Maybe this was obvious, but I will spell it out for slow people like myself: 60 Hz = 60 cycles per second = 3600 cycles per minute A simple 2 pole AC motor sp

Maybe this was obvious, but I will spell it out for slow people like myself: 60 Hz = 60 cycles per second = 3600 cycles per minute A simple 2 pole AC motor spins 1 rotation per cycle, so 3600 RPM. AC is a sine wave cycle of current. Current flows one way to attract it to one pole, the current flows the other way to attract it to the other pole. For a big mainframe disc drive, it sounds like the obvious choice. Why they stuck with it after switching to DC, who knows. Maybe they didn't want to redesign the controller circuits.