Abstract:
In an AC generator of a motor vehicle, having a positive polarity brush from which current flows into a corresponding slip ring and a negative polarity brush into which current flows from the corresponding slip ring, the brushes or the springs of the brushes are configured respectively differently, to achieve substantially identical rates of wear for the brushes. This is done for example by increasing the area of contact between the negative polarity brush and its slip ring, lengthening the negative polarity brush, or decreasing the amount of force applied by the spring of the negative polarity brush.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based on and incorporates herein by reference Japanese Patent Application No. 2005-296797filed on Oct. 11, 2005.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of Application  
         [0003]     The present invention relates to an AC generator, generally referred to as an alternator, of a vehicle such as an automobile or truck. In particular, the invention relates to the brush apparatus of such an alternator.  
         [0004]     2. Description of Related Art  
         [0005]     The brush apparatus of the alternator of a vehicle incorporates a pair of brushes held in sliding contact and in electrical contact with respective slip rings that are mounted on the rotor of the generator. Current flows from one of these brushes (referred to in the following as the positive polarity brush), from a high (positive) power supply potential, into the corresponding slip ring, and the current flows from the other brush (referred to in the following as the negative polarity brush) into the corresponding slip ring, and hence to the low power supply potential. The slip rings corresponding to the positive polarity brush and the negative polarity brush will be respectively referred to as the positive polarity slip ring and negative polarity slip ring.  
         [0006]     Hitherto, such a pair of brushes have been generally formed of identical materials and with identical configurations, for example as described in Japanese patent publication No. 9-182353 (pages 4-8), FIGS. 1-8).  
         [0007]     Over a long period of use, wear occurs at the area of contact between such a brush and the corresponding slip ring. The operating life of a brush (more accurately, the operating life of the combination of that brush with the corresponding slip ring) until the brush becomes defective as a result of such contact wear, will be referred to in the following as the wear-down lifetime of the brush. Due to the fact electrical current flows in respectively opposite directions through the pair of brushes, the negative polarity brush and positive polarity brush have respectively different wear-down lifetimes. Thus there is the problem that when one of the brushes (more specifically, the combination of a brush and the corresponding slip ring) reaches the end of its wear-down lifetime, the alternator as a whole becomes inoperable. Hence the operating life of the alternator is shortened.  
         [0008]     In Japanese patent publication No. 2005-261153, it is proposed to overcome this problem by forming the slip rings from respectively different materials, with the slip ring corresponding to the brush having the more rapid rate of wear being formed of a metallic material that has greater resistance to wear than the material of the other slip ring. However such a method has basic disadvantages.  
         [0009]     Firstly, if the metallic material having greater resistance to wear is for example stainless steel, then the material has a substantially higher electrical resistivity than a material such as copper. This will result in an amount of lowering of the output power of the alternator, to an extent that cannot be ignored.  
         [0010]     Secondly, with the usual method of manufacture of the slip rings for an alternator, a single elongated ring is formed in a mold, then a central part of the ring is removed by machining, to form the negative polarity-side slip ring and positive polarity-side slip ring. However if the slip rings are formed of respectively different materials, then it becomes necessary to provide separate molds for the pair of slip rings, and to form these in the molds to a high degree of accuracy of axial concentricity. Hence, the molding process becomes slower and more complex, and the manufacturing cost will thereby be increased.  
         [0011]     Thirdly, it is necessary to electrically connect the slip rings to the field winding of the generator by welding or soldering attachment of connecting leads to the slip rings. However if the negative polarity-side slip ring and positive polarity-side slip ring are of respectively different metallic materials, then it becomes necessary to provide two separate installations for performing such attachment operations, since the requirements for performing the welding or soldering work will be respectively different for the two types of metallic material. Thus since the work cannot be performed by using a single installation for performing the welding or soldering attachment operations, the manufacturing cost will be increased.  
         [0012]     Fourthly, if the negative polarity-side slip ring and positive polarity-side slip ring are of respectively different metallic materials, then it becomes necessary to provide two separate installations for performing machining work to form the slip rings, since the machining requirements will be respectively different for the two types of metallic material. Hence, the manufacturing cost will be increased.  
       SUMMARY OF THE INVENTION  
       [0013]     It is an objective of the present invention to overcome the above problem, by providing an AC generator for a motor vehicle whereby the operating life of the AC generator can be lengthened without a reduction of the power output level of the generator and without an increase in manufacturing cost.  
         [0014]     To achieve the above objective, the invention provides a vehicle AC generator for a vehicle, having a field winding that is wound on a rotor iron core, a rotor shaft with a negative polarity slip ring and a positive polarity slip ring mounted circumferentially thereon and respectively electrically connected to opposite ends of said field winding, a negative polarity brush and a positive polarity brush that are held in sliding contact with the respective outer circumferences of said negative polarity slip ring and positive polarity slip ring, and a negative polarity spring and positive polarity spring for respectively urging the negative polarity brush and positive polarity brush against the corresponding slip rings. With such an AC generator, the operating life of each brush is determined by the degree of wear that occurs due to said sliding against the corresponding slip ring. According to a first aspect, the invention is characterized in that each of said negative polarity slip ring and said positive polarity slip ring are formed of an identical material, while said negative polarity brush and said positive polarity brush are constituted respectively differently, such that said negative polarity brush and positive polarity brush substantially identical operating lifetimes.  
         [0015]     This can be achieved by forming the negative polarity brush and positive polarity brush to have respectively different sizes of contact area with the corresponding slip rings. In that way, the respective wear-down lifetimes of the brushes can be made substantially identical, i.e., by increasing the size of the slip ring contact area of the one of the brushes which would otherwise wear more rapidly, the wear-down lifetime of that brush can be appropriately extended, so that the operating life of the alternator as a whole can be correspondingly extended  
         [0016]     Preferably, the negative polarity brush is shaped such as to increase the contact area between that brush and the negative polarity slip ring, by comparison with the contact area between the positive polarity brush and the positive polarity slip ring.  
         [0017]     That increase in contact area can be achieved by increasing the length of the negative polarity brush by comparison with that of the positive polarity brush, such that the wear-down lifetime of the negative polarity brush is made substantially identical to that of the positive polarity brush.  
         [0018]     As a second alternative, the objective can be achieved by forming the brushes from respectively different materials. Specifically, the negative polarity brush is preferably formed from a material having a higher resistance to wear than does the material of the positive polarity brush. When the brushes are formed of a material that includes a proportion of copper, the objective can be achieved by making the proportion of copper contained in the material of the negative polarity brush higher than that contained in the material of the positive polarity brush.  
         [0019]     As another alternative, the objective of the invention can be achieved by arranging that the level of force applied to the negative polarity brush by a spring which urges that brush against the negative polarity slip ring is made lower than the corresponding level of force applied by a spring that acts on the positive polarity brush. This can be done (without otherwise altering the configuration and components of the alternator) by increasing the pitch of the coils in the negative polarity spring in relation to that of the positive polarity spring, or forming the negative polarity spring of material that is more flexible than that of the positive polarity spring, for example.  
         [0020]     As summarized above, the objective of the invention can be achieved simply by altering the shape or the constituent material of one of the brushes or of one of the springs, and the slip rings can each be formed of an identical material. Hence the objective can be achieved without a lowering of the power output capability or a significant increase in manufacturing cost of the alternator, in particular by comparison with an alternator in which the slip rings are formed of respectively different materials. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]      FIG. 1  shows a side view of an embodiment of an alternator for a motor vehicle, taken partially in cross-section;  
         [0022]      FIG. 2  is an expanded view of a portion of  FIG. 1 , showing details of a brush apparatus;  
         [0023]      FIG. 3  conceptually illustrates a manner of establishing respectively different sizes of an area of contact between brush and slip ring, for the brushes in the apparatus of  FIG. 2 ;  
         [0024]      FIGS. 4 and 5  conceptually illustrate alternative manners of establishing respectively different sizes of an area of contact between brush and slip ring, for the brushes in the apparatus of  FIG. 2 ; and  
         [0025]      FIG. 6  conceptually illustrates a manner of applying respectively different values of spring force to the brushes in the apparatus of  FIG. 2 . 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0026]      FIG. 1  shows the overall configuration of an embodiment of an alternator for a vehicle, seen in side view in partial cross-section. The alternator  1  includes a stator  2 , a rotor  3 , a frame  4 , a brush apparatus  5 , a rectifier apparatus  6  and a rear cover  7 .  
         [0027]     The stator  2  is made up of a stator iron core  22 , a stator winding  23  and an insulator  24  which is located between the stator iron core  22  and stator winding  23  for electrical insulation. The rotor  3  includes a field winding  31  which is a tubular concentric wave-wound coil formed of insulation-covered copper wire, which is enclosed at opposing ends of the rotor shaft  33  by opposing pole cores  32 . The pole cores  32  function as the rotor iron core, and each is formed with six claw-shaped portions. A cooling fan  35  is fixedly attached by welding, etc., to the front side of the front-end one of the pole cores  32 , for impelling a flow of cooling air that is drawn in through the front end (i.e., left-side end, as seen in  FIG. 1 ) of the alternator  1 , with the air moving along axial and radial directions. Similarly, a cooling fan  36  is fixedly attached by welding, etc., to the rear side of the rear-end one of the pole cores  32 , for impelling a flow of cooling air that is drawn in through the rear end of the alternator  1 , with the air moving along a radial direction.  
         [0028]     A negative polarity slip ring  37  and a positive polarity slip ring  38 , each formed of an identical material, are mounted on the circumference of the rotor shaft  33  near the rear end of that shaft and are respectively connected to opposing ends of the field winding  31  for supplying an excitation current to the field winding  31 , with that current flowing through the slip rings  37  and  38  and a brush apparatus  5 .  
         [0029]     The frame  4  encloses the stator  2  and the rotor  3 , with the rotor shaft  33  of the rotor  3  being rotatably supported by the frame  4  and with a fixed air gap established between the stator  2  and the outer circumference of the pole cores  32  of the rotor  3 . The frame  4  is formed with intake apertures  41  and outlet apertures  42 , to pass the aforementioned flows of cooling air.  
         [0030]     The brush apparatus  5  serves to supply a flow of excitation current from the rectifier apparatus  6  to the field winding  31  of the rotor  3 . The slip rings  37  and  38  are respectively fixedly attached circumferentially on the rotor shaft  33  of the rotor  3 , with the negative polarity brush  51  and the positive polarity brush  52  being respectively held in sliding contact with the slip rings  37  and  38 . The structure of the brush apparatus  5  will be described in detail hereinafter.  
         [0031]     The rectifier apparatus  6  is a 3-phase rectifier for rectifying a 3-phase output voltage that is generated by the stator winding  23 . The rear cover  7  covers components of the alternator  1  including the brush apparatus  5 , the rectifier apparatus  6 , the IC regulator  9 , etc., that are mounted on the outer side of the rear-end part of the frame  4 , to protect these components.  
         [0032]     The structure of the brush apparatus  5  is illustrated in the expanded partial cross-sectional view of  FIG. 2 , which shows the brush apparatus  5  installed in the alternator  1 . The brush apparatus  5  is made up of a brush holder  60  which is formed of molded plastic and has contact terminals embedded therein, a negative polarity brush  51  and a positive polarity brush  52 , pigtails  63  which electrically connect the brushes  51 ,  52  to the contact terminals, a negative polarity spring  64  and a positive polarity spring  65 .  
         [0033]     An output voltage (i.e., unsmoothed DC voltage) that is applied to the positive polarity brush  2  from the rectifier apparatus  6  is higher (more positive) than the voltage applied to the negative polarity brush  51 . Each of the negative polarity brush  51  and positive polarity brush  52  is of oblong rectangular form, with the face that is in contact with the corresponding one of the slip rings  37  and  38  being formed with a curve that matches the outer circumference of the slip ring. The negative polarity spring  64  and a positive polarity spring  65  are respectively disposed opposite the curved contact faces of the negative polarity brush  51  and positive polarity brush  52 , for urging the corresponding brush into contact with the corresponding slip ring with a predetermined amount of force.  
         [0034]     Each of the negative polarity brush  51  and positive polarity brush  52  is formed of carbon powder as a main constituent, into which a proportion of copper powder is mixed.  
         [0035]     The present invention provides four methods for ensuring that the respective wear-down lifetimes (as defined hereinabove) of the negative polarity brush  51  and positive polarity brush  52  can be made substantially identical.  
       METHOD 1  
       [0036]     With the first method, the area of contact between the negative polarity brush  51  and the negative polarity slip ring  37  is made different from that between the positive polarity brush  52  and the positive polarity slip ring  38 . By increasing the area of a brush face that is in contact with a slip ring, the rate of wear of the brush, due to rubbing against the slip ring, can be reduced. Specifically, the contact area between the negative polarity brush  51  and the negative polarity slip ring  37  (i.e., the area of the curved face of the negative polarity brush  51 ) is made greater than the contact area between the positive polarity brush  52  and the positive polarity slip ring  38 .  
         [0037]     As illustrated in  FIG. 3 , this can be accomplished by increasing a width dimension of the negative polarity brush  51  (which otherwise will have a higher rate of wear than the positive polarity brush  52 , as described hereinabove) as measured along a direction parallel to the axis of the rotor shaft  33 , i.e., a direction at right angles to the direction of force applied to the negative polarity brush  51  by the negative polarity spring  64 . The axial length of the curved face of the negative polarity brush  51  is thereby increased, so that the contact area between the negative polarity brush  51  and negative polarity slip ring  37  is increased.  
         [0038]     Alternatively as illustrated in  FIG. 4  this can be accomplished by increasing a width dimension of the negative polarity brush  51 , as measured along a direction at right angles to the axis of the rotor shaft  33  and at right angles to the direction of force applied to the negative polarity brush  51  by the negative polarity spring  64 . The circumferential length of the curved face of the negative polarity brush  51  is thereby increased, so that the contact area between the negative polarity brush  51  and negative polarity slip ring  37  is increased. By increasing that contact area by an appropriate amount in relation to that of the positive polarity brush  52 , the wear-down lifetime of the negative polarity brush  51  can be made substantially identical to that of the positive polarity brush  52 .  
         [0039]     As a further alternative, it would be possible to increase both of the above dimensions of the negative polarity brush  51 , to increase the contact area by an appropriate amount.  
       METHOD 2  
       [0040]     With the second method, the lengths (i.e., dimension measured along the direction in which force is applied by the corresponding one of the springs  64 ,  65 ) of the negative polarity brush  51  and positive polarity brush  52  are made respectively different. Specifically, the length of the negative polarity brush  51  is made greater than that of the positive polarity brush  52 , as illustrated in  FIG. 4 . By increasing the length of the negative polarity brush  51  by an appropriate amount in relation to that of the positive polarity brush  52 , the wear-down lifetime of the negative polarity brush  51  can be made substantially identical to that of the positive polarity brush  52 .  
       METHOD 3  
       [0041]     With the third method, the brushes  51 ,  52  are formed of respectively different materials, with one material having a higher resistance to wear than the other material. Specifically, the negative polarity brush  51  is preferably formed of a material having a higher resistance to wear than the material of the positive polarity brush  52 .  
         [0042]     For example when the brush material consists of a substance such as graphite with an admixture of copper powder, then the proportion of copper in the material used to constitute the negative polarity brush  51  is made greater than the proportion of copper in the material of the positive polarity brush  52 . In that way, by appropriately increasing the resistance to wear of the material used for the negative polarity brush  51 , by comparison with that of the material of the positive polarity brush  52 , the wear-down lifetime of the negative polarity brush  51  can be made substantially identical to that of the positive polarity brush  52 .  
       METHOD 4  
       [0043]     With the fourth method, the springs corresponding to the negative polarity brush  51  and positive polarity brush  52  are manufactured to exert respectively different levels of force. Specifically, the spring that acts on the negative polarity brush  51  is formed such as to exert a smaller amount of force than the spring corresponding to the positive polarity brush  52 .  
         [0044]     As illustrated in  FIG. 6 , this can for example be accomplished by increasing the pitch of the coils of the spring  64 , corresponding to the negative polarity brush  51 , by comparison with that of the spring  65  corresponding to the positive polarity brush  52 , i.e., the number of turns per unit length of the spring  65  is made greater than for the spring  64 .  
         [0045]     Alternatively, it is possible to achieve the same result by using an identical coil pitch for each of the springs  64 ,  65 , but making the diameter of the spring  64  smaller than that of the spring  65 .  
         [0046]     As a further alternative, the springs  64 ,  65  can be of identical configurations, but with the spring  64  formed of a material that is more flexible than the material used to form the spring  65 .  
         [0047]     By using one or a combination of the above techniques to appropriately decrease the level of spring force applied to the negative polarity brush  51  by comparison with the spring force applied to the positive polarity brush  52 , the wear-down lifetime of the negative polarity brush  51  can be made substantially identical to that of the positive polarity brush  52 .  
         [0048]     As can be seen from the above embodiments, the objectives of the invention can be achieved by a simple modification of the shape or constituent material of a brush or a spring of the alternator, while each of the slip rings can be formed of an identical material, so that that there will be no reduction of power output or any significant increase in manufacturing cost, by comparison with a conventional alternator.  
         [0049]     It should be understood that although various different methods of rending the wear-down lifetime of the negative polarity brush  51  substantially identical to that of the positive polarity brush  52  have been described separately above, it would possible to combine two or more of these methods to achieve the objectives of the invention. The above description should thus be understood in a descriptive and not in a limiting sense.