Single-phase semiconverter feeding a highly inductive load with a freewheeling diode across the load What is the typical input (source) current waveform drawn from the AC supply under continuous-current conditions?

Introduction / Context:
Power electronic rectifiers seldom draw sinusoidal current unless special conditioning is used. A single-phase semiconverter (half-controlled bridge) supplying a highly inductive load with a freewheeling diode is a classic case where the load current is nearly constant, yet the source current is discontinuous and distinctly non-sinusoidal. Understanding the input current shape is vital for estimating harmonics, power factor, and line-current distortion.

Given Data / Assumptions:

• Single-phase semiconverter (two SCRs and two diodes) on an AC source.
• Load is highly inductive, so load current is approximately constant (continuous conduction).
• A freewheeling diode is connected across the load.
• Commutation overlap is neglected for conceptual clarity.

Concept / Approach:

With high inductance, the load current changes slowly and can be approximated as constant over a half cycle. When the controlled devices conduct (from firing angle α to near the line-current zero), the source must supply approximately that constant current. When the freewheeling diode conducts, the load current circulates locally and the source current drops to zero, creating a pulsed input current. The approximate constancy of current during conduction makes the pulses nearly rectangular in magnitude.

Step-by-Step Solution:

Verification / Alternative check:

Textbook waveforms for semiconverters with freewheeling diodes show source current as rectangular pulses whose width is determined by firing angle and load current continuity. Harmonic analysis confirms high harmonic content typical of such pulsed currents.

Why Other Options Are Wrong:

Sinusoidal is incorrect because the current does not follow the voltage due to controlled conduction and freewheeling. Triangular would require a linearly changing current, not present with large inductance. Saying “sinusoidal or rectangular depending only on firing angle” ignores the dominant role of load inductance and freewheeling in fixing the pulse shape.

Common Pitfalls:

Confusing rectifier source current with load current; assuming any AC-fed circuit draws sinusoidal current; forgetting that freewheeling eliminates source current during portions of the cycle.

Final Answer:

Rectangular (pulsed, near-constant magnitude during conduction)