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 ?
Voltage/Current
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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.,
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.,
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
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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.
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.
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.
cirdan
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Early electrification in Italy and also on some mountain lines in Switzerland used three phase AC, so typically you had two overhead lines, each with one phase, and the third phase using the ground return. The AC was typically at low frequency and was applied directly to the motors. In some cases motor speed could even be doubled/halved by switching the poles (so if you had say a 6 pole motor, you would apply the three phases P1,P2,P3,P1,P2,P3 at low speed and switch to P1,P1,P2,P2,P3,P3 for high speed). You could do star-delta switches to control the power, and had starting resistors. The fixed speed feature was quite useful for example on long uphill climbs on rack railways where you typically hold a constant speed regardless of everything else. These early examples used large diameter squirrel cage motors running at low rpm (sometimes the entire locomotive body was just the motor). Squirrel cage motors allow some phase slippage (and actually torque increases with phase slippage) so you could use that to accelerate.
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Interesting. The PRR FF1 I mentioned above also used 3 phase motors, but was just a single phase feed from the 11KV 25 HZ catenary. They converted to 3 phase using motor generator sets onboard the locomotive.
Forgive my ignorance here. So, in some cases the AC from the catenary is rectified to DC, then inverted back to AC for the motors ? What exactly is the purpose of that intermediate step ? why go through DC before going back to AC ?
Control. Keep in mind that your inverters are providing power to the traction system typically through some type of VFD/IGBT arrangement but also (in passenger rail) typically sending 3 phase 480 down the train for HEP which in turn will be dropped where necessary for utilization.
Besides what PVD said above, it is also a different type of AC in terms of voltage and frequency etc. and I suspect it is harder to convert one type of AC directly to another, easier to go through the step of converting to DC then inverting back to AC.
The ability to invert, rectify, and control have been advanced considerably by the march forward of technology. Modern solid state equipment has made things that were theoretically doable into the practical and commonplace. In very long distance transmission HVDC is now common, (transform the generated AC to a very high voltage, rectify to DC for transmission on 2 instead of 3 conductors, invert back to AC at the destination grid connection) Much more doable with today's equipment.
Even in traction, a VFD style control provides better control and can save large amounts of energy compared to controlling dc motors with resistance.
Even in traction, a VFD style control provides better control and can save large amounts of energy compared to controlling dc motors with resistance.
You especially see this in Europe where over the years each country evolved their own electrical standard so a longer distance train might encounter 3 or 4 voltage / frequency /AC-DC changes over its trip. In the old days this would require changing locomotives at each border. Today's locomotives can handle these changes and can be run through. Although for high speed lines, 25 KV 50 HZ AC has more or less become the standard that everyone uses.
Amtrak ACS-64 from Boston to Washington, right here....
1500V DC motors were common even in the early years. That is how 1200-1500V DC became a standard.
This happens to be for GEs bid to replace the Milwaukee Road Electrics in 1969.
