pulse duration fan modulation?

[pybyr]

Just got to thinking as I was putting some wood in my EBW-150 (which has the whole big old farmhouse very cozy on two cloth shopping bags worth of hard maple- I get so much more heat per unit wood than ever before (pre-gasifier)--

for combustion control, instead of trying to vary the speed of an AC motor, which is kind of klutzy and imprecise (other than maybe inverter control, which is complex) why not do a pulse duration control-

fan runs full time until furnace nears needed temperature-

-then controls begin to do on-off cycling of the fan, with the duration of the ons and offs varied as the temperature climbs, until (if the boiler is exceeding max desired temp) the fan shuts off, perhaps with only a brief "on" every so often to keep the wood/coals from going out.

Such a system could have some sort of "fuzzy logic" that might learn/ anticipate/ respond to operation over time, or factoring into account outside temperature & actual resulting likely BTU load.

Wondering why this hasn't been tried yet (or maybe it has and I just am not aware)?

 

[sgschwend]

How about a variable pitch fan? Or like in a automobile a clutch fan either thermal or electromagnetic.

I always thought that fuzzy logic was invented by folks that weren't sure of what they are doing.

 

[DaveBP]

I think that is the method of variable speed control used by Tekmar controls like the 157 and some of their more sophisticated controls. Other makes,too, I'm sure.

Pulse Width Modulation is the acronym I remember as PWM. I think that it does not work efficiently or at all on different types of motors. The permanent split capacitor type motors used in small circulators and fans are typically controlled this way. Power factors go down (less efficient) as you throttle the motor slower but it's a cheap way to vary the speed without damaging it.

Some of the contols out there are quite clever but they are pretty pricey.

 

[ewdudley]

...

for combustion control, instead of trying to vary the speed of an AC motor, which is kind of klutzy and imprecise (other than maybe inverter control, which is complex) why not do a pulse duration control-

fan runs full time until furnace nears needed temperature-

-then controls begin to do on-off cycling of the fan, with the duration of the ons and offs varied as the temperature climbs, until (if the boiler is exceeding max desired temp) the fan shuts off, perhaps with only a brief "on" every so often to keep the wood/coals from going out.

Such a system could have some sort of "fuzzy logic" that might learn/ anticipate/ respond to operation over time, or factoring into account outside temperature & actual resulting likely BTU load.

...

A.K.A "time proportioning control" or "time proportion control".

It would be interesting to see 'how low can you go' and still keep the secondary combustion chamber hot enough to fire-up cleanly.

I'd be afraid that the typical combustion fan motor might not start efficiently at all, or it might not hold up under the abuse of running, for example, 20 seconds out of a 30 second cycle. I do know of three-phase condenser fan motors that can do this successfully for years on end without having been specifically designed for it.

Check out the 'T7100' 'TET1700' 'TET 7100' JLD7100 relay controller. It should do exactly what you want for less than 4.0 kilocents plus a thermocouple.

http://www.google.com/url?sa=t&source=web&ct=res&cd=1&ved=0CAkQFjAA&url=http://users.rcn.com/erics/PID/PID_Manual_1.doc&rct=j&q='tet7100"+|+"tet+1700"&ei=DgIiS7GkHdWelAfbybT7CQ&usg=AFQjCNGZ0QsIP_INqLKUTBOKTH_Zs9egLQ

--ewd

 

[in hot water]

A PWM pulse width modulation control should do this. I have a prototype control on my EKO this year. It has two outputs that can be configured as variable speed control. Right now one runs my boiler to tank pump, the second shuffles to my radiant/ DHW tank. Both operate on a differential function but one can be configured as a thermostat function.

The control includes a data logger and SD card to transfer data. The control can be configured on a laptop then the card transfers the program. I will post some data this weekend.

Here is how my system is piped, and a picture of the control. I can send a pdf of the control manual to anyone that is interested.

hr

 

[Nofossil]

I suppose that classic PWM and your proposed 'duty cycle modulation' are really the same thing on different time scales. PWM varies the effective power delivered to the load by changing the percentage of time that power is applied - generally over time periods measured in milliseconds. If I understand you correctly, you're thinking about actually letting the fan stop for some percentage of the time.

I suspect that you will lose some combustion efficiency doing it that way as you'll lose secondary combustion or at least lose the carefully tuned primary / secondary ratio during the time the fan is off.

That having been said, I'm actually implementing exactly that approach for circulator control. Most circs don't respond predictably at low speed settings, so duty cycle appears to be a better choice for flow rates below about 30% of rated output.

 

[pybyr]

I suppose that classic PWM and your proposed 'duty cycle modulation' are really the same thing on different time scales. PWM varies the effective power delivered to the load by changing the percentage of time that power is applied - generally over time periods measured in milliseconds. If I understand you correctly, you're thinking about actually letting the fan stop for some percentage of the time.

I suspect that you will lose some combustion efficiency doing it that way as you'll lose secondary combustion or at least lose the carefully tuned primary / secondary ratio during the time the fan is off.

That having been said, I'm actually implementing exactly that approach for circulator control. Most circs don't respond predictably at low speed settings, so duty cycle appears to be a better choice for flow rates below about 30% of rated output.

Yes, I was definitely thinking of full pauses in the fan, of varying on to off ratios (varying duty cycle rather than true PWM with constant power feed), rather than trying to do speed control, as it seems as if you could actually achieve better control with ratio of on/off than with efforts to vary speed at low speed.