A 2500/250 V, 25 kVA transformer has a core loss of 130 W and full-load copper loss of 320 W. Calculate its efficiency at full load, 0.8 pf. The transformer is now connected as an autotransformer to give 2500/2750 V. Calculate its kVA rating and efficiency at full load, 0.8 pf. Compare with the two-winding kVA rating and efficiency.

(i) Two-winding Transformer:

Power output = 25 × 0.8 = 20 kW

Losses = 130 + 320 = 450 W

$\eta = \frac{20}{20.45}\times 100=97.87\mathrm{\%}$

(ii) Autotransformer: With reference to Fig. 8.29.

$I_2=\frac{25\times 1000}{250}=100\mathrm{A}$

$I_1-I_2 =\frac{25\times 1000}{2500}=10\mathrm{A}$

$\therefore I_1=110 \mathrm{A}$
kVA rating $=\frac{2500\times 110}{1000}=\frac{2750\times 100}{1000}=275$

Power output = 2.75 × 0.8 = 220 kW

$\eta =\frac{220}{220+0.45}=99.87\mathrm{\%}$

It is seen that when a two-winding transformer is connected as an autotransformer, its rating goes up from 25 kVA to 275 kVA and its efficiency from 97.8% to 99.8%.
This is possible because a large part of its kVA is transported conductively.