In the thread "4-axle units" traisessive1 mentions those types of units could be called "four-axle" power. Which got me to wondering...
Why would any railroad bother with such an arrangement? Yes, I know part of the reason is to save a few $ by having less traction motors, but if that is the case, why not just buy a new (and presumably cheaper) 4-axle unit?
If not getting to use all the HP you've paid for is what you're after, then the units make sense; otherwise--what a stupid concept!
Buying such a unit would be something I could see a railroad like NS or CSX doing, but not a supposedly 'smart' railroad like BNSF.
Opinions or thoughts......?
It should be noted that BNSF uses the four-motor power primarily in expedited service and FEC has no real grades. The horsepower is still getting used in either case.
The main reason for not using B-B type locomotives, but rather using A1A-A1A or on EMDs B1-1B powered axle arrangements is weight. Either you sacrifice fuel to hold the weight to allowable limits or engine size, or perhaps both. Both the GP60M and B40-8W locomotives were right at the maximum allowable weights, and both had reduced fuel capacity, and remember these locomotives were built to Tier 0 emissions standards. So imagine what enlarged cooling capacity carrying more water with more weight is going to do.
zardoz why not just buy a new (and presumably cheaper) 4-axle unit?
I'm not that knowledgeable about this, but I kinda remember that it has to do with weight. New locomotives are too heavy for four axles.
(edit) Oops, beaulieu beat me to it.
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"A stranger's just a friend you ain't met yet." --- Dave Gardner
I think the original theory was that BNSF's intermodal traffic was always powered highly enough that the trains wouldn't drag up grades at speeds low enough to require the tractive effort of 6 AC motors, so eliminating 2 reduced cost.
Of course they show up with some regularity on everything from coal trains to heavy manifests, but...
HP to the traction motor normally = Speed.
A GEVO with A-1-A trucks is putting 4400 hp into 4 traction motors - 1100 HP each. That much HP at a traction motor tends to make the engine 'slippery' in drag tonnage situations. With 'suitable' tonnage in a intermodal situation the train should easily maintain track speed.
To my mind, if the 'idler' axle is allowed to carry it's weight on rail, the engine would be even more 'slippery' in drag tonnage situations. Never having had any first hand dealings with these locomotives, I don't have the real answers.
Never too old to have a happy childhood!
zardozbut if that is the case, why not just buy a new (and presumably cheaper) 4-axle unit?
What new 4 axle unit? I'm guessing (but have no clue if right) cheaper to do what they did and strip down an existing 6-axle than to engineer a whole new 4-axle.
The opinions expressed here represent my own and not those of my employer, any other railroad, company, or person.
An ES44C4 costs about the same as an ES44DC. About $1,000,000 less than an ES44AC. That is a chunk of change per locomotive. I have read that BNSF engineers say that they are slippery, but he did not state in what service the locomotive was being used.
Don’t the ES44C4 units have, for lack of a better term, ”levers” that are supposed to raise the idler axles slightly so that more weight is on the powered axles to improve tractive effort when needed?
They are not leavers, but pistons. If you look closely at a picture of one of the trucks, you will see cylinder on the outside of the center axle. That is the piston that raises and lowers the center wheels a little.
It's not a million dollars difference these days between a C4 (Or ES44DC when they were still making them) and an ES44AC. Back around 1995, Trains would regularly quote a half million dollars difference between something like the SD70M and SD70MAC (Or Dash 9 and AC4400CW).
And that gap has closed a lot which is one reason why most of the Class 1's just buy all six motor AC road power these days (And why a C4 with just two less AC traction motors and two less inverters can be sold for approximetely what an ES44DC would cost).
I bet we're talking about ~$250,000 or less these days for what BNSF saves by buying a C4 instead of a six motor unit.
caldreamer They are not leavers, but pistons. If you look closely at a picture of one of the trucks, you will see cylinder on the outside of the center axle. That is the piston that raises and lowers the center wheels a little.
caldreamer, thanks for that clarification!
kgbw49 Don’t the ES44C4 units have, for lack of a better term, ”levers” that are supposed to raise the idler axles slightly so that more weight is on the powered axles to improve tractive effort when needed?
"Tractive effort when needed"--when is tractive effort NOT needed? Granted, not as much is needed at higher speeds, but those higher speeds still need to be maintained.
Having locomotives with unpowered axles seems akin to having a "B" unit--sure, it saved a few bucks at purchase, but the operational limitations soon doomed them to obscurity. And BNSF hardly needed to penny-pinch.
Somebody correct me if I'm wrong but I remember reading years ago that in a fast freight/intermodal context, a 4 motor locomotive will go faster than a 6 motor of the same horsepower since more current is fed to each motor so that it can rotate at a higher speed.
People have been known to say that, but no reason to believe it. If cutting out two traction motors gave a C-C more tractive effort at 70 mph, it would be easy enough to include that feature on the engine. Then if you wanted even more TE, you could cut out two more motors. Think a 1A1-1A1 would be a champ at 70 mph?
BackshopSomebody correct me if I'm wrong but I remember reading years ago that in a fast freight/intermodal context, a 4 motor locomotive will go faster than a 6 motor of the same horsepower since more current is fed to each motor so that it can rotate at a higher speed.
Voltage, not current, determines the rotational speed of one of these motors. Think about why transition is used. And about why field-weakening to reduce back EMF (which is voltage) works to increase speed.
Now, the losses in six motors will be higher than in four in most cases, and of course the capital costs of the added motors, cases and gears are non-trivial, so if you have relatively high horsepower and can control slip adequately there are cost advantages to 4 over 6 since the limiting factor over most required road operation is imposed by fixed horsepower. Fuel and lower emissions, as said, make longer chassis with weight distribution essential... the question then becoming why we don't see intermediate "BoCo" designs for that sort of reason, following the same logic as the FL9s and 5-axle FMs when weight went up and carrier idler axle was added.
A big piece of this is improvements in C truck design, notably the HTCR radial and the 'rollerblade' wing-box primary suspension on GEs, in a world where really good track at only moderate speed has become a 'norm'. It might be interesting to see how effective the Alco Hi-Ad C truck would be once no longer subject to dreadful harmonic-rock effects... but we have better ones cheaper now.
Fine, I'm not an engineer (TG) but do I have the concept correctly? The same amount of total power fed to fewer motors allows them to turn faster?
beaulieu The main reason for not using B-B type locomotives, but rather using A1A-A1A or on EMDs B1-1B powered axle arrangements is weight. Either you sacrifice fuel to hold the weight to allowable limits or engine size, or perhaps both. Both the GP60M and B40-8W locomotives were right at the maximum allowable weights, and both had reduced fuel capacity, and remember these locomotives were built to Tier 0 emissions standards. So imagine what enlarged cooling capacity carrying more water with more weight is going to do.
They want that jumbo sized fuel tank you get with a 6 axle frame! If you could jam all the equipment, crash-worthy cab, emissions controls/cooling and that jumbo fuel tank into a four axle package with the same axle weight and without making the locomototive out something other than cheap, easy to deal with steel, the RRs would be on it.
The real difference between a six motor six axle and a four motor six axle is between roughly 12 and 18 mph. If, at full HP, you aren't going to drop below 18 mph, you don't need those other two motors for anything. Might as well not have them along for the trip.
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
BackshopFine, I'm not an engineer (TG) but do I have the concept correctly? The same amount of total power fed to fewer motors allows them to turn faster?
Again, no, you're not right. Let me try plainer or better English.
"Current" is not the same thing as horsepower; it's just a measure of the number of electrons being moved. It is measured in amps, and a take-home message most places in power engineering is that you want to minimize this in conductors because its square shows up in heat losses.
Voltage equates to pressure, and for reasons I'll let Erik express in English, it determines how fast a typical DC traction motor will turn ... which in turn determines the thing you were trying to ask, how fast the motors and gearing will move the locomotive.
Grossly simplifying for a moment, power in watts (which is how much 'twist' the motor exerts on a load) is amps x volts, and dividing kilowatts by ¾ roughly gives you horsepower. Now in the sense that more torque will overcome more resistance, more current will give you more power, and if there are fewer losses in 4 motors vs. 6, there will be more power available for torque, hence more acceleration and perhaps a higher balancing speed shy of the voltage-determined speed. But the actual power the locomotive as a whole exerts is already limited by the effective prime-mover power.
Note that if you had said 'makes the locomotive move faster' you'd have been correct, and perhaps that is what you meant. But this is not because current makes the motors 'turn faster' but because it makes them turn 'stronger'.
To put it simply, for DC current times voltage equals power in the same way that torque times rotation speed equals power.
In a DC electric motor the torque varies with the product of armature current and field, while the voltage across the terminals (often referred to as back EMF (ElectroMotive Force) is proportional to the field time the rotational speed. In a typical DC series traction motor, the field is proportional to armature current at low current and approaches a limit at high current due to saturation of the magnet pathways of the motor (it is possible to use more iron in the motor frame to avoid saturation, but the motor would be too heavy to use). The onset of saturation is usually about the continuous current rating of the motor.
A simple rule to keep in mind is that for a given torque (drawbar pull), the terminal voltage of a motor will be proportional to the speed, while current remains roughly constant. Another thing to keep in mind is that at high speeds, the field needs to be weakened to keep the commutator from flashing.
Back in the days of DC traction generators, there were limits on how much voltage could be generated before the commutator started flashing (really bad news) and how much current could be generated before the windings overheated. Similar issues exist with the motors, where too much voltage will cause the commutator to flash. For low speeds and high tractive effort where the motors are drawing a lot of current, the solution is to connect the motors in series and for a given traction generator current, 6 motors in series will give 50% more drawbar pull than 4 in series, albeit at 2/3rds the speed. Conversely at high speeds, the motors are connected in parallel to limit the voltage required from the traction generator. (This is what transition was all about).
An additional aside about traction generator (and traction alternators for DC motors). The windings for these beasts were set up so that voltage would decrease with increasing voltage in a way that the current times voltage was roughly constant, i.e. presenting a constant load to the prime mover. This was developed by Hermann Lemp in the early 1920's and was effectively the first infinitely variable automatic transmission.
There is a case where 6 motors can run faster than 4 motors - with 6 motors in parallel, each motor is drawing 2/3rds of the current as in the 4 motor case, which means the field with be lower which then means that the speed can be higher before the motor or generator voltage limits are reached. The other workaround was to use field shunts to reduce the field, which reduced torque for a given armature current but allowed for a higher speed (as in upshifting gears).
Slight edit required re the Lemp control.
Here is J. Parker Lamb on the application to motor locomotives.
The list of Lemp's patents is here, with the relevant one being US1216237A.
OK, I understand the DC motors, very well explained, Now please explain the AC motors and why they can pull at full force for very long times and to not overheat like DC motors.
caldreamerOK, I understand the DC motors ... Now please explain the AC motors and why they can pull at full force for very long times and to not overheat like DC motors.
The first thing to rememberis that the DC motors in most American freight power are not like model-railroad or treadmill motors -- they have a wound armature and also a wound (and not permanent-magnet) field. This wound field's power varies only in strength, with the more power giving the more 'magnetism' but also greater heating due to the resistance in the field windings. Think of this as what the rotating armature 'pulls and pushes against' to create torque.
Now, more current passes through some of the armature windings, in the correct direction to produce torque in the intended direction of rotation. This would be relatively little rotation indeed before the magnetic fields aligned; instead, the device known as the commutator switches the connection to the windings as the armature rotates, so the armature poles keep having to turn to try to line up with the field and the motor develops torque. Again, there are heat losses as the current passes through the armature winding, and there can be relatively high peak current if the commutator 'makes' and 'breaks' connections and the magnetic field induced in particular coils lapses (generating high peak current just as opening circuit on a capacitor induces high peak voltage across the gap). Meanwhile, for functional reasons keeping the air gap inside the motor between the field pole faces and the armature pole faces should be kept to a minimum for best magnetic efficiency.
What this implies is that there is a fair amount of heat generated in one of these motors, and when it is to work in a confined space protected from road dirt and water it will need to dissipate that heat reasonably well. This is why most diesel-electrics have capable traction-motor blowers: to air-cool through the gap regardless of whether the motor is turning fast enough that it can drive a cooling fan arrangement.
Now, the limit in heating is largely determined by the softening point of the material used to insulate the turns of the armature and field coils from each other ... modern materials like Kapton surviving larger temperature for a given thickness, or conversely allowing more 'copper' for a higher output within given coil dimensions ... and the advent of modern welding (TIG) used to attach armature coil wire to commutator bars gets rid of the solder that so famously affected, say, the DL109 testing on ATSF, so higher temperatures are now possible than were allowable in older motors. There are still limits on how hot the motors can become, though. (Permanent-magnet motors have much more stringent restrictions on their temperature, for reasons involving the Curie point of the magnet material close to the gap and nonreversible changes in magnet power as a result of overheating, and those do matter for modern trains that use them, but that's a different subject.)
In an AC motor, there are three 'phases' of electrical supply, 120 degrees apart in phase, and these are applied to the field windings in a motor so they create a 'rotating' field, like the effect of chase lights in a theatre marquee. The changing magnetic field can induce current flow, and hence magnetic attraction, in plain bars of nonmagnetic material set in an armature 'drum', and the rotating field tends to pull the bars around, developing torque without running energy through connections or windings in the rotor at all. When the rotor is locked, all that happens is that the magnetic phase continues rotating and exerting what amounts to a 'drag' on the rotor; conversely, the speed with which the rotating field 'turns' can be varied.
I'll let Erik explain the induction of back EMF in wound-armature motors, but note that a good locomotive induction motor has essentially none, meaning that the maximum synchronized rotational speed of the motor is determined only by characteristics of the modulated field current, and for a synthesized AC drive this can be relatively fast without counter-losses related to speed.
Note that you can't just connect main-generator 'juice' to an AC induction motor and have it turn ... instead, what locomotives do is generate AC at variable frequency (because an alternator is more robust and capable than a 'generator'), rectify and filter this to relatively-well-smoothed DC at what's called the DC-link voltage, and then 'synthesize' AC, which need not be a sine-wave or at powerline frequency as in HEP, at appropriate frequency and phase to be used for traction. You can deduce some of the advantages for and against one inverter per truck vs. one per motor, and we've have some discussions about this that may make more sense to you now.
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oltmanndThe real difference between a six motor six axle and a four motor six axle is between roughly 12 and 18 mph. If, at full HP, you aren't going to drop below 18 mph, you don't need those other two motors for anything. Might as well not have them along for the trip.
zardoz oltmannd The real difference between a six motor six axle and a four motor six axle is between roughly 12 and 18 mph. If, at full HP, you aren't going to drop below 18 mph, you don't need those other two motors for anything. Might as well not have them along for the trip. True; however, much of the time freight IS moving under 18mph. In addition, under 18mph is where the most tractive effort is needed. And even if the C4 locomotives do have fancy wheelslip controls, when the computer detects slipping it REDUCES power, which leads us back to my main point.
oltmannd The real difference between a six motor six axle and a four motor six axle is between roughly 12 and 18 mph. If, at full HP, you aren't going to drop below 18 mph, you don't need those other two motors for anything. Might as well not have them along for the trip.
True; however, much of the time freight IS moving under 18mph. In addition, under 18mph is where the most tractive effort is needed. And even if the C4 locomotives do have fancy wheelslip controls, when the computer detects slipping it REDUCES power, which leads us back to my main point.
You have this a bit sideways. Any train climbing the ruling grade with C4 locomotives at less than 14 mph is very likely to stall.
The maximum TE on AC locmotives is adhesion limited. A six motor locomotive will produce about 150,000# TE. A four motor unit, about 100,000# TE. A 4400 HP six motor unit will hit this limit at roughly 9 mph. A four motor unit, at 13.5 mph. (my 12 and 18 mph guessimates were off a bit. A remnant from the days of GP40s and SD40s)
Lets put these locomotives on a 1.5% grade with a train at 1 HP per ton. A 4400 ton train needs 132,000# TE to climb that grade. A six motor unit can do it and will climb the grade at ~10 mph. A four motor unit will stall with that train unless adhesion conditions allow 47% adhesion - not likely.
The C4 locomotives are good matches for the modern DC fleet. You can mix and match them with DC locomotives without the DC unit seriously limiting the performance of the AC locomotive. At the point the DC units are approching "being in the red", the C4 units are approaching their adhesion limit.
Please correct me if I am incrrect. The way I understand the expanatiuon of why AC will not overhet like DC units is because they do not have motor windings, but instead have non magnetic bars that spin and creae the torque as they spin.
caldreamerThe way I understand the explanation of why AC will not overheat like DC units is because they do not have motor windings, but instead have non magnetic bars that spin and create the torque as they spin.
You have to visualize exactly where the 'nonmagnetic bars' are, what they're hooked up to do, and what the geometry of their 'spinning' is.
There is no motor action in these large motors without magnetic attraction. Therefore, the 'non-magnetic' bars have to be made "susceptible" to being attracted by the rotating magnetic field before any 'spinning' will take place. This is why they are electrically connected as they are. Part of the action involved concerns 'eddy currents' which you might want to read about.
The non-magnetic bars that have had magnetism induced in them are arranged around the outside of a rotating armature that performs the same function a wound-rotor armature does. The 'bars' replace the pole faces in such an armature. They are very rigidly fixed in slots in the rotor drum, and shouldn't move at all, let alone 'spin', individually. It is the whole armature, with the set of bars fixed around its periphery, that spins.
And of course there is a pinion arrangement on the end of the drum-armature shaft that communicates with a bull gear on an axle in the 'usual' way for nose suspended motors, at least in most of these AC freight locomotives.
Oddly, a great many of the Internet resources on induction traction motors either wildly overcomplicate the perceived math or leave out key details of 'how the trick actually works' that laymen can understand. Here is a site that contains a number of interesting observations once you get past all those scary formulae. Note in particular the discussions of the different regimes of motor heating.
oltmanndYou have this a bit sideways. Any train climbing the ruling grade with C4 locomotives at less than 14 mph is very likely to stall.
zardoz Perhaps I did not make my point very well (even my wife says I ramble on sometimes). I agree with what you posted. The C4's might have their place, but a versatile locomotive they are not.
Perhaps I did not make my point very well (even my wife says I ramble on sometimes). I agree with what you posted. The C4's might have their place, but a versatile locomotive they are not.
And so it should come as no surprise that the only Class I railroad to buy them was also the largest buyer of the Dash-9 and ES44DC locomotives before them.
caldreamer Please correct me if I am incrrect. The way I understand the expanatiuon of why AC will not overhet like DC units is because they do not have motor windings, but instead have non magnetic bars that spin and creae the torque as they spin.
"Squirrel Cage" induction motors have large conducting bars in the rotor, the slip between the rotating field (stator windings) induces currents to flow in the bars which interacts with the rotating field to generate torque. These bars are not embedded in iron slots as in the case of a DC motor, and also do not need insulation. Both of these help with heat transfer and the lack of insulation also allows the bars to operate at higher temperatures. A more subtle issue is that the currents in the bars are at a lower frequency than in a DC motor, which takes care of skin effect and proximity effect.
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