There has been a couple good thoughts as to why a variable frequency drive (VFD) would fail after powering back up, moisture will be a big player in the operation of a VFD but I will caution you about blowing air across a VFD. The components are a little picky when it comes to static and that can be all it takes to kill it. I have a couple MCC rooms that the VFDs are covered with powder and that's a concern but the greater concern is getting the powder off without incident. We have been successful with an electronics vacuum on a powered down VFD, our only causality was a couple HIM's. You could pressurize the panel with a few Psi of air if you have good door seals and that will help keep the dust and moisture out.
If power can be kept to the variable frequency drive that will be very helpful in maintaining the VFD temperature and moisture level also keeps the capacitors charged. Depending on the VFD, say an AB 1336 classic (20+ years old) the components are rock solid in comparison to a 1336 ++. This all has to do with how much silicone is used in the construction of the VFD parts. The 1336 + and ++ seem to have issues with the capacitors failing after several years of operation but only become an issue after they have had a power down condition usually followed by a small light show depending on the size of the VFD. The 700 series VFD seems to be even more sensitive than the older generations. The 700 series are nice because they have that control card which is very easy to replace but is a little fragile and the 1336 (plus and plus plus) VFDs have the L-6 board which can be a tricky one troubleshoot.
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