Patent Publication Number: US-2021175761-A1

Title: Rotor, 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/JP2018/018085 filed on May 10, 2018, the disclosure of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a rotor, a motor, a compressor, and an air conditioner. 
     BACKGROUND 
     With an increase in capacity of a compressor for a large-sized air conditioner such as a commercial air conditioner, it is required to increase a rotation speed of a motor. When the rotation speed of the motor is increased, a frequency of a current flowing through a winding of the motor increases. In a permanent-magnet-embedded motor having a rare earth magnet as a permanent magnet, an eddy current may be generated in the permanent magnet at a high frequency range, and the motor efficiency may be reduced. Thus, it is an aim to reduce the eddy current. 
     Patent Reference 1 discloses a rotor core having a magnet hole into which a permanent magnet is inserted. On an inner surface of the magnet hole, a part in contact with the permanent magnet and a part not in contact with the permanent magnet are alternately provided in the axial direction. Patent Reference 2 discloses a rotor having permanent magnets finely divided in the axial direction and circumferential direction. 
     PATENT REFERENCE 
     [Patent Reference 1]: Japanese Patent Application Publication No. 2015-116105 (see FIG.  4 ) 
     [Patent Reference 2]: Japanese Patent Application Publication No. 2005-354899 (see FIG.  5 ) 
     However, in a configuration described in Patent Reference 1, there are many portions where the inner surface of the magnet hole and the permanent magnet do not contact each other. Thus, the magnetic flux of the permanent magnet is less likely to reach a stator through the rotor core. Thus, the magnetic flux effectively interlinking with the winding of the stator decreases, which leads to a reduction in the magnetic force. In a configuration described in Patent Reference 2, a leakage magnetic flux is more likely to occur between the permanent magnets divided in the circumferential direction, which also leads to a reduction in the magnetic force. 
     SUMMARY 
     The present invention is intended to solve the above described problems, and an object of the present invention is to reduce an eddy current loss while suppressing the reduction in the magnetic force. 
     A rotor of the present invention includes a rotor core having an annular shape about an axis, the rotor core having a first core part and a second core part in a direction of the axis, the first core part having a first magnet insertion hole, and the second core part having a second magnet insertion hole, a first permanent magnet disposed in the first magnet insertion hole and being formed of a rare earth magnet, and a second permanent magnet disposed in the second magnet insertion hole and being formed of a rare earth magnet. A width of the first magnet insertion hole in a radial direction about the axis is wider than a width of the second magnet insertion hole in the radial direction. The first core part has one or more slits, a number of which is N1, on an outer side of the first magnet insertion hole in the radial direction. The number N1 is greater than or equal to one. Each of the slits has a length in the radial direction longer than a length in a circumferential direction about the axis. The second core part has no, one or more slits, a number of which is N2, on an outer side of the second magnet insertion hole in the radial direction. The number N2 is greater than or equal to zero. Each of the slits has a length in the radial direction longer than a length in the circumferential direction. N1&gt;N2 is satisfied. A ratio of a length of the second core part in the direction of the axis to a length of the rotor core in the direction of the axis is greater than or equal to 70% and less than 100%. 
     In the present invention, the rotor core includes the first core part having the slits the number of which is N1, and the second core parts having the slits the number of which is N2 (&lt;N1), and the ratio of the length of the second core part in the axial direction to the length of the rotor core in the axial direction is greater than or equal to 70% and less than 100%. Thus, the eddy current loss can be reduced. Further, a leakage magnetic flux can be reduced, and thus the reduction of the magnetic force can be suppressed, as compared to a case where the permanent magnet is divided in the circumferential direction. In addition, the second permanent magnet can be formed to be thinner than the first permanent magnet, and thus the manufacturing cost can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1(A)  is a cross-sectional view showing a motor of a first embodiment and  FIG. 1(B)  is a schematic view showing a cross-sectional structure of a winding. 
         FIG. 2  is a cross-sectional view showing a portion corresponding to one magnetic pole of the motor of the first embodiment. 
         FIG. 3  is a perspective view schematically showing a portion corresponding to one magnetic pole of a rotor of the first embodiment. 
         FIG. 4  is a longitudinal sectional view taken along a line IV-IV in  FIG. 3  as seen in a direction indicated by arrows. 
         FIG. 5  is a cross-sectional view taken along a line V-V in  FIG. 3  as seen in a direction indicated by arrows. 
         FIG. 6  is a longitudinal sectional view taken along a line VI-VI in  FIG. 3  as seen in a direction indicated by arrows. 
         FIG. 7  is a graph showing a relationship between a ratio of a length of a second core part in an axial direction to that of a rotor core and a volume of a permanent magnet. 
         FIG. 8  is a graph showing a relationship between the ratio of the length of the second core part in the axial direction to that of the rotor core and an eddy current loss in the permanent magnet. 
         FIG. 9  is a graph showing a relationship between a ratio of the length of the second core part in the axial direction to that of the rotor core and a generated torque. 
         FIG. 10  is a graph showing a relationship between the ratio of the length of the second core part in the axial direction to that of the rotor core and a torque ripple. 
         FIG. 11  is a cross-sectional view of a second core part of a rotor of a first modification of the first embodiment. 
         FIG. 12  is a cross-sectional view of a second core part of a rotor of a second modification of the first embodiment. 
         FIG. 13  is a cross-sectional view of a second core part of a rotor of a third modification of the first embodiment. 
         FIG. 14  is a perspective view schematically showing a portion corresponding to one magnetic pole of a rotor of a second embodiment. 
         FIG. 15  is a longitudinal sectional view showing the rotor of the second embodiment. 
         FIG. 16  is a perspective view schematically showing a portion corresponding to one magnetic pole of a rotor of a third embodiment. 
         FIG. 17  is a longitudinal sectional view showing the rotor of the third embodiment. 
         FIG. 18  is a diagram showing a compressor to which the motor of each embodiment is applicable. 
         FIG. 19  is a diagram showing an air conditioner using the compressor shown in  FIG. 18 . 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
     (Configuration of Motor) 
       FIG. 1(A)  is a cross-sectional view showing a motor  100  of a first embodiment. The motor  100  shown in  FIG. 1(A)  is incorporated inside a cylindrical shell  5 . The shell  5  is a part of a container of a compressor  500  ( FIG. 18 ) in which the motor  100  is incorporated. 
     The motor  100  includes a rotatable rotor  2  and a stator  1  provided so as to surround the rotor  2 . The stator  1  is incorporated inside the above-described shell  5 . An air gap (i.e., a gap) G of, for example, 0.5 mm is provided between the stator  1  and the rotor  2 . 
     Hereinafter, a direction of an axis C1, which is a rotation axis of the rotor  2 , is referred to as an “axial direction”. A direction along a circumference about the axis C1 is referred to as a “circumferential direction”. A radial direction about the axis C1 is referred to as a “radial direction”.  FIG. 1  is a sectional view (i.e., a cross-sectional view) taken along a plane perpendicular to the axis C1. 
     (Configuration of Stator) 
     The stator  1  includes a stator core  10  and a winding  15  wound on the stator core  10 . The stator core  10  is formed of a plurality of stack elements which are stacked in the axial direction and fastened by crimping or the like. The stack element is a punched electromagnetic steel sheet having a thickness of, for example, 0.25 to 0.5 mm. 
     The stator core  10  includes a yoke  11  having an annular shape about the axis C1 and a plurality of teeth  12  protruding inward in the radial direction from the yoke  11 . The number of teeth  12  is 18 in this example, but is not limited to 18. The winding  15  is wound around the tooth  12  of the stator core  10  via a not shown insulating portion (insulator). Slots  13  for accommodating the winding  15  are formed each between two teeth  12  adjacent to each other in the circumferential direction. 
       FIG. 1(B)  is a schematic diagram showing a cross-sectional structure of the winding  15 . The winding  15  includes a conductor  15   a  formed of aluminum or copper, and a refrigerant-resistant insulating film  15   b  covering a circumference of the conductor  15   a . The winding  15  is in contact with the refrigerant inside the compressor  500  ( FIG. 18 ), and the conductor  15   a  is protected by the refrigerant-resistant insulating film  15   b . A method for winding the winding  15  may be either distributed winding in which the winding is wound across the plurality of teeth  12  or concentrated winding in which the winding is wound around each tooth  12 . 
     A plurality of abutting surfaces  17  which are cylindrical surfaces about the axis C1 and a plurality of cutout portions  16  which are flat surfaces parallel to the axis C1 are formed on an outer circumference of the stator core  10 . The plurality of abutting surfaces  17  and the plurality of cutout portions  16  are alternately formed in the circumferential direction. Each of the number of abutting surfaces  17  and the number of cutout portions  16  is six in this example, but is not limited to six. 
     The abutting surfaces  17  are fitted to an inner circumferential surface  51  of the shell  5 . A clearance is formed between the cutout portion  16  and the inner circumferential surface  51  of the shell  5 . The clearance serves as a refrigerant flow passage through which refrigerant in the compressor  500  flows in the axial direction. 
     (Configuration of Rotor) 
     The rotor  2  includes a rotor core  20  having an annular shape about the axis C1. The rotor core  20  has an outer circumferential surface which is cylindrical about the axis C1. A shaft hole  24  is formed at a center of the rotor core  20  in the radial direction. A rotational shaft  25  is fixed into the shaft hole  24  by press-fitting. 
     The rotor  2  has a plurality of magnetic poles in the circumferential direction. The number of magnetic poles is equal to the number of first permanent magnets  22 A to be described later, and also equal to the number of second permanent magnets  22 B ( FIG. 6 ). In this example, the number of magnetic poles of the rotor  2  is six. The number of magnetic poles of the rotor  2  is not limited to six, and it is sufficient that the number of magnetic poles of the rotor  2  is two or more. 
       FIG. 2  is a cross-sectional view showing a portion corresponding to one magnetic pole of the rotor  2  and a part of the stator  1  facing this portion via the air gap. In  FIG. 2 , a straight line in the radial direction that passes through a center of the magnetic pole of the rotor  2  is defined as a magnetic pole center line M1. A boundary between adjacent magnetic poles (i.e., an inter-pole portion) is denoted by reference character M2. 
       FIG. 3  is a perspective view schematically showing a portion corresponding to one magnetic pole of the rotor  2 .  FIG. 4  is a cross-sectional view taken along a line IV-IV in  FIG. 3  as seen in a direction indicated by arrows, i.e., a longitudinal sectional view of the rotor  2 . In  FIG. 3 , the first permanent magnets  22 A and the second permanent magnet  22 B of the rotor  2  are shown by solid lines. 
     As shown in  FIGS. 3 and 4 , the rotor core  20  includes two first core parts  20 A and one second core part  20 B in the axial direction. More specifically, the one second core part  20 B is disposed between the two first core parts  20 A in the axial direction. Each first core part  20 A has a length L1 in the axial direction, while the second core part  20 B has a length L2 in the axial direction. 
       FIG. 5  is a cross-sectional view taken along a line V-V in  FIG. 3  as shown in a direction indicated by arrows, i.e., a cross-sectional view of the first core part  20 A. The first core part  20 A is formed of stack elements which are stacked in the axial direction and fastened by crimping or the like. The stack element is a punched electromagnetic steel sheet having a thickness of, for example, 0.25 to 0.5 mm. 
     First magnet insertion holes  21 A are formed along an outer circumference of the first core part  20 A. Each first magnet insertion hole  21 A passes through the first core part  20 A in the axial direction. In this example, six first magnet insertion holes  21 A (see  FIG. 1 ), the number of which is the same as the above described number of magnetic poles of the rotor  2 , are formed at equal intervals in the circumferential direction. 
     The magnetic pole center line M1 described above passes through a center of the first magnet insertion hole  21 A in the circumferential direction. The first magnet insertion hole  21 A extends linearly in a direction perpendicular to the magnetic pole center line M1. The first magnet insertion hole  21 A has an outer end edge  201  which is an end edge on the outer side in the radial direction, and an inner end edge  202  which is an end edge on the inner side in the radial direction. 
     First permanent magnets  22 A are inserted into the first magnet insertion holes  21 A. Each first permanent magnet  22 A constitutes one magnetic pole. The first permanent magnet  22 A is in the form of a flat plate and has a plate surface perpendicular to the magnetic pole center line M1. 
     The first permanent magnet  22 A is magnetized so that the first permanent magnet  22 A has different magnetic poles on the outer side and on the inner side in the radial direction of the first core part  20 A. Magnetizing directions of the first permanent magnets  22 A of the adjacent magnetic poles are opposite to each other. 
     The first permanent magnet  22 A is formed of a rare earth magnet (more specifically, a rare earth sintered magnet) that contains as main components, neodymium (Nd), iron (Fe) and boron (B). A surface of the first permanent magnet  22 A is covered with an insulating film. The rare earth magnet has a high residual flux density and a high coercive force, and thus the motor efficiency and demagnetization resistance are enhanced. In order to further enhance the coercive force, dysprosium (Dy) or terbium (Tb) may be added to the rare earth magnet. 
     A flux barrier  23  is formed on each of both sides of the first magnet insertion hole  21 A in the circumferential direction. Each flux barrier  23  is a hole extending in the radial direction from an end of the first magnet insertion hole  21 A in the circumferential direction toward the outer circumference of the first core part  20 A. The flux barrier  23  is provided to reduce leakage magnetic flux between adjacent magnetic poles (i.e., magnetic flux flowing through the inter-pole portion M2). 
     In the first core part  20 A, a side slit  35  is formed on an inner side of each of the two flux barriers  23  in the circumferential direction. The side slit  35  has a length in the circumferential direction longer than a length in the radial direction and extends along the outer circumference of the first core part  20 A. 
     The side slit  35  serves to increase a magnetic resistance at the flux barrier  23 , thereby enhancing the effect of reducing the leakage magnetic flux between the adjacent magnetic poles. Due to the function of reducing the leakage magnetic flux by the side slits  35  and the flux barriers  23 , the magnetic flux (effective magnetic flux) interlinking with the teeth  12  of the stator  1  increases, and thus the motor efficiency is enhanced. 
     The first core part  20 A has one or more slits, the number of which is N1 (where N1 is an integer greater than or equal to one), on the outer side of the first magnet insertion hole  21 A in the radial direction. In this example, two of each of the slits  31 ,  32 ,  33 , and  34  are provided so that the slits  31 ,  32 ,  33 , and  34  are arranged from the center toward each side of the first magnet insertion hole  21 A in the circumferential direction. The number N1 of the slits is eight. The number N1 of the slits is not limited to eight, and it is sufficient that the number N1 is one or more. It is noted that the number N1 does not include the number of side slits  35 . 
     Each of the slits  31 ,  32 ,  33 , and  34  has a shape such that a length in the radial direction is longer than a length in the circumferential direction. The slits  31 ,  32 ,  33 , and  34  are provided to reduce a torque ripple. In order to enhance the effect of reducing the torque ripple, the slits  31 ,  32 ,  33 , and  34  are desirably formed symmetrically with respect to the magnetic pole center line M1. The expression “formed symmetrically” means that they are symmetric in terms of shape and arrangement. 
     The longitudinal directions of the slits  31 ,  32 ,  33 , and  34  are desirably parallel to the magnetic pole center line M1. By arranging the slits  31 ,  32 ,  33 , and  34  parallel to the magnetic pole center line M1, the magnetic flux from the first permanent magnet  22 A can be guided to the stator  1  at the shortest distance. The longitudinal directions of the slits  31 ,  32 ,  33 , and  34  may be inclined with respect to the magnetic pole center line M. In such a case, the slits  31 ,  32 ,  33  and  34  are desirably inclined symmetrically with respect to the magnetic pole center line M1. 
     The first core part  20 A has first through holes  26 , a second through hole  27 , and a third through hole  28  on the inner side of each first magnet insertion hole  21 A in the radial direction. These through holes  26 ,  27 , and  28  are refrigerant flow passages. 
     Two first through holes  26  are formed for each magnetic pole so that one first through hole  26  is formed on each of both sides of the magnetic pole center line M1. One second through hole  27  is formed for each magnetic pole, and is located on the inner side with respect to the first through hole  26  in the radial direction and on the magnetic pole center line M1. One third through hole  28  is formed for each magnetic pole, and is located on the inner side with respect to the second through hole  27  in the radial direction and on the inter-pole part M2. All the through holes  26 ,  27 , and  28  are not necessarily provided, and it is sufficient that at least one of the through holes  26 ,  27 , and  28  is provided. In  FIG. 1(A)  described above, the through holes  26 ,  27 , and  28  are omitted. 
       FIG. 6  is a cross-sectional view taken along a line VI-VI in  FIG. 3  as seen in a direction indicated by arrows, i.e., a cross-sectional view of the second core part  20 B. The second core part  20 B is formed of stack elements which are stacked in the axial direction and fastened by crimping or the like. The stack element is a punched electromagnetic steel sheet having a thickness of, for example, 0.25 to 0.5 mm. 
     Second magnet insertion holes  21 B are formed along an outer circumference of the second core part  20 B. Each second magnet insertion hole  21 B passes through the second core part  20 B in the axial direction. In this example, six second magnet insertion holes  21 B, the number of which is the same as the above described number of magnetic poles of the rotor  2 , are formed at equal intervals in the circumferential direction. The first magnet insertion hole  21 A ( FIG. 5 ) and the second magnet insertion hole  21 B are formed continuously with each other in the axial direction. 
     The magnetic pole center line M1 described above passes through the center of the second magnet insertion hole  21 B in the circumferential direction. The second magnet insertion hole  21 B extends linearly in a direction perpendicular to the magnetic pole center line M1. The second magnet insertion hole  21 B has an outer end edge  203  which is an end edge on the outer side in the radial direction, and an inner end edge  204  which is an end edge on the inner side in the radial direction. 
     Second permanent magnets  22 B are inserted into the second magnet insertion holes  21 B. Each second permanent magnet  22 B constitutes one magnetic pole. The second permanent magnet  22 B is in the form of a flat plate and has a plate surface perpendicular to the magnetic pole center line M1. The second permanent magnet  22 B is magnetized in a similar manner to the first permanent magnet  22 A adjacent thereto in the axial direction. 
     The second permanent magnet  22 B is formed of a rare earth magnet (more specifically, a rare earth sintered magnet) that contains as main components, neodymium, iron, and boron, as is the case with the first permanent magnet  22 A. The second permanent magnet  22 B has a surface covered with an insulating film. In order to further enhance the coercive force, dysprosium or terbium may be added to the rare earth magnet. 
     A flux barrier  23  is formed on each of both sides of the second magnet insertion hole  21 B in the circumferential direction. The flux barrier  23  is continuous to the flux barrier  23  ( FIG. 5 ) of the first core part  20 A in the axial direction. The shapes and arrangement of the flux barriers  23  of the second core part  20 B are the same as those of the flux barriers  23  ( FIG. 5 ) of the first core part  20 A. 
     A side slit  35  is formed on an inner side of each of the two flux barriers  23  in the circumferential direction. The side slits  35  are continuous to the side slits  35  ( FIG. 5 ) of the first core part  20 A in the axial direction. The shapes and arrangement of the side slits  35  of the second core part  20 B are the same as those of the side slits  35  ( FIG. 5 ) of the first core part  20 A. 
     The second core part  20 B has no, one or more slits, the number of which is N2 (where N2 is an integer greater than or equal to zero and less than N1), on the outer side of the second magnet insertion hole  21 B in the radial direction. In this example, the number N2 is zero. That is, in the second core part  20 B, no slit is provided on the outer side of the second magnet insertion hole  21 B in the radial direction. The number N2 is not limited to zero and may be greater than or equal to one as long as the number N2 is less than the number N1. It is noted that the number N2 does not include the number of side slits  35 . 
     The second core part  20 B has first through holes  26 , a second through hole  27 , and a third through hole  28  on the inner side of each second magnet insertion hole  21 B in the radial direction. The first through holes  26  are continuous to the first through holes  26  ( FIG. 5 ) of the first core part  20 A in the axial direction. The second through hole  27  is continuous to the second through hole  27  ( FIG. 5 ) of the first core part  20 A in the axial direction. The third through hole  28  is continuous to the third through hole  28  ( FIG. 5 ) of the first core part  20 A in the axial direction. 
     With reference to  FIG. 4  again, the first magnet insertion hole  21 A and the second magnet insertion hole  21 B are continuous to each other in the axis direction. A width W1 of the first magnet insertion hole  21 A in the radial direction is wider than a width W2 of the second magnet insertion hole  21 B in the radial direction. A width of the first permanent magnet  22 A in the radial direction is wider than a width of the second permanent magnet  22 B in the radial direction. 
     The outer end edges  201  and  203  of the magnet insertion holes  21 A and  21 B are located at the same position in the radial direction. Meanwhile, the inner end edge  202  of the first magnet insertion hole  21 A is located on the inner side in the radial direction with respect to the inner end edge  202  of the second magnet insertion hole  21 B. The magnet insertion holes  21 A and  21 B are not limited to such a configuration. Alternatively, the inner end edges  202  and  204  may be located at the same position in the radial direction, while the outer end edge  201  may be located on the inner side in the radial direction with respect to the outer end edge  203 . 
     The rotor core  20  is formed by stacking, in the axial direction, a plurality of electromagnetic steel sheets each of which is punched into the shape of the first core part  20 A shown in  FIG. 5  and a plurality of electromagnetic steel sheets each of which is punched into the shape of the second core part  20 B shown in  FIG. 6 . The positions in the circumferential direction of both ends of the first magnet insertion hole  21 A ( FIG. 5 ) in the circumferential direction are the same as the positions in the circumferential direction of both ends of the second magnet insertion hole  21 B ( FIG. 6 ) in the circumferential direction. 
     In the rotor core  20 , the first core part  20 A having the wider first magnet insertion holes  21 A is disposed on each of both sides in the axial direction of the second core part  20 B having the narrower second magnet insertion holes  21 B. Thus, the second permanent magnet  22 B can be inserted into the second magnet insertion hole  21 B from one side of the rotor core  20  in the axial direction, and then the first permanent magnets  22 A can be inserted into the first magnet insertion holes  21 A from both sides of the rotor core  20  in the axial direction. 
     As described above, since the outer end edges  201  and  203  of the magnet insertion holes  21 A and  21 B are located at the same position in the radial direction, the permanent magnets  22 A and  22 B which are inserted are guided by the outer end edges  201  and  203 . Thus, the permanent magnets  22 A and  22 B can be easily inserted into the magnetic insertion holes  21 A and  21 B. 
     (Operation) 
     Next, an operation of the motor  100  of the first embodiment will be described. Each of the permanent magnets  22 A and  22 B is formed of a rare earth magnet and thus has electrical conductivity. A magnetic flux (i.e., stator magnetic flux) generated in the winding  15  of the stator  1  passes through the permanent magnets  22 A and  22 B. An eddy current flows in the permanent magnet  22 A in accordance with a change over time (dΦ/dt) in the stator magnetic flux Φ passing through the permanent magnet  22 A, and an eddy current flows in the permanent magnet  22 B in accordance with a change over time (dΦ/dt) in the stator magnetic flux Φ passing through the permanent magnet  22 B. The eddy current causes a loss (i.e., eddy current loss), and results in reduction in the motor efficiency. Furthermore, temperatures of the permanent magnets  22 A and  22 B increase due to Joule heat, which causes high-temperature demagnetization of the permanent magnets  22 B and  22 B. 
     In general, as the number of slits on the outer side of the magnetic insertion hole in the radial direction increases, the stator magnetic flux is more likely to concentrate on a region between the slit and the magnet insertion hole (that is, the magnetic flux density increases). Thus, an inductive electromotive force is generated in the permanent magnet due to fluctuations in the magnetic flux, and the eddy current is more likely to flow in the permanent magnet. In the first embodiment, the eddy current loss is reduced by decreasing the number N2 of the slits on the outer side of the second magnet insertion hole  21 B of the second core part  20 B in the radial direction. 
     Meanwhile, when the number of slits on the outer side of the magnet insertion hole in the radial direction is small, the eddy current loss is reduced, but a torque ripple (torque pulsation) increases, which causes noise and vibration of the motor  100 . In the first embodiment, since the first core part  20 A has the slits  31  to  34 , the number of which is N1 (&gt;N2), on the outer side of the first magnet insertion hole  21 A in the radial direction, the torque ripple can be reduced, and the noise and vibration of the motor  100  can be reduced. 
     As the number of slits on the outer side of the magnetic insertion hole in the radial direction increases, the stator magnetic flux is more likely to be guided to the permanent magnet along the slits, and thus demagnetization of the permanent magnet is more likely to occur. In the first embodiment, since the number N1 of the slits  31  to  34  in the first core part  20 A is greater than the number N2 of the slits in the second core part  20 B, the second permanent magnet  22 B is less likely to be demagnetized (i.e., has higher demagnetization resistance) than the first permanent magnet  22 A. 
     In this regard, the demagnetization resistance of the entire rotor  2  is identical to a lower one of the demagnetization resistances of the first permanent magnet  22 A and the second permanent magnet  22 B. Thus, in order to enhance the demagnetization resistance of the entire rotor  2 , it is necessary to enhance the demagnetization resistance of the first permanent magnet  22 A. 
     For this reason, in the first embodiment, the width W1 of the first magnet insertion hole  21 A is wider than the width W2 of the second magnet insertion hole  21 B (W1&gt;W2). Thus, the width of the first permanent magnet  22 A inserted into the first magnet insertion hole  21 A is wider than the width of the second permanent magnet  22 B inserted into the second magnet insertion hole  21 B. Consequently, the concentration of the stator magnetic flux in the first permanent magnet  22 A is relieved, and thereby the demagnetization of the first permanent magnet  22 A is less likely to occur. That is, the demagnetization resistance of the first permanent magnet  22 A can be made closer to that of the second permanent magnet  22 B. 
     As the first core part  20 A and the second core part  20 B are combined in this way, the eddy current loss can be reduced without increasing the torque ripple and without reducing the demagnetization resistance. Since the eddy current loss is reduced, the motor efficiency can be enhanced. Further, since heat generation in the permanent magnets  22 A and  22 B is suppressed, the high-temperature demagnetization can be prevented. 
     Since the second permanent magnets  22 B can be formed to be thinner than the first permanent magnet  22 A, the material cost can be reduced, and the manufacturing cost of the motor  100  can be reduced. In addition, the leakage magnetic flux which may occur in a case where the permanent magnet is divided in the circumferential direction is less likely to occur, and thus the reduction of the magnetic force can be suppressed. 
     Next, a ratio of a length of the second core part  20 B in the axial direction to that of the rotor core  20  will be described. The ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  is a ratio (%) of the length L2 of the second core part  20 B in the axial direction to a length (L1×2+L2) of the rotor core  20  in the axial direction, and is expressed as L2/(L1×2+L2)×100. 
     The ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  may be considered to be a ratio of a volume of the second core part  20 B to a volume of the rotor core  20 , or a ratio of a weight of the second core part  20 B to a weight of the rotor core  20 . 
       FIG. 7  is a graph showing a relationship between the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  and a total volume of the permanent magnets  22 A and  22 B. A reference value (100%) of the total volume of the permanent magnets  22 A and  22 B is defined by the total volume of the permanent magnets  22 A and  22 B obtained when the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  is zero, that is, when the rotor core  20  is formed of only the first core part  20 A. The total volume of the permanent magnets  22 A and  22 B is represented as a relative value to this reference value. 
     From  FIG. 7 , it is understood that as the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  increases, the total volume of the permanent magnets  22 A and  22 B decreases. This is because the width W2 of the second magnet insertion hole  21 B in the second core part  20 B is narrower than the width W1 of the first magnet insertion hole  21 A in the first core part  20 A. 
       FIG. 8  is a graph showing a relationship between the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  and an eddy current loss. A reference value (100%) of the eddy current loss is defined by an eddy current loss caused when the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  is zero, that is, when the rotor core  20  is formed of only the first core part  20 A. The eddy current loss is represented as a relative value to this reference value. 
     From  FIG. 8 , it is understood that as the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  increases, the eddy current loss decreases. In particular, when the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  is greater than or equal to 70%, it is understood that the eddy current loss is reduced to 50% or less of the eddy current loss caused when the rotor core  20  is formed of only the first core part  20 A. 
     This reduction effect (50%) of the eddy current loss is generally equivalent to that obtained by dividing the permanent magnet into two parts in the circumferential direction. That is, it is understood that the same effect as that obtained by dividing the permanent magnet into two parts in the circumferential direction can be obtained by using the rotor core  20  in which the first core part  20 A and the second core part  20 B are combined in the axial direction and setting the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  to be greater than or equal to 70%. 
     When the permanent magnet is divided into two parts in the circumferential direction, the eddy current loss is reduced, but a leakage magnetic flux is generated between the permanent magnet parts divided in the circumferential direction. When the leakage magnetic flux is generated, a torque constant (i.e., a constant K in the expression T=K×I, where T denotes a generated torque, and I denotes a current) decreases. In the first embodiment, it is not necessary to divide the permanent magnet in the circumferential direction, and thus the reduction of the toque constant due to the leakage magnetic flux can be suppressed. In other words, it is possible to suppress the reduction of the magnetic force due to the leakage magnetic flux that occurs in a case where the permanent magnet is divided into two parts in the circumferential direction. 
     An upper limit of the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  is a value when the first core part  20 A is formed of only one electromagnetic steel sheet. Thus, the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  is greater than or equal to 70% and less than 100%. 
       FIG. 9  is a graph showing a relationship between the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  and a generated torque. A reference value (100%) of the generated torque is defined by a torque generated when the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  is zero, that is, when the rotor core  20  is formed of only the first core part  20 A. The generated torque is represented as a relative value to this reference value. 
     From  FIG. 9 , it is understood that as the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  increases, the generated torque increases. This is because of the following reason. Since the number N2 of slits in the second core part  20 B is small (specifically, zero), there is few obstacle that interrupts the magnetic flux from the second permanent magnet  22 B to the stator  1 , and thus the magnetic flux (effective magnetic flux) interlinking with the teeth  12  increases. 
       FIG. 10  is a graph showing a relationship between the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  and a torque ripple. The torque ripple is defined by (T max −T min )/T ave ×100, based on a maximum torque value T min , a minimum torque value T min , and an average torque value T ave  in one cycle of electrical angle. For example, the expression “torque ripple of 100%” means that a difference (T max −T min ) between the maximum torque value and the minimum torque value is equal to the average torque value T ave . 
     From  FIG. 10 , it is understood that as the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  increases, the torque ripple also increases. This is because an increase in the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  causes a decrease in the ratio of the length in the axial direction of the first core part  20 A including the slits  31  to  34  for reducing the torque ripple to that of the rotor core  20 . 
     The torque ripple reaches a maximum value (55%) when the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  is 100%, that is, when the rotor core  20  is formed of only the second core part  20 B. When the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  is 90%, the torque ripple decreases by 10% with respect to the maximum value (i.e., by 5% on the vertical scale in  FIG. 10 ). A decrease in the torque ripple of 10% corresponds to the reduction in noise of 1 dB. Thus, in order to obtain the effect of reducing the noise by 1 dB, the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  is desirably less than or equal to 90%. 
     Effects of Embodiment 
     As described above, in the rotor  2  of the first embodiment, the rotor core  20  includes the first core part  20 A having the first magnet insertion holes  21 A and the second core part  20 B having the second magnet insertion holes  21 B. The permanent magnets  22 A and  22 B formed of rare earth magnets are disposed in the magnet insertion holes  21 A and  21 B, respectively. The width W1 of the first magnet insertion hole  21 A in the radial direction is wider than the width W2 of the second magnet insertion hole  21 B in the radial direction. The first core part  20 A has the slits  31  to  34 , the number of which is N1 (1≤N1), on the outer side of the first magnet insertion hole  21 A in the radial direction, and the slits are elongated in the radial direction. The second core part  20 B has the slits, the number of which is N2 (0≤N2&lt;N1), on the outer side of the second magnet insertion hole  21 B in the radial direction, and the slits are elongated in the radial direction. The ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  is greater than or equal to 70% and less than 100%. 
     Since the number N2 of the slits in the second core part  20 B is smaller than the number N1 of the slits in the first core part  20 A, and the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  is greater than or equal to 70% and less than 100%, the eddy current loss can be reduced. Since the width W1 of the first magnet insertion hole  21 A is wider than the width W2 of the second magnet insertion hole  21 B, the demagnetization resistances of the first permanent magnets  22 A and  22 B can be made closer to each other. Further, the second permanent magnet  22 B can be formed to be thinner than the first permanent magnet  22 A, and thus the manufacturing cost can be reduced. Moreover, the leakage magnetic flux that occurs when the permanent magnet is divided in the circumferential direction is less likely to occur, and thus the reduction of the magnetic force can be suppressed. 
     By setting the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  to be less than or equal to 90%, noise caused by the torque ripple can be reduced. 
     There is provided two first core part  20 A, one on each of both sides of the second core part  20 B in the axial direction. Thus, the permanent magnets  22 A and  22 B are easily inserted into the magnet insertion holes  21 A and  21 B, and the weight balance of the rotor  2  in the axial direction is enhanced. 
     The first magnet insertion hole  21 A and the second magnet insertion hole  21 B are continuous to each other in the axial direction. Thus, the second permanent magnet  22 B can be inserted into the second magnet insertion hole  21 B through the first magnet insertion hole  21 A, and thus an inserting operation is facilitated. 
     In addition, since the outer end edges  201  and  203  (or inner end edges  202  and  204 ) of the first magnet insertion hole  21 A and the second magnet insertion hole  21 B are located at the same position in the radial direction, the permanent magnets  22 A and  22 B are guided by the outer end edges  201  and  203  (or inner end edges  202  and  204 ) during insertion, and thus the inserting operation is further facilitated. 
     Since the plurality of slits  31 ,  32 ,  33 , and  34  in the first core part  20 A are formed symmetrically with respect to the magnetic pole center line M1, spatial harmonics of the magnetic flux generated in the air gap can be suppressed, and the torque ripple and an excitation force in the radial direction (a force with which the rotor core  20  is attracted by a stator magnetic field) can also be suppressed. 
     Further, the first core part  20 A has the side slit  35  on each of both ends of the first magnet insertion hole  21 A in the circumferential direction, while the second core part  20 B has the side slit  35  on each of both ends of the second magnet insertion hole  21 B in the circumferential direction. Thus, the leakage magnetic flux between the adjacent magnetic poles can be reduced. 
     The magnet insertion holes  21 A and  21 B extend linearly and perpendicularly to the magnetic pole center line M1, and thus core portions on the outer side of the magnet insertion holes  21 A and  21 B in the radial direction can be made smaller. Consequently, a centrifugal force applied to the core portions on the outer side of the magnet insertion holes  21 A and  21 B in the radial direction can be reduced, and thus the durability of the rotor core  20  can be enhanced. 
     The through holes  26 ,  27 , and  28  are provided to pass through the rotor core  20  in the axial direction, and thus the rotor  2  can be cooled by refrigerant flowing through the through holes  26 ,  27 , and  28 . Thus, the high-temperature demagnetization of the permanent magnet  22 A and  22 B can be suppressed. 
     The cutout portions  16  are provided on the outer circumference of the stator core  10 , and thus the motor  100  can be cooled by the refrigerant flowing through between the cutout portions  16  and the shell  5 . 
     Since the winding  15  of the stator  1  includes the conductor  15   a  made of copper or aluminum and the insulating film  15   b  covering the surface of the conductor  15   a , corrosion of the winding  15  can be prevented, for example, in the refrigerant of the compressor  500 . 
     First Modification 
       FIG. 11  is a cross-sectional view of a second core part  20 B of a rotor  2  of a first modification of the first embodiment. In the second core part  20 B of the first modification, two of each of the slits  32 ,  33 , and  34  are arranged on the outer side of each second magnet insertion hole  21 B in the radial direction. That is, the number N2 of slits is six. The number N1 of slits for each first magnet insertion hole  21 A in the first core part  20 A is eight as described above, and thus N1&gt;N2 is satisfied. 
     The shapes and arrangement of the slits  32 ,  33 , and  34  in the second core part  20 B are the same as, for example, those of the slits  32 ,  33 , and  34  among the eight slits  31 ,  32 ,  33 , and  34  in the first core part  20 A. However, the slits are not limited to such a configuration and it is sufficient that six slits are provided on the outer side of each second magnet insertion hole  21 B in the radial direction. 
     The slits  32 ,  33 , and  34  in the second core part  20 B are desirably formed symmetrically with respect to the magnetic pole center line M1. With this arrangement, spatial harmonics of the magnetic flux generated in the air gap is suppressed, and thus the torque ripple and the excitation force in the radial direction can be reduced. The longitudinal directions of the slits  32 ,  33 , and in the second core part  20 B are desirably parallel to the magnetic pole center line M1. With this arrangement, the magnetic flux from the second permanent magnet  22 B can be guided to the stator  1  by the shortest distance. 
     Also in the first modification, the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  is greater than or equal to 70% and less than 100%. The width W1 of the first magnet insertion hole  21 A in the first core part  20 A is wider than the width W2 of the second magnet insertion hole  21 B in the second core part  20 B. 
     The rotor  2  of the first modification has the same configuration as the rotor  2  of the first embodiment except that the second core part  20 B has the slits  32 ,  33 , and  34  on the outer side of the magnet insertion hole  21 B in the radial direction. 
     Second Modification 
       FIG. 12  is a cross-sectional view of a second core part  20 B of a rotor  2  of a second modification of the first embodiment. In the second core part  20 B of the second modification, two of each of the slits  33  and  34  are formed on the outer side of each second magnet insertion hole  21 B in the radial direction. That is, the number N2 of slits is four. The number of slits N1 for each first magnet insertion hole  21 A in the first core part  20 A is eight as described above, and thus N1&gt;N2 is satisfied. 
     The shapes and arrangement of the slits  33  and  34  in the second core part  20 B are the same as, for example, those of the slits  33  and  34  among the eight slits  31 ,  32 ,  33 , and  34  in the first core part  20 A. However, the slits are not limited to such a configuration and it is sufficient that four slits are provided on the outer side of each second magnet insertion hole  21 B in the radial direction. 
     The slits  33  and  34  in the second core part  20 B are desirably formed symmetrically with respect to the magnetic pole center line M1. The longitudinal directions of the slits  33  and  34  in the second core part  20 B are desirably parallel to the magnetic pole center line M1. 
     Also in the second modification, the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  is greater than or equal to 70% and less than 100%. The width W1 of the first magnet insertion hole  21 A in the first core part  20 A is wider than the width W2 of the second magnet insertion hole  21 B in the second core part  20 B. 
     The rotor  2  of the second modification has the same configuration as the rotor  2  of the first embodiment except that the second core part  20 B has the slits  33  and  34  on the outer side of the second magnet insertion hole  21 B in the radial direction. 
     Third Modification 
       FIG. 13  is a cross-sectional view of a second core part  20 B of a rotor  2  of a third modification of the first embodiment. In the second core part  20 B of the third modification, two slits  34  are formed on the outer side of each second magnet insertion hole  21 B in the radial direction. That is, the number N2 of slits is two. The number of slits N1 for each first magnet insertion hole  21 A in the first core part  20 A is eight as described above, and thus N1&gt;N2 is satisfied. 
     The shapes and arrangement of the slits  34  in the second core part  20 B are the same as, for example, those of the slits  34  among the eight slits  31 ,  32 ,  33 , and  34  in the first core part  20 A. However, the slits are not limited to such a configuration and it is sufficient that two slits are provided on the outer side of the second magnet insertion hole  21 B in the radial direction. 
     The slits  34  in the second core part  20 B are desirably formed symmetrically with respect to the magnetic pole center line M1. The longitudinal direction of the slit  34  in the second core part  20 B is desirably parallel to the magnetic pole center line M1. 
     Also in the third modification, the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  is greater than or equal to 70% and less than 100%. The width W1 of the first magnet insertion hole  21 A in the first core part  20 A is wider than the width W2 of the second magnet insertion hole  21 B in the second core part  20 B. 
     The rotor  2  of the third modification has the same configuration as the rotor  2  of the first embodiment except that the second core part  20 B has the slits  34  on the outer side of the second magnet insertion hole  21 B in the radial direction. 
     In the first to third modifications ( FIGS. 11 to 13 ), the second core part  20 B has the slits on the outer side of the second magnet insertion hole  21 B in the radial direction, but the number N2 of these slits is smaller than the number N1 of slits in the first core part  20 A (N2&lt;N1). Thus, the eddy current loss in the permanent magnets  22 A and  22 B can be reduced. Since the widths W1 and W2 of the magnet insertion holes  21 A and  21 B satisfy W1&gt;W2, the demagnetization resistances of the first permanent magnets  22 A and  22 B can be made closer to each other. 
     The second core part  20 B has the slits on the outer side of the second magnet insertion hole  21 B in the radial direction, and thus the effect of reducing the torque ripple can be enhanced as compared to the first embodiment. 
     Second Embodiment 
     Next, a second embodiment of the present invention will be described.  FIG. 14  is a perspective view showing a portion corresponding to one magnetic pole of a rotor  2  in a motor of a second embodiment.  FIG. 15  is a cross-sectional view taken along a line XV-XV in  FIG. 14  as seen in a direction indicated by arrows, i.e., a longitudinal sectional view of the rotor  2 . 
     In the above-described first embodiment, two first core parts  20 A are provided, one on each of both sides of the second core part  20 B in the axial direction. In contrast, in the second embodiment, two second core parts  20 B are provided, one on each of both sides of the first core part  20 A in the axial direction. 
     The first core part  20 A has the same configuration as the first core part  20 A ( FIG. 5 ) of the first embodiment and is disposed at the center of the rotor core  20  in the axial direction. The second core part  20 B has the same configuration as the second core part  20 B ( FIG. 6 ) of the first embodiment and is disposed on each of both ends of the rotor core  2  in the axial direction. 
     As shown in  FIG. 15 , the first core part  20 A has a length L3 in the axial direction, while each second core part  20 B has a length L4 in the axial direction. The width W1 of the first magnet insertion hole  21 A is wider than the width W2 of the second magnet insertion hole  21 B. The ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  is expressed as L4×2/(L3+L4×2)×100. The ratio is greater than or equal to 70% and less than 100%. 
     Although the second core part  20 B has no slit on the outer side of the second magnet insertion hole  21 B in the radial direction, the second core part  20 B may have slits, the number of which is N2 (&lt;N1), on the outer side of the second magnet insertion hole  21 B in the radial direction as described in the modifications ( FIGS. 11 to 13 ). 
     In the second embodiment, the second magnet insertion hole  21 B having the narrower width W2 is located on each of both sides of the first magnet insertion hole  21 A having the wider width W1 in the axial direction. For this reason, it is necessary to insert the first permanent magnet  22 A into the first magnet insertion hole  21 A before the rotor core  20  is completed. 
     Thus, the first permanent magnet  22 A is inserted into the first magnet insertion hole  21 A at the stage when the first core part  20 A is formed by stacking the electromagnetic steel sheets. Thereafter, electromagnetic steel sheets are stacked on both sides of the first core part  20 A in the axial direction to form the second core parts  20 B, and then the second permanent magnets  22 B are inserted into the second magnet insertion holes  21 B. 
     The rotor  2  of the second modification has the same configuration as the rotor  2  of the first embodiment except that the rotor core  20  has two core parts  20 B, one on each of both sides of the first core part  20 A in the axial direction to that of the rotor core. 
     As described above, also in the second embodiment, the number N2 of the slits in the second core part  20 B is smaller than the number N1 of the slits in the first core part  20 A, and the ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  is greater than or equal to 70% and less than 100%. Thus, the eddy current loss can be reduced. Since the width W1 of the first magnet insertion hole  21 A is wider than the width W2 of the second magnet insertion hole  21 B, the demagnetization resistances of the first permanent magnets  22 A and  22 B can be made closer to each other. Further, the second permanent magnet  22 B can be formed to be thinner than the first permanent magnet  22 A, the manufacturing cost can be reduced. 
     Since the second core part  20 B is provided on each of both sides of the first core part  20 A in the axial direction, the weight balance of the rotor  2  in the axial direction can be enhanced. 
     Third Embodiment 
     Next, a third embodiment of the present invention will be described.  FIG. 16  is a perspective view showing a portion corresponding to one magnetic pole of a rotor  2  in a motor of a third embodiment.  FIG. 17  is a cross-sectional view taken along a line XVII-XVII in  FIG. 16  as seen in a direction indicated by arrows, i.e., a longitudinal sectional view of the rotor  2 . 
     The rotor core  20  of the above-described first embodiment includes two first core parts  20 A and one second core part  20 B, while the rotor core  20  of the second embodiment includes two second core parts  20 B and one first core part  20 A. 
     In contrast, the rotor core  20  of the third embodiment includes one first core part  20 A and one second core part  20 B. The first core part  20 A has the same configuration as the first core part  20 A ( FIG. 5 ) of the first embodiment, while the second core part  20 B has the same configuration as the second core part  20 B ( FIG. 6 ) of the first embodiment. The first core part  20 A and the second core part  20 B are provided to be adjacent to each other in the axial direction. 
     As shown in  FIG. 17 , the first core part  20 A has a length L5 in the axial direction, while the second core part  20 B has a length L6 in the axial direction. The width W1 of the first magnet insertion hole  21 A is wider than the width W2 of the second magnet insertion hole  21 B. The ratio of the length of the second core part  20 B in the axial direction to that of the rotor core  20  is expressed as L6/(L5+L6). The ratio is greater than or equal to 70% and less than 100%. 
     Although the second core part  20 B has no slit on the outer side of the second magnet insertion hole  21 B in the radial direction, the second core part  20 B may have slits, the number of which is N2 (&lt;N1), on the outer side of the second magnet insertion hole  21 B in the radial direction as described in the modifications ( FIGS. 11 to 13 ). 
     The rotor core  20  is formed by stacking, in the axial direction, electromagnetic steel sheets each of which is punched into the shape of the first core part  20 A shown in  FIG. 5 , and electromagnetic steel sheets each of which is punched into the shape of the second core part  20 B shown in  FIG. 6 . The first permanent magnet  22 A is inserted into the first magnet insertion hole  21 A of the first core part  20 A from one side (a lower side in  FIG. 17 ) of the rotor core  20 . The second permanent magnet  22 B is inserted into the second magnet insertion hole  21 B of the second core part  20 B from the other side (an upper side in  FIG. 17 ) of the rotor core  20 . 
     The rotor  2  of the third embodiment has the same configuration as the rotor  2  of the first embodiment except that the rotor core  20  includes one first core part  20 A and one second core part  20 B. 
     As described above, according to the third embodiment, since the rotor core  20  includes one first core part  20 A and one second core part  20 B, the configuration of the rotor core  20  is simple and its assembly process is also simple, in addition to the effects described in the first embodiment. Thus, the manufacturing cost of the motor  100  can be enhanced. 
     In the above-described first to third embodiments and the modifications, the magnet insertion holes  21 A and  21 B extend linearly and perpendicularly to the magnetic pole center line M1, but the magnet insertion holes  21 A and  21 B are not limited to such an example. Specifically, the magnetic insertion hole  21 A or  21 B may extend in a V-shape such that a center in the circumferential direction protrudes inward in the radial direction. Moreover, a plurality of permanent magnets may be disposed in each magnet insertion hole. 
     (Scroll Compressor) 
     Next, a compressor to which the motor of each of the first to third embodiments and the modifications described above is applicable will be described.  FIG. 18  is a cross-sectional view showing a configuration of a compressor  500  that includes the motor  100  of the first embodiment. Instead of the motor  100  of the first embodiment, the motor of the second or third embodiment or each modification may be used. 
     The compressor  500  is a scroll compressor and includes, in a storage container  502 , a compression mechanism  510 , the motor  100  that drives the compression mechanism  510 , a main shaft  501  that connects the compression mechanism  510  and the motor  100 , a sub-frame  503  that supports an end portion (sub-shaft portion) of the main shaft  501  opposite to the compression mechanism  510 , and a refrigeration machine oil  504  stored in an oil reservoir  505  at a bottom of the storage container  502 . 
     The compression mechanism  510  includes a fixed scroll  511  and an orbiting scroll  512  which are combined to form a compression room between plate-shaped scroll teeth, an Oldham ring  513 , a compliant frame  514 , and a guide frame  515 . 
     A suction pipe  506  passing through the storage container  502  is press-fitted to the fixed scroll  511 . A discharge pipe  507  is provided to pass through the storage container  502 . The discharge pipe  507  allows high-pressure refrigerant gas discharged from a discharge port of the fixed scroll  511  to be discharged to the outside (refrigeration cycle). 
     The storage container  502  includes a cylindrical shell  5  into which the motor  100  is fitted by shrink-fitting. In addition, a glass terminal  508  for electrically connecting the stator  1  of the motor  100  to a drive circuit is fixed to the storage container  502  by welding. 
     The motor  100  of each of the first to third embodiments and the modifications described above has the motor efficiency enhanced by reducing the eddy current loss. Thus, by using the motor  100  as a power source of the compressor  500 , the operation efficiency of the compressor  500  can be enhanced, and thus consumption energy can be reduced. 
     Herein, the scroll compressor is described as an example of the compressor, but the motor of each embodiment and modification described above may be applied to any compressor other than the scroll compressor. 
     (Air Conditioner) 
     Next, an air conditioner  400  including the compressor  500  described above will be described.  FIG. 19  is a diagram showing a configuration of the air conditioner  400 . The air conditioner  400  shown in  FIG. 19  includes a compressor  401 , a condenser  402 , a throttle device (a decompressor)  403 , and an evaporator  404 . The compressor  401 , the condenser  402 , the throttle device  403 , and the evaporator  404  are connected together by a refrigerant pipe  407  to constitute a refrigeration cycle. That is, refrigerant circulates through the compressor  401 , the condenser  402 , the throttle device  403 , and the evaporator  404  in this order. 
     The compressor  401 , the condenser  402 , and the throttle device  403  are provided in an outdoor unit  410 . The compressor  401  is constituted by the compressor  500  illustrated in  FIG. 18 . The outdoor unit  410  is provided with an outdoor fan  405  that supplies outdoor air to the condenser  402 . The evaporator  404  is provided in an indoor unit  420 . The indoor unit  420  is provided with an indoor fan  406  that supplies indoor air to the evaporator  404 . 
     An operation of the air conditioner  400  is as follows. The compressor  401  compresses sucked refrigerant and sends out the compressed refrigerant. The condenser  402  exchanges heat between the refrigerant flowing from the compressor  401  and the outdoor air to condense and liquefy the refrigerant, and sends out the liquefied refrigerant to the refrigerant pipe  407 . The outdoor fan  405  supplies the outdoor air to the condenser  402 . The throttle device  403  changes its opening degree to adjust the pressure and the like of the refrigerant flowing through the refrigerant pipe  407 . 
     The evaporator  404  exchanges heat between the refrigerant brought into a low-pressure state by the throttle device  403  and the indoor air to cause the refrigerant to take heat from the indoor air and evaporate (vaporize), and then sends out the evaporated refrigerant to the refrigerant pipe  407 . The indoor fan  406  supplies indoor air to the evaporator  404 . Thus, cooled air deprived of heat at the evaporator  404  is supplied into the room. 
     The motor  100  described in each of the first to third embodiments and the modifications is applicable to the compressor  401  (the compressor  500  in  FIG. 18 ). Thus, the operation efficiency of the compressor  401  during operation of the air conditioner  400  can be enhanced, and the operational stability of the air conditioner  400  can be enhanced. 
     The compressor  500  to which the motor described in each of the first to third embodiments and the modifications is applied is not limited to the air conditioner  400  shown in  FIG. 19 , but may be used in other types of air conditioners. 
     Although the desirable embodiments of the present invention have been specifically described, the present invention is not limited to the above-described embodiments, and various modifications or changes can be made to those embodiments without departing from the scope of the present invention.