Powerflex Undervoltage Fault (F4) when supply is normal

Hello all, I have a trouble recently and would like to share and ask for your advices:

I have an MCC which has 4 pcs of PF700 (3x72A and 1x205A) supply from a common busbar. Yesterday when we are running normally, suddenly we had all of the 3pcs of 72A PF700 Fault (Undervoltage (F4)) when nothing happen with the 205A PF700 . I have check the history from the power monitoring system and found nothing abnormal. This is the fist time this happen since the factory started up 3 years ago.

I think the posibility of real undervoltage is low (with the power monitoring data and the 205A PF700 didnt has error). Do you guy have any idea about how this can happen. Thank you.

It really sounds like your supply voltage dropped for a very short time.



You can trip an VFDs MCP by hand and you will see the VFD will not fault right away. It may be a second or longer before the F4 fault comes up.
The VFD holds a charge for a while. You can turn the MSP off, wait half a second, turn it back on again and the VFD wont notice it.


A 205hp VFD will have larger capacitors and hold a charge longer after it is "turned off" (or the power dips).

It sounds like the power was off long enough for the smaller 72A VFDs to discharge enough to trigger the F4 fault. The 205A vfd held its charge through the voltage drop event and never faulted.


I would not trust the power monitoring system.
You need to check for yourself at the VFDs supply bus bar... nowhere else.
A multimeter with a MIN setting should be able to catch and hold a voltage drop reading.

Thank you GTUnit. I will test it next week as your advice and share the result. On another hand, do you think undervoltage fault is needed? What happenned if I disable this fault and the real undervoltage happened (I am using PF700 with ENET communication).

That fault can also be triggered by excessive DC bus ripple, which happens for instance when you lose one phase while the drive is under load, or one phase has a high resistance connection somewhere that appears normal until there is significant current through it.

Are all of the VFDs on the same MCC bus? If so, it might indicate a loose bus bar connector somewhere, usually a splice kit. If it was taking place down stream of the power monitor, the PM would not see the problem, but the VFDs would.

Thank you jraef for your idea. Yes, they are on the same MCC bus. I will take chance to inspect these busbar when shutdown period come.