NODIA AND COMPANY. GATE SOLVED PAPER Electrical Engineering POWER ELECTRONICS. Copyright By NODIA & COMPANY

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1 No art of this ublication may be reroduced or distributed in any form or any means, electronic, mechanical, hotocoying, or otherwise without the rior ermission of the author. GATE SOLVED PAPER Electrical Engineering Coyright By NODIA & COMPANY Information contained in this book has been obtained by authors, from sources believes to be reliable. However, neither Nodia nor its authors guarantee the accuracy or comleteness of any information herein, and Nodia nor its authors shall be resonsible for any error, omissions, or damages arising out of use of this information. This book is ublished with the understanding that Nodia and its authors are sulying information but are not attemting to render engineering or other rofessional services. NODIA AND COMPANY B-8, Dhanshree Tower Ist, Central Sine, Vidyadhar Nagar, Jaiur Ph : enquiry@nodia.co.in

2 YEAR 03 TWO MARKS Q. Thyristor T in the figure below is initially off and is triggered with a single ulse of width 0 m s. It is given that L 00 b m H l and C 00 b m F l. Assuming latching and holding currents of the thyristor are both zero and the initial charge on C is zero, T conducts for (A) 0 m s (C) 00 m s (B) 50 ms (D) 00 ms Q. The searately excited dc motor in the figure below has a rated armature current of 0 A and a rated armature voltage of 50 V. An ideal choer switching at 5kHz is used to control the armature voltage. If L a 0. mh, R a W, neglecting armature reaction, the duty ratio of the choer to obtain 50% of the rated torque at the rated seed and the rated field current is (A) 0.4 (B) 0.5 (C) 0.6 (D) 0.7 Common Data For Q. 3 and 4 In the figure shown below, the choer feeds a resistive load from a battery source. MOSFET Q is switched at 50 khz, with duty ratio of 0.4. All elements of the circuit are assumed to be ideal

3 Q. 3 The Peak to Peak source current rile in ams is (A) 0.96 (B) 0.44 (C) 0.9 (D) 0.8 Q. 4 The average source current in Ams in steady-state is (A) 3/ (B) 5/3 (C) 5/ (D) 5/4 Statement for Linked Answer Questions: 5 and 6 The Voltage Source Inverter (VSI) shown in the figure below is switched to rovide a 50 Hz, square wave ac outut voltage V o across an RL load. Reference olarity of V o and reference direction of the outut current i o are indicated in the figure. It is given that R 3 ohms, L 9.55 mh. Q. 5 In the interval when V0 < 0 and i0 > 0 the air of devices which conducts the load current is (A) Q, Q (B) Q 3, Q 4 (C) D, D (D) D, D Q. 6 Aroriate transition i.e., Zero Voltage Switching ^ZVSh/Zero Current Switching ^ZCSh of the IGBTs during turn-on/turn-off is (A) ZVS during turn off (B) ZVS during turn-on (C) ZCS during turn off (D) ZCS during turn-on 3 4 YEAR 0 ONE MARK Q. 7 A half-controlled single-hase bridge rectifier is sulying an R-L load. It is oerated at a firing angle a and the load current is continuous. The fraction of cycle that the freewheeling diode conducts is (A) / (B) ( - a/ ) (C) a/ (D) a/ Q. 8 The tyical ratio of latching current to holding current in a 0 A thyristor is (A) 5.0 (B).0 (C).0 (D) 0.5

4 YEAR 0 TWO MARKS Q. 9 In the circuit shown, an ideal switch S is oerated at 00 khz with a duty ratio of 50%. Given that D i c is.6 A eak-to-eak and I 0 is 5A dc, the eak current in S, is (A) 6.6 A (C) 5.8 A (B) 5.0 A (D) 4. A Common Data for Questions 0 and In the 3-hase inverter circuit shown, the load is balanced and the gating scheme is 80c conduction mode. All the switching devices are ideal. Q. 0 The rms value of load hase voltage is (A) 06. V (C). V (B) 4.4 V (D) 8.8 V Q. If the dc bus voltage V d 300 V, the ower consumed by 3-hase load is (A).5 kw (B).0 kw (C).5 kw (D) 3.0 kw YEAR 0 ONE MARK Q. A three hase current source inverter used for the seed control of an induction motor is to be realized using MOSFET switches as shown below. Switches S to S 6 are identical switches.

5 The roer configuration for realizing switches S to S 6 is Q. 3 Circuit turn-off time of an SCR is defined as the time (A) taken by the SCR turn to be off (B) required for the SCR current to become zero (C) for which the SCR is reverse biased by the commutation circuit (D) for which the SCR is reverse biased to reduce its current below the holding current YEAR 0 TWO MARKS Q. 4 A voltage commutated choer circuit, oerated at 500 Hz, is shown below. If the maximum value of load current is 0 A, then the maximum current through the main ( M ) and auxiliary ( A ) thyristors will be (A) (B) (C) (D) i max M A and i A max 0 A i max M A and i A max A i max M 0 A and i A max A i max M 0 A and i A max 8A

6 Statement for Linked Answer Questions: 5 & 6 A solar energy installation utilize a three hase bridge converter to feed energy into ower system through a transformer of 400 V/400 V, as shown below. The energy is collected in a bank of 400 V battery and is connected to converter through a large filter choke of resistance 0 W. Q. 5 The maximum current through the battery will be (A) 4 A (B) 40 A (C) 80 A (D) 94 A Q. 6 The kva rating of the inut transformer is (A) 53. kva (B) 46.0 kva (C).6 kva (D) 7.5 kva YEAR 00 ONE MARK Q. 7 The ower electronic converter shown in the figure has a single-ole double-throw switch. The ole P of the switch is connected alternately to throws A and B. The converter shown is a (A) ste down choer (buck converter) (B) half-wave rectifier (C) ste-u choer (boost converter) (D) full-wave rectifier Q. 8 Figure shows a comosite switch consisting of a ower transistor (BJT) in series with a diode. Assuming that the transistor switch and the diode are ideal, the I -V characteristic of the comosite switch is

7 Q. 9 The fully controlled thyristor converter in the figure is fed from a single-hase source. When the firing angle is 0c, the dc outut voltage of the converter is 300 V. What will be the outut voltage for a firing angle of 60c, assuming continuous conduction (A) 50 V (C) 300 V YEAR 009 (B) 0 V (D) 00 V ONE MARK Q. 0 An SCR is considered to be a semi-controlled device because (A) It can be turned OFF but not ON with a gate ulse. (B) It conducts only during one half-cycle of an alternating current wave. (C) It can be turned ON but not OFF with a gate ulse. (D) It can be turned ON only during one half-cycle of an alternating voltage wave. YEAR 009 TWO MARKS Q. The circuit shows an ideal diode connected to a ure inductor and is connected to a urely sinusoidal 50 Hz voltage source. Under ideal conditions the current waveform through the inductor will look like.

8 Q. The Current Source Inverter shown in figure is oerated by alternately turning on thyristor airs (T,T ) and (T 3,T 4 ). If the load is urely resistive, the theoretical maximum outut frequency obtainable will be (A) 5 khz (C) 500 khz (B) 50 khz (D) 50 khz Q. 3 In the choer circuit shown, the main thyristor (T M ) is oerated at a duty ratio of 0.8 which is much larger the commutation interval. If the maximum allowable realied dv/ dt on T M is 50 V/ m s, what should be the theoretical minimum value of C? Assume current rile through L 0 to be negligible. (A) 0. m F (C) m F (B) 0.0 mf (D) 0 mf Q. 4 Match the switch arrangements on the to row to the steady-state V -I characteristics on the lower row. The steady state oerating oints are shown by large black dots. (A) P-I, Q-II, R-III, S-IV (C) P-IV, Q-III, R-I, S-II (B) P-II, Q-IV, R-I, S-III (D) P-IV, Q-III, R-II, S-I

9 YEAR 008 ONE MARK Q. 5 In the single hase voltage controller circuit shown in the figure, for what range of triggering angle ( a ), the inut voltage ( V 0 ) is not controllable? (A) 0c < a < 45c (B) 45c < a < 35c (C) 90c < a < 80c (D) 35c < a < 80c Q. 6 A 3-hase voltage source inverter is oerated in 80c conduction mode. Which one of the following statements is true? (A) Both ole-voltage and line-voltage will have 3 rd harmonic comonents (B) Pole-voltage will have 3 rd harmonic comonent but line-voltage will be free from 3 rd harmonic (C) Line-voltage will have 3 rd harmonic comonent but ole-voltage will be free from 3 rd harmonic (D) Both ole-voltage and line-voltage will be free from 3 rd harmonic comonents YEAR 008 TWO MARKS Q. 7 The truth table of monoshot shown in the figure is given in the table below : Two monoshots, one ositive edge triggered and other negative edge triggered, are connected shown in the figure, The ulse widths of the two monoshot oututs Q and Q are T ON and T ON resectively. The frequency and the duty cycle of the signal at Q will resectively be (A) f, D (B) f TON, D T + T 5 T + T T + T ON ON ON ON ON ON (C) f TON, D T T + T ON ON ON (D) f TON, D T T + T ON ON ON

10 Q. 8 A single hase fully controlled bridge converter sulies a load drawing constant and rile free load current, if the triggering angle is 30c, the inut ower factor will be (A) 0.65 (B) 0.78 (C) 0.85 (D) Q. 9 A single-hase half controlled converter shown in the figure feeding ower to highly inductive load. The converter is oerating at a firing angle of 60c. If the firing ulses are suddenly removed, the steady state voltage ( V 0 ) waveform of the converter will become Q. 30 A single hase source inverter is feeding a urely inductive load as shown in the figure. The inverter is oerated at 50 Hz in 80c square wave mode. Assume that the load current does not have any dc comonent. The eak value of the inductor current i 0 will be (A) 6.37 A (B) 0 A (C) 0 A (D) 40 A

11 Q. 3 A three hase fully controlled bridge converter is feeding a load drawing a constant and rile free load current of 0 A at a firing angle of 30c. The aroximate Total harmonic Distortion (%THD) and the rms value of fundamental comonent of inut current will resectively be (A) 3% and 6.8 A (B) 3% and 7.8 A (C) 66% and 6.8 A (D) 66% and 7.8 A Q. 3 A single hase fully controlled converter bridge is used for electrical braking of a searately excited dc motor. The dc motor load is reresented by an equivalent circuit as shown in the figure. Assume that the load inductance is sufficient to ensure continuous and rile free load current. The firing angle of the bridge for a load current of I 0 0 A will be (A) 44c (B) 5c (C) 9c (D) 36c Q. 33 In the circuit shown in the figure, the switch is oerated at a duty cycle of 0.5. A large caacitor is connected across the load. The inductor current is assumed to be continuous. The average voltage across the load and the average current through the diode will resectively be (A) 0 V, A (B) 0 V, 8 A (C) 40 V A (D) 40 V, 8 A YEAR 007 ONE MARK Q. 34 A single-hase fully controlled thyristor bridge ac-dc converter is oerating at a firing angle of 5c and an overla angle of 0c with constant dc outut current of 0 A. The fundamental ower factor (dislacement factor) at inut ac mains is (A) 0.78 (B) 0.87 (C) (D) 0.9 Q. 35 A three-hase, fully controlled thyristor bridge converter is used as line commutated inverter to feed 50 kw ower 40 V dc to a three-hase, 45 V(line), 50 Hz ac mains. Consider dc link current to be constant. The rms current of the thyristor is (A) 9.05 A (B) A (C) A (D) A

12 Q. 36 A single hase full-wave half-controlled bridge converter feeds an inductive load. The two SCRs in the converter are connected to a common DC bus. The converter has to have a freewheeling diode. (A) because the converter inherently does not rovide for free-wheeling (B) because the converter does not rovide for free-wheeling for high values of triggering angles (C) or else the free-wheeling action of the converter will cause shorting of the AC suly (D) or else if a gate ulse to one of the SCRs is missed, it will subsequently cause a high load current in the other SCR. Q. 37 Six MOSFETs connected in a bridge configuration (having no other ower device) must be oerated as a Voltage Source Inverter (VSI). This statement is (A) True, because being majority carrier devices MOSFETs are voltage driven. (B) True, because MOSFETs hav inherently anti-arallel diodes (C) False, because it can be oerated both as Current Source Inverter (CSI) or a VSI (D) False, because MOSFETs can be oerated as excellent constant current sources in the saturation region. YEAR 007 TWO MARKS Q. 38 A single-hase voltages source inverter is controlled in a single ulse-width modulated mode with a ulse width of 50c in each half cycle. Total harmonic distortion is defined as THD Vrms V - 00 V # where V is the rms value of the fundamental comonent of the outut voltage. The THD of outut ac voltage waveform is (A) 65.65% (B) 48.4% (C) 3.83% (D) 30.49% Q. 39 A three-hase, 440 V, 50 Hz ac mains fed thyristor bridge is feeding a 440 V dc, 5 kw, 500 rm searately excited dc motor with a rile free continuos current in the dc link under all oerating conditions, Neglecting the losses, the ower factor of the ac mains at half the rated seed is (A) (B) 0.37 (C) 0.90 (D) Q. 40 A single-hase, 30 V, 50 Hz ac mains fed ste down transformer (4:) is sulying ower to a half-wave uncontrolled ac-dc converter used for charging a battery ( V dc) with the series current limiting resistor being 9.04 W. The charging current is (A).43 A (B).65 A (C). A (D).0 A

13 Q. 4 In the circuit of adjacent figure the diode connects the ac source to a ure inductance L. The diode conducts for (A) 90c (C) 70c (B) 80c (D) 360c Q. 4 The circuit in the figure is a current commutated dc-dc choer where, Th M is the main SCR and Th AUX is the auxiliary SCR. The load current is constant at 0 A. Th M is ON. Th AUX is trigged at t 0. Th M is turned OFF between. (A) 0ms < t # 5ms (B) 5 ms < t # 50 ms (C) 50 ms < t # 75 ms (D) 75 ms < t # 00 ms Common Data for Question 43 and 44. A : Pulse Transformer (PT) is used to trigger the SCR in the adjacent figure. The SCR is rated at.5 kv, 50 A with IL 50 ma, IH 50 ma, and IG max 50 ma, IG min 0 0 ma. The SCR is connected to an inductive load, where L 50 mh in series with a small resistance and the suly voltage is 00 V dc. The forward dros of all transistors/diodes and gate-cathode junction during ON state are.0 V Q. 43 The resistance R should be (A) 4.7 kw (C) 47 W (B) 470 kw (D) 4.7 W

14 Q. 44 The minimum aroximate volt-second rating of ulse transformer suitable for triggering the SCR should be : (volt-second rating is the maximum of roduct of the voltage and the width of the ulse that may alied) (A) 000 mv-s (B) 00 mv-s (C) 0 mv-s (D) mv-s YEAR 006 ONE MARK Q. 45 The seed of a 3-hase, 440 V, 50 Hz induction motor is to be controlled over a wide range from zero seed to.5 time the rated seed using a 3-hase voltage source inverter. It is desired to kee the flux in the machine constant in the constant torque region by controlling the terminal voltage as the frequency changes. The inverter outut voltage vs frequency characteristic should be Q. 46 A single-hase half wave uncontrolled converter circuit is shown in figure. A -winding transformer is used at the inut for isolation. Assuming the load current to be constant and V V sin wt, the current waveform through diode D will be m

15 YEAR 006 TWO MARKS Q. 47 A single-hase inverter is oerated in PWM mode generating a single-ulse of width d in the centre of each half cycle as shown in figure. It is found that the outut voltage is free from 5 th harmonic for ulse width 44c. What will be ercentage of 3 rd harmonic resent in the outut voltage ( V / ) o3 V o max? (A) 0.0% (B) 9.6% (C) 3.7% (D) 53.9% Q. 48 A 3-hase fully controlled bridge converter with free wheeling diode is fed from 400 V, 50 Hz AC source and is oerating at a firing angle of 60c. The load current is assumed constant at 0 A due to high load inductance. The inut dislacement factor (IDF) and the inut ower factor (IPF) of the converter will be (A) IDF 0.867; IPF 0.88 (B) IDF 0.867; IPF 0.55 (C) IDF 0.5; IPF (D) IDF 0.5; IPF 0.38 Q. 49 A voltage commutation circuit is shown in figure. If the turn-off time of the SCR is 50 msec and a safety margin of is considered, then what will be the aroximate minimum value of caacitor required for roer commutation? (A).88 mf (B).44 mf (C) 0.9 mf (D) 0.7 mf Q. 50 A solar cell of 350 V is feeding ower to an ac suly of 440 V, 50 Hz through a 3-hase fully controlled bridge converter. A large inductance is connected in the dc circuit to maintain the dc current at 0 A. If the solar cell resistance is 0.5 W,then each thyristor will be reverse biased for a eriod of (A) 5c (B) 0c (C) 60c (D) 55c

16 Q. 5 A single-hase bridge converter is used to charge a battery of 00 V having an internal resistance of 0. W as shown in figure. The SCRs are triggered by a constant dc signal. If SCR gets oen circuited, what will be the average charging current? (A) 3.8 A (C).9 A (B) 5 A (D) 3.54 A Q. 5 An SCR having a turn ON times of 5 msec, latching current of 50 A and holding current of 40 ma is triggered by a short duration ulse and is used in the circuit shown in figure. The minimum ulse width required to turn the SCR ON will be (A) 5 msec (C) 00 msec (B) 50 msec (D) 5 msec Common Data For Q. 53 and 54 A voltage commutated choer oerating at khz is used to control the seed of dc as shown in figure. The load current is assumed to be constant at 0 A Q. 53 The minimum time in msec for which the SCR M should be ON is. (A) 80 (B) 40 (C) 70 (D) 0 Q. 54 The average outut voltage of the choer will be (A) 70 V (B) 47.5 V (C) 35 V (D) 0 V

17 YEAR 005 ONE MARK Q. 55 The conduction loss versus device current characteristic of a ower MOSFET is best aroximated by (A) a arabola (B) a straight line (C) a rectangular hyerbola (D) an exonentially decaying function Q. 56 A three-hase diode bridge rectifier is fed from a 400 V RMS, 50 Hz, three-hase AC source. If the load is urely resistive, then eak instantaneous outut voltage is equal to (A) 400 V (B) 400 V (C) V (D) 400 V 3 Q. 57 The outut voltage waveform of a three-hase square-wave inverter contains (A) only even harmonics (B) both odd and even harmonic (C) only odd harmonics (D) only trile harmonics YEAR 005 TWO MARKS Q. 58 The figure shows the voltage across a ower semiconductor device and the current through the device during a switching transitions. If the transition a turn ON transition or a turn OFF transition? What is the energy lost during the transition? VI (A) Turn ON, ( t + t ) (B) Turn OFF, VI( t + t ) VI (C) Turn ON, VI( t + t ) (D) Turn OFF, ( t + t ) Q. 59 An electronics switch S is required to block voltage of either olarity during its OFF state as shown in the figure (a). This switch is required to conduct in only one direction its ON state as shown in the figure (b) Which of the following are valid realizations of the switch S?

18 (A) Only P (C) P and R (B) P and Q (D) R and S Q. 60 The given figure shows a ste-down choer switched at khz with a duty ratio D 05.. The eak-eak rile in the load current is close to (A) 0 A (C) 0.5 A (B) 0.5 A (D) 0.5 A Q. 6 Consider a hase-controlled converter shown in the figure. The thyristor is fired at an angle a in every ositive half cycle of the inut voltage. If the eak value of the instantaneous outut voltage equals 30 V, the firing angle a is close to (A) 45c (B) 35c (C) 90c (D) 83.6c Q. 6 An electric motor, develoing a starting torque of 5 Nm, starts with a load torque of 7 Nm on its shaft. If the acceleration at start is rad/sec, the moment of inertia of the system must be (neglecting viscous and coulomb friction) (A) 0.5 kg-m (B) 0.5 Nm (C) 4 kg-m (D) 4 Nm YEAR 004 ONE MARK Q. 63 A biolar junction transistor (BJT) is used as a ower control switch by biasing it in the cut-off region (OFF state) or in the saturation region (ON state). In the ON state, for the BJT (A) both the base-emitter and base-collector junctions are reverse biased (B) the base-emitter junction is reverse biased, and the base-collector junction is forward biased (C) the base-emitter junction is forward biased, and the base-collector junction is reverse biased (D) both the base-emitter and base-collector junctions are forward biased

19 Q. 64 The circuit in figure shows a full-wave rectifier. The inut voltage is 30 V (rms) single-hase ac. The eak reverse voltage across the diodes D and D is (A) 00 V (B) 00 V (C) 50 V (D) 50 V Q. 65 The triggering circuit of a thyristor is shown in figure. The thyristor requires a gate current of 0 ma, for guaranteed turn-on. The value of R required for the thyristor to turn on reliably under all conditions of V b variation is (A) 0000 W (B) 600 W (C) 00 W (D) 800 W Q. 66 The circuit in figure shows a 3-hase half-wave rectifier. The source is a symmetrical, 3-hase four-wire system. The line-to-line voltage of the source is 00 V. The suly frequency is 400 Hz. The rile frequency at the outut is (A) 400 Hz (B) 800 Hz (C) 00 Hz (D) 400 Hz

20 YEAR 004 TWO MARKS Q. 67 A MOSFET rated for 5 A, carries a eriodic current as shown in figure. The ON state resistance of the MOSFET is 0.5 W. The average ON state loss in the MOSFET is (A) 33.8 W (C) 7.5 W (B) 5.0 W (D) 3.8 W Q. 68 The triac circuit shown in figure controls the ac outut ower to the resistive load. The eak ower dissiation in the load is (A) 3968 W (C) 7935 W (B) 590 W (D) 0580 W Q. 69 Figure shows a choer oerating from a 00 V dc inut. The duty ratio of the main switch S is 0.8. The load is sufficiently inductive so that the load current is rile free. The average current through the diode D under steady state is (A).6 A (B) 8.0 A (B) 6.4 A (D) 0.0 A Q. 70 Figure shows a choer. The device S is the main switching device. S is the auxiliary commutation device. S is rated for 400 V, 60 A. S is rated for 400 V, 30 A. The load current is 0 A. The main device oerates with a duty ratio of 0.5. The eak current through S is (A) 0 A (C) 30 A (B) 0 A (D) 40 A

21 Q. 7 A single-hase half-controlled rectifier is driving a searately excited dc motor. The dc motor has a back emf constant of 0.5 V/rm. The armature current is 5 A without any rile. The armature resistance is W. The converter is working from a 30 V, single-hase ac source with a firing angle of 30c. Under this oerating condition, the seed of the motor will be (A) 339 rm (B) 359 rm (C) 366 rm (D) 386 rm Q. 7 A variable seed drive rated for 500 rm, 40 Nm is reversing under no load. Figure shows the reversing torque and the seed during the transient. The moment of inertia of the drive is (A) kg-m (B) km-m (C) kg-m (D) 0.8 kg-m YEAR 003 ONE MARK Q. 73 Figure shows a thyristor with the standard terminations of anode (A), cathode (K), gate (G) and the different junctions named J, J and J3. When the thyristor is turned on and conducting (A) J and J are forward biased and J3 is reverse biased (B) J and J3 are forward biased and J is reverse biased (C) J is forward biased and J and J3 are reverse biased (D) J, J and J3 are all forward biased Q. 74 Figure shows a MOSFET with an integral body diode. It is emloyed as a ower switching device in the ON and OFF states through aroriate control. The ON and OFF states of the switch are given on the V - I lane by DS S

22 Q. 75 The seed/torque regimes in a dc motor and the control methods suitable for the same are given resectively in List-II and List-I List-I List-II P. Field Control. Below base seed Q. Armature Control. Above base seed 3. Above base torque 4. Below base torque Codes: (A) P-, Q-3 (B) P-, Q- (C) P-, Q-3 (D) P-, Q-4 Q. 76 A fully controlled natural commutated 3-hase bridge rectifier is oerating with a firing angle a 30c, The eak to eak voltage rile exressed as a ratio of the eak outut dc voltage at the outut of the converter bridge is (A) 0.5 (B) 3/ (C) 3 c - m (D) 3 - YEAR 003 TWO MARKS Q. 77 A hase-controlled half-controlled single-hase converter is shown in figure. The control angle a 30c The outut dc voltage wave shae will be as shown in

23 Q. 78 A choer is emloyed to charge a battery as shown in figure. The charging current is 5 A. The duty ratio is 0.. The choer outut voltage is also shown in the figure. The eak to eak rile current in the charging current is (A) 0.48 A (C).4 A (B). A (D) A Q. 79 An inverter has a eriodic outut voltage with the outut wave form as shown in figure When the conduction angle outut voltage is (A) 0.78 V (C) 0.90 V a 0c, the rms fundamental comonent of the (B).0 V (D).7 V Q. 80 With reference to the outut wave form given in above figure, the outut of the converter will be free from 5 th harmonic when (A) a 7c (B) a 36c (C) a 50c (D) a 0c

24 Q. 8 An ac induction motor is used for a seed control alication. It is driven from an inverter with a constant Vf / control. The motor name-late details are as follows (no. of oles ) V:45 V VPh:3 V f:50 Hz N:850 rm The motor runs with the inverter outut frequency set at 40 Hz, and with half the rated sli. The running seed of the motor is (A) 400 rm (B) 80 rm (C) 340 rm (D) 790 rm ***********

25 SOLUTION Sol. Sol. Given, L 00 m H C 00 m F When the circuit is triggered by 0 m s ulse, the thyristor is short circuited and so, we consider I C Im sin wt Therefore, voltage stored across caacitor is V C C # I C dt Vm ^ -cos wth where w is angular frequency obtained as w 4 0 LC 00 # -6 b # 0 l So, T 00 ms f w As I C Im sin wt oscillates between - ve and - ve half cycle so, circuit is conducting for only half of cycle and thyristor is oen after half cycle. i.e., the conduction eriod T 00 ms Otion (D) is correct. Given, the rated armature current I arated ^ h 0 A as rated armature voltage V arated ^ h 50 volt Also, for the armature, we have L a 0. mh, R a W and T 50% of T rated ^T " Torqueh So, we get I 6 Ia^rotatedh@ ^05. h 0A N Nrated, I f I frated " rated field current At the rated conditions, E V-Ia ^ rated h Ra 50-0^h 30 volt For given torque, V E+ IaRa 30 + ^0h^h 40 V Therefore, choer outut 40 V or, D^00h 40 or, D (D " duty cycle)

26 Sol. 3 Sol. 4 Sol. 5 Here, as the current from source of V is the same as that ass through inductor. So, the eak to eak current rile will be equal to eak to eak inductor current. Now, the eak to eak inductor current can be obtained as I L (Peak to Peak) Vs DT s L where, V s " source voltage volt, L " inductance 00mH 0-4 H, D " Duty ration 04., T S " switching time eriod of MOSFET and f s " switching frequency 50 khz Therefore, we get I L^Peak to Peakh # # A 50 # 0 This is the eak to eak source current rile. Otion (B) is correct. Here, the average current through the caacitor will be zero. (since, it is a boost converter). We consider the two cases : Case I : When MOSFET is ON i c - i 0 (i 0 is outut current) (since, diode will be in cut off mode) Case II : When MOSFET is OFF Diode will be forward biased and so i c Is - i0 (I s is source current) Therefore, average current through caacitor ic I c I cavg, + DTs^- ioh+ ^ -DhTs^I s-ioh & 0 (D is duty ratio) Solving the equation, we get I i0 s...() ^ - D h Since, the outut load current can be given as i V0 Vs/ -D / A R R 0 Hence, from Eq. () I i0 s A D Otion (D) is correct. We consider the following two cases : Case I : When Q, Q ON In this case the + ve terminal of V 0 will be at higher voltage. i.e. V0 > 0 and so i0 > 0 (i.e., it will be + ve). Now, when the Q, Q goes to OFF condition we consider the second case. Case II : When Q 3, Q 4 ON and Q, Q OFF : In this condition, - ve terminal of alied voltage V 0 will be at higher otential i.e., V0 < 0 and since, inductor ooses the change in current so, although the olarity of voltage V 0 is inversed, current remains same in inductor i.e. I > 0 fs 0.

27 Sol. 6 Sol. 7 This is the condition when conduction have been asked. In this condition ^V0> 0, I0> 0h since, IGBT s can t conduct reverse currents therefore current will flow through D 3, D 4 until I D becomes negative. Thus, D 3 and D 4 conducts. Otion (D) is correct. When Q 3, Q 4 is switched ON, initially due to the reverse current it remain in OFF state and current asses through diode. In this condition the voltage across Q 3 and Q 4 are zero as diodes conduct. Hence, it shows zero voltage switching during turn-on Otion (D) is correct. The circuit of a single-hase half controlled bridge rectifier with RL load and free wheel diode is shown as below. The voltage current wave forms are shown in figure below. We note that, for continuous load current, the flywheel diode conducts from to + a in a cycle. Thus, fraction of cycle that freewheel diode conducts is a/.

28 Sol. 8 Sol. 9 Sol. 0 Thus fraction of cycle that freewheel diode conducts is a/. Otion (B) is correct. The latching current is higher than the holding current. Usually, latching current is taken two to three times the holding currents. I S I + Ti A 0 Otion (B) is correct. For a three-hase bridge inverter, rms value of outut line voltage is c Sol. Sol. Sol. 3 Sol. 4 Sol. 5 V L V 3 dc # V dc 300 V 4.4 V Otion (D) is correct. P VL 3 ( 4. 4) # 3 R # - 3kW 0 Only otion C allow bi direction ower flow from source to the drive Once the SCR start conducting by an forward current, the gate has no control on it and the device can be brought back to the blocking state only by reducing the forward current to a level below that of holding current. This rocess of turn-off is called commutation. This time is known as the circuit turn-off time of an SCR. Otion (A) is correct. Maximum current through main thyristor I M ( max) I V C 0 + s L Maximum current through auxiliary thyristor I A ( max) I 0 A Otion (A) is correct. Outut voltage of 3-hase bridge converter Maximum outut V 0 ( V ) max V h 0 Vh cos a 3 3 cos a # # V 3 Resistance of filter choke is 0 W, So 0. # 0 3 # 0-6 A

29 Sol. 6 Sol. 7 Sol. 8 Sol. 9 ( V 0 ) max E+ IR chock I( 0) I - 4 A Otion (D) is correct. kva rating 3 VI L L # # #4 7.5 kva Otion (A) is correct. The figure shows a ste down choer circuit. a V out DV in where, D Duty cycle and D < Given figure as The I -V characteristic are as Since diode connected in series so I can never be negative. When current flows voltage across switch is zero and when current is zero than there may be any voltage across switch. Otion (A) is correct. Given fully-controlled thyristor converter, when firing angle a 0, dc outut voltage V dc0 300 V If a 60c, then V dc? For fully-controlled converter V V dc0 dc cos a a a 0, V dc0 300 V At a V 300 dc cos 0 c V dc c, V dc? V dc 300 # cos 60c # V Sol. 0 SCR has the roerty that it can be turned ON but not OFF with a gate ulse, So SCR is being considered to be a semi-controlled device.

30 Sol. Otion (D) is correct. Current wave form for i L v L i L Ldi L dt vdt L # Sol. for 0 < w t +, v L v 0 sin t di L in w dt i L vdt L - cos 00t+ C # at 00t /, i L 0, C 0 i L i L( eak) -00 cos t Am for < w t v v 0 In CSI let T 3 and T 4 already conducting at t 0 At triggering T and T, T 3 and T 4 are force cumulated. Again at t T, T and T are force cumulated. This comletes a cycle. L in Sol. 3 Time constant t RC 4# 0. 5 m sec Frequency f t - khz # 0 Otion (A) is correct. duty ratio T M 08. Maximum dv on TM dt Minimum value of C? 50 V/ msec

31 Given that current rile through L 0 is negligible. Current through T M Im duty ratio# current 0.8 #.5 0 A a I m C dv dt 0 C 50 # -6 0 C # m F Sol. 4 Characteristics are as Sol. 5 Otion (A) is correct. R+ jxl j ` tan f w R L f 45c so, firing angle a must be higher the 45c, Thus for 0 < a < 45c, V 0 is uncontrollable.

32 Sol. 6 Sol. 7 Sol. 8 Sol. 9 Sol. 30 Otion (D) is correct. A 3-f voltage source inverter is oerated in 80c mode in that case third harmonics are absent in ole voltage and line voltage due to the factor cos( n / 6). so both are free from 3 rd harmonic comonents. Otion (B) is correct. In this case f TON+ TON TON and, D T + T ON ON Otion (B) is correct. Given a 30c, in a -f fully bridge converter we know that, Power factor Distortion factor # cos a Otion (A) is correct. Outut of this D.f. (Distortion factor) I / I 0.9 s(fundamental) ower factor 09. # cos30c 078. Here the inductor makes T and T 3 in ON because current assing through T and T 3 is more than the holding current. Inut is given as s Here load current does not have any dc comonent ` Peak current occur at ( / w) ` V s L di dt Here So di ( max ) 00. di 0# dt di akb 50 l # 00 # 0 0 A.

33 Sol. 3 Sol. 3 Sol. 33 Otion (B) is correct. Total rms current I a A 3 # where ` Fundamental current I a 0.78 # A THD - DF DF Ia # Ia 0 86 # 0 THD b - 3% l Here for continuous conduction mode, by Kirchoff s voltage law, average load current ` 0 A, So I V- Ia V + 50 V m I a V - 30 V cos a -30 # # 30 cos a -30c a 9c In the given diagram when switch S is oen I0 IL 4 A, Vs 0 V when switch S is closed ID 0, V0 0 V Duty cycle 05. so average voltage is V - s d Average current am Average voltage 0 40 V

34 Sol. 34 Otion (A) is correct. Firing angle a 5c Overla angle m 0c so, I 0 [ cos a- cos( a+ m)] wls V m ` 0 30 [ cos 5c- cos( 5c+ 0c)] # 50Ls ` Ls H Sol. 35 Sol. 36 Sol. 37 Sol. 38 Sol. 39 V V m cos LsI 0 0 a - w -3 # 30 cos 5c - # 3. 4 # 50 # 4. 5 # 0 # c Dislacement factor VI VI s s 30 # # Given that P 50 # 000 W V d 40 So P Vd# Id I d 50 # RMS value of thyristor current Otion (B) is correct. Single hase full wave half controlled bridge converter feeds an Inductive load. The two SCRs in the converter are connected to a common dc bus. The converter has to have free wheeling diode because the converter does not rovide for free wheeling for high values of triggering angles. Otion (D) is correct. If we connect the MOSFET with the VSI, but the six MOSFETs are connected in bridge configuration, in that case they also oerated as constant current sources in the saturation region so this statement is false. Given that, total harmonic distortion THD Vrms V - 00 V # Pulse width is 50c Here V rms V 50 b Vs 0.9Vs 80 l THD Otion (A) is correct. When losses are neglected, 04. Vs sin 75c V s # (. 09Vs) - (. 087Vs) 3. 9% (. 087V) V rms(fundamental) s

35 Sol. 40 Sol. 4 3 # # 440 cos a K 750 m # 60 # Here back emf e with f is constant e V 0 K m w m 440 K 500 m # # 60 at this firing angle K m 8. cos a 037. V t 3 # 440 # (0.37) 9.85 V I a I sr I / f. VI t s VI s sr Otion (D) is correct. V s Here charging current I a V m sin q q radian V m V e V There is no ower consumtion in battery due to ac current, so average value of charging current. I av(charging) [ Vm cos q-e( -q)] # [ Vm cos ( )] # # # q - - q.059 W/A Conduction angle for diode is 70c as shown in fig. Sol. 4 Sol. 43 Otion ( ) is correct. Here, V m maximum ulse voltage that can be alied so 0 7 V Here V dro is in rimary transistor side, so that we get 9V ulse on the

36 Sol. 44 secondary side. Again there are V dro in diode and in gate cathode junction each. I gmax 50 ma So R Vm W I 50 ma g max Otion (A) is correct. We know that the ulse width required is equal to the time taken by i a to rise uto i L so, V s L di + R ( V 0) dt i T. Sol. 45 Sol. 46 i 00 a [ -t -e ] /. 05 Here also t T, ia il 0.5 -T/ [ -e ] - T.876 # 0 4 Width of ulse 87.6 ms Magnitude of voltage 0 V V sec rating of P.T. 0 # 87.6 ms 87.6 ms 867 mv-s is arox to 000 mv-s Otion (D) is correct. If we varying the frequency for seed control, Vf / should be ket as constant so that, minimum flux density (B m ) also remains constant So, V 4.44NBmAf Otion (D) is correct. In first half cycle D will conduct and D will not and at q 0 there is zero voltage. So current wave form is as following Sol. 47 Otion (B) is correct. In the PWM inverter V 0 outut voltage of inverter V 4 0 sin sin sin / n V s / 3 nd nwt n n So the ulse width d 44c V 4Vs 0 sin 7c sin wt V 03 4V s sin 3 7 sin 3 t 3 # c w ^ h

37 Sol. 48 Sol. 49 Sol. 50 Sol. 5 so, V bv 03 0max 4V sin( ) 3 s # 3# 7c l 9. 6% 4Vs sin 7c Given that 400 V, 50 Hz AC source, a 60c, IL 0 so, Inut dislacement factor cos a 0. 5 and, inut ower factor D.F. # cos a I distortion factor s(fundamental) I so, inut ower factor # s A 4 # 0 sin 60 # c 0 # / 3 Otion (A) is correct. We know that T RC ln So C R T mf # # Otion (A) is correct. Let we have 0.5 W, I 0 A R solar 0 so V s # V ` # 440 # cos a cos a 55c So each thyristor will reverse biased for 80c- 55c 5c. In this circuitry if SCR gets oen circuited, than circuit behaves like a half wave rectifier. So I avg Average value of current # -q ( V sin t E) d m w - q R q

38 Sol. 5 a I 0(avg) cos ( ) R Vm q E q 6 - [ ( 30 ) cos 00( )] # # # q- - q q sin - V E - b l sin Rad m c c 30 # m ` I 0avg) ( [ 30 cos 38 00( 0.66)] # # c - - # Otion (B) is correct..9 A Sol. 53 Sol. 54 Sol. 55 Sol. 56 In this given circuit minimum gate ulse width time Time required by i a rise u to i L i ma 5# 0 i 00 [ -e ] 0-40t ` anode current I I + I [ -e -40t ] [ -e -40t ] 40t - e T 50 ms Otion (B) is correct. Given I L 0 A. So in the + ve half cycle, it will charge the caacitor, minimum time will be half the time for one cycle. so min time required for charging LC w # # 0 # 0 40 m sec 0 Given T on 40 m sec Average outut T V T on # total T total /f 03 msec -6 so average outut 40 # 0-3 # V # 0 Otion (A) is correct. The conduction loss v/s MOSFET current characteristics of a ower MOSFET is best aroximated by a arabola. Otion (B) is correct. In a 3-f bridge rectifier V rms 400 V, f 50 Hz This is urely resistive then instantaneous voltage V 0 V 400 V rms

39 Sol. 57 Sol. 58 Sol. 59 Sol. 60 Sol. 6 Sol. 6 Sol. 63 Sol. 64 A 3-f square wave (symmetrical) inverter contains only odd harmonics. Otion (A) is correct. In Ideal condition we take voltage across the device is zero. average ower loss during switching VI ( t + t ) (turn ON) So in P thyristor blocks voltage in both olarities until gate is triggered and also in R transistor along with diode can do same rocess. Duty ratio a 05. here T sec # 0 T a 40 R L 00 mh msec 5 -att / s -( -a) TT / a ( )( ) Rile Vs -e -e R -TT / s G - e ( T I) Vs max fL 4 # 0 # 00 # A Otion (B) is correct. We know that V rms 30 V so, V m 30 # V If whether a 90c Then V eak V sin a 30 m 30 sin a 30 sin a angle a 35c T st T L 5 Nm 7Nm a rad/sec T Ia so T Tst - TL 8 Nm I 8 4 kgm Otion (D) is correct. When we use BJT as a ower control switch by biasing it in cut-off region or in the saturation region. In the on state both the base emitter and base-collector junction are forward biased. Otion (A) is correct. Peak Inverse Voltage (PIV) across full wave rectifier is V m V m 50 V so, PIV 00 V

40 Sol. 65 Sol. 66 Otion (D) is correct. V b! 4 V V b max 6 V V b min 8V Vb ( min) Required value of R I -3 W g 0 # 0 Sol. 67 Sol. 68 Sol. 69 Rile frequency 3 f 3 # Hz So from V 0 rile frequency 00 Hz Given that R 05W. I 5 A So average ower losses w / # I Rdt ( / w) 0 w / # # # w 7.5 W Otion (D) is correct. Outut dc voltage across load is given as following V dc V ( ) sin a a ; - + a & 0E sin / 30 # # ' a k b l G 37.8 V losses V dc ( 37. 8) 000 W R 00 V s 00 V, duty ratio 0.8, R 0 W Sol. 70 Sol. 7 Sol. 7 So average current through diode Otion (D) is correct. Peak current through S av s R 0.8 # 00 8 A 0-6 I I0 + VS C/ L # 0-00 # 0 Otion ( ) is correct A

41 Sol. 73 Sol. 74 Sol. 75 Sol. 76 Sol. 77 Sol. 78 so and T 40 Nm a 500 -(-500) ; 05. E # rad/sec T Ia I T 40 # kgm a Otion (D) is correct. When thyristor turned on at that time J junction will break. So J, J, J3 all are in forward bias. Otion (D) is correct. The ON-OFF state of switch is given on V - I lane as following When VDS + ve, diode conducts and IS 0 VDS - ve, diode oens, but IS 0, D "- ve otential. Otion (B) is correct. P. Field control-above base seed Q. Armature control-below base torque Otion (A) is correct. As we know in fully controlled rectifier. V PP Vm- Vmcos( / 6 + a) a a 30c or V PP V m [ - cos( / c)] or VPP 05. V m Otion ( ) is correct. Otion (A) is correct. In the choer during turn on of choer V -t area across L is, # Ton Ton Vdt L L di imax # b dt Ldi 0 0 dt l # Li ( max - imin) L^DIh imin DS S V -t are alied to L is (60 - )T on 48T on

42 Sol. 79 So now volt area Otion (A) is correct. D I -3 48T on 48 # 0. # L -3 0 # 0 A Sol. 80 Sol. 8 Outut voltage V 4 0 sin sin sin / n V S / 3 b nd nwt n n 35,, l^ h^ h^ h ` RMS value of fundamental comonent V 4VS rms(fundamental) sin d # a 0c, d 0c & d 60c V rms(fundamental) 4V S # sin 60c 0.78VS 0.78 V Otion (A) is correct. After removing 5 th harmonic 5 d 0,, ` Pulse width d a 0,, 4 0c, 7c, 44c 5 5 N Sa 3000 rm N a 850 rm S FL where by ( Vf / ) control N sb b 50 l 400 rm ` N new running seed of motor b - l 340 rm ***********