# A triac based single-phase voltage regulator feeds an R-L load as shown in the figure. What is the range of triggering angle for which the output voltage Vo is not controllable?

### Right Answer is:

0° ≤ α ≤ 45°

#### SOLUTION

**Concept:**

For controlling the load voltage, the minimum value of firing angle must be greater than the load phase angle.

That is, ${\alpha _{min}} > \varphi$

Where

$\varphi = {\tan ^{ – 1}}\left( {\frac{{\omega L}}{R}} \right)$

Therefore, firing angle for which the output voltage is uncontrollable is

$0 \le \alpha \le \varphi$

For controlling the load, the minimum value of firing angle α = load phase angle.

Phase angle φ = tan^{−1}(ωL/R)

φ = tan^{−1}(10/10) = 45°

The firing angle for which the output voltage is uncontrollable is

**0 ≤ α _{min} ≤ φ**

**0° ≤ α ≤ 45°**