AC TRACTION MOTORS

[Greenhornet]

I was wondering in the AC traction motors, how many phases are they? It must be at least three. Thanks all.

[Dutchrailnut]

It is, my friend or if it were more, than a lot more wiring would be needed

[jrw249]

Yes, they are three phase.

[Mookie]

I know what a traction motor is and where it is.  Someone want to take a minute and explain the rest to me? 

I would appreciate it.

Mookie

[jrw249]

In simple terms a locomotive has a diesel engine (except electric locos) which is connected to a alternator which makes electric to run the electric traction motors connected to the axles to turn the steel wheels.

[Mookie]

and that is the 3 phases? 

Evidently I knew that, just didn't know the terminology.

Thanx

[CopCarSS]

Mooks,

Three Phase refers to how the main generator generates electricity. There are three seperate windings in the generator. Each is off-set by 120 degrees from the other. Consequently, if you look at the sine waves that they produce when they reverse polarity, they'll be out of step with each other by 120 degrees.

Three phase power has some distinct advantages in applications like large motors (traction motors for example). Perhaps Randy can fill us in on the details, but I believe that the rotating magnetic field produced in three phase motors is preferable for starting and accelerating.

Hopefully that explains it a little bit. It's been awhile since I dealt with any of this, so any filler and corrections would be appreciated.

[jrw249]

Three phases is the type of electricity used.  For example, the electric  used in your house is single phase AC. A battery is DC like used in flashlights. Traction motors  can be DC or AC depending on what the constomer wants. AC traction is more expensive but can be an advantage hauling slow heavy  loads.

[Mookie]

I think I have it...  And the railroads have gone mostly to AC currently (sorry...).  Do they still even make new locomotives with DC say within the major 5?

[BaltACD]

I think I have it...  And the railroads have gone mostly to AC currently (sorry...).  Do they still even make new locomotives with DC say within the major 5?

Both GE and EMD are constructing locomotives in both AC and DC forms, as ordered by the carriers...In round terms a DC locomotive sells for approximately $1.5M, an AC locomotive sells for approximately $2.0M.  You can get more locomotives for your dollar by buying DC.  You can lug more tonnage per locomotive by buying AC.  Some carriers are sold on AC locomotives for all their new needs, some on DC and some buy some of each to service individual segments of their product line.

[Modelcar]

....J:  Those traction motors also have another duty to perform ....Downgrade, will find them turning into generators {electrically}, and become "brakes" in simple terms, to help to keep the train {speed}, under control.

[timz]

So-called "AC" diesels generate three-phase AC and immediately rectify it to DC, which then goes to the inverters to be converted to the proper frequency of AC. The diesel engine itself (the FDL or GEVO or 710 or whatever) is running at a constant speed, so it's producing constant-frequency AC-- but the motors need a frequency that's (almost) matched to their speed at the moment.

[tree68]

The careful (fully electronic) control of the AC to the traction motors is what allows an AC locomotive to crawl along at single digit speeds in notch 8.  That would fry a DC motor.

[MichaelSol]

The careful (fully electronic) control of the AC to the traction motors is what allows an AC locomotive to crawl along at single digit speeds in notch 8.  That would fry a DC motor.

It's that crawling part that raises some questions.

From an industrial engineering prof:

"The story we read about the AC motors on diesels is that, while more expensive than DC motors, the latest designs seem to cope better with very demanding loads at very low speeds than do DC motors. In a nutshell, they enable fewer diesel locomotive units on a train, albeit operating at slower train speeds. The operations opportunity: one can feasibly handle a heavier load while running more slowly if diesels are equipped with AC motors. The contemporary switch to AC motors was driven by the Powder River Coal business and BN management in particular. Powering of their coal trains evolved from 5 3,000 HP DC units to 3 4,000 HP AC units. Train speeds slowed down (and so car cycles stretched out somewhat and therefore freight car costs went up), but locomotive and fuel costs per trip went down.

"Knowing what I know about railroad management costing systems, I suspect the profit gain from this change was overestimated. I suspect the profits lost from slowing down all the trains, particularly on lines with mixed traffic, were underestimated."

Intentionally planning a lower operating speed -- and paying a higher price for that "ability" -- has a significant system cost penalty.

 

[joemcspadden]

The careful (fully electronic) control of the AC to the traction motors is what allows an AC locomotive to crawl along at single digit speeds in notch 8.  That would fry a DC motor.

It's that crawling part that raises some questions.

From an industrial engineering prof:

"The story we read about the AC motors on diesels is that, while more expensive than DC motors, the latest designs seem to cope better with very demanding loads at very low speeds than do DC motors. In a nutshell, they enable fewer diesel locomotive units on a train, albeit operating at slower train speeds. The operations opportunity: one can feasibly handle a heavier load while running more slowly if diesels are equipped with AC motors. The contemporary switch to AC motors was driven by the Powder River Coal business and BN management in particular. Powering of their coal trains evolved from 5 3,000 HP DC units to 3 4,000 HP AC units. Train speeds slowed down (and so car cycles stretched out somewhat and therefore freight car costs went up), but locomotive and fuel costs per trip went down.

"Knowing what I know about railroad management costing systems, I suspect the profit gain from this change was overestimated. I suspect the profits lost from slowing down all the trains, particularly on lines with mixed traffic, were underestimated."

Intentionally planning a lower operating speed -- and paying a higher price for that "ability" -- has a significant system cost penalty.

 

Michael--what you wrote and quoted above is certainly reflected in the philosophy
of Norfolk Southern, which of course hauls a lot of coal over very demanding
terrain. The only ac motors on the roster, as I understand it, are a few SD80-macs
inherited from Conrail, and the word is they will be eliminating all of these as time
goes on.

Regards, Joe

[MichaelSol]

Michael--what you wrote and quoted above is certainly reflected in the philosophy of Norfolk Southern, which of course hauls a lot of coal over very demanding
terrain. The only ac motors on the roster, as I understand it, are a few SD80-macs
inherited from Conrail, and the word is they will be eliminating all of these as time
goes on.

That's interesting Joe, thanks for pointing that out.

I read comments like this above "I think I have it...  And the railroads have gone mostly to AC currently (sorry...).  Do they still even make new locomotives with DC say within the major 5?"

I wonder where these comments come from. My impression is that not only is DC the most popular traction motor, but that AC passed a sort of peak -- as railroads began to add in the collateral costs of operation based on experience. 

By itself, the AC traction motor is a remarkably robust piece of engineering, heads and shoulders above the old DC traction motors I grew up with.

But the cost comparison is not located in the traction motor, it is located in the inverter, an expensive piece of additional electrical equipment, and based on my long-ago experience of pricing of inverters for railroad use and my current wondering of how any savings could justify the cost of those inverters, I am wondering how the overall investment can be justified since it is based on the AC advantages at very low speeds -- which not only incurs the additional cost of inverters, but increases overall system costs as well.

I would like to see some numbers ...

 

[JayPotter]

One approach to justifying the cost of AC-traction is increased productivity expressed in terms of horsepower-per-ton.  Exemplar numbers are contained on page 44 of the November 2006 issue of TRAINS. 

[Mookie]

Michael - I live in BNSF territory and don't see DCs any more.  And the power run-throughs that we see here are mostly AC.  I know some of the smaller lines will run DC - probably because it is cheaper to purchase, but would the major railroads run AC since (I understand) that the speeds between point A and point D are usually traversed at the lower speeds. 

If I was say - BNSF - I would spend the extra money for what I can get from an AC and make up the difference in another place.  I think the advantages definitely outweigh the disadvantages - especially in this part of the country.  I can't speak to NS since that is a part of the country I have never visited. 

[joemcspadden]

Michael - I live in BNSF territory and don't see DCs any more.  And the power run-throughs that we see here are mostly AC.  I know some of the smaller lines will run DC - probably because it is cheaper to purchase, but would the major railroads run AC since (I understand) that the speeds between point A and point D are usually traversed at the lower speeds. 

If I was say - BNSF - I would spend the extra money for what I can get from an AC and make up the difference in another place.  I think the advantages definitely outweigh the disadvantages - especially in this part of the country.  I can't speak to NS since that is a part of the country I have never visited. 

Mookie--once again, I have to disagree. Norfolk Southern currently has 3841 locomotives
on its roster. 18 of them are ac, and they will be getting rid of those, by all accounts.
Their most recent order (currently in the process of being delivered) are 170 Gevos--
and they are all dc.

Joe

[Mookie]

So Joe, why did NS go with DC and BNSF seems to have gone with AC?  I must be missing something here.

[joemcspadden]

So Joe, why did NS go with DC and BNSF seems to have gone with AC?  I must be missing something here.

I don't think it's all that unusual for two different railroads to examine
the same data and reach different conclusions regarding the economics
of one technology vs. another. Part of the answer may well reside in
what Michael Sol wrote above. I've also read that NS feels that dc motors
are more reliable and less subject to breakdown & expensive repairs. I
wouldn't know.

But another similar contemporary example immediately springs to mind:
when ordering Gevos, dash 9s, etc., I believe the western class one roads
have usually elected the 4400 hp option. NS, for a variety of similar reasons,
has decided to stick mostly with the 4000 hp versions.

I don't believe there is a "right" or "wrong" in either case.

Regard, Joe

[selector]

I don't believe there is a "right" or "wrong" in either case.

Regard, Joe

Maybe the more appropriate term would be "best fit?"  And that would be an internal decision based on numerous factors.

[tree68]

It's all in the application. 

MS mentions speeds - it's possible that BNSF needs the low speed pulling capability enough that it's worth their while - perhaps cheaper in the long run than maintaining/using pushers.  Possible scenario - hard pull out of a valley followed by relatively flat running.  The speeds would average out.

NS may opt to put more power on to raise the minimum train speed (a necessity with DC anyhow).

Too, we can't really have a meaningful discussion without looking at things like average train length/weight and overall grades.

 

[oltmannd]

It's all in the application. 

MS mentions speeds - it's possible that BNSF needs the low speed pulling capability enough that it's worth their while - perhaps cheaper in the long run than maintaining/using pushers.  Possible scenario - hard pull out of a valley followed by relatively flat running.  The speeds would average out.

NS may opt to put more power on to raise the minimum train speed (a necessity with DC anyhow).

Too, we can't really have a meaningful discussion without looking at things like average train length/weight and overall grades.

 

Maybe you shouldn't give NS too much credit!

About 10 years ago, I asked an NS middle-to-upper level Mech Dept staff guy, "Why no ACs?"  The main reason was that "NS ran mostly two units per trains, so no unit replacement benefit with AC".  As it turns out, that really isn't a true statement.  There are lots of applications on  ACs would give good unit replacement ratios while still keeping HP/ton at required levels.  I suspect the truth was that the Mech Dept couldn't be bothered to do the training and the hassle of keeping the fleet segregated was more than anyone wanted to take on, and now there's a lot of inertia and, perhaps some crow to eat if ACs were to be purchased.

I think some of this same "logic" is the reason NS has derated 4400 HP locomotives....

...and which RR was last to dieselize, and which RR was last to purchase GP9s, etc, etc.

[joemcspadden]

It's all in the application. 

MS mentions speeds - it's possible that BNSF needs the low speed pulling capability enough that it's worth their while - perhaps cheaper in the long run than maintaining/using pushers.  Possible scenario - hard pull out of a valley followed by relatively flat running.  The speeds would average out.

NS may opt to put more power on to raise the minimum train speed (a necessity with DC anyhow).

Too, we can't really have a meaningful discussion without looking at things like average train length/weight and overall grades.

 

Maybe you shouldn't give NS too much credit!

About 10 years ago, I asked an NS middle-to-upper level Mech Dept staff guy, "Why no ACs?"  The main reason was that "NS ran mostly two units per trains, so no unit replacement benefit with AC".  As it turns out, that really isn't a true statement.  There are lots of applications on  ACs would give good unit replacement ratios while still keeping HP/ton at required levels.  I suspect the truth was that the Mech Dept couldn't be bothered to do the training and the hassle of keeping the fleet segregated was more than anyone wanted to take on, and now there's a lot of inertia and, perhaps some crow to eat if ACs were to be purchased.

I think some of this same "logic" is the reason NS has derated 4400 HP locomotives....

...and which RR was last to dieselize, and which RR was last to purchase GP9s, etc, etc.

Oh. . .maybe this is a fair analysis, maybe not. One thing's obvious, though,
if you look at how railroads behave, rather than what they say:

Runthrough power is becoming more and more common every day. UP & BNSF
4400ACs are now a commonplace sight on NS and CSX roads; by the same
token, a lot of NS catfish (D9-40CWs) are running every day on BNSF and UP
tracks out west. It seems obvious that the differences between AC and DC are
perceived as relatively minor among the class ones. You don't hear BNSF
saying to NS, "you can't come out here on our lines with that junk."

Regards, Joe

[beaulieu]

So Joe, why did NS go with DC and BNSF seems to have gone with AC?  I must be missing something here.

Mookie BNSF splits their orders between AC and DC locomotives. The new BNSF locomotives numbered in the 7xxx series are DC motored, while the new 6xxx series are AC motored. Currently I think the scorecard goes like this.

BNSF splitting between AC and DC

Only buying DC  

CN and NS

Only buying AC

UP, CP, and KCS

Jury still out on CSX because they got into the same problem that UP did, the severe need to replace large numbers of older increasingly unreliable power as quickly as possible. Once UP accomplished that they went back to buying exclusively AC motored power. Indications that I have heard is that the ES44DCs have satisfied the urgent need for more reliable power on CSX and future orders will be for AC motored power only.

 

2006 Actual production by builder and railroad, note because this is actual production it doesn't match orders. Some orders partially built in 2005 or 2007.

 

BNSF      GE    292 ES44AC locomotives

 

CN          GE   35 ES44DC locomotives 

             EMD   5 SD70M-2 locomotives 

 

CP           GE   80 ES44AC locomotives

 

CSX         GE  102 ES44DC locomotives

 

FEC          EMD  4 SD70M-2 locomotives

 

FXE          EMD  15  SD70ACe locomotives

               GE     60 ES44AC locomotives

 

KCS          EMD   5 SD70ACe locomotives

KCSM        GE    22 ES44AC locomotives

 

NS            EMD  76 SD70M-2 locomotives

                GE    62 ES40DC locomotives

 

U P           GE    100 ES44AC locomotives

               EMD  100 SD70ACe locomotives

 

Totals       AC    674

               DC    284

 

The production for 2007 will be close to an even split, as BNSF and CSX are taking delivery of ES44DCs this year. In 2008 the balance is expected to swing back heavily to AC as both BNSF and CSX have large orders. 

 

 

[MichaelSol]

Michael - I live in BNSF territory and don't see DCs any more.  And the power run-throughs that we see here are mostly AC. 

As best I can gather from the company magazines, through "iffy" tangential references, BN has about 1,000 AC locomotives, and 5,300 DC locomotives. The AC's appear to be almost entirely coal service, and recent purchases have been directed to that type because of the coal demand.

[timz]

One approach to justifying the cost of AC-traction is increased productivity expressed in terms of horsepower-per-ton.  Exemplar numbers are contained on page 44 of the November 2006 issue of TRAINS. 

For those that don't have that issue handy-- it just points out that 4400 hp AC GEs are rated for 2900 tons apiece up Cranberry, which is a lower hp-per-ton than their predecessors. Nothing beyond that.

[Mookie]

Fascinating.  I must take some time and digest all this good information!

[JayPotter]

For those that don't have that issue handy-- it just points out that 4400 hp AC GEs are rated for 2900 tons apiece up Cranberry, which is a lower hp-per-ton than their predecessors. Nothing beyond that.

For something beyond the basic numbers, you need to read the article.  Its basic premise -- in the context of our discussion here -- is that a railroad will have a sufficient number of units in a given consist to produce enough horsepower to move its train at whatever speed is desired across its route; however if the railroad has to add even more units in order for the consist to produce enough tractive effort to keep the train from stalling on one or more short segments of that route, the railroad is probably wasting horsepower and should at least consider AC traction.