Maker Pro
Maker Pro

Running 120v 60hz motors on 120v 50hz

I recently did a test with a box fan plugged in to a ups on battery
I was able to get the ups to produce 120v 60hz and 120v 50hz.
The box fan for some reason ran faster on 50hz (yes on 50hz) than it
did at 60hz.

Has anyone ever did a test like this? If, so please describe.