An enclosure is often needed for additional connections, fusing, power disconnect and sometimes for a braking resistor (depending on the type). A fan cooling system may be needed, depending on the VFD efficiency/dissipation, cabinet material, volume/radiating surface area and operating conditions. My mill 3Hp VFD has a factory steel cabinet with no venting that is 12x22x8, the fan comes on when the VFD is running the machine motor/off when idle. I have seen sizing a VFD cabinet for no ventilation quoted something in the range of ~4sq in/W of dissipation for a sealed steel enclosure or ~1 sq ft/Hp, but there are other factors that need to be considered. It is a simple matter to cut a few holes in the cabinet and add a filter/fan to the VFD cabinet when using a smaller cabinet. The last 3Hp VFD mill system build I did, I used a 12x20x8 metal cabinet with no venting, anything smaller I would have added cooling vents/fan. IF adding a fan, then a plastic enclosure is a bit easier to cut holes in using a large hole saw or jig saw.
Non Ventilated Enclosure
Non ventilated enclosures rely on the heat being dissipated through the walls of the enclosure. The better heat conduction of the enclosure the more heat dissipated. For this reason metal enclosures are far better at dissipating heat than plastic enclosures.
The power that can be dissipated in a given exposed surface area is:
PESA = k x S x △T
PESA: Power dissipated from within the enclosure via exposed surface area in W (Watts)
k: Heat transfer coefficient [sheet metal ~ 5.5W/m2K, plastic ~ 3.5W/m2K ]
S: Corrected enclosure surface area of the enclosure, in m2 in accordance with IEC890.
△T: Temperature differential (inside enclosure - outside ambient), in °C
Due to the distance of the VFD cabinet to the machine, I would recommended shielded cables at least for the control inputs/speed and do not tie them to the motor cable.