Voltage/Current

[Aristotle]

I have a question about electrical systems. I'm in Japan at the moment and have been taking a lot of trains, both high-speed and "local" (which is what non-high-speed trains are often called here, whether inter- or intra- city), and I notice that while the Shinkansen high-speed runs at 25kv AC, almost all locals run at 600-1500 v DC, and some of these locals are hitting around 80 mph. Both are mostly overhead catenary, including subway trains. I know that AC is more efficient for long-distance use, but given the difference in voltages, is it the case that motive power is mostly determined by current rather than voltage ?

 

[BlackDiamond]

The power output of a locomotive (or train in the case of an EMU) is a combination of current and voltage. The lower the voltage, the more current is needed to achieve a given number of Watts.

The main reason that AC is more efficient for longer distances is that it can be transmitted at a high voltage then transformed down to the voltage needed to run the train. For example the power supply for Amtrak's NEC is transmitted at 138 KV then transformed down to 12 KV at substations. For DC you can distribute it to the substations as AC but then have to transform and rectify it from AC to DC which is a more complex process, then feed the catenary with fairly heavy feeders since the voltage is lower.

Most DC systems are legacy because in the early days of electrification the technology wasn't there yet for motor control of 50/60 HZ AC. For local and suburban trains it still works pretty well. A number of European countries still use DC for example Italy and Belgium use 3 KV DC except for certain high speed lines.,