RIC-TO2 Session 2014-15 CI ‘S.No.20-09-14 I CI India’s Best Institute for IES, GATE & PSUs RANKIMPROVEMENT COURSE ELECTRICAL ENGINEERING Test - 2 | Power Electronics Duration : 2% Hours Maximum Marks : 150 Read the following instructions carefully Sectlon-A : Objective Questions Section-B : Conventional Questions Maximum Marka :80 © Tima: Hr Meximum Merks: 100 © Times: 1%Hrs. No. of Negative || 1 Candidate should attempt any FOUR quetions ou of uestions | TYPY | Maree | markcng Five, T 2, Eachaueetion cares 35 mats, at-to | Mca 10 0.33 || 3. Eachquestion may have two ar three sub cvslon. 4, The number of marks carta by each subalusion of a an-so| mea 40 ase ‘questonis indicated athe end of the question 5,_Anowers must be writen only In INGLIS Total Number of Questions : 30 Total Number of Quastiona :3 | ¢Alempt any FOUR] Objective Questions must be answered on Objective Response Sheet (ORS) by darkening the appropriate bubble (marked A, 8, C, D) using HB pencil against the question number. Each question has only one correct answer, In case you wish to change an answer, erase the old answer completely using a good soft eraser, 2, There will bea seperate answer sheet for Conventional questions, 3. Write your name & Roll No. at the specified locations on the right half of the ORS as well as conventional answer sheet re | Bhubaneswar 1145124612, 9958995830re] En MADE ERSY rts noite | shops | Hyserabad | pur | tuenow | indore | Bhubaneswar RICEET4 | jective Questions QNo. 1 to Q.No. 10 carry 1 markeach Q.1 Inthe circuit shown below, ifthe Latching current of the thyristor is 4 mA then the minimum width of the gate pulse required to properly turn on the SCR is oan 88. of 400 Va (a) tus (b) 2us (c) Sus (6) 4us Q.2_ In the circuit shown below, the circuit turn off time for main thyristor (T,) is 30 us. 1, = 50 A and source voltage is V, = 50 V. For reliable operation assume 50% tolerance on circuit turn off time of main thyristor The value of commutating capacitor ‘C’is (a) 30uF (b) 40 mF (©) 45uF (d) SopF Q.3. In two transistor model of a thyristor shown below in figure WADE EASY, New Dein © Gop (21+ 49) fg + losoe @ 1.= Ty (aq +09) ely + Ieaor + Fosae ©) a= te [a;+a) G2ly + Iea01+ !oo00 (©) I= fa, +a) (d) None of the above Q.4 Foravoltage source inverter shown in figure In the interval when V, <0 and i, > 0, the pair of devices which conducts the load curentis @ aa ©) a. (©) DD, (d) Dy, D, 2+ “POWERELECTRONICS wwwamadeeasy.in | © cps MADE ERSYGQ MADE ERY cats sits | shopat | terabed | ape | Lucnow | indore | thbarenrr | RICEET4 Q5 A 1-6 full wave ac voltage controller is {@) line voltage with 120° fring shown in figure, (b) load phase voltage with 120° fring (6) line voltage with 180° fring (@) load phase voltage with 180° fring Q.8 Inthe circuit shown below, sa0vims) eo He é The delay angles of thyristors 7, and T, are equal a,=a,=0= 2 The rms current of thyristors is (@ 4a 5A (6A (7A V,, Vp and V, are zero center PMMC voltmeters, The circultis intially relaxed. Switch sis closed at t= 0. In steady state, readings of voltmeters V, V, and V, are respectively, (a) 100 V, 100 v,-100V Q.6 Astar-connected load of 15 Q/phase is fed from 420 V de through a 3+) bridge inverter. The rms value of load current is "180° conduction mode], produced or utlleed in any form without th (@) 116A (b) 12.48 (©) 104A (@) 13.24 : (©) 100,020 2 (©) -100 V, 0, 200 Q.7 Figure (a) shows a 3-6 inverter fed by a | ?| {d) 100, 0, 100 V constant votage source V,andconnected | & toabalanced resistive load atthe output, | 3] 0-8 Inthe circuit shown below, Each switching device may conduct for | £ 120° of for 120°. The waveform shown in Ee Ee, figure (o)'s, z c : Ve yp wy 8f sy a ee gem a Hw |S Lf} - * LWW] 2 If V, = 200 V, R, = 10 @ and A, 4) yy u thon the poak current through T, is 2 (a) 36A (b) 40 @ 3 (c) 42A (@) 454 =| Q.10 A single-phase full bridge inverter with We E square wave outpul voltage will have in 2 its output waveform a filth harmonic % 3) Conporent Tha mavivurvalasofthe th = harmonic component is equal to x 3 percentage of the source voltage, the value ° ofxis (a) 20.02% — (b) 24.12% ® (e) 22.79% (a) 25.46% 3. + “POWERELECTRONICS www.madeeasy.in | ©ceprah MADE ERSYGQ MADE ERY cats sits | shopat | terabed | ape | Lucnow | indore | thbarenrr | RICEET4 (QNo.11 to Q.No. 30 carry 2 marks each The exact minimum average output . voltage is Q.11. Asingle phase full converter, connected fa) 30 (sev to 230 V, 60 Hz source, is feeding a load R=10Qin series with a large inductance that makes the load current ripple free. For a firing angle of 45° the reactive power input is (@) 2001 Var (b) 2143 VAr (©) 2315 Var (d) 2418 Vr (c) 34V (d) 36V Q.16 An SCRhas V,z1, characteristics given as V, = 1.5 +81, Ina certain application, the gale voltage consists of rectangular pulses of 12 V and of duration 50 ys with duty cycle 0.2. The value of R, series resistor in gate circuit to limit the peak power dissipation in the gate to 5 watts is é Q.12. A thyristor has thermal capacity of 0.1 J/ °C and a thermal resistance of 0.9°CW. fa) $2 (o) 62 ‘The permissible short-time power loss for 7a (80 the temperature rise of 40° C over 0.01 sec is. Q.17 A 220 V, 1000 rpm, 60 A separately- (@) 422W (&) 412. excited de motor has an armature (©) 451W (6) 440w resistance of 0.1 2. Itis fed from a single: phase full converter with an ac source Q.13 A single-phase bridge-type cycloconverter Voltage of 220 V, 50 Hz. Assuming has input voltage of 280 V, 50 Hz and load continuous conduetion, the firing angle for of R= 10, Output frequency is one-third rated motor torque at (500) rpms, of input frequency. For a firing angle delay fa) 1176" (192 of 30°, The rms current of each thyristor produced or utlleed in any form without th 3| (c) 122.4" (a) 190.1" s ? (@ 113A (0) 2A 8 | @.18 A thyristor has a foward characteristics @ 142A 5A 8 which may be approximated over its £ normal working range to the straight line Q.14 Inthe circuit shown below 2 ‘as shown in figure 150m a 7 rats. pi 2 wesy oly a0 g = 60 | 6 z 8 ‘The average output current / 4 Bo switching frequency is 25 kHz. 3 n State Voltage () The peak to peak ripple current of Inductor is 5 (a) 0.724 (0) 0.778 3 ‘The mean power loss for a half sine wave (2 0898 (@) 0948 3] ofmeanvaue 1B Ais Q.15. Inavoltage commulated chopper, voltage | z (a) 30.14W input V= 250, constantload current /,= | = (o) 94.2 20 A, switching frequency is 250 Hz, | 3) (c) 28.26W ° (a) 326W commulating components L = 1.25 mH and C= 8 pF. 4 + “POWERELECTRONICS www.madeeasy.in | ©ceprah MADE ERSYre] MADE ERSY con | nos | anopat Fderbad | per RICEE14 Lucow | Indore | Bhubaneswar 2.19 2.20 21 if the Converter shown in figure has a Q.22 Asingle-phase full bridge inverter has RLC_ purely resistive load of R and the delay load of R= 4, L = 35 mH and C= 155 LF. The de input voltage is 230 V and angle is a= output frequency is 50 Hz. The 3'¢ 8 harmonic component in load current is 2 (a) 16A (b) 222A a | 2 (c) 1.98.4 (d) 261A £| Q.23. The figure shows one period of the output, : voltage of an inverter. a should be chosen ‘The rectfcatoneiclency and Form Factor | § such that 60° < ee < 80° rms value of (FP) respectively are 2 fundamental components 60 V, then o: in (@) 18.12% and 1.6 = degrees is (6) 20.24% and 2.22 S| ov wav tay (c) 22.04% and 1.22 5 q (6) 30.14% and 11 4 wo | foe |, na single phase full wave diode bridge | = ore fee | tease] 589) (doare) rectifier, the diodes have a reverse | 5 LW recovery time of 40 us, For an ac input | & oo mov 120 voltage of 280 V and 2500 Hz, the |2 (7715) eae" percentage reduction in average output | 2 (c) 83.26" (@) 88.14 voltage due to the effect of reverse | 8 tocovery times 3 2.24 Inas-phase bridge rection fed rom the (a oot ©) 955% c star-connected secondary of a (ose) 128% : transformer, let the phase voltage of A- Z phase be V,, sinwt (phase sequence three-phase half-wave controlled rectifier | = A.B,C). At the instant when the voltage of circuit is shown in the figure. Itis operated | 3 ‘Acphase is maximum, the output voltage from 3-9 slar-connected, supply |2 at the rectifier terminals will be transformer with a line to line ac supply | 2 voltage of 440 volts rms, at 50 Hz. The |& (@) 18, (0) V8 Vy thyristor are triggered at a delay angle of | 3 a = 30°. Assume continuous ripple free | 2: OB (@) ¥, current. a r 2) Q.25 Compared to a single-phase halt-bridge Ro— pe 3 inverter. the output power ofa single-phase hs = full-bridge inverter is higher by a factor of a ms! é (a2 (b) 3 h % & (4 de se Vo Zaaze0 | @.26 Consider the statements given below, we -{ 3) Usecrireounesing dove a (9) Reduce the reactive power demand The average output cutrentis 3 of the converter (a) 12.864 (bo) 14.244 (ii) Improves the fundamental power (c) 15.12A (d) 16.714 factor POWERELECTRONICS wwwmadeeasy.in | ©copnge MADE ERSYre] MADE ERSY con | nos | anopat Fderbad | per Lucow | Indore | Bhubaneswar RICEE14 27 2.28 6 (ii) Decreases the current distortion factor (coF) (iv) Increases (Total Harmonic Distortion) THD which of the above statements are not correct (@) (0, (il) and tiv) (b) (i) and (i) (6) only (i) (@) (i) and (vy A six pulse eyclo-converter, fed from 3 phase, 400 V, 50 Hz source, is delivering a load current of 40 A to a single-phase resistive load. The source has an inductance of 1.2mH per phase. The rms, value of load voltage for firing angle delay of 0° is (@) 320.42V (c) 346.12V (b) 371.78 (d) 399.24 inthe chopper circuit shown in figure, the input de voltage has a constant value V, The output voltage Vy is assumed ripple free. The switch Sis operated with a switching time period Tand a duty ratio D.Whattis the value of critical inductance (Lg) at the boundary of continuous and discontinuous conduction of the inductor current i,? é produced or utlleed in any form without th ofthis book may be "ASY, New Delhi. No p 0 MADE ° Q.29 Q.30 f@) f= Set) 0 Bh Ve(210) Lgx leo) ne OE fo) Lg= WA Dale (tga Mae") ‘3-6 dual converter, operating in the circulating current mode, has the following data: per phase supply voltage = 230 V, f= 50 Hz, e,, = 60°, current limiting reactor, L = 15mb The peak value of circulating currentis (a) 22.144 (b) 25.19 (0) 27744 (a) 29.82 single phase voltage controller has input voltage of 280 V, 50 Hz anda load of R= 15 Q. For 8 cycles ON and 6 cycles OFF the average and rms thyristor currents respectively are (2) 42Aand9.16A (0) 39Aanda.13A (c) 824A and 6.13 (0) 64 Aand 426A ——— End of Section - A —— POWER ELECTRONICS www.madeeasy.in | ©ceprah MADE ERSYMIDE ERSY 025 | 01s |p pce | ap | oo |e | those | RICEETA | Eine on Gace (Answer any FOUR out of FIVE) Q.1 (a) (i) Asingle-phase controlled rectifier bridge consists of one SCR and three diodes as shown in figure. Sketch output voltage waveform for afiing angle efor the SCR and hence obtain an expression for the average output voltage under the assumption of constant current ‘Show the conduction of various components as well (ii) Draw waveforms of current through T,, D,, D, and D, assuming constant load current (ii) For an ac source voltage of 280 V, 50 Hz and firing angle of 45°, find the average output current and power delivered to battery in case load consists of A= 52, L = 8 mH and E= 100. [15 marks] (b) A3-phase M-3 converter is operated from 3-phase, 230 V, 50 Hz supply with load resistance R= 10.9 An average output voltage of 50% of the maximum possible output voltage is required. Determine the firing angle [10 marks] @.2 (a) The buck regulator shown below has an input voltage of V, = 12V. The required average output voltage is V, = § V at A = 500 2 and the peak-to-peak output ripple voltage is 20 mV. The switching frequency is 25 kHz. Ifthe peak-to-peak ripple cutrent of inductors limited 10 0.8 A, determine (i) the duty cycle k, (i) the filter inductance L, and (ii the fiter capacitor C, and (iv) the critical values of L and C. [15 marks] (b) AnRLEloadis operating in a choppe! 0.06 R= Oand constant &. Fora duty cycle of 0.2, find the chopping frequency to limit the amplitude of load current excursion to 10 A [10 marks} 7 + POWERELECTRONICS wwwamadeeasy.in | © cps MADE ERSYMADE ERY can | seis | shopat | yeerabae | per | Lidoow | indore | shasareswar | RICEET4 130 V de, T= 1 ms. The load consists of RLC in series Q.3. Fora single-phase full bridge inverter, V, 1 with R= 19, ol = 6 @and 55-72 (@)_ Sketch the waveforms for load voltage v,, fundamental component of load current ig,, Source current, and voltage across thyristor’. Indicate the devices under conduction during different intervals on one eycle. (i) Find the power delivered to load due to fundamental component. (iti) Check whether forced commutation is reauited or not, Take thyristor turn-off time as 100 ps. [25 marks] 4A single-phase voltage controller is employed for controling the power flow from 230 V, 50 Hz source into a load circuit consisting of A= 3 @ and wl. = 4 @. Calculate: (9 the control range of fring angle, (i) the maximum value of rms load current, (iiithe maximum power and power factor, (js) the maximum values of average and rms thyristor currents, (») the maximum possible value of difdt that may occur in the thyristor and (wi the conduction angle for « = 0° and «= 120° assuming a gate pulse of duration m-radian [25 marks] Q.5 (a) A S-phase to single-phase cyclo-converter employs 3-pulse positive and negative group converters. Each converters supplied from detta/star transformer with per phase turns ratio of 2:1. The supply voltage is 400 V, 50 Hz. The RL load has R= 2.8 and at low output frequency, 1.8. In order to account for commutation overlap and thyristor turn-off time, the fring angie in the invsersion mode should not exceed 160°. Compute: (@)_ the value of the fundamental rms output voltage (i) rms output current and [15 marks] (b)_ Asoparately excited de motor, operating trom a single-phase hal-controlled bridge ata speed of 1400 rpm, has an input voltage of 390 sin314¢ and a back emf 80 V. The SCRs are fred symmetrically at c= 30° in every half cycle and the armature has a resistance of 4. Calculate the average armature current and the motor torave. [10 marks} End of Paper For Answer key and Detailed Solutions, visit : www.madeeasy.in 8 + “POWERELECTRONICS www.madeeasy.in | ©ceprah MADE ERSY