Patent Publication Number: US-2016244073-A1

Title: Method for controlling a drive-off process of a railway vehicle

Description:
The invention concerns a method for controlling a drive-off process of an electrically driven vehicle, with which a holding torque necessary for preventing the vehicle from rolling back is determined. 
     Electric motors, in particular alternating current motors, of electrically driven vehicles, are frequently supplied by a converter. The converter produces an output voltage from an input voltage, the output voltage being output in the form of pulses with adjustable pulse duration and/or adjustable pulse height to an electric motor of the vehicle. Said output voltage is preferably a three-phase system with variable frequency and voltage amplitude. 
     The motor revolution rate of a three-phase motor is—for a predetermined load characteristic curve—dependent on the frequency and voltage of the power supply. The motor revolution rate increases in particular with rising frequency of the output voltage. Therefore, the motor revolution rate can be controlled by regulating the frequency and the voltage of the output voltage produced by the converter. 
     It is the object of the present invention to disclose a method with which a machine-friendly drive-off process of an electrically driven vehicle is enabled. 
     This object is achieved by a method of the aforementioned type, with which, according to the invention, while a determined motor revolution rate is less than a predetermined first revolution rate limit, a traction torque is limited by a control unit of the vehicle to a torque limit dependent on the holding torque and the traction torque is only increased by the control unit to above the torque limit if the motor revolution rate is greater than the first revolution rate limit. 
     The invention is based on the consideration that with a drive-off process of the vehicle a high thermal load on the converter can occur, in particular on a semiconductor component of the converter, which can reduce the service life of the converter. 
     The invention is further based on the consideration that at low frequencies of the output voltage produced by the converter and hence also at low motor revolution rates, such as occur during a drive-off process of a vehicle, a conducting phase of the semiconductor component of the converter is relatively long. Consequently, the semiconductor component can heat up for a relatively long time in a conducting phase and can reach a high temperature if the motor revolution rate is low. A maximum temperature of the semiconductor component that is reached during a conducting phase of the semiconductor component can depend on the traction torque that is set for the respective motor revolution rate. Said temperature can increase with increasing traction torque. 
     At a higher frequency of the output voltage produced by the converter, and hence also at a higher motor revolution rate, a conducting phase of the semiconductor component is shorter, as a result of which the semiconductor component heats up for less time in a conducting phase. As a result, it is possible that a maximum temperature of the semiconductor component that is reached during a conducting phase of the semiconductor component at a higher motor revolution rate adopts a lower value than for a low motor revolution rate—even in the case of a higher set traction torque. 
     That part of the drive torque that is produced by one or more electric motors of a vehicle that acts in total on wheels of the vehicle and can contribute to a transfer of traction force to a road surface can be considered to be the traction torque. 
     The vehicle can comprise inter alia a single carriage. It can however also comprise a plurality of carriages that are coupled together. At least one carriage of the vehicle comprises an electric drive, wherein the electric drive comprises at least one electric motor that is supplied by means of a converter. 
     The vehicle can be for example a railway vehicle or a motor vehicle. If the vehicle is a motor vehicle, then the vehicle can for example comprise an electrically driven truck or automobile as well as one or a plurality of carriages without their own drives that are coupled to the truck or automobile. If the vehicle is a railway vehicle, then the vehicle can comprise one or a plurality of driven carriages and one or a plurality of carriages without their own drives. 
     The vehicle is fitted with an electric drive that comprises one or a plurality of electric motors. The same are supplied from one or a plurality of converters, wherein each electric motor can be supplied by a converter or a plurality of electric motors can be supplied by one converter. 
     A holding torque can be the minimum traction torque to be applied by the electric motor in order to prevent rolling back by the vehicle, for example on an upslope. In general, rolling against a direction of travel specified by a driver or an automatic control system can be considered to be rolling back. 
     The point in time at which the traction torque is increased or a brake torque is reduced after stopping the vehicle—whichever occurs first—is defined as the start of a drive-off process. 
     A brake torque can be a torque produced by one or a plurality of brake systems that acts in total on wheels or wheel axles of the vehicle. The purpose of the brake torque can for example be to hold the vehicle at a standstill, in particular on an upslope or a downslope. A brake system can comprise one or a plurality of brakes, in particular a brake per wheel or wheel axle of the vehicle. In particular, if the vehicle is a railway vehicle, the vehicle can comprise further brakes that can produce braking forces directly between the vehicle and the track surface. When using said further brakes, the brake torque can be a sum of a torque produced by the further brakes and the torque produced by the brake system that acts on the wheels or wheel axles of the vehicle. 
     Before the start of the drive-off process, the traction torque is advantageously zero, hence unnecessary heating up of the electric motor and/or of the converter can be avoided. Moreover, before the start of the drive-off process the brake torque is usefully at least as large as the holding torque, so that rolling back of the vehicle is prevented. 
     A control unit can be understood to mean a device that is configured for controlling the traction torque. The control unit can in particular comprise automatic traction control. The control unit can also be configured for controlling one or a plurality of brake systems, in particular for applying and/or releasing one or a plurality of brakes. 
     A torque limit can be a value of the traction torque to which the traction torque is limited by the control unit, wherein the value can be dependent inter alia on the determined holding torque. The torque limit is advantageously greater than the holding torque, so that the vehicle can be accelerated from rest in the direction of travel, in particular without rolling back in doing so. 
     A motor revolution rate is advantageously determined repeatedly, in particular at fixed time intervals. A motor revolution rate sensor can be used to determine the motor revolution rate for example. A vehicle speed can be proportional to the motor revolution rate; in this respect the speed can be measured additionally or alternatively, wherein for simplicity a speed measurement is also understood below as determining a motor revolution rate. 
     A revolution rate limit can be a design-related revolution rate value. The revolution rate limit can in particular depend on the converter design. The revolution rate limit can also depend on the torque limit. The revolution rate limit can in particular be greater, the greater is the torque limit. 
     The higher a semiconductor component temperature that is reached during a drive-off process in a conducting phase of a semiconductor component of the converter, the shorter the semiconductor component service life can be. The traction torque is therefore advantageously controlled by the control unit so that a maximum semiconductor component temperature reached during a conducting phase of a semiconductor component of the converter remains below a specified temperature value. 
     Said feature of the method can for example be implemented by storing a semiconductor component temperature calculated as a function of a plurality of parameters in the control unit as an especially multi-dimensional table. A set of possible parameter values can be provided in the table for each of the parameters on which the calculated temperature depends. 
     The calculated temperature can be a function of: the holding torque, the traction torque, the brake torque, the motor revolution rate and/or design-related converter parameters. 
     The calculated semiconductor component temperature can be acquired at specified time intervals using the known and/or determined parameter values in the table. The calculated temperature can be compared with the specified temperature value. The torque limit can then be increased or reduced by the control unit. 
     The temperature of the semiconductor component can for example relate to a temperature on a contact surface with a bonding wire. The bonding wire can be arranged to connect the semiconductor component electrically conductively to one or a plurality of components, in particular to connections of a chip housing that can enclose the semiconductor component. The bonding wire is preferably soldered or welded to the semiconductor component. 
     The semiconductor component and the bonding wire can comprise different materials. The bonding wire can for example consist essentially of aluminum. The semiconductor component preferably consists essentially of silicon. 
     Different, in particular material-dependent coefficients of thermal expansion of the bonding wire and of the semiconductor component can, following a defined number of switching cycles of the semiconductor component, result in a crack on the contact surface of the bonding wire and as a result in a failure of the converter. The number of switching cycles following which a failure of the converter can occur can be dependent on: a material of the bonding wire, a material of the semiconductor component, a geometry of the bonding wire, a geometry of the semiconductor component and/or operating parameters of the converter. 
     The semiconductor component can in particular be a bipolar transistor with an insulated gate electrode (insulated-gate bipolar transistor). 
     In an advantageous embodiment of the invention, the torque limit is less than the maximum of 1.3 times the holding torque and 0.3 times the maximum traction value that can be set by the control unit. At these values converter-friendly driving off can be achieved even on a steep upslope or when driving off on a hill. 
     Furthermore, the torque limit is preferably above the maximum of 1.2 times the holding torque and 0.2 times the maximum of the traction value that can be set by the control unit. In this way it is achieved that the torque limit is not too low and, despite the converter protection, rapid driving off is enabled. 
     As a result of the torque limit being determined by means of a maximum function, it can be achieved that the torque limit can be set by the control unit to a fixed predetermined value for low values of the holding torque. Furthermore, it can be achieved that for high values of the holding torque the torque limit is set by the control unit to a value that is dependent on the holding torque, and that in particular is greater than the holding torque. In this way, an advantageous compromise between converter protection and rapid driving off can be achieved. 
     In one version of the invention, a band of torques with a maximum value and a minimum value of the traction torque can be stored in the control unit. Preferably, the traction torque can only be set within said band of torques if the determined motor revolution rate is less than the predetermined first revolution rate limit. In this way it can be prevented that the traction torque is set to a value that is damaging to the converter by an external intervention. In this case the traction torque can be adjusted by the driver or by an external controller for example. 
     The maximum value of the band of torques can in particular be the maximum of 1.3 times the holding torque and 0.3 times the maximum of the traction value that can be set by the control unit. The minimum value of the band of torques can in particular be the maximum of 1.2 times the holding torque and 0.2 times the maximum of the traction value that can be set by the control unit. 
     If such a band of torques is stored in the control unit, in the absence of an external intervention, for example by the driver or by the external controller, the traction torque can be held constant by the control unit at a predetermined value, as long as the determined motor revolution rate is less than the first revolution rate limit. The predetermined value can in particular be the average of the maximum value and the minimum value. 
     The traction torque is advantageously held by the control unit above a minimum torque, preferably between the minimum torque and the torque limit, once the traction torque is at least as great as the minimum torque. In this way it can be achieved that a dwell period of the vehicle in a state with a low motor revolution rate is reduced. It is useful for the minimum torque to be less than the torque limit. 
     The minimum torque can be dependent on the holding torque. The minimum torque is preferably greater than the holding torque. As a result, despite any inaccuracies in determining the holding torque or despite the effect of further influences, such as for example a headwind or friction, the vehicle can be prevented from rolling back—even without a holding action of a brake. 
     The minimum torque can be greater than the holding torque by a predetermined percentage value of the holding torque, for example 10%. If, however, the determined holding torque is less than a specified threshold value, in particular if the holding torque is zero, the minimum torque can be a fixed predetermined value, for example 15% of the maximum of the traction value that can be set by the control unit. 
     A further version of the invention provides that the torque limit and/or the minimum torque can be set by the driver, in particular in stages. Advantageously, it is possible for the driver to completely cancel limiting of the traction torque, in particular in the presence of an operational exception situation. 
     In a preferred development of the invention, once the motor revolution rate is greater than the first revolution rate limit the traction torque is increased by the control unit until a predetermined second revolution rate limit is reached. As a result, a higher speed and/or acceleration of the vehicle can be achieved. At the same time, despite an increase of the traction torque, the maximum temperature of the semiconductor component that is reached during a conducting phase of the semiconductor component can be lower than in the period of time in which the motor revolution rate is less than the first revolution rate limit owing to a shorter duration of the conducting phase. 
     The traction torque is preferably increased linearly, in particular proportionally, relative to the motor revolution rate, once the motor revolution rate is greater than the first revolution rate limit and as long as the motor revolution rate is less than the second revolution rate limit. 
     Preferably, on reaching the second revolution rate limit the traction torque adopts the maximum of the traction value that can be set by the control unit. After adopting the maximum of the traction value that can be set by the control unit, the traction torque is preferably held constant by the control unit until the maximum motor power is reached at said traction value. 
     The first revolution rate limit is advantageously set by the control unit depending on the holding torque. As a result, it is possible that a range of revolution rates, within which the traction torque is limited to protect the converter, can be adapted depending on the situation. In particular, the first revolution rate limit can be set by the control unit to a value that is greater, the greater is the holding torque. 
     The ratio of the second revolution rate limit to the first revolution rate limit is preferably equal to the ratio of the maximum of the traction value that can be set by the control unit to the torque limit. 
     In an advantageous version of the invention, a gradient parameter dependent on a track slope angle is determined. The track slope angle is usefully related to a track segment on which the vehicle is located. The track slope angle can in particular be a value that is averaged over an entire length of the vehicle. 
     The gradient parameter can for example be the track slope angle itself. Alternatively, the gradient parameter can be a component of the acceleration due to gravity that is dependent on the track slope angle and that is oriented parallel to the track segment. A component of the acceleration due to gravity that acts as a downhill acceleration on the vehicle can be determined from the gradient parameter in a simple manner. 
     A positive value of the gradient parameter can represent a positive track slope angle, wherein the positive track slope angle can occur on an upslope. A negative gradient parameter can represent a negative track slope angle, wherein the negative track slope angle can occur on a downslope. Upslope and downslope are also understood to relate to the direction of travel of the vehicle. 
     The gradient parameter can be determined using an accelerometer for example. In particular, the accelerometer can be an element of an inertial measurement unit that comprises, besides the accelerometer, at least one further accelerometer and/or at least one rate of turn sensor. In order for example to achieve greater accuracy when determining the gradient parameter, the gradient parameter can be determined using a plurality of accelerometers. In addition, at least one rate of turn sensor can be used when determining the gradient parameter. 
     It is also advantageous if a mass of the vehicle is determined. If the vehicle comprises a pneumatic suspension system, then the mass can for example be determined from a measurement of an air pressure. If the vehicle comprises a suspension system with coil springs, then the mass can for example be determined from measurements of axial lengths of the coil springs. 
     The holding torque is advantageously calculated from the mass of the vehicle and from the gradient parameter. The control unit is advantageously set up to calculate the holding torque from the mass and the gradient parameter. 
     Advantageously, a respective carriage gradient parameter is determined for each carriage of the vehicle. As a result, it can be taken into account that the carriage can be standing on track segments with different track slope angles. In order to be able to determine the respective carriage gradient parameter, each carriage can be fitted with at least one dedicated accelerometer. 
     A carriage mass is preferably determined for each carriage of the vehicle. A total mass of the vehicle can be calculated from the carriage masses. Advantageously, the control unit is set up to calculate the total mass of the vehicle. 
     A carriage holding torque is preferably calculated from the respective carriage mass and from the respective carriage gradient parameter for each carriage of the vehicle. The holding torque of the vehicle is advantageously calculated by summing up all determined carriage holding torques. 
     In an advantageous embodiment version, a substitute value calculation is provided in the control unit, which refers to substitute values and/or substitute algorithms if one or a plurality of individual values is lost. Thus, for example, if a carriage gradient parameter is not available, for example because of an accelerometer defect, a carriage gradient parameter can be extrapolated or interpolated from one or a plurality of other carriage gradient parameters. If there is a carriage positioned both in front of and behind the carriage, the carriage gradient parameter of which is not available, then the missing carriage gradient parameter can be set equal to the average of the carriage gradient parameters of said two carriages. If the carriage, the carriage gradient parameter of which is not available, is only adjacent to one carriage, then the missing carriage gradient parameter can be set equal to the carriage gradient parameter of the adjacent carriage. 
     It is also advantageous if the substitute value calculation refers to substitute values and/or substitute algorithms if one or a plurality of individual values is lost during the determination of the carriage masses. Thus, for example, if the carriage mass of a carriage is not available, a maximum mass, in particular a maximum permissible maximum mass, of the carriage can be set as the carriage mass. 
     The respective carriage-holding torque can inter alia be greater than zero if a component of a weight force of the respective carriage is acting against the vehicle direction of travel, i.e. for example if the carriage is on an upslope in relation to the direction of travel. The respective carriage holding torque can inter alia be less than zero if a component of the weight force of the respective carriage is acting in the direction of travel of the vehicle, i.e. for example if the carriage is on a downslope in relation to the direction of travel. 
     Summing all determined carriage holding torques for the calculation of the holding torque of the vehicle usefully takes place while taking into account a sign of the respective carriage holding torque. If the determined holding torque is less than zero, it is advantageously set to zero by the control unit. 
     The gradient parameter is or the carriage gradient parameters are advantageously determined repeatedly, in particular at fixed time intervals. The holding torque of the vehicle is usefully calculated from the last determined gradient parameter or from the last determined carriage gradient parameters. As a result, changes of the gradient parameter or the carriage gradient parameters that occur during the drive-off process can be taken into account and the current holding torque can always be calculated. 
     Advantageously, a brake is released by the control unit for driving off. The brake torque then reduces from an initial brake torque to zero. A further advantageous embodiment of the invention provides that the increase in the traction torque is carried out as a function of a reducing brake torque. The control unit thus controls the increase of the traction torque as a function of brake torque, the variation of which with time can be stored in the control unit, for example by storing the variation against time of a release command as a function of an initial brake torque. Owing to such synchronization, heating up of the converter can be kept low. 
     The traction torque advantageously increases at least over a time segment to the extent that the brake torque reduces. The time segment comprises here at least half the time required to fully release the brake. 
     It is advantageous if the brake is released by the control unit before the traction torque is increased by the control unit, in particular starting at zero. As a result, an unnecessarily long holding action by the brake can be avoided. 
     The holding action of the brake is for example unnecessary at the latest from the point in time at which the traction torque is as large as the torque limit, since it is possible that the holding action of the brake from said point in time only hinders driving off, but is no longer necessary for preventing the vehicle from rolling back. 
     It is advantageous if the traction torque is controlled by the control unit such that the traction torque first reaches the torque limit when the brake torque reaches the value zero. As a result, it can be prevented that the brake torque counteracts the traction torque and as a result hinders driving off when the traction torque reaches the torque limit. 
     In a preferred development of the invention, the traction torque is increased by the control unit such that the sum of the traction torque and the brake torque remains constant, in particular greater than the holding torque. The sum of the traction torque and the brake torque is usefully constant only from the point in time at which the traction torque is increased. 
     Furthermore, the traction torque can be increased by the control unit such that the sum of the traction torque and the brake torque remains constant and at least as great as the torque limit, in particular equal to the torque limit. 
     The sum of the traction torque and the brake torque can in particular relate to the magnitude of a vector sum, because the traction torque and the brake torque can act in different directions during the drive-off process. 
     It is also advantageous if the traction torque is increased, in particular starting from zero, once the brake torque falls below the torque limit. This makes it possible that the sum of the traction torque and the brake torque remains at least as great as the torque limit despite a decrease of the brake torque. 
     In an advantageous embodiment of the invention, a first point in time at which the brake torque has fallen to zero is pre-calculated. A preceding second point in time can be calculated from the first point in time. The traction torque at the second point in time, in particular starting from zero, preferably reaches the torque limit at the first point in time by rising at the maximum allowed rate. This allows a period of time of counteracting traction torque and brake torque to be reduced. 
     The maximum allowed rate at which the traction torque is increased can be lower than a technically maximum possible rate at which the traction torque can be increased. The maximum allowed rate can be a limited rate for reasons of passenger comfort, in particular in relation to avoiding sudden jolts, and/or to protect a drive train of the vehicle. 
     If the motor revolution rate is already greater than the first revolution rate limit and if the motor revolution rate, for example owing to braking, has fallen below the first revolution rate limit, then it is advantageous if the traction torque is limited by the control unit to the torque limit, as long as the motor revolution rate is less than the first revolution rate limit. In particular, if the motor revolution rate falls below the first revolution rate limit the traction torque can be held at the torque limit by the control unit as long as the motor revolution rate is less than the first revolution rate limit. It is also advantageous if the traction torque is increased by the control unit to above the torque limit once the motor revolution rate is again greater than the first revolution rate limit. 
     In a further advantageous version of the invention, the traction torque is a negative traction torque that is preferably used to brake the vehicle electrically. In relation to the advantageous developments of the invention described above, a magnitude of the negative traction torque is decisive for controlling the traction torque by the control unit. As a result, converter-friendly electrical braking can be achieved. 
     It is advantageous if control of the traction torque by the control unit can be deactivated by the driver during electrical braking or automatically deactivated in the event of emergency braking, so that rapid deceleration of the vehicle is possible, in particular to a standstill. 
     The invention further concerns a control system for an electrically driven vehicle with at least one electric motor, a converter for supplying the electric motor and a control unit for controlling the converter that is set up to determine the holding torque necessary for preventing the vehicle from rolling back. 
     A converter-friendly control system is achieved according to the invention by setting up the control unit to control the converter such that, as long as a determined motor revolution rate is less than a predetermined first revolution rate limit, a traction torque of the vehicle is limited to a torque that is dependent on the holding torque and the traction torque is only increased to above the torque limit if the motor revolution rate is greater than the first revolution rate limit. 
     The description given above of advantageous embodiments of the invention contains numerous features that are partly reproduced in the individual dependent claims to form a plurality of combined features. Said features can, however, also usefully be considered individually and can be combined to form useful further combinations. In particular, said features can each be combined individually and in any suitable combination with the method according to the invention and the device according to the invention. 
     The properties, features and advantages of said invention described above, as well as the manner in which they are achieved, will become more apparent and clearly comprehensible in combination with the following description of the exemplary embodiments, which are described in detail in connection with the figures. The exemplary embodiments are used to describe the invention and do not limit the invention to the combination of features presented therein, and also not in relation to functional features. Moreover, suitable features of any exemplary embodiment can also be specifically considered in isolation, removed from an exemplary embodiment, introduced into another exemplary embodiment to extend it and/or combined with any of the claims. 
    
    
     
       In the figures: 
         FIG. 1  shows an electrically driven vehicle with three carriages on three different track segments with different track slope angles, 
         FIG. 2  shows exemplary time profiles of a traction torque and a brake torque for a drive-off process for which the holding torque is zero, 
         FIG. 3  shows exemplary time profiles of the traction torque as well as of the brake torque for a drive-off process for which the holding torque is greater than zero, 
         FIG. 4  shows an exemplary profile of the traction torque as a function of a motor revolution rate for the drive-off process of  FIG. 2 , 
         FIG. 5  shows an exemplary profile of the traction torque as a function of the motor revolution rate for the drive-off process of  FIG. 3 , 
         FIG. 6  shows an exemplary time profile of a temperature of a bipolar transistor of a converter and 
         FIG. 7  shows a further exemplary time profile of the temperature of the bipolar transistor for a higher motor revolution rate. 
     
    
    
       FIG. 1  shows a schematic representation of an electrically driven vehicle  2  with three carriages  4 . The vehicle  2  is a railway vehicle. The right carriage  4  as seen by the observer is implemented as a driven carriage  4  and the other two carriages  4  are implemented without their own drives. 
     The vehicle  2  comprises two electric motors  6  implemented in the form of alternating current motors that are supplied by means of a converter  8 . The converter  8  comprises a bipolar transistor that is not shown in  FIG. 1  with an isolated gate electrode. 
     The vehicle  2  also comprises a control system  9  that comprises a control unit  10  that is set up for controlling a traction torque of the vehicle  2 . The control unit  10  is in particular configured for controlling the traction torque by controlling the converter  8 . Furthermore, the vehicle  2  comprises a motor revolution rate sensor  12  for each of the electric motors  6  thereof that is configured for measuring a motor revolution rate of the respective electric motor  6 . 
     The three carriages  4  of the vehicle  2  are each fitted with a pneumatic suspension system that is not shown in  FIG. 1 . Moreover, each carriage  4  comprises two brake systems  13  that can be controlled by the control unit  10 . Each of the brake systems  13  comprises two brakes, which are not shown in  FIG. 1  for clarity. 
     Each of the three carriages  4  comprises an accelerometer  14  that is configured to measure an acceleration of the carriage  4  that is oriented parallel to a track segment  16 . The accelerometers  14  are connected to the control unit  10  by means of a data line system that is not shown in  FIG. 1  and that is configured for transmission of the determined accelerations to the control unit  10 . 
     The three carriages  4  of the vehicle  2  are each fitted with a pressure sensor  20  that is configured for measuring a pressure prevailing in the pneumatic suspension system of the respective carriage  4 . The pressure sensors  20  are connected by means of the data line system to the control unit  10  and are configured for the transmission of the determined pressure to the control unit  10 . 
     In relation to a direction of travel  22 , the driven carriage  4  is on a track segment  16  comprising a downslope—and hence a negative track gradient angle  24 . The forward of the two carriages  4  without a dedicated drive is located on a flat track segment  16 . The rear of the two carriages  4  without a dedicated drive is located on a track segment  16  comprising an upslope—and hence a positive track gradient angle  24 . The track gradient angle  24  of the track segment  16  with the upslope is greater in magnitude than the track gradient angle  24  of the track segment  16  with the downslope. 
     In  FIG. 1 , changes of track gradient angle  24  between the respective track segments  16  are abrupt and the track gradient angle  24  in the downslope or in the upslope is greater than it may actually be for adhesion railways, which is only used as an illustration. 
     An acceleration of the respective carriage  4  that is oriented parallel to the track segment  16  on which the carriage  4  is located and that is dependent on the track gradient angle  24  is determined by the accelerometers  14  of the three carriages  4  at fixed time intervals. The acceleration is a component of the acceleration due to gravity that is acting as a downhill acceleration. The determined accelerations are then transmitted to the control unit  10 . The motor revolution rates of the electric motors  6  are determined by means of the two motor revolution rate sensors  12  in the same time intervals. 
     The pressures prevailing in the pneumatic suspension systems of the respective carriages  4  are determined by the pressure sensors  20  of the three carriage  4  and transmitted to the control unit  10 . The control unit  10  calculates a mass of the respective carriage  4  from the transferred pressures. Furthermore, the control unit  10  calculates the total masses of the vehicle  2  from the individual carriage masses. 
     A carriage holding torque is calculated by the control unit  10  for each carriage  4  from the three calculated carriage masses and from the transmitted accelerations of the three carriages  4 , and a holding torque necessary to prevent the vehicle  2  from rolling back is calculated by summing all calculated carriage holding torques taking into account their respective signs. 
     For driving the vehicle  2  off, the control unit  10  controls the brake systems  13  such that the brakes of the brake systems  13  are released. Consequently, the brake torque produced by the brake systems  13  at wheels  26  of the vehicle  2 , starting from an initial value that is greater than the determined holding torque, is reduced to zero. Moreover, the control unit  10  controls the converter  8  such that while the brake torque is being reduced the traction torque acting on the wheels  26  of the vehicle  2  is increased from zero. 
       FIG. 2  shows a diagram in which exemplary schematic time profiles of a traction torque M T  as well as of a brake torque M B  during a drive-off process of the railway vehicle described in  FIG. 1  are illustrated. 
     The diagram comprises an ordinate axis and an abscissa axis. A torque M is plotted on the ordinate axis. A time t is plotted on the abscissa axis. 
     Furthermore, the diagram concerns a drive-off process during which the railway vehicle—in contrast to  FIG. 1 —is on a level track segment, i.e. a determined holding torque M F  is zero. 
     The holding torque is zero during the entire illustrated period of time, since with adhesion railways changes of a track gradient angle take place on large length scales in relation to typical carriage lengths of railway vehicles, whereas by contrast the railway vehicle in the illustrated period of time only covers a distance of a few carriage lengths. 
     Initially, the traction torque M T  is zero and a brake torque M B  produced by the brake systems  13  of the railway vehicle is constant at an initial value, which is greater than zero. 
     The drive-off process starts at the point in time t 0  at which the control unit  10  activates the brake systems  13  of the vehicle  2  such that the brake systems  13  release their brakes. From the point in time t 0  the brake torque M B  decreases starting from the initial value. For simplicity, in  FIG. 2  the brake torque M B  decreases at a constant rate. The rate at which the brake torque M B  decreases can also fluctuate with time however. 
     A first point in time t 2 , at which the brake torque M B  will reduce to zero, is precalculated by the control unit  10 . A second point in time t 1  is calculated based on the first point in time t 2 . Said second point in time t 1  is characterized in that the traction torque M T , increased at a maximum allowed rate, reaches a torque limit M G  at the first point in time t 2  if the traction torque M T  at the point in time t 1  is increased starting from zero. 
     From the point in time t 1  the traction torque M T  is increased by the control unit  10  starting from zero. As precalculated, the brake torque M B  decreases to zero at the point in time t 2  and the traction torque M T  reaches the torque limit M G  at the point in time t 2 . Once the traction torque M T  is greater than the brake torque M B  and in addition frictional resistances in bearings of the vehicle  2  are overcome, i.e. between the point in time t 1  and the point in time t 2 , the railway vehicle starts to drive in the direction of travel and a motor revolution rate of the two electric motors  6  increases starting at zero. 
     The torque limit M G  is set by the control unit  10  such that a maximum temperature of the bipolar transistor that is reached during a conducting phase of the bipolar transistor of the converter  8  remains below a specified temperature value. In the present case, the torque limit M G  is equal to 0.25 times the maximum of the traction value M end  that can be set by the control unit  10 . 
     The traction torque M T  is increased at an average rate, which is equal to a maximum allowed rate, up to the point in time t 2 , wherein said maximum allowed rate is less than a technically possible maximum rate at which the traction torque M T  can increase. The maximum allowed rate is rather a rate limited for passenger comfort reasons, in particular in relation to avoiding sudden jolts, as well as for protecting a drive train of the vehicle  2 . 
     The average rate at which the traction torque M T  is increased is greater in magnitude than the rate at which the brake torque M B  decreases. 
     At the start of the increase in the traction torque M T  at the point in time t 1 , a small, practically instantaneous jump in the traction torque M T  of height of approx. 5% of the maximum of the traction value M end  that can be set by the control unit  10  takes place and is used to increase the traction torque M T  more rapidly. In the event of a jump in the traction torque M T  of such a small height, owing to damping by a suspension system of the vehicle  2 , neither detectable jolts nor significant wear on the drive train of the vehicle  2  occurs. 
     After exceeding a minimum torque M min , the traction torque M T  is held above the minimum torque M min  by the control unit  10  for the rest of the drive-off process, wherein the minimum torque M min  in the present case equals 0.15 times the maximum of the traction value M end  that can be set by the control unit  10 . 
     From the point in time t 2 , at which the traction torque M T  is of the same magnitude as the torque limit M G , the traction torque M T  is held constant by the control unit  10  at the torque limit M G  until the motor revolution rate reaches a predetermined first revolution rate limit. 
     The predetermined first revolution rate limit is reached at the point in time t 3 . From said point in time the traction torque M T  is increased by the control unit  10 , in particular in proportion to the motor revolution rate, until the motor revolution rate reaches a predetermined second revolution rate limit. 
     The predetermined second revolution rate limit is reached at the point in time t 4 . At said point in time, the traction torque M T  takes the maximum traction value M end  that can be set by the control unit  10 . The ratio of the second revolution rate limit to the first revolution rate limit equals the ratio of the maximum traction value M end  that can be set by the control unit  10  to the torque limit M G . 
     From the point in time t 4 , the traction torque M T  is held constant at the maximum traction value M end  that can be set by the control unit  10  until reaching maximum motor power at point in time t 5 . 
     The following descriptions of the other figures are essentially each limited to the differences from the immediately previously described figure. 
       FIG. 3  shows a diagram in which further exemplary time profiles of the traction torque M T  as well as of the brake torque M B  are schematically illustrated. The diagram concerns a drive-off process during which the railway vehicle is on an upslope, thus the determined holding torque M F  is greater than zero. 
     For simple comparability of  FIG. 3  and  FIG. 2 , the scale of the ordinate axes and the abscissa axes is the same in both figures. 
     The initial value of the brake torque M B , which is exactly the same as in  FIG. 2 , is greater than the determined holding torque M F . In the present case, the magnitude of the holding torque M F  is approx. 0.5 times the maximum of the traction value M end  that can be set by the control unit  10 . The torque limit M G  is equal to 1.25 times the holding torque M F  and the minimum torque M min  is equal to 1.1 times the holding torque M F . The increase in traction torque M T  starts not from the precalculated point in time t 1  but from a preceding point in time t 1 ″. The average rate at which the traction torque M T  is increased from the point in time t 1 ′ is in this case selected as equal in magnitude to the rate at which the brake torque M B  decreases, so that a magnitude of a vector sum of the brake torque M B  and the traction torque M T  remains approximately constant from the point in time t 1 ″ to the point in time t 2 . 
     An increase of the traction torque M T  at the maximum allowed rate from the precalculated point in time t 1  that is analogous to  FIG. 2  would result in the brake torque M B  already being below the torque limit M G  and possibly even below the holding torque M F  immediately before the point in time t 1 , which is still before the traction torque M T  has built up. As a result, rolling back of the vehicle  2  could not be safely prevented. 
     The railway vehicle starts to drive in the direction of travel and the motor revolution rate of the electric motors  6  increases once a difference of the traction torque M T  and the holding torque M F  is greater than the brake torque M B  and frictional resistances in bearings of the vehicle  2  are also overcome, i.e. between the point in time t 1 ″ and the point in time t 2 . 
     Because in the present case the torque limit M G  is greater than in  FIG. 2 , and the average rate with which the traction torque M T  is increased until reaching the torque limit M G  is also less than in  FIG. 2 , in the present case there is a period of time to reach the torque limit M G  from the start of the increase of the traction torque M T  from the value zero that is longer than in  FIG. 2 . 
     As can be seen from a comparison of  FIG. 3  and  FIG. 2 , a period of time in which the traction torque M T  is held at the torque limit M G  is longer in  FIG. 3  than in  FIG. 2 . This is because in the present case the torque limit M G  is greater than in  FIG. 2  and consequently longer duration limiting of the traction torque M T  takes place in order to protect the converter  8 . 
     Furthermore, it can be seen from the comparison of  FIG. 3  and  FIG. 2  that a period of time in which a traction torque M T  increase that is proportional to the motor revolution rate takes place is shorter in  FIG. 3  than in  FIG. 2 , which is because the increase proportional to the motor revolution rate starts at a higher traction torque M T . 
       FIG. 4  shows a diagram in which an exemplary profile of the traction torque M T  is illustrated schematically as a function of the motor revolution rate n. The diagram concerns the time profile of the traction torque M T  that is illustrated in  FIG. 2 , as well as the drive-off situation that is described in connection with  FIG. 2 . 
     The diagram comprises an ordinate axis and an abscissa axis. A torque M is plotted on the ordinate axis. The motor revolution rate n is plotted on the abscissa axis. 
     While the motor revolution rate n is less than the predetermined first revolution rate limit n 1 , the traction torque M T  is set by the control unit  10  to the torque limit M G , which is 0.25 times the maximum of the traction value M end  that can be set by the control unit  10 . As described in connection with  FIG. 2 , the railway vehicle starts to drive in the direction of travel once the traction torque M T  is greater than the brake torque M B  and frictional resistances in bearings of the vehicle  2  are also overcome, i.e. still before the point in time t 2  at which the traction torque M T  is equal to the torque limit M G . As traction has already built up before the railway vehicle starts to drive in the direction of travel, with the motor revolution rate at zero the traction torque M T  is already at a value greater than zero, but less than the torque limit M G . Because the motor revolution rate n increases in proportion to the speed of the vehicle  2 , the traction torque M T  increases with rising revolution rate n, in particular linearly with revolution rate n, until reaching the torque limit M G  at the point in time t 2 . 
     After reaching the torque limit M G , the traction torque M T  is held constant by the control unit  10  at the torque limit M G  until the motor revolution rate n reaches the predetermined first revolution rate limit n 1  at the point in time t 3 . Once the motor revolution rate n has exceeded the predetermined first revolution rate limit n 1  and while the motor revolution rate n is less than the predetermined second revolution rate limit n 2 , the traction torque M T  is increased by the control unit  10  in proportion to the motor revolution rate n. 
     On reaching the second revolution rate limit n 2  at the point in time t 4 , the traction torque M T  equals the maximum of the traction value M end  that can be set by the control unit  10 . As long as the maximum motor power has not yet been reached, the traction torque M T  is held constant at the traction value M end  that can be set by the control unit  10 . From the point in time t 5  at which the maximum motor power is reached, the traction torque M T  is conversely reduced in proportion to the motor revolution rate n, whereas the maximum motor power is maintained. 
       FIG. 5  shows a diagram in which a further exemplary profile of the traction torque M T  as a function of the motor revolution rate n is schematically illustrated. The diagram concerns the time profile of the traction torque M T  that is illustrated in  FIG. 3  as well as the drive-off situation that is described in connection with  FIG. 3 . 
     For simple comparability of  FIG. 5  and  FIG. 4 , the scale of the ordinate axes and the abscissa axes is the same in both figures. 
     With the motor revolution rate at zero, the traction torque M T  is already at a value that is greater than the holding torque M F , but less than the torque limit M G . 
     The torque limit M G , to which the traction torque is set by the control unit  10 , equals 1.25 times the determined holding torque M F , wherein the holding torque M F  is approx. 0.5 times the maximum of the traction value M end  that can be set by the control unit  10 . The torque limit M G  in the present case is thus larger than in  FIG. 4 . Accordingly, the predetermined first revolution rate limit n 1 , until reaching which the traction torque M T  is held at the torque limit M G , is set by the control unit  10  to a larger value than in  FIG. 4  to protect the converter  8 . 
     The predetermined second revolution rate limit n 2 , until reaching which the traction torque M T  is increased in proportion to the motor revolution rate n after exceeding the predetermined first revolution rate limit n 1 , by contrast is set by the control unit  10  to the same value as in  FIG. 4 . 
     The torque limit to which the traction torque is limited while the motor revolution rate n is less than the predetermined first revolution rate limit n 1 , can be adjusted by the driver in stages, in particular in three setting steps. 
     The first setting step is set by default. Selection of the second or third setting step is limited to the existence of an operational exception situation and must be enabled by the driver by operating an unlocking lever or an unlocking switch. 
     In the first setting step the limiting of the traction torque is carried out as previously described, i.e. the limiting of the traction torque M T  to the torque limit M G  described in connection with  FIGS. 2 through 5  concerns the first setting step. In the second setting step the torque limit is set such that a difference between the maximum of the traction value M end  that can be set by the control unit  10  and the torque limit is halved compared to the corresponding difference in the first setting step. 
     In the third step by contrast no traction torque limiting occurs. 
     If there is an operational exception situation, faster driving off of the railway vehicle can be achieved by selecting the second or the third setting step. 
     In  FIG. 2  an exemplary time profile of the traction torque M T ′ for selection of the second setting step is illustrated using a dashed line. 
     In the second setting step the torque limit M G ′ is set to a value amounting to approx. 0.62 times the maximum of the traction value M end  that can be set by the control unit  10 . As a result, the difference between the maximum of the traction value M end  that can be set by the control unit  10  and the torque limit M G ′ is halved compared to the difference between the maximum of the traction value M end  that can be set by the control unit  10  and the torque limit M G  in the first setting step. 
     Because the torque limit M G ′ in the second setting step is greater than the torque limit M G  in the first setting step, the precalculated point in time t 1 ′ in the second setting step is before the precalculated point in time t 1  in the first setting step. The traction torque M T ′ that is increased from the point in time t 1 ′ with a maximum allowed rate reaches the torque limit M G ′ at the point in time t 2 . 
     Up to the point in time t 3 ′ at which the motor revolution rate n reaches the first revolution rate limit n 1 , the traction torque M T ′ is held constant by the control unit  10  at the torque limit M G ′. From the point in time t 3 ′ an increase of the traction torque M T  takes place in proportion to the motor revolution rate n until the traction torque M T ′ reaches the maximum of the traction value M end  that can be set by the control unit  10  at the point in time t 4 ′. From that point until the maximum motor power is reached at point in time t 5 , the traction torque M T ′ remains constant at the maximum of the traction value M end  that can be set by the control unit  10 . 
     In the second setting step, a period of time in which the traction torque M T ′ is held at the torque limit M G ′ is longer than in the first setting step. This is because the torque limit M G ′ is greater in the second setting step than in the first setting step, and consequently longer duration limiting of the traction torque M T ′ occurs in order to protect the converter  8 . 
     Furthermore, in the second setting step a period of time in which the increase of the traction torque M T ′ in proportion to the motor revolution rate n occurs is shorter than in the first setting step, because the increase in proportion to the motor revolution rate n starts at a higher traction torque M T ′ than in the first setting step. 
     The variation against time of the traction torque M T ′ in the second setting step described in connection with  FIG. 2  can be transferred analogously to  FIG. 3 . 
       FIG. 6  shows a diagram in which an exemplary time profile of a temperature of the bipolar transistor of the railway vehicle that is described in  FIG. 1  is illustrated schematically. The diagram comprises an ordinate axis and an abscissa axis. A temperature T is plotted on the ordinate axis. A time t is plotted on the abscissa axis. 
     The illustrated temperature is the temperature at a contact surface of the bipolar transistor on which a bonding wire is soldered or welded onto the bipolar transistor. Said temperature can for example be the temperature to which the control unit  10  refers when controlling the traction torque M T . 
     Furthermore, the illustrated temperature profile relates to a period of time in which a motor revolution rate n of the two electric motors  6  is less than the predetermined first revolution rate limit n 1 , and hence a frequency of the output voltage produced by the converter  8  is low. The illustrated period of time is so short that the motor revolution rate n in said period of time is considered to be approximately constant. 
     During a conducting phase of the bipolar transistor, the bipolar transistor heats up and the temperature at the contact surface increases. Accordingly, the bipolar transistor cools down during a non-conducting phase of the bipolar transistor and the temperature at the contact surface decreases. 
     A periodic fluctuation of the temperature between a minimum temperature and a maximum temperature is shown in  FIG. 6 . The maximum temperature is a maximum temperature T max  reached at the contact surface during a bipolar transistor conducting phase. Said temperature is reached at the end of the conducting phase. The minimum temperature is a minimum temperature T min  at the contact surface reached during a non-conducting phase of the bipolar transistor. Said temperature is reached at the end of the non-conducting phase. Consequently, the value of the minimum temperature T min  reached at the contact surface depends inter alia on a duration of the non-conducting phase of the bipolar transistor. Accordingly, the value of the maximum temperature T max  reached at the contact surface depends inter alia on a duration of the conducting phase of the bipolar transistor. 
     The temperature profile is illustrated in a simplified form and is only intended to illustrate a relationship between the motor revolution rate n dependent on the frequency of the output voltage and the maximum temperature T max  reached at the contact surface during a conducting phase of the bipolar transistor. For the illustration of the temperature profile it was assumed that an average temperature at the contact surface—in relation to a time average of the temperature over a plurality of temperature periods—has been set to a constant value and does not increase with time t. 
       FIG. 7  shows a diagram in which a further exemplary time profile of the temperature of the bipolar transistor of the converter  8  is illustrated. The illustrated temperature profile relates to a period of time in which the motor revolution rate n is greater than the first predetermined revolution rate limit n 1 , and thus the frequency of the output voltage produced by the converter  8  is higher than in  FIG. 6 . 
     For simple comparability of  FIG. 6  and  FIG. 7 , the scale of the ordinate axes and the abscissa axes is the same in both figures. 
     The duration of a conducting phase of the bipolar transistor is inversely proportional to the frequency of the output voltage produced by the converter  8 . Therefore, the duration of a conducting phase of the bipolar transistor is shorter the higher is the motor revolution rate n. Accordingly, the bipolar transistor heats up for a shorter period at a higher motor revolution rate n. As a result, the maximum temperature T max  reached during a conducting phase of the bipolar transistor at the contact surface can be lower for a higher motor revolution rate n than for a lower motor revolution rate n—even though a rate at which the temperature rises at the higher motor revolution rate n can be higher, for example since the traction torque M T  is greater. 
     This fact is apparent when comparing  FIG. 6  and  FIG. 7 . Thus in  FIG. 7  the rate at which the temperature rises is greater than in  FIG. 6 . But because the duration of a conducting phase in  FIG. 7  is shorter than in  FIG. 6 , the maximum temperature T max  reached at the contact surface during a conducting phase of the bipolar transistor is lower in  FIG. 7  than in  FIG. 6 . 
     At a higher motor revolution rate n, in addition the duration of a non-conducting phase of the bipolar transistor is also shorter. Therefore, the bipolar transistor cools down for less time at a higher motor revolution rate n and the minimum temperature T min  reached at the contact surface during a non-conducting phase of the bipolar transistor can be greater for a higher motor revolution rate n than for a lower motor revolution rate n. For simplicity it can be assumed that the duration of a non-conducting phase of the bipolar transistor in  FIG. 7  is so long that an increase in the minimum temperature T min  that is reached in  FIG. 7  compared to the minimum temperature T min  that is reached in  FIG. 6  is negligible. 
     Although the invention has been illustrated and described in detail using the preferred exemplary embodiment, the invention is not limited by the disclosed example and other variations can be derived therefrom by a person skilled in the art without departing from the scope of protection of the invention.