Sizing parallel thyristors with derating A 750 A DC circuit uses identical thyristors each rated at 175 A. With a derating factor of 25% (i.e., each device carries only 75% of its nameplate current), how many thyristors must be placed in parallel?

Introduction / Context:
When paralleling thyristors, current sharing is imperfect due to tolerances and temperature coefficients. A derating factor is applied so each device is loaded below its nominal rating, ensuring reliability. Calculating the required count is a straightforward but essential design step.

Given Data / Assumptions:

• Total circuit current Itotal = 750 A.
• Device nameplate Irated = 175 A.
• Derating 25% → usable per device Iusable = 0.75 * Irated.

Concept / Approach:

Number of devices in parallel n must satisfy n * Iusable ≥ Itotal. Use ceiling to ensure sufficient devices.

Step-by-Step Solution:

Verification / Alternative check:

Check margin: 6 * 131.25 = 787.5 A ≥ 750 A → acceptable.

Why Other Options Are Wrong:

3, 4, or 5 do not meet the required current with derating; 10 is over-conservative for the stated parameters.

Common Pitfalls:

Forgetting to apply derating; using rounding down instead of ceiling.

Final Answer:

6