Use Victron’s Auto Transformer for three main purposes: step up, step down, and split phase output balancing. While the step up and step down functions are straightforward, split phase balancing requires more attention.

Understanding Split Phase Balancing
Consider a 30A 120/240V split phase supply, which could come from the grid, a generator, or two stacked inverters. Some connected loads are 240V, while others are 120V. Each 120V leg should not exceed 30A. However, 120V loads on the two legs will often be unbalanced. For example, a 120V 1200W hairdryer will draw 10A from one leg, and a 120V washing machine could draw over 20A from one leg. This results in a current difference of 20A or more, meaning the 30A supply isn’t fully utilized. When one leg draws 30A, the other may only draw 10A, leading to an overload if the 240V load increases, while the other leg still has capacity.

Theoretical maximum power from a 30A 120/240V supply is 30 x 240 = 7.2 kVA. With a 20A unbalance, the practical maximum is 30 x 120 + 10 x 120 = 4.8 kVA, or 67% of the theoretical maximum.

The Autotransformer Solution
The solution is to use an autotransformer. By not using the neutral of the split phase supply and connecting an autotransformer to create a new neutral, load unbalance is absorbed. For a 30A supply, the load can be increased to 7.2 kVA. A 20A load unbalance will result in one leg supplying 40A and the other 20A. The 20A difference flows through the neutral and the autotransformer windings, balancing the current through both 120V wires to 30A each.

Ground Relay for Inverter/Chargers
When operating in inverter mode, the neutral output of the inverter/charger must be grounded to ensure proper GFCI function. In a split phase supply, the neutral must be grounded. A grounding relay built into the autotransformer’s enclosure, controlled by the 230/240V Multi or Quattro, achieves this (the internal grounding relay in the 230/240V Multi or Quattro must be disabled).

Temperature Protection
The autotransformer is temperature-protected and disconnects from the supply if it overheats. It requires manual reset.

Alternative to Stacked Inverters
Instead of stacking two 120V inverters for a 120/240V split phase supply, you can use a 240V inverter with an autotransformer. Two stacked 120V 3kVA inverters supply up to 25A to each 120V leg, but if one leg’s load is under 25A, the other leg’s maximum load is still limited to 25A.

A single 240V 5kVA inverter with a 32A autotransformer supplies up to 21A of balanced load to each 120V leg. If one leg has fewer loads, more power is available on the other leg, with a maximum unbalance of 32A. This allows up to 38.5A on one leg if the other leg has no more than 3.5A (maximum unbalance: 35A). For expected load unbalance, a lower power 240V inverter with an autotransformer is preferable to the stacked inverter solution.