Abstract:
An electric generator is disclosed, which comprises: at least a magnet, each having more than two poles; and at least a claw-pole set, each being composed of an inner claw-pole and an outer claw-pole; wherein, the inner claw-pole and the outer claw-pole are interlaced arranged and used for guiding magnetic flux; the inner claw-pole is connected to an iron core whose outer diameter is smaller than the magnet and thus the loop of the inner claw-pole and the outer claw-pole is conducted; the core is winded by a solenoid coil; the number of claws of the inner claw-pole is the half of the pole number of the magnet while the outer claw-pole is the same, so that, as the magnet is move relative to the claw-pole set, the magnetic flux passing through the solenoid coil will change continuously and thus an induction electromotive force is generated.

Description:
FIELD OF THE INVENTION 
   The present invention relates to an electric generator, and more particularly, to a compact, ease-to-fabricated electric generator with simplified design, which is substantially a flat multi-pole magnet of disc shape or other geometrical shape, being integrally formed with an iron core working cooperatively with a claw-pole set arranged wrapping a solenoid coil, so that its voltage output is satisfactory even when operating at low rotation speed and thus is suitable to be used in applications and manufacturing relating to electric generators. 
   BACKGROUND OF THE INVENTION 
   As the call for reducing battery usage is becoming popular, the need for miniaturized high-performance electric generators has emerged. One such device is the hub-type dynamo for bicycles, which is considered to be an artistic electric generator design producing reasonable small resistance. According to specifications relating to bicycle illumination, the dynamo, received in the limited space of a bicycle hub, is required to drive a 12 ohm light bulb to illuminate slightly when the bicycle is cruising at a low speed, i.e. about 5 km/h, and to suppress the raising of voltage outputted when cruising at a higher speed. Thus, when designing a hub-type dynamo, multi-pole structure is most popular. 
   The common hub-type dynamo is originated from the coaxial multi-pole hub design of Sturmey-Archer bicycle hub. One such design can be seen in both U.S. Pat. No. 5,769,750, entitled “Epicyclic change gear system”, and U.S. Pat. No. 5,813,937, entitled “Epicyclic change speed gear system”, respectively claimed priority to GB Pat. No. 9409844 and 9304189. Such coaxial multi-pole hub is characterized in that: a plurality of poles are provided and arranged in an alternating manner while enabling the plural poles to be wrapped by a tube-like magnet, and is vastly implemented by various electric generators. One such application is seen in TW Pat. No. 94109740, entitled “Hub-type dynamo and bicycle”, also claimed priority to JP Pat. No. 2004-190663, in which a hub-type dynamo  10  is disclosed, as seen in  FIG. 1  and  FIG. 2 . The hub-type dynamo  10  is comprised of: a permanent magnet  14 , arranged inside a shell  12  of the dynamo  10 ; and a spindle  11 ; wherein the spindle further comprise: a plurality of poles  74 ,  75 , arranged in a manner enabling the polar of each pole to be faced toward that of the permanent magnet  14 ; two stator pins  16 ,  17 , arranged on the circumference thereof; and a coil  22 , arranged between the two stator pins  16 ,  17 . By the use of two positioning components  30 ,  31 , arranged on the spindle  11 , to hold and fix the stator pins  16 ,  17  in respective, the two stator pins  16 ,  17  are fixed to the spindle  11 . Moreover, the fixing of the two stator pins  16 ,  17  on the spindle  11  is characterized in that: there is a recess hole  15  formed in each stator pin  16 ,  17  while enabling each recess hole  15  to channel with a hole  13  bored through the spindle  11 ; and there is an insulator arranged between each stator pin  16 ,  17  and its corresponding positioning component  30 ,  31  for preventing the crossing of electricity from an electricity-conducting member, placed next to each positioning component  30 ,  31 , to the stator pins  16 ,  17 . The aforesaid coaxial multi-pole electric generator is further characterized in that: by the forming of such recess hole  15 , the generation of eddy current can be prevented and thus the efficiency of the electric generator is improved. However, although the generators with coaxial multi-pole design are popular and vastly adopted, it has shortcomings listed as following: 
   (1) The magnet is shaped like a tube and is warping around the stator pins that cause the resulting electric generator to be bulky and costly. 
   (2) The plural poles will guide and cause the corresponding magnetic lines to defect more than twice that cause the magnetic flux passing through the coil to drop. 
   (3) As each stator pin shall have a specific 3-D shape and each magnetic line is deflected more than twice, and moreover, the magnetic fields between the plural poles are easily to interfere with each other, the overall electric generation efficiency is low. 
   Please refer to  FIG. 3 , which is a hub-type dynamo disclosed in TW Pat. No. 92137088. The hub-type dynamo of  FIG. 3  is comprised of a hub  2 , a coil seat  3 , a coil structure  4 , an iron core  5 , a casting set  6 , two magnetic blocks  7  and a bearing axle  8 . Moreover, a conventional hub-type dynamo  1  is also disclosed in TW Pat. No. 92137088, as shown in  FIG. 4 , which is comprised of a hub  11 , a coil seat  12 , a coil structure  13 , an iron core  14 , a casting set  15 , a magnetic ring  16  and a bearing axle  17 . As disclosed in the Taiwan patent, the conventional hub-type dynamo  1  has two shortcomings. One of which is that the size of the conventional hub-type dynamo  1  can not be reduced effectively since the iron core  14 , the coil structure  12 , the coil seat  13 , the casting set  15  and the magnetic ring  16  are being received in the hub  11  from inside out and in a layer-by-layer manner. Another is that, since the iron core  14  is substantially a silicon steel lamination that is formed by stacking a plurality of silicon steel sheets  141  in a one-by-one manner, the formation and installation of the iron core  14  is complicated that it is time-consuming and uneconomical. With reference to the shortcomings of the conventional hub-type dynamo  1 , the hub-type dynamo  2  adopted the two magnetic blocks  7 , instead of using a conventional tube-like magnet, while enabling the two to be placed along the axial direction X respectively at the two sides of the casting set  6 , so that the diameter of the hub  2  and volume thereof can be reduced. It is noted that the reduced diameter should be twice the thickness of the magnet. In addition, comparing the hub-type dynamo of  FIG. 3  with that of  FIG. 4 , not only the magnetic ring  16  is replaced and substituted by the two magnetic blocks  7 , but also the appearances of the two casting sets  6 ,  17  are totally different. Although each component of the casting set  6  of  FIG. 3  is constructed with a plurality of radially extended claws, each of the plural claws is not bended. That is, as the two components of the casting set  6  is placed respectively at the two sides of the coil structure  4  along the axial direction, the claws of one component will not interlace with those of another component, which has nothing in common with the conventional casting set  15 . For those skilled in the art, the design of the aforesaid casting set  6  is serious defected and is not realistic. It is noted that as the way the claws being arranged, only half the surface area of each magnetic block  7  can be utilized, moreover, there will be circuits happening between the unused magnetic block  7  and the casting set  6 , and thus the efficiency of electricity generation is severely reduced. As for the formation and installation of the conventional iron core  14 , it is solved in the Taiwan patent by previously using a pin  52  to hold and position the plural steel sheets  51  into an iron core  5  so that the installation of the iron core  5  can be facilitated. However, for those skilled in the art, the aforesaid solution is also not realistic. As seen in  FIG. 4 , the direction of the stacking of the plural silicon steel sheets  141  to form the conventional iron core  14  is perpendicular to the magnetic lines, i.e. the X direction, by which the iron core  14  can have good permeance and such stacking is common in devices such as transformers and motors, etc. However, the direction of the stacking of the plural silicon steel sheets  51  to form the iron core  5  is parallel to the X direction, by which the hysteresis loss and eddy current loss are increased. Therefore, it is not a good idea to cause a serious efficiency drop just for reducing volume and simplifying assembly, as the hub-type dynamo disclosed in TW Pat. No. 92137088. 
   Please refer to  FIG. 5 , which shows a flat rotary electric generator disclosed in U.S. Pub. No. 20040135452. In  FIG. 5 , as a toroidal coil structure  1  is sandwiched between two matching disc-shaped magnetic pole structures  2  and as the dimension of the winding of the toroidal coil structure  1  is restricted and limited, when multi-pole design is adopted while enabling each pole to be a section of one disc-shaped magnetic pole structures  2  bounded by two radii, the volume enclosed within the two matching sections respectively of the two matching disc-shaped magnetic pole structures  2  is also restricted and must be considered. Therefore, the overall diameter of the flat rotary electric generator can not be reduced effectively. In addition, the overall size is required to be increase when it is intended to have high efficiency. Thus, the aforesaid flat rotary electric generator can not be miniaturized while increasing power density. 
   From the above description, it is noted that as the hub-type dynamo can be easily integrated with the roller brake that is suitable to be applied in the mass production of bicycle, the improvement of the efficiency of the hub-type dynamo while reducing the cost thereof can be a great boost for bicycle industry, as well as other applications requiring portable power generator. Moreover, as hub-type dynamo is common in the magnetic resistance system of currently available fitness bicycle, it is preferred to have a highly efficient hub-type dynamo in the fitness bicycle since not only the power generating efficiency is improved, but also the cost can be reduced. In addition, a miniature power generator, being the improvement over the hub-type dynamo, is in great need, since it can be received in pocket, shoe sole, glasses, watch, etc., to be used as backup or emergency power for those portable electronic devices, such as RF radio, or cellular phone, and so on. Therefore, it is required to have a cheap, small-sized electric generator capable of generating sufficient power. 
   SUMMARY OF THE INVENTION 
   In view of the disadvantages of prior art, the primary object of the present invention is to provide a compact, ease-to-fabricated electric generator with simplified design, which is substantially a flat multi-pole magnet of disc shape or other geometrical shape, being integrally formed with an iron core working cooperatively with a claw-pole set arranged wrapping a solenoid coil, so that its voltage output is satisfactory even when operating at low rotation speed. 
   To achieve the above object, the present invention provides an electric generator, which comprises: 
   at least a magnet, each having more than two poles; 
   at least a claw-pole set, each being composed of an inner claw-pole and an outer claw-pole; 
   an iron core, connected to each inner claw-pole for enabling loops of the inner claw-poles and the outer claw-poles to be conducted; and 
   a solenoid coil, winding on the outside of the iron core; 
   wherein, the inner claw-pole and the outer claw-pole are interlaced arranged and used for guiding magnetic flux. 
   Preferably, each magnet is a flat disc with a plurality of poles, each being arranged at a radial section while surrounding the center of the disc. 
   Preferably, the outer diameter of the iron core is smaller than that of the magnet and each inner claw-pole is extending outwardly and radially from the iron core. 
   Preferably, each magnet is independently arranged, and an interfacing part is substantially a ring structure having an inner side connected to the iron core and an outer side connected to the outer claw-pole, while the inner side and the outer side are interconnected with each other, thereby, a magnetic circuit is formed by the magnet, the iron core, the inner claw-pole and the outer claw-pole. 
   Preferably, a space is formed between the inner side and the outer side of the interfacing ring and used for receiving the solenoid coil. 
   Preferably, each magnet is a multi-pole magnet of biased pole magnetization. 
   Preferably, each magnet is composed of two magnetic pieces, being spaced apart by an interval while enabling each to correspond to one claw-pole set. 
   Preferably, one of the two magnetic pieces is connected to an axial end of a hollow tube-like first connecting part while another magnetic piece is connected to another axial end, so that the two magnetic pieces can be driven to rotate synchronously by the first connecting part. 
   Preferably, an axial end of a second connecting part is connected to the outer claw-pole of one claw-pole set of the at least a claw-pole set while another axial end of the second connecting part is connected to the outer claw-pole of another claw-pole set of the at least a claw-pole set, so that the two outer claw-poles can be driven to rotate synchronously by the second connecting part. Moreover, the outer diameter of the second connecting part is smaller than the inner diameter of the first connecting part. 
   Preferably, a space is formed between the second connecting part and the iron core and used for receiving the solenoid coil. 
   Preferably, each claw-pole set is made of a material selected from the group consisting of iron, silicon iron, silicon steel, and the combination thereof. 
   Preferably, the iron core is made of a material selected from the group consisting of iron, silicon iron, silicon steel, and the combination thereof. 
   Preferably, the inner claw-pole and the outer claw-pole are integrally formed/stacking formed with an inner/outer tube by a metallic process selected from the group consisting of a stamping process, a casting process. 
   Preferably, any one of the outer claw-pole and the inner claw-pole is a stacking of a plurality of silicon steel sheets. 
   Preferably, the width, length and thickness of different inner/outer claw-poles are different; and the width, length and thickness of the inner claw-pole and the outer claw-pole of the same claw-pole set can be different. 
   Preferably, the outer claw-pole is a cone-shape part tapering from the edge of the outer tube toward the axial center of the same; and the inner claw-pole is a fan-shaped part radially expanding from the edge of the inner tube. 
   Preferably, the magnet is a multi-pole magnet made of a permeance material selected from the group consisting of NdFeB, SmCo, Hard Ferrite, AlNiCo, and the like. 
   Preferably, the number of claws of the inner claw-pole is the half of the pole number of the magnet while the outer claw-pole is the same. 
   Preferably, the magnet can be a single-sided magnet or a dual-sided magnet. 
   Preferably, the geometrical shape of each pole of the magnet is conforming to that of the inner/outer claw-pole. 
   Preferably, when the inner/outer claw-pole is rotated radially by a helix angle, each pole of the magnet is twisted for magnetizing the pole with respect to the radius and angle of each pole. 
   Preferably, the electric generator further comprises at least a back panel set. Each back panel set further comprises: 
   an iron back, arranged at a side of the magnet opposite to that proximate to the claw-pole set, for enabling the closing of magnetic lines; 
   a fixation cap, axially arranged at the center of the iron back and the magnet while enabling an end of the fixation cap to abut against the iron core; 
   a bearing, ensheathing the fixation cap; and 
   a bearing cap, wrapping the bearing; 
   wherein, the bearing cap is screw-fixed to the iron back by screws, and thus the magnet, the iron back, the bearing, the bearing cap, and the fixation cap are assembled. 
   Preferably, the fixation cap is axially extending by a specific length for enabling the same to abut against the iron core while maintaining the magnet to be spaced from the inner/outer claw-pole by a specific distance. 
   Preferably, the iron back is made of a permeance material selected from the group consisting of iron, ferro-cobalt alloy, Ni—Fe alloy, silicon iron and the combination thereof. 
   Preferably, the at least one claw-pole set are serially connected while the at least one back panel are axially arranged at the two outer sides of the magnet. 
   Preferably, the iron back and the bearing cap are integrally formed. 
   Preferably, a side of the claw-pole set opposite to the magnet is embedded with slender metal bars for attracting magnetic lines of the claw-pole set to flow therethrough and thus causing comparatively stronger magnetic flux density to the solenoid coil. 
   Preferably, a plurality of connecting parts, being interconnected with each other serially or in parallel, are arranged between the outer claw-pole and the iron core for enabling electricity generated by the electric generator to be outputted. 
   Preferably, each connecting part is made of a material selected from the group consisting of iron, silicon iron, silicon steel and the combination thereof. 
   Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
       FIG. 1  is a sectional view of a conventional hub-type dynamo. 
       FIG. 2  is a left side view of a stator of the conventional hub-type dynamo shown in  FIG. 1 . 
       FIG. 3  shows a hub-type dynamo disclosed in TW Pat. No. 92137088. 
       FIG. 4  shows a conventional hub-type dynamo disclosed in TW Pat. No. 92137088. 
       FIG. 5  shows a flat rotary electric generator disclosed in U.S. Pub. No. 
       FIG. 6  is an explode diagram depicting an electric generator according to a first preferred embodiment of the invention. 
       FIG. 7  is a sectional view of  FIG. 6 . 
       FIG. 8  is a front view of an electric generator of the invention depicting the interlacing of outer claw-poles and inner claw-poles. 
       FIG. 9  is a schematic diagram illustrating the winding of a solenoid coil according to a preferred embodiment of the invention. 
       FIG. 10  is a sectional diagram depicting an electric generator according to a second preferred embodiment of the invention. 
       FIG. 11A  is a simulated diagram showing the distribution of magnetic flux density on a single-sided magnet of an electric generator of the invention while the electric generator is operating at 30 RPM. 
       FIG. 11B  is a simulated diagram showing the distribution of magnetic flux density on a single-sided magnet of an electric generator of the invention while the electric generator is operating at 120 RPM as the size and claw-pole number of the electric generator is restricted by an exemplification. 
       FIG. 12A  shows a waveform of induced voltage, whereas a single-sided magnet of an electric generator of the invention is operating at 30 RPM. 
       FIG. 12B  shows a waveform of induced voltage, whereas a single-sided magnet of an electric generator of the invention is operating at 120 RPM. 
       FIG. 13A  is a simulated diagram showing the distribution of magnetic flux density on a dual-sided magnet of an electric generator of the invention while the electric generator is operating at 60 RPM. 
       FIG. 13B  is a simulated diagram showing the distribution of magnetic flux density on a dual-sided magnet of an electric generator of the invention while the electric generator is operating at 90 RPM. 
       FIG. 14A  shows a waveform of induced voltage, whereas a dual-sided magnet of an electric generator of the invention is operating at 60 RPM. 
       FIG. 14B  shows a waveform of induced voltage, whereas a dual-sided magnet of an electric generator of the invention is operating at 90 RPM. 
       FIG. 15A  is a table showing the performance comparison of various electric generators of the invention. 
       FIG. 15B  shows performances of an electric generator of single-side magnet and another electric generator of dual-side magnet by depicting the changing of the induced voltages with respect to rotation speed. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   To further understand and recognize the fulfilled functions and structural characteristics of the invention, several preferable embodiments cooperating with detailed description are presented as the follows. 
   Please refer to  FIG. 6  to  FIG. 8 , which show an electric generator according to a first embodiment of the invention. The electric generator  10  is a single-sided magnet structure, which is primarily comprised of: a claw-pole set  20 , a shaft  30 , an iron core  40 , an interfacing part  50 , a solenoid coil  60 , a magnet  70  and a back panel  80 . 
   The claw-pole set  20  is composed of an outer claw-pole part  21  and an inner claw-pole part  22 . Wherein, the outer claw-pole part  21  is used for constructing an outer magnetic circuit and is further comprised of an outer tube  211  and a plurality of outer claw-poles  212 . The plural outer claw-poles are arranged at an axial end of the outer tube  211  while each extending from the edge of the outer tube  211  toward the axis of the same. In addition, the inner claw-pole part  22  is used or constructing an inner magnetic circuit and is further comprised of an inner tube  221  and a plurality of inner claw-poles  222 . In this first preferred embodiment, the iron core  40  is integrally formed with the inner tube  222  that the iron core  40  can be treated as the inner tube  222  of the inner claw-pole part  22 . The plural inner claw-poles  222  are arranged corresponding to the plural outer claw-poles  212  and are arranged at an axial end of the inner tube  221  while each extending outwardly and radially from the axle of the inner tube  221  for interlacing with the plural outer claw-poles  212 . With regard to the shape and number of the inner/out claw-pole, please refer to  FIG. 8 . In  FIG. 8 , there are ten outer claw-poles  212  and ten inner claw-poles  222 , whereas each outer claw-pole  212  is a cone-shape part tapering from the edge of the outer tube  211  toward the axial center of the same, and each inner claw-pole  222  is a fan-shaped part radially expanding outwardly from the edge of the inner tube  221 , and thereby, the outer claw-poles  212  and the inner claw-poles  222  can be placed and oriented to interlace and fit to each other. It is noted that the shapes and dimensions, i.e. width, length and thickness, of each inner/outer claw-pole  212 ,  222  can be varied with respect to the voltage waveform intended to be generated and its manufacturing method. For instance, each claw can be shaped like a rectangular with its edge being rounded, or the thickness of each claw can be varied along the flowing direction of magnet flux in a manner that the thickness is reducing from the end thereof to the tip, or the thickness is maintained the same. As the claw-poles shown in  FIG. 7 , the thickness of each outer/inner claw-pole  212 ,  222  is gradually reducing from the portion connecting to the outer/inner tube  212 ,  222 . Moreover, any of the inner claw-pole part  22  and outer claw-pole part  21  can be made of a material selected from the group consisting of iron, silicon iron, carbon steel, silicon steel and the combination thereof. Take one outer claw-pole  212  for instance, it can be integrally formed with the outer tube  211  by a processing method, such as stamping and casting, or it can be formed independent to the formation of the outer tube  211  whereas the two can thereafter be assembled by compactly ensheathing one inside the other. In another aspect, the outer claw-pole  212  can be a stacking of a plurality of silicon steel sheets that is lately integrally formed with the outer tube  211  by a process of plastic injection. The afore description is also true to the inner claw-pole  222  and the inner tube  221 , only if the object formed therewith can allow magnetic flux to be conducted between two poles of opposite polarities. 
   As seen in  FIG. 6  and  FIG. 7 , both the out tube  211  of the outer claw-pole part  21  and the inner tube  221  of the inner claw-pole part  22  are bored through the shaft  30 . As the iron core  40  is axially connected to the inner tube  221 , the iron core  40  can be mounted and ensheath the shaft  30 . In addition, as the inner diameter of the outer tube  211  is larger than the outer diameter of the iron core  40 , not only the interference between magnetic flux can be prevented, but also a space can be formed between the outer tube  211  and the iron core  40  to be used for receiving the solenoid coil  60  wrapping around the iron core  40 . 
   Furthermore, the an interfacing part  50  can be substantially a ring structure which comprises: an inner side  52 , axially connected to an axial end of the iron core  40  opposite to the inner claw-pole  222 ; and an outer side  51 , axially connected to the an axial end of the out tube  211  opposite to the outer claw-pole  212 ; In addition, as the inner side  52  and the outer side  51  are interconnected with each other, the outer claw-pole part  21  is connected to the inner claw-pole part  11 . It is noted that the interfacing part  50  can be integrally formed with the out tube  211  and the iron core  40 , and then the integrated structure can be bonded with the outer claw-pole  212  and then the inner claw-pole  222 . In a preferred aspect, a fixation cap  53  is arranged on a surface of the interfacing part  50 , not proximate to the claw-pole set  20 . By fixedly securing the fixation cap  53  onto the shaft  30  while enabling an end of the fixation cap  53  to abut against the interfacing part  50 , the interfacing part  50 , the outer tube  22  and the iron core  40  are forced to closely contact with each other. 
   As seen in  FIG. 6  to  FIG. 8 , the magnet  70  is mounted on the shaft  30  at a position proximate to the outer/inner claw-poles  212 ,  222 . It is noted that the magnet  70  can be substantially a flat magnet of disc shape or other geometrical shape, that the magnet  70  has more than two poles. As the dotted area shown in  FIG. 8 , the magnet  70  can be equiangularly divided into twenty poles  71 , whereas there are ten north (N) poles  711  and ten south (S) poles  712 , arranged in an alternating manner. In addition, the geometrical shape of each pole of the magnet  70  is conforming to that of the inner/outer claw-pole  212 ,  222 , and the number of claws of the inner claw-pole  222  is the half of the number of the pole  71  while the outer claw-pole  212  is the same, i.e. ten outer claw-poles  212  and ten inner claw-poles  222 . The magnet  40  is a multi-pole magnet made of a permeance material selected from the group consisting of NdFeB, SmCo, Hard Ferrite, AlNiCo, and the like. In addition, in order to avoid the adverse affect caused by the solenoid coil  60  being wrapped at a position not equally distant from the N pole  711  and the S pole  712 , the magnet  70  is a multi-pole magnet of biased pole magnetization. Moreover, for those skilled in the art, the magnet  70  can be a single-sided magnet or a dual-sided magnet. When the inner/outer claw-pole  212 ,  222  is rotated radially by a helix angle, each pole  71  of the magnet  70  is twisted for magnetizing the pole with respect to the radius and angle of each pole. Further, a side of the claw-pole set  20  opposite to the magnet  70  is embedded with slender metal bars for attracting magnetic lines of the claw-pole set  20  to flow therethrough and thus causing comparatively stronger magnetic flux density to the solenoid coil  60 . 
   In  FIG. 6  and  FIG. 7 , a back panel  80  is arranged outside the magnet  70 , which includes a flat disc-shape iron back  81 . The iron back  81  is arranged at a side of the magnet  70  opposite to the outer/inner claw-pole  212 ,  222 . Moreover, a fixation cap  82  is arranged at the center of integrated structure of the iron back  81  and the magnet  70 , and is secured axially to the shaft  30 . In addition to the fixation cap  82 , a bearing  83  is further mounted on the shaft  30  while the bearing is further covered by a bearing cap  84 , also being mounted on the shaft  30 . The bearing cap  84  is screw-fixed to the iron back  81  by screws  85 , and thus the magnet  70 , the iron back  81 , the bearing  83 , the bearing cap  84 , and the fixation cap  82  are assembled. An end of the fixation cap  82  facing toward the claw-pole set  20  is abutted against the inner tube  222  of the inner claw-pole part  22 . As seen in  FIG. 7 , the fixation cap  82  is designed with an axial-extending length L, by which a distance D can be maintained between the magnet  70  and the outer/inner claw-pole  212 ,  222  when the fixation cap  82  is abutted against the inner tube  222 . Thus, as the iron back  81  is arranged at a side of the magnet  70  opposite to that proximate to the claw-pole set  20 , the iron back  81  is capable of enabling the closing of magnetic lines and thus reducing magnetic flux loss so that the magnetic flux density at the side of the claw-pole set  20  is increased. The iron back  81  is made of a permeance material selected from the group consisting of iron, ferro-cobalt alloy, Ni—Fe alloy, silicon iron and the combination thereof. By the disposition of the bearing  83 , the magnet  70  is pivotally connected to the shaft  30 . In addition, the coil  60 , a plurality of the claw-pole set  20  and the magnet  70  can be serially mounted on the shaft  30  while arranging the iron back  81  outside the magnet  70  where it is far away from the interfacing part  50 . Moreover, the iron back  81  and the bearing cap  84  can be integrally formed for simplicity. 
   By the combination of aforesaid components, a magnet circuit can be constructed within the magnet  70 , the claw-pole set  20 , and the iron core  40 . When the magnet  70  is rotated relative to the rotation of the claw-pole set  20 , the flowing direction of the magnet flux within the solenoid coil  60  is constantly changing between forward flowing and reverse flowing as the relative positions of the outer/inner claw-poles  212 ,  222  and the poles  70  of the magnet  70  are changing correspondingly. In a preferred aspect, when the outer/inner claw-poles  212 ,  222  complete one rotation with respect to the rotating magnet  70 , the number of direction change of the magnet flux flowing inside the solenoid coil  60  is equal to the pole number of the magnet  70 . 
   Please refer to  FIG. 9 , which is a schematic diagram illustrating the winding of a solenoid coil according to a preferred embodiment of the invention. In  FIG. 9 , a plurality of connecting parts  41 , being interconnected with each other serially or in parallel by the coils wrapped respectively thereon, are arranged between the outer tube  211  and the iron core  40  for enabling electricity generated by the electric generator to be outputted. It is noted that each coil  61  is functioning similar to the solenoid coil  60 , moreover, a portion of each coil  60  can be wrapped on a corresponding outer claw-pole  212 , as the out claw-pole  212  shown in  FIG. 6 , so that the utilization of outer magnetic circuit is enhanced and thus the electricity generation is increased. 
   Please refer to  FIG. 10 , which is a sectional diagram depicting an electric generator according to a second preferred embodiment of the invention. In  FIG. 10 , the electric generator  100 , being a dual-sided magnet structure, is an extension of the single-sided magnet structure  10  of  FIG. 6 . The electric generator  100  is primarily comprised of: a claw-pole set  200 , a shaft  300 , an iron core  400 , a solenoid coil  600 , two magnetic pieces  700  and two back panels  800 . The functions of the aforesaid components are similar to those shown in  FIG. 6 , and thus are not described further herein. However, in this second preferred embodiment, each magnetic piece  700  is correspond to a set of out claw-poles  2120  and a set of inner claw-poles  2220 , whereas the two sets of inner claw-poles  2220  are respectively connected axially to the two axial ends of the iron core  400  while one of the two magnetic pieces  700  is connected to an axial end of a hollow tube-like first connecting part  720  and another magnetic piece  700  is connected to another axial end, so that the two magnetic pieces  700  can be driven to rotate synchronously by the first connecting part  720 ; and an axial end of a second connecting part  2110  is connected to one of the two set of outer claw-poles  2120  while another axial end of the second connecting part  2110  is connected to another set of outer claw-poles  2120 , so that the two outer claw-pole sets can be driven to rotate synchronously by the second connecting part  2110 . In addition, a space is formed between the second connecting part  2110  and the iron core  400  for receiving the solenoid coil  600 . 
   In this second preferred embodiment, the inner claw-pole sets  220  are mounted on the two axial ends of the iron core  400  by an ensheathing manner. However, they can be integrally formed with the iron core  400  as those shown in  FIG. 6 , in which the iron core  400  is acting as an inner tube shared by the two inner claw-pole sets  220 . As for the interlacing arrangement of the outer claw-poles and the inner claw-poles, it is similar to that shown in  FIG. 8 . In addition, a plurality of the coil  600 , the claw-pole sets  200  and the magnetic pieces  700  can be serially mounted on the shaft  300  while sandwiching one magnetic piece  700  between one claw-pole set  200  and it corresponding solenoid coil set  600  and only arranging one iron back  810  outside the outer-most magnetic piece  700  while no iron back  810  is needed for those magnetic pieces  700  in the middle. 
   Please refer to  FIG. 11A  to  FIG. 14B , which are simulations performed by ANSOFT for evaluating the performance of an electric generator of the invention under different rotation speed. As seen in  FIG. 11A  and  FIG. 11B , which are simulated diagrams showing the distributions of magnetic flux density on a single-sided magnet of an electric generator of the invention while the electric generator is operating at 30 RPM and at 120 RPM, the magnetic flux density at the iron core is far more saturated comparing to those at the claw-poles.  FIG. 12A  and  FIG. 12B  show respectively a waveform of induced voltage, whereas a single-sided magnet of an electric generator of the invention is operating at 30 RPM and a waveform of 120 RPM. Moreover, as seen in  FIG. 13A  and  FIG. 13B , which are simulated diagrams showing the distributions of magnetic flux density on a dual-sided magnet of an electric generator of the invention while the electric generator is operating at 60 RPM and 90 RPM, the magnetic flux densities at the iron core on both account are all saturated which demonstrates that the pat of magnetic flux of a dual-sided magnetic structure is comparatively shorter and thus the transmission efficiency is preferred.  FIG. 14A  and  FIG. 14B  show respectively a waveform of induced voltage, whereas a single-sided magnet of an electric generator of the invention is operating at 60 RPM and a waveform of 90 RPM. From the simulation disclosed above, the efficiency of the electric generator of the invention can be verified. 
   Please refer to  FIG. 15A  and  FIG. 15  B, which are respectively a table showing the performance comparison of various electric generators of the invention and a diagram showing performances of an electric generator of single-side magnet and another electric generator of dual-side magnet by depicting the changing of the induced voltages with respect to rotation speed. In these two figures, it is noted that, under the same rotation speed, the induced voltages obtained from a single-sides magnetic structure and a dual-sided magnetic structure are not the same. Nevertheless, both is quite capable of achieving a required voltage, but under different rotation speeds. In  FIG. 15B , curve L 1  represents the variation of total induced voltage acquired from a single-sided magnetic structure operating at different rotation speed; and curve L 2  represents the variation of total induced voltage acquired from a dual-sided magnetic structure operating at different rotation speed. While applying the electric generator as a hub-type dynamo of a bicycle, the power density can achieve 45 mW/cm3 while operating at 150 RPM. While applying the electric generator on a fitness bicycle, the power density can achieve 40 mW/cm3 while operating at 500 RPM. 
   To sum up, the electric generator of the invention has advantages list as following: 
   (1) As the magnetization direction of a flat disc-shaped magnet of the invention is parallel to the axial direction of the iron core, the corresponding magnetic lines are directed to the iron core effectively as they are deflected only once after the magnetic lines are received into the inner/outer claw-poles, so that less magnet is required in the electric generator of the invention, and thus the overall appearance of the invention can be flattened that is suitable for certain specific applications of limited space available. 
   (2) In the electric generator of the invention, the magnetic lines are guided effectively to flow inside the solenoid coil by the design of the flat disc-shaped magnet and the inner/outer claw-pole sets, and the flowing direction of the magnet flux within the solenoid coil is constantly changing between forward flowing and reverse flowing as the relative positions of the outer/inner claw-pole sets and the poles of the multi-pole magnet are changing correspondingly to the rotation of the multi-pole magnet, the magnetic flux of the solenoid coil is changed with respect to time that is totally different to the conventional method of cutting the magnetic line perpendicularly, as that shown in  FIG. 3 , by which not only the induced voltage output efficiency is increase, but also the magnetic resistance is reduced. 
   (3) As all the magnetic lines passing the claw-poles are all being guided into the iron core, the number of solenoid coils required can be reduced. Moreover, by the use of only a single solenoid coil, the residue magnetic flux on the surface of the magnet can be utilized effectively. 
   (4) The flat disc-shape magnet can be magnetized easily. 
   (5) The electric generator of the invention can have low resistance and high conversion efficiency. 
   (6) The electric generator of the invention can have good voltage performance even while operating at comparatively low rotation speed. 
   (7) The electric generator of the invention is compact and light-weighted. 
   (8) The electric generator of the invention is simple in structure and low cost. 
   While the preferred embodiment of the invention has been set forth for the purpose of disclosure, modifications of the disclosed embodiment of the invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the invention.