Patent Publication Number: US-2012025644-A1

Title: Electric motor having speed change function

Description:
TECHNICAL FIELD 
     This invention relates to an electric motor having a speed change function, which is capable of varying a ratio between a rotational speed of a rotor and a rotational speed of an output shaft. 
     BACKGROUND ART 
     Output characteristics of an electric motor vary depending on a size and the kind of the electric motor, and the electric motor have various kinds of needs. For example, some of the conventional electric motors may fulfill a requirement of output characteristics but may not fulfill a requirement of size. In order to fulfill those requirements, according to the prior art, an electric motor is provided with a transmission arranged in an outer case holding a rotor and a stator, and configured to output the torque of the rotor from the output shaft while varying the torque according to a speed change ratio. 
     For example, Japanese Patent Laid-Open No. 2007-269129 discloses a wheel rotating device, in which a stator is disposed inwardly in radial directions of the wheel coupled to a hub, a cylindrical rotor is disposed inwardly in radial directions of the stator in a rotatable manner, and a planetary gear mechanism is disposed inwardly in radial directions of the rotor. According to the teachings of Japanese Patent Laid-Open No. 2007-269129, the rotor is connected with a sun gear of the planetary gear mechanism, a ring gear is attached to a case integral with the stator and fixed, and a carrier is connected with the hub. Therefore, the planetary gear mechanism serves as a speed reducer, and a speed reducing ratio is unambiguously governed by a gear ratio of the planetary gear mechanism. 
     In addition, Japanese Patent Laid-Open No. 6-328950 discloses a hybrid vehicle in which a planetary gear mechanism and a clutch adapted to integrate the planetary gear mechanism are arranged in an inner circumferential side of a motor. A stator of the motor is fixed to an inner circumferential face of a transmission housing, and the rotor is connected with the ring gear. The carrier is connected with an output shaft, and the clutch is adapted to connect the sun gear with the carrier selectively. Therefore, in the hybrid vehicle taught by Japanese Patent Laid-Open No. 6-328950, a speed change ratio is shifted between two stages. 
     Further, Japanese Patent Laid-Open No. 2008-75878 discloses a continuously variable transmission, in which a plurality of spherical rolling member is individually held in a rotatable manner by a spindle tiltable with respect to a rotational canter axis of support member. A driving member and a driven member are arranged on an outer circumferential side of the rolling member and those driving member and driven member are opposed to each other across the rolling member. More specifically, an axial end face of each of the driving member and the driven member is pushed onto an outer face of the rolling member. Therefore, a rotation radius of the rolling member between a point to which the driving member is contacted (and the spindle), and a rotation radius of the rolling member between a point to which the driven member is contacted (and the spindle), are varied when the spindle as a rotational center of the rolling member is tilted. As a result, circumferential velocities of those contact points, that is, rotational speeds of the driving member and the driven member are varied. Therefore, a speed change ratio is varied continuously or steplessly. In addition, a power is transmitted to the driving member thorough a pulley arranged integrally and coaxially with the driving member. 
     According to the device taught by Japanese Patent Laid-Open No. 2007-269129, a ratio between a rotational speed of the rotor and a rotational speed of the wheel has to be governed by the planetary gear mechanism. Therefore, in case of driving a vehicle by driving a wheel directly by the motor, the motor has to be driven at a speed at which energy efficiency is low depending on a vehicle speed. Thus, the device taught by Japanese Patent Laid-Open No. 2007-269129 has to be improved to optimize the energy efficiency. 
     As described, the hybrid vehicle taught by Japanese Patent Laid-Open No. 6-328950 is capable of shifting the speed change ratio between two stages. However, a rotational speed of the motor has to be fixed to high speed or low speed according to the speed change ratio. That is, the hybrid vehicle taught by Japanese Patent Laid-Open No. 6-328950 is incapable of driving the motor at an intermediate speed, as well as at a higher speed and a lower speed. Thus, according to the teachings of Japanese Patent Laid-Open No. 6-328950, the motor has to be driven at a speed at which energy efficiency is low depending on a vehicle speed, as in the device taught by Japanese Patent Laid-Open No. 2007-269129. Therefore, the hybrid vehicle taught by Japanese Patent Laid-Open No. 6-328950 is also necessary to be improved to optimize the energy efficiency. As to the continuously variable transmission taught by Japanese Patent Laid-Open No. 2008-75878, the power is transmitted to the driving member through the pulley as described above. Therefore, according to the teachings of Japanese Patent Laid-Open No. 2008-75878, a drive unit has to be enlarged entirely. For this reason, the continuously variable transmission taught by Japanese Patent Laid-Open No. 2008-75878 is difficult to be mounted on a vehicle. 
     DISCLOSURE OF THE INVENTION 
     The present invention has been conceived noting the technical problems thus far described, and its object is to provide a compact electric motor which is capable of improving energy efficiency. 
     In order to achieve the above-mentioned object, according to the present invention, there is provided an electric motor having a speed change function, which is configured to differentiate a rotational speed of a rotor from a rotational speed of an output shaft for outputting a torque transmitted from the rotor, characterized by comprising: a stator coil, which is arranged on an inner circumferential face of a cylindrical outer case; a cylindrical rotor, which is arranged in an inner circumferential side of the stator coil, and which is adapted to generate a torque by receiving a magnetic force generated by the stator coil; and a continuously variable transmission mechanism, which is arranged in an inner circumferential side of the rotor, and which is adapted to continuously vary a ratio between a rotational speed of an input member connected with the rotor, and a rotational speed of an output member connected with the output shaft and rotated by the torque from the input member. 
     Specifically, the continuously variable transmission mechanism comprises: a rolling member, which is configured to tilt a rotational center axis thereof, and whose outer face is formed into a smooth curved face; a rotating shaft whose outer circumferential face is contacted with the rolling member situated in an outer circumferential side thereof; and two rotary members contacted with the outer circumferential face of the rolling member in a torque transmittable manner from both sides of the rolling member in a direction along a rotational center axis of the rolling member. In the continuously variable transmission mechanism, one of the rotating shaft and the two rotary members is connected with the rotor, and another one of the rotating shaft and the two rotary members serves as an output member. 
     According to the present invention, a permanent magnet is attached to the rotor, and the aforementioned two rotary members are contacted individually with the outer circumferential face of the rolling member in the rotor side. For example, the rolling member comprises a magnetic body attracted by the permanent magnet toward the rotor side. 
     As explained above, a permanent magnet is attached to the rotor, and the aforementioned two rotary members are contacted individually with the outer circumferential face of the rolling member in the rotor side. However, it is also possible to form the rolling member using nonmagnetic material. 
     The outer case comprises a cylindrical portion in which the stator coil is attached to an inner circumferential face thereof; and an end plate formed integrally with one of axial ends of the cylindrical portion. More specifically, one of the rotary members situated closer to the end plate is connected with the rotor, and the other rotary member is connected with the output shaft protruding in a direction opposite to the end plate. 
     In addition, the electric motor of the present invention comprises: a cam mechanism, which is interposed between the rotor and one of the rotary members, and which is adapted to convert a torque acting between the rotor and said one of the rotary members into a thrust force in the axial direction thereby pushing said one of the rotary members onto the rolling member. 
     The electric motor of the present invention further comprises: another cam mechanism, which is interposed between the output shaft and the other rotary member functioning as an output member, and which is adapted to convert a torque acting between the output shaft and the other rotary member thereby pushing the other rotary member onto the rolling member. 
     In addition, the electric motor of the present invention further comprises: a bearing member, which is adapted to hold the rotating shaft in a rotatable manner, and to reciprocate in the axial direction of the output shaft; a support shaft, which penetrates the rolling member along a rotational center axis of the rolling member thereby supporting the rolling member in a rotatable manner; an arm attached individually to both ends of the support shaft, and extends individually toward both sides of the bearing member; and a convex curve formed on at least one of the bearing member and the arm, at which the bearing member and the arm are contacted locally or linearly. According to the invention, the support shaft is tilted together with the rolling member by moving the bearing member in the axial direction to push the arm. 
     According to the present invention, a torque is generated by supplying a current to the stator coil and the rotor is rotated by the torque. As described, the continuously variable transmission mechanism is situated between the rotor and the output shaft. Therefore, the rotational speed of the rotor can be kept to the speed at which energy efficiency is optimized by varying the speed change ratio of the continuously variable transmission mechanism arbitrarily. Specifically, the continuously variable transmission mechanism is arranged in the inner circumferential side of the rotor. That is, the stator coil, the rotor and the continuously variable transmission mechanism are arranged concentrically. Therefore, in addition to the above-explained advantage, the electric motor of the present invention can be downsized entirely and the energy efficiency of the electric motor can be improved. 
     Moreover, since the speed change ratio is varied continuously by tilting the rotational center axis of the rolling member, the rotational speed of the rotor can be kept easily to the speed at which energy efficiency is optimized. 
     In case the rotor is provided with a permanent magnet and the rolling member is formed of magnetic material entirely or partially, the contact pressure between the rolling member and said two of the rotary member can be enhanced by a magnetic force attracting the rolling member toward the rotor. 
     To the contrary, in case the rotor is provided with a permanent magnet but the rolling member is made of nonmagnetic material, the rolling member will not be attracted by the magnetic force. Therefore, in this case, generation of togging torque can be prevented. 
     According to the present invention, a cylindrical case can be used as the outer case. In this case, the end plate is fixed to one of the axial ends of the outer case, and the output shaft is protruded from the other axial end. That is, the end plate can be utilized to fix the electric motor. Therefore, fixing strength of the electric motor can be enhanced. 
     According to the present invention, when the torque is transmitted between the rotor and the output member, the torque being transmitted will act between the rotor and one of the rotary members, or between the other rotary member and the output member. Then, the torque is converted into a thrust force in the axial direction by the cam mechanism, and at least one of the rotary members is pushed onto the rolling member by the thrust force thus converted. As a result, the contact pressure between the rolling member and the rotary member can be ensured, and the transmission torque capacity is thereby maintained. 
     In addition to the above-explained advantage, according to the present invention, a ratio between the rotational speeds of the rotor and the output member, that is, the speed change ratio can be varied continuously by moving the bearing member holding the rotating shaft rotatably in the axial direction to tilt the rolling member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side sectional view showing one example of an electric motor according to the present invention. 
         FIG. 2  is a front sectional view showing the electric motor shown in  FIG. 1 . 
         FIG. 3  is a front view showing a shift shaft and a shift key. 
         FIG. 4  is a perspective sectional view explaining a structure of a carrier. 
         FIG. 5  is a partial view showing one example of a cam mechanism. 
         FIG. 6  is a graph indicating a relation between a tilt angle and a speed change ratio (i.e., speed ratio). 
         FIG. 7  is a perspective sectional view showing an example to use the electric motor as an in-wheel motor. 
         FIG. 8  is a partial view showing another example of a structure of a stator shaft. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Next, this invention will be described in more detail. An example of an electric motor  1  is shown in  FIGS. 1 and 2 . In the electric motor  1  shown therein, a continuously variable transmission mechanism  3  is arranged inside of an inner circumference of a motor part  2 . The motor part  2  comprises a stator coil  4  and a rotor  5  arranged in an inner circumferential side of the stator coil  4 . The stator coil  4  is formed by arranging a plurality of coils twining around an iron core in a circular manner, and fixed to an inner face of a cylindrical portion  7  of an outer case  6 . The outer case  6  is a bottomed cylindrical member. Specifically, the outer case  6  comprises an end plate  8  as a circular plate, and the cylindrical portion  7  is integrated with a circumference of the end plate  8 . 
     The rotor  5  is also a cylindrical member, and a (not shown) plurality of permanent magnets are attached to an outer circumference of the rotor  5 . Thus, the motor part  2  is formed as a permanent magnet type synchronous electric motor. A length of the rotor  5  is substantially identical to that of the cylindrical portion  7 , and the rotor  5  is housed in the outer case  6 . Flange portions  9  and  10  are formed to extend from both axial ends of the rotor  5  toward an inner circumferential side (i.e., toward a center). Specifically, the flange portion  9  of the end plate  8  side is relatively shorter than the flange portion  10  of the opposite side. In other words, the flange portion  10  of the outer case  6  side is relatively longer than the flange portion  9 . In addition, bearing  11  is arranged in an inner circumferential side of the flange portion  9 , and a bearing  12  is arranged in an inner circumferential side of the flange portion  10 . That is, the rotor  5  is held by those bearings  11  and  12  in a rotatable manner. More specifically, the bearing  11  is fitted onto a protrusion formed on an inner face of the end plate, and the bearing  12  is fitted onto an output shaft  13  to be explained below. 
     Next, here will be explained the continuously variable transmission mechanism  3 . A stator shaft  14  as a fixed shaft extends from a center of the end plate  8  while protruding the outer case  6 . The stator shaft  14  is formed coaxially with a center axis of the cylindrical portion  7 , and one of end portions thereof is integrated with the end plate  8 . A method to integrate the stator shaft  14  with the end plate  8  should not be limited to a specific method. For example, the stator shaft  14  can be integrated with the end plate  8  by a shrinkage fitting method, welding, a bolt etc. As shown in  FIG. 1 , a hollow portion is formed in the stator shaft  14  from the end portion of the end plate  8  side to a portion slightly above an axially intermediate portion. In addition, two slits  15  of predetermined length are formed on the axially intermediate portion of the stator shaft  14 . Specifically, those slits  15  are formed on symmetric portions of the stator shaft  14  across a center axis of the stator shaft  14  while penetrating through the stator shaft  14  from an outer face to the hollow portion. 
     A shift shaft  16  is inserted into the hollow portion in the stator shaft  14  in a rotatable manner, and one of the end portions of the shift shaft  16  protrudes from the end portion of the stator shaft  14  of the end plate  8  side. In addition, an external thread  17  is formed on an intermediate portion of the shift shaft  16 , more accurately, on a portion corresponding to the slit  15 , and a shift key  18  is screwed onto the external thread  17 . The shift shaft  16  and the shift key  18  are schematically shown in  FIG. 3 . As shown in  FIG. 3 , the shift key  18  comprises a cylindrical internal thread  19  screwed onto the external thread  17 , and two keys  20  protruding diametrically from the internal thread  19 . The key  20  protrudes toward the outer circumferential side of the stator shaft  14  through the slit  15 . Therefore, the key  20  of the shift key  18  is moved back and forth in the axial direction of the stator shaft  14  by turning the shift key  18 . 
     A bearing member  21  is fitted loosely onto the stator shaft  14  in a manner to reciprocate in the axial direction. Specifically, the bearing member  21  is an annular member, and a periphery of the bearing member  21  is depressed entirely at an axially intermediate portion thereof. In addition, a leading end of the key  20  is individually engaged with the bearing member  21  at diametrically symmetric two portions of the bearing member  21 . Therefore, the bearing member  21  can be moved by the key  20  in the axial direction. 
     A portion in the vicinity of a bottom of the depressed portion of the bearing member  21  is formed into a smooth concave curve whose cross section is arcuate, and a bearing ball  22  is situated thereon. An idle roller  23  is held by the bearing ball  22  in a rotatable manner at a central portion of the bearing member  21 . Specifically, the idle roller  23  is a cylindrical member, and both ends of an inner circumferential face thereof are contacted with the bearing balls  22  to be held by the bearing member  21  through the bearing balls  22  in a rotatable manner. Therefore, the idle roller  23  is moved back and forth in the axial direction together with the bearing member  21 . 
     A plurality of planet balls  24  corresponding to the rolling member of the present invention are arranged around the idle roller  23 . Not only a magnetic material but also a nonmagnetic material can be used to form the planet ball  24 , and the planet ball  24  is preferably shaped into a perfect sphere. However, an ellipsoidal member like a rugby ball having a smooth outer surface and whose sectional shape is oval may also be used as the planet ball  24 . In the example shown in  FIG. 2 , eight planet balls  24  are provided. In fact, each planet ball  24  shown therein is not contacted with the adjacent planet ball  24 . That is, in order not to generate a drag torque between the adjacent planet balls  24  when the planet balls  24  are rotated, a predetermined clearance is maintained individually between the adjacent planet balls  24 . 
     Each of the planet balls  24  is supported in a rotatable manner by a support shaft  25  penetrating the planet ball  24  through a center of the planet ball  24 . For example, a bearing is interposed between an outer circumferential face of the support shaft  25  and the planet ball  24 , and the planet ball  24  is allowed to rotate by the bearing. As shown in  FIG. 1 , the support shafts  25  are arranged parallel to the stator shaft  14 . More specifically, each of the support shafts  25  is arranged parallel to the stator shaft  14 , and adapted to oscillate (i.e., to tilt) in a direction of the plane including a center axis of the stator shaft  14 . 
     Both end portions of the support shaft  25  protrude from the planet ball  24 , and an arm  26  is attached to each of the end portions of the support shaft  25 . Specifically, the arm  26  is adapted to apply a force for tilting the support shaft  25  penetrating through the planet ball  24 . In the example shown in  FIG. 1 , each of the arms  26  extends from the support shaft  25  toward the stator shaft  14 , that is, extends radially toward the center of the electric motor  1 . As shown in  FIG. 1 , a leading end of the arm  26  is tapered (toward the rotational center). The arms  26  thus attached to both ends of the support shaft  25  are individually contacted with side faces (i.e., outer faces) of the bearing member  21  thereby clamping the bearing member  21 . For this purpose, an inner face of the leading end of the arm  26  is tapered in a manner to widen a clearance between the opposing inner faces of the arms  26  toward the stator shaft  14 . Meanwhile, an outer face of the bearing member  21  is formed into a convex face  27 . Therefore, the arm  26  is contacted with the bearing member locally or linearly. Therefore, when the bearing member  21  is moved in the axial direction of the stator shaft  14 , the arm  26  is pushed diagonally outwardly by the bearing member  21  at a contact point between the tapered face of the arm  26  and the convex face  27  of the bearing member  21 . That is, when the bearing member  21  is moved in the axial direction of the stator shaft  14 , the support shaft  25  penetrating through the planet ball  24  as a rotational center axis of the planet ball  24  is inclined with respect to the stator shaft  14  in a plane including the rotational center axis of the stator shaft  14 . 
     An assembly of the support shaft  25  penetrating the planet ball  24  and the arms  26  is held in a manner not to move in the axial direction of the stator shaft  14 . In order to hold the support shaft  25 , the planet ball  24  and the arms  26 , the electric motor  1  is provided with a carrier  28 . As shown in  FIGS. 1 and 4 , the carrier  28  is a member like a cage formed by connecting a pair of circular plates  29  by a plurality of connecting shafts  30 . On each opposed face of the circular plate  29 , a plurality of radial grooves  31  are formed from the center of the circular plate  29  to an outer circumference of the circular plate  29 . Specifically, a number of the radial grooves  31  is same as the number of the arms  26 , and a width of the radial groove  31  is substantially identical to that of the arm  26 . Accordingly, each of the arms  26  is fitted into the radial groove  31  in a manner to incline the support shaft  25  as explained above. Therefore, the assembly of the arms  26  and the support shaft  25  penetrating through the planet ball  24  is not rotated (or revolved) around the stator shaft  24 . 
     Thus, the carrier  28  is configured as a cage. That is, a periphery of the carrier  28  is not closed. Therefore, the planet balls  24  protrude slightly outwardly from the carrier  28 . To the portions of outer faces of the planet balls  24  thus protruding from the carrier  28 , two rotary members such as an input disc  32  and an output disc  33  are contacted in a torque transmittable manner. Those input disc  32  and output disc  33  are annular members, and arranged on both right and left side of the planet balls  24  in  FIG. 1 . Both faces of the input disc  32  and the output disc  33  contacted with the outer face of the planet ball  24  are individually formed into a concave face whose curvature is identical to that of the outer face of the planet ball  24 . 
     More specifically, an outer diameter of the input disc  32  is slightly shorter than an inner diameter of the rotor  5 , and the input disc  32  is arranged to be opposed to the comparatively shorter flange portion  9  integrated with the rotor  5 . In addition, a cam mechanism  34  is interposed between the flange portion  9  and a back face of the input disc  32 . Meanwhile, the output disc  33  corresponding to the output member of the present invention is arranged on the opposite side of the input disc  32  across the planet balls  24 . The output disc  33  is connected with an output shaft  13  through a cam mechanism arranged on a back side of the output disc  33 . 
     The output shaft  13  is a hollow shaft and the stator shaft  14  is inserted therein. Specifically, two bearings  36  are fitted onto the stator shaft  14 , and the output shaft  13  is fitted onto those bearings  36  in a rotatable manner. One of the end portions of the output shaft  13  protrudes out of the outer case  6 , and a flange portion  37  is integrally formed on the other end portion of the output shaft  13  to protrude radially outwardly therefrom. An outer diameter of the flange portion  37  is slightly smaller than the inner diameter of the rotor  5 , and substantially identical to those of the aforementioned input disc  32  and the output disc  33 . Further, a cylindrical portion is formed from an outer end of the flange portion  37 . The cylindrical portion extends outer circumferential side of the carrier  28  toward the output disc  33 , and a cam mechanism  35  is interposed between a leading end face of the cylindrical portion and a back face of the output disc  33 . 
     The cam mechanisms  34  and  35  will be explained in more detail hereinafter. Both of the cam mechanisms  34  and  35  are configured to generate a thrust force in the axial direction according to the torque. A principle thereof is shown in  FIG. 5 . As shown in  FIG. 5 , a first rotary member  38  and a second rotary member  39  are arranged to be opposed to each other on a common axis, and a torque is to be transmitted therebetween. Further, a cam face  40  is formed on an opposed face of at least one of the rotary members  38  and  39 . The cam face  40  is inclined in a manner to decrease a clearance between the rotary members  38  and  39  gradually in the circumferential direction. In other words, a thickness of at least one of the rotary members  38  and  39  is thickened gradually in the circumferential direction. Thus, the clearance between the rotary members  38  and  39  is varied in the circumferential direction, and a cam roller  41  is interposed therebetween. 
     Specifically, in case a torque is applied to any of the rotary members  38  and  39  in a direction to narrow the clearance between the rotary members  38  and  39  where the cam roller  41  is interposed, the rotary members  38  and  39  are integrated by the cam roller  41 . As a result, thrust forces are generated according to the torque and an inclination of the cam face  40 . Specifically, a load Ft at a contact point between the cam roller  41  and the rotary member  38  or  39  in the circumferential (or tangential) direction can be expressed by the following formula: 
         Ft=Tin /( n·r ); 
     where Tin represents an input torque, n represents a number of cam rollers  41 , and r represents a radius of the rotary member  38  or  39  at a portion where the cam roller  41  is situated. Further, a thrust force Fa acting in the axial direction can be calculated by: 
         Fa=Ft /tan(α/2);
 
     where α represents an inclined angle of the cam face  40 . 
     Therefore, when a predetermined torque is applied to the rotor  5 , the torque is transmitted to the input disc  32  through the cam mechanism  34 , and at the same time, a thrust force is generated in the axial direction according to the torque thus transmitted. Consequently, the input disc  32  is pushed onto the planet balls  24  by the thrust force. Meanwhile, when the torque is applied to the output disc  33  contacted with the planet ball  24 , the torque is then transmitted to the output shaft  13  thorough the cam mechanism  35 , and at the same time, a thrust force is generated in the axial direction according to the torque thus transmitted. Consequently, the output disc  33  is pushed onto the planet balls  24  by the thrust force. Thus, the discs  32  and  33  are pushed onto the planet balls  24  according to the torque being transmitted, and a transmission torque capacity is determined in accordance with the thrust force and a coefficient of friction. In addition, a thrust bearing  42  is interposed between the flange portion  37  of the output shaft  13  and the flange portion  10  of the rotor  5 . 
     In the electric motor  1  thus has been explained, the torque is applied to the rotor  5  by supplying a controlled alternating current to the stator coil  4 . As described, the rotor  5  is held by the bearings  11  and  12  in a rotatable manner. Therefore, the rotor  5  is rotated when the torque is applied thereto. As also described, the cam mechanism  34  is interposed between the relatively shorter flange portion  9  (i.e., the flange portion  9  of the right side in  FIG. 1 ) and the input disc  32 . Therefore, in this case, the rotor  5  is connected with the input disc  32  by the cam mechanism  34  in a manner to rotate integrally, and the input disc  32  is pushed onto the planet balls  24  by the thrust force in the axial direction generated according to the torque of the rotor  5 . 
     As a result, the torque is transmitted from the input disc  32  to the planet balls  24  by the frictional force acting therebetween. As described, each of the planet balls  24  is held in a rotatable manner by the support shaft  25  penetrating therethrough and the idle roller  23 , therefore, the planet balls  24  are rotated by the torque transmitted from the input disc  32 , and in this case, the idle roller  23  is also rotated. As also described, the output disc  33  is also contacted with the planet balls  24 , therefore, the torque is transmitted form the planet balls  24  to the output disc  33  by the frictional force acting therebetween. 
     As also described, the cam mechanism  35  is interposed between the output disc  33  and the flange portion  37  of the output shaft  13 . Therefore, the output disc  33  is connected with the output shaft  13  by the cam mechanism  35  in a manner to rotate integrally, and in this situation, a thrust force is generated in the axial direction according to the torque of the output disc  33 . Consequently, the output disc  33  is pushed onto the planet balls  24  by the generated thrust force, and a transmission torque capacity is determined in accordance with the contact pressure therebetween. Thus, the torque of the rotor  5  is transmitted to the output shaft  13  through the continuously variable transmission mechanism  3 , and the torque thus transmitted is outputted from the output shaft  13  to a predetermined external equipment. 
     As described, the torque thus transmitted from the rotor  5  to the output shaft  13  is determined by the transmission torque capacity of the continuously variable transmission mechanism  3 , and the transmission torque capacity is governed mainly by the contact pressure between the planet balls  24  and the input disc  32 , and the contact pressure between the planet balls  24  and the output disc  33 . In case the planet balls  24  are magnetic bodies, the planet balls  24  are attracted by a magnetic force of the permanent magnet arranged in the rotor  5  to be adhered to the input disc  32  and the output disc  33 . In this case, therefore, the contact pressure between the planet balls  24  and the input disc  32 , and the contact pressure between the planet balls  24  and the output disc  33  are increased so that the transmission torque capacity of the continuously variable transmission mechanism  3  is increased. To the contrary, in case the planet balls  24  are not made of magnetic material, a magnetic attraction will not act on the planet balls  24 . However, since the planet balls  24  are not permanent magnets, each clearance between the planet balls  24  will not be varied intermittently even if the planet balls  24  are arranged at regular intervals. Therefore, in this case, generation of cogging torque can be avoided. 
     More specifically, the torque of the rotor  5  is increased or deceased depending on a speed change ratio of the continuously variable transmission  3 , and transmitted to the output shaft  13 . The speed change ratio of the continuously variable transmission  3  is varied according to a tilt angle of the support shaft  25  penetrating through the planet ball  24 . For example, provided that a radius of the input disc  32  is identical to that of the output disc  33 , a rotation radius of the planet ball  24  between a point contacted with the input disc  32  and the rotational center thereof, and a rotation radius of the planet ball  24  between a point contacted with the output disc  33  and the rotational center thereof are identical to each other if the support shaft  25  is kept parallel to the stator shaft  14 . Therefore, in this case, the speed change ratio of the continuously variable transmission mechanism  3  is “1”. 
     In case the support shaft  25  is tilted, one of the rotation radius of the planet ball  24  between the point contacted with the input disc  32  and the rotational center thereof, and the rotation radius of the planet ball  24  between the point contacted with the output disc  33  and the rotational center thereof is increased depending on the tilt angle of the support shaft  25 . At the same time, the other rotational radius of the planet ball  24  between the point contacted with the input disc  32  or the output disc  33  and the rotational center thereof is decreased depending on the tilt angle of the support shaft  25 . Therefore, a rotational speed of the input disc  32  and a rotational speed of the output disc  33  are varied in accordance with the above explained change in the rotational radii of the planet ball  24 . As a result, the speed change ratio as a ratio between the rotational speeds of the input disc  32  and the output disc  33  is varied according to the tilt angle of the support shaft  25 . Such change in the speed change ratio is depicted in  FIG. 6 . Specifically, the rotational speed of the output disc  33  with respect to the rotational speed of the input disc  32  on the assumption that the rotational speed of the input disc  32  is “1” is shown in  FIG. 6 . In  FIG. 6 , a change in the above-explained rotational speed of the output disc  33  with respect to a change in the tilt angle of the support shaft  25  is plotted and connected with a line. 
     The planet ball  24  is tilted together with the support shaft  25  by turning the shift shaft  16  using a not shown shifting device such as a motor and a linkage mechanism etc. Specifically, when the shift shaft  16  is turned, the shift key  18  screwed onto the external thread  17  is moved in the axial direction. In this situation, since the shift key  18  is engaged with the bearing member  21 , the bearing member  21  is moved together with the shift key  18 . As described, the convex face  27  is formed on each side face of the bearing member  21 , and arms  26  are contacted with the convex faces  27  locally or linearly. As also described, the arms  26  are held in a manner not to move in the axial direction but allowed to be inclined. Therefore, when the bearing member  21  is moved in the axial direction, the arm  26  is pushed diagonally outwardly by the bearing member  21  at the contact point between the tapered face of the arm  26  and the convex face  27  of the bearing member  21 , and the arm  26  is thereby inclined. As a result, the planet ball  24  held rotatably by the support shaft  25  penetrating therethrough is inclined together with arms  26 , and a speed change ratio is set according to the tilt angle of the planet ball  24 . 
     Thus, the electric motor  1  of the present invention is capable of outputting the torque generated by the rotor  5  from the output shaft  13  while increasing and decreasing according to the speed change ratio which can be varied continuously (i.e., steplessly). To the contrary, in case of fixing the rotational speed of the output shaft  13  to a constant speed, the rotational speed of the rotor  5  can be varied according to the speed change ratio of the continuously variable transmission mechanism  3 . Therefore, according to the electric motor  1  of the present invention, the rotational speed of the rotor  5  can be set to the speed at which energy efficiency is optimized. Here, such speed at which the energy efficiency is optimized can be obtained from a characteristic diagram or the like. As described, the motor part  2  is configured to generate a torque by supplying a current thereto, and the continuously variable transmission mechanism  3  is configured to vary a speed change ratio thereof continuously which changes the torque of the motor part  2 . According to the present invention, the electric motor  1  is formed by fitting the motor part  2  together with the continuously variable transmission mechanism  3  arranged concentrically therewith in the outer case  6 . Therefore, in addition to the above-explained advantages, the electric motor  1  can be downsized easily and entirely. 
     Taking advantage of the above-explained benefits, the electric motor  1  of the present invention can be used for many purposes. For example, as schematically shown in  FIG. 7 , the electric motor  1  can be used as an in-wheel motor of a vehicle. As shown in  FIG. 7 , a tire  44  is put on a wheel  43 . A through hole is formed in the center of the wheel  43 , and the electric motor  1  is attached to the wheel  43  by fitting the output shaft  13  into the through hole. On the other hand, the electric motor  1  is fixed to a not shown vehicle body. Specifically, the electric motor  1  is fixed to the vehicle body by fixing the end plate  8  of the outer case  6  to an appropriate portion of the vehicle body. By thus fitting the end plate  8  to the vehicle body, a junction area between the electric motor  1  and the vehicle body can be ensured widely so that a fixing strength between the electric motor  1  and the vehicle body can be enhanced. 
     As described, in the electric motor  1 , one of the axial ends of the outer case  6  is closed by the end plate  8 , and the output shaft  13  protrudes from the other axial end. Therefore, when the output shaft  13  outputs a torque therefrom, a bending load and a shearing load are applied to the output shaft  13  and the stator shaft  14  supporting the output shaft  13  from the inner circumferential side. In order to bear the loads thus applied thereto, for example, the stator shaft  14  is preferably structured as shown in  FIG. 8 . In the example shown in  FIG. 8 , an outer diameter d 1  of a portion of the stator shaft  14  protruding from the carrier  28  is larger than an outer diameter d 2  of an end side of the stator shaft  14  fixed with the outer case  6 . In addition, in the diametrically larger portion of the stator shaft  14 , a flange portion  45  is formed to be contacted tightly with a side face of the carrier  28 . Thus, according to the example shown in  FIG. 8 , the outer diameter of the portion of the stator shaft  14  protruding from the carrier  28 , that is, the outer diameter of the potion of the stator shaft  14  to which the bending load is applied is enlarged to enhance a stiffness thereof. Therefore, a support stiffness of the stator shaft  14  can be enhanced. 
     The present invention should not be limited to the example thus has been explained. For example, the motor part may also be structured other than the synchronous electric motor. In the above-explained example, one of the discs contacted with the planet balls held by the idle roller serves as the input member, and the other disc serves as the output member. However, the above-explained continuously variable transmission mechanism is a transmission comprising three rotary elements such as the idle roller and the pair of discs. Therefore, one of those three elements may be used as the idler, and one of the other two elements may be used as the input element, and remaining element may be used as the output element. In addition, in case the rolling member is configured to be attracted by the magnetic force, the rolling member may also be formed of magnetic material partially instead of forming the rolling member using the magnetic material entirely. In this case, for example, it is also possible to form only an outer surface of the rolling member using the magnetic material.