Auto isolation transformer boosting an already high voltage

DIY
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I have a Victron system on my sailboat that includes an auto switching isolation transformer. The boat is a 230V/50Hz boat. Power is coming into the shore power connecter at 127V, being doubled, plus the extra 5% provided by the isolation transformer, to become 267V.

The voltage is briefly observed to be over 270V by our dual Multiplus configuration, so the switch-over to shore power does not happen.

While the intent of boosting 5% is a excellent solution in a lot of cases, it is a killer in other uses where incoming voltage is already higher than it should be.

If this were not a smart, auto switching transformer, swapping connections around to instead create a 5% buck would be straightforward. Since this model is more complex, including the ability to sense and adapt to the incoming voltage as well as use a soft-start feature (which probably doesn’t actually do anything when feeding a MultiPlus), that change is not so obvious, especially without a real schematic.

If the toroid transformer inside has a dual primary windings and dual secondary winding (with 5% more number of turns, swapping each input wire for each corresponding output wire should accomplish what I’m looking for. But, again, without having the schematic, it is not possible to know for sure.

Can anyone help on this topic so I can correctly change my autotransformer to a 5% buck?

Thanks,
Mike

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Okay, some more internet sleuthing and measurements of my isolation transformer has confirmed what I have been thinking. To change from a Boosting (+4.3%) configuration to a Bucking (-4.3%) configuration, the following changes are required within the case.

There are two sets of 4 wires, covered in white anti-chafe insulation. On the left side of the enclosure are two blues and two browns, labeled (J18, J15) and (J1x, J11). Each Brown and Blue make up a coil with a number of windings equal to some number ‘N’, each.
On the right side are two orange and two black, labeled (J30, J27) and (J28, J26). Each red and black pair make up a coil with 1.0435N windings each.

Since the intended output is known, the black jumpers are connected as per the manual to make the winding connections in order to get the desired output.
J29 is the “top” end of output coil number 1, routed on board to the Output Line connection
J25 is the “bottom” end of output coil number 2, routed on board to the Output Neutral connection.

When you want 230V output, you short together J31 and J32,
J31 is the “bottom” end of the output coil number 1.
J32 is the “top” end of the output coil number 2.
This effectively creates one “long” secondary coil that has 2.087*N windings.

When you want 115V output, you short together J31-J25 and J29-J32.
This is placing the two output coils in parallel with each other, effectively creating one “thicker” secondary coil hat have 1.0435*N windings (and can handle twice the current).

The beauty of this device is that you don’t need to know the incoming voltage, the device will figure this out for you. So, those same type of connections (making one long primary coil with 2*N windings, or one thicker primary coil with N windings) are being figure out and then made via relays on the control board.

In the central lower section of the control board is a wide-input range offline power supply (runs off AC) and likely some voltage comparators that are used to determine if the input is in the “good” low or “good” high range, and light LEDs and switch relays appropriately.

All that so say, that the reversing the connections of the the toroid transformer will not affect any of the operations of the unit.

So, my research (and years of electrical engineering experiences) leads me to believe that to change this device to be a ~-5% buck configuration, the following wires should be swapped:

J18 ↔ J30
J15 ↔ J27
J16 ↔ J28
J11 ↔ J26

This would result in the wire numbering, from left to right to be:
J30 J27 J28 J26 J18 J15 J16 J11

Normal (Boost) configuration:

image
image1280×960 141 KB

Altered (Buck) configuration:

image
image1920×2292 371 KB

Can anyone confirm or refute these findings?
Mike

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Well, this change has been made and has been working great for 2 weeks now and solved the problem of having too high a voltage on the input to the inverter/chargers. I plan on making a video about this and if I do will post a link here.

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Did you ever make a video of this?

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Hi bob,

Video is recorded and I just have to edit it. I have that on my short list. I will also add an entry on my blog with a link to download the document to attach to the inside of the transformer so future you (or the next boat owner) knows what’s going on.

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