IEEE 388-1992 (R2007) pdf free.IEEE Standard for Transformers and Inductors in Electronic Power Conversion Equipment.
IEEE 388 Ratio of Transformation.The ratio of transformation of two magnetically coupled wind-ings is the transformer parameter determined primarily by the ratio of the number of turns ineach winding.By convention, it is measured as the ratio of voltages induced across each wind-ing by a common exciting current, with the windings connected “series aiding.”(See the com-mentary under A1.1.) Thus, the ratio of transformation can be described as the forwardvoltage transfer ratio of the transformer with the currents in the two windings being identical.
For an ideal transformer with windings of N and N turns, the ratio of tran sformation equalsthe turns ratio.
Due to the relationship between the number of turns and the inductance of windings, theratio of tran sfor m ation, in an ideal tran sformer, is also equal to the square root of the windingself-inductances.
These relationships are valid only where the coefficient of coupling is unity and the resistivecomponents of winding impedances have the same ratio as the square of the turns ratio.(Thisis also used in the tran sformer equivalents to represent the ideal section of the transformercoil.)
For more details, see IEEE Std 389-1990[6].
The recommended method for ratio of tran sformation measurements is the bridge circuitshown in the Fig A2.The two windings under test are series aiding connected to permit thesame current to pass through both, with the variable resistance, R, used to equalize the phaseangle of the winding impedances. Then, the voltages appearing across the two windings canbe compared using potentiometric methods. The current through the windings must be lim-ited to make the core and copper losses negligible.
Inductance (Impedance)Unbalance.Inductance unbalance is the percent deviation inthe inductance of two magnetically coupled windings and is normally specified for windingswith equal numbers of turns.
For an ideal transfor mer, i.e.,when the coefficient of coupling between the two windingsequals (or is approximately) one, the inductance unbalance is equal to the unbalance in turnssquared.IEEE 388 pdf download.