Power electronics MCQ #9

1) Third harmonic can be eliminated if pulse signal width is equal to ____ in single pulse modulation of PWM inverter.

      (a) 30 degree

      (b) 60 degree

      (c) 120 degree

      (d) 150 degree

2) In a thyristor, the holding current is

      (a) more than latching current

      (b) less than latching current

      (c) equal to latching current

      (d) equal to zero

3) An SCR can be turned off by

      (a) interrupting its anode current

      (b) reversing the polarity of its anode-cathode voltage

      (c) low current drop-out

      (d) all of the above

4) The dv/dt effect in SCR can result in

      (a) low capacitive charging current

      (b) false triggering

      (c) increased junction capacitance

      (d) high rate of rise of anode voltage

ANSWER: | 1-C | 2-B | 3-A | 4-B |

5) UJT is known as

      (a) voltage control device

      (b) current control device

      (c) relaxation oscillator

      (d) none of the above

6) A power chopper converts

      (a) ac to dc

      (b) dc to dc

      (c) dc to ac

      (d) ac to ac

7) An advantage of using freewheeling diode with bridge type ac/dc converter is

      (a) regenerative breaking

      (b) reliable speed control

      (c) improved power factor

      (d) reduced cost of the system

8) In buck-boost converter what is the maximum value of the switch utilization factor?

      (a) 1

      (b) 0.75

      (c) 0.5

      (d) 0.25

ANSWER: | 5-a | 6-b | 7-c | 8-b |

9) What is the waveform of the current following through the diode in a buck-boost converter?

      (a) square wave

      (b) trapezoidal wave

      (c) triangular wave

      (d) sinusoidal wave

10) AC voltage regulators are widely used in

      (a) traction drive

      (b) slip power recovery scheme of slip ring induction motor

      (c) synchronous motor drives

      (d) fan drive

11) In a current source inverter, if frequency of voltage is f, then frequency of output voltage is

      (a) f

      (b) f/2

      (c) f/4

      (d) f/8

12) In case of voltage source inverter, freewheeling diode can be avoided for the load of

      (a) inductive nature

      (b) capacitive nature

      (c) resistive nature

      (d) back emf nature

ANSWER: | 9-b | 10-d | 11-a | 12-a |

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