A long time ago (late 2015/early 2016 I believe), I had the brief opportunity to step inside one of the ACS-64's. One thing I remember clearly was a lock-shaped mechanism on one of the walls of the "engine corridor." It looked like one of the locks you'd see in an elevator that people use to shut it down for maintenance or need to keep it open for long periods without setting an alarm off.
Anyway, so the engineer saw me looking at this lock and what he told me is that is was basically a speed limiter for the locomotive to run. I remember there being a key-notch for 70 mph, 90 or 100 mph, 110 mph, 125 mph, and 135 mph. That one got me the most surprised, since at the time I only thought the Acelas went faster than 125 mph. Only in later years did I know that the Sprinters were designed for a top speed of 135 mph, but were limited to 125 mph in service. Is this common for passenger locomotives now, hence why the P42's which were limited to run 79 mph in service were almost readily able to run tests at 110 mph, their design max speed, in Michigan?
Now if I'm right about this, this leads me into another question. I'm aware of the railroads like the Pennsy and Erie Lackawanna regearing some of their passenger diesels for freight trains. Be it for lack of reliability compared to EMD's units, or just less passenger trains to pull. I thought I read somewhere that these different gearings changed the loco's performance a bit. I know the ratio of gears really determines the speed of the geartrain, but can such re-gearings also alter the engine's tractive effort or maximum speed? Similarly, what was the purpose of regearing the CSX F40's for "freight" speeds when they could likely keep line speed on the first few notches and have only been used on OCS and other special passenger runs. And on another side note, why would you decrease the horsepower of a locomotive when regearing it for freight service. I mean I understand freight diesels generally have less horsepower than their passenger contemporaries, but why derate them?
Yes I'm aware I'm asking a lot, feel free to take your time in answering
RailfanGXY ....what was the purpose of regearing the CSX F40's for "freight" speeds....
Almost certainly a parts inventory issue. A traction motor on the F40 goes belly-up, you just plop another 62:15 geared traction motor combo in. DC traction motors are one of the top things that fail on locomotives. About 1/3 of the cost of maintaining a DC motored locomotive is traction motor related.
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
RailfanGXYI'm aware of the railroads like the Pennsy and Erie Lackawanna regearing some of their passenger diesels for freight trains. Be it for lack of reliability compared to EMD's units, or just less passenger trains to pull. I thought I read somewhere that these different gearings changed the loco's performance a bit. I know the ratio of gears really determines the speed of the geartrain, but can such re-gearings also alter the engine's tractive effort or maximum speed?
The "taller" the gearing, the higher the minimum continuous speed and the higher the top speed. The traction motor can only turn so fast before it will come apart. For a locomotive with "freight" gearing (and 40" wheels), standard 62:15 gearing will get you 65-70 mph max (depending how much the RR is willing to risk). It will also get you minimum continous full HP speed around 18 mph for a 3000 HP four axle. If you want a higher top speed, you sacrifice how much tonnage you can pull up the ruling grade.
Amtrak's SDP40Fs were all geared 57:20, so were supposed to be good for ... 102 mph? But no RR authorized them to run more than 90, and only UP and SFe (and ICG?) allowed them more than 79.
timz Amtrak's SDP40Fs were all geared 57:20, so were supposed to be good for ... 102 mph? But no RR authorized them to run more than 90, and only UP and SFe (and ICG?) allowed them more than 79.
So, you buy 102 mph gearing to give you a margin over 90 mph - there was still some "cheating" going on back then. If you can get up and over the ruling grade with the HP/ton you need for schedule, no need to change gearing. Also, same combos for F40s - it's a pain to have to store various gear ratios.
I don't believe EL, SAL and others that ended up using passenger engines in freight service regeared them. The used them with their passenger gearing on their 'premier' freight services that relied on speed and not tonnage to be fulfill the carriers desires. Most of the photographs I have seen, the passenger engines are being used on intermodal runs, not in unit bulk commodity trains where maximum tonnage per locomotive is the key to making money.
Never too old to have a happy childhood!
BaltACDI don't believe EL, SAL and others that ended up using passenger engines in freight service regeared them. The used them with their passenger gearing on their 'premier' freight services that relied on speed and not tonnage to be fulfill the carriers desires.
It would take me more time than I have to go through and confirm all the different references, but my strong impression 'over the years' was that any railroad that had the money changed out the gearing when putting 'passenger' engines in freight service. (And note that "had the money" is a relative thing, as you have to be hard up or have very special service offerings indeed to use a first-generation passenger locomotive as freight power in the first place...)
PRR very famously ran into a problem outside the issue of final-drive gearing when they started 'repurposing' good E units as higher-speed freight engines, e.g. on TrucTrain services. Most of the PRR freight engines had automatic back transition, and the freight crews were used to this. Many of the E units did not, and this would result in dramatic flashovers and associated damage fairly quickly when anyone 'forgot'.
Part of the problem with taller numerical gearing is that the slipping problem, especially on older units with rudimentary if indeed any antislip control other than idiot lights, is greater for the same 'horsepower to the rail' in a given speed range. So unless you have very fast trains indeed that have full priority to keep running at the very fast speeds, and don't have to stop at any unexpected point in inclement conditions, you'll have a much more flexible locomotive at a 'saner' geared speed -- 65 to 70mph proving ample for almost everyone that could prove a n actual revenue enhancement from the achieved speed. (That certainly didn't seem to prove to be the case for any railroad that tried 90mph-plus peak speeds... even in the greatest days of UP, SP, and ATSF high-speed merchandise train attempts ... or for the Z train experiments with Genesis locomotives around the turn of the 21st Century)
Overmod BaltACD I don't believe EL, SAL and others that ended up using passenger engines in freight service regeared them. The used them with their passenger gearing on their 'premier' freight services that relied on speed and not tonnage to be fulfill the carriers desires. It would take me more time than I have to go through and confirm all the different references, but my strong impression 'over the years' was that any railroad that had the money changed out the gearing when putting 'passenger' engines in freight service. (And note that "had the money" is a relative thing, as you have to be hard up or have very special service offerings indeed to use a first-generation passenger locomotive as freight power in the first place...) PRR very famously ran into a problem outside the issue of final-drive gearing when they started 'repurposing' good E units as higher-speed freight engines, e.g. on TrucTrain services. Most of the PRR freight engines had automatic back transition, and the freight crews were used to this. Many of the E units did not, and this would result in dramatic flashovers and associated damage fairly quickly when anyone 'forgot'. Part of the problem with taller numerical gearing is that the slipping problem, especially on older units with rudimentary if indeed any antislip control other than idiot lights, is greater for the same 'horsepower to the rail' in a given speed range. So unless you have very fast trains indeed that have full priority to keep running at the very fast speeds, and don't have to stop at any unexpected point in inclement conditions, you'll have a much more flexible locomotive at a 'saner' geared speed -- 65 to 70mph proving ample for almost everyone that could prove a n actual revenue enhancement from the achieved speed. (That certainly didn't seem to prove to be the case for any railroad that tried 90mph-plus peak speeds... even in the greatest days of UP, SP, and ATSF high-speed merchandise train attempts ... or for the Z train experiments with Genesis locomotives around the turn of the 21st Century)
BaltACD I don't believe EL, SAL and others that ended up using passenger engines in freight service regeared them. The used them with their passenger gearing on their 'premier' freight services that relied on speed and not tonnage to be fulfill the carriers desires.
PRR very famously ran into a problem outside the issue of final-drive gearing when they started 'repurposing' good E units as higher-speed freight engines, e.g. on TrucTrain services. Most of the PRR freight engines had automatic back transition, and the freight crews were used to this. Many of the E units did not, and this would result in dramatic flashovers and associated damage fairly quickly when anyone 'forgot'. Part of the problem with taller numerical gearing is that the slipping problem, especially on older units with rudimentary if indeed any antislip control other than idiot lights, is greater for the same 'horsepower to the rail' in a given speed range. So unless you have very fast trains indeed that have full priority to keep running at the very fast speeds, and don't have to stop at any unexpected point in inclement conditions, you'll have a much more flexible locomotive at a 'saner' geared speed -- 65 to 70mph proving ample for almost everyone that could prove a n actual revenue enhancement from the achieved speed. (That certainly didn't seem to prove to be the case for any railroad that tried 90mph-plus peak speeds... even in the greatest days of UP, SP, and ATSF high-speed merchandise train attempts ... or for the Z train experiments with Genesis locomotives around the turn of the 21st Century)
The B&O obtained a series of F3 engines (both A and B units) for passenger service (view them on publicity photos of the Strata-dome equipped, streamlined Columbian). Those engines were geared for a 98 MPH maximum speed. After a decade of mostly passenger use, they were all assigned to the Akron-Chicago Division for freight use between Chicago and Willard, in this service they retained their passenger gearing. The ruling grade between Chicago and Willard is Tiffin Hill at 0.30%. They were among the first engines traded in for 2nd generation diesel power.
Remember, at the time carriers were using passenger power in freight service, costs were being held to a minimum - regearing a in service locomotive increases costs in replacing a working traction motor/wheel set with another working traction motor/wheel set. Then name of the game was use the locomotives with minimum maintenance expenditures until the end of their lease or trust - then get them off the company roster. In the late 60's all carriers were in severe financial stress.
B&O had some passenger GP7s and GP9s that were regeared when they were renumbered into the freight-locomotive series during 1964 and 1958, respectively. I suspect that the B&O passenger F3s were not regeared in 1958, when they were renumbered into the freight-locomotive series, because they continued to be used occasionally in passenger service. Although some of them -- along with all of B&O's first freight F3s -- were traded in on GP30s in 1962, others survived until 1964, when they and a number of F7s were traded in on GP35s.
Locomotive design is a careful matching of weight, torque, horsepower and speed. DC motors have max torque at zero rpm and the torque drops nearly linearly to zero at some maximum rpm. (it is a straight line graph -- nearly). Since power is approximately rpm times torque, power is maximum somewhere in the middle of the torque curve. Higher rpm and power drops. That limits the power a locomotive can produce. Additionally, power is limited by the amount of power it can transfer to the rail. The steel to steel coefficient of friction is about 25% so once the pull on the coupler exeeds about 25 tons per 100 ton loco the wheels slip. Addtionally, the coupler will break at about 100 tons of pull. (That is why the max of about 4 locos coupled together.)
So you gear the motors so their maximum power occurs at the max speed one expects along their route. There are a number of ratios but most are about 4 to 1. If the maximum power on a motor occurs at 2000 rpm and and you have a 40" wheel (diameter) each wheel rotation is 10 feet of travel. Then the wheel goes at 500 rpm * 10 foot/revolution = 5000 feet/minute. That is 1 mile per minute or 60 mph. So roughly the motors are designed for max torque in the range of 2000 rpm so the loco's power is max at the designed speed of 60 mph. Interestingly if you try to speed up further by raising the rpm of the electric motors you are limited as the motor torque drops but all limiting forces (wind, friction...) increase and the train can't go faster.
For DC series motors, torque is a non-linear (though monotonic) function of current, and terminal voltage is a function of current and motor speed. Ultimate speed limit for a given tractive effort is where the terminal votage reaches the ratings for that motor ASSUMING that the prime mover + traction generator/alternator can source that amount of power. This was one on the interesting things about the Milwaukee Little Joes, they could produce maximum tractive effort at 23 - 24 mph provided that the catenary could supply the current (which it could near the Janney and East Portal substations).
DC motor current is a balance between the generator and motor. As the motor speeds up its EMF (retarding voltage due to magnetic field rotation) increases. To match that, the generator's voltage has to increase. At some point the generator (or the overhead catenary) voltage is maximum and the motor speed is limited. As the load increases, supply voltage drops due to generator internal load (or catenary loss -- which is just another distant generator). So not only are the motors limted by their torque curve but the generators are limted by their internal loss. The idea is to make them all match at the maximum design speed. If you cannot do that (due to design limits on power systems), you have to take what you get. If the maximum power off the catenary occurs at a train speed of 24 mph, that is what you get. No amount of gearing or ratios can help that. Power is controlled by train size and the grade.
FINALLY someone introduces the back EMF coherently into the discussion (although I'd note that you have to say the whole thing, 'back EMF' or 'counter-EMF' in this context, as EMF is just 'electromotive force', a fancy way to say 'voltage')
How the back EMF is generated in a DC motor, and why winding shunts address it in ways permitting higher rotational speed, is an interesting discussion and I recommend it to all of your attention. Some of the design of series/parallel transition is involved with this as well as with current limitation; it is why some EMDs have extended transition (into field shunting) above full parallel.
A 'fun' thing to look into is why E8 generators flash over if run on freight and proper manual 'back-transition' isn't made when it should be...
petitnjDC motors have max torque at zero rpm and the torque drops nearly linearly to zero at some maximum rpm.
petitnj DC motor current is a balance between the generator and motor. As the motor speeds up its EMF (retarding voltage due to magnetic field rotation) increases.
DC motor current is a balance between the generator and motor. As the motor speeds up its EMF (retarding voltage due to magnetic field rotation) increases.
That's assuming a relatively constant field. Back in the days before power electronics provided a low cost high efficiency variable DC voltage, one way to vary the speed of a DC motor was to design it for field control (usually "shunt" wound). The armature would normally be directly connected across the supply lines, with resistance used only on starting and starting would typically be done with maximum field (lowest speed). Decreasing the field would result in a lower EMF (voltage) which would cause armature current (and torque) to increase which then would increase speed and resulting armature EMF.
The EMF of a series motor is a function of both current and rotational, though more accurately the magnetic flux density at the pole faces and rotational speed. Traction motors are usually designed for minimal weight (relatively speaking) and will frequently operate with a saturated magnetic circuit (flux density is NOT a linear function of field current).
The limiting factor for EMF is commutation, with a typical limit of 20V between commutator bars on a motor with interpoles. For a motor, terminal voltage will be the EMF plus the I*R voltage, for a generator terminal voltage will be EMF minus the I*R voltage.
The Lemp control system for traction generators does a faily good approximation of providing a constant power output when coupled to a series motor, i.e. a continuously variable transmission. If you look at the tractive effort vs speed curves for a diesel electric locomotive, at the lowest speeds, tractive effort is adhesion limitied and horspower limited at higher speeds.
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