Question

The full load slip of 400 V, 3-phase, squirrel cage induction motor is 4%, full load current is circulated when 100 volts is applied between the lines with locked rotor. The necessary tapping on an auto transformer to limit the starting current to 1.5 times the full load current of the motor is _____%.

• A. 61.24

Answer 1

The correct option is A 61.24
$I_{sc}=\left(\frac{400}{100}\right)I_{fl}=4I_{fl}$

$I_{sc}$ is the current flowing through the winding when full rated voltage is applied in auto transformer starting,

$I_{st}=x^2{I}_{sc}$

$x^2\left(4I_{fl}\right)=1.5I_{fl}$

$x^2=\frac{1.5}{4}$

$x=\sqrt{\frac{1.5}{4}}$

$=0.6124or61.24\%$