POWER ELECTRONICS
Things have changed in the electronics world. This is the signalling cabin at Edgeware Road, London in 1926
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 forms of 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.
REPLACING RESISTORS 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
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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 applications, engineers should also know that this option isn’t always available. As well as considering alternative resistor options, engineers should also consider how to implement them – as replacement resistors can often be the most effective way to revolutionise. ●
Simone Bruckner is Managing Director at Cressall Resistors.
www.cressall.com
IMAGE: COURTESY TFL
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