Abstract:
An electric machine for a vehicle, in particular for a utility vehicle, includes a rotor mounted rotatably on a shaft and a stator surrounding the rotor, at least one device of the electric machine located in at least one pressure chamber which is subjected at least intermittently to an overpressure which is elevated with respect to the ambient pressure. The over pressure in at least one pressure chamber protects against ingress of water

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority of DE 10 2014 000 364.7, filed Jan. 16, 2014, which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to an electric machine for a vehicle, in particular for a utility vehicle, and to a method for protection against ingress of water for an electric machine. 
         [0003]    Electric machines installed in vehicles, in particular in utility vehicles, may be designed to be protected against splash water. Depending on the field of application of the vehicle concerned, generators or other electric machines installed in vehicles may not only be exposed to splash water but may be partially or completely surrounded by water so that, in particular, sensitive electrical devices of the electric machine concerned may be impaired or even rendered inoperative. For example, if water penetrates the region of the wiper contacts of the commutator of an electric machine, electrical contact between wiper contacts and slip rings may be temporarily or completely interrupted, causing malfunction. In addition, the penetrating water can cause corrosion damage to the devices concerned. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    An object of the invention to develop an electric machine in such a way that water-sensitive devices of the electric machine can be optimally protected against ingress of water and/or continue to function even under water. 
         [0005]    According to an embodiment of the invention, at least one device located in the housing of the electric machine is arranged in at least one pressure chamber which is subjected, preferably intermittently, via a compressed air connection to an overpressure which is elevated with respect to the ambient pressure of the medium surrounding the electric machine. As a result of the overpressure built up in the pressure chamber, the device located therein is protected against water ingress. The electronic elements of a frequency converter, a commutator or other electronic components may be arranged in such a pressure chamber. It is especially advantageous to arrange the slip rings arranged on the rotor shaft and the wiper contacts resting against the slip rings in such a pressure chamber, since this area is especially problematic with respect to ingress of water. 
         [0006]    In order to be pressurized, the pressure chamber concerned may be connected to a compressed air reservoir present in any case in a motor vehicle. It is also possible, however, to connect the pressure chamber to a compressor or to the overpressure side of an intake tract of an internal combustion engine. The compressed air supply for the pressure chamber therefore does not require additional devices, since devices of a motor vehicle present in any case can be used for this purpose. 
         [0007]    In the preferred embodiment of an electric machine, the pressure chamber or pressure chambers do not need to be pressurized constantly. Rather, a control unit which controls the compressed air supply to the respective pressure chamber as a function of environmental parameters or other parameters may be provided. For example, a water sensor may detect immersion of the electric machine, so that the control unit then triggers pressurization of the pressure chambers concerned. By controlling the pressurization as a function of environmental parameters or other parameters, the ingress of water to water-sensitive devices of the electric machine is avoided in a specified manner, without the need to maintain a constant overpressure in the respective pressure chambers. However, it may also be possible to trigger the pressurization manually. 
         [0008]    On a pressure chamber, a compressed air supply line may be connected directly to a compressed air connection. However, compressed air channels leading to one or more pressure chambers may also be arranged on the electric machine and connected to an external compressed air reservoir via a common compressed air connection. The compressed air channels arranged in the electric machine make it possible for compressed air to be supplied to a compressed air connection located remotely from the pressure chamber, independently of the spatial location of the pressure chamber concerned. 
         [0009]    It may be especially advantageous to arrange at least one outlet opening with a small opening cross section on each pressure chamber, the opening cross section being selected such that a suitable overpressure can be built up in the pressure chamber using the existing pressure reservoir or using other pressure generators present. The outlet opening has the advantage that the pressure chamber has defined ventilation, whereby any moisture present can be removed from the pressure chamber. If the outlet opening is located in a lower drainage region of the pressure chamber, water may also flow out from the outlet opening automatically. 
         [0010]    The electric machine according to the invention is preferably arranged in a utility vehicle suitable for deployment in water, the pressure chamber or chambers of which is/are connected to a compressed air reservoir or to a compressed air generator present in the utility vehicle, via a control unit. The utility vehicle may be, for example, an amphibious vehicle or a recovery vehicle which may be used in flooded areas. It is a further object of the invention to specify a method for improving protection against ingress of water for devices of electric machines. 
         [0011]    This further object is achieved in that a pressure chamber in which the devices to be protected from water are located is pressurized at least intermittently. Here, the pressurization is effected as a function of environmental parameters or other operating parameters of the electric machine and/or of the vehicle in which the electric machine is arranged. Humidity data from sensors or data of a global positioning system may be used as parameters. However, a pressurization may take place, for example, every time the vehicle starts and/or stops, in order to effect removal of moisture from the pressure chamber by means of an associated ventilation function. Alternatively or additionally, manual activation is also possible. 
         [0012]    Activation of the brakes of the vehicle may also be used as a braking parameter for pressurization; the pressurization may then be maintained for a given time period. However, the vehicle speed may also be used as a parameter in order, for example, to switch off the pressurization at relatively high vehicle speeds. It is assumed here that deployment in water does not take place at relatively high vehicle speeds. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The invention is explained in more detail below with reference to exemplary embodiments represented in the drawing, in which: 
           [0014]      FIG. 1  is a block diagram of devices for pressurizing pressure chambers provided on an electric machine, and 
           [0015]      FIG. 2  is a partial view of an electric machine in longitudinal section in the region of a pressure chamber surrounding slip rings arranged on a rotor shaft. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    The block diagram of  FIG. 1  shows schematically an electric machine  1  having a rotor  3  mounted rotatably on a shaft  2  and a stator  4  surrounding the rotor  3 . Two pressure chambers  6 ,  7 , in which devices  8 ,  9 , represented only indicatively, are located, are arranged in the housing  5  of the electric machine  1 . 
         [0017]    The two pressure chambers  6 ,  7  are connected to a compressed air reservoir  12  via pressure lines  10 ,  11 . 
         [0018]    A control unit  13  with an actuator  14  and a processor  15  is located between the pressure lines  10  and  11 , which processor  15  controls the actuator  14  as a function sensor data of a water sensor  16  and as a function of further parameters of an engine control unit  17 . The engine control unit  17  may, for example, transmit to the processor  15  speed data of the vehicle in which the entire arrangement represented in  FIG. 1  with the electric machine  1  is arranged. The engine control unit  17  may also transmit GPS data (position data) to the processor  15  as operating parameters of the vehicle. With reference to all the operating parameters and environmental parameters received, the processor  15  causes activation of the actuator  14  in order to pressurize the pressure chambers  6  and  7  intermittently with pressure from the compressed air reservoir  12 . In this case the pressure chambers  6 ,  7  are subjected to an overpressure such that a pressure higher than the ambient pressure prevails in the pressure chambers  6 ,  7 . This is especially important if the electric machine  1  is completely immersed in water, in order to ensure that water cannot enter the pressure chambers  6 ,  7 . 
         [0019]    The pressure chamber  6  contains the device  8  to be protected from water, which comprises essentially slip rings and wiper contacts, as shown in more detail in  FIG. 2 . A frequency converter with its electronic components may, for example, be arranged in the pressure chamber  7  as a device  9  to be protected from water; diode rectifiers, for example, may be provided in conjunction with generators. 
         [0020]    The two pressure chambers  6 ,  7  have a compressed air connection  18  to which a respective pressure line  10  is connected. In addition, the two pressure chambers  6 ,  7  have outlet openings  19  provided in the lower region, from which compressed air as well as water can be discharged in the direction of the down-arrows marked. 
         [0021]    In the enlarged partial view of  FIG. 2 , the pressure chamber  6 , represented only indicatively in  FIG. 1 , is shown in more detail. The end of the shaft  2  of the rotor  3 , on which slip rings  20  are arranged, projects into the pressure chamber  6 . The shaft  2  is mounted rotatably in the housing  5  of the electric machine  1  by means of a ball bearing  21 . In the region of the pressure chamber  6 , the housing  5  is configured as a cover  22  which is sealed with respect to the rest of the housing  5  by means of a sealing ring  23 . In addition, the compressed air connection  18  is moulded integrally on the cover  22  in the form of a connecting piece. 
         [0022]    Wiper contacts  24 , which are pressed against the slip rings  20  by means of compression springs  25 , are located in the upper region of the pressure chamber  6 . Electric lines  26  from the wiper contacts  24  lead out from the upwardly extended housing part  27  of the cover  22 . A compressed air connection  18 , via which compressed air can be fed into the pressure chamber  6  in the direction of the associated arrow, is also arranged on the upwardly oriented housing part  27 . 
         [0023]    Compressed air is also fed into the compression chamber  6  at the compressed air connection  18  projecting to the right from the cover  22  in the direction of the arrow  28  marked, so that a corresponding overpressure is produced in the pressure chamber  6 . 
         [0024]    The outlet opening  19 , also shown in  FIG. 1 , is arranged in the lower region of the pressure chamber  6 , the cross-sectional area of which outlet opening  19  is selected such that an overpressure can be built up in the pressure chamber  6  by means of the compressed air supply at the compressed air connections  18 . The outlet opening  19  has a ventilation function and also a water drainage function, in case water should penetrate the pressure chamber  6 , for example in switched-off mode. 
         [0025]    It is noted that the wiper contacts  24  to be seen in the exemplary embodiment represented in  FIG. 2  are in the form of rod-shaped carbon brushes. Instead of the slip rings  20 , a commutator which cooperates with wiper contact elements other than the wiper contacts  24  shown here, may be arranged on the shaft  2  of the rotor, in order to establish the required electrical connection to the rotor winding of the rotor. 
         [0026]    The pressurization according to the invention in the region of devices which are to be protected against ingress of water may also be used with brushless electrical machines and, in general, also with other electrical units or other devices to be protected from water. 
       LIST OF REFERENCES 
       [0000]    
       
           1  Electric machine 
           2  Shaft 
           3  Rotor 
           4  Stator 
           5  Housing 
           6 ,  7  Pressure chamber 
           8 ,  9  Device 
           10 ,  11  Pressure lines 
           12  Compressed air reservoir 
           13  Control unit 
           14  Actuator 
           15  Processor 
           16  Water sensor 
           17  Engine control unit 
           18  Compressed air connection 
           19  Outlet opening 
           20  Slip rings 
           21  Ball bearing 
           22  Cover 
           23  Sealing ring 
           24  Wiper contacts