Patent Publication Number: US-2023163670-A1

Title: Electromagnetic device and aircraft in which electromagnetic device is used

Description:
TECHNICAL FIELD 
     The present disclosure relates to an electromagnetic device and an aircraft using the electromagnetic device. 
     BACKGROUND ART 
     Conventionally, regarding a stator core surrounded by two opposed movable parts in a rotary electric machine which is an electromagnetic device, it is disclosed that the stator core is formed by stacking thin sheets in a direction parallel to two surfaces facing the movable parts which are rotors and perpendicular to the movable direction of the movable parts, and retention members are fitted to holes provided so as to penetrate in the stacking direction, thereby retaining the stator core (for example, Patent Document  1 ). 
     Also, it is disclosed that, in a rotary electric machine, a stator core is formed by stacking thin sheets in a direction parallel to the two opposed movable parts and substantially parallel to the movable direction of the movable parts, bolt holes for retaining the stacked sheets are provided in a direction parallel to two surfaces of the stator core facing the movable parts which are rotors and perpendicular to the movable direction of the movable parts, and bolts are fastened, thereby retaining the stator core (for example, Patent Document  2 ). 
     CITATION LIST 
     Patent Document 
     
         
         Patent Document  1 : Japanese Laid-Open Patent Publication No. 2019-37084 
         Patent Document  1 : Japanese Laid-Open Patent Publication No. 2018-85886 
       
    
     SUMMARY OF THE INVENTION 
     Problems to Be Solved by the Invention 
     In Patent Document  1 , an electromagnetic force due to operation of the rotary electric machine acts in a direction to cause shear between the stacked sheets of the stator core, and therefore the retention members are fitted to the holes penetrating in the stacking direction, so as to retain the stator core. Thus, the retention members interfere with a magnetic path of the stator core, leading to size increase of the device and reduction in efficiency. 
     On the other hand, in Patent Document 2, since the stator core needs to be retained by applying a clamping force in the stacking direction, the magnetic property of the stator core is deteriorated. In addition, also in another example in Patent Document 2, the stator core is pressed by fitting portions and thus the magnetic property is deteriorated. 
     The present disclosure has been made to solve the above problem, and an object of the present disclosure is to provide an electromagnetic device in which a stator core is retained without increasing the size of the device and without deteriorating the magnetic property, and an aircraft using the electromagnetic device. 
     Solution to the Problems 
     An electromagnetic device according to the present disclosure includes: two movable parts movable in parallel or antiparallel to each other; and a stator core arranged with two surfaces thereof respectively opposed to the two movable parts. At least part of the stator core is formed by stacking thin sheets in a movable direction of the movable parts and is retained with tensile stress applied thereto in a direction parallel to the two surfaces opposed to the two movable parts and perpendicular to the movable direction of the movable parts. 
     Effect of the Invention 
     In the electromagnetic device according to the present disclosure, since the stator core is retained with tensile stress applied thereto, it becomes possible to retain the stator core without increasing the size of the device and without deteriorating the magnetic property of the stator core. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a sectional view along a rotary shaft of a rotary electric machine according to embodiment 1. 
         FIG.  2    is a sectional view along the rotary shaft of the rotary electric machine according to embodiment 1 and is a partial enlarged view of  FIG.  1   . 
         FIG.  3    is a partial sectional view along a direction perpendicular to the rotary shaft of the rotary electric machine according to embodiment 1. 
         FIG.  4    shows the structure of a stator core of the rotary electric machine according to embodiment 1. 
         FIG.  5    is a sectional view along a direction perpendicular to a rotary shaft of a rotary electric machine according to embodiment 2. 
         FIG.  6    is a sectional view along a rotary shaft of a rotary electric machine according to embodiment 3. 
         FIG.  7    is a partial sectional view along a direction perpendicular to the rotary shaft of the rotary electric machine according to embodiment 3. 
         FIG.  8    is a partial perspective view showing the structure of a stator of a rotary electric machine according to embodiment 4. 
         FIG.  9    is a partial perspective view showing the structure of the stator core of the rotary electric machine according to embodiment 4. 
         FIG.  10    is a partial perspective view showing the structure of the stator core of the rotary electric machine according to embodiment 4 and is a partial enlarged view of  FIG.  9   . 
         FIG.  11    is a perspective view showing a method for assembling the stator core of the rotary electric machine according to embodiment 4. 
         FIG.  12    is a partial perspective view showing the structure of another stator of the rotary electric machine according to embodiment 4. 
         FIG.  13    is a sectional view along a rotary shaft of a rotary electric machine according to embodiment 5. 
         FIG.  14    is a partial sectional view along a direction perpendicular to the rotary shaft of the rotary electric machine according to embodiment 5. 
         FIG.  15    is a sectional view along a rotary shaft of a rotary electric machine according to embodiment 6. 
         FIG.  16    is a sectional view along a direction perpendicular to the rotary shaft of the rotary electric machine according to embodiment 6 and shows the structure of a stator. 
         FIG.  17    is a sectional view along a rotary shaft of a magnetic gear according to embodiment 7. 
         FIG.  18    is a sectional view along a direction perpendicular to the rotary shaft of the magnetic gear according to embodiment 7. 
         FIG.  19    is a sectional view along a movable axis, showing the structure of a linear motor according to embodiment  8 . 
         FIG.  20    is a schematic view showing an aircraft using an electromagnetic device according to embodiment 9. 
         FIG.  21    is another schematic view showing another aircraft using an electromagnetic device according to embodiment 9. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments will be described with reference to the drawings. The drawings are schematically shown and some configurations are omitted or simplified for convenience of description. The relationship of sizes and positions of components and the like shown in different drawings are not necessarily precisely shown, and may be changed as appropriate. In the following description, the same constituent elements are denoted and shown by the same reference characters, and also they are the same in names and functions. Therefore, the detailed description thereof may be omitted for avoiding redundant description. 
     In the embodiments, a rotary electric machine, a magnetic gear, and a linear motor are sequentially described as examples of an electromagnetic device, but the electromagnetic device is not limited thereto. 
     Embodiment 1 
     Hereinafter, a rotary electric machine as an electromagnetic device according to embodiment 1 will be described with reference to the drawings. 
       FIG.  1    is a sectional view along a rotary shaft, showing the structure of the rotary electric machine according to embodiment 1. In  FIG.  1   , a rotary electric machine  1  includes two rotors which are an inner rotor  10  and an outer rotor  20 , and a stator  30  provided between the two rotors in the radial direction, and is formed as a radial gap motor of a double-rotor type. 
     The inner rotor  10  includes a shaft  2 , a boss  12  press-fitted and fixed to the shaft  2 , and a permanent magnet  14  adhered and fixed to the radially outer side of the boss  12 . 
     The outer rotor  20  includes an outer shaft  21  fixed to the shaft  2 , and a permanent magnet  22  adhered and fixed to the radially inner side of the outer shaft  21 . 
     In the stator  30 , a stator core  31  is provided in the axial direction between a load-side retention member  35  attached to a load-side base  34 , and a non-load-side base  33 , and a stator coil  32  is wound around the stator core  31 . The stator  30 , which is provided between the inner rotor  10  and the outer rotor  20 , rotatably retains the inner rotor  10  and the outer rotor  20  by a load-side inner bearing  3 , a load-side outer bearing  5 , a non-load-side inner bearing  4 , and a non-load-side outer bearing  6 . 
       FIG.  2    is an enlarged view in areas X 1  and X 2  enclosed by broken lines in  FIG.  1   . 
     Of the stator core  31 , both axial-direction ends near the radial-direction center extend in the axial direction and protrude from the wound stator coil  32  parts. One distal end portion  31   c  on the load side is fixed to the load-side retention member  35  by a bolt  38 , and another distal end portion  31   d  on the fixed side is fixed by being caught in a T-shaped groove  33   a  provided in the non-load-side base  33 . In the drawing, a position where the stator core  31  is fixed by the bolt  38  is defined as a load-side retention surface  36 , and the bottom surface of the groove  33   a  where the stator core  31  is fixed in the T-shaped groove  33   a  is defined as a fixed-side retention surface  37 . In this case, the natural length of a length Ls between the fixed part of the stator core  31  with the bolt  38  and the end fixed to the groove  33   a  is smaller than a length L between the load-side retention surface  36  and the fixed-side retention surface  37 . Therefore, the stator core  31  is extended in the axial direction with tensile stress applied thereto. 
       FIG.  3    is a sectional view along a direction perpendicular to the rotary shaft of the rotary electric machine  1  and is a partial sectional view along an A-A line direction in  FIG.  1   . In the drawing, the rotary electric machine  1  has a concentrated-winding configuration with 48 poles and 72 slots. The stator core  31  is formed by stacking, substantially in the circumferential direction, thin steel sheets that are long in the axial direction. The inner rotor  10  and the outer rotor  20  rotate at the same angular velocity counterclockwise in the drawing. That is, the inner rotor  10  and the outer rotor  20  are movable parts. 
       FIG.  4    shows the structure of the stator core  31 . The stator core  31  is formed from thin steel sheets that are long in the axial direction and are each formed such that one axial-direction end protrudes from a body portion  31   a  so as to form the distal end portion  31   c  having a bolt fastening hole and another axial-direction end protrudes so as to form the distal end portion  31   d  having a T shape. In an order indicated by arrows in  FIG.  4 A , both side portions  31   b  along the axial direction are bent. Then, as shown in  FIG.  3   , the steel sheets are stacked in a state in which the respective side portions  31   b  of the stator core  31  opposed to the inner rotor  10  and the outer rotor  20  are bent toward the rotation-direction lagged side at an angle smaller than 90°. The thin steel sheets of the stator core  31  are electromagnetic steel sheets manufactured by rolling, and are arranged such that the rolling direction coincides with the radial direction of the stator core and a direction connecting the side portions  31   b  and also coincides with the opposing direction to the inner rotor  10  and the outer rotor. As shown in  FIG.  1    and  FIG.  2   , the stator core  31  is attached such that the distal end portion  31   d  is fixed in the T-shaped groove  33   a  provided in the non-load-side base  33  and the distal end portion  31   c  is fixed by the bolt  38  with the stator core  31  pulled in the axial direction. 
     With the above structure, the stator core  31  is fixed with tensile stress applied thereto. Thus, magnetic property deterioration due to compressive stress does not occur and efficiency can be enhanced. In addition, since the radial-direction center parts of the stator core  31  are fixed, it is easy to apply tensile stress equally to the stator core  31  and thus efficiency can be easily enhanced. 
     Further, since the distal end portions  31   c ,  31   d  of the stator core  31  are thinner than the body portion  31   a  thereof, the axial-direction sectional areas of the distal end portions  31   c ,  31   d  are small, and the stator core  31  is fixed at positions protruding from the wound parts of the stator coil  32 . Thus, the influence on the magnetic path is also small. 
     In addition, even if the load-side retention member  35  and the non-load-side base  33  are made of a magnetic material, as described above, since the distal end portions  31   c ,  31   d  of the stator core  31  are thinner than the body portion  31   a  thereof, the axial-direction sectional areas of the distal end portions  31   c ,  31   d  are small, and thus the influence on the magnetic path is also small. On the other hand, if the load-side retention member  35  and the non-load-side base  33  are made of a non-magnetic material, a closed magnetic path to the stator core  31  at each individual circumferential-direction part is not formed and thus the influence on the magnetic path can be eliminated. 
     In addition, since the side portions  31   b  along the axial direction of the stator core  31  are bent toward the rotation-direction lagged side, magnetic fluxes from the movable inner rotor  10  and outer rotor  20  can be readily collected to the stator core  31 . 
     In the above structure of the stator core  31 , at both side end portions along the axial direction, both side portions  31   b  on the inner rotor  10  side and the outer rotor  20  side are bent toward the rotation-direction lagged side. However, as shown in  FIG.  4 B  and  FIG.  4 C , only one of the side portions  31   b  may be bent, whereby the same effects are obtained. 
     Both side portions  31   b  along the axial direction of the stator core  31  are bent at the same angle. However, the bending angles may be gradually increased toward the radial-direction lagged side. Thus, a larger amount of magnetic flux can be readily collected, whereby torque of the rotary electric machine  1  can be enhanced. 
     As described above, in the rotary electric machine according to embodiment 1, the stator  30  is provided between the inner rotor  10  and the outer rotor  20  which are movable with the shaft  2  as an axis, and the stator  30  includes the stator core  31  formed by stacking thin sheets in the rotation direction and is retained with tensile stress applied thereto in the axial direction, whereby the stator core  31  can be retained without deteriorating the magnetic property thereof. Thus, efficiency and torque of the rotary electric machine can be enhanced. 
     Embodiment 2 
     Hereinafter, a rotary electric machine as an electromagnetic device according to embodiment 2 will be described with reference to the drawings. 
       FIG.  5    is a partial sectional view along a direction perpendicular to the rotary shaft of the rotary electric machine  1  according to embodiment 2. 
     In  FIG.  5   , the stator core  31  of the rotary electric machine  1  is formed by stacking, substantially in the circumferential direction, i.e., the rotation direction, thin steel sheets that are long in the axial direction. Both of the inner rotor  10  and the outer rotor  20  rotate at the same angular velocity counterclockwise in the drawing. At the side portions  31   b  which are end portions along the axial direction of the stator core  31  and are opposed to the inner rotor  10  side and the outer rotor  20  side, thin steel sheet surfaces are bent at an angle smaller than 90°. On the rotation-direction lagged side and advanced side, respectively, the side portions  31   b  are bent such that the bending angles gradually increase toward the radial-direction lagged side and advanced side, within an angle range of smaller than 90°. Although gaps are formed between the bent thin steel sheets, the gaps are filled with resin. The other configurations are the same as in embodiment 1. 
     The above configuration also provides the same effects as in embodiment 1. The side portions  31   b  of the stator core  31  are bent toward both of the radial-direction lagged side and advanced side in the rotation direction. Therefore, in a case where the rotation direction can become both directions equally, torque can be enhanced for rotations toward both sides. 
     In addition, since gaps between the thin sheets at the side portions  31   b  of the stator core  31  are filled with resin, the thin sheets are prevented from being bent or vibrated by the electromagnetic force, and thus the strength can be improved. 
     As described above, the rotary electric machine according to embodiment 2 provides the same effects as in embodiment 1. Further, since the side portions  31   b  of the stator core  31  are bent toward both of the radial-direction lagged side and advanced side in the rotation direction, it becomes possible to achieve torque enhancement of the rotary electric machine in both cases of two-direction rotations. 
     Embodiment 3 
     Hereinafter, a rotary electric machine as an electromagnetic device according to embodiment 3 will be described with reference to the drawings. 
       FIG.  6    is a sectional view along the rotary shaft, showing the rotary electric machine  1  according to embodiment 3. In  FIG.  6   , the stator core  31  is formed such that, at both ends in the axial direction thereof, both side portions on the inner rotor  10  side and the outer rotor  20  side extend and protrude in the axial direction from the body portion, the distal end portions  31   c  thereof on the load side are bent inward in L shapes, and the distal end portions  31   d  thereof on the fixed side are bent outward in L shapes. 
     In the same manner as described in embodiment 1, the natural length between both ends of the stator core  31  is smaller than the length between the fixed-side retention surfaces  37  of grooves in the non-load-side base  33  and the load-side retention surfaces  36  of L-shaped grooves provided at outer side parts of the load-side retention member  35 , where the L-shaped parts of the distal end portions  31   c ,  31   d  at both ends of the stator core  31  are fixed. Accordingly, the L-shaped parts of the distal end portions  31   d  on one end side in the axial direction of the stator core  31  are fixed by being caught in the L-shaped grooves provided in the non-load-side base  33 , and the L-shaped parts of the distal end portions  31   c  on the other end side in the axial direction of the stator core  31  are retained by being fitted to the grooves in the load-side retention member  35 , so that tensile stress is applied. 
     The axial-direction sectional area of the stator core  31  described above is smaller at both distal end portions  31   c ,  31   d  than at the body portion wound with the stator coil  32 , and the stator core  31  is fixed at positions extending from the body portion. Thus, the influence on the magnetic path is small as in embodiment 1. 
       FIG.  7    is a sectional view along a direction perpendicular to the rotary shaft of the rotary electric machine  1  and is a partial sectional view along a B-B line direction in  FIG.  6   . In  FIG.  7   , the stator core  31  is formed by stacking, substantially in the circumferential direction, i.e., the rotation direction, thin steel sheets that are long in the axial direction. Both of the inner rotor  10  and the outer rotor  20  rotate at the same angular velocity counterclockwise in the drawing, and both side portions on the inner rotor  10  side and the outer rotor  20  side which are both side portions along the axial direction of the stator core  31  are not bent and thus are along the same plane as the body portion corresponding to the winding part of the stator core  31 . The other structures are the same as in embodiment 1. 
     As described above, according to embodiment 3, the same effects as in embodiment 1 are provided. That is, the stator  30  is provided between the inner rotor  10  and the outer rotor  20  which are movable with the shaft  2  as an axis, and the stator  30  includes the stator core  31  formed by stacking thin sheets in the rotation direction and is retained with tensile stress applied thereto in the axial direction, whereby the stator core  31  can be retained without deteriorating the magnetic property thereof. Thus, efficiency and torque of the rotary electric machine can be enhanced. 
     Embodiment 4 
     Hereinafter, a rotary electric machine as an electromagnetic device according to embodiment 4 will be described with reference to the drawings. 
       FIG.  8    is a perspective view showing part of the structure of the stator  30  of the rotary electric machine according to embodiment 4,  FIG.  9    is a perspective view showing the structure of the stator core  31 , and  FIG.  10    is a partial enlarged view in a broken-line area X 3  in  FIG.  9   . In embodiments 1 to 3, the stator core  31  is formed by stacking, substantially in the circumferential direction, i.e., the rotation direction, thin steel sheets that are long in the axial direction. In the stator core  31  of embodiment 3, at both ends in the axial direction thereof, both side portions on the inner rotor  10  side and the outer rotor  20  side extend and protrude in the axial direction from the body portion, the distal end portions  31   c  thereof on the load side are bent inward in L shapes, and the distal end portions  31   d  thereof on the non-load side are bent outward in L shapes. On the other hand, the stator core  31  of embodiment 4 is different in that the stator core  31  includes side portions  311  respectively opposed to the inner rotor  10  and the outer rotor  20 , and a retention portion  312  retaining both side portions  311  and wound with the stator coil  32 . 
     As shown in  FIG.  9    and  FIG.  10   , both side portions  311  of the stator core  31  are formed by stacking, substantially in the circumferential direction, i.e., the rotation direction, thin electromagnetic steel sheets that are long in the axial direction. On the other hand, the retention portion  312  of the stator core  31  is formed by stacking, in the axial direction, electromagnetic steel sheets having a predetermined shape, to a height corresponding to the permanent magnets  14 ,  22  of the inner rotor  10  and the outer rotor  20 . The retention portion  312  has substantially a rectangular shape and has cutouts  312   a  at center parts on the inner circumferential side which is the inner rotor side and the outer circumferential side which is the outer rotor side. In the cutouts  312   a , both side portions  311  are inserted and retained. In addition, flanges  312   b  may be provided in the circumferential direction from the cutout  312   a , as shown in the drawings. Both side portions  311  of the stator core  31  are retained in close contact with the retention portion  312  so that a magnetic flux in a direction perpendicular to the rotary shaft flows seamlessly. Both side portions  311  extend toward both sides in the axial direction from the retention portion  312  so as to have distal end portions  31   c ,  31   d  bent in L shapes. 
     The stator core  31  of embodiment 4 which includes both side portions  311  stacked in the circumferential direction and the retention portion  312  stacked in the axial direction is also retained with tensile stress applied thereto in the axial direction, as in embodiment 3. That is, in relation to  FIG.  6    in embodiment 3, the natural lengths of both side portions  311  of the stator core  31  are smaller than the length between the fixed-side retention surfaces  37  of grooves in the non-load-side base  33  and the load-side retention surfaces  36  of L-shaped grooves provided at outer side parts of the load-side retention member  35 , where the L-shaped parts at the distal ends of both side portions  311  of the stator core  31  are fixed. Accordingly, the L-shaped parts of the distal end portions  31   c  on the other end side in the axial direction of the stator core  31  are retained by being fitted to the grooves in the load-side retention member  35 , so that tensile stress is applied. 
     In  FIG.  9   , the L-shaped parts at both ends in the axial direction of both side portions  311  are both bent inward toward the retention portion  312  side. However, as shown in  FIG.  6   , the ends on the fixed side may be bent outward in L shapes. 
     As in both side portions  311  of the stator core  31  in embodiment 4, parts of the stator core  31  are formed by stacking, substantially in the circumferential direction, i.e., the rotation direction, thin electromagnetic steel sheets that are long in the axial direction, and are retained with tensile stress applied thereto. Thus, magnetic property deterioration due to compressive stress is suppressed and efficiency can be enhanced. 
     Next, the retention portion  312  for retaining the side portions  311  of the stator core  31  will be described. In embodiments  1  to  3 , the stator core  31  is formed by stacking, substantially in the circumferential direction, i.e., the rotation direction, thin steel sheets that are long in the axial direction. On the other hand, in embodiment 4, the side portions  311  are formed on both of the inner circumferential side and the outer circumferential side by stacking, substantially in the circumferential direction, i.e., the rotation direction, thin steel sheets that are long in the axial direction as in embodiments 1 to 3, and the retention portion  312  for retaining both side portions  311  is formed by stacking electromagnetic steel sheets in the axial direction. Therefore, as described above, in a direction perpendicular to the axis, a substantially rectangular piece corresponding to each of electromagnetic steel sheets composing the retention portion  312  is present and thus it becomes easy to perform working into a desired shape. As shown in  FIG.  9    and  FIG.  10   , the retention portion  312  has substantially a rectangular shape and has, at center parts on the inner circumferential side and the outer circumferential side, the cutouts  312   a  in which both side portions  311  are inserted and retained. The flanges  312   b  are provided in the circumferential direction from the cutout  312   a . Working into such a shape also becomes easy. 
     Meanwhile, as shown in  FIG.  7   , in the axial-direction cross-section of the rotary electric machine  1 , the stator  30  is provided between the inner rotor  10  and the outer rotor  20 , and the stator cores  31  are arranged at constant intervals. Therefore, a magnetic flux due to the structure in which permeability is not constant as seen from the gaps between the stator  30 , and the inner rotor  10  and the outer rotor  20 , is generated. The generated magnetic flux is called a spatial harmonic, leading to loss. 
     In the present embodiment, the retention portions  312  have the flanges  312   b  serving to fill spaces between circumferential-direction adjacent parts of the stator  30 , whereby a spatial harmonic can be reduced. In addition, since the retention portion  312  is stacked in the rotary shaft direction, the area of the conductor interlinked by a circumferential-direction interlinkage magnetic flux is small. Thus, the resistance of the conductor increases and eddy current can be reduced. 
     Next, a method for attachment between the side portions  311  and the retention portion  312  will be described. 
     In  FIG.  11   ,  FIG.  11 A  shows the retention portion  312  stacked in the axial direction and  FIG.  11 B  shows both side portions  311  stacked in the circumferential direction. The cutout  312   a  of the retention portion  312  includes two types of cutouts, i.e., shallowly-cut cutouts  312   a   1  and deeply-cut cutouts  312   a   2 , and the retention portion  312  is stacked such that the cutouts  312   a   1  and  312   a   2  have predetermined thicknesses d2, d1, respectively. On both of the inner circumferential side and the outer circumferential side, the stacked parts at the shallowly-cut cutouts  312   a   1  project relative to the stacked parts at the deeply-cut cutouts  312   a   2 . 
     The side portions  311  are arranged such that the inner circumferential side and the outer circumferential side thereof are opposed to each other, and have, on the retention portion sides, protruding parts  311 A and recessed parts  311 B sequentially formed in lengths corresponding to the thicknesses d1, d2, respectively. A level difference d3 between the protruding part  311 A and the recessed part  311 B corresponds to the difference between the cutting depths of the shallowly-cut cutout  312   a   1  and the deeply-cut cutout  312   a   2  of the retention portion  312 . The retention portion  312  in  FIG.  11 A  and both side portions  311  in  FIG.  11 B  are fitted such that the stacked part at the shallowly-cut cutout  312   a   1  and the recessed part  311 B are fitted to each other and the stacked part at the deeply-cut cutout  312   a   2  and the protruding part  311 A are fitted to each other. That is, the recess and protrusion shapes of both members are fitted to each other in the direction of arrows in the drawing, thus forming the stator core  31  as shown in  FIG.  9   . 
     As described above, since the recess and protrusion shapes formed on the cutouts  312   a  of the retention portion  312  and the recess and protrusion shapes formed on both side portions  311  are fitted to each other, the stator core  31  can be formed in a state in which the thin-sheet-shaped electromagnetic steel sheets stacked in directions different from each other are in close contact with each other without coming apart. 
     The recess and protrusion shapes formed on the cutouts  312   a  of the retention portion  312  and the recess and protrusion shapes formed on both side portions  311  are not limited to the above ones. For example, shapes not only for fitting in the radial direction but also for fitting or engaging in the axial direction may be adopted. 
       FIG.  12    shows a modification of  FIG.  8    and is a perspective view showing the configuration of another stator  30  according to embodiment 4. Difference from  FIG.  8    is that, in both side portions  311  of the stator core  31 , parts on the rotation-direction advanced side in  FIG.  7    are formed by structural bodies  311   b  which are not stacked electromagnetic steel sheets and are made of a non-metal or non-magnetic material. Parts on the rotation-direction lagged side are formed by stacked bodies  311   a  of electromagnetic steel sheets. 
     In electromagnetic steel sheets stacked in the circumferential direction, in-plane eddy current is generated and the generated eddy current tends to be greater on the rotation-direction advanced side. Therefore, if the parts on the rotation-direction advanced side are formed by members that are not electromagnetic steel sheets as shown in  FIG.  12   , loss due to eddy current on the rotation-direction advanced side can be reduced. In the present embodiment, high-strength resin is used as the structural bodies  311   b  made of a non-metal or non-magnetic material. The shapes of the structural bodies  311   b  and the stacked bodies  311   a  of electromagnetic steel sheets are not limited to the shown ones, and they may be different in size in the circumferential direction, or the like. 
     As described above, according to embodiment 4, both side portions  311  composing the stator core  31  are formed by stacking, substantially in the circumferential direction, i.e., the rotation direction, thin electromagnetic steel sheets that are long in the axial direction, and tensile stress is applied thereto. Thus, as in embodiments 1 to 3, magnetic property deterioration due to compressive stress is suppressed and efficiency can be enhanced. 
     In addition, the retention portion  312  of the stator core for retaining both side portions  311  is formed by stacking substantially-rectangular electromagnetic steel sheets in the axial direction, and has flange shapes on the inner circumferential side and the outer circumferential side. Such a structure contributes to suppression of a spatial harmonic and eddy current, thus achieving efficiency enhancement. 
     Further, in both side portions  311  composing the stator core  31 , parts on the rotation-direction advanced side are formed by the structural bodies  311   b  which are not stacked bodies of electromagnetic steel sheets and are made of a non-metal or non-magnetic material. Thus, it becomes possible to reduce loss due to eddy current. 
     Embodiment 5 
     Hereinafter, a rotary electric machine as an electromagnetic device according to embodiment 5 will be described with reference to the drawings. 
       FIG.  13    is a sectional view along the rotary shaft, showing the structure of the rotary electric machine  1  according to embodiment 5. In  FIG.  13   , difference from embodiment 1 is that the outer shaft  21  is not fixed to the shaft  2 . Therefore, the inner rotor  10  and the outer rotor  20  of the rotary electric machine  1  are not connected to each other, and for example, the inner rotor  10  and the outer rotor  20  rotate in directions opposite to each other, and the outer rotor  20  rotates at an angular velocity that is ½ of the angular velocity of the inner rotor  10 . 
       FIG.  14    is a sectional view along a direction perpendicular to the rotary shaft of the rotary electric machine  1  and is a partial sectional view along a C-C line direction in  FIG.  13   . In  FIG.  14   , the inner rotor  10  rotates counterclockwise and the outer rotor  20  rotates clockwise. The stator core  31  is formed by stacking, substantially in the circumferential direction, thin steel sheets that are long in the axial direction. At the side portions  31   b  which are both end portions along the axial direction of the stator core  31  and are opposed to the inner rotor  10  and the outer rotor  20 , thin electromagnetic steel sheet surfaces are bent toward the rotation-direction lagged side at an angle smaller than 90°. That is, since the rotation direction is opposite to that of the outer rotor  20  in embodiment 1, it is found that the side portion  31   b  of the stator core  31  on the outer rotor  20  side is bent toward the opposite side as compared to  FIG.  3   , so as to be directed toward the rotation-direction lagged side. 
     The other configurations are the same as in embodiment 1, and the stator core  31  in embodiment 5 is also attached with tensile stress applied thereto. 
     In the above description, the example in which the inner rotor  10  and the outer rotor  20  rotate in directions opposite to each other and the angular velocity of the outer rotor  20  is ½ of that of the inner rotor  10 , has been shown. However, the rotation directions of the inner rotor  10  and the outer rotor  20  may be the same and their rotation speeds may be set independently of each other. In a case where the rotation directions of the inner rotor  10  and the outer rotor  20  are the same, the side portions  31   b  of the stator core  31  on the inner rotor  10  side and the outer rotor  20  side may be bent toward the same side so as to be directed toward the rotation-direction lagged side. 
     As described above, according to embodiment  5 , the same effects as in embodiment 1 are provided. That is, the stator  30  is provided between the inner rotor  10  and the outer rotor  20  which are movable with the shaft  2  as an axis, and the stator  30  includes the stator core  31  formed by stacking thin sheets in the rotation direction and is retained with tensile stress applied thereto in the axial direction, whereby the stator core  31  can be retained without deteriorating the magnetic property thereof. Thus, efficiency and torque of the rotary electric machine can be enhanced. 
     In addition, since the inner rotor  10  and the outer rotor  20  of the rotary electric machine  1  are not connected to each other, the rotation directions and the rotation speeds of the respective rotors can be set independently of each other. Therefore, even in a case where the respective rotors drive different targets, control can be performed with their rotation directions and rotation speeds set respectively. 
     Embodiment 6 
     Hereinafter, a rotary electric machine as an electromagnetic device according to embodiment 6 will be described. 
       FIG.  15    is a sectional view along the rotary shaft, showing the structure of a rotary electric machine  1 A according to embodiment 6. In  FIG.  15   , the rotary electric machine  1 A includes two rotors which are a non-load-side rotor  10 A and a load-side rotor  20 A, and a stator  30 A provided between the two rotors in the radial direction, and is formed as an axial gap motor of a double-rotor type. 
     The non-load-side rotor  10 A includes a shaft  2 A, a non-load-side boss  12 A press-fitted and fixed to the shaft  2 A, and a permanent magnet  14 A adhered and fixed to the load side of the non-load-side boss  12 A. 
     The load-side rotor  20 A includes a load-side boss  21 A fixed to the shaft  2 A, and a permanent magnet  22 A adhered and fixed to the non-load side of the load-side boss  21 A. 
     The stator  30 A is provided between the non-load-side rotor  10 A and the load-side rotor  20 A, and rotatably retains the non-load-side rotor  10 A and the load-side rotor  20 A by a load-side inner bearing  3 A and a non-load-side inner bearing  4 A. 
     In the stator  30 A, a stator core  31 A is provided between a radially-outer-side retention member  35 A and a radially-inner-side retention member  33 A, and a stator coil  32 A is wound around the stator core  31 A. The radially-outer-side retention member  35 A is attached to a non-load-side base  39 A and a load-side base  39 B provided separately from the shaft  2 A, thus surrounding the two rotors, i.e., the non-load-side rotor  10 A and the load-side rotor  20 A. The shaft  2 A rotatably protrudes from a center part of the load-side base  39 B and is separate from the load-side base  39 B. 
     The stator core  31 A is formed by stacking, substantially in the circumferential direction, i.e., the rotation direction, thin electromagnetic steel sheets that are long in the radial direction. At both ends in the radial direction, both radial-direction side portions of the stator core  31 A on the non-load-side rotor  10 A side and the load-side rotor  20 A side extend and protrude. A distal end portion  31 A c  on the radially outer side has an L shape bent outward in the axial direction, and this L-shaped part is engaged with a groove  35 A a  in the radially-outer-side retention member  35 A. A distal end portion  31 A d  on the radially inner side of the stator core  31 A has an L shape bent inward in the axial direction, and is fixed by being engaged with an L-shaped groove  33 A b  provided in the radially-inner-side retention member  33 A. 
     The natural length between the engaged parts of both distal end portions  31 A c ,  31 A d  in the radial direction of the stator core  31 A is smaller than a length L between the bottom surface of the groove  35 A a  in the radially-outer-side retention member  35 A and the bottom surface of the L-shaped groove  33 A b  provided at an outer side part of the radially-inner-side retention member  33 A. Therefore, the stator core  31 A is extended in the radial direction with tensile stress applied thereto. 
       FIG.  16    is a sectional view showing the structure of the stator  30 A and is a partial sectional view along a D-D line direction in  FIG.  15   . In the drawing, the rotary electric machine  1 A has a concentrated-winding configuration with  10  poles and  12  slots. The stator core  31 A is formed by stacking, substantially in the circumferential direction, thin steel sheets that are long in the radial direction, as described above. The non-load-side rotor  10 A and the load-side rotor  20 A rotate at the same angular velocity. 
     The thin steel sheets of the stator core  31 A are electromagnetic steel sheets manufactured by rolling, and are arranged such that the rolling direction coincides with the axial direction of the stator core  31 A, i.e., the direction in which the non-load-side rotor  10 A and the load-side rotor  20 A are opposed to each other. 
     As described above, according to embodiment  6 , even in the rotary electric machine forming an axial gap motor of a double-rotor type, since the stator core  31 A is fixed with tensile stress applied thereto as in embodiments 1 to 5, magnetic property deterioration due to stress does not occur and torque is improved. Thus, a high-efficiency rotary electric machine can be provided. 
     Embodiment 7 
     Hereinafter, a magnetic gear as an electromagnetic device according to embodiment 7 will be described. 
       FIG.  17    is a sectional view along the rotary shaft, showing the structure of a magnetic gear  1 B according to embodiment 7, and  FIG.  18    is a partial sectional view along an E-E line direction. The magnetic gear  1 B does not include the stator coil  32 , and  FIG.  17    corresponds to a case where the stator coil  32  is not wound around the stator core  31  in  FIG.  13    in embodiment 5. As shown in  FIG.  18   , the stator core  31  is flat and does not have bent portions at both side portions in the radial direction. For the inner rotor  10  and the outer rotor  20 , the stator cores  31  serve as pole pieces for modulating magnetic fluxes of the inner rotor  10  and the outer rotor  20  in accordance with greatness/smallness of magnetic resistance based on presence/absence of the stator cores  31  in the circumferential direction. 
     In  FIG.  17    and  FIG.  18   , the inner rotor  10  and the outer rotor  20  of the magnetic gear  1 B are not connected to each other. The inner rotor  10  rotates counterclockwise in the drawing, and the outer rotor  20  rotates clockwise in the drawing at the same electric angular velocity as the inner rotor  10 . Here, for example, it is assumed that the number of poles of the outer rotor  20  is  60  and the number of poles of the inner rotor  10  is  12 , so that the number of poles of the outer rotor  20  is 5 times the number of poles of the inner rotor  10 . In this case, the magnetic gear having a speed reduction ratio of  5  can be obtained. While the number of poles of the outer rotor  20  which is a low-speed rotor is  60  and the number of poles of the inner rotor  10  which is a high-speed rotor is  12 , the number of the stator cores  31  which are pole pieces in the circumferential direction is set to satisfy (number of pole pieces) = (number of poles of low-speed rotor) ± (number of poles of high-speed rotor). Therefore, the number of the stator cores  31  is 60 ± 12 = 72 or 48, and in this example, is set to 48. 
     The other configurations are the same as in embodiment 5. That is, also in the present embodiment, the stator core  31  is extended in the axial direction and fixed with tensile stress applied thereto. 
     In the above description, it is described that the rotation directions of the inner rotor  10  and the outer rotor  20  are the same, but their rotation directions may be opposite to each other. In this case, the number of the stator cores  31  which are pole pieces may be 72 instead of 48. In addition, the numbers of poles of the inner rotor  10  and the outer rotor  20  may be set in accordance with the change gear ratio of the magnetic gear for targets to be driven by the inner rotor  10  and the outer rotor  20 . 
     As described above, according to embodiment 7, in the magnetic gear  1 B, the stator core  31  is extended in the axial direction and fixed with tensile stress applied thereto, whereby the stator core  31  can be retained without deteriorating the magnetic property thereof. Thus, efficiency and torque of the magnetic gear can be enhanced. 
     Embodiment 8 
     Hereinafter, a linear motor as an electromagnetic device according to embodiment 8 will be described. 
       FIG.  19    is a sectional view along a movable axis of a linear motor  1 C according to embodiment 8. In  FIG.  19   , the linear motor  1 C includes a stator  30 C provided between two movable elements  10 C,  20 C. The first movable element  10 C on one side of the stator  30 C and the second movable element  20 C on the other side thereof are retained movably in an arrow direction in the drawing by a linear guide (not shown). 
     The first movable element  10 C is formed such that a permanent magnet  14 C is pasted to a first movable base  12 C. The second movable element  20 C is formed such that a permanent magnet  22 C is pasted to a second movable base  21 C. 
     The stator  30 C includes a stator core  31 C and a stator coil  32 C wound around the stator core  31 C. The stator core  31 C is extended in a direction perpendicular to the movable direction of the first movable element  10 C and the second movable element  20 C and parallel to the first movable element  10 C and the second movable element  20 C, and is retained at both end portions. Both end portions of the stator core  31 C are retained and fixed by, for example, bolts, with tensile stress applied thereto in the extended direction of the stator core  31 C. As in embodiments 1 to 6, the stator core  31 C is formed by stacking thin sheets. Specifically, the stator core  31 C is formed by stacking, in the movable direction of the first movable element  10 C and the second movable element  20 C, thin sheets rolled such that the rolling direction coincides with the opposing direction to the first movable element  10 C and the second movable element  20 C. 
     As described above, according to embodiment 8, the same effects as in embodiment 1 are provided. That is, the stator core  31  can be retained without deteriorating the magnetic property thereof. Thus, efficiency and torque of the linear motor can be enhanced. 
     Embodiment 9 
     Hereinafter, an aircraft according to embodiment 9 will be described. 
       FIG.  20    shows an example of an aircraft  100  according to embodiment 9, and the electromagnetic device described in each embodiment 1 to 7 is provided thereto. In  FIG.  20   , in an engine case  210  of the aircraft  100 , a fan  230 , the rotary electric machine  1 ,  1 A, the magnetic gear  1 B, and an engine  220  are arranged and connected via a shaft. The rotary electric machine  1 ,  1 A is a motor and is used for driving the fan  230 . The magnetic gear  1 B is used as a transmission for increasing/reducing the speed. 
     In the case where the rotary electric machine  1 ,  1 A is provided, although not shown, a gear for changing the number of revolutions may be provided between the fan  230  and the rotary electric machine  1 ,  1 A and/or between the rotary electric machine  1 ,  1 A and the engine  220 . In this case, the gear may be a mechanical gear such as a spur gear or a planetary gear, or may be the magnetic gear  1 B. 
     In the case where the rotary electric machine  1 ,  1 A is provided, in  FIG.  20   , the rotary electric machine  1 ,  1 A and the engine  220  are arranged coaxially with the fan  230 . However, they may be arranged with different axes via a gear or the like, whereby the same effects are provided. 
     In the rotary electric machine shown in each embodiment 1 to 6, since tensile stress is applied to the stator core, the stator core can be assuredly retained without deteriorating the magnetic property and high torque output can be obtained. Therefore, the rotary electric machine is suitably applied to a rotation target provided to the aircraft. 
     In the magnetic gear shown in embodiment 7, the stator core can be assuredly retained without deteriorating the magnetic property and a part subjected to wear is not present as compared to a mechanical gear. Therefore, the magnetic gear is suitably applied to a mechanism component provided to the aircraft. 
       FIG.  21    shows another example of the aircraft  100  according to embodiment 9.  FIG.  21 A  shows the aircraft  100  having a fan case  240  at a tail, and  FIG.  21 B  is a schematic enlarged view of the fan case  240 . In the drawings, similarly, the electromagnetic device described in each embodiment 1 to 7 is provided. In  FIG.  20   , the electromagnetic device is stored in the same engine case  210  as the engine  220 , whereas, as shown in  FIG.  21   , the electromagnetic device may be stored in a case different from the case for the engine  220 , to drive a driving target.  FIG.  21    shows the example in which the rotary electric machine  1 ,  1 A or the magnetic gear  1 B is connected via a shaft to the fan  230  in the fan case  240  at the tail. In the case where the magnetic gear  1 B is connected, the rotary electric machine  1 ,  1 A or the engine  220  is further connected to perform driving. 
     Alternatively, without having the engine  220 , the aircraft  100  may have the rotary electric machine  1 ,  1 A which is the electromagnetic device, as a drive motive-power source. Although not shown, the rotary electric machine  1 ,  1 A may be attached to a blade of a helicopter, a multicopter having a plurality of rotor blades, or the like, instead of the aircraft  100  having fixed wings, so as to be used as a drive source. 
     As described above, according to embodiment 9, the electromagnetic device shown in each embodiment 1 to 7 is applied to an aircraft. Thus, the stator core can be assuredly retained without deteriorating the magnetic property and high torque output can be obtained, whereby the flight range per fuel can be improved. 
     Modification and Supplementary Note of Embodiments 
     In the above embodiments, it is desirable that the stator core is retained with tensile stress applied thereto at a level not exceeding 100 MPa, but the tensile stress level may be such a level as not to cause breakdown or breakage by the stress. 
     Although the disclosure is described above in terms of various exemplary embodiments and implementations, it should be understood that the various features, aspects, and functionality described in one or more of the individual embodiments are not limited in their applicability to the particular embodiment with which they are described, but instead can be applied, alone or in various combinations to one or more of the embodiments of the disclosure. 
     It is therefore understood that numerous modifications which have not been exemplified can be devised without departing from the scope of the present disclosure. For example, at least one of the constituent components may be modified, added, or eliminated. At least one of the constituent components mentioned in at least one of the preferred embodiments may be selected and combined with the constituent components mentioned in another preferred embodiment. 
     In the above embodiments, the rotary electric machines  1 ,  1 A are described as a motor. However, the same effects are provided even when the rotary electric machines  1 ,  1 A operate as an electric generator. 
     DESCRIPTION OF THE REFERENCE CHARACTERS 
     
         
           1 ,  1 A rotary electric machine 
           1 B magnetic gear 
           1 C linear motor 
           2 ,  2 A shaft 
           3 ,  3 A load-side inner bearing 
           4 ,  4 A non-load-side inner bearing 
           5  load-side outer bearing 
           6  non-load-side outer bearing 
           10  inner rotor 
           10 A non-load-side rotor 
           10 C first movable element 
           12  boss 
           12 A non-load-side boss 
           12 C first movable base 
           14 ,  14 A,  14 C permanent magnet 
           20  outer rotor 
           20 A load-side rotor 
           20 C second movable element 
           21  outer shaft 
           21 A load-side boss 
           21 C second movable base 
           22 ,  22 A,  22 C permanent magnet 
           30 ,  30 A,  30 C stator 
           31 ,  31 A,  31 C stator core 
           31   a  body portion 
           31   b  side portion 
           31   c ,  31   d  distal end portion 
           31 A c ,  31 A d  distal end portion 
           32 ,  32 A,  32 C stator coil 
           33  non-load-side base 
           33 A radially-inner-side retention member 
           33 A b  groove 
           33   a  groove 
           34  load-side base 
           35  load-side retention member 
           35 A radially-outer-side retention member 
           35 A a  groove 
           36  load-side retention surface 
           37  fixed-side retention surface 
           38  bolt 
           39 A non-load-side base 
           39 B load-side base 
           100  aircraft 
           210  engine case 
           220  engine 
           230  fan 
           240  fan case 
           311  side portion 
           311 A protruding part 
           311 B recessed part 
           311   a  stacked body 
           311   b  structural body 
           312  retention portion 
           312   a ,  312   a   1 ,  312   a   2  cutout 
           312   b  flange