Abstract:
The disclosed is a spindle motor most suitable for magnetic disk drives, optical disk drives or the like capable of constraining undesirable cogging torques to a low level and of winding a coil regularly. In particular, the inward periphery of magnetic pole of iron core for a stator includes of an arc shaped surface concentrically to the outward periphery of rotor magnet and a pair of flat surfaces, generally perpendicular to the centerline of magnetic pole, attached to both sides of the arc shaped surface peripherally. The configuration can reduce the cogging torques without any decrease in motor efficiency.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a spindle motor for use in hard disk drives, optical disk drives or the like and in particular, to an iron core capable of winding a coil regularly with ease and of constraining cogging torques in the spindle motor.  
       BACKGROUND ART  
       [0002]     In recent years, various kinds of information equipment have been widely used which require disks an increased storage capacity, boosting to develop devices with a higher storage density, a compact sizing and low profile designing. Moreover, along with advances of downsizing and low profiling of the devices, a wire diameter for the coil wound on iron core of the stator of spindle motor has decreased and various winding methods for the wire has been developed.  
         [0003]     Next, an iron core of a stator for conventional spindle motor used in hard disk drives, optical disk drives or the like is described schematically with reference to drawings.  FIG. 9  shows a schematic cross sectional view taken along a plane perpendicular to a rotation center of a portion of stator and rotor magnet of a conventional spindle motor.  
         [0004]     In  FIG. 9 , an iron core comprises: a plurality of magnetic poles  94  having pole-tops  91  at both sides peripherally on the top facing rotation center  1  inwardly and straight portion  93  to wind a coil; and pole base  95  to join respective magnetic poles  94  outwardly. A plurality of iron layers made of for instance silicon steel plates or the like are laminated to form iron core  96 ; core  96  and coil  92  forms stator  97 . The outward periphery of rotor magnet  98  including a plurality of magnetized sections faces the inward periphery of magnetic poles  94  of stator  97  across an air gap. Upon energizing, as well known, coil  92  generates magnetic fluxes allowing rotor magnet  98  including a plurality of magnetized sections to rotate. In such configuration having stator  97  and rotor magnet  98 , the air gap between the inward periphery of magnetic pole  94  and rotor magnet  98  varies abruptly in the vicinity of clearance between pole-tops  91  of neighboring magnetic poles  94 , or slots  99 , causing the magnetic flux density to vary abruptly. Consequently, attractive forces between magnetic poles  94  and rotor magnet  98  vary abruptly causing cogging torques and ripple occurs in motor rotations. To constrain the abrupt variations in magnetic flux density between magnetic pole  94  and rotor magnet  98  in the vicinity of slots  99 , pole-top  91  is formed such that a distance to surface  91   a  from rotation center  1  becomes smaller as it extends peripherally.  
         [0005]     Additionally, examples of iron core design to constrain the generation of cogging are:  
         [0006]     an inward periphery for a magnetic pole of the iron core has a larger radius of curvature than a distance between an intersection of the inward periphery with the centerline of the magnetic pole through the rotation center,  
         [0007]     or an inward periphery for a magnetic pole of the core has a plane perpendicular to the centerline of the magnetic pole (for instance, see Japanese Patent Unexamined Publications No. H8-111968 and H11-987920). Such core configurations can widen the air gap between the magnetic core and rotor magnet gradually from the center of inward periphery toward both ends of the pole-top. This results in gradual variations in attractive forces between magnetic poles  94  and rotor magnet  98  enabling motor to reduce cogging torque at rotation.  
         [0008]     However, the conventional iron core configuration for the spindle motor has the problems of decreases in motor efficiency, as the air gap between the magnetic core and rotor magnet widens gradually from the center of inward periphery toward both ends of pole-top and that a distance to the surface of projection facing the pole-base from the rotation center becomes smaller as it extends peripherally.  
         [0009]     Additionally, along with the progress in downsized and low profiled devices, the iron core of spindle motor requires a very thin wires to wind a coil and the regular winding technology using thin wires has become of great importance. However, in the iron core configuration of the stators shown in  FIG. 9 , surface  91   a  of pole-top  91 , facing pole-base  95 , intersects with straight portion  93  obtusely, causing difficulties to wind coils  92  using very thin wires on straight portion  93  of magnetic pole  94  regularly. Even if wound regularly, the regular windings of coil  92  is broken or likely to be broken due to a slight slack of the winding in the processes of winding, assembling after winding or at motor operation, causing difficulties to dispose the coils in a predetermined position properly. The problem is that in an extreme case coil  92  touches rotor magnet  98  owing to the broken coil windings to cause failures such as damaging the insulation layers of coil or the like resulting in a poor reliability of the motor operation.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention aims at, to solve the aforementioned problems, providing a spindle motor with a stator capable of reducing cogging torques significantly without any decrease in motor efficiency and of winding a coil regularly with ease using thin wires, preventing the coiled style from being broken. To accomplish the purposes the spindle motor disclosed comprises:  
         [0011]     a rotary spindle having a rotation center;  
         [0012]     a rotor body provided with a circular rotor secured to the rotary spindle and magnets secured to the external periphery of the circular rotor;  
         [0013]     an iron core provided with a plurality of magnetic poles having coiling portions and pole-tops provided on both sides of the coiling portion extending from the coiling portion peripherally and a pole-base to join magnetic poles;  
         [0014]     a stator having coils wound around a plurality of magnetic poles;  
         [0015]     a bearing sleeve holding the bearing to secure the rotary spindle rotatably; and  
         [0016]     a housing to secure the stator and the bearing sleeve, wherein the rotor magnet rotates around the rotation center facing the inward periphery of the iron core of stator. The inward periphery of the iron core includes a concentrically shaped surface to the outward periphery of the rotor and a pair of flat surfaces, generally perpendicular to the centerline of the magnetic pole, attached to both sides peripherally.  
         [0017]     Moreover, angle α that respective intersections of the arc shaped surface of the inward periphery of the iron core with a pair of flat surfaces make to the rotation center, and  
         [0018]     angle β that respective intersections of the coiling portion of the magnetic pole with the pole-top make to rotation center, are in a relation to satisfy the following equation: α≦β.  
         [0019]     In this configuration, the inward periphery of pole-top has a surface concentrically to the outward periphery of the rotor magnet ranging generally same width of coiling portion of the magnetic pole causing no decrease in motor efficiency.  
         [0020]     Moreover, the air gap between rotor magnet  4  and iron core  8  widens gradually over a pair of flat surfaces  31   b,  causing energy fluctuations from the gap to decrease gradually as away from centerline  23 , thereby enabling the fluctuations of flux density passing inward periphery  31  to approximate a sine wave and to reduce cogging torques.  
         [0021]     Additionally, in the spindle motor disclosed, surface of the projection facing the pole-base and surface of the pole base facing the pole-top are in parallel with each other and are generally perpendicular to the centerline of the magnetic pole. The length of the parallel surface of pole-base is equal to or longer than the length of the parallel surface of the projection.  
         [0022]     The configuration can provide coiling portion of the magnetic pole with a regular winding easily preventing the coil style from being broken. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  illustrates a schematic cross-sectional view showing main parts of the spindle motor used in preferred embodiment 1 of the present invention.  
         [0024]      FIG. 2  illustrates a cross-sectional plan view taken along the plane A-A of the spindle motor shown in  FIG. 1 .  
         [0025]      FIG. 3  illustrates a exploded view showing a magnetic pole and a portion joined to the pole-base of an iron core for a stator of the spindle motor.  
         [0026]      FIG. 4  illustrates a graph showing an example of cogging torques generated in the spindle motor.  
         [0027]      FIG. 5  illustrates a exploded view showing the shape of a magnetic pole of the iron core in the spindle motor used in preferred embodiment 1 of the present invention.  
         [0028]      FIG. 6  illustrates a exploded view showing a pole-top of magnetic pole formed inwardly on the iron core for the spindle motor stator used in preferred embodiment 2 of the present invention.  
         [0029]      FIG. 7   a  illustrates a exploded view showing an example of a notch provided in the projection of a pole-top of magnetic pole.  
         [0030]      FIG. 7   b  illustrates a exploded view showing another example of a notch provided in the projection of a pole-top of magnetic pole.  
         [0031]      FIG. 7   c  illustrates a exploded view showing still another example of a notch provided in the projection of a pole-top of magnetic pole.  
         [0032]      FIG. 8  illustrates a exploded view showing a pole-top of magnetic pole formed inwardly on an iron core for the spindle motor stator used in preferred embodiment 3 of the present invention.  
         [0033]      FIG. 9  illustrates a schematic cross-sectional view showing a stator and a rotor magnet used in a conventional spindle motor. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]     Now, the preferred embodiments of the present invention are described with reference to drawings.  
       Preferred Embodiment 1  
       [0035]     The spindle motor used in preferred embodiment 1 is described with reference to FIGS.  1  to  5 .  FIGS. 1, 2 ,  3 ,  4  and  5  are to describe the configuration of spindle motor used in preferred embodiment 1 and cogging torques.  FIG. 1  illustrates a schematic cross-sectional view showing main parts of the spindle motor used in preferred embodiment 1 of the present invention,  FIG. 2  illustrates a cross-sectional plan view taken along the plane A-A of the spindle motor shown in  FIG. 1 ,  FIG. 3  illustrates a exploded view showing a magnetic pole and a portion joined to the pole-base of an iron core for a stator of the spindle motor,  FIG. 4  illustrates a graph showing an example of cogging torques generated in a spindle motor, and  FIG. 5  illustrates a exploded view showing the shape of a magnetic pole of an iron core in a spindle motor to study cogging torques.  
         [0036]     As shown in  FIGS. 1 and 2 , rotor  3  is secured to rotary spindle  2  rotating around rotation center  1  by a known method such as press fitting, adhesive bonding or the like. Rotor magnet  4  is secured to a bottom surface of flange  3   a  of rotor  3  by a known method such as press fitting, adhesive bonding or the like to form rotor body  5  including rotary spindle  2 , rotor  3  and rotor magnet  4 . Needless to say, spindle  2  and rotor  3  can be incorporated instead of formed from different materials individually.  
         [0037]     On the other hand, bearing sleeve  7  securing ball bearing  6  that holds rotary spindle  2  rotatably and stator  10  having coil  9  wound on iron core  8  laminated a plurality of iron layers  8   a  are secured to chassis  11  by a known method such as press fitting, adhesive bonding, crimping, welding or the like. Stator  10  is mounted in spindle motor  12  such that inward periphery of iron core  8  faces outward periphery of rotor magnet  4  secured to rotor  4 .  
         [0038]      FIG. 1  illustrates a rotary spindle type bearing system in which rotary spindle  2  rotates in bearing sleeve  7  fixed on chassis  11 . Needless to say, however, a fixed spindle type bearing system can be adopted in which bearing sleeve fixed on rotor  3  rotates around the rotation center  1  fixed to chassis  11 . The bearing is not limited to the ball bearing only but a well known dynamic fluid bearing would be acceptable.  
         [0039]     As shown in  FIG. 2 , iron core  8  formed of a plurality of laminated iron layers includes a plurality of magnetic poles  21  and pole-base  22  to join magnetic poles  21  radially, and magnetic pole  21  comprises coiling portion  21   a  to wind coil  9  and inward pole-top  21   b.    
         [0040]     Magnetic pole  21  is shaped such that the width of coiling portion  21   a  perpendicular to centerline  23  is equal to or larger in pole-base  22  side than in pole-top  21   b  side.  
         [0041]     As shown in  FIG. 3 , inward periphery  31  of pole-top  21   b  of iron core  8  faces outward periphery of rotor magnet (not shown) across a small air gap, and in the vicinity of centerline  23  of magnetic pole  21 , arc shaped surface  31   a  concentrically to outward periphery of rotor magnet  4  intersects with a pair of flat surfaces  31   b  in both outer sides peripherally.  
         [0042]     At that time, angle α that a pair of intersections B and B′ of arc shaped surface  31   a  with flat surfaces  31 B make to rotation center  1 , should be at least smaller than angle β that intersections C and C′ of coiling portion  21   a  of magnetic pole  21  with pole-top  21   b  make to rotation center  1 , or they are in the relation to satisfy the following equation: 
 
α≦β  (equation 1) 
 
         [0043]     On the other hand, surfaces  32   a,  back side of projections  32  extending peripherally to both side, are formed to be generally perpendicular to centerline  23  of magnetic pole  21 . Surfaces  33  on pole-base  22  are formed generally in parallel with surfaces  32   a,  or to be perpendicular to centerline  23  of magnetic pole  21 . The width between both ends of surfaces  32   a  is generally equal to the width of inward periphery  31  of pole-top  21   b.  Surfaces  33 , on pole-base  22 , generally in parallel with surfaces  32   a  has a length equal to or longer than the length of surfaces  32   a,  and moreover a little bit longer than a thickness of coil  9  wound on coiling portion  21   a  of magnetic pole  21 .  
         [0044]     In spindle motor  12 , inward periphery  31  of pole-top  21   b  facing rotor magnet  4  includes an arc shaped surface  31   a  concentrically to the outward periphery of rotor magnet  4  attached to a pair of flat surfaces  31   b  generally perpendicular to centerline  23 . The configuration provides inward periphery  31  of pole-top  21   b  with an arc shaped surface  31   a  concentrically to the outward periphery of rotor magnet  4  generally ranging the width of coiling portion  21   a,  preventing spindle motor  12  from decreasing in motor efficiency. Moreover, the air gap between rotor magnet  4  and iron core  8  widens gradually over a pair of flat surfaces  31   b,  causing energy fluctuations from the gap to decrease gradually as away from centerline  23 , thereby enabling the fluctuations of flux density passing inward periphery  31  to approximate a sine wave. Consequently, cogging torques in the spindle motor can be improved.  
         [0045]      FIG. 4  illustrates a graph showing an example of cogging torques generated in a spindle motor. In  FIG. 4 , curve  41  (thick solid line) shows an example of cogging torques generated in the spindle motor of preferred embodiment 1, and curve  42  (thin solid line) shows an example of cogging torques generated in a conventional spindle motor. Here,  FIG. 5  illustrates the shape of iron core in a spindle motor used for the study in preferred embodiment 1.  
         [0046]     Dimensions of the iron core elements:  
         [0047]     D 0 =φ21 mm: outer diameter (not shown)  
         [0048]     D=φ11 mm: inner diameter  
         [0049]     T=0.35 mm: thickness (not shown)  
         [0050]     n=9: number of magnetic pole (number of slot)  
         [0051]     W=2.4 mm: width of magnetic pole  
         [0052]     L 2 =1.1 mm: clearance between neighboring magnetic poles  
         [0053]     L 1 =5.42 mm: distance between flat surface  31   b  and centerline  51  through rotation center  1  and parallel to surface  31   b    
         [0054]     α=21.3 degree  
         [0055]     β=23.4 degree.  
         [0056]     Dimensions of the rotor magnet element (not shown in  FIG. 5 ):  
         [0057]     Outer diameter=φ10.7 mm  
         [0058]     Inner diameter=φ7.8 mm  
         [0059]     Thickness=0.35 mm  
         [0060]     Residual flux density=1.36 T  
         [0061]     Number of pole=12 poles  
         [0062]     anisotropic magnetized.  
         [0063]     Compared with the conventional iron core having an arc shaped surface for the inward periphery of the pole-top, the spindle motor having the pole-top with aforesaid shaping can reduce the cogging torques to about half as shown in  FIG. 4   
         [0064]     The kinds of magnet have great influences on cogging torques generated, curves  41  and  42  in  FIG. 4  are, therefore, only to show an example of result of study on cogging torques by the spindle motor having aforesaid iron core configuration.  
         [0065]     Generally, motor efficiency increases when angle α nears to angle β. However, as angle α decreases motor efficiency decreases and cogging torques decrease as well. Optimum values for angle α and β, therefore, should be determined to balance the motor efficiency and cogging torques according to the property required for the motor.  
         [0066]     Additionally, surfaces  32   a,  back side of projections  32 , and surfaces  33  on pole-base  22  are formed generally in parallel with each other and are perpendicular to centerline  23  of magnetic pole  21 . The configuration can provide coiling portion  21   a  of magnetic pole  21  with a regular winding easily preventing the coil style from being broken.  
         [0067]     The shape of magnetic pole  21  that the width of coiling portion  21   a  perpendicular to centerline  23  is equal to or larger in pole-base  22  side than in pole-top  21   b  side provides magnetic pole  21  with a low magneto-resistance enabling to increase magnetic fluxes and thereby to constrain vibrations generated by the motor rotation in magnetic pole  21  to a lower level.  
         [0068]     The present invention is not so limited to the spindle motor having  9  slots as described in preferred embodiment 1 as an example shown in  FIG. 2  As described in preferred embodiment 1, the inward periphery of pole-top, facing the rotor magnet, includes an arc shaped surface concentrically to the outward periphery of the rotor magnet attached to a pair of flat surfaces generally perpendicular to the centerline of the magnetic pole. The configuration can provide the spindle motor with an excellent rotation performance as energy fluctuations in the air gap between rotor magnet and iron core at motor rotation are lowered, enabling the fluctuations of flux density passing the inward periphery to approximate a sine wave to reduce cogging torques without any decrease in motor efficiency.  
         [0069]     Moreover, surface of the projection facing the pole-base and surface on the pole-base facing the pole-top are formed to be generally in parallel with each other and are perpendicular to the centerline of magnetic pole. The configuration can provide the coiling portion of magnetic pole with a regular winding easily preventing the coil style from being broken, resulting in a downsized spindle motor with a high reliability.  
       Preferred Embodiment 2  
       [0070]     The configuration of the spindle motor used in preferred embodiment 2 is described with reference to  FIGS. 6, 7   a,    7   b  and  7   c.    FIG. 6  is a exploded view showing a pole-top of magnetic pole formed inwardly on an iron core for the spindle motor stator used in preferred embodiment 2 of the present invention,  FIG. 7   a,    7   b  and  7   c  are exploded views showing respective examples of notches provided in the projections of pole-top of magnetic pole. In  FIG. 6 , the similar elements described previously in  FIGS. 1, 2  and  3  have the same reference marks. The main parts of the spindle motor used in preferred embodiment 2 are similar to preferred embodiment 1 such:  
         [0071]     that projection surfaces, against pole-base  22 , of pole-top  21   b  of magnetic pole  21  and surfaces of pole-base  22  against the slot are generally in parallel with each other and are generally perpendicular to centerline  23  of magnetic pole  21 ,  
         [0072]     and that a width of coiling portion  21   a  perpendicular to centerline  23  of magnetic pole  21  is equal to or larger in pole-base  22  side than in pole-top  21   b  side,  
         [0073]     and still that inward periphery  31  of pole-top  21   b  magnetic pole  21  of iron core  8  has a concentrically shaped surface  31   a  to the outward periphery of rotor magnet  4  and a pair of flat surfaces  31   b,  generally perpendicular to centerline  23  of magnetic pole  21 , attached to both sides peripherally, and is formed to satisfy the equation (1) described previously.  
         [0074]     Main points different from preferred embodiment 1 are the shapes provided on projections of the pole-tops.  
         [0075]     In  FIG. 6 , a rectangular shaped cutout  63  is removed off from projection  61  to form a branch including thin portion  61   a  and thick portion  61   b.  The width of thick portion  61   b  against centerline  23  is smaller than the width of thin portion  61   a  against the pole-base (not shown). Cutout surface  61   c  intersects with centerline  23  at an angle γ that satisfies the following equation: 
 
γ≦90 degree  (equation 2) 
 
         [0076]     Moreover, surface  62  of thick portion  61   b  against the pole-base is generally perpendicular to centerline  23  of magnetic pole  21 . In projection  61 , surfaces of thin portion  61   a  and thick portion  61   b  are attached sequentially. Instead of the sequential attaching lines, the cutout can be formed such that outside end of surface  62  peripherally attaches to outside end of inward periphery continuously by a generally straight line, arc shaped curve, elliptical shaped curve or the like as shown in  FIGS. 7   a,    7   b  and  7   c  respectively.  
         [0077]     Like aforesaid preferred embodiment 1, the cogging torques can be reduced significantly without any decrease in motor efficiency by the configuration that inward periphery  31 , facing rotor magnet  4 , of pole-top  21   b  of respective magnetic poles  21  of the iron core includes of a concentrically shaped surface to the outward periphery of rotor magnet  4  and a pair of flat surfaces  31   b,  generally perpendicular to centerline  23  of magnetic pole  21 , attached to both sides peripherally.  
         [0078]     Moreover, projection  61  of pole-top  21   b  on magnetic pole  21  is provided with a branch including thin portion  61   a  and thick portion  61   b .  Thin portions  61   a  provided at both ends of inward periphery  31  facing rotor magnet  4  receive less fluxes from rotor magnet  4  enabling the fluctuations of flux density passing inward periphery  31  to approximate a sine wave, thereby causing cogging torques in the spindle motor to improve further.  
         [0079]     As described above, preferred embodiment 2 can provide the effects similar to preferred embodiment 1. Moreover, the branch provided on the projection of pole tops can reduce energy fluctuations in the air gap between rotor magnet and iron core at motor rotation to constrain cogging torques without any decrease in motor efficiency causing flux density passing inward periphery to approximate a sine wave, thereby causing cogging torques to reduce further and can realize the spindle motor with an excellent rotation performance and a high reliability.  
       Preferred Embodiment 3  
       [0080]     The configuration of a spindle motor used in preferred embodiment 3 is described with reference to  FIG. 8 .  FIG. 8  is a exploded view showing a pole-top formed inwardly on a magnetic pole of an iron core for the spindle motor stator used in preferred embodiment 3 of the present invention. In  FIG. 8 , the similar elements described previously in  FIGS. 1, 2  and  3  have the same reference marks. The main parts of the spindle motor used in preferred embodiment 3 are similar to preferred embodiment 1 and 2 such:  
         [0081]     that inward periphery  31  of pole-top  21   b  of magnetic pole  21  of iron core  8  has a concentrically shaped surface  31   a  to the outward periphery of rotor magnet  4  and a pair of flat surfaces  31   b,  generally perpendicular to centerline  23  of magnetic pole  21 , attached to both sides peripherally, and is formed so as to angle α that a pair of intersections B and B′ of arc shaped surface  31   a  with flat surfaces  31   b  make to rotation center  1 , and angle β that intersections C and C′ of coiling portion  21   a  with pole-top  21   b  make to rotation center  1 , will satisfy the equation (1) described previously,  
         [0082]     and that projection surfaces  81 , against pole-base  22 , of pole-top  21   b  of magnetic pole  21  and surfaces  89  of pole-base  22  against slot  24  are generally in parallel with each other and are generally perpendicular to centerline  23  of magnetic pole  21 , and a width of coiling portion  21   a  perpendicular to centerline  23  of magnetic pole  21  is equal to or larger in pole-base  22  side than in pole-top  21   b  side.  
         [0083]     Main points different from preferred embodiment 1 and 2 are the shapes provided on projections of the pole-tops. Only the differences will be described here.  
         [0084]     In  FIG. 8 , the points differ from preferred embodiment 1 and 2 significantly are:  
         [0085]     that the width of surface  81  of pole-top  21   b  against pole-base  22  perpendicular to centerline  23  is generally equal to the width of pole-top  21   b  of inward periphery  31  perpendicular to centerline  23 , and  
         [0086]     that projection  82  of pole-top  21   b  is provided with notch  83  between flat surface  31   b  of pole-top  21   b  and surface  81 .  
         [0087]     Next, the size and shape of the notch provided on projection  82  is described. The width  85  perpendicular to centerline  23  between both bottom surfaces of notches provided on both projections  82  is at least larger than the distance between the intersections C and C′ of coiling portion  21   a  with pole-top  21   b,  or the width  84  perpendicular to centerline  23  of coiling portion  21   a  against pole-top  21   b.  Additionally, radial width of thin portion  87  of notch  83  between end surface  86  and flat surface  31   b  becomes at least thinner as it goes further peripherally, and on the other hand end surface  88  of notch  83  against surface  81  is generally in parallel with surface  81 . End surface  88  of notch  83  is not necessarily in parallel with surface  81  but may be acceptable to make an obtuse angle with end surface  86 .  
         [0088]     As described in preferred embodiment 2, notch  83  provided on projection  82  between flat surface  31   b  and surface  81  reduces magnetic fluxes received from rotor magnet  4  in both sides of pole-top  21   b  peripherally. The fluctuation of flux density passing inward periphery  31  of pole-top  21   b  of magnetic pole  21  at rotation of rotor magnet can be approximated to a sine wave, causing cogging torques to reduce further. To avoid overlapping, however, a detailed description is omitted here.  
         [0089]     In addition to the aforesaid effects, as the width of surface  81  of pole-top  21   b  is formed to have the same value of the width of inward periphery  31  perpendicular to centerline  23 , coils (not shown) can be wound easily and can increase number of winding layer to contribute a downsized designing of the spindle motor.  
         [0090]     As mentioned above, the spindle motor in preferred embodiment 3 has the configuration that the inward periphery of pole-top of the iron core includes a concentrically shaped surface to the outward periphery of the rotor magnet and a pair of flat surfaces, generally perpendicular to a centerline of the magnetic pole, attached to both sides peripherally, and additionally, the notch is provided in the projection of pole-top of magnetic pole. The configuration can show effects similar to preferred embodiment 1 and 2, and can increase the number of coiling layers for product downsizing to provide the spindle motor with an excellent rotation performance and a high reliability.