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dennisuello avatar image

Quattro/autotransformer for off-grid system in US

I am in the process of designing my off-grid power system. PV will be the main source, with a backup generator. I have mostly figured out the PV side, using SmartSolar 150/85 charge controllers. I will have a shop, with a small apartment, and a house will be build a few years later.

I need 240V split-phase to power my shop equipment (welder, plasma cutter and air compressor). Two of them might be running at the same time, so I need quite a bit of surge capacity, as I also will have a 240V well pump. I am looking at the following:

Quattro 48/15000/200-100/100 230V (set 240V/60Hz)

into 100A Autotransformer feeding my main panel in the shop and then eventually feeding a subpanel in the future house.

I'm just trying to run this by a few knowledgeable people before I write the check.

Another question I had, does anyone know how efficient is the autotransformer? What's it power consumption?

MultiPlus Quattro Inverter ChargerAutotransformer
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2 Answers
wkirby avatar image
wkirby answered ·

There are quite a few things to think about here.
How will your loads be distributed over the two phases? The Autotransformer has a passthrough current of 100A, but the neutral current is limited to 28A continuously. So if your 120V loads are balances well across the two phases then your neutral current will be lower.

If you have a lot of 240V tools, then no neutral current is created in the Autotransformer and the power passes right through it.
A toroidal transformer of the size used in the Victron Autotransformer is in the order of about 98% efficient.

There is not much to go on without specifics of the tools you are using. Compressors have high startup currents, but how much varies my the size and type of compressor. I can guess that it is almost a certainty that the compressor will start while plasma cutting, so that needs to be considered. Again, how powerful is the plasma cutter and also the welder. Would another person be using the welder while the plasma cutter and compressor are running?

More questions than answers, but it's really difficult to say yes or no without gathering the specific information about the tools.

My small 3KVA Multiplus works just fine for me in my shop, but I only have a small lathe, milling machine, compressor and drilling machine.

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dennisuello answered ·

Then what really is the benefit of an autotransformer, vs. just getting two 5k Quattros and create a split phase 240v directly from them. I know I'd have to be careful about balancing loads between legs in the panels.

Currently, my shop is running fine off a subpanel, that's fused at 50Amps at the main. I've never tripped that breaker. Plasma and air compressor absolutely run at the same time, most of the time I deal with metal 3/16th or thinner, so I'm never close to the max of the machine. It's just me in the shop, so only lights would be on. However, in this off-grid set-up, I would also be powering my house from the same inverter, so I'm trying to up-size it so I never have to worry about inverter shutting down in the middle of a big CNC plasma table job.

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