2. A single phase half wave converter is used to charge a battery of 50
V. The thyristor is continuously fired by dc signal. Input voltage is v = Vm sin ωt. The thyristor will conduct
Correct : B. when v > 50 V
3. In a 3 phase semiconverter, the firing frequency is
Correct : B. 3 times the line frequency
4. A thyristor has its gate current zero. To start conduction the forward voltage must exceed
Correct : B. forward breakover voltage
5. On load tap changer using thyristors can have a maximum of two
taps.
Correct : B. False
6. In a single phase full wave ac regulator, the firing angles in the two half cycles
Correct : A. are always equal
7. As regards single phase dual converter
Correct : B. the change in operation from one quadrant to the other is faster in circulating current mode
8. To protect an SCR against high di/dt we connect
Correct : D. an inductance in series with SCR
9. In a parallel inverter
Correct : A. the commutating capacitor C is across the primary of transformer and the load is across the secondary
10. A single phase fully controlled bridge rectifier is fed by a source having inductance. The output voltage during overlap period is
Correct : C. zero
11. Which of these commutation methods is used in AC-DC converters?
Correct : D. Class F
12. When thyristors are connected in series and parallel, it may be
necessary to have :
Correct : C. both current and voltage derating
13. A single phase half wave regulator uses
Correct : C. 1 diode and 1 thyristor
14. A high frequency series inverter consists of
Correct : B. a number of series inverters in parallel
15. In an SCR, the anode current consists of
Correct : C. both electrons and holes
16. A 3 phase semiconverter when supplying a certain load has a rectification efficiency of 0.6 and transformer utilisation factor of 0.35. When a 3 phase full converter is supplying the same load
Correct : A. rectification efficiency will be more than 0.6 and TUF will be more than 0.35
17. In a fully controlled bridge converter feeding a resistive load the waveshapes of output voltage and output current
Correct : A. are always similar
18. Natural commutation can be used in :
Correct : B. AC circuits only
19. In a thyristor the gate lead is connected to the n layer near the p layer to which anode is connected.
Correct : B. False
20. McMurray Bedford full bridge inverter uses
Correct : C. complementary commutation
21. Which of these commutation methods uses an external pulse for commutation?
Correct : C. Class E
22. A relaxation oscillator uses
Correct : B. UJT
23. In a 3 phase bridge inverter with 180° mode of operation the numbers of thyristors conducting at one time are
Correct : C. 3
24. A single phase full converter can operate in
Correct : C. 2 quadrants
25. In a 3 phase semiconverter, the freewheeling diode comes into operation only if firing angle
Correct : C. is more than 60°
26. The two-transistor model of a thyristor consists of two transistors :
Correct : A. one n-p-n and other p-n-p
27. In a thyristor, the forward breakover voltage
Correct : C. decreases as gate current is increased
28. The gate lead, in a thyristor is welded to :
Correct : B. n layer nearest to the anode terminal
29. In a 3 phase full converter, the firing angle is less than 60°. The instantaneous output voltage
Correct : A. will have positive part only
30. In a stud mounted SCR, the stud is
Correct : C. in electrical and thermal contact with heat sink
31. In a full bridge inverter, the waveshape of load current depends on
Correct : A. relative values of load parameters R, L and C
32. The characteristics of which device has peak point and valley point
Correct : B. PUT
33. In an ordinary thyristor
Correct : C. turn off time is more than turn on time
34. Thyristors connected in series need :
Correct : C. both static and dynamic equalizing circuits
35. If 4 quadrant operation is required we need
Correct : A. dual converter
36. Which of the following devices exhibit negative resistance characteristics?
Correct : C. Both UJT and PUT
37. As compared to BJT, MOSFET has
Correct : A. lower switching losses and higher conduction losses
38. A single phase bridge type cycloconverter uses eight thyristors.
Correct : A. True
39. In the basic series inverter circuit, the source supplies load
Correct : A. only in positive half cycle
40. In a parallel inverter
Correct : A. each thyristor is turned on twice during each cycle
41. The specified maximum junction temperature of thyristors is about
Correct : C. 120°C
42. A half bridge inverter
Correct : A. needs a 3 wire dc supply
43. A 3 phase series inverter has
Correct : B. 6 thyristors
44. Surge current rating of an SCR is
Correct : B. the maximum non-repetitive surge current in which the SCR can withstand
45. Which of the following devices has metal-silicon junction?
Correct : B. Schottky diode
46. In a series inverter
Correct : B. the commutating elements L and C are in series with load
47. In thermal ohm's law, temperature and power loss are analogous to
Correct : B. I and V respectively of ohm's law
48. A string of series connected thyristors needs equalising circuit to
Correct : C. ensure proper voltage division
49. The minimum gate current which can turn on SCR is called :
Correct : A. trigger current
50. Which of these devices can conduct in both directions?
Correct : C. SBS
51. A 3 phase dual converter consists of
Correct : C. two 3 phase full converters connected in antiparallel
52. Which inverter circuit uses coupled inductors?
Correct : D. Both (b) and (c)
53. When an SCR is in forward blocking mode, the applied voltage
Correct : A. appears across only one junction
54. In sinusoidal PWM for inverters the width of different pulses are not
the same.
Correct : A. True
55. The symbol in figure is for
Correct : B. PUT
56. Mc Murray full bridge inverter uses
Correct : C. 8 thyristors and 4 diodes
57. In a 3 phase full converter, the instantaneous output voltage can
have negative part if
Correct : C. firing angle is more than 60°
58. The circuit in figure is for
Correct : D. complementary commutation
59. The main utility of SCR is due to
Correct : A. gate triggering
60. The symbol in figure is for
Correct : C. Shockley diode
61. he circuit in figure is for
Correct : B. self commutation
62. A single phase half bridge inverter required to feed R-L loads needs
Correct : C. 2 thyristors and two diodes
63. In a single phase semi-converter the number of thyristors is
Correct : D. 2
64. Which of these commutation methods uses two leads?
Correct : D. Class D
65. The difference between resonant commutation and self commutation is
Correct : A. both these methods are based on entirely different principles
66. In a series inverter
Correct : A. the commutating elements carry full load current
67. Which of these devices is used in relaxation oscillator?
Correct : A. UJT
68. Two thyristors of the same rating and specifications
Correct : D. may have equal or different on state voltage drops and equal or different forward blocking voltage's for the same current
69. A single phase full bridge inverter for R-L loads needs
Correct : B. 4 thyristors and 4 diodes
70. In the basic series inverter circuit, the source current has
Correct : B. high harmonic content
71. Thyristors connected in series need static and dynamic stabilizing
circuits.
Correct : A. True
72. When a UJT is used as a relaxation oscillator the current should be
Correct : C. more than Ip but less than Iv
73. In a single phase full wave regulator feeding a resistive load, the
average current over one full cycle
Correct : A. is always zero
74. Which of these devices have four layers?
Correct : B. SCR
75. When UJT is used for thyristor gate signal, the output of UJT is
Correct : C. saw tooth wave
76. In rectifier circuits, the rms value of ac component of output voltage is equal to
Correct : D. (Vrms2- Vdc2)0.5
77. A UJT exhibits negative resistance characteristics when
Correct : C. Current is more than peak current but less than valley current
78. In rectifier circuits, ripple factor RF and form factor FF are related as