Managed to snatch up an old MR issue (1995 I do believe) with an article introducing Santa Fe’s brand new EMD SD75I locomotives. The article briefly touched on why ATSF went with a DC traction motor model at a time when AC motors were becoming ever more popular
So, it brought up the question, why did railroads, in general or specific reason, choose AC traction or DC traction?
Note: I am referring to the traction motors only, not the generators.
Railroads are by and large a conservative lot, so some stuck with DC traction motors mainly for the familiarity and ease of maintence despite the advantages of AC traction. After all, they weren’t repalcing the entire fleet, just adding some new locos, and ever OTHER loco would be DC traction. Others were more willing to gamble and try out the nefangled stuff.
–Randy
AC pulls a bit harder than DC. The AC traction motors are induction motors with all the windings in the stator and no brushes (a wear item like brake shoes) AC costs more than DC due to the solid state controls. The difference isn’t great, maybe 10% or so. Lotta guys can fix the motors of either kind. Fewer guys can fix the solid state electronics. If I had an existing fleet of DC locomotives, I’d be inclined to order DC should I need to get more locos , just to keep the fleet all the same. Makes stocking spare parts and training people easier if all the locomotives are the same.
Some links on AC motors.
http://en.wikipedia.org/wiki/Electric_locomotive
http://cs.trains.com/trn/f/741/t/148586.aspx
I am quite sure if you search the Trains.com site, you will find many discussions of AC and DC.
Rich
When Santa Fe bought their SD75I’s, AC traction was quite a bit more expensive. The BN SD70MAC AC traction motor locomotives(early 90’s) were about 20-25% more expensive than a comparable DC traction motor engine. At that time BN was quoted about 1.5M for a SD70M and about 2.0M for a SD70MAC. The other factor for Santa Fe was that these engines were to be used on premium Intermodal trains between Chicago and the West Coast. There was no need for the ability to lug at low speed as with a unit coal train.
After the BNSF merger, the buying pattern stayed the same. DC traction motors for Merchandise trains, and AC traction motors for heavy unit coal trains. Over the years, the cost of AC traction has dropped, but not enough for BNSF to go all AC traction. They were still buying DC traction motor ES44DC ‘Gevo’ engines from GE for merchandise trains, and AC traction motor ES44AC & SD70ACe’s for unit mineral/coal trains. A few years ago, GE introduced a new AC traction motor model(ES44C4) that only has 4 AC traction motors and 4 inverters. The center axles have idler wheels. This got the cost of AC traction down to DC pricing, and still produce similar continuous tracktive effort curves. This converted BNSF to all AC traction even for merchandise/Intermodal trains. Currently BNSF has hundreds of the ES44C4 engines in general freight service, and they will be getting a similar EMD SD70ACe with only 4 powered axles within the next year.
Depending on options, large freight locomotive can cost anywhere from 2.25-2.5M each. Saving even $100,000 each on a 100 unit order is $10M bucks!
Jim