Abstract:
A system and method for reducing brush temperature associated with an electric motor or alternator is disclosed as having at least one brush box for housing at least one brush biased radially toward an opening toward the commutator. The brush boxes have at least one or a plurality of heat sink fins associated therewith for thermally conducting heat away from the brushes.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to motors or alternators and, more particularly, to a direct current motor or alternator including a brush holder plate having a plurality of brush boxes for holding brushes and which have at least one or a plurality of heat sink fins associated with the brush boxes.  
           [0003]    2. Description of Related Art  
           [0004]    As is well known, an electric motor or alternator includes a rotor and a stator. The rotor generally includes an armature with windings, as well as a shaft extending from the armature and a commutator. The stator, which has magnets or coils therein, generally surrounds the rotor in the form of a yoke or cylindrical frame. Brushes are also provided and supported from the stator by a brush holder plate. By maintaining contact between the brushes and the commutator, current is transferred to the armature, which inspires an electromagnetic field. The interaction between the electromagnetic field of the armature and that of the stator causes rotation of the rotor to, thereby, perform work in a motor; this interaction also generates electricity in an alternator when the rotor is turned by external force.  
           [0005]    In order to maintain the brushes in firm contact with the commutator, they have generally been positioned in a brush box situated on the brush holder plate and, typically, positively biased toward the center thereof by springs or the like. A recurrent problem with such motors and alternators has been the heat generated by the brushes increases the brush temperature, which increases the brush wear rate, which reduces the brush life and, thereby, the life of motor or alternator.  
           [0006]    It is, therefore, an object of the invention to provide a brush box which can facilitate reducing the temperature of the brushes and, thereby improve the life of the motor or alternator.  
         SUMMARY OF THE INVENTION  
         [0007]    It is, therefore, a primary object of the invention to provide an electric motor or alternator having means for reducing the temperature of the brushes.  
           [0008]    Another object of the invention is to provide a brush box having a heat sink in the form of at least one or a plurality of heat sink fins associated therewith in order to facilitate reducing the temperature of the brushes.  
           [0009]    Still another object of the invention is to provide a heat sink in the form of a plurality of heat sink fins shaped and arranged in a way which reduces air flow resistance of heat sink fins and obtains high heat transfer coefficient.  
           [0010]    In one aspect, this invention comprises an electric motor or alternator comprising a stator comprising a pair of magnets or coils for providing an electrical field, a rotor mounted on an armature shaft and rotatably positioned in the stator, the rotor comprising a commutator positioned on an armature shaft, a brush holder plate positioned around the commutator and comprising a plurality of brush boxes each capable of receiving at least one brush, and the brush box comprising a heat sink for dissipating heat generated by the brushes to lower a brush temperature.  
           [0011]    In another aspect, this invention comprises a method for decreasing brush temperature of an electric motor or alternator comprising the steps of providing a motor or alternator having a housing comprising a stator comprising at least two magnets or coils for providing an electromagnetic field, providing a rotor mounted on an armature shaft, the rotor comprising a commutator positioned on an armature shaft, providing a brush holder plate having a plurality of brush boxes each capable of receiving at least one brush, and providing a heat sink on the brush box for dissipating heat generated by the brush.  
           [0012]    In still another aspect, this invention comprises a brush holder plate having an aperture therethrough for receiving a commutator, the brush holder plate further comprising a plurality of brush boxes each having at least one brush therein for contacting the commutator when the commutator is positioned in the aperture, and the brush box comprising a heat sink.  
           [0013]    Other objects and advantages of the inventions will be apparent from the following description, the accompanying drawings, and the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is an exploded perspective view of a DC motor including a brush holder plate having two brush boxes with heat sink fins in accordance with one embodiment of the invention;  
         [0015]    [0015]FIG. 2 is a front view of the brush holder plate of the present invention;  
         [0016]    [0016]FIG. 3 is a cross-sectional view of the brush holder plate of FIG. 2, taken along line A-A, where two brush boxes are shown having three heat sink fins each to facilitate reducing brush temperature;  
         [0017]    [0017]FIG. 4 is a rear view of the brush holder plate shown in FIG. 2; and  
         [0018]    [0018]FIG. 5 is a view of a predicted air velocity field around brush holder plate showing the heat sink fins are arranged generally along the predicted streamlines of the air flow around the brush holder plate. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]    Referring now to FIG. 1, an electric motor  10  in accordance with one embodiment of the invention is shown. In the embodiment being described, the electric motor  10  is a direct-current (DC) motor comprising a stator  12  and a rotor  14 . The rotor  14  includes an armature  16 , an armature shaft  18  which extends through armature  16  and a commutator  20  positioned on armature shaft  18  adjacent to armature  16 . The stator  12 , as best illustrated in FIG. 1, has a housing  22  containing magnets  13  therein and an end cap  24  to which the housing  22  is connected. It will be understood that when DC motor  10  is assembled, rotor  14  will be surrounded by stator  12 .  
         [0020]    A brush holder plate  26  is provided which is positioned within housing  22  of stator  12 . It should be appreciated that brush holder plate  26  has a central opening  28  therethrough defined by an annular wall  30  (as illustrated in FIGS. 2 and 4). At least one, and preferably two or more brush boxes  32  are provided on brush holder plate  26  for housing brushes  34 , which are biased radially inward towards central opening  28 . As is known in the art, brushes  34  may be biased by spring members  36 , which are preferably attached at one end to a connector  38  extending from brush holder plate  26  (FIG. 2). In this way, the free end of spring member  36  is able to bias brush  34  towards central opening  28 . It should also be appreciated that the brush holder plate  26  includes four leg members  42  each having a rubber cap  44  (FIG. 1). The leg members  42  enable brush holder plate  26  to be seated within housing  22  of stator  12 .  
         [0021]    A terminal  46  (FIG. 1) is provided in order to permit an electrical connection between electrical power source (not shown) and brushes  34  by means of leads  48  and  52  (FIG. 4), as well as act as a media for holding two inductors  49 . Accordingly, it should be appreciated that the current from the electrical power source is then able to be supplied to brushes  34 , whereupon such current is transferred to commutator  20 . The current received by commutator  20  then acts upon the windings of armature  16  so as to induce an electromagnetic field. The interaction between the electromagnetic field of armature  16  and that of the magnets  13  in stator  12  causes rotor  14  to rotate.  
         [0022]    Armature shaft  18  is preferably engaged with a bearing  50  within end cap portion  24  of stator  12  and another bearing  51  within housing  22 , whereby rotation of rotor  14  is able to provide useful work. With respect to the present application, DC motor  10  is specifically utilized as a power blower mechanism for providing an air blower for an automobile, but it should be understood that DC motor  10  could be used in any number of applications and is not limited thereto. For example, the DC motor  10  could be utilized as an engine cooling fan mechanism, a power lift mechanism, a windshield wiper mechanism or other useful device.  
         [0023]    As best illustrated in FIGS. 1 and 2, the brush boxes  32  each receive the at least one brush  34 . During operation, the at least two brushes  34  cooperate with the commutator during operation and are heated up to a high temperature. The high brush temperature increases the wear rate of both brushes  34  and commutator  20 , and therefore reduces the life of motor  10 .  
         [0024]    Accordingly, the invention comprises a plurality of heat sink fins  60  which are generally elongated, as best illustrated in FIGS. 2, 3 and  4 . In the embodiment being described, the plurality of fins  60  are provided on a top surface  32   a , but are not provided on the bottom surface  32   b  (FIG. 3). The plurality of fins  60  may comprise at least one contoured or curved fins  65  (FIG. 5). It should be understood, however, that the plurality of fins  60  can be provided on both top surface  32   a  and other surfaces  32   b ,  32   c  and  32   d  (FIGS. 2 and 3) and that more or fewer fins could be provided on such surfaces. Also, the plurality of fins  60  may be situated on only one of the boxes  32 .  
         [0025]    In the embodiment being described, the plurality of fins  60  are integrally formed into brush boxes  32  and both brush boxes  32  and the plurality of fins  60  are copper. To reduce the resistance of heat conduction, the thickness of the plurality of fins  60  is 1 mm. It should be understood, that brush boxes  32  and the plurality of fins  60  can also be made from other metals with high thermal conductivity, such as aluminum. Note, as illustrated in FIGS. 2 and 5, that the plurality of fins  60  are substantially parallel to each other and arranged along the streamline of the air flow around brush holder plate  26  to reduce air flow resistance and increase heat transfer coefficient of the heat sink. FIG. 5 shows a predicted velocity field around the brush holder plate  26 , illustrating the direction of air flow. This predicted velocity field was obtained from a simulation that will now be described.  
         [0026]    The simulation was performed using a Model No. BL9000 blower motor available from the Assignee of the present invention. This computational fluid dynamics and conjugate heat transfer analysis was conducted using the STAR-CD CFD package, available from CD adapco Group. Three fins were added to the brush box  32  in the manner illustrated in FIGS.  2 - 4 . An operating point of the motor  10  was 303.8 watts input power, 72.5% efficiency, 3423 RPM, 0 and 3 cubic feet per minute (cfm) ventilation flow rate and 20 degrees Celsius ambient temperatures. The results are listed in Table I below:  
                                                           TABLE I                           Simulation Results                    Heat Sink       Heat Sink           Copper   Copper   Copper   Copper           Brush Box   Brush Box   Brush Box   Brush Box           (no fins)   (with fins 60)   (no fins)   (with fins 60)                        Brush   143-151   121-135   114-130   89-102       Temperature       (Degree       Celsius)       Vent Flow   0 CFM   0 CFM   3 CFM   3 CFM       Rate       Case File   b19a/01   b19d/01   b19b   b19f                  
 
         [0027]    Thus, the simulation shows that there is more than twenty degree centigrade temperature reduction on the brushes  34 . To further substantiate this idea, an experimental study was conducted and those test results will now be described.  
         [0028]    The tests were conducted using the Model No. BL9000 motor/GMT-800 module available from the Assignee of the present invention. The system was operating at 14 volts and by adjusting the opening on the air flow outlet, the current was set at about 22 amps. The modified brush holder box  34  is similar to the one illustrated in FIG. 2. The copper fins  60  were soldered onto the copper brush box  32 , but it should be appreciated that they could be integrally formed, separately mounted or otherwise associated with the brush box  32  with a good thermal connection in order to dissipate heat as described herein. In the test module, the vent flow inlet  66  was located very close to one of the brush boxes  32  (FIG. 1). In order to facilitate reducing blockage of vent air flow into the motor, the plurality of fins  60  were added to only one side of the brush box  32  which is located away from vent inlet  66 . The following Table II summarizes the test results:  
                                                             TABLE II                           Test Results                TEMPERATURE               (DEGREES CELSIUS)                Test A   Test B                            #1 Brush   83.5   73           #2 Brush   82   78           Motor casing   36.6   33.8           Ambient Temperature   21.1   20.3           Voltage (volt)   13.99   14.00           Current (amps)   22.4   22.4                      
 
         [0029]    In Test A, brush boxes were made of copper without fins. Test B was a repeat run of Test A, except that there were heat sink fins  60  on the brush box of #1 brush. Thus, when the copper brush box #1 was combined with the heat sink fins  60 , the #1 brush temperature decreased by 9.7 degrees Celsius after the change in the ambient temperature T ∞  is deducted. It should be appreciated that the temperature reduction margin is less than that predicted by the numerical simulations illustrated relative to Table I above. The major reason is that the surface contact between the brushes  34  and the brush box  32  is relatively poor, but at least good enough to achieve the temperature reduction mentioned in Table II. This inefficiency in heat transfer is a result of the brushes  34  having to be able to move freely inside the brush boxes  32  to insure good electrical contact between the brushes  34  and the commutator  20 . Therefore, only a very small percentage of a surface of the brush  34  is actually in contact with the brush box  32  and the normal pressure in this contact area is very low. Consequently, very poor thermal connection between the brushes  34  and brush box  32  is resulted, and typically, there is a fifteen degree Celsius temperature difference between them.  
         [0030]    Advantageously, this system provides means for reducing a brush temperature by 9.7 degrees Celsius in the embodiment being described.  
         [0031]    In the embodiment being described, a method for decreasing brush temperature of an electric motor comprises the steps of providing a motor  10  having the aforementioned characteristics, providing the rotor  14  mounted on an armature shaft  18  and comprising the commutator  20 . The method also contemplates providing the brush holder plate  26  having the aforementioned brush boxes  32  each capable of receiving at least one brush  34 . The method further facilitates providing heat sink fins  60  on the brush boxes  32  for dissipating heat generated by the brushes  34 .  
         [0032]    As mentioned earlier herein, at least one or a plurality of fins  60  may be situated on one or more of the brush box  32  to achieve the desired heat dissipation characteristics.  
         [0033]    While the methods herein described, and the forms of apparatus for carrying these methods into effect, constitute preferred embodiments of this invention, it is to be understood that the invention is not limited to these precise methods and forms of apparatus, and that changes may be made without departing from the scope of the invention, which is defined in the appended claims.