Phase control in controlled rectifiers As the firing (triggering) angle of a thyristor in a controlled rectifier is delayed further into the cycle, how does the average DC output voltage change?

Introduction / Context:
Controlled rectifiers (phase-controlled converters) regulate output voltage by delaying the firing angle of thyristors relative to the AC source. Understanding the relationship between firing angle and average output voltage is fundamental in drives and DC power supplies.

Given Data / Assumptions:

• Single-phase or three-phase line-commutated converter.
• Ideal source and device drops neglected for the basic trend.
• Firing angle denoted by alpha (α), measured from the natural commutation point.

Concept / Approach:
In a controlled rectifier, the average DC output voltage is proportional to the cosine of the firing angle. For example, for a single-phase full-wave bridge, Vdc = (2*Vm/π) * cos α (ideal). As α increases (firing delayed), cos α decreases; beyond 90°, Vdc becomes negative (inversion).

Step-by-Step Solution:
At α = 0°, conduction begins immediately at each half-cycle’s voltage peak alignment, producing maximum positive Vdc.As α increases, conduction starts later, reducing the area under the rectified waveform and lowering average Vdc.Therefore, delaying firing angle decreases the average output voltage; at α > 90°, the converter can enter inversion with Vdc < 0.

Verification / Alternative check:
Plot Vdc against α using Vdc = K * cos α; the monotonic decrease with α is evident from 0° to 180°.

Why Other Options Are Wrong:

• “False” would imply Vdc rises with α, contradicting the cosine law of phase control.

Common Pitfalls:

• Confusing instantaneous output with average output; instantaneous may be high at peaks, but average falls with greater α.
• Ignoring device drops—these shift values slightly but not the trend.

Final Answer:
True