Patent Publication Number: US-9899884-B2

Title: Motor armature and motor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a motor armature and a motor. 
     2. Description of the Related Art 
     As causes of inefficiency in driving motors, there have been known copper losses generated by a resistance component of a winding, core losses generated by physical properties of a core constituting an armature, and mechanical losses generated by friction in rotational motion. While the core losses can be further classified into hysteresis losses and eddy current losses, the eddy current losses account for a large proportion. Therefore, it is necessary to reduce the eddy current losses in order to improve motor efficiency by reducing the core losses. Eddy currents are currents generated in the core by an electromagnetic induction action. When a magnetic flux in the core changes, the eddy currents flow in a direction to oppose the change in direction of the magnetic flux. The eddy current losses are energy lost as Joule heat by the flow of the eddy currents. 
     A motor core is normally formed by stacking a plurality of electromagnetic steel sheets. Each of the electromagnetic steel sheets is defined by a planar body where an insulating layer is formed on a surface. An eddy current loss Pe in a planar conductor is known to be expressed by the following expression 1 using a thickness h of the conductor, a frequency f, a maximum magnetic flux density Bm, and a resistivity ρ of a magnetic material. According to the expression, it is understood that the eddy current loss becomes ¼ by decreasing a thickness of the electromagnetic steel sheet used for the core to ½. That is, it is understood that the eddy current loss is reduced by decreasing the thickness of the electromagnetic steel sheet. 
     
       
         
           
             
               
                 
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     Also, a method for reducing the core losses by devising a shape of a stator core has been conventionally proposed (See, for example, Japanese Patent Laid-Open Nos. 2011-160578 and 2012-135123). 
     Japanese Patent Laid-Open No. 2011-160578 discloses a core in which a plurality of slits parallel to a compressive stress are provided in a back yoke portion. It is described that the compressive stress by shrink fitting is relaxed, and the core losses can be reduced by employing the configuration. 
     However, when the thickness of the electromagnetic steel sheet is decreased in order to reduce the eddy current losses, a mechanical strength of the electromagnetic steel sheet is disadvantageously lowered, and it disadvantageously becomes difficult to machine the electromagnetic steel sheet. 
     When the slits are provided in the back yoke portion in order to reduce the eddy current losses, the mechanical strength of the electromagnetic steel sheet is also disadvantageously lowered. 
     SUMMARY OF THE INVENTION 
     In accordance with a preferred embodiment of the present invention, a motor armature includes a core including a stack of two or more electromagnetic steel sheets, the core including a tooth portion extending in a radial direction of the motor; and a conductive wire that is wound around the tooth portion in a circumferential direction, wherein two or more first recessed portions extending in the radial direction are located in a surface of the tooth portion of each of the electromagnetic steel sheets. 
     In accordance with preferred embodiments of the present invention, motor efficiency is improved by reducing eddy current losses. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view schematically illustrating one configuration example of a motor according to Preferred Embodiment 1 of the present invention. 
         FIG. 2  is a sectional view illustrating a section of a stator  12  in  FIG. 2 . 
         FIG. 3  is an enlarged view of a tooth portion  12 T in  FIG. 2  as viewed from an axial direction. 
         FIG. 4  is an enlarged view of the tooth portion  12 T in  FIG. 1  as viewed from a circumferential direction. 
         FIG. 5  is a view illustrating one example of a detailed configuration of a core  120  according to a preferred embodiment of the present invention. 
         FIG. 6  is a perspective view including a section of the core  120  taken along a cutting line B-B in  FIG. 5 . 
         FIG. 7  is a perspective view illustrating another example of the section of the core  120 . 
         FIG. 8  is a view schematically illustrating an armature for evaluation  200  according to a preferred embodiment of the present invention. 
         FIG. 9  is a view schematically illustrating an armature for comparison  210 . 
         FIG. 10  is a view schematically illustrating a relationship between first recessed portions  51  and crystal grain layers of an electromagnetic steel sheet  122  according to a preferred embodiment of the present invention. 
         FIG. 11  is a view illustrating one example of the detailed configuration of the core  120  according to Preferred Embodiment 2 of the present invention. 
         FIG. 12  is a perspective view including a section of the core  120  taken along a cutting line C-C in  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, preferred embodiments of the present invention are described with reference to the drawings. Although the present specification is described by assuming that a direction parallel or substantially parallel to a center axis J of a motor is an up-down direction for the sake of convenience, a position of the motor in use in the present invention is not limited by the assumption. Also, it is assumed herein that the direction parallel or substantially parallel to the center axis J of the motor is referred to simply as an “axial direction”, and a radial direction and a circumferential direction centering on the center axis J are referred to simply as a “radial direction” and a “circumferential direction”, respectively. It is similarly assumed herein that directions of an armature and a core of the armature corresponding to the axial direction, the radial direction, and the circumferential direction of the motor in a state in which the armature and the core are incorporated in the motor are also referred to as an “axial direction”, a “radial direction”, and a “circumferential direction”, respectively. 
     Preferred Embodiment 1 
       FIG. 1  is a view illustrating one configuration example of a motor according to one exemplary preferred embodiment of the present invention, and shows a section taken along a plane including the center axis J. The motor is preferably used, for example, as a driving source of a driving apparatus for home electric appliances, business machines, medical instruments, automobiles or the like. The motor preferably includes a stator portion that is fixed to a frame body of the driving apparatus, and a rotor portion that is rotatably supported by the stator portion. The rotor portion includes a shaft  10  and a rotor  11 . On the other hand, the stator portion preferably includes a stator  12 , a circuit board  13 , bearings  14 , a bracket  15 , and a spacer  16 . In the following, the respective components are described in detail. 
     The shaft  10  is a columnar or substantially columnar member that extends in the axial direction (the up-down direction). The shaft  10  is supported by the two bearings  14 , and rotates about the center axis J. 
     The rotor  11  is a member that rotates together with the shaft  10 . The rotor  11  rotates relative to the stator  12 . The rotor  11  preferably includes a rotor holder  110  and a rotor magnet  111 . The rotor holder  110  preferably includes a cylindrical portion  11 C and a bottom plate portion  11 B, and also has a bottomed cylindrical shape including an opening on an axially upper side. The cylindrical portion  11 C has a cylindrical or substantially cylindrical shape, and is located on a radially outer side of the stator  12 . The bottom plate portion  11 B is a planar member that extends radially inward from a lower end of the cylindrical portion  11 C. The bottom plate portion  11 B is located below the stator  12  and fixed to the shaft  10 . The rotor magnet  111  is a magnet that is fixed to an inner circumferential surface of the cylindrical portion  11 C of the rotor holder  110 . A pole surface facing the stator  12  is provided on an inner circumferential surface of the rotor magnet  111 . The pole surface is magnetized such that an N pole region and an S pole region are alternately defined in the circumferential direction. 
     The stator  12  is an armature of the motor. The stator  12  preferably has an annular or substantially annular ring shape, and is fixed to the bracket  15 . The stator  12  is also located on a radially inner side of the rotor  11 . An outer circumferential surface of the stator  12  radially faces the rotor magnet  111  with a gap therebetween. 
     The circuit board  13  is a board on which an electronic circuit (not shown) that supplies a drive current to a coil  121  is mounted. The circuit board  13  is preferably provided by a planar body having a circular or substantially circular shape. The circuit board  13  is located on an axially upper side of the rotor  11 , and faces the axially-upper opening of the rotor holder  110 . The circuit board  13  also preferably includes a through hole corresponding to the bracket  15 . 
     The bearings  14  are members that rotatably support the shaft  10 . For example, a ball bearing is preferably used; however, any other desirable type of bearing could be used instead. Both of the two bearings  14  are preferably fixed to the shaft  10  by, for example, press fitting, and are loosely fitted to the bracket  15 . The bearings  14  are accommodated in the bracket  15  by interposing the spacer  16  fixed within the bracket  15  between the bearings  14  and the bracket  15 . 
     The bracket  15  is a bracket holding portion that accommodates the bearings  14 , and is preferably obtained by pressing a metal sheet such as a galvanized steel sheet. The bracket  15  is preferably defined by a cylindrical portion  15 C that is fixed to an inner circumferential surface of the stator  12  by press fitting, and a flange portion  15 F that extends radially outward from an upper end of the cylindrical portion  15 C. The spacer  16  having a cylindrical or substantially cylindrical shape is accommodated in the cylindrical portion  15 C. By press-fitting the spacer  16  into the bracket  15 , the two bearings  14  are supported by the bracket  15 . 
       FIGS. 2 to 4  are views illustrating a detailed configuration of the stator  12  in  FIG. 1 .  FIG. 2  is a sectional view illustrating a section of the stator  12  taken along a cutting line A-A in  FIG. 1 .  FIGS. 3 and 4  are enlarged views illustrating a portion of the stator  12  in an enlarged scale.  FIG. 3  shows an appearance of a tooth portion  12 T as viewed from the axial direction, and  FIG. 4  shows an appearance of the tooth portion  12 T as viewed from the circumferential direction. 
     A core  120  is preferably a laminate member defined by stacking two or more electromagnetic steel sheets  122  in the axial direction (the up-down direction). For example, 20 silicon steel sheets each having a thickness of about 350 μm may be stacked to obtain the core  120 . Also, the core  120  is defined by the electromagnetic steel sheets  122  having the same thickness and the same planar shape. Therefore, the planar shape of the electromagnetic steel sheets  122  corresponds to that of the core  120 . The planar shape of the core  120  is preferably defined by a ring-shaped core back portion  12 B, a plurality of tooth portions  12 T that extend radially outward from an outer circumferential edge of the core back portion  12 B, and a plurality of umbrella portions  12 U that are located at radially outer ends of the tooth portions  12 T, respectively. 
     The core back portion  12 B extends in the circumferential direction. The core back portion  12 B shown in  FIGS. 2 to 4  has an annular ring shape. The center of the annular ring shape corresponds to the center axis J. A radial width of the annular ring shape is preferably constant or substantially constant. An inner circumferential surface of the annular ring shape faces an outer circumferential surface of the cylindrical portion  15 C of the bracket  15 . 
     Each of the tooth portions  12 T extends in the radial direction. The core back portion  12 B is located at one end of the tooth portion  12 T, and the umbrella portion  12 U is preferably located at the other end of the tooth portion  12 T. The tooth portion  12 T shown in  FIGS. 2 to 4  is located on the radially outer side of the core back portion  12 B. That is, the core back portion  12 B is located at a radially inner end of the tooth portion  12 T, and the umbrella portion  12 U is located at a radially outer end of the tooth portion  12 T. The tooth portion  12 T also has a rectangular or substantially rectangular shape that linearly extends in the radial direction while maintaining a constant circumferential width. 
     Each of the umbrella portions  12 U is located at one end of the tooth portion  12 T, and extends in the circumferential direction. A circumferential width of the umbrella portion  12 U is larger than that of the tooth portion  12 T. A radial length of the umbrella portion  12 U is smaller than that of the tooth portion  12 T. The umbrella portion  12 U shown in  FIGS. 2 to 4  is provided at the radially outer end of the tooth portion  12 T. An outer circumferential surface of the umbrella portion  12 U faces an inner circumferential surface of the rotor  11 . Therefore, a magnetic flux produced by the rotor  11  partly enters the core  120  through the outer circumferential surface of the umbrella portion  12 U, and generates a magnetic field in the radial or substantially radial direction in the umbrella portion  12 U. 
     The coil  121  is preferably defined by a conductive wire wound around the core  120 . When an electric drive current is passed through the conductive wire, a magnetic flux is generated in the core  120  as a magnetic core. Therefore, circumferential torque is generated between the umbrella portion  12 U and the rotor magnet  111 , and the shaft  10  is caused to rotate about the center axis J. The conductive wire of the coil  121  is circumferentially wound around the tooth portion  12 T. Thus, a radial magnetic flux is generated in the tooth portion  12 T according to a change amount of the drive current. The magnetic flux flows within the umbrella portion  12 U and the core back portion  12 B located at the both ends of the tooth portion  12 T. 
     Note that the conductive wire of the coil  121  is wound around the laminate formed by stacking the two or more electromagnetic steel sheets  122 . Although the coil  121  is wound around the tooth portion  12 T by interposing an insulator, for example, made of electrically insulating resin between the coil  121  and the tooth portion  12 T so as to prevent electrical conduction with the core  120 , the description is omitted in the present preferred embodiment for the sake of convenience. Note that electrical insulation may be effected by powder coating in which an insulating powder film is provided on the core  120  instead of using the insulator. 
     When a magnetic flux density in the core  120  changes, eddy currents flow in the core  120 . The eddy currents are currents which flow in a loop in a plane perpendicular to the magnetic flux, and flow in a direction to oppose the change in the magnetic flux density. Since the respective electromagnetic steel sheets  122  defining the core  120  are insulated from each other, a path for the eddy currents is provided in each of the electromagnetic steel sheets  122 , and is not provided across the separate electromagnetic steel sheets  122 . That is, the path for the eddy currents is defined in a region within each of the electromagnetic steel sheets  122 , and in a plane perpendicular to the magnetic flux. 
       FIGS. 5 and 6  are views illustrating one example of a detailed configuration of the core  120 .  FIG. 5  is an enlarged view illustrating the core  120  in a state in which the coil  121  is not attached, and shows an appearance as viewed from the axial direction similarly to  FIG. 3 .  FIG. 6  is a perspective view including a section of the core  120  taken along a cutting line B-B in  FIG. 5 . 
     Recessed portions  51  to  53  are preferably provided in a surface of each of the electromagnetic steel sheets  122  constituting the core  120 . The recessed portions  51  to  53  are preferably defined in each of the tooth portion  12 T, the umbrella portion  12 U, and the core back portion  12 B of the electromagnetic steel sheet  122 , respectively. A depth d of the recessed portions  51  to  53  is smaller than a thickness h of the electromagnetic steel sheet  122 . The recessed portions  51  to  53  do not axially penetrate all the way through the electromagnetic steel sheet  122 . For example, the recessed portions  51  to  53 , the depth d of which preferably is about 100 μm, are defined in the electromagnetic steel sheet  122 , the thickness h of which preferably is about 350 μm. The recessed portions  51  to  53  also have an elongated opening in the surface of the electromagnetic steel sheet  122 . In the above description, “elongated” means that a length in a direction in which the recessed portion extends is larger than a groove width of the recessed portion. That is, the recessed portions  51  to  53  are defined as groove portions of the electromagnetic steel sheet  122 . 
     The eddy currents predominantly flow in the vicinity of the surface of the electromagnetic steel sheet  122 . Therefore, the path for the eddy currents is caused to meander by forming the recessed portions  51  to  53  in the electromagnetic steel sheet  122 , so that a length of the path is extended. As a result, an electrical resistance R in the path will be increased, which causes a corresponding decrease in an eddy current I (this is because voltage V=IR. Since V remains constant, if R is increased, I must be decreased). An eddy current loss is a loss by Joule heat generated by the flow of the eddy currents, and is generally provided as R×I 2 . Therefore, when the recessed portions  51  to  53  are provided in the electromagnetic steel sheet  122 , the eddy current loss is significantly reduced or minimized, and efficiency of the motor is improved. Moreover, as compared to a case in which a slit penetrating the electromagnetic steel sheet  122  is provided, a mechanical strength of the electromagnetic steel sheet  122  is ensured. 
     The recessed portions  51  to  53  preferably have a shape extending in the same or substantially the same direction as a direction of the magnetic flux flowing in the electromagnetic steel sheet  122 . Since the eddy currents flow in the plane perpendicular to the magnetic flux, the recessed portions  51  to  53  are crossed with the eddy currents when viewed in plan view from the axial direction by matching or substantially matching the extension direction of the recessed portions  51  to  53  with the direction of the magnetic flux. The generation of the eddy currents is thus effectively significantly reduced or minimized. Also, when each of the recessed portions  51  to  53  includes three or more recessed portions that are disposed parallel or substantially parallel to each other at the same or substantially the same interval between the adjacent recessed portions, the eddy currents are more effectively reduced or minimized. 
     The recessed portions  51  to  53  are preferably formed, for example, by irradiating the electromagnetic steel sheet  122  with laser light. Since the core  120  is the laminate of the two or more electromagnetic steel sheets  122 , a variation in the core  120  may be increased when slight distortion occurs in each of the electromagnetic steel sheets  122 . Thus, it is desirable to use a short pulse laser to machine the recessed portions  51  to  53 . Particularly, by using a picosecond pulse laser or a femtosecond pulse laser, it is possible to prevent an occurrence of distortion in the electromagnetic steel sheet  122  at the time of machining the recessed portions  51  to  53 . 
     It is desirable that the width of the recessed portions  51  to  53  be as small as possible in view of ensuring the mechanical strength of the electromagnetic steel sheet  122  since the width of the recessed portions  51  to  53  hardly affects the eddy current loss. Thus, an axial sectional shape of the recessed portions  51  to  53  is preferably provided as a V shape where the width becomes smaller with distance from the surface of the electromagnetic steel sheet  122 . Particularly, it is desirable that an opening width w of the recessed portions  51  to  53  is smaller than the depth d, and the sectional shape of the recessed portions  51  to  53  is defined as a V shape. For example, a section of the recessed portions  51  to  53  is defined as a V shape in which the depth d=100 μm and the opening width w=20 μm. That is, the depth d of a first recessed portion or a second recessed portion described below is larger than the circumferential width w. It is also desirable to arrange the recessed portions  51  to  53  so as to have a volume ratio of about 1% or less to the electromagnetic steel sheet  122 , for example. 
     Each of the recessed portions  51  to  53  can be defined in an upper surface and a lower surface facing each other of the electromagnetic steel sheet  122 . In the present specification, the recessed portions  51  to  53  defined in the upper surface of the electromagnetic steel sheet  122  are referred to as a first upper recessed portion, a second upper recessed portion, and a third upper recessed portion, respectively, and the recessed portions  51  to  53  defined in the lower surface of the electromagnetic steel sheet  122  are referred to as a first lower recessed portion, a second lower recessed portion, and a third lower recessed portion, respectively. Although the example in which each of the recessed portions  51  to  53  is provided in the upper surface and the lower surface of the electromagnetic steel sheet is described in the present preferred embodiment, the recessed portions  51  to  53  may be provided in at least one of the upper surface and the lower surface of the electromagnetic steel sheet  122 . 
     The first recessed portion  51  is a recessed portion that is defined in the tooth portion  12 T of the electromagnetic steel sheet  122 . The first recessed portion  51  preferably includes an opening in the surface of the electromagnetic steel sheet  122 . In the opening, a circumferential width is smaller than a radial length, and the opening extends in the radial direction. That is, the first recessed portion  51  has a groove shape extending in the radial direction. Also, the two or more first recessed portions  51  are disposed in the circumferential direction in the tooth portion  12 T. 
     The magnetic flux radially flows in the tooth portion  12 T. Therefore, by radially extending the first recessed portion  51 , the first recessed portion  51  is crossed with the eddy currents when viewed in plan view from the axial direction, so that the eddy current loss is significantly reduced or minimized. Preferably, by disposing the two or more first recessed portions  51  in the circumferential direction, the eddy currents are further reduced. Moreover, by disposing the three or more first recessed portions  51  at the same or substantially the same interval, the eddy currents are more effectively significantly reduced or minimized. 
     First upper recessed portions  51 A in  FIG. 6  are the first recessed portions  51  provided in the upper surface of the electromagnetic steel sheet  122 , and first lower recessed portions  51 B in  FIG. 6  are the first recessed portions  51  provided in the lower surface of the electromagnetic steel sheet  122 . That is, the first recessed portions  51  are defined in the upper surface and the lower surface of the electromagnetic steel sheet  122 . 
     When the first upper recessed portions  51 A and the first lower recessed portions  51 B are located at different positions, the mechanical strength of the electromagnetic steel sheet  122  is further increased, and the length of the path for the eddy currents is further extended as compared to a case in which the first upper recessed portions  51 A and the first lower recessed portions  51 B are located at the same positions. Therefore, it is desirable that the first upper recessed portions  51 A and the first lower recessed portions  51 B are located at circumferentially different positions. For example, it is desirable that at least one of the first upper recessed portions  51 A is arranged between the two first lower recessed portions  51 B adjacent to each other in the circumferential direction, or at least one of the first lower recessed portions  51 B is arranged between the two first upper recessed portions  51 A adjacent to each other in the circumferential direction. Therefore, at least one of the first recessed portions located in the upper surface or the lower surface of the electromagnetic steel sheet is located between the two adjacent first recessed portions located in the surface on the axially opposite side. Furthermore, it is more desirable that all of the first upper recessed portions  51 A and all of the first lower recessed portions  51 B are located at circumferentially different positions. That is, the first recessed portions are alternately located in the upper surface and the lower surface of the electromagnetic steel sheet in the circumferential direction. It is particularly desirable that the first upper recessed portions  51 A and the first lower recessed portions  51 B are alternately defined in the circumferential direction as shown in  FIG. 6 . 
     Furthermore, it is desirable that each of the first upper recessed portions  51 A is located in or substantially in the center between the two first lower recessed portions  51 B adjacent to each other, and each of the first lower recessed portions  51 B is located in or substantially in the center between the two first upper recessed portions  51 A adjacent to each other. It is also desirable that an interval s between the first recessed portions  51  is smaller than a winding pitch p of the conductive wire constituting the coil  121 . The interval s between the first recessed portions  51  is a distance between opening edge portions of the first recessed portions  51  adjacent to each other. The winding pitch p is a distance between the centers of the conductive wires adjacent to each other. 
       FIG. 7  is a view illustrating another example of the detailed configuration of the core  120 .  FIG. 7  is a perspective view including a section of the core  120  taken along a cutting line B-B in  FIG. 5  similarly to  FIG. 6 . In  FIG. 7 , the first upper recessed portions  51 A and the first lower recessed portions  51 B are located at circumferentially different positions. The first upper recessed portions  51 A are arranged between the two first lower recessed portions  51 B adjacent to each other, and the first lower recessed portions  51 B are arranged between the two first upper recessed portions  51 A adjacent to each other. That is, although the first upper recessed portions  51 A and the first lower recessed portions  51 B are not alternately defined in the circumferential direction, a combination of the two first upper recessed portions  51 A adjacent to each other and a combination of the two first lower recessed portions  51 B adjacent to each other are alternately defined in the circumferential direction. 
     The second recessed portion  52  is preferably a recessed portion that is defined in the umbrella portion  12 U of the electromagnetic steel sheet  122 . The second recessed portion  52  preferably includes an opening in the surface of the electromagnetic steel sheet  122 . In the opening, a circumferential width is smaller than a radial length, and the opening extends in the radial or substantially radial direction. That is, the second recessed portion  52  has a groove shape extending in the radial or substantially radial direction. Also, the two or more second recessed portions  52  are disposed in the circumferential direction in the umbrella portion  12 U. 
     The second recessed portion  52  is arranged so as to be matched or substantially matched with the direction of the magnetic flux flowing in the umbrella portion  12 U. By matching or substantially matching the extension direction of the second recessed portion  52  with the direction of the magnetic flux, the second recessed portion  52  is crossed with the eddy currents when viewed in plan view from the axial direction, so that the eddy current loss is effectively significantly reduced or minimized. Also, by disposing the two or more second recessed portions  52  in the circumferential direction, the eddy currents are further reduced. 
     While the second recessed portions  52  extend in the radial or substantially radial direction, an interval between the adjacent second recessed portions  52  is increased toward the radially outer side. To be more specific, the second recessed portions  52  extend linearly in the radial direction at a position around the circumferential center of the umbrella portion  12 U. On the other hand, the interval between the adjacent second recessed portions  52  is increased toward the rotor  11  at a position around opposite circumferential ends. That is, the two or more second recessed portions are located in the surface of the umbrella portion, and the interval between the adjacent second recessed portions is increased toward the rotor. 
     The second recessed portions  52  are provided in the upper surface and the lower surface of the electromagnetic steel sheet  122 . It is desirable that second upper recessed portions defined in the upper surface and second lower recessed portions defined in the lower surface are located at different positions from each other. For example, at least one of the second upper recessed portions is arranged between the two second lower recessed portions adjacent to each other in the circumferential direction, or at least one of the second lower recessed portions is arranged between the two second upper recessed portions adjacent to each other in the circumferential direction. Also, it is more desirable that all of the second upper recessed portions and all of the second lower recessed portions are located at circumferentially different positions. It is particularly desirable that the second upper recessed portions and the second lower recessed portions are alternately defined in the circumferential direction. Furthermore, it is desirable that each of the second upper recessed portions is located in or substantially in the center between the second lower recessed portions adjacent to each other in the circumferential direction, and each of the second lower recessed portions is located in substantially the center between the second upper recessed portions adjacent to each other in the circumferential direction. 
     The umbrella portion  12 U is provided at the radially outer end of the tooth portion  12 T, and faces the rotor  11 . Therefore, the magnetic flux generated by the drive current of the coil  121 , and the magnetic flux generated by the magnet  111  of the rotor  11  exist in the umbrella portion  12 U. Moreover, when magnetic flux densities are compared, a magnetic flux density of the latter one is larger in most cases. Therefore, a larger magnetic field is defined in the umbrella portion  12 U than in the tooth portion  12 T, so that a larger eddy current loss could occur. Thus, it is desirable that the interval between the second recessed portions  52  adjacent to each other is smaller than the interval between the first recessed portions  51  adjacent to each other. For example, it is desirable that a minimum value of the interval between the second recessed portions  52  is smaller than a minimum value of the interval between the first recessed portions  51 . It is also desirable that an average value of the interval between the second recessed portions  52  is smaller than an average value of the interval between the first recessed portions  51 . 
     The third recessed portion  53  is preferably a recessed portion that is defined in the core back portion  12 B. The third recessed portion  53  has an opening extending in the substantially circumferential direction. In the opening, a radial width is smaller than a circumferential length, and the opening extends in the circumferential direction. That is, the third recessed portion  53  has a groove shape extending in the circumferential direction. The magnetic flux circumferentially flows in the core back portion  12 B. Therefore, by circumferentially extending the third recessed portion  53 , the third recessed portion  53  is crossed with the eddy currents when viewed in plan view from the axial direction, so that the eddy current loss is significantly reduced or minimized. 
     By disposing the two or more third recessed portions  53  in the radial direction, the eddy currents are further significantly reduced or minimized. Moreover, by disposing the three or more third recessed portions  53  at the same or substantially the same interval, the eddy currents are more effectively significantly reduced or minimized. 
     The third recessed portions  53  are defined in the upper surface and the lower surface facing each other of the electromagnetic steel sheet  122 . It is desirable that third upper recessed portions defined in the upper surface and third lower recessed portions defined in the lower surface are located at different positions from each other. For example, it is desirable that at least one of the third upper recessed portions is arranged between the third lower recessed portions adjacent to each other in the radial direction, or at least one of the third lower recessed portions is located between the third upper recessed portions adjacent to each other in the radial direction. Also, it is more desirable that all of the third upper recessed portions and all of the third lower recessed portions are located at radially different positions. It is particularly desirable that the third upper recessed portions and the third lower recessed portions are alternately defined in the radial direction. Furthermore, it is desirable that each of the third upper recessed portions is located in or substantially in the center between the third lower recessed portions adjacent to each other in the radial direction, and each of the third lower recessed portions is located in or substantially in the center between the third upper recessed portions adjacent to each other in the radial direction. 
     Simulation Result 
       FIGS. 8 and 9  are views schematically illustrating a simulation result of the eddy current loss.  FIG. 8  shows an armature for evaluation  200  including an electromagnetic steel sheet  122  according to a preferred embodiment of the present invention, and  FIG. 9  shows an armature for comparison  210  including a conventional electromagnetic steel sheet  211  in which no recessed portion  51  is formed. 
     In the armature for evaluation  200  in  FIG. 8 , a conductive wire is wound around the single electromagnetic steel sheet  122 . The electromagnetic steel sheet  122  is the electromagnetic steel sheet shown in  FIG. 6 . The first upper recessed portions  51 A are defined in the upper surface of the electromagnetic steel sheet  122 . The first lower recessed portions  51 B are defined in the lower surface of the electromagnetic steel sheet  122 . The first upper recessed portions  51 A and the first lower recessed portions  51 B are alternately defined in a right-left direction in  FIG. 6 . Also, the thickness h of the electromagnetic steel sheet  122  is 350 μm. In the first upper recessed portions  51 A and the first lower recessed portions  51 B, the depth d is 100 μm, and the opening width w is 20 μm. The first upper recessed portions  51 A and the first lower recessed portions  51 B are formed by emitting laser light from a short pulse laser, and removing approximately 1% of a volume of the electromagnetic steel sheet  122 . 
     In the armature for comparison  210  in  FIG. 9 , a conductive wire is wound around the single electromagnetic steel sheet  211 . The electromagnetic steel sheet  211  is a conventional electromagnetic steel sheet used for a general motor armature, and is the same as the electromagnetic steel sheet  122  according to the present preferred embodiment except that the first upper recessed portions  51 A and the first lower recessed portions  51 B are not provided. 
     When a clockwise drive current  201  flows through the coil  121 , a magnetic field  202  is formed from a near side to a far side perpendicular to the paper surface in each of the electromagnetic steel sheets  122  and  211 . When the magnetic field  202  changes, an eddy current  203  flows to significantly reduce or prevent the change. For example, when the magnetic field  202  increases, the eddy current  203  flows in a counterclockwise direction. 
     Eddy current losses generated in the armature for evaluation  200  and the armature for comparison  210  under the same conditions were obtained by a simulation, and it was discovered that the eddy current loss in the armature for evaluation  200  was reduced by 30% as compared to that in the armature for comparison  210 . 
     The eddy current  203  flows through a loop path defined in each of the electromagnetic steel sheets  122  and  211 . The eddy current  203  also mainly flows in the vicinity of the surface of each of the electromagnetic steel sheets  122  and  211 . Therefore, a path for the eddy current  203  is defined as a smooth curved path extending along the surface of the electromagnetic steel sheet  211  in  FIG. 9 . In contrast, in  FIG. 8 , since the first upper recessed portions  51 A and the first lower recessed portions  51 B are provided in the upper surface and the lower surface of the electromagnetic steel sheet  122 , a flow path for the eddy current  203  meanders so as to avoid the recessed portions, so that a length of the flow path is extended. As a result, the eddy current  203  is reduced, and the eddy current loss is also reduced. 
     The electromagnetic steel sheet  122  according to the present preferred embodiment can also be considered as equivalent to an electromagnetic steel sheet that is shaped so as to have the thickness h of about 350 μm by repetitively bending an electromagnetic steel sheet having the thickness h of about 250 μm, for example. That is, it is possible to ensure a larger mechanical strength while achieving an eddy current loss equal to that of the electromagnetic steel sheet having the thickness of about 250 μm, for example. 
     It is also possible to significantly reduce or prevent the eddy current loss while avoiding an increase in cost caused by decreasing the thickness of the electromagnetic steel sheet  122 . When the thinner electromagnetic steel sheet  122  is used, the number of the electromagnetic steel sheets  122  required to fabricate the core  120  having the same axial length is increased. Thus, if the thickness of the electromagnetic steel sheet  122  is decreased in order to significantly reduce or minimize the eddy current loss, machining man-hours required to fabricate the core  120  is increased. However, since the electromagnetic steel sheet  122  according to the present preferred embodiment significantly reduces or minimizes the eddy current loss without decreasing the thickness of the electromagnetic steel sheet  122 , the eddy current loss is significantly reduced or minimized without notably increasing the cost. 
     Crystal Grain Layers of Preferred Embodiments 
       FIG. 10  is a view schematically illustrating a relationship between the first recessed portions  51  and crystal grain layers of the electromagnetic steel sheet  122 , and shows the section taken along B-B in  FIG. 5  in an enlarged scale. The electromagnetic steel sheet  122  preferably includes a first crystal grain layer  31 , and second crystal grain layers  32 A and  32 B on an upper side and a lower side of the first crystal grain layer  31 . Note that hatching is given to the second crystal grain layers  32 A and  32 B in  FIG. 10 . 
     The first crystal grain layer  31  is a layer including crystal grains that are not exposed through the upper surface and the lower surface of the electromagnetic steel sheet  122  before the recessed portions  51  to  53  are formed. The second crystal grain layers  32 A and  32 B are layers including crystal grains that are exposed from the upper surface or the lower surface of the electromagnetic steel sheet  122 . The crystal grain is a mass of crystals whose crystal orientations are aligned. The crystal grains of the second crystal grain layers  32 A and  32 B have a smaller grain size than the crystal grains of the first crystal grain layer  31 . 
     The first recessed portions  51  are configured so as to reach the first crystal grain layer  31 . That is, the depth d of the first recessed portions  51  is preferably larger than a thickness of the second crystal grain layers  32 A and  32 B. The crystal grains of the first crystal grain layer  31  define a portion of an inner surface of each of the first recessed portions  51 . Since a relationship between each of the second and third recessed portions  52  and  53 , and the crystal grain layers  31 ,  32 A, and  32 B is the same as that of the case of the first recessed portions  51 , the overlapping description is omitted. 
     Preferred Embodiment 2 
     In Preferred Embodiment 1, the example in which the recessed portions  51  to  53  preferably define the groove portions of the electromagnetic steel sheet  122  has been described. In contrast, a case in which each of the recessed portions  51  to  53  is a plurality of micro recessed portions  54  disposed in a line is described in the present preferred embodiment. 
       FIGS. 11 and 12  are views illustrating one example of the detailed configuration of the core  120  according to Preferred Embodiment 2 of the present invention.  FIG. 11  is an enlarged view illustrating a portion of the core  120  in an enlarged scale, and shows an appearance as viewed from the axial direction similarly to  FIG. 5 .  FIG. 12  is a perspective view including a section of the core  120  taken along a cutting line C-C in  FIG. 11 . 
     Each of the recessed portions  51  to  53  preferably includes the plurality of micro recessed portions  54 . The depth d of the micro recessed portions  54  is smaller than the thickness h of the electromagnetic steel sheet  122 . The micro recessed portions  54  do not axially penetrate all the way through the electromagnetic steel sheet  122 . For example, the micro recessed portions  54 , the circumferential width of which is preferably about 20 μm and the depth d of which is preferably about 100 μm, are defined in the electromagnetic steel sheet  122 , the thickness h of which is preferably about 350 μm. A volume of the micro recessed portions is preferably about 1% or less of the volume of the electromagnetic steel sheet. Each of the micro recessed portions  54  has an opening preferably having a circular or oval shape in the surface of the electromagnetic steel sheet  122 . The openings of the plurality of micro recessed portions  54  are disposed in a line on the surface of the electromagnetic steel sheet  122 . The linear recessed portions  51  to  53  are defined as the lines of the micro recessed portions  54  in the surface of the electromagnetic steel sheet  122 . 
     The micro recessed portions  54  are formed preferably by irradiating the electromagnetic steel sheet  122  with laser light. For example, a short pulse laser can be used. Particularly, by using a picosecond pulse laser or a femtosecond pulse laser, it is possible to prevent the occurrence of distortion in the electromagnetic steel sheet  122  at the time of machining the micro recessed portions  54 . 
     It is desirable that the opening width w of the micro recessed portions  54  is smaller than the depth d, and a section of the micro recessed portions  54  has a V shape. For example, a sectional shape of the micro recessed portions  54  is defined as a V shape in which the depth d=100 μm and the opening width w=20 μm. It is also desirable to form the micro recessed portions  54  by removing a volume ratio of about 1% or less to the electromagnetic steel sheet  122  in view of ensuring the mechanical strength. The opening width w is an opening length of the micro recessed portions  54  in a width direction of the recessed portions  51  to  53 . 
     The first recessed portion  51  is a recessed portion that is defined in the tooth portion  12 T of the electromagnetic steel sheet  122 , and is defined by the plurality of micro recessed portions  54 . By disposing the micro recessed portions  54  in a line on the surface of the electromagnetic steel sheet  122 , the linear first recessed portion  51  extending in the radial direction is formed. 
     The first upper recessed portion  51 A in  FIG. 12  is the first recessed portion  51  provided in the upper surface of the electromagnetic steel sheet  122 , and is defined by the plurality of micro recessed portions  54  defined in the upper surface of the electromagnetic steel sheet  122 . Similarly, the first lower recessed portion  51 B in  FIG. 12  is the first recessed portion  51  provided in the lower surface of the electromagnetic steel sheet  122 , and is defined by the plurality of micro recessed portions  54  defined in the lower surface of the electromagnetic steel sheet  122 . 
     Although examples of an outer rotor type motor have been described in the above preferred embodiments, the present invention is not limited to the motor as described above. That is, various preferred embodiments of the present invention can be applied to a motor including an armature in which the core  120  is defined by stacking the electromagnetic steel sheets  122 , the tooth portion  12 T of the core  120  extends in the radial direction, and the conductive wire is circumferentially wound around the tooth portion  12 T. 
     For example, the preferred embodiments of the present invention can be applied to an inner rotor type motor in which the rotor  11  is located on the radially inner side of the stator  12 . In the case of the inner rotor type motor, the tooth portion  12 T is located on the radially inner side of the core back portion  12 B. That is, the core back portion  12 B is located at the radially outer end of the tooth portion  12 T, and the umbrella portion  12 U is located at the radially inner end of the tooth portion  12 T. 
     Also, although the example of the brushless type motor in which the stator  12  is defined by the armature has been described in the above preferred embodiments, the present invention is not limited to the motor as described above. That is, preferred embodiments of the present invention can be applied to a motor with a brush in which the rotor is defined by the armature. 
     The core  120  according to the preferred embodiments of the present invention may be a straight core (developed core) in which the core back portion  12 B is provided with a ring shape by folding the core after attaching the coil  121  thereto. The core  120  according to the preferred embodiments of the present invention may be a core not including the umbrella portion  12 U. The core  120  according to the preferred embodiments of the present invention may be also defined by two or more split cores. That is, the core  120  may be formed by connecting two or more split cores to which the coil is previously attached. 
     The example of the case in which the core back portion  12 B has an annular ring shape has been described in the above preferred embodiments of the present invention. However, as long as the core back portion  12 B extends in the circumferential direction, the core back portion  12 B may have another shape. For example, the core back portion  12 B may have a ring shape having polygonal inner edges or outer edges. Also, the width of the core back portion  12 B may not be constant. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.