Morning, PF755 on the main chiller pump, 75hp, 480VAC feed. It's faulted on input phase loss twice this week, both times mid afternoon when it's 95 outside and the whole plant is on air. Reset it, runs fine, load steady at about 90 A.
I measured at the breaker with the drive off and got 479, 481, 480 phase to phase, all three there. Then we called the utility and they told us there's nothing on their end.
So either they're sagging a phase when the load's up and won't admit it, or the rectifier in the drive is on its way out. Has anyone seen a 755 rectifier fail like that, one phase at a time? Is there a way to get the 755 to log what it saw at the moment it tripped, so I've got something to take back to them?
You measured with the drive off, at the breaker. Measure under load, at the drive's input terminals. And what's between the breaker and the drive, fuses?
Three class J fuses in a fused disconnect between the breaker and the drive. Locked out to get the cover off, then back on and measured at the line terminals with it running at 90 A, full arc flash kit for that one. 478, 480, 455. The low one is on the C fuse. Pulled that fuse after and it reads continuity fine on the meter, so I'm not sure that proves much. Bit stuck now.
Continuity isn't the test. A fuse can read zero ohms on a meter and still have half its element gone. High resistance under 90 A drops volts, and it's worse when the afternoon heat is sitting in the panel. The drive sees ripple on the DC bus off the uneven phase and calls it phase loss, which is what it is from where it sits.
Not the rectifier. Not the utility. Lock out the disconnect, prove it dead, pull all three fuses and fit a new set of the same class and rating, not one. Then look hard at the fuse clips and the lugs on that C phase for brown marks. A loose clip will cook the next fuse too.