Abstract:
The purpose of the present invention is to provide a electric generator which has a simple structure while sufficiently suppressing cogging torque. A electric generator is constituted by a rotor section comprising a plurality of protrusions which is arranged linearly with constant pitch at mutually separated positions sandwiching permanent magnets, and a stator section which has a stator member comprising stator yokes, permanent magnets facing the permanent magnets, and protrusions which protrude closer to the rotor section than the permanent magnets and are arranged linearly with constant pitch at mutually separated positions sandwiching the permanent magnets, the stator member being configured such that the protrusions and the protrusions are shifted from each other by a half pitch between adjoining stator members. Further, the rotor section-side protrusions may be shifted by a half pitch instead of the stator section-side protrusions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Stage Application of International Application No. PCT/JP2012/082566 filed on Dec. 14, 2012, and published in Japanese as WO 2013/089248 A1 on Jun. 20, 2013. This application claims priority to Japanese Application No. 2011-275628 filed on Dec. 16, 2011. The entire disclosures of the above applications are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates to an electric generator. 
       BACKGROUND ART 
       [0003]    An electric generator comprising a rotor having a magnet such as a permanent magnet and a stator having a coil may be improved in power generation efficiency as the number of turns of the coil is large. However, increasing of the number of turns of the coil enlarges size of the electrostatic generator. 
         [0004]    On the other hand, JP 2006-340425 A, for example, discloses a motor reduced in size and improved in output by arranging the magnet of the rotor side and the coil of the stator side in a predetermined positional relationship. Therefore, it is considered to obtain an electric generator with high power generation efficiency by adopting the positional relationship between the magnet and coil of such a motor. 
         [0005]    Further, in the electric generator as described above, it is known that when the rotor rotates cogging torque is generated in relation to the core. Such cogging torque is caused by attractive force or repulsive force generated between the magnetic poles of the rotor and the core and would cause cogging on rotation of the rotor. Therefore, the rotation of the rotor becomes unstable by the cogging torque. Regarding such a point, JP 2006-101695 A, for example, discloses a mean for suppressing cogging by making the shape of the part for mounting the magnet of the rotor into a predetermined shape to moderate the change in magnetic flux at the rotor rotating. 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0006]    However, the conventional technologies described above have problems that the structures are complicated and cannot suppress cogging sufficiently. 
         [0007]    Accordingly, this invention aims to provide a power electric generator which has not a complex structure and achieves sufficient suppression of cogging. 
       Solution to Problem 
       [0008]    An electric generator according to this invention comprises a rotor portion and a stator portion in which: the rotor portion includes: a rotor yoke which made of soft magnetic material and which has a cylindrical non-magnetic portion and one or more pairs of grooves towards the non-magnetic portion; annular rotor side permanent magnets each of which is arranged in each of the grooves and magnetized as north and south poles in an axial direction; and a plurality of rotor side protrusions which protrude over the rotor side permanent magnet to the stator portion side and are arranged linearly with a constant pitch at mutually separated positions sandwiching the rotor side permanent magnets; the stator portion includes a stator member made of soft magnetic material having: a stator yoke; annular stator side permanent magnets being opposed to the rotor side permanent magnets; and stator side protrusions arranged linearly with the constant pitch at mutually separated positions sandwiching the stator side permanent magnets; wherein the stator members are arranged to shift the stator side protrusions by a half of the pitch between the neighboring stator members. 
         [0009]    It is preferable for the electric generator according to this invention that non-magnetic portions a number of which is equal to the multiple of two are provided instead of the non-magnetic portion wherein each of the non-magnetic portions extends in axial direction to connect between neighboring ones of the rotor side permanent magnets and has a length longer than the axial distance between the both end faces of the corresponding both rotor side permanent magnets. 
         [0010]    According to another aspect of this invention, an electric generator comprises a rotor portion and a stator portion in which: the rotor portion includes: a rotor yoke which made of soft magnetic material and which has a cylindrical non-magnetic portion and one or more pairs of grooves towards the non-magnetic portion; annular rotor side permanent magnets each of which is arranged in each of the grooves and magnetized as north and south poles in an axial direction; and a plurality of rotor side protrusions which protrude over the rotor side permanent magnet to the stator portion side and are arranged at mutually separated positions sandwiching the rotor side permanent magnets; wherein one of the rotor side protrusions located between two of the rotor side permanent magnets is shift by a half of the pitch at the position where the length of the one of the rotor side protrusions in the axial direction is equally divided by two; and in which: the stator portion includes a stator member made of soft magnetic material having: a stator yoke; annular stator side permanent magnets being opposed to the rotor side permanent magnets; and stator side protrusions arranged linearly with the constant pitch at mutually separated positions sandwiching the stator side permanent magnets. 
         [0011]    It is also preferable for the electric generator according to the another aspect of this invention that non-magnetic portions a number of which is equal to the multiple of two are provided instead of the non-magnetic portion wherein each of the non-magnetic portions extends in axial direction to connect between neighboring ones of the rotor side permanent magnets and has a length longer than the axial distance between the both end faces of the corresponding both rotor side permanent magnets. 
         [0012]    In these configurations, it is preferable that areas of the opposing parts of the rotor protrusions and the stator side protrusions and non-opposing part thereof are set constant respectively regardless of the positional relationship between the rotor portion and the stator portion in the rotation direction. 
         [0013]    According to further aspect of this invention, non-magnetic portions are provided instead of the stator side permanent magnets. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]      FIG. 1  shows main parts of a preferred embodiment according to this invention by a cross section along the central axe of a rotor portion. 
           [0015]      FIG. 2  shows a perspective view of an arrangement of the electric generator shown in  FIG. 1  with cutting out partly in conjunction with the rotor and the inserting direction thereof. 
           [0016]      FIG. 3  shows a perspective view of the rotor portion of the electric generator shown in  FIG. 1  with cutting out partly. 
           [0017]      FIG. 4  shows a positional relationship in a circumferential direction (a rotation direction) between rotor side protrusions (solid lines) of the rotor portion shown in  FIG. 1  and stator side protrusions (dashed lines) of the stator portion shown in  FIG. 2 . 
           [0018]      FIG. 5  shows a predetermined positional relationship S 1  between the rotor side protrusions (solid lines) of the rotor portion shown in  FIG. 1  and the stator side protrusions (dashed lines) of the stator portion shown in  FIG. 2 . 
           [0019]      FIG. 6  schematically shows a state of magnetic paths in the electric generator in the state S 1  of the predetermined relationship shown in  FIG. 5 . 
           [0020]      FIG. 7  shows a state S 2  where the rotor side protrusions (solid lines) of the rotor portion shown in  FIG. 1  and the stator side protrusions (dashed lines) of the stator portion shown in  FIG. 2  have another predetermined positional relationship. 
           [0021]      FIG. 8  schematically shows a state of magnetic paths in the electric generator in the state S 2  of the predetermined relationship shown in  FIG. 7  with cutting along the A-A line shown in  FIG. 7 . 
           [0022]      FIG. 9  shows a state S 3  where the rotor side protrusions (solid lines) of the rotor portion shown in  FIG. 1  and the stator side protrusions (dashed lines) of the stator portion shown in  FIG. 2  have another predetermined positional relationship. 
           [0023]      FIG. 10  schematically shows a state of magnetic paths in the electric generator in the state S 3  of the predetermined relationship shown in  FIG. 9 . 
           [0024]      FIG. 11  shows changes of the relationships between the rotor side protrusions (solid lines) of the rotor portion shown in  FIG. 1  and the stator side protrusions (dashed lines) of the stator portion shown in  FIG. 2  as time advances (times t 1  to t 5 ). 
           [0025]      FIG. 12  schematically shows a current occurring state on two wound portions in the electric generator as time advances (times t 1  to t 5 ). 
           [0026]      FIG. 13  schematically shows, as comparative example, a magnetic path in an electric generator which has a bypassing portion in a rotor side yoke. 
           [0027]      FIG. 14  shows a perspective view of an arrangement of a rotor portion and a stator portion of an electric generator according to another embodiment with cutting out partly. 
           [0028]      FIG. 15  shows main parts of an electric generator according to another embodiment by a cross section along the central axe of a rotor portion. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0029]    Preferred embodiments of the present invention will now be described with reference to the drawings. In the following description, the term “flux” means a flux of magnetic field lines and the term “magnetic path” means a path of the magnetic field lines. Further, notation of “a magnetic path is formed by the magnetic flux” may be read as “a magnetic path is formed by the magnetic field lines.” 
         [0030]    As shown in  FIGS. 1 to 3 , an electric generator  1  according to an embodiment of the present invention comprises a rotor portion  2  and a stator portion  3 . The rotor portion  2  comprises a rotor yoke  4  made of a soft magnetic material of a cylindrical shape. In the rotor yoke  4 , there are provided a cylindrical hollow portion  5  having a square cross-section hollow and two grooves  6 U and  6 L penetrating from an outer periphery “a” of the rotor yoke  4  to a non-magnetic portion of the hollow portion  5  in the circumferential direction. An annular permanent magnet  7 U is provided so as to be sandwiched between the wall surfaces of the groove  6 U and an annular permanent magnet  7 L is provided also so as to be sandwiched between the wall surfaces of the groove  6 L. 
         [0031]    The rotor yoke  4  has a plurality of rectangular-shaped protrusions  8 U made of magnetic material at the axially upward side of the permanent magnet  7 U shown in  FIGS. 1-3 , a plurality of rectangular-shaped protrusions  8 L made of magnetic material at the axially downward side of the permanent magnet  7 L shown in  FIGS. 1-3 , and a plurality of rectangular-shaped protrusions  9  made of magnetic material between the permanent magnet  7 U and  7 L. Respective outer surfaces of the protrusions  8 U,  9  and  8 L are arranged so as to form equally-spaced angular intervals respectively about the center axis C of the rotor yoke  4  and so as to be linearly arranged in the axial direction. 
         [0032]    The outer periphery “a” has an arc-shaped cross section for each of the protrusions  8 U,  9  and  8 L. Supposing the length of a part of the outer periphery “a” for one protrusion being L, the span of a virtual outer periphery “a” with the neighboring protrusion  8 U,  9  or  8 L is set as L. At the center of the rotor yoke  4 , a rotation shaft  10  is provided. The portion the rotation shaft  10  contacts with the rotation shaft  10  is joined so that the rotor yoke  4  can rotate integrally in accordance with the rotation shaft  10  rotating. 
         [0033]    In a housing of the electric generator  1 , which is not show in  FIGS. 1-3 , the rotor portion  2  and the stator portion  3  are accommodated such that slight gaps are maintained between the protrusions  8 U,  9  and  8 L of the rotor portion  2  and respective protrusions  16 U 1 ,  16 U 2 ,  16 L 1  and  16 L 2  of the stator unit  3  described below. On the outside of the housing, the rotation shaft  10  is extending directly, or indirectly through a gear train or else so that the rotor portion  2  can rotate together with the rotation shaft  10  at the inside of the stator portion  3  when a rotation torque is applied on the rotation shaft  10  from outside. The protrusions  8 U,  9  and  8 L may be manufactured by integral molding together with the main body of the rotor yoke  4 . The protrusions  16 U 1 ,  16 U 2 ,  16 L 1  and  16 L 2  described below may be also manufactured by integral molding together with stator yokes  12 U and  12 L. 
         [0034]    In the example shown in  FIGS. 1-3 , the rotor part  2  is configured as the upper end face of the upper-side permanent magnet  7 U takes the north pole, the lower end face of the same takes the south pole, the upper end face of the lower-side permanent magnet  7 L takes the south pole, and the lower end face of the same takes the north pole. That is, the annular permanent magnets  7 U and  7 L are magnetized in the axial direction together and are arranged such that the opposing faces of the permanent magnets  7 U and  7 L have the same polarity. 
         [0035]    On the rotor portion  2  itself, the magnetic flux of the upper-side permanent magnet  7 U is to pass from the lower end of the projecting portions  8 U through the outside of the rotor part  2 , direct to the upper end of the protrusions  9  shown in  FIG. 1 , and return to the south pole. At the same time, the magnetic flux of the lower-side permanent magnet  7 L is to pass from the upper end of the protrusions  8 L through the outside of rotor portion  2 , direct to the lower end of the protrusions  9  shown in  FIG. 1 , and return to the south pole. 
         [0036]    In the rotor portion  2  according to the preferred embodiment of the present invention having such a configuration, magnetic flux always directs from the protrusions  8 U,  9  and  9 L to themselves. By forming such protrusions  8 U,  9  and  8 L, it is possible to increase the density of the magnetic flux to increase the attractive force. 
         [0037]    The stator portion  3  has a double decker construction of stator yokes  12 U and  12 L being piled in the axial direction, wherein the stator yokes  12 U and  12 L are made of a soft magnetic material in a cylindrical shape with a cylindrical hole portion  11  in the center for inserting the rotor portion  2 . In this description, it is explained, for the purpose of illustration, that the stator yokes  12 U and  12 L are constructed as the “double decker construction.” Practically, these may be constructed from one yoke member to form one part corresponding to the stator yoke  12 U and another part corresponding to the stator yoke  12 L. Two stator yokes  12 U and  12 L may be formed separately and joined in double decker. 
         [0038]    In the stator yoke  12 U and  12 L of the stator portion  3 , there are provided annular hollow portions  13 U and  13 L each having a square cross-section, and annular grooves  14 U and  14 L penetrating from the inner periphery (facing to the hole portion  11 ) of the stator yoke  12 U and  12 L to the hollow portions  13 U and  13 L. An annular permanent magnet  15 U is attached on the groove  14 U of the stator yoke  12 U and an annular permanent magnet  15 L is attached on the groove  14 L of the stator yoke  12 L. Protrusions  16 U 1 ,  16 U 2 ,  16 L 1  and  16 L 2  made f magnetic material arranged in two stages by two longitudinally are provided on the hole portion  11  or the inner-side surface of the stator yoke  12 U and  12 L so as to sandwich the permanent magnet  15 U and  15 L in the axial direction. The inner periphery “b” of the protrusions  6 U 1 ,  16 U 2 ,  16 L 1  and  16 L 2  are configured so as to be opposed to the outer periphery “a” of the rotor yoke  4  and each part of the inner periphery “b” for the protrusions  6 U 1 ,  16 U 2 ,  16 L 1  and  16 L 2  is slightly longer than the length L of the outer periphery “a” in the circumferential direction. 
         [0039]    Protrusions  16 U 1  and  16 U 2  of the stator yoke  12 U and protrusions  16 L 1  and  16 L 2  of the stator yoke  12 L are arranged so as to shift by a half pitch each other in the circumferential direction with respect to the central axis of the stator yokes  12 U and  12 L. The protrusions  16 U 1 ,  16 U 2 ,  16 L 1  and  16 L 2  are also arranged so as to form equally-spaced angular intervals along the inner-side face of the stator yokes  12 U and  12 L in the circumferential direction about the center axis of the stator yokes  12 U and  12 L and be spaced in the same length with the width of each inner periphery “b” of the protrusions  16 U 1 ,  16 U 2 ,  16 L 1  and  16 L 2 . In the hollow portions  13 U and  13 L, there are provided wound portions  17 U and  17 L respectively. The wound portion  17 U has a wound in the circumferential direction multiply and both ends of the wire (not shown) are drawn from the stator yoke  12 U to the outsid. The wound portion  17 L similarly has a wound and both ends of the wire (not shown) are drawn from the stator yoke  12 L to the outside. Each of the wires leads electric current generated by the electric generator  1  to the outside. 
         [0040]    In the example shown in  FIGS. 1 and 2 , it is configured that the upper end face of the permanent magnet  15 U of the stator yoke  12 U takes a south pole and the lower end face of the same takes a north pole. If the stator yoke  12 U is in a condition that the rotor portion  2  is not inserted in the hole  11 , a magnetic path is generally formed on the stator yoke  12 U starting from the north pole of the lower end face of the permanent magnet  15 U, passing the inner periphery and the inside of the stator yoke  12 U, directing to the lower side of the stator yoke  12 U, turning in the vicinity of the boundary between the stator yoke  12 U and the stator yoke  12 L, directing from the center of the stator yoke  12 U to the outside, turning at a point of the outer periphery side, directing along the outer periphery and through the inside of the upper end to the inside in the radial direction, turning at a point of the inner periphery side to downward again, and returning to the south pole of the permanent magnet  15 U (not shown in the drawings). In addition to this magnetic path, a portion of the magnetic flux forms a magnetic path of directing from the stator yoke  12 U near the north pole of the permanent magnet  15 U to the hole  11  side and returning to the stator yoke  12 U near the south pole of the permanent magnets  15 U. 
         [0041]    At the same time, in the example shown in  FIGS. 1 and 2 , it is configured that the lower end face of the permanent magnet  15 L of the stator yoke  12 L takes a south pole and the upper end face of the same takes a north pole. If the stator yoke  12 L is in a condition that the rotor portion  2  is not inserted in the hole  11 , a magnetic path is generally formed on the stator yoke  12 L starting from the north pole of the upper end face of the permanent magnet  15 L, passing the inner periphery and the inside of the stator yoke  12 L, directing to the upper side of the stator yoke  12 L, turning in the vicinity of the boundary between the stator yoke  12 L and the stator yoke  12 U, directing from the center of the stator yoke  12 L to the outside, turning at a point of the outer periphery side, directing along the outer periphery and through the inside of the lower end to the inside in the radial direction, turning at a point of the inner periphery side to upward again, and returning to the south pole of the permanent magnet  15 L (not shown in the drawings). In addition to this magnetic path, a portion of the magnetic flux forms a magnetic path of directing from the stator yoke  12 L near the north pole of the permanent magnet  15 L to the hole  11  side and returning to the stator yoke  12 L near the south pole of the permanent magnets  15 L. 
         [0042]    The protrusions  16 U 1  and  16 U 2  of the stator portion  3  shown in  FIG. 1  are not opposed to the protrusions  8 U and  9  of the rotor portion  2 . On the other hand, the protrusions  16 L 1  and  16 L 2  of the stator unit  3  shown in  FIG. 1  IG  1   1 ,  16 L  2  are opposed to the protrusions  9  and  8 L of the rotor portion  2 . In this electric generator  1  in this way, the protrusions  8 U,  9  and  8 L of the rotor yoke  2  take a condition that there is no portion being opposed to the protrusions  16 U 1  and  16 U 2  of the stator yoke  12 U when being opposed to the protrusions  16 L 1  and  16 L 2  of the stator yoke  12 L entirely. 
         [0043]    In the electric generator  1 , as shown in  FIG. 4 , the protrusions  8 U,  9  and  8 L of the rotor portion  2  pass across the protrusions  16 U 1 , 16 U 2 , 16 L 1  and  16 L 2  of the stator portion  3  in the circumferential direction in accordance with the rotor yoke  4  of the rotor portion  2  rotating along with rotation of the rotation shaft  10 . It causes a change of the magnetic flex and this change induces an electric current on each of the wound portions  17 U and  17 L to generate electricity. The electric generator  1  may be used for storing of electricity and/or driving a load by extracting the electric current flowing in the wound portions  17 U and  17 L. 
         [0044]    As shown in  FIG. 5 , for example, a state S 1  is defined as the protrusions  16 U 1  and  16 U 2  of the stator unit  3  are not opposed to any of the protrusions  8 U,  9  and  8 L, a lower half of the protrusion  9  of the rotor portion  2  is opposed to the protrusion  16 L 1  of the stator portion  3 , and the protrusion  8 L of the rotor  2  is opposed to the protrusion  16 L 2  of the stator portion  3 . 
         [0045]    In such a state S 1 , since the stator yoke  12 U of the stator portion  3  is not affected so much by the attractive force from the protrusions  8 U and  9  of the rotor portion  2 , most of the magnetic flux from the permanent magnet  15 U of the stator yoke  12 U forms, as shown in  FIG. 6 , a magnetic path M 1  (shown by dashed-dotted lines) starting from the north pole of the permanent magnet  15 U, passing around the hollow portion  13 U and returning to the south pole of the permanent magnet  15 U. At this time, most of the magnetic flux of the permanent magnet  7 U of the rotor portion  2  is made direct to the south pole side of the stator yoke  12 L because of strong affection of the protrusions  16 L 1  and  16 L 2  of the stator yoke  12 L which have the inner periphery “b” of the stator portion  3  being closer to the outer periphery “a” of the stator portion  3  than the protrusions  16 U 1  and  16 U 2  of the stator yoke  12 U. Since the inner periphery “b” of the protrusions  16 L 1  and  16 L 2  of the stator portion  3  comes very close to the outer periphery “a” of the protrusions  9  and  8 L of the rotor portion  2 , most of the magnetic flux of the permanent magnet  7 L of the rotor portion  2  directs from the protrusions  9  and  8 L of the rotor portion  2  into the stator yoke  12 L side while most of the magnetic flux of the permanent magnet  15 L of the stator portion  3  directs to the rotor portion  2 . 
         [0046]    Consequently, as shown in  FIG. 6 , in the rotor portion side, a magnetic path M 2  is formed starting from the north pole of the upper end face of the permanent magnet  7 U, passing around the vicinity of the upper end face of the rotor yoke  4 , once entering into the stator yoke  12 L side at the vicinity of the lower end face of the rotor yoke  4 , entering into the rotor part  2  side again at the vicinity of the upper end face of the stator yoke  12 , and returning to the south pole of the lower end face of the permanent magnet  7 U. A magnetic path M 3  is also formed such that most of the magnetic flux of the permanent magnet  15 L of the stator yoke  12 L of the stator portion  3  directs to the protrusion  9  of the rotor portion  2  while most of the magnetic flux of the permanent magnet  7 L of the rotor portion  2  directs to the protrusion  16 L 2  of the stator portion  3 . As is formed the magnetic path M 2 , the magnetic flux of the permanent magnet  7 U of the rotor portion affect the stator portion  3  little and it makes the magnetic path M 3  between the permanent magnet  7 L of the rotor portion  2  and the permanent magnet  15 L of the stator portion  3  more efficient. 
         [0047]    In the state S 1  as described above, changes are happened on the wound portions  17 U and  17 L such that most of the magnetic flux of the permanent magnet  15 L having passed around the wound portion  17 L until just before the state S 1  is absorbed into the rotor portion  2  side to become weak and that the magnetic flux of the permanent magnet  15 U having directed to the rotor portion  2  side until just before the state S 1  returns to the wound portion  17  U of the stator portion  3  to become intensive. Accordingly, an electric current flows through the wound portions  17 U and  17 L so as to generate magnetic lines counteracting the changes of the magnetic flux. It will be describe in detail about the current flowing through the wound portion  17 U and  17 L. 
         [0048]    As shown in  FIG. 7 , a state S 2  is defined as each half of the protrusions  8 U,  9  and  8 L in view of the circumferential direction is opposed to each half of the protrusions  16 U 1 ,  16 U 2 ,  16 L 1 , and  16 L 2  in the same direction. In such a state S 2 , as shown in  FIG. 8  illustrating a cross section along the line A-A in  FIG. 7 , in comparison with the state S 1 , a part of magnetic flux of the permanent magnet  15 U of the stator yoke  12 U directs to the rotor portion  1  side and directs from the protrusion  16 U 2  of the stator portion  3  to the rotor portion  2  side. On the other hand, most of the magnetic flux of the permanent magnet  7 U of the rotor portion  2  side directs to the stator portion  3  side. That is, it directs from the protrusion  8 U of the rotor portion  2  to the stator portion  3  side. As a result, a magnetic path M 4  is formed directing from the north pole of the permanent magnet  7 U of the rotor portion  2  side to the south pole of the permanent magnet  15 U of the stator portion  3  side and directing from the north pole of the permanent magnet  15 U of the stator portion  3  side to the south pole of the permanent magnet  7 U of the rotor portion  2  side. According to this, the magnetic flux of the magnetic path M 5  passing around the wound portion  17 U is weakened as compared with the magnetic path M 1  in the state S 1  (shown as S 1 : thick dashed-dotted lines to S 2 : thin dashed-dotted lines). 
         [0049]    In the state S 2 , a part of magnetic flux of the permanent magnet  7 L of the rotor portion  2 , which has directed to the stator yoke  12  side of the stator portion  3  in the state S 1 , passes through the hollow portion  5  to return the rotor  2  side. A part of the magnetic flux of the permanent magnet  15 L also becomes to pass around the wound portion  17 L. That is, compared to the state S 1 , a magnetic path M 6  is formed by weak magnetic lines directing from the north pole of the permanent magnet  7 L of the rotor portion  2  side to the south pole of the permanent magnet  15 L of the stator portion  3  side and from the north pole of the permanent magnet  15 L of the stator portion  3  side to the south pole of the permanent magnet  7 L of the rotor portion  2  side. The magnetic flux of the magnetic path M 7  passing around the wound portion  17  is made intensive as compared to the state S 1  (shown as S 1 : thick dashed-dotted lines to S 2 : thin dashed-dotted lines). The magnetic path M 2  described above is yielded in the rotor portion  2  opposing to the protrusions  16 L 1  and  16 L 2  of the stator portion  3  and its intensity becomes weak. In the state S 1 , a magnetic path M 9  described below is also yielded and its intensity is becoming intensive. 
         [0050]    In the state S 2  as described above, compared to the state S 1 , changes are happened such that the magnetic flux passing around the wound portion  17 U is made weak and that the magnetic flux passing around the wound portion  17 L is made intensive. so that an electric current flows so as to generate magnetic lines counteracting L the changes of the magnetic flux. Since the permanent magnets  7 U and  7 L of the rotor portion  2  are arranged for opposing portions to be different and to be connected by the hollow portion  5 , the magnetic flux of the respective permanent magnets  7 U and  7 L directs to the stator portion  3  effectively. 
         [0051]    As shown in  FIG. 9 , a state S 3  is defined as the protrusion  16 L 1  and  16 L 2  are not opposed to any of the protrusions  8 U,  9  and  8 L of the rotor portion  2 , an upper half of the protrusion  9  of the rotor portion  2  is opposed to the protrusion  16 U 2  of the stator portion  3  and the protrusion  8 U of the rotor portion  2  is opposed to the protrusion  16 U 1  of the stator portion  3 . 
         [0052]    As shown in  FIG. 10 , the state S 3  can be interpreted as an upside down state of the state S 1  shown in  FIG. 6 . That is, since the stator yoke  12 L of the stator portion  3  is not affected so much by the attractive force from the protrusions  9  and  8 L of the rotor portion  2 , the magnetic flux from the noth pole of the permanent magnet  15 L forms a magnetic path M 8  (shown by dashed-dotted lines) passing around the hollow portion  13 L and returning to the south pole of the permanent magnet  15 L. At this time, since the magnetic flux of the permanent magnet  7 L of the rotor portion  2  is strongly affected by the protrusions  16 U 1  and  16 U 2  of the stator yoke  12 U of the stator portion  3  rather than the protrusions  16 L 1  and  16 L 2  of the stator yoke  12 L, most of the magnetic flux of rotor portion  2  is directed to the stator yoke  12 U side. Since the protrusions  16 U 1  and  16 U 2  of the stator portion  3  become very close to the protrusions  8 L and  9 L of the rotor portion  2 , most of the magnetic flux of the permanent magnet  7 U of the rotor portion  2  directs from the protrusion  8 U of the rotor portion  2  to the stator yoke  12 U side. Thus, as shown in  FIG. 10 , a magnetic path M 9  is formed in the rotor portion  2  side starting from the north pole of the lower end face of the permanent magnet  7 L, passing around the hollow portion  5  from the vicinity of the lower end face of the rotor yoke  5 , once entering into the stator yoke  2  side at the vicinity of the upper end face of the rotor yoke  4 , entering into the stator yoke  12 U side at the vicinity of the lower end face of the stator yoke  12 U, and returning to the noth pole of the upper end face of the permanent magnet  7 L. 
         [0053]    A magnetic path M 10  is also formed such that the magnetic flux of the permanent magnet  15 U of the stator yoke  12 U of the stator portion  3  directs to the protrusion  9  of the rotor portion  2  and the magnetic flux of the permanent magnet  7 U of the rotor portion  2  directs from the protrusion  8 U of the rotor portion  2  to the protrusion  16 U 1  of the stator portion  3 . 
         [0054]    In the state S 3  as described above, changes are happened on the wound portions  17 U and  17 L such that most of the magnetic flux of the permanent magnet  15 U having passed around the wound portion  17 U until just before the state S 3  is absorbed into the rotor portion  2  side to become weak and that the magnetic flux of the permanent magnet  15 L having directed to the rotor portion  2  side until just before the state S 3  returns to the magnetic path passing around the wound portion  17  U of the stator portion  3  to become intensive to form a magnetic path M 8 . Accordingly, an electric current flows through the wound portions  17 U and  17 L so as to generate magnetic lines counteracting the changes of the magnetic flux. 
         [0055]      FIG. 11  shows a positional relationship between the protrusions  8 U,  9  and  8 L of the rotor portion  2  and the protrusions  16 U 1 ,  16 U 2 ,  16 L 1  and  16 L 2  of the stator portion  3 , along with a process of transitions of the states S 1 , S 2  and S 3  from S 1  through S 2  to S 3  over passing time of t 1  to t 5 .  FIG. 12  shows a state of the electric current generated on the wound portions  17 U and  17 L of the electric generator  1  at each time of t 1  to t 5 . As shown in  FIG. 12 , an electric current occurs in a sinusoidal wave form by rotating the rotor portion  2  in a constant rate. In the example shown in  FIG. 12 , the current is generated in the opposite directions at the wound portions  17 U and  17  L because these wires are wound in a same direction and the directions of the magnetic lines on the wound portions  17 U and  17 L are opposing. If the winding directions of the wound portions  17 U and  17  L are made opposite, it can be generate the current in a same direction on the wound portions  17 U and  17  L. 
         [0056]    In  FIG. 12 , the time t 1  corresponds to the state S 1  shown in  FIG. 6 . At the time t 1 , since a lower half of the protrusion  9  of the rotor portion  2  and the protrusion  8 L of the same are opposed to the protrusions  16 L 1  and  16 L 2  of the stator portion  3 , and the magnetic flux around the wound portion  17 U becomes a maximum and then decreases, an electric current starts to flow in a direction through the wire so as to generate magnetic lines counteracting the change of the magnetic flux. Defining this direction as positive, a positive current is starting to flow. It is shown in  FIG. 12  by a point W 11 . On the other hand, since the magnet flux around the wound portion  17 L becomes a minimum and then increases, an electric current starts to flow through the wire so as to generate magnetic lines counteracting the change of the magnetic flux. This direction is the reverse of that of the wound portion  17 U. This reverse direction is defined as negative in opposition to the positive described above. It is shown in  FIG. 12  by a point W 12 . At this time, the lower part of the rotor portion  2  is generating a maximum attractive force with the stator portion  3 . 
         [0057]    Therefore, at time t 1 , as shown in  FIG. 12 , since reversal of the direction of current flow occurs on the wound portion  17 U and  17 L  17 L, it takes an intermediate point of positive and negative values in the AC curve. That is, at the time t 1 , as shown in  FIG. 12 , neither positive current nor negative current flows at the wound portions  17 U and  17 L (minimum current value=0 amp). 
         [0058]    In  FIG. 12 , the time t 5  corresponds to the state S 3  shown in  FIG. 10 . At the time t 5 , since an upper half of the protrusion  9  of the rotor portion  2  and the protrusion  8 U of the same are opposed to the protrusions  16 U 1  and  16 L 2  of the stator portion  3 , and the magnetic flux around the wound portion  17 L becomes a maximum and then decreases, an electric current starts to flow in a direction through the wire so as to generate magnetic lines counteracting the change of the magnetic flux. This direction may be defined as negative in accordance with the above mentioned basis. It is shown in  FIG. 12  by a point W 12 . On the other hand, since the magnet flux around the wound portion  17 U becomes a minimum and then increases, an electric current starts to flow through the wire so as to generate magnetic lines counteracting the change of the magnetic flux. This direction is the reverse of that of the wound portion  17 L. This direction is positive in relation to the negative described above. It is shown in  FIG. 12  by a point W 22 . At this time, the upper part of the rotor portion  2  is generating a maximum attractive force with the stator portion  3 . 
         [0059]    Therefore, at the time t 5  as shown in  FIG. 12 , a current of the positive maximum (W 12 ) is generated at the wound portion  17 U and a current of the negative maximum (W 22 ) is generated at the wound portion  17 L. 
         [0060]    The time t 3  corresponds to the state S 2  shown in  FIG. 8 . At the time t 3 , each half of the protrusions  8 U,  9  and  8 L of rotor portion  2  in the circumferential direction is opposed to each half of the protrusions  16 U 1 ,  16 U 2 ,  16 L 1  and  16 L 2  in the same direction. The point of the time t 3  is an intermediate point of the transition from the state S 1  to the state S 5 . Therefore, at the time as shown in  FIG. 12 , a current is generated in a medial value between the maximum and minimum (0 amp) values in positive or negative at each of the wound portions  17 U and  17 L. 
         [0061]    The times t 2  and t 4  correspond to transitions from the state S 1  to the state S 2  and from the state S 2  to the state S 3  respectively. Therefore, at the time t 2  as shown in  FIG. 12 , a current is generated in a medial value between the current values generated at the state S 1  (0 amp) and at the state S 2 . On the other hand, at the time t 4  as shown in  FIG. 12 , a current is generated in a medial value between the current values generated at the state S 2  and at the state S 1  (0 amp). 
         [0062]    In this way, at the times t 1  to t 5 , a current occurs in a quarter cycle of a sinusoidal waveform to be generated on the wound portions  17 U and  17 L. That is, when the rotation shaft  10  of the electric generator  1  is rotating in a constant rate, one cycle part of the sinusoidal current waveform to be generated on the wound portion  17 U and  17 L is completed in four times periods of the times t 1  to t 5 . 
         [0063]    Here, considering the reason why cogging is generated in a general electric generator, cogging would be caused by changing of attractive or repulsive force between a rotor side and a stator side according to the rotor position. According to the electric generator  1  of this embodiment on the other hand, the attractive force will be constant between protrusions  8 U,  8  and  8 L of the rotor portion  2  and protrusions  16 U 1 ,  16 U 2 ,  16 L 1  and  16 L 2  of the stator portion  3  in any positional relationship of the rotor portion  2  and the stator portion  3  at the times t 1  to t 5 . 
         [0064]    For example, at the time t 1  (the state S 1 ), the lower half of the protrusion  9  of the rotor portion  2  and the protrusion  8 L of the same are fully opposed to the protrusions  16 L 1  and  16 L 2  of the stator portion  3  and are attracting them in the maximum attractive force. On the other hand, the protrusion  16 U 1  and  16 U 2  of the stator portion  3  are not opposed to any of the protrusions  8 U,  9  and  8 L of the rotor portion  2  and the attractive force therebetween becomes a minimum. Therefore, it can be considered that the strength of the attractive force between the protrusions  8 U,  9  and  8 L of the rotor portion  2  side and the protrusions  16 U 1 ,  16 U 2 ,  16 L 1  and  16 L 2  of the stator portion  3  side depend on the total of the strengths of the opposing and non-opposing areas therebetween. 
         [0065]    Supposing Q cm 2  (square centimeter) as a maximum value of an area where the protrusion  16 U 1 ,  16 U 2 ,  16 L 1  or  16 L 2  of the stator portion  3  is opposed to the protrusions  8 U,  9 , and  8 L of the rotor portion  2 , and P 1  as strength of attractive force per 1 cm 2  unit, then, at the time t 1 , the opposing area between the lower half of the protrusion  9  of the rotor portion  2  and the protrusion  16 L 1  of the stator portion  3  becomes Q cm 2  and the attractive force becomes Q×P 1 . The opposing area between the protrusion  8 L of the rotor portion  2  and the protrusion  16 L 2  of the stator portion  3  also becomes Q cm 2  and the attractive force becomes QxP 1 . The non-opposing area between the protrusion  8 L of the rotor portion  2  and the protrusion  16 U 1  of the stator portion  3  and the non-opposing area between the upper half of the protrusion  9  of the rotor portion  2  and the protrusion  16 U 2  of the stator portion  3  are Q cm 2  respectively and the attractive forces become Q×P 2  respectively where P 2  is strength of attractive force per 1 cm 2  unit. Therefore, the attractive force between the rotor portion  2  and stator portion  3  becomes 2Q×P 1 +2Q×P 2 =2Q(P 1 +P 2 ) at the time t 1  on each of the protrusions  8 U,  9 ,  8 L,  16 U 1 ,  16 U 2 ,  16 L 1  and  16 L 2 . 
         [0066]    Similarly, at the time t 2 , the protrusion  8 U of the rotor portion  2  is opposed to the protrusions  16 U 1  of the stator portion  3  at respective quarters in the circumferential direction, the protrusion  9  of the rotor  2  is opposed to the protrusion  16 U 2  of the stator portion  3  at a quarter in the circumferential direction of the upper half of the former and a quarter in the same direction of the latter, the protrusion  9  of the rotor  2  is opposed to the protrusion  16 L 1  of the stator portion  3  at three quarters in the circumferential direction of the lower half of the former and three quarters in the same direction of the latter, and the protrusion  8 L of the rotor  2  is opposed to the protrusion  16 L 2  of the stator portion  3  at three quarters in the circumferential direction of the former and three quarters in the same direction of the latter to attract each other. 
         [0067]    In this case, the opposing areas are (¼)Q cm 2  for between the protrusion  8 U of the rotor portion  2  and the protrusion  16 U 1  of the stator  3  and for between the upper half of the protrusion  9  of the rotor portion  2  and the protrusion  16 U 2  of the stator  3  respectively, and (¾)Q cm 2  for between the lower half of the protrusion  9  of the rotor portion  2  and the protrusion  16 L 1  of the stator  3  and for between the protrusion  8 L of the rotor portion  2  and the protrusion  16 L 2  of the stator  3  respectively. Therefore, at the time t 1 , the total attractive force between the rotor portion  2  and stator portion  3  on the opposing parts of each of the protrusions  8 U,  9 ,  8 L,  16 U 1 ,  16 U 2 ,  16 L 1  and  16 L 2  becomes as follows: 
         [0000]    
       
         
           
             
               
                 
                   ( 
                   
                     1 
                     / 
                     4 
                   
                   ) 
                 
                  
                 Q 
                  
                 
                     
                 
                  
                 
                   cm 
                   2 
                 
                 × 
                 P 
                  
                 
                     
                 
                  
                 1 
               
               + 
               
                 
                   ( 
                   
                     1 
                     / 
                     4 
                   
                   ) 
                 
                  
                 Q 
                  
                 
                     
                 
                  
                 
                   cm 
                   2 
                 
                 × 
                 P 
                  
                 
                     
                 
                  
                 1 
               
               + 
               
                 
                   ( 
                   
                     3 
                     / 
                     4 
                   
                   ) 
                 
                  
                 Q 
                  
                 
                     
                 
                  
                 
                   cm 
                   2 
                 
                 × 
                 P 
                  
                 
                     
                 
                  
                 1 
               
               + 
               
                 
                   ( 
                   
                     3 
                     / 
                     4 
                   
                   ) 
                 
                  
                 Q 
                  
                 
                     
                 
                  
                 
                   cm 
                   2 
                 
                 × 
                 P 
                  
                 
                     
                 
                  
                 1 
               
             
             = 
             
               
                 
                   ( 
                   
                     8 
                     / 
                     4 
                   
                   ) 
                 
                  
                 Q 
                  
                 
                     
                 
                  
                 
                   cm 
                   2 
                 
                 × 
                 P 
                  
                 
                     
                 
                  
                 1 
               
               = 
               
                 2 
                  
                 Q 
                  
                 
                     
                 
                  
                 
                   cm 
                   2 
                 
                 × 
                 P 
                  
                 
                     
                 
                  
                 1 
               
             
           
         
       
     
         [0000]    On the other hand, the attractive force of the non-opposing parts can be similarly calculated to become 2Qcm 2 ×P 1 . Therefore, the attractive force at the time  2  becomes 2Q×P 1 +2Q×P 2 =2Q(P 1 +P 2 ) same as that at the time t 1 . 
         [0068]    Similarly, at time t 3 , the protrusion  8 U of the rotor portion  2  is opposed to the protrusions  16 U 1  of the stator portion  3  at respective halves in the circumferential direction, the protrusion  9  of the rotor  2  is opposed to the protrusion  16 U 2  of the stator portion  3  at a half in the circumferential direction of the upper half of the former and a half in the same direction of the latter, the protrusion  9  of the rotor  2  is opposed to the protrusion  16 L 1  of the stator portion  3  at a half in the circumferential direction of the lower half of the former and a half in the same direction of the latter, and the protrusion  8 L of the rotor  2  is opposed to the protrusion  16 L 2  of the stator portion  3  at respective halves in the circumferential direction to attract each other. 
         [0069]    In this case, the opposing areas are (¼)Q cm 2  for between the protrusion  8 U of the rotor portion  2  and the protrusion  16 U 1  of the stator  3 , for between the upper half of the protrusion  9  of the rotor portion  2  and the protrusion  16 U 2  of the stator  3 , for between the lower half of the protrusion  9  of the rotor portion  2  and the protrusion  16 L 1  of the stator  3  and for between the protrusion  8 L of the rotor portion  2  and the protrusion  16 L 2  of the stator  3  respectively. Therefore, at the time t 3 , the total attractive force between the rotor portion  2  and stator portion  3  on the opposing parts of each of the protrusions  8 U,  9 ,  8 L,  16 U 1 ,  16 U 2 ,  16 L 1  and  16 L 2  becomes as follows: 
         [0000]    
       
         
           
             
               
                 
                   ( 
                   
                     1 
                     / 
                     2 
                   
                   ) 
                 
                  
                 Q 
                  
                 
                     
                 
                  
                 
                   cm 
                   2 
                 
                 × 
                 P 
                  
                 
                     
                 
                  
                 1 
               
               + 
               
                 
                   ( 
                   
                     1 
                     / 
                     2 
                   
                   ) 
                 
                  
                 Q 
                  
                 
                     
                 
                  
                 
                   cm 
                   2 
                 
                 × 
                 P 
                  
                 
                     
                 
                  
                 1 
               
               + 
               
                 
                   ( 
                   
                     1 
                     / 
                     2 
                   
                   ) 
                 
                  
                 Q 
                  
                 
                     
                 
                  
                 
                   cm 
                   2 
                 
                 × 
                 P 
                  
                 
                     
                 
                  
                 1 
               
               + 
               
                 
                   ( 
                   
                     1 
                     / 
                     2 
                   
                   ) 
                 
                  
                 Q 
                  
                 
                     
                 
                  
                 
                   cm 
                   2 
                 
                 × 
                 P 
                  
                 
                     
                 
                  
                 1 
               
             
             = 
             
               
                 
                   ( 
                   
                     4 
                     / 
                     2 
                   
                   ) 
                 
                  
                 Q 
                  
                 
                     
                 
                  
                 
                   cm 
                   2 
                 
                 × 
                 P 
                  
                 
                     
                 
                  
                 1 
               
               = 
               
                 2 
                  
                 Q 
                  
                 
                     
                 
                  
                 
                   cm 
                   2 
                 
                 × 
                 P 
                  
                 
                     
                 
                  
                 1 
               
             
           
         
       
     
         [0000]    On the other hand, the attractive force of the non-opposing parts, which are parts of the stator portion  3  other than the protrusions  16 U 1 ,  16 U 2 ,  16 L 1  and  16 L 2  being opposed to the protrusions  8 U,  8  and  8 L, becomes 2Qcm 2 ×P 2  as the area of the non-opposing parts becomes Q2 cm 2 . Therefore, the total attractive force at the time t 3  becomes 2Q(P 1 +P 2 ) same as that at the times t 1  and t 2 . 
         [0070]    Similarly, at time t 4 , the protrusion  8 U of the rotor portion  2  is opposed to the protrusions  16 U 1  of the stator portion  3  at respective three quarters in the circumferential direction, the protrusion  9  of the rotor  2  is opposed to the protrusion  16 U 2  of the stator portion  3  at three quarters in the circumferential direction of the upper half of the former and three quarters in the same direction of the latter, the protrusion  9  of the rotor  2  is opposed to the protrusion  16 L 1  of the stator portion  3  at a quarter in the circumferential direction of the lower half of the former and a quarter in the same direction of the latter, and the protrusion  8 L of the rotor  2  is opposed to the protrusion  16 L 2  of the stator portion  3  at respective quarters in the circumferential direction to attract each other. 
         [0071]    In this case, the opposing areas are (¾)Q cm 2  for between the three quarters of the protrusion  8 U of the rotor portion  2  and the three quarters of the protrusion  16 U 1  of the stator  3  and for between the three quarters of the upper half of the protrusion  9  of the rotor portion  2  and the three quarters of the protrusion  16 U 2  of the stator  3  respectively, and (¼) Q cm 2  for between the quarter of the lower half of the protrusion  9  of the rotor portion  2  and the quarter of the protrusion  16 L 1  of the stator  3  and for between the quarter of the protrusion  8 L of the rotor portion  2  and the quarter of the protrusion  16 L 2  of the stator  3  respectively. Therefore, at the time t 4 , the total area of the fully opposing parts of the protrusions  8 U,  9 ,  8 L,  16 U 1 ,  16 U 2 ,  16 L 1  and  16 L 2  between the rotor portion  2  and stator portion  3  is as follows: 
         [0000]    
       
         
           
             
               
                 
                   ( 
                   
                     3 
                     / 
                     4 
                   
                   ) 
                 
                  
                 Q 
                  
                 
                     
                 
                  
                 
                   cm 
                   2 
                 
               
               + 
               
                 
                   ( 
                   
                     3 
                     / 
                     4 
                   
                   ) 
                 
                  
                 Q 
                  
                 
                     
                 
                  
                 
                   cm 
                   2 
                 
               
               + 
               
                 
                   ( 
                   
                     1 
                     / 
                     4 
                   
                   ) 
                 
                  
                 Q 
                  
                 
                     
                 
                  
                 
                   cm 
                   2 
                 
               
               + 
               
                 
                   ( 
                   
                     1 
                     / 
                     4 
                   
                   ) 
                 
                  
                 Q 
                  
                 
                     
                 
                  
                 
                   cm 
                   2 
                 
               
             
             = 
             
               
                 
                   ( 
                   
                     8 
                     / 
                     4 
                   
                   ) 
                 
                  
                 Q 
                  
                 
                     
                 
                  
                 
                   cm 
                   2 
                 
               
               = 
               
                 2 
                  
                 Q 
                  
                 
                     
                 
                  
                 
                   cm 
                   2 
                 
               
             
           
         
       
     
         [0000]    Therefore, the attractive force caused by the protrusions being opposed each other becomes 2Q×P 1 . On the other hand, the area of the protrusions  8 U,  9  and  8 L of the rotor portion  2  being opposed to the protrusions  16 U 1 ,  16 U 2 ,  16 L 1  and  16 L 2  of the stator portion  3  becomes 2Qcm 2  so that the total attractive force becomes 2Q(P 1 +P 2 ) same as that at the times t 1 , t 2  and t 3 . 
         [0072]    Similarly, at time t 3  (the state S 3 ), the upper half of the protrusion  9  of the rotor  2  and the protrusion  8 U of the same is opposed to the protrusions  16 U 1  and  16 U 2  of the stator portion  3  and attract therebetween, but the protrusions  16 L 1  and  16 L 2  of the stator portion  3  are not opposed to any of the protrusions  8 U,  9  and  8 L of the rotor portion  2 . In this case, the opposing areas are Q cm 2  for between the upper half of the protrusion  9  of the rotor portion  2  and the protrusion  16 U 2  of the stator  3  and for between the protrusion  8 U of the rotor portion  2  and the protrusion  16 U 1  of the stator  3  respectively. Therefore, at the time t 5 , the total area of the fully opposing parts of the protrusions  8 U,  9 ,  8 L,  16 U 1 ,  16 U 2 ,  16 L 1  and  16 L 2  between the rotor portion  2  and stator portion  3  is as follows: 
         [0000]        Q cm 2   +Q cm 2 =2 Q cm 2    
         [0073]    Therefore, the attractive force caused by the protrusions being opposed each other becomes 2Q×P 1 . On the other hand, the area of the protrusions  8 U,  9  and  8 L of the rotor portion  2  being opposed to the protrusions  16 U 1 ,  16 U 2 ,  16 L 1  and  16 L 2  of the stator portion  3  becomes 2Qcm 2  so that the total attractive force becomes 2Q(P 1 +P 2 ) same as that at the times t 1 , t 2 , t 3  and t 3 . 
         [0074]    As described above, the total attractive force between the rotor portion  2  and the stator portion  3  on each of the protrusions  8 U,  9 ,  8 L,  16 U 1 ,  16 U 2 ,  16 L 1  and  16 L 2  does not change at any of the times t 1  to t 5  shown in  FIGS. 11 and 12 . The current waveform occurring at the times t 1  to t 5  is a waveform of a quarter cycle of the sinusoidal waveform to be generated on the wound portions  17  U and  17 L. A sinusoidal wave is to be continued by the quarter cycle of the current waveform with changing the increasing and decreasing direction and the positive and negative direction. Therefore, it is obvious that the attractive force becomes constant at all times between the protrusions  8 U,  9  and  8 L of the rotor portion  2  and the protrusions  16 U 1 ,  16 U 2 ,  16 L 1  and  16 L 2  of the stator portion  3  at any part of the sinusoidal current waveform to be generated on the wound portions  17 U and  17  L. This means that the attractive force is always constant between the rotor portion  2  and the stator portion  3  even how the positional relationship thereof is. Therefore it is obvious that no cogging occurs in the electric generator  1 . 
         [0075]    The electric generator in which no cogging occurs as described above can generate power in high efficiency because rotation torque applied on the rotation shaft  10  from outside is not reduced by cogging torque and most of it is used a torque for electric generation. 
         [0076]    In the electric generator  1 , the hollow portion  5  of the rotor portion  2  is continuously passing through the two permanent magnets  7 U and  7 L. According to this, as shown in  FIGS. 6 and 10 , the magnetic flux of the two permanent magnets  7 U and  7 L of the rotor portion  2  forms one magnetic path M 2  or M 9  without the respective magnetic fluxes of the permanent magnets  7 U and  7 L being divided. On one side of the permanent magnets  7 U and  7 L, the magnetic fluxes are direct into the stator portion  3  integrally so that magnetic flux passing around the wound portion  17 U or  17 L can be reduced almost perfectly while, on the other side the permanent magnets  7 U and  7 L, less of the magnetic flex direct into the stator portion  3  so that the flux passing around the permanent magnet  7 U or  7 L can be used up to the maximum. Consequently, power generating efficiency can be further improved. 
         [0077]      FIG. 13  shows, as a comparative example, a rotor portion  2 A comprising two hollow portions  5 U and  5 L.  FIG. 13  shows an example in the state S 1  same as  FIG. 6 , wherein loops of magnetic fluxes of the respective permanent magnets  7 U and  7 L of the rotor portion  2 A are divided each other and magnetic paths M 11  and M 12  are formed as individual closed loops. As a result, a magnetic path M 13  occurs around the wound portion  17 U and a magnetic path M 14  occurs around the wound portion  17 L. It hardly changes at any state of the times t 1  to t 5  shown in  FIG. 11 . It makes the difference between the maximum and the minimum of the magnetic path occurring around the wound portion  17 U and  17 L small so that the electric generator  1 A cannot fully function for a generator. However, some extent of magnetic flux may occur in this configuration, this configuration may be adopted. 
       Other Embodiments 
       [0078]    The embodiment described above can be modified in various ways without departing from the scope of the invention. For example, the hollow portion  5  may not be hollow and be embedded by non-magnetic material such as aluminum or resin material. Although it has been described that each of the protrusions  8 U,  9  and  8 L is formed integrally with the body of the yoke  4  and each of the protrusions  16 U 1 ,  16 U 2 ,  16 L 1  and  16 L 2  is formed integrally with the body of the yoke  12 U and  12 L, each protrusion may not be formed integrally but be made as a separated member and fixed on the respective body by such as glue. 
         [0079]    In the embodiment shown in  FIG. 14 , a rotor portion  2 B comprises four protrusions  8 U 1 ,  8 U 2 ,  8 L 1  and  8 L 2  along the axial direction and the protrusions  8 U 1  and  8 U 2  are arranged to shift by a half pitch with the protrusions  8 L 1  and  8 L 2  on the axial direction. On the other hand, the stator portion  3 B comprises three protrusions  16 U,  16 M and  16 L along the axial direction. Configuring such an electric generator  1 B, it can be realize the function same as that of the electric generator  1  according to the above described embodiment. 
         [0080]    In this case, the rotor portion  2 B may be configured by arranging to pile one rotor member having protrusions  8 U 1  and  8 U 2  along the central axis direction with a same pitch and another rotor member having protrusions  8 L 1  and  8 L 2  with the same pitch linearly arranged in the axial direction, in two stages, such that the protrusion  8 U 1  and  8 U 2  are shifted a half pitch with the protrusions  8 L 1  and  8 L 2 . The stator portion  3 B may also be configured by arranging to pile one stator member having protrusions  16 U and  16 M 1  with a same pitch linearly arranged in the axial direction and another stator member having protrusions  16 M 2  and  16 L with the same pitch linearly arranged in the axial direction, in two stages, such that the protrusion  16 U,  16 M 1 ,  16 M 2  and  16 L are linearly arranged in the axial direction and have the same pitch. The rotor portion  2 B and/or the stator portion  3 B may be made from on yoke member by, for example, cutting out. 
         [0081]    Although it has been described that each of the electric generators  1  and  1 B comprises the stator portion  3  having a two stage piled structure of the stator yokes  12 U and  12 L, the number of the piled stages may be any even number. In this case, the rotor portion part  2  preferably has a construction in that the structure shown in  FIG. 14  is piled along the axial direction. Similarly, although it has been described that the electric generator  1 B comprises the rotor portion  2 B consisting of a rotor member of a two stage piled structure, the rotor portion  2 B may be preferred to be piled same structures and the number of the piled stages may be any even number. 
         [0082]    Although it has been described that each of the electric generators  1  and  1 B is an inner rotor type generator, an outer rotor type generator may be adopted. Although the rotor yoke  4  and the stator yokes  12 U,  12 L are preferable made of soft magnetic material, these may be made by simple magnetic material. Grooves  6 U,  6 L,  14 U and  14 L may not be grooved if each of them is formed integrally with the permanent magnet but these are called as “grooves” including ones formed integrally. Although the axial length of the hollow portion  5  as a non-magnetic portion is set equal or longer than the axial distance between the outer ends of the combination of the permanent magnetics  7 U and  7 L, it may be equal to, slightly longer or slightly shorter that the axial distance between the outer ends of the permanent magnetics  7 U and  7 L. Although the width of each protrusion and the distance between protrusions are preferably equal for cogging to be suppressed, the width the distance may not be equal and be set slightly different by another requirement. 
         [0083]    As shown in  FIG. 15 , an electric generator  1 C may be configured by comprising stator yokes  12 CU and  12 CL on which the permanent magnets  15 U, and  15 L are not provided. That is, the electric generator  1 C is configured so as to use the spaces where the permanent magnetics  15 U, and  15 L are removed from the stator portion  3  of the electric generator  1  as a non-magnetic portion. This space may be filled by resin, aluminum which is a non-magnetic material, or the like. Although the electric generator operates similar to the electric generators  1 ,  1 B according to the embodiments described above, the current waveform shown in  FIG. 12  should be inversed at the wound portions respectively because the permanent magnets  15 U and  15 L are not provided ion the stator portion  3 C. 
         [0084]    The state shown in  FIG. 15 , for example, corresponds to the state S 1  in the electric generator  1  shown in  FIG. 6 . Considering the case of the state S 1  in the electric generator  1  shown in  FIG. 6 , since the stator yoke  12 L has the permanent magnet  15 L, the magnetic flux of the permanent magnet  15 L is sucked into the rotor portion  2  side so that the magnetic flux around the wound portion  17 L becomes weak. On the other hand, in the electric generator  1 C, since the permanent magnet  15 L is not provided on the stator yoke  12 CL, the magnetic flux of the permanent magnet  7 L of the rotor portion  2  extends through the opposing protrusions  9 ,  8 L and  16 L 1 ,  16 L 2  into the stator yoke  12 CL side. As a result, a magnetic path M 15  is formed and the magnetic flux around the wound portion  17 L becomes intensive. 
         [0085]    Similarly, in the electric generator  1 , since the permanent magnet  15 U is provided on the stator yoke  12 U, the magnetic flux of the permanent magnet  15 U forms the magnetic path M 1  around the wound portion  17 U on the state S 1  so that the magnetic flux around the wound portion becomes intensive. On the other hand, in the electric generator  1 C, since the permanent magnet  15 U is not provided on the stator yoke  12 CU, the magnetic flux of the permanent magnet  7 U of the rotor portion  2 U extends into the stator yoke  12 CU side to form the magnetic path M 16  but the protrusions  16 U 1  and  16 U 2  of the stator portion  3 C are not opposd to the protrusions  8 U and  9  so that the magnetic flux around the wound portion  17 U becomes weak. 
         [0086]    As described above, it can been seen that the intensity states of the magnetic flux on the wound portion  17 U and  17 L are reversed between the electric generators  1  and  1 C. Thus, the waveform shown in  FIG. 12  is reversed on the wound portions respectively between the electric generators  1  and  1 C.