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.,

 

[Aristotle]

Thanks. That clarifies some things for me. I think I didn't take into account, also, the difference between what is sent through the catenary system and what voltage/current actually hits the electric motors. After that 138KV gets stepped down at a substation on the NEC, does it hit the motors at 12kv or is it stepped down again ? Also, since DC motors are typically using lower voltages are they therefore larger and heavier than AC motors ?

 

[cirdan]

Traction motors typically work at voltage between 500V and 800V, exceptionally up to 1500 or 2000V. Higher voltages would require more insulation, which engenders geometrical problems (railroad traction motors are typically more compact than say industrial motors of similar ratings). If you have 25kV on the catenary, this needs to be stepped down and converted.

 

[BlackDiamond]

Way back when, in the days of the GG-1 and older motors, they transformed the AC down to 600 V or so but still ran the traction motors off of AC. This is one reason why they used 25 HZ as it worked better with the motors of the day.

Post WW2 with the rise of Diesel Electric and use of 600 VDC traction motors, the trend was to rectify the AC to DC and apply that to DC motors. This is how the PRR E44 freight locomotives and Silverliner fleets were powered. Originally using Ignitron rectifiers later converted to solid state.

Nowadays we have moved back to AC motors because modern AC motors can be built without commutators and brushes and such high maintenance items. To do this the DC is then inverted back to AC. This way the motor can be controlled by varying the voltage and frequency. If they ran them directly off the AC they would be stuck at one speed like a synchronous motor. As a matter of fact the Pennsy in 1917 built a locomotive the FF1 with AC synchronous motors which could only run at 2 speeds 10.3 and 20.6 mph and therefore was limited to use as a pusher on grades.