On load tap changer why does voltage change on the primary side?

On-load tap changers (or circuit tap changers), as the name suggests, permit tap changing and hence voltage regulation with the transformer on-load. Tap changing is usually done on the HV winding for two reasons:

1. Because the currents are lower, the tap changer contacts, leads, etc., can be smaller.
2. As the HV winding is wound outside the LV winding, it is easier to get the tapping connections out to the tap changer.

To avoid placing moving contacts in high current path. 

For ON Load Tap changer the current on the HV side is very low comparing to current on LV side. So while tap changing on Load conditions ARC may arise due to Contact changing and it's easy to control tap changing in low current side. So to achieve safe Tap changing operation without any ARCing or other hazards and to achieve proper regulation Tap changers are always connected in HV side than on LV side.

On high voltage/primary side, current is less.

The OLTC selector arrangement for 16 steps and 17 positions. In the diagram, the on load tap changer is connected to tapping 4 while changing from tap position 4 to tap position 5, M1 is opened first and this transfers the load current via A1 with resistor R1in series. Then A2 closed and two resistors R1 and R2 are in series across tapping 4 and 5. A circulating current will flow through these resistors because of step up voltage between tapping 4 and 5. After that A1 opens and interrupts the circulating current and the load current is transferred to tapping 5 passing through resistor R2. Finally M2closes and takes load current, completing the tap changing. For a tap changing in opposite direction, the sequence is reverse. When the tap changer has several tappings it is advantageous to halve the length of tapping winding and to introduce a reversing or changeover selector.

DUE TO THE CHANGE IN NO. OF TURNS OF COIL.

N1/N2 = V1/V2