Firstly, it is considered a transformer that has a tap changer on the HV side, with a voltage source of 100 V on the HV.

If the tap is on the position where the primary turns are np=50 then the winding ratio is 50/10, or 5:1. The primary voltage is Vp=100 V and the secondary voltage is therefore Vs=100/5= 20V. If the tap is on the position where the primary turns are np=100 then the winding ratio is 100/10, or 10:1. The primary voltage is Vp=100 V and the secondary voltage is therefore Vs=100/10=10V.

Thus, for an HV tap the LV or secondary voltage decreases when the number of primary windings are increased. Secondly it is considered a transformer with the tap on the LV side. If the tap is on the position where the secondary turns are np=5 then the winding ratio is 100/5, or 20:1. The primary voltage is Vp=100 V and the secondary voltage is therefore Vs=100/20= 5V.

If the tap is on the position where the secondary turns are np=10 then the winding ratio is 100/10, or 10:1. The primary voltage is thus Vp=100 V and the secondary voltage is therefore Vs=100/10= 10V.

Thus, for an LV tap the LV or secondary voltage increases when the number of secondary windings are increased.