Abstract:
A stator structure which enables coils to be fitted over teeth while the circularity of the stator core and the parallelism between the end surfaces of the stator core are maintained. A stator structure is provided with: a stator core which is formed by stacking steel plates and which comprises a yoke and teeth; and coils which are fitted over the teeth so as to surround the teeth. A cut is formed only in one part of the yoke, and the cut is opened. Opening the cut allows the coils to be fitted over the teeth while the circularity of the stator core and the parallelism between the end surfaces of the stator core are maintained.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a 371 national phase application of PCT/JP2010/060355 filed on 18 Jun. 2010, the entire contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a stator structure including a stator core made of laminated steel plates and provided with a yoke part and teeth parts and coils wound around and mounted on the teeth parts. 
     BACKGROUND OF THE INVENTION 
     Some stator cores are configured such that coils are mounted or fitted on teeth parts of the stator core. The circularity of the stator core (stator-core circularity) and the parallelism of end faces (end-face parallelism) of the stator core are determined depending on press accuracy. For a one-piece stator core which can be produced by a single press work to steel plates, therefore, the stator-core circularity and the end-face parallelism can be easily enhanced as compared with a split stator core produced by assembling split core pieces individually. 
     The one-piece stator core is manufactured for example by mounting edgewise coils on teeth parts in sequence. However, for a stator core  100 , as shown in  FIG. 14 , when a last coil  109  is to be mounted on a teeth part  108 , the last coil  109  interferes with a firstly mounted coil  105  and a coil  107  which are to be adjacent to the coil  109 . Specifically, as illustrated in  FIG. 15  showing an enlarged view of a part indicated by a chain line R in  FIG. 14 , the mounting width S of the coil  109  is wider than the mountable width U between the adjacent coils  105  and  107 . Thus, the last coil  109  could not be mounted on the teeth part  108 . In the case where the coils are mounted on the teeth parts in sequence, therefore, the last coil  109  could not be mounted on the teeth part  108 . 
     As this type of technique, there is conventionally a stator core  200  described in Patent Document 1 shown in  FIG. 16 . As shown in  FIG. 16 , stator parts  201  of the stator core  200  are formed, on an inner peripheral, with teeth parts  203 . Slits  206  are formed on both sides of each stator part  201 . When bundled coil wires are to be mounted on the teeth parts  203  of the stator parts  201 , the outer periphery of each stator part  201  is pressed. This causes deformation of each stator part  201  allowed by the slits  206  formed on both sides of each stator part  201 . When one stator part  201  is externally pressed, thereby widening the slits  206 , the corresponding teeth part  203  is made to protrude inward. Since each teeth part  203  is caused to protrude inward, the bundled coil wires are allowed to be mounted thereon. 
     As above, the stator parts  201  are pressed externally in sequence so that the bundled coil wires are mounted on the teeth parts  203 . 
     RELATED ART DOCUMENTS 
     Patent Documents 
     
         
         Patent Document 1: JP-A-2000-245081 
         Patent Document 2: JP-A-2001-251819 
         Patent Document 3: JP-A-2006-352991 
       
    
     SUMMARY OF INVENTION 
     Problems to be Solved by the Invention 
     However, the conventional arts have the following disadvantages. Specifically, the stator core  200  has a problem that the stator parts  201  have to be pressed individually for each teeth part  203  to mount the bundled coil wires on the teeth parts  203 . Therefore, in a case of the teeth parts  203  provided at twelve places in the whole circumference, the outer periphery of the stator parts  201  have to be pressed twelve times so that the coil is mounted on the teeth parts  203 . Such twelve-times pressing of the outer periphery of the stator parts  201 , increasing the number of steps, results in deteriorated productivity. 
     In the stator core  200 , the slits  206  are formed over the whole circumference of the inner periphery. Due to the slits  206  formed in the whole circumference, the circularity of the stator core  200  and the parallelism of end faces of the stator core  200  are deteriorated. 
     The stator-core circularity represents the circularity of a hollow cylindrical stator core. The end-face parallelism represents the parallelism of an end face forming an inner wall of the hollow part of the hollow cylindrical stator core with respect to a central axis. Based on the stator-core circularity and the end-face parallelism, the stator core and a rotor set in the hollow part of the stator core are controlled in three dimensions to avoid the stator core and the rotor from bumping or colliding with each other. 
     If the stator-core circularity and the end-face parallelism are low, the rotor has to be reduced in size to prevent bumping or colliding with the stator core. Such a size-reduced rotor generates a wide gap between the stator core and the rotor and thus a magnetic flux density between the stator core and the rotor lowers. This results in a decrease in motor power. 
     The circularity and the end-face parallelism of the stator core  200  are deteriorated for the following reasons. When the outer periphery of the stator part  201  is pressed, the stator part  201  is allowed to be deformed by the slits  206 . When pressing on the stator part  201  is stopped, the stator part  201  returns to an original position by a restoring force. Although each stator part  201  returns to its original shape by the restoring force, the stator parts  201  actually have different restoring forces because of the slits  206  arranged over the whole circumference. Therefore, the circularity and the end-face parallelism of the stator core  200  are deteriorated by the total of differences in restoring force depending on the slits  206 . 
     The present invention has been made to solve the above problems and has a purpose to provide a stator structure in which coils can be mounted on teeth parts while keeping circularity and end-face parallelism of a stator core. 
     Means of Solving the Problems 
     To achieve the above purpose, one aspect of the invention provides a stator structure configured as below. 
     (1) In a stator structure including a stator core made of laminated steel plates and provided with a yoke part and teeth parts and coils wound around and mounted on the teeth parts, the yoke part is formed with a cut section only in one place, and the coil is mounted on the teeth part by opening the cut section in a circumferential direction of the stator core. 
     (Deleted) 
     (2) In the stator structure in (1), an engagement protrusion is formed in one end of the both end portions of the cut section, and an engagement recess is formed in the other end of the both end portions, the engagement recess being engageable with the engagement protrusion.
 
(3) In the stator structure in (2), the engagement protrusion is formed in a lamination direction, and the engagement recess is formed in the lamination direction.
 
(4) In the stator structure in (2), the engagement protrusion is formed in a radial direction, and the engagement recess is formed in the radial direction.
 
(5) In a stator manufacturing method including mounting coils on a stator core made of laminated steel plates and provided with a yoke part and teeth parts so that the coils are wound around the teeth parts, the yoke part is formed with a cut section only in one place, and the method includes a step of mounting the coil on the teeth part by opening the cut section in a circumferential direction of the stator core.
 
(6) In the stator manufacturing method in (5), an engagement protrusion is formed in one end of the both end portions of the cut section, and an engagement recess is formed in the other end of the both end portions, the engagement recess being engageable with the engagement protrusion.
 
(7) In the stator manufacturing method in (5), the engagement protrusion is formed in a lamination direction, and the engagement recess is formed in the lamination direction.
 
(8) In the stator manufacturing method in (5), the engagement protrusion is formed in a radial direction, and the engagement recess is formed in the radial direction.
 
     Effects of the Invention 
     The operations and advantageous effects of the above stator structure will be explained. 
     (1) In a stator structure including a stator core made of laminated steel plates and provided with a yoke part and teeth parts and coils wound around and mounted on the teeth parts, the yoke part is formed with a cut section only in one place. Accordingly, the coils can be mounted on the teeth parts while maintaining the circularity and the end-face parallelism. The reason is as follows. When a coil is to be mounted on a last teeth part, on which a coil could not be mounted in a conventional art, the cut section is opened or split to allow the coil to be mounted on the last teeth part. Opening the cut section is conducted within the elastically deformable range of the stator core. This is because, as long as an open width of the cut section is within the elastically deformable range of the stator core, the stator core can return to its original shape with high circularity and high end-face parallelism by the elasticity without causing plastic deformation of the cut section. 
     Since the cut section is formed in one place, the cut section does not need to be opened when the coils excepting a coil to be mounted on a last teeth part are to be mounted on teeth parts. Thus, the coils can be mounted on the teeth parts while the cut section remains in an original state. Accordingly, the cut section has only to be opened only one time to mount the coil on the last teeth part, so that the circularity and the end-face parallelism substantially remain unchanged. Furthermore, since the cut section has only to be opened only one time, an assembling efficiency is high and a manufacturing cost can be reduced. 
     (2) The coil is mounted on the teeth part by opening the cut section. Accordingly, the coil can be easily mounted on the last teeth part. The reason is as follows. The stator consists of laminated steel plates, which has low rigidity. This allows the cut section to be opened easily by a few of millimeters within the elastically deformable range. Since the cut section formed in the yoke part beside the last teeth part is allowed to open, it is possible to generate a gap with a coil mounting width needed to mount a coil. 
     When the cut section is opened by a few of millimeters within the elastically deformable range and then is to be returned to its original position in the case where the cut section, the cut section will return to its original shape by the elasticity of the yoke part. Thus, the cut section can easily return to the original shape without needing application of a returning force thereto. This can reduce a manufacturing cost. 
     (3) The one-end protrusion is formed in one end of both end portions of the cut section and an other-end protrusion in the other end of the both end portions of the cut section, the one-end protrusion and the other-end protrusion protruding from an outer periphery of the yoke part. Accordingly, the cut section of the stator core can be opened while maintaining the circularity and the end-face parallelism. The reason is as follows. When the yoke part is directly grasped with a handle or the like and a force to open the cut section is exerted on the yoke part, the yoke part may be deformed by the handle or the like and thus the circularity and the end-face parallelism could not be maintained. However, when the force to open the cut section is directly applied to the one-end protrusion and the other-end protrusion formed on the outer periphery of the yoke part, the yoke part needing to maintain the circularity and the end-face parallelism is not directly grasped and therefore the yoke part is not deformed. This can keep the circularity of the stator core and the parallelism of the end face. 
     Separating the one-end protrusion and the other-end protrusion formed on the outer periphery of the yoke part is easier than opening the cut section by the opening force directly applied to the yoke part. This is because, when the cut section is to be opened, the outer peripheral portion formed distantly from the stator core only needs a smaller force. Accordingly, by separating the one-end protrusion and the other-end protrusion both formed on the outer peripheral part, the cut section formed in the yoke part can be easily opened. This makes it possible to enhance an assembling efficiency to insert a coil on the last teeth part and thus reduce a manufacturing cost. 
     (4) The contact surfaces of the one-end protrusion and the other-end protrusion contacting with each other are formed with gap-forming recesses. Accordingly, they can be easily separated. Specifically, a tool is allowed to be inserted in the gap-forming recesses, and thus these recesses can be separated easily with the tool to open the cut section.
 
(5) The gap-forming recesses are elliptic. Accordingly, the cut section can be easily opened. Specifically, a tool having an elliptic shape smaller than the elliptic gap-forming recesses is inserted in the recess. This elliptic tool has an elliptic shape having a major axis and a minor axis. Therefore, when the elliptic tool is turned 90°, the gap-forming recesses are separated by a distance corresponding to a value obtained by subtracting the minor axis from the major axis of the elliptic shape. Thus, the cut section can be easily opened by simply turning the elliptic tool 90°.
 
     By turning the elliptic tool by 90°, it is possible to accurately open the cut section by a fixed distance. To be concrete, owing to the elliptic tool has the elliptic shape having the major axis and the minor axis, the gap-forming recesses are separated by the distance corresponding to a value obtained by subtracting the minor axis from the major axis of the elliptic shape. This allows the cut section to open accurately open by a fixed distance. 
     (6) The engagement protrusion is formed in one end of the both end portions of the cut section, and the engagement recess is formed in the other end of the both end portions, the engagement recess being engageable with the engagement protrusion. Accordingly, when the cut section is opened and then fully returns to its original shape by the elasticity, the cut section of the stator core can be returned to the original shape while keeping the circularity and the end-face parallelism. Specifically, while the opened cut section is returning fully to its original shape, the engagement protrusion and the engagement recess serve as a guide for the returning motion. Owing to these protrusion and recess serving as a guide for returning of the cut section, the cut section can return to its original shape. Therefore, the stator core can return to a shape having high circularity and high end-face parallelism without causing plastic deformation. 
     The engagement protrusion and the engagement recess are designed to have a larger step (difference in length) between the protrusion and the recess than a range allowing the cut section to be opened during coil assembling. Thus, the protrusion and the recess can serve as a guide without causing the stator core from disassembling. 
     (7) The engagement protrusion is formed in a lamination direction, and the engagement recess is formed in the lamination direction. Accordingly, this can prevent a displacement of the cut section in the lamination direction. Since the thickness of the stator core is larger in the lamination direction than in the radius direction, the engagement protrusion and the engagement recess may be formed in two or more places in the lamination direction. Two or more engagement protrusions and recesses allow the cut section to more reliably return to its original position when the cut section is to fully return.
 
(8) The engagement protrusion is formed in a radial direction, and the engagement recess is formed in the radial direction. Accordingly, this can prevent a displacement of the cut section in the radius direction. Since the stator core is formed with the engagement protrusion and engagement recess in the radius direction, they can be produced by use of a press die used for shaping one steel plate. This is because the stator core can be manufactured by laminating steel plates having the same shape. This can more reduce the manufacturing cost than in an engagement portion is formed in the lamination direction.
 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a step ( 1 ) of mounting a coil in a stator core in a first embodiment of the invention; 
         FIG. 2  is a diagram showing a step ( 2 ) of mounting the coil in the stator core in the first embodiment of the invention; 
         FIG. 3  is a partial enlarged view of a part enclosed by a chain line P in  FIG. 1  in the first embodiment of the invention; 
         FIG. 4  is a partial enlarged view of a part enclosed by a chain line Q in  FIG. 2  in the first embodiment of the invention; 
         FIG. 5  is a diagram showing a step ( 3 ) of mounting the coil in the stator core in the first embodiment of the invention; 
         FIG. 6  is a partial enlarged view of a shape ( 1 ) of a cut section in a second embodiment of the invention; 
         FIG. 7  is a partial enlarged of a shape ( 2 ) of the cut section in the second embodiment of the invention; 
         FIG. 8  is a perspective external view of a stator core in a third embodiment of the invention; 
         FIG. 9  is a partial enlarged view of a part enclosed by a chain line D in  FIG. 8  in the third embodiment t of the invention; 
         FIG. 10  is a front view of a stator core in a fourth embodiment of the invention; 
         FIG. 11  is a partial enlarged view ( 1 ) of a part enclosed by a chain line E in  FIG. 10  in the fourth embodiment of the invention; 
         FIG. 12  is a partial enlarged view ( 2 ) of the part enclosed by the chain line E in  FIG. 10  in the fourth embodiment of the invention; 
         FIG. 13  is a perspective external view of a coil in the first embodiment of the invention; 
         FIG. 14  is a diagram showing a step of mounting a coil in a stator core in a conventional art; 
         FIG. 15  is a partial enlarged view of a part enclosed by a chain line R in  FIG. 14  in the conventional art; and 
         FIG. 16  is a partial enlarged view of a stator core in a Patent Document. 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
     &lt;Whole Structure of Stator Core&gt; 
       FIG. 1  is a diagram showing a step ( 1 ) of mounting a coil C in a stator core  1 .  FIG. 2  is a diagram showing a step ( 2 ) of mounting the coil C in the stator core  1 .  FIG. 3  is a partial enlarged view of a part enclosed by a chain line P in  FIG. 1 .  FIG. 4  is a partial enlarged view of a part enclosed by a chain line Q in  FIG. 2 .  FIG. 5  is a diagram showing a step ( 3 ) of mounting the coil C in the stator core  1 . 
     The stator core  1  in  FIG. 1  is made of a plurality of thin steel plates not shown laminated or stacked in two or more layers and in a hollow cylindrical shape. In the present embodiment, the stator core  1  has a diameter of 200 mm. On the inner peripheral surface of the stator core  1 , there are formed twelve teeth parts T at a predetermined pitch. The twelve teeth parts T are referred to as a first teeth part T 1 , a second teeth part T 2 , . . . , and a twelfth teeth part T 12 . 
     The teeth parts T support twelve coils C each of which is formed of a conductor wire having a flat rectangular cross section and wound in more than one turn. In the present embodiment, the coils C include twelve coils C in correspondence with the twelve teeth parts T. The twelve coils C are referred to as a first coil C 1 , a second coil C 2 , . . . , and a twelfth coil C 12 . 
     (Configuration of Cut Section) 
     As shown in  FIG. 1 , the stator core  1  is formed with a cut section  50  extending in a radial direction. The cut section  50  is formed through all the thin steel plates. When a pull force is applied to the stator core  1 , the stator core  1  is elastically deformed as shown in  FIG. 2 , thus opening or splitting the cut section  50 . The cut section  50  includes one end  51  formed on a first teeth part T 1  side of the yoke part  12  and the other end  52  formed on a twelfth teeth part T 12  side of the yoke part  12 . Opening of the cut section  50  therefore means that the one end  51  and the other end  52  are separated from each other. While no force is applied to the cut section  50 , as shown in  FIGS. 1 and 3 , the one end  51  and the other end  52  are in contact with each other. 
     When the stator core  1  is elastically deformed by application of the pull force, the cut section  50  is opened in an elastically deformable range of the core  1 , generating a gap L as shown in  FIG. 4 . The width of the gap L is defined as a distance from the one end  51  to the other end  52  of the cut section  50 . The width of the gap L in the present embodiment is about 3 mm. Because this about 3-mm width of the gap L corresponds to the elastically deformable range and falls in a range that does not have any influence on circularity and parallelism of end faces (end-face parallelism) after the stator core  1  returns to its original shape by its elasticity (elastic force). Further, when the gap L is generated by a width of about 3 mm, it can provide a distance long enough to insert the last twelfth coil C 12  on the last twelfth teeth part T 12 . In the stator core  1  having a diameter of 200 mm, such a mere about 3-mm width of the gap L hardly influences the circularity and the end-face parallelism. 
     The cut section  50  can be formed by cutting when the thin steel plates are formed by press. Alternatively, the cut section  50  may be formed simultaneously with press work. 
     Although the width of the gap L is set to about 3 mm in the present embodiment, the width of the gap L is not limited to about 3 mm as long as it is in the elastically deformable range and in a region in which the stator core  1  is not plastically deformed. Specifically, the elastically deformable range may be changed according to the materials of the stator core  1  and also according to the size of the stator core  1 . Thus, the width of the gap L is not limited to about 3 mm set in the present embodiment. 
     Furthermore, the width of the gap L has only to be determined as a width allowing the last twelfth coil C 12  to be mounted on the last twelfth teeth part T 12 . According to the cases where the number of teeth parts is increased to 18, 24, etc. or decreased to 9 or 6 as alternatives to the present embodiment including twelve teeth parts, the width of the gap L is changed. 
     In  FIGS. 2 and 4 , the width of the gap L to open the cut section  50  is illustrated to be large as a conceptual diagram to facilitate understanding thereof. Actually, the width of the gap L is as small as about 3 mm. 
     (Configuration of Coil) 
       FIG. 13  is an external perspective view of the first coil C 1 . Although  FIG. 13  describes the first coil C 1 , the second coil C 2  to the twelfth coil C 12  are also configured similarly. As shown in  FIG. 13 , the first coil C 1  is a coil made by edgewise bending a flat rectangular conductor wire by use of an edgewise bending wiring device not shown. 
     The first coil C 1  has a first end portion C 101   a  and a second end portion C 101   b . One of the first end portion C 101   a  and the second end portion C 101   b  is a winding starting end and the other is a winding ending end. The first coil C 1  is formed of a wire wound in a nearly trapezoidal shape so that short sides are gradually longer as the wire is wound to the first end portion C 101   a  side. 
     In the present embodiment, an edgewise coil is explained as the finished first coil C 1 . The same applies to any other types of coils having finished shapes, irrespective of what shape the cross section has, circular or rectangular. 
     &lt;Method of Mounting Coil in Stator Core&gt; 
     (First Step) 
     As shown in  FIG. 1 , the coils C are sequentially mounted on the teeth parts T of the stator core  1 . To be concrete, the first coil C 1  is mounted on the first teeth part T 1  formed in one end of the cut section  50 , the second coil C 2  is mounted on the second teeth part T 2 , . . . , so that eleven coils C are mounted sequentially on eleven teeth parts T. 
     After eleven coils C are mounted on eleven teeth parts T as shown in  FIGS. 1 and 3 , the twelfth coil C 12  is not allowed to be mounted on the twelfth teeth part T 12  formed in the other end of the cut section  50 . 
     Specifically, as shown in  FIG. 3 , a mounting width H which is defined as a mounting width of the twelfth coil C 12  on a first end portion C 12   a  side is wider than a mountable width J defined from the second end portion C 1   b  of the first coil C 1  to a second end portion C 11   b  of the eleventh coil C 11  to receive a coil. Thus, the first coil C 1  and the eleventh coil C 11  interfere with the twelfth coil C 12  to be mounted. 
     (Second Step) 
     To mount the twelfth coil C 12  on the twelfth teeth part T 12 , a pull force in a circumferential direction is applied to the cut section  50  of the stator core  1 . To be concrete, both end portions of the cut section  50  of the yoke part  12  are grasped from above and below and moved apart from each other in the circumferential direction. The circumferential pull force exerted on the cut section  50  elastically deforms the stator core  1 . When the stator core  1  is elastically deformed as shown in  FIG. 4 , the cut section  50  is opened in the elastically deformable range, thereby generating the gap L. The width of the gap L in the present embodiment is a distance of about 3 mm. 
     When the gap L is generated as shown in  FIG. 4 , the width from the second end portion C 1   b  of the first coil C 1  to the second end portion C 11   b  of the eleventh coil C 11  is widened from the mountable width J to a mountable width K. The distance determined by subtracting the mountable width J from the mountable width K is proportional to the width of the gap L. 
     The mountable width K from the second end portion C 1   b  of the first coil C 1  to the second end portion C 11   b  of the eleventh coil C 11  is larger than the mounting width H of the twelfth coil C 12  on the side of the first end portion C 12   a . Therefore, the twelfth coil C 12  can be mounted on the twelfth teeth part T 12  without being interfered by the first coil C 1  and the eleventh coil C 11 . 
     (Third Step) 
     After the twelfth coil C 12  is mounted on the twelfth teeth part T 12 , the circumferential pull force exerted on the cut section  50  is removed. Upon removal of the pull force, the stator core  1  returns by its elasticity to its original state shown in  FIG. 5 . When the stator core  1  returns to the original state shown in  FIG. 5 , the one end  51  and the other end  52  of the cut section  50  are brought in contact with each other, and the gap L disappears. Since the gap L disappears, the first coil C 1  and the twelfth coil C 12  approach each other. 
     The stator core  1  comes to the state shown in  FIG. 5  by the elasticity and therefore does not cause plastic deformation. Because of no plastic deformation, the stator core  1  can maintain the circularity and the end-face parallelism of the original stator core  1 . 
     (Opening Cut Section) 
     The details of opening or splitting the cut section  50  in the second and third steps will be explained. 
     When the pull force in the circumferential direction is applied to the cut section  50  to generate the gap L in the stator core  1 , the gap L can be generated without affecting the circularity and the end-face parallelism of the stator core  1 . The reason thereof is as below. Since the stator core  1  is applied with the pull force in the elastically deformable range, the stator core  1  will return to its original shape by its elasticity. Therefore, the circularity and the end-face parallelism of the stator core  1  remain unchanged from those obtained before application of the pull force. When the pull force is to be applied to the stator core  1 , in the present embodiment, the pull force is exerted in such a range as not to make the gap wider than about 5 mm. If the gap is wider than about 5 mm, the stator core  1  is plastically deformed and thus does not return to its original shape by the elasticity. Accordingly, the pull force in the range causing no plastic deformation is applied to the stator core  1 . 
     With the above configuration, all the coils C can be mounted on the teeth parts T without affecting the circularity and the end-face parallelism of the stator core  1 . 
     As explained in detail above, according to the stator core  1  in the first embodiment, the following advantageous effects can be provided. 
     Since the cut section  50  is formed only in one place in the yoke part  12 , it is possible to mount the coils C on the teeth parts T while maintaining the circularity and the end-face parallelism. The reason is as below. In the conventional stator core  100  shown in  FIG. 14 , the last coil  109  could not be mounted on the last teeth part  108 . In contrast, according to the present embodiment, the cut section  50  is opened to allow mounting of the twelfth coil C 12  on the last twelfth teeth part T 12 . When the cut section  50  is to be opened, the cut section  50  is opened in the elastically deformable range of the stator core  1 . As long as the opening width is in the elastically deformable range of the stator core  1 , the cut section  50  is not plastically deformed and the stator core  1  can return, by the elasticity, to its original shape having high circularity and high end-face parallelism. 
     Since the cut section  50  is formed only in one place, it is possible to directly mount the first coil C 1  and others on the first teeth part T 1  and others without opening the cut section  50  except for the case where the twelfth coil C 12  is to be mounted on the twelfth teeth part T 12 . Accordingly, the cut section  50  has only to be opened only once in order to mount the twelfth coil C 12  on the twelfth teeth part T 12 , so that the circularity and the end-face parallelism remain unchanged. Only one-time opening the cut section  50  makes it possible to enhance an assembling efficiency and reduce a manufacturing cost. 
     Opening the cut section  50  allows the twelfth coil C 12  to be easily mounted on the twelfth teeth part T 12 . The reason is as below. The stator core  1  is made of laminated steel plates and thus has low rigidity. This allows the cut section  50  to be easily opened by a few of millimeters in the elastically deformable range. Since the cut section  50  formed in the yoke part  12  beside the twelfth teeth part T 12  is allowed to be easily opened, the gap corresponding to the mountable width J needed to mount the twelfth coil C 12  can be generated. 
     In the case where the cut section  50  is opened by about three millimeters in the elastically deformable range, when the cut section  50  is to be allowed to return to its original shape, the cut section  50  will naturally return to the original shape by the elasticity of the yoke part  12 . Accordingly, the cut section  50  can easily return to the original shape without needing application of a returning force thereto. This can reduce a manufacturing cost. 
     Second Embodiment 
     A stator core  2  in a second embodiment is different from the stator core  1  in the first embodiment only in that a cut section  20  of the stator core  2  is different in shape from the cut section  50  of the stator core  1 . The second embodiment is identical to the first embodiment except for the cut section and therefore will be explained with a focus on the cut section  20  without repeating the explanation of other parts or components. 
     The second embodiment in which other parts or components are not explained can also provide the same operations and advantageous effects as those in the first embodiment. 
     Modified Example of Shape of Cut Section in Radial Direction 
       FIG. 6  is a partial enlarged view of a shape ( 1 ) of the cut section  20  of the stator core  2 . As shown in  FIG. 6 , the stator core  2  is formed with the cut section  20  extending in a radial direction. The cut section  20  is formed through all the thin steel plates. When a pull force is applied to the stator core  2 , therefore, the cut section  20  is opened or split. The cut section  20  includes one end  21  formed on the first teeth part T 1  side of the yoke part  12  and the other end  22  formed on the twelfth teeth part T 12  side of the yoke part  12 . Opening the cut section  20  therefore means that the one end  21  and the other end  22  are separated from each other. While no force is applied to the cut section  20 , the one end  21  and the other end  22  are in contact with each other. 
     The one end  21  may be formed with an engagement protrusion  23  having a curved surface at a distal end and the other end  22  may be formed with an engagement recess  24  having a curved surface engageable with the engagement protrusion  23 . The protrusion  23  and the recess  24  are formed in a radial direction X. 
     The protrusion  23  has a length N longer than a width of the gap L at which the cut section  20  is opened. A depth of the recess  24  engaging with the protrusion  23  is equal to the length N of the protrusion  23 . For instance, if the width of the gap L is about 3 mm, the length N of the protrusion  23  and the depth of the recess  24  are respectively set to be 4 mm or more. 
       FIG. 7  is a partial enlarged view of a shape ( 2 ) of the cut section  20  of the stator core  2 . Furthermore, the shape of the cut section  20  is not limited to the shape having such a curved end as shown in  FIG. 6  and may be a shape having a triangular end as shown in  FIG. 7 . Specifically, as shown in  FIG. 7 , the cut section  20  may be formed with an engagement protrusion  25  having a triangular protruding shape and an engagement recess  26  having a triangular recessed shape engageable with the protrusion  25 . These protrusion  25  and recess  26  are formed in a radial direction X. 
     The protrusion  25  has a length N longer than a width of the gap L at which the cut section  20  is opened. A depth of the recess  26  engaging with the protrusion  25  is equal to the length N of the protrusion  25 . For instance, if the width of the gap L is about 3 mm, the length of the protrusion  25  and the depth of the recess  26  are respectively set to be 4 mm or more. 
     (Operations and Advantageous Effects of Shape of Cut Section in Radial Direction) 
     With the engagement protrusion  23  and the engagement recess  24  shown in  FIG. 6 , it is possible to restrain a displacement of the stator core  2  shown in  FIG. 6  in the radial direction when the stator core  2  is elastically deformed. By restraining the displacement in the radial direction, the stator core  2  can return to its original shape having high circularity and high end-face parallelism. 
     The reason thereof is as below. While the cut section  20  having been opened is fully returning to its original shape by the elasticity, the protrusion  23  and the recess  24  serve as a guide for the returning motion. Owing to the protrusion  23  and the recess  24 , the cut section  20  can return to its original position. Since the cut section  20  can return to the original position, the stator core  2  can return to a shape having high circularity and high end-face parallelism without being plastically deformed. 
     Furthermore, the length N of the engagement protrusion  23  is set to 4 mm or more, which is longer than the width of the gap L of about 3 mm for opening the cut section  20 , so that the protrusion  23  does not disengage from the recess  24  during coil assembling. Accordingly, the protrusion  23  and the recess  24  can serve as a guide to prevent the stator core  2  from disassembling. 
     The stator core  2  having the protrusion  23  and the recess  24  in the radial direction X can be made by use of a single press die used for shaping a steel plate. Since the stator core  2  having the protrusion  23  and the recess  24  can be manufactured by use of the single press die, a manufacturing cost can be reduced than in the case where engagement portions are formed in a lamination direction. To manufacture the stator core having the engagement portions in the lamination direction, steel plates have to be made in at least two patterns. 
     In the case where the engagement protrusion  25  and the engagement recess  26  shown in  FIG. 7  are formed, they can provide the same effects as the engagement protrusion  23  and the engagement recess  24  having curved end faces shown in  FIG. 6 . Since the protrusion  25  and the recess  26  in  FIG. 7  can provide the same effects, their explanations are omitted. 
     Third Embodiment 
     A stator core  3  in a third embodiment is different from the stator core  1  in the first embodiment only in that the shape of a cut section  30  of the stator core  3  is different from the shape of the cut section  50  of the stator core  1 . The third embodiment is identical to the first embodiment except for the cut section and thus is explained with a focus on the cut section  30  without repeating the explanation of other parts or components. 
     The third embodiment in which other parts or components are not explained can also provide the same operations and advantageous effects as those in the first embodiment. 
     Modified Example of Shape of Cut Section in Lamination Direction 
       FIG. 8  is an external perspective view of the stator core  3  in the third embodiment.  FIG. 9  is a partial enlarged view of a part of the stator core  3  enclosed by a dashed-chain line D in  FIG. 8 . 
     The stator core  3  is formed with the cut section  30  extending in a radial direction as shown in  FIG. 8 . The cut section  30  is formed through all the thin steel plates and thus is opened or split when a pull force is applied to the stator core  3 . The cut section  30  includes one end  31  formed on the first teeth part T 1  side of the yoke part  12  and the other end  32  formed on the twelfth teeth part T 12  side of the yoke part  12 . Opening the cut section  30  therefore means that the one end  31  and the other end  32  are separated from each other. While no force is applied to the cut section  30 , the one end  31  and the other end  32  are in contact with each other as shown in  FIG. 8 . 
     As shown in  FIG. 9 , the one end  31  may be formed with an engagement protrusion  33  and the other end  32  may be formed with an engagement recess  34  engageable with the protrusion  33 . The protrusion  33  and the recess  34  are formed in a lamination direction Y. 
     The protrusion  33  has a length M longer than a width of the gap L at which the cut section  30  is opened. The recess  34  engaging with the protrusion  33  has a depth equal to the length M of the protrusion  33 . For instance, in the case where the width of the gap L is about 3 mm, the length M of the protrusion  33  and the depth of the recess  34  are respectively set to be 4 mm or more. 
     (Operations and Advantageous Effects of Shape of Cut Section in Lamination Direction) 
     Owing to the presence of the engagement protrusion  33  and the engagement recess  34  formed as shown in  FIGS. 8 and 9 , it is possible to restrain a displacement of the stator core  3  in the lamination direction Y in  FIG. 9  when the stator core  3  is elastically deformed. Since the displacement in the lamination direction Y is prevented, the stator core  3  can return to its original shape with high circularity and high end-face parallelism. The reason is as below. The protrusion  33  and the recess  34  serve as a guide to fully return the opened cut section  30  to an original position. With those protrusion  33  and recess  34 , the cut section  30  can return completely to the original position. Consequently, the stator core  3  can return to the shape having high circularity and high end-face parallelism without causing plastic deformation. 
     Since the length M of the engagement protrusion  33  is longer than the width of the gap L for opening the cut section  30  during coil assembling, the protrusion  33  does not disengage from the recess  34 . Therefore, the protrusion  33  and the recess  34  can serve as a guide to prevent the stator core  3  from disassembling. 
     Furthermore, the thickness of the stator core  3  in the lamination direction is larger than the thickness in the radial direction, so that the protrusion  33  and the recess  34  can be formed in two or more places in the lamination direction. Specifically, the third embodiment includes the protrusion  33  and the recess  34  each in one place but may include a plurality of engagement protrusions and a plurality of engagement recesses. In the case of including the engagement protrusions and the engagement recesses, the cut section  30  can fully return to the original position more reliably. 
     Fourth Embodiment 
     A stator core  4  in a fourth embodiment is different from the stator core  1  in the first embodiment only in that the stator core  4  is formed with a one-end protrusion  41  and an other-end protrusion  42  each protruding outward from the yoke part  12 . Thus, the fourth embodiment is explained with a focus on a cut section  70  without repeating explanation of other parts or components. The fourth embodiment in which explanations of other parts or components are omitted can provide the same operations and advantageous effects as those in the first embodiment. 
     (Configuration of One-End Protrusion and Other-End Protrusion) 
       FIG. 10  is a front view of the stator core  4  in the fourth embodiment.  FIG. 11  is a partial enlarged view ( 1 ) of a part enclosed by a chain line E in  FIG. 10  in the fourth embodiment.  FIG. 12  is a partial enlarged view ( 2 ) of the part enclosed by the chain line E in  FIG. 10  in the fourth embodiment. As shown in  FIG. 12 , the one-end protrusion  41  and the other-end protrusion  42  are formed each protruding outward from the outer periphery of the yoke part  12 . The one-end protrusion  41  is formed on a side of one end  71  of both end portions of a cut section  70 , while the other-end protrusion  42  is formed on a side of the other end  72  of both end portions of the cut section  70 . As shown in  FIG. 10 , the one-end protrusion  41  and the other-end protrusion  42  constitute an outward protruding portion  40 . 
     A one-end gap forming recess  43  of a semielliptic shape is formed in a contact surface of the one-end protrusion  41  that contacts with the other-end protrusion  42 . An other-end gap forming recess  44  of a semielliptic shape is formed in a contact surface of the other-end protrusion  42  that contacts with the one-end protrusion  41 . When the one-end protrusion  41  and the other-end protrusion  42  are placed in contact relation, their recesses  43  and  44  form a through hole of a hollow elliptic cylindrical shape. In the present embodiment, the recesses are provided as through holes, but may be formed in a concave or recessed shape, not a through hole shape. 
     (Operations and Advantageous Effects of One-End Protrusion and Other-End Protrusion) 
     The one-end protrusion  41  and the other-end protrusion  42  are used in the second step to open the cut section  70  from a closed position shown in  FIG. 11  to generate a gap L as shown in  FIG. 12 . The gap L in the fourth embodiment is about 3 mm. The stator core  4  in the fourth embodiment is formed with the one-end gap forming recess  43  of a semielliptic shape in the contact surface of the one-end protrusion  41  that contacts with the other-end protrusion  42  and the other-end gap forming recess  44  of a semielliptic shape in the contact surface of the other-end protrusion  42  that contacts with the one-end protrusion  41 . 
     As shown in  FIGS. 10 and 11 , an elliptic cylindrical tool  60  is inserted in the hollow elliptic cylindrical though hole defined by the recesses  43  and  44 . The tool  60  has a size smaller than the elliptic cylindrical through hole and thus can be inserted in the through hole. As shown in  FIG. 12 , the tool  60  is inserted in the through hole and then rotated 90° about a center point F. By this 90°-rotation of the tool  60 , the one-end protrusion  41  and the other-end protrusion  42  are separated from each other by a distance corresponding to a value obtained by subtracting a minor axis  60 B from a major axis  60 A of the elliptic shape. Accordingly, the simple 90°-rotation of the elliptic cylindrical tool  60  makes it easy to open the cut section  70 . 
     Furthermore, by the 90°-rotation of the elliptic cylindrical tool  60 , the one-end protrusion  41  and the other-end protrusion  42  can be separated accurately by the distance corresponding to the value obtained by subtracting the minor axis  60 B from the major axis  60 A of the elliptic shape. 
     Accordingly, by use of the tool  60 , it is possible to accurately apply a force to the stator core  4  in an elastically deformable range. The tool  60  can therefore be returned to its original position without causing plastic deformation of the stator core  4 . Thus, the stator core  4  can maintain the circularity and the end-face parallelism at the same level as before the use of the tool  60 . 
     By using the tool  60 , the one-end protrusion  41  and the other-end protrusion  42  are moved apart from each other. The cut section  70  can therefore be easily opened. 
     Furthermore, since the cut section  70  is formed with the one-end protrusion  41  and the other-end protrusion  42 , the cut section  70  can be opened while keeping the circularity and the end-face parallelism of the stator core  4 . The reason is as below. The one-end protrusion  41  and the other-end protrusion  42  are formed on the outer periphery of the yoke part  12  needing to maintain the circularity and end-face parallelism. Therefore, the cut section  70  can be opened while keeping the circularity and the end-face parallelism of the yoke part  12  more reliably in the case where the one-end protrusion  41  and the other-end protrusion  42  formed on the outer periphery of the yoke part  12  are separated than in the case where the yoke part  12  needing to maintain the circularity and end-face parallelism is directly opened. 
     When a force is directly applied to the yoke part  12 , the yoke part  12  may be deformed, resulting in that the circularity and end-face parallelism cannot be maintained. However, when a force is directly applied to the one-end protrusion  41  and the other-end protrusion  42  formed on the outer periphery of the yoke part  12 , the yoke part  12  needing to maintain the circularity and the end-face parallelism is not deformed. Thus, the circularity and the end-face parallelism can be kept. 
     Furthermore, separating the one-end protrusion  41  and the other-end protrusion  42  formed on the outer periphery of the yoke part  12  is easier than directly separating the yoke part  12  by application of a force thereto. This is because the outward protruding portion  40  formed distantly from the stator core  4  needs only a small force to open the cut section  70 . Accordingly, opening the outward protruding portion  40  can easily open the cut section  70  formed in the yoke part  12 . This can enhance the assembling efficiency to mount the twelfth coil C 12  on the last twelfth teeth part T 12  and also reduce the manufacturing cost. 
     The present invention is not limited to the above embodiments and may be embodied in other specific forms without departing from the essential characteristics thereof. 
     For instance, it may combine the engagement protrusion and the engagement recess in the radial direction in the second embodiment with the engagement protrusion and the engagement recess in the lamination direction in the third embodiment. This combination of both configurations allows the cut section to fully return to the original position more reliably when returning by the elasticity. 
     For instance, the features of the stator cores in the first to fourth embodiments may be combined. This combined configuration can provide operations and advantageous effects obtainable from respective features. 
     REFERENCE SIGNS LIST 
     
         
           1  Stator core 
         C Coil 
         C 1  to C 12  First coil to twelfth coil 
         T Teeth part 
         T 1  to T 12  First teeth part to twelfth teeth part 
           12  Yoke part 
           50  Cut section 
           51  One end of cut section 
           52  Other end of cut section