AC vs. DC traction

[Anonymous]

Good thread. Couple of thoughts…

I broke some old time hoggers in on AC while I was an EMD field engineer; it was not easy convincing them that they would not burn up the motors. In one case, a road foreman had to run.

Depending on the hill, you sometimes needed th. 4, 5, or 6 to hold a heavy train. If you goof, the train slides back and the brakes apply. After waiting for another try, we had to notch out to 6 before releasing the brakes. THAT was really hard for them to grasp (at first). Imagine being at almost full throttle before leasing the brakes!

Its all about tractive effort, AC does more but comes with a $500k-$700k premium. [DC’s were around $1.5M and AC $2 to $2.2M]. Just look at two 5,000 HP SD80MAC’s replacing four C30’s for Conrail on the B&A, or three SD70MAC’s doing the work of five SD40-2’s for BN. It pays for itself but in the right application. HP equals speed, but AC can pull more (low end torque). Put both together and you have 6000 HP AC’s that can do it all. BTW, DC’s could not put 6000 HP to the rails, they would slip. Its simple (well sort of…) physics. The locomotives would need to weigh too much.

AC invertors are like traction control in a car, which is why you have to take AC from a constant speed engine/alternator set, clean it up (DC), and convert it back to AC with a computer fast enough to make the adjustments. It is like a fly-by-wire jet. Only a computer can get that much HP to the rails.

[joecool1212]

With all the advantages of AC over DC, It must be simple economics that keeps NS from switching to AC traction. Any comments? Joe A.

[CSSHEGEWISCH]

NS has long been very conservative when it comes to motive power: high noses, AAR control stands, etc. They may be trying to keep parts inventories reasonably simple and other similar ways of controlling costs.

[oltmannd]

QUOTE: Originally posted by joecool1212

With all the advantages of AC over DC, It must be simple economics that keeps NS from switching to AC traction. Any comments? Joe A.

NS believes a 4000 HP DC locomotive is the "one size fits all" solution them. No doubt there are applications where AC would be more economical, but they believe the utilization penalty would outweigh the benefit.

Once upon a time, I was talking with an NS mech dept guy (back when the mech dept ruled the roost at NS w.r.t. loco purchases). He said ACs were a poor choice because nearly all NS trains could be handled by two DC units. That may have been true then, but is certainly isn't the case now.

Someday, NS will purchase AC units, but it doesn't look like it'll be real soon.

[CSSHEGEWISCH]

If NS believes that the C40-9W/ES40DC is pretty much a one-size-fits-all road locomotive, it would be similar to the motive power policy of pre-1964 N&W, which was overwhelmingly GP9/18's and some RS11/36's.

[Anonymous]

It's funny how this transition in technology doesn't trade anything but money.

Usually all new technologies lacked something that old ones had. Like transition from records to CD's (records had a different warm texture), transition from technicolor to eastmancolor, transition from film photography to digital photography (film is still alive though), transition from steamers to diesel etc.

All of these transitions left someone unhappy, but this time there is nothing to complain about. it seems.

[chad thomas]

About mixing AC & DC locos. It's my understanding (not always correct) is that the reason the AC4400CTE is designated CTE (controlled tractive effort) is that those units can automatically derate themselves when MUed to DC units.

[Anonymous]

Maybe NS sees a benefit in keeping the number of loco. models to a minimum. Recall that Southwest Airlines uses only 737's, even though there would be times (vacations, Xmas, etc.) when a bigger plane would fill to capacity. But using only one type of airliner means the mechanics are well familiar with them, also that they're easier--or at least more convenient--to repair. Possibly true for NS, or am I just speculating?