240V Generator to 120V Inverter - Potential Inverter Backfeed w/ Current Set-up

[PaleBlueDotFarm]

My current 120V inverter supplies a PSX-240 transformer, which results in 240V at the PV/Generator transfer switch.

Once on generator (240V) power, I turn ON a 120V circuit from the house panel, which supplies the inverter for battery charging. This diagram shows the situation. It seems atypical and dangerous, but I'm new at this. However, I like the complete isolation of the generator from the inverter.

In replacing my existing inverter with another 120V model, do I have other options to supply the inverter with 120V power for charging from the 240V generator?

Thank you!

 

[RCinFLA]

A 240/120vac split phase synchronous generator can supply 120vac from either hot 240vac leg (black or red wires) to center neutral (white wire) of generator however for a for a synchronous generator it will only be able to supply a little over half the generator kVA rating. Think of the generator as a center tapped 240vac transformer output where the center tap is the neutral connection.

For a 240/120vac inverter-generator, they are actually two separate three phase alternators on engine drive shaft feeding a rectified high voltage DC to two separate 120vac inverters run in series for 240/120vac output. The rectified high voltage DC runs between 200-320 vdc. The variable speed ECO mode modulates engine rpm for given load on generator to keep the HV DC at, to slightly above, 200 vdc. The 200 vdc runs into the sinewave synthesis high frequency PWM H-bridge chopper (IGBT power devices) and is L-C filtered to create the AC output. This is just like a HF inverter. For 120vac you only get half the rated power from inverter-generator on each 120vac half.

You can get full generator 120vac power with a step down transformer from 240vac to 120vac of appropriate VA capability. This can be a full isolation or auto-transformer.

Some generators, both synchronous and inverter-generators, have a series/parallel switch that configures the two split phase 120vac halves in parallel for 120vac only operation at full generator kVA capability.

 

[PaleBlueDotFarm]

Thanks for the detailed description. Just when I think I'm starting to understand this stuff...!

So it sounds like using this 120V circuit for charging through the inverter sends reduces quality of charging compared to using a transformer. Given that this is a rare use-case for us, is it harmful in any way?

I imagine bad things would happen if I had the 120V charging circuit was ON to the inverter if the inverter was supplying the house panel. But if it is isolated in this charging procedure, is it harmful?

 

[RCinFLA]

Running 120vac output from generator is not harmful, just potentially not being able to get full generator capacity output from a 240/120vac generator. Again, some generators have a 120vac only switch that allow full generator power to be delivered only to 120vac.

A generator loaded greater than 25-35% of rated VA has optimum fuel efficiency. At less than 20% generator loading, the generator mechanical overhead to run pistons up and down, open/close valves, and run flywheel air cooling becomes sizable percentage of losses in fuel consumption efficiency.

Generator AC input pass-through inverter to AC output has no detrimental effect other than consuming some of generator capacity that might be needed for charging battery. Supplying AC output loads normally has priority over charging for the AC input power from generator.