Throttle Commands

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  • Member since
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Throttle Commands
Posted by JPS1 on Monday, September 25, 2017 9:02 AM

When the engineer of a diesel electric locomotive opens the throttle, is he or she telling the electric motors to go, which in turn tells the diesel engine and generator to provide the required electric energy or is it the other way around?

Rio Grande Valley, CFI,CFII

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Posted by Overmod on Monday, September 25, 2017 9:13 AM

JPS1
When the engineer of a diesel electric locomotive opens the throttle, is he or she telling the electric motors to go, which in turn tells the diesel engine and generator to provide the required electric energy or is it the other way around?

On older locomotives, he is telling the solenoids on the engine Woodward governor to open in a particular way, which moves the fuel rack and runs the engine to a higher speed (determined by fuel delivery).  The motors only "see" the current the rest of the electrical control system sends them, and that includes the connections involved in series/parallel transition to make higher motor speed more economical.

In some more modern AC systems, road speed is in part a function of synthesized inverter frequency and in part the current (amperage) sent to the motors, both of which are essentially controlled by computer.  The nominal throttle setting is an input to the computer in this respect, not a positive mechanical connection to an engine governor.  These diesel engines are likely to have some kind of electronic fuel injection rather than mechanical governor, so there is no longer a 'hard' relationship between throttle position and prime-mover response.  I do think that in these situations the engine 'follows' the electrical demand the inverter draws from the DC link, but it's still more complicated than 'telling the electric motors to go' (more like telling them to turn at a more-or-less exact speed with predetermined torque).

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Posted by oltmannd on Monday, September 25, 2017 9:30 AM

The throttle varies the electrical horsepower output.  

In EMD pre-Dash2's it would vary some with fuel temperature (hot fuel has less energy per unit volume) and the the auxiliary load, e.g. how many cooling fans were turned on.  50 series and up did away with the variation by fuel temperature.

Late Dash 7s and newer are similar.  In fact, GE's CHEC excitation stood for "Constant Horsepower Excitation Control".

The change in HP between the notches isn't linear.  The change is small between the lower notches and greater at the higher notches.

-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/

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Posted by oltmannd on Monday, September 25, 2017 9:43 AM

Overmod is correct about the decoupling of diesel engine speed schedules from throttle notch.  Currently, it's all about fuel economy.  Earlier, it was all about smoke abatement.  GE U series often had a 1-5-8 engine speed schedule  - just three engine speeds for eight notches. Later, they had a "skip 3, double 6." speed schedule.  

And, then there were the U25's "half notches" which allowed the engineer to advance the diesel engine speed before advancing the load for that notch.  That was about getting quicker loading when the engineer wanted it - but was mostly a gimmick and was removed for simplicity's sake.

-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/

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