Early, full-height 8 inch floppy drives had synchronous spindle motors. We added CP/M systems at field sites that had 10KVA UPSes to support a redundant minicomputer system. Some of the backup disks from field sites were unreadable at the home office. Turns out that these 1970s-vintage UPSes had free-running oscillators and were not crystal-controlled or phase-locked to the mains. The frequency could be off by a couple of hertz, changing the spindle speed proportionately.
We set up an audio oscillator, a 100-watt public-address amplifier, and a transformer to power one of the system's 2 drives, tweaked the frequency until the drive could read reliably, and PIPed the data to a fresh disk in the other drive, which remained powered by the utility.
The later, half-height drives ran on DC and did not have the issue.