Patent Publication Number: US-2020291954-A1

Title: Centrifugal Pump

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Japanese patent application serial number 2019-048250, filed Mar. 15, 2019, which is hereby incorporated herein by reference in its entirety for all purposes. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND 
     This disclosure relates generally to centrifugal pumps. 
     One type of conventional centrifugal pump includes a housing defining a pump chamber therein and an impeller, which is housed in the pump chamber and has a plurality of blades. When the centrifugal pump is running, a fluid is suctioned into the pump chamber via an inlet port and supplied toward a central portion of the impeller. Most of the fluid flows from a space just above the central portion of the impeller into spaces between the blades through openings, each of which is defined by radially inner ends of circumferentially adjacent pairs of the blades. Then, the fluid is forced by the blades from the pump chamber to the outside via an outlet port. 
     BRIEF SUMMARY 
     In one aspect of this disclosure, a centrifugal pump includes a housing and an impeller configured to be rotated about a rotational axis in a rotational direction. The housing defines a discharge passage and a suction passage therein. The impeller is housed in the housing and is coaxially aligned with the suction passage. The impeller includes a main plate, a plurality of first blades extending across the main plate, and a plurality of second blades extending across the main plate. The main plate has a circular shape and a top surface facing the suction passage. The first blades extend radially along the top surface of the main plate. The second blades extend radially along the top surface of the main plate. The radial length of each first blade is equal to the radial length of each second blade. Each second blade has a low blade part and a high blade part extending radially outward from a radially outer end of the low blade part. The low blade part of each second blade has a height measured axially from the main plate that is less than a height of each first blade measured axially from the main plate when comparing the heights of the first blades and the heights of the low blade parts of the second blades at an equal radial distance from the rotational axis. The high blade part of each second blade has a height measured axially from the main plate that is the same as the height of each first blade when comparing the heights of the first blades and the heights of the high blade parts of the second blades at an equal radial distance from the rotational axis. 
     In accordance with this aspect, a difference between the amount of the fluid forced by each first blade and the amount of the fluid forced by each second blade can be reduced, while ensuring a relatively large area of each opening of the impeller. Accordingly, the pump efficiency of the centrifugal pump can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a detailed description of the preferred embodiments of the present teaching, reference will now be made to the accompanying drawings. 
         FIG. 1  is a cross-sectional view of a first embodiment of a centrifugal pump in accordance with the principles described herein. 
         FIG. 2  is a perspective view of the impeller of the centrifugal pump of  FIG. 1 . 
         FIG. 3  is a plan view of the impeller of the centrifugal pump of  FIG. 1 . 
         FIG. 4  is a cross-sectional view of the impeller of  FIG. 3  taken along line IV-IV in  FIG. 3 . 
         FIG. 5  is a cross-sectional view of the impeller of  FIG. 3  taken along line V-V in  FIG. 3 . 
         FIG. 6  is a plan view of another embodiment of an impeller in accordance with the principles described herein. 
         FIG. 7  is a perspective view of a third embodiment of an impeller in accordance with the principles described herein. 
         FIG. 8  is a plan view of the impeller of  FIG. 7 . 
         FIG. 9  is a cross-sectional view of the impeller of  FIG. 8  taken along line IX-IX in  FIG. 8 . 
         FIG. 10  is a cross-sectional view of the impeller of  FIG. 8  taken along line X-X in  FIG. 8 . 
         FIG. 11  is a perspective view of a fourth embodiment of an impeller in accordance with the principles described herein. 
         FIG. 12  is a plan view of the impeller of  FIG. 11 . 
         FIG. 13  is a cross-sectional view of the impeller of  FIG. 12  taken along line in  FIG. 12 . 
         FIG. 14  is a cross-sectional view of the impeller of  FIG. 12  taken along line XIV-XIV in  FIG. 12 . 
         FIG. 15  is a side view of a second blade of the impeller of  FIG. 12  viewed along line XV in  FIG. 12 . 
         FIG. 16  is a side view of a first blade of the impeller of  FIG. 12  viewed along line XVI in  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION 
     In general, the shape of the blades of a centrifugal pump affects the pumping efficiency. Consequently, various types of blades have been provided for the purpose of improving the pump efficiencies of centrifugal pumps. For example, in Japanese Laid-Open Patent Publication No. H11-218097, an impeller includes a plurality of first blades and a plurality of second blades that are shorter than the first blades. The first blades extend radially from a central portion of the impeller to a radially outer periphery of the impeller. The second blades extend radially from a region spaced from the central portion of the impeller to a position proximal the radially outer periphery of the impeller. In such centrifugal pumps, the second blades do not extend to or proximal the central portion of the impeller. As a result, an opening is defined between the radially inner ends of each pair of circumferentially adjacent first blades. The area of each such opening can be increased to improve the pump efficiency. However, because the second blades are shorter than the first blades, the volume of a fluid forced by each second blade is generally less than the volume of the fluid forced by each first blade. Therefore, there has been a need for an improved centrifugal pump. 
     A centrifugal pump generally includes an impeller and a housing forming a pump chamber for housing the impeller therein. The impeller has a main plate with a substantially circular shape and a plurality of blades extending radially along a top surface of the main plate from proximal a center of the impeller to a radially outer periphery of the impeller. When the centrifugal pump is operated, the impeller is rotated about a rotational axis such in a rotational direction such that a fluid is forced forward by the blades of the impeller relative to the rotational direction in the pump chamber so as to flow radially outward. As a result, the centrifugal pump suctions the fluid into the pump chamber and discharges the fluid from the pump chamber. 
     One conventional method for improving the pump efficiency of a centrifugal pump is to increase the number of blades. However, when too many conventional blades are provided on the main plate of the impeller, a space near the central portion of the impeller is crowded with radially inner ends of the blades. Thus, an area of each opening, through which the fluid flows from a space just above the central portion of the impeller into spaces between the blades, is relatively small. In such state, the flow of fluid from the space just above the central portion of the impeller into the spaces between the blades is limited, so that the pump efficiency cannot be improved effectively. Each opening is defined by the radially inner ends of each pair of circumferentially adjacent blades and the top surface of the main plate near the center of the impeller. In this disclosure, the area of the opening may also be referred to as “opening area.” 
     To improve the pump efficiency of the centrifugal pump, embodiments described herein are directed to impellers having two kinds of blades for simultaneously increasing both the opening area and the number of the blades on the main plate. More specifically, as shown in  FIG. 6 , the impeller includes a plurality of long blades and a plurality of short blades. The long blades are longer than the short blades in a plan view of the impeller. The long blades and the short blades extend radially inward from the outer circumferential periphery of the impeller and are alternately arranged in a circumferential direction of the impeller. A radial distance between the central, rotational axis of the impeller and a radially inner end of each long blade is less than a radial distance between the central, rotational axis of the impeller and a radially inner end of each short blade in the plan view of the impeller. In this case, the radially inner ends of the short blades do not extend radially to a central portion of the impeller, so that each opening at the central portion of the impeller is defined by the radially inner ends of each pair of circumferentially adjacent long blades. Thus, the opening area of each opening can be increased in comparison with a conventional impeller with the same total number of blades, but with each blade extending radially from or proximal the central portion of the impeller to the outer periphery of the impeller. 
     As described above, regarding the example of the impeller shown in  FIG. 6 , the radial distance between the rotational axis and the radially inner end of each short blade is greater than the radial distance between the rotational axis and the radially inner end of each long blade. Thus, in a space within a predetermined radial distance from the rotational axis of the impeller in the plan view, the fluid is forced by the long blades only, and is not forced by the short blades. Accordingly, the amount of the fluid forced by the long blades is significantly greater than that of the short blades. Therefore, there has been a need for an improved centrifugal pump. 
     The following discussion is directed to various exemplary embodiments. However, one skilled in the art will understand that the examples disclosed herein have broad application, and that the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to suggest that the scope of the disclosure, including the claims, is limited to that embodiment. 
     A first embodiment will be described with reference to  FIGS. 1 to 5 . In the first embodiment, a centrifugal pump  10  is used as a purge pump mounted on a vehicle, such as an automobile, for supplying a deficiency of a purge flow from a canister to an intake passage of an internal combustion engine. In each drawing, X direction shows the rightward direction, Y direction shows the forward direction, and Z direction shows the upward direction, such that X direction, Y direction, and Z direction are perpendicular to each other. However, these directions do not limit the orientation or the mounting direction of the centrifugal pump  10  on the vehicle. 
     As shown in  FIG. 1 , the centrifugal pump  10  includes a motor  12 , an impeller  140  configured to be rotated by the motor  12 , and a housing  16  that houses both the motor  12  and the impeller  140  therein. The motor  12  may be a brushless motor. The motor  12  includes a rotor  18 , a shaft  20 , and a stator  22 . The rotor  18  has a hollow cylindrical shape. The shaft  20  has a lower end that is coaxially inserted into the rotor  18 . The stator  22  surrounds an outer circumference of the rotor  18 . The rotor  18  includes a plurality of permanent magnets, such that the rotor  18  shows magnetic poles aligned in a circumferential direction relative to a central axis of the shaft  20 . The stator  22  includes a plurality of coils surrounding the outer circumference of the rotor  18  by a predetermined distance. 
     The shaft  20  has an upper end coaxially inserted into a central hole  142  of the impeller  140 , such that the shaft  20  is configured to transmit torque, generated between the rotor  18  and the stator  22 , to the impeller  140 . The housing  16  includes a first housing part  24  and a second housing part  26 . Each of the first housing part  24  and the second housing part  26  may be made from a resin material. The first housing part  24  and the second housing part  26  are coupled to each other to define a pump chamber  28 . The impeller  140  is housed in the pump chamber  28 , such that the impeller  140  can rotate in the pump chamber  28  without coming into contact with an inward facing surface of the housing  16 . The first housing part  24  has a suction part  30  having a hollow cylindrical shape extending upward. The suction part  30  defines a suction passage  32  therein. The suction part  30  includes an inlet port  34  at an upstream end of the suction passage  32 , i.e. at an opposite end to the pump chamber  28 . The suction passage  32  provides fluid communication between the pump chamber  28  and the exterior of the centrifugal pump  10  via the inlet port  34 . The first housing part  24  has a discharge part  36  extending in a tangential direction from an outer periphery of the impeller  140  (rightward in  FIG. 1 ). The discharge part  36  defines a discharge passage  38  therein. The discharge part  36  includes an outlet port  40  at a downstream end of the discharge passage  38 , i.e. at an opposite end to the pump chamber  28 . The discharge passage  38  provides fluid communication between the pump chamber  28  and the exterior of the centrifugal pump  10  via the outlet port  40 . 
     The centrifugal pump  10  includes bearings  42 ,  44 . Each of the bearings  42 ,  44  is composed of a ball bearing having both an outer ring and an inner ring, such that the outer ring is fixedly inserted into the second housing part  26  by press-fitting and that the inner ring is fixed on the shaft  20 . Due to this configuration, the bearings  42 ,  44  support the shaft  20  while allowing the shaft  20  to rotate. 
     The second housing part  26  houses a control unit  46  at a lower end part thereof. The control unit  46  is coupled to a connector configured to be connected to an external power source, such as a battery mounted on the vehicle. The control unit  46  is configured to receive electric power from the external power source and to supply it to the stator  22 . 
     Next, the impeller  140  will be described in detail. As shown in  FIG. 3 , the impeller  140  is configured to rotate about a central, rotational axis C in a rotational direction R that is clockwise in a plan view of the impeller  140 . As shown in  FIGS. 1 to 3 , the impeller  140  includes a main plate  144  having a substantially circular shape, a plurality of first blades  146  extending from the main plate  144 , and a plurality of second blades  148  extending from the main plate  144 . The main plate  144  has a top surface facing the suction passage  32 . The first blades  146  and the second blades  148  are formed on the top surface of the main plate  144 . As shown in  FIGS. 2 to 5 , the main plate  144  has a projection part  152  protruding upward at a central portion thereof and a flat part  150  extending radially outward from an outer periphery of the projection part  152 , such that the outer periphery of the projection part  152  is contiguous with a radially inner periphery of the flat part  150 . The flat part  150  is oriented perpendicular to the rotational axis C of the impeller  140 . As shown in  FIGS. 4 and 5 , the projection part  152  has an inclined surface  154  continuously increasing the height thereof moving radially inward from the flat part  150 . 
     The first blades  146  and the second blades  148  extend radially along the top surface of the impeller  140  on both the projection part  152  and the flat part  150 . As shown in  FIG. 3 , in the plan view along the rotational axis C of the impeller  140 , each of the first blades  146  has substantially the same shape and the same radial length as each of the second blades  148 . In this disclosure, a radial length of a blade is the length of the blade in a radial direction and may be calculated by subtracting the radius of the central hole  142  from the radial distance between the rotational axis C and the radially outer end of the blade in the plan view. The first blades  146  and the second blades  148  are alternately arranged at regular intervals in the circumferential direction. The first blades  146  and the second blades  148  protrude perpendicularly upward from the top surface of the main plate  144 . 
     As shown in  FIG. 3 , in the plan view of the impeller  140 , each of the blades  146 ,  148  is gently curved, such that the radially inner end thereof is positioned forward of a virtual line passing through the rotational axis C and the radially outer end of the corresponding blade relative to the rotational direction R of the impeller  140 . 
     The second blades  148  have the same shape as each other, so that one of the second blades  148  will be described for convenience of explanation. As shown in  FIG. 5 , the second blade  148  has a radially inner low blade part  156  and a radially outer high blade part  158 . The low blade part  156  is positioned proximal the central hole  142  of the main plate  144 . Regarding the height from the main plate  144 , the low blade part  156  of the second blade  148  is less than the first blades  146  when comparing them to each other at any given radial distance from the rotational axis C. The high blade part  158  is positioned radially outward of the low blade part  156 . Regarding the height from the main plate  144 , the high blade part  158  of the second blade  148  is equal to the first blades  146  at any given radial distance from the rotational axis C. In this embodiment, the low blade part  156  is formed on the projection part  152  only, whereas the high blade part  158  extends along both the projection part  152  and the flat part  150 . Thus, as shown in  FIG. 5 , a boundary, which is identified with a boundary line B, between the low blade part  156  and the high blade part  158  is positioned just above the projection part  152 . The height of the low blade part  156  continuously increases moving radially toward the boundary line B. In this disclosure, the height of each blade from the main plate is the distance measured from the main plate to an upper end point of the blade in a direction along a virtual line, passing the upper end point and being normal to the top surface of the main plate, in a cross-sectional view of the blade along a plane parallel to the rotational axis C. 
     Next, an operation of the centrifugal pump  10  will be described with reference to  FIG. 1 . When the control unit  46  supplies electric power to the stator  22 , the stator  22  produces a magnetic field. The rotor  18  is rotated by the magnetic field, such that the shaft  20 , the inner rings of the bearings  42 ,  44 , and the impeller  140  are integrally rotated with the rotor  18  about rotational axis C in rotational direction R. Due to rotation of the impeller  140 , a fluid, i.e. purge gas is suctioned into the suction passage  32  via the inlet port  34  in a suction direction Y 1  and flows through the suction passage  32  toward the central portion of the impeller  140 . Then, the fluid is forced by the first blades  146  and the second blades  148  in the rotational direction R of the impeller  140  while flowing radially outward along the top surface of the main plate  144 . As a result, the fluid is pressurized and is discharged from the outlet port  40  via the discharge passage  38  in a discharge direction Y 2 . 
     In accordance with the first embodiment, regarding the height from the main plate  144 , the low blade part  156  of each second blade  148  is less than each first blade  146  when comparing the first blades  146  and the second blades  148  to each other at an equal distance from the rotational axis C. Thus, an opening area of each opening of the impeller  140  can be increased in comparison with a case where the height of each second blade  148  is the same as that of each first blade  146  over the entire radial length thereof. Further, in a space near the central portion of the impeller  140 , most of the fluid flows radially outward due to inclination of the inclined surface  154 , so that the amount of the fluid moved by the low blade part  156  of each second blade  148  is substantially the same as, and in particular, slightly less than the amount of the fluid moved by each first blade  146 . Thus, the amount of the fluid forced by the second blades  148  can be increased, thereby decreasing a difference between the amount of the fluid forced by the first blades  146  and the amount of the fluid forced by the second blades  148 . Such difference may cause pulsations in the fluid flow. Accordingly, the pump efficiency of the centrifugal pump  10  can be improved, while preventing the pulsations in the fluid flow. 
     The height of the low blade part  156  of each second blade  148  from the main plate  144  continuously increases moving radially outward from the radially inner end to the radially outer end thereof. Due to this configuration, the impeller  140  can be easily produced by using molds. 
     The main plate  144  includes the projection part  152  and the flat part  150  extending radially outward from the projection part  152 . In a radially inner space positioned above the projection part  152 , the fluid is not pressurized sufficiently by the first blades  146  and the second blades  148 , and thus, the fluid flows along the inclined surface  154 . In a radially outer space positioned above the flat part  150 , the fluid is forced and sufficiently pressurized by the first blades  146  and the second blades  148  to be moved forward relative to the rotational direction R and radially outward. Accordingly, each second blade  148 , having the low blade part  156  on the projection part  152  and the high blade part  158  extending over the entire radial length of the flat part  150 , can sufficiently force the fluid to flow above the flat part  150 . 
     The first blades  146  and the second blades  148  are alternately arranged at regular intervals in the circumferential direction of the impeller  140 . Thus, the opening area of each opening of the impeller  140  can be held substantially constant in the circumferential direction of the impeller  140 . Further, the difference between the amounts of the fluid forced by each of the first blades  146  and the second blades  148  can be decreased. 
     A second embodiment will be described with reference to  FIGS. 7 to 10 . The second embodiment is substantially the same as the first embodiment described above, with some differences regarding the shape of an impeller  240 . Thus, while the differences will be described, similar configurations will be described shortly or will not be described in the interest of conciseness. 
     As shown in  FIG. 7 , the impeller  240  includes a main plate  244  having a substantially circular shape, a plurality of first blades  246  extending along the main plate  244 , and a plurality of second blades  248  extending along the main plate  244 . The main plate  244  has a central hole  242  at a central portion thereof and a top surface facing upward. The first blades  246  and the second blades  248  are formed on the top surface of the impeller  240 . The main plate  244  has a projection part  252  protruding upward at a central portion of the main plate  244 , and a flat part  250  extending radially outward from a radially outer periphery of the projection part  252 . As shown in  FIGS. 9 and 10 , the projection part  252  has an inclined surface  254  that continuously increasing in axial height moving radially inward. 
     As shown in  FIG. 7 , the first blades  246  and the second blades  248  extend radially along the projection part  252  and the flat part  250 . More specifically, each of the first blades  246  and the second blades  248  extends radially from a radially inner periphery of the inclined surface  254  of the projection part  252  to a radially outer periphery of the flat part  250 . The radial length of each first blade  246  is equal to that of each second blade  248 . The first blades  246  and the second blades  248  are alternately arranged on the main plate  244  at regular intervals in the circumferential direction of the impeller  240 . The first blades  246  and the second blades  248  are oriented perpendicular to the main plate  244  (parallel to the rotational axis C). 
     As shown in  FIG. 8 , in a plan view along the rotational axis C of the impeller  240 , each of the first blades  246  and the second blades  248  is positioned along a virtual line extending radially through the rotational axis C. Thus, an inlet angle of each of the first blades  246  and the second blades  248  is 90 degrees. An outlet angle of each of the first blades  246  and the second blades  248  is also 90 degrees. 
     The first blades  246  have the same shape as each other, so that one of the first blades  246  will be described in the interest of conciseness. The second blades  248  also have the same shape as each other, so that one of the second blades  248  will be described for convenience of explanation. As shown in  FIG. 8 , the first blade  246  has a radially inner thin blade part  260  and a radially outer thick blade part  262 . The thin blade part  260  extends radially outward from a radially inner periphery of the projection part  252 . The thin blade part  260  of the first blade  246  is thinner than the second blade  248  when comparing them to each other at an equal distance from the rotational axis C. The thick blade part  262  extends radially outward from a radially outer end of the thin blade part  260 . The thickness of the thick blade part  262  is equal to that of the second blade  248  when comparing them to each other at an equal distance from the rotational axis C. The thin blade part  260  is formed on the projection part  252  of the main plate  244  only, whereas the thick blade part  262  is formed on both the projection part  252  and the flat part  250 . Thus, a boundary between the thin blade part  260  and the thick blade part  262  is positioned just above the projection part  252 . As shown in  FIG. 8 , the thickness of the second blade  248  is constant over the whole radial length thereof. The thickness of the second blade  248  may be 1 mm. The thickness of the thin blade part  260  of the first blade  246  is preferably half of the thickness of the thick blade part  262 . The thickness of the thin blade part  260  may be 0.5 mm. In this disclosure, the thickness of each blade refers to a dimension of the blade in the front-rear direction relative to the rotational direction R (i.e., the circumferential direction). 
     As shown in  FIGS. 7 and 10 , the second blade  248  has a radially inner low blade part  256  and a radially outer high blade part  258 . The low blade part  256  extends radially outward from the radially inner periphery of the inclined surface  254 . The high blade part  258  extends radially outward from a radially outer end of the low blade part  256 . As shown in  FIGS. 9 and 10 , the height of the low blade part  256  from the main plate  244  is less than that of the first blade  246  when comparing them to each other at an equal distance from the rotational axis C. The height of the high blade part  258  from the main plate  244  is equal to that of the first blade  246  when comparing them to each other at an equal distance from the rotational axis C. As shown in  FIG. 10 , the low blade part  256  is formed on the projection part  252  of the main plate  244  only, whereas the high blade part  258  is formed on both the projection part  252  and the flat part  250 . Thus, a boundary, which is identified by a boundary line B, between the low blade part  256  and the high blade part  258  is positioned just above the projection part  252 . The height of the second blade  248  from the main plate  244  increases at the boundary line B between the low blade part  256  and the high blade part  258  in a stepped manner moving radially outward. More specifically, the height of the second blade  248  from the main plate  244  drastically increases at the boundary line B. The stepped shape may have a projecting part and a recessed part, each forming an angle 85 to 95 degrees in a cross-sectional view of the second blade  248 , taken along a plane including both the rotational axis C and a longitudinal axis of the second blade  248 . 
     In accordance with the second embodiment, the first blades  246  and the second blades  248  have the same radial length as each other. Each of the second blades  248  has the low blade part  256  and the high blade part  258 . Each low blade part  256  is formed proximal the central portion of the impeller  240 . The height of each low blade part  256  from the main plate  244  is less than that of each first blade  246  when comparing the first blades  246  and the second blades  248  to each other at an equal distance from the rotational axis C. Each high blade part  258  extends radially outward from the radially outer end of the corresponding low blade part  256 . The height of each high blade part  258  from the main plate  244  is equal to that of each first blade  246  when comparing them to each other at an equal distance from the rotational axis C. Due to this configuration, the difference between the amount of the fluid forced by the first blades  246  and the amount of the fluid forced by the second blades  248  can be reduced, while increasing an opening area of each opening of the impeller  240 . Accordingly, the pump efficiency of the centrifugal pump  10  can be improved. 
     The height of each second blade  248  from the main plate  244  increases at the boundary line B between the low blade part  256  and the high blade part  258  in the stepped manner. Thus, the height of each low blade part  256  can be sufficiently lowered over the whole radial length thereof. 
     Each second blade  248  includes the low blade part  256  formed on the projection part  252  only and the high blade part  258  extending radially over the whole radial length of the flat part  250 . Thus, each second blade  248  has a sufficient height to pressurize and force the fluid to flow radially outward on the flat part  250 . 
     Each first blade  246  has the thin blade part  260  and the thick blade part  262 . The thin blade part  260  of each first blade  246  is thinner than each second blade  248  when comparing them to each other at an equal distance from the rotational axis C. The thickness of each thick blade part  262  is equal to that of each second blade  248  when comparing them to each other at an equal distance from the rotational axis C. Thus, the opening area of each opening of the impeller  240  can be increased in comparison with a case where each first blade  246  has the thickness same as the thick blade part  262  over the whole radial length thereof. 
     Each first blade  246  has the thin blade part  260  and the thick blade part  262 , which extends radially outward from the radial outer end of the thin blade part  260  and is thicker than the thin blade part  260 . Thus, the strength of each first blade  246  can be increased in comparison with a case where each first blade  246  has the thickness same as the thin blade part  260  over the whole radial length thereof. 
     The first blades  246  and the second blades  248  are alternately arranged in the circumferential direction of the impeller  240 . Thus, the opening area of each opening of the impeller  240  can be held to be substantially constant in the circumferential direction of the impeller  240 . Further, the difference between the amounts of the fluid forced by each of the first blades  246  and the second blades  248  can be decreased. 
     A third embodiment will be described with reference to  FIGS. 11 to 16 . The third embodiment is substantially the same as the first embodiment described above, with some differences regarding the shape of an impeller  340 . Thus, while the differences will be described, similar configurations will be described shortly or will not be described in the interest of conciseness. 
     As shown in  FIGS. 11 and 12 , the impeller  340  is configured to be rotated about a central, rotational axis C in the rotational direction R that is clockwise direction in a plan view along the rotational axis C of the impeller  340 . The impeller  340  includes a main plate  344  having a substantially circular shape, a plurality of first blades  346  extending along the main plate  344 , and a plurality of second blades  348  extending along the main plate  344 . The main plate  344  has a central hole  342  at a central portion thereof and a top surface facing upward. The first blades  346  and the second blades  348  are formed on the top surface of the main plate  344 . The main plate  344  has a projection part  352  protruding upward and a flat part  350  extending radially outward from a radially outer periphery of the projection part  352 . The projection part  352  has an inclined surface  354  that continuously increases in axial height moving radially inward. 
     Each of the first blades  346  and the second blades  348  extends radially from a radially inner periphery of the inclined surface  354  of the projection part  352  to a radially outer periphery of the flat part  350 . As shown in  FIG. 12 , in the plan view along the rotational axis C of the impeller  340 , the radial length of each first blade  346  is equal to that of each second blade  348 . The first blades  346  and the second blades  348  are alternately arranged at regular intervals in the circumferential direction of the impeller  340 . 
     The first blades  346  have the same shape as each other, and the second blades  348  also have the same shape as each other. Thus, one of the first blades  346  and one of the second blades  348  will be described below in the interest of conciseness. Regarding each of the first blade  346  and the second blade  348 , in the plan view along the rotational axis C, a radially inner end thereof is positioned, relative to the rotational direction R, in front of a virtual line extending radially from the rotational axis C to a radially outer end thereof. In the plan view, an upper edge of each of the first blades  346  and the second blades  348  is gently curved rearward relative to the rotational direction R moving radially outward. 
     As shown in  FIGS. 11 and 12 , the first blade  346  is divided into a first inner blade part  364  and a first outer blade part  366 . The first inner blade part  364  extends radially outward from the radially inner periphery of the inclined surface  354 . The first outer blade part  366  extends radially outward from a radially outer end of the first inner blade part  364  to the radially outer periphery of the impeller  340 . 
     As shown in  FIG. 12 , in the plan view along the rotational axis of the impeller  340 , the first inner blade part  364  is positioned, relative to the rotational direction R, in front of a radial reference line N 1 , extending radially and passing through the rotational axis C and a connection part between the first inner blade part  364  and the first outer blade part  366 . The connection part between the first inner blade part  364  and the first outer blade part  366  corresponds to a radially inner end of the first outer blade part  366 . The first outer blade part  366  has a front surface  366 F facing forward and a rear surface  366 R facing rearward relative to the rotational direction R. In some embodiments, the radial reference line N 1  may pass the front surface  366 F of the first outer blade part  366  at the radially inner end of the first outer blade part  366 . 
     As shown in  FIG. 11 , the first inner blade part  364  has a front surface  364 F facing forward and a rear surface  364 R facing rearward relative to the rotational direction R The front surface  364 F and the rear surface  364 R extend perpendicular to the top surface of the main plate  344 . In some embodiment, an angle formed between the top surface of the main plate  344  and each of the front surface  364 F and the rear surface  364 R may be between 85 to 95 degrees. 
     The front surface  366 F of the first outer blade part  366  extends from the top surface of the main plate  344  obliquely rearward relative to the rotational direction R of the impeller  340 . The front surface  366 F is contiguous with the front surface  364 F of the first inner blade part  364 . As shown in  FIGS. 13 and 14 , the front surface  366 F extends linearly between an upper end and a lower end thereof in a cross-sectional view perpendicular to a longitudinal axis of the first outer blade part  366 . The front surface  366 F may be gently curved in a concave or convex manner in some embodiments. 
     In a cross-sectional view perpendicular to the longitudinal axis of the first outer blade part  366 , an angle θ formed between the front surface  366 F of the first outer blade part  366  and a first vertical reference line L, which extends parallel to the rotational axis C and passes through the upper end of the front surface  366 F, is acute and continuously increases moving radially outward. 
     As shown in  FIG. 12 , in the plan view along the rotational axis C of the impeller  340 , a lower portion of the front surface  366 F of a radially outer end of the first outer blade part  366  is positioned in front of the radial reference line N 1  relative to the rotational direction R. And, an upper portion of the front surface  366 F of the radially outer end of the first outer blade part  366  is positioned rearward of the radial reference line N 1  relative to the rotational direction R. This configuration is illustrated in  FIGS. 13 and 14  more clearly. In  FIGS. 13 and 14 , the lower portion of the front surface  366 F is positioned, relative to the rotational direction R, in front of a second vertical reference line N 1 ′, which extends vertically and perpendicular to the radial reference line N 1 . The upper portion of the front surface  366 F is positioned rearward of the second vertical reference line N 1 ′ relative to the rotational direction R. As shown in  FIG. 11 , a radially outer surface of the first outer blade part  366  is flush with the radially outer periphery of the main plate  344 . 
     As shown in  FIGS. 13 and 14 , the thickness T of the first outer blade part  366  in a front-rear direction relative to the rotational direction R (i.e., circumferential direction) continuously increases from an upper end toward a lower end thereof. A rear surface  366 R of the first outer blade part  366  extends perpendicular to the top surface of the main plate  344 . As shown in  FIG. 11 , the rear surface  366 R of the first outer blade part  366  is contiguous with the rear surface  364 R of the first inner blade part  364 . The thickness of the first inner blade part  364  in the front-rear direction relative to the rotational direction R is constant between an upper end and a lower end thereof. 
     As shown in  FIG. 12 , the second blade  348  is divided into a second inner blade part  368  and a second outer blade part  370 . The second inner blade part  368  extends radially outward from the radially inner periphery of the projection part  352 . The second outer blade part  370  extends radially outward from a radially outer end of the second inner blade part  368  to the radially outer periphery of the main plate  344 . 
     In the plan view along the rotational axis of the impeller  340 , the second inner blade part  368  is positioned, relative to the rotational direction R, in front of a radial reference line N 2 , which extends radially and passes through the rotational axis C and a connection part between the second inner blade part  368  and the second outer blade part  370 . The connection part between the second inner blade part  368  and the second outer blade part  370  corresponds to a radially inner end of the second outer blade part  370 . As shown in  FIG. 11 , the second outer blade part  370  has a front surface  370 F facing forward and a rear surface  370 R facing rearward relative to the rotational direction R. The radial reference line N 2  may pass the front surface  370 F of the second outer blade part  370  at the radially inner end of the second outer blade part  370  in some embodiments. 
     As shown in  FIG. 11 , the second inner blade part  368  has a front surface  368 F facing forward and a rear surface  368 R facing rearward relative to the rotational direction R. The front surface  368 F and the rear surface  368 F extend perpendicular to the top surface of the main plate  344 . An angle formed between the top surface of the main plate  344  and each of the front surface  368 F and the rear surface  368 R may be between 85 to 95 degrees in some embodiments. 
     The front surface  370 F of the second outer blade part  370  extends from the top surface of the main plate  344  obliquely rearward relative to the rotational direction R of the impeller  340 . The front surface  370 F is contiguous with the front surface  368 F of the second inner blade part  368 . The front surface  370 F extends linearly between an upper end and a lower end thereof in a cross-sectional view perpendicular to a longitudinal axis of the second outer blade part  370 . The front surface  370 F may be gently curved in a concave or convex manner in some embodiments. 
     Although not illustrated, in a cross-sectional view perpendicular to the longitudinal axis of the second outer blade part  370 , an angle formed between the front surface  370 F of the second outer blade part  370  and a vertical reference line, which extends parallel to the rotational axis C and passes the upper end of the front surface  370 F, is acute and continuously increases from the radially inside toward the radially outside. 
     As shown in  FIG. 12 , in the plan view along the rotational axis C of the impeller  340 , a lower portion of the front surface  370 F of a radially outer end of the second outer blade part  370  is positioned in front of the radial reference line N 2  relative to the rotational direction R. An upper portion of the front surface  370 F of the radially outer end of the second outer blade part  370  is positioned rearward of the radial reference line N 2  relative to the rotational direction R. As shown in  FIG. 11 , a radially outer surface of the second outer blade part  370  is flush with the radially outer periphery of the main plate  344 . 
     As shown in  FIG. 11 , the thickness of the second outer blade part  370  in the front-rear direction relative to the rotational direction R (i.e., circumferential direction) gradually increases from an upper end toward a lower end thereof. A rear surface  370 R of the second outer blade part  370  extends perpendicular to the top surface of the main plate  344  in a cross-sectional view perpendicular to the longitudinal axis of the second blade  348 . The rear surface  370 R of the second outer blade part  370  is contiguous with the rear surface  368 R of the second inner blade part  368 . The thickness of the second inner blade part  368  in the front-rear direction relative to the rotational direction R (i.e., circumferential direction) is constant between an upper end and a lower end thereof. 
     As shown in  FIGS. 11 and 15 , the second blade  348  has a low blade part  356  and a high blade part  358 . The low blade part  356  extends radially outward from the radially inner periphery of the inclined surface  354 . The high blade part  358  extends radially outward from a radially outer end of the low blade part  356 . As shown in  FIGS. 11, 15 and 16 , the height of the low blade part  356  of the second blade  348  from the main plate  344  is less than that of the first blade  346  when comparing them to each other at an equal distance from the rotational axis C. The height of the high blade part  358  from the main plate  344  is equal to that of the first blade  346  when comparing them to each other at an equal distance from the rotational axis C. The low blade part  356  is formed on the projection part  352  of the main plate  344  only, whereas the high blade part  358  extends on both the projection part  352  and the flat part  350 . Thus, a boundary between the low blade part  356  and the high blade part  358 , which is near a boundary line B in  FIG. 15 , is positioned just above the projection part  352 . The height of the low blade part  356  continuously increases moving radially from the inner end of the low blade part  356  toward the boundary line B between the low blade part  356  and the high blade part  358 . 
     In accordance with the third embodiment, the first blades  346  and the second blades  348  have the same radial length as each other. Each second blade  348  has the low blade part  356  and the high blade part  358 . Each low blade part  356  is formed near the central portion of the impeller  340 . The height of each low blade part  356  from the main plate  344  is less than that of each first blade  346  when comparing them to each other at an equal distance from the rotational axis C. Each high blade part  358  extends radially outward from the radially outer end of the corresponding low blade part  356 . The height of each high blade part  358  from the main plate  344  is equal to that of each first blade  346  when comparing them to each other at an equal distance from the rotational axis C. Due to this configuration, the difference between the amount of the fluid forced by the first blades  346  and the amount of the fluid forced by the second blades  348  can be reduced, while increasing the opening area of each opening of the impeller  340 . Accordingly, the pump efficiency of the centrifugal pump  10  can be improved. 
     The height of the low blade part  356  of each second blade part  348  from the main plate  344  continuously increases moving radially outward. Thus, the impeller  340  can be easily produced by using molds. 
     The low blade part  356  of each second blade  348  is formed on the projection part  352  only, whereas the high blade part  356  of each second blade  348  extends radially over the whole radial length of the flat part  350 . Thus, each second blade  348  has a sufficient height to pressurize and force the fluid to flow radially outward on the flat part  350 . 
     The first blades  346  and the second blades  348  are alternately arranged in the circumferential direction of the impeller  340 . Thus, the opening area of each opening of the impeller  340  can be held to be substantially constant in the circumferential direction of the impeller  340 . Further, the difference between the amounts of the fluid forced by each of the first blades  346  and the second blades  348  can be decreased. 
     As mentioned above, the apparatuses and methods disclosed herein are not limited to the above-described embodiments. For example, the centrifugal pump may be used for pumping various fluids, such as air, water, or the like. The motor may be composed of a brushed motor. The main plate may have additional blades or grooves along a lower surface thereof. The projection part may be omitted, such that the main plate may have a flat top surface having a circular shape. The height of the low blade part may increase toward the radially inside. 
     The thickness of the thin blade part of each second blade may gradually increase toward the radial outside. Each second blade may have a thin blade part and a thick blade part instead of the first blades. 
     The impeller may include a plurality of third blades each having a low blade part and a high blade part. In such case, the height of the low blade part of each third blade from the main plate is lower or higher than that of each second blade when comparing them to each other at an equal distance from the rotational axis of the impeller. 
     The numbers of the first blades, the second blades, and the third blades may be different from each other. However, the first blades, the second blades, and the third blades are preferably arranged on the main plate in the circumferential direction on the basis of a repetitive order pattern. 
     The blades may not be arranged at regular intervals in the circumferential direction. However, the blades are preferably positioned on the main plate on the basis of a predetermined regularity. For example, in a case where the impeller includes first blades, second blades, and third blades repeatedly, a first predetermined circumferential distance between the first blade and the second blade may be greater than a second predetermined circumferential distance between the second blade and the third blade in each repeating unit.