OK, so looking at gearboxes, 30:1 gets me 140 sfm, 20:1 gets me 213 sfm, and 15:1 gets me 290 sfm.
The largest feasible pulley diameter is probably 11", and the smallest 1.5"; that difference gets me down to 569 sfm without using an intermediate pulley (which would mean an axle and a mount, doable but again more complexity means more procrastination).
So, it looks like there are two approaches:
1. gearbox to reduce the top speed to around 300 sfm and a VFD for variable speed from 0 to top speed
2. pulley to reduce the top speed of roughly 600 sfm, and a VFD for variable speed from 0 to 600 sfm
Is #1 correct? Should the gearbox instead be used to reduce the speed to say 140 sfm in order to preserve torque at the low-end speed, with the VFD only used to *increase* speed?
Of these two, the pulley is vastly cheaper. A gearbox, vfd, and motor all cost about the same (~150), so adding a gearbox into the mix increases the cost by 50%.
Is there a low-cost option to have variable speed without a VFD, excluding of course step pulleys?