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Well, after spending the afternoon farting around with PWM on the Raspberry-Pi, I got the variable fan speed working, however I discovered that the fan motor was overheating my servo driver, and slowing the fan down even a bit really made the heating coils glow red.
Yep, you can't control one without the other. You'd need to ensure that heater output was maintained within an acceptable range, either by cascading air flow and thermal output together or setting thermal output limits to the air reaching the bean mass. The fun is in working it all out...
You could find a way to redirect some of the flow between the heating element and the roast chamber. Could aim to maintain approximately the same total flow (so no change in air temperature).
Well, after spending the afternoon farting around with PWM on the Raspberry-Pi, I got the variable fan speed working, however I discovered that the fan motor was overheating my servo driver, and slowing the fan down even a bit really made the heating coils glow red.
If you wanted to do anything with the fan output, would probably be better to pulse the output from a low of say 50% to a high of 80-90% (only guessing) during the latter stages of the roast and then leave on high between 1st and 2nd-Crack. Experimentation would would confirm what the optimum duty cycle would be. This would allow you better control of moisture retention whilst still allowing for the exhausting of chaff.
Some of the up-market pro roasters (equipment) allow for it to be done this way, so might be worth thinking about...
Mal.
Ooooo... "up-market pro roasters". Sounds like a challenge. I like the idea of 50%+
You could always control the airflow the old fashioned way - with some kind of slide/louvre/butterfly valve. You could even actuate it with a stepper motor controlled by the pi.
If you wanted to do anything with the fan output, would probably be better to pulse the output from a low of say 50% to a high of 80-90% (only guessing) during the latter stages of the roast and then leave on high between 1st and 2nd-Crack. Experimentation would would confirm what the optimum duty cycle would be. This would allow you better control of moisture retention whilst still allowing for the exhausting of chaff.
Some of the up-market pro roasters (equipment) allow for it to be done this way, so might be worth thinking about...
The reason I asked was that my Roast-Pi runs the heat gun fan flat out all the time, and PWM adjusts the heating coils. I was wondering if I would be better off ramping the fan speed as well, so as to "cook" rather than "dry". It sounds as if I am in the right ball-park, so I don't think I'll bother.
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