Out of interest, why is your system ungrounded.?
It must be relatively common for it to have a special mention in the manual.
As far as commonality my experience has been on systems built before the 80's. See Kev77's response below. The first ground fault is not a problem (before modern electronics). In theory this allowed a 24/7 365 facility the ability to continue operating. The classic application would be a modern war ship where the ability to continue operations was more important than saving a few lives from shock hazard.
A lot of older manufacturing systems were ungrounded probably because they are more fault tolerate. I'm sure someone on here can elaborate on the real reason.
For example, we had large sump pumps that would ground out and still work. We had ground lights showing we had a. Issue but the pump would not trip at the first sign of a fault. A grounded system would trip the main GFI and you would be down until it was corrected. I think newer systems have high resistive grounds that do somewhat the same thing.
Where I work we have the high resistance ground systems on 3 of our 480V systems. These are Wye systems with a large resistor tied to the X0 terminal then to ground. the resistor limits the ground fault to around 5 amps. We also have a delta system with a zig zag transformer attached to the three corners of the delta and this also limits the ground current to around 5 amps.
One of the consequences of the high resistance ground is you need to remove the jumpers that connect the metal oxide varsistors to ground because if you don't they will try to become the ground point (during a ground fault episode) for the entire system sacrificing themselves in the process, then spraying their guts on the rest of the components in the VFD's
you need a maintenance crew that understands it.
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One of the stranger things I have noticed is the zig zag transformer system seems to make the PF 755's think that the are being feed with single phase.
bendanator,
Several of the previous respondents have suggested many things to check out. Some might not seem obvious to you.
We had a similar problem when new drives were installed on our ungrounded system and the phase to ground MOV jumper was left in.
He is suggesting if the panel was installed with jumpers removed did you remove the jumpers when you replaced the drives?
If it’s the DC bus caps, do you have a line contactor ahead of the drive by chance?
The period of powerup in a VFD is a very stressful time. If the contactor is cycled very often the front end of the drive will be short lived. Another problem happens when the drive is running and there is a short time of no or low power input, the precharge circuit may not have time to reset and when power returns the caps will draw an almost infinite amount of power thus destroying some of the components, wires, printed circuit traces, caps, diodes. Most times this can be solved by a line choke/ filter on the VFD power input.
Although somewhat unlikely, is it possible all of the replacements were purchased around 2005? If so then this may be relevant.
http://www.plctalk.net/qanda/showthread.php?t=67658
