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06-13-2011, 01:07 PM #1
3-phase speed control for fan motor
I would like to add a speed control to the fan for my recently installed paint booth. Awhile back I purchased a VFD just to have handy for a situation like this, but, as it turns out, a few months ago I moved my shop to a facility that already has real 3-phase in place.
My understanding is that VFD's are only for single phase-in. What type of speed control should I look for? I'd like to be able to accomplish two things -- having the ability to manipulate air flow and, more importantly, being able to create a "soft start" to keep my peak usage to a minimum.
06-13-2011, 01:11 PM #2
VFDs are most often, for threep-phase in, and generate three phase out.
SOME VFDs are set up strictly for single-phase in, and in that circumstance, just hook two legs of your three-phase supply to it. It'll work fine.
Your current VFD should be able to do absolutely everything you want, without looking for some other product. Put it to work!
06-14-2011, 06:57 AM #3
06-15-2011, 10:02 AM #4
Are you going to keep the vfd?
06-15-2011, 11:52 AM #5
I only see connections for two lines in and three out. If that's alright to do so, than I'll keep it. Never would have guessed on my own that it could be hooked up that way. I'll consult the manual and see about how to proceed, if it covers that scenario.
So in that case, I'll only have two going in and two coming out?
06-15-2011, 03:05 PM #6
06-16-2011, 06:12 AM #7
06-16-2011, 10:08 AM #8Originally Posted by rons
I'm going to address both of these comments in one post because I think they go hand-in-hand. My comprehension on this subject is rather poor, so I apologize for any frustration I might cause.
Jaref, I think I understand your point. Taking what Dave said in post no. 2, what he's saying it to just leave the 3rd leg of my 3-phase circuit in limbo while I run my current VFD as single phase in? So, your point that the current draw has to be multiplied by 1.732 kind of makes my goal of reducing consumption by dialing down the fan a counterproductive effort, correct? (By the way, it does happen to be a 10a motor on the fan).
So, where rons recommends a surplus commercial 3ph. in / 3 ph. out VFD, will I get around the consumption issue? I'm a business, not a hobbyist, and my shop has 3 ph. service in place already. I'm not trying to generate it, I'm just trying to manipulate my fan. Is obtaining one of these units the logical way to go for my application?
06-16-2011, 10:30 AM #9
sounds like u got it now.
u shud post the motor nameplate data and the vfd nameplate data including mfgr and all ratings listed.
this way folks can verify it is a good match, as well as help answer the questions which sound like will follow soon after you hook it up.
06-16-2011, 12:06 PM #10
That's a good suggestion. The next time I climb up there I'll snap a shot of the data plate on the motor. I'd like to find one of those surplus 3 ph. units mentioned earlier along with some guidance on what to look for. This isn't a high priority since the booth is operable as-is, but I would like to keep chipping away at the details while it's fresh on my mind.
06-16-2011, 12:41 PM #11
A VFD only saves energy with a fan in one way. IF you can take advantage of a reduction in flow of your air delivery, AND you were doing it with a VAV box (Variable Air Volume), such as dampers, THEN if you remove the dampers and use the VFD to slow the motor down, it will use less energy because you have removed the losses in the turbulence created by the dampers. That's it, despite all the hogwash people spew about saving incredible amounts of energy with VFDs, it ONLY saves in that specific situation. Now mind you if you were using dampers and you run a lot, it does add up to a lot of energy. Usually a VFD in that situation can pay for itself in 18 months or so. But if you run the fan at a constant flow all the time, the VFD is going to WASTE more energy compared to just fixing any inefficiency in your system.
That inefficiency might be the result of an engineer being sloppy and saying "I could do it with 2HP but a 3HP motor will mean I don't get called back, so put in a 3HP". That happens a lot. In that case you could put in a VFD and dial the flow back to the equivalent of 2HP, saving you energy. But you would save MORE energy by just putting in a 2HP fan.
I know it's probably not what some VFD salesman told you, but it's the truth. I used to sell VFDs, I can attest to the fact that most salesmen have no clue about these issues, they are drinking the kool-aid and they pass it on.