[PLUG-TALK] fan speed control..

[Keith Lofstrom]

On Fri, Jul 01, 2005 at 11:38:54AM -0700, Brent Rieck wrote:
> 
>   Does anybody know if I can safely use a standard incandescent light
> dimmer to control the speed of the fan?  I built a little test rig using
> parts from my electrical scrap bin and it works to control the speed of
> the fan, but I have safety concerns - will the switch overheat and catch
> fire, will the fan overheat and catch fire, will either crap out while
> I'm out of town for a week and cause all my machines to melt down, etc,
> etc.

No - I'm not kidding, NO.  

What an incandescent light dimmer does is chops the waveform into the
load - rather than a sine wave, the light sees the front edge of the
waveform chopped out.  Since an incandescent light is just a dumb
resistor (actually, a thermistor, the resistance is lower when it is
cold) it does just fine with this kind of supply.  Of course, this 
kind of dimmer creates all sorts of crap on the power line, and may
interfere with your tube audio gear and your test equipment, and 
violate European power factor rules, but we're 'Murcans, who cares?

However, motors, computers, and fluorescent lights are not dumb
resistors.  They expect to see a sine wave, always, and can misbehave
or croak if they don't get one.  Motors in particular.  Most 
appliance and fan motors are "shaded pole induction motors", and
are designed to turn just a little more slowly than the frequency
of the power line (or a submultiple sometimes).  Coils in the motor
(the stator) set up a spinning magnetic field, and a rotating cylinder
of iron in there (the rotor) tries to keep up.  It doesn't quite -
that is where the shaded pole comes in, it is something that helps
start the motor from a dead stop - but when the motor is going much
slower than the power frequency, it is operating very inefficiently
and gets hotter than in normal operation.  This is called "high slip".   

Inefficient == gets hot, draws excessive current.  This isn't too bad
if you are talking about a startup time of a second or two, but it is
bad news if the motor runs continuously this way.  It draws high peak
currents (blows up the controller) and it gets hot, which dries up the
bearing librication, which increases drag, which makes the motor hotter,
ad infinitum.

We had a fan motor over the stove that got in this condition;  it took
a while to start, because it had to heat up enough to melt the accumulated
gunk.  Someday, it might have caught fire, though stoves and hoods are
designed for that.  Not so good if the fan is not above a stove.

Note that for a given diameter fan, the turbulence noise goes up as
the fourth power (!!!) of the rotation speed, while the air volume
moved goes up as the first power.   So if you make the fan twice the
diameter, you can run it at one-quarter the rotation speed to move the
same air, and you cut down noise (other things equal) by a factor of 64.

So, I suggest you find a motor with a speed control.  Since you have
a lot of room, you might even find one of those "Casablanca" fans,
and trim down the blades to fit your cabinet.   Or just use one of
those box window fans with the three-way controller, though they are
faster and may be a little too noisy on even the lowest setting.  
Don't bypass the built-in controller, of course, it is designed to
work with that motor. 

Or just put the fan in another room, and suck the air through some
wide flexible ducting, for that desirable "Brazil" look.   :-)

Bottom line - there are motors designed for speed control, and speed
controllers designed for ordinary induction motors.  Don't use a
light dimmer, unless you have good insurance and wear fireproof
jammies. :-/

Keith

-- 
Keith Lofstrom          keithl at keithl.com         Voice (503)-520-1993
KLIC --- Keith Lofstrom Integrated Circuits --- "Your Ideas in Silicon"
Design Contracting in Bipolar and CMOS - Analog, Digital, and Scan ICs