DickDV,
You obviously have a lot of experience and knowledge about drive systems. You also have a lot of well-deserved respect on this forum. I've read many of your other posts and know you are more knowledgeable than I am, when it comes to VFD's.
I'm just a guy who's seen too many things that are "not possible" cause systems to shutdown and drives to fail.
I don't want to sound like an argumentative upstart, but I object to your blanket statement that unequal output line lengths cannot be the cause of this failure (admittedly overly paraphrased, but essentially your point).
I agree that we need more information, but the EE who suggested the line lengths has been on site and knows more than I know about the system, and presumably has taken some measurements or performed some calculations to support his statement concerning line lengths.
Things we don't know about this system, for example:
Is it a voltage source or a current source drive? Is there any dv/dt or di/dt protection? What is the drive operating mode? Have there been any custom changes done to apply this drive to this load?
What kind of cables are on the drive? How are they run to the motors? What are the ambient conditions where the motors are operating? What are the motors' load characteristics? Is there any additional impedance anywhere in the system?
What horsepower rating (now we know, but didn't originally) is the overall system?
Is there a non-drive-related system fault before the drive failure? Is there a dynamic brake or other sink to absorb any excess energy caused by the failure? Are the load conditions changing, and, if so, is there any relationship between these changing load conditions and the coincident drive failures?
What type of motors are on the drive? Are there output contactors between the drive and the motors? Is there any data record to see the sequence of failures or shutdowns? What other protections are provided for on this system? Assuming there are other protective devices: Where are they monitoring the signals, what signals are they monitoring, and what actions do they take when they detect an abnormality?
I tend to initially believe what I hear from the field until the evidence disputes the suspicion. As Reagan said, "Trust but verify."
I agree with everything you said, if this is a modern PWM voltage-source drive system, if there are no filters, if there are no contactors opening at the wrong time in the wrong place, if there is no additional energy being supplied to the drive because of the shutdown, if... You understand my point: There are just too many "if's."
I'd also like to hear the engineer's EXACT statement before we condemn it out of hand. Perhaps Mordred has not accurately reported the EE's statement. I am not suggesting that Mordred is lying or obfuscating. I am suggesting that maybe the EE didn't say exactly what Mordred thought he heard.
The symptom reported here is repeated drive failures. The presumably qualified man on site thinks unequal motor line lengths may be the cause. Until I see evidence to the contrary, I will treat his suspicions with due respect.
Again, I'm not trying to be argumentative. I just think we can't dismiss the on-site suggestion without knowing more than we currently know.
Having said everything above, I find it hard to believe that unequal line lengths are the cause of these drive failures. Much of my previous response's conjecture was done trying to justify the EE's reported statement. Before I'd spend a lot of time and money changing cables, I'd like to try to pinpoint the cause of the failure, and then correct the situation.
Mordred,
"...none of those write ups even mention any concern for line distance."
I completely disagree with this. You'll kill a motor quicker than you can spit if your leads are too long on a PWM system.
http://www.electram.com/Rex_reactor_dvdt_filters.htm
That is a sale's pitch for their dv/dt filters, but the first link that I found with my google search on this topic. Long leads can kill motors or drives, unless adequate precautions are taken, such as the filter in the link above.
I have no connection with electram. It was just the first link that came up that seems to cover this situation.