CROSS REFERENCE TO RELATED APPLICATION 
   This application is a division of application Ser. No. 10/638,298, filed Aug. 12, 2003, now U.S. Pat. No. 6,787,966, the entire contents of which is hereby incorporated by reference in this application. 


   The present application is based on and claims priority from Japanese Patent Application 2002-261846, filed Sep. 6, 2002, the contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an electric motor and a method and an apparatus for manufacturing such a motor. 
   2. Description of the Related Art 
   Usually, a motor has a stator that includes a plurality of permanent magnets circumferentially disposed on the inside surface thereof to provide magnetic poles and a rotor that is disposed in the space surrounded by the magnetic poles. For example, JP-A-10-145990 discloses a motor in which the rotor is constituted of a center core, a plurality of coil cores on which concentrated coils or solenoid coils are mounted. The center core is disposed to be coaxial with the rotating axis of the rotor, and the coil cores are disposed around the center core. Each of the concentrated coils is wound around an insulator. Thus, a high space factor, which is a ratio of the conductor&#39;s volume to the motor&#39;s total volume, can be provided. 
   In the disclosed motor, the center core and the coil cores are fixed by means of concavity-convexity connection. In other words, projections are formed in one of the center core and the coil cores, and recesses are formed in the other so that the projections can be fitted into the recesses in the radial direction when they are assembled. 
   However, it is difficult to insert a plurality of projections into the recesses at one time, and each coil core has to be fixed to the center core. 
   SUMMARY OF THE INVENTION 
   A main object of the invention is to provide an improved rotor of a motor that is easy to be assembled. 
   According to a main feature of the invention, a motor includes a permanent magnet unit disposed at a circumference for providing a plurality of magnetic poles, a rotor disposed inside the circumference to be coaxial with the permanent magnet unit. The rotor is constituted of a center core disposed to be coaxial with a center axis of the rotor, a plurality of coil cores disposed at peripheral portions of the rotor and a plurality of concentrated coils respectively mounted on the coil cores. In assembling, each coil core is fitted to the center core by sliding in an axial direction of the center core. 
   In the above featured motor, one of the center core and the plurality of coil cores has axially extending engagement grooves at peripheral portions thereof, and the other has axially extending engagement projections at peripheral portions thereof. The engagement projections project toward the engagement grooves to be fitted to the engagement grooves when the engagement projections move or slide in the axial direction. Preferably, the engagement groove has a generally trapezoidal cross-section having a smaller parallel side being open at the peripheral portion, and the engagement projection has a generally trapezoidal cross-section that is complementary to the trapezoidal cross-section of the engagement groove. 
   In the above featured motor, an axial end of the engagement groove or an axial end of the engagement projection is chamfered. 
   Another object of the invention is to provide a method of manufacturing such an improved motor. 
   Another main feature of the invention, a method of manufacturing the above featured motor includes a step of holding a plurality of coil cores disposed in the same position as the rotor so that the periphery thereof can be on a prescribed circle, and a step of moving the center core in the axial direction thereof to insert the same into the inside surfaces of the coil cores. 
   In the above featured method of manufacturing, the step of moving further includes a step of restricting the coil cores to move in radially outward directions. Preferably, the step of restricting includes a step of pressing the coil cores in radially inward directions. The step of holding may include a step of restricting the coil cores to move in an axial direction of the prescribed circle, and the step of moving may include a step of pressing an axial end of the center core in the same axial direction to insert the center core into the coil cores. 
   Another object of the invention is to provide an apparatus for manufacturing such an improved motor. 
   According to another feature of the invention, an apparatus for manufacturing the above featured motor includes means for holding the plurality of coil cores disposed in the same position as the rotor so that the periphery thereof can be on a prescribed circle, and means for moving the center core in the axial direction thereof to insert the same into the inside surfaces of the coil cores. In the above featured apparatus, the means for of moving may include means for restricting the coil cores to move in radially outward directions. In the above featured apparatus, the step of restricting may include means for pressing the coil cores in radially inward directions. The means for holding may include means for restricting the coil cores to move in an axial direction of the prescribed circle, and the means for moving may include means for pressing an axial end of the center core in the same axial direction to insert the center core into the coil cores. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and characteristics of the present invention as well as the functions of related parts of the present invention will become clear from a study of the following detailed description, the appended claims and the drawings. In the drawings: 
       FIG. 1  is a schematic view of an apparatus for manufacturing a motor according to a preferred embodiment of the invention; 
       FIG. 2  is a cross-sectional side view of a fuel pump including the motor according to the preferred embodiment; 
       FIG. 3A  is a cross-sectional view of the motor according to the preferred embodiment cut along line III—III in  FIG. 2 , and  FIG. 3B  is a fragmentary enlarged view of the motor shown in  FIG. 3A ; 
       FIG. 4A  is an enlarged perspective view of the center core shown in  FIG. 2 , and  FIG. 4B  is a cross-sectional side view of the center core cut along line IVB—IVB in  FIG. 4A ; 
       FIG. 5A  and  FIG. 5B  illustrate steps of assembling a coil core and a solenoid coil of the rotor according to the preferred embodiment; 
       FIG. 6A  and  FIG. 6B  illustrate steps of chamfering the center core; 
       FIG. 7A  is a schematic diagram illustrating an apparatus for chamfering the center core, and  FIG. 7B  is a cross-sectional view cut along line IIVB—IIVB in  FIG. 7A ; 
       FIG. 8A  and  FIG. 8B  illustrate steps of assembling the center core and the coil cores into the rotor; 
       FIG. 9A  is a cross-sectional view of an apparatus for assembling the center core and the coil cores into the rotor cut along line IXA—IXA in  FIG. 9B , and  FIG. 9B  is a cross-sectional view of the apparatus cut along line IXB—IXB; 
       FIG. 10  is a schematic diagram illustrating a cross-section of a plunger of the apparatus for assembling; and 
       FIG. 11  is a cross-sectional side view of another fuel pump including the motor according to the preferred embodiment of the invention. 
   


   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A fuel pump that includes a motor according to a preferred embodiment of the invention will be described with reference to FIGS.  1 – FIGS. 3A and 3B . 
   The fuel pump  10  is an in-tank type pump that is mounted in a fuel tank of a vehicle. The fuel pump  10  includes a housing  12 , a fuel-inlet cover  14 , a pump casing  16 , a fuel-outlet cover  18 , an impeller  20 , a permanent magnet unit  30 , a pair of bearings  34  and  35 , a shaft  38  and a rotor  40 . The covers  14  and  18  are respectively fixed to the opposite ends of the housing  12  by crimping. The pump casing  16  is disposed between the fuel-inlet cover  14  and the housing  12 . A pump channel  17  is formed between the fuel-inlet cover  14  and the pump casing  16 . The impeller  20  is rotatably held between the fuel-inlet cover  14  and the pump casing  16 . The impeller  20  has a disk plate and a plurality of blade-and-ditches formed on the periphery of the disk plate. When the impeller  20  rotates together with the rotor  10 , a pressure difference is generated between front and rear sides of blade-and-ditches. As the pressure difference is repeatedly generated by the plurality of blade-and ditches, the fuel in the pump channel  17  is pressurized. When the impeller rotates, the fuel in the fuel tank is pumped into the pump channel  17  through a fuel inlet (not shown) of the fuel-inlet cover  14 . Then the fuel passes through a motor chamber  22  and a fuel outlet to be discharged into an engine. 
   The permanent magnet unit  30  is constituted of four permanent magnets  31 . Each permanent magnet  31  has an arc of about a quarter of a circumference of the inner surface of the housing so that four permanent magnets  31  can be mounted thereon. Each of the permanent magnets  31  has a smooth arc-shaped concave inner surface  31   a,  which forms one of magnetic poles. The inner surface  31   a  of one of the permanent magnets  31  provides a magnetic pole different from the inner surface  31   a  of another permanent magnet  31  adjacent thereto in the circumferential direction. 
   The bearings  34 ,  35  are respectively fixed to the pump casing  16  and the fuel-inlet cover  18 . The shaft  38  is disposed at the center of the permanent magnet unit  30 . The shaft  38  is supported by the bearings  34 ,  35  to rotate about an axis  0 . 
   The rotor  40  is rotatably disposed inside the permanent magnet unit  40  around the shaft  38 . The rotor  40  has a center core  42  at the center thereof and six coil cores  52  at the radially outer portion  43  of the center core  42 . Incidentally, the axial direction of the rotor  40  will be referred to as the axial direction, and the circumferential direction of the rotor will be referred to as the circumferential direction, without notice. 
   The center core  42  is a generally hexagonal column having six side surfaces  43   a  and has open engagement grooves  44  on the respective side surfaces  43   a,  which face respective coil cores  52 . Each groove  44  extends from one core end  45  to the other core end  46  in the axial direction so that it opens at opposite ends of the center core  42 . Each of the engagement grooves  44  has a bottom  47  and a pair of side walls  48  and  49 . The distance between the side walls  48  and  49  becomes shorter as the pair of walls  48  and  49  comes closer to the opening or the side surface  43   a.  That is, the cross-section of the engagement grooves  44  is a trapezoid that has a smaller parallel side located at the opening of the groove  44 . As shown in  FIGS. 4A and 4B , all the corners  50  of the side surfaces  43   a  and the core ends  45 ,  46  except for the bottoms  47  of the grooves  44  are chamfered. The chamfered corners  50  have inclined surface of about 45 degrees in angle to the side surfaces  43   a  and the core ends  45 ,  46 . 
   The six coil cores  52  are respectively covered by bobbins  60  with solenoid coils and fixed to the radially outer portions  43  of the center core  42 . Each of the coil cores  52  is a T-shape member having a semi-lunar peripheral portion  53  and a spoke-like coil portion  54 . The peripheral portion  53  has a smooth convex arc-shaped surface that forms a uniform air gap between the inner surface  31   a  of the permanent magnet  31 . The coil portion  54  has a trapezoidal engagement projection  56  that projects radially inward from the radially inner surface  54   a,  which faces the center core  42 . The engagement projection  56  extends from end to end of the coil portion  54  in the axial direction and fitted into each of the engagement grooves  44 . The engagement projection  56  has a pair of side walls  57  and  58 , and the distance between the side walls becomes longer as the pair of walls  57  and  58  extends more radially inward. That is, the cross-section of the engagement projection  56  is a trapezoid that is complementary with the cross section of the engagement groove  44 . Therefore, the pair of side walls  57  and  58  of the engagement projection  56  is sandwiched and supported by the pair of side walls  48  and  49  of the engagement groove  44 . As a result, the coil cores  52  are prevented from dropping out from the center core  42 . 
   The bobbin  60  is made of a resin and covers the coil core  52  except the peripheral surface  53   a  of the peripheral portion  53  and the projection  56 . The bobbin  60  magnetically insulates the peripheral portions of adjacent coil cores  52  from each other. The bobbin  60  sandwiches the coil portion  54  at the cross-section perpendicular to the axis  0  and provides a trapezoidal coil space which narrows from the peripheral portion  53  toward the center core  42 . A solenoid coil  62  of magnet wire is accommodated in the coil space of each bobbin  60 . Electric current is supplied to the coil  62  from a terminal via brushes  64  and commutator  66 . The permanent magnet unit  30 , the shaft  38 , the rotor  40 , etc. constitute an electric motor. 
   The rotor of such a motor is manufactured in the following steps. 
   (1) After magnet wires are wound around the bobbins  60  to form the coils  62 , the coil cores  52  are respectively inserted to the bobbins  60  from the coil portions  54 , as shown in  FIG. 5 . 
   (2) As shown in  FIGS. 6A and 6B , unprocessed piece  42 ′ of the hexagonal-column-shape center core  42  is provided, and all the corners  50  of the side surfaces  43   a  and the core ends  45 ,  46  except for the bottoms  47  of the grooves  44  are chamfered by a chamfering apparatus  80  shown in  FIG. 7A . 
   The chamfering apparatus  80  is constituted of a base  81 , a moving head  81 , a punch  83 , three holders  84 , etc. The piece  42 ′ is positioned on the base  81  so that the axis thereof can extends in the vertical direction. The shaft  38  is force-fitted to the center of the piece  42 ′ to project downward from the lower end of the piece  42 ′ into a hole  85  of the base  81 . The moving head  82  is disposed above the base  81  to move in the vertical direction. The punch  83  has a hexagonal cross-section and is fixed to the moving head  82  to project toward the base  81 . When the punch  83  moves toward the base  81 , the upper end of the shaft  38  that projects from the upper end of the piece  42 ′ enters the inside of the punch  83 . The three holders  84  are disposed on the upper surface of the base at equal intervals in the circumferential direction so that each holder  84  can contact one of the side surfaces  43   a  of the piece  42 ′, as shown in  FIG. 7B . Therefore, the three holders  84  jointly restrict the piece  42 ′ to move in the radial, circumferential and axially downward directions. 
   As shown in  FIG. 7B , the peripheral portions  43  of the piece  42 ′ is held by the three holders  84 . Then, the moving head  82  is driven to move downward toward the base  81  to hit the punch  83  on the piece  42 ′. As a result, corners  50  of the outer portions  43  of the piece  42 ′ are chamfered except for the grooves  44 , as shown in  FIGS. 6B and 7B . 
   (3) As shown in  FIGS. 8A and 8B , the six coil cores  52  together with the bobbins  60  and the coils  62  (hereinafter referred to as the coil cores  52 ) are fixed to the peripheral portions  43  of the center core  42  at one time by an assembling apparatus  100 , which is shown in  FIGS. 9A  and  9 B. 
   The assembling apparatus  100  includes a base  101 , a moving head  102 , a punch  103 , a cylindrical holder member  105 , a support pin  106 , a plurality of plungers  107 , etc. 
   The holder member  105  has a cylindrical wall and a bottom  108  and is vertically fixed to the base  101  to support the six coil cores  52  from outside. The coil cores  52  are disposed in the holder member  105  so that they can have a common imaginary circle S that is coaxial with the holder member  105  and so that the peripheral surfaces  53   a  can be disposed opposite the inner surface of the holder member  105  at a small uniform clearance. The lower end surfaces of the coil cores  52  are in contact with the bottom  108  of the holder member  105 , so that the coil cores  52  are restricted to move in the center axis of the imaginary line. Incidentally, the axis O is the center axis of the imaginary circle S, as shown in  FIGS. 1 and 9 . 
   The support pin  106  is a tube member having a hexagonal cross-section. In other words, the support pin  106  has six side surfaces  106   a  each of which has an engagement groove III that has the same cross-sectional shape as the engagement groove  44  of the center core  42 . The support pin  106  is inserted in a bore  109  of the base  101  to move up and down and vertically penetrates the bottom  108  of the holder member  105 . The lower end of the support pin  106  is biased upward by a coil spring  110 , which is fixed to the base 101 . When the support pin  106  projects into the inside of the holder member  105 , the coil cores  52  are supported from inside. Then, the inner surface  54   a  of each coil core  52  contacts one of the six side surfaces  106   a  of the support pin  106 , and the engagement projections  56  of the coil cores  52  are respectively fitted to the engagement grooves  111 . Thus, the coil cores  52  are fixed in the circumferential and radial directions of the imaginary circle S. 
   Six plungers  107  are disposed on the circumference of the holder member  105  at equal intervals. Each plunger  107  presses one of the coil cores  52  from outside. As shown in  FIG. 10 , the plunger  107  is a ball type plunger that is constituted of a coil spring  112  and a ball  113 . The plunger  107  biases the peripheral surface  53   a  of the coil cores  52  from outside by the ball  113 , which is biased by the spring  112 . Therefore, the coil cores  52  are restricted to move in the radially outward directions. 
   The moving head  102  is disposed above the base  101  to move up and down. The punch  103  is a cylindrical member that has an outside diameter smaller than the circumscribed circle of the hexagonal inner surface of the center core  42  and larger than the outside diameter of the shaft  38 . The punch  103  is mounted on the moving head  102  to project along the same axis as the imaginary circle S. The punch  103  can go into the inside space of the coil cores  52  when it moves toward the base  101 . 
   In assembling the coil cores  52  to the center core  42 , six coil cores  52  are put between the holder member  105  and the support pin  106 , which projects into the inside of the holder member, so that all the coil cores  52  are held on the imaginary circle S. Accordingly, the plungers  107  bias the outer peripheries of the respective coil cores  52  from outside to hold the coil cores  52  in a prescribed position of a rotor, as shown in  FIG. 9B . 
   Then, the center core  42  is mounted on the upper surface of the coil cores  52 , as shown in  FIG. 9A . The center core  42  is set to align with the center axis O of the imaginary circle S with the core end  45  thereof being down so that the lower end of the shaft  38  can be inserted in the inside of the support pin  106 . Further, the circumferential position of center core  42  is adjusted so that the engagement grooves  44  can confront the engagement projections  56  of the coil cores. 
   As shown in  FIG. 1 , the moving head  102  is moved down toward the base  101  so that the punch  103  presses the center core  42  downward. Accordingly, as the center core  42  moves downward relative to the coil cores  52 , the engagement projections  56  are guided by the chamfered corners  50  and inserted into the engagement grooves  44 . The support pin  106  is pushed by the lower core end  45  of the, center core  42  and moves down against the biasing force of the spring  110  until the center core  42  is completely force-fitted into the inside space of the six coil cores  52  to form a rotor  40 . 
   In the above described step, it is possible to move the coil cores  52  relative to the center core  42 . The chamfered corners  50  may be replaced by rounded corners or may be formed on the engagement projections  56  instead of the engagement grooves  44 . The engagement projections  56  may be formed on the center core  42  instead of the engagement grooves  44 , which have to be formed on the coil cores  52  in that case. 
   The shaft  38  can be  14  fixed to the fuel-inlet cover  14  and the fuel-outlet cover  18  instead of the center core  42 , as shown in  FIG. 11 . In this case, the rotor  40  is disposed around the shaft  38  via a pipe  39  to rotate relative to the shaft  38 . In  FIG. 11 , the same reference numeral indicates the same or substantially the same part or component as the previously described embodiment. 
   In the foregoing description of the present invention, the invention has been disclosed with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made to the specific embodiments of the present invention without departing from the scope of the invention as set forth in the appended claims. Accordingly, the description of the present invention is to be regarded in an illustrative, rather than a restrictive, sense.
A method of manufacturing a motor that includes a permanent magnet unit disposed at a circumference for providing a plurality of magnetic poles, and a rotor disposed inside the circumference to be coaxial with the said permanent magnet unit. The rotor includes a center core coaxial with the center axis of the rotor, a plurality of coil cores disposed at peripheral portions of the rotors, and a plurality of concentrated coils respectively mounted on the coil cores. The method includes simultaneously: holding all of the coil cores by peripheral portions thereof so that outer peripheries of the held coil cores are on a prescribed circle, punching corners of one end of the center core to form chamfered corners; and moving the held coil cores from the end of the center core where the chamfered corners are formed so that the center core can be completely fitted to the held coil cores.