Condition for oscillatory commutation in a series inverter (R load) A series inverter supplies a resistive load R with commutating components L and C in series. For the current to be oscillatory (so that natural current zero occurs for thyristor turn-off), the components must satisfy which condition?

Introduction / Context:
In a series inverter, commutation relies on the natural oscillation of the series RLC path to force the current through zero. Ensuring underdamped behavior is essential for achieving these current zero crossings within each cycle.

Given Data / Assumptions:

• Series RLC path with load resistance R.
• Thyristor needs a natural current zero for turn-off.

Concept / Approach:

The series RLC response is underdamped if the damping factor α = R/(2L) is less than the undamped natural frequency ω0 = 1/√(LC). Under this condition, the current oscillates and crosses zero, enabling reliable natural commutation.

Step-by-Step Solution:

Verification / Alternative check:

Equivalently, R < 2 * √(L/C). Either form ensures an oscillatory response and a natural current zero for commutation.

Why Other Options Are Wrong:

Equality implies critical damping; the inequality reversed gives overdamping with no oscillation. The other forms do not consistently capture the underdamped criterion for all L, C, R.

Common Pitfalls:

Forgetting that it is the current zero crossing (not voltage) that commutates the thyristor; choosing components that yield overdamped response will prevent proper turn-off.

Final Answer:

1/√(L*C) > R/(2L)