Patent Publication Number: US-2019190326-A1

Title: Stator, stator manufacturing method and motor

Description:
This application claims the benefit of priority to Japanese Patent Application No. 2016-195185 filed on Sep. 30, 2016 and is a Continuation Application of PCT Application No. PCT/JP2017/035113 filed on Sep. 28, 2017. The entire contents of each application are hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a stator, a stator manufacturing method, and a motor. 
     2. Description of the Related Art 
     A stator of a motor includes a plurality of teeth radially installed thereon, and an annular part connecting radially outer sides of the teeth in an annular shape. In the stator, an inclined part is formed on an end portion of each core piece of each divided laminate core, and pairs of core pieces with different shapes are alternately laminated with one another. 
     Since the stator of the conventional shape has the inclined part, it is possible to easily join the divided laminate cores adjacent to each other, while the divided laminate cores thus joined is easily detached. 
     SUMMARY OF THE INVENTION 
     According to an exemplary embodiment of the present disclosure, a stator includes a core in an annular shape having a center that is a vertically extending central axis, and a conductive wire that is wound around the core. The core includes core pieces in which at least a first laminate member and a second laminate member are laminated. The first laminate member includes a first tooth portion extending in a radial direction and a first core back portion connected to a radially outer side of the first tooth portion and extending in a circumferential direction. The first core back portion includes a first protrusion on one side thereof in the circumferential direction and a first recess on the other side thereof in the circumferential direction. The second laminate member includes a second tooth portion extending in a radial direction and a second core back portion connected to a radially outer side of the second tooth portion and extending in a circumferential direction. The second core back portion includes a second recess on one side thereof in the circumferential direction and a second protrusion on the other side thereof in the circumferential direction. Positions of two circumferential ends of the first core back portion are different from positions of two circumferential ends of the second core back portion. In the first protrusion, the one side in the circumferential direction is thicker in a lamination direction than the other side in the circumferential direction, and in the second protrusion, the other side in the circumferential direction is thicker in the lamination direction than the one side in the circumferential direction. 
     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 cross-sectional view of a motor according to an exemplary embodiment of the present disclosure. 
         FIG. 2  is a plane view of a laminate member of a core piece according to an exemplary of the present disclosure. 
         FIG. 3  is a plane view of laminate members of laminated core pieces according to an exemplary embodiment of the present disclosure. 
         FIG. 4  is a plane view of annularly connected core pieces according to an exemplary embodiment of the present disclosure. 
         FIG. 5  is an enlarged view of a connection portion of adjacent core pieces according to an exemplary embodiment of the present disclosure. 
         FIG. 6  is a plane view showing an area, in which core back portions of adjacent core pieces overlap each other in a lamination direction according to an exemplary embodiment of the present disclosure. 
         FIG. 7  is a cross-sectional view of a connection portion of adjacent core pieces according to an exemplary embodiment of the present disclosure. 
         FIG. 8  is a graph showing a relationship between an average distance and a magnetic property of the area in which core back portions of adjacent core pieces overlap each other in a lamination direction according to an exemplary embodiment of the present disclosure. 
         FIG. 9  is a plane view of a core piece according to a modified embodiment according to an exemplary embodiment of the present disclosure. 
         FIG. 10  is a cross-sectional view of a connection portion of the core piece according to the modified embodiment according to an exemplary embodiment of the present disclosure. 
         FIG. 11  is a flowchart showing a process of manufacturing a stator according to an exemplary embodiment of the present disclosure. 
         FIG. 12  is a view showing a laminate member formed on a plate member used in a process of manufacturing a stator according to an exemplary embodiment of the present disclosure. 
         FIG. 13  is a view showing core pieces in which laminate members are laminated in the process of manufacturing a stator according to an exemplary embodiment of the present disclosure. 
         FIG. 14  is a view showing a divided stator having a coil formed by winding a conductive wire around teeth of a core piece in the process of manufacturing a stator according to an exemplary embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, exemplary embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The exemplary embodiments described below are only exemplary examples of the present disclosure, but the technical scope is not limited thereby. Further, the same reference numerals may be assigned to the same components, and the descriptions thereof may be omitted. 
     The exemplary embodiments of the present disclosure relate to a configuration of a stator (referred to as a “core piece”) used in a motor and a method of manufacturing the stator. In the description, a direction parallel to a central axis of the motor and the stator is referred to as an “axial direction,” a direction orthogonal to the central axis is referred to as a “radial direction,” and a direction along an arc centered around the central axis is referred to as a “circumferential direction.” In the description, a circumferentially inner side refers to a side close to a connection portion between a core back portion and a tooth portion of a core piece, and a circumferentially outer side refers to a side distant from the connection portion between the core back portion and the tooth portion of the core piece. In the description, the term “core piece” refers to an element including a tooth portion around which a conductive wire is wound and an annularly connected core back portions. The term “core” refers to a group of a plurality of annularly connected core pieces. The term “divided stator” refers to a core piece around which the conductive wire is wound. The term “stator” refers to a group of a plurality of divided stators annularly connected. Further, each layer of the core piece, which defines the core by being laminated, refers to a “laminate member.” Further, the term “laminate member” does not indicate only a first member of members composing the core piece, but may include a plurality of members having the same or similar shapes and consecutively laminated. 
     Further, for convenience of description in the specification, in laminate members laminated in a manufacturing process, a direction in which the laminate members are laminated refers to an “upper side” or an “upper direction,” and a direction in which laminate members, which are already laminated, are positioned refers to a “lower side” or a “lower direction.” In most cases, the lower side opposing the upper side is positioned on a lower side in a gravity direction. Further, a direction in which the laminate members of the core piece are laminated refers to a “lamination direction.” In the following description, the lamination direction is parallel to a central axis of rotation of the motor, but the lamination direction and the central axis are not necessarily parallel to each other. 
       FIG. 1  is a cross-sectional view of a motor  80  of one embodiment of the present disclosure. As shown in  FIG. 1 , the motor  80  preferably includes a shaft  81 , a rotor  82 , a stator  83 , a housing  84 , a bearing holder  85 , a first bearing  86 , a second bearing  87 , an insulator  88 , a coil-drawing line  89 , a coil  90 , and the like. The shaft  81  and the rotor  82  are preferably integrated with each other by, for example, the shaft  81  being press fit through the rotor  82 . The shaft  81  has a cylindrical shape having a center that is a central axis extending in one direction. The rotor  82  is positioned at a middle of the shaft  81 . The rotor  82  is rotatable about the stator  83 . The stator  83  is disposed to surround the rotor  82  in an axial direction. The stator  83  includes the coil  90  which is preferably by winding a conductive wire around the core of the stator  83 . The housing  84  is engaged with an outer circumferential surface of the stator  83  and accommodates the shaft  81 , the rotor  82 , the stator  83 , the bearing holder  85 , the first bearing  86 , the second bearing  87 , the insulator  88 , the coil-drawing line  89 , and the coil  90  which compose the motor  80 . The bearing holder  85  supports the second bearing  87 . The bearing holder  85  is engaged with the housing  84 . The first bearing  86  is preferably disposed at a lower portion of the housing  84  and supports one side of the shaft  81 . The second bearing  87  supports the other side of the shaft  81 . The insulator  88  is disposed between the stator  83  and a conductive wire of the coil  90  to insulate the stator  83  and the conductive wire of the coil  90 . 
       FIG. 2  is a plane view of one laminate member  10   a  of a core piece  10  which defines the stator  83 .  FIG. 3  is a plan view of the laminated core pieces  10 .  FIG. 4  is a plan view of a core  1  in a state in which the core pieces  10  are annularly connected. 
     As shown in  FIG. 4 , a center point of a circle of an outer circumferential surface or an inner circumferential surface defined by the core  1  is C 1 . Straight lines A 1 , A 2 , and A 3  shown in  FIGS. 2 and 3  each are lines extending in a radial direction through the center point C 1 . An inner angle between the straight line A 1  and the straight line A 2  and an inner angle between the straight line A 1  and the straight line A 3  are preferably about 15°, for example. An inner angle between tooth portions  40  of adjacent core pieces  10  is preferably about 30°, for example. An inner angle between the tooth portions  40  of the adjacent core pieces  10 , an inner angle between the straight lines A 1  and A 2 , and an inner angle between the straight lines A 1  and A 3  vary according to the number of core pieces  10  forming the core  1 . The core  1  according to the present preferred embodiment of the present disclosure is preferably includes the twelve core pieces  10 , and thus, as described above, each of the inner angles between the tooth portions  40  of the adjacent core pieces  10  is preferably about 30°. Further, the number of core pieces  10  composing the core  1  may be arbitrarily changed as desired. 
     As shown in  FIG. 2 , the laminate member  10   a  of the core piece  10  includes the tooth portion  40  and the core back portion  20 . The core piece  10  is formed by laminating the plurality of laminate members  10   a  with a predetermined thickness. The tooth portion  40  is linearly symmetrical with respect to the straight line A 1  passing through the center point C 1 . The tooth portion  40  has a shape in which an end on an inner side in a radial direction extends in a circumferential direction, and has an inner circumferential surface  41  on the inner side in the radial direction. 
     As shown in  FIG. 3 , one laminate member and another laminate member of the core piece  10  are laminated so that the tooth portion  40  does not protrude. Since circumferential lengths of one circumferential end of one laminate member and another circumferential end of another laminate member are different from each other, one side protrudes from another side. 
     The core back portion  20  is an element defining an annular portion of the core  1 . The core back portion  20  is connected with a radially outer side of the tooth portion  40  and has a shape extending in a circumferential direction. 
     The core back portion  20  includes a circular arc-shaped protrusion  21  and a radially straight portion  22  formed at one end thereof in the circumferential direction. The radially straight portion  22  has a shape of a straight line extending in a radial direction through the center point C 1 . The radially straight portion  22  protrudes outward from the straight line A 1  in a circumferential direction. The circular arc-shaped protrusion  21  preferably has a shape protruding circumferentially outward of a radially straight line passing through the center point C 1  and the radially straight portion  22 . The circular arc-shaped protrusion  21  preferably has a circular arc shape partially overlapping a circle having a center that is an intersection point C 2  between the straight line A 2  and an outer circumferential recess  26   b  of the core back portion  20 . An end on a circumferential inner side of the circular arc-shaped protrusion  21  is connected with an end on the circumferential outer side of the radially straight portion  22 , and the circular arc-shaped protrusion  21  and the circumferential end of the radially straight portion  22  become one circumferential end of the core back portion  20 . 
     Further, the circular arc-shaped protrusion  21  may not necessarily have a circular arc shape if so desired. For example, the core back portion  20  may be a protrusion with an arc shape of an ellipse or a gently curved protrusion instead of the circular arc-shaped protrusion  21 . But a portion corresponding to the circular arc-shaped protrusion  21  of one end of the core back portion  20  is in contact with a contact portion  23  of an adjacent core piece at one point. 
     The core back portion  20  includes the contact portion  23  and a radially straight portion  24  provided at the other end thereof in the circumferential direction. Like the radially straight portion  22 , the radially straight portion  24  has a shape extending in a radial direction through the center point C 1 . Unlike the radially straight portion  22 , the radially straight portion  24  has a shape of being recessed circumferentially inward of the straight line A 3 . The contact portion  23  preferably has a straight shape with an inclined surface recessed circumferentially inward of the radially straight portion  24 . An inner angle between the radially straight portion  22  and the contact portion  23  is preferably about 135°. An end on a circumferential inner side of the contact portion  23  is connected with an end on a circumferential outer side of the radially straight portion  24 , and the contact portion  23  and one circumferential end of the radially straight portion  24  become the other circumferential end of the core back portion  20 . 
       FIG. 5  is an enlarged view of a connection portion of laminate members  10   a  and  11   a  of the core pieces  10  and  11  adjacent to each other. As shown in  FIG. 5 , an inner angle P 2  between the radially straight portion  24  and the contact portion  23  is preferably 135°. 
     Further, the contact portion  23  may not necessarily have a straight line shape. For example, the contact portion  23  may be a shape of a circular arc-shaped protrusion or recess or a curved portion. But a portion corresponding to the contact portion  23  of the other end of the core back portion  20  is in contact with the circular arc-shaped protrusion  21  of the adjacent core piece at one point. The contact portion  23  is also referred to as a linear recess as a representation corresponding to the circular arc-shaped protrusion. In the core back portion  20 , one portion in the circumferential direction which has the circular arc-shaped protrusion  21  and the radially straight portion  22  is an example of the “protrusion” in the present disclosure. In the core back portion  20 , the other portion in the circumferential direction which has the contact portion  23  and the radially straight portion  24  is an example of the “recess” of the present disclosure. 
     As shown in  FIG. 5 , one end of the laminate member  10   a  of the core piece  10  is preferably in contact with the other end of the laminate member  11   a  of the adjacent core piece  11 . Specifically, the circular arc-shaped protrusion  21  of the core piece  10  and the contact portion  23  of the core piece  11  are in contact with each other at one contact point P 1 . The radially straight portion  22  of the core piece  10  and the radially straight portion  24  of the core piece  11  are spaced apart from each other. But the radially straight portion  22  of the core piece  10  and the radially straight portion  24  of the core piece  11  are not necessarily spaced apart from each other and may be in contact with each other. 
     As described above, in the core piece  10  and the core piece  11  which are adjacent to each other, the circular arc-shaped protrusion  21  of the laminate member  10   a  of the core piece  10  and the contact portion  23  of the laminate member  11   a  of the core piece  11  are in contact with each other at one point. When the core piece  10  rotates outward of the radial direction with respect to the core piece  11 , the radially straight portion  22  and the radially straight portion  24  are not in contact with each other, but the circular arc-shaped protrusion  21  and the contact portion  23  are in contact with each other at one point. Even when the core piece  11  and the core piece  10  relatively rotate, the core piece  10  and the core piece  11  are in contact with each other at one point, and thus a frictional resistance between the core piece and the core piece  11  decreases. Therefore, compared to a configuration in which core pieces adjacent to each other are in surface contact with each other or in contact with each other at a plurality of points as in the conventional art, the core pieces can rotate while connected with each other. 
     Further, when the core piece  10  rotates with respect to the core piece  11 , a center of rotation is a center C 2  of a circular arc of the circular arc-shaped protrusion  21 . In the laminate members of the core piece  10 , since the center C 2  coincides with a lamination direction, the core piece  10  may smoothly rotate about the center C 2  as an axis. 
     Further, in the laminate members  10   a  and  11   a  of the core pieces  10  and  11 , an inner angle between the radially straight portion  24  and the contact portion  23  is preferably about 135°, and thus the core piece  10  may rotate within a wide range when rotating with respect to the core piece  11  while being in contact with the core piece  11  at one point. Further, the inner angle P 2  is not necessarily limited to about 135° and may be changed within a range of about 130° to about 140°. Even when the inner angle P 2  is an arbitrary angle in a range of about 130° to about 140°, the core pieces can be rotated in a sufficiently wide range while being in contact with each other at one point. 
     An outer circumferential surface of the core back portion  20  is engaged with a housing (not shown) when a motor is assembled. The core back portion  20  includes a central recess  29 , outer circumferential surfaces  25   a  and  25   b , and outer circumferential recesses  26   a  and  26   b  provided at an outer circumferential part thereof. 
     The central recess  29  which is recessed inward in the radial direction is arranged at a position at which an outer circumferential surface of the core back portion  20  and the straight line A 1  intersect with each other. The central recess  29  extends in a groove shape in a vertical direction in which the laminate members are laminated. 
     Each of the outer circumferential surfaces  25   a  and  25   b  preferably has a circular arc shape including a center that is the center point C 1 . The outer circumferential surfaces  25   a  and  25   b  are connected with both circumferential sides of the central recess  29 . The outer circumferential surfaces  25   a  and  25   b  are portions which are in contact with the inner circumferential surface of the housing while the stator including the core  1  around which the conductive wire is wound is engaged with an inner side of the housing. 
     The outer circumferential recesses  26   a  and  26   b  are connected with circumferential end sides on the outer circumferential surfaces  25   a  and  25   b . The outer circumferential recesses  26   a  and  26   b  are recessed from the outer circumferential surfaces  25   a  and  25   b  inward in a radial direction. The outer circumferential recesses  26   a  and  26   b  include a circular arc shape having a smaller diameter than that of the outer circumferential surfaces  25   a  and  25   b  and having the center point C 1  the same as that of the outer circumferential surfaces  25   a  and  25   b . When the stator is fitted to an inner side of the housing, the outer circumferential recesses  26   a  and  26   b  are not in contact with an inner circumferential surface of the housing, and thus gaps are defined between the inner circumferential surface of the housing and the outer circumferential recesses  26   a  and  26   b.    
     The outer circumferential surface of the core back portion  20  of the core piece  10  is preferably engaged with the housing as a stator, as described above, the outer circumferential surfaces  25   a  and  25   b  are in contact with an inner circumferential surface of the housing, and the central recess  29  and the outer circumferential recesses  26   a  and  26   b  are not in contact with the inner circumferential surface of the housing. Therefore, accuracy of a size of the outer circumferential surface of the core back portion  20  can increase. Further, the core back portion  20  may not necessarily have the outer circumferential recesses  26   a  and  26   b . When the core back portion  20  is formed in a shape having the outer circumferential recesses  26   a  and  26   b , dimensions of the outer circumferential surfaces  25   a  and  25   b  more effectively increase. 
     The core back portion  20  preferably includes inner circumferential surfaces  27   a  and  27   b  and inner circumferential recesses  28   a  and  28   b  provided on an inner circumferential surface thereof. The inner circumferential surfaces  27   a  and  27   b  have a circular arc shape having a center that is the center point C 1 . The inner circumferential surfaces  27   a  and  27   b  are connected with both circumferential sides of the tooth portion  40 . The inner circumferential recesses  28   a  and  28   b  are connected with circumferential end sides of the inner circumferential surfaces  27   a  and  27   b . The inner circumferential recesses  28   a  and  28   b  are recessed from the inner circumferential surfaces  27   a  and  27   b  outward in the radial direction. The inner circumferential recesses  28   a  and  28   b  preferably include a circular arc shape having an inner diameter smaller than that of the inner circumferential surfaces  27   a  and  27   b  having the center that is the center point C 1  the same or substantially the same as that of the inner circumferential surfaces  27   a  and  27   b.    
     As shown in  FIG. 3 , when the core piece  10  including a plurality of laminate members which are laminated is viewed from above, since positions of both circumferential ends of the core back portion  20  are different from each other among the laminate members, the laminate member disposed on a lower side is partially shown. When viewed from above, a circular arc-shaped protrusion  121 , a radially straight portion  122 , an outer circumferential recess  126   a , and an inner circumferential recess  128   a  of the laminate member disposed below the laminate member disposed on the top are shown at the contact portion  23 , which is defined short in a circumferential direction of the core back portion  20 , and a circumferential outer side of the radially straight portion  24 . The circular arc-shaped protrusion  121 , the radially straight portion  122 , the outer circumferential recess  126   a , and the inner circumferential recess  128   a  of the laminate members of the core piece  10  overlap an adjacent core piece in a lamination direction. 
       FIG. 6  is a view showing the core back portions  20  of the core pieces  10  and  11  adjacent to each other overlap each other in a lamination direction, and particularly, a view showing an overlapping area. A circular arc-shaped protrusion  221 , a radially straight portion  222 , an outer circumferential recess  226   a , and an inner circumferential recess  228   a  of the laminate member of the core piece  11  are preferably laminated on the circular arc-shaped protrusion  121 , the radially straight portion  122 , the outer circumferential recess  126   a , and the inner circumferential recess  128   a  of the laminate member of the core piece  10 . The laminate member of the core piece  10  is disposed under the laminate member of the core piece  11 . As shown in  FIG. 6  with inclined lines, the core piece  10  and the core piece  11  overlap in an area R. A boundary of the area R is determined by the circular arc-shaped protrusion  221 , the radially straight portion  222 , the outer circumferential recess  226   a , and the inner circumferential recess  228   a , which are laminate members of the core piece  11  positioned on an upper side, and the circular arc-shaped protrusion  121 , the radially straight portion  222 , the outer circumferential recess  226   a , and the inner circumferential recess  228   a , which are laminate members of the core piece  10  positioned on a lower side. But the outer circumferential recess  226   a  and the inner circumferential recess  228   a , the outer circumferential recess  226   a , and the inner circumferential recess  228   a  preferably overlap each other in the lamination direction. 
     For example, an area of the area R is greater than an area of a circumferentially cross-sectional area of the core back portion  20  at a position of the straight line A 3 . Further, the cross-section of the core back portion  20  is calculated by multiplying a circumferential length of the core back portion  20  and a thickness of the laminate member. The reason why the area R is formed as described above is as follows. 
     One circumferential end of each of the laminate members of the core piece  10  is in contact with the other circumferential end of each of the laminate members of the core piece  11  at one point. For this reason, as compared with when one circumferential end of the core piece  10  is in surface contact with the other circumferential end of the core piece  11 , a magnetic path is defined circumferential ends of the core pieces  10  and  11  so that an amount of magnetic flux flowing therein is narrow. Therefore, the area greater than or equal to the magnetic path which is narrowed due to the area R is able to be secured. Further, since the radially straight portion  22  and the radially straight portion are not in contact with each other in a circumferential direction in an assembled state, the magnetic path is not provided at a position at which the radially straight portion  22  and the radially straight portion  24  are not in contact with each other. 
     However, even in the case of adopting a configuration in which one circumferential end of each of the laminate members of the core piece  10  is not in contact, or is in surface contact, or is in contact at a plurality of points, with the other circumferential end of each of the laminate members of the core piece  11  adjacent thereto, the magnetic path is defined in the area R, and thus the magnetic property is improved. Here, the magnetic property is an amount of the magnetic flux flowing through a portion where an uneven part of an end of the core piece  10  and an uneven part of an end of the core piece  11  are engaged with each other. 
     Further, it is preferable that the area R be less than or equal to about 5 times the circumferential cross-sectional area of the core back portion  20 . Therefore, an area in which the core back portions  20  of the adjacent core piece  10  overlap in the lamination direction is sufficiently secured, and thus a sufficient magnetic path is able to be secured. Further, because a frictional resistance is prevented from being excessively generated in the lamination direction of the core back portion  20  of the adjacent core piece  10 , the adjacent core pieces are able to rotate in a manufacturing process. 
       FIG. 7  is a cross-sectional view of the connection portion of the core pieces  10  and  11  adjacent to each other. As shown in  FIG. 7 , the core piece  10  is preferably defined by laminate members  10   a  to  10   d  which are laminated. The core piece  11  is preferably defined by laminate members  11   a  to  11   d  which are laminated. Ends of the core piece  10  and the core piece  11  face each other and have uneven parts. Ends of the laminate members  10   a  and  10   c  and the laminate members  11   b  and  11   d  are protrusions, and ends of the laminate members  10   b  and  10   d  and the laminate members  11   a  and  11   c  are recesses. Uneven parts of the end of the core piece  10  and uneven parts of the end of the core piece  11  are engaged with each other to connect the core pieces  10  and  11 . 
     Ends  31   a  to  31   d  are provided at circumferential ends of the laminate members  10   a  to  10   d  of the core piece  10 , respectively. The ends  31   a  and  31   c  are ends of the circular arc-shaped protrusions  21  or the radially straight portions  22 . The ends  31   b  and  31   d  are ends of the contact portion  23   s  or the radially straight portions  24 . On the other hand, ends  32   a  to  32   d  are provided at circumferential ends of the laminate members  11   a  to  11   d  of the core piece  11 , respectively. The ends  32   a  to  32   d  face the ends  31   a  to  31   d , respectively. The ends  32   b  and  32   d  are ends of the circular arc-shaped protrusions  21  or the radially straight portions  22 . The ends  31   b  and  31   d  are ends of the contact portions  23  or the radially straight portions  24 . As shown in  FIG. 7 , a gap  61  at a portion on the circumferentially inner side of the end  32   b  is wider than a gap  62  at a portion on the circumferentially outer side of the end  32   b . The ends  32   b  and  32   d  are formed to be thicker in the lamination direction from the circumferentially inner side toward the circumferentially outer side. In other words, thicknesses of the ends  32   b  and  32   d  increase in the laminating direction from the circumferentially inner side toward the circumferentially outer side. 
     More specifically, upper surfaces  33   b  and  33   d  of the ends  32   b  and  32   d  are inclined upward toward the circumferentially outer side. Lower surfaces  34   b  and  34   d  of the ends  32   b  and  32   d  are inclined downward toward the circumferentially outer side. A lower surface  34   a  faces the upper surface  33   b , an upper surface  33   c  faces the lower surface  34   b , and a lower surface  34   c  faces the upper surface  33   d . Each of an upper surface  33   a , the lower surface  34   a , the upper surface  33   c , and the lower surface  34   c  extends in a straight shape toward the circumferentially outer side without inclination. In this manner, in a portion where the core back portions  20  of the adjacent core pieces  10  and  11  are laminated, a distance in the lamination direction varies depending on its circumferential position. 
     As described above, the end of the circular arc-shaped protrusion  21  or the radially straight portion  22  has a shape in which its thickness in the lamination direction increases in the circumferential direction, that is, a circumferentially outwardly thickening shape. In the core pieces connected to each other, it is possible to fix adjacent core pieces to each other and it is possible to prevent them from becoming detached. Especially, as in the manufacturing method which will be described later, in the case of adopting a manufacturing method in which lamination progresses as adjacent core pieces are laminated to overlap each other, it is particularly effective because the connection between the adjacent core pieces is not released. 
       FIG. 8  is a diagram showing a result of calculating the relationship between an average distance of a region where the core back portions  20  of the adjacent core pieces overlap in the lamination direction and a magnetic property in a motor, using software for magnetic analysis. The region where the core back portions  20  of the adjacent core pieces overlap in the lamination direction is indicated as a region from the gap  61  to the gap  62  in  FIG. 7 . The horizontal axis of the graph of  FIG. 8  represents the average distance in the region where the core back portions  20  of the adjacent core pieces overlap each other. The vertical axis of the graph of  FIG. 8  represents a magnetic property relative to the case where a magnetic property in a motor using a stator in a state in which the core back portions  20  of the adjacent core pieces are in contact with each other in the entire region where they overlap in the lamination direction is taken as 100%. As shown in  FIG. 8 , when the magnetic property in the motor using the stator in the state in which the core back portions  20  of the adjacent core pieces are in contact with each other in the entire region where they overlap in the lamination direction is taken as 100%, the magnetic property of the motor decreases as the average distance of the region where the core back portions  20  overlap in the lamination direction becomes larger. 
     For example, as shown in  FIG. 8 , the magnetic property in the motor using the stator in which the average distance of the region where the core back portions  20  of the adjacent core pieces overlap each other in the lamination direction is 10 μm is about 99% as compared with the magnetic property in the motor using the stator in which the core back portions  20  of the adjacent core pieces are in contact with each other in the entire region where they overlap in the lamination direction. Also, the magnetic property in the motor using the stator in which the average distance of the region where the core back portions  20  of the adjacent core pieces overlap each other in the lamination direction is 20 μm is about 98% as compared with the magnetic property in the motor using the stator in which the core back portions  20  of the adjacent core pieces are in contact with each other in the entire region where they overlap in the lamination direction. Also, the magnetic property in the motor using the stator in which the average distance of the region where the core back portions  20  of the adjacent core pieces overlap each other in the lamination direction is 50 μm is about 97% as compared with the magnetic property in the motor using the stator in which the core back portions  20  of the adjacent core pieces are in contact with each other in the entire region where they overlap in the lamination direction. 
     Therefore, if the average distance of the region where the adjacent core pieces overlap each other in the lamination direction is set to 50 μm or less, it is possible to suppress the deterioration of the magnetic property at the time of driving the motor using the stator to about 3%. Also, if the average distance of the region where the adjacent core pieces overlap each other in the lamination direction is set to 20 μm or less, it is possible to suppress the deterioration of the magnetic property at the time of driving the motor using the stator to about 2%. Also, if the average distance of the region where the adjacent core pieces overlap each other in the lamination direction is set to 10 μm or less, it is possible to suppress the deterioration of the magnetic property at the time of driving the motor using the stator to about 1%. In addition, it is preferable to select the average distance of the region where the adjacent core pieces overlap each other in the lamination direction depending on a specification of a motor to be manufactured, required simplicity of a manufacturing process, or the like. For example, in the case of increasing the magnetic property of a motor, the average distance is set to 10 μm, and in the case of reducing contact resistance in order to facilitate rotation of core pieces in a conductive wire winding process in a manufacturing method of a motor which will be described later, the average distance is set to 50 μm. 
     Further, the upper surface  33   b  and the lower surface  34   b  of the end  32   b  and the upper surface  33   d  and the lower surface  34   d  of the end  32   d  are not necessarily inclined, and may have a shape in which thicknesses are different on one side and the other side thereof in the circumferential direction. For example, the upper surface  33   b  and the lower surface  34   b  of the end portion  32   b , and the upper surface  33   d  and the lower surface  34   d  of the end portion  34   d  may have a shape in which steps are formed intermittently so that the thickness of the core back portion  20  changes. 
     Furthermore, the ends  31   a  and  31   c  may also be formed to be larger in the lamination direction from the circumferentially inner side toward the circumferentially outer side. With this configuration, the distance in the lamination direction also varies depending on a circumferential position in the portion where the core back portions  20  of the adjacent core pieces  10  and  11  are laminated. In addition, this configuration also makes it possible to connect adjacent core pieces  10  and  11  more strongly. 
     A stator, a core, and a core piece of the present disclosure are not limited to the above-described preferred embodiment, and various forms made based on the preferred embodiment may be included. For example, the stator, the core, and the core piece of the present disclosure may be elements including the modified preferred embodiments described below. Further, the same elements as those in the above-described preferred embodiment will be designated with the same name or numeral references, and the description thereof may be omitted. 
       FIG. 9  is a plan view of laminate members  12   a  defining a core piece  12  as a modified preferred embodiment according to the present disclosure. As shown in  FIG. 9 , the shapes of both circumferential ends of the laminate member  12   a  of the modification are different from those of the laminate member  10   a  (see  FIG. 2 ) according to the above-describe preferred embodiment of the present disclosure. 
     Specifically, the laminate member  12   a  has a circular arc-shaped protrusion  21   a  provided at one circumferential end of the core back portion  20   a  thereof. The laminate members  12   a  preferably include a contact portion  23   a  formed at the other circumferential end of the core back portion  20   a . The laminate member  12   a  of the modification does not have radially straight portions formed at both ends thereof. 
     Even in the case of this configuration, ends in a circumferential direction of the adjacent core pieces are in contact with each other at one point, and the same effect as that of the above-described embodiment is obtained. The core piece  12  of the modification is used, and thus the laminate members of the core piece are able to be easily manufactured. 
     However, as described in the above-described embodiment, when the laminate member includes the radially straight paths  22  and  24 , and one core piece is rotated in a direction in which an inner side in the radial direction gets close to the other core piece, the radially straight paths  22  and  24  come into contact with each other. Therefore, one core piece is able to be prevented from rotating in a direction in which the radially inner side gets close to the other core piece. 
       FIG. 10  is a cross-sectional view of a connection portion of core pieces  13  and  14  in a modified preferred embodiment according to the present disclosure. As shown in  FIG. 10 , the core pieces  13  and  14  of the present modified preferred embodiment are defined by laminating laminate members  13   a  to  13   d  and  14   a  to  14   d , respectively. Ends of the core piece  13  and the core piece  14  are opposed to each other, and uneven parts are defined thereon. The laminate members  13   a  to  13   d  include ends  35   a  to  35   d  in the circumferential direction. The laminate members  14   a  to  14   d  have ends  36   a  to  36   d  in the circumferential direction. 
     Compared to the core pieces  10  and  11  in the embodiment shown in  FIG. 7 , the core pieces  13  and  14  of present modified preferred embodiment are different in that the ends  36   b  and  36   d  get thinned in the lamination direction from the circumferentially inner side toward the circumferentially outer side. In other words, the thicknesses of the ends  36   b  and  36   d  in the lamination direction decrease from the circumferentially inner side toward the circumferentially outer side. As shown in  FIG. 10 , a gap  63  at a portion on the circumferentially inner side of the end  36   b  is narrower than a gap  64  at a portion on the circumferentially outer side. The ends  36   b  and  36   d  correspond to the ends  32   b  and  32   d  in the preferred embodiment, respectively. 
     More specifically, upper surfaces  37   b  and  37   d  of the ends  36   b  and  36   d  are inclined downward toward the circumferentially outer side. Lower surfaces  38   b  and  38   d  of the ends  36   b  and  36   d  are inclined upward toward the circumferentially outer side. A lower surface  38   a  faces the upper surface  37   b , an upper surface  37   c  faces the lower surface  38   b , and a lower surface  38   c  faces the upper surface  37   d . Each of the upper surface  37   a , the lower surface  38   a , the upper surface  37   c , and the lower surface  38   c  extends in a straight shape toward the circumferentially outer side without inclination. In this way, a distance in the lamination direction varies depending on a position in the circumferential direction in a portion where the core back portions  20  of the adjacent core pieces  13  and  14  are laminated. 
     As described above, the ends of the circular arc-shaped protrusion  21  or the radially straight portion  22  have a shape in which the thickness in the lamination direction decreases in the circumferential direction, that is, a circumferentially outwardly tapered or substantially circumferentially outwardly tapered shape. When separated core pieces are connected to each other, the core pieces can be easily connected to each other. 
     In the stator of the present modified preferred embodiment, similarly to the preferred embodiment, the average distance and the magnetic property of the region where the core back portions  20  of the adjacent core pieces overlap in the lamination direction have a relationship as shown in  FIG. 8 . That is, when the magnetic property in the motor using the stator in the state in which the core back portions  20  of the adjacent core pieces are in contact with each other in the entire region where they overlap in the lamination direction is taken as 100%, the magnetic property of the motor decreases as the average distance of the region where the core back portions  20  overlap in the lamination direction becomes larger. Therefore, in the stator of the present modified preferred embodiment, it is preferable in view of the magnetic property that the average distance of the region where the adjacent core pieces overlap each other in the lamination direction be 50 μm or less similarly to the stator of the preferred embodiment. Also, it is more preferable in view of the magnetic property that the average distance of the region where the adjacent core pieces overlap each other in the lamination direction be 20 μm or less. Also, in the stator of the present modified preferred embodiment, it is even more preferable in view of the magnetic property that the average distance of the region where the adjacent core pieces overlap each other in the lamination direction be 10 μm or less. However, it is preferable to select the average distance of the region where the adjacent core pieces overlap each other in the lamination direction depending on a specification of a motor to be manufactured, required simplicity of a manufacturing process, or the like. For example, in the case of increasing the magnetic property of a motor, the average distance is set to 10 μm, and in the case of reducing contact resistance in order to facilitate rotation of core pieces in a conductive wire winding process in a manufacturing method of a motor which will be described later, the average distance is set to 50 μm. 
     Further, the upper surface  37   b  and the lower surface  38   b  of the end  36   b  and the upper surface  37   d  and the lower surface  38   d  of the end  36   d  are not necessarily inclined, and may have a shape in which thicknesses are different on one side and the other side thereof in the circumferential direction. 
     Furthermore, the ends  35   a  and  35   c  may also be defined to be smaller in the lamination direction from the circumferentially inner side toward the circumferentially outer side. With this configuration, the distance in the lamination direction also varies depending on a circumferential position in the portion where the core back portions  20  of the adjacent core pieces  13  and  14  are laminated. 
     A method of manufacturing a stator of an exemplary embodiment of the present disclosure will be described with referent to  FIGS. 11 to 14 . Further, although a plurality of the stacked laminate plate members are arranged in a circumferential direction, to form of annularly connected cores in practice, only a portion of them are shown in  FIGS. 12 to 14 , and the others are omitted for the sake of simplicity. Hereinafter, in a plane which is horizontal to a gravity direction, a direction horizontal to a transfer direction of the plate member refers to a “transverse direction.” 
       FIG. 11  is a flowchart showing a process of manufacturing a stator according to an exemplary embodiment of the present disclosure. In the process of manufacturing the stator, a process of separating a laminate member from a plate member, which is a base material, (S 100 ) is performed first. When the laminate member is separated, the separated laminate member is laminated on the laminate member (S 110 ). 
       FIG. 12  is a view showing laminate members  101   a ,  101   b ,  101   c ,  101   d ,  102   a ,  102   b ,  102   c ,  102   d ,  103   a ,  103   b ,  103   c ,  103   d ,  104   a ,  104   b ,  104   c , and  104   d  of core pieces provided on a plate member  2 . The laminate members  101   a    101   b ,  101   c , and  104   d  are arranged in each lamination layer. The laminate members  101   a    101   b ,  101   c , and  104   d  are arranged in a first layer, the laminate members  102   a ,  102   b ,  102   c , and  102   d  are arranged in a second layer, the laminate members  103   a ,  103   b ,  103   c , and  103   d  are arranged in a third layer, and the laminate members  104   a ,  104   b ,  104   c , and  104   d  are arranged in a fourth layer, and thus the core piece is formed. In the process of separating the laminate members, the laminate members in the same layer are simultaneously or sequentially separated. 
     When all of the laminate members are not laminated (N of S 120 ), the plate member  2  is transferred in a transfer direction S (see  FIG. 12 ), then the laminate members to be laminated are transferred to a separation position (S 130 ). For example, before separation of the laminate members  102   a ,  102   b ,  102   c , and  102   d  in the second layer is performed, the laminate members  102   a ,  102   b ,  102   c , and  102   d  formed on the plate member  2  are positioned right above the separated laminate members  101   a ,  101   b ,  101   c , and  104   d  in the first layer. Further, a separation of the laminate members  102   a ,  102   b ,  102   c , and  102   d  is performed (S 100 ) so that the laminate members  102   a ,  102   b ,  102   c , and  102   d  are laminated on the laminate members  101   a  to  104   d.    
       FIG. 13  is a view showing core pieces in which laminate members are laminated in a process of manufacturing a stator. When all of the laminate members are laminated (Y of S 120 ), as shown in  FIG. 13 , core pieces  15   a ,  15   b ,  15   c , and  15   d  in which the laminate members are laminated are arranged in a transverse direction. In this state, conductive wires are wound around tooth portions  40  of the core pieces  15   a ,  15   b ,  15   c , and  15   d , and thus a coil  70  is formed (S 140 ). When the conductive wires are wound around the tooth portions  40  of the core pieces  15   a ,  15   b ,  15   c , and  15   d , the core pieces  15   a ,  15   b ,  15   c , and  15   d  may be rotated in a direction in which tooth portions  40  of the adjacent core pieces are spaced apart from each other, and thus a wide space provided around the tooth portions  40  allows the conductive wires to be easily wound around the tooth portion  40 . In this case, the circular arc-shaped protrusion  21  and the contact portion  23  of the adjacent core pieces are in contact with each other at one point, and the core pieces are rotated about a center C 2  while changing a contact position.  FIG. 13  is a view showing divided stators on which a coil  70  is formed by winding a conductive wire around tooth portions  40  of core pieces  15   a ,  15   b ,  15   c , and  15   d . When the conductive wires are wound around the tooth portions  40 , the divided stators of the core pieces  15   a  to  15   d  around which the conductive wires are wound are rotated, and the core back portions  20  are annularly connected (S 150 ). Thus, the stator having the core  1 , on which the conductive wire is wound, shown in  FIG. 4  is formed. 
     If a stator using the core pieces of the embodiment as shown in  FIG. 7  is adopted, even when the divided stator of the core pieces  15   a ,  15   b ,  15   c , and  15   d  around which the conductive wires are wound is rotated as described above, it is possible to smoothly rotate the divided stator while preventing release of the connection between the core pieces. 
     Further, the plate member  2  used in a manufacturing configuration may not be necessarily one plate member but may be two or more plate members if so desired. 
     The present disclosure may be used as, for example, a stator for a motor. 
     Features of the above-described preferred embodiments and the modifications thereof may be combined appropriately as long as no conflict arises. 
     While preferred embodiments of the present invention 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 invention. The scope of the present invention, therefore, is to be determined solely by the following claims.