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attic exhaust

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Posted by Dodo on August 10, 2003, 7:24 am
 
I'd like to use a solar panel to run two AC 3.4amp attic exhaust fans. What
sort of components would I need to do this? I know I'd need the panel and an
inverter, but I'm not sure if I'd need anything else. I'd also appreciate
advice on exactly what models of equipment would suit my needs.



Posted by Kevin M on August 10, 2003, 2:05 pm
 
That's a question I've been wanting to learn about, too, but most of
the fans seem to be 24VDC rather than AC.  There are commercial
systems available for 3-400 $US, and I haven't been able to see a way
to build one for less than $00.  Assuming I could figure out the
amps, volts, etc.




Posted by Joe Fischer on August 10, 2003, 7:27 pm
 : That's a question I've been wanting to learn about, too, but most of
: the fans seem to be 24VDC rather than AC.

       Which makes sense for the following reason;

AC motors will be damaged by voltage less than rated,

while DC motors will just rotate slower at lower voltages
without overheating or damage.    (There are many types
of motors, but DC motors commonly have brushes and either
a slip ring with small brushes carrying the field current
to the rotor, or have large brushes carrying the armature
current).

      There are many low cost motors, mostly 12 volt
in auto junk yards, they are the heater and air
conditioner fan motors.
      They are attached to squirrel cage blowers,
which may not be high volume fans.

      Some of them have permanent magnetic stators
with rotating field coils and small brushes.
      The squirrel cage blowers do have a disadvantage
of being noisy.

      There are also DC motors used for radiator
fans for many newer cars, and that have regular
fan blades.  

      Any of these, 12 volt or 24 volt from trucks
or RVs, could be used with just diodes and solar
panels provided they are fused to protect in case
of bearing failure.
      The most destructive problem with motors is
that the bearings bind up and lock the rotor, which
often means the motor overheats, smokes badly, and
burns the windings out if not fused.

      The good thing is that 12 volt panels can
be used with either 12 or 24 volt motors if wired
correctly.
      The permanent motor in my cars draws a lot
of amps, at least 10 or more.    This is not near
the equivalence of 3.4 amps on 120 volts, but it
is considerable.  

      The radiator fan motors would be easier to
use for ventilation because they have the right
type of fan blade already installed and may have
the motor support bracket.
      But more than one or two may be needed,
although any ventilation on a hot sunny day
will provide some comfort, lower cooling bills
and help protect the roofing materials from
drying out.

Joe Fischer

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3

Posted by SpiderG on August 11, 2003, 11:48 pm
 Do a Google search for solar attic fans - I got dozens of hits for both
through the roof setups and gable end versions. Some can be outfitted with a
klixon temp switch too (makes sense for winter use).

I personally put a through the roof unit in a few weeks ago. Although a
little pricey, it only took 30 min. on the roof - and no power cords to run.

Good Luck!

SpiderG


Posted by Nick Pine on August 12, 2003, 3:08 am
 

There's also much to be said for low and high vents, with no motors.

Nick


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