Patent Publication Number: US-2019195235-A1

Title: Centrifugal fan

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2017-250034 filed on Dec. 26, 2017. The entire contents of this application are hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a centrifugal fan. 
     2. Description of the Related Art 
     Japanese Unexamined Patent Application Publication No.  2017 - 78393  discloses a centrifugal fan. Impellers included in the centrifugal fan include a first impeller and a second impeller. The first impeller includes a first surface, a second surface located on the opposite side to the first surface, and an opening portion that is provided in a central portion and that opens to the first surface and the second surface. The second impeller includes a third surface facing the second surface, a fourth surface located on the opposite side to the third surface, and a plurality of blades having first ends on the third surface and second ends that are located on the opposite side to the first ends and that are connected to the second surface. The first surface of the first impeller has a first flat portion provided on an inner circumferential surface on an opening portion side. The fourth surface of the second impeller has a second flat surface portion provided at an outer edge portion. Minus balance adjustment is performed by removing a portion of at least one of the flat portions among the first flat portion and the second flat portion. 
     In the configuration disclosed in Japanese Unexamined Patent Application Publication No. 2017-78393, it is difficult to perform balance adjustment with only one of an upper surface side and a lower surface side of the impellers, and, as a result, it may be necessary to perform balance adjustment of both the upper surface side and the lower surface side of the impellers. In addition, in a thin centrifugal fan, because the thickness of the flat portions on which the minus balance adjustment is performed becomes small, it is difficult to secure sufficient space for performing balance adjustment of the upper surface side and lower surface side of the impellers. 
     SUMMARY OF THE INVENTION 
     A centrifugal fan according to an exemplary embodiment of the present invention includes a motor, an impeller, and a housing. The motor includes a rotary portion that rotates about a center axis extending up and down. The impeller is fixed to the rotary portion and rotates together with the rotary portion. The housing accommodates the motor and the impeller. The rotary portion includes a rotor holder in which a magnet is fixed. The impeller includes a plurality of blade portions and a blade support portion. The plurality of blade portions are disposed at intervals in a circumferential direction. The blade support portion supports the plurality of blade portions on a radial-direction outer side. The housing includes an inlet port and an outlet port. The inlet port is provided on an upper surface of the housing and penetrates the housing in an axial direction. The outlet port is provided on a side surface of the housing and penetrates the housing in a radial direction. The blade support portion includes an accommodating portion that opens upward and that accommodates a balance adjustment member. The accommodating portion is disposed on a radial-direction inner side of the blade portions and between an upper end and a lower end of the blade portions in the axial direction. 
     The above and other elements, features, steps, characteristics and advantages of the present disclosure 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 illustrating a configuration of a centrifugal fan according to an exemplary embodiment of the present invention. 
         FIG. 2  is a vertical sectional view of a centrifugal fan according to an exemplary embodiment of the present invention. 
         FIG. 3  is a vertical sectional view of a rotor holder. 
         FIG. 4  is a perspective view of an impeller. 
         FIG. 5  is an enlarged view illustrating a portion of  FIG. 2  in an enlarged manner. 
         FIG. 6  is a view for explaining a preferred arrangement of an accommodating portion and a balance adjustment member. 
         FIG. 7  is a schematic plan view illustrating a portion of a fixing structure of the impeller that fixes the impeller to the rotor holder. 
         FIG. 8  is a schematic sectional view taken along the line A-A in  FIG. 7 . 
         FIG. 9  is a schematic sectional view illustrating a modification example of the fixing structure of the impeller. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the drawings. Further, in this specification, a direction parallel to a center axis C of a motor  1  included in a centrifugal fan  100  illustrated in  FIG. 2  will be referred to as “axial direction”, a direction perpendicular to the center axis C will be referred to as “radial direction”, and a direction along an arc about the center axis C will be referred to as “circumferential direction”. In addition, in the present specification, the shape and positional relationship of each element will be described with the axial direction taken as the up-down direction and with a side provided with an inlet port  33  illustrated in  FIG. 1  as an upper side with respect to the motor  1 . However, in practicality, there is no intention to limit the orientation of the centrifugal fan  100  according to the present invention to this up-down direction definition. 
       FIG. 1  is a perspective view illustrating a configuration of the centrifugal fan  100  according to an embodiment of the present invention.  FIG. 2  is a vertical sectional view of the centrifugal fan  100  according to the embodiment of the present invention. As illustrated in  FIG. 1  and  FIG. 2 , the centrifugal fan  100  includes the motor  1 , an impeller  2 , and a housing  3 . Further, the housing  3  includes, in detail, an upper housing  31  and a lower housing  32 .  FIG. 2  illustrates the centrifugal fan  100  from which the upper housing  31  has been removed. 
     The motor  1  is of an outer rotor type. The motor  1  includes a rotary portion  11 . The motor  1  further includes a stationary portion  12 . 
     The rotary portion  11  rotates about the center axis C extending in the up-down direction. The rotary portion  11  includes a rotor holder  111 . The rotary portion  11  further includes a shaft  112 , a thrust plate  113 , and a magnet  114 . The shaft  112  is disposed centered on the center axis C. 
       FIG. 3  is a vertical sectional view of the rotor holder  111 . As illustrated in  FIG. 2  and  FIG. 3 , the rotor holder  111  is a capped cylindrical member centered on the center axis C. The magnet  114  is fixed in the rotor holder  111 . Specifically, the rotor holder  111  includes a rotor cylinder portion  111   a.  The rotor cylinder portion  111   a  has a cylindrical shape. The magnet  114  is fixed to an inner circumferential surface of the rotor cylinder portion  111   a.  The magnet  114  is fixed to the inner circumferential surface of the rotor cylinder portion  111   a  by, for example, an adhesive agent. The rotor holder  111  further includes a rotor lid portion  111   b.  The rotor lid portion  111   b  is located on the upper side of the rotor cylinder portion  111   a  and is connected to the rotor cylinder portion  111   a.  The rotor lid portion  111   b  extends to a radial-direction outer side from an upper end portion of the shaft  112 . The rotor lid portion  111   b  has, on a lower surface thereof, a rotor annular portion  111   c  surrounding the shaft  112 . 
     Further, the configuration of the rotor holder  111  is not limited to this, and, for example, the rotor cylinder portion  111   a  may be a separate member disposed on the radial-direction outer side of the rotor lid portion  111   b.  In this case, the rotor cylinder portion  111   a  is fixed to the impeller  2  by insert molding. In other words, the rotor cylinder portion  111   a  is disposed on the radial-direction outer side of a radial-direction outer end of the rotor lid portion  111   b  with a gap therebetween. 
     Further, in the present embodiment, the rotor holder  111  and the shaft  112  are a single member. For example, the rotor holder  111  and the shaft  112  are formed by cutting a metal member. However, the shaft  112  may be a separate member from the rotor holder  111 . In this case, the upper end portion of the shaft  112  is fixed to the rotor lid portion  111   b.    
     The thrust plate  113  is a disk-like member extending in the radial direction. The thrust plate  113  is formed of, for example, a metal. The thrust plate  113  is fixed to a lower end portion of the shaft  112 . An upper surface of the thrust plate  113  faces a lower surface of a sleeve  122   a  (described later) in the axial direction. The thrust plate  113  may be a single member with the shaft  112 . 
     The magnet  114  fixed to the inner circumferential surface of the rotor cylinder portion  111   a  has an annular shape. However, the magnet  114  may be composed of a plurality of magnet pieces disposed at intervals in the circumferential direction. 
     The stationary portion  12  includes a stator  121 , a bearing portion  122 , and a bush  123 . 
     The stator  121  is an annular member centered on the center axis C. The stator  121  is disposed on a radial-direction inner side of the magnet  114 . The stator  121  is an armature that generates a magnetic flux according to a drive current. The stator  121  includes a stator core, an insulator, and coils. The stator core is a magnetic body. The stator core is, for example, formed by laminating electromagnetic steel plates. The stator core has an annular core back and a plurality of teeth. An inner circumferential surface of the core back is fixed to an outer circumferential surface of the bush  123 . The plurality of teeth protrude to the radial-direction outer side from the core back. The insulator is an insulating body. As a material of the insulator, for example, a resin may be used. The insulator covers at least a portion of the stator core. The coils are formed by winding a conductive wire around the teeth with the insulator therebetween. 
     The bearing portion  122  is disposed on the radial-direction inner side of the stator  121 . The bearing portion  122  includes the sleeve  122   a  and a sleeve housing  122   b.  The sleeve  122   a  has a cylindrical shape centered on the center axis C. The sleeve  122   a  is, for example, a metal sintered body and is impregnated with a lubricating oil. The sleeve housing  122   b  has a housing cylinder portion and a housing cap. The sleeve housing  122   b  is formed of, for example, a metal. The housing cylinder portion has a cylindrical shape centered on the center axis C. The sleeve  122   a  is fixed to an inner circumferential surface of the housing cylinder portion. The housing cap is fixed to a lower end portion of the housing cylinder portion. The housing cap closes a lower portion of the housing cylinder portion. 
     The shaft  112  passes through the sleeve  122   a  and is located on the radial-direction inner side of the sleeve  122   a.  A gap where the lubricating oil is present is formed between an outer circumferential surface of the shaft  112  and an inner circumferential surface of the sleeve  122   a  in the radial direction. A gap where the lubricating oil is present is formed between a lower surface of the thrust plate  113  and an upper surface of the housing cap of the sleeve housing  122   b  in the axial direction. 
     The bush  123  is a cylindrical member. The bush  123  is formed by, for example, cutting a metal member. An inner circumferential surface of the bush  123  is fixed to a lower region of an outer circumferential surface of the sleeve housing  122   b.  The bush  123  is inserted and fixed in a lower housing hole portion  32   a  provided in the lower housing  32  and penetrating in the axial direction. 
     By supplying a driving current to the stator  121 , a rotational torque is generated between the magnet  114  and the stator  121 . As a result, the rotor holder  111  rotates with respect to the stator  121 , and the impeller  2  fixed to the rotor holder  111  also rotates about the center axis C. 
       FIG. 4  is a perspective view of the impeller  2 .  FIG. 4  is a view of the impeller  2  as seen diagonally from above. The impeller  2  is fixed to the rotary portion  11  and rotates together with the rotary portion  11 . More specifically, as illustrated in  FIG. 2 , the impeller  2  is fixed to the rotor holder  111 . The impeller  2  is located on the radial-direction outer side of the rotor holder  111 . The impeller  2  is formed of a resin. However, the impeller  2  may be formed of another member such as a metal. 
     As illustrated in  FIG. 2  and  FIG. 4 , in detail, the impeller  2  has a plurality of blade portions  21  and a blade support portion  22 . The plurality of blade portions  21  and the blade support portion  22  are a single member. The plurality of blade portions  21  are disposed at intervals in the circumferential direction. In detail, the plurality of blade portions  21  are disposed at equal intervals in the circumferential direction about the center axis C. The shapes of the plurality of blade portions are the same. The blade support portion  22  supports the plurality of blade portions  21  on the radial-direction outer side. The blade support portion  22  is provided in an annular shape. Specifically, the blade support portion  22  has an annular shape centered on the center axis C. At least a portion of the blade support portion  22  is located on the radial-direction outer side of the rotor holder  111 . In the present embodiment, the blade support portion  22  is located on the radial-direction outer side of the rotor cylinder portion  111   a  and is fixed to the rotor holder  111 . In the present embodiment, the blade support portion  22  is fixed to the rotor holder  111  by using an adhesive agent. Each of the blade portions  21  extends to the radial-direction outer side from a radial-direction outer end of the blade support portion  22 . Each of the blade portions  21  extends to the radial-direction outer side while curving. In the present embodiment, a lower surface of the blade portions  21  and a lower surface of the blade support portion  22  are located on the same plane. Further, note that the lower surface of the blade portions  21  may be located on the upper side or lower side of the lower surface of the blade support portion  22 . 
     The housing  3  accommodates the motor  1  and the impeller  2 . The housing  3  is formed of, for example, a resin or a metal. The upper housing  31  and the lower housing  32  may be formed of the same material or may be formed of different materials. The upper housing  31  has a cylindrical shape centered on the center axis C. Specifically, the upper housing  31  has a first cylinder portion  31   a  and a second cylinder portion  31   b  having different outer diameters. The second cylinder portion  31   b  having a small diameter is disposed on the first cylinder portion  31   a  having a large diameter, and both the first cylinder portion  31   a  and the second cylinder portion  31   b  are formed as a single connected member. The lower housing  32  is in the form of a flat plate extending in the radial direction from the center axis C. The stationary portion  12  of the motor  1  is fixed to the lower housing  32 . The stator  121  is disposed on an upper surface of the lower housing  32 . 
     The housing  3  has the inlet port  33  and an outlet port  34 . The inlet port  33  is provided on an upper surface of the housing and penetrates the housing in the axial direction. The outlet port  34  is provided on a side surface of the housing and penetrates the housing in the radial direction. In the present embodiment, an upper end opening of the second cylinder portion  31   b  forms the inlet port  33 . The inlet port  33  has a circular shape. The outlet port  34  penetrates the first cylinder portion  31   a  in the radial direction. The outlet port  34  extends in the circumferential direction and has a rectangular shape in plan view from the radial direction. Further, the shapes of the inlet port  33  and the outlet port  34  are not limited to these. For example, the upper housing  31  need not have the second cylinder portion  31   b,  and an upper end opening of the first cylinder portion  31   a  may form the inlet port  33 . In addition, the outlet port  34  may be formed by providing a gap between the upper housing  31  and the lower housing  32  in the axial direction. Furthermore, a cylinder portion extending to an axial-direction upper side may be formed in the lower housing  32 , and the outlet port  34  may be formed by penetrating the cylinder portion in the radial direction. 
     By the rotation of the impeller  2 , air is sucked into the housing  3  from the inlet port  33 . The air sucked into the housing  3  is swirled in the housing  3  in the circumferential direction by the rotation of the impeller  2  and then discharged from the outlet port  34 . 
     Next, a balance adjustment structure of the impeller  2  included in the centrifugal fan  100  will be described.  FIG. 5  is an enlarged view illustrating a portion of  FIG. 2  in an enlarged manner. As illustrated in  FIG. 2 ,  FIG. 4  and  FIG. 5 , the blade support portion  22  has an accommodating portion  23  that accommodates a balance adjustment member  4  (see  FIG. 6  described later). The balance adjustment member  4  is a member disposed for adjusting the rotational balance of the impeller  2 . In the case where it is necessary to adjust the rotational balance of the impeller  2 , the balance adjustment member  4  is disposed at a place where balance adjustment of the accommodating portion  23  is required. The balance adjustment member  4  is not disposed in a place where it is not necessary to adjust the rotational balance of the impeller  2 . Further, in the case where the adjustment of the rotational balance of the impeller  2  is not required at all, the balance adjustment member  4  is not disposed in the accommodating portion  23 . The balance adjustment member  4  is, for example, an adhesive agent or a solid weight or the like. 
     The accommodating portion  23  is disposed on the radial-direction inner side of the blade portions  21  and between an upper end  21   a  and a lower end  21   b  of the blade portions  21  in the axial direction. In the present embodiment, an upper surface and a lower surface of the blade portions  21  are flat. Therefore, the upper surface of the blade portions  21  is the upper end  21   a  of the blade portions  21  and the lower surface of the blade portions  21  is the lower end  21   b  of the blade portions  21 . However, at least one of the upper surface and the lower surface of the blade portions  21  may be a curved surface such as a projecting surface or a recessed surface. In the case of such a curved surface, the upper end  21   a  of the blade portions  21  is a portion of the upper surface of the blade portions  21 , and the lower end  21   b  of the blade portions  21  is a portion of the lower surface of the blade portions  21 . In other words, the accommodating portion  23  is disposed within the maximum axial-direction length of the blade portions  21 . 
     In the present embodiment, the accommodating portion  23  is provided between the upper end  21   a  and the lower end  21   b  of the blade portions  21  without providing an accommodating portion for accommodating the balance adjustment member  4  in the upper portion and lower portion of the blade portions  21 . As a result, the impeller  2  can be made thinner. In addition, according to the present embodiment, it is not necessary to dispose the balance adjustment member  4  separately for the upper portion and lower portion of the blade portions  21  and the load of the balance adjustment work can be reduced. Further, more preferably, the accommodating portion  23  is disposed in the middle between the upper end  21   a  and the lower end  21   b  of the blade portions  21  in the axial direction. As a result, it is possible to adjust the balance of the impeller  2  without offsetting the blade portions  21  vertically. 
     In the present embodiment, the accommodating portion  23  opens upward. More specifically, the accommodating portion  23  is a groove portion that is on an upper surface of the blade support portion  22  and that is recessed to an axial-direction lower side. According to this, because the accommodating portion  23  has a bottom surface, it is possible to easily dispose the balance adjustment member  4  in the accommodating portion  23 . In addition, in the case where a member having fluidity such as an adhesive agent is used as the balance adjustment member  4 , it is possible to suppress the balance adjustment member  4  from flowing out downward from the accommodating portion  23  during operation. Further, the shape of the accommodating portion  23  is not particularly limited. The accommodating portion  23  may have, for example, a U shape, a V shape, or the like when viewed in vertical section. 
     The accommodating portion  23  extends in the circumferential direction. In the present embodiment, the accommodating portion  23  is provided annularly about the center axis C. Specifically, the accommodating portion  23  has an annular shape. However, a plurality of accommodating portions  23  may be disposed at intervals in the circumferential direction. As illustrated in  FIG. 4 , a plurality of ribs  24  extending in a direction including a radial-direction component are disposed inside the accommodating portion  23 , which is annular. In the present embodiment, the plurality of ribs  24  are disposed at equal intervals in the circumferential direction. Each of the ribs  24  extends in the radial direction. However, each of the ribs  24  may extend diagonal to the radial direction. Although it is preferable that the plurality of ribs  24  and the blade support portion  22  be a single member, the plurality of ribs  24  may be separate members from the blade support portion  22 . The rigidity of the impeller  2  can be improved by providing the plurality of ribs  24  in the accommodating portion  23 . Therefore, it is possible to make the impeller  2  thin. 
     Further, the accommodating portion  23  may be a through hole penetrating in the axial direction. In this case, it is preferable that a plurality of accommodating portions  23  be disposed at intervals in the circumferential direction. The plurality of through holes may have an arcuate shape, a circular shape, or the like. The through holes forming the accommodating portions  23  may be configured such that an opening diameter on the lower side is smaller than that on the upper side. According to this configuration, it is possible to suppress the adhesive agent from flowing out downward until the adhesive agent is cured by the action of the surface tension in the through holes. In addition, the accommodating portion  23  may be a groove portion on the lower surface of the blade support portion  22  that is recessed to the axial-direction upper side. 
     The accommodating portion  23  overlaps in the radial direction at least a portion of an impeller fixing portion  5  in which an inner circumferential surface of the blade support portion  22  and an outer circumferential surface of the rotor holder  111  are fixed. More specifically, the accommodating portion  23  overlaps in the radial direction at least a portion of the impeller fixing portion  5  in which the inner circumferential surface of the blade support portion  22  and an outer circumferential surface of the rotor cylinder portion  111   a  are fixed. In the present embodiment, the impeller fixing portion  5  has a configuration in which the inner circumferential surface of the blade support portion  22  and the outer circumferential surface of the rotor cylinder portion  111   a  are fixed with an adhesive agent. The accommodating portion  23  is located on the radial-direction outer side of the impeller fixing portion  5 . In the impeller fixing portion  5 , an imbalance tends to occur, for example, due to variations in the amount of adhesive agent used for fixing and variations in component dimensions. According to this configuration, the accommodating portion  23  that accommodates the balance adjustment member  4  is provided at a position overlapping at least a portion of the impeller fixing portion  5  in the radial direction. As a result, it is possible to perform balance adjustment near the position at which an imbalance occurs and it is possible to improve the accuracy of balance adjustment. Further, in the present embodiment, the accommodating portion  23  overlaps a portion of the impeller fixing portion  5  in the radial direction. According to this, the accommodating portion  23  does not become overly large and it is possible to suppress a decrease in the strength of the blade support portion  22 . 
     Further, in the case where the rotor cylinder portion  111   a  is a separate member disposed on the radial-direction outer side of the rotor lid portion  111   b,  the inner circumferential surface of the blade support portion  22  is fixed to an outer circumferential surface of the rotor lid portion  111   b  or an outer circumferential surface of the rotor annular portion  111   c.  At this time, the impeller fixing portion  5  is formed such that the inner circumferential surface of the blade support portion  22  and the outer circumferential surface of the rotor lid portion  111   b  or the outer circumferential surface of the rotor annular portion  111   c  are fixed with an adhesive agent. The accommodating portion  23  is located on the radial-direction outer side of the impeller fixing portion  5 . 
     The accommodating portion  23  overlaps in the radial direction at least a portion of a magnet fixing portion  6  in which the inner circumferential surface of the rotor cylinder portion  111   a  and an outer circumferential surface of the magnet  114  are fixed. The accommodating portion  23  is located on the radial-direction outer side of the magnet fixing portion  6 . In the magnet fixing portion  6 , for example, an imbalance tends to occur due to variations in the amount of adhesive agent used for fixing and variations in component dimensions. According to this configuration, the accommodating portion  23  that accommodates the balance adjustment member  4  is provided at a position overlapping at least a portion of the magnet fixing portion  6  in the radial direction. For this reason, it is possible to perform balance adjustment near the position at which an imbalance occurs and it is possible to improve the accuracy of balance adjustment. Further, in the present embodiment, the accommodating portion  23  overlaps a portion of the magnet fixing portion  6  in the radial direction. According to this, the accommodating portion  23  does not become overly large, and it is possible to suppress a decrease in the strength of the blade support portion  22 . 
     As a preferred embodiment, as illustrated in  FIG. 5 , the blade support portion  22  has a first inclined surface  22   a  that has an axial-direction height that increases from the radial-direction outer side to the radial-direction inner side. The first inclined surface  22   a  may be a flat surface or a curved surface such as a projecting surface or a recessed surface. According to this, it is possible to make the air introduced from the inlet port  33  smoothly flow toward the blade portions  21  by the first inclined surface  22   a.  The accommodating portion  23  is located on the radial-direction inner side of the first inclined surface  22   a.  In the present embodiment, the accommodating portion  23  is adjacent to the first inclined surface  22   a.  The accommodating portion  23  is located on the radial-direction inner side of the first inclined surface  22   a.  Therefore, it is possible to suppress the flow of air along the first inclined surface  22   a  from being hindered by the accommodating portion  23 . That is, according to the present configuration, it is possible to efficiently send the air introduced from the inlet port  33  to the outlet port  34 . 
     In a preferred form, as illustrated in  FIG. 5 , the rotor holder  111  has a second inclined surface  111   d  that has an axial-direction height that increases from the radial-direction outer side toward the radial-direction inner side at an end portion of an upper portion of the rotor holder  111  on the radial-direction outer side. The second inclined surface  111   d  is located on the axial-direction upper side of an upper end of the blade support portion  22 . In the present embodiment, the blade support portion has a flat portion  22   b  parallel to a horizontal plane perpendicular to the axial direction on the radial-direction inner side of the accommodating portion  23 . The flat portion  22   b  corresponds to the upper end of the blade support portion  22 . The second inclined surface  111   d  may be a flat surface or a curved surface such as a projecting surface or a recessed surface. 
     By providing the second inclined surface  111   d,  it is possible to widen the flow path of the air flowing into the housing  3  from the inlet port  33 . Because the second inclined surface  111   d  is located on the upper side of the upper end of the blade support portion  22 , it is possible to suppress the flow of air passing through the second inclined surface  111   d  toward the blade portions  21  from being hindered by the blade support portion  22 . 
       FIG. 6  is a view for explaining a preferable arrangement of the accommodating portion  23  and the balance adjustment member  4 . In a preferred form, as illustrated in  FIG. 6 , the accommodating portion  23  is located below an upper end T of the outer circumferential surface of the rotor holder  111 . Specifically, the accommodating portion  23  is located below an upper end T of the outer circumferential surface of the rotor cylinder portion  111   a.  As a result, it is possible to suppress the flow of the air flowing in through the inlet port  33  from being hindered by the accommodating portion  23  and to efficiently send the air to the outlet port  34 . 
     As a more preferable form, as illustrated in  FIG. 6 , the accommodating portion  23  is located on the lower side of a straight line X connecting a connection point CP of the upper surface of the blade support portion  22  and an end portion of the blade portions  21  on the radial-direction inner side and a radial-direction outer end OE of an upper end portion of the rotor holder  111 . That is, the groove portion is located on the lower side of a straight line X connecting the connection point CP of the upper surface of the blade support portion  22  and the end portion of the blade portions  21  on the radial-direction inner side and the radial-direction outer end OE of the upper end portion of the rotor holder  111 . According to this, because the height position of the accommodating portion  23  can be set to be low, it is possible to suppress the accommodating portion  23  from obstructing the flow of the air flowing from the inlet port  33 . Further, in the case where the second inclined surface  111   d  is not provided and the rotor holder  111  has a corner portion at an upper portion outer circumferential end, the radial-direction outer end OE of the upper end portion of the rotor holder  111  coincides with the upper end T of the outer circumferential surface of the rotor cylinder portion  111   a.  In addition, similarly, in the case where the rotor holder  111  has an R shape at the upper portion outer circumferential end, the radial outer end OE of the upper end portion of the rotor holder  111  coincides with the upper end T of the outer circumferential surface of the rotor cylinder portion  111   a.    
     As illustrated in  FIG. 6 , an upper end of the inner circumferential surface of the blade support portion  22  is preferably located on the lower side of the straight line X connecting the connection point CP of the upper surface of the blade support portion  22  and the end portion of the blade portions on the radial-direction inner side and the radial-direction outer end OE of the upper end portion of the rotor holder  111 . As a result, it is possible to suppress the flow of the air flowing in through the inlet port  33  from being obstructed by the inner circumferential surface of the blade support portion  22  and to efficiently send the air to the outlet port  34 . In the present embodiment, a radial-direction inner end of the flat portion  22   b  located on the radial-direction inner side of the accommodating portion  23  forms the upper end of the inner circumferential surface of the blade support portion  22 . 
     It is preferable that the balance adjustment member  4  be accommodated without extending out from the accommodating portion  23  to the upper side. However, the balance adjustment member  4  may project to the upper side from the accommodating portion  23 . In consideration of this point, it is preferable that an upper end of the balance adjustment member  4  accommodated in the accommodating portion  23  be located below the upper end of the outer circumferential surface of the rotor holder  111 . Specifically, as illustrated in  FIG. 6 , the upper end of the balance adjustment member  4  accommodated in the accommodating portion  23  is preferably located below the upper end T of the outer circumferential surface of the rotor cylinder portion  111   a.  As a result, it is possible to suppress the flow of the air flowing in through the inlet port  33  from being obstructed by the balance adjustment member  4  and to efficiently send the air to the outlet port  34 . 
     As illustrated in  FIG. 6 , the upper end of the balance adjustment member  4  accommodated in the accommodating portion  23  is preferably located on the lower side of the straight line X connecting the connection point CP of the upper surface of the blade support portion  22  and the end portion of the blade portions on the radial-direction inner side and the radial-direction outer end OE of the upper end portion of the rotor holder  111 . According to this, because the height position of the balance adjustment member  4  can be set to be low, it is possible to suppress the balance adjustment member  4  from obstructing the flow of the air flowing from the inlet port  33 . 
     Next, the fixing structure of the impeller  2  that fixes the impeller  2  to the rotor holder  111  in the centrifugal fan  100  will be described in detail.  FIG. 7  is a schematic plan view illustrating a portion of the fixing structure of the impeller  2  that fixes the impeller  2  to the rotor holder  111 .  FIG. 8  is a schematic sectional view taken along the line A-A in  FIG. 7 . 
     As illustrated in  FIG. 7  and  FIG. 8 , at least a portion of the inner circumferential surface of the blade support portion  22  faces the outer circumferential surface of the rotor holder  111  in the radial direction with a gap therebetween in which an adhesive agent  7  is present at least in a portion of the gap. In detail, at least a portion of the inner circumferential surface of the blade support portion  22  faces the outer circumferential surface of the rotor cylinder portion  111   a  in the radial direction with a gap therebetween in which the adhesive agent  7  is present at least in a portion of the gap. In the present embodiment, a portion of the inner circumferential surface of the blade support portion  22  is in contact with the outer circumferential surface of the rotor cylinder portion  111   a.  This point will be touched upon later. However, the inner circumferential surface of the blade support portion  22  need not be in contact with the outer circumferential surface of the rotor cylinder portion  111   a.  It suffices that the adhesive agent  7  fix the blade support portion and the rotor cylinder portion  111   a.  For this reason, the adhesive agent  7  may be contained in the entirety of the gap provided between the inner circumferential surface of the blade support portion  22  and the outer circumferential surface of the rotor cylinder portion  111   a  in the radial direction or it may be contained only in a portion of the gap. 
     At least one of the outer circumferential surface of the rotor holder  111  and the inner circumferential surface of the blade support portion  22  has a recessed portion  8 , which are recessed in the radial direction and in which the adhesive agent  7  is present. Specifically, at least one of the outer circumferential surface of the rotor cylinder portion  111   a  and the inner circumferential surface of the blade support portion  22  has the recessed portion  8 , which are recessed in the radial direction and in which the adhesive agent  7  is present. In this embodiment, the recessed portion  8  is provided on both the outer circumferential surface of the rotor cylinder portion  111   a  and the inner circumferential surface of the blade support portion  22 . Details of this point will be described later. The adhesive agent  7  may be contained in the entirety of the recessed portion  8  or may be contained in a portion of the recessed portion  8 . By providing the recessed portion  8 , for example, it is possible to absorb the influence of volume change associated with curing of the adhesive agent  7  in the recessed portion  8 . Therefore, it is possible to suppress deformation of the impeller  2  and the rotor holder  111  when fixing the rotor cylinder portion  111   a  and the blade support portion  22  using the adhesive agent  7 . In addition, because the adhesion area can be increased by providing the recessed portion  8 , the impeller  2  and the rotor holder  111  can be firmly fixed. For this reason, it is possible to firmly fix the impeller  2  to the rotor holder  111  without increasing the axial-direction length of the impeller fixing portion  5 . 
     In the present embodiment, as illustrated in  FIG. 8 , the outer circumferential surface of the rotor cylinder portion  111   a  has the recessed portion  8  overlapping at least a portion of the magnet  114  in the radial direction as a preferable configuration. In the case where the impeller  2  is press-fitted and fixed to the rotor holder  111 , deformation of the rotor cylinder portion  111   a  may occur due to the press fitting and there is a possibility that the arrangement of the magnet  114  or the like may be affected. In the present embodiment, the impeller  2  and the rotor holder  111  are fixed by the adhesive agent  7 . In such a configuration, by adopting a configuration in which the recessed portion  8  is disposed at a position overlapping the magnet  114  in the radial direction, it is possible to reduce the possibility that the influence of volume change upon curing of the adhesive agent  7  adversely affects the magnet  114 . That is, according to the present embodiment, by fixing the impeller  2  to the rotor holder  111 , the possibility of an adverse effect on the magnet  114  can be reduced. 
     As illustrated in  FIG. 8 , the outer circumferential surface of the rotor holder  111  has a flange portion  111   e  extending to the radial-direction outer side. More specifically, the outer circumferential surface of the rotor cylinder portion  111   a  has the flange portion  111   e  extending to the radial-direction outer side. Either one of the upper surface and the lower surface of the blade support portion  22  faces the flange portion  111   e  in the axial direction. In the present embodiment, the lower surface of the blade support portion  22  faces the flange portion  111   e  in the axial direction. In this configuration, the impeller  2  is fitted into the rotor holder  111  from the axial-direction upper side to lower side. 
     The lower surface of the blade support portion  22  may be in contact with an upper surface of the flange portion  111   e.  In this case, the position of the impeller  2  in the axial direction with respect to the rotor holder  111  can be set by the flange portion  111   e.  In addition, the adhesive agent  7  may be present at least in a portion between the lower surface of the blade support portion  22  and the upper surface of the flange portion  111   e.  As a result, it is possible to firmly fix the impeller  2  and the rotor holder  111 . 
       FIG. 9  is a schematic sectional view illustrating a modification example of the fixing structure of the impeller. As illustrated in  FIG. 9 , an upper surface of a blade support portion  22 A may face a flange portion  111   e A. In this configuration, an impeller  2 A is fitted into a rotor holder  111 A from the axial-direction lower side to upper side. Also in the configuration of the modification example, it is possible to set the position of the impeller  2 A in the axial direction with respect to the rotor holder  111 A. Also in the configuration of the modification example, it is possible to firmly fix the impeller  2 A and the rotor holder  111 A by interposing an adhesive agent on at least a portion between the impeller  2 A and the rotor holder  111 A in the axial direction. 
     Further, in the case where the rotor cylinder portion  111   a  is a separate member disposed on the radial-direction outer side of the rotor lid portion  111   b,  the inner circumferential surface of the blade support portion  22  is fixed to an outer circumferential surface of the rotor lid portion  111   b  or an outer circumferential surface of the rotor annular portion  111   c.  At this time, the impeller fixing portion  5  is formed such that the inner circumferential surface of the blade support portion  22  and the outer circumferential surface of the rotor lid portion  111   b  or the outer circumferential surface of the rotor annular portion  111   c  are fixed with an adhesive agent. Furthermore, at this time, at least one of the outer circumferential surface of the rotor lid portion  111   b,  the outer circumferential surface of the rotor annular portion  111   c,  and the inner circumferential surface of the blade support portion  22  has the recessed portion  8 , which are recessed in the radial direction and in which the adhesive agent  7  is present. In addition, the outer circumferential surface of the rotor lid portion  111   b  or the outer circumferential surface of the rotor annular portion  111   c  has the flange portion  111   e  extending to the radial-direction outer side. Either one of the upper surface and the lower surface of the blade support portion  22  faces the flange portion  111   e  in the axial direction. 
     As illustrated in  FIG. 8 , the rotor holder  111  has a rotor holder irregular portion  115  in which irregularities in which the radial-direction position of the outer circumferential surface of the rotor holder  111  changes are formed at least once in the axial direction. The rotor holder  111  has the rotor holder irregular portion  115  in which irregularities in which the radial-direction position of the outer circumferential surface of the rotor cylinder portion  111   a  changes are formed at least once in the axial direction. In the rotor holder irregular portion  115  of the present embodiment, the irregularities in which the radial-direction position of the outer circumferential surface of the rotor cylinder portion  111   a  changes are formed twice in the axial direction. From top to bottom, a projection, a recess, a projection, and a recess are formed in order. The number of times the irregularities are formed may be 1 time or 3 times or more. The shapes of the recesses and the projections are not particularly limited, and may be, for example, a U shape, a V shape, or the like when viewed in cross section. 
     The rotor holder irregular portion  115  is provided on the same axial-direction side as the blade support portion  22  with respect to the flange portion  111   e.  In the present embodiment, the rotor holder irregular portion  115  is provided on the upper side of the flange portion  111   e.  Further, in the modification example illustrated in  FIG. 9 , a rotor holder irregular portion  115 A is provided under the flange portion  111   e A. 
     The rotor holder irregular portion  115  has a first recessed portion  8   a  facing the flange portion  111   e  and included in the recessed portion  8 . In the present embodiment, the first recessed portion  8   a  is located on the upper side of the flange portion  111   e.  The first recessed portion  8   a  extends in the circumferential direction. The first recessed portion  8   a  may be provided all around the outer circumferential surface of the rotor cylinder portion  111   a.  In addition, a plurality of first recessed portions  8   a  may be disposed at intervals in the circumferential direction. The first recessed portion  8   a  overlaps a part of the magnet  114  in the radial direction. 
     In a configuration in which the first recessed portion  8   a  is not provided, in the case where the adhesive agent  7  accumulates more than necessary at a position where a radial-direction inner end of the flange portion  111   e  and the outer circumferential surface of the rotor cylinder portion  111   a  are connected to each other, the impeller  2  becomes easily inclined with respect to the rotor holder  111 . According to the configuration of the present embodiment, because the adhesive agent  7  enters the first recessed portion  8   a,  the inclination of the impeller  2  with respect to the rotor holder  111  can be suppressed by the impeller  2  riding over the adhesive agent  7 . In addition, according to the configuration of this embodiment, it is possible to rigidly fix the impeller  2  and the rotor holder  111  by increasing the adhesion area by the first recessed portion  8   a.    
     In a preferred embodiment, as illustrated in  FIG. 8 , the rotor holder irregular portion  115  further includes a second recessed portion  8   b  included in the recessed portion  8  at a position further away from the flange portion  111   e  in the axial direction than the first recessed portion  8   a . In the present embodiment, the second recessed portion  8   b  is located on the upper side of the first recessed portion  8   a . A projection among the irregularities forming the rotor holder irregular portion  115  is present between the first recessed portion  8   a  and the second recessed portion  8   b  in the axial direction. The second recessed portion  8   b  extends in the circumferential direction. The second recessed portion  8   b  may be provided all around the outer circumferential surface of the rotor cylinder portion  111   a.  In addition, a plurality of second recessed portions  8   b  may be disposed at intervals in the circumferential direction. The second recessed portion  8   b  overlaps a part of the magnet  114  in the radial direction. The adhesion area can be further increased by the second recessed portion  8   b,  and the impeller  2  and the rotor holder  111  can be more firmly fixed to each other. 
     As illustrated in  FIG. 8 , the blade support portion  22  has a first blade support portion irregular portion  25  in which irregularities in which the radial-direction position of the inner circumferential surface of the blade support portion  22  changes are formed at least once in the axial direction. In the first blade support portion irregular portion  25  of the present embodiment, the irregularities in which the radial-direction position of the inner circumferential surface of the blade support portion  22  changes are formed once in the axial direction. From top to bottom, a projection and a recess are formed in order. The number of times the irregularities are formed may be two or more times. The shapes of the recesses and the projections are not particularly limited, and may be, for example, a U shape, a V shape, or the like when viewed in cross section. 
     The first blade support portion irregular portion  25  has a third recessed portion  8   c  facing the flange portion  111   e  and included in the recessed portion  8 . In the present embodiment, the third recessed portion  8   c  is located on the upper side of the flange portion  111   e.  The third recessed portion  8   c  extends in the circumferential direction. The third recessed portion  8   c  may be provided over the entire circumference of the inner circumferential surface of the blade support portion  22 . In addition, a plurality of third recessed portions  8   c  may be disposed at intervals in the circumferential direction. By providing the third recessed portion  8   c,  the adhesive agent  7  can enter the third recessed portion  8   c . As a result, it is possible to suppress the inclination of the blade support portion  22  riding on the adhesive agent  7 . In addition, by providing the third recessed portion  8   c,  the adhesion area can be increased and the impeller  2  and the rotor holder  111  can be firmly fixed. 
     As illustrated in  FIG. 7 , the blade support portion  22  has a second blade support portion irregular portion  26  in which irregularities in which the radial-direction position of the inner circumferential surface of the blade support portion  22  changes are formed in the circumferential direction repeatedly. The shapes of the recesses and the projections are not particularly limited, and may be, for example, a U shape, a V shape, or the like in plan view from the axial direction. In the present embodiment, a plurality of projecting portions  9  of the second blade support portion irregular portion  26  are in contact with the outer circumferential surface of the rotor holder  111 . More specifically, the plurality of projecting portions  9  of the second blade support portion irregular portion  26  are in contact with the outer circumferential surface of the rotor cylinder portion  111   a.  Because the plurality of projecting portions  9  come into contact with the outer circumferential surface of the rotor cylinder portion  111   a,  the impeller  2  can be easily arranged coaxially with respect to the rotor holder  111 . In the present embodiment, an upper surface of each of the projecting portions  9  is located on the same plane as the upper surface of the blade support portion  22 . On the other hand, a lower end of each of the projecting portions  9  is located on the upper side of the lower surface of the blade support portion  22 . This is because the blade support portion  22  has the third recessed portion  8   c . However, the third recessed portion  8   c  may be omitted, and the lower surface of the projecting portions  9  and the lower surface of the blade support portion  22  may be located on the same plane. 
     Further, it should be noted that the entirety of the projecting portions  9  need not contact the outer circumferential surface of the rotor cylinder portion  111   a.  The adhesive agent  7  may be interposed between the projecting portions  9  and the outer circumferential surface of the rotor cylinder portion  111   a  in the radial direction. The projecting portions  9  and the outer circumferential surface of the rotor cylinder portion  111   a  need not be in contact with each other and the adhesive agent  7  may be interposed therebetween in the radial direction over the entirety thereof. In addition, the upper surface of each of the projecting portions  9  may be located on the lower side of the upper surface of the blade support portion  22 . 
     As illustrated in  FIG. 7 , it is preferable that the ribs  24  overlap the projecting portions  9  of the second blade support portion irregular portion  26  in the radial direction. According to this configuration, because the ribs  24  are provided at positions where a force is easily applied when the blade support portion  22  is attached to the rotor holder  111 , deformation or the like of the blade support portion  22  at the time of assembly can be suppressed. 
     Further, in the above description, the blade support portion  22  is provided with the first blade support portion irregular portion  25  and the second blade support portion irregular portion  26 . However, only one of the first blade support portion irregular portion  25  and the second blade support portion irregular portion  26  has to be provided. In addition, in the case where the recessed portion  8  is provided on the outer circumferential surface of the rotor cylinder portion  111   a,  a configuration in which neither the first blade support portion irregular portion  25  nor the second blade support portion irregular portion  26  is provided in the blade support portion  22  may be used. On the contrary, in the case where the recessed portion  8  is provided on the inner circumferential surface of the blade support portion  22 , the rotor holder irregular portion  115  need not be provided. 
     The present invention can be applied to, for example, a centrifugal fan used for a range hood fan, a ventilating fan for a duct, a heat exchanging unit, paper suction for a printing device, facilitation of inhalation and exhaustion for a mask, and the like. 
     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 disclosure 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 disclosure. The scope of the present disclosure, therefore, is to be determined solely by the following claims.