Question 1

Half-bridge inverter DC supply requirement A single-phase half-bridge inverter uses two main switches and a DC midpoint. Which statement best describes its DC supply requirement?

Accepted Answer

Introduction / Context: The single-phase half-bridge inverter produces a bipolar output by referencing each leg alternately to the mid-point of the DC link. Understanding its DC requirement (true three-wire supply or two-wire formed into three-wire via split capacitors) is fundamental in power electronics design. Given Data / Assumptions: Topology: half-bridge with two active switches. Midpoint is required to generate +Vdc/2 and −Vdc/2 levels. Midpoint can be provided by a real three-wire source or by two series DC capacitors (virtual three-wire). Concept / Approach: A half-bridge inherently requires a central reference (midpoint) to create two equal and opposite voltages across the load. Even if the DC bus is originally two-wire, designers split it with two series electrolytics to create a neutral, still functioning as a three-wire DC supply. Thus, the accurate statement is that a three-wire supply (real or synthesized) is needed. Step-by-Step Solution: Identify required output levels: +Vdc/2, −Vdc/2 → needs midpoint.Realize midpoint via center-tapped source or split capacitors.Therefore, functionally the inverter needs a 3-wire DC supply. Verification / Alternative check: Datasheets/application notes show half-bridge DC link drawn with two capacitors in series and their junction as the neutral node. Why Other Options Are Wrong: Stating “two-wire” alone omits the need for a midpoint; claims about higher voltage/efficiency are secondary and not inherent to needing three wires. Common Pitfalls: Confusing half-bridge (needs midpoint) with full-bridge (does not need midpoint). Final Answer: needs a 3 wire dc supply

Question 2

Three-phase series inverter switch count A 3-phase series inverter (thyristor-based commutation) typically requires how many thyristors for three-phase output generation?

Accepted Answer

Introduction / Context: Different inverter topologies (voltage-source, current-source, series/parallel commutated) use different device counts. A 3-phase series inverter generates three-phase output with controlled commutation of thyristors. The device count question checks familiarity with practical implementations comparable to the classic three-phase bridge structure. Given Data / Assumptions: Each phase needs positive and negative half-cycle generation through controlled devices. Commutation is provided via series (resonant) networks. We consider a symmetrical three-phase output with conventional legs. Concept / Approach: The functional arrangement maps to a six-device structure akin to a three-phase bridge: each phase leg requires an upper and a lower controlled path. In series (resonant) inverters, commutation circuits are added, but the number of main thyristors to synthesize three-phase power remains six in common configurations. Step-by-Step Solution: Three phases × two controlled devices per phase → 6 main thyristors.Auxiliary commutation components do not change the main device count.Therefore, typical requirement is six thyristors. Verification / Alternative check: Standard references draw three-phase thyristor inverters with six controlled devices, regardless of the exact commutation method (series or parallel resonant auxiliary networks). Why Other Options Are Wrong: 3 thyristors cannot synthesize all three legs; 12 refers to special multi-level or redundant schemes; “3 or 6” is ambiguous and not standard. Common Pitfalls: Confusing auxiliary commutation switches with main devices; counting diodes as thyristors. Final Answer: 6 thyristors

Question 3

Single-phase converter with center-tapped transformer and highly inductive load For a single-phase full converter using a center-tapped secondary and a highly inductive (continuous current) load, with firing angle α, each thyristor conducts for what…

Accepted Answer

Introduction / Context: Full-controlled single-phase converters commonly use either a bridge or a center-tapped secondary with two thyristors. With a highly inductive load, current is continuous and each device conducts for a fixed span independent of the firing angle, apart from overlap. The question targets recognition of conduction intervals under continuous current. Given Data / Assumptions: Single-phase full control with center-tapped transformer secondary. Load inductance is large enough to maintain continuous current. Overlap and source impedance are neglected for simplicity. Concept / Approach: In continuous conduction, once fired, a thyristor continues to conduct until current is naturally transferred to the other device. In the ideal case, each device conducts for 180 degrees (π radians). Firing angle α shifts the current transfer instant but does not reduce the total conduction angle per device for a continuous current scenario. Step-by-Step Solution: Assume continuous current: current never falls to zero within a half-cycle.At each half-cycle, the newly fired SCR takes over and conducts until the opposite half-cycle ends.Conduction interval per device → 180° = π radians. Verification / Alternative check: Waveforms in standard texts show rectangular device current pulses of width π when load inductance is high. Why Other Options Are Wrong: α or π − α would apply if current were discontinuous; π/2 is for special commutation intervals; 2π − α is not meaningful for single device conduction in one cycle. Common Pitfalls: Assuming conduction width shrinks with larger α even under continuous current; forgetting that inductance sustains current. Final Answer: π

Question 4

Voltage across thyristors: half-wave vs full-wave (M-2) converters with resistive load A single-phase half-wave controlled converter feeding an R load and a single-phase full-wave (M-2) converter feeding an R load are both fired at angle α. How do t…

Accepted Answer

Introduction / Context: Converter topology determines device stress and voltage blocking patterns. Even with the same firing angle α and purely resistive loads, the thyristor voltages in half-wave (one device conduction per positive half-cycle) and full-wave M-2 (two devices conducting alternately in each half-cycle) differ significantly. Recognizing these waveform differences is essential for device selection and snubber design. Given Data / Assumptions: Both loads are purely resistive. Firing angle α is the same in both circuits. Ideal devices; source inductance neglected. Concept / Approach: In the half-wave circuit, the controlled device sees full reverse half-cycle voltage stress and blocks for the entire negative half. In the M-2 full-wave circuit (center-tapped), two SCRs alternately conduct; each device’s blocking and conduction intervals are mirrored across half-cycles. Voltage across each thyristor thus has a distinct pattern compared with the single-device half-wave case, leading to non-similar waveforms. Step-by-Step Solution: Half-wave: one SCR; device blocks from ωt = 0 to α, then conducts to π; blocks entire negative half-cycle.M-2: two SCRs; each fires at α in alternate half-cycles; blocking/forward intervals alternate between the two devices.Device voltage waveforms differ in polarity, duration, and timing → not similar. Verification / Alternative check: Plotting v_SCR(t) for both topologies demonstrates different blocking intervals and polarities even for the same α. Why Other Options Are Wrong: “Similar” or “identical only at α = 0” misrepresents fundamental topology differences; “may be similar” is vague and incorrect for ideal cases. Common Pitfalls: Comparing output load voltage waveforms instead of device voltage stress; ignoring the second device in M-2. Final Answer: are not similar

Question 5

HVDC system applicability beyond long distance Is the statement “HVDC systems are preferable only for long distances” correct?

Accepted Answer

Introduction / Context: High Voltage Direct Current (HVDC) transmission is known for economic advantages over long distances due to lower line losses and no reactive power flow. However, its applicability extends well beyond distance economics. The question probes broader engineering reasons for choosing HVDC. Given Data / Assumptions: HVDC link vs HVAC alternatives. Consider submarine cables, asynchronous interties, power flow control. No quantitative cut-off distance is provided; focus on principle. Concept / Approach: HVDC is preferred for: (1) long-distance overhead lines; (2) very long submarine cables (where HVAC capacitive charging makes AC infeasible); (3) asynchronous interconnection between grids of different frequencies; (4) precise and rapid power flow control and damping of oscillations; (5) connecting remote renewable generation. Hence saying “only for long distances” is false. Step-by-Step Solution: Identify non-distance drivers: submarine cable viability, grid decoupling, controllability.Compare with AC: HVAC suffers from reactive power and stability constraints in the above cases.Conclude the statement is incorrect: HVDC is preferred for several reasons not limited to distance. Verification / Alternative check: Real-world installations (e.g., cross-sea interconnectors, back-to-back HVDC stations) validate HVDC selection where distances may be short but technical needs dominate. Why Other Options Are Wrong: Options that qualify “only for overhead” or “only above 800 kV” miss other major use cases and technologies (VSC-HVDC, LCC back-to-back). Common Pitfalls: Equating HVDC strictly with very long lines; ignoring cable charging issues and asynchronous ties. Final Answer: False

Question 6

Half-bridge inverter feeding an R–L load A single-phase half-bridge inverter must feed an R–L load (current continuity required). What minimum set of power devices is needed in the inverter leg(s)?

Accepted Answer

Introduction / Context: Inductive loads store energy and force current to continue flowing when the applied voltage reverses or devices turn off. Inverters for R–L loads therefore require freewheeling paths to maintain current continuity and avoid overvoltage across switches. This is fundamental to safe commutation and waveform quality. Given Data / Assumptions: Topology: single-phase half-bridge. Load: R–L, so current lags voltage and must freewheel when switches commutate. Switches are thyristors; diodes can provide antiparallel freewheeling paths. Concept / Approach: Each active device needs an antiparallel diode to allow reverse or freewheel current when the device is off. In a half-bridge with two controlled devices, two antiparallel diodes are sufficient to provide current paths during the opposite half-cycle or device turn-off, ensuring current continuity and limiting overvoltage. Step-by-Step Solution: Identify two main thyristors in half-bridge (upper and lower).Add one antiparallel diode across each thyristor.This provides necessary freewheel paths → total: 2 thyristors + 2 diodes. Verification / Alternative check: Standard inverter schematics for inductive loads show antiparallel diodes across each controlled device (also inherent body diodes in MOSFETs/IGBTs provide this in other technologies). Why Other Options Are Wrong: Only thyristors (no diodes) leave no path for inductive current → dangerous; using four thyristors is full-bridge, not half-bridge; omitting diodes with MOSFETs is unrealistic because body diodes exist and are required. Common Pitfalls: Confusing half-bridge with full-bridge; ignoring the need for freewheel diodes for inductive loads. Final Answer: 2 thyristors and two diodes

Question 7

Steady-state current in an R–L–E load fed by a DC chopper When a DC chopper supplies an R–L–E load (resistance R, inductance L, back EMF E), how does the steady-state load current behave?

Accepted Answer

Introduction / Context: In chopper-fed DC drives and supplies, the input switch alternates between on and off states. An R–L–E load includes a source EMF (e.g., motor back EMF), resistance, and inductance. Understanding current ripple under PWM operation is essential for sizing L, estimating ripple, and predicting torque pulsations in motors. Given Data / Assumptions: Ideal chopper with duty ratio D. Load has finite inductance L (not infinite). Periodic steady state reached. Concept / Approach: With finite L, current cannot change instantaneously but changes gradually. During the switch ON interval, the net applied voltage across L may be (Vdc − E − iR), leading to rising current; during OFF, the freewheel path applies approximately (−E − iR) (or similar, depending on topology), causing current to fall. Hence the current oscillates between a minimum and maximum around an average value determined by D, R, L, and E. Step-by-Step Solution: ON interval: di/dt ≈ (Vdc − E − iR) / L → current increases.OFF interval: di/dt ≈ (−E − iR) / L (with freewheel) → current decreases.Result: periodic ripple between I_min and I_max, not perfectly constant. Verification / Alternative check: In the limit L → ∞ (theoretical), ripple tends to zero and current approaches constant; for finite L, ripple persists and is a design parameter set by switching frequency and inductance. Why Other Options Are Wrong: “Remains constant” requires infinite L; “constant if R is constant” is incorrect since L dictates ripple; current is not zero during off-time because the inductor freewheels current. Common Pitfalls: Assuming zero current during off time; forgetting that inductor enforces current continuity via freewheel path. Final Answer: varies between maximum and minimum values

Question 8

Single-phase full-wave AC voltage controller using two thyristors For a single-phase full-wave AC regulator that uses two antiparallel thyristors only, how should the two gate drive circuits be arranged with respect to electrical isolation?

Accepted Answer

Introduction / Context: Antiparallel thyristor AC regulators (two SCRs connected in opposite directions across the load) are widely used for single-phase voltage control. Proper gate drive isolation is critical for safety, reliable triggering, and avoiding unintended current paths through the control circuit. Given Data / Assumptions: Two SCRs connected antiparallel across a single-phase source and load. Independent gate triggering required on positive and negative half cycles. Typical control uses pulse transformers or opto-isolated drivers. Concept / Approach: Each SCR anode/cathode swaps roles every half cycle. The gate emitter reference for one SCR sits at a different instantaneous potential than the other. Without isolation, a shared low-voltage controller can experience large common-mode voltages or create fault paths, risking mis-triggering or damage. Step-by-Step Solution: Identify that each SCR is referenced to a different mains potential.Note that the gate circuit must withstand line potential differences.Provide galvanic isolation (pulse transformer or optocoupler) for each gate circuit → safe, clean, phase-selective triggering. Verification / Alternative check: Industrial regulator schematics consistently show isolated gate drives to avoid circulating currents and to meet insulation requirements. Why Other Options Are Wrong: May or may not be isolated / preferably isolated: understates safety and EMC needs. Should not be isolated: unsafe and prone to mis-triggering. Common Pitfalls: Sharing a ground between both gates and the control PCB; ignoring common-mode dv/dt; insufficient isolation ratings of optocouplers. Final Answer: must be isolated

Question 9

Device physics – which device uses a metal–silicon junction as its principle? Identify the semiconductor device that relies on a metal–silicon (Schottky) junction for its operation.

Accepted Answer

Introduction / Context: Semiconductor junctions determine device behavior. P–N junctions dominate traditional diodes and SCRs, while metal–semiconductor junctions form Schottky barriers with distinctive electrical traits (low forward drop and fast switching). Given Data / Assumptions: Metal–silicon junction creates a Schottky barrier. Comparing typical power devices. Concept / Approach: Schottky diodes use a metal contact on N-type silicon, forming a rectifying barrier. No minority-carrier charge storage exists, enabling very fast recovery and low Vf, at the expense of higher reverse leakage and lower reverse voltage ratings than P–N diodes. Step-by-Step Solution: Identify device using metal–semiconductor junction → Schottky diode.Other listed devices rely primarily on P–N junctions (power diode, SCR) or MOS structures (MOSFET gate oxide plus body diode but not a main metal–silicon rectifying junction). Verification / Alternative check: Datasheets specify “Schottky barrier” and show low stored charge and negligible reverse recovery. Why Other Options Are Wrong: General-purpose power diode: P–N junction. SCR: multiple P–N layers (pnpn). MOSFET: insulated gate; body diode is P–N, not metal–silicon rectification. Common Pitfalls: Confusing the MOSFET’s metal gate with a rectifying junction; the gate is insulated by SiO2 and does not form a conduction junction to silicon. Final Answer: Schottky diode

Question 10

SCR usefulness – key reason What is the primary reason for the widespread utility of a silicon controlled rectifier (SCR) in power control applications?

Accepted Answer

Introduction / Context: SCRs dominate line-commutated converters and AC controllers because they combine high power handling with controllable turn-on via the gate. Understanding what differentiates SCRs from uncontrolled diodes clarifies their value in practical circuits. Given Data / Assumptions: SCR structure: pnpn with gate control. Line-commutated; turn-off by current zero or forced commutation. Concept / Approach: The standout feature is controllable turn-on using a low-power gate signal, enabling phase control and power regulation. While SCRs also have high voltage and current ratings, those are not unique; the core advantage over diodes is the ability to trigger conduction at chosen instants. Step-by-Step Solution: Compare to a diode: diode turns on when forward biased; SCR waits for a gate pulse.Phase control → adjust firing angle → regulate average output voltage and power.Therefore, “gate triggering” is the defining utility. Verification / Alternative check: Basic AC regulator and controlled rectifier examples show power control via adjustable firing angle using the gate. Why Other Options Are Wrong: High temperature triggering: not a standard operating mode and would be unsafe. “Triggering” (vague): lacks specificity; “gate triggering” is precise. High breakdown voltage: valuable but not the main reason for controllability. Common Pitfalls: Assuming high ratings alone justify SCR use; without gate control, a power diode has similar ratings but no controllability. Final Answer: gate triggering

Question 11

Half-wave vs full-wave controlled rectifiers – efficiency and ripple (Assertion-Reason) Assertion (A): A half-wave controlled rectifier has poorer efficiency and a higher ripple factor than a full-wave controlled rectifier. Reason (R): Adding a free…

Accepted Answer

Introduction / Context: Converter performance is judged by efficiency, ripple, and power factor. Half-wave circuits utilize only one half cycle, leading to inferior performance relative to full-wave circuits that exploit both half cycles. Given Data / Assumptions: Comparison at similar load and firing conditions. Freewheeling diode considered where applicable for inductive loads. Concept / Approach: Full-wave converters process both half cycles, delivering higher average output power for the same peak values and significantly lower ripple. Half-wave converters inherently waste half the source cycle and impose larger DC offsets and ripple on the load. Step-by-Step Solution: Assess Assertion: Half-wave has lower efficiency and higher ripple → True.Assess Reason: A freewheeling diode provides a path for inductive load current during source reversals, smoothing current and improving displacement power factor → True.Causality: The reason speaks to improving a half-wave circuit, not to why half-wave is worse than full-wave. Hence it does not explain the assertion’s comparison to full-wave. Verification / Alternative check: Waveform sketches show continuous load current with a freewheel path, yet average power remains lower than full-wave because negative half cycles are still not rectified to the load. Why Other Options Are Wrong: A: Implies correct causal link, which is absent. C/D: Would deny a correct statement (either A or R), which they are not. Common Pitfalls: Confusing “improvement” within a half-wave circuit with “equivalence” to a full-wave circuit. Even with freewheeling, half-wave utilization remains inferior. Final Answer: Both A and R are correct but R is not correct explanation of A

Question 12

Fully controlled bridge with RLE load – effect of reversing the battery EMF A fully controlled bridge converter feeds an R–L–E load. With one battery polarity, the required firing angle is 47°. If the battery connections are reversed (E changes sign…

Accepted Answer

Introduction / Context: In an R–L–E load, E represents a back EMF or battery. The average DC output of a fully controlled bridge is Vdc = Vd0 * cos α (for continuous current), opposed or aided by E depending on its polarity. Reversing E alters the required α for the same current direction and magnitude. Given Data / Assumptions: Line-commutated 1-phase bridge (concept extends similarly to 3-phase with different Vd0). Continuous current approximation. Initial α = 47° for given E. Concept / Approach: Converter must supply Vdc to overcome R*I plus the opposing EMF. If E reverses sign so that it opposes converter output more strongly (i.e., acts against power flow), the bridge must reduce its average output (more retarded firing) or even go toward inversion to keep current conditions, which means a larger α is needed. Step-by-Step Solution: Original: Vd0 cosα = E_original + I R.After reversal: Vd0 cosα′ = (−E_original) + I R (harder to drive current).Thus cosα′ < cosα → α′ > α → greater than 47°. Verification / Alternative check: Limit case: if E is large and opposing, α may need to exceed 90° (inversion region) to hold current constraints. Why Other Options Are Wrong: Equal or less than 47° would imply the converter output need not compensate the added opposition, which is incorrect. “Either” ignores the defined direction of opposition after reversal. Common Pitfalls: Forgetting that cos α decreases as α increases; mixing up aiding vs opposing EMF effects. Final Answer: more than 47°

Question 13

Approximate efficiency of a chopper (DC–DC converter) What is a typical overall efficiency figure for a practical chopper operating under normal conditions?

Accepted Answer

Introduction / Context: DC–DC choppers use high-frequency switching to minimize dissipation and achieve high efficiency. Understanding realistic efficiency helps with thermal design and power budgeting for drives and power supplies. Given Data / Assumptions: Modern switching devices with low conduction and switching losses. Reasonable magnetics and minimal control losses. No extreme low-load or poorly designed cases. Concept / Approach: Losses arise from conduction (I^2R or Vce(sat)·I), switching (overlap and device capacitances), gate drive, magnetics, and ripple. With proper design, these are a small fraction of output power, yielding high efficiency well above 80% and often 90–95% for medium to high power converters. Step-by-Step Solution: Estimate conduction loss fraction small with low Rds_on or low Vce(sat).Switching at suitably chosen frequency controls switching loss.Ancillary losses (gate drive, control) are minor at scale. Verification / Alternative check: Datasheets and application notes routinely report efficiencies exceeding 85–95% under nominal load. Why Other Options Are Wrong: 50%, 20%, 5% are characteristic of linear regulators or faulty designs, not switching choppers. Common Pitfalls: Ignoring cooling and layout; excessive switching frequency without need; operating deep in light-load where fixed overheads dominate can reduce efficiency but rarely to very low values. Final Answer: 80% or more

Question 14

Bidirectional conduction capability Which of the following devices can inherently conduct current in both directions between its main terminals (bilateral conduction)?

Accepted Answer

Introduction / Context: Understanding whether a device is unilateral or bilateral is key to AC control. Some switches conduct in only one direction (like SCRs), while others are constructed to conduct in both (SBS, TRIAC). Given Data / Assumptions: Focus on conduction directionality between main terminals. Gate/control terminals ignored for this property. Concept / Approach: The SBS is a symmetrical triggering device that conducts in either polarity once its breakover threshold is reached in that polarity. It is used to trigger TRIACs or for symmetrical relaxation oscillator behavior. By contrast, an SCR is unilateral, and a SUS is explicitly unilateral, while a UJT is a transistor used as a trigger/oscillator, not a bilateral power switch. Step-by-Step Solution: Identify bilateral device → SBS.Confirm that SCR, SUS are unilateral; UJT is not a bilateral switch. Verification / Alternative check: Characteristic curves show symmetrical breakover in both polarities for SBS devices. Why Other Options Are Wrong: SCR: conducts in forward direction only until current falls below holding value. SUS: “unilateral” by definition. UJT: a triggering device, not a bilateral power conduction element. Common Pitfalls: Confusing SBS with TRIAC (also bilateral); both are bilateral, but TRIAC is a higher-power AC switch while SBS is a trigger/low-power device. Final Answer: SBS (silicon bilateral switch)

Question 15

McMurray full-bridge inverter – device count In a classic McMurray full-bridge inverter implementation, how many thyristors and diodes are typically used in the main power circuit?

Accepted Answer

Introduction / Context: McMurray family inverters employ forced commutation networks. The full-bridge version duplicates the half-bridge commutation legs, increasing the device count to realize bidirectional load voltage with reliable commutation. Given Data / Assumptions: Classic McMurray full-bridge (not McMurray-Bedford variant). Symmetrical commutation in both legs. Concept / Approach: A half-bridge McMurray stage uses a commutation pair per leg. Converting to a full bridge doubles the number of controlled devices while adding clamp/feedback diodes for energy recovery. A typical arrangement ends up with eight thyristors and four diodes in the main bridge paths, with auxiliary commutation components (capacitors, inductors) not counted here as “diodes”. Step-by-Step Solution: Half-bridge: 4 SCRs + 2 diodes.Full-bridge: two such legs → 8 SCRs + 4 diodes. Verification / Alternative check: Reference topologies for McMurray full bridge show duplicated commutation legs yielding the cited device count. Why Other Options Are Wrong: 4-device answers correspond to half-bridge or simpler bridges. 8 thyristors and 8 diodes overstates the diode count for the basic scheme. Common Pitfalls: Mixing McMurray and McMurray-Bedford counts or including auxiliary recovery diodes differently from main path diodes. Final Answer: 8 thyristors and 4 diodes

Question 16

Series inverter output frequency with commutation dead time In a series inverter, let 0.5T be the natural half-oscillation time of the load commutation (each conduction interval), and let T_off be the dead time between turning off one thyristor and …

Accepted Answer

Introduction / Context: Series inverters rely on LC commutation to produce quasi-sinusoidal current bursts through the load. Between successive half-cycles, a dead time T_off is inserted to allow device recovery and safe commutation. This dead time stretches the electrical period and lowers the output frequency compared to the ideal LC period. Given Data / Assumptions: Each conduction interval spans a natural half-oscillation = 0.5T. After each half-cycle, there is a dead time T_off before the opposite device conducts. The output period consists of two half-cycles plus two dead times. Concept / Approach: One full cycle requires positive half-cycle conduction, a dead time, negative half-cycle conduction, and another dead time. Summing durations yields the total period; the reciprocal gives frequency. Step-by-Step Solution: Half-cycle 1 duration = 0.5T.Dead time 1 = T_off.Half-cycle 2 duration = 0.5T.Dead time 2 = T_off.Total period = 0.5T + T_off + 0.5T + T_off = T + 2T_off.Therefore, f = 1 / (T + 2T_off). Verification / Alternative check: In the ideal limit T_off → 0, f → 1/T, which matches the LC natural frequency period. Increasing T_off reduces f, as expected. Why Other Options Are Wrong: 1/(T + T_off) and 1/(0.5T + T_off) omit one of the dead times. 1/(2T + 2T_off) double counts the oscillation period. Common Pitfalls: Confusing the natural LC period with the practical output period that includes mandatory recovery times. Final Answer: f = 1 / (T + 2T_off)

Question 17

AC component (ripple) of a rectifier output – RMS value For a rectifier output with measured total RMS value Vrms and DC component Vdc, what is the RMS value of the AC (ripple) component alone?

Accepted Answer

Introduction / Context: Rectifier outputs contain a DC component plus ripple (AC component). Quantifying ripple is essential for filter design and performance evaluation. RMS values add in a mean-square sense when components are orthogonal (DC vs AC). Given Data / Assumptions: Total RMS of output voltage = Vrms. DC (average) component = Vdc. AC component is the residual after removing the DC level. Concept / Approach: Because the DC component is constant and the AC component has zero average, the mean of the square of the sum equals the sum of the means of the squares: Vrms^2 = Vdc^2 + (Vac_rms)^2. Therefore, Vac_rms = sqrt(Vrms^2 − Vdc^2). Step-by-Step Solution: Start with decomposition: v(t) = Vdc + v_ac(t), with average of v_ac(t) = 0.Compute RMS: Vrms^2 = (1/T)∫[Vdc + v_ac(t)]^2 dt = Vdc^2 + (1/T)∫[v_ac(t)]^2 dt.Hence Vac_rms = √(Vrms^2 − Vdc^2). Verification / Alternative check: Check limiting cases: If ripple is zero, Vrms = Vdc → Vac_rms = 0. If Vdc = 0 (pure AC), Vac_rms = Vrms, as expected. Why Other Options Are Wrong: Vrms includes DC; not just ripple. 0.5 Vrms is arbitrary and incorrect. √(Vrms − Vdc) is dimensionally wrong and not based on mean-square addition. Common Pitfalls: Subtracting RMS magnitudes directly instead of subtracting power (square) quantities; forgetting that RMS is a quadratic metric. Final Answer: (Vrms^2 - Vdc^2)^0.5

Question 18

Half-bridge vs. full-bridge inverters (same inverter family) Do half-bridge and full-bridge inverters of the same general type necessarily use different commutation methods?

Accepted Answer

Introduction / Context: In power electronics, bridge topology (half-bridge or full-bridge) determines how devices are arranged across the DC link, while commutation method (how devices are turned off) is a separate design choice. This question checks whether topology inherently forces a different commutation method. Given Data / Assumptions: Same inverter “type” means same switching family (for example, line-commutated vs. forced-commutated resonant, McMurray/McMurray–Bedford, PWM, etc.). Devices are thyristors/MOSFETs/IGBTs depending on application; DC link and load can be similar across topologies. Concept / Approach: Commutation refers to how switching devices turn off and transfer current. Forced commutation networks (auxiliary capacitors/inductors), self-commutated PWM with transistors, and resonant techniques can be implemented in either a half-bridge or a full-bridge arrangement. The choice of half-bridge vs. full-bridge is primarily about available DC-link voltage swings and device count, not an intrinsic change in commutation physics. Step-by-Step Solution: Identify the role of topology: half-bridge yields approximately ±Vdc/2 pole voltages; full-bridge yields ±Vdc.Recognize commutation independence: the same commutation network (e.g., complementary gating with dead-time for IGBT PWM, or McMurray forced commutation for SCRs) can be applied to both topologies.Conclude: there is no necessity that half-bridge and full-bridge of the same “type” use different commutation. They can use the same method. Verification / Alternative check: Industrial inverters are often available in both half- and full-bridge forms using the same switching devices and control (e.g., SPWM with IGBTs). Differences show up in voltage utilization and current paths, not in mandatory commutation changes. Why Other Options Are Wrong: “True” implies a forced difference that does not exist. Options about DC-link voltage or load power factor mix system constraints with commutation method selection; they do not mandate different commutation just due to half vs. full bridge. Common Pitfalls: Assuming more devices in a full-bridge implies a new commutation scheme. In reality, switching and commutation principles can be retained across both topologies. Final Answer: False

Question 19

Three-phase to single-phase full-wave bridge cycloconverter How many thyristors are used in a 3-phase to 1-phase full-wave bridge cycloconverter (positive and negative converter groups included)?

Accepted Answer

Introduction / Context: A cycloconverter produces a low-frequency AC output directly from a higher-frequency AC supply using phase-controlled thyristor bridges. Device counting is a common exam check for understanding converter structure and gating requirements. Given Data / Assumptions: Input: three-phase AC supply. Output: single-phase AC (both positive and negative half-cycles synthesized). Full-wave bridge operation in each group (positive and negative). Concept / Approach: To synthesize a single-phase output, a cycloconverter typically uses two three-phase controlled rectifier bridges: one bridge generates the positive portions (positive converter), and another generates the negative portions (negative converter). Each three-phase full-wave controlled bridge requires 6 thyristors. Using two such bridges results in 6 + 6 = 12 devices, often linked via intergroup reactors to prevent circulating currents. Step-by-Step Solution: One 3-phase controlled bridge → 6 SCRs.Two bridges (positive and negative groups) → 2 * 6 = 12 SCRs total.Therefore, the cycloconverter uses 12 thyristors. Verification / Alternative check: Standard textbooks depict two 6-pulse bridges feeding a single-phase load through intergroup reactors. This architecture is ubiquitous in low-frequency large drives. Why Other Options Are Wrong: 8 devices correspond to some single-phase bridges; 18 or 36 devices match higher-pulse or multioutput arrangements, not the basic 3-phase to 1-phase full-wave case. Common Pitfalls: Confusing 3-phase to 3-phase cycloconverters (which require more devices) with the 3-phase to 1-phase variant. Final Answer: 12

Question 20

Device tolerances in thyristors Two thyristors of the same rating and specification are tested. What can be said about their on-state voltage drops and forward blocking characteristics?

Accepted Answer

Introduction / Context: Even with identical part numbers, semiconductor devices exhibit parameter spread due to manufacturing tolerances. Understanding this spread is critical when paralleling or series-connecting devices and when estimating losses. Given Data / Assumptions: Same part number and data-sheet limits. Normal production tolerances; devices not bin-sorted. Measurements under same test conditions. Concept / Approach: Data sheets specify ranges or typical values (with max/min limits) for forward drop, leakage, and blocking capability. Two nominally identical devices can have slightly different forward drops and leakage due to doping variations, die size, and packaging. Hence equal or different values are both possible within specified limits. Step-by-Step Solution: Recognize specified ranges → parameters vary between devices.Conclude that either equality or difference is possible while still meeting specification. Verification / Alternative check: Manufacturers often provide statistical distributions. Engineering practice uses equalizing resistors/capacitors for series/parallel operation to mitigate mismatch. Why Other Options Are Wrong: Options asserting “always equal” or “always different” are too absolute and conflict with real-world tolerance bands. Common Pitfalls: Assuming part number implies exact equality; overlooking temperature dependence that can widen measured differences. Final Answer: They may be equal or different in on-state drop and in forward-blocking characteristics for the same current

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