Highly inductive load with thyristor conduction In the circuit shown (load is highly inductive), the average value of the load current is 15 A and current is essentially continuous. What is the peak (maximum instantaneous) current through a conducting thyristor?

Introduction / Context:
In phase-controlled rectifiers feeding highly inductive loads, the inductor smooths current ripple so strongly that the load (and device) current becomes practically constant over a cycle. Understanding how this affects device current ratings is essential for selecting proper thyristors and estimating losses.

Given Data / Assumptions:

• Average load current = 15 A.
• Load is 'highly inductive', implying continuous, nearly ripple-free current.
• Conduction is by pairs of thyristors (or one device in midpoint), each device carrying the instantaneous load current during its conduction interval.

Concept / Approach:

With a very large inductance, di/dt is small and the current waveform is almost flat (constant). In this limiting case, the instantaneous current through the conducting path is essentially equal to the DC (average) value. Hence the peak current through a conducting thyristor equals the nearly constant load current value.

Step-by-Step Solution:

Verification / Alternative check:

If small ripple exists, the ripple typically oscillates around 15 A by a few percent; the peak would still be very close to 15 A for a 'highly inductive' description. RMS may differ from average, but peak is governed by ripple which is minimal here.

Why Other Options Are Wrong:

15√2 A and 30 A assume sinusoidal or pulsed current peaks, not valid for strongly smoothed current. 22.5 A and 12 A are arbitrary and do not follow from the constant-current assumption.

Common Pitfalls:

Confusing device current with the instantaneous AC source current or transformer secondary current. The thyristor in series with an RLE load carries the load current during conduction, which is near-constant with a large inductor.

Final Answer:

15 A