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 = AH represents 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=

=v_rcosθ±v_xsinθ (1.42)

where

and