Abstract:
An alternator exhibits uniform temperature distribution in a rectifier, thereby preventing a locally hot spot from being developed. Positive-side diodes and negative-side diodes are arranged in a zigzag pattern in a circumferential direction, and formed of diodes on an inside diameter side and diodes on an outside diameter side.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an alternator equipped with a rectifier for rectifying alternating current generated at a stator coil into direct current. 
     2. Description of the Related Art 
     FIG. 10 is a sectional view of a conventional automotive alternator, FIG. 11 is an electrical circuit diagram of the alternator, FIG. 12 is a front view of a rectifier  12  of FIG. 10 when the rectifier  12  is observed from inside, and FIG. 13 is a front view of a rear bracket  2  of FIG.  10 . 
     The automotive alternator includes: a case  3  composed of an aluminum front bracket  1  and an aluminum rear bracket  2 ; a shaft  6  rotatably disposed in the case  3  and which has a pulley  4  secured to one end thereof; a Lundell-type rotor  7  secured to the shaft  6 ; a stator  8  secured to an inner wall of the case  3 ; a slip ring  9  secured to the other end of the shaft  6  and which supplies electric current to the rotor  7 ; a pair of brushes  10  that slide in contact with the slip ring  9 ; a brush holder  11  accommodating the brushes  10 ; a rectifier  12  electrically connected to the stator  8  and which rectifies alternating current generated in the stator  8  into direct current; a heat sink  17  fitted in the brush holder  11 ; and a regulator  18  adhesively fastened to the heat sink  17  and which adjusts an alternating voltage generated in the stator  8 . 
     The rotor  7  is equipped with a rotor coil  13  for generating magnetic flux on passage of electric current, and a pole core  14  covering the rotor coil  13  in which magnetic poles are produced by the magnetic flux. The pole core  14  has a pair of a first pole core assembly  21  and a second pole core assembly  22  that intermesh with each other. Centrifugal fans  5  for cooling are welded on axial end surfaces of the first pole core assembly  21  and the second pole core assembly  22 . 
     The stator  8  is provided with a stator core  15  through which a rotating magnetic field generated by the rotor  7  passes, and a stator coil  16  in which alternating current is generated by the rotating magnetic field. The stator coil  16  is constructed by a first stator coil section  16   a  and a second stator coil section  16   b , each being formed of three coils whose conductors are wound around the stator core  15  and which are in Y-connection. 
     FIG. 14 is a front view of the rectifier  12 , and FIGS. 15 and 16 are exploded front views of the rectifier  12  of FIG.  14 . The rectifier  12  includes positive-side diodes  26  and negative-side diodes  28  connected to output ends of the first stator coil section  16   a  and the second stator coil section  16   b , and neutral point diodes  30  connected to neutral points  31  of the first stator coil section  16   a  and the second stator coil section  16   b.    
     The rectifier  12  further includes: a circular strip-shaped positive-side heat sink  24  having, on its surface, six positive-side diodes  26  and two neutral point diodes  30  equidistantly provided on the same circumference; a circular strip-shaped negative-side heat sink  27  disposed radially outside of and on the same plane as the positive-side diodes  26 , and has, on its surface, six negative-side diodes  28  and two neutral point diodes  30  equidistantly provided on the same circumference; and a circuit board  29  electrically connecting the diodes  26 ,  28 , and  30 , and the stator coil  16 . 
     The surfaces of the positive-side heat sink  24  and the negative-side heat sink  27  have recesses  32  and  33  for accommodating the columnar positive-side diodes  26 , the negative-side diodes  28 , and the neutral point diodes  30 . FIG. 17 shows the positive-side heat sink  24  and the negative-side heat sink  27  of the FIG.  15  as observed from a rear side. On the rear surfaces of the heat sinks  24  and  27 , protuberances  34  and  35  are formed at the same time when the recesses  32  and  33  are formed. 
     The positive-side diodes  26 , the negative-side diodes  28 , and the neutral point diodes  30  are fixed to the recesses  32  and  33  of the heat sinks  24  and  27  by soldering. Perpendicularly extending lead wires  36  and  37  of the diodes  26 ,  28 , and  30  are electrically connected to terminals  38  and  39  of the circuit board  29 . 
     The positve-side heat sink  24  is retained on the negative-side heat sink  27  via a holder  40 . The positive-side heat sink  24 , the negative-side heat sink  27 , and the circuit board  29  are fixed in the case  3  by screws (not shown) attached to the rear bracket  2  via through holes  41 . Furthermore, the negative-side heat sink  27  is grounded by being directly attached to the rear bracket  2 . 
     In the automotive alternator having the construction set forth above, electric current is supplied from a battery (not shown) to the rotor coil  13  via the brushes  10  and the slip ring  9 , generating magnetic flux. The pulley  4  is driven by an engine, and the rotor  7  is rotated by the shaft  6 . This causes a rotating magnetic field to be imparted to the stator coil  16 , so that an electromotive force is generated in the stator coil  16 . The alternating electromotive force is converted into direct current through the positive-side diodes  26  and the negative-side diodes  28  of the rectifier  12 , a magnitude thereof is adjusted by the regulator  18 , and the battery is recharged. 
     The rotor coil  13 , the stator coil  16 , the positive-side diodes  26 , the negative-side diodes  28 , and the regulator  18  constantly generate heat during power generation. When an alternator of, for example, a 100A rated output current class, runs at a speed generating a high temperature, the rotor coil  13  generates a calorific value of 60W, the stator coil  16  generates a caloric value of 500W, the positive-side diodes  26  and the negative-side diodes  28  together generate a calorific value of 120W, and the regulator  18  generates a calorific value of 6W. Excessive heat generation causes deteriorated performance of the alternator and shortens lives of components. 
     As countermeasures for the heat generation, the fans  5  rotate as the rotor  7  rotates. The rotation of the fans  5  causes outside air to be introduced into the case  3  through an aperture A of the case  3 , and to flow as indicated by arrows a of FIG. 10 to thereby cool the negative-side heat sink  26 , the negative-side diodes  28 , the positive-side heat sink  24 , and the positive-side diodes  26 . The outside air is then led radially outward by the fans  5  to cool a coil end of the stator coil  16  at the rear side, and exhausted to open air through an aperture B. 
     Furthermore, the rotation of the fans  5  causes outside air to be introduced into the case  3  also through an aperture C. The outside air flows as indicated by arrows β of FIG. 10 to cool a power transistor of the regulator  18 . The outside air is then led radially outward by the fans  5  to cool a coil end of the stator coil  16  at the rear side, and exhausted to open air through an aperture D. 
     Similarly, outside air introduced through apertures E of the front bracket  1  is directed radially outward by the fans  5  to cool an end of the stator coil  16  at a front side. The outside air is then exhausted out of the case  3  through an aperture F. 
     In the automotive alternator having the construction described above, the lead wires  36  and  37  of the positive-side diodes  26  and the negative-side diodes  28 , which extend in an axial direction of the shaft  6 , are directly abutted against the terminals  38  and  39  of the circuit board  29 . For this reason, the six positive-side diodes  26  and the two neutral point diodes  30  are disposed at equal intervals on the circumference of the positive-side heat sink  24 , and the six negative-side diodes  28  and the two neutral point diodes  30  are disposed at equal intervals on the circumference of the negative-side heat sink  27 . Therefore, the positive-side diodes  26 , in particular, on the positive-side heat sink  24  on an inside diameter side are close to each other. When a distance between central points of the positive-side diodes  26  is denoted as W, and a diameter of the positive-side diodes  26  is denoted as D, a value expressed as W/D≅1.5 is obtained. A study of temperature distribution of the rectifier  12  has revealed that the temperature rises toward a center in a circumferential direction of the positive-side heat sink  24 , a difference between temperature extremes being approximately 13 degrees Celsius, while the temperature rises inward in a radial direction, a difference between temperature extremes being approximately 3 degrees Celsius. There has been a problem in that the positive-side diode  26  at the center of the positive-side heat sink  24  where the temperature is the highest reaches a locally high temperature, 125 degrees Celsius. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made with a view toward solving the problems described above, and it is an object thereof to provide an alternator that exhibits uniform temperature distribution in a rectifier, thereby preventing a locally hot place from being developed. 
     To this end, according to one aspect of the present invention, there is provided an alternator having at least a first diode assembly or a second diode assembly composed of diodes on an inside diameter side and diodes on an outside diameter side arranged in a zigzag pattern in a circumferential direction. 
     In a preferred form of the alternator in accordance with the present invention, the diodes on the inside diameter side and the diodes on the outside diameter side are provided in recessions of a heat sink having a recessed surface, and protuberances associated with the recesses are formed in a rear surface of the heat sink. 
     In a preferred form of the alternator in accordance with the present invention, one of the diode on the inside diameter side and the diode on the outside diameter side is disposed such that it is partly included in a region of an air detachment portion produced on a peripheral surface by cooling air that collides with the other of the diode on the inside diameter side and the diode on the outside diameter side. 
     In another preferred form of the alternator in accordance with the present invention, if a distance between a central point of a columnar diode on the inside diameter side and a central point of its adjacent columnar diode on the outside diameter side is denoted as W, and a diameter of the diode on the inside diameter side and the diode on the outside diameter side is denoted as D, then (W/D)&lt;2. Furthermore, if an angle at which a line connecting a central point of the diode on the inside diameter side and a central point of its adjacent diode on the outside diameter side crosses a line connecting a central axis of a shaft and the central point of the diode on the outside diameter side or the diode on the inside diameter side is denoted as θ, then angle θ is 100°&lt;θ&lt;140°. 
     In a preferred form of the alternator according to the present invention, diodes on the outside diameter side that are secured to the second heat sink are disposed so as to oppose the diodes on the outside diameter side that are secured to the first heat sink. 
     In a preferred form of the alternator according to the present invention, the diodes on the outside diameter side and the diodes on the inside diameter side that are secured to the second heat sink are disposed away from radial lines of the diodes on the outside diameter side and the diodes on the inside diameter side that are secured to the first heat sink. 
     In a preferred form of the alternator according to the present invention, the first heat sink and the second heat sink are disposed on different vertical planes with respect to axes. 
     In a preferred form of the alternator according to the present invention, the first heat sink is a positive-side heat sink, the first diode assembly is a positive-side diode assembly, the second heat sink abutted against the case is a negative-side heat sink, and the second diode assembly is a negative-side diode assembly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front view of a rectifier of an automotive alternator according to a first embodiment of the present invention. 
     FIG. 2 is a front view of the rectifier shown in FIG. 1, from which a circuit board has been removed. 
     FIG. 3 is a rear view of the rectifier shown in FIG.  2 . 
     FIG. 4 is an enlarged view of an essential section of the rectifier shown in FIG.  2 . 
     FIG. 5 is a chart showing a local Nusselt number Nux of a column placed at right angles to a flow. 
     FIG. 6 is a front view of a rectifier of an automotive alternator according to a second embodiment of the present invention. 
     FIG. 7 is a front view of a rectifier of an automotive alternator according to a third embodiment of the present invention. 
     FIG. 8 is a front view of a rectifier of an automotive alternator according to a fourth embodiment of the present invention. 
     FIG. 9 is a sectional view of an automotive alternator according to a sixth embodiment of the present invention. 
     FIG. 10 is a sectional side elevation of a conventional automotive alternator. 
     FIG. 11 is an electrical circuit diagram of the automotive alternator of FIG.  10 . 
     FIG. 12 is a front view of a rectifier of the automotive alternator shown in FIG. 10, as observed from inside thereof. 
     FIG. 13 is a front view of a rear bracket of the automotive alternator shown in FIG.  10 . 
     FIG. 14 is a front view of the rectifier shown in FIG.  10 . 
     FIG. 15 is a front view of the rectifier shown in FIG. 10, from which a circuit board has been removed. 
     FIG. 16 is a front view of the circuit board shown in FIG.  14 . 
     FIG. 17 is a rear view of the rectifier shown in FIG.  15 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An automotive alternator according to a first embodiment of the present invention will be described. Components that are the same as or equivalent to those shown in FIG.  10  through FIG. 17 will be assigned the same reference numerals in the descriptions. 
     First Embodiment 
     FIG. 1 is a front view of a rectifier  50  of an automotive alternator according to the first embodiment of the present invention, and FIG. 2 is a front view of the rectifier  50 , from which a circuit board  56  shown in FIG. 1 has been removed. 
     The rectifier  50  includes: positive-side diodes  51   a  and  51   b , which are first diodes, and negative-side diodes  52   a  and  52   b , which are second diodes, the first and second diodes being connected to output ends of a first stator coil section  16   a  and a second stator coil section  16   b , respectively; and neutral point diodes  53  connected to neutral points  31  of the first stator coil section  16   a  and the second stator coil section  16   b.    
     The rectifier  50  further includes: a circular strip-shaped positive-side heat sink  54  which is a first heat sink and has, on its surface, six positive-side diodes  51   a  and  51   b  and two neutral point diodes  53 ; a circular strip-shaped negative-side heat sink  55 , which is a second heat sink, disposed a radially outside of and on the same plane as the positive-side heat sink  54 , and has, on its surface, six negative-side diodes  52   a  and  52   b , and two neutral point diodes  53 ; and a circuit board  56  electrically connecting the diodes  51   a ,  51   b ,  52   a ,  52   b , and  53 , and a stator coil  16 . 
     The surfaces of the positive-side heat sink  54  and the negative-side heat sink  55  have recesses  57  and  58  formed to accommodate the columnar positive-side diodes  51   a  and  51   b  (the diodes  51   a  are located on an inside diameter side, while the diodes  51   b  are located on an outside diameter side), the negative-side diodes  52   a  and  52   b  (the diodes  52   a  are located on the inside diameter side, while the diodes  52   b  are located on the outside diameter side), and the neutral point diodes  53 . FIG. 3 shows the positive-side heat sink  54  and the negative-side heat sink  55  of the FIG. 2, as observed from a rear side. On the rear surfaces of the heat sinks  54  and  55 , protuberances  59  and  60  are formed at the same time when the recesses  57  and  58  are formed. 
     The positive-side diodes  51   a  and  51   b , the negative-side diodes  52   a  and  52   b , and the neutral point diodes  53  are fixed to the recesses  57  and  58  of the heat sinks  54  and  55  by soldering. Lead wires  61 ,  62 , and  63  of the diodes  51   a ,  51   b ,  52   a ,  52   b , and  53  are electrically connected to terminals  64  and  65  of the circuit board  56 . 
     Positive-side diodes  51   a  and  51   b  on the positive-side heat sink  54  are alternately arranged on the inside diameter side and the outside diameter side of the positive-side heat sink  54  in a zigzag pattern in a circumferential direction. Similarly, negative-side diodes  52   a  and  52   b  on the negative-side heat sink  55  are alternately arranged on the inside diameter side and the outside diameter side of the negative-side heat sink  55  in a zigzag pattern in a circumferential direction. Hence, a distance L 1  between the positive-side diode  51   a  and the negative-side diode  52   b  that are radially adjacent is different in the circumferential direction from a distance L 2  between the positive-side diode  51   b  and the negative-side diode  52   a  that are radially adjacent. Therefore, when electrically connecting the lead wires  61 ,  62 , and  63  of the diodes  51   a ,  51   b ,  52   a ,  52   b , and  53  to the terminals  64  and  65  of the circuit board  56 , the lead wires  61 ,  62 , and  63  are bent in the middle thereof for the greater distance L 2 . 
     The positive-side heat sink  54  is retained on the negative-side heat sink  55  via a holder  40 . The positive-side heat sink  54 , the negative-side heat sink  55 , and the circuit board  56  are fixed in a case  3  by screws (not shown) attached to a rear bracket  2  via through holes  41 . Furthermore, the negative-side heat sink  55  is grounded by being directly attached to the rear bracket  2 . 
     In this embodiment, the positive-side diodes  51   a  and  51   b  are disposed so that, when a distance between central points of the positive-side diodes  51   a  and  51   b  is denoted as W, and a diameter of the positive-side diodes  51   a  and  51   b  is denoted as D, a value expressed as W/D≅1.5 is obtained, and an angle at which a line  100  connecting a center point of the diode  51   b  on the outside diameter side (hereinafter referred to simply as “the outer diode  51   b ”) on the positive-side heat sink  54  and a center point of the rectifier  50  intersects with a line  101  connecting the center point of the outer diode  51   b  and a center point of the diode  51   a  on the inside diameter side (hereinafter refereed to simply as “the inner diameter  51   a ”) adjacent to the diode  51   b  is 112.5 degrees. 
     A study of temperature distribution of the rectifier  50  under the same conditions as those of a conventional rectifier has revealed that the temperature rises toward a center in a circumferential direction, a difference between temperature extremes being approximately 10 degrees Celsius, meaning a temperature drop of 5 degrees Celsius. Furthermore, the temperature rises inward in a radial direction, a difference between temperature extremes being approximately 1 degree Celsius, meaning a temperature drop of 2 degrees Celsius. Temperatures of the positive-side diodes  51   a  and  51   b  at a central portion of the positive-side heat sink  54 , which are the highest temperatures, are 120 degrees Celsius, which indicates a temperature drop of 5 degrees Celsius. 
     Thus, the temperature distribution of the rectifier  50  has become even, and the maximum temperature of the positive-side diodes  51   a  and  51   b  has dropped. The improved cooling efficiency is considered to be due to the following reason. 
     FIG. 4 is a partial enlarged view of the positive-side heat sink  54 , which is the first heat sink, and shows a flow of air that collides against the columnar outer diode  51   b  on the positive-side heat sink  54  and moves along side surfaces thereof. When the air collides against the diode  51   b , it branches onto the two sides thereof, and the flow is blocked at a branching point, resulting in zero velocity. Behind that point, namely, a stagnation point  200 , a laminar boundary layer is formed along the side surfaces, then the branched flow is detached from the side surfaces. At the rear, a detached air portion  201  wherein a vortex or backflow is generated is formed. The inner diodes  51   a  are disposed most closely to the detached air portion  201 . Thus, it is considered that the inner diodes  51   a  are partly involved in the detached air portion  201 , which is a turbulent area, leading to promoted heat transfer on peripheral wall surfaces of the inner diodes  51   a.    
     FIG. 5 shows local Nusselt number Nux of a column placed at right angles to a flow (αxD/λ, where αx denotes a local heat transfer coefficient on the column, D denotes a diameter of the column, and λ denotes thermal conductivity of a fluid. The values of diameter D and thermal conductivity λ are fixed, so that a larger value of Nux means better local heat transfer). This information is provided on page 168 of “Thermal Conductology” published by Rikogaku. In the chart, the axis of abscissa indicates an angle θ (the angle θ is 180 degrees at a position opposing the stagnation point) from the stagnation point  200  to a predetermined position on a side surface of the column, the stagnation point  200  being zero, and the axis of ordinates indicates the Nusselt number at the predetermined position. As can be understood from the chart, the Nusselt number shows larger values when the angle θ is in a range of 100 to 140 degrees. In other words, it is considered that the Nusselt number shows especially larger values due to the generated detached air portion  201 . 
     Accordingly, setting the angle θ to the range of 100 to 140 degrees in FIG. 4 causes an air layer of the peripheral wall surfaces of the adjoining inner diodes  51   a  to be disturbed due to influences exerted by the detached air portion  201  produced by the outer diode  51   b . This permits the positive-side diode  51   b  to be efficiently cooled. 
     In order to cause the adjoining inner diodes  51   a  to be influenced by the detached air portion  201 , the outer diode  51   b  must be close to the inner diodes  51   a  to a certain extent. Influences exerted by a flow around the column have been disclosed by many examples of experiments carried out in the past. Results of such experiments have revealed that the influences are exerted on adjoining columns when (W/D)&lt;2 (refer to, for example, VIII-INTERFERENCE DRAG 8-2). 
     Thus, in the automotive alternator according to the first embodiment set forth above, the inner diodes  51   a  and the outer diodes  51   b  on the positive-side heat sink  54  are arranged in the zigzag pattern in the circumferential direction, and the inner diodes  51   a  are subjected to the influences of the detached air portion  201  generated at the outer diode  51   b . This arrangement ensures efficient cooling of the inner diodes  51   a . Similarly, the protuberances  59  on the rear side of the positive-side heat sink  54  produces the detached air portion, and the heat transfer of the protuberance  59  adjacent to the detached air portion is promoted, also causing the rear side of the positive-side heat sink  54  to be efficiently cooled. 
     In addition, the negative-side diodes  52   a  and the negative-side diodes  52   b  on the negative-side heat sink  55 , which is the second heat sink, are also arranged in the zigzag pattern in the circumferential direction. Hence, a distance between adjoining negative-side diodes  52   a  and  52   b  is greater than it would if they were arranged on the same circumference, resulting in a reduced ventilation drag. This leads to increased ventilation with consequent higher efficiency of cooling the entire rectifier  50 . 
     Second Embodiment 
     FIG. 6 is a front view of a rectifier  70  of an automotive alternator according to a second embodiment of the present invention, a circuit board thereof having been removed. The construction of the second embodiment is the same as that of the first embodiment except that a layout of negative-side diodes  71   a  and  71   b , which are second diodes, is different. 
     The second embodiment is adapted to enhance influences of a detached portion  201  from an outer diode  51   b   1  adjacent to an inner diode  51   a   1  (a central portion of the positive-side heat sink  54 , which is a first heat sink) where temperature is the highest. More specifically, in order to increase a volume of air colliding with the outer diode  51   b   1  adjacent to the inner diode  51   a   1  at the central portion, a distance L between the outer diode  51   b   1  and an outer diode  71   b   1  radially located on an outer side is increased by disposing the outer diode  71   b   1  on an outer diameter side of a negative-side heat sink  55 , which is a second heat sink. 
     Third Embodiment 
     FIG. 7 is a front view of a rectifier  90  of an automotive alternator according to a third embodiment of the present invention, a circuit board thereof having been removed therefrom. The construction of the third embodiment is the same as that of the second embodiment except that a layout of negative-side diodes  91   a  and  91   b , which are second diodes, is different. 
     In the third embodiment, the outer diodes  91   b  and the inner diodes  91   a  are disposed away from a radial line G of outer diodes  51   b  and inner diodes  51   a  secured to a positive-side heat sink  54 , which is a first heat sink. This arrangement increases a volume of cooling air that flows inward in a radial direction and collides with the outer diodes  51   b  and the inner diodes  51   a  secured to the positive-side heat sink  54 , thus further efficiently cooling the outer diodes  51   b  and the inner diodes  51   a.    
     Fourth Embodiment 
     FIG. 8 is a sectional view of an automotive alternator according to a fourth embodiment of the present invention. The construction of the fourth embodiment is the same as that of the first embodiment except that a positive-side heat sink  81  and a negative-side heat sink  82  are disposed on different vertical planes with respect to an axis of a shaft  6 . 
     In this embodiment, the positive-side heat sink and the negative-side heat sink are not on the same plane, so that ventilation drag on the planes is reduced, and a volume of air introduced through an aperture A is increased. This makes it possible to suppress a rise in temperature of positive-side diodes  51  and negative-side diodes  52 . 
     Fifth Embodiment 
     In the embodiments described above, the columnar diodes project from the front surfaces of the heat sinks, and protuberances are formed on the rear surfaces to promote heat transfer by disturbance in the detached air portions on both surfaces. Alternatively, however, the inner diodes and the outer diodes may be accommodated in the recesses of the heat sinks so that the front surfaces of the diodes are flush with the front surfaces of the heat sinks. 
     More specifically, the protuberances jutting out of the rear surfaces of the heat sinks associated with the recesses housing the inner diodes may be disposed so that they are partly included in the areas of the detached air portion produced by collision against the protuberances jutting out of the rear surfaces of the heat sinks associated with the recesses housing the outer diodes. In this case, the heat transfer promotion effect from the disturbance in the detached air portion is obtained at the rear surfaces of the heat sinks. 
     Sixth Embodiment 
     FIG. 9 is a sectional view of an automotive alternator according to a sixth embodiment of the present invention. 
     In the above embodiments, the rectifier  50 ,  70 , or  90  is housed in the case  3 , while in the sixth embodiment, a rectifier  100  is housed in a cover  102  adjacent to a rear bracket  101 . As in the case of the first embodiment, the rectifier  100  has inner and outer positive-side diodes  103  arranged in a zigzag pattern in a circumferential direction, and inner and outer negative-side diodes  104  arranged in the zigzag pattern in the circumferential direction. The inner positive-side diodes  103  are disposed so that they are partly included in an area of a detached air portion generated on a circumferential surface by cooling air that collides against the outer positive-side diodes  103 . A positive-side heat sink  105  and a negative-side heat sink  106  are disposed on vertical planes that are different with respect to axes. 
     In the sixth embodiment, the inner positive-side diodes  103  are subjected to the influences of a detached air portion generated at the outer positive-side diodes  103 , so that the inner positive-side diodes  103  are efficiently cooled, as in the case of the first embodiment. Furthermore, the negative-side heat sink  106  and the positive-side heat sink  105  are not on the same plane, so that ventilation drag on the planes is reduced, and a volume of air introduced through an aperture H of the cover  102  is increased. This makes it possible to suppress a rise in temperature of the positive-side diodes  103  and the negative-side diodes  104 . 
     In the above embodiments, the negative-side heat sink is disposed on the outside diameter side, and the positive-side heat sink is disposed on the inside diameter side. Obviously, however, the present invention can be applied to a rectifier wherein the negative-side heat sink is disposed on the inside diameter side, and the positive-side heat sink is disposed on the outside diameter side. 
     The rectifier in each of the above embodiments is equipped with neutral point diodes connected to the neutral points, and four diodes are disposed in each heat sink. Alternatively, however, only three diodes per heat sink, which is a number of diodes required for three-phase full-wave rectification, may be used. Obviously, the present invention is also applicable to a case wherein a number of diodes disposed on the outer heat sink is greater than a number of diodes disposed on the inner heat sink. 
     Furthermore, only the positive-side diodes or the negative-side diodes may be arranged in the zigzag pattern in the circumferential direction. 
     The shape of the positive-side diodes and the negative-side diodes is not limited to the columnar shape; it may alternatively be a rectangular or polygonal shape. 
     In the above embodiments, the positive-side diodes are disposed so that only the inner positive-side diodes are disposed to be partly included in the area of the detached air portion. Alternatively, however, the negative-side diodes may be disposed so that the outer negative-side diodes are partly included in the area of the detached air portion. 
     Furthermore, in the above embodiments, the descriptions have been given of a case wherein the cooling air flows inward in the radial direction. The present invention, however, may also be applied to a case wherein the cooling air flows in the vicinity of the shaft into the case and moves outward in the radial direction. In this case, therefore, the outer diodes are efficiently cooled by the heat transfer promotion effect by the disturbance in the detached air portion caused by the inner diodes. 
     It is also obvious that the application of the present invention is not limited to an automotive alternator. 
     As described above, the present invention provides the following advantages. 
     As described above, in an alternator according to one aspect of the present invention, at least either the first diodes or the second diodes is composed of diodes on an inside diameter side and diodes on an outside diameter side that are arranged in a zigzag pattern in a circumferential direction. Therefore, the drag of the cooling air flowing into the rectifier is reduced, so that a cooling flow is increased, resulting in higher cooling efficiency of the rectifier. There is another advantage in that the rectifier can be made compact, and a degree of freedom for disposing the diodes is increased. 
     According to another form of the alternator, the diodes on the inside diameter side and the diodes on the outside diameter side may be provided in recessions of a heat sink having a recessed surface, and protuberances associated with the recesses may be formed in a rear surface of the heat sink. Therefore, An area of contact between the heat sinks and the cooling air is increased, leading to higher cooling efficiency of the diodes. 
     According to still another form of the alternator, one of the diode on the inside diameter side and the diode on the outside diameter side may be disposed such that it is partly included in a region of an air detachment portion produced on a peripheral surface by cooling air that collides with the other of the diode on the inside diameter side and the diode on the outside diameter side. Therefore, One of the inner diode or the outer diode is efficiently cooled by the heat transfer promotion effect by the disturbance in the detached air portion that takes place in the other of the inner diode or the outer diode. 
     According to still another form of the alternator, if a distance between a central point of a columnar diode on the inside diameter side and a central point of its adjacent columnar diode on the outside diameter side is denoted as W, and a diameter of the diode on the inside diameter side and the diode on the outside diameter side is denoted as D, then (W/D)&lt;2, and if an angle at which a line connecting a central point of the diode on the inside diameter side and a central point of its adjacent diode on the outside diameter side crosses a line connecting a central axis of a shaft and the central point of the diode on the outside diameter side or the diode on the inside diameter side is denoted as H, then angle θ is 100°&lt;θ&lt;140°. Therefore, one of the inner diode or the outer diode is efficiently cooled by securely benefiting from the heat transfer promotion effect by the disturbance in the detached air portion that takes place in the other of the inner diode or the outer diode. 
     According to still another form of the alternator, the diodes on the outside diameter side that are secured to the second heat sink may be disposed so as to oppose the diodes on the outside diameter side that are secured to the first heat sink. Therefore, A space between opposing diodes can be securely provided, making it possible to reduce the ventilation drag and increase the volume of air that collides against the outer diodes or the inner diodes. 
     According to still another form of the alternator, the diodes on the outside diameter side and the diodes on the inside diameter side that are secured to the second heat sink may be disposed away from radial lines of the diodes on the outside diameter side and the diodes on the inside diameter side that are secured to the first heat sink. Therefore, the volume of cooling air that collides against the diodes increases, permitting the diodes to be cooled further efficiently. 
     According to still another form of the alternator, the first heat sink and the second heat sink may be disposed on different vertical planes with respect to an axis. Therefore, The ventilation drag on the same plane is reduced, permitting the first diodes and the second diodes to be cooled further efficiently. 
     According to still another form of the alternator, the first heat sink may be a positive-side heat sink, the first diodes may be positive-side diodes, the second heat sink abutted against the case may be a negative-side heat sink, and the second diodes may be negative-side diodes. Therefore, the heat of the negative-side diodes is transmitted to the case due to heat conduction, permitting the negative-side diodes to be cooled further efficiently.