Power Electronics - QB

Department o f electrical and electronics engineering

QUESTION BANK

Subject: EE 2301 Power Electronics

UNIT I: POWER SEMICONDUCTOR DEVICES

Part-A

1. State the advantages of IGBT over MOSFET.
2. What is the function of snubber circuit?
3. What is the latching current of SCR?

4.   Compare power MOSFETs with BJTs.

5. In Traics,  which of the modes the sensitivity of gate is high.
6. Define the term pinch off voltage of MOSFET.
7. Draw the turn-on characteristics of SCR and marks the timings t_d, t_r and t_on.
8. Why are IGBT becoming popular in their application to controlled converters?
9. Draw the V-I characteristics of SCR and mark the holding and latching current.
10. What are the advantages GTO over BJT?
11. What is the holding current SCR?
12. What is reverse recovery time?
13. What are the requirements of gate drive?
14. Why is pulse triggering preferred over RC triggering?

15.   Differentiate natural and forced commutation.

Part-B

16.   Draw and explain the switching characteristics of a thyristor during its

       turn- on and turn-off process.

     17.    Explain the various protection schemes commonly encountered in thyristor.

     18.    (i)  Describe the switching characteristics of power MOSFETs.

       (ii) Describe any one MOSFET gate drive circuit in detail.

     19.    (i)  Draw and explain the switching characteristics of GTO.

      (ii)  Give the merits and demerits of a GTO as compared to a conventional  

     thyristor.

20.     Describe the different modes of operation of a thyristor with the help of its static

 V-I characteristics.

21.    Explain the switching performance of BJT with relevant waveforms indicate clearly

  turn-on  and turn-off times and their components.  Also define the term SOA.

22.   (i)  Draw and explain the V-I characteristics of TRAIC.

       (ii) Explain why triac is rarely operated in I quadrant with negative gate                                                              current and in III quadrant with positive gate current.

23. Explain the class C commutation with waveform.

24.  Briefly discuss the RC triggering and synchronized UJT triggering of SCR.

UNIT II: PHASE CONTROLLED CONVERTERS

Part A

1. Why is power factor of semiconverter better than full converters?
2. What is the inversion mode of rectifiers?
3. Write the expression for output dc voltage in a single phase  fully controlled

       converter.  Also draw its variation with firing angle.  

4. What is total harmonic distortion and  displacement factor?
5. List the various applications of phase controlled converters.
6. Under what conditions a single phase fully controlled converter gets operated

       as an inverter.

7. State the principle of phase control in ac-dc converters.
8. It is  required to operate a single phase full converter in the inverter mode with

       RLE  load.  Should the average output voltage be more or less than E during

              inverter operation? Why ?

9. A single phase half wave rectifier is used to supply power to a load impedance of

10 Ω from 230 V, 50 Hz ac supply at the firing angle of 30.  Calculate the load

       current.

10. What is the delay angle of phase controlled rectifier?
11. What is the function of freewheeling diode in semiconverter?
12. What is the effect of inductive load in the performance of a three phase bridge

Rectifier?

Part B

13. Describe the working of single-phase fully controlled bridge converter in rectifying mode and inversion mode.Skecth the waveforms for delay angle, load voltage, load current and thyristor voltage.
14. Explain the operation of 3-phase, 6 pulse bridge converter with resistive load.

              Draw the output voltage, voltage across T₁, and current waveform of T₁ for

              α=0. List the firing sequence of SCRs.

15. (i) Explain the operation of single phase semiconverter and derive the expressions for its average and rms output voltages.

              (ii)  Derive the expressions for harmonic factor, displacement factor and

              factor of a single  phase full converter from the fundamental principle.

16. A singe phase full converter is supplied from 230 V, 50 Hz source.  The load consists of R=10 Ω and large inductance so as to render the load current constant.  For a firing angle delay of 30 determine (i) Average output voltage (ii) average output current (iii) Average and rms values of thyristor currents (iv) the power factor.
17. (i)  Explain about battery charger.

              (ii) Reactive power control of converters

18. Explain dual converter with circulating and non-circulating modes.

UNIT  III : DC-DC CONVERTERS

Part A

1. What are the control strategies of chopper?
2. Mention the disadvantages of FM scheme used in choppers.
3. What are the methods of controlling the output voltage of a chopper?
4. What are the advantages and disadvantages of buck/boost regulator?
5. Define the term duty cycle of dc choppers.
6. Differentiate between constant frequency and variable frequency control strategies of varying the duty cycle of dc choppers.
7. What is meant by time ratio control in dc-dc converters?
8. What are the classifications of dc-dc converters depending upon the directions of current and voltages?
9. What is meant by buck regulator?
10. What is a dc chopper?

Part B

11. (i)  Describe the principle of step-up chopper.  Derive an expression for the   average output voltage in terms of input dc voltage and duty cycle.  State the assumptions made.

            (ii)A step-up chopper supplies a load of 480 V from 230 V dc supply.  

            Assuming the non conduction period of the thyristor to be 50 μs, find the

            on time of the thyristor.         

12.  With the help of power circuit diagrams, discuss the operation of class-C and class-D types of two quadrant choppers.
13. (i) Draw the circuit diagram of a buck converter and explain its operation with equivalent circuit for different modes and waveforms.

     (ii)Discuss the need for resonant switching in SMPS.

14. (i) Discuss the principle of operation of dc-dc step down chopper with

      suitable waveforms.  Derive an expression for its average dc output volatage.

      (ii) A Step down dc chopper has input voltage of 230 V with 10 Ω load   

      resistor connected voltage drop across chopper is 2 V when it is ON.  For a

      duty cycle of 0.5, calculate average, rms values of output voltage and power

      delivered to the load.

15. Draw the circuit of cuk regulator and explain its working principle with necessary waveforms in detail.

UNIT IV: INVERTERS

Part A

1.       List the four applications of CSI.

2.       Distinguish between VSI and CSI.

3.       A single phase full bridge inverter has a resistive load of R =10 Ω and the input voltage Vdc of 100 V.  Find the rms output voltage at frequency.

4.       What is PWM? List its various techniques.

5.       Define modulation index of PWM.  What is its use?

6.       What is the purpose of connecting diode in antiparallel with thyristors in inverters?

7.       What is the need for voltage control in an inverter?

8.       Mention any two advantages of CSI.

9.       Define the term inverter gain.

Part B

10.    Describe the operation of series inverter.  What are its advantages?

11.    State different methods of voltage control in inverters.  Describe about PWM control in inverter.

12.    (i) Describe with a neat sketch and waveforms the operation of a single phase half bridge inverter supplying RL load.

      (ii) Describe the working of a single phase full bridge inverter supplying R,RL

       loads with relevant circuit and waveforms.

13.    Explain the operation of a single phase capacitor commutated CSI with R load with equivalent circuit and output waveform.

14.    Explain the principle of operation of three phase inverter operating in 180 conduction mode with necessary waveforms.  Also obtain the expression for line to line voltages.

15.    Explain the following PWM techniques used in inverter.

      (i) Sinusoidal PWM   (ii) Multiple PWM

16.   Explain harmonic control in inverter.

UNIT V: AC TO AC CONVERTERS

Part A

1.       What is an ac voltage controller?

2.       Compare on-off and phase-angle ac voltage controllers.

3.       What are the effects of load inductance on the performance of ac voltage controllers?

4.       List the merits and demerits of ac voltage controller?

5.       What is a cycloconverter?

6.       List the applications of cycloconverter.

7.       What are the applications of ac voltage controller?

8.       Write the output voltage equation of cycloconverter.

Part B

9.       (i) Explain the operation of single phase controller with inductive load.

      (ii) Draw the possible configuration of a single phase ac voltage controller and  

      compare them.

10.    With necessary circuit and waveforms explain the principle of operation of single phase ac voltage controller having only thyristors feeding resistive load by on-off control and phase control.  Derive the expression for rms value of output voltage in both case.

11.    Explain about the matrix converters.

12.    Discuss about the multistage sequence control.

13.    (i) Explain single phase cycloconverter.

      (ii) Explain power factor control.

14.    (i) Explain single phase to single phase step up cycloconverter.

      (ii) Explain single phase to single phase step down cycloconverter.

15.    Explain three phase to single phase cycloconverter.

16.    Explain three phase to three phase cycloconverter.

Sri Krishna Engineering College

Panapakkam,Chennai-601 301.

Department o f electrical and electronics engineering

Classs test –I

Part A

1. What is the function of snubber circuit?
2. What is the latching current of SCR?
3.  Compare power MOSFETs with BJTs.
4. In Traics, which of the modes the sensitivity of gate is high.
5. What is meant by syncrhonisation?

                                                                   Part B

6. (i) Describe the different modes of operation of a thyristor with the help of its static

 V-I characteristics.   [7]

                (ii)Describe the switching characteristics of power MOSFETs. [8]