Thanks everyone for your input... lots of very knowledgeable people on this site..
Besides, "notch 8" wouldn't mean anything any more (ie, full throttle).
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...
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RWM has it right. I'll just add that the number of notches is 8 instead of 7 of 9 or 10 because that's how many unique combinations you can make from 3 solenoids. The 4th, "D" solenoid is in the governor to actuate an engine shutdown.
The governor is basically a flyball type and the three solenoid act on a triangular plate with a geometry such that each solenoid is "worth" either 1, 2 or 4 notches. (I think A is worth 1, B is worth 4 and C is worth 2) The plate pushes against a spring in the opposite direction from that of the flyball mechanism.
The D solenoid is woth -2 (it doesn't act on the triangular plate directly) and is is picked up along with C in notches 5 and 6 to get it some exercise so it won't be stuck when it's called on for a shut down. It is also used for low idle (D + A = -1, not enough for a shut down)
If you added another solenoid, you could get 16 speed settings.
The fulcrum nut sets the speed.
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
You nailed it, RWM! Now I understand how a diesel-electric throttle works (in principle, anyway). Hope you don't mind if I cut and save your response for my personal RR info file. - a.s.
Locomotives have to have a system to maintain speed of the engine regardless of load placed on the engine, a function which is provided by a governor. Automobiles do not need this system because they do not have an electrical transmission but have a mechanical transmission.
EMD began by designing its locomotives with eight throttle steps plus idle. It could have chosen 10, 16, 7, 9, or 25, but chose eight because (1) that was a sufficient level of "fineness" to meet users needs for the rather narrow rpm range capability of the engine without subjecting the user to needless complexity that would drive up purchase price and maintenance price, and (2) it was convenient to use 8 because of the electromechanical technology of the day. The Woodward governor applied to an EMD engine used four solenoids to effect speed control. Each throttle setting energized a different combination of solenoids, as follows (for a 16-567C):
Someone more versed in the byzantine workings of governors and electrical control systems can probably do a far better job of explaining how governor's work. Suffice it for me to say that "8" throttle notches is what we had at first and that became the convention. Today's locomotives with microprocessor controls could have a continuously variable throttle, and in effect the microprocessors govern the engine in that fashion anyway. But the 8 notches remain because it is convenient to operate in that fashion. Speed control on a locomotive has to be varied continuously anyway, in many operating conditions, in order to maintain the train as close as possible but not over the maximum authorized track speed. Thus removing the detents doesn't save any work but it does make operating memory and practice harder to replicate.
RWM
That's interesting...you mentioned "the same amp rate for a given rpm" for each power setting. So the power setting determines the rpm which in turn determines the current output of the generator.
Because you don’t need the ability to vary the throttle.
As Tree pointed out, you have to MU locomotives, so they need to all have the same basic settings…in that a given RPM produces a set amp rate to the traction motors.
Notch 5 on a Dash 9 is the same as notch 5 on a SD70.
Also, you would be fatigued beyond any thing you have experienced if you used a foot throttle on a locomotive…driving a automobile or a large truck is totally different.
You have varying terrain or road surfaces, and the need to accelerate and decelerate quickly for turns and stops, neither of which really applies to what a train does.
Keep in mind that a Dash 9 weighs in around 440,000lbs...if you "stomped on the gas" in one, it would slip the wheels, and something that weighs that much needs a constant steady acceleration, just like what is provided by "notches" as opposed to a variable throttle.
But a constant setting works best for how trains handle…you don’t need to stop and start often, so why go through the mechanical additions to create a variable throttle that’s not needed.
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Searched "throttle settings"...found nothin
Seems like we talked about this a while back.
I think part of it has to do with MU'ing locomotives together. Something like three or four wires can give you enough combinations to do all eight steps.
I don't think its the electric motor...those 200 ton dump trucks used in mines are diesel-electric also..and they have a continuously variable foot throttle much like a car does.
I've wondered that for years. Does it have to do with the fact that the actual traction comes from an electric motor? Trolley cars have (used to have) notches on their throttles.
In a road vehicle the throttle can be adjusted from zero to maximum throttle by simply varying the pressure on the gas peddle. Why aren't locomotives designed this way...with a continuously variable throttle?.. i.e. why the eight settings?
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