Why does my UPS output power go off when switching from battery to mains?

I have a small 600VA UPS(includes battery) that I use to power my Denon AVR.

I have noticed that the UPS switches smoothly to battery when the mains power is interrupted. and the AVR does not switch off. However, when the mains is restored, the AVR goes off and then turns on again immediately.

Is my UPS defective, or do I need to buy one with better technology like double conversion.

It sounds defective or perhaps the battery is failing, also check/clean the terminals. IDK what double conversion is but ones I got 20 years ago work both ways just fine.

Single conversion means that the UPS runs the devices off of the incoming AC until that AC fails or goes out of spec. The UPS then switches over to its internally generated AC that comes from its batteries and an inverted.

Double conversion means that the UPS converts the incoming AC to DC, and then runs that DC through an inverter to generate its own AC output. If the incoming AC fails, then it switches over to its batteries instead of the AC-generated DC, which then goes through the same inverted to generate its AC output. Since there is an AC to DC conversion along with the DC to AC conversion, that’s there the “double conversion” name comes from.

Double conversion provides better isolation from the incoming AC so any power noise on the incoming AC won’t make it into your protected equipment. Since there is no direct connection from the incoming AC to the output AC, you also naturally get better protection from surges and power spikes (though surge protection is a different issue). The downside is that double conversion costs more.

I dunno. It may take more time to switch back to the incoming AC than it does to switch to batteries, but you would have to measure its switching time to see if it is within the 4 to 8 ms that the manufacturer specifies to know if it is defective or not. Your AVR might just need something with a smaller guaranteed transfer time.

In other words, if it takes 4 ms to switch from AC to batteries but 8 ms to switch back, it’s within the manufacturer’s specs and isn’t defective. It’s just that your AVR needs something faster than 8ms (note - I have no idea what your AVR can actually tolerate for transfer times, this is just an example).

The transfer time of a double conversion UPS tends to be faster than a single conversion. 4 to 8 ms isn’t bad for a single conversion. Double conversion is often faster than 3 ms.

I would think the double conversion UPS switching time would be instantaionus. AC input to DC buss from the DC buss to the AC output. But withthe batteries floating on the DC buss. If the AC goes off there would be no need of switching because the batteries would keep the voltage on the DC bus from dropping. That was how it was explained to me.

Also there was a feed back from the AC output to the AC input keeping the AC output in sync with the input. This way to work on the up all that was necessary was to close a bypass and open the input breaker, output breaker, and the battery breaker.

It depends.

Many newer UPSs have an energy efficient “eco mode” that operates by bypassing and shutting down the inverters so the output is directly powered from the incoming AC. They are often advertised as being “green”. If the incoming power goes out of spec (like during a brownout) then they can switch into double-conversion mode. If you don’t care so much about energy efficiency and need the zero transfer time then you can manually turn off eco mode.