Abstract:
A motor includes: a rotor comprising: a rotary shaft; a magnetic body rotatable together with the rotary shaft; and first and second permanent magnets fixed on an outer circumference or an inner circumference of the magnetic body, and a stator comprising: an iron core arranged around the rotor; and a coil for exciting the iron core.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2011-013467, filed on Jan. 25, 2011, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     (i) Technical Field 
     The present invention relates to a motor. 
     (ii) Related Art 
     There is known a motor including: a rotor; a stator arranged around the rotor. The rotor includes: a rotary shaft; a magnetic body fixed to the rotary shaft; and plural permanent magnets fixed on an outer circumference of the magnetic body. The stator includes: an iron core; and a coil for exciting the iron core. Such a magnetic body may have projections for positioning the permanent magnets. Such a projection is sandwiched between the adjacent permanent magnets. Japanese Patent Application Publication No. 8-65929 and Japanese Utility Model Application Publication No. 59-99686 disclose techniques relevant to such a motor. 
     Magnetic field lines have emanated from one of the adjacent permanent magnets may partially extend to the other thereof through the projection. Such magnetic field lines may not contribute to a magnetic attractive force or a magnetic repulsive force generated between the permanent magnets and the iron core. This may reduce the torque of the rotor. 
     SUMMARY 
     It is therefore an object of the present invention to provide a motor in which a reduction in torque is suppressed. 
     A motor includes: a rotor comprising: a rotary shaft; a magnetic body rotatable together with the rotary shaft; and first and second permanent magnets fixed on an outer circumference or an inner circumference of the magnetic body, and a stator comprising: an iron core arranged around the rotor; and a coil for exciting the iron core, wherein the first and second permanent magnets are arranged such that different polarities of the first and second permanent magnets respectively face the stator, the magnetic body comprises first and second projections protruding to the first and second permanent magnets side and respectively abutting with a side surface of the first permanent magnet and a side surface of the second permanent magnet, a groove is formed between the first and second projections adjacent to each other, and a bottom of the groove overlaps a virtual extension line extending from an inner surface of the first permanent magnet abutting with the magnetic body or is located radially inward with respect to the virtual extension line, when viewed from an axial direction of the rotary shaft. 
     The bottom of the groove formed between the first and second projections is identical to the virtual extension line extending from the inner surface of the permanent magnet or is located radially inward with respect to the virtual extension line. Therefore, an air layer can be sufficiently ensured within the groove. This suppresses the magnetic field lines that have emanated from the first permanent magnet from extending to the second permanent magnet instead of the iron core. It is thus possible to suppress a reduction in torque. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a section view of a motor according to the present embodiment; 
         FIG. 2A  is a side view of a rotor, and  FIG. 2B  is a view of the rotor when viewed in its axial direction; 
         FIG. 3  is an enlarged view of a groove; 
         FIGS. 4A to 4C  are explanatory views of projections and grooves according to variations; 
         FIGS. 5A to 5C  are explanatory views of projections and grooves according to variations; and 
         FIG. 6  is a view of the rotor into which a cover is assembled. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a section view of a motor M according to the present embodiment. The motor M includes: a housing H; and a rotor and a stator housed in the housing H. The rotor includes: a rotary shaft  10  rotatably supported by bearings  62  and  64 ; a magnetic body  20  fixed to the rotary shaft  10 , and permanent magnets  30   c  and  30   f  fixed on an outer circumference of the magnetic body  20 . The permanent magnets  30   c  and  30   f  will be described later in detail. The magnetic body  20  is made of a magnetic material such as a magnetic steel. The stator includes: iron cores  40  arranged around the rotor; and coils  50  respectively wound around the iron cores  40 . The coil  50  is energized to excite the iron core  40  so as to have predetermined polarities. The rotor is rotated relative to the stator by the magnetic attraction force and the magnetic repulsive force generated between the iron cores  40  and the permanent magnets  30   c  and  30   f . This rotates the rotary shaft  10 . 
       FIG. 2A  is a side view of the rotor.  FIG. 2B  is a view of the rotor when viewed in its axial direction. Plural permanent magnet  30   a  to  30   f  are fixed on an outer circumferential surface of the magnetic body  20 . The magnetic body  20  has a substantially hexagonal shape when viewed in the axial direction. The magnetic body  20  is formed with a through hole into which the rotary shaft  10  is press-fitted. The adjacent permanent magnets are arranged such that different polarities thereof respectively face the radial outside of the rotary shaft  10 . In other words, the adjacent permanent magnets are arranged such that different polarities thereof respectively face the stator. The magnetic body  20  is provided with projections  23   a  to  23   f  which respectively abut with and position the side surfaces of the permanent magnets  30   a  to  30   f . The projections  23   a  to  23   f  protrude radially outward. In other words, the projections  23   a  to  23   f  respectively protrude to the permanent magnets  30   a  to  30   f  sides. The permanent magnet  30   a  is positioned to be sandwiched by two projections  23   a . The permanent magnets  30   a  to  30   f  are respectively fixed on plane surfaces of the outer circumference of the magnetic body  20 . After the permanent magnets  30   a  to  30   f  are positioned by the projections  23   a  to  23   f , the permanent magnets  30   a  to  30   f  are adhered to the magnetic body  20  with an adhesive. Additionally, the adjacent permanent magnets function as a pair having an N pole and an S pole, and the pair has tow or more poles. 
     The groove G is formed between the adjacent projections  23   a  and  23   b . Also, the groove G is formed between the other adjacent projections. These projections  23   a  to  23   f  are formed to extend in the axial direction of the magnetic body  20 , as illustrated in  FIG. 2A . Each length of the projections  23   a  to  23   f  and the grooves G in the axial direction are the same as each length of the permanent magnets and the magnetic body  20 . 
       FIG. 3  is an enlarged view of the groove G. The permanent magnet  30   a  includes: an outer surface  31   a  facing radially outward; an inner surface  32   a  facing radially inward and abutting with the magnetic body  20 ; and a side surface  33   a  abutting with the projection  23   a . Likewise, the permanent magnet  30   b  includes: an outer surface  31   b  facing radially outward; an inner surface  32   b  facing radially inward and abutting with the magnetic body  20 ; and a side surface  33   b  abutting with the projection  23   b . For example, the outer surface  31   a  of the permanent magnet  30   a  is magnetized to have the N pole, the inner surface  32   a  is magnetized to have the S pole, the outer surface  31   b  of the permanent magnet  30   b  is magnetized to have the S pole, and the inner surface  32   b  is magnetized to have the N pole. Thus, the outer surface  31   a  of the permanent magnet  30   a  and the outer surface  31   b  of the permanent magnet  30   b  are magnetized to have different polarities. 
     A length of the projection  23   a  in the radially outward direction is substantially the same as the thickness of the permanent magnet  30   a . The projection  23   a  abuts with the whole side surface  33   a . This applies to the projection  23   b . The width of the groove G in the circumferential direction of the magnetic body  20  is greater in the radially outward direction. A bottom B of the groove G is located radially inward with respect to a virtual extension line La of the inner surface  32   a  and a virtual extension line Lb of the inner surface  32   b , when the rotor is viewed in the axial direction. 
     In a case where the magnetic body is partially interposed between the side surfaces  33   a  and  33   b  without providing the groove G therebetween, the magnetic field lines that have emanated from the outer surface  31   a  of the permanent magnet  30   a  may partially extend to the outer surface  31   b  of the permanent magnet  30   b  through the magnetic body  20  instead of the iron core  40 . Such magnetic field lines may not contribute to the magnetic attractive force or the magnetic repulsive force generated between the permanent magnets  30   a  and  30   b  and the iron core  40 . 
     This reduces the torque. 
     In the present embodiment, the groove G is provided between the adjacent projections  23   a  and  23   b , in other words, between the side surfaces  33   a  and  33   b . The provision of the groove G between the side surfaces  33   a  and  33   b  forms an air layer therebetween. Herein, the magnetic permeability of the air is lower than that of the magnetic body  20 . This suppresses the magnetic field lines that have emanated from the outer surface  31   a  of the permanent magnet  30   a  from directly extending to the outer surface  31   b  of the permanent magnet  30   b  through the magnetic body  20 . This suppresses a reduction in torque of the rotor. Further, the projections  23   a  to  23   f  respectively position the permanent magnets  30   a  to  30   f , thereby suppressing a reduction in torque while ensuring the performance of assembling the permanent magnets  30   a  to  30   f  into the magnetic body  20 . 
     Furthermore, as mentioned above, the bottom B of the groove G is located radially inward with respect to the virtual extension line La from the inner surface  32   a  and the virtual extension line Lb from the inner surface  32   b , when the rotor is viewed in the axial direction. Therefore, the air layer is sufficiently provided between the permanent magnets  30   a  and  30   b . This suppresses a reduction in torque. 
     Also, the inner surface of the groove G spreads in the radially outward direction. Thus, the distance between the projections  23   a  and  23   b  is greater in the radially outward direction. 
     Next, variations will be described.  FIGS. 4A to 5C  are explanatory views of variations of the projections and grooves. Additionally, similar components will be designated with similar reference numerals and the duplication descriptions thereof will be omitted here. 
     As illustrated in  FIG. 4A , the length of projections  23   a   1  and  23   b   1  of the magnetic body  20  in the radial direction is smaller than each thickness of the permanent magnets  30   a  and  30   b . Thus, the projections  23   a   1  and  23   b   1  respectively abut with parts of bases of the side surfaces  33   a  and  33   b , and respectively expose the remaining parts of the side surfaces  33   a  and  33   b . Also, a bottom B 1  of the groove G 1  formed between the projections  23   a   1  and  23   b   1  is located radially inward with respect to the virtual extension lines La and Lb. Such a shape of the groove G 1  suppresses a reduction in torque. 
     As illustrated in  FIG. 4B , a width of the groove G 2  formed between projections  23   a   2  and  23   b   2  of a magnetic body  202  is constant in the radially outward direction. Also, a bottom B 2  of the groove G 2  is located radially inward with respect to the virtual extension lines La and Lb. The bottom B 2  has a plane shape. Such a shape of the groove G 2  suppresses a reduction in torque. 
     As illustrated in  FIG. 4C , each length of projections  23   a   3  and  23   b   3  of a magnetic body  203  in the radial direction is smaller than each thickness of the permanent magnets  30   a  and  30   b . Thus, the projections  23   a   3  and  23   b   3  respectively abut with parts of bases of the side surfaces  33   a  and  33   b , and respectively expose the remaining parts of the side surfaces  33   a  and  33   b . Also, a bottom B 3  of the groove G 3  is formed in such a position to overlap the virtual extension lines La and Lb. Such a shape of the groove G 3  suppresses a reduction in torque. 
     As illustrated in  FIG. 5A , a bottom B 4  of a groove G 4  formed between projections  23   a   4  and  23   b   4  of a magnetic body  204  has a curved shape. Also, the bottom B 4  is located radially inward with respect to virtual extension lines La and Lb. Such a shape of the groove G 4  suppresses a reduction in torque. 
     As illustrated in  FIG. 5B , surfaces of a magnetic body  205  where permanent magnets  30   a   1  and  30   b   1  are respectively fixed each have a curved shape. Also, the permanent magnets  30   a   1  and  30   b   1  each have a thickness substantially uniform. A bottom B 5  of a groove G 5  formed between projections  23   a   5  and  23   b   5  is formed in such a position to overlap the virtual extension lines La 1  and Lb 1 . Such a shape of the groove G 5  suppresses a reduction in torque. 
     As illustrated in  FIG. 5C , a bottom B 6  of a groove G 6  formed between projections  23   a   6  and  23   b   6  of a magnetic body  206  is located radially inward with respect to the virtual extension lines La 1  and Lb 1 . Such a shape of the groove G 6  suppresses a reduction in torque. 
       FIG. 6  is a view of the rotor into which a cover  80  is assembled. The cover  80  has a substantially cylindrical shape, and is formed of a non-magnetic body. The cover  80  is formed with fitting portions P which are respectively insertable into the grooves G. The plural fitting portions P are formed in the circumferential direction and protrude in the radially inward direction. Thus, the cover  80  is assembled into the rotor, thereby preventing the permanent magnets  30   a  to  30   f  from being disengaging from the magnetic body  20 . Also, the fitting portions of the cover  80  are respectively inserted into the grooves G of the magnetic body  20 , thereby preventing the cover  80  from rotating relative to the magnetic body  20  and preventing the cover  80  from rattling relative to the magnetic body  20 . Additionally, the cover  80  does not influence the magnetic forces of the permanent magnets  30   a  to  30   f  because the cover  80  is the non-magnetic body. 
     While the exemplary embodiments of the present invention have been illustrated in detail, the present invention is not limited to the above-mentioned embodiments, and other embodiments, variations and modifications may be made without departing from the scope of the present invention. 
     Additionally, the size, in the radial direction, of the groove G (the size of the air layer) may be extremely small. Herein, in consideration of press working performed in manufacturing the magnetic body  20  and workability in adhering the permanent magnet to the magnetic body  20 , it is preferable that the size, in the radial direction, of the groove G is 0.1 mm or more. Since an increase in the size, in the radial direction, of the groove G reduces the torque output, a reduction in the size of the permanent magnet will further reduce the torque. Thus, a reduction in thickness, in the circumferential direction, of the projection is conceivable without reducing the size of the permanent magnet  30 . However, in a case where the projection is too thin in the circumferential direction, the projection may be easily bent in positioning the permanent magnet and may not play a role as a positioning member. 
     The above embodiment has described the example where the six permanent magnets  30   a  to  30   f  are fixed to the single magnetic body  20 . However, the number of the permanent magnets is not limited to six. 
     The above embodiment has described the inner rotor type. However, the present invention may be an outer rotor type.