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258 Braking Force, kN 250 200 150 100 50 FIGURE 9.29 Dynamic brake characteristics. 0 Current Limited Voltage Limited 0 20 40 60 80 100 120 Velocity, kph dependent on velocity, the use of dynamic brakes gave unpredictable results unless amental note was made of locomotive velocity and the driver was aware of what performance to expect. Extended range systems, which involved switching resistor banks, greatly improved dynamic brake usability ondiesel electric locomotives. More recent locomotive packages have provided large regions of maximum retardation at steady force levels. The performance ofthe dynamic brake is limited at higher speeds by current, voltage, and commutator limits. Performance at low speeds is limited by the motor field. Designers now try to achieve full dynamic brake force at as low a velocity as possible. Recent designs have achieved the retention of maximum dynamic braking force down to 10 km/h. Dynamic braking is usually controlled as acontinuous level rather than anotch, but again some locomotives may provide discrete control levels. The way in which the control level affects the braking effort differs for different locomotive traction packages. Four different dynamic brake characteristics have been identified, but further variations are not excluded, Figure 9.30 and Figure 9.31. Laterdesigns (shown on the left in Figure9.30 and Figure9.31)provide larger rangesofspeed where anear constant braking effort can be applied.Modelling of the characteristic can be achieved by fitting apiecewise linear function to the curve, representing 100% dynamic braking force. The force applied to the simulation can then be scaled linearly in proportion to the control setting. In some configurations it will be necessary to truncate the calculated value bydifferent amounts, Braking Force, kN 250 200 150 100 50 0 0 20 40 60 80 100 120 Velocity, kph 250 200 150 100 50 Early Comm. Limited Extended Modern FIGURE 9.30 Dynamic brake characteristics —diesel electric locomotives. © 2006 by Taylor & Francis Group, LLC Braking Force, kN Handbook of Railway Vehicle Dynamics 0 0 20 40 60 80 100 120 Velocity, kph
Longitudinal Train Dynamics 259 Braking Force, kN 250 200 150 100 50 0 0 20 40 60 80 100 120 Velocity, kph FIGURE 9.31 Dynamic brake characteristics —electric locomotives. see characteristics on the right hand side of Figure 9.30 and both characteristics inFigure 9.31. In these cases acombination of look up tables and mathematical functions will be required. D . P NEUMATIC B RAKE M ODELS 0 0 20 40 60 80 100 120 The modelling of the brake system requires the simulation of afluid dynamic system that must run in parallel with the train simulation. The output from the brake pipe simulation is the brake cylinder force, which is converted bymeans of rigging factors and shoe friction coefficients into aretardation force that is one term of the sum of retardation forces F r . Modelling of the brake pipe and triple valvesystems is asubject in itselfand therefore will not be treated in this chapter beyond characterising the forces that can be expected and the effect of these forces on train dynamics. The majority of freight rollingstock still utilises brake pipe-based control of the brake system. The North Americansystem differs in design from the Australian/U.K. systems, but both apply brakes sequentially starting from the point where the brake pipe is exhausted. Both systems depend on the fail-safe feature whereby the opening of the brake valve in the locomotive, or the facture of the brake pipe allowing loss of brake pipe pressure, results in the application of brakes in the train. The implications for train dynamics is that the application of brakes can be accompanied by severe slack action as the brakes nearest the lead of the train, closest to the brake control, apply brakes first.For brakes applied at the leadofagroup of wagons 700 mlong, the initiationofbraking at the last wagon typically lags the lead application by , 5sec, Figure 9.32. The brake system shown in Figure 9.32 benefits from distributed locomotives allowing the release ofair at the lead and mid train positions. The response at the mid point of the first wagon group (Vehicle 26) is faster Brake Pipe Pressure, kPa 600 500 400 300 200 100 0 40 60 80 100 Time,s Brake Cylinder Pressure,kPa 600 500 400 300 200 100 Braking Force, kN 250 200 150 100 50 Velocity, kph 0 40 60 80 100 Time,s FIGURE 9.32 Brake pipe and cylinder responses —emergency application. © 2006 by Taylor & Francis Group, LLC Vehicle 3 Vehicle 26 Vehicle 77 Vehicle 105
Page 1 and 2: 9 CONTENTS Longitudinal Train Dynam
Page 3 and 4: Longitudinal Train Dynamics 241 ass
Page 5 and 6: Longitudinal Train Dynamics 243 By
Page 7 and 8: Longitudinal Train Dynamics 245 Pol
Page 9 and 10: Longitudinal Train Dynamics 247 For
Page 11 and 12: Longitudinal Train Dynamics 249 If
Page 13 and 14: Longitudinal Train Dynamics 251 var
Page 15 and 16: Longitudinal Train Dynamics 253 For
Page 17 and 18: Longitudinal Train Dynamics 255 Com
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Page 35 and 36: Longitudinal Train Dynamics 273 Res
Page 37 and 38: Longitudinal Train Dynamics 275 ope
Page 39: Longitudinal Train Dynamics 277 V f

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