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158 Build Your Own Electric VehicleDC and AC Working TogetherThe series DC motor provides the best starting torque, and the AC induction motor isthe most efficient at speed. By using them together with some mechanism to switchback and forth (for example, at around 30 mph on the level with a kickdown for hills),you get the best of both worlds. Only about 96 volts are needed to get the DC motorstarted in your average utility commuter EV. When the AC motor takes over, you canadd the rest of the battery pack toward powering it—120, 144, 192 volts, or whatever.Darwin emphasized, “Pick a good core material with some nickel in it to achieve 96percent efficiency, and remember size and weight go down as the frequency does up.”TuningElectrical and mechanical systems deliver better performance when balanced and tunedup. Race car mechanics set up their cars for the average case, then tune and adjust themfor maximum performance. Should you do anything less with your EV? Match theimpedance of the motor to the controller for maximum power transfer, and go througheverything else with a fine-tooth comb.Keep It SimpleWhat’s the simplest controller that can implement this approach? Do you even need touse solid-state designs? How few batteries can you get by with? Which are the smallestmotors you can use? Can you do something innovative with their placement? How canyou simplify the drivetrain? Can you do something better with the tires? You get theidea.ConclusionCurrently, the most economical (and indeed common) option for electric vehicles isseries DC technology. However, it is likely that all commutated motors will be phasedout over the next decade or two, since a commutator’s functionality can be replaced byclever electronics (which is getting cheaper every day), and with improved reliabilityand efficiency. At present, AC induction and permanent magnet brushless DC are thebest technologies available, with efficiencies up to 98 percent, silent operation, andalmost never requiring any servicing. They each have various advantages anddisadvantages over one another. It will be interesting to see which one becomes the newstandard in the years to come.