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Hooking up inverter to center tap transformer
#1
Just hypothetically...

Would it be possible to hook up a European 230-240v inverter to a center tap transformer to get 2x 115-120v (each 180 out of phase)?  I'm not planning on doing this I'm just wondering out of curiosity if it's possible for a home user to do practically (and without spending outrageously) since the power company seems to have no problem doing this at the pole outside my house.
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#2
Yes, that would work just fine. There's someone here that actually got an old large transformer (in the 10's of kW if I remember correctly) to do just that. The transformer is similar to the ones that are hung on the pole in shape, but scaled down to about 2ft tall. I don't recall who it was, though.

The biggest issue with doing this though is that you will need to make sure to try to keep both 120V legs balanced with each other as much as you can. Don't load down 1 leg with 30A and the other 5A and run it that way for prolonged periods. This will cause the transformer to overheat as only "part" of it is functioning at high output. The overheating won't happen evenly and could cause warping or short circuit. It might take a long while for this to happen, but why kill a perfectly good transformer? Unless it's "for science" and documenting the process of what happens Wink
(the above amps were an example. Large deviations of amps between the legs is the point)

And, I know there's more to it than just this simple, but this is just the quick answer.
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#3
And I'm guessing the utility (FPL for both of us probably) doesn't have to worry about this cause of the statistical probabilities that thousands of customer devices usage will balance each other out on both legs?
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#4
At the utility side, they alternate who's on which phase. This is why you could have power and your neighbor next door or 2 down the street can have power.

And yes, because there are so many, the loads aren't as much of an issue, and they are operating at a voltage of at least 14,400 volts. It's not really the volts causing the imbalancing heat, but the amps.
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#5
The USA type centre tapped transformers high voltage side is a single winding, single phase connection.
The low voltage (house side) is effectively one continuous winding with a centre tap.
So that's how both of these low side windings are 180 degrees out of phase as you mentioned (not like 3 phase where they're each 120 deg out).
Like Korishan said it's good practice to balance your house's loads across the two windings as this affects the flux & heating in the windings.
Yes IMHO you could feed a grid-tie type inverter (theoretically anyway, supply co. permitting, etc) into the transformer.
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#6
Yes, I didn't clarify that the transformer was only using 1 leg from mains, thanks Redpacket.

But to hook up a transformer to the inverter, you'd use both hot legs, one on each end of the primary winding.

Ohhhh, also to note, the transformer for doing this needs to be a 1:1 winding, not a 1:2. The "center" tap would be the 1:2 winding. Otherwise you won't get 2x 120V on the output secondary, you'll get 1x 120V with a 60V center. (or is that 2:1 winding?)
The reason is so that you can still have 240V on the secondary winding to power your heavy loads (dryer, electric water heater, etc)
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#7
(01-31-2019, 12:47 PM)Korishan Wrote: Yes, I didn't clarify that the transformer was only using 1 leg from mains, thanks Redpacket.

But to hook up a transformer to the inverter, you'd use both hot legs, one on each end of the primary winding.

Ohhhh, also to note, the transformer for doing this needs to be a 1:1 winding, not a 1:2.  The "center" tap would be the 1:2 winding. Otherwise you won't get 2x 120V on the output secondary, you'll get 1x 120V with a 60V center. (or is that 2:1 winding?)
The reason is so that you can still have 240V on the secondary winding to power your heavy loads (dryer, electric water heater, etc)

Do they even do 1x 120V hot to hot with a 60V centre??? Man you've got some complications compared to our 240V down here!

Yep agree, both hot legs to 240V inverter. 

The inverters output needs to be isolated as well, not ground referenced/connected at all.
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#8
Probably not generally made that way. I figured I'd mention that in case the OP was making the transformer.
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#9
Connecting a large transformer to an inverter will possibly blow the inverter as the inrush when the transformer is first connected can be very high, just like connecting an AC motor.

If you get a transformer with two separate 120V secondaries (not center tapped) then you can also join one pair of wires together to get a center tap (check them so they are out of phase not in phase as they connection would otherwise be in parallel...
Primary : 240V 0V
Secondary : 120V 0V 120V 0V (isolated secondaries - can be put in series or parallel)
Secondary : 120V 0V 120V (auto transformer secondary)

"This will cause the transformer to overheat as only "part" of it is functioning at high output." : not entirely correct, as long as the wires are not overloaded either phase can be run 100%. The issue with running them 100% is a factor of the I^2 R losses unless your running it at a too high voltage and the saturation losses will then also add to the heating..

The issue over grounding is partly resolved by the isolation of primary/secondary in the transformer, it will not work with a full autotransformer and the result may be your battery -ve wire could end up at 120V if the 0V export line of the inverter is grounded to the case..
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#10
No I'm not planning on making any transformer. Just interested from an academic curiosity perspective. Thanks guys.
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