Abstract:
A claw pole dynamo includes an annular member having a magnet extending in a circumferential direction thereof for rotation around an axis; a first stator yoke having a first annular portion and a plurality of first pole shoes extending in a direction of the axis and in close proximity to the magnet, wherein the first annular portion defines a plurality of first discontinuities (such as slots) in a circumferential direction thereof for suppressing eddy currents in the first annular portion; a second stator yoke having a second annular portion and a plurality of second pole shoes extending in the direction of the axis and in close proximity to the magnet, wherein the plurality of first pole shoes are interleaved with the plurality of second pole shoes in the circumferential directions of the first and second annular portions; a core yoke for magnetically coupling the first stator yoke and the second stator yoke and for transmitting a magnetic flux in the direction of the axis; and a coil disposed around the core yoke. The plurality of discontinuities suppress the eddy currents in the first stator yoke. If desired, the second stator yoke could have a similar plurality of discontinuities in the second annular portion thereof to suppress eddy currents in the second annular portion. A first separation member with a plurality of discontinuities may be disposed between the first stator yoke and the core yoke to further suppress eddy currents, and the core yoke may include a plurality of sections disposed at different positions in a circumferential direction of the core yoke to further reduce eddy currents.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to power generating devices for bicycles and, more particularly, to claw-pole dynamos mounted in a bicycle hub. 
     An example of a conventional claw-pole power generator is described in Japanese Unexamined Patent Application (Kokai) 8-192784. In the power generator 10 described therein, two iron-core members 15 are combined with each other such that the pole shoes 15a and 15b disposed along the corresponding outer peripheries are adjacent to each other. The central portions of these iron-core members 15 are drawn (as shown in FIG. 2 of the Application), and the inner peripheral ends of the two combined iron-core members 15 are brought into contact with each other. Magnetic flux is thereby transmitted by the inner peripheral portions of the iron-core members located inside an annular coil 14, thus causing an electric current to pass through the coil 14 under the action of the alternating flux induced therein. 
     Considerable eddy currents are induced by the axial alternating flux in the two iron-core members when the inner peripheral portions of the iron-core members are narrowed, brought into contact with each other and coupled magnetically, as described above. Much of the power generated is therefore consumed as core loss, resulting in diminished power output. 
     Linking the inner peripheral portions of both iron-core members with separate members have been suggested for obtaining a structure that possesses reduced core loss. In such a structure, however, eddy currents are induced inside the iron-core members and the separate members, further lowering the efficiency of power generation. In the particular case of a power generator in which the input rotation has a low speed, such as a power supply for a bicycle lamp or a wind-powered generator for household use, a reduction in the power generation efficiency would be disadvantageous in that it would require a bulkier power generator or a stronger rotational force for power generation. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a claw-pole dynamo wherein eddy current are suppressed during power generation so as to improve the power generation efficiency of the dynamo. In one embodiment of the present invention, a claw pole dynamo includes an annular member having a magnet extending in a circumferential direction thereof for rotation around an axis; a first stator yoke having a first annular portion and a plurality of first pole shoes extending in a direction of the axis and in close proximity to the magnet, wherein the first annular portion defines a plurality of first discontinuities (such as slots) in a circumferential direction thereof; a second stator yoke having a second annular portion and a plurality of second pole shoes extending in the direction of the axis and in close proximity to the magnet, wherein the plurality of first pole shoes are interleaved with the plurality of second pole shoes in the circumferential directions of the first and second annular portions; a core yoke for magnetically coupling the first stator yoke and the second stator yoke and for transmitting a magnetic flux in the direction of the axis; and a coil disposed around the core yoke. The plurality of discontinuities suppress the eddy currents in the first stator yoke. If desired, the second stator yoke could have a similar plurality of discontinuities in the second annular portion thereof. 
     In another embodiment of the present invention, a claw pole dynamo includes an annular member having a magnet extending in a circumferential direction thereof for rotation around an axis; a first stator yoke having a first annular portion and a plurality of first pole shoes extending in a direction of the axis and in close proximity to the magnet; a second stator yoke having a second annular portion and a plurality of second pole shoes extending in the direction of the axis and in close proximity to the magnet; wherein the plurality of first pole shoes are interleaved with the plurality of second pole shoes in the circumferential direction; a core yoke for magnetically coupling the first stator yoke and the second stator yoke and for transmitting a magnetic flux in the direction of the axis; a coil disposed around the core yoke; and a first separation member disposed between the first stator yoke and the core yoke. If desired, the first separation member may include a plurality of discontinuities in a circumferential direction thereof to suppress eddy currents. A second separation member with or without a similar plurality of discontinuities may be disposed between the second stator yoke and the core yoke to further suppress eddy currents. 
     In yet another embodiment of the present invention, a claw pole dynamo may include an annular member having a magnet extending in a circumferential direction thereof for rotation around an axis; a first stator yoke having a first annular portion and a plurality of first pole shoes extending in a direction of the axis and in close proximity to the magnet; a second stator yoke having a second annular portion and a plurality of second pole shoes extending in the direction of the axis and in close proximity to the magnet; wherein the plurality of first pole shoes are interleaved with the plurality of second pole shoes in the circumferential direction; a core yoke for magnetically coupling the first stator yoke and the second stator yoke and for transmitting a magnetic flux in the direction of the axis; wherein the core yoke includes a plurality of sections disposed at different positions in a circumferential direction of the core yoke; and a coil disposed around the core yoke. The plurality of sections in the core yoke help to suppress eddy currents in the core yoke. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial cross-sectional view of a particular embodiment of a claw-pole dynamo according to the present invention; 
     FIG. 2 is a view taken along line II—II in FIG. 1; 
     FIG. 3 is a plan view of a particular embodiment of a stator yoke according to the present invention; 
     FIG. 4 is a view taken along line IV—IV in FIG. 3; 
     FIG. 5 is a plan view of particular embodiments of a bobbin and a core yoke according to the present invention; 
     FIG. 6 is a view taken along line VI—VI in FIG. 5; 
     FIG. 7 is a plan view of a particular embodiment of a core yoke section according to the present invention; 
     FIG. 8 is a view taken along line VIII—VIII of FIG. 7; 
     FIG. 9 is a plan view of a particular embodiment of a separation disk according to the present invention; 
     FIG. 10 is an exploded cross-sectional view of the stator yoke, core yoke and separation disks used in the dynamo shown in FIG. 1; 
     FIG. 11 is a cross-sectional view showing the relationship between the magnet and claw poles of the dynamo shown in FIG. 1; 
     FIG. 12 is a plan view of alternative embodiments of a bobbin and a core yoke according to the present invention; 
     FIG. 13 is a plan view of additional alternative embodiments of a bobbin and a core yoke according to the present invention; 
     FIG. 14 is a plan view of additional alternative embodiments of a bobbin and a core yoke according to the present invention; and 
     FIG. 15 is a side view of a particular embodiment of a bicycle that incorporates the dynamo shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 1 is a partial cross-sectional view of a particular embodiment of a claw-pole dynamo  1  according to the present invention, FIG. 2 is a view taken along line II—II in FIG. 1, and FIG. 15 is a side view of a particular embodiment of a bicycle  101  that incorporates the dynamo  1  shown in FIG.  1 . Bicycle  101  comprises a frame  102  having front-wheel forks  98 ; a handle  104 ; a drive unit  105  composed of a chain, pedals, and the like; a front wheel  106  having spokes  99 ; and a rear wheel  107 . Power can be supplied to a headlamp, rear lamp, or the like by integrating the hub dynamo  1  into the bicycle  101  in the power generator. 
     The hub dynamo  1  shown in FIG. 1 is integrated into the hub of the front wheel  106  of the bicycle  101 . More specifically, the two end portions of a hub axle  20  are fixed to the right and left front-wheel forks  98 , and spokes  99  are fixed to the two flanges  11   a  and  12   a  of an external rotor assembly. The axis O-O shown in FIG. 1 is the axis of rotation of the front wheel  106  of the bicycle, and the external rotor assembly rotates together with the front wheel  106  about the axis O-O. 
     The hub dynamo  1  comprises an internal stator assembly and an external rotor assembly. The internal stator assembly (internal stator) comprises, as shown in FIG. 1, the hub axle  20 , two stator yokes  31  and  32 , a bobbin  41  with a wound coil  40 , a cylindrical core yoke  50 , and two separation disks  61  and  62  (see FIG. 8) integrated as shown in FIG.  1 . The internal stator is fixed to the front wheel forks  98  by the hub axle  20 . Both ends of the hub axle  20  are fixed to the front wheel forks  98  by clamp nuts  2  or lock nuts  3 , and the stator yokes  31  and  32 , cylindrical core yoke  50 , and separation disks  61  and  62  are fixed to this hub axle  20 . Each of the members constituting the internal stator assembly will be described in detail below. 
     The external rotor assembly (external rotor) comprises a first frame  11 , a second frame  12 , and a cap  13  integrated as shown in FIG.  1 . The integrated external rotor assembly is rotatably fixed to the hub axle  20  with the aid of bearings  21  and  22 . A plurality of spokes  99  of the front wheel  106  are fixed to a flange  11   a  formed on the outer peripheral portion of the first frame  11  and to a flange  12   a  formed on the outer peripheral portion of the second frame  12 . A permanent magnet  14  comprising four magnets spaced at equal intervals in the circumferential direction is fixed to the cap  13 , as shown in FIG.  1 . In this permanent magnet  14 , N and S poles are intermittently formed at equal intervals, and a total of 28 poles of each type face the yokes (pole shoes)  31   b  and  32   b  of the stator yokes  31  and  32  described below (see FIG.  11 ). 
     FIG. 3 is a plan view of a particular embodiment of the stator yokes  31  and  32  according to the present invention, and FIG. 4 is a view taken along line IV—IV in FIG.  3 . In this embodiment, the stator yoke  31 / 32  is made of high-machinability magnetic steel sheets (soft magnetic iron) based on pure iron. As shown in FIGS. 3 and 4, the stator yokes  31 / 32  comprises a disk portion  31   a / 32   a  and claws  31   b / 32   b . More specifically, fourteen claws  31   b / 32   b  are formed at equal intervals in the circumferential direction, and these claws extend in the direction of the axis O-O from the outer peripheral end of the corresponding disk portion  31   a / 32   a . The circumferential dimensions of the gaps between the claws  31   b / 32   b  are set somewhat larger than the circumferential dimensions of the claws  31   b / 32   b  so that the claws  31   b  and  32   b  on the two assembled stator yokes  31 / 32  are positioned at equal intervals in the circumferential direction and equal gaps are formed therebetween, as shown in FIG.  11 . As shown in FIG. 11, the permanent magnet  14  faces the outside of the claws  31   b  and  32   b  in the radial direction. 
     As shown in FIG. 3, the disk portion  31   a / 32   a  is provided with a round hole  31   c / 32   c  for insertion of the hub axle  20 , and with slits  31   d / 32   d  and  31   e / 32   e . The round hole  31   c / 32   c  is formed in the center of the disk portion  31   a / 32   a . The slits  31   d / 32   d  extend radially outward from the round hole  31   c / 32   c  to the intermediate portion between the outer peripheral end of the disk portion  31   a / 32   a  and the round hole  31   c / 32   c . The slit  31   e / 32   e  extends radially outward from the round hole  31   c / 32   c  to the outer peripheral end of the disk portion  31   a / 32   a . The seven slits  31   d / 32   d  and one slit  31   e / 32   e  are arranged at equal intervals in the circumferential direction. 
     FIG. 5 is a plan view of particular embodiments of bobbin  41  and core yoke  50  according to the present invention, and FIG. 6 is a view taken along line VI—VI in FIG.  5 . In this embodiment, the bobbin  41  is an annular resin member in which a groove  41   a  for winding and holding the coil  40  is wound on the outer peripheral portion, and a notch  41   b  having a stepped portion for engaging the cylindrical core yoke  50  is formed on the inner peripheral portion. The two ends of the coil  40  wound in the groove  41   a  are brought out through the hole  41   c  shown in FIG.  5  and through a hole (not shown) located on the opposite side in FIG.  5 . 
     The cylindrical core yoke  50  includes 12 sectional assemblies  51  which engage the notch  41   b  of the bobbin  41  to achieve mounting inside this bobbin  41 . Each of the sectional assemblies  51  is obtained by fitting together four sections shaped as rectangular thin sheets, as shown in FIGS. 7 and 8, wherein a single sectional assembly  51  comprises three sections  52  and one section  53 . The sections  52  are provided with four concavities  52   a  and one convexity  52   b , and the section  53  is provided with four round holes  53   a . The concavities  52   a  and the convexity  52   b  are joined together, as are the round holes  53   a  and the convexity  52   b , and are thus assembled into a sectional assembly  51  (see FIG.  8 ). Each of the sections  52  and  53  is manufactured from silicon-containing steel sheets with a thickness of 1 mm or less. 
     Fitting such sectional assemblies  51  into the notch  41   b  of the bobbin  41  in the manner shown in FIG. 5 will allow these 12 sectional assemblies  51  to form a cylindrical core yoke  50  provided with an internal space that is square in cross section and that accommodates the hub axle  20 . The cylindrical core yoke  50  is obtained by stacking the sections  52  and  53  parallel to the direction of the axis O-O. As shown in FIG. 6, the length of the cylindrical core yoke  50  in the direction of the axis O-O is greater than the length of the bobbin  41  in the direction of the axis O-O, thus creating a situation in which the two end faces of the cylindrical core yoke  50  extend somewhat beyond the two end faces of the bobbin  41 . 
     FIG. 9 is a plan view of a particular embodiment of a separation disk  61  or  62  according to the present invention. In this embodiment, the separation disks  61 / 62  are fabricated from silicon-containing steel sheets having the same thickness as the sections  52  and  53 . The separation disks  61 / 62  are provided with round holes  61   b / 62   b  for insertion of the hub axle  20 , and with slits  61   c / 62   c  and  61   d / 62   d . The round holes  61   b / 62   b  are formed in the center and have substantially the same diameter as the round holes  31   c / 32   c  of the stator yokes  31 / 32 . The slits  61   c / 62   c  extend radially outward from the round holes  61   b / 62   b  to the area in the vicinity of the outer peripheral ends. The slits  61   d / 62   d  extend radially outward from the round holes  61   b / 62   b  to the outer peripheral ends. These seven slits  61   c / 62   c  and one slit  61   d / 62   d  are arranged at equal intervals in the circumferential direction. 
     As shown in FIG. 10, the separation disks  61 / 62  is positioned between the stator yokes  31 / 32  and the bobbin  41 /cylindrical core yoke  50  in a sandwich configuration. In the assembled state shown in FIG. 1, the separation disks  61 / 62  separate the cylindrical core yoke  50  and the disk portions  31   a / 32   a  of the stator yokes  31 / 32  while in contact therewith, such that direct contact between the two is prevented. When the silicon-containing steel sheets are assembled in order to accommodate the magnetic flux, the two stator yokes  31  and  32  will assume a state in which the inner peripheral portions thereof are magnetically coupled with each other through the agency of the cylindrical core yoke  50  and the separation disks  61  and  62 . In addition, the slits  61   c / 62   c  and  61   d / 62   d  are designed narrower than the slits  31   d / 32   d  and  31   e / 32   e  of the stator yokes  31 / 32  to ensure that the cylindrical core yoke  50  and the stator yokes  31 / 32  are separated in a more secure fashion. A small gap exists between the bobbin  41  and the separation disks  61  and  62  because the two end faces of the cylindrical core yoke  50  extend somewhat beyond the two end faces of the bobbin  41 , as shown in FIGS. 6 and 10. 
     Generation of power by the hub dynamo  1  will now be described. 
     When the spokes  99  are rotated in relation to the front wheel forks  98  of a moving bicycle  101 , the external rotor assembly, which is fixed to the spokes  99  and is allowed to rotate on the bearings  21  and  22  in relation to the internal stator assembly fixed to the front wheel forks  98 , rotates in relation to the internal stator assembly. When this happens, the permanent magnet  14  rotates around the outside of the claws  31   b  and  32   b  of the stator yokes  31  and  32  (see FIG.  11 ). Due to this arrangement, one of the claws  31   b  and  32   b  receives a magnetic flux from an S or N pole of the permanent magnet  14  when the other receives a magnetic flux from an N or S pole, respectively. More specifically, the permanent magnet  14  rotating on the outside of the claws  31   b  and  32   b  causes a first state, in which the stator yoke  31  acts as an N pole and the stator yoke  32  acts as an S pole, and a second state, in which the stator yoke  31  acts as an S pole and the stator yoke  32  acts as an N pole, to occur repeatedly, thus inducing an alternating magnetic flux in the direction of the axis O-O in the cylindrical core yoke  50 , to which the two yokes  31  and  32  are magnetically coupled. 
     A current is produced in the coil  40 , and power is generated by the alternating magnetic flux produced on the inside of the coil  40 . More specifically, an alternating magnetic flux is induced and power is generated in the cylindrical core yoke  50  that is disposed inside the coil  40  and that links the two stator yokes  31  and  32 . However, an eddy current is induced in addition to the alternating magnetic flux during such power generation. Such an eddy current lowers the power generation efficiency, but this current can be suppressed in the present hub dynamo  1  because the slits  31   d / 32   d  and  31   e / 32   e  are provided to the disk portions  31   a / 32   a  of the stator yokes  31 / 32 . More specifically, an eddy current is induced in the disk portions  31   a / 32   a  in the circumferential direction, but the presence of the slits  31   d / 32   d  and  31   e / 32   e , which extend radially outward from the round holes  61   b / 62   b , breaks the main path of the eddy current, thus making it more difficult for the current to flow through the disk portions  31   a / 32   a  in the circumferential direction. The eddy current is therefore reduced, and the power generation efficiency is enhanced. The efficiency with which the eddy current is reduced in this embodiment is high because the slits  31   d / 32   d  and  31   e / 32   e  are centered around the inner peripheral part of the disk portion  31   a / 32   a , which is characterized by strong eddy currents. The eddy current be reduced with even higher efficiency because a plurality of slits  31   d / 32   d  are provided. 
     Experiments were conducted to compare the power output produced when the external rotor assembly and the internal stator assembly rotated in relation to each other at 110 rpm, which corresponded to a case in which the bicycle  101  was traveling at a speed of 15 km/h. It was found that the power output of the hub dynamo  1  had increased about 26% over that produced when the slits  31   d / 32   d  and  31   e / 32   e  were dispensed with, and about 6% over that produced when the slits  31   e / 32   e  (but not the slits  31   d / 32   d ) were formed. 
     In this embodiment, the stator yoke  32  is fabricated from magnetic steel sheets based on pure iron that are easier to machine than carbonaceous materials, thus making it easier to mold the curved portions of the disk portion  31   a / 32   a  and the claw  31   b / 32   b . This, in turn results in lower manufacturing costs. On the other hand, fabricating the stator yoke  32  from magnetic steel sheets based on pure iron in such a manner reduces electric resistance and increases iron loss due to an eddy current, but providing the disk portion  31   a / 32   a  with the slits  31   d / 32   d  and  31   e / 32   e  suppresses the eddy current and makes it possible to maintain the desired power generation efficiency. 
     Additionally, the eddy current induced during power generation is inhibited and the power generation efficiency is increased because a separation disk  61 / 62  is placed between the stator yoke  31 / 32  and the cylindrical core yoke  50 . More specifically, an eddy current is induced in the stator yokes  31 / 32  and the cylindrical core yoke  50  due to the formation of an alternating magnetic flux, but since separation disks  61 / 62  made of high-resistance, silicon-containing magnetic steel sheets are interposed therebetween, it is possible to reduce the eddy current induced in the area between the stator yokes  31 / 32  and the cylindrical core yoke  50 . The eddy current is therefore reduced, and higher power generation efficiency is achieved. Also, providing the separation disk  61 / 62  with the slits  61   c / 62   c  and  61   d / 62   d  makes it more difficult for an eddy current to flow through the separation disk  61 / 62 . The eddy current is thus reduced even further, raising the power generation efficiency. 
     Experiments were also conducted to compare the power outputs produced when the external rotor assembly and the internal stator assembly rotated in relation to each other at 120 rpm, which corresponded to a case in which the bicycle  101  was traveling at a speed of 15 km/h. It was found that the power output of the hub dynamo  1  was about 6% higher than that produced when the separation disks  61  and  62  were dispensed with. 
     Additionally, the eddy current is suppressed and the power generation efficiency is increased because the cylindrical core yoke  50  comprises a plurality of sections  52  and  53 . More specifically, an eddy current is induced in the cylindrical core yoke  50  by the creation of an alternating magnetic flux, but the magnitude of this eddy current is reduced because the sections  52  and  53  are placed at different positions in the circumferential direction, and the areas separating these sections  52  and  53  are introduced such that an eddy current flowing in the circumferential direction are disrupted. This makes it more difficult for the eddy current to flow through the cylindrical core yoke  50  in the circumferential direction, reduces the eddy current, and raises the power generation efficiency. In this embodiment, the sections  52  and  53  comprising the cylindrical core yoke  50  are fabricated from silicon-containing magnetic steel sheets, so the cylindrical core yoke  50  has higher electric resistance and the eddy current induced in the cylindrical core yoke  50  are further reduced. As noted above, the cylindrical core yoke  50  is made from sectional assemblies  51  obtained by superposing and integrating in advance four thin-sheet sections  52  and  53 . This facilitates assembly and makes it possible to improve installation during the final assembly of the hub dynamo  1 . Additionally, it is more difficult for the sections  52  and  53  to shift in relation to each other because the sections  52  are provided with concavities  52   a  and a convexity  52   b , and the section  53  is provided with round holes  53   a , and these are used to form the sectional assembly  51 . 
     Experiments were conducted to compare the power outputs produced when the external rotor assembly and the internal stator assembly rotated in relation to each other at 120 rpm, which corresponded to a case in which the bicycle  101  was traveling at a speed of 15 km/h. It was found that the power output of the hub dynamo  1  had increased about 33% over that produced when the cylindrical core yoke  50  was made of undivided cylindrical magnetic soft iron (magnetic steel sheets based on pure iron). 
     In the first embodiment described above, sectional assemblies  51  comprising a plurality of sections  52  and  53  were stacked parallel to the direction of the axis O-O to obtain a cylindrical core yoke  50 , but it is also possible to replace the cylindrical core yoke  50  with a cylindrical core yoke  70  such as that shown in FIG.  12 . In the cylindrical core yoke  70 , 190 thin silicon-containing steel sheets (sections)  72  are aligned in the circumferential direction, as shown in FIG. 12 (for easier understanding, a smaller number of sections is shown in FIG.  12 ). Placing them inside a bobbin  42  yields a centrally located space for accommodating a hub axle  20 . 
     The bobbin  42  with a wound coil  40  is provided with a cylindrical space enclosed within a circumferential surface  42   b  along the inner peripheral portion thereof such that engagement with the cylindrical core yoke  70  is achieved. The outer peripheral surface of the cylindrical core yoke  70  is caused to engage the inner peripheral surface  42   b  of the bobbin  42 . The coil  40  is wound inside a groove  42   a  formed in the outer peripheral portion of the bobbin  42 . 
     Since the hub dynamo of the present embodiment has radially extending thin sheets  72 , the areas separating adjacent thin sheets  72  prevent the circumferential flow of the eddy currents moving in the circumferential direction inside the cylindrical core yoke  70 . The eddy currents flowing through the cylindrical core yoke  70  are therefore inhibited, and the power generation efficiency is raised. The extent to which power generation efficiency is improved is the same as in the case of the above-described hub dynamo  1  of the first embodiment. 
     In the first embodiment described above, the sectional assemblies  51  composed of a plurality of sections  52  and  53  were stacked parallel to the direction of the axis O-O to produce a cylindrical core yoke  50 , but it is also possible to replace the cylindrical core yoke  50  with a cylindrical core yoke  80  such as that shown in FIG.  13 . In the cylindrical core yoke  80 , 16 magnetic soft iron blocks  82  are aligned in the circumferential direction, as shown in FIG.  13 . Placing them inside a bobbin  42  yields a centrally located space for accommodating a hub axle  20 . 
     The bobbin  42  with a wound coil  40  is provided with a cylindrical space enclosed within a circumferential surface  42   b  along the inner peripheral portion thereof such that engagement with the cylindrical core yoke  80  is achieved. The outer peripheral surface of the cylindrical core yoke  80  is caused to engage the inner peripheral surface  42   b  of the bobbin  42 . The coil  40  is wound inside a groove  42   a  formed in the outer peripheral portion of the bobbin  42 . 
     In the hub dynamo of this embodiment, the areas separating adjacent blocks  82  extend in a radial configuration in the manner shown in FIG. 13, inhibiting the circumferential flow of the eddy currents moving in the circumferential direction inside the cylindrical core yoke  80 . The eddy currents flowing through the cylindrical core yoke  80  are therefore inhibited, and the power generation efficiency is raised. 
     Experiments were conducted to compare the power outputs produced when the external rotor assembly and the internal stator assembly rotated in relation to each other at 120 rpm, which corresponded to a case in which the bicycle  101  was traveling at a speed of 15 km/h. It was found that the power output of this hub dynamo had increased about 29% over that produced when the cylindrical core yoke  80  was composed of undivided cylindrical magnetic soft iron (magnetic steel sheets based on pure iron). 
     In the first embodiment described above, sectional assemblies  51  comprising a plurality of sections  52  and  53  were stacked parallel to the direction of the axis O-O to obtain a cylindrical core yoke  50 , but it is also possible to replace the cylindrical core yoke  50  with a cylindrical core yoke  90  such as that shown in FIG.  14 . In the cylindrical core yoke  90 , about 100 magnetic soft iron rods  92  extending in the direction of the axis O-O are bundled in the circumferential direction, as shown in FIG.  14 . Placing them inside a bobbin  42  yields a centrally located space for accommodating a hub axle  20 . 
     The bobbin  42  with a wound coil  40  is provided with a cylindrical space enclosed within a circumferential surface  42   b  along the inner peripheral portion thereof such that engagement with the cylindrical core yoke  90  is achieved. The outer surface of the cylindrical core yoke  90  is caused to engage the inner peripheral surface  42   b  of the bobbin  42 . The coil  40  is wound inside a groove  42   a  formed in the outer peripheral portion of the bobbin  42 . 
     Because of the structure of the cylindrical yoke  90  in the hub dynamo of the present embodiment, a magnetic flux can easily propagate in the direction of the axis O-O, but the flow of eddy currents in the circumferential direction is impaired. The eddy currents flowing through the cylindrical core yoke  90  are thus inhibited, and the power generation efficiency is increased. 
     Experiments were conducted to compare the power outputs produced when the external rotor assembly and the internal stator assembly rotated in relation to each other at 120 rpm, which corresponded to a case in which the bicycle  101  was traveling at a speed of 15 km/h, and it was found that the power output of this hub dynamo had increased about 18% over that produced when the cylindrical core yoke  90  was composed of undivided cylindrical magnetic soft iron (magnetic steel sheets based on pure iron). 
     While the above is a description of various embodiments of the present invention, further modifications may be employed without departing from the spirit and scope of the present invention. For example, the size, shape, location or orientation of the various components may be changed as desired. The functions of one element may be performed by two, and vice versa. It is not necessary for all advantages to be present in a particular embodiment at the same time. Every feature which is unique from the prior art, alone or in combination with other features, also should be considered a separate description of further inventions by the applicant, including the structural and/or functional concepts embodied by such feature(s). Thus, the scope of the invention should not be limited by the specific structures disclosed or the apparent initial focus on a particular structure or feature.