[Stoves] Variable Speed Stove Fan

[Tom Miles]

Paul,

Thanks. 

Do you remember where you found the kits in Canada?

What fans have you been using?

Tom


-----Original Message-----
From: Paul S. Anderson [mailto:psanders at ilstu.edu] 
Sent: Friday, December 01, 2006 2:05 AM
To: Discussion of biomass cooking stoves; Tom Miles
Cc: 'Discussion of biomass cooking stoves'
Subject: Re: [Stoves] Variable Speed Stove Fan

>>> Subject: Re: [Stoves] Variable Speed Stove Fan
>>> On Nov 29, 2006, at 12:46 PM, Tom Miles wrote:
>>>
>>>> Kyle's comment reminded me of a conversation between Charlie Sellers
>>>> and Paul Anderson at Stoves Camp this summer. Charlie was describing
>>>> an electronic  "chopper circuit" to adjust the speed of a DC fan. Has
>>>> anyone built one? Can anyone provide the circuit, list of components
>>>> (and
>>>> sources)
>>>> or a step by step how-to (with pictures) for building one? Or is there
>>>> an internet source?
>>>>

Solution:

Charlie told me about "Pulse Width Modulator" (PWM) technology.  I
eventually
found a correct-size small unit from a distributor in Canada.  I bought 2 of
the kits (unassembled for about US$8 each, I think) and paid for him to
assemble them ($10 to do both), plus shipping, became almost $40.

But well worth it!!!!!   I turn the knob-stem and the 12-volt blower 
that I have
goes from stand-still to full speed.  But it does have a modest humming
noise
all the time that the current is connected.

I learned that the kits are from Thailand, so now that I am in Cambodia I
will
be trying to get assembled units.

Sorry I do not have the purchase details right now, but I am just finishing
a
3-day holiday at Angkor ruins.  Next week  I get started on stove
implimentation projects here with GERES (French) and CFSP (Cambodian) 
agencies.

The explanation by William Carr (below) sounds just right.  Google Pulse
width
modulator (or see Wikipedia?) for diagrams of what William has described.  I
have not heard it called "type D", so I cannot comment about that name
except
that "pmw" is part of the Internet link that William has provided.

Wish you were here!!!!   :-))

Paul

>>> This sounds like a "type D" amplifier.
>>>
>>> It came up in High School, when I was interested in controlling the
speed
>>> of
>>> small DC motors.
>>>
>>>
>>> (Turns out it's also the only way to 'dim' fluorescent lights.
>>>
>>> You see, a fluorescent tube won't fire without full voltage.   Lower
>>> the voltage, no light at all.)
>>>
>>>
>>>
>>> You can control the speed of a DC motor fairly well by dialing down the
>>> voltage, that trick works somewhat with an AC motor, but it won't work
at
>>> all with a fluorescent tube.
>>>
>>>
>>>
>>> So instead of turning down the voltage, you want to dial down the
wattage
>>> instead without affecting the voltage.
>>>
>>>
>>>
>>> The only way to do that is with a type D amplifier, by converting the
>>> input
>>> signal into a squarewave (AC) or a series of pulses (DC).
>>>
>>>
>>> If you look at the sinewave of standard 60 cycle AC,  you see a trace
>>> going
>>> up, and down, up, and down.
>>>
>>> The circuit is getting peak power at the tops of the wave, of course,
but
>>> it's still getting SOME power on the way up and on the way down.
>>>
>>> At any given point you can calculate the amount of power by a process
>>> called
>>> "Root-Mean-Square", a term that still triggers my math aversion 24 years
>>> later....
>>>
>>> By generating a squarewave output, you can provide the full voltage at
the
>>> correct frequency.
>>>
>>>
>>> But if you cleverly reduce the width of the squarewave, you will be
still
>>> supplying the correct voltage, at the correct frequency, but you'll
starve
>>> the circuit on wattage.
>>>
>>> Thus, the motor slows down in a nice linear fashion as you decrease the
>>> squarewave width, and a fluorescent tube dims gradually.
>>>
>>> The method works with DC also, just by making the squarewave a series of
>>> DC
>>> pulses.
>>>
>>>
>>> Here's a link to controlling your DC motor:
>>>
>>> http://www.solorb.com/elect/pwm/index.html
>>>
>>> William Carr
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
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-- 
Paul S. Anderson, Ph.D., Geography professor - Emeritus
Telephone:  USA-309-452-7072 (residence and office)
Internet site:  www.ilstu.edu/~psanders
For my gasifier stoves info, go to:
http://bioenergylists.org/contributors#Paul_Anderson




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