Almost all variable AC voltage sources will use a variac like talked about already here. The primary reason is that it is fairly cost effective and simple technology to build. Since these things use a wire wound core with part of the winding open without insulation so that a sliding contact can move from winding to winding you will find that the resolution of these devices is limited by the number of turns in the winding. If an auto-transformer type is needed and you know the input is 120 VAC rms then you would need a unit with 120 or 240 winding turns to achieve 1 volt steps. To control the output to say 0.1 volt resolution then you would probably need one with a 1200 turn winding.

Here is another picture of a motor controlled 3-phase unit.



Michael Karas