Abstract:
An electric machine includes a stator, a rotor interacting magnetically with the stator, a housing surrounding the stator and rotor, and a hollow shaft provided for arrangement of the rotor and mounted on the housing. A radial fan is mounted rotationally fixed on the hollow shaft on the ventilation side. A section of a fan blade of the radial fan extends axially away from the housing to a greater extent than the hollow shaft. A guide element with radially extending plate is arranged in the hollow shaft, wherein the plate is arranged axially further away from the housing than the end side of the hollow shaft on the ventilation side. An inner coolant flow can thus be delivered from the section of the fan blade out of the hollow shaft through a passage between the end side of the hollow shaft on the ventilation side and the plate radially outwards.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is the U.S. National Stage of International Application No. PCT/EP2012/062417, filed Jun. 27, 2012, which designated the U.S. and has been published as International Publication No. WO 2013/004559 and which claims the priority of German Patent Application, Serial No. 102011078784.4, filed Jul. 7, 2011, pursuant to 35 U.S.C. 119(a)-(d). 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an electric machine having a stator, a rotor, which interacts magnetically with the stator, a housing, which surrounds the stator and the rotor, and a hollow shaft on which the rotor is arranged and which is supported on the housing. 
     A stator of an electric machine, which heats due to losses, can be relatively easily cooled directly by means of an air or water cooling system. The rotor of an electric machine can only be cooled directly if the motor housing is open. This requirement of an open motor housing nevertheless in some instances contravenes a special protection system of the electric machine. In particular, electric machines with explosion protection require the housing to be closed. In this case, a direct cooling of the rotor can then not be realized with previously known techniques. 
     The thermal energy produced due to losses in the rotor can also be output indirectly to the motor housing by transmission using air. This indirect cooling principle is however not as efficient as a direct cooling system. Furthermore, the stator is heated due to the rotor losses. A consequence of this heating-up of the electric machine is inter alia that the grease service life and thus the bearing service life is reduced. 
     Highly efficient cooling systems have to date been achieved for instance in open-circuit ventilated motors. On account of the open housing, only the protective system IP 23 can be retained here. With closed housings, an efficient cooling system can be achieved for instance by means of a so-called ‘thermisiphon’ in the rotor. This guides heat effectively outwards, without it being necessary to open the housing. 
     Furthermore, the publication U.S. Pat. No. 4,574,210 A describes a motor with an external rotor and a corresponding cooling system. The internal stator has a hollow shaft, through which coolant can flow. Furthermore, the coolant flows around the external rotor. A tube with a flange-type plate is placed in the hollow shaft of the stator. Cooling air then flows through the tube into the hollow shaft and back outwards between the tube and the inner wall of the hollow shaft. There the coolant is carried along by the main cooling flow, which flows past the plate to the external rotor. 
     A similar electric machine is described in the publication U.S. Pat. No. 3,445,696 A. There the coolant flows into the hollow shaft of a rotor, reverses inside the hollow shaft and is drawn by an injection nozzle radially outwards into the cooling flow which cools the external stator. 
     SUMMARY OF THE INVENTION 
     The object of the present invention thus consists in more effectively cooling the rotor of an electric machine. 
     In accordance with the invention, this object is achieved by an electric machine having
         a stator   a rotor which interacts magnetically with the stator,   a housing, which surrounds the stator and the rotor and   a hollow shaft, on which the rotor is arranged and which is mounted on the housing, wherein   a radial fan is arranged in a rotationally fixed fashion on the hollow shaft on the ventilation side,   a section of a fan blade of the radial fan extends axially away from the housing to a greater extent than the hollow shaft and   a guide element with a radially extending plate is arranged in the hollow shaft, wherein   the plate is arranged axially further away from the housing than the end side of the hollow shaft on the ventilation side, so that   an inner coolant flow can thus be delivered from the section of the fan blade of the radial fan out of the hollow shaft through a passage between the end side of the hollow shaft on the ventilation side and the plate radially outwards.       

     A radial fan which extends axially over the hollow shaft is therefore advantageously arranged on the ventilation side. The plate thus creates a gap between it and the shaft end, through which a coolant flow, which comes out of the hollow shaft, is delivered radially outwards into the radial fan. A frequently unused region of the radial fan is thus used to guide an inner coolant flow for the rotor. 
     In one embodiment the hollow shaft is closed on the drive side, and coolant can enter the hollow shaft at the end side on the ventilation side. Here the hollow shaft is to reach through the entire rotor and a tube of the guide element is disposed in the hollow shaft, through which tube coolant is delivered from the ventilation side through the rotor and between the outer wall of the tube and the inner wall of the hollow shaft back to the ventilation side. This is advantageous in that no space has to be provided on the drive side for the supply of coolant. Instead, the coolant inflow takes place completely from the ventilation side. Furthermore, coolant passes completely through the rotor, even when the coolant flows into the hollow shaft on the ventilation side and flows out again on the ventilation side. 
     A number of axially running cooling channels can be embodied between the outer wall of the tube and the inner wall of the hollow shaft. The distribution of the coolant on the internal periphery of the hollow shaft can thus be optimized. 
     Furthermore, the radial fan may be embodied so as to deliver an axially arriving outer coolant flow, for cooling the housing, radially outwards. The radial fan is used here to deliver two coolant flows, namely the inner coolant flow and the outer coolant flow. 
     Furthermore, the guide element may comprise fan blades aligned radially with respect to the hollow shaft. These ensure that the inner coolant flow is delivered more strongly radially outwards. 
     It is particularly advantageous if the guide element is a part which is separate from the hollow shaft and is inserted herein from the ventilation side. A modular guide element is thus present, which can be used and/or retrofitted if necessary. 
     It is furthermore favorable if the guide element is manufactured from plastic. Such a plastic part can be easily produced in a complex structure as an injection molded part and has a low weight so that the inertia of the electric machine is as a result barely influenced. 
     It is likewise advantageous if the hollow shaft is not embodied to be hollow on a section on the drive side of the electric machine. This means that the hollow shaft on the drive side has a solid journal which is sufficiently stable to fasten a gearbox thereto for instance. 
     The housing around the stator and the rotor can be closed. Consequently, the cooling of the shaft of the electric machine ensures an adequate rotor cooling, without the electric machine having to be open-circuit ventilated. 
     In a special embodiment, the housing forms an explosion protection around the stator and the rotor. Despite efficient rotor cooling, the electric machine can thus be assigned to a high protection system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The present invention is now explained in more detail with the aid of the appended drawings, in which: 
         FIG. 1  shows a longitudinal section through an electric motor according to the present invention; 
         FIG. 2  shows a view of a guide element, 
         FIG. 3  shows a longitudinal section through the guide element in  FIG. 2  and 
         FIG. 4  shows an end side view of the guide element in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The exemplary embodiments shown in more detail below represent preferred embodiments of the present invention. 
     The example in  FIG. 1  relates to an electric motor or generator having a stator  1  and a rotor  2 . The rotor  2  is rotatably mounted on a hollow shaft  3  within the stator  1 . 
     In this example, the stator  1  and the rotor  2  are accommodated in a closed housing  4 . The housing  4  has a drive-side bearing shield  4  on the drive side A and a ventilation-side bearing shield  6  on the ventilation side B. The hollow shaft  3  is mounted in the bearing shields  5  and  6 . The housing  4  seals the interior of the motor including the stator  1  and the rotor  2  so that explosion protection is ensured for instance. 
     The hollow shaft  3  protrudes here through the entire housing  4 , i.e. it protrudes from the drive-side bearing shield  5  as well as from the ventilation-side bearing shield  6 . Furthermore, the hollow shaft  3  has a blind hole  7 , which is open on the ventilation side B, i.e. the shaft end side  9  (end side of the hollow shaft). The blind hole  7  passes through the ventilation-side bearing shield  6  and the entire rotor  2 . The hollow shaft  3  is closed on the drive-side shaft end side  8 , i.e. the end side of the hollow shaft  3  on the drive side A. A shaft journal  3 ′ which is part of the hollow shaft  2  protrudes overall from the bearing shield  5  on the drive side A. The shaft journal  3 ′ is solid and therefore has increased stability compared with the remaining part of the hollow shaft  3 . In particular, it is thus suited to driving a gearbox or suchlike. 
     A radial fan  10  is disposed on a stub shaft  3 ″ which is part of the hollow shaft  3  and protrudes from the ventilation-side bearing shield  6 . Compared with the hollow shaft  3 , it has fan blades  11  disposed radially outwards. They are fastened to a hub  12  which is mounted on the stub shaft  3 ″. 
     To reduce the inertia of the electric machine, the stub shaft  3 ″ is embodied shorter. This means that it does not protrude to the outermost axial end of the radial fan  10 . Instead, the stub shaft  3 ″ ends clearly upstream of the outermost axial end of the radial fan  10 , so that a section  13  of each fan blade  11  protrudes axially past the hollow shaft  3  and/or the stub shaft  3 ″. Radial fans embodied in such a way are customary and are used to deliver a main cooling flow  14  radially outwards. This main cooling flow  14  firstly strikes a fan hood  15 , which surrounds the radial fan  10  and is fastened to the housing  4 . The fan hood  15  has breakthroughs  16  on the front side, through which the main cooling flow  14  can pass to the radial fan  10 . The main cooling flow  14  through the radial fan  10  retains a radial component, so that it is guided radially outwards to the casing of the fan hood  15  and/or to the casing of the housing  4 . 
     A guide element  17  is inserted into the blind hole  7 . The guide element  17  provides for an inner coolant flow  18 , which is introduced into the hollow shaft  3  on the ventilation side B, routed through the rotor  2  and guided back radially outwards to the fan blades  11  on the ventilation side B. 
     A guide element  17 , which can be manufactured from a light plastic so as to reduce the inertia of the electric machine is shown in  FIG. 2 . The guide element  17  is used to guide the inner coolant flow  18  inside the blind hole  7  and when flowing out, out of the hollow shaft  3 . A longitudinal section of the guide element  17  is shown in  FIG. 3  and an end side view is shown in  FIG. 4 . 
     The guide element  17  has a shank  171  and a plate section  172 . The shank  171  here has a completely continuous tube  173 . Star-shaped ribs  174  are molded radially outwards on the tube  173  (see  FIG. 3  and  FIG. 4 ). The present example shows three such ribs  174  running in the axial direction. If the guide element  17  is inserted into the blind hole  7  of the hollow shaft  3 , coolant channels form through the star-shaped ribs  174  between the inner wall of the blind hole  7  and the outer wall of the tube  173  in each case separated by the ribs  174 . In the present example, three such coolant channels result, into which the inner coolant flow  18  flows back to the ventilation side B from the base of the blind hole  7 . 
     The plate section  172  is molded on the one end of the shank  171 . It has a plate  175 , which essentially extends radially outwards. The plate  175  has a through-opening  176  in its center, into which the tube  173  opens. The edge of the plate  175  is aligned somewhat axially rearwards toward the shank  171 . It thus follows the curve of the outer edges of the fan blade  11  (see  FIG. 1 ). 
     In particular, the plate  175  joins flush with the exteriors of the protruding sections  13  of the fan blades  11 . 
     The plate section  172  has a radially protruding fan blades  177  directly on the plate  175 . These extend at the outermost radial edge in the axial direction according to the section  13  of each fan blade  11 . The fan blades  177  thus rest against the shaft end face  9  and/or the front face of the hub  12 , and at the same time the plate  175  is flush with the outer contour of the fan blades  11 . 
     The guide element  17  thus represents an additional component, which can be used in a modular manner. It is used here as an additional rotor fan and is inserted into the rotor bore. The guide element  17  can in principle also be equipped without the fan blades  177 . 
     The guide element has the following functions:
     a) Coolant and/or fresh air is taken into the tube  173  from the outside on the ventilation side B.   b) The coolant and/or air is guided through the rotor  2  in the tube  173 .   c) The coolant (inner coolant flow  18 ) flows out of the end of the tube  173  in the vicinity of the base of the blind hole  7 . The inner coolant flow  18  is axially deflected at the base of the blind hole  7 , so that it flows back to the ventilation side B.   d) The axial reversal of the inner coolant flow  18  takes place in one or a number of channels, which are separated in the peripheral direction by the ribs  174  of the guide element  17 . The reversal takes place outside of the tube  173 , so that the inner coolant flow  18  can absorb heat from the rotating shaft  3  (i.e. heat losses from the motor, mainly those of the rotor) across the inner shaft surface.   e) The guide element  17  finally has the function of expelling the coolant in the region of the radial fan  10 . Here the inner coolant flow  18  of the rotor cooling is discharged into the outer main cooling flow  14  of the radial fan  10 .   

     The function of the rotor fan (here radial fan  10 ) is assisted in the outflow area by the coolant flow of the main fan  10 . A support of the inner coolant flow  18  according to the injector principle (venturi nozzle) namely develops in the transition region between the guide element  17  and the fan blades  11  in section  13 . 
     The coolant flow for the rotor is here taken in from the ventilation side B and is also discharged again via the ventilation side after being heated up in the rotor. 
     The intake and discharge of the coolant in the above example takes place on the ventilation side B. In an alternative embodiment, the coolant can also flow via the drive-side shaft end side  8 . In this case, an axial deflection of the coolant flow within the shaft is not required. The guide element  17  does not then need to have any tube  173 . Optionally, it can naturally have the star-shaped protruding ribs  174 , which bound the axial flow channels inside the shaft. The main object of the guide element  17  is then the deflection of the inner coolant flow  18  in the radial direction to the axially protruding sections  13  of the fan blade  11 . 
     The afore-cited examples relate to open cooling systems. The inventive cooling principle can however also be applied to a rotor cooling in a closed system. 
     It is the current prior art that the shafts  3  are set back in the region of the fan seat in order to save on shaft steel material. This space is now used in accordance with the invention for the fan blades  177  of the internal rotor fan. As a result, a modular use and/or a retrofitting of the electric machine is possible with an internal rotor fan, since all components remain uninfluenced in terms of their dimensions. Provision is only made for a bore and/or blind hole in the shaft. The rotor dynamics and the natural bending frequency is only influenced to a very limited degree by the measures in the shaft. 
     Since the interior of the motor and/or the electric machine is not opened, the protective system remains uninfluenced by the additional rotor cooling system. The ventilation principle can thus also be applied to explosion-protected motors. 
     It is also advantageous, as indicated already, that the construction volume and add-on volume is not changed by retrofitting the additional rotor cooling system. In such cases only a hole is to be made and/or retrofitted in the shaft and the internal rotor fan (guide element  17 ) is to be inserted into the shaft. An axial fixing of the guide element  17  takes place for instance by means of a snap-on closure. The fixing is to permit disassembly of the internal rotor fan. 
     The degree of motor efficiency can advantageously be increased by reducing the stator and rotor temperatures using the inventive fan concept. The winding service life, and also the bearing and grease service life can also be increased in the process.