How are the speeds matched of the locos in multiple unit operation?
Electric motors are not geared to each other. Each traction motor is geared to its own wheel set. The motor will move the wheel set as fast as the power fed to the traction motor and the load placed upon the wheel set will permit. If a traction motor/wheel set has more power than load - it will slip - thus locomotives that have electric motor have some form of wheel slip control to prevent continued wheel slippage.
There are hundreds if not thousands of books that can detail every facet of this means of electrical traction.
At each notch, a locomotive puts out the same percentage of its total output as the others do, so they all go approximately the same speed.
That works as long as these locomotives have a similar speed scale, with each notch position corresponding approximately to the same speed as other types.
Things start to become ‘interesting’ if you mix freight and passenger locomotives in the consist, as you can imagine…
N.F.
Yes-freight locomotives all tend to have very similar gear ratios unless they are for specific special jobs. UP tried to equip some “Fast Forties” with faster ratios but gave up as it was not flexible enough.
Back in the 50sand 60s SP had passenger engines with several different gear ratios. The E-units and PAs were geared a little above the FP7s, which in turn were geared higher than the boiler-equipped SD7/9s and GP9s. FP7 and F7 units were tryed with compromise 61:16 gearing before settling on 60:17. By the late 1960s all of the remaining non-commute passenger power including the new SDP45s had been regeared to 60:17, which was supposed to be good for 78 MPH, slightly above SP’s 75MPH limit on most lines. Even the SD9E pair rebuilt in 1974 for “reserve passenger service” got 60:17 gears. (The trailing unit in last month’s SP office car story was one of those). There were supposedly rules on which units could be mixed. In practice it worked out that the E’s and PAs stayed together, the GPs and SDs worked alone, and the FP7/F7Bs stayed together. After 1968 the SDP45s often partnered with FP7s and F7Bs.
Having noted that, SP often mixed other units in, usually with 65MPH 62:15 gears. SSW 9389 in the office car story is one of them. I rode the Zephyr over Donner once with 9389 (in red and gray) leading a pair of Amtrak SDP40Fs as far as Sparks.
Towards the end of their service lives, Amtrak leased out some F40PH’s to UP and other freight carriers. They wound up being restricted to intermodal service because of their high-speed gearing.
Problems with locomotive that have different gearing manifest themselves in two fashions. A ‘passenger’ locomotive being used in freight service has a higher ‘minimum continuous speed’ - the speed at which the maximum amperage can be supplied to the traction motor and not have the traction motor overheat and damage the electrical components of the traction motor. There are ‘short time ratings’ which allow more amperage to go to the traction motors for a defined amount of time. These kinds of restrictions apply to DC traction, not AC traction.
The other side of the gearing issue is that a ‘passenger’ locomotive can operate at speeds that will have ‘freight’ geared locomotive suffering mechanical damage to their traction motors account excessive rotational speeds. In current freight and passenger locomotives the traction motors are physically geared to their own wheelset and there is no ‘neutral’ - if a wheel turns a traction motor turns.
Back in the day, passenger locomotives could be geared for operat
Balt has it right. Gear ratio only is an issue at lower and higher speeds. In between it just manifests itself by the voltage and amperage for a given speed.
One big difference would be older DC electrics where different classes would be incapable of MU’ing. Examples were the GE boxcabs and Little Joes on the Milwaukee and the boxcabs vs 201-202 on the BA&P.
Boy, you guys make things so complicated!
The best analogy that I have ever seen was to think of this as a Tug-of-War. On one side of the rope we have the train, on the other side of the rope we have the engines.
When one person pulls on the rope with all of their might, nothing moves. When another person grabs ahold and pulls with all of their might, nothing moves, when another grabs the rope and starts pulling, things begin to move and so on and so forth. Everyone pulls the best that they can and the train moves. Eezy-peezy!
Chicago Rapid Transit, the precursor to today’s CTA, ran mixed consists from various eras and body types (40 or 50MPH top speed, wood and steel bodied) in trains of up to eight cars, with plenty of tugging and shoving. Rube Goldberg adaptations of mismatched control systems didn’t help a whole lot.
CRT tenant Chicago Aurora & Elgin leased and later bought wooden cars from the North Shore Line that would trainline with their own, but the speed mismatch between the series ended the practice of mixing them pretty quickly.
That’s cute, butit’s already the wrong analogy. We’re concerned with what’s happening between the MUed units, not behind the drawbar of the last one (except in a more incidental sense)
But the situation here is more complicated. What if your first person is pulling away and the second person yanks the rope harder, or pulls with more acceleration on the rope and bangs into the first person? Or when the first person takes his time ‘warming up’ to pull hard, while the second person goes right to it and yanks? Those are the kinds of analogy that apply in this situation, and you are not discussing them.
You could also include (appropriately) some considerations like maximum pull coming only above a certain rope speed, or time at full pull being limited by endurance or excessive temperature, or leverage producing greater slipping or differential force when different ‘notches’ of exertion are selected by a simple system (or when the analogue of ‘making transition’ factors into pulling on the rope)
The analogy itself is a reasonable one (if you include some method of incorporating active buff and inertia into the tug-o
There is / was an A&SAB alcoa road switcher on display in Panama City, Fl that had just a 19 (?) pin MU receptacle not the standard 27 one.
Based on Jerry Pinkepank’s article, “Lash 'Em Up”, in TRAINS some years back, 27-pin MU cables were the standard with a two-cable arrangement of 12 and 21 pin cables being a major alternate. It also mentioned that not every pin carried a function. There were also some other variants.
Until about the mid-1960’s, locomotives stayed almost entirely on their home roads so MU incompatibility was not a major issue.
[quote user=“RME”]
BigJim
railfan50036
How are the speeds matched of the locos in multiple unit operation?
Boy, you guys make things so complicated!
The best analogy that I have ever seen was to think of this as a Tug-of-War. On one side of the rope we have the train, on the other side of the rope we have the engines.
That’s cute, butit’s already the wrong analogy. We’re concerned with what’s happening between the MUed units, not behind the drawbar of the last one (except in a more incidental sense)
When one person pulls on the rope with all of their might, nothing moves. When another person grabs ahold and pulls with all of their might, nothing moves, when another grabs the rope and starts pulling, things begin to move and so on and so forth. Everyone pulls the best that they can and the train moves. Eezy-peezy!
But the situation here is more complicated. What if your first person is pulling away and the second person yanks the rope harder, or pulls with more acceleration on the rope and bangs into the first person? Or when the first person takes his time ‘warming up’ to pull hard, while the second person goes right to it and yanks? Those are the kinds of analogy that apply in this situation, and you are not discussing them.
You could also include (appropriately) some considerations like maximum pull coming only above a certain rope speed, or time at full pull being limited by endurance or excessive temperature, or leverage producing greater slipping or differential force when different ‘notches’ of exertion are selected by a simple system (or when the analogue of ‘making transition’ factors into pulling on the rope)
The analogy itself is a reasonable one (if you include some method of incorporating activ
I think part of this is that we’re using very different definitions of what he means by ‘matching the speeds’.
Rather obviously, railroads make some attempt to keep locomotives with similar loading response and gear ratio in consists … this avoids the very real issues I was discussing.
I had thought this was clear, but in retrospect I see it was an assumption: there is no real attempt to “match” locomotive speeds with any kind of adjustment once the consist is together and on a train. (I have seen some articles about tinkering with things like transition relays when those ‘popped’ in inconvenient ways, this in a sense and indirectly being related to gear ratio, but I doubt these involved actual “engineering” or perhaps even management- or craft-sanctioned activities.)
One thing to remember, though, is that even in steady-state the loading that locomotives take up based on MU control may not be the ‘pull-on-the-rope’ sum of all the nominal TEs that, for example, applies to steam multiple-headers, although (as Big Jim notes) the difference is usually not of meaningful importance in running trains. If you look at comparable points on the constant-power graph for locomotives with different final-drive ratios (leaving out conditions of low-power motor rating or high-speed birdsnesting risk) you can easily see the difference in effective drawbar pull at speed dictated by the digital – and yes, it is digital – ‘run’ setting of the MU control. And in essence this is the situation you’d have if you had a steam doubleheader … with a common throttle and reverse setting for both locomotives that couldn’t be ‘tweaked’. (That of course is much more extreme th
I gotta agree with BigJim that everyone is getting carried away with all kinds of details. We need to hear from Railfan exactly what he meant by his question. I assumed he was asking about how throttle setting was passed from one unit to another.
But that’s defined by the MU settings for the engine governor, and has nothing whatsoever to do with motor gearing (or transition or much of anything else of relevance to the issue of “matching” units).
Since there is evidently little interest in ways of achieving better ‘fine matching’ of locomotive unit performance, here is one reference that answers Paul’s assumed question.
One example of benefit from an enhanced control interpretation is David Cook’s patent application 20140365049. There are obvious versions of this approach that could address mismatches in gearing, transition, etc. but we can take them up in a different thread where there is interest.
There is an old saying:
Perception means everything, reality means nothing.
Folks in my forty+ years of railroading, the reality is, speed matching is a perception…a non-issue…a bunch of bunk!
You have GOT to remember, real locomotive axles are NOT geared together in a group like toy trains, so, they don’t fight one another. Each axle has its own traction motor, independently geared. They are free wheeling and compliment one another, so they do not care what the other units are doing.
Each unit will only do what is asked of it. AND keep in mind that a throttle notch does not ask for speed, but, that certain horsepower rating that it is designed to give for each particular notch.