Generally the variable frequency drive (VFD) operating temperature range is -10 degrees C to 40 degrees C. I would not be too concerned about the upper operating temperature limit (40 degrees C) as it is rather simple to derate the VFD for high thermal limits. The same goes for the high elevation application as the VFD will have some additional derating due to the lower thermal mass of air at high altitude. One might check with the VFD manufacturer to determine if that variable speed drive has an upper elevation limit due to the lower dielectric value of air at high elevations.
Most VFDs have a lower operating temperature of -10 degrees C (≈ 14 degrees F) which will inhibit the VFD from functioning if below that level. Two issues need to be addressed here. The first is the enclosure will require a heat source to bring the VFD up to the temperature range before it is enabled. How large a heater is dependent on how fast you would like the VFD to be enabled from a "cold start" and the volume of the enclosure and the R value of the enclosure itself. The second is the enclosure will need some means to regulate the amount of ambient air permitted to enter (and exhaust) the enclosure at very low temperatures. One does not want to subject the variable frequency drive to extreme levels of thermal cycling that may occur if a cooling fan is cycled to introduce large amounts of very cold air. I typically solve this issue by utilizing a 3 phase intake fan that is controlled by a small VFD (like a 1 hp VFD) and provided air via an electrically actuated louver. On the opposite side of the enclosure, the exhaust opening is covered by a self-closing gravity louver. When heating the enclosure the cooling fan (again controlled via a small VFD) is inhibited and will only function when the internal temperature warrants it. One would require a better definition of the "wash down" requirement to make sure that proper baffles are installed to prevent water from entering the enclosure.
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