My question is the star point common for the line and load side of the neutral. I'm not 100% sure on this one.
I've since managed to make it back and connect the input side neutral just to be sure!
Cheers for the response
If you have 3 windings connected in star on a 3-phase 400 V supply; there can only be 230 V (approx) across each of them.
The 400 V is between phases, not from each phase to earth. L-E voltage is 230 V.
Therefore the line terminals should be labelled as 230 V, not 400 V
Any tappings on the windings will be at less than 230 V - presumably in this case 110 V (not 230 V as shown).
Without the N connection from supply to star point, the load currents have to be considered as combinations of loads in series between each pair of phases. Which is fine as long as the load between each pair of phases is the same.
But if there is any variation - for example, having a number of single-phase loads;
variations in connected load on any phase will result in the actual voltage applied to the equipment varying; up to a maximum 400 V worst case.
Plenty of cases where failing to recognise this has resulted in damaged equipment.
OP stated the load is unbalanced; therefore connecting the incoming N is crucial.
I hope that you have worked through the fault and SC protection requirements for the 110 V circuits;
ie can the tx supply enough fault current for operation within time limits.
Maybe thinking about it this way?
400V phase to phase for primary.
230V phase to phase for secondary
115V phase to neutral secondary
Yes, the fault protection needs to be correct.
But for the unbalanced secondary load, it will be 115V load which was unbalanced, and having a secondary neutral connected to the star point should ensure that the voltage remains correct.
Or is my brain still in lockdown and I've totally screwed this up?
If it's 230V phase to phase on the secondary then it's 230 ÷ √3 to neutral which is 132V.
So I'm not sure why there's a neutral point at all in that case, or where the "unbalanced load" could come into it.
Hard to tell without seeing more of the wiring diagram.
Therefore will be designed for operation on 60 Hz supply.
auto transformer (any transformer) will provide 50Hz output; which may not be suitable.
The need for neutral reference becomes clear if you forget about the transformer altogether; and just consider a selection of single-phase loads on a three-phase supply.
With N, as loads vary (being switched on or off) the out-of-balance current flows in N, and voltage in each phase remains steady.
Without N, switch off all loads in one phase, and the loads on the other 2 phases become effectively 2 sets of loads in series on a 400 V supply.
The voltage across each lot of "single phase" loads will be proportional to the impedance of each. Only if both sets of loads are equal will the voltage across each group be the same, and as individual loads are switched on or off the voltages will be constantly varying.
With all three phases