Patent Publication Number: US-2020300250-A1

Title: Centrifugal Pump

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a National Phase entry of, and claims to the benefit of, PCT Application No. PCT/JP2018/042599 filed Nov. 19, 2018, which claims priority to Japanese Patent Application No. 2017-222615 filed Nov. 20, 2017, each of which is incorporated herein by reference in its entirety for all purposes. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND 
     The disclosure generally relates to centrifugal pumps. 
     One type of a centrifugal pump has a pump and a motor (see, e.g. Japanese Laid-Open Patent Publication No. 2017-61919). The pump includes an impeller configured to forcibly transfer a fluid and defines a pump chamber for housing the impeller therein. The motor includes a rotational shaft engaged with the impeller. The centrifugal pump forcibly feeds the fluid, due to the rotation of the impeller caused by the operation of the motor. In Japanese Laid-Open Patent Publication No. 2017-61919, an outward surface of a casing is provided with a plurality of fins, and the rotational shaft of the motor has a cooling fan. Due to this configuration, the casing is cooled by exposing the fins to the air from the cooling fan, thereby suppressing an increase in the temperature of the motor. 
     SUMMARY 
     In one aspect of this disclosure, a centrifugal pump includes a pump and a motor. The pump includes an impeller for forcibly transferring a fluid and forms a pump chamber for housing the impeller therein. The motor includes a rotational shaft and a casing defining a motor chamber that houses the rotational shaft therein. The rotational shaft is hollow and is configured to rotate the impeller. The rotational shaft defines a discharge passage therein and has a first end engaged with the impeller and a second end where one end of the discharge passage opens. The one end of the discharge passage is in fluid communication with a low pressure area in the pump chamber via the other end of the discharge passage and is in fluid communication with a high pressure area in the pump chamber via an introduction passage formed by the casing 
     In accordance with this aspect, embodiments described herein offer the potential to improve the cooling performance of the motor of the centrifugal pump, thereby suppressing deterioration and reduction in strength of the components of the motor. 
    
    
     
       BRIEF DESCRIPTION OF THE 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 an enlarged partial cross-sectional view of the centrifugal pump of  FIG. 1  schematically illustrating a fluid flow in a principal part thereof. 
         FIG. 3  is an enlarged partial cross-sectional view of a principal part of a second embodiment of a centrifugal pump in accordance with the principles described herein. 
         FIG. 4  is an enlarged cross-sectional view of a spiral groove of a rotational shaft of a third embodiment of a centrifugal pump in accordance with the principles described herein. 
         FIG. 5  is an enlarged partial cross-sectional view of a principal part of a fourth embodiment of a centrifugal pump. 
         FIG. 6  is a cross-sectional view of a flow control valve of the centrifugal pump of  FIG. 5  at a low temperature. 
         FIG. 7  is a cross-sectional view of the flow control valve of the centrifugal pump of  FIG. 5  at a high temperature. 
     
    
    
     DETAILED DESCRIPTION 
     As previously described, Japanese Laid-Open Patent Publication No. 2017-61919 disclosing a casing that may be cooled by exposing fins to air from the cooling fan. However, the interior of the motor is hardly cooled. Accordingly, components of the motor may deteriorate or lose strength. Therefore, there has been a need for improved centrifugal pumps. 
     Embodiments will be described below with reference to drawings. 
     A first embodiment of a centrifugal pump  10  that is used as a purge pump is shown in  FIG. 1 . Centrifugal pump  10  can be mounted on a vehicle, such as an automobile, and may be configured to make up for an insufficient amount of a purge flow from a canister to an intake passage of an internal combustion engine (also referred to as an engine).  FIG. 1  is a cross-sectional view of the centrifugal pump  10 . Although upward, downward, rightward, and leftward directions of the centrifugal pump  10  are defined based on  FIG. 1 , these directions do not limit installation orientations of the centrifugal pump on the vehicle. 
     As illustrated in  FIG. 1 , the centrifugal pump  10  includes a pump  12  and a motor  14 . The pump  12  and the motor  14  are aligned in an axial direction (the vertical direction) along a rotational shaft  52  that will be described later. A casing  16 , corresponding to an outer shell structure of the centrifugal pump  10 , is divided into three parts, including a first casing part  17 , a second casing part  18 , and a third casing part  19  generally arranged one on top of the other in the axial direction (the vertical direction in  FIG. 1 ). The first casing part  17 , the second casing part  18 , and the third casing part  19  are fastened to each other by a plurality of bolts  20  or the like. A first O-ring  21  is disposed between the first casing part  17  and the second casing part  18  for sealing therebetween. A second O-ring  22  is disposed between the second casing part  18  and the third casing part  19  for sealing therebetween. 
     The first casing part  17  and the second casing part  18  form a pump casing for the pump  12 . The first casing part  17  and the second casing part  18  define a pump chamber  23  having a hollow disk shape. The first casing part  17  includes an inlet port  24  having a hollow cylindrical shape extending axially outward (upward in  FIG. 1 ). The inlet port  24  defines an inlet passage  25  allowing fluid communication between the interior and the exterior of the pump chamber  23 . The first casing part  17  includes an outlet port  26  having a hollow cylindrical shape. The outlet port  26  extends in a direction tangential to a circle that is formed along an outer periphery of a base plate  34  of an impeller  33 , which will be described later, in the rightward direction in  FIG. 1 . The outlet port  26  defines an outlet passage  27  allowing fluid communication between the interior and the exterior of the pump chamber  23 . 
     An inner cylindrical part  29  and an outer cylindrical part  30 , each having a hollow cylindrical shape, are formed at a lower surface of the second casing part  18 . The outer cylindrical part  30  is coaxially aligned with the inner cylindrical part  29  disposed within the outer cylindrical part  30 . The inner cylindrical part  29  defines a through-hole extending along the axial direction. The inner cylindrical part  29  and the outer cylindrical part  30  form an annular space  31  therebetween. The second casing part  18  has introduction holes  32 , each of which is a through hole penetrating the second casing part  18  in the vertical direction. Each of the introduction holes  32  is in fluid communication with the annular space  31 . The introduction holes  32  are uniformly circumferentially spaced. 
     The impeller  33  is housed in the pump chamber  23  of the pump  12 . The impeller  33  includes the base plate  34  having a circular plate shape, a plurality of blades  35  uniformly circumferentially spaced on an upper surface of the base plate  34 , and a boss  36  having a hollow cylindrical shape coaxially formed with and located at a lower surface of the base plate  34 . The impeller  33  is rotatably disposed in the pump chamber  23 . The boss  36  is rotatably inserted into the inner cylindrical part  29  of the second casing part  18 . A discharge hole  37 , which is a through-hole having a smaller diameter than an inner diameter of the boss  36 , is formed at a central portion of the base plate  34 . The discharge hole  37  is positioned near the inlet passage  25 , which corresponds to a lower pressure area of the pump chamber  23 . 
     The motor  14  may be a brushless motor configured to rotate the impeller  33 . The second casing part  18  and the third casing part  19  form a motor casing for the motor  14 . The third casing part  19  has a hollow cylindrical shape with a closed bottom. The third casing part  19  includes a cylindrical wall  39  having a hollow cylindrical shape, and a bottom wall  40  closing a lower opening of the cylindrical wall  39 . A step-shaped recessed part  42  is formed at an inner periphery of an upper end surface of the cylindrical wall  39 . The outer cylindrical part  30  of the second casing part  18  is fitted into the step-shaped recessed part  42 . A lower end surface of the outer cylindrical part  30  is positioned away from a bottom surface of the step-shaped recessed part  42  by a predetermined distance. 
     The second casing part  18  and the third casing part  19  define a motor chamber  43 . The motor chamber  43  is in fluid communication with the pump chamber  23  via the annular space  31  and the introduction holes  32  of the second casing part  18 . A lower end portion of the inner cylindrical part  29  of the second casing part  18  is loosely inserted into an inner circumferential surface of the cylindrical wall  39  of the third casing part  19 . The inner cylindrical part  29  and the cylindrical wall  39  define a predetermined gap, which is referred to as a first space S 1 , between mutually facing surfaces thereof in the radial direction. A support recessed part  45  having a hollow cylindrical shape with a closed bottom is coaxially formed with and located at the bottom wall  40  of the third casing part  19 . A retainer  46  having a hollow cylindrical shape with a closed bottom is provided in the support recessed part  45 . 
     The motor  14  includes a rotor  48 , a stator  50 , and other components. The rotor  48  includes the rotational shaft  52  and permanent magnets  53 . The rotational shaft  52  comprises of a hollow shaft. The permanent magnets  53  are disposed at a center portion of the rotational shaft  52 , such that a plurality of magnetic poles are arranged in the circumferential direction. The permanent magnets  53  are positioned by a pair of upper and lower positioning plates  54  fixed to the rotational shaft  52 . 
     The rotor  48  is housed in the motor chamber  43 . An end part, i.e. an upper end part of the rotational shaft  52  is rotatably supported in the inner cylindrical part  29  of the second casing part  18  via a bearing, which is referred to as a first bearing  56 . The first bearing  56  may be a ball bearing including an inner ring fixed to the rotational shaft  52  and an outer ring fixed in the inner cylindrical part  29  of the second casing part  18 . The boss  36  of the impeller  33  is supported on the inner ring of the first bearing  56 . Due to this, the base plate  34  of the impeller  33  and the second casing part  18  define a predetermined gap, which is referred to as a second space S 2 , between mutually facing surfaces thereof in the axial direction. The upper end part of the rotational shaft  52  extends through the first bearing  56 . An upper end of the rotational shaft  52  is inserted into, i.e. engaged with the boss  36  of the impeller  33  in an integrally rotatable manner. 
     The other end part, i.e. a lower end part, of the rotational shaft  52  is rotatably supported in the retainer  46  of the third casing part  19  via a bearing, which is referred to as a second bearing  57 . The second bearing  57  may be a barrel-shaped ball bearing including an inner ring fixed on the rotational shaft  52  and an outer ring loosely fitted into the retainer  46 . Thus, the retainer  46  and the outer ring of the second bearing  57  form a predetermined gap, which is referred to as a third space S 3 , between mutually facing surfaces thereof in the radial direction. A hollow inner space  58  of the rotational shaft  52  is in fluid communication with the discharge hole  37  of the impeller  33 . A communication chamber  60 , which is in fluid communication with both the hollow inner space  58  of the rotational shaft  52  and the third space S 3 , is formed in a lower part of the retainer  46 . The one end part, i.e. the upper end part, of the rotational shaft  52  may be herein referred to as a first end part. The other end part, i.e. the lower end part, of the rotational shaft  52  may be herein referred to as a second end part. 
     The rotational shaft  52  of the rotor  48  extends axially, i.e. vertically, within the casing  16 . The rotor  48  is capable of rotating about the central axis of the rotational shaft  52  in the casing  16 . The impeller  33  rotates with the rotor  48 . The rotor  48  and the impeller  33  are collectively referred to as a rotation unit  62 . 
     The stator  50  includes a core  64 . The core  64  includes a core body  65 , coils  67  wound around the core body  65 , and a bobbin  66  disposed between the core body  65  and the coils  67 . The core body  65  is composed of a plurality of core plates stacked axially, i.e. vertically. The bobbin  66  is made from a resin material. The core  64  is entirely covered with a resin layer, which forms the cylindrical wall  39  of the third casing part  19 . The core  64  is axially aligned and radially opposed to the permanent magnets  53  of the rotor  48 . A combination of the casing  16 , the retainer  46 , and the stator  50  may be referred to as a fixed unit  68 . 
     The cylindrical wall  39  of the third casing part  19  and the rotor  48  define a predetermined gap, which is referred to as a fourth space S 4 , between mutually facing surfaces thereof in the radial direction. The fourth space S 4  is in fluid communication with the third space S 3 . 
     The hollow inner space  58  of the rotational shaft  52  and the discharge hole  37  of the impeller  33  form a discharge passage  70 . The discharge passage  70  is formed in the rotation unit  62  such that one end of the discharge passage  70  opens at an end part of the rotational shaft  52 , which is opposite to the impeller  33 , i.e. a lower end part, and that the other end of the discharge passage  70  opens at the low pressure area in the pump chamber  23 . 
     The introduction holes  32  of the second casing part  18 , the annular space  31 , the first space S 1 , the fourth space S 4 , the third space S 3  in the motor chamber  43 , and the communication chamber  60  collectively define an introduction passage  72 . The introduction passage  72  is formed in the motor chamber  43  of the fixed unit  68 , such that one end thereof opens at a high pressure area in the pump chamber  23  and that the other end is in fluid communication with the other end of the discharge passage  70 . 
     A control circuit (not shown), which provides power feed control to the motor  14 , is disposed in a lower part of the third casing part  19 . The third casing part  19  has a connector  74 . A terminal  75  linked to the control circuit is disposed in the connector  74 . An external connector (not shown) coupled to an external power source (not shown), such as a battery mounted on the vehicle, is connected to the connector  74 . The control circuit receives electric power from the external power source and supplies it to the motor  14 . 
     The motor  14  is driven by power supplied from the external power source. As a result, the rotor  48  is rotated such that the rotation unit  62 , including the impeller  33 , is rotated so as to forcibly move the fluid. More specifically, the fluid, i.e. a purge gas in this embodiment is suctioned into the pump chamber  23  via the inlet passage  25  due to rotation of the impeller  33  (see an arrow Y 1  in  FIG. 1 ). The fluid is pressurized in the pump chamber  23  by rotation of the impeller  33 , and then is discharged from the outlet passage  27  (see an arrow Y 2  in  FIG. 1 ). At this time, in the pump chamber  23 , the pressure of the fluid on the downstream side, i.e. the outlet passage  27  side is higher than that on the upstream side, i.e. the inlet passage  25  side. In other words, differential pressure is generated in the pump chamber  23 . An area near the inlet passage  25  of the pump chamber  23  corresponds to the low pressure area in this disclosure. An outer circumferential area of the pump chamber  23  corresponds to the high pressure area in this disclosure. 
     Due to the differential pressure generated in the pump chamber  23 , a part of the fluid in the pump chamber  23  is introduced from the high pressure area in the pump chamber  23  into the introduction passage  72  through the second space S 2 . Then, the fluid flows through the discharge passage  70  and is discharged into the low pressure area in the pump chamber  23 . More specifically, the fluid in the second space S 2  of the pump chamber  23  flows through the introduction holes  32 , the annular space  31 , the first space S 1 , the fourth space S 4 , and the third space S 3  of the introduction passage  72  into the communication chamber  60  (see the solid arrows in  FIG. 2 ). Then, the fluid in the communication chamber  60  flows through the hollow inner space  58  and the discharge hole  37  of the discharge passage  70  into the low pressure area in the pump chamber  23  (see the dot line arrows in  FIG. 2 ). 
     A part of the fluid in the second space S 2  flows toward the fourth space S 4  via a radial gap between the inner cylindrical part  29  of the second casing part  18  and the boss  36  of the impeller  33  and a gap between components of the first bearing  56 , such as a space between the inner ring and the outer ring, a space between the inner ring and the balls, and a space between the outer ring and the balls. A part of the fluid in the fourth space S 4  flows toward the communication chamber  60  via a gap between components of the second bearing  57  such as a space between the inner ring and the outer ring, a space between the inner ring and the balls, and a space between the outer ring and the balls. 
     In accordance with the centrifugal pump  10  of the first embodiment, the fluid flowing through both the introduction passage  72  and the discharge passage  70  absorbs heat from both the fixed unit  68  and the rotation unit  62  of the motor  14 . The heat is then transferred to the fluid in the low pressure area of the pump chamber  23 . As a result, the cooling performance of the motor  14  can be improved, thereby suppressing deterioration and a reduction in strength of the components of the motor  14 . 
     The fluid flowing through the fourth space S 4  between the rotor  48  and the fixed unit  68  can efficiently cool mutually facing portions of both the rotor  48  and the fixed unit  68 . 
     The fluid flowing through the hollow inner space  58  of the rotational shaft  52  can efficiently cool the rotational shaft  52  from the inside. 
     The fluid flowing through the third space S 3  between the retainer  46  and the second bearing  57  can efficiently cool the second bearing  57 . 
     The fluid flowing through the gap between the components of the first bearing  56  can efficiently cool the first bearing  56 . The fluid flowing through the gap between the components of the second bearing  57  can efficiently cool the second bearing  57 . 
     The cooling performance for the motor  14  is improved, so that heat stresses at peripheral parts of the bearings  56 ,  57  can be decreased. Accordingly, a failure risk of the motor  14  due to heat can be decreased. An increase in the resistance of the coils  67  of the stator  50  can be suppressed, thereby inhibiting a decrease in motor efficiency. A decrease in the life of the motor  14  caused by heat deterioration thereof can be suppressed, thereby increasing the life of the centrifugal pump  10 . 
     A second embodiment of a centrifugal pump similar to centrifugal pump  10  shown in  FIG. 1  with some differences is shown in  FIG. 3 . The differences will be described, and repetitive explanations will be omitted.  FIG. 3  is the cross-sectional view of a principal part of the second embodiment of the centrifugal pump. As illustrated in  FIG. 3 , a bypass passage  78  bypassing the fourth space S 4  is formed in the fixed unit  68 . The bypass passage  78  includes a longitudinal passage  79 , a transverse passage  80 , and a communication hole  81 . The longitudinal passage  79  and the transverse passage  80  are formed in the third casing part  19 . The communication hole  81  is formed in the retainer  46 . 
     The longitudinal passage  79  is formed in the cylindrical wall  39  of the third casing part  19 , so as to extend in the longitudinal direction, i.e. the vertical direction, near the radially outer portion of the stator  50 . One end part, i.e. an upper end part, of the longitudinal passage  79  opens at a bottom surface of the step-shaped recessed part  42 , and thus, is in fluid communication with the annular space  31  via an axial gap between the outer cylindrical part  30  of the second casing part  18  and the step-shaped recessed part  42 . The transverse passage  80  is formed in the bottom wall  40  of the third casing part  19  to extend near a lower portion of the stator  50  in the horizontal direction, in particular the radial direction of the bottom wall  40 . One end part, i.e. an outer end part, of the transverse passage  80  is in fluid communication with the other end part, i.e. a lower end part, of the longitudinal passage  79 . The communication hole  81  is formed in the retainer  46  to allow fluid communication between the other end part, i.e. an inner end part of the transverse passage  80  and the third space S 3 . The third casing part  19  may be referred to herein as “a wall of a casing.” The bypass passage  78  may be referred to herein as “a part of an introduction passage.” 
     In accordance with the second embodiment, a part of the fluid in the annular space  31  flows toward the third space S 3  via the bypass passage  78  (see the dash-dot line arrows in  FIG. 3 ). Accordingly, the fluid flowing through the bypass passage  78  can efficiently cool the stator  50 . 
     A third embodiment of a centrifugal pump similar to centrifugal pump  10  shown in  FIG. 1  with some differences is shown in  FIG. 4 . The differences will be described, and repetitive explanations will be omitted.  FIG. 4  is a cross-sectional view of a spiral groove of the rotational shaft. As illustrated in  FIG. 4 , a spiral groove  83  like a screw groove is formed on an inner facing surface of the rotational shaft  52 . The inner facing surface of the rotational shaft  52  corresponds to a wall surface defining the hollow inner space  58 . A winding direction of the spiral groove  83  is set as a direction capable of promoting a fluid flow, by using the rotation of the rotation unit  62 . 
     In accordance with the third embodiment, the spiral groove  83  of the rotational shaft  52  can promote fluid flow through the hollow inner space  58  by using the rotation of the rotational shaft  52 . Thus, the amount of the fluid flowing through the discharge passage  70  and the introduction passage  72  (see  FIG. 1 ) is increased, thereby improving the cooling performance for the motor  14 . The spiral groove  83  may also be formed on an inner circumferential surface of the discharge hole  37  of the impeller  33 . 
     A fourth embodiment of a centrifugal pump similar to centrifugal pump shown in  FIG. 3  with some differences is shown in  FIG. 5 . The differences will be described, and repetitive explanations will be omitted. As illustrated in  FIG. 5 , the transverse passage  80  of the bypass passage  78  is provided with a bimetallic type valve  85  configured to control a size of a passage area depending on the fluid temperature. The bimetallic type valve  85  may use extension/contraction of bimetallic member  86  depending on temperature changes so as to control the size of the passage area. The bimetallic member  86  is a plate having an arc-shape, such that one end, i.e. a base end, of the bimetallic member  86  is fixedly engaged with a passage wall surface of the transverse passage  80  of the bypass passage  78 , i.e. the bottom wall  40  of the third casing part  19 . When the bimetallic member  86  contracts at lower temperatures, the bimetallic type valve  85  decreases the size of the passage area (see  FIG. 6 ). When the bimetallic member  86  extends at higher temperatures, the bimetallic type valve  85  increases the size of the passage area (see  FIG. 7 ). The bimetallic type valve  85  may be herein referred to as “flow control valve” or “temperature-sensitive flow control valve.” 
     In accordance with the fourth embodiment, the bimetallic type valve  85 , which is configured to control the size of the passage area depending on the fluid temperature, decreases the size of the passage area of the transverse passage  80  of the bypass passage  78  at lower temperatures (see  FIG. 6 ). Thus, the bimetallic type valve  85  reduces the amount of fluid flowing therethrough. Accordingly, a decrease in pump efficiency can be suppressed at lower temperatures. The size of the passage area of the transverse passage  80  of the bypass passage  78  is increased at higher temperatures (see  FIG. 7 ), so that the amount of fluid flowing therethrough is also increased. Accordingly, the cooling performance for the motor  14  can be improved at higher temperatures. 
     The bimetallic type valve  85  may alternatively be provided in the longitudinal passage  79  of the bypass passage  78 . The bimetallic type valve  85  may alternatively be disposed in the introduction passage  72  or the discharge passage  70 . Other temperature-sensitive flow control valves, such as a bellows valve, a wax valve or the like, may be used instead of the bimetallic type valve  85 . The flow control valve may be composed of an electromagnetic flow control valve instead of the temperature-sensitive flow control valve. 
     The aspects disclosed herein are not limited to the above-described embodiments and can be carried out in other various embodiments. For example, the centrifugal pump  10  may be used as a pump for forcibly transferring a variety of fluids, e.g. gases, such as air, or liquids, such as water or fuel, other than the above described purge gas. The centrifugal pump  10  is not limited to the above described purge pump and may be used as a water pump configured to circulate cooling water for an internal combustion engine. The brushless motor of the motor  14  may be replaced with a brushed motor. 
     Various configurations of the aspects are disclosed herein. A first configuration is a centrifugal pump including a pump and a motor. The pump includes an impeller for forcibly transferring a fluid and forms a pump chamber housing the impeller therein. The motor includes a hollow rotational shaft configured to rotate the impeller and a casing defining a motor chamber that houses the rotational shaft therein. The rotational shaft defines a discharge passage therein and has a first end engaged with the impeller and a second end where one end of the discharge passage opens. The one end of the discharge passage is in fluid communication with a low pressure area in the pump chamber via the other end of the discharge passage. The one end of the discharge passage is in fluid communication with a high pressure area in the pump chamber via an introduction passage formed by the casing. 
     In accordance with the first configuration, the impeller of the pump is rotated due to being driven by the motor, so as to forcibly transfer the fluid. At that time, due to differential pressure generated in the pump chamber, a part of the fluid in the high pressure area of the pump chamber is introduced into the introduction passage. The part of the fluid then flows through the discharge passage, and then is discharged into the low pressure area. Accordingly, the fluid flowing through both the introduction passage and the discharge passage absorbs heat from the motor, in particular the casing and a peripheral area around the rotational shaft. The fluid then transmits the heat to the fluid in the low pressure area of the pump chamber. Consequently, the cooling performance of the motor can be improved, thereby suppressing deterioration and a reduction in strength of the components of the motor. 
     A second configuration corresponds to the centrifugal pump of the first configuration. The motor includes a rotor. A part of the introduction passage is an empty space between the rotor and the casing. 
     In accordance with the second configuration, the fluid flowing through the empty space between the rotor and the casing can efficiently cool mutually facing portions of the rotor and the casing. 
     A third configuration corresponds to the centrifugal pump of the first configuration or the second configuration. The motor includes a stator. The casing includes a wall covering the stator. A part of the introduction passage is formed in the wall. 
     In accordance with the third configuration, the fluid flowing in the wall of the casing covering the stator can efficiently cool the stator. 
     A fourth configuration corresponds to the centrifugal pump of any one of the first to third configurations. An inner circumferential surface of the discharge passage has a spiral groove promoting a fluid flow therethrough by using a rotation of the rotational shaft. 
     In accordance with the fourth configuration, the spiral groove promotes the fluid flow by using the rotation of the rotational shaft. Accordingly, the amount of the fluid flowing through both the discharge passage and the introduction passage can be increased, thereby improving the cooling performance of the motor. 
     A fifth configuration corresponds to the centrifugal pump of any one of the first to fourth configurations. At least one of the introduction passage and the discharge passage includes a flow control valve configured to control a size of a passage area depending on a fluid temperature. 
     In accordance with the fifth configuration, due to the flow control valve configured to control the passage area depending on the fluid temperature, the size of the passage area is decreased when the fluid temperature is low, such that the flow amount is also reduced. Accordingly, a decrease in pump efficiency can be suppressed. However, when the fluid temperature is high, the size of the passage area is increased, such that the flow amount is also increased. Accordingly, a cooling performance for the motor can be improved.