Hi-Res PDF - CRCnetBASE

256
TractionForce, kN
450
400
350
300
250
200
150
100
50
0
0 20 40 60 80 100
Velocity, kph
Notch =2
Notch =4
Notch =6
Notch =8
FIGURE 9.26 Typical tractive effort performance curves —diesel electric.
Handbook of Railway Vehicle Dynamics
Voltage Limited
Else F t = db ¼ðN 2 = 64Þ P max= v ð 9 : 19Þ
where N is the throttle setting in notches, 0to8; P max is the maximum locomotive traction
horsepower, W; Temax is the maximum locomotive traction force, N ;and k f is the torque reduction,
N/(m/sec).
While areasonablefittothe publishedpower curves may be possible with asimpleequation of
the form P ¼ F t/dbv ,itmay be necessary to modify this model toreflect further control features or
reflectchanges in efficiency or thermal effects at different train speeds. It is common for the traction
performance characteristic to fall below the power curve P max ¼ F t/dbv at higher speeds due to
limits imposed bythe generator maximum voltage. Enhanced performance closer to the power
curve at higher speeds is achieved on some locomotives by adding amotor field weakening
control. 13 It can be seen that accurate modelling of locomotives, even without the need to understand
the electrical detail, can become quite complicated. In all cases the performance curves
should beviewed and as much precise detail as possible should be obtained about the control
features to ensure the development of asuitable model.
It is typical for locomotive manufacturers to publish both the maximum tractive effort and
the maximum continuous tractive effort. The maximum continuous tractive effort is the traction
force delivered at full throttle notch after the traction system has heated to maximum operating
temperature. As the resistivity of the windings increase with temperature, motor torque, which
is dependent on current, decreases. As traction motors have considerable mass, considerable
time is needed for the locomotive motors to heat and performance levels drop to maximum
continuous tractive effort. Atypical thermal derating curve for amodern locomotive isshown in
Figure 9.27.
Manufacturer’s data from which performance curves such as in Figure 9.26 are derived can
usually be taken to be maximum rather than continuous values. If the longitudinal dynamics
problem under study has severe grades, and locomotives are delivering large traction forces for
long periods, it will be necessary to modify the simple model represented in Figure 9.26 with a
further model adding these thermal effects.
Arecent innovation in locomotive control is the inclusion of apower application rate limit.
The effect of this control is that the power (or dynamic brake) can be applied no faster than apreset
rate by the manufacturer irrespective of how fast the driver sweeps the control. Opinions differ as
to whether this system was included as an innovation to reduce train dynamics ordue to engine
design considerations. Records inthe Australian patent office identify the system as apollution
© 2006 by Taylor & Francis Group, LLC