b, c, and also to appropriately assure the cross sectional area through which the magnet magnetic flux and the stator magnetic flux are allowed to be pass. Since the stator magnetic flux having passed through the q-axis salient pole width Wq 1  is divided by the cooling hole  42  into two directions, it is possible to assure the cross sectional area through which the stator magnetic flux is to pass, owing to the arrangement in which the q-axis magnetic-flux passage width Wq 2  is smaller than the q-axis salient pole width Wq 1  and is not smaller than the half of the q-axis salient pole width Wq 1 . 
     While the preferred embodiments of the present invention have been described in detail by reference to the drawings, it is to be understood that the embodiments described above are given for illustrative purpose only, and that the present invention may be embodied with various modifications and improvements which may occur to those skilled in the art. 
     NOMENCLATURE OF ELEMENTS 
     
         
           10 : rotating electric machine 
           12 : rotor for rotating electric machine 
           20 : rotor shaft 
           22 : rotor core 
           24 , A, a, B, b, C, c: magnets 
         b, c: magnets (first magnets) 
         B,C: magnets (second magnets) 
           40   a ,  40   b ,  40   c ,  40   d : flux barriers 
           40   c ,  40   d : pair of flux barriers 
           42 : cooling hole (void portion) 
         O: axis 
         q: q-axis 
         Pv: V-shaped arrangement pattern 
         fs: magnetic field line of stator magnetic flux (stator magnetic flux) 
         fr 1 , fr 2 , fr 3 , fr 4 : magnetic field line of magnet magnetic flux (magnet magnetic flux) 
         S 1 ,S 2 : vertexes 
         Lm: imaginary line 
         Ls: shortest line segment 
         P: intersection point 
         Wq 1 : q-axis salient pole width 
         Wq 2 : q-axis magnetic-flux passage width