EX-502 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 different types of methods to turn on the Thyristor.
   How to answer

   • Define thyristor turn-on and need for gate triggering
   • Explain forward voltage, gate, dv/dt, temperature and light triggering
   • Give triggering current/voltage conditions and circuit
   • Compare methods with applications
2. 7 Marks
   bDescribe the process of conduction that takes place in MOSFET of n-channel type.
   How to answer

   • Describe n-channel enhancement MOSFET structure
   • Explain channel formation with Vgs > Vth
   • Describe drift of electrons from source to drain
   • Explain output characteristics and conduction control
3. 7 Marks
   aExplain rectification mode and inversion mode in a single phase fully controlled converter.
   How to answer

   • Define single-phase fully controlled bridge converter circuit
   • Explain rectification mode with alpha < 90 deg, positive Vdc
   • Explain inversion mode with alpha > 90 deg, negative Vdc with source
   • Give waveforms and power flow direction
4. 7 Marks
   bDraw voltage and current waveform for continuous load currents in a single phase full bridge rectifier with RLE load.
   How to answer

   • Show single-phase full bridge with RLE load circuit
   • Assume continuous conduction with large inductance
   • Sketch source voltage, output voltage, load current waveforms
   • Explain thyristor conduction intervals
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

   • Write RMS output voltage vs firing angle formula for R load
   • Equate load power P = Vo_rms^2 / R to find alpha
   • Calculate firing angle delay
   • Calculate input power factor = P/(Vs*Is)
6. 7 Marks
   bDescribe the working of a two-stage sequence control of voltage controller for RL load.
   How to answer

   • Define sequence control concept with two stages
   • Show circuit with two series controllers/transformer taps
   • Explain firing sequence for power factor improvement
   • Give waveforms and advantages for RL load
7. 7 Marks
   aExplain the different types of over current and over voltage protection in SCR.
   How to answer

   • Need for protection in SCR circuits
   • Explain over-current causes and fuse/circuit breaker protection
   • Explain over-voltage causes, dv/dt and snubber/crowbar protection
   • Summarize coordination of protections
8. 7 Marks
   bExplain following power devices and their application

   1. i)Power Mosfet
   2. ii)IGBT
   3. iii)GTO
   4. iv)DIAC
   How to answer

   • Give structure/symbol and characteristics of each device
   • Explain turn-on/turn-off operation principle
   • List voltage/current ratings and switching speed
   • List typical applications of each
9. 7 Marks
   aDiscuss the working of a three phase bridge inverter operating as a $180^{\circ}$ conduction mode.
   How to answer

   • Show three-phase bridge inverter circuit with six switches
   • Explain 180-degree conduction sequence and gating
   • Draw phase and line voltage waveforms
   • Derive output voltage levels and features
10. 7 Marks
    bExplain the principle of operation of McMurray-Bedford inverter.
    How to answer

    • Identify McMurray-Bedford as forced-commutated inverter
    • Explain circuit with auxiliary commutation components
    • Explain charging/discharging and SCR commutation steps
    • Give waveforms and limitations
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 : $R = 1\Omega$, $L = 5mH$ and $E = 24V$.

    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

    • Calculate duty ratio D=Ton/T and time constant
    • Check continuity condition using current expressions
    • Calculate average output current formula
    • Calculate Imax and Imin steady-state values
12. 7 Marks
    bExplain the working of Type-C chopper with circuit and waveform.
    How to answer

    • Show Type-C two-quadrant chopper circuit with two switches/diodes
    • Explain Class-C operation as chopper and boost
    • Sketch output voltage/current waveforms
    • List applications in DC drives
13. 7 Marks
    aExplain the working of Step Up Chopper.
    How to answer

    • Show step-up chopper circuit with L, switch, diode
    • Explain energy storage in inductor during ON
    • Explain energy transfer to load during OFF
    • Give output voltage relation Vo = Vs/(1-D)
14. 7 Marks
    bDifferentiate between Voltage source and Current source Inverter.
    How to answer

    • Define VSI with voltage stiff source and CSI with current stiff source
    • Compare source, commutation, load dependence on criteria
    • Give circuit examples and waveforms
    • Compare applications and advantages
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

    • Show single-phase bridge cycloconverter with positive/negative converters
    • Explain phase control to synthesize f/2 output
    • Sketch input f waveform and output f/2 waveform for R load
    • Explain switching sequence for resistive load
16. 7 Marks
    bExplain the working of three phase to single phase cycloconverter.
    How to answer

    • Show three-phase to single-phase cycloconverter configuration
    • Explain three-phase input converters operation
    • Explain output frequency synthesis and control
    • List applications and limitations