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Thread: VFD nuisance tripping

  1. #1
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    VFD nuisance tripping

    We are facing nuisance tripping after installing new VFD to control an old motor
    The VFD is Schneider ATV61HD75N4 75KW, 3P, 380-480V, 50/60Hz.
    The Motor is US Motor Catalogue H100E2ES Model #R925.
    The connection between the VFD and the motor is copper cable (3X70mm+35mm) in RGS conduit and it is about 40Meter. The fourth core 35mm of the cable used as grounding.
    The error message on the VFD says Impedance Sh Circuit.
    Otherwise that VFD is much noisier than others with lower Wattage.
    How we can solve the nuisance tripping problem?
    Does the Motor Choke VW3A5104 will solve it?
    Awaiting your comments
    Attached Images Attached Images

  2. #2
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    I thought a quick google search would bring up something. That was frustrating. You may have to wait for the smart guys to show up.
    Tom
    TBLO

  3. #3
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    What does the VFD manual say this error message means?
    Bob

  4. #4
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    It means you have a short circuit either inside of the VFD, i.e. one of the transistors is shorted, or you have a short circuit in the motor circuit somewhere. You can run a diagnostic of the transistor status from the front keypad, in parameter section 1.10. If they all check out OK, then you have a problem in the wiring to the motor or the motor windings themselves. My money is on the motor windings.

    One thing that happens a lot with installing VFDs in old motors, is that the old motor insulation was leaking to ground or phase-to-phase, but the short was of high enough impedance that the Across-the-Line (DOL to you) starter did not trip out because the total current was still below the overload relay trip threshold (often exacerbated by people thinking the OL is bad and "tweaking" it so that is quits tripping).

    But the VFD has to be much more careful about things like that and will not tolerate it because it can damage the transistors. Hence the VFD becomes a "fault finder". The problem was already there, you just were not aware of it yet.
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  5. #5
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    Quote Originally Posted by Jraef View Post
    If they all check out OK, then you have a problem in the wiring to the motor or the motor windings themselves. My money is on the motor windings.
    One thing that happens a lot with installing VFDs in old motors, is that the old motor insulation was leaking to ground or phase-to-phase, but the short was of high enough impedance that the Across-the-Line (DOL to you) starter did not trip out because the total current was still below the overload relay trip threshold (often exacerbated by people thinking the OL is bad and "tweaking" it so that is quits tripping).
    I have also heard the assertion that the pulsed waveform from a VFD can cause a greater strain on the motor insulation causing a breakdown which would not happen with sinusoidal voltage. Have you experienced that or is it a wive's tale?

  6. #6
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    Quote Originally Posted by GoldDigger View Post
    I have also heard the assertion that the pulsed waveform from a VFD can cause a greater strain on the motor insulation causing a breakdown which would not happen with sinusoidal voltage. Have you experienced that or is it a wive's tale?
    Sort of. Under the right circumstances, the high speed of the transistor fired DC pulses can interact with the output cables in a capacitor-like fashion to create charges of high voltage that reflect back and forth between the drive and the motor in waves, it's called the "reflected (or standing) wave phenomenon". It's well studied and accepted, yet not completely understood yet. What we do know however is that if there is enough distance, the reflected waves build up a voltage that eventually exceeds the dielectric withstand voltage of the motor insulation and finds the weakest point. That is usually the "first turn" in the windings, where the insulation first gets stretched a little thinner as it bends. What happens then is that you get microscopic "punch-throughs" of voltage between the adjacent phase windings and it gets worse and worse over time until the windings short completely and fail. This can happen in as little as 2 weeks (my shortest experience) or could take years, there are a lot of factors that go into it. There are several things you can do to mitigate this, one of which is to always use shielded motor lead cable. We here in the US have only recently started this, but in IEC world they have been for a long time, because they tended to not use conduit as much as we do. If you but "inverter duty" motors now, most mfrs have upped the ante on the motor winding insulation as well. in the past, the voltage rating was 2X the maximum voltage level, so 600V class motors had 1300-1800V insulation. The voltage spikes can however exceed 2200V now on a 460V rated motor run by a VFD, so a lot of magnet wire mfrs have begun selling it with 2400-4000V insulation.

    In this case though, he used RGS conduit and what he referred to as "multi-core" cable, but maybe not shielded. Still the damage would not be instantaneous. My interpretation was that they had an old motor and just installed a new VFD, but the VFD is tripping right away. In that case, I think the motor winding damage was already there.
    __________________________________________________ ____________________________
    "Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
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  7. #7
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    Quote Originally Posted by GoldDigger View Post
    I have also heard the assertion that the pulsed waveform from a VFD can cause a greater strain on the motor insulation causing a breakdown which would not happen with sinusoidal voltage. Have you experienced that or is it a wive's tale?
    It's real.
    Been there. Done that. In real life.

  8. #8
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    Quote Originally Posted by GoldDigger View Post
    I have also heard the assertion that the pulsed waveform from a VFD can cause a greater strain on the motor insulation causing a breakdown which would not happen with sinusoidal voltage. Have you experienced that or is it a wive's tale?
    You can do the math and answer this yourself also....

    480vac across the line = 480*1.4=672v Peak across any winding every cycle.

    480v thru vfd =480*1.4*2= 1344v Peak across any winding every cycle.

    Long cable lengths on vfd and 480*1.4*3.x= 2000+ Peak across any winding every cycle.

    Does this strain your motor insulation?

    I have seen motor insulation failure in less than 5 minutes of on time, many times (many = more than 10).

  9. #9
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    At double peak voltage you would think all 600v insulation would fail fairly quickly wouldn't you?

    Every drive in our plant is running standard motors from 10hz to 120hz on VFDs and no insulation failures would I attribute to the VFD in the past 15 years.

    Put me down for "wives tale" on this one. I'm sure on paper it makes sense, in the field I haven't seen it happen or it took so long I didn't attribute a failure to it. Most of our motors cook from lack of cooling and overloading but that's still a multiple year scenario.

    Does the motor meg out? The leaking current theory not being noticed on the DOL is what I would look at. You mentioned the ground so I'm assuming this is not an ungrounded system.

  10. #10
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    Quote Originally Posted by oregonshooter View Post
    Put me down for "wives tale" on this one.
    Then you'd be incorrect.

    I have come across the problem in the field on a few occasions.

    Here's a couple of excerpts from a report I wrote for one site:

    Rate of change of voltage (dv/dt)
    Rate of change of voltage was very fast. In all cases it was either outside the limits of the IEC TS60034-17 technical specification, or marginal. For the rise times measured the maximum dv/dt should not be greater than approximately 1600V/us. The average dv/dt measured at the pump house was 2500V/us.


    The motors in question were dying like flies. Within weeks.

    Conclusion
    The insulation failures have the appearance of partial discharge failures such as would be created by the very high electric field intensity caused by the very fast dv/dt. The recorded dv/dt is as high as any we have measured at or close to the motor terminals.
    In summary, the installation of dv/dt filters to reduce the rate of change of voltage applied to the motor is highly recommended.


    The recommended filters were installed. And the problem went away.

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