Horsepower 101

In another thread, a statement was made about engine horsepower and that it would not produce full horsepower for a small light train. This is incorrect.

The engine will produce full horespower for the thottle setting selected. The load regulator does not bring the alternator load up until the engine has reached the RPM for the desired thottle position. At 100% LR at 100% horsepower the locomotive will accelerate until the feed back current from the traction motors equalizes with the alternator output. This the the top speed of the traction motor (excluding physical limitations of the armature etc). When this is reached, the engine will still try to produce horsepower to overcome the feedback which it can't.

ok Paul now explain it for a AC locomotive, and engine RPM has nothing to do with HP.

I am not an electrician, and I am not up to snuff on this AC stuff yet. but I do have an enquiring mind and I will talk with someone who is in the know and post a responce.
My next thought is how does engine rpm NOT affect horsepower? Do you think an EMD 12-710 engine could develop 3000 hp at idle (320 rpm)? If you are talking about a constant rpm engine, then horsepower would be controlled though excitation of the alternator for a given thottle position. Union Pacific tried this several years ago where the engine ran at a constant 902 rpm and locomotive speed was contolled by the excitaion system (like what GE does with the P units in hotel mode). All it did was wear out some 16 cylinder engines prematurely. BTY, I once got 3700hp out of a 12-710 on "load test 2" and bringing up the rpm to manually trip the overspeed. Alco took the 1800hp 12-251 to 2000 hp by raising the rpm to 1050rpm, early EMD 645s were rated around 800+ rpm.

I have a question. When a locomotive is rated at a certain Horsepower is it the HP of the prime mover or the traction motors. For example the DE/DM30s have 3000hp. Does that mean that the diesel power plant generates 3,000 hp or does it mean that that the electric traction motors generate 3000hp
Thanks.

what the Diesel motor generates so HEP & Traction & Auxiliaries.

and Paul your source is not in the know, a Diesel of any kind of hp rating is in 4th notch equalibrium, it won't accelerate or decelerate. It realy does not matter if you got 1 or 10 cars behind the diesel as long as you do not exeed the hp rating of the Locomotive.
Yes a 3000 hp locomotive on a short train can develop higher traction, but by yanking out the throttle you only get to cruising speed sooner and you will be coasting at about 500 hp for two cars in 4th notch , not 3000 or 1500 like you suggest.
Your car does not redline in cruise control either and a diesel is just the same. after 5.4 million miles looking at loadmeters and computer screens in last 15 years I should get a bit of an Idea about it.

All of the engine performance curves (dozens) I have ever seen for Cummins, Caterpillar and Detroit Diesel show that horsepower increases as RPM increases. Torque, on the other hand, peaks around one third of the way toward maximum RPM.

Horsepower in North American railroads is the horsepower available for traction. That is the gross horsepower minus losses for cooling, etc. The losses seem to run around 10% of the total HP. Thus a locomotive rated at 3600HP actually produces about 3900HP. Starting in the late 1980's GE and EMD started using in computers in locomotives to maximize tractive effort and adhesion. This allowed at least GE to have the HP rating for traction much closer to the gross horsepower. The lost HP tends now to be more around 150HP.

Horsepower for passenger locos is trickier. Trains tend to need about 700HP for electric light, heat and air conditioning.

Railroads on other continents generally measure locomotive power in kilowatts.

"...at 100% horsepower the locomotive will accelerate until the feed back current from the traction motors equalizes with the alternator output."

That wording could stand some improvement. If it were just a matter of currents equalizing, why would the train accelerate when it moved from level track onto a downgrade (after having reached its top speed on the level)? The currents don't change on the downgrade.

timz wrote:why would the train accelerate when it moved from level track onto a downgrade (after having reached its top speed on the level)? The currents don't change on the downgrade.

I believe it's called the acceleration effects of gravity, which are completely independent of any current flow.

<i>Railroads on other continents generally measure locomotive power in kilowatts.</i>

Actually, MEGAwatts these days. And that's traction motor input. HP available for traction conviently ignores generation/rectification/inversion losses, which are not 0% (otherwise the generator would be infinitely small, and ditto for the inverters and wires. It is only because they're not 100% efficient that they need to be of any size - think about it.). Realistically, you don't even see the whole hp available for traction at the wheel.

Traction motor input's more accurate (though there are motor and gearbox losses). Electrics are rated that way simply because it's easy to do. I'm guessing diesels are rated in HP available for traction for the same reason - it's easy. (not to mention, it was a bigger number, thus sounded better anyway).

BRN: "Coasting with a train" is not what we are talking about. I have yet to see a locomotive (computer screen or not) produce rated horsepower at anything other then the 8th notch. After 18 years of watching turbo boost gauges, sticking penciles in load regulators, flash fielding a main generators with a 6v lantern batteries, and even a few computor screens I too have gained some sort of limited knowedge. I also work on locomotives that may be a little more advanced then the ones you worked on. When was the last time you worked on a 4,500HP 12 cylinder? I did, this morning at about three a.m. The engine is rated at X horsepower and that is what it will produce when asked by the thottle. If the engineer reaches track speed and rquires only to buzz along in the third notch to maintain track speed, then the locomotive, will produce the horsepower for the third notch. Also, dont forget, the EMD EM2000 microprocessor will not allow full horepower to the rail until a certain speed is obtained. I stand by my statement, and my skills. I will enquire about the A.C.question of motor feed back.