Total Approximate Voltage Drop Of A Transformer
During no-load condition, induced voltages at the primary and secondary windings are equal to the applied voltage and secondary terminal voltage respectively. If 0V2 be the secondary terminal voltage at no load, we can write E2 = 0V2. Let V2 be the secondary voltage on load. Figure 1.40 shows the phasor diagram of a transformer referred to as secondary.
In Figure 1.40, R02 and X02 are the equivalent resistance and reactance of the transformer, respec-tively, referred to as secondary side. With O as centre, an arc is drawn in Figure 1.40, which intersects the extended OA at H. From C, a perpendicular is drawn on OH, which intersects it at G. Now AC = AHrepresents the actual drop and AG represents the approximate voltage drop. BF is drawn perpendicular to OH. BE is drawn parallel to AG, which is equal to FG.
The approximate voltage drop
= AG = AF+ FG = AF+ BE
= I2R02cosθ+I2X02sinθ (1.39)
Figure 1.40 Phasor Diagram of a Transformer Referred to as Secondary
This approximate voltage drop shown in Equation (1.39) is for lagging power factor only.
For leading power factor, the approximate voltage drop will be
= I2R02cosθ–I2X02sinθ (1.40)
where ‘+’ sign is for lagging power factor and ‘-’ sign is for leading power factor.
The above calculation is referred to as secondary. It may be noted that voltage drop referred to as primary is
I1R01cosθ±I1X01sinθ (1.41)
∴% voltage drop in secondary is= 
=vrcosθ±vxsinθ (1.42)
where
and 