Just checked out a 1:1 600VA isolation transformer. The no load is a boost of 1.03 on the secondary. Check the output voltage on an old school AC adapter if you have one around. The no load voltage on those can be high by as much as 50%. Load it down to the rated amp and the voltage drops down to something in line with the rated voltage. Step load regulation is more pronounced on tiny transformers. It's for this reason one neighbor's AC is much more likely to cause another neighbor's lights to dim noticeably every time their stat cycles when they're in a rural area that is fed from a tiny pole piglet.
In theory transformers conform exactly to the ratio but out in the real world, they're made of real world materials with resistance and needs compensation. We regularly ignore things like air friction, wiring resistance and such in the classroom for the sake of simplicity so the focus remains on the principle. If the wire you wind around a nail was an ideal coil, you could attach a battery, then short the wires together and it would stay on indefinitely like an MRI machine.
This surprised me a bit at first too but the worst inrush occurs when its switched on at zero crossing and the least when its turned on at the peak.
Last edited by Electric-Light; 01-11-17 at 03:21 PM.
Returning to you original query... "Impact of reverse-feeding on transformer losses!" Perhaps my answer using a single-phase xfmr was too elementary for some of the posters! So, following is additional detail.
While the Xfmr is considered a ‘simple’ electrical apparatus, the mfg’r has practical problems to consider. For example, consider a 3-core (or 3-limb or 3-leg) xfmr! Most assume it is symmetrical regarding development of magnetic flux density and excitation current! The simple fact is flux-density in each of the 3-cores is uneven!
It is not by accident the Xfmr is described as having a primary-winding and secondary-winding! Then design parameters are directed toward having the supply-source connected to the primary-winding! Why?
Unmatching magnetic flux-densities in the 3-cores result in uneven induced-voltages in the secondary-winding! Then, the mfg’r, ‘adjusts’ the effective turns-ratio of each phase so that secondary output-voltages are better matched! Usually, the ‘adjustment’ is made to the turns-ratio in the winding having a smaller wire, hence, more latitude in location and accessibility!
Now you can see why reversing-feed could affect the primary-winding’s voltage if used as the output, resulting in the Caveat to contact the Mfg’r!
Reur comment on ‘Inrush’… I suggest searching MHF files! There is a wealth of info! Here’s a hint… determining ’Inrush-current’ is not simple. The process is quite complicated! Any ‘Inrush-current’ problem must consider two components… the transient-period (the first few cycles, including the first-cycle-peak) and then the steady-state region (a few to many seconds)! Furthermore, the 3-phase currents never coincide together!
Gentle people! Please do not misconstrue the above as Xmrs-101!
Regards, Phil Corso
Last edited by Phil Corso; 01-11-17 at 03:53 PM.