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

  1. #101
    Join Date
    Mar 2016
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    The Motor City, Michigan USA
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    708
    Quote Originally Posted by Redwood Infrared View Post
    ... - it is not feasible to pull the wire and put a twist in it without replacing it or adding a splice; there's just not enough slack in the line to do that. ...
    There's probably enough slack for a gentle twist -- one turn per five or ten feet? -- if you neatly lay the conductors, eliminate "braids", and bundle them tightly. One or two turns per segment of conduit is probably sufficient. (if it's an electromagnetic problem at all)

  2. #102
    Join Date
    Jun 2018
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    Arcata, CA, USA
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    47
    ggunn:

    I have already strongly recommended the structural support measure you suggest here and

    Quote Originally Posted by ggunn View Post
    Why would a tenant's UPS have any effect on the solar installation? The solar only runs when the grid is up and the UPS only runs when the grid is down. The UPS should also have a transfer switch that disconnects it from the rest of the service (and therefore the PV) when it is running.

    My guess is that the conductors are vibrating a bit, and over time the vibration has loosened the conduit attachments so that they are now rattling. Didn't you say that the conduit has fewer supports than it should and that was going to be corrected? Add the new supports and tighten the old ones and see what that gets you.
    But beyond that, we are detecting strong magnetic fields along the length of the conduit that we believe are anomalous and likely point to the root cause of the vibration. Increasing structural integrity might prove to be a mitigation but not resolution of a condition the building owner finds undesirable. They are a regional leader in green building development so truly understanding matters of this nature is important to them.

    And as far as what the UPS has to do with it, I am told there have been cases of similar situations where it turned out that THD introduced into a building's power distribution network by a UPS was definitively identified as the root cause of undesirable vibration and noise in cable ducts. As such I have not yet concluded anything but I will absolutely be running an experiment to see if it makes any difference when we take the UPS offline while the PV array is producing and the undesirable noise condition exists. If the noise stops when the UPS goes offline, then we will have identified our root cause and can develop more meaningful mitigations from there.

  3. #103
    Join Date
    Nov 2016
    Location
    Kennewick
    Posts
    106
    This is exactly why I prefer microinverters installed on the roof. 500 volts dc coming off of panels to a main inverter. Which then had to have a bunch of labels on the conduit. The conduit has to be EMT or RMC. Also you could be getting vibrations from the inverters themselves.


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  4. #104
    Join Date
    Apr 2008
    Location
    Ann Arbor, Michigan
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    6,843
    180619-2130 EDT

    Redwood Infrared:

    I think you go off in two many different tangents instead of focusing on the very likely causes..

    You have a high sound level. That means some reasonable amount of acoustic power. Possibly much greater than 10 W, and possibly not as high as 100 W. This is a lot of random power and quite likely means it originates from a high power source. Quite possibly your sound radiator is is the conduit, but you could also have energy mechanically coupled to other radiators.

    It is possible that a mechanical resonance is part of the problem.

    I don't have big pipe to work with for experiments. However, I did some tests on 1/2" thinwall steel conduit. My experiments are not well controlled and values may be in substantial error, but the results have some degree of consistency.

    With the conduit clamped at one end to my drill press table and the other end resting on the edge of a tool box and the accelerometer taped to the middle I measured 5.9 Hz.

    Next two cantilevered tests. First, was 5 ft (60"), and result was 7.7 Hz. Second, was 90", and result was 3.3 Hz. (90/60)^2 = 2.25 . The 3.3 * 2.25 = 7.4 . Looks to vary as the square of the ratio.

    When the member is initially excited to oscillate there are harmonics superimposed on the fundamental. These die out fairly quickly.

    I would suggest that you see where your 4" conduit tends to vibrate. Use a span length equal to the support distance in the building.

    .

  5. #105
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    Apr 2008
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    Ann Arbor, Michigan
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    6,843
    180619-2241 EDT

    In a mechanical system, if mass remains constant and the support spring gets stiffer, then the resonant frequency increases.

    If two wires carry the same current, then as the wires come closer together the force between the wires increases.

    In an AC circuit the force between the wires will have a constant value with a superimposed double frequency force component.

    Current direction in one wire relative to the other will determine whether the force is attractive, or repulsive.

    Two parallel wires close together with current flowing in the same direction produce additive magnetic fields, and thus as you move away from the wires there is a substantial magnetic field.

    When the currents are in opposite directions, then the external magnetic fields tend to cancel.

    You have 120 Hz sound energy. That comes from synchronized 60 Hz currents.

    .

  6. #106
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    Apr 2008
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    18062445 EDT

    With the two point suspension I get a substabtial 8th harmonic superimposed on the fundamental.

    .

  7. #107
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    Apr 2008
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    Ann Arbor, Michigan
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    180620-2425 EDT

    New experiment to demonstrate electricql excitation of mechanical vibration of 10' 1/2" thinwall.

    The conduit is c-clamped at one end to a drill press table. The other end freely rests on a moderately sharp toolbox edge. The accelerometer is at the 5' point. A hairpin loop of #16 (not much mass) extends thru the entire conduit length.

    A 16 V pulse of energy from a 50,000 ufd capacitor thru the wire loop is used to shock excite mechanical oscillation of the conduit. For a short time 50 Hz showed up with about 1/8 this (50/8) for a longer time. Correlates with mechanical excitation in prior experiments.

    Note: this is an impulse excitation of a tuned circuit. I don't have a high current source with variable frequency to continuously excite the mechanical system. I suspect your conduit system has a mechanical resonance around 120 Hz, but likely would not be the fundamental resonance.

    .

  8. #108
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    Apr 2008
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    Ann Arbor, Michigan
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    180621-1432 EDT

    Another experiment. This time with 3/4 thinwall.

    Fundamental about 7.6 Hz, next major spike at 63.4 Hz, ratio 8.34 from FFT. Not an integer ratio.

    When viewing the y-t trace (y axis vs time) my best count ratio between the two superimposed waveforms is 8.5 .

    The 3/4 tubing is a higher resonant frequency than the 1/2 as I think I would expect.

    A more controlled experiment on the 1/2 and using a cursor to measure frequency instead of eyeball the results are --- 6.6 Hz, 49.1 Hz, ratio 7.44; 6.6 Hz, 86.6 Hz, ratio 13.0 .

    What happens when you go to 4"? My expectation is a higher frequency.

    If I take my above diameter ratio (0.75/0.5 = 1.5), then take the sq-root of 1.5 the value is 1.225 . Multiply 6.6 by 1.225 and the result is 8.08 . This is close to 8.34, but may not be a good predictor.

    4 divided by 1/2 = 8 . The sq-root of 8 is 2.82. Multiply 6.6 by 2.83 and the result is 18.7 . Then by 8 and the result is 149. High compared to 120, but my approach is just a wild estimate. Your support points are greater than 10 ft and that lowers the frequency. Need to know if your supported conduit resonates near 120 Hz.

    From the previous experiments with a single turn loop it is fairly clear you can excite the tube to vibrate.

    Exciting at resonance can produce a large audio signal.

    .

  9. #109
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    Sep 2004
    Location
    Vancouver, WA
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    687

    Magnetic Flux in Transformer?

    I just stumbled on this thread and have been involved in electrical acoustic noise for 30+ years, off an on. In general, it is iron that vibrates due to magnetostriction. A step-up/down or buck/boost transformer core will "hum" at 120Hz with strong 240Hz and 480Hz tones. However, magnetistriction is due to the magnetic flux created by voltage, not the current, so it should be constant. Are there any transformers in the building that could be the source of the noise?

    If the current harmonics from the inverters cause a strong change in voltage, due to a "weak" utility source, this harmonic voltage will cause every transformer, iron core ballast, and motor to be noisy. A harmonic filter can lessen the current harmonics, thus lowering the voltage harmonics, thus reducing the acoustic noise.

    Also, are there any VFDs in the building driving motors? A VFD creates a messy waveform which causes the iron in a motor to vibrate noisily.
    e^(i pi) = -1

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