Patrick Handscombe writes in 071203 MMDigest:

> Nowadays multi-stage turbine pumps are commonly used in industry,
> often belt driven by an external AC motor.  The suction head is
> roughly proportional to the speed of rotation, the diameter of the
> fans and the number of stages in series.

Just a correction -- the suction head goes up as the square of the
RPM and fan diameter, so a given percentage increase in RPM gives
double that percentage boost in head.

> To achieve high suction with reasonable flow in a small volume as in
> domestic vacuum cleaners usually means two stages in series of fans
> no bigger than about 5" diameter run at about 20,000 rpm, and the
> series wound "universal" brushed motors needed to turn them will
> always be extremely difficult to make quiet -- as in the Motora...

Yes, brush type motors scream and whine, and they are also noisy
electrically: the sparking brushes introduce glitches into the power
line and radiate as RF, and can be bothersome to MIDI or other
electronic systems nearby.

I do have a couple blowers with high-speed brushless induction motors,
so such motors are possible, though not as common and cheap as the
brushed vacuum cleaner motors.

Mike Knudsen