Abstract:
A direction in which cooling air generated by a cooling fan flows to be discharged is perpendicular or nearly perpendicular to a direction in which diffusers are inclined. Thus, airflow resistance is increased by the diffusers, and the cooling air cannot be efficiently discharged from discharge ports provided circumferentially in an outer peripheral portion of a housing. Further, a stator coil cannot be sufficiently cooled. An electrical rotating machine includes: a rotor having a cooling fan on an end face of the rotor; a stator that is arranged opposite the rotor and has a stator coil with a rotation clearance provided between the stator and the rotor; a housing that holds the rotor and the stator; a plurality of intake ports that are provided in a side wall portion of the housing; a plurality of discharge ports that are provided circumferentially in an outer peripheral portion of the housing; and a plurality of diffusers that are provided circumferentially on an inner wall surface of a corner portion of the housing, the diffusers inclining toward the side opposite to a rotational direction of the rotor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electrical rotating machine including a cooling fan. 
     2. Description of the Related Art 
     In order to reduce the size of an electrical rotating machine and increase power of the electrical rotating machine, an electrical rotating machine with a built-in fan grows in demand. A conventional electrical rotating machine cannot compensate for a temperature increased by self-heat generation. 
     To avoid this, JP-9-172752-A (page 6 [0041-0046], FIGS. 1 and 2) describes a generator having a plurality of rectifying guides (hereinafter referred to as diffusers) on an inner wall surface of a corner portion of a frame (housing) that forms a stator. 
     In FIGS. 1 and 2 of JP-9-172752-A, adjacent pairs of the rectifying guides are arranged at equal intervals, and the rectifying guides are inclined at a predetermined angle toward a rotational direction of a cooling fan with respect to a radial direction of a shaft, thereby improving the effect of adjusting cooling air. 
     SUMMARY OF THE INVENTION 
     According to JP-9-172752-A, the diffusers are inclined toward a rotational direction of a rotor. That is, the cooling air generated by the cooling fan is discharged toward the outside of the generator in a direction nearly perpendicular to the direction in which the diffusers are inclined. Thus, airflow resistance is increased by the diffusers. The cooling air cannot be efficiently discharged from a plurality of discharge ports circumferentially arranged on outer peripheral portions of front and rear housings. Therefore, a stator coil cannot be sufficiently cooled. 
     An object of the present invention is to provide an electrical rotating machine that uses a built-in cooling fan to exhibit improved performance in cooling a stator coil. 
     A desirable aspect of the present invention is described below to solve the aforementioned problem. 
     The electrical rotating machine includes: a rotor having a cooling fan on an end face of the rotor; a stator that is arranged opposite the rotor and has an stator coil with a rotation clearance provided between the stator and the rotor; a housing that holds the rotor and the stator; a plurality of intake ports that are provided in a side wall portion of the housing; a plurality of discharge ports that are provided circumferentially in an outer peripheral portion of the housing; and a plurality of diffusers that are provided circumferentially on an inner wall surface of a corner portion of the housing, the diffusers inclining toward the side opposite to a rotational direction of the rotor. 
     The present invention provides the electrical rotating machine that uses a built-in cooling fan to exhibit improved performance in cooling a stator coil. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical cross sectional view of an alternator for a vehicle. 
         FIG. 2  is a diagram showing a back surface of a front housing (when viewed from the side opposite to the side of a pulley). 
         FIG. 3  is a diagram showing a back surface of a rear housing (when viewed from the side of the pulley). 
         FIG. 4  is a vertical cross sectional view of the alternator. 
         FIG. 5  is a left side view of main parts of the alternator (when viewed from the side of the pulley). 
         FIG. 6  is a right side view of main parts of the alternator (when viewed from the opposite side of the pulley). 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention is described below with reference to  FIGS. 1 to 6 . 
     As shown in  FIG. 1 , each of a front housing  1  and a rear housing  2  has an inner storage space and a bottom surface and is formed in a cylindrical shape or bowl shape. The front housing  1  has a fixing section  3  integrated with the front housing  1 . The rear housing  2  has a fixing section  4  integrated with the rear housing  2 . Each of the fixing sections  3  and  4  has a fixing hole. The fixing section  3  protrudes on an outer circumferential side of the front housing  1  in a radial direction of the front housing  1 . The fixing section  4  protrudes on an outer circumferential side of the rear housing  2  in a radial direction of the front housing  2 . The fixing sections  3  and  4  are attached to a vehicle by means of bolts (not shown). Each of the front and rear housings  1  and  2  is made of an aluminum alloy and molded by die casting. 
     A rear cover  5  is attached to one of end portions (separated from each other in the direction of a rotational axis of the rear housing  2 ) of the rear housing  2  and is thinner than each of the front and rear housings  1  and  2 . The rear cover  5  has an inner storage space and a bottom surface and is formed in a cylindrical shape or bowl shape in a similar fashion to the housings. The rear cover  5  has a plurality of intake ports (openings)  5   a  on inner and outer circumferential sides thereof. Air flows through the intake ports  5   a . A terminal  6  to be connected to a battery is attached to the outer circumferential side of the rear cover  5 . The rear cover  5  is made of resin or an aluminum alloy. 
     A ball bearing  7   a  is attached to one of outer end portions (separated from each other in the direction of a rotational axis of the front housing  1 ) of the front housing  1  and located near the radial center of the front housing  1 , while a ball bearing  7   b  is attached to one of outer end portions (separated from each other in the direction of the rotational axis of the rear housing  2 ) of the rear housing  2  and located near the radial center of the rear housing  2 . The diameter of the ball bearing  7   a  is larger than the diameter of the ball bearing  7   b.    
     A shaft  8  extends through an inner ring of the ball bearing  7   a  and an inner ring of the ball bearing  7   b . The shaft  8  is capable of rotating relative to the front and rear housings  1  and  2  and held by the ball bearings  7   a  and  7   b.    
     A pulley  9  is fixed to the shaft  8  by a bolt on the side of the front housing  1  so that the shaft  8  and the pulley  9  are capable of rotating in an integrated manner. The pulley  9  serves as a rotation transfer member. A rotation of an engine (not shown) is transferred to a crank pulley. Then, the rotation of the engine is transferred from the crank pulley to the pulley  9  through a belt that serves as an endless transfer belt. The shaft  8  is rotated by the rotation of the pulley  9 . The rotational rate of the shaft  8  is proportional to the rotational rate of the engine and a pulley ratio of the pulley  9  and the crank pulley. 
     Two slip rings  10  are attached to an end portion of the shaft  8  on the side of the rear housing  2  so that the slip rings  10  and the shaft  8  are capable of rotating in an integrated manner. Power is supplied through two brushes  11  to the slip rings  10 . The brushes  11  slide while being pushed by the respective slit rings  10 . 
     A front rotor core  12 F and a rear rotor core  12 R join the shaft  8  in a serrated manner at a substantially central portion (in the direction of a rotational axis of the shaft  8 ) of the shaft  8  so that the rotor cores  12 F and  12 R and the shaft  8  are capable of rotating in an integrated manner. In this case, the front rotor core  12 F joins a portion of the shaft  8 , while the rear rotor core  12 R joins another portion of the shaft  8 . Each of the rotor cores  12 F and  12 R is made of a magnetic material and molded. An outer end portion of the front rotor core  12 F plastically flows in an annular groove  8   a  formed in the shaft  8  so that a movement of the front rotor core  12 F in the direction of the rotational axis of the shaft  8  is restricted under the condition that the rotor cores  12 F and  12 R face each other in the direction of the rotational axis of the shaft  8  and are in contact with each other. An outer end portion of the rear rotor core  12 R plastically flows in an annular groove  8   b  formed in the shaft  8  so that a movement of the rear rotor core  12 R in the direction of the rotational axis of the shaft  8  is restricted under the condition that the rotor cores  12 F and  12 R face each other in the direction of the rotational axis of the shaft  8  and are in contact with each other. The front-side and rear rotor cores  12 F and  12 R fixed to the shaft  8  in the above-described way form a rotor  12 . 
     A plate-shaped front fan  13 F and a plate-shaped rear fan  13 R are attached to respective end faces (separated from each other in the direction of a rotational axis of the rotor  12 ) of the rotor  12 . The front fan  13 F has a plurality of blades on an outer circumferential side of the front fan  13 F. The rear fan  13 R has a plurality of blades on an outer circumferential side of the rear fan  13 R. The fans  13 F and  13 R and the rotor  12  rotate in an integrated manner. 
     Each of the front rotor core  12 F and the rear rotor core  12 R includes a shaft portion  12   a  and a plurality of rotor core claw magnetic poles  12   b . The shaft portions  12   a  are located on inner sides of the rotor cores  12 F and  12 R. The rotor core claw magnetic poles  12   b  are located on outer sides of the rotor cores  12 F and  12 R. Each of the rotor core claw magnetic poles  12   b  has an L shape in a cross section taken along a radial direction of the rotor  12 . One of ends (separated from each other in the direction of the rotational axis of the rotor  12 ) of the shaft portion  12   a  of the rotor member  12 F faces and is in contact with one of ends (separated from each other in the direction of the rotational axis of the rotor  12 ) of the shaft portion  12   a  of the rotor member  12 R so that the front rotor core  12 F and the rear rotor core  12 R form a Lundell-type iron core. A field coil  14  is provided between outer circumferences of the shaft portions  12   a  and inner circumferences of the rotor core claw magnetic poles  12   b  and wound around the rotational axis of the rotor  12 . Both ends of the field coil  14  extend along the shaft  8  and are connected to the respective two slip rings  10 . The brushes  11  supply field currents through the slip rings  10  to the field coil  14 . The currents (that are to be supplied to the field coil  14 ) are controlled on the basis of the state of the battery so that when a generated voltage is higher than a voltage of the battery included in the vehicle, power generation starts. An IC regulator (not shown) controls the generated voltage so that a voltage applied to the terminal  6  is constant. The IC regulator is located in a rectification circuit  15  (described later) provided in the rear cover  5  and serves as a voltage control circuit. 
     A stator  17  is sandwiched between the front housing  1  and the rear housing  2  and fixed to the front housing  1  and the rear housing  2 . An inner circumference of the stator  17  faces outer circumferences of the rotor core claw magnetic poles  12   b  while there are small gaps between the inner circumference of the stator  17  and the outer circumferences of the rotor core claw magnetic poles  12   b.    
     The stator  17  includes a stator core  17   a  and a stator coil  17   b . The stator core  17   a  is made of a magnetic material. The stator coil  17   b  is wound around the stator core  17   a . The stator coil  17   b  of each phase is connected to the rectification circuit  15  located in the rear cover  5 . The rectification circuit  15  is connected to the battery through the terminal  6 . 
     The rectification circuit  15  includes a plurality of rectifier. The rectifiers form a three-phase coil. Thus, six rectifiers perform full-wave rectification. 
     Next, the rotor  12  is described below in detail. As shown in  FIG. 1 , the front rotor core  12 F and the rear rotor core  12 R form the rotor  12 . Each of the rotor cores  12 F and  12 R includes the plurality of rotor core claw magnetic poles  12   b  (specifically, six claw magnetic poles  12   b ) arranged in the circumferential direction of the rotor  12 . The rotor core claw magnetic poles  12   b  are located on the outer sides of the shaft portions  12   a  and have an L shape in the cross section taken along the radial direction of the rotor  12 . The rotor core claw magnetic poles  12   b  (extending from the front rotor core  12 F) and the rotor core claw magnetic poles  12   b  (extending from the rear rotor core  12 R) are alternately arranged in the circumferential direction of the rotor  12 . The number of all the rotor core claw magnetic poles  12   b  is 12 in the present embodiment. 
     The front rotor core  12 F and the rear rotor core  12 R are fixed to the shaft  8  under the condition that: the field coil  14  is located between the shaft portions  12   a  and the rotor core claw magnetic poles  12   b;  the rotor core claw magnetic poles  12   b  (extending from the front rotor core  12 F) and the rotor core claw magnetic poles  12   b  (extending from the rear rotor core  12 R) are alternately arranged in the circumferential direction of the rotor  12 ; and the end of the shaft portion  12   a  of the rotor member  12 F is in contact with the end of the shaft portion  12   a  of the rotor member  12 R. 
     The front fan  13 F serves as a cooling fan and is attached to one of outer ends (separated from each other in the direction of the rotational axis of the rotor  12 ) of the front rotor core  12 F by welding or the like. The rear fan  13 R serves as a cooling fan and is attached to one of outer ends (separated from each other in the direction of the rotational axis of the rotor  12 ) of the rear rotor core  12 R by welding or the like. The front fan  13 F and the rear fan  13 R are arranged so that air flows toward a central portion of the rotor  12  due to the rotation of the rotor  12 . The front fan  13 F has the blades integrated with the front fan  13 F. The rear fan  13 R has the blades integrated with the rear fan  13 R. Each of the blades is a protruding portion and made of a metal plate. In addition, the blades included in the front fan  13 F are arranged in the circumferential direction of the front fan  13 F, while the blades included in the rear fan  13 R are arranged in the circumferential direction of the rear fan  13 R. The blades arranged in a part of the circumference of each of the fans  13 F and  13 R are formed in a substantially arc shape and bent at a substantially right angle by pressing and have a surface inclined to the radial direction of the fan. The front fan  13 F is fixed to the outer end (of the two outer ends separated from each other in the direction of the rotational axis of the rotor  12 ) of the front rotor core  12 F in an integrated manner by welding. The rear fan  13 R is fixed to the outer end (of the two outer ends separated from each other in the direction of the rotational axis of the rotor  12 ) of the rear rotor core  12 R in an integrated manner by welding. The front fan  13 F, the rear fan  13 R, and the rotor  12  allow air to flow. 
     Next, the stator  17  is described in detail. As shown in  FIG. 1 , the stator core  17   a  has thin plates that are laminated and formed in a coil-like shape. Each of the thin plate included in the stator core  17   a  is made of a magnetic material. The stator core  17   a  also has a plurality of slots (not shown) in an inner circumferential surface of the stator core  17   a . The number of the slots are determined on the basis of the number of the rotor core claw magnetic poles  12   b  and arranged at equal intervals. The three-phase pre-wound stator coil  17   b  is inserted in the slots and connected by Y-connection or triangle connection. Insulator sheets (that are insulating members) are inserted in the slots to prevent the stator coil  17   b  inserted in the slots from being exposed to the inner circumferential surface of the stator core  17   b.    
     In the present embodiment, the number of magnetic poles that are provided for one phase and included in the stator  17  is 12 and the same as the number of the magnetic poles included in the rotor  12 . 
     The surface of the stator coil  17   b  is coated by varnish or the like to cause the stator coil  17   b  to be insulated. A terminal of the stator coil  17   b  passes through the rear housing  2  and is connected to a terminal  15   a  of the rectification circuit  15 . An insulating paper (that is an insulating member) may be arranged between the stator core  17   a  and the stator coil  17   b.    
     The following describes the configuration of diffusers that are inclined to improve a cooling effect. 
     The front housing  1  has a plurality of intake ports  1   c  formed in a side wall portion of the front housing  1 , and the rear housing  2  has a plurality of intake ports  2   c  formed in a side wall portion of the rear housing  2 , as shown in  FIGS. 1 to 3 . Air flows through the intake ports  1   c  and the intake ports  2   c.    
     As shown in  FIG. 4 , an outer radius φD 11  indicates a distance between the rotational axis of the rotor  12  and a point (that is the farthest point from the rotational axis of the rotor  12  among all points of each intake port  1   c ) of each of the intake ports  1   c , and a outer radius φD 21  indicates a distance between the rotational axis of the rotor  12  and a point (that is the farthest point from the rotational axis of the rotor  12  among all points of each intake port  2   c ) of each of the intake ports  2   c . In addition, a radius φD 12  indicates an inner radius of each of the blades included in the front fan  13 F (attached to the end face of the rotor  12  having the housings  1  and  2  that face each other). A radius φD 22  indicates an inner radius of each of the blades included in the rear fan  13 R (attached to the other end face of the rotor  12  having the housings  1  and  2  that face each other). The radius φD 11  is equal to or smaller than the radius φD 12 . The radius φD 21  is equal to or smaller than the radius φD 22 . 
     The front housing  1  has a plurality of discharge ports (openings)  1   d  on the outer circumferential side of the front housing  1 . The discharge ports  1   d  are arranged in the circumferential direction of the front housing  1 . The rear housing  2  has a plurality of discharge ports (openings)  2   d  on the outer circumferential side of the rear housing  2 . The discharge ports  2   d  are arranged in the circumferential direction of the rear housing  2 . In  FIG. 4 , a radius φD 13  of a bottom end of each discharge port  1   d  indicates a distance between the rotational axis of the rotor  12  and a point (that is the closest point from the rotational axis of the rotor  12  among all points of the discharge port  1   d ) of the discharge port  1   d , and a radius φD 23  of a bottom end of each discharge port  2   d  indicates a distance between the rotational axis of the rotor  12  and a point (that is the closest point from the rotational axis of the rotor  12  among all points of the discharge port  2   d ) of the discharge port  2   d . A radius φD 14  indicates an outer radius of each blade included in the front fan  13 F. A radius φD 24  indicates an outer radius of each blade included in the rear fan  13 R. The radius φD 13  is equal to or larger than the radius φD 14 . The radius φD 23  is equal to or larger than the radius φD 24 . 
     A plurality of diffusers  1   a  circumferentially provided are formed integrally with an inner wall surface  1 E of a corner portion of the front housing  1 . Likewise, a plurality of diffusers  2   a  circumferentially provided are formed integrally with an inner wall surface  2 E of a corner portion of the rear housing  2 . 
     As shown in  FIGS. 2 ,  3 ,  5 , and  6 , the diffusers  1   a  are inclined at a predetermined angle θ 1 , and the diffusers  2   a  are inclined at a predetermined angle θ 2 . The diffusers  1   a  and  2   a  are inclined toward the side opposite to the rotational direction of the rotor  12 . That is, the diffusers  1   a  are inclined toward the same direction as an inclination (angle) θ 3  of each of the blades included in the front fan  13 F, and the diffusers  2   a  are inclined toward the same direction as an inclination (angle) θ 4  of each of the blades included in the rear fan  13 R. The angle θ 1  is determined on the basis of the inclination angle θ 3 , and the angle θ 2  is determined on the basis of the inclination angle θ 4 . 
     The diffusers  1   a  are located on the outer side of the front housing  1  with respect to the outer radius φD 14  of the front fan  13 F. The diffusers  2   a  are located on the outer side of the rear housing  2  with respect to the outer radius φD 24  of the rear fan  13 R. Reference symbol θp 1  denotes the interval at which the diffusers  1   a  are arranged, as shown in  FIG. 5 . Reference symbol θp 2  denotes the interval at which the diffusers  2   a  are arranged, as shown in  FIG. 6 . The front housing  1   a  has ribs  1   b  that form the discharge ports  1   d , while the rear housing  2   a  has ribs  2   b  that form the discharge ports  2   d . The discharge ports  1   d  are located circumferentially on the outer peripheral portion of the front housing  1 . Likewise, the discharge ports  2   d  are located circumferentially on the outer peripheral portion of the rear housing  2 . The diffusers  1   a  extend to the respective ribs  1   b  as shown in  FIG. 5 , while the diffusers  2   a  extend to the respective ribs  2   b  as shown in  FIG. 6 . The ribs  1   b  are farther from the rotational axis of the rotor  12  than the bottom ends of the discharge ports  1   d , while the ribs  2   b  are farther from the rotational axis of the rotor  12  than the bottom ends of the discharge ports  2   d . If an obstacle such as a hole or a screw, which disturbs installation of the diffusers  1   a , is present and overlaps any one or more of the diffusers  1   a , then the diffusers  1   a  may be arranged at different intervals in the circumferential direction of the front housing  1 . If an obstacle such as a hole or a screw, which disturbs installation of the diffusers  2   a , is present and overlaps any one or more of the diffusers  2   a , then the diffusers  2   a  may be arranged at different intervals in the circumferential direction of the rear housing  2 . 
     The interval θp 1  of the diffusers  1   a  is determined on the basis of the ribs  1   b  and the interval of the blades included in the front fan  13 F. The interval θp 2  of the diffusers  2   a  is determined on the basis of the ribs  2   b  and the interval of the blades included in the rear fan  13 R. 
     Next, operations of the generator according to the present embodiment are described. 
     When the engine starts, the rotation of the engine is transferred from a crank shaft through the belt to the pulley  9 . Thus, the pulley  9  rotates the rotor  12  through the shaft  8 . When the brushes  11  supply a direct current through the slip rings  10  to the field coil  14  located in the rotor  12 , a magnetic flux is generated around inner and outer circumferences of the field coil  14 . Thus, a north pole or a south pole is alternately formed at the rotor core claw magnetic poles  12   b  (included in the rotor  12 ) in the circumferential direction of the rotor  12 . The magnetic flux generated by the field coil  14  circles from the rotor core claw magnetic poles  12   b  (north poles) included in the front rotor core  12 F to the stator coil  17   b  included in the stator  17 . Then, the magnetic flux reaches the rotor core claw magnetic poles  12   b  (south poles) included in the rear rotor core  12 R. Thus, the magnetic flux forms a magnetic circuit extending to the rotor  12  and the stator  17 . Since the magnetic flux generated by the rotor  12  intersects the stator coil  17   b , an alternating current induced voltage is produced across the U-phase, V-phase, and W-phase stator coil  17   b . Thus, the three-phase alternating current induced voltage is produced. 
     The alternating current voltage produced in the aforementioned way is full-wave rectified and converted into a direct current voltage by the rectification circuit  15 . The IC regulator (not shown) controls a current to be supplied to the field coil  14  so that the rectified direct current voltage is approximately 14.3 V and constant. 
     When the rotor  12  rotates, the front fan  13 F and the rear fan  13 R rotate with the rotation of the rotor  12 . Thus, airflow shown by arrows (broken lines) of  FIG. 1  is formed. The air flow is directed from the outside of the generator into the generator in the axial direction and directed to the outside of the generator in the outer circumferential direction. 
     The front fan  13 F rotates to take air from the outside of the generator in the axial direction through the intake ports  1   c  (located in an outer circumferential portion of the ball bearing  7   a  attached to the front housing  1 ). When the air taken by the front fan  13 F flows toward the outer circumference of the front housing  1  due to a centrifugal force generated by the blades of the front fan  13 F, the flow of the air is adjusted by the diffusers  1   a  as shown by arrows (broken lines) of  FIG. 5 . The air is then discharged from the discharge ports  1   d  located circumferentially in the outer peripheral portion of the front housing  1 . 
     One of side surfaces of the stator  17  and an outer surface of the stator  17  are fixed under the condition that the side surface and outer surface of the stator  17  are in contact with the front housing  1 . Thus, heat generated by the stator  17  is transferred to the front housing  1  and released from the surface of the front housing  1 . The heat released from the front housing  1  is discharged to the outside of the generator by the air that flows toward the discharge ports  1   d . Thus, the stator coil  17   b  included in the stator  17  can be cooled. The diffusers  1   a  also serves as heat releasing fins. In the present embodiment, since the diffusers  1   a  are inclined toward the side opposite to the rotational direction of the rotor  12 , the cooling effect is improved. 
     The rear fan  13 R rotates to take air from the outside of the generator in the axial direction through the intake ports  5   a  (formed in an outer end portion of the rear cover  5 ), holes (that are open and formed in end faces (separated from each other in the axial direction) of the rear cover  5  on the inner circumferential side), the rectification circuit  15 , and the intake ports  2   c  (formed in an outer circumferential portion of the ball bearing  7   b  attached to the rear housing  2 ). When the air taken by the rear fan  13 R flows toward the outer circumference of the rear housing  2  due to a centrifugal force generated by the blades of the rear fan  13 R, the flow of the air is adjusted by the diffusers  2   a  as shown by arrows (broken lines) of  FIG. 6 . The air is then discharged from the discharge ports  2   d  that are located circumferentially in the outer peripheral portion of the rear housing  2 . Thus, the heat generated by the stator  17  and the heat transferred to the rear housing  2  are released from the surface of the rear housing  2  in the same way of the front housing  1 . Thus, the stator coil  17   b  included in the stator  17  can be cooled by the air that flows toward the discharge port  2   d . The diffusers  2   a  also serves as heat releasing fins. In the present embodiment, since the diffusers  2   a  are inclined toward the side opposite to the rotational direction of the rotor  12 , the cooling effect is improved. 
     Rotation of the fans  13 F and  13 R produces a difference between pressure applied to the front fan  13 F and pressure applied to the rear fan  13 R. The pressure difference causes air to flow between the magnetic poles of the rotor  12  and between the rotor  12  and the stator  17 . In the present embodiment, since the pressure applied to the rear fan  13 R becomes large, air flows from the side of the front housing  1  through a gap between the rotor  12  and the stator  17  and a gap between the magnetic poles included in the rotor  12  toward the side of the rear housing  2  so that the rotor  12  and the stator  17  are cooled.