as part of a lab course, my group was asked to "renovate" an old experiment meant to teach students about air compressors (characteristics, flow rate, pressure...) the "old" installation was comprised of one electric (i think AC) motor connected via a belt to one of two compressors, both air pumps, one water cooled, the other air cooled. you could only operate one compressors at a time, and had to stop the motor, loosen some bolts, tie the belt over the other compressor, and tighten the whole installation. for the "new" experiment installation, we're required to look up possible solutions to drive the two compressors without having to dismantle or take apart bolts or screws. my question: what is easier/economical; drive each compressor with a separate motor, or use a single motor to drive both compressors through a certain differential system that drives one compressor at a time. some more points: the two compressors are around 2HP, and need to be driven at variable speeds through user control, which begs the other question i have; AC or DC motors?