Matching CFM for scrubbers and fans?

[qbert]

1. If I understand how these fan speed controllers work (I'm a fair tinkerer with DC, less knowledgeable about AC) - then I think this is fine.

I'll put out my explanation - which may in fact be quite wrong (hopefully someone more knowledgeable can clarify):

Without going over too much background (google or wikipedia if interested), AC voltage is a sine wave. It oscillates smoothly between full voltage one direction to the other.

The solid state speed controllers use the variable resistor (knob) to create a threshold voltage below which a device called an SCR will not turn on and not apply power to the circuit. Instead of limiting the maximum voltage, they keep the circuit off until the voltage reaches a particular threshold (say 20v instead of your fan getting juice the instant we pass 0v).

So instead of sweeping from 0-110v, powering the fan the whole time, you get a break of total offness followed by a sweep from 20v (or whatever, based on knob position) to 110v. This is conceptually sort of similar to pulse width modulation as a DC voltage fan speed control.

I believe this method of speed control is what limits these to single-phase AC motors.


So, picture your existing control limiting on time from 20v-110v. With the temp controller at 100% full power goes to the existing control and you then still only get 20v-110v. As the temp controller reduces from 100%, nothing really changes until it starts chopping things higher than 20v.

I can't claim to have a real solid grasp of how the variable resistor/SCR circuit works, but I think that they are sort of like a zener diode and its absolute voltage that matters, so the fact that the temp controller is already chopping the output would not matter to your existing control - it will still work exactly the same as if it was seeing a full signal (that is, the speed drops are not additive).

However, you almost certainly would NOT want the simple contoller ahead of the temp controller, since the temp controller is stepping down and rectifying AC to DC to run the "smarts" circuitry. You will want a normal full AC sine wave provided to that device.


I wouldn't mind testing this if I could think of a cheaper end device (not a $100+ S&P fan), since the only devices at risk would be the simple controller ($20) and whatever's at the end - maybe a cheap house fan or something? Not sure that an incandescent bulb would work - suppose an oscilloscope would show it clearly, but I unfortunately don't have one these days.


2. I'll bet the speed controller buffers itself a bit so its not constantly adjusting. It would be pretty poor design not to.

 

[qbert]

A little more googling seems to suggest frequency-based speed control (varying the 60Hz) could be used, too, so perhaps the more advanced temp controller uses that (hard to guess), in which case having the speedster or whatever after it should still work just fine (more confident about that), since there's no way chopping the start of the waveform for speed control could be interfered with by a frequency-based speed control earlier on the line.


But again, I'm FAAAR from an expert on this. Just a little background knowledge and a willingness to extrapolate.

 

[Batboy]

Thanks for great detail, even if most of it was beyond me. I too hope that some other knowledgeable people jump in. I do have a question about this statement:

However, you almost certainly would NOT want the simple contoller ahead of the temp controller, since the temp controller is stepping down and rectifying AC to DC to run the "smarts" circuitry. You will want a normal full AC sine wave provided to that device.

Because of this belief, wouldn't my suggestion not work? I guess it depends on how you are using the term "ahead". Right now I've got an S&P fan wired with a cord, but the 'hot' lead of the cord is run through an S&P "simple" speed controller. If I left this untouched, but simply plugged the cord into the new controller instead of an outlet, would you consider the "simple" controller ahead of the new controller? I don't see how this could be rearranged.

 

[qbert]

That would in fact put the temp controller ahead (in my terms) of the simple controller, which I am thinking may very well work.

Outlet -> temp controller -> simple rotary controller -> fan.


What I think we definitely want to avoid is having the simple controller between the outlet and the temp controller (because then the simple controller would affect the voltage that the temp controller sees).

 

[qbert]

Here's one of the better speed controller explanations I've found, but there's a lot of various pieces of info out there, and its not really clear if this is actually what we're working with when it comes to these particular fans & controllers:

http://en.allexperts.com/q/Electronics-3923/2008/12/Regulating-AC-motors-1.htm

 

[ScrubNinja]

I'm too high to understand any of that but have you guys seen the nano plc thread?

Basically, there is so much demand in industry for high levels of functionality in a small package that, in this case we can benefit from it. With the proper Input and Output components, we can make one of these function as A: CO2 Monitor/Controller, Lighting Controller/Flip-Flop (Bring room on in stages to avoid large current spikes etc), High Temperature Cutoff, Ventilation/Fan Controller, Nutrient PH/EC Monitor/Controller ETC.

We could even combine several of these functions into ONE package IE: A Self Contained Growroom Controller All you need is the proper model relay for what you are looking to accomplish and the proper Input and Output components to perform the operations.

Maybe that's what you were talking about already. Peace.

 

[qbert]

I'm too high to understand any of that but have you guys seen the nano plc thread?



Maybe that's what you were talking about already. Peace.

oooohhhhh.... plc's. I'm a computer programmer by trade, so that appeals to my geek side (and the fact that i saw "plc" and already knew what it stood for before clicking through to the thread, lol).