Question Bank for Power Electronics.

  1. Explain the conventional method to get 12V from a 30V battery without use of power electronics.
  2. How to convert 30V to 12V using power electronics.
  3. Define Power Electronics. What are the different losses in semiconductor switches ?
  4. A single-phase half-wave uncontrolled rectifier feeding power to RL load. Draw output voltage, output current, voltage across device. Explain the voltage across R and L separately. Derive the expression of average output voltage.
  5. A single-phase half-wave controlled rectifier feeding power to RLE load. Draw output voltage, output current, and voltage across thyristor for α = 60°. Mark all the quantities and points in waveform.
  6. Explain the operation of Single-phase Mid-point controlled converter with RL load with discontinuous current. Draw output voltage. voltage across both the SCRs. Derive expression of output voltage.
  7. Explain the operation of single phase fully-controlled converter with FWD feeding power to RLE load for α = 120° assuming continuous conduction, Draw output voltage waveform.
  8. A three-phase half wave controlled rectifier feeding power to RL load at α = 60° with continuous current. Draw output voltage across device and input current waveform of any one phase.
  9. Explain operation of Three-phase fully controlled converter with R load for α = 90°.
  10. Write short Notes on following
    (a) Define firing angle for 1-phase and 3-phase converters.
    (b) Compare different half-controlled (single-phase) converters.
    (c) Effect of Source Impedance in single-phase and three-phase converters.
    (d) Importance of V-I characteristics.
  11. Explain working of a three-phase half wave controlled rectifier with resistive load for continuous as well as discontinuous conduction. Derive expressions for average voltage and rms voltage in both the cases.
  12. A 3-phase half wave converter is operated from a 3-phase star connected 240-V, 60Hz supply and the load resistance is R=12 Ω. If it is required to obtain an average output voltage of 50% of the maximum possible output voltage, calculate:
    • (a) the delay angle α ,
    • b) the rms and average output currents.
    • c) the average and rms thyristor currents.
  13. Explain the two transistor model of a thyristor. Based upon that explain the different turn on methods.
  14. The capacitance of reverse biased junction J₂ in a thyristor is CJ2 = 20pF and can be assumed to be independent of the off-state voltage. The limiting value of the charging current to turn on the thyristor is 18mA. Determine the critical value of dv/dt.
  15. Explain dv/dt protection for a SCR.
  16. A circuit consists of a thyristor in series with a resistance of 6 ohms. A DC supply of 200 volts has to be impressed across this system. Required dv/dt is is 100V/µ seconds and the discharge current is to be limited to 120A. The thyristor is operated at a frequency of 2kHz. Determine:
    • (a) the values of C, and R,
    • (b) the snubber loss,
    • (c) the power rating of the snubber circuit.
  17. Explain working of a single phase full wave controlled rectifier with resistive load and find out values of all the performance parameters.
  18. Write short notes on:
    • a) Freewheeling and feedback.
    • b) parallel operation of thyristor.
    • c) dynamic characteristic of thyristor.
    • d) Single phase semi converter.
    • e) hotspots
  19. Explain the working of a 1-ɸ full wave fully controlled rectifier with a resistive load. Calculate values of VDC, Vrms, IDC, Irms, Form factor, ripple factor, rectification efficiency, transformer utilization factor and peak inverse voltage for firing angle 60 degree.
  20. Explain the working of a 3-ɸ half-wave-controlled rectifier with resistive load. Derive expressions for VDC, IDC, Vrms, Irms, for continuous as well as discontinuous conduction. What is the condition for continuous conduction?
  21. A 3-ɸ full wave converter is operated from a 3-ɸ star connected 270-V, 60Hz supply and the load resistance is R =10 Ω. If it is required to obtain an average output voltage of 50% of the maximum possible output voltage, calculate:
    • (a) the delay angle α.
    • (b) the rms and average output currents.
    • (c) the average and rms thyristor currents.
  22. Explain working of a three-phase inverter with 180° conduction for balanced 3-ɸ star connected resistive load. Draw the waveforms for phase voltages and line voltages. Give one merit and one demerit of 180° conduction in comparison to 120° conduction.
  23. Why do we need series operation of a thyristor. What are the associated problems and how do we solve them. Explain properly and derive expression for static equalizing resistance and dynamic equalizing capacitance.
  24. Write notes on:
    • a) Static (VI) characteristic of SCR
    • b) Dynamic Characteristic of SCR
    • c) feedback and freewheeling
    • d) two transistor model of thyristor
    • e) class E (four quadrant) chopper
  25. Define and explain firing angle in respect of single phase and three phase converters.
  26. What are the different types of single phase half controlled converters compare them.
  27. A single-phase bridge converter feeding power to resistive load is triggered at an angle α = 120°. Draw load voltage, load current, and thyristor voltage waveform.
  28. A single-phase bridge converter feeding power to resistive load is triggered at an angle α = 120°, if load has some inductance, draw load voltage and load current waveform (assume current to be discontinuous).
  29. Explain effect of source inductance on performance of single phase bridge converter.
  30. Explain single phase half wave controlled rectifier with RLE load. Draw output voltage, output current waveforms for α = 60°.
  31. A three-phase half wave controlled rectifier feeding power to RL load at α = 60°. Draw output voltage and input current waveform of any one phase.
  32. Explain V-I Characteristics of SCR.
  33. Short notes on following:
    • (a) Cosine firing scheme
    • (b) Class-C commutation
    • (c) Performance parameters of Half wave rectifier.
  34. Explain V-I characteristic of thyristor. Show all the three modes distinctly and explain.      
  35. Explain dv/dt and di/dt protection of thyristor.
  36. Explain working of a single phase full bridge inverter with inductive load and Derive expression for vo, io.
  37. What do you understand by 
    • a) HFn
    • b) THD
    • c) DF
    • d) LOH 
  38. A single phase  full bridge inverter has a resistive load  of R=2.5 ohms and the dc voltage input of is 50 V. Determine:
    • a) The rms output voltage at fundamental frequency Vo1,
    • b) the output power Po,
    • c) the average and peak currents of each transistor,  
    • d)the peak blocking voltage VBR of each transistor,
    • e) the THD, 
    • f)the DF and
    • g) the LOH.      
  39. A string of thyristors is connected in series to withstand a dc voltage of 15kV. The maximum leakage current and recovery charge differences of thyristors are 10 mA and 150µC, respectively. A derating factor of 20% is applied for the steady state and transient voltage sharing of thyristors. If the maximum steady state voltage sharing is 1000V, determine:
    • a) the steady state equalizing resistance
    • b) the dynamic equalizing capacitance. Derive expressions used. 
  40. Explain different speed control methods of a dc motor. How do we use controlled rectifier circuits for dc motor control in all four quadrants.
  41. Explain working of a three phase inverter with 120° conduction for balanced 3-ɸ  star connected resistive load.  Give the line and phase voltages for delta connected load as well. 
  42. Give one merit and one demerit of 180° conduction in comparison to 120° conduction.
  43. Explain following regarding a thyristor:        
    • a) Static characteristic
    • b) Dynamic Characteristic
    • c) Gate Characteristic
  44. A step down chopper is feeding an RL load with V = 240 volts, R = 6 ohms, L= 15 mH, f = 1kHz, k =0 .5 and E = 0 volts. Calculate the peak to peak ripple, the average value of load current, and the effective input resistance. 
  45. Explain working of a step up chopper and derive formula for average voltage.
  46. Explain the following:
    • a) Resistance triggering
    • b) UJT triggering
    • c) Complementary commutation 
  47. Explain working of a three-phase half wave controlled rectifier with resistive load for continuous as well as discontinuous conduction. Derive expressions  for average voltage and rms voltage in both the cases. 
  48. A 3-ɸ half wave  converter is operated  from a 3-ɸ star connected 240-V, 60Hz supply and the  load resistance is R =12Ω. If it is required to obtain an average output voltage of 50% of the  maximum possible output voltage, calculate:
    • (a) the delay angle α,
    • b) the rms and average output  currents
    • c) the average and rms thyristor currents. 
  49. Explain working of a step down chopper with RLE load. Derive expression for peal to peak ripple. Also find out condition for maximum ripple and continuous current.  
  50. Why step down chopper is also known as 1st  quadrant chopper?  
  51. Why do we need dv/dt and di/dt protection of a thyristor. How do we design the two protection circuits ?
  52. The capacitance of reverse-biased junction J2 in a thyristor is Cj2  = 20 pF and can be assumed to be independent of the off-state voltage. The limiting value of the charging current to turn on the thyristor is 20 mA. Determine the critical value of dv/dt. 
  53. Explain the working of a 3-ɸ full-wave controlled rectifier with resistive load. Derive expressions for VDC , IDC, Vrms, Irms. for continuous as well as discontinuous conduction. What is the condition for continuous conduction ?
  54. A 3-ɸ full wave  converter is operated  from a 3-ɸ star connected 270-V, 60Hz supply and the  load resistance is R =10Ω. If it is required to obtain an average output voltage of 50% of the  maximum possible output voltage, calculate
    • (a) the delay angle α,
    • b) the rms and average output  currents
    • c) the average and rms thyristor currents.
  55. Explain working of a three-phase inverter with 120° conduction for balanced 3-ɸ star connected resistive load.  Give the line and phase voltages for delta connected load as well.
  56. Give one merit and one demerit of 180° conduction in comparison to 120° conduction.
  57. Explain static and dynamic characteristic of thyristor.   
  58. A  circuit consists of a thyristor in series with a resistance of 6 ohms. A DC supply of 200 volts has to be impressed across this system. Required dv/dt is is 100V/µseconds and the discharge current is to be limited to 100A. The thyristor is operated at a frequency of 2kHz. Determine
    • (a) the values of Cs and Rs,
    • (b) the snubber loss, and
    • (c) the power rating of the snubber circuit.  
  59. Explain working of a single phase full wave controlled rectifier with inductive load for continuous conduction as well as discontinuous conduction. Derive expressions for Vdc and Vrms.    
  60. How things change in case of single phase semiconverters. Do the derivations in this case also. 
  61. Why do we need series operation of a thyristor. What are the associated problems and how do  we solve them. Explain properly and derive expression for static equalizing resistance. 
  62. Ten thyristors are used in a string to withstand a dc voltage of Vs =  15 kV. The maximum leakage current and recovery charge differences of thyristors are 10 mA and 180 µC respectively. Each thyristor has a voltage-sharing resistance of 54 kΩ and capacitance of C1 = 0.6 µF. Determine
    • (a) the maximum steady-state voltage sharing VDS(max) ,
    • (b) the steady- state voltage-derating factor,
    • (c) the maximum transient voltage sharing VDT(max), and
    • (d) the transient voltage-derating factor.  
    • Explain the working of a 1-ɸ full wave fully controlled rectifier with a R-L load. Derive expressions for VDC , Vrms, IDC, Irms, Form factor, ripple factor, rectification efficiency, transformer utilization factor  and peak inverse voltage for continuous conduction.                
  63. How do you convert  above circuit in a semi converter (give one method)?How the voltage and  current waveforms of the load will be affected? Give one advantage and one disadvantage of  this circuit in comparison to circuit in part(a).
  64. For a 3-ɸ controlled rectifier even for resistive load current can be either continuous or  discontinuous. How ?
  65. Explain two transistor model of thyristor.
  66. Explain five voltage control techniques of a 1-phase inverter.  
  67. Explain advanced modulation techniques of a single phase inverter. 
  68. Explain working of a single phase full wave controlled rectifier with inductive load for continuous conduction as well as discontinuous conduction. Derive expressions for Vdc and Vrms in both cases. 
  69. What are the different performance parameters for a controlled rectifier circuit and what will be their values for abovementioned circuit ?    
  70.   Name and explain six speed control methods for an Induction Motor and how do we use different power electronic circuits for them.
  71. Explain seven forced commutation techniques for a thyristor.
  72. What are the different classifications of chopper ? Explain.  How do we make use of different classes of chopper for dc motor speed control ? 
  73. Why do we need series and parallel operation of thyristors ? What are the associated problems? Explain clearly with the help of VI characteristics. How do we solve these problems? Derive worst case voltage for static as well as dynamic case.
  74. For an ac voltage controller with inductive load firing angle α should be either greater than or equal to power factor angle θ. Why ?
  75. Explain and also derive expression for Vrms.
  76. What are the three circuit options for bidirectional control? Explain with relative merits and demerits. 
  77. Explain static (V-I) characteristic of thyristor. Show all the three modes distinctly and explain. Also explain its dynamic characteristic.
  78. Why do we need dv/dt protection of a thyristor. How do we design the protection circuit (Snubber circuit)?
  79. 20/6
  80. b) A circuit consists of a thyristor in series with a resistance of 6 ohms. A DC supply of 200 volts has to be impressed across this system. Required dv/dt is is 100V/µseconds and the discharge current is to be limited to 120A. The thyristor is operated at a frequency of 2kHz. Determine (a) the values of C_{i} and R_{m} (b) the snubber loss, and (c) the power rating of the snubber circuit.
  81. Explain working of a three-phase semi converter with RL load. Draw output voltage, input current waveform of any one phase for a = 300, 90°
  82. Explain the working of a 1-phase full wave fully controlled rectifier with a R-L. load. Derive expressions for VDC , Vrms , IDC, Irms, Form factor, ripple factor, rectification efficiency, transformer utilization factor and peak inverse voltage for continuous conduction.
  83. Short Notes:
    • 1) dv/dt and di/dt protection
    •  2) Freewheeling and feedback
    •  3) 1 phase semi converter
    •  4) Two-transistor model of transistor
    •  5) Step down chopper
    • 6) Dynamic characteristics of SCR
    • 7) Parallel operation of thyristors
    • 8) Thyristor turn on methods
    • 9) Series operation of thyristors (no derivations)
    • 10) Hotspots
    • 11) Four-quadrant chopper operation    
    •  12) Three-phase semi converter
    • 13) Step-up chopper
    • 14) Transformer tap changing
    • 15) Slip power recovery
    • 16) Snubber circuits and Hotspots
    • 17) Pulse width modulation
    • 18) Cycloconverter
    • 19) V/F control of induction motor
    • 20) Thyristor turn-on methods
    • 21) Parallel operation of thyristors
    • 22) Natural commutation and forced commutation with examples
    • 23) Four-quadrant chopper operation 
    •  24) Freewheeling and feedback
    • 25) 3 phase semi converter
    • 26) Two-transistor model of transistor
  84. What is a Power Diode ? Determine the Incremental Resistance of a Silicon p-n junction for V = 0.15V T = 2900 K, J0 = 1.1A/m2 and Diode Junction Area = 3 x 10-6 m2.
  85. Draw a TRIAC Phase Control Circuit, and explain its Principle of Operation.
  86. What is the necessity of the Collector Drift Region in a Power BJT ? Explain.
  87. Explain the Working of a UJT Relaxation Oscillator. Discuss the Factors, which affect the Frequency of Oscillation.
  88. Discuss the Design Procedure of the Snubber Circuits, Give applications of Snubber Circuits.
  89. In a Single-phase Full Wave Rectifier, the Load is Resistive with R = 5 Ω. If the Peak Value of the Input Supply Voltage is 200 V and f = 50 Hz., find the Average Value of the Output Voltage, Average Value of Output Current and the DC Output Power.
  90. A Step-up Chopper is used to deliver Load Voltage of 500V from a 220 V DC source. If the Blocking Period of the Thyristor is 80 micro seconds, find the Pulse Width.
  91. Sketch the Circuit of Switching Mode Regulators and Explain its Working.
  92. Derive the expressions for the Average and rms Values of the Load Current and the Load Voltage for single-phase full-wave Rectifier with R-L emf type Load, when the Load Current is Discontinuous.
  93. With the help of neat diagram, explain the operation of resistance firing circuit. Also, draw and explain the associated waveforms.
  94. Discuss three important regions in Current-Voltage Characteristics of a Diode.
  95. A DC Chopper connected to a 100V, DC source supplies an inductive load having 40 mH in series with a resistance of 5 Ω. A freewheeling Diode is placed across the Load. The Load Current varies between the limits of 10 A and 12 A. Determine, the Time Ratio of the Chopper.
  96. A three phase, six-pulse, 50kVA, 415 V Cycloconverter is operating at a firing angle of 450 and supplying load of 0.8 power factor. Determine the input current to the Converter.
  97. Design a Series Inverter Circuit for operation in the frequency range 1 to 5 kHz. The load resistance may vary from 25 Ω to 100 Ω. The peak load current is limited to 3 A, and the supply voltage is 100V.
  98. What are 2/4 -quadrant chopper drives ?
  99. Draw and Explain, the Control Circuit Block Diagram for a Cycloconverter with Non-circulating Current Mode.
  100. Explain working of a PWM Inverter.
  101. Design a Self-commutated Inverter circuit to operate at a frequency of 3 kHz with an optimum distortion. The load specifications are as following: R = 5 Ω, L = 5mH, Edc = 100V. Also, find the output power.
  102. Discuss the Basic Principle of Operation of a Ring-connected Cycloconverter Circuit. Also, draw the associated waveforms for the ideal case.
  103. Design an OTT-filter required for a parallel inverter to meet the following specifications: Output voltage EL = 230V, Frequency f = 50Hz Load Current IL =1.5A and Edc = 40V.
  104. How Harmonic Reduction is done in 1-phase and 3-phase Cycloconverter ?
  105. Draw a neat diagram of parallel inverter employing feedback-diodes. Explain the working of inverter with the help of voltage and current waveforms What care should be taken to avoid commutation failure ?
  106. Explain the operation of B-6 and M-6 full controlled circuits with R and R – L loads.
  107. Describe three-phase to three-phase Cycloconverter with relevant circuit arrangement using 18 Thyristors and 36 Thyristors. What are the advantages of three-phase bridge Cycloconverter circuit over three-phase to three-phase Cycloconverter circuit consisting of 18 SCRs ?
  108. What is the Softness Factor of a Diode ?
  109. Give typical ratings of a general purpose Power Diode.
  110. What is Transformer Utilization Factor ?
  111. What is Overdrive Factor of a BJT ?
  112. How can a LASCR be turned OFF ?
  113. Discuss 2-Transistor analogy.
  114. What is V/f Control ?
  115. List Limitations of Series Inverter.
  116. Compare VSI and CSI.
  117. What are Transients ? Why these occur ?