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OT (sort of) VFD on a RO filter

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  • OT (sort of) VFD on a RO filter

    He guys,

    I am doing the engineering inspection on a water plant rehab. The other day the electrical contractor made the comment that the #2 RO train had been sitting idle for so long (maybe 5 years or more) that the VFD running the main pump motor (20hp 480V 3phase) had probably discharged the capacitors in the VFD so that if it was powered up would likely blow up the VFD.

    Any of you guys that are experienced with VFD's think this is a likely problem? Or is this guy blowing the proverbial magic smoke?

    Thanks,

    Tim

  • #2
    What's a #2 RO train?

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    • #3
      Oops sorry,

      Reverse Osmosis filter

      Tim

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      • #4
        Usually not a problem but to play on safe side consider "reforming" the caps
        https://library.e.abb.com/public/cf2...ming_Rev_G.pdf

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        • #5
          Thank you MattiJ,

          I guess the electrician wasn't just blowing smoke. It will be at his discretion as to weather he changes the VFD or reforms the capacitors.

          I do appreciate the quick and thorough response.

          Tim

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          • #6
            They will probably be OK but there is a chance you could pop a cap on powerup. Pretty slim chance, but it is still there. I would just power it up. I am sure it will be fine.

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            • #7
              Caps don't last forever an long periods of no charge is reputed to shorten their lives. Back in the day, long unused caps could be recovered by "forming" that is, by charging then very slowly in stages. Makes sense,they are electrolytic and the dialectric film is a product of forming thin non conductive films of the aluminum plates. It makes even more sense to simply change them out for new once of like rating and form factor.

              Call it a periodic maintenance procedure. Electrolytic capacitors have a finite working life generally years - longer than the service life of the equipment in which they are used. Not a big project nor are the caps a big ticket item. Change them all out for that matter even the little ones in the logic boards. It's like a new suit of hoses for an excavator: PITA, not that expensive, but the replacement ensures against almost certain trouble.

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              • #8
                You might be surprised how expensive some of the large low ESR capacitors are. And I have some capacitors that are almost 50years old, and seem to be still good.
                http://pauleschoen.com/pix/PM08_P76_P54.png
                Paul , P S Technology, Inc. and MrTibbs
                USA Maryland 21030

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                • #9
                  VFD often come with instructions on how to reform the capacitors. Basically stepped voltage over a hour or two. I did it the easy way - put a 60W light bulb in series with the vfd on 120v, then an hour later 240v. That limits the current in case of crap happening.

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                  • #10
                    Second the light bulb idea. Put a set of low wattage say 20w lamps equal to the input voltage in series with one phase, disconnect one phase and apply power. Lights should go bright momentarily then quickly dim. If the lights go out or very dim within a minute you are good to go. Remain bright and you have a short, usually a cap but sometimes a rectifier. Input circuit is a three phase bridge rectifier feeding the caps thru a surge limiting circuit thus the need to disconnect one phase or full voltage will be provided thru that phase. Simple in concept but fairly complex in application.

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