Power Electronics MCQs Quiz Multiple Choice Questions & Answers Page 2

51. The square wave operation of 3 phase VSI lines contains the harmonics. The amplitudes are

1. Inversely proportional to their harmonic order
2. Not related to their harmonic order
3. Directly proportional to their harmonic order
4. None of these

52. The switching function of semiconductor devices can be characterized with

1. Frequency only
2. Duty ratio and frequency
3. Duty ratio only
4. Duty ratio, frequency and time delay

53. The triac is .....................

1. A four-terminal device
2. Not a thyristor
3. Like a bidirectional SCR
4. Answers (1) and (2)

54. The UJT may be used as ............................

1. A sawtooth generator
2. A rectifier
3. Am amplifier
4. None of the above

55. The V-I characteristics for a triac in the first and third quadrants are essentially identical to those of ........................ in its first quadrant

1. SCR
2. UJT
3. Transistor
4. none of the above

56. To turn on UJT, the forward bias on the emitter diode should be ...................... the peak point voltage

1. Equal to
2. More than
3. Less than
4. None of the above

57. TRIAC is a semiconductor power electronic device which contains

1. Two SCRs connected in parallel
2. Two SCRs connected in series
3. Two SCRs connected in reverse parallel
4. Two BJTs connected in series

58. Under harmonic free load voltages, the 3 phase VSI

1. Does not contains third harmonic
2. Does not contains fifth harmonic
3. Does not contains second harmonic
4. Does not contains seventh harmonic

59. Unipolar modulation is generally used in

1. AC - DC converters
2. DC - AC converters
3. AC - AC converters
4. DC - DC converters

60. Very large values of modulation index (greater than 3.24 lead to)

1. Sine AC output voltage
2. Triangular AC output voltage
3. Square AC output voltage
4. Trapezoidal AC output voltage

61. Voltage commutation circuit can be converted into a current commutation by interchanging the positions of

1. Capacitor and SCR
2. Inductor and capacitor
3. Diode and capacitor
4. Capacitor and load

62. When a UJT is turned ON, the resistance between emitter terminal and lower base terminal.................................

1. Is decreased
2. Is increased
3. Remains the same
4. None of the above

63. When the emitter terminal of a UJT is open, the resistance between the base terminal is generally .......................

1. Low
2. Extremely low
3. High
4. None of the above

64. When the temperature increases, the inter-base resistance (RBB) of a UJT ......................

1. Decreases
2. Remains the same
3. Increases
4. None of the above

65. When the temperature increases, the intrinsic stand off ratio ....................

1. Decreases
2. Essentially remains the same
3. Increases
4. None of the above

66. Which of the following is not a characteristic of UJT?

1. Negative resistance
2. Peak-point voltage
3. Intrinsic stand off ratio
4. Bilateral conduction

67. With increase in firing angle

1. Harmonic distortion increases and quality of input current decreases
2. Harmonic distortion decreases and quality of input current increases
3. Both harmonic distortion and quality of input current increases
4. Both harmonic distortion and quality of input current decreases

68. A crowbar is a circuit which is used to protect a

1. current sensitive load from excessive dc power supply output voltages
2. voltage sensitive load from excessive ac power supply output voltages
3. voltage sensitive load from excessive dc power supply output voltages
4. current sensitive load from excessive ac power supply output voltages

69. A diac has ................... terminals

1. Three
2. Four
3. Two
4. None of the above

70. A diac has …………… pn junctions

1. Two
2. Three
3. Four
4. None of the above

71. A diac has ……………… semiconductor layers

1. Two
2. Four
3. Three
4. None of the above

72. A diac is …………………. switch

1. A d.c.
2. A mechanical
3. An c.
4. None of the above

73. A diac is simply ………………

1. A three junction device
2. A triac without gate terminal
3. A single junction device
4. None of the above

74. A diac is turned on by …………………

1. Gate voltage
2. Gate current
3. A breakover voltage
4. None of the above

75. A full wave rectifier with resistive load produces

1. Third harmonic
2. Fifth harmonic
3. Second harmonic
4. Do not produce harmonics

76. A triac can pass a portion of …………… half-cycle through the load

1. Only negative
2. Both positive and negative
3. Only positive
4. None of the above

77. A triac has …………….. semiconductor layers

1. Three
2. Four
3. Two
4. Five

78. A Triac has three terminals viz ………………

1. Two main terminal and a gate terminal
2. Cathode, anode, gate
3. Drain, source, gate
4. None of the above

79. A triac is a …………. switch

1. Unidirectional
2. Mechanical
3. Bidirectional
4. None of the above

80. A triac is equivalent to two SCRs …………..

1. In series
2. In inverse-parallel
3. In parallel
4. None of the above

81. A UJT has ……………….

1. One pn junction
2. Three pn junctions
3. Two pn junctions
4. None of the above

82. A UJT is sometimes called …………. diode

1. High resistance
2. Single-base
3. Low resistance
4. Double-base

83. AC power in a load can be controlled by using

1. two SCRs in series
2. three SCRs in series
3. two SCRs in parallel opposition
4. four SCRs in series

84. After peak point, the UJT operates in the ……………. region

1. Saturation
2. Negative resistance
3. Cut-off
4. None of the above

85. An SCR can be used

1. for power control
2. for speed control of dc shunt motor
3. as static conductor
4. all of these

86. An SCR is made up of silicon because

1. silicon has small leakage current than germanium
2. silicon has small leakage voltage than germanium
3. silicon has large leakage current than germanium
4. silicon has large leakage voltage than germanium

87. Between the incoming and outgoing devices in voltage commutation

1. Small overlapping operation
2. No overlapping operation
3. Large overlapping takes place
4. None of these

88. Between the peak point and the valley point of UJT emitter characteristics we have ………….. region

1. Negative resistance
2. Cut-off
3. Saturation
4. None of the above

89. Double fourier series analysis of PWM is

1. Three dimensional functions
2. One dimensional functions
3. Two dimensional functions
4. All of these

90. During the commutation period in 3 phase converter, overlap time is

1. Dependent on the voltage
2. Dependent on both the load current and load voltage behind the short circuit current
3. Dependent on the load current
4. Independent on both the load current and load voltage

91. For a buck converter to reduce the conduction losses in diode

1. A low on – resistance switch can be added in parallel
2. A high on – resistance switch can be added in series
3. A high on – resistance switch can be added in parallel
4. A low on – resistance switch can be added in series

92. For bidirectional operation of converters

1. A parallel combination of controllable switch and capacitor is used
2. A series combination of controllable switch and a diode is used
3. A parallel combination of controllable switch and a diode is used
4. A series combination of controllable switch and a capacitor is used

93. For similar carrier and modulating signals, the line current used in CSI is

1. Identical to line current in VSI
2. Identical to phase voltage in VSI
3. Identical to line voltage in a VSI
4. Identical to phase voltage in CSI

94. For sweptfrequency measurements, the input impedance of the mismatched transmission line would vary with frequency as the electrical length ofthe transmission line would

1. Remains same with change in frequency
2. Increase with frequency
3. Decrease with frequency
4. Either (a or b)

95. Harmonics in 3 phase inverters can be reduced by using

1. Active filter
2. Both passive and active filters
3. Passive filter
4. None of these

96. In a 3 phaseCSI, if the required line output voltages are balanced and 120 degreeout of phase, then the chopping angles are used to eliminate only theharmonics at frequencies

1. 7
2. 11
3. 5
4. All of these

97. In a load commutated DC – DC chopper, the capacitor has a

1. Symmetric rectangular voltage across itself
2. Symmetric trapezoidal voltage across itself
3. Symmetric triangular voltage across itself
4. Symmetric sinusoidal voltage across itself

98. In a three phase converter, the number of notches per cycle is

1. Three
2. Six
3. One
4. Nine

99. In a UJT, the p-type emitter is ……………. doped

1. Heavily
2. Moderately
3. Lightly
4. None of the above

100. In bipolar modulation, the carrier is symmetric about zero with amplitude equal to Cm, the PWM output

1. Switches between – 1 / 2 and + 1 / 2
2. Switches between – 1 and + 1
3. Zero
4. Switches between 0 and + 1