Re: Central Vac overload

[Art Todesco <actodesco@xxxxxxxxx>]

On 4/1/2010 8:13 AM, B Fuhrmann wrote:
>> External air output pipe ran free when unblocked but dropped to very low
>> flow when the hose was blocked.  If there's an internal check valve, it's
>> venting to someplace other than the vacuum's output pipe.
>
> No internal check valve on most vacuums.  They expect that the increased
> noise will let you know.  With a check valve, almost no change in sound.
>
>> Another theory.  It takes more work to create a vacuum than maintain it,
>> so
>> when the hose has contracted as much as it can, the motor has less load on
>> it and thus the current drops.  With an open vacuum, the motor is
>> constantly
>> sucking and never achieving even a partial vacuum within the tubing.
>
> A vacuum does it's work by pumping air.  If it is not pumping air, it is not
> doing as much work.
>
> The amount of vacuum goes up when it is clogged because air is not flowing
> into the system.
>
>
Yup.  I learned this many years ago in
working on pipe organs.  Normally, when
the blower is running, it is basically
plugged.  The pressure is at its normal
operating pressure and no air is being
moved, except to any leaks.  And in pipe
organs there are usually many tiny leaks
which eventually do add up.  But, if a
wind line is opened, the blower current
goes up because it is now moving air and
thus, doing more work.  In an organ
blower, the motor is usually an
induction motor.  It doesn't speed up as
in a vacuum cleaner.  In a vacuum
cleaner, the motor is usually a series
(sometimes called universal) motor.
These motors will actually generate a
back EMF.  When the motor is not doing
an work, i.e. sealed off, this back EMF
will actually "self power" the motor a
little, making it turn faster.  Back in
high school shop, they said that a
series motor with no load, will keep
going faster and faster until it
literally breaks apart.  I've run a
small sewing machine motor, no load, and
indeed it kept going faster and faster.
  I was afraid to keep going, but I
suspect it would eventually find a top
speed as even the bearings and cooling
fan blade do some loading, albeit small.