Power Electronics MCQs 2026

Chopper used in DC motor speed control varies

1. Aaverage armature voltage by changing duty cycle✓
2. Bfield flux only without armature voltage effect ever for speed control in shunt motor context both matter but armature chopper is classic✓
3. Cair gap length physically without electronics✓
4. Dnumber of poles without electrical control✓

Regenerative chopper operation returns energy when

1. Amotor always motoring without reversal of power flow ever✓
2. Bduty cycle is zero always with switch permanently open without regeneration path design✓
3. Cfuse blows on overload only without energy return✓
4. Dload drives current back to source during braking✓

Current ripple in chopper output depends on

1. Aonly input DC voltage magnitude without L or fsw✓
2. Bonly PID proportional band without power stage✓
3. Cinductance, switching frequency and load current✓
4. Donly insulator creepage distance only✓

Discontinuous conduction mode in buck chopper occurs when

1. Ainductor current never reaches zero ever in all loads without exception✓
2. Binductor current falls to zero before next switching cycle✓
3. Cinput voltage equals output always✓
4. Dduty cycle equals unity always in DCM definition boundary only at boundary case✓

Thyristor chopper requires

1. Aonly natural commutation from AC line zero always on DC bus chopper without AC zero✓
2. Bno commutation because SCR turns off automatically on DC without design ever✓
3. Conly gate pulse removal without commutation network on DC✓
4. Dforced commutation circuit to turn off SCR✓

IGBT/MOSFET choppers operate at

1. Ahigher switching frequencies than SCR choppers with simpler control✓
2. Bonly line frequency 50 Hz without PWM ever as defining advantage of modern devices✓
3. Czero frequency DC without switching ever✓
4. Donly sub-hertz without filters ever as typical modern design point✓

Input filter on chopper reduces

1. Aoutput voltage ripple only at load without affecting input✓
2. Bmotor slip without electrical connection✓
3. Ccurrent ripple reflected to DC source✓
4. Dsymmetrical component unbalance only on AC lines without DC chopper relevance✓

Buck-boost chopper output voltage polarity is

1. Asame polarity always as input without inversion ever in ideal buck-boost✓
2. Balways positive without inversion regardless of topology name✓
3. Cindependent of duty cycle always✓
4. Dinverted compared to input and magnitude depends on D✓

Forced commutation is required in

1. AAC diode rectifier on resistive load only without SCR✓
2. Bpassive RC snubber only without switches✓
3. CDC choppers and inverters using SCRs✓
4. Donly Buchholz relay operation✓

dv/dt rating of SCR specifies

1. Amaximum rate of rise of anode voltage without spurious turn-on✓
2. Bmaximum gate current only✓
3. Cmaximum on-state voltage drop only✓
4. DPID derivative gain only✓

di/dt rating limits

1. Aonly reverse recovery of diode without relation✓
2. Bonly line sag calculation✓
3. Conly distance relay reach only✓
4. Drate of rise of anode current at turn-on to protect device✓

Gate trigger current is

1. Asame as holding current always✓
2. Bequal to anode latching current always✓
3. Cminimum gate current to switch SCR from off to on state✓
4. Dindependent of junction temperature always without derating✓

Two SCRs in anti-parallel on AC switch line allow

1. Aonly half-wave rectification without diodes✓
2. Bonly DC chopper operation without commutation✓
3. Conly open-loop step response measurement only✓
4. Dcontrol of power in both half cycles✓

RC snubber across SCR protects against

1. Aonly di/dt at turn-on without capacitance✓
2. Bdv/dt and voltage transients✓
3. Conly gate oxide breakdown from overcurrent only without voltage effect✓
4. Donly motor slip measurement✓

On-state voltage drop of high-power SCR is typically

1. Aabout 1 to 2 V depending on current rating✓
2. Bhundreds of volts always when conducting✓
3. Czero always✓
4. Dequal to reverse peak voltage always✓

SCR is preferred over transistor in high-power AC line applications when

1. Aswitching frequency exceeds 1 MHz always without IGBT✓
2. Bonly microampere currents are switched✓
3. Cline commutation is available and high voltage/current rating needed economically✓
4. Donly battery voltage below 1 V✓

Reverse conducting SCR integrates

1. Aanti-parallel diode in same package for commutation and freewheeling✓
2. BIGBT with anti-parallel SCR only without diode✓
3. Conly vacuum interrupter contacts✓
4. Donly PID integral windup clamp only✓

Light activated SCR (LASCR) is triggered by

1. Aonly mechanical vibration of heat sink without photons✓
2. Bonly distance relay carrier signal✓
3. Conly symmetrical component calculator only✓
4. Dincident light on sensitive gate region✓

Half-wave rectifier with resistive load produces

1. Aoutput with one conduction half cycle per input cycle✓
2. Bfull DC without ripple without filter✓
3. Cpure sinusoidal AC at output always✓
4. Dthree-phase balanced output always✓

Full-wave centre-tap rectifier uses

1. Aone diode only on three-phase supply without transformer✓
2. Bfour SCRs in bridge without AC input✓
3. Conly inductor filter without diodes✓
4. Dtwo diodes and centre-tapped transformer secondary✓

Bridge rectifier with four diodes gives

1. Ahalf-wave only always✓
2. BDC with frequency equal to input without ripple✓
3. Crequires forced commutation for diodes always✓
4. Dfull-wave output without centre-tapped transformer✓

Average DC voltage of single-phase full-wave bridge (ideal) is

1. AVm/π only✓
2. B2Vm/π where Vm is peak AC voltage✓
3. C2Vm only without division✓
4. DVm/2 only✓

Three-phase half-wave rectifier (three diodes) has ripple frequency

1. Aequal to supply frequency only✓
2. Bsix times supply frequency always✓
3. Cthree times supply frequency✓
4. Dzero ripple without filter always✓

Three-phase full-wave bridge rectifier uses

1. Atwo diodes only on single phase always✓
2. Bone SCR without diodes on DC bus only✓
3. Csix diodes or SCRs for two pulses per phase per cycle✓
4. Donly RC snubber without switches✓

Output ripple frequency of six-pulse rectifier is

1. Athree times only always✓
2. Bsix times the supply frequency✓
3. Csame as supply without harmonics✓
4. Dzero with any load always✓

Power factor of uncontrolled three-phase rectifier is

1. Aless than unity and depends on load and firing angle✓
2. Balways unity without harmonics✓
3. Czero always✓
4. Dindependent of load current always✓

Controlled rectifier uses SCRs to adjust

1. Aonly frequency of AC input without inversion✓
2. Baverage output voltage by varying firing angle α✓
3. Conly power factor to unity always without control✓
4. Donly filter inductance without delay angle✓

As firing angle α increases in controlled rectifier, average output voltage

1. Aincreases linearly without limit above input peak always✓
2. Bdecreases✓
3. Cremains constant always✓
4. Dbecomes independent of AC supply✓

Extinction angle β in line-commutated inverter mode must be

1. Azero for safe operation always✓
2. Bnegative without limit✓
3. Cequal to firing angle only without overlap✓
4. Dgreater than recovery time of devices plus safety margin✓

Overlap angle μ in rectifiers occurs due to

1. Asource inductance causing simultaneous conduction of phases✓
2. Bonly load capacitance without source impedance✓
3. Conly PID derivative kick✓
4. Donly Buchholz gas pressure alone✓

Isolation transformer at inverter output provides

1. Agalvanic isolation and voltage matching✓
2. Bforced commutation for SCR always without electronics✓
3. Csymmetrical component transformation only without galvanic isolation benefit✓
4. Ddistance protection reach setting only✓

Overmodulation in PWM inverter occurs when

1. Afiring angle is below zero in rectifier only without PWM context✓
2. BDC link is unregulated without any voltage ripple ever✓
3. Creference exceeds linear range and low-order harmonics increase✓
4. Dgate dead time is infinite always✓

UPS inverter must maintain

1. Aonly rectifier operation without battery ever for offline UPS definition✓
2. Bstable voltage and frequency during supply outage using stored energy✓
3. Conly line sag calculation on EHV lines without battery✓
4. Donly Buchholz alarm without static switch✓

Multilevel inverter reduces

1. Aneed for any switching devices entirely without semiconductors✓
2. Bdv/dt stress and harmonic content by stepping output voltage levels✓
3. CDC link voltage to zero always without capacitors✓
4. Dmotor slip to zero always without load torque consideration✓

DC chopper converts

1. Afixed DC voltage to variable average DC voltage by switching✓
2. BAC to fixed DC only without switching✓
3. CDC to AC directly without inversion stage terminology confusion—chopper is DC-DC✓
4. DAC to AC at same frequency without electronic switching only using transformer taps without switches✓

Step-down (buck) chopper output voltage is

1. Alower than input DC voltage✓
2. Balways higher than input without exception✓
3. Cequal to input always without switching✓
4. Dunrelated to duty cycle always✓

Step-up (boost) chopper can produce output voltage

1. Aonly lower than input always✓
2. Bhigher than input DC source✓
3. Cexactly zero without inductor ever in all operating modes✓
4. Dwithout any inductor or energy storage element ever in ideal boost✓

Duty cycle D of chopper is defined as

1. AOFF time to period only✓
2. Binput voltage to output voltage always without switching✓
3. Cpeak current to average current only without time definition✓
4. Dratio of ON time to total switching period✓

Average output voltage of ideal buck chopper is

1. AVin/D always✓
2. B(1-D)×Vin for buck—note boost is different✓
3. CD × Vin✓
4. Dzero regardless of D always✓

Average output voltage of ideal boost chopper is

1. AVin / (1 − D)✓
2. BD × Vin which is buck relation not boost✓
3. CVin × (1 − D) always✓
4. Dindependent of D always✓

Freewheeling diode in chopper circuit

1. Ablocks inductor current always when switch OFF causing infinite voltage spike always without diode✓
2. Bprovides path for inductor current when switch is OFF✓
3. Creplaces main switching device entirely without switch✓
4. Dmeasures fault impedance for distance relay only✓

Silicon controlled rectifier (SCR) is a

1. Atwo-terminal diode only without control✓
2. Bfour-layer PNPN device with three terminals anode, cathode and gate✓
3. Cvacuum triode only✓
4. Dtransformer core only✓

SCR turns ON when

1. Acathode is positive with respect to anode always✓
2. Bgate alone turns off device in DC circuit without commutation✓
3. Creverse voltage alone triggers conduction✓
4. Dgate pulse is applied while anode is positive with respect to cathode and forward biased✓

Latching current of SCR is

1. Aminimum anode current to maintain ON state after gate pulse removal✓
2. Bgate current only✓
3. Creverse leakage only✓
4. Dholding current always greater than latching✓

Holding current is

1. Aalways equal to latching current exactly in all datasheets without exception✓
2. Bgate trigger current only✓
3. Cminimum anode current below which SCR turns OFF✓
4. Dpeak repetitive forward voltage only✓

In forward blocking state SCR

1. Aconducts heavily without gate✓
2. Bblocks current with anode positive until triggered✓
3. Cconducts in reverse direction freely✓
4. Dbehaves as short circuit always✓

SCR in reverse bias

1. Ablocks current like a reverse-biased diode until breakdown rating exceeded✓
2. Bconducts like a forward diode always✓
3. Cturns on with gate pulse in reverse conduction mode normally✓
4. Dhas no peak inverse voltage rating✓

SCR cannot be turned OFF by removing gate signal alone in DC circuit because

1. Agate has no role in turn-on✓
2. Banode-cathode becomes superconducting permanently✓
3. Cdevice is unipolar only✓
4. Dregenerative internal feedback sustains conduction once latched✓

Natural commutation of SCR occurs in

1. AAC circuits when current passes through zero✓
2. Bpure DC bus without auxiliary circuit always✓
3. Conly open-loop control systems✓
4. Donly transformer oil only✓

Interphase transformer in dual three-phase rectifiers helps

1. Aeliminate all harmonics completely without any remaining ripple✓
2. Breplace all SCRs with diodes only on DC side without AC✓
3. Cmeasure distance to fault on line only✓
4. Doperate two 6-pulse bridges with 30° phase shift for 12-pulse operation✓

Filter inductor on DC side of rectifier

1. Aincreases ripple amplitude always✓
2. Bblocks DC while passing only AC ripple without inductance effect✓
3. Csmooths current and reduces ripple✓
4. Dreplaces need for diodes entirely✓

Filter capacitor on DC side of rectifier

1. Asmooths only current without affecting voltage ripple✓
2. Beliminates need for transformer always✓
3. Csmooths voltage ripple✓
4. Dprovides forced commutation for SCR in all DC circuits automatically without design✓

Diode rectifier does not require

1. AAC input voltage ever✓
2. Btransformer or source impedance ever✓
3. Cany heat sinking on diodes ever✓
4. Dgate firing or commutation circuit✓

Harmonics generated by rectifiers can cause

1. Aimproved power factor without filters always✓
2. Bdistortion, heating and relay misoperation on the supply system✓
3. Czero neutral current in all cases always✓
4. Dautomatic voltage regulation without STATCOM always✓

Dual converter uses two bridges to provide

1. Aonly half-wave rectification without reversal✓
2. Bregenerative four-quadrant DC control✓
3. Conly open-loop motor speed without feedback✓
4. Donly fuse protection without thyristors✓

Voltage source inverter (VSI) has

1. ADC voltage source with capacitor at input and switches create AC output✓
2. BDC current source at input with large inductor always as defining feature without alternative✓
3. Cno switching devices✓
4. Donly diode rectifier without switches on AC side✓

Current source inverter (CSI) has

1. Acapacitive DC link without inductor as defining element✓
2. Blarge DC inductor maintaining nearly constant DC current✓
3. Conly passive diodes without controlled switches on output always✓
4. Donly SCR without commutation in all modern CSI designs without exception✓

PWM inverter controls output voltage and harmonic content by

1. Afixed square wave only at 50 Hz without modulation ever in all drives✓
2. Bvarying pulse widths of switched DC voltage✓
3. Canalog potentiometer on DC link only without switches✓
4. Donly transformer tap changer without switching✓

Single-phase H-bridge inverter uses

1. Atwo diodes only without switches✓
2. Bone SCR only on DC positive rail without return path switch✓
3. Cfour switches to synthesize AC from DC✓
4. Dsix diodes in three-phase bridge only without inversion capability✓

Third harmonic injection in three-phase inverter PWM helps

1. Aeliminate all switching losses completely without trade-off✓
2. Bincrease DC bus utilization by extending linear modulation range✓
3. Creplace PID controller in outer loop always✓
4. Dmeasure symmetrical components only without PWM change✓

Dead time in inverter gate signals prevents

1. Aovermodulation only without shoot-through risk ever✓
2. Bnatural commutation in AC line only✓
3. Cshoot-through short circuit of DC bus through upper and lower switches✓
4. DFerranti effect on transmission line only✓

Line-commutated inverter (LCI) feeds power from DC to AC when

1. Afiring angle is zero always in rectifier only without reversal✓
2. BDC voltage is zero always✓
3. CSCR firing angle exceeds 90° into inverter mode with sufficient extinction angle✓
4. Dload is purely resistive without AC source✓

Forced-commutated inverter (e.g., IGBT based) can

1. Aonly work with synchronous motor at fixed speed without control✓
2. Bnot control frequency or voltage✓
3. Cnot use PWM ever✓
4. Doperate from DC source without relying on AC line for turn-off✓

Output frequency of variable frequency drive inverter is controlled by

1. Aonly DC link voltage without changing switch rate ever for frequency change✓
2. Bonly transformer turns ratio on output without electronics✓
3. Conly fuse rating without switches✓
4. Dmodulating switch timing / PWM carrier and reference frequency✓

Harmonic filter at inverter output may be required to

1. Ameet grid code THD limits✓
2. Bincrease THD deliberately✓
3. Celiminate DC link capacitor always✓
4. Dreplace all IGBTs with SCRs without commutation ever in all cases✓

Space vector modulation (SVM) in three-phase inverters

1. Auses only single-phase H-bridge always without three phases✓
2. Boptimizes switching to maximize DC bus usage and reduce harmonics✓
3. Celiminates all switching losses without heat sink ever✓
4. Dreplaces motor windings entirely✓

Regenerative braking with inverter drive returns energy by

1. Ainverting DC link power back to AC supply when motor acts as generator✓
2. Bdissipating only in rotor copper without electronics always✓
3. Copening fuse links only✓
4. Dincreasing line sag only on overhead conductors✓

Square wave inverter output contains

1. Aonly DC without any AC component ever✓
2. Bonly even harmonics without fundamental ever✓
3. Cfundamental plus odd harmonics✓
4. Dpure sinusoid without harmonics always without filter✓

Chopper efficiency is improved by

1. Amaximizing dead time without limit always causing shoot-through if too small but excessive dead time hurts output—still minimizing losses is key✓
2. Busing only linear regulators without switching always for high power efficiency claim✓
3. Copening fuse on every cycle for current limit as efficiency strategy✓
4. Dminimizing switching and conduction losses in devices✓

Soft switching techniques in choppers aim to

1. Aincrease dv/dt stress on SCR always deliberately✓
2. Beliminate inductor from circuit always without replacement✓
3. Cswitch at zero voltage or zero current to reduce losses✓
4. Dreplace PID with bang-bang only without semiconductor benefit✓