Conventional Trams with
Intermittent Overhead Power Supply
By J.H. Crawford
The parent document, Fuel-Cell Trams, should be read first.
As explained in the parent document, the great problem with conventional trams is the high cost (and ugliness!) of the necessary overhead power delivery system. What is needed is a way to eliminate overhead power, or at least to great reduce its costs.
It is not yet clear that fuel cells for transport applications can be developed for an economical price. (It is already clear that they work.) There are also concerns about the availability of sufficient quantities of the precious-metal catalysts that are required by the most suitable fuel cell designs. If it turns out that the fuel cell does not become available for widespread transport application, there is another approach.
There is no inherent reason why the overhead power system must be installed the full length of the tracks of an urban light-rail system. When halts are closely spaced, as is universal in street-running urban systems, then there is no real need for overhead power supply between the stations. If we assume that the top speed in urban running is limited to 50 km/hr (about 30 MPH), that speed can be attained in a distance of just under 50 meters (about 165 feet).If the tram is 50 meters long and has pantographs at each end, then an overhead wire no longer than the tram itself is sufficient to accelerate and brake the tram, assuming a relatively high rate of acceleration, 0.2 G, a rate that was actually slightly exceeded by the PCC trams of the 1930s. The tram can simply coast to the next stop. As the lead pantograph makes contact with the overhead wires, braking begins, slowing the tram to a stop just as the trailing pantograph comes into contact with the overhead wire. Trams are so efficient that very little speed would be lost between the stations. However, hills would still have to be provided with overhead wires - this arrangement would work best in cities without hills. Similarly, if the tram is required to take a curve at reduced speed, wires will be required to permit the tram to accelerate again, once clear of the curve.
This does, of course, assume that trams are never forced to stop between stations. While a close approach to this condition can be made by simply removing competing traffic from the street, emergencies may still force a tram to stop between stations. If a dozen conventional automotive batteries were installed to supply power for lights and control equipment while the tram was between the stations, these batteries could also supply enough power to allow the tram to limp into the next station. The batteries would be recharged while the tram was under wires (8 seconds for braking, 15 seconds for the stop, and a further 8 seconds during acceleration, for a total of 31 seconds; the coasting phase lasts no more than 30 seconds).
The only diversions from conventional practice are the installation of guides at the beginning and end of the overhead wire, to bring the pantograph into gentle contact with the overhead wire, and the provision of a modest battery bank. A pilot test of this system could be made with an old double-ended PCC streetcar and a 400 meter section of track fitted with overhead wires for 50 meters at each end.
Finally, because the section with wires is so short (and could be integrated into a covered station), it would be possible to provide 3-phase power (requiring three conductors), which permits the use of small, light, efficient, and easily-controlled 3-phase motors.
Please consult the parent document, Fuel-Cell Trams.
Copyright © 2001 J. Crawford. This page may be freely
reproduced provided that J.H. Crawford is acknowledged as the author, his
copyright is maintained, and the full text is reproduced.
By publication of this work, J. Crawford places in the
public domain any original ideas contained in the work.