Them’s the brakes

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From diesel to electric locomotives, the railway industry has seen numerous advancements since the opening of the Liverpool and Manchester Railway in 1830 marked the dawn of steam-powered rail travel. Simone Bruckner, managing director of Cressall Resistors, discusses how braking resistors have helped revolutionise the railway industry.

Around half a century ago, when diesel locomotives were replacing steam engines, dynamic braking was implemented to make rail operations safer and more efficient. Although safety will always be of paramount importance, as the industry advances, there are an increasing number of factors that must also be optimised. As fuel costs and environmental impacts assume greater importance, so does the need for options to increase energy efficiency and reduce emissions.

Initially, dynamic braking was seen as a tool for mountainous territory, where freight-car wheels were prone to overheating on long downgrades. Diesel locomotives for trains operating in level territory, relatively light trains like passenger trains, and slow-movers like yard engines did not have dynamic braking. 

However, larger railroads, like the those of Pennsylvania and Santa Fe, 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 flexibility 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 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 difference 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 electrified third rail. Alternatively, it can be stored onboard through the use of a flywheel, 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 flowing through it by changing the resistance. For the case of rheostatic braking, this resistance provides a force against which work may be done. While regenerative braking leads to a more efficient 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 electrified. For this reason, rheostatic dynamic braking is favoured. 

Making the metro efficient

Although rail and tram are among the most efficient 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 tons is huge, so any increase in energy efficiency will have significant benefits. 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.

This ability to balance the power needed to accelerate the trains with the power needed to stop them, makes metro systems one of the most energy-efficient of all forms of urban mass transportation. However, this success isn’t always shared. When there are no other trains on the track, or the distances between trains is too great, it may not be possible to use all the regenerated power. 

In these cases, the energy is dissipated in brake resistors, mounted either on the trains themselves or at fixed locations alongside the track. Resistors such as Cressall’s expanded mesh resistors are particularly suitable for this application, as they are convection cooled and therefore silent, with no moving or wearing parts of any kind and capable of dissipating very high powers in a compact space.

Resistor replacement

As braking resistors for traction applications are in high demand, engineers need to consider the most efficient method of implementing them. For reasons of speed, simplicity and cost, it is usually more economical to replace old resistors rather than to take out a whole drive system and replace it with modern drives. This means that resistor providers should hold and offer extensive records of the railway resistors supplied for all types of electric and diesel electric locomotives, electric multiple units and metro cars. 

In many cases where original equipment designs are not available or no longer manufactured, it is also useful for resistor manufacturers to design functionally equivalent replacements. If necessary, these should be retested to ensure that they will meet the same type-test criteria for electrical, thermal and vibration performance as the original equipment designs. Beyond the obvious need to match resistance values, it can be equally important to ensure that the active mass, type of material used and the electrical creepages and clearances are all appropriate. 

The industry has come a long way since the Liverpool and Manchester Railway, just as dynamic braking is no longer limited to mountainous journeys. Although regenerative braking offers the most energy efficient option of dynamic braking in many railway applications, engineers should also know that this option isn’t always available. As well as considering alternative resistor options, railway engineers should also consider how they go about implementing them – as replacement resistors can often be the most effective way to revolutionise.