Phase rotation Beware of phase rotation between the inverter and AC in. When wired in the wrong rotation, the system will not accept the mains input and only operates in inverter mode. In that case swap two phases to correct it. A quick way to fix phase rotation is to swap 2 random phases and see if now the inverter system will accept AC in. In case the system is mobile it is likely that, at some point, there will be a generator or grid connection with incorrectly wired phase rotation and the inverter/charger system will reject the input and stay in inverter mode, consequently draining the batteries. Mounting a simple changeover switch that can swap two of the phases is a nice solution which instantly fixes the phase-rotation issue, without stalling the event. Besides manual switching there are also automatic devices available to do this. AC input and AC output cabling Do not over-dimension the AC cabling. Using extra thick cabling has negative side effects. Technical background: In a parallel system the AC current should be evenly distributed through all paralleled units. But small differences in the internal resistance of the back-feed contactors in each inverter/charger may result in AC current being diverted from one unit to another. When the resistance in the cabling is very low, a small difference in resistance results in a large relative difference. And this results in bad current distribution.
An exaggerated example: To units are connected in parallel, Unit A and unit B. If extremely thick and short cabling is used, one might achieve a very low wiring resistance. But a slight internal resistance difference might exist between the two units back feed contactors. In this scenario a total resistance for Unit A is 0.1 mΩ and a total resistance for Unit B is 0.2mΩ. This will result in Unit A carrying twice as much current as Unit B although the resistance difference is very small. Now if we use the same 2 units in parallel and we use thinner and longer cables one might end up with a total resistance for Unit A of 15Ω and a total resistance for Unit B of 16Ω. This will result in a much better current distribution (Unit A will carry 1.066 times more current than Unit A) even if the absolute difference in resistance is much bigger than in the previous example (1Ω vs 0.1 mΩ). Recommendation : Use AC long cables of similar length. Look in the manual for recommended AC output cable lengths Do not increase the cross section of the AC cabling more than is recommended in the manual!
For example: the voltage drop tolerance of a 100A back feed contactor is about 20mV at 100A. The total cable resistance (input + output) should therefore be larger than R = 60mV/100A = 6mΩ. System check: The best way to check for these type of wiring issues is to fully load the system and then measure (current clamp) each units individual AC currents. They should very similar. If there are big differences, then there is an issue with wiring (or with a connection).
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