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AC vs. DC traction
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Another thought should be taken into consideration: what AC means. Alternating Current switches polarity several times per second. Most table saws run the same speed because they all face 60 cycle per second household current. On an AC locomotive, the power plant generates power with an AC cycle based on the RPM of the alternator. This is then converted to DC which is then converted back to AC, only at a different frequency. Control of an AC motor requires setting both the freqency AND the voltage. The rule is: 105% acceleration. That means the inverters respond to an acceleration command from the throttle by supplying 5% more than what is being used at the moment. <br /> <br />The result is that both the power and the frequency are set to that optimum setting. On a DC locomotive, a Run 8 throttle setting could be supply 200% or more of the power that's needed to just go faster. This extra power is wasted off as heat and the destruction of the traction motors. AC traction motors only ever get about 5% more than what they are already handling, never mind what the engineer is asking for. The speed increases and so does the 5% figure until either the locomotive goes the speed the engineer wants or something breaks. <br /> <br />The computer that tends to all of this also looks at how fast each axle is going. On a DC loco, an axle that goes overspeed (wheel slip) is either cut back or cut out, depending on the wheel slip management. On an AC locomotive, that axle won't ever speed up beyond 5% more than the others because of the power applied. Then the computer looks at this overspeed and pulls it back to exactly the same as the other five axles. If all six can't speed up, the unit lays down, just as a DC unit would do, only without spinning wheels or overheated motors. <br /> <br />Thanks to the computerize inverters, money saving from AC unit not only comes from nominal fuel savings but also lesser maintenance expenses, greater locomotive availability (in theory), and more flexible assignment. <br /> <br />John
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