Patent Publication Number: US-2019173366-A1

Title: Stator core and motor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a stator core, and a motor including the stator core. 
     2. Description of the Related Art 
     Conventionally, a technique for reducing cogging torque has been proposed for a brushless motor. For example, in a brushless motor, a protrusion portion is provided at a center of a pole tooth portion of each salient pole of a stator core, and a pair of notch portions is provided on both sides of the protrusion portion. Accordingly, the cogging torque of the brushless motor is reduced. 
     However, in an engine cooling fan or the like, a motor having a large number of poles and slots is used for increasing an output torque of the motor. Even in such a multi-pole and multi-slot motor, it is required to reduce a cogging torque while suppressing reduction in the output torque of the motor. 
     SUMMARY OF THE INVENTION 
     An exemplified embodiment of the present disclosure is a stator core, which includes an annular core back; and a plurality of teeth extending radially from the core back to a rotor magnet including a plurality of magnets. A ratio of a number of slots, which is equal to a number of the plurality of teeth, to a number of poles, which is equal to a number of the plurality of magnets of the rotor magnet is  3 : 4 . A slot open ratio which is a ratio of a slot open angle between two adjacent teeth in a peripheral direction to an angle between center lines of the two teeth is 0.5 or more. When, in the rotor magnet, a ratio of a peripheral angle of one magnet to an angle between center positions of two adjacent magnets in the peripheral direction is set as a magnet ratio, a ratio of the slot open ratio to the magnet ratio is 0.6 to 0.7. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a motor according to an exemplified embodiment of the present disclosure. 
         FIG. 2  is a perspective view of the motor. 
         FIG. 3  is a longitudinal cross-sectional view of the motor. 
         FIG. 4  is a plan view of a stator core. 
         FIG. 5  is a longitudinal cross-sectional view of a shaft and a rotor. 
         FIG. 6  is a diagram illustrating an arrangement of a plurality of magnets. 
         FIG. 7  is a longitudinal cross-sectional view of an axial flow fan. 
         FIG. 8  is a diagram illustrating a slot open ratio and a magnet width ratio at which a cogging torque is minimized. 
         FIG. 9  is a diagram illustrating a slot open ratio and a magnet width ratio at which a cogging torque is minimized. 
         FIG. 10  is a diagram illustrating a slot open ratio and a magnet width ratio at which a cogging torque is minimized. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a perspective view showing an appearance of a motor  1  according to one exemplary embodiment of the present disclosure. The motor  1  is an external rotor brushless motor. The motor  1  is used to, for example, rotate an impeller in an axial flow fan.  FIG. 2  is a perspective view of the motor  1  as viewed from a direction different from that of  FIG. 1 .  FIG. 3  is a longitudinal cross-sectional view of the motor  1 . Parallel slanting lines in the detailed cross-section are omitted in  FIG. 3 . Moreover, a configuration on a far side from the cross-section and a partial outer surface of the motor are also drawn in  FIG. 3 . 
     In this specification, an upper side and a lower side in a direction of a center axis J 1  of the motor  1  in  FIG. 3  are simply referred to as an “upper side” and a “lower side”, respectively. The upper and lower sides in this specification do not indicate upper and lower sides in a gravity direction when installed into the actual equipment. Hereinafter, a peripheral direction around the center axis J 1  is simply referred to as a “peripheral direction”, and a radial direction around the center axis J 1  is simply referred to as a “radial direction.” Moreover, a direction parallel to the center axis J 1  is referred to as a “vertical direction” or an “axial direction.” 
     The motor  1  includes a stationary portion  2 , a rotating portion  3 , and a bearing mechanism  4 . The bearing mechanism  4  rotatably supports the rotating portion  3  with respect to the stationary portion  2 . The stationary portion  2  includes a bracket  21 , an armature  22 , and a bus bar unit  26 . The rotating portion  3  includes a shaft  31  and a rotor  32 . The bearing mechanism  4  includes a lower ball bearing  41  and an upper ball bearing  42 . 
     The bracket  21  includes the cylindrical portion  212 . The cylindrical portion  212  is a substantially cylindrical portion centered on the center axis J 1  oriented in the vertical direction. The bearing mechanism  4  is fixed to an inner peripheral surface of the cylindrical portion  212  of the bracket  21 . In particular, the lower ball bearing  41  of the bearing mechanism  4  is fixed to an inner peripheral surface of a lower portion of the cylindrical portion  212 . Moreover, the upper ball bearing  42  is fixed to an inner peripheral surface of an upper portion of the cylindrical portion  212 . 
     The armature  22  is fixed to an outer peripheral surface of the cylindrical portion  212  of the bracket  21 . The armature  22  is disposed radially outward of the bearing mechanism  4 . The armature  22  is electrically connected to the bus bar unit  26 . The armature  22  is electrically connected to an external power supply (not shown) via the bus bar unit  26 . 
     The armature  22  includes a stator core  220 , an insulator  223 , and a plurality of coils  224 . The stator core  220  includes a core back portion  221  and a plurality of teeth  222 . The insulator  223  is an insulator covering surfaces of the plurality of teeth  222 . The plurality of coils  224  are formed by winding a conductive wire from above the insulator  223  to the plurality of teeth  222 . In this embodiment, the plurality of coil  224  are three-phase coils. 
       FIG. 4  is a plan view of the stator core  220 . The core back portion  221  is an annular portion centered on the center axis J 1 . The core back portion  221  is fixed to the outer peripheral surface of the cylindrical portion  212  of the bracket  21 . The plurality of teeth  222  eradiate radially outward from the core back portion  221 . The plurality of teeth  222  are arranged at substantially equal angular intervals in the peripheral direction. Each tooth  222  includes a tooth body portion  226  and a tooth tip portion  227 . The tooth body portion  226  extends substantially linearly and radially outward from the core back portion  221 . The tooth tip portion  227  extends to both peripheral sides from a radial outer end portion of the tooth body portion  226 . The core back portion  221  and the plurality of teeth  222  are, for example, a single member made of metal. 
     In the example illustrated in  FIG. 4 , the number of slots, which is the number of the plurality of teeth  222 , is 18. An angle θ1 in the peripheral direction between center lines of two adjacent teeth  222  in the peripheral direction is 20 degrees. The center line of the tooth  222  is a straight line passing through the center axis J 1  and a center in the peripheral direction of the tooth  222  in a plan view. Hereinafter, the angle θ1 is referred to as an “inter-teeth angle θ1.” Furthermore, the angle θ2 in the peripheral direction of a slot open between the two adjacent teeth  222  in the peripheral direction is simply referred to as a “slot open angle θ2.” The slot open is a space between tip portions of the two adjacent teeth  222  in the peripheral direction. In the example illustrated in  FIG. 4 , the slot open angle θ2 is an angle between a straight line passing through the center axis J 1  and contacting a peripheral end portion of the tooth tip portion  227  of one tooth  222 , and a straight line passing through the center axis J 1  and contacting a peripheral end portion of the tooth tip portion  227  of the other tooth  222 , in a plan view. 
     Hereinafter, a ratio of the slot open angle θ2 to the inter-teeth angle θ1 is referred to as a “slot open ratio.” In the stator core  220  of the motor  1 , the slot open ratio is 0.5 or more. 
     The shaft  31  exemplified in  FIG. 3  is a substantially columnar or cylindrical member, which is centered on the center axis J 1 . The shaft  31  is made of, for example, metal. The shaft  31  is formed by, for example, an aluminum alloy. As shown in  FIG. 3 , the shaft  31  is rotatably supported together with the rotor  32  by the bearing mechanism  4 . In particular, the lower ball bearing  41  of the bearing mechanism  4  supports a lower portion of the shaft  31 . The upper ball bearing  42  is disposed higher than the lower ball bearing  41  and supports the shaft  31 . The impeller of the axial flow fan, for example, is attached to an upper end portion of the shaft  31 . 
       FIG. 5  shows longitudinal cross-sectional views of the shaft  31  and the rotor  32 . The rotor  32  is connected to the shaft  31 . The rotor  32  is a substantially cylindrical covered member centered on the center axis J 1 . The rotor  32  opens downward. The rotor  32  includes a rotor lid portion  321 , a rotor side wall portion  322 , a rotor magnet  341 , and a rotor yoke  342 . 
     The rotor lid portion  321  is connected to the shaft  31 . The rotor lid portion  321  is a substantially disk-like portion centered on the center axis J 1 . The rotor side wall portion  322  extends downward from an outer edge portion of the rotor lid portion  321 . The rotor side wall portion  322  is a substantially cylindrical portion centered on the center axis J 1 . In the example illustrated in  FIG. 5 , the rotor lid portion  321  and the rotor side wall portion  322  are a single member made of resin. Moreover, the rotor lid portion  321  and the rotor side wall portion  322  are integrally formed with the shaft  31  by the insert molding. 
     The rotor magnet  341  is fixed to an inner peripheral surface of the rotor side wall portion  322 . The rotor magnet  341  includes a plurality of magnets  343  arranged in the peripheral direction. The rotor magnet  341  is radially opposed to the armature  22  radially outward of the armature  22 . In the example illustrated in  FIG. 5 , the rotor yoke  342  is disposed between a magnet  343  of the rotor magnet  341  and the rotor side wall portion  322 . In other words, the rotor magnet  341  is indirectly fixed to the inner peripheral surface of the rotor side wall portion  322  via the rotor yoke  342 . The rotor yoke  342  is made of metal. Alternatively, the rotor yoke  342  may be excluded and the rotor magnet  341  may be directly fixed to the inner peripheral surface of the rotor side wall portion  322 , in the motor  1 . 
       FIG. 6  is a diagram illustrating an arrangement of a plurality of magnets  343  of the rotor magnet  341 . The rotor magnet  341  includes the plurality of magnets  343 . The plurality of magnets  343  are disposed at substantially equal angular intervals in the peripheral direction around the center axis J 1 . In the example illustrated in  FIG. 6 , the number of poles, which is the number of the plurality of magnets  343 , is  24 . Therefore, a ratio of the number of slots to the number of poles is 3:4. In the rotor magnet  341 , an angle θ3 in the peripheral direction between center positions of two adjacent magnets  343  in the peripheral direction is 15 degrees. The center position of the magnet  343  is a position of a center of the magnet  343  in the peripheral direction in a plan view. Hereinafter, the angle θ3 is referred to as an “inter-magnet angle θ3.” Moreover, an angle θ4 of one magnet  343  in the peripheral direction is referred to as a “magnet angle θ4.” The magnet angle θ4 is an angle between a straight line passing through the center axis J 1  and contacting one peripheral end portion of the magnet  343 , and a straight line passing through the center axis J 1  and contacting the other peripheral end portion of the magnet  343 , in a plan view. 
     Hereinafter, a ratio of the inter-magnet angle θ3 and the magnet angle θ4 is referred to as a “magnet width ratio.” In the magnet  341  of the motor  1 , a ratio of the slot open ratio to the magnet width ratio is 0.6 to 0.7. In the motor  1 , it is preferable the magnet width ratio is 0.75 to 0.90. 
     In the motor  1 , a current is supplied to the coil  224  via the bus bar unit  26 , whereby a torque is generated between the coil  224  and the rotor magnet  341 . Therefore, the rotating portion (that is, the shaft  31  and the rotor  32 ) rotates around the center axis J 1 . The rotating portion  3  is rotatable in both a clockwise direction and a counterclockwise direction in a plan view. 
       FIG. 7  is a cross-sectional view illustrating the axial flow fan  10  using the motor  1  described above. The axial flow fan  10  includes the motor  1  and an impeller  11 . The impeller  11  is attached to an upper end portion of the shaft  31  above the rotor  32 . The impeller  11 , for example, sends wind downward from an upper side of the rotor  32 . 
       FIG. 8  is a diagram illustrating, when the slot open ratio of the stator core  220  and the magnet width ratio of the rotor magnet  341  are variously tailored, the magnet width ratio in which a cogging torque is minimized for the respective slot open ratio, which is obtained by the simulation with the motor  1  having  18  slots and  24  poles. Additionally,  FIG. 8  also illustrates a ratio of the slot open ratio to the magnet width ratio in which the cogging torque is minimized. 
     Examples  1  to  5  illustrated in  FIG. 8  show results of the simulations in a case where the slot open ratio is 0.6, 0.575, 0.55, 0.525 and 0.5, respectively. Comparative Examples 1 and 2 show results of the simulations in a case where the slot open ratio is 0.45 and 0.4, respectively. 
     In Comparative Examples 1 and 2, a size of a winding wound around the teeth  222  is limited since the slot open ratio is relatively small. Meanwhile, in Examples 1 to 5, the slot open is relatively large since the slot open ratio is 0.5 or more, thus the winding can be quite largely around the teeth  222 . Consequently, it is possible to increase an output torque of the motor  1 . Moreover, the ratio of the slot open ratio to the magnet width ratio is 0.6 to 0.7 as shown in  FIG. 8 , thus another advantageous effect of decreasing the cogging torque can also be obtained. In Examples 1 to 5, the magnet width ratio is 0.75 to 0.90. 
     The motor  1  may have different numbers of slots and poles as long as the ratio of the number of slot to the number of poles is 3:4.  FIG. 9  is a diagram illustrating, when the slot open ratio of the stator core  220  and the magnet width ratio of the rotor magnet  341  are variously tailored, the magnet width ratio in which a cogging torque is minimized for the respective slot open ratio, which is obtained by the simulation with the motor  1  having  12  or  24  slots. Additionally,  FIG. 9  also illustrates a ratio of the slot open ratio to the magnet width ratio in which the cogging torque is minimized. 
     Examples 6 to 9 illustrated in  FIG. 9  show results of the simulations in a case where the number of slots is  12  while the slot open ratio is 0.6, 0.567, 0.533, and 0.5, respectively. Examples 10 to 13 show results of the simulations in a case where the number of slots is  24  while the slot open ratio is 0.6, 0.567, 0.533, and 0.5, respectively. 
     In Examples 6 to 13, the slot open ratio is 0.5 or more, and the ratio of the slot open ratio to the magnet width ratio is 0.6 to 0.7, as in Examples 1 to 5. Therefore, the output torque of the motor  1  can be increased while decreasing the cogging torque. Moreover, in Examples 6 to 13, the magnet width ratio is 0.75 to 0.90. 
     As described above, the stator core  220  includes the annular core back portion  221  and the plurality of teeth  222 . The plurality of teeth  222  extend radially from the core back portion  221  to the rotor magnet  341  having the plurality of magnets  343 . The ratio of the number of slots to the number of poles is 3:4. The number of slots is the number of the plurality of teeth  222 , and the number of poles is the number of magnets  343  of the rotor magnet  341 . In the stator core  220 , the slot open ratio is a ratio of the slot open angle θ2 between the two adjacent teeth  222  in the peripheral direction to the angle θ1 between center lines of the two teeth  222 , which is 0.5 or more. Moreover, in the rotor magnet  341 , when the ratio of the peripheral angle θ4 of one magnet  343  to the angle θ3 between center positions of two adjacent magnets  243  in the peripheral direction is set as the magnet ratio, the ratio of the slot open ratio to the magnet ratio is 0.6 to 0.7. Therefore, as described above, the output torque of the motor  1  can be increased while decreasing the cogging torque. 
     In the stator core  220 , the magnet width ratio is 0.75 to the 0.90. Consequently, the cogging torque can be further decreased. 
     The motor  1  includes the armature  22  including the stator core  220 , the rotor  32  including the rotor magnet  341 , and the bearing mechanism  4  rotatably supporting the rotor  32 . The rotor magnet  341  is disposed radially outward of the plurality of teeth  222  of the armature  22 . The number of slots in the motor  1  is, for example, 12, 18 or 24. Therefore, it is possible to provide the external rotor motor  1  with the increased output torque and the reduced cogging torque. 
     In the example as described above, the motor  1  is an external rotor motor, but the simulation was also carried out for an internal rotor motor. Examples 14 to 16 illustrated in  FIG. 10  show results of the simulations for the internal rotor motor in a case where the number of slots is 18 while the slot open ratio is 0.6, 0.55, and 0.5, respectively. Additionally,  FIG. 10  also illustrates a ratio of the slot open ratio to the magnet width ratio in which the cogging torque is minimized. 
     In the internal rotor motor, the ratio of the number of slots to the number of poles is  3 : 4 , the slot open ratio is 0.5 or more, and the ratio of the slot open ratio to the magnet width ratio is 0.6 to 0.7, similar to other examples. Therefore, the output torque of the motor can be increased while decreasing the cogging torque. Moreover, the magnet width ratio is 0.75 to 0.90, thus the cogging torque can be further decreased. 
     The internal rotor motor includes an armature including a stator core, a rotor including a rotor magnet, and a bearing mechanism rotatably supporting the rotor. The rotor magnet is disposed radially inward of a plurality of teeth of the armature. The number of slots in the internal rotor motor is, for example,  18 . Therefore, it is possible to provide the internal rotor motor with the increased output torque and the reduced cogging torque. 
     The stator core  220 , the external rotor motor  1  and the internal rotor motor can be modified in various ways. 
     In the stator core  220 , the magnet width ratio may be less than 0.75, or alternatively may exceed 0.90. Furthermore, in the stator core  220 , the number of slots is not limited to 12, 18 or 24. The numbers of slots and poles may be appropriately altered. 
     The shape and structure of the external rotor motor  1  can be appropriately altered from the examples described above, and it is the same for the internal rotor motor. 
     The stator core according to the present disclosure can be used for a motor with various structures and for various purposes. Furthermore, the motor according to the prevent disclosure can be used as a motor for various purposes. 
     Features of the above-described preferred embodiments and the modifications thereof may be combined appropriately as long as no conflict arises. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.