Patent Publication Number: US-10312755-B2

Title: Motor

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present invention claims priority under 35 U.S.C. § 119 to Japanese Application No. 2015-039135 filed Feb. 27, 2015, the entire content of which is incorporated herein by reference. 
     FIELD OF THE DISCLOSURE 
     The present disclosure relates to a motor. 
     BACKGROUND 
     For example, Japanese Utility Model Application Publication No. S63-70275 describes a permanent magnet generator having a plurality of plate shape permanent magnets radially arranged on an outer circumference of a non-magnetic axis, and a magnetic pole formed by disposing a core, which has a fan shape in cross section, between the plate shape permanent magnets. The permanent magnet generator disclosed in Japanese Utility Model Application Publication No. S63-70275 has an object of reducing leakage of magnetic flux that passes between adjacent poles through the non-magnetic shaft, and a V-shaped notch portion is provided on an inner diameter side of the magnetic pole core. 
     However, in the permanent magnet generator (motor) as described above, the circumferential width of the magnetic core (core piece part) is smaller at an end portion on a radially inner side of the V-shaped notch portion. For this reason, a technical problem has been raised since the magnetic flux is saturated inside the magnetic pole core, and thereby the magnetic flux is easily leaked to the V-shaped notch portion side. Accordingly, in the permanent magnet generator as described above, the magnetic flux, which is leaked to a radially inner side of the magnetic pole core, could not be sufficiently reduced. 
     SUMMARY 
     One example of the present disclosure is a motor comprising a rotor which has a shaft having its center on a vertically extending center axis, a stator which is disposed at a radially outer side of the rotor, and a bearing which supports the shaft. The rotor has a plurality of core piece parts which are arranged along a circumferential direction, and a plurality of permanent magnets which are respectively disposed between the neighboring core piece parts in the circumferential direction, and magnetize the core piece parts. The permanent magnet has two magnetic poles arranged along the circumferential direction. The magnetic poles of the circumferentially neighboring permanent magnets are arranged such that magnetic poles with identical polarity face each other in the circumferential direction. The core piece part has a concave portion where a surface on a radially inner side is recessed toward a radially outer side, and two protrusion portions which are disposed on both circumferential sides of the concave portion and protrude radially inward respectively along each of the neighboring permanent magnets. When a segment connecting an end portion on a radially inner side of a surface of the protrusion which faces the permanent magnet in the circumferential direction and an end portion on a radially outer side of the concave portion is set to a first segment, the protrusions have a portion located on a radially inner side from the first segment when viewed in an axial direction. 
     According to one example of a motor according to the present disclosure, leakage flux can be reduced. 
     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 embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures, in which: 
         FIG. 1  is a cross-sectional view of a motor according to an embodiment. 
         FIG. 2  illustrates a rotor according to the embodiment, and shows a cross-sectional view taken along the II-II line in  FIG. 1 . 
         FIG. 3  is a cross-sectional view which illustrates a portion of the rotor according to the embodiment, and is a partially enlarged view of  FIG. 2 . 
         FIG. 4  is a partially enlarged view which illustrates another example of a rotor according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Herein, a motor according to an exemplary embodiment of the present disclosure will be described with reference to the drawings. However, it should be noted that the scope of the present disclosure is not limited to the below embodiment, and can be appropriately altered without departing from the scope of the technical concept of the present disclosure. Also, in the following drawings, each structure may have different dimensions or sizes from the actual structure, in order to clearly illustrate each constitution. 
     In addition, in the drawings, an X-Y-Z coordinate system is provided as an appropriate 3-dimensional orthogonal coordinates system. In the X-Y-Z coordinate system, a direction parallel to the axial direction of the center axis J shown in  FIG. 1  is referred to as the z-axis direction. A direction perpendicular to the z-axis direction, that is, the left and right directions of  FIG. 1 , is referred to as the x-axis direction. A direction perpendicular to both the x-axis direction and the z-axis direction is referred to as the y-axis direction. Also, a circumferential direction having its center on the center axis J is referred to as the θZ direction. As to the θZ direction, a clockwise direction when viewed toward the +Z side from the −Z side is referred to as the positive direction, and a counter clockwise direction when viewed toward the +Z side from the −Z side is referred to as the negative direction. 
     Moreover, herein, a direction in which the center axis J extends (z-axis direction) is referred to as the vertical direction. The positive side of the z-axis direction (+Z side) is referred to as the “upper side,” and the negative side of the z-axis direction (−Z side) is referred to as the “lower side.” It is to be understood that the descriptions of the vertical direction, the upper side and the lower side are used for explanation only, and they do not limit the actual positional relation or direction. Also, unless otherwise explained, a direction parallel to the center axis J (z-axis direction) is simply referred to as the “axial direction,” a radial direction having its center on the center axis J is simply referred to as the “radial direction,” and a circumferential direction having its center on the center axis J (θZ direction), that is, the axial circumference of center axis J, is simply referred to as the “circumferential direction.” 
     Further, a direction along the positive direction of the θZ direction (+θZ side, one circumferential direction) is referred to as the “driving side,” and the direction along the negative direction of the θZ direction (−θZ side, the other circumferential direction) is referred to as the “counter driving side.” Also, it is to be understood that the descriptions of the driving and counter driving sides are used for explanation only, and they do not limit the actual driving direction. 
     Furthermore, herein, descriptions such as being axially extended do not only refer to a case of strictly being extended in the axial direction (z-axis direction), but it may also include a case of being extended in a direction inclined at less than 45° relative to the axial direction. Also, descriptions such as being radially extended do not only refer to a case of strictly being extended in the radial direction, that is, the direction perpendicular to the axial direction (z-axis direction), but it may also include a case of being extended in a direction inclined at less than 45° relative to the radial direction. 
       FIG. 1  is a cross-sectional view of a motor  10  of this embodiment. As shown in  FIG. 1 , the motor  10  includes a housing  20 , a rotor  30  having a shaft  31 , a stator  40 , a lower bearing  51 , an upper bearing  52 , and a bus bar unit  60 . 
     The housing  20  is a casing having a cylindrical portion. The housing  20  receives the rotor  30 , the stator  40 , the lower bearing  51 , the upper bearing  52 , and the bus bar unit  60 . The housing  20  has a lower housing  21 , and an upper housing  22 . The lower housing  21  has a cylindrical shape which is open on both sides in the axial direction (±Z side). The upper housing  22  is fixed to an end portion of the upper side (+Z side) of the lower housing  21 . The upper housing  22  covers upper sides of the rotor  30  and the stator  40 . 
     The stator  40  is retained inside the lower housing  21 . The stator  40  is disposed at a radially outer side of the rotor  30 . The stator  40  has a core back portion  41 , a teeth portion  42 , a coil  43 , and a bobbin  44 . The core back portion  41  has, for example, a cylindrical shape concentric with the center axis J. The outer surface of the core back portion  41  is fixed to the inner surface of the lower housing  21 . 
     The teeth portion  42  extends from the inner surface of the core back portion  41  toward the shaft  31 . Although it is omitted from the drawings, a plurality of teeth portions  42  are provided and arranged at equal intervals in the circumferential direction. The bobbin  44  is mounted on each teeth portion  42 . The coil  43  is wound around each teeth portion  42  via the bobbin  44 . In this embodiment, the core back portion  41  and the teeth portion  42  are made of a stack of laminated steel plates which is formed by laminating a plurality of electromagnetic steel plates. 
     The bus bar unit  60  is disposed at an upper side (+Z side) of the stator  40 . The bus bar unit  60  has a connector portion  62 . An external power source, which is not shown in the drawings, is connected to the connector portion  62 . The bus bar unit  60  has a wiring member which is electrically connected with the coil  43  of the stator  40 . One end of the wiring member is exposed to the exterior of the motor  10  via the connector portion  62 . Accordingly, power is supplied from the external power source to the coil  43  through the wiring member. The bus bar unit  60  has a bearing support portion  61 . 
     The lower bearing  51  and the upper bearing  52  support the shaft  31 . The lower bearing  51  is disposed at a lower side (−Z side) than the stator  40 . The lower bearing  51  is retained in the lower housing  21 . The upper bearing  52  is disposed at an upper side (+Z side) than the stator  40 . The upper bearing  52  is retained in the bearing support portion  61  of the bus bar unit  60 . 
     The rotor  30  has a shaft  31 , and a rotor body unit  32 . The shaft  31  has its center on the center axis J which extends in the vertical direction (z-axis direction). In this embodiment, the shaft is a member having a cylindrical shape. The shaft may be a solid type member or a hollow type cylindrical member. The rotor body unit  32  is disposed at a radially outer side of the shaft  31 . In this embodiment, the rotor body unit  32  is fixed to an outer circumferential surface of the shaft  31 . In this embodiment, the rotor  30  rotates, for example, in the counter clockwise direction about the center axis J when viewed from the upper side (+Z side), that is, from the counter driving side (−θZ side) to the driving side (+θZ side). 
       FIG. 2  illustrates the rotor  30 , and shows a cross-sectional view taken along the II-II line from  FIG. 1 .  FIG. 3  is a partially enlarged view of  FIG. 2 . The rotor body unit  32  has, as shown in  FIG. 2 , a plurality of permanent magnets  33 A,  33 B, a plurality of core piece parts  34 N,  34 S, and a mold resin portion  35 . That is, the rotor  30  has the plurality of permanent magnets  33 A,  33 B, the plurality of core piece parts  34 N,  34 S, and the mold resin portion  35 . The rotor body unit  32  is formed by molding, for example, in which the core piece parts  34 N,  34 S and the permanent magnets  33 A,  33 B are arranged in a mold and then resin is inserted therein. 
     The mold resin portion  35  is disposed between the plurality of core piece parts  34 N,  34 S. The mold resin portion  35  is made of resin. In this embodiment, the plurality of core piece parts  34 N,  34 S are retained by the mold resin portion  35 . 
     Also, herein, the description that the mold resin portion is disposed between the plurality of core piece parts, includes the case that at least a portion of the mold resin portion is located on a line that connects any two core piece parts of the plurality of core piece parts. The two core piece parts of the plurality of core piece parts are not particularly limited, and they may be two circumferentially neighboring core piece parts, or two core piece parts that face each other in the radial direction across the shaft  31 . 
     The permanent magnets  33 A,  33 B magnetize the core piece parts  34 N,  34 S. In this embodiment, the core piece parts  34 N,  34 S are made of a stack of laminated steel plates which is formed by laminating a plurality of electromagnetic steel plates. The electromagnetic steel plate is a type of magnetic material. The permanent magnet  33 A and the permanent magnet  33 B are alternately arranged in the circumferential direction. The permanent magnets  33 A,  33 B are respectively disposed between the core piece parts  34 N,  34 S in the circumferential direction. 
     The permanent magnets  33 A,  33 B respectively have two magnetic poles arranged in the circumferential direction. The permanent magnet  33 A has, for example, the N-pole on the driving side (+θZ side), and the S-pole on the counter driving side (−θZ side). The permanent magnet  33 B has, for example, the S-pole on the driving side (+θZ side), and the N-pole on the counter driving side (−θZ side). Accordingly, the magnetic poles of the circumferentially neighboring permanent magnets  33 A,  33 B are arranged such that magnetic poles with identical polarity face each other in the circumferential direction. 
     The permanent magnet  33 A and the permanent magnet  33 B are configured in the same way, except that their arrangement of the magnetic poles in the circumferential direction is different from each other. For this reason, in the following descriptions, the permanent magnet  33 A may be sometimes explained as a representation of the permanent magnets, and explanations related to the permanent magnet  33 B may be omitted. 
     As shown in  FIG. 3 , the permanent magnet  33 A is, for example, in direct contact with the core piece part  34 N and the core piece part  34 S which are disposed respectively on both sides in the circumferential direction. As a result, the permanent magnet  33 A is securely attached to the core piece part  34 N and the core piece part  34 S by magnetic force. The permanent magnet  33 A extends in the radial direction. The shape of the cross section perpendicular to the axial direction (Z-axis direction) of the permanent magnet  33 A is, for example, quadrangular. As shown in  FIG. 2 , the number of the permanent magnet  33 A provided in this embodiment is, for example, seven. The number of the permanent magnet  33 B is also, for example, seven. That is, the number of the permanent magnet  33 A is identical to that of the permanent magnet  33 B. The number of the permanent magnets  33 A,  33 B may be appropriately altered in accordance with the specification of the motor. 
     The core piece parts  34 N,  34 S are arranged along the circumferential direction at a radially outer side of the shaft  31 . The core piece part  34 N and the core piece part  34 S are alternately arranged along the circumferential direction. The core piece part  34 N is disposed between the N-pole of the permanent magnet  33 A and the N-pole of the permanent magnet  33 B. Accordingly, the core piece part  34 N is magnetized to the N-pole. The core piece part  34 S is disposed between the S-pole of the permanent magnet  33 A and the S-pole of the permanent magnet  33 B. Accordingly, the core piece is magnetized to the S-pole. 
     In this embodiment, the core piece parts  34 N,  34 S are connected to each other only by the mold resin portion  35 . That is, other than the mold resin portion  35 , the rotor  30  does not have any other portion to connect the core piece parts  34 N,  34 S to each other, and the core piece parts  34 N,  34 S are separated. Accordingly, when the mold resin portion  35  is formed by molding, for example, the resin easily flows between the core piece parts  34 N,  34 S. Also, the magnetic flux in the core piece parts  34 N,  34 S can be inhibited from leaking to a radially inside of the core piece parts  34 N,  34 S. 
     The number of the core piece part  34 N provided in this embodiment is, for example, seven. The number of the core piece  34 S is also, for example, seven. That is, the number of the core piece part  34 N and the number of the core piece part  34 S are identical. The number of the permanent magnets  33 A,  33 B is identical to the number of the core piece parts  34 N,  34 S. Further, the number of the core piece parts  34 N,  34 S may be appropriately altered, as long as they match the number of the permanent magnets  33 A,  33 B. 
     The core piece part  34 N and the core piece part  34 S are configured in the same way, except that they are magnetized to have different polarities. For this reason, in the following descriptions, the core piece part  34 N may be sometimes explained as a representation of the core piece parts, and the core piece part  34 S may not be explained in detail. 
     As shown in  FIG. 3 , the core piece part  34 N has a substantially fan shape, in which the circumferential width thereof increases from a radially inner side toward a radially outer side. Corner parts on a radially outer side of the core piece part  34 N are, for example, chamfered in both circumferential directions. The chamfering may be angular chamfering or round chamfering. In the example shown in  FIG. 3 , the corner parts on the radially outer side of the core piece parts  34 N are angular-chamfered. 
     The core piece part  34 N has a concave portion  36 , and two protrusion portions. The two protrusion portions respectively have a driving side protrusion  37  and a counter driving side protrusion  38 . The concave portion  36  is a portion where a surface on a radially inner side of the core piece part  34 N is recessed toward a radially outer side. When viewed in the axial direction, the concave portion  36  has a shape in which the circumferential width thereof is reduced from a radially inner side toward a radially outer side. 
     The driving side protrusion  37  and the counter driving side protrusion  38  are disposed on both circumferential sides of the concave portion  36 , respectively. More specifically, the driving side protrusion  37  is disposed on the driving side (+θZ side) of the concave portion  36 . The counter driving side protrusion  38  is disposed on the counter driving side (−θZ side) of the concave portion  36 . The driving side protrusion  37  is adjacent to the counter driving side protrusion  38  via the concave portion  36 . 
     The driving side protrusion  37  and the counter driving side protrusion  38  respectively protrude radially inward along each of the neighboring permanent magnets  33 A,  33 B. More specifically, the driving side protrusion  37  protrudes radially inward along the side surface of the permanent magnet  33 A adjacent to the driving side (+θZ side) of the core piece part  34 N. The counter driving side protrusion  38  protrudes radially inward along the side surface of the permanent magnet  33 B adjacent to the counter driving side (−θZ side) of the core piece part  34 N. 
     A first driving side protrusion lateral surface  37   a  is a surface of the driving side protrusion  37  which faces the permanent magnet  33 A in the circumferential direction. A magnet counter driving side surface  33 Aa is a surface of the permanent magnet  33 A which faces the core piece part  34 N in the circumferential direction. The first driving side protrusion lateral surface  37   a  is substantially parallel to the magnet counter driving side surface  33 Aa. The first driving side protrusion lateral surface  37   a  is in contact with the magnet counter driving side surface  33 Aa. A first counter driving side protrusion lateral surface  38   a  is a surface of the counter driving side protrusion  38  which faces the permanent magnet  33 B in the circumferential direction. A magnet driving side surface  33 Ba is a surface of the permanent magnet  33 B which faces the core piece part  34 N in the circumferential direction. The first counter driving side protrusion lateral surface  38   a  is parallel to the magnet driving side surface  33 Ba. The first counter driving side protrusion lateral surface  38   a  is in contact with the magnet driving side surface  33 Ba. 
     For example, within the core piece part  34 N, a magnetic flux passing through the permanent magnet  33 A and the core piece part  34 N and a magnetic flux passing through the permanent magnet  33 B and the core piece part  34 N are in close proximity. For this reason, within the core piece part  34 N, there exist a magnetic flux that flows toward a radially outer side of the core piece part  34 N from the permanent magnets  33 A,  33 B through the core piece part  34 N, and a magnet flux that flows toward a radially inner side of the core piece part  34 N from the permanent magnets  33 A,  33 B through the core piece part  34 N. That is, within the core piece part  34 N, there may exist a magnetic path extending to a radially outer side of the core piece part  34 N from the permanent magnets  33 A,  33 B through the core piece part  34 N, and a magnetic path extending to a radially inner side of the core piece part  34 N from the permanent magnets  33 A,  33 B through the core piece part  34 N. The magnetic path extending to a radially inner side of the core piece part  34 N is likely to occur at a portion closer to a radially inner side of the core piece part  34 N. 
     The magnetic flux extending to a radially inner side of the core piece part  34 N passes through a radially inner side of the permanent magnet  33 A or a radially inner side of the permanent magnet  33 B, and extends to the neighboring core piece part  34 S. The magnetic flux which passes through this magnetic path, that is, the magnetic flux which passes through a radially inner side of the permanent magnet  33 A or a radially inner side of the permanent magnet  33 B, and flows in between the neighboring core pieces  34 N,  34 S, does not contribute to the generation of torque, and neither to the rotation of the rotor  30 . That is, among the magnetic flux of the permanent magnets  33 A,  33 B, the proportion that does not contribute to the generation of torque increases. 
     As to the specific flows of the magnetic fluxes in the core peace part  34 N of this embodiment, for example, the magnetic fluxes emitted from the N-poles of the permanent magnets  33 A,  33 B facing the core piece part  34 N are in close proximity inside the core piece part  34 N. The flows of the magnetic fluxes that are in close proximity avoid each other and flow toward a radially outer side of the core piece part  34 N, or a radially inner side of the core piece part  34 S. Also, the magnetic flux in the core piece  34 S flows in the reverse direction of the magnetic flux in the core piece part  34 N. 
     In addition, in the following descriptions, the magnetic path which passes through a radially inner side of the permanent magnet  33 A or a radially inner side of the permanent magnet  33 B, and connects the neighboring core piece parts  34 N,  34 S may be referred to as the magnetic path passing through a radially inner side of the permanent magnets  33 A,  33 B. Also, herein, the magnetic flux which flows in the magnetic path passing through a radially inner side of the permanent magnets  33 A,  33 B may be referred to as the leakage flux. 
     According to this embodiment, the concave portion  36  is provided on the inside surface of the core piece part  34 N. Also, the driving side protrusion  37  and the counter driving side protrusion  38  are provided on both sides of the concave portion  36  in the circumferential direction, respectively. For this reason, the magnetic flux passing through the permanent magnet  33 A and the driving side protrusion  37  and the magnetic flux passing through the permanent magnet  33 B and the counter driving side protrusion  38  can be inhibited from approaching each other at a portion where the concave portion  36  is provided, that is, at a portion closer to a radially inner side of the core piece part  34 N. Accordingly, the formation of a magnetic path which passes through a radially inner side of the permanent magnets  33 A,  33 B can be suppressed. As a result, according to this embodiment, the leakage flux in the rotor  30  can be reduced. 
     In this embodiment, a segment which connects an end portion P 1  on a radially inner side of the first driving side protrusion lateral surface  37   a  and an end portion P 2  on a radially outer side of the concave portion  36  is referred to as a first segment S 1 . When viewed from the axial direction, the driving side protrusion  37  has a protruding inner portion  37   c  which is a portion disposed on a radially inner side from the first segment S 1 . 
     For example, when the circumferential width of the driving side protrusion  37  and the circumferential width of the counter driving side protrusion  38  are relatively small, there is possibility that magnetic saturation may occur in the driving side protrusion  37  and the counter driving side protrusion  38 . As a result, there is a possibility that the magnetic flux is easily leaked radially inward from the driving side protrusion  37  and the counter driving side protrusion  38 , so that the leakage flux cannot be sufficiently reduced. 
     To the contrary, according to this embodiment, the circumferential width of the core piece part  34 N, that is, the circumferential width of the driving side protrusion  37  can be made relatively bigger at a portion closer to a radially inner side of the core piece part  34 N, by providing the protruding inner portion  37   c . With this, magnetic saturation is inhibited from occurring in the driving side protrusion  37 . Accordingly, the magnetic flux can be inhibited from leaking radially inward from the driving side protrusion  37 . Thus, according to this embodiment, the motor  10  having a structure capable of reducing leakage flux is obtained. 
     When viewed in the axial direction, the counter driving side protrusion  38  has a protruding inner portion  38   c  which is a portion disposed on a radially inner side from a segment connecting an end portion on a radially inner side of the first counter driving side protrusion lateral surface  38   a  and an end portion P 2  on a radially outer side of the concave portion  36 . Accordingly, the magnetic flux can be inhibited from leaking radially inward from the counter driving side protrusion  38 . Thus, according to this embodiment, the motor  10  having a structure capable of further reducing leakage flux is obtained. 
     A first curve portion  38   e , a second curve portion  38   f , and a linear portion  38   d  are provided to the exterior of a second counter driving side protrusion lateral surface  38   b , which is a surface on the concave portion  36  side (+θZ side) of the counter driving side protrusion  38 , when viewed in the axial direction. 
     The first curve portion  38   e  is a curved portion located at an end portion of a radially inner side of the counter driving side protrusion  38 . For this reason, when the rotor body unit  32  is molded, the resin may easily flow in between the counter driving side protrusion  38  and the driving side protrusion  37 , that is, into the concave portion  36 . The center of curvature of the first curve portion  38   e  is located at a radially outer side than the first curve portion  38   e.    
     The second curve portion  38   f  is a curved portion located at an end portion on a radially outer side of the counter driving side protrusion  38 . For this reason, when the rotor body unit  32  is molded, the resin may easily flow in between the counter driving side protrusion  38  and the driving side protrusion  37 , that is, into the concave portion  36 . The center of curvature of the second curve portion  38   f  is located at a radially inner side than the second curve portion  38   f.    
     The linear portion  38   d  is a straight-lined portion connecting the first curve portion  38   e  and the second curve portion  38   f . For this reason, for example, it is easier to form a mold for punching out the core piece part  34 N, when compared to the case in which the entire contour of the second counter driving side protrusion lateral surface  38   b  has a curved shape. 
     The linear portion  38   d  is inclined in a direction away from the magnet driving side surface  33 Ba of the permanent magnet  33 B, as it goes toward a radially outer side from a radially inner side. For this reason, the magnetic path which passes through the permanent magnet  33 B and the counter driving side protrusion  38  can easily become a magnetic path which extends to a radially outer side of the core piece part  34 N along the linear portion  38   d . Accordingly, the formation of a magnetic path which passes through a radially inner side of the permanent magnets  33 A,  33 B can be suppressed. As a result, the leakage flux in the rotor  30  can be reduced. 
     In this embodiment, a first curve portion  38   e , a second curve portion  38   f , and a linear portion  38   d  are also provided to the exterior of a second driving side protrusion lateral surface  37   b , which is a surface on the concave portion  36  side (−θZ side) of the driving side protrusion  37 , in the same manner as the counter driving side protrusion  38 . Accordingly, identical effects to the effects by the first curve portion  38   e , the second curve portion  38   f  and the linear portion  38   d  can also be acquired for the driving side protrusion  37 . Therefore, the detailed descriptions thereof are omitted herein. 
     In this embodiment, the two protrusions, that is, the driving side protrusion  37  and the counter driving side protrusion  38  are, for example, axisymmetric with respect to a radial line RL 1  which passes through the circumferential center of the core piece part  34 N. 
     For this reason, it is possible to equally increase the circumferential width of the driving side protrusion  37  and the circumferential width of the counter driving side protrusion  38 . Therefore, it is easy to further inhibit the occurrence of magnetic saturation in the driving side protrusion  37  and the counter driving side protrusion  38 . Accordingly, the magnetic flux can be inhibited from leaking from the driving side protrusion  37  and the counter driving side protrusion  38 . 
     Since the driving side protrusion  37  and the counter driving side protrusion  38  are axisymmetric, the magnetic flux flowing through the driving side protrusion  37  and the magnetic flux flowing through the counter driving side protrusion  38  are also axisymmetric with respect to the radial line RL 1 . For this reason, the effects of the magnetic flux which the core piece part  34 N applies on the stator  40  can be inhibited from being unbalanced in the circumferential direction of the core piece part  34 N. As a result, the rotation of the rotor  30  can be stabilized. 
     An end portion on a radially inner side of the driving side protrusion  37  is located at a radially outer side than an end portion on a radially inner side of the permanent magnet  33 A. For this reason, the distance from the end portion on a radially inner side of the driving side protrusion  37  to reach the core piece part  34 S located on the driving side (+θZ side) of the permanent magnet  33 A through a radially inner side of the permanent magnet  33 A increases. Therefore, the magnetic flux can be inhibited from flowing in between the neighboring core piece parts  34 N,  34 S through a radially inner side of the permanent magnet  33 A. Accordingly, in this embodiment, the leakage flux can be further reduced. 
     An end portion on a radially inner side of the counter driving side protrusion  38  is located at a radially outer side than an end portion on a radially inner side of the permanent magnet  33 B. For this reason, as in the driving side protrusion  37 , the magnetic flux can be inhibited from flowing in between the neighboring core piece parts  34 N,  34 S through a radially inner side of the permanent magnet  33 B. Therefore, according to this embodiment, the leakage flux in the rotor  30  can be further reduced. 
     In this embodiment, the shortest segment which connects the end portions P 3 , P 4  on a radially inner side of the circumferentially neighboring permanent magnets  33 A,  33 B is referred to as a second segment S 2 . The radial distance L 1  from the end portion P 2  on a radially outer side of the concave portion  36  to the second segment S 2  is longer than the length L 2  of the second segment S 2 . For this reason, the radial width of the concave portion  36  can be longer. Therefore, the magnetic flux can be inhibited from approaching a portion closer to a radially inner side of the core piece part  34 N. Thus, according to this embodiment, the leakage flux in the rotor  30  can be further reduced. 
     Further, the present embodiment may additionally employ the following constitutions. 
     In this embodiment, the driving side protrusion  37  and the counter driving side protrusion  38  may not be axisymmetric with respect to the radial line RL 1 . In this case, only one of the protruding inner portion  37   c  or the protruding inner portion  38   c  may be provided. 
     Also, the shapes of the concave portion  36 , the driving side protrusion  37  and the counter driving side protrusion  38  may not be limited to a particular shape, as long as a protruding inner portion, that is, the protruding inner portion  37   c  or the protruding inner portion  38   c  may be formed on respective one of the two protrusions. 
     In addition, the linear portion  38   d  may be parallel to the magnet driving side surface  33 Ba of the permanent magnet  33 B, or may be inclined in a direction approaching the magnet driving side surface  33 Ba, as it goes toward a radially outer side from a radially inner side. 
     Further, an end portion on a radially inner side of the driving side protrusion  37  may be located at an identical position as the end portions of the permanent magnets  33 A,  33 B in the circumferential direction, or at a radially inner side than the end portions of the permanent magnets  33 A,  33 B. The same applies to the counter driving side protrusion  38 . 
     Furthermore, the distance L 1  may be equal to or smaller than the length L 2 . 
     Moreover, the present embodiment may also employ the configuration as shown in  FIG. 4 .  FIG. 4  is a partially enlarged cross-sectional view of another example of a rotor  130 . As shown in  FIG. 4 , the rotor  130  has a rotor body unit  132 . The rotor body unit  132  has a plurality of permanent magnets  33 A,  33 B, a plurality of core piece parts  134 N,  134 S, and a mold resin portion  135 . The mold resin portion  135  is identical to the mold resin portion  35  shown in  FIG. 3 . 
     The core piece part  134 N has a concave portion  136 , a driving side protrusion  137 , and a counter driving side protrusion  138 . In this configuration, the shape of the concave portion  136  is quadrangular, when viewed in the axial direction. Other structures of the concave portion  136  are identical to the concave portion  36  shown in  FIG. 3 . 
     In this configuration, a segment which connects an end portion P 5  on a radially inner side of a first driving side protrusion lateral surface  137   a , which is a surface facing the permanent magnet  33 A of the driving side protrusion  137  in the circumferential direction, and an end portion P 6  on a radially outer side of the concave portion  136  is referred to as a first segment S 3 . Here, the end portion on a radially outer side of the concave portion  136  is not limited to a single portion. In this case, the end portion P 6  refers to the portion closest to the permanent magnet  33 A among the end portions on a radially outer side of the concave portion  136 . 
     When viewed in the axial direction, the driving side protrusion  137  has a protruding inner portion  137   c  which is a portion located at a radially inner side on the first segment S 3 . For this reason, the magnetic flux which is leaked from the driving side protrusion  137  can be reduced. The counter driving side protrusion  137  has the same configuration, and therefore detailed explanation is not repeated here. 
     When viewed in the axial direction, the contour of a second counter driving side protrusion lateral surface  138   b , which is a surface on the concave portion  136  side (+θZ side) of the counter driving side protrusion  138 , has a linear shape. For this reason, when compared to a case in which the contour of the second counter driving side protrusion lateral surface  138   b  includes a curved portion, it is easier to make the counter driving side protrusion  138  when manufacturing the motor. The second driving side lateral surface  137   b , which is a surface on the concave portion  136  side (−θZ side) of the driving side protrusion  137 , has the same contour, and therefore detailed explanation is not repeated here. 
     The driving side protrusion  137  and the counter driving side protrusion  138  are axisymmetric with respect to a radial direction line RL 2  which passes through the circumferential center of the core piece part  134 N. The core piece part  134 S and the core piece part  134 N are configured in the same way, except for the polarity in which they are magnetized. Other structures of the rotor  130  are identical to those of the rotor  30  shown in  FIG. 1  to  FIG. 3 . 
     While 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. 
     While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention. 
     The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.