Victron Autotransformer 120/240VAC-32A

The autotransformer: for boosting, step-down and balancing of separate phases

An autotransformer can be used for voltage boosting, voltage reduction and balancing of separate phase output voltages. While the step-up and step-down functions are really simple, the balancing of separate phase output voltages may require a little more attention.

Consider for example a power supply with separate phases at 120/240V 30A. The power supply could be from the grid, from a generator or from two inverters on top of each other.
Some of the connected loads are 240V, others are 120V. On each of the 120V output strings the load should not exceed 30A. The problem is that as soon as the 120V loads are connected, the two outputs will have a different amperage current. This is because the 120V loads on the two output strings will never be balanced. A 1200W hair dryer at 120V, for example, will draw a current of 10A from one branch. A 120V clothes washer can draw even more than 20A from one branch. Between the two strings the current difference, or current unbalance, will therefore often be 20A or more. This means that the 30A supply will not be used to its limit. While 30A is drawn from one string, no more than 10A can be drawn from the other string, and increasing the load to 240V, for example, will result in an overload on one string while the other string still has spare capacity.

Theoretically, the total power that can be obtained from a 120/240V power supply at 30A is 30 x 240 = 7.2kVA. In case of a 20A unbalance, the maximum power in practice will be 30 x 120 + 10 x120 = 4.8kVA, or 67% of the theoretical maximum.

The solution is an autotransformer.

By leaving the neutral of the phase-separated power supply unused, and connecting an autotransformer to create a new neutral, as shown in Figure 1, any load imbalance is "absorbed" by the autotransformer. In the case of a 30A supply, the load can be increased to 7.2kVA, and an unbalance of 20A will result in one string drawing 40A and the other string 20A. The 20A difference will pass through the neutral and the autotransformer windings. The current through both wires of the 120V power supply with separate phases will be 30A.

Includes a grounding relay for use with Multi or Quattro type chargers/inverters.

When operating in inverter mode, the neutral output of the charger/inverter must be grounded to ensure proper operation of a GFCI. In case of a power supply with separate phases the neutral must be earthed. For this purpose a grounding relay is built into the autotransformer housing. The relay is controlled by the Multi or Quattro at 230/240V. (The internal earthing relay must be disconnected in the Multi or Quattro at 230/240V).

Protection against high temperatures

In case of excessive overheating, the autotransformer is disconnected from the power supply. Resetting is manual.

An alternative to overlapping inverters

The alternative to superimposing two 120V inverters to provide a 120/240V power supply with separate phases is a 240V inverter with an additional autotransformer. Two overlapping 120V 3kVA inverters will provide 25A to each 120V string. Even if the load on one string is less than 25A, the maximum load on the other string will be limited to 25A. A 5kVA 240V inverter with a 32A autotransformer will provide a balanced load of up to 21A to each of the 120V strings. A lower load on one string will however result in more power being available on the other string, with a maximum unbalance of 32A. Therefore, the load can be up to 38.5A on one string if the load is no more than 3.5A on the other string (maximum unbalance: 38.5 - 3.5 = 35A).

If an unbalance of loads is foreseeable, a lower power 240V inverter with autotransformer is therefore preferable to a superimposed inverter solution.