Patent Publication Number: US-2023155431-A1

Title: Motor

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a U.S. national stage of application No. PCT/JP2021/010210, filed on Mar. 12, 2021, with priority under 35 U.S.C. § 119(a) and 35 U.S.C. § 365(b) being claimed from Japanese Patent Application No. 2020-062835, filed on Mar. 31, 2020, the entire disclosures of which being hereby incorporated herein by reference. 
    
    
     1. FIELD OF THE INVENTION 
     The present disclosure relates to a motor. 
     2. BACKGROUND 
     Typically, a motor includes a rotor and a stator. The rotor includes at least one magnet. 
     It is conceivable to suppress a cogging torque and a torque ripple in order to reduce the vibration and noise generated by the motor. Conventionally, a motor that reduces a cogging torque by providing step skew in a rotor or a stator is known. 
     Since a core of the rotor is configured by stacking electromagnetic steel plates in an axial direction, it is difficult to improve the dimensional accuracy in the axial direction. Further, as the core, an inner core and an outer core may be stacked in a radial direction and used. On the other hand, the magnet of the rotor needs to have a sufficient size in the axial direction in order to ensure sufficient magnetic characteristics. Therefore, it is assumed that the magnet and the outer core protrude to a one axial side with respect to the inner core in a case where an actual dimension of the inner core in the axial direction becomes small within a tolerance of a design dimension. If the rotor partially protrudes in the axial direction in a case where a plurality of the rotors are stacked in the axial direction, such as a case where step skew is provided in the rotors, there is a possibility that protruding portions interfere with each other, the overall axial dimension increases, so that it is difficult to obtain desired characteristics. 
     SUMMARY 
     Example embodiments of the present disclosure provide motors each capable of suppressing each portion of a rotor from protruding in an axial direction. 
     A motor according to an example embodiment of the present invention includes a rotor rotatable about a central axis and a stator opposing the rotor in a radial direction. The rotor includes an inner core extending along the central axis, magnetic pole portions radially outward of the inner core and arranged along a circumferential direction, and a holder holding the inner core and the magnetic pole portions. At least a portion of the magnetic pole portions includes two layers including a magnet and an outer core located radially outward or inward of the magnet and extending along the central axis. The holder includes a flange portion located on a one axial side of the inner core and the magnetic pole portions. The flange portion includes a first opposing surface that opposes an end surface opposing the one axial side of the inner core, a second opposing surface that opposes an end surface opposing the one axial side of the outer core, and a third opposing surface that opposes an end surface opposing the one axial side of the magnet. The second opposing surface is located on the one axial side with respect to the first opposing surface and the third opposing surface. 
     The above and other elements, features, steps, characteristics and advantages of the present disclosure will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic sectional view of a motor of an example embodiment of the present disclosure in a cross section taken along a central axis. 
         FIG.  2    is a partial sectional view of the motor of the example embodiment in a cross section orthogonal to the central axis. 
         FIG.  3    is a perspective view of a rotor of the example embodiment. 
         FIG.  4    is a sectional view of the rotor of the example embodiment in a cross section passing through the central axis and an embedded magnetic pole portion. 
         FIG.  5    is a sectional view of the rotor of the example embodiment in a cross section passing through the central axis and an exterior magnetic pole portion. 
         FIG.  6    is a perspective view of the rotor of the example embodiment, and illustrates a state in which one embedded magnetic pole portion has been removed. 
         FIG.  7    is a perspective view of a rotor coupling body of the example embodiment. 
         FIG.  8    is a graph illustrating a waveform of cogging torque of the motor of the example embodiment. 
         FIG.  9    is a graph illustrating a waveform of a torque ripple of the motor of the example embodiment. 
         FIG.  10    is a sectional view of a rotor of a modification of an example embodiment of the present invention in a cross section passing through a central axis and an exterior magnetic pole portion. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, an axial direction of a central axis J, that is, a direction parallel to the vertical direction, is simply referred to as the “axial direction”, a radial direction around the central axis J is simply referred to as the “radial direction”, and a circumferential direction around the central axis J is simply referred to as the “circumferential direction”. In the following example embodiment, a lower side (−Z) corresponds to a one axial side, and an upper side (+Z) corresponds to the other axial side. Note that the vertical direction, the upper side, and the lower side are simply names for describing a relative positional relationship of each portion, and an actual arrangement relationship or the like may be an arrangement relationship other than the arrangement relationships indicated by these names. 
       FIG.  1    is a schematic sectional view of a motor  1  in a cross section along the central axis J.  FIG.  2    is a partial sectional view of the motor  1  in a cross section orthogonal to the central axis J. 
     As illustrated in  FIG.  1   , the motor  1  of the present example embodiment includes a rotor coupling body  2 , a stator  30 , a plurality of bearings  15 , and a housing  11  accommodating these. The bearing  15  rotatably supports a shaft  21  of the rotor coupling body  2 . The bearing  15  is held by the housing  11 . 
     The stator  30  has an annular shape centered on the central axis J. The rotor coupling body  2  is arranged radially inward of the stator  30 . The stator  30  opposes a pair of rotors  20  of the rotor coupling body  2  in the radial direction. 
     The stator  30  includes a stator core  31 , an insulator  32 , and a plurality of coils  33 . The stator core  31  includes a plurality of electromagnetic steel plates stacked along the axial direction. 
     Specifically, the stator core  31  includes a substantially annular core back  31   a  and a plurality of teeth  31   b . In the example embodiment, the core back  31   a  has an annular shape centered on the central axis J. The teeth  31   b  extend radially inward from a radially inner surface of the core back  31   a . An outer peripheral surface of the core back  31   a  is fixed to an inner circumferential surface of a circumferential wall of the housing  11 . The plurality of teeth  31   b  are arranged at intervals in the circumferential direction on the radially inner surface of the core back  31   a . In the example embodiment, the plurality of teeth  31   b  are arranged at regular intervals in the circumferential direction. 
     The insulator  32  is attached to the stator core  31 . The insulator  32  includes a portion covering the teeth  31   b . For example, an insulating material such as a resin is used as a material for the insulator  32 . 
     The coil  33  is attached to the stator core  31 . The plurality of coils  33  are mounted to the stator core  31  with the insulator  32  interposed therebetween. The plurality of coils  33  are configured by winding a conductive wire around each of the teeth  31   b  with the insulator  32  interposed therebetween. 
     The rotor coupling body  2  includes the shaft  21 , the pair of rotors  20  fixed to the shaft  21 , a spacer  9  arranged between the pair of rotors  20 , and a cover  25 . The rotor coupling body  2  rotates about the central axis J. That is, the shaft  21 , the pair of rotors  20 , and the spacer  9  rotate about the central axis J. The shaft  21  has a columnar shape centered on the central axis J and extending in the axial direction. The cover  25  has a tubular shape centered on the central axis J. The cover  25  surrounds the pair of rotors  20  from the radially outside. The cover  25  is made of a non-magnetic material such as an aluminum alloy or a resin material. 
       FIG.  3    is a perspective view of the rotor  20 . 
     The rotor  20  includes an inner core  22 , a plurality of magnetic pole portions  27  and  28  located radially outward of the inner core  22  and arranged along the circumferential direction, and a holder  40 . Note that the pair of rotors  20  of the rotor coupling body  2  has the same configuration. 
     The inner core  22  extends along the central axis J. The inner core  22  has a substantially polygonal shape as viewed from the axial direction. The inner core  22  is provided with a central hole  22   h  and a plurality of hole portions  22   d  penetrating in the axial direction. The central hole  22   h  is located at the center as viewed from the axial direction. The plurality of hole portions  22   d  are arranged around the central hole  22   h . The shaft  21  is inserted into and fixed to the central hole  22   h . The hole portion  22   d  is provided to lighten the inner core  22  to reduce the weight of the inner core  22 . 
     A plurality of (eight) flat portions  22   a  and  22   b  arranged along the circumferential direction and a plurality of (eight) grooves  22   c  located among the flat portions  22   a  and  22   b  are provided on an outer peripheral surface of the inner core  22  facing radially outward. The groove  22   c  extends over the entire axial length of the inner core. The groove  22   c  is open radially outward. The groove  22   c  has a wedge shape with a groove width decreasing toward the radially outside. 
     The flat portions  22   a  and  22   b  have flat shapes perpendicular to the radial direction. The flat portion  22   a  extends over the entire axial length of the inner core  22  in the axial direction. The eight flat portions  22   a  and  22   b  are classified into four first flat portions  22   a  and four second flat portions  22   b . The first flat portion  22   a  and the second flat portion  22   b  are alternately arranged along the circumferential direction. The first flat portion  22   a  is arranged radially outward of the second flat portion  22   b.    
     The eight magnetic pole portions  27  and  28  are classified into four exterior magnetic pole portions (first magnetic pole portions)  27  and four embedded magnetic pole portions (second magnetic pole portions)  28 . The exterior magnetic pole portion  27  is arranged on the first flat portion  22   a , and the embedded magnetic pole portion  28  is arranged in the second flat portion  22   b . That is, the exterior magnetic pole portion  27  and the embedded magnetic pole portion  28  are alternately arranged along the circumferential direction of the central axis J. 
     The exterior magnetic pole portion  27  has an exterior magnet (magnet)  23   a  exposed to a radially outer surface. On the other hand, the embedded magnetic pole portion  28  has an embedded magnet (magnet)  23   b  and the outer core  24  covering the embedded magnet  23   b  from the radially outside. The exterior magnet  23   a  and the embedded magnet  23   b  are permanent magnets. 
     Note that the expression “the magnet is exposed radially outward” in the present specification means that the magnet is magnetically exposed radially outward. That is, it means that a member that affects flow of a magnetic flux of the magnet is not arranged between the magnet and the stator located radially outward of the magnet. Thus, the cover made of the non-magnetic material may be arranged between the magnet and the stator as illustrated in the present example embodiment. 
     As illustrated in  FIG.  2   , the exterior magnet  23   a  is arranged on a radially outer surface (the first flat portion  22   a ) of the inner core  22  in the exterior magnetic pole portion  27 . The exterior magnet  23   a  is exposed radially outward. The exterior magnetic pole portion  27  can be referred to as a magnetic pole portion of a surface permanent magnet (SPM). 
     The exterior magnet  23   a  has a plate shape. The exterior magnet  23   a  has a quadrangular shape as viewed from the radial direction. The exterior magnet  23   a  has an arc shape in which a radially inner surface is linear and a radially outer surface projects radially outward as viewed from the axial direction. Thus, a radial thickness of the exterior magnet  23   a  increases from both circumferential ends toward the central side (circumferential inside). The radial inner surface of the exterior magnet  23   a  has a flat shape extending in a direction perpendicular to the radial direction. The radially outer surface of the exterior magnet  23   a  has a curved shape that is convex radially outward as viewed in the axial direction. 
     In the embedded magnetic pole portion  28 , the embedded magnet  23   b  is arranged on a radially outer surface (the second flat portion  22   b ) of the inner core  22 , and the outer core  24  is arranged on the radially outer surface of the embedded magnet  23   b . That is, in the embedded magnetic pole portion  28 , the embedded magnet  23   b  and the outer core  24  are arranged in this order from the second flat portion  22   b  to the radially outer side. The embedded magnet  23   b  is covered by the outer core  24 , and the outer core  24  is exposed radially outward. Positions of both circumferential ends of the embedded magnet  23   b  and positions of both circumferential ends of the outer core  24  are arranged to overlap each other as viewed from the radial direction. The embedded magnetic pole portion  28  can be referred to as a magnetic pole portion of an interior permanent magnet (IPM). 
     The embedded magnet  23   b  has a plate shape. The embedded magnet  23   b  has a quadrangular plate shape. The embedded magnet  23   b  has a rectangular shape in which a length along the circumferential direction is larger than a length in the radial direction as viewed from the axial direction. Each of the radially inner surface and the radially outer surface of the embedded magnet  23   b  has the flat shape extending in the direction perpendicular to the radial direction. 
     The outer core  24  has a plate shape. The outer core  24  has a quadrangular shape as viewed from the radial direction. The outer core  24  has an arc shape in which a radially inner surface is linear and a radially outer surface is convex radially outward as viewed from the axial direction. Thus, a radial thickness of the outer core  24  increases from both the circumferential ends toward the central side (circumferential inside). The radially inner surface of the outer core  24  is a flat shape extending in the direction perpendicular to the radial direction. The radially outer surface of the outer core  24  has a curved surface convex radially outward as viewed in the axial direction. 
     As illustrated in  FIG.  3   , the holder  40  holds the inner core  22  and magnetic pole portions  27  and  28  to be embedded. The holder  40  is made of a resin material. In the present example embodiment, the holder  40  is molded by insert molding in which a part of the inner core  22  is embedded. Further, the plurality of magnetic pole portions  27  and  28  are fixed to the holder  40 . In a process of molding the holder  40 , the inner core  22  is held in a mold in a state where an upper end surface (end surface opposing the other axial side)  22   j  is in contact with the mold. 
     The holder  40  includes a flange portion  41  and a plurality of (eight in the present example embodiment) holding portions  48 . The flange portion  41  is located on the lower side (one axial side) of the inner core  22  and the plurality of magnetic pole portions  27  and  28 . The holding portion  48  extends in a columnar shape from the flange portion  41  toward the upper side (other axial side). The plurality of holding portions  48  are arranged at equal intervals along the circumferential direction. The exterior magnetic pole portion  27  or the embedded magnetic pole portion  28  is arranged between the holding portions  48  adjacent to each other in the circumferential direction. 
     As illustrated in  FIG.  2   , the holding portion  48  includes an anchor portion  48   a  and movement suppressing portions  48   b . The groove  22   c  is filled with the molten resin and solidified, thereby forming the anchor portion  48   a . A circumferential width of the anchor portion  48   a  increases toward the radially inner side. The movement suppressing portion  48   b  is located radially outward of the anchor portion  48   a  and connected to the anchor portion  48   a . The movement suppressing portion  48   b  is arranged at a radially outer end of the holding portion  48 . The movement suppressing portions  48   b  protrude from the anchor portion  48   a  toward both circumferential sides (one side and the other side), respectively. The movement suppressing portion  48   b  has a plate shape in which a plate surface is directed the radial direction. 
     According to the present example embodiment, the magnetic pole portion (the exterior magnetic pole portion  27  or the embedded magnetic pole portion  28 ) is press-fitted between the holding portions  48  arranged along the circumferential direction. That is, the plurality of holding portions  48  hold each of the magnetic pole portions  27  and  28  from both sides in the circumferential direction. According to the present example embodiment, since the wedge-shaped groove  22   c  is provided on the radially outer surface of the inner core  22 , the holding portion  48  is suppressed from moving radially outward, and the holding portion  48  can be caused to function. Furthermore, the holding portion  48  can press the exterior magnetic pole portion  27  and the embedded magnetic pole portion  28  from the radially outer side by the movement suppressing portion  48   b  and can suppress the magnetic pole portions  27  and  28  from moving radially outward. 
       FIGS.  4  and  5    are sectional views of the rotor  20  in a cross section along the central axis J. The cross section in  FIG.  4    passes through the central axis J and the embedded magnetic pole portion  28 . Further, the cross section in  FIG.  5    passes through the central axis J and the exterior magnetic pole portion  27 . Note that the illustration of the hole portion  22   d  provided in the inner core  22  is omitted in  FIGS.  4  and  5   . 
     As illustrated in  FIG.  4   , the flange portion  41  has a first opposing surface  41   a , a second opposing surface  41   b , and a third opposing surface  41   c  which face upward (the other axial side) in the cross section passing through the central axis J and the embedded magnetic pole portion  28 . In the cross section passing through the central axis J and the embedded magnetic pole portion  28 , the first opposing surface  41   a , the third opposing surface  41   c , and the second opposing surface  41   b  are arranged in this order from the inner side to the outer side in the radial direction. 
     The first opposing surface  41   a  overlaps the inner core  22  as viewed from the axial direction. The first opposing surface  41   a  faces the lower end surface (end surface opposing the one axial side)  22   k  of the inner core  22 . The holder  40  of the present example embodiment embeds the lower end surface  22   k  of the inner core  22 . Thus, the first opposing surface  41   a  is in contact with the lower end surface  22   k.    
     The second opposing surface  41   b  overlaps the outer core  24  as viewed from the axial direction. The second opposing surface  41   b  faces a lower end surface (end surface opposing the one axial side)  24   k  of the outer core  24 . The second opposing surface  41   b  may be in contact with or separated from the lower end surface  24   k  of the outer core  24 . The second opposing surface  41   b  is located on the lower side (one axial side) of the first opposing surface  41   a  and the third opposing surface  41   c.    
       FIG.  6    is a perspective view of the rotor  20 , and is a view illustrating a state in which one embedded magnetic pole portion  28  has been removed. As illustrated in  FIG.  6   , a part of the second opposing surface  41   b  extends to a region located immediately below the embedded magnet  23   b.    
     The second opposing surface  41   b  is provided with a protrusion  42  protruding upward (to the other axial side). That is, the flange portion  41  has the protrusion  42  protruding upward from the second opposing surface  41   b . The protrusion  42  is arranged at a radially inner end of the second opposing surface  41   b . The protrusion  42  is located at the circumferential center of the second opposing surface  41   b . The protrusion  42  has a semicircular shape as viewed from the axial direction. The third opposing surface  41   c  is provided on an upper surface of the protrusion  42 . That is, the third opposing surface  41   c  is located at an upper tip (tip on the other axial side) of the protrusion  42 . 
     As illustrated in  FIG.  4   , the third opposing surface  41   c  opposes a lower end surface (end surface opposing the one axial side)  23   k  of the embedded magnet  23   b . The third opposing surface  41   c  may be in contact with or separated from the lower end surface  23   k  of the embedded magnet  23   b.    
     In the present example embodiment, the inner core  22  includes a plurality of electromagnetic steel plates  22   t  stacked along the axial direction of the central axis J. Similarly, the outer core  24  has a plurality of electromagnetic steel plates  24   t  stacked along the axial direction of the central axis J. As a result, magnetic characteristics of the inner core  22  and the outer core  24  in a desired direction can be enhanced. In the present example embodiment, design dimensions of thicknesses of the electromagnetic steel plates  22   t  and  24   t  of the inner core  22  and the outer core  24  are the same. In addition, the number of stacked electromagnetic steel plates  22   t  in the inner core  22  and the number of stacked electromagnetic steel plates  24   t  in the outer core  24  are the same. 
     The electromagnetic steel plates  22   t  and  24   t  are formed by press working. Therefore, it is necessary to set dimensional tolerances of the inner core  22  and the outer core  24  to be large by piling up dimensional errors of thicknesses of base materials of the electromagnetic steel plates  22   t  and  24   t  in the axial direction. Further, in general, the electromagnetic steel plates  22   t  and  24   t  of the inner core  22  and the outer core  24  are made of the same type of steel plates, the dimensional tolerances of the inner core  22  and the outer core  24  are similarly set. 
     According to the present example embodiment, the second opposing surface  41   b  is located below the first opposing surface  41   a . Therefore, even when an axial dimension of the outer core  24  is larger than an axial dimension of the inner core  22 , an upper end surface  24   j  of the outer core  24  can be suppressed from protruding above the upper end surface  22   j  of the inner core  22 . 
     A tolerance of an axial dimension of the embedded magnet  23   b  can be set to be smaller than those of the inner core  22  and the outer core  24 . However, the embedded magnet  23   b  preferably has a certain axial dimension or more in order to ensure sufficient magnetic characteristics, and is unlikely to have a negative tolerance with respect to the inner core  22  and the outer core  24 . 
     According to the present example embodiment, the third opposing surface  41   c  is located on the upper side of the second opposing surface  41   b . Since the embedded magnet  23   b  can have a smaller dimensional tolerance in the axial direction as compared with the outer core  24 , even when the third opposing surface  41   c  is arranged on the upper side of the second opposing surface  41   b , the upper end surface  23   j  of the embedded magnet  23   b  can be suppressed from protruding above the upper end surface  22   j  of the inner core  22 . Furthermore, since the third opposing surface  41   c  is located on the upper side of the second opposing surface  41   b , the embedded magnet  23   b  can be arranged to overlap a wide range of the second flat portion  22   b  of the inner core  22 , so that flow of a magnetic flux between the inner core  22  and the embedded magnet  23   b  can be made smoother. 
     According to the present example embodiment, the third opposing surface  41   c  is provided on the protrusion  42 . Therefore, an area of the third opposing surface  41   c  can be reduced, and the dimensional accuracy of the entire third opposing surface  41   c  can be easily improved. Furthermore, a thickness of the flange portion  41  is not increased on the lower side of the third opposing surface  41   c , and the generation of a sink mark in the flange portion  41  can be suppressed. 
     The third opposing surface  41   c  illustrated in  FIG.  4    is located on the lower side of the first opposing surface  41   a . Since the first opposing surface  41   a  is a surface that embeds the lower end surface  22   k  of the inner core  22 , a relative axial position between the second opposing surface  41   b  and the third opposing surface  41   c  changes depending on an actual dimension of the inner core  22 . Therefore, it is also conceivable that the third opposing surface  41   c  is located on the upper side of the first opposing surface  41   a.    
     Here, the tolerances of the axial dimensions of the inner core  22  and the outer core  24  are each defined as ±D, and the tolerance of the axial dimension of the embedded magnet  23   b  is defined as ±d. Note that the dimensional tolerance of the embedded magnet  23   b  can be set to be smaller than those of the inner core  22  and the outer core  24 , a relationship of D&gt;d is established. 
     In a case where the actual axial dimension of the inner core  22  is the minimum within the tolerance, the third opposing surface  41   c  is arranged at a position of D+d on the lower side with respect to the first opposing surface  41   a . In this case, the second opposing surface  41   b  is arranged at a position of 2D on the lower side with respect to the first opposing surface  41   a.    
     When the actual dimension of the inner core  22  in the axial direction is the maximum within the tolerance, the third opposing surface  41   c  is arranged at the position D−d on the upper side with respect to the first opposing surface  41   a . Further, in this case, the second opposing surface  41   b  is arranged at a position substantially coinciding with the first opposing surface  41   a.    
     Note that a positional relationship between the second opposing surface  41   b  and the third opposing surface  41   c  in the axial direction is set such that the third opposing surface  41   c  is always arranged on the upper side of the second opposing surface  41   b  by d+D regardless of the actual dimension of the inner core  22 . 
     The embedded magnetic pole portion  28  is press-fitted between the holding portions  48  arranged in the circumferential direction in the state of overlapping the outer core  24  and the embedded magnet  23   b  in the radial direction. In a process of press-fitting the embedded magnetic pole portion  28 , the outer core  24  and the embedded magnet  23   b  are press-fitted until any one of the lower end surfaces  24   k  and  23   k  comes into contact with the flange portion  41 . In a case where such a press-fitting process is adopted, at least one of the outer core  24  and the embedded magnet  23   b  comes into contact with the flange portion  41 . When such a press-fitting process is adopted, the press-fitting process can be easily performed. 
     Note that a press-fitting process of press-fitting the outer core until the upper end surfaces  24   j  and  23   j  of the outer core  24  and the embedded magnet  23   b  reach the upper end surface  22   j  of the inner core  22  may be adopted. In this case, the lower end surfaces  24   k  and  23   k  of the outer core  24  and the embedded magnet  23   b  are separated from the flange portion  41 . In a case where such a press-fitting process is adopted, the overlapping area among the inner core  22 , the outer core  24 , and the embedded magnet  23   b  as viewed from the radial direction can be increased, and the flow of the magnetic flux can be made smooth. 
     Assuming that the upper end surface  22   j  of the inner core  22  is a reference surface in the present example embodiment, the upper end surfaces (surfaces opposing the other axial side)  24   j  and  23   j  of the outer core  24  and the embedded magnet  23   b  are located on the lower side (one axial side) of the reference surface  22   j . According to the present example embodiment, the outer core  24  and the embedded magnet  23   b  do not protrude upward from the reference surface  22   j  of the inner core  22  on the opposite side of the flange portion  41 . Thus, when another member is arranged on the upper side of the rotor  20  with reference to the reference surface  22   j , interference between the other member and each of the outer core  24  and the embedded magnet  23   b  can be suppressed. 
     More specifically, when the spacer  9  is arranged in contact with the reference surface  22   j , interference between the spacer  9  and each of the outer core  24  and the embedded magnet  23   b  can be suppressed, and an increase in axial dimension of the rotor coupling body  2  can be suppressed. 
     As illustrated in  FIG.  5   , in the cross section passing through the central axis J and the exterior magnetic pole portion  27 , the flange portion  41  has the first opposing surface  41   a  and a fourth opposing surface  41   d  facing upward (the other axial side). In the cross section passing through the central axis J and the exterior magnetic pole portion  27 , the first opposing surface  41   a  and the fourth opposing surface  41   d  are arranged in this order from the inner side to the outer side in the radial direction. 
     The fourth opposing surface  41   d  overlaps the exterior magnet  23   a  as viewed from the axial direction. The fourth opposing surface  41   d  opposes a lower end surface of the exterior magnet  23   a . The fourth opposing surface  41   d  may be in contact with or separated from the lower end surface of the exterior magnet  23   a . The fourth opposing surface  41   d  is located on the lower side of the first opposing surface  41   a . According to the present example embodiment, it is possible to suppress the exterior magnet  23   a  from protruding upward from the upper end surface (reference surface)  22   j  of the inner core  22  on the opposite side of the flange portion  41 . Thus, when the spacer  9  is arranged in contact with the reference surface  22   j , interference between the spacer  9  and the exterior magnet  23   a  can be suppressed, and the increase in the axial dimension of the rotor coupling body  2  can be suppressed. 
       FIG.  7    is a perspective view of the rotor coupling body  2  of the present example embodiment. 
     In the rotor coupling body  2 , the pair of rotors  20  are stacked in the axial direction with the flange portions  41  arranged on the opposite axial sides. Further, the spacer  9  is arranged between the pair of rotors  20 . 
     In the following description, in a case where the pair of rotors  20  are distinguished from each other, one arranged on the upper side is referred to as a first rotor  20 A, and the other arranged on the lower side is referred to as a second rotor  20 B. In the first rotor  20 A, the flange portion  41  of the holder  40  is arranged on the upper side of the inner core  22  and the magnetic pole portions  27  and  28 . On the other hand, in the second rotor  20 B, the flange portion  41  of the holder  40  is arranged on the lower side of the inner core  22  and the magnetic pole portions  27  and  28 . 
     As illustrated in  FIG.  7   , the first rotor  20 A and the second rotor  20 B are arranged such that the exterior magnetic pole portion  27  and the embedded magnetic pole portion  28  are shifted in the axial direction. The embedded magnetic pole portion  28  of the second rotor  20 B is arranged on the lower side of the exterior magnetic pole portion  27  of the first rotor  20 A. Further, the exterior magnetic pole portion  27  of the second rotor  20 B is arranged on the lower side of the embedded magnetic pole portion  28  of the first rotor  20 A. That is, the exterior magnetic pole portion  27  of one of the pair of rotors  20  and the embedded magnetic pole portion  28  of the other of the pair of rotors  20  are arranged side by side in the axial direction. A circumferential center of the exterior magnetic pole portion  27  of one rotor  20  and a circumferential center of the embedded magnetic pole portion  28  of the other rotor  20  are arranged so as to overlap each other. In this manner, the magnets (the exterior magnet  23   a  and the embedded magnet  23   b ) of the present example embodiment are arranged straight in the axial direction without applying skew. 
     In the same rotor  20 , the exterior magnetic pole portion  27  and the embedded magnetic pole portion  28  have mutually different magnetic poles facing radially outward. Further, the exterior magnetic pole portion  27  and the embedded magnetic pole portion  28  arranged in the axial direction have the same magnetic pole facing radially outward. For example, the exterior magnetic pole portion  27  of the first rotor  20 A and the embedded magnetic pole portion  28  of the second rotor  20 B have an N pole facing radially outward, and the embedded magnetic pole portion  28  of the first rotor  20 A and the exterior magnetic pole portion  27  of the second rotor  20 B have an S pole facing radially outward. 
       FIG.  8    is a graph illustrating a waveform of cogging torque of the motor  1  of the present example embodiment.  FIG.  9    is a graph illustrating a waveform of a torque ripple of the motor  1  of the example embodiment. As illustrated in  FIG.  8    and  FIG.  9   , in the example embodiment, opposite phases can be generated in the cogging torque without applying skew to the magnets (the exterior magnet  23   a  and the embedded magnet  23   b ). That is, the cogging torque generated in a first rotor  20 A and the cogging torque generated in a second rotor  20 B are generated with phases opposite to each other, and thus, cancel each other, so that a fluctuation range of a combined cogging torque waveform (a difference between a maximum value and a minimum value of the combined cogging torque) can be kept small. The opposite phase can be generated in the torque ripple. That is, because the torque ripple generated in the first rotor  20 A and the torque ripple generated in the second rotor  20 B are generated with phases opposite to each other, the torque ripple generated in the first rotor  20 A and the torque ripple generated in the second rotor  20 B cancel each other, and a fluctuation range of a combined torque ripple waveform (the difference between the maximum value and the minimum value of the combined torque ripple) can be suppressed to be small. Thus, the cogging torque can be reduced while suppressing the torque reduction, and the torque ripple can be reduced. Then, the vibration and noise generated by the motor  1  can be reduced. 
     Modification 
     In the above-described example embodiment, the exterior magnetic pole portion  27  includes only the exterior magnet  23   a . However, an exterior magnetic pole portion  127  may include an outer core  124  located radially inward of an exterior magnet  123   a  as illustrated in  FIG.  10    as a modification. 
     Note that a constituent element of the identical aspect to that of the above-described example embodiment is denoted by the same reference sign, and the description thereof will be omitted. 
     As illustrated in  FIG.  10   , a holder  140  of a rotor  120  of the modification has, on a flange portion  141 , a first opposing surface  141   a , a second opposing surface  141   b , and a third opposing surface  141   c . In a cross section passing through the central axis J and the exterior magnetic pole portion  127 , the first opposing surface  141   a , the second opposing surface  141   b , and the third opposing surface  141   c  are arranged in this order from the radially inner side to the radially outer side. The first opposing surface  141   a  opposes a lower end surface  22   k  of the inner core  22 . The second opposing surface  141   b  opposes a lower end surface of the outer core  124 . The third opposing surface  141   c  opposes a lower end surface of the exterior magnet  123   a . Further, the flange portion  141  has a protrusion  142  protruding upward from the second opposing surface  141   b , and the third opposing surface  141   c  is located at an upper tip of the protrusion  142 . The second opposing surface  141   b  is located on the lower side of the first opposing surface  141   a  and the third opposing surface  141   c.    
     According to the present modification, the second opposing surface  141   b  is located on the lower side of the first opposing surface  141   a . Therefore, even when an axial dimension of the outer core  124  is larger than an axial dimension of the inner core  22 , an upper end surface of the outer core  124  can be suppressed from protruding above the upper end surface  22   j  of the inner core  22 . Further, the third opposing surface  141   c  is located on the upper side the second opposing surface  141   b  according to the present modification. Since the exterior magnet  123   a  can have a smaller dimensional tolerance in the axial dimension as compared with the outer core  124 , even when the third opposing surface  141   c  is arranged on the upper side of the second opposing surface  141   b , the upper end surface of the exterior magnet  123   a  can be suppressed from protruding above the upper end surface  22   j  of the inner core  22 . Furthermore, since the third opposing surface  141   c  is located on the upper side of the second opposing surface  141   b , the exterior magnet  123   a  can be arranged to overlap a wide range of the first flat portion  22   a  of the inner core  22 , so that flow of a magnetic flux between the inner core  22  and the exterior magnet  123   a  can be made smoother. 
     According to the present modification, the exterior magnet  123   a  does not protrude upward from the upper end surface (reference surface)  22   j  of the inner core  22  on the opposite side of the flange portion  141 . Thus, when a spacer  109  is arranged in contact with the reference surface  22   j , interference between the spacer  109  and each of the outer core  124  and the exterior magnet  123   a  can be suppressed, and an increase in axial dimension of the rotor coupling body  2  can be suppressed. Note that an outer diameter of the spacer  109  of the present modification is smaller than an outer diameter of the exterior magnet  123   a . In this manner, a shape of the spacer  109  is not limited as long as having a size that overlaps the magnet and the outer core of each magnetic pole portion as viewed from the axial direction. 
     As illustrated in the above-described example embodiment and modification, it suffices that the magnetic pole portion opposing the second opposing surface and the third opposing surface has two layers including the magnet and the outer core that is located on the outer side and the inner side of the magnet in the radial direction and extends along the central axis. 
     Although the example embodiment of the present invention and the modification thereof have been described above, the respective configurations and combinations thereof in the example embodiment and the modification are merely examples, and therefore addition, omission, substation and other variations of the configurations can be made within the scope not departing from the gist of the present invention. Further, the present invention is not to be limited by the example embodiment and the modification thereof. 
     For example, the shapes of the magnets and the shapes of the outer cores are not limited to the examples described in the above-described example embodiment and modification. Further, the number of poles of the rotor and the number of slots of the stator are not limited to those of the above-described example embodiment. Additionally, features of the above-described preferred embodiments and the modifications thereof may be combined appropriately as long as no conflict arises. 
     While example embodiments of the present disclosure have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present disclosure. The scope of the present disclosure, therefore, is to be determined solely by the following claims.