Auto transformer ?

[Bossrox]

If using say a 6kw split phase inverter, normally the limit is 3 kw off each leg. The ? I have is with an auto transformer, can it combine the total 6kw output onto a single 120v branch without over amping either leg coming off the inverter? I tried doing this with a simple 10kw 220 to 120 transformer but the transformer itself hogged a significant amount of power maxing out a 6kw inverter on a 4kw load, not very efficient.

 

[Solar Guppy]

Auto transformers are more efficient than isolation transformers and yes, you can pull full rated power on a single leg, bypassing the common complaint of split phase inverters ( really two inverters out of phase ).

My Solis 5G HVES has no neutral and it is required to have a neutral forming, and it is nice not to worry about overloading one leg

 

[timselectric]

Yes, just size it according to the needed output.
You turn your split-phase inverter into a single phase inverter, by not using its neutral. Then use the autotransformer to create the split-phase output.

 

[RCinFLA]

Most of what are called 5kVA autotransformers are actually 2.5 kVA autotransformers. A 28 lbs 60Hz transformer is a 2.5 kVA autotransformer.

Transformers can take a 2x overload for about 20-30 minutes before you can boil water on them.

 

[Solar Guppy]

Most of what are called 5kVA autotransformers are actually 2.5 kVA autotransformers. A 28 lbs 60Hz transformer is a 2.5 kVA autotransformer.

Transformers can take a 2x overload for about 20-30 minutes before you can boil water on them.

The Solis a true 6000 VA rated autotransformer which can pull full rated VA Split Phase Imbalance. It's one of the reasons I decided to go with this product / manufacture

 

[Bossrox]

The Solis a true 6000 VA rated autotransformer which can pull full rated VA Split Phase Imbalance. It's one of the reasons I decided to go with this product / manufacture

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I'm glad you revealed all that. After looking more into these transformers, the only 2 I could find were victron & growatt. Growatt's 100 amp model stats say it can only do 5kw continuous, less than half it's rated intermittent current. The victron doesn't give any stats on continuous current so no telling if it can do 12kw non stop. But you say this brand can do the full current non stop without any trouble?

 

[Solar Guppy]

But you say this brand can do the full current non stop without any trouble?

Not me, the specifications list clearly it is full VA rated to deliver to single "phase" you can also look at the weight difference. I have had it running for 2+ years, with any concerns.

There is one down side and it is in the Solis 5G HVES specifications when having an auto transformer on the output of the inverter, the grid to backup switching takes a bit longer as the inverter has to deal with the winding inductance as it switches to internal voltage output, I see typically 60-80 milliseconds ( specification is 200 ms maximum ) to get a stable sinewave on the power failure. It's not a Solis thing, all inverters that use an auto transformer have this "issue".

This is why inverters that do split phase with internally a pair of inverters can have ups like switching ( 20 ms ), so pick your poison, single leg overloads well below the rated output is reached OR slower switching on grid power failure ( we talking about 10's of milliseconds added ). I prefer the later so one gets full rated output regardless of the phase distribution.

I have a few small low cost ups's on important circuits ( computers, NAS server, security DVR ) to deal with the longer switch times

 

[Vigo]

Most of what are called 5kVA autotransformers are actually 2.5 kVA autotransformers. A 28 lbs 60Hz transformer is a 2.5 kVA autotransformer.

Transformers can take a 2x overload for about 20-30 minutes before you can boil water on them.

Great info.

I actually repurposed (and hardwired) a large "5000 watt" travel converter (i.e. portable autotransformer in a box with outlets and a carrying handle) to serve as the autotransformer in my system because A: it was cheap, and B: the growatt version was not yet released and other cheap over the counter solutions were out of stock. This thing was ~$120 on Amazon with one day shipping.

In my case i just added PC case fans and a small temp controller to mine to keep it under ~50c. I suspect that's extremely conservative but I also dont like anything in that 'power room' space to be so hot that i can't keep my hand on it, so it's admittedly just a preference.

The weight and time figures you posted are very helpful context for someone trying to figure out what an autotransformer's 'real' capabilities are.