Topic: Power Electronics
In a single-phase fully-controlled bridge converter feeding a highly inductive load, what happens if the firing angle is increased from $30^{\circ}$ to $120^{\circ}$?
- A. Output voltage increases, power factor improves
- B. Output voltage decreases, power factor improves
- C. Output voltage decreases, power factor worsens
- D. Output voltage increases, power factor worsens
Answer: C. Output voltage decreases, power factor worsens
Explanation: For a fully-controlled bridge converter with a highly inductive load, the average output voltage is given by $V_{avg} = \frac{2V_m}{\pi} \cos\alpha$. As the firing angle $\alpha$ increases from $30^{\circ}$ to $120^{\circ}$, $\cos\alpha$ decreases from a positive value ($+0.866$) to a negative value ($-0.5$), meaning the output voltage algebraically decreases (entering inversion mode). The fundamental power factor is proportional to $\cos\alpha$. Since its magnitude drops and enters the negative region, the power factor is considered to worsen.