The locomotive guys at work tell me that it is the other way around, for units in equal service.
I was going to wait for some more answers before my comment, but my time is short here, so I will comment and catch up later.
IF - DC’s are lower maintenance - then it would be a good deal. IF AC’s are - then maybe you get what you pay for?
Michael,
I am not sure this is valid. I would speculate that the application of AC locomotives also allowed these trains to increase in length (from 115 cars to 135 cars for example). This seems to be overlooked. Also, have car cycles slowed down because of longer hauls, such as those to the southeast ?
If you discuss railroad topics with arbfbe, I would trust his views over this “industrial engineering prof”, IF that person is not involved with railroads full time.
Well, I missed the Company picnic this year, and so didn’t get a chance to talk to arbfe. But, the prof is well experienced in the rail industry. I’m not sure you could get both: the slow speeds and high tractive effort on a comparable train by reducing the number of units, and then add additional tonnage as well. Well, you could, but at some point the fluidity of the system just begins to break down. AC is not designed to move the trains faster, but only pays off when trains can move slower, and while that might pay off on a grade here and there, it still creates a bottleneck.
A single line system with an average running speed of 20 mph, 8 mile siding distances, has a capacity of 27 trains per day. Three AC locomotives replacing 5 DC locomotives (or 4 DC locomotives with helper districts) will slow the transit time. That is the point of AC power – it permits that to happen without damage to the equipment and is the justification for the investment – as nearly as I can tell.
Lowering the average speed as little as 2 mph can reduce the capacity to 24 trains per day – three trainloads of revenue. Increasing the carloads to 130 lowers the average speed yet again, because AC doesn’t change the fundamental TE curves, just allows operation at a lower range and if the heavier trains result in another reduction of just 2 mph because of the heavier train, track capacity drops to 22 trains per day – a total loss of
…If a DC consist of power is forced down to a slow crawl speed…What is the result if they are forced to operate in that range for an extended period of time…And if it is greatly harmful to the traction motors…what can they do about it when they are already on the hill that is causing such a condition…?
We’ve all seen heavy trains crawling up grade…some just barely moving…{Example}: Horseshoe curve…and I’m speaking back some years ago when I’m rather sure no AC traction motors were involved. In fact, I heard part of the conversation on my radio from the engineer, that he believed he was going to stall…But I continued to watch and the train did make it up around that curve, but just at a walking speed at best. A bit farther up the hill is a spot a wee bit steeper and I don’t know what happened there.
DC motors will burn up after a certain amount of abuse. As for your example you have either one of two options, either slug it out and hope you make it before the motors burn up, or stop and wait for a helper.
Apparently, the AC traction motor offers more than just being able to operate at lower speeds at high tractive efforts as explained below.
This text is from http://www.republiclocomotive.com_ac_traction_vs_dc_traction.html/
The AC (alternating current) Drive, also known as Variable Frequency Drive, has been the standard in industry for many years. While it has been used in locomotives for over two decades (especially in Europe), it has only been recently that the price of the drives has allowed them to be used in most of the new diesel-electric locomotives in the United States.
AC traction for locomotives is a major improvement over the old DC systems. The primary advantages of AC traction are adhesion levels up to 100% greater than DC and much higher reliability and reduced maintenance requirements of AC traction motors.
The tractive effort of a locomotive (whether AC or DC) is defined by the equations:
Tractive effort = Weight on drivers x Adhesion
Adhesion = Coefficient of friction x Locomotive adhesion variable
The friction coefficient between wheel and rail is usually in the range of .40 to .45 for relatively clean, dry rail in reasonable condition and is essentially the same for all locomotives. The locomotive adhesion variable represents the ability of the locomotive to convert the available friction into usable friction at the wheel rail interface. It varies dramatically from about .45 for old DC units to about .90 for modern AC units. This variable incorporates many factors including electrical design, control systems, truck type and wheel conditions.
First generation DC locomotives such as SW1200s, GP9
I ran some numbers base on the information from the Republic web site.
Assume a 400,000 lb locomotive with 4,000 hp.
At 10 mph, an AC locomotive with 37% adhesion can produce 150,000 lb of tractive effort, which also corresponds to max power output (14.7 ft/sec x 150,000 lb x 1 hp/550 ft-lb/sec).
A DC locomotive with 26% adhesion would produce 104,000 lb of tractive effort, which equates to 2,773 hp at 10 mph.
These results suggest that the DC locomotive must derate at low speeds not only to protect overheating the traction motors, but to prevent wheelslip. Thus, maximum power cannot be realized with the DC locomotive at this operating point.
The AC locomotive produces an additional 46,000 lb of tractive effort at 10 mph. This is equivalent to an additional 1,150 tons (about 9 or 10 carloads) when operating up a 2% grade.
How do these results compare to the real-world of railroading?
Thanks
Anthony V.
So, spend a half a million dollars instead of dropping some sand?
This is baloney. Cost involves the inverter, not the traction motor. Further, I’ve seen the operating results in real operation – not test results – of DC traction motors operating at beyond 40% adhesion. Depends on a variety of factors. Please review: note above that the most profitable railroads seem to be sticking with DC.
Thyristor controls.
Please explain your theory in economic terms: it makes those roads look like idiots.
They aren’t.
SOOOooo, if I may curl around to something close the original topic: Is DC doomed?
A few years ago the notion was that DC was okay for not-too-heavy intermodal and quick-off-the dime applications, like commuter-train engines.
Is that the feeling today? Or is the AC /DC/ AC system of power generation bound to dominate?
Should I make this a new post?
PS: Does Metra use AC or DC? - al
I would guess that the AC traction motor requires much less maintenance because there are no carbon brushes, but that has to be balanced against the higher cost. Don’t know if the AC drive electronics are a maintenance item in terms of having to fix power electronics.
As to the 40 percent adhesion, whether it is achieved with AC or DC, don’t you have to put in a margin of safety for less than ideal conditions of rain or debris on the rails? If you are counting on 40 percent adhesion in something like a passenger locomotive (AEM-7’s pioneered advance wheel slip control in the U.S. I believe), and if you don’t get 40 percent, well, you are going to accelerate a little slower and may be a minute or two late to the next station. If you are counting on 40 percent adhesion to make it up the ruling grade with a coal unit train, there may be times when you just get stuck fast.
In terms of the capacity argument, one of the points of friction between Amtrak and the host railroads is the one of a 50 MPH average speed train trying to make its way through 20 MPH traffic on a single-track line. The general belief is that the host railroads are making haphazard use of their capacity with all traffic running as unscheduled “extras” and Amtrak is accused of making haphazard use of the “traffic slots” assigned to it regarding its dispatching of trains, accounting for Amtrak trains held in sidings to let freights go by. But if there is a science to quantifying the capacity of different modes of rail traffic for single track, double track, CTC, etc., is there a scientific quantification of how much of a line’s capacity is utilized by daily Amtrak service or by more frequent “corridor” trains, and is any of this taken into account in what the host railroads are paid, either in terms of standard rates and or performance bonuses or penalties?
For example, the Canadian Pacific hosts the 7-times daily Hiawatha train from Chicago to Milwau
That’s an interesting question that I would like an answer to as well. I do know that NICTD (the South Shore Line’s owner/operator) overhauled its entire fleet to use AC Traction (so now DC Power from the catenary is inverted to AC power for the traction motors). I wonder what the benefits would be for commuter cars to use AC traction? The South Shore isn’t exactly in mountainous territory…
Brian
One thing’s certain–DC power won’t be “doomed” until Norfolk Southern and
Canadian National stop ordering it. And it doesn’t look like that’s going to
happen any time soon.
Joe
[:)]
Fascinating discussion. Reminds me of the Westinghouse/Edison battles over 100 years ago.
My opinion is that the numbers are so close that organizational culture and personal preference can swing decisions. When it’s as close as what appears above, any analyst, or group of analysts, can make any study come out the way the bosses want. If the advantage were, say, 1.5 to 1 in favor of one type, there would be no controversy. Clearly, the purported advantages are much closer than that, so we find different companies favoring different solutions.
Having said that, here’s my opinion as a retired electrical engineer.
AC motors require much less maintenance. I’m sure this is a big equalizer on life-cycle costs.
There is no reason builders can’t provide DC motors with control systems that have slip control just as good as what we see on the AC motors. And there’s no reason DC motors can’t have cooling that enables them to pull just as hard at low speeds.
When making comparisons, it’s important to compare equivalent generations of locomotives. I.e., make sure to compare a DC-motored locomotive of the same era as the AC-motored locomotive.
[:)] [:)]
This leaves me with a question about DC traction control systems. In a situation in which power to one DC traction motor is reduced in order to regain adhesion on its axle, how would the control system be able to compensate for that by increasing the power to those traction motors that have not lost adhesion in order to maximize total unit tractive effort?
On DC locomotives the wheelslip will drop the output of the generator, it does not control individual traction motors.
On a AC powered locomotive the wheelslip is controlled on each traction motor so a 4 axle locomotive is basicly 4 small single axle locomotives in one car body powered by one big diesel.
The AC traction motors the power to coils inside the motor are in the stator and only two small carbon brushes(they last a year or two) and slip rings are needed for a auxhilliary field.
On DC motors the field is in strator but the High current coils are in the rotor requireing 4 sets of 4 big carbon brushes to feed the rotor current, these brushes need replacement about every 180 days.
To me, this is the point!! It’s a close call. Some of the posts in this thread make it
sound like railroads who have made the switch to AC power have discovered the key
to modern efficient operations. It ain’t necessarily so.
Norfolk Southern is a major coal hauler, and there is no U.S. railroad terrain
any more challenging than the Appalachian and Piedmont regions where this
activity takes place. Do they employ lots of helpers and distributed power?
Of course–but it is my understanding that UP and BNSf still use these techniques
on their mountainous divis
Good point. One wonders how much of the added cost is the control system which powers each axle seperately,and if the manufacturers have attempted that with the DC drives, just to see if they can gain additional adhesion by not having to reduce power to all axles in a single truck…
One possible reason the transit system quoted above transitioned from DC to AC may have to do with getting power to the train…with a DC system, unless you have your own dedicated generators, you must first convert the line power to the proper voltage AC with a transformer then convert to DC using rectifier banks…when using AC, all that would be needed is a transformer.
That is primarily the reason AC won out over DC in the Edison(DC)/Tesla(AC) argument…for a DC system, at the time it was envisioned each neighborhood would have its own “Dynamo”, so power would not have to be transmitted very far. At the time, the up/down voltage conversion for transmitting DC power long distances incurred high losses.
(For the non double Es in the crowd, power is voltage times current, and losses are current times resistance. so for the same distance, if you increase the voltage, you dont have to send as much current over the same line resistance, so your losses are smaller. You just have to get the voltages down to safe/useable levels for the consumer at the end of the trip…)
Something interesting which I found out a few years ago, there are parts of our power grid now which use DC transmission lines. Seems as if they have solved the high loss in conversion issue…
Conversation between Milwaukee Road Electrical Engineer and EMD, 9/4/72, re: DC Traction Motors:
"This suggests the possibility of developing a solid state module that could go in parallel with an individual traction motor. This could supply additional controlled energy to each axle modulated to result in the 25% adhesion which ASEA has attained (see schematic).
"With individual controlled added power to each axle, rapid dropping of this power to prevent slipping would tend to throw the load over to the diesel generator I believe.
"This alternative would have to be justified on the basis of reduced engine maintenance and ownership cost.
"Further feasibility requires estimating cost for such a unit and projected maintenance cost.
“A chopper controlled electric engine that could achieve 25% adhesion is still the most promising possibility.”
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 better investment.
A hypothetical grade profile ranging from 0 to 1.2%, 80 100-ton cars, requires from 55,000 lbs to 670,000 lbs of Tractive Effort. At 15 mph five SD40-2s supply 311,250 lbs of Tractive Effort, if I have my math correct. That will get the train up a 1.2% grade at about 14.4 mph. On a 6 degree half mile curve, followed by a 4 degree curve, on that 1.2% grade, however, the train would have to slow to 6.9 mph to generate the Tractive Effort necessary to overcome the resistance. That could take over four minutes to get that train through the 6 degree curve. My recollection is that 11 mph was the “oh gosh” point on SD40-2s in terms of heating up the traction motors, and that the engineer didn’t want to spend too much time below that. How much time depended on the profile.
Adding DC power, one additional SD40-2 gets that minimum speed up to 8.3 mph, yet another, up to 9.6 – seven locomotives. It takes 8 locomotives to