Question

A single-phase, 50 kVA, 2200 V/400 V, 50 Hz transformer is tested for open and short-circuit test with below shown inferences:
Open circuit test : hv open circuited
V = 400 V, I = 1.2 A, W = 240 W
Short circuit test : Iv side short circuited
V= 100 V, I = rated current, W = 280 W
The value of half full load efficiency of transformer operating at unity power factor will be ____%
(upto 2 decimal place).

• A. 98.78

Answer 1

The correct option is A 98.78
$\text{For open circuit test,}{P}_{oc}=P_i=240W$

$\text{For short circuit test,}{P}_{cu}=280W$

$P_{cu}\text{at half load,}{P}_{cu\left(half\right)}={\left(\frac{1}{2}\right)}^2\times 280=70W$

$\therefore \text{Efficiency at half load}$,

${\eta }_{1/2}=\frac{50000\times 0.5}{50000\times 0.5+losses}$

$\text{losses = 70 + 240 = 310}$

${\eta }_{1/2}=\frac{25000}{25000+310}=0.9878or98.78\%$