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2003 Course Advanced Power Electronics

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Total No. of Questions : 12] P1352 [Total No. of Pages : 4 [3764] - 243 B.E. (Electronics) ADVANCED POWER ELECTRONICS (2003) 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) b) Draw the circuit diagram of a three-phase fully-controlled converter feeding a highly inductive (level) active load and the following waveforms for firing angle = 120 . i) Supply phase voltages. ii) Supply line voltages. iii) Output voltage. iv) Phase B line current. [8] A three-phase fully-controlled converter operates from the 415V, 50Hz mains and feeds a highly inductive (level) active load having E = 400V and R = 20 . If the firing angle is 120 , calculate : i) Average (DC) load voltage. ii) Average (DC) load current. iii) RMS line current. iv) Power supplied by the battery of the active load. [10] v) Power fed back to the mains assuming the converter to be lossless. P.T.O. OR Q2) a) b) Q3) a) b) Why are dual converters important? With the help of a neat circuit diagram and relevant waveforms, explain the operation of a three-phase dual mode dual converter. [10] With the help of a neat circuit diagram, explain the technique for achieving equal voltage sharing of two series connected diodes. Derive the relationship between the current sharing resistors in terms of the diode parameters. [8] With the help of a neat circuit diagram, relevant waveforms and mode equivalent circuits, explain the operation of a three-phase, 180 mode, voltage source inverter feeding a balanced, star-connected resistive load. Also derive an expressions for the following : i) RMS line-to-neutral output voltage. ii) RMS SCR current. [10] Explain the MMSR technique of selective harmonic elimination for inverters. [6] OR Q4) a) With the help of a neat circuit diagram, relevant waveforms and mode equivalent circuits, explain the operation of a three-phase, ASCSI feeding an induction motor load. [10] b) Briefly explain any one technique for output voltage control and harmonic reduction in inverters. [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] b) Enumerate the different methods of sensing DC current in power electronic circuits and explain any one in detail. How would you modify this method to sense DC voltage? [8] [3764]-243 2 OR Q6) a) With the help of a neat circuit diagram, waveforms of capacitor voltage & inductor current and mode equivalent circuits, explain the operation of a ZVS resonant DC-DC converter. [10] b) What are the advantages of resonant converters over switched-mode converters? [6] SECTION - II Q7) a) Compare the inherent properties of a typical DC stepper motor and DC servo motor with respect to any four of the following : i) Speed control. ii) Torque-speed behaviour. iii) Shaft-torque and input power. iv) Angular position control. v) b) Rotor inertia. vi) Start-stop operation at high speed. [10] Explain half-stepping & micro-stepping modes of operation for stepper motors. [8] OR Q8) a) With the help of a neat circuit diagram and relevant waveforms, explain the operation of a single-phase semiconverter-based separately excited DC motor drive. Derive expressions for the motor armature current and torque in terms of the firing angle, motor speed, field current and motor parameters. [12] b) A 230V, 1000rpm, 10A separately excited DC motor is fed from a single-phase semiconverter operating from the 230V, 50Hz mains. If Ra = 0.75 , calculate the firing angle to obtain rated torque for a motor speed of 500rpm. [3764]-243 [6] 3 Q9) Explain any two of the following methods for speed control of induction motors : [16] a) Variable frequency PWM-VSI drive. b) Stator voltage control. c) Slip power. OR Q10)a) With the help of the appropriate torque-speed characteristics, explain how electromagnetic braking is achieved for three-phase induction motors fed from a constant V/f variable voltage variable frequency drive. [8] b) A 4 pole, 415V, 50Hz, three-phase induction motor has a rated speed of 1460rpm. Calculate its speed, slip and slip frequency under constant V/f control for a stator frequency of 40Hz, the load torque being equal to 60% of rated motor torque. [8] Q11)a) What are the different types of transients present in the power system? What are the sources of these transients? [8] b) What are the different types of industrial and residential/ commercial loads which contribute to harmonics in the power supply? Explain the effect of these harmonics on the working of power systems and connected equipment. [8] OR Q12)a) b) Write a short note on Energy Audit. [8] Enumerate the different types of short-duration power-line voltage disturbances and explain any two in detail. vvvv [3764]-243 4 [8]

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