An official note on OC faults that trip only on ramp-up, because the usual answer is not the drive.
Case: a 3-axis router with an air-cooled spindle, VFD fed single-phase through a step-up. The drive faults OC about a second into ramp-up and never under load, cold or warm. The spindle turns clean by hand.
The order we work through:
Accel time against the motor rated current. A short ramp trips on inrush almost every time.
Drive parameters against the nameplate — rated current and pole count. A drive still holding the previous motor’s numbers will overshoot the winding limit before it can throttle back.
The spindle leads meggered to ground, and the connector checked for moisture.
The drive output scoped, where you have the equipment. Phase imbalance shows up here.
The one that catches people out is 2. A drive configured for a larger motor is the most common cause we see of a fault that appears only during ramp-up.
If the log and the ramp curve do not explain it on your machine, open a ticket with the model and the alarm code and our support team will take it from there.
When you check the VFD parameters, did you confirm it was programmed for the motor that is actually on the machine, or left on whatever generic setup someone slapped on? I cannot picture how that would trip OC on ramp-up. Motor-specific VFD settings are new to me and I still mix up generic vs custom.
Pull the motor nameplate before anything else. Does it actually read what someone punched into the drive, or did whoever set it up just take the previous motor’s numbers?
I chased an OC on ramp-up for three days once. Spun it by hand, checked the drive, meggered the leads — all good. The tech before me had programmed the drive for a bigger motor’s rated current. The ramp current limit sat too high, so it spiked past what the winding could take before the drive could throttle back.
Get the nameplate, enter the actual rated current and pole count, plus service factor if it is listed. Everything downstream depends on that being right.
One addition from our side, because this one cost us a spindle.
We hit an OC-on-ramp-up on a demo router last year. The first instinct was to stretch the accel ramp, and the fault went away — until the spindle failed two weeks later. The ramp was the symptom, not the cause: the winding was already on its way out and a longer ramp simply hid it.
That is why we would treat the nameplate check as the one not to skip. If the drive is still sitting on the previous motor’s numbers, everything downstream is a guess; a drive told the wrong rated current will overshoot the real winding limit however gentle the ramp is.
For a machine that turns clean by hand and still trips a second in, pull the fault log before touching parameters.
The ramp-curve detail sits with our engineering team rather than with me — support can take that one.