I know someone is going to say how stupid of a question this is, but: is a slug deriving from a D.C. traction unit–say a GP-40 that NS has been converting lately–compatible with an A.C. unit–say an SD-70ACe?
I would presume not. But?
Thanks,
Gabe
Not possible. Locomotives with Asynchronous 3-phase AC drives cannot power slugs, that’s not to say that you couldn’t engineer something from the drawing board to do that. but you for sure cannot simply convert something like that. They would have to reprogram the computer system, and completely redesign the traction inverters.
I think it would be possible to do an AC slug, but it would be a more expensive proposition than a DC slug. The slog would need its own set of inverters, but could be powered off of the traction alternator of the mother unit.
Not sure if it would make much sense.
- Erik
P.S. AC slugs would be relatively easy if the locomotives used wound rotor induction motors (e.g. the GN 3 phase electrics). One big selling advantage of the VV-VF inverters is that squirrel cage motors can be used, and those are cheaper, lighter, more reliable and more rugged than wound rotor motors.
Not impossible but not probable. It would be complex. It would need to:
A. Have AC traction motors with a set of electronic controls for each, or
B. Have DC motors with a rectifier and electronic controls for each.
There is also the increased hazard of transmitting AC current from one to another.
The DC slug is fairly simple. The DC output is just shared although you would want wheelslip control for each motor.
Since a AC loco has about twice the adhesion of a DC loco there is little need for a slug. Slugs are for low speed operation only.
Gabe ,
Look for some A/C slugs in the very near future.
Randy
Thanks, Randy.
I was hoping you would lend your insight. I am about as far away from being an expert on this as you could imagine–but I could see some reasons for having AC slugs. Many of which related to the reasons supporting NS’ recent ambitious slug program.
Gabe
I think the way to do it would be to run the DC buss back to the slug and put the inverters for the slug traction motors in the slug. It would be a pretty expensive proposition…
Don is correct , the DC bus will be run through jumpers. As far as price , ASEA Brown Boveri (ABB traction) is making some very nice and affordable (somewhat) packages.
Randy,
Any idea of how the slug will interact with the mother to share the load on the DC bus? The power circuitry changes to support an AC slug are much simpler than for DC locomotives, but there needs to be some adjustment to the inverter control circuitry to avoid overloading the prime mover and traction alternator.
My understanding is that the GE locomotives use a constant voltage DC bus (~800V), which makes busing the power straightforward.
I’d guess that two applications for an AC slug would be hump yard duty and coping with lousy rail conditions (e.g. wet leaves covering the rails).
- Erik
P.S. Another thought is making a battery slug. The GE hybrid locomotive prototype has a enough battery capacity to provide 1/2 power for an hour, the slug should be able to handle 4X the battery size allowing an hour of full power for both mother and slug. Might be enough to handle the energy regenerated from going down the “17 Mile Grade”.
The alternator is still a constant kilowatt machine. The current and voltage will divide .
Even though the AC traction motors are very good and the locomotives are efficient there still is only a fraction of the horsepower getting to the rail, slugs increase the locomotives footprint hence higher tractive effort. An AC slug would be a great drag engine and help accelerate a train to track speed.
Randy
While we are on the subject of ‘Slugs’ Road and otherwise, back in 1941 EMD produced a two off set of switchers. They were known as a "Cow&Calf’ based on the 1350HP F-units of the time.
The Two sets found homes in the IC RR roundhouses. and lasted a pretty lengthy career of some 25 years (1941-1966). It seems like a pretty good idea, back then and in these modern times. Why were there only two sets built(?). Was the compatibility within a Locomotive Fleet that problematic?
Now it seems that the lastest version is the CSX’s ‘Mother & Slug’ concept. Paring a power unit with a parasitic second unit to utilize the electrical output to increase unit tractive effort by a factor of two. These Road Slug units are appearing all over, not only in switch yards on-line, but in interchange service with other railroads.
NS is building all sorts of different critters from Hybrid Power to Road Slug sets.(?)
I’ve not seen any of the Road Slugs out here on BNSF, but in SE Kansas there always seemed to be several sets a week that were on UP trains over around Parsons,Ks. Sometimes they were even in the lead.
What are the chances of down the road seeing Road Slugs made with some of the more recent GE or EMD Power ?
How compatible is the Mother Slug concept mixed in to a consist with other higher HP locomotives, are they a help or a henderance?
Just curious.
Experience with the CSX Mother-Slug road units - they are the equivalent to two 4 axle engines up to 25 MPH when the slug is electrically disconnected from the Mother. The reality in operation is that the maximum speed for a Mother-slug combo is 25-28 MPH. When the slug is cut out, there is not enough horsepower in the mother to continue to accelerate any train that is near the tonnage limit for the Mother-Slug combo.
Is that true for both GE and EMD locomotives? I had a vague (i.e. not necessarily reliable) recollection that GE used a constant voltage bus with their IGBT inverters. Given that the IGBT’s have to be rated to handle both the maximum voltage and current, there isn’t a pressing need to have the bus voltage follow the output voltage. Bear in mind that the inverter when generating a low output voltage can have an average output current higher than the average input current.
There would be some benefits of bus voltage following output voltage. Switching losses would be lower, both for turn-on and turn-off, though the latter is the only one that varies with current. Conduction losses would be about the same, the IGBT would take less of a hit and the free-wheeling diodes would get hit harder in the constant bus voltage case.
On the other hand there would be benefits to the constant bus voltage design. Control would be a bit easier as it wouldn’t have to allow for large variations in bus voltage. Perhaps more importantly, the bus decoupling capacitors would be a lot more effective at a constant bus voltage - energy storage goes up with the square of the voltage and the caps have to be rated for max bus voltage.
While I haven’t worked on traction inverters, I did have one project using IGBT’s to produce 100+KW at 2 to 2.5 kHz.
- Erik
P.S. A constant power alternator (alternator equivalent of the Lemp system for DC generators) would allow for power sharing as doubling the current draw from having two inverters would cause the output voltage to drop by half.
Which is exactly why I do not see why any carrier would be at all interested in those new B-B AC units.
Sure, wheel-slip tech is lots better these days, but when I first heard of the new high-horse AC units with only 4 axles, I was really surprised. It would seem to me that having only 4 axles on an AC locomotives would defeat the purpose of going to AC in the first place. I cannot believe that the short-term cost savings of purchasing a locomotive having 2 less traction motors would even come close to the advantage of having 50% more traction per locomotive.
Would it not take 3 units with 4 traction axles each to equal 2 units with 6 traction axles each? Where is the savings? Certainly not in fuel consumption.
The slug might be useful for pulling tonnage up a grade, and then might not be needed on level ground.
Question: During manufacturing, could a slug be configured for dynamic braking?
I am not certain, but I believe on the CSX Road Slugs the dynamic brake is operative on both the mother and slug. Moving trains on a grade requires two functions … getting the load up the grade and controlling the load down the grade. Dynamic braking is vital in controlling trains down a grade.
Are the slugs you referring to de-motored (and de-cabbed) hood units? If so, then they might still retain the dynamic braking parts from when it was fully functional.
CSX Road slugs retain the operational cab…they are coupled back to back with the Mother and allow operation fro
As for the IC units, possibly it was more power than other roads wanted to put into switchers at the time. This was before the car and train sizes grew to where they are today. The last cataloged EMD end cab switcher, the MP15AC, ended up being pretty similar, 1500 hp and road trucks.
The older road slugs as used by MILW, CNW, BM and others had one slug between 2 mothers, usually higher horsepower for the time. CNW had 2500 hp mothers (GP35s and C425s), BM 3000 hp (GP40-2s). This enabled roughly the pull of 6-axle engines, with less beating of the track. MILW had high HP mothers (U30Bs) on some, and lower (F7s) on others. The low-HP sets were used on steeper grades with tight turns, roughly 2 SD7s without beating the track as much.
The CSX sets end up being similar to a pair of GP15Ts I wonder if it would have been more prudent to rebuild with turbo 8-645s than stripping everything out and having non-powered slugs. Would be another engine to maintain, but might be better off if something happens to the mother, then 2 pieces of power are OOS. If one GP15T goes down, the other one might get them back in.