In recent years, we have been working on driverless light rail systems like this at Strukton Rolling Stock. We signed a long term agreement in 2014 in which we were asked to develop the traction drive system for an entirely new metro vehicle that will be the new generation of the VAL; a light rail, driverless vehicles for which we have already provided new traction systems for the existing Toulouse and Chicago fleet.
This is a nice incentive for us, since the series involves very stringent requirements. The customer fits the vehicles with engines that are quieter and more compact, yet deliver at least as much power as previous versions. And since mechanical ventilation requires a lot of maintenance, the customer prefers a traction system that is natural air cooled during operation.
The light rail system will be powered by permanent magnet motors. We are designing the converter, which converts 750V direct current into alternating current for the engines by quickly switching between positive and negative charges according to a specific pattern.
Watch below what Wim has to say about this project and converters:
This quick switching between positive and negative is done with the aid of large semiconductors (a kind of transistors) that have been bolted onto a radiator block. This block is cooled by moving air during operation. This is important, because all this switching releases a lot of energy, in the form of heat, while the internals of the semiconductors are specified for a maximum of 125 °C. Which is already quite hot of course!
With a switching frequency of several thousands of hertz, it’s difficult to design an air-cooled system based solely on the wind passing over the heat sink during movement. We have come up with a cunning solution for significantly reducing the heat released during operation: We have designed a number of circuits that allow for low-loss switching (soft switching). Microchip manufacturers are currently working on the development of semiconductors made from different materials to limit power losses during switching. But these new semiconductors are not suitable yet for this kind of operation and performance.
Watch below Wim's detailed guided tour of a converter:
Stringent safety requirements
Naturally, passengers need to feel just as safe in fully-automated light rail vehicles as in conventional systems with a driver. This means that special attention needs to be paid to safety aspects. This already starts in the design phase: all steps (incl.drawings and calculations) are traceable. Each consecutive stage in the design and production process is accounted for. These measures are all necessary to comply with IRIS certification requirements. Consequently, the safety standards adopted for the design and production of rolling stock increasingly resemble the requirements that are set for the design of aircraft and spacecraft.
First step towards a prototype
At this point, we have nearly rounded off the theoretical phase involving calculations and simulations. We will deliver a functioning prototype early 2016 and will consequently start the serial deliveries.