Power Electronics Multiple Choice Questions and Answers

61. Unipolar modulation is generally used in

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

62. 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

63. 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

64. 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

65. 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

66. 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

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

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

68. 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

69. 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

70. 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