About Power Electronics

Following are some of the multiple choice questions on the Power Electronics with answers that will help the students in developing their knowledge.

Power Electronics MCQ

1. Which device can be used in a chopper circuit?

  • BJT
  • MOSFET
  • GTO
  • ALL THE ABOVE

2. Which of the following statements is correct in connection with the inverters?

  • VSI and CSI both require feedback diodes
  • only CSI require feedback diodes
  • GTO is used in current source inverter
  • only VSI requires feedback diodes

3. The major advantage of using dual converters is that

  • it is cheaply available
  • no mechanical switch is required to change the mode of operation
  • it has better pf
  • its operating frequency is very high

4. In AC voltage controllers the

  • variable ac with fixed frequency is obtained
  • variable ac with variable frequency is obtained
  • variable dc with fixed frequency is obtained
  • variable dc with variable frequency is obtained

5. The values of duty cycle (α) lies between..........

  • 1<α<100
  • 0<=α<=1
  • 0>α>-1
  • 0<α<1

6. The shape of the output voltage waveform in a single PWM is

  • quasi-square wave
  • sine wave
  • triangular wave
  • square wave

7. In an inverter, if the fundamental frequency is 50 Hz, if third harmonic is eliminated, then frequencies of other components in the output voltage wave, in Hz, would be

  • 250, 350, 500, higher frequencies
  • 50, 250, 350, 500
  • 50, 250, 350, 550
  • none of these

8. The TRIAC is most sensitive in the _________ quadrants

  • 1st & 3rd with positive gate current
  • 1st with positive gate current & 3rd with negative gate current
  • 3rd with positive gate current & 1rd with negative gate current
  • 1st & 3rd with negative gate current

9. All the modern AC-DC converters are using GTOs instead of SCRs because

  • emperature is an issue in SCRs
  • Switching losses are high in SCRs
  • GTOs have reliable commutation
  • GTOs are cheaply available today

10. In case of TRC (Time Ratio Control), _________ is varied

  • firing angle
  • load cycle
  • duty cycle
  • load angle

11. A voltage source inverter is normally employed when

  • source inductance is large and load inductance is small
  • source inductance is small and load inductance is large
  • both source and load inductance are large
  • both source and load inductance are small

12. In the ___________ type of chopper, two stage conversions takes place.

  • AC-DC
  • AC link
  • DC link
  • None of the mentioned

13. Dual converters provide

  • two quadrant operation
  • three quadrant operation
  • four quadrant operation
  • none of the mentioned

14. In voltage source inverters

  • load voltage Vo depends on load impedance but load current Io does not depend on load impedance Z
  • both Vo and Io depends on Z
  • Io depends on Z but Vo does not depend
  • none of these

15. In the three-phase bridge inverter, each step consists of

  • 30°
  • 60°
  • 90°
  • will depend on firing angle

16. The load voltage of a chopper can be controlled by varying the..................

  • duty cycle
  • firing angle
  • reactor position
  • extinction angle

17. What are rheostats used for?

  • Control large amounts of heat
  • Vary large amounts of current
  • Control of a circuit's power rating
  • Vary a circuit's tolerance

18. A single-phase semi-converter is having continuous conduction, as such each thyristor will conduct for an angle of

  • α
  • π
  • α+π
  • π-α

19. In the continuous conduction mode if the firing angle of the converter is increased beyond _________ degrees the converter operates in the _______ mode.

  • 90, rectifier
  • 180, inverter
  • 90, inverter
  • 180, rectifier

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

  • Remains the same
  • Is decreased
  • Is increased
  • None of the above

21. The series-inverter control method is a/an..........

  • external frequency control method
  • external voltage control method
  • internal voltage control method
  • none of the mentioned

22. What is the duty cycle of a chopper ?

  • Ton/Toff
  • Ton/T
  • T/Ton
  • Toff x Ton

23. In a half wave bridge inverter circuit, the power delivered to the load by each source is given by

  • 2(Vs x Is)
  • Vs x Is
  • (Vs x Is)/2
  • None of the mentioned

24. For a single phase thyristor circuit with R load & firing angle α, the conduction angle can be given by

  • π+α
  • 2π+α
  • π-α
  • α

25. In a UJT, the p-type emitter is ___________ doped

  • Lightly
  • Heavily
  • Moderately
  • None of the above

26. What is the purpose of a resistor?

  • control current flow
  • allow current flow
  • prevent current flow
  • allow heat to build up

27. Cycloconverter converts

  • ac voltage to dc voltage
  • dc voltage to dc voltage
  • ac voltage to ac voltage at same frequency
  • ac voltage at supply frequency to ac voltage at load frequency

28. A PWM switching scheme is used with a three phase inverter to

  • reduce THD with modest filtering
  • minimize the load on DC side
  • increase the life of the batteries
  • reduce low order harmonics and increase high order harmonics

29. A UJT has ___________.

  • Two pn junctions
  • One pn junction
  • Three pn junctions
  • None of the above

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

  • High
  • Low
  • Extremely low
  • None of the above

31. In case of circulating current type dual converters, the reactor is inserted between

  • no reactor is used in case of circulating type dual converter
  • across the load
  • supply and converter
  • between the converters

32. The ____________ region in a power diode increases its reverse voltage blocking capacity.

  • pn junction
  • drift
  • potential barrier
  • depletion layer

33. The UJT may be used as ___________.

  • Am amplifier
  • A sawtooth generator
  • A rectifier
  • None of the above

34. The Gate-Drain transfer capacitance of a MOSFET has large value in the ________________ region and small value in the ________________ region.

  • saturation, ohmic
  • ohmic, saturation
  • triode, linear
  • ohmic, active

35. A diac has ___________ semiconductor layers

  • Three
  • Two
  • Four
  • None of the above

36. The device that does not have the gate terminal is ___________.

  • Triac
  • FET
  • SCR
  • Diac

37. A single phase half wave controlled converter always operates in the ________ conduction mode.

  • continuous
  • discontinuous
  • both (a) and (b)
  • none of the above

38. The four quadrant operation of dual converters can be obtained by

  • moving the mechanical lever
  • changing the firing angle value
  • adding inductance to the circuit
  • none of the mentioned

39. A single phase full bridge inverter can operate in load commutation mode if load consists of

  • RLC overdamped
  • RLC critically damped
  • RLC underdamped
  • RLC undamped

40. A single full converter alone can given a

  • two quadrant operation
  • three quadrant operation
  • four quadrant operation,
  • none of the mentioned

41. A UJT is sometimes called ___________ diode.

  • Low resistance
  • High resistance
  • Single-base
  • Double-base

42. 1. In case of three-phase dual converter, one of the converter circuits is fired at an angle of 60°. For both the converter circuits to have equal average output voltage, what is the value of the firing angle for the other converter circuit?

  • 60°
  • 120°
  • 100°
  • Insufficient data

43. The process of commutating a SCR by applying a reverse voltage to an SCR through a previously charged capacitor is called as

  • capacitor commutation
  • forced commutation
  • voltage commutation
  • current commutation

44. How does resistance affect current?

  • less resistance - less current
  • more resistance - less current
  • more resistance - more current
  • resistance does not affect current

45. The normal way to turn on a diac is by ___________.

  • Gate current
  • Gate voltage
  • Breakover voltage
  • None of the above

46. Circulating current flows

  • from load to converters
  • from one converter to another converter
  • in the whole circuit
  • none of the mentioned

47. In non-circulating current mode dual converters, the circulating current is avoided by

  • connecting a series reactor
  • maintaining α1 + α2 = 180°
  • adding an extra SCR
  • operating only one converter

48. In pulse width modulation scheme, _________ is kept constant

  • ton
  • toff
  • Fs
  • Ts

49. A chopper may be thought as a

  • Inverted with DC input
  • DC equivalent of an AC transformer
  • Diode rectifier
  • DC equivalent of an induction motor

50. In the principle of phase control

  • control is achieved by adjusting the firing angle of the devices
  • the load is on for some cycles and off for some cycles
  • control is achieved by adjusting the number of on off cycles
  • control cannot be achieved

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