Abstract:
Disclosed are a multilayered wound coil, a stator, and a manufacturing method therefor. The stator is provided with: a stator core comprising laminated steel sheets; and a coil which is wound around a teeth section formed on the stator core and has a plurality of layers formed in the circumferential direction of the stator core. The winding of the coil proceeds in either the radial direction or the circumferential direction of the stator core, whichever direction has fewer adjacent conductors, and doubles back at the end in said direction. This reduces the difference in potential between adjacent conductors, making it possible to ensure insulation between adjacent conductors even with a thinner insulating film.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a 371 national phase application of PCT/JP2010/057401 filed on 27 Apr. 2010, claiming priority to Japanese Patent Application No. 2009-153627 filed 29 Jun. 2009, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a technique for winding a coil in a stator to be used in a motor and more particularly to a technique of forming a multilayered coil from a conductor wire. 
       BACKGROUND OF THE INVENTION 
       [0003]    Of motors for vehicles, a motor to be used for driving a car is demanded for reduction in size and increase in output power. Accordingly, the use of a flat rectangular conductor effective in improvement of a space factor has been discussed. 
         [0004]    However, when the rectangular conductor is to be used for a coil, it is hard to wind the rectangular conductor in a coiled form due to its wide cross-sectional area. Although a wider cross-sectional area of the rectangular conductor can lead to an increase in current density, a problem with an eddy current arises. Therefore, in forming a coil by winding the rectangular conductor, various reviews or studies have been made. 
         [0005]    Patent Document 1 discloses a technique related to a rectangular wire structure, a winding method for rectangular wire, and a winding device. 
         [0006]    In the case of winding the rectangular conductor in a multilayered form, using a winding device, a rectangular wire may be laterally dislocated or displaced during winding. In Patent Document 1, to avoid such a problem, the rectangular conductor is partially formed with a recessed or protruding retaining portion and is wound. With this configuration, it is possible to prevent displacement of the rectangular conductor to be wound as second and subsequent layers on the rectangular conductor. 
         [0007]    Patent Document 2 discloses a technique related to a winding structure of an electric motor, a winding method and a winding apparatus. 
         [0008]    Positioning means is provided to position a rectangular conductor so as to incline a cross section of the conductor at a predetermined angle relative to a line passing the center of a teeth portion of a stator core. The rectangular conductor is wound in a multilayered configuration to form a coil. This positioning means is constituted of an insulator with a stepped surface to retain the rectangular conductor at a slant along the stepped surface. With this configuration, it is possible to prevent positional displacement of the rectangular conductor. 
         [0009]    Patent Document 3 discloses a technique related to a stator structure for a rotary electric machine and a manufacturing method of the stator structure. 
         [0010]    A pair of rectangular conductors are wound on a teeth portion of a stator core to form a two-layered coil. At that time, assuming that the number of rectangular conductors to be supplied to each teeth portion is P, the number of slots of an entire stator is T, and the number of neutral points is S, the winding wire is twisted between teeth portions at intervals of N pieces that satisfy the relation: T=3×S×P×N. 
         [0011]    With such configuration, a pair coil can be made from a rectangular conductor. This makes it possible to suppress loss of cyclic currents and others and reduce the cross-sectional area per one rectangular conductor, thus preventing the occurrence of eddy currents or the like. 
         [0012]    However, the method of forming a coil by winding a rectangular conductor as disclosed in Patent Documents 1 and 2 may involve the following problems. 
         [0013]    There is first adopted a method of winding a conductor sequentially from a base side of a teeth portion of a stator core basically along the teeth surface to the inside in a radiation direction of the core, and then returning back at a distal end side of the teeth portion to form a second layer. This is regarded as a most general method to wind a coil by use of a winding device. 
         [0014]    However, in this winding manner, a returning point of a second layer is located on the base side of the teeth portion. Thus, a winding start portion of the first layer and a winding end portion of the second layer overlap one on the other. A potential difference between a winding start portion and a winding end portion becomes highest when a coil is supplied with currents. Accordingly, in a two-layered coil, a potential difference is highest between a winding start portion of a first layer and a winding end portion of a second layer. 
         [0015]    Therefore, a rectangular conductor has to be applied with insulating coating enough to withstand this potential difference and thus it is conceivable that the rectangular conductor needs to be coated with thick insulating coating. 
         [0016]    As the thickness of the insulating coating is thicker, however, the space factor decreases, which may inhibit the increase in output power of a motor. 
         [0017]    On the other hand, in the stator of Patent Document 3 with the rectangular conductor being wound in pairs, such a problem as in Patent Documents 1 and 2 will not occur. However, a mechanism of a winding device for pair winding is apt to be complex and also the peripheral length of the pair coil on an outer circumferential side is longer than on an inner circumferential side. This causes a resistance difference, which may greatly generate heat in the coil. 
         [0018]    Consequently, it may inhibit the increase in output power of a motor. 
         [0019]    As a method to solve the problem with potential difference in Patent Documents 1 to 3, the use of a winding method as disclosed in Patent Document 4 is conceivable. 
         [0020]      FIG. 10  shows a cross section of a coil in Patent Document 4. 
         [0021]    A coil  200  is a wound coil in two layers and four rows (2-layer×4-row) as shown in  FIG. 10  by winding a wire to sequentially form an outer layer, an inner layer, another inner layer, and another outer layer. As a result of such winding, a potential difference of adjacent conductors merely occurs by an amount corresponding to four turns. Thus, a reduction in potential difference can be achieved. 
         [0022]    As the potential difference is lower, the thickness of the insulating coating to be provided around the conductor can be made thinner. This can realize compact size and high power output of the coil  200 . 
       RELATED ART DOCUMENTS 
     Patent Documents 
       [0023]    Patent Document 1: JP 2001-359250A 
         [0024]    Patent Document 2: JP 2007-244115A 
         [0025]    Patent Document 3: JP 2008-109829A 
         [0026]    Patent Document 4: JP 2005-85560A 
       SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
       [0027]    However, Patent Document 4 conceivably causes the problems mentioned below. 
         [0028]      FIG. 11  shows a first row of a wound coil and  FIG. 12  shows a second row of the wound coil. 
         [0029]    In the case where the winding method of Patent Document  4  is actually performed, assuming that the order of winding the coil  200  is a side A, a side B, a side C, and a side D, a wire starts to be wound from the side A of a second layer (Layer  2 ) (outer side) and then along the sides B, C, and D of Layer  2 , and shifts to the side A of a first layer (Layer  1 ) (inner side). The wire is successively wound along the sides B and C of the first layer (inner side) and shifts to the side D. In the side D of the first layer, a bridging portion is formed to be continuous from the first row to a second row. 
         [0030]    Then, the wire is wound first along a side A of a first layer of the second row and then a side B, a side C, and a side D of the first layer, and shifts to a second layer. This winding is problematic after the sides B and C of the second layer are formed. 
         [0031]    Since the wire has already been wound in the side D of the second row, it is necessary for winding the coil  200  in two-layered configuration to form a bridging portion in the side C of the second layer of the second row to connect to a third row. 
         [0032]    However, the wire in the sides A and C has to be inserted in slots of a stator. Thus, a bridging portion if formed in the side C is likely to deteriorate the space factor in the slots. 
         [0033]    In the method of Patent Document 4, specifically, even when the coil is formed by winding a wire in two layers, this is less likely to contribute to an increase in space factor of the stator. 
         [0034]    The present invention has been made to solve the above problems and has a purpose to provide a multilayered wound coil capable of suppressing the thickness of an insulating coated layer, a stator, and a manufacturing method therefor. 
       Means of Solving the Problems 
       [0035]    To achieve the above purpose, one aspect of the invention provides the following configurations. 
         [0036]    (1) In a stator including a stator core formed with a teeth portion and a slot, and a coil made of a conductor in a wound state including a plurality of layers (n layers) in a circumferential direction of the stator core when the coil is inserted in the slot, the coil includes a first row wound from an outer layer toward an inner layer, a second row wound from the inner layer toward the outer layer, a third row wound from the outer layer toward the inner layer, with respect to the number n of layers of an insertion portion of the coil inserted in the slot, the number of layers in a coil end portion on at least one side is n+1 or more. and the coil includes: a first bridging portion formed in an innermost layer in the coil end on the lead side to connect the first row and the second row, and a second bridging portion formed in an outermost layer in the coil end on the lead side to connect the second row and the third row. 
         [0037]    (2) In the stator of (1), preferably, at least one of the first bridging portion and the second bridging portion includes a first end section, a second end section, and a lane change section positioned between the first end section and the second end section, the first end section and the second end section are formed to conform to the conductor in an adjacent layer, and the conductor is lane-changed to an adjacent row in the lane change section. 
         [0038]    To achieve the above purpose, a multilayered wound coil in another aspect of the invention provides the following configurations. 
         [0039]    (3) In a multilayered wound coil including a conductor wound in a plurality of layers (n layers), the conductor is wound from an outer layer toward an inner layer in a first row, from the inner layer toward the outer layer in a second row, and from the outer layer toward the inner layer in a third row, and the number of layers in a coil end on at least one side is n+1 or more, the coil includes: a first bridging portion formed in an innermost layer in the coil end on the one side to connect the first row and the second row, and a second bridging portion formed in an outermost layer in the coil end on the one side to connect the second row and the third row. 
         [0040]    (4) In the coil of (3), preferably, at least one of the first bridging portion and the second bridging portion includes a first end section, a second end section, and a lane change section positioned between the first end section and the second end section, the first end section and the second end section are formed to conform to the conductor in an adjacent layer, and the conductor is lane-changed to an adjacent row in the lane change section. 
         [0041]    To achieve the above purpose, a stator manufacturing method in another aspect of the invention provides the following configurations. 
         [0000]    (5) In a method of manufacturing a stator including a coil wound in a plurality of layers (n layers) in a circumferential direction of a stator core, the coil being inserted in a slot portion formed in the stator core, the coil is formed by winding a conductor from an outer layer toward an inner layer in a first row, from the inner layer toward the outer layer in a second row, and from the outer layer toward the inner layer in a third row, and the number n of layers in a coil end on a lead side of the coil is n+1 or more, and the coil includes: a first bridging portion formed in an innermost layer in the coil end on the lead side to connect the first row and the second row, and a second bridging portion formed in an outermost layer in the coil end on the lead side to connect the second row and the third row. 
         [0042]    (6) In the stator manufacturing method of (5), preferably, a bridging portion to connect adjacent rows is formed in the coil end on the lead side by pressing the bridging portion in an axial direction of the coil with a forming jig. 
       EFFECTS OF THE INVENTION 
       [0043]    The stator in one aspect of the invention configured as above can provide the following operations and effects. 
         [0044]    The aspect (1) of the invention provides a stator including a stator core formed with a teeth portion and a slot, and a coil made of a conductor in a wound state including a plurality of layers (n layers) in a circumferential direction of the stator core when the coil is inserted in the slot, wherein the coil includes a first row wound from an outer layer toward an inner layer, a second row wound from the inner layer toward the outer layer, a third row wound from the outer layer toward the inner layer, with respect to the number n of layers of an insertion portion of the coil inserted in the slot, the number of layers in a coil end portion on a lead side is n+1 or more, and the coil includes: a first bridging portion formed in an innermost layer in the coil end on the lead side to connect the first row and the second row, and a second bridging portion formed in an outermost layer in the coil end on the lead side to connect the second row and the third row. 
         [0045]    The coil of the invention is formed in such a way that, assuming that a coil is for example wound in two layers in a circumferential direction of a stator core and six rows in a radial direction of the stator core, in the case of starting winding from a second layer-first row, the winding proceeds in a first layer-first row, and then to the first layer-second row, the second layer-second row, the second layer-third row, and the first layer-third row, . . . 
         [0046]    In a coil end on the lead side, the layers are formed in the number of layers: n+1, i.e., three layers in the aforementioned example, more than the layers of each of other sides. This is not disclosed in Patent Document 4. 
         [0047]    According to the aspect of the invention, even when a coil is formed in a two-layered configuration, this two-layered coil can be formed without contradiction by forming a three-layered part. This is because the winding proceeds from the side C of the second layer shown in  FIG. 12  to a side D of a third layer, so that the wire can avoid interfering with the wire in the side D of the second layer. 
         [0048]    As a result, the insulating coating used for the conductor has only to be thick so as to withstand a potential difference between the second layer-first row and the second layer-second row. An example is assumed that a coil is formed according to the method conducted by winding a conductor sequentially from a base side of a teeth portion of the stator core to the inside of the core in the radial direction along the teeth surface, and then returning back at the distal end of the teeth portion to form a second layer. In this example, when a voltage of 100V is applied to the coil from a winding start portion to a winding end portion, the coil must withstand a potential difference of 100V. In contrast, in the stator disclosed in (1) of the invention, a first turn and a fourth turn are located adjacently. Thus, the coil has only to withstand a potential difference about one-third of that in the aforementioned example. As the number of turns increases, the potential difference can be decreased. 
         [0049]    As above, since the thickness of the insulating coating used for the conductor can be suppressed. This can improve the space factor when the conductor is wound around the stator core. Further, a reduction in cost can also be achieved simply because the thickness of the insulating coating can be thin. 
         [0050]    Furthermore, since the bridging portion of the coil is formed in the coil end, there is no longer necessary to avoid interference between wire portions inserted in each slot. This contributes to improvement of the space factor in the slot. 
         [0051]    According to the above configuration (2) of the invention, in the stator of (1), at least one of the first bridging portion and the second bridging portion includes a first end section, a second end section, and a lane change section positioned between the first end section and the second end section, the first end section and the second end section are formed to conform to the conductor in an adjacent layer, and the conductor is lane-changed to an adjacent row in the lane change section. 
         [0052]    Since the bridging portion is provided with the first end section and the second end section and it undergoes lane change to an adjacent row in the lane change section, it is possible to suppress an increase in lamination thickness of the coil caused by deformation of the conductor due to lane change. 
         [0053]    In the multilayered coil having two or more layers, a turn involving the lane change and another turn involving no lane change are arranged adjacently depending on the winding manner. The winding in the aspect disclosed in (1) corresponds to this case. In such a case, if the conductor is deformed for lane change, even a side of the conductor adjacent to another side including the lane change section is likely to be deformed due to lane change, resulting in an increase in lamination thickness of the coil. 
         [0054]    However, the lane change section is provided between the first end section and the second end section and these first and second end sections are shaped to extend along the adjacent layer. Accordingly, it is possible to suppress the influence of deformation of the conductor of the lane change section from reaching the adjacent side. 
         [0055]    As a result, the lamination thickness of the coil is not increased. This can contribute to an increase in space factor of the stator. 
         [0056]    The multilayered wound coil in another aspect of the invention configured as above can provides the following operations and effects. 
         [0057]    According to the configuration (3) of the invention, in a multilayered wound coil including a conductor wound in a plurality of layers (n layers), the conductor is wound from an outer layer toward an inner layer in a first row, from the inner layer toward the outer layer in a second row, and from the outer layer toward the inner layer in a third row, and the number of layers in a coil end on a lead side is n+1 or more, the coil includes: a first bridging portion formed in an innermost layer in the coil end on the lead side to connect the first row and the second row, and a second bridging portion formed in an outermost layer in the coil end on the lead side to connect the second row and the third row. 
         [0058]    It is therefore possible to reduce a potential difference between adjacently located portions of the conductor when supplied with currents and thus realize a coil with a small thickness of the insulating coating used for the conductor. 
         [0059]    Further, it is possible to reduce a potential difference between adjacently located portions of the conductor when supplied with currents as in (2) and thus realize a coil with a small thickness of the insulating coating used for the conductor. 
         [0060]    According to the configuration (4) of the invention, in the coil of (3), at least one of the first bridging portion and the second bridging portion includes a first end section, a second end section, and a lane change section positioned between the first end section and the second end section, the first end section and the second end section are formed to conform to the conductor in an adjacent layer, and the conductor is lane-changed to an adjacent row in the lane change section. 
         [0061]    As with the stator disclosed in (2), therefore, the first end section and the second end section are provided and the lane change section for making lane change of the conductor to an adjacent row is formed between the first and second end sections. Accordingly, it is possible to suppress the influence of deformation of the conductor in the lane change section from reaching the adjacent part (side) provided with the lane change section and the adjacent part. 
         [0062]    The stator manufacturing method in another aspect of the invention configured as above can provide the following operations and effects. 
         [0063]    According to the configuration (5) of the invention, in a method of manufacturing a stator including a coil wound in a plurality of layers (n layers) in a circumferential direction of a stator core, the coil being inserted in a slot portion formed in the stator core, the coil is formed by winding a conductor from an outer layer toward an inner layer in a first row, from the inner layer toward the outer layer in a second row, and from the outer layer toward the inner layer in a third row, and the number n of layers in a coil end on a lead side of the coil is n+1 or more, the coil includes: a first bridging portion formed in an innermost layer in the coil end on the lead side to connect the first row and the second row, and a second bridging portion formed in an outermost layer in the coil end on the lead side to connect the second row and the third row. 
         [0064]    When the coil with a low potential difference between adjacently located portions of the conductor is formed and then is inserted in the stator core, the stator can be manufactured with a high space factor. 
         [0065]    According to the configuration (6) of the invention, in the stator manufacturing method of (5), a bridging portion to connect adjacent rows is formed in the coil end on the lead side by pressing the bridging portion in an axial direction of the coil with a forming jig. 
         [0066]    Since the manufacturing method achieved by pressing the bridging portion in the axial direction of the coil by use of the forming jig, it is possible to form a coil by sequentially winding the conductor with the forming jig installed in a coil winding device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0067]      FIG. 1  is a perspective view of a coil in a first embodiment; 
           [0068]      FIG. 2  is a cross sectional view of the coil mounted on a split type stator core in the first embodiment; 
           [0069]      FIG. 3  is a perspective view of the stator in the first embodiment; 
           [0070]      FIG. 4  is an exploded perspective view of the coil in the first embodiment; 
           [0071]      FIG. 5  is a cross sectional view of a lead side of the coil in the first embodiment; 
           [0072]      FIG. 6  is a schematic view of a device in the first embodiment; 
           [0073]      FIG. 7  is a schematic diagram of a winding step 1 in the first embodiment; 
           [0074]      FIG. 8  is a schematic diagram of a winding step 2 in the first embodiment; 
           [0075]      FIG. 9  is a cross sectional view showing a relationship between a coil and a teeth portion in the first embodiment; 
           [0076]      FIG. 10  is a cross sectional view of a coil in Patent Document 4; 
           [0077]      FIG. 11  is a schematic diagram showing the order of winding wind a first row of the coil in Patent Document 4; 
           [0078]      FIG. 12  is a schematic diagram showing the order of winding a second row of the coil in Patent Document 4; 
           [0079]      FIG. 13  is a plan view of a coil in a second embodiment; 
           [0080]      FIG. 14  is a cross sectional side view of the coil in the second embodiment; 
           [0081]      FIG. 15  is a cross sectional side view of the coil in the second embodiment; 
           [0082]      FIG. 16  is a side view of the coil in the second embodiment; 
           [0083]      FIG. 17  is a schematic diagram showing a manner of forming a lane change section in the second embodiment; 
           [0084]      FIG. 18  is a perspective view of a coil formed by a coil winding method without forming a lane change section, shown for comparison; and 
           [0085]      FIG. 19  is a schematic cross sectional view of a coil formed by a coil winding method without forming a lane change section, shown for comparison. 
       
    
    
     DETAILED DESCRIPTION 
       [0086]    A detailed description of a first preferred embodiment of the present invention will now be given. 
       First Embodiment 
       [0087]      FIG. 1  is a perspective view of a coil in a first embodiment.  FIG. 2  is a cross sectional view of the coil mounted on a split type stator core. In  FIG. 2 , the coil is shown with eight turns for convenience of explanation.  FIG. 3  is a perspective view of a stator. 
         [0088]    A coil  10  is formed by winding a flat rectangular conductor  20 . This conductor  20  is a metal wire superior in electric conduction property, such as copper, having an outer surface applied with an insulating coating film  21 . The rectangular conductor  20  has a rectangular cross section. 
         [0089]    This rectangular conductor  20  is wound in two layers and six rows (2-layer x b-row), resulting in a coil  10  shown in  FIG. 1 . 
         [0090]    Assuming that the row is defined in the radial direction of a split core unit  25  and the layer is defined in the circumferential direction of the same, the coil  10  is formed by winding the rectangular conductor  20  by twelve turns with two layers and six rows. 
         [0091]    A stator core piece  30  is a member constituting the split core unit  25  and is made of laminated electric steel sheets. Each core piece  30  is formed with a teeth portion  31 . The core pieces  30  are arranged in a cylindrical shape to form the core unit  25 . 
         [0092]    The coil  10  is mounted on the teeth portion  31  by interposing an insulator  26  therebetween. The core pieces  30  in this state are arranged in the cylindrical shape and then an outer ring  27  is fitted around the core pieces  30 , thus completing a stator  50 . In  FIG. 3 , each core piece  30  is resin molded for the purpose of taking measures against vibration or the like. 
         [0093]    Winding steps of the coil  10  will be explained below. 
         [0094]      FIG. 4  is an exploded perspective view of the coil. 
         [0095]    A winding for the coil  10  in  FIG. 4  starts to first form a second layer (LAYER  2 )-first row (ROW  1 ) in  FIG. 2  as seen by comparison with  FIG. 2 . Thus, a winding start portion  10 A is located in LAYER  2 -ROW  1 . The numerals given at the center of each cross section of the rectangular conductor  20  in  FIG. 2  represent the number of turns. The number of turns is defined by counting, as one turn, one winding of a coil  10  by a winding device  100  mentioned later. 
         [0096]    In a first turn of the coil  10 , the conductor is wound by one turn to form LAYER  2 -ROW  1  which is an outer circumferential side of the coil  10 . In a second turn, the conductor is wound to form LAYER  1 -ROW  1  which is an inner circumferential side of the coil  10 . In other words, the winding proceeds inward in the circumferential direction of the split core unit  25  from the first turn to the second turn. 
         [0097]    At a lead side  10 X of the coil  10 , a first bridging portion  10 C 1  is formed to extend from LAYER  1 -ROW  1  to LAYER  1 -ROW  2 , then the conductor shifts to a third turn. 
         [0098]    In a third turn, the conductor is wound to form LAYER  1 -ROW  2  which is the inner circumferential side of the coil  10 . At the lead side  10 X, the conductor shifts to LAYER  2 -ROW  2 . In a fourth turn, the conductor is wound to form LAYER  2 -ROW  2  which is the outer circumferential side of the coil  10 . 
         [0099]    At the lead side  10 X of the coil  10 , a second bridging portion  10 C 2  is formed to extend from LAYER  2 -ROW  2  to LAYER  2 -ROW  3  to shift to a fifth turn. In other words, the winding is turned from the second turn and proceeds outward in the circumferential direction of the core unit  25  to form the third turn and the fourth turn. 
         [0100]    In the fifth turn, the conductor is wound to form LAYER  2 -ROW  3  which is the outer circumferential side of the coil  10 . In a sixth turn, the conductor is wound to form LAYER  1 -ROW  3  which is the inner circumferential side of the coil  10 . 
         [0101]    At the lead side  10 X of the coil  10 , a third bridging portion  10 C 3  is formed to extend from LAYER  1 -ROW  3  to LAYER  1 -ROW  4  and shift to a seventh turn. In other words, the winding is turned from the fourth turn and proceeds inward in the circumferential direction of the core unit  25  to form the fifth turn and the six turn. 
         [0102]    In a seventh turn, the conductor is wound to form LAYER  1 -ROW  4  which is the inner circumferential side of the coil  10 . In an eighth turn, the conductor is wound to form LAYER  2 -ROW  4  row which is the outer circumferential side of the coil  10 . 
         [0103]      FIG. 5  is a cross sectional view of a lead side of a coil, viewed along arrows A in  FIG. 1 . 
         [0104]    By the above winding steps, the rectangular conductor  20  is wound in two layers at an opposite-lead side  10 Y of the coil  10  and on both sides thereof. At the lead side  10 X of the coil  10 , the conductor  20  is formed in three layers. 
         [0105]    This is because it is necessary to form the second bridging portion  10 C 2  and a fourth bridging portion  10 C 4  on an upper side to detour around the conductor  20  wound inside as shown in  FIG. 5 , and the first bridging portion  10 C 1 , the third bridging portion  10 C 3 , and a fifth bridging portion  1005  on a lower side to detour the conductor  20  wound outside. 
         [0106]    As a result, the odd-numbered turns of the coil  10  are formed in the second layer while the even-numbered turns are formed in the first layer or the third layer. 
         [0107]    A winding device of the coil  10  will be briefly explained below. 
         [0108]      FIG. 6  is a schematic view of the device.  FIG. 7  is a schematic plan view of a winding step 1.  FIG. 8  is a schematic plan view of a winding step 2. 
         [0109]    The winding device  100  includes an uncoiler  140 , a feed mechanism  120 , a damper  130 , and a winding mechanism  150 . 
         [0110]    The feed mechanism  120  is a device to feed the rectangular conductor  20 . This mechanism  120  is configured to grasp the rectangular conductor  20  with a feed damper  121  and then draw the conductor  20  from the uncoiler  140 . The feed damper  121  is controlled to move by a predetermined amount by a ball screw  23  connected to a servo motor  122 . The damper  130  is provided with a retaining damper  131  and a feed roller  132 . The retaining damper  131  and the feed damper  121  alternately clamp the conductor  20 . 
         [0111]    The winding mechanism  150  is configured to edgewise bend the rectangular conductor  20  to form the coil  10 . 
         [0112]    An inner circumferential jig  151  is a member for holding a surface of the rectangular conductor  20  which will be the inner circumferential side of the coil  10  on completion. A first rotating jig  152  and a first bending jig  154  are moved by rotation as shown in  FIGS. 7 and 8  to edgewise bend the conductor  20 . A second rotating jig  153  is also used to edgewise bend the conductor  20 . 
         [0113]    A first guide  155  and a second guide  156  are jigs for guiding the opposite surface of the conductor  20  to the surface contacting the first rotating jig  152  and the second rotating jig  153  when the conductor  20  is edgewise bent. The guides  155  and  156  are therefore appropriately retractable when the coil  10  is wound. 
         [0114]    An upper support  157  is a guide member to support an upper surface of the coil  10  and configured to gradually move upward as winding of the coil  10  proceeds. A fixed guide  159  is intended to support the rectangular conductor  20  moving straight. Those mechanisms are arranged on a base  158 . 
         [0115]    With those mechanisms, the rectangular conductor  20  is edgewise bent as shown in  FIGS. 7 and 8 , thereby forming the coil  10 . The detailed explanation thereof is omitted. 
         [0116]    The coil  10  in the first embodiment exhibiting the above configuration and operation can provide the advantages described below. 
         [0117]    As a first advantage, the rectangular conductor  20  with a reduced thickness can be used. 
         [0118]    The stator  50  in the first embodiment includes the stator core pieces  30  each formed with the teeth portions  31  and the slots  32 , and the coils  10  each inserted in the slots  32  while the conductor in a wound state forms a plurality of layers (two layers) in the circumferential direction of the core pieces  30 . In each coil  10 , the conductor is wound from an outer layer to an inner layer in the first row, from the inner layer to the outer layer in the second row, and from the outer layer to the inner layer in the third row. The number of layers of a portion of the coil  10  inserted in a slot  32  is two, while the number of layers in a coil end portion on at least one side is three or more. 
         [0119]    The coil  10  is configured, as shown in  FIGS. 1 and 2 , to have two layers in the circumferential direction of the core unit  25  and six rows (four rows in  FIG. 2  by omission) in the radial direction of the core unit  25 . At the lead side  10 X, three layers are formed. 
         [0120]    Accordingly, the number of the conductor sections is smaller in the circumferential direction of the core unit  25 . The winding of the coil  10  proceeds in the circumferential direction of the core unit  25 . After the winding is performed by two layers, it proceeds in an inverse direction. The winding device  100  stores in advance a program for winding the coil  10 . Accordingly, the feed mechanism  120 , the damper  130 , the uncoiler  140 , and the winding mechanism  150  of the winding device  100  are operated according to the program to wind the coil  10 . 
         [0121]    By winding conducted as above, a potential difference between the adjacent portions of the rectangular conductor  20  only corresponds to three turns; specifically, between a first turn and a fourth turn, between a third turn and a fifth turn, between a sixth turn and an eighth turn, and so on. 
         [0122]    As shown in Patent Documents 1 and 2, when winding proceeds in a direction in which the number of the conductor sections is larger, if a potential difference between a winding start portion  10 A and a winding end portion  10 B of a coil  10  with twelve turns is 100V, the start portion  10 A and the end portion  10 B of the rectangular conductor  20  are adjacently located. Accordingly, the conductor  20  has to be covered with an insulating coating film  21  resistant to a potential difference of 100V. 
         [0123]    In contrast, according to the method of the first embodiment, in the case of a coil  10  with twelve turns, a potential difference is as small as one-third of the above. Thus, the thickness of the insulating coating film  21  can be made thinner by just that much. 
         [0124]    Since the thickness of the insulating coating film  21  can be reduced as above, the film  21  occupies only a space at a small ratio in each slot of the stator  50 , thus enabling improvement of the space factor. 
         [0125]    Further, the thinner thickness of the film  21  can also contribute to cost reduction by just such a decreased film thickness. 
         [0126]    Patent Document 4 mentions the coil  200  but fails to disclose the processing of a coil end. Thus, the method of Patent Document 4 could not directly manufacture the coil  200 . This method seems to be uncompleted. In contrast, the present embodiment of the invention discloses a manufacturing method of the coil  10  in detail. Specifically, for the processing of the lead side  10 X of the coil  10 , the inner-layer-side bridging portions such as the first bridging portion  10 C 1  and the third bridging portion  10 C 3  and the outer-layer-side bridging portions such as the second bridging portion  10 C 2  are formed to provide a three-layered configuration at the lead side  10 X to enable winding of the coil  10 . 
         [0127]    Further, the coil  10  and the core unit  25  are insulated from each other by the insulator  26 . For insulation between the coil ends at the lead side  10 X and the non-lead side  10 Y formed on both end faces of the core unit  25 , a method of using an interphase sheet or other method are conceivable. It is therefore only necessary to simply determine the thickness of the insulating coating film  21  according to the potential difference between adjacently located portions of the rectangular conductor  20 . 
         [0128]    The winding start position of the coil  10  is set in an outermost position in a circumferential direction of the core unit  25 . This can achieve a reduction in the number of rows. 
         [0129]    In the coil  10  of the present embodiment, the winding start portion  10 A is on an outer circumferential side of the coil  10  and also the winding end portion  10 B is on the outer circumferential side of the coil  10 . Thus, the start portion  10 A and the end portion  10 B do not interfere with the coil  10 . 
         [0130]    If the winding start portion  10 A or the winding end portion  10 B is located on the first layer side of the coil  10 , for instance, the rectangular conductor  20  to be located at the lead side  10 X has to be wound by detouring the start portion  10 A or the end portion  10 B. In this case, the rectangular conductor  20  has to detour outward or inward of the coil  10  by a distance corresponding to the thickness of the conductor  20 . Therefore, the coil  10  is apt to involve redundant thickness. 
         [0131]    Since the coil  10  is a two-layered coil, it can reduce the thickness of the stator  50 . 
         [0132]    In the case where the rectangular conductor  20  is edgewise bent by the winding device  100 , it is hard to edgewise bend the conductor  20  with a bend or curve having a diameter equal to or smaller than the width of the conductor  20 . 
         [0133]    Accordingly, it is possible to maintain the width of a coil end more equally than in edgewise bending a rectangular conductor  20  having a double width in a single layer configuration. 
         [0134]      FIG. 9  is a cross sectional view showing a relationship between a coil and a teeth portion. 
         [0135]    A section of the coil  10  to be inserted in a slot  32  between teeth portions  31  needs to be straight. 
         [0136]    Accordingly, the thickness of a coil end is R+width X in a single-layered coil and R+width X×3 in a two-layered coil. Bending R is equal to the width X and hence the single-layered coil needs a thickness of 2X and the two-layered coil needs a thickness of 4X at each coil end. Assuming that the width X is 10 mm, the thickness of the single-layered coil at the coil end is 20 mm and that of the two-layered coil is 20 mm. Thus, the thickness of the lead side  10 X is theoretically the same in the single-layered coil and the two-layered coil. 
         [0137]    At the non-lead side  10 Y, the single-layered coil has a thickness of 2X and the two-layered coil has a thickness of 3X. The coil end of the two-layered coil can be thinner than that of the single-layered coil. 
         [0138]    Consequently, from the viewpoint of the entire stator  50 , the two-layered coil  10  can more contribute to a reduction in thickness of the stator  50 . 
         [0139]    A second embodiment of the invention will be described below. 
       Second Embodiment 
       [0140]    The second embodiment of the invention is slightly different from the first embodiment in the number of turns of a coil  10  and the shape of a bridging portion. The following explanation is focused on such differences. 
         [0141]      FIG. 13  is a plan view of a coil in the second embodiment.  FIG. 14  is a cross sectional side view of the coil taken along a line B-B in  FIG. 13 .  FIG. 15  is a cross sectional side view of the coil taken along a line C-C in  FIG. 13 . 
         [0142]    The coil  10  in the second embodiment, as with the coil  10  of the first embodiment, is formed by winding a flat rectangular conductor  20  in two layers so that a lead side  10 X which is one of the coil ends provides a three-layered configuration. In this respect, the second embodiment is identical to the first embodiment except that the coil  10  of the second embodiment has two layers and eight rows (2-layer×8-row), i.e., a total of sixteen turns. 
         [0143]    Accordingly, a winding starts from the winding start portion  10 A to form a first turn T 1  for LAYER  2 -ROW  1 , a second turn T 2  for LAYER  1 -ROW  1 , a first bridging portion  10 C 1  extending from LAYER  1 -ROW  1  to LAYER  1 -ROW  2 , and shifts to a third turn T 3  for LAYER  1 -ROW  2 . Then, the winding makes a fourth turn T 4  for LAYER  2 -ROW  2  and a second bridging portion  10 C 2  extending from LAYER  3 -ROW  2  to LAYER  3 -ROW  3 , and shifts to a fifth turn T 5  for LAYER  2 -ROW  3 . 
         [0144]    A sixth turn T 6  following the fifth turn T 5  is formed for LAYER  1 -ROW  3 . A third bridging portion  10 C 3  is formed to extend from LAYER  1 -ROW  3  to LAYER  1 -ROW  4 , thereby forming a seventh turn T 7  for LAYER  1 -ROW  4 . An eighth turn T 8  is formed for LAYER  2 -ROW  4 , a fourth bridging portion  10 C 4  is formed to extend from LAYER  3 -ROW  4  to LAYER  3 -ROW  5 , thereby forming a ninth turn T 9  for LAYER  2 -ROW  5 . 
         [0145]    A tenth turn T 10  following the ninth turn T 9  is formed for LAYER  1 -ROW  5 , a fifth bridging portion  10 C 5  is formed to extend from LAYER  1 -ROW  5  to LAYER  1 -ROW  6 . Then, an eleventh turn T 11  is formed for LAYER  1 -ROW  6 . Sequentially, a twelfth turn T 12  is formed for LAYER  2 -ROW  6 , a sixth bridging portion  1006  is formed to extend from LAYER  3 -ROW  6  to LAYER  3 -ROW  7 , and then a thirteenth turn T 13  is formed for LAYER  2 -ROW  7 . 
         [0146]    A fourteenth turn T 14  following the thirteenth turn T 13  is formed for LAYER  1 -ROW  7 , a seventh bridging portion  1007  is formed to extend from LAYER  1 -ROW  7  to LAYER  1 -ROW  8 , and then a fifteenth turn T 15  is formed for LAYER  1 -ROW  8 . A sixteenth turn T 16  is formed for LAYER  2 -ROW  8 , extending to the winding end portion  10 B. 
         [0147]      FIG. 16  is a side view of the coil, viewed along arrows D in  FIG. 13 . 
         [0148]    The second bridging portion  10 C 2 , the fourth bridging portion  10 C 4 , and the sixth bridging portion  1006  are formed on an outer side of the lead side  10 X as shown in  FIG. 16 , that is, in the third layer, while the first bridging portion  10 C 1 , the third bridging portion  10 C 3 , the fifth bridging portion  10 C 5 , and the seventh bridging portion  10 C 7  are similarly formed on an inner side of the lead side  10 X of the coil not shown, that is, in the first layer. 
         [0149]    At the non-lead side  10 Y of the coil  10 , on the other hand, only two layers are formed as mentioned above as with other two sides as shown in  FIG. 15  and no bridging portion is formed. 
         [0150]    The first bridging portion  10 C 1  to the seventh bridging portion  10 C 7  each include three regions; a first end section  10 D 1 , a second end section  10 D 2 , and a lane change section  10 D 3  joining the first end section  10 D 1  and the second end section  10 D 2 . 
         [0151]      FIG. 17  is a schematic view showing a manner of forming a lane change section. 
         [0152]    The lane change section  10 D 3  is formed by use of an upper die  181  and a lower die  182  provided in a base  158  of the winding device  100  shown in  FIG. 6 . The upper die  181  is formed with a first die surface  181  a to form the lane change section  10 D 3  while the lower die  182  is formed with a second die surface  182   a.    
         [0153]    The edgewise-bent rectangular conductor  20  is pressed by the upper die  181  and the lower die  182  sandwiching therebetween the conductor  20 , thereby forming the lane change section  10 D 3  between the first end section  10 D 1  and the second end section  10 D 2 . This lane change section  10 D 3  is formed on a short side of the coil  10  wound in a rectangular form. 
         [0154]    The upper die  181  and the lower die  182  are connected to a thrust power generator not shown which has a pressure function by moving from above and below in  FIG. 17  to hold the rectangular conductor  20  therebetween. The upper die  181  and the lower die  182  are configured to be retract to regions where the dies are not interfere with the conductor  20  in winding the conductor  20  to form a coil  10 . 
         [0155]    The first end section  10 D 1  and the second end section  10 D 2  can exhibit their functions as long as they have a width of about several millimeters. Accordingly, depending on the necessary width to form the lane change section  10 D 3 , the width of the first end section  10 D 1  and the second end section  10 D 2  is determined. 
         [0156]    The first end section  10 D 1  and the second end section  10 D 2  are formed with a shape conforming to a portion of the rectangular conductor  20  forming an adjacent layer. Specifically, as shown in  FIG. 16 , the first end section  10 D 1  of the first bridging portion  10 C 1  has a shape conforming to the connection side between the first turn T 1  in the second layer and the second turn T 2 , which are adjacent to each other in a short side SSC of the lead side  10 X. The second end section  10 D 2  of the first bridging portion  10 C 1  has a shape conforming to the connection side between the third turn T 3  in the second layer and the fourth turn T 4 , which are adjacent to each other in the short side SSC of the lead side  10 X. 
         [0157]    The coil  10  made by winding the rectangular conductor  20  in the configuration as shown in the second embodiment provides the following operations and advantages. 
         [0158]    As a first advantage, the wound coil can have a reduced thickness, resulting in an increased space factor of the stator  50 . 
         [0159]      FIG. 18  is a perspective view of a coil produced by a coil winding method that forms no lane change section.  FIG. 19  is a schematic cross sectional view of the coil produced by the coil winding method that forms no lane change section. 
         [0160]    In the stator  50  in the second embodiment, at least one of the first bridging portion  10 C 1  and the second bridging portion  10 C 2  includes the first end section  10 D 1 , the second end section  10 D 2 , and the lane change section  10 D 3  between the first end section  10 D 1  and the second end section  10 D 2 . These first and second end portions  10 D 1  and  10 D 2  are formed along the adjacent layer, that is, the second layer of the rectangular conductor  20 . The conductor  20  is lane-changed to an adjacent row (e.g., from the first row to the second row for the first bridging portion  10 C 1 ) in the lane change section  10 D 3 . 
         [0161]    With the lane change section  10 D 3  in each of the first bridging portion  10 C 1  to the seventh bridging portion  10 C 7 , the thickness of the coil  10  can be minimized. 
         [0162]    If the rectangular conductor  20  is wound to form a coil  10  without providing the lane change section  10 D 3 , this coil  10  will be widened by a bulging X 2  as shown in  FIG. 18  depending on the shape of the bridging portions. The thickness of the lead side  10 X provided with the bridge is a thickness X 3 , while the thickness of the non-lead side  10 Y is a thickness X 1 . Thus, the thickness of the lead side  10 X is thicker by a difference from that of the non-lead side  10 Y. If the coil bulges in this way, it becomes a factor of deteriorating the space factor when the coil is inserted in the core unit  25 . 
         [0163]    The conceivable reason thereof results from interference between the portions of the rectangular conductor  20  caused by the processing of the bridging portions. 
         [0164]    Each of the first bridging portion  10 C 1  to the seventh bridging portion  10 C 7  is formed in the short side SSC of the coil  10 . However, the bridging portions may come into such a state as shown in  FIG. 19  depending on the material of the rectangular conductor  20  and the length of the short side SSC. An upper interference region Z 1  and a lower interference region Z 2  come about in a long side LSC, causing a bulging portion at the lead side  10 X. 
         [0165]    To be concrete, if the first bridging portion  10 C 1  is formed in the short side SSC as shown in  FIG. 19 , a portion of the rectangular conductor  20  in the long side LSC is also twisted. This may generate the upper interference region Z 1  and the lower interference region Z 2  in the long side LSC. These upper and lower interference regions Z 1  and Z 2  are regions that may cause interference with adjacently located portions of the rectangular conductor  20  and may be formed mainly in the long side LSC close to the lead side  10 X. 
         [0166]    These upper and lower interference regions Z 1  and Z 2  have only a little influence on the adjacently located portions of the conductor  20 , but the influence becomes remarkably larger as the number of turns increases. 
         [0167]    Further, the upper and lower interference regions Z 1  and Z 2  cause problems only in a multilayered wound coil  10 . 
         [0168]    Specifically, as shown in  FIG. 19 , the side A of the first turn T 1  is formed horizontal from the winding start portion  10 A and also the side C is formed horizontal. The side A of the second turn T 2  is also formed horizontal. However, the side C of the second turn T 2  continues to the first bridging portion  10 C 1  connecting to the side A of the third turn T 3  formed in an adjacent row. Thus, the side C of the second turn T 2  and the side A of the third turn T 3  are twisted. 
         [0169]    The side C of the third turn T 3  is formed horizontal and the side A of the fourth turn T 4  A is also formed horizontal. The side C of the fourth turn T 4  and the side A of the fifth turn T 5  are twisted because they are continuous to the second bridging portion  10 C 2 . 
         [0170]    As a result, the horizontally formed sides and the twisted sides are both contained in the long side LSC. Thus, the upper interference region Z 1  and the lower interference region Z 2  cause problems, resulting in an increased thickness of the lead side  10 X at which twisting influence is present. Such a problem is less likely to occur in a single-layered coil including a wire is wound in a uniform shape in all rows. This problem is considered as being specific to a multilayered coil. 
         [0171]    However, the coil  10  formed with the lane change section  10 D 3  as in the second embodiment can solve such problems. 
         [0172]    The reason thereof is as follows. With the first end section  10 D 1  and the second end section  10 D 2  formed on both sides of the lane change section  10 D 3 , the lane change can be completed in concentric manner in the lane change section  10 D 3 . This results in no influence of the lane change on the long side LSC. 
         [0173]    In other words, the first end section  10 D 1  and the second end section  10 D 2  are formed in a shape conforming to the adjacent layers, so that the influence of the lane change section  10 D 3  on the long side LSC can be suppressed. Thus, the coil  10  can be wound with a reduced thickness. 
         [0174]    A conceivable method is achieved by twisting the long side or twisting the connection portion between the long side LSC and the short side SSC to make the long side LSC longer than the width of the core unit  25 , thereby minimizing the influence of twisting in the slot of the core unit  25 , that is, the influence of the upper interference region Z 1  and the lower interference region Z 2 . However, this configuration results in a longer coil end, which will disturb reduction in size. This configuration also needs a longer rectangular conductor to be used for a coil  10 , which will disturb reduction in cost. 
         [0175]    Specifically, the first end section  10 D 1  and the second end section  10 D 2  in the bridging portion even though they are short can also contribute to reduction in size and cost. 
         [0176]    The present invention is explained along the above embodiments but is not limited thereto. The present invention may be embodied in other specific forms without departing from the essential characteristics thereof. 
         [0177]    For instance, the material of the coil  10  and the material of the insulating coating and others may be any changed from the exemplified materials. Furthermore, the winding using the winding device  100  is just an example. Another type of winding device may be used to form a coil  10 . 
         [0178]    The number of turns of the coil  10  may be increased or decreased. In particular, the coil ends on the lead side and the non-lead side are preferably small but do not give any limit to an increase in the number of layers. The winding method shown in the above embodiments should be selected appropriately according to the width of a slot  32  and the necessary number of turns of a coil  10 . 
       DESCRIPTION OF THE REFERENCE SIGNS 
       [0000]    
       
           10  Coil 
           10 A Winding start portion 
           10 B Winding end portion 
           10 C 1  First bridging portion 
           10 C 2  Second bridging portion 
           10 C 3  Third bridging portion 
           10 C 4  Fourth bridging portion 
           10 C 5  Fifth bridging portion 
           10 X Lead side 
           10 Y Non-lead side 
           20  Flat rectangular conductor 
           21  Insulating coating film 
           25  Split core unit 
           26  Insulator 
           27  Outer ring 
           30  Stator core piece 
           31  Teeth portion 
           32  Slot 
           50  Stator 
           100  Winding device