Abstract:
A rotary apparatus, in which a rotor can rotate stably when it rotates at a high speed, can rotate at a relatively high speed by the torque of a highly reliable induction motor. The rotary apparatus comprises a rotor shaft and an induction motor. The induction motor includes a motor rotor core fixed to the rotor shaft, conductors disposed in the motor rotor core and a motor end ring for assembling and connecting the conductors, and can rotate the rotor shaft at a high speed by the torque. The rotor shaft is provided with a member that covers the motor end ring.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a rotary apparatus in which a rotor shaft, to which a rotor of an induction motor is fixed, rotates at a relatively high speed by the torque of the induction motor. 
         [0003]    2. Description of the Related Art 
         [0004]    A turbomolecular pump is an example of a rotary apparatus in which a rotor shaft rotates at a relatively high speed.  FIG. 1  is a cross-sectional diagram showing a conventional turbomolecular pump (see Japanese Patent Laid-Open Publication No. 2002-286036). The turbomolecular pump includes a rotor shaft  11  to which are integrally fixed a motor rotor  13  of an induction motor  12 , targets  15 ,  15  of a radial magnetic bearing  14 , sensingobjectportions  17 ,  17  of aradial displacement sensor  16 , a target  19  of an axial magnetic bearing  18 , and a sensing object portion (not shown) of an axial displacement sensor. 
         [0005]    A rotor (impeller)  64  having rotaryblades  60  and a threaded groove portion  62  is secured to an upper end of the rotor shaft  11 . Fixed blades  68 , arranged alternately with the rotary blades  60 , are provided on an inner surface of a pump casing  66 . A blade exhaust portion L 1 , which exhausts a gas by the interaction of the rotary blades  60  rotating at a high speed and the stationary fixed blades  68 , is thus constructed. A threaded groove spacer  70  is disposed such that it surrounds the threaded groove portion  62 . A threaded groove exhaust portion L 2 , which exhausts a gas by the drag effect of the threaded groove  62   a  of the threaded groove portion  62  rotating at a high speed, is thus constructed. With the threaded groove exhaust portion L 2  provided downstream of the blade exhaust portion L 1 , the turbomolecular pump can deal with a wide range of flow rate. 
         [0006]    In the conventional turbomolecular pump, the motor rotor  13  and rotor spacers  20  for axial positioning are in contact with motor end rings  13   b,    13   b  and the targets  15 ,  15  in the axial direction, as shown in  FIG. 2 . The motor end ring  13   b  is used to assemble and connect conductors disposed in a core  13   a  of the motor rotor  13  of the induction motor  12 . A cast pure aluminum material is generally used for the motor end ring  13   b.  The specific gravity, tensile strength, longitudinal elastic modulus and linear expansion coefficient of cast pure aluminum generally used for motor end rings are as follows: 
         [0007]    Specific gravity: 2.7 
         [0008]    Tensile strength: 68 MPa 
         [0009]    Longitudinal elastic modulus: 68.6 MPa 
         [0010]    Linear expansion coefficient: 2.4×10 −5 /0C 
         [0011]    It is possible that the strength of a motor end ring can restrict the permissible rotating speed of a rotor when rotating it at a high speed. 
       SUMMARY OF THE INVENTION 
       [0012]    In the conventional turbomolecular pump, the motor end ring  13   b  is cantilevered, as shown in  FIG. 2 . Such a motor end ring  13   b,  when rotated at a high speed, elastically deforms by centrifugal force, etc., as shown by broken lines  100  in FIG.  3 . In order to reduce the radial deformation of the motor end ring  13   b,  the end surface of the motor end ring  13   b  is in contact with the end surface of the rotor spacer  20 . Reducing the deformation at the end portion of the motor ring  13   b  can also reduce stress which acts on that portion. However, the motor rotor  13  generates heat when carrying out an operation that places a load on the induction motor  12 , such as introduction of a gas into the pump. Because the motor end ring  13   b,  made of aluminum, has a larger expansion coefficient than other members made of other materials, an axial internal stress acts on the motor end ring  13   b  and the rotor spacer  20  when the motor end ring  13   b  generates heat. The internal stress (which causes the motor end ring  13   b  and the motor spacer  20  to compress each other) brings about a change in the natural frequency of the entire rotor, hindering stable rotation of the rotor. 
         [0013]    The present invention has been made in view of the above situation in the related art. It is therefore an object of the present invention to provide a rotary apparatus in which a rotor can rotate stably when it rotates at a high speed and which rotates at a relatively high speed by the torque of a highly reliable induction motor. 
         [0014]    In order to achieve the object, the present invention provides a rotary apparatus comprising: a rotor shaft; and an induction motor including a motor rotor core fixed to the rotor shaft, conductors disposed in the motor rotor core and a motor end ring for assembling and connecting the conductors, and capable of rotating the rotor shaft at a high speed by the torque. The rotor shaft is provided with a member that covers the motor end ring. 
         [0015]    With the provision, to the rotor shaft, of the member that covers the motor end ring which assembles and connects conductors disposed in the motor rotor core, it becomes possible to prevent radial deformation of the motor end ring upon high-speed rotation, thereby preventing breakage of the motor end ring. 
         [0016]    In a preferred aspect of the present invention, the member that covers the motor end ring, in its portion lying outside the outer periphery of the motor end ring, is in axial contact with the motor rotor core. 
         [0017]    In a preferred aspect of the present invention, the member that covers the motor end ring, in its portion lying inside the inner periphery of the motor end ring, is in axial contact with the motor rotor core. 
         [0018]    Because the member that covers the motor end ring, in its portion lying either outside the outer periphery or inside the inner periphery of the motor end ring, is in axial contact with the motor rotor core, axial positioning of the motor rotor core, etc. can be performed irrespective of the motor end ring having a large thermal expansion coefficient. This can suppress the action of an internal stress due to thermal expansion of the motor end ring, thereby preventing a change in the natural frequency of the entire rotor. 
         [0019]    In a preferred aspect of the present invention, the end of the motor end ring on the side opposite the motor rotor core is not in contact with the member that surrounds the motor end ring, with an axial gap being formed between them. 
         [0020]    This can prevent an increase in internal stress due to thermal expansion of the motor end ring, and thus can prevent a change in the natural frequency of the entire rotor caused thereby. 
         [0021]    In a preferred aspect of the present invention, the motor end ring is not in radial contact with an inner peripheral surface of the member that surrounds the motor end ring, with a radial gap being formed between them. 
         [0022]    In a preferred aspect of the present invention, the motor end ring has a tapered cross-sectional shape whose radial thickness decreases with distance from the motor rotor core. 
         [0023]    The use of such a tapered cross-sectional shape can reduce deformation of the motor end ring caused by its rotation. 
         [0024]    According to the present invention, the provision to the rotor shaft of the member that covers the motor end ring makes it possible to prevent displacement of the motor end ring upon its rotation, thereby preventing breakage of the motor end ring. Thus, the present invention can provide a highly reliable rotary apparatus which is excellent in high-speed rotation stability. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0025]      FIG. 1  is a cross-sectional diagram showing a conventional turbomolecular pump; 
           [0026]      FIG. 2  is a cross-sectional diagram showing a shaft assembly of the turbomolecular pump of  FIG. 1 ; 
           [0027]      FIG. 3  is an enlarged view of a portion of  FIG. 2 ; 
           [0028]      FIG. 4  is a cross-sectional diagram showing an embodiment of a rotor for use in a rotary apparatus according to the present invention; 
           [0029]      FIG. 5  is an enlarged view of a portion of  FIG. 4 ; 
           [0030]      FIG. 6  is a cross-sectional diagram showing another embodiment of a rotor for use in a rotary apparatus according to the present invention; 
           [0031]      FIG. 7  is an enlarged view of a portion of  FIG. 6 ; 
           [0032]      FIG. 8  is a cross-sectional diagram showing yet another embodiment of a rotor for use in a rotary apparatus according to the present invention; and 
           [0033]      FIG. 9  is a cross-sectional diagram showing yet another embodiment of a rotor for use in a rotary apparatus according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]    Preferred embodiments of the present invention will now be described with reference to the drawings.  FIG. 4  is a cross-sectional diagram showing an embodiment of a rotor for use in a rotary apparatus according to the present invention, and  FIG. 5  is an enlarged view of a portion of  FIG. 4 . The rotor  10  of this embodiment is a shaft assembly of a turbomolecular pump. The rotor  10  includes a rotor shaft  11 . A motor rotor  13  of an induction motor, and targets  15 ,  15  of a radial magnetic bearing, disposed on both sides of the motor rotor  13 , are fixed to the rotor shaft  11  and arranged in the axial direction with rotor spacers  20 ,  20  interposed between the targets  15 ,  15  and a motor rotor core  13   a.  The motor rotor  13  has the motor rotor core  13   a  in which conductors are disposed, and motor end rings  13   b,  which assemble and connect the conductors, are disposed on both sides of the motor rotor core  13   a.    
         [0035]    The rotor spacer  20  is a cylindrical shape and has, in its interior, a space having such an inner diameter that it covers an outer periphery of the motor end ring  13   b.  The motor rotor core  13   a,  the rotor spacers  20 ,  20  and the targets  15 ,  15  of the radial magnetic bearing are axially positioned such that the rotor spacers  20 ,  20  are interposed between the motor rotor core  13   a  and the targets  15 ,  15  disposed on both sides of the motor rotor core  13   a.  The rotor spacers  20 ,  20  each cover the radial periphery of the motor end ring  13   b.  In particular, the rotor spacer  20 , at its one end (the end on the side opposite the motor rotor core  13   a ), is fit to the rotor shaft  11  and, at the other end, in its portion lying outside the outer periphery of the motor end ring  13   b,  is in axial contact with the end surface of the motor rotor core  13   a.  A predetermined gap g 1  is formed between the end surface of the motor end ring  13   b  on the side opposite the motor rotor core  13   a  and the inner end surface of the rotor spacer  20 , i.e., the end surface of the motor end ring  13   b  is not in contact with the rotor spacer  20 . 
         [0036]    By thus providing the gap g 1  between the end surface of the motor end ring  13   b  and the inner surface of the rotor spacer  20  so that the motor end ring  13   b  is not in contact with the rotor spacer  20  in the axial direction, it becomes possible to prevent an increase in internal stress due to thermal expansion of the motor end ring  13   b,  thus preventing a change in the natural frequency of the rotor caused thereby. Further, by constructing the rotor spacer  20  such that it covers the radial periphery of the motor end ring  13   b,  it becomes possible to prevent radial deformation of the motor end ring  13   b  due to centrifugal force, etc., thereby preventing breakage of the motor end ring  13   b  caused by the deformation. The radial gap g 2  between the motor end ring  13   b  and the inner surface of the rotor spacer  20 , covering the motor end ring  13   b,  may be sufficient if it is formed to such an extent as to enable assembling of the rotor  10 . 
         [0037]    In order to avoid direct contact with the motor end ring  13   b,  the rotor spacer  20 , in its portion lying outside the outer periphery of the motor end ring  13   b,  is made to be in contact with the motor rotor core  13   a  when positioning the motor rotor  13  and the targets  15 ,  15  of the of the radial magnetic bearing in the axial direction, as shown in  FIG. 4 . Thus, the rotor spacers  20 ,  20  are in direct contact with the both end surfaces of the motor rotor core  13   a  in the axial direction. Silicon steel, which is a ferromagnetic material, may be used as a material for the motor rotor core  13   a.  A stainless steel (SUS) alloy or a titanium alloy is suitably used as a material for the rotor spacer  20 . The linear expansion coefficients of a stainless steel alloy and a titanium alloy are smaller than the linear expansion coefficient of aluminum, and are relatively near the linear expansion coefficient of silicon steel. Accordingly, the use of such materials in combination for the motor rotor core  13   a  and for the rotor spacer  20  will reduce an increase in internal stress upon thermal expansion of the members, and thus reduce a change in the natural frequency of the entire rotor  10 . 
         [0038]    Table 1 below shows specific examples of materials usable for the rotor spacer  20 , i.e., the member that covers the motor end ring  13   b,  and the properties of the materials (specific gravity, tensile strength [Mpa], longitudinal elastic modulus [Gpa], linear expansion coefficient×10 −5 /□ c, and specific strength=tensile strength/specific gravity). 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 SUS 
                 SUS 
                 SUS 
                 SUS 
                 TAF 
               
               
                   
                 304 
                 403 
                 420 
                 630 
                 6400 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Specific gravity 
                 7.93 
                 7.75 
                 7.75 
                 7.75 
                 4.42 
               
               
                 Tensile strength [Mpa] 
                 520 
                 440 
                 540 
                 930 
                 890 
               
               
                 Longitudinal elastic 
                 193 
                 200 
                 200 
                 196 
                 113 
               
               
                 Modulus [GPa] 
               
               
                 linear expansion 
                 17.2 
                 9.9 
                 10.3 
                 10.8 
                 8.8 
               
               
                 coefficient × 10 −5 /□C 
               
               
                 Specific strength 
                 65.6 
                 56.8 
                 70 
                 120 
                 201 
               
               
                   
               
               
                 Specific strength = tensile strength/specific gravity 
               
             
          
         
       
     
         [0039]    As shown in Table 1, SUS  304 , SUS  403 , SUS  420 , SUS  630  and TAF  6400  are examples of materials usable for the member (rotor spacer  20 ) that covers the motor end ring  13   b.    
         [0040]    Because the rotor spacer  20  constrains radial deformation of the motor end ring  13   b,  the rotor spacer  20  itself slightly deforms radially, as shown by dotted lines  101  in  FIG. 4 . In view of the slight radial deformation of the rotor spacer  20 , it is desirable that the outer diameter Ds of the rotor spacer  20  be made not more than the outer diameter Dc (Ds≦Dc) of the other members of the rotor  10 , such as the motor rotor core  13   a  and the target  15  of the radial magnetic bearing (see  FIG. 4 ), as shown in  FIG. 5 . 
         [0041]      FIG. 6  is a cross-sectional diagram showing another embodiment of a rotor for use in a rotary apparatus according to the present invention, and  FIG. 7  is an enlarged view of a portion of  FIG. 6 . As shown in the Figures, in this embodiment, the member that covers the motor end ring  13   b,  i.e., the rotor spacer  20 , in its portion lying inside the inner periphery of the motor end ring  13   b,  is in axial contact with the end surface of the motor rotor core  13   a.  Further, a space having such an inner diameter that it covers the outer periphery of the motor end ring  13   b  is formed in the rotor spacer  20  at its end on the side opposite the motor rotor core  13   a.  The motor rotor core  13   a,  the rotor spacers  20 ,  20  and the targets  15 ,  15  of the radial magnetic bearing are axially positioned such that the rotor spacers  20 ,  20  are interposed between the motor rotor core  13   a  and the targets  15 ,  15  disposed on both sides of the motor core  13   a.  The rotor spacers  20 ,  20  each cover the end portion of the periphery of the motor end ring  13   b  on the side opposite the motor rotor core  13   a.  Further, a predetermined gap gl is formed between the end surface of the motor end ring  13   b  on the side opposite the motor rotor core  13   a  and the inner end surface of the rotor spacer  20 . 
         [0042]    By thus providing the gap g 1  between the end surface of the motor end ring  13   b  and the inner end surface of the rotor spacer  20  so that the motor end ring  13   b  is not in contact with the rotor spacer  20  in the axial direction, it becomes possible to prevent an increase in internal stress due to thermal expansion of the motor end ring  13   b,  thus preventing a change in the natural frequency of the rotor caused thereby. Further, by constructing the rotor spacer  20  such that it covers the end portion of the periphery of the motor end ring  13   b  on the side opposite the motor rotor core  13   a,  it becomes possible to prevent radial deformation of the motor end ring  13   b  due to centrifugal force, etc., thereby preventing breakage of the motor end ring  13   b  caused by the deformation. 
         [0043]    Because the rotor spacer  20  constrains radial deformation of the motor end ring  13   b,  the rotor spacer  20  itself slightly deforms radially, as shown by dotted lines  102  in  FIG. 6 . In view of the slight radial deformation of the rotor spacer  20 , it is desirable that the outer diameter Ds of the rotor spacer  20  be made not more than the outer diameter Dc (Ds≦Dc) of the other members of the rotor  10 , such as the motor rotor core  13   a  and the target  15  of the radial magnetic bearing (see  FIG. 6 ), as shown in  FIG. 7 . 
         [0044]      FIG. 8  is a cross-sectional diagram showing yet another embodiment of a rotor for use in a rotary apparatus according to the present invention. As shown in  FIG. 8 , the motor end ring  13   b  has a tapered cross-sectional shape whose radial thickness decreases with distance from the motor rotor core  13   a.  The rotor spacer  20  has, in its interior, a space having a tapered cross-sectional shape and covering the periphery of the tapered motor end ring  13   b.  Thus, the motor end ring  13   b  is disposed in the tapered space, and the rotor spacer  20  covers the periphery of the motor end ring  13   b.  Further, a predetermined gap g 1  is formed between the end surface of the motor end ring  13   b  on the side opposite the motor rotor core  13   a  and the inner end surface of the rotor spacer  20 . 
         [0045]    The use of such a tapered cross-sectional shape can reduce deformation of the motor end ring  13   b  caused by centrifugal force during rotation of the motor end ring  13   b.  Further, because of an increase in the cross-sectional area of the base portion of the motor end ring  13   b,  the structural strength of the motor end ring  13   b  can be increased. The motor end ring  13   b  collects and connects secondary currents flowing in the conductors in the motor rotor core  13   a.  If the cross-sectional conduction area of the motor end ring  13   b  is the same as that shown in  FIG. 2 , the electric resistance is the same and thus the performance of the induction motor is the same. Insofar as the same cross-sectional conduction area can be maintained, any shape can be employed for the motor end ring  13   b.    
         [0046]    By providing the gap g 1  between the end surface of the motor end ring  13   b  and the inner end surface of the rotor spacer  20  so that the motor end ring  13   b  is not in contact with the rotor spacer  20  in the axial direction, as described above, it becomes possible to prevent an increase in internal stress due to thermal expansion of the motor end ring  13   b,  thus preventing a change in the natural frequency of the rotor caused thereby. Further, by constructing the rotor spacer  20  such that it covers the radial periphery of the motor end ring  13   b,  it becomes possible to prevent radial deformation of the motor end ring  13   b  due to centrifugal force, etc., thereby preventing breakage of the motor end ring  13   b  caused by the deformation. The radial gap g 2  between the motor end ring  13   b  and the inner surface of the rotor spacer  20 , covering the motor end ring  13   b,  may be sufficient if it is formed to such an extent as to enable assembling of the rotor  10 . 
         [0047]      FIG. 9  is a cross-sectional diagram showing yet another embodiment of a rotor for use in a rotary apparatus according to the present invention. As shown in the  FIG. 9 , in this embodiment, the member that covers the motor end ring  13   b,  i.e., the rotor spacer  20 , in its portion lying inside the inner periphery of the motor end ring  13   b,  is in axial contact with the end surface of the motor rotor core  13   a.  The motor end ring  13   b  has a tapered cross-sectional shape whose radial thickness decreases with distance from the motor rotor core  13   a.  A space having such an inner diameter that it covers the outer periphery of the motor end ring  13   b  is formed in the rotor spacer  20  at its end on the side opposite the motor rotor core  13   a.  The rotor spacer  20 , interposed between the motor rotor core  13   a  and the target  15  of the radial magnetic bearing, covers the end portion of the periphery of the motor end ring  13   b  on the side opposite the motor rotor core  13   a.  The rotor thus constructed has the same technical effect as the rotor having the construction shown in  FIG. 8 . 
         [0048]    A rotary apparatus having the above-described rotor  10  can be exemplified by a turbomolecular pump as shown in  FIG. 1 , which drives a rotor at a rotating speed of tens of thousands of revolutions per minute. The rotor  10  can also be applied, e.g., in a molecular drag pump that exhausts a larger flow rate than a turbomolecular pump. While the use of a magnetic bearing has been described, it is also possible to use a mechanical bearing, a kinetic pressure bearing or the like. 
         [0049]    While the present invention has been described with reference to the embodiments thereof, it will be understood by those skilled in the art that the present invention is not limited to the particular embodiments described above, but it is intended to cover modifications within the inventive concept. For example, though in the above-described embodiments the rotor spacer  20  also serves as a member that covers the periphery of the motor end ring  13   b,  it is also possible to provide a member, which covers the periphery of the motor end ring  13   b,  separately from the rotor spacer  20 .