Power Electronics (Sample Questions) Module-1

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1 Module-1 Short Questions (Previous Years BPUT Questions 1 to 18) 1. What are the conditions for a thyristor to conduct? di 2. What is the common method used for protection? dt 3. What is the importance of snubber circuit? 4. What is the difference between enhance type MOSFET and depletion type MOSFET? 5. What is a freewheeling diode, and what is its purpose? 6. Comparison between Power MOSFET and BJT. 7. List the name of the methods adopted for triggering a thyristor. 8. Thyristor A has rated gate current of 2A and thyristor B, a rated current of 100mA. Which one is SCR and which one is GTO? 9. Which device has negative resistance characteristics? Draw the characteristics and identify the negative resistance area. 10. Why R-C triggering method is preferred over resistance triggering? 11. Give the merits of GTO as compared to a conventional SCR. 12. What will happen if one SCR in a parallel connected SCR string turns on first? 13. What are the protections to be provided for an SCR? 14. What is the necessity of pulse transformer in firing scheme of SCR? 15. Why the MOSFETs do not required negative voltage during turn off? 16. Define latching current and holding currents as applicable to an SCR. 17. How many regions of operation are there for BJTs? Explain briefly. 18. For thyristors why pulse triggering is preferred to dc triggering? 19. What are the types of power diodes? Discuss the differences between p-n junction diodes and Schottky diodes. 20. What do you mean by ODF? 21. What is the significance of FBSOA and RBSOA? 22. What is a UJT? Define intrinsic stand-off ratio of UJT. 23. What are the base drive techniques for increasing switching speeds of BJTs? 24. What is a totem pole arrangement? 25. Draw the static V-I characteristic of thyristors. 26. Draw the gate characteristics of thyristor. 27. What is the function of varistor in a thyristor protection circuit? 28. Draw the static V-I characteristic of Triac. 29. The gate-cathode characteristic of an SCR has a straight line slope 0f 130. For trigger source voltage of 15V and allowable gate power dissipation of 0.5W, compute the gate-source resistance. 30. What is I 2 t rating? 31. A thyristor is triggered by a pulse train of 5 khz. The duty ratio of 0.4 if the allowable average power is 100W. What is the maximum allowable gate drive power? Prepared by: Mr. Satish Choudhury (Asst. Professor, Dept. of. EICE, ITER, BBSR) Page 1 of 7

2 Long Questions (Previous Years BPUT Questions 1 to 10) 1. Describe RC full wave triggering circuit for a single SCR. 2. Draw and discuss turn-on and turn-off characteristics of an SCR. 3. What do you mean by commutation of an SCR. Discuss about forced and line commutation briefly? 4. What are the protections to be provided for an SCR? 5. Draw and explain the transfer and output characteristics of an IGBT. 6. Enumerate the various mechanisms by which thyristors can be triggered into conduction. Discuss briefly the techniques which result in random turn-on. But the others leading to reliable turn-on thyristors should be discussed in detail. 7. Draw and explain the output characteristics of n-channel enhancement mode MOSFET. 8. Can a forward voltage be applied to a SCR after its anode current has fallen to zero value? Explain. 9. Justify the statement higher the gate current lower is the forward break over voltage. 10. How does a GTO differ from a conventional SCR? Give the circuit symbol and static V-I characteristics. 11. Describe the different modes of operation of a thyristor with the help of its static V-I characteristics. 12. Compare an UJT firing circuit with R and RC firing circuits. 13. Explain the working principle of an oscillator employing an UJT. Derive expressions for the frequency of triggering and firing angle delay in terms of eta, charging resistance etc. 14. Explain the V-I characteristics of UJT. 15. A relaxation oscillator using an UJT is to be designed for triggering an SCR. The UJT has the following data: 0.7, I P 0.5mA, V p 15V, V v 0.8V, I V 2mA, R BB 6k.Normal leakage current with emitter open is 3mA. The firing frequency is 1.5 khz. For C=0.05μF, compute the values of charging resistor and the external resistors connected in the base circuits. Take forward voltage drop of E-B 1 junction as zero. Prepared by: Mr. Satish Choudhury (Asst. Professor, Dept. of. EICE, ITER, BBSR) Page 2 of 7

3 Module-2 Short Questions (Previous Years BPUT Questions 1 to 20) 1. Why the power factor of semi converter is better than that of full converter? 2. What are the advantages and disadvantages of AC voltage Controller? 3. What is the efficiency of 3-phase full wave rectifier? 4. What is the input displacement factor for a single phase semiconductor feeding a ripple free current of 10 A to R-L load and having a firing angle of 60? Hints: DF cos What is the power factor of a single phase full converter with highly inductive R-L Load for 45? 2 2 Hints: pf cos 6. What are the advantages of freewheeling diode connected in a controlled rectifier? 7. A single phase half wave circuit is supplied at 120V. Determine the average load voltage for firing angle 0, 30 assuming the load current to be continuous and level with constant 2.5V drop of the thyristor. 8. In a 3-phase semi-converter for firing angle to 120 and excitation angle equal to 110. What is the conduction period of the freewheeling diode? 9. What is peak inverse voltage of thyristor connected in the three phase, six pulse bridge circuit having input voltage of 415V and voltage safety factor of 2. Hints: PIV Voltage safety factor 2 rms input voltage 10. In a 3 phase full converter if the load current is I and ripple free, what will be the rms value of thyristor current? 11. What type of commutation used in step up cyclo-converter? 12. A single phase fully controlled bridge is supplied at 120V. Determine the average voltage drop for firing angle of 30, assuming each thyristor to have a voltage drop of 0.78V. 13. In a 3-phase semi converter the firing angle is 90 and for continuous conduction what is the conduction period of the freewheeling diode? 14. A single phase one pulse controlled rectifier has 400 sin 314t volts as the input voltage and resistor R as the load. For firing angle of 60 for the SCR, what is the average output voltage? 15. In a three phase semi converter for firing angle less than or equal to 60 what is the conduction angle for each diode and thyristor? 16. What is inversion mode of converters? 17. Does the input power factor of converters depend on the load power factor? 18. In a three phase full converter what is the firing interval of the three SCRs pertaining to one group? 19. Why the power factor of semi converters is better than full converters? 20. What are the applications of cyclo-converter? 21. What is extinction angle control of converters? Hints: Rashid: pg: What is the principle of phase control? 23. What are the effects of removing the free-wheeling diode in single phase semi-converter? 24. Does the input power factor of converters depend on the delay angle? Prepared by: Mr. Satish Choudhury (Asst. Professor, Dept. of. EICE, ITER, BBSR) Page 3 of 7

4 25. Define Modulation index? 26. What is the principle of burst firing and how it helps in AC voltage controller? 27. Which applications basically need cycloconverter? 28. What are the gate signal requirements of thyristors for voltage controllers with RL load? 29. What are the advantages and disadvantages of cyclo-converters? 30. What are three possible arrangements for a single phase full wave ac voltage controller? Prepared by: Mr. Satish Choudhury (Asst. Professor, Dept. of. EICE, ITER, BBSR) Page 4 of 7

5 Long Questions (Previous Years BPUT Questions 1 to 4) 1. Derive expressions for the average and rms values of the output voltage of single phase full converter. 2. A single phase full bridge rectifier having a supply voltage of V m sin t has a purely resistive load of R. Determine the efficiency, the form factor, the ripple factor, the transformer utilization factor, the peak inverse voltage of a Diode, and the crest factor of the input current. 3. Describe the working of a single phase fully controlled bridge converter in the rectifying mode and inversion mode. Also sketch the following waveform for 45 and 120. i) Load voltage waveform ii) Load current waveform iii) Thyristor current and voltage waveforms iv) Supply voltage and current waveforms 4. A 3-ph fully controlled converter is employed to change a battery with an emf 95V and an internal resistance of 0.1. The supply rms voltage is 110V line-to-line and sufficient inductance is induced output circuit to maintain the current virtually constant at 20A.Determine the firing angle α, power factor of supply. 5. A single phase full wave converter is connected to a 120V, 60Hz supply. The load current I a is continuous and its ripple content is negligible. The turns ratio of the transformer is unity. Determine the HF of the input current, DF, and input PF. Calculate V dc, V n, V rms, HF, DF and pf. 6. A three phase half wave converter is operated from a three phase star connected 220V, 60Hz supply and the load resistance is R=10. If the average output voltage is 25% of the maximum possible average output voltage, calculate the delay angle, the rms and average output currents, the average and rms thyristor currents, the rectification efficiency, the transformer utilization factor, the input power factor. 7. A three phase full wave converter is operated from a three phase star connected 220V, 60Hz supply and the load resistance is R=10. If the average output voltage is 25% of the maximum possible average output voltage, calculate the delay angle, the rms and average output currents, the average and rms thyristor currents, the rectification efficiency, the transformer utilization factor, the input power factor. 8. Explain the principle of operation of Burst firing. 9. Explain the principle of operation of single phase bidirectional controller with RL load with the help of neat wave form and also write the expression for output voltages. 10. Explain the principle of operation step up cyclo converter with neat diagram. 11. Explain the principle of operation of three phase full wave converter with RL load with the help of relevant diagram at α=60, 90 and Repeat the Q.N.11 for three phase semi-converter. 13. Explain the principle of operation line commutated inverter. 14. A single phase full converter charges a battery which offers a constant value of E. A resistor R is inserted to limit the battery charging current. Derive the expression for the average charging current in terms of V m, E, R etc. on the assumption that each pair of SCRs if fired continuously in each half cycle. 15. Describe the working of a single phase converter in the rectifier mode with RLE load. Discuss how one pair of SCRs is commutated by an incoming pair of SCRs, Illustrate your answer with waveforms for source voltage, E, output voltage and current, source current, current through and voltage across one SCR. 16. Repeat Q.N.15 in inverter mode. Prepared by: Mr. Satish Choudhury (Asst. Professor, Dept. of. EICE, ITER, BBSR) Page 5 of 7

6 Module-3 Short Questions 1. What are the differences between half bridge and full bridge inverters? 2. Comparison between 120 and 180 conduction mode three phase inverter. 3. Comparison between ZVS and ZCS converters. 4. What is a switching mode regulator? 5. What are the four basic types of switching mode regulators? 6. What are the advantages and disadvantages of fly back, push-pull, forward converters? 7. What is the general arrangement of UPS systems? 8. What are the advantages of HVDC transmission over HVAC transmission system? 9. Comparison between buck-boost regulator and Cứk regulator. 10. What are the types of HVDC transmission systems? 11. What do you mean by duty cycle? Prepared by: Mr. Satish Choudhury (Asst. Professor, Dept. of. EICE, ITER, BBSR) Page 6 of 7

7 Long Questions 1. Explain the principle of operation of four-quadrant chopper or Type-E chopper. 2. Find the expression for minimum and maximum value of load current for a step down chopper with RLE load. 3. Show that the maximum value of ripple current is inversely proportional to chopping frequency and the circuit inductance in a step down chopper. 4. Describe M-type and L-type ZCS resonant converter with relevant circuits and waveforms. 5. Describe ZVS resonant converter with appropriate circuits and waveforms. 6. Explain 180 mode VSI with appropriate circuits and wave forms and also write the expression for line voltages and phase voltages. 7. Explain 120 mode VSI with appropriate circuits and wave forms and also write the expression for line voltages and phase voltages. 8. Explain buck regulator and boost regulator. 9. What is SMPS? Explain Push pull converter with neat circuit diagram and relevant waveforms. 10. Short notes (a)buck regulator, (b)boost regulator, (c)buck-boost regulator, (d)cứk regulator, (e) Forward Converter, (f) fly back converter, (g) Half-bridge and Full bridge Converter, (h) UPS, (i) HVDC system. 11. The buck regulator has an input voltage, V s =15V. The required average output voltage V a =5V and peak to peak output ripple voltage is 10mV. The switching frequency is 20KHz. The peak-to-peak ripple current of inductor is limited to 0.5A. Determine the duty cycle k, the filter inductance L, the filter capacitor C, and the critical value of L and C. 12. The boost regulator has an input voltage V s =6V. The average output voltage V a =15V and average load current is 0.5A. The switching frequency is 20 khz. If L=250μH and C=440μF, determine the duty cycle k, the ripple current of inductor, the peak current of inductor, the ripple voltage of filter capacitance and the critical values of L and C. 13. The step down converter has load resistance 0.2, input voltage 220V, and battery voltage of 10V. the average load current is 200A and the chopping frequency is 200Hz. Use the average output voltage to calculate the value of load inductance L, which would limit the maximum load current to 5% of load current. 14. Solve more problems on Step up chopper, step down chopper, buck regulator, boost regulator. ****ALL THE BEST**** Prepared by: Mr. Satish Choudhury (Asst. Professor, Dept. of. EICE, ITER, BBSR) Page 7 of 7

INSTITUTE OF AERONAUTICAL ENGINEERING (Autonomous) Dundigal, Hyderabad

I INSTITUTE OF AERONAUTICAL ENGINEERING (Autonomous) Dundigal, Hyderabad-000 DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING TUTORIAL QUESTION BANK Course Name : POWER ELECTRONICS Course Code : AEE0