Patent Publication Number: US-6707204-B2

Title: Rotational unit

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a rotational unit having a mechanical rotational device; an electric rotational device, which is coupled to the rotary shaft of the mechanical rotational device and functions as at least one of a motor and a generator; and a rotational member, which is coupled to the rotary shaft and has a power transmitting mechanism located at the periphery of the rotational member for transmitting power between the rotational unit and an external device. 
     Japanese Laid-Open Utility-Model Publication No. 6-87678 discloses such a rotational unit. 
     In the rotational unit disclosed in the publication, the rotary shaft of a mechanical rotational device (the compression mechanism of a hybrid compressor) is coupled to an electric rotational device (a motor). A rotational member (a pulley) is also coupled to the rotary shaft for transmitting power from an external device (an engine). An electromagnetic clutch is located between the rotational member and the rotary shaft to selectively transmit power. 
     As the electromagnetic clutch is engaged and disengaged, the mechanical rotational device is driven by the force of the engine and the rotor of the electric rotational device is rotated to generate electricity, and the mechanical rotational device is driven by the force of the electric rotational device. 
     The rotational member is coupled to a power transmitting mechanism. A belt is engaged with the power transmitting mechanism to transmit power of the engine to the rotational member. The electric rotational device is displaced from the power transmitting mechanism in the axial direction of the rotary shaft. 
     The rotor of the electric rotational device includes permanent magnets. The electric rotational device also includes a stator part, which is formed with a conductor wire. The electric rotational device is driven by electricity supplied from a battery connected to the conductor wire. Also, the battery is charged with electricity generated by the electric rotational device. 
     Although the electric rotational device is axially displaced from the power transmission, the radial dimension of the electric rotational device is not increased to increase the power. Also, since the electromagnetic clutch is formed with relatively large members such as electromagnets, the size of the rotational member is increased. When the electromagnetic clutch is engaged or disengaged, the clutch is controlled by external electric signals, which complicates the structure. 
     When the mechanical rotational device is driven by the engine, the rotor is dragged along and rotated. At this time, since the rotor includes permanent magnets and magnetic force of the rotor acts on the stator, heat is generated due to excitation loss of the stator, which causes energy loss. When the rotor is dragged along and rotated, the force between the permanent magnets and the stator changes due to changes in distances between the poles of the permanent magnets and the poles of the stator. This fluctuates the torque acting on the rotary shaft and thus generates rotational vibration. 
     Current generated by the electric rotational device may be smoothed. To smooth the current, a capacitor may be connected to the battery in parallel. Even if the battery is disconnected from the capacitor by a relay when the battery need not be charged, the electricity continues to be generated as long as the rotor is dragged along and rotated. Accordingly, the voltage between the terminals of the capacitor becomes excessive, which may damage the capacitor. Therefore, the voltage between the terminals of the capacitor needs to be controlled such that it does not become excessive, which complicates the structure. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a first objective of the present invention to provide a compact and simple rotational unit that permits the size of an electric rotational device to be increased regardless of the size of a power transmitting mechanism to increase the power of the electric rotational device. A second objective of the present invention is to provide a rotational unit that reduces energy loss when a mechanical rotational device is driven by an external drive source and suppresses rotational vibrations of a rotary shaft. 
     To achieve the foregoing and other objectives and is accordance with the purpose of the present invention, a rotational unit having a mechanical rotational device, a rotary shaft, an electric rotational device, a rotational member, a one-way clutch is provided. The mechanical rotational device has a housing. The housing includes a front wall. The rotary shaft has an end portion that protrudes from the front wall of the housing. The electric rotational device is coaxial with the rotary shaft. The electric rotational device is coupled to the end portion of the rotary shaft and functions as at least one of a motor and a generator. The rotational member is coupled to the rotary shaft and has a power transmitting mechanism for transmitting power between the rotational unit and an external device. The a one-way clutch is located in a power transmitting path between the rotary shaft and the rotational member. The one-way clutch is located inward of the rotational member. The electric rotational device is located on or forward of the front wall of the housing. At least part of the electric rotational device is located outside the outer dimension of the power transmitting mechanism. 
     Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
     FIG. 1 is a cross-sectional view illustrating a rotational unit according to a first embodiment of the present invention; 
     FIGS.  2 ( a ) and  2 ( b ) are enlarged partial cross sectional views illustrating the one-way clutch used in the rotational unit shown in FIG. 1; 
     FIG. 3 is an enlarged cross-sectional view illustrating a rotational unit according to a second embodiment of the present invention; and 
     FIG. 4 is an enlarged cross-sectional view illustrating a rotational unit according to a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A rotational unit according to a first embodiment of the present invention will now be described. The rotational unit includes a mechanical rotational device, which is a variable displacement swash plate type compressor C for a refrigeration circuit (refrigeration cycle) of a vehicular air conditioner in this embodiment. In FIG. 1, the left end is defined as the front end, and the right end defined as the rear end. 
     As shown in FIG. 1, the mechanical rotational device, or the compressor C, includes a cylinder block  11 , a front housing member  12  coupled to the front end of the cylinder block  11 , and a rear housing member  14  coupled to the rear end of the cylinder block  11 . A valve plate assembly  13  is located between the rear housing member  14  and the cylinder block  11 . The cylinder block  11 , the front housing member  12 , the valve plate assembly  13 , and the rear housing member  14  form the housing of the compressor C. 
     A crank chamber  15  is defined between the cylinder block  11  and the front housing member  12 . A rotary shaft, which is a drive shaft  16  in this embodiment, extends through the crank chamber  15  and is rotatably supported by the housing. 
     The front end portion of the drive shaft  16  is supported by the front housing member  12  with a radial bearing  12 B. A cylindrical support wall  41  is formed in the front wall  12 A of the front housing member  12 . The front end portion of the drive shaft  16  is located in the support wall  41 . The front end portion of the drive shaft  16  is coupled to an external device, or an external drive source, by a rotational member (pulley  17 ), and a belt  18  engaged with the rotational member. In this embodiment, the external drive source is a vehicle engine E, and the rotational member is a pulley  17 . A seal member  12 C is located between the front end portion of the drive shaft  16  and the front wall  12 A. The seal member  12 C is located outside of the radial bearing  12 B. The seal member  12 C prevents gas from flowing between the interior of the housing and the exterior. 
     The pulley  17  is connected to an electric rotational device, which is a motor generator MG in this embodiment. The motor generator MG is located in the power transmitting path between the engine E and the drive shaft  16 . Also, the motor generator MG is partially outside the outer dimension of the pulley  17 . When the engine E is running, the pulley  17  always transmits power from the engine E to the drive shaft  16  and the motor generator MG. At this time, the motor generator MG functions as a generator. When the air conditioner need be operated when the engine E is not running, the motor generator MG functions as a motor and drives the drive shaft  16 . 
     A lug plate  19  is located in the crank chamber  15  and is secured to the drive shaft  16  to rotate integrally with the drive shaft  16 . A cam plate, which is a swash plate  20  in this embodiment, is located in the crank chamber  15 . The swash plate  20  slides along the drive shaft  16  and inclines with respect to the axis of the drive shaft  16 . The swash plate  20  is coupled to the lug plate  19  by a hinge mechanism  21 . The hinge mechanism  21  causes the swash plate  20  to rotate integrally with the lug plate  19  and the drive shaft  16 . The hinge mechanism  21  also permits the swash plate  20  to slide along and incline with respect to the axis of the drive shaft  16 . 
     A snap ring  22  is secured to the drive shaft  16 . A spring  23  extends between the snap ring  22  and the swash plate  20 . The snap ring  22  and the spring  23  determine the minimum inclination angle of the swash plate  20 . The minimum inclination angle of the swash plate  20  refers to an angle at which the angle defined by the axis of the drive shaft  16  and the swash plate  20  is closest to ninety degrees. 
     Cylinder bores  24  (only one is shown) are formed in the cylinder block  11 . The cylinder bores  24  extend parallel to the axis of the drive shaft  16 . A single headed piston  25  is reciprocally accommodated in each cylinder bore  24 . The front and rear opening of each cylinder bore  24  is covered by the corresponding piston  25  and the valve plate assembly  13 . A compression chamber, the volume of which varies in accordance with the reciprocation of the corresponding piston  25 , is defined in each bore  24 . The front end of each piston  25  is connected to the periphery of the swash plate  20  through a pair of shoes  26 . The rotation of the swash plate  20  is converted into reciprocation of the pistons  25 . 
     The drive shaft  16 , the lug plate  19 , the swash plate  20 , the hinge mechanism  21 , the pistons  25 , and the shoes  26  form a piston type compression mechanism. 
     A suction chamber  27  and a discharge chamber  28  are defined in the rear housing member  14 . The front ends of the suction chamber  27  and the discharge chamber  28  are covered by the valve plate assembly  13 . Sets of suction ports  29  and suction valve flaps  30  and sets of discharge ports  31  and discharge valve flaps  32  are formed in the valve plate assembly  13 . Each set of the suction port  29  and the corresponding suction valve flap  30  and each set of the discharge port  31  and the corresponding discharge valve flap  30  correspond to one of the cylinder bores  24  (compression chamber). When each piston  25  moves from the top dead center position to the bottom dead center position, refrigerant gas in the suction chamber  27  flows into the corresponding cylinder bore  24  via the corresponding suction port  29  and suction valve flap  30 . When each piston  25  moves from the bottom dead center position to the top dead center position, refrigerant gas in the corresponding cylinder bore  24  is compressed to a predetermined pressure and is discharged to the discharge chamber  28  via the corresponding discharge port  31  and discharge valve flap  32 . 
     The suction chamber  27  is connected to the discharge chamber  28  through an external refrigerant circuit (not shown). Refrigerant discharged from the discharge chamber  28  flows to the external refrigerant circuit, in which heat exchange by using the refrigerant takes place. Refrigerant discharged from the external refrigerant circuit is drawn into the cylinder bores  24  through the suction chamber  27 , and is then compressed. 
     A shaft chamber  33  is defined in the cylinder block  11  to accommodate the rear portion of the drive shaft  16 . A connecting passage  34  is formed in the drive shaft  16  to communicate the front portion of the crank chamber  15  and the shaft chamber  33 . A communication passage  35  is formed in the valve plate assembly  13  to communicate the suction chamber  27  with the shaft chamber  33 . The shaft chamber  33 , the connecting passage  34 , and the communication passage  35  form a bleed passage connecting the crank chamber  15  with the suction chamber  27 . 
     A supply passage  36  is formed in the compressor housing to connect the discharge chamber  28  with the crank chamber  15 . A control valve  37  is provided in the supply passage  36  to adjust the opening degree of the supply passage  36 . 
     The degree of opening of the control valve  37  is changed for controlling the relationship between the flow rate of high-pressure gas flowing into the crank chamber  15  through the supply passage  36  and the flow rate of gas flowing out of the crank chamber  15  through the bleed passage. The crank chamber pressure Pc is determined accordingly. In accordance with a change in the crank chamber pressure Pc, the difference between the crank chamber pressure Pc and the pressure in the compression chambers is changed, which alters the inclination angle of the swash plate  20 . As a result, the stroke of each piston  25 , that is, the discharge displacement, is controlled. 
     As shown in FIG. 1, a hub  42  is rotatably supported by the support wall  41  with a bearing  43 . The hub  42  is secured to the drive shaft  16  to rotate integrally with the drive shaft  16 . 
     The hub  42  is shaped like a cup having a flange  46  at the open end. That is, the hub  42  has an inner cylinder  44 , which is coupled to the drive shaft  16 , an outer cylinder  45 , the flange  46 , and a rubber ring  47 . The rubber ring  47  is located between the inner cylinder  44  and the outer cylinder  45  and functions as a torque fluctuation reduction member. The hub  42  is secured to the drive shaft  16  by threading the inner cylinder  44  to the front end portion of the drive shaft  16 . The flange  46  is integrally formed with the outer cylinder  45 . The rubber ring  47  couples the inner cylinder  44  with the outer cylinder  45 . The rubber ring  47  reduces fluctuations of torque transmitted between the inner cylinder  44  and the outer cylinder  45  and prevents the life of the bearings  12 B,  43  from being shortened by displacement of the axis of the outer cylinder  45  from the axis of the drive shaft  16 . 
     The pulley  17  has a substantially cylindrical shape and is rotatably supported by the outer cylinder  45  of the hub  42  with a bearing  48 . The pulley  17  rotates relative to the hub  42  and the front housing member  12 . The circumference of the pulley  17  functions as a power transmitting mechanism, which is a belt holder  49  in this embodiment. The belt holder  49  has a saw-tooth cross section. A belt  18 , which is connected to the engine E, is wound about the belt holder  49 . 
     A one-way clutch  50  is arranged between the pulley  17  and the outer cylinder  45  of the hub  42 . In other words, the one-way clutch  50  is located inward of the pulley  17 . An outer clutch member  51  is fixed to the inner circumference of the pulley  17 . An annular inner clutch member  52  is fixed to the outer circumference of the outer cylinder  45  of the hub  42 . The inner clutch member  52  is surrounded by the outer clutch member  51 . 
     As shown in FIGS.  2 ( a ) and  2 ( b ), recesses  53  are formed in the inner circumference of the outer clutch member  51 . The recesses  53  are arranged at equal angular intervals about the axis of the drive shaft  16 . A cam surface  54  is formed on the right end, or the clockwise end, of each recess  53  as viewed in FIGS.  2 ( a ) and  2 ( b ). A roller  55 , which extends parallel with the drive shaft  16 , is accommodated in each recess  53 . Each roller  55  can be moved from a position where the roller  55  is engaged with the cam surface  54  as shown in FIG.  2 ( a ) to a position where the roller  55  is disengaged from the cam surface  54  as shown in FIG.  2 ( b ). 
     A spring seat  56  is provided in each recess  53  at the end opposite to the cam surface  54 . A spring  57  is arranged between each spring seat  56  and the corresponding roller  55 . Each spring  57  urges the corresponding roller  55  toward the corresponding cam surface  54 . 
     As shown in FIG.  2 ( a ), when the pulley  17  is rotated by the power transmission from the engine E in the direction indicated by an arrow, each roller  55  is urged toward the corresponding cam surface  54  by the corresponding spring  57 . Then, the rollers  55  transmit power between the cam surfaces  54  and the outer circumference of the inner clutch member  52 , which rotates the hub  42  in the same direction as the rotation of the pulley  17 . That is, when the engine E is running, the force of the engine E is transmitted to the drive shaft  16  through the hub  42 . Thus, the drive shaft  16  is always rotated when the engine E is running. 
     If the hub  42  is rotated in the direction indicated by the arrow in FIG.  2 ( b ) when the engine E is not running and the pulley  17  is not rotating, the pulley  17  is rotated in the opposite direction relative to the hub  42 . Therefore, each roller  55  is disengaged from the corresponding cam surface  54 . Thus, the hub  42  runs idle with respect to the pulley  17 . 
     The motor generator MG is formed of an induction machine, which functions as a rotational electric device having no permanent magnets. As shown in FIG. 1, part of the motor generator MG is located axially between the belt holder  49  of the pulley  17  and the front wall  12 A of the compressor housing. 
     The motor generator MG includes the outer cylinder  45  of the hub  42 , a stator  61  and a rotor  62 . The stator  61  is fixed to the front surface of the front wall  12 A of the front housing member  12 . The stator  61  is located at the outermost position in the radial direction of the drive shaft  16  without radially protruding outward from the outer circumference (the maximum diameter portion) of the front housing member  12 . The stator  61  includes a stationary iron core and a coil wound about the core. 
     The rotor  62  of the motor generator MG is fixed to the peripheral portion of the flange  46  of the hub  42  to face the stator  61 . Like the stator  61 , the rotor  62  is located at the outermost position in the radial direction of the drive shaft  16  without radially protruding outward from an imaginary cylinder that extends axially from the circumference (the maximum diameter portion) of the front housing member  12 . The rotor  62  includes a rotational iron core and a rotary conductor, which is fixed to the rotational core. 
     The coil of the stator  61  is connected to a battery (not shown) by a drive circuit (not shown) having an inverter and a converter. Based on commands from a controller (not shown), the drive circuit controls charging of electricity from the coil to the battery and supply of electricity from the battery to the coil. 
     The drive circuit is controlled by the controller. When the battery need to be charged while the engine E is running, the drive circuit causes the motor generator MG to function as an induction generator for generating electricity. That is, when the hub  42  (the rotor  62 ) is rotated by the engine E, electricity is generated in the coil. The generated electricity is sent to the battery through the drive circuit to charge the battery. 
     When the battery does not need to be charged while the engine E is running, the drive circuit causes the motor generator MG not to generate electricity. Specifically, the drive circuit is controlled by the controller such that no excitation current is supplied to the motor generator MG, which functions as an induction machine. 
     In this state, no magnetic force exists between the stator  61  and the rotor  62 . Therefore, even if the rotor  62  is rotated by the force of the engine E, energy loss, such as heat due to excitation loss of the stator  61  and the rotor  62 , does not occur. Also, even if the rotor  62  is being rotated by the force of the engine E, torque fluctuations of the drive shaft  16  due to magnetic force are not produced. 
     When the controller judges that air conditioning (cooling) is needed based on external information, the drive circuit causes the motor generator MG to function as an induction motor. That is, the drive circuit supplies electricity to the coil to generate rotational force in the rotor  62 . The rotational force is transmitted to the drive shaft  16  through the hub  42 . This permits the passenger compartment to be air conditioned even if the engine E is not running. 
     When the motor generator MG functions as a motor and rotates the hub  42 , the one-way clutch  50  prevents power from being transmitted between the hub  42  and the pulley  17 . Therefore, the power of the motor generator MG is not transmitted to the engine E. 
     The compressor C, the bearing  43 , the hub  42 , the bearing  48 , the one-way clutch  50 , the pulley  17 , the motor generator MG, the drive circuit, the battery, and the controller form the rotational unit. 
     This embodiment has the following advantages. 
     (1) The motor generator MG is coaxial with the drive shaft  16  and is located at the front side of the front wall  12 A of the front housing member  12 . Also, part of the motor generator MG is radially outside of the belt holder  49 . Compared to a case where the motor generator MG is located about the compressor housing and at the rear side of the front wall  12 A, the first embodiment decreases the size of the rotational unit in the axial direction. The size of either of the motor generator MG or the belt holder  49  does not limit the size of the other. This permits the size of the motor generator MG to be increased and the size of the belt holder  49  to be decreased. Therefore, for example, the power of the motor generator MG can be easily increased while minimizing the size of the belt holder  49 . 
     (2) Part of the motor generator MG is located axially between the belt holder  49  and the compressor housing. Compared to a case where the entire motor generator MG is located at the opposite side of the belt holder  49 , the first embodiment permits the size of the rotational unit to be axially reduced. 
     (3) Part of the motor generator MG is located between the front wall  12 A of the front housing member  12  and the belt holder  49  such that the motor generator MG does not protrude radially outward from the compressor housing. Therefore, compared to a case where part of or the motor generator MG or the entire motor generator MG is located on the outer circumference of the compressor housing, the first embodiment permits the size of the rotational unit to be decreased in the radial direction. 
     Also, in the first embodiment, the stator  61  and the rotor  62  are located at the outermost position without protruding radially outward from the outer circumference of the front housing member  12 . Therefore, the power of the motor generator MG can be increased while minimizing the radial dimension of the rotational unit. 
     (4) The one-way clutch  50  is located between the drive shaft  16  and the pulley  17 , which are in the power transmission path. Compared to a case where an electromagnetic clutch is located between the drive shaft  16  and the pulley  17 , the parts used in the mechanism for disconnecting the power transmission between the drive shaft  16  and the pulley  17  are light. This decreases the size of the pulley  17  and minimizes the size and the weight of the rotational unit. Further, since there is no need to perform a control procedure for disengaging an electromagnetic clutch, the structure of the rotational unit is simple. 
     (5) The rubber ring  47  is located between the inner cylinder  44  and the outer cylinder  45 . The rubber ring  47  reduces the torque fluctuations between the inner cylinder  44  and the outer cylinder  45 . The rubber ring  47  also hinders the life of the bearings  12 B,  43  from being shortened by displacement of the axis of the outer cylinder  45  from the axis of the drive shaft  16 . 
     (6) The motor generator MG is an induction machine having no permanent magnet. Compared to a case where a motor generator having permanent magnets is used, the first embodiment reduces the cost. 
     This structure permits magnetic force between the stator  61  and the rotor  62  to be eliminated. Therefore, when the rotor  62  is rotated by the force of the engine E, energy loss such as heat due to excitation loss of the stator and the rotor  62  is prevented. 
     Since the magnetic force between the stator  61  and the rotor  62  can be eliminated, torque fluctuations in the drive shaft  16  due to magnetic force are prevented when the rotor  62  is rotated by external force. Therefore, rotational vibration of the drive shaft  16  is suppressed. 
     This structure can prevent the motor generator MG from generating electricity even if the rotor  62  is being rotated by the force of the engine E. The structure therefore has the following advantages. For example, suppose a condenser is connected to the battery in parallel for smoothing electricity that is generated by the motor generator MG and is then commutated. In this case, the battery is disconnected from the condenser when, for example, the battery need not be charged. At this time, even if the rotor  62  is being rotated by the force of the engine E, the condenser is prevented from being damaged by excessive voltage between the terminals of the condenser. The structure for preventing the voltage between the condenser terminals from being excessive is simple, which simplifies the structure of the rotational unit. 
     FIG. 3 illustrates a second embodiment according to the present invention. The second embodiment has the same construction as the first embodiment except for the location and the structure of a motor generator MG 2  and the structure of a hub  81 . Thus, like or the same reference numerals are given to those components that are like or the same as the corresponding components of the first embodiment. 
     As shown in FIG. 3, a cylindrical support wall  41  is formed at the front wall  12 A of the front housing member  12 . The support wall  41  of the second embodiment extends further forward as compared to that of the first embodiment. 
     The hub  81  is located between the pulley  17  and the drive shaft  16  in the power transmission path. The hub  81  includes a first hub member  82  and a second hub member  83 . 
     The first hub member  82  includes a small cylinder  84 , a large cylinder  85 , and a flange  86 . The small cylinder  84  is fitted about the outer ring of the bearing  43  and is located of the pulley  17 . The inner diameter of the large cylinder  85  is greater than the maximum outer diameter of the pulley  17 . The flange  86  couples the small cylinder  84  to the large cylinder  85 . 
     The pulley  17  is rotatably supported by the small cylinder  84  with the bearing  48  and the one-way clutch  50  and rotates relative to the hub  81 . The second hub member  83  includes an inner hub member  83 A, a disk-like outer hub member  83 B and a torque fluctuation reduction member, which is a rubber ring  83 C. The rubber ring  83 C is located between the boss  83 A and the outer hub member  83 B. A boss is formed in the center of the inner hub member  83 A. The second hub member  83  is secured to the drive shaft  16  by threading the boss to the front end portion of the drive shaft  16 . The rubber ring  83 C is located between the inner hub member  83 A and the outer hub member  83 B to couple the members  83 A,  83 B to each other. The diameter of the circumference of the outer hub member  83 B is equal to the inner diameter of the large cylinder  85  of the first hub member  82 . The second hub member  83  is detachably attached to the first hub member  82  to cover the front opening of the large cylinder  85 . 
     In the state where the second hub member  83  is secured to the first hub member  82 , the hub members  83 ,  82  rotate integrally. The rubber ring  83 C reduces fluctuations of torque transmitted between the inner hub member  82 A and the outer hub member  83 B. Further, in the state where the outer hub member  83 B is attached to the first hub member  82 , the rubber ring  83 C prevents the life of the bearings  12 B,  43  from being shortened by the displacement of the axis of the outer hub member  83 B from the axis of the drive shaft  16 . 
     The main part of an electric rotational device, which is the motor generator MG 2  in this embodiment, is located at the opposite side of the pulley  17  from the compressor housing. The motor generator MG 2  includes the first hub member  82 , stator supports  87 , a stator  88 , and a rotor  91 . Therefore, part of the motor generator MG 2  is outside the outer dimension of the pulley  17 . 
     The stator supports  87  (only two of them are shown in FIG. 3) are fixed to the distal end of the support wall  41 . The stator supports  87  extend outward in the radial direction of the drive shaft  16 . The stator  88  is secured to the distal ends of the stator supports  87 . The stator  88  includes a stationary iron core  89  and a coil  90  wound about the core  89 . 
     The rotor  91  is mounted on the inner circumference of the large cylinder  85  of the first hub member  82  to face the stator  88 . The rotor  91  includes a rotational iron core and a rotary conductor fixed to the iron core. 
     As in the first embodiment, the coil  90  is connected to a battery (not shown) by a drive circuit (not shown) having an inverter and a converter. Based on commands from a controller (not shown), the drive circuit controls charging electricity from the coil  90  to the battery and supply of electricity from the battery to the coil  90 . 
     The compressor C, the bearing  43 , the hub  81 , the bearing  48 , the one-way clutch  50 , the pulley  17 , the motor generator MG 2 , the drive circuit, the battery, and the controller form the rotational unit. 
     The rotational unit of the second embodiment has the advantages (1), (4), and (6) of the rotational unit of the first embodiment. Additionally, the rotational unit of the second embodiment has the following advantages. 
     (7) The motor generator MG 2  (except the small cylinder  84 ) is located on the opposite side of the belt holder  49  from the compressor housing. Therefore, the compressor C does not hamper the maintenance of the motor generator MG 2 . That is, the structure of the second embodiment improves the efficiency of maintenance, which is performed from, for example, the front side after detaching the second hub member  83  from the first hub member  82 . 
     (8) The rubber ring  83 C is located between the inner hub member  83 A and the outer hub member  83 B. The rubber ring  83 C reduces the torque fluctuations between the inner hub member  83 A and the outer hub member  83 B. When the outer hub member  83 B is attached to the first hub member  82 , the rubber ring  83 C hinders the life of the bearings  12 B,  43  from being shortened by displacement of the axis of the outer hub member  83 B from the axis of the drive shaft  16 . 
     FIG. 4 illustrates a third embodiment according the present invention. The third embodiment has the same construction as the second embodiment except for the location of the motor generator MG 2  and the structure of the pulley  17 . Thus, like or the same reference numerals are given to those components that are like or the same as the corresponding components of the second embodiment. 
     As shown in FIG. 4, the diameter of a pulley  17  of the third embodiment is greater than the pulley  17  of the second embodiment. An annular recess  71  is formed on the front side of the pulley  17 . The recess  71  is formed radially inward of the belt holder  49 . 
     The rear portion of the motor generator MG 2  is located radially inward of the belt holder  49 . In other words, part of the motor generator MG 2  that includes the stator  88  and the rotor  91  overlaps the belt holder  49  in the axial direction. The maximum outer diameter of the motor generator MG 2  (the diameter of the circumference of the rotor  91 ) is smaller than the maximum outer diameter of the pulley  17 . The radially outer portion of the flange  86  bulges rearward from the radially inner portion so that part of the motor generator MG 2  is located radially inward of the bulging portion. 
     In addition to the advantages (4), (6), (7), and (8), the third embodiment has the following advantage. 
     (9) The motor generator MG 2  is coaxial with the drive shaft  16  and is located forward of the front wall  12 A of the front housing member  12 . Part of the motor generator MG 2  overlaps the belt holder  49  in the axial direction. Compared to a case where the motor generator MG is located rearward of the front wall  12 A and about the housing of the compressor housing, the third embodiment reduces the size of the rotational unit in the radial direction of the drive shaft  16 . Also, compared to a case where the motor generator MG 2  is located outside the outer dimension of the belt holder  49  in the axial direction, the third embodiment reduces the size of the rotational unit in the axial direction. 
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the invention may be embodied in the following forms. 
     In the first embodiment, the stator  61  and the rotor  62  are located at the outermost position without protruding radially outward from the circumference of the front housing member  12 . However, the stator  61  and the rotor  62  may be located radially inward of the positions of the first embodiment. 
     In the first embodiment, the rubber ring  47  is fixed to the inner cylinder  44  and the outer cylinder  45 . However, the rubber ring  47  may be replaced with a detachable member that discontinues power transmission between the inner cylinder  44  and the outer cylinder  45  when an excessive load torque acts on the cylinders  44 ,  45 . 
     In the first embodiment, the rubber ring  47  may be omitted, and the inner cylinder  44  may be directly coupled to the outer cylinder  45 . 
     In the second embodiment, the inner diameter of the large cylinder  85  of the hub  81  may be smaller than the maximum diameter of the pulley  17 . 
     In the second embodiment, the rubber ring  83 C is fixed to the inner hub member  83 A and the outer hub member  83 B. However, the rubber ring  83 C may be replaced with a detachable member that discontinues power transmission between the inner hub member  83 A and the outer hub member  83 B when an excessive load torque acts on the hub members  83 A and  83 B. 
     In the second embodiment, the rubber ring  83 C may be omitted, and the inner hub member  83 A may be directly coupled to the outer hub member  83 B. 
     In the third embodiment, the outer diameter of the front portion of the motor generator MG 2  may be partly greater than the maximum diameter of the pulley  17 . 
     In the illustrated embodiments, the rubber rings  47 ,  83 C are used as a torque fluctuation reduction member. The rubber rings  47 ,  83 C may be replaced by any structure as long as the structure reduces torque fluctuations. 
     In the illustrated embodiments, the one-way clutch  50  having the outer clutch member  51 , the inner clutch member  52 , and the rollers  55  is used. However, the one-way clutch  50  may be replaced by any structure as long as the structure permits power transmission from the pulley  17  to the drive shaft  16  and prevents power transmission from the motor generator MG to the pulley  17 . 
     In the illustrated embodiment, the present invention is applied to the motor generators MG, which include an induction machine having no permanent magnets. However, the present invention may be applied to a motor generator having permanent magnets. Compared to a motor generator having no permanent magnets, a motor generator having permanent magnets can produce greater power. 
     In the illustrated embodiments, the electric rotational device is an induction machine having no permanent magnets. However, the electric rotational device may be a reluctance motor having no permanent magnets. Although not capable of generating electricity, a reluctance motor having no permanent generates a relatively great starting torque as compared to an induction machine having no permanent magnets. That is, the reluctance motor is advantageous in generating a greater torque. The reluctance motor may be, for example, a switched reluctance motor (SR motor) or a variable reluctance motor (VR motor). 
     In the illustrated embodiments, an induction machine having no permanent magnets is used as the electric rotational device. However, the present invention may be applied to a stepping motor having no permanent magnets. Since a stepping motor generates a greater starting torque compared to an induction machine, the stepping motor is therefore advantageous in generating greater torque. 
     In the illustrated embodiments, the mechanical rotational device is applied the compressor C having single headed pistons, which compresses refrigerant at one side of each piston. However, the mechanical rotational device may be a double-headed piston type compressor. A double-headed piston type compressor has pairs or front and rear cylinder bores. Each piston corresponds to one of the pairs of the front and rear cylinder bores and compresses gas in the corresponding cylinder bores. 
     In the illustrated embodiments, the present invention is applied to the compressor C, in which the cam plate (swash plate  20 ) rotates integrally with the drive shaft  16 . However, the present invention may be replaced with a wobble type compressor, in which a cam plate rotates relative to a drive shaft. 
     In the illustrated embodiments, the present invention may be applied to a fixed displacement compressor, in which the stroke of pistons is not variable. 
     In the illustrated embodiment, the present invention is applied to the piston type compressor C, in which pistons  25  reciprocate. However, the present invention may be applied to a rotary compressor such as a scroll type compressor. 
     In the illustrated embodiment, the present invention is applied to the compressor C. However, the present invention may be applied to any type of rotary apparatus as long as the apparatus drives a rotary shaft by using driving force transmitted through a rotor or by using driving force of an electric rotational device. 
     Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.