Throttle (notch) setting vs power needs

[wayne]

Not sure I know how to state the question(s), but I'll try. Empty grain trains over Stampede Pass, WA sometimes have 2-unit consists , but more often 3. Apparently 2 unit are enough, so is it safe to assume that with 2 units each is working harder and the train perhaps going slower than if there were 3? Does that mean a difference in notch settings if there are 2 or 3?

Also, much of the division is nearly level. Are one or more (if 3 units) not used (at idle) when less power is needed or are all of them run at a lower notch level when not on a grade?

Which brings me to a 3rd question: What is the connection between the notch settings and the traction motors?--I know the amps go up as the locomotive accelerates in each notch, but are there gears in the motors or is it that as the motors turn faster they need less juice to have enough power to turn the wheels?

[tree68]

Well, the experts will either affirm my limited knowledge or give the right answer....

It's all about trailing tonnage. The railroads have formuli for computing how much power they need on the point. Barring extenuating circumstances (power shortage, moving power around), you can bet that those two or three units have similar HP/ton numbers.

As for the notch - think of it like you do the gas pedal in your vehicle. If you have a big load (or are climbing a steep hill) you'll be getting less speed for a given amount of pedal pressure. If you have a V8 instead of a 6 (ie, more units) you won't use as much pedal to get the same results.

Based on what I've read here in the past, I doubt that they would isolate a unit if the HP requirement is less - they'll just let off the gas (use lower notches). If the train is moving from a division that needs lots of horsepower to one that needs less, they may very well simply cut the unit out. That would be a place where they would likely be using distributed power (or even manned helpers).

Mr. Stahl will have to address the third question. I think I have an idea, but not good enough to try to answer.

[wabash1]

the amps come down as the unit speeds up. when a unit changes transition it amps raise but as speed increases the amps decrease. in most cases you need the speed down before the amps rise even pulling a grade. EMD units will give more amps sooner to keep moving GE units wont load til the speed is very low.

[adrianspeeder]

You can't force amps into any electrical device. The load (in this case motors) will draw as much as they need. As the motors have an easier time at higher speeds, they draw less amps. (my over simplified overview)

Adrianspeeder

[Anonymous]

As I understand electrical devices, as speed increases, current goes down, and voltage increases....

Voltage is like horsepower, as torque is to current...

basic ohms law...

[daveklepper]

Voltage times current is power, including both watts and horsepower. (Someone else can probide the constant conversion factor, it has slipped my mind.) Torgue is roughly proportional to current but varies with the specific motor characteristics.

Generally, the notch determines how much horsepower you want from the diesel engine and this roughly, in most cases, corresponds to an engine speed, and how much fuel per second is being fed the diesel engine. Then there is load regular circuitry, in the old days using relays and resistors and now mostly electronic, that determines how much field current the generator(s) or alternator(s) receive and this corresponds to a particular power output for the engine and generator/alternator speed. There can also be individual control of field currents of the motors, as well as with dc motors, series or parallel connections. In all locomotives since the FT, the engineer doesn't have to worry about all this stuff, it is all handled by the load regular control circuitry, except of course that he/she does not want the ameter reading to go into the red, a problem occasionally still to be avoided with dc traction motors but very rarely if at all with ac. (I'm not sure that ac-motor equipped locomotives still have ameters. Do they?)

I speak from some very long ago practical experience when I was a student engineer with EMD and a locomotive test engineer with the B&M, 1952-1953. I also got to run a freight train with a GP-7. Others can add corrections or details as required.

[mvlandsw]

AC locomotives do not have amp meters. Their output is displayed on the computer screens as thousands of pounds of pulling force.

[Hugh Jampton]

QUOTE: Originally posted by daveklepper

Voltage times current is power, including both watts and horsepower. (Someone else can probide the constant conversion factor, it has slipped my mind.) Torgue is roughly proportional to current but varies with the specific motor characteristics.

I was taught the conversion of Watts to HP is Jumbo -1,, 746 Watts = 1 HP.

[zardoz]

Since a diesel engine is most efficient when operating at maximum speed, ideally an engineer would isolate a unit if the power required did not exceed the 5th notch. But there are two reasons to not do this (at least on older locomotives; I'm not sure about the designs later than the SD40-2). First, when a unit is isolated, there is no wheel-slip warning function on the isolated unit; therefore, if a pinion gear broke on a trailing unit, and the wheels started to slide, there would be no indication of this condition sent to the lead locomotive. Secondly, to isolate a unit would require someone to physically go to that unit to flip the switch. That would require someone to walk the catwalk along the unit's hood, as well as negotiate the walkways between the units, presumably while moving at speed; a dangerous operation best avoided if possible.

On the CNW SD40-2's, they installed a "fuel-saver" switch. What this did was keep the unit on-line, but limited the locomotive to running in only the first notch (idle). This way you had the wheel-slip protection, but did not waste fuel running in the third-to-fifth notch positions.

[jchnhtfd]

Notches... why... I suspect, very simply to keep the throttle from drifting out of position under vibration (there is some vibration in locomotive cabs... !). Why 8? It isn't always 8 -- I think some GEs have or had 16 -- and some newer engines don't have notches at all; the throttle won't vibrate out of position for other reasons.

As to voltage, amperage, and power -- as the speed of any electric motor increases, the voltage (for a given output power) also increases, and the amperage drops. That's built into the physics of electric motors. The transitions are to keep both current and voltage in the generator or alternator within a reasonable range over a wide speed range in the motors. The 'notch' -- throttle setting -- does govern the power output from the diesel; if all goes well, the transitions etc. take care of themselves.

[chad thomas]

Wayne

Check out some of the essays at Al Krugs website:
http://www.alkrug.vcn.com/rrfacts/rrfacts.htm