Row of eight PowerFlex 525s in the packaging MCC, 480VAC, all on the same bus. Two or three times a week four or five of them fault F004 UnderVoltage in the same second. Never all eight, mostly the top four in the row. Reset them and they run fine for days after.
I measured the incoming at the MCC, 478 steady the whole time I stood there with the meter on it. Then I swapped the drive that faults most for a spare, on the idea that one sick drive was dragging the rest down with it. The spare faults too.
four drives going together on a bus that measures fine has me stuck. Is four of them going in the same second telling me it's the bus and not the drives, because I'm not sure what else to put a meter on?
Same second means the supply, not a drive. What else starts at those times? Anything big on the same transformer, a compressor, a chiller, an oven?
Checked the timestamps against the compressor room. The 200hp backup compressor starts across the line within a few seconds of every one of them. It kicks in when the main unit can't keep up.