Abstract:
A brushless AC generator is mounted on a vehicle such as a truck, a farming vehicle or a construction vehicle. The generator is composed of a housing, stator fixedly contained in the housing, a rotor, for supplying a magnetic field to the stator, rotatably supported in the stator, and other associated components including a voltage regulator and a rectifier. The housing is composed of a front housing and a rear housing. An outside fan is connected to a rotor shaft and disposed at a front side. Cooling air is introduced into the generator by the outside fan through windows formed in the rear housing. A fringe surface of the window facing a shoulder portion of the rotor is sloped, so that the cooling air is smoothly introduced along the sloped surface thereby to improve cooling efficiency.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is based upon and claims benefit of priority of Japanese Patent Application No. 2007-194389 filed on Jul. 26, 2007, the content of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a brushless AC generator to be mounted on an automotive vehicle such as a truck, a construction vehicle or a farming vehicle. 
   2. Description of Related Art 
   Ambient temperature in which an AC generator is used is becoming high because an engine has been modified to improve exhaust gas quality. Further, electric power required for a vehicle is increasing to secure driving safety and comfort. A brushless AC generator has been used in vehicles such as trucks which are driven in a long distance and construction or farming vehicles which are used in a dusty environment. This is because the brushless AC generator is resistive to dust and has a long lifetime. 
   Generally, in such an AC generator, no window for introducing cooling air is formed in a housing to protect components from being damaged by foreign particles or water entering into the generator. Electric components such as a rectifier are protected by a sidewall of the housing and are cooled by a outside fan fixed to a rotor shaft. To meet a requirement for increasing an output of the generator, various measures, such as increasing a diameter and a thickness of a stator and decreasing resistance in windings, have been taken. Further, a counter measure has to be taken against a temperature rise due to increase in the generator power. 
   In the brushless AC generator, it is difficult to increase a diameter of the outside fan to increase its cooling ability because there is a limitation in the diameter of a size in a radial direction. There is also a limitation in increasing the number of blades and its area in the axial direction because a mechanical strength of the blades has to be taken into consideration. It is conceivable to make narrower a gap between cooling fins of a rectifier and a sidewall of the housing to increase cooling efficiency. However, if the gap is made narrow, a possibility of accumulation of foreign particles in the gap increases. 
   An example of the brushless AC generator disclosed in JP-A-2001-292550 includes cooling windows formed in a rear housing, and cooling air is introduced through the cooling windows into the generator by an outside fan disposed at a pulley side. However, there is a problem that cooling air is not smoothly introduced into the housing because a flow of the cooling air is disturbed or stagnated by a step or a gap between the rear housing and an axial end of a rotor. Further, in the case where such a gap is exposed to the cooling window, water or other liquid may enter the gap thereby causing various troubles. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved brushless AC generator, in which water or other liquid is prevented from entering into the generator while improving cooling efficiency. 
   The brushless AC generator is often used in a vehicle such as a truck, a farming vehicle or a construction vehicle. When the generator is used in a dusty environment, it is important to prevent foreign particles such as dusts and water from entering into the generator. The brushless AC generator includes a housing composed of a front housing and a rear housing, a stator is functioning as an armature contained in a housing, and a rotor rotatably supported in the stator for supplying a magnetic field to the stator. 
   The rotor has a rotor shaft, to which an outside fan for sucking cooling air into the housing and a pulley for driving the rotor by an engine are connected. The rear housing, which is disposed at a rear side opposite to a front side where the outside fan is positioned, includes a disc portion formed around a center hole, spokes extending in the radial direction from the disc portion, a cylindrical portion connected to an outer periphery of the spokes, and windows for introducing cooling air. The windows are defined by an outer peripheral end surface of the disc portion, the spokes and the cylindrical portion. 
   The rotor includes claw-shaped rotor poles which are circularly connected to each other, forming a shoulder portion. The disc portion of the rear housing faces a rear end surface of the shoulder portion, forming a small gag therebetween. The outer peripheral end surface of the disc portion is sloped, so that its diameter L 2  at the front side is larger than its diameter L 1  at the rear side. The cooling air introduced into the housing through the windows of the rear housing smoothly flows along the sloped surface toward the front side of the housing. 
   Preferably, the diameter L 2  is set to fall in a range between an outer diameter Φ 1  of the rear end surface of the shoulder portion and an outermost diameter Φ 2  of the shoulder portion, i.e., Φ 1 ≦L 2 ≦Φ 2 . In this manner, a gap or step between the outer peripheral end surface of the disc portion and the rear end surface of the shoulder portion can be further reduced thereby to attain a smoother flow of the cooling air. The sloped surface may be formed by attaching a separate member to the outer peripheral end surface of the disc portion. The separate member may be extended in the axial direction beyond the rear end surface of the shoulder portion to thereby further reduce gap between the outer peripheral end surface of the disc portion and the rear end surface of the shoulder portion. 
   According to the present invention, cooling efficiency of the cooling air is improved while preventing water and other liquid from entering into the brushless AC generator. Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment described below with reference to the following drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view showing an entire structure of a brushless AC generator according to the present invention; 
       FIG. 2  is a plan view showing the brushless AC generator, viewed from its rear side indicated in  FIG. 1 ; 
       FIG. 3  is a side view (partially cross-sectioned) showing a rotor used in the generator; 
       FIG. 4  is a plan view showing a rear housing used in the generator; 
       FIG. 5  is a partial cross-sectional view showing a relation between a disc portion of the rear housing and a shoulder portion of the rotor; 
       FIG. 6  is a partial cross-sectional view showing a relation between a disc portion of the rear housing and a shoulder portion of a rotor in a conventional brushless AC generator; 
       FIG. 7  is a graph showing an amount of cooling air versus rotational speed of the rotor of the brushless AC generator; 
       FIG. 8  is a similar view as shown in  FIG. 5 , showing a modified form  1  of the embodiment of the present invention; 
       FIG. 9  is a similar view as shown in  FIG. 5 , showing a modified form  2  of the embodiment of the present invention; 
       FIG. 10  is a similar view as shown in  FIG. 5 , showing a modified form  3  of the embodiment of the present invention; and 
       FIG. 11  is a similar view as shown in  FIG. 5 , showing a modified form  4  of the embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A preferred embodiment of the present invention will be described with reference to  FIGS. 1-7 . As shown in  FIGS. 1 and 2 , a brushless AC generator  1  includes a stator  2 , a rotor  3 , a field winding  4 , a front housing  51 , a rear housing  52 , a rectifier  6 , a regulator  7  and a rear cover  8 . The stator  2  having a stator core  21  and a stator winding  22  wound on the stator core  21  functions as an armature. 
   The rotor  3  generates a magnetic field for the stator  2  and is rotatably disposed in the stator  2 . As shown in  FIGS. 1 and 3 , the rotor  3  includes rotor yokes  32 ,  33  rotating together with a rotor shaft  31 , claw-shaped rotor poles  34  integrally formed with the rotor yoke  33 , and claw-shaped rotor poles  36  connected to the claw-shaped rotor yokes  34  by a ring-shaped support member  35 . The claw-shaped rotor poles  36  are connected to each other in a circumferential direction, forming a ring-shaped shoulder portion  36   a . The rotor shaft  31  is rotatably supported by a rear bearing  37  held in the rear housing  52  and a front bearing  38  held in the front housing  51 . A field winding  4  is wound on a cylindrical yoke  41  that is fixedly connected to the rear housing  52  with bolts  42 . A magnetic field generated by the field winding  4  is supplied to the rotor  3 . 
   The stator  2  is fixedly held by the front housing  51  and the rear housing  52 . One end of the rotor shaft  31  extends to the front side of the front housing  51 , and a cooling fan  53  is connected to the extended portion of the rotor shaft  31  with a nut  54 . A pulley (not shown in  FIG. 1 ) is also connected to the extended portion of the rotor shaft  31 . 
   The rectifier  6  having cooling fins and the regulator  7  are mounted on the rear housing  52 . The rectifier  6  rectifies alternating current generated in the stator winding  22  into direct current. The regulator  7  controls an output voltage of the brushless AC generator  1  by controlling an amount of excitation current supplied to the field winding  4 . The amount of excitation current is controlled by turning on or off the excitation current. 
   When the rotor  3  and the cooling fan  53  are rotated by the engine, cooling air is introduced into the generator through windows formed in the rear cover  8 . The cooling air cools the rectifier  6  and the regulator  7 , and then the cooling air is further introduced into the inner space of the generator through windows  52   f  (refer to  FIG. 4 ) formed in the rear housing  52 . The stator  2  and the rotor  3  are cooled by the introduced cooling air. Then, the cooling air is blown out from the front side of the generator  1 . 
   Now, the rear housing  52  and a relation between the rear housing  52  and the rotor  3  will be described in detail. As shown in  FIG. 4 , the rear housing  52  includes a disc portion  52   a  formed around a center hole, plural spokes  52   d  extending in the radial direction from the disc portion  52   a , and a cylindrical portion  52   e  extending in the axial direction from the spokes  52   d . The rectifier  6  is mounted on the rear surface of the disc portion  52   a , while the cylindrical yoke  41  is connected on the front surface of the disc portion  52   a . Plural windows  52   f  for introducing cooling air are defined by an outer peripheral end surface  52   b  of the disc portion  52   a  and spokes  52   d.    
   As shown in  FIG. 5 , the outer peripheral end surface  52   b  of the disc portion  52   a , which forms an inner fringe of the window  52   f , is tapered. That is, its rear side diameter L 1  is made smaller than its front side diameter L 2  (L 2 &gt;L 1 , also refer to  FIG. 4 ). Further, the front side diameter L 2  is made between an outer diameter Φ 1  of a rear end surface of a shoulder portion  36   a  and an outermost diameter Φ 2  of the shoulder portion  36   a  (Φ 1 ≦L 2 ≦Φ 2 ). As shown in  FIG. 3 , the shoulder portion  36   a  of the rotor  3  is a ring-shaped portion connecting the claw-shaped rotor poles  36  in the circumferential direction. 
   By forming the outer peripheral end surface  52   b  in the tapered shape as shown in  FIG. 5 , cooling air introduced inside the rear housing  52  smoothly flows in the axial direction. For further explain the advantage of the tapered surface, the structure according to the present invent shown in  FIG. 5  is compared with that of a conventional structure shown in  FIG. 6 . In the conventional structure, a step is formed between an outer peripheral end surface  52   b ′ and a rear end surface  36   b ′. Therefore, cooling air introduced inside through a window does not flow smoothly because the step exists between  52   b ′ and  36   b ′, and the cooling air stagnates at the step portion, as shown in  FIG. 6 . Further, part of the cooling air enters into a gap formed between the disc portion of the rear housing and the rear end surface of the rotor. 
     FIG. 7  shows a result of a comparison test between the conventional generator having the structure shown in  FIG. 6  and the generator according to the present invention. Rotational speed of the rotor is shown on the abscissa and an amount of cooling air on the ordinate. A solid line shows the amount of cooling air in the generator according to the present invention, while a dotted line shows that of the conventional generator. As seen in the graph, an amount of the cooling air increases 10-15% at a high speed region by applying the present invention. 
   The shape of the outer peripheral end surface  52   b  may be variously modified to attain the same or similar advantage as that attained in the embodiment described above. Some modified forms will be described below with reference to  FIGS. 8-11 . 
   In a modified form  1  shown in  FIG. 8 , a thickness of the disc portion  52   a  is increased at a portion close to the outer peripheral end surface  52   b , so that a gap “g” between the front surface of the disc portion  52   a  and the rear end surface  36   b  of the shoulder portion  36   a  becomes small. The slope of the outer peripheral end surface  52   a  is made to coincide with the slope of the shoulder portion  36   a . In this manner, the cooling air flows into the generator  1  further smoothly, avoiding stagnation of the cooling air flow. 
     FIG. 9  shows a modified form  2 . In this modified form, the outer peripheral end surface  52   b  is made parallel to the axial direction (not tapered), and the gap g is narrowed in the same manner as in the modified form  1 . The diameter L 2  is made to fall between the diameter Φ 1  and the diameter Φ 2  (Φ 1 ≦L 2 ≦Φ 2 ). In this manner, disturbance of the cooling air flow caused by hitting the rear end surface  36   b  is reduced, thereby improving cooling efficiency. By making the gap g smaller, the cooling air and water entering inside the claw-shaped rotor poles  36  can be reduced, thereby preventing formation of rust on the rotor  3 . 
     FIG. 10  shows a modified form  3 . In this modified form, a projected portion  52   g  is additionally formed on the outer peripheral end surface  52   b  which is tapered. The projected portion  52   g  extends in the axial direction beyond the rear end surface  36   b  of the shoulder portion  36   a , making a small gap between  52   g  and  36   b . In this manner disturbance in the airflow is alleviated, and the air and water are prevented from entering an inside portion of the rotor  3 . 
     FIG. 11  shows a modified form  4 . In this modified form, a cover member  52   h  having a tapered surface is connected to the disc portion  52   a  by the bolt  42  that connects the yoke  41  to the disc portion  52   a . Namely, the cover member  52   h  is made separately from the rear housing  52  and connected thereto by the bolt  42 . The tapered portion of the cover member  52   h  overlapping with the tapered surface of the shoulder portion  36   a  may be shortened not to overlap therewith. 
   While the present invention has been shown and described with reference to the foregoing preferred embodiment and modified forms, it will be apparent to those skilled in the art that changes in form and detail may be made therein without departing from the scope of the invention as defined in the appended claims. For example, though the diameter L 2  is set to fall in a range between Φ 1  and Φ 2  as shown in  FIG. 5 , it may be possible to set the diameter L 2  smaller than Φ 1 . The tapered surface improves smoothness of the cooling airflow in this case, too.