EC-604-C Power Electronics - Dec 2024 Question Paper

Under each question, How to answer lists the points an answer should cover, outlined by CampusPrep from the syllabus. Check the details against your textbook.

1. 7 Marks
   aExplain the basic structure and V-I characteristics of a power diode.
   How to answer

   • Define power diode as high-power PN junction device and describe basic p-n structure with drift region
   • Explain forward-biased conduction and reverse blocking operation
   • Sketch and explain V-I characteristics including forward drop, reverse leakage, breakdown
   • Mention ratings like current, voltage, switching speed
2. 7 Marks
   bDescribe the basic structure and V-I characteristics of a DIAC. How does a DIAC operate and in what applications is it commonly used?
   How to answer

   • Describe DIAC as bidirectional two-terminal thyristor with three-layer structure
   • Explain symmetric V-I characteristics and breakover voltage in both directions
   • Explain triggering operation without gate
   • List applications like TRIAC triggering, lamp dimmer, motor control
3. 7 Marks
   aExplain the basic structure of a thyristor and describe its static and dynamic characteristics.
   How to answer

   • Describe four-layer P-N-P-N structure with anode, cathode, gate
   • Explain static V-I characteristics: forward blocking, latching, holding current
   • Explain dynamic characteristics: turn-on, turn-off, dv/dt, di/dt
   • Mention gate triggering and commutation
4. 7 Marks
   bAnalyze the operation of a single-phase half-wave rectifier with resistive (R) and inductive ($R_L$) loads. What are the key differences in their performance?
   How to answer

   • Describe single-phase half-wave rectifier circuit with R load and waveforms
   • Analyze operation with R-L load including freewheeling and conduction extension
   • Derive/compare average output, ripple, power factor
   • Summarize key differences in performance
5. 7 Marks
   aFor a three-phase full-wave fully controlled rectifier with a peak line voltage $V_L = 440V$ and a firing angle $\alpha = 60^\circ$. Calculate the average output voltage of the rectifier.
   How to answer

   • State average output formula for 3-phase fully controlled bridge $V_{dc} = (3\sqrt{3}/\pi)V_{m,ph}\cos\alpha$
   • Convert given peak line voltage to required RMS/peak phase value
   • Substitute $\alpha = 60^\circ$ and compute numerical value
   • State final average voltage with units
6. 7 Marks
   bDescribe the operation of a line frequency phase-controlled rectifier using SCRs. How is the output voltage controlled?
   How to answer

   • Describe SCR-based line-frequency phase-controlled rectifier topology
   • Explain firing-angle control of SCR conduction
   • Show how varying alpha varies average DC output
   • Mention waveforms and output voltage control
7. 7 Marks
   aExplain the basic operation of a single-phase bridge inverter. Discuss the advantages of using a bridge configuration over parallel configurations.
   How to answer

   • Describe single-phase bridge inverter circuit with four switches
   • Explain 180-degree conduction sequence and output square waveform
   • List advantages: higher output, better utilization, no centre-tap
   • Compare with parallel inverter configuration
8. 7 Marks
   bWhat are harmonics in the context of inverters and why is it important to reduce them? Discuss two techniques for harmonics reduction in inverter circuits.
   How to answer

   • Define harmonics as multiples of fundamental in inverter output
   • Explain effects: heating, distortion, motor torque pulsation
   • Describe filter technique for harmonic reduction
   • Describe PWM / selective harmonic elimination technique
9. 7 Marks
   aDescribe the working principle of a single-phase Pulse Width Modulated (PWM) inverter. How does PWM control improve the performance of inverters?
   How to answer

   • Explain PWM principle comparing reference and carrier to generate pulses
   • Describe single-phase PWM inverter operation
   • Show how pulse width controls voltage and harmonics
   • Explain performance improvement in THD and control
10. 7 Marks
    bExplain the principles of On-Off and phase controls in AC voltage controllers. How do these methods differ in regulating the output voltage?
    How to answer

    • Define On-Off (integral cycle) control principle
    • Define phase-angle control principle
    • Explain output voltage regulation in each method
    • Compare harmonics, efficiency and applications
11. 7 Marks
    aExplain the operation of a three-phase to single-phase cycloconverter. Discuss its advantages and limitations compared to other power conversion techniques.
    How to answer

    • Describe three-phase to single-phase cycloconverter topology and switching
    • Explain frequency conversion without DC link
    • List advantages like direct conversion, low-speed drives
    • List limitations like harmonics, limited frequency range
12. 7 Marks
    bDiscuss the principle of operation of a single-phase transformer taps changer. How does it control the output voltage in power distribution systems?
    How to answer

    • Explain tap changer concept on transformer secondary/primary
    • Describe thyristor/triac tap switching for voltage regulation
    • Explain control of turns ratio to maintain output
    • Mention use in power distribution systems
13. 7 Marks
    aDescribe the working principle of buck regulators. How do they step down the input voltage to provide a regulated output?
    How to answer

    • Explain buck regulator chopper operation with switch, diode, LC filter
    • Show duty-cycle relation $V_o = D V_{in}$ for step-down
    • Explain inductor current and regulation via feedback
    • Mention continuous conduction and efficiency
14. 7 Marks
    bDescribe the operation of a step-up chopper. Provide a detailed explanation of how it increases the output voltage compared to the input voltage.
    How to answer

    • Describe step-up chopper circuit with inductor, switch, diode
    • Explain energy storage in inductor during ON period
    • Explain voltage boost during OFF period $V_o = V_{in}/(1-D)$
    • Explain role of duty cycle in output increase
15. 14 Marks
    Write a short note on any two

    1. a)Static and dynamic characteristics of Thyristor
    2. b)Switched-mode power supply
    3. c)Basic structure and V-I characteristics of an IGBT.
    4. d)Three-phase bridge inverter
    How to answer

    • Select any two topics from a-d for short notes
    • Give structure, characteristics/diagram and operation for each
    • Mention key formulas and applications
    • Conclude with uses in power electronics