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Re: fan wiring

On Sun, 14 Jan 2001 16:22:12 EST, K9AUB at aol_com wrote:

< K9AUB is quoting Rod Hanks >

> >  cheapest light dimmer I could find at Home Depot and hooked
> >  it up to one of the four inch RadioShack AC cooling fans (they only
> >  sell one model that fits this description).  It did work.  However,
> >  I found that I could only control the fan in a narrow range of
> >  speeds, and that there was some sort of noise from the fan when
> >  I slowed it very much at all.  The net effect was that it did not
> >  make it very quiet.

>  I suspect if you switch to DC motor fans (such as are used in your 
> computer power supply), you'll get better speed control and quiet-running 
> fans.  I believe Radio Shack sells little 12v DC power supplies with variable 
> voltage, although they many not be the cheapest things you've ever bought.  
> Expect to pay $40-100 for a small variable voltage DC supply.  However, you 
> can also expect very quiet and controllable fan operation as well.

I'd just like to add my own 2 cents worth:

Speed control for motors is a little like PMDD in that there
are some simple ways to get good results and there are other 
ways that seem to require a graduate degree to get anywhere.
To get an idea of the complexity involved, try typing "motor
speed control inductive load" into your favorite search engine.

Rod - I don't know anything about the dimmer you bought, but
you'll want to use one which is rated for non-linear or inductive
loads. For your purpose, avoid dimmers rated for incandescent
or linear loads only.  You'll want the kind of dimmer which is 
sold for ceiling fans or range hood fans.  Even with an appropriate
dimmer, there is still no guarantee that your fan is going to like
being dimmed.  You could read the technical info on the fan or 
call the fan's manufacturer to see if it is compatible with triac-
based speed control.  But even with a dimmer that is rated for 
inductive loads and a triac-dimmable fan, if you reduce
the speed of your AC motor to less than about 1/3 of its undimmed 
speed, you're probably going to get humming anyway.  When 
dimmed, some motors hum much louder than others.  There are 
hum-free ways to reduce the speed of an AC motor, but
AFAIK, they are going to be prohibitively expensive or 
time-consuming for your project.

A quiet DC fan (like those available for computers) might be
cheaper than buying a new AC dimmer, and might be quiet enough
to let run as is.  Plus, if you later decide to reduce the speed of a
DC fan, you're going to have more options than with an AC fan.
You could use a potentiometer.  You could buy an infinitely
variable DC voltage supply like K9AUB mentions.  But you might
get along just as well with a wall-wart "transformer" that lets you
pick voltages between 1.5 and 12 volts DC at 1.5 volt intervals.
You could get one of these for less than $15, maybe less than $10.  

If you do decide to go with a DC fan, don't even think about 
installing a dimmer before the transformer.  Transformers
are (or at least can be) inductive loads just like motors, so you
really have to do your homework before attempting to "dim" one.


Disclaimer:  Although I have successfully assembled speed controls for
both AC and DC motors, I'm just a tinkerer and have no formal electrical
training.  With 120 VAC especially, proceed at your own risk.