Case in point: a handful of years ago we upgraded a machine to give it vfd control of the motor instead of a soft starter so they could control speed. The motor ran just fine from the soft starter. No obvious issues from what anyone could tell and it didn’t look like the motor was all that old. However, when we took out the soft starter and replaced it with a drive, as soon as we tried to run the motor from the drive the first time the drive kept tripping. Something about a faulty motor connection or ground fault if I remember correctly from the fault code.
So we grabbed our DMMs and did some basic checks. Everything seemed just fine. Winding resistances seemed reasonable, and no detectable shorts to ground. So we connected it back up to the drive. Same thing. Kept tripping and refusing to run.
So then we brought in our megger, and it ends up showing us that the winding insulation in the motor was starting to fail. Only a hundred or so mega-ohm to ground I believe. We showed this to the customer, they bought a new motor, and then everything ran great on the new vfd.
If it wasn’t for what the megger could show us that a DMM couldn’t, I might have had a hard time explaining why this high end vfd wasn’t able to start a motor that ran just fine from the soft starter.