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# Making SD40-2s Tick

#### Stolen shamelessly from somewhere on the Internet

<< Don't the 3000 hp units lose part of their power
below a certain speed? Transition and all that?
>>

I don't think the SD40s "Power Limit," but they
might. It's the 3600 hp units that
Power Limit to about 2300 hp below 18 mph and then gradually apply it from
there to 23 mph. This does not affect
their pulling power, though. It just
makes them less potentially slippery. At
15 mph an SD40 and SD45 will both pull about 900-1000 amps, which is about all
they can handle before slipping. The
difference is, when an SD40 gains speed, still wide open, that amperage will
begin to drop as the speed picks up. With
Power Limiting on an SD45, between 18 and 23 mph, the train will accelerate with
NO drop in amperage, and it will emerge at 23 where the SD40 was at 18 --carrying
higher amps at the higher speed -- which is the pulling power.

When I was delivering SD45s on the Cotton Belt, engineers
who had never seen Power Limiting couldn't believe that they could still
accelerate and not have that amp needle drop!

The AC units today calculate tractive effort to give the hoghead the same kind of reference.

The secret to the diesel-electric's success is a simple
formula:

W = V x A

Watts (the diesel/generator output) = Volts (speed) times
Amps (pulling power).

When the speed is low, the V is very small, and almost
any W (output) results in nearly infinite power (amps). Adhesion is the only limitation. That's why a 1000 hp switcher can slowly
move a 100-car train that it takes four SD45s to get over the road. As the V (speed) increases with the throttle
wide open, the Amps (power) MUST go down, according to the formula. And it works exactly that way. Power limiting simply holds some of the W
(output) in reserve and won't let the engineer get at it (where he would just
overpower and spin the wheels) until the speed is high enough to absorb the
input.

That little formula is what killed steam. A diesel-electric (the secret is actually the
electric motor) begins its power curve at infinity when standing and decreases
as it speeds up. A steam engine starts
at ZERO and climbs. That's why steam
engines have a hard time starting a big train. They say that a steam engine could get over
the road with anything it can start. T'aint so, with
a diesel.They can start almost
anything but very quickly run out of steam (so to speak). With diesels, the only way to get more W is to
add more units or go for bigger diesel engines.

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