EX-502 Power Electronics - Nov 2023 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 two transistor analogy of SCR with equivalent circuit and give three applications.
   How to answer

   • Two-transistor (p-n-p and n-p-n) equivalent circuit of SCR
   • Regenerative feedback explanation of latching
   • Equivalent circuit diagram
   • Three applications e.g. controlled rectifier, chopper, inverter, AC controller
2. 7 Marks
   bExplain the operation of IGBT with VI and switching characteristics.
   How to answer

   • Structure and operation of IGBT
   • VI output and transfer characteristics
   • Switching turn-on / turn-off characteristics
   • Sketch of characteristics
3. 7 Marks
   aDraw and explain the necessity of static and dynamic equalizing circuit for series connected SCR'S? Derive relations used for determining the values of shunt resistor R and capacitor C in this circuit.
   How to answer

   • Need for voltage sharing in series SCRs - static and dynamic unbalance
   • Static equalizing resistor circuit and derivation of R
   • Dynamic equalizing RC snubber circuit and derivation of C
   • Circuit diagram with shunt R and RC
4. 7 Marks
   bDerive the output voltage and current expressions for firing angle of 45 degrees.
   How to answer

   • Assumed converter / phase-control circuit and firing angle 45 degrees
   • Derivation of average / rms output voltage expression
   • Derivation of load current expression
   • Substitution for $\alpha = 45^\circ$
5. 7 Marks
   aExplain the operation of single phase fully controlled converter with RL load.
   How to answer

   • Circuit of single phase fully controlled bridge with RL load
   • Operation in different modes with thyristor conduction
   • Load voltage and current waveforms
   • Expression for average output
6. 7 Marks
   bExplain the effect of source impedance on the performance of single phase fully controlled bridge converter.
   How to answer

   • Source inductance and overlap concept
   • Effect on commutation and output voltage drop
   • Waveforms with overlap angle
   • Expression for voltage loss due to source impedance
7. 7 Marks
   aA three phase fully controlled bridge converter is connected to three phase ac supply of 400V, 50Hz and operates with a firing angle $\alpha = \pi/4$. The load current is maintained constant at 10A and the load voltage is 360V, compute:

   1. i)Source inductance Ls
   2. ii)Load resistance R
   3. iii)Overlap angle, μ
   How to answer

   • Formula for three-phase fully controlled output with overlap: $V_o = 1.35 V_{LL}\cos\alpha - 3\omega L_s I_o/\pi$
   • Use given $V_o=360V$, $I_o=10A$, $\alpha=\pi/4$ to find $L_s$
   • Load resistance $R = V_o/I_o$
   • Overlap angle from $\cos\alpha - \cos(\alpha+\mu) = \sqrt{2}\omega L_s I_o / V_{ph,peak}$
8. 7 Marks
   bExplain the operation of single pulse modulation of inverter with neat diagram.
   How to answer

   • Principle of single pulse modulation in inverter
   • Control by varying pulse width / delay angle
   • Circuit and gating sequence
   • Neat diagram with output waveform
9. 7 Marks
   aDescribe the operation of 3-phase bridge inverter circuit diagram with resistive load in $120^{\circ}$ conduction mode.
   How to answer

   • 3-phase bridge inverter topology with resistive load
   • 120 degree conduction mode switching sequence
   • Phase and line voltage waveforms
   • Circuit diagram
10. 7 Marks
    b3-phase bridge inverter is fed from a d.c. source of 200 V. If the load is star-connected of $10\Omega$/phase resistance, Estimate the RMS load current and load power (in watt) if it is operated in $120^{\circ}$ conduction mode.
    How to answer

    • Line / phase voltage for 120-degree mode from $V_{dc}=200V$
    • RMS load current $I_{rms}=V_{ph,rms}/R$
    • Load power $P=3 V_{ph,rms} I_{rms}$
    • Numerical substitution for star 10 ohm load
11. 7 Marks
    aWrite short notes on :

    1. i)Jones chopper
    2. ii)Current limit control of dc-dc converter
    How to answer

    • Jones chopper - circuit and principle
    • Current limit control - principle and waveform
    • Comparison / applications
12. 7 Marks
    bA dc on-off chopper operating at 1 kHz and duty cycle of 10% is supplied from a 200V source. If the load inductance is 10 mH and resistance $10\Omega$. Compute the maximum and minimum circuit in the load.
    How to answer

    • Chopper period $T=1/f$, ON time $T_{on}=D T$
    • Current expressions with R-L load
    • Maximum current at end of ON / minimum at end of OFF steady state
    • Substitute $Vs=200V, R=10\Omega, L=10mH, f=1kHz, D=0.1$
13. 7 Marks
    aDerive an expression for rms value of output voltage of single phase bridge type ac voltage controller.
    How to answer

    • Single phase bridge AC voltage controller circuit
    • Integration of output over conduction angle
    • RMS voltage formula $V_{or}=V_s\sqrt{(\pi-\alpha+\sin2\alpha/2)/\pi}$
    • Final expression
14. 7 Marks
    bDraw the circuit of single phase voltage controller with anti-parallel connection of two thyristors and an R-L load. Explain its working. Sketch load voltage and load current waveforms. Derive an expression for output voltage.
    How to answer

    • Circuit with two anti-parallel thyristors and R-L load
    • Working for positive and negative half cycles with firing angle
    • Load voltage and current waveforms
    • Derivation of output voltage expression
15. 14 Marks
    Write short notes on any three:

    1. a)AC voltage controllers
    2. b)Harmonics and their reduction
    3. c)GTO and its VI characteristics
    4. d)Forced commutation technique of thyristor
    How to answer

    • Principle / circuit of chosen topic
    • Key characteristics / waveforms
    • Advantages / applications / control methods
    • Neat diagram where applicable