Question

A 200/100 V, single-phase transformer is rated 10 kVA. This transformer is connected as an auto-transformer across a 50-V supply (the winding designed for 100 V should be connected across 50 V supply) for getting maximum output voltage. If the load current is now 8 A, then determine the input current of the auto transformer.

• A. 20 A
• B. 18 A
• C. 26 A
• D. 24 A

Answer 1

The correct option is D 24 A

$For1-\varphi T/F,given\frac{N_2}{N_1}=\frac{100}{200}=\frac{1}{2}$

For this single phase transformer, if we give 50 V supply for the winding designed for 100 V (i.e. low voltage side), then secondary voltage of $1-\varphi$ transformer is $\frac{2}{1}\times 50=100V$

The equivalent auto transformer for getting maximum output voltage is as shown below. For getting maximum output voltage, two windings should be connected with series aiding polarities.

$\therefore V_2=100+50=150V$

Given, load current = 8A

$\therefore OutputVA=150\times 8=1200VA$

Input current to the auto transformer is
$=\frac{1200}{50}=24A$