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Help me understand VFD's

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  • Help me understand VFD's

    Ok, well I mostly understand them, but I still have a couple questions.

    Actually, I'm trying to do something stupid, and I need some advice.

    I've got an old 25hp Allen Bradley drive (1333-GAA Series B), and I want to hook it up to a 3hp motor.. Mostly because I have this drive out in the garage.

    There's more. I want to run it on single-phase input.

    Normally this isn't a problem, you just have to de-rate the drive to about half of what it would normally do. That part makes sense to me, since you want to maintain the max input current but only have 2 hot wires.

    I checked out the manual (Warning: 4MB pdf), which says I need a 125A breaker. That's more than I've got going to the whole shop.

    Am I correct in thinking that it'll never draw 54A, because I'm never pulling 31A out of it? I know that motors only draw a fraction of their nameplate amps with no load.. do vfd's work the same way?

    Is there a way to figure out how many amps it'll draw if I limit the output to 10A?

    And my last question.. I could be wrong, but it seems like this thing would run cooler and last longer if I use it to drive a small motor. Since I'm only running 8 or 10A through circuits designed for 60, the drive should be happy. Does that make sense?

  • #2
    The problem I see here is that you have a drive that is WAY too big for your application, Chances are that it will not track as low as you want, if it does, I have concerns as to the stability of it.

    My personal opinion is that you will end up "frying" what ever is hooked up to it and likley the drive itself.

    Not trying to scare you, but this is just my opinion.

    REgards and good luck,

    Comment


    • #3
      If you'd like I can pose your ? to one of our EE's at work.

      regards,
      tgomez

      Comment


      • #4
        Originally posted by billie_bob View Post
        If you'd like I can pose your ? to one of our EE's at work.

        regards,
        tgomez
        That would be great. Thank you.

        Comment


        • #5
          I'll won't be back to work until Monday; I will pose them your ? at that time.

          Comment


          • #6
            I will try to regurgitate what our EE told me about your question. So if some of this is obvious to you; I ask to please be patent with my lack of understanding the topic. First, he told me it appears this is an older VFD; which is specifically sized for a 25? HP motor (which i suspect you already knew); therefore, he said it is likely it will over amp your 3 hp motor. I realize you had stated that your maximum current draw would still be below the maximum amps your motor can draw at 140% of rating. However, he did state that there was still a risk that your motor would not handle the sudden surge of current supplied by the VFD. He came to this conclusion since most newer VFD are circuit driven to meter the current required by a motor. He stated most newer VFD require the user program the motor amp to hp curve. Once the VFD has been programmed the electronics do the rest. He also noted from the PFD manual; your VFD will can handle a single phase input; however, the output to the motor is still 3 phase. So, if you motor is a single phase motor the VFD will more than likely kill it fairly quickly. In fact, he said if you supply 3 phase power to the single motor phase you should detect arcing immediately. If your motor is 3 phase then it may be able to handle the over amp surge a few times; however, he suspects the motor life will be quickly accelerated.

            I hope this makes sense; I am sure there a folks out there with several opinions, but I can assure you I have been working with these guys for over ten years. Base on my experience with them I would not be too quick to discount their input. Good luck; if you do decide to try it I would be very interested in finding out what happens.

            Regards,
            tgomez

            Comment


            • #7
              Thanks, Billybob. Yes, it's an older drive, and I'm using it on a 3-phase motor.

              The part of your post that I'm not completely clear about is how i'm going to over-amp the motor if it's not physically overloaded. It's true that this antique drive doesn't have a programmable current limit like the new ones, but I don't understand how that's any different than wiring it straight to the panel. There's 100 amps available there, but the motor still only pulls 8 or 10.

              What if I put some 10A fuses between the motor & drive?

              Comment


              • #8
                I suspect if you install a breaker before the motor you should be fine. In response to your first question; according to my sources they said during the initial startup the VFD will provide the amps required to turn a 25 hp motor. It is during this time when you may very likely overamp the motor. If you install the motor protection before the motor you should be fine assuming the starting amps required are below your breaker rating.

                regards,
                tgomez

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