Quearn

Share

### Related Questions

- 42. Which of the following is one of the unique attributes of Cloud Computing? A. utility type of delivery ...
- 35. What is Relaxed Layered Style? A. each layer can be constrained to use only the layer directly below it ...
- 426. The usual BUS structure used to connect the I/O devices is A. star bus structure B. multiple ...
- 'Fair employment legislation' is applicable to which of the following? A. Collective bargaining B. Planning & selection C. Training & ...
- Initially the personnel department was only involved in: A. Recruitment & selection B. Compensation & benefits C. Training & development ...

From 0 to α there is no conduction. SCRs are gated at α and than naturally commutated at π. Hence, conduction angle = π-α.

Explanation: All the above given statements are correct. They all are the major drawbacks of using phase controlled converters. 3. Consider the following statement: The overlap angle of a phase controlled converter will increase ___________ i) if the firing angle is increased ii) if the supply frequency is increased ii) if the supply voltage magnitude is lowered Of these statements a) all are correct b) only the first and the last one are correct c) only the first one is incorrect d) all are incorrect

All the modern AC-DC converters are using GTOs instead of SCRs because Answer: c Explanation: GTOs unlike conventional SCRs can be turned off by providing a negative gate pulse. Hence, using GTOs the turn off as well as the turn-on time can be controlled.

⇒Find the average output dc voltage of a single-phase semi-converter with Vs=230 V and firing angle of 30°. The converter is operating under continuous conduction. a) 193 V b) 256 V c) 0 V d) 230 V Answer: a Explanation: Vo = √2Vs (1+cosα)/π.

→In the below given circuit, the average value of output voltage is 191 V. Find the value of the average current through the given load if E = 100 V and R = 5 Ω. Assume that the load current is continuous. a) 0 A b) 10.2 A c) 18.2 A d) 29.2 A Answer: c Explanation: Vo = 191 V Io = Vo-E/R = 191-100/5 = 18.2 A.

In the below given circuit, the average value of output voltage is 191 V. Find the value of the power delivered to the voltage source E. Take E = 100 V and R = 5 Ω. Assume that the load current is continuous. a) 1.82 kW b) 18.2 kW c) 0.182 kW d) 18.2 W Answer: a Explanation: Note that as the load current is continuous, L can be neglected. Vo = 191V. Io = Vo-E/R = 191-100/5 = 18.2A P = E x Io = 100 x 18.2 = 1.82 kW

A single-phase semi-converter is operated from a 240 V, 60 Hz, AC source. It is fired at an angle of 45°. Find the value of average output voltage. a) 176 V b) 184 V c) 167 V d) 148 V Answer: b Explanation: For a semi-converter, the output voltage Vo = √2Vs (1+cosα)/π.

A single-phase semi-converter is operated from a 230 V, 60 Hz, AC source. A DMM connected at its output terminals read 219.3 V. Find the value of FF (form factor). a) 1.24 b) 0.735 c) 1.11 d) 1 Answer: a Explanation: A DMM always reads the RMS value. Therefore, Vrms = 219.3 V Vo = √2Vs (1+cosα)/π = 176.72 V FF = Vrms/Vo = 1.241.

A three-phase, three-pulse, M-3 type controlled converter uses ____________ Answer: c Explanation: It uses three SCRs with a three-phase transformer. M-3 type 3-pulse converters are not practically used.

A three-phase, three-pulse, M-3 type controlled converter has firing angle for one of the SCRs set as 15°. This SCR would start conducting at a) 0° b) 15° c) 30° d) 45° Answer: d Explanation: In a three phase controller, the actually conduction starts at 30° + α. Hence, ωt = 30+15 = 45°.

n a three-phase, three-pulse, M-3 type controlled converter T1 starts to conduct at 30 + n°. At what angles do T2 and T3 start to conduct? Assume that the conduction sequence is T1-T2-T3. a) 2n° and 3n° b) 150 + n° and 270 + n° c) n° each d) 30 + n° and 60 + n° Answer: b Explanation: In three-phase three pulse converter the conduction can start only after 30°. As each SCR conducts from 120°, T2 would conduct on 30+120+n° = 150+n° and so on.

14. A three-phase three pulse type controlled converter is constructed using 3 SCR devices. The circuit is supplying an R load with α < 30°. As such, each SCR device would conduct for a) 60° each cycle b) 120° each cycle c) 180° each cycle d) 360° each cycle Answer: b Explanation: Each conduct for 120° per cycle is the firing angle is less than 30°. 120 x 3 = 360°.

15. Find the expression for average output voltage at R for the below given configuration. Take firing angle as α = 15°, transformer ratio as 1:1:1 and Vmp as the maximum value of phase voltage at the supply. a) (3Vmp/2π) x cosα b) (3√3Vmp/2π) x cosα c) (3√3Vmp/2π) x sinα d) (3Vmp/2π) x sinα Answer: b Explanation: The circuit is that of a three-pulse M-3 connection. The firing angle is less than 30°. Therefore, each device conducts for an angle of 120°. Vo = 3 x [ 1/2π ∫ Vmp sinωt d(ωt) ] Where, the integration runs from α+π/6 to α+5π/6. Vo = (3√3/2π) x Vmp x cosα.

16. In the circuit shown below, SCR T1 conducts first. If T1 is fired at an angle of α > 30°, then T1 would conduct from a) α to 180° b) 30 + α to 180° c) 30 + α to 150° d) 30 + α to 120° Answer: b Explanation: When firing angle is more than 30°, T1 would conduct from 30 + α to 180°. Irrespective of the firing angle, T1 will be turned on at 180° because it conducts first which means it is connected to the R phase and the phase sequence is R-Y-B. As R starts at 0° its value is 0 at 180° which reverse biases the SCR T1.

⇒17. Find the expression for average output voltage for the given circuit if firing angle is greater than 30°. Take Vmp = secondary side maximum value of phase voltage. a) (3√3Vmp/2π) x cosα b) (3√3Vmp/2π) x (1+cosα) c) (3√3Vmp/2π) x [1+cos(30+α)]. d) (3√3Vmp/2π) x [3+cos(30+α)]. Answer: c Explanation: Vo = 3 x [ 1/2π ∫ Vmp sinωt d(ωt) ] Where, the integration runs from α+π/6 to π. Because conduction takes place from 30 + α to 180° for T1 and than the waveform is symmetrical for all other SCRs. Vo = (3√3/2π) x Vmp x [1+cos(30+α)].

18. A three-phase three-pulse converter would operate as a line commutated inverter a) 30° < α < 60° b) 90° < α <180° c) 90° > α d) it can never operate as a line commutated inverter Answer: b Explanation: The output voltage is proportional to cosine of the firing angle α. If α goes above 90° then the output voltage is negative, meaning that it is operating as an inverter.

19. A three-phase M-3 converter is operated from a 3-phase, 230 V, 50 Hz supply with load resistance R = 10 Ω. Find the value of firing angle if an average output voltage of 50% of the maximum possible output voltage is required. Hint: α > 30°. a) 92.7° b) 67.7° c) 45° d) 75.7° Answer: b Explanation: We need , Vo = 0.5 Vom. α>30° hence we use the equation Vo = (3√3/2π) x Vmp x [1+cos(30+α)] √3Vmp = Vml = √2×230 Therefore, Vo = (3/2π) x √3 Vml x [1+cos(30+α)] = 0.5Vom (1/√3) x [1+cos(30+α)] = Vo x 2π/3Vml = Vo/Vom = 1/2 α = 67.7°.

20. A three-phase half-wave controlled converter is fed from a 3-phase, 400 V source and is connected to a load which takes a constant current of 36 A. Find, the value of average output voltage and average current rating of SCR for a firing angle of 30°. a) 234 V, 36 A b) 234 V, 12 A c) 135 V, 36 A d) 135 V, 12 A Answer: b Explanation: Vo = (3√3/2π) x Vmp x cosα = 233.874 V. Ia = Io/3 = 12 A.

→21. A three phase full converter will require __________ number of SCRs. a) 3 b) 6 c) 9 d) 2 Answer: b Explanation: Three legs having two SCRs each, six in total.

22. A three phase six pulse full converter works as a ac to dc converter for firing angles in the range a) α > 90 b) 90 < α < 180 c) 0 < α < 90 d) 0 < α < 360 Answer: c Explanation: When α is less than 90°, the SCRs conduct for 120° and the current and voltage are positive on an average hence, the power flows from AC source to DC load.

⇒23. For the below given circuit, α = 60°. T2 will start conduction at ωt = __________ a) 60° b) 120° c) 90° d) 150° Answer: d Explanation: Assuming the phase sequence is R-Y-B. T1 would start conducting at 30+60 = 90°, T2 at 90+210/2 = 150°. This is because after T1, T3 would conduct from the upper group, as T2 belongs to the lower group it will start to conduct exactly between T1 and T3 i.e. between 90 and 210(90+120) which is 150°.

24. For a three phase full controlled converter, with 3 thyristors in the upper or positive group and 3 thyristors in the lower or negative group, at any given time a) two thyristors are conducting from each group b) one thyristor is conducting from each group c) one thyristor is conducting from either of the groups d) all 6 thyristors are conducting at a time Answer: b Explanation: Let’s say T1, T3 and T5 belong to the positive group and T2, T4 and T6 to the negative group. At any given time one SCR from each group conducts. e.g. T1 and T6 or T1 and T2.

25. In case of a three phase full controlled converter with 6 SCRs, commutation Answer: b Explanation: Every SCR conducts for 120°. This means that the SCRs from the positive group are fired 120° among themselves, same is true for SCRs from negative group. For example, if T1 starts conducting at 90° it will conduct till 90+120 = 210°. But while T1 is conducting, half of the time i.e. from 90 to 150, T6 is conducting and another half of the time T2 is conducting. Hence, commutation (change in the SCR which is conducting) takes place every 60 degrees irrespective of the firing angle. Construct the firing sequence table for better understanding.

⇔26. For the below given circuit, the conduction sequence for the negative group of a) T4-T6-T2 b) T1-T2-T3 c) T2-T6-1 d) T2-T4-T6 Answer: d Explanation: The negative group of SCRs has T2, T4 and T6. The conduct as T2-T4-T6, as T2 is connected to the B phase, T4 to the R phase and like-wise.

28. A three-phase full converter charges a battery from a three-phase supply of 230 V. The battery emf is 200 V and the internal resistance of the battery is 0.5 Ω. Find the value of the continuous current which is flowing through the battery if its terminal voltage is 210 V a) 10 A b) 20 A c) 0.5 A d) 25 A Answer: b Explanation: Vo = 210 V Vo = E + Io x R 210 = 200 + 0.5 x Io Io (Current through the battery) = 20 A.

30. A three-phase full converter charges a battery from a three-phase supply of 230 V. Find the value of the power delivered to the load if a continues current of 20A is flowing through the battery of emf 200 V and internal resistance of 0.5 Ω. a) 0 W b) 5600 W c) 4200 W d) 1040 W Answer: c Explanation: Iavg = Irms = 20 A P = E x Iavg + Irms2 x R = 4200 W.

32. A three-phase full converter is driving a DC motor. If a continues current of Im amperes is flowing through the motor load, then find the rms value of supply current drawn by the converter to drive the motor. a) Im/√2 b) Im2 /3 c) √2Im/√3 d) √2Im/3 Answer: c )2 (IS = (1/π) x (Im)2 Explanation: The RMS value of the supply current IS over π radians would be x (2π/3) = Im√2/√3.

→34. In the below given circuit, each SCR and diode conduct for a) 60° and 120° respectively b) 120° and 60° respectively c) 120° d) 60° Answer: c Explanation: At any given time, one SCR and one diode is conducting, each conduct for 120° per cycle.

→35. In the below given circuit, __ and __ conduct along with T2. a) T1, T3 b) D1, D2 c) D1, D3 d) T1, T2 Answer: c Explanation: When one SCR conducts, a diode conducts along with it at a time to provide the path of current flow. . For example, if T2 starts conducting at 90° it will conduct till 90+120 = 210°. But while T2 is conducting, half of the time i.e. from 90 to 150 D1 is conducting and another half of the time D3 is conducting. T2 and D2 cannot conduct together as it will cause a short circuit. Hence, T2-D1 conduct for 60° and then T2-D3 conduct for another 60°.

→36. In a three-phase semi-converter, at a time one SCR and one diode conduct simultaneously. With SCR T1 conducting which diode(s) is most likely to conduct along with T1? a) D2 only b) D3 only c) D1 and D2 d) D2 and D3 Answer: d Explanation: T1 and D1 together will cause a S.C. D2 or D3 any of these two can conduct along with T1 depending on which phase voltage is currently active RB or RY.

37. What is the value of voltage at the output terminal when the freewheeling diode (FD) is conducting? a) Zero b) Maximum c) E d) It could be anything depending on α Answer: a Explanation: When FD is conducting it will short circuit the load terminal resulting in zero voltage. It won’t be E because the terminals are shorted. It can be E when none of the devices are conducting (This can happen only when α > 120°)

→38. A 3-phase full converter feeds power to an R load of 10 Ω. For a firing angle delay of 30° the load takes 5 kW. An inductor of large value is also connected to the load to make the current ripple free. Find the value of per phase input voltage. a) 133 V b) 230/√3 V c) 191/√3 V d) 298/√3 V Answer: c Explanation: Ior = Vo/R = (3Vml/Rπ) cos α P = 5 kW = Ior2 x R = [(3Vml/π) cos α]2 x 1/R] Therefore, Vs (line) = √50000 x (π/√2 x 3 x cos30) = 191.22 V Vs (phase) = 191/√3 V.

39. A three-phase semi-converter circuit is given a supply of 400 V. It produces at the output terminals an average voltage of 381 V. Find the rectification efficiency of the converter circuit. a) 99.65 % b) 95.25 % c) 91 % d) 86.5 % Answer: b Explanation: Rectification efficiency = Pdc/Pac Pdc = Vo x Io . . . (both average values of output current and voltage) Pac = Vrms x Irms . . . (both rms values of input current and voltage) For a semi-converter Irms = Io Therefore, Rectification efficiency = 381/400 = 95.25 %.

40. In a 3-phase semi-converter, for firing angle less than 60° the freewheeling diode Answer: d Explanation: In case of a semi-converter operating with α < 60°, FD does not comes into play, as the voltage never falls to zero and gives no chance for the inductor to discharge

⇒41. In a 3-phase full wave converter, if V is the maximum value of line voltage at the input, then each SCR is subjected to a peak negative voltage of a) V b) 3V c) √3V d) V/2 Answer: c Explanation: PIV = √3 Vml in case of a semi-converter.

43. The effect of source inductance on the performance of a 3-phase controlled Answer: b Explanation: It reduces the average value of the output voltage by introducing a overlap delay μ.

44. Which of the following converter circuits would require a neutral point? a) 3-phase semi-converter b) 3-phase full converter c) 3-phase full converter with freewheeling diode d) 3-phase half wave converter Answer: d Explanation: Half wave converter would require delta-star transformer, the secondary winding star connection requires a neutral point.

46. A 3-phase bridge converter is given a three-phase supply in the phase sequence R-Y-B. Let the neutral to R phase voltage be Vm sinωt. The first SCR (connected to R phase) is fired at an angle of 15°. What is the maximum value at the output terminals at this instant? a) Vm/√2 b) Vm c) 1.5 Vm d) 3 Vm Answer: c Explanation: In case of a 3 phase bridge converter, the maximum value of voltage at the output terminal is always 1.5 Vm.

48. Each SCR of a 3-phase 6-pulse converter conducts for Answer: a Explanation: A 3-phase 6-pulse converter is nothing but the 3-pulse full controlled converter using 6 devices each conducting for 120°.

49. A 3-phase full converter has an average output voltage of 365 V for zero degree firing and resistive load. For a firing angle of 90 degree, the output voltage would be a) 125 V b) 569 V c) 365 V d) zero Answer: d Explanation: Cos 90 = 0.

→50. Semi-converters are a) single quadrant converters b) double quadrant converters c) three quadrant converters d) none of the mentioned Answer: a Explanation: Semi-converters are single quadrant converters, because the voltage and current can only be both positive due to the diodes connected.

51. In a 3 phase M-6 controlled converter for continuous conduction mode, each a) 2π/6 radians b) 30 degrees c) 3π/2 radians d) 120 degrees Answer: a Explanation: Each SCR conducts for 60° or 2π/6 radians. 6 x 60 = 360°.

52. In a 3 phase, 12-pulse controlled converter for continuous conduction mode, a) π/6 radians b) 60 degrees c) 3π/2 radians d) 12 degrees Answer: a Explanation: Each SCR conducts for 30° or π/6 radians. 12 x 30 = 360°.

⇒53. A 3-phase full converter delivers a ripple free load current of 10 A with a firing angle delay of 45°. The input voltage is 3-phase, 400 V, 50 Hz. The source current is given by the following relation. Find the fundamental component of the source current amplitude. a) 11.03 b) 2.205 c) 11.03 sin 45 d) 46.98 Answer: a Explanation: Put n = 1 (fundamental component) and the rest of the given values in the above given equation.

54. A 3-phase full converter delivers a ripple free load current of 10 A with a firing angle delay of 45°. Find the DF (distortion factor). a) 1.414 b) 0 c) 0.707 d) 0.569 Answer: c Explanation: DF = cosα = cos45 = 0.707 Note that ripple free current does not mean that DF = 0. It means the current is continues in magnitude and direction, current will always contain harmonics (distortions) whether it is rippled or ripple free.

56. The commutation period when both incoming and outgoing SCRs are conducting Answer: b Explanation: Due to source inductance, SCRs cannot start and stop conducting immediately, hence a time occurs when both incoming and outgoing SCRs are conducting together. This delay is called as commutation angle or overlap period (μ↓

57. A 3-phase full converter delivers a ripple free load current of 10 A with a firing angle delay of 45°. The input voltage is 3-phase, 400 V, 50 Hz. The source current is given by the following relation. Find the value of 2nd harmonic source current amplitutue. a) 11.25 A b) 0.256 A c) 2.69 sin (ωt – 45) A d) 0 A Answer: d Explanation: 2nd harmonics are absent in a 3-phase full converter, it has only odd number of harmonics i.e. 3rd, 5th etc. →↓

59. Which of the below mentioned converter can operate in both 3-pulse and 6-pulse Answer: c Explanation: Semi-converters can operate in 6-pulse modes gating the SCRs at appropriate intervals.

60. A M-6 controlled converter or 6-pulse half-wave controlled converter is obtained by using a transformer having a) a double delta connected secondary winding b) a double star connected secondary winding c) a double delta connected primary winding d) 6-windings on both primary and secondary sides Answer: b Explanation: M-6 requires a transformer having a delta connected primary and a double star connected secondary such that 6 SCRs are connected to it on the secondary side.

61. Dual converters provide a) two quadrant operation b) three quadrant operation c) four quadrant operation d) none of the mentioned Answer: c Explanation: Dual converters provide four quadrent operation, which means voltage can be positive or negative and so can be the current. Hence, AC-DC, DC-AC any converter configuration can be used.

62. A dual converters has a) two full converters in series b) two half converters in series c) two full converters in anti-parallel d) two half converters in anti-parallel Answer: c Explanation: Dual converters have two full converters connected in anti-parallel which provides a four quadrant operation.

→63. The major advantage of using dual converters is that a) it is cheaply available b) it has better pf c) no mechanical switch is required to change the mode of operation d) its operating frequency is very high Answer: c Explanation: No mechanical arrangement is required to change from inverter to converter and converter to inverter, which was required in earlier methods

64. The four quadrant operation of dual converters can be obtained by Answer: c Explanation: The four quadrant operation can be obtained simply by adjusting appropriate values of firing angles for both the connected converters.

65. A single full converter alone can given a a) four quadrant operation b) three quadrant operation c) two quadrant operation d) none of the mentioned Answer: c Explanation: A single full converter alone gives two quadrent operation, hence for all four quadrant operation two full converter circuits are connected in anti-parallel.67. In the below given circuit, the right side converter C2 operates in the ___ and ___ a) second, fourth b) first, fourth c) second, third d) first, third Answer: c Explanation: The C2 converter will supply the load current in direction opposite to that supplied by the converter C1. For converter C2, when α> 90 it operates in 2nd quadrant and if α<90 both current and voltage are negative, C2 is in inverter mode and operates in 3rd quadrant.

67. In the below given circuit, the right side converter C2 operates in the ___ and ___ a) second, fourth b) first, fourth c) second, third d) first, third Answer: c Explanation: The C2 converter will supply the load current in direction opposite to that supplied by the converter C1. For converter C2, when α> 90 it operates in 2nd quadrant and if α<90 both current and voltage are negative, C2 is in inverter mode and operates in 3rd quadrant.

67. In the below given circuit, the right side converter C2 operates in the ___ and ___ a) second, fourth b) first, fourth c) second, third d) first, third Answer: c Explanation: The C2 converter will supply the load current in direction opposite to that supplied by the converter C1. For converter C2, when α> 90 it operates in 2nd quadrant and if α<90 both current and voltage are negative, C2 is in inverter mode and operates in 3rd quadrant.

↓68. Name the below given circuit. a) Single-phase dual converter circulating current type b) Single-phase dual converter non-circulating current type c) Three-phase dual converter non-circulating current type d) Three-phase dual converter circulating current type Answer: b Explanation: The circuit is a single phase dual converter circuit. As the there is no reactor (inductor) in series, it is a non-circulating type.

69. For a single-phase dual converter, with converters C1 and C2 connected in antiparallel, which relation among the following is true to keep the average voltages from C1 and C2 equal? C1 and C2 have firing angles α1 and α2 respectively. a) α1 = α2 b) α1 + α2 = 360° c) α1 + α2 = 180° d) none of the mentioned Answer: c Explanation: By maintaining α1 + α2 = 180°, one converter can operate as converter and another as an inverter hence, the average output voltages are equal . This can be proved as follows Vom cos α1 = Vom cos α2 Vom = 2Vm/π . . (Vm for both the converters is same) cos α1 = cos α2 cos α1 = cos (180 – α2) α1 + α2 = 180°.

by a) connecting a series reactor 70. In non-circulating current mode dual converters, the circulating current is avoided b) maintaining α1 + α2 = 180° c) operating only one converter d) adding an extra SCR Answer: c Explanation: Reactor is added in circulating current mode not in non-circulating mode. The circulating current is avoided by using only one of the converters.

→71. In circulating current mode dual converters, the circulating current is avoided by a) connecting a series reactor b) maintaining α1 + α2 = 180° c) operating only one converter d) adding an extra SCR Answer: a Explanation: Reactor is added in circulating current mode not in non-circulating mode. The reactor limits the current to a reasonable value.

72. Choose the correct statement a) Circulating current exists only in circulating current mode b) Circulating current exists only in non-circulating current mode c) Circulating current exists in both the circulating and non-circulating current mode d) none of the mentioned Answer: c Explanation: The circulating current does exist in both the converter circuits, but it is avoided by using a reactor in non-circulating type and by operating only one circuit in case of circulating type.

73. What causes circulating current in dual converters? c) Out of phase voltages from both the converters d) none of the mentioned Answer: c Explanation: In case of practical dual converters, the voltages from both the converter circuits though equal in magnitude is out of phase. This indifference in voltages causes circulating currents to flow.

74. Name the below given circuit. a) Single-phase dual converter with circulating current type b) Single-phase dual converter with non-circulating current type c) Three-phase dual converter with non-circulating current type d) Three-phase dual converter with circulating current type Answer: c Explanation: The circuit is a three phase dual converter circuit. As the there is no reactor (inductor) in series, it is a non-circulating type.

75. In case of three-phase dual converter, one of the converter circuits is fired at an angle of 60°. For both the converter circuits to have equal average output voltage, what is the value of the firing angle for the other converter circuit? a) 60° b) 120° c) 100° d) Insufficient data Answer: b Explanation: For equal average output voltage, α1 + α2 = 180°

76. In case of circulating current type dual converters, the reactor is inserted between d) no reactor is used in case of circulating type dual converter Answer: c Explanation: Reactor (inductance) is added in circulating current mode between both the converters. The reactor limits the current to a reasonable valu