Paralleling multiple inverter's outputs

[lifer]

I've read few threads about stacking multiple inverters (mainly grid-tie) with no viable solutions.

Reading infos about DC-DC inverters I came to this idea: to rectify each inverter's output, to sum up those DC voltages then to convert DC to AC using a full-bridge stage driven by a 50-100 khz PWM signal (50/60Hz modulated).

I know, I'm going to build more than a half of a pure sine wave inverter this way, but I see some advantages:

1. I already have 5 small inverters (700-1500W, pure sine wave) - but I need some 3kW peek power for an electric oven
2. The inverters could be a cheaper ones (modified sine wave or square wave) as their ouput will be rectified anyway
3. You can add/remove inverters at any time
4. You can shutdown "unused" inverters depending on load
5. I can't afford a commercial 4-5kW pure sine wave inverter

I know there will be some losses in the process but a DC to AC inverter (mentioned earlier) can have over 95% efficiency so it's not a big problem.

I need some advices regarding the best way to sum up those rectified outputs. A diode and a small (value) resistor will be enaugh to compensate voltage differentials?

Any negative vibes?!

[DamonHD]

You'd need to have a monster cap or battery or other energy store on each output to ensure that you never have a moment when (eg) one of the component outputs is not instantaneously zero for example.  Plus you'll have isolation issues with big chunks of stuff floating well away from ground potential.

Rgds

Damon

[OperaHouse]

There is an answer that worked for about everything........If you really need to ask, you probably can't do it.
Most things work on fundamentally simple principles.  The devil is in the details.

I love inverters and keep a stock of defunct ones around for projects.  Many have damaged H bridges.  I just tear that section out and run 140V DC to run CFL lamps with electronic ballast.  I also run VCR and DVD decks on 140V DC.  A lot of line powered electronics will run just fine on 140V DC.  Others will go up in smoke.  One old HF inverter has multiple inverters that produce about 32V each that they just add up.  That seems very useful but I haven't found a use for it yet.

Many inverters employ multiple mini inverters in parallel to obtain the DC current needed.  These are all synced to each other and function on the same control voltage.  That allows them to share current equally.  Parallel separate inverters and they will have different supply voltages and not share.  These inverters will also likely have floating neutrals that a diode will not isolate
the output. 

Some heater controls work on waveform slope and may not function on MSW. You could possibly operate the oven heaters on DC.  Depending on temperatures needed 130-160V may be enough.  Most contacts do not like switching DC, but there are ways to control dedicated inverters.

Is this a conventional stove or an industrial oven?  I started to modify a nice gas oven to work on 12V DC.  The burner igniters were easy enough to run on a cheapie 100W inverter.  The clock and oven computer could run on a 5V buck inverter with a little stinger attached to supply 50V for the display.  24V relays could be switched to 12V or just run a FET.  The oven actually had an electric broiler that ran every time you ran the gas oven till it got near temperature.  That was disconnected.  What finally killed the project was the "pilot light".  This was a glow element that consumed 400V and ran all the time the gas burner was on.  That was more than my solar array put out mid day and I bake in the evening.  My system only has a little more than one car battery.   And as a safety feature the gas valve ran on the igniter current.  Would have to build a high current low voltage inverter to run that.  Then would have to built a flame detect for safety.  Flames actually conduct electricity.

So, nothing is ever simple.  There should be lots of inverter projects here, but fieldlines is a technology desert.  Your chance of getting sufficient help are minimal.

[lifer]

You're both right. On second thought, I think it's very hard to control/limit the output current of each separate inverter (because you can't control their output voltage) so using a simple resistor limiter it's out of question.

@Damon: I forgot about low frequency (50hz) and high power filters. Monster caps, you name it!

@OperaHouse: My electric oven it's a commercial one (Whirepool) and it only needs those 3kW for catalytic self-cleaning, I suppose. Anyway, I don't think I could make it work with DC (btw, it's 230V mains here).

That's it, I'm gonna build the entire 3-4kW pure sine inverter from scratch. I know it's a heck of a job but I just can't wait to live off-grid!

[boB]

I think your original idea of rectifying and summing the outputs (with filter cap of course) could work.

The inverters are supposed to self limit.  Of course, the DC summed polarity would have to be symetrically
loaded so that each inverter did not see an offset DC load.  They don't always like that.

But if it were loaded plus to minus on that summed output, it would probably work OK as a large-ish high
voltage power supply.

boB

[Xan]

Here's a guy that actually did it. It probably helped that he had two identical inverters....!   

Part 1: https://www.youtube.com/watch?v=pwGBt2QB8Qg
Part 2: https://www.youtube.com/watch?v=GJhcsJdcQ4o