Discussion
A half-wave thyristor converter supplies a purely inductive load, as shown in Fig. If the triggering angle of the SCR is 120°, the extinction angle will be :
[The figure shows a single SCR in series with an inductor L across an ac supply Vin sin ωt.]
Answer: A
The extinction angle is 240°.
With a purely inductive load, \(v = L\,di/dt\). After the SCR fires at \(\alpha = 120^{\circ}\), the current rises while the supply is positive, reaches its peak at 180°, and then falls while the supply is negative. It returns to zero when the area under the voltage curve after 180° equals the positive area from 120° to 180°:
\[ \begin{aligned} \int_{\alpha}^{\beta} \sin\omega t\, d(\omega t) &= 0 \\ \cos\alpha - \cos\beta &= 0 \\ \beta &= 360^{\circ} - \alpha = 240^{\circ} \end{aligned} \]
180° would apply to a resistive load, where current stops when the voltage reaches zero. 120° is the firing angle itself, and 200° does not satisfy the condition.
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