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Thread: air compressor VFD control

  1. #1
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    air compressor VFD control

    So I am moving to a shop without 3 phase and the VFD I bought works, but it must soft start for 2 full seconds, in the meantime it must not have any back pressure (any shorter and the VFD will overload). The problem is unless the tank pressure is under 60 psi, the compressor will not fully start and will just linger at a slow speed which I am sure is bad for the motor.

    What I found work is remove the unloader valve completely, and this would allow the compressor to spin up, and then close the valve to allow the tank to fill up.

    What I need is some kind of relay controlled air switch that will basically bleed air until the motor spins up fully, then close it to allow the tank to fill up.

    What am I looking for? Do such a beast exist, and can be triggered with the VFD using some kind of a control output?

    Or I guess another thing I can do is to hook the hose from the unloader valve to a smallish (1 gallon) tank that would come up to pressure slowly enough for the motor to spin to full speed, then the unloader valve will bleed the tank dry once the pressure switch shuts off.
    Last edited by Tai Fu; 09-15-2021 at 12:29 AM.
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  2. #2
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    cheeper than a vfd/ and any further work may have been a 220 motor and starter.

  3. #3
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    I think the problem I am running into is that the VFD won't ever provide enough juice to get a motor started under load (as it seems for an air compressor the load gets higher and higher until a certain RPM then the load goes down). I guess a motor must be drawing like 40 amps for the first second or so to start up. But once it's at speed it can pump as normal like it's on a 3 phase supply... Or I don't know how to allow the VFD to ignore protection for just a second or two to allow enough juice to get in there...

    Problem is single phase motor costs much more than the VFD.
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  4. #4
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    Was the VFD sized properly for this horsepower?

    I dont know how invested you are but I normally pay about 450-500 for a VFD startup. They run through all settings putting them where they need to be. If you search for Greenheck fan or B&G pump dealers they often have service techs that do VFDs only. If its a standard brand VFD they are guarenteed to have answers. You could also pose the question to these guys over the phone and probably get some answers for free.

    Im not an expert, but I buy a bunch for work for pumps and fans. And usually the only critical item is input power and HP

  5. #5
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    Yes it is, it says it's "3KW" but really it has an output amp of 14 amps, which matches my motor's full load amp. But I can't start the motor hard on that VFD at all without it overloading, it will only do it soft start, but the problem is it does not want to soft start under load, and it looks like for an air compressor the load gets higher and higher the faster you spin, however for the motor it only develops full HP after about 50 hz or so, and so soft start must be done without any load, thus it is necessary for me to auto bleed the air until the motor is at its rated frequency/speed.

    The motor is 5HP, or 4kw. Once the motor is spinning at its full speed it pumps just fine but it just can't start up.
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  6. #6
    I have just skimmed the above, and so will not try to address the load and capacity issue(s). All I can suggest is that you carefully consider the best way to deliver proper power to each of the components.

    As for control, most VFDs have relay outputs that could provide a discrete signal to a solenoid valve. I don't see any specifics for make or model of VFD that your currently use, but I would assume you could energize the VFD's relay once the RPM approaches full speed. You will need to carefully read the manual to determine if this logic can be configured and which parameters are involved.

    Presumably, the relay could be wired to a solenoid valve replacing the OEM unloader.

  7. #7
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    I asked the seller... he said I need vector control type VFD for compressors... The one I got wasn't that type...

    I'm returning them because someone is selling me a rotary phase converter with a 10HP motor in it cheap. I think that will do.
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  8. #8
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    For most uses 3 seconds is the minimum. recommended startup acceleration time. For you I would just set it to 6-10 seconds and see if that works. Set the deceleration to freewheel down to off.
    Bill D.

  9. #9
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    Tried that, it didn't work. I played around with various startup times from 1.0 seconds all the way to 15 seconds, and if I don't bleed the output so the motor starts under no load, it will either start, then .2 seconds later cut off with an error, or it will start but refuse to get up to full speed (running at 20hz at MOST and can't go faster than that). So either I must try to run two or three VFD in tandem and see if this works, or return this thing and try the 10HP RPC that a neighbor is selling me cheap.
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  10. #10
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    Update: Got the RPC moved into the new shop. Thing is ****ing heavy, weights at least as much as the compressor itself, if not more heavy. If this won't drive the compressor not sure what else will.
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  11. #11
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    I'm a little bit confused but that's not unusual for me. As far as I know the function of the unloader is to discharge any pressurized air between the compressor and storage tank. That make sure that there is no pressure for the piston to start against. I had an unloader fail and the motor would just hum, would not turn at all. I don't think this is your case but many consumer compressors have the unloader as part of the pressure switch. I replaced the pressure switch with a Square D without the unloader and installed a separate unloader in the line between the compressor and tank. It has worked fine for years.
    Last edited by Curt Harms; 09-16-2021 at 11:31 AM.

  12. #12
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    The thing is, because with a VFD (especially one that is a bit undersized) the VFD can't supply the necessary current during starting since the cylinder pushes back quite hard with a few rotation. It works with 3 phase line voltage as the motor can have a large inrush current that can overcome this, but the VFD can't.

    Why is it that air compressors don't just use a solenoid to unload the cylinders during motor start to make the motor start without any load?
    Last edited by Tai Fu; 09-16-2021 at 4:40 PM.
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  13. #13
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    Actually I do have a question...

    So let's say you got a rotary phase converter running. But you want to start something that may have a higher load. I heard of people using multiple idler motors with a timer relay to turn one on, then the other, and then both motors spinning at the same time would basically share the load between them. So let's say I got a 5HP idler motor going, and then I got a 5HP lathe too, but wants to start a 10HP motor. Could I just turn on the RPC, lathe, then the 10HP motor? Essentially the lathe would be just another idler motor... It's all hypothetical though as I don't really have a lathe... and the RPC I just bought is most likely a 10HP one (it's heavy as hell... couldn't get it onto a truck without mechanical assistance and that's with 2 people lifting it)
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  14. #14
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    The extra idler motor is best if it has no extra moving parts to make noise or wear so a dust collector or a big disk sander is often the choice.
    A big compressor will have unloader valves so once pressure is reached the pump is doing no real work. It continues to rotate and intake air but it just exhausts it by the unloader at zero pressure. These are often powered by oil pressure so they do not close until the pump is up to rpm and making oil pressure to close the valves.
    This method is also used on smaller compressors running for gasoline engines.
    They can be identified by small cans sitting on top of each cylinder, often with small tubing going into them for pressure sensing.
    Bill D

  15. #15
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    So essentially the more equipment you got hooked up to your RPC the more powerful it becomes? Essentially every spinning motor becomes its own 3 phase generator?
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