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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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