Acal looks to bring clean power to the rail industry

Peter Creely is applications manager for power at Acal BFi, a company that has diversified from its primary role as a specialist distributor to a company that can offer a broader range of engineering services – principally those that rely on specialist expertise. Acal BFi's power business is a prime example. Power supply engineers are a scarce commodity and, according to Creely, is a luxury that most SMEs cannot afford. "Why would they retain full time power engineers if their expertise is in a completely different area," he suggested. "If they do employ a power supply designer, then they need a high volume product to justify that expense. Every engineering project needs power, so you have got a very small source of expertise being chased by a huge market demand." Creely believes one reason for this lack of expertise comes from the skill sets that are taught, or more importantly those which are not taught, in universities. He said: "There are three key skills relevant to power supply design in which there doesn't seem to be academic interest. EMC, which is very much a "learned experience" based upon hands on testing. Analogue engineering which in this digital age is generally not taught in great detail, and finally control theory, which involves a significant part of the power supply, is not widely taught. Even thermal management, which on the surface looks straight forward, can have serious implications on the reliability of the product if not considered in great detail. Most electronics engineers leaving university today have not been schooled in these crucial areas of design..." Companies lacking the in-house expertise therefore use experts like Acal BFi. This may involve developing a power supply solution for a new product. Creely pointed to an interesting project in the rail industry, a semi-custom power solution, which was needed to resolve an in-situ problem on an existing air conditioning system. The rail industry has very specific problems. Electrically it is a very noisy environment. With heavy currents being switched, the interconnecting cables are subjected to high voltage transients. There are also communications networks within the train that must be protected from these effects. It is a tough EMC environment. Mechanically, there is also a high degree of vibration, a wide operating temperature range and a need for abrasive cleaning techniques. Creely added: "They also have a very specific source of power. It is not like utility power or telecoms power, rail power is very variable, can be a wide range of DC inputs and has a lot of switchgear associated with it. All these challenges need to be understood, and a solution delivered in a short time scale at an agreed price." Previously the air conditioning systems were powered directly from the vehicle's primary power source, which lacked stability. This meant that there was variable air flow from the boiler where the air was heated by burning diesel fuel. The consequences of the variation in input voltage to the fan, and therefore air speed, were twofold. Firstly it was inefficient in terms of the amount of fuel used, and secondly it created a major maintenance problem. Incomplete burning resulted in carbon deposits to the extent where there was a disruptive amount of maintenance and cleaning required ultimately reducing the availability of the vehicle. The objective of the project was to take the original voltage source as the input, which varied from 19 to 34VDC, and provide a 200W stabilised 24VDC supply to the boiler fan control system. Having already used a design based on market leading power modules, the customer was aware of Acal BFi's capabilities, but was not so sure about the complexities of the problem in hand. "We helped them define the specification so that we could deliver a product that is clearly understood and defined. We also worked with them throughout the development because nothing is straightforward. In this case we spent time with them on the EMC side, and supported their engineers at the test house to make sure the product was compliant." Customers want a 'Black box solution' for their power needs. However every application will have different demands in terms of communication and control, EMC compliance, thermal management, reliability, customer interface and many other variables, so an off the shelf solution invariably leads to design compromise in some key areas of the customers end product. In this railway case, Creely based the solution on standard power modules, selected for their relevance to the rail industry. "We have a range of power module products available and we select the product to suit the customer's application and the environment within which the product will be used. The actual unit at the heart of our custom design was a 200W mini – brick module with 24V output. The next stage of the custom design, Creely claimed was where the expertise of the power specialist came into play: "The basic unit self-protects thermally, and self-protects for overload currents, but that is it. EMC noise will be generated on switching and the temperature will have to be managed, heatsinking will be required. The module technology used internally in our design is changing all the time as well. Modules have come a long way from the brick-style modules to the smaller chip designs that exist today offering improved power density. Part of our expertise is selecting the correct internal components for our design." However in this case the main challenge turned out to be EMC compliance. Creely outlined the situation: "The EMC environment is defined for the rail industry and our product has to operate within that environment. So there are the two things – susceptibility and radiation. In this case the customer couldn't take their current product to market because it was outside the specification on radiated emissions. The problem was the cable they had attached to our power supply was radiating emissions, something you can't identify easily in a product but in this application it produced a problem. If you look at the specification it is not clear where the responsibility was - with the cable or the supply - but it is part of our role to come up with a total solution, so we took the cable and the supply to our test area for evaluation." A power supply has noise in its input and output. A normal specification defines where the system interfaces with the outside world, where there is a common source of power that needs to be kept clean. Between the power supply and a dedicated load is not usually an issue. In this case the cable was common at both input and output. Creely said: "What was happening was the cable was radiating back into the common source. The cable they had was railway standard but it was the cable length they specified that caused us problems. What we had to do to fix that particular problem was to get inside the power supply so that our output cable was clean and didn't couple into the shared primary supply. "EMC itself is a difficult thing to specify and design for, a lot of it is experience, measurement, trial and error, and testing. Fortunately with the equipment we have, we can do that quite quickly and we have much of the equipment to evaluate power supplies like this. We can compare the plots the test house produces and so can feel confident to go back to the test house and pass the tests at the first attempt." The equipment Creely refers to is a 3m long anechoic chamber. Acal BFi customers can use this space at short notice enabling designers to test quickly and avoid having to reapply for compliant test house space - an expensive business in its own right.