failed drives

graywolf

Member
Location
United States
I have had three power flex 70 frame 8 drives fail,lost bus caps and output transistors. Have not had any warnings or trips prior to failures. Have monitored line for 6 months no problems found. Anyone else have like problems:dunce:
 

kwired

Electron manager
Location
NE Nebraska
I have had three power flex 70 frame 8 drives fail,lost bus caps and output transistors. Have not had any warnings or trips prior to failures. Have monitored line for 6 months no problems found. Anyone else have like problems:dunce:
More details of the driven load may help answer the question.

Does the drive provide much breaking or reversing duties? Those kinds of activities would probably tax most of the components you mentioned and the use of breaking resistors is recommended to help dissipate some of the energy that may be a problem.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I don't know of any issue of high failure rates on PowerFlex drives, in fact they have arguably the lowest failure rate in the industry. Of the less than 1% that fail, roughly 80+% of field failures are attributable to heat related issues, 15% to power quality issues and 4% to application issues. The defect rate is a fraction of a percent.

So assuming you are comfortable with the cooling system, and the application is correct (per kwired's response), power quality is a likely candidate, especially if the caps are involved. On the large drives like that, the big caps have what are called "balancing resistors" across them, which help to even out the stresses the caps see from grid transients that come right through the bridge rectifier. Years of severe transients will take out those resistors, which is followed shortly thereafter by cap failure, followed shortly thereafter by transistor failure. Regular preventative maintenance programs will include inspection and periodic replacement of those balancing resistors. Ignoring PM on drives leaves those resistors (and other things) as ticking time bombs.

Usually when a cap fails first though, the drive will fault in high DC bus ripple to prevent transistor damage. That would lead me to suspect that the failure happened the other way around, that a transistor failed first. There are protection systems to help avoid cascading failures in that direction too, but often, without load reactors, a serious event on the load side happens too fast for the systems to kick in. Whenever people ask me if load reactors are necessary, I always explain that no, they are not. Neither is health insurance (can't say that any more though), but when it turns out you needed it, you will wish you had it. The main benefit of a load reactor is to slow down the rise time of a load side transient event, such as a motor winding or lead cable short, to where the drive can act in time to limit the damage.

You say you have 3 that failed? All at the same time? If so, that points back to a transient event on the line side again. It only takes one significant event to do damage across multiple units.

Another issue, what is your power grid like? Is it an ungrounded delta or a high resistance grounded wye? If so, there is a jumper on the incoming rectifier section that had to be removed to prevent MOV damage. Was that done? If it is a solidly grounded wye system, did someone inadvertently remove that jumper? Those Frame 8 drives usually also have common mode chokes that need the jumper removed under the same conditions, and people used to doing it on smaller drives that don't have common mode chokes sometimes miss it on the larger ones that do. Over time, that can lead to damage too.
 

electric_cal

Member
Location
California
I don't know of any issue of high failure rates on PowerFlex drives, in fact they have arguably the lowest failure rate in the industry. Of the less than 1% that fail, roughly 80+% of field failures are attributable to heat related issues, 15% to power quality issues and 4% to application issues. The defect rate is a fraction of a percent.

So assuming you are comfortable with the cooling system, and the application is correct (per kwired's response), power quality is a likely candidate, especially if the caps are involved. On the large drives like that, the big caps have what are called "balancing resistors" across them, which help to even out the stresses the caps see from grid transients that come right through the bridge rectifier. Years of severe transients will take out those resistors, which is followed shortly thereafter by cap failure, followed shortly thereafter by transistor failure. Regular preventative maintenance programs will include inspection and periodic replacement of those balancing resistors. Ignoring PM on drives leaves those resistors (and other things) as ticking time bombs.

Usually when a cap fails first though, the drive will fault in high DC bus ripple to prevent transistor damage. That would lead me to suspect that the failure happened the other way around, that a transistor failed first. There are protection systems to help avoid cascading failures in that direction too, but often, without load reactors, a serious event on the load side happens too fast for the systems to kick in. Whenever people ask me if load reactors are necessary, I always explain that no, they are not. Neither is health insurance (can't say that any more though), but when it turns out you needed it, you will wish you had it. The main benefit of a load reactor is to slow down the rise time of a load side transient event, such as a motor winding or lead cable short, to where the drive can act in time to limit the damage.

You say you have 3 that failed? All at the same time? If so, that points back to a transient event on the line side again. It only takes one significant event to do damage across multiple units.

Another issue, what is your power grid like? Is it an ungrounded delta or a high resistance grounded wye? If so, there is a jumper on the incoming rectifier section that had to be removed to prevent MOV damage. Was that done? If it is a solidly grounded wye system, did someone inadvertently remove that jumper? Those Frame 8 drives usually also have common mode chokes that need the jumper removed under the same conditions, and people used to doing it on smaller drives that don't have common mode chokes sometimes miss it on the larger ones that do. Over time, that can lead to damage too.
Excellent and concise post:thumbsup:
 

augie47

Moderator
Staff member
Location
Tennessee
LOL, I have never been accused of being "concise" before... so thanks.
but almost, if not, always "excellent" posts IMO.

The recent loss of Laslo serves to remind me how fortunate we are that have folks as knowledgeable as yourself on the Forum.
 
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graywolf

Member
Location
United States
Some more back ground, I'am 2 months on job and inherited problem.
drive one failed one year ago #2 three months ago and #3 two months ago similar damage on all.
I was told power was monitored for 6 months no issues were found. Line is solid ground Y, all jumpers are in place except common choke i am not sure. please clarify jumper on or off. loads are submerged pumps no problems found on that end.
 

Besoeker

Senior Member
Location
UK
If no drives failed during your monitoring period, then you need to keep monitoring. You likely have an intermittent voltage transient.
I don't buy the voltage transient explanation.

The input circuit comprises a plain rectifier followed by a BIG DC link capacitor. That would absorb anything that would be considered a transient without appreciable voltage change.
The diodes might experience an over voltage that could exceed their PIV capability and that could lead to their failure but such failures are not reported.

Sustained over voltage, maybe. But the OP says the supply is being monitored. So maybe that's another one to be crossed off the list. Unless the monitoring missed specific events.

What's left? We don't know details of the load. If there is any way it could well regenerate back into the DC link, excessive deceleration rate for example, that could overvolt the DC link, destroy capacitors and IGBTs.
 

Besoeker

Senior Member
Location
UK
drive set to ramp down 10s, with contactor on output. Can you recommend a test to capture regen
Monitor the DC link voltage although I would expect most VFDs to drop out on fault with DC link over voltage.
But if that doesn't drop the output contactor the load will continue to geed the DC link.
If that's the problem.
 
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electric_cal

Member
Location
California
When the power study was performed, at what point was the measurement taken? Was it taken at the utility source, or close to the drive connection points?

Do you possibly have a large load that is ahead of the drives in your power line up that could be a source of voltage sags and spikes?
 

graywolf

Member
Location
United States
power study done by third party and close to drives, large facility with many large loads.
I've requested equipment to do another in house study.
i will try to check for regen on a normal shut down tomorrow.
Thanks to all
 

Besoeker

Senior Member
Location
UK
Ding ding ding! We may have a winner!

1) Why is there a contactor on the output?
2) What is controlling that contactor? and
3) What is the logic behind when it opens and closes?
FWIW, we (almost) always fit a contactor on the output. It is mandatory for safety reasons to have a positive disconnect between the drive and the motor.
It's interlocked with the pilot relay that drives such the drive is disabled before the contactor opens.
Graywolf's 10 second ramp down infers that the contactor remains closed during that period.
 

StarCat

Senior Member
Location
Moab, UT USA
Contactor?

Contactor?

Field installed, outside of the case, and not part of the drive???
That raises questions similar to a disconnect in the same position.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
FWIW, we (almost) always fit a contactor on the output. It is mandatory for safety reasons to have a positive disconnect between the drive and the motor.
It's interlocked with the pilot relay that drives such the drive is disabled before the contactor opens.
Graywolf's 10 second ramp down infers that the contactor remains closed during that period.
Yes, that's how YOU do it, because you know what you are doing. But for all the time I have been in the drives business, I can attest to the fact that contactors on the output tend to do more harm than good. It isn't always that the systems are designed wrong, it's that the original intent gets lost in time and the third or fourth person responsible makes changes they think are OK, not understanding the ramifications. Given that graywolf has inherited responsibility for these pumps, I'd be willing to bet that he does not yet know the answers to my questions, because nobody explained it to him. And that is likely because whomever should be responsible likely does not know either! Hopefully he will get an answer and we can either eliminate this possibility or suggest a solution if it is the culprit, but I'm willing to bet this will not be easy for him. There may be nobody at his facility that can answer them. I know it's not right, it's just what I experience day in and day out.

An output contactor on a submersible pump is, in my opinion, not only pointless but risky. Opening or worse, chattering, that contactor under load can lead to blowing the transistors, which would explain the cap failure. A low voltage condition on the control circuit can cause contactor chatter. But more often what happens is that these contactors are tied to Emergency Stop circuits, and lazy "Bubbah" operators will slap an E-Stop to do a normal shut down almost every time. Three times when I have suspected that, and found it by connecting a PLC input to record when the E-Stop was hit, I have found that operators hit it EVERY time they needed to shut down. Why? One told me because he wanted reassurance that it worked. So instead of going to the HMI and commanding an orderly shutdown, then opening the disconnect switch to service the pump or work on a valve etc., he slapped the E-Stop button. The others were explained as "being easier", which I think is closer to the truth, if you substitute "easy" with "lazy".

A simple solution as you know Besoker, is to use an early break aux contact on the contactor or E-stop that is wire to a drive input that turns off the VFD output before the main contacts open. But I cannot count the number of very otherwise qualified engineers that do not know this. WE know it because we are in the Drives industry. Some engineers only see a few drives in their entire career, if at all.
 

junkhound

Senior Member
Location
Renton, WA
Did not see anyone yet mention harmonics.

Triplett harmonics can saturate common mode inductors, leading to noise problems and possible upset of control circuits.

More than once, have seen line harmonics resonate with the LC EMI input filtering and boost the DC link voltage to higher than cap ratings, even on systems with a boost PFC circuit.
 
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