Serious efforts to use AC (alternating current) diesel-electric engines began when General Motors introduced its 4,000-hp SD-60MAC in 1991-92. While the AC power increases an engine's adhesion, allowing it to start far heavier trains than a DC-powered locomotive of the same horsepower, the heavier load prevents AC-powered engines from reaching track speed. To solve that problem, locomotive builders have raced to design and deliver AC engines with ever-larger horsepower. In 1995 GM introduced the 5,000-hp SD-80MAC, which uses a 20-cylinder version of EMD's 710-series prime mover to generate extra power.
While they were impressed with GM's results, most railroads chose to wait for the 6,000-hp SD-90MAC, but the railroads that opted to wait for the SD-90MACs initially found themselves with less power when their -90s were delivered in 1996. GM opted to deliver the engines with 4,300-hp prime movers that were designed to be replaced with 6,000-hp engines when they became available. Now that they are operating at full power with a GM16V265H engine, the SD-90MACs offer 170,000 pounds of continuous and 200,000 pounds of starting tractive effort, as well as 115,000 pounds of dynamic braking effort.
Did You Know? GM claims that one SD-90MAC can replace two SD40-2's in both heavy haul and high speed freight operations.
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