As AC traction motor technology improved in the 1990s, the SD70MAC emerged as ideal power for heavy trains.
Billed as the biggest technological development since the switch from steam to diesel, improvements in AC traction motor technology promised major changes in how railroads ran trains and assigned power.
Diesels with conventional DC traction motors can develop a lot of power at low speed, but only for a short period of time before the motors will overheat. AC traction motors on the other hand can run at full power without overheating for a long time. This gives AC motored locomotives an advantage in hauling heavy drag freights like coal trains. After testing EMD’s SD60MAC prototypes, Burlington Northern determined they could replace five aging SD40-2 and C30-7 locomotives with just three new SD70MACs on their Powder River Basin coal trains. Burlington Northern’s initial order for 350 SD70MACs brought AC traction…
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