Abstract:
A detailed listing of all claims that are, or were, in the present application, irrespective of whether the claim(s) remain(s) under examination in the application is presented below. The claims are presented in ascending order and each includes one status identifier. Those claims not cancelled or withdrawn but amended by the current amendment utilize the following notations for amendment: 1. deleted matter is shown by strikethrough for six or more characters and double brackets for five or fewer characters; and 2. added matter is shown by underlining.

Description:
PRIORITY CLAIM 
       [0001]    The present application is a National Phase entry of PCT Application No. PCT/JP2009/0069743, filed Nov. 20, 2009, which claims priority from Japanese Patent Application Number 2008-298514, filed Nov. 21, 2008, the disclosures of which are hereby incorporated by reference herein in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a motor including a common brush, a low-speed drive brush, and a high-speed drive brush, and a method for setting the widths and locations of the three brushes. 
       BACKGROUND ART 
       [0003]    A motor that includes three brushes, which are a common brush, a low-speed drive brush, and a high-speed drive brush, and is driven at two speeds, a low-speed and a high-speed, is used to drive a vehicle wiper in the prior art. Patent literature  1  describes an example of a motor including two magnets, which are fixed to an inner circumferential surface of a yoke housing, and an armature, which is rotatably arranged at an inner side of the two magnets. The armature includes a rotation shaft, which is supported by the yoke housing, an armature core, which is fixed to the rotation shaft and has a plurality of coils wound thereon, and a commutator, which is also fixed to the rotation shaft. The commutator includes a plurality of segments arranged on the outer circumferential surface of the commutator along a circumferential direction. A common brush, a low-speed drive brush, and a high-speed drive brush are arranged near the commutator. Each brush has a distal portion slidably pressed against the outer circumferential surface of the commutator. The common brush and the low-speed drive brush are arranged at an interval of 180° in the circumferential direction of the commutator. The high-speed drive brush is arranged at a position shifted from the low-speed drive brush by a predetermined angle in the circumferential direction. The armature rotates at a low speed when supplied with current via the common brush and the low-speed drive brush and rotates at a high speed when supplied with current via the common brush and the high-speed drive brush. 
         [0004]    Patent literature 2 also describes such a motor including three brushes. The publication describes that the motor can be miniaturized by using magnets forming four or more poles. The motor described in the publication includes four magnets, which are fixed to an inner circumferential surface of a yoke housing, and an armature, which is rotatably arranged at an inner side of the magnets. An armature core including sixteen teeth, which extend radially, and a commutator, which has sixteen segments arranged in the circumferential direction, are fixed to the rotation shaft of the armature. Sixteen coils are wound in an overlapping manner around the armature core. In each pair of segments arranged at a 180° interval, the segments are short-circuited so that they have the same potential in the commutator. 
       PRIOR ART LITERATURE 
       [0005]    Patent Literature 
         [0006]    [Patent Literature 1] 
         [0007]    Japanese National Phase Laid-Open Patent Publication No. 10-503640 
         [0008]    [Patent Literature 2] 
         [0009]    Japanese Laid-Open Patent Publication No. 2007-143278 
       SUMMARY OF THE INVENTION 
       [0010]    However, in the motor described in patent literature 2, the change in the number of effective coils (number of coils that generate a magnetic field when supplied with current) becomes large depending on whether or not the brushes short-circuit segments that are adjacent to each other in the circumferential direction of the rotation shaft. For example, in the motor described in patent literature 2, all sixteen coils are effective when none of the three brushes short-circuit two segments that are adjacent to each other in the circumferential direction. When the three brushes each simultaneously short-circuit two segments that are adjacent to each other in the circumferential direction, six of the coils are undergoing commutation and the remaining ten are effective coils. Therefore, in the motor described in patent literature 2, the number of effective coils is either sixteen or ten. 
         [0011]      FIGS. 24(   a ) and  24 ( b ) show another example of a motor. The motor includes a magnet, which has four magnetic pole portions, an armature core, which has fourteen teeth with fourteen coils wound thereon in an overlapping manner, and a commutator, which includes fourteen segments arranged in the circumferential direction. In each pair of segments arranged at a 180° interval, the segments are short-circuited. The three brushes of the motor are laid out in the same manner as the brushes of the motor described in patent literature 1 that includes the magnets forming two magnetic pole portions. In this motor, referring to  FIG. 24(   a ), when a common brush  101  short-circuits two segments  111 , which are adjacent to each other in the circumferential direction of the rotation shaft, two coils  112  indicated by broken lines undergo commutation, and twelve coils  112  indicated by solid lines are effective. Furthermore, referring to  FIG. 24(   b ), when the common brush  101 , a low-speed drive brush  102 , and a high-speed drive brush  103  each short-circuit two segments  111 , which are adjacent in the circumferential direction of the rotation shaft, six coils  112  indicated by broken lines undergo commutation, and eight coils  112  indicated by solid lines are effective. In this manner, the number of effective coils is either twelve or eight. 
         [0012]    When the change in the number of effective coils is large, the resistance fluctuation in the coils of the armature increases. This fluctuates the value of the supplied current and thereby fluctuates the torque. The torque fluctuation vibrates the motor and generates noise. 
         [0013]    A first object of the present invention is to suppress resistance fluctuation in the coils of a motor including magnets forming four or more magnetic pole portions, a common brush, a low-speed drive brush, and a high-speed drive brush. 
         [0014]    A second object of the present invention is to provide a brush configuration method that suppresses resistance fluctuation in the coils of a motor including magnets forming four or more magnetic pole portions, a common brush, a low-speed drive brush, and a high-speed drive brush. 
         [0015]    To achieve the first object, one aspect of the present invention provides a motor including at least one magnet, an armature, a common brush, a low-speed drive brush, and a high-speed drive brush. The magnet forms four or more magnetic pole portions. The armature includes a rotation shaft, an armature core fixed to the rotation shaft and having a plurality of teeth, a coil wound around the teeth, and a commutator fixed to the rotation shaft and having a plurality of segments arranged in a circumferential direction of the rotation shaft. The common brush, low-speed drive brush, and high-speed drive brush are spaced apart and arranged along the circumferential direction of the rotation shaft to slidably contact the commutator, with each brush having a width in the circumferential direction of the rotation shaft. The number of the teeth is the same as the number of segments. When the number of the magnetic pole portions formed by the magnet is represented by P and the number of the teeth is represented by S, (2S/P) is an odd number. The width and location of each brush in the circumferential direction of the rotation shaft are set so that the three types of the brushes, which are the common brush, the low-speed drive brush, and the high-speed drive brush, all do not simultaneously short-circuit two of the segments that are adjacent to each other in the circumferential direction. 
         [0016]    In this structure, the common brush, the low-speed drive brush, and the high-speed drive brush all do not simultaneously short-circuit two of the segments that are adjacent to each other in the circumferential direction. Accordingly, changes in the number of effective coils are suppressed, and resistance fluctuation of the coil is suppressed. As a result, fluctuation in the value of the supplied current is suppressed, torque fluctuation is suppressed, and motor vibration caused by torque fluctuation is suppressed. Further, a decrease in the number of effective coils is suppressed. 
         [0017]    Preferably, the width of each brush and the location of the brush in the circumferential direction of the rotation shaft are set so that the common brush, the low-speed drive brush, and the high-speed drive brush sequentially short-circuit two of the segments that are adjacent to each other in the circumferential direction. 
         [0018]    In this structure, the common brush, the low-speed drive brush, and the high-speed drive brush sequentially short-circuit two segments that are adjacent to each other in the circumferential direction. Thus, sudden changes in the number of effective coils are suppressed. Accordingly, the generation of vibration in the motor is suppressed. 
         [0019]    Preferably, rotation of the armature repeats a short-circuit state, in which a maximum of two of the three types of the brushes, which are the common brush, the low-speed drive brush, and the high-speed drive brush, short circuit two of the segments that are adjacent to each other in the circumferential direction, and a non-short-circuit state, in which none of the three types of the brushes short circuit two of the segments that are adjacent to each other in the circumferential direction. 
         [0020]    In this structure, when in a short-circuit state, a maximum of only two types of the brushes simultaneously short circuit two segments that are adjacent to each other in the circumferential direction. Thus, by repeating the short-circuit state and a non-short-circuit state, changes in the number of effective coils may be minimized, and resistance fluctuation of the coil may be minimized. As a result, fluctuation in the value of the supplied current is minimized, and resistance fluctuation of the coil is minimized. Further, in the short-circuit state, the coils are all effective coils. Thus, the coils may be used effectively. 
         [0021]    Preferably, rotation of the armature repeats a double type short-circuit state, in which two of the three types of the brushes, which are the common brush, the low-speed drive brush, and the high-speed drive brush, short circuit two of the segments that are adjacent to each other in the circumferential direction, a single-type short circuit state, in which one type of the brushes short circuit two of the segments that are adjacent to each other in the circumferential direction, and the non-short-circuit state so that the single-type short circuit state is performed before and after the double type short-circuit state. 
         [0022]    In this structure, the double type short-circuit state, the single type short-circuit state, and the non-short-circuit state are repeated with the single type short-circuit state being in between the double type short-circuit states. Thus, the non-short-circuit state to the double short-circuit state or the double short-circuit state to the non-short-circuit state are reached in a stepped manner with the single type short-circuit state being in between. Accordingly, even though the double type short-circuit state is present, the number of effective coils does not suddenly change, and sudden resistance fluctuation of the coil is further suppressed. As a result, sudden fluctuations in the value of the supplied current are minimized, torque fluctuation is further suppressed, and vibration of the wiper motor caused by torque fluctuation is further suppressed. 
         [0023]    Preferably, when the number of magnetic pole portions of the magnet is represented by P, with respect to the circumferential direction of the rotation shaft, when the segments each have a width represented by L 1 , adjacent ones of the segments are spaced apart by an interval represented by L 2 , the width of the common brush is represented by B 1 , the width of the low-speed drive brush is represented by B 2 , the width of the high-speed drive brush is represented by B 3 , and the width of a layout area in which the three brushes are arranged is represented by A; L 2 &lt;B 1 &lt;(L 1 +2×L 2 ), L 2 &lt;B 2 &lt;(L 1 +2×L 2 ), L 2 &lt;B 3 &lt;(L 1 +2×L 2 ), and A&lt;(n×L 1 +(n+1)×L 2 ) are satisfied (where the number of the segments is divided by the number P of magnetic pole portions of the magnet, and the quotient is rounded up to an integer to obtain n). 
         [0024]    In this structure, by setting each value so as to satisfy the above conditions, a short-circuit state in which a maximum of two of the three types of brushes, which are the common brush, the low-speed drive brush, and the high-speed drive brush, short-circuit two segments that are adjacent to each other in the circumferential direction, and a non-short-circuit state, in which none of the brushes short-circuit two segments that are adjacent to each other in the circumferential direction, is easily realized. 
         [0025]    Preferably, rotation of the armature repeats a single type short-circuit state, in which one of the three types of the brushes, which are the common brush, the low-speed drive brush, and the high-speed drive brush, short circuit two of the segments that are adjacent to each other in the circumferential direction, and the non-short-circuit state. 
         [0026]    In this structure, two or more types of the brushes never simultaneously short-circuit two segments that are adjacent to each other in the circumferential direction. Accordingly, changes in the number of effective coils are further suppressed, and resistance fluctuation of the coil is further suppressed. As a result, fluctuation in the value of the supplied current is suppressed, torque fluctuation is suppressed, and motor vibration caused by torque fluctuation is suppressed. 
         [0027]    Preferably, when the number of magnetic pole portions of the magnet is represented by P, with respect to the circumferential direction of the rotation shaft, when the segments each have a width represented by L 1 , adjacent ones of the segments are spaced apart by an interval represented by L 2 , the width of the common brush is represented by B 1 , the width of the low-speed drive brush is represented by B 2 , the width of the high-speed drive brush is represented by B 3 , the width of a layout area in which the three brushes are arranged is represented by A, the common brush and the low-speed drive brush located at opposite sides of the high-speed drive brush are spaced apart by an interval represented by D 1 , the common brush and the high-speed drive brush are spaced apart by an interval represented by D 2 , and the high-speed drive brush and the low-speed drive brush are spaced apart by an interval represented by D 3 ; B 1 &gt;L 2 , B 2 &gt;L 2 , B 3 &gt;L 2 , A&lt;(n×L 1 +(n+1)×L 2 ), D 1 &gt;((n−1)×L 1 +(n−2)×L 2 ), D 2 &gt;(n 1 ×L 1 +(n 1 −1)×L 2 ), D 3 &gt;(n 2 ×L 1 +(n 2 −1)×L 2 ), n=n 1 +n 2 +1 are satisfied (where the number of the segments is divided by the number P of magnetic pole portions of the magnet, and the quotient is rounded up to an integer to obtain n, with n 1  and n 2  being positive integers). 
         [0028]    In this structure, the non-short-circuit state, in which none of the three types of brushes, which are the common brush, the low-speed drive brush, and the high-speed drive brush, short-circuit two segments that are adjacent to each other in the circumferential direction, and the single type short-circuit state, in which only one type of the brushes short-circuit two segments that are adjacent to each other in the circumferential direction, is easily repeated. 
         [0029]    Preferably, when the number of magnetic pole portions of the magnet is represented by P, with respect to the circumferential direction of the rotation shaft, when the segments each have a width represented by L 1 , adjacent ones of the segments are spaced apart by an interval represented by L 2 , the width of the common brush is represented by B 1 , the width of the low-speed drive brush is represented by B 2 , the width of the high-speed drive brush is represented by B 3 , the width of a layout area in which the three brushes are arranged is represented by A, the common brush and the low-speed drive brush located at opposite sides of the high-speed drive brush are spaced apart by an interval represented by D 1 , the common brush and the high-speed drive brush are spaced apart by an interval represented by D 2 , and the high-speed drive brush and the low-speed drive brush are spaced apart by an interval represented by D 3 ; B 1 &gt;L 2 , B 2 &gt;L 2 , B 3 &gt;L 2 , A&lt;(n×L 1 +(n+1)×L 2 ), D 1 &gt;((n− 1 )×L 1 +(n− 2 )×L 2 ), D 2 &gt;((n−2)×L 1 +(n−3)×L 2 ), and D 3 &gt;L 1  are satisfied (where n is a value that is the same as the number of segments arranged in a range of (360°/P)). 
         [0030]    In this structure, by setting each value so as to satisfy the above conditions, the common brush, the low-speed drive brush, and the high-speed drive brush alternately and singly short-circuit two segments that are adjacent to each other in the circumferential direction. 
         [0031]    Preferably, the common brush, the low-speed drive brush, and the high-speed drive brush are arranged in the order of the common brush, the high-speed drive brush, and the low-speed drive brush from a rear side to a front side in a rotation direction of the commutator. 
         [0032]    In this structure, the brushes are arranged in the order of the common brush, the high-speed drive brush, and the low-speed drive brush from the rear side to the front side with respect to the rotation direction of the commutator. When the wiper motor is driven, the induced voltage generated by the rotation of the armature shifts the magnetic center of the armature (the center in the circumferential direction of the pole formed when the coil is supplied with current) slightly toward the rear in the rotation direction of the commutator. Thus, when the brushes are arranged in the order of the common brush, the high-speed drive brush, and the low-speed drive brush from the rear side to the front side in the rotation direction of the commutator, the magnetic center formed in the armature when supplying current with the high-speed drive brush is located near the magnetic center formed in the armature when supplying power with the low-speed drive brush. As a result, the motor may be used more effectively without adversely affecting the performance of the wiper motor when the wiper motor is driven at a low speed and when driven at a high speed. 
         [0033]    Preferably, a slot, through which the coil extends, is formed between adjacent ones of the teeth in the circumferential direction of the rotation shaft, and a value obtained by dividing the total number of slots by two is an odd number. 
         [0034]    In this structure, the short-circuiting of two segments that are adjacent to each other in the circumferential direction with the low-speed drive brush and the high-speed drive brush becomes further easier. 
         [0035]    To achieve the second object, a further embodiment of the present invention provides a brush configuration method for setting a width and a location for a plurality of brushes in a motor. The motor includes at least one magnet, which forms four or more magnetic pole portions, and an armature. The armature includes a rotation shaft, an armature core, a coil, and a commutator. The armature core is fixed to the rotation shaft and has a plurality of teeth. The coil is wound around the teeth. The commutator is fixed to the rotation shaft and has a plurality of segments arranged in a circumferential direction of the rotation shaft. A common brush, a low-speed drive brush, and a high-speed drive brush are spaced apart and arranged along the circumferential direction of the rotation shaft in slidably contact with the commutator. Each brush has a width in the circumferential direction of the rotation shaft. The method includes setting the number of the teeth to be the same as the number of the segments, setting the number of the magnetic pole portions formed by the magnet and represented by P and the number of the teeth represented by S so that (2S/P) is an odd number, and setting the width of each brush and location of the brush in the circumferential direction of the rotation shaft so that the three types of the brushes, which are the common brush, the low-speed drive brush, and the high-speed drive brush, all do not simultaneously short-circuit two of the segments that are adjacent to each other in the circumferential direction. 
         [0036]    In this method, the common brush, the low-speed drive brush, and the high-speed drive brush all do not simultaneously short-circuit two of the segments that are adjacent to each other in the circumferential direction. Accordingly, changes in the number of effective coils are suppressed, and resistance fluctuation of the coil is suppressed. As a result, fluctuation in the value of the supplied current is suppressed, torque fluctuation is suppressed, and motor vibration caused by torque fluctuation is suppressed. Further, a decrease in the number of effective coils is suppressed. 
         [0037]    Preferably, the width of each brush and location of the brush in the circumferential direction of the rotation shaft are set so that the common brush, the low-speed drive brush, and the high-speed drive brush sequentially short-circuit two of the segments that are adjacent to each other in the circumferential direction. 
         [0038]    In this method, the common brush, the low-speed drive brush, and the high-speed drive brush sequentially short-circuit two segments that are adjacent to each other in the circumferential direction. Thus, sudden changes in the number of effective coils are suppressed. Accordingly, the generation of vibration in the motor is suppressed. 
         [0039]    Preferably, the width of each brush and location of the brush in the circumferential direction of the rotation shaft are set so that rotation of the armature repeats a short-circuit state, in which a maximum of two of the three types of the brushes, which are the common brush, the low-speed drive brush, and the high-speed drive brush, short circuit two of the segments that are adjacent to each other in the circumferential direction, and a non-short-circuit state in which none of the three types of the brushes short circuit two of the segments that are adjacent to each other in the circumferential direction. 
         [0040]    In this method, when in a short-circuit state, a maximum of only two types of the brushes simultaneously short circuit two segments that are adjacent to each other in the circumferential direction. Thus, by repeating the short-circuit state and a non-short-circuit state, changes in the number of effective coils may be minimized, and resistance fluctuation of the coil may be minimized. As a result, fluctuation in the value of the supplied current is minimized, and resistance fluctuation of the coil is minimized. Further, in the short-circuit state, the coils are all effective coils. Thus, the coils may be used effectively. 
         [0041]    Preferably, the width of each brush and location of the brush in the circumferential direction of the rotation shaft are set so that rotation of the armature repeats a double type short-circuit state, in which two of the three types of the brushes, which are the common brush, the low-speed drive brush, and the high-speed drive brush, short circuit two of the segments that are adjacent to each other in the circumferential direction, a single-type short circuit state, in which one type of the brushes short circuit two of the segments that are adjacent to each other in the circumferential direction, and the non-short-circuit state so that the single-type short circuit state is performed before and after the double type short-circuit state. 
         [0042]    In this method, the double type short-circuit state, the single type short-circuit state, and the non-short-circuit state are repeated with the single type short-circuit state being in between the double type short-circuit states. Thus, the non-short-circuit state to the double short-circuit state or the double short-circuit state to the non-short-circuit state are reached in a stepped manner with the single type short-circuit state being in between. Accordingly, even though the double type short-circuit state is present, the number of effective coils does not suddenly change, and sudden resistance fluctuation of the coil is further suppressed. As a result, sudden fluctuations in the value of the supplied current are minimized, torque fluctuation is further suppressed, and vibration of the wiper motor caused by torque fluctuation is further suppressed. 
         [0043]    Preferably, when the number of magnetic pole portions of the magnet is represented by P, with respect to the circumferential direction of the rotation shaft, when the segments each have a width represented by L 1 , adjacent ones of the segments are spaced apart by an interval represented by L 2 , the width of the common brush is represented by B 1 , the width of the low-speed drive brush is represented by B 2 , the width of the high-speed drive brush is represented by B 3 , and the width of a layout area in which the three brushes are arranged is represented by A, each value is set to satisfy L 2 &lt;B 1 &lt;(L 1 +2×L 2 ), L 2 &lt;B 2 &lt;(L 1 +2×L 2 ), L 2 &lt;B 3 &lt;(L 1 +2×L 2 ), and A&lt;(n×L 1 +(n+1)×L 2 ) (where the number of the segments is divided by the number P of magnetic pole portions of the magnet, and the quotient is rounded up to an integer to obtain n). 
         [0044]    In this method, by setting each value so as to satisfy the above conditions, a structure that repeats the non-short-circuit state, in which none of the three types of brushes, the common brush, the low-speed drive brush, and the high-speed drive brush, short-circuit two segments that are adjacent to each other in the circumferential direction, and the single type short-circuit state, in which only one type of the brushes short-circuit two segments that are adjacent to each other in the circumferential direction, is easily realized. 
         [0045]    Preferably, the width of each brush and location of the brush in the circumferential direction of the rotation shaft are set so that rotation of the armature repeats a single type short-circuit state, in which one of the three types of the brushes, which are the common brush, the low-speed drive brush, and the high-speed drive brush, short circuit two of the segments that are adjacent to each other in the circumferential direction, and the non-short-circuit state. 
         [0046]    In this method, two or more types of the brushes never simultaneously short-circuit two segments that are adjacent to each other in the circumferential direction. Accordingly, changes in the number of effective coils are further suppressed, and resistance fluctuation of the coil is further suppressed. As a result, fluctuation in the value of the supplied current is suppressed, torque fluctuation is suppressed, and motor vibration caused by torque fluctuation is suppressed. 
         [0047]    Preferably, when the number of magnetic pole portions of the magnet is represented by P, with respect to the circumferential direction of the rotation shaft, when the segments each have a width represented by L 1 , adjacent ones of the segments are spaced apart by an interval represented by L 2 , the width of the common brush is represented by B 1 , the width of the low-speed drive brush is represented by B 2 , the width of the high-speed drive brush is represented by B 3 , the width of a layout area in which the three brushes are arranged is represented by A, the common brush and the low-speed drive brush located at opposite sides of the high-speed drive brush are spaced apart by an interval represented by D 1 , the common brush and the high-speed drive brush are spaced apart by an interval represented by D 2 , and the high-speed drive brush and the low-speed drive brush are spaced apart by an interval represented by D 3 , each value is set to satisfy B 1 &gt;L 2 , B 2 &gt;L 2 , B 3 &gt;L 2 , A&lt;(n×L 1 +(n+1)×L 2 ), D 1 &gt;((n−1)×L 1 +(n−2)×L 2 ), D 2 &gt;(n 1 ×L 1 +(n 1 −1)×L 2 ), D 3 &gt;(n 2 ×L 1 +(n 2 −1)×L 2 ), n=n 1 +n 2 +1 (where the number of the segments is divided by the number P of magnetic pole portions of the magnet, and the quotient is rounded up to an integer to obtain n, with n 1  and n 2  being positive integers). 
         [0048]    In this method, the non-short-circuit state, in which none of the three types of brushes, which are the common brush, the low-speed drive brush, and the high-speed drive brush, short-circuit two segments that are adjacent to each other in the circumferential direction, and the single type short-circuit state, in which only one type of the brushes short-circuit two segments that are adjacent to each other in the circumferential direction, is easily repeated. 
         [0049]    Preferably, when the number of magnetic pole portions of the magnet is represented by P, with respect to the circumferential direction of the rotation shaft, when the segments each have a width represented by L 1 , adjacent ones of the segments are spaced apart by an interval represented by L 2 , the width of the common brush is represented by B 1 , the width of the low-speed drive brush is represented by B 2 , the width of the high-speed drive brush is represented by B 3 , the width of a layout area in which the three brushes are arranged is represented by A, the common brush and the low-speed drive brush located at opposite sides of the high-speed drive brush are spaced apart by an interval represented by D 1 , the common brush and the high-speed drive brush are spaced apart by an interval represented by D 2 , and the high-speed drive brush and the low-speed drive brush are spaced apart by an interval represented by D 3 , each value is set to satisfy B 1 &gt;L 2 , B 2 &gt;L 2 , B 3 &gt;L 2 , A&lt;(n×L 1 +(n+1)×L 2 ), D 1 &gt;((n−1)×L 1 +(n−2)×L 2 ), D 2 &gt;((n−2)×L 1 +(n−3)×L 2 ), and D 3 &gt;L 1  (where n is a value that is the same as the number of segments arranged in a range of (360°/P)). 
         [0050]    In this method, by setting each value so as to satisfy the above conditions, the common brush, the low-speed drive brush, and the high-speed drive brush alternately and singly short-circuit two segments that are adjacent to each other in the circumferential direction. 
         [0051]    Preferably, the common brush, the low-speed drive brush, and the high-speed drive brush are arranged in the order of the common brush, the high-speed drive brush, and the low-speed drive brush from a rear side to a front side in a rotation direction of the commutator. 
         [0052]    In this method, the brushes are arranged in the order of the common brush, the high-speed drive brush, and the low-speed drive brush from the rear side to the front side with respect to the rotation direction of the commutator. When the wiper motor is driven, the induced voltage generated by the rotation of the armature shifts the magnetic center of the armature (the center in the circumferential direction of the pole formed when the coil is supplied with current) slightly toward the rear in the rotation direction of the commutator. Thus, when the brushes are arranged in the order of the common brush, the high-speed drive brush, and the low-speed drive brush from the rear side to the front side in the rotation direction of the commutator, the magnetic center formed in the armature when supplying current with the high-speed drive brush is located near the magnetic center formed in the armature when supplying power with the low-speed drive brush. As a result, the motor may be used more effectively without adversely affecting the performance of the wiper motor when the wiper motor is driven at a low speed and when driven at a high speed. 
         [0053]    Preferably, a slot, through which the coil extends, is formed between adjacent ones of the teeth in the circumferential direction of the rotation shaft, and the number of the teeth is set such that a value obtained by dividing the total number of slots by two is an odd number. 
         [0054]    In this method, the short-circuiting of two segments that are adjacent to each other in the circumferential direction with the low-speed drive brush and the high-speed drive brush becomes further easier. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0055]      FIG. 1  is a schematic diagram showing a wiper motor according to first to third embodiments of the present invention; 
           [0056]      FIG. 2  is a cross-sectional view of the wiper motor taken along line  2 - 2  in  FIG. 1 ; 
           [0057]      FIG. 3  is a schematic diagram in which a motor unit of the first embodiment is laid out along a plane; 
           [0058]      FIGS. 4(   a ) to  4 ( e ) are diagrams showing the positional relationship of a commutator and three brushes in the first embodiment; 
           [0059]      FIGS. 5(   a ) to  5 ( d ) are diagrams showing the positional relationship of the commutator and the three brushes in the first embodiment; 
           [0060]      FIGS. 6(   a ) and  6 ( b ) are diagrams showing changes in the number of effective coils in the wiper motor according to the first embodiment; 
           [0061]      FIG. 7  is a schematic diagram in which a motor unit of the second embodiment is laid out along a plane; 
           [0062]      FIGS. 8(   a ) to  8 ( d ) are diagrams showing the positional relationship of a commutator and three brushes in the second embodiment; 
           [0063]      FIGS. 9(   a ) to  9 ( d ) are diagrams showing the positional relationship of the commutator and the three brushes in the second embodiment; 
           [0064]      FIG. 10  is a schematic diagram in which a motor unit of the third embodiment is laid out along a plane; 
           [0065]      FIGS. 11(   a ) to  11 ( d ) are diagrams showing the positional relationship of a commutator and three brushes in the third embodiment; 
           [0066]      FIGS. 12(   a ) to  12 ( d ) are diagrams showing the positional relationship of the commutator and the three brushes in the third embodiment; 
           [0067]      FIGS. 13(   a ) to  13 ( d ) are diagrams showing the positional relationship of the commutator and the three brushes in the third embodiment; 
           [0068]      FIGS. 14(   a ) to  14 ( d ) are diagrams showing the positional relationship of the commutator and the three brushes in the third embodiment; 
           [0069]      FIGS. 15(   a ) to  15 ( d ) are diagrams showing the width and location of the brushes in the circumferential direction in a further example; 
           [0070]      FIGS. 16(   a ) to  16 ( d ) are diagrams showing the width and location of the brushes in the circumferential direction in a further example; 
           [0071]      FIGS. 17(   a ) to  17 ( d ) are diagrams showing the width and location of the brushes in the circumferential direction in a further example; 
           [0072]      FIGS. 18(   a ) to  18 ( d ) are diagrams showing the width and location of the brushes in the circumferential direction in a further example; 
           [0073]      FIGS. 19(   a ) to  19 ( d ) are diagrams showing the width and location of the brushes in the circumferential direction in a further example; 
           [0074]      FIGS. 20(   a ) to  20 ( d ) are diagrams showing the width and location of the brushes in the circumferential direction in a further example; 
           [0075]      FIGS. 21(   a ) to  21 ( d ) are diagrams showing the width and location of the brushes in the circumferential direction in a further example; 
           [0076]      FIGS. 22(   a ) to  22 ( d ) are diagrams showing the width and location of the brushes in the circumferential direction in a further example; 
           [0077]      FIGS. 23(   a ) to  23 ( d ) are diagrams showing the width and location of the brushes in the circumferential direction in a further example; and 
           [0078]      FIGS. 24(   a ) and  24 ( b ) are diagrams showing changes in the number of effective coils in a motor of the prior art. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       [0079]    A wiper motor according to a first embodiment of the present invention will now be discussed with reference to the drawings. 
         [0080]      FIG. 1  is a schematic diagram showing a wiper motor of the present embodiment. The wiper motor is used as a drive source for a vehicle wiper (not shown), which wipes the windshield or the like of a vehicle, and includes a motor unit  1  and a reduction gear  2  coupled to the motor unit  1 . 
         [0081]    The motor unit  1  includes a tubular yoke housing  11  having a bottom  11   a.  At least one magnet  12  forming four magnetic pole portions (two N poles and two S poles) is fixed to an inner circumferential surface of the yoke housing  11 . Thus, the motor unit  1  includes two magnetic circuits. The N poles and the S poles of the magnet  12  are alternately arranged in the circumferential direction of the yoke housing  11 . 
         [0082]    An armature  21  is rotatably arranged at a radially inner side of the magnet  12 . The armature  21  includes a rotation shaft  22 , an armature core  24 , and a commutator  25 . The rotation shaft  22  has a basal end located near the bottom  11   a  of the yoke housing  11  and supported by a bearing  23 , which is arranged at a central part of the bottom  11   a.  Further, the rotation shaft  22  has a distal end extending from an opening of the yoke housing  11  towards the reduction gear  2 . 
         [0083]    The armature core  24  is arranged facing the magnet  12  in the radial direction and is fixed to the rotation shaft  22  so as to be integrally rotatable with the rotation shaft  22 . The armature core  24  includes fourteen teeth  24   a,  which extend radially outward about the rotation shaft  22 . A slot  24   b  is formed between the teeth  24   a  that are adjacent to each other in the circumferential direction of the armature core  24  (see  FIG. 3 ). 
         [0084]    The commutator  25  is fixed to the rotation shaft  22  so as to be integrally rotatable with the rotation shaft  22  at a portion closer to the reduction gear  2  than the armature core  24 . As shown in  FIG. 2 , the commutator  25  includes a cylindrical insulator  26 , which is formed from an insulative resin material and externally fitted to the rotation shaft  22 , and fourteen segments  27  fixed to the outer circumferential surface of the insulator  26 . Each segment  27  is formed by a rectangular plate elongated in the axial direction of the rotation shaft  22  (see  FIG. 1 ) and curved along the outer circumferential surface of the insulator  26 . The fourteen segments  27  are arranged so as to form a cylindrical shape in overall. Adjacent ones of the segments  27  are spaced apart from each other in the circumferential direction of the rotation shaft  22 . The segments  27  that are spaced apart from each other by an interval of 180° are paired so that every first segment  27  is paired with the eighth segment  27  in the present embodiment. Thus, there are seven pairs of the segments  27 . A short-circuit line  28 , such as a conductive wire, short-circuits the segments  27  of each pair (refer to  FIG. 3 ). In  FIG. 3 , the fourteen segments  27  are numbered in order from “1” to “14” along the circumferential direction. 
         [0085]    Fourteen coils  29  are wound in an overlapping manner about the teeth  24   a.  Each coil  29  extends through the slot  24   b  and across two teeth  24   a.  Each coil  29  has an initial winding end connected to one of the segments  27  and a terminal winding end connected to another one of the segments  27 , which is the adjacent one in the circumferential direction of the rotation shaft  22 . The fourteen coils  29  are thus connected in series to form a single loop as a whole. When short-circuiting the segments  27  of each pair arranged at 180° intervals, two closed loops are formed. Each loop includes seven coils  29  (refer to  FIG. 6(   a )). 
         [0086]    As shown in  FIG. 1 , the reduction gear  2  includes a gear housing  31 , which is fixed to the open end of the yoke housing  11 . The gear housing  31  has an opening facing toward the yoke housing  11 . A brush holder  32  is fastened by a screw  33  to the gear housing  31  in the opening and arranged at the outer side of the commutator  25 . The brush holder  32  is annular and formed from an insulative resin material. 
         [0087]    As shown in  FIG. 2 , three holding portions  32   a  are defined on the surface of the brush holder  32  facing toward the motor unit  1 . Each holding portion  32   a  has the form of a tetragonal tube that extends in the radial direction. A power supply brush  40  having the form of a tetragonal rod is inserted into each holding portion  32   a.  Each power supply brush  40  is urged towards the commutator  25  by a spring or the like (not shown), which is accommodated in the holding portion  32   a,  and has a distal portion slidably pressed against the outer circumferential surface of the commutator  25  (i.e., surface at radially outward side of each segment  27 ). The power supply brush  40  located at the right as viewed in  FIG. 2  functions as a common brush  41 . The power supply brush  40  located at the left as viewed in  FIG. 2  functions as a low-speed drive brush  42 , which supplies current to the armature  21  with the common brush  41  to rotate the armature  21  at a low speed. The power supply brush  40  located in the middle as viewed in  FIG. 2  functions as a high-speed drive brush  43 , which supplies current to the armature  21  with the common brush  41  to rotate the armature  21  at a high speed. The three brushes  41 ,  42 , and  43  are arranged in the order of the common brush  41 , the high-speed drive brush  43 , and the low-speed drive brush  42  from the rear side to the front side with respect to the rotation direction of the commutator  25 . That is, the high-speed drive brush  43  is arranged at the front side of the common brush  41  in the rotation direction commutator  25 , and the low-speed drive brush  42  is arranged at the front side of the high-speed drive brush  43  in the rotation direction of the commutator  25 . The common brush  41  functions as a cathode brush, and the low-speed drive brush  42  and high-speed drive brush  43  function as anode brushes. Each power supply brush  40  is connected to a pigtail  44 . The pigtails  44  supply current to the power supply brushes  40 . 
         [0088]    Further, as shown in  FIG. 1 , a cylindrical bearing seat  31   a  extending along the axial direction of the rotation shaft  22  is arranged in the opening of the gear housing  31 . An annular bearing  34 , which is received in the bearing seat  31   a,  axially supports the middle part of the rotation shaft  22 . The bearing  34  cooperates with the bearing  23  to axially support the rotation shaft  22 . The rotation shaft  22  extends through the bearing  34  into the gear housing  31 . A spirally threaded worm  51  is formed on the portion of the rotation shaft  22  located in the gear housing  31 . A disk-shaped worm wheel  52  engaged with the worm  51  is accommodated in the gear housing  31 . The worm  51  and the worm wheel  52  form a reduction gear mechanism  53  for reducing the rotation speed of the rotation shaft  22 . 
         [0089]    A cylindrical output shaft  54 , which extends in the axial direction of the worm wheel  52  and rotates integrally with the worm wheel  52 , is arranged at the central part of the worm wheel  52 . The output shaft  54  has a distal portion projecting out of the gear housing  31  and fixed to a basal portion of a crank arm  55 . The crank arm  55  has a distal portion coupled by a link mechanism (not shown) to the vehicle wiper (not shown). 
         [0090]    In the wiper motor, when current is supplied to the armature  21  via the common brush  41  and the low-speed drive brush  42 , the armature  21  rotates at a low speed. Further, when current is supplied to the armature  21  via the common brush  41  and the high-speed drive brush  43 , the armature  21  rotates at a high speed. When the armature  21  rotates, the rotation of the rotation shaft  22  is decelerated by the worm  51  and the worm wheel  52  and output from the output shaft  54 . This pivots the vehicle wiper, which is coupled to the crank arm  55  by the link mechanism, in a reciprocating manner. 
         [0091]    The widths and locations of the three power supply brushes  40  (i.e., common brush  41 , low-speed drive brush  42 , and high-speed drive brush  43 ) in the circumferential direction of the rotation shaft  22  will now be discussed. Referring to  FIG. 4(   a ), in the wiper motor of the present embodiment, the width and location of each of the brushes  41  to  43  in the circumferential direction are set so that the common brush  41 , the low-speed drive brush  42 , and the high-speed drive brush  43  alternately and singly short-circuit the two segments  27 , which are adjacent to each other in the circumferential direction, at different timings. 
         [0092]    First, the number P of magnetic pole portions formed by the magnet  12  is set to a value satisfying the expression of P≧4. The number of teeth  24   a  and the number of segments  27  are set to be the same in the wiper motor. If the number of teeth  24   a  (i.e., number of segments  27 ) is S, the value of S is set such that (2S/P) is an odd number. In the present embodiment, the expressions of P=4 and S=14 are satisfied. Thus, the expression (2S/P) results in a value of “7”, which is an odd number. 
         [0093]    With respect to the circumferential direction of the rotation shaft  22 , the width of the segment  27  is represented by L 1 , the interval between adjacent ones of segments  27  is represented by L 2 , the width of the common brush  41  is represented by B 1 , the width of the low-speed drive brush  42  is represented by B 2 , the width of the high-speed drive brush  43  is represented by B 3 , the width of the layout area of the three brushes  41  to  43  in which the high-speed drive brush  43  is located at the middle is represented by A, the interval between the common brush  41  and the low-speed drive brush  42  located at opposite sides of the high-speed drive brush  43  is represented by D 1 , the interval between the common brush  41  and the high-speed drive brush  43  is represented by D 2 , and the interval between the high-speed drive brush  43  and the low-speed drive brush  42  is represented by D 3 . In this case, these values are each set to satisfy the conditions shown below. 
         [0000]      B1&gt;L2, B2&gt;L2, B3&gt;L2, 
         [0000]        A &lt;( n×L 1+( n+ 1)× L 2),
 
         [0000]        D 1&gt;(( n− 1)× L 1+( n− 2)× L 2),  D 2&gt;(( n− 2)× L 1+( n− 3)× L 2),
 
         [0000]      D3&gt;L1 
         [0094]    Here, “n” is a value that is the same as the number of segments  27  arranged in an angular range of (360°/P). In the present embodiment, P is 4 and the number of segments  27  is “14”. Thus, n is a value satisfying (360°/14)×(n−1)≦360° /4≦(360°/14)×n, where n=4. 
         [0095]    In a setting that satisfies the above conditions, the high-speed drive brush  43  of the present embodiment has a width in the circumferential direction of the rotation shaft  22  that is smaller than that of the common brush  41  and the low-speed drive brush  42 . 
         [0096]    The relationship of the commutator  25  and the brushes  41  to  43  when the commutator  25  is rotated in the wiper motor of the present embodiment, which has the common brush  41 , the low-speed drive brush  42 , and the high-speed drive brush  43  arranged near the commutator  25  so as to satisfy the above conditions, will now be discussed with reference to  FIGS. 4 and 5 . In  FIGS. 4 and 5 , the rotation direction of the commutator  25  is indicated by an arrow. 
         [0097]    In  FIGS. 4(   a ) and  5 ( a ), the common brush  41  starts to short-circuit the number “5” and “6” segments  27 , which are adjacent to each other in the circumferential direction. In this state, the low-speed drive brush  42  has one circumferential end (front end in the rotation direction of the commutator  25 ) arranged between the number “1” and “2” segments  27 , which are adjacent to each other in the circumferential direction. Thus, the low-speed drive brush  42  contacts only the number “2” segment  27  and does not contact the number “1” segment  27 . Further, the high-speed drive brush  43  contacts only the number “4” segment  27  and has two circumferential ends that are both arranged inward from the two circumferential ends of the number “4” segment  27 . When the common brush  41  is short-circuiting two circumferentially adjacent segments  27 , the low-speed drive brush  42  and the high-speed drive brush  43  each contact only one segment  27  and do not short-circuit two circumferentially adjacent segments  27 . 
         [0098]    When shifting from the state shown in  FIGS. 4(   a ) and  5 ( a ) to the state shown in  FIGS. 4(   b ) and  5 ( b ), the high-speed drive brush  43  starts to short-circuit the number “3” and “4” segments  27 , which are adjacent to each other in the circumferential direction. In this state, the common brush  41  ends the short-circuiting of two adjacent segments  27  and has one circumferential end (front end in the rotation direction of the commutator  25 ) arranged between the number “5” and “6” segments  27 . The common brush  41  contacts only the number “6” segment  27  and does not contact the number “5” segment  27 . Further, the low-speed drive brush  42  has one circumferential end (rear end in the rotation direction of the commutator  25 ) arranged between the number “2” and “3” segments  27 , which are adjacent to each other in the circumferential direction. The low-speed drive brush  42  contacts only the number “2” segment  27  and does not contact the number “3” segment  27 . As shown in  FIG. 4C , when the high-speed drive brush  43  is short-circuiting two circumferentially adjacent segments  27 , the common brush  41  and the low-speed drive brush  42  each contact only one segment  27  and do not short-circuit two circumferentially adjacent segments  27 . 
         [0099]    Then, when shifting to the state shown in  FIGS. 4(   d ) and  5 ( c ), the low-speed drive brush  42  starts to short-circuit the number “2” and “3” segments  27 , which are adjacent to each other in the circumferential direction. In this state, the high-speed drive brush  43  ends the short-circuiting of two adjacent segments  27  and has one circumferential end (front end in the rotation direction of the commutator  25 ) arranged between the number “3” and “4” segments  27 . The high-speed drive brush  43  contacts only the number “4” segment  27  and does not contact the number “3” segment  27 . The common brush  41  has one circumferential end (front end in the rotation direction of the commutator  25 ) arranged between the number “5” and “6” segments  27 . The common brush  42  contacts only the number “6” segment  27  and does not contact the number “5” segment  27 . When the low-speed drive brush  42  is short-circuiting two circumferentially adjacent segments  27 , the high-speed drive brush  43  and the low-speed drive brush  42  each contact only one segment  27  and do not short-circuit two circumferentially adjacent segments  27 . 
         [0100]    When the commutator  25  is further rotated to the state shown in  FIGS. 4(   e ) and  5 ( d ), the low-speed drive brush  42  ends the short-circuiting of the two circumferentially adjacent segments  27 , and the common brush  41  starts to short-circuit two circumferentially adjacent segments  27  adjacent in the circumferential direction in the same manner as the state shown in  FIGS. 4(   a ) and  5 ( a ). Subsequently, as the commutator  25  rotates, states similar to the state shown in  FIGS. 4(   a ) to  4 ( e ) and  FIGS. 5(   a ) to  5 ( d ) are repeated. In other words, in the order of the common brush  41 , the high-speed drive brush  43 , and the low-speed drive brush  42 , the brushes  41  to  43  alternately and singly short-circuit circumferentially adjacent segments  27 . Thus, when one of the three brushes  41  to  43  is short-circuiting two circumferentially adjacent segments  27 , the remaining two contact only one segment  27 . 
         [0101]    As shown in  FIG. 6(   a ), when the common brush  41  of the three brushes  41  to  43  is short-circuiting two circumferentially adjacent segments  27 , among the fourteen coils  29 , two are short-circuited as indicated by the broken lines, and the remaining twelve are effective coils. In the same manner, as shown in  FIG. 6(   b ), when the high-speed drive brush  43  is short-circuiting two circumferentially adjacent segments  27 , among the fourteen coils  29 , two are short-circuited as indicated by the broken lines, and the remaining twelve are effective coils. Further, when the low-speed drive brush  42  is short-circuiting two circumferentially adjacent segments  27 , two are short-circuited, and the remaining twelve are effective coils. Thus, the number of effective coils does not change. This minimizes resistance fluctuation of the coils  29 . 
         [0102]    The first embodiment has the advantages described below. 
         [0103]    (1) The common brush  41 , the low-speed drive brush  42 , and the high-speed drive brush  43  do not short-circuit two circumferentially adjacent segments  27  at the same time. More specifically, the common brush  41 , the low-speed drive brush  42 , and the high-speed drive brush  43  alternately and singly short-circuit two circumferentially adjacent segments  27 . Further, two or more brushes do not short-circuit two circumferentially adjacent segments  27  at the same time. Thus, the number of effective coils does not change, and the resistance fluctuation of the coils  29  is minimized. As a result, fluctuation in the value of the supplied current is minimized, and torque fluctuation is minimized. This prevents the wiper motor from vibrating and thereby keeps the generated noise subtle. 
         [0104]    (2) The widths and locations of the common brush  41 , the low-speed drive brush  42 , and the high-speed drive brush  43  in the circumferential direction are set based on the conditions of B 1 &gt;L 2 , B 2 &gt;L 2 , B 3 &gt;L 2 , A&lt;(n×L 1 +(n+1)×L 2 ), D 1 &gt;((n−1)×L 1 +(n−2)×L 2 ), D 2 &gt;((n−2)×L 1 +(n−3)×L 2 ), D 3 &gt;L 1 . By setting each value so as to satisfy the above conditions, a structure in which the common brush  41 , the low-speed drive brush  42 , and the high-speed drive brush  43  alternately and singly short-circuit two circumferentially adjacent segments  27  is easily realized. 
         [0105]    (3) The common brush  41 , the low-speed drive brush  42 , and the high-speed drive brush  43  are arranged in the order of the common brush  41 , the high-speed drive brush  43 , and the low-speed drive brush  42  from the rear side to the front side with respect to the rotation direction of the commutator  25 . When the wiper motor is driven, the induced voltage generated by the rotation of the armature  21  shifts the magnetic center of the armature  21  (the center in the circumferential direction of the pole formed when the coil  29  is supplied with current) slightly toward the rear in the rotation direction of the commutator  25 . Thus, when the brushes  41  to  43  are arranged in the order of the common brush  41 , the high-speed drive brush  43 , and the low-speed drive brush  42  from the rear side to the front side in the rotation direction of the commutator  25 , the magnetic center formed in the armature  21  when supplying current with the high-speed drive brush  43  is located near the magnetic center formed in the armature  21  when supplying power with the low-speed drive brush  42 . As a result, the wiper motor may be used more effectively without adversely affecting the performance of the wiper motor when the wiper motor is driven at a low speed and when driven at a high speed as compared to when the brushes  41  to  43  are arranged in the order of the low-speed drive brush  42 , the high-speed drive brush  43 , and the common brush  41  from the rear side to the front side with respect to the rotation direction of the commutator  25 . 
         [0106]    (4) The armature core  24  includes fourteen slots  24   b,  and the number obtained by dividing the total number of slots (i.e., “14”) by two is an odd number (i.e., “7”). In other words, the number of slots per one magnetic circuit is an odd number. As a result, in the wiper motor of the present embodiment including the magnet  12  that forms four magnetic pole portions, a structure in which the common brush  41 , the low-speed drive brush  42 , and the high-speed drive brush  43  alternately and singly short-circuit two circumferentially adjacent segments  27  is easily realized. 
       Second Embodiment 
       [0107]    A wiper motor according to a second embodiment of the present invention will now be discussed with reference to the drawings. In the second embodiment, like or same reference numerals are given to those components that are the same as the corresponding components of the first embodiment. Such components will not be described. 
         [0108]      FIG. 7  is a schematic diagram in which a motor unit and power supply brushes of the second embodiment are laid out along a plane. As shown in  FIG. 7 , the wiper motor of the second embodiment has a motor unit  61  including a yoke housing  11 , which is similar to that of the motor unit  1  in the first embodiment, and at least one magnet forming four magnetic pole portions (two N poles and two S poles) fixed to an inner circumferential surface of the yoke housing  11  (refer to  FIG. 1 ). An armature  62  is rotatably arranged at a radially inner side of the magnet  12 . The armature  62  includes a rotation shaft  22 , an armature core  63 , which is fixed to the rotation shaft  22  in an integrally rotatable manner, a commutator  64 , which is fixed to the rotation shaft  22  in an integrally rotatable manner at a location closer to the reduction gear  2  than the armature core  63 , and a plurality of coils  65 , which are wound around the armature core  63 . 
         [0109]    The armature core  63  is arranged facing the magnet  12  in the radial direction and is fixed to the rotation shaft  22  so as to be integrally rotatable with the rotation shaft  22 . The armature core  63  includes eighteen teeth  63   a,  which extend radially outward about the rotation shaft  22 . A slot  63   b  is formed between the teeth  63   a  that are adjacent to each other in the circumferential direction of the armature core  63 . 
         [0110]    As shown in  FIG. 9(   a ), the commutator  64  includes an insulator  26  and eighteen segments  27  fixed to the outer circumferential surface of the insulator  26 . The eighteen segments  27  are arranged at equal angular intervals in the circumferential direction, and adjacent ones of the segments  27  are spaced apart from each other in the circumferential direction of the rotation shaft  22 . As shown in  FIG. 7 , the eighteen segments  27  are numbered in order from “1” to “18” along the circumferential direction. 
         [0111]    As shown in  FIG. 7 , eighteen coils  65  are wound in an overlapping manner about the teeth  63   a.  Each coil  65  is wound around the armature core  63  extending through the slots  63   b  across four teeth  63   a,  which are arranged successively in the circumferential direction. Each coil  65  has an initial winding end connected to one of the segments  27  and a terminal winding end connected to another one of the segments  27 , which is the adjacent one in the circumferential direction. 
         [0112]    In the same manner as the motor unit  1  of the first embodiment, the motor unit  61  is coupled to a reduction gear  2 . In the second embodiment, a brush holder  32  (refer to  FIG. 2 ) of the reduction gear  2  holds six power supply brushes  70 , each having the form of a tetragonal rod extending in the radial direction. Among the six power supply brushes  70 , two are common brushes  71 , two are low-speed drive brushes  72 , and the remaining two are high-speed drive brushes  73 . As shown in  FIG. 9(   a ), the six power supply brushes  70  are arranged in the order of the common brush  71 , the high-speed drive brush  73 , the low-speed drive brush  72  and so on from the rear side to the front side in the rotation direction of the commutator  64  (i.e., the rotation direction of the armature  62 ). In  FIG. 9(   a ), the arrows show the rotation direction of the commutator  64 . Further, the two common brushes  71  are arranged in symmetry at opposite sides of the center axis L of the commutator  64  (i.e., in a 180° interval in the circumferential direction). The two low-speed drive brushes  72  and the two high-speed drive brushes  73  are also arranged in symmetry at opposite sides of the center axis L 
         [0113]    Each power supply brush  70  is urged towards the commutator  64  by a spring or the like (not shown) and has a distal portion slidably pressed against the outer circumferential surface of the commutator  64  (i.e., surface at radially outward side of each segment  27 ). The common brushes  71  function as anode brushes, and the low-speed drive brushes  72  and high-speed drive brushes  73  function as cathode brushes. Each power supply brush  70  is connected to a pigtail (not shown) in the same manner as the power supply brushes  40  of the first embodiment. The pigtails supply current to the power supply brushes  70 . 
         [0114]    The widths and locations of the six power supply brushes  70  (i.e., the two common brushes  71 , the two low-speed drive brushes  72 , and the two high-speed drive brushes  73 ) in the circumferential direction of the rotation shaft  22  will now be described in detail. Referring to  FIG. 7 , in the wiper motor of the present embodiment, the width and location of each of the brushes  71  to  73  in the circumferential direction are set so as to repeat a single type short-circuit state (short-circuit state) and a non-short-circuit state. In the single type short-circuit state, just one of the three types of power supply brushes  70 , which are the common brushes  71 , the low-speed drive brushes  72 , and the high-speed drive brushes  73 , short-circuit two segments  27  that are adjacent to each other in the circumferential direction. In the non-short-circuit state, none of the brushes  70  short-circuit two segments  27  that are adjacent to each other in the circumferential direction. 
         [0115]    First, the number P of magnetic pole portions formed by the magnet  12  is set to a value satisfying the expression of P≧4. The number of teeth  63   a  and the number of segments  27  are set to be the same in the wiper motor. If the number of teeth  63   a  (i.e., number of segments  27 ) is S, the value of S is set such that (2S/P) is an odd number. In the present embodiment, the expressions of P=4 and S=18 are satisfied. Thus, the expression (2S/P) results in a value of “9”, which is an odd number. 
         [0116]    With respect to the circumferential direction of the rotation shaft  22 , the width of the segment  27  is represented by L 1 , the interval between adjacent ones of segments  27  is represented by L 2 , the width of the common brush  71  is represented by B 1 , the width of the low-speed drive brush  72  is represented by B 2 , the width of the high-speed drive brush  73  is represented by B 3 , the width of the layout area of each set of the three brushes  71  to  73  in which the high-speed drive brush  73  is located at the middle is represented by A, the interval between the common brush  71  and the low-speed drive brush  72  located at opposite sides of the high-speed drive brush  73  is represented by D 1 , the interval between the common brush  71  and the high-speed drive brush  73  is represented by D 2 , and the interval between the high-speed drive brush  73  and the low-speed drive brush  72  is represented by D 3 . In this case, these values are each set to satisfy the conditions shown below. 
         [0000]      B1&gt;L2, B2&gt;L2, B3&gt;L2, 
         [0000]        A &lt;( n×L 1+( n+ 1)× n 2),
 
         [0000]        D 1&gt;(( n− 1)× L 1+( n− 2)× L 2),
 
         [0000]        D 2&gt;( n 1 ×L 1+( n 1−1)× L 2),
 
         [0000]        D 3&gt;( n 2× L 1+( n 2−1)× L 2),
 
         [0000]        n=n 1+ n 2+1 
         [0117]    Here, “n” is a value that is the same as the number of segments  27  arranged in an angular range of (360°/P). In other words, “n” is a quotient obtained by dividing the number of segments  27  (i.e., equal to the number S of the teeth  63   a ) in the motor unit  61  by the number P of magnetic pole portions. When the quotient is not an integer, the rounded up number is used as “n”. Further, “n 1 ” and “n 2 ” are positive integers satisfying “n=n 1 +n 2 +1”. Like in the present embodiment, when the number P of magnetic pole portions formed in the magnet is “4” and the number of segments  27  is “14”, for example, the setting are N=5, N 1 =2, and N 2 =2 
         [0118]    In the present embodiment, the circumferential widths and locations of the common brushes  71 , the low-speed drive brushes  72 , and the high-speed drive brushes  73  are set so that the circumferential widths B 1 , B 2 , and B 3  are of the same value and narrower than the circumferential width of the segments  27 . Further, the common brush  71  and the low-speed drive brush  72  are arranged on opposite sides of the high-speed drive brush  73  at a 90° interval, which is the same as the angular interval between adjacent ones of the magnetic pole portions formed in the magnet  12 . 
         [0119]    The relationship of the commutator  64  and the brushes  71  to  73  when the commutator  64  is rotated in the wiper motor of the present embodiment, which has the common brushes  71 , the low-speed drive brushes  72 , and the high-speed drive brushes  73  arranged near the commutator  25  so as to satisfy the above conditions, will now be discussed with reference to  FIGS. 8(   a ) to  9 ( d ).  FIGS. 8(   a ) to  8 ( d ) are schematic diagrams showing portions taken from  FIG. 7  that require to be described. Further,  FIGS. 8(   a ) to  8 ( d ) show only one set of the brushes  71  to  73  but the relationship is the same for the remaining set of the three brushes  71  to  73  and the commutator  64 . In  FIGS. 8(   a ) to  9 ( d ), the rotation direction of the commutator  64  is indicated by an arrow. 
         [0120]    In  FIGS. 8(   a ) and  9 ( a ), the common brush  71  short-circuits the number “2” and “3” segments  27 , which are adjacent to each other in the circumferential direction. In  FIGS. 8 and 9 , the power supply brush  70  that is short-circuiting two segments  27  that are adjacent to each other in the circumferential direction are shown by hatching lines. In this state, the high-speed drive brush  73  has one circumferential end (rear end in the rotation direction of the commutator  64 ) arranged between the number “5” and “6” segments  27 , which are adjacent to each other in the circumferential direction. The high-speed drive brush  73  contacts only the number “6” segment  27  and does not contact the number “5” segment  27 . Further, the low-speed drive brush  72  contacts only the number “7” segment  27 . In this manner,  FIGS. 8(   a ) and  9 ( a ) show a single type short-circuit state, in which just the common brushes  71  short-circuit two segments  27  that are adjacent to each other in the circumferential direction. Thus, among the eighteen coils  65 , only two coils  65  undergo commutation (refer to the coil  65  shown by broken lines in  FIG. 8(   a )) and the remaining sixteen are supplied with current. In this state, there are sixteen effective coils. 
         [0121]    When shifting from the state shown in  FIGS. 8(   a ) and  9 ( a ) to the state shown in  FIGS. 8(   b ) and  9 ( b ), the high-speed drive brush  73  short-circuits the number “5” and “6” segments  27 , which are adjacent to each other in the circumferential direction. In this state, the common brush  71  ends the short-circuiting of two adjacent segments  27  and has one circumferential end (front end in the rotation direction of the commutator  64 ) arranged between the number “2” and “3” segments  27 . The common brush  71  contacts only the number “2” segment  27  and does not contact the number “3” segment  27 . Further, the low-speed drive brush  72  has one circumferential end (rear end in the rotation direction of the commutator  64 ) arranged between the number “6” and “7” segments  27 , which are adjacent to each other in the circumferential direction. The low-speed drive brush  72  contacts only the number “7” segment  27  and does not contact the number “6” segment  27 . In this manner,  FIGS. 8(   b ) and  9 ( b ) show a single type short-circuit state, in which just the high-speed drive brushes  73  short-circuit two segments  27  that are adjacent to each other in the circumferential direction. Thus, among the eighteen coils  65 , only two coils  65  undergo commutation (refer to the coil  65  shown by broken lines in  FIG. 8(   b )) and the remaining sixteen are supplied with current. In this state, there are sixteen effective coils. 
         [0122]    Then, when shifting to the state shown in  FIGS. 8(   c ) and  9 ( c ), the low-speed drive brush  72  short-circuits the number “6” and “7” segments  27 , which are adjacent to each other in the circumferential direction. In this state, the high-speed drive brush  73  ends the short-circuiting of two adjacent segments  27  and has one circumferential end (front end in the rotation direction of the commutator  64 ) arranged between the number “5” and “6” segments  27 . The high-speed drive brush  73  contacts only the number “5” segment  27  and does not contact the number “6” segment  27 . Further, the common brush  71  contacts only the number “2” segment  27 . In this manner,  FIGS. 8(   c ) and  9 ( c ) show a single type short-circuit state, in which just the low-speed drive brushes  72  short-circuit two segments  27  that are adjacent to each other in the circumferential direction. Thus, among the eighteen coils  65 , only two coils  65  undergo commutation (refer to the coil  65  shown by broken lines in  FIG. 8(   b )) and the remaining sixteen are supplied with current. In this state, there are sixteen effective coils. 
         [0123]    Next, when shifting to the state shown in  FIGS. 8(   d ) and  9 ( d ), the low-speed drive brush  72  ends the short-circuiting of two adjacent segments  27  and has one circumferential end (front end in the rotation direction of the commutator  64 ) arranged between the number “6” and “7” segments  27 . The low-speed drive brush  72  contacts only the number “6” segment  27  and does not contact the number “7” segment  27 . Further, the common brush  71  has one circumferential end (rear end in the rotation direction of the commutator  64 ) arranged between the number “1” and “2” segments  27 . The common brush  71  contacts only the number “2” segment  27  and does not contact the number “1” segment  27 . Further, the high-speed drive brush  73  contacts only the number “5” segment  27 . In this manner,  FIGS. 8(   d ) and  9 ( d ) show a non-short-circuit state, in which none of the common brushes  71 , low-speed drive brushes  72 , and high-speed drive brushes  73  short-circuit two segments  27  that are adjacent to each other in the circumferential direction. Thus, in this state, there are eighteen effective coils. 
         [0124]    When the commutator  64  further rotates and shifts to the state shown in  FIGS. 8(   a ) and  9 ( a ), rotation of the armature  62  repeats states similar to the states shown in  FIGS. 8(   a ) to  8 ( d ) and  FIGS. 9(   a ) to  9 ( d ). In other words, rotation of the armature  62  repeats the single type short-circuit state in which just the two common brushes  71  short-circuit two segments  27  that are adjacent to each other in the circumferential direction, the single type short-circuit state in which just the two high-speed drive brushes  73  short-circuit two segments  27  that are adjacent to each other in the circumferential direction, the single type short-circuit state in which just the two low-speed drive brushes  72  short-circuit two segments  27  that are adjacent to each other in the circumferential direction, and the non-short-circuit state in which none of the common brushes  71 , low-speed drive brushes  72 , and high-speed drive brushes  73  short-circuit two segments  27  that are adjacent to each other in the circumferential direction. Thus, among the three types of the common brushes  71 , low-speed drive brushes  72 , and high-speed drive brushes  73 , two types never simultaneously short-circuit two segments  27  that are adjacent to each other in the circumferential direction, and all of the brushes never simultaneously short-circuit two segments  27  that are adjacent to each other in the circumferential direction. Further, as shown in  FIGS. 8  and  9 , in the present embodiment, the power supply brushes  70  (brushes  71  to  73 ) that short-circuit two segments  27  adjacent to each other in the circumferential direction shift in the same direction as the rotation direction of the commutator  64  (toward the front in the rotation direction). 
         [0125]    In addition to advantages (3) and (4) of the first embodiment, the second embodiment has the advantages described below. 
         [0126]    (1) The common brushes  71 , low-speed drive brushes  72 , and high-speed drive brushes  73  never all simultaneously short-circuit two segments  27  that are adjacent to each other in the circumferential direction. Thus, the number of effective coils does not change, and the resistance fluctuation of the coils  29  is minimized. As a result, fluctuation in the value of the supplied current is minimized, and torque fluctuation is minimized. This prevents the wiper motor from vibrating and thereby keeps the generated noise subtle. Further, the number of effective coils is minimized. 
         [0127]    (2) The common brushes  71 , low-speed drive brushes  72 , and high-speed drive brushes  73  sequentially short-circuit two segments  27  adjacent to each other in the circumferential direction. Thus, the number of effective coils is prevented from suddenly changing. This prevents the wiper motor from vibrating. 
         [0128]    (3) The single type short-circuit state and non-short-circuit state are repeated. Thus, two or more types of the brushes never simultaneously short-circuit two segments  27  adjacent to each other in the circumferential direction. Accordingly, changes in the number of effective coils are further decreased, and the resistance fluctuation of the coil  65  is further decreased. As a result, changes in the value of the supplied current are further suppressed, and torque fluctuation is further suppressed. Further, in a non-short-circuit state, all of the coils  65  are effective coils. Thus, the coils  65  may be used effectively. 
         [0129]    (4) The widths and locations of the common brushes  71 , the low-speed drive brushes  72 , and the high-speed drive brushes  73  in the circumferential direction are set based on the conditions of B 1 &gt;L 2 , B 2 &gt;L 2 , B 3 &gt;L 2 , A&lt;(n×L 1 +(n+1)×L 2 ), D 1 &gt;((n−1)×L 1 +(n−2)×L 2 ), D 2 &gt;(n 1 ×L 1 +(n 1 −1)×L 2 ), D 3 &gt;(n 2 ×L 1 +(n 2 −1)×L 2 ), n=n 1 +n 2 +1. By setting each value so as to satisfy the above conditions, a structure in which the non-short-circuit state and the single type short-circuit state are repeated is easily realized. 
       Third Embodiment 
       [0130]    A wiper motor according to a third embodiment of the present invention will now be discussed with reference to the drawings. In the third embodiment, like or same reference numerals are given to those components that are the same as the corresponding components of the first embodiment. Such components will not be described. 
         [0131]      FIG. 10  is a schematic diagram in which a motor unit and power supply brushes of the third embodiment are laid out along a plane. As shown in  FIG. 10 , in the third embodiment, a brush holder  32  (refer to  FIG. 2 ) of the reduction gear  2  holds six power supply brushes  80 , each having the form of a tetragonal rod extending in the radial direction. Among the six power supply brushes  80 , two are common brushes  81 , two are low-speed drive brushes  82 , and the remaining two are high-speed drive brushes  83 . As shown in  FIG. 13(   a ), the six power supply brushes  80  are arranged in the order of the common brush  81 , the high-speed drive brush  83 , and the low-speed drive brush  82  from the rear side to the front side in the rotation direction of the commutator  64  (i.e., the rotation direction of the armature  62 ). In  FIG. 13(   a ), the arrows show the rotation direction of the commutator  64 . Further, the two common brushes  81  are arranged in symmetry at opposite sides of the center axis L of the commutator  64  (i.e., in a 180° interval in the circumferential direction). The two low-speed drive brushes  82  and the two high-speed drive brushes  83  are also arranged in symmetry at opposite sides of the center axis L 
         [0132]    Each power supply brush  80  is urged towards the commutator  64  by a spring or the like (not shown) and has a distal portion slidably pressed against the outer circumferential surface of the commutator  64  (i.e., surface at radially outward side of each segment  27 ). The common brushes  81  function as anode brushes, and the low-speed drive brushes  82  and high-speed drive brushes  83  function as cathode brushes. Each power supply brush  80  is connected to a pigtail (not shown) in the same manner as the power supply brushes  40  of the first embodiment. The pigtails supply current to the power supply brushes  80 . 
         [0133]    The widths and locations of the six power supply brushes  80  (i.e., the two common brushes  81 , the two low-speed drive brushes  82 , and the two high-speed drive brushes  83 ) in the circumferential direction of the rotation shaft  22  will now be described in detail. Referring to  FIG. 10 , in the wiper motor of the present embodiment, the width and location of each of the brushes  81  to  73  in the circumferential direction are set so as to repeat a double type short-circuit state (short-circuit state), a single type short-circuit state (short-circuit state), and a non-short-circuit state. In the double type short-circuit state, two of the three types of power supply brushes  80 , which are the common brushes  81 , the low-speed drive brushes  82 , and the high-speed drive brushes  83 , short-circuit two segments  27  that are adjacent to each other in the circumferential direction. In the single type short-circuit state, just one type of the power supply brushes  80  short-circuit two segments  27  that are adjacent to each other in the circumferential direction. In the non-short-circuit state, none of the brushes  80  short-circuit two segments  27  that are adjacent to each other in the circumferential direction. 
         [0134]    First, the number P of magnetic pole portions formed by the magnet  12  is set to a value satisfying the expression of P≧4. The number of teeth  63   a  and the number of segments  27  are set to be the same in the wiper motor. If the number of teeth  63   a  (i.e., number of segments  27 ) is S, the value of S is set such that (2S/P) is an odd number. In the present embodiment, the expressions of P=4 and S=18 are satisfied. Thus, the expression (2S/P) results in a value of “9”, which is an odd number. 
         [0135]    With respect to the circumferential direction of the rotation shaft  22 , the width of the segment  27  is represented by L 1 , the interval between adjacent ones of segments  27  is represented by L 2 , the width of the common brush  81  is represented by B 1 , the width of the low-speed drive brush  82  is represented by B 2 , the width of the high-speed drive brush  83  is represented by B 3 , the width of the layout area of each set of the three brushes  81  to  83  in which the high-speed drive brush  83  is located at the middle is represented by A. In this case, these values are each set to satisfy the conditions shown below. 
         [0000]        L 2&lt; B 1&lt;( L 1+2× L 2),
 
         [0000]        L 2&lt; B 2&lt;( L 1+2× L 2),
 
         [0000]        L 2&lt; B 3&lt;( L 1+2× L 2),
 
         [0000]        A &lt;( n×L 1+( n+ 1)× L 2).
 
         [0136]    Here, “n” is a value that is the same as the number of segments  27  arranged in an angular range of (360°/P). In other words, “n” is a quotient obtained by dividing the number of segments  27  (i.e., to the number S of the teeth  63   a ) in the motor unit  61  by the number P of magnetic pole portions. When the quotient is not an integer, the rounded up number is used as “n”. 
         [0137]    In the present embodiment, the circumferential widths and locations of the common brushes  81 , the low-speed drive brushes  82 , and the high-speed drive brushes  83  are set so that the circumferential widths B 1 , B 2 , and B 3  are of the same value and narrower than the circumferential width of the segments  27 . Further, the common brush  81  and the low-speed drive brush  82  are arranged on opposite sides of the high-speed drive brush  83  at a 90° interval, which is the same as the angular interval between adjacent ones of the magnetic pole portions formed in the magnet  12 . 
         [0138]    The relationship of the commutator  64  and the brushes  81  to  83  when the commutator  64  is rotated in the wiper motor of the present embodiment, which has the common brushes  81 , the low-speed drive brushes  82 , and the high-speed drive brushes  83  arranged near the commutator  25  so as to satisfy the above conditions, will now be discussed with reference to  FIGS. 11(   a ) to  14 ( d ).  FIGS. 11(   a ) to  12 ( d ) are schematic diagrams showing portions taken from  FIG. 10  that require to be described. Further,  FIGS. 11(   a ) to  12 ( d ) show only one set of the brushes  71  to  73  but the relationship is the same for the remaining set of the three brushes  81  to  83  and the commutator  64 . In  FIGS. 11(   a ) to  14 ( d ), the rotation direction of the commutator  64  is indicated by an arrow. 
         [0139]    In  FIGS. 11(   a ) and  13 ( a ), the common brush  81  short-circuits the number “3” and “4” segments  27 , which are adjacent to each other in the circumferential direction. In  FIGS. 11(   a ) to  14 ( d ), the power supply brush  80  that is short-circuiting two segments  27  that are adjacent to each other in the circumferential direction are shown by hatching lines. In this state, the high-speed drive brush  83  has one circumferential end (front end in the rotation direction of the commutator  64 ) arranged between the number “6” and “7” segments  27 , which are adjacent to each other in the circumferential direction. The high-speed drive brush  83  contacts only the number “6” segment  27  and does not contact the number “7” segment  27 . Further, the low-speed drive brush  82  contacts only the number “8” segment  27 . In this manner,  FIGS. 11(   a ) and  13 ( a ) show a single type short-circuit state, in which just the common brushes  81  short-circuit two segments  27  that are adjacent to each other in the circumferential direction. Thus, among the eighteen coils  65 , only two coils  65  undergo commutation (refer to the coil  65  shown by broken lines in  FIG. 11(   a )) and the remaining sixteen coils  65  are supplied with current. In this state, there are sixteen effective coils. 
         [0140]    When shifting from the state shown in  FIGS. 11(   a ) and  13 ( a ) to the state shown in  FIGS. 11(   b ) and  13 ( b ), the common brush  81  short-circuits the number “3” and “4” segments  27 , which are adjacent to each other in the circumferential direction, in a manner similar to the state shown in  FIGS. 11(   a ) and  13 ( a ). Further, the high-speed drive brush  83  contacts only the number “6” segment  27 , and the low-speed drive brush  82  contacts only the number “8” segment  27 . Thus, the single type short-circuit state is maintained, in which just the common brushes  81  short-circuit two segments  27  that are adjacent to each other in the circumferential direction. 
         [0141]    As the commutator  64  further rotates and shifts to the states shown in  FIGS. 11(   c ) and  13 ( c ), the common brush  81  ends the short-circuiting of two adjacent segments  27  and has one circumferential end (front end in the rotation direction of the commutator  25 ) arranged between the number “3” and “4” segments  27 . The common brush  81  contacts only the number “3” segment  27  and does not contact the number “4” segment  27 . Further, the high-speed drive brush  83  contacts only the number “6” segment  27 . The low-speed drive brush  82  has one circumferential end (rear end in the rotation direction of the commutator  64 ) arranged between the number “7” and “8” segments  27 , which are adjacent to each other in the circumferential direction. The low-speed drive brush  82  contacts only the number “8” segment  27  and does not contact the number “7” segment  27 . In this manner,  FIGS. 11(   c ) and  13 ( c ) show a non-short-circuit state, in which none of the common brushes  81 , the low-speed drive brushes  82 , and the high-speed drive brushes  83  short-circuit two segments  27  that are adjacent to each other in the circumferential direction. Thus, all eighteen coils  65  are supplied with current. In this state, there are eighteen effective coils. 
         [0142]    Then, when shifting to the state shown in  FIGS. 11(   d ) and  13 ( d ), the low-speed drive brush  82  short-circuits the number “7” and “8” segments  27 , which are adjacent to each other in the circumferential direction. In this state, the common brush  81  has one circumferential end (front end in the rotation direction of the commutator  64 ) arranged between the number “3” and “4” segments  27 . The common brush  81  contacts only the number “3” segment  27  and does not contact the number “4” segment  27 . Further, the high-speed drive brush  83  has one circumferential end (rear end in the rotation direction of the commutator  64 ) arranged between the number “5” and “6” segments  27 . The high-speed drive brush  83  contacts only the number “6” segment  27  and does not contact the number “5” segment  27 . In this manner,  FIGS. 11(   d ) and  13 ( d ) show a single type short-circuit state, in which just the low-speed drive brushes  82  short-circuit two segments  27  that are adjacent to each other in the circumferential direction. Thus, among the eighteen coils  65 , only two coils  65  undergo commutation (refer to the coil  65  shown by broken lines in  FIG. 11(   d )) and the remaining sixteen coils  65  are supplied with current. In this state, there are sixteen effective coils. 
         [0143]    When shifting from the state shown in  FIGS. 11(   d ) and  13 ( d ) to the state shown in  FIGS. 12(   a ) and  14 ( a ), the low-speed drive brush  82  short-circuits the number “7” and “8” segments  27 , which are adjacent to each other in the circumferential direction, in a manner similar to the state shown in  FIGS. 11(   d ) and  13 ( d ). Further, the common brush  81  contacts only the number “3” segment  27 , and the high-speed drive brush  83  contacts only the number “6” segment  27 . Thus, the single type short-circuit state is maintained, in which just the low-speed drive brushes  82  short-circuit two segments  27  that are adjacent to each other in the circumferential direction. 
         [0144]    Next, when shifting to the state shown in  FIGS. 12(   b ) and  14 ( b ), in addition to the low-speed drive brush  82 , the high-speed drive brush  83  short-circuits the number “5” and “6” segments  27 , which are adjacent to each other in the circumferential direction. In this state, the common brush  81  has one circumferential end (rear end in the rotation direction of the commutator  64 ) arranged between the number “2” and “3” segments  27 . The common brush  81  contacts only the number “3” segment  27  and does not contact the number “2” segment  27 . In this manner,  FIGS. 12(   b ) and  14 ( b ) show a double type short-circuit state, in which two types of brushes, namely, the low-speed drive brushes  82  and the high-speed drive brushes  83 , short-circuit two segments  27  that are adjacent to each other in the circumferential direction. Thus, among the eighteen coils  65 , only four coils  65  undergo commutation (refer to the coils  65  shown by broken lines in  FIG. 12(   b )) and the remaining fourteen coils  65  are supplied with current. In this state, there are fourteen effective coils. 
         [0145]    Then, when shifting to the state shown in  FIGS. 12(   c ) and  14 ( c ), the low-speed drive brush  82  ends the short-circuiting of two adjacent segments  27  and has one circumferential end (front end in the rotation direction of the commutator  64 ) arranged between the number “7” and “8” segments  27 . The low-speed drive brush  82  contacts only the number “7” segment  27  and does not contact the number “8” segment  27 . Further, the high-speed drive brush  83  remains short-circuited to the number “5” and “6” segments  27  that are adjacent to each other in the circumferential direction, and the common brush  81  remains in contact with only the number “3” segment  27 . In this manner,  FIGS. 12(   c ) and  14 ( c ) show a single type short-circuit state, in which just the high-speed drive brushes  83  short-circuit two segments  27  that are adjacent to each other in the circumferential direction. Thus, among the eighteen coils  65 , only two coils  65  undergo commutation (refer to the coil  65  shown by broken lines in  FIG. 11(   d )) and the remaining sixteen coils  65  are supplied with current. In this state, there are sixteen effective coils. 
         [0146]    When shifting to the state shown in  FIGS. 12(   d ) and  14 ( d ), in addition to the high-speed drive brush  83 , the common brush  81  short-circuits the number “2” and “3” segments  27 , which are adjacent to each other in the circumferential direction. In this state, the low-speed drive brush  82  has one circumferential end (rear end in the rotation direction of the commutator  64 ) arranged between the number “7” and “8” segments  27 . The low-speed drive brush  82  contacts only the number “7” segment  27  and does not contact the number “8” segment  27 . In this manner,  FIGS. 12(   d ) and  14 ( d ) show a double type short-circuit state, in which two types of brushes, namely, the common brushes  81  and the high-speed drive brushes  83 , short-circuit two segments  27  that are adjacent to each other in the circumferential direction. Thus, among the eighteen coils  65 , only four coils  65  undergo commutation (refer to the coils  65  shown by broken lines in  FIG. 12(   d )) and the remaining fourteen coils  65  are supplied with current. In this state, there are fourteen effective coils. 
         [0147]    When the commutator  64  further rotates and shifts to the state shown in  FIGS. 11(   a ) and  13 ( a ), rotation of the armature  62  repeats states similar to the states shown in  FIGS. 11(   a ) to  11 ( d ),  FIGS. 12(   a ) to  12 ( d ),  FIGS. 13(   a ) to  13 ( d ), and  FIGS. 14(   a ) to  14 ( d ). In other words, rotation of the armature  62  repeats the single type short-circuit state in which just the two common brushes  81  short-circuit two segments  27  that are adjacent to each other in the circumferential direction, the non-short-circuit state in which none of the three types of brushes  81  to  83  short-circuit two segments  27  that are adjacent to each other in the circumferential direction, the single type short-circuit state in which just the two low-speed drive brushes  82  short-circuit two segments  27  that are adjacent to each other in the circumferential direction, the double type short-circuit state in which two types of brushes, namely, the two low-speed drive brushes  82  and the two high-speed drive brushes  83 , short-circuit two segments  27  that are adjacent to each other in the circumferential direction, the single type short-circuit state in which just the two high-speed drive brushes  83  short-circuit two segments  27  that are adjacent to each other in the circumferential direction, and the double type short-circuit state in which two types of brushes, namely, the two common brushes  81  and the two high-speed drive brushes  83 , short-circuit two segments  27  that are adjacent to each other in the circumferential direction. Thus, all of the common brushes  81 , low-speed drive brushes  82 , and high-speed drive brushes  83  never simultaneously short-circuit two segments  27  that are adjacent to each other in the circumferential direction. Further, as shown in  FIGS. 11 and 14 , in the present embodiment, the power supply brushes  80  (brushes  81  to  83 ) short-circuit segments  27  adjacent to each other in the circumferential direction shift in the direction opposite to the rotation direction of the commutator  64  (toward the rear in the rotation direction). 
         [0148]    In addition to advantages (3) and (4) of the first embodiment, the third embodiment has the advantages described below. 
         [0149]    (1) A maximum of only two types of brushes simultaneously short-circuit two segments that are adjacent to each other in the circumferential direction. Thus, by repeating the double type short-circuit state, the single type short-circuit state, and the non-short-circuit state, change in the number of effective coils is decreased, and resistance fluctuation of the coils  65  is decreased. As a result, changes in the value of the supplied current are minimized, and torque fluctuation is minimized. Further, in a non-short-circuit state, all of the coils  65  are effective coils. Thus, the coils  65  may be used effectively. 
         [0150]    (2) The double type short-circuit state, the single type short-circuit state, and the non-short-circuit state are repeated with the single type short-circuit state being in between the double type short-circuit states. Thus, the non-short-circuit state to the double short-circuit state and the double short-circuit state to the non-short-circuit state are reached in a stepped manner with the single type short-circuit state being in between. Accordingly, even though the double type short-circuit state is present, the number of effective coils do not suddenly change, and sudden resistance fluctuation of the coils  65  is further suppressed. As a result, sudden fluctuations in the value of the supplied current are minimized, torque fluctuation is further suppressed, and vibration of the wiper motor caused by torque fluctuation is further suppressed. 
         [0151]    (3) The widths and locations of the common brushes  81 , the low-speed drive brushes  82 , and the high-speed drive brushes  83  in the circumferential direction are set based on the conditions of L 2 &lt;B 1 &lt;(L 1 +2×L 2 ), L 2 &lt;B 2 &lt;(L 1 +2×L 2 ), L 2 &lt;B 3 &lt;(L 1 +2×L 2 ), and A&lt;(n×L 1 +(n+1)×L 2 ). By setting each value so as to satisfy the above conditions, a structure in that repeats short-circuit states in which a maximum of two types of brushes simultaneously short-circuit two segments that are adjacent to each other in the circumferential direction (i.e., single type short-circuit state and double type short-circuit state) and a non-short-circuit state are easily repeated. 
         [0152]    (4) In the wiper motor of the present embodiment, as the armature  62  rotates, the power supply brushes  80  (brushes  81  to  83 ) short-circuit segments  27  adjacent to each other in the circumferential direction shift in the direction opposite to the rotation direction of the commutator  64  (toward the rear in the rotation direction). This generates magnetic flux in a direction opposite to the rotation direction of the armature  62 . Thus, in contrast to a wiper motor in which the power supply brushes that short-circuit segments  27  adjacent in the circumferential direction shift in the same direction as the rotation direction of the commutator  64  (toward the front in the rotation direction), the wiper motor of the present embodiment functions in a direction that suppresses vibration caused by magnetic flux fluctuation. 
         [0153]    The present invention may be embodied in the following forms. 
         [0154]    In each of the above-described embodiments, the common brushes  41 ,  71 , and  81 , the high-speed drive brushes  43 ,  73 , and  83 , and the low-speed drive brushes  42 ,  72 , and  82  are arranged in order from the rear to the front in the rotation direction of the commutator  64 . However, the low-speed drive brushes  42 ,  72 , and  82 , the high-speed drive brushes  43 ,  73 , and  83 , and the common brushes  41 ,  71 , and  81  may be arranged in order from the rear to the front in the rotation direction of the commutator  64 . 
         [0155]    In the first embodiment, the high-speed drive brush  43  is formed to have a smaller circumferential width than the common brush  41  and the low-speed drive brush  42 . However, the circumferential widths of the common brush  41 , the low-speed drive brush  42 , and the high-speed drive brush  43  may be changed as required as long as the conditions of B 1 &gt;L 2 , B 2 &gt;L 2 , B 3 &gt;L 2 , A&lt;(n×L 1 +(n+1)×L 2 ), D 1 &gt;((n−1)×L 1 +(n−2)×L 2 ), D 2 &gt;((n−2)×L 1 +(n−3)×L 2 ), D 3 &gt;L 1  are satisfied. For instance, as shown in  FIG. 15(   a ), the circumferential widths of the common brush  41 , the low-speed drive brush  42 , and the high-speed drive brush  43  may be set to be the same (i.e., B 1 =B 2 =B 3 ). In this case, as shown in  FIGS. 15(   a ) to  15 ( d ), the brushes  41  to  43  would also alternately short-circuit two circumferentially adjacent segments  27  in the order of the low-speed drive brush  42 , the high-speed drive brush  43 , and the common brush  41  as the commutator  25  rotates. Therefore, the same advantages as the above-described embodiment are obtained. In addition, a common brush may be used as the three brushes  41  to  43  since they have the same shape. This reduces the manufacturing cost. 
         [0156]    In the third embodiment, the widths and locations of the common brushes  81 , the low-speed drive brushes  82 , and the high-speed drive brushes  83  in the circumferential direction may be changed as long as the conditions of L 2 &lt;B 1 &lt;(L 1 +2×L 2 ), L 2 &lt;B 2 &lt;(L 1 +2×L 2 ), L 2 &lt;B 3 &lt;(L 1 +2×L 2 ), and A&lt;(n×L 1 +(n+1)×L 2 ) are satisfied. 
         [0157]    For example, the widths and locations of the brushes  81  to  83  in the circumferential direction may be changed as shown in  FIGS. 16(   a ) to  19 ( d ). In such a case, rotation of the commutator  64  would sequentially repeat a single type short-circuit state in which just the common brushes  81  short-circuit adjacent segments  27  (refer to  FIGS. 16(   a ) to  18 ( a )), a double type short-circuit state in which two types of brushes, namely, the two low-speed drive brushes  82  and the two high-speed drive brushes  83 , short-circuit adjacent segments  27  (refer to  FIGS. 16(   b ) and  18 ( b )), a single type short-circuit state in which just the high-speed drive brushes  83  short-circuit adjacent segments  27  (refer to  FIGS. 16(   c ) and  18 ( c )), a double type short-circuit state in which two types of brushes, namely, the low-speed drive brushes  82  and the high-speed drive brushes  83 , short-circuit adjacent segments  27  (refer to  FIGS. 16(   d ) and  18 ( d )), a single type short-circuit state in which just the two low-speed drive brushes  82  short-circuit adjacent segments  27  (refer to  FIGS. 17(   a ),  19 ( a ),  17 ( b ), and  19 ( b )), a non-short-circuit state in which none of the three types of brushes  81  to  83  short-circuit adjacent segments  27  (refer to  FIGS. 17(   c ) and  19 ( c )), and a single type short-circuit state in which just the common brushes  81  short-circuit adjacent segments  27  (refer to  FIGS. 17(   d ) to  19 ( d )). This obtains advantages similar to advantages (1) to (3) of the third embodiment. 
         [0158]    The widths and locations of the brushes  81  to  83  in the circumferential direction may be changed as shown in  FIGS. 20(   a ) to  23 ( d ). As shown in  FIG. 20(   a ), in this example, the width of the high-speed drive brush  83  in the circumferential direction is set to be greater than the widths of the common brush  81  and the low-speed drive brush  82  in the circumferential direction. Further, rotation of the commutator  64  would sequentially repeat a double type short-circuit state in which two types of brushes, namely, the common brushes  81  and the two high-speed drive brushes  83 , short-circuit adjacent segments  27  (refer to  FIGS. 20(   a ),  22 ( a ),  20 ( b ), and  22 ( b )), a non-short-circuit state in which none of the three types of brushes short-circuit adjacent segments  27  (refer to  FIGS. 20(   c ) and  22 ( c )), a double type short-circuit state in which two types of brushes, namely, the low-speed drive brushes  82  and the high-speed drive brushes  83 , short-circuit adjacent segments  27  (refer to  FIGS. 20(   d ),  22 ( d ),  21 ( a ),  23 ( a ),  21 ( b ), and  23 ( b )), a single type short-circuit state in which just the high-speed drive brushes  83  short-circuit adjacent segments  27  (refer to  FIGS. 21(   c ) and  23 ( c )), and a double type short-circuit state in which two types of brushes, namely, the common brushes  81  and the high-speed drive brushes  83 , short-circuit adjacent segments  27  (refer to  FIGS. 21(   d ) and  23 ( d )). This obtains advantages similar to advantages (1) and (3) of the third embodiment. 
         [0159]    In the second and third embodiments, every two segments  27  arranged in a 180° interval may be short-circuited by a short-circuit line. In this case, in the second embodiment, one set of the brushes  71  to  73  may be eliminated. In the third embodiment, one set of the brushes  81  to  83  may be eliminated. 
         [0160]    In the first embodiment, the common brush  41  functions as a cathode brush, and the low-speed drive brush  42  and high-speed drive brush  43  function as anode brushes. Instead, the common brush  41  may function as an anode brush, and the low-speed drive brush  42  and high-speed drive brush  43  may function as cathode brushes. In this case, by reversing the structure of the magnet  12  (i.e., switching the N pole and S pole in  FIG. 3 ), the input of current may be reversed. The same applies for the second and third embodiments. 
         [0161]    The wiper motor (motor unit  1  and  61 ) is not limited to the structure of the above-described embodiments. The widths and locations of the brushes in the circumferential direction may be set so that the number P of magnetic pole portions formed by the magnet  12  is “4” or greater, the number S of the teeth  24   a  and  63   a  is the same as the number of segments  27 , “2S/P” is an odd number, and the common brushes  41 ,  71 , and  81 , the low-speed drive brushes  42 ,  72 , and  82 , and the high-speed drive brushes  43 ,  73 , and  83  do not all simultaneously short-circuit two adjacent segments  27 . For example, the wiper motor may have a structure that repeats the double type short-circuit state and the non-short-circuit state. In such a case, the number of effective coils may be suppressed to suppress the generation of vibration in the wiper motor. 
         [0162]    In each of the above-described embodiments, the present invention is applied to a wiper motor, which is used as a drive source for a vehicle wiper. However, the present invention may be applied to a motor that is not a wiper motor. The present invention may be embodied in a motor that does not include the reduction gear mechanism  53 . 
         [0163]    The magnet  12  may be formed from a magnet  12  that forms four or more magnetic pole portions or includes a plurality of magnets  12 , with each forming a magnetic pole. 
         [0164]    When the number of magnetic pole portions is P, three types of brushes, in which a common brush, low-speed drive brush, and high-speed drive brush form a single set, are located at positions corresponding to magnetic pole portions and arranged in angular intervals (720/P)°. 
         [0165]    Further, the short-circuit line that reduces the number of brushes may connect P/2 segments at a (720/P)° interval. In such a case, the number of brushes is adjustable within a range of three brushes or more (3P/2). For example, when the number of magnetic pole portions is six, short-circuit lines connect three segments at a 120° interval, and the brushes are adjustable within a range of three to nine. When the number of magnetic pole portions is eight, short-circuit lines connect four segments at a 90° interval, and the brushes are adjustable within a range of three to twelve.