Monday, October 13, 2025

The voltage control in inverters can be done by

 41. The commutation in which current is transferred from one thyristor to another is known as

A) Natural commutation
B) Load commutation
C) Current commutation
D) Complementary commutation
Answer: D) Complementary commutation

42. The turn-off time of SCR mainly depends on

A) Gate current
B) Junction temperature
C) Reverse voltage
D) Forward current
Answer: B) Junction temperature

43. The free-wheeling diode in a converter reduces

A) Harmonics in supply current
B) Voltage spikes across SCR
C) Load current ripple
D) All of these
Answer: D) All of these

44. The device that combines the features of MOSFET and BJT is

A) TRIAC
B) DIAC
C) IGBT
D) GTO
Answer: C) IGBT

45. In class B commutation, the capacitor is charged

A) Externally
B) Through supply
C) By load current
D) By forced current
Answer: C) By load current

46. The main purpose of using PWM inverters is to

A) Increase current
B) Reduce harmonic distortion
C) Reduce frequency
D) Decrease output voltage
Answer: B) Reduce harmonic distortion

47. The average output voltage of a single-phase full converter is

A) Vavg=2Vmπcos⁡αV_{avg} = \frac{2V_m}{\pi} \cos\alpha
B) Vavg=Vmπ(1+cos⁡α)V_{avg} = \frac{V_m}{\pi} (1+\cos\alpha)
C) Vavg=Vm2cos⁡αV_{avg} = \frac{V_m}{2} \cos\alpha
D) Vavg=Vmcos⁡αV_{avg} = V_m \cos\alpha
Answer: A) 2Vmπcos⁡α\frac{2V_m}{\pi} \cos\alpha

48. In a full converter with α = 60°, the output voltage is

A) Positive
B) Zero
C) Negative
D) Maximum
Answer: A) Positive

49. The power factor of a converter becomes leading when

A) α < 90°
B) α = 90°
C) α > 90°
D) α = 0°
Answer: C) α > 90°

50. The class F commutation is also known as

A) Load commutation
B) Resonant pulse commutation
C) Current commutation
D) Voltage commutation
Answer: B) Resonant pulse commutation

51. The on-state voltage drop of a power diode is typically

A) 0.1 V
B) 0.7 V
C) 1–1.2 V
D) 5 V
Answer: C) 1–1.2 V

52. The current rating of SCR is limited by

A) Junction temperature
B) Reverse recovery time
C) dv/dt
D) di/dt
Answer: A) Junction temperature

53. The device that can operate in all four quadrants is

A) TRIAC
B) SCR
C) GTO
D) BJT
Answer: A) TRIAC

54. The storage time of a BJT is important because it affects

A) ON-state losses
B) Turn-off switching speed
C) Gate triggering
D) Thermal stability
Answer: B) Turn-off switching speed

55. The dynamic dv/dt rating of a thyristor is improved by

A) Using snubber circuit
B) Increasing junction area
C) Reducing gate resistance
D) Increasing frequency
Answer: A) Using snubber circuit

56. The inverter output voltage waveform is ideally

A) Pure sine wave
B) Square wave
C) Triangular
D) Pulse train
Answer: B) Square wave

57. The ripple factor of a single-phase full-wave rectifier is

A) 0.482
B) 1.21
C) 0.707
D) 0.812
Answer: A) 0.482

58. The control ratio of a DC chopper is

A) Vo/Vs=αV_o/V_s = \alpha
B) Vo/Vs=1−αV_o/V_s = 1 - \alpha
C) Vo/Vs=fV_o/V_s = f
D) None
Answer: A) Vo/Vs=αV_o/V_s = \alpha

59. The current in a step-down chopper is continuous if

A) Load inductance is large
B) Supply voltage is low
C) Frequency is high
D) Both A and C
Answer: D) Both A and C

60. The control circuit of an SCR provides

A) Power to the load
B) Triggering pulses
C) Reverse blocking
D) Cooling
Answer: B) Triggering pulses

61. The operation of a full converter in inverter mode requires

A) α < 90°
B) α = 90°
C) α > 90°
D) α = 0°
Answer: C) α > 90°

62. The main drawback of line commutation is

A) Requires AC supply
B) Not applicable to DC circuits
C) Low speed
D) Both A and B
Answer: D) Both A and B

63. The turn-on time of an SCR is mainly controlled by

A) Gate current amplitude
B) Gate voltage
C) Anode current
D) Reverse voltage
Answer: A) Gate current amplitude

64. A class E chopper operates in

A) First quadrant only
B) Second and fourth quadrants
C) All four quadrants
D) Third quadrant only
Answer: C) All four quadrants

65. The average output voltage of a step-up chopper is

A) Vo=Vs/(1−α)V_o = V_s/(1 - \alpha)
B) Vo=VsαV_o = V_s \alpha
C) Vo=Vs(1−α)V_o = V_s(1 - \alpha)
D) Vo=0V_o = 0
Answer: A) Vo=Vs/(1−α)V_o = V_s/(1 - \alpha)

66. In an inverter, commutation failure occurs due to

A) High dv/dt
B) Low supply voltage
C) Low current
D) Short circuit
Answer: B) Low supply voltage

67. The term “latching current” in SCR means

A) Minimum current to maintain conduction
B) Minimum current to initiate conduction
C) Reverse leakage current
D) None
Answer: B) Minimum current to initiate conduction

68. The device suitable for high voltage and low frequency operation is

A) SCR
B) MOSFET
C) IGBT
D) GTO
Answer: A) SCR

69. The forced commutation circuit requires

A) Energy storage element
B) Pure resistor
C) Only capacitor
D) None
Answer: A) Energy storage element

70. The voltage control of a DC motor using a chopper gives

A) High efficiency
B) Low efficiency
C) Constant speed
D) None
Answer: A) High efficiency

71. The device used in snubber circuit for SCR is

A) Inductor
B) Capacitor
C) Resistor
D) Both B and C
Answer: D) Both B and C

72. The frequency of operation of SCR is limited by

A) Turn-off time
B) Gate current
C) Load
D) Voltage rating
Answer: A) Turn-off time

73. The inverter which uses commutation by load current is

A) Voltage source inverter
B) Current source inverter
C) Resonant inverter
D) Single-phase inverter
Answer: B) Current source inverter

74. The purpose of soft switching in inverters is to

A) Reduce switching losses
B) Increase ripple
C) Increase voltage
D) Reduce load
Answer: A) Reduce switching losses

75. In a full converter, if load current is continuous, then each SCR conducts for

A) 90°
B) 120°
C) 180°
D) 360°
Answer: C) 180°

76. The firing angle in converters is measured from

A) Zero crossing of load current
B) Zero crossing of supply voltage
C) Peak voltage
D) None
Answer: B) Zero crossing of supply voltage

77. The output of a three-phase bridge rectifier is

A) Pure DC
B) Rippled DC
C) AC
D) Pulsating AC
Answer: B) Rippled DC

78. The input displacement factor of a converter equals

A) cos(α)
B) sin(α)
C) tan(α)
D) 1
Answer: A) cos(α)

79. The most suitable device for high switching frequency is

A) MOSFET
B) IGBT
C) SCR
D) TRIAC
Answer: A) MOSFET

80. The IGBT gate drive is similar to that of

A) BJT
B) MOSFET
C) SCR
D) TRIAC
Answer: B) MOSFET

81. The commutation circuits in inverters are used to

A) Turn on thyristors
B) Turn off thyristors
C) Change frequency
D) Control firing angle
Answer: B) Turn off thyristors

82. A current source inverter is best suited for

A) Induction motor drives
B) DC drives
C) Lighting loads
D) Heating loads
Answer: A) Induction motor drives

83. The conduction angle in a single-phase inverter with square wave output is

A) 60°
B) 90°
C) 120°
D) 180°
Answer: D) 180°

84. The maximum efficiency of a full converter occurs when

A) Firing angle = 0°
B) Firing angle = 90°
C) Firing angle = 180°
D) Current = 0
Answer: A) Firing angle = 0°

85. The major limitation of a GTO is

A) High gate current requirement for turn-off
B) Low voltage rating
C) Slow switching
D) Small power capacity
Answer: A) High gate current requirement for turn-off

86. In a chopper circuit, if duty cycle = 0.6, then output voltage =

A) 0.6 × Vs
B) 0.4 × Vs
C) 1.6 × Vs
D) None
Answer: A) 0.6 × Vs

87. The frequency of chopping is

A) 50 Hz
B) 60 Hz
C) Variable depending on control
D) Always fixed
Answer: C) Variable depending on control

88. The main function of commutation circuit is to

A) Turn OFF SCR
B) Turn ON SCR
C) Increase current
D) Decrease voltage
Answer: A) Turn OFF SCR

89. The inverter used for adjustable speed control of induction motor is

A) VSI
B) CSI
C) Both A and B
D) None
Answer: C) Both A and B

90. The diode used in inverter circuits is

A) Free-wheeling diode
B) Zener diode
C) Varactor diode
D) Photodiode
Answer: A) Free-wheeling diode

91. The feedback diode in a chopper provides

A) Path for load current during OFF period
B) Filtering
C) Protection
D) Reverse voltage
Answer: A) Path for load current during OFF period

92. The conduction losses are minimized by

A) Increasing ON resistance
B) Decreasing ON resistance
C) Reducing switching frequency
D) None
Answer: B) Decreasing ON resistance

93. The advantage of PWM over square wave inverter is

A) Less harmonic distortion
B) Simpler control
C) Higher current
D) None
Answer: A) Less harmonic distortion

94. The main advantage of current source inverter is

A) Short-circuit protection
B) Constant current output
C) Simpler commutation
D) All of these
Answer: D) All of these

95. The operation of a cycloconverter is based on

A) Phase control
B) Voltage control
C) Frequency control
D) Both A and C
Answer: D) Both A and C

96. The main application of cycloconverter is

A) Variable frequency AC drives
B) Battery charging
C) Lighting
D) Welding
Answer: A) Variable frequency AC drives

97. A GTO can be turned off by

A) Reverse voltage
B) Negative gate current
C) Forced commutation
D) Both A and B
Answer: B) Negative gate current

98. The turn-on losses are more significant in

A) SCR
B) MOSFET
C) IGBT
D) GTO
Answer: D) GTO

99. The voltage control in inverters can be done by

A) PWM
B) Cycloconverter
C) Phase control
D) Resonant circuits
Answer: A) PWM

100. The main disadvantage of PWM inverter is

A) High switching losses
B) High harmonics
C) Low efficiency
D) Poor voltage regulation
Answer: A) High switching losses

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