Abstract:
The purpose of the invention is to provide an inverter-driven dynamo electric machine and system for the same having high reliability and high efficiency such that even in the case of inverter pulse voltages having high dv/dt, generation of shaft voltages and generation of shaft currents causing electric corrosion of a bearing with the generation of the shaft voltages are suppressed, thereby keeping the bearing free of electric corrosion. 
     The purpose of the invention is achieved by the following method. That is, the purpose is achieved by an inverter-driven dynamo electric machine and system for the same including at least one machine support bearing which supports a shaft of a rotor, and one electric discharge bearing which discharges the voltage generated in the shaft of the rotor, wherein a bearing having a lower dielectric breakdown voltage between an inner ring and an outer ring than that of the machine support bearing is used as the electric discharge bearing. 
     Accordingly, this can provide an inverter-driven dynamo electric machine and system for the same having high reliability and high efficiency such that the bearing is not electrically corroded with respect to the inverter pulse voltage.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a dynamo electric machine driven by an inverter and a system therefor. 
         [0002]    From the aspect of saving of energy, a variable-speed operation of a dynamo electric machine using an inverter power supply is actively performed recently in various fields such as power, industry, automobile, railroad, and household appliances. However, in the dynamo electric machine, there arise various problems such as bearing electric corrosion, insulation, and EMI/EMC along with inverter drive, and a development of a countermeasure technique to the above problems is performed. 
         [0003]    With regard to a countermeasure technique of the bearing electric corrosion of dynamo electric machines or driving force transmission devices along with the inverter drive, [Patent literature 1], [Patent literature 2], [Patent literature 3], and [Patent literature 4] are conventionally disclosed. 
       CITATION LIST 
     Patent Literature  
       [0004]    Patent literature 1: Japanese Laid-open Patent Publication No. 2008-45697 
         [0005]    Patent literature 2: WO 01/036832 
         [0006]    Patent literature 3: Japanese Laid-open Patent Publication No. 09-291943 
         [0007]    Patent literature 4: Japanese Laid-open Patent Publication No. 2003-324489 
       SUMMARY OF INVENTION 
     Technical Problem  
       [0008]    Incidentally, in the above-described countermeasure techniques, there is a problem that a shaft voltage and a shaft current (or, a bearing current) causing bearing electric corrosion fail to be suppressed. A conductive sealing material disclosed in [Patent literature 1], conductive grease disclosed in [Patent literature 2], a conductive flexible material disclosed in [Patent literature 3], and a method for using a frictional contact pressurization spring and reducing a shaft voltage disclosed in [Patent literature 4], for example, in the case of an inverter pulse voltage in which a voltage change rate dv/dt is large, a large displacement current i=C·dv/dt flows through the above conductive members or contact resistance portions of a pressurization spring. A voltage of V=r·i is further generated on these resistance portions (resistance value r), and as a result, a shaft voltage generated in a shaft of a rotor cannot be suppressed. Since the shaft voltage cannot be suppressed, a voltage is applied between an inner ring and an outer ring of bearing positioning between a shaft and a housing (earth). As a result, an oil film of grease on the bearing is dielectrically broken and an arc discharge shaft current generated along with dielectric breakdown of an oil film, namely, a shaft current causing electric corrosion of the bearing cannot be suppressed. 
         [0009]    Incidentally, since a switching loss is reduced in an inverter and a converter efficiency is improved, dv/dt of a power device to be used as the inverter tends to be raised recently. As a result, it is feared that in the future, in a conventional bearing electric corrosion measure, the bearing electric corrosion cannot be prevented and it becomes difficult to provide a reliable dynamo electric machine in which the bearing electric corrosion is not generated. Against the above-described problem, measures to limit dv/dt of the power device of the inverter are considered. However, it is feared that since converter efficiency of the inverter cannot be improved, efficient inverter and inverter-driven dynamo electric machine system cannot be provided in the future. 
         [0010]    In view of the foregoing, it is an object of the present invention to provide a reliable and efficient inverter-driven dynamo electric machine and system therefor which suppress generation of a shaft voltage and that of a shaft current generated along with the above, and in which the bearing is not electrically corroded also with respect to an inverter pulse voltage having high dv/dt. 
       Solution to Problem  
       [0011]    To attain the problem, in a dynamo electric machine of the invention, at least one bearing or bearing group having different electrical discharge characteristic and mechanical characteristic is used in left and right of a rotor shaft. 
         [0012]    A dynamo electric machine to be driven by an inverter includes at least one machine support bearing configured to support a rotor shaft, and at least one electric discharge bearing configured to discharge a voltage generated in the rotor shaft, wherein the electric discharge bearing includes a bearing having a lower dielectric breakdown voltage between an inner ring and an outer ring than the dielectric breakdown voltage of the machine support bearing. 
         [0013]    In the dynamo electric machine, grease having a dielectric breakdown voltage lower than that of the machine support bearing is used for the electric discharge bearing. 
         [0014]    In the dynamo electric machine, unevenness is provided between an inner ring and outer ring of the electric discharge bearing, and a dielectric breakdown voltage between the inner ring and outer ring of the electric discharge bearing is set to be lower than that between the inner ring and outer ring of the machine support bearing. 
         [0015]    The dynamo electric machine further includes at least one machine support bearing configured to support the rotor shaft, and at least one electric discharge bearing configured to discharge a voltage generated in the rotor shaft, wherein grease having relative permittivity higher than that of the machine support bearing is used for the electric discharge bearing. 
         [0016]    In the bearing for a dynamo electric machine, a bearing discharges a voltage generated in a rotor shaft of the dynamo electric machine. 
         [0017]    In the bearing for a dynamo electric machine, unevenness is provided on a surface of any of an inner ring and an outer ring, or facing surfaces of both of the inner ring and the outer ring. 
         [0018]    In the bearing for a dynamo electric machine, needle-like projections are provided on a surface of any of an inner ring and an outer ring, or facing surfaces of both of the inner ring and the outer ring. 
         [0019]    In the bearing for a dynamo electric machine, groove projections are provided in a circumferential direction on a surface of any of an inner ring and an outer ring, or facing surfaces of both of the inner ring and the outer ring. 
         [0020]    In the bearing for a dynamo electric machine, an electric discharge bearing and a machine support bearing are integrated. 
         [0021]    In an end bracket, the bearing for a dynamo electric machine is built in. 
         [0022]    An inverter-driven dynamo electric machine system includes the above-described dynamo electric machine, bearing, and end bracket. 
       Advantageous Effects of Invention  
       [0023]    The present embodiments provide an inverter-driven dynamo electric machine, a system, bearing, and end bracket for the same having high reliability and high efficiency such that the bearing is not electrically corroded with respect to the inverter pulse voltage. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0024]      FIG. 1  illustrates a dynamo electric machine according to a first embodiment; 
           [0025]      FIG. 2  illustrates a common mode voltage, a shaft voltage, and a shaft current at the time of rotating a conventional dynamo electric machine and the dynamo electric machine according to a first embodiment; 
           [0026]      FIG. 3  illustrates an electric discharge bearing of a dynamo electric machine according to a first embodiment; 
           [0027]      FIG. 4  illustrates another mode of an electric discharge bearing; 
           [0028]      FIG. 5  illustrates another mode of an electric discharge bearing; 
           [0029]      FIG. 6  illustrates another mode of an electric discharge bearing; 
           [0030]      FIG. 7  illustrates another mode of an electric discharge bearing; 
           [0031]      FIG. 8  illustrates another mode of an electric discharge bearing; 
           [0032]      FIG. 9  illustrates another mode of an electric discharge bearing; 
           [0033]      FIG. 10  illustrates a structure in which an electric discharge bearing and a machine support bearing are integrated; 
           [0034]      FIG. 11  illustrates a hybrid bearing in which a function of an electric discharge bearing and that of a machine support bearing are integrated; 
           [0035]      FIG. 12  illustrates a dynamo electric machine according to a second embodiment; 
           [0036]      FIG. 13  illustrates a dynamo electric machine according to a third embodiment; 
           [0037]      FIG. 14  illustrates a dynamo electric machine according to a fourth embodiment; 
           [0038]      FIG. 15  illustrates a dynamo electric machine according to a fifth embodiment; 
           [0039]      FIG. 16  illustrates a dynamo electric machine according to a sixth embodiment; 
           [0040]      FIG. 17  illustrates a dynamo electric machine according to a seventh embodiment; 
           [0041]      FIG. 18  illustrates a dynamo electric machine according to an eighth embodiment; 
           [0042]      FIG. 19  illustrates an external end bracket in which an electric discharge bearing is built and a dynamo electric machine according to a ninth embodiment; 
           [0043]      FIG. 20  illustrates an external end bracket in which an electric discharge bearing is built and a dynamo electric machine according to a tenth embodiment; 
           [0044]      FIG. 21  illustrates a characteristic of grease of a conventional technology; and 
           [0045]      FIG. 22  illustrates a characteristic of grease according to the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0046]    Preferred embodiments of the present invention will now be described in detail below with reference to the accompanying drawings. 
       First Embodiment  
       [0047]      FIG. 1  illustrates a dynamo electric machine according to a first embodiment. The dynamo electric machine  1  includes a stator  5  storing a stator winding  6  and a rotor  7  rotating according to a rotating magnetic field. These units are stored in a housing  2  and end brackets  3  and  4 . A shaft  8  of the rotor  7  is mechanically supported by machine support bearings  9  and  10  attached to the end brackets  3  and  4 . Since the machine support bearings  9  and  10  mechanically support the shaft  8  of the rotor  7  radially and axially, a mechanical stress is not applied to an electric discharge bearing  11  provided on the end bracket  3  of a non-load side. 
         [0048]    On the other hand, for the electric discharge bearing, there is used grease having a dielectric breakdown voltage lower than that to the machine support bearings  9  and  10 , preferably, grease which is low by 0.1 V or more in the range of the rotation number to be used. Before broken dielectrically in the machine support bearing, an oil film is set to be broken dielectrically in the electric discharge bearing. That is, an electrical stress caused by an inverter (not illustrated) which drives the dynamo electric machine is set to be supported by the electric discharge bearing and not to be applied to the machine support bearing. 
         [0049]      FIG. 2  illustrates an inverter common mode voltage applied to a dynamo electric machine, a shaft voltage generated between the shaft  8  of the dynamo electric machine and a grounding wire, and a shaft current flowing through the machine support bearing at the time of rotating by an inverter a conventional dynamo electric machine and the dynamo electric machine according to the first embodiment. In the case where the same common mode pulse voltage of the inverter is applied, a shaft current is generated in a voltage change part (a rising edge and a falling edge) of the common mode voltage in the conventional dynamo electric machine. At the time when this shaft voltage is larger than an oil film dielectric breakdown voltage  110  of the machine support bearing, a pulse shaft current along with the oil film dielectric breakdown flows through the machine support bearing. On the contrary, in the dynamo electric machine according to the first embodiment, a shaft voltage is suppressed by the oil film dielectric breakdown voltage  111  of the electric discharge bearing. Since the shaft voltage does not reach the oil film dielectric breakdown voltage  110  of the machine support bearing, the shaft current does not flow through the machine support bearing. As a result, electric corrosion is not generated in the machine support bearing and, also at the time of rotating the dynamo electric machine by the inverter for a long time, washboard-shape grooves are provided on a race surface of the machine support bearing. There are solved a problem that at the time of rotating the rotor  7 , the shaft  8  is vibrated to cause noises, and further, a problem that the electric corrosion progresses to peel off a race surface of the machine support bearing, and the machine support bearing and the dynamo electric machine using the same break down. On the other hand, a shaft current along with an oil film breakdown is generated or electric corrosion on a bearing along with the shaft current occurs on the electric discharge bearing. However, since the mechanical stress is not applied, also at the time of rotating the rotor  7 , there is no problem that the shaft  8  is vibrated to generate noises, or a race surface is peeled off. 
         [0050]      FIG. 3  illustrates a detailed configuration diagram of the electric discharge bearing according to the first embodiment. The electric discharge bearing  20  includes an inner ring  22  and an outer ring  21 , and a space  24  between both of them is filled with grease. In addition, for preventing an outflow of the grease, seal plates  23  are attached to the left and right sides. As grease, there is used grease having a dielectric breakdown voltage lower than that of the grease used in the machine support bearing. However, since characteristics except the dielectric breakdown voltage may be freely selected, grease which is usually unusable in the machine support bearing in terms of mechanical loss may be widely used. 
         [0051]      FIGS. 4 to 9  each illustrate a configuration example of another electric discharge bearing. In the electric discharge bearing  30  of  FIG. 4 , when a race surface  33  of an inner ring  32  is mesh-processed in an outer ring  31  and the inner ring  32 , electric field concentration points on a surface are increased and a dielectric breakdown voltage of an oil film is reduced. A surface is polished, a surface is polished by using abrasive powder, or a surface is plasma-processed, thereby implementing the mesh processing. As compared with the above, in the electric discharge bearing  40  of  FIG. 5 , a race surface  43  of an outer ring  41  is mesh-processed in the outer ring  41  and an inner ring  42 . Through the process, the electric field concentration points on a surface are increased and a dielectric breakdown voltage of an oil film is reduced. Further, in the electric discharge bearing  50  of  FIG. 6 , when race surfaces  53  of both an outer ring  51  and an inner ring  52  are mesh-processed, the electric field concentration points on a surface are increased and a dielectric breakdown voltage of an oil film is reduced. 
         [0052]    On the other hand, in an electric discharge bearing  60  of  FIG. 7 , when uneven grooves are provided on a race surface  63  of an outer ring  61  in the outer ring  61  and an inner ring  62 , a concentration electric field on a surface is raised and the dielectric breakdown voltage of an oil film is reduced. In the same manner, in an electric discharge bearing  70  of  FIG. 8 , when uneven grooves are provided on a race surface  73  of an outer ring  71  in the outer ring  71  and an inner ring  72 , a concentration electric field on a surface is raised and a dielectric breakdown voltage of an oil film is reduced. Further, in an electric discharge bearing  80  of  FIG. 9 , when uneven grooves are provided on both race surfaces  83  of an outer ring  81  and an inner ring  82 , a concentration electric field on a surface is raised and a dielectric breakdown voltage of an oil film is reduced. In  FIGS. 7 to 9 , grooves are provided, and further needle-like projections may be provided on a surface. 
         [0053]    In the above-described bearings of  FIGS. 4 to 9 , when uneven grooves are processed to a race surface of the bearing, the dielectric breakdown voltage of an oil film may be controlled and various types of grease may be used in the electric discharge bearing. Specifically, grease having a dielectric breakdown voltage lower than that of the machine support bearing ought to be used in the bearing of  FIG. 3 . However, as in  FIGS. 4 to 9 , when processing is applied to a race surface of the bearing, grease having a dielectric breakdown voltage the same as or higher than that of the machine support bearing may be used. 
         [0054]      FIG. 10  illustrates a bearing in which an electric discharge bearing and a machine support bearing are integrated. In the bearing of  FIG. 10 , the electric discharge bearing  90  and the machine support bearing  91  are held by a ring  92  on the outer ring side. In  FIG. 1 , the electric discharge bearing and the machine support bearing are separated from each other, and a jig for matching a center of the outer ring of the mechanically-free electric discharge bearing with that of the machine support bearing is necessary. However, when using a hybrid bearing, a center of the outer ring of the electric discharge bearing and that of the machine support bearing are easily matched with each other. 
         [0055]      FIG. 11  illustrates the hybrid bearing in which a function of the electric discharge bearing and that of the machine support bearing are integrated. In the hybrid bearing  100 , a groove  101  for concentrating an electric field is provided in the inner ring and outer ring of the machine support bearing. When adopting the above-described method, since two types of bearings need not be used, an axial direction thickness of the end bracket of the dynamo electric machine is reduced. 
       Second Embodiment  
       [0056]      FIG. 12  illustrates a dynamo electric machine according to a second embodiment. In the first embodiment, the electric discharge bearing is provided in the end bracket of the non-load side; however, the electric discharge bearing is provided in the end bracket of the load side in the second embodiment. Specifically, the dynamo electric machine  121  includes a stator  125  storing a stator winding  126  and a rotor  127  rotating according to a rotating magnetic field. The above-described units are stored in a housing  122  and the end brackets  123  and  124 . A shaft  128  of the rotor  127  is mechanically supported by machine support bearings  129  and  1210  attached to the end brackets  123  and  124 . Since the machine support bearings  129  and  1210  mechanically support the shaft  128  of the rotor  127  radially and axially, a mechanical stress is not applied to an electric discharge bearing  1211  provided on the end bracket  124  of the load side. 
         [0057]    Since a mechanical stress is not applied to the electric discharge bearing of the invention, the electric discharge bearing may be provided on the load side as in the second embodiment. In the case where an interval between the dynamo electric machine and a machine load (not illustrated) connected to the shaft  128  is wide, when the electric discharge bearing is provided on the end bracket of the load side as described above, a size of the entire inverter-driven dynamo electric machine system including the machine load is made the same as a conventional size. 
       Third Embodiment  
       [0058]      FIG. 13  illustrates a dynamo electric machine according to a third embodiment. In the first embodiment, the electric discharge bearing is externally provided in the end bracket of the non-load side; however, the electric discharge bearing is internally provided in the end bracket of the non-load side in the third embodiment. Specifically, the dynamo electric machine  131  includes a stator  135  storing a stator winding  136  and a rotor  137  rotating according to a rotating magnetic field. The above-described units are stored in a housing  132  and the end brackets  133  and  134 . A shaft  138  of the rotor  137  is mechanically supported by machine support bearings  139  and  1310  attached to the end brackets  133  and  134 . Since the machine support bearings  139  and  1310  mechanically support the shaft  138  of the rotor  137  radially and axially, a mechanical stress is not applied to the electric discharge bearing  1311  provided on the end bracket  133  of the non-load side. 
         [0059]    In the dynamo electric machine in which a space between the end bracket  133  of the non-load side and both the stator winding  136  and the stator  135  is wide, the electric discharge bearing may be provided in the dynamo electric machine as described above. 
       Fourth Embodiment  
       [0060]      FIG. 14  illustrates a dynamo electric machine according to a fourth embodiment. In the third embodiment, the electric discharge bearing is internally provided in the end bracket of the non-load side; however, the electric discharge bearing is internally provided in the end bracket of the load side in the fourth embodiment. Specifically, the dynamo electric machine  141  includes a stator  145  storing a stator winding  146  and a rotor  147  rotating according to a rotating magnetic field. The above-described units are stored in a housing  142  and the end brackets  143  and  144 . A shaft  148  of the rotor  147  is mechanically supported by machine support bearings  149  and  1410  attached to the end brackets  143  and  144 . Since the machine support bearings  149  and  1410  mechanically support the shaft  148  of the rotor  147  radially and axially, a mechanical stress is not applied to the electric discharge bearing  1411  provided on the end bracket  144  of the load side. 
         [0061]    In the dynamo electric machine in which a space between the end bracket  144  of the load side and both the stator winding  146  and the stator  145  is wide, the electric discharge bearing may be provided in the dynamo electric machine as described above. 
       Fifth Embodiment  
       [0062]      FIG. 15  illustrates a dynamo electric machine according to a fifth embodiment. In the first to fourth embodiments, the dynamo electric machine on which two machine support bearings are provided is disclosed, respectively. In a dynamo electric machine capable of supporting a shaft by one machine support bearing, an electric discharge bearing may be provided on an end bracket different from that on which the machine support bearing is provided. Specifically, the dynamo electric machine  151  includes a stator  155  storing a stator winding  156  and a rotor  157  rotating according to a rotating magnetic field. The above-described units are stored in a housing  152  and the end brackets  153  and  154 . A shaft  158  of the rotor  157  is mechanically supported by machine support bearing  1510  attached to the end bracket  154 . Since the machine support bearing  1510  mechanically supports the shaft  158  of the rotor  157  radially and axially, a mechanical stress is not applied to the electric discharge bearing  1511  provided on the end bracket  153  of the non-load side. 
         [0063]    In the dynamo electric machine capable of supporting a shaft by one machine support bearing as described above, or also in the dynamo electric machine capable of supporting a shaft by two machine support bearings conventionally, a machine support bearing enough to bear a mechanical force radially and axially is used. In this case, the fifth embodiment permits an electric discharge bearing to be provided on the other end bracket and a size of a dynamo electric machine to be made the same as that of a conventional dynamo electric machine. 
       Sixth Embodiment  
       [0064]      FIG. 16  illustrates a dynamo electric machine according to a sixth embodiment. In the fifth embodiment, the machine support bearing is provided in the end bracket of the load side, and the electric discharge bearing is provided in the end bracket of the non-load side, and vice versa. Specifically, the dynamo electric machine  161  includes a stator  165  storing a stator winding  166  and a rotor  167  rotating according to a rotating magnetic field. The above-described units are stored in a housing  162  and the end brackets  163  and  164 . A shaft  168  of the rotor  167  is mechanically supported by the machine support bearing  169  attached to the end bracket  163 . Since the machine support bearing  169  mechanically supports the shaft  168  of the rotor  167  radially and axially, a mechanical stress is not applied to the electric discharge bearing  1611  provided on the end bracket  154  of the load side. 
         [0065]    In general, a machine support bearing is provided on an end bracket of a load side in terms of balance. In the case where the machine support bearing is balanced with a bearing of a machine load, the machine support bearing may be provided on the non-load side, and the electric discharge bearing may be provided on the load side. Through the process, a shaft voltage is reduced in a position near to the machine support bearing and the bearing of the machine load may be protected. 
       Seventh Embodiment 
       [0066]      FIG. 17  illustrates a dynamo electric machine according to a seventh embodiment. In the first to sixth embodiments, one electric discharge bearing is provided and further two electric discharge bearings may be provided. Specifically, the dynamo electric machine  171  includes a stator  175  storing a stator winding  176  and a rotor  177  rotating according to a rotating magnetic field. The above-described units are stored in a housing  172  and end brackets  173  and  174 . A shaft  178  of the rotor  177  is mechanically supported by machine support bearings  179  and  1710  attached to the end brackets  173  and  174 . Since the machine support bearings  179  and  1710  mechanically support the shaft  178  of the rotor  177  radially and axially, a mechanical stress is not applied to electric discharge bearings  1711  and  1712  provided on the end brackets  173  and  174  of the load side. 
         [0067]    A shaft voltage or shaft current to become problematic in an inverter-driven dynamo electric machine may become large when a size of the dynamo electric machine or a capacity thereof becomes large depending on the type or generation mechanism. In this case, when providing two electric discharge bearings, a shaft current flowing through one electric discharge bearing may be reduced to half and a life of the electric discharge bearing may be enlarged. A plurality of electric discharge bearings may be further provided from the same reason. In addition, when the plurality of electric discharge bearings are provided on the load and non-load sides of the dynamo electric machine as described above, a shaft current flowing in cycles through a shaft impossible to cope with by one electric discharge bearing is not allowed to flow through a machine support bearing but allowed to flow through an electric discharge bearing, thereby coping with the above-described problem. 
       Eighth Embodiment  
       [0068]      FIG. 18  illustrates a dynamo electric machine according to an eighth embodiment. In the seventh embodiment, the electric discharge bearing is externally provided in the end bracket, and further may be internally provided in the end bracket. Specifically, the dynamo electric machine  181  includes a stator  185  storing a stator winding  186  and a rotor  187  rotating according to a rotating magnetic field. The above-described units are stored in a housing  182  and the end brackets  183  and  184 . A shaft  188  of the rotor  187  is mechanically supported by machine support bearings  189  and  1810  attached to the end brackets  183  and  184 . Since the machine support bearings  189  and  1810  mechanically support the shaft  188  of the rotor  187  radially and axially, a mechanical stress is not applied to the electric discharge bearings  1811  and  1812  provided on the end brackets  183  and  184  of the load side. 
         [0069]    In the case where a space between the end brackets  183  and  184  and both the stator winding  186  and stator  185  of the dynamo electric machine  181  is empty, the electric discharge bearings  1811  and  1812  may be internally provided on the end brackets  183  and  184  as described above, and a size of the dynamo electric machine may be made the same as that of a conventional dynamo electric machine. 
       Ninth Embodiment  
       [0070]      FIG. 19  illustrates a dynamo electric machine according to a ninth embodiment. In the first to eighth embodiments, the electric discharge bearing is provided in the end bracket of the dynamo electric machine. As compared with the above, in the ninth embodiment, the end bracket in which the electric discharge bearing is provided is additionally provided from the outside of the dynamo electric machine. Specifically, the dynamo electric machine has a hole  197  into which a bolt  192  for fixing an external end bracket is inserted. Toward an extension drive shaft  198  of the dynamo electric machine  191 , the end bracket  194  on which the electric discharge bearing  193  is provided is pressed from the non-load side and clamped by using washers  196 , nuts  195 , and bolts  192 . In addition, a direction in which the bolts  192 , the washers  196 , and the nuts  195  are inserted may be any of the load side and the non-load side. 
         [0071]    As described above, when the end bracket on which the electric discharge bearing is provided is attached externally, electric corrosion preventive measures of the bearing are implemented also to the existing dynamo electric machine. 
       Tenth Embodiment  
       [0072]      FIG. 20  illustrates a dynamo electric machine according to a tenth embodiment. In the ninth embodiment, the end bracket on which the electric discharge bearing is provided is additionally provided from the non-load side of the dynamo electric machine. As compared with the above, in the tenth embodiment, the end bracket on which the electric discharge bearing is provided is additionally provided from the load side of the dynamo electric machine. Specifically, the dynamo electric machine has a hole  207  into which a bolt  202  for fixing the external end bracket is inserted. Toward the shaft  208  of the dynamo electric machine  201 , the end bracket  204  on which the electric discharge bearing  203  is provided is pressed from the load side and clamped by using washers  206 , nuts  205 , and bolts  202 . In addition, a direction in which the bolts  202 , the washers  206 , and the nuts  205  are inserted may be any of the load side and the non-load side. 
         [0073]    In the case where an interval between the dynamo electric machine and a machine load connected to the shaft  208  is wide, the external end bracket is provided on the load side as described above and electric corrosion preventive measures of the bearing are implemented also to the existing dynamo electric machine. 
       Eleventh Embodiment 
       [0074]      FIGS. 21 and 22  illustrate the present embodiment of characteristics of grease to be used for a dynamo electric machine of the invention. A structure of the dynamo electric machine is applicable to any of the first to tenth embodiments. In the first to tenth embodiments, a shaft voltage is suppressed by an oil film breakdown of the electric discharge bearing. In addition, high-frequency impedance is reduced, thereby suppressing a shaft voltage. In short, as illustrated in a conventional example of  FIG. 21 , there is used grease having high impedance to high frequency for keeping insulation characteristic along with lubricity of oil films as indicated in  210  in a normal bearing. As illustrated in  FIG. 22 , in the eleventh embodiment of the invention, the electric discharge bearing is newly provided apart from the machine support bearing. When grease having high relative permittivity to the machine support bearing is used for the electric discharge bearing, impedance is reversely reduced to high frequency and a shaft voltage is suppressed to less than oil film dielectric breakdown voltage of the machine support bearing. Therefore, with relation to an inverter pulse voltage having high dv/dt and large high frequency component occupied in a voltage waveform, oil film dielectric breakdown is not caused by the machine support bearing and electric corrosion of the bearing may be prevented. 
       INDUSTRIAL APPLICABILITY 
       [0075]    The present invention is applicable to a dynamo electric machine driven by an industrially applicable inverter and a system for the same. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           1  Dynamo electric machine 
           2  Housing 
           3 ,  4  End bracket 
           5  Stator 
           6  Stator winding 
           7  Rotor 
           8  Shaft 
           9 ,  10  Machine support bearing 
           11  Electric discharge bearing