http://www.altoonaworks.info/rebuilds/rjc_gp38-2.html

Why would they replace generators with alternators, when the current has to be rectified back to DC for the traction motors?

For the best efficiency (or least power wasted, to put it the other way around) in controlling a motor, it's generally easier to perform the necessary slicing, dicing, bending, folding, spindling, and mutilating of the electric power when it's in the form of AC. Because AC has "life" or "motion", so to speak, you can use various components (passive ones, such as capacitors and inductors, and active ones, such as SCRs and rectifiers) to shape the current into something you want, and usually with very little loss. Once you've got the waveform you want, you run the current through the rectifiers to feed the motors.

DC you really can't do much with-- it's "dead" or "motionless" in the sense that it doesn't vary with time, and so it's difficult to manipulate. If you want to throttle back the current, about the only thing you can do with it is run it through a big honkin' rheostat, with the attendant loss of power in the form of heat.

My two cents' worth.

Its really a matter of maintenance. In either a DC generator equipped locomotive, or a locomotive with an alternator and rectifiers, power (voltage and current) is controlled by changine the engine speed. Using an alternator and rectifiers gets rid of the brushes in the main generator. A GE GT-581 main generator has 40 brushes; a GT-564 has 50. Thats a lot of brushes to not have to change-

If the AC / DC transformation magical technology used in the data center where I work is any example, DC doesn't have any franchise on creating waste heat. I spent yesterday doing PMs on huge air handlers, each of which keeps just one UPS cool and functional (ironically, an air handler motor overheating was the only mechanical breakdown I encountered). We're talking TONS of waste heat here, guys....and this is static equipment, nowhere near as robust as what would be necessary for railroad use.

We also use VFD's to control the speed of our motors, by changing the frequency of the AC sent to them. They also waste heat, and the wave forms they emit - not exactly the pleasant bell curves we all learned about in junior high physics.

Everything buzzes, rattles, and occasionally shreiks. I miss my quiet locomotives............DC battery rooms are the quiet place to go for as bit of peaceful meditation and problem solving.

I chalk it up (in railroading anyhow) to the desire to include complex solid state electronic controls into systems which were perfectly controllable with manual systems historically in locomotives. Perhaps the idea is to reduce human requirements for operators, or to try to squeeze out another iota or two of efficiency. Certainly there is less to maintain in terms of brushes, linkages, etc but when those things wore out or broke any idiot could see what was wrong - not so much in a big SCR or a flaw on a bit of a circuit board. Try to jam a stick in a tiny modular sealed control relay and limp home........

dave

For exactly the same reason your automobile uses an alternator to generate AC and then a rectifier to convert it to DC to charge your battery, rather than the old DC battery chargers. With the advent of solid state rectifiers, using an alternator/rectifier package to generate DC is cheaper, probably lighter in weight, lower maintenance, and more reliable then using a DC generator.

Just to add:

The GP38s were built with generators and the GP38ACs were built with alternators.

Stock GP38-2s were built with alternators, so NS's rebuild program just brought the former GP38s up to the same standard as the rest of their fleet.

I think you are horribly behind the times. 15 or 20 years ago - yes, AC motor control was still a bit iffy. Today, however, things are much improved.

The efficiency of AC converters and switch mode power supplies is astounding. Sure, the UPS gets hot - but I bet your server farm is sucking far more juice now, compared with even a big water cooled mainframe 20 years ago. Blame the internet. If you look at the power data, I bet you will find that your nicely efficient solid state UPS is far nicer than a big motor generator set from years past. Scale up an MG set to fit your requirements today, and you will have a pizza oven that will quickly shame your air handlers.

Also, modern AC technology can be extremely rugged. It is "relatively easy"* to design an AC switch mode power unit with tons of redundancy and fault tolerance. If a module burns up, perhaps in your UPS, hot swap it out and the rest of the system will not even notice. I am not sure if modern locomotives are built with this much redundancy, but it is certainly an option that the electrical engineers can pursue.

* still hard!

It's interesting to see the longevity of the GP38 and GP38-2s, long after many of their more "modern" siblings have met the scrapper's torch. I guess the locomotives remain popular with both Class I and smaller users due to the lack of turbo, and it is just the "right" amount of horsepower.

It's the GP9 with a 645 engine, the "second generation Geep". Basically, it's the smallest, most comfortable general-purpose engine out there that still has an engine a major or minor railroad is willing to deal with. Alcos are secondary-market, there aren't enough GE's of that size you can get parts for, etc. The other alternative is "green" power like gen-sets, and that's still like a hybrid today--outside the comfort zone of a great many drivers, technology-wise.

On a similar vein, NS rebuilt some SD50s into "SD40-3" units, which I found interesting. Many of the "new" technologies of the SD50 do not appear destined for longevity, and as such, railroads such as NS are choosing to rebuilt the units into a more long-living package.

On a similar vein, NS rebuilt some SD50s into "SD40-3" units, which I found interesting. Many of the "new" technologies of the SD50 do not appear destined for longevity.


From what I understand, the SD-40E's as they are now being called, are as much "refurbishment" or "refresh" as rebuild. They derate the 645 prime mover to 3000 HP as the key feature. They are pretty common around Altoona (shades of other engines getting a second life plodding up horseshoe curve).

Of course, NS has ordered a lot of NEW locomotives already derated, such as its Dash-9's @ 4000 Hp, instead of 4400. It would be pretty interesting to see how lowering the horsepower increases service life, reliability,availability and fuel consumption.

NS is also looking for some "stimulus" money, er corporate welfare to redo some GP-38's in to "GP-22ECOs".

They are apparently going to redo SD-60's into "SD-60Es", I'm not sure what that entails, but I think the SD50's had more reliability issues with 645 PMs than 60's with 710's.

I think I read that one of the execs just said there would be no CAPEX on locomotives for 2010, perhaps because they are rebuilding.

These are the threads that make me wish I spent more time on Calc and Physics than Pac-man.