Application example: Heavy-duty locomotives in Australia
A very good example of extreme utilization of tractive effort is provided by the heavy-duty locomotives in Australia. The goal was to increase their tractive effort in order to enable three locomotives to provide the same tractive effort as five. This was accomplished so successfully that now it is even possible to haul 124 wagons instead of the specified 120.
The starting conditions
The Australian rail operator Queensland Rail currently deploys class 3100 and 3200 locomotives with a starting tractive effort of 375 kN, a locomotive weight of 110 tons, and a coefficient of friction of 0.29. In an extensive conversion program, a total of 63 locomotives were to be converted to a three-phase drive system. The converted class 3700 locomotives needed to attain a coefficient of friction of 0.4 when deployed for the same purpose, in other words, a starting tractive effort of 500 kN with a locomotive weight of 126 tons.
The central question was whether the three locomotives would be able to start the loaded train consisting of 120 wagons, each weighing 106 tons – a total weight of 12,720 tons – on the 10 ‰ reference gradient. Initial tests were unsuccessful because the drives caused vibration of the bogies, preventing the high tractive effort from being utilized effectively. The bogies, which were originally designed for the DC technology of the Class 3100/3200, were unable to transfer the high tractive effort to the rail.
An analysis identified the interaction of the primary and secondary suspension and excitation of vibrations by the drive system as the cause of the inefficient utilization of the tractive effort. Two options were considered as a solution: a new combination of springs and dampers, or attenuation of the vibrations using drive control software. After suitable modification of the software, the performance was successfully tested and the main requirement of the project – replacement of five old locomotives with three new ones – was fulfilled.
The result – requirement more than fulfilled
After further optimization of the software, it is possible to haul even heavier trains up the critical gradient. The locomotives now haul trains with 124 wagons instead of 120, four more than originally planned.