[Billfred]

This experience comes from R/C cars, and it may or may not be applicable. Just thought I'd warn in advance.

In R/C cars, you've got manual and electronic speed controllers. ESCs are just like Victors in FIRST (except usually designed for 7.2 volts, standard radio gear, and are cheaper), and basically blip the throttle hundreds to thousands of times a second, so fast that it seems like you're driving it at various amounts of power. MSCs are basically a servo sweeping the electrical connection across two, one, or no ceramic resistors, thus giving varying amount of resistance and changing the speed of the drive motor. (This is what I think you're talking about by variable resistors...correct me if I'm wrong) It gets the job done, but there's lots of energy lost to heat. Most people immediately ditch the MSC at their earliest chance for an ESC...if they can afford it.

Hope this helps!