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cjmaclean (279 posts)
Sep-02-01, 11:31 AM (EST) "AC or DC - Which is better?" Hi all,
I was wondering what the advantages and disadvantages of both AC and DC railway power supply (catenary) are.
Here in Germany we have 15 kV, 16 2/3 Hz AC catenary current, while in France they have both an AC (25 kV, 50 Hz) AND a DC (1.5 kV) network.Can anybody enlighten me?
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Subject Author Message Date ID RE: AC or DC - Which is better? fhornuk Sep-02-01 1 RE: AC or DC - Which is better? Walt Sep-02-01 2 RE: AC or DC - Which is better? derekmorton Sep-02-01 3 RE: AC or DC - Which is better? Matt Vince Sep-03-01 4 RE: AC or DC - Which is better? cjmaclean Sep-03-01 5 RE: AC or DC - Which is better? rcp27 Sep-03-01 6 RE: AC or DC - Which is better? cjmaclean Sep-03-01 7 RE: AC or DC - Which is better? rcp27 Sep-03-01 8 RE: AC or DC - Which is better? cjmaclean Sep-03-01 9 RE: AC or DC - Which is better? zeeland174 Sep-03-01 10 RE: AC or DC - Which is better? Walt Sep-03-01 11 RE: AC or DC - Which is better? cjmaclean Sep-03-01 12 RE: AC or DC - Which is better? angus987 Sep-07-01 13

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fhornuk (40 posts)
Sep-02-01, 12:16 PM (EST) 1. "RE: AC or DC - Which is better?" I'm not an electrical engineer, so I'm not sure of the science! But I think it has something to do with the number of sub-stations required to provide adequate power supply. Also (in the UK at least) ALL street running systems (trams and light rail mainly) MUST have DC power supply (normally about 750v) because DC current is far less likely to arc than AC current. Remove | Alert Edit | Reply | Reply With Quote | Top
Walt (121 posts)
Sep-02-01, 04:27 PM (EST) 2. "RE: AC or DC - Which is better?" My answer applies to the days before electronic controls, which is when most systems were designed. It might not be correct for the past 20 years or so.Basically, AC was preferred for railroads because of the efficiency of AC transmission of high current over relatively long distances, while DC was preferred for trolley cars and subways because the control devices were much smaller and simpler and the transmission distances were relatively short.AC can be stepped up to relatively high voltages for transmission, and if you are familiar with Ohm's law you will understand the relationship between the current loss in the transmission wires and high or low voltage. The down side to AC propulsion was that it required large, bulky equipment in the locomotive to step it down and provide control for the motors. The insides of those big electric locomotives are mostly filled with transformers and/or rotary converters for control purposes.DC has to be converted from AC at the substation, usually with a rotary converter (motor-generator set), or occasionally with rectifier tubes. Then it is transmitted to the trolleys at relatively low (typically 600V) voltage, which required very heavy feeder wires and a voltage loss as the cars move away from the substation location. On the plus side, DC to the motors is controlled by resistors and switch gear that is relatively compact and can be mounted under the floor of the car.There were some hybrid systems. Virginian Railway, New Haven, and PRR had some Electric locos with DC motors running on an AC distribution system. The conversion was handled in the locomotive by a rectifier tube system. Milwaukee Road used DC for the final distribution, with substations only 15 miles apart to reduce the transmission loss.Modern solid state chopper controls have completely changed the situation, and the factors affecting the choice between AC and DC is much more complex today. Remove | Alert Edit | Reply | Reply With Quote | Top
derekmorton (101 posts)
Sep-02-01, 07:23 PM (EST) 3. "RE: AC or DC - Which is better?" As you have noted the AC voltages tend to be higher, this means less current and therefore allows smaller wires and longer distances between substations.
AC transmission can supply more current on the same size wire as DC transmission, and it is also simpler to implement as rectifiers are not required.Derek Morton
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Matt Vince (29 posts)
Sep-03-01, 06:55 AM (EST) 4. "RE: AC or DC - Which is better?" Walt - effectivly true, although the use of 'hybrid' locomotives with AC supply and DC motors is more widespread. Certainly since the 60s, the British rail network has used 25kV AC for overhead, but with DC traction motors. This required intitially Mercury Arc rectifiers (don't ask me how they work) then later solid-state rectifiers. This may be because we were using proven technology, or BR just didn't have the money. However, DC motors can be powerful, the BR Class 90 has phenomenal acceleration. DC is, however, the ONLY option in 3rd rail electrification. To answer another part of the initial question, the 1.5kV DC was used in France because it was cheaper initially in the late 1940s/early 1950s when it all really started, and because technology had not yet been devised for 25kV AC. By the late 1950s that had changed, and 25kV AC found favour, and has remained so ever since. Remove | Alert Edit | Reply | Reply With Quote | Top
cjmaclean (279 posts)
Sep-03-01, 07:18 AM (EST) 5. "RE: AC or DC - Which is better?" LAST EDITED ON Sep-03-01 AT 07:18 AM (EST)Here in Germany our newer electric locos (e.g. class 120) are equipped with three-current ("Drehstrom" in German) traction motors even.
Those articles that I have read seem to suggest these motors can be designed much simpler than AC or DC motors and are the most efficient of all.
However, I haven't understood yet how three-current motors are built up or in what ways exactly they are different from conventional motors.
Also, what equipment is required to convert the monophase 15 kV, 16 2/3 Hz overhead current into three-current?
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rcp27 (5 posts)
Sep-03-01, 09:38 AM (EST) 6. "RE: AC or DC - Which is better?" I believe what you are refering to is terms 3 phase AC in english. It has been known for a long time that this is more efficient than either single phase (ordinary) AC or DC, indeed 3 phase AC is used in most of the world's electricity generation and transmission systems for this reason. In order transmit 3 phase, 3 conductors are required, which is immensly difficult for railway trains (some have done it, using 2 overhead lines and the track as the third). Modern solid state electronics have alowed either single phase AC, as is the case in Germany, or DC (on the modern 3rd rail trains in Britain) to be converted to 3 phase AC in a small enough device that it can be carried on board a train. It has several great benefits, such as much more effective regenerative breaking, and better accelleration characteristics.Robin Remove | Alert Edit | Reply | Reply With Quote | Top
cjmaclean (279 posts)
Sep-03-01, 12:46 PM (EST) 7. "RE: AC or DC - Which is better?" Right, it is "three-phase AC" ...
Thanks for the information. In fact, the first serious line trials of electric traction here in Germany were conducted with 3-phase AC, back in 1901. They used 10 kV, 50 Hz three-phase current on a 33 km leg between Marienfelde and Zossen. However, the 3-phase AC required a three-wire catenary, which was erected by the side of the track (not above, as it is today). This arrangement proved unsatisfactory in the end, and finally led to the overhead wire/roof pantograph arrangement as we know it today. Somewhere I saw a photo of this early electric locomotive, and if I'm not completely wrong the "pantograph" was in fact a fairly high mast on top of the machine, from which three pickup shoes extended to the wires by the side of the line.
I also remember that somewhere in Switzerland there is a rack railway going up into the mountains which uses three-phase AC until today. They have two parallel overhead wires and two pantographs side by side on top of their engines, and the rails are the third pole. Was that what you meant with your statement?
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rcp27 (5 posts)
Sep-03-01, 01:31 PM (EST) 8. "RE: AC or DC - Which is better?" That was one of them that I know of. Another example from history was the Metropolitain Railway. When it was elecrifying it's lines under London, they proposed (and I think electrifyed a stretch for test purposes) using 2 wire overhead, but in order to run through with the District round the circle, they both had to use the same system, and the District won out in the end, with the present London Underground 3rd/4th rail DC system (outer power rail at +420V, inner at -210V) Remove | Alert Edit | Reply | Reply With Quote | Top
cjmaclean (279 posts)
Sep-03-01, 01:48 PM (EST) 9. "RE: AC or DC - Which is better?" Does anybody know if there were ever railroads which used power transmission through the main rails themselves (as on two-conductor DC model railroads)? It's probably a stupid question, because I definitely wouldn't do that on a real railroad, but I wanted to ask anyway ...
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zeeland174 (18 posts)
Sep-03-01, 02:13 PM (EST) 10. "RE: AC or DC - Which is better?" i dont think so. that would be difficult to realise because the wheels of the cars would have to be insulated. otherwise the axle would short-circuit the rails.
(man that would give some sparks. and it wil cost a fortune in fuses ) Remove | Alert Edit | Reply | Reply With Quote | Top
Walt (121 posts)
Sep-03-01, 03:55 PM (EST) 11. "RE: AC or DC - Which is better?" >Does anybody know if there were
>ever railroads which used power
>transmission through the main rails
>themselves (as on two-conductor DC
>model railroads)? I have an interesting story to go with this question. The original electrified street railway in the U.S. was in Richmond, Va., and initially used 600 VDC via the rails. It only lasted a few days because of the hazards, and was converted to overhead wires, but in that short time the horses in the city quickly learned to step over the rails or they would get a nasty shock.After about 20 years, someone noticed that the horses still avoided stepping on the rails, even though any that had been shocked had been put out to pasture long ago. A group of horses was brought in from outlying farms, and by the next day they too refused to step on the rails. The first theory was that the horses were simply imitating what they saw the others doing, so another experiment was set up.Another group was shipped in from New York City, which by that time had an extensive trolley system, and the horses there had no fear of the rails. They were kept in isolation and only brought out late at night. The first night they had no hesitation about stepping on the rails. Then they were placed in a barn with Richmond horses, and the following night they refused to step on the rails!! The conclusion was that horses had a way of communicating with each other, and is still the subject of academic discussions and experiments.
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cjmaclean (279 posts)
Sep-03-01, 04:39 PM (EST) 12. "RE: AC or DC - Which is better?" Interesting story about these horses. Anyway, here I have a drawing of the loco used on the Marienfelde-Zossen trial line:The three-wire catenary on the right side of the track can be clearly seen, as well as the "mast" pantograph.Another drawing of one of the electric railcars used for these trials as well shows an arrangement of THREE separate pantographs of different heights that could be rotated to both sides. This was necessary because the catenary would be found only on the outer side of the track. As far as my information tells me, there was another set of three pantographs on the other end of the railcar as well.
It's definitely fascinating to see how technology has evolved from roots as simple as those!
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angus987 (26 posts)
Sep-07-01, 05:48 AM (EST) 13. "RE: AC or DC - Which is better?"
Just got to say that AC/DC are the geatest rock band that ever walked the Earth. Hence the nick. And they're Australian!Adam Remove | Alert Edit | Reply | Reply With Quote | Top

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