Likely component fault on Powerflex 4 VFD?
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    Default Likely component fault on Powerflex 4 VFD?

    I have an Allen-Bradley Powerflex 4 VFD sitting on my desk that failed earlier in the week. Every time I'd try to start the motor, it would come up with fault F005, which is an over-voltage fault. The other two VFD in the same cabinet that run from the same power feed displayed the same buss voltage, 330-340 vdc. This one was made in December of 2005. It doesn't get heavy use, but it does sit powered up pretty much all the time.

    Does this sound like a main filter cap fault that is allowing excess ripple on the DC buss, or is it not something I could probably actually fix? I've already purchased a replacement drive, so if I can fix this it would just be for my own use at home. Replacing the caps wouldn't be a problem for me, but I don't want to waste my time or money if that probably isn't it. A quick visual inspection doesn't show any obvious problems, no burn marks, no bulged or leaky caps. All Nichicon parts too, not offbrand junk.

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    Hard to say, unless one is very familiar with that unit.

    I checked the manual to see if that is actually an excess ripple voltage fault, but that would be F003, not F005. So cap failure is not as likely a cause.

    The cause may be a fault in the voltage measuring circuit. If you dug around in the unit you might find a bad connection or other fairly simple fault on the power board that is presenting an incorrect high voltage input to the controls.

    Some of the units have an integrated power module that includes some of that circuitry, and is not repairable.

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    Most likely the little internal relay that is used to short out the pre-charge resistor is welded closed, so the resistor is not there to perform it's current limiting function when you power up the drive and the inrush drawn by the capacitors charging themselves up causes a ringing effect on the bus voltage that triggers the fault. You could try to take the drive apart and see if you can de-solder the relay, find a replacement, solder it back in etc. etc., might take you 3-4 hours if you are an average Joe bench technician, not counting the time to find the exact relay. So then the problem is, even after fixing that, you may find that just the few times you energized it trying to figure out what's happening has now damaged the capacitors and the drive is junk anyway.

    How that relay often gets fried / welded is that something "chattered" the voltage coming into the VFD, i.e. you turned line power off and back on rapidly and/or repeatedly.

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    Have not seen that version of the problem, but it is possible.

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    Quote Originally Posted by JST View Post

    The cause may be a fault in the voltage measuring circuit. If you dug around in the unit you might find a bad connection or other fairly simple fault on the power board that is presenting an incorrect high voltage input to the controls.
    I was using the internal display functions to read the buss voltage. I had not actually poked a meter in there. Would the display voltage likely be read by different components than what would set the fault code? I can also confirm the buss voltage with a meter to see if the internal matches.


    Quote Originally Posted by Jraef View Post
    Most likely the little internal relay that is used to short out the pre-charge resistor is welded closed,

    I can meter that out easy enough. Very possible its trash anyway but poking a meter in there to verify welded contacts won't cost anything. I'm not planning to spend any real time or money on this, just was hoping it might be something cheap and simple to fix.

    Thanks for the input, I'll give it a look and possibly deposit it in the circular file if thats where it belongs. This is the fourth drive unit to fail on me, and all of them have had the same fault. All around 10 years old too.

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    I'd suspect the same components do both, but that does not guarantee it will show up. Does the display not show a voltage that SHOULD be an overvoltage? If it shows 340V, then something else is fouled up, or else the two functions do NOT share parts..

    The one reason not to necessarily suspect the ringing problem is that it occurs when you start the motor, which is long after you power up the drive. The precharge relay should have operated way before that, and connection to the mains would also have happened long before. So that even if the relay was "welded shut", by the time you start the motor the drive should be just the same condition as if it were all perfectly correct.

    Dies the motor do anything before the error shows up? maybe jerk a little, or the like?

    Does the fault come up instantly or is there a couple seconds of delay?

    The drive may not check the status until asked to run. If it latched an overvolt status from back at turn-on, that would explain it, but usually the controls do not come on for a couple seconds after power is on the DC bus, as the power supply has to start up, and the micro controllers have to start up and verify communication from the interface to the actual power board. Untiil the micro is "on" and ready, it is generally not looking for voltage or doing anything other than booting up.

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    Display ran between 330 and 340, I presume because of line fluctuation. Over-voltage according to the manual is 405 volts, equivalent to 150 vac input. Power line voltage here runs right about 118 vac. This is a little 1hp single phase to 208 3 phase inverter, driving a 1/2 hp motor in a stretch wrap machine.

    Motor does absolutely nothing, it won't even try to budge. Hit the go button and the fault is instant. I did try resetting to factory defaults just in case it was some program issue, and that makes no difference. I can reset the fault and it will stay clear as long as the drive is not being commanded to do anything.

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    Well, then unless you see something obvious when looking inside, some obviously bad part, I don't know.

    I have seen cases where a condition sets several faults, and the one that actually appears o the display is just the first one in line, so it really may not be the true basic problem.


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