Can anyone tell me why there is so much more reliance on ac units on drag trains?
If there is a marked advantage why do some roads still split their orders between ac & dc?
Thanks
Norfolk Southern has not “purchased” any of AC’s that I know of. They did inherit some from Conrail (13 I think). I not sure what they thought of them.
AC traction motors (and ac motors in general) require a lot less maintenance than DC motors. Modern electronics have made excellent low speed control possible with an AC motor. Prior to industrial electronics good low speed control was only available with DC motors. Thanks to modern inventions like TRIACS and diodes we can now do the same thing with an AC motor as a DC motor (without the nasty maintenance expenses).
And what it all comes down to is whether or not the railroad calculates that the increased purchase cost will be offset by lower maintenance costs and higher productivity over the lifetime of the locomotive. NS and CN with their type of operations and track profile don’t believe that they will, CP believes that they will, and BNSF, UP, and CSX think they will IF they can have them spend enough of their time in heavy haul service.
There are also benifits from low speed running. Most DC motors can not be under 9 miles an hour and at full power for a long time or they will burn up. However that is not an issue with AC motors, they can crawl along forever without them burning up.
Bert
Bingo. AC costs a whole lot more up front. Railroads, believe it or not, are not inclined to spend money they don’t need to spend. They’ll buy AC when they need it, for the drags, and not when they don’t.
One interesting development in the industry is the Union Pacific’s use of their latest AC’s (ES44AC and SD70ACe) primarily in intermodal or general merchandise freights rather than coal (the coal trains on the UP still use the slightly older AC’s - AC4400, SD9043AC). It seems the UP is trying to test the “economics” of using AC’s in services where their low-speed tractive effort is not absolutely essential - which is exactly how EMD has been trying to market these locomotives. Although the new purchase price of a DC locomotive will always be less than a comparable AC locmotive, these latest offerings from EMD and GE may be better able to strike a balance between initial purchase price (which had always favored DC locomotives) and standardization on as few different models as possible (which would obviously favor AC’s, assuming some traffic is very heavy coal trains and/or other relatively heavy tonnages on tougher grades.
If one goes to railpictures.net and does a custom search for the ES44AC or SD70ACe on the Union Pacific, you’ll see very few pictures of these locomotives on coal trains…Should be interesting to see when (or even if) Union Pacific buys any more of the DC versions (ES44DC or SD70M-2).