EMD SD80MAC and SD90MAC series official thread

[Justin B]

What I wonder is why the ACe has more tractive effort across the board. It was my understanding that tractive effort was simply a function of weight and adhesion. Do AC motors allow a more effective use of traction control (i.e. creep, etc.), or are the ACe's just ballasted heavier and this is all a marketing ploy?? EMD does not list weights for either unit, so I would wager that the ACe is at least a teenie bit heavier to give it a boost in its numers. But I only speculate...

[trainiac]

What I wonder is why the ACe has more tractive effort across the board. It was my understanding that tractive effort was simply a function of weight and adhesion.

That's true for starting tractive effort--which is why the difference there between the SD70ACe and SD70M-2 is fairly small. However, for continuous tractive effort there are many more factors that come into play--horsepower (in high speed situations) but also the power and efficiency of the motors, the effectiveness of the wheelslip-control system and the design of the trucks. It's advances in these areas that give an SD70M substantially more continuous tractive effort over an SD40-2 of exactly the same weight.

Do AC motors allow a more effective use of traction control (i.e. creep, etc.), or are the ACe's just ballasted heavier and this is all a marketing ploy?? EMD does not list weights for either unit, so I would wager that the ACe is at least a teenie bit heavier to give it a boost in its numers. But I only speculate...

The speed of AC motors can be regulated extremely precisely, whereas with DC motors, it's just a matter of reducing the power input to the motor until torque is reduced enough to prevent slipping. I would think therefore that it's easier to allow "creep" (at which point maximum tractive effort is produced) with AC motors. As for weight, the "Features" subsection of the EMD pages puts the two at equals with each other--408,000 lbs for the SD70ACe and 407,000 lbs for the SD70M-2.

Another reason for the bigger difference with the maximum continuous tractive effort rating might be because it can be developed at a lower speed with AC motors. In continuous run-8 service, the DC motors have to be turning fast enough to prevent overheating--which works out to (I'm guessing) about 12 mph. At that speed, neither the motor torque nor wheel-rail adhesion are at their maximum.

[Jay Potter]

The amount of tractive effort that an SD70ACe will produce between zero and about eight miles per hour depends on (1) the ability of its adhesion-management system to minimize the extent to which rail conditions reduce adhesion and (2) the upper limit of tractive effort that the adhesion management system will allow the traction motors to produce. In other words, if rail conditions are ideal the SD70ACe can produce as much tractive effort as its traction control software will allow it to produce. Above about eight miles per hour, tractive effort increasingly becomes a function of speed, with tractive effort decreasing as speed increases.

The primary difference between DC-traction adhesion management and AC-traction adhesion management is that when one wheel on a six-motor DC-traction unit begins to slip, power is reduced to all six motors. However when one wheel on a six-motor EMD AC-traction unit begins to slip, power is reduced only to the three motors on that wheel's truck. And when one wheel on a six-motor GE AC-traction unit begins to slip, power is reduced only to the motor for that particular wheel.

[Justin B]

That makes sense, thanks for clearing that up for me.

[slchub]

The AC's are very nice especially when you have a heavy manifest and your put into the hole on a grade. The AC's will give you dynamic braking practically to 0.00 mph whereas the DC's will start to fade at about 4-5 mph.

My question that nobody seems to know is why do the AC units have windows which are rectangular and the DC units have a teardrop in the corner of the windshield? I like the AC's but prefer the DC's when I have to do a setout or pickup as I can see the conductor without having to stick my head out the window to see if he is on the motor

[trainiac]

My question that nobody seems to know is why do the AC units have windows which are rectangular and the DC units have a teardrop in the corner of the windshield? I like the AC's but prefer the DC's when I have to do a setout or pickup as I can see the conductor without having to stick my head out the window to see if he is on the motor

The window design isn't a question of AC vs. DC models, but of old vs. new. Older SD70M's and SD70MAC's have the "teardrop" windows. The cab was redesigned with simpler forms and rectangular windows for lower production costs and parts standardization, and the new cab (along with many other new features) is used on both the SD70M-2 and the SD70ACe.

[Steve F45]

the windshields are the same. the only cosmetic difference i've seen is the sd70ace has the headlights on the nose whereas the sd70m-2 has them above the windows between number boards.

[UPRR engineer]

That's true for starting tractive effort--which is why the difference there between the SD70ACe and SD70M-2 is fairly small. However, for continuous tractive effort there are many more factors that come into play--horsepower (in high speed situations) but also the power and efficiency of the motors, the effectiveness of the wheelslip-control system and the design of the trucks. It's advances in these areas that give an SD70M substantially more continuous tractive effort over an SD40-2 of exactly the same weight.

The speed of AC motors can be regulated extremely precisely, whereas with DC motors, it's just a matter of reducing the power input to the motor until torque is reduced enough to prevent slipping. I would think therefore that it's easier to allow "creep" (at which point maximum tractive effort is produced) with AC motors. As for weight, the "Features" subsection of the EMD pages puts the two at equals with each other--408,000 lbs for the SD70ACe and 407,000 lbs for the SD70M-2.

Another reason for the bigger difference with the maximum continuous tractive effort rating might be because it can be developed at a lower speed with AC motors. In continuous run-8 service, the DC motors have to be turning fast enough to prevent overheating--which works out to (I'm guessing) about 12 mph. At that speed, neither the motor torque nor wheel-rail adhesion are at their maximum.

For everyone who reads your posts buddy. Where's all this knowledge about motive power coming from?...... and what is it you do for money?

[trainiac]

For everyone who reads your posts buddy. Where's all this knowledge about motive power coming from?...... and what is it you do for money?

I'll answer the last question first: I'm beginning a mechanical engineering program at university (a little money from there, and some more from mowing lawns ).

My knowledge of train operation pales in comparison to the people who actually work for railroads. I'm basing what I'm saying on what I've read on other forums, in books, on other websites and in conversations with railfans and railroad workers--because I've always been curious about how things work (especially anything train-related). Some of it has come through making locomotive drawings, because I try to figure out what all the wires and pipes and attachments that I'm drawing actually are (that's how I discovered traction motor cables and air dryers). A lot of it has also come out of practical experience (for example, physics courses and HO models taught me how a DC motor works--and to test what I knew, I tried building one afterwards out of an empty armature). I try to avoid making assumptions--but I know I'm not immune to doing so!

Hope this clears things up! I can clarify specifically on the post you quoted if need be.

[mp15ac]

the windshields are the same. the only cosmetic difference i've seen is the sd70ace has the headlights on the nose whereas the sd70m-2 has them above the windows between number boards.

The headlight location is a choice of the railroad. Either version (AC or DC) can have the headlights either on the nose, or above the windows.

Stuart

[UPRR engineer]

I'll answer the last question first: I'm beginning a mechanical engineering program at university (a little money from there, and some more from mowing lawns ).

OK buddy, i wonder where guys get there info from, that helped. Thanks for sharing.

[nickleinonen]

The AC's will give you dynamic braking practically to 0.00 mph whereas the DC's will start to fade at about 4-5 mph.

yep.. one shift when i was working as a hostler at my diesel shop, we had a csx ac 4400 and i ended up using it as a taxi for some shop moves.. i tried out the DB on it and i was impressed. i've tested the DB on our cn loco's with the dc motors, and the db at slow speeds is not near as effective. that db worked almost like independent brakes on the ac motored unit..

[GOLDEN-ARM]

How did this go from a question about SD-80ACE's, to SD-70 ACE's? HUGE differences between the AC/DC locos, and the technology on those ACE's certainly costs more money. Headlite locations and positions, and horn placement, etc., are mere cosmetic variations, dictated by the purchasing carrier, and would certainly not be any way of accurately identifying a loco. Try looking on the side of the loco, as most larger carriers identify the model number, somewhere externally. A "lightning-bolt" on the loco, near the number on the cab, is also a good indication of an AC motor. The AC's are preferred power for engineers, as they will accept more "abuse" without damage. (imagine holding the train on a grade, with the throttle, instead of the air......) I myself would not care if I ever saw another Dash-9, or any of the other locos with the so called "wide-cab" (Why the hell do people call them wide cabs? the cab is the same width as older equipment. Perhaps they mean wide nose.......) An SD-40, 50 or 60 is fine with me, and I wouldn't care if I never operated a GE product again, excepting some of their very tiny switchers. Just my opine, though, yours may differ dramatically...............

[mp15ac]

Golden-Arm,

The reason why the thread went from SD80ACe to SD70ACe is very simple. There is no such locomotive as an SD80ACe. The only 80-series locmotive was the SD80MAC.

Stuart

[Steve F45]

it was a complete mistype on my part. I screwed up the title but in the first paragraph i posted it said the sd70ACe.