Dear reader, I have a question about a single-phase auto transformer which is connected to a three-phase electricity grid. The windings of the auto transformer are on a common iron package.

The circuit is planned to be used in a solar power wall of a house. In normal operation, figure A applies, see below. If the grid fails then the inverter of the power wall takes over and will supply energy from batteries and then figure B applies.

Figure A shows how the auto-transformer is connected to the three-phase grid. The auto-transformer gives a 2:1 centertap and divides the L1-L3 phase voltage of 220(380) volts into two. This type of transformer is also called a SplitPhase transformer. I need to connect the centre tap of the auto-transformer to the neutral of the three-phase grid or star point.I have included the values of the voltages for 127V and 220V grids in drawing A.

Figure B shows how the hybrid inverter takes over from the power grid when it fails. The grid is then isolated from the Inverter and the house using an Auto-Tranfer-Switch (ATS) and the Inverter then starts supplying power to a part of the house.

Here is my question en doubt: When I connect the center tap of the auto-transformer to the neutral of the three-phase grid I have a problem explaining the voltages. The three-phase grid tells me that between the neutral and L1 there should be a voltage of 127 (220) volts, but according to the operation of the auto-transformer I would expect 110 (190) volts there.If the voltage were to follow the phases then in the winding the magnetic flux should differ in phase but this cannot be done in a common iron package.What mistake am I making here or am I blowing up an auto transformer here.

I hope someone can explain to me what is happening here and hopefully explain me that nothing is going wrong.


(sorry the picture is in Dutch but i assume it will explain it self.

"Driefasen net met auto transformator" means: Three Phase grid with auto Transformer

"Inverter als energie bron met de auto transformator" means: Inverter as energy source with auto transformer.)

  • \$\begingroup\$ Please explain where the pictures came from. Are you copying them from somewhere or is this a circuit proposal? Why do you state two voltages: 127 (220)? What does this mean? What is L2 meant to be connected to? \$\endgroup\$
    – Andy aka
    Commented Oct 7, 2023 at 18:11
  • \$\begingroup\$ @Andyaka L2 is Sir Not Appearing in this Diagram. It exists because we have 3-phase distribution, but it's used somewhere else. \$\endgroup\$
    – hobbs
    Commented Oct 7, 2023 at 18:31
  • \$\begingroup\$ Andy Aka I have the original of the picture, I made it last night. The two voltage are for existing grid voltages in the world Uses 220V US and Latin America uses 127V. Since I don't know who read this post, I added the values for the two grid types. \$\endgroup\$
    – wogoos
    Commented Oct 7, 2023 at 18:51
  • \$\begingroup\$ @hobbs I see an L1-L2-L3 indicating the three phases of the gird \$\endgroup\$
    – wogoos
    Commented Oct 7, 2023 at 18:53

1 Answer 1


N of the supply is at the center of all three phases, L1, L2, and L3.

The center tap of the autotransformer (let's call it C instead of N) is halfway between L1 and L3, which is not the same thing. If we say that L1 is 0°, L2 is 120°, and L3 is 240°, then (L1 + L3) / 2 is at 300°. That's 180° away from L2 (the phase you didn't use), and its magnitude is half the magnitude of a single phase. If |L1-N| is 127VAC, then |C-N| is 63.5VAC.

So if we remove the wire in your diagram between C and N, those two points differ by 63.5VAC. If we connect that wire, the transformer tries to supply a ton of reactive current to pull N to C, and then it overheats and catches fire.

You haven't said exactly what the application is here (looks like whole-house backup to me), but I don't think it makes any sense. You could simply disconnect the two neutrals, and then the transformer wouldn't blow up, but if we assume that N is grounded, then that means that C can't be grounded, and there's a dangerous potential between your house's "neutral" and ground.

In North America, where split-phase residential power is commonly used with 3-phase distribution, the transformers used are isolation transformers, not autotransformers (they're also usually step-down transformers, but that's beside the point). Since the two sides of the transformer are isolated, the center tap of the output can be safely grounded, and then the two output phases are symmetrical with respect to ground.

  • \$\begingroup\$ Hi Hobbs, Yes you are right is a Huawei backup system for a three phase house in mexico city. The problem is that they ordered a single phase system. The hybrid Inverter has a 240V floating output which in normal mode is connected with L1 And L2 of the 3phase network. Till here that not a problem.When there is a power outage a \$\endgroup\$
    – wogoos
    Commented Oct 7, 2023 at 22:38
  • \$\begingroup\$ Continue ..... When there is a power outage a so called Backup Box filled with Relays :-( and a timer will isolate the L1 and L2 of the house from the gird. Then the Inverter kicks in and starts feeding assigned groups in the house using on batteries with 240V. Here the problem starts With a spiltphase tansformer I can create the 2x120V but I also need to use the Center point of the transform to act as a neutral for the house, If I hard wire this center point to the neutral of the grid I have that voltage conflict. \$\endgroup\$
    – wogoos
    Commented Oct 7, 2023 at 22:49

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