4. The load voltage of a chopper can be controlled by varying the
Correct : A. duty cycle
5. The values of duty cycle (α) lies between
Correct : C. 0<=α<=1
6. If T is the time period for a chopper circuit and α is its duty cycle, then the
chopping frequency is
Correct : D. α/Ton
7. AC voltage controllers convert
Correct : C. Fixed ac to variable ac
8. In AC voltage controllers the
Correct : A. variable ac with fixed frequency is obtained
9. The GTO (gate turn-off thyristor) is a
Correct : A. p-n-p-n device
10. The GTO can be turned off
Correct : B. by a negative gate pulse
11. In GTO The anode current is ideally limited by the
Correct : C. load Impedance
12. In a GTO the n+ layer forms the
Correct : C. cathode
13. The turn-off gain βoff of the GTO is given by
Correct : B. Ia/Ig
14. A GTO can be represented by two transistors T1 & T2. The current gain of
both transistors are α1 and α2 respectively. A low value of gate current requires
Correct : B. low value of α1 and high value of α2
15. Gold doped GTOs have _____________ as compared to the conventional
GTOs
Correct : B. low negative gate current requirement
16. Latching current for the GTOs is ________ as compared to CTs
(Conventional thyristors).
Correct : A. more
17. In case of the two-transistor model (T1 & T2) of GTO with anode-short,
the anode-short is placed between the
Correct : C. emitter of T1 & base of T1
18. Choose the correct statement:
GTOs have _________ as compared to the CTs.
Correct : D. faster switching speed
19. A single-phase voltage controller, using one SCR in anti parallel with a diode, feeds a load R and Vs = 230 V. For a firing angel of 90° for the SCR, the PMMC voltage connected across R would read
Correct : C. –51.8 V
20. A single phase voltage controller has input of 230 V and a load of 15 Ω resistive. For 6 cycles on and 4 cycles off, determine the average value of SCR current.
Correct : C. 4.14 A
21. An IGBT has three terminals called:
Correct : D. Collector, Emitter and Gate
22. The function of snubber circuit connected across the SCR is to:
Correct : A. Suppress dv/dt
23. An UJT exhibits negative resistance region:
Correct : B. Between peak and valley point
24. For dynamic equalizing circuit used for series connected SCRs, the
choice of C is based on:
Correct : A. Reverse recovery characteristics
25. A four quadrant operation require:
Correct : B. Two full converters connected back to back
26. In a circulating-current type of dual converter, the nature of the
voltage across the reactor is:
Correct : A. Alternating
27. The frequency of the ripple in the output voltage of 3-phase semiconductor depends on:
Correct : D. Firing angle and the supply frequency
28. A single-phase full bridge inverter can operate in load-commutation mode in case load consists of:
Correct : B. RLC underdamped
29. Practical way of obtaining static voltage equalization in series connected SCRs is by the use of:
Correct : C. Resistors of the same value across each SCR
30. A resistor connected across the gate and cathode of an SCR in a
circuit increases its
Correct : C. Noise Immunity
31. In thyristor, holding current is:
Correct : B. less than the latching current
32. When a thyristor turn on, the gate drive:
Correct : A. can be turned down but thyristor remains in on position
33. A thyristor can be termed as:
Correct : A. DC switch
34. During forward blocking state, a thyristor is associated with:
Correct : B. low current and large voltage
35. Turn-off time of an SCR is measured from the instant:
Correct : A. anode current becomes zero
36. A forward voltage can be applied to an SCR after its:
Correct : B. gate recovery time
37. In a thyristor, the magnitude of the anode current will:
Correct : D. no change with the variation of the gate current
38. In a thyristor, ratio of latching current to holding current is:
Correct : C. 2.5
39. Once SCR starts conducting a forward current, its gate losses
control over:
Correct : C. anode circuit voltage and current
40. On state voltage drop across thyristor used in a 230 V supply
system is of the order:
Correct : C. 1- 1.5 V
41. Silicon controlled rectifier can be turned on
Correct : A. By applying a gate pulse and turned off only when current becomes zero
42. TRIAC is a semiconductor power electronic device which
contains
Correct : A. Two SCR’s connected in reverse parallel
43. An SCR can be used
Correct : D. all of these
44. A crowbar is a circuit which is used to protect a
Correct : A. voltage sensitive load from excessive dc power supply output voltages
45. The input current waveform of a bridge controlled rectifier when the
load is perfectly filtered is
Correct : B. Square wave
46. The advantage of using free – wheeling diode in half controlled bridge converter is that
Correct : A. There is always a path for the dc current independent of the ac line
47. Harmonics in 3 phase inverters can be reduced by using
Correct : C. Both passive and active filters
48. The most suited gate pulses given to the AC regulator with R – L load can be in the form of
Correct : C. A train of pulses
49. With increase in firing angle,
Correct : B. Harmonic distortion increases and quality of input current decreases
50. The power MOSFET device is a
Correct : B. Voltage controlled unipolar device
51. A static induction thyristor
Correct : B. is a minority carrier device
52. A single phase step up cycloconverter changes 50 Hz to 100 Hz.
Then one half wave of input will give rise to
Correct : B. one full wave of output
53. Which of these triggering methods for series connected SCRs use LASCR in the gate circuit?
Correct : D. Optical triggering
54. A 3 phase ac regulator uses
Correct : B. 6 thyristors
55. The average load current supplied by a thyristor depends on
Correct : A. firing angle
56. In a 3 phase full wave regulator feeding a star connected resistance load the input voltage is 400 V line to line. The firing angle is 160°. The line to line output voltage would be
Correct : D. Zero
57. A thyristor is reverse biased. A positive gate pulse is applied. The
thyristor
Correct : C. will not turn on
58. A single phase series converter is used for
Correct : A. high voltage output
59. A single phase half wave controlled rectifier circuit has an R-L load.
A freewheeling diode is also in the circuit. When freewheeling diode is conducting the SCR
Correct : B. is reverse biased
60. An SBS can conduct is one direction only.
Correct : B. False
61. The number of doped regions in a SCR is
Correct : C. 4
62. All inverters use forced commutation.
Correct : B. False
63. In 180° mode of operation of a 3 phase bridge inverter, two thyristors conduct at one time.
Correct : B. False
64. A reactor is used in a dual converter operating in non-circulating current mode.
Correct : B. False
65. Which of these commutation methods uses an auxiliary SCR?
Correct : C. Class C
66. A thyristor needs protection against
Correct : C. both high and high
67. In integral cycle control the period of on-off control should be more than mechanical time constant of load.
Correct : B. False
68. A single phase half wave converter is feeding a resistive load. Just before triggering, the voltage across thyristor is
Correct : C. same as input voltage
69. In a 3 phase full wave regulator feeding resistance load connected in star, the possible range of firing angle a is
Correct : B. 0 ≤ ≤ 150°
70. A 3 phase bridge inverter is fed by 400 V battery. The load is star connected and has a resistance of 10 Ω per phase. The peak value of load current is
Correct : B. 20 A
71. In an SCR, the anode current is controlled by
Correct : B. external circuit only
72. The rated current of a thyristor is 40 A. The holding current will
be about :
Correct : D. 10 mA
73. The number of leads in a power semiconductor diode are
Correct : A. 2
74. If ton and toff are the turn on and turn off times of a thyristor, then
Correct : C. toff >> ton
75. UJT can be used for
Correct : A. firing an SCR
76. In a full wave converter (M - 2 connection) feeding an RLE load, the peak thyristor current is 15 A. For fully controlled bridge circuit and same load current, the peak thyristor current will be
Correct : C. 15 A
77. During forward blocking state of SCR, the voltage and current
respectively are
Correct : C. high and low
78. In a 3 phase full converter the average load current is 150 A.
The average thyristor current is
Correct : C. 50 A
79. The two modes of operation of three phase bridge inverters are
known as
Correct : C. 120° mode and 180° mode
80. In a 3 phase fully controlled converter the firing frequency is
Correct : B. 6 times the line frequency
81. Two thyristor of same rating and same specifications
Correct : C. may have equal or unequal turn on and turn off periods
82. The amount of dielectric heating is inversely proportional to
frequency.
Correct : B. False
83. A semiconverter feeding an RLE load operates in
Correct : A. first quadrant only
84. The phenomenon of overlap in converter operation due to
Correct : B. source inductance
85. An SCR is triggered at 40° in the positive half cycle only. The average anode current is 50 A. If the firing angle is changed to 80°, the average anode current is likely to be
Correct : C. less than 50 A but more than 25 A
86. In a single phase full wave regulator, the firing angles in the
positive and negative half cycles are generally
Correct : A. Equal
87. Current derating may be necessary when thyristors are
operating in
Correct : B. Parallel
88. A 3 kV circuit uses SCR of 800 V rating. If derating is 25%, the number of SCRs in series is
Correct : B. 5
89. The terminals of a power MOSFET are called
Correct : B. source, gate, drain
90. In a thyristor the gate current is increased, then
Correct : C. anode current will remain constant
91. Thyristors are not suitable for logic circuits.
Correct : B. False
92. The number of leads in an SCR are :
Correct : B. 3
93. A thyristor has a turn on time of 6 μs. If the anode circuit is
inductive, the turn on time will be
Correct : C. more than 6 μs
94. The total number of leads in reverse conducting thyristor is
Correct : B. 3
95. Assertion (A): A TRIAC is a bidirectional SCR.
Reason (R): A TRIAC is a four layers 3 terminal device and can conduct in both directions.
Correct : A. Both A and R are correct and R is correct explanation of A
96. A semi converter operates in two quadrants.
Correct : B. False
97. A single phase half wave rectifier is feeding a resistive load. When a = 0, dc output voltage is 127 V. When a = 30°, the dc output voltage will be about
Correct : B. 120 V
98. A single phase full wave regulator feeds R-L load. The best gating signal is
Correct : C. pulse train
99. A 3 phase induction motor is fed by a 3 phase ac regulator to change the stator voltage. The variation in speed will be more if
Correct : B. load torque increases with increase in speed
100. The waveshape of output voltage of half bridge inverter is