[Stoves] Variable Speed Stove Fan SIMPLIFIED

Kevin Chisholm kchisholm at ca.inter.net
Mon Dec 4 11:59:20 CST 2006 

Dear Tom

Even better than a paper bag is a garbage bag, and better still is the very 
thin polyethylene cover that Dry Cleaners use to cover their finished work.

The volume of these bags can be measured easily by filling the bag to the 
shape it takes on when filled with air, using the light "foamed plastic 
popcorn" that is used for packing goods for shipment. Then dump the foamed 
plastic popcorn into a rectangular box and measure it, to get an accurate 
indication of volume.

Best wishes,

Kevin


----- Original Message ----- 
From: "Thomas Reed" <tombreed at comcast.net>
To: "Discussion of biomass cooking stoves" <stoves at listserv.repp.org>
Sent: Monday, December 04, 2006 1:36 PM
Subject: Re: [Stoves] Variable Speed Stove Fan SIMPLIFIED


> Dear All:
>
> While it is nice to turn a know to adjust the fan or blower speed, the
> simplest and most direct way is a piece of paper (etc.) over the air
> inlet.
>
> While we're at it, you can measure the output of the blower/fan
> measuring the inflation time of a very light weight newspaper bag of
> know volume.  (I learned that from Fred Hottenroth, originator of the
> Sierra Stove, about 1990.)
>
> TOM REED     The Simplifier
>
>
>
> Paul S. Anderson wrote:
>>>>> 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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>>>>>
>>>>>
>>>>>
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>>>>>
>>>>>
>>>>>
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