Patent Abstract:
First and second stators are arranged on mutually spaced positions of an imaginary common axis, and first and second rotors are coaxially and rotatably arranged on mutually spaced positions of the imaginary common axis between the first and second stators.

Full Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates in general to electric motors and more particularly to the electric motors of an axial gap type which comprises a rotor shaft that is rotatable about its axis, at least one rotor that is fixed to the rotor shaft to rotate therewith and at least one stator that is disposed around the rotor shaft and axially spaced from the rotor. 
   2. Description of the Related Art 
   Hitherto, various radial gap electric motors have been proposed and put into practical use particularly in the field of power generators that need higher power density and lower heat generation. Some of them are disclosed in Japanese Laid-open Patent Applications, Tokkaihei 11-341758 and Tokkai 2000-224836. 
   The motor shown by 11-341758 comprises a cylindrical stator, an outer rotor rotatably disposed around the cylindrical stator and an inner rotor rotatably disposed in the cylindrical stator. While, the motor shown by 2000-224836 comprises a cylindrical stator, an outer rotor rotatably arranged in a diametrically outer side in the cylindrical stator and an inner rotor rotatably arrange in a diametrically inner side in the cylindrical stator. 
   In both of the motors mentioned hereinabove, by feeding the stator with a compound current, the outer and inner rotors are forced to rotate independently from each other. 
   In the motor of the latter reference, viz., 2000-224836, it is considerably difficult to provide the inner rotor with a sufficient size due to the inevitably limited space defined in the diametrically inner area of the cylindrical stator, and thus, it is difficult to expect a sufficient torque from such small sized inner rotor. In view of this drawback, electric motors of the type of the former reference, viz., 11-341758 have been widely used in these days. 
   SUMMARY OF THE INVENTION 
   Besides the above-mentioned radial gap electric motors, axial gap electric motors are also known, some of which are of a double rotor type having two rotors that are rotatable independently from each other. In this type electric motor, the two rotors are rotatably arranged relative to a fixed single stator. Two output shafts or the like are used for the respective two rotors, that are coaxially installed for receiving the respective torque of the two rotors. 
   When the axial gap electric motors of the above-mentioned two rotor type are constructed to be powered by a compound current, it is necessary to arrange the two rotors at axially opposed sides of the stator respectively. However, in this case, stable supporting or holding of the stator relative to a motor case is quite difficult because of a complicated arrangement that is inevitably needed by the two rotors, the two output shafts and the single stator on a common axis. 
   Accordingly, it is an object of the present invention to provide an axial gap electric motor of two rotor type, which is free of the above-mentioned drawbacks. 
   That is, in accordance with the present invention, there is provided an axial gap electric motor which comprises two stators that are axially arranged and two rotors that are axially arranged between the two stators. 
   In accordance with a first aspect of the present invention, there is provided an axial gap electric motor which comprises first and second stators that are arranged on mutually spaced positions of an imaginary common axis in a manner to face each other; and a plurality of rotors that are coaxially and rotatably arranged on mutually spaced positions of the imaginary common axis between the first and second stators. 
   In accordance with a second aspect of the present invention, there is provided an axial gap electric motor which comprises annular first and second stators that are coaxially arranged around an imaginary common axis; annular first and second rotors that are coaxially and rotatably arranged on the imaginary common axis between the first and second stators; a hollow first rotor shaft that is rotatable about the imaginary common axis and has one axial end secured to a center portion of the first rotor to rotate therewith; a second rotor shaft that is rotatably received in the hollow first rotor shaft and has one axial end secured to a center portion of the second rotor to rotate therewith; and a case that houses therein the first and second stators, the first and second rotors, and the first and second rotor shafts. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematically illustrated sectional view of an axial gap electric motor of a first embodiment of the present invention; 
       FIG. 2A  is a schematically illustrated plan view of a part of a first rotor employed in the electric motor of the first embodiment of  FIG. 1 ; 
       FIG. 2B  is a development provided by developing given portions of the first rotor of  FIG. 2A  in a circumferential direction; 
       FIG. 3A  is a view similar to  FIG. 2A , but showing a part of a second rotor employed in the electric motor of the first embodiment of  FIG. 1 ; 
       FIG. 3B  is a development provided by developing given portions of the second rotor of  FIG. 3A  in a circumferential direction; 
       FIG. 4A  is a view similar to  FIG. 2A , but showing a part of a first rotor employed in an axial gap electric motor of a second embodiment of the present invention; 
       FIG. 4B  is a development provided by developing given portions of the first rotor of  FIG. 4A  in a circumferential direction; 
       FIG. 5A  is a view similar to  FIG. 4A , but showing a part of a second rotor employed in the electric motor of the second embodiment of the present invention; 
       FIG. 5B  is a development provided by developing given portions of the second rotor of  FIG. 5A  in a circumferential direction; 
       FIG. 6  is a view similar to  FIG. 1 , but showing an axial gap electric motor of a third embodiment of the present invention; 
       FIG. 7A  is a schematically illustrated plan view of a part of a first or second rotor employed in the electric motor of the third embodiment; 
       FIG. 7B  is a development provided by developing given portions of the rotor of  FIG. 7A  in a circumferential direction; 
       FIG. 8A  is a schematically illustrated plan view of a part of another first or second rotor that is employable in the electric motor of the third embodiment; and 
       FIG. 8B  is a development provided by developing given portions of the rotor of  FIG. 8A  in a circumferential direction. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   In the following, various embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
   For ease of understanding, the following description includes various directional terms, such as, right, left, upper, lower, rightward and the like. However, such terms are to be understood with respect to a drawing or drawings on which a corresponding part or portion is shown. Throughout the specification, substantially the same parts or portions are denoted by the same numerals. 
   Referring to  FIG. 1 , there is shown in a sectioned manner an axial gap electric motor  100  which is a first embodiment of the present invention. 
   Motor  100  comprises a hollow first rotor shaft  21  and a second rotor shaft  22  that is concentrically and rotatably received in first rotor shaft  21 , as shown. 
   First and second circular rotors  31 A and  32 A are concentrically connected to right ends of first and second rotor shafts  21  and  22  respectively, so that first rotor  31 A and first rotor shaft  21  rotate like a single unit, and second rotor  32 A and second rotor shaft  22  rotate like another single unit. 
   An annular first stator  41  and an annular second stator  42  are coaxially arranged around a common axis of first and second rotor shafts  21  and  22  in such a manner as to put therebetween first and second circular rotors  31 A and  32 A. As will be described in detail hereinafter, first and second stators  41  and  42  are secured to axially opposed portions of a motor case  5  respectively. 
   Motor case  5  comprises generally a circular left wall portion  51 , a circular right wall portion  52  and a cylindrical intermediate wall portion  53  that extends between left and right wall portions  51  and  52 , as shown. 
   As shown in the drawing, first stator  41  is located at a left position of first circular rotor  31 A to face a left surface of rotor  31 A, and second stator  42  is located at a right position of second circular rotor  32 A to face a right surface of rotor  32 A. 
   As shown, first rotor shaft  21  is rotatably held by motor case  5  by means of two bearings  61 . While, second rotor shaft  22  is rotatably held by motor case  5  by means of three bearings  62 . Two of bearings  62  are used for a relative rotation between first and second rotor shafts  21  and  22 , as shown. 
   Each of first and second circular rotors  31 A and  32 A comprises a rotor back core  71  or  72 , a plurality of magnets  81  or  82 , a plurality of rotor cores  91  or  92  and an outer frame  101  or  102 , as will be described in detail hereinafter. 
   For tight connection between first rotor shaft  21  and first rotor  31 A, screw bolts  121  are used that extend between a raised annular portion  111  formed on first rotor shaft  21  and a base portion of rotor back core  71 . More specifically, after passing through a hole formed in the base portion of rotor back core  71 , each screw bolt  121  is screwed into a threaded bore formed in raised annular portion  111 . 
   For tight connection between second rotor shaft  22  and second core  32 , screw bolts  122  are used that extend between a raised annular portion  112  formed on the second rotor shaft  22  and a base portion of rotor back core  72 . More specifically, after passing through a hole formed in the base portion of rotor back core  72 , each screw bolt  122  is screwed into a threaded bore formed in raised annular portion  112 . 
   Each of first and second stators  41  and  42  comprises a stator back core  131  or  132 , a plurality of stator cores  141  or  142  and a plurality of stator coils  151  or  152 . 
   For tight connection between first stator  41  and motor case  5 , stator back core  131  is secured to the left wall surface of motor case  5 , and for tight connection between second stator  42  and motor case  5 , stator back core  132  is secured to the right wall surface of motor case  5 , as shown. 
   As shown, around a left end portion of first hollow rotor shaft  21 , there is arranged a first encoder device  161  that senses an angular position of first rotor shaft  21 . Around a right end portion of second rotor shaft  22 , there is arranged a second encoder device  162  that senses an angular position of second rotor shaft  22 . 
   Motor case  5  is formed with a water jacket  17  in and through which cooling water flows to cool the motor  100 . 
   Each of stator cores  141  and  142  is a member in and through which magnetic fluxes flow in a direction of the common axis of first and second rotor shafts  21  and  22 . For producing such magnetic fluxes, each stator coil  151  or  152  is put around the corresponding stator core  141  or  142 . 
   Stator back core  131  or  132  functions to orient the magnetic fluxes of stator cores  141  or  142  around the common axis and force the magnetic fluxes to shift toward another stator core  141  or  142 . 
   It is to be noted that the number of magnetic poles of magnets  81  that constitute first rotor  31 A differs from the number of magnetic poles of magnets  82  that constitute second rotor  32 . Thus, first rotor  31 A and second rotor  32 A can rotate at different rotation speeds independently when first and second stators  41  and  42  are fed with a compound current, like in the above-mentioned radial gap electric motor. 
   The detail of the compound current is described in U.S. Pat. No. 6,291,963 granted to Masaki Nakano on Sep. 18, 2001. 
   When, in operation, first and second stators  41  and  42  are fed with a compound current, first and second rotors  31 A and  32 A are forced to rotate independently. Rotation of first rotor  31 A is transmitted to an external element (not shown) through first rotor shaft  21 , and rotation of second rotor  32 A is transmitted to another external element (not shown) through second rotor shaft  22 . 
   In the following, the construction of first and second rotors  31 A and  32 A will be described in detail with reference to  FIGS. 2A ,  2 B,  3 A and  3 B. 
   Referring to  FIGS. 2A and 2B , there is shown in detail first rotor  31 A. 
     FIG. 2A  is a partial plan view of first rotor  31 A, and  FIG. 2B  is a development provided by developing, in a circumferential direction, portions that show a radius “r 1 ” from a center “C 1 ” of first rotor  31 A. 
   As is seen from these drawings, first rotor  31 A comprises a rotor back core  71 , a plurality of magnets  81  that are put on opposed surfaces of rotor back core  71  in a manner to have magnetic surfaces in an axial direction, rotor cores  91  each being arranged between adjacent two of magnets  81  while piercing rotor back core  71 , and an outer frame  101  that tightly holds magnets  81  and rotor cores  91  relative to rotor back core  71 . 
   As shown, magnets  81  are arranged in a manner to alternatively change the N and S poles in a circumferential direction. 
   Rotor cores  91  are constructed of a magnetic material. 
   In the illustrated first embodiment, magnets  81  are arranged to constitute six pairs of magnet groups. 
   Rotor back core  71  and each rotor core  91  are constructed of a plurality of flat magnetic steel sheets that are put on one another. However, if desired, such core  71  and rotor core  91  may be constructed of a pressed powder magnetic material. 
   As is seen from these drawings, particularly  FIG. 2A , flat magnetic steel sheets of rotor core  91  are piled in a radial direction with respect to center “C 1 ” of first rotor  31 A. 
   Thus, as is understood from the arrows illustrated in  FIG. 2B , under operation of motor  100 , there are produced loops of magnetic flux each flowing from a surface of stator  41  into rotor  31 A and flowing through rotor  31 A in an axial direction. 
   Due to the nature of the magnetic steel sheets piled in the above-mentioned manner, a tendency of shifting flowing of the magnetic flux toward a periphery of rotor  31 A is increased. Accordingly, penetration of the magnetic flux through rotor  31 A (or  32 A) is carried out under the magnetic resistance being reduced in magnitude. Furthermore, due to the same reason, loops of reluctance torque are obtained, which brings about increase in torque of the motor  100  by a degree corresponding to the reluctance torque. 
   Referring to  FIGS. 3A and 3B , there is shown in detail second rotor  32 A. 
     FIG. 3A  is a partial plan view of second rotor  32 A, and  FIG. 3B  is a development provided by developing, in a circumferential direction, portions that show a radius “r 1 ” from a center “C 2 ” of second rotor  32 A. 
   As is seen from the drawings, like in the above-mentioned first rotor  31 A, second rotor  32 A comprises rotor back core  72 , a plurality of magnets  82  that are put on opposed surfaces of rotor back core  72  in a manner to have magnetic surfaces in an axial direction, rotor cores  92  each being arranged between adjacent two magnets  82  while piercing rotor back core  72 , and outer frame  102  that tightly holds magnets  82  and rotor cores  92  relative to rotor back core  72 . Rotor cores  92  are constructed of a magnetic material. 
   In the illustrated first embodiment  100 , magnets  82  are arranged to constitute three pairs of magnet groups. 
   Other constructional features of this second rotor  32 A are substantially the same as those of the above-mentioned first rotor  31 A, and thus, explanation of such constructional features will be omitted. 
   As is seen from  FIGS. 3A and 3B , particularly  FIG. 3A , flat magnetic steel sheets of rotor core  92  are piled in a radial direction with respect to center “C 2 ” of second rotor  32 A. Thus, as is understood from the arrows illustrated in  FIG. 3B , there are produced loops of magnetic flux each flowing from a surface of stator  42  into rotor  32 A and flowing through rotor  32 A in an axial direction. Due to nature of the magnetic steel sheets piled in the above-mentioned manner, a tendency of shifting flowing of the magnetic flux toward a periphery of rotor  32 A is increased, like in the above-mentioned first rotor  31 A. Thus, penetration of the magnetic flux through rotor  32 A is carried out under the magnetic resistance being reduced in magnitude. Furthermore, due to the same reason, loops of reluctance torque are obtained, which induces increase in torque of the motor  100  like in case of first rotor  31 . 
   Referring to  FIGS. 4A and 4B , and  5 A and  5 B, there are shown first and second rotors  31 B and  32 B that are employed in a second embodiment  200  of the present invention. 
   For clarifying a positional relationship between first or second rotor  31 B or  32 B and corresponding first or second stator  41  or  42 , stator cores SC 1 , SC 2 , SC 3 , SC 4 , SC 5  and SC 6  of the stator  41  or  42  are illustrated in  FIG. 4A  or  5 A by broken lines. 
   Referring to  FIGS. 4A and 4B , there is shown first rotor  31 B. 
     FIG. 4A  is a partial plan view showing first rotor  31 B as viewed behind first stator  41  illustrated by broken lines, and  FIG. 4B  is a development provided by developing, in a circumferential direction, portions that show a radius “r 1 ” from a center “C 1 ” of first rotor  31 B. 
   As is seen from these drawings, first rotor  31 B comprises a rotor back core  71 , a plurality of magnets  81  that are put on opposed surfaces of rotor back core  71  in a manner to have magnetic surfaces in an axial direction and an outer frame  101  that tightly holds magnets  81  relative to rotor back core  71 . 
   Also, in this second embodiment, magnets  81  are arranged to constitute six pairs of magnet groups, like in the case of the first embodiment. 
   As is understood from the above, in this second embodiment  200 , means that corresponds to rotor cores  91  employed in the above-mentioned first embodiment  100  is not employed. Thus, as is seen from  FIG. 4B , loops of reluctance torque are not produced and thus generation of reluctance torque is not expected from first rotor  31 B of this second embodiment  200 . 
   Referring to  FIGS. 5A and 5B , there is shown second rotor  32 B. 
     FIG. 5A  is a partial plan view of second rotor  32 B, and  FIG. 5B  is a development provided by developing, in a circumferential direction, portions that show a radius “r 1 ” from a center “C 2 ” of second rotor  32 B. 
   As is seen from the drawings, like in the above-mentioned first rotor  31 B, second rotor  32 B comprises a rotor back core  72 , a plurality of magnets  82  that are put on opposed surfaces of rotor back core  72  in a manner to have magnetic surfaces in an axial direction, and an outer frame  102  that tightly holds magnets  82  relative to rotor back core  72 . 
   In the illustrated second embodiment  200 , magnets  82  are arranged to constitute three pairs of magnet groups. 
   Other constructional features of this second rotor  32 B are substantially the same as those of the above-mentioned first rotor  31 B, and thus, explanation of such constructional features will be omitted. Because of lack of means that corresponds to rotor cores  91 , generation of reluctance torque is not expected from second rotor  32 B of this second embodiment  200 . 
   Referring to  FIG. 6 , there is shown in a sectional manner an axial gap electric motor  300  which is a third embodiment of the present invention. 
   Since motor  300  of this third embodiment is similar in construction to the above-mentioned motor  100  of the first embodiment of  FIG. 1 , only first and second rotors  31 C and  32 C that are different from those of the first embodiment  100  will be described in detail in the following. 
   Referring to  FIGS. 7A and 7B , there is shown first rotor  31 C employed in motor  300  of the third embodiment. 
   As will be described hereinafter, the construction of first rotor  31 C may be used in second rotor  32 C. 
     FIG. 7A  is a partial plan view of first rotor  31 C (or second rotor  32 C), and  FIG. 7B  is a development provided by developing, in a circumferential direction, portions that show a radius “r 1 ” from a center “C 1 ” of first rotor  31 C (or second rotor  32 C). 
   As is seen from the drawings, that is, from  FIGS. 7A and 7B , first rotor  31 C comprises a rotor back core  71 , a plurality of magnets  81  that are put on one surface of rotor back core  71  in a manner to have magnetic surfaces in an axial direction, rotor cores  91  each being arranged between adjacent two magnets  81  while being embedded at one end in core back core  71 , and an outer frame  101  that tightly holds magnets  81  and rotor cores  91  relative to rotor back core  71 . 
   Rotor back core  71  and each of rotor cores  91  are constructed of a plurality of flat magnetic steel sheets that are put on one another. However, if desired, such core  71  and rotor core  91  may be constructed of a pressed powder of magnetic material. Furthermore, if desired, rotor cores  91  may be removed like in the above-mentioned second embodiment  200 . 
   It is to be noted that second rotor  32 C may employ the construction of the above-mentioned first rotor  31 C. 
   Under operation of motor  300 , there are produced loops of magnetic flux as is shown by arrows in  FIG. 7B . 
   Referring to  FIGS. 8A and 8B , there is shown first and second rotors  31 D employable in motor  300  of the third embodiment. 
   As will be described hereinafter, the construction of first rotor  31 D may be used in second rotor  32 D. 
     FIG. 8A  is a partial plan view of first rotor  31 D (or second rotor  32 D), and  FIG. 8B  is a development provided by developing, in a circumferential direction, portions that show a radius “r 1 ” from a center “C 1 ” of first rotor  31 D (or second rotor  32 D). 
   As is seen from these drawings, that is, from  FIGS. 8A and 8B , first rotor  31 D comprises a rotor back core  71 , a plurality magnets  81  that are put on one surface of rotor back core  71  in a manner to have magnetic surfaces in an axial direction, and a plurality of rotor cores  91  each being arranged between adjacent two magnets  81  while piercing rotor back bore  71 . It is to be noted that first rotor  31 D has no means corresponding the above-mentioned outer frame  101  ( FIG. 7A ). 
   As shown in  FIGS. 8A and 8B , each rotor core  91  has at a radially outer end thereof an enlarged flange  91   a  that is snugly received in a recess formed in an annular supporting member  21  that is attached to the other surface of core back core  71 . 
   Each rotor core  91  is welded to annular supporting member  21 . Preferably, annular supporting member  21  is constructed of a non-magnetic metal. 
   As is described hereinabove, the construction of first rotor  31 D may be applied to second rotor  32 D. 
   Due to provision of annular supporting member  21 , first rotor  31 D or second rotor  32 D has a much increased mechanical strength. 
   If both first and second rotors  31 D and  32 D have the above-mentioned construction with annular supporting member  21 , the flow of the magnetic flux between the two rotors  31 D and  32 D is much smoothed. If annular supporting member  21  is constructed of a non-magnetic metal, an eddy-current loss caused by permeation of magnetic flux can be reduced. 
   As will be understood from the foregoing description, in accordance with the present invention, there is provided an axial gap electric motor in which two rotors are independently arranged between two stators. 
   The number of the magnets held by one rotor may be different from that of the magnets held by the other rotor. With this type arrangement, the two rotors can be independently driven while producing substantially the same torque. Because of the two stators are arranged outside of the two rotors, fixing of the two stators to the motor case is easily made. 
   In the above-mentioned embodiments  100 ,  200  and  300 , two rotors, that is, first and second rotors ( 31 A,  32 A), ( 31 B,  32 B), ( 31 C,  32 C) or ( 31 D,  32 D) are arranged between first and second stators  41  and  42 , more than two rotors may be arranged between the two stators  41  and  42 . 
   The entire contents of Japanese Patent Application 2004-230725 filed Aug. 6, 2004 are incorporated herein by reference. 
   Although the invention has been described above with reference to the embodiments of the invention, the invention is not limited to such embodiments as described above. Various modifications and variations of such embodiments may be carried out by those skilled in the art, in light of the above description.

Technology Classification (CPC): 7