Question

A 400 V, 50 Hz, 4 pole, 1440 rpm star connected squirrel cage induction motor is controlled by V/f control and frequency is reduced to 30 Hz. The percentage slip at 30 Hz frequency is _______%

• A. 6.67

Answer 1

The correct option is A 6.67
$N_s=\frac{120f}{P}=\frac{120\times 50}{4}=1500rpm$

$s=\frac{N_s-N_r}{N_s}=\frac{1500-1440}{1500}$

$s=\frac{60}{1500}=4\%$

$N_s=\frac{120f}{P}=1500rpm$

$s{N}_s=60rpm$

$N_s$ at $30Hz=\frac{120\times 30}{4}=900rpm$

$\text{The slip speed}\left(s{N}_s\right)\text{remains constant in}V/f\text{control}$

$\therefore N_r=900-60=840rpm$

$\text{Slip}=\frac{60}{900}\times 100=\frac{20}{3}$

$=6.67\%$