BNSF has joined the majority of Class I railroads by ordering nothing but AC drive locomotives, that leaves only CN among the Class I railroads as still buying DC drive mainline locomotives with DC traction motors. How long will that last. Sean Graham-White has reported that this years BNSF order for 227 locomotives will consist entirely of GE ES44C4s.
Could you explain briefly to someone who is not an engineer or technician the advantages and disadvantages of an AC vs DC locomotive? Thanks!
AC traction is more expensive and better for prolonged pulling at low speeds.
Sam1: there are actually three possibilities, and BNSF is the only RR so far to choose the third one. Other RRs have some six-axle engines with six AC motors (best pullers but expensive) and also some cheaper six-axle engines with six DC motors. BNSF apparently thinks if you want a cheaper engine the best choice is six axles and four AC motors; us fans have no idea how much cheaper they are, or how well they pull.
I have no idea of BNSF locomotive strategy…
My carrier exclusively bought AC’s for a number of years…for several years in the middle of this decade the only locomotive orders were for ES44DC’s that have been down rated to ES40DC’s, the subsequent orders have been exclusively for AC’s.
People making such decisions change over time and the ideas of what is ‘right’ change with the personnel. Any comptent top level manager can make any set of facts prove any bias he may have on a particular point…both for and against.
DC the only advantage is the initial purchase price.
AC advantages
-
Either a lighter motor for a given amount of power or a more powerful motor for a given amount of weight
-
The potential to reduce the amount of unsprung mass. The greater the unsprung mass the greater the hammering of the track.
-
inherent advantage with both the 3-phase asynchronous and permag synchronous motors for superior traction control. The motor will rotate at a speed determined by the frequency of the power supplied, and will resist turning either faster or slower.
-
no need for brushes and commutators with their limitations and maintenance requirements.
-
Greater rotational speed range for the traction motors.
The price differential has been decreasing since the introduction of the SD70MAC in 1994. I expect that within a couple of years DC motored locomotives will cost more due to low production levels. It is almost certain that the only DC locomotives produced this year will be the already completed CN order for SD70M-2s.
I think DC-traction, C-C mainline diesels are going the way of DC-generator-equipped diesel locos in the 60s and 70s and spartan cab mainline locos in the 90s. Even NS is buying brand-new AC motive power from GE and EMD.
*Author of a 2007 book on “GE Evolution Locomotives” - see:
http://www.amazon.com/GE-Evolution-Locomotives-Sean-Graham-White/dp/0760322988
This almost overlooked point is almost as significant as the move to AC motors - namely, that after almost 2 years of experience with the original order of 25, BNSF is now jumping in for them in a big way, with over 9 times as many in this order. To be clear, these locomotives ride on essentially A1A trucks, but hey are quite different from those trucks of old. In particular, the center unpowered axle has GE’s “Dynamic Weight Management System” to transfer up to 25% or so of the weight on it to the adjoining powered axles when needed for additional tractive effort. Although this concept was greeted with considerable skepticism when it was first unveiled 2 years ago, BNSF is now “putting its money where its mouth is” with this large follow-on order, so I have to presume that the exper
Now I know or at least I have a good idea of the differences. Thanks for your excellent response.
What about the minority builders (MPI, NRE, Progress, Railpower, etc)? It appears that they’re still pretty big on DC traction motors.
I understand that most contemporary AC railroad traction variable frequency induction motors are practically immune from the overload/ overheating limitations of DC traction motors, which have restrictions on how many amps they can draw/ how much power they can produce for ‘short-time ratings’ such as 1 hr., 30 mins., 15 mins., 5 mins., etc. In contrast, an AC motor can be near a stall while at full power and almost sit on a grade that way for all day, and it won’t overheat and burn off the insulation and melt the windings, etc., which kind of abuse would do exactly that to a DC motor.
But some have questioned the usefulness of that attribute as a practical matter, because a train at very low speeds is clogging up the main line and not making the best utilization of the more expensive and capable AC-motored locomotive. The DC-motored locomotive would have to keep moving at 15 MPH or so to avoid overheating, and that also keeps the main line more fluid. So the real advantages of the AC motor might be in the factors listed above by others - this one is just '“the frosting on the cake”, another useful but not primary benefit.
- Paul North.
In situations that would ‘kill’ a DC engine and leave it’s train DEAD looking for additional power, the AC engine could keep the train moving to a point of clearance. That is the practical difference on a day to day basis of AC traction in today’s railroad enviornment. There is nothing worse that a DC powered train to have Ground Relay issues that require a traction motor or two to be cut out on the ruling grade and the train then being ‘dead in the water’ until additional power can be secured to get the ‘cork’ out of the bottle and let traffic move again.
An AC induction motor is almost invariably cheaper to make than DC motor of the same capacity and synchronous motors aren’t much more expensive than induction motors for a large enough motor size. On the other hand, it may have taken s short while for economies of production to ramp up for AC traction motors, but I suspect that happened several years ago.
I think it is safe to say that the inverters are responsible for most of that price differential. A lot of progress has been made in large scale power electronics in the last 20 years. When EMD and Siemens first collaborated on AC drives, the only suitable component for the inverter were GTO’s (Gate Turn-Off thyristors), which were a bit clumsy to work with. GE entered the market when suitable IGBT’s came into production, which are much easier to work with. The price of these components has been coming down.
- Erik
[Edit] Note that all of the production electric cars use AC traction motors.
That is not true that BNSF has gone to all AC traction units. They are still ordering ES44DCs and ES44C4s (a A1A version of the ES44DC), along with of course the ES44ACs. The ACs are supposed to stay on coal trains while the DCs and C4s are supposed to be on intermodals.
BNSF’s entire locomotive order for 2011 consists of GE ES44C4s, which is an A1A version of the ES44AC not the ES44DC. These locomotives have 4 AC traction motors on a six axle locomotive. They have AC motors to enable them to produce the same amount of tractive effort as a ES44DC with six DC traction motors.
While I can’t speak to BNSF’s plans (I’d say this order is strong anecdotal evidence that they’re considering leaving DC orders behind despite very recent purchases of them, but only time will tell), it simply isn’t true that only Canadian National is still in the DC camp.
Norfolk Southern is still firmly committed to DC traction and has ordered plenty of modern DC power in recent years with clearly no plans to change when they discuss their recent foray into modern AC power. So it’s incorrect to assume such a thing just because they’ve concluded that AC power is advantageous enough to justify the higher price tag in some specialized situations finally. And CSX has also made significant orders of the latest DC power in recent times.
The only Class 1 carriers clearly committed to AC exclusivity for new purchases are Union Pacific, Kansas City Southern, and Canadian Pacific. BNSF has bought over 700 ES44DC units in very recent times and Canadian National and CSX have both bought large numbers of the latest DC units.
Just because there might be no order on the books currently for DC units is hardly evidence that a line has decided to focus exclusively on AC purchases in the future. Things always change, it wouldn’t even be shocking to see a line like Union Pacific even start buying DC power again after buying extremely large numbers of SD70M’s a few short years ago. They might’ve just felt like their large fleet has been adequate for their needs for a few years so we might all be wrong to assume they’ve taken an all AC course for future purchases.
NS has ordered 25 SD70ACe and 25 ES44AC locomotives each for 2011, 2012, 2013, and 2014. They may place supplemental orders, but they have no DC motored locomotives on order at all. I would say that also makes them now AC only. Like all the other Class I railroads they are nervous about Tier 4 coming in 2015.
No, it only makes them AC Only if they only have AC locomotives. Thats like calling the Apache Railway “EMD Only” because they only have EMD’s on order, not ALCos.
What are you going to claim when there is a day when Norfolk Southern doesn’t have a locomotive order announced or under construction? That they’re ceasing buying new locomotives and will be sticking with their current fleet or second hand purchases permanently?
It’s the same logic you’re using to make that claim.
Several top individuals in Norfolk Southern’s mechanical department have stated otherwise. Norfolk Southern isn’t going to just be placing orders for AC locomotives in the future. That they don’t currently have anything on order is lousy evidence to make such a bold claim.
Come back when there is some evidence to support such a thing (For example, a period of several years since they last bought a DC locomotive, a word from an official at the company, or just something instead of the nothing you’re basing it off now). Right now, their huge DC orders in recent years, their slow testing of the waters with AC orders, and their statements all point to continued investment in new DC locomotives in the future for the bulk of their orders.