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Of all the sinewave inverters I have used, they contain a switch at the back labelled Normal and UPS. When you put the inverter in normal mode, there is some 40-50 ms switchover time, enough to make my PC reboot. Now when I put it to UPS mode, the turnover time is 8-10 ms according to the specs, the computer doesn't reboot, and the lights don't even flicker. But the only caveat is the input voltage is narrowed from 100V-280V in normal to 180V-260V in UPS mode.

So my question is, if the relay and the sinewave generator circuit is indeed capable of starting a sinewave within 8ms, why does the Normal Mode wait till 50ms to start the inverter circuit? Or is it that in UPS mode, the inverter circuit is always on and when main power goes off, it immediately switches over to the inverter output? In this case, the UPS mode will increase wear and tear of the inverter circuit, right, because it's always on?

enter image description here Table from the Exide Inverterz GQP

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I would agree. It sounds like the inverter is running all the time when in UPS mode & either handling the entire system load or switching the load from mains to inverter output on power failure.

Transformer already energized = less transfer time

The increased minimum input voltage in UPS mode suggests the primary AC-DC is having to both charge batteries & supply power to generate AC.

Due to inverter conversion losses you would need more power input overall to run in "UPS" mode, hence more current at the lower voltage rating. possibly exceeding some limiting rating... (speculation here)

I am unsure why the maximum input V would lower.

All of that is assuming some hybrid topology between a double conversion on-line UPS & standby UPS... Double conversion offline perhaps?

Topology info taken from:

https://download.schneider-electric.com/files?p_File_Name=SADE-5TNM3Y_R7_EN.pdf&p_Doc_Ref=SPD_SADE-5TNM3Y_EN

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