Various aspects of home brew inverters

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Joined: 09/08/2007
Location: Australia
Posts: 4406
Posted: 10:02pm 11 Jun 2018      

A UPS is exactly that, non interrupted, and it absolutely must be phase locked to the grid. Power output must be completely non disturbed and continuous for it to be called a UPS.

Only three ways to do that that I know of. In ancient times a rotary UPS used an alternator to supply the load, coupled to a grid powered synchronous electric motor, with a battery powered dc motor on the same shaft. The dc motor could be switched in if the mains failed, and a huge flywheel kept the whole thing going during the momentary switch over.

Something a bit more modern, but still in use uses a ferroresonant transformer which essentially does the same thing but without any moving parts. Sola Basic are an Aussie company that have been building those for over a hundred years. I was a design engineer for a while at Sola Basic in Melbourne, and actually designed one of their commercial UPS's.

Another way is to constantly run a really big inverter from a grid powered rectifier, backed up with a battery. Pretty much like an off grid system, except the rectifier replaces the solar panels.

Anything else is called a momentary break power supply, and its not a true UPS.
With something like that there is no requirement for phase locking an inverter. The power goes off for a few tens of milliseconds then comes back on. Its a hard switch over, usually done with relays or contactors and is perfectly adequate for most things.
Cheers,  Tony.