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. 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

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

  • BJT
  • MOSFET
  • GTO
  • ALL THE ABOVE

3. 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

4. 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

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

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

6. 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

7. Dual converters provide

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

8. 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

9. What is the purpose of a resistor?

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

10. 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

11. 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

12. 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

13. 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

14. How does resistance affect current?

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

15. AC voltage regulator converts

  • fixed mains voltage to fixed ac voltage
  • fixed mains voltage directly to variable ac voltage without any change in frequency
  • fixed mains voltage directly to variable ac voltage with change in frequency
  • none of these

16. A dual converters has

  • two full converters in series
  • two half converters in series
  • two full converters in anti-parallel
  • two half converters in anti-parallel

17. What are power resistors used for?

  • As general purpose resistors
  • To obtain an exact value of resistance
  • Vary a circuit's power rating
  • Control large amounts of current

18. Which resistor will prevent the most current flow?

  • 1 ohm
  • 100 ohms
  • 1000 ohms
  • 0.5k ohms

19. What are the color codes for a 470k ohm resistor?

  • Yellow, Violet, Yellow
  • Orange, Grey, Orange
  • Yellow, Violet, Orange
  • Orange, Grey, Yellow

20. The gate-source voltage at which the ________________ layer in a MOSFET is formed is called the ________________ voltage.

  • drift layer, breakover
  • depletion layer, threshold
  • inversion, threshold
  • inversion, breakover

21. The minimum frequency of the output voltage harmonic in a single phase fully controlled bridge converter is _________ the input supply frequency.

  • four times
  • three times
  • two times
  • same

22. The Gate-Source capacitance of a MOSFET is the ________________ among all three capacitances.

  • smallest
  • constant
  • largest
  • differential

23. 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

24. For the same load resistance as the vgE of an IGBT is increased it enters __________________ region.

  • triode
  • active
  • linear
  • saturation

25. An IGBT has small __________________ temperature coefficient of on state voltage drop.

  • constant
  • negative
  • positive
  • variable

26. From the input side the IGBT behaves essentially as a __________________.

  • BJT
  • Thyristor
  • MOSFET
  • Diode

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

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

28. A UJT has ___________.

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

29. The input displacement factor of a single phase fully controlled bridge converter in the continuous conduction mode is equal to the cosine of the ________ angle.

  • extinction
  • firing
  • conduction
  • load

30. In the discontinuous conduction mode the load current remains __________ for a part of the input cycle.

  • maximum
  • half
  • same
  • zero

31. 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

32. A half controlled converter has better output voltage ___________________ compared to a fully controlled converter.

  • ripple factor
  • power factor
  • load factor
  • form factor

33. For the same firing angle and input voltage the half controlled converter gives ___________________ output voltage form factor compared to a fully controlled converter.

  • lower
  • higher
  • constant
  • variable

34. At the boundary between continuous and discontinuous conduction the value of the output current at ωt = α is ___________________.

  • maximum
  • constant
  • zero
  • half

35. Single phase half controlled converters are most suitable for loads requiring ___________________ voltage and current.

  • bidirectional
  • unidirectional
  • unidirectional as well as bidirectional
  • none of the above

36. In a three phase fully controlled converter each device conducts for an interval of __________ degrees.

  • 180 degrees
  • 90 degrees
  • 120 degrees
  • 150 degrees

37. The input ac current of a three phase fully controlled converter contains only _________ harmonics but no _________ harmonic.

  • odd, triplen
  • odd, even
  • even, odd
  • even, triplen

38. In ___________ current dual converter only one converter conducts at any time.

  • circulating
  • load
  • source
  • non-circulating

39. The input current of a three phase half controlled converter does not have ________________ cycle symmetry.

  • full
  • quarter
  • half
  • none of the above

40. Length of the overlap period depends on the valve of the source inductance and load ______________

  • voltage
  • current
  • inductance
  • power factor

41. The control method used for PWM dc - dc converter is

  • voltage mode control
  • hysteresis mode control
  • current mode control
  • all of the above

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

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

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

  • ton
  • toff
  • Fs
  • Ts

44. In case of a constant frequency system, Ton = (1/4)T. If the chopping frequency 2 kHz, find the value of Toff.

  • 2/8 microsecond
  • 3/8 microsecond
  • 4/8 microsecond
  • 5/8 microsecond

45. In the current limit control method, the chopper is switched off when

  • load current reaches the zero value
  • load current reaches the lower limit
  • load current reaches the upper limit
  • none of the mentioned

46. Find the output voltage for a step-up chopper when it is operated at a duty cycle of 50 % and Vs = 240 V.

  • 560
  • 1200
  • 480
  • 720

47. In dc choppers, the waveforms for input and output voltages are respectively

  • continuous, discontinuous
  • discontinuous, continuous
  • both continuous
  • both discontinuous

48. If the chopper frequency is 200 Hz and on time is 2 ms, then the duty ratio would be

  • 0.4
  • 0.6
  • 0.8
  • none of these

49. In a step down chopper, the ratio Vo/Vs is given by

  • D
  • 1-D
  • 1/ 1-D
  • D/ 1-D

50. 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

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