Re: @YetAnotherLocksmith
er.. the common or garden UPS has been doing exactly that for years. Mains goes off - UPS alternates independently. Mains goes on - UPS slews the phase of the generated output to match the mains in, then switches over to live input.
I had an old APC unit which would cold-start without a power input. It would run at 60hz. When the mains was connected, you could hear it hunting around to achieve 50hz, mains phase before switching to live.
Biting the hand that feeds IT
