Abstract:
An object is to provide a multi-stage electric centrifugal compressor including an electric motor and free from risk of breakdown of an operation control part due to heat generated by low-pressure stage and high-pressure stage compressors. A multi-stage electric centrifugal compressor includes: an electric motor; a pair of centrifugal compressors coupled to either side of the electric motor, the pair of centrifugal compressors comprising a low-pressure stage compressor and a high-pressure stage compressor connected in series; a heat-shielding plate disposed between an end portion on a low-pressure-stage-compressor side of the electric motor and an end portion on a motor-housing side of the low-pressure stage compressor, and configured to shield heat generated by the low-pressure stage compressor; and a bending portion disposed in middle of the heat-shielding plate, and extending along a rotational shaft of the electric motor so as to surround an outer periphery of the rotational shaft. An inner surface of the bending portion faces the rotational shaft via a clearance part, and the bending portion functions as a shaft sealing portion which prevents leakage of intake air from the low-pressure stage compressor.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to a multi-stage electric centrifugal compressor including an electric motor and compressors disposed on either side of a rotational shaft extending from either side of the electric motor. 
       BACKGROUND ART 
       [0002]    Engines, an example of an internal combustion engine, have been reduced in size, and there are growing needs for an increased low-speed torque and improved responsiveness. A multi-stage centrifugal compressor is attracting attention as an approach to meet such needs (see Patent Document 1). A multi-stage centrifugal compressor has a rotational shaft extending from either side of a rotary driving unit, a low-pressure stage compressor disposed on one end of the rotational shaft, and a high-pressure stage compressor connected to the opposite end of the rotational shaft and configured to re-compress intake air compressed by the low-pressure stage compressor. 
         [0003]    If an electric motor is employed as the rotary driving unit of the above multi-stage centrifugal compressor, when the electric motor operates to drive the low-pressure stage compressor and the high-pressure stage compressor, intake air compressed by the low-pressure stage compressor has its temperature increased and generates heat, and so does intake air compressed by the high-pressure stage compressor. Accordingly, heat is accumulated in the multi-stage centrifugal compressor, and the electric motor may break down. 
         [0004]    Thus, a motor housing that retains an electric motor is normally equipped with a plurality of heat-dissipating plates. Further, a centrifugal compressor utilizing a centrifugal force can be easily reduced in size, and thus an operation control part that controls operation of an electric motor is sometimes provided accommodated in a centrifugal compressor. 
       CITATION LIST 
     Patent Literature 
       [0005]    Patent Document 1: JP2004-11440A 
       SUMMARY 
     Problems to be Solved 
       [0006]    In recent years, besides a turbo assist function for the purpose of improvement of responsiveness at a low engine speed, turbo assist is also required during normal operation, which makes a usage environment of engines increasingly severe. Accordingly, even if heat generated by an electric motor driving a centrifugal compressor is dissipated through heat-dissipating plates, heat generated by a low-pressure stage compressor and a high-pressure stage compressor may not be dissipated sufficiently from the heat-dissipating plates, and may accumulate in a multi-stage centrifugal compressor. As a result, an operation control part, which is an electric component, may break down due to accumulated heat. 
         [0007]    In view of the above, an object of at least some embodiments of the present invention is to provide a multi-stage electric centrifugal compressor which includes an electric motor but does not have a risk of breakdown of an operation control part due to heat generated by a low-pressure stage compressor and a high-pressure stage compressor. 
       Solution to the Problems 
       [0008]    A multi-stage electric centrifugal compressor according to some embodiments of the present invention comprises: an electric motor; a pair of centrifugal compressors coupled to either side of the electric motor, the pair of centrifugal compressors comprising a low-pressure stage compressor and a high-pressure stage compressor connected in series; a heat-shielding plate disposed between an end portion on a low-pressure-stage-compressor side of the electric motor and an end portion on a motor-housing side of the low-pressure stage compressor, and configured to shield heat generated by the low-pressure stage compressor; and a bending portion disposed in middle of the heat-shielding plate, and extending along a rotational shaft of the electric motor so as to surround an outer periphery of the rotational shaft. An inner surface of the bending portion faces the rotational shaft via a clearance part, and the bending portion functions as a shaft sealing portion which prevents leakage of intake air from the low-pressure stage compressor. 
         [0009]    In the above multi-stage electric centrifugal compressor, the heat-shielding plate for shielding heat generated by the low-pressure stage compressor is disposed between the end portion of the electric motor on the side of the low-pressure stage compressor and the end portion of the low-pressure stage compressor on the side of the motor housing, and thereby it is possible to prevent heat, generated by intake air with an increased temperature from flowing through the low-pressure stage compressor, from propagating toward the electric motor. Thus, it is possible to obtain a multi-stage electric centrifugal compressor capable of protecting an electric component disposed on a motor housing from heat generated by a low-pressure stage compressor. Further, the bending portion is disposed in the middle of the heat-shielding plate, and extending along the rotational shaft so as to surround the outer periphery of the rotational shaft of the electric motor, with the inner surface of the bending portion facing the rotational shaft via the clearance part, so that the bending portion functions as a shaft sealing portion which prevents leakage of intake air from the low-pressure stage compressor. Accordingly, the bending portion functioning as a shaft sealing portion reduces leakage of intake air that may flow through the low-pressure stage compressor and inside the bending portion to leak out toward a bearing that supports the rotational shaft during operation of the low-pressure stage compressor. Thus, it is possible to reduce a risk of accumulation of heat in the multi-stage electric centrifugal compressor, which makes it possible to position electric components in the multi-stage electric centrifugal compressor, and to prevent a risk of damage to a bearing that supports the rotational shaft due to uneven arrangement of grease in the bearing. Further, the bending portion can utilize the inner surface of the bending portion as a guide member that determines the position during assembly of the multi-stage electric centrifugal compressor. 
         [0010]    In some embodiments, an operation control part is disposed on the low-pressure-stage-compressor side of the motor housing, and configured to control operation of the electric motor. 
         [0011]    In this case, the operation control part is disposed on the low-pressure-stage-compressor side of the motor housing, and thus positioned remote from the high-pressure stage compressor. Accordingly, it is possible to reduce an influence of heat generated by intake air that flows to the high-pressure stage compressor and gets heated. Further, while the operation control part is disposed near the low-pressure stage compressor, the heat-shielding plate is disposed between the operation control part and the low-pressure stage compressor, and thereby the heat-shielding plate shields heat generated by intake air that flows to the low-pressure stage compressor and gets heated, which reduces influence from heat on the operation control part. Thus, it is possible to obtain a multi-stage electric centrifugal compressor capable of protecting an operation control part from heat generated by a high-pressure stage compressor and a low-pressure stage compressor. Moreover, the low-pressure stage compressor normally generates heat of a lower temperature than the high-pressure stage compressor during operation, and thus it is desirable to position the operation control part, which is an electric component, on the side of the low-pressure stage compressor of a lower temperature. 
         [0012]    Further, in some embodiments, the operation control part is disposed to have a gap from the heat-shielding plate. 
         [0013]    In this case, the operation control part is disposed to have a gap from the heat-shielding plate, and thus it is possible to prevent effectively propagation of heat of the heat-shielding plate to the operation control part. 
         [0014]    In some embodiments, the multi-stage electric centrifugal compressor further comprises: a seal-member fitting portion disposed on an outer periphery of the rotational shaft which faces the inner surface of the bending portion of the heat-shielding plate; and a ring disposed on an outer peripheral surface of the seal-member fitting portion and configured to slide relative to the inner surface of the bending portion. 
         [0015]    In this case, the ring is disposed on the outer peripheral surface of the seal-member fitting portion and configured to slide relative to the inner surface of the bending portion, and thereby the outer peripheral surface of the seal-member fitting portion and the inner surface of the bending portion are in slide contact via the ring. Accordingly, during operation of the low-pressure stage compressor, it is possible to prevent leakage of intake air even more securely with the ring, even if intake air flowing through the low-pressure stage compressor passes through the bending portion and tries to leak out toward the bearing disposed on the rotational shaft. Thus, it is possible to prevent infiltration of high-temperature intake air into the electric motor more effectively, and to dispose electric components (operation control part) inside the multi-stage electric centrifugal compressor, which makes it possible to obtain a multi-stage electric centrifugal compressor free from risk of uneven arrangement of grease in a bearing that supports a rotational shaft. Herein, the seal-member fitting portion may be formed integrally with the rotational shaft, or may be a cylindrical sleeve fitted onto the rotational shaft. 
         [0016]    In some embodiments, a plurality of the rings is disposed on the outer peripheral surface of the seal-member fitting portion, spaced from one another in an axial direction of the rotational shaft. 
         [0017]    In this case, a plurality of the rings is disposed on the outer peripheral surface of the seal-member fitting portion, spaced from one another in the axial direction of the rotational shaft, and thereby the outer peripheral surface of the seal-member fitting portion and the inner surface of the bending portion are in contact with each other via the plurality of rings. Accordingly, the rings and the inner surface of the bending portion contact each other via a larger contact area, and thus it is possible to enhance the sealing function. Accordingly, during operation of the low-pressure stage compressor, it is possible to prevent leakage of intake air securely with the rings, even if intake air flowing through the low-pressure stage compressor passes through the bending portion and tries to leak out toward the bearing. Thus, it is possible to prevent infiltration of high-temperature intake air into the electric motor, and to prevent accumulation of heat in the multi-stage electric compressor securely, as well as to achieve a multi-stage electric centrifugal compressor free from risk of uneven arrangement of grease in a bearing. 
         [0018]    In some embodiments, the low-pressure stage compressor is configured to have a lower compression ratio than the high-pressure stage compressor. 
         [0019]    In this case, the low-pressure stage compressor is configured to have a lower compression ratio than the high-pressure stage compressor, and thereby it is possible to suppress a temperature increase in the vicinity of the operation control part and to reduce a pressure in the vicinity of the bending portion. Accordingly, it is possible to obtain a multi-stage electric centrifugal compressor with a reduced risk of breakdown of an operation control part. 
       Advantageous Effects 
       [0020]    According to at least some embodiments of the present invention, it is possible to provide a multi-stage electric centrifugal compressor including an electric motor and free from risk of breakdown of an operation control part due to heat generated by a low-pressure stage compressor and a high-pressure stage compressor. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0021]      FIG. 1A  is a cross-sectional view of a multi-stage electric centrifugal compressor, and  FIG. 1B  is a partial enlarged view of a section indicated by arrow A in  FIG. 1A . 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    Embodiments of the multi-stage electric centrifugal compressor of the present invention will now be described with reference to  FIGS. 1A and 1B . The embodiments will be described referring to, as an example, a multi-stage electric centrifugal compressor including an electric motor and a pair of compressors disposed on either side of the electric motor. It is intended, however, that unless particularly specified, dimensions, materials, shapes, relative positions and the like of components described in the embodiments shall be interpreted as illustrative only and not intended to limit the scope of the present invention. 
         [0023]    As depicted in  FIG. 1A  (cross-sectional view), the multi-stage electric centrifugal compressor  1  includes a rotational shaft  3  supported rotatably, a low-pressure stage impeller  11  mounted to the first end of the rotational shaft  3 , a high-pressure stage impeller  21  mounted to the second end of the rotational shaft  3 , and an electric motor rotor  30  mounted to a middle section of the rotational shaft  3  in a longitudinal direction. 
         [0024]    The low-pressure stage impeller  11  is disposed inside a low-pressure stage compressor  10  disposed on the first end of the multi-stage electric centrifugal compressor  1 . The low-pressure stage compressor  10  includes the low-pressure stage impeller  11  mounted to the first end of the rotational shaft  3 , and a low-pressure stage housing  16  surrounding the low-pressure stage impeller  11 . The low-pressure stage housing  16  defines a space part  17  that accommodates the low-pressure stage impeller  11  rotatably. An inlet  17   a  for intake of intake air is disposed on the first end side of the space part  17 , and a flow channel  17   c  is formed in a radial direction of the space part  17 , the flow channel  17   c  communicating with the inlet  17   a  and curving in the circumferential direction of the low-pressure stage compressor  10 . Further, an outlet  17   b  communicating with the flow channel  17   c  is disposed on an end portion on one side in the width direction of the low-pressure stage housing  16 , i.e., on an end portion depicted in front of the page of  FIG. 1A . Intake air enters through the inlet  17   a,  has its temperature increased by being compressed by the low-pressure stage impeller  11 , flows through the flow channel  17   c,  and then exits through the outlet  17   b.    
         [0025]    An insertion opening  18  of a circular shape is disposed on the second end side of the low-pressure stage housing  16  in a side view, and the low-pressure stage impeller  11  can be inserted into the insertion opening  18 . The insertion opening  18  is an opening larger than the low-pressure stage impeller  11 , so that a part of the flow channel  17   b  is exposed. A side face  16   a  of the low-pressure stage housing  16  on the side of the insertion opening  18  has a flat shape and is formed in an annular shape in a side view. 
         [0026]    A heat-shielding plate  35  is disposed on the second end side of the low-pressure stage compressor housing  16 , and mounted to the side face  16   a  of the low-pressure stage compressor housing  16  so as to close the flow channel  17   c  being exposed. The heat-shielding plate  35  will be described below in detail. A motor housing  45  which retains the electric motor rotor  30  and a bearing  40 R is mounted to a high-pressure-stage-compressor- 20  side of the heat-shielding plate  35 . The motor housing  45  will be described below in detail. 
         [0027]    The low-pressure stage impeller  11  includes a back plate  12  of a disc shape, a boss portion  13  formed into a truncated conical shape and disposed integrally with the back plate  12  so as to protrude from a surface of the back plate  12  in a direction orthogonal to the surface of the back plate  12 , and a plurality of vanes  14  formed integrally from an outer circumferential surface of the boss portion  13  to the back plate  12 . A through hole  13   a  is disposed through the center of the boss portion  13 , and the rotational shaft  3  is inserted into the through hole  13   a,  and thereby the low-pressure stage impeller  11  is mounted to the rotational shaft  3  via a nut  15 . The low-pressure stage impeller  11  has a diameter smaller than that of the high-pressure stage impeller  21  of the high-pressure stage compressor  20 , which will be described below. Thus, the low-pressure stage compressor  10  has a smaller pressure ratio than the high-pressure stage compressor  20 . 
         [0028]    The high-pressure stage compressor  20  has a configuration similar to that of the low-pressure stage compressor  10 , and includes the high-pressure stage impeller  21  mounted to the second end side of the rotational shaft  3 , and a high-pressure stage housing  26  surrounding the high-pressure stage impeller  21 . The high-pressure stage housing  26  defines a space part  27  that accommodates the high-pressure stage impeller  21  rotatably. An inlet  27   a  for intake of intake air is disposed on the second end side of the space part  27 , and a flow channel  27   c  is formed in a radial direction of the space part  27 , the flow channel  27   c  communicating with the inlet  27   a  and curving in the circumferential direction of the high-pressure stage compressor  20 . Further, an outlet  27   b  communicating with the flow channel  27   c  is disposed on an end portion on one side in the width direction of the high-pressure stage housing  26 , i.e., on an end portion depicted in front of the page of  FIG. 1A . Intake air enters through the inlet  27   a,  has its temperature increased by being compressed by the high-pressure stage impeller  21 , flows through the flow channel  27   c,  and then exits through the outlet  27   b.    
         [0029]    The inlet  27   a  of the high-pressure stage housing  26  is in communication with the outlet  17   c  of the low-pressure stage housing  16  via an intake-air communication passage  29 . 
         [0030]    An insertion opening  28  of a circular shape is disposed on the first end side of the high-pressure stage housing  26  in a side view, and the high-pressure stage impeller  21  can be inserted into the insertion opening  28 . The insertion opening  28  is an opening larger than the high-pressure stage impeller  21 , so that a part of the flow channel  27   c  is exposed. A side face  26   a  of the high-pressure stage housing  26  on the side of the insertion opening  28  has a flat shape and is formed in an annular shape in a side view. 
         [0031]    The high-pressure stage impeller  21  has a configuration similar to that of the low-pressure stage impeller  11 , and includes a back plate  22  of a disc shape, a boss portion  23  formed into a truncated conical shape and disposed integrally with the back plate  22  so as to protrude from a surface of the back plate  22  in a direction orthogonal to the surface of the back plate  22 , and a plurality of vanes  24  formed integrally from an outer circumferential surface of the boss portion  23  to the back plate  12 . A through hole  23   a  is disposed through the center of the boss portion  23 , and the second end side of the rotational shaft  3  is inserted into the through hole  23   a,  and thereby the high-pressure stage impeller  21  is mounted to the second end side of the rotational shaft  3  via a nut  15 . Accordingly, the low-pressure stage impeller  11  is mounted to the first end side of the rotational shaft  3 , and the high-pressure stage impeller  21  is mounted to the second end side of the rotational shaft  3 , so that the low-pressure stage impeller  11  and the high-pressure stage impeller  21  rotate integrally with the rotational shaft  3 . 
         [0032]    The high-pressure stage impeller  21  has a diameter larger than the above mentioned diameter of the low-pressure stage impeller  11 . Thus, the high-pressure stage compressor  20  has a larger pressure ratio than the low-pressure stage compressor  10 . 
         [0033]    A pair of bearings  40 R,  40 L is disposed on either side of the rotational shaft  3  extending from either side of the electric motor rotor  30 . The bearings  40 R,  40 L are roller bearings of grease type. The bearing  40 L on the side of the high-pressure stage compressor  20 , from among the bearings  40 R,  40 L, is disposed in a bearing housing  50 . 
         [0034]    The bearing housing  50  is formed into an annular shape, and has an insertion hole  50   a  in the middle, into which the rotational shaft  3  can be inserted. A bearing mounting hole  50   b  is disposed on a low-pressure-stage-compressor- 10  side of the insertion hole  50   a,  and has a larger diameter than the insertion hole  50   a.  The bearing  40 L is mounted to the bearing mounting hole  50   b,  and the rotational shaft  3  is inserted into the bearing  40 L, and thereby the rotational shaft  3  is supported rotatably via the bearing  40 L. A protruding stepped portion  51  having an annular shape in a side view is disposed on an end portion of the bearing housing  50  on the side of the high-pressure stage compressor  20 , being fittable into the insertion opening  28  of the high-pressure stage housing  26 , and a surface portion  52  of an annular shape is disposed radially outside the protruding stepped portion  51 , facing and contacting the side face  26   a  of the high-pressure stage housing  26 . The bearing housing  50  is fixed integrally to the high-pressure stage housing  26  via a bolt  53  inserted through the high-pressure stage housing  26 . 
         [0035]    A side face  54  of the bearing housing  50  disposed on the side of the low-pressure stage compressor  10  has an engaging recess portion  54   a  having a circular shape in a side view. 
         [0036]    An end portion of the motor housing  45  disposed on the side of the high-pressure stage compressor  20  is inserted into the engaging recess portion  54   a.    
         [0037]    Meanwhile, the motor housing  45  has an insertion hole  45   a  into which the rotational shaft  3  is to be inserted, disposed on the first end side of the motor housing  45 . Further, a rotor space part  45   b  that surrounds the electric motor rotor  30  rotatably is disposed on the second end side of the motor housing  45 , and a bearing mounting hole  45   c  to mount the bearing  40 R is disposed between the insertion hole  45   a  and the rotor space part  45   b.  With the rotational shaft  3  inserted through the electric motor rotor  30  and the bearing  40 R while the electric motor rotor  30  is disposed in the rotor space part  45   b  and the bearing  40 R is disposed in the bearing mounting hole  45   c,  the rotational shaft  3  is rotatably supported and is rotatable in response to a driving force from the electric motor rotor  30 . A plurality of fins  46  extending radially outward is disposed on an outer periphery of the motor housing  45 , which makes it possible to dissipate heat generated by the electric motor rotor  30  and the bearing  40 R, for instance. 
         [0038]    The electric motor rotor  30  is a rotor of an electric motor, configured to rotate the rotational shaft  3  in response to a driving force with a motor coil (not depicted), and is capable of rotating at a high speed. Operation of the electric motor rotor  30  and the motor coil is controlled by an operation control part  47  described below. 
         [0039]    The heat-shielding plate  35  for shielding heat generated by the low-pressure stage compressor  10  is disposed between an end portion of the motor housing  45  disposed on the side of the low-pressure stage compressor  10  and an end portion of the low-pressure stage compressor  10  disposed on the side of the motor housing. The heat-shielding plate  35  is formed into a disc shape, and a flange portion  35   a  formed into an annular shape is disposed on a rim part of the heat-shielding plate  35 . The flange portion  35   a  is fixed to the low-pressure stage housing  16  via a bolt  36  while being in contact with a rim part of the low-pressure stage housing  16 , and is fixed to the motor housing  45  via a bolt (not depicted) while being in contact with a rim part of the motor housing  45 . 
         [0040]    The heat-shielding plate  35  is formed to have a smaller thickness at the inside thereof than at the flange portion  35   a.  The inside of the heat-shielding plate  35  extends along the side face  16   a  of the low-pressure stage housing  16  so as to close the insertion opening  18  of the low-pressure stage housing  16 . A bending portion  35   b  of a tubular shape is disposed in the middle of the heat-shielding plate  35 , bending toward the bearing  40 R to form an L shape and extending along an outer peripheral surface of the rotational shaft  3 , in a side view. An inner surface  35   c  of the bending portion  35   b  is formed as a through hole into which the rotational shaft  3  is to be inserted. As depicted in  FIG. 1B , the diameter φk of the inner surface  35   c  of the bending portion  35   b  is larger than the diameter φs of the rotational shaft  3 . 
         [0041]    Thus, a clearance part  39  is formed between the inner surface  35   c  of the bending portion  35   b  and the rotational shaft  3 . A seal-member fitting portion  37  of a cylindrical shape is disposed on the clearance part  39 , being fit onto an outer periphery of the rotational shaft  3 . A piston ring  38  is mounted to an outer peripheral surface of the seal-member fitting portion  37 , so as to slide relative to the inner surface  35   c  of the bending portion  35   b.  Two piston rings  38  are disposed, spaced from each other in the axial direction of the rotational shaft  3 . 
         [0042]    As depicted in  FIG. 1A , the operation control part  47  for controlling operation of the electric motor rotor  30  is disposed on the low-pressure-stage-compressor- 10  side of the motor housing  45 . The operation control part  47  is housed inside the end portion of the motor housing  45  on the side of the low-pressure stage compressor  10 , and a side face of the operation control part  47  disposed on the side of the low-pressure stage compressor  10  is spaced from the heat-shielding plate  35  via a gap. 
         [0043]    Next, operation of the multi-stage electric centrifugal compressor  1  will be described. When the electric motor rotor  30  is driven, the low-pressure stage impeller  11  and the high-pressure stage impeller  21  rotate along with rotation of the rotational shaft  3 . In response to rotation of the low-pressure stage impeller  11 , intake air enters through the inlet  17   a  of the low-pressure stage compressor  10 , has its temperature increased by being compressed by the low-pressure stage impeller  11 , flows through the flow channel  17   c  inside the low-pressure stage compressor  10  to reach a predetermined pressure, and then exits through the outlet  17   b.    
         [0044]    Intake air discharged from the outlet  17   b  flows through the intake-air communication passage  29  to flow into the high-pressure stage compressor  20  through the inlet  27   a  of the high-pressure stage compressor  20 . Intake air having flowed into the high-pressure stage compressor  20  has its temperature increased by being compressed by the high-pressure stage impeller, flows through the flow channel  27   c  to reach a predetermined pressure, and then exits through the outlet  27   b.    
         [0045]    Herein, the operation control part  47  is disposed on the low-pressure-stage-compressor- 10  side of the motor housing  45 , and thus positioned remote from the high-pressure stage compressor  20 . Accordingly, it is possible to reduce influence of heat generated by intake air that flows to the high-pressure stage compressor  20  and gets heated. Further, while the operation control part  47  is disposed near the low-pressure stage compressor  10 , the heat-shielding plate  35  is disposed between the operation control part  47  and the low-pressure stage compressor  10 , and thereby the heat-shielding plate  35  shields heat generated by intake air that flows to the low-pressure stage compressor  10  and gets heated. Accordingly, heat of intake air flowing through the low-pressure stage compressor  10  also has little influence on the operation control part  47 . Further, in general, an increased temperature is lower in the low-pressure stage compressor  10  than in the high-pressure stage compressor  20 , and thus electric components are desired to be disposed on the side of the low-pressure stage compressor  10 . In view of this, in the present embodiment, the operation control part  47  is disposed on the side of the low-pressure stage compressor  10 . Further, the operation control part  47  is disposed with a gap  48  provided between the heat-shielding plate  35  and the side face of the operation control part  47  on the side of the low-pressure stage compressor  10 , and thereby it is possible to prevent more effectively heat of the heat-shielding plate  35  from propagating to the operation control part  47 . Thus, it is possible to achieve the multi-stage electric centrifugal compressor  1  capable of protecting the operation control part  47  from heat generated by the high-pressure stage compressor  20  and the low-pressure stage compressor  10 . 
         [0046]    Further, while intake air taken into the low-pressure stage compressor  10  flows through the flow channel  17   c  inside the low-pressure stage compressor  10  to be discharged through the outlet  17   b,  intake air may flow along the inner surface  35   c  of the heat-shielding plate  35  to leak out, in the middle of the flow channel  17   c.  In this regard, the bending portion  35   b  of a tubular shape is disposed in the middle of the heat-shielding plate  35  to bend toward the bearing  40 R and extend along the outer peripheral surface of the rotational shaft  3 , with the seal-member fitting portion  37  of a cylindrical shape fitted to the outer periphery of the rotational shaft  3  on the side of the inner surface  35   c  of the bending portion  35   b,  and with the plurality of piston rings  38  disposed on the outer peripheral surface of the seal-member fitting portion  37  to slide relative to the inner surface  35   c  of the bending portion  35   b.  Accordingly, during operation of the low-pressure stage compressor  10 , the piston rings  38  and the seal-member fitting portion  37  can securely prevent leakage of intake air that may leak through a through hole  35   b   1 . Therefore, it is possible to prevent infiltration of high-temperature intake air into the electric motor, and to prevent securely a risk of damage due to galling of the bearing  40 R caused by grease shifting inside the bearing  40 R and leaking out of the bearing  40 R. 
         [0047]    The embodiments of the present invention have been described above. However, the present invention is not limited thereto, and various modifications may be applied as long as they do not depart from the object of the present invention. For instance, some of the above described embodiments may be combined upon implementation. 
       DESCRIPTION OF REFERENCE NUMERALS 
       [0048]      1  Multi-stage electric centrifugal compressor 
         [0049]      3  Rotational shaft 
         [0050]      10  Low-pressure stage compressor 
         [0051]      11  Low-pressure stage impeller 
         [0052]      12 ,  22  Back plate 
         [0053]      13 ,  23  Boss portion 
         [0054]      13   a,    23   a  Through hole 
         [0055]      14 ,  24  Vane 
         [0056]      15  Nut 
         [0057]      16  Low-pressure stage housing 
         [0058]      16   a,    26   a,    54  Side face 
         [0059]      17 ,  27  Space part 
         [0060]      17   a,    27   a  Inlet 
         [0061]      17   b,    27   b  Flow channel 
         [0062]      17   c,    27   c  Outlet 
         [0063]      18 ,  28  Insertion opening 
         [0064]      20  High-pressure stage compressor 
         [0065]      21  High-pressure stage impeller 
         [0066]      26  High-pressure stage housing 
         [0067]      29  Intake-air communication passage 
         [0068]      30  Electric motor rotor 
         [0069]      35  Heat-shielding plate 
         [0070]      35   a  Flange portion 
         [0071]      35   b  Bending portion 
         [0072]      35   b   1 ,  55   a  Through hole 
         [0073]      35   c  Inner surface 
         [0074]      36 ,  53  Bolt 
         [0075]      37  Seal-member fitting portion 
         [0076]      38  Piston ring (ring) 
         [0077]      39  Clearance part 
         [0078]      40 R,  40 L Bearing 
         [0079]      45  Motor housing 
         [0080]      45   a,    50   a  Insertion hole 
         [0081]      45   b  Rotor space part 
         [0082]      45   c,    50   b  Bearing mounting hole 
         [0083]      46  Fin 
         [0084]      47  Operation control part 
         [0085]      48  Gap 
         [0086]      50  Bearing housing 
         [0087]      51  Protruding stepped portion 
         [0088]      52  Surface portion 
         [0089]      54   a  Engaging recess portion 
         [0090]    φk, φs Diameter