Patent Publication Number: US-2023136107-A1

Title: Stator, motor, fan, vacuum cleaner, and winding method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of application Ser. No. 16/094,767 filed on Oct. 18, 2018, which is a U.S. national stage application of International Patent Application No. PCT/JP2016/070871 filed on Jul. 14, 2016, the disclosures of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a stator, a motor, a fan, a vacuum cleaner, and a winding method 
     BACKGROUND 
     Conventionally, there is known a motor of an inner rotor type made by attaching a permanent magnet to a rotor on an inner side and attaching a coil to a stator on an outer side. In order to wind the coil around the stator at high density, it is desirable to wind the coil regularly (regular winding). 
     For this reason, a stator made by connecting a plurality of split cores, each including a core piece in an arc-like shape and a tooth, is disclosed in Patent Reference 1, for example. A coil is wound around each tooth in a state where the split cores are expanded in a band shape, and thereafter the split cores are combined into an annular shape so that the stator is obtained. 
     PATENT REFERENCE 
     Patent Reference 1: Japanese Patent Application Publication No. 2000-184631 (see  FIG.  1   ) 
     However, since it is necessary to provide a space for arranging the coil inside the stator, there is a problem that it is difficult to reduce a distance from a center to an outer circumference of the motor, that is, a distance from a rotation center of a rotor to an outer circumference of the stator. 
     SUMMARY 
     The present invention is made to solve the above described problem, and an object of the present invention is to provide a stator capable of providing a space for arranging a coil and capable of being downsized. 
     A stator according to the present invention includes a yoke extending in a circumferential direction about an axis line, a tooth extending from the yoke in a first direction toward the axis line, and a coil wound around and fixed to the tooth. The yoke has an inner wall surface facing the axis line. The tooth has a root part connected to the yoke. The inner wall surface of the yoke is a flat surface extending from an end of the root part of the tooth in the circumferential direction to an inner circumferential side relative to a plane passing through the end and perpendicular to the first direction. 
     A winding method according to the present invention includes a step of preparing a stator core having a yoke extending in a circumferential direction about an axis line and a tooth extending from the yoke in a first direction toward the axis line, and a step of winding a plurality of layers of a coil around the tooth by repeating a step of winding one layer of the coil around the tooth and a step of securing the wound one layer of the coil. 
     According to the present invention, a stator capable of providing a space for arranging a coil and capable of being downsized can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a longitudinal sectional view showing a configuration of a motor in a first embodiment. 
         FIG.  2    is a cross-sectional view showing the configuration of the motor in the first embodiment. 
         FIG.  3    is a cross-sectional view showing a configuration of a stator in the first embodiment. 
         FIG.  4    is an enlarged view showing a part around a tooth in the stator in the first embodiment. 
         FIGS.  5 (A),  5 (B) and  5 (C)  are a plan view and cross-sectional views for explaining a manufacturing process of the motor in the first embodiment. 
         FIGS.  6 (A),  6 (B),  6 (C) and  6 (D)  are cross-sectional views for explaining a winding process in the first embodiment. 
         FIGS.  7 (A),  7 (B) and  7 (C)  are cross-sectional views for explaining the manufacturing process of the motor in the first embodiment. 
         FIG.  8    is an enlarged view showing a part around a tooth in a stator in a first modification of the first embodiment. 
         FIG.  9    is an enlarged view showing a part around a tooth in a stator in a second modification of the first embodiment. 
         FIG.  10    is an enlarged view showing a part around a tooth in a stator in a third modification of the first embodiment. 
         FIG.  11    is a cross-sectional view showing a configuration of a fan to which the motor in the first embodiment is applied. 
         FIG.  12    is a schematic view showing a configuration of a vacuum cleaner including the fan of  FIG.  11   . 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
     (Configuration of Motor) 
     First, a motor according to a first embodiment of the present invention will be described.  FIG.  1    is a longitudinal sectional view showing a configuration of a motor  100  in the first embodiment. The motor  100  in the first embodiment is a brushless DC motor, for example. 
     The motor  100  includes a rotor  5 , a stator  2  in an annular shape arranged around the rotor  5 , a frame (housing)  80  in which the stator  2  is housed, bearings  85  and  86 , and a spring  87 . 
     The frame  80  is divided in the direction of a rotation axis (an axis line Ax) of the rotor  5  into a first frame part  81  and a second frame part  82 . The first frame part  81  has a cylindrical shape, and the stator  2  is inserted in the first frame part  81 . The first frame part  81  has a bearing holding part  81   a  at an end in an axial direction. The bearing  85  is mounted in the bearing holding part  81   a.  Further, the first frame part  81  has a flange part  81   b  at an end on the second frame part  82  side. 
     The bearing  86  is mounted in the second frame part  82 , and the second frame part  82  has a flange part  82   b  at an end on the first frame part  81  side. The flange parts  81   b  and  82   b  of the first frame part  81  and the second frame part  82  are fixed to each other by means of adhesion, screw fastening or welding. 
     The bearings  85  and  86  rotatably support a shaft  6  of the rotor  5 . The shaft  6  penetrates the second frame part  82  in the axial direction and projects outside. For example, an impeller  91  ( FIG.  11   ) is attached to a tip end of the shaft  6 . 
     The spring  87  for applying pressure in the axial direction to the bearing  85  is arranged between the bearing  85  and the bearing holding part  81   a  of the first frame part  81 . The spring  87  is formed of a wave washer or the like, for example. 
     In the following description, a direction of the axis line Ax as the rotation axis of the rotor  5  (i.e., a center axis of the shaft  6 ) will be referred to as an “axial direction”. Further, a rotational circumferential direction about the axis line Ax (i.e., a direction along an outer circumference of the rotor  5  or the stator  2 ) will be referred to as a “circumferential direction”. Furthermore, a rotational radial direction about the axis line Ax (i.e., a radial direction of the rotor  5  or the stator  2 ) will be referred to as a “radial direction”. 
       FIG.  2    is a cross-sectional view showing the configuration of the motor  100  in the first embodiment.  FIG.  2    corresponds to a cross-sectional view at a line segment II-II in  FIG.  1    viewed in a direction of arrows. Incidentally, the frame  80  ( FIG.  1   ) is omitted in  FIG.  2   . 
     As shown in  FIG.  2   , the rotor  5  includes the shaft  6  and a permanent magnet  7  provided on an outer circumferential side of the shaft  6 . The permanent magnet  7  includes two arc-shaped magnet parts  71  and two arc-shaped magnet parts  72 , and is formed in an annular shape as a whole. Thus, the number of permanent magnets  7  (i.e., the number of poles) is four. The permanent magnet  7  (the magnet parts  71  and  72 ) is fixed to an outer circumferential surface of the shaft  6  by means of adhesion or the like. 
     Each magnet part  71  is magnetized so that its outer circumferential surface serves as a north pole. Each magnet part  72  is magnetized so that its outer circumferential surface serves as a south pole. A boundary between adjacent magnet parts  71  and  72  of the permanent magnet  7  is an inter-pole part. 
     Incidentally, the configuration of the rotor  5  is not limited to the example described above. For example, the rotor  5  may also be configured by attaching plate-like or semi-cylindrical permanent magnets to an outer circumferential surface or magnet insertion holes of the rotor core. Further, the shaft  6  is not limited to one having a circular cross section, and a part of the shaft  6  to which the permanent magnet  7  is attached may have a polygonal cross section. 
       FIG.  3    is a cross-sectional view showing a configuration of the stator  2 . The stator  2  includes a stator core  1 , an insulator  3  provided on the stator core  1 , and a coil  4  (winding) wound around the stator core  1  via the insulator  3 . 
     The stator core  1  is formed by punching a plurality of electromagnetic steel sheets  50  ( FIG.  5 (A) ) each having a thickness of 0.1 to 0.7 mm, stacking the punched electromagnetic steel sheets  50  in the axial direction, and securing the electromagnetic steel sheets  50  together by means of crimping or the like. This point will be described later. 
     The stator core  1  includes a yoke  9  in an annular shape about the axis line Ax that is the rotation axis of the rotor  5  (the center axis of the shaft  6 ) and four teeth  12  each of which extends from the yoke  9  in a direction toward the axis line Ax (the first direction). The four teeth  12  are arranged at equal intervals in the circumferential direction about the axis line Ax. A slot is formed between teeth  12  adjacent to each other in the circumferential direction. 
     Each tooth  12  has a winding surface including a pair of side surfaces  12   a,  which are both end surfaces in the circumferential direction, and both end surfaces (not shown) in the axial direction. The coil  4  is wound around this winding surface. A center line C of each tooth  12  is a straight line in the radial direction passing through the above described axis line Ax. Each side surface  12   a  of the tooth  12  is a flat surface parallel to the center line C. 
     An inner circumferential surface  12   b  in an arc-like shape is formed at a tip end of the tooth  12  on an inner side in the radial direction, and the inner circumferential surface  12   b  faces an outer circumferential surface of the rotor  5  ( FIG.  2   ). Further, a flange part  13  projecting toward both sides in the circumferential direction is famed at the tip end of the tooth  12 . 
     The yoke  9  includes first yoke parts  10  each connected to the tooth  12  and second yoke parts  11  adjacent to the first yoke parts  10  in the circumferential direction. Four first yoke parts  10  and four second yoke parts  11  are arranged alternately in the circumferential direction. 
     Further, the yoke  9  has split surfaces  21  at positions where each tooth  12  is sandwiched between two of the split surfaces  21  from both sides in the circumferential direction. Since the number of the teeth  12  is four, eight split surfaces  21  are provided. Each split surface  21  of the yoke  9  is formed between a center of the first yoke part  10  in the circumferential direction (the center line C of the tooth  12 ) and a center of the second yoke part  11  in the circumferential direction. In this example, each split surface  21  of the yoke  9  is formed at a boundary between the first yoke part  10  and the second yoke part  11 . 
     In addition, a joint surface  23  is famed at an end in the circumferential direction of one of the four second yoke parts  11  of the yoke  9  (the second yoke part  11  located on the upper left in  FIG.  3   ). This joint surface  23  is made of end surfaces (abutting parts) that are welded together when split cores (explained later) expanded in a band shape are combined into the stator core  1 . 
     In this example, the joint surface  23  is formed at a center of one second yoke part  11  in the circumferential direction in order to inhibit damage to an insulation coating of the coil  4  caused by heat at the time of welding. As will be explained later, the abutting parts of the split cores are abutted against each other and welded to a predetermined depth in bead-like form or at some points. 
       FIG.  4    is an enlarged view showing the tooth  12  of the stator core  1  and the vicinity of the tooth  12 . As described above, the yoke  9  includes the first yoke parts  10  and the second yoke parts  11  and the teeth  12  extend inward in the radial direction (direction toward the axis line Ax) from the first yoke parts  10 . 
     The tooth  12  has an inclined surface  15  on each side of its root part (part connected to the first yoke part  10 ) in the circumferential direction. The inclined surface  15  is inclined so that its distance from the axis line Ax increases with increase in distance from the side surface  12   a  of the tooth  12  in the circumferential direction. 
     The second yoke part  11  has an inner wall surface  11   a  on an inner circumferential side (i.e., a side facing the axis line Ax) and an outer wall surface  11   b  ( FIG.  3   ) on an outer circumferential side (i.e., a side opposite to the axis line Ax). Both of the inner wall surface  11   a  and the outer wall surface  11   b  are flat surfaces. 
     The inner wall surface  11   a  of the second yoke part  11  extends linearly from an end G of the root part of the tooth  12  in the circumferential direction in a plane perpendicular to the axis line Ax. In this example, the split surface  21  of the yoke  9  is arranged at a boundary between the inner wall surface  11   a  of the second yoke part  11  and the inclined surface  15  of the tooth  12 . 
     Specifically, the inner wall surface  11   a  of the second yoke part  11  is a flat surface extending from the end G in the circumferential direction of the root part of the tooth  12 , and extending inward in the radial direction (to the axis line Ax side) relative to a plane V passing through the end G and perpendicular to the center line C. The outer wall surface  11   b  of the second yoke part  11  is a flat surface extending in parallel with the inner wall surface  11   a.    
     With this configuration, a distance from the axis line Ax to the outer circumference (the outer wall surface  11   b ) of the second yoke part  11  can be reduced as compared with a case where the inner wall surface  11   a  and the outer wall surface  11   b  ( FIG.  3   ) of the second yoke part  11  are cylindrical surfaces about the axis line Ax. 
     An angle B formed by the inner wall surface  11   a  of the second yoke part  11  and the side surface  12   a  of the tooth  12  (parallel to the center line C) is 60 degrees, for example. However, the angle B is not limited to 60 degrees, and it is sufficient that the angle B is less than 90 degrees. 
     As shown in  FIG.  3   , a flat surface  18  is formed at a center of the outer wall surface  11   b  of the second yoke part  11  in the circumferential direction, and the flat surface  18  extends along the circumferential direction about the axis line Ax and forms an angle of 45 degrees with respect to the above described plane V. By forming this flat surface  18 , the distance from the axis line Ax to the outer circumference of the second yoke part  11  can be reduced further. 
     As shown in  FIG.  4   , an inclined surface  13   a  facing the inner wall surface  11   a  of the second yoke part  11  is formed on the flange part  13  at the tip end of the tooth  12 . The inclined surface  13   a  is a flat surface and is famed in parallel with the inner wall surface  11   a  of the second yoke part  11 . 
     A region surrounded by the inner wall surface  11   a  of the second yoke part  11 , the inclined surface  15  of the tooth  12 , the side surface  12   a  of the tooth  12  and the inclined surface  13   a  of the flange part  13  is a region in which the coil  4  wound around the tooth  12  is arranged. 
     When the coil  4  is wound around the tooth  12 , the inclined surface  15  of the tooth  12  and the inclined surface  13   a  of the flange part  13  described above respectively make contact with end parts of a first layer of the coil  4  on the outer circumferential side and the inner circumferential side, and thus displacement of the coil  4  is prevented. In other words, the inclined surface  15  of the tooth  12  and the inclined surface  13   a  of the flange part  13  function as stoppers when the coil  4  is wound around the tooth  12 . 
     A distance H from the side surface  12   a  of the tooth  12  to the split surface  21  in the circumferential direction is desired to be within a range of 0.5 times to 1.5 times a diameter of the coil  4 , i.e., a wire diameter D. If the distance H is within this range, the first layer of the coil  4  can be properly guided so as to prevent displacement when a plurality of layers of the coil  4  are wound around the tooth  12 , and a sufficient space for winding work can be provided on both sides of the tooth  12  in the circumferential direction. 
     The coil  4  is wound at high density in a trefoil shape, for example. Winding in a trefoil shape means a winding method in which a coil part (wire) of an upper-side layer of the coil  4  is placed in a concave part between two adjacent coil parts (wires) of a lower-side layer of the coil  4 . 
     An angle B formed by the inner wall surface  11   a  of the second yoke part  11  and the side surface  12   a  of the tooth  12  (parallel to the center line C) is, most preferably, 60 degrees. When the angle B is 60 degrees, the inner wall surface  11   a  of the second yoke part  11  is a flat surface extending along an envelope of the coil  4  wound around the tooth  12 . With this configuration, a larger amount of coil  4  can be wound around the stator  2  having a smaller outer diameter. 
     Since the coil  4  can be wound at high density as described above, it is possible to increase a ratio of a cross-sectional area of the coil  4  to a cross-sectional area of the slot (a winding space factor) and thereby reduce copper loss. 
     The insulator  3  is provided so as to cover parts of the yoke  9  and the tooth  12  of the stator core  1 , i.e., a part around which the coil  4  is wound and a part which faces the coil  4 , and electrically insulates the coil  4  and the stator core  1  from each other. 
     Specifically, the insulator  3  includes inclined surface parts  30  covering the inclined surfaces  15  of the tooth  12 , inner wall surface parts  31  covering the inner wall surfaces  11   a  of the second yoke parts  11 , winding surface parts  32  covering the side surfaces  12   a  of the tooth  12  and both end surfaces of the tooth  12  in the axial direction (i.e., surfaces around which the coil  4  is wound), inclined surface parts  33  covering the inclined surfaces  13   a  of the flange part  13 , and flange end surface parts  34  covering end surfaces of the flange part  13  in the circumferential direction. The inclined surface part  30  and the inner wall surface part  31  are adjacent to each other in the circumferential direction across the split surface  21 . 
     The insulator  3  is famed of insulating material. More specifically, the insulator  3  is formed of resin such as PPS (polyphenylene sulfide) or PET (polyethylene terephthalate), for example. 
     The insulator  3  can be formed integrally with the stator core  1  by setting the stator core  1  in a mold and filling the mold with resin. Alternatively, it is also possible to fit previously formed resin molded bodies onto the stator core  1 . 
     In the case of the resin molded bodies, the inclined surface parts  30 , the winding surface parts  32 , the inclined surface parts  33  and the flange end surface parts  34  of the insulator  3  (i.e., parts other than the inner wall surface parts  31  provided on the second yoke parts  11 ) may be famed integrally. 
     (Manufacturing Method of Motor) 
     Next, a manufacturing method of the motor  100  will be described.  FIG.  5 (A)  is a plan view for explaining a manufacturing process of the motor  100 .  FIGS.  5 (B) and  5 (C)  are cross-sectional views for explaining the manufacturing process of the motor  100 . 
     First, as shown in  FIG.  5 (A) , an electromagnetic steel sheet  50  is punched into a shape in which a plurality of split cores  51  are connected together in a band shape. In this example, the split cores  51  include four split cores  52  each famed of the tooth  12  and the first yoke part  10  and four split cores  53  each formed of the second yoke part  11 . The split cores  52  and the split cores  53  are alternately connected together in the band shape to form a split core array. Both ends of the split core array are made by dividing one split core  53  famed of the second yoke part  11  into two. 
     A thin-wall part  22  is formed between adjacent split cores  51 , that is, between the first yoke part  10  (the split core  52 ) and the second yoke part  11  (the split core  53 ). The thin-wall part  22  is formed at a position between the first yoke part  10  and the second yoke part  11  and in an edge part on the outer circumferential side. The thin-wall part  22  deforms plastically when the split cores  51  are connected together into an annular shape ( FIGS.  7 (A) and  7 (B)  which will be explained later). 
     In  FIG.  5 (A) , in order to punch as many split core arrays as possible out of the electromagnetic steel sheet  50 , two split core arrays are punched in such a manner that the teeth  12  in one array face the second yoke parts  11  in the other array. However, the punching pattern of the electromagnetic steel sheet  50  is not limited to this example. 
     A plurality of punched electromagnetic steel sheets  50  are stacked in the axial direction and are secured together by means of crimping, welding, adhesion or the like, so that the stator core  1  shown in  FIG.  5 (B)  is foiled. At this stage, the split cores  51  constituting the stator core  1  are not connected in the annular shape but are expanded in the band shape (linearly). 
     Subsequently, the insulator  3  is famed on the yoke  9  (the first yoke parts  10  and the second yoke parts  11 ) and the teeth  12  of the stator core  1 . As described above, the insulator  3  is formed integrally with the stator core  1  by setting the stator core  1  in a mold and filling the mold with resin, or formed by fitting previously formed resin molded bodies onto the stator core  1 . 
     The inclined surface part  30  of the insulator  3  is foiled to cover the inclined surface  15  ( FIG.  4   ) of the root part of the tooth  12 , and the inner wall surface part  31  is famed to cover the inner wall surface  11   a  ( FIG.  4   ) of the second yoke part  11 . The winding surface part  32  is formed to cover the winding surface of the tooth  12 . The inclined surface part  33  ( FIG.  4   ) of the insulator  3  is formed to cover the inclined surface  13   a  of the flange part  13 , and the flange end surface part  34  ( FIG.  4   ) is formed to cover the end surface of the flange part  13  in the circumferential direction. 
     Subsequently, as shown in  FIG.  5 (C) , the coil  4  is wound around and fixed to the insulator  3  (more specifically, the winding surface part  32 ) around each tooth  12 . In short, the winding work of the coil  4  is carried out. Since the stator core  1  is expanded in the band shape, there are sufficient spaces on both sides of each tooth  12 , and the winding process of the coil  4  can be carried out easily. 
       FIGS.  6 (A),  6 (B),  6 (C) and  6 (D)  are schematic views for explaining the winding process of the coil  4  shown in  FIG.  5 (C) . First, as shown in  FIG.  6 (A) , a first layer of the coil  4  (referred to as a first layer  41 ) is closely wound on the insulator  3  around the tooth  12 . 
     The winding of the first layer  41  of the coil  4  is started at a position where the first layer  41  is in contact with the inclined surface  15  of the tooth  12 , for example. A tail end of the first layer  41  of the coil  4  is in contact with the inclined surface  13   a  of the flange part  13 . Since the first layer  41  of the coil  4  is guided from both sides by the inclined surface  15  of the tooth  12  and the inclined surface  13   a  of the flange part  13  as described above, the displacement of the first layer  41  of the coil  4  can be prevented. 
     After winding the first layer  41  of the coil  4 , an ultraviolet-curable adhesive agent is applied to (for example, dropped onto) the first layer  41  of the coil  4 , and further the adhesive agent is irradiated with ultraviolet rays as indicated by arrows E. The adhesive agent is cured by the ultraviolet irradiation and the first layer  41  of the coil  4  is secured to the insulator  3  around the tooth  12 . Further, adjacent coil parts (wires) of the first layer  41  of the coil  4  are also secured to each other. 
     Subsequently, as shown in  FIG.  6 (B) , a second layer of the coil  4  (referred to as a second layer  42 ) is wound on the first layer  41  of the coil  4 . In this step, winding is carried out in the above described trefoil shape, that is, in such a manner that each coil part (wire) of the second layer  42  of the coil  4  enters the concave part between two adjacent coil parts of the first layer  41  of the coil  4 . 
     Since the first layer  41  of the coil  4  is positioned by the inclined surface  15  of the tooth  12  or the like as described above, the second layer  42  of the coil  4  can be placed on the first layer  41  of the coil  4  in a stable condition. Incidentally, the second layer  42  of the coil  4  does not contact the inclined surface  15  of the tooth  12  but contacts the inclined surface  13   a  of the flange part  13 . 
     After winding the second layer  42  of the coil  4 , the ultraviolet-curable adhesive agent is applied, and further the adhesive agent is irradiated with ultraviolet rays as indicated by arrows E. The adhesive agent is cured by the ultraviolet irradiation and the second layer  42  of the coil  4  is secured to the first layer  41  of the coil  4 . Further, adjacent coil parts (wires) of the second layer  42  of the coil  4  are also secured to each other. 
     Subsequently, as shown in  FIG.  6 (C) , a third layer of the coil  4  (referred to as a third layer  43 ) is wound on the second layer  42  of the coil  4  in the trefoil shape. After winding the third layer  43  of the coil  4 , the ultraviolet-curable adhesive agent is applied, and further the adhesive agent is irradiated with ultraviolet rays as indicated by arrows E. The adhesive agent is cured by the ultraviolet irradiation and the third layer  43  of the coil  4  is secured to the second layer  42  of the coil  4 . Further, adjacent coil parts of the third layer  43  of the coil  4  are also secured to each other. 
     Subsequently, as shown in  FIG.  6 (D) , a fourth layer of the coil  4  (referred to as a fourth layer  44 ) is wound on the third layer  43  of the coil  4  in the trefoil shape. Since the fourth layer  44  of the coil  4  is not guided by the inclined surface  13   a  of the flange part  13 , the winding of the fourth layer  44  of the coil  4  is carried out for a smaller number of turns than the third layer  43 . After winding the fourth layer  44  of the coil  4 , the ultraviolet-curable adhesive agent is applied, and further the adhesive agent is irradiated with ultraviolet rays as indicated by arrows E. The adhesive agent is cured by the ultraviolet irradiation and the fourth layer  44  of the coil  4  is secured to the third layer  43  of the coil  4 . Further, adjacent coil parts of the fourth layer  44  of the coil  4  are also secured to each other. 
     Thereafter, winding of a fifth layer to a preset number of the coil  4  is carried out (see  FIG.  4   ), and the ultraviolet irradiation is carried out each time one layer of the coil  4  is wound. Specifically, the winding of the coil  4 , the application of the adhesive agent, and the ultraviolet irradiation are repeatedly carried out each time one layer of the coil  4  is wound. Regarding the fourth layer  44  and its subsequent layers, the number of turns per layer is decremented by one each time one layer of the coil  4  is wound. 
     Since the coil  4  is secured each time one layer of the coil  4  is wound as above, the coil  4  does not collapse during the winding work, and the coil  4  can be wound regularly at high density as shown in  FIG.  4   . Incidentally, while  FIGS.  2  to  4    show that the number of winding layers of the coil  4  is eight for convenience of illustration, the number of layers of the coil  4  is not limited to this example. 
     Further, since the inclined surface  15  of the tooth  12  and the inclined surface  13   a  of the flange part  13  function as stoppers (contact surfaces) for the coil  4  ( FIG.  6 (A) ) as described above, the displacement of the coil  4  can be inhibited during the winding process and even after the winding. Accordingly, productivity can be enhanced and manufacturing cost can be reduced. 
     Incidentally, it is desirable to wind the coil  4  while applying appropriate tension to the coil  4 . This is because excessively high tension may cause the coil  4  to be stretched and deteriorate an insulation coating on the surface of the coil  4  or may increase electrical resistance due to a decrease in the cross-sectional area of the coil  4 , while excessively low tension may cause the coil  4  to be deflected and make the regular winding difficult. 
     Therefore, in a coil winding machine for winding the coil  4 , a winding nozzle for controlling the position of the coil  4  has to be appropriately positioned in a direction perpendicular to the side surface  12   a  of the tooth  12  with respect to a winding target position. For this purpose, spaces on both sides of the side surfaces  12   a  of the tooth  12  in the circumferential direction have to be opened. 
     Since each split surface  21  of the yoke  9  is arranged between the center of the first yoke part  10  in the circumferential direction and the center of the second yoke part  11  in the circumferential direction as described above, the spaces on both sides of the side surfaces  12   a  of the tooth  12  in the circumferential direction can be widely opened in the state where the stator core  1  is expanded in the band shape. 
     Incidentally, while the ultraviolet-curable adhesive agent is used for securing the coil  4  in the above explanation, the type of the adhesive agent is not limited to this example. For example, it is also possible to use an adhesive agent that is cured by heating, or an adhesive agent that is cured by a reaction of two types of liquids. 
       FIGS.  7 (A),  7 (B) and  7 (C)  are cross-sectional views for explaining a manufacturing process of the motor  100  after completion of the winding of the coil  4 . After the winding of the coil  4  is completed, the stator core  1  expanded in the band shape is bent into the annular shape as shown in  FIG.  7 (A) . Thus, each thin-wall part  22  between the first yoke part  10  and the second yoke part  11  adjacent to each other deforms plastically. 
     Subsequently, as shown in  FIG.  7 (B) , the annular stator core  1  is famed by abutting the abutting parts (parts to become the joint surface  23 ) on both ends of the stator core  1  against each other and welding the abutting parts together to a predetermined depth in bead-like form or at some points. Accordingly, the stator  2  including the stator core  1 , the insulator  3  and the coil  4  is obtained. Thereafter, the stator  2  is press-fitted into the first frame part  81  of the frame  80  shown in  FIG.  1   . Incidentally, the frame  80  (the first frame part  81 ) is omitted in  FIG.  7   . 
     On the other hand, in regard to the rotor  5 , the permanent magnet  7  is attached to the shaft  6  and thereafter the bearings  85  and  86  ( FIG.  1   ) are attached to the shaft  6 . Then, the rotor  5  is inserted into inside of the stator core  1  of the stator  2  as shown in  FIG.  7 (C) . Thereafter, the frame  80  ( FIG.  1   ) is formed by attaching the second frame part  82  to the first frame part  81 . Accordingly, the motor  100  is completed. 
     Incidentally, while the stator core  1  formed continuously via the thin-wall parts  22  is used in this example, it is also possible to form the split cores constituting the stator core  1  as members independent from each other, combine the split cores into the annular shape, and weld the split cores to each other. 
     Effects of Embodiment 
     As described above, in the first embodiment of the present invention, the yoke  9  of the stator core  1  includes the first yoke part  10  and the second yoke part  11 , and the inner wall surface  11   a  of the second yoke part  11  extends from the end G in the circumferential direction of the root part of the tooth  12 , and extends in a plane inward in the radial direction relative to the plane V passing through the end G and perpendicular to the center line C. With this configuration, the distance from the axis line Ax to the outer circumference of the stator core  1  (more specifically, the outer circumference of the second yoke part  11 ) can be reduced and the space for arranging the coil  4  can be provided. Namely, downsizing of the motor  100  can be achieved. 
     In addition, since the yoke  9  has the split surface  21  between the center of the first yoke part  10  in the circumferential direction and the center of the second yoke part  11  in the circumferential direction, the spaces on both sides of the side surfaces  12   a  of the tooth  12  in the circumferential direction can be widely opened in the state where the stator core  1  is expanded in the band shape. Accordingly, the winding work of the coil  4  using the coil winding machine can be carried out easily and accuracy. 
     Further, since the stator core  1  includes the four teeth  12  arranged at equal intervals in the circumferential direction, downsizing of a motor required to rotate at high speed, such as a motor for a fan of a vacuum cleaner can be achieved. 
     Further, since the coil  4  is fixed using the adhesive agent, the coil  4  wound around the tooth  12  can be held in a stable condition and stability of an operation of the motor  100  can be enhanced. Further, since the ultraviolet-curable adhesive agent is used as the adhesive agent, the process of curing the adhesive agent by ultraviolet irradiation can be repeated each time one layer of the coil  4  is wound. Accordingly, collapse of the coil  4  during the winding work can be prevented and the regular winding at high density becomes possible. 
     Further, when the distance H from the side surface  12   a  of the tooth  12  to the split surface  21  is greater than or equal to 0.5 times the diameter D of the coil  4 , the first layer  41  of the coil  4  is inhibited from climbing over the inclined surface  15  of the tooth  12  in the winding process. In other words, the inclined surface  15  of the tooth  12  serves as a stopper for the first layer  41  of the coil  4 , and thus the coil  4  can be wound around the tooth  12  at high density. 
     Further, when the distance H from the side surface  12   a  of the tooth  12  to the split surface  21  is too long, the second yoke parts  11  may enter the spaces on both sides of the side surfaces  12   a  of the tooth  12  in the circumferential direction in a state where the stator core  1  is expanded in the band shape and may interfere with the winding work. When the distance H is less than or equal to 1.5 times the diameter D of the coil  4 , the second yoke parts  11  do not enter the spaces on both sides of the side surfaces  12   a  of the tooth  12  in the circumferential direction, and thus the winding work can be carried out smoothly. 
     Further, since the split surface  21  of the yoke  9  is formed at the boundary between the first yoke part  10  and the second yoke part  11 , the configuration of each of the first yoke part  10  and the second yoke part  11  can be simplified. 
     Further, since the inclined surface  15  of the tooth  12  has an inclination such that its distance from the axis line Ax increases with increase in distance from the tooth  12  in the circumferential direction, the inclined surface  15  suitably functions as the stopper when the coil  4  is wound. 
     Further, since the plurality of layers of the coil  4  are wound so that the number of turns decreases with increase in distance from the tooth  12 , the winding can be carried out in such a manner that the envelope of the wound coil  4  extends along the inner wall surface  11   a  of the second yoke part  11 . 
     Further, since the outer wall surface  11   b  of the second yoke part  11  extends in parallel with the inner wall surface  11   a  of the second yoke part  11 , the distance from the axis line Ax to the outer circumference (i.e., the outer wall surface  11   b ) of the second yoke part  11  of the stator core  1  can be reduced effectively. 
     First Modification 
       FIG.  8    is an enlarged schematic view showing a part around a tooth  12  in a stator  2  in a first modification of the first embodiment. In the above described first embodiment, the split surface  21  of the yoke  9  is famed at the boundary between the first yoke part  10  and the second yoke part  11 . In contrast, in this first modification, a split surface  24  of the yoke  9  is formed at a position shifted in the circumferential direction from a boundary (represented by reference character P) between the first yoke part  10  and the second yoke part  11 . 
     In this example, the split surface  24  is formed in the first yoke part  10 . The rest of the configuration is as described in the first embodiment. Specifically, the shape and arrangement of the inner wall surface  11   a  of the second yoke part  11  are the same as those in the first embodiment. Further, the distance H from the side surface  12   a  of the tooth  12  to the split surface  24  in the circumferential direction is desired to be within the range of  0 . 5  times to 1.5 times the diameter D of the coil  4 . 
     Incidentally, while the split surface  24  is formed in the first yoke part  10  in the example shown in  FIG.  8   , it is also possible to form the split surface  24  in the second yoke part  11 . Also in this case, the distance H from the side surface  12   a  of the tooth  12  to the split surface [[ 21 ]] 24  in the circumferential direction is desired to be within the range of 0.5 times to 1.5 times the diameter D of the coil  4 . 
     Also in this first modification, the distance from the axis line Ax to the outer circumference of the stator core  1  (more specifically, the outer circumference of the second yoke part  11 ) can be reduced similarly to the first embodiment. Further, by setting the distance H from the side surface  12   a  of the tooth  12  to the split surface [[ 21 ]] 24  in the circumferential direction within the range of 0.5 times to 1.5 times the diameter D of the coil  4 , the displacement of the coil  4  can be inhibited, and smooth winding work can be achieved by widely opening the spaces on both sides of the side surfaces  12   a  of the tooth  12  in the circumferential direction. 
     Second Modification 
       FIG.  9    is an enlarged schematic view showing a part around a tooth  12  in a stator  2  in a second modification of the first embodiment. In the above described first embodiment, the inclined surface  15  ( FIG.  4   ) of the tooth  12  and the inner wall surface  11   a  of the second yoke part  11  are adjacent to each other. In contrast, in this second modification, another inner wall surface  16  is formed between the inclined surface  15  of the tooth  12  and the inner wall surface  11   a  of the second yoke part  11 . 
     The inclined surface  15  is configured as described in the first embodiment and serves as a stopper for the first layer of the coil  4  in the winding process. The another inner wall surface  16  extends further outward in the circumferential direction from an outer end of the inclined surface  15  in the circumferential direction. 
     The inner wall surface  11   a  of the second yoke part  11  is a flat surface extending from an outer end G of the inner wall surface  16  in the circumferential direction, and extending inward in the radial direction relative to a plane V passing through the end G and perpendicular to the center line C. Similarly to the first embodiment, the distance H from the side surface  12   a  of the tooth  12  to the split surface  21  in the circumferential direction is desired to be within the range of 0.5 times to 1.5 times the diameter D of the coil  4 . 
     Also in this second modification, the distance from the axis line Ax to the outer circumference of the stator core  1  (more specifically, the outer circumference of the second yoke part  11 ) can be reduced similarly to the first embodiment. Further, the displacement of the coil  4  can be inhibited, and smooth winding work can be achieved by widely opening the spaces on both sides of the side surfaces  12   a  of the tooth  12  in the circumferential direction. 
     Third Modification 
       FIG.  10    is an enlarged schematic view showing a part around a tooth  12  in a stator  2  in a third modification of the first embodiment. In the above described first embodiment, the first yoke part  10  has the planar inclined surface  15  ( FIG.  4   ). The first yoke part  10  in this third modification has a curved surface  17  instead of the planar inclined surface  15 . 
     The curved surface  17  is curved so that its distance from the axis line Ax increases with increase in distance from the side surface  12   a  of the tooth  12  in the circumferential direction. Accordingly, the curved surface  17  functions as a stopper (a contact surface) for the first layer of the coil  4  in the winding process. 
     The inner wall surface  11   a  of the second yoke part  11  is a flat surface extending from an outer end G of the curved surface  17  in the circumferential direction, and extending inward in the radial direction relative to a plane V passing through the end G and perpendicular to the center line C. Similarly to the first embodiment, the distance H from the side surface  12   a  of the tooth  12  to the split surface  21  in the circumferential direction is desired to be within the range of 0.5 times to 1.5 times the diameter D of the coil  4 . 
     Also in this third modification, the distance from the axis line Ax to the outer circumference of the second yoke part  11  can be reduced similarly to the first embodiment. Further, the displacement of the coil  4  can be inhibited, and smooth winding work can be achieved by widely opening the spaces on both sides of the tooth  12  in the circumferential direction. 
     (Fan) 
     Next, a fan  110  to which the motor in the first embodiment or each modification described above is applied will be described.  FIG.  11    is a sectional view showing the fan  110  to which the motor in the first embodiment or each modification is applied. The fan  110  is employed for a vacuum cleaner  200  ( FIG.  12   ), for example. 
     The fan  110  includes a blower unit  90  mounted on the second frame part  82  of the motor  100 . The blower unit  90  includes a main plate  92  having a through hole  92   a  allowing the shaft  6  to penetrate, an impeller  91  attached to the tip end of the shaft  6  penetrating the through hole  92   a  of the main plate  92 , and a fan cover  93  covering the impeller  91  from outside. 
     An air intake port  93   a  is famed at a center of the fan cover  93 . A channel (air channel) for air flowing in through the air intake port  93   a  is formed between the main plate  92  and the fan cover  93 . 
     When the rotor  5  of the motor  100  rotates, the impeller  91  attached to the shaft  6  of the rotor  5  rotates. When the impeller  91  rotates, air flows in through the air intake port  93   a,  flows in the air channel between the main plate  92  and the fan cover  93  toward an outer circumferential side, and is discharged through an air outlet (not shown) formed on the outer circumferential side. 
     The motor  100  in the first embodiment or each modification described above is capable of reducing the distance from the center to the outer circumference of the stator core  1 . Thus, by applying this motor  100  to the fan  110 , a discharge air channel from the blower unit  90  can be formed on an outer side of the motor  100  and an outer diameter of the fan  110  can be reduced. 
     (Vacuum Cleaner) 
     Next, a vacuum cleaner  200  including the fan  110  to which the motor in the first embodiment or each modification described above is applied will be described.  FIG.  12    is a schematic diagram showing the vacuum cleaner  200  including the fan  110  to which the motor in the first embodiment or each modification is applied. 
     The vacuum cleaner  200  includes a cleaner main body  201 , a pipe  203  connected to the cleaner main body  201 , and a suction part  204  connected to a tip end part of the pipe  203 . The suction part  204  is provided with a suction port  205  for sucking in air containing dust. A dust collection container  202  is arranged in the cleaner main body  201 . 
     Further, the fan  110  for sucking in air containing dust from the suction part  204  to the dust collection container  202  is arranged in the cleaner main body  201 . The cleaner main body  201  is provided with a grip part  206  to be gripped by a user, and the grip part  206  is provided with an operation part  207  such as an on/off switch. 
     When the user grips the grip part  206  and operates the operation part  207 , the fan  110  is driven. When the fan  110  is driven, suction wind occurs and dust is sucked in together with air via the suction port  205  and the pipe  203 . The dust sucked in is stored in the dust collection container  202 . 
     Since this vacuum cleaner  200  employs the small-sized fan  110  described above, downsizing of the vacuum cleaner  200  becomes possible. 
     While the preferred embodiments of the present invention have been described specifically above, the present invention is not limited to the above described embodiments and a variety of improvements or modifications are possible within the range not departing from the subject matter of the present invention.