Rectifiers & Converters MCQ [Free PDF] – 11 Objective Questions with Answers

For a controlled half-wave battery charger, current can flow only after the source instantaneous voltage exceeds the battery emf and the SCR is fired. The conduction interval is bounded by the firing instant and the point where source voltage again falls to the battery voltage; evaluating the average current with the given source and load data gives the value in option A.

Source inductance prevents current from transferring instantaneously between rectifier devices. During the overlap angle μ, outgoing and incoming devices conduct simultaneously, reducing average DC output voltage. Key relation: Overlap introduces a commutation reactance drop; Vdc_actual < ideal Vdc.

Exam focus: More source inductance/current → more overlap. Common trap: Ignoring source inductance overestimates rectifier DC output.

A non-circulating-current dual converter prevents the two converter bridges from conducting simultaneously. Only one converter is enabled at a time; an interlocking/dead-time scheme blocks the other, so there is no closed path for circulating current.

A freewheeling diode gives inductive current a path when the main source/switch is removed. It clamps otherwise large inductive overvoltage, keeps current continuous and can improve efficiency and waveform quality in DC converter/rectifier circuits. Key relation: Inductor relation v=L di/dt explains the voltage spike if current path is interrupted.

Exam focus: Always provide a current path for stored inductive energy. Common trap: Freewheeling diodes are not limited to AC circuits.

Firing angle controls average DC voltage; the AC-side current is non-sinusoidal and its fundamental is displaced approximately by α. Vdc=(2√2×230/π)cos30°≈179.3 V. With R=10 Ω and highly inductive continuous current, Id≈17.93 A. Key relation: 1φ: Vdc=(2Vm/π)cosα≈0.9Vrms cosα; 3φ: Vdc≈1.35V_LL cosα.

Exam focus: Large load inductance → nearly constant DC current. Common trap: Distinguish displacement factor cosα from total power factor including distortion.

D — Remains unchanged. With continuous current each thyristor pair conducts for 180° until the next pair is fired; changing α only shifts the conduction interval, not its length.

An ideal half-wave rectifier passes one half-cycle of a sinusoidal input and blocks the other, creating pulsating DC. Vm=√2×100≈141.4 V and ideal half-wave average Vdc=Vm/π≈45.0 V. Key relation: V_m=√2 V_rms; V_DC=V_m/π; η_max≈40.6%.

Exam focus: Remember π in half-wave average; full-wave uses 2Vm/π. Common trap: Do not confuse diode ideality with 100% rectification efficiency.

D — Only during the positive half-cycle after triggering. The SCR is forward-biased only in the positive half-cycle and conducts from the firing angle α until the current falls to zero.

In three-phase bridges each thyristor conducts for 120 electrical degrees under continuous current. Vdc≈1.35×415×cos60°≈280.1 V. Displacement factor=cos60°=0.5. Key relation: 1φ: Vdc=(2Vm/π)cosα≈0.9Vrms cosα; 3φ: Vdc≈1.35V_LL cosα.

Exam focus: Large load inductance → nearly constant DC current. Common trap: Distinguish displacement factor cosα from total power factor including distortion.

B — AC to DC. A three-phase full-wave converter is a rectifier that converts three-phase AC input into DC output (six-pulse).

The primary is delta-connected, so its phase voltage is 1100 V. With a per-phase turns ratio of 5, the star-secondary phase voltage is 1100/5 = 220 V rms. The three-pulse rectifier's maximum load voltage is the phase-voltage peak, so I_max = √2×220/10 = √2×22 A.