> However, my old electrician pal warned
> me off -- he said the 3450 RPM motor had fewer poles and thus
> less start-up
> torque even when "geared" back to the same RPM by a pulley
IMO it's a myth, at least as stated. AFAIK, all other things being equal,
the 3450 rpm motor should develop half the starting torque of the 1725,
which would make it the same once it was geared back to the same compressor
rpm. But there might be some other factor at work, perhaps the 1725 you
were looking at was not designed for as much starting torque or whatever.
Some motors will overheat when used for cyclic loads like air compressors,
since they do have a higher peak-to-average power ratio than other services.
"Compressor duty" is one step beyond "continuous duty".
It's easy enough to reduce the starting torque requirement for an air
compressor anyway, all you do is increase the volume between the pump and
the check valve into the tank. This volume is bled down to zero pressure
after each run, so the pump starts with no load. A bigger volume gives the
motor more time to start.
Of course, achieving that 2:1 reduction in drive ratio might be a problem
too ... most compressors already have a 3:1 or 4:1 ratio. Don't have my
design handbooks handy, but 8:1 sounds like an awfully big reduction for a
single belt. There's also a minimum radius below which belts aren't as
efficient, so you might need to increase the pulley on the compressor
instead, which would be another PITA.