Abstract:
Provided are a rotating electrical machine and a method for manufacturing the rotating electrical machine, wherein the tapered surface of a plate-side tapered section and the tapered surface of a shaft-side tapered section are bonded with pressure by having forces operate between a plurality of plate-side protruding sections and a plurality of shaft-side protruding sections in the directions wherein the plate-side protruding sections and the shaft-side protruding sections are separated from each other. Thus, a ring core is fixed to a shaft.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a rotary electric machine including a ring core made up of a plurality of stacked ring-shaped core plates each comprising a plurality of divided core plates, a shaft inserted through the ring core, and a plurality of magnets inserted in magnet insertion holes defined in the divided core plates. 
       BACKGROUND ART 
       [0002]    There are known rotary electric machines for use as parts in electric motors or the like. The rotary electric machines mainly include a ring core made up of a plurality of stacked ring-shaped core plates, a shaft inserted through the ring core, and a plurality of magnets disposed in the ring core. Known technologies for fitting the ring core over the shaft include a shrink-fitting process and a press-fitting process (for example, Japanese Laid-Open Patent Publication No. 07-022168). According to Japanese Laid-Open Patent Publication No. 07-022168, a hollow cylindrical rotor ( 1 ) is heated to increase its inside diameter, and a shaft ( 7 ) is inserted therein. The rotor ( 1 ) is then cooled to reduce the inside diameter thereof to fit over the shaft ( 7 ) (for example, see paragraph [0031] and  FIG. 3(   b ) of Japanese Laid-Open Patent Publication No. 07-022168). 
         [0003]    There is also known a technology wherein each of a plurality of ring-shaped core plates that make up a ring core comprises a plurality of divided core plates (for example, Japanese Laid-Open Patent Publication No. 2002-262496). In addition, Japanese Laid-Open Patent Publication No. 2002-262496 discloses that internal involute splines ( 11 ) are formed on the inner circumferential surface of divided cores ( 1 ), external involute splines ( 18 ) are formed on the outer circumferential surface of a shaft ( 17 ), and they are brought into mesh with each other to fasten a rotor ( 16 ) to the shaft ( 17 ) (for example, see paragraphs [0020], and  FIGS. 11 ,  12 , and  14  of Japanese Laid-Open Patent Publication No. 2002-262496). 
       SUMMARY OF INVENTION 
       [0004]    According to Japanese Laid-Open Patent Publication No.  07 - 022168 , since the rotor ( 1 ) is fixed to the shaft ( 7 ) by the shrinkage of the rotor ( 1 ), if the torque applied to the shaft ( 7 ) increases, then the rotor ( 1 ) may possibly be spaced from the shaft ( 7 ) under centrifugal forces, failing to transmit the torque sufficiently. 
         [0005]    Even with the meshing structure disclosed in Japanese Laid-Open Patent Publication No. 2002-262496, inasmuch as the internal involute splines ( 11 ) and the external involute splines ( 18 ) engage perpendicularly to each other, when centrifugal forces are applied to the rotor ( 16 ) upon rotation of the shaft ( 17 ), the rotor ( 16 ) may possibly be displaced in a direction away from the shaft ( 17 ). At this time, magnets disposed in the rotor ( 16 ) are also displaced, and the rotor ( 16 ) tends to be brought into contact with the stator, damaging the rotary electric machine. 
         [0006]    The present invention has been made in view of the above problems. It is an object of the present invention to provide a rotary electric machine which is capable of efficiently transmitting a torque from a shaft to a ring core and also of preventing itself from contacting a stator while the rotary electric machine is rotating at a high speed, and a method of manufacturing such a rotary electric machine. 
         [0007]    A rotary electric machine according to the present invention comprises a ring core made up of a plurality of stacked ring-shaped core plates each comprising a plurality of divided core plates, a shaft inserted through the ring core, and a plurality of magnets inserted in magnet insertion holes defined in the divided core plates, wherein each of the ring-shaped core plates has on an inner circumferential surface thereof a plurality of plate-side protrusions projecting toward the shaft, and the shaft has on an outer circumferential surface thereof a plurality of shaft-side protrusions projecting toward the divided core plates, each of the plate-side protrusions has a plate-side tapered portion having a width progressively greater toward the shaft, and each of the shaft-side protrusions has a shaft-side tapered portion having a width progressively greater toward the divided core plates, and a tapered surface of the plate-side tapered portion and a tapered surface of the shaft-side tapered portion are pressed against each other, securing the ring core to the shaft, under a force acting in a direction to move the plate-side protrusions and the shaft-side protrusions away from each other. 
         [0008]    According to the present invention, the ring core is secured to the shaft by pressing the tapered surface of the plate-side tapered portion which is progressively wider toward the shaft and the tapered surface of the shaft-side tapered portion which is progressively wider toward the divided core plates, against each other. Therefore, a torque can efficiently be transmitted from the shaft to the ring core. Even when the rotary electric machine rotates at a high speed, applying centrifugal forces to the ring-shaped core plates, the ring-shaped core plates are prevented from increasing in diameter. Consequently, the rotary electric machine is capable of performing as desired while rotating at a high speed. 
         [0009]    The plate-side tapered portions may be disposed in phase with the magnets. Since the plate-side tapered portion is held in press-contact with the shaft-side tapered portion, the position in phase with the plate-side tapered portion is relatively hard to displace while the rotary electric machine is in rotation. Consequently, the rotary electric machine is prevented from being damaged due to contact with the stator while the rotary electric machine is in high speed rotation. 
         [0010]    The plate-side tapered portion may comprise a plate-side trapezoidal region in the shape of an inverted isosceles trapezoid having a width progressively greater toward the shaft, and the shaft-side tapered portion comprises a shaft-side trapezoidal region in the shape of an inverted isosceles trapezoid having a width progressively greater toward the ring-shaped core plates. 
         [0011]    Each of an angle formed between two slant lines interconnecting upper and lower bottoms of the plate-side trapezoidal region and an angle formed between two slant lines interconnecting upper and lower bottoms of the shaft-side trapezoidal region may be in the range from 60° to 120° inclusive. The angle which is equal to or greater than 60° makes it easy to inhibit the relative displacement between the plate-side tapered portion and the shaft-side tapered portion and the displacement of the ring-shaped core plates with respect to the shaft while the rotary electric machine is in rotation. The angle which is equal to or smaller than 120° makes it easy to fit the ring-shaped core plates over the shaft. 
         [0012]    A space defined between adjacent ones of the plate-side protrusions may be greater than the shaft-side tapered portion as viewed in plan. 
         [0013]    The coefficient of thermal expansion of the shaft may be equal to or greater than the coefficient of thermal expansion of the divided core plates. 
         [0014]    The rotary electric machine may further comprise a plurality of securing pins inserted in the ring-shaped core plates along the directions in which the ring-shaped core plates are stacked, securing the ring-shaped core plates together, wherein the ring-shaped core plates may have a plurality of pin holes defined therein for receiving the securing pins inserted therein, and the pin holes may be disposed in positions in which the magnetic flux density of the magnets is lowest and which are in phase with the magnets. Therefore, it is possible to inhibit a reduction in the performance of the rotary electric machine due to the securing pins inserted into the pin holes. 
         [0015]    The pin holes may be disposed in positions which are spaced from the magnet insertion holes by the thickness of one magnet. It is thus possible to inhibit a reduction in the performance of the rotary electric machine. 
         [0016]    Each of the ring-shaped core plates may have a plurality of dowels deformed along the directions in which the ring-shaped core plates are stacked, the dowels may be disposed along a circle that is concentric to the rotational axis of the shaft, and have a U-shaped cross section along lines tangential to the circle, and the dowels may have longitudinal directions parallel to the lines tangential to the circle. The ring-shaped core plates are thus prevented from being deformed while the rotary electric machine is in rotation. 
         [0017]    According to the present invention, a method of manufacturing a rotary electric machine including a ring core made up of a plurality of stacked ring-shaped core plates each comprising a plurality of divided core plates, a shaft inserted through the ring core, and a plurality of magnets inserted in magnet insertion holes defined in the divided core plates, comprises the heating step of heating the shaft, the fitting step of fitting the ring core over the heated shaft, and the cooling step of cooling the shaft to integrally combine the shaft and the ring core with each other, after the fitting step, wherein each of the ring- shaped core plates has on an inner circumferential surface thereof a plurality of plate-side protrusions projecting toward the shaft, and the shaft has on an outer circumferential surface thereof a plurality of shaft-side protrusions projecting toward the divided core plates, each of the plate-side protrusions has a plate-side tapered portion having a width progressively greater toward the shaft, and each of the shaft-side protrusions has a shaft- side tapered portion having a width progressively greater toward the divided core plates, and in the fitting step, the plate-side protrusions are brought into fitting engagement with the shaft-side protrusions which are thermally expanded, and, in the cooling step, the shaft shrinks to bring a tapered surface of the plate-side tapered portion and a tapered surface of the shaft-side tapered portion into intimate contact with each other. 
         [0018]    The plate-side tapered portion may comprise a plate-side trapezoidal region in the shape of an inverted isosceles trapezoid having a width progressively greater toward the shaft, and the shaft-side tapered portion may comprise a shaft-side trapezoidal region in the shape of an inverted isosceles trapezoid having a width progressively greater toward the ring-shaped core plates, each of an angle formed between two slant lines interconnecting upper and lower bottoms of the plate-side trapezoidal region and an angle formed between two slant lines interconnecting upper and lower bottoms of the shaft-side trapezoidal region may be in the range from 60° to 120° inclusive, and a space defined between adjacent ones of the plate-side protrusions may be greater than the shaft-side tapered portion which is heated in the heating step, as viewed in plan. 
         [0019]    The coefficient of thermal expansion of the shaft may be equal to or greater than the coefficient of thermal expansion of the divided core plates. 
         [0020]    According to the present invention, a method of manufacturing a rotary electric machine including a ring core made up of a plurality of stacked ring-shaped core plates each comprising a plurality of divided core plates, a shaft inserted through the ring core, and a plurality of magnets inserted in magnet insertion holes defined in the divided core plates, comprises the cooling step of cooling the ring core, the fitting step of fitting the ring core which is cooled over the shaft, and the normal temperature restoring step of restoring the ring core to normal temperature after the fitting step, wherein each of the ring-shaped core plates has on an inner circumferential surface thereof a plurality of plate-side protrusions projecting toward the shaft, and the shaft has on an outer circumferential surface thereof a plurality of shaft-side protrusions projecting toward the divided core plates, each of the plate-side protrusions has a plate-side tapered portion having a width progressively greater toward the shaft, and each of the shaft-side protrusions has a shaft-side tapered portion having a width progressively greater toward the divided core plates, and in the fitting step, the plate-side protrusions which are cooled to shrink are brought into fitting engagement with the shaft-side protrusions, and, in the normal temperature restoring step, the ring-shaped core plates are thermally expanded to bring a tapered surface of the plate-side tapered portion and a tapered surface of the shaft-side tapered portion into intimate contact with each other. 
         [0021]    The plate-side tapered portion may comprise a plate-side trapezoidal region in the shape of an inverted isosceles trapezoid having a width progressively greater toward the shaft, and the shaft-side tapered portion may comprise a shaft-side trapezoidal region in the shape of an inverted isosceles trapezoid having a width progressively greater toward the ring-shaped core plates, each of an angle formed between two slant lines interconnecting upper and lower bottoms of the plate-side trapezoidal region and an angle formed between two slant lines interconnecting upper and lower bottoms of the shaft-side trapezoidal region may be in the range from 60° to 120° inclusive, and a space defined between adjacent ones of the plate-side protrusions may be greater than the plate-side tapered portion which is cooled in the cooling step, as viewed in plan. 
         [0022]    The coefficient of thermal expansion of the divided core plates may be equal to or greater than the coefficient of thermal expansion of the shaft. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0023]      FIG. 1  is an exploded perspective view of a rotor as a rotary electric machine according to an embodiment of the present invention; 
           [0024]      FIG. 2  is an exploded perspective view of a portion of a rotor core of the rotor; 
           [0025]      FIG. 3  is a plan view of the rotor; 
           [0026]      FIG. 4  is an enlarged fragmentary plan view of the rotor shown in  FIG. 3 ; 
           [0027]      FIG. 5  is a flowchart of a method of manufacturing the rotor according to the embodiment; 
           [0028]      FIG. 6  is a view showing one state in the method of manufacturing the rotor; 
           [0029]      FIG. 7  is an enlarged fragmentary plan view showing the manner in which a shaft is expanded due to heat in  FIG. 4 ; 
           [0030]      FIG. 8  is a view showing another state in the method of manufacturing the rotor; 
           [0031]      FIG. 9  is an enlarged fragmentary plan view of a modification of the rotor; and 
           [0032]      FIG. 10  is a flowchart of a modification of the method of manufacturing the rotor. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     Arrangement of the Embodiment 
       [0033]      FIG. 1  is an exploded perspective view of a rotor  10  as a rotary electric machine according to an embodiment of the present invention. The rotor  10  according to the present embodiment cooperates with a stator, etc., not shown, in making up an electric motor. 
         [0034]    The rotor  10  includes a rotor core  12  (ring core) made up of a plurality of (e.g., one hundred sheets of) stacked ring-shaped core plates  14 , a shaft  16  inserted through the rotor core  12 , and a plurality of magnets  18  inserted in the rotor core  12 . The shaft  16  according to the present embodiment has a higher coefficient of thermal expansion than each of the ring-shaped core plates  14 . 
         [0035]    Each of the ring-shaped core plates  14  comprises a predetermined number (3 in the present embodiment) of thin, sectorial divided core plates  20  arranged circumferentially into a ring shape. If it is assumed that the position where two divided core plates  20  of the ring-shaped core plate  14  in the lowermost layer (first layer) abut against each other is indicated by an arrow E 1 , then the position where two divided core plates  20  of the ring-shaped core plate  14  in the layer (second layer) above the lowermost layer abut against each other is indicated by an arrow E 2 . Similarly, the corresponding abutting position in the ring-shaped core plate  14  in the third layer is indicated by an arrow E 3 , the corresponding abutting position in the ring-shaped core plate  14  in the fourth layer by an arrow E 4 , and the corresponding abutting position in the ring-shaped core plate  14  in the fifth layer by an arrow E 1  (the abutting position in the fifth layer is the same as the abutting position in the first layer). The ring-shaped core plates  14  in layers above the fifth layer are stacked in the same sequence. As can be understood from  FIG. 1 , the arrows El through E 4  are shifted 30° out of phase with each other. The abutting positions in the ring-shaped core plates  14  in each layer, e.g., in the first layer, are angularly spaced by the same angles as the angle of the arc represented by a single divided core plate  20 . These abutting positions are located as a total of three positions angularly spaced by 120° from the reference position indicated by the arrow E 1 . The abutting positions in the other layers are similarly angularly spaced apart. 
         [0036]    Specifically, as shown in  FIG. 2 , the ring-shaped core plate  14  in the first layer has a total of three positions E 1  where the ends (abutting surfaces) of two divided core plates  20  abut against each other, angularly spaced by a predetermined angle θ 1  (120° in the present embodiment). The ring-shaped core plate  14  in the second layer has three positions E 2  where the ends of two divided core plates  20  abut against each other, angularly spaced by a predetermined angle θ 2  (30° in the present embodiment) from the positions E 1 . The ring-shaped core plate  14  in the third layer has three positions E 3  where the ends of two divided core plates  20  abut against each other, angularly spaced by the predetermined angle θ 2  (30° in the present embodiment) from the positions E 2 . The abutting positions in the upper layers are similarly angularly spaced apart. With the rotor core  12 , therefore, the ring-shaped core plates  14  in the respective layers are stacked in the positions that are angularly spaced by the predetermined angle θ 2 )(30°). 
         [0037]    Each of the divided core plates  20  that make up the ring-shaped core plates  14  has magnet holes  22  (magnet insertion holes) for receiving magnets  18  inserted therein. The magnet holes  22  are defined at equal angular intervals in the circumferential directions of the rotor core  12  and are positioned in phase with each other through the ring-shaped core plates  14  with respect to the rotational axis Ax of the rotor  10 . That the magnet holes  22  are positioned in phase with each other means that the magnet holes  22  are positioned in line through the ring-shaped core plates  14  with respect to the rotational axis Ax as viewed in plan ( FIG. 3 ). The magnet holes  22  positioned in phase with each other through the ring-shaped core plates  14  jointly define slots  24  for housing therein the magnets  18  each substantially in the form of a rectangular parallelepiped. 
         [0038]    Each of the divided core plates  20  has pin holes  26  defined therein in phase with the magnet holes  22 . When securing pins  28  are inserted into the pin holes  26  along the directions in which the ring-shaped core plates  14  are stacked, the ring-shaped core plates  14  are secured to each other. Each of the pin holes  26  is defined in a position which is spaced from the corresponding magnet hole  22  (in phase in the pin hole  26 ) toward the rotational axis Ax of the rotor  10  by the thickness of one magnet  18 . The magnetic flux density of the magnet  18  is lowest in the position where each of the pin holes  26  is defined. 
         [0039]    Dowels  30  are disposed on both sides of each pin hole  26 . Each of the dowels  30  is defined by a convexity in one of the directions in which the ring-shaped core plates  14  are stacked and a concavity in the other of the directions in which the ring-shaped core plates  14  are stacked. All the dowels  30  are arranged along a circle that is concentric to the rotational axis Ax, and have a U-shaped cross section along the directions of a line tangential to the circle that is concentric to the rotational axis Ax. The dowels  30  are longer in the directions of the line tangential to the circle than in the directions of a line perpendicular to the line tangential to the circle. When the ring-shaped core plates  14  are stacked together, the dowels  30  of adjacent ones of the ring-shaped core plates  14  engage each other. 
         [0040]    Each of the ring-shaped core plates  14  (the divided core plates  20 ) has tapered keys  32  (plate-side protrusions) disposed on an inner side thereof (shaft  16  side) at respective positions that are in phase with the magnets  18  and the pin holes  26 . As shown in  FIGS. 1 and 3 , the shaft  16  has a plurality of tapered keys  34  (shaft-side protrusions) disposed on an outer circumferential surface thereof and held in mesh with the tapered keys  32  of the divided core plates  20 . In other words, each of the tapered keys  32  of the divided core plates  20  is disposed in a space  36  defined between adjacent ones of the tapered keys  34  of the shaft  16 . Stated otherwise, each of the tapered keys  34  of the shaft  16  is disposed in a space  38  defined between adjacent ones of the tapered keys  32  of the divided core plates  20 . 
         [0041]    As shown in  FIG. 4 , each of the tapered keys  32  of the divided core plates  20  includes a proximal portion  50  (plate-side proximal portion) having a constant width, an intermediate portion  52  (plate-side tapered portion) disposed more closely to the shaft  16  than the proximal portion  50  and having a progressively greater width, and a distal end portion  54  disposed more closely to the shaft  16  than the intermediate portion  52  and having a constant width. The width Wp 1  of the distal end portion  54  is greater than the width Wp 2  of the proximal portion  50 . The intermediate portion  52  is in the shape of an inverted isosceles trapezoid, and includes two sides interconnecting upper and lower bottoms thereof and defined by two tapered surfaces  56  which are angularly spaced by an angle θp of about 100°. 
         [0042]    Each of the tapered keys  34  of the shaft  16  includes a proximal portion  60  (shaft-side proximal portions) having a width progressively greater toward the divided core plates  20 , an intermediate portion  62  (shaft-side tapered portion) disposed more closely to the divided core plates  20  than the proximal portion  60  and having a width which increases at a larger rate of change than the width of the proximal portion  60 , and a distal end portion  64  disposed more closely to the divided core plates  20  than the intermediate portion  62  and having a width which increases at a smaller rate of change than the width of the intermediate portion  62 . The minimum width Ws 1  of the distal end portion  64  is greater than the maximum width Ws 2  of the proximal portion  60 . Side surfaces  66  which face adjacent ones of the proximal portions  60  lie parallel to each other (with a constant distance Ds 1  between adjacent side surfaces  66 ). Side surfaces  68  which face adjacent ones of the distal end portions  64  lie parallel to each other (with a constant distance Ds 2  between adjacent side surfaces  68 ). The intermediate portion  62  is in the shape of an inverted isosceles trapezoid, and includes two sides interconnecting upper and lower bottoms thereof and defined by two tapered surfaces  70  which are angularly spaced by an angle θs of about 120°. 
         [0043]    As shown in  FIG. 4 , the distance Ds 1  between the proximal portions  60  of the shaft  16  is greater than the width Wp 1  of the distal end portion  54  of the divided core plates  20 . The distance Ds 2  between the distal end portions  64  of the shaft  16  is greater than the width Wp 2  of the proximal portion  50  of the divided core plates  20 . Furthermore, the tapered surfaces  56  of the intermediate portion  52  and the tapered surfaces  70  of the intermediate portion  62  which face the tapered surfaces  56  lie parallel to each other. In addition, the side surfaces  66  of the proximal portion  60  of the shaft  16  and the side surfaces  72  of the distal end portion  54  of the divided core plates  20  lie parallel to each other. The side surfaces  68  of the distal end portions  64  of the shaft  16  and side surfaces  74  of the proximal portion  50  of the divided core plates  20  lie parallel to each other. 
         [0044]    The tapered keys  32  of the divided core plates  20  and the tapered keys  34  of the shaft  16  are of the structure described above. As shown in  FIG. 4 , the intermediate portions  52  of the tapered keys  32  and the intermediate portions  62  of the tapered keys  34  are held in intimate contact with each other through the tapered surfaces  56 ,  70 . According to the present embodiment, as described later, the shaft  16  is heated to thermally expand in its entirety (see  FIG. 7 ), then the tapered keys  32  and the tapered keys  34  are positioned, and thereafter the shaft  16  is cooled to shrink in its entirety. At normal temperature, the tapered keys  34  of the shaft  16  keep the tapered keys  32  of the divided core plates  20  pulled toward the rotational axis Ax of the rotor  10 , thereby securely coupling the divided core plates  20  to the shaft  16 . 
         [0045]    While the intermediate portions  52 ,  62  are being held in intimate contact with each other, the distal end portions  54  of the tapered keys  32  have distal end surfaces  76  kept out of contact with the shaft  16 , and the distal end portions  64  of the tapered keys  34  have distal end surfaces  78  kept out of contact with the divided core plates  20 . 
         [0046]    [Method of Manufacturing a Rotor] 
         [0047]    A method of manufacturing the rotor  10  according to the present embodiment will be described below. 
         [0048]      FIG. 5  is a flowchart of a method of manufacturing the rotor  10 . In step S 1 , the shaft  16  is heated to a prescribed temperature (e.g., several hundreds ° C.). In step S 2 , the heated shaft  16  is set in a jig  80  (see  FIG. 6 ). At this time, the tapered keys  34  of the shaft  16  are thermally expanded as indicated by the two-dot-and-dash lines in  FIG. 7 . 
         [0049]    In step S 3 , the rotor core  12  at normal temperature is fitted over the shaft  16  (see  FIGS. 6 and 8 ). Since the tapered keys  34  of the shaft  16  which is heated are thermally expanded, as described above, the rotor core  12  can be fitted over the shaft  16  without the tapered keys  32  and the tapered keys  34  being brought into contact with each other. 
         [0050]    In step S 4 , the shaft  16  and the rotor core  12  which is heated by the heat of the shaft  16  are cooled. As a result, the tapered keys  32 ,  34  shrink. At normal temperature, the tapered keys  34  of the shaft  16  keep the tapered keys  32  of the divided core plates  20  pulled toward the rotational axis Ax of the rotor  10 , thereby securely coupling the divided core plates  20  to the shaft  16 . 
       Advantages of the Present Embodiment 
       [0051]    According to the present embodiment, as described above, the tapered surfaces  56  of the tapered keys  32  of the divided core plates  20  and the tapered surfaces  70  of the tapered keys  34  of the shaft  16  are held in intimate contact with each other, securing the divided core plates  20  to the shaft  16 . Therefore, a torque can efficiently be transmitted from the shaft  16  to the rotor core  12 . Even when the rotor  10  rotates at a high speed, applying centrifugal forces to the divided core plates  20 , the divided core plates  20  are prevented from increasing in diameter. Consequently, the rotor  10  is prevented from being damaged due to an increase in diameter while rotating at a high speed. 
         [0052]    According to the present embodiment, the tapered keys  32  of the divided core plates  20  are disposed in phase with the magnets  18 . Since the tapered keys  32  and the tapered keys  34  are held in pressed contact with each other, the positions in phase with the tapered keys  32  are relatively hard to displace while the rotor  10  is in rotation. 
         [0053]    According to the present embodiment, each of the angle θp of the tapered keys  32  and the angle θs of the tapered keys  34  is in the range from 60° to 120° inclusive. The angle range makes it easy to inhibit the relative displacement between the tapered keys  32  and the tapered keys  34  and the displacement of the divided core plates  20  with respect to the shaft  16  while the rotor  10  is in rotation, and also to shrink-fit the divided core plates  20  over the shaft  16 . 
         [0054]    According to the present embodiment, the rotor core  12  can be fitted over the shaft  16  without contacting the shaft  16  by shrink fitting. It is thus possible to prevent demerits (e.g., scoring on the rotor core  12  and the shaft  16  when the rotor core  12  is fitted over the shaft  16 ) caused if the rotor core  12  is fitted over the shaft  16  by press fitting. 
         [0055]    According to the present embodiment, the pin holes  26  are located at the position where the magnetic flux density of the magnet  18  is the lowest, i.e., the position which is spaced from the magnet hole  22  by the thickness of one magnet  18 . Therefore, it is possible to inhibit a reduction in the performance of the rotor  10  due to the securing pins  28  inserted into the pin holes  26 . 
         [0056]    According to the present embodiment, the dowels  30  of the U-shaped cross section have their longitudinal directions parallel to the lines tangential to the circle which is concentric to the rotational axis Ax of the rotor  10 , for thereby preventing the ring-shaped core plates  14  from being deformed while the rotor  10  is in rotation. 
         [0057]    According to the present embodiment, the divided core plates  20  are divided at angular intervals of 120°. The ring-shaped core plates  14  in adjacent layers are stacked such that the abutting positions of the divided core plates  20  are angularly spaced by the predetermined angle θ 2 )(30°). Since the ring-shaped core plates  14  are stacked such that the abutting positions of the divided core plates  20  are angularly spaced, the divided core plates  20  are prevented from being positionally displaced. 
         [0058]    [Modifications] 
         [0059]    The present invention is not limited to the above embodiment, but may adopt various arrangements based on the contents of the present description. For example, the present invention may adopt the following arrangements: 
         [0060]    In the above embodiment, the tapered keys  32 ,  34  are of linear shapes as viewed in plan. However, the tapered keys  32 ,  34  are not limited to linear shapes, but, as shown in  FIG. 9 , may have round edges at corners (e.g., the bases of the proximal portions  50 ,  60  and the boundaries between the proximal portions  50 ,  60  and the intermediate portions  52 ,  62 ), for thereby making the tapered keys  32 ,  34  more rigid. 
         [0061]    In the above embodiment, the numbers of the tapered keys  32 ,  34  are as shown in  FIGS. 1 and 3 . However, the numbers of the tapered keys  32 ,  34  are not limited to those illustrated, but may be changed according to design. 
         [0062]    In the above embodiment, the angles θp, Os of the tapered keys  32 ,  34  are as shown in  FIG. 4 . However, the angles θp, Os of the tapered keys  32 ,  34  may be of other values. If the angles θp, θs are equal to or greater than 60°, then it is easy to inhibit relative displacement between the tapered keys  32  and the tapered keys  34  and displacement of the ring-shaped core plates  14  with respect to the shaft  16  while the rotor  10  is in rotation. If the angles θp, Os are equal to or smaller than 120°, then it is easy to shrink-fit or cooling-fit the ring-shaped core plates  14  over the shaft  16 . 
         [0063]    In the above embodiment, each of the tapered keys  32  comprises the proximal portion  50 , the intermediate portion  52 , and the distal end portion  54 , and each of the tapered keys  34  comprises the proximal portion  60 , the intermediate portion  62 , and the distal end portion  64 . However, insofar as each of the tapered keys  32 ,  34  has only a region corresponding to the intermediate portions  52 ,  62 , it may dispense with other regions. In the above embodiment, each of the intermediate portions  52 ,  62  is in the shape of an inverted isosceles trapezoid. However, each of the intermediate portions  52 ,  62  is not limited to the shape of an inverted isosceles trapezoid, but may be of other shapes. For example, each of the intermediate portions  52 ,  62  may be of a trapezoidal shape including only one tapered surface  56  or  70 . 
         [0064]    In the above embodiment, the shaft  16  is set in the jig  80  after the shaft  16  is heated. However, the jig  80  may have a heating means, and the shaft  16  may be heated after it is set in the jig  80 . While the rotor core  12  is shrink-fitted over the shaft  16  while only the shaft  16  is being heated in the above embodiment, the rotor core  12  may be shrink-fitted over the shaft  16  while both the shaft  16  and the rotor core  12  are being heated provided that the coefficient of thermal expansion of the shaft  16  is higher than the coefficient of thermal expansion of the rotor core  12 . 
         [0065]    The rotor core  12  may be fitted over the shaft  16  by cooling fitting rather than shrink fitting. 
         [0066]      FIG. 10  is a flowchart of a method of manufacturing the rotor  10  using a cooling fitting process. According to the manufacturing method shown in  FIG. 10 , the coefficient of thermal expansion of the divided core plates  20  should preferably be equal to or higher than the coefficient of thermal expansion of the shaft  16 . 
         [0067]    In step S 11 , the shaft  16  at normal temperature is set in the jig  80 . Then, in step S 12 , the rotor core  12  is cooled. The rotor core  12  thus shrinks in its entirety, with its inside diameter reduced. As a result, the tapered keys  32  of the divided core plates  20  are displaced toward the rotational axis Ax of the rotor  10 . It is thus possible to fit the rotor core  12  over the shaft  16  without the tapered keys  32  and the tapered keys  34  being brought into contact with each other. 
         [0068]    Then, in step S 13 , the cooled rotor core  12  is fitted over the shaft at normal temperature. Thereafter, in step S 14 , the rotor core  12  and the shaft  16  which is cooled by contacting the shaft  16  are left to stand or heated to normal temperature. As a result, the tapered keys  32 ,  34  are thermally expanded. The tapered keys  32  of the divided core plates  20  keep the tapered keys  34  of the shaft  16  pulled away from the rotational axis Ax of the rotor  10 , thereby securely coupling the divided core plates  20  to the shaft  16 . 
         [0069]    The rotor core  12  may be fitted over the shaft  16  by press fitting rather than shrink fitting or cooling fitting.