First, this arrangement only gives you two speeds, corresponding to twelve and 24 volts; with an electronic PWM control the speed selection is continuous as on a gasoline-powered car. Even at 4 or more speeds, with more elaborate switching, the vehicle’s acceleration would be very jerky given the power levels I’ve specified for my project.

Second, though your switch may handle “high amps” the problem isn’t just current handling, it’s voltage. I will be running a 348VDC system, with currents running up to 2000 amps on the motor side, and briefly up to 1600 amps on the battery side. (I need to put up a page about the controller, but currently it’s being borrowed for a somewhat notorious drag racing postal van). Breaking 1600 battery amps is not easy for any mechanical switch, and the very high voltage makes the situation much worse as it requires a much bigger gap. The massive sealed, hydrogen-filled contactor in this post is only expected to break that kind of current a few times, and trying to make contact under that kind of load would likely weld the contactor the very first time.

There is one thing that “contactor controllers” are good at, and that is efficiency. A friend of mine has an original Henney Kilowatt, which is a late 1950s conversion of a Renault Dauphine produced by the Henney Coach Company in partnership with the Eureka Vacuum Cleaner Company (believe it or not) here in the States. It is based on a contactor controller which gives four levels of power, by mechanically rearranging the battery pack. On any level except the first (which uses a huge resistor in series) there is nothing in the circuit but the batteries, switches and wiring. An electronic control always imposes a small amount of resistance, in the case of IGBTs it’s a constant voltage drop of about 2 volts when switched on. However, the losses are not extreme. Very little heat is generated by a modern IGBT-based controller during normal street driving and therefore little cooling is required.