Patent Publication Number: US-2022239171-A1

Title: Rotor, electric motor, blower, air conditioner, and manufacturing method for rotor

Description:
TECHNICAL FIELD 
     The present invention relates to a rotor for use in an electric motor. 
     BACKGROUND ART 
     As a rotor for use in an electric motor, a rotor having two types of magnets is generally employed (see, for example, Patent Reference 1). In Patent Reference 1, permanent magnets having high magnetic force (also referred to as first permanent magnets) form the entire outer peripheral surface of the rotor, and permanent magnets having lower magnetic force than that of the first permanent magnets (also referred to as second permanent magnets) are disposed inside the first permanent magnets. In this rotor, since the first permanent magnets form the entire outer peripheral surface of the rotor, magnetic force of the rotor can be effectively enhanced. 
     PRIOR ART REFERENCE 
     Patent Reference 
     
         
         Patent Reference 1: Japanese Patent Application Publication No. 2005-151757 
       
    
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     In the case where the first permanent magnets having high magnetic force form the entire outer peripheral surface of the rotor, however, sufficient magnetic force of the rotor can be obtained, but since magnets having high magnetic force are generally expensive, costs for the rotor increase disadvantageously. 
     It is therefore an object of the present invention to obtain sufficient magnetic force of a rotor even in the case of reducing the amount of a first permanent magnet having high magnetic force. 
     Means of Solving the Problem 
     A rotor according to an aspect of the present invention is a rotor having 2n (n is a natural number) magnetic poles, and the rotor includes: 
     at least one first permanent magnet forming part of an outer peripheral surface of the rotor and magnetized to have polar anisotropy; and 
     at least one second permanent magnet that is of a different type from the at least one first permanent magnet, is adjacent to the at least one first permanent magnet in a circumferential direction of the rotor, has lower magnetic force than magnetic force of the at least one first permanent magnet, and is magnetized to have polar anisotropy, wherein 
     the at least one second permanent magnet has 3×2n magnetic poles. 
     A rotor according to another aspect of the present invention is a rotor having 2n (n is a natural number) magnetic poles and including a plurality of layered magnets including two to m (m is a natural number and a divisor of n) layers that are stacked in an axial direction, wherein 
     each layered magnet of the plurality of layered magnets includes 
     at least one first permanent magnet forming part of an outer peripheral surface of the rotor and magnetized to have polar anisotropy, and 
     at least one second permanent magnet that is of different type from the at least one first permanent magnet, is adjacent to the at least one first permanent magnet in a circumferential direction of the rotor, has lower magnetic force than magnetic force of the at least one first permanent magnet, and is magnetized to have polar anisotropy, 
     the at least one second permanent magnet has 3×2n magnetic poles, and 
     in each first permanent magnet of the plurality of layered magnets, supposing one cycle is an angle formed by adjacent north poles in a plane orthogonal to the axial direction of the rotor, positions of north poles of two first permanent magnets adjacent to each other in the axial direction are shifted from each other by n/m cycles in the circumferential direction. 
     Effects of the Invention 
     According to the present invention, even when the amount of first permanent magnets having high magnetic force is reduced, sufficient magnetic force of the rotor can be obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view schematically illustrating a structure of a rotor according to a first embodiment of the present invention. 
         FIG. 2  is a plan view schematically illustrating the structure of the rotor. 
         FIG. 3  is a diagram illustrating orientation of first permanent magnets in the rotor. 
         FIG. 4  is a diagram illustrating a structure of the first permanent magnets and positions of magnetic poles in the first permanent magnets. 
         FIG. 5  is a cross-sectional view schematically illustrating a structure of a second permanent magnet. 
         FIG. 6  is a diagram illustrating a structure of the second permanent magnet and positions of magnetic poles in the second permanent magnet. 
         FIG. 7  is a flowchart depicting an example of a process for fabricating a rotor. 
         FIG. 8  is a cross-sectional view schematically illustrating a structure of a rotor according to a first comparative example. 
         FIG. 9  is a diagram illustrating a structure of first permanent magnets and orientation of the first permanent magnets in a rotor according to a second comparative example. 
         FIG. 10  is a diagram illustrating a structure and orientation of a second permanent magnet in the rotor according to the second comparative example. 
         FIG. 11  is a diagram illustrating a structure and orientation of the rotor according to the second comparative example. 
         FIG. 12  is a graph showing changes in surface magnetic flux density. 
         FIG. 13  is a cross-sectional view schematically illustrating a structure of a rotor according to a first variation. 
         FIG. 14  is a plan view schematically illustrating a structure of a rotor according to a second variation. 
         FIG. 15  is a side view schematically illustrating the structure of the rotor according to the second variation. 
         FIG. 16  is a cross-sectional view schematically illustrating the structure of the rotor according to the second variation. 
         FIG. 17  is a plan view schematically illustrating a structure of a rotor according to a third variation. 
         FIG. 18  is a side view schematically illustrating the structure of the rotor according to the third variation. 
         FIG. 19  is a cross-sectional view schematically illustrating the structure of the rotor according to the third variation. 
         FIG. 20  is a cross-sectional view schematically illustrating a structure of a rotor according to a fourth variation. 
         FIG. 21  is a side view schematically illustrating the structure of the rotor according to the fourth variation. 
         FIG. 22  is a cross-sectional view schematically illustrating a structure of a rotor according to a fifth variation. 
         FIG. 23  is a side view schematically illustrating the structure of the rotor according to the fifth variation. 
         FIG. 24  is a partial cross-sectional view schematically illustrating a structure of an electric motor according to a second embodiment of the present invention. 
         FIG. 25  is a diagram schematically illustrating a structure of a fan according to a third embodiment of the present invention. 
         FIG. 26  is a diagram schematically illustrating a configuration of an air conditioner according to a fourth embodiment of the present invention. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     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 a rotor  2 , 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 rotation center of the rotor  2 . The axis Ax also represents an axis of an electric motor  1  described later. A direction parallel to the axis Ax is also referred to as an “axial direction of the rotor  2 ” or simply as an “axial direction.” The “radial direction” refers to a radial direction of the rotor  2  or a stator  3 , and is a direction orthogonal to the axis Ax. An xy plane is a plane orthogonal to the axial direction. An arrow D 1  represents a circumferential direction about the axis Ax. The circumferential direction of the rotor  2  or the stator  3  will be also referred to simply as a “circumferential direction.” 
     In some drawings, “N” and “S” respectively represent a north pole and a south pole in the rotor  2  (including variations thereof). 
       FIG. 1  is a side view schematically illustrating a structure of a rotor  2  according to a first embodiment of the present invention. In  FIG. 1 , broken lines represent positions of magnetic poles (north poles or south poles) of the rotor  2 . 
       FIG. 2  is a plan view schematically illustrating the structure of the rotor  2 .  FIG. 2  is a plan view taken along line C 2 -C 2  in  FIG. 1 . In  FIG. 2 , arrows on the rotor  2  represent directions of main magnetic flux. 
     The rotor  2  is used for an electric motor (e.g., an electric motor  1  described later). 
     The rotor  2  includes at least one first permanent magnet  21  and at least one second permanent magnet  22  that is of a different type from the first permanent magnet  21 . 
     The “at least one first permanent magnet  21 ” includes two or more first permanent magnets  21 . The “at least one second permanent magnet  22 ” includes two or more second permanent magnets  22 . 
     The rotor  2  has 2n (n is a natural number) magnetic poles. In this embodiment, n is four, and the rotor  2  has eight magnetic poles. The rotor  2  includes the plurality of first permanent magnets  21  and one second permanent magnet  22 . In this embodiment, the rotor  2  includes 2n first permanent magnets  21  and one second permanent magnet  22 . Thus, in this embodiment, the rotor  2  includes eight first permanent magnets  21  and one second permanent magnet  22 . 
     For example, as illustrated in  FIG. 1 , north poles of the first permanent magnets  21  and south poles of the first permanent magnets  21  are alternately arranged on the outer peripheral surface of the rotor  2 . It should be noted that the plurality of first permanent magnets  21  may be coupled to each other by, for example, ring-shaped coupling parts, and the second permanent magnet  22  may be divided into a plurality of parts. 
       FIG. 3  is a diagram illustrating orientation of the first permanent magnets  21 , that is, directions of magnetic flux from the first permanent magnets  21 , in the rotor  2 . 
       FIG. 4  is a diagram illustrating a structure of the first permanent magnets  21  and positions of magnetic poles in the first permanent magnets  21 . 
     The first permanent magnets  21  form part of the outer peripheral surface of the rotor  2 . As illustrated in  FIG. 3 , the first permanent magnets  21  are magnetized to have polar anisotropy. In other words, the first permanent magnets  21  are magnetized such that the rotor  2  has polar anisotropy. In this embodiment, as illustrated in  FIG. 3 , one pair of first permanent magnets  21  (i.e., 2n first permanent magnets  21 ) form 2n magnetic poles. The first permanent magnets  21  are rare earth magnets. For example, each first permanent magnet  21  is a bonded magnet as a mixture of a rare earth magnet and a resin, that is, a rare earth bonded magnet. Each first permanent magnet  21  has higher magnetic force than that of the second permanent magnet  22 . 
     In the xy plane, the inner peripheral surfaces and the outer peripheral surfaces of the first permanent magnets  21  are concentrically formed. That is, the thickness of the first permanent magnets  21  in the xy plane is uniform in the circumferential direction. 
     The rare earth magnet is, for example, a magnet containing neodymium (Nd), iron (Fe), and boron (B) or a magnet containing samarium (Sm), iron (Fe), and nitrogen (N). The resin is, for example, a nylon resin, a polyphenylene sulfide (PPS) resin, or an epoxy resin. 
     The second permanent magnet  22  is adjacent to the first permanent magnets  21  in the circumferential direction of the rotor  2 , and forms part of the outer peripheral surface of the rotor  2 . Specifically, part of the second permanent magnet  22  is adjacent to the first permanent magnets  21  in the circumferential direction of the rotor  2 , and another part of the second permanent magnet  22  is located at the inner side of the first permanent magnets  21  in the radial direction of the rotor  2 . Thus, the second permanent magnet  22  is a ring-shaped magnet. 
     In the examples illustrated in  FIGS. 1 and 2 , on the outer peripheral surface of the rotor  2 , the plurality of first permanent magnets  21  and a plurality of parts of the second permanent magnet  22  are alternately arranged in the circumferential direction of the rotor  2 . 
       FIG. 5  is a cross-sectional view schematically illustrating a structure of the second permanent magnet  22 .  FIG. 5  is a cross-sectional view taken along line C 5 -C 5  in  FIG. 1 . In  FIG. 5 , arrows on the second permanent magnet  22  represent directions of main magnetic flux. 
       FIG. 6  is a diagram illustrating a structure of the second permanent magnet  22  and positions of magnetic poles in the second permanent magnet  22 . 
     As illustrated in  FIG. 5 , the second permanent magnet  22  is magnetized to have polar anisotropy. In other words, the second permanent magnet  22  is magnetized such that the rotor  2  has polar anisotropy. In this embodiment, the second permanent magnet  22  is a single structure, that is, one magnet. The second permanent magnet  22  constitutes magnetic poles in the rotor  2  together with the first permanent magnets  21 . 
     The second permanent magnet  22  is a magnet that is of a different type from the first permanent magnets  21 . The second permanent magnet  22  is a ferrite magnet. For example, the second permanent magnet  22  is a bonded magnet as a mixture of a ferrite magnet and a resin, that is, a ferrite bonded magnet. The resin is, for example, a nylon resin, a polyphenylene sulfide (PPS) resin, or an epoxy resin. The second permanent magnet  22  has lower magnetic force than that of each first permanent magnet. 
     The second permanent magnet  22  has 3×2n magnetic poles. That is, the second permanent magnet  22  has easy axes of magnetization to have 3×2n magnetic poles. Thus, in this embodiment, the second permanent magnet  22  has 24 magnetic poles, and at least 24 easy axes of magnetization. 
     Orientation of the rotor  2  indicated by arrows in  FIG. 2  is a synthesis of orientation of the first permanent magnets  21  illustrated in  FIG. 3  and orientation of the second permanent magnet  22  illustrated in  FIG. 5 . Consequently, the surface magnetic flux density of the rotor  2 , that is, the magnetic flux density on the outer peripheral surface of the rotor  2 , is at maximum at the boundary between each first permanent magnet  21  and the second permanent magnet  22 . 
     Part of a plurality of magnetic poles (e.g., some north poles) of the second permanent magnet  22  is adjacent to each first permanent magnet  21 . Accordingly, the magnetic flux density on the outer peripheral surface of the rotor  2  is at maximum at the boundary between each first permanent magnet  21  and the second permanent magnet  22 . 
     An example of a method for fabricating the rotor  2  will be described. 
       FIG. 7  is a flowchart depicting an example of a process for fabricating the rotor  2 . 
     In first step S 1 , a mold for a second permanent magnet  22  is filled with a material for the second permanent magnet  22 . 
     In second step S 2 , a second permanent magnet  22  is molded, and the second permanent magnet  22  is oriented. For example, a magnetic field of polar anisotropy is generated inside the mold for the second permanent magnet  22  by using a magnet for magnetization. Accordingly, the second permanent magnet  22  is molded and oriented. The second permanent magnet  22  is molded by, for example, injection molding. In this embodiment, the second permanent magnet  22  is molded to have orientation of polar anisotropy and have 3×2n magnetic poles. In other words, easy axes of magnetization are formed in the second permanent magnet  22  such that the second permanent magnet  22  has 3×2n magnetic poles. 
     First step S 1  and second step S 2  may be performed at the same time. In this case, a magnetic field of polar anisotropy is previously generated inside the mold for the second permanent magnet  22  by using a magnet for magnetization, for example. In the state where the field of polar anisotropy is generated inside the mold for the second permanent magnet  22 , the mold for the second permanent magnet  22  is filled with a material for the second permanent magnet  22  by injection molding. Accordingly, the second permanent magnet  22  is molded and oriented. 
     In third step S 3 , the second permanent magnet  22  in the mold is cooled. 
     In fourth step S 4 , the second permanent magnet  22  is taken out of the mold. 
     Since a mold corresponding to the shape of each of the first permanent magnets  21  is formed in the mold for the second permanent magnet  22 , the shape of each of the first permanent magnets  21  is molded on the outer peripheral surface of the second permanent magnet  22  concurrently with obtainment of the second permanent magnet  22 . 
     In fifth step S 5 , the second permanent magnet  22  is demagnetized. For example, the second permanent magnet  22  is demagnetized by a demagnetizer. 
     In sixth step S 6 , the second permanent magnet  22  is placed in a mold for the first permanent magnets  21 . 
     In seventh step S 7 , the mold for the first permanent magnets  21  is filled with a material for the first permanent magnets  21 . 
     In eighth step S 8 , the first permanent magnets  21  are molded and oriented. For example, a magnetic field of polar anisotropy is generated inside the mold for the first permanent magnets  21  by using a magnet for magnetization. Accordingly, the plurality of first permanent magnets  21  are molded and oriented. Each of the first permanent magnets  21  is molded by, for example, injection molding. In this embodiment, 2n first permanent magnets  21  are formed on the outer peripheral surface of the second permanent magnet  22  to form part of the outer peripheral surface of the rotor  2 , and the first permanent magnets  21  are molded to have orientation of polar anisotropy. 
     Seventh step S 7  and eighth step S 8  may be performed at the same time. In this case, a magnetic field of polar anisotropy is generated inside the mold for the first permanent magnets  21  beforehand by using a magnet for magnetization, for example. In the state where the field of polar anisotropy is generated inside the mold for the first permanent magnets  21 , the mold for the first permanent magnets  21  is filled with a material for the first permanent magnets  21  by injection molding. Accordingly, each of the first permanent magnets  21  is molded and oriented at the same time. 
     In ninth step S 9 , the first permanent magnets  21  in the mold are cooled. 
     In tenth step S 10 , the first permanent magnets  21  and the second permanent magnet  22  are taken out of the mold. 
     In eleventh step S 11 , the first permanent magnets  21  are demagnetized. For example, the first permanent magnets  21  are demagnetized by a demagnetizer. 
     In twelfth step S 12 , the first permanent magnets  21  and the second permanent magnet  22  are magnetized. For example, the first permanent magnets  21  and the second permanent magnet  22  are magnetized by a magnetizer such that the first permanent magnets  21  and the second permanent magnet  22  have polar anisotropy. 
     In this manner, the rotor  2  is obtained. 
     Advantages of the rotor  2  according to the first embodiment will be described. 
       FIG. 8  is a cross-sectional view schematically illustrating a structure of a rotor  200  according to a first comparative example. In  FIG. 8 , arrows on the rotor  200  represent directions of main magnetic flux. In the rotor  200  according to the first comparative example illustrated in the  FIG. 8 , a ring-shaped rare earth bonded magnet  201  having higher magnetic force than that of a cylindrical ferrite bonded magnet  202  is disposed on the outer peripheral surface of the ferrite bonded magnet  202 . The ring-shaped rare earth bonded magnet  201  extends in the circumferential direction of the rotor  200 , and the thickness of the rare earth bonded magnet  201  in the xy plane is uniform in the axial direction of the rotor  200 . That is, the ring-shaped rare earth bonded magnet  201  forms the entire outer peripheral surface of the rotor  200 . 
     On the other hand, the rotor  2  according to the first embodiment includes the plurality of first permanent magnets  21 . The first permanent magnets  21  form part of the outer peripheral surface of the rotor  2 , and do not form the entire outer peripheral surface of the rotor  2 . Accordingly, the amount of the first permanent magnets  21  having high magnetic force can be reduced, as compared to the rotor  200  according to the first comparative example. In a case where the first permanent magnets  21  are expensive rare earth bonded magnets, the amount of rare earth bonded magnets can be reduced as compared to the rotor  200  according to the first comparative example, and thus, costs for the rotor  2  can be reduced. 
       FIG. 9  is a diagram illustrating a structure of first permanent magnets  301  and orientation of the first permanent magnets  301  in a rotor  300  according to a second comparative example. 
       FIG. 10  is a diagram illustrating a structure and orientation of a second permanent magnet  302  in the rotor  300  according to the second comparative example. 
       FIG. 11  is a diagram illustrating a structure and orientation of the rotor  300  according to the second comparative example. 
       FIG. 12  is a graph showing changes in surface magnetic flux density. In  FIG. 12 , the vertical axis represents a surface magnetic flux density [a.u.] (specifically, a surface magnetic flux density at a position indicated by line C 5  in  FIG. 1 ), and the horizontal axis represents a mechanical angle [degrees]. In  FIG. 12 , “A” represents a surface magnetic flux density of the rotor  2  according to the first embodiment, “B” represents a surface magnetic flux density of the rotor  200  according to the first comparative example, and “C” represents a surface magnetic flux density of the rotor  300  according to the second comparative example. 
     In the rotor  200  according to the first comparative example, the rare earth bonded magnet  201  and the ferrite bonded magnet  202  are respectively different in shape from each of the first permanent magnets and the second permanent magnet  22  of the rotor  2  according to the first embodiment. 
     In the rotor  300  according to the second comparative example, the first permanent magnets  301  and the second permanent magnet  302  are the same as the first permanent magnets and the second permanent magnet  22  of the rotor  2  according to the first embodiment in terms of shape, but the number of magnetic poles of the second permanent magnet  302  of the rotor  300  according to the second comparative example is different from the number of magnetic poles of the second permanent magnet  22  of the rotor  2  according to the first embodiment. The number of magnetic poles of the second permanent magnet  22  of the rotor  2  according to the first embodiment is 24, and the number of magnetic poles of the second permanent magnet  302  of the rotor  300  according to the second comparative example is 8. 
     As shown in  FIG. 12 , in the rotor  200  according to the first comparative example indicated by broken line B, a sine wave that is uniform in the circumferential direction is formed. On the other hand, in the rotor  3  according to the second comparative example indicated by broken line C, an irregular sine wave is formed. Thus, in the rotor  300  according to the second comparative example, vibrations and noise are large during rotation of the rotor  300 , as compared to the first comparative example. 
     On the other hand, in the rotor  2  according to the first embodiment, the second permanent magnet  22  has 3×2n magnetic poles (24 magnetic poles in this embodiment), and the magnetic flux density on the outer peripheral surface of the rotor  2  is at maximum at the boundary between each first permanent magnet  21  and the second permanent magnet  22 . Accordingly, as shown in  FIG. 12 , a relatively uniform sine wave is formed. That is, in the rotor  2  according to the first embodiment, an abrupt change in surface magnetic flux density is suppressed, as compared to the second comparative example. Accordingly, vibrations and noise can be reduced during rotation of the rotor  2 , as compared to the second comparative example. 
     As described above, in the rotor  2  according to the first embodiment, the amount of the first permanent magnets  21  having high magnetic force can be reduced, as compared to the rotor  200  according to the first comparative example. Specifically, in the rotor  2  according to the first embodiment, since the first permanent magnets  21  form part of the outer peripheral surface of the rotor  2 , the amount of the first permanent magnets  21  can be reduced by about 20%, as compared to the rotor  200  according to the comparative example. In general, a material unit price of rare earth magnets is greater than or equal to 10 times that of ferrite magnets. Thus, in a case where magnets including rare earth magnets (e.g., rare earth bonded magnets) are used as the first permanent magnets  21  and a magnet including a ferrite magnet (e.g., a ferrite bonded magnet) is used as the second permanent magnet  22 , even when the amount of the second permanent magnet  22  is large, costs for the first permanent magnets  21  can be significantly reduced. As a result, costs for the rotor  2  can be significantly reduced. 
     In addition, as described above, in the rotor  2  according to the first embodiment, even when the amount of the first permanent magnets  21  having high magnetic force is reduced, an abrupt change in surface magnetic flux density is suppressed. Accordingly, vibrations and noise can be reduced during rotation of the rotor  2 , as compared to the second comparative example. 
     With the method for fabricating the rotor  2 , the rotor  2  having the advantages described above can be fabricated. 
     First Variation 
       FIG. 13  is a cross-sectional view schematically illustrating a structure of a rotor  2   a  according to a first variation. 
     In the xy plane, an angle A 1  formed by two lines T 11  passing through a rotation center (i.e., an axis Ax) of the rotor  2   a  and both ends P 11  of the inner peripheral surface of each first permanent magnet  21  is larger than an angle A 2  formed by two lines T 12  passing through the rotation center of the rotor  2   a  and both ends P 12  of the outer peripheral surface of the first permanent magnet  21 . The inner peripheral surfaces of the first permanent magnets  21  are the radially inner surfaces of the first permanent magnets  21 . The outer peripheral surfaces of the first permanent magnets  21  are the radially outer surfaces of the first permanent magnets  21 . 
     In the xy plane, the inner peripheral surface of each first permanent magnet  21  is longer than the outer peripheral surface of the first permanent magnet  21 . Accordingly, a centrifugal force generated during rotation of the rotor  2   a  can prevent detachment of the first permanent magnets  21  from the second permanent magnet  22 . 
     In the xy plane, an angle A 3  is smaller than an angle A 4 . Accordingly, a centrifugal force generated during rotation of the rotor  2   a  can prevent detachment of the first permanent magnets  21  from the second permanent magnet  22 . In the xy plane, the angle A 3  is an angle formed by two lines T 22  passing through opposed ends P 13  of the inner peripheral surfaces of two first permanent magnets  21 , and these end ends P 13  face each other in the circumferential direction of the rotor  2 . In other words, the two ends P 13  are adjacent to each other in the circumferential direction of the rotor  2 . In the xy plane, the angle A 4  is an angle formed by two lines T 21  passing through both ends P 21  of the outer peripheral surface of the second permanent magnet  22  between two first permanent magnets  21 . The outer peripheral surface of the second permanent magnet  22  is the surface of the second permanent magnet  22  facing outward in the radial direction. 
     The rotor  2   a  according to the first variation has the same advantages as the rotor  2  according to the first embodiment. 
     Second Variation 
       FIG. 14  is a plan view schematically illustrating a structure of a rotor  2   b  according to a second variation. 
       FIG. 15  is a side view schematically illustrating the structure of the rotor  2   b  according to the second variation. 
       FIG. 16  is a cross-sectional view schematically illustrating the structure of the rotor  2   b  according to the second variation. Specifically,  FIG. 16  is a cross-sectional view taken along line C 16 -C 16  in  FIG. 14 . 
     In the rotor  2   b  according to the second variation, the first permanent magnet  21  is a single structure. The first permanent magnet  21  includes a plurality of bodies  21   a  and at least one ring-shaped portion  21   b . In the example illustrated in  FIG. 15 , the first permanent magnet  21  has two ring-shaped portions  21   b . The plurality of bodies  21   a  correspond to the first permanent magnets  21  in the first embodiment (e.g., the first permanent magnets  21  illustrated in  FIG. 1 ). Thus, the bodies  21   a  form part of the outer peripheral surface of the rotor  2   b , and are magnetized to have polar anisotropy. Part of a second permanent magnet  22  is present between two bodies  21   a  adjacent to each other in the circumferential direction. 
     In the example illustrated in  FIG. 15 , the two ring-shaped portions  21   b  are integrated as a single member (also referred to as a single structure) with the plurality of bodies  21   a . Thus, in the second variation, the rotor  2   b  includes one first permanent magnet  21  and one second permanent magnet  22 . In the example illustrated in  FIG. 15 , the ring-shaped portions  21   b  are located at both ends of the first permanent magnet  21  in the axial direction. It should be rioted that the ring-shaped portions  21   b  may be located at one end of the first permanent magnet  21  in the axial direction. The ring-shaped portions  21   b  cover the whole or part of end portions of the second permanent magnet  22  in the axial direction of the rotor  2   b.    
     As illustrated in  FIG. 16 , each ring-shaped portion  21   b  may include at least one projection  21   c  or at least one recess  21   d . Each ring-shaped portion  21   b  may include both of the at least one projection  21   c  and the at least one recess  21   d . The projection  21   c  projects toward the second permanent magnet  22 . For example, the projection  21   c  is engaged with a recess formed on the second permanent magnet  22 . For example, the recess  21   d  is engaged with a projection formed on the second permanent magnet  22 . 
     In general, when the temperature of the rotor changes, magnets deform in some cases. In such cases, one of two types of magnets might be detached from the rotor because of a difference in thermal shrinkage. In the second variation, since the rotor  2   b  has the ring-shaped portions  21   b , when the temperature of the rotor  2   b  changes, even in a case where the first permanent magnet  21  or the second permanent magnet  22  deforms because of a difference in thermal shrinkage, it is possible to prevent detachment of the first permanent magnet  21  (especially the bodies  21   a ) from the second permanent magnet  22 . In addition, a centrifugal force generated during rotation of the rotor  2   b  can prevent detachment of the first permanent magnet  21  (especially the bodies  21   a ) from the second permanent magnet  22 . 
     Furthermore, since each ring-shaped portion  21   b  has at least one projection  21   c  to be engaged with the second permanent magnet  22 , the first permanent magnet  21  can be firmly fixed to the second permanent magnet  22 . Accordingly, detachment of the first permanent magnet  21  (especially the bodies  21   a ) from the second permanent magnet  22  can be effectively prevented. 
     Moreover, since each ring-shaped portion  21   b  has at least one recess  21   d  to be engaged with the second permanent magnet  22 , the first permanent magnet  21  can be firmly fixed to the second permanent magnet  22 . Accordingly, detachment of the first permanent magnet  21  (especially the bodies  21   a ) from the second permanent magnet  22  can be effectively prevented. 
     The rotor  2   b  according to the second variation has the same advantages as the rotor  2  according to the first embodiment. 
     Third Variation 
       FIG. 17  is a plan view schematically illustrating a structure of a rotor  2   c  according to a third variation. 
       FIG. 18  is a side view schematically illustrating the structure of the rotor  2   c  according to the third variation. 
       FIG. 19  is a cross-sectional view schematically illustrating the structure of the rotor  2   c  according to the third variation. Specifically,  FIG. 19  is a cross-sectional view taken along line C 19 -C 19  in  FIG. 17 . 
     The rotor  2   c  according to the third variation further includes at least one resin  25 . For example, the resin  25  can be molded integrally with a rib for fixing a shaft in the rotor  2   c.    
     In the example illustrated in  FIG. 18 , the resin  25  is fixed to both ends of each first permanent magnet  21  in the axial direction of the rotor  2   c . That is, in the example illustrated in  FIG. 18 , the rotor  2   c  includes two resins  25 . It should be noted that the resins  25  fixed to both ends of each first permanent magnet  21  in the axial direction of the rotor  2   c  may be integrated as a single member. One resin  25  may be fixed to one end of each first permanent magnet  21  in the axial direction of the rotor  2   c . In the example illustrated in  FIG. 17 , each resin  25  is a ring-shaped resin in the xy plane. Each resin  25  covers end portions of the first permanent magnets  21  in the axial direction of the rotor  2   c  and the whole or part of end portions of the second permanent magnet  22  in the axial direction. 
     As illustrated in  FIG. 19 , each resin  25  may include at least one projection  25   a  or at least one recess  25   b . Each resin  25  may include both of the at least one projection  25   a  and the at least one recess  25   b . The projection  25   a  projects toward the second permanent magnet  22 . For example, the projection  25   a  is engaged with a recess formed on the first permanent magnet  21  or the second permanent magnet  22 . For example, the recess  25   b  is engaged with a projection formed on the first permanent magnet  21  or the second permanent magnet  22 . 
     In general, when the temperature of a rotor changes, magnets deform in some cases. In such cases, one of two types of magnets might be detached from the rotor because of a difference in thermal shrinkage. In the third variation, since the rotor  2   c  includes the resins  25 , when the temperature of the rotor  2   c  changes, even in the case where the first permanent magnet  21  or the second permanent magnet  22  deforms because of a difference in thermal shrinkage, it is possible to prevent detachment of the first permanent magnet  21  from the second permanent magnet  22 . In addition, a centrifugal force generated during rotation of the rotor  2   c  can prevent detachment of the first permanent magnet  21  from the second permanent magnet  22 . 
     Furthermore, since each resin  25  includes at least one projection  25   a  to be engaged with the first permanent magnet  21  or the second permanent magnet  22 , the resin  25  can be firmly fixed to the first permanent magnet  21  or the second permanent magnet  22  with the resin  25  covering the first permanent magnet  21 . Accordingly, detachment of the first permanent magnet  21  from the second permanent magnet  22  can be effectively prevented. 
     Furthermore, since each resin  25  includes at least one recess  25   b  to be engaged with the first permanent magnet  21  or the second permanent magnet  22 , the resin  25  can be firmly fixed to the first permanent magnet  21  or the second permanent magnet  22  with the resin  25  covering the first permanent magnet  21 . Accordingly, detachment of the first permanent magnet  21  from the second permanent magnet  22  can be effectively prevented. 
     Moreover, since the rotor  2   c  according to the third variation includes at least one resin  25 , the amount of the first permanent magnet  21  can be reduced, as compared to the rotor  2   b  according to the second variation. 
     The rotor  2   c  according to the third variation has the same advantages as the rotor  2  according to the first embodiment. 
     Fourth Variation 
       FIG. 20  is a cross-sectional view schematically illustrating a structure of a rotor  2   d  according to a fourth variation. Specifically,  FIG. 20  is a cross-sectional view taken along line C 20 -C 20  in  FIG. 21 . 
       FIG. 21  is a side view schematically illustrating the structure of the rotor  2   d  according to the fourth variation. 
     The rotor  2   d  according to the fourth variation includes at least one first permanent magnet  21 , one second permanent magnet  22 , at least one third permanent magnet  23 , and at least one fourth permanent magnet  24 . In the example illustrated in  FIG. 21 , the structure of each third permanent magnet  23  is the same as the structure of the first permanent magnet  21 , and magnetic properties of each third permanent magnet  23  are the same as magnetic properties of each first permanent magnet  21 . The structure of each fourth permanent magnet  24  is the same as the structure of the second permanent magnet  22 , and magnetic properties of each fourth permanent magnet  24  are the same as magnetic properties of each second permanent magnet  22 . 
     The third permanent magnet  23  may be a single structure or may be divided into a plurality of parts. The fourth permanent magnet  24  may be a single structure or may be divided into a plurality of parts. 
     As illustrated in  FIG. 21 , the third permanent magnet  23  and the fourth permanent magnet  24  are stacked on the first permanent magnet  21  and the second permanent magnet  22  in the axial direction of the rotor  2   d.    
     That is, each third permanent magnet  23  forms part of the outer peripheral surface of the rotor  2   d , and is magnetized to have polar anisotropy. Each third permanent magnet  23  is, for example, a bonded magnet as a mixture of a rare earth magnet and a resin, that is, a rare earth bonded magnet. Each third permanent magnet  23  has higher magnetic force than that of the fourth permanent magnet  24 . The rare earth magnet is, for example, a magnet containing neodymium (Nd)-iron (Fe)-boron (B) or a magnet containing samarium (Sm)-iron (Fe)-nitrogen (N). The resin is, for example, a nylon resin, a polyphenylene sulfide (PPS) resin, or an epoxy resin. 
     The fourth permanent magnet  24  is adjacent to the third permanent magnet  23  in the circumferential direction of the rotor  2   d , and forms part of the outer peripheral surface of the rotor  2   d . Specifically, part of the fourth permanent magnet  24  is adjacent to the third permanent magnet  23  in the circumferential direction of the rotor  2   d , and another part of the fourth permanent magnet  24  is located at the inner side of the third permanent magnet  23  in the radial direction of the rotor  2   d . Thus, the fourth permanent magnet  24  is a ring-shaped magnet. 
     The fourth permanent magnet  24  is magnetized to have polar anisotropy. The fourth permanent magnet  24  is a magnet that is of a different type from the third permanent magnet  23 . Specifically, the fourth permanent magnet  24  is, for example, a bonded magnet as a mixture of a ferrite magnet and a resin, that is, a ferrite bonded magnet. The resin is, for example, a nylon resin, a polyphenylene sulfide (PPS) resin, or an epoxy resin. The fourth permanent magnet  24  has lower magnetic force than that of each third permanent magnet. The fourth permanent magnet  24  has 3×2n magnetic poles, in a manner similar to the second permanent magnet  22 . 
     In the rotor  2   d  according to the fourth variation, the first permanent magnet  21  is a single structure. The first permanent magnet  21  includes a plurality of bodies  21   a , at least one ring-shaped portion  21   b  (also referred to as a first ring-shaped portion in the fourth variation). The plurality of bodies  21   a  correspond to the first permanent magnets  21  in the first embodiment (e.g., the first permanent magnets  21  illustrated in  FIG. 1 ). Thus, the bodies  21   a  form part of the outer peripheral surface of the rotor  2   d  and are magnetized to have polar anisotropy. Part of the second permanent magnet  22  is present between two bodies  21   a  adjacent to each other in the circumferential direction. 
     The ring-shaped portion  21   b  is integrated with the plurality of bodies  21   a  as a single member. Thus, in the fourth variation, the rotor  2   d  includes one first permanent magnet  21  and one second permanent magnet  22 . In the example illustrated in  FIG. 21 , the ring-shaped portion  21   b  is formed at an end portion of the first permanent magnet  21  in the axial direction. The ring-shaped portion  21   b  covers an end portion of the second permanent magnet  22  in the axial direction of the rotor  2   d.    
     In the rotor  2   d  according to the fourth variation, the third permanent magnet  23  is a single structure. The third permanent magnet  23  includes a plurality of bodies  23   a , at least one ring-shaped portion  23   b  (also referred to as a second ring-shaped portion in the fourth variation). The plurality of bodies  23   a  correspond to the first permanent magnets  21  in the first embodiment (e.g., the first permanent magnets  21  illustrated in  FIG. 1 ). Thus, the bodies  23   a  form part of the outer peripheral surface of the rotor  2   d  and are magnetized to have polar anisotropy. Part of the fourth permanent magnet  24  is present between two bodies  23   a  adjacent to each other in the circumferential direction. 
     The ring-shaped portion  23   b  is integrated with the plurality of bodies  23   a  as a single member. Thus, in the fourth variation, the rotor  2   d  includes one third permanent magnet  23  and one fourth permanent magnet  24 . In the example illustrated in  FIG. 21 , the ring-shaped portion  23   b  is formed at an end portion of the third permanent magnet  23  in the axial direction. The ring-shaped portion  23   b  covers an end portion of the fourth permanent magnet  24  in the axial direction of the rotor  2   d.    
     In the axial direction of the rotor  2   d , the ring-shaped portion  21   b  faces the ring-shaped portion  23   b . Accordingly, the proportion of the first permanent magnet  21  and the third permanent magnet  23  can be increased in a center portion of the rotor  2   d  in the axial direction. As a result, in an electric motor, the amount of magnetic flux flowing from the rotor  2   d  into a stator increases, and thus an output of the electric motor can be increased. 
     In the electric motor, the length of the rotor  2   d  in the axial direction is preferably larger than the length of the stator in the axial direction. Accordingly, leakage of magnetic flux from the rotor  2   d  can be reduced. Specifically, in the electric motor, the amount of magnetic flux flowing from the rotor  2   d  into the stator increases, and thus an output of the electric motor can be increased. 
     In the fourth variation, the rotor  2   d  includes two layers of magnets. In other words, the rotor  2   d  is divided into two layers. Specifically, the rotor  2   d  includes a first layer constituted by the first permanent magnet  21  and the second permanent magnet  22 , and a second layer constituted by the third permanent magnet  23  and the fourth permanent magnet  24 . Thus, since the rotor  2   d  includes the plurality of layers, an eddy-current loss in the rotor  2   d  can be reduced. 
     In the xy plane, the magnetic pole center position (e.g., the position of a north pole) of the first permanent magnet  21  preferably coincides with the magnetic pole center position (e.g., the position of a north pole) of the third permanent magnet  23 . Accordingly, a magnetic flux density at each magnetic pole center position of the rotor  2   d  can be increased, and thus, a larger amount of magnetic flux flows from the rotor  2   d  into the stator in the electric motor, and an output of the electric motor can be enhanced. Magnetic pole center positions of the first permanent magnet  21  and magnetic pole center positions of the third permanent magnet  23  are positions indicated by the broken line in  FIG. 21 . 
     The rotor  2   d  according to the fourth variation has the same advantages as the rotor  2  according to the first embodiment. 
     Fifth Variation 
       FIG. 22  is a cross-sectional view schematically illustrating the structure of a rotor  2   e  according to a fifth variation.  FIG. 22  is a cross-sectional view taken along line C 22 -C 22  in  FIG. 23 . 
       FIG. 23  is a side view schematically illustrating the structure of the rotor  2   e  according to the fifth variation. 
     The rotor  2   e  according to the fifth variation has 2n (n is a natural number) magnetic poles, as in the first embodiment and the variations thereof described above. In addition, the rotor  2   e  includes a plurality of layered magnets  20  from two to m (m is a natural number and a divisor of n) layers stacked in the axial direction. In the example illustrated in  FIG. 23 , n=4 and m=2. That is, in the example illustrated in  FIG. 23 , the rotor  2   e  includes two layers of layered magnets  20 . 
     Each layered magnet  20  of the plurality of layered magnets  20  includes at least one first permanent magnet  21  and one second permanent magnet  22 . 
     As illustrated in  FIG. 23 , the plurality of layered magnets  20  are stacked in the axial direction of the rotor  2   e . As described above, the rotor  2   e  includes two layers of magnets. In other words, the rotor  2   e  is divided into two layers. Thus, since the rotor  2   e  includes the plurality of layers, an eddy-current loss in the rotor  2   e  can be reduced. 
     In the axial direction of the rotor  2   e , a ring-shaped portion  21   b  of each first permanent magnet  21  faces a ring-shaped portion  21   b  of another first permanent magnet  21 . Accordingly, a proportion of the first permanent magnets  21  can be increased in a center portion of the rotor  2   e  in the axial direction. As a result, in an electric motor, a larger amount of magnetic flux flows from the rotor  2   e  into a stator, and thus an output of the electric motor can be thereby increased. 
     In each first permanent magnet  21  of the plurality of layered magnets  20 , supposing one cycle is an angle between adjacent north poles in the xy plane, positions of north poles of two first permanent magnets  21  adjacent to each other in the axial direction are shifted from each other by n/m cycles in the circumferential direction. Positions of south poles of two first permanent magnets  21  adjacent to each other in the axial direction are also shifted from each other by n/m cycles in the circumferential direction. Accordingly, even in a case where the layered magnets  20  have variations in orientation, uniform orientation can be obtained in the rotor  2   e . As a result, in a manner similar to the example indicated by “A” in  FIG. 12 , an abrupt change in the flux density can be suppressed in the circumferential direction in the entire rotor  2   e , and vibrations and noise in the electric motor can be reduced. 
     The rotor  2   e  according to the fifth variation has the same advantages as the rotor  2  according to the first embodiment. 
     Second Embodiment 
       FIG. 24  is a partial cross-sectional view schematically illustrating a structure of an electric motor  1  according to a second embodiment of the present invention. 
     The electric motor  1  includes the rotor  2  according to the first embodiment, and a stator  3 . Instead of the rotor  2 , the rotors  2   a  through  2   j  according to the variations of the first embodiment are applicable to the electric motor  1 . 
     The electric motor  1  includes the rotor  2 , the stator  3 , a circuit board  4 , a magnetic sensor  5  for detecting a rotation position of the rotor  2 , a bracket  6 , bearings  7   a  and  7   b , a sensor magnet  8  as a magnet for detecting a rotation position of the rotor  2 , and a shaft  37  fixed to the rotor  2 . The electric motor  1  is, for example, a permanent magnet synchronous 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 rotor  2  rotates about an axis Ax. 
     Since the electric motor  1  according to the second embodiment includes the rotor  2  according to the first embodiment (including the variations thereof), the same advantages as those of the rotor  2  described in the first embodiment (including advantages of the variations thereof) can be obtained. 
     The electric motor  1  according to the second embodiment includes the rotor  2  according to the first embodiment, and thus, efficiency of the electric motor  1  can be increased. 
     Third Embodiment 
       FIG. 25  is a diagram schematically illustrating a structure of a fan  60  according to a third embodiment of the present invention. 
     The fan  60  includes a blade  61  and an electric motor  62 . The fan  60  is also referred to as a blower. The electric motor  62  is the electric motor  1  according to the second embodiment. The blade  61  is fixed to a shaft of the electric motor  62 . The electric motor  62  drives the blade  61 . When the electric motor  62  is driven, the blade  61  rotates to generate an airflow. In this manner, the fan  60  is allowed to send air. 
     In the fan  60  according to the third embodiment, the electric motor  1  described in the second embodiment is applied to the electric motor  62 , and thus, the same advantages as those described in the second embodiment can be obtained. In addition, efficiency of the fan  60  can be enhanced. 
     Fourth Embodiment 
     An air conditioner  50  (also referred to as a refrigeration air conditioning apparatus or a refrigeration cycle apparatus) according to a fourth embodiment of the present invention will be described. 
       FIG. 26  is a diagram schematically illustrating a configuration of the air conditioner  50  according to the fourth embodiment. 
     The air conditioner  50  according to the fourth embodiment includes an indoor unit  51  as a blower (first blower), a refrigerant pipe  52 , and an outdoor unit  53  as a blower (second blower) connected to the indoor unit  51  through the refrigerant pipe  52 . 
     The indoor unit  51  includes an electric motor  51   a  (e.g., the electric motor  1  according to the second embodiment), an air blowing unit  51   b  that supplies air when being driven by the electric motor  51   a , and a housing  51   c  covering the electric motor  51   a  and the air blowing unit  51   b . The air blowing unit  51   b  includes, for example, a blade  51   d  that is driven by the electric motor  51   a . For example, the blade  51   d  is fixed to a shaft of the electric motor  51   a , and generates an airflow. 
     The outdoor unit  53  includes an electric motor  53   a  (e.g., the electric motor  1  according to the second embodiment), an air blowing unit  53   b , a compressor  54 , and a heat exchanger (not shown). When the air blowing unit  53   b  is driven by the electric motor  53   a , the air blowing unit  53   b  supplies air. The air blowing unit  53   b  includes, for example, a blade  53   d  that is driven by the electric motor  53   a . For example, the blade  53   d  is fixed to a shaft of the electric motor  53   a , and generates an airflow. The compressor  54  includes an electric motor  54   a  (e.g., the electric motor  1  according to the second embodiment), a compression mechanism  54   b  (e.g., a refrigerant circuit) that is driven by the electric motor  54   a , and a housing  54   c  covering the electric motor  54   a  and the compression mechanism  54   b.    
     In the air conditioner  50 , at least one of the indoor unit  51  or the outdoor unit  53  includes the electric motor  1  described in the second embodiment. Specifically, as a driving source of an air blowing unit, the electric motor  1  described in the second embodiment is applied to at least one of the electric motors  51   a  or  53   a . That is, the indoor unit  51  or the outdoor unit  53  may include the electric motor  1  described in the second embodiment, and each of the indoor unit  51  and the outdoor unit  53  may include the electric motor  1  described in the second embodiment. In addition, the electric motor  1  described in the second embodiment may be applied to the electric motor  54   a  of the compressor  54 . 
     The air conditioner  50  is capable of performing a cooling operation of sending cold air from the indoor unit  51  or a heating operation of sending hot air, for example. In the indoor unit  51 , the electric motor  51   a  is a driving source for driving the air blowing unit  51   b . The air supply unit  51   b  is capable of sending conditioned air. 
     In the air conditioner  50  according to the fourth embodiment, the electric motor  1  described in the second embodiment is applied to at least one of the electric motors  51   a  or  53   a , and thus, the same advantages as those described in the second embodiment can be obtained. In addition, efficiency of the air conditioner  50  can be enhanced. 
     Furthermore, with the use of the electric motor  1  according to the second embodiment as a driving source of a blower (e.g., the indoor unit  51 ), the same advantages as those described in the second embodiment can be obtained. Accordingly, efficiency of the blower can be enhanced. The blower including the electric motor  1  according to the second embodiment and the blade (e.g., the blade  51   d  or  53   d ) driven by the electric motor  1  can be used alone as a device for supplying air. This blower is also applicable to equipment except for the air conditioner  50 . 
     In addition, the electric motor  1  according to the second embodiment is used for a driving source of the compressor  54 , thereby obtaining the same advantages as those described in the second embodiment. Moreover, efficiency of the compressor  54  can be enhanced. 
     The electric motor  1  described in the second embodiment can be mounted on equipment including a driving source, such as a ventilator, a household electrical appliance, or a machine tool, as well as the air conditioner  50 . 
     Features of the embodiments and features of the variations described above can be combined as appropriate. 
     DESCRIPTION OF REFERENCE CHARACTERS 
       1  electric motor,  2  rotor,  3  stator,  21  first permanent magnet,  22  second permanent magnet,  23  third permanent magnet,  24  fourth permanent magnet,  25  resin,  50  air conditioner,  51  indoor unit,  51   d ,  61  blade,  53  outdoor unit,  60  fan (blower).