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Thread: VFD tripping on instantaneous overcurrent

  1. #1
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    VFD tripping on instantaneous overcurrent

    I have a Powerflex700S 480V drive running a 350hp motor connected to a process fan. I appears that every once in a while (mostly at night for some reason) the drive trips on an instantaneous trip.

    When looking at this paramater it states that this trip occurs when the motor current reaches 214% of full load current. Full load current is 392A so the drive must see 838A in order to trip on this fault.

    Our DCS does not show any increased current or any trend upward in the current before it trips. Even if there was an upward trend due to process conditons or something the drive would hit its "overload" or "overcurrent' fault before the instantaneous, is that correct?

    I have put a power meter on the drive output to watch and see if I can recordd any transients, since our DCS may not be quick enough. My question is what on a fan can cause the current to be driven so high so quickly that it causes an instantaneous fault? The only thing I can think of is some of intermittent cable or motor fault? The system is ungrounded (zig-zag xfmr for ground detection) so this pretty much rules out any ground fault current.

    Is it possible that the drive is being fooled into thinking there is this much current for some reason? Noise, harmonics, etc..?

  2. #2
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    It's a good bet your DCS won't be able to record the instantaneous current that is tripping the drive out. The drive shuts off very fast, like in microseconds.

    Is this happening on startup? If so, it's possible the fan is moving when the VFD tries to start, and that can create a very high current. Usually the drive itself can do DC injection braking to keep the fan motionless when it is off.

    If it is not something happening on startup, I would be looking at what else is going on at the same time. It's not likely that the motor current would spike fast enough to trip on instant OL.

    Think of the instant settings as the magnetic part of a thermal magnetic breaker, and the overload setting as the thermal side. One is instantaneous and the other is an I^2T calculation.

    ETA:
    Does this thing have an encoder on it?
    Last edited by petersonra; 06-22-10 at 05:30 PM.
    Bob

  3. #3
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    You indicate the problem is intermittent. You may want to try replacing the CT control card. While, my experience is with ABB (SAMI and MegaStar) units, I do believe the control concepts are transferable.

  4. #4
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    Quote Originally Posted by petersonra View Post

    Is this happening on startup? If so, it's possible the fan is moving when the VFD tries to start, and that can create a very high current. Usually the drive itself can do DC injection braking to keep the fan motionless when it is off.ETA:
    Does this thing have an encoder on it?

    I agree with peter. I had 2 100 HP air handlers on vfd's in a hospital surgery suite. When the power failed these motors would still be spooling down when the emergency generator picked up the non critical gear. I installed some time delay relays that also energized the braking feature upon transfer of power, both from normal to emergency but the other way too. Solved the problem.
    Some people are like slinkies. They serve absolutely no useful purpose. But still put a smile on your face when pushed down a flight of stairs.

  5. #5
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    Quote Originally Posted by petersonra View Post
    If it is not something happening on startup, I would be looking at what else is going on at the same time. It's not likely that the motor current would spike fast enough to trip on instant OL.
    No this is not happening on startup but rather while the motor is running. The motor will run for several hours at steady current under full load, and then all of the sudden appears to trip. I will look at process conditions to see what is happening at the same time, but I really cant see any process condition that could happen this quickly that would cause this amount of current without first causing an overload condition. With a fan I think the only things that can happen are a damper opening somewhere allowing more airflow to move for a given fan speed, or the air becoming more dense with dust or some other material. Even in both these cases however I would not expect this to happen this quickly.

    Quote Originally Posted by petersonra View Post
    ETA:
    Does this thing have an encoder on it?
    No this motor does not have an encoder on it.


    If there is a short circuit on the motor (L-L) will the motor trip on an instantaneous as such. Is the drive quick enough to prevent damage? Can the drive interrupt fault currents at the motor or is it the primary protection (breaker, fuses, etc..) on the drive that have to interrupt fault current on load side of drive?

    Although in this case gnd fault current shouldn't be an issue, typically if there is a ground fault will the drive detect this and display it as such or will it simply show this as an instantaneous overcurrent condition?

  6. #6
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    Most high end drives have short circuit protection for the motor built-in. But short circuits in the windings tend to not be fleeting, they either happen or not. Things I have seen in the field that have caused phantom SC trips though:

    • Water in the feeder conduit; uncommon, but possible. Usually the damage is worse though.
    • Bad connection at the pecker head, i.e. Kearney bolts vibrating against each other and wearing through the insulating tape.
    • Intermittent short duration power loss, i.e. a quick dip. This is the thing I have seen more often that anything else with this trip. Check to see if the drive is configured for "Flying Restart", a feature that detects the decaying field so that the drive output can be made to match it when power is restored and re-accelerate to the desired speed; it helps to prevent that very thing. masterinbama's fix is what you do for a drive that isn't capable of flying restart, but I'd be surprised if the Power Flex was not.

  7. #7
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    Looking into it, the PowerFlex is capable of Flying Restart. But you had better take a look at this! Pay particular attention to the bottom paragraph of page 3:
    Frame 12 Drives Experienced Unstable Operation and Trip on IOC, Over Speed, or Over Load Faults

    You may just need a firmware upgrade.
    Last edited by Jraef; 06-23-10 at 12:07 AM.

  8. #8
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    Quote Originally Posted by Jraef View Post
    Most high end drives have short circuit protection for the motor built-in. But short circuits in the windings tend to not be fleeting, they either happen or not. Things I have seen in the field that have caused phantom SC trips though:

    • Intermittent short duration power loss, i.e. a quick dip. This is the thing I have seen more often that anything else with this trip. Check to see if the drive is configured for "Flying Restart", a feature that detects the decaying field so that the drive output can be made to match it when power is restored and re-accelerate to the desired speed; it helps to prevent that very thing. masterinbama's fix is what you do for a drive that isn't capable of flying restart, but I'd be surprised if the Power Flex was not.
    Some drives call this "ride though" capability, as in riding through a short duration power loss.
    Bob

  9. #9
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    Quote Originally Posted by philly View Post
    If there is a short circuit on the motor (L-L) will the motor trip on an instantaneous as such. Is the drive quick enough to prevent damage? Can the drive interrupt fault currents at the motor or is it the primary protection (breaker, fuses, etc..) on the drive that have to interrupt fault current on load side of drive?

    Although in this case gnd fault current shouldn't be an issue, typically if there is a ground fault will the drive detect this and display it as such or will it simply show this as an instantaneous overcurrent condition?
    Most drives these days have ground fault protection built in.

    Drives can typically shut themselves off fast enough to avoid damge if a short is seen downstream. The semiconductors can switch off very fast.
    Bob

  10. #10
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    This problem has again resurfaced after some time. We ran fine for a while without these issues but they seem to be appearing again.

    Is it possible that we are overloading the fan motor? Maybe by moving to much air through it?

    If there is an overload condition would we expect to see an overload fault from the dirve before we saw an instantaneous fault or can an overload happen quick enough to just send the drive into an instantaneous fault? I would expect that we would see an overload fault first but am not sure.

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