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2003 Course Advanced Power Electronics

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Total No. of Questions :12] P1069 [Total No. of Pages : 4 [3864] - 243 B.E. (Electronics) ADVANCED POWER ELECTRONICS (2003 Course) Time : 3 Hours] [Max. Marks : 100 Instructions to the candidates: 1) Answer Q1 or Q2, Q3 or Q4, Q5 or Q6 from section I and Q7 or Q8, Q9 or Q10, Q11 or Q12 from section II. 2) Answers to the two sections should be written in separate books. 3) Neat diagrams must be drawn wherever necessary. 4) Figures to the right indicate full marks. 5) Use of logarithmic tables, slide rule, Mollier charts, electronic pocket calculator and steam tables is allowed. 6) Assume suitable data, if necessary. SECTION - I Q1) a) Draw the circuit diagram of a three-phase semi-converter feeding a highly inductive (level) active load and the following waveforms for firing angle = 60o: i) ii) Supply line voltages. iii) Output voltage. iv) b) Supply phase voltages. Phase A line current. [8] A three-phase semi-converter operates from the 415V, 50Hz mains and feeds a highly inductive (level) active load having R = 20 . If the firing [10] angle is 60o, calculate: i) Average (DC) load voltage. ii) Average (DC) load current. iii) RMS line current. OR P.T.O. Q2) a) With the help of a neat circuit diagram and relevant waveforms, explain the operation of a three-phase circulating-current type dual converter. Derive an expression for the peak circulating current in terms of the [10] firing angle. b) A series-parallel bank of thyristors consists of 5 parallel strings, with each string containing 6 thyristors in series. The rating of all thyristors is identical with a voltage rating of 1600V and current rating of 1000A. During testing it was found that the maximum voltage withstand capability and maximum current carrying capacity of the bank was 8 KV and 4 KA, respectively. Calculate the series-string efficiency and parallel-string efficiency. [8] Q3) a) With the help of a neat circuit diagram, relevant waveforms and mode equivalent circuits, explain the operation of a three-phase, 120o mode, voltage source inverter feeding a balanced, star-connected resistive load. Also derive an expression for the RMS phase output voltage and RMS SCR current. [10] b) The above inverter operates from a 400V DC supply and feeds a balanced, star-connected resistive load of 25 per-phase. Calculate i) RMS phase output voltage. ii) RMS phase current. iii) RMS SCR current. [6] OR Q4) a) What are the different output voltage control and harmonic reduction [10] techniques in inverters? Explain any one technique in detail. b) Cross-conduction ( shoot-through ) faults are likely to occur in a 180o mode three-phase VSI but unlikely when the same inverter is operated in 120o mode. Justify. How would you modify the gate pulse waveforms of a 180o mode threephase VSI so as to eliminate cross-conduction? [6] Q5) a) With the help of a neat circuit diagram and relevant waveforms, explain the Symmetrical Angle Control (SAC) technique for power factor improvement in AC-DC converters. [8] [3864]-243 2 b) Explain how the following parameters are sensed in power electronic circuits: [8] i) DC current. ii) AC current. OR Q6) a) b) Q7) a) b) Q8) a) b) With the help of a power circuit diagram, control circuit block diagram and waveforms, explain the operation of a single-phase active waveshaping circuit for power factor improvement. [10] Compare ZVS & ZCS converters. [6] SECTION - II Compare servomotors with conventional DC motors and stepper motors. [8] With the help of a neat circuit diagram and relevant waveforms, explain the operation of an unipolar-voltage chopper drive for PM & hybrid stepper motors. [10] OR With the help of a neat circuit diagram and relevant waveforms, explain the operation of a three-phase LCC based separately excited DC drive having a full converter in the armature circuit and a semi-converter in the field circuit. Also derive an expression for the motor speed in terms of armature converter firing angle, field converter firing angle, motor torque and supply voltage. [10] A 400V, 1500rpm, 15A separately excited DC motor with Ra = 0.2 , Rf = 225 , Kv = 3.97/ V/A-rad/sec, is fed from a three-phase full converter operating from the 415V, 50Hz mains. The field circuit is fed from a three-phase semi-converter. Speed variation below base speed is obtained by armature voltage control with the field converter set at Vf = 450V, whereas speed variation above base speed is obtained by field voltage control with the armature converter set at Va = 450V. Calculate. i) The armature converter firing angle for a motor speed of 900rpm if the motor torque is 30N/m. ii) The field converter firing angle for a motor speed of 1800rpm if the motor current is 10A. [8] [3864]-243 3 Q9) a) b) Q10)a) b) Q11)a) b) Q12)a) b) With the help of a neat circuit diagram and appropriate torque-speed curves, explain the slip power recovery scheme for speed control of induction motors in both sub-synchronous and super-synchronous speed regions. [8] Explain the operation of a three-phase brushless DC motor drive with the help of a neat circuit diagram and relevant waveforms. [8] OR Explain the operation (any two) of the following motor protections: i) Field failure in DC motors. ii) Soft start for DC motors. iii) Phase failure for three-phase AC motors. [8] A 6 pole, 380V, 60Hz, three-phase induction motor has a rated speed of 1160rpm. Calculate its speed, slip and slip frequency during electromagnetic braking under constant V/f control for a stator frequency of 45Hz, the braking torque (-ve) being equal to 75% of rated motor torque. [8] What is meant by supply voltage waveform distortion in a power system? What are the causes and effects of harmonics and inter-harmonics in the power system? [8] What are the different types of voltage transients present in a power system? List the mitigation techniques for these voltage transients. [8] OR Explain the terms Voltage sag (dip) and Voltage swell (rise) . What [8] are the different sources / causes of voltage sags and swells? Explain how an Energy Audit helps in controlling energy losses in a typical production plant. [8] jjj [3864]-243 4

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