Question 1

Multiphase (interleaved) chopper operation In a multiphase chopper system designed to reduce output ripple, how do the individual chopper units operate?

Accepted Answer

Introduction / Context: Multiphase (interleaved) choppers are used in high-power DC supplies and motor drives to reduce current ripple, decrease filter size, and share device stress. Understanding the operating mode of each phase is fundamental for ripple calculations and controller design. Given Data / Assumptions: Multiple identical chopper legs (phases) feeding a common DC output. Common input DC source, usually with proper input/output filtering. Objective: minimize ripple by phase interleaving. Concept / Approach: Interleaving staggers the switching instants among parallel legs. Each leg carries a fraction of the load current and turns on at a unique phase angle within the switching period. This produces ripple cancellation at the output, effectively increasing the ripple frequency and lowering ripple amplitude. Therefore, legs operate in parallel but not simultaneously in time—each fires in turn (one at a time in a time-shifted sense). Step-by-Step Solution: Identify topology: multiple choppers feeding the same output → parallel power paths.Interleave gate signals by 360°/m (for m phases).Ripple components partially cancel, reducing filter requirements. Verification / Alternative check: Ripple-frequency multiplication equals m times the per-phase switching frequency; this is only achieved when phases are interleaved, not when all switch simultaneously. Why Other Options Are Wrong: (a) Series connection is uncommon for choppers intended for ripple reduction. (c) Simultaneous operation forfeits ripple cancellation benefits. (d) Mixed behavior contradicts the design objective; consistent interleaving is required. Common Pitfalls: Equating 'parallel' with 'simultaneous' switching. In interleaving, they are parallel paths but time-shifted for ripple minimization. Final Answer: In parallel and one at a time (interleaved time-sharing)

Question 2

Dielectric heating power stage using thyristors In industrial dielectric heating systems requiring high-frequency AC for heating, the thyristor-based power stage typically consists of:

Accepted Answer

Introduction / Context: Dielectric heating (also called radio-frequency heating) requires high-frequency alternating electric fields to heat insulating materials. Mains frequency (50/60 Hz) is too low, so power-electronic conversion is used to produce the required high-frequency AC. Given Data / Assumptions: Supply is standard AC mains. Load needs high-frequency AC (tens of kHz commonly). Thyristor-based conversion stages are considered. Concept / Approach: The most common approach is AC → DC → high-frequency AC. A controlled rectifier (or diode bridge with DC link) first produces DC. An inverter then synthesizes high-frequency AC to drive the dielectric heating electrodes. This two-stage 'rectifier–inverter' chain allows control of output frequency and power level independently from the grid frequency. Step-by-Step Solution: Use a rectifier to convert grid AC to DC (with filtering).Feed a high-frequency inverter (e.g., series/parallel resonant topology) with the DC link.Match the load through an RF tank/network to deliver controllable HF power. Verification / Alternative check: Industrial RF heaters and induction heaters commonly use rectifier–inverter chains; an AC regulator alone cannot change frequency, and a chopper works on DC only (no HF AC generation). Why Other Options Are Wrong: (a) Chopper does not create AC. (b) Controlled rectifier alone gives DC output. (c) AC regulator only varies RMS at line frequency. (e) Cycloconverters are typically used for low-frequency outputs, not high-frequency dielectric heating. Common Pitfalls: Confusing dielectric heating (needs high frequency) with resistance heating (mains frequency is acceptable). Final Answer: Rectifier–inverter combination

Question 3

Firing (gating) frequency in a three-phase semiconverter For a 3-phase semiconverter (three thyristors and three diodes), the required firing frequency of the thyristor gating pulses is:

Accepted Answer

Introduction / Context: In three-phase rectifiers, ripple frequency and pulse number differ from the firing (gating) frequency per device. Knowing how often each thyristor must receive a firing pulse helps design gate drivers and synchronizing circuits. Given Data / Assumptions: Three-phase semiconverter: 3 SCRs + 3 diodes. Balanced three-phase supply. Continuous conduction assumed for conceptual discussion. Concept / Approach: In a semiconverter, only the positive group (say, anode-connected to phases) uses controllable devices, each naturally commutated at a line-frequency rate. Each SCR is triggered once per cycle of its phase. Thus the per-device firing frequency equals the line frequency. Note that the output ripple frequency relates to pulse number p = 3 for a semiconverter (3f ripple), but this is distinct from gating frequency per thyristor. Step-by-Step Solution: Identify that each SCR conducts for a portion of the cycle once per phase period.Therefore, each receives one firing pulse per cycle → firing frequency = f_line.Ripple frequency (3f) should not be confused with gating frequency. Verification / Alternative check: Conduction tables for three-phase semiconverters show one controlled firing per phase period, with diodes providing the complementary paths. Why Other Options Are Wrong: Options (b), (c), and (d) confuse ripple/pulse frequency with gate firing rate. Option (e) has no basis in natural-commutated operation. Common Pitfalls: Mixing up output ripple frequency and required gate trigger repetition per SCR. Final Answer: Equal to the line frequency

Question 4

Single-phase fully controlled bridge with source inductance During the commutation (overlap) interval caused by source inductance, the instantaneous output voltage of the rectifier equals:

Accepted Answer

Introduction / Context: Source inductance in line-commutated rectifiers creates commutation overlap, during which outgoing and incoming thyristor pairs conduct simultaneously. Understanding the effect on output voltage is important for predicting performance, especially at high load current and low firing angles. Given Data / Assumptions: Single-phase fully controlled bridge (four SCRs). Finite source inductance causing overlap. Ideal devices aside from commutation overlap. Concept / Approach: During overlap, both thyristor pairs are on. The AC source effectively short-circuits through the two conducting legs, and the instantaneous output voltage becomes the difference of the two phase voltages applied by each leg. In a single-phase bridge, these two are equal and opposite in sign at the commutation instant, yielding a net of approximately zero at the DC terminals during the overlap interval. Step-by-Step Solution: At commutation, pair 1 (outgoing) and pair 2 (incoming) conduct together.The output is the average of two opposite half-cycle source voltages → cancellation.Hence, v_o during overlap ≈ 0 (neglecting small resistive drops). Verification / Alternative check: Classical expressions show v_o is reduced by overlap such that the average output voltage V_dc is decreased by a term proportional to overlap angle and source inductance; instantaneously, v_o ≈ 0 while both pairs share current. Why Other Options Are Wrong: Options (a), (b), and (e) assume single-pair conduction. Option (d) is not the correct instantaneous relation for a full bridge during overlap. Common Pitfalls: Confusing average voltage reduction over a cycle with instantaneous voltage during the overlap interval; mixing half-controlled and full-controlled behavior. Final Answer: Zero

Question 5

Triggering a thyristor without gate current If the gate current of a thyristor is zero, conduction can still start only when the forward anode-to-cathode voltage exceeds:

Accepted Answer

Introduction / Context: A silicon-controlled rectifier (SCR) is designed to turn on via a gate trigger. However, in the absence of gate current, it may still turn on if the anode-cathode voltage is raised high enough to avalanche the central junction—this is the forward breakover phenomenon. Knowing this is important for protection and dv/dt design. Given Data / Assumptions: Gate current = 0 (no intentional trigger). Device is forward biased at the anode-cathode terminals. Static characteristics as per a standard SCR. Concept / Approach: With no gate drive, the SCR remains blocking until the anode-cathode voltage reaches the forward breakover voltage V_BO, where junction avalanche and regenerative feedback force the device into conduction. Under normal operation one avoids reaching V_BO to prevent uncontrolled turn-on; instead, a gate pulse is used at a lower forward voltage. Step-by-Step Solution: Set I_G = 0.Increase V_AK from zero: the device remains off until V_AK ≈ V_BO.At V_BO, regenerative action latches the SCR on → conduction begins. Verification / Alternative check: Datasheets depict forward blocking and breakover regions; V_BO reduces with higher temperature or high dv/dt but is well above the on-state voltage. Why Other Options Are Wrong: 0.5 V and on-state voltage describe conduction after turn-on, not the triggering threshold. 'Peak working off-state forward voltage' is a rating below breakover, not the trigger point. Common Pitfalls: Confusing breakover with latching/holding parameters; mistaking low on-state voltage for the trigger threshold. Final Answer: The forward breakover voltage

Question 6

Dual converter operating quadrants State whether the following is true or false: “In a dual converter, both converters operate as rectifiers simultaneously.”

Accepted Answer

Introduction / Context: A dual converter consists of two controlled bridges connected in anti-parallel across a DC load to provide four-quadrant operation. Correct understanding of their operating roles (rectifier or inverter) is vital for setting firing angles and ensuring smooth current reversal. Given Data / Assumptions: Dual converter with two fully controlled bridges (Converter-1 and Converter-2). Four-quadrant DC operation is intended. Either circulating-current or non–circulating-current scheme may be used. Concept / Approach: At any given time, one bridge functions as a rectifier (delivering power from AC to DC), while the other—if active—must behave as an inverter (returning power from DC to AC) when both are biased. In non–circulating-current mode, only one bridge is gated at a time (rectifier or inverter). In circulating-current mode, both may be biased simultaneously but one will be set to a slightly higher DC voltage and the other slightly lower, creating controlled circulating current; still, both are not acting as rectifiers at the same time. Step-by-Step Solution: Identify power flow direction in each bridge.In four-quadrant operation, simultaneous dual rectification is impossible; one side would necessarily invert to balance DC voltage.Hence the statement is false. Verification / Alternative check: Control strategies always ensure complementary firing angles: alpha_1 + alpha_2 ≈ 180 degrees to set opposite DC polarities in circulating-current systems. Why Other Options Are Wrong: Options suggesting conditional truth are incorrect; the bridges cannot both draw power from AC to DC together while connected antiparallel at a common DC bus. Common Pitfalls: Assuming 'both on' equals 'both rectifying'; in reality, one must act as an inverter for controlled current circulation or reversal. Final Answer: False

Question 7

Output voltage waveform of a single-phase half-bridge inverter (unmodulated) What is the basic shape of the line-to-neutral output voltage for a half-bridge inverter using equal DC-link capacitors and 180° conduction (no PWM)?

Accepted Answer

Introduction / Context: The half-bridge inverter is a fundamental single-phase topology that produces an AC output from a DC link. Without pulse-width modulation (PWM) or filtering, the raw output waveform is set by the switching pattern rather than the load alone. Given Data / Assumptions: Two switches with complementary gating (180° conduction). Split DC link with midpoint to form the neutral. No output filter; unmodulated operation. Concept / Approach: When the top device conducts, the output node is tied to +Vdc/2; when the bottom device conducts, it is tied to −Vdc/2. With ideal devices and hard switching, the output toggles between two constant levels. Thus the time-domain output voltage is a two-level square wave (line-to-neutral). A sinusoidal output would require modulation plus filtering. Step-by-Step Solution: Enumerate switching states: upper ON → +Vdc/2, lower ON → −Vdc/2.Sequence the states with 50% duty for 180° conduction.Observe the rectangular alternation → square waveform. Verification / Alternative check: Fourier analysis of the square wave shows odd harmonics; adding an LC filter or PWM can approximate a sinusoid, but the native inverter output remains square. Why Other Options Are Wrong: (a) and (c) would require specific modulation or filtering; (d) incorrectly attributes shape to load; (e) sawtooth is not produced by standard half-bridge switching. Common Pitfalls: Confusing inverter output before filtering with the filtered load voltage; assuming load inductance alone makes it sinusoidal. Final Answer: Square

Question 8

Carrier types in SCR anode current In a conducting SCR (thyristor), the anode current consists of which charge carriers?

Accepted Answer

Introduction / Context: An SCR is a four-layer PNPN device. When it conducts, the internal conductivity is enhanced (conductivity modulation) due to high-level injection, and both types of carriers participate in current transport. Knowing this clarifies why the on-state voltage is low despite relatively thick silicon needed for blocking capability. Given Data / Assumptions: SCR operating in its forward-conducting state. Charge transport through PN junctions within a multilayer device. High-level injection conditions. Concept / Approach: In the on-state, both P-type and N-type regions inject minority carriers into adjacent regions. The resulting carrier plasma reduces resistivity significantly (conductivity modulation). Consequently, anode current is carried by electrons and holes, much like in a forward-biased large-area diode but with regenerative thyristor action sustaining conduction. Step-by-Step Solution: Forward-bias the device so that all three internal junctions are effectively conducting.Carrier injection occurs from P to N and from N to P regions.Total current = electron component + hole component. Verification / Alternative check: Two-transistor (PNP–NPN) equivalent model shows current sharing between transistor branches, implying both carrier types participate. Why Other Options Are Wrong: Options (a) and (b) are incomplete; (d) suggests exclusivity by operating quadrant, which does not apply to the on-state of an SCR; (e) is impossible for conduction. Common Pitfalls: Assuming a unipolar conduction similar to metals; neglecting the role of conductivity modulation in reducing on-state drop. Final Answer: Both electrons and holes

Question 9

Single-phase full-wave AC voltage controller (AC regulator) using two antiparallel SCRs In practical operation, the firing angles used in the positive and negative half-cycles of the input supply ___.

Accepted Answer

Introduction / Context: A single-phase full-wave AC voltage controller (AC regulator) typically uses two SCRs in antiparallel (or a TRIAC) to control the RMS output voltage delivered to a load by phase-angle control. A common doubt is whether the firing angles applied in the positive and negative half-cycles must always be identical. Given Data / Assumptions: Single-phase supply and a pair of antiparallel SCRs (or an equivalent TRIAC). Gate pulses can be independently timed each half-cycle. Load may be resistive or inductive. Concept / Approach: For ideal symmetrical control, designers choose equal firing angles in both half-cycles so that the average output (DC component) is zero and electromagnetic effects are minimized. However, the controller hardware and control algorithm can impose different firing delays in each half-cycle (intentional or due to tolerances), so equality is not a hard requirement of operation—only of good practice when symmetry is desired. Step-by-Step Solution: Define α+ as the firing angle in the positive half-cycle and α− in the negative half-cycle.Symmetrical operation: α+ = α− to avoid DC offset and reduce transformer/core saturation issues.Asymmetrical operation: α+ ≠ α− can occur due to gating circuit design, sensor phase errors, or deliberate control strategies.Therefore, in general, the firing angles may be equal or unequal; equality is preferred but not mandatory. Verification / Alternative check: Waveform simulations with α+ ≠ α− show unequal positive/negative half-cycle areas, producing a DC component. Controllers are usually calibrated to set α+ ≈ α− for power-quality reasons, but operation is still possible otherwise. Why Other Options Are Wrong: Always equal / never equal: Too absolute; both can occur. Sometimes equal: Vague and not as general as “may be equal or unequal.” Must differ by 90°: No such general rule exists. Common Pitfalls: Assuming equality is enforced by physics. It is a control choice to ensure symmetry and reduce harmonics. Final Answer: may be equal or unequal

Question 10

Single-phase dual converter (back-to-back converters) If α₁ and α₂ are the firing angles of the two converters operating in circulating-current mode, which relation must hold for equal and opposite average voltages?

Accepted Answer

Introduction / Context: A single-phase dual converter consists of two line-commutated converters connected back-to-back to provide reversible DC output. In circulating-current mode, both converters can conduct simultaneously through an interconnecting reactor, and their average output voltages are controlled to be equal in magnitude and opposite in polarity. Given Data / Assumptions: Two fully controlled single-phase converters: Converter 1 with firing angle α₁, Converter 2 with firing angle α₂. Circulating current reactor present; steady operation requires equal and opposite average DC voltages. Concept / Approach: For a line-commutated single-phase full converter, the average DC output voltage (neglecting overlap) is proportional to cos(α). Equal and opposite average voltages imply cos(α₁) = −cos(α₂), which yields α₁ + α₂ = π. Step-by-Step Solution: Let Vd1 ∝ cos(α₁) for Converter 1.Let Vd2 ∝ cos(α₂) for Converter 2.Condition for equal and opposite averages: Vd1 = −Vd2 ⇒ cos(α₁) = −cos(α₂).Hence α₁ + α₂ = π (principal solution for 0 ≤ α ≤ π). Verification / Alternative check: Phasor and waveform analysis of the dual converter confirm this relationship for zero net average across the circulating reactor. Why Other Options Are Wrong: α₁ − α₂ = 0: Would make average voltages equal, not opposite. α₁ − α₂ = π / α₁ + α₂ = 2π: Not the standard balance condition in 0–π operating range. Common Pitfalls: Confusing inversion relations; remember the average voltage is proportional to cos(α). Final Answer: α₁ + α₂ = π

Question 11

DC chopper design for current ripple limit A step-down DC chopper (V = 500 V) feeds a DC series motor. Chopping frequency is 400 Hz. What series inductance L is required to limit the peak-to-peak current ripple to 10 A (assume worst-case timing near…

Accepted Answer

Introduction / Context: DC choppers control the average voltage applied to a motor by rapid switching. The series inductance (motor inductance plus any added inductor) limits current ripple. Designers often set a maximum allowable peak-to-peak ripple ΔI for performance and EMI reasons. Given Data / Assumptions: Input DC voltage V = 500 V. Chopping frequency f = 400 Hz ⇒ switching period T = 1 / 400 = 0.0025 s. Allowed ripple ΔI = 10 A (peak-to-peak). Worst-case ripple occurs near mid duty where the on-time and off-time are comparable. For estimation, take Ton ≈ 0.001 s (close to 0.4 of T), a common conservative assumption in preliminary sizing. Concept / Approach: For a step-down chopper with negligible back-EMF during the small ripple interval, the rising current during Ton is approximately linear: ΔI ≈ (V / L) * Ton. Rearranging gives L ≈ (V * Ton) / ΔI. Step-by-Step Solution: T = 1 / f = 1 / 400 = 0.0025 s.Assume Ton ≈ 0.001 s (near mid-duty used for worst-case ripple sizing).Use ΔI = (V / L) * Ton ⇒ L = (V * Ton) / ΔI.L = (500 * 0.001) / 10 = 0.05 H = 50 mH (rounded to about 51 mH). Verification / Alternative check: If Ton were slightly different (e.g., 0.00125 s at exact 50% duty), L would be 62.5 mH, confirming that a design choice around 50–60 mH is reasonable. Selecting the offered value near 50 mH matches the target ripple limit with a safety margin handled elsewhere. Why Other Options Are Wrong: 20 mH / 31 mH: Too small; would allow larger ripple than 10 A under the stated assumptions. 92 mH: Overly conservative for the target; larger than necessary per the basic sizing. Common Pitfalls: Ignoring the duty-dependent Ton in ripple estimation; forgetting that motor back-EMF can reduce ripple but should not be relied upon for worst-case design. Final Answer: about 51 mH

Question 12

UJT (uni-junction transistor) characteristic Between which characteristic points does a UJT exhibit its negative-resistance region?

Accepted Answer

Introduction / Context: The UJT is widely used in relaxation oscillators and triggering circuits because of its negative-resistance region in the V–I characteristic. Recognizing the interval over which negative resistance exists is key to designing timing and triggering networks. Given Data / Assumptions: Standard UJT emitter characteristic with peak point (Vp, Ip) and valley point (Vv, Iv). Device biased with emitter referenced to the base resistor network. Concept / Approach: As the emitter voltage is increased to the peak point, the device enters conduction; beyond this point, the emitter voltage actually drops with increasing current until the valley point—this is the negative-resistance region. After the valley point, the device behaves in a low-resistance regime. Step-by-Step Solution: Increase Ve from zero to Vp: current rises slowly to Ip (no negative resistance yet).From peak (Vp, Ip) to valley (Vv, Iv): dV/dI is negative—negative-resistance behavior.Beyond the valley point: the device transitions to a quasi-ohmic low-resistance state. Verification / Alternative check: Typical datasheets and textbooks plot the V–I curve showing the negative-resistance stretch explicitly between peak and valley. Why Other Options Are Wrong: Before peak / after valley only: Regions are not negative-resistance. After peak only (without valley limit): Incomplete; the region ends at the valley point. Common Pitfalls: Confusing the trigger point (peak) with the entire operating range; stable oscillation relies on designing around the peak–valley segment. Final Answer: between peak and valley points

Question 13

Gate current requirement for SCR turn-on In typical power SCR applications, the gate current needed to turn on an SCR is of the order of:

Accepted Answer

Introduction / Context: The SCR (silicon-controlled rectifier) is a latching device that turns on when properly forward-biased and when a sufficient gate current is applied. Understanding the correct magnitude of gate current avoids overstressing gate drivers and ensures reliable triggering. Given Data / Assumptions: Commercial SCRs often specify gate trigger current Igt in the range of a few milliamps to a few tens of milliamps. Anode current capability may be tens to hundreds of amperes, much larger than the gate current. Concept / Approach: The gate current only initiates conduction by injecting carriers; it does not have to sustain the load current. Once latched, the SCR continues conducting as long as the anode current exceeds the holding current. Step-by-Step Solution: Check relative magnitudes from datasheets: Igt is typically mA, not A.Conclude that “a few mA” is the correct order for gate drive. Verification / Alternative check: Application notes recommend gate pulses of a few times Igt for noise immunity, still within mA to tens of mA. Why Other Options Are Wrong: Ampere-level or fractions of anode current: Far in excess of what is required and could damage the gate. Common Pitfalls: Confusing gate trigger current with holding current or latching current; those are device-specific but still far below anode load currents. Final Answer: a few mA

Question 14

Single-phase semiconverter feeding a resistive load If the firing angle is α (in radians), each controlled device (thyristor) conducts for how long per half-cycle?

Accepted Answer

Introduction / Context: A single-phase semiconverter (half-controlled bridge) with a resistive load produces a chopped sinusoidal output. The conduction interval of each thyristor depends on the firing angle α and the natural commutation at the next current zero. Given Data / Assumptions: Resistive (R) load, negligible inductance. Thyristor is fired at ωt = α in each relevant half-cycle. Natural commutation occurs at the next zero crossing of source voltage/current at ωt = π. Concept / Approach: In an R-load, current and voltage are in phase. Once the thyristor is triggered at α, it conducts until the source current naturally reaches zero at the end of the half-cycle (ωt = π). Therefore, the conduction interval equals (π − α). Step-by-Step Solution: Trigger at ωt = α.Conduction continues to ωt = π (current goes to zero).Thus, conduction duration = π − α. Verification / Alternative check: Plotting instantaneous current for an R load confirms conduction ends at the natural current zero coinciding with the voltage zero. Why Other Options Are Wrong: π radians: Would imply conduction over the full half-cycle regardless of α. (π + α): Physically impossible within a half-cycle. (π − 0.5 α) or α: Do not match natural commutation boundary at π. Common Pitfalls: Confusing inductive load behavior (where conduction can extend) with purely resistive load behavior. Final Answer: (π − α) radians

Question 15

Gate pulse duration versus SCR turn-on time An SCR has a specified turn-on time of 4 μs. Approximately how long should the gate pulse be applied to ensure reliable turn-on under practical conditions?

Accepted Answer

Introduction / Context: Gate pulses must be sufficiently wide and strong to guarantee that an SCR latches on under real-world conditions (component tolerances, temperature, source impedance). The specified turn-on time t_on is a device characteristic measured under idealized conditions. Given Data / Assumptions: SCR turn-on time t_on = 4 μs. Standard practice uses a gate pulse wider than t_on (often 3–5 times) to ensure latching. Concept / Approach: A margin ensures that the anode current exceeds the latching current during the pulse and that the device does not drop out due to line notches or minor disturbances. Therefore, a pulse around 20 μs (≈5×) is conservative and widely adopted. Step-by-Step Solution: Choose pulse width τ_g ≥ k * t_on.For robust design, take k ≈ 5.τ_g ≈ 5 * 4 μs = 20 μs. Verification / Alternative check: Many application notes recommend ≥10 μs for small SCRs and even longer for large devices; 20 μs satisfies this guidance. Why Other Options Are Wrong: 2 μs, 5 μs: Too close to or below t_on, marginal under real conditions. 50 μs: Acceptable but unnecessarily long for this specification, potentially increasing drive losses. Common Pitfalls: Using pulses just equal to t_on without safety margin can cause misfiring under temperature and supply variations. Final Answer: 20 μs

Question 16

Comparing 3-phase semiconverter and 3-phase full converter (same load) If a 3-phase semiconverter gives rectification efficiency ≈ 0.6 and transformer utilization factor (TUF) ≈ 0.35 for a given load, how do these figures change when a 3-phase full …

Accepted Answer

Introduction / Context: Three-phase rectifiers are evaluated by rectification efficiency (conversion of AC to useful DC) and transformer utilization factor (how effectively the transformer VA rating is used). A full converter uses six controlled devices and both halves of the AC waveform more effectively than a semiconverter. Given Data / Assumptions: Same load conditions for fair comparison. Semiconverter: rectification efficiency ≈ 0.6, TUF ≈ 0.35. Full converter: six-pulse, controlled on both polarities. Concept / Approach: The full converter provides a higher average DC output for a given RMS transformer current and spreads conduction among phases more uniformly, improving both rectification efficiency and transformer utilization. Step-by-Step Solution: Compare output DC levels: full converter achieves larger controllable average DC voltage magnitude.Compare transformer loading: full converter’s conduction pattern yields better TUF.Thus both metrics improve over the semiconverter. Verification / Alternative check: Standard tables for power converters list higher TUF and efficiency for full converters than for semiconverters under comparable conditions. Why Other Options Are Wrong: Mixed better/worse claims: Do not reflect typical performance trends for the same load. Common Pitfalls: Confusing “efficiency” with power factor; power factor behavior differs, but the question asks rectification efficiency and TUF. Final Answer: rectification efficiency will be more than 0.6 and TUF will be more than 0.35

Question 17

Saturable reactor behavior When the magnetic core of a saturable reactor is driven into saturation, what happens to its inductance?

Accepted Answer

Introduction / Context: Saturable reactors exploit the nonlinear magnetization of a core to control effective inductance with a DC control current. Understanding how inductance changes near saturation is crucial for control and protection. Given Data / Assumptions: Inductance L ∝ μ (effective permeability) for a given geometry. Core B–H curve shows μ dropping significantly in saturation. Concept / Approach: As the core approaches saturation, incremental permeability decreases sharply, which reduces the incremental inductance. Therefore, in saturation, the inductive reactance drops, allowing more AC current to flow—this is the very principle used in magnetic amplifiers and saturable reactors. Step-by-Step Solution: Unsaturated: μ is high ⇒ L is high.Drive into saturation: μ falls ⇒ L falls.Hence, the statement “inductance is high in saturation” is false. Verification / Alternative check: Differential inductance measured from the slope of the B–H curve near saturation is small, matching the expected drop. Why Other Options Are Wrong: Inductance infinite/unchanged: Not supported by magnetic material physics. “True” to the original claim: Opposite of actual behavior. Common Pitfalls: Confusing unsaturated high-μ behavior with the saturated regime; they are opposite in effect on L. Final Answer: False

Question 18

SCR conduction characteristics and junction temperature Assertion (A): When a silicon controlled rectifier (SCR) is conducting, the on-state voltage drop across it is approximately 1 V. Reason (R): When an SCR is conducting, the junction temperature…

Accepted Answer

Introduction / Context: This item checks practical knowledge of SCR on-state behavior and allowable junction temperature. In power electronics design, knowing typical on-state voltage and realistic thermal limits is essential for loss and reliability calculations. Given Data / Assumptions: Silicon controlled rectifier operating in on-state (conduction region). Normal current densities and adequate gate drive. Commercial device ratings and data-sheet conventions. Concept / Approach: In conduction, an SCR behaves like a forward-biased PNPN device with an on-state drop typically around 1 to 2 V depending on current and die temperature. Junction temperature ratings for most SCRs are commonly in the 110–150°C range; designing for ~125°C is typical. A blanket statement of “about 200°C” exceeds normal maximum junction ratings. Step-by-Step Solution: Check Assertion: On-state voltage ≈ 1 V to 2 V → “about 1 V” is acceptable as a typical value → A is correct.Check Reason: Typical Tj(max) is not 200°C; it is usually ≤ 150°C → R is false.Conclusion: A is true; R is false → Option “A is correct but R is wrong”. Verification / Alternative check: Representative SCR data sheets list V_T(on) ≈ 1.2–1.8 V at rated current and Tj(max) around 125–150°C. These corroborate A and contradict R. Why Other Options Are Wrong: Options implying R is correct conflict with standard Tj ratings. “Both wrong” is invalid because the on-state drop statement is broadly correct. Common Pitfalls: Confusing case temperature with junction temperature. Assuming on-state drop is negligible (it is not at high currents). Final Answer: A is correct but R is wrong

Question 19

Gate-drive choice for a single-phase full-wave AC regulator feeding an R–L load Which gate signal strategy is best suited for reliable firing?

Accepted Answer

Introduction / Context: In AC voltage regulators using antiparallel SCRs feeding an R–L load, the instantaneous device voltage can vary rapidly and the anode current lags due to inductance. Gate-drive strategy must ensure the SCR actually turns on when anode-cathode voltage becomes forward biased. Given Data / Assumptions: Single-phase full-wave AC regulator (two SCRs back-to-back). Load contains significant inductance L. Gate trigger threshold varies with temperature and device. Concept / Approach: Because inductive loads shift current zeroes and create uncertain forward-bias timing, a single narrow pulse may miss the exact forward-biased moment. A pulse train (a series of short pulses spanning a few milliseconds) greatly increases the probability of successful turn-on exactly when the device becomes forward-biased, while keeping average gate power low. Step-by-Step Solution: Inductance → current lags → forward bias may occur slightly after the planned α instant.Single short pulse may occur too early → misfire risk.Pulse train straddling α ensures at least one pulse coincides with proper forward bias → reliable triggering. Verification / Alternative check: Industrial gate drivers commonly use pulse trains for AC controllers, exactly to accommodate line and load variations and reduce gate heating. Why Other Options Are Wrong: Short pulse or long single pulse is less robust to timing uncertainty. DC gate bias wastes power and can stress the gate without improving reliability. “Either (a) or (b)” is inferior to a well-timed pulse train. Common Pitfalls: Assuming the voltage zero cross equals immediate forward bias on the intended device. Under-estimating gate power with continuous drive. Final Answer: A pulse train (repetitive pulses) around the firing instant

Question 20

Cycloconverter frequency capability A cycloconverter cannot provide an output frequency higher than the input supply frequency. Is this statement correct?

Accepted Answer

Introduction / Context: Cycloconverters directly synthesize low-frequency waveforms from a higher-frequency AC source by phase-controlled switching, without an intermediate DC link. Understanding their inherent frequency limit is fundamental in drives and large low-speed machine applications. Given Data / Assumptions: Standard grid-commutated cycloconverters (line-commutated). Phase control of segments of the input waveform. No forced-commutation high-frequency synthesis stage is used. Concept / Approach: Classic cycloconverters are step-down frequency converters; they construct each output half-cycle from many appropriately phased segments of the input. Since they select portions of the existing input cycles, the output fundamental cannot exceed the supply frequency. Step-by-Step Solution: Input frequency = f_s.Cycloconverter output is assembled by gating segments of the input → output fundamental f_o is a submultiple of f_s.Therefore f_o ≤ f_s, typically f_o ≪ f_s in practice (e.g., 10–20 Hz from 50/60 Hz). Verification / Alternative check: Applications such as large synchronous motor drives use cycloconverters specifically for low frequencies; for higher frequencies, DC-link inverters are preferred. Why Other Options Are Wrong: “False”: contradicts the direct selection principle. Transformer turns ratio does not change frequency. Load type or system phase count does not raise the maximum output frequency. Common Pitfalls: Confusing cycloconverters with matrix converters or PWM voltage-source inverters which can synthesize higher frequencies from a DC link. Final Answer: True

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