Abstract:
The invention relates to a traction drive for the driving and generative braking of a rail vehicle or a combination of rail vehicles, a permanent-field synchronous motor and a traction current converter being respectively associated with at least two axles of the rail vehicle or combination of rail vehicles. The traction current converter includes at least one pulse current converter on the engine side, and the clamps of the permanent-field synchronous motor are connected to a change-over switch such that the permanent-field synchronous motor can be connected to a load circuit containing at least one load element, in order to drive the pulse current converter or for generative braking. According to the invention, the load circuits connected to the permanent-field synchronous motors for generative braking are designed such that the brake characteristic lines of the permanent-field synchronous motors differ in terms of characteristic features such as the position of the maximum of the brake torque according to the rotational speed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application claims the benefit of priority to International Application No. PCT/EP2007/009258 filed 25 Oct. 2007, which claims priority to German Patent Application No. 10 2006 051 337.1 filed 31 Oct. 2006, the contents of which are incorporated herein by reference in their entirety. 
     
    
     BACKGROUND  
       [0002]    The invention is based on a traction drive for driving, and for performing regenerative braking of a rail vehicle or a combination of rail vehicles, wherein at least two axles of the rail vehicle or of the combination of rail vehicles are respectively assigned a permanently excited synchronous motor and a traction current converter, and the traction current converter has at least one machine-side pulse current converter, and the permanently excited synchronous motor is connected at its terminals to a changeover switch in such a way that the permanently excited synchronous motor can be connected to a load circuit containing at least one load element in order to drive the pulse current converter or to perform regenerative braking. 
         [0003]    The objective when equipping rail vehicles is to use ever more effective and lightweight drive machines. Presently, inverter-fed asynchronous machines are used as the standard drive machine. However, these machines offer little potential for further development in terms of reducing their mass and torque density and, when used in rail vehicles, these machines usually require a transmission. For this reason, increasing efforts are currently being made to develop and use permanently excited synchronous machines as vehicle drives. By virtue of its high torque density, this type of machine permits direct drives to be implemented while eliminating the transmission means; this allows the mass of the drive train to be reduced to a very high degree. 
         [0004]    Permanently excited synchronous machines have a number of particular features compared to asynchronous technology owing to their permanent excitation. Therefore, in the case of a rotating machine, for example, it is possible to achieve a braking effect in addition to the inverter-regulated generator mode using purely passive components. The braking effect which is achieved by a permanently excited synchronous machine by connecting braking resistors is disclosed in DE 101 60 612, which defines the generic type. 
         [0005]    When the rotating, permanently excited synchronous machine with braking resistors is connected into the circuit, a characteristic torque curve or characteristic force curve, also referred to below as a natural braking characteristic curve, is obtained as a function of the rotational speed of the synchronous motor and, therefore, also of the speed of the vehicle. The braking characteristic curve has a maximum value in its profile when plotted against the rotational speed/speed. 
         [0006]    The problem when using the regenerative braking effect of a permanently excited synchronous motor is, therefore, that the natural braking characteristic curve does not have a constant braking torque profile when plotted against the rotational speed. This constitutes a disadvantage in the braking behavior compared to the technology with a regulated braking force which is conventionally customary. 
       SUMMARY  
       [0007]    The invention is based on the object of developing a traction drive of the above-mentioned type in such a way that the braking behavior can be influenced selectively. 
         [0008]    The basic idea of the invention is that the load circuits that are connected to the permanently excited synchronous motors in order to perform regenerative braking are embodied in such a way that the brake characteristic curves of the permanently excited synchronous motors are different in terms of characteristic features such as the position of the maximum braking torque as a function of the rotational speed. 
         [0009]    In other words, despite the design of the motors being identical the braking characteristic curves of the permanently excited synchronous motors have braking torques of different magnitude when the rotational speed is the same owing to the different load circuits. If, for example, the load circuit of one of the synchronous motors gives rise to a maximum braking torque at low rotational speeds, this can compensate a braking torque which is relatively low in comparison at low rotational speeds and is generated by another synchronous motor of the rail vehicle owing to a different load circuit. In an analogous fashion, the other synchronous motor with a maximum braking torque at relatively high rotational speeds can compensate the braking torque of the synchronous motor which is then lower. 
         [0010]    Two or more permanently excited synchronous motors that are identical but are provided with different load circuits can then be combined with one another synchronous motor in such a way that, by superimposing the braking characteristic curves (which are then different), a braking torque profile (which is as balanced or constant as possible when plotted against the rotational speed) is obtained for a bogie, a rail vehicle component, an individual vehicle (such as a wagon) or a combination of rail vehicles. This is achieved by adjusting the passive load circuits of the individual permanently excited synchronous drives. 
         [0011]    The more load circuits of the individual drives are matched to one another, the better the degree by which the braking torque profile can be approximated to a constant braking torque. In the simplest case, as described in the following exemplary embodiment, this can be done by using different resistance values of the braking resistors. However, any desired connection of resistors, capacitors and/or inductors such as choking coils is also conceivable. 
         [0012]    Advantageous developments and improvements of the invention specified in the independent claims are possible by virtue of the measures specified in the dependent claims. 
         [0013]    At least two axles, each with at least one permanently excited synchronous motor, a machine-side traction current converter, a changeover switch and a load circuit may be assigned to a bogie of the rail vehicle or of the combination of rail vehicles. An approximately constant braking torque profile can then be obtained for each bogie. 
         [0014]    Generally, the load circuits of the permanently excited synchronous motors assigned to the at least two axles can differ in load elements and/or in having different connections of their load elements. A particularly simple embodiment is obtained if the load circuits of the at least two axles merely have different braking resistors, wherein the braking resistors of the load circuits are embodied, for example, in such a way that the maximum braking torque of the braking characteristic curve of the permanently excited synchronous motor assigned to the one axle occurs at a lower rotational speed than the maximum braking torque of the braking characteristic curve of the permanently excited synchronous motor assigned to the other axle. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0015]    An exemplary embodiment of the invention is illustrated in the figures and will be described in more detail in the following description. In the figures: 
           [0016]      FIG. 1  is a schematic illustration of a traction drive for an alternating current vehicle according to an embodiment of the invention. 
           [0017]      FIG. 2  is a schematic illustration of a bogie having two axles, each of which being equipped with a traction drive in the manner of  FIG. 1 . 
           [0018]      FIG. 3  shows braking characteristic curves of the traction drive of the bogie in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    In  FIG. 1 , a traction drive  1  for an alternating current vehicle, also referred to as an AC rail vehicle, is illustrated wherein, a traction transformer is denoted by  2 , a traction current converter by  4 , a permanently excited synchronous motor by  6  and a brake device by  8 . The traction transformer  2  has a primary winding  10  and a plurality of secondary windings  12  (only two secondary windings  12  of which being illustrated). The traction current converter  4  has two four-quadrant choppers  14 , an absorption circuit  16 , a capacitor battery  18 , an overvoltage protection device  20  and a machine-side pulse current converter  22 . The two four-quadrant choppers  14  are each linked on the alternating voltage side to a secondary winding  12  of the traction transformer  2  and are connected electrically in parallel on the direct voltage side. The absorption circuit  16 , the capacitor battery  18 , the overvoltage protection device  20  and the direct-voltage-side input connections of the machine-side pulse current converter  22  are connected electrically parallel to the two direct-voltage-side connections  24  and  26  of this feed circuit. On the output side, the machine-side pulse current converter  22  can be connected to connections of the permanently excited synchronous motor  6 . 
         [0020]    The brake device  8  is composed, per phase, of the permanently excited synchronous motor  6 , a braking resistor  28  and a changeover switch  30 . These braking resistors  28  are connected electrically in, for example, a star configuration and each have, for example, a constant resistance value. A triangular circuit is alternatively also conceivable. The changeover switches  30  are linked in such a way to the outputs of the machine-side pulse current converter  22  and to the inputs of the permanently excited synchronous motor  6  such that the inputs of the permanently excited synchronous motor  6  can be connected on one side to the braking resistor  28  and on the other side to the outputs of the machine-side pulse current converter  22 . 
         [0021]    These changeover switches  30 , which are also referred to as failsafe switches, can be activated electrically, mechanically or pneumatically. As soon as these changeover switches  30  have moved from the “drive” operating position, i.e., the terminals of the permanently excited synchronous motor  6  are connected to the outputs of the machine-side pulse current converter  22 , into the “brake” operating position, i.e., the terminals of the permanently excited synchronous motor  6  are connected to the braking resistors  28  which are connected in a star; the permanently excited synchronous motor  6  also generates a braking torque, which changes in accordance with the profile of the braking characteristic curve as the speed of the rail vehicle is reduced. Neither the machine-side pulse current converter  22  nor any kind of regulating means is required to generate the braking torque. 
         [0022]    Such a traction drive  1  is described in detail in DE 101 60 612, mentioned above. For this reason, no further reference will be made to the structure of functionality of the traction drive. 
         [0023]    At least two axles of the AC rail vehicle are assigned such a permanently excited synchronous motor  6  with traction current converter  4  and the further components according to  FIG. 1 . In this context, the braking resistors  28  form a load circuit  32 , which is assigned to the synchronous motor  6  and to which it is connected by means of the changeover switch  30  to perform regenerative braking. In the regenerative braking mode, such a synchronous motor  6  has a braking characteristic curve  36  such as the one illustrated, for example, in  FIG. 3  by means of the dashed line. 
         [0024]    A braking characteristic curve, as explained herein, is understood to be the profile of the braking torque M or of the braking force F plotted against the rotational speed n of the synchronous motor  6 , which is identical to the axle rotational speed in the present case of direct drive. This axle rotational speed n is proportional to the velocity of the AC rail vehicle. As is apparent from  FIG. 3 , the braking torque M firstly increases steeply as the rotational speed n rises from zero, before said braking torque M drops again after a maximum value M max  has been reached. A rotational speed n Mmax  is assigned to this maximum braking torque M max . The rotational speed n Mmax  represents a typical profile of a braking characteristic curve of a permanently excited synchronous motor  6 , to which a load circuit  32  with braking resistors  28  is connected in the regenerative braking mode. 
         [0025]    The load circuits  32  of the synchronous motors  6 , which are assigned to the various axles, are embodied in such a way that their braking characteristic curves are different with respect to the rotational speed n Mmax  at which the maximum braking torque M max  respectively occurs. Two or more permanently excited synchronous motors  6 , which are identical but are provided with different load circuits  32 , may be combined with one another in such a way that, by superimposing the braking characteristic curves that are then different, a braking torque profile which is as balanced as possible when plotted against the rotational speed n is obtained for a bogie, a rail vehicle component, an individual vehicle (such as a wagon) or a combination of rail vehicles. This is done by adjusting the passive load circuits  32  of the individual permanently excited synchronous motors  6 . This adjustment is carried out here by correspondingly selecting and connecting passive load elements such as, for example, resistors, choking coils and/or capacitors. 
         [0026]    Using braking resistors  28  whose value can be continuously adjusted or switched simplifies adjustment of the load circuits  32 . Within the scope of the invention, the respective resistance value of the load circuit  32  of a permanently excited synchronous motor  6  is adjusted with respect to the load circuits  32  of the other permanently excited synchronous motors  6  before the regenerative brake operates so that it can no longer be changed during the braking mode. 
         [0027]    According to one exemplary embodiment according to  FIG. 2 , a bogie  34  of the AC rail vehicle has two axles, each with at least one permanently excited synchronous motor  6   a,    6   b,  one machine-side traction current converter  4   a,    4   b,  one changeover switch  30   a,    30   b  and one load circuit  32   a,    32   b.  Each of the two drive shafts of the two axles of the bogie  34  are driven by a permanently excited synchronous motor  6   a,    6   b , which synchronous motors  6   a,    6   b  are preferably identical, as are also the traction current converters  4   a,    4   b.  However, the respective load circuits  32   a,    32   b  may differ. Resistors  28   a  with resistance values which are higher than the resistance values of the resistors  28   b  of the load circuit  32   b  of the other permanently excited synchronous motor  6   b  are may be installed in the load circuit  32   a  of the one permanently excited synchronous motor  6   a.    
         [0028]    The influence of different resistors on the position of the maximum braking torque M max  or the maximum braking force when plotted against the rotational speed n within the braking characteristic curve is as follows. As the braking resistance values decrease, the maximum braking torque M max  is displaced in the direction of relatively low rotational speeds n or speeds until the short circuit occurs, i.e., only the internal resistance of the drive continues to act. Conversely, as the braking resistance values increase the maximum braking torque M max  is displaced in the direction of relatively high rotational speeds n or speeds. However, the absolute value of the maximum braking torque M max  of the braking characteristic curve remains the same provided that the inductors and capacitors in the brake circuit do not change. 
         [0029]    The one permanently excited synchronous motor  6   a  then has the maximum braking torque M max  of its braking characteristic curve  36  (dashed line) at a lower rotational speed n Mmax a  than the other permanently excited synchronous motor  6   b  whose maximum braking torque M max  of its braking characteristic curve  38  (dotted line) is at a higher rotational speed n Mmax b . The maximum braking torque M max  is in each case of equal size since a variation in the resistance values displaces the maximum braking torque M max  but does not change its magnitude. 
         [0030]    If the braking resistors  28   a,    28   b  are connected simultaneously to the corresponding permanently excited synchronous motors  6   a,    6   b  by the changeover switches  30   a,    30   b,  a resulting braking characteristic curve  40  is obtained for the bogie  34  by superimposing the braking characteristic curves  36  and  38 ; this resulting braking characteristic curve  40  has an approximately constant profile over a large rotational speed range starting at low rotational speeds n just above zero, as is apparent from  FIG. 3 . Alternatively, by superimposing suitable braking characteristic curves, i.e., by means of correspondingly adapted load circuits, it would also be possible to obtain a braking torque profile which, when plotted against the rotational speed n, increases, decreases or is predefined in any desired way. 
         [0031]    In addition to resistors  28  or instead of them, it is also possible for a load circuit  32  to contain other passive electronic components such as capacitors and/or inductors, for example, in the form of choking coils. If a resistor  28  and a capacitor are connected in parallel, for example, the maximum braking torque M max  is increased. In contrast, by means of an inductor, which is connected in series with the resistor  28 , it is possible to reduce the maximum braking torque M max  of the assigned, permanently excited synchronous motor  6 . Generally, a large number of load circuits  32  are conceivable, either by varying the resistance values, capacitance values or inductance values and/or by varying the connection of the individual components (parallel or serial), the combination of which brings about desired properties of the superimposed braking characteristic curve  40 , such as, for example an approximately constant braking torque profile when plotted against the rotational speed n. 
         [0032]    The invention is not restricted to traction drives of vehicles which are fed by an alternating current circuit; rather, the invention can also be applied in traction drives of vehicles having permanently excited synchronous motors, which are fed by a direct current power system, such as that described, for example, in DE 101 60 612.