Question

$A100kVA,1000V/400V,1-\varphi$ transformer when excited at rated voltage on h. v. side, draws a no load current of 3 A at 0.75 lagging power factor. If it is excited from the l.v. side at rated voltage, the no load current and power factor will be respectively,

• A. 7.5 A, 0.75 lag
• B. 6.5 A, 0.75 lag
• C. 7.5 A, 0.6 lag
• D. 6.5 A, 0.70 lag

Answer 1

The correct option is A 7.5 A, 0.75 lag

$\text{When h.v. side is excited,}$
$\text{core loss}$ $=VIcos\varphi$
$=1000\times 3\times 0.75=2250W$

$\text{When l.v. side is excited,}$
$\text{core loss}$ $=VIcos\varphi =2250W$

$I=\frac{2250}{V\times cos\varphi }=\frac{2250}{400\times 0.75}=7.5A$

$\text{Power factor is same for a transformer.}$