Activity: draw and/or note:
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the block diagram of the vehicle control,
relation between driving torque M and the tractive effort,
changes in the value of μ in function of the creeping speed,
computer aided drive system of a locomotive that has slide and spin protection,
calculation and equivalence of electric and mechanical power of power
transmission,
changes in hyperbolas of tractive and braking efforts in diesel locomotives, the
curve of travelling resistance,
limits of the vehicle’s traction power.
1.
Drive control, vehicle control
The function of a vehicle is to travel a specified distance with a specifiable velocity function,
for which driving and/or braking are needed, depending on the road or track conditions. Its
briefing is shown in figure 1. below:
Figure 1. The task of the vehicle control is to travel a specified distance on the basis of a
specified velocity-time function. Signals released by the driver are realised through the values
of the velocity and the distance.
Describing in detail the external effects influencing the driver or the automatic vehicle
controlling system and the effects of the output of the control, the following diagram can be
obtained, figure2.
Figure 2. Factors influencing the vehicle control will operate via the driver. The vehicle
equipment implements tractive and braking efforts Fv and Ff corresponding to control signals
uv and fv.
Labels in the figure:
c: external factors (traffic control etc.),
τ: the driver’s provisions on the time delay or advances,
r: accidental disturbances in the traffic,
Fv (uv , v) and Ff (uf, v): the velocity functions of the tractive effort Fv and braking effort Ff
implemented by the vehicle equipment on the driven wheels according to the driver’s actions,
ResistanceF : travelling resistances,
m(1+ γ) : mass value, considering also the effect of the rotating masses; γ generally means a
mass excess ranging between 3 and 10 %,
a: acceleration,
v: velocity,
s: distance.
Control signals uv and fv in the functional block diagram are released by the driver to the
internal control system of the vehicle. The energy converter and traction motors as well as the
electrical and pneumatic braking equipment of the vehicle implement tractive and braking
efforts Fv and Ff corresponding to control signals u v and fv included in blocks Fv (uv, v) and Ff
(uf, v), producing the required accelerating and decelerating effects, by which the task
expected from the vehicle can be performed.
2. Connection between the wheel and the road, limits of transferring longitudinal forces.
Curves of tractive and braking forces. The traction power of the vehicle.
Connection between the vehicle wheel and the surface of the road of the railway rail limit the
transfer of the tractive efforts, which can be characterised by the notions of μ friction and road
adhesion factors and the factor of force connections, and from the aspect of its content, it can
be interpreted as Fn ratio of forces of possible direction to forces orthogonal to the surface,
figure…
.
Figure 3.Driving torque M develops tractive effort Fv on the circumference of wheel of radius
R, which cannot exceed the product of the normal force orthogonal to the surface and the
friction or force connection factor.
It was demonstrated for the connection between the rail and the driven wheel that force
connection factor μ increases to a possible limit in the range of the so-called micro-slips, then,
after exceeding this limit the value of the factor decreases, figure 4. By not decreasing the
driving torque developed wheel spin takes place, where the circumferential speed of the wheel
will be significantly higher than the rate of travel. That is, continuous efforts require durable
creeping and relatively small speed difference between the contact surface of the wheel and
the rail. Excessive rising of the speed difference will result in a decreasing μ value after
exceeding a certain limit value.
In the case of road vehicles a similar situation occurs.
Figure 4. Changes in the value of μ in function of the creeping speed of the wheel of the vehicle.
One of the most important issues of the torque control of vehicles with electrical drives is the
detection of the spin and reduction of the driving torque to an essential extent. Several
technical solutions have been developed for this, and the operation follows similar principles
in railway and road vehicles; even the manufacturers are the same. These detectors mainly
detect the changes in the angular velocity or, recently, in the angular acceleration, and when
non-admissible angular accelerations appear they initiate interventions that is rapid but a
slight reduction of the motor current, after the shortest possible delay.
In road and railway vehicles running with electrical drives, reduction of the driving torque in
vehicles driven by AC motors can be handled as a process with a sufficiently low time
constant and maintaining the tractive and braking efforts is really effective in recent vehicles
– refer to the description relating to synchronous motor hybrid vehicle drives.
In the existing DC motor (railway) vehicles, admissibly rapid response can only be given to
the appearing wheel spin by means of advanced electronic equipment, due to the properties of
the torque controlled system that has long since been applied. The rate of motor current
reduction in vehicles running without such equipment is not sufficient due to the high
electrical time constants; thus, elimination of the wheel spin is delayed and can take place
only through an excessively high level of current reduction.
In the braking operation, it is of the same importance. The detail on the left bottom part of the
figure shows the limit of the occurrence of the blocking, if the limit for the slip has been
exceeded due to over braking. Macro-slip occurring at this time brings a higher, several
percents of speed difference compared to the rate of travel, which leads to unnecessary wear
and lower braking power than in the case of braking with a speed difference, belonging to the
preceding state and with higher μ value, which could be performed by a lower braking torque.
It can be said that the maximum value of μ can be obtained in the range of micro-slip and
considering a low speed difference, nearly proportional to the value of μ as well as exceeding
the maximum of μ, the value of μ will decrease and the creeping speed will increase. For the
return, the torque of the traction motor must be decreased; then this value can be increased
again.
Similarly to cars, this function is fulfilled by a unit with an independent processor but in this
case it is not the brake-pressure that is changed but the motor current.
Wheel drives controlled to a constant torque have good acceleration properties, but the control
sensitivity of such a drive is lower for the intention to change upon the detection of over
braking related blocking, therefore the detection of the blocking and the intervention must
take place with a very short delay.
When the cars travel through a curve, extent and quickness of interventions made after
detecting the risk of skidding are of similar importance in the case of hybrid vehicles, when
electric and mechanical driving torques are present simultaneously. An intervention with a
more rapid effect can be expected from the torque reduction of the electric motor; in this way
using the rubbing brake can be avoided to stop the wheel from starting to slide, although its
operating process is already well-developed and efficient.
The response readiness of synchronous motors, as it is well-known from robot drives, is more
rapid than in any other type of motors. Asynchronous motors are used in drives of bigger
vehicle masses, from trolley buses to electric locomotives, where the inertial moment of the
rotating masses is higher by one order of magnitude than in passenger cars, thus interventions
with less rapid responses can also be efficient.
The following figure 5.shows the control related block diagram of a diesel-electric
locomotive; also marked are the tasks of handling spinning and sliding events of the wheelrail connections.
Figure 5.Computer controlled drive system of a diesel-electric locomotive, equipped with slide and
spin protection
The curves of tractive and braking efforts of the locomotive (figure 6.) represent that content
of the efficiency reverses when the motor is rotated from the direction of the vehicle, during
generator operation. All of the losses increase the numerical value of the braking power from
the side of the vehicle, being the driving side, under the same electric power. The curve of the
braking operation is not allowed to go above the line of the constant-indexed current, thus the
value of the braking force is fixed from 40 km/h to a standstill.
3. Traction power transferred to the drive of the vehicle and its limits
The upper starting section of the curve of the tractive effort indicates a highly decreasing
tractive effort nature for the increasing speed, which follows the nature of the road adhesion
and friction factor. The process of developing the tractive effort is controlled on the basis of
the trend and it follows the dropping function line shown in the figure.
Equivalence of electrical and mechanical power of the power train
Power PD delivered by a diesel locomotive with specified setting corresponds to the power of
an electric generator with the same numerical value, which can also be obtained with the
product 3 z UmI mcosφ ηm of the motor voltage and current after the inverter, where z is the
number of the motors and it is equal to the product of the speed and the tractive effort, if value
ηtpe of the transmission efficiency is:
PD = MD ωD = 3 UgI g cosφ ηg = 3 z UmI mcosφ ηm = Fv v ηtpe = Ptract.
Losses of the inverters were taken into account at efficiency η m of the motors.
At the maximum of the starting tractive effort of a standing vehicle, some 1/6 to 1/4 of the
rated power is necessary to cover the total loss and since the speed is zero, the tractive power
and the efficiency will be zero too.
Exceeding a speed of several km/h, the efficiency already reaches 75 to 80 % and this value
will still increase upon the decrease of the currents and the increase of the motor voltage. The
highest value of the total efficiency of the power train η tpe varies around 80 to 85%, which is
about 2/3 of the nominal speed of the vehicle.
The internal voltage of the motors, when the flux is constant, is proportional to the speed, so
to the frequency:
Ub = 4.44 f N Φ ξ,
Thus, the inverters must produce higher voltage as terminal voltage with dropping voltage on
the electrical impedances but basically speed-proportional voltage is needed. While
accelerating along a hyperbolic type curve of the tractive effort, the current decreasing
according to the decreasing tractive effort and torque will decrease the copper loss and the
power loss of the main units of the system.
Ploss = I2R.
Owing to this, the overall efficiency increases but, at increasing speeds, the value of the
mechanical losses, which increases partly proportionally to this and partly with a squared
increase, slightly deteriorates it.
The power can be drawn as a line parallel with the abscissa above the initial speed range,
after the product of the speed and the tractive effort has reached the value of the specified and
constant power, which constitutes a hyperbola with the factors of the tractive force F v and
speed v, which is shown in the figure.
Figure 6. The nominal power related hyperbolas of tractive and braking efforts in diesel
locomotives with asynchronous motor drive, whose potential power is limited by that of the
diesel motor. The curve drawn for the travelling resistance allows for speeds that reach 115
km/h.
The start-up and acceleration section that reaches the hyperbolic curve of the tractive force
shows decreasing values for the tractive force, which is adjusted to the reduction of value μ.
This takes place along with the increase in the rate of travel. A part of the decrease is
attributable to the increase in the vertical movement of the wheels due to the faults on the
track, which occurs when the speed increases. The rate of the field of decreasing forces
increases in the velocity function of forces orthogonal to the track, which means there is a risk
of spinning and leads to a decrease in the average value of μ.
The traction power of the diesel locomotives can be considerably lower than that of the
installed resource if the locomotive is expected to provide train heating and air-conditioning.
These operations are supplied by the synchronous generator, which is installed separately but
driven from the diesel motor.
The relating power demand is 30 to 40 kW for one passenger carriage today, which may
require 600 to 1000 kW power. An internal computer-controlled vehicle control system
analyses the conditions and the transport task then makes a decision if the traction power
following the driver’s intention or priority of the passenger transport should prevail and to
what extent.
The asynchronous traction motors of electric locomotives called multi-current machines,
which are capable of travelling under two or several kinds of overhead wire systems are
operated from one kind of inverter but the intermediate direct voltage circuit that is before the
inverter can be supplied directly from two kinds of alternating voltage transformers and two
kinds of overhead wires, i.e., from 1500 V or from 3000 V.
It can be seen in the following figure (figure 7.) that the value of the traction power delivered
by the traction motors depends on the voltage of the overhead wire, since the voltage of the
inverters and the motors are of nearly 2800 V AC, therefore the power consumable from the
network of 1500 V voltage is limited.
The speed range can be expanded according to the dashed lines through changing the
gearwheels and switching them over during standstill or through using traction motors
capable of considerable field weakening, whose use permits the avoidance high motor voltage
at high speeds. The synchronous motors follow this pattern, where it is possible to avoid the
over-sizing of the motor.
Figure 7. Traction force curves of a four-current electric locomotive
According to the foregoing description, limits of the driving power include:
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the highest possible tractive effort depending also on the mass of the vehicle,
permitted axle load,
dimensions of the installable traction motor,
friction or road adhesion factor decreasing with the speed,
at higher speeds, limits of the power of the available resources,
in the aggregate.