For the reasons for the requirement of insulation, the low voltage winding is placed near the core while the HV winding is placed outside. Hence practically it is easier and simpler to provide tapings on high-voltage winding.
Why are transformer windings tapped in the middle?
Electrical connection-wise, tapings are provided on HV winding at its ends. However, the physical placement of these tapped ends is done such that they are positioned in the middle. This is done to eliminate axial forces.
There are two forces at play because of the interaction between the two windings: radial and axial.
The radial force which is bursting in nature and acts outwards does not compress the two windings since they carry currents in opposite directions. The radial force is taken care of by mass inertia of winding and using proper mechanical bracing. Unbalanced axial force can cause dislocation of winding along the core limb especially during inrush and short circuit conditions because of large mmf to unbalance.
If tapped ends of HV winding are somehow physically positioned at the center, the axial forces F1 and F2 acting on the tapped HV winding become balanced, and the problem is resolved. This is because the number of active parts of HV windings are present in almost equal amounts above and below the tapped ends. Hence, F1≈F2 is maintained for all tap
In the case of a step-down transformer, it is an added advantage to provide tapings on the HV side. At light loads, the lv side voltage increases. It is required to decrease this voltage by adjusting the tapping on the HV side to a position where the number of turns is large. With a large number of turns, the flux and flux density increase.
Tapped transformers are used to get different voltages from the output terminal. Tapings are done on the secondary side of the transformer or at the load side of the transformer. Tapings are also done in autotransformer. tapping can be changed during on-load operation and during off-load operation also. the secondary winding is tapped at the middle to make the axial and radial force equal at two halves of the center tapped winding and create the mechanical balance of the secondary winding. different tapings provide different currents at the load terminal