From above figure it is clear that current through inductor starts increasing from 110° to 180° due to positive value of voltage across it ( LΔI = vxΔt). Now after w
t = 180°, inductor is subjected to negative voltage. Due to symmetry of positive & negative voltage across inductor, the current through inductor decreases at the same rate at each instant after w
t = 180, as it was increasing during w
t = 110° to w
t = 180°. Hence the current through inductor will become zero at w
t = 180 + 70 = 250°. Note that Thyristor can’t turnoff until current through it becomes zero & negative voltage appears across it. Hence Thyristor will turnoff at w
t = 250°. So it conducts for q
= 70 x 2= 140°.