At a grain elevator, I have a 480 volt 7.5 HP motor on a VFD. This motor is running a unload pit drag. Lately, it has been intermintently tripping out the drive overloads on start up (at times under load, other times no load). A week ago I showed up and opened up the drive to find it absolutely FULL of grain dust. We blew it out, cleaned it up and it started to run just fine. Now I'm being told it is doing the same thing. I'm on my way out the door to go troubleshoot it...this time I will disconect and meg the windings to check that out, I will also isolate the drive and check out the amperage. I'm wondering if the drive is shot? Any ideas and comments are much appreciated!
Lets try to summarize it.
20 year old drive: I am surprised the capacitors are still alive. Perhaps time to replace those, although the drives are sooo cheap that it may not be the cost effective solution.
It was full with dust. Two issues here. It may have already cooked some of your chips and now they are pre-destruction failure mode and behaving erratically. Grain dust, when heated, becomes carbon - a conductor - and I don't care how well one thinks the 'blowing out' cleaned out the dust, the carbon tracking on the circuit broards can produce the unrepeateable intermittent faults. These may not even be visible to the casual observer as wery low voltages and currents involved to produce these 'leaks'.
Check the namaplate currents of the motor AND the drive and see if the drive manufacturer constant torque recommended ratios are met.
Distance between motor and drive, motors suitability for inverter use, cable type used, are all potential issues.
It maybe time for a new drive.