The turn ratio measurement can be made using two different methods;
a. Bridge method
b. By measuring the voltage ratios of the windings
Measurement of turn ratio is based on, applying a phase voltage to one of the windings using a bridge (equipment) and measuring the ratio of the induced voltage at the bridge. The measurements are repeated in all phases and at all tap positions, sequentially. During measurement, only the turn ratio between the winding couples which have the same magnetic flux can be measured, which means the
turn ratio between the winding couples which have the parallel vectors in the vector diagram can be measured.
Theoretical turn ratio = HV winding voltage / LV winding voltage The theoretical no-load turn ratio of the transformer is adjusted on the equipment by an adjustable transformer, it is changed until a balance occurs on the % error indicator. The value read on this error
indicator shows the deviatons of the transformer from real turn ratio as % .
The voltages at the winding couples to be measured can be measured at the same time and the ratio can be determined, or digital instruments which are manufactured for this purpose can be used in the voltage ratio measurement method. By using such instruments, in addition to measuring the turn ratio, also determining the connection group (with three-phase measuring instrument) and measuring
the currents during measurement are also possible. The method of comparing the vector couple voltages also allows measuring the angle (phase slip) between vectors at the same time The no-load deviation of the turn ratios should be ≤ % 0,5 .