Patent Publication Number: US-2021167641-A1

Title: Core, stator, and rotating electrical machine

Description:
TECHNICAL FIELD 
     The present disclosure relates to a core, a stator, and a rotating electrical machine. 
     The present application claims priority to Japanese Patent Application No. 2018-141841 filed on Jul. 27, 2018, and the entire contents of the Japanese patent application are incorporated herein by reference. 
     BACKGROUND ART 
     Patent Literature 1 discloses an axial-gap rotating electrical machine (electric motor or electric generator) in which a rotor and a stator are disposed to face each other in a direction along the rotating shaft of the rotor. The stator used for the rotating electrical machine includes an armature core (core) having a back yoke (yoke) and a plurality of teeth, and a coil disposed at each of the teeth. The yoke is a circular-ring-shaped member. Each of the teeth is a block-shaped member protruding from the yoke in the direction of the rotating shaft of the rotor. 
     The armature core of Patent Literature 1 is constituted by coupling the teeth and the yoke, which have been fabricated separately, to each other. More specifically, the yoke and the teeth are coupled to each other by fitting a columnar protruding portion provided at each of the teeth to a recessed portion (through hole or recess) provided in the yoke. Moreover, in Patent Literature 1, a multilayer steel sheet constitutes the yoke, and a dust core (powder compact) constitutes the teeth. 
     CITATION LIST 
     Patent Literature 
     PTL 1: International Publication No. 2007/114079 
     SUMMARY OF INVENTION 
     A core according to the present disclosure is 
     a core included in a rotor or a stator of an axial-gap rotating electrical machine, in which 
     the core includes a block-shaped first member and a plate-shaped second member that are constituted by powder compacts, 
     the first member includes a first surface that faces the second member, and a first coupling portion that is formed at the first surface, 
     the second member includes a second surface that faces the first surface, and a second coupling portion that is formed at the second surface and that is coupled to the first coupling portion, 
     one of the first coupling portion and the second coupling portion is constituted by a protrusion, and the other one is constituted by a recess having a shape corresponding to the protrusion, and 
     a front shape of the first coupling portion when seen in a direction orthogonal to the first surface and a front shape of the second coupling portion when seen in a direction orthogonal to the second surface are ring shapes or discontinuous ring shapes that are partly discontinuous. 
     A stator according to the present disclosure includes 
     the core according to the present disclosure; and 
     a coil that is disposed at each tooth included in the core. 
     A rotating electrical machine according to the present disclosure is 
     an axial-gap rotating electrical machine in which a rotor and a stator are disposed side by side in an axial direction of a rotating shaft of the rotor, in which 
     the stator is the stator according to the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a top view of a stator included in an axial-gap rotating electrical machine according to a first embodiment. 
         FIG. 2  is a perspective view illustrating a portion of the stator in  FIG. 1 . 
         FIG. 3  is a sectional view taken along line in  FIG. 2 . 
         FIG. 4  is a bottom view of a tooth included in the stator in  FIG. 1 . 
         FIG. 5A  is a sectional view taken along line V-V in  FIG. 4 . 
         FIG. 5B  is an enlarged view of a portion circled in  FIG. 5A . 
         FIG. 6  is a sectional view taken along line VI-VI in  FIG. 4 . 
         FIG. 7  is a top view of a yoke included in the stator in  FIG. 1 . 
         FIG. 8A  is a sectional view taken along line VIII-VIII in  FIG. 7 . 
         FIG. 8B  is an enlarged view of a portion circled in  FIG. 8A . 
         FIG. 9  is a sectional view taken along line IX-IX in  FIG. 7 . 
         FIG. 10  is a fragmentary vertical sectional view of the axial-gap rotating electrical machine of the first embodiment. 
         FIG. 11  is a fragmentary sectional view describing the coupling state of a tooth and a yoke of a rotating electrical machine according to a second embodiment. 
         FIG. 12A  is a fragmentary sectional view describing the coupling state of a tooth and a yoke of a rotating electrical machine according to a third embodiment. 
         FIG. 12B  is an enlarged view of a portion circled in  FIG. 12A . 
         FIG. 13A  is a fragmentary sectional view describing the coupling state of a tooth and a yoke of a rotating electrical machine according to a fourth embodiment. 
         FIG. 13B  is an enlarged view of a portion circled in  FIG. 13A . 
         FIG. 14  is a fragmentary sectional view describing the coupling state of a tooth and a plate-shaped piece of a rotating electrical machine according to a fifth embodiment. 
         FIG. 15  is a fragmentary sectional view describing the coupling state of a tooth and a plate-shaped piece of a rotating electrical machine according to a sixth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Problems to be Solved by Present Disclosure 
     It is desired to improve the magnetic characteristics of a core and to improve the productivity of the core. To attain this, it is considered effective that both a yoke and teeth are constituted by high-density powder compacts. To increase the density of a powder compact, a soft magnetic powder needs to be compressed and molded under a high molding pressure (surface pressure). However, as a result of the studies by the inventors, it has been found that the yoke and the teeth having the shapes described in Patent Literature 1 are likely to have a portion with a partly low density. It has been also found that the yoke and the teeth having the shapes described in Patent Literature 1 may not be completed by one-time compression molding. 
     An object of the present disclosure is to provide a core of a powder compact having a high density as a whole and being excellent in productivity. In addition, another object of the present disclosure is to provide a stator including the above-described core. Furthermore, still another object of the present disclosure is to provide a rotating electrical machine including the above-described stator. 
     Advantageous Effects of Present Disclosure 
     The core of the present disclosure has a high density as a whole and being excellent in productivity. Moreover, the stator of the present disclosure is excellent in magnetic characteristics and productivity. Furthermore, the rotating electrical machine of the present disclosure is excellent in output characteristics and productivity. 
     Description of Embodiments of Present Disclosure 
     The inventors have studied problems expected when a block-shaped tooth and a plate-shaped yoke are each formed of a powder compact. Consequently, it has been found that a situation in which the recessed portion and the protruding portion that couple the tooth and the yoke to each other have simple shapes as described in Patent Literature 1 leads to a problem. When the tooth having the protruding portion is fabricated by compression molding, the soft magnetic powder hardly reaches a portion that becomes a corner of the protruding portion, and the density of the protruding portion (in particular, the corner of the protruding portion) is likely to be lower than that of the other portion. When the density of the protruding portion is low, the strength of the protruding portion decreases, and the protruding portion is likely to be broken. Thus, the tooth and the yoke may be no longer coupled to each other. Moreover, when the yoke having the recessed portion is fabricated by compression molding, the soft magnetic powder hardly reaches a portion that becomes an edge of the recessed portion. Thus, to fabricate the yoke having the recessed portion, compression molding to form the whole shape of the yoke is performed, and then compression molding to complete the recessed portion has to be performed. That is, it is not able to complete the yoke having the recessed portion by one-time compression molding. Regarding these problems, the inventors have found that the above-described problems can be solved by forming the recessed portion and the protruding portion into specific shapes. 
     Based on the above-described findings, embodiments of the present disclosure are listed below. 
     &lt;1&gt; A core according to an embodiment is a core included in a rotor or a stator of an axial-gap rotating electrical machine, in which 
     the core includes a block-shaped first member and a plate-shaped second member that are constituted by powder compacts, 
     the first member includes a first surface that faces the second member, and a first coupling portion that is formed at the first surface, 
     the second member includes a second surface that faces the first surface, and a second coupling portion that is formed at the second surface and that is coupled to the first coupling portion, 
     one of the first coupling portion and the second coupling portion is constituted by a protrusion, and the other one is constituted by a recess having a shape corresponding to the protrusion, and 
     a front shape of the first coupling portion when seen in a direction orthogonal to the first surface and a front shape of the second coupling portion when seen in a direction orthogonal to the second surface are ring shapes or discontinuous ring shapes that are partly discontinuous. 
     The above-described core has almost no portion with a locally low density of the soft magnetic powder, and hence the core has a high density as a whole. A portion with a locally low density is not formed in the core because the front shapes of the protrusion and the recess that constitute the first coupling portion and the second coupling portion are the ring shapes or the discontinuous ring shapes (hereinafter, both the ring shapes and the discontinuous ring shapes may be collectively merely referred to as ring shapes). When the protrusion has the ring shape, a difference in height appears between the protrusion, and an inner portion and an outer portion of the ring shape of the protrusion. Due to the difference in height, the soft magnetic powder easily flows between the protrusion, and the inner portion and the outer portion that are lower than the protrusion during compression molding. Moreover, when the recess has the ring shape, a difference in height appears between the recess, and an inner portion and an outer portion of the ring shape of the recess. Due to the difference in height, the soft magnetic powder easily flows between the recess, and the inner portion and the outer portion that are higher than the recess during compression molding. As long as the fluidity of the soft magnetic powder in the vicinities of the protrusion and the recess is improved, a decrease in density in the vicinities can be suppressed. 
     The above-described core is excellent in productivity. As described above, this is because the fluidity of the soft magnetic powder is high in the vicinities of the protrusion and the recess during compression molding of the core, and the core with a high density as a whole can be fabricated by one-time compression molding. Moreover, since the fluidity of the soft magnetic powder is high, the rate of occurrence of a defective part having a portion with a locally low density decreases. 
     &lt;2&gt; As an aspect of the core according to the embodiment, there may be provided an aspect, in which 
     the first member is a tooth, and 
     the second member is a yoke. 
     When the tooth and the yoke are separate members, a metal mold for fabricating the core can be formed in a simple shape. Thus, the densities of the tooth and the yoke are easily made uniform. Moreover, a plurality of teeth included in the core can be fabricated using a metal mold. Thus, the productivity of the core can be improved. 
     &lt;3&gt; As an aspect of the core according to the embodiment, there may be provided an aspect, in which 
     the first member is a tooth having a flange portion, and 
     the second member is a yoke. 
     When the flange portion is provided at an end surface of the tooth, detachment of the coil disposed at the tooth from the tooth hardly occurs. 
     &lt;4&gt; As an aspect of the core according to the embodiment, there may be provided an aspect, in which 
     the first member is an integral object of a tooth and a yoke, 
     the second member is a plate-shaped piece that is provided separately from the yoke, and 
     the plate-shaped piece is disposed on an end surface of the tooth on a side opposite to the yoke and includes a flange portion that protrudes from an outline of the end surface. 
     When the flange portion is provided at the end surface of the tooth, detachment of the coil disposed at the tooth from the tooth hardly occurs. When the plate-shaped piece including the flange portion and the tooth are separate members, the flange portion can be formed after the coil is disposed at the tooth. Thus, the coil can be easily disposed at the core. 
     &lt;5&gt; As an aspect of the core according to the embodiment, there may be provided an aspect, in which 
     the first member is a tooth, and 
     the second member is a yoke and a plate-shaped piece that is provided separately from the yoke, and 
     the plate-shaped piece is disposed on an end surface of the tooth on a side opposite to the yoke and includes a flange portion that protrudes from an outline of the end surface. 
     This configuration is, namely, a configuration in which the tooth, the yoke, and the plate-shaped member having the flange portion are separate components. With this configuration, a variation in density of the components constituted by powder compacts can be suppressed. 
     &lt;6&gt; As an aspect of the core according to the embodiment, there may be provided an aspect, in which 
     the front shape of the first coupling portion and the front shape of the second coupling portion are non-circular-ring shapes. 
     With the above-described configuration, after the first member and the second member are coupled to each other, relative rotation of both members on the first surface (second surface) can be suppressed. 
     &lt;7&gt; As an aspect of the core according to the embodiment, there may be provided an aspect, in which 
     a joint between a bottom surface and an inner wall surface of the recess and a joint between the inner wall surface and the first surface or the second surface of the recess are rounded, and 
     a joint between a top surface and an outer wall surface of the protrusion and a joint between the outer wall surface and the first surface or the second surface of the protrusion are rounded. 
     Rounding both the above-described joints can improve the fluidity of the soft magnetic powder near the protrusion and the recess during compression molding. 
     &lt;8&gt; As an aspect of the core according to the embodiment, there may be provided an aspect, in which 
     a depth of the recess and a height of the protrusion are 0.5 mm or more and 30% or less of a thickness of one of the first member and the second member having a smaller thickness. 
     When the depth of the recess and the height of the protrusion are 0.5 mm or more, the coupling strength between the protrusion and the recess can be sufficiently secured. Moreover, when the depth of the recess and the height of the protrusion are 30% or less of the thickness of the one of the first member and the second member having the smaller thickness, a decrease in fluidity of the soft magnetic powder and a burden on the metal mold during compression molding can be suppressed. 
     &lt;9&gt; As an aspect of the core according to the embodiment, there may be provided an aspect, in which 
     a width of the recess and a width of the protrusion are 0.5 mm or more and 10 mm or less. 
     When the width of the recess and the width of the protrusion are 0.5 mm or more, the coupling strength between the protrusion and the recess can be sufficiently secured. In particular, when the width of the protrusion is 1.0 mm or more, the mechanical strength of the protrusion can be sufficiently secured. Moreover, when the width of the recess and the width of the protrusion are 10 mm or less, a decrease in fluidity of the soft magnetic powder during compression molding can be suppressed. 
     &lt;10&gt; As an aspect of the core according to the embodiment, there may be provided an aspect, in which 
     a joint between a bottom surface and an inner wall surface of the recess and a joint between the inner wall surface and the first surface or the second surface of the recess are rounded, 
     a joint between a top surface and an outer wall surface of the protrusion and a joint between the outer wall surface and the first surface or the second surface of the protrusion are rounded, and 
     a radius of curvature of roundness of each of the joints is 0.5 mm or more and 4.0 mm or less. 
     Rounding the joints between surfaces in the protrusion and the recess can improve the fluidity of the soft magnetic powder during compression molding of the first member and the second member. Consequently, a portion with a locally low density hardly appears in the first member and the second member. In particular, when the radii of curvature of the roundness of the joints are 0.5 mm or more and 4.0 mm or less, the fluidity is easily improved. 
     &lt;11&gt; As an aspect of the core according to the embodiment, there may be provided an aspect, in which 
     the first coupling portion is a recess, and the second coupling portion is a protrusion. 
     The second member with the second coupling portion formed has a plate shape. Thus, when the second coupling portion is the protrusion, the mechanical strength of the second member as a whole can be improved. 
     &lt;12&gt; As an aspect of the core according to the embodiment, there may be provided an aspect, in which 
     the first coupling portion is a protrusion, and the second coupling portion is a recess. 
     The second member with the second coupling portion formed has a plate shape. Thus, when the second coupling portion is the recess, workability of assembly is improved when the first member and the second member are bonded to each other. 
     &lt;13&gt; As an aspect of the core according to the embodiment, there may be provided an aspect, in which 
     the front shape of the first coupling portion and the front shape of the second coupling portion each are a racetrack shape or any shape selected from a triangle, a rectangle, a trapezoid, and a rhombus whose vertices are rounded. 
     With the above-described shapes, the front shapes of the first coupling portion and the second coupling portion do not become excessively complicated. Thus, the mechanical strength of the first coupling portion and the second coupling portion is hardly decreased. Moreover, the above-described configuration has an advantage that the metal molds for fabricating the first member and the second member are easily fabricated. 
     &lt;14&gt; As an aspect of the core according to the embodiment, there may be provided an aspect, in which 
     when an area of the first surface is 100%, an area on an inner side with respect to an outline of an outer periphery of the first coupling portion is 10% or more and 80% or less, and 
     when an area of the second surface is 100%, an area on an inner side with respect to an outline of an outer periphery of the second coupling portion is 10% or more and 80% or less. 
     The area of the first surface (second surface) is the planar area of the first surface (second surface) when seen in the direction orthogonal to the first surface (second surface). That is, the area of the first surface (second surface) includes the area on the inner side with respect to the outline of the outer periphery of the first coupling portion (second coupling portion). With the above-described configuration, the first member including the first coupling portion and the second member including the second coupling portion can be rigidly coupled to each other. 
     &lt;15&gt; As an aspect of the core according to the embodiment, there may be provided an aspect, in which 
     the first member includes a third coupling portion that is formed on an inner side of a ring shape of the first coupling portion at the first surface, 
     the second member includes a fourth coupling portion that is formed on an inner side of a ring shape of the second coupling portion at the second surface, 
     the third coupling portion has a shape protruding or recessed on a side opposite to the first coupling portion with respect to the first surface, and 
     the fourth coupling portion has a shape corresponding to the third coupling portion. 
     Coupling the third coupling portion and the fourth coupling portion to each other in addition to the coupling between the first coupling portion and the second coupling portion can further rigidly couple the first member and the second member to each other. 
     &lt;16&gt; A stator according to an embodiment includes 
     the core according to any one of the above-described &lt;1&gt; to &lt;15&gt;; and 
     a coil that is disposed at each tooth included in the core. 
     The above-described stator is excellent in magnetic characteristics. This is because the core included in the stator is the core with a high density according to the embodiment. Moreover, the above-described stator is excellent in productivity. This is because the core included in the stator is the core being excellent in productivity according to the embodiment. 
     &lt;17&gt; A rotating electrical machine according to an embodiment is 
     an axial-gap rotating electrical machine in which a rotor and a stator are disposed side by side in an axial direction of a rotating shaft of the rotor, in which 
     the stator is the stator according to the above-described &lt;16&gt;. 
     The rotating electrical machine is excellent in output characteristics. This is because the stator included in the rotating electrical machine is the stator being excellent in magnetic characteristics. Moreover, the above-described rotating electrical machine is excellent in productivity. This is because the stator included in the rotating electrical machine is the stator being excellent in productivity. 
     Details of Embodiments of Present Disclosure 
     Specific examples of a core, a stator, and a rotating electrical machine according to embodiments of the present disclosure are described with reference to the drawings. The same reference sign in the drawings indicates the same or corresponding part. Note that the present invention is not limited by these illustrative examples but is defined by the scope of the claims. It is intended to cover the meaning equivalent to the claims and all modifications within the scope of the claims. 
     First Embodiment 
     Rotating Electrical Machine 
     In a first embodiment, an axial-gap rotating electrical machine  1  illustrated in  FIG. 10  is described as an example. The rotating electrical machine  1  may be an electric generator or an electric motor (motor). The rotating electrical machine  1  includes a rotor  2  and a stator  3  disposed in a housing  10 . 
     Rotor 
     The rotor  2  includes a plurality of flat-plate-shaped magnets  22  and a circular-ring-shaped holding plate  21  that supports the magnets  22 . The holding plate  21  is fixed to a shaft  20  and rotates together with the shaft  20 . The magnets  22  are embedded in the holding plate  21 . The magnets  22  are disposed to be spaced apart in the circumferential direction of the shaft  20 . Moreover, the magnets  22  are magnetized in the direction of the rotating shaft of the rotor  2  (the axial direction of the shaft  20 ). The magnetization directions of the magnets  22  adjacent to each other in the circumferential direction of the shaft  20  are opposite to each other. 
     Stator 
     The stator  3  includes a core  30  and a coil  31  disposed at a tooth  4  of the core  30 . The stator  3  is disposed to face the rotor  2  in the axial direction of the shaft  20  and is fixed to the housing  10 . A bearing  23  is disposed between the stator  3  and the shaft  20 , and the stator  3  does not rotate. The rotating electrical machine  1  according to the present embodiment has a feature in the stator  3 , or more particularly in the core  30  included in the stator  3 . 
     Core 
     The core  30  illustrated in  FIGS. 1 to 3  includes teeth  4  and a yoke  5 . In this example, 12 teeth  4  are formed at the core  30 . The number of teeth  4  is not particularly limited. In the case of the axial-gap rotating electrical machine  1 , adjacent two teeth  4  form a magnetic circuit via the yoke  5 , and hence the number of teeth  4  is preferably 2 n (n is a natural number). In this example, the teeth  4  each serve as a block-shaped first member, the yoke  5  serves as a plate-shaped second member, and the teeth  4  and the yoke  5  are fabricated separately. The teeth  4  and the yoke  5  fabricated separately are coupled to each other by a coupling mechanism  9  illustrated in  FIG. 3 . The details of the coupling mechanism  9  will be described later. 
     Teeth 
     The teeth (first member)  4  are described mainly with reference to  FIGS. 4, 5A, 5B , and  6 . Each of the teeth  4  in this example is a substantially trapezoidal columnar member having a flange (flange portion  45 ) at an end portion on the side opposite to the yoke  5  ( FIG. 3 ). The shape of the tooth  4  is not particularly limited. For example, the tooth  4  may have a substantially triangular columnar shape. For another example, the tooth  4  may have a circular columnar shape or a quadrangular columnar shape. 
     The tooth  4  includes a first surface  40 , a peripheral surface  41 , and an end surface  42 . The first surface  40  is a flat surface and is a lower surface of the tooth  4  facing the yoke  5  ( FIG. 3 ). The end surface  42  is an upper surface of the tooth  4  on the side opposite to the first surface  40 . The peripheral surface  41  is a surface that connects the first surface  40  and the end surface  42  to each other. A first coupling portion  91  of the coupling mechanism  9 , which will be described later, is formed at the first surface  40  of the tooth  4 . 
     The flange portion  45  included in the tooth  4  may be omitted. However, in the case of the axial-gap rotating electrical machine  1  ( FIG. 10 ), a larger facing area of the tooth  4  facing the rotor  2  is more advantageous to improve the performance. Thus, the flange portion  45  that protrudes in a direction orthogonal to a protruding direction of the tooth  4  is formed at an end portion of the tooth  4  on the side opposite to the yoke  5  in this example. The outline of the outer periphery of the flange portion  45  in this example is substantially similar to the outline of the outer periphery of the first surface  40 . The flange portion  45  also has a role of suppressing detachment of the coil  31  disposed at the tooth  4  from the tooth  4 . 
     The tooth  4  is a powder compact obtained by compression molding a soft magnetic powder. The soft magnetic powder is an aggregate of soft magnetic particles. Examples of the soft magnetic powder include pure iron (purity is 99% by mass or more), and at least a kind of powder selected from iron-based alloys of an Fe—Si—Al-based alloy (sendust), an Fe—Si-based alloy (silicon steel), an Fe—Al-based alloy, and an Fe—Ni-based alloy (permalloy). The soft magnetic particles preferably have an insulating coating on the surfaces thereof. When the insulating coating is formed on the surfaces of the soft magnetic particles, electric insulation can be secured among the soft magnetic particles. Thus, an iron loss of the tooth  4  caused by an eddy current loss can be decreased. The insulating coating may be, for example, a phosphate coating or a silica coating. 
     The average particle size of the soft magnetic particles is preferably 10 μm or more and 300 μm or less. When the average particle size of the soft magnetic particles is 10 μm or more, the fluidity of the soft magnetic powder is not decreased, and increases in coercivity and hysteresis loss of the powder compact can be suppressed. In contrast, when the average particle size of the soft magnetic particles is 300 μm or less, the eddy current loss of the powder compact that is generated in a high frequency range can be effectively decreased. The average particle size of the soft magnetic particles is more preferably 40 μm or more and 260 μm or less. In this case, the average particle size represents the particle size of particles when the sum of mass of particles in the order from a smaller particle size reaches 50% of the total mass in the histogram of particle sizes, that is, D50 particle size. 
     The relative density of the powder compact is preferably 90% or more. Increasing the density can improve the magnetic characteristics of the powder compact. The relative density is more preferably 93% or more. The relative density represents a proportion (%) of the density of a powder compact with respect to the true density of the powder compact (soft magnetic powder). 
     Yoke 
     The yoke  5  is described mainly with reference to  FIGS. 7, 8A, 8B, and 9 . As illustrated in  FIG. 7 , the yoke  5  is a circular-ring-shaped member. The yoke  5  in this example is constituted by a member. The yoke  5  may be constituted by combining a plurality of divided pieces. For example, the circular-ring-shaped yoke  5  may be formed by connecting fan-shaped divided pieces to each other. 
     The yoke  5  includes a second surface  50 , a back surface  52 , an inner edge surface  53 , and an outer edge surface  54 . The second surface  50  is a surface that faces the first surface  40  as illustrated in  FIG. 3 . The second surface  50  is a flat surface that is parallel to the first surface  40 . Thus, the entire surface of the first surface  40  of each tooth  4  comes into surface contact with the second surface  50 . The back surface  52  of the yoke  5  is a lower surface of the yoke  5  that is parallel to the second surface  50 . The edge surface  53  is a surface that connects the second surface  50  and the back surface  52  to each other on the inner side of the circular ring of the yoke  5 . A through hole  55  through which the shaft  20  ( FIG. 10 ) penetrates is formed on the inner side of the edge surface  53 . The edge surface  54  is a surface that connects the second surface  50  and the back surface  52  to each other on the outer side of the circular ring of the yoke  5 . A second coupling portion  92  of the coupling mechanism  9 , which will be described later, is formed at the second surface  50  of the yoke  5 . 
     The yoke  5  is constituted by a powder compact similarly to the tooth  4 . The composition of the powder compact that constitutes the yoke  5  may be the same as or may differ from the composition of the powder compact that constitutes the tooth  4 . The density of the yoke  5  may be the same as or may differ from the density of the tooth  4 . 
     Coupling Mechanism 
     The coupling mechanism  9  includes the first coupling portion  91  and the second coupling portion  92 . The first coupling portion  91  is a recess that is formed in the first surface  40 , and the second coupling portion  92  is a protrusion protruding from the second surface  50 . The internal shape of the first coupling portion  91  (recess) is a shape corresponding to the external shape of the second coupling portion  92  (protrusion). Thus, by fitting the second coupling portion  92  to the first coupling portion  91 , the outer peripheral surface of the second coupling portion  92  comes into surface contact with the inner peripheral surface of the first coupling portion  91 . Consequently, the tooth  4  is coupled to the yoke  5 . 
     First Coupling Portion 
     The shape of the first coupling portion  91  (recess) is described in detail with reference to  FIGS. 4, 5A, and 5B . As illustrated in  FIG. 4 , the front shape of the first coupling portion  91  when seen in a direction orthogonal to the first surface  40  is a ring shape. The front shape of the first coupling portion  91  in this example is a racetrack shape. The front shape of the first coupling portion  91  may be a circular-ring shape or may be a non-circular-ring shape including the racetrack shape in this example. Examples of the non-circular-ring shape other than the racetrack shape include a polygonal shape whose vertices are rounded, such as a triangle, a rectangle (including a square), a trapezoid, or a rhombus; and a discontinuous ring shape, such as a C shape or a U shape in which the ring is not partly connected. When the front shape of the first coupling portion  91  is a non-circular-ring shape, the tooth  4  can be inhibited from rotating relative to the yoke  5  on the first surface  40  (second surface  50 ) after the tooth  4  is coupled to the yoke  5 . Alternatively to the illustrative example in  FIG. 4 , when the first coupling portion  91  is divided into two on the inner side and the outer side in the radial direction of the yoke  5  ( FIG. 1 ), an upper portion and a lower portion of the sheet sandwiching a division line (see a two-dot chain line in  FIG. 4 ) may be asymmetric to each other. In this case, the direction of the tooth  4  with respect to the yoke  5  is limited, and hence the tooth  4  is no longer misaligned. 
     The inner side of the ring shape of the first coupling portion  91  (recess) has the same height as the height of the first surface  40 . Alternatively to this example, the inner side of the ring shape of the first coupling portion  91  may be recessed with respect to the first surface  40 , or may protrude from the first surface  40  like a third embodiment which will be described later. 
     In this example, the number of the first coupling portion  91  at the tooth  4  is one. A plurality of first coupling portions  91  may be provided at a tooth  4 . When the plurality of first coupling portions  91  are formed, some of the first coupling portions  91  may be recesses and the others may be protrusions. 
     The width of the first coupling portion  91  (recess) in this example gradually decreases toward the deep side in the depth direction as illustrated in  FIG. 5B . That is, the width of a bottom surface  9   d  of the first coupling portion  91  is smaller than the width of an opening portion of the first coupling portion  91 . Moreover, an inner wall surface  9   i  is inclined in a direction to expand from the bottom surface  9   d  toward the opening portion. The recess having such a shape contributes to improvement in the fluidity of the soft magnetic powder during compression molding. Needless to say, the width of the recess may be uniform in the depth direction. In this case, the width of the first coupling portion  91  is a length of the first coupling portion  91  in a direction orthogonal to the circumferential direction of the ring shape of the first coupling portion  91  (the thickness direction of the sheet of  FIG. 5B ). 
     The joint between the bottom surface  9   d  and the inner wall surface  9   i  of the first coupling portion  91  (recess) is rounded. The joint between the inner wall surface  9   i  and the first surface  40  is also rounded. Rounding both the joints can improve the fluidity of the soft magnetic powder during compression molding. For example, the radius of curvature of the roundness can be 0.5 mm or more and 4.0 mm or less. The radius of curvature is more preferably 1.0 mm or more and 3.0 mm or less. 
     A width w 1  of the first coupling portion  91  (recess) is preferably 0.5 mm or more and 10 mm or less. The width w 1  is the width of the opening portion of the recess. When the width w 1  is 0.5 mm or more, the coupling strength between the first coupling portion  91  (recess) and the second coupling portion  92  (protrusion) can be sufficiently secured. When the width w 1  is 10 mm or less, a decrease in fluidity of the soft magnetic powder during compression molding can be suppressed. The width w 1  is more preferably 0.5 mm or more and 4 mm or less. The width w 1  is further preferably 1.0 mm or more and 3.0 mm or less. 
     A depth d 1  of the first coupling portion  91  (recess) is preferably 0.5 mm or more and 30% or less of the thickness of one of the first member and the second member having a smaller thickness (in this example, the yoke  5 ). The depth d 1  is a length of a perpendicular from the first surface  40  to the bottom surface  9   d.  When the depth d 1  is 0.5 mm or more, the coupling strength between the first coupling portion  91  (recess) and the second coupling portion  92  (protrusion) can be sufficiently secured. When the depth d 1  is 30% or less of the thickness of the yoke  5 , a decrease in fluidity of the soft magnetic powder during compression molding can be suppressed. The depth d 1  is more preferably 1.0 mm or more and 25% or less of the thickness of one of the first member and the second member having a smaller thickness. 
     The size of the outline of the outer periphery (see  FIG. 4 ) of the first coupling portion  91  at the first surface  40  can be appropriately selected. For example, when the area of the first surface  40  is 100, the area on the inner side with respect to the outline of the outer periphery of the first coupling portion  91  can be 10% or more and 80% or less. When the proportion of the area of the external shape of the first coupling portion  91  in the first surface  40  is set to 10% or more and 80% or less, the tooth  4  and the yoke  5  can be rigidly coupled to each other. The proportion of the area is more preferably 20% or more and 70% or less. 
     Second Coupling Portion 
     The shape of the second coupling portion  92  (protrusion) is described in detail with reference to  FIGS. 7 and 8 . As described above, the second coupling portion  92  has a shape corresponding to the first coupling portion  91 . As illustrated in  FIG. 7 , the front shape of the second coupling portion  92  when seen in a direction orthogonal to the second surface  50  has the same shape and the same size as those of the front shape of the first coupling portion  91 . 
     The inner side of the ring shape of the second coupling portion  92  (protrusion) has the same height as that of the second surface  50 . Alternatively to this example, the inner side of the ring shape of the second coupling portion  92  may protrude with respect to the second surface  50 , or may be recessed with respect to the second surface  50  like a fourth embodiment which will be described later. 
     The width of the second coupling portion  92  (protrusion) in this example gradually decreases toward a top surface  9   t  as illustrated in  FIG. 8B . That is, the width of the top surface  9   t  of the second coupling portion  92  is smaller than the width of the base of the second coupling portion  92 . Moreover, an outer wall surface  90  is inclined in a direction to narrow from the base toward the top surface  9   t.  The protrusion having such a shape contributes to improvement in the fluidity of the soft magnetic powder during compression molding. In this case, the width of the second coupling portion  92  is a length of the second coupling portion  92  in a direction orthogonal to the circumferential direction of the ring shape of the second coupling portion  92  (the thickness direction of the sheet of  FIG. 8B ). 
     The joint between the top surface  9   t  and the outer wall surface  90  and the joint between the outer wall surface  90  and the second surface  50  of the second coupling portion  92  (protrusion) are rounded to meet the shape of the first coupling portion  91  (recess). Rounding both the joints of the protrusion can improve the fluidity of the soft magnetic powder during compression molding. 
     A width w 2  of the second coupling portion  92  (protrusion) is the same as the width w 1  of the first coupling portion  91  (recess) in  FIG. 5B . The width w 2  is the width of the base of the protrusion. A height h 2  of the second coupling portion  92  (protrusion) is the same as the depth d 1  of the first coupling portion  91  (recess) in  FIG. 5B . The height h 2  is a length of a perpendicular from a plane extending from the second surface  50  to the top surface  9   t.  The limitations on the width w 2  and the height h 2  of the protrusion contribute to improvement in the mechanical strength of the protrusion. The limitations on the width w 2  and the height h 2  of the protrusion contribute to improvement in the fluidity of the soft magnetic powder near the protrusion during compression molding. 
     Advantageous Effects of Present Embodiment 
     The core  30  ( FIG. 1 ) of the embodiment has almost no portion with a locally low density of the soft magnetic powder, and hence the core  30  has a high density as a whole. A portion with a locally low density is not formed in the core  30  because the front shapes of the protrusion and the recess that constitute the first coupling portion  91  and the second coupling portion  92  are the ring shapes or the discontinuous ring shapes. 
     The core  30  of the embodiment is excellent in productivity. This is because the fluidity of the soft magnetic powder is high in the vicinities of the protrusion and the recess during compression molding of the core  30 , and the core  30  with a high density as a whole can be fabricated by one-time compression molding. Moreover, since the fluidity of the soft magnetic powder is high, the rate of occurrence of a defective part having a portion with a locally low density decreases. This is also the cause of improvement in the productivity of the core  30 . 
     In this example, the tooth  4  and the yoke  5  are separate members. Thus, the metal mold for fabricating the core  30  can be formed in a simple shape. Consequently, the densities of the tooth  4  and the yoke  5  are easily made uniform. Moreover, the plurality of teeth  4  included in the core  30  can be fabricated using a metal mold. Consequently, the productivity of the core  30  can be improved. 
     In this example, the second coupling portion  92  formed at the yoke  5  is the protrusion. Thus, a decrease in the mechanical strength of the yoke  5  due to the provision of the second coupling portion  92  can be suppressed. 
     The stator  3  ( FIG. 10 ) including the core  30  of the above-described embodiment is excellent in magnetic characteristics. This is because the density of the magnetic powder of the stator  3  is high. Moreover, the stator  3  is excellent in productivity. This is because the productivity of the core  30  included in the stator  3  is high. 
     The rotating electrical machine  1  including the stator  3  of the above-described embodiment is excellent in output characteristics. This is because the magnetic characteristics of the stator  3  included in the rotating electrical machine  1  is high. Moreover, the rotating electrical machine  1  is excellent in productivity. This is because the productivity of the stator  3  included in the rotating electrical machine  1  is high. 
     Second Embodiment 
     In a second embodiment, a core  30  that differs from the core  30  in the first embodiment is described with reference to  FIG. 11 . The way of viewing  FIG. 11  is the same as that in  FIG. 3  of the first embodiment. 
     In the core  30  in  FIG. 11 , a first coupling portion  91  of a tooth  4  is a protrusion, and a second coupling portion  92  of a yoke  5  is a recess. The shapes and sizes of the protrusion and the recess may be the same as those of the first embodiment. Also with the configuration in this example, advantageous effects similar to those of the first embodiment can be obtained. 
     Third Embodiment 
     In a third embodiment, a core  30  in which a tooth  4  has a third coupling portion  93  and a yoke  5  has a fourth coupling portion  94  is described with reference to  FIGS. 12A and 12B . 
     The third coupling portion  93  is a protrusion that is formed on the inner side of the ring shape of the first coupling portion  91  at the first surface  40  and that protrudes from the first surface  40 . A height h 3  of the third coupling portion  93  in this example is larger than the height h 2  of the second coupling portion  92  (protrusion). The height h 3  of the third coupling portion  93  is a length of a perpendicular from the first surface  40  to a top surface of the protrusion. The preferable range of the height h 3  is the same as the preferable range of the height h 2 . 
     As illustrated in  FIG. 12A , the fourth coupling portion  94  is a recess that is formed on the inner side of the ring shape of the second coupling portion  92  at the second surface  50  and that is lowered with respect to the second surface  50 . The internal shape of the fourth coupling portion  94  (recess) is a shape corresponding to the external shape of the third coupling portion  93  (protrusion). Thus, as illustrated in  FIG. 12B , a depth d 4  of the fourth coupling portion  94  (recess) in this example is the same as the height h 3  of the third coupling portion  93  (protrusion). 
     With the configuration in this example, the tooth  4  and the yoke  5  can be coupled to each other more rigidly compared with the core  30  according to any one of the first and second embodiments. 
     Fourth Embodiment 
     In a fourth embodiment, a core  30  that differs from the core  30  in the third embodiment is described with reference to  FIGS. 13A and 13B . 
     As illustrated in  FIG. 13A , a third coupling portion  93  in this example is a recess that is formed on the inner side of the ring shape of the first coupling portion  91  at the first surface  40 . In contrast, a fourth coupling portion  94  is a protrusion that is formed on the inner side of the ring shape of the second coupling portion  92  at the second surface  50 . 
     Also in this example, the internal shape of the third coupling portion  93  (recess) is a shape corresponding to the external shape of the fourth coupling portion  94  (protrusion). Thus, as illustrated in  FIG. 13B , a depth d 3  of the third coupling portion  93  in this example is the same as a height h 4  of the fourth coupling portion  94 . 
     Fifth Embodiment 
     In a fifth embodiment, a core  30  in which a tooth  4  and a plate-shaped piece  6  that is disposed on the side of the tooth  4  opposite to a yoke  5  are coupled to each other is described with reference to  FIG. 14 . 
     The tooth  4  in this example is integrally formed with the yoke  5 . The tooth  4  and the yoke  5  may be separately provided like the core  30  according to any one of the first to fourth embodiments. In this case, although the number of components that constitute the core  30  increases, the entire density of the core  30  is easily made uniform. 
     The plate-shaped piece  6  is provided on an end surface  42  of the tooth  4  on the side opposite to the yoke. The plate-shaped piece  6  is fabricated separately from the tooth  4  and is coupled to the tooth  4 . That is, the tooth  4  is a first member, and the plate-shaped piece  6  is a second member. Moreover, the end surface  42  of the tooth  4  (first member) is also a first surface  40  that faces the plate-shaped piece  6  (second member). A facing surface  60  of the plate-shaped piece  6  that faces the first surface  40  is a second surface  50 . 
     A first coupling portion  91  constituted by a ring-shaped recess is formed in the first surface  40  of the tooth  4 . A second coupling portion  92  constituted by a ring-shaped protrusion is formed at the second surface  50  of the plate-shaped piece  6 . 
     With the configuration in this example, a flange portion  45  can be formed at the tooth  4  after the coil  31  ( FIG. 10 ) is disposed at the tooth  4 . In this case, the coil  31  is very easily disposed. 
     Sixth Embodiment 
     In a sixth embodiment, a core  30  that differs from the core  30  in the fifth embodiment is described with reference to  FIG. 15 , as a configuration including a plate-shaped piece  6  that is disposed on the side of a tooth  4  opposite to a yoke  5 . 
     In this example, a first coupling portion  91  constituted by a ring-shaped protrusion is formed at a first surface  40  of the tooth  4 . Moreover, a second coupling portion  92  constituted by a ring-shaped recess is formed in a second surface  50  of the plate-shaped piece  6 . Even with the configuration in this example, a flange portion  45  can be formed at the tooth  4  after the coil  31  ( FIG. 10 ) is disposed at the tooth  4 . 
     Seventh Embodiment 
     The core  30  according to any one of the first to sixth embodiments is used for a stator. The core may be used for a rotor. 
     REFERENCE SIGNS LIST 
     
         
           1  rotating electrical machine 
       
    
       10  housing
       2  rotor   

       20  shaft,  21  holding plate,  22  magnet,  23  bearing
       3  stator   

       23  core,  31  coil
       4  tooth (first member)   

       40  first surface,  41  peripheral surface,  42  end surface,  45  flange portion
       5  yoke (second member)   

       50  second surface,  52  back surface,  53  inner edge surface,  54  outer edge surface,  55  through hole
       6  plate-shaped piece (second member)   

       60  facing surface
       9  coupling mechanism   

       91  first coupling portion,  92  second coupling portion,  93  third coupling portion,  94  fourth coupling portion 
       9   d  bottom surface,  9   i  inner wall surface,  9   o  outer wall surface,  9   t  top surface