The "AC brush motors" are actually series wound DC motors. In fact any wound field type DC motor will work on AC. Why? Because the magnetic field polarity on the armature and the stator field are changing together. The result is the vector forces are in the same direction. If you hook a battery up to a universal motor with leads reversed or not, it will still rotate the same direction. Now if you make the armature or stator magnetic field stationary by not changing the polarity on one or the other, then it will change direction of rotation of the motor if you change polarity on which ever part you did not make the polarity stationary for.
Honestly though, we should not be using brushed motors in power tools anymore for 2014. We should have already switched to 400 Hertz, 3-phase AC induction motors because with today's chip integration, it should be possible to make a single chip solution that only requires a DC filter capacitor (full wave diode bridge built into the the chip) and a heatsink to cool this chip. The other components would be a tachometer to get RPM speed of the motor for closed looped feedback for precise torque control and a linear travel potentiometer for the trigger button when variable speed is needed.
In fact I even drew up a chip outline for what such a chip would have for pins that I submitted to all the major power tool makers. I doubt they'll switch. Could be a conspiracy to keep the tools with brushed motors so that they will have a short life expectancy. After all, we live in a disposable society.....
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