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.
A 200hp across the line on the same transformer is your brownout. Six times FLA for a second or two, the transformer sags, the DC bus on the drives drops below the F004 trip. The top four are probably the loaded ones, the idle ones have a full bus to spend.
Nothing wrong with your drives. Log it first, recording meter on the MCC bus, min and max, so you have the depth and the length of the sag. Land the leads during an outage, or it's a live 480 bus with the door open and full arc flash kit. The cheap answer is a soft starter on that compressor. The 525 has a ride through setting but it buys milliseconds.
Log the sag first. A soft starter is an easy sell with a trace. And don't tune the drives to hide it.
Recording meter on the MCC bus for a week, leads landed on the Sunday shutdown. Sags to about 390VAC for a good second on every backup compressor start. Not much on paper, but a loaded drive can't hold its bus through it. Trace attached.
Soft starter for the compressor is quoted and goes in at the next outage.
It was the compressor dragging the whole MCC down every time it came in across the line, and the drives were just the first thing to notice. Nothing to fix on our side of the bus.
The top four versus bottom four thing, they're on the same bus bar. Could be just which ones were loaded when it hit, like ed says, but I never proved it. Thanks ed, dan.
Top four running, bottom four idle with a full bus. That's the whole difference.