Your question is rather vague. If you are asking what do you call a group of single-phase Transformers, connected to supply three phase, then the answer is a 'three-phase transformer bank'.
By having a transformer with 3 phase input and single phase out put
This depends on the voltage, and whether it is a three phase or single phase transformer.
Harmonics are really not needed in single phase transformers.
Yes, you can use three single phase transformers on three phase applications and interconnect them to serve as a single transformer or you could use one single phase transformer fed from two of the three lines in a step down application.
A single-phase transformer works with a single-phase supply, while a 3-phase transformer is used with a 3-phase supply. A single-phase transformer has 2 wires on the primary and secondary (ignoring taps) while a 3-phase tansformer has 3 or 4 wires on the primary and secondary.
The XO denotes the joined secondary terminal connection of three single coils of a three phase transformer. It is the common return on any unbalanced current of a three phase system.
By design are you going to wind the transformer yourself? In your design you need a 5:1 ratio. On the output side of the transformer any two legs of a three phase transformer is considered single phase voltage. Good luck on your project.
If it's a split-phase system with centre-tapped transformer, then two ammeters are needed, one on each side.
with an meg ohm meter
A three-phase transformer bank is often used in power stations because it is easier to construct and transport very large single-phase transformers, compared with constructing and transforming an equivalent-capacity three-phase transformer.
..the questions does not say wether the 30kVA transformer is 3 phase transformer or single phase transformer..but it is implying that a single phase welding load at 16A per phase is to be connect to it, it is assumed then that the transformer is 3 phase transformer..we assume load is rated 240V.. ..though not much details is given about the transformer voltage specs. but if is rated 3 phase 415/240V, the approx full load current per phase is given by 30KVAx1.3912=41.736A.. ..but if the transformer was rated single phase 240V say, the approx full load current FLC=30000/240V gives 125ampers.. ..if you further devide the FLC by the intended load current.. ..Recommended no of welders per for a 3 phase 415V transformer and assuming single welding sets =41.736/16=2.6..so you can connect max two welding sets to this transformer per phase,..max 6 weld sets can connect at an approx load factor of 76%.. ..for the single phase transformer, FLC/16=125/16, gives 7.8 but is also recommended not to connect more than 6 welding sets to such transformer for the same reasons..
If the loads are balanced, the neutral conductor in a single-phase 240V system with a center-tapped transformer would carry no current, as the currents flowing in opposite directions would cancel each other out. If the loads are unbalanced, the neutral conductor would carry the difference in currents between the two lines.