Commutation dynamics – underdamped cases In which forced-commutation class(es) does the overall commutation circuit operate in an underdamped (oscillatory) manner?

Introduction / Context:
Forced commutation methods use auxiliary L–C networks to momentarily reverse-bias and turn off a conducting thyristor. The damping of the L–C network—underdamped, critically damped, or overdamped—governs how current and voltage waveforms evolve during commutation.

Given Data / Assumptions:

• Class A (load commutation / resonant commutation): relies on a resonant L–C (with the load) to produce current reversal.
• Class B (parallel or impulse commutation): uses an L–C branch to force a reverse current pulse through the thyristor.
• Ideal components for conceptual analysis.

Concept / Approach:

In both Class A and Class B commutation, an oscillatory current is intentionally created in an L–C path. This implies an underdamped response (sinusoidal or pseudo-sinusoidal). The natural frequency ω0 = 1/√(LC) sets the time scale over which the thyristor is reverse biased. Correct sizing of L and C ensures that reverse current exceeds the load current for at least the device turn-off time (tq).

Step-by-Step Solution:

Verification / Alternative check:

Waveforms in standard references show sinusoidal current reversal (Class A) and oscillatory capacitor current pulses (Class B), confirming underdamped behavior.

Why Other Options Are Wrong:

• Class C and others use different mechanisms (e.g., complementary current paths) and are not generically defined by an underdamped resonant turn-off loop.

Common Pitfalls:

Equating “forced” with “non-oscillatory”; many forced-commutation schemes purposely exploit oscillation to achieve reliable reverse bias.

Final Answer:

Both Class A and Class B