The rotor resistance and standstill reactance per phase of a 3-phase slip-ring induction motor are 0.5Ω and 1Ω respectively. What should be the Value of external resistance per phase to be inserted in the rotor circuit to give maximum torque at starting,?

Correct option is B

​$$T_{st} = \frac{T_{max}}{2 S_{max}} \\1 = \frac{2 S_{max}}{S_{max}^2 + 1} \\S_{max} - 2 S_{max} + 1 = 0 \\S_{max} = 1 \\1 = \frac{r_{ext} + 0.5}{1} \\= 0.5 \, \Omega$$​​