the MRH Forum

Notching and percent of applied power General discussion
Reply
Frequent poster 969 posts Mustangok Frequent poster 969 posts
Mustangok

My locomotives are controlled at present by a Digitrax Zephyr Express. When the throttle is advanced it indicates a percentage of power applied via its 2.4" LCD screen. It's a straight rheostat type run from 0 - 100% that must depend on the decoder for power regulation to simulate notching if I get the CVs tweaked right.

 Here is a great link discussing prototype throttle notching:

https://model-railroad-hobbyist.com/node/24130

This made me curious about the correlation with notched throttles on the real thing, and led to some interesting information that generated more questions for me. For example, is it the idea that I divide 100 by 8 notches and imagine every 12.5% is another notch?

This item from Digitrax made it sound like there are only 7 notches associated with actual movement down the rails:

https://www.digitrax.com/tsd/KB311/diesel-notches/

So do I divide 100 by 7 notches and imagine every 14.28571428571429% is another notch? Har! I crack myself up.

Previously I simply set a speed that looked right for conditions and all was well. But now I am experimenting with sound, and the matching or lagging of same with engine revolutions and speed down the rails. If you operate with Proto Throttle I guess it is moot, but with mine you are only going to be shown percent power and have to listen for the ill-defined notching to be provided as the decoder simulates it, if it will.

Many modelers here have experience with several varieties of controllers and throttles. What do you see on them that indicates application of power? Are there any besides Proto Throttle that provide feedback to the user as if notching were its thing? I see reference in model railroading articles and videos like "run it up to notch 4" or whatever. How does this modeler know what notch he is in if he's using some other type throttle?

Bonus link for modern era modelers with a short explanation of AC vs DC traction:

http://www.republiclocomotive.com/ac-traction-vs-dc-traction.html

Kent B
Reply 0
Milt Spanton mspanton Super User 979 posts
Milt Spanton mspanton

When I had Soundtrax decoders

When I had Soundtrax decoders I set the “notches per speed step” so that it achieved Run 8 at half throttle. I set the speed curve to reach 50 percent at the same time, and then the speed curve was flat to keep engineers from “speeding”. I also set cv3 accel very high. This allowed me to set the throttle at half speed to get Run 8 sound as a loco started to pull the train. Then you can throttle back, giving a prototypical sound. 
To me, it is more about sound and visually appropriate train speeds. Now all the diesels are Loksound, and I can do a lot of this using Drive Hold. 
As your references listed have pointed out, you can’t really equate throttle steps to train speed and sound very well. Train dynamics completely overrule that ideal. 
I like to “ model” running a train by making it sound like I have observed in real life. 

- Milt
The Duluth MISSABE and Iron Range Railway in the 50's - 1:87
Reply 0
greg ciurpita gregc Super User 1,159 posts
greg ciurpita gregc

notches vs power vs speed

below is  a table relating notch and power.  (i think the speed refers to the generator RPM)   but power doesn't equate to speed.   it doesn't even equate to force.

i hope it's obvious that that throttles on real trains and automobiles don't dictate a speed.  it takes time to accelerate to some speed, at which point the driver backs off on the throttle to maintain the speed.

Newton's equations, F=ma or  a=F/m, relates acceleration, force and mass.   acceleration is proportional to force, but inversely proportion to mass (weight).    the same force will accelerate a short train to some speed in less time than a longer heavier train

Horsepower is ft-lbF/min or lbF / (ft/min), in other words force / speed, which means for constant HP, force decreases with speed.   maximum force is at zero speed.

a more realistic model railroad throttle could select a notch determining the HP.    that HP setting determines a force depending on the speed.    that force determines an acceleration depending on the length (weight) of the train.   that acceleration determines how quickly the train increases speed with each passing second

a high notch might be selected when starting a train to accelerate it quickly.    but the power must be limited to avoid slip.    at some desired speed, the notch will be reduced.

25 mph is 36.7 ft/sec.   it would take a train 10 min to reach 25 mph if accelerating at 0.06 ft/sec/sec

of course grades have a tremendous effect on acceleration.     the force produced by the engine typically needs to exceed the force of the grade which is approximately the weight of the train * grade/100.    if at a particular HP and speed and engine does not produce enough force, the force will increase as the speed decreases.   when the sum of all the forces is zero, acceleration will be zero and speed will be maintained.

 

greg - LaVale, MD     --   MRH Blogs --  Rocky Hill Website  -- Google Site
Reply 0
David Husman dave1905 MRH Author 9,891 posts
David Husman dave1905

Speed

Speed is not linear on a train.  A train can be in run 8 and only going 10 mph.  or it can be be in run 3 and doing 50 mph.  The "notch" controls the speed of the diesel engine which controls the amperage the generator/alternator is putting out.  The speed is determines by the weight of the train, the grade and what the locomotive is doing.  Throw in "transition" (changing the motors to being connected in parallel or series) and the speed of the diesel engine (the "notch" ) doesn't match the ground speed.

Dave Husman

Visit my website :  https://wnbranch.com/

Blog index:  Dave Husman Blog Index
Reply 0
Reply