a system for compressing air includes : a variable speed driver having a drive shaft ; at least one pre - main air compressor having at least one stage and adapted to receive and compress a flow of ambient or other air and to be driven by the drive shaft of the variable speed driver ; at least one main air compressor having at least one stage on a first pinion shaft and adapted to receive and further compress a flow of compressed air transmitted from the at least one pre - main air compressor ; at least one booster air compressor having at least one stage on a second pinion shaft and adapted to receive and further compress a flow of further compressed air transmitted from the at least one main air compressor ; and another driver adapted to drive at least one of the first pinion shaft and the second pinion shaft through at least one transmission .