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Thread: Mysterious vibration in long conduit between solar inverters and main service panel

  1. #61
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    Quote Originally Posted by gar View Post
    Also note that acoustic tubes can create resonances. A tight wire might be a useful sound transmitter. Mechanics use screwdrivers to couple engine or other noises to their ear.
    My mechanic friend gave me a stethoscope that has a metal rod in place of the disc that's on a medical one. I used it to find a clattering harmonic balancer on my car.

  2. #62
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    Quote Originally Posted by ggunn View Post
    My mechanic friend gave me a stethoscope that has a metal rod in place of the disc that's on a medical one. I used it to find a clattering harmonic balancer on my car.
    a cheap piezo mic (batt operated with pre-amp, etc) makes for a great contact tool for "listening" in to "see" the vibrations. can be had from ebay for like $2

    60Hz does "beat" as 120x per sec. it's likely resonance.

  3. #63
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    [QUOTE=junkhound;1922634]
    Quote Originally Posted by Redwood Infrared View Post
    ...did a test ....QUOTE]

    Think we all are breathlessly awaiting anyof that test data to be shown. Sure would be helpful to share an audio waveform, voltage and current waveforms.

    Need to have something factual on which to base conjectures vs. just that it drives folks out of the office.

    All we have so far factual is that something makes noise and it is related to the level of sunshine and the inverter model number.

    Would be great if you could get a release to share the THD, voltage, and current measurements taken by the poco.

    I'm working on that but don't hold your breath. With so many stakeholders I have to be careful not to stomp on any toes. I might be able to get away with a couple of screenshots but really want to get it cleared first.

  4. #64
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    Quote Originally Posted by ATSman View Post
    Great discussion. I would like to know how this all plays out in the end.
    One thing I have learned thru the years (48) in T-shooting problems is that my mind tends to think of the most complicated reasons for the cause of the failure. When all is said and done it usually turns out to be something simple. I would like to know if this is a normal reaction and have others felt the same way.
    But I will not rule out the times when I have pulled my hair out on some weird problems....ha!
    I'm still holding out hope that in the end simple thermography will have saved the day. For 2 years nobody thought to look for a heat signature until I showed up and within 10 seconds I was able to point out what didn't look right. This is just one of those cases where due to the vibration I don't have any idea whether I'm looking at a symptom or a root cause. It's entirely conceivable that a minor connection issue led to a voltage or current unbalance that set up an initial vibration and we got where we are through a positive feedback loop. We should have a good idea if that's the case once the electrician redresses terminal ends of the conductors and installs some hardware to improve all those connections. If we remove the heat but still have vibration, or if even after replacing switchgear those connections are still getting hot, well then we'll know there's something fundamentally wrong with the circuit that's inducing these crazy fields. But I'll look like a flippin' hero if it turns out it was just bad connections getting hot.

  5. #65
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    Bad connections getting hot mean higher than normal voltage drop, which in turn means unbalanced current in parallel runs. And that can cause vibration, so your odds are pretty good.

  6. #66
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    based on you freq measurements of ~ 60, 120, 240 imo not mechanical, ie, motor or compressor
    that would be much higher

    get some eh rated gloves
    when humming grab and hold at noisest point
    both hands or 1 hand same conduit
    preferably with eh shoes or insulated ladder
    see if it dampens
    The difference between genius and stupidity is that genius has its limits.

  7. #67
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    Quote Originally Posted by Ingenieur View Post
    based on you freq measurements of ~ 60, 120, 240 imo not mechanical, ie, motor or compressor
    that would be much higher

    get some eh rated gloves
    when humming grab and hold at noisest point
    both hands or 1 hand same conduit
    preferably with eh shoes or insulated ladder
    see if it dampens
    We've already tried that. So far no efforts to dampen it have been very effective.

  8. #68
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    Quote Originally Posted by GoldDigger View Post
    Bad connections getting hot mean higher than normal voltage drop, which in turn means unbalanced current in parallel runs. And that can cause vibration, so your odds are pretty good.
    Normally I'd absolutely agree, just can't understand why we're not seeing significant voltage drop anywhere.

    Although I have to admit we didn't have good lighting on the inverter combiner breaker box and today I got a much closer look at it and sure enough, there is visible evidence of minor arcing on the tabs where the breakers are bolted to the bus bars. I couldn't measure any voltage drop under 70% conditions with things running about 50°F rise over ambient but perhaps there's a point when the system hits peak output that it gets hot enough the resistance increases exponentially, and so far that's only been transient.

  9. #69
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    try this when at full output
    shutdown
    disconect 1/2 the capacity, inverters, etc
    disconnect one set of the conductors
    turn the remaining 50% on, see if it hums
    The difference between genius and stupidity is that genius has its limits.

  10. #70
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    Quote Originally Posted by Redwood Infrared View Post
    Normally I'd absolutely agree, just can't understand why we're not seeing significant voltage drop anywhere.

    Although I have to admit we didn't have good lighting on the inverter combiner breaker box and today I got a much closer look at it and sure enough, there is visible evidence of minor arcing on the tabs where the breakers are bolted to the bus bars. I couldn't measure any voltage drop under 70% conditions with things running about 50°F rise over ambient but perhaps there's a point when the system hits peak output that it gets hot enough the resistance increases exponentially, and so far that's only been transient.
    The reason that you are not seeing significant voltage drop anywhere could well be that when you have two low resistance paths in parallel it does not take very much of an added voltage drop in one of the two paths to produce a very unequal division of current between the two paths.

    Example:

    If you have two 119.9 ohm loads fed in parallel from a 120V source and the current split between two parallel wires each with a resistance of .2 ohms, the total circuit resistance will be 120 ohms, the current will be 1A and that 1A will be divided equally between the two wires.
    If instead one path (A) has a wire resistance of .2 ohm and the other (B) has a resistance of .3 ohm the total current will still be about 1A but the current will be divided 600ma on one wire (A) and 400ma on the other (B). Note that the voltage drop measured across each of the two paths will still be identical because they are in parallel and the currents will not be equal.
    The overall heating in the B path because of the extra .1 ohm resistance will in fact be lower than in the A path because of the unequal current division. If the added .1 ohm is in the form of a bad connection, you will see a voltage drop of .04 volts across that bad connection and the local heating will be only 0.016W.

    Run the calculation with more reasonable numbers to match your situation and you will see similar results. A very small added voltage drop can produce wildly different currents between the two paths.

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