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Trolleys standardized on a 600 Volt Direct Current (VDC) power system. Their traction motors, their main electrical component, could be driven directly from this power source. Early LRV's switched to 750 VDC to have more power available, and their traction motors could also be driven directly from this voltage. Alternating Current (AC) power is an inherently more efficient scheme for conveying high power over long distances. This is witnessed by the exclusive use of AC in city power provided by the power companies. To supply the transit systems, AC city power needs to be converted to 600 or 750 VDC. This is often done at substations along the way. All of the LRV's discussed in this website use 750 VDC as their standard power input, with some of them supporting 600 VDC or other voltages as options.
Modern LRV's use both DC and AC components, and electronic technology can now efficiently and economically convert from one form of current to the other. It is conceivable that in the future power sources to LRV's would use AC. The Japanese High Speed train, for example, uses a 25000 VAC source.
Wireless Power Delivery
Overhead wiring is the most economical form of power delivery system to LRV's in most cases. However, the wiring is unsightly, particularly in historical areas. Wireless systems of power delivery are also used, sometimes in combination with wired sections.
In cases where the rail is protected and not accessible to pedestrians, a third rail between the vehicle rails has been used to deliver power to LRV's. Obviously there is a great safety risk if people can touch this rail. A great technical innovation has improved this situation considerably. It consists of switched third rail, where power is only applied to the third rail when the vehicle is on top of it, being sensed through electronic means. The vehicle itself then protects the powered rail from pedestrian access.
The newest power delivery technique is an electromagnetic inductive power transfer system:
An induction coil under the road or track carries a high-frequency alternating current (AC), thereby creating a magnetic field. This field induces a voltage in the vehicle-side inductive power receiver (pick-up), which is used to charge and power the vehicle. Thanks to the vehicle detection system, wayside components activate a given inductive segment only when the vehicle is positioned directly above it.
One could think of the coil under the road and the vehicle receiver as the two windings of a transformer. The lack of physical contact should have benefits in terms of safety, wear and immunity to weather.
As indicated in the Vehicle Technology section, energy storage components onboard the vehicle can significantly change the power delivery system design by limiting power transfers to only sections of the line and relying on storage to "bridge" the power gaps.
Rights Of Way
As indicated in the Introduction, one of the important characteristics of modern Light Rail systems is their flexibility in using a variety of rights of way, allowing a wide range of cost/performance trade-offs. One of the most natural arrangements is the use of existing railroad rights of way. This includes several variations, from the actual sharing of freight tracks, as in the case of the San Diego Trolley, to the use of tracks no longer in use by a railroad such as in the Portland MAX or the use of space parallel to railroad tracks such as in Calgary and some sections of San Diego. Another common design is to use existing streets, either shared with other traffic or dedicated for exclusive transit use, often called transit malls in downtown areas. Transit malls generally include use by buses and emergency vehicles, but not private vehicles, encouraging the use of public transit in selected areas. The most costly and generally highest performance implementations are dedicated, grade-separated new designs, which may include underground and elevated sections, or the use of the median or the side of a highway. The ability to combine all or some of these arrangements for use by the same vehicles is perhaps the greatest advantage of Light Rail.
This website is not a professional guide, but an editing of existing referenced material for educational purposes. The website author assumes no responsibility for any problems resulting from using the material presented in this website.
 Margarita Novales, Light Rail Systems Free of Overhead Wires, Transportation Research Record, Vol. 2219, pp 30-37.