Paul_D_North_JrHow 'bout a little help here in understanding the 'back EMF' = 'back ElectroMotive Force' concept, also known as 'counter-electromotive force' ? [And no, it's not an ailment peculiar to GM's diesels, or one that GE's are immune from . . . ]
Paul,
An analogy that might help is to think of a water turbine - the "back EMF" of a motor corresponds to the "back pressure" of a water turbine that corresponds to the work transmitted to the output shaft - with electric current being equivalent to water flow. The power available from water under pressure (conversely the power required from a pump to generate water under pressure) is the flow times the pressure drop, 1 cubic foot per second at 1,000 psi has the same potential power as 1,000 cubic feet per second at 1 psi. Similarly, DC power is volts times amps.
Getting on to DC motors, the EMF is proportional to the armature speed times the strength of the magnetic field produced by the field windings. The magnetic field strength is proportional to the field current up until the iron in the motor frame starts saturating. The torque from a motor is proportional to the armature current times the magnetic field generated by the field windings.
The series motor has the field current equal to the armature current, so for a constant armature voltage (assuming no field saturation), as speed increases, the current through the armature needs to decrease to maintain the same EMF. Since diesel locomotives operate in a constant power more for a given prime mover speed, this means that the generator voltage needs to increase with increasing track speed. One problem is that there are limits as to how high the voltage can go before the generator (or motor) starts having troubles (e.g. arcing). One way of getting around that is to re-arrange the series/parallel connections of the motors - which reduces the strain on the generator. Field shunting is another means - this allows the armature current to increase without increasing the field (hence without increasing the back EMF).
Hope this helps,
- Erik
timzPaul_D_North_Jra 600 volt DC generator and C-C trucks, the voltage across each motor would then change from 100 volts (600 volts/ 6 motors) to 300 volts (600 volts / 2 sets of 3 motors each) to 600 volts (600 volts across each motor). That's the general idea, except that AFAIK no road diesel ever connected all its motors up in one series string-- no need to do so, I assume. Some C-Cs did go from two strings of three motors, to three pairs, to straight parallel, but none? of the post-1965 models needed that initial two-strings-of-three stage. Since circa 1980? all locomotives use parallel connection of the motors all the time; the reconnection (if any) is done in the alternator/generator instead. The other possibility was field shunting-- not used on any locomotive since... 1972?
Paul_D_North_Jra 600 volt DC generator and C-C trucks, the voltage across each motor would then change from 100 volts (600 volts/ 6 motors) to 300 volts (600 volts / 2 sets of 3 motors each) to 600 volts (600 volts across each motor).
The other possibility was field shunting-- not used on any locomotive since... 1972?
The first loco with gen transition I can remember was the SD50.
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
As far as I know, EMD ALWAYS used field shunting, as well switching from series to parallel motor connections, on all locomotives that used dc generators. (Possibly some switchers may have been exceptions.) This may have been changed once they started with alternators and rectifiers, and certainly was dropped with any ac motor locomotives.
Paul Milenkovic I suppose that book is out of print and hard to get and, oh well.
Paul Milenkovic a 4500 HP six-axle AC-motored microprocessor-wheel-slip-controlled Diesel is probably a match for anything steam in both HP and lugging ability.
Wasn't Paul Kiefer the Chief of Motive Power on the New York Central or some high post? I suppose that book is out of print and hard to get and, oh well.
I think that the narrative was that from a HP standpoint, a single Superpower steam locomotive -- Northern, Challenger, and so on -- matched not just a Diesel but a multi-unit Diesel consist, to within certain bounds. That is, back in the day when an FT was what, 1300 HP and an E7 was 2000 HP.
The (multi-unit) Diesel had the steam locomotive beat hands down on weight on powered axles and probably on lugging ability, but as far as I could tell, you could "flog" a steam locomotive until you lost your steam (by using too much) whereas you could do expensive damage to a Diesel consist by exceeding short-time ratings.
But these days, a 4500 HP six-axle AC-motored microprocessor-wheel-slip-controlled Diesel is probably a match for anything steam in both HP and lugging ability. If they were able to pull that consist with a Challenger, they should be able to manage it with a single modern AC Diesel. That they have at least 2 (or maybe 3) such units on a modern intermodal is that they want some reserve for an on-road unit failure, be able to run through without helpers or locomotive changes for grades on certain divisions, have acceleration to meet schedules.
If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
In Paul Kiefer's book, A Practical Evaluation of Railroad Motive Power, the only reference I saw was either 2-unit or 3-unit E7's in comparison to a Niagara. IIRC, it took three units to equal the performance of a single Niagara, but two units were close enough, and more economical.
Paul_D_North_Jrtimz You've got the wrong idea about back EMF, which is just nature's way of keeping us from getting something for nothing. We want to get mechanical work out of a motor, so we have to put electrical work into it-- that is, we have to shove the current past the "back EMF". An SD40-2 succeeds in doing that at 60-70 mph just as well as it does at 20-30 mph. [snip] How 'bout a little help here in understanding the 'back EMF' = 'back ElectroMotive Force' concept, also known as 'counter-electromotive force' ? [And no, it's not an ailment peculiar to GM's diesels, or one that GE's are immune from . . . ] I've done a little Internet research on this, but it's hard to find something that applies directly to railroad locomotive motors. I gather that the back EMF - mainly in the form of voltage - results from the motor's armature spinning in the magnetic field that's created by the electric current running through the stator coils. It's also proportional to the motor speed - slow speed, little back EMF; fast speed, lots of back EMF. So it's also a 'feedback loop' or 'check-and-balance' kind of arrangement on an unloaded motor just going faster and faster . . . I also understand that the rearrangement of the motor circuits from the series to series-parallel and then to full parallel is a method to boost the voltage to overcome this - for a 600 volt DC generator and C-C trucks, the voltage across each motor would then change from 100 volts (600 volts/ 6 motors) to 300 volts (600 volts / 2 sets of 3 motors each) to 600 volts (600 volts across each motor). Is that correct, or not ? Feel free to correct anything that's wrong, not quite right, or needs further explanation, whatever is most convenient. Thanks. - Paul North.
timz You've got the wrong idea about back EMF, which is just nature's way of keeping us from getting something for nothing. We want to get mechanical work out of a motor, so we have to put electrical work into it-- that is, we have to shove the current past the "back EMF". An SD40-2 succeeds in doing that at 60-70 mph just as well as it does at 20-30 mph. [snip]
How 'bout a little help here in understanding the 'back EMF' = 'back ElectroMotive Force' concept, also known as 'counter-electromotive force' ? [And no, it's not an ailment peculiar to GM's diesels, or one that GE's are immune from . . . ]
I've done a little Internet research on this, but it's hard to find something that applies directly to railroad locomotive motors. I gather that the back EMF - mainly in the form of voltage - results from the motor's armature spinning in the magnetic field that's created by the electric current running through the stator coils. It's also proportional to the motor speed - slow speed, little back EMF; fast speed, lots of back EMF. So it's also a 'feedback loop' or 'check-and-balance' kind of arrangement on an unloaded motor just going faster and faster . . .
I also understand that the rearrangement of the motor circuits from the series to series-parallel and then to full parallel is a method to boost the voltage to overcome this - for a 600 volt DC generator and C-C trucks, the voltage across each motor would then change from 100 volts (600 volts/ 6 motors) to 300 volts (600 volts / 2 sets of 3 motors each) to 600 volts (600 volts across each motor). Is that correct, or not ?
Feel free to correct anything that's wrong, not quite right, or needs further explanation, whatever is most convenient. Thanks.
- Paul North.
There is no way to get around the laws of physics, even in railroading.
Yardmaster01 The sweet spot of diesel electric locomotives tends to be in the range of 5 to 35 mph, over 35 and the back EMF becomes ever greater and horsepower transmission efficiency decreases thereby lowering effective horsepower and tractive effort.
Like most railfans, you have basically no idea what speed an SD40-2 would make on a given grade with a given tonnage-- for whatever reason, railfans in this country aren't much interested in that. (Not interested enough to go out a take a proper look, anyway.) But various railroads have long steady grades with parallel highways, where you can pace the trains in your car and get a good idea of their speed. If by chance you live in California you can pace SFe trains up a steady 1.0% grade eastward from Essex; I fear few? no? freights climb that hill at 60+ mph now, but SD40-2s used to take the 991 train up there at that speed, overcoming the back EMF just as they're supposed to.
Yardmaster01 The NYC found that it took 2 FT's to replace one Niagara at 70 mph, an FT consisting of four 1200 hp permanently coupled units or EIGHT total units.
Yardmaster01studies the NYC did that found their Niagara's were more thermally efficient AT SPEED than the EMD FT locomtives
Thanks for the feedback GP 40-2, can't fault your logic. It sucks to admit I'm turning into an antique like the machines I love. 1977 seems like yesterday..... At least back then I could stand trackside and watch the occasional F-unit or Alco roar by.
Yardmaster01...I have my old college engineering texts with the relevent data but I'm not computer savvy enough to figure out how to get them posted. (Principles of Electric Generation and Use, McMillan Publishing, 4th ed. 1977, page 279-283)
I'm sure a engineering book from 1977 does not delve into microprocessor controlled frequency drive AC motors. Which is to say a lot has changed in diesel-electric locomotive performance since the time of the SD40-2s, or the time of the FTs.
Given feltonhill's accurate statement that a Challenger is a 4600 HP locomotive @ 40 mph, that gives it a 500 hp advantage over a 4400 HP AC @ 40 MPH and about 900 HP less than an AC6000 @ 40 MPH.
The "sweet spot" on an AC is actually at higher speeds, which is why CSX has used ACs on intermodal from the get go (a trend that I now see the other Class 1s engaging in).
You will get no argument from me that the late steam designs were indeed powerful (not as powerful as some railfans like to pretend they are), and it wasn't until the last 15 years or so that D-E technology has progressed to the point of being able to give both high low speed tractive effort and high speed horsepower from a single unit.
Sorry guys, didn't mean to ruffle any feathers. When I referred to "high horse power" diesels I was thinking of SD 40-2's, (They're putting these things in museums already!) shows how old I'm getting. The Challenger at speed is probably the equivalent of 1 1/2 or maybe 2 at best of today's most modern power. At start the king of tractive effort and horsepower is still the gear reduced electric motor, be it straight electric or diesel electric.
The fact remains that the more you try to push an electric motor the more it pushes back. Effective horse power drops off following a descending logarithmic curve. I have my old college engineering texts with the relevent data but I'm not computer savvy enough to figure out how to get them posted. (Principles of Electric Generation and Use, McMillan Publishing, 4th ed. 1977, page 279-283)
Back EMF (Electro- Mechanical Force) is the major impediment in the usage of electric propulsion. Engineers try to design electric propulsion systems to stay in the "sweet spot" of the electric motor so that the motor isn't running too far along the bottom of the curve. The sweet spot of diesel electric locomotives tends to be in the range of 5 to 35 mph, over 35 and the back EMF becomes ever greater and horsepower transmission efficiency decreases thereby lowering effective horsepower and tractive effort.
Staight electric locomotives overcome the back EMF problem somewhat by having a relatively large amount of available horsepower to drive the traction motors and can then run at a higher speed. As far as their traction motors are concerned, they "see" the available horsepower as virtually infinite. This principle allows today's high speed trains to generate the speeds that they do.
Southern Pacific tried to get around the back EMF problem by trialing the Krause-Maffei diesel hydraulic locomotives. While they did indeed have tremendous low end pulling power and maintained that power farther into the speed band, they leaked like sieves and tended to overheat.
Steam locomotives develop their highest tractive effort and horsepower farther along the speed curve than the previously mentioned types and there-in actually lies their greatest detriment as far as most efficient utillization. Most railroads never ran their trains at a high enough speed to take advantage of the potential horsepower, although the Norfolk and Western was the main exception to that rule. They had the Y class for low speed lugging, the superlative A class for high speed heavy tonnage and the 600 class 4-8-4's for low tonnage high speed passenger trains. Railroads like UP and DM&IR wasted the huge horsepower potential of their large simple articulateds lugging high tonnage trains up steep grades at low speeds.
The diesel on the other hand had the advantage of utillizing ALL of it's horsepower right from the start. If you wanted to go faster, you just added more units. This is still true today. When you get right down to it, railroading is a relatively low speed undertaking with most of today's diesels operating in the low end of the speed band and thus being most efficiently used.
As far as saying that the Challenger was using less btus to do the job, that was an extrapollation on my part from studies the NYC did that found their Niagara's were more thermally efficient AT SPEED than the EMD FT locomtives it took to replace them. The NYC found that it took 2 FT's to replace one Niagara at 70 mph, an FT consisting of four 1200 hp permanently coupled units or EIGHT total units. Now one might postulate that the Challenger is a larger locomotive than the Niagara and so it would take more diesel electric horsepower to replace it at speed. I neglected to take into account the higher efficiency of today's modern power and if I was wrong on this will gladly concede the point.
Didn't mean to get so long winded and as said before, didn't mean to ruffle any feathers. Hope some more people chime in on this and correct any inaccuracies. This thread has been fascinating!
Maybe where he went wrong was reading this
CAZEPHYRfive GE's had pulled the train into Cheyenne from the west.
and concluding the 4-6+6-4 could equal five GEs. Hopefully CAZEPHYR would disavow any such implication.
We could clarify the situation if somebody had clocked the five GEs up the west side of Sherman Hill with the stack train. Given the train's tonnage, it would be easy to calculate that if they were really only good for 1000 hp per unit at 40 mph they couldn't possibly make the speed they actually did make up the 0.82%.
Yardmaster01Some calculations even suggest that the Challeger uses less btus than than diesels to get the job done but that's another subject entirely.
Yardmaster01...That's why a Challenger putting out 5000 hp at 40mph is the equivalent of four or five high horse power diesels, because those diesels have only about 5000 effective hp and similar tractive effort at that speed.
...That's why a Challenger putting out 5000 hp at 40mph is the equivalent of four or five high horse power diesels, because those diesels have only about 5000 effective hp and similar tractive effort at that speed.
Perhaps you would be so kind as to show us EMD or GE tractive effort curves for the above mentioned units at 40 MPH to confirm your statement.
All this talk about comparing steam locomotives to diesels is all very interesting but all of you seem to have forgotten the most important aspect of all, the respective power curves of both power types. A diesel electric developes maximum horsepower and tractive effort starting out and both rapidly diminish as speed increases due to the fact that the traction motors (all electric motors for that matter) act as generators. As the speed of the traction motors increases so does the amount of back voltage and it takes more and more horse power to counteract this force. That's why it takes so many diesels to make a train go fast.
A steam locomotive on the other hand has a far different horse power and tractive effort curve. As the steam locomotive starts to pull at low speed it develops its lowest tractive effort and horsepower, that's why a steam locomotive's starting tractive effort number is so important. It determines the tonnage of the train it can pull without needing helpers to get going. As the speed increases so does the tractive effort and horse power. At what speed these level out and then fall off is determined by driver diameter, boiler pressure, rate of steam production,cylinder stroke, valve gear timing and the weight of the locomotive.
That's why a Challenger putting out 5000 hp at 40mph is the equivalent of four or five high horse power diesels, because those diesels have only about 5000 effective hp and similar tractive effort at that speed. Some calculations even suggest that the Challeger uses less btus than than diesels to get the job done but that's another subject entirely.
It all comes down to something very simple: A steam locomotive can pull a train at speed that it can't start on it's own, a diesel locomotive can start a train on it's own that it can't pull at speed. Hope this helps.
P.S. That was a great video that started this thread and thanks for posting it!
Thanks for the video Paul.
What's kinda sad is that those particular SD40-2's are also rather familiar to me...saw them many many times.
How fast the time flies.
John
On that we agree. From an aesthetic standpoint, I'd trade about 10,000 diesels for another Challenger.
UP 4-12-2 [snip] Sorry, I've spent many nights, and Saturday and Sunday afternoons at Horseshoe Curve or at ALTO Tower during the 1980's. I still can and will forever be able to see Conrail's ex-EL SD45-2 helper engines in my mind's eye . . . [snip]
Then you might enjoy the following little video - not mine, I just found it elsewhere on the 'Net about 2 weeks ago - supposedly from 1991 of a minor derailment on the Curve, and the sounds of the rear helpers throttling back to come to a stop at the western calk of the mountain there - it's about 5-1/2 minutes long, and well worth viewing and listening - takes me right back as if I was there. It's notable as an actual recorded instance when railfans were able to notify a train crew of a problem - read the caption notes for the full story. The rear helpers - SD40-2's 6387 and 6384 if I read and identify them right - go by from 1:10 to 1:20, and have come to a stop by 2:30. The derailed car goes by at about from 0:17 to 0:22:
http://www.youtube.com/watch?v=MwwVsRlS2eU
Ok, fine, some modern diesels might roughly match the performance of a single Challenger, or exceed it at slower speeds--but they will not provide the sheer drama provided by that Challenger.
Sorry, I've spent many nights, and Saturday and Sunday afternoons at Horseshoe Curve or at ALTO Tower during the 1980's. I still can and will forever be able to see Conrail's ex-EL SD45-2 helper engines in my mind's eye (including "Satan's Engine" 6666), but diesels are just plain boring when compared to steam, whether real or in model form. My sons could give two hoots about diesels, but put an MTH Challenger in front of them with the sound, lights, and smoke, and they both (even the one who no longer plays much with trains at all) are captivated.
There simply is no other machine that offers the same visual and sensory impact of working steam power.
All probably very true, fellas, and I sure appreciate the way all of you ran with this for me.
I will say this, though...if there had been a crowd witnessing a contest from a full stop, with two equal consists, one pulled by a single AC4400 or whatever, and one pulled by #3985, I wonder which would eliciit the stronger cheering section.
-Crandell
A 'heavy' = 420,000 lb. = 210 tons or so C-C with 70,000 lb. axles loads at only a 33 % coefficient of friction = 3.0 factor of adhesion will produce that 140,000 lbs. of tractive effort that's cited above for the Challenger - at the 40 % which is sometimes cited for the AC-drive locomotives it would be 168,000 lbs., which I believe would top all of the steamers.
HorsePower is different - the production diesels appear to be practically limited to about 4,400 HP, since most of the 6,000 HP units appear to have been retired, so the big steam locomotives would be able to top that figure - but only a few could reach over the 6,000 HP threshold. And since above a certain speed usually in the 15 to 20 MPH range the TE starts to drop in inverse proportion to the speed due to the HP being limited - which doesn't happen quite as much or in the same way for steamers - a steam engine might be able to produce more TE than the diesel at the same and higher speeds, which would mean that the steam locomotive is also a higher HP machine at those faster speeds.
So in terms of sheer lugging power, the AC unit would win. But at speed, it would be a closer contest, probably depending on the specific train resistance and grade, and speed range.
One advantage that a pair of 4-8-2's would have over a single articulated is that they would have 2 fireboxes, 2 large grate areas, 2 throat sheets, 2 shorter sets of flue tubes - albeit maybe almost the same total length, and 2 stacks for exhaust - though a few articulateds had those too, as I recall. As such, I believe that the pair of 4-8-2's would have a much higher total fuel / coal consumption rate and easier drafting than the single articulated - hence it follows that they would be able to produce more power than it.
And if we view a Challenger through modern lenses - isn't its driving wheel arrangement really similar to a a C-C diesel - only it's technically a 2-C+C-2 instead ? And on the PRR they called that wheel arrangement a GG1 - which in their prime I'd like to see matched against said Challenger, the pair of Mountains, or any other modern C-C diesel unit . . . Would the PRR have classified a Challenger as a 'GG2' ?
There really is not a single individual diesel locomotive being bought in any quantity today that is really the equal of a Challenger.
.
There really is not a single individual diesel locomotive being bought in any quantity today that is really the equal of a Challenger. The equivalent power and/or tractive effort would take two or more units. But that is probably more of virtue for diesel power than a liability, because it allow better matching of the power to the job at hand.
But that doesn't detract in any way from the sheer beauty and enjoyment of seeing a well-desigend and well-maintained and well-operated steam locomotive hauling a heavy modern freight train of a type undreamed when the locomotive was built over well-maintained and well-despatched track. The UP and its employees can be very very proud of their continuation of mainline steam heritage operations.
And the skill of the CSX T-1 engineer in recovering from the slips is also to be applauded/
selectorUP, we seem to have slipped on some terms, and hence your confusion (I think it's yours...forgive me if not...). GP said that, as long as we agree that we could MU either diesels or steamers, then two Mountain-types could out-perform a single articulated of any kind. Two of them, not one. To back up, the person before GP said that the diesels have an advantage in terms of their configuration as a hauling unit. Steamers less so. If....IF...we were to group in like ways, MU'd diesels and MU'd steamers, GP says that there would hardly have been the need to develop any single articulated because two Mountains, if MU'd as diesels were, would have been the defining requirement in steam. I hope I have accurately reflected GP's thoughts. -Crandell
UP, we seem to have slipped on some terms, and hence your confusion (I think it's yours...forgive me if not...). GP said that, as long as we agree that we could MU either diesels or steamers, then two Mountain-types could out-perform a single articulated of any kind. Two of them, not one.
To back up, the person before GP said that the diesels have an advantage in terms of their configuration as a hauling unit. Steamers less so. If....IF...we were to group in like ways, MU'd diesels and MU'd steamers, GP says that there would hardly have been the need to develop any single articulated because two Mountains, if MU'd as diesels were, would have been the defining requirement in steam.
I hope I have accurately reflected GP's thoughts.
The most modern 4-8-2s, built in the late steam era, were 4000 HP machines, capable of nearly 70,000 lbs TE, and 100 mph speeds.
MU two of them and you have an 8000 HP, 140,000 lbs TE lash up that is still capable of moving tonnage at 100 MPH. You might find a low speed articulated that can produce more than 140,000 lbs starting TE, but it isn't going to produce 8000 HP at speed, and it surely isn't going run at 100 mph.
This Baldwin R1d, built in 1941 for the B&M is a good example. With a 495 sq.ft. firebox area, 2000 sq.ft. Superheater, and a total heating surface of 6,500 sq.ft., it was a serious, no-fooling-around high HP speed machine.
Ok, Crandell, I misread the part about mu'd as diesels, ie, only one crew. Sorry.
However, the argument still doesn't totally work for me, because the big articulated offers some efficiencies that you just don't get with multiple smaller steam engines.
GP40-2CSSHEGEWISCH While I will concede that no single unit can match the performance of a single Challenger... Then my friend, you have no idea what a single AC6000 is capable of at speed. You do bring up good point about the MU ability of diesel-electrics. If it was possible for the railroads to MU steam engines under one crew, the development of the steam locomotive would have stopped with the 4-8-2 Mountain locomotive. A pair of good 4-8-2s will out perform any single large articulated ever made.
CSSHEGEWISCH While I will concede that no single unit can match the performance of a single Challenger...
While I will concede that no single unit can match the performance of a single Challenger...
You do bring up good point about the MU ability of diesel-electrics. If it was possible for the railroads to MU steam engines under one crew, the development of the steam locomotive would have stopped with the 4-8-2 Mountain locomotive. A pair of good 4-8-2s will out perform any single large articulated ever made.
What?
I'm not perfect, and certainly misunderstood one poster on this thread--but to say two 4-8-2's would outperform any single large articulated ever made?
Sorry, I don't think so.
Which one 4-8-2 was capable of 85000 pounds starting tractive effort???? That's what you need to match or beat the performance of a Y-6B.
How about a 4-8-2 at 88000 pounds starting tractive effort? That's what you need to match the performance of Virginian's A-E class 2-10-10-2, which had a starting tractive effort of 176,600 pounds, and which had a nice, long, healthy service career, lasting into the 1950's.
The Union Pacific Big Boy and other large articulateds actually got over 6000 horsepower to the drawbar (over 7000 for the 2-6-6-6). How many 4-8-2's produced 3500 horsepower?--Oh, and by the way--they still had two crews to one crew for the large articulated.
Please don't try to tell me that twice as much manpower is more efficient than that large articulated.
Over on steamlocomotive.com, they have the NYC Mohawk ranked as the best 4-8-2 ever built. Though it could run 80 mph, the highest tractive effort version produced a puny (compared to articulateds) 60617 pounds of starting tractive effort. Your'e going to need three of them to narrowly exceed Virginian's 2-10-10-2 in a slugfest from a standing stop.
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