Question

The rotor resistance and reactance per phase of a 4 pole, 50 Hz, 3-phase induction motor are 0.03 $\Omega$ and 0.18 $\Omega$ respectively. The value of additional rotor resistance/phase required to give 80% the maximum torque at starting is

• A. 0.06 $\Omega$
• B. 0.12 $\Omega$
• C. 0.09 $\Omega$
• D. 0.03 $\Omega$

Answer 1

The correct option is A 0.06 $\Omega$

$T_{st}=0.8T_{max}$
0.8 = $\frac{2s_{max}}{s_{max}^2+1}$
$s_{max}^2-\frac{2}{0.8}{s}_{max}+1=0$
$s_{max}^2-2.5s_{max}+1=0$
$s_{max}=0.5$
$\therefore$ $0.5=\frac{0.03+R_{ext}}{0.18}$
$0.09=0.03+R_{ext}$
$R_{ext}=0.09-0.03=0.06\Omega$