IES - Electrical Engineering - Power Electronics

www.gatehelp.com IES Electrical Engineering Topic wise Questions Power Electronics YEAR 2008 MCQ 1 For a single a.c. to d.c. controlled rectifier to operate in regenerative mode, which of the following conditions should be satisfied ? (A) Half-controlled bridge, α < 90c, source of e.m.f. in load (B) Half-controlled bridge, α > 90c, source of e.m.f. in load (C) Full-controlled bridge, α > 90c, source of e.m.f. in load (D) Full-controlled bridge, α < 90c, source of e.m.f. in load MCQ 2 A half-controlled bridge converter is operating from an r.m.s input voltage of 120 V. Neglecting the voltage drops, what are the mean load voltage at a firing delay angle of 0c and 180c, respectively ? (A) 120 # 2 2 V and 0 π (B) 0 and 120 # 2 2 V π (C) 120 2 V and 0 π (D) 0 and 120 2 V π MCQ 3 For a step-down d.c. chopper operating with discontinuous load IES EE Topic wise 2001-2008 Power Electronics www.gatehelp.com current, what is the expression for the load voltage ? ( K is duty ratio of chopper) (A) V0 = Vd.c. # K (B) V0 = Vd.c. /K (C) V0 = Vd.c. / (1 − K) (D) V0 = Vd.c. (1 − K) MCQ 4 An ideal chopper is operating at a frequency of 500 Hz from a 60 V battery input. It is supplying a load having 3 Ω resistance and 9 mH inductance. Assuming the load is shunted by a perfect commutating diode and assuming battery is low less, what is the mean load current at an on/off ratio of 1/1 ? (A) 10 A (B) 15 A (C) 20 A (D) None of the above MCQ 5 Matching List I (Device) with List II (Monolithic construction of) and select the correct answer using the code given below the lists : List I A. Triac List II 1. Two thyristors in anti-parallel B. Reverse conducting thyristor 2. A thyristor and a diode in anti-parallel C. Diac Codes : A (A) (B) (C) (D) MCQ 6 Page 2 3. Two diodes in anti-parallel B 2 2 3 1 C 3 1 1 2 1 3 2 3 www.gatehelp.com A two-quadrant d.c. to d.c. chopper can operate with which of the following load conditions ? 1. + ve voltage, + ve current 2. 3. 4. − ve voltage, + ve current − ve voltage, − ve current + ve voltage, − ve current IES EE Topic wise 2001-2008 Power Electronics Select the correct answer using the code given below : (A) 1 only (B) 1 and 2 (C) 1 and 4 (D) 3 and 4 MCQ 7 A single-phase current source inverter is connected with capacitive load only the waveform of the output voltage across the capacitor for constant source current will be (A) sine wave (B) square wave (C) triangular wave (D) step function MCQ 8 A modern power semiconductor device characteristics of BJT and MOSFET is (A) GTO (B) FCT (C) IGBT (D) MCT MCQ 9 that combines the What are the advantage of switching power supplies over linear power supplies ? 1. The devices operate in linear/active region. 2. 3. The devices operate as switches. Power losses are less. Page 3 IES EE Topic wise 2001-2008 Power Electronics www.com Select the correct answer using the code given below : (A) 1 and 3 (B) 2 and 3 (C) 1 and 2 (D) 1. Loads requiring starting performance good MCQ 11 A cycloconverter-fed induction motor drive is most suitable for which one of the following ? (A) Compressor drive (B) Machine tool drive (C) Paper mill drive (D) Cement mill drive Page 4 . 2 and 3 MCQ 10 Match List I (Controller) with List II (Type of load) and select the correct answer using the code given below the lists : List I List II A. Blowers and compressors D.c. voltage controller 3. Centrifuges in sugar industry cascade in the rotor circuit of an induction motor C. Very low speed. 3-phase a. Sub-synchronous converter. Chopper controlled resistance 1. Cycloconverter Codes : A (A) (B) (C) (D) 3 3 4 4 B 4 4 3 3 C 2 1 1 2 D 1 2 2 1 4.gatehelp.2. high-power reversible drive in the rotor circuit of an induction motor B. converter ? (A) 3-phase fully controlled bridge converter (B) 3-phase fully controlled bridge converter with free wheeling diode (C) 3-phase half-controlled bridge converter (D) A pair of 3-phase converters in sequence control MCQ 13 A single-phase full-bridge inverter is connected to a load of 2. What is the r.s.www. input voltage is 48 V. motor is required to control the speed of blower from a 3-phase a. source.4 Ω .gatehelp.c.com MCQ 12 IES EE Topic wise 2001-2008 Power Electronics A large d.m. What is the duty cycle of the regulator ? (A) 5/12 (B) 12/5 (C) 5/2 (D) 6 YEAR 2007 MCQ 15 Which one of the following is the correct statement ? In a two quadrant converter working in the 1st and 2nd quadrants (A) load current and load voltage are always positive (B) load current is always negative Page 5 .c. to d.c. output at fundamental frequency ? (A) 4 # 48 V 2π (B) 2 # 48 V 2π (C) 4 # 48 V π (D) 2 # 48 V π MCQ 14 A buck regulator has an input voltage of 12 V and the required output voltage is 5 V.c.c. What is the most suitable a. The d. com (C) load current can be positive or negative (D) load current and load voltage are always negative MCQ 16 The circuit shown in the below figure will work as which one of the following ? (A) Buck-Boost converter (B) Buck converter (C) Boost converter (D) Dual converter MCQ 17 What is the maximum output voltage of a 3-phase bridge rectifier supplied with the line voltage of 440 V ? (A) 528 V (B) 396 V (C) 594 V (D) 616 V MCQ 18 A single phase ac voltage controller is controlling current in a purely inductive load. what will be the conduction angle of the SCR ? (A) π (B) (π − α) (C) (2π − α) (D) 2π MCQ 19 Page 6 .IES EE Topic wise 2001-2008 Power Electronics www. If the firing angle of the SCR is α .gatehelp. www. 3. 4. 180c respectively (B) 180c. regulator (D) Cycloconverter MCQ 21 Consider the following statements : 1. Which of the statements given above is/are correct ? (A) 1 only (B) 2 and 3 (C) 3 and 4 (D) 4 only MCQ 22 Which one of the following is the main advantage of SMPS over linear power supply ? (A) No transformer is required (B) Only one stage of conversion (C) No filter is required Page 7 . minimum output voltage and maximum output voltage are obtained at which conduction angles ? (A) 0c.c. 0c respectively (C) 0c. Only voltage source inverter requires feedback diodes.gatehelp.c. GTOs can not be used in a current source inverter. 2. 180c respectively MCQ 20 IES EE Topic wise 2001-2008 Power Electronics The pulse-width modulated inverter for the control of an a. 0c respectively (D) 180c.com In a single phase full wave controlled bridge rectifier. Only current source inverter requires feedback diodes. motor is fed from which one of the following ? (A) Controlled rectifier (B) Uncontrolled rectifier (C) a. Both voltage source inverter and current source inverter require feedback diodes. TRIAC Codes : A (A) (B) (C) (D) 3 3 1 1 B 1 1 2 2 C 4 2 3 4 D 2 4 4 3 List II 1. UJT D.IES EE Topic wise 2001-2008 Power Electronics www. (A) Both A and R are true and R is the correct explanation of A (B) Both A and R are true but R is not the correct explanation of A (C) A is true but R is false (D) A is false but R is true MCQ 24 Match List I (Type of Device) with List II (Characteristic/Application) and select the correct answer using the code given below the lists : List I A.gatehelp. Bi-directional switching 3.com (D) Low power-dissipation MCQ 23 Assertion (A) : Power electronic converters are extensively used in adjustable speed drives. MOSFET B. Turn off by negative gate 2. High speed switching 4. Reason (R) : Power electronic converters do not produce harmonic distortion. Triggering circuit MCQ 25 Snubber circuits are used to protect thyristor from which of the following ? (A) High di and low dv dt dt (B) High dv and low di dt dt Page 8 . GTO C. It has high input impedance.www. Which of the statements given above are correct ? (A) 1. MCQ 29 Page 9 . 3 and 4 (B) 1.gatehelp. 2. 2 and 4 (C) 1. 3. It combines the attributes of MOSFET and BJT. It has low forward voltage drop. 2.com (C) Low dv and low di dt dt (D) High dv and high di dt dt MCQ 26 IES EE Topic wise 2001-2008 Power Electronics Consider the following statements in respect of IGBT : 1. Its switching speed is very much lower than that of MOSFET. 2 and 3 (D) 3 and 4 YEAR 2006 MCQ 27 A resistor connected across the gate and cathode of an SCR (A) increases dv/dt rating of SCR (B) increases holding current of SCR (C) increases noise immunity of SCR (D) increases turn-off time of SCR MCQ 28 Turn-on of a thyristor takes place when (A) anode to cathode voltage is positive (B) anode to cathode voltage is negative (C) there is a positive current pulse at the gate (D) the anode to cathode voltage is positive and there is a positive current pulse at the gate. 4. Voltage source inverter B.IES EE Topic wise 2001-2008 Power Electronics www. Phase controlled a. but increases the response time (B) allows smooth reversal of load current with improved speed of response (C) does not allow smooth reversal of load current. Current source inverter List II 1. Inverter limit converter D. the circulating current (A) allows smooth reversal of load current. to d. Small source inductance MCQ 31 In a dual converter. 3.gatehelp.com Which is the most suitable power device for high frequency (>100 KHz) switching application ? (A) Power MOSFET (B) Bipolar junction transistor (C) Schotty diode (D) Microwave transistor MCQ 30 Match List I with List II and select the correct answer using the code given below the lists : List I A. Larger source inductance 2.c. but reduces the response time (D) flows if there is no interconnecting inductor MCQ 32 Page 10 . Poor power factor C. Cycloconverter Codes : A (A) (B) (C) (D) 4 3 4 3 B 2 1 1 2 C 3 4 3 4 D 1 2 2 1 4.c. www. the output line voltage is free from (A) 3rd harmonic (B) 7th harmonic (C) 11th harmonic (D) 13th harmonic MCQ 36 Page 11 .c. current into the load is 30c.c.gatehelp.which one of the following has larger peak inverse voltage of the thyristor ? (A) Single phase full wave centre tapped circuit (B) Single phase full wave bridge circuit (C) Three phase full wave bridge circuit (D) Three phase full wave centre tapped circuit MCQ 35 In a three phase voltage source inverter operating in square wave mode.5 (C) (D) 3 3 2 MCQ 34 For the same voltage output. what is the displacement factor of the rectifier ? (A) 1 (B) 0.com What is the power factor of a single phase a. regulator feeding a resistive load ? (A) (Per unit power) 2 (B) (Per unit power) 1/2 (C) (D) (Per unit power) 2 2 (Per unit power) 1/2 2 IES EE Topic wise 2001-2008 Power Electronics MCQ 33 When the firing angle α of a single phase fully controlled rectifier feeding constant d. com A fully controlled line commutated converter functions as an inverter when firing angle (α) is in the range (A) 0c − 90c (B) 90c − 180c (C) 90c − 180c only when there is a suitable d.c. load MCQ 37 Which of the following devices should be used as a switch in a low power switched mode power supply (SMPS) ? (A) GTO (B) MOSFET (C) TRIAC (D) THYRISTOR YEAR 2005 MCQ 38 A converter which can operate both in 3-pulse and 6-pulse modes is a (A) 1-phase full converter (B) 3-phase half-wave converter (C) 3-phase semi converter (D) 3-phase full converter MCQ 39 A gate turn off (GTO) thyristor has capacity to (A) Amplify the gate-current (B) Turn-off when positive current pulse is given at the gate (C) Turn-off when a gate-pulse is given at the gate even though it is reverse biased (D) Turn-off when a negative current pulse is given at the gate MCQ 40 Power electronic device with poor turn-off gain is (A) A symmetrical thyristor Page 12 .IES EE Topic wise 2001-2008 Power Electronics www.f.gatehelp. source in the load (D) 90c − 180c only when it supplies a back e.m. gatehelp.www. The peak voltage limit Secondary breakdown limit Power dissipation limit Peak current limit 1. B. C. 3. List II PQ QR RS ST Codes : A (A) (B) (C) (D) 2 4 2 4 B 1 3 3 1 C 4 2 4 2 D 3 1 1 3 MCQ 42 Which one of the following statements is not correct ? (A) Power MOSFETs are so constructed as to avoid punch through Page 13 . 4. D. 2.com (B) A conventional thyristor (C) Power bipolar junction transistor (D) Gate turn-off thyristor MCQ 41 IES EE Topic wise 2001-2008 Power Electronics Match List I (Limiting factor) with List II (Safe Operating Area Portion) and select the correct answer using the code given below the lists : List I A. (B) Smoothen the waveform of circulating current (C) Limit the circulating current (D) Increase the circulating current MCQ 46 In a single-phase semiconvector with discontinuous conduction and extinction angle β < π . di/dt protection is achieved by the use of (A) An inductance L in series with the thyristor (B) A resistor in series with the thyristor (C) RC in series with the thyristor (D) RL in series with the thyristor MCQ 45 The circulating current inductor is required in a dual converter to (A) Improve the p.gatehelp.com (B) In a power MOSFET.IES EE Topic wise 2001-2008 Power Electronics www. freewheeling action takes place for (A) α (B) π − β (C) β − π (D) zero degree Page 14 .f. the channel length is relatively large and channel width is relatively small (C) Power MOSFETs do not experience any minority carrier storage (D) Power MOSFETs can be put in parallel to handle larger currents MCQ 43 Carrier frequency gate drive is used for turn-on of a thyristor to reduce (A) di/dt (B) Turn-on time (C) dv/dt (D) size of pulse transformer MCQ 44 In a thyristor. what is the power factor ? (A) Unity for all values of firing angle 1/2 (B) . 1 &(π + α) − 1 sin 2α 0E π 2 Where α id firing angle measured from voltage zero. voltage controller feeding a resistive load. 1 &(π − α) + 1 sin 2α 0E 2 π 1/2 (C) . 1 &(π + α) + 1 sin 2α 0E π 2 1/2 (D) . What is the value of firing delay angle α .www.c. MCQ 49 Triacs cannot be used in AC voltage regulator for a (A) Resistive load (B) Back emf load (C) Inductive load (D) Resistive inductive load MCQ 50 A motor armature supplies through phase controlled SCRs receives a smoother voltage shape at (A) High motor speed (B) Low motor speed (C) Rated motor speed (D) None of the above Page 15 .com MCQ 47 IES EE Topic wise 2001-2008 Power Electronics A three pulse converter is feeding a purely resistive load. which dictates the boundary between continuous and discontinuous mode of current conduction ? (A) α = 0c (B) α = 30c (C) α = 60c (D) α = 150c MCQ 48 For a single phase a.gatehelp. IES EE Topic wise 2001-2008 Power Electronics www. What is the percentage of the fifth harmonic component in relation to the fundamental component ? (A) 40 % (B) 30 % (C) 20 % (D) 10 % MCQ 53 Which one of the following statements is correct ? The turn off times of converter grade SCRs are normally in the range of (A) 1 to 2 microseconds (B) 50 to 200 microseconds (C) 500 to 1000 microseconds (D) 1 to 2 milliseconds MCQ 54 Which one of the following statements is correct ? A triac is a (A) 2 terminal switch Page 16 .gatehelp.com MCQ 51 Which one of the following statements is correct ? A voltage source inverter is normally employed (A) when the source has low impedance and load has high reactance (B) when the source has high impedance and load has low reactance (C) when both the source and load have high values of impedance and reactance respectively (D) when both the source and load have low values of impedance and reactance respectively YEAR 2004 MCQ 52 A single-phase inverter has square wave output voltage. -d.gatehelp.c. converter ? (A) BJT (B) GTO (C) MOSFET (D) Thyristor MCQ 56 Which one of the following is correct ? In a switched capacitor network for VAr compensation the SCRs are commutated by (A) forced commutation (B) resonant commutation (C) natural commutation (D) delayed commutation MCQ 57 Which one of the following shows current fold back characteristics curve for an SCR controlled shunt regulated power supply ? Page 17 .c.www.com (B) 2 terminal bilateral switch (C) 3 terminal unilateral switch (D) 3 terminal bidirectional switch IES EE Topic wise 2001-2008 Power Electronics MCQ 55 Which one of the following is the most suitable device for a d. gatehelp.IES EE Topic wise 2001-2008 Power Electronics www. the holding current IH is Page 18 . the reverse saturation current ICO (A) doubles for every 10cC rise in temperature (B) doubles for every 1cC rise in temperature (C) increases linearly with temperature (D) decreases linearly with temperature MCQ 59 Which one of the following statements is correct ? In a thyristor.com MCQ 58 Which one of the following statements is correct ? In a transistor. tr 2 dt MCQ 62 In the buck-boost converter. dv/dt protection is achieved through the use of (A) RL in series with SCR (B) RC across SCR (C) L in series with SCR (D) RC in series with SCR MCQ 61 A power diode is in the forward conduction mode and the forward current is now decreased.gatehelp.75 (C) 0.50 (D) 0.tr dt (B) 1 b di l . What is the stored charge ? (A) b di l . The reverse recovery time of the diode is tr and the rate of fall of the diode current is di/dt .t r2 2 dt (C) b di l .www.25 MCQ 63 Page 19 . what is the maximum value of the switch utilization factor ? (A) 1.t r2 dt (D) 1 b di l .00 (B) 0.com (A) more than the latching current IL (B) less than IL (C) equal to IL (D) equal to zero MCQ 60 IES EE Topic wise 2001-2008 Power Electronics Which one of the following statements is correct ? For an SCR. gatehelp. the heat sinks on which the thyristors are mounted. are made of (A) aluminium (B) copper (C) nickel (D) stainless steel MCQ 64 Which one of the following capacitor is suitable for compensation of harmonic and reactive power ? (A) Mica capacitor (B) Glass capacitor (C) Polypropylene capacitor (D) Electrolytic capacitor MCQ 65 Which one of the following statements is correct ? The following is bleeder resistor in a power supply is (A) to ensure a minimum current drain in the current (B) to increase the output d.c. voltage (C) to increase the output current (D) same as that of a load resistor MCQ 66 Which one of the following is used as the main switching element in a switched mode power supply operating in 20 kHz to 100 kHz range ? (A) Thyristor (B) MOSFET (C) Triac (D) UJT YEAR 2003 Page 20 .IES EE Topic wise 2001-2008 Power Electronics www.com Which one of the following statements is correct ? In order to get best results per unit cost. Rate of rise of gate 3. Is the sum of delay and rise times C. Chopper C. Fixed DC to variable DC 3. Inverter D.gatehelp. Turn-on time of thyristor 1. Depends on junction capacitances B. Thyristor switches back to Offcurrent in thyristor state Page 21 . Cycloconverter Codes : A (A) (B) (C) (D) 2 3 2 3 B 3 2 3 2 C 1 4 4 1 D 4 1 1 4 List II 1.www. Fixed DC to variable voltage and variable frequency AC 2.com MCQ 67 IES EE Topic wise 2001-2008 Power Electronics Match List I (Converters) with List II (Type of conversion) and select the correct answer : List I A. Controlled Rectifier B. Turn-on time of transistor 2. Fixed AC to variable frequency AC MCQ 68 Turn-on and turn-off times of transistor depend on (A) static characteristic (B) junction capacitances (C) current gain (D) None of the above MCQ 69 Match List I (Characteristic/Action) with List II (Observations) and select the correct answer using the codes given below the list : List I List II A. Fixed AC to variable DC 4. gatehelp. (assuming that load is shunted by a perfect commutating diode and battery is loss-less) is (A) 240 V (B) 48 V (C) 15 V (D) 4 V MCQ 71 The characteristic features of discontinuous conduction compared to continuous conduction in a four-pulse. supplies a load having resistance of 3 ohms and inductance of 9 mH from a 60 V battery. Page 22 .m. The mean value of the load voltage for on/off ratio of 4/1. single-phase bridge converter are (A) Larger average value of load voltage and larger ripple-content (B) Larger average value of load voltage and smaller ripple-content (C) Smaller average value of load voltage and smaller ripple-content (D) Smaller average value of load voltage and larger ripple-content MCQ 72 If the r. half-wave controlled rectifier. less than latching current Codes : A (A) (B) (C) (D) 2 1 2 1 B 1 2 1 2 C 4 3 3 4 D 3 4 4 3 MCQ 70 An ideal chopper operating at a frequency of 500 Hz. the minimum and maximum values of firing angles for a single-phase.com D.s source voltage is V volts.IES EE Topic wise 2001-2008 Power Electronics www. Affect the delay time removed. If the gate pulse is 4. when the thyristor is carrying a current. The basic three-phase bridge inverter can be controlled.c.gatehelp.com supplying a load with a back e. The angle through which each switch conducts. of 40 volts are (A) 0cand 180c (C) α = sin−1 (40/ 2 V) and 9π − sin−1 _40/ 2 V iC (D) 0c and 8π − sin−1 (40/ 2 V)B MCQ 73 IES EE Topic wise 2001-2008 Power Electronics (B) α = sin−1 (40/ 2 V) and 180c An AC voltage-regulator using back-to-back connected SCRs is feeding an RL load. The SCR firing angle α < φ ( φ is power factor angle of the load). and at any instant the number of switches conducting simultaneously are. respectively (A) 120c and 02 (B) 120c and 03 (C) 180c and 02 (D) 180c and 04 MCQ 76 Page 23 . the output load-voltage waveform will be (A) Symmetrical chopped a.c.f. source is switched in steps to synthesize the three-phase output. voltage (B) Half-wave rectified (C) Full-wave rectified (D) Sinusoidal MCQ 74 In a self-controlled synchronous motor fed from a variable frequency inverter (A) The rotor poles invariably have damper windings (B) There are stability problems (C) The speed of the rotor decides stator frequency (D) The frequency of the stator decides the rotor speed MCQ 75 A d.www.m. If SCRs are fired using short-duration gate pulses. 4 Ω . full-bridge inverter supplying power to a highly inductive load as shown above. half-bridge inverter has input voltage of 48 V DC.gatehelp. Inverter is feeding a load of 2. The duty cycle is (A) 3/2 (B) 2/3 (C) 5/2 (D) 15/2 MCQ 79 Page 24 . the correct sequence of operation of switches and diodes is (A) S1 S 4 − S 3 S2 − S1 S 4 − S 3 S2 (B) S1 S2 − D1 D2 − S 3 S 4 − D 3 D 4 (C) S1 D 3 − S1 S 4 − S 4 D2 − D2 D 3 (D) S2 D 4 − D 4 D1 − D1 D 3 − S 3 S2 MCQ 77 A single-phase.com For a single-phase. The rms output voltage at fundamental frequency is (A) 2 # 48 V π (B) 2 # 48 V 2π (C) 2 # 48 V π (D) 2 # 48 V 2 2π MCQ 78 A boost-regulator has an input voltage of 5 V and the average output voltage of 15 V.IES EE Topic wise 2001-2008 Power Electronics www. at which the device just goes into conduction without a gate pulse. Select your answers to these items using the codes given below : Codes : (A) Both A and R are true and R is the correct explanation of A (B) Both A and R are true but R is NOT the correct explanation of A (C) A is true but R is false (D) A is false but R is true MCQ 80 IES EE Topic wise 2001-2008 Power Electronics Assertion (A) : The critical rate of change of forward-voltage is the value of dv/dt .www. You are to examine these two statements carefully and decide if the Assertion A and the Reason R are individually true and if so. even before the break-forward voltage limit is reached.gatehelp.com An inter-group reactor is used in a single-phase cycloconverter circuit to (A) Reduce current-ripples (B) Reduce voltage-ripples (C) Limit circulating current (D) Limit di/dt in the semiconductor switch Direction : The following item consist of two statements. one labelled as Assertion ‘A’ and the other labelled as Reason ‘R’. Reason (R) : Thyristors go to a state of conduction with the application of sharp rate of change of forward-voltage in the absence of gate pulse. YEAR 2002 MCQ 81 When cathode of a thyristor is made more positive than its anode (A) all the junctions are reverse biased (B) outer junctions are reverse biased and central one is forward biased (C) outer junctions are forward biased and central one is reverse biased Page 25 . whether the Reason is a correct explanation of the Assertion. 6 μs (B) 100 μs (C) 150 μs (D) 200 μs MCQ 85 A thyristor controlled reactor is used to get (A) variable resistance (B) variable capacitance (C) variable inductance (D) improved reactor power factor MCQ 86 Page 26 .c.IES EE Topic wise 2001-2008 Power Electronics www. voltage of 220 volts. chopper with an input d.-d.c. if the output voltage required is 330 volts and the non-conducting time of thyristor-chopper is 100 μs .gatehelp.c. the ON time of thyristor-chopper would be (A) 66.com (D) all the junctions are forward biased MCQ 82 The sharing of the voltages between thyristors operating in series is influenced by the (A) di/dt capabilities (B) dv/dt capabilities (C) junction temperatures (D) static v -i characteristics and leakage currents MCQ 83 R -C snubber is used in parallel with the thyristor to (A) reduce dv/dt across it (B) reduce di/dt through it (C) limit current through the thyristor (D) ensure its conduction after gate signal is removed MCQ 84 For a step up d. For a firing delay angle of 60c. The load resistance is 15. Input to converter is from an ideal 230 V. to d.com A single-phase full-bridge convertre with a free-wheeling diode feeds an inductive load. 50 Hz single phase source. the average value of diode current is (A) 10 A (B) 8.774 A (D) 3. converter.53 Ω and it has a large inductance providing constant and ripple free d.33 A MCQ 87 IES EE Topic wise 2001-2008 Power Electronics The operation of an inverter fed induction motor can be shifted from motoring to regenerative braking by (A) reversing phase sequence (B) reducing inverter voltage (C) decreasing inverter frequency (D) increasing inverter frequency MCQ 88 In a three-phase full wave a.c. currnet.165 A (B) 5. the ratio of output ripple-frequency to the supply-voltage frequency is (A) 2 (B) 3 (C) 6 (D) 12 MCQ 89 A 6-phase bridge converter feeds a purely resistive load. The delay angle α is measured from the point of natural-commutation. The effective control of natural-commutation.www.c. The effective control of voltage can be obtained when α lies in the range (A) 0 # α # 105c (B) 0 # α # 120c (C) 0 # α # 150c (D) 0 # α # 180c Page 27 .gatehelp.c. 200 Hz (B) 500 Hz .50 kHz MCQ 94 Page 28 . a d.c.c. after conversion of a. voltage regulators are widely used in (A) traction drives (B) fan drives (C) synchronous motor drives (D) slip power recovery scheme of slip-ring induction motor MCQ 91 When fed from a fully controlled rectifier. can operate in (A) forward motoring and reverse braking mode (B) forward motoring and forward braking mode (C) reverse motoring and reverse braking mode (D) reverse motoring and forward braking mode MCQ 92 Compared to a single-phase half-bridge inverter. driving an active load. motor.IES EE Topic wise 2001-2008 Power Electronics www. voltage.c.1 kHz (C) 2 kHz .com MCQ 90 a. The frequency of the pulses is typically in the range of (A) 100 Hz .gatehelp.5 kHz (D) 20 kHz . the output power of a single-phase full-bridge inverter is higher by a factor of (A) 12 (B) 8 (C) 4 (D) 2 MCQ 93 In a switched-mode power supply (SMPS).c. supply to a highly filtered d. a switching transistor is switched ON and OFF at a very high speed by a pulse width modulator (PWM) which generates very-high frequency square pulses. Vpeak /π (1 + cos α) 2. Semi-contorlled Codes : A (A) (B) (C) 3 1 3 B 2 4 4 C 4 2 2 D 1 3 1 Page 29 List II 1. load is 220 V.φ Rectifier topology-feeding resistive load) with List II (Average output voltage) and select the correct answer : ( α is the firing angle) List I A.12 V (C) 266 V (D) 220 V YEAR 2001 MCQ 96 Match List I (1. Uncontrolled-half wave B. 60 A at a power factor of 0.c. The single phase a. The rms value of input voltage per phase required is (A) 376.gatehelp.2 V (B) 311. Vpeak /2π (1 + cos α) . 2Vpeak /π cos α 3. Vpeak /π 4.www.com How many switches are used to construct a three-phase cycloconverter ? (A) 3 (B) 6 (C) 12 (D) 18 MCQ 95 IES EE Topic wise 2001-2008 Power Electronics A 3-phase cycloconverter is used to obtain a variable-frequency single-phase a.c. Controlled-half wave C. output. Controlled-full wave D.6 lagging. Codes : (A) Both A and R are true and R is the correct explanation of A (B) Both A and R are true but R is NOT the correct explanation of A (C) A is true but R is false (D) A is false but R is true MCQ 97 Assertion (A) : High frequency power supplies are lightweight.IES EE Topic wise 2001-2008 Power Electronics www. MCQ 98 The latching current in the above circuit is 4 mA.com (D) 1 2 4 3 Direction : The following item consist of two statements.gatehelp. You are to examine these two statements carefully and decide if the Assertion A and the Reason R are individually true and if so. The minimum width of the gate pulse required to turn on the thyristor is (A) 6 μs (B) 4 μs (C) 2 μs (D) 1 μs MCQ 99 Triac cannot be used in (A) ac voltage regulators Page 30 . whether the Reason is a correct explanation of the Assertion. Select your answers to these items using the codes given below. one labelled as Assertion ‘A’ and the other labelled as Reason ‘R’. Reason (R) : Transformer size get reduced at high frequency. com (B) cycloconverters (C) solid state type of switch (D) inverter MCQ 100 IES EE Topic wise 2001-2008 Power Electronics The snubber circuit is used in the thyristor circuits for (A) triggering (B) dv/dt protection (C) di/dt protection (D) phase shifting MCQ 101 It is prefereable to use a train of pulse of high frequency for gate triggering of SCR in order to reduce (A) dv/dt problem (B) di/dt problem (C) the size of the pulse transformer (D) the complexity of the firing circuit MCQ 102 A four quadrant chopper cannot be operated as (A) one quadrant chopper (B) cycloconverter (C) inverter (D) bi-directional rectifier MCQ 103 Match List I (Waveforms) with List II (Descriptions) and select the correct answer : Page 31 .gatehelp.www. com List II 1. . Phase voltage of a three phase inverter with 180c conduction and input dc voltage E .IES EE Topic wise 2001-2008 Power Electronics www. 3. 2. Page 32 Single phase fully controlled ac to dc converter Voltage commutated dc to dc chopper with input dc voltage E .gatehelp. com 4. Three-phase diode bridge rectifier. the firing angle of thyristor is to be controlled in the range of (A) 0c to 90c (B) 0c to 180c (C) 90c to 180c (D) 90c to 270c Page 33 .8 (C) 0. 5.894 (D) Difficult to estimate because of insufficiency of data MCQ 106 In a thyristor-controlled rectifier. Line voltage of a three phase inverter with 120c conduction and input dc voltage E .gatehelp. The input power factor of the controller is (A) 0. IES EE Topic wise 2001-2008 Power Electronics Codes : A (A) (B) (C) (D) 3 5 3 5 B 4 1 1 4 C 1 4 4 1 D 5 2 5 2 MCQ 104 The total harmonic distortion (THD) of ac supply input current of rectifiers is maximum for (A) single-phase diode rectifier with dc inductive f filter (B) 3-phase diode rectifier with dc inductive filter (C) 3-phase thyristor with inductive filter (D) Single-phase diode rectifier with capacitive filter MCQ 105 A single phase ac voltage controller feeding a pure resistance load has a load voltage of 200 V(rms) when fed from a source of 250 V(rms).64 (B) 0.www. free-wheeling can be needed for the load of (A) inductive nature (B) capacitive nature (C) resistive nature (D) back emf nature MCQ 110 PWM switching is preferred in voltage source inverters for the purpose of (A) controlling output voltage (B) output harmonics (C) reducing filter size (D) controlling output voltage.IES EE Topic wise 2001-2008 Power Electronics www. The average resistance in the rotor circuit for the copper frequency of 200 Hz is (A) 26/5 Ω (B) 24/5 Ω (C) 18/5 Ω (D) 16/5 Ω MCQ 108 The most suitable solid state converter for controlling the speed of the three-phase cage motor at 25 Hz is (A) cycloconverter (B) current source inverter (C) voltage source inverter (D) load commutated inverter MCQ 109 In case of voltage source inverter.com MCQ 107 A 3-phase wound rotor induction motor is controlled by a choppercontrolled resistance in its rotor circuit. A resistance of 2 Ω is connected in the rotor circuit and a resistance of 4 Ω is additionally connected during OFF periods of the chopper. output harmonics and reducing filter size Page 34 .gatehelp. 128 Hz ********* Page 35 .gatehelp.com MCQ 111 IES EE Topic wise 2001-2008 Power Electronics Which one of the following is not the advantage of solid state switching of ac capacitors into ac supply over relay-based switching ? (A) low transients (B) low losses (C) fast response (D) long life MCQ 112 The most suitable device for high frequency inversion in SMPS is (A) BJT (B) IGBT (C) MOSFET (D) GTO MCQ 113 The quality of output ac voltage of a cycloconverter is improved with (A) increase in output voltage at reduced frequency (B) increase in output voltage at increased frequency (C) decrease in output voltage at reduced frequency (D) decrease in output voltage at increased frequency MCQ 114 A cycloconverter is operating on a 50 Hz supply.www.16 Hz (B) 0 .64 Hz (D) 0 . The range of output frequency that can be obtained with acceptable quality. is (A) 0 .32 Hz (C) 0 .