Abstract:
An electric machine, particularly a generator for motor vehicles, includes a housing and a casing enclosing the housing concentrically, the casing, together with the housing, bounding an annular space that is sealed in a fluid-tight manner and is connected to a coolant outflow and a coolant inflow. To achieve a uniform circumflow of the housing with sufficient cooling capacity, even given low volumetric flow of the coolant, a plurality of axial guide bars are arranged in the annular space, so that the coolant restrictedly flows in an axially wide-strip manner through the annular space within space segments arranged in a row in the circumferential direction, with an opposing flow direction in successive space segments.

Description:
FIELD OF THE INVENTION 
   The present invention relates to an electric machine, particularly a generator for motor vehicles. 
   BACKGROUND INFORMATION 
   In a conventional water-cooled generator for motor vehicles, for example as described in French Patent Application No. 2 717 638, the stator and rotor are accommodated in a pot-shaped housing which, on one hand, is inserted into a pot-shaped encasing in such a manner that an annular space remains between the covering of the pot-shaped encasing and the outer surface of the housing, the annular space being covered in a fluid-tight manner on one side by the bottom of the encasing, and on the other side, by a cover mounted on the pot edge of the encasing and housing. The annular space, on sides turned away from each other, includes an inflow orifice and a discharge orifice which are each enclosed by a connecting piece, projecting radially on the encasing, for a water pipe. The water circulated by a pump in a circulation circuit enters into the annular space through the inflow orifice, circumflows the housing, and exits the annular space again via the discharge orifice. The heat emitted by the generator is absorbed by the cooler water and carried away. 
   SUMMARY OF THE INVENTION 
   The electric machine of the present invention may provide that, due to the forced guidance of the coolant by the guide bars, a uniform circumflow of the housing of the electric machine by the coolant is ensured, so that the formation of so-called “hot spots” due to locally insufficient flow velocity of the coolant, for example, due to the development of recirculation bubbles, is prevented. Due to the multitude of guide bars, which at the same time act as cooling vanes, the housing surface available for heat transfer to the coolant increases, so that for an equal cooling capacity, the volumetric flow of the coolant may be reduced by the factor by which the housing surface is increased. Due to the wide-strip axial flow of the coolant, which in each case is turned around at the end sides of the annular space, at the location where the stator of the machine supplies a large input of heat through the metallic housing, the surface for the heat transfer and the flow velocity of the coolant are also great, so that the local cooling capacity is well adapted to the local heat input. The spacings of the guide bars may be dimensioned accordingly for this purpose, as well. Short-circuit losses due to leakage currents over the guide bars are low, since due to the configuration which is favorable for the flow, no large pressure differences result across the small gaps present between the covering and the outer surface of the guide bars. 
   All in all, in the example machine of the present invention, sufficiently great cooling capacity is furnished for heat dissipation in all space segments, and indeed also for the cases when only a limited volumetric flow of the coolant is available. 
   According to an example embodiment of the present invention, the guide bars in each space segment form a plurality of parallel flow channels having in each case an inflow end and an outflow end. In the annular space, at each end of the flow channels, coolant collecting sections are formed, extending in the circumferential direction, of which in each case one inflow collecting section extends over the inflow ends and one outflow collecting section extends over the outflow ends of each space segment. At each end of the flow channels, adjoining the discharge collecting section of the one space segment in the circumferential direction is an inlet collecting section of the following space segment, each inlet collecting section being separated from the discharge collecting section of the space segment following in the flow direction. This parallel arrangement of the guide bars in a few, e.g., five, space segments distributed over the periphery of the housing reduces the length of the cooling channel and the resistance to flow. 
   According to an example embodiment of the present invention, the coolant collecting sections are formed in such a manner that the cross-section of the inlet collecting sections decreases in the flow direction, and the cross-section of the discharge collecting sections increases in the flow direction. Due to this structural formation, the flow velocity in the region of the re-routing of the coolant flow is reduced, and coolant is uniformly distributed to the parallel flow channels, accompanied by uniform flow velocity in the flow channels. 
   In an example embodiment of the present invention, the coolant collecting sections at the ends of the flow channels are formed in the manner that the guide bars have an equal length, and that within a space segment, successive guide bars in the circumferential direction are shifted axially—e.g., by equal amounts—in the same direction relatively to each other. In so doing, the last guide bar in the space segment is shifted so far that it is brought forward with one bar end to one of two annular bars terminating the annular space at the end face. Due to this structural configuration, the desired decrease and increase of cross-section in the coolant collecting sections and the separation of the inflow collecting sections from the outflow collecting sections in the following space segment may be implemented from the standpoint of production engineering. 
   According to an example embodiment of the present invention, an inflow channel extending in the axial direction and connected to the coolant inflow, and an outflow channel extending in the circumferential direction and connected to the coolant outflow are formed between the first guide bar of the first space segment in the flow direction and the last guide bar of the last space segment. The inflow and outflow channels are separated from each other by a separating bar extending in the circumferential direction from the front end of the last guide bar in the last space segment up to the first guide bar in the first space segment. In this context, an inflow connecting piece for the coolant inflow mounted on the casing is aligned with its axis in such a manner that the axis forms an obtuse angle with the axial flow direction of the coolant in the inflow channel, and an outflow connecting piece for the coolant outlet mounted on the casing is implemented so that its axis forms an obtuse angle with the more or less tangential flow direction in the outflow channel. Due to this guidance of the inflow and outflow of the coolant, the pressure at the separating bar is reduced on the high-pressure side by the volumetric flow directed away from the separating bar. On the low-pressure side, the pressure at the separating bar is increased by the dynamic pressure of the upwardly discharging coolant. All in all, the result thereby on one hand is a reduction of the total pressure over the separating bar, so that only extremely low leakage losses occur, and on the other hand, a feed and discharge of the coolant which is favorable to the flow, thus helping to reduce the flow resistance. 
   According to an example embodiment of the present invention, the guide bars, one annular bar and the separating bar, are integrally molded on the housing, which is produced using pressure diecasting or injection molding techniques. Due to the bar formation, which is selected to be favorable from a standpoint of production engineering, the pressure or injection mold is able to be drawn off in the axial direction. The casing, produced separately with integrally molded annular bar, is slid onto the housing thus produced, and the annular space is sealed by two O-rings between the housing and the casing in the region of the annular bars. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a longitudinal section of a generator for motor vehicles in a cutaway view. 
       FIG. 2  shows a perspective view of the generator in the direction of arrow II in  FIG. 1  in a cutaway view. 
       FIG. 3  shows a plan view of the generator in  FIG. 1 , with the casing drawn off, in a perspective representation. 
       FIG. 4  shows a developed view of the housing of the machine in FIGS.  1 - 3 . 
   

   DETAILED DESCRIPTION 
   The generator, shown in longitudinal section in a cutaway view in  FIG. 1 , for a motor vehicle as an example embodiment for a general electric machine includes, a stator  11 , accommodated in a housing  10 , including a stator winding  12 , and a rotor  14 , enclosed concentrically by stator  11  leaving an air gap  13 , which sits in a rotationally fixed manner on a rotor shaft  15  rotationally mounted in housing  10 . Housing  10  is pot-shaped, including a base part  101  and a cylindrical part  102 . On cylindrical part  102 , at the end facing away from base part  101 , a radial flange  103  is integrally formed, upon which a bearing cover  16  is secured, sealing housing  10  at the front end. Rotor shaft  15 —as not shown in FIG.  1 —is in each case accommodated in a pivot bearing integrated in base part  101  and in bearing cover  16 . 
   Directly adjacent to flange  103 , an annular bar  104  is integrally molded, projecting radially from the surface of cylindrical part  102 . From the other end of cylindrical part  102 , a separately manufactured, hollow-cylindrical casing  17  is slid onto housing  10  and, with its front end in the push-in direction, overreaches annular bar  104  on one side, and abuts against flange  103  on the other side. At the rear end of casing  17 , a radially inwardly projecting annular bar  171  is integrally formed, whose radial height corresponds to the radial height of annular bar  104  on housing  10 . In this manner, an annular space  18  is bounded between casing  17  and cylindrical part  102  of housing  10  and is sealed at the end faces by the two annular bars  104  and  171 . In the region of annular bars  104  and  171 , in each case a ring seal  19  in the form of an O-ring is disposed between casing  17  and cylindrical part  102  of housing  10 , in each instance a ring seal  19  fitting in an annular groove  20  cut into cylindrical part  102  of housing  10 . Annular space  18  is connected to a coolant inflow  21  and a coolant outflow  22 , which are each assigned an inflow connecting piece  23  and outflow connecting piece  24 , respectively, arranged outside on casing  17 . 
   To ensure a uniform circumflow of the housing by the coolant, a restricted guidance of the coolant is implemented in annular space  18  from coolant inflow  21  to coolant outflow  22 . In this context, the coolant is restrictedly guided by a plurality of axial guide bars  25 , so that it flows in an axially wide-strip manner through annular space  18  within segments of the annular space  18  that are arranged in a row in the circumferential direction, hereinafter known as space segments  26 , with opposing flow direction in successive space segments  26 . Guide bars  25  have a radial bar height corresponding to the radial width of annular space  18 , and in the example embodiment, having the same cross-section and same axial length, are arranged parallel and equidistant to each other, as may be seen in  FIGS. 3 and 4 . In the example embodiment described here, guide bars  25  are divided over a total of five space segments  26 ; however, the number of space segments  26  may be selected as desired. In each space segment  26 , guide bars  25  form between themselves a plurality of parallel flow channels  27  including an inlet end  271  and a discharge end  272 , which have an equal flow cross-section. Guide bars  25  may also be configured and arranged in such a manner, for example, with differently sized distances between them, that flow channels  27  enclosed by them have different flow cross-sections. In this context, the different flow cross-sections are spatially allocated in adaptation to locally different heat input into housing  10 . Within each space segment  26 , inlet ends  271  lead into an inlet collecting section  28  and discharge ends  272  lead into a discharge collecting section  29 , so that at each end of flow channels  27 , adjoining in each case a discharge collecting section  29  of the one space segment  26  in the circumferential direction is an inlet collecting section  28  of the following or preceding space segment  26 . In inlet collecting sections  28  and discharge collecting sections  29 , known altogether as coolant collecting sections, the coolant is distributed to individual flow channels  27  in space segment  26 , i.e., the coolant emerging from flow channels  27  is combined and turned around in its flow direction. To avoid short-circuit currents or leakage currents from inlet collecting section  28  of the one space segment  26  to discharge collecting section  29  of space segment  26  following in the flow direction, in each case inlet collecting section  28  of the one space segment  26  is separated from discharge collecting section  29  of space segment  26  following in the flow direction. As  FIGS. 3 and 4  show, coolant collecting sections  28 ,  29  are configured in such a manner that each cross-section of inlet collecting sections  28  decreases in the flow direction, and each cross-section of discharge collecting sections  29  increases in the flow direction. The coolant collecting sections with their cross-sections changing in the flow direction are realized in the manner that parallel guide bars  25  following one another in the circumferential direction within a space segment  26  are shifted axially by an amount, e.g., by the same amount in each case, in the same direction relative to each other. In so doing, the last guide bar  25  in each space segment  26  is shifted so far that with one of its two bar ends, it abuts against one of annular bars  104  and  171 , respectively, and consequently separates inlet collection section  28  from discharge collecting section  29  of space segment  26  following in the flow direction. 
   As may be seen in  FIGS. 3 and 4 , between first guide bar  251  of first space segment  261  in the flow direction and last guide bar  252  of last space segment  262  in the flow direction, an inflow channel  30  is formed that is connected to coolant inflow  21  and extends in the axial direction, and an outflow channel  31  is formed that is connected to coolant outflow  22  and extends in the circumferential direction. Inflow channel  30  and outflow channel  31  are separated from each other by a separating bar  32  extending in the circumferential direction from the front end of last guide bar  252  in last space segment  262 , up to first guide bar  251  in first space segment  261 . To reduce the total pressure prevailing at separating bar  32 , and consequently to avoid possible leakage losses via the small gap which may form between the bar surface of separating bar  32  and the inner surface of casing  17 , or to keep the leakage losses very small, inflow connecting piece  23  is formed in such a manner that its axis forms an obtuse angle α with the axial flow direction of the coolant in inflow channel  30 , as is symbolized in  FIG. 3  by inflow arrow  33  (see also FIG.  2 ). On the other hand, outflow channel  31  is formed in such a manner that its axis runs more or less tangentially with respect to housing  10 , or in its circumferential direction, and forms an obtuse angle β with the flow direction in outflow channel  31 , as is symbolized in  FIG. 3  by outflow arrow  34  (see also FIG.  2 ). 
   The arrangement of guide bars  25  and flow channels  27  formed between guide bars  25  may be seen very well in the developed view of housing  10  shown in FIG.  4 . The flow of the coolant in individual flow channels  27  and in the coolant collecting sections at flow ends  271  and  272  of flow channels  27  is symbolized by the flow arrows drawn in in FIG.  4 . 
   Housing  10 , together with parallel guide bars  25 , separating bar  32  and annular bar  104 , may be produced using the pressure diecasting or injection molding method. Due to the structural configuration of housing  10 , the pressure or injection mold is then able to be drawn off axially, and after the generator is completed, separately produced casing  17  may be slid so far onto housing  10  that the front end face of casing  17  in the push-in direction strikes against flange  103  formed on cylindrical part  102  of housing  10 . Housing  10  and casing  17  are held together by fastening screws  35  which are screwed through flange  103  into the end face of casing  17 . At the same time, screws  35  also affix bearing cover  16  on flange  103  (FIGS.  1  and  2 ).