POWER ELECTRONICS - B.Tech 5th Semester Exam., 2020 (New Course)

2020•Semester 2•Civil-CA•End Semester
Bihar Engineering University, Patna
B.Tech 5th Semester Exam., 2020 (New Course)

POWER ELECTRONICS

Time: 03 HoursCode: 100506Full Marks: 70

Instructions:

  1. The marks are indicated in the right-hand margin.
  2. There are NINE questions in this paper.
  3. Attempt FIVE questions in all.
  4. Question No. 1 is compulsory.
Q.1 Choose the correct answer of the following (any seven):[14]

  1. It is preferable to use a train of pulse of high frequency for gate triggering of SCR in order to reduce

    1. \( dv/dt \) problem
    2. \( di/dt \) problem
    3. the size of the pulse transformer
    4. the complexity of the firing circuit

  2. Reverse recovery current in a diode depends upon

    1. forward field current
    2. storage charge
    3. temperature
    4. PIV

  3. In a 3-phase full-wave a.c. to d.c. converter, the ratio of output-ripple frequency to the supply-voltage frequency is

    1. 2
    2. 3
    3. 6
    4. 12

  4. A fully controlled natural commutated 3-phase bridge rectifier is operating with a firing angle \( \alpha = 30^{\circ} \). The peak to peak voltage ripple expressed as a ratio of the peak output d.c. voltage to the output of the converter bridge is

    1. 0.5
    2. \( 1 - (\sqrt{3}/2) \)
    3. \( \sqrt{3} - 1 \)
    4. \( \sqrt{3}/2 \)

  5. Natural commutation of a thyristor takes place when

    1. anode current becomes zero
    2. gate current becomes zero
    3. voltage across the device becomes zero
    4. voltage across the device becomes negative

  6. A four-quadrant chopper cannot be operated as

    1. one-quadrant chopper
    2. cycloconverter
    3. inverter
    4. bi-directional rectifier

  7. A boost regulator has an input voltage of 5 V and the average output voltage of 15 V. The duty cycle is

    1. \( 3/2 \)
    2. \( 2/3 \)
    3. \( 5/2 \)
    4. \( 15/2 \)

  8. PWM switching is preferred in voltage source inverter for the purpose of

    1. controlling output voltage
    2. output harmonics
    3. reducing filter size
    4. controlling output voltage, output harmonics and reducing filter size

  9. Compared to a single-phase half-bridge inverter, the output power of a single-phase full-bridge inverter is higher by a factor of

    1. 12
    2. 8
    3. 4
    4. 2

  10. How many switches are used to construct a 3-phase to 3-phase cycloconverter?

    1. 3
    2. 6
    3. 12
    4. 18
Q.2 Solve both questions :[10+4=14]

  1. A single-phase bridge-type cycloconverter feeds a load R. For an output frequency equal to one-third of the input frequency, sketch output voltage waveform for a firing angle of about \( 30^{\circ} \). Also derive an expression for the r.m.s. output voltage.

  2. Explain the processes of \( dv/dt \) triggering and temperature triggering.

Q.3 Solve both questions :[8+6=14]

  1. A 3-phase bridge inverter delivers power to a resistive load from a 450 V d.c. source. For a star-connected load \( 10~\Omega \) per phase, determine the following for \( 120^{\circ} \) mode of operation:
    (i) r.m.s. value of output phase and line voltages
    (ii) r.m.s. value of transistor current
    (iii) r.m.s. value of load current
    (iv) Load power

  2. Describe the structural features of power diodes. How do these differ from signal diodes?

Q.4 Solve both questions :[5+9=14]

  1. A single-phase full-converter delivers a constant load current \( I_0 \). Express its source current in Fourier series.

  2. Derive the expressions for the following performance parameters with the help of waveforms and Fourier series expression in Question No. 4(a): Input displacement factor, Current distortion factor, Power factor, Total harmonic distortion, Voltage ripple factor, Active and Reactive power inputs.

Q.5 Solve both questions :[10+4=14]

  1. A battery is charged through a resistor R. If \( v_s = 220\sqrt{2} \sin \omega t \), \( E = 100 \text{ V} \) (d.c.) and \( R = 20~\Omega \), calculate the battery charging current and power supplied to the battery.

    Question Diagram

  2. Write down any four applications of cycloconverters.

Q.6 Solve both questions :[10+4=14]

  1. Discuss with relevant waveforms, class-B (resonant-pulse commutation) type of commutation employed for thyristor circuits.

  2. What are forward blocking losses and gate power loss in a thyristor?

Q.7 Solve both questions :[10+4=14]

  1. For step-down chopper circuit, source voltage \( V_s = 220 \text{ V} \), chopping period \( T = 2000~\mu s \), on period \( = 600~\mu s \), load circuit parameters \( R = 1~\Omega \), \( L = 5 \text{ mH} \) and \( E = 24 \text{ V} \).
    (i) Find whether load current is continuous or not.
    (ii) Calculate the value of average output current.
    (iii) Compute the maximum and minimum values of steady state output currents.

  2. Draw the circuit diagram of step-down chopper, also draw the waveforms of source voltage \( V_s \) and output voltage \( V_0 \) for duty ratio \( \alpha = 0.4 \) and time period \( T = 10~\mu s \).

Q.8 Solve both questions :[10+4=14]

  1. Describe unipolar-voltage switching in a single-phase full-bridge inverter by suitable waveforms for carrier wave, reference wave and output voltage. Show the turn-on and turn-off of various switches in waveforms.

  2. Justify why it is called unipolar-voltage switching inverter or three-level PWM inverter.

Q.9 Solve both questions :[7+7=14]

  1. Explain the operation of boost converter with voltage and current waveforms across the boost inductor. Assume continuous conduction.

  2. Derive its output voltage equation in terms of duty cycle and input voltage.