Q: Integral-cycle (on–off) control of an AC regulator with a resistive load If the duty cycle is a (0 < a ≤ 1), what is the overall input power factor seen by the supply?

Introduction / Context: Integral-cycle control (burst firing) turns whole cycles of the AC waveform on and off to control average power. With a purely resistive load, current and voltage are in phase during each “on” cycle, yet the waveform is discontinuous over long intervals, affecting the overall power factor seen by the supply. Given Data / Assumptions: Resistive load (displacement factor during “on” cycles = 1). Duty cycle a is the fraction of time the regulator delivers full cycles to the load. “On” cycles are ideal (no device drops); “off” cycles draw zero current. Concept / Approach: Overall power factor PF is real power divided by apparent power over the averaging interval. Real power scales directly with the duty cycle a (since only a portion of cycles deliver power). Apparent power scales with the overall RMS current, which is reduced by √a because current flows only during an a-fraction of time. Combining these yields PF = √a for a resistive load under integral-cycle control. Step-by-Step Solution: Let P_full be real power if always on; then P_avg = a * P_full.Let I_rms,full be RMS current if always on; overall RMS current I_rms,tot = √a * I_rms,full (current present only a fraction of time).Apparent power S = V_rms * I_rms,tot = V_rms * √a * I_rms,full.PF = P_avg / S = (a * V_rms * I_rms,full) / (V_rms * √a * I_rms,full) = √a. Verification / Alternative check: Check limits: if a = 1 (always on), PF = 1 (unity). If a = 0.25, PF = 0.5, matching the intuitive reduction due to burst operation. Why Other Options Are Wrong: PF = a or a^2 overestimates the reduction; PF = 1 ignores the discontinuous nature of current and is only valid for a = 1. Common Pitfalls: Confusing displacement factor (which remains 1 in “on” cycles) with overall power factor; forgetting to compute RMS over the entire time window including “off” intervals. Final Answer: √a

Q: Single-phase fully controlled bridge feeding an R–L–E load (continuous current) Given v_s(t) = Vm sin(ωt), firing angle α, and average current I0, which relation is correct?

Introduction / Context: For a fully controlled bridge supplying an R–L–E load (e.g., a DC motor back-emf), the average DC output voltage depends on firing angle α and equals the sum of the average resistive drop and the back-emf. This relation is the cornerstone for speed and torque control in drives. Given Data / Assumptions: Input: v_s(t) = Vm sin(ωt). Fully controlled bridge (four SCRs), continuous current. Ideal devices, no overlap; average load current = I0; series R and back-emf E. Concept / Approach: For continuous current and no overlap, the average output voltage is V_avg = (2Vm/π) cos α. KVL for the DC side gives V_avg = R I0 + E. Combining these two expressions yields the desired relation for cos α in terms of R, I0, E, and Vm. Step-by-Step Solution: Use converter average: V_avg = (2Vm/π) * cos α.Use DC KVL: V_avg = R I0 + E.Equate: (2Vm/π) * cos α = R I0 + E.Solve for cos α: cos α = (π/(2Vm)) * (R I0 + E). Verification / Alternative check: Limit checks: with E = 0 and α = 0°, cos α = 1 ⇒ V_avg = 2Vm/π, the well-known maximum. Increasing α reduces V_avg linearly with cos α as expected. Why Other Options Are Wrong: Options with incorrect proportionality (e.g., multiplied by 2Vm/π instead of divided) or sign errors (E − R I0) do not satisfy both the converter equation and DC KVL simultaneously. Common Pitfalls: Mixing up Vm (peak) with V_rms; forgetting continuous conduction assumption; ignoring the back-emf term. Final Answer: cos α = (π/(2Vm)) * (R I0 + E)

Q: Three-phase semiconverter (half-controlled) output continuity Beyond what firing angle does the DC output voltage become discontinuous (gaps appear) for a resistive/weakly inductive load?

Introduction / Context: The three-phase semiconverter (one quadrant, half-controlled) uses three controlled devices and three diodes. Its output voltage waveform quality and continuity depend strongly on the firing angle α and load smoothing. Designers must know where natural gaps begin to appear so that filter sizing and performance expectations are realistic. Given Data / Assumptions: Three-phase semiconverter feeding a load with limited smoothing. Ideal devices, negligible overlap for concept. Focus on onset of discontinuity as α increases. Concept / Approach: In a semiconverter, each device pair supplies segments of the line-to-line wave. As α increases, the conduction intervals shrink relative to the 60° spacing of line EMFs. When α exceeds 60°, there are intervals where no device pair can sustain positive DC voltage, leading to notches or zero output spans—i.e., discontinuity in the average DC voltage waveform for lightly filtered or resistive loads. Step-by-Step Solution: At α ≤ 60°, conduction sequences overlap sufficiently to keep the output positive across phases.At α > 60°, conduction windows retreat and gaps appear where the rectified line-to-line voltage would otherwise be available.Hence the DC output becomes discontinuous beyond α = 60° (for weak smoothing). Verification / Alternative check: Phase diagram timing confirms the loss of continuous transfer when the trigger is delayed beyond the 60° inter-phase spacing typical of a three-phase system. Why Other Options Are Wrong: 30° is too low; 90° and 120° overstate the continuity range for a semiconverter with weak smoothing. Common Pitfalls: Confusing semiconverters with fully controlled bridges; assuming large output inductance which can maintain current continuity even when voltage has gaps. Final Answer: α > 60°

Question 1

Applications of thyristors – assertion and reason Assertion (A): Thyristors are used in controlled heating, alternator excitation systems, motor speed control, and HVDC transmission. Reason (R): Static VAR systems using thyristors are very commonly …

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Introduction / Context: Thyristors (SCRs and related devices) are ubiquitous in high-power conversion and control. Evaluating assertion–reason statements helps reinforce where and why thyristors are applied across industry sectors such as drives, power transmission, and reactive power control. Given Data / Assumptions: Applications listed: controlled heating, alternator excitation, motor control, HVDC. Static VAR compensators (SVC) or STATCOMs include thyristor-switched capacitors/reactors. Concept / Approach: A is correct: All listed applications are standard for thyristor technology due to high current capability, robustness, and latching behavior. R is also correct: SVCs widely use thyristors to modulate reactive power in AC networks. However, R does not explain A; it merely cites one common application area rather than providing the causal justification for the broad versatility in A. Step-by-Step Solution: Assess A: Valid—these are classic thyristor applications.Assess R: Valid—thyristor-based SVCs are common in HV AC grids.Causality: R does not logically explain A; it is an additional fact. Verification / Alternative check: Power electronics textbooks and utility practice confirm both statements; propulsive drives, HVDC links, and thermal processing all employ thyristors, and utilities deploy SVCs using thyristors for voltage stabilization and power factor correction. Why Other Options Are Wrong: Stating R explains A misattributes causality. Declaring either A or R false conflicts with established practice. Common Pitfalls: Confusing SVCs (thyristor-switched) with STATCOMs (converter-based) and assuming one invalidates the other; both exist. Final Answer: Both A and R are correct but R is not correct explanation of A

Question 2

Raising speed of a dc series motor fed through a full converter A DC series motor drives a load whose torque is proportional to speed (T_load ∝ ω). It is supplied from a single-phase AC source via a full-controlled converter. At 500 rpm, the firing …

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Introduction / Context: Armature-controlled DC drives using single-phase full converters regulate average armature voltage by changing the firing angle α. For a series motor with a speed-proportional load torque, determining the direction of change of α to raise speed is a core control concept. Given Data / Assumptions: DC series motor; T_load ∝ ω. Full-controlled (bridge) rectifier, average armature voltage Va ∝ cos α (for continuous conduction). At 500 rpm, α = 45°; desire higher speed at same load law. Concept / Approach: For a fixed supply, decreasing α raises the average armature voltage Va, increasing current and electromagnetic torque. Since T_load rises with speed, the operating point is where electromagnetic torque equals load torque. A higher Va shifts the motor’s speed–torque curve upward, producing a new intersection at a higher speed. Step-by-Step Solution: Average armature voltage (ideal): Va ≈ (2V_m/π) cos αDecrease α → cos α increases → Va increases.Higher Va → greater Ia and developed torque → equilibrium at higher ω for T_load ∝ ω. Verification / Alternative check: Drive characteristics in standard texts show speed increases monotonically as α is reduced, barring field-weakening nuances (not in series field case here). Practical controllers decrease α to accelerate. Why Other Options Are Wrong: Increase α (option b): reduces Va, lowering speed. Option c: ambiguous; the required change is specifically to reduce α. Decrease load torque (option d): not a converter control action; and the question asks about firing angle adjustment. Common Pitfalls: Confusing displacement factor effects with average voltage control; the dominant effect on speed is Va via α. Final Answer: Decrease the firing angle

Question 3

Cycloconverter input-side power factor In practical operation, the input (supply-side) power factor of a cycloconverter is generally:

Accepted Answer

Introduction / Context: Cycloconverters synthesize low-frequency output directly from the AC supply using phase-controlled thyristor bridges. Their supply current is highly non-sinusoidal and displaced, affecting the input power factor seen by the grid and influencing transformer sizing and utility penalties. Given Data / Assumptions: Line-commutated cycloconverter (no DC link). Phase-controlled bridges with significant harmonic content. Typical industrial load (e.g., large synchronous or induction machine at low output frequency). Concept / Approach: Phase control introduces both displacement (due to firing angle) and distortion (harmonics) components. The net effect is a low overall power factor. Moreover, the fundamental input current typically lags the voltage (lagging PF) because the bridges draw current when line voltage is favorable, analogous to rectifier behavior, and reactive power is drawn due to commutation and harmonics. Step-by-Step Solution: Firing at α > 0 → displacement between fundamental current and voltage.Non-sinusoidal currents → large distortion factor reduction.Combined effect → low, lagging input PF. Verification / Alternative check: Utility experiences and textbooks report typical input PF values well below unity for cycloconverters, motivating filters or VAR support. Why Other Options Are Wrong: “High” PF options (c, d) contradict heavy distortion and phase delay. “Leading” (a) is not characteristic of line-commutated bridges absent overcompensation or capacitive correction. Common Pitfalls: Assuming motor power factor dictates supply PF; in cycloconverters, the converter dominates. Final Answer: Low and lagging

Question 4

Modified McMurray full-bridge inverter – device count A modified McMurray full-bridge inverter employs which combination of main thyristors and diodes?

Accepted Answer

Introduction / Context: McMurray and modified McMurray inverters are classic forced-commutated topologies using commutation capacitors and auxiliary devices. The “modified” versions add energy recovery paths and additional diodes to improve efficiency and device stress handling. Given Data / Assumptions: Full-bridge single-phase inverter framework. Forced commutation via commutation capacitors and auxiliaries. Energy recovery diodes present in modified topology. Concept / Approach: The original McMurray full-bridge commonly totals 8 thyristors and 4 feedback diodes (one antiparallel diode per main leg). The modified McMurray extends the commutation and recovery network, introducing additional diodes to route commutation energy back to the source or capacitors, bringing the count to 8 thyristors and 8 diodes. Step-by-Step Solution: Count main devices: full bridge needs 4 main thyristors per half-cycle pair → total 8 for full commutation scheme.Add feedback (antiparallel) and recovery diodes per leg and auxiliary paths → aggregate diodes increase to 8.Hence, modified McMurray full-bridge: 8 thyristors + 8 diodes. Verification / Alternative check: Typical device-count tables in power electronics references list higher diode counts for “modified” topologies due to energy recovery additions, versus the original McMurray–Bedford implementations. Why Other Options Are Wrong: Lower diode counts align with simpler or non-modified versions. Common Pitfalls: Confusing McMurray, McMurray–Bedford, and modified variants; each has distinct commutation and recovery networks affecting device counts. Final Answer: 8 thyristors and 8 diodes

Question 5

Fully controlled bridge converter feeding a highly inductive load (assume nearly constant DC load current) What is the typical input (source) current waveform drawn from the AC supply?

Accepted Answer

Introduction / Context: A fully controlled bridge converter supplying a highly inductive load is a staple case in power electronics. With large inductance, the load current can be assumed nearly constant (continuous conduction). The question focuses on identifying the shape of the input (mains) current waveform under these conditions, which is essential for estimating harmonics, power factor, and filter requirements. Given Data / Assumptions: Topology: single-phase fully controlled bridge (four SCRs) or an equivalent fully controlled bridge. Load: highly inductive, so i_load is approximately constant within a cycle. Continuous conduction, device drops and overlap neglected for concept. Freewheeling occurs inherently via the bridge commutations. Concept / Approach: When the bridge devices conduct, the source must supply approximately the constant DC load current. During the alternate half cycle, the current reverses direction in the supply (because a different pair of SCRs conducts), but its magnitude remains nearly the same while conducting. Between commutations, depending on control angle, the source current is zero. Thus, the input current is a sequence of flat-topped pulses rather than a sinusoid. Step-by-Step Solution: Assume i_load ≈ I0 (constant).From α to α+π, one device pair conducts ⇒ source current ≈ ±I0 (flat segment).Outside that conduction window, source current ≈ 0.Therefore the line current waveform is pulsed and nearly rectangular during conduction. Verification / Alternative check: Standard waveforms in converter texts show rectangular line current pulses for continuous-current operation with inductive loads. Harmonic analysis confirms significant odd harmonics typical of rectangular pulses. Why Other Options Are Wrong: Sinusoidal is incorrect because phase control and the DC load current flatten the conduction current. DC is impossible since the AC source enforces alternating polarity. Triangular would require linearly changing current, which contradicts the constant-current assumption due to large inductance. Common Pitfalls: Confusing the DC load current with AC source current; assuming any AC-fed circuit draws sinusoidal current; overlooking the effect of large inductance making the current during conduction nearly constant. Final Answer: Rectangular (pulsed, approximately constant magnitude during conduction)

Question 6

Silicon controlled rectifier (SCR) characteristic Does applying a gate signal reduce the forward breakover voltage of an SCR?

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Introduction / Context: The forward breakover voltage of an SCR is the voltage at which the device will turn on without a gate signal. Understanding how the gate affects this threshold is central to reliable triggering and protection against accidental turn-on. Given Data / Assumptions: SCR in forward blocking mode (anode positive with respect to cathode). A gate signal can be applied to initiate conduction. Device operates within its safe temperature range. Concept / Approach: Applying a gate current injects carriers into the SCR’s inner junctions, reducing the required anode-to-cathode voltage to reach regenerative action. Thus, the forward breakover voltage is effectively lowered when a suitable gate signal is present, enabling controlled turn-on at a commanded phase angle in AC circuits. Step-by-Step Solution: Start with SCR forward blocking at V_AK below intrinsic breakover.Apply gate current: additional carriers enhance transistor action in the two-transistor model.Regenerative feedback starts at a lower V_AK ⇒ conduction begins earlier. Verification / Alternative check: Datasheets show breakover curves shifting with gate current (I_G). Higher I_G corresponds to lower breakover voltage and lower required V_AK for turn-on. Why Other Options Are Wrong: Temperature changes influence parameters but the fundamental effect of the gate applies across normal ranges. Reverse blocking mode is unrelated to forward breakover reduction by the gate. Common Pitfalls: Assuming removal of the gate will turn the SCR off (it will not, once latched); confusing latching current and holding current with breakover behavior. Final Answer: True

Question 7

Relationship between ripple factor (RF) and form factor (FF) in rectifier outputs Choose the correct formula linking RF and FF.

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Introduction / Context: Ripple factor and form factor are standard quality measures for rectifier outputs. Form factor quantifies the ratio of RMS value to average value, while ripple factor quantifies the AC component relative to the DC component in a rectified waveform. Given Data / Assumptions: Let V_rms be total RMS of the rectified output. Let V_dc be the average (DC) value. Let V_ac,rms be the RMS value of the ripple component alone. Concept / Approach: By definition: FF = V_rms / V_dc and RF = V_ac,rms / V_dc. Also, V_rms^2 = V_dc^2 + V_ac,rms^2. Rearranging these gives the direct relationship between RF and FF. Step-by-Step Solution: Start: V_rms^2 = V_dc^2 + V_ac,rms^2.Divide both sides by V_dc^2 ⇒ (V_rms/V_dc)^2 = 1 + (V_ac,rms/V_dc)^2.Thus, FF^2 = 1 + RF^2.Therefore, RF = (FF^2 − 1)^(1/2). Verification / Alternative check: Plug common rectifier values (e.g., half-wave, full-wave) to confirm known RF values are recovered from the corresponding FF values. Why Other Options Are Wrong: Linear relationships RF = FF − 1 or RF = FF^2 − 1 ignore the Pythagorean relationship from RMS summation. The square-root of (FF − 1) is dimensionally inconsistent relative to the derivation. Common Pitfalls: Confusing total RMS with ripple RMS; forgetting that RMS components add in power (square) not linearly. Final Answer: RF = (FF^2 − 1)^(1/2)

Question 8

Integral-cycle (on–off) control of an AC regulator with a resistive load If the duty cycle is a (0 < a ≤ 1), what is the overall input power factor seen by the supply?

Accepted Answer

Introduction / Context: Integral-cycle control (burst firing) turns whole cycles of the AC waveform on and off to control average power. With a purely resistive load, current and voltage are in phase during each “on” cycle, yet the waveform is discontinuous over long intervals, affecting the overall power factor seen by the supply. Given Data / Assumptions: Resistive load (displacement factor during “on” cycles = 1). Duty cycle a is the fraction of time the regulator delivers full cycles to the load. “On” cycles are ideal (no device drops); “off” cycles draw zero current. Concept / Approach: Overall power factor PF is real power divided by apparent power over the averaging interval. Real power scales directly with the duty cycle a (since only a portion of cycles deliver power). Apparent power scales with the overall RMS current, which is reduced by √a because current flows only during an a-fraction of time. Combining these yields PF = √a for a resistive load under integral-cycle control. Step-by-Step Solution: Let P_full be real power if always on; then P_avg = a * P_full.Let I_rms,full be RMS current if always on; overall RMS current I_rms,tot = √a * I_rms,full (current present only a fraction of time).Apparent power S = V_rms * I_rms,tot = V_rms * √a * I_rms,full.PF = P_avg / S = (a * V_rms * I_rms,full) / (V_rms * √a * I_rms,full) = √a. Verification / Alternative check: Check limits: if a = 1 (always on), PF = 1 (unity). If a = 0.25, PF = 0.5, matching the intuitive reduction due to burst operation. Why Other Options Are Wrong: PF = a or a^2 overestimates the reduction; PF = 1 ignores the discontinuous nature of current and is only valid for a = 1. Common Pitfalls: Confusing displacement factor (which remains 1 in “on” cycles) with overall power factor; forgetting to compute RMS over the entire time window including “off” intervals. Final Answer: √a

Question 9

Single-phase fully controlled bridge feeding an R–L–E load (continuous current) Given v_s(t) = Vm sin(ωt), firing angle α, and average current I0, which relation is correct?

Accepted Answer

Introduction / Context: For a fully controlled bridge supplying an R–L–E load (e.g., a DC motor back-emf), the average DC output voltage depends on firing angle α and equals the sum of the average resistive drop and the back-emf. This relation is the cornerstone for speed and torque control in drives. Given Data / Assumptions: Input: v_s(t) = Vm sin(ωt). Fully controlled bridge (four SCRs), continuous current. Ideal devices, no overlap; average load current = I0; series R and back-emf E. Concept / Approach: For continuous current and no overlap, the average output voltage is V_avg = (2Vm/π) cos α. KVL for the DC side gives V_avg = R I0 + E. Combining these two expressions yields the desired relation for cos α in terms of R, I0, E, and Vm. Step-by-Step Solution: Use converter average: V_avg = (2Vm/π) * cos α.Use DC KVL: V_avg = R I0 + E.Equate: (2Vm/π) * cos α = R I0 + E.Solve for cos α: cos α = (π/(2Vm)) * (R I0 + E). Verification / Alternative check: Limit checks: with E = 0 and α = 0°, cos α = 1 ⇒ V_avg = 2Vm/π, the well-known maximum. Increasing α reduces V_avg linearly with cos α as expected. Why Other Options Are Wrong: Options with incorrect proportionality (e.g., multiplied by 2Vm/π instead of divided) or sign errors (E − R I0) do not satisfy both the converter equation and DC KVL simultaneously. Common Pitfalls: Mixing up Vm (peak) with V_rms; forgetting continuous conduction assumption; ignoring the back-emf term. Final Answer: cos α = (π/(2Vm)) * (R I0 + E)

Question 10

Three-phase semiconverter (half-controlled) output continuity Beyond what firing angle does the DC output voltage become discontinuous (gaps appear) for a resistive/weakly inductive load?

Accepted Answer

Introduction / Context: The three-phase semiconverter (one quadrant, half-controlled) uses three controlled devices and three diodes. Its output voltage waveform quality and continuity depend strongly on the firing angle α and load smoothing. Designers must know where natural gaps begin to appear so that filter sizing and performance expectations are realistic. Given Data / Assumptions: Three-phase semiconverter feeding a load with limited smoothing. Ideal devices, negligible overlap for concept. Focus on onset of discontinuity as α increases. Concept / Approach: In a semiconverter, each device pair supplies segments of the line-to-line wave. As α increases, the conduction intervals shrink relative to the 60° spacing of line EMFs. When α exceeds 60°, there are intervals where no device pair can sustain positive DC voltage, leading to notches or zero output spans—i.e., discontinuity in the average DC voltage waveform for lightly filtered or resistive loads. Step-by-Step Solution: At α ≤ 60°, conduction sequences overlap sufficiently to keep the output positive across phases.At α > 60°, conduction windows retreat and gaps appear where the rectified line-to-line voltage would otherwise be available.Hence the DC output becomes discontinuous beyond α = 60° (for weak smoothing). Verification / Alternative check: Phase diagram timing confirms the loss of continuous transfer when the trigger is delayed beyond the 60° inter-phase spacing typical of a three-phase system. Why Other Options Are Wrong: 30° is too low; 90° and 120° overstate the continuity range for a semiconverter with weak smoothing. Common Pitfalls: Confusing semiconverters with fully controlled bridges; assuming large output inductance which can maintain current continuity even when voltage has gaps. Final Answer: α > 60°

Question 11

Static excitation system in synchronous generators Which components are typically used in a static excitation system?

Accepted Answer

Introduction / Context: Excitation systems provide the field current for synchronous generators. Static exciters replaced rotating exciters in many modern plants for faster response and reduced maintenance. Identifying their building blocks helps understand control loops and reliability considerations. Given Data / Assumptions: Static system means no rotating exciter machines. Field current is obtained by rectifying an AC source. Fast controllability is a design aim. Concept / Approach: In a static excitation system, a power transformer feeds a controlled rectifier (thyristor bridge). The rectified output provides adjustable DC to the generator field through slip rings or brushless arrangements with stationary power electronics. Thyristors enable rapid, high-power control of field current under AVR (automatic voltage regulator) commands. Step-by-Step Solution: AC source stepped to suitable voltage by transformer.Controlled rectifier (thyristor bridge) converts AC to adjustable DC.DC supplied to field; AVR modulates firing angle to regulate generator terminal voltage. Verification / Alternative check: Plant schematics and standards (IEEE/IEC) categorize excitation systems as static when they employ stationary power electronics rather than rotating exciters. Why Other Options Are Wrong: Options including AC or DC exciters imply rotating machines; those are not “static” systems. A standalone transformer with a DC exciter is not a static exciter either. Common Pitfalls: Confusing brushless exciters (which are rotating but with rotating rectifiers) with fully static systems; ignoring the thyristor bridge as the controllable element. Final Answer: Transformer and thyristors (controlled rectifier)

Question 12

Semi-converter feeding a purely resistive load How do the output voltage and output current waveforms compare?

Accepted Answer

Introduction / Context: A single-phase semi-converter (half-controlled bridge) is often used for DC outputs with reduced reactive effects. With a purely resistive load, the current follows the instantaneous applied voltage without phase shift. Recognizing this is key for predicting power factor and waveform quality. Given Data / Assumptions: Semi-converter topology supplying a purely resistive load. Negligible device voltage drops for conceptual clarity. Instantaneous current i(t) = v_out(t) / R. Concept / Approach: For a resistive load, there is no energy storage. Therefore, the current through the load is proportional to and in phase with the applied voltage at every instant. Regardless of the firing angle, whenever the converter applies a positive voltage segment, the current mirrors that shape exactly, scaled by 1/R. When the converter output is zero, the current is also zero. Step-by-Step Solution: During conduction window: v_out(t) is a clipped/controlled sinusoid; i_out(t) = v_out(t)/R has identical shape.During non-conduction: v_out(t) = 0 ⇒ i_out(t) = 0.Thus, waveforms are identical in shape and phase, differing only by the constant scale factor 1/R. Verification / Alternative check: Oscilloscope traces for R loads show current and voltage crossing zero simultaneously with identical wave shapes during each conduction interval. Why Other Options Are Wrong: Claiming they can never be similar contradicts resistive behavior. Limiting similarity to small firing angles is unnecessary. Ambiguous dependence on conduction mode is irrelevant for a strictly resistive load. Common Pitfalls: Confusing R loads with RL loads (where current lags and shapes differ). Assuming device commutation effects change the fundamental R-proportionality. Final Answer: They are exactly similar (same shape and phase)

Question 13

McMurray full-bridge inverter Which commutation principle is employed in the McMurray full-bridge inverter topology?

Accepted Answer

Introduction / Context: McMurray and McMurray-Bedford inverter families are classic forced-commutated topologies used prior to widespread high-frequency PWM methods. The full-bridge version uses a specific commutation approach that pairs switches in a complementary manner using commutating inductors and capacitors. Given Data / Assumptions: Voltage-source inverter, full-bridge configuration. Designed for self-commutation (not reliant on utility zero crossings). Commutation network includes coupled inductors and capacitors. Concept / Approach: Complementary commutation uses energy exchange between commutating components to transfer current from one device to its complementary partner, turning the outgoing device off while turning the incoming device on. This is distinct from auxiliary commutation (separate auxiliary switch per device) and from natural commutation (line-commutated converters). Step-by-Step Solution: During transition, commutating L–C elements force current reversal across the outgoing SCR.The complementary device assumes current seamlessly, achieving turn-off of the outgoing switch.This process repeats each half-cycle, implementing complementary commutation across the bridge legs. Verification / Alternative check: Canonical diagrams and waveforms in inverter textbooks label the McMurray full-bridge as a complementary-commutated inverter. Why Other Options Are Wrong: Auxiliary commutation is associated more with half-bridge McMurray types; natural commutation belongs to line-commutated converters, not self-commutated inverters. “Any of the above” is too vague and incorrect. Common Pitfalls: Confusing the McMurray half-bridge (auxiliary) with the full-bridge (complementary); assuming modern PWM methods imply the same commutation principle. Final Answer: Complementary commutation

Question 14

n-stage high-frequency series inverter (total time period T) For an inverter composed of n identical series-resonant stages operating in sequence within one period T, how long is each stage active?

Accepted Answer

Introduction / Context: Multi-stage high-frequency series inverters distribute the switching and resonant intervals among several identical stages to increase effective output frequency and reduce component stress. Understanding timing allocation per stage is key for design and control. Given Data / Assumptions: Total inverter period is T. There are n identical resonant stages. Stages operate sequentially and non-overlapping within T. Concept / Approach: If the stages share the cycle equally and operate one after another without overlap, the simplest time division gives each stage an equal slice of T. Hence, each stage remains active for T/n while the others are idle or preparing for the next interval. This evenly apportions resonant energy exchange and switch stress among stages. Step-by-Step Solution: Assume uniform time-division multiplexing of resonant actions.Total period T = sum of n equal active intervals.Therefore each active interval per stage = T / n. Verification / Alternative check: Design examples and timing diagrams demonstrate equal-length gating windows to maintain symmetry and balance among identical stages. Why Other Options Are Wrong: T/2n implies overlaps or double counting; T means only one stage would operate; T/(n − 1) has no basis in equal partitioning for n stages. Common Pitfalls: Assuming unequal sharing without a specified reason; overlooking dead-time which, if present, is typically small and still leads to approximately T/n per stage. Final Answer: T/n

Question 15

Assertion (A): Gate triggering is the most commonly used method to turn on a Silicon Controlled Rectifier (SCR). Reason (R): Even a small gate current is sufficient to drive an SCR into conduction (latching) and is easier to control than other metho…

Accepted Answer

Introduction: Silicon Controlled Rectifiers (SCRs) are thyristor devices widely used in power control and conversion. Triggering methods include gate triggering, light triggering, dv/dt triggering, and thermal triggering. The question tests understanding of why gate triggering is preferred and whether a small gate current is sufficient to turn an SCR on. Given Data / Assumptions: Device: SCR (thyristor) Gate terminal available for control Objective: evaluate correctness of Assertion (A) and Reason (R) Concept / Approach: In practice, gate triggering is preferred because it is reliable, repeatable, and allows precise timing. The SCR requires a brief gate current pulse to create enough carriers to start conduction; once anode current exceeds the latching current, the device remains on without further gate drive. Step-by-Step Solution: 1) Gate triggering is the industry standard: it uses low-power electronics and provides precise control over firing angle.2) A small gate current (above the specified I_G trigger) initiates conduction; after latching, the anode current maintains the ON state.3) Alternative methods like dv/dt or thermal are either undesirable (can cause false turn-on) or impractical for routine control.4) Therefore A is true. R states a key reason: small, easily generated gate current is sufficient, which directly supports why gate triggering is common. Verification / Alternative check: Datasheets specify I_GT (gate trigger current) typically in mA, confirming that modest pulses reliably fire SCRs under rated conditions. Why Other Options Are Wrong: B: Suggests explanation is unrelated; it is directly related. C: R is not wrong; it reflects standard SCR behavior. D: A is not wrong; gate triggering is indeed the most common. Common Pitfalls: Confusing holding current with latching current; assuming continuous gate drive is needed (it is not once latched). Final Answer: Both A and R are correct and R is the correct explanation of A.

Power Electronics Questions