Can someone explain to me what are the pros and cons of each of the traction systems, and for what kind of jobs would it be preferable to use one over the other?
In general terms:
DC drive
Good : Cheaper (currently), simpler, well understood, generates less interference with signalling etc
Bad : Not as good performance as AC for really heavy-haul freight or for high-speed passenger use. Motors have brushgear which needs maintenance and is a weak point if abused.
AC drive
Good : Better wheelslip control than DC so ideal for really heavy-haul freight (also better dynamic braking), and AC traction motors can withstand abuse that would wreck DC motors. AC motors are lighter and smaller than DC so it’s possible to get more power in the same space (which is probably why the GE/EMD 6000hp units are AC drive only) - this is also important for for high-speed passenger trains.
Bad : More expensive than DC (but getting cheaper) due to complex electronics, can create interference problems with signalling etc.
The railway world in general is going AC drive, so as soon as the cost of AC drives matches that of DC the DC drive will become obsolete for new rolling stock (my prediction [:)]). We’ve pretty much reached that stage in Europe, certainly for electric locomotives/multiple units.
Tony
Tony-
Good answer. I’ll just add:
AC: better wheelslip = higher adhesion = more tractive effort, which is the stuff you need to haul heavy trains such as unit trains
The reason you don’t see 6000 HP DC locomotives is that about the most the frt RRs ever need from a locomotive with 100,000 to 120,000# TE is 4000 - 4500 HP. More than this and you’re just wasting fuel. A six axle AC and do 150,000 to 175,000# TE which is a good match for 5000 - 6000 HP.
I read an account in TRAINS some time ago that said that an AC was in run 8 at about a half mile per hour, pulling a sizeable train up a grade. Can’t do that with DC. (Would have been interesting to see the result when they broke a drawbar, which I believe the story said they did).
Good explanations
Yea, thank you all, good readings.
So then, if you had a bunch of AC’s and bunch of DC’s in your fleet, how would you assign jobs to them? I mean it seems to me like AC’s would get all the job, cause they are good for both high speed trains and for slow heavy crawls.
Godd Answers! Question: Can AC & DC be MU’d?
Yes they can but when they are mue’d use lose they low speed advantage of the ac units.
I have seen the BN burn up alot of DC motors on pushers at Crawford Hill.
Allan.
The AC’s go to coal and grain where slow speed heavy haul is important. The DC’s go to manifest, intermodal and local service.
Dave H.
It was that story in Trains that promted my “AC traction motors can withstand abuse that would wreck DC motors” comment above - one of those ‘wish I could have been there to see the show’ stories [:)]
Incidentally, does anyone know what the AC/DC price differential is for the latest generation EMD/GE diesels ?
Tony
I’ve been told the DC’s are about half a million dollars cheaper but can’t verify.
Any takers?
A half a million sounds right.
I suppose the next question is how much does an SD70ACe etc cost anyway ?
(I’d guess 2-3 million USD but I could be wildly out [:)])
Tony
afair in large orders a ACe loco is about 1.2-1.5 million a piece.
The reason for the higher traction ratings for AC motors is that they do not seem to have an amperage limit rating. Amperage (current flow rate) heats up circuit wires. At a given horsepower setting in a diesel or electric locomotive, the slower the engine moves (when held back by a load) the higher the current flow (amperage) experienced by the traction motors. The tractive effort rating for a locomotive is based on the maximum current that the dc motors will tolerate on a continuous basis. Electric Locomotives can accelerate faster than diesels because they can temporarily overload their motors to twice their steady state rating for short periods of time. AC motors, having no amperage limit, are rated by the absolute maximum tractive effort they can generate, even with slightly slipping wheels. Comparing a 6000 hp AC engine to a 4400 hp AC engine, the 6000 hp engine will be able to pull a given train at a higher speed only if you are traveling above at 28 mph. Below that speed, assuming both engines weigh the same, adhesion limits prevent the stronger engine from generating more tractive effort that the 4400 hp locomotive. Consequently, both engines have the same start-up train weight limits (i.e. if the 4400 hp engine can start a 44 car train, the 6000 hp will not be able to handle a longer train).
I could see where DC locomotives are well worth having. Given the data here, a DC loco goes for between 60 percent and two-thirds the cost of the comparable AC model. But I do have some dumb questions (I’ll start a new thread if you like but these are still in topic):
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I know it may be impossible to compare since AC and DC’s have comparative advantages, but “all things being equal” which gets the better fuel economy?
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Who assigns locomotives, the number, electrical mode and tractive power? The yardmaster? Regardless of whom, are there computer formulas that list the most efficient use with variables set for speed, length of train and tare weight? If computer designs aren’t used or aren’t always the most helpful, can somone like a yardmaster chose AC or DC, etc., from his experience and seat-of-the-pants reckoning?
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Finally, I notice that CP’s 2004 Fact Book is proud of the fact that more than half of its locomotives are now AC and gave the distinct impression that DC was going to be attritioned out. Of course, a lot of CP’s traffic is in heavy commodities, but proportionately the road hauls more intermodal than CN.
SOOooo, would it be a good idea to keep DC’s since that power mode has been specifically mentioned here as good for fast freight and intermodals? -
A lot has been written about burning out DC’s. Do DC locomotives have a shorter useful life than their AC brothers?
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Does CP’s inclination toward AC echo that of the other large carriers?
My, he asks a lot of questions. [8D]
Thanks, Al Smalling
1> if all things are equal then so would the mileage??
2> We had a power desk, and the guy there knew which locos were in the redy track, which way they were facing, which were mued to what, etc. He also had a bit of paper which gave the loco ratings over each subdivision depending on the ruling grade. When a train was to be run he’d look at the total tons and and assign enough tractive effort to pull that train. This was before AC traction and I suppose it’s all computer based now though.
3>No, DC traction is good at this stuff, but I’d say AC is better.
4> Not necessarily, some parts have a shorter service lfe, but this stuff can (and is) replaced when it goes wrong beyond economic repair. Lots of 30 yar old DC locos about still, in 30 years ther’ll be lots of AC locos about still.
5> Yes, CP has always been a forward lookin railway, and this basically means that they are less risk averse than the competition. It could have gone all wrong for them (can you say Edsel). As the price of the locos come down and the reliability improves as the technology matures more railroads will move to AC traction. In a few years DC locos will go the way of the steam engine.
Nowt wrong with asking questions.
Generally speaking, AC drive locomotives are 5%-7% more efficient in converting the cankshift HP of the diesel to drawbar horsepower.
The current question is it worth spending $500,000 more on a locomotive for this efficiency increase. When diesel fuel was very cheap (in relative terms) a few years ago, some RRs, like the NS, didn’t feel it was.
With the manufacturing costs of AC going down, and fuel prices going up, we very well may see new DC traction go the way of steam locomotives.
If AC units are only for Coal Trains. Then why are AC units on “other” Trains as well?
Allan.