Question

A 50 kVA, 440 V, 3-phase induction motor is provided with a 3-phase step down auto transformer starter which steps down the voltage to 50% of the input. Given that the starting current of the motor on rated voltage is 6 times the rated full load current. The starting kVA drawing by the auto transformer is

• A. 70 kVA
• B. 50 kVA
• C. 78 kVA
• D. 75 kVA

Answer 1

The correct option is D 75 kVA

$\text{Given,}{I}_{st}=6\times I_{fL}$

$I_{fL}=\frac{\text{Power in kVA}}{\sqrt{3}\times V_L}=\frac{50\times {10}^3}{\sqrt{3}\times 440}=65.608A$

Current drawn by auto transformer from the mains at starting
$I_{st}=x^2{I}_{st\left(motor\right)}=x^2\times 6\times I_{fl}$
$=(0.5{)}^2\times 6\times 65.608$
$=98.412A$

Starting kVA drawn by the auto transformer
$=\sqrt{3}{V}_L{I}_{st\left(auto\right)}$
$=\sqrt{3}\times 440\times 98.412$
$=75kVA$