Abstract:
When accommodating a cage stator coil in a stator core made up of distributed cores, an end side distributed core composing end side core at an end side in axial direction is made larger than a central side distributed core composing central side core. The cage stator coil is formed by compressing a central portion of an original cage stator coil. The end side segment core composing the end core is set at a central portion in an axial direction of the cage stator coil then the end side coil is moved to an end portion in the axial direction. The central side segment core composing the central core is set at the central portion in the axial direction of the cage stator coil thereafter.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims the benefit of priority from earlier Japanese Patent Application No. 2008-110784 filed Apr. 21, 2008, the description of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     [1. Technical Field of the Invention] 
     The present invention relates to a segment-core type stator for radially-gapped inner-rotor type rotary electric machines and a method for manufacturing the stator. 
     [2. Related Art] 
     Recently, it is essential for the automotive industry to give serious consideration to the environment and power saving. Under such circumstances, EVs (electric vehicles) and HVs (hybrid vehicles) have been researched and put to practical use. 
     In vehicles, such as EVs and HVs, it is important to reduce vehicle weight that has strong positive correlation with fuel consumption. In this regard, there has been a strong demand for enhancing output per unit weight (kW/kgw) of a high-current motor (e.g., drive motor) loaded on such a heavy vehicle. It has been known that, in a stator incorporated in such a motor, the increase in a space factor of slots, i.e. the cross-sectional area of the coil conductor per unit cross-sectional area of slots, in the stator, has a strong positive correlation with the output per unit weight of the motor (kW/kgw). 
     One known method for winding a coil about a stator involves use of a coil winder. In the case of a conventional winding method, i.e. in the case of winding a fine round wire about one slat, using such a coil winder, it is necessary to ensure a space in the slot, to which the coil winder is inserted. In addition, the cross section of the coil conductor has a circular shape. For these reasons, the space factor of slots (hereinafter referred to “slot space factor”) of a stator about which a coil is wound has been as small as about 40%. 
     It should be appreciated that, throughout the specification, when a term “cross section” or “cross-sectional area” is used for a winding or a wire, the term refers to a cross section or a cross-sectional area perpendicular to the longitudinal direction. 
     If a flat wire having a large cross-sectional area can be used as a stator coil, the slot space factor may be remarkably improved. However, such a large-size flat wire cannot be wound about a stator with the use of a coil winder, unlike the case of winding a normal fine round wire. On the contrary, bending, per se, has been difficult in winding such a large-size flat wire. Therefore, when a stator is manufactured using a large-size flat conductor wire, the method may preferably include a step of bending the large-size flat conductor wire for preparation of a stator coil in advance, followed by a step of inserting portions of the conductor wire of the coil to be accommodated in slots (herein after referred to as “slot-accommodated portions”) into the respective slots of the stator core, for completion of a stator. 
     Stator coils are known to have concentrated winding or distributed winding. Comparing with the latter, the former is known to have an advantage, for example, of reducing torque ripple. A stator coil having a distributed winding of a large-size flat conductor wire will have the cage shape (hereinafter referred to as a “cage stator coil” or a “cage coil”). The outer diameter of such a cage stator coil will be approximately equal to that of the bottom surface of the slots of the stator core. For this reason, the stator coil cannot be easily inserted into a cylindrical stator core. Specifically, a cage stator coil prepared using a large-size flat square conductor wire of large cross-sectional area will have a very large rigidity. Therefore, it will be very difficult to position the slot-accommodated portions, as they are, radially inside the stator core having a relatively small inner diameter. 
     An approach known for avoiding the difficulty mentioned above in mounting a cage stator coil in the slots of a stator core, may be to use a segment core structure. Specifically, in this approach, a stator is divided into a plurality of segment cores. Then, the slot-accommodated portions of the cage stator coil are inserted into the slots of each segment core. After that, the segment cores are mechanically connected to complete a stator. Owing to the combination of such a segment-core type stator with a cage stator coil, an insulation-coated flat conductor wire having a large cross-sectional area can be used as a coil conductor to realize a radially-gaped inner-rotor type rotary electric machine having a good slot space factor. 
     Examples of such a segment-core type stator are disclosed, for example, in Japanese Patent Application Laid-Open Nos. 2002-141230 and 2003-235187. 
     In the segment-core type stator as mentioned above, the larger number of divisions of the stator core may more facilitate the assemblage of the stator core with the cage stator coil. However, the larger the number of divisions is, the smaller each of the segment cores may become. This, in turn, may complicate the process of mechanically connecting a number of segment cores, each having slots into which the slot-accommodated portions have been inserted. 
     Stator cores, meanwhile, are required to be ensured with good mechanical rigidity against magnetic vibration and external forces. However, the large number of segment cores with a large number of mechanical connecting portions may unavoidably deteriorate the mechanical rigidity of the stator core. In addition, the large number of segment cores may necessitate the increase of the magnetic resistance of the stator core, leading to deterioration in the output and the torque. 
     On the other hand, a segment-core type stator having a less number of divisions, such as two, may make it difficult for a cage stator coil with a relatively large diameter, to be inserted into the slots in the segment cores. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in light of the above problems, and has as its object to provide a segment-core type stator for inner-rotor type rotary electric machines, in which stator, the increase in the number of divisions of the stator core is suppressed, while complication and difficulties are also suppressed in assembling a cage stator coil into the stator core, and to provide a method for manufacturing the stator. 
     In order to achieve the aforementioned object, a segment-core type stator for inner-rotor type rotary electric machines having a cage stator coil made up of distributed winding coils, in the cage stator coil a plurality of slot accommodated portions to be accommodated in slots of the stator are disposed at positions a predetermined distance away from a center of an axis in a circumferential direction with a predetermined pitch and a cylindrical stator core composed of a plurality of segment cores wherein the slot accommodated portions of the cage stator coil are accommodated in the slots, wherein a central side segment core composing a cylindrical central core which forms a part of a central portion in an axial direction of the stator core has a larger occupying angle than an end side segment core composing a cylindrical end core which forms a part of an end portion in the axial direction of the stator core. 
     In particular, the stator core of the invention at least has the end cores disposed near the axial end portions and the center core disposed near the axial center portion. Further, each of the plurality of segment cores for configuring the center core is formed in a circumferentially large scale than each of the plurality of segment cores for configuring each of the end cores. In other words, the number of divisions of the cylindrical core, for the center core, is less than that of each of the end cores. 
     With this configuration, the difficulty in fitting the segment cores to the stator coil can be eliminated, while the number of divisions of the stator core can be reduced. Therefore, the rigidity of the stator can be enhanced, and the durability can also be enhanced against magnetic noise and external impacts. In addition, this configuration can contribute to simplification of the processes for manufacturing the stator. Further, since the magnetic resistance of the stator core can be reduced, the efficiency, in turn, can be enhanced, leading to the increase of the output and the torque. 
     In a preferred mode of the present invention, an edge in the circumferential direction of the central side segment core composing the central core and an edge in the circumferential direction of the end side segment core composing the end core are disposed in different positions in the circumferential direction to each other. 
     Thus, magnetic resistance can be further reduced, and thus, local increase of the magnetic resistance can be reduced. Accordingly, magnetic vibration and magnetic noise can also be reduced. 
     In a preferred mode of the present invention, an edge in the circumferential direction of the end side segment core composing a first end core and an edge in the circumferential direction of the end side segment core composing a second end core are disposed in different positions in the circumferential direction each other. 
     Thus, magnetic resistance can be further reduced, and thus, local increase of the magnetic resistance can be reduced. Accordingly, magnetic vibration and magnetic noise can also be reduced. 
     In order to achieve the object, as another aspect, a method for manufacturing a segment-core type stator for inner-rotor type rotary electric machine having a cage stator coil made up of distributed winding coils is disclosed. In this method, in the cage stator coil a plurality of slot accommodated portions to be accommodated in slots of the stator are disposed at positions a predetermined distance away from a center of an axis in a circumferential direction with a predetermined pitch and a cylindrical stator core composed of a plurality of segment cores wherein the slot accommodated portions of the cage stator coil are accommodated in the slots, wherein the method comprises steps of preparing a cage stator coil made up of distributed winding coils, in the cage stator coil, a plurality of slot accommodated portions to be accommodated in slots of the stator are disposed at positions predetermined distance away from a center of an axis in a circumferential direction with a predetermined pitch, forming a cage stator coil by compressing a central portion of an original cage stator coil, setting an end side segment core composing an end core at a central portion in an axial direction of the cage stator coil then moving the end side coil to an end portion in the axial direction, an d setting an central side segment core composing an central core at the central portion in the axial direction of the cage stator coil thereafter. 
     This may facilitate the insertion of the stator coil into the slots of the end cores each having less number of divisions. 
     In a preferred mode of the present invention, the method for manufacturing the segment-core type stator for inner-rotor type rotary electric machines further comprises a step of accommodating a compressed portion of the cage stator coil in the slot by expanding the compressed portion of the cage stator coil to outwards of a diameter direction after the central side segment core has been set at the central portion in the axial direction of the cage stator coil. 
     In a preferred mode of the present invention, the method for manufacturing the segment-core type stator for inner-rotor type rotary electric machines further comprises a step of accommodating a compressed portion of the cage stator coil in the slot by expanding the compressed portion of the cage stator coil to outwards of a diameter direction before the central side segment core is set at the central portion in the axial direction of the cage stator coil. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a side view illustrating an example of a stator structure used for an embodiment of the present invention; 
         FIG. 2  is a perspective view illustrating the shape of the stator coil of the stator illustrated in  FIG. 1 ; 
         FIG. 3  is a schematic axial cross-sectional view of the stator core of the stator illustrated in  FIG. 1 ; 
         FIG. 4  is a plan view illustrating an end portion of the stator core illustrated in  FIG. 3 ; 
         FIG. 5  is a plan view illustrating a center portion of the stator core illustrated in  FIG. 3 ; 
         FIG. 6  is a schematic perspective view illustrating a stator coil used in a method for manufacturing a stator, according to a first embodiment of the present invention; 
         FIG. 7  is a schematic axial cross-sectional view illustrating a step of radially compressing the stator coil, according to the first embodiment; 
         FIG. 8  is a schematic axial cross-sectional view illustrating an initial stage at a step of fitting a core with a cage coil, according to the first embodiment; 
         FIG. 9  is a schematic axial cross-sectional view illustrating a final stage at the step of fitting the core with the cage coil, according to the first embodiment; 
         FIG. 10  is a schematic axial cross-sectional view illustrating a step of inserting slot-accommodated portions into slots, according to the first embodiment; 
         FIG. 11  is a schematic axial cross-sectional view illustrating an initial stage at a step of fitting a core with a cage coil and at a step of inserting slot-accommodated portions into slots, according to a second embodiment of the present invention; and 
         FIG. 12  is a schematic axial cross-sectional view illustrating a state where a step of inserting slot-accommodated portions into slots has been completed, according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the drawings, hereinafter will be specifically described some preferred embodiments of the present invention. 
     The preferred embodiments deal with a structure of a stator for a rotary electric machine, which stator is configured by fitting a cage stator coil of distributed winding into segment cores (hereinafter may also be referred to as “divided cores”), and a method for manufacturing the stator. It should be appreciated that the embodiments provided below are just intended to exemplify preferred modes of the present invention and that, as a matter of course, the technical concept of the present invention may be implemented, being combined with other known techniques. 
     (First Embodiment) 
     Referring to a side view shown in  FIG. 1 , an example of a stator structure is described. 
       FIG. 1  shows a stator  3  for a radially-gaped inner-rotor type rotary electric machine. The stator  3  includes a stator coil  1 , a stator core  2  into which the stator coil  1  is fitted. 
     The stator coil  1  is configured by star-connecting three-phase windings, each of which is obtained by winding in a distributed manner a large-size flat conductor wire having a large cross-sectional area. Thus, the stator coil  1 , as a whole, has the shape of a cage. 
     The stator coil  1  includes a number of slot-accommodated portions  11  which are conductive parts to be inserted into respective slots of the stator core  2 . The stator coil  1  also includes coil end portions  12  which are conductive parts projected axially outward from both ends of the respective slot-accommodated portions, or in other words, projected from both end faces of the stator core  2 . In  FIG. 1 , the slot-accommodated portions are hidden by the stator core  2 . Each of the coil end portions  12  is formed by bending a conductor wire (a large-size flat conductor wire) into a shape of a staircase to provide steps in the substantially axial and circumferential directions. With this bending, the length of the axial projection of the coil end portions  12  is reduced. In the figure, a reference numeral  13  indicates output terminals and neutral points. 
     Referring to  FIG. 2 , the cage stator coil  1  is explained.  FIG. 2  is a perspective view illustrating the stator coil  1 . The stator coil  1  consists of the slot-accommodated portions  11  that axially pass through the slots of the stator core  2 , and coil ends each of which corresponds to a group of the coil end portions  12 . In each of the coil ends, each coil end portion  12  connects two slot-accommodated portions  11 ,  11  distanced from each other by an electrical angle of “π”. In the present embodiment, the cage stator coil  1  is configured, being imparted with wave winding. The wave-winding structure, per se, of the stator coil  1  will be briefly explained, although it is well known today. 
     Each slot accommodates a total of eight slot-accommodated portions  11 , concurrently forming eight overlaps in the radial direction and one step in the circumferential direction. A group of two radial overlaps of the slot-accommodated portions  11  configure one turn of a three-phase wave-winding partial coil. Accordingly, the stator coil  1  is configured by serially connecting a total of four turns of three-phase wave-winding partial coils. Two slots are arranged for every pole and every phase. In other words, the stator core  2  has twelve slots per electrical angle of “2π”. 
     Referring to  FIG. 3 , hereinafter is explained the stator core  2  made up of laminated electromagnetic steel plates.  FIG. 3  is a schematic axial cross-sectional view of the stator  3 . As can be seen, in the stator  3 , the stator core  2  is in the state of being fitted with the stator coil  1 . 
     The stator core consists of end portions  21 ,  22  of the core (hereinafter referred to as “end cores  21 ,  22 ”) and a center portion  23  of the core (hereinafter referred to as “center core  23 ”). The end cores  21 ,  22  are arranged axially sandwiching the center core  23 . Indicated by numeral  24  is a plane including tip end faces of the teeth of the end cores  21 ,  22  and the center core  23 , which plane corresponds to the inner peripheral surface of the stator core  2 . Indicated by numeral  25  is a plane including the bottom surfaces of slots  26  of the end cores  21 ,  22  and the center core  23 . 
     The slot-accommodated portions  11  of the stator coil  1  are accommodated in the slots  26  of the end cores  21 ,  22  and the center core  23 . Indicated by  27  are lines indicative of the end faces of insulator sheets accommodated in the respective slots  26 . Each of the ends of each slot-accommodated portion  11  continues into each of the coil end portions  12  that form each of the coil ends. In  FIG. 3 , the axial length of each of the end cores  21 ,  22  is made considerably larger than that of the center core  23 . However, as a matter of course, the proportion of the lengths can be adequately determined. 
     As schematically shown in  FIG. 4 , each of the end cores  21 ,  22  consists of two half-cylindrical segment cores  4 ,  4 . In other words, the two half-cylindrical segment cores  4 ,  4  are combined with each other to impart a cylindrical shape to each of the end cores  21 ,  22 . 
     As schematically shown in  FIG. 5 , the center core  23  is made up of a combination of a total of “N” number of arc segment cores  5  obtained by dividing a cylindrical core into the “N” number of pieces in a rotational symmetrical manner. In other words, the “N” number of arc segment cores  5  are combined with each other to impart a cylindrical shape to the center core  23 . 
     In  FIG. 5 , eight arc segment cores  5  are cylindrically combined to form the center core  23 . However, the circumferential angle occupied by each of the arc cores  5  may be appropriately changed only if the occupying angle is less than 180°. Also, the shape of a radial cross section, in other words, segment edges, of each half-cylindrical segment core  4  may also be appropriately freely designed. However, for the convenience of the manufacturing processes, all the arc segment cores  5  may preferably have the same shape. 
     The structural feature of the stator  3  lies in that, of the end cores  21 ,  22  and the center core  23  adjacently disposed in the axial direction to form the stator core  2 , the center core  23  is made up of a greater number of segment cores than in the end cores  21 ,  22 . For example, when each of the end cores  21 ,  22  is configured by 120-degree arc segment cores obtained by evenly dividing a cylinder into three, the center core  23  may be configured by less-than-90-degree arc segment cores  4  obtained by evenly dividing a cylinder into four or more. Thus, in the stator core  2 , the end portions of the core, i.e. the end cores  21 ,  22 , have a less number of segments. As a result, the rigidity of the stator core  2  is increased, the iron loss is decreased, and the number of assembling processes can be reduced. 
     Hereinafter will be described a method for manufacturing the stator  3 . 
     First, a step of forming the cage stator coil  1  mentioned above is carried out to configure the stator coil  1  shown in  FIG. 6 . The cross section of the coil conductor perpendicular to the longitudinal direction thereof, configuring the stator coil  1 , has a rectangular shape. Specifically, the stator coil  1  is made up of a so-called flat wire having a large cross-sectional area. The step, per se, of forming the cage stator coil  1  using the flat wire does not constitute the gist of the present invention. Therefore, the explanation on the specific method of manufacture is omitted. 
     Then, at the subsequent step, the axial center portion of the stator coil  1  is radially compressed to reduce the diameter of the stator coil  1 . This step of radial compression is more specifically described with reference to  FIG. 7 . 
     First, a radial compression device is explained, referring to  FIG. 7 . The radial compression device includes a pair of divided cores  6 ,  6  and a mold punch  7 . Each counterpart core  6  has a tapered cylindrical shape and has a base end portion whose diameter substantially coincides with the inner diameter of the stator coil  1 . In particular, each counterpart core  6  has a top end portion whose diameter is slightly smaller than that of the base end portion, by a predetermined length. The divided cores  6 ,  6  are individually inserted into the stator coil  1  from the lateral sides of the coil for the mating of the top end portions of the divided cores  6 ,  6 . As a result, the tapered divided cores  6 ,  6 , as a whole, provide a cylindrical shape, with the diameter at the center portion being reduced. 
     The mold punch  7  plays a roll of plastically deforming the stator coil  1  for radial compression. The mold punch  7  includes a rotary shaft  71  and a thick disk-shaped puncher  72  which is fitted and secured to the rotary shaft  71 . The axial cross section of the puncher  72  has a shape which is substantially equal to the axial cross section of the small-diameter portion of the divided cores  6 ,  6  with the addition of the radial thickness of the stator coil  1 . The maximum-diameter portion of the puncher  72  and the minimum-diameter portion of the divided cores  6 ,  6  are located at the same axial position. The rotary shaft  71  is adapted to be displaceable in the direction of radial compression, shown in  FIG. 7 , of the stator coil. The mold punch  7  is also provided with a rotating device for rotating the rotary shaft  71  and an advancing/retreating device for advancing/retreating the rotary shaft  71  in the direction of radial compression of the stator coil  1  (see  FIG. 7 ). However, since these devices are generally used ones, they are omitted from the illustration. 
     The procedure of the radial compression is explained below. First, the divided cores  6 ,  6  are individually inserted into the stator coil  1  from the respective axially lateral sides of the coil to have the top end faces of the divided cores  6 ,  6  been in closely contact with each other. Then, the mold punch  7  is rotated, and at the same time is moved toward the stator coil in the radial direction of the stator coil. Thus, the mold punch  7  radially compresses the stator coil  1  except for the lateral end portions of the stator coil  1 . 
     In this way, the stator coil  1  is pressed against the outer peripheral surfaces of the divided cores  6 ,  6  by the mold punch  7  and plastically deformed into the shape of a Japanese hand drum “tsuzumi” as shown in  FIG. 7 . In other words, the amount of radial compression of the stator coil  1  is maximized at the axial center portion of the coil, and is continuously decreased toward its lateral end portions. 
     The tsuzumi-shaped stator coil  1  is then removed from the mold punch  7  and the divided cores  6 ,  6 , followed by the fitting of the end cores  21 ,  22  and the center core  23 . Referring to  FIG. 8 , the step of the fitting is explained. In the present embodiment, in addition to the end cores  21 ,  22  and the center core  23 , the stator core  2  is provided with intermediate cores  21 ′,  22 ′ each made up of the half-cylindrical segment cores  4 ,  4 , as in the end cores  21 ,  22 . The cylindrical partial cores, i.e. the end cores  21 ,  22 , the center core  23  and the intermediate cores  21 ′,  22 ′, are axially arranged, so that in the state of completion, these cylindrical partial cores are axially arranged in the order of  21 ,  21 ′,  23 ,  22 ′ and  22  (see  FIG. 9 ). 
     First, the half-cylindrical segment cores  4 ,  4  for configuring the end core  21  are arranged at the axial center portion, i.e. the minimum-diameter portion, of the tsuzumi-shaped stator coil  1  so as to be coaxial with the stator coil  1  to thereby form the cylindrical end core  21 . After that, the end core  21  is axially moved toward the front end (to the left side in  FIG. 8 ) of the stator coil  1 . In the course of the axial movement of the end core  21 , the slot-accommodated portions  11  of the stator coil  1 , whose end portions have not been radially compressed, are accommodated in the slots  26  of the half-cylindrical segment cores  4 ,  4  (see  FIG. 9 ). Then, in the manner similar to the above fitting process of the end core  21 , the end core  22  is fitted to the stator coil  1  so as to be located at the opposite end portion of the stator coil  1 . Then, the intermediate cores  21 ′,  22 ′ are sequentially fitted in the similar manner. 
     As a result, except for the axial center portion, the end cores  21 ,  22  and the intermediate cores  21 ′,  22 ′ are fitted to the stator coil  1  and located radially outer side of the stator coil  1 . As shown in  FIG. 9 , the slot-accommodated portions  11  of the stator coil  1  are almost fully accommodated in the slots  26  of the end cores  21 ,  22 . However, as to the slots  26  of the intermediate cores  21 ′,  22 ′, due to the radial compression of the stator coil  1 , the slot-accommodated portions  11  are only partially accommodated. 
     In this way, the space for the center core  23  (also referred to as “center space”) is defined in the outer side of the axial center portion of the stator coil  1 . Then, the eight arc segment cores  5  for configuring the center core  23  are arranged axially outer side of the center space. The eight arc segment cores  5  are moved radially inward for press-fitting into the center space. Thus, the stator core  2  consisting of the end cores  21 ,  22 , the center core  23  and the intermediate cores  21 ′,  22 ′ is arranged axially outer side of the stator coil  1  (see  FIG. 9 ). 
     Then, after mutually bonding the segment cores of the stator core  2  by welding or the like, the subsequent step is carried out. At the subsequent step, the slot-accommodated portions  11  corresponding to the radially compressed portion of the stator coil  1  are biased radially outward. In the present embodiment, a push-in head  8  schematically shown in  FIG. 10  is used for biasing the slot-accommodated portions  11  concerned. 
     The push-in head  8  is adapted to axially advance/retreat radially inner side of the slot-accommodated portions  11  of the stator coil  1 , along the axial direction. The push-in head  8  has a frusto-conical shape, with the outer diameter of its top end portion being formed smaller than the minimum inner diameter of the tsuzumi-shaped stator coil  1 , and the outer diameter of its base end portion being formed slightly smaller than the inner diameter of the stator core  2 . With the axial movement of the push-in head  8 , the slot-accommodated portions  11  concerned, corresponding to the radially compressed portion, of the stator coil  1  are substantially pushed into the slots  26 . After that, the slot-accommodated portions  11  concerned are biased radially outward, whereby the entire slot-accommodated portions  11  are completely accommodated in the slots  26  to complete the stator  3 . 
     According to the present embodiment, the half-cylindrical segment cores  4  for configuring the end cores  21 ,  22  and the intermediate cores  21 ′,  22 ′ are located outer side of the stator coil  1 , i.e. at the axial center portion corresponding to the minimum diameter portion of the slot-accommodated portions  11  of the stator coil  1 . Therefore, the size of each of the half-cylindrical segment cores  4 ,  4  may be increased so as to have a shape of a large-angle arc. In other words, the number of divisions of the end cores  21 ,  22  and the intermediate cores  21 ′,  22 ′ can be decreased. Thus, in the stator  3 , iron loss may be decreased, torque may be increased, and rigidity may be enhanced. 
     Further, the radially outward biasing, as explained above, of the slot-accommodated portions  11  corresponding to the radially compressed portion of the stator coil  1  may contribute to increasing work hardening, and thus the rigidity of the slot-accommodated portions  11  concerned will be enhanced. Thus, the stator coil  1  can be prevented from projecting out of the opening portions of the slots into the inside of the stator core  2 . Thus, no new step has to be added to thereby avoid complication of the manufacture. 
     (First Modification) 
     Preferably, the circumferential positions of the mutually bonded faces of the half-cylindrical segment cores  4 ,  4  for configuring each of the end cores  21 ,  22  may be circumferentially offset from the circumferential positions of the mutually bonded faces of the arc segment cores  5  for configuring the center core  23 . Thus, magnetic resistance can be reduced. 
     (Second Modification) 
     In the above embodiment, the intermediate cores  21 ′,  22 ′ have been arranged between the end cores  21 ,  22  and the center core  23 , and each of the intermediate cores  21 ′,  22 ′ has been configured by the half-cylindrical segment cores  4 ,  4 , similar to the end cores  21 ,  22 . In this case as well, it is preferable that the circumferential positions of the mutually bonded faces of the half-cylindrical segment cores  4 ,  4  for configuring each of the end cores  21 ,  22  may be circumferentially offset from the circumferential positions of the mutually bonded faces of the half-cylindrical segment cores  4 ,  4  for configuring each of the intermediate cores  21 ′,  22 ′. Thus, magnetic resistance can be decreased. 
     (Third Modification) 
     In the above embodiment, the intermediate cores  21 ′,  22 ′ have been arranged between the end cores  21 ,  22  and the center core  23 , and each of the intermediate cores  21 ′,  22 ′ have been configured by the half-cylindrical segment cores  4 ,  4 , similar to the end cores  21 ,  22 . Alternatively, the intermediate cores  21 ′,  22 ′ may have more number of divisions than the end cores  21 ,  22 . However, in this case, the number of divisions of the intermediate cores  21 ′,  22 ′ may preferably be less than the number of divisions of the center core  23 . In this case as well, it is preferable that the circumferential positions of the mutually bonded faces of the half-cylindrical segment cores  4 ,  4  for configuring each of the end cores  21 ,  22  may be circumferentially offset from the circumferential positions of the mutually bonded faces of the half-cylindrical segment cores  4 ,  4  for configuring each of the intermediate cores  21 ′,  22 ′. Thus, magnetic resistance can be decreased. 
     (Second Embodiment) 
     Referring now to  FIGS. 10 to 12 , hereinafter is described a method for manufacturing the stator  3 , according to a second embodiment. In the second embodiment, the identical or similar components to those in the first embodiment described above are given the same reference numerals for the sake of omitting explanation. 
     The second embodiment is different from the first embodiment in that the steps involved in fitting the core have been changed. The steps of forming and radially compressing the stator coil  1  are the same as those in the first embodiment. 
     With reference to  FIG. 8 , the steps of fitting the core according to the second embodiment are explained. In the present embodiment as well, the stator core  2  has the end cores  21 ,  22  and the center core  23 , as well as the intermediate cores  21 ′,  22 ′ configured by the half-cylindrical segment cores  4 ,  4 . 
     First, the end cores  21 ,  22  and the intermediate cores  21 ′,  22 ′ are fitted to the stator coil  1  so as to be located radially outer side of the stator coil  1 , in the same manner as in the first embodiment. Specifically, the half-cylindrical segment cores  4 ,  4  are located at the axial center portion, i.e. the minimum-diameter portion, of the stator coil  1  so as to be coaxial with the stator coil  1 . Then, the segment cores  4 ,  4  are axially moved to thereby configure the end cores  21 ,  22  and the intermediate cores  21 ′,  22 ′. 
     Then, at the next step, the slot-accommodated portions  11  of the tsuzumi-shaped stator coil  1  are radially outwardly biased (see  FIG. 11 ). This biasing is carried out using the push-in head  8 , for example, used in the first embodiment. Thus, the slot-accommodated portions  11  are accommodated in the slots of the end cores  21 ,  22  and the intermediate cores  21 ′,  22 ′. 
     Subsequently, eight arc segment cores  5  for configuring the center core  23  are arranged at the space for the center core  23  (center space) defined in the outer side of the axial center portion of the stator coil  1 . The eight arc segment cores  5  are radially moved inward for press-fitting into the center space. At this occasion, the slot-accommodated portions  11  corresponding to the center core  23  are inserted into the slots of the arc segment cores  5  (see  FIG. 12 ). Thus, the slot-accommodated portions  11  of the stator coil  1  are entirely accommodated in the slots of the stator core  2 . After that, the segment cores of the stator core  2  are mutually bonded by welding or the like to thereby complete the stator  3 . According to the present embodiment, the advantages similar to those in the first embodiment can be attained.