Throughout the diesel age, locomotives worked according to a simple principle: a diesel engine turned a generator that produced alternating electrical current, which was then converted to direct current to run the traction motors that drove the axles. The leap forward that made possible that pull up the Toponas grade depended on a fundamental shift in technology during the 1990s from DC motors to AC motors. This change has been enabled by the availability of fast, inexpensive microprocessors.
Power for both a DC locomotive and an AC locomotive starts its path to the wheels in the same way. In both types, a diesel engine turns a generator that produces AC power, which is then converted to DC. (The starting AC power, at a constant 60 cycles per second, could run the locomotive at only one speed.) Here, though, the technologies diverge. In a DC locomotive, the DC power goes directly to motors that turn the wheels. In an AC motor, the direct current passes through a series of computer-controlled components called inverters, which “chop” the DC power into AC power. This AC is in turn fed to the motors.
Computer chips make AC motors practical by regulating the flow of power with a precision impossible by any other means. The chips monitor and control the DC entering the inverters and make sure that they deliver the proper amount of AC to the traction motors. This is no small feat: each inverter may require as many as 500 on-off commands per second to regulate the AC flow. And while 500 commands per second may seem unimpressive in a day of gigahertz chips, the proper comparison is not with other computers but with human beings. Imagine a train engineer trying to make 500 changes in throttle position every second.
AC motors are more robust than their DC cousins. They’ve been put through brutal tests that demanded maximum possible power production, sometimes for days on end. Those tests went far beyond anything the worst railroad environment could produce, and the motors never came close to overheating, according to Michael E. Iden, Union Pacific’s general director of car and locomotive engineering. As long as the equipment is operating properly, AC motors “really should never burn out,” Iden says. Many railroads are even using AC locomotive power-instead of air brakes-to hold trains stationary on heavy grades, Iden says. This technique, which avoids the time-consuming process of pumping off air brakes, would fry a DC motor in minutes.
Beyond their ability to pull heavier loads, AC motors improve overall efficiency. Each locomotive wheel makes contact with an area of rail no larger than a nickel. The percentage of weight on that wheel that is converted into pulling power is called “adhesion.” While the best DC motors can muster an adhesion of about 30 percent, AC locomotives take advantage of precise computer control of the traction motors to achieve adhesion averaging 34 to 38 percent; each percentage point gain in adhesion provides the pulling power for five additional fully loaded coal cars.