Abstract:
An electric motor includes a stator, at least one rotor, an output shaft engaged with the at least one rotor, and a contact area changing device configured to change an area of contact between the at least one rotor and the output shaft, wherein the area of contact between the at least one rotor and the output shaft affects a characteristic resonant frequency of the at least one rotor. A method to reduce vibrations in an electric motor includes engaging an output shaft into at least one rotor, rotating the output shaft and the at least one rotor with respect to a stator, changing an area of contact between the at least one rotor and a radial protrusion of the output shaft, and shifting the resonant frequency of the at least one rotor with the changed area of contact.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority from Japanese Patent Application No. 2007-239268, filed Sep. 14, 2007. The contents of the priority application are hereby incorporated by reference in their entirety. 
       BACKGROUND OF THE DISCLOSURE 
       [0002]    1. Field of the Disclosure 
         [0003]    The present disclosure relates to a technique for preventing an axial gap-type electric motor, which arranges a stator and a disk-shaped rotor to be opposite each other along a direction of an axis of an output shaft, from being resonated by a surface vibration. 
         [0004]    2. Description of the Related Art 
         [0005]    It is known in the art that when a rotational speed of a rotor of a electrical motor reaches a characteristic resonant frequency of the electrical motor, the rotor vibrates, which causes the electrical motor to resonate. 
         [0006]    In this regard, Japanese Laid-Open Patent Publication No. (Hei) 8-107650 (“Patent Document”) discloses a technique for reducing the resonance of the electrical motor. 
         [0007]    The electrical motor disclosed in the above reference changes installation rigidity by adjusting a fastening force of an installation rigidity adjusting bolt, which installs a heat exchanger in a frame of the electrical motor. This changes a characteristic frequency of the entire electrical motor. The technique disclosed in said reference is directed to enable quiet operation of the electric motor by avoiding resonation, even if the installation rigidity of the electrical motor differs from the evaluation conducted at the time of shipment from a factory. 
         [0008]    However, the characteristic frequency of the rotor itself cannot be even by the conventional electrical motor. Particularly, in an axial gap-type electrical motor having a disk-shaped rotor, the thickness of the rotor is thin compared to its radius. Thus, the rigidity of the rotor is low and a surface vibration, which is when the ends of the rotor are bent or distorted with respect to a virtual plane perpendicular to the axis of the output shaft and the ends rapidly oscillate from one side of the virtual plane to another, tends to occur. 
         [0009]    When the rotational speed of the rotor conforms to the characteristic frequency of the rotor itself, the resonance of the rotor is considerably increased, thereby disturbing the quiet operation of the electrical motor. 
         [0010]    Further, a reinforcing member such as a rib may be installed in the rotor. However, the reinforcing member may inhibit the functionality of a magnetic circuit of an electromagnetic force for driving the rotor. The rib may also increase the volume of the rotor itself, and thus, the density of a magnetic flux along the rotor may be weakened, thereby decreasing the rotor output torque. 
       SUMMARY OF THE CLAIMED SUBJECT MATTER 
       [0011]    In one aspect, the present disclosure relates to an electric motor including a stator, at least one rotor, an output shaft engaged with the at least one rotor, and a contact area changing device configured to change an area of contact between the at least one rotor and the output shaft, wherein the area of contact between the at least one rotor and the output shaft affects a characteristic resonant frequency of the at least one rotor. 
         [0012]    In another aspect, the present disclosure relates to a method to reduce vibrations in an electric motor including engaging an output shaft into at least one rotor, rotating the output shaft and the at least one rotor with respect to a stator, changing an area of contact between the at least one rotor and a radial protrusion of the output shaft, and shifting the resonant frequency of the at least one rotor with the changed area of contact. 
         [0013]    In another aspect, the present disclosure relates to an electric motor including a stator, at least one rotor means, an output shaft means engaged with the at least one rotor means, and a contact area changing means for changing an area of contact between the at least one rotor and the output shaft, wherein the area of contact between the at least one rotor and the output shaft affects a characteristic resonant frequency of the at least one rotor means. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]    Features of the present disclosure will become more apparent from the following description in conjunction with the accompanying drawings. 
           [0015]      FIG. 1  is a cross-sectional view schematically showing a structure of an axial gap-type electrical motor operating in a first operational state in accordance with a first exemplary embodiment of the present disclosure. 
           [0016]      FIG. 2  is a cross-sectional view schematically showing a structure of the axial gap-type electrical motor operating in a second operational state in accordance with the first exemplary embodiment of the present disclosure. 
           [0017]      FIG. 3  is a cross-sectional view schematically showing a structure of the axial gap-type electrical motor operating in a third operational state in accordance with the first exemplary embodiment of the present disclosure. 
           [0018]      FIG. 4(   a ) is a graph showing a relationship between amplitude of the surface vibration of the rotor and a rotational speed of the rotor in accordance with the first exemplary embodiment of the present disclosure. 
           [0019]      FIG. 4(   b ) is a graph showing a relationship between a change of a contact area and a resonance rotational speed. 
           [0020]      FIG. 5  is a cross-sectional view schematically showing a structure of an axial gap-type electrical motor operating in a first operational state in accordance with a second exemplary embodiment of the present disclosure. 
           [0021]      FIG. 6  is a cross-sectional view schematically showing a structure of an axial gap-type electrical motor operating in a second operational state in accordance with the second exemplary embodiment of the present disclosure. 
           [0022]      FIG. 7  is a cross-sectional view schematically showing a structure of an axial gap-type electrical motor operating in a third operational state in accordance with the second exemplary embodiment of the present disclosure. 
           [0023]      FIG. 8  is a graph showing a relationship between amplitude of a surface vibration of a rotor and a rotational speed of a rotor in accordance with the second exemplary embodiment of the present disclosure. 
           [0024]      FIG. 9  is a cross-sectional view schematically showing a structure of an axial gap-type electrical motor operating in a first operational state in accordance with a third exemplary embodiment of the present disclosure. 
           [0025]      FIG. 10  is a cross-sectional view schematically showing a structure of an axial gap-type electrical motor operating in a second operational state in accordance with the third exemplary embodiment of the present disclosure. 
           [0026]      FIG. 11  is a cross-sectional view schematically showing a structure of an axial gap-type electrical motor in accordance with a fourth exemplary embodiment of the present disclosure. 
           [0027]      FIG. 12  is a graph showing a relationship between the phase current and the attraction force of the axial gap-type electrical motors in accordance with the first to fourth exemplary embodiments. 
           [0028]      FIG. 13  is a schematic view of a relative position relationship in a circumferential direction of a stator iron core and a rotor permanent magnet. 
           [0029]      FIG. 14  is a cross-sectional view schematically showing a structure of an axial gap-type electrical motor operating in a second operational state in accordance with a fifth exemplary embodiment of the present disclosure. 
           [0030]      FIG. 15  is a cross-sectional view schematically showing a structure of an axial gap-type electrical motor operating in a first operational state in accordance with the fifth exemplary embodiment of the present disclosure. 
           [0031]      FIG. 16  is a graph showing a relationship between amplitude of a surface vibration of a rotor and a rotational speed of a rotor in accordance with the fifth exemplary embodiment of the present disclosure. 
           [0032]      FIG. 17  is a cross-sectional view schematically showing a structure of an axial gap-type electrical motor in accordance with a sixth exemplary embodiment of the present disclosure. 
           [0033]      FIG. 18  is a cross-sectional view schematically showing a structure of an axial gap-type electrical motor in accordance with a seventh exemplary embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    Example embodiments of the present disclosure will be explained in detail based on the provided drawings. However, it should be understood that the embodiments disclosed herein are exemplary, and that nothing should preclude additional embodiments from being considered within the scope of the attached claims. 
       First Exemplary Embodiment 
       [0035]      FIGS. 1 to 3  are cross-sectional views schematically showing a structure of an axial gap-type electrical motor in accordance with a first exemplary embodiment of the present disclosure. 
         [0036]    A stator  1  of an axial gap-type electrical motor  11  comprises a stator core  2 , a stator iron core  5  and an armature winding  7 . 
         [0037]    The stator core  2  formed of electromagnetic materials has a circular ring shape and an outer periphery of the stator core  2  is supported by and fixed to a motor case  8 . Further, a center hole  3  is formed in a center portion of the stator core  2  to receive an output shaft  4 . The output shaft  4  is rotatably supported at a bearing installed in the center hole  3 . The stator core  2 , includes several stator iron cores  5  which are disposed at an equal distance from each other and arranged to correspond to the circumference of the stator core  2 . Each stator iron core  5  may be formed by stacking steel plates, and protrudes from the stator core  2  in the axial direction substantially parallel to an axis  0  of the output shaft  4 . Each stator iron core may also include a leading end  5   s  facing the rotor  6 , and armature windings  7  may be wound around each stator iron core  5  between the stator core  2  and the leading end  5   s . As shown, leading end  5   s  has a height larger than that of the stator core  2  so that the armature windings may be retained between the leading end  5   s  and the stator core  2 . 
         [0038]    The rotor  6  may have a disk shape and may include several permanent magnets  10  on the surface of the rotor  6  facing stator core  2 . Each of the permanent magnets may be disposed at an equal circumferential distance from each other in locations corresponding to each of the stator iron cores  5  and are may further be arranged to correspond to the circumference of the rotor  6 . The rotor  6  and the leading end  5   s  of the stator iron winding are shown spaced apart via an air gap  9 . A back yoke  12  is shown embedded at an inner portion of the rotor  6  and coupled to the permanent magnet  10 . The permanent magnet  10  may be magnetized along the direction of the axis  0  and the back yoke  12  may form a magnetic circuit along which a magnetic field may flow. An alternating current may flow through the armature winding  7  which generates an electromagnetic force between the stator  1  and the rotor  6 , causing the permanent magnet  10  to rotate the rotor  6  so that the axial gap-type motor  11  functions as a synchronous motor. 
         [0039]    The output shaft  4  passes through a center of the rotor  6  and may engage the rotor  6  through a spline engagement so that the output shaft  4  and rotor  6  rotate about axis  0  together. At a position along the output shaft  4  corresponding generally to the stator iron cores  5 , a ring-shaped radial protrusion  4   d  may extend from an outer peripheral surface of the output shaft  4 . The radial protrusion  4   d  may have an end surface  4   t  substantially perpendicular to the axis  0  facing the rotor  6 . As shown, the end surface  4   t  includes a recess  4   r  such that a ring-shaped stopper  13  may be installed there within. 
         [0040]    As shown in  FIG. 1 , the stopper  13  includes a contact surface where the front surface of the rotor  6  contacts the stopper  13 . The contact surface includes a straight surface  17   a  extending substantially perpendicular to the axis  0  and a linear tapered surface  17   b  protruding from the end surface  4   t  downward toward the output shaft  4  until it meets the straight surface  17   a , and outward in the direction of the axis  0  toward the rotor  6 . While contact surface ( 17   a  and  17   b ) is shown as a combination of straight surface  17   a  and tapered surface  17   b , the relative proportions of straight surface  17   a  and tapered surface  17   b  may be varied. Additionally, contact surfaces entirely straight (e.g.,  17   a ) or entirely tapered (e.g.,  17   b ) should be considered within the scope of the present disclosure and attached claims. 
         [0041]    In selected embodiments, stopper  13  may be formed of light metal characterized by a low Young&#39;s modulus (for a metal) or a hard rubber or resin characterized by a high Young&#39;s modulus (for rubbers and resins), the stopper  13  may be configured to slightly deform if rotor  6  is thrust thereupon along axis  0 . Such thrusting (and deformation of stopper  13 ) may change the surface area of the contact surface ( 17   a  and  17   b ) contacting the rotor  6 . 
         [0042]    Around the output shaft on the side of the rotor  6  opposite the stopper  13 , a ring-shaped collar  14 , a crown washer  15  and a lock nut  16  may be threaded upon output shaft  4  so as not to be loosened along the direction of the axis  0 . The collar  14  may prevent the rotor  6  from being removed from the output shaft  4  in a direction away from the radial protrusion  4   d.    
         [0043]    Thus, output shaft  4 , stopper  13 , rotor  6 , stator  1 , collar  14 , washer  15  and lock nut  16  interactively operate to constitute a mechanism for changing the contact area (i.e., contact surfaces  17   a  and  17   b ) between rotor and protrusion  4   d  of output shaft  4 . 
         [0044]    Next, a function of the axial gap-type electrical motor  11  will be explained. While the electromagnetic force is generated between the stator  1  and the rotor  6  by the magnetic circuit causing the rotor to rotate, an attraction force is generated between the stator  1  and the rotor  6 . Because the stator  1  and the radial protrusion  4   d  face the same surface of the rotor  6 , the attraction force, which becomes a magnitude of the electromagnetic force in the direction of the axis  0 , may exert a thrust on the stopper  13  between the radial protrusion  4   d  and the rotor  6 . 
         [0045]    When the attraction force is low and, and thereby the thrust of the direction of the axis  0  upon the stopper  13  is low, because the stopper  13  is hardly deformed, a radius of the contact surface of the stopper  13  and the rotor  6  may be R 1 , as shown in  FIG. 1 . 
         [0046]    However, when the attraction force is greater than the value shown in  FIG. 1 , the component of the electromagnetic force indicated by a thin arrow F 2  in  FIG. 2  may pull the rotor  6  closer to protrusion  4   d  than in  FIG. 1 . By doing so, because the stopper  13  may be deformed by the thrust, the length of the linear taper is decreased and the radius of the contact area of the contact surface of the stopper  13  may also be increased. The radius of the contact area at this time becomes R 2 , as shown in  FIG. 2 , where R 1 &lt;R 2 . 
         [0047]    When the attraction force is greater than the value shown in  FIG. 2 , the component of the electromagnetic force indicated by a thick arrow F 3  in  FIG. 3  pulls the rotor  6  closer to protrusion  4   d  than in  FIG. 2 . By doing so, because the stopper  13  is further deformed by the thrust, the linear taper disappears and the radius of the contact surface of the stopper  13  and the rotor  5  becomes R 3 , as shown in  FIG. 3 , where R 1 &lt;R 2 &lt;R 3 . In such a case, the end surface  4   t  of the radial protrusion  4   d  contacts the rotor  6 , thereby continuing the contact surface of stopper  13 . Because the end surface  4   t  and the rotor  6  are made of steels having the Young&#39;s modulus higher than the stopper  13 , the rigidity of the support of the rotor  6  provided by the radial protrusion  4   d  is maximized. 
         [0048]    The radius of the contact surface of the output shaft  4  and the rotor  6  may continuously change from R 1  to R 2  to R 3  by the magnitude of the electromagnetic force in the direction of the axis  0 . This changes the characteristic frequency wherein the rotor  6  is resonated by the surface vibration. If the attraction force is decreased from the state shown in  FIG. 3 , then the radius of the contact surface is decreased from R 3  and may be returned to R 1  or R 2  as shown in  FIGS. 1 and 2 . 
         [0049]      FIG. 4(   a ) is a graph showing a relationship between amplitude of the surface vibration of the rotor  6  and a rotational speed of the output shaft  4  in accordance with the rotor  6 . When the radius of the contact surface is R 1 , the characteristic frequency wherein the amplitude of the surface vibration is maximized (i.e., the “resonant” frequency of rotor  6  and output shaft  4 ) may be fR 1 . That is, when the rotational speed of the rotor  6  is at fR 1 , the rotor  6  may be at the point of maximum natural vibration. 
         [0050]    Because the characteristic frequency and the rotational speed of the rotor  6  are movements in a unit time, both are indicated in a horizontal axis of  FIG. 4(   a ). 
         [0051]    However, when the radius of the contact surface is R 3 , which is greater than R 1 , the characteristic resonant frequency of the rotor  6  wherein the amplitude of the surface vibration of the rotor  6  is maximized becomes fR 3 , which is also greater than fR 1 . As the radius of the contact surface is increased, the contact area may be increased as well. Accordingly, because the rigidity of the support of the rotor  6  provided by the radial protrusion  4   d  is increased, the characteristic frequency may be similarly increased. 
         [0052]    Because the radius of the contact surface is changed between R 1  and R 3  as indicated by the arrow in  FIG. 4(   a ) corresponding to the magnitude of the attraction force, the characteristic frequency and the surface vibration of the rotor  6  may also be changed, thereby deviating from the rotational speed of the rotor  6 . For example, when the rotational speed of the rotor  6  is fR 1 , the attraction force may be small (e.g., as indicated by the small arrow in  FIG. 2) , causing the radius of the contact surface to be R 1 , thereby deviating the characteristic frequency of the rotor  6  from fR 3 . Further, when the rotational speed of the rotor  6  is fR 3 , the attraction force may be large (e.g., as indicated by the small arrow in  FIG. 3 ), causing the radius of the contact surface to be R 3 , thereby deviating the characteristic frequency of the rotor from fR 1 . As a result, the resonant frequency (e.g., fR 1 , fR 3 ) may vary such that it is never the same as the present rotational frequency of rotor  6 . Therefore it may be possible to construct an axial gap-type motor having no “achievable” surface vibration resonant frequency. 
         [0053]    The characteristic frequency of the rotor  6  shown in  FIG. 4(   a ) may further be illustrated in the map provided in  FIG. 4(   b ). A horizontal axis of  FIG. 4(   b ) represents a diameter of an engagement portion wherein the radius of the contacting portion is doubled. Further, a longitudinal axis is a resonance rotational speed of the rotor  6 , which is resonated by the surface vibration and equals to the characteristic frequency shown in  FIG. 4(   a ). As shown in  FIG. 4(   b ), when the diameter of the engagement portion is increased, the resonance rotational speed is increased. In the first exemplary embodiment, the resonance rotational speed of the rotor  6  is deviated from an actual rotational speed by changing the effective diameter of the engagement portion. By doing so, it may be possible to prevent the resonance of the surface vibration of the rotor  6  so that quiet operation is possible. 
       Second Exemplary Embodiment 
       [0054]      FIGS. 5 to 7  are cross-sectional section views schematically showing a structure of an axial gap-type electrical motor  21  in accordance with a second exemplary embodiment of the present disclosure.  FIG. 5  shows a state when the radius of the contact surface is small (R 1 ).  FIG. 6  shows a state when the radius of the contact surface is greater than that shown in  FIG. 5  (R 2 ).  FIG. 7  shows a state when the radius of the contact surface is greater than that shown in  FIG. 6  (R 4 ). Because the basic constitution of the second exemplary embodiment is similar to that of the first exemplary embodiment, similar elements are denoted by the same reference numerals and explanations thereof are omitted herein. 
         [0055]    An axial gap-type electrical motor  21  in accordance with the second exemplary embodiment has a ring-shaped stopper  23  through which the output shaft  4  passes. Although a material of the stopper  23  may be the same as the stopper  13  of the first exemplary embodiment, the contact surface of the stopper  23  may be constructed with a non-linear taper and may instead be formed as a convex arcuate taper. As such the convex arcuate taper of the stopper  23  may protrude toward the rotor  6  as an arcuate surface (e.g., a spherical or hyperbolic surface) protruding from the end surface  4   t  downward toward the output shaft  4  until it meets the outer peripheral surface of the output shaft  4  and outward toward the rotor  6 . Thus, the stopper  23  may contact the front surface of the rotor  6  at the point the stopper  23  meets the output shaft  4 . As such, the movement of the rotor  6  along the direction of the axis  0  may be changed without rattling. 
         [0056]    Moreover, the outer peripheral portion of the stopper  23  proximate to the wall  4   k  of the recess of the radial protrusion  4   d  may be located inward of the end surface  4   t  with respect to the rotor  6 , as shown in  FIG. 5 . 
         [0057]    When the attraction force along the direction of the axis  0  to attract the stopper  23  towards the stator iron core  5  is small, a radius of a contact surface of the stopper  23  and the rotor  6  may be R 1 , as shown in  FIG. 5  from negligible deformation of the stopper  23 . 
         [0058]    When the attraction force (i.e., the electromagnetic force) is greater than the value shown in  FIG. 5 , a magnitude of the electromagnetic force in the direction of the axis  0  indicated by a thin arrow in  FIG. 6  pulls the rotor  6  more closely to protrusion  4   d . By doing so, because the stopper  23  may be deformed by the thrust and the arcuate surface may therefore be reduced as well, the radius of the contact surface of the stopper  23  and the rotor  6  may be increased. Thus, the radius may become R 2 , as shown in  FIG. 6 , wherein R 1 &lt;R 2 . 
         [0059]    When the attraction force (i.e., the electromagnetic force) is even greater than the value shown in  FIG. 6 , the magnitude of the electromagnetic force in the direction of the axis  0  indicated by a thick arrow in  FIG. 7  may pull the rotor  6  closer to protrusion  4   d  than in  FIG. 6 . By doing so, because the stopper  23  may be further deformed by the thrust and the arcuate surface may therefore nearly disappear, the radius of the contact surface of the stopper  23  and the rotor  6  may be increased. Thus, the radius may approach R 4  (minus a small portion of the arcuate surface near the wall  4   k  of the recess) as shown in  FIG. 7 , where R 1 &lt;R 2 &lt;R 4 . In such a case, the end surface  4   t  of the radial protrusion  4   d  at the outer peripheral side compared to the stopper  23  contacts the rotor  6 . Because the end surface  4   t  and the rotor  6  have a higher Young&#39;s modulus and a higher rigidity than the stopper  23 , the rigidity of the support of the rotor may be maximized. 
         [0060]      FIG. 8  is a graph showing a relationship between amplitude of the surface vibration of the rotor  6  and a rotational speed of the rotor  6  in accordance with the second exemplary embodiment of the present disclosure. When the radius of the contact surface is R 1 , the characteristic resonant frequency where the amplitude of the surface vibration of the rotor  6  is maximized may be described as fR 1 . That is, when the rotational speed is R 1  and the contact surface of the rotor is R 1 , the surface vibration of the rotor  6  will be maximized. 
         [0061]    When the radius of the contact surface is R 4 , which is greater than R 1 , the characteristic resonant frequency where the amplitude of the surface vibration of the rotor  6  becomes maximized changes to fR 4 , a speed greater than fR 1 . Thus, as the diameter of the contact surface is increased, the rigidity of the support of the rotor  6  may also increase. Thus, the characteristic frequency may also be increased. 
         [0062]    Because the radius of the contact surface changes between R 1  and R 4  as indicated by the arrow in  FIG. 8 , the characteristic frequency caused by the surface vibration of the rotor  6  may change, thereby deviating from the rotational speed of the rotor  6 . For example, when the rotational speed of the rotor approaches fR 4 , the radius of the contact surface may be R 1  to thereby deviate the characteristic frequency of the rotor from fR 4 . Further, when the rotational speed of the rotor approaches fR 1 , the radius of the contact surface may be R 4  to thereby deviate the characteristic frequency of the rotor from fR 1 . By doing so, it may be possible to prevent the resonance of the surface vibration of the rotor  6  and quiet operation of the axial gap-type motor  21  may be possible. 
         [0063]    In particular, the contact surface of the rotor  6  and the output shaft  4  may be a combination of the stopper  23 , which may be the arcuate surface protruded toward the direction of the axis  0 , and the front surface of the rotor  6  for supporting such an arcuate surface in the second exemplary embodiment. As such, the change in the characteristic frequency of the rotor  6  with regard to the attraction force may differ from the stopper  13 , which is the linear taper and straight surface, in the first exemplary embodiment. Further, because the small portion of the arcuate surface near the wall  4   k  of the recess is provided which does not contact the rotor  6  even when the radius is R 4 , the characteristic frequency of the rotor may be discontinuously changed. 
       Third Exemplary Embodiment 
       [0064]      FIGS. 9 and 10  are cross-sectional section views schematically illustrating a structure of an axial gap-type electrical motor  31  in accordance with a third exemplary embodiment of the present disclosure.  FIG. 9  shows a state when the radius of the contact surface is relatively small (R 1 ).  FIG. 10  shows a state when the radius of the contact surface is relatively large (i.e., greater than  FIG. 9 ) (R 3 ). Because the basic constitution of the second exemplary embodiment is similar to that of the first exemplary embodiment, similar elements are denoted by the same reference numerals and explanations thereof are omitted herein. 
         [0065]    An axial gap-type electrical motor  31  in accordance with the third exemplary embodiment comprises a stopper  33  through which the output shaft  4  is passes. Although a material of the stopper  33  may be the same as stopper  13  of the first exemplary embodiment, stopper  33  is shown such that its contact surface is neither a linear taper or an arcuate taper. Rather, stopper  33  includes a planar surface substantially perpendicular to the axis  0  and contacting the front surface of the rotor  6 . As such, rotor  6  may be constructed such that its movement along the direction of the axis  0  may occur without rattling. 
         [0066]    When an attraction force (i.e., an electromagnetic force) between the stator  1  and the rotor  6  is low and a magnitude of the electromagnetic force in the direction of the axis  0  is low, a radius of the contact surface of the stopper  33  and the rotor  6  may be R 1 . As such, stopper  33  (as shown in  FIG. 9 ) is hardly deformed. 
         [0067]    When the attraction force (i.e., the electromagnetic force) is greater than the value shown in  FIG. 9 , the magnitude of the electromagnetic force in the direction of the axis  0  may be large (as indicated by a thick arrow in  FIG. 10 ). Such larger attraction force may pull the rotor  6  closely against protrusion  4   d  such that the contact surface of the rotor  6  may be R 3 . In such a case, the end surface  4   t  of the radial protrusion  4   d  additionally contacts the rotor  6 . Because the end surface  4   t  and the rotor  6  have a higher Young&#39;s radius and a higher rigidity than the stopper  33 , the rigidity of the support of the rotor  6  is maximized. 
         [0068]    Because the radius of the contact surface changes between R 1  and R 3 , the characteristic resonant frequency caused by the surface vibration of the rotor  6  may also change, thereby deviating from the rotational speed of the rotor  6 . For example, when the rotational speed of the rotor  6  approaches fR 3 , the radius of the contact surface may be R 1  to deviate the characteristic frequency of the rotor  6  away from fR 3 . 
         [0069]    Further, when the rotational speed of the rotor  6  approaches fR 1 , the radius of the contact surface may be R 3  to thereby deviate the characteristic frequency of the rotor  6  away from fR 1 . By doing so, it may be possible to prevent the axial gap-type motor  31  from achieving a speed associated with the characteristic resonance frequency of the rotor  6 . 
         [0070]    In particular, because the contact surface of the rotor  6  and the output shaft  4  may be a combination of the stopper  33 , which is a planar surface perpendicular toward the direction of the axis  0 , and the front surface of the rotor  6  for supporting such a planar surface, the change in the characteristic frequency of the rotor  6  with regard to the attraction force may differ from the stopper  13  (i.e., a linear taper) in the first exemplary embodiment and the stopper  23  (i.e., a arcuate taper) in the second exemplary embodiment. 
       Fourth Exemplary Embodiment 
       [0071]      FIG. 11  is a cross-sectional view schematically showing a structure of an axial gap-type electrical motor  41  in accordance with a fourth exemplary embodiment.  FIG. 11  shows a state when the radius of the contact surface is small (R 1 ). Because the basic constitution of the fourth exemplary embodiment is similar to that of the first exemplary embodiment, similar elements are denoted by the same reference numerals and explanations thereof are omitted herein. 
         [0072]    In an axial gap-type electrical motor  41  in accordance with the fourth exemplary embodiment, the end surface  4   t  perpendicular to the axis  0  of the output shaft radial protrusion  4   d  may directly contact the front surface of the rotor  6 . As such, the rotor  6  may be configured such that its position along the direction of the axis  0  may be changed without rattling. As such, the front surface of the rotor  6  may form a taper extending inward toward the radial protrusion  4   d  and downward toward the output shaft  4 , as shown in  FIG. 11 . As such, front tapered surface of rotor  6  may be deformable in a manner similar to stoppers  13 ,  23 , and  23  of the first, second, and third exemplary embodiments. 
         [0073]    When an attraction force (i.e., an electromagnetic force) is greater than the value shown in  FIG. 11 , a magnitude of the electromagnetic force in the direction of the axis  0  may pull the rotor  6  toward protrusion  4   d . In such a case, because the front surface of the rotor  6  may be deformed by the thrust, the taper may be decreased. Further, the radius of the contact area between radial protrusion  4   d  and the rotor  6  may also be increased. 
         [0074]    As such, the radius of the contact surface of the output shaft  4  and the rotor  6  may be changed by the magnitude of the electromagnetic force in the direction of the axis  0 . The characteristic resonant frequency where the rotor  6  is resonated by the surface vibration may be changed. As such, because it may be possible to prevent the resonance of the surface vibration of the rotor  6 , quiet operation of the axial gap-type motor  41  becomes possible. 
         [0075]    In the axial gap-type electrical motors  11 ,  21 ,  31  and  41  of the first to fourth exemplary embodiments as described above, the radius of the contact surface of the output shaft  4  and the rotor  6  may be changed by the magnitude of the electromagnetic (attraction) force in the direction of the axis  0 . The electromagnetic force may be controlled based on a relationship between the alternating current and the attraction force shown in  FIG. 12 . 
         [0076]    In  FIG. 12 , the horizontal axis is an effective value (or maximum value) of a phase current flowing through the armature winding, and the longitudinal axis is the attraction force, which becomes the magnitude of the electromagnetic force in the direction of the axis  0  generated between the stator  1  and the rotor  6 . Further, in  FIG. 12 , a dash line indicates when a current phase angle β becomes −90 degrees (i.e., a strong field system). Also, a solid line indicates when the current phase angle β is 0 (zero) degrees and a chain line indicates when the current phase angle β is +90 degree (i.e., a weak field system). 
         [0077]    Generally, the axial gap-type electrical motors  11 ,  21 ,  31  and  41  may be driven with the phase current equal to or less than Ia. Further, a weak field system control is performed wherein a polarity of the current phase angle β is on the positive side. As such, the attraction force is decreased. Thus, as shown in  FIG. 12 , an area used at the time of driving in the general operation may be indicated by being surrounded by a line when β is 0 (zero) degree, a line when the current phase angle β is +90 degree and a line when the phase current is Ia. 
         [0078]    As for the area used at the time of driving in the general operation, when the characteristic frequency of the rotor  6  is changed as shown in  FIGS. 4 and 8  to be deviated from the rotational speed of the rotor  6 , the axial gap-type electrical motors  11 ,  21 ,  31  and  41  may be driven with the phase current equal to or more than Ia. Further, a strong field system control is performed wherein a polarity of the current phase angle β is on the negative side. As such, the attraction force is increased. As indicated by a solid line ellipse,  FIG. 12  shows a characteristic value variable control area in the operation of spacing the characteristic frequency from the rotational speed. 
         [0079]    The weak field system control and the strong field system control will be explained herein while referring to  FIG. 13 .  FIG. 13  is a schematic view showing a relative position relationship in a circumferential direction of the stator iron core  5  and the magnet  10 , illustrating when the current phase angle β is 0 (zero) degree. In  FIG. 13 , when a polarity of the current phase angle β is on the negative side, because a permanent magnet  10   b  having a south pole front surface becomes closer to the stator iron core  5  having the north pole leading end  5   s , it can be understood that the attraction force is increased. 
       Fifth Exemplary Embodiment 
       [0080]      FIGS. 14 and 15  are cross-sectional views schematically showing a structure of an axial gap-type electrical motor  51  in accordance with a fifth exemplary embodiment of the present disclosure.  FIG. 14  shows a state when the radius of the contact surface is large (R 3 ).  FIG. 15  shows a state when the radius of the contact surface is smaller than the value shown in  FIG. 14  (R 1 ). Because the basic constitution of the fifth exemplary embodiment is similar to that of the first exemplary embodiment, similar elements are denoted by the same reference numerals and explanations thereof are omitted herein. 
         [0081]    An axial gap-type electrical motor  51  in accordance with the fifth exemplary embodiment may comprise an actuator to change a contact area of the rotor  6  and the output shaft  4 . Such an actuator may be a piston mechanism for displacing the rotor  6  in the direction of axis  0 . 
         [0082]    A ring-shaped disc spring  52  through which the output shaft  4  passes may be arranged adjacent to the rear surface of the rotor  6 . A collar  14  may prevents the disc spring  52  from moving away from the rotor  6  in the direction of the axis  0 . As shown, the disc spring  52  presses the rotor  6  toward the stator  2  in the direction of the axis  0  and pushes the rotor  6  into contact with the radial protrusion  4   d  of the output shaft  4   d  as shown in  FIG. 14 . 
         [0083]    As shown, the radial protrusion  4   d  may contain a piston  53  which is configured to be slidably moveable along the direction of the axis  0  and a cylinder  54  for housing the piston  53 . The cylinder  54  may be in communication with hydraulic lines  55  installed in an inner portion of the output shaft  4 . While the rotor  6  is in contact with the radial protrusion  4   d  and the contact area has a large radius (R 3 ), the piston  53  is completely housed within the cylinder  54 , as shown in  FIG. 14 . 
         [0084]    If a hydraulic pressure is supplied from the through hydraulic line  55  to the cylinder  54 , piston  53  may be forced to protrude from the end surface  4   t  of the radial protrusion  4   d  and may displace the rotor  6  towards the disc spring  52 . As such, the radius of the contact surface between the rotor  6  and the output shaft radial protrusion  4   d  may be reduced to R 1  as shown in  FIG. 15 . 
         [0085]    Further, if the hydraulic pressure of the cylinder  54  is reduced, then the piston  53  may retract into end surface  4   t  by a biasing force of the disc spring  52  pushing the rotor  6  in a direction toward the radial protrusion  4   d . As such, the radius of the contact surface may be optionally changed to either R 1  or R 3  by activating or deactivating piston  53  in cylinder  54  by changing the pressure within hydraulic line  55 . 
         [0086]    In the fifth exemplary embodiment, the output shaft  4 , the collar  14 , a washer  15 , the disc spring  52 , a lock nut  16 , and the piston  53  may be interactively operated to constitute a device for changing the contact area between rotor  6  and protrusion  4   d.    
         [0087]    As mentioned above, when the radius of the contact surface is R 3 , the characteristic frequency resonated by the surface vibration (rotational speed) is fR 3 , as shown in  FIG. 16 . When the radius of the contact surface is R 1 , the characteristic frequency resonated by the surface vibration (rotational speed) is fR 1 . If the contact surface of the rotor  6  and the output shaft  4  has a large radius (R 3 ), because the rigidity of the support of the rotor  6  is high, the characteristic frequency fR 3  becomes greater than the characteristic frequency fR 1 , as shown in  FIG. 16 . To make the characteristic frequency small as indicated by the arrow in  FIG. 16 , the radius of the contact surface may be decreased from R 3  to R 1  by supplying the hydraulic pressure to the cylinder  54 . 
         [0088]    Further, because the rotor  6  may be displaced along the direction of the axis  0  by using the piston mechanism in the axial gap-type electrical motor  51  in accordance with the fifth exemplary embodiment, an amount of displacement of the rotor  6  along the direction of the axis  0  may be increased more than would be possible using the axial gap-type electrical motors  11 ,  21 ,  31 , and  41  in accordance with the first to fourth exemplary embodiments. As a result, air gap  9  may be increased. Thus, when the rotational speed of the rotor exceeds a predetermined value, an induced voltage disadvantageous for a high speed rotation as a motor may be reduced by increasing the air gap  9 . 
         [0089]    As described above, the axial gap-type electrical motors  11 ,  21 ,  31 ,  41  and  51  in accordance with the first to fifth exemplary embodiments are depicted having only one rotor. However, it should be understood that with axial gap-type electrical motors having two (or more) rotors  6  and one (or more) stator  2  explained below in reference to  FIGS. 17 and 18 , it may still be possible to modify the characteristic resonant frequency of the motor by changing the contact area between rotors  6  and the output shafts  4 . 
       Sixth Exemplary Embodiment 
       [0090]      FIG. 17  is a cross-sectional view schematically showing a structure of an axial gap-type electrical motor  61  in accordance with a sixth exemplary embodiment of the present disclosure, showing when the radius of the contact surface is small (R 1 ). The basic constitution of the sixth exemplary embodiment is substantially similar to that of the first exemplary embodiment with the exception that two rotors  6  are symmetrically arranged about a single stator  1 . 
       Seventh Exemplary Embodiment 
       [0091]      FIG. 18  is a cross-sectional view schematically showing a structure of an axial gap-type electrical motor  71  in accordance with a seventh exemplary embodiment of the present disclosure, showing when the radius of the contact surface is large (R 3 ). The basic constitution of the seventh exemplary embodiment is substantially similar to that of the fifth exemplary embodiment of  FIG. 13  with the exception that two rotors  6  are symmetrically arranged about a single stator  1 . 
         [0092]    Advantageously, the present disclosure provides a technique for effectively preventing the resonance of the rotor without installing a separate reinforcing member in the rotor. In order to achieve such an advantage, an axial gap-type electrical motor of the present disclosure may comprise a disk-shaped rotor arranged opposite a stator, wherein the rotor and the stator are spaced apart axially along an output shaft which engages to the rotor, and a device for changing a contacting area between the rotor and the output shaft depending on a rotational speed of the rotor. 
         [0093]    As such, it may be possible to deviate a characteristic resonant frequency at a particular rotational speed by modifying the joining state between the rotor and the output shaft engaged to the rotor. As such, it may be possible to conduct a quiet operation by preventing the resonance of the rotor itself during an operation of the electrical motor. 
         [0094]    While the disclosure has been presented with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments may be devised which do not depart from the scope of the present disclosure. Accordingly, the scope of the disclosure should be limited only by the attached claims.