Abstract:
Embodiments describe a motor. The motor includes a stator, and a rotor, which is arranged within the stator. An end part of at least one air-gap slot of the rotor has an offset with a predetermined distance and/or a predetermined angle relative to a main body part adjacent immediately to the end part. With the offset of a predetermined distance and/or a predetermined angle configured at the end part of at least one air-gap slot of the rotor, ripple torque of the motor is effectively lower down while complexity of the motor, stator or rotor will not be increased.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    Applicant hereby claims foreign priority benefits under U.S.C. §119 from Chinese Patent Application No. 201511022730.2 filed on Dec. 29, 2015, the content of which is incorporated by reference herein. 
       TECHNICAL FIELD 
       [0002]    The present invention relates generally to a motor, and more particularly, to a permanent magnet assist synchronous reluctance motor. 
       BACKGROUND 
       [0003]    An induction motor or a reluctance motor is generally used to replace a rare earth permanent magnet motor, in order to reduce costs of a variable speed motor. The induction motor will lower down motor efficiency although it can reduce the costs of the variable speed motor. The reluctance motor requires larger current, which will increase costs of a frequency converter, and thereby the total costs of the variable speed motor and the frequency converter will be increased. 
         [0004]    In addition, as shown in  FIGS. 1 and 2 , there are several air-gap slots  21  arranged on the rotor  20  of the existing reluctance motor. Each air-gap slot  21  has an end part  21   a , and a main body part  21   b  which is immediately adjacent to the end part  21   a . As shown in  FIGS. 1 and 2 , the end part  21   a  of the air-gap slot  21  and the main body part  21   b  corresponding to the end part  21   a  are aligned with each other, which will greatly limit optimization of motor design and will make it impossible to further reduce torque ripple of the reluctance motor. 
         [0005]    Non-uniform air gaps are generally used in order to reduce the torque ripple of a motor. Even though the non-uniform air gap can reduce the torque ripple of the motor, it will increase the complexity of a stator or rotor of the motor, and it is hard to measure the air gap of the motor. 
       SUMMARY 
       [0006]    In view of the above, in an aspect, a motor is provided, whose ripple torque is effectively reduced while whose rotor&#39;s or stator&#39;s complexity is not increased. 
         [0007]    In another aspect, a motor is provided, which can enable effective decrease in costs of a variable-speed motor and can have a higher efficiency. 
         [0008]    In an aspect, a motor includes a stator, and a rotor, which is arranged within the stator; an end part of at least one air-gap slot of the rotor has an offset with a predetermined distance and/or a predetermined angle relative to a main body part adjacent immediately to the end part. 
         [0009]    In an exemplary embodiment, the rotor includes multiple groups of air-gap slots, the multiple groups being separately distributed around a center of the rotor; each group of air-gap slots includes multiple air-gap slots which are arranged separately along a radial direction of the rotor. 
         [0010]    In an exemplary embodiment, at least one end part of at least one air-gap slot in said each group of air-gap slots has the offset with the predetermined distance and/or the predetermined angle relative to a main body part of the at least one air-gap slot. 
         [0011]    In an exemplary embodiment, end parts of any air-gap slot in said each group of air-gap slots, except for an air-gap slot located at the outmost in the radial direction of the rotor, have an offset with a predetermined distance and/or a predetermined angle relative to a corresponding main body part of said any air-gap slot. 
         [0012]    In an exemplary embodiment, the at least one end part of the at least one air-gap slot in said each group of air-gap slots has the offset in a direction towards or away from an adjacent group of air-gap slots. 
         [0013]    In an exemplary embodiment, the at least one air-gap slot is approximately U-shaped, V-shaped or circular arc-shaped; or, 
         [0014]    each air-gap slot in each group of air-gap slots has a same shape or different shapes; or, 
         [0015]    the rotor includes four, six or eight groups of air-gap slots; or, 
         [0016]    said each group of air-gap slots includes two or three air-gap slots, the two or three air-gap slots being separately arranged along the radial direction of the rotor. 
         [0017]    In an exemplary embodiment, a first end part of a first air-gap slot in said each group of air-gap slots has an offset distance or angle equal to or different from an offset distance or angle that a second end part of a second air-gap slot in said each group of air-gap slots; or, 
         [0018]    the first end part of the first air-gap slot in said each group of air-gap slots has an offset direction the same as or different from an offset direction that the second end part of the second air-gap slot in said each group of air-gap slots; or, 
         [0019]    two end parts of a same air-gap slot have a same offset distance or angle, or different offset distances or angles; or, 
         [0020]    the two end parts of the same air-gap slot have a same offset direction or different offset directions; or, 
         [0021]    two adjacent groups of air-gap slots are symmetrical or asymmetrical to each other; or, 
         [0022]    an end part of an air-gap slot in one of the two adjacent groups has an offset distance or angle the same as or different from an offset distance or angle that a corresponding end part of a corresponding air-gap slot in the other of the two adjacent groups has; or, 
         [0023]    the end part of the air-gap slot in one of the two adjacent groups has an offset direction the same as or different from an offset direction that the corresponding end part of the corresponding air-gap slot in the other of the two adjacent groups has. 
         [0024]    In an exemplary embodiment, the end part of the at least one air-gap slot is connected with the main body part adjacent immediately to the end part or is spaced apart by a predetermined distance from the main body part adjacent immediately to the end part; where the predetermined distance is more than or equal to 0.5 mm and less than or equal to 0.8 mm. 
         [0025]    In an exemplary embodiment, the main body part adjacent immediately to the end part of the at least one air-gap slot extends along an arc-shaped line, and the end part has the offset with the predetermined distance and/or predetermined angle relative to a tangent line of an edge part of the arc-shaped line of the main body part adjacent immediately to the end part. 
         [0026]    In an exemplary embodiment, a distance (W) between respective vertexes of two end parts of respective innermost air-gap slots in two adjacent groups of air-gap slots, a radius (R) of the rotor and the number (2p) of said groups of air-gap slots meet a relation: 
         [0000]      0.065≦ W /(2π R/ 2 p )≦0.09.
 
         [0027]    In an exemplary embodiment, the maximum electrical degree θ of an included angle between a first line and a second line meets a relation of 124°≦θ≦140°, where the first line is between a center point of an end part of a magnetic flux path of the rotor and an axial center point of the rotor, and the second line is between a center point of the other end part of the magnetic flux path of the rotor and the axial center point of the rotor. 
         [0028]    In an exemplary embodiment, magnetic filler is filled in at least one air-gap slot of the rotor. 
         [0029]    In an exemplary embodiment, one or more pieces of the magnetic filler is/are filled in a same air-gap slot. 
         [0030]    In an exemplary embodiment, the magnetic filler is a ferrite magnet containing a rare earth element and/or sintered neodymium-iron-boron permanent magnet. 
         [0031]    In an exemplary embodiment, the motor is a motor applicable to a variable speed compressor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  shows a schematic diagram of a rotor of a reluctance motor in the prior art, where no magnetic fillers are filled in air-gap slots of the rotor. 
           [0033]      FIG. 2  shows a schematic diagram of the rotor of the reluctance motor in the prior art, where magnetic fillers are filled in the air-gap slots of the rotor. 
           [0034]      FIG. 3  shows a schematic diagram of a rotor of a reluctance motor according to a first embodiment of the present invention, where no magnetic fillers are filled in air-gap slots of the rotor. 
           [0035]      FIG. 4  shows a schematic diagram of the rotor of the reluctance motor according to the first embodiment of the present invention, where magnetic filler is filled in the air-gap slots of the rotor. 
           [0036]      FIG. 5  shows a schematic diagram of the reluctance motor according to the first embodiment of the present invention. 
           [0037]      FIG. 6  shows a schematic diagram of a rotor of a reluctance motor according to a second embodiment of the present invention, where magnetic filler is filled in air-gap slots of the rotor. 
           [0038]      FIG. 7  shows the rotor of  FIG. 6 , indicating the maximum width of the middle portion of the air-gap slots in the rotor and the maximum width of side portions of the air-gap slots in the rotor. 
           [0039]      FIG. 8  shows a schematic diagram of a rotor of a reluctance motor according to a third embodiment of the present invention, where magnetic filler is filled in air-gap slots of the rotor. 
           [0040]      FIG. 9  shows a schematic diagram of a rotor of a reluctance motor according to a fourth embodiment of the present invention, where magnetic filler is filled in air-gap slots of the rotor. 
           [0041]      FIG. 10  shows a schematic diagram of a rotor of a reluctance motor according to a fifth embodiment of the present invention, where magnetic filler is filled in the air-gap slots of the rotor. 
           [0042]      FIG. 11  shows a schematic diagram of a rotor of a reluctance motor according to a sixth embodiment of the present invention, where magnetic filler is filled in air-gap slots of the rotor. 
           [0043]      FIG. 12  shows the rotor of  FIG. 11 , indicating a distance between end vertexes of two closest air-gap slots, where the two closest air-gap slots respectively belong to two adjacent groups of air-gap slots, and indicating the radius of the rotor. 
           [0044]      FIG. 13  shows the rotor of  FIG. 11 , indicating the maximum electrical degree of an included angle between a first line and a second line, where the first line is between a middle point of an end part of a magnetic flux path in the rotor and an axial point of the rotor and the second line is between a middle point of the other end part of the magnetic flux path in the rotor and the axial point of the rotor, and also indicating respective center points of the two end parts of the magnetic flux path. 
           [0045]      FIG. 14  shows relationship between motor torque ripple and parameters including the number of groups of air-gap slots, rotor radius and a distance between end vertexes of two closest air-gap slots, where the two closest air-gap slots respectively belong to two adjacent groups of air-gap slots. 
           [0046]      FIG. 15  shows a schematic diagram of a rotor of a reluctance motor according to a seventh embodiment of the present invention, where magnetic filler is filled in e air-gap slots of the rotor. 
           [0047]      FIG. 16  shows a schematic diagram of a rotor of a reluctance motor according to an eighth embodiment of the present invention, where magnetic filler is filled in air-gap slots of the rotor. 
           [0048]      FIG. 17  shows a schematic diagram of a rotor of a reluctance motor according to a ninth embodiment of the present invention, where magnetic filler is filled in air-gap slots of the rotor. 
           [0049]      FIG. 18  shows a schematic diagram of a rotor of a reluctance motor according to a tenth embodiment of the present invention, where magnetic filler is filled in air-gap slots of the rotor. 
           [0050]      FIG. 19  shows a schematic diagram of a rotor of a reluctance motor according to an eleventh embodiment of the present invention, where magnetic filler is filled in air-gap slots of the rotor. 
           [0051]      FIG. 20  shows a schematic diagram of a rotor of a reluctance motor according to a twelfth embodiment of the present invention, where magnetic filler is filled in air-gap slots of the rotor. 
       
    
    
     DETAILED DESCRIPTION 
       [0052]    The present invention will be further described below with reference to the accompanying drawings and specific embodiments. The same or similar reference signs in the description indicate the same or similar parts. The following description of the embodiments of the present invention with reference to the accompanying drawings is intended to explain the general inventive concept of the present invention, but should not be construed as a limitation of the present invention. 
         [0053]    In addition, specific details are described in the following detailed description in order to enable a comprehensive understanding of the embodiments. It is obvious, however, that one or more embodiments may be implemented without these specific details. In other instances, some conventional structures and devices are shown in the schematic diagrams to simplify the drawings. 
       Embodiment 1 
       [0054]      FIG. 3  shows a schematic diagram of a rotor  120  of a reluctance motor according to a first embodiment of the present invention, where no magnetic filler  122  is filled in air-gap slots  121  of the rotor  120 ;  FIG. 4  shows a schematic diagram of the rotor  120  of the reluctance motor according to the first embodiment of the present invention, where the magnetic filler  122  is filled in the air-gap slots  121  of the rotor  120 ;  FIG. 5  shows a schematic diagram of the reluctance motor according to the first embodiment of the present invention. 
         [0055]    In an exemplary embodiment of the present invention, the reluctance motor is provided. As shown in  FIG. 5 , the reluctance motor mainly includes a stator  110 , a rotor  120  and a coil  130 . The rotor  120  is configured within a containing chamber of the stator  110 , and the coil  130  is configured within a coil slot in a side wall of the containing chamber of the stator  110 . 
         [0056]    As shown in  FIG. 3 , in the illustrated embodiment, end parts at least one air-gap slot  121  of the rotor  120  has end parts  121   a  which have an offset of a predetermined distanced from a main body part  121   b  adjacent immediately to the end parts  121   a.    
         [0057]    In the embodiment shown in  FIGS. 3 and 4 , the rotor  120  includes four groups of air-gap slots which are distributed separately around the center of the rotor  120 , and each group of air-gap slots includes three air-gap slots which are separately arranged along a radial direction of the rotor. It should be noted, however, that the present invention should not be limited to the illustrated embodiment, the rotor may include four, six, eight or more groups of air-gap slots, and each group of air-gap slots may include two, four or more air-gap slots. 
         [0058]    In the embodiment shown in  FIGS. 3 and 4 , end parts except for the outmost air-gap slot in each group of air-gap slots, end parts  121   a  of any of the other air-gap slots  121  in each group of air-gap slots have an offset of predetermined distanced from a main body part  121   b  adjacent immediately to the end parts  21   a.    
         [0059]    As shown in  FIGS. 3 and 4 , in the embodiment, the outmost air-gap slot in each group of air-gap slots is V-shaped as a whole, and the other air-gap slots  121  in each group of air-gap slots, except for the outmost air-gap slot, are U-shaped as a whole. In the embodiment, the outmost air-gap slot in each group of air-gap slots are disconnected in the middle and spaced apart by materials of the rotor. Thus, it can ensure sufficient mechanical strength for the rotor. 
         [0060]    As shown in  FIGS. 3 and 4 , in the embodiment, the respective offset distances d at the respective end parts  121   a  of different air-gap slots  121  in each group of the air-gap slots are the same; the respective offset directions at the respective end parts of different air-gap slots in each group of the air-gap slots are the same, for example, a direction towards an adjacent group of air-gap slots. 
         [0061]    As shown in  FIGS. 3 and 4 , in the embodiment, two adjacent groups of air-gap slots are symmetrical to each other. 
         [0062]    As shown in  FIGS. 3 and 4 , in the embodiment, an end part of an air-gap slot  121  in a group of air-gap slots has the same offset distance as an offset distance that an end part of a corresponding air-gap  121  in an adjacent group of air-gap slots has. 
         [0063]    As shown in  FIGS. 3 and 4 , in the embodiment, the end part of the air-gap slot  121  in a group of air-gap slots has the same offset direction as an offset direction that the end part of the corresponding air-gap  121  in the adjacent group of air-gap slots has. 
         [0064]    As shown in  FIGS. 3 and 4 , in the embodiment, the end parts  121   a  of an air-gap slot  121  are connected with a main body part  121   b  adjacent immediately to the end parts  121   a.    
         [0065]    As shown in  FIGS. 3 and 4 , in the embodiment, the main body part  121   b  adjacent immediately to the end parts  121   a  of an air-gap slot  121  extends along a straight line. 
         [0066]    As shown in  FIGS. 3, 4 and 5 , magnetic filler  122  is filled in at least one air-gap slot  121  of the rotor  120 . In the embodiment, magnetic filler  122  is filled only in two inner air-gap slots  121  in each group of air-gap slots of the rotor  120 . 
         [0067]    As shown in  FIGS. 3, 4 and 5 , one or more magnetic filler  122  is filled in one air-gap slot  121 . In an embodiment of the present invention, the magnetic filler may be a ferrite magnet containing a rare earth element and/or a sintered neodymium-iron-boron permanent magnet. The sintered neodymium-iron-boron permanent magnet with little or no Dy could be used, for example, a Dy content may be 3% or less. 
         [0068]    In an embodiment of the present invention, the motor shown in  FIGS. 3-5  could be a motor applicable to a variable-speed compressor. 
       Embodiment 2 
       [0069]      FIG. 6  shows a schematic diagram of a rotor  220  of a reluctance motor according to a second embodiment of the present invention, where magnetic filler  222  is filled in air-gap slots  221  of the rotor  220 ;  FIG. 7  shows the rotor of  FIG. 6 , indicating the maximum width of the middle portion of the air-gap slots  221  in the rotor  220  and the maximum width of side portions of the air-gap slots  221  in the rotor  220 . 
         [0070]    As shown in  FIG. 6 , in the embodiment, end parts  221   a  of at least one air-gap slot  221  of the rotor  220  have an offset with a predetermined distance d and a predetermined angle α relative to a main body part  221   b  adjacent immediately to the end parts  221   a.    
         [0071]    In the embodiment shown in  FIGS. 6 and 7 , the rotor  220  includes four groups of air-gap slots which are separately distributed around the center of the rotor  220 , and each group of air-gap slots includes three air-gap slots which are separately arranged along a radial direction of the rotor. 
         [0072]    In the embodiment shown in  FIGS. 6 and 7 , end parts  221   a  of a middle air-gap slot  221  in each group of air-gap slots have an offset with a predetermined distance d and a predetermined angle α relative to a main body part  221   b  adjacent immediately to the end parts  221   a . In the embodiment shown in  FIGS. 6 and 7 , the end parts of an innermost air-gap slot in each group of air-gap slots have an offset only with a predetermined angle α, without a predetermined distance d, relative to a main body part adjacent immediately to the end parts. End parts of an outmost air-gap slot in each group of air-gap slots have no offset. 
         [0073]    As shown in  FIGS. 6 and 7 , in the embodiment, the outmost air-gap slot in each group of air-gap slots is in a circular arc shape as a whole, and the other air-gap slots  221 , except for the outmost air-gap slot, in each group of air-gap slots are U-shaped as a whole. 
         [0074]    In the embodiment, as shown in  FIGS. 6 and 7 , the outmost air-gap slot is disconnected in its middle and the innermost air-gap slot is also disconnected in its middle, either is spaced apart by materials of the rotor. Thus, it can ensure sufficient mechanical strength for the rotor. 
         [0075]    As shown in  FIGS. 6 and 7 , in the embodiment, respective end parts  221   a  of different air-gap slots  221  in each group of the air-gap slots have the same offset distance d and the offset angle α; the respective end parts of different air-gap slots in each group of the air-gap slots have the same offset directions, for example, towards an adjacent group of air-gap slots. After the offset, the ratio between Q1, the minimum distance between the end parts of different air-gap slots, and Q2, the distance between main body parts of different air-gap slots, should be larger than or equal to 0.95, i.e., Q1/Q2≧0.95, in order to ensure the saturation of a magnetic path at q axis as an offset. 
         [0076]    As shown in  FIGS. 6 and 7 , in the embodiment, the two adjacent groups of air-gap slots could be symmetrical to each other. 
         [0077]    As shown in  FIGS. 6 and 7 , in the embodiment, corresponding end parts of two corresponding air-gap slots  221  from two adjacent groups of air-gap slots may have the same offset distance d and the same offset angles α. 
         [0078]    As shown in  FIGS. 6 and 7 , in an embodiment, the corresponding end parts  221   a  of the two corresponding air-gap slots  221  from the two adjacent groups of air-gap slots have the same offset direction. 
         [0079]    As shown in  FIGS. 6 and 7 , in an embodiment, end parts  221   a  of an air-gap slot  221  are connected with a main body part  221   b  which is adjacent immediately to the end parts  221   a.    
         [0080]    As shown in  FIGS. 6 and 7 , in an embodiment, a main body part  221   b  which is adjacent immediately to end parts  221   a  of an air-gap slot extend along a straight line. 
         [0081]    As shown in  FIGS. 6 and 7 , magnetic filler  222  is filled in at least one air-gap slots  221  of the rotor  220 . In an embodiment, magnetic filler  222  are filled only in a middle air-gap slot  221  in each group of air-gap slots of the rotor  220 . 
         [0082]    As shown in  FIGS. 6 and 7 , one or more pieces of magnetic fillers  222  may be filled in one air-gap slots  221 , for example, two pieces of magnetic filler  222  are respectively filled in two sides of a U-shaped air-gap slot. In an embodiment of the present invention, the magnetic filler may be a ferrite magnet containing a rare earth element and/or sintered neodymium-iron-boron permanent magnet. The motor shown in  FIGS. 6 and 7  may be a motor applicable to a variable-speed compressor. 
         [0083]    As shown in  FIG. 7 , in an embodiment, the maximum width of the middle portion of a middle air-gap slot  221  in each group of air-gap slots of the rotor  220  is h1, and the maximum width of side portions is h2. In an embodiment of the present invention, the middle air-gap slot  221  should meet the following relation: 1.5≦h1/h2≦2.5. In the embodiment, magnetic filler  222  is filled only in two side portions of a middle air-gap slot  221  in each group of air-gap slots. 
       Embodiment 3 
       [0084]      FIG. 8  shows a schematic diagram of a rotor  320  of a reluctance motor according to a third embodiment of the present invention, where magnetic filler is filled in air-gap slots  321  of the rotor  320 . 
         [0085]    The third embodiment shown in  FIG. 8  differs from the second embodiment shown in  FIGS. 6 and 7  is the structure of an air-gap slot. 
         [0086]    In the third embodiment shown in  FIG. 8 , in an innermost air-gap slot  321  in each group of air-gap slots, a main body part  321   b  which is immediately adjacent to end parts  321   a  of the innermost air-gap slot  321  extends along an arc-shaped line, and the end parts  321   a  have an offset with a predetermined angle α relative to a tangent line of an edge part of the arc-shaped line of the main body part  321   b  corresponding to the end parts  321   a . Except for the above, the third embodiment shown in  FIG. 8  is basically the same as the second embodiment shown in  FIGS. 6 and 7 . 
       Embodiment 4 
       [0087]      FIG. 9  shows a schematic diagram of a rotor  420  of a reluctance motor according to a fourth embodiment of the present invention, where magnetic filler  422  is filled in air-gap slots  421  of the rotor  420 . 
         [0088]    The fourth embodiment shown in  FIG. 9  differs from the first embodiment shown in  FIGS. 3 and 4  in the number of air-gap slots in each group. 
         [0089]    In the fourth embodiment shown in  FIG. 9 , each group of air-gap slots includes two air-gap slots  421 , and magnetic fillers  422  is filled in every air-gap slot  421 . 
       Embodiment 5 
       [0090]      FIG. 10  shows a schematic diagram of a rotor  520  of a reluctance motor according to a fifth embodiment of the present invention, where magnetic filler is filled in air-gap slots  521  of the rotor  520 . 
         [0091]    The fifth embodiment shown in  FIG. 10  differs from the first embodiment shown in  FIGS. 3 and 4  in the structure of an air-gap slot. 
         [0092]    In the fifth embodiment shown in  FIG. 10 , an outmost air-gap slot and a middle air-gap slot in each group of air-gap slots are V-shaped as a whole, and an innermost air-gap slot in each group of air-gap slots are U-shaped as a whole. 
         [0093]    In the embodiment, as shown in  FIG. 10 , an air-gap slot may be disconnected in the middle, and spaced apart by the materials of the rotor. Thus, it can ensure sufficient mechanical strength for the rotor. 
       Embodiment 6 
       [0094]      FIG. 11  shows a schematic diagram of a rotor  620  of a reluctance motor according to a sixth embodiment of the present invention, where magnetic filler is filled in air-gap slots  621  of the rotor  620 ;  FIG. 12  shows the rotor  620  of  FIG. 11 , indicating the distance W between end vertexes of two closest air-gap slots, where the two closest air-gap slots respectively belong to two adjacent groups of air-gap slots, and indicating the radius R of the rotor  620 ;  FIG. 13  shows the rotor of  FIG. 11 , indicating the maximum electrical degree θ of an included angle between a first line and a second line, where the first line is between a middle point of an end part of a magnetic flux path in the rotor and an axial point of the rotor and the second line is between a middle point of the other end part of the magnetic flux path in the rotor and the axial point of the rotor, and also indicating respective center points A of the two end parts of the magnetic flux path. 
         [0095]    The sixth embodiment shown in  FIGS. 11-13  differs from the first embodiment shown in  FIGS. 3 and 4  in the structure of an air-gap slot. 
         [0096]    In the sixth embodiment shown in  FIGS. 11-13 , an external edge of an end part  621   a  of an middle air-gap slot  621  in each group of air-gap slots has an offset with a first offset distance d1 and a first offset angle α1 relative to an external edge of a main body part  621   b  adjacent immediately to the end part  621   a , and an inner edge of the end part  621   a  of the middle air-gap slot  621  in each group of air-gap slots has an offset with a second offset distance d2 and a second offset angle α2 relative to an inner edge of the main body part  621   b  adjacent immediately to the end part  621   a . An external edge of an end part  621   a  of an innermost air-gap slot  621  in each group of air-gap slots has an offset with a third offset distance d3 and a third offset angle α3 relative to an external edge of a main body part  621   b  adjacent immediately to the end part  621   a  of the innermost air-gap slot  621 , and an inner edge of the end part  621   a  of the innermost air-gap slot  621  in each group of air-gap slots has an offset with a fourth offset distance d4 and a fourth offset angle α4 relative to an inner edge of the main body part  621   b  adjacent immediately to the end part  621   a  of the innermost air-gap slot  621 . 
         [0097]    In an embodiment of the present invention, the first offset distance d1 may be equal or unequal to the third offset distance d3, and the second offset distance d2 may be equal or unequal to the fourth offset distance d4. The first offset angle α1 may be equal or unequal to the third offset angle α3, and the second offset angle α2 may be equal or unequal to the fourth offset angle α4. 
         [0098]    In an embodiment of the present invention, the first offset distance d1, the second offset distance d2, the third offset distance d3 and the fourth offset distance d4 may be equal or unequal to each other. The first offset angle α1, the second offset angle α2, the third offset angle α3 and the fourth offset angle α4 may also be equal or unequal to each other. 
         [0099]    In an embodiment of the present invention, the above offset distances d1, d2, d3 and d4 should respectively meet the following relations: 0&lt;d1≦0.5 mm, 0&lt;d2≦1.5 mm, 0&lt;d3≦1.5 mm, and 0&lt;d4≦1.5 mm. The above offset angles α1, α2, α3 and α4 should respectively meet the following relations: 0&lt;α1≦30°, 0&lt;α2≦30°, 0&lt;α3≦30°, 0&lt;α4≦30°. 
         [0100]    In an embodiment of the present invention, as shown in  FIGS. 12 and 13 , by properly designing the above offset distances d1, d2, d3, d4 and the above offset angles α1, α2, α3, α4, the distance W, the radius R and the number of air-gap slot groups 2p can meet the following relation: 0.065≦W/(2πR/2p)≦0.09; and/or the maximum electrical degree θ of the included angle can meet the following relation: 124°≦θ≦140°. 
         [0101]    In the embodiment, respective offset directions of the above offset distances d1, d2, d3, d4 are the same, and respective offset directions of the above offset angles α1, α2, α3, α4 are the same. 
       Embodiment 7 
       [0102]      FIG. 15  shows a schematic diagram of a rotor  720  of a reluctance motor according to a seventh embodiment of the present invention, where magnetic filler is filled in air-gap slots  721  of the rotor  720 . 
         [0103]    The seventh embodiment shown in  FIG. 15  differs from the first embodiment shown in  FIGS. 3 and 4  in the structure of an air-gap slot. 
         [0104]    In the seventh embodiment shown in  FIG. 15 , one of two end parts  721   a  of the innermost air-gap slots  721  has an offset with a predetermined distance and/or a predetermined angle relative to the main body part  721   b  adjacent immediately to the end part, and the other end part of the two end parts  721   a  has no offset; none of end parts of the other air-gap slots, except for the innermost air-gap slots  721 , has an offset. 
       Embodiment 8 
       [0105]      FIG. 16  shows a schematic diagram of a rotor  820  of a reluctance motor according to an eighth embodiment of the present invention, where magnetic filler is filled in air-gap slots  821  of the rotor  820 . 
         [0106]    The eighth embodiment shown in  FIG. 16  differs from the first embodiment shown in  FIGS. 3 and 4  in the structure of an air-gap slot. 
         [0107]    In the eighth embodiment shown in  FIG. 16 , one of two end parts  821   a  of a middle air-gap slot  821  has an offset distance and/or offset angle relative to a main body part  821   b  adjacent immediately to the end part  821   a , and the other of the two end parts  821   a  has a different offset distance and/or offset angle. That is, the two end parts  821   a  of the middle air-gap slot  821  have different offset distances or offset angles. 
       Embodiment 9 
       [0108]      FIG. 17  shows a schematic diagram of a rotor  920  of a reluctance motor according to a ninth embodiment of the present invention, where magnetic filler is filled in air-gap slots  921  of the rotor  920 . 
         [0109]    The ninth embodiment shown in  FIG. 17  differs from the first embodiment shown in  FIGS. 3 and 4  in the structure of an air-gap slot. 
         [0110]    In the ninth embodiment shown in  FIG. 17 , one of two end parts  921   a  of a middle air-gap slot  921  has an offset towards the external of the rotor  920 , and the other has an offset towards the internal of the rotor  920 ; that is, respective offset directions of the two end parts  921   a  of the middle air-gap slot  921  are different. 
       Embodiment 10 
       [0111]      FIG. 18  shows a schematic diagram of a rotor  1020  of a reluctance motor according to a tenth embodiment of the present invention, where magnetic filler is filled in air-gap slots  1021  of the rotor  1020 . 
         [0112]    The tenth embodiment shown in  FIG. 18  differs from the first embodiment shown in  FIGS. 3 and 4  in the structure of an air-gap slot. 
         [0113]    In the tenth embodiment shown in  FIG. 18 , two adjacent groups of air-gap slots are asymmetrical to each other. As shown in  FIG. 18 , respective end parts  1021   a  of two corresponding middle air-gap slots  1021  respectively from two adjacent groups of air-gap slots have different offset distances, different offset angles and different offset directions. 
       Embodiment 11 
       [0114]      FIG. 19  shows a schematic diagram of a rotor  1120  of a reluctance motor according to an eleventh embodiment of the present invention, where magnetic filler is filled in air-gap slots  1121  of the rotor  1120 . 
         [0115]    The eleventh embodiment shown in  FIG. 19  differs from the first embodiment shown in  FIGS. 3 and 4  in the structure of an air-gap slot. 
         [0116]    In the eleventh embodiment shown in  FIG. 19 , both end parts  1121   a  of a middle air-gap slot  1121  have an offset with a first offset distance d1 and a first offset angle α1. Both end parts  1121   a  of an innermost air-gap slot  1121  have an offset only with a second offset angle α2. 
         [0117]    In the embodiment, the first offset angle α1 may be equal or unequal to the second offset angle α2. 
       Embodiment 12 
       [0118]      FIG. 20  shows a schematic diagram of a rotor  1220  of a reluctance motor according to a twelfth embodiment of the present invention, where magnetic filler is filled in air-gap slots  1221  of the rotor  1220 . 
         [0119]    The twelfth embodiment shown in  FIG. 20  differs from the first embodiment shown in  FIGS. 3 and 4  in the structure of an air-gap slot. 
         [0120]    In the twelfth embodiment shown in  FIG. 20 , end parts  1221   a  of a middle air-gap slot  1221  are disconnected from their corresponding main body part  1221   b , and are spaced apart by a predetermined distance; similarly, end parts  1221   a  of an innermost air-gap slot  1221  are disconnected from their corresponding main body part  1221   b , and are spaced apart by a predetermined distance. The predetermined distance is more than or equal to 0.5 mm and less than or equal to 0.8 mm. Thus, it can ensure sufficient mechanical strength for the rotor. In addition, magnetic leakage can also be avoided. 
         [0121]    The foregoing only provides some embodiments of the present invention, and persons of ordinary skill in the art shall understand that changes may be made to these embodiments without departing from the principle of the general inventive concept; the scope of the present invention is defined by the claims and their equivalents. 
         [0122]    It should also to be noted that the word “comprising/comprise” does not exclude other elements or steps, and the word “a” or “an” does not exclude a plurality. In addition, any reference signs to the elements of the claims should not be construed as a limitation to the scope of the invention.