Abstract:
A cooling structure for a rotating electric machine is proposed which displays high cooling performance with a simple structure, does not undergo reductions in efficiency when the rotating electric machine is operating at high rotation speeds and has high reliability. The rotating shaft of the rotating element comprises a hollow structure and an inner cylindrical section which rotates together with the rotating shaft is provided with a space in an inner section of the rotating shaft. Coolant flows in an annular gap between the rotating shaft and the inner cylindrical section. In this manner, the rotating element is effectively cooled with a small amount of coolant.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to a cooling structure for a rotating element of a rotating electric machine.  
         BACKGROUND OF THE INVENTION  
         [0002]    Tokkai Hei 9-46973 published by the Japanese Patent Office in 1997 discloses a cooling structure for a rotating element of a rotating electric machine. The cooling structure as disclosed in this publication is provided with an injection pipe for cooling liquid which is fixed to the case. One end of the injection pipe is inserted into an open end of a hollow rotation shaft of a rotating element. Cooling liquid is transferred to the center of the rotating element from the outside.  
         SUMMARY OF THE INVENTION  
         [0003]    However since cooling liquid in the conventional technique fills the gap between the hollow rotation shaft which rotates with the rotating element and the fixed injection pipe, the efficiency of the rotating electric machine is particularly reduced at high rotation speeds as a result of the viscosity resistance of the cooling liquid. Furthermore since the radius of the bearing of the rotation shaft is large due to the existence of the injection pipe inserted into the hollow rotation shaft, the bearing loss is large.  
           [0004]    It is therefore an object of this invention to provide a simple cooling structure for a rotating electric machine which displays highly efficient cooling performance and does not result in reductions in efficiency at high rotation speeds.  
           [0005]    In order to achieve above object, this invention provides a cooling structure for a rotating electric machine having a rotating element which is provided with a rotating shaft having a hollow structure, the cooling structure comprising: an inner cylindrical section disposed in the rotating shaft and rotating together with the rotating shaft; and an annular gap provided between the inner peripheral surface of the rotating shaft and the outer peripheral surface of the inner cylindrical section. Coolant flows in the annular gap.  
           [0006]    The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1A is a sectional view of a rotating element according to a first embodiment of this invention and FIG. 1B is a sectional view of an alternative rotating element according to a first embodiment of this invention.  
         [0008]    [0008]FIG. 2 shows a first cylindrical section of a hollow rotation shaft: FIG. 2A is a front view and FIG. 2B is a sectional view along the line  2 B- 2 B in FIG. 2A.  
         [0009]    [0009]FIG. 3 shows an inner cylindrical section of the hollow rotation shaft: FIG. 3A is a front view and FIG. 3B is a lateral view.  
         [0010]    [0010]FIG. 4 is an enlarged sectional view of the hollow rotation shaft in proximity to the inner cylindrical section.  
         [0011]    [0011]FIG. 5 shows the first cylindrical section of the hollow rotation shaft according to another embodiment: FIG. 5A is a front view and FIG. 5B is a sectional view along the line  5 B- 5 B.  
         [0012]    [0012]FIG. 6 is a sectional view of a rotating element according to another embodiment of this invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]    Referring to FIG. 1A, a rotating element  1  of a rotating electric machine is provided with a hollow rotating shaft  2  having a hollow structure, a plurality of magnetized steel plates  3  which are provided on an outer periphery of the hollow rotating shaft  2  and are laminated in a direction of the rotational axis of the rotating element  1 , and two endplates  4  fixed to the hollow rotating shaft  2 . The two endplates  4  sandwich the plurality of laminated magnetized steel plates  3 , namely the core of the rotating element  1 . The rotating electric machine is operated for example as a motor.  
         [0014]    The hollow rotating shaft  2  comprises a first cylindrical section  5   a  positioned in a central section of the hollow rotating shaft  2  and two second cylindrical sections  5   b  which are stepped and disposed on both ends of the first cylindrical section  5   a . The first cylindrical section  5   a  has a constant radius in a direction of the rotation axis and is inserted and fitted into the plurality of magnetized steel plate  3 . Each second cylindrical section  5   b  projects outwardly from each end plate  4 , and across the whole length it has an outer radius and inner radius which are smaller than the outer radius and inner radius of the first cylindrical section  5   a , respectively. The first cylindrical section  5   a  is integrated with the plurality of magnetized steel plates  3  of the rotating element  1  and the second cylindrical section  5   b  is fixed at both ends of the first cylindrical section  5   a . A section of the second cylindrical section  5   b  is supported to rotate freely on the casing through a seal and bearing (not shown).  
         [0015]    An inner cylindrical section  6  is inserted into the first cylindrical section  5   a . The inner cylindrical section  6  is thin-walled and hollow. A narrow annular gap  7  is formed between the inner peripheral surface of the first cylindrical section  5   a  and the outer peripheral surface of the inner cylindrical section  6 . The second cylindrical section  5   b  is fitted to the first cylindrical section  5   a  from both sides after assembling the inner cylindrical section  6  into the inner section of the hollow rotating shaft  2  of the rotating element  1  so that the inner cylindrical section  6  is fixed to the rotating element  1 . In this manner, the hollow rotating shaft  2  is provided with the second cylindrical section  5   b  on each end.  
         [0016]    The inner section of the second cylindrical section  5   b  comprises an inlet passage  15   a  and an outlet passage  15   b . Coolant, which is introduced from the outside of the hollow rotating shaft  2  to the inlet passage  15   a,  flows through the annular gap  7  and cools the inner section of the rotating element  1 . Thereafter the coolant is discharged from the outlet passage  15   b  to the outside of the hollow rotating shaft  2 .  
         [0017]    When the sectional area of the inlet passage  15   a  perpendicular to a direction of the rotating axis is taken to be Ai and the sectional area of the annular gap  7  perpendicular to a direction of the rotating axis is taken to be Ac, the following relationship is established. 
         Ai≧Ac 
         [0018]    Consequently, the flow speed of coolant in the annular gap  7  is increased compared with the flow speed of the coolant in the inlet and outlet passage  15   a ,  15   b , which increases the cooling efficiency in turn.  
         [0019]    The inner cylindrical section  6  is divided into two sections  6   a ,  6   b . The sections  6   a ,  6   b  of the inner cylindrical section  6  are open at one end and closed at the other end. The sections  6   a ,  6   b  of each inner cylindrical section  6  are housed in the first cylindrical section  5   a  so that the open ends are mutually opposed.  
         [0020]    The sections  6   a ,  6   b  of the inner cylindrical section  6  are pressed into contact from both closed ends by the two second cylindrical sections  5   b  so that the inner cylindrical section  6  rotates together with the rotating element  1 . The closed end of the sections  6   a ,  6   b  of the inner cylindrical section  6  comprises a conical end wall  8 , namely a protruding end wall. The conical end wall  8  is positioned in front of a conical surface  9 . The conical end wall  8  faces a conical surface  9  constituting an enlarging section of the passages  15   a ,  15   b  on an inner section of the second cylindrical section  5   b.  The shape of the end wall  8  is not limited to a conical shape and may be a convex shape which gradually narrows.  
         [0021]    Referring now to FIG. 2, a plurality of splines  10  are formed at equal intervals in a peripheral direction on the inner peripheral surface of the first cylindrical section  5   a . The plurality of splines  10  extends in an axial direction of the first cylindrical section  5   a , in other words in a direction of the rotation axis. The plurality of splines  10  make contact with the outer peripheral surface of the inner cylindrical section  6  to maintain the annular gap  7  and divide the annular gap  7  between the first cylindrical section  5   a  and the inner cylindrical section  6  into several equal portions. The splines  10  also have the function of increasing the cooling efficiency by increasing the surface area of the inner peripheral surface of the first cylindrical section  5   a.    
         [0022]    Referring now to FIG. 3 and FIG. 4, a plurality of tiny projections  11  are formed on the conical end wall  8  of the inner cylindrical section  6 . The tiny projections  11  have a small size in comparison with the conical end wall  8 . A tiny annular space  14  is formed between the conical end wall  8  and the conical surface  9  because the tiny projections  11  come into contact with the conical surface  9  of the second cylindrical section  5   b.    
         [0023]    The splines  10  can be manufactured in a cost-effective manner by an extraction process. The tiny projections  11  can also be simply manufactured by a pressing process.  
         [0024]    The opposed pair of sections  6   a ,  6   b  in the inner cylindrical section  6  are integrated by being pressed and gripped by the two second cylindrical sections  5   b . However a slit  12  with an extremely small width is naturally formed along the contact surface of the sections  6   a ,  6   b  of the inner cylindrical section  6  because the processed contact surface normally comprises tiny undulations. Thus the width of the slit is so small as not to be apparent to the naked eye. Since the inner space  13  of the inner cylindrical section  6  is connected with the outer annular gap  7  because of the slit  12 , a portion of the coolant also fills the inner space  13 .  
         [0025]    Referring to FIG. 1B, when the inner cylindrical section  6  is not divided into two sections, the inner cylindrical section  6  can be manufactured as an integrated component. In this case, the inner space  13  of the inner cylindrical section  6  can be connected with the outer annular gap  7  by providing a hole  17  in the inner cylindrical section  6 .  
         [0026]    Referring now to FIG. 4, the difference in the inclination of the conical surface  9  of the second cylindrical section  5   b  and the conical end wall  8  of the inner cylindrical section  6  will be described. Due to the tiny projections  11 , the angle θa subtended by the inner conical face  9  and the rotation axis  30  is smaller than the angle θi subtended by the conical wall face  8  and the rotation axis  30 . The width of the annular space  14  gradually reduces towards the annular gap  7 . Consequently, when coolant flows into the annular space  14  between the conical end wall  8  and the conical surface  9  from the inlet passage  15   a , the cross-sectional area of the passage of the coolant is gradually reduced. Thus, the coolant flowing towards the annular gap  7  undergoes rapid acceleration and does not display a large pressure loss. This result has the effect of reducing pressure loss in the pump which supplies coolant.  
         [0027]    The above structure allows flow of coolant from the inlet passage  15   a  into the annular gap  7  between the inner cylindrical section  6  and the first cylindrical section  5   a . A portion of the coolant fills the inner space  13  of the inner cylindrical section  6  from the tiny slit  12  which is formed between sections  6   a ,  6   b  of the inner cylindrical section  6  in order to cool the rotating element  1  from the inside. Thereafter the coolant is discharged from the outlet passage  15   b . Since the coolant has a high flow speed when flowing through the annular gap  7 , the coolant removes heat from the rotating element  1  in an efficient manner. When a general-purpose lubricating oil is used as a coolant, it is preferred that the width d 1  of the annular gap  7  is greater than 0.3 mm. This setting improves the cooling efficiency and does not generate excessive pressure loss. The upper limit of the width d 1  of the annular gap  7  are determined from the relationship Ai≧Ac of the cross-sectional area Ai of the inlet passage  15   a  with the cross sectional area Ac of the annular gap  7 .  
         [0028]    The annular gap between the first cylindrical section  5   a  and the inner cylindrical section  6  is maintained at equal intervals by the splines  10 . Thus the coolant displays a constant flow rate and as a result an equal cooling effect is obtained on the entire periphery of the first cylindrical section  5   a.    
         [0029]    The hollow rotating shaft  2  of the rotating element  1  is formed from a first cylindrical section  5   a  with a large radius and a second cylindrical section  5   b  with a small radius. The thin-walled inner cylindrical section  6  is disposed in the inner section of the first cylindrical section  5   a . Since a large inner space  13  is formed in the hollow rotating shaft  2 , the weight of the hollow rotating shaft  2  is low and the inertia of the hollow rotating shaft  2  is conspicuously low in comparison to a rotating shaft without an inner space  13 . Consequently, the rotating performance and vibration characteristics of the rotating electric machine are improved.  
         [0030]    The inner space  13  of the inner cylindrical section  6  is connected to the annular gap  7  through a slit  12  and coolant also fills the inner space  13  of the inner cylindrical section  6 . Consequently, even when the temperature or pressure of the coolant varies, the pressure differential between the inner and outer sections of the inner cylindrical section  6  is small. As a result, although the inner cylindrical section  6  which enters the first cylindrical section  5   a  is extremely thin, deformation of the inner cylindrical section  6  is avoided. In other words, it is possible to prevent the width of the annular gap  7  from being unnecessarily enlarged and the annular gap  7  from being closed. Consequently improved stable cooling performance is maintained at all times.  
         [0031]    Furthermore since the bearing on the casing grips the second cylindrical section  5   b  with a small radius, bearing loss is reduced and the rotating element  1  displays excellent rotation performance.  
         [0032]    Referring to FIG. 5, a second embodiment of this invention will be described. In the second embodiment, the number of splines  10  is increased in comparison to the first embodiment. Consequently, the increase in the heat radiating surface makes the heat radiation effect higher than that in the first embodiment. The splines  10  are divided at a plurality of positions in an axial direction by notches  10   a.  The cooling effect is increased because the coolant displays turbulent flow at the divided positions.  
         [0033]    Referring to FIG. 6, a third embodiment of this invention will be described. The hollow rotating shaft  2  is provided with a first cylindrical section  5   a  with a large radius, a second cylindrical section  5   b  with a small radius disposed upstream side of coolant flow and a stepped shaft  5   c  disposed on the downstream side of coolant flow. The stepped shaft  5   c  has a small radius in comparison with the outer radius of the first cylindrical section  5   a , over its entire length. The stepped shaft  5   c  is provided with a small-radius section  21  and a flange  22 . The flange  22  is engaged with the inner periphery of the first cylindrical section  5   a  and is fixed in a position making contact with the inner cylindrical section  6 . The small-radius section  21  is retained on a bearing  20 . A plurality of coolant outlets  23  connected with the annular gap  7  are formed on the flange  22 . A baffle plate  24  is disposed in the passage connecting the inlet passage  15   a  of the second cylindrical section  5   b  with the annular gap  7 . This allows the circulation of coolant to simply follow the rotation of the hollow rotating shaft  2 . Furthermore, in this embodiment, the inner space in the inner cylindrical section  6  is sealed to prevent coolant from entering.  
         [0034]    In the third embodiment, the coolant flowing in the annular gap  7  does not undergo a large resistance because it is discharged to the outside of the rotating element  1  from a coolant outlet  23 . As a result, it is possible to reduce the pressure loss in the coolant in comparison to the first embodiment in which coolant flowing through the annular gap  7  is recirculated to the outlet passage. In addition, the discharged coolant can be used in order to lubricate the bearing  20 .  
         [0035]    The entire contents of Japanese Patent Applications P2001-187589 (filed Jun. 21, 2001) and P2002-49439 (filed Feb. 26, 2002) are incorporated herein by reference.  
         [0036]    Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the above teachings. The scope of the invention is defined with reference to the following claims.