http://electrical-mentor.blogspot.in/ POWER ELECTRONICS Question Bank by Shankar Shankar Shankar Shankar Version: PEQBTNC06 Conventional, Objective and Interview questions in Power Electronics for GATE |IES | All PSUs Version Code: PEQBTNC06Page 2 22 2 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Version Code: PEQBTNC06Page 3 33 3 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 PREFACE I would like to present this Question bank on Power electronics to my student community at free of cost. I have prepared both conventional and objective questions in the subject of Power Electronics from various sources and knowledge gained from my teaching experience over a span of 7 years. The content of this Question bank is mainly useful for GATE and Engineering Service (ESE) aspirants to gain in depth analysis into the subject. As previous GATE and ESE papers are available in various modes, I have not repeated those questions here.It is expected that the reader must have basic knowledge in the area of Power Electronics and its applications at under graduate level before solving this booklet. This booklet contains the following sections: Conventional Questions: By solving these questions, the reader can enhance his/her basic concepts in Power electronics and can establish the link between other branches of electrical engineering. By solving these types of questions, I am sure your confidence levels in the subject will increase which is the key thing for success in any competitive exam and in career as well. I have provided answers for around 90% of questions and remaining 10% is left as open for the readers so that they can sharpen their knowledge. I will address these questions in the next release of this booklet based on response and will provide some more open questions in subsequent releases Objective Questions: In the present trend, every exam is based on Objective questions. After solving the conventional questions, the reader can test his/her understanding in the concepts by taking 4 practice tests based on objective questions Interview Questions: These questions are collected from various interviews like M.Tech admissions in IITs, OCES & DGFS interviews in BARC etc from student community itself. In fact, these questions are not my creation and collected from various students. If you attend any interview, you can also share your experience for the benefit of your next generation And then I have given practical approach for compensator design for PE converter After solving this booklet, I am expecting you can face any exam, or interview very confidently especially in the field of Power Electronics. With initial thoughts in my mind, this booklet came out. I am planning to update this booklet based on feedback received and will revise in regular intervals and need basis Finally I would like to express my sincere thanks to Mr Saida (my student) for his valuable suggestions and efforts in the drafting corrections If you have any suggestions for further development of this booklet, if you find any mistakes or corrections required, please feel free to write an email toelectrical.mentor@gmail.com by referring version codeVersion Code: PEQBTNC06Page 4 44 4 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 INDEX Sl. NoDescription Page No 0Preface3 1Conventional Questions5 2Objective Questions - Practice Test 135 3Objective Questions - Practice Test 247 4Objective Questions - Practice Test 360 5Objective Questions - Practice Test 468 6Interview Questions76 7Answers for Conventional Questions89 8Answers for Objective Questions92 10Compensator Design93 11Useful units for Electrical Engineering114 12Useful Mathematical Formulae116 Version Code: PEQBTNC06Page 5 55 5 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Power Electronics Conventional Questions Q1. Inapowerelectronicslaboratory,anexperimentisconductedtofindcircuit componentvalueanditscircuitdiagramisshowninFig.Thevoltageandcurrent waveforms for periodic time of 20 ms are captured from oscilloscope are also shown below. Find out what could be the circuit element and its value

Fig for Q1 Q2. Inapowerelectronicslaboratory,anexperimentisconductedtofindcircuit componentvalueanditscircuitdiagramisshowninFig.Thevoltageandcurrent waveforms for periodic time of 10 ms are captured from oscilloscope are also shown below. Find out what could be the circuit element and its value Fig for Q2 Q3. In a power electronics laboratory, the impedance Z(s) diagram (bode plot) for a pure inductor is captured using network analyzer as shown in Fig ii vi10V1A t10V2TsVi(t)i1(t) Ii(t)v(t)viIi 10A1Vt2TsVersion Code: PEQBTNC06Page 6 66 6 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Fig for Q3 (a) If 0 = 100 rad/s, find the value of inductance (b) If 1 = 50 rad/s and 2 = 200 rad/s then find the value of Z (s) in dB and at = 1000 rad/s Q4.In a power electronics laboratory, the impedance Z (s) diagram (bode plot) for a pure capacitor is captured using network analyzer as shown in Fig Fig for Q4 (a) If C = 10F then find the values of 0, 1 and 2 in rad/s (These frequencies are in decade fashion) (b) Find the frequency in rad/s when Z (s) = 2 Q5. The current through and the voltage across a power semi conductor switch is shown in Fig. 6 dB0 dB6 dB102log10()Z(s) dB 20 dB0 dB 20 dB102log10()Z(s) dB Version Code: PEQBTNC06Page 7 77 7 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Fig for Q5 Evaluate, (a) The average current and the RMS current rating of the device.(b) The conduction loss in the device

Q6.The approximate wave shape of a capacitor current in a commutation circuit is shown in Fig. The capacitor has an equivalent series resistance (ESR) of 20 m. Fig for Q6 Evaluate the power dissipation in the capacitor Q7. In an inverter, the current through the active device is measured and found to be as shown in Fig. The switching frequency may be considered very high compared to the fundamental frequency of the output current. Fig for Q7 20A Version Code: PEQBTNC06Page 8 88 8 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Determine,(a) The average and RMS current rating of the switch. (b) If the power device is a power transistor with a Vce drop of1.2V, evaluate the conduction loss Q8. TheSCRisusedinanapplicationcarryinghalfsinusoidalcurrentofperiod1ms and a peak of 100 A as shown in Fig. The SCR may be modeled during conduction to have a constant voltage drop of 1.1 V and a dynamic resistance of 8 m. Calculate the average conduction loss in the device for this application Fig for Q8 Q9.Theperiodiccurrentthroughapower-switchingdeviceinaswitchingconverter application is shown in Fig. Fig for Q9 (a) Evaluate the average current through the device. (b) Evaluate the RMS current through the device. (c)ABJTwithadevicedropof1.2VandaMOSFETwithanof150mare considered for this application. Evaluate the conduction loss in the device in either case. Q10. Apowerdiode(idealinblockingandswitching)showninFig,iscapableof dissipating75W.Forsquarewaveoperation,itisratedforpeakcurrentof100A and 135 A at duty ratios 0.5 and 0.33 respectively.Version Code: PEQBTNC06Page 9 99 9 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Fig for Q10 (a)EvaluatetheONstatemodelofthediode(Thisprocedureisknownaspiece-wise modeling of semiconductor device). (b) The above diode while dissipating 40W at an ambient temperature of 350 C, is running with a case temperature of 750 C and 1250 C respectively. Evaluate the thermal resistances of the device Q11. The diode (20ETS08) is a 20 A, 800 V rectifier diode. It has a voltage drop of 0.8 V at 2 A and 1.2 V at 30 A. (a)Findapiece-wiselinearmodelforthisdiodeconsistingofacut-involtageand dynamic resistance.(b) With this piece-wise model evaluate its conduction loss for a 30 A peak half sine wave of current. Q12. Apower-switchingdeviceisratedfor600Vand30A.Thedevicehasanonstate voltage drop of 1.5 V to 2.4 V for conduction current in the range of 15 to 30 A. The device has a leakage current of 5 mA while blocking 600 V.Evaluate (a) The maximum conduction loss, (b) The maximum blocking loss, and (c) The ratio of the conduction and blocking loss with maximum possible power that may be controlled by this switch and make your comment on the result. Q13. A composite switch used in a power converter is shown in Fig. The periodic current through the switch is also shown.Version Code: PEQBTNC06Page 10 10 10 10 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Fig for Q13 Evaluate, (a) The average current and RMS current through the composite switch. (b) The power loss in the MOSFET and the diode of the composite switch. Q14. A power MOSFET has an Rds(on) of 50 m. The device carries a current as shown in Fig. Consider the switching process to be ideal and evaluate the conduction loss in thedevice.(ExploreifyoucansimplifytheevaluationofRMSvaluebyapplying superposition). Fig for Q14 Q15. A power-switching device is ideal in conduction and blocking (0 V during conduction and 0 A in blocking). It is used in a circuit with switching voltages and currents as shown. The switching waveforms under resistive loading and inductive loading are shown in Fig. The switching times tr and tf are 100 ns and 200 ns respectively. Evaluate, (a) The switch-on and switch-off energy loss (in joule) for resistive loading (b) The switch-on and switch-off energy loss (in joule) for inductive loading (c) The resistive and inductive switching losses in watt for a switching frequency of 100 kHz. Version Code: PEQBTNC06Page 11 11 11 11 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Fig for Q15 Q16. ThecurrentthroughandthevoltageacrossaswitchingdeviceisgiveninFig. Evaluate the approximate switch-off and switch-on energy loss in the device. Fig for Q16 Q17. AdisctypeThyristorisshownwithitscoolingarrangementinFig.Thedeviceis operating with a steady power dissipation of 200 W. Fig for Q17 Version Code: PEQBTNC06Page 12 12 12 12 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Various thermal resistances are defined as below: =0.3 C/W; =0.3 C/W; =0.05 C/W; =0.05 C/W; =0.5 C/W;=0.4 C/W; Evaluate the steady state temperature rise of the junction Q18. A composite switch (Q1 and Q2 in parallel) carrying a load current of 10 A is shown in Fig. The switches may be considered ideal in switching. The on-state resistances of the devices Q1 and Q2 are respectively 0.8 and 0.2 . The devices are mounted on a common heat sink held at a temperature of 800 C. Fig for Q18 Evaluate, (a) RMS values of I1 and I2 (b) The average power dissipation (P1 and P2) in Q1 and Q2. (c) The junction temperatures of Q1 and Q2 (Note: RJC1 and RJC2 is thermal resistances from Junction to case of Q1 and Q2). Q19. Fig for Q19 Version Code: PEQBTNC06Page 13 13 13 13 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Thevoltageacrossacapacitorusedforapowerelectronicapplicationisshownin Fig.Thecapacitancevalueis2.5F.Thecapacitorhasanequivalentseries resistance (ESR) of 10 m. The dielectric of the capacitor has a thermal resistance of 0.2 0C/W to the ambient. (a) Sketch the current waveform through the capacitor for one cycle (b) Evaluate the losses in the capacitor (c) Evaluate the temperature rise in the dielectric of the capacitor Q20. A power electronic capacitor is specified to have the following values. Capacitance = 10F;ESR=30m;ESL=75nH;Sketchtheimpedanceofthecapacitorasa functionoffrequencyinthedBvslog.Determinetherangeoffrequencyfor which the capacitor may be idealized to be a pure capacitance of 10 F Q21. ThecurrentthroughadiodeisshowninFig.Considerthefollowingdatafor waveform analysis. t1 = 100 s, t2 = 350 s, t3 = 500 s, f = 250 Hz, fs = 5 kHz, Im = 450 A and Ia = 150 A Determine, (a) Average diode current and (b) RMS diode current Fig for Q21 *Q22. Visit a manufacturer's website, identify a controlled power switching device (BJT, or MOSFET, or IGBT etc) of rating > 10A and > 600V. Download the datasheet and fill in the following. (a) Manufacturer (b) Device and Type No (c) On-state voltage (V) (d) ON-state current (A) (e) Transient switching times (s) (f) Maximum junction temperature (K) (g) Recommended drive conditions (?) (h) Conduction loss at rated current (W) (i) Blocking loss at rated voltage (W) (j) Switching energy loss (J). t1t2t3Tti2i1=Imsinst Im IaT=f1 Version Code: PEQBTNC06Page 14 14 14 14 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Q23. The magnetic circuit of a coupled inductor is shown in Fig. The magnetic material of the core may be assumed to be ideal.N1= 100 T; N2 = 200 T; Ag1= Ag2 = 40mm2; Ag = 80mm2; lg1 = 1mm; lg2 = 2mm;lg = 1.5mm Fig for Q23 Evaluate the inductances L1; L2; L12; L21 *Q24. The following figures (a, b, and c) show three magnetic circuits with an exciting windingoneachhaving100turns.Thecorein(c)isobtainedbyassembling together one each of cores shown in (a) and (b). The magnetic material for the core maybeconsideredtohaveverylargepermeabilitywithsaturationfluxdensityof 0.2 T. (a) Evaluate the expression for flux linkages (N) for cores (a) and (b) as a function of the exciting current ia and ib. (b) Plot the characteristics N vs i for the cores (a) and (b). (c) From the above plot N vs i for the composite core (c). (d) Comment on the inductance of the circuit (c). Fig for Q24 Version Code: PEQBTNC06Page 15 15 15 15 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 *Q25. Fig for Q25 (a)ForthewaveformsshowninFig,calculatetheiraveragevalue,RMSvaluesof the fundamental and the harmonic frequency components (b) For the waveforms shown in Fig, consider A = 100 and = 600 where applicable. Calculate their total RMS values (c) For the waveforms shown from a to d in Fig shown, calculate the ratio of (i) the fundamentalfrequencycomponenttothetotalRMSvalueand(ii)thedistortion component to the total RMS value (d)ForthewaveformsshownfrometofinFigshown,calculatetheratioofthe average value to the total RMS value (form factor) 0 0 A A 2 0 0 2 2 2 2 00 0 2 2 2 2 A A 0 0 tt t A A AA DT T (a) (b) (c) (d) (e) (f) Version Code: PEQBTNC06Page 16 16 16 16 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 (e) For some common rectifiers, the line currents may be like the waveforms shown in a to b of Fig 2 with = 600. The need for power per phase is the same in the two cases i.e, the RMS value of the fundamental component the line currents are 100 A in both cases.(i) Calculate the amplitude and the RMS value for waveform a in Fig shown (ii) Calculate the amplitude and the RMS value for waveform b in Fig shown (iii) Comment on the above answers Q26.Aninductiveloadconnectedtoa120V,60Hzacsourcedraws1kWatapower factor of 0.8. Calculate the capacitance required in parallel with the load in order to bring the combined pf to be 0.95 lag Q27.A 110 V/220 V, 60 Hz single phase 1 kVA transformer has a leakage reactance of 4 %. Calculate its total leakage inductance referred to (a) the 110 V side and (b) 220 V side *Q28.An input voltage of a repetitive waveform is filtered and the applied across the load resistanceasshowninFig.Considerthesystemtobeinsteadystate.Itisgiven that L = 5 H and Pload = 250 W Fig for Q28 (a) Calculate the average output voltage V0 (b) Assume that C so that vo (t) = V0. Calculate Iload and the RMS value of the capacitor current ic (c) In part (b), plot vo and iL *Q29.The voltage v across load and current i into the positive polarity are as follows (1 3) = + 2 + 2 + 2V iloadR (load)v0iciLvi+++(Fig 3)vi15V06 st4 sVersion Code: PEQBTNC06Page 17 17 17 17 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 = +2 +2 A Calculate the following: (a) The average power P supplied to the load (b) The RMS value of and (c) The power factor at which the load is operating Q30. Asinglephasehalfwavedioderectifierisdesignedtosupplydcoutputvoltageof 200 V and load resistance of 10 . Calculate the average and RMS current ratings of diode, PIV of diode and transformer for this circuit arrangement Q31. (a)InthecircuitshowninFig,ThePMMCammeterreads10A.Findthe inductance value. Also find volt meters reading if they are PMMC type Fig for Q31 (b) If all the meters in part (a) are replaced with MI type instruments, then find the meter readings Q32. (a) In the circuit shown in Fig, Ideal PMMC instruments are placed. Find voltmeter readings Fig for Q32 V1A L V2220V50Hz V1A 1F V2220V50Hz C Version Code: PEQBTNC06Page 18 18 18 18 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 (b) In case voltmeter 2 in part (a) is replaced by MI type, then find its reading Q33. Abatteryistobechargedbyasinglephasehalfwavedioderectifier.Thesupply voltage is 30 V, 50 Hz and the battery emf is constant at 6 V. Determine,(a)Theresistancetobeinsertedinserieswiththebatterytolimitthecharging current to 4 A. Take a voltage drop of 1 V across the diode when it is ON (b) PIV of diode (c) In case battery capacity is 100 W.h, find the charging time in hours Q34. Findthetimerequiredtodeliverachargeof200A.hthroughasinglephasehalf wavedioderectifierwithanoutputcurrentof100A(RMS)andwithsinusoidal input voltage. Assume diode conduction over a half cycle. *Q35. (a) A dc battery is to be charged through a resistor R from a single phase half wave uncontrolledrectifier.Foranacsourcevoltageof230V50Hz,findthevalueof average charging current and supply power factor for R = 8 and E = 150 V (b) In case, if diode is replaced by SCR and fired continuously through a constant dc signal, the repeat part (a) (c) In case, SCR in part (b) is triggered after 1 ms from its forward bias point. Then repeat part (b) (d) Comment on all the calculations Q36. Fig for Q36 (a) For the single phase half wave rectifier shown find out the PIV

rating of diode Version Code: PEQBTNC06Page 19 19 19 19 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 (b)WilltherequiredPIV ratingchangeifainductorisplacedbetweenthediode andcapacitor (c)WhatwillbetherequiredVRRM ratingifthecapacitorisremoved?Assumea resistive load.(d) The source of the single phase rectifier circuit has an internal resistance of 2 . FindouttherequiredNonrepetitivepeaksurgecurrentratingofthediode.Also find the i2t rating of the protective fuse to be connected in series with the diode. *Q37.AsinglephasemidpointconverterisshowninFig,whereweassumethe transformeristobeidealandthedcsideloadtoberepresentedbyacurrentstiff load.CalculatetheVAratingofthetransformerasaratiooftheaveragepower supplied to the load. Fig for Q37 Q38. A single bridge consists of one SCR and three diodes operating with a firing angle of 450. Find the average load current and power delivered to the load in case the load consistsofR=8.356,L=8mHandE=100V.Assumetheloadcurrentis constant in the entire working range Q39. A single phase full converter feeds power to RLE load with R = 10 , L = 6 mH and E = 60 V. The ac source voltage is 220 V, 50 Hz. In case one of the four SCRs gets open circuited due to fault, find the average value of load current by assuming the load current as continuous and firing angle is 450. Q40. A three phase half wave phase controlled rectifier delivers power to a resistive load of10.Inputtotherectifieris400V,50Hzthreephaseacsupply.Findpower delivered to the load for a firing angle of (a) 150 and (b) 600 Vp=Vmsint n : 1 : 1 D1IdD2Version Code: PEQBTNC06Page 20 20 20 20 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Q41. A three phase half wave phase controlled rectifier is operated from a 3-ph 230 V, 50 Hzsupplywithloadresistanceof10.Anaverageoutputvoltageof50%ofthe maximumpossibleoutputvoltageisrequired.Determine,(a)Firingangleofthe converter (b) Average and RMS values of load current Q42. Athreephasehalfwavephasecontrolled rectifierisfedfroma3-ph,400V50Hz source and is connected to load taking a constant current of 30 A. SCRs are having a voltage drop of 1.9 V during their conduction. Calculate, (a) Average value of load voltage for a firing angle of 300 and 600 (b) Average and RMS current ratings of SCRs as well as PIV of SCRs (c) Power loss in each SCR (d)Incase,iffreewheelingdiode(FD)isconnectedacrossload,findtheaverage valueofoutputvoltage,averageandRMSvalueofFDcurrentforfiringanglesof 300 and 600 Q43. A three phase half wave phase controlled rectifier is operating from a 3-ph, 400 V 50 Hz and delivers power to the armature of a dc motor with negligible resistance and largeinductorinthedcbus.ThesourcetransformerhasDY-11connectionwith unity phase turns ratio. Back emf of the motor is 300 V. Find the firing angle of the rectifier Q44. A three phase fully controlled rectifier is delivers a ripple free load current of 10 A withafiringangleof300.Theaverageoutputvoltageis400V.Findactiveand reactive power input to the bridge and input power factor of the converter Q45.A battery consists of R = 5 and E = 150 V is charging through a three phase half wave phase controlled rectifier. Input voltage to the converter is 230 V (RMS) from anylinetoneutralandfiringangleforSCRsis300.Findaveragecurrentflowing through the battery Q46. Athreephasefullconverterisfedfrom230V,50Hzsupplyhavingsource inductance of 4 mH per phase. The load current is 10 A and ripple free (a) Calculate the voltage drop in dc output voltage due to source inductance (b) If dc output voltage is 210 V, find firing angle and overlap period (c)Incase,thebridgeismadetooperateasalinecommutatedinverterwithdc voltage of 210 V, find firing angle for the same load current Version Code: PEQBTNC06Page 21 21 21 21 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Q47. Athreephasehalfwavedioderectifierdeliverspowertoaninductiveloadwhich takes ripple free current of 100 A. The source voltage to the bridge is 3-ph 440 V, 50 Hz. Determine, (a) The average and RMS current ratings of diode (b) PIV of diode (c) RMS value of source current *Q48. A battery with a nominal voltage of 200 V and internal resistance of 10 m has to bechargedatconstantcurrentof20Afroma3-phase220V,50HzACpower supply.Whichofthefollowingconvertercircuitwillgivebetterperformancein termsofDistortionfactorinsourcecurrent,fundamentalpowerfactor,andtotal input power factor? (i) 3- Full converter (ii) 3- Semi converter Q49.(a) For the same average DC output voltage of 100 V, calculate the PIV of SCR for the following configurations (Consider = 00) (i) 1- full wave center tap converter (midpoint converter) (ii) 1- full converter (iii) 1- semi converter (iv) 3- half wave converter (v) 3- full converter (vi) 3- semi converter (b)Fromtheabovecalculations,whichconfigurationishavingmaximumand minimum PIV rating for SCR Q50. Fig for Q50 Version Code: PEQBTNC06Page 22 22 22 22 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Twosixpulseconverters,usedforabipolarHVDCtransmissionsystem(shownin figure)areratedat1000MW,200kV.Evaluate,theRMScurrentandpeak reverse voltage ratings for each of the SCRs Q51. A buck converter has an input voltage of 16 V. The required average output voltage is 8 V and peak to peak ripple in output voltage is 10 mV. The switching frequency of the converter is 25 kHz. If the peak to peak ripple in inductor current is limited to 0.7 A. Determine, (a) Duty cycle ratio (b) Filter inductance (c) Filter capacitance Q52.The input voltage to a boost converter is 8 V. The required average output voltage is 16Vandtheaverageoutputloadcurrentis0.5A.Theswitchingfrequencyofthe converter is 30 kHz. If L = 160 H and C = 380 F, calculate, (a) Duty cycle ratio (b) Thepeaktopeakrippleininductorcurrent(c)Thepeakcurrentoftheswitch(d) The ripple voltage in capacitor Q53.The input voltage to a buck- boost converter is 10 V. The switch is operating with a duty ratio of 0.3 and the switching frequency is 25 kHz. The filter inductance is 150 H and filter capacitance is 220 F. The average load current is 1.2A. Determine, (a)Theaverageoutputvoltage(b)Thepeaktopeakrippleinoutputvoltage(c) The peak to peak ripple in inductor current (d) The peak and average current of the switch *Q54.AswitchedmodepowerconverterisshowninFig.TheswitchesSareONduring DTs and the switches S are ON during (1-D)Ts Fig for Q54 (a)Evaluatethesteadystateperformanceofthecircuit.Assumetheswitches, inductors and capacitors are to be ideal(b) Indicate how the voltage conversion ratio will be modified if the inductor has a resistance of RL S L SV0+V0I0RC SSVg+Ig IL+ILVersion Code: PEQBTNC06Page 23 23 23 23 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 *Q55. Consider the circuit given in Fig. Carry out the steady state analysis for the same and evaluate the following Fig for Q55 (a) Output voltage (b) Average input current (c) Output power (d) Efficiency (e) Power dissipation in the MOSFET and the diode *Q56. Figure P13 shows a boost converter cascaded by a buck converter. The switches S and Sare ON during DTS and (1-D)TS respectively. Fig for Q56 (a) Evaluate the steady state currents in L1 and L2 in terms of I0 and D. (b) Evaluate the steady state voltages across C1 and C2 in terms of Vg and D (c) Evaluate the current ripples in L1 and L2 (d) Evaluate the voltage ripple in C1 and C2 Q57. ADC-DCconvertercircuitisshowninFig.Itconsistsofonactiveswitch(S1)and threepassiveswitchesD1,D2andD3.Ithasfourenergystorageelements-two inductors(L1,L2)andtwocapacitors(C1,C2).Considerthatthecurrentsthrough the inductor and voltage across the capacitors are all continuous. The switch S1 is on duringTonandoffduringToff.ThedutyratioofS1maybedesignatedasD.The switch drops may be taken to be zero during conduction. Version Code: PEQBTNC06Page 24 24 24 24 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Fig for Q57 (a) Indicate the duty ratios of the three diodes D1, D2 and D3. (b)Evaluatethesteady-stateinductorcurrents(I1,I2)andthesteadystate capacitor voltages (VC1, VC2). (c) Evaluate the voltage conversion ratio Vo/Vg. (d) Sketch the steady-state waveforms of (I1, I2; VC1, and VC2). (e) Evaluate the ripple currents I1 and I2 in terms of Vg, D, L1, L2 and R. (f) Evaluate the ripple voltages VC1 and VC2 in terms of Vg, D, L1, L2, C1, C2, and R. (g) Calculate L1, L2, C1 and C2 by considering the circuit data as Vg = 100 V, D = 0.6, R = 12 and Ts = 20 s.Assume ripple in capacitor voltage is 1% of its average value and ripple in inductor current is 10 % of its average value Q58. InthebuckconvertershownthediodeDhasaleadinductanceof0.2Handa reverse recovery change of 10 C at iD =10A. Fig for Q58 Find peak current through active switch. Q59. The following Figure shows a PI controller and its asymptotic magnitude bode plot. Select R1, R2, and C. make any suitable assumptions if necessary Version Code: PEQBTNC06Page 25 25 25 25 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Fig for Q59 *Q60. A fly back converter operating at a duty ratio of 0.3 is shown in the following Fig. The transistor ON state drop is 1 V. The diode ON state drop is 0.7 V. The resistance of the inductor windings is 0.5 and 0.25 for the primary and secondary respectively. Fig for Q60 Evaluate the voltage conversion ratio and efficiency of the converter Q61 Inaflybackconverter,therequiredoutputvoltageis100Vforanominalinput voltage of 12 V. If the switch is operating at D = 0.5(a) Find the turns ratio of flyback transformer.Assume voltage drop across switch is 0.8 V and diode is 0.8 V (b) Find minimum and maximum values of D, if input voltage varies from 10 to 14 V, by maintaining V0 be constant. Assume the switching frequency of 2 kHz (c)FindthevalueofLsonsecondarywindingsothatsecondarycurrentisjust continuousattheminimumvalueofDcalculatedinpart(b).Considerload resistance of 100 Version Code: PEQBTNC06Page 26 26 26 26 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Q62.A fly-back converter is to be designed to operate in just-continuous conduction mode when the input dc is at its minimum expected voltage of 200 volt and when the load drawsmaximumpower.Theloadvoltageisregulatedat16volts.Whatshouldbe the primary to secondary turnsratio (N1/N2) of the transformer if the switch duty ratio is limited to 80 %. Neglect ON-state voltage drop across switch and diodes Q63. The average output voltage flyback converter is 24 V at a resistive load of 0.8 . The duty cycle ratio is 0.5 and switching frequency is 1 kHz. The ON state voltage drops of BJT and Diode are VT = 1.2V and VD = 0.7V. The turns ratio of transformer is = 0.25. Find the efficiency of the converter Q64. Find maximum voltage stress of the switch in the primary winding and diode in the tertiary winding if the forward converter-transformer has 10 primary turns and 15 tertiary turns and the maximum input dc voltage is 300 V Q65.Iftheturnsratiooftheprimaryandtertiarywindingsoftheforwardtransformer are in the ratio of 1:2, what is the maximum duty ratio at which the converter can be operated? Corresponding to this duty ratio, what should be the minimum ratio of secondary to primary turns if the input dc supply is 400 V and the required output voltage is 15 V. Neglect switch and diode conduction voltage drops. Q66. Fig for Q66 Version Code: PEQBTNC06Page 27 27 27 27 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Aforwardconverterisoperatingattheboundaryofcontinuousanddiscontinuous conduction. Theswitch is operating at 100 kHz.Assume =for the core so that energy recovery winding is ignored. A load of 10 A at 20 V is being supplied. (a) Find the inductance value (b) Find peak to peak ripple in output voltage as % of average output voltage () *Q67. A forward converter operating at a duty ratio of 0.3 is shown in the following Fig. The transistor while ON drops a voltage of 1.0 V, and the diode while ON drops a voltage of 0.7 V. Fig for Q67 Evaluate the output voltage and efficiency of the converter. *Q68. Aforwardconverteroperatingatadutyratioof0.4isshowninthefollowingFig. Assume the components to be ideal. Fig for Q68 Version Code: PEQBTNC06Page 28 28 28 28 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Sketch the following waveforms under steady state. (a) Inductor current. (b) Secondary current. (c) Primary current. (d) Output voltage. Q69. A single phase full bridge VSI is fed from 230 V dc. In the output voltage waveform, only fundamental component of voltage is considered.(a) Determine the RMS currentratings of switches and diodeof the bridge for the following types of loads: (i) R = 2 (ii) L = 2 (b) Find also the repetitive peak voltage that may appear across switches in part (a) Q70. A single phase full bridge VSI delivers power to RLC load with R = 3 andXL = 12 .Thebridgeoperateswithperiodicityof0.2ms.CalculatethevalueofCsothat load commutation is achieved for the SCRs. Turn off time for thyristors is 12 s and considerfactorofsafety2.Assumethatloadcurrentcontainsonlyfundamental component. Q71. A single phase full bridge VSI delivers power at 50 Hz to RLC load with R = 5 , L = 0.3 H and C = 50 F. The dc input voltage is 220 V.Evaluate, (a) Expression for load current up to 5th harmonic (b) Power delivered to load and fundamental power (c) The RMS and peak currents of each switch(d)Conductiontimeofswitchesanddiodesbyconsideringonlyfundamental components Q72. A single phase full bridge VSI delivers power to a load of R = 12 and L = 0.04 H from a 400 V DC source. If the inverter operates at a frequency of 50 Hz, determine the power delivered to the load for(a) Square wave operation (b) Quasi square wave operation with an on period of 0.6 of a cycle (c)Twosymmetricallyspacedpulsesperhalfcyclewithanonperiodof0.6ofa cycle Q73. Asinglephasecurrentsourceinverter(CSI)withidealswitcheshasthefollowing data: Source current = 30 A, frequency = 500 Hz, and pure capacitive load = 20 F For this inverter, Evaluate: Version Code: PEQBTNC06Page 29 29 29 29 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 (a) The circuit turn off time (b) The maximum value of reverse voltage that appears across switches Q74. A single phase capacitor commutated CSI connected to the load R has the following data: R = 40 , C = 50 F, Source current = 40 A, frequency = 500 Hz. Evaluate, (a) Express the load current as function of time and its value at t = 0 and t = T/2 (b) The circuit turn off time Q75. A3-phase1200modeinverterfeedsastarconnectedloadofR=5.DCsource voltage is 230 V and output frequency is 50 Hz.(a)Expressthelinetolineoutputvoltage,linetoneutraloutputvoltageandline current in fourier series up to 11th harmonic components. (b) RMS values of line to line and line to neutral voltages (c) RMS values of line to line and line to neutral voltages at fundamental frequency (d) THD for line current (e) Load power and average value of source current (f) Average and RMS value of switch currents Q76. SCRTinthefigurebelowisinitiallyOFFandistriggeredwithasinglepulseof width10s.Itisgiventhat =H and =F.Assumelatchingand holding currents are zero and initial conditions L and C are zero.(a) Evaluate the conduction time of SCR T (b) Voltage across device and capacitor after SCR is turned OFF Fig for Q76 Q77.A circuit employing current commutation as shown below has C = 20 F and L = 3 H. Initially capacitor is charged towards the source voltage (=230 V dc).Version Code: PEQBTNC06Page 30 30 30 30 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014

Fig for Q77 DeterminetheconductiontimeforauxiliarySCR(TA)andcircuitturnofftimefor main SCR (TM) in case constant load current is (a) 300 A and (b) 60 A Q78. In the circuit shown in the Figure below, has commutating elements L = 20 H and C = 40 F are connected in series with the load resistance of R = 1 . Fig for Q78 Check whether self commutation or load commutation, would occur or not. Find also conduction time of SCR Q79. ForthecircuitshowninFig,(dv/dt)ratingofthysristorTis400V/s.andits junction capacitance is 25 pF. Switch S is closed at t = 0. Version Code: PEQBTNC06Page 31 31 31 31 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Fig for Q79 (a) Calculate the value of Cs so that thyristor T is not turned on due to dv/dt (b) In case maximum current through thyristor in above circuit is limited to 40A, determine the value of Rs Q80. Forillustratingcomplementarycommutation,thefollowingcircuitisemployed where Vdc = 200 V and R1 = 10 . Fig for Q80 (a) Find the value of capacitor so that T1 is commutated in 50 s.(b) It is required that SCR T2 is turned off naturally when current through it falls below holding current of 4 mA. Find the value of R2. Q81. In the complementary scheme of commutation, Source voltage is 200 V dc, R1 = 10 and R2 = 100 . Evaluate, (a) Peak value of current through SCRs T1 and T2 (b)CapacitancevalueCifeachSCRhasturnofftimeof40s.Takeafactorof safety 2. Justify if you make any assumptions Version Code: PEQBTNC06Page 32 32 32 32 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 *Q82. (a) A separately excited dc motor is represented in block diagram form as show in Fig. Fill all the blocks with usual and your convenient notation Fig for Q82-a (b) From the block diagram given in part (a), find G1 (s) and G2 (s) as per the definitions given below = = (c) Now, express G1 (s) in terms of machine mechanical and electrical time constants which are defined as below: = = Where, KT and KE are torque and electrical constant (d) The dc motor under consideration is having the following data.T rated = 10 N-m ; Nrated = 3700 rpm KT = 0.5 N-m/A ; KE = 53V/1000 rpm;Ra = 0.37 , e = 4.05ms, m = 11.7 ms If this motor is controlled from a power electronic converter, Evaluate the terminal voltage required (in steady state) if motor is required to deliver a torque of 5 Nm at a speed of 1500 rpm (e) If G1(s) in the given statement is expressed as =/ Then find the values of D and n by using data given in part (d). Now, plot the asymptotic magnitude and phase plot of G1 (s) by means of bode plot. Then find out Phase margin and Gain margin with approximate hand calculations or using MATLAB Vt(s)Q(s)?? ? ?Ia(s)Te(s)TWL-Ea(s)Version Code: PEQBTNC06Page 33 33 33 33 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 (f) A PI controller is used in the speed control loop (shown in Fig) to obtain the transfer function of the following form: Fig for Q82-f ==11 + 2 + Where D = 0.5 and n = 300 rad/s (g) Draw the bode plot of closed loop transfer function = if the gain KP = 60 is used for the proportional position regulator. (h) What is the bandwidth of the above closed loop systems (Hint: This is indication of speed of the response) Q83. A 200 V, 1450 rpm, 100 A separately excited dc motor has an armature resistance of 0.04 . The machine is driven by a 3- half controlled rectifier operating from a 3- 220V,50Hzsupply.Themotoroperatesatratedspeedandratedloadtorque. Assumingcontinuousconductiontoevaluate(a)Firingangleoftheconverter(b) RMS value of fundamental input current (c) Fundamental power factor (d) THD in source current Q84. A3-fullconverterisfeedinga100HP,400V,1500rpmseparatelyexciteddc motorhavingarmatureresistanceof0.1andfilterchokewhichisconnectedin series with armature to maintain constant current of 175 A. The bridge is connected to a 3-, 400 V, 50 Hz supply. The source has an inductance of 0.5 mH and back emf constant of the machine is 0.25 V/rpm. Evaluate (a) Firing angle (b) Overlap angle Q*(s)Kp Fw(s) *(s) (s) s1 Q(s) Version Code: PEQBTNC06Page 34 34 34 34 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Q85. A 200 V, 875 rpm, 150 A separately excited dc motor has an armature resistance of 0.06 . It is fed from a single phase fully controlled rectifier with an ac source voltage of 220 V, 50 Hz. Assuming continuous conduction calculate, (a) Firing angle for rated motor torque and at 750 rpm (b) Firing angle for rated motor torque and at (-500) rpm (c) Motor speed for = 1600 and rated torque Q86. A220V,1500rpm,50Aseparatelyexciteddcmotorwitharmatureresistanceof 0.5 is fed from a 3- full converter. Available ac source has a line voltage of 440 V, 50 Hz. A Y- connected transformer is used to feed the full converter so that motor rated terminal voltage equals the rated voltage when converter firing angle is zero. (a)Calculatetheturnsratiooftransformer(betweenphasewindingsofprimary and secondary) (b)Determinethefiringangleoftheconverterwhen(i)motorisrunningat1200 rpm and rated torque (ii) motor is running at -800 rpm and twice the rated torque Q87. A230V,960rpmand200Aseparatelyexciteddcmotorwitharmatureresistanceof 0.02.Themotorisfedfrombasicchoppercircuitwhichprovidesbothmotoring andbakingoperation.Thesourcehasavoltageof230V.Assumingcontinuous conduction (a)Calculatethedutyratioofthechopperformotoringoperationatratedtorque and 350 rpm (b) Calculate the duty ratio of the chopper for braking operation at rated torque and 350 rpm (c)Ifmaximumdutyratioofchopperislimitedto0.95andmaximumpermissible motorcurrentistwicetherated.Calculatemaximumpermissiblemotorspeed obtainable without field weakening and power fed to the source (d)If motor field is also controlled in part (c), calculate filed current as a fraction of its rated value for a speed of 1200 rpm Q88. Afourpole,10HP,460VInductionmotorissupplyingitsratedpowertoa centrifugal pump type of load at a 60 Hz frequency. Its rated speed is 1746 rpm (a) Calculate its speed, slip frequency, and slip when it is supplied by a 230 V, 30 Hz source (b) If the starting torque is required to be 150 % of the rated torque for a constant air gap flux, find the starting frequency that need to apply to the motor Version Code: PEQBTNC06Page 35 35 35 35 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Objective Questions Practice Test - 1 Q1. Whichofthefollowingpowersemiconductordeviceispopularlyusinginwindandsolar power converters? (A) SCR(B) GTO(C) IGBT(D) BJT Q2. Which of the following MOSFET is most suitable as power electronic switch? (A)N channel depletion MOSFET (B)P channel depletion MOSFET (C)N channel enhancement MOSFET (D)P channel enhancement MOSFET Q3. Consider the following statements and choose the correct option. Statements about ideal switches: 1.In OFF state, current flowing through ideal switch is zero. 2. In ON state, voltage across ideal switch is zero. 3. The ideal switch need finite energy to switch ON/OFF or OFF/ON. 4. The switch can be turned ON and OFF instantaneously (A) All statements are true(B) Only 1, 2 and 4 true (C) Only 1, 2 and 3 are true(D) None 04. MatchListI(Transfercharacteristics)andListII(Devices)andselectthecorrect option. List IList II PQRSPQRS (A) 132 4(B) 21 43 (C) 4132(D) 1234 Q5. A BJT with a device drop of 1.2 V is carrying a current which is shown below. VGs ID(1) VGs ID(2) VGs ID(3) VGs ID(4) (P)DGS(Q)GSD(R) GSD (S)GSi10A05 15 2030(s)Version Code: PEQBTNC06Page 36 36 36 36 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 tonCE ictoftICstTVCC(A) 5 W(B) 5.5 W(C) 6 W(D) 8 W Q6. Which of the following device (s) will be considered as bipolar and unidirectional switch. 1. SCR 2. Symmetrical GTO 3. Asymmetrical GTO 4. BJT in series with diode (A) Only 1, 2, 3(B) 1, 3 and 4 (C) 1, 2, 3, 4(D) 1, 2 and 4 Q7. During forward conduction, a thyristor has static V I characteristic as shown by a straight lineingivenfigure.Findtheaveragepowerlossinthethyristorincasethyristoris carrying a constant current of 80 A for one half cycle. (A) 70.4 W (B) 11.6 W (C) 40 W (D) 50 W Q8. The switching waveform for a power transistor are shown in below figure In case, Ics=80A, Vcc=220V, ton=1.5 s and toff=4 s then power loss in the transistor during turn ON is (A) 8.8 W (B) 23.46 W (C) 32.6 W (D) None Q9. For a power diode, the reverse recovery time is 3.9 s and the rate of diode current decay is 50 A/s. For a softness factor of 0.3, Find the storage charge. (A) 350 C(B) 292.5 C (C) 150 C(D) 200 C Q10. Turn on time of an SCR in series with RL circuit can be reduced by (A) Increasing R(B) Decreasing R (C) Increasing L(D) Decreasing L Ia100A0.8V 2.0VVa Version Code: PEQBTNC06Page 37 37 37 37 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Q11. Fordynamicequalizingcircuitusedforseries connectedSCRs, theselectionofCisbased on (A) Reverse recovery characteristics(B) Turn on characteristics (C) Turn off characteristics (D) Rise time characteristics Q12.The circuit symbol for GTO is (A) P, Q and S are correct(B) Only P & Q are correct (C) Only Q & S are correct(D) All are symbols are GTO Q13. Input power factor of any rectifier circuit can be defined as below: (A)rectifier the to input power Apparentrectifier the to input power ActualIPF=(B) IPF= (distortion factor) (displacement factor) (C) ( )2THD 1DPFIPF+=(D) All the above Q14. A single phase semi converter is operated from 230 V, 50 Hz AC supply and operated with a firing angle of 3. The load on the converter is highly inductive with a resistance of 15.53 and load current is ripple free. RMS value of the fundamental source current will be. (A) 10A(B) 9A (C) 5A (D) 7.8A Q15. A single phase fully controlled rectifier is supplying power to a purely resistive load of 100 andthebridgeistriggeredwith=90.Thesourcevoltageis200 V,50Hz.The power delivered to the load is given by (A) 1 kW(B) 2 kW(C) 3 kW(D) 4 kW Q16. The DC equivalent circuit of a single phase full converter is shown below. The net average outputvoltageisavailableacrossterminalsXandY.Findthesourceinductanceby assuming input frequency of 50 Hz. AGG(P)A KG(R)A K G(S)A KG(Q)I0 V0=200V x y V 220cosV 2m= Version Code: PEQBTNC06Page 38 38 38 38 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 (A) 1 H(B) 0.2 H(C) 0.01 H(D) none Q17. Match List I (1- bridge configuration) with list II (Average output voltage) and choose the correct option. (Assume I0 is constant) List I (1- bridge)List II [Average output voltage] (P) with 4 SCRs 1. =m0V 2V(Q) with 2 SCRs + 2 diodes 2. =m0V 2V cos (R) with 1 SCR + 3 Diodes 3.] cos 3 [20+ =mVV(S) with 4 diodes 4.] cos 1 [0+ =mVV PQRSPQRS (A)2413(B) 2431 (C) 4231(D) 4213 Q18. WhichofthefollowingPEconvertercircuitscanbeusedtooperatetheDCmachinein regenerative braking mode? Select your choices from the given options. 1. 1- full converter 2. 1- semi converter 3. 3- full converter4. 3- semi converter 5. Type E chopper6. 1- full bridge VSI Options: (A) 1, 3, 5 and 6 only (B) 1, 2, 3, 4 only (C) 1, 3 and 5 only (D) All can be used Q19. In controlled rectifiers the nature of load current i.e. whether load current is continuous or discontinuous (A)does not depend on type of load and firing angle delay (B)depends on type of load and firing angle delay (C) depends only on the type of load (D)depends only on the firing angle delay Q20. If =90o in the following circuit, the RMS value of the output voltage is (A) 230 V(B) 230 2 V(C) 115 V(D) 115 2 V Version Code: PEQBTNC06Page 39 39 39 39 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Q21. A 230 V, 50 Hz one-pulse SCR controlled converter is triggered at a firing angle of 40o and loadcurrentextinguishesatanangleof210o.Findthecircuitturnofftime(insecond)if load R=5 and L=2 mH (A) 120 (B) (1/120)(C) 60(D) (1/60) Q22. A four quadrant operation of the DC machine requires (A)Two full converters is series (B)Two full converters connected back to back (C)Two full converters connected in parallel(D)Two semi converters connected back to back. Q23. The output ripple frequency of 3- semi converter depends on (A) Input frequency(B) firing angle (C) source inductance(D) both A&B Q24. A 3- full converter feeds power to a resistive load of 10 . For a firing angle of 45o, the load takes 1 kW. Find the magnitude of the line Voltage (A) 100 2 V(B) 100 3 V(C) 200 V(D) 200 3 V Q25. A3-halfwavecontrolledrectifierisdeliveringpowertohighlyinductiveloadwith continuous load current. Which of the following equation can be used to find average output voltage when firing angle is 450? (A) [ ] += cos 12V 3 3Vmp0(B)= cos2V 3 3Vmp0 (C)((

||

\| + +=6cos 12V 3Vmp0(D) ((

||

\| + +=6cos 12V 3 3Vmp0 Q26. A3-fullconverterissupplyingpowertoaninductiveloadandcurrentof31.42A.The RMS value of fundamental input current will be (A) 24.5 A(B) 31.42A(C) 25.65A(D) None D3D2D4D1T2T1230V50Hz R 0+ Version Code: PEQBTNC06Page 40 40 40 40 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Q27. In 12 pulse rectifier, the lowest harmonic in the input current will be (A) 3rd(B) 5th(C) 7th(D) 11th Q28. Which of following rectifier can only be used as both 3-pulse and 6-pulse rectifier? (A) 1- full converter (B) 3- full converter (C) 3- half wave converter(D) 3- half controlled converter Q29. In step up chopper (boost converter), the average output voltage when duty cycle ratio=1. (A)infinity(B)zero (C)equal to source voltage (D)less than infinity but greater than source voltage Q30. InwhichofthefollowingDC-DCconverter,thenatureofpowerconversionwillbeof current to voltage type (A) Buck converter(B) Boost converter (C) Buck-Boost converter(D) fly back converter (Isolated type) Q31. A chopper circuit shown in the figure is operating at a switching frequency of 25 kHz with a duty cycle ratio of 0.25 The peak to peak ripple in the inductor current will be (A) 1 A(B) 1.8 A(C) 0.5 A(D)0.8 A Q32. Thefundamentaldifferencebetweentransformerandinductorusedinpowerconversion circuits is(A)both have same purpose (B)Inductorisusedtosmoothenenergyflowwhereastransformerwillprovide electrical isolation and voltage levels matching. (C)Inductor will store energy whereas transformer will not store energy (D)both B & C are correct Q33. A type A chopper is operating at 2 kHz from a 100 V dc source has a load time constant of 5msand load resistanceof10 .Find the maxvalueofinductor current fora meanload voltage of 50V. (A) 5.25A(B) 5.025A (C) 5.125A(D) 4.875A 12Vsw120H +V0Version Code: PEQBTNC06Page 41 41 41 41 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 300V1005mH 1F230Vsw L=0.2mHC=10FDVdc D T1 Q34. AdiodeisconnectedinserieswithLCcircuitasshowninfig.Assumethecapacitoris initially charged to a voltage of -50 V. The voltage across capacitor at the time diode turns off is (A) 230 V(B) -280 V(C) -220 V(D) 510 V Q35. FindtheconductiontimeofSCRinthefollowingcircuit.AssumeLandCareinitially relaxed (A) 0.314 ms(B) 0.314 s(C) 3.14 ms(D) none Q36. A complementary commutation scheme is shown in fig. If Vdc = 200V, R1 = 10 and R2 = 100 determine peak value of current through T2. (A) 24A(B) 42A(C) 21A(D) 12A Q37. Inthecircuitshowninfigure,ifVdc=200V, C=4FandL=16HandR=20 .The peak value of current through T1 and D can respectively be VdcT1 R1T2R2Version Code: PEQBTNC06Page 42 42 42 42 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 100VAB (A) 110 A, 100 A (B) 110 A, 10 A(C) 10 A, 110 A(D) 100 A, 110 A Q38. In3-fullbridgeVSI,whichofthefollowingvoltagewillhave3rdharmonicin180 operation (A) line voltage(B) phase voltage(C) pole voltage(D) none Q39. A single phase full bridge VSI is feeding power to a resistive load of 5 . The fundamental output voltage is found to be 200 V (rms). Find the rms value of switch and diode currents at fundamental frequency (A) Isw,1 = 20 A 2 and ID,1 =A 2 10(B) Isw,1 = 20 A 2 and ID,1 = 0A (C) Isw,1 = 10 A 2 and ID,1 = 0A (D) Isw,1 = 10 A 2 and ID,1 =A 2 5 Q40. Thesourcevoltageofa3fullbridgeVSIis200V.Thermsvalueofphasevoltagein 120 operation will be (A) 40.82V(B) 20.41V(C) 81.64V(D) 141.42V Q41. AsinglephasefullbridgeVSIisoperatingin180squareoperation.Thephaseangle between the pole voltages is 45. The RMS value of the output voltage between two poles is (A) 100 V(B)V4100(C) 200 V(D) 50 V Q42. AsinglephasefullbridgeVSIhasasourcevoltageof200V.TheloadconsistsofRLCin series where R = 1, L = 7 and =6C1. Identify the fundamental component of load current from the following. (A) 180 sin (t + 45) (B) 180 sin(t - 45) (C) 127.3 sin(t-45)(D) 127.3 sin(t+45) Version Code: PEQBTNC06Page 43 43 43 43 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Q43. The operating points of three power electronic switches on VI plane is shown below Consider the following statements regarding the switches P, Q, R 1. P is most suitable for VSI 2. P is most suitable for CSI 3. Q is the most suitable for VSI 4. Q is most suitable for CSI 5. P, Q and R can be used in either VSI or CSI Now, select the correct option from the following (A) only 2 & 3 are correct (B) only 1 & 4 are correct (C) 2, 3 and 5 are correct (D) All are correct Q44. InsinglepulsemodulationusedinPWMinverters,Vdcistheinputdcvoltage.For eliminatingthirdharmonic,themagnitudesofrmsvalueoffundamentalcomponentof output voltage and pulse width are respectively (A) o dc60 ,V 4 (B) odcV 120 ,6 (C) odc60 , V2 2(D) odc120 , V4 Q45. A PWM inverter is capable of producing the following type of output voltage (A)variable in magnitude and frequency Version Code: PEQBTNC06Page 44 44 44 44 of 120 120 120 120 http://electrical-mentor.blogspot.in/ Copyright Reserved 2014 Load(1) LoadSCR(2) LoadMOSFET/BJT/ IGBT (4) Load TRIAC (3) (B)variable voltage, fixed frequency (C)Fixed voltage, variable frequency (D)Fixed voltage, fixed frequency Q46. Consider the following circuits From the above circuits which one can be considered as AC voltage regulator (A) only 1 & 3 (B) only 1, 2 and 3(C) only 1(D) All circuits Q47. An induction motor is required to run at a very low speed around 25 to 40 rpm from 50 Hz source. Which of the following circuit is most suitable for this application (A) step up cycloconverter(B) inverter (C) step down cycloconverter(D) All the above Q48. A load consisting of R = 10 and L = 10 is being fed from 230 V, 50 Hz source through a 1- AC voltage controller. For a firing angle delay of 45, the rms value of load current will be (A) 23A(B)A223(C)A223> (D)A223