“rated” output cannot be exceeded, my experience is with gen sets not locomotives, but the answer is the same, rated output cannot be exceeded. If load increases beyond rated output, exitation is lowered or stopped depending on design. This is called “dropping the load”. It is necessary to design a unit this way to protect itself from overload damage.
An engine’s rpm can drop as the unit is being loaded, but that is a function of governor design. A “droop” governor falls from “high idle” - maximum no load speed, a predetermined amount to “rated speed” - full load rpm. A governor without droop is called an isochronus governor.
So, when you are load testing, you are looking to achieve a certain number, if you get it, your job is done. If you exceed that number, something is wrong, just the same as if you couldn’t get the number.
All of the components of the locomotive and the test equipment have been designed to handle so much current, heat, stress, etc. If any of this equipment is operated beyond what the manufacturer says it can operate at there is a risk of serious damaging or destroying the equipment or injuring or killing the operator. Unless as Mr. Ruppert points out it is designed (most likely the computer program the controls the equipment) to not allow operation beyond safe limits.
There is usually a factor of safety built into the design, however, most people will not know what it is and therefore should not exceed the operating limits.
So the testing is just basically verifing the the engine can put out the hp the manufacturer says it can and not really testing to see the maximum hp it can put out.
J ruppert is right , you should not be able to exeed the rated out put of the machine, however I have seen 4400 horse power SD45s, of course there is something wrong with the locomotive and needs an electrician to straighten it out before the prime mover is ruined. Here’s a simple question for you guys: You are building a load test cell out of locomotive dynamic brake grids, each grid is .43 ohms, how many do you need to test 3000 hp locomotives?
Randy
If you don’t care about damaging the prime mover, don’t worry about it, on a locomotive with a horse power problem on the high end I usually shut them down when I blow off a loadbox cable or if some thing else blows to hell. You can’t fix the problem if you can’t load em up and put your meter to it.
Randy
and Karn – in both cases what you want to know – and are finding out – is the useful output of the prime mover. The losses in a railroad alternator are very small (relatively speaking) and the two values would be quite close. There are, incidentally, some marine applications where one looks at the KW from the alternators rather than ‘flywheel’ horsepower – a surprising number of ships are diesel electric drive, even some very big ones (e.g. Queen Mary 2, the biggest passenger liner in the world![:D])
Using 1 HP = 745.7 W, 3000 HP = 2237100 W
Assume the voltage is 1250 V
Use W=(V^2)/R to get R=W/(V^2)
R=2237100/(1250^2)=0.6984 Ohms
To make sure that the current does not exceed the current for 3000HP and 1250V the resistance cannot drop below the above value. Therefore 2 grids are used.
Back to the original question, the “Traction HP” for a locomotive is the shaft horsepower going into the main generator (or alternator) that will be converted to electrical HP for traction. It is to be measured at AAR std conditions, 28.86" Hg (1000 ft elevation) 60 deg F ambient air and fuel temp, 7.043 #/gal fuel density and 19350 BTU/# HHV.
It is engine brake HP less external auxillaries (such as TM blower, cooling fans, air compressor). The brake HP does not include internal auxillaries such as water pumps and lube oil pumps.
In practice, you measure the electrical output from the main generator and adjust for generator efficiency (that is why Randy divides by 700 instead of 743), calculate the auxillaries, add them up, then adjust results back to standard conditions.
Ok Randy. Since i’m a machinist and not a " sparky ", when I would put a SD40-2 in self-load test, I was only using 2 of the 6 availible DB grids???[%-)]
ericsp had the right idea i commend him for the thought on the matter. Each grid section is .43 ohms each grig has two .43 sections for a total of .86 ohms each locomotive (SD40-2) has 2 grids in series for a total of 1.72 ohms , in self load the grids are arranged in series parellell in pairs. That means voltage will be set at 1204 and current will be 700 amps per grid or 4200 amps plenty of capacity to test a 5057 horse power locomotive. On an SD45 the voltage limit settings are 900 volts and 4000 amps total 3600 horsepower.
Randy