Patent Publication Number: US-2005121989-A1

Title: Brushless motor having claw pole type stator

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application is based on and incorporates herein by reference Japanese Patent Application No. 2003-407633 filed on Dec. 5, 2003 and Japanese Patent Application No. 2004-327690 filed on Nov. 11, 2004.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a brushless motor, and particularly to a brushless motor, which has a claw pole type stator.  
      2. Description of Related Art  
      One type of brushless stepping motor includes a claw pole type stator and a rotor. The claw pole type stator has a plurality of pole teeth, which are made by, for example, processing a magnetic sheet metal material. The rotor includes a plurality of permanent magnets, which are opposed to the stator. In the stator, coil bobbins, around which coils are wound, are axially arranged one after another. In this stepping motor, the pole teeth are made through the sheet metal processing, so that the manufacturing costs can be made low. Also, the coils can be easily wound around the coil bobbins by open winding.  
      Japanese Examined Utility Model Publication No. 2559692 discloses one such a stepping motor, which is an outer rotor type. In this stepping motor, the coil bobbins, around which the coils are wound, are axially arranged one after another. Furthermore, an outer yoke of the stator covers outer peripheral surfaces of the coil bobbins. The outer yoke is formed by rolling a magnetic plate material into a cylindrical shape, and a plurality of slits is made in a peripheral wall of the outer yoke to form the pole teeth in the outer yoke. Ring shaped permanent magnets are coaxially arranged at radially outward of the outer yoke. An inner magnetic pole surface of each permanent magnet is opposed to the pole teeth of the outer yoke in such a manner that a small gap is provided between the inner magnetic pole surface of the permanent magnet and the pole teeth of the outer yoke.  
      In general, in the claw pole type stepping motor, the respective bobbins are axially clamped by two metal components, each of which is made through the sheet metal processing and each of which has pole teeth. At this time, the two metal components are opposed to each other and clamp the coil bobbins therebetween in such a manner that the pole teeth of one of the two metal components and the pole teeth of the other one of the two metal components are alternately arranged in the circumferential direction. In contrast, in the stepping motor of Japanese Examined Utility Model Publication No. 2559692, the outer peripheral surfaces of the two coil bobbins are covered by the cylindrical magnetic material. Thus, the structure is relatively simple.  
      However, when the above stepping motor is used as, for example, a drive source, such as an electric fan motor, which continuously rotates, the stepping motor would be desynchronized. The desynchronization occurs more often at a high rotational speed, which is equal to or greater than 1000 rpm. To address the above disadvantage, Japanese Unexamined Patent Publication No. 2001-78392 discloses another type of stepping motor, which has two position sensors to control the rotation of the motor through a closed loop control operation. In the stepping motor of Japanese Unexamined Patent Publication No. 2001-78392, coils are wound around coil bobbins, which are axially arranged one after another, and the coil bobbins are held by yokes or yoke parts made of a magnetic material. Hall elements, which serve as the sensors, are provided at two predetermined circumferential positions, which are axially opposed to end surfaces of permanent magnets of an inner rotor. With this structure, phase detection can be relatively accurately performed to limit desynchronization.  
      However, the stepping motor recited in Japanese Unexamined Patent Publication No. 2001-78392 is intended to precisely rotate a predetermined angle at a low speed, which is equal to or smaller than 500 rpm. Also, the coil bobbins are displaced one half pitch from each other and are held by the two yokes. The two Hall elements are provided to sense the displacement of the one half pitch. Therefore, the structure is relatively complicated, and the manufacturing costs are relatively high. For example, when the stepping motor of Japanese Unexamined Patent Publication No. 2001-78392 is used in the electric fan motor, which does not require the high positional accuracy, manufacturing costs of an electric fan system, which has the electric fan motor, are disadvantageously increased.  
     SUMMARY OF THE INVENTION  
      The present invention addresses the above disadvantages. Thus, it is an objective of the present invention to provide a brushless motor of continuously rotating type, which has a claw pole type stator and is structurally simple to achieve low manufacturing costs.  
      To achieve the objective of the present invention, there is provided a brushless motor, which includes a stator, a rotor and a single magnetic position sensor. The stator includes first and second yokes and first and second coils. Each of the first and second yokes is a claw pole type and has a plurality of pole teeth, which extend in an axial direction. The first and second yokes are axially opposed to each other in such a manner that the pole teeth of the first yoke and the pole teeth of the second yoke are alternately arranged in a circumferential direction. The first and second coils are circumferentially wound to form first and second phases, respectively, and are arranged between the first yoke and the second yoke. The rotor includes at least one rotor magnet, which provides a plurality of magnetic poles. The at least one rotor magnet is radially opposed to the plurality of pole teeth of the first yoke and the plurality of pole teeth of the second yoke. The single magnetic position sensor senses a rotational position of the rotor and outputs a position measurement signal, which indicates the sensed rotational position of the rotor. A half-wave electric current is alternately supplied to the first and second coils based on the position measurement signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention, together with additional objectives, features and advantages thereof, will be best understood from the following description, the appended claims and the accompanying drawings in which:  
       FIG. 1  is a schematic cross sectional view of a brushless motor according to an embodiment of the present invention;  
       FIG. 2  is a front view of a rotor of the brushless motor;  
       FIG. 3  is an exploded perspective view of a stator of the brushless motor provided with a shaft and a bearing;  
       FIG. 4  is a partial enlarged view of a pole tooth of one of yokes of the stator;  
       FIG. 5  is a circuit diagram of a control circuit of the brushless motor;  
       FIG. 6  is a descriptive view showing supply of electric current to coils of the stator;  
       FIG. 7  is a deployed view of the yokes and rotor magnets of the brushless motor;  
       FIG. 8A  is a partial enlarged view showing a modification of the pole tooth;  
       FIG. 8B  is a partial enlarged view showing another modification of the pole tooth;  
       FIG. 8C  is a partial enlarged view showing another modification of the pole tooth;  
       FIG. 8D  is a partial enlarged view showing another modification of the pole tooth;  
       FIG. 8E  is a partial enlarged view showing another modification of the pole tooth;  
       FIG. 9  is a descriptive view showing arrangement of a Hall IC of the brushless motor;  
       FIG. 10  is a deployed view similar to  FIG. 7 , showing a modification of the rotor magnets;  
       FIG. 11  is a deployed view similar to  FIG. 7 , showing another modification of the rotor magnets; and  
       FIG. 12  is a deployed view similar to  FIG. 7 , showing a further modification of the rotor magnets. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      One embodiment of the present invention will be described with reference to the accompanying drawings.  
      In the following embodiment, a brushless motor M of the present invention is embodied in a drive unit of an electric fan of a vehicle. With reference to  FIG. 1 , the brushless motor M is an outer rotor motor and includes a rotor  10  and a stator  20 . The stator  20  is arranged radially inward of the rotor  10 . The rotor  10  includes a case  11 , a shaft  13  and permanent magnets (rotor magnets)  12 . The case  11  includes a circular flat portion  11   a  and a cylindrical peripheral wall portion  11   b . The peripheral wall portion  11   b  extends axially from an outer peripheral edge of the flat portion  11   a . The shaft  13  is received through a through hole, which penetrates through a center of the flat portion  11   a , and the shaft  13  is secured to the flat portion  11   a . The rotor magnets  12  form magnetic poles, respectively, and are secured to an inner peripheral surface of the peripheral wall portion  11   b . A fan  5  is connected to a distal end of the shaft  13 . The fan  5  is rotated in one direction upon rotation of the rotor  10 .  
      As shown in  FIG. 2 , each rotor magnet  12  is formed as a thin magnet, which is curved along an arc and which forms the magnetic pole. Furthermore, each rotor magnet  12  is magnetized in such a manner that a magnetic flux of the rotor magnet  12  is oriented in a radial direction. The rotor  10  of the present embodiment has four magnetic poles. Thus, four separate rotor magnets  12  are secured to the inner peripheral surface of the peripheral wall portion  11   b  to form the four magnetic poles. The rotor magnets  12  of opposite polarities are arranged alternately in a circumferential direction in such a manner that a direction of the magnetic flux of one of respective adjacent two of the rotor magnets  12  relative to a rotational axis of the shaft  13  is opposite from a direction of the magnetic flux of the other one of the respective adjacent two of the rotor magnets  12  relative to the rotational axis.  
      Although the rotor  10  of the present embodiment includes the separate rotor magnets  12 , the present invention is not limited to this arrangement. For example, in place of the separate rotor magnets  12 , a single annular rotor magnet can be press fitted into the case  11 . In such a case, the single annular rotor magnet should be magnetized to have a plurality of magnetic poles in such a manner that the direction of the respective magnetic flux changes every predetermined angular interval (in the case of the four magnetic poles, the predetermined angular interval is about 90 degrees).  
      The stator  20  includes a cylindrical spacer  22 , two yokes (first and second yokes of a claw pole type)  21 , and two coil bobbins (first and second coil bobbins)  24 . The spacer  22  is made of a non-magnetic material, such as a synthetic resin material. Each yoke  21  is arranged radially outward of the spacer  22  and includes two pole teeth  21   a , which extend in the axial direction and are opposed to the rotor magnets  12 . The coil bobbins  24  are arranged radially inward of the pole teeth  21   a  and are made of a non-magnetic material. Two coils (first and second coils)  25   a ,  25   b  are circumferentially wound around the coil bobbins  24 , respectively. A through hole  22   a  is formed through a center of the spacer  22 , and a bearing  2  is securely press fitted to one end of the spacer  22 . The other end of the spacer  22  is secured to an end plate  1 , and a bearing  3  is provided to the end plate  1 . The bearings  2 ,  3  rotatably support the shaft  13 , which is received through the through hole  22   a.    
      In the stator  20  of the present embodiment, the two coil bobbins  24  are stacked one after another in the axial direction. The coil  25   a  conducts an A-phase electric current and is wound around one of the two coil bobbins  24 , and the coil  25   b  conducts a B-phase electric current and is wound around the other one of the two coil bobbins  24 . The coils  25   a ,  25   b  are wound in opposite directions, and the A-phase electric current (a first-phase electric current) and the B-phase electric current (a second-phase electric current) are supplied to the coils  25   a ,  25   b , respectively, in a common direction. Thus, at the time of energizing the coils  25   a ,  25   b , magnetic fields of opposite directions are generated. The stacked coil bobbins  24  are secured in the stator  20  in such a manner that the coil bobbins  24  are clamped between the two yokes  21 .  
      Each yoke  21  is made of a magnetic material and includes the two pole teeth  21   a , an inner yoke portion  21   b  and an annular portion  21   c . The pole teeth  21   a  serve as outer yoke portions, which cover outer peripheral surfaces of the coil bobbins  24 . The inner yoke portion  21   b  covers an inner peripheral surface of the adjacent coil bobbin  24 . The annular portion  21   c  connects between the inner yoke portion  21   b  and the pole teeth  21   a  and covers an end surface of the adjacent coil bobbin  24 . The two yokes  21  are integrally connected to one another in the axial direction in such a manner that the inner yoke portions  21   b  of the yokes  21  are fitted to each other. Each yoke  21  is made through sheet metal processing in such a manner that the two pole teeth  21   a  are circumferentially displaced 180 degrees from one another and extend from an outer peripheral edge of the annular portion  21   c . Each pole tooth  21   a  of each yoke  21  has a decreasing circumferential width, which decreases toward its distal end, i.e., toward the annular portion  21   c  of the other yoke  21 . In other words, each pole tooth  21   a  is tapered toward the annular portion  21   c  of the other yoke  21 . A magnetic pole surface of each symmetrical ones (described later) of the pole teeth  21   a  has a circumferential width, which is the same as a pole width of the rotor magnet  12 . As shown in  FIG. 7 , when the pole tooth  21   a  is deployed in a plane, i.e., is unbent to extend in the plane, the magnetic pole surface of the pole tooth  21   a  has a generally trapezoidal shape the annular portion  21   c  side of the pole tooth  21   a  is a long side of the trapezoidal shape, and the distal end side of the pole tooth  21   a  is a short side of the trapezoidal shape. The pole teeth  21   a  are arranged in opposed relationship to the rotor magnets  12  to make a magnetic interaction with the rotor magnets  12 . Furthermore, as described above, each pole tooth  21   a  is formed into the trapezoidal shape, which has the decreasing circumferential width that decreases toward its distal end. Thus, when the two yokes  21  are axially assembled together, the pole teeth  21   a  of one of the yokes  21  do not physically interfere with the pole teeth  21   a  of the other one of the yokes  21 .  
      As shown in  FIG. 3 , at the time of assembling the stator  20 , the coil bobbins  24 , around which the coils  25   a ,  25   b  are respectively wound, are received in one of the yokes  21 . Then, the other one of the yokes  21  is coaxially installed to the one of the yokes  21  in opposed relationship in such a manner that a phase of each of the pole teeth  21   a  of the other one of the yokes  21  is shifted about 90 degrees from a phase of an adjacent one of the pole teeth  21   a  of the one of the yokes  21 . Then, the spacer  22  is press fitted into the yokes  21 . Thus, the stator  20  includes the four pole teeth  21   a , which form the four magnetic poles. The brushless motor M (more specifically, the rotor  10  of the brushless motor M) of the present embodiment is normally continuously rotated at a high speed, which is equal to or greater than 1,000 rpm, so that it is not required to set fine step angles. Therefore, the structure of the brushless motor M is relatively simple.  
      Furthermore, as shown in  FIG. 4 , at least one (but not all) of the four pole teeth  21   a  is made to have a non-symmetrical magnetic pole surface, which is radially opposed to the rotor magnet(s)  12  and is non-symmetrical about a center axis L (i.e., the rotational axis of the shaft  13 ) while the rest (the symmetric pole teeth  21   a ) of the four pole teeth  21   a  has a symmetrical magnetic pole surface, which is symmetrical about the center axis L. In the present embodiment, one circumferential end of the one non-symmetrical pole tooth  21   a  has a notch  21   aa , from which a generally triangular shaped end part is notched. In the brushless motor M of the present embodiment, the number of the magnetic poles of the rotor magnets  12  is four, and the number of the magnetic poles of the pole teeth  21   a  is also four. When the number of the magnetic poles of the rotor magnets  12  is equal to the number of the magnetic poles of the pole teeth  21   a , the rotor magnets  12  and the pole teeth  21   a  could be held in a magnetically balanced state at the time of stopping the brushless motor M. Thus, even when the electric current is resupplied to the coils  25   a ,  25   b  of the stator  20 , an electromotive force, which is required to rotate the rotor  10  of the brushless motor M, is not generated in the magnetically balanced state.  
      In the stator  20  of the present embodiment, the at least one of the pole teeth  21   a  is made to be slightly non-symmetrical about the center axis L, as described above. Thus, at the time of supplying the electric current, the corresponding rotor magnet  12  of the rotor  10  is slightly circumferentially shifted from this non-symmetrical pole tooth  21   a . Therefore, at the time of supplying the electric current to the stator  20 , the electromotive force is directed to one circumferential direction, and thereby rotation of the rotor  10  can be initiated. That is, although the pole teeth  21   a  are arranged at generally equal intervals in the circumferential direction, the formation of the notch  21   aa  in the non-symmetrical pole tooth  21   a  causes a reduction in the magnetic interaction of the non-symmetrical pole tooth  21   a  with the corresponding rotor magnet  12 . Thus, one circumferential part of the stator  20 , in which the non-symmetrical pole tooth  21   a  is provided, becomes magnetically unbalanced, so that a rotational force is generated in the one circumferential direction to initiate the rotation of the rotor  10 .  
      The brushless motor M of the present embodiment is constructed to initiate the rotation in the one circumferential direction with the above-described simple structure. In the brushless motor M, as discussed above, then number of the magnetic poles of the rotor  10  is four, and then number of the magnetic poles of the stator  20  is also four. With such minimum numbers of the magnetic poles, the structure of the brushless motor M is simplified. It should be noted that then number of the magnetic poles in each of the rotor  10  and the stator  20  is not limited to four and can be changed to 2n where “n” is a natural number, which is equal to or greater than 2. Furthermore, the shape of the notch  21   aa  is not limited to the generally triangular shape and can be change to any other suitable shape, such as a rectangular shape, an arcuate shape.  
      Furthermore, as shown in  FIG. 1 , a printed circuit board  30  is provided to an inner surface of the end plate  1 . A Hall IC  31 , which serves as single magnetic position sensor, is provided to the printed circuit board  30 . When each rotor magnet  12  is rotate to axially oppose the Hall IC  31 , the Hall IC  31  is axially opposed to the rotor magnet  12  in such a manner that a predetermined gap is formed between the Hall IC  31  and an adjacent axial end surface of the opposed rotor magnet  12 . At the time of rotating the rotor  10 , the Hall IC  31  senses the magnetism of the corresponding rotor magnet(s)  12  and outputs a position measurement signal, which indicates a rotational position of the rotor  10 , to a controller  40  ( FIG. 5 ). A switching point from the currently sensed magnetism sensed by the Hall IC  31  to the next magnetism corresponds to a switching point from the current rotor magnet  12  to the next rotor magnet  12 . Accordingly, the controller  40  outputs a control signal to a control circuit of the printed circuit board  30  based on the position measurement signal to rotate the brushless motor M at the predetermined rotational speed.  
      As shown in  FIG. 5 , the control circuit of the printed circuit board  30  includes transistors  32   a ,  32   b . The transistor  32   a  is connected to the coil  25   a , which is for the A phase (the first phase). Furthermore, the transistor  32   b  is connected to the coil  25   b , which is for the B phase (the second phase). When a pulse signal is inputted as the control signal from the controller  40  to a base terminal of a corresponding transistor  32   a ,  32   b , a predetermined electric current flows through the corresponding coil  25   a ,  25   b  to generate a corresponding magnetic field. As shown in  FIG. 6 , the stator  20  of the present embodiment is supplied with a half-wave electric current, which is supplied in such a manner that the electric current of the A phase and the electric current of the B phase do not overlap with one another. Since the half-wave electric current is supplied to the stator  20 , the electrical circuit construction is relatively simple.  
      Furthermore, as described above, the coil  25   a  is wound in the direction opposite from that of the coil  25   b . Thus, when the half-wave electric current is alternately supplied to the A phase and the B phase, respective adjacent two pole teeth  21   a , which respectively have opposite polarities, will change their polarities (N and S poles) from time to time to make the magnetic interaction with the corresponding rotor magnets  12 . When the control signal is supplied from the controller  40  to the printed circuit board  30  at predetermined timing, the rotor  10  is continuously rotated in the single direction.  
       FIG. 7  is a deployed view of the pole teeth  21   a  of the yokes  21  and the rotor magnets  12 , which are deployed in the plane along the circumferential direction and are seen in the radial direction. When each pole tooth  21   a  is deployed in the plane, the pole tooth  21   a  has the generally trapezoidal shape, in which the annular portion  21   c  side of the pole tooth  21   a  forms the long side of the generally trapezoidal shape, and the distal end side of the pole tooth  21   a  forms the short side of the generally trapezoidal shape. An average circumferential width of the magnetic pole surface (or a circumferential width of the axial center of the magnetic pole surface) of each symmetrical pole tooth  21   a  is set to be generally the same as the pole width of each rotor magnet  12  (or a circumferential width of the axial center of each rotor magnet  12 ). When the average circumferential width of the magnetic pole surface of the pole tooth  21   a  is set to be generally the same as the pole width of the rotor magnet  12 , the magnetic flux generated therebetween can be most effectively used.  
       FIG. 7  shows the state, in which each pole tooth  21   a  is most significantly opposed to the corresponding rotor magnet  12 , so that a radially overlapping surface area of the pole tooth  21   a , which is overlapped with the corresponding rotor magnet  12  in the radial direction, is maximized. In other words, a radially overlapping total surface area of the pole teeth  21   a  of the yokes  21  relative to the rotor magnets  12  is maximized. In this state, the circumferential center of each symmetrical pole tooth  21   a  coincides with the circumferential center of the corresponding rotor magnet  12  in the radial direction.  
      The Hall IC  31  is arranged near a circumferential gap of the pole teeth  21   a  of the two yokes  21 . More specifically, at the above rotational position of the rotor  10 , in which each pole tooth  21   a  is most significantly overlapped with the corresponding rotor magnet  12  in the radial direction, the Hall IC  31  is arranged to overlap with the circumferential end of one of the rotor magnets  12  in the axial direction.  
      With this arrangement of the Hall IC  31 , when the rotor  10  is rotated to the above rotational position, in which the overlapping surface area of each pole tooth  21   a  with the opposed rotor magnet  12  is maximized, i.e., when the maximum magnetic interaction is made between the pole tooth  21   a  and the opposed rotor magnet  12  (i.e., the time of generating the largest attractive or repulsive force), the Hall IC  31  senses the switching of the magnetism and outputs the corresponding signal, which indicates the switching of the rotor magnet  12 , to the controller  40 . The controller  40  can determine the time point of this switching upon receiving the above signal. The controller  40  switches the supply of the half-wave electric current between the A phase and the B phase at the time point of the switching (i.e., at a leading edge of the change in the magnetic flux measured through the Hall IC  31 ).  
      As described above, in the brushless motor M of the present embodiment, the supply of the half-wave electric current is switched at the above rotational position of the rotor  10 , in which the maximum magnetic interaction occurs between each pole tooth  21   a  and the opposed rotor magnet  12 . Therefore, the large drive force can be generated at the maximum efficiency.  
      Furthermore, the circumferential ends of each pole tooth  21   a  are slanted in the circumferential direction with respect to the axial direction, which is generally parallel to the axis of the shaft  13 . The circumferential ends of the magnetic pole of each rotor magnet  12  of the present embodiment are generally parallel to the axial direction. In this way, in the brushless motor M of the present embodiment, when the rotor  10  is rotated in the predetermined direction, a degree of the magnetic interaction between each pole tooth  21   a  and the corresponding rotor magnet  12  can be gradually changed. Therefore, torque ripple of the brushless motor M can be reduced at the time of rotating the brushless motor M.  
      The circumferential width of each rotor magnet  12  is set to be larger than the circumferential width of the distal end (the short side) of each pole tooth  21   a  and is shorter than the base end (the long side where the annular portion  21   c  is located) of the pole tooth  21   a . In this way, when the rotor  10  is rotated, the overlapping surface area of each pole tooth  21   a  with the corresponding rotor magnet  12  in the radial direction is progressively changed at the circumferential ends of the pole tooth  21   a . In this way, the magnetic interaction between the pole tooth  21   a  and the corresponding rotor magnet  12  does not rapidly change, so that the torque ripple of the brushless motor M generated at the time of rotating the brushless motor M can be reduced.  
      As discussed above, the brushless motor M of the present embodiment has the stator  20 . In the stator  20 , the coil bobbins  24 , around which the coils  25   a ,  25   b  are wound, are stacked one above the other, and the yokes  21  axially clamp the coil bobbins  24 . The stator  20  has the claw pole structure, in which the pole teeth  21   a  extend in the yokes  21  to cover the outer peripheral surfaces of the two-phase coil bobbins  24 . Thus, unlike the previously proposed brushless motor, in the stator  20  of the brushless motor M of the present embodiment, each coil bobbin  24  is not individually clamped by the corresponding two yokes, each of which has the pole teeth. Specifically, the two stacked coil bobbins  24  are integrally clamped by the two yokes  21  in the stator  20  of the brushless motor M of the present embodiment. More specifically, each pole tooth  21   a  extends over the two-phase coil bobbins  24 . Therefore, the number of components of the stator  20  is minimized with the simple structure, and thereby the manufacturing costs can be minimized. Furthermore, the coils  25   a ,  25   b  are supplied with the half-wave electric current. Thus, the control circuit is relatively simple.  
      Furthermore, the two-phase coils  25   a ,  25   b  are supplied with the half-wave electric current, and the rotational position of the rotor  10  is sensed with the Hall IC  31 . Then, the Hall IC  31  outputs the position measurement signal to the controller  40 . In turn, the controller  40  controls the rotation of the brushless motor M. Thus, the brushless motor M can be continuously rotated without making the desynchronization. Furthermore, the half-wave electric current is alternately supplied to the two-phase coils  25   a ,  25   b , so that only the one Hall IC  31  needs to be provided in the circumferential direction of the rotor  10 .  
      Furthermore, in the brushless motor M of the present embodiment, although the number (four in the present embodiment) of the magnetic poles of the stator  20  is the same as the number (four in the present embodiment) of the magnetic poles of the rotor  10 , the at least one of the pole teeth  21   a  of the stator  20  is made non-symmetrical about the center axis L to improve the startability of the brushless motor M. Therefore, the startability of the brushless motor M can be advantageously improved with the above simple structure.  
      The present embodiment can be modified as follows.  
      In the above embodiment, the one of the pole teeth  21   a  is made non-symmetrical about the center axis L by notching the one circumferential end of the pole tooth  21   a . However, the present invention is not limited to this. For example, this pole tooth  21   a  can be modified to any other suitable shape, as shown in  FIGS. 8A-8E . In  FIG. 8A , a slit  21   d  is formed at a location adjacent to one of the circumferential ends of the pole tooth  21   a  to achieve the magnetic unbalance. In  FIG. 8B , a curved portion  21   e , which is radially slightly curved, is formed in the one of the circumferential ends of the pole tooth  21   a . In  FIG. 8C , a notch  21   f  is formed by largely notching the one of the circumferential ends of the pole tooth  21   a . In  FIG. 8D , a thin wall portion  21   g  is formed by radially thinning a wall of the one of the circumferential ends of the pole tooth  21   a . In  FIG. 8E , the pole tooth  21   a  is symmetrical. However, the circumferential width of the pole tooth  21   a  is shorter than the other pole teeth  21   a , and thus the center is shifted a predetermined angle in the left direction in  FIG. 8E . Even with the above modifications, the pole tooth  21   a  is still magnetically asymmetrical about the center axis L in the circumferential direction. Thus, the startability of the brushless motor M can be improved. It is only required to have at least one asymmetrical pole tooth  21   a , which is shown in, for example,  FIGS. 8A  to  8 E, to make the stator  20  magnetically asymmetrical with respect to the rotor  10  in the circumferential direction.  
      In the above embodiment, the Hall IC  31  is axially opposed to the axial end surface of the respective rotor magnet  12 . However, the present invention is not limited to this. For example, the Hall IC  31  can be arranged in a manner shown in  FIG. 9 . Specifically, when one of the pole teeth  21   a  of one of the yokes  21  is rotated to a position shown in  FIG. 9 , the Hall IC  31  faces an axial space between the distal end of the one of the pole teeth  21   a  of the one of the yokes  21  and the annular portion  21   c  of the other one of the yokes  21 . With this arrangement, the Hall IC  31  can sense a radial magnetic flux. In this way, a relatively large magnetism, which is larger than the axial magnetic flux, can be sensed, so that accuracy of sensing of the rotational position is improved. Furthermore, the Hall IC  31  is positioned to face the space between the two yokes  21 , i.e., the axial space, which is defined between the pole teeth  21   a  of one of the yokes  21  and the other one of the yokes  21 . Therefore, a loss of the magnetic flux, which is caused by the Hall IC  31  and would otherwise contribute to the rotation of the rotor  10 , is reduced or minimized.  
      In the above embodiment, the outer rotor brushless motor M is described. However, the present invention is not limited to this. Alternatively, the present invention can be implemented in an inner rotor brushless motor. In the above embodiment, the number of the rotor magnets  12  is four, and the number of the pole teeth  21   a  is also four. However, as long as the number of the rotor magnets  12  and the number of the pole teeth  21   a  are even numbers and are equal to each other, any other appropriate number can be selected.  
      Furthermore, in the above embodiment, the coil  25   a  of the A-phase electric current and the coil  25   b  of the B-phase electric current are wound around the separate bobbins  24 , respectively. However, the present invention is not limited to this. For example, the coils  25   a ,  25   b  may be wound around a single bobbin  24 .  
      Furthermore, in the above embodiment, the circumferential width of each rotor magnet  12  is generally the same as the circumferential width of the axial center of the symmetrical pole tooth  21   a , which is measured at the axial center of the pole tooth  21   a , and the circumferential ends of the rotor magnet  12  are generally parallel to the axial direction. However, the present invention is not limited to this. For example, each rotor magnet  12  may be modified to any other appropriate shape, as shown in FIGS.  10  to  12 .  
      In  FIG. 10 , the pole width of the rotor magnet  12  is set to be smaller than the circumferential width of the axial center of the symmetrical pole tooth  21   a , which is measured at the axial center of the pole tooth  21   a . In this example, the pole width of the rotor magnet  12  is set to be generally the same as the circumferential width of the distal end of the symmetrical pole tooth  21   a . As shown in  FIG. 10 , even in this case, the Hall IC  31  is arranged to generally axially coincide with the circumferential end of the corresponding rotor magnet  12  at the time where the overlapping surface area of the pole tooth  21   a  with the rotor magnet  12  in the radial direction is maximized. When the Hall IC  31  is arranged in this way, it is possible to sense the rotational position of the rotor  10  where the maximum magnetic interaction occurs between the pole tooth  21   a  and the rotor magnet  12 . Upon receiving this signal, the controller  40  can make the switching of the supply of the half-wave electric current to the coils  25   a ,  25   b  at the suitable timing, at which the maximum drive force is generated.  
       FIG. 11  shows the exemplary rotor magnets  12 , each of which has a generally parallelogram shape in the deployed state. In  FIG. 11 , each rotor magnet  12 , which is slanted, i.e., is skewed in the circumferential direction, is arranged to have a maximum overlapping surface area that overlaps with the corresponding pole tooth  21   a . As shown in  FIG. 11 , in this case, the Hall IC  31  is arranged to generally axially coincide with the circumferential end (the end at the lower side in this case) of the corresponding rotor magnet  12 . When the Hall IC  31  is arranged in this way, it is possible to sense the rotational position where the maximum magnetic interaction occurs between the pole tooth  21   a  and the rotor magnet  12 .  
       FIG. 12  shows another case where the pole width of each rotor magnet  12  of  FIG. 11  is maximized. In other words, in  FIG. 12 , a circumferential space between respective adjacent rotor magnets  12  is eliminated or is substantially eliminated. Even in this case, when the Hall IC  31  is arranged to generally axially coincide with the circumferential end of the corresponding rotor magnet  12 , it is possible to sense the rotational position of the rotor  10  where the maximum magnetic interaction occurs between the pole tooth  21   a  and the rotor magnet  12 .  
      As discussed above, when the Hall IC  31  is arranged in the manner shown in  FIG. 11  or  FIG. 12 , it is possible to sense the rotational position of the rotor  10  where the maximum magnetic interaction occurs between the pole tooth  21   a  and the rotor magnet  12 . Thus, the controller  40 , which receives the position measurement signal that indicates the above rotational position where the maximum magnetic interaction occurs, can make the switching of the supply of the half-wave electric current to the coils  25   a ,  25   b  at the suitable timing, at which the maximum drive force is generated.  
      Furthermore, by skewing the rotor magnets  12 , the magnetic interaction between each pole tooth  21   a  and the corresponding rotor magnet  12  can be progressively changed during the rotation of the rotor  10 . Therefore, torque ripple of the brushless motor M can be reduced at the time of rotating the brushless motor M.  
      Furthermore, it is preferred that a slant angle of each circumferential end edge of the rotor magnet  12  in the circumferential direction is made larger than a corresponding slant angle of an adjacent circumferential end edge of the corresponding pole tooth  21   a . With this arrangement, the torque ripple of the brushless motor M can be more reduced to achieve more smooth rotation of the brushless motor M.  
      Additional advantages and modifications will readily occur to those skilled in the art. The invention in its broader terms is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described.