Hi.
From behind my spider webs junk, connected a 230VAC fan out of a defunct compressor unit, to 115VAC. Runs slower as I wanted, quieter as I wanted, capacitor is cold, motor runs barely warm ~40C considered operating temperature. What to observe/consider before putting it permanently in the attic to vent the hot air ?

 

I don't see a problem as long as it starts reliably.

 

I agree with sghioto.
The motor will be running with a lot of slip, but that shouldn't be a problem as long as it starts okay and doesn't overheat.

 

Keeping the rain out should be your number-1 priority.
Number-2 is a Thermostat, to prevent moisture build-up in the Attic during cooler weather.
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Rain is part of the environment these motors run at. Keeping it dry could cause a failure

 

Keeping the rain out should be your number-1 priority.

Externet you get rain inside your attic?

 

The protection of the connection requirements for the wiring are quite different for an attic fan versus one installed in an outside unit. That will be the main concern. And there may be a local requirement to add some sort of fire detection cutout to the power feed, not for a fan problem, but in case of a fire, the fan should switch off. Check you local electrical code for the actual requirements in your area. OR put it in a window as a portable fan and avoid all of that. (Just an idea, not a recommendation.)

 

The one I installed had a rain proof hood of course, it also had a thermostat attached that sensed attic temp, in order that it did not run by inadvertently left on by a switch, as it cannot normally be heard in rooms below.

 

The rental property that had such a fan, where I was asked to install a speed control to reduce the noise. At full speed it was easy to hear through all of the house. When running as slow as the controller would keep it running, it was still easily heard throughout the house..
When I was tenyears old, my father added one to our house, at the top of the stairway to the upstairs. It moved air very rapidly and certainly could be heard. It was used a few times that summer, covered over for the winter, and not uncovered again that I recall. Some things seem like a god idea until they are actually experienced.

 

The one I installed, a factory, roof intended model, could hardly be heard in the Attic !!