AC4400 Diesel Locomotive

Prototype Information

EMD and GE would soon square off in the development of Alternating-Current, or A.C., traction for locomotives. Prior production models from both builders utilized Direct-Current (D.C.) traction motors for propulsion, as was done since the very first diesel-electric locomotives were produced. A.C. traction motors demonstrated significant advantages over their D.C. counterparts by allowing for much greater continuous tractive effort, as well as reduced maintenance. They were also much more resilient than D.C. motors, and were able to endure much higher short-time ratings (the ability of the traction motors to withstand high amperage at low speeds for a set period of time without incurring damage), giving them extraordinary lugging ability.

A.C. units showed their ability to replace their D.C. predecessors on a 3-to-5 basis, being able to walk away with heavy trains from a dead stop on an upgrade, in scenarios that were nearly impossible with an equivalent number of D.C. units. This wizardry required substantial changes to the locomotive’s electrical system, including a system of electrical inverters to “refine” the current being fed to the traction motors and an extensive microprocessor control system to manage it all. While these changes increased per-unit cost, most railroads found the benefits to be more than worth the expense. With that, EMD would introduce the SD70MAC, and GE the AC4400CW. The AC4400CW was based upon GE’s highly successful Dash 9-44CW design, and would become one of the hottest selling locomotives of the 1990s and beyond.