Yes, they are three phase.
I know what a traction motor is and where it is. Someone want to take a minute and explain the rest to me?
I would appreciate it.
Mookie
She who has no signature! cinscocom-tmw
and that is the 3 phases?
Evidently I knew that, just didn't know the terminology.
Thanx
Mooks,
Three Phase refers to how the main generator generates electricity. There are three seperate windings in the generator. Each is off-set by 120 degrees from the other. Consequently, if you look at the sine waves that they produce when they reverse polarity, they'll be out of step with each other by 120 degrees.
Three phase power has some distinct advantages in applications like large motors (traction motors for example). Perhaps Randy can fill us in on the details, but I believe that the rotating magnetic field produced in three phase motors is preferable for starting and accelerating.
Hopefully that explains it a little bit. It's been awhile since I dealt with any of this, so any filler and corrections would be appreciated.
-ChrisWest Chicago, ILChristopher May Fine Art Photography"In wisdom gathered over time I have found that every experience is a form of exploration." ~Ansel Adams
Mookie wrote:I think I have it... And the railroads have gone mostly to AC currently (sorry...). Do they still even make new locomotives with DC say within the major 5?
Never too old to have a happy childhood!
....J: Those traction motors also have another duty to perform ....Downgrade, will find them turning into generators {electrically}, and become "brakes" in simple terms, to help to keep the train {speed}, under control.
Quentin
Larry Resident Microferroequinologist (at least at my house) Everyone goes home; Safety begins with you My Opinion. Standard Disclaimers Apply. No Expiration Date Come ride the rails with me! There's one thing about humility - the moment you think you've got it, you've lost it...
tree68 wrote:The careful (fully electronic) control of the AC to the traction motors is what allows an AC locomotive to crawl along at single digit speeds in notch 8. That would fry a DC motor.
It's that crawling part that raises some questions.
From an industrial engineering prof:
"The story we read about the AC motors on diesels is that, while more expensive than DC motors, the latest designs seem to cope better with very demanding loads at very low speeds than do DC motors. In a nutshell, they enable fewer diesel locomotive units on a train, albeit operating at slower train speeds. The operations opportunity: one can feasibly handle a heavier load while running more slowly if diesels are equipped with AC motors. The contemporary switch to AC motors was driven by the Powder River Coal business and BN management in particular. Powering of their coal trains evolved from 5 3,000 HP DC units to 3 4,000 HP AC units. Train speeds slowed down (and so car cycles stretched out somewhat and therefore freight car costs went up), but locomotive and fuel costs per trip went down."Knowing what I know about railroad management costing systems, I suspect the profit gain from this change was overestimated. I suspect the profits lost from slowing down all the trains, particularly on lines with mixed traffic, were underestimated."
Intentionally planning a lower operating speed -- and paying a higher price for that "ability" -- has a significant system cost penalty.
MichaelSol wrote: tree68 wrote:The careful (fully electronic) control of the AC to the traction motors is what allows an AC locomotive to crawl along at single digit speeds in notch 8. That would fry a DC motor.It's that crawling part that raises some questions.From an industrial engineering prof:"The story we read about the AC motors on diesels is that, while more expensive than DC motors, the latest designs seem to cope better with very demanding loads at very low speeds than do DC motors. In a nutshell, they enable fewer diesel locomotive units on a train, albeit operating at slower train speeds. The operations opportunity: one can feasibly handle a heavier load while running more slowly if diesels are equipped with AC motors. The contemporary switch to AC motors was driven by the Powder River Coal business and BN management in particular. Powering of their coal trains evolved from 5 3,000 HP DC units to 3 4,000 HP AC units. Train speeds slowed down (and so car cycles stretched out somewhat and therefore freight car costs went up), but locomotive and fuel costs per trip went down."Knowing what I know about railroad management costing systems, I suspect the profit gain from this change was overestimated. I suspect the profits lost from slowing down all the trains, particularly on lines with mixed traffic, were underestimated."Intentionally planning a lower operating speed -- and paying a higher price for that "ability" -- has a significant system cost penalty.
joemcspadden wrote:Michael--what you wrote and quoted above is certainly reflected in the philosophy of Norfolk Southern, which of course hauls a lot of coal over very demandingterrain. The only ac motors on the roster, as I understand it, are a few SD80-macsinherited from Conrail, and the word is they will be eliminating all of these as timegoes on.
That's interesting Joe, thanks for pointing that out.
I read comments like this above "I think I have it... And the railroads have gone mostly to AC currently (sorry...). Do they still even make new locomotives with DC say within the major 5?"
I wonder where these comments come from. My impression is that not only is DC the most popular traction motor, but that AC passed a sort of peak -- as railroads began to add in the collateral costs of operation based on experience.
By itself, the AC traction motor is a remarkably robust piece of engineering, heads and shoulders above the old DC traction motors I grew up with.
But the cost comparison is not located in the traction motor, it is located in the inverter, an expensive piece of additional electrical equipment, and based on my long-ago experience of pricing of inverters for railroad use and my current wondering of how any savings could justify the cost of those inverters, I am wondering how the overall investment can be justified since it is based on the AC advantages at very low speeds -- which not only incurs the additional cost of inverters, but increases overall system costs as well.
I would like to see some numbers ...
Michael - I live in BNSF territory and don't see DCs any more. And the power run-throughs that we see here are mostly AC. I know some of the smaller lines will run DC - probably because it is cheaper to purchase, but would the major railroads run AC since (I understand) that the speeds between point A and point D are usually traversed at the lower speeds.
If I was say - BNSF - I would spend the extra money for what I can get from an AC and make up the difference in another place. I think the advantages definitely outweigh the disadvantages - especially in this part of the country. I can't speak to NS since that is a part of the country I have never visited.
Mookie wrote:Michael - I live in BNSF territory and don't see DCs any more. And the power run-throughs that we see here are mostly AC. I know some of the smaller lines will run DC - probably because it is cheaper to purchase, but would the major railroads run AC since (I understand) that the speeds between point A and point D are usually traversed at the lower speeds. If I was say - BNSF - I would spend the extra money for what I can get from an AC and make up the difference in another place. I think the advantages definitely outweigh the disadvantages - especially in this part of the country. I can't speak to NS since that is a part of the country I have never visited.
Mookie wrote:So Joe, why did NS go with DC and BNSF seems to have gone with AC? I must be missing something here.
joemcspadden wrote:I don't believe there is a "right" or "wrong" in either case.Regard, Joe
I don't believe there is a "right" or "wrong" in either case.Regard, Joe
Maybe the more appropriate term would be "best fit?" And that would be an internal decision based on numerous factors.
It's all in the application.
MS mentions speeds - it's possible that BNSF needs the low speed pulling capability enough that it's worth their while - perhaps cheaper in the long run than maintaining/using pushers. Possible scenario - hard pull out of a valley followed by relatively flat running. The speeds would average out.
NS may opt to put more power on to raise the minimum train speed (a necessity with DC anyhow).
Too, we can't really have a meaningful discussion without looking at things like average train length/weight and overall grades.
tree68 wrote: It's all in the application. MS mentions speeds - it's possible that BNSF needs the low speed pulling capability enough that it's worth their while - perhaps cheaper in the long run than maintaining/using pushers. Possible scenario - hard pull out of a valley followed by relatively flat running. The speeds would average out.NS may opt to put more power on to raise the minimum train speed (a necessity with DC anyhow).Too, we can't really have a meaningful discussion without looking at things like average train length/weight and overall grades.
Maybe you shouldn't give NS too much credit!
About 10 years ago, I asked an NS middle-to-upper level Mech Dept staff guy, "Why no ACs?" The main reason was that "NS ran mostly two units per trains, so no unit replacement benefit with AC". As it turns out, that really isn't a true statement. There are lots of applications on ACs would give good unit replacement ratios while still keeping HP/ton at required levels. I suspect the truth was that the Mech Dept couldn't be bothered to do the training and the hassle of keeping the fleet segregated was more than anyone wanted to take on, and now there's a lot of inertia and, perhaps some crow to eat if ACs were to be purchased.
I think some of this same "logic" is the reason NS has derated 4400 HP locomotives....
...and which RR was last to dieselize, and which RR was last to purchase GP9s, etc, etc.
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
oltmannd wrote: tree68 wrote: It's all in the application. MS mentions speeds - it's possible that BNSF needs the low speed pulling capability enough that it's worth their while - perhaps cheaper in the long run than maintaining/using pushers. Possible scenario - hard pull out of a valley followed by relatively flat running. The speeds would average out.NS may opt to put more power on to raise the minimum train speed (a necessity with DC anyhow).Too, we can't really have a meaningful discussion without looking at things like average train length/weight and overall grades. Maybe you shouldn't give NS too much credit! About 10 years ago, I asked an NS middle-to-upper level Mech Dept staff guy, "Why no ACs?" The main reason was that "NS ran mostly two units per trains, so no unit replacement benefit with AC". As it turns out, that really isn't a true statement. There are lots of applications on ACs would give good unit replacement ratios while still keeping HP/ton at required levels. I suspect the truth was that the Mech Dept couldn't be bothered to do the training and the hassle of keeping the fleet segregated was more than anyone wanted to take on, and now there's a lot of inertia and, perhaps some crow to eat if ACs were to be purchased. I think some of this same "logic" is the reason NS has derated 4400 HP locomotives.......and which RR was last to dieselize, and which RR was last to purchase GP9s, etc, etc.
Mookie BNSF splits their orders between AC and DC locomotives. The new BNSF locomotives numbered in the 7xxx series are DC motored, while the new 6xxx series are AC motored. Currently I think the scorecard goes like this.
BNSF splitting between AC and DC
Only buying DC
CN and NS
Only buying AC
UP, CP, and KCS
Jury still out on CSX because they got into the same problem that UP did, the severe need to replace large numbers of older increasingly unreliable power as quickly as possible. Once UP accomplished that they went back to buying exclusively AC motored power. Indications that I have heard is that the ES44DCs have satisfied the urgent need for more reliable power on CSX and future orders will be for AC motored power only.
2006 Actual production by builder and railroad, note because this is actual production it doesn't match orders. Some orders partially built in 2005 or 2007.
BNSF GE 292 ES44AC locomotives
CN GE 35 ES44DC locomotives
EMD 5 SD70M-2 locomotives
CP GE 80 ES44AC locomotives
CSX GE 102 ES44DC locomotives
FEC EMD 4 SD70M-2 locomotives
FXE EMD 15 SD70ACe locomotives
GE 60 ES44AC locomotives
KCS EMD 5 SD70ACe locomotives
KCSM GE 22 ES44AC locomotives
NS EMD 76 SD70M-2 locomotives
GE 62 ES40DC locomotives
U P GE 100 ES44AC locomotives
EMD 100 SD70ACe locomotives
Totals AC 674
DC 284
The production for 2007 will be close to an even split, as BNSF and CSX are taking delivery of ES44DCs this year. In 2008 the balance is expected to swing back heavily to AC as both BNSF and CSX have large orders.
Mookie wrote: Michael - I live in BNSF territory and don't see DCs any more. And the power run-throughs that we see here are mostly AC.
Michael - I live in BNSF territory and don't see DCs any more. And the power run-throughs that we see here are mostly AC.
As best I can gather from the company magazines, through "iffy" tangential references, BN has about 1,000 AC locomotives, and 5,300 DC locomotives. The AC's appear to be almost entirely coal service, and recent purchases have been directed to that type because of the coal demand.
JayPotter wrote:One approach to justifying the cost of AC-traction is increased productivity expressed in terms of horsepower-per-ton. Exemplar numbers are contained on page 44 of the November 2006 issue of TRAINS.
For those that don't have that issue handy-- it just points out that 4400 hp AC GEs are rated for 2900 tons apiece up Cranberry, which is a lower hp-per-ton than their predecessors. Nothing beyond that.
Fascinating. I must take some time and digest all this good information!
timz wrote: For those that don't have that issue handy-- it just points out that 4400 hp AC GEs are rated for 2900 tons apiece up Cranberry, which is a lower hp-per-ton than their predecessors. Nothing beyond that.
For something beyond the basic numbers, you need to read the article. Its basic premise -- in the context of our discussion here -- is that a railroad will have a sufficient number of units in a given consist to produce enough horsepower to move its train at whatever speed is desired across its route; however if the railroad has to add even more units in order for the consist to produce enough tractive effort to keep the train from stalling on one or more short segments of that route, the railroad is probably wasting horsepower and should at least consider AC traction.
JayPotter wrote:...enough horsepower ... even more units ... keep the train from stalling ... should at least consider AC traction.
All you're saying (so far) is that moving a given train from A to B in a given time requires both X rated horsepower and Y continuous-or-whatever rated tractive effort. If two GP9s produce enough horsepower but not enough TE, you can switch to SD9s. If two GP40s don't produce enough TE, you can switch to SD40s. Nothing new there. But now there's no such thing as a B44-9, so you could say the DC C44-9 is the starting point, and the only way to get more TE is to go AC (unless GE starts selling D44-9s, or C33-9s-- or C22-9s).
timz wrote: continuous-or-whatever rated tractive effort
That's correct, the issue is continuous tractive effort, with the emphasis on "continuous". Regardless of whether the locomotives in a DC-traction consist are four-motored or six-motored, the consist has to have enough horsepower to keep train speed from dropping too far below the units' minimum continuous speed for too long a period of time. As a practical matter, that's not a consideration with a consist of AC-traction units. If a DC-traction consist encounters a grade that causes train speed to decrease too much, the units will derate themselves to avoid damaging their traction motors. An AC-traction consist won't do that. So since an AC-traction consist doesn't have to maintain the speed levels that a DC-traction consist would have to maintain, the AC-traction consist doesn't have to have the level of horsepower that the DC-traction consist would have to have.
joemcspadden wrote:Folks, I am not trying to prove anything with the following statement,so don't jump all over me. But it is interesting to note that the twomost profitable class one railroads (in terms of the best operatingratios) are the two roads who have chosen to align themselves withDC power.Regards, Joe
That's an interesting observation. To the extent that higher average operating speeds means less congestion, greater capacity, higher efficiency and greater productivity, the higher investment in a machine designed to return that investment only if used at very low operating speeds does seem to present a bit of a conundrum regarding investment in AC power.
In the example offered by my industrial engineering colleague above, put in some numbers:
DC, 3000 hp, $1,500,000/unit, 5 per train: 15,000 hp. At 7%, total annual financing cost (P+I) of motive power on an 800 mile railroad running 18 trains per day with 120 mile divisions: $118,342,969.
AC, 4400 hp, $2,300,000, 3 per train: 13,200 hp. Total cost same railroad configuration: $108,875,531. It's cheaper to go AC.
If there are two hypothetical "slow spots" that need the 15,000 hp of the DC engines to maintain sufficient speed to avoid overheating, what does the configuration look like by reducing the DC train power and adding two helper districts?
In that instance: DC, 4 per train. 12,000 hp. Total annual financing cost, including 2 helpers -- two units each (and crew at $94,000 salary including benefit costs, two man crews, three crews per 24 hr cycle), total hp on grade of 18,000 hp, total annual financing cost (plus extra crew): $96,591,328.14. Cheaper to go DC.
The addition of helper districts saves, in that instance, $12,284,203.13 in annual financing charges over the cost of AC equipment, especially where the helper districts apply considerably more hp to the train movement, upping the average speed -- better overall efficiency, etc, etc. -- additional savings over the cost of financing.
In the real world of differing train tonnages, there may be a mix of AC and DC that provides an optimum cost benefit, but that would require all the details.
I wouldn't suggest that a couple of slow spots on a coal train run justifies the investment in AC power compared to putting on a couple of helpers where needed.
MichaelSol wrote: I wouldn't suggest that a couple of slow spots on a coal train run justifies the investment in AC power compared to putting on a couple of helpers where needed.
That might be true in some circumstances; but let me change the scenario to reflect CSXT's circumstances. Instead of discussing "slow" spots, we need to discuss "helper" spots. In other words, we're dealing with route segments on which helpers are going to be required regardless of whether the locomotives in use are AC-traction or DC-traction. That's because a train of significant length can't be moved across the curves and grades of those segments without employing a helper to reduce in-train forces. So helper-related expenses approach being fixed costs. In that kind of high-cost situation, it seems particularly appropriate for the railroad to maximize the length of its trains as much as the capabilities of the assigned locomotive consists will allow. And because of the AC-traction characteristics that we've been discussing, the trains can be longer if AC-traction units are assigned to them than they can if an equivalent amount of DC-traction horsepower is assigned to them.
I guess this brings us back to the issue of whether smaller faster trains are more profitable than longer slower trains. CSXT believes, with regard to its tonnage traffic, that length is more important than speed. That's not saying that speed is irrelevant; it's just saying that there are financial advantages to including, in tonnage trains, the additional cars that AC-traction locomotive consists allow CSXT to add.
JayPotter wrote:Regardless of whether the locomotives in a DC-traction consist are four-motored or six-motored, the consist has to have enough horsepower to keep train speed from dropping too far below the units' minimum continuous speed for too long a period of time.
JayPotter wrote:since an AC-traction consist doesn't have to maintain the speed levels that a DC-traction consist would have to maintain, the AC-traction consist doesn't have to have the level of horsepower that the DC-traction consist would have to have.
I guess that's the first question to ask, when wondering whether AC makes sense over a given piece of RR: if your trains climb the ruling grade at 7 mph, will your crews cover their run in 12 hours? If not, forget AC.
(But if 6000 hp units can be made to work ...)
Brian (IA) http://blhanel.rrpicturearchives.net.
Mookie wrote:Do the DC's require more maintenance over time? Is that too broad a question?
The subscription DVD which is being sent by Trains to subscribers, in an attempt to get them started in a DVD of the month club (Discussed in this thread) is called 'Ultimate Railroading DVD Series: Big Power'.
It has a nice discussion of the merits of AC vs DC power.
It sounds to me a little like the Ford vs Chevy argument for some engineeering departments...
joemcspadden wrote: Mookie wrote:Do the DC's require more maintenance over time? Is that too broad a question? I believe the theory is that DC will require less maintenance over time. I'm not sure the final chapter has been written on this yet.Joe
The locomotive guys at work tell me that it is the other way around, for units in equal service.
An "expensive model collector"
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?
MichaelSol wrote: From an industrial engineering prof: The contemporary switch to AC motors was driven by the Powder River Coal business and BN management in particular. Powering of their coal trains evolved from 5 3,000 HP DC units to 3 4,000 HP AC units. Train speeds slowed down (and so car cycles stretched out somewhat and therefore freight car costs went up), but locomotive and fuel costs per trip went down.
The contemporary switch to AC motors was driven by the Powder River Coal business and BN management in particular. Powering of their coal trains evolved from 5 3,000 HP DC units to 3 4,000 HP AC units. Train speeds slowed down (and so car cycles stretched out somewhat and therefore freight car costs went up), but locomotive and fuel costs per trip went down.
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.
nanaimo73 wrote: 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 5 trains per day of capacity. Notwithstanding the heavier trains, carloads moved drops from 3,095 carloads per day to 2,860 carloads per day. At $1,600 per carload, that's a loss of $376,000 per day, or $137,240,000 per year. Ouch.
I would say that, obviously, this has all been looked at and the numbers apparently came out in favor of the AC for specific uses based upon some similar analysis using different operation-specific parameters. On the other hand, historical experience has sometimes offered different lessons in that regard.
In any case, it is interesting to model the scenarios, as that provides perhaps some insight into the decision making process.
.....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.
Modelcar wrote: .....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, GP9s, SD40s, and GE center cabs typically have adhesion levels of 18% to 20%. More modern units with adhesion control such as SD60s and Dash 8s have adhesion levels of 25% to 27%. The newer AC traction units such as the SD80MAC and the C44AC are usually rated at 37% to 39% adhesion. Thus, the newer locomotives have about twice the adhesion of the older units and the Class I railroads are, in fact, typically replacing two older units with a single new AC unit.
There are three primary reasons that AC traction offers so much more adhesion. First, in a standard DC drive, if wheel slip occurs, there is a tendency for the traction motor to speed up and run away, even to the point of mechanical failure if the load is not quickly reduced. As the wheel slippage increases, the coefficient of friction also drops rapidly to a level of 0.10 or less, and because all the motors are connected together, the load to the entire locomotive must be reduced. Therefore, maximum adhesion is obtained by operating at a level with a comfortable margin of safety below the theoretical maximum. More modern DC systems incorporate a wheel slip control which senses the beginning of a slip and automatically modulates the power in order to retain control. This allows the locomotive to operate safely at a point closer to its theoretical maximum.
The AC system, however, operates in a very different fashion. The variable frequency drive creates a rotating magnetic field which spins about 1% faster than the motor is turning. Since the rotor cannot exceed the field speed, any wheel slip is minimal (less than 1%) and is quickly detected by the drive which instantly reduces load to the axle.
Next, the DC locomotive typically has a number of throttle settings with a set power level for each one. While this sytem is simple and effective, it does not produce a constant motor torque since power is the product of torque and speed. Therefore, the tractive effort varies significantly for each throttle setting depending on speed, making it impossible to obtain maximum adhesion.
The AC locomotive, however, can control to a specific motor torque level allowing the tractive effort to be essentially constant at the higher range of available adhesion. Ths fast acting wheel slip control can counteract any wheel slip so that the torque level can be set close to the upper limits.
The third way that AC traction provides improved adhesion is through weight transfer compensation. When a locomotive is pulling a load, weight tends to transfer from the front axle to the rear axle of each truck. At maximum tractive effort, the weight on the lead axle may be reduced by about 20%. Since the tractive effort is proportional to the weight on drivers, then in a DC system where the motors are fed from a common source, the tractive effort will be determined by the lightest axle. Thus, in effect, the equivalent locomotive weight is reduced by about 20%. With an AC system, however, the drive is able to compensate for the weight transfer. When the lead axle goes light, the AC drive system will reduce power to that axle and apply more power to the rear axle without incurring wheelspin.
The combination of eliminating wheel slip and compensating for weight transfer gives the AC traction system an adhesion of 37% to 39% versus the 18% to 20% of the older DC systems. Therefore, a locomotive with AC traction can provide the same tractive effort as a DC locomotive weighing twice as much or can give twice as much tractive effort for the same weight.
GE and EMD added AC traction to their mainline units and were then able to replace two older DC units with one new AC locomotive. Republic locomotive took a different approach and decided to make a lighter, less costly unit for industrial switching. The DC powered SW9/SW1200, produced in large quantities from 1951 to 1965 and used for heavy yard switching as well as branch line service, was taken as the performance standard. At 230,000 to 240,000 pounds these units are typically rated at about 40,000 pounds tractive effort continuous (somewhat higher intermittent but limited by traction motors and generators). The AC traction RX500 at 144,000 pounds and a conservative 35% adhesion level is rated at 50,400 pounds tractive effort continuous.
With AC traction, it is also important to consider braking. As with traction, braking is a function of weight on drivers. Therefore, when using standard friction braking (tread brakes) the braking capability of the locomotive (excluding train braking) is proportional to the locomotive weight. With AC traction, however, the braking can be much higher because the drive system in braking acts just like the drive does in traction thus eliminating wheel slip. The drive converts the motors to generating mode (dynamic braking) and the electricity produced is dissipated in the braking resistors. Thus the motors are slowing the locomotive without using the air brakes. Again, the adhesion levels are much higher so the locomotive can again be significantly lighter for the same amount of braking. The dynamic braking in AC traction locomotives also allows full braking down to zero speed, unlike DC dynamic braking.
All in all, the AC traction locomotive offers about twice the amount of adhesion as a DC unit. Therefore, a modern lightweight AC locomotive such as the RX500 can provide as much or more tractive effort than an old style DC unit like the SW1200 which weighs 60% more.
© 2007 Republic Locomotive
This suggests that AC power can pull more tonnage at the same speed as DC power when operating below maximum power output, not just by slowing down.
Anthony V.
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
AnthonyV wrote: First generation DC locomotives such as SW1200s, GP9s, SD40s, and GE center cabs typically have adhesion levels of 18% to 20%. More modern units with adhesion control such as SD60s and Dash 8s have adhesion levels of 25% to 27%. The newer AC traction units such as the SD80MAC and the C44AC are usually rated at 37% to 39% adhesion. Thus, the newer locomotives have about twice the adhesion of the older units and the Class I railroads are, in fact, typically replacing two older units with a single new AC unit.
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 Milwaukee on double-track with crossovers CTC main line, and this service is regarded as having the best on-time performance on the Amtrak network. CP is dropping broad hints that if people want to increase the service or extend the service to Watertown, somebody paying for more crossovers would help. Part of the success of the California trains is that not only did they raise in-state money to get their own passenger cars and locomotives, even more money was spent on infrastructure improvements -- not a completely new passenger line, but strategically-placed improvements to relieve congestion.
I get the sense that the Canadian National is none too happy about the impact of the new Illinois trains on their lines, but there was maybe not much forethought of the impact of the new passenger trains on the host railroad traffic situation and now there is a lot of finger pointing about chronically-late trains.
Has the payment Amtrak makes to their host railroads had an assessment in terms of how much capacity is being used, or is the payment a kind of legacy matter that the host railroads agreed to as part of the original Amtrak founding that allowed the railroads to discontinue their own passenger trains?
If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
al-in-chgo wrote: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
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
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.
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.
cordon wrote: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.
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?
JayPotter wrote: cordon wrote: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. 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.
cordon wrote: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.
cordon wrote: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.
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...
Dutchrailnut wrote: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.
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."
joemcspadden wrote: cordon wrote: 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.Norfolk Southern is a major coal hauler, and there is no U.S. railroad terrainany more challenging than the Appalachian and Piedmont regions where thisactivity takes place. Do they employ lots of helpers and distributed power?
cordon wrote: 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.
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 keep the minimum speed of 11 mph on that grade and curvature. If there were enough combinations of that grade and curvature, over a long enough profile, the short time ratings would be exceeded and the DC locomotives would simply have to be maximized to keep the train moving.
By comparison, four 4400 hp AC locomotives could keep the minimum speed at 8 mph, three AC locomotives could keep it moving at 6 mph. If the railroad didn't mind poking along at 5 mph, they could get by with just two AC locomotives. At 2.5 mph, one AC locomotive could still keep the train moving if it didn't slip. The relative costs of the equipment start to become real clear at that point as to which is the better purchase. That doesn't address congestion or opportunity costs, but, it does clarify the equipment cost.
For the five DC SD40-2 locomotives vs three 4400hp AC locomotives postulated earlier, the DC would transit a given 212 mile distance in 6 hours and ten minutes, the AC in 6 hours and 39 minutes. Dropping to two of the AC units, however, the transit time stretches out to 8 hours and 58 minutes -- takes up a lot of track space and time, and time is money.
I would guess therefore that the decision to purchase AC or DC is very profile specific and that NS's analytical approach is identical to the UP approach -- but their profiles generate different results.
Dutchrailnut wrote: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.
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.
joemcspadden wrote:Norfolk Southern is a major coal hauler, and there is no U.S. railroad terrainany more challenging than the Appalachian and Piedmont regions where thisactivity takes place.
No US climb is more "challenging" than the N&W 1.4% grades?
MichaelSol wrote:It takes 8 [SD40-2] locomotives to keep the minimum speed of 11 mph on that [1.2%] grade and curvature....At 2.5 mph, one AC locomotive could still keep the train moving if it didn't slip.
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?
MichaelSol wrote: Please review: note above that the most profitable railroads seem to be sticking with DC.
A very misleading statement. The kind of power bought by the railroads have little to do with the profits of said railroads.
n012944 wrote: MichaelSol wrote: Please review: note above that the most profitable railroads seem to be sticking with DC.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.
timz wrote: MichaelSol wrote:It takes 8 [SD40-2] locomotives to keep the minimum speed of 11 mph on that [1.2%] grade and curvature....At 2.5 mph, one AC locomotive could still keep the train moving if it didn't slip.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 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 ...
MichaelSol wrote: n012944 wrote: MichaelSol wrote: Please review: note above that the most profitable railroads seem to be sticking with DC.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.
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.
MichaelSol wrote:A TE curve is pretty steep at the lower speeds.
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.
timz wrote: joemcspadden wrote:Norfolk Southern is a major coal hauler, and there is no U.S. railroad terrainany more challenging than the Appalachian and Piedmont regions where thisactivity takes place.No US climb is more "challenging" than the N&W 1.4% grades?
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.
AnthonyV wrote: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.
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.
broncoman wrote: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.
nanaimo73 wrote: MichaelSol wrote: n012944 wrote: MichaelSol wrote: Please review: note above that the most profitable railroads seem to be sticking with DC.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. 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.
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.
Originally posted by MichaelSolAnd 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.... I would guess therefore that the decision to purchase AC or DC is very profile specific and that NS's analytical approach is identical to the UP approach -- but their profiles generate different results.
...
Indeed, if what the gentleman said had any merit at all -- "The kind of power bought by the railroads have little to do with the profits of said railroads." -- railroads would not be spending any time looking at merits of AC v DC and we would not be having this conversation on this thread.
It was a baseless remark -- 'highly misleading".
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.
JayPotter wrote: MichaelSol wrote:A TE curve is pretty steep at the lower speeds.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.
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?
MichaelSol wrote: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 wrote: AnthonyV wrote: 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.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.
AnthonyV wrote: 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.
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.
GP40-2 wrote:On a GE locomotive with a 4400 nominal traction HP rating, The AC version produces nearly 400 more rail HP @70 mph than the DC version. I've discussed this in past posts using test data from CSX locomotives.
How much does that additional horsepower cost?
timz wrote: MichaelSol wrote:It takes 8 [SD40-2] locomotives to keep the minimum speed of 11 mph on that [1.2%] grade and curvature....At 2.5 mph, one AC locomotive could still keep the train moving if it didn't slip. 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?
MichaelSol wrote: nanaimo73 wrote: MichaelSol wrote: n012944 wrote: MichaelSol wrote: Please review: note above that the most profitable railroads seem to be sticking with DC.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. 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.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.It was a baseless remark -- 'highly misleading".
I didn't say that it had nothing to do with profitability, it just has very little to do with it. There are many reasons for the NS and CN operating ratio, and locomotive choice is but a very small reason.
timz wrote: timz wrote: MichaelSol wrote:It takes 8 [SD40-2] locomotives to keep the minimum speed of 11 mph on that [1.2%] grade and curvature....At 2.5 mph, one AC locomotive could still keep the train moving if it didn't slip. 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?So that is what you meant to say? On further reflection, doesn't that strike you a bit unlikely? The eight SD40-2s produce 650,000+ lb of TE at 11 mph, don't they?
A little low, but depending on measured rail hp that could be about right. My numbers came from a GE locomotive test program which has a little more to it (and also dates from pre-traction motor software limitation days), but if you want to walk through the basic TE equation, the single unit AC (recall, SD40-2, 3000 engine hp v GE AC 4400 engine hp, and the AC generates a higher rail hp compared to its engine rating than the DC) at 2.5 mph would generate 660,000 lbs TE but for the software restriction. That is pretty much what is described by the TE curves I posted above.
TE=(hp*375)/speed
In round numbers, using engine hp:
Eight locomotives x 3000 hp x 375 / 11 mph = 818,182 lbs TE
One locomotive x 4400 hp x 375 / 2 mph = 825,000 lbs TE
What is unlikely?
selector wrote:Michael, you are asking real-time lifetime? Initial outlay? Both?
Well, we don't have much on the lifetime outlays yet, so I am thinking purchase price per hp.
MichaelSol wrote:One locomotive x 4400 x 375 / 2 mph = 825,000 lbs TE What is unlikely?
timz wrote: MichaelSol wrote:One locomotive x 4400 x 375 / 2 mph = 825,000 lbs TE What is unlikely?So what do you figure for the AC's tractive effort at 0.1 mph?
You keep returning to this like you want to argue about something. I have posted TE graphs for your perusal, and offered you the fairly straightforward TE equation that you can play with with any numbers you want. Why do you want me to answer this question?
What's your point?
MichaelSol wrote:I have posted TE graphs for your perusal, and offered you the fairly straightforward TE equation that you can play with with any numbers you want. Why do you want me to answer this question? What's your point?
n012944 wrote: I didn't say that it had nothing to do with profitability, it just has very little to do with it. There are many reasons for the NS and CN operating ratio, and locomotive choice is but a very small reason.
If a railroad earns a 7% return, and motive power accounts for 25% of a railroad's operating expenses, and AC ( or sticking with DC) can provide a mere 5% improvement in overall motive power expenses -- only 1.25% of all operating expenses -- it goes right to the bottom line and that changes that railroad's profitability by 1.25%, from 7% to 8.25%.
A single management decision that could produce that kind of a change in profitability is a very large reason, not a small one, as it would represent an 18% increase in profitability over your competitor who did not make that decision.
In the real world, that's not small.
timz wrote: MichaelSol wrote:I have posted TE graphs for your perusal, and offered you the fairly straightforward TE equation that you can play with with any numbers you want. Why do you want me to answer this question? What's your point?The fairly straightforward TE equation says the AC (or any other 4400 hp locomotive) will produce 16,000,000 lb of TE at 0.1 mph. And 160,000,000 lb at 0.01 mph. In other words, we can't expect the formula to "work" in that speed range. And it doesn't at 2 mph, either. There is no chance that a single six-axle AC will maintain 2.5 mph (or any other mph greater than zero) with a train that brings eight SD40-2s down to 11 mph.
Well, you've been playing a game here, the usual stuff that comes up on these threads. If you disagree, just simply say so like a grown-up, get it off your chest, and cut out the little "gotcha" string of posts. There are professional ways of approaching these things and pretending you can't do your own math isn't one of them. You didn't need me to do your math for you. I know what the equation says. So do you.
Some of the early AC tests marvelled at how single units could pull heavy trains at, literally, inches per hour, obviously without the software limitations. But, you've got it all figured out. It "can't work". On an unregulated, hypothetical AC or DC locomotive -- and my example was clearly identified as a hypothetical -- what do you think the TE is at 2.5 mph? Why do you think the forumula is in error? What is your formula for the TE on that relevant range? What tests are you relying on? Where is the study that says the curve is in error over part of its range? Is it in error on the high end too? There's not much TE left there. Why?
I think you do just want to argue. Good luck.
MichaelSol wrote:Why do you think the formula is in error?
timz wrote: MichaelSol wrote:Why do you think the formula is in error?The formula isn't "in error"-- we just can't use it in this speed range in the "straightforward" way you want to use it. (Which is to say, we can't expect an AC44 to produce 4400 hp in this speed range.) I guess you'll agree an AC44 can't actually produce 160,000,000 lb TE at 0.01 mph-- right? Despite the formula?
You're playing games. This is an interesting thread on an adult topic and just doesn't need this "oh yeah well what about ..." attitude. If you think it is wrong, ditch the attitude, go out, measure it, and come back with your results that show the formula is wrong.
What I will concede is that at 0.01 mph I don't particularly care what you think is actually produced.
MichaelSol wrote:If you disagree, just simply say so like a grown-up, get it off your chest, and cut out the little "gotcha" string of posts. There are professional ways of approaching these things and pretending you can't do your own math isn't one of them.
JayPotter wrote: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.
The 9.78 mph figure should actually be approximately 8 mph. The 9.78 mph figure is the speed at which a standard AC4400CW should be able to produce 145,000 pound of TE regardless of rail conditions. If rail conditions are ideal and speed decreases, TE will increase to 180,000 pounds at around 8 mph; but, because of software-imposed limitations, further speed reduction will not produce any increase in TE.
timz wrote: MichaelSol wrote:If you disagree, just simply say so like a grown-up, get it off your chest, and cut out the little "gotcha" string of posts. There are professional ways of approaching these things and pretending you can't do your own math isn't one of them.I unprofessionally and un-grownupedly figured that once you gave the matter a bit more thought you'd see there had to be a flaw in your calculation. Should I give up on that possibility?
It's an industry formula, it's not "my" calculation. There "had" to be a flaw? Well, what is it? You don't like it?
Let's clarify this. I relied on accepted industry formula. You don't "think" or "like" or "accept" the results. That doesn't put the burden on me. It's not my formula to concede one way or the other that there is a "flaw" in it, nor my "job" to defend an accepted approach.
The industry has used the formula for a good number of years without apparently understanding that it is wrong. Why don't you write a paper on how the formula is wrong, and provide the correct formula that the rail industry obviously needs? Then you can play "gotcha" to your heart's content with whoever designed the original formula and forgot to put parameters on it for you. The engineering design puts its own parameters on it, and that no doubt varies with individual designs or even software limitations, as pointed out.
Until then, drop the attitude. I don't need it, the thread doesn't need it, and Trains forums don't need it.
MichaelSol wrote: n012944 wrote: I didn't say that it had nothing to do with profitability, it just has very little to do with it. There are many reasons for the NS and CN operating ratio, and locomotive choice is but a very small reason. If a railroad earns a 7% return, and motive power accounts for 25% of a railroad's operating expenses, and AC ( or sticking with DC) can provide a mere 5% improvement in overall motive power expenses -- only 1.25% of all operating expenses -- it goes right to the bottom line and that changes that railroad's profitability by 1.25%, from 7% to 8.25%. A single management decision that could produce that kind of a change in profitability is a very large reason, not a small one, as it would represent an 18% increase in profitability over your competitor who did not make that decision.In the real world, that's not small.
Lots of what ifs in that argument. BTW please show me a railroad that has 25% of its operating budget tied up in the purchase price of locomotives.
MichaelSol wrote:It's an industry formula, it's not "my" calculation. There "had" to be a flaw? Well, what is it?
Nobody said there's a flaw in the formula-- the formula is just a slightly-modified restatement of the definition of the horsepower, and can't be far wrong. It's your calculation-- your misuse of the formula-- that's gone off the deep end. When you said
"It takes 8 [SD40-2] locomotives to keep the minimum speed of 11 mph on that [1.2%] grade and curvature....At 2.5 mph, one AC locomotive could still keep the train moving if it didn't slip."
I was naturally incredulous that you could believe such a thing. Now you're demanding test results that disprove your calculation-- as if tests existed that support your calculation. None do, and none ever will. (Unfortunately, no test will ever disprove it either, since nobody would bother arranging such a test. No need.)
I think that I can explain the problem with the TE formula.
When the industry began applying the formula to AC-traction units, the figures stayed basically the same (EMD, for some reason, began to use 323 instead of 375); however it appears to me that the purpose of the formula changed. It ceased being used to compute TE for any speed and began to be used to compute (or maybe a better term would be "illustrate") something called "nominal continuous TE". This is the amount of TE that an AC-traction unit can be relied upon to produce under any rail conditions. In other words, if a railroad wanted to know what dispatching criteria it should apply to a given AC-traction model, the formula could explain that the unit would produce at least X amount of TE and would be moving at Y speed when doing so.
In other words, "nominal continuous speed" is the AC-traction counterpart to the DC-traction "minimum continuous speed". The former means the speed at which an AC-traction unit produces the maximum TE that it can be relied upon to produce, regardless of rail conditions. The latter means the speed which a DC-traction unit must maintain in order to avoid derating because of thermal limitations.
So the formula is basically a carryover from the DC-traction-only era; and it can't be applied as broadly to AC-traction units as it was to DC-traction units.
n012944 wrote: Lots of what ifs in that argument. BTW please show me a railroad that has 25% of its operating budget tied up in the purchase price of locomotives.
What is with some of you people today. Now, this changes the argument considerably. Previously, AC was alleged superior because of a combination of purchase price and lower maintenance -- that the operating benefits -- which goes to a variety of costs -- and maintenance -- which isn't purchase price -- and crew costs -- slower movement of trains -- fuel cost -- outweighs or exacerbates the higher purchase price of the units, but perhaps less, in combination, than the cost of equivalent DC power. All of which goes into the operating ratio.
Now, if the argument is just about purchase price alone. No, I doubt any railroad spends 25% of its operating budget on the purchase price alone, but that wasn't the proposition.
Tell you what, you show me how locomotives -- purchase, finance, maintenance, operating implications -- "have little to do with the profits of said railroads" or any other railroad.
Show how little it is and offer a basis for your contention.
timz wrote: MichaelSol wrote:It's an industry formula, it's not "my" calculation. There "had" to be a flaw? Well, what is it?Nobody said there's a flaw in the formula-- the formula is just a slightly-modified restatement of the definition of the horsepower
Nobody said there's a flaw in the formula-- the formula is just a slightly-modified restatement of the definition of the horsepower
Timz is exactly right - It is based on the definition power (actually drawbar horsepower) derived straight from mechanics.
Power = force x velocity = TE x velocity
In consistent conventional English units
Power = TE in pounds x velocity in ft/sec which yields power in ft-lb/sec.
Express velocity in mph by multiplying by 5280 ft/mile/3600 sec/hr
Convert power in ft-lb/sec to HP by dividing by 550 ft-lb/sec/HP
This results in
HP = TE x V x 5280/(3600 x 550)
or HP =TE x V/375
solving for TE yields
TE=375*HP/V
It represents the IDEAL tractive effort vs speed behavior for a given drawbar horsepower. As others have pointed out it can't be used to characterize actual locomotive tractive effort behavior at low speeds because of the characteristic of derating power below a critical speed to prevent wheel slip and/or traction motor overheating.
timz wrote: MichaelSol wrote:It's an industry formula, it's not "my" calculation. There "had" to be a flaw? Well, what is it?Nobody said there's a flaw in the formula-- the formula is just a slightly-modified restatement of the definition of the horsepower, and can't be far wrong. It's your calculation-- your misuse of the formula-- that's gone off the deep end. When you said"It takes 8 [SD40-2] locomotives to keep the minimum speed of 11 mph on that [1.2%] grade and curvature....At 2.5 mph, one AC locomotive could still keep the train moving if it didn't slip."I was naturally incredulous that you could believe such a thing. Now you're demanding test results that disprove your calculation-- as if tests existed that support your calculation. None do, and none ever will. (Unfortunately, no test will ever disprove it either, since nobody would bother arranging such a test. No need.)
"Misuse," "incredulous," "deep end". Wow! It took you what, how many posts to finally get it off your chest?
Why didn't you just disagree?
The fact is TE is a measure. As TE drops off, does it take more locomotive hp to supply the same TE? Yes, it does.
All the way across the curve.
Whether a locomotive not limited by its software can generate the same or more TE than several locomotives at five or six times the speed is not remarkable. I did not invent the TE curve, nor "misuse" it nor go off "the deep end" by using it within the published range of its use.
And you should be no more "incredulous" about it at two miles an hour compared to 11, or 12 miles an hour compared to 70, particularly when I said it "could" generate the TE if it didn't "slip" -- i.e. didn't have the weight to keep the wheels down or didn't fly apart from the stress.
And there is no reasonable basis to dispute it at 2 miles an hour, as at 12, or 20, or 40.
However, as many posts now suggest, you could have stated it in one post, instead of making it a little playground game by pretending you couldn't do the math yourself, and stringing this along for whatever people like you get out of this stuff.
AnthonyV wrote: Timz is exactly right -...TE=375*HP/V
Timz is exactly right -
Actually, you're dead wrong. He didn't say that. I did. Exactly in that form. Indeed, he claimed it was my "calculation," and refused to take any credit for it at all.
My reason for offering the equation is that it is the basis of the conversation and explains the TE curves, which I also posted.
If there is some point in jumping in to belabor the obvious, please explain that point.
As I pointed out as well, slip and overheating are the problems at low speeds, and notwithstanding my "example" specifically designed to show the TE advantages of AC at slow speeds compared to DC which doesn't operate well at low speeds because of overheating, and I specifically referenced the slip making the slowest speeds unlikely, however, by simply ignoring what I said, this is degenerating into the usual dogs chasing their tails trying to make something out of it.
If someone wants to show that overheating is a problem for DC at slower speeds, or that slip (or software restrictions) might make high TE unlikely at very slow speeds, well have at it. I've already said it. If someone wants to actually read what I said, it's already there. Somebody wants to play games if they want to suggest I did not qualify those remarks.
However, the high TE available at low speeds offers AC a comparative, not a total, advantage over the DC motors. And I stand by the statements. The point was to suggest how much of an advantage is gained by operating below 11 mph, or wherever a DC motor might overheat to the point that it shuts down. And that is because TE is higher at slower speeds, and the TE curve is relatively steep below 10 mph.
Timz and Anthony V. can play games with that all next week or all eternity for all I care, since that seems to be the point, the only point, of what are becoming neverending posts on the topic and somewhat symptomatic of certain posters attempting to misrepresent an "example" to prove some elusive point that is apparently personal.
MichaelSol wrote: AnthonyV wrote: Timz is exactly right -...TE=375*HP/VActually, you're dead wrong. He didn't say that. I stated that equation. Indeed, he claimed it was my "calculation," and refused to take any credit for it at all. My reason for offering the equation is that it is the basis of the conversation and explains the TE curves, which I also posted.If there is some point in jumping in to belabor the obvious, please explain that point.As I pointed out as well, slip and overheating are the problems at low speeds, and notwithstanding my "example" specifically designed to show the TE advantages of AC at slow speeds compared to DC which doesn't operate well at low speeds because of overheating, and I specifically referenced the slip making the slowest speeds unlikely, however, by simply ignoring what I said, this is degenerating into the usual dogs chasing their tails trying to make something out of it. If someone wants to show that overheating is a problem for DC at slower speeds, or that slip (or software restrictions) might make high TE unlikely at very slow speeds, well have at it. I've already said it. If someone wants to actually read what I said, it's already there. Somebody wants to play games if they want to suggest I did not qualify those remarks. However, the high TE available at low speeds offers AC a comparative, not a total, advantage over the DC motors. And I stand by the statements. The point was to suggest how much of an advantage is gained by operating below 11 mph, or wherever a DC motor might overheat to the point that it shuts down. And that is because TE is higher at slower speeds, and the TE curve is relatively steep below 10 mph.Timz and Anthony V. can play games with that all next week or all eternity for all I care, since that seems to be the point, the only point, of what are becoming neverending posts on the topic and somewhat symptomatic of certain posters attempting to misrepresent an "example" to prove some elusive point that is apparently personal.
Actually, you're dead wrong. He didn't say that. I stated that equation. Indeed, he claimed it was my "calculation," and refused to take any credit for it at all.
It is pointless to use the idealized TE equation as you did - it doesn't apply.
AnthonyV wrote: It is pointless to use the idealized TE equation as you did - it doesn't apply.
And how do you actually know this?
Please, be specific: why does the TE equation accepted by the industry give the wrong results at 2.5 mph? How about 4 mph? What about 6? Where do you think the curve misrepresents reality, and why that specific point? Given the misrepresentations you make above, even about who posted the TE equation, I expect you to give a specific answer. Particularly since I am relying on an accepted methodology, and you are the one objecting to it. Ball's in your court: what is wrong with the standard TE curve generated by the rail industry and generally agreed on by it? Again, be specific. You could break new ground here and show the industry has been wrong for ... how many years?
Indeed, you laboriously derived the equation to show why it is correct ["timz is correct"] and identical to the one I posted, yet you say the results obtained by that same equation are wrong, but didn't offer that "timz is wrong" based on that same equation using an actual number -- in this case 2.5 mph.
Well, which is it? Why bother to go through all that math, and then say it isn't correct? Why bother to triumphantly announce how the equation is derived, then argue that it doesn't work with some numbers, but works with others? Is your derivation wrong? Give us all the benefit of deriving the correct equation instead. Why do you refuse to do that?
And since I used it as an "example" of why AC has advantages below 10 mph, what is your current investment in this conversation? Do you disagree that AC has those advantages? Or that the example, idealized or otherwise, represents those advantages? Or are you trying to claim something else?
If I had the time, I would go back and find an early AC test, in which the locomotive was chugging up the grade, grossly overloaded, at a few inches at a time. The "test" that timz claims hasn't, can't, be done, was actually done, and ... probably many times. Are there mechanical or engineering limits? You betcha. That's not the curve's fault, and doesn't change it one bit. Recall, that was a "hypothetical" and plainly identified as such.
But, that tells me what he thinks he knows.
The TE was in fact measured at the rate representing an extremely slow speed. It may have been reported in Modern Railroads, it may have even been in Trains. However, if I took the time to find it, I know exactly what you would do with it -- you would change the subject, and pretend that something else was said. There is no point in wasting the time with characters that are here just to argue -- particularly when they misrepresent statements, and even contradict themselves, to do so.
Another thread ....
The key words seem to be "if it doesn't slip." If I were to say also, "and if it doesn't overheat," then the same curve would apply to the DC motors. With both caveats in place, both motors would achieve the same TE at equivalent speeds.
I think we should all agree that it will slip at those low speeds and we should get back to the key questions, which I think are:
Why, in current locomotive practice, do AC traction motors appear to have better low-speed performance than DC traction motors?
Why, in the face of all the information in previous posts indicating that AC traction and control is better than DC, do some railroads continue to buy and use locomotives with DC motors?
Why is cooling an issue? It seems that designers should be able to provide whatever cooling DC motors need.
Why have designers of locomotives with DC motors not provided slip control independently to each axle, which appears to be common, if not universal, practice among locomotives with AC motors?
Most of this discussion has been about low-speed performance. How do the two approaches compare at medium and high speeds?
How do they compare in dynamic braking?
I have spent several hours without success trying to find some illuminating information. I think there must be something we haven't discussed yet. Otherwise, it seems that no one should be buying DC motors.
cordon wrote:Why have designers of locomotives with DC motors not provided slip control independently to each axle, which appears to be common, if not universal, practice among locomotives with AC motors?
I suspect -- but don't really know -- that configuring DC-traction units with single-axle control wouldn't be cost effective; however U.S. Patent 6,634,303 does relate to that concept.
EMD AC-traction units do not have single-axle control either. That's a GE-only feature.
cordon wrote:The key words seem to be "if it doesn't slip." If I were to say also, "and if it doesn't overheat," then the same curve would apply to the DC motors. With both caveats in place, both motors would achieve the same TE at equivalent speeds.I think we should all agree that it will slip at those low speeds and we should get back to the key questions, which I think are: Why, in current locomotive practice, do AC traction motors appear to have better low-speed performance than DC traction motors? Why, in the face of all the information in previous posts indicating that AC traction and control is better than DC, do some railroads continue to buy and use locomotives with DC motors?Why is cooling an issue? It seems that designers should be able to provide whatever cooling DC motors need. Why have designers of locomotives with DC motors not provided slip control independently to each axle, which appears to be common, if not universal, practice among locomotives with AC motors?Most of this discussion has been about low-speed performance. How do the two approaches compare at medium and high speeds?How do they compare in dynamic braking? I have spent several hours without success trying to find some illuminating information. I think there must be something we haven't discussed yet. Otherwise, it seems that no one should be buying DC motors.
cordon wrote:How do they compare in dynamic braking? ...Why is cooling an issue? It seems that designers should be able to provide whatever cooling DC motors need. Why have designers of locomotives with DC motors not provided slip control independently to each axle, which appears to be common, if not universal, practice among locomotives with AC motors?
How do they compare in dynamic braking? ...
It might have been another thread, but I believe somewhere above in this thread a statement was made that Dynamic Braking on AC units is actually effective down to slower speeds than DC units. I dont recall reading anything about amount of breaking effort...I would guess it is at least as effective as DC, since you can change the control voltages to increase or decrease the amount of effort...
Individual axle control and cooling are possible, just send money. The relative simplicity of the DC control system is part of why they are less expensive. If you start adding complexity in order to acheive the advantage of the AC units, the cost will go up.
Didnt SP have Tunnel Motors in order to get cooling air to the traction motors in tunnels? If it were wildly successfull, wouldnt it have appeared in other and later generation DC units? So getting cooling air to the DC traction motors may not be an inexpensive proposition...it would be tough to get big fans and duct work down to the motors, and a refrigeration system adds complexity and cost.
And when you spend that money, you may find that the new DC units which exhibit the same slow speed pulling characteristics as the AC units are just as expensive...
Without getting into formulae or curves, my own understanding of the situation (and I have an SB and SM degrees from MIT and worked for a short time for EMD IN 1952) is that AC motors can provide higher continuous low-speed tractive effort by handling higher current because the nature of the current is not continuous, the coils and their insulation are not subject to steady heat but to intermittant heat. The statement that dc motors could be supplied with all the cooling necessary would be correct if there were actually liguid cooling associated with the motors, but the motor mechanical features are pretty much unchanged from Frank Sprague's original Richmond, Virginia, streetcars (1886-7), with one side of the motor suspended from the truck frame and the other side supported by the gear or gears meshing with gear or gears on the axle. These motors are "self ventilating" with built-in fans, and no external cooling is provided on the typical locomotive. So obviously, the amount of cooling air supplied is directly dependent on the speed of revolution of the motor. Supplimentary fans are employed, but there is still a limitation on the amount of air that can be blown passed the motor case and be effective, since heat transfer through the metal from the coil isn't instantanous or perfect.
Three phase motors have non-continous current in the coils, but the nature of sine waves and thus the nature of three phase power is that the power of the motor is continuous, with the peak power of one phases occuring when the other two combine at a minimum. So you don't get the AC motor sound that was familiar to all who rode Pennsy and New Haven commuter trains or GG-1'S and New Haven electrics, other than the Jets and ex-Virginians which had dc motors.
There is one other advantage of ac three-phase non-synchronouos hysterisis motors: No carbon brushes to wear out and require replacement.
...Dave: Do I understand you correctly, stating a traction motor has no external cooling fans....? I personally don't know but thought they were set up with high speed fans to provide cooling air thru duct work to keep temps under control....??
http://www.conrailcorp.com/mssd80mac.html
Here is an interesting article about Conrails SD80MACs. There is an interesting part explaining how the AC locomotives will save the company money.
The first graphic in that link is pretty interesting, it detailed some data collection that was envisioned as part of the test....would love to see that!
Technology Review (Cambridge, Mass.), March 2002 v105 i2 p74(5)
Digital rail-road: An injection of computing power is turning trains into smart machines for precisely controlled hauling of heavy loads.
By Don Phillips.
No one had intended to make railroad history on May 5,1998. It's just that there was a shortage of locomotives in Phippsburg, CO. Instead of the usual five locomotives, only four were available to pull a 108-car coal train up Union Pacific Railroad's steep Toponas grade on the western slope of the Rocky Mountains. What followed is, among locomotive builders, legendary.
The locomotives were brand new General Electric behemoths with a twist: their traction motors operated on alternating current rather than direct current. Climbing the Toponas grade that day, the trains slowed to a barely perceptible six meters per minute. No self-respecting engineer would have tried such a foolhardy trick with conventional direct-current motors: wheels would have slipped, the train would have stalled, and the motors themselves would have been fried like an egg. But none of those things happened. Indeed, later investigation showed that the locomotives had been producing more pulling power than was thought possible at that speed. This feat of strength initiated a radical transformation of railroading--a revolution that stems directly from advances in information technology.
[excerpt]...
Now, the speed measured here was 0.22 mph -- substantially less than the 2.5 mph that for some reason became an issue here. Well, apparently these locomotives can move at that speed -- this train was moving at that speed because that's where it could develop enough tractive effort to keep moving. It could not go faster because, according to the standard TE curve, the locomotives would have lost tractive effort, it's tractive effort would have fallen below that necessary to overcome the resistance of journals, flange, grade and curvature -- and the train would have stalled.
The train had to operate at 0.22 mph because that is the point on the TE curve where it could develop enough TE to move.
It was neither "pointless" nor "idealized" on that particular day.
MichaelSol wrote: Technology Review (Cambridge, Mass.), March 2002 v105 i2 p74(5) Digital rail-road: An injection of computing power is turning trains into smart machines for precisely controlled hauling of heavy loads. By Don Phillips. No one had intended to make railroad history on May 5,1998. It's just that there was a shortage of locomotives in Phippsburg, CO. Instead of the usual five locomotives, only four were available to pull a 108-car coal train up Union Pacific Railroad's steep Toponas grade on the western slope of the Rocky Mountains. What followed is, among locomotive builders, legendary. The locomotives were brand new General Electric behemoths with a twist: their traction motors operated on alternating current rather than direct current. Climbing the Toponas grade that day, the trains slowed to a barely perceptible six meters per minute. No self-respecting engineer would have tried such a foolhardy trick with conventional direct-current motors: wheels would have slipped, the train would have stalled, and the motors themselves would have been fried like an egg. But none of those things happened. Indeed, later investigation showed that the locomotives had been producing more pulling power than was thought possible at that speed. This feat of strength initiated a radical transformation of railroading--a revolution that stems directly from advances in information technology. [excerpt]...Now, the speed measured here was 0.22 mph -- substantially less than the 2.5 mph that for some reason became an issue here. Well, apparently these locomotives can move at that speed -- this train was moving at that speed because that's where it could develop enough tractive effort to keep moving. It could not go faster because, according to the standard TE curve, the locomotives would have lost tractive effort, it's tractive effort would have fallen below that necessary to overcome the resistance of journals, flange, grade and curvature -- and the train would have stallThe train had to operate at 0.22 mph because that is the point on the TE curve where it could develop enough TE to move. It was neither "pointless" nor "idealized" on that particular day.
Now, the speed measured here was 0.22 mph -- substantially less than the 2.5 mph that for some reason became an issue here. Well, apparently these locomotives can move at that speed -- this train was moving at that speed because that's where it could develop enough tractive effort to keep moving. It could not go faster because, according to the standard TE curve, the locomotives would have lost tractive effort, it's tractive effort would have fallen below that necessary to overcome the resistance of journals, flange, grade and curvature -- and the train would have stall
I notice you have not presented any data for tractive effort actually produced by these locomotives at these speeds.
Your assertion that the idealized formula applies indicates that you believe each locomotive (assuming 4,400 hp each) was producing 7,500,000 lb of tractive effort at 0.22 mph.
Also, the formula predicts an infinite starting tractive effort (i.e., at 0 mph).
You better get on the phone with GE because they are under the false assumption that their Evolution AC locomotive produces a starting tractive effort of about 180,000 lb and a maximum continuous tractive effort around 166,000 lb.
AnthonyV wrote:I notice you have not presented any data for tractive effort actually produced by these locomotives at these speeds.
Each of the units would have been producing 180,000-or-less pounds of TE. The extent to which those TE levels approached 180,000 pounds would have depended on the levels of adhesion that rail conditions allowed the units to produce.
AnthonyV wrote: I notice you have not presented any data for tractive effort actually produced by these locomotives at these speeds.
Anthony, you are going to "notice" things until you turn blue. And not answer a single question posed to you in the earlier post. Let's look at this: "Your assertion that the idealized formula applies indicates that you believe each locomotive (assuming 4,400 hp each) was producing 7,500,000 lb of tractive effort at 0.22 mph."
First, we would need to know how much TE was necessary to move the train. We would need to know the resistance: the grade, the curvature, and the tonnage of the train. The article does not provide that information, and so I can't generate the numbers. But, I do ask that you cease saying that I am making "an assertion." I pointed out that a generally accepted TE curve exists. I didn't create it. Get that through your head. I also applied the TE curve to a "hypothetical" AC locomotive to show the potential of AC traction motors -- and immediately qualified that hypothetical as unlikely. Get that through your head.
I don't "believe" each locomotive was producing 7.5 million lbs of TE. I don't even particularly care what each locomotive was producing. This bizarre argument has already gone on far too long, and is representative of what is wrong with these forums.
I believe that the locomotives were producing enough to move the train. TE is indeed a function of speed. After I posted the accepted formula, you then spent the time to undertake a derivation, arrived at exactly the same formula, pronounced that it vindicated timz -- "timz is right" -- and then denounced the results of the formula you had just arrived at. What was that all about?
I really don't follow the contradictions in whatever it is you are getting at, but you are making a career out of it, just as you did with the Brown thread and several others. The TE curve has long been accepted by the industry. You don't like the results. You can't seem to explain why. I do accept that more tractive effort is generated at lower speeds, and that this provides an advantage for AC. I am not going to lose sleep over the fact that the principle underlying the curve is a sound one, and I will leave it to better engineers to come up with a better equation if there is one.
You haven't.
The equation is clearly purely physics. There are mechanical limitations as Jay Potter pointed out. I pointed out that I doubted a single unit would produce the curve TE because of "slip" which was my inarticulate description of whatever happens when a machine reaches the point that it cannot physically produce the work product predicted by theory. I should have included drawbars yanked and door handles flying off to boot, had I realized at the time that my hypothetical had to be rigorously defined in case someone decided it was worth 20 or 30 posts to nitpick. For some reason you can't seem to accept that I distinguished what the curve predicted from what would happen in the real world, and I did so in the single reference that I made to a low speed use of the TE curve -- as a cautionary about that end of the curve.
I think it was an intentional misreading, allowing you to continue these endless nitpicking sessions of self-contradictory posts. That's the problem with guys playing "gotcha" -- they don't read too well.
"You better get on the phone with GE because they are under the false assumption that their Evolution AC locomotive produces a starting tractive effort of about 180,000 lb and a maximum continuous tractive effort around 166,000 lb."
Get on the phone yourself. I understand the GEVO is software limited. Place the limitation on the TE curve and you can have any starting tractive effort you want if the machine will support it. Pick a spot. The question you might as is why there is a software limitation.
Because those motors want to generate more TE than the physical framework will safely permit. DC motors tried to do the same thing and overheated -- and shut down or burned up. Might be because those motors want to follow that curve. Ya think? So GEVO has a software limitation. Does that mean the curve is wrong? No. It means that engineers cannot economically harness the lower end of that curve nor is there a particular need to.
However, since I already pretty much said that in my initial post on the topic, these endless efforts of yours to prove the curve wrong are misguided. The technology simply can't/hasn't/doesn't need to reach the TE potential offered by low speed operation. The advantage of the AC motor is that it exploits the low speed TE in a fashion that the DC motors could not. And as the TE curve shows, there is an enormous potential there to exploit.
Now, there must be something you can put your good mind to that is more productive than beating a horse that was already dead -- because the remark concerning the single unit AC locomotive had already been qualified at the outset as improbable, but offered an intriguing insight into the possibilities of AC compared with DC traction motors.
AnthonyV wrote:Also, the formula predicts an infinite starting tractive effort (i.e., at 0 mph).
The speed is the denominator or divisor in the equation. Formulas that divide by zero are undefined operations. It does not predict "infinity." It predicts nothing.
MichaelSol wrote: I don't even particularly care what each locomotive was producing.
I don't even particularly care what each locomotive was producing.
Of course you don't.
AnthonyV wrote: MichaelSol wrote: I don't even particularly care what each locomotive was producing.Of course you don't.Anthony V.
And, as this post finally shows, for you this whole exchange wasn't really about TE either ... otherwise you would not have had to misrepresent my remarks in the first instance, in order to create a spurious and superficial issue for you to argue about.
MichaelSol wrote:[quoting the article]...on May 5,1998. It's just that there was a shortage of locomotives in Phippsburg, CO. Instead of the usual five locomotives, only four were available to pull a 108-car coal train up Union Pacific Railroad's steep Toponas grade on the western slope of the Rocky Mountains. What followed is, among locomotive builders, legendary.
Thanks for looking that up. I gather you hope it supports your earlier claim, which needs repeating:
It takes 8 [SD40-2] locomotives to keep the minimum speed of 11 mph on that [1.2%] grade and curvature....At 2.5 mph, one AC locomotive could still keep the train moving if it didn't slip.The Toponas article will indeed support that claim, as long as the grade there is around 7%. Anybody happen to remember if it is?
The Toponas article will indeed support that claim, as long as the grade there is around 7%. Anybody happen to remember if it is?
selector wrote:Yup, they don't call it an "irrational" number fer nuthin'.
ya but....
an irrational number is not obtained by attempting to divide by zero...
an irrational number is a number that cannot be exactly expressed as a fraction...(such as Pi)
Irrational Number
an irrational number might also have something to do with some of the posts in this thread ...
timz wrote: Thanks for looking that up. I gather you hope it supports your earlier claim, which needs repeating: It takes 8 [SD40-2] locomotives to keep the minimum speed of 11 mph on that [1.2%] grade and curvature....At 2.5 mph, one AC locomotive could still keep the train moving if it didn't slip.The Toponas article will indeed support that claim, as long as the grade there is around 7%. Anybody happen to remember if it is?
"Could" and "if" are the operative words. And they mean it couldn't happen. I wish I had realized earlier that this is some weird agenda for you and not an honest discussion. I would, indeed, have taken more time to precisely explain what was intended to be a hypothetical demonstrating the potential extremes available to AC motors regarding TE.
Note my post that the GE AC units resemble steam engines -- a lower limit on TE. Look at the steam engine TE curve I posted. You will see exactly what I mean, and if you are able to absorb a context -- that's it. You will see a lower limit on the TE curve put there by engineering limitations implemented by software restrictions. It's just not as hard to figure out as you are making it.
This has become a real obsession with you two, hasn't it?
As I said earlier, if you disagree, it might be easier to just say so and join the adults instead of stringing this out to endless posts attempting, oh so hard, to misconstrue what I said by carefully avoiding both content and context. By that method, I am sure you will get what you seek. However, why don't you leave the rest of us to an honest discussion of an interesting topic.
JSGreen wrote:It might have been another thread, but I believe somewhere above in this thread a statement was made that Dynamic Braking on AC units is actually effective down to slower speeds than DC units. I dont recall reading anything about amount of breaking effort...I would guess it is at least as effective as DC, since you can change the control voltages to increase or decrease the amount of effort...
It is my understanding as well that dynamic braking works ar lower speeds on AC units than on DC units - though somewhere around 1 mph no net power will be produced by the motors and the heat will be disapated in the squirrel cage of the motor. It should be possible to make a chopper control system for a DC locomotive to allow dynamic braking at lower speeds (the chopper acts to step up the voltage from the motor) - this freature is inherent with the variable-voltage-variable-frequency inverters on the AC locomotives.
I also suspect that AC braking effort is greater than DC braking due to the stiffer speed/torque characteristics of the AC motor.
daveklepper wrote:Without getting into formulae or curves, my own understanding of the situation (and I have an SB and SM degrees from MIT and worked for a short time for EMD IN 1952) is that AC motors can provide higher continuous low-speed tractive effort by handling higher current because the nature of the current is not continuous, the coils and their insulation are not subject to steady heat but to intermittant heat.
Without getting into formulae or curves, my own understanding of the situation (and I have an SB and SM degrees from MIT and worked for a short time for EMD IN 1952) is that AC motors can provide higher continuous low-speed tractive effort by handling higher current because the nature of the current is not continuous, the coils and their insulation are not subject to steady heat but to intermittant heat.
I was thinking along similar lines wrt AC vs DC motors. The rotor windings on a DC motor are only active for a fraction of the time and are not sized to continuously handle the rated motor current. At high speeds, the "on" pulse is short enough that the heating can be averaged over the whole on/off cycle for that winding. At low speeds, the "on" pulse may approach or exceed the thermal time constant of the winding - leading to burning out the winding at lower currents than at high speeds.
FWIW, I have a BSEE and an MSNE from UC Berkeley - the EE course work included an electrical machinery class.
MichaelSol wrote: AnthonyV wrote:Also, the formula predicts an infinite starting tractive effort (i.e., at 0 mph). The speed is the denominator or divisor in the equation. Formulas that divide by zero are undefined operations. It does not predict "infinity." It predicts nothing.
Folks, no one would ever really try to divide by zero. What the writer meant was that the value of the tractive effort in the formula grows large without bound as one makes the speed approach zero. Stating it in this way avoids the "divide by zero" issue while still making the point that one cannot apply the formula by itself to a real machine without some other condition.
We all understand that. We all understand that we don't actually divide by zero. Because of that, it neither meaningful nor helpful to comment that the formula does not predict infinity.
By the same token, it is equally non-meaningful and not helpful to make the comment that "the formula predicts an infinite starting tractive effort (i.e., at 0 mph)."
Please, let's stop the picking and get back to trying to find answers.
cordon wrote:By the same token, it is equally non-meaningful and not helpful to make the comment that "the formula predicts an infinite starting tractive effort (i.e., at 0 mph)."
The moral of the story was, live by the sword, die by the sword. If you think you are good enough to distort an avowed hypothetical for personal reasons -- you can't go around demonstrating mathematical ignorance for all to see -- and expect to have credibility.
These two characters trying to play "gotcha" by distorting statements and content are not good enough to not set themselves up for gotcha ... especially when they make egregious, basic math errors, coupled with labored distortions of someone's remarks to try and force an awkward crtique. It's the kind of attitude that has taken these threads down before -- some folks don't get it. As I mentioned -- how many posts ago? -- there is an adult, professional way to disagree. These guys ain't it ... and, as I also pointed out, this is an interesting topic -- and remarkably unenlightened by timz's game playing or AVs snide remarks.
Just agree to disagree, guys, and then we can all discuss AC motors ...
Sorry, I did say supplementary fans are employed, but what I meant is there is no external liquid cooling (as is used on some European light rail and diesel electric and electic bus wheel motors). Yes, there are car-body supplemental fans.
I made one other mistake. The real reason there is less overheating with ac motors, in addition to the non-steady interruption of the current that I did mention, is that dc motors employ field coils and rotating armature coils, which are the coils receiving electricity via commutators and carbon brushes, the latter needing regular replacement. The non-synchornous ac hysterises motors, whether three-phase as on modern locomotives, or single-phase as on certain consumer products, have no armature coils. Instead, they have slanted aluminum or copper bars, in which current is set up by the field coils and then reacted to magnetically by the computer controlled changes in which coils are powered and the polarity. Obviously, cooling armature coils within the case, rotating on the shaft, is more difficult than cooling the field coils, which are the only coils on the type of ac motors used. The slanted bars can get hot as blazes without damage.
These motors are similar in construction to the familiar ac synchronous induction motor, used on most home applliances these days, except the bars are slanted, not straight across, and there are many many more poles and coils around the circumferance of the motor.
This type of motor was made possible only with computer technology development and electronic control of current voltage, polarity, and phase (time). The Amtrak AEM-7 pioneered it on USA railroads and then it was applied to diesels. And DC locomotives today still have AC generators in general and then rectify the current to dc to save on generator brush wear. Current from the ac three-phase generator comes off of slip rings, where there is constant contact between the ring and the "brush", rather than segments with insulartion as on dc commutators in dc generators and motors (and ac commutator motors as on New Haven, PRR, and Great Northen electrics.)
daveklepper wrote: The real reason there is less overheating with ac motors, in addition to the non-steady interruption of the current that I did mention, is that dc motors employ field coils and rotating armature coils, which are the coils receiving electricity via commutators and carbon brushes, the latter needing regular replacement.
I understand that there are "brushless" DC motors. Do they have any potential application here?
I am not familiar with any used on locomotives.
There are inside-out variations of both dc motors and the ac non-synchronouse hysterises motor. These are called wheel-motors or hub-motors. The largest manufacturer is Magnet Motor of Germany and these are widely applied on Swiss and other European trolleybuses, dual-mode buses, diesel electric buses, and airport diesel electric movers, all low floor. In addition, Stadler uses some (perhaps made by Brown Brovori under liscense) on rail cars of several types. Rotatling permanent magnets, like the bars of the ac hysterisis motor revolve around computer controlled current fixed armature coils attached to a fixed axle. These don't require brushes because the armature is fixed and the field rotates! I was the first to think up the same approach (1998) to the ac non-synchronous motors, and Alstom uses the idea without paying me a dime. Never applied for a patent. Used on the Max Line in Las Vages (guided diesel electric buses). Didn't know about the dc idea when publishing the idea, but Posche apparently thought up the dc brushless motor before WWII. Anoother manufacture of the dc brushless is Stored Enerngy Systems of Derby England which markets them in the the Rail Cat, a clever tiny remote controlled shop switcher with storage battery power. Use by many British and European light rail and rapid transit systems, possibly now some railroads. Want to enter the light rail market, particularly "Ultra Light Rail."
MichaelSol wrote:"Could" and "if" are the operative words. And they mean it couldn't happen.
Finally we've got that straightened out.
Good. I just "noticed," if I may use the word, that the person who drew the curves did not extend the curves for Diesel engines below about one mile per hour (MPH). We can be sure that that's not an editorial error because the curves for the steam engine go all the way to zero MPH.
The brushless DC motor is actually a solid-state-circuit-controlled synchronous AC motor. It is "DC" in the sense that the user supplies it with DC, but inside it creates and uses polarity-reversing pulses of current to drive the armature. The armature most commonly has a constant magnetic field created by permanent magnets, but there are cases of wound-field armatures. These can receive current either through brushes and slip rings, in which case it is not really brushless, or through a small alternator/rectifier mounted on the shaft.
So, the brushless DC motor and the solid-state-controlled AC motor are sort of cousins.
The common industrial AC induction motor, as pointed out above, usually has a very robust armature that can get very hot without damage, and, as Anthony pointed out way back on Page 3, the key feature of the induction motor is that it doesn't spin up to high speed when it slips. This may be the critical difference.
Even though a control system may be possible for a DC motor that provides similar performance, the worse slip characteristics of the DC motor may make such a control system more expensive. This is a supposition on my part. I would feel more comfortable if someone could verify it.
cordon wrote: Good. I just "noticed," if I may use the word, that the person who drew the curves did not extend the curves for Diesel engines below about one mile per hour (MPH). We can be sure that that's not an editorial error because the curves for the steam engine go all the way to zero MPH.
Nice catch. Looking at it more closely, it looks like the curve ends about 2 mph, perhaps 2.5. The relationship is otherwise reasonably linear until down to about 2.5 mph. It looks as though below 2.5 mph the curve begins to change radically, and the numbers do indeed become incomprehensible -- particularly since every train moves through those numbers every trip, DC and AC alike. Reaching below 2.5 mph, to 0.1 mph for instance, to show how absurd the numbers are, indeed, "goes off the deep end" since the relationship breaks down and it is, literally, "off the chart" and, like dividing by zero, has no definition at all. As you point out, the chart actually shows that.
Makes sense.
Click on the graphic to enlarge.
I suppose DC motors have the ability to use different types of windings, series or parallel for example to multiply torque like a car transmission, but in every other respect the AC would have to be superior. It wouldn't "wind up" so much in a sudden loss of traction. Less parts that "wear". The motor itself might be more efficient but factor in the control circuits and it may not - no idea how efficient they are. They could be smaller - the 40HP, 400-cycle motors that run air cycle machines in transport aircraft are about the size of a 32 OZ beer can. Then again, the inverter circuits may not be THAT complex. You speed up the diesel, up goes the cyclic rate of AC, speed up the cyclic rate, up can go the motor HP because it will keep it's torque at a higher speed. That's power. I wonder what kind of capacitance those rectifiers have, must be big-time. Could be dangerous. 600 or more volts, tons of current, might throw a spark the width of the train. Talk about Grand Funk Railroad (no apologies).
Just thoughts. I don't know much about locomotives.
I believe cordon has the best explanation of the differences in motors and explanation for the ruggedness of the induction motor.
I also believe the AC traction motor is a good-ol-3-phase induction motor, also called a squirrel-cage motor on account of the geometry of the copper bars running through the iron rotor. There is a reluctance motor, working on the principle of a solenoid where a piece of iron exerts a force to line up with a magnetic path -- don't know what a hysteresis motor is -- but the traction motor works on the induction motor principle instead where the magnetic field induces electric current in the copper bars embedded in the rotor. The explanation that the rotor has no brushes or slip rings and is just a solid iron rod with bars of copper embedded in it, that such a rotor can take much higher thermal loads than the rotor of a DC motor that requires thin layers of insulation to hold up, that explanation makes more sense than pulses vs constant currents.
There is no reason that a DC motor can't have an electronic control system to reduce wheel slip, but it also makes sense that the AC motor is easier to control because the frequency of the rotating magnetic field needs to be only a little higher than the wheel rotation rate to get torque while the DC motor needs much higher voltage than the back EMF voltage to get high torque and will have a greater tendency to "run away" once the wheels break free.
As to the torque curve, the model has torque HP-limited above a certain speed and adhesion limited below that speed. The adhesion may not be flat -- there may be better grip as you get down to ultra-low speeds. The train creeping at .25 MPH is probably a combination of the somewhat higher adhesion than at 2.5 MPH but also the somewhat lower train resistance at .25 MPH than 2.5 MPH, but one is still operating at the bitter edge of stalling the train completely.
Pulling power is an admirable quality in a locomotive, but using the pulling power advantage of AC to underpower trains is the real nub of the issue. One is in effect buying expensive locomotives to go slower for the advantage of saving fuel. The reduction in locomotives through reduction in horsepower is an illusory saving if you end of going slower and getting less trip cycles out of a consist and a train. So the real downside to AC is lower equipment and track utilization and tying up your mainline with drag freights creeping up your ruling grade and perhaps being underpowered to make good speed on the flats.
Some time ago when Mark Hemphill was our resident iconoclast, I offered the opinion that the move from steam to Diesels was one of underpowering trains on account of the superior lugging ability of MU Diesels to steam with weight spread over leading and trailing and tender axles, and that AC was continuing this trend of underpowering trains and resulting in sluggish rail service. I was wrong on believing John Kneiling's old advice about fuel being cheap compared to capital costs on freight cars as I was told a new coal hopper could be had for a mere $50,000 and at that price, you could run that hopper car slow to go after fuel savings. I was impressed that a new 100 ton hopper car would be that cheap -- a friend was just quoted $35,000 for a ground water heat pump for his house, but who is to argue with the former editor of Trains. I was also told that the concept of "fast freight" in the steam era was overstated and that trains crept up ruling grades as they do today.
But I think it ironic that some of the highest of high tech -- and what can be more high-tech than an AC-drive locomotive given that it relies on the latest in high-power semiconductor devices, electronic circuit and control system design, and microprocessor technology -- that the application of this high tech is to plod up the ruling grade even slower.
When I was a child, a favorite story was "The Little Engine That Could" about a railway engine that had to make it up a ruling grade. These days railroaders are all saying "I think I can, I think I can", from a crew trying to make their mileage before their hours run out, to a dispatcher trying to get traffic over a line, to a railroad CEO trying to pay a shareholder dividend, to a sales rep from GE peddling locomotives.
Yes Michael, the discussion is about tractive effort and I did not distort a word. On page three you presented results of your analysis as follows:
MichaelSol wrote: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.......By comparison, four 4400 hp AC locomotives could keep the minimum speed at 8 mph, three AC locomotives could keep it moving at 6 mph. If the railroad didn't mind poking along at 5 mph, they could get by with just two AC locomotives. At 2.5 mph, one AC locomotive could still keep the train moving if it didn't slip.
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.......
By comparison, four 4400 hp AC locomotives could keep the minimum speed at 8 mph, three AC locomotives could keep it moving at 6 mph. If the railroad didn't mind poking along at 5 mph, they could get by with just two AC locomotives. At 2.5 mph, one AC locomotive could still keep the train moving if it didn't slip.
Note that your qualifier "if it didn't slip" applies only to the 2.5 mph statement.
Applying the idealized formula as you did results in the following:
Speed Calculated TE Theoretical #Loco's
8 206,250 lb 3.2 (4)
6 275,000 lb 2.4 (3)
5 330,000 lb 2 (2)
2.5 660,000 lb 1 (1)
These calculations represent the performance of an idealized locomotive producing 4,400 drawbar horsepower. Actual locomotive performance is far from ideal below 10 to 15 mph depending on the locomotive.
From the GE website, the maximum continuous tractive effort for the GE Evolution AC locomotive is 166,000 lb. As Jay pointed out, the tractive effort curve is flat from about 10 mph and below due to the practical limitations discussed by others more knowedgeable than I am. Note that the maximum continuous tractive effort for the DC version is about 109,000 lb for speeds below about 15 mph.
The differences between tractive effort calculated using the idealized formula and actual continuous tractive effort are obvious. Any analysis of locomotive performance and economics must take this into account.
AnthonyV wrote: Yes Michael, the discussion is about tractive effort and I did not distort a word. On page three you presented results of your analysis as follows: MichaelSol wrote: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.......By comparison, four 4400 hp AC locomotives could keep the minimum speed at 8 mph, three AC locomotives could keep it moving at 6 mph. If the railroad didn't mind poking along at 5 mph, they could get by with just two AC locomotives. At 2.5 mph, one AC locomotive could still keep the train moving if it didn't slip.Note that your qualifier "if it didn't slip" applies only to the 2.5 mph statement.
"On page 3 you said ..." And this is page 8? And you are still beating this poor horse?
If you don't understand by now that I proposed a hypothetical, to demonstrate the potential of AC motors, after multiple posts stating exactly that, you never will. Exactly how long are you going to keep this going?
As I mentioned, had I anticipated that a couple of little kids would have piled on, I would have qualified it in much, much more detail. You have taken on the tone of "oh yeah, but you said, you said, you said." It is just like a little kid. There are 12 year old rail fans that show more maturity on something like this than you are able to muster. This has been personal with you on three different topics now, dating back to Steam v Diesel, inventing controversies from scratch.
As Jay pointed out, the tractive effort curve is flat from about 10 mph and below due to the practical limitations discussed by others more knowedgeable than I am. Note that the maximum continuous tractive effort for the DC version is about 109,000 lb for speeds below about 15 mph.
And in addition to math, please go back and get some physics. IF AC traction had a "flat" TE curve below 10 mph, then they could all go 10 mph right? That coal train in Colorado had no business going .22 mph -- it gained no more TE acccording to you, and so why wasn't it going 10 mph. And for DC being flat below 15, then 15 would be the minimum they would have to go, since they could develop no more TE by going slower, right? "Maximum continuous" is an equipment limitation on TE over time -- not a measure of TE at any given point in time.
The TE curves say nothing about being "continuous" ratings -- because the theory cannot possibly know how the motors are constructed. Indeed, that was the point of my comparison between hypothetical NS and UP circumstances -- the short time needs on NS might well justify the DC motors.
You misread what Jay said.
As you misread what I said.
Repeatedly.
MichaelSol wrote:You misread what Jay said.
Maybe; but I'm not sure that he did.
Anyway, I'll try a different approach.
I view the AC-traction (i.e. AC4400CW in this discussion) TE curve as being divided into two main parts. On the right is the "horsepower" segment, which has a slope. This encompasses the speed range within which TE is primarily a function of speed. In other words, if you choose a speed on the horizontal axis, the corresponding point on the curve will show, on the vertical TE axis, approximately how much TE will be produced at that speed. The other segment, to the left, is the "adhesion" segment. It's a horizontal (i.e. no slope) line at the 180K level on the vertical TE axis.
I refer to the latter part of the curve as the "adhesion" segment because, at the low speeds which it encompasses, the amount of TE that the locomotive will produce does not depend on its rate of speed but, rather, on its level of adhesion. In other words, that portion of the curve cannot be used to relate a particular rate of speed to a particular level of TE. The amount of TE produced at speeds within that segment will not exceed 180K because of the software limitation. How close it will come to 180K depends on the ability of the adhesion-management system to deal with the rail conditions. So, depending on rail conditions, the locomotive might be able to produce the full 180K at 7 mph; or it might not be able to produce the full 180K until speed drops to 2 mph; or it might not be able to produce the full 180K regardless of how low the speed drops. However, and this brings us back to one of the primary advantages of AC-traction, the locomotive can spend a whole lot of time at those low speeds producing whatever level of TE rail conditions will allow it to produce.
JayPotter wrote: I refer to the latter part of the curve as the "adhesion" segment because, at the low speeds which it encompasses, the amount of TE that the locomotive will produce does not depend on its rate of speed but, rather, on its level of adhesion.
I refer to the latter part of the curve as the "adhesion" segment because, at the low speeds which it encompasses, the amount of TE that the locomotive will produce does not depend on its rate of speed but, rather, on its level of adhesion.
Which is exactly why, on page 3 no less, even with the hypothetical of what an AC traction motor "could" do, I qualified the condition of AC TE by the specific reference to "slip" -- an adhesion characteristic at 2.5 mph that qualifies and limits what the locomotive can do at 2.5 mph -- or at slower speeds in general, and that is further qualified by drawbars and other physical limitations of the equipment.
It amazes me that I can qualify a hypothetical of what an AC motor is capable of, by a specific reference to probable adhesion limitations on TE at slow speeds, and five pages later we are still arguing that I did or didn't say such a thing, and that it even matters after repeated restatements to that effect.
It is likely, in the "legendary" Colorado 1998 AC locomotive event, that the TE put out by each machine exceeded 180,000 lbs. If the grade there was 2%, and the train was going through the equivalent of two or more curves totaling 19 degrees or more, each locomotive would have had to produce more than 180,000 lbs of TE, perhaps explaining the remark that "later investigation showed that the locomotives had been producing more pulling power than was thought possible at that speed." Strictly using the TE curve, that was more than possible. The limits on those engines from a software perspective, I do not know.
The advantage of the AC motor is that it permits the locomotive to reach a mechanical or adhesion limit, rather than be restricted by a thermal limit imposed by electrical considerations.
JayPotter wrote:I suppose that one source of confusion might be that, within the "adhesion" segment, the curve doesn't show predicted approximate TE but, rather, maximum possible TE.
I have no doubt that this is where some of the confusion rests. As I pointed out above, "'Maximum continuous' is an equipment limitation on TE over time -- not a measure of TE at any given point in time," that is, maximum possible TE is what can occur at a given point, whereas "maximum continuous" TE has little to do with the TE curve but is a limitation imposed entirely by the design, manufacture and condition of the equipment being used -- with the TE curve merely offering the upper boundary of possible TE.
From what I am used to for DC electrical equipment in general, having maximum capacity of 2-3 times the continuous capacity is not unusual, and which is why there are one hour, thirty minute, and even shorter time periods associated with such overloads, and electric locomotives, which had the advantage of relatively unlimited power supply to the traction motors if they could use it, could generate very high TE -- limited by heat and slip.
An electrical engineer was on board in one instance when two Little Joes had to power a train that was otherwise equipped with 3 SD40-2s, when the diesels all quit at once for some reason. Now, the Joes carried an equivalent diesel hp rating of 6,000, but the electrical engineer authorized proceeding with just the two Joes, on Butte Hill. These GE 750 traction motors in the Little Joes had an "OK" overload capacity, nothing like the old Boxcabs, but they'll take a load. The Engineer calculated the rail hp at 7,000 each (7,636 diesel equivalent hp). They were reportedly operating at about 8 mph. Adhesion had to have been very high, I assume sand was used. Using 7,636 diesel equivalent horsepower, the tractive effort of each Joe was about 358,000 lbs of TE. That would be the equivalent of 206,250 lbs of TE on an equivalent horsepower basis with the 4400 hp AC locomotives.
But the AC GEVO locomotives are limited to ... 180,000 lbs TE?
Are the GEVO's more limited by their software than the earlier 4400 hp AC models? Why was an old-fashioned DC traction motor, without any of the benefits of modern electronic control technology, able in that instance to apply more TE than a modern AC traction motor? In that instance, 2 DC straight-electric locomotives did the job (generating 716,000 lbs TE) that would have required four GE AC 4400 hp locomotives (720,000 lbs TE).
At the slower speeds, the DC traction motors produced, in that limited instance, up to 30% more TE than the software limited AC traction motor -- and applied it to the rail! Now, how could that happen?
Do AC locomotives use sand?
MichaelSol wrote: the tractive effort of each [MILW Little] Joe was about 358,000 lbs
MichaelSol wrote:It looks as though below 2.5 mph the [TE-vs-speed] curve begins to change radically, and the numbers do indeed become incomprehensible ... the relationship breaks down ... has no definition at all.
The curve isn't doing anything radical or incomprehensible, of course. We typically start by assuming constant horsepower, which implies TE that increases without limit as speed decreases. In reality TE can't increase without limit, even if adhesion permitted-- so we can't expect a constant-HP curve to predict TE at low speeds. So how do we predict TE at 1 mph? That's the $64 question, all right. How about it, Jay? Did GE or CSX tell you what an AC's ultimate TE would be if they geared the wheels to the rail and dropped all software limitations?
When the Milwaukee tested the demonstrator Little Joe, GE-750, the highest tractive effort recorded was 35%. This was recorded under idea test conditions on the 2.2% grade near Beverly, Washington on the Coast Division. That level of adhesion with a Little Joe equals 152,390 pounds of tractive effort. Since the one hour rating for a Little Joe was 85,500 pounds, that pull could not have been sustained for very long without burning up the traction motors.
MichaelSol wrote:It is likely, in the "legendary" Colorado 1998 AC locomotive event, that the TE put out by each machine exceeded 180,000 lbs. If the grade there was 2%, and the train was going through the equivalent of two or more curves totaling 19 degrees or more, each locomotive would have had to produce more than 180,000 lbs of TE
Did the train just climb to Toponas with the four ACs? Or did it continue to the Moffat Tunnel? Looks like they tried to make the latter climb continuous 2.0% compensated for about 4 1/2 miles; the climb to Toponas is a bit less, maybe 1.85% compensated at the steepest.
By the way-- looks like you're doing the curve compensation wrong. We don't want to clutter this thread up with that discussion, but if you're interested drop me a line and I'll explain.
nanaimo73 wrote:When the Milwaukee tested the demonstrator Little Joe, GE-750, the highest tractive effort recorded was 35%. This was recorded under idea test conditions on the 2.2% grade near Beverly, Washington on the Coast Division. That level of adhesion with a Little Joe equals 152,390 pounds of tractive effort. Since the one hour rating for a Little Joe was 85,500 pounds, that pull could not have been sustained for very long without burning up the traction motors. -The Milwaukee Electrics, second edition, page 285
I am familiar with the book, the author, the locomotives. That was prior to adding 40 tons of concrete to the engines. I assume that, rather than tractive effort, you mean highest adhesion was 35%. That's pretty good for a DC locomotive with a long wheelbase considering the curvature on the Beverly Grade -- the infamous "Heartbreak Curves". Too, the horsepower rating of the Joes was higher during that episode than during the Demonstrator tests.
In the instance of the Butte Hill, the train tonnage substantially exceeded the limitations of the tonnage chart. It happened that the chief electrical engineer, Barry Kirk, was on board, and approved the attempt to make it up the hill, otherwise the train would have waited for the Helper assigned to that grade. The point of the anecdote -- and I admit I am really tiring of attempting any contributions at all to this thread -- is that the Electrical Engineer was in a position of both expertise and experience -- and authority -- to authorize and observe an interesting "test" with results substantially beyond what was reasonably expected based on exactly the testing you describe from Noel Holley's book. Substantially so.
And to report authoritatively as to what he observed, and to do so because it exceeded expectations.
Now, if you want to apply a 35% adhesion limitation to both sets of calculations, and compare the results ...
timz wrote: MichaelSol wrote:It looks as though below 2.5 mph the [TE-vs-speed] curve begins to change radically, and the numbers do indeed become incomprehensible ... the relationship breaks down ... has no definition at all.Sounds like a problem, all right. So there's no way even to guess what the engine's tonnage rating will be at 1 mph?The curve isn't doing anything radical or incomprehensible, of course. We typically start by assuming constant horsepower, which implies TE that increases without limit as speed decreases. In reality TE can't increase without limit, even if adhesion permitted-- so we can't expect a constant-HP curve to predict TE at low speeds. So how do we predict TE at 1 mph? That's the $64 question, all right. How about it, Jay? Did GE or CSX tell you what an AC's ultimate TE would be if they geared the wheels to the rail and dropped all software limitations?
The graph clearly shows that we don't assume that -- the relationship breaks loose between speed and TE at somewhere between 2-3 mph. As Cordon pointed out, the creator of the graphs understood that and intended not to show a relationship there -- and you had missed that when you demanded a calculation at 0.1 mph. Unless someone wants to operate at 1 mph all the time, there's really no point in "guessing" what the tonnage limit might be -- it's probably defined by the drawbar -- but it won't be any less than at 5, 10, 20, or 50, and will in fact be some margin greater since the tonnage constraint -- resistance -- is the least at the slowest speeds, all other things being equal. I mean, what's the point, particularly if the curve goes flat because of software restrictions?
Interesting thread. Needs to lighten up a bit.
I would leave electric locomotives out, though. They have unlimited power. Instead of a 16-cylinder diesel engine amid ships, they are motivated by some turbine the size of a schoolbus with 4 or 6 boilers behind it.
With the right DC motors and drawing on power from the Hoover dam, a single electric with (really) good traction could pull Mt. Ranier into Oregon. Actually that might be a good idea, as Oregon needs a good ski area.
MichaelSol wrote:The AC GEVO locomotives are limited to ... 180,000 lbs TE?Are the GEVO's more limited by their software than the earlier 4400 hp AC models?Do AC locomotives use sand?
The AC GEVO locomotives are limited to ... 180,000 lbs TE?
Are the GEVO's more limited by their software than the earlier 4400 hp AC models?
A standard ES44AC is limited to 180K TE; however GE offers "optional equipment" that will allow a maximum of 198K TE. I presume that refers to the HTE software. CSXT's ES44ACs will have a maximum TE of 200K, the same as its AC4400CWs with HTE software. So no, the ES44ACs are no more software-limited than their AC4400CW predecessors. Yes, sand is used on AC-traction units.
timz wrote:So how do we predict TE at 1 mph? That's the $64 question, all right. How about it, Jay? Did GE or CSX tell you what an AC's ultimate TE would be if they geared the wheels to the rail and dropped all software limitations?
I really don't know the theoretical answer to the "ultimate TE" question. However GE's and CSXT's practical answer is that, at least for the time being, the ultimate TE is 200K. That's because a TE level in excess of that would risk damaging GE's traction motors and CSXT's track structure and, when produced by two-unit consists, would break couplers.
So a prediction of AC4400CW TE at 1 mph for a CSXT unit with HTE software is 200K, rail conditions permitting; and a counterpart prediction for other AC4400CWs is 180K, rail conditions permitting.
MichaelSol wrote:Why was an old-fashioned DC traction motor, without any of the benefits of modern electronic control technology, able in that instance to apply more TE than a modern AC traction motor? In that instance, 2 DC straight-electric locomotives did the job (generating 716,000 lbs TE) that would have required four GE AC 4400 hp locomotives (720,000 lbs TE).
I wouldn't even venture a guess to that question. If, as I understand, the train didn't wait for a helper, then both electrics were apparently on the head end of the train. If so, their combined 716K TE would have all been applied at the same point in the train. What I don't understand is how a train that required that level of TE to move could be moved, without breaking couplers or derailing the train in a curve, with all of that TE applied to one end of the train, as opposed to being distributed to multiple points.
JayPotter wrote: What I don't understand is how a train that required that level of TE to move could be moved, without breaking couplers or derailing the train in a curve, with all of that TE applied to one end of the train, as opposed to being distributed to multiple points.
What I don't understand is how a train that required that level of TE to move could be moved, without breaking couplers or derailing the train in a curve, with all of that TE applied to one end of the train, as opposed to being distributed to multiple points.
Well, I do have to go back and point out that the TE curve is, as you also mentioned, a maximum, and includes everything under the curve. Calculating the max doesn't mean it happened, since everything under the curve includes all of the accompanying conditions. That's probably a basic problem in attempting any calculation without all the data ....
I found this report, which discusses HP, adhesion, weight, and TE. It also covers maximum drawbar pull for couplers. Unfortunately, it doesn't get into comparisons of AC and DC motors.
There are lots of numbers here.
MichaelSol wrote:Calculating the max doesn't mean it happened, since everything under the curve includes all of the accompanying conditions. That's probably a basic problem in attempting any calculation without all the data ....
I'm sorry; but I guess that I didn't understand the example. I thought that we were dealing with a situation that actually did happen (i.e. "2 DC straight electric locomotives did the job [generating 716,000 lbs TE]") and that the issue was why this job, which was done by only two DC electrics, "would have required four GE AC 4400 hp locomotives" (720,000 lbs TE)".
In other words, I didn't think that we were "calculating the max" of anything. I thought that we were comparing what two DC electrics actually did against what four AC4400CWs might have been able to do if rail conditions were ideal.
Since I know about zero regarding DC electrics, the only thing that I can add to the discussion is that if four AC4400CWs have to move a train that might require each of the units to produce TE levels anywhere near the 180K maximum, I would expect that the four locomotives would be split into at least two consists and positioned at different points in the train to minimize the risk of generating excessive in-train forces.
Since I've been having some difficulty distinguishing scenarios that involve hypothetical calculations from scenarios that involve actual performance, I decided to offer a for-whatever-it-may-be-worth example that deals entirely with actual performance. I think that it illustrates some of the concepts that we've been discussing.
The train involved was a loaded coal train in excess of 15,000 tons powered by three AC4400CWs with HTE software that allowed each of them to produce 200K TE, instead of the standard 180K. Two of the units were on the head end; and the third unit was shoving on the rear.
The train was ascending a grade at a speed in excess of 10 mph when the lead unit stopped loading. Its TE dropped from something above 100K to zero; the train began to decelerate; and the TE of each of the other two units began to increase to compensate for the reduction in the lead unit's TE. The trailing unit in the head-end consist attained 169K TE. I presume -- but don't actually know -- that this was because rail conditions would only allow approximately 39% adhesion. However the helper (which was shoving on rail that had been conditioned by the passage of two locomotives and 110 cars) was able to attain approximately 46% adhesion and produced its full 200K TE. The deceleration stopped; and the train crested the grade at 3 mph.
Since I got lost somewhere between the curve and the graph - finally something I can understand! Thanx Jay!
Mookie wrote:Since I got lost somewhere between the curve and the graph - finally something I can understand! Thanx Jay!
A few points I haven't seen mentioned in this tread:
1. Adhesion limit vs. thermal limit on DC locomotives. On most DC locomotives, the practical, day-in-day out adhesion limit is very nearly the same as the continuous thermal limit of the motors. For example, an SD40's adhesion limit at 18% adhesion is around 12 mph. The thermal limit of the motors is around 13 mph. The improvements in adhesion control were pretty well matched with improvments in the traction motors and TM cooling system. One improvement was no good without the other. (Note that these improvments also came with HP increases, by and large, keeping the balance of HP and max dependable TE about the same)
2. Sand, wheelslip control, wheel creep control. Before EMD Super Series and GE Sentry wheelslip controls systems, wheel slip was measured and controlled so that the wheels didn't slip at all. Sand was the first response to wheelslip detection. Super Series and Sentry changed all that. It was found that greater adhesion is possible if the wheel is allowed to creep slightly faster than ground speed. Sand is the enemy of wheel creep, so is a last resort and much less is use. The control system works to maximize the traction motor current rather than minimize slip. (The radar on EMDs and TM speed sensors on GEs are only used to detect slip beyond the allowable creep limits.)
3. Adhesion on DC vs AC: You control wheelslip on a DC unit by regulating the excitation on the main generator. It's a big inductive machine, so fast changes are physically impossible. On an AC unit, you regulate the frequency out of the invertors, which you can change almost instantaneously. So, you can get much higher adhesion on an AC on a day in and day out basis. Single axle wheelsip control on DC units never caught on, I think, because once you've outfitted a unit with all the electronics, you're half way to having inverters, so might as well plop in those rugged squirrel cage motors to make it a full AC unit.
4. Grade C knuckles are only good for 250-300,000#, Grade E - 350-400,000#. These are above the yield strength of the pieces, so you're looking a finite fatigue life. Two AC units at 180,000# TE each on the head end of a regular old box car train makes me wince!
oltmannd wrote: 4. Grade C knuckles are only good for 250-300,000#, Grade E - 350-400,000#. These are above the yield strength of the pieces, so you're looking a finite fatigue life. Two AC units at 180,000# TE each on the head end of a regular old box car train makes me wince!
And while I am making a complete fool of myself - while we generally run DPU (usually 1-2 head and 1 rear) I never see DPU on any freight - just 2-3 motors on the headend.
Just my ramblings for today.
cordon wrote: I found this report, which discusses HP, adhesion, weight, and TE. It also covers maximum drawbar pull for couplers. Unfortunately, it doesn't get into comparisons of AC and DC motors.
A useful and interesting summary. Both Al Krugg and Jay Potter above mention "curvature" as an element in resistance calculations and drawbar strain -- defining the TE need -- yet, as timz also points out above ... what about compensated curves/grades? While in the absence of information, I might default to a calculation using uncompensated grades, aren't most mainline grades compensated these days?
MichaelSol wrote:aren't most mainline grades compensated these days?
JayPotter wrote: [Each of the units would have been producing 180,000-or-less pounds of TE. The extent to which those TE levels approached 180,000 pounds would have depended on the levels of adhesion that rail conditions allowed the units to produce.
[Each of the units would have been producing 180,000-or-less pounds of TE. The extent to which those TE levels approached 180,000 pounds would have depended on the levels of adhesion that rail conditions allowed the units to produce.
Jay:
You hit the nail on the head regarding AC locomotive performance at 0.2 mph in Colorado.
According to the November 1998 Trains Magazine titled "AC Rules", page 67:
"The event recorder from the lead unit, Southern Pacific AC4400 202, reveals that the locomotive was producing between 170,000 and 180,000 pounds of tractive effort the entire time."
Also, I am confused as to the definitions of starting tractive effort and maximum continuous tractive effort. Could you clarify them for me?
Thanks.
Mookie wrote: Since I got lost somewhere between the curve and the graph - finally something I can understand! Thanx Jay!
Way over my head too .... for me it's simple , I don't want to pull trains slower than 2 MPH , I don't want to pay BIG money for AC motors and drives . I don't want to pay the BIG money to repair that expensive motor when it dies. I don't want the shop downtime when you have a failure in the comlicated electronics (yes , although the motors run cool , the electronics don't). I still have a diesel engine to maintain , still have wheels that are going to burn off faster when I'm at the limits of adhesion. I certainly don't want to be recrewing trains just so the locomotives can show off thier slow speed pulling ability.
Randy Stahl wrote: Mookie wrote: Since I got lost somewhere between the curve and the graph - finally something I can understand! Thanx Jay!Way over my head too .... for me it's simple , I don't want to pull trains slower than 2 MPH , I don't want to pay BIG money for AC motors and drives . I don't want to pay the BIG money to repair that expensive motor when it dies. I don't want the shop downtime when you have a failure in the comlicated electronics (yes , although the motors run cool , the electronics don't). I still have a diesel engine to maintain , still have wheels that are going to burn off faster when I'm at the limits of adhesion. I certainly don't want to be recrewing trains just so the locomotives can show off thier slow speed pulling ability.
Never mind about the definitions - I found your previous post in which you explained clearly the definition of maximum continuous TE.
....It is my understanding the grade up around Horseshoe {Altoona}, drops to about 1.45 % while the rest of the climb is around 1.8%....This was constructed and opened in 1854, but I have no idea if it was done with the compensation then, or much later. {The old main line Pennsy}.
Joemcs - we will sit over here on the sidlines and visit a minute.
Pulled out my roster of "seens" - and can tell you that we have a lot of different types running around.
Lots of 8's and 9's yet - some two axle 8's - GP's from 7-50 and our switchers are SW10 and SW15. Yard engine is an SD9.
We have had B23-7's go through here and the 60 Power Units. SD40 is still very popular.
And - we only watch on weekends, so hard to tell what we miss during the week that comes and goes.
We have a diesel shop very close to watch site, so we do see some "oddities" from other railroads every so often.
Now if I just had a little more knowledge on each one, besides the sightings. Working on it, slowly!
Mook
cordon wrote: I found this report, which discusses HP, adhesion, weight, and TE. It also covers maximum drawbar pull for couplers. Unfortunately, it doesn't get into comparisons of AC and DC motors. There are lots of numbers here.
A few more numbers:
EMD's "all weather" adhesion rating for their locomotives (99% of the time, conditions will allow at least this adhesion level). They're a bit lower than those Krug cites and closer to the ones Conrail used for their drag tonnage ratings. (Except Conrail used 18% for the SD40-2s because of the Flexicoil trucks)
Mookie wrote:Joemcs - we will sit over here on the sidlines and visit a minute. Pulled out my roster of "seens" - and can tell you that we have a lot of different types running around. Lots of 8's and 9's yet - some two axle 8's - GP's from 7-50 and our switchers are SW10 and SW15. Yard engine is an SD9. We have had B23-7's go through here and the 60 Power Units. SD40 is still very popular. And - we only watch on weekends, so hard to tell what we miss during the week that comes and goes. We have a diesel shop very close to watch site, so we do see some "oddities" from other railroads every so often.Now if I just had a little more knowledge on each one, besides the sightings. Working on it, slowly!Mook
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