AC vs DC traction motors - pros/cons?

Until the pros get here....

 

In my research making a scratch build, I noticed that some RRs opted for the DC model. I think it was CN and even NS?? But not many....

 

I'm pretty sure that the others would order the AC model that buy that brand. Maybe the GE DC version is sold to other RR's more? We should look at the "who bought them spreadsheet"...

http://www.trainweb.org/emdloco/sd70.htm

(bottom of page)

From wikipedia:

Total produced

SD70: 122
SD70M: 1,646
SD70I: 26
SD70MAC: 1,109
SD70ACe: 1,034
SD70ACe/lc: 64
SD70ACS: 25
SD70M-2: 331

 

 

Norfolk Southern has 130 of the -2

And The FEC has 8. intresting note: The FEC has never rostered a GE product-- it's always been a EMD powered RR-- and has never used an AC version---

The way things agr going, after a while EMD/CAT and GE will NOT offer a DC unit.  Even NS and CN are ordering AC units!

Then I expect that the FEC will get some more used SD40-2's and some more SD70M-2's from RR's that don't want them anymore, as the GP 40's that they have are getting very old.

Concerning your subject title, I'm not aware that railroads EVER purchased locomotives out of "love".

I think the real question here is, why would a RR purchase as AC loco over a DC loco?

I am curious as well what are the pros and cons of AC vs. DC?

Chris

In just simple terms, the biggest two advantages of AC traction motors over DC traction motors are:

 

1) The ability to stall the AC motor and do absolutely no damage to it. In fact, one can use the AC traction locomotive to hold a heavy train on a grade, release the air brakes on the train, and then start the train, without doing any heat damage to the AC motors. For heavy haul freight service, there is NOTHING to compare to the AC three phase, induction traction system!

 

2) Unlike a DC series wound, traction motor, with its commutator, brushes, and brush holders, which all require maintenance, the AC three phase induction motor has only one moving part; the rotor. Plus its torque capabilities are much greater than the DC motor, and does NOT overheat during very slow speeds, since very high DC current is NOT a function of a three phase induction motor. With an AC motor, it is all about voltage AND frequency!

 So then,.... RRs that chose the DC models must be left with a cost decision? Why else would they choose DC?

Have you been taking lessons from Dominic Mazoch on how to ask rhetorical questions?

 

Of course only those executives on those railroads could answer such questions.

Wyhog,

 

Perhaps the question may be re-phrased as, What are the advantages of DC traction motors over AC traction motors?

 

ChipR

Beside the cost of the entire AC traction system, there are no advantages of the DC traction motor over the AC motor. 

That's the reason why BNSF has been buying ES44C4's.  With only four AC traction motors the price is comparable to a six motor ES44DC.

 

Stuart

Thank you for the explanation HW the winner is clear to me. 

 

Chris

HW,  Thank you.

 

ChipR

Here are some various comments I read at Trainorders.com that were interesting (in addition to discussions indicating that there is quite a bit of variation among individual railroads as to how they choose to use or not use AC and DC engines):

 

"No one really questions the great advantages of AC traction.  It's just that it's much more expensive so it seems that at least since its introduction into the North American railroad scene up until now it has only been found to be cost effective for heavy haul applications."

 

"A DC traction diesel-electric, on the other hand, has no power electronics, except for the rectifiers, and to go to AC traction requires the addition of the electronics package.  That is where the price difference comes in, plus there is a slight fuel consumption penalty in higher speed operations with AC traction, further adding to cost."

 

"Upfront cost is a good heck of a lot more for AC than DC. Considering some railroads cheapened out on buying isolated cabs for crews, the much bigger cost of AC when it is unneeded would definitely be avoided."  

"On CSX there is a powered axle rule. No train, except for the Pope Creek coal trains in Maryland and the Q090-Q091 produce express train I believe, may have more than 24 powered axles. A DC unit is counted straight up (i.e a six-axle ES44DC is six axles), but an AC locomotive is counted at 1.5 axles per physical axle (a six-axle SD70ACe counts as 9 axles). This limits CSX to having at most two AC locomotives running on power on one train (9+9=18, which is good, but 9+9+9=27, not good). A DC locomotive may be added with the two AC locomotives (i.e. an ES44DC-AC4400CW-AC4400CW [6+9+9=24, good], which I mention because I have physically seen it). However this wastes the advantage of the AC motors, as the DC motor cannot operate at the low speed of the AC motors without burning up, so this would be used mostly on faster trains such as intermodals and auto racks, which wastes the slow speed capability of the AC motors. The best thing for the railroad is to buy AC for slower coal trains and things like that, and use the cheaper DC where the AC does not have advantages, such as over-the-road intermodals."

 

quote:

 A DC unit is counted straight up (i.e a six-axle ES44DC is six axles)

Breezy,

Only conventional DC units are counted straight up. High adhesion units, like the ES44DC, are counted at 1.3 times the conventional DC traction motor, then AC units at the even higher rate.

From a T&E standpoint, I've never been a fan of the -2s. For a new engine, the cab is awfully noisy. Those things rattle and hum a ton! The ACes with the isolated cab..... much better!

Ok, so why is the limit on CSX "24" powered axles?  BNSF runs 4 units, 6 axles each, on almost every thing through Denver.

 

ChipR

I can tell you why didn't love the SD70's. If it wasn't one of those back breaking desktop models, it had a conventional control stand. BUT, the dummies went and put a screen and desktop in front of you anyway! Just try to stretch out with that stuff in your way.

Then, they crammed the toilet in such a position that you couldn't stand on the floor to open the door. You had to stand on the first step in order to get the door open.

EMD's display screen sucked compared to a Dash 9 (and Dash 9 only, they too screwed the screens up beginning with the EVO series).

The dynamic brake handle on the control stand was placed so far back toward the brake stand that it you couldn't quickly get a good hold on it.

Those straight up&down front windows caused quite a bit of unwanted reflections from outside lights at night.

 

Put a GE Dash 9 cab on an SD70 and you might have something!

Yeah, but are they all on line? It has to do with having so much power that knuckles and drawheads start breaking.

 That was a great answer on why RRs would choose DC still. I didn't realize that AC didn't matter as much at higher speeds and used more fuel. It's good to have all the facts.

Who ever said that an AC traction unit "used more fuel" when compared to a DC unit, had better be able to back that statement up with documented test data!  

Big Jim,

 

They are as near as i can tell from the exhaust.  A hard climb from Denver to Palmer Lake.

 

ChipR

Hot Water,

 

The comment about fuel consumption arose from the following:

 

"  That is where the price difference comes in, plus there is a slight fuel consumption penalty in higher speed operations with AC traction, further adding to cost."

 

Posted by Breezinup.

 

ChipR

So,,,,,more "experts". Or possibly some of the "experts" over on Trainorders.com?

OK. I wasn't making the comment, though. As I said, I was just repeating some items from a discussion on Trainorders that seemed interesting and on point to this discussion of AC and DC engines and why DC engines are used at all.

 In my opinion ;if your going point A to point B on a road train or even a local that does not involve  much stopping and setting cars out online,AC all the way.They keep a nice smooth train ,have great dynamic once you get over the wheel chatter,but really stink for flood loading or stop and go type work.

 

 I like a good DC motor for about any job on the railroad that has to do with switching,flood loading or local jobs that have several industry stops.

 

 I'm sure in the pencil pushing world an AC looks better,but they are a temperamental beast for the throttle pushing Engineer's  

 

 

 

There is no truth to the statement that an AC traction motored locomotive uses for fuel than a DC motored locomotive, at least for GE products. The system efficiency of an AC locomotive is higher.

I also do not understand the negative comment re use of AC locomotives for flood loading. GE has had slow speed control for its locomotives for at least 15 years, and this slow speed control will control train speeds within 0.1 mph. I believe that EMD also has a similar system.

I also do not understand the characterization of AC locomotives as "temperamental". They are head and shoulders better than any DC unit ever built, regardless of the intended service. That is why almost all North American railroads have standardized on AC traction.

 

I agree. GE & EMD cruise controls are not compatible. Trying to get a matching set is very hard too. Even with three units, the odd type will more times than not wind up in the middle of the consist and the signal won't pass through.

mackb4,

 

I have been involved with crew training in the Powder River Basin, on BNSF and have watched/ridden probably hundreds of "flood loading" operations with pure SD70MAC consists, and never encountered problems. Different mines had different speed requirements, depending on the size of the loading silo and the gradient of the balloon tracks. One location out of Gallup, New Mexico even flood loads on a fairly steep DOWN GRADE. Prior to the receipt of AC traction units, the Santa Fe/BNSF had to load the train in "plug motor" with pacesetter, since dynamic brake obviously simply doesn't work on DC units at flood loading speeds of .5 to .8MPH.

 

Once the Engineering folks got the inverters and computers on the AC units to provide VERY STRONG dynamic brake at speeds down to below .1MPH, they no longer had to use "plug motor".

 

I wonder what is so different in that Kenova operation?

 Maybe that's the problem ,the programming from the NS on this.We have all had this problem up there at Colmont and when possible will turn an AC so it's not the leader when loading.

 Our 2500-2700 DC's do the same thing as far as " running away " with the cruise on

 

  Our mine at Colmont is very steep especially at the load out.A few years ago the mine had considered moving it down hill about 50 car lengths to try and make it easier.It's nothing for a seasoned veteran to get a car or two by the load out especially when it's raining and usually it's with a set of AC's.

 

 I agree,they are great and I've mentioned it in my earlier post,when your going point A to point B and are on mostly level track,but in the flood loading I deal with,they are not the best.Most of us at Kenova agree that the DC's load better in are situation.Sometimes I'd rather handload with a set of SD-50 or 60's like our 6500 and 6600's than load with the 8000 GE's or 1000 EMD's.

 

 The best units I think the NS has to load with are the 9000 series GE's or the 7500-7700's.And not all units like Jim mentioned will load in unison .And a EMD that will load with a GE will beat you to death if it's the trailing unit.I've even had that happen on the road when at speed.

 

 Hot Water do you know if each carrier modifies or somehow calibrates their locos to their own needs as far as the computers ? Maybe that is the problem ? 

Not to my knowledge, at least prior to my retirement at the end of 1998. All the SD70Mac, and SD75 units between the BN & AT&SF prior to the BNSF merger, were pretty much the same concerning the software versions. Naturally, the Electrical Control Group of the EMD Engineering Department was always "improving" the software, based on the unusual operating problems during the winter months in Wyoming. We also had so early "battles" withe Semons Engineering people in Germany. The people from Europe simple could NOT comprehend the 20 to 30 degrees below zero temperatures in the Powder River Basin, and by the time the unit coal train arrived at the big power plant near Houston, Texas, the temperature could be in the 80s with VERY high humidity.

 

We finally had to have the BN put together one or two business cars on the head end of one of the Texas bound coal trains and let the Germans have a fast of Wyoming and two days later step off the train in the Huston area!!! They FINALLY realized what we had to deal with in the field. 

  I wish everyone could experience Wayne Branch and the mining operation at Colmont (East Lynn,WV). It truly is a different if not a step back in time in railroading.

 

 Two hard to find books on Wayne Branch and East Lynn are out there. "Last Train to Dunlow " and " East Lynn Booming ".

 

 Now give that Colmont was developed by Mobil Gas in the 1970's and shutdown just a year or two later then reopened in 1991,it is a very modern and extremely large complex.Just a bear to load at.

 

Before we used retainer valves and some air or  handbrakes ,before cruise was added to the units and a lot of independent brake fanning,but now we use air and cruise.

 The electric brake cars are wonderful up there because you can add or reduce percentage as needed and not have a run away on your hands   

 

 

 

mackb4,

 

How steep is the grade?

 

ChipR