UNIVERSITY QUESTIONS. Unit-1 Introduction to Power Electronics

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1 UNIVERSITY QUESTIONS Unit-1 Introduction to Power Electronics 1. Give the symbol and characteristic features of the following devices. (i) SCR (ii) GTO (iii) TRIAC (iv) IGBT (v) SIT (June 2012) 2. What are the important applications of power electronics? (Dec 2011, Jun 2011, Jun 2010) 3. Mention and explain the different types of power electronics converter systems. Draw there input/output characteristics. (Dec 2011, Jun 2011, Dec 2010, June 2012) 4. Mention and explain the classification of power semiconductor switching device, on the bases of control characteristics. Give an examples. (Jun 2011, Dec 2010) 5. Explain peripheral effects of power converter system. (Dec 2011, Dec 2012) 6. What is the difference between thyristor and Triac? (Dec 2012) Unit-2 Power Transistor 1. With the necessary waveforms, explain the switching characteristics of a power Transistor. (Dec 2011, Dec 2010) 2. Compare the BJT and MOSFET. (Dec 2011) 3. Discuss the different methods of providing isolation of gate drive circuits from power circuit. (Dec 2011, June 2012) 4. What is the necessity of base drive control in power transistors? Explain proportional base control. (Jun 2011) 5. For the transistor switch as shown in figure (Jun 2011) a. Calculate forced beta, f of transistor. SJBIT/Dept of ECE Page 2

2 b. If the manufacturers specified is in the range of 8 to 40, calculate the minimum overdrive factor (ODF). c. Obtain power loss P T in the transistor. 6. For the transistor switch as shown in figure (Dec 2010) Calculate forced beta, of transistor. If the manufacturers specified is in the range of f 8 to 40, calculate the minimum overdrive factor (ODF). Obtain power loss P T in the transistor. V 10 V, R 0.75, B V 1.5 V, R 11, BE sat V 1 V, V 200V CE sat B C CC 7. The of a bipolar transistor varies from 12 to 75. The load resistance is RC 1.5. The dc supply voltage is V CC =40V and the input voltage base circuit is V B =6V. If V CE(sat) =1.2V, V BE (sat) =1.6V and R B =0.7 determine a. The overdrive factor ODF. b. The forced f. c. Power loss in transistor P T (Jun 2010) 8. Explain the steady state characteristics of Power MOSFET and compare this with power BJT. (Jun 2010) 9. With necessary sketches, explain briefly the switching characteristics of IGBT. (Jun 2011) 10. What is a bipolar transistor and explain how many types and give their region of operation. (Dec 2012) 11. Compare BJT s, MOSFET and IGBT. (Dec 2012) SJBIT/Dept of ECE Page 3

3 Unit -3 Introduction to Thysistors 1. Draw the transistor model of a thyristor and derive an expression for the anode current in terms of the common base current gain of the transirtor. (Dec 2011, Dec 2010, June 2012) 2. Write a short note on dv/dt and di/dt protection. (Dec 2011, June 2012) 3. Sketch the gate characteristics of SCR and explain different regions of gate characteristics. Also indicate different regions, different voltages, and different currents on gate characteristics. (Jun 2011) 4. Explain Resistance triggering. (Jun 2011) 5. Distinguish between : (i) Latching current and holding current. (ii) Converter grade thyristor and inverter grade thyristor. (iii) (iii) Thyristor turn off time and circuit turn off time. (Dec 2010, Jun 2010) 6. With the help of neat circuit diagram and waveforms, explain RC firing circuit used with half controlled rectifier. (June 2010) 7. With a neat circuit diagram and waveforms explain the UJT relaxation oscillator. (June 2012) 8. What is the difference between SCR and a TRIAC? Explain its characteristics? (Dec 2012) Unit-4 Controlled Rectifiers 1. With the necessary circuit and waveforms, explain the principle of operation of single phase full converter with R-L load, Derive the expression for the average output voltage. (Dec 2010, Jun 2011, June 2012) 2. With the circuit diagram, and waveform, explain the principle of operation of dual converter, with and without circulating current. (Dec 2010) 3. What are the advantages and drawbacks of circulating current mode of operation of a dual converter? (Dec 2010) 4. Explain with the help of wave forms, 1-φ semi converter (half bridge converter) with R load. Derive an expression for V o(avg) and V o(rms). (Dec 2011, June 2012) 5. Explain the operation of single phase semi converter with highly inductive load. Derive the expression for V (dc) and V (rms). (Jun 2010) 6. How do you classify Phase control converters? Explain. (Dec 2012) SJBIT/Dept of ECE Page 4

4 Unit-5 Commutation 1. What is commutation? Explain Complementary commutation. (June 2012) 2. What do mean by commutation? Explain briefly the different types of commutation (Dec 2010, Dec 2012) 3. Explain the self commutation with the help of neat sketch and obtain the expression for the capacitor voltage and current. (Dec 2011, Dec 2010, Jun 2010) 4. Calculate the values of RL and C to be used for commutating the main SCR in the circuit shown in figure When it is conducting a full load current of 25 A flows. The minimum time for which the SCR has to be reverse biased for proper commutation is 40sec. Also find R 1, given that the auxiliary SCR will undergo natural commutation when its forward current falls below the holding current value of 2 ma. (Dec 2011) 5. An impulse commutated thyristor circuit is shown in figure Determine the available turn off time of the circuit if V = 200 V, R = 10 and C = 5 F. Voltage across capacitor before T 2 is fired is V volts with polarity as shown. (Dec 2010, Jun 2010) 6. With a neat circuit diagram and waveforms explain external pulse commutation. (June 2012) Unit-6 AC Voltage Controller 1. Draw the circuit diagram of a single phase AC voltage controller and explain the principle of ON-OFF control, with the help of relevant wave forms. Derive the expression for rms output voltage in terms of rms supply voltage. (Dec 2011, June 2012) 2. Distinguish between ON-OFF control and phase control of AC voltage controller. (Jun 2010,Dec 2012) 3. Explain the single phase bidirectional AC voltage controller with resistive load with waveform. (Dec 2011) 4. A single phase full wave controller has an input voltage of 230 V (RMS) and a load resistance of 10 ohm. The firing angle of thyristor is 2. Find a. Average output voltage. b. RMS output voltage. c. The Average thyristor current. SJBIT/Dept of ECE Page 5

5 d. The rms current value of the thyristor. e. Diode average current. f. Diode rms current. (Dec 2010) 5. What is an ac voltage controller? With the help of circuit diagram and waveform, explain the principle of phase control. (Jun 2011, Jun 2012) 6. With the necessary waveforms explain the single phase Full wave controller with inductive load. Derive the expression for the rms output voltage. (Jun 2011) 7. What are the advantages and disadvantages of ON-OFF control? (Dec 2012) Unit-7 DC Chopper 1. Give the classification of choppers. Explain class E chopper with circuit and quadrant diagram. (Dec 2010, Jun 2010) 2. Explain the principle of operation of a step up chopper with suitable circuit diagram and waveforms. Derive the expression for average output voltage of step up chopper. (Dec 2011, Dec 2010, Jun 2010) 3. A Chopper circuit is operating on TRC at a frequency of 2 khz on a 460 V supply. If the load voltage is 350 volts, calculate the conduction period of the thyristor in each cycle. (Dec 2011,Dec 2012) 4. With neat circuit diagram and waveform, explain the operation of Jone s chopper. (Jun 2011,Jun 2012) 5. A dc chopper in figure 2.25 has a resistive load of R 10 and input voltage of V = 200 V. When chopper is ON, its voltage drop is 2 V and the chopping frequency is 1 khz. If the duty cycle is 60%, determine Average output voltage RMS value of output voltage Effective input resistance of chopper Chopper efficiency. (Jun 2011) 6. Explain the different control strategies used in choppers. (Jun 2011,Jun 2012) 7. What is the principle of stepdown operation of chopper? (Dec 2012) SJBIT/Dept of ECE Page 6

6 Unit-8 Inverters 1. Explain the performance of inverters. (Dec 2011,Jun 2012,Dec 2012) 2. With a neat circuit diagram, explain the principle of variable DC link. (Dec 2011, Dec 2010) 3. Write a short note on CSI (current source inverter). (Dec 2011, Jun 2011) 5. Explain the principle of operation of single phase half bridge inverter. (Dec 2010) 6. Write and explain the performance parameters of an inverter. (Dec 2010,Jun 2010) 7. What is an inverter? Explain its operation with a neat diagram and waveforms. (Dec 2012) 8. How to differentiate single phase and three phase inverters? (Dec 2012) SJBIT/Dept of ECE Page 7

6. Explain control characteristics of GTO, MCT, SITH with the help of waveforms and circuit diagrams.

6. Explain control characteristics of GTO, MCT, SITH with the help of waveforms and circuit diagrams. POWER ELECTRONICS QUESTION BANK Unit 1: Introduction 1. Explain the control characteristics of SCR and GTO with circuit diagrams, and waveforms of control signal and output voltage. 2. Explain the different