TECHNOLOgy REvIEW


Understanding the choices of power solutions for transportation applications


With so many manufacturers offering such extensive ranges of power supply products it can be quite a challenge knowing just where to look first. Inevitably, as with most selection processes, it is a case of narrowing down the options based on requirements that are already defined or can readily be determined. For the types of power supply required by transportation applications the starting point is usually the style of housing and mounting method, like rugged Euro cassette or DIN rail. Only then can we begin to consider the more usual specifications such as output load requirements and whether operation will be from an AC or DC source.


Answers to these questions though are only the first step and there are many secondary factors to consider including the harsh environmental conditions these power supplies will need to endure. So not just operating temperature range but the ability to withstand vibration and shock as well as specific requirements for moisture or dust sealing. Safety or other regulatory issues also have to be accounted for, e.g. does the unit need to provide input/output isolation, meet a UL fire-retardant specification or comply with EMC rules? Unfortunately not all applications will have known requirements for these different selection criteria so the goal of this article is to provide an appreciation of these factors to enable a better informed decision in choosing a suitable power supply.


selecting by enclosure type Transportation applications, particularly those associated with commercial passenger or freight vehicles for road or rail systems demand rugged solutions. Alongside power supply requirements for trains, trams, metros/subways, buses and trucks are other industrial and agricultural vehicles with similar needs, such as forklifts, cherry pickers, cranes, excavators, dump trucks, tractors, belt-conveyors and fire/rescue service vehicles.


While all these applications can be


classified as ‘heavy industrial’, some are more demanding than others and supplies will


philip lechner of Avnet AbAcus considers power conversion solutions for the hArsh, dusty And difficult conditions found in trAins, trucKs And other trAnsport systems.


need to be designed and tested to meet the relevant specifications. A bus or light-rail tram may just need a suitable temperature range and a sufficiently rugged construction to resist physical shock and vibration but a tractor or excavator may need much more protection against the ingress of dust or moisture. Power supplies for subway trains may need to operate at higher temperature and comply with more stringent safety regulations.


These and other factors are important to consider but, in this industry sector, power supplies are typically classified by the style of housing and their mounting method, as reflected in manufacturers’ catalogues and websites. Consequently, in attempting to understand the selection process for choosing a power supply for transport applications, it makes sense to review examples of units within each of these physical formats, as detailed in the following sections.


din rAil power supplies The classification of ‘DIN rail’ refers to a method for mounting power supply units (PSUs) inside an equipment rack. DIN rails are metal tracks fixed horizontally to a vertical flat surface and used for the convenient and flexible clip-fit mounting of electrical equipment such as circuit breakers and control switches. There are different sizes and profiles of DIN rail but the most common type, as used by both the supplies reviewed below, has a ‘top hat’ cross- section profile, measuring 35mm by 7.5mm, in accordance with EN60715.


As DIN rails are normally associated with AC electrical installations, it is not surprising to find that most PSUs that use this mounting method are AC-DC converter types. The CliQ and CliQ II ranges from Delta PSU include both single- and


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three-phase units and cater for output voltages of 12V, 24V and 48V with power ratings from 15W through to 480W. The DRP024V480W1BA model, shown in figure 1, is a 24V 480W output AC-DC converter that operates from a single- phase input of 85-264VAC (nominally 100- 240VAC, 50/60Hz) but can also operate from a DC input between 120-375V. CliQ II provides detachable IP20 connectors with better performance coupled with higher efficiencies.


Figure 1: Delta’s CliQ II series 24V 480W single- phase AC-DC converter


Delta’s CliQ PSUs have been designed to meet worldwide safety standards and provide reliable operation in hazardous environments using a compact, rugged case that allows easy handling and installation. The units are specified for an ambient temperature range of -20oC to +75o, at up to 95% relative humidity (RH), and are convection cooled. They comply with the Class 1 Div. 2 hazardous location requirement for safe operation (where flammable gases/liquids may be present under abnormal conditions) and meet shock and vibration standards IEC60068- 2-27 and IEC60068-2-6 respectively. These PSUs have a10 year life expectancy (>300,000 hours), provide protection for overvoltage, overcurrent, over-temperature and short-circuit conditions and are compliant with a range of other safety directives and EMC/emissions standards. Although the cases of CliQ PSUs are not sealed, conformal coating of the circuit boards protects components against moisture, dust and chemicals to avoid problems with degraded electrical performance and early life failure.


continues p 8 > focus magazine - issue 15


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