Thanks, Art, for summing it up so well. I wouldn't have asked the
question
if it wasn't a problem. I was downstairs, right near the vac cabinet so I
could hear the motor noise change, but my wife, vacuuming upstairs,
didn't.

Based on some other comments, I need to do some testing, especially
current
draw, to see what's really happening when the vacuum becomes clogged.

Current sensing (for slightly low current, fans spinning air take less power
than ones moving air) is one way but is likely to be more expensive than a
vane on a switch to watch for no air flow on the exhaust.
Another possibility is to look for a lower pressure (higher vacuum) just
ahead of the main unit.