EX-502 Power Electronics - May 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
   aDescribe LASCR. Give its industrial applications. With neat circuit diagram explain how UJT firing circuit will generate pulse for an SCR.
   How to answer

   • Definition and structure of LASCR - light activated SCR
   • Industrial applications - HVDC, static VAR, light-triggered switching
   • UJT relaxation oscillator firing circuit diagram and components
   • Working - capacitor charging to peak point, UJT turn-on, pulse to gate via pulse transformer
   • Output gate pulse waveform for SCR triggering
2. 7 Marks
   bWhat are "dv/dt" and "di/dt" ratings of SCRs? What happens if these ratings are exceeded?
   How to answer

   • Define dv/dt rating - rate of rise of forward voltage
   • Define di/dt rating - rate of rise of anode current
   • Protection methods - snubber for dv/dt, series inductor for di/dt
   • Consequences of exceeding - false triggering / thermal breakdown and device failure
   • Permissible limits and design care
3. 7 Marks
   aSCRs with a rating of 1000 V and 200 A are available to be used in a sting to handle 6 kV and 1 kA. Calculate the number of series and parallel units required. Assume the derating factor is 0.1.
   How to answer

   • Formula for series string with derating factor
   • Calculate number of series SCRs for 6 kV from 1000 V rating
   • Formula for parallel branches with derating
   • Calculate number of parallel paths for 1 kA from 200 A rating
   • Total devices and string arrangement
4. 7 Marks
   bExplain the working of three phase full converter for a firing delay angle of 45 degree.
   How to answer

   • Circuit of three-phase full bridge converter
   • Firing sequence and conduction of thyristors at alpha = 45 degree
   • Output voltage waveform and modes
   • Effect of delay angle on average output and operation as rectifier
   • Equations for average output voltage
5. 7 Marks
   aA single phase full converter bridge is connected to RLE load. The supply voltage is 230 V, 50 Hz. The average load current of 10 A is constant over the working range. For R = 0.4 Ohms and L = 2 mH, compute the firing angle delay for E = 120 V.
   How to answer

   • Average output voltage formula Vo = (2Vm/pi)cos alpha for full converter
   • KVL for RLE load: Vo = E + Io*R for continuous constant current
   • Substitute Vs=230 V, Io=10 A, R=0.4 ohm, E=120 V
   • Solve for cos alpha and alpha
   • Check continuous conduction assumption
6. 7 Marks
   bDescribe how a freewheeling diode improves power factor in a converter system?
   How to answer

   • Connection of freewheeling diode across inductive load
   • Conduction during negative source period preventing negative voltage
   • Improvement in average output and reduction in reactive power
   • Effect on power factor: reduced phase lag between V and I
   • Waveforms with and without freewheeling diode
7. 7 Marks
   aWhat is an inverter ? list a few industrial applications of inverter?
   How to answer

   • Definition of inverter - DC to AC conversion
   • Principle of switching to produce AC output
   • Industrial applications - UPS, drives, induction heating, solar, HVDC
   • Classification brief
8. 7 Marks
   bDraw and explain the operation of three-phase series-inverter circuit.
   How to answer

   • Circuit diagram of three-phase series inverter with commutating components
   • Role of series RLC and thyristors
   • Sequence of operation and commutation
   • Output frequency and waveforms
   • Advantages/limitations
9. 7 Marks
   aExplain in detail about $180^{\circ}$ Conduction Mode of 3 - $\phi$ Voltage Source Inverters.
   How to answer

   • Circuit of 3-phase bridge VSI and 180 degree conduction rule
   • Conduction pattern - three devices ON at a time
   • Switching sequence over 360 degrees in 60 degree steps
   • Phase and line voltage waveforms
   • Expression for output voltage
10. 7 Marks
    bA square-wave inverter has a dc source of 125 V, an output frequency of 60 Hz and an RL series load with R = 20 $\Omega$ and L = 25 mH, Determine

    1. i)an expression for load current
    2. ii)rms load current
    3. iii)average source current
    How to answer

    • Square-wave voltage +/-Vs applied to R-L load, Fourier / transient solution
    • Expression for load current i(t) using exponential terms
    • Calculate impedance, time constant and rms load current
    • Average source current from power balance
    • Numerical substitution for 125 V, 60 Hz, 20 ohm, 25 mH
11. 7 Marks
    aExplain the operation of step-up chopper with a neat circuit diagram and necessary output waveforms and also derive expression for output voltage.
    How to answer

    • Circuit of step-up chopper with L, chopper, diode, capacitor and load
    • ON period energy storage in inductor, OFF period delivery to load
    • Voltage and current waveforms
    • Volt-second balance derivation Vo = Vs/(1-D)
    • Conditions for step-up operation
12. 7 Marks
    bThe step-down dc chopper has a resistive load, R = 20 $\Omega$ and input voltage, $V_{s}$ = 220 V. When the chopper remains on, its voltage drop, $V_{ch}$ = 1.5 V and chopping frequency, f = 10 kHz. If the duty cycle is 80%. Estimate the :

    1. i)average output voltage
    2. ii)rms output voltage
    3. iii)Chopper efficiency
    How to answer

    • Average output Vo = D*(Vs - Vch) for step-down with device drop
    • RMS output Vor = sqrt(D)*(Vs - Vch) approx
    • Output power Po = Vor^2/R and input power
    • Chopper efficiency = Po/Pin
    • Substitute R=20 ohm, Vs=220 V, Vch=1.5 V, D=0.8, f=10 kHz
13. 7 Marks
    aDescribe the Morgan chopper with associated voltage and current waveforms.
    How to answer

    • Circuit of Morgan chopper - current commutated chopper
    • Components - main and auxiliary thyristors, capacitor, inductor, diode
    • Sequence of commutation and conduction modes
    • Voltage and current waveforms across load and capacitor
    • Applications
14. 7 Marks
    bDescribe the operating principle of single-phase to single-phase step-up cyclo converter with the help of mid-point and bridge type configuration. Illustrate your answer with appropriate circuit and waveforms.
    How to answer

    • Principle of cycloconverter - direct AC-AC frequency changing
    • Mid-point configuration circuit and operation for step-up
    • Bridge configuration circuit and operation
    • Output voltage and frequency waveforms showing step-up
    • Firing control of positive and negative groups
15. 7 Marks
    aExplain the various modes of operation of TRIAC with the help of equivalent circuits and relevant waveforms.
    How to answer

    • Structure of TRIAC as two antiparallel SCRs
    • Four triggering modes - I+, I-, III+, III- with gate polarity
    • Equivalent circuits for each mode
    • V-I characteristics and relevant waveforms
    • Sensitivity comparison of modes
16. 7 Marks
    bA single phase half wave AC voltage controller has a resistive load of R = 40 ohms and the input voltage is Vs = 230V, 50Hz. The Delay angle of thyristor is 50 degrees. Determine

    1. i)The rms value of output voltage $V_{o}$
    2. ii)The input power factor
    3. iii)The average input current
    How to answer

    • Formula for rms output of single-phase half-wave controller with R load
    • Substitute Vs=230 V, alpha=50 degrees to get Vo_rms
    • Input power factor = Po/(Vs*Is_rms)
    • Average input current / output current calculation
    • Numerical results