I agree with this concept in regard to GE units, which have one inverter per axle; but I wouldn’t think that it would apply to EMD units, which have one inverter per truck.
For what it’s worth, just a few more numbers comparing AC and DC performance.
Data published in Trains’ Locomotive special issue for the GE Evolution and EMD 70 series locomotives is as follows:
GE: 4,400 hp,
AC max continuous tractive effort 166,000 lb
DC max continuous tractive effort 109,000 lb
EMD: 4,300 hp
AC max continuous tractive effort: 157,000 lb
DC maximum continuois tractive effort: 113,100 lb
Converting to speed at which maximum traction horsepower is produced yields the following:
GE: AC produces 4,400 traction hp at 9.94 mph vs. 15.2 mph for DC
EMD: AC produces 4,300 traction hp at 10.3 mph vs. 14.3 mph for DC
AC power has a distinct advantage over DC power beyond just slowing down to a crawl to take advantage of its superior resistance to overheating and riding up the tractive effort curve. AC units have full power available for traction above about 10 mph. DC units cannot produce full power for traction until about 15 mph. This consistent with the notion that AC power seems to be assigned to heavy haul coal service.
The performance characteristics of the two types converge at about 15 mph, above which the performance characteristics are problably very similar. This is consistent with the notion of assigning DC power to high-speed intermodal and other freight.
Anthony V.
No US climb is more “challenging” than the N&W 1.4% grades?
Have I quoted you correctly? You need eight SD40-2s to maintain 11 mph, but one AC can maintain 2.5 mph with the same train on the same railroad-- long as it doesn’t slip?
A very misleading statement. The kind of power bought by the railroads have little to do with the profits of said railroads.
Considering motive power is one of the single largest categories of capital investment as well as operating costs, I would say your statement is highly misleading.
In any case, the original comment was:
“But it is interesting to note that the two most profitable class one railroads (in terms of the best operating ratios) are the two roads who have chosen to align themselves with DC power.” Joe made the statement my post referred to it as I thought it was an interesting observation. It may be just a coincidence.
I am sure that management choices on significant items of investment and cost have nothing to do with profits.
A TE curve is pretty steep at the lower speeds. This is where the phrase comes from “a diesel can’t pull what it can start, etc. …”.
Click on these images to bring them up in a readable window …
Actually, it is the other way around. Railroads like CP and MRL that have to move heavy trains over mountains have the higher operating costs, and they are forced to buy AC locomotives to try and keep those operating costs down.
CN has the best operating ratio because of their route stucture. They have three central hubs, which are connected to each other and the three coasts with 6 mainlines, which are virtually gradeless. CN does not need AC locomotives because their operating costs are low.
Theoretically it is; but in actuality it’s horizontal at low speed (on AC-traction units at speeds from around 10 mph and below) because the unit’s adhesion-management software limits the tractive effort that each traction motor can produce. For example, an AC4400CW with standard software operating under ideal rail conditions will begin to produce 180,000 pounds of TE when its speed falls to 9.78 mph; but TE will not increase as speed continues to drop. That’s because each traction motor is software-limited to 30,000 pounds of TE.
Actually, just think of the now-inactive Saluda grade. It’s almost three times as steep.
As for the old Norfolk & Western, I don’t mind it or the new NS being characterized as an Appalachian hauler. But not all their routes are creepy, crawly or twisty. In TRAINS last year I read that the Chattanooga - Knoxville - Bristol - Roanoke - Lynchburg line (pre-merger the Bristol-to-Lynchburg segment was N&W’s) is getting more business than ever, much of it stacks, from the Southern’s former territory. Although mostly single-tracked, the route is a good way to slip between the cracks in the Appalachian ridges (my characterization, not TRAINS’).
It’s good to remember that not all the old N&W lines were deep-coal lines; and I’m happy to see that the old “Pelican” route is proving its use again. In fact, that line runs right by my old high school in Glade Spring! - a. s.
I thought my statement was self-explanatory. i said nothing about a
“single climb.” i was referring to the fact that NS operates trackage in
Virginia, North Carolina, West Virginia, Kentucky, and Tennessee that
totals thousands of miles–much of it with tunnels, trestles, steep
grades, etc., etc. And one heck of a lot of long and heavy coal trains are
operated in this challenging region.
Michal Sol is absolutely correct in pointing out that the proflle of this
region differs significantly from some of the challenging western
divisions and may well dictate different engine choices–even when
the same methods of analysis are used by the various railroads.
All I was trying to say was that NS hasn’t chosen DC power because
it only has easy level track to deal with or because it doesn’t haul
much coal. To the contrary.
Joe
The characteristics of AC and DC motors never converge as far as rail horsepower is concerned. The AC induction motor has a 5% to 7% advantage in rail horsepower produced over a DC motor for a given nominal traction horsepower rating. The AC motors efficiency actually increases at higher speed. This is well documented by CSX’s use of C60ACs and C44ACs in intermodal service.
Forgive me if this question seems absurd, but I live near the line over Donner so I only see UP and BNSF power regularly. Does the ES44DC come in a CONTROLLED TRACTIVE EFFORT version? It would seem this is a AC only ability at this time. Do the DC units lend themselves to unmanned helper operations? It sounds like from the conversation that the DC unit heavy roads use dedicated helpers as opposed to unmanned helpers. This may be an incorrect assumption. I would be curious if NS would have a different loco makeup if it operated the UP or BNSF roads.
The CTE software package limits AC motored locomotives to TE ratings similar to DC locomotives. This ability is used on manifest and similar mixed consist trains where too much push by the DPU locomotives at low speeds could push light weight cars off the tracks on curves.
Your final sentence cuts to the very heart of the matter doesn’t it? Or,
better yet, stand the question on its head: would the NS territory have
a different locomotive makeup if operated by UP or BNSF? Okay, people–
what do you think? i would LOVE to hear everyone’s answer to these
questions!
Joe
The comment was addressed to the notion that motive power purchases don’t have anything to do with profitability. Didn’t say anything about specifics – merely that the gentleman’s general contention is haywire.
As to “the other way around”, my earlier post, walking through some numbers to see what popped out, did, indeed support the notion that Western roads and Eastern railroads have different needs generated by profile differences and that these differences appear to readily justify different power choices.
[quote]
Originally posted by MichaelSol
And perhaps that is a key. For what little I know about the Appalachians, the ups and downs are considerably more condensed than the long grade profiles of Western railroads. A DC Traction Motor can certainly take its share of overheating for a short period, and for so long as that demand on the motor is limited to short intervals, a DC locomotive is a bette
Has there been studies done so far on the durabilty of AC vs DC units? The AC4400 has been in service for close to if not more than 10yrs now as has the SD70/80/90. I would be curious as the the cost per mile.
Now, this is interesting. It makes the TE curve look more like that of … a steam engine!
What is the purpose of this software limitation?
Between 180K and 200K to avoid excessive mechanical stress within the traction motor; and above 200K both to avoid that stress and to avoid coupler failures when operating two-unit consists.
GP40-2
Interesting - I was not aware there was that much of a difference between the two at higher speeds.
Do you have a graph showing the tractive effort vs speed curve for AC and DC? It would be useful if we all understood the performance differences between the two types.
Thanks
Anthony V.