Patent Publication Number: US-2022231554-A1

Title: Rotor, electric motor, compressor, and air conditioner

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a U.S. national stage application of International Patent Application No. PCT/JP2019/025334 filed on Jun. 26, 2019, the disclosure of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a rotor for use in an electric motor. 
     BACKGROUND 
     Rotors having magnet insertion holes provided with flux barrier parts (also referred to as “flux barriers”), which are space, have been used. In such a rotor, leakage flux can be reduced, and thus motor efficiency can be enhanced. However, because of the presence of thin portions between the outer peripheral surface of the rotor and the flux barrier parts, stress tends to be concentrated on these thin portions during rotation of the rotor. As the rotation speed of the rotor increases, this stress increases, and as a result, the rotor, especially the thin portions, is easily deformed. In view of this, a rotor having a center rib (also simply referred to as a “rib”) between two magnet insertion holes is proposed (see, for example, Patent Reference 1). In the rotor having the center rib, a part of stress occurring in the rotor is dispersed to the center rib, and thus stress generated on the thin portions is reduced. Accordingly, deformation of the rotor can be prevented. 
     PATENT REFERENCE 
     
         
         Patent Reference 1: Japanese Patent Application Publication No. 2017-192211 
       
    
     With a conventional technique, however, in the case where the center rib is present between two magnet insertion holes, the strength of the rotor to a centrifugal force increases, but magnetic flux passing through the center rib, that is, leakage flux, increases, and power of an electric motor decreases, disadvantageously. 
     SUMMARY 
     An object of the present invention is to enhance power of an electric motor. 
     A rotor according to an aspect of the present invention includes: a rotor core including a first magnet insertion hole, a second magnet insertion hole, and a center lib between the first magnet insertion hole and the second magnet insertion hole; 
     a first permanent magnet disposed in the first magnet insertion hole, the first permanent magnet being W1 mm long in a longitudinal direction in a plane perpendicular to an axial direction; and 
     a second permanent magnet disposed in the second magnet insertion hole, the second permanent magnet being W1 mm long in a longitudinal direction in the plane, wherein 
     the first magnet insertion hole and the second magnet insertion hole are disposed in a V shape so that a center of one pair of the first magnet insertion hole and the second magnet insertion hole projects toward a center of the rotor core in the plane, 
     the first magnet insertion hole includes 
     a first magnet disposition part in which the first permanent magnet is disposed, 
     a first flux barrier part communicating with the first magnet disposition part, 
     a first outside opening part located on an outer side with respect to the first magnet disposition part in a radial direction of the rotor core, the first outside opening part having a radius of curvature of R1 mm in the plane, and 
     a first inside opening part located on an inner side with respect to the first magnet disposition part in the radial direction, the first inside opening part having a radius of curvature of R2 mm in the plane, and 
     the second magnet insertion hole includes 
     a second magnet disposition part in which the second permanent magnet is disposed, 
     a second flux barrier part communicating with the second magnet disposition part, 
     a second outside opening part located on an outer side with respect to the second magnet disposition part in the radial direction, the second outside opening part having a radius of curvature of R1 mm in the plane, and 
     a second inside opening part located on an inner side with respect to the second magnet disposition part in the radial direction, the second inside opening part having a radius of curvature of R2 mm in the plane, and 
     the rotor satisfies R1&gt;R2 and 0&lt;(R1+R2)/W1&lt;0.082. 
     An electric motor according to another aspect of the present invention includes: 
     a stator; and 
     the rotor disposed inside the stator. 
     A compressor according to yet another aspect of the present invention includes: 
     a closed container; 
     a compression device disposed inside the closed container; and 
     the electric motor to drive the compression device. 
     An air conditioner according to still another aspect includes: 
     the compressor; and 
     a heat exchanger. 
     According to the present invention, power of the electric motor can be enhanced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view schematically illustrating a structure of an electric motor according to a first embodiment of the present invention. 
         FIG. 2  is a plan view schematically illustrating a structure of a rotor. 
         FIG. 3  is an enlarged view illustrating a region constituting one magnetic pole of the rotor. 
         FIG. 4  is an enlarged view illustrating a peripheral structure of a center lib. 
         FIG. 5  is an enlarged view illustrating the peripheral structure of the center lib. 
         FIG. 6  is a graph showing a relationship between a centrifugal force [p.u.] generated during rotation of the rotor and a ratio of radii of curvature. 
         FIG. 7  is a graph showing a relationship between demagnetization resistance [p.u.] and a ratio of radii of curvature. 
         FIG. 8  is a graph showing a relationship between demagnetization resistance [p.u.] and a ratio of radii of curvature. 
         FIG. 9  is an enlarged view illustrating a peripheral structure of an outside opening part. 
         FIG. 10  is a graph showing a relationship between a centrifugal force [p.u.] and a ratio of radii of curvature. 
         FIG. 11  is a graph showing a relationship between a demagnetization resistance [p.u.] and a ratio of radii of curvature. 
         FIG. 12  is a cross-sectional view schematically illustrating a structure of a compressor according to a second embodiment of the present invention. 
         FIG. 13  is a diagram schematically illustrating a configuration of a refrigerating and air conditioning apparatus according to a third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
     In an xyz orthogonal coordinate system shown in each drawing, a z-axis direction (z axis) represents a direction parallel to an axis Ax of an electric motor  1 , an x-axis direction (x axis) represents a direction orthogonal to the z-axis direction (z axis), and a y-axis direction (y axis) represents a direction orthogonal to both the z-axis direction and the x-axis direction. The axis Ax is a center of a stator  3 , and is also a rotation center of a rotor  2 . A direction parallel to the axis Ax is also referred to as an “axial direction of the electric motor  1 ,” an “axial direction of the rotor  2 ,” or simply as an “axial direction.” The “radial direction” is a radial direction of the rotor  2  or the stator  3 , and is a direction orthogonal to the axis Ax. An xy plane is a plane perpendicular to the axial direction. An arrow Dl represents a circumferential direction about the axis Ax. The circumferential direction of the rotor  2  or the stator  3  will be also simply referred to as a “circumferential direction.” 
     &lt;Structure of Electric motor  1 &gt; 
       FIG. 1  is a plan view schematically illustrating a structure of the electric motor  1  according to a first embodiment of the present invention. 
     The electric motor  1  includes the rotor  2  and the stator  3 . 
     In this embodiment, the electric motor  1  is, for example, a three-phase synchronous motor. Specifically, the electric motor  1  is a permanent magnet synchronous motor (also called a brushless DC motor) such as a permanent magnet-embedded motor. 
     The rotor  2  is rotatably disposed inside the stator  3 . An air gap is formed between the rotor  2  and the stator  3 . The air gap is, for example, from 0.3 mm to 1 mm. The rotor  2  rotates about an axis Ax. The rotor  2  includes a rotor core  21 , at least one permanent magnet  22 , and a shaft  24 . 
     The stator  3  is disposed outside the rotor  2 . The stator  3  includes, for example, an annular stator core  31  and a stator winding wound around the stator core  31 . In the example illustrated in  FIG. 1 , the stator  3  includes a yoke  35  extending in the circumferential direction of the stator  3 , and a plurality of teeth  34  extending radially from the yoke  35 . In the example illustrated in  FIG. 1 , the stator core  31  includes 18 teeth  34 . Space between the teeth  34  is at least one slot  33  in which the stator winding is disposed. 
     The stator winding used for the stator  3  is, for example, a winding in which an insulation film is formed around a conductor such as copper or aluminium. The stator winding forms a coil for generating a rotation magnetic field. When a current flows in the stator winding, a rotation magnetic field occurs. The number of windings and the diameter of the stator winding are set in accordance with, for example, a voltage applied to the stator winding, the rotation speed of the electric motor  1 , or the cross-sectional area of the slot. 
     The stator core  31  of the stator  3  is constituted by, for example, annular electromagnetic steel sheets stacked in the axial direction. Each of the electromagnetic steel sheets is punched in a predetermined shape beforehand. The thickness of each electromagnetic steel sheet of the stator  3  is, for example 0.1 mm to 0.7 mm. In this embodiment, the thickness of each electromagnetic steel sheet of the stator  3  is 0.35 mm. The electromagnetic steel sheets are fixed together by swaging. 
     A structure of the rotor  2  will be described specifically. 
       FIG. 2  is a plan view schematically illustrating the structure of the rotor  2 . 
       FIG. 3  is an enlarged view illustrating a region constituting one magnetic pole of the rotor  2 . 
     In the example illustrated in  FIG. 2 , the rotor  2  includes the rotor core  21 , a plurality of permanent magnets  22  embedded in the rotor core  21 , and the shaft  24  fitted in a shaft hole  215  of the rotor core  21 . The rotor  2  includes two or more magnetic poles. Two or more permanent magnets  22  constitute one magnetic pole of the rotor  2 . In this embodiment, the electric motor  2  is a permanent magnet-embedded electric rotor. 
     The rotor core  21  is an annular rotor core. The rotor core  21  is formed of, for example, a plurality of electromagnetic steel sheets. The rotor core  21  includes at least one pair of magnet insertion holes  210 , at least one center rib  213 , and at least one thin portion  214 . 
     The rotor core  21  of the rotor  2  is constituted by, for example, annular electromagnetic steel sheets stacked in the axial direction. Each of the electromagnetic steel sheets is punched in a predetermined shape beforehand. The thickness of each electromagnetic steel sheet of the rotor  2  is, for example 0.1 mm to 0.7 mm. In this embodiment, the thickness of each electromagnetic steel sheet of the rotor  2  is 0.35 mm. The electromagnetic steel sheets are fixed together by swaging. 
     The pair of magnet insertion holes  210  includes a first magnet insertion hole  211  and a second magnet insertion hole  212 . In the xy plane, the center of one pair of magnet insertion holes  210  projects toward the center (i.e., the axis Ax) of the rotor core  21 . That is, one pair of magnet insertion holes  210  (i.e., the first magnet insertion hole  211  and the second magnet insertion hole  212 ) is disposed in a V shape in the xy plane. 
     The center lib  213  is a part of the rotor core  21 , and extends in the radial direction. The center lib  213  is disposed between the first magnet insertion hole  211  and the second magnet insertion hole  212 . Thus, the center lib  213  is located at a magnetic pole center part of the rotor  2 . 
     As long as the rotor core  21  includes the center lib  213 , arrangement of the pair of magnet insertion holes  210  (i.e., the first magnet insertion hole  211  and the second magnet insertion hole  212 ) is not limited to a V shape. 
     The first magnet insertion hole  211  includes a magnet disposition part  211   a  (also referred to as a first magnet disposition part) in which the permanent magnet  22  as a first permanent magnet is disposed, a flux barrier part  211   b  (also referred to as a first flux barrier part) that is space communicating with the magnet disposition part  211   a , an outside opening part  211   c  (also referred to as a first outside opening part), and an inside opening part  211   d  (also referred to as a first inside opening part). 
     The second magnet insertion hole  212  includes a magnet disposition part  212   a  (also referred to as a second magnet disposition part) in which the permanent magnet  22  as a second permanent magnet is disposed, a flux barrier part  212   b  (also referred to as a second flux barrier part) that is space communicating with the magnet disposition part  212   a , an outside opening part  212   c  (also referred to as a second outside opening part), and an inside opening part  212   d  (also referred to as a second inside opening part). 
     The flux barrier part  211   b  is located at an inter-pole part of the rotor  2  or near the inter-pole part. Similarly, the flux barrier part  212   b  is located at an inter-pole part of the rotor  2  or near the inter-pole part. 
     The thin portion  214  between the outer peripheral surface of the rotor core  21  and the first magnet insertion hole  211  will be also referred to as a “first thin portion.” The thin portion  214  between the outer peripheral surface of the rotor core  21  and the second magnet insertion hole  212  will be also referred to as a “second thin portion.” In this case, the first thin portion is a region between the flux barrier part  211   b  and the outer peripheral surface of the rotor core  21 , and the second thin portion is a region between the flux barrier part  212   b  and the outer peripheral surface of the rotor core  21 . 
     A minimum width of each thinner portion  214  in the radial direction is, for example, greater than or equal to the thickness of one electromagnetic steel sheet of the rotor core  21 . The minimum width of each thinner portion  214  in the radial direction is preferably substantially equal to the thickness of one electromagnetic steel sheet of the rotor core  21 . In this case, an increase of leakage flux in each thin portion  214  can be effectively suppressed. 
     In the example illustrated in  FIG. 2 , the rotor core  21  includes six magnet insertion holes  210 , six center ribs  213 , and twelve thin portions  214 , and the shaft hole  215 . The six magnet insertion holes  210  are arranged in the circumferential direction of the rotor  2 . Each first magnet insertion hole  211  and each second magnet insertion hole  212  extend in the axial direction. 
     The permanent magnet  22  as the first permanent magnet is placed in each first magnet insertion hole  211 . The permanent magnet  22  as the second permanent magnet is placed in each second magnet insertion hole  212 . 
     The shaft  24  is fixed to the shaft hole  215  by a technique such as shrink fitting or press fitting. When a current flows in the stator winding of the stator  3 , the rotor  2  (specifically, the rotor core  21 ) rotates, and rotation energy of the rotor core  21  is transferred to the shaft  24 . 
     Each permanent magnet  22  is, for example, a flat-plate permanent magnet. Each permanent magnet  22  is, for example, a rare earth magnet containing neodymium (Nd) and dysprosium (Dy). The rare earth magnet has a high residual flux density and a high coercive force. Thus, in the case of using rare earth magnets as the permanent magnets  22 , efficiency of the electric motor  1  can be enhanced. As the permanent magnets  22 , magnets except for rare earth magnets, such as ferrite sintered magnets, may be used. 
     Each permanent magnet  22  is magnetized in a direction perpendicular to the longitudinal direction of the permanent magnet  22  in the xy plane. That is, each permanent magnet  22  is magnetized in the lateral direction of the permanent magnet  22  in the xy plane. 
     In the xy plane, each permanent magnet  22  is W1 mm long in the longitudinal direction. The length of W1 is a maximum length of each permanent magnet  22  in the longitudinal direction. It should be noted that the length of each permanent magnet  22  in the longitudinal direction in the first magnet insertion hole  211  may be different from the length of each permanent magnet  22  in the longitudinal direction in the second magnet insertion hole  212 . 
     One pair of magnet insertion holes  210  is associated with one magnetic pole of the rotor  2 . Specifically, two permanent magnets  22  (i.e., the first permanent magnet and the second permanent magnet) placed in one pair of magnet insertion holes  210  constitute one magnetic pole (i.e., a north pole or a south pole) of the rotor  2 . The number of magnetic poles of the rotor  2  only needs to two or more. In this embodiment, the rotor  2  has six magnetic poles. 
     Each of the magnetic poles (“each magnetic pole” or “magnetic pole”) refers to a region serving as a north pole or a south pole of the rotor  2 . 
     In general, while a rotor rotates, a centrifugal force is exerted on a rotor core. Thus, if the rotor core does not include a center lib, large stress is applied to thin portions between the outer peripheral surface of the rotor core and magnet insertion holes (specifically, flux barrier parts). If this stress is large, the rotor core (especially, the thin portions) is easily deformed. On the other hand, in this embodiment, since the rotor core  21  includes the center ribs  213 , part of the stress generated in the rotor  2  is dispersed to the center ribs  213 , and thus stress applied to the thin portions  214  is alleviated. Accordingly, deformation of the rotor core  21 , especially the thin portions  214 , can be prevented. As a result, the electric motor  1  can rotate at high speed and consequently power of the electric motor  1  can be enhanced. 
       FIGS. 4 and 5  are enlarged views each illustrating a peripheral structure of the center lib  213 . 
     As illustrated in  FIG. 4 , the first magnet insertion hole  211  includes the magnet disposition part  211   a , the flux barrier part  211   b  ( FIG. 3 ), the outside opening part  211   c , and the inside opening part  211   d . The second magnet insertion hole  212 , includes the magnet disposition part  212   a , the flux barrier part  212   b  ( FIG. 3 ), the outside opening part  212   c , and the inside opening part  212   d.    
     The outside opening part  211   c  is located on an outer side of the magnet disposition part  211   a  in the radial direction of the rotor core  21 , and adjacent to the center lib  213 . The outside opening part  211   c  has a radius of curvature of R1 in the xy plane. Thus, the outside opening part  211   c  projects from the magnet disposition part  211   a  outward in the radial direction. 
     The inside opening part  211   d  is located on an inner side of the magnet disposition part  211   a  in the radial direction of the rotor core  21 , and adjacent to the center lib  213 . The inside opening part  211   d  has a radius of curvature of R2 in the xy plane. Thus, the inside opening part  211   d  projects from the magnet disposition part  211   a  inward in the radial direction. 
     Since the rotor core  21  includes the outside opening part  211   c  and the inside opening part  211   d , demagnetization of the permanent magnets  22  (especially, the permanent magnets  22  in the first magnet insertion holes  211 ) generated by a magnetic field from the stator winding of the stator  3  can be reduced. As a result, power of the electric motor  1  can be enhanced. 
     The outside opening part  212   c  is located on an outer side of the magnet disposition part  212   a  in the radial direction of the rotor core  21 , and adjacent to the center lib  213 . The outside opening part  212   c  has a radius of curvature of R1 in the xy plane. Thus, the outside opening part  212   c  projects from the magnet disposition part  212   a  outward in the radial direction. 
     The inside opening part  212   d  is located on an inner side of the magnet disposition part  212   a  in the radial direction of the rotor core  21 , and adjacent to the center lib  213 . The inside opening part  212   d  has a radius of curvature of R2 in the xy plane. Thus, the inside opening part  212   d  projects from the magnet disposition part  212   a  inward in the radial direction. 
     Since the rotor core  21  includes the outside opening part  212   c  and the inside opening part  212   d , demagnetization of the permanent magnets  22  (especially, the permanent magnets  22  in the second magnet insertion holes  212 ) generated by a magnetic field from the stator winding of the stator  3  can be reduced. As a result, power of the electric motor  1  can be enhanced. 
     In this embodiment, the radius of curvature of R1 is 0.5 mm, the radius of curvature of R2 is 0.3 mm, and the length of W1 of each permanent magnet  22  is 21.5 mm. It should be noted that the radius of curvature of R1, the radius of curvature of R2, and the length of W1 of each permanent magnet  22  are not limited to these examples. The shape of the first magnet insertion hole  211  may be different from the shape of the second magnet insertion hole  212 . 
     As illustrated in  FIG. 4 , the rotor core  21  includes, at each magnetic pole, an outside curved part  216   a  (also referred to as a first outside curved part), an outside curved part  216   b  (also referred to as a second outside curved part), an inside curved part  217   a  (also referred to as a first inside curved part), an inside curved part  217   b  (also referred to as a second inside curved part), an outside support part  218   a  (also referred to as a first outside support part), an outside support part  218   b  (also referred to as a second outside support part), an inside support part  219   a  (also referred to as a first inside support part), an inside support part  219   b  (also referred to as a second inside support part), an outside connection part  220   a  (also referred to as a first outside connection part), an outside connection part  220   b  (also referred to as a second outside connection part), an inside connection part  221   a  (also referred to as a first inside connection part), and an inside connection part  221   b  (also referred to as a second inside connection part). 
     The outside curved part  216   a  defines the outside opening part  211   c . The outside curved part  216   a  has a curvature of 1/R1 in the xy plane. Accordingly, the outside opening part  211   c  has a radius of curvature of R1 in the xy plane. 
     The outside curved part  216   b  defines the outside opening part  212   c . The outside curved part  216   b  has a curvature of  1 /R1 in the xy plane. Accordingly, the outside opening part  212   c  has a radius of curvature of R1 in the xy plane. 
     The inside curved part  217   a  defines the inside opening part  211   d . The inside curved part  217   a  has a curvature of 1/R2 in the xy plane. Accordingly, the inside opening part  211   d  has a radius of curvature of R2 in the xy plane. 
     The inside curved part  217   b  defines the inside opening part  212   d . The inside curved part  217   b  has a curvature of 1/R2 in the xy plane. Accordingly, the inside opening part  212   d  has a radius of curvature of R2 in the xy plane. 
     The outside support part  218   a  supports the permanent magnet  22  in the first magnet insertion hole  211 . The outside support part  218   a  defines the magnet disposition part  211   a.    
     The outside support part  218   b  supports the permanent magnet  22  in the second magnet insertion hole  212 . The outside support part  218   b  defines the magnet disposition part  212   a.    
     The inside support part  219   a  supports the permanent magnet  22  in the first magnet insertion hole  211 . The inside support part  219   a  defines the magnet disposition part  211   a.    
     The inside support part  219   b  supports the permanent magnet  22  in the second magnet insertion hole  212 . The inside support part  219   b  defines the magnet disposition part  212   a.    
     The outside connection part  220   a  is located between the outside curved part  216   a  and the outside support part  218   a , and connects the outside curved part  216   a  and the outside support part  218   a . The outside connection part  220   a  has a radius of curvature of R3 in the xy plane. In other words, the outside connection part  220   a  has a curvature of 1/R3 in the xy plane. 
     The outside connection part  220   b  is located between the outside curved part  216   b  and the outside support part  218   b , and connects the outside curved part  216   b  and the outside support part  218   b . The outside connection part  220   b  has a radius of curvature of R3 in the xy plane. In other words, the outside connection part  220   b  has a curvature of 1/R3 in the xy plane. 
     The inside connection part  221   a  is located between the inside curved part  217   a  and the inside support part  219   a , and connects the inside curved part  217   a  and the inside support part  219   a . The inside connection part  221   a  has a radius of curvature of R4 in the xy plane. In other words, the inside connection part  221   a  has a curvature of 1/R4 in the xy plane. 
     The inside connection part  221   b  is located between the inside curved part  217   b  and the inside support part  219   b , and connects the inside curved part  217   b  and the inside support part  219   b . The inside connection part  221   b  has a radius of curvature of R4 in the xy plane. In other words, the inside connection part  221   b  has a curvature of 1/R4 in the xy plane. 
     The relationship between the radii of curvature of R1 and R2 satisfies R1&gt;R2. Accordingly, stress concentrated on the center libs  213  is dispersed, and thus, mechanical strength of the rotor core  21  to a centrifugal force generated in the rotor  2  can be increased, and deformation of the rotor core  21 , especially the thin portions  214 , and be prevented. Consequently, the electric motor  1  can rotate at high speed, and thus power of the electric motor  1  can be enhanced. In addition, a magnetic resistance becomes large in the outside opening part  211   c  and the outside opening part  212   c , and thus, demagnetization in each permanent magnet  22  can be reduced. On the other hand, magnetic resistance in the inside opening part  211   d  and the inside opening part  212   d  is smaller than that in the outside opening part  211   c  and the outside opening part  212   c , and thus a magnetic force of each permanent magnet  22  can be effectively used. That is, if the rotor  2  satisfies R1&gt;R2, demagnetization of the permanent magnets  22  can be reduced, and power of the electric motor  1  can be enhanced. 
     The relationship between the radii of curvature of R3 and R4 satisfies R3&gt;R4. Accordingly, it is possible to achieve both reduction of demagnetization of the permanent magnets  22  and increase in power of the electric motor  1 . Specifically, as the radius of curvature of R3 increases, the lengths of the outside support part  218   a  and the outside support part  218   b  decrease. Accordingly, the outside opening part  211   c  and the outside opening part  212   c  become larger, and magnetic resistance increases. As a result, demagnetization in each permanent magnet  22  can be reduced. On the other hand, as the radius of curvature of R4 decreases, the lengths of the inside support part  219   a  and the inside support part  219   b  increase. Accordingly, the inside opening part  211   d  and the inside opening part  212   d  become smaller, and thus a magnetic force of each permanent magnet  22  can be effectively used. As a result, it is possible to achieve both reduction of demagnetization of the permanent magnets  22  and increase in power of the electric motor  1 . 
       FIG. 6  is a graph showing a relationship between a centrifugal force [p.u.] generated during rotation of the rotor  2  and a ratio between radii of curvature (specifically, a ratio of radius of curvature of R1+R2 to the length of W1 of each permanent magnet). 
     As shown in  FIG. 6 , the rotor  2  preferably satisfies 0&lt;(R1+R2)/W1&lt;0.082. Accordingly, a centrifugal force generated during rotation of the rotor  2  can be reduced, and deformation of the rotor core  21 , especially the thin portions  214 , can be prevented. The rotor  2  more preferably satisfies 0.02&lt;(R1+R2)/W1&lt;0.082. Accordingly, a centrifugal force generated during rotation of the rotor  2  can be effectively reduced, and deformation of the rotor core  21 , especially the thin portions  214 , can be effectively prevented. 
       FIG. 7  is a graph showing a relationship between a demagnetization resistance [p.u.] and a ratio between radii of curvature (specifically, a ratio of radii of curvature of R1+R2 to the length of W1 of each permanent magnet). In this embodiment, the demagnetization resistance refers to the magnitude of a current flowing in the stator winding of the stator  3  when the amount of magnetic flux from the permanent magnets  22  decreases by 1%. That is, in  FIG. 7 , as the demagnetization resistance increases, the value of current flowing in the stator winding of the stator  3  can be increased. Accordingly, as the demagnetization resistance increases, power of the electric motor  1  can be enhanced. 
     As shown in  FIG. 7 , the rotor  2  preferably satisfies 0&lt;(R1+R2)/W1&lt;0.082. As a result, demagnetization resistance increases, and power of the electric motor  1  can be enhanced. 
     As the proportion of the radii of curvature of R1 and R2 to the length of W1 of each permanent magnet  22  increases, the area of the permanent magnet  22  facing the outside opening part  211   c  and the area of the permanent magnet  22  facing the outside opening part  212   c  increase. Accordingly, an effective amount of magnetic flux from each permanent magnet  22  decreases, and power of the electric motor  1  decreases. On the other hand, as the proportion of the radii of curvature of R1 and R2 to the length of W1 of each permanent magnet  22  decreases, the permanent magnets  22  are more easily demagnetized. Accordingly, the rotor  2  more preferably satisfies 0.02&lt;(R1+R2)/W1&lt;0.06. As a result, demagnetization resistance further increases, and power of the electric motor  1  can be enhanced. 
     In addition, if the rotor  2  satisfies R1&gt;R2 and 0&lt;(R1+R2)/W1&lt;0.082, demagnetization of the permanent magnets  22  can be further reduced, and power of the electric motor  1  can be further enhanced. 
     If the rotor  2  satisfies R1&gt;R2 and 0.02&lt;(R1+R2)/W1&lt;0.082, demagnetization of the permanent magnets  22  can be further reduced, and power of the electric motor  1  can be further enhanced. 
     If the rotor  2  satisfies R1&gt;R2 and 0.02&lt;(R1+R2)/W1&lt;0.06, demagnetization of the permanent magnets  22  can be further reduced, and power of the electric motor  1  can be further enhanced. 
       FIG. 8  is a graph showing a relationship between a demagnetization resistance [p.u.] and a ratio R3/R4 of a radius of curvature. 
     As shown in  FIG. 8 , the rotor  2  preferably satisfies 0&lt;R3/R4&lt;4.45. As a result, demagnetization resistance increases, and power of the electric motor  1  can be enhanced. The rotor  2  more preferably satisfies 0&lt;R3/R4&lt;4. As a result, demagnetization resistance further increases, and power of the electric motor  1  can be enhanced. 
     The radii of curvature of R3 and R4 may be the same. In this case, the radii of curvature of R3 and R4 are, for example, 0.9 mm, and the rotor  2  satisfies R3/R4=1. As a result, demagnetization resistance can be enhanced, as compared to a conventional rotor (i.e., R3=R4=0). 
       FIG. 9  is an enlarged view illustrating a peripheral structure of the outside opening part  211   c.    
     As illustrated in  FIG. 9 , the radius of curvature of R3 may be larger than the radius of curvature of R1. As the radius of curvature of R3 increases, the outside support part  218   a  and the outside support part  218   b  become shorter. Accordingly, the outside opening part  211   c  and the outside opening part  212   c  become larger, and magnetic resistance increases. As a result, demagnetization in each permanent magnet  22  decreases, and power of the electric motor  1  can be further enhanced. 
       FIG. 10  is a graph showing a relationship between a centrifugal force [p.u.] and a ratio of R1/R3 of a radius of curvature. 
     As shown in  FIG. 10 , the rotor  2  preferably satisfies 0&lt;R1/R3&lt;3. Accordingly, a centrifugal force generated during rotation of the rotor  2  can be reduced, and deformation of the rotor core  21 , especially the thin portions  214 , can be prevented. 
       FIG. 11  is a graph showing a relationship between a demagnetization resistance [p.u.] and a ratio of R1/R3 of a radius of curvature. 
     As shown in  FIG. 11 , the rotor  2  preferably satisfies 0&lt;R1/R3&lt;3. As a result, demagnetization resistance increases, and power of the electric motor  1  can be enhanced. 
     The rotor  2  more preferably satisfies 0.5&lt;R1/R3&lt;3. Accordingly, a centrifugal force generated during rotation of the rotor  2  can be further reduced, and deformation of the rotor core  21 , especially the thin portions  214 , can be further prevented. 
     In the examples shown in  FIGS. 10 and 11 , the radius of curvature of R1 is, for example, 0.5 mm, and the radius of curvature of R3 is, for example, 0.6 mm, and the rotor  2  satisfies 0&lt;R1/R3&lt;3. As a result, demagnetization resistance can be enhanced, as compared to a conventional rotor (i.e., R1=R3=0). 
     Second Embodiment 
     A compressor  300  according to a second embodiment of the present invention will be described. 
       FIG. 12  is a cross-sectional view schematically illustrating a structure of the compressor  300 . 
     The compressor  300  includes an electric motor  1  serving as an electric element, a closed container  307  serving as a housing, and a compression mechanism  305  serving as a compression element (also referred to as a compression device). In this embodiment, the compressor  300  is a scroll compressor. It should be noted that the compressor  300  is not limited to a scroll compressor. The compressor  300  may be a compressor other than the scroll compressor, for example, may be a rotary compressor. 
     The electric motor  1  in the compressor  300  is the electric motor  1  described in the first embodiment. The electric motor  1  drives the compression mechanism  305 . 
     The compressor  300  also includes a subframe  308  supporting a lower end (i.e., an end opposite to the compression mechanism  305 ) of the shaft  24 . 
     The compression mechanism  305  is disposed inside the closed container  307 . The compressor mechanism  305  includes a fixed scroll  301  having a spiral portion, a swing scroll  302  having a spiral portion forming a compression chamber between the spiral portion of the swing scroll  302  and the spiral portion of the fixed scroll  301 , a compliance frame  303  holding the upper end of the shaft  24 , and a guide frame  304  fixed to the closed container  307  and holding the compliance frame  303 . 
     A suction pipe  310  penetrating the closed container  307  is press fitted in the fixed scroll  301 . The closed container  307  is provided with a discharge pipe  306  that discharges a high-pressure refrigerant gas discharged from the fixed scroll  301 , to the outside. The discharge pipe  306  communicates with an opening disposed between the compressor mechanism  305  of the closed container  307  and the electric motor  1 . 
     The electric motor  1  is fixed to the closed container  307  by fitting the stator  3  in the closed container  307 . A configuration of the electric motor  1  has been described above. To the closed container  307 , a glass terminal  309  for supplying electric power to the electric motor  1  is fixed by welding. 
     When a current flows in a stator winding  32  of the electric motor  1 , the electric motor  1  is driven. When the electric motor  2  rotates, this rotation is transferred to the swing scroll  302 , and the swing scroll  302  swings. When the swing scroll  302  swings, the volume of the compression chamber formed by the spiral portion of the swing scroll  302  and the spiral portion of the fixed scroll  301  changes. Then, a refrigerant gas is sucked from the suction pipe  310 , compressed, and then discharged from the discharge pipe  306 . 
     The compressor  300  includes the electric motor  1  described in the first embodiment, and thus, obtains advantages described in the first embodiment. 
     In addition, since the compressor  300  includes the electric motor  1  described in the first embodiment, the efficient compressor  300  can be provided. 
     Third Embodiment 
     A refrigerating and air conditioning apparatus  7  serving as an air conditioner and including a compressor  300  according to a third embodiment will be described. 
       FIG. 13  is a diagram schematically illustrating a configuration of the refrigerating air conditioning device  7  according to the third embodiment. 
     The refrigerating and air conditioning apparatus  7  is capable of performing cooling and heating operations, for example. A refrigerant circuit diagram illustrated in  FIG. 13  is an example of a refrigerant circuit diagram of an air conditioner capable of performing a cooling operation. 
     The refrigerating and air conditioning apparatus  7  according to the third embodiment includes an outdoor unit  71 , an indoor unit  72 , and a refrigerant pipe  73  connecting the outdoor unit  71  and the indoor unit  72 . 
     The outdoor unit  71  includes a compressor  300 , a condenser  74  as a heat exchanger, a throttling device  75 , and an outdoor air blower  76  (first air blower). The condenser  74  condenses a refrigerant compressed by the compressor  300 . The throttling device  75  decompresses the refrigerant condensed by the condenser  74  to thereby adjust a flow rate of the refrigerant. The throttling device  75  will be also referred to as a decompression device. 
     The indoor unit  72  includes an evaporator  77  as a heat exchanger, and an indoor air blower  78  (second air blower). The evaporator  77  evaporates the refrigerant decompressed by the throttling device  75  to thereby cool indoor air. 
     A basic operation of a cooling operation in the refrigerating and air conditioning apparatus  7  will now be described. In the cooling operation, a refrigerant is compressed by the compressor  300  and the compressed refrigerant flows into the condenser  74 . The condenser  74  condenses the refrigerant, and the condensed refrigerant flows into the throttling device  75 . The throttling device  75  decompresses the refrigerant, and the decompressed refrigerant flows into the evaporator  77 . In the evaporator  77 , the refrigerant evaporates, and the refrigerant (specifically a refrigerant gas) flows into the compressor  300  of the outdoor unit  71  again. When the air is sent to the condenser  74  by the outdoor air blower  76 , heat moves between the refrigerant and the air, and similarly, when the air is sent to the evaporator  77  by the indoor air blower  78 , heat is moved between the refrigerant and the air. 
     The configuration and operation of the refrigerating and air conditioning apparatus  7  described above are examples, and the present invention is not limited to the examples described above. 
     The refrigerating and air conditioning apparatus  7  according to the third embodiment has the advantages described in the first and second embodiments. 
     In addition, since the refrigerating and air conditioning apparatus  7  according to the third embodiment includes the compressor  300  according to the second embodiment. Thus, the refrigerating and air conditioning apparatus  7  has high efficiency. 
     Features of the embodiments and features of the variations described above can be combined as appropriate.