[kragen]

I don't think this is correct. Drills use "universal" motors which don't care if they're running on AC or DC, because 60Hz AC motors are limited to 3600rpm, which isn't nearly fast enough for a drill, and also because it's not okay if the drill stops working if it hits resistance and slows down. (Most AC motors run at a fraction of that.) You can run a cheap electric drill off 120Vdc just as easily as 120Vac. Getting it to run on 48Vdc or 30Vdc involves rewinding the motor with the same amount of copper in the form of thicker wire.

Fancy drills already have a lot of electronics that do care about the polarity of the applied voltage, but they usually want it to be DC. Once you get anything more sophisticated than phase-angle control with a TRIAC, you're using MOSFETs anyway, and you can often use half as many of them if you're using DC, because MOSFETs like DC.