DIAGNOSIS A burning smell indicates something is overheating. The most common causes are a failing motor bearing (causing friction) or an electrical component on the control board starting to burn out.
FIX Stop using the fan immediately and unplug it. This is a potential fire hazard. You need to inspect it. 1) After it cools, disassemble it enough to see the motor and the control board. 2) Look for visible signs: melted wire insulation, a darkened or bubbled area on the green circuit board, or black dust around the motor bearings. 3) If you see damage on the board, it must be replaced. If the motor itself is the source (you’ll often feel excessive heat or hear a screeching bearing), the entire motor assembly needs replacement.
IF THAT DIDN'T WORK If you find no obvious visual damage, the smell could be from dust packed against a hot component over a long time. Give the entire interior a thorough cleaning with compressed air as described in the cleaning question, then monitor it closely on a short test run.
HEADS UP Do not ignore this smell or continue to run the fan. Electrical fires start this way. If you’re not confident in diagnosing this, it’s safer to replace the entire unit.