AC vs. DC motor comments

[Tadman]

I have read a few threads where engineers mention that AC motors vibrate more than DC - any comments from those with experiance or knowledge? I know when I ride the CTA elevated or the South Shore (both electric MU's) the cars with AC motors ride much smoother than the old DC motor units, so the comments here surprised me.

Thanks.

[Jay Potter]

I hesitate to interpret comments that I haven't seen; however I suspect that they were referring to the tendency of an AC-traction locomotive to vibrate when it's starting a train or moving it upgrade at low speed. The vibration occurs because the adhesion-management system is making multiple rapid adjustments of wheel speed in order to maximize tractive effort.

[Guest]

I hesitate to interpret comments that I haven't seen; however I suspect that they were referring to the tendency of an AC-traction locomotive to vibrate when it's starting a train or moving it upgrade at low speed. The vibration occurs because the adhesion-management system is making multiple rapid adjustments of wheel speed in order to maximize tractive effort.

This would be correct Jay.

A passenger train, in particular, lighter wieght MU type cars react differently than a 440,000 pound AC propelled locomotive trying to pull 12,000 tons. When you start that kind of tonnage, no matter what kind of traction motors, there will be some wheel slip. With the AC type locomotives, the computers as Jay said compensate for the slip. One note is that while EMD type AC systems have one inverter per truck, the GE's have an inverter per axle. When the computer trys to compensate on the EMD, it will do the same thing to the entire truck, whereas the GE will only compensate to the single axle which is effected. In theory, this makes the GE seem a bit smoother. Both however will jump when starting to move.

It is a lot easier to see what is going on with a GE since the data of each axle can be seen on the computer screen in real time. EMD's do not have this feature.

[498]

I was looking at Tadman's question yesterday and thinking about how you could answer it clearly and understandably. Jay and RRKen, you both provided superb answers!

[UPRR engineer]

Wanna know the best way to get an AC motor to shake and vibrate. Some of you railfans who love locomotives arent gonna like this story.

I was taking stundent engineer trips with this good hog that i loved to work with, he let me do anything i wanted. So every day i would try to run a different way or test my skills. All air one day no dyno, all dyno the next, grandstand, run slow, anything i wanted. So one day he said " what ya gonna do today kid". "We are gonna test the aint roll back feature". When we were coming down from the plant i slowed her down to about 5 or 6 mph, threw the reverser back and put it in the 1st run. It stopped alright but about threw us both out of the seat. The conductor came up see what the hell was going on, i was laughing my ass off. I did this with both versons an 8000 the 1st time and then again with a 7000, same results with both.

Not alot of hogs would have done that one eather.

[Nelson Bay]

Wanna know the best way to get an AC motor to shake and vibrate. Some of you railfans who love locomotives arent gonna like this story.

I was taking stundent engineer trips with this good hog that i loved to work with, he let me do anything i wanted. So every day i would try to run a different way or test my skills. All air one day no dyno, all dyno the next, grandstand, run slow, anything i wanted. So one day he said " what ya gonna do today kid". "We are gonna test the aint roll back feature". When we were coming down from the plant i slowed her down to about 5 or 6 mph, threw the reverser back and put it in the 1st run. It stopped alright but about threw us both out of the seat. The conductor came up see what the hell was going on, i was laughing my ass off. I did this with both versons an 8000 the 1st time and then again with a 7000, same results with both.

Not alot of hogs would have done that one eather.

Isn't what you did called plugging the motors? I always thought, except in cases of dire emergency, that "plugging" is against the rules due to the potential for equipment & track damage and personal injury.

[Jay Potter]

I've been under the impression that "plugging the motor" refers to the practice of using the reverser to reverse the direction of the current to traction motors on DC-traction units and that this was used by some railroads to control the movement of coal trains that were flood loading on a downgrade with the head end toward the bottom of the grade. Consequently that loading procedure was referred to as "plug loading"

Today, at least on CSXT, the term "plug loading" also refers to the procedure of using the dynamic brakes on AC-traction units to control the movement of coal trains in the same situation, i.e. being flood loaded on a downgrade. One of the problems that CSXT encountered in its AC-traction program was an incompatibility between the software related to this procedure on EMD units and the corresponding software on GE units. This problem was resolved during the second half of last year, which explains why consists made up of both EMD and GE AC-traction units have been appearing more frequently in CSXT coal service.

[Nelson Bay]

Jay

So on CSX it's ok to throw the reverser in the opposite direction of movement while running a train?

[Jay Potter]

On CSXT it's ok to (1) use the dynamic brakes on AC-traction units to control the movement of coal trains that are being flood-loaded on a downgrade and (2) refer to this procedure as "plug loading" even though (3) this procedure is not "plugging the motor".

[UPRR engineer]

On older units you need a pace setter box to plug load, some of the newer units have them already installed. Normal slow speed coal loading is done on flat track. I think CSX and old SP used the plug method because they were pulling down hill. Plus style is more like a weak dynamic brake but it uses power on the traction motor in the opposite direction. Kinda like using power to hold yourself on a hill.

Me throwing the reverser and opening the trottle was nothing like plug loading.

[Nelson Bay]

Jay

Is what I call, possibly incorrectly, "plugging the motor/motors OK on CSX?


UPRR engineer

Is it ok on UP?


It just seems the practice has to damage something?

[UPRR engineer]

You dont throw the reverser in plug loading like i did with those AC's. At least i dont think you do. A box or computer now a days is used. Im sure some where on the old SP they still plug load coal trains. I do sometimes have to load coal, its on flat track so we use the slow speed control.

[Jay Potter]

Nelson's question about CSXT rules really needs to be answered by someone who works for CSXT; but my best-guess answer follows.

CSXT requires that locomotive speed changes be made in ways that permit in-train forces to adjust to those changes; and its rules for emergency stopping do not include a procedure for stopping by moving the reverser opposite to the direction of travel. So, if I'm correct in those two respects, then "plugging the motor", as Nelson understands that concept, would not be acceptable unless the locomotive were moving extremely slowly.

That having been said, I have a feeling that plugging the motor does not really apply to AC-traction units. For instance, the operating manual for the SD70MAC-T1 discusses plugging the motor on DC-traction locomotives but states, with regard to the SD70MAC-T1: "It is not possible to plug the traction motors on this locomotive. If plugging is attempted, opposite direction brake (maximum dynamic brake) applies."

[Nelson Bay]

Jay

Thanks for the operators manual info. Does the manual mention anything about what can/cannot be done when a DC motor unit trails an AC unit in a consist?

[Jay Potter]

The operators manual doesn't deal with consist make-up in the context of "plugging the motor"; and I don't believe that it addresses the issue of consist make-up even generally.

On CSXT, at least, guidance for consist make-up is provided by guidelines that divide locomotives into categories and specify the order in which categorized locomotives should be placed in multiple. As one would expect, the guidelines generally call for AC-traction units to trail DC-traction units (and for microprocessor DC-traction units to trail earlier DC-traction units). However there are exceptions to this for trains having sufficiently high horsepower-to-tonnage ratios.