Jim Wilkins wrote:
"jim" " wrote in message
...
Tim Wescott wrote:
On Tue, 28 Apr 2015 19:51:06 -0400, Bob Engelhardt wrote:

A neighbor has a Festool shop vac, er ... "dust extractor" (at
$650, it
wouldn't sell as a "shop vac" G).

It has been shutting itself off due to the motor overheating.
It's
spec'ed as using 2.9 - 8.3A (depending upon the speed, I assume).
So I
put an ammeter on it and measured 9A, at low speed! Ah-ha,
something to
work with. Suspecting a blocked air path was putting an extra
load on
it, I first removed the dust bag, then the HEPA filter, and then
took
the motor-fan out of the housing. No help: in free air, at low
speed it
used 7.4A and at high speed 10A!

The rotor turns easily enough, but doesn't spin when given a push.
The
brushes are fine and there is no arcing at the commutator.

So, the only thing that _might_ be an abnormality is the
less-than-free
rotor. Could that be the basis for such increased current (7.5 vs
2.9)?
The assembly is not meant to be serviced (they'll sell you a
new
motor-fan for $95+). I suppose that I don't have anything to lose
by
trying to break it down, but I'd like there to be something else
to try.

I'm not sure of the exact mechanism, but on home-style vacuum
cleaners
when you block the vacuum line the motor speeds up. I assume this
means
the motor is unloaded, and I assume it's because the turbine is
suddenly
working in rarefied air.

I'd be interested in hearing truth from one who knows.


That is one way of looking at it. Normally the air is
very dense on the leading edge of the vanes when its
plugged it is more rarefied. Its kind of like when you
are rowing a canoe and you miss the water. Its a whole
lot easier when not pushing against something dense.

You could see if there's a difference between blocking the inlet and
the outlet.


Both will lower the load on the motor but not the same amount.
The load goes down when the pressure differential on either
side of the vane goes down. When the flow is blocked the air
just goes round and round and there is not much pressure
differential. When the inlet is blocked the air is more
rarefied so it does have slightly less resistance.

Also there is probably some air flow through the motor for cooling.
If that is on the discharge side blocking the discharge won't
stop all flow and blocking the inlet may still cause overheating
because the motor cooling flow is also stopped.