Abstract:
A vehicle AC generator capable of enhancing a cooling effect by increasing an air volume of cooling air is obtained. The AC generator includes: a casing having a ventilation port on an outer periphery; a stator installed in the casing; a rotor supported in the stator in a rotatable manner; a fan installed oppositely to the ventilation port in the casing and rotated integrally with the rotor; and a protrusion formed on a surface of the casing opposing a tip end of the fan at a position adjacent to an outer periphery of the ventilation port of the casing.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an AC generator, and more particularly, to a vehicle AC generator that charges an in-vehicle battery and supplies an in-vehicle electric device with power. 
         [0003]    2. Description of the Related Art 
         [0004]    Ina vehicle AC generator in the related art, as described in International Publication No. WO 2008-037294, a suction port of a fan provided to a bracket has a circular cross section. A diameter of the suction port decreases smoothly toward the fan and then gradually increases to a position adjacent to the fan in closest proximity. 
         [0005]    In the vehicle AC generator in the related art, however, because the diameter of the suction port gradually increases toward the fan, a range in which momentum is given to the cooling air by the fan is reduced and so is an amount of pressure rise by the fan. 
         [0006]    Consequently, even when a pressure loss at an outlet port side of the fan is lessened by providing a space at the tip end of the fan, the vehicle AC generator in the related art has a problem that an increase in air volume of cooling air decreases in a case where there is a certain amount of pressure loss in an air passage upstream of the fan. 
         [0007]    Also, a pressure of airflow flowing out from the fan is high whereas a pressure is low in the vicinity of the suction port of the fan. Hence, a leakage flow of the cooling air is generated in a space in the vicinity of the tip end of the fan from downstream of the fan to the suction port of the fan. However, because the structure is such that the suction port of the fan gradually expands to the position adjacent to the fan in closest proximity, resistance against the leakage flow of the cooling air is so low that a flow rate of the leakage flow of the cooling air readily increases. Given that an amount of work by the fan is constant, an air volume of the discharged cooling air decreases when there is a considerable leakage flow of the cooling air.
   [Patent Document 1] International Publication No. WO 2008/037294   
 
         [0009]    As has been described, the vehicle AC generator in the related art described in Patent Document 1 has a problem that a temperature of a component that needs cooling by airflow generated by the fan rises due to a decrease in air volume of the cooling air. 
       SUMMARY OF THE INVENTION 
       [0010]    The invention was devised to solve the problems of the AC generator in the related discussed above, and has an object to enhance a cooling effect on a stator winding and rectifier elements by lessening a pressure loss in an air passage of airflow generated by a fan of a vehicle AC generator and also by increasing an air volume of cooling air. 
         [0011]    An AC generator according to an aspect of the invention includes: a casing having a ventilation port on an outer periphery; a stator installed in the casing; a rotor supported in the stator in a rotatable manner; a fan installed oppositely to the ventilation port in the casing and rotated integrally with the rotor; and a protrusion formed on a surface of the casing opposing a tip end of the fan at a position adjacent to an outer periphery of the ventilation port of the casing. 
         [0012]    According to the AC generator configured as above, by providing the protrusion on a surface of the casing opposing the tip end of the fan at a position adjacent to the outer periphery of the ventilation port of the casing, an area of an air passage of the cooling air can be increased and hence an air volume generated by the fan can be increased. It thus becomes possible to enhance a cooling effect on a stator winding and rectifier elements in the AC generator. 
         [0013]    The foregoing and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a cross section of a vehicle AC generator according to a first embodiment of the invention; 
           [0015]      FIG. 2  is a rear end view showing a protection cover unattached state of the vehicle AC generator according to the first embodiment of the invention; 
           [0016]      FIG. 3  is a perspective view of a fan of the vehicle AC generator according to the first embodiment of the invention; 
           [0017]      FIG. 4  is a perspective view of a rear bracket of the AC generator according to the first embodiment of the invention; 
           [0018]      FIG. 5  is a cross section of a major portion in an AC generator according to a second embodiment of the invention; and 
           [0019]      FIG. 6  is a cross section of a major portion in an AC generator according to a third embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       [0020]      FIG. 1  is a cross section of a vehicle AC generator according to a first embodiment of the invention.  FIG. 2  is a rear end view showing a protection cover unattached state of the vehicle AC generator according to the first embodiment of the invention. 
         [0021]    Hereinafter, a configuration and an operation of the AC generator of the first embodiment will be described in detail with reference to the drawings. Referring to  FIG. 1  and  FIG. 2 , the vehicle AC generator  1  includes a casing  4  formed of a front bracket  2  and a rear bracket  3  both of which are of a cup shape and made of aluminum. 
         [0022]    The AC generator  1  also includes a shaft  6  supported on the casing  4  in a rotatable manner via a pair of bearings  5 , a pulley  7  firmly fixed to the shaft  6  at an end extending frontward with respect to the casing  4 , and a rotor  8  fixed to the shaft  6  and installed in the casing  4 . 
         [0023]    The AC generator  1  further includes fans  11  fixed to the rotor  8  on both end faces in an axial direction  6   a , a stator  12  fixed to the casing  4  so as to surround the rotor  8 , a pair of slip rings  15  fixed to the shaft  6  in an extending portion extending rearward with respect to the casing  4  and supplying the rotor  8  with a current, a pair of brushes  16  sliding on surfaces of the respective slip rings  15 , and a brush holder  17  in which to store the brushes  16 . 
         [0024]    The AC generator  1  furthermore includes a voltage adjustor  22  installed on the rear side of the rear bracket  3  and adjusting magnitude of an AC voltage generated at the stator  12 , a connector  24  installed on the rear side of the rear bracket  3 , through which signals are inputted and outputted between the voltage adjustor  22  or the like and an outside device (not shown), a rectifier device  28  installed on the rear side of the rear bracket  3  and rectifying an AC voltage generated at the stator  12  to a DC voltage, and a protection cover  50  attached to the rear bracket  3  so as to cover the brush holder  17 , the voltage adjustor  22 , and the rectifier device  28 . 
         [0025]    The rotor  8  is a Lundell type rotor and includes a field winding  9  generating a flux when an excitation current is flown through and a pole core  10  provided so as to cover the field winding  9  and on which magnetic poles are generated by the flux. 
         [0026]    The stator  12  includes a cylindrical stator core  13  and a stator winding  14  coiled around the stator core  13  and generating AC with a variance of the flux from the field winding  9  in association with rotations of the rotor  8 . Further, the stator  12  is installed so as to surround the rotor  8  while the stator core  13  is pinched at opening ends of the front bracket  2  and the rear bracket  3  from both sides in the axial direction. 
         [0027]    The rectifier device  28  includes a positive-electrode-side heat sink  29  on which multiple positive-electrode-side rectifier elements  30  are mounted, a negative-electrode-side heat sink  31  on which multiple negative-electrode-side rectifier elements  32  are mounted, and a circuit board  33 . The circuit board  33  is sandwiched between the positive-electrode-side heat sink  29  and the negative-electrode-side heat sink  31  substantially in the shape of a capital C. The circuit board  33  is molded from insulating resin, such as PBT. 
         [0028]    The positive-electrode-side rectifier elements  30  and the negative-electrode-side rectifier elements  32  are connected so as to form a predetermined bridge circuit via an electric conductor insert-molded in the circuit board  33 . 
         [0029]    An operation of the vehicle AC generator  1  will now be described. The vehicle AC generator  1  operates as a three-phase AC generator with 12 magnetic poles and 36 slots. It should be appreciated, however, that the numbers of the magnetic poles and the slots are not limited to those specified above. 
         [0030]    In the vehicle AC generator  1 , a flux is generated as a current is supplied from a battery (not shown) to the field winding  9  of the rotor  8  via the brushes  16  and the slip rings  15 . With this flux, N-poles and S-poles are generated on an outer peripheral surface of the pole core  10  alternately in a circumferential direction. 
         [0031]    On the other hand, the rotor  8  is rotated as a rotational torque of the engine is transmitted from an output shaft of the engine to the shaft  6  via a belt and the pulley  7 . Hence, a rotational field is given to the stator winding  14  of the stator  12  and an electromotive force is generated at the stator winding  14 . An AC current generated by this electromotive force is rectified by the rectifier device  28  and charged to the battery or supplied to an electric load. 
         [0032]    The fans  11  rotate in association with rotations of the rotor  8 . On the front side, cooling air is sucked into the front bracket  2  from a front-side inlet  2   a  and flown in the axial direction to the vicinity of the rotor  8 . Then, the cooling air is bent in a centrifugal direction by the fan  11  in the vicinity of the rotor  8  and discharged from a front-side outlet  2   b.    
         [0033]    On the rear side, the cooling air is sucked into the protection cover  50  from multiple outlets  53  provided to the protection cover  50  and flown to the rear bracket  3  by passing through spaces among radiation fins provided to the positive-electrode-side heat sink  29  and the negative-electrode-side heat sink  31 . Subsequently, the cooling air is sucked into the rear bracket  3  from a rear-side inlet  3   a  and flown in the axial direction  6   a  to the vicinity of the rotor  8 . Then, the cooling air is bent in the centrifugal direction by the fan  11  in the vicinity of the rotor  8  and discharged from a rear-side outlet  3   b.    
         [0034]    A part of heat generated at the stator  12  is released from a coil end  12   a  by the cooling air flowing out from the fans  11  and headed for the front-side outlet  2   b  and the rear-side outlet  3   b.    
         [0035]    Also, another part of heat at the stator  12  is transmitted to the front bracket  2  and the rear bracket  3  and released to the cooling air from multiple front-side ribs  51  of the front-side outlet  2   b  and rear-side ribs  52 , so that the stator is cooled. In addition, the positive-electrode-side rectifier elements  30  and the negative-electrode-side rectifier elements  32  are cooled by the cooling air flowing through spaces among the radiation fins provided to the positive-electrode-side heat sink  29  and the negative-electrode-side heat sink  31 . 
         [0036]    A structure and an operation on the periphery of the rear bracket  3  and the fan  11  will now be described. There is a clearance  60  between a tip end  11   a  of the rotating fan  11  and the stationary rear bracket  3 . Herein, a pressure is low on the side of the shaft  6  of the fan  11  and a pressure is high on the side of the rear bracket outlet  3   b . A leakage flow is therefore generated in the clearance  60  from the rear-side outlet  3   b  toward the shaft  6 . 
         [0037]    As an amount of the leakage flow increases, an amount of air cooling the stator  12 , the positive-electrode-side rectifier elements  30 , and the negative-electrode-side rectifier elements  32  decreases. Hence, temperatures of the stator  12 , the positive-electrode-side rectifier elements  30 , and the negative-electrode-side rectifier elements  32  rise and an amount of power generation of the AC generator  1  decreases. 
         [0038]    In order to reduce an amount of the leakage flow in the clearance  60 , the rear bracket  3  and the tip end  11   a  of the fan  11  are opposed in parallel to each other so that the clearance  60  is reduced to the minimum. This configuration can reduce an amount of the leakage flow. However, this configuration has a problem that a high pressure side and a low pressure side are formed on a vane surface of the rotating fan  11 , and pressures on these sides propagate to the rear bracket  3  and give rise to a fluctuation of the pressure on the surface of the rear bracket  3 , which results in large noises. 
         [0039]    On the other hand, in the typical vehicle AC generator  1 , a lead  14   a  is connected from the coil end  12   a  of the stator  12  to the rectifier device side, and the presence of the lead  14   a  reduces an area of a passage of air flowing out from the fan  11 . The tip end of the coil end  12   a  also reduces the area of the passage of air flowing out from the fan  11  in some cases due to its location closer to the tip end  11   a  of the fan  11  in the axial direction  6   a  than to a bottom surface  11   b  of the fan  11 . When the area of the passage of air flowing out from the fan  11  is reduced, an amount of air to cool the stator  12 , the positive-electrode-side rectifier elements  30 , and the negative-electrode-side rectifier elements  32  is decreased. Hence, temperatures of the stator  12 , the positive-electrode-side rectifier elements  30 , and the negative-electrode-side rectifier elements  32  rise and an amount of power generation of the vehicle AC generator  1  decreases. 
         [0040]    A structure on the periphery of the rear bracket  3  and the fan  11  according to the first embodiment of the invention will now be described. 
         [0041]      FIG. 3  is a perspective view of a fan of the vehicle AC generator according to the first embodiment of the invention. Referring to  FIG. 3 , the fan  11  includes a fan main body  83  and multiple vanes (blades)  84  provided to the fan main body  83 . Each of the multiple vanes  84  has a portion  81  having a maximum vane height somewhere between an inner periphery  80  and an outer periphery  82  of the fan main body  83 . The vane  84  also has a constant height from the portion  81  having the maximum vane height to the outer periphery  82 . 
         [0042]      FIG. 4  is a perspective view of the rear bracket of the vehicle AC generator according to the first embodiment of the invention. The rear bracket  3  includes multiple inlets  3   a  to let the cooling air in. Also, the rear bracket  3  includes outlets  3   b  partitioned by the ribs  52 . A diameter  85  of the outer periphery of the multiple inlets  3   a  of the rear bracket  3  is formed to be smaller than a diameter of the outer periphery of the fan  11 . Also, the vane height of the fan  11  at a position in a radial direction  6   b  of the outer periphery of the inlets  3   a  has the portion  81  having the maximum vane height. The diameter  85  of the outer periphery of the inlets  3   a  of the rear bracket  3  is formed to be larger than a diameter of a periphery of the fan  11 , which forms the portion  81  having the maximum vane height. 
         [0043]    Referring to  FIG. 1  again, in addition to the configuration described above, the AC generator  1  of the first embodiment includes a protrusion  72  on a surface of the rear bracket  3  opposing the tip end  11   a  of the fan  11  at a position adjacent to the outer periphery of the inlets  3   a  of the rear bracket  3 . 
         [0044]    An end portion  70  of the rear bracket  3  on the side of the fan  11  is parallel to the axial direction  6   a  of the shaft  6  and in closest proximity to the fan  11 . The protrusion  72  inclines from the end portion  70  to the radial direction  6   b  so as to move away from the fan  11 . Owing to the configuration as above, the protrusion  72  having a right-angled triangular cross section is formed on the surface of the rear bracket  3  opposing the tip end  11   a  of the fan  11  at a position adjacent to the outer periphery of the inlets  3   a  of the rear bracket  3 . 
         [0045]    By configuring in this manner, a distance between the tip end  11   a  on the outer periphery of the fan  11  and the rear bracket  3  can be increased and hence the area of the outlets  3   b  can be increased. 
         [0046]    Accordingly, the area of the air passage of the cooling air blocked by the lead  14   a  and the coil end  12   a  can be increased. Hence, a pressure loss is lessened and an air volume generated by the fan  11  is increased. It thus becomes possible to lower temperatures of the positive-electrode-side rectifier elements  30  and the negative-electrode-side rectifier elements  32 . 
         [0047]    Also, momentum is given to the cooling air by the fan  11  while flowing from the major diameter of the inlets  3   a  to the major diameter of the fan  11  and a pressure rises as a result. By aligning the outer side surface (outer peripheral side surface) of the inlets  3   a  parallel to the axial direction  6   a , a distance between the major diameter of the inlets  3   a  and the major diameter of the fan  11  can be increased and the pressure can be raised as a result. Hence, even in a case where there is a large pressure loss at the heat sink due to the cooling of the positive-electrode-side rectifier elements  30  and the negative-electrode-side rectifier elements  32 , the pressure can be raised by the fan  11 . It thus becomes possible to lower the temperatures of the positive-electrode-side rectifier elements  30  and the negative-electrode-side rectifier elements  32 . 
         [0048]    Also, by configuring in such a manner as in this embodiment that the tip end  11   a  of the fan  11  and the rear bracket  3  are in close proximity to each other in the vicinity of the end portion  70  whereas a distance between the tip end  11   a  and the rear bracket  3  increases toward the outer periphery of the fan  11 , a fluctuation of the pressure on the surface of the rear bracket  3 , which is the source of noises, can be reduced. It thus becomes possible to increase an air volume of the cooling air by suppressing an amount of the leakage flow without increasing noises. 
         [0049]    A structure on the periphery of the front bracket  2  and the fan  11  of the first embodiment will now be described. Although the lead  14   a  is absent on the front side, there is the coil end  12   a  that reduces an area of the outlet port of the fan. Constituent members such that reduce the area of the outlet port of the fan are fewer on the front side than on the rear side. It should be noted, however, that a cooling performance can be enhanced by a structure that increases the area of the outlet port of the fan on the front side in the same manner as on the rear side. 
         [0050]    The structure of the fan  11  on the front side is the same as the structure of the fan on the rear side shown in  FIG. 3 . Herein, the description of the structure of the fan shown in  FIG. 3  is not repeated. It should be appreciated, however, that the multiple vanes (blades)  84  of the fan  11  may have a constant vane height from the inner periphery  80  to the outer periphery  82  of the fan  11 . 
         [0051]    A diameter of the outer periphery of the inlets  2   a  of the front bracket  2  is formed to be smaller than the diameter of the outer periphery of the fan  11 . A vane height of the fan  11  at a position in the radial direction  6   b  of the outer periphery of the inlets  2   a  has a portion having the maximum vane height. 
         [0052]    Referring to  FIG. 1  again, the outer peripheral side surface of the inlets  2   a  of the front bracket  2  is parallel to the axial direction  6   a  of the shaft  6 . Hence, an end portion  71  of the inlets  2   a  in the outer peripheral surface on the side of the fan  11  is in closest proximity to the fan  11 . The protrusion  73  gradually inclines from the end portion  71  to the radial direction  6   b  so as to move away from the fan  11 . Owing to the configuration as above, a protrusion  73  is formed on a surface of the front bracket  2  opposing a tip end  11   c  of the fan  11  at a position adjacent to the inlets  2   a  on the periphery of the front bracket  2 . The protrusion  73  is of a right-angled triangular shape in cross section. 
         [0053]    When configured in this manner, a distance between the tip end  11   c  on the outer periphery of the fan  11  and the front bracket  2  can be increased and hence the area of the outlets  2   b  can be increased. 
         [0054]    Accordingly, the area of the air passage of the cooling air blocked by the coil end  12   a  can be increased. Hence, a pressure loss is reduced and an air volume generated by the fan  11  is increased. It thus becomes possible to lower temperatures of the bearings  5  and the stator  12 . 
         [0055]    Also, momentum is given to the cooling air by the fan  11  while flowing from a major diameter of the inlets  2   a  to the major diameter of the fan  11  and a pressure rises as a result. By aligning the outer side surface of the inlets  2   a  parallel to the axial direction  6   a , a distance between the major diameter of the inlets  2   a  and the major diameter of the fan  11  is increased and a pressure can be raised as a result. 
         [0056]    Accordingly, even in a case where the tip end of the coil end  12   a  extends in the axial direction  6   a  and is positioned flush with the fan tip end  11   c  or where the area of the inlets  2   a  is reduced due to multiple ribs provided to the inlets  2   a , a pressure can be raised by the fan  11 . It thus becomes possible to lower the temperatures of the bearings  5  and the stator  12 . 
         [0057]    The tip end  11   c  of the fan  11  and the front bracket  2  are in close proximity to each other in the vicinity of the end portion  71  whereas a distance between the tip end  11   c  and the front bracket  2  increases toward the outer periphery of the fan  11 . Owing to this configuration, a fluctuation of the pressure on the surface of the front bracket  2 , which is the source of noises, can be reduced. It thus becomes possible to increase an air volume of the cooling air by suppressing an amount of the leakage flow without increasing noises. 
       Second Embodiment 
       [0058]      FIG. 5  is a cross section of a major portion in an AC generator of a second embodiment. Reference numerals of  FIG. 5  same as those of the first embodiment above denote the counterparts of the first embodiment above and a description is omitted herein. Also, an arrow indicates a flow of air from inlets  3   a  to outlets  3   b.    
         [0059]    On the rear side, in a case where there is a large pressure loss at the heat sink due to the cooling of positive-electrode-side rectifier elements  30  and negative-electrode-side rectifying members  32 , a pressure generated by the fan is increased, and so is a pressure difference between the vicinity of the center and the vicinity of the outlet port of the fan. When a pressure difference is increased, a leakage flow from the outlets  3   b  toward the inlets  3   a  of the rear bracket  3  in the vicinity of the clearance  60  is increased and hence an air volume of the cooling air is reduced in some cases. 
         [0060]    Herein, a description will be given to a structure on the periphery of the fan  11  in a case where a pressure generated by the fan is large with reference to  FIG. 5 . The fan  11  includes a fan main body  83  and multiple vanes (blades)  84  provided to the fan main body  83 . Each of the multiple vanes  84  has a portion  81  having a maximum vane height somewhere between an inner periphery  80  and an outer periphery  82  of the fan main body  83 . Also, the vane  84  has a constant height from the portion  81  having the maximum vane height to the outer periphery  82 . 
         [0061]    The rear bracket  3  includes multiple inlets  3   a  to let the cooling air in. Also, the rear bracket  3  includes outlets  3   b  partitioned by ribs  52 . A diameter of an outer periphery of the inlets  3   a  of the rear bracket  3  is formed to be smaller than a diameter of an outer periphery of the fan  11 . 
         [0062]    Also, the vane height of the fan  11  at a position in a radial direction  6   b  of the outer periphery of the inlets  3   a  has the portion  81  having the maximum vane height. An outer peripheral side surface of the inlets  3   a  of the rear bracket  3  is parallel to the axial direction  6   a  of the shaft  6 . Hence, an end portion  70  of the inlets  3   a  in the outer peripheral surface on the side of the fan  11  is in closest proximity to the fan  11 . From the end portion  70  to the radial direction  6   b , the protrusion  72  has a portion parallel to the tip end  11   a  of the fan  11  and a portion inclined so as to move away from the fan  11 . When configured in this manner, a protrusion  72  having a trapezoidal cross section is formed on the outer periphery of the rear bracket  3  adjacently to the inlets  3   a.    
         [0063]    In the first embodiment above, the clearance  60  reaches the minimum in the vicinity of the end  70  of the rear bracket  3  whereas the clearance  60  increases to the radial direction  6   b . In the second embodiment, the clearance  60  is constant from the end portion  70  of the rear bracket  3  to the radial direction  6   b  and the clearance  60  starts to increase from a position within the diameter of the outer periphery of the fan  11 . 
         [0064]    A range in which the clearance  60  is the minimum is extended in the radial direction  6   b  in comparison with the first embodiment above. Hence, a resistance is increased against a flow flowing backward from a high-pressure region at the outlet port of the fan  11  to a low-pressure region at the inlets  3   a  of the rear bracket  3  by passing the clearance  60 . A flow rate of the back flow can be thus suppressed. 
         [0065]    Consequently, an air volume of the cooling air is increased and it becomes possible to lower temperatures of the positive-electrode-side rectifier elements  30  and the negative-electrode-side rectifier elements  32 . 
         [0066]    When configured in this manner, a distance between the tip end  11   a  on the outer periphery of the fan  11  and the rear bracket  3  can be increased. Consequently, a length of the outlets  3   b  of the rear bracket  3  in the axial direction  6   a  is extended and hence an area of the outlets  3   b  can be increased. 
         [0067]    Accordingly, an area of the air passage of the cooling air blocked by the lead  14   a  and the coil end  12   a  can be increased. Hence, a pressure loss is lessened and an air volume generated by the fan  11  is increased. It thus becomes possible to lower the temperatures of the positive-electrode-side rectifier elements  30  and the negative-electrode-side rectifier elements  32 . 
         [0068]    Also, momentum is given to the cooling air by the fan  11  while flowing from the outer periphery of the inlets  3   a  to the outer periphery of the fan  11  and a pressure rises as a result. By aligning the outer side surface of the inlets  3   a  parallel to the axial direction  6   a , a distance between the diameter of the outer periphery of the inlets  3   a  and the major diameter of the fan  11  can be increased and the pressure can be raised as a result. 
         [0069]    Accordingly, even in a case where there is a large pressure loss at the heat sink due to the cooling of the positive-electrode-side rectifier elements  30  and the negative-electrode-side rectifier elements  32 , a pressure can be raised by the fan  11 . It thus becomes possible to lower the temperatures of the positive-electrode-side rectifier elements  30  and the negative-electrode-side rectifier elements  32 . 
         [0070]    In the second embodiment, the tip end  11   a  of the fan  11  and the inlets  3   a  of the rear bracket  3  are in close proximity to each other in the vicinity of the end portion  70  whereas a distance between the tip end  11   a  and the rear bracket  3  increases toward the outer periphery of the fan  11 . Owing to this configuration, a fluctuation of the pressure on the surface of the rear bracket  3 , which is the source of noises, can be reduced. It thus becomes possible to increase an air volume of the cooling air by suppressing an amount of the leakage flow without increasing noises. 
         [0071]    The above has described as to a shape of the rear-side inlets  3   a  on the side of the fan  11  that the protrusion  72  having a trapezoidal cross section is formed on the outer periphery of the inlets  3   a . It should be appreciated, however, that the same advantages can be obtained even when the cross section is formed in a rectangular shape. 
       Third Embodiment 
       [0072]      FIG. 6  is a cross section of a major portion in an AC generator according to a third embodiment of the invention. Reference numerals same as those of the first embodiment above denote the counterparts of the first embodiment above and a description is omitted herein. Also, an arrow indicates a flow of air from inlets  2   a  to outlets  2   b.    
         [0073]    The second embodiment above has described a case where a high pressure is required on the rear side. On the other hand, fins (not shown) are provided on the front side in order to increase the strength of the inlets  2   a  and the outlets  2   b  and also to release heat. When the number of the fins is small and the fins are short, a pressure is not required on the front side in comparison with the rear side. A structure to increase the cooling air on the front side will now be described with reference to  FIG. 6 . 
         [0074]    In a case where a pressure is not required, no pressure difference is generated between the vicinity of the center and the outlet port of the fan. Hence, there is substantially no leakage flow from the outlet port toward the vicinity of the center of the fan. Accordingly, there is no need to provide a difference like a relation between the diameter of the outer periphery of the inlets and the diameter of the outer periphery of the fan as described in the first embodiment above. Each of the multiple vanes  84  of the fan  11  has a portion  81  having a maximum vane height somewhere between an inner periphery  80  and an outer periphery  82  of the fan main body  83 . Also, the vane  84  has a constant height from the portion  81  having the maximum height to the outer periphery  82 . 
         [0075]    Also, in the third embodiment, as is shown in  FIG. 6 , a diameter of the outer periphery of the inlets  2   a  of the front bracket  2  is formed to be larger than a diameter of the outer periphery of the fan  11 . An outer peripheral side surface of the inlets  2   a  of the front bracket  2  is parallel to the axial direction  6   a  of the shaft  6 . Accordingly, an end portion  71  of the inlets  2   a  in the outer peripheral surface on the side of the fan  11  is in closest proximity to the fan  11 . From the end portion  71  to the radial direction  6   b , the protrusion  73  has a portion parallel to the tip end  11   c  of the fan  11  and a portion inclined so as to move away from the fan  11 . 
         [0076]    When configured as above, a protrusion  73  having a trapezoidal cross section is formed on the outer periphery of the front bracket  2  adjacently to the inlets  2   a  of the front bracket  2  on the side of the fan  11 . 
         [0077]    Owing to this configuration, an area of the outlets  2   b  can be increased and also it becomes difficult for the cooling air flowing out from the fan  11  to flow toward the inlets  2   a . Consequently, an air volume of the cooling air is increased and it becomes possible to lower temperatures of the bearings  5  and the stator  12 . The third embodiment has described the protrusion  73  having a trapezoidal cross section. It should be appreciated, however, that the same advantages can be obtained even when the protrusion  73  has a triangular cross section. 
         [0078]    It should be understood that the respective embodiments of the invention can be combined without restrictions and modified and omitted when the need arises within the scope of the invention. 
         [0079]    Various modifications and alterations of this invention will be apparent to those skilled in the art without departing from the scope and spirit of this invention, and it should be understood that this is not limited to the illustrative embodiments set forth herein.