Re: Central Vac overload

["Robert Green" <robert_green1963@xxxxxxxxx>]

"John Gilmer" <jlgilmer@xxxxxxxxxxxx> wrote in message
news:JbydnR3XQL2o7i_WnZ2dnUVZ_tidnZ2d@xxxxxxxxxxxxxxxxxx
>
> "Robert Green" <robert_green1963@xxxxxxxxx> wrote in message
> news:hosmv1$fp7$1@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> > Some of you might remember my question about building a silencing
cabinet
> > for a central vac.  Well, it worked, but too well.  The problem is now
> > that
> > if something gets stuck in the hose, we can no longer hear the immediate
> > increase in the motor load the way we used to.  On occasion, the unit's
> > motor even shuts down from the overload.
>
> Believe it or not, the "motor load" is usually at a minimum when the vac
is
> completely blocked off.

It's counter-intuitive, but I guess the motor RPM would decrease while
overloaded, not obviously increase, which does make for a sort of "runaway
Toyota" feel.

> If you want to monotor for a blocked inlet you might want to check flow in
> the exhaust (after the bag).   Most central vacs exhaust through a tube
> which can be connected outside if your don't want to "re-cycle" the dirt
the
> filter bag misses.

Mine is connected to an outlet pipe and muffler to the outside, so it's
doable, but hopefully not necessary with Art's relief valve.

<stuff snipped>

> In many/most central vacs (and even some "shop vacs") the cooling air for
> the motor is separate from the air used to suck up dirt.    If your's
isn't
> separate then a blockage would definitely cause it to overheat.
Otherwise,
> you may find that it actually runs slightly cooler when the inlet is
> blocked.

Not sure about this issue.  I suppose I could test it.  For now, I am going
to mount a temperature sensor in the unit and hook it up to my home
automation controller to shut the unit down if the temperature exceeds a
certain level.  That will probably have to be a trial and error sort of
thing.

> Frankly, sensing overheat isn't a bad way of seeing if something has gone
> wrong.

Exactly.  Which is why the temperature sensor might be a good thing to add
in addition to the relief valve.  In fact, what I should do is hook up a
recording thermometer to a probe inside the silencing box to monitor a few
sample runs to try to estimate the temperature rise and where it plateaus.

> Modern motors "run hot" so don't worry about overheating shortening the
>life of the motor.

I've burned up enough gear accidentally (a few AMD CPUs, a few Fujitsu
tablet PCs, a Sony receiver and many, many many more) that I am now hard
over in the other direction, adding cooling fans to stereos and equipment
racks to make sure everything gets good airflow.  I believe heat really
hastens the aging process and fans help reverse it.  Now only if I could fan
my bad knees and get them working again!

> If anything, the life limiting is as likely to be the motor bearings as
the
> windings.

And those failures would be heat-related, as in winding insulation and
bearing lube.

> If you like to worry about blockage, you can quickly check to see whether
it > still "picks up" a piece of pocket litter.

The point that Art made applies here.  An electrically driven motor brush
head will appear to be picking up dust but in reality is just sucking it
into the brush head.  If there's not much dirt, it's very easy to think
you're vacuuming when you're not.  The motorized carpet sweeper head make
enough noise that you really can't hear the change in the motor RPM or the
lack of vacuum hiss.

--
Bobby G.