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Old 05-26-2011, 04:16 AM   #8
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I think the bigger question, is why has Norfolk Southen, a railroad that has traditionally been an advocate of DC traction, actually purchased several AC locomotives?

Here are some basics. Any electric motor (such as a traction motor) requires the reversal of a magnetic field within the motor. There are two ways to achieve this.

A Direct Current locomotives uses commutators on the rotor that are in physical contact with a brush to achieve the field reversal. Because of this physical contact, these locomotives require more physical maintenance on them. DC traction motors also tend to "burn" themselves up at low speed, making DC locomotives less than ideal at low speeds.

Alternating Current locomotives use the basic nature of the current running through them to achieve field reversal. Because of this, no physical contact is requires between the rotor and anything else. Consequently they require less physical maintenance on the locomotive. AC locomotives perform very well in low speed service.

However, despite the fact that AC locomotives sound like a panacea to your locomotive problems, there is a drawback: $$$$. All of this requires more complex systems, and they have a significantly higher first cost. They can also be more of a pain to maintain due to the complex systems.

So, the principal economic debate going on is whether they'd rather pay significantly higher up-front cost, or just pay the cost of replacing the commutators. Each road seems to have their own philosophy. BNSF seems to have more or less standardized on AC Power on their low speed coal trains, but DC power on their fast intermodal trains (the new ES44C4s notwithstanding). UP seems to be putting the ACs on everything. NS (mostly) and CN have decided that it's far cheaper and easier to continue replacing the commutators. Which is right? No one is really sure yet. Despite the fact that the first AC locomotive came out nearly 20 years ago, I don't think I've seen a definite study on whether AC locomotives are worth the money.

PM me with your e-mail and I'll send you a few presentations and lectures from one of my rail engineering courses that might help you out.
-Mike W.
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Last edited by Slopes09; 05-26-2011 at 04:27 AM.
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