AC vs. DC traction

[Anonymous]

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?

[owlsroost]

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

[oltmannd]

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.

[tree68]

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).

[daveklepper]

Good explanations

[Anonymous]

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.

[daveallisonpe]

Godd Answers! Question: Can AC & DC be MU'd?

[edbenton]

Yes they can but when they are mue'd use lose they low speed advantage of the ac units.

[Anonymous]

I have seen the BN burn up alot of DC motors on pushers at Crawford Hill.
Allan.

[dehusman]

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.

[owlsroost]

QUOTE: 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.

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

[Anonymous]

I've been told the DC's are about half a million dollars cheaper but can't verify.

Any takers?

[chad thomas]

A half a million sounds right.

[owlsroost]

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

[Anonymous]

afair in large orders a ACe loco is about 1.2-1.5 million a piece.

[Leon Silverman]

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).

[Anonymous]

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):

1. 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?

2. 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?

3. 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?

4. A lot has been written about burning out DC's. Do DC locomotives have a shorter useful life than their AC brothers?

5. Does CP's inclination toward AC echo that of the other large carriers?

My, he asks a lot of questions. [8D]


Thanks, Al Smalling

[Hugh Jampton]

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.

[GP40-2]

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.

[Anonymous]

If AC units are only for Coal Trains. Then why are AC units on "other" Trains as well?
Allan.

[GP40-2]

QUOTE: Originally posted by BNSF railfan.

If AC units are only for Coal Trains. Then why are AC units on "other" Trains as well?
Allan.

That's not true. AC locomotives are used for everything from coal to high speed intermodal.

[Anonymous]

Thanks, guys, I'm enjoying the answers and learning.

allen smalling

[daveklepper]

AC locomotive's greatest advantage is in low speed drag service, pusher service, mountain railroading, but they have an advantage in all kinds of service as well because of generally lower maintenance costs and greater fuel economy. In fact, today, probably if you figure total costs over the life of the engine, the AC would come out ahead despite greater first costs.

[Anonymous]

For all you engineers, is AC easier to operate in a locomotive, or DC, or no diff??
Say, would an engineer who is used to DC have to learn a new style to operate AC?

Is it true DC has the drawback that it can be burned out?

Is the ES44DC well thought of? My best friend has as his wallpaper a photo of a BNSF locomotive, and the shot is so close-up the stenciling with ES44DC can be seen.

Inquiring minds, etc.
Thanks

allen

[Anonymous]

It seems to me that the inverter on an AC unit would put out a modified sine wave, which would have a short interval of no voltage between half cycles. If the power factor is reasonably close to unity, the current would die out quickly enough that a resistance measurement could be made on the motor windings so as to indicate the instantaneous temperature. This would allow an "intelligent" inverter to apply the maximum power that could be used at the moment without damaging the motor.

It never fails to surprise me that the engine-generator set puts out AC, which is then rectified to DC, then is inverted back to AC for the traction motors, and finally winds up more efficient than the old-time straight DC rigs! Ah, the wonders of modern solid-state electronics! If the economics were such that the price of AC locomotives would plummet like computers, we would all wonder why DC was ever considered.

[Anonymous]

Another thought should be taken into consideration: what AC means. Alternating Current switches polarity several times per second. Most table saws run the same speed because they all face 60 cycle per second household current. On an AC locomotive, the power plant generates power with an AC cycle based on the RPM of the alternator. This is then converted to DC which is then converted back to AC, only at a different frequency. Control of an AC motor requires setting both the freqency AND the voltage. The rule is: 105% acceleration. That means the inverters respond to an acceleration command from the throttle by supplying 5% more than what is being used at the moment.

The result is that both the power and the frequency are set to that optimum setting. On a DC locomotive, a Run 8 throttle setting could be supply 200% or more of the power that's needed to just go faster. This extra power is wasted off as heat and the destruction of the traction motors. AC traction motors only ever get about 5% more than what they are already handling, never mind what the engineer is asking for. The speed increases and so does the 5% figure until either the locomotive goes the speed the engineer wants or something breaks.

The computer that tends to all of this also looks at how fast each axle is going. On a DC loco, an axle that goes overspeed (wheel slip) is either cut back or cut out, depending on the wheel slip management. On an AC locomotive, that axle won't ever speed up beyond 5% more than the others because of the power applied. Then the computer looks at this overspeed and pulls it back to exactly the same as the other five axles. If all six can't speed up, the unit lays down, just as a DC unit would do, only without spinning wheels or overheated motors.

Thanks to the computerize inverters, money saving from AC unit not only comes from nominal fuel savings but also lesser maintenance expenses, greater locomotive availability (in theory), and more flexible assignment.

John

[CORNELIUSSEON]

All of the above is true, but the Transit industry has come up with an alternative. New York's MTA has placed orders for large fleets of Commuter and Rapid Transit cars that are powered by DC, but utilize the power as AC. That means that the investment in DC transmission is kept in place, while the trains get the benefits of AC power. One of the additional benefits of this AC/DC arrangement has been the restoration of Dual Power locomotives to the American rail industry. Historians here will remember the EMD FL-9 locomotive on the New Haven, and earlier Dual Power locomotives from GE in the formative days of the Diesel. Thanks to the research done by New York's MTA and the manufacturers who had large potential orders as incentive, AMTRAK now has Dual Power locomotives they use on the New York - Albany route as well. As before, the Dual Power status is Diesel prime movers, this time producing AC Power, and DC Third Rail Shoes, and the switching equipment necessary to select AC Power from the two sources. For the first time in a long time, AMTRAK and the two commuter roads can run trains from deep in Diesel territory direct into Penn Station and Grand Central Terminal without stopping to change locomotives.

[CORNELIUSSEON]

All of the above is true, but the Transit industry has come up with an alternative. New York's MTA has placed orders for large fleets of Commuter and Rapid Transit cars that are powered by DC, but utilize the power as AC. That means that the investment in DC transmission is kept in place, while the trains get the benefits of AC power. One of the additional benefits of this AC/DC arrangement has been the restoration of Dual Power locomotives to the American rail industry. Historians here will remember the EMD FL-9 locomotive on the New Haven, and earlier Dual Power locomotives from GE in the formative days of the Diesel. Thanks to the research done by New York's MTA and the manufacturers who had large potential orders as incentive, AMTRAK now has Dual Power locomotives they use on the New York - Albany route as well. As before, the Dual Power status is Diesel prime movers, this time producing AC Power, and DC Third Rail Shoes, and the switching equipment necessary to select AC Power from the two sources. For the first time in a long time, AMTRAK and the two commuter roads can run trains from deep in Diesel territory direct into Penn Station and Grand Central Terminal without stopping to change locomotives.

[Anonymous]

Regarding MU'g AC and DC locos, I believe the earlier mentioned article in Trains discussed that they had to be careful MU'g the two types, trying to keep the DC loco as the lead loco. The reason for the concern was that the AC could be redlining all day long without burning out the traction motors while the trailing DC unit's TM's would be fried after like 30-60 minutes. That was back when AC was new on the freight scene, before the train crews understood AC very well or were trained to understand that what was OK/going on in the front AC unit may not be so OK for any trailing DC unit(s). Early on several DC locos got fried in such mixed consists.

I don't know if it's still an issue or not. The crews now have a lot more experience with and a far better understanding of AC locos now that there are so many running in the fleets, so they may be trained to factor for that. Perhaps even the newer DC locos have software programming or hardware to prevent redlining them beyond certain limits, causing them to automatically cut out? Does anybody know?

Thanks,

Greg

[Anonymous]

Well, I have heard that all of the above is true and that the engineers can put the loco's in notch one or two to help hold a train on a hill and not stall burn the motors like DC. All I know is that I changed out a lot more DC motors than AC.