[saltcured]

I wonder if modern motor and power control tech could be adapted to make a desk-side motor-generator set that is efficient enough to rival an always-active, dual conversion AC-DC-AC UPS.

How efficient could a small AC->motor->generator->AC chain be with a modest flywheel mass to provide cycle-to-cycle stability?

Could it ever make sense to put one of these after a standby UPS so the output is always filtered by the motor-generator but the UPS only has to kick in for outages?

[mrob]

One advantage of a motor-generator set is that it's relatively easy to get high-voltage isolation using an insulating shaft. It might be possible to build something that could survive nearby lightning strikes to the incoming AC line. I don't think any standard UPS can do this.