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  1. #1
    W_Higgins's Avatar
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    Default 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.

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    DaveKamp is offline Titanium
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    Thumbs up Nope...

    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!

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    Quote Originally Posted by DaveKamp View Post
    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!
    LOL, I'm sorry, but this is just too funny! Most of the time we have to explain over and over to people that a 3 phase VFD can be made to work on 1 phase power, this is exactly the opposite!

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    Shawn Ghormley is offline Stainless
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    Default

    Are you going to keep the vfd?

  5. #5
    W_Higgins's Avatar
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    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?

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    Jraef's Avatar
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    Quote Originally Posted by W_Higgins View Post
    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?
    No. A VFD uses the incoming AC power as a raw material only. It converts it to DC so it really doesn't care if the incoming power is single phase or 3 phase, it ends up DC anyway. Then it converts (inverts) it again back to 3 phase AC for the motor. There is no direct connection of the input to the output AC. So you will have 2 wires going in, 3 going out to the motor. The only thing you have to remember is that the single phase power going in will draw more current than the motor nameplate says by a factor of the sq. root of 3 (1.732). So for example if your fan motor says 10.0 amps FLC on the nameplate (it won't, I'm just keeping the math simple), the current draw by the VFD at full load on the motor will be 10.0A x 1.732 or 17.32A and THAT then is what you would base your fusing on ahead of the VFD.

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    rons is offline Stainless
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    Quote Originally Posted by W_Higgins View Post
    My understanding is that VFD's are only for single phase-in.
    Most of the surplus VFD's I see use 3 phase input. They are larger in size than new ones
    but are built solid with nice heat sinks. $95 - $125 for like a 1hp unit. You don't anything
    more than a VFD with Volt/frequency control for just controlling a exhaust fan.

  8. #8
    W_Higgins's Avatar
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    Quote Originally Posted by Jraef View Post
    No. A VFD uses the incoming AC power as a raw material only. It converts it to DC so it really doesn't care if the incoming power is single phase or 3 phase, it ends up DC anyway. Then it converts (inverts) it again back to 3 phase AC for the motor. There is no direct connection of the input to the output AC. So you will have 2 wires going in, 3 going out to the motor. The only thing you have to remember is that the single phase power going in will draw more current than the motor nameplate says by a factor of the sq. root of 3 (1.732). So for example if your fan motor says 10.0 amps FLC on the nameplate (it won't, I'm just keeping the math simple), the current draw by the VFD at full load on the motor will be 10.0A x 1.732 or 17.32A and THAT then is what you would base your fusing on ahead of the VFD.
    Quote Originally Posted by rons
    Most of the surplus VFD's I see use 3 phase input. They are larger in size than new ones
    but are built solid with nice heat sinks. $95 - $125 for like a 1hp unit. You don't anything
    more than a VFD with Volt/frequency control for just controlling a exhaust fan.

    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?

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    mike_kilroy is offline Stainless
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    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.

  10. #10
    W_Higgins's Avatar
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    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.

  11. #11
    Jraef's Avatar
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    Quote Originally Posted by W_Higgins View Post
    ... 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?
    You pay for POWER, not current. Current is a component of power, but power is power is power, you cannot alter that fact. So you have a 10A 230V 3 phase motor, that must be 3HP. 3HP = 2.24kW of POWER. Regardless of how you get there, you are going to pay for that 2.24kW of power, it could be 17A at 230V single phase, 10A at 230V 3 phase or 170A at 24V single phase, it all comes out the same (I'm leaving out a lot of math to keep it simple). Feeding it with single phase or 3 phase will make absolutely no difference in your POWER consumption.

    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.

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