On constant flux tap changer method the tap changer is located on high voltage side –connected to high voltage windings.The number of high voltage winding turns –including tap changer winding- is changed so that voltage per turn remains constant.
On variable flux tap changer method the tap changer is located on low voltage side-connected to low voltage winding.As primary voltage increase the voltage per turn will increase also.Then, in order to maintain the low voltage constant one has to decrease the number of low voltage turns per windings.
The advantages of this method are:
The level of insulation for tapped winding and for tap changer is low.
A Wye connected three-phase tap changer can be used.
The current in common winding is lower than current in LV line terminal ,so a smaller tap changer can be selected.
The disadvantage is that as the losses, exciting current,impedance and the sound level vary with the tap position a bigger core must be selected and the transformer is bigger and heavier.
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