Patent Publication Number: US-9885367-B2

Title: Centrifugal fan

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a centrifugal fan, and more particularly, to a centrifugal fan capable of reducing noise caused due to air blowing. 
     2. Description of the Related Art 
       FIG. 17  is a perspective view showing an example of a related-art centrifugal fan.  FIG. 18  is a side sectional view showing the example of the related-art centrifugal fan. 
     As shown in  FIGS. 17 and 18 , a centrifugal fan  801  includes a casing  810  having an inlet opening  813  ( 833 ) and outlet openings  819 , and an impeller  830  accommodated in the casing  810 . The impeller  830  includes a plurality of blades  851  around a rotary shaft of a motor  860 . The centrifugal fan  801  introduces air taken from the inlet opening  813  ( 833 ), from a center of the impeller  830  towards between the blades  851  and discharges the air in a radially outward direction of the impeller  830  with a hydrodynamic force generated by a centrifugal action resulting from rotation of the impeller  830 . The air which is discharged outwards from an outer periphery of the impeller  830  is then discharged through the outlet openings  819  of the casing  810 . Each blade  851  has a vertical end edge portion at a side of the inlet openings  813 . 
     As shown in  FIG. 18 , the centrifugal fan  801  is thin. The centrifugal fan  801  includes the motor  860  for rotating the impeller  830  at a substantially center part of the casing  810 . The motor  860  is an outer rotor brushless motor having a rotor yoke  863  attached to the impeller  830 . 
     The centrifugal fan  801  is widely used for an electrical household appliance, an OA equipment, cooling, ventilation and air conditioning of an industrial equipment, a vehicular blower and the like. The blowing performance and noise of the centrifugal fan  801  are highly influenced by a blade shape of the impeller  830  and a shape of the casing  810  (a structure of the centrifugal fan  801 ). 
     In order to reduce the noise and to improve the blowing performance, the shape of the impeller and the structure of the casing have been optimized, and a variety of suggestions have been made. 
     For example, a centrifugal fan has been suggested which optimizes a blade shape to thereby reduce the noise (for example, refer to JP-A-S63-289295). 
     In this thin-type centrifugal fan  801 , however, the noise is apt to occur. That is, there is a limitation on the thinning of the motor  860  for rotating the impeller  830 . Therefore, when the centrifugal fan  801  is made to be thin, a height of the motor  860  becomes relatively larger than that of the centrifugal fan  801 . Thus, as shown in  FIG. 18 , the rotor yoke  863  of the motor  860  protrudes to the center part of the inlet opening  813  ( 833 ) of the centrifugal fan  801 . The rotor yoke  863  protruding to the inlet opening  813  ( 833 ) disturbs flowing of the air from the inlet opening  813  ( 833 ) towards the outlet openings  819 , so that the noise is generated. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a thin centrifugal fan capable of reducing noise. 
     According to an illustrative embodiment of the present invention, there is provided a centrifugal fan comprising: an impeller; a lower casing which is provided below the impeller; and a motor which is mounted to the lower casing and configured to rotate the impeller. The impeller includes an upper shroud, a lower shroud, and a plurality of blades arranged in a circumference direction between the upper shroud and the lower shroud. The centrifugal fan is configured to discharge a fluid taken from an inlet opening formed at an upper part of the upper shroud to a lateral side of the impeller as the impeller is rotated. A rotor of the motor is attached to an attachment part of the lower shroud. In a section passing a rotary shaft of the impeller, both the rotor and the attachment part are provided at a more outer side than a circle, the circle having a center on a line passing an upper end portion of the inlet opening at an inner side and parallel to the rotary shaft of the impeller, and having a radius of 80% of a distance from the upper end portion of the inlet opening at the inner side to a surface of the lower shroud below the upper end portion, and the circle being tangent to the surface of the lower shroud or passing an end edge portion of the lower shroud. 
     In the above centrifugal fan, the motor may be an outer rotor motor, the rotor may be provided in the inlet opening of the impeller to protrude upwards toward an outside of the inlet opening, and a height of the rotor in an upper-lower direction may be a half or smaller of a height of the centrifugal fan in the upper-lower direction. 
     In the above centrifugal fan, the lower casing may be made using a metal plate. 
     In the above centrifugal fan, waffle flattening processing may be performed on at least a part of the lower casing. 
     In the centrifugal fan, a center part of the lower casing may be formed with a recess part which is recessed downwards, and at least a driving circuit of the motor may be provided in the recess part. 
     In the above centrifugal fan, a surface of the lower casing, which faces the impeller, may become a part of a wall surface which is configured to guide the fluid taken from the inlet opening. 
     In the above centrifugal fan, the lower shroud may be provided to only a rotary shaft side of the impeller such that at least an outer periphery part of each blade faces an upper surface of the lower casing. 
     The above centrifugal fan may further comprise an upper casing which is provided above the impeller, the upper casing may be formed with a circular opening when seen from a plan view such that the fluid is taken into the inlet opening, and a ratio of a height from a lower end of each blade to the inlet opening in an upper-lower direction with respect to an inner diameter of the circular opening may be within a range of 15% or larger and 25% or smaller. 
     In the above centrifugal fan, a position of the upper end portion of the inlet opening in the upper-lower direction may be same as or higher than an upper end portion of the circular opening. 
     The above centrifugal fan may further comprise an upper casing which is provided above the impeller, the upper casing may be formed with an opening such that the fluid is taken into the inlet opening, and a position of the upper end portion of the inlet opening in the upper-lower direction may be same as or higher than an upper end portion of the opening of the upper casing. 
     In the above centrifugal fan, the lower casing may be formed with an aperture part, to which a connector for feeding power to the motor is fixed. 
     According to the above configuration, both the rotor and the attachment part are provided at the more outer side than the predetermined-size circle which has the center on the line passing the upper end portion of the inlet opening at the inner side and parallel to the rotary shaft of the impeller, and which is tangent to the surface of the lower shroud or passes the end edge portion of the lower shroud. Therefore, the centrifugal fan capable of reducing the noise can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a plan view showing a centrifugal fan according to an illustrative embodiment of the present invention; 
         FIG. 2  is a sectional view taken of the centrifugal fan along a line A-A of  FIG. 1 ; 
         FIG. 3  is a partially enlarged view of  FIG. 2 ; 
         FIG. 4  is an explanatory view of a shape of an impeller; 
         FIG. 5  is a graph showing a relation between a shape of a lower shroud and performance of the centrifugal fan; 
         FIG. 6  shows an exemplary configuration of a centrifugal fan according to a comparative example; 
         FIG. 7  is a perspective view of a lower casing, which is seen from the lower side; 
         FIG. 8  is a side sectional view of the centrifugal fan along a line different from  FIG. 2 . 
         FIG. 9  is a graph showing a vibration occurrence situation when using the lower casing which is made of a metal plate having a thickness of 1.6 mm, and to which waffle flattening processing is performed; 
         FIG. 10  is a graph showing a vibration occurrence situation when using the lower casing which is made of a metal plate having a thickness of 1.6 mm, and to which waffle flattening processing is not performed; 
         FIG. 11  is a graph showing a vibration occurrence situation when using the lower casing which is made of a metal plate having a thickness of 1.2 mm, and to which waffle flattening processing is performed; 
         FIG. 12  is a graph showing a vibration occurrence situation when using the lower casing which is made of a metal plate having a thickness of 1.2 mm, and to which the waffle flattening processing is not performed; 
         FIG. 13  is a side sectional view showing a configuration of a centrifugal fan according to a modified example of the illustrative embodiment; 
         FIG. 14  is a side sectional view showing a configuration of a centrifugal fan according to another modified example of the illustrative embodiment; 
         FIG. 15  is an explanatory view of a shape of an impeller of the centrifugal fan; 
         FIG. 16  is a perspective view of a lower casing of a centrifugal fan according to a further modified example of the illustrative embodiment, which is seen from the lower side; 
         FIG. 17  is a perspective view showing an example of a related-art centrifugal fan; and 
         FIG. 18  is a side sectional view showing the example of the related-art centrifugal fan. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, illustrative embodiments of the present invention will be described. 
       FIG. 1  is a plan view showing a centrifugal fan according to an illustrative embodiment of the present invention.  FIG. 2  is a sectional view of the centrifugal fan taken along a line A-A of  FIG. 1 .  FIG. 3  is a partially enlarged view of  FIG. 2 . 
     Referring to  FIGS. 1 to 3 , a centrifugal fan  1  includes a casing  10 , an impeller  30  and a motor  60 . The centrifugal fan  1  has a rectangular parallelepiped shape of a substantial square except for a part to which the motor  60  is attached, when seen from the plan view. The centrifugal fan  1  is a thin centrifugal fan having a relatively small size (height) in an upper-lower direction. The impeller  30  is attached to a rotor  63  which rotates together with a shaft  61  of the motor  60 . The centrifugal fan  1  rotates the impeller  30  by the motor  60 . As the impeller  30  rotates, the centrifugal fan  1  discharges an air (an example of a fluid) taken from an inlet opening  33  thereof to lateral sides of the impeller  30 . That is, the air taken from the inlet opening  33  passes between blades  51  of the impeller  30  and is discharged in a radially outward direction of the impeller  30  with a hydrodynamic force generated by a centrifugal action resulting from the rotation of the impeller  30 . The air is discharged from outlet openings  19  of the casing  10 , which are provided at lateral sides of the impeller  30 . 
     The motor  60  is an outer rotor brushless motor, for example. The motor  60  is mounted to a center part of a lower casing  21  by a fastening member such as screw, bolt and the like. The motor  60  has the rotor  63  which has a shape of a cup, and the rotor  63  opens downwards. An annular magnet  65  is attached on an inner surface of a side periphery of the rotor  63 . The shaft  61  is attached to a center part of the rotor  63 . 
     The shaft  61  is rotatably supported by a pair of bearings  66   a  mounted to a bearing holder  66 . An outer periphery part of the bearing holder  66  is provided with a stator  67 . The stator  67  is configured by a stacked stator core, an insulator mounted to the stator core and having a coil wound thereon or the like. The stator  67  is arranged to face the magnet  65  at a predetermined gap in a radial direction (in a left-right direction in  FIG. 2 ). The stator  67  is connected to a circuit board  69 . The circuit board  69  is a printed wiring board, for example. The circuit board  69  is mounted thereon with an electronic component for controlling the motor  60 , and the like, and a driving circuit of the motor  60  is mounted thereon. 
     The casing  10  is configured by an upper casing  11  and a lower casing  21 . Specifically, the upper casing  11  and the lower casing  21  are assembled to each other by using screws  14  positioned at four corners, when seen from the plan view, so that the casing  10  is configured. The screw  14  is a bolt which is inserted from the lower casing  21 , for example. The upper casing  11  and the lower casing  21  are assembled to each other with pillars being interposed therebetween at parts at which the screws  14  are arranged, for example. Meanwhile, at this time, the pillars may be integrated with any one of the upper casing  11  and the lower casing  21 . The outlet openings  19  are provided at lateral sides of the casing  10 , except for the fastened parts of the upper casing  11  and the lower casing  21  using the screws  14 , and between the upper casing  11  and the lower casing  21 . 
     The impeller  30  is accommodated in the casing  10 . The impeller  30  has a disc shape as a whole. The upper casing  11  is provided above the impeller  30 , and the lower casing  21  is provided below the impeller  30 . That is, the centrifugal fan  1  is configured such that the impeller  30  is sandwiched and held between the upper casing  11  and the lower casing  21 . 
     The impeller  30  includes an upper shroud  31 , a lower shroud  41  and a plurality of blades  51  which are arranged between the upper shroud  31  and the lower shroud  41 . The impeller  30  is formed at its center part with an inlet opening  33  which opens upwards. The inlet opening  33  is surrounded by an upper end portion  35  of the upper shroud  31  at an inner side. As shown in  FIG. 1 , the blades  51  are arranged at an appropriate interval in a circumference direction. 
     The lower shroud  41  in which the rotor  63  is fitted is arranged at the center part of the impeller  30 . The lower shroud  41  is provided at its center part with a cylindrical part (an example of an attachment part)  43  formed such that the rotor  63  is provided thereto. The rotor  63  is fitted to the cylindrical part  43 , which is provided at the center part of the lower shroud  41 , and holds the impeller  30 . The rotor  63  is provided in the inlet opening  33  to protrude upwards towards an outside of the inlet opening  33 . As shown in  FIG. 3 , a height of a part of the rotor  63  holding the cylindrical part  43  in an upper-lower direction is set to be a half or smaller of a height H of the centrifugal fan  1  in the upper-lower direction. Thereby, the centrifugal fan  1  can be made relatively thin while the air taken from the inlet opening  33  is not disturbed by the rotor  63 . 
     Each blade  51  has the same curved shape. The blade  51  is a backward inclined blade, which is a so-called turbo type. The shape of the blade  51  is backward inclined as regards the rotating direction thereof. The upper shroud  31 , the lower shroud  41  and the blades  51  are integrally molded using a synthetic resin, for example. 
     The upper casing  11  is formed of a resin such as engineering plastic and the like. The upper casing  11  is formed at its center part with an opening  13 . The opening  13  has a circular shape, when seen from the plan view. The opening  13  is formed such that the air is taken into the inlet opening  33  of the impeller  30 . The opening  13  has an inner diameter slightly larger than the diameter of the inlet opening  33  which is configured by the upper shroud  31 . That is, in this illustrative embodiment, the size of the opening  13  is substantially equivalent to the size of the inlet opening  33 . 
     The lower casing  21  is made of a metal plate such as iron. The lower casing  21  is formed at its center part with a recess part  23  which is recessed downwards. The recess part  23  has a bowl shape. As shown in  FIG. 2 , in this illustrative embodiment, the motor  60  and a driving circuit of the motor  60  such as the circuit board  69  and the like are housed in the recess part  23 . The motor  60  is mounted to the lower casing  21  by a fastening member such as screw, bolt and the like. However, the motor  60  may be mounted to the lower casing  21  by crimping a lower part of the bearing holder  66  to the recess part  23 , instead of the fastening member. 
     An outer periphery part of the lower casing  21  forms a side plate which is bent in an axial direction (upper-lower direction in  FIG. 2 ). The rigidity of the lower casing  21  is increased by providing the side plate. 
     A part of an upper surface of the lower casing  21 , which is around the recess part  23 , forms a partition wall part  29  which faces a lower surface of the impeller  30 . The partition wall part  29  is formed into a plane shape to be close to the lower surface of the impeller  30 . 
     As shown in  FIG. 2 , the lower shroud  41  of the impeller  30  is provided to only a side of the shaft (the rotary shaft of the impeller  30 )  61  such that at least an outer periphery part of each blade  51  faces the partition wall part  29 . That is, the respective blades  51  are exposed at the parts of the impeller  30  facing the partition wall part  29 . The surface of the lower casing  21  facing the impeller  30  becomes a part of a wall surface which laterally guides the air, which is taken from the inlet opening  33 . The blades  51  are arranged to face the partition wall part  29  at a predetermined gap in the axial direction. In the meantime, at least a portion of the lower part of each blade  51  may be exposed to the partition wall part  29  or the entire thereof may be exposed to the partition wall part  29 . 
     In the meantime, an outer diameter size of the impeller  30  which is accommodated in the casing  10  is set to be smaller than a size of one lateral side of the casing  10 . Thereby, the impeller  30  which is rotating does not protrude beyond an outer edge of the casing  10 , so that contact of the impeller  30  with the other member and damage resulting from the contact can be prevented. 
     The lower casing  21  has functions as a main plate which guides the air in the impeller  30  and as a base plate of the casing  10 . Thus, a setting of the gap between the impeller  30  and the partition wall part  29  may be important. When the gap is too large, the air taken from the inlet opening  33  passes between the blades  51  and also flows into the gap. As a result, a pressure of the air discharged from the impeller  30  is reduced, so that a blowing characteristic is deteriorated. On the other hand, when the gap is too small, a following problem occurs. That is, when there occurs non-uniformity as regards size precision of each component, the blades  51  may contact the partition wall part  29 . In order to prevent the contact, it is necessary to manage the size of each component in high precision, so that the cost of the centrifugal fan  1  is increased. Thus, the gap is appropriately set, taking into consideration the above aspects. 
     A ratio of a height h from a lower end of each blade  51  to an upper end portion of the inlet opening  33  in the upper-lower direction with respect to an inner diameter (which is denoted with φd in  FIG. 2 ) of the opening  13  is within a range of 15% or larger and 25% or smaller. That is, a relation between the height h and the inner diameter φd of the opening  13  is expressed as follows.
 
0.15 ≦h/φd≦ 0.25
 
     In this illustrative embodiment, as shown in  FIG. 3 , an upper surface of the lower shroud  41  is a curved surface  49  which is a downward convex arc-shaped curve, in a side section. An outer periphery end portion  45  of the lower shroud  41  is positioned in the vicinity of a part vertically below the upper end portion  35  of the upper shroud  31 . Also, an inner periphery end portion  47  of the lower shroud  41 , which is an upper end portion of the cylindrical part  43 , is an upper end portion of the lower shroud  41 . The inner periphery end portion  47  is positioned in the vicinity of an outer periphery upper end portion  63   a  of the rotor  63 . The curved surface  49  is formed between the outer periphery end portion  45  and the inner periphery end portion  47 . The lowest part of the curved surface  49  is the outer periphery end portion  45  and the highest part thereof is the inner periphery end portion  47 . 
       FIG. 4  is an explanatory view of the shape of the impeller  30 . 
       FIG. 4  is a sectional view in the same section as that shown in  FIG. 2 . The shape of the lower shroud  41  is further described with reference to  FIG. 4 . That is, in this illustrative embodiment, the curved surface  49  is formed into an arc shape which has a center on a line L passing the upper end portion  35  of the inner side of the inlet opening  33  and parallel to the rotary shaft of the impeller  30 , in a section passing the rotary shaft of the impeller  30  (hereinafter, simply referred to as the section). In the section, the curved surface  49  is formed to be provided at a more outer side than a circle C 2  shown in  FIG. 4 . In other words, the rotor  63  and the lower shroud  41  including the cylindrical part  43  are provided at the more outer side than the circle C 2  (not overlap the circle C 2 ) in the section. Thereby, the air flow path from the inlet opening  33  to the outlet opening  19  between the upper shroud  31  and the lower shroud  41  is configured such that the air easily flows. 
     The circle C 2  is a circle which has a center A 2  positioned on the line L and passes the outer periphery end portion  45 , which is the end edge of the lower shroud  41 . A radius R 2  of the circle C 2  is 80% of a radius R of the circle C 1 . The circle C 1  is a circle having the upper end portion  35  as a center A 1  thereof. The radius R of the circle C 1  is a distance from the upper end portion  35  to the outer periphery end portion  45  which is positioned in the vicinity of a part of the surface of the lower shroud  41  vertically below the upper end portion  35 . That is, in this illustrative embodiment, the radius R of the circle C 1  is substantially the same as the height h of the flow path of the air taken from the inlet opening  33  in the upper-lower direction. 
     In the meantime, the curved surface  49  may be an arc shape having a center which is not positioned on the line L in the section. Also, the curved surface  49  may be non-circular in the section. The curved surface may be elliptical or other curved shape. In any case, the lower shroud  41  including the curved surface  49  is provided at the more outer side than the circle C 2  which has a radius of 80% of the distance from the upper end portion  35  to the surface of the curved surface  49  and the center positioned on the line L, and which is tangent to the curved surface  49  in the section. The rotor  63  is also provided at the more outer side than the circle C 2 . Meanwhile, in this case, the circle C 2  may be a circle which has the center A 2  positioned on the line L and the radius R 2  of 80% of the radius R of the circle C 1 , and which is tangent to the surface of the lower shroud  41 . 
     In this illustrative embodiment, the lower shroud  41  has the above-described shape, so that following effects can be obtained. That is, in the centrifugal fan  1 , it is difficult to make the motor  60  thin, which rotates the impeller  30 . Thus, in the related-art technique, the rotor part of the motor is made to protrude the center part of the inlet opening, which disturbs the air flow and causes the noise. However, according to the configuration of this illustrative embodiment, while the centrifugal fan  1  is made to be thin, the air flow path is configured such that the air can smoothly flow. Therefore, the generation of the noise can be suppressed. 
       FIG. 5  is a graph showing a relation between the shape of the lower shroud  41  and performance of the centrifugal fan  1 . 
     In  FIG. 5 , an arc size in a horizontal axis indicates a radius of the arc of the curved surface  49 , which has a center on a vertical line passing the upper end portion  35  (upper shroud inner periphery part) and which passes the outer periphery end portion of the lower shroud  41  in the section, as a ratio of the radius to the height h of the air flow path where when the radius is the same as the height, the ratio is 100%. 
     As shown in  FIG. 5 , the larger the arc size is, a noise value is monotonically decreased. Compared to cases where the arc size is 60% and 70%, when the arc size is 80%, the noise value is remarkably reduced. Compared to the case where the arc size is 80%, when the arc size is 90%, the noise value is reduced but an amount of the reduction is relatively small. 
     The maximum static pressure is little changed, irrespective of the arc sizes. The maximum current, which flows through the motor  60  when the centrifugal fan  1  operates, is smallest when the arc size is about 80%, is increased a little when the arc size is 90% and is considerably increased when the arc size becomes 70% or 60%. 
     From the shown relation between the lower shroud  41  and the performance of the centrifugal fan  1 , it can be said that when the arc size is larger than 80%, the noise value can be considerably reduced while making the current flowing through the motor  60  small. That is, in this illustrative embodiment, the curved surface  49  of the lower shroud  41  has the shape as described above, so that the noise value of the centrifugal fan  1  can be made relatively small. 
       FIG. 6  shows an exemplary configuration of a centrifugal fan according to a comparative example. 
     A centrifugal fan  901  shown in  FIG. 6  is a comparative object of the centrifugal fan  1  of this illustrative embodiment. Similarly to the centrifugal fan  1 , the centrifugal fan  901  includes an upper casing  911 , a lower casing  921 , an impeller  930 , a motor  960  and the like. The motor  960  has a shaft  961 , a rotor  963  and the like. The impeller  930  includes an upper shroud  931 , a lower shroud  941  and a plurality of blades  951 . A part of each blade  951  facing an inlet opening  933  has a taper shape where it becomes lower downwards as it is closer to a center part of the impeller  930 . 
     Compared to the centrifugal fan  801  shown in  FIG. 18  (related-art centrifugal fan  801 ) or centrifugal fan  901  shown in  FIG. 6  (centrifugal fan  901  of the comparative example), the centrifugal fan  1  of this illustrative embodiment can be driven with lower noise values. That is, the noise values which are generated when the centrifugal fans  1 ,  801 ,  901  are respectively driven at predetermined speed are as follows. 
     That is, the noise value of the related-art centrifugal fan  801  is 58 dBA, for example. 
     The noise value of the centrifugal fan  901  of the comparative example is 55 dBA, for example. 
     On the other hand, the noise value of the centrifugal fan  1  of this illustrative embodiment is 52 dBA, for example. 
     That is, in this illustrative embodiment, the curved surface  49  of the lower shroud  41  has the shape as described above, so that the noise value of the centrifugal fan  1  can be made smaller, compared to the related-art centrifugal fan or centrifugal fan of the comparative example. 
     Meanwhile, in this illustrative embodiment, the following effects can be obtained, in addition to the effects obtained by improving the air flow path, as described above. 
     That is, in the structure of the related-art technique shown in  FIG. 18 , the lower shroud of the impeller extends to the outer periphery part. Thus, when it is intended to integrally configure the impeller, a complicated mold such as slide-type is required and the productivity of the impeller is remarkably reduced. Compared to this related-art structure, in this illustrative embodiment, the impeller  30  is configured such that at least a part of the blade  51  is exposed to the partition wall part  29  and faces the partition wall part  29 . Hence, a configuration of the mold for integrally forming the impeller  30  can be simplified, so that the productivity of the impeller  30  can be improved. 
     Also, in the structure of the related-art technique, a part of the casing positioned below the impeller is made of resin. Thus, the rigidity of the centrifugal fan is low and the vibration or noise is relatively large. Compared to this related-art structure, in this illustrative embodiment, since the lower casing  21  is configured using the metal plate, the rigidity of the centrifugal fan  1  can be increased, compared to the case where the resin is used. Thus, the vibration or noise of the centrifugal fan  1  can be reduced. Further, since the number of resin components having relatively complex shapes is reduced, the manufacturing cost of the centrifugal fan  1  can be reduced. 
     The casing  10  which accommodates therein the impeller  30  is not provided with a sidewall, except for the pillar parts interposed between the upper casing  11  and the lower casing  21 . That is, since the side parts of the casing  10  are opened and the openings become the outlet openings  19 , the air discharged in the radially outward direction of the impeller  30  is not scattered by the sidewall of the casing  10 . Thus, the noise can be considerably reduced, which is caused due to the scattering of the air upon the blowing. Also, since the casing  10  does not have the sidewall and has the substantially same size as the outer diameter size of the impeller  30 , the size of the centrifugal fan  1  can be reduced. 
     Here, the lower casing  21  of the centrifugal fan  1  has following features. 
       FIG. 7  is a perspective view of the lower casing  21 , which is seen from the lower side.  FIG. 8  is a side sectional view of the centrifugal fan  1  along a line different from  FIG. 2 . 
     As shown in  FIG. 7 , in this illustrative embodiment, the lower casing  21  is formed with an aperture part  25 . The aperture part  25  is formed at a side of the part of the recess part  23 , to which the bearing holder  66  is attached. The aperture part  25  is an aperture having a substantially rectangular shape, when seen from the plan view. The lower casing  21  is formed at four corners with screw holes  14   b  in which the screws  14  are provided. 
     As shown in  FIG. 8 , a connector  71  is attached to the aperture part  25 . The connector  71  is used to feed power to the motor  60  and is connected to the motor  60  at the inner side of the recess part  23 . A user can easily put the centrifugal fan  1  into a drivable state by connecting a cable for power feeding to the connector  71 . 
     Also, a lower surface of the partition wall part  29  of the lower casing  21  is subject to waffle flattening processing, so that a plurality of small recess portions is formed. Thereby, the lower surface of the partition wall part  29  is formed with small concavity and convexity. That is, the waffle flattening processing is performed on the partition wall part  29 , so that the rigidity of the partition wall part  29  is improved, the distortion is corrected and the flatness is thus increased. The waffle flattening processing may be performed on the upper and lower surfaces of the partition wall part  29  and may be performed on a part other than the partition wall part  29 . The waffle flattening processing is preferably performed on at least the partition wall part  29  of the lower casing  21 . 
     Here, the waffle flattening processing is performed on a part of the lower casing  21 , so that the vibration or noise can be made relatively small, which is generated upon the driving of the centrifugal fan  1 . When a thickness of the metal plate configuring the lower casing  21  is made to be thin, a degree of the generation of the vibration or noise is generally increased. However, in this illustrative embodiment, the waffle flattening processing is performed, so that the generation of the vibration or noise can be suppressed while using the thinner metal plate. Thereby, a weight of the centrifugal fan  1  can be reduced. Also, the manufacturing cost of the centrifugal fan  1  can be reduced. 
       FIG. 9  is a graph showing vibration occurrence situations when using the lower casing  21  which is made of a metal plate having a thickness of 1.6 mm, and to which the waffle flattening processing is performed.  FIG. 10  is a graph showing vibration occurrence situations when using the lower casing  21  which is made of a metal plate having a thickness of 1.6 mm, and to which the waffle flattening processing is not performed.  FIG. 11  is a graph showing vibration occurrence situations when using the lower casing  21  which is made of a metal plate having a thickness of 1.2 mm, and to which the waffle flattening processing is performed.  FIG. 12  is a graph showing vibration occurrence situations when using the lower casing  21  which is made of a metal plate having a thickness of 1.2 mm, and to which the waffle flattening processing is not performed. 
       FIGS. 9 to 12  show measurement results which are obtained on the assumption that the centrifugal fan  1  is configured by the same members, except for the lower casing  21 . The waffle flattening processing (which may be simply referred to as the processing) is performed with a pitch of 2 mm and a depth of 0.5 mm on the lower surface of the lower casing  21 . 
     As can be seen from the comparison of  FIGS. 9 and 10 , when the processing is performed, a magnitude of the vibration which is generated around a frequency of 120 Hz is remarkably reduced, compared to the case where the processing is not performed. As can be seen from the comparison of  FIGS. 12 and 10 , when the thickness of the lower casing  21  is made to be thinner (1.2 mm), the generation of the vibration is increased if the processing is not performed. However, as can be seen from the comparison of  FIGS. 10 and 11 , even though the thickness of the lower casing  21  is made to be thinner, the generation of the vibration can be considerably reduced if the processing is performed, compared to the case where the lower casing is made of the thicker metal plate but the processing is not performed. When the processing is performed, the vibration is a little increased even though the lower casing  21  is made to be thinner. However, compared to the case where the processing is not performed, the vibration can be considerably reduced. Hence, the generation of the vibration of the centrifugal fan  1  can be suppressed while configuring the lower casing  21  with the relatively thin metal plate. 
     Modified Examples 
     In the meantime, the shape of the rotor is not limited to the above shape. 
       FIG. 13  is a side sectional view showing a configuration of a centrifugal fan  101  according to a modified example of the above illustrative embodiment. 
     As shown in  FIG. 13 , the centrifugal fan  101  is different from the centrifugal fan  1  of the above illustrative embodiment, in that the centrifugal fan  101  has a motor  160  having a rotor  163  having a different shape. The other configurations are similar in the centrifugal fan  101  and the centrifugal fan  1 . 
     In the rotor  163 , a length of the side part in the upper-lower direction, which is attached to the lower shroud  41 , is the same as that of the rotor  63 . However, the rotor  163  has a cone shape on an upper surface, which is upwards higher as it is closer to the center part thereof. According to this structure, the motor  160  has a shaft  161  which is slightly longer than the shaft  61 . 
     The upper part of the rotor  163  is formed into the cone shape, so that air guiding effect can be obtained for the air taken from the inlet opening  33 . The air taken into the impeller  30  is guided to a side flow path between the upper shroud  31  and the lower shroud  41  as it is closer to the rotor  163 . Therefore, the air can be caused to smoothly flow. 
     In the meantime, the above guiding effect may be obtained without forming the upper part of the rotor  163  into the cone shape. For example, a cone-shaped cover may be attached to the rotor  63  having a flat upper part, so that the rotor  63  and the cover form a shape, like the shape of the rotor  163  of this modified example. 
       FIG. 14  is a side sectional view showing a configuration of a centrifugal fan  201  according to another modified example of the above illustrative embodiment. 
     As shown in  FIG. 14 , the centrifugal fan  201  is different from the centrifugal fan  1  of the above illustrative embodiment, in that the centrifugal fan  201  has an impeller  230  having a different shape. The other configurations are similar in the centrifugal fan  201  and the centrifugal fan  1 . 
     The impeller  230  has an upper shroud  231 , a lower shroud  41  and the blades  51 . A shape of an upper part of the upper shroud  231  is different from that of the upper shroud  31  of the above illustrative embodiment. In this modified example, an upper end portion of the upper shroud  231  is positioned above the upper end portion of the opening  13  of the upper casing  11 . That is, in the centrifugal fan  201 , the position of an upper end portion  235  of the inlet opening  33  in the upper-lower direction is higher than the opening  13 . 
       FIG. 15  is an explanatory view of a shape of the impeller  230  of the centrifugal fan  201 . 
     In  FIG. 15 , the impeller  230  is partially enlarged, like  FIG. 4 . Also in this modified example, the curved surface  49  is formed into an arc shape which has a center on a line L 2  passing the upper end portion  235  of the inner side of the inlet opening  33  and parallel to a rotary shaft of the impeller  230  in a section passing the rotary shaft of the impeller  230 . In the section, the curved surface  49  is provided at a more outer side than the circle C 2 . In other words, the rotor  63  and the lower shroud  41  including the cylindrical part  43  are provided at the more outer side than the circle C 2  in the section. Here, also in this modified example, the circle C 2  is a circle which has a center on the line L 2  and the radius R 2  of 80% of the radius R of the circle C 1 . The circle C 1  is a circle which has the upper end portion  235  as the center A 1  and passes the outer periphery end portion  45  which is positioned in the vicinity of a part of the surface of the lower shroud  41  vertically below the upper end portion  235 . 
     The impeller  230  is configured as described above, so that the air flow path from the inlet opening  33  to the outlet opening between the upper shroud  231  and the lower shroud  41  is configured to cause the air to easily flow. Thus, the generation of the noise in the centrifugal fan  201  can be suppressed. 
     In the meantime, the position of the upper end portion  235  of the inlet opening  33  in the upper-lower direction may be the same as the upper end portion of the opening  13 . 
       FIG. 16  is a perspective view of a lower casing  421  of a centrifugal fan according to a further modified example of the above illustrative embodiment, which is seen from the lower side. 
     The lower casing may not be provided with an aperture part for attaching a connector. That is, as shown in  FIG. 16 , the lower casing  421  is not formed with the aperture part  25 , unlike the lower casing  21 . The lower casing  421  is formed with a wiring hole  425  at the recess part  23 , for example. The wiring hole  425  is provided so as to pull out a lead wire  471 , which is connected to the driving circuit of the motor  60 , from the inside of the centrifugal fan to the outside. The other structures of the lower casing  421  are the same as those of the lower casing  21 . 
     [Others] 
     The shape of the casing is not limited to the substantial square shape, when seen from the plan view. The casing may have an arbitrary shape including polygonal, circular and asymmetrical shapes. The fastening parts of the upper and lower casings are not limited to the inner sides of the four corners of the upper casing, when seen from the plan view. For example, screws or pillars for fastening the upper and lower casings may be provided at parts that are juncturally provided to the upper casing so that they protrude outwards from an outer peripheral edge forming a substantial square shape, when seen from the plan view of the upper casing. 
     In the meantime, when the pillars are provided between the upper casing and the lower casing at the fastening parts of the upper and lower casings, the pillars are preferably shaped as follows. That is, the pillar has preferably a substantially cylindrical shape having a size enabling a screw for fastening the upper and lower casings to pass therethrough. By using the pillars having the shape, the air discharged from the impeller is discharged outwards from the lateral sides of the casing while the resistance is little applied thereto. As a result, the noise of the centrifugal fan can be reduced. 
     The lower casing may be formed of a material other than the metal plate, such as resin material. The upper and lower casings may be integrally formed. 
     It should be considered that the above illustrative embodiments are just exemplary and are not limitative. The scope of the present invention is defined by the claims, not by the above descriptions, and includes all changes in the meaning and scope equivalent to the claims.