When I converted my lathe to 3-phase & a VFD I used a TEFC motor. Worrying about cooling at low speeds, I removed the motor's fan & attached a 120v muffin fan, that would run continuously.
Well, that fan was from my junk box & failed after a while. I just unplugged it & resolved to keep a close check on the motor temp. I don't do long stretches of heavy work & the motor never got more than warm.
Tonight I worked it harder than I ever have & probably as heavy as I ever will: I turned down a 14" long 2-1/2" diam piece of mild steel to
2". Not a big deal, I know, but as heavy as I do. The motor got too hot to keep my hand on for more than 5 seconds or so. Which is still not nearly the 105 degree C that even lowest rated motor can run at.YMMV, of course, but for me, I'm going to take the failed fan off the motor and not replace it. (I shoulda' left the original fan on.)
Bob