Coal drag sd70s

[shadesman89]

Nowadays I see most sd70aces on coal drag duty can anyone explain why?

[bpetit]

AC traction better for the mountainous grades as they can pull better.

[Ericmopar]

The nature of A.C. traction helps keep heat buildup down in the traction motors, so they can lug a heavy load at slower speeds.
It's partly the nature of A.C. itself, and partly because A.C. motors have a lighter armature and therefor can use lower gearing. IE a lighter armature can spin up faster and farther, which is why the A.C. motors can use the lower gearing, for the same effective top speed. The lower gearing is part of the reason why they can pull a heavier load at a lower speed than a D.C. loco.
I found out there is more too it than that. Apparently the new generations of U.S. engines, are also using a higher working voltage, than the old 600V engines. That in turn means less current for the same load as well, which also helps control heat buildup.

It's interesting stuff to read. Apparently they generate A.C. then rectify it to D.C. and then they run it through inverter/VFDs, to convert it into multi frequency A.C.
At least that's the way I understand it, after reading an article about it.

[dfcfu342]

The merits of AC vs DC have been discussed ad nauseam and tends to make people get all defensive over which one is better for some reason.

As EricMopar and bpetit said AC is better suited to pulling coal drags because it has better low speed tractive effort, produces less heat, and doesn't wear on traction motors due to their nature. If you want to read the where's and why's just Google it

[BNSFdude]

AC is better in all applications, but its a matter of cost.

[Rich_S]

The nature of A.C. traction helps keep heat buildup down in the traction motors, so they can lug a heavy load at slower speeds.
It's partly the nature of A.C. itself, and partly because A.C. motors have a lighter armature and therefor can use lower gearing. IE a lighter armature can spin up faster and farther, which is why the A.C. motors can use the lower gearing, for the same effective top speed. The lower gearing is part of the reason why they can pull a heavier load at a lower speed than a D.C. loco.
I found out there is more too it than that. Apparently the new generations of U.S. engines, are also using a higher working voltage, than the old 600V engines. That in turn means less current for the same load as well, which also helps control heat buildup.

It's interesting stuff to read. Apparently they generate A.C. then rectify it to D.C. and then they run it through inverter/VFDs, to convert it into multi frequency A.C.
At least that's the way I understand it, after reading an article about it.

Plan and simple the advantage of AC traction is there is not short term rating on the traction motors. As you mentioned, DC traction motors when put under a heavy load with high amperage create enough heat to damage the windings in the motor even with the traction motor blowers operating at high speed. Since AC motors use induction, they do not produce enough heat to damage the motor.

To answer your second question, all North American locomotives today have Alternators, but the reason the AC is rectified to DC then inverted back to AC is to control the frequency. In other words, to make a DC motor spin faster you simply supply it with more voltage. To control the speed of a AC motor you have to control it's frequency and that is the job of the inverter.

One of the biggest noticeable difference between DC traction motors and AC traction motors are the cable sizes. DC traction motor cables are in the 1100 AWG to 1600 AWG size, while AC traction motors use 700 AWG cable. All neat stuff, also dynamic braking is handled differently between DC traction motors and AC traction motors.

[FourEightFour]

I have heard of a study done by either BN or C&S up in Wyoming concerning AC vs. DC on the Powder River trains. One thing found out during the test was that AC traction tends to spin the wheel an infinitesimal amount faster than the actual speed of the locomotive under a heavy load. This caused a surprising amount of pull on the rails. Has anyone else heard of this before or is this common knowledge nowadays?

[Ericmopar]

I have heard of a study done by either BN or C&S up in Wyoming concerning AC vs. DC on the Powder River trains. One thing found out during the test was that AC traction tends to spin the wheel an infinitesimal amount faster than the actual speed of the locomotive under a heavy load. This caused a surprising amount of pull on the rails. Has anyone else heard of this before or is this common knowledge nowadays?

I've heard of it, and know it's a traction enhancer built into the locos computer system.
I wonder if the genius' in railroading, have thought about what that will do to rail and wheel wear?

Here it is in action on EMD built Indian engines.
It begs the question... Just because it can be done, should it be done?

[Rich_S]

I have heard of a study done by either BN or C&S up in Wyoming concerning AC vs. DC on the Powder River trains. One thing found out during the test was that AC traction tends to spin the wheel an infinitesimal amount faster than the actual speed of the locomotive under a heavy load. This caused a surprising amount of pull on the rails. Has anyone else heard of this before or is this common knowledge nowadays?

I've heard of it, and know it's a traction enhancer built into the locos computer system.
I wonder if the genius' in railroading, have thought about what that will do to rail and wheel wear?

Here it is in action on EMD built Indian engines.
It begs the question... Just because it can be done, should it be done?

Actually what you are seeing in that video is the Super Series handling wheel slip. Radar has to see ground speed before the super series will use wheel creep. That video is a testament to AC traction motors and EMD super series handling a stall condition with wheel slip. Because of the speed probes on the traction motors along with current transducers and the radar feedback, super series knows the wheels are slipping so it's using a combination of sanding and power de-rating to try and over come the stall.

Super Series Wheel Creep occurs when the super series see ground speed from radar. It then tries to spin the wheels just a tiny bit faster than normal rotation for the ground speed, which yields a increase in tractive effort. If a wheel slip occurs, wheel creep is turned off and the wheel slip system takes over until better traction conditions resume.

[dfcfu342]

One thing found out during the test was that AC traction tends to spin the wheel an infinitesimal amount faster than the actual speed of the locomotive under a heavy load. This caused a surprising amount of pull on the rails. Has anyone else heard of this before or is this common knowledge nowadays?

The A/C traction software will spin the wheels up to 2% faster than rail speed which actually provides greater power transfer to the rail than maintaining 100% adhesion. So yes it's common knowledge and actually programmed into the locomotive.

[FourEightFour]

The more you know. I'm a steam guy. I can bore one with technicals of steam, yet when it comes to diesels, "I no nothing".