POWER ELECTRONICS
where freight-car wheels were prone to overheating on long downgrades. Diesel locomotives for trains operating in level territory, relatively light trains such as passenger trains, and slow-movers such as yard engines did not have dynamic braking. However, larger railroads such as those of Pennsylvania and Santa Fe in the USA began to request dynamic braking for diesel locomotives to help combat overheating. As the development of diesel progressed, railroads began purchasing dynamic braking units in greater numbers, shedding the original notion that they should only be used in mountainous territory. This helped give them greater operational fl exibility by allowing power to roam the system wherever it was needed, instead of being restricted to a particular region.
REGENERATIVE AND RHEOSTATIC Dynamic braking refers to the use of an electric motor as a generator to dissipate energy and is more precisely
described by two terms - regenerative and rheostatic braking. The diff erence between the two types of dynamic braking is what is done with the electricity after it has been produced. In regenerative braking, the electricity is either immediately reused by other locomotives, or it is stored for later use. This electricity can be transmitted through overhead wires or, in the case of electric locomotives, an electrifi ed third rail. Alternatively, it can be stored onboard through the use of a fl ywheel, battery or other energy storage system. Rheostatic braking occurs when the electrical energy produced is run through resistors and dissipated as heat energy. A rheostat is a device that regulates the current fl owing through it by changing the resistance. For the case of rheostatic braking, this resistance provides a force against which work may be done. Although regenerative braking leads to a more effi cient system because of the reuse of energy, the infrastructure that it requires is not always available. Diesel-electric locomotives run primarily on track that
has not been electrifi ed. For this reason, rheostatic dynamic braking is favoured.
IMPROVING THE METRO Although rail and tram are among the most effi cient means of public transport, they still consume a large amount of energy - especially during acceleration. The amount of energy required to accelerate a vehicle weighing hundreds of tonnes is huge, so any increase in energy effi ciency will have considerable benefi ts. Regenerative techniques - in which braking energy is reused for acceleration - hold the potential to improve this. The majority of metro and
underground electric trains employ regenerative braking systems to feed power generated by the traction motors back into the line when coming to a halt at stations. On intensively used networks, such as the London Underground or Paris Metro, the braking power from the stopping trains is consumed and recycled by the other trains on the track.
Bombardier engineers inspecting an undercarriage at London’s Old Oak Common depot
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IMAGE: COURTESY TFL
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