[Amps] Muffin Fans and hot exhaust

[Steve Katz]

> Steve,
> 
> I have a 120 cfm boxer type fan helping the 60 cfm blower in my Alpha 77.
> The Alpha has several 50 ohm/50 watt resistors in series with the AC to
> reduce the RPM of the blower.
> 
> //Why in the world would they do that?  Sounds like a waste of energy, and
> a way to dissipate additional power as heat, maybe creating the
> self-fulling prophecy that more cooling will always be required...
> 
>   The muffin fan helps "pull" additional air
> into the powersupply.  Muffins will not create backpressure to any extent,
> but will move CFM, more than the blower requires.
> 
> //The fan isn't creating the backpressure; the differential in inlet vs.
> outlet aperture, as interrupted by components in the way and turbulences
> created by the enclosure design is what creates the backpressure.  The fan
> creates pressure, to overcome that backpressure.
> 
>   The backpressue is
> created by the rotating squirrel cage blower, against a restriction (like
> the tube and tube socket)..  What Alpha did in the 77 was to have a heat
> sensor in the outlet airflow that would short out the series resistors
> feeding the blower..  The extra muffin fan just helps to keep the total
> CFM
> moving, not create backpressure.
> 
> //As I recall, the 77 had a squirrel cage blower for the tube and a muffin
> fan for the power supply.  It wasn't rocket science.  I suppose they used
> resistors and switches, etc, in lieu of simply using variable speed DC
> fan(s) because maybe at the time the variable speed DC fans weren't
> available or cost too much (or something).  Nowadays, variable speed DC
> fans with tachometry cost about $7 each brand new, for high-quality
> brushless designs with ball bearings and 30,000 hour MTBF.  Possibly when
> the 77 was designed, those didn't exist yet, or at least not at that
> price.
> 
> -WB2WIK/6
> 
> 
> Thanks and 73's,
> Dudley
> WA5QPZ
> jhurry@austin.rr.com
> 
> 
> 
> 
> >
> >
> >
> > > I buy those 5 inch 240 VAC muffin fans at Dayton and
> > > just wire them into the 120 VAC wall outlet.  They run
> > > at half the speed and are extremely quiet.  I use one on
> > > top of my 2 meter mobile rig in the shack, which likes
> > > to run hot, and used to have one on top of my amp to
> > > help exhaust the hot air.  Phil  KB9CRY
> > >
> > [Steve Katz]  I realize that this works to some extent; however, I
> > have been unsuccessful in quantifying actual results because the AC
> motors
> > run on reduced voltage have a great reduction in torque and thus cannot
> spin
> > well against back pressure.  When I run a standard 240V 125cm (4.25")
> > "Boxer" type fan on 120V, I can stall it with the tip of my finger very
> > easily, no damage done to finger.  Same fan run on 240V cannot be
> stalled
> in
> > this manner.  The obvious decrease in torque is that -- very obvious.
> As
> > such, when I run the "reduced voltage" fan against the back pressure of
> a
> > tightly integrated enclosure, it develops almost no air flow, as
> compared
> > with the fan run on its intended voltage.  So, I guess my question is:
> Has
> > anyone qualified rpm and torque vs. airflow w/backpressure, say 0.2" or
> 0.3"
> > (what might be typically found in an amateur amp using linear flow
> cooling
> > above chassis)?  Thanks!  -WB2WIK/6
> >
> >
> >
> > > > At 08:23 AM 7/22/02 -0700, Steve Katz wrote:
> > > > >  //What "reduced voltage" are you talking about?  I'd like to hear
> > > > >more about operating AC fans on reduced voltage, I was never
> successful
> > > > >doing this.  (I do operate DC brushless fans on "reduced voltage"
> to
> > > reduce
> > > > >rpm and noise, and this works, with reduced airflow, of course.)
> > > >
> > > > I just wired them in series.  Definitely slows them down and quiets
> > > them.
> > > >
> > > >
> > > > 73, Pete N4ZR
> > > >
> > > > Check out the World HF
> > > > Contest Station Database at
> > > > www.pvrc.org
> > > >
> > > >
> > > >
> > > >
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