Thanks! But why they don't like it? UPS, as a black box, should harvest whatever energy is in the source, regardless is it sine wave or freaking square mess. Even DC or RF is present :) Why so picky?
That's true to an extent for online UPSes, which converts from AC to DC and then from DC to AC, completely isolating the input current from the output, but these are the most expensive types of UPSes.
Your run-of-the-mill UPS is likely to either be offline, which forwards the input to the output, or line-interactive, which can compensate to an extent under or over voltage conditions with a regulator. If the input current characteristics are outside allowable tolerances, they can't compensate and must switch the load to battery to continue powering it.
An offline UPS will switch to battery if it doesn't like the input. What qualifies as acceptable input depends on the design and specifications of the UPS. If it expects a real grid-like sine wave and doesn't see one it will reject it, regardless if the load would like it or not.
> If it expects a real grid-like sine wave and doesn't see one it will reject it, regardless if the load would like it or not.
But why would it require grid-like sine wave and not go along with whatever is the source, provided source can still be used to charge its batteries? I saw no answers yet, and this "why" is the very key to the discussion.
For example, a common topology in offline UPS is that the inverter and the charger are the same circuit driven differently[0] - so you can't charge the battery while the inverter is carrying load. This is popular at the low-end because you have literally half as much UPS, but makes what you're describing impossible.
Another common issue at the low end is that the inverter isn't thermally sized to run non-stop, they know they can cut a corner because your battery presents a finite and known duty cycle.
There are ways around this, but at some point you end up fixing the wrong problem - eg, it's cheaper, safer, and more resilient to buy a transfer switch instead of uprating two UPS to be capable of daisy-chaining.
It's not sarcasm. When nobody tells you a why item and you think that's because literally everyone else is dumber than you, it's likely there are odd numbers of sign errors in your heuristics.
> If the input current characteristics are outside allowable tolerances
This is the key to this whole discussion. I guess it boils down to existing line-interactive designs, why they can't work with simulated sine as a source.
A transfer-switch based UPS still needs to protect against brownouts, so it too has line behavior parameters outside of which it will switch to battery. It's an interesting question if/why a modified sine wave input would trigger that or not. Probably one of those "it depends on the design" things. (modified sine -> approximately an oscillating square wave.)
A dual-conversion (online) UPS is almost certainly more robust as far as what kinds of inputs it can accept (though as GP noted, they're more expensive, and they're also less efficient due to the additional rectification->inverter).
