EX-502 Power Electronics - Jun 2025 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 different commutation techniques used to turn off the Thyristor.
   How to answer

   • Define commutation and need to turn-off thyristor
   • Classify: natural/line, load, forced - voltage/current commutation
   • Explain Class A, B, C, D, E, F with circuit operation
   • Waveforms and conditions for successful commutation
2. 7 Marks
   bDescribe the advantages of IGBT over other power electronics devices.
   How to answer

   • Brief overview of power devices: SCR, MOSFET, BJT, GTO
   • IGBT structure combining MOS gate and bipolar conduction
   • Advantages: high input impedance, low on-state loss, high voltage/current rating, fast switching
   • Comparison table of ratings and applications
3. 7 Marks
   aExplain the working of a single phase fully controlled converter with its waveform.
   How to answer

   • Circuit diagram of single-phase full bridge with 4 thyristors
   • Operation in two modes with firing angle alpha for R/RL load
   • Output voltage and current waveforms, average voltage formula
   • Effect of source inductance / continuous vs discontinuous conduction
4. 7 Marks
   bDraw voltage and current waveform for single phase half bridge rectifiers with RLE load.
   How to answer

   • Circuit of single-phase half-bridge / midpoint rectifier with RLE load
   • Voltage waveform across load showing positive and freewheeling periods
   • Current waveform with inductive smoothing and battery E effect
   • Modes of conduction explanation
5. 7 Marks
   aA single phase full wave ac voltage controller feeds a load of $R = 5 \Omega$ with an input voltage of $230 V$, $50 Hz$. If load power is $5$ kW, find -

   1. i)firing angle delay of thyristor
   2. ii)input power factor
   How to answer

   • Formula for RMS output voltage Vo = Vs*sqrt((pi-alpha)/pi + sin2alpha/2pi) for R load
   • Load power P = Vo^2/R to solve for firing angle alpha
   • Calculate displacement and distortion factors for input power factor
   • Numerical values for alpha and power factor
6. 7 Marks
   bDescribe the working of a two-stage sequence control of voltage controller for RL load.
   How to answer

   • Need for sequence control to improve power factor
   • Circuit with two series controllers / transformer taps
   • Operating sequence: first stage varied then second
   • Waveforms and power factor improvement for RL load
7. 7 Marks
   aExplain the working of GTO with its VI characteristics.
   How to answer

   • Structure of GTO vs conventional thyristor
   • Turn-on and turn-off operation with gate current
   • VI characteristics: forward, reverse blocking and switching
   • Applications and comparison with SCR
8. 7 Marks
   bExplain different types of SCR firing circuits.
   How to answer

   • Need for gate triggering with isolation
   • R-C firing, UJT relaxation oscillator, cosine control
   • Pulse transformer and opto-isolator circuits
   • Comparison of firing angle control accuracy
9. 7 Marks
   aDiscuss the working of a three phase bridge inverter operating as a $120^{\circ}$ conduction mode.
   How to answer

   • Circuit of three-phase bridge inverter with 6 switches
   • Switching sequence for 120-degree conduction, only two devices on
   • Phase and line voltage waveforms and operation steps
   • Advantages and limitations vs 180-degree mode
10. 7 Marks
    bExplain the working of self cumulated inverters.
    How to answer

    • Definition of self-commutated / load-commutated inverter
    • Circuit with commutating components L-C
    • Commutation process and waveform
    • Applications
11. 7 Marks
    aFor type-A chopper circuit, source voltage $V_s = 220\,V$, chopping period $T = 2000\,\mu s$, on period $T_{on} = 600\,\mu s$, load circuit parameters : $R = 1 \Omega, L = 5\,mH$ and $E = 24\,V$.

    1. i)Find whether load current is continuous or not.
    2. ii)Calculate the value for average output current.
    3. iii)Compute the maximum and minimum values of steady state output current.
    How to answer

    • Check continuity: compare Ton, T with time constant L/R and back emf
    • Formula for average output voltage and current Io_avg = (D*Vs - E)/R for continuous
    • Formulas for Imax and Imin using exponential transient
    • Numerical substitution for Vs=220V, T=2000us, Ton=600us, R=1ohm, L=5mH, E=24V
12. 7 Marks
    bExplain the working of Type-C chopper with circuit and waveform.
    How to answer

    • Type-C as two-quadrant / Class C chopper combining A and B
    • Circuit with two switches/diodes for motoring and regeneration
    • Operation in quadrants I and II with current paths
    • Waveforms for output voltage and current
13. 7 Marks
    aExplain the working of Step Up/Step Down Chopper.
    How to answer

    • Topology of step-up vs step-down chopper
    • Energy storage in inductor for voltage boosting/bucking
    • Duty cycle relation Vo = D*Vs and Vo = Vs/(1-D)
    • Waveforms and applications
14. 7 Marks
    bDescribe the working of current source inverter.
    How to answer

    • Principle of constant current source feeding inverter
    • Circuit with large source inductance and thyristors/capacitors
    • Commutation and output current waveforms
    • Comparison with voltage source inverter
15. 7 Marks
    aExplain the working of a single phase bridge type cycloconverter using a simple diagram. Draw the input waveform of frequency $f$ and output wave of frequency $f/2$ for a resistive load.
    How to answer

    • Bridge cycloconverter with positive and negative converters
    • Control to synthesize output frequency f/2 from input f
    • Input sine at f and stepped output at f/2 for R load
    • Waveform sketch and firing sequence
16. 7 Marks
    bExplain the working of single phase full wave controller with RL load.
    How to answer

    • Circuit of single-phase full-wave AC controller with 2 thyristors
    • Operation for RL load with firing angle and extinction angle
    • Voltage and current waveforms showing phase lag
    • Effect of inductance on control range