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2003 Course Advanced Power Electronics

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Total No. of Questions : 12] P988 [Total No. of Pages : 4 [3664]-199 B.E. (E&TC) ADVANCED POWER ELECTRONICS (2003 Course) Time : 3 Hours] [Max. Marks : 100 Instructions to the candidates: 1) Answer Q.1 or Q.2, Q.3 or Q.4, Q.5 or Q.6 from Section - I and Q.7 or Q.8, Q.9 or Q.10, Q.11 or Q.12 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) Explain the effect of source inductance on the performance of a single phase full converter with waveforms. [10] Derive expression for its output voltage interms of maximum voltage (vm), firing angle , & overlap angle . Explain how two 3- full converters can be connected back to back to form a circulating current type of dual converter. Explain its operation with the help of voltage waveform across 1) Each converter. 2) Load. 3) Reactor. [8] OR Q2) a) b) Draw the circuit diagram of 3- fully controlled converter operating with highly inductive load & explain operation with following waveforms for = 60 . i) Load voltage. ii) Load current. iii) Supply current (any one phase). iv) Current though SCR (any one). [12] Define string efficiency and state its significance in connection with series and parallel operation of power devices. [6] P.T.O. Q3) a) b) With the help of a neat circuit diagram, relevant waveforms, explain the operation of three-phase 180 mode voltage source inverter feeding a balanced, star-connected resistive load. [8] Three phase bridge inverter is operating with dc supply of V volts. The load is star connected and it is purely resistive. The load resistance per phase is R ohms. Calculate the device utilization factor (DUF) for 180 and 120 mode and comment on the result. [8] OR Q4) a) With the help of circuit diagram, explain the circuit of boost inverter [6] circuit. b) Briefly explain any one techniques for output voltage control and harmonic reduction in inverters. [6] c) Compare between voltage source inverter and current source inverter.[4] Q5) a) With the help of neat circuit diagram, relevant waveforms, explain the operation of a class E resonant converter. [8] b) How will you measure : [8] i) Sinusoidal voltage and current. ii) Nonsinusoidal voltage and current. OR Q6) Explain the concept of power factor. On which parameters does the power factor depend? Enlist various power factors improvement techniques for line commutated converters. Explain any one method of power factor improvement in details. Compare it with other techniques. [16] SECTION - II Q7) a) b) Draw and explain the power circuit of semiconverter feeding a separately excited d.c motor. Explain with typical voltage and current waveforms, the operation in both continuous armature current and discontinuous armature current mode. [10] What are the effects of discontinuous armature current for d.c motor drive. [6] [3664]-199 2 OR Q8) a) A separately excited d.c motor is fed from a 230V, 50Hz supply via a single-phase half controlled bridge rectifier. Armature parameters are inductance 0.06 H, resistance 0.3 , the motor voltage constant is ka = 0.9V/Arad/sec and the field resistance is RF = 104 . The field current is also controlled by a semiconverter and is set to the maximum possible value. The load torque is TL = 50N-m at 800 rpm. The inductances of the armatures and field circuits are sufficient enough to make the armature and field currents continuous and ripple free compute. [10] i) The field current Ff, ii) The firing angle of the converter in the armature circuit and iii) The input power factor of the armature circuit converter. b) Draw the circuit diagram and explain the working of Chopper drive (unipolar) for stepper motor. [6] Q9) a) Explain the induction motor operation, when the V/f ratio is held constant. Also derive the expression for the maximum torque. [8] b) Explain briefly the following methods of braking a d.c. motor i) Regenerative braking. ii) [6] Dynamic braking. iii) Plugging. c) What do you understand by soft start? State and explain the soft start methods employed for motors. [4] OR Q10)a) Draw and explain the operation of 3- brushless d.c motor drive also explain the related waveforms. [8] b) Justify The speed range of an induction motor is restricted to about 30% of full range while operating with slip power regulation system . [4] c) Explain the following speed control method for an induction motor Stalar voltage control. [3664]-199 [6] 3 Q11)a) What is power quality? Why it is required? Explain different types of power line disturbances. [8] b) Define the term sag? Explain different sources of sags & interruptions.[8] OR Q12)a) b) What is energy audit? Explain types of energy audit. [8] Explain probable preventive solutions to control the factors contributing [8] the power quality distortions. vvvv [3664]-199 4

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