An electric fan here at work was getting really hot and making a funny
sound when it was plugged in to a UPS when the power went out.
Most basic UPS's output an approximated sinewave. Instead of a smooth
shape, it's in steps. Different models or brands may have different step
levels. Higher-end UPS's can output real sinewaves.
The stepped-sine is mainly designed for use with the switching power
supplies used by most computers. They may also work, to some extent, with
transformers, but could cause extra heat and/or noise.
A number of years ago, during a long power outage, I used a small UPS to
run a fan for a few hours. It did hum quite a bit and run a bit slower,
but still works fine to this day.