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I want to hook up two 3kw inverter generators together to power my house in case of power loss. They only have 120V not 240V. How can I make this work while supplying power to all circuits 120V & 240V?

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  • What make/model of generator are we talking about here? Commented Mar 2, 2021 at 12:43
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    Sell two unsuitable generators, buy one suited to the job.
    – Ecnerwal
    Commented Mar 2, 2021 at 13:05

2 Answers 2

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Two identical 12V car batteries could deliver 24V, and two identical 120V~ transformers at the same net could deliver 240V~.

But two 120V~ generators can not simply be connected in series to get 240V~.

They must have a special interface/circuit to offer that option, because both generators must be locked to exactly the same frequency and must be locked to a constant phase difference of exactly Pi, e.g. by a phase locked loop circuit.

In case of the car batteries and the 2 transformers, this condition is automatically ensured without special circuits. Both car batteries deliver exactly the same frequency (0.000000 Hertz) and can simply be connected with exactly Pi difference ( + pole connected to -).

Both transformers are fed by exactly the same frequency (from the grid) and can also be simply connected with Pi difference.

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    Just a little note for the mathematically challenged: Pi radians is the same measurement as 180 degrees.
    – brhans
    Commented Mar 2, 2021 at 12:32
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Doesn't work. They're not in phase

This is part of why Thomas Edison hated AC power. It's very hard to understand, what with the flux vectors, phasing and all that jazz. Tesla was a genius, and Edison was merely persistent.

The upshot is that 240V is a single source with a center-tap (giving 120V each direction of center. In fact, when Edison designed it, it was +120V DC and -120V DC. And had Edison won the War of the Currents, your idea would have worked.

Here's the deal. Draw a 1.20 inch arrow on a sheet of paper. Now spin the paper at 60 revolutions per second, but that's hard, so simply spin the entire universe also at 60 Hz, so that the paper appears stationary to you. Just sitting on your desk.

Now, draw a second 1.20 inch arrow on another piece of paper. Connect the arrows nose to tail, so that they are 2.40 inches long. Problem solved, right?

OK, except the second arrow is spinning at 59.9 revolutions per second. So from your perspective, it's actually spinning at 0.1 revolution per second. It starts out aiming nose to tail, but soon it's at a cattywumpus angle to the other arrow. If you measure the distances between both arrows, it isn't 2.40 inches anymore, it's less. At 2/3 angle it's 2.08 inches, at 1/3 angle it's 1.20 inches, and soon they're pointing against each other and it's 0.00 inches!

And it's constantly changing.

That's what you get if you try to series-wire two generators.

There's nothing you can do with neutrals and grounds to make this work either... if you tie the neutrals (as you certainly should for safety, one arrow will still be spinning around the tail of the other arrow, doing the exact same thing to the exact same effect.

240V loads won't work. Multi-wire branch circuits will explode. This is bad.

How to do this properly

In 1988 I'd have said you're out of luck. However, now, especially after countless weather disasters where people who had bought solar systems thought those solar systems would keep their lights on and got a rude surprise about UL 1741 (but I digress)... there's a great deal of interest and available materiél in the solar/battery space. And from that, this is easy.

Because we can DC-couple this thing. We can have the generators make DC, which you can easily parallel... and then, use a 240V DC/AC converter to make your proper 120/240V split-phase.

But we can do even better than that. These systems include a battery (for capacitance and surge if nothing else; not least the battery picks up surge/startup load so the generator doesn't need to be wildly oversized just to start a motor). We don't need the battery for much as long as the generator array exceeds the continuous system draw. But why not?

Use the battery! Upsize the battery so it can do some of the "heavy lifting" and sustain most of a use session. Now you can get the deal done with one generator instead of 2. At that point you have a system that is primarily battery, and the generator is simply used to keep the battery topped up. This means you can get away with a much smaller generator too.

At that point, you run the generator at a time of your choosing, and the battery/inverter carries the house's small loads all night. No sleeping to generator hum; no getting up at 4am to refuel.

An example

A company called DC Solar got caught up in a Ponzi scheme (lying about how many units they had built to attract investors)... But one thing they did right is make absolutely fantastic, top-shelf portable generator+inverter setups on trailers. We're talking "power a big jobsite or small house" size. 5 KWH battery pack, inverters, solar panels + charge controllers (the solar was actually the most rinkydink and seems like it was bolted on as an afterthought; it can be ignored), and a computer that would automatically start a Kubota generator to keep the batteries topped up (if the solar wasn't doing the job, obviously).

Anyway there's a lot of info on Youtube about how these things were set up, and it's pretty much a masterclass in how to use batteries + generator + aux solar to make a top-shelf backup power system. Their technical people were top drawer, it's a shame the business people were crooks.

It's sad; the bankruptcy court "in their infinite wisdom" is having a fire-sale, dumping thousands of these on the marketplace at the same time. Which obviously is going to depress their price madly. Most of them are just going to be parted out, too.

Anyway, a most superb 5 KWH battery pack can be had from a $1000 Tesla module, and you can substitute the usual cheap Chinese for the chargers and inverters, if you're into that sort of thing.

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