U.S. patent application number 15/025957 was filed with the patent office on 2016-08-25 for method for controlling a drive-off process of a railway vehicle.
The applicant listed for this patent is SIEMENS AKTIENGESELLSCHAFT. Invention is credited to TILL FOERSTER, STEFAN HASSLER, THORSTEN STUETZLE.
Application Number | 20160244073 15/025957 |
Document ID | / |
Family ID | 51485593 |
Filed Date | 2016-08-25 |
United States Patent
Application |
20160244073 |
Kind Code |
A1 |
FOERSTER; TILL ; et
al. |
August 25, 2016 |
METHOD FOR CONTROLLING A DRIVE-OFF PROCESS OF A RAILWAY VEHICLE
Abstract
A method controls a drive-off process of an electrically driven
vehicle, the electric motor of which is fed via a converter. A
holding torque necessary to prevent the vehicle from rolling back
is determined. By using sensors for determining carriage masses and
sensors for determining route inclinations, the holding torque can
be precisely determined. In order to achieve a drive-off process of
the electrically driven vehicle that is gentle on the electric
motor and as long as a determined rotational motor speed is less
than a specified limit rotational speed, a traction torque is
limited by a control unit of the vehicle to a limit torque
dependent on the holding torque, and the traction torque is
increased beyond the limit torque by the control unit only once the
rotational motor speed is greater than the limit rotational
speed.
Inventors: |
FOERSTER; TILL; (FUERTH,
DE) ; HASSLER; STEFAN; (KIRCHENTHUMBACH, DE) ;
STUETZLE; THORSTEN; (NUERNBERG, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SIEMENS AKTIENGESELLSCHAFT |
Munchen |
|
DE |
|
|
Family ID: |
51485593 |
Appl. No.: |
15/025957 |
Filed: |
September 1, 2014 |
PCT Filed: |
September 1, 2014 |
PCT NO: |
PCT/EP2014/068468 |
371 Date: |
March 30, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B61C 15/00 20130101;
B60L 2200/26 20130101; Y02T 10/72 20130101; B60L 2260/26 20130101;
B61H 9/006 20130101; B60L 2240/26 20130101; B61C 3/00 20130101;
B60L 2240/525 20130101; B60L 2240/642 20130101; B60L 2260/22
20130101; Y02T 90/16 20130101; Y02T 10/64 20130101; B60L 15/2027
20130101; B60L 2260/44 20130101; Y02T 10/645 20130101; Y02T 10/7275
20130101; B60L 15/2081 20130101; Y02T 10/7291 20130101 |
International
Class: |
B61C 15/00 20060101
B61C015/00; B61H 9/00 20060101 B61H009/00; B61C 3/00 20060101
B61C003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 30, 2013 |
DE |
10 2013 219 743.8 |
Claims
1-15. (canceled)
16. A method for controlling a drive-off process of an electrically
driven vehicle, with which a holding torque necessary for
preventing the electrically driven vehicle from rolling back is
determined, which comprises the steps of: limiting a traction
torque via a control unit of the electrically driven vehicle to a
torque limit dependent on the holding torque if a determined motor
revolution rate is less than a predetermined first revolution rate
limit; and raising the traction torque above the torque limit by
the control unit if the determined motor revolution rate exceeds
the predetermined first revolution rate limit.
17. The method according to claim 16, wherein the torque limit is
less than a maximum of 1.3 times the holding torque and 0.3 times a
maximum traction value that can be set by the control unit.
18. The method according to claim 16, wherein the torque limit
exceeds the maximum of 1.2 times the holding torque and 0.2 times
the maximum traction value that can be set by the control unit.
19. The method according to claim 16, which further comprises
increasing the traction torque linearly in relation to the
determined motor revolution rate by the control unit until a
predetermined second revolution rate limit is reached once the
determined motor revolution rate is greater than the predetermined
first revolution rate limit.
20. The method according to claim 16, which further comprises
setting the predetermined first revolution rate limit via the
control unit depending on the holding torque.
21. The method according to claim 19, wherein a ratio of the
predetermined second revolution rate limit to the predetermined
first revolution rate limit equals a ratio of a maximum of a
traction value that can be set by the control unit to the torque
limit.
22. The method according to claim 16, which further comprises:
determining for each carriage of the electrically driven vehicle a
respective carriage mass and a carriage gradient parameter
dependent on a track gradient angle; calculating a carriage holding
torque from the respective carriage mass and the carriage gradient
parameter; and calculating the holding torque of the electrically
driven vehicle by summing all determined carriage holding
torques.
23. The method according to claim 16, wherein a brake is released
by the control unit for driving off and a rise of the traction
torque takes place in dependence on a reducing brake torque.
24. The method according to claim 16, wherein a brake is released
by the control unit for driving off before the traction torque is
increased by the control unit starting from zero.
25. The method according to claim 16, wherein a brake is released
by the control unit for driving off and the traction torque is
controlled by the control unit so that the traction torque first
reaches the torque limit when a brake torque reaches zero.
26. The method according to claim 16, which further comprises
increasing the traction torque via the control unit so that a sum
of the traction torque and a brake torque remains constant.
27. The method according to claim 16, which further comprises:
precalculating a first point in time at which a brake torque will
have fallen to zero; and calculating a preceding second point in
time from the first point in time, so that, rising at a maximum
allowed rate from zero at the preceding second point in time, the
traction torque reaches the torque limit at the first point in
time.
28. The method according to claim 16, wherein if the determined
motor revolution rate falls below the predetermined first
revolution rate limit the traction torque is held by the control
unit at the torque limit while the determined motor revolution rate
is less than the predetermined first revolution rate limit, and the
traction torque is increased by the control unit to above the
torque limit once the determined motor revolution rate exceeds the
predetermined first revolution rate limit.
29. The method according to claim 16, wherein the traction torque
is a negative traction torque for electrical braking.
30. The method according to claim 16, which further comprises
increasing the traction torque via the control unit so that a sum
of the traction torque and a brake torque is equal to the torque
limit.
31. A control system for an electrically driven vehicle having at
least one electric motor and a converter for supplying the electric
motor, the control system comprising: a control unit for
controlling the converter, said control unit configured to
determine a holding torque that is necessary to prevent rolling
back of the electrically driven vehicle, said control unit further
configured to control the converter so that a determined motor
revolution rate is less than a predetermined first revolution rate
limit, and a traction torque of the vehicle is limited to a torque
limit that is dependent on the holding torque and the traction
torque is only then raised above the torque limit if the determined
motor revolution rate is greater than the predetermined first
revolution rate limit.
Description
[0001] 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.
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] The semiconductor component can in particular be a bipolar
transistor with an insulated gate electrode (insulated-gate bipolar
transistor).
[0029] 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.
[0030] 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.
[0031] 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.
[0032] 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.
[0033] 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.
[0034] 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.
[0035] 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.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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.
[0041] 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.
[0042] 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.
[0043] 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.
[0044] 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.
[0045] 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.
[0046] 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.
[0047] 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.
[0048] 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.
[0049] 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.
[0050] 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.
[0051] 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.
[0052] 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.
[0053] 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.
[0054] 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.
[0055] 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.
[0056] 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.
[0057] 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.
[0058] 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.
[0059] 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.
[0060] 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.
[0061] 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.
[0062] 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.
[0063] 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.
[0064] 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.
[0065] 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.
[0066] 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.
[0067] 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.
[0068] 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.
[0069] 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.
[0070] 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.
[0071] 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.
[0072] 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.
[0073] 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.
[0074] 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.
[0075] 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.
[0076] In the figures:
[0077] FIG. 1 shows an electrically driven vehicle with three
carriages on three different track segments with different track
slope angles,
[0078] 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,
[0079] 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,
[0080] 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,
[0081] 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,
[0082] FIG. 6 shows an exemplary time profile of a temperature of a
bipolar transistor of a converter and
[0083] FIG. 7 shows a further exemplary time profile of the
temperature of the bipolar transistor for a higher motor revolution
rate.
[0084] 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.
[0085] 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.
[0086] 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.
[0087] 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.
[0088] 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.
[0089] 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.
[0090] 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.
[0091] 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.
[0092] 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.
[0093] 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.
[0094] 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.
[0095] 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.
[0096] FIG. 2 shows a diagram in which exemplary schematic time
profiles of a traction torque M.sub.T as well as of a brake torque
M.sub.B during a drive-off process of the railway vehicle described
in FIG. 1 are illustrated.
[0097] 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.
[0098] 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.sub.F is
zero.
[0099] 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.
[0100] Initially, the traction torque M.sub.T is zero and a brake
torque M.sub.B produced by the brake systems 13 of the railway
vehicle is constant at an initial value, which is greater than
zero.
[0101] The drive-off process starts at the point in time t.sub.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.sub.0 the brake torque M.sub.B decreases
starting from the initial value. For simplicity, in FIG. 2 the
brake torque M.sub.B decreases at a constant rate. The rate at
which the brake torque M.sub.B decreases can also fluctuate with
time however.
[0102] A first point in time t.sub.2, at which the brake torque
M.sub.B will reduce to zero, is precalculated by the control unit
10. A second point in time t.sub.1 is calculated based on the first
point in time t.sub.2. Said second point in time t.sub.1 is
characterized in that the traction torque M.sub.T, increased at a
maximum allowed rate, reaches a torque limit M.sub.G at the first
point in time t.sub.2 if the traction torque M.sub.T at the point
in time t.sub.1 is increased starting from zero.
[0103] From the point in time t.sub.1 the traction torque M.sub.T
is increased by the control unit 10 starting from zero. As
precalculated, the brake torque M.sub.B decreases to zero at the
point in time t.sub.2 and the traction torque M.sub.T reaches the
torque limit M.sub.G at the point in time t.sub.2. Once the
traction torque M.sub.T is greater than the brake torque M.sub.B
and in addition frictional resistances in bearings of the vehicle 2
are overcome, i.e. between the point in time t.sub.1 and the point
in time t.sub.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.
[0104] The torque limit M.sub.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.sub.G is equal to 0.25 times the
maximum of the traction value M.sub.end that can be set by the
control unit 10.
[0105] The traction torque M.sub.T is increased at an average rate,
which is equal to a maximum allowed rate, up to the point in time
t.sub.2, wherein said maximum allowed rate is less than a
technically possible maximum rate at which the traction torque
M.sub.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.
[0106] The average rate at which the traction torque M.sub.T is
increased is greater in magnitude than the rate at which the brake
torque M.sub.B decreases.
[0107] At the start of the increase in the traction torque M.sub.T
at the point in time t.sub.1, a small, practically instantaneous
jump in the traction torque M.sub.T of height of approx. 5% of the
maximum of the traction value M.sub.end that can be set by the
control unit 10 takes place and is used to increase the traction
torque M.sub.T more rapidly. In the event of a jump in the traction
torque M.sub.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.
[0108] After exceeding a minimum torque M.sub.min, the traction
torque M.sub.T is held above the minimum torque M.sub.min by the
control unit 10 for the rest of the drive-off process, wherein the
minimum torque M.sub.min in the present case equals 0.15 times the
maximum of the traction value M.sub.end that can be set by the
control unit 10.
[0109] From the point in time t.sub.2, at which the traction torque
M.sub.T is of the same magnitude as the torque limit M.sub.G, the
traction torque M.sub.T is held constant by the control unit 10 at
the torque limit M.sub.G until the motor revolution rate reaches a
predetermined first revolution rate limit.
[0110] The predetermined first revolution rate limit is reached at
the point in time t.sub.3. From said point in time the traction
torque M.sub.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.
[0111] The predetermined second revolution rate limit is reached at
the point in time t.sub.4. At said point in time, the traction
torque M.sub.T takes the maximum traction value M.sub.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.sub.end that can be set by the control
unit 10 to the torque limit M.sub.G.
[0112] From the point in time t.sub.4, the traction torque M.sub.T
is held constant at the maximum traction value M.sub.end that can
be set by the control unit 10 until reaching maximum motor power at
point in time t.sub.5.
[0113] The following descriptions of the other figures are
essentially each limited to the differences from the immediately
previously described figure.
[0114] FIG. 3 shows a diagram in which further exemplary time
profiles of the traction torque M.sub.T as well as of the brake
torque M.sub.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.sub.F is greater than
zero.
[0115] 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.
[0116] The initial value of the brake torque M.sub.B, which is
exactly the same as in FIG. 2, is greater than the determined
holding torque M.sub.F. In the present case, the magnitude of the
holding torque M.sub.F is approx. 0.5 times the maximum of the
traction value M.sub.end that can be set by the control unit 10.
The torque limit M.sub.G is equal to 1.25 times the holding torque
M.sub.F and the minimum torque M.sub.min is equal to 1.1 times the
holding torque M.sub.F. The increase in traction torque M.sub.T
starts not from the precalculated point in time t.sub.1 but from a
preceding point in time t.sub.1''. The average rate at which the
traction torque M.sub.T is increased from the point in time
t.sub.1' is in this case selected as equal in magnitude to the rate
at which the brake torque M.sub.B decreases, so that a magnitude of
a vector sum of the brake torque M.sub.B and the traction torque
M.sub.T remains approximately constant from the point in time
t.sub.1'' to the point in time t.sub.2.
[0117] An increase of the traction torque M.sub.T at the maximum
allowed rate from the precalculated point in time t.sub.1 that is
analogous to FIG. 2 would result in the brake torque M.sub.B
already being below the torque limit M.sub.G and possibly even
below the holding torque M.sub.F immediately before the point in
time t.sub.1, which is still before the traction torque M.sub.T has
built up. As a result, rolling back of the vehicle 2 could not be
safely prevented.
[0118] 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.sub.T and the
holding torque M.sub.F is greater than the brake torque M.sub.B and
frictional resistances in bearings of the vehicle 2 are also
overcome, i.e. between the point in time t.sub.1'' and the point in
time t.sub.2.
[0119] Because in the present case the torque limit M.sub.G is
greater than in FIG. 2, and the average rate with which the
traction torque M.sub.T is increased until reaching the torque
limit M.sub.G is also less than in FIG. 2, in the present case
there is a period of time to reach the torque limit M.sub.G from
the start of the increase of the traction torque M.sub.T from the
value zero that is longer than in FIG. 2.
[0120] As can be seen from a comparison of FIG. 3 and FIG. 2, a
period of time in which the traction torque M.sub.T is held at the
torque limit M.sub.G is longer in FIG. 3 than in FIG. 2. This is
because in the present case the torque limit M.sub.G is greater
than in FIG. 2 and consequently longer duration limiting of the
traction torque M.sub.T takes place in order to protect the
converter 8.
[0121] 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.sub.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.sub.T.
[0122] FIG. 4 shows a diagram in which an exemplary profile of the
traction torque M.sub.T is illustrated schematically as a function
of the motor revolution rate n. The diagram concerns the time
profile of the traction torque M.sub.T that is illustrated in FIG.
2, as well as the drive-off situation that is described in
connection with FIG. 2.
[0123] 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.
[0124] While the motor revolution rate n is less than the
predetermined first revolution rate limit n.sub.1, the traction
torque M.sub.T is set by the control unit 10 to the torque limit
M.sub.G, which is 0.25 times the maximum of the traction value
M.sub.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.sub.T is greater
than the brake torque M.sub.B and frictional resistances in
bearings of the vehicle 2 are also overcome, i.e. still before the
point in time t.sub.2 at which the traction torque M.sub.T is equal
to the torque limit M.sub.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.sub.T is already at a value greater than zero, but less than the
torque limit M.sub.G. Because the motor revolution rate n increases
in proportion to the speed of the vehicle 2, the traction torque
M.sub.T increases with rising revolution rate n, in particular
linearly with revolution rate n, until reaching the torque limit
M.sub.G at the point in time t.sub.2.
[0125] After reaching the torque limit M.sub.G, the traction torque
M.sub.T is held constant by the control unit 10 at the torque limit
M.sub.G until the motor revolution rate n reaches the predetermined
first revolution rate limit n.sub.1 at the point in time t.sub.3.
Once the motor revolution rate n has exceeded the predetermined
first revolution rate limit n.sub.1 and while the motor revolution
rate n is less than the predetermined second revolution rate limit
n.sub.2, the traction torque M.sub.T is increased by the control
unit 10 in proportion to the motor revolution rate n.
[0126] On reaching the second revolution rate limit n.sub.2 at the
point in time t.sub.4, the traction torque M.sub.T equals the
maximum of the traction value M.sub.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.sub.T is held constant at the
traction value M.sub.end that can be set by the control unit 10.
From the point in time t.sub.5 at which the maximum motor power is
reached, the traction torque M.sub.T is conversely reduced in
proportion to the motor revolution rate n, whereas the maximum
motor power is maintained.
[0127] FIG. 5 shows a diagram in which a further exemplary profile
of the traction torque M.sub.T as a function of the motor
revolution rate n is schematically illustrated. The diagram
concerns the time profile of the traction torque M.sub.T that is
illustrated in FIG. 3 as well as the drive-off situation that is
described in connection with FIG. 3.
[0128] 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.
[0129] With the motor revolution rate at zero, the traction torque
M.sub.T is already at a value that is greater than the holding
torque M.sub.F, but less than the torque limit M.sub.G.
[0130] The torque limit M.sub.G, to which the traction torque is
set by the control unit 10, equals 1.25 times the determined
holding torque M.sub.F, wherein the holding torque M.sub.F is
approx. 0.5 times the maximum of the traction value M.sub.end that
can be set by the control unit 10. The torque limit M.sub.G in the
present case is thus larger than in FIG. 4. Accordingly, the
predetermined first revolution rate limit n.sub.1, until reaching
which the traction torque M.sub.T is held at the torque limit
M.sub.G, is set by the control unit 10 to a larger value than in
FIG. 4 to protect the converter 8.
[0131] The predetermined second revolution rate limit n.sub.2,
until reaching which the traction torque M.sub.T is increased in
proportion to the motor revolution rate n after exceeding the
predetermined first revolution rate limit n.sub.1, by contrast is
set by the control unit 10 to the same value as in FIG. 4.
[0132] 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.sub.1, can be adjusted by the driver
in stages, in particular in three setting steps.
[0133] 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.
[0134] 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.sub.T to the torque limit M.sub.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.sub.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.
[0135] In the third step by contrast no traction torque limiting
occurs.
[0136] 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.
[0137] In FIG. 2 an exemplary time profile of the traction torque
M.sub.T' for selection of the second setting step is illustrated
using a dashed line.
[0138] In the second setting step the torque limit M.sub.G' is set
to a value amounting to approx. 0.62 times the maximum of the
traction value M.sub.end that can be set by the control unit 10. As
a result, the difference between the maximum of the traction value
M.sub.end that can be set by the control unit 10 and the torque
limit M.sub.G' is halved compared to the difference between the
maximum of the traction value M.sub.end that can be set by the
control unit 10 and the torque limit M.sub.G in the first setting
step.
[0139] Because the torque limit M.sub.G' in the second setting step
is greater than the torque limit M.sub.G in the first setting step,
the precalculated point in time t.sub.1' in the second setting step
is before the precalculated point in time t.sub.1 in the first
setting step. The traction torque M.sub.T' that is increased from
the point in time t.sub.1' with a maximum allowed rate reaches the
torque limit M.sub.G' at the point in time t.sub.2.
[0140] Up to the point in time t.sub.3' at which the motor
revolution rate n reaches the first revolution rate limit n.sub.1,
the traction torque M.sub.T' is held constant by the control unit
10 at the torque limit M.sub.G'. From the point in time t.sub.3' an
increase of the traction torque M.sub.T takes place in proportion
to the motor revolution rate n until the traction torque M.sub.T'
reaches the maximum of the traction value M.sub.end that can be set
by the control unit 10 at the point in time t.sub.4'. From that
point until the maximum motor power is reached at point in time
t.sub.5, the traction torque M.sub.T' remains constant at the
maximum of the traction value M.sub.end that can be set by the
control unit 10.
[0141] In the second setting step, a period of time in which the
traction torque M.sub.T' is held at the torque limit M.sub.G' is
longer than in the first setting step. This is because the torque
limit M.sub.G' is greater in the second setting step than in the
first setting step, and consequently longer duration limiting of
the traction torque M.sub.T' occurs in order to protect the
converter 8.
[0142] Furthermore, in the second setting step a period of time in
which the increase of the traction torque M.sub.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.sub.T' than
in the first setting step.
[0143] The variation against time of the traction torque M.sub.T'
in the second setting step described in connection with FIG. 2 can
be transferred analogously to FIG. 3.
[0144] 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.
[0145] 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.sub.T.
[0146] 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.sub.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.
[0147] 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.
[0148] 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.sub.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.sub.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.sub.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.sub.max reached at the contact surface depends inter alia on a
duration of the conducting phase of the bipolar transistor.
[0149] 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.sub.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.
[0150] 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.sub.1,
and thus the frequency of the output voltage produced by the
converter 8 is higher than in FIG. 6.
[0151] 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.
[0152] 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.sub.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.sub.T is
greater.
[0153] 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.sub.max
reached at the contact surface during a conducting phase of the
bipolar transistor is lower in FIG. 7 than in FIG. 6.
[0154] 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.sub.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.sub.min that is
reached in FIG. 7 compared to the minimum temperature T.sub.min
that is reached in FIG. 6 is negligible.
[0155] 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.
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