I was wondering in the AC traction motors, how many phases are they? It must be at least three. Thanks all.
It is, my friend or if it were more, than a lot more wiring would be needed
Yes, they are three phase.
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
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.
and that is the 3 phases?
Evidently I knew that, just didn’t know the terminology.
Thanx
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.
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.
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.
…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.
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.
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
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 …
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.
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
So Joe, why did NS go with DC and BNSF seems to have gone with AC? I must be missing something here.