The induction deduction

Experimental electric trams and buses that pick up power by induction, either from cables beneath the track or from coils beneath where they stop, are being trialled in Germany, New Zealand, Italy and China. The concept potentially does away with many of the traditional causes of travel delay such as ice on conductor rails or overhead line problems. It would also make tramways less unsightly. The idea itself has been around for years, and is used to power automatic guided vehicles in factories. These can be made in such a way that the pickup coils are very near the floor, or fit around conducting cables. But, public service vehicles running over rail tracks or highways need to have a larger air gap, so it has taken a time to solve the problems of how to transfer power efficiently. At the present time, there are three separate solutions, but no clear indication of which is best just yet. The company most established at building inductively powered electric vehicles for factory use is Wampfler, part of the Delachaux Group, with headquarters in Weil am Rhein in Germany. It offers two systems. Both of these operate at around 20kHz and rely on the use of a pair of cables with current travelling through them in opposite directions. The first system has two cables that are held on protrusions extending from a wall. These are suitable for rail mounted vehicles with 'E' shaped 2.5 kW pickups on the vehicles. The other, uses a pair of cables buried in the floor. But, trams being developed by Austria based Bombardier Transportation use a very different system. The company is reluctant to give details, but it has published diagrams that show cables laid in a series of long loops, with one side buried deep on one side the track, and the other running beneath the tram. We imagine that the purpose of this is to extend the distance over which a significant magnetic field from the power cable can be picked up by coils in the tram. A 30m long "Flexity" tram equipped with this system is being trialled on a 1km test track in Bautzen, Germany at speeds up to 40 km/h and on gradients of 6%. "It operates at a frequency higher than 50Hz," says Emma Gallaway, marketing manager at Bombardier. "However, for Intellectual Property reasons we are unable to give the exact frequency at this stage. Efficiency is the same as a transformer, between 95% and 98%. We have already tested the system against our own signalling products with no adverse effects. However, further testing in a real environment is necessary." Single cable supply has been used in some guided vehicles but is not presently the norm. One of the striking aspects of the Bombardier system, designated 'Primove', is that the system is rated at 250kW, 500kW peak. Despite energy being stored in ultracapacitors, the transfer of more than 100kW inductively from a cable is no easy task. Because the frequency has to be a lot higher than 50kHz, there is a strong skin effect so that supply cables have to be made up of bundles of fine wires, known as 'Litz' cables. The ultracapacitor units are made up of a large number of individual 'Boostcap' cells each rated at 3,000 Farads, 2.7V. Ian Corfield, director and project management at Bombardier plant in Derby, says: "A commercial version of the tram will be available in the second half of the year. And the use of the ultracapacitor units resulted in 30% energy savings." An institution that has done much to advance the science of using inductive power transfer for buses is the University of Auckland's Power Electronic Research Group in New Zealand, led by Professor John Boys and Professor Grant Covic. This has resulted in the development of five inductively charged bus systems. These use a 20kW charging station to charge the 30, 12V, batteries in each bus in under 20 minutes. But, Wampfler has also developed a bus charging system that is being used in the Porto Antico area of Genoa, Italy. Three buses with 56, 6V batteries are inductively charged at 60kW for 10 minutes in each hour. And, these buses have to kneel down to no more than 30mm above the primary coils to receive power. We have also heard of an experimental Chinese bus, that kneels down to inductively receive charge at each bus stop, storing its charge in ultracapacitors. This would allow charging to be undertaken very quickly, but evidently requires the bus stops be fairly close together. Similarly, Bombardier estimate the power stored in its tram and train ultracapacitors is only sufficient to drive around 500m. The Auckland team are currently developing the technology further and are looking at other possibilities that include the possibility of embedding cables in roads, so that cars could draw power on the move. There was also a European Union project, EVIAC – Electric Vehicle Inductive Automatic Charging – that established that public inductive charger units for electric cars were both viable and safe. Pointers * Non contact inductive powering of trams has been demonstrated as practical and is to be offered commercially later this year * There are inductive charging systems for battery powered buses in use in several locations round the world * Wide use of inductively charged and/or powered vehicles is also a practicable possibility