# EE AND ECE IMPORTANT MCQ-Power Electronics 3

1)   The output current in PWM DC – DC converters is equal to

a. Average value of the output inductor current
b. Product of an average inductor current and a function of duty ratio
c. Either (a) or (b)
d. None of these
 ANSWER: Either (a) or (b)

2)   The opposite of susceptibility is

a. Immunity
b. Emission
c. Interference
d. Electromagnetic compatibility
 ANSWER: Immunity

3)   The ability of an electronic
system to function properly in its intended electromagnetic environment
and should not be a source of pollution to that electromagnetic
environment is known as

a. Susceptibility
b. Emission
c. Interference
d. Electromagnetic compatibility
 ANSWER: Electromagnetic compatibility

4)   If all the SCR’s of 3 phase PAC is replaced by diodes, they would be triggered

a. 120 degree after the zero crossing of the corresponding line voltages
b. 60 degree after the zero crossing of the corresponding line voltages
c. 120 degree before the zero crossing of the corresponding line voltages
d. 60 degree before the zero crossing of the corresponding line voltages
 ANSWER: 60 degree after the zero crossing of the corresponding line voltages

5)   For commutation in three phase PAC, normally balanced three phase voltages VR, VY and VB are connected to the three legs of the converter via

a. Three inductances
b. Three capacitances
c. Three resistance
d. Three transistors
 ANSWER: Three inductances

6)   In square wave operation mode of 3 phase VSI, the VSI

a. Can control the load voltage
b. Cannot control the load voltage
c. Cannot control the load voltage except by means of dc link voltage
d. Cannot control the load voltage except by means of dc link current
 ANSWER: Cannot control the load voltage except by means of dc link voltage

7)   In a 3 phase VSI SPWM to use
a single carrier signal and preserve the features of PWM technique, the
normalized carrier frequency should be

a. Multiple of two
b. Odd multiple of three
c. Odd multiple of five
d. Odd multiple of seven
 ANSWER: Odd multiple of three
8)   Regulator sampling PWM is usually used in

a. High power inverters
b. Rectifiers
c. Low power inverters
d. Only (a) and (b)
 ANSWER: Only (a) and (b)

9)   A combination of synchronized leading edge and trailing edge modulation has also been used to control a

a. Boost single – phase power factor converter
b. A buck dc – dc converter to reduce ripple in the intermediate dc bus capacitor
c. Both (a) and (b)
d. None f these
 ANSWER: Both (a) and (b)

10)   If energy is taken from the AC side of the inverter and sends it back into the DC side, then it is known as

a. Motoring mode operation
b. Braking mode operation
c. Regenerative mode operation
d. None of these
 ANSWER: Regenerative mode operation

11)   The ac output voltage waveform of VSI and AC output current waveform of CSI respectively is composed of

a. High dv / dt, low di / dt
b. Low dv / dt, low di / dt
c. Low dv / dt, high di / dt
d. High dv / dt, high di / dt
 ANSWER: High dv / dt, high di / dt

12)   Advantages of Cuk converter is / are

a. Large number of reactive component
b. Low stress on switch
c. Low stress on capacitor
d. None of these
 ANSWER: None of these

13)   In a flyback converter, the inductor of the buck-boost converter has been replaced by a

a. Flyback capacitor
b. Flyback resistor
c. Flyback transformer
d. Flyback transistor
 ANSWER: Flyback transformer

14)   In a push – pull converter, the filter capacitor can be obtained as

a. Cmin = V / ( Vr L f2 )
b. Cmin= ( 1 – D ) V / ( Vr L f2 )
c. Cmin= ( 1 – 2 D ) V / 32 ( Vr L f2 )
d. Cmin= ( 1 – 2 D ) V / 42 ( Vr L f2 )
 ANSWER: Cmin= ( 1 – 2 D ) V / 32 ( Vr L f2 )

15)   MOSFET stands for

a. Metal – oxide semiconductor field effect transistor
b. Molybdenum – oxide semiconductor field effect transistor
c. Metal – oxide silicon field effect transistor
d. Metal – oxide semiconductor field effect transmitter
 ANSWER: Metal – oxide semiconductor field effect transistor

16)   A MOSFET, for its conduction uses

a. Only minority carriers
b. Only majority carriers
c. Both minority and majority carriers
d. None of these
 ANSWER: Only majority carriers

17)   An RC snubber network used in BJT

a. Divert the collector current during turn – off
b. Improves the reverse bias safe operating area
c. Dissipates a fair amount of switching power
d. All of these
 ANSWER: All of these
18)   The ac voltage controller can be used for

a. Lighting and heating control
b. On – line transformer tap changing
c. Soft starting
d. All of these
 ANSWER: All of these

19)   The phase angle of gate signal in TRIAC can be shifted by using

a. A capacitor
b. A variable resistor
c. An inductor
d. Only (a) and (b)
 ANSWER: Only (a) and (b)

20)   A TRIAC can be turned on with

a. Positive voltage at the gate terminal
b. Negative voltage at the gate terminal
c. Either (a) or (b)
d. None of these
 ANSWER: Either (a) or (b)

21)   DIAC and TRIAC both are semiconductors devices and conduct in

a. DIAC conducts in forward direction and TRIAC conducts in reverse direction
b. Both conducts in forward direction
c. Both conducts in reverse direction
d. Both conducts in either direction
 ANSWER: Both conducts in either direction

22)   The Graetz bridge makes excellent use of

a. Current transformer
b. Potential transformer
c. Power transformer
d. SCR
 ANSWER: Power transformer

23)   If the firing angle becomes negative, then the rectifier begins to work as

a. A rectifier
b. An inverter
c. A chopper
d. A regulator
 ANSWER: An inverter

24)   In a 3 phase half wave rectifier, when firing angle is less than 90 degree, then the average dc output voltage becomes

a. Positive
b. Negative
c. Zero
d. None of these
 ANSWER: Positive

25)   Transformer utilization factor is a measure of the merit of a rectifier circuit. It is the ratio of

a. AC input power to the transformer volt – amp rating required by secondary
b. AC input power to the transformer volt – amp rating required by primary
c. DC output power to the transformer volt – amp rating required by secondary
d. DC output power to the transformer volt – amp rating required by primary
 ANSWER: DC output power to the transformer volt – amp rating required by secondary

26)   Ripple factor is the ratio of

a. Rms value of the ac component of load voltage to the dc voltage
b. Average value of the ac component of load voltage to the peak value of voltage
c. Average value of the dc component of load voltage to the ac voltage
d. Peak value of the dc component of load voltage to the ac voltage
 ANSWER: Rms value of the ac component of load voltage to the dc voltage

27)   Form factor of a rectifier is the ratio of

a. Root mean square value of voltage and current to its peak value
b. Root mean square value of voltage and current to its average value
c. Average value of current and voltage to its root mean square value
d. Peak value of current and voltage to its root mean square value
 ANSWER: Root mean square value of voltage and current to its average value

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28)   The
curve between V and I of SCR when anode is positive w.r.t cathode and
when anode is negative w.r.t cathode are known as

a. both as forward characteristics
b. both as reverse characteristics
c. former as forward characteristics and later as reverse characteristics
d. former as reverse characteristics and later as forward characteristics
 ANSWER: former as forward characteristics and later as reverse characteristics

29)   With gate open, the maximum anode current at which SCR is turned off from ON condition is called

a. breakdown voltage
b. peak reverse voltage
c. holding current
d. latching current
 ANSWER: holding current

30)   Bidirectional semiconductor device is

a. Diode
b. BJT
c. SCR
d. TRIAC
 ANSWER: TRIAC

31)   An ideal diode is

a. Unidirectional
b. Bidirectional
c. Fixed voltage polarity
d. Only (a) and (c)
 ANSWER: Only (a) and (c)

32)   The IGBT resulted in higher switching speed and lower energy losses. It can be used for

a. Uninterruptible power supplies
b. Induction heating system
c. Constant voltage and frequency power supplies
d. All of these
 ANSWER: All of these

33)   To detect an over – current fault condition, the most reliable method is to connect a

a. Current sensor across IGBT
b. Voltage sensor across IGBT
c. Current sensor in series with IGBT
d. Voltage sensor in series with IGBT
 ANSWER: Current sensor in series with IGBT

34)   The on – state voltage drop of IGBT consists of

a. Drop across the collector junction
b. Drop across the drift region
c. Drop across MOSFET portion
d. All of these
 ANSWER: All of these

35)   The maximum firing angle in the half wave controlled regulator is

a. 180 degree
b. 190 degree
c. 200 degree
d. 210 degree
 ANSWER: 210 degree

36)   In a phase controlled 3 phase ac voltage controller, the harmonic present is

a. 2nd
b. 3rd
c. 5th
d. 7th
 ANSWER: 2nd

37)   In dual converters,

a. Both rectifiers provides positive current to the load
b. Both rectifiers provide negative current to the load
c. One rectifiers provide positive current to the load and the other negative current
d. One rectifier provide positive current to the source and the other negative current to the load
 ANSWER: Both rectifiers provides positive current to the load
38)   Advantages of HVDC transmission over AC system is / are

a. Reversal of power can be controlled by firing angle
b. Very good dynamic behavior
c. They can link two AC system operating un synchronized
d. All of these
 ANSWER: All of these

39)   In rectifiers, load current flow is

a. Unidirectional
b. Bidirectional
c. Either (a) or (b)
d. Non directional
 ANSWER: Unidirectional

40)   Due to non sinusoidal waveform of the input current, the power factor of the rectifier is

a. Negatively affected by firing angle
b. Negatively affected by distortion of the input current
c. Positively affected by both firing angle and distortion of the input current
d. Both (a) and (b)
 ANSWER: Both (a) and (b)

41)   The man made noise sources is / are

a. Motors
b. Switches, radio interferences
c. Computers, digital electronics
d. All of these
 ANSWER: All of these

42)   Any electrical signal present in a circuit other than the desired signal is known as

a. Noise
b. Distortion
c. Interference
d. All of these
 ANSWER: Noise

43)   In commutation PAC stands for

a. Permanent angle converter
b. Phase angle converter
c. Phase angle commutation
d. Phase and commutation
 ANSWER: Phase angle converter

44)   In the operation and control
of both naturally commutated and forced commutated SCR base converter,
commutation plays an important role. The converters is / are

a. AC – DC
b. DC – DC
c. DC – AC
d. All of these
 ANSWER: All of these

45)   In a 3 phase VSI out of eight valid states, the number of valid states that produce zero ac line voltages is/are

a. One
b. two
c. Three
d. Four
 ANSWER: two

46)   In a three phase voltage source inverters

a. Only amplitude of voltage is controllable
b. Only phase is controllable
c. Both amplitude and phase is controllable
d. Amplitude, phase and frequency of voltages should always be controllable
 ANSWER: Amplitude, phase and frequency of voltages should always be controllable

47)   Double edge modulation eliminates certain harmonics when the reference is a

a. Sine wave
b. Square wave
c. Triangular wave
d. Trapezoidal wave
 ANSWER: Sine wave
48)   The carrier which are commonly used in constant – frequency PWM is

a. Sawtooth carrier
b. Inverted sawtooth carrier
c. Triangle carrier
d. All of these
 ANSWER: All of these

49)   A capacitive load in voltage source inverters generates

a. Small current spikes and can be reduced by using an inductive filter
b. Large current spikes and can be increased by using an inductive filter
c. Small current spikes and can be increased by using an inductive filter
d. Large current spikes and can be reduced by using an inductive filter
 ANSWER: Large current spikes and can be reduced by using an inductive filter

50)   The main objective of static power converters is to

a. Obtain an dc output waveform from a dc power supply
b. Obtain an ac output waveform from a dc power supply
c. Obtain an dc output waveform from a dc power supply
d. Obtain an ac output waveform from a ac power supply
 ANSWER: Obtain an ac output waveform from a dc power supply

51)   The control method used for PWM dc – dc converter is

a. Voltage mode control
b. Current mode control
c. Hysteric control
d. All of these
 ANSWER: All of these

52)   The average value of the output voltage in a step – down dc chopper is given by

a. V 0 = V s
b. V 0 = D V s
c. V 0 = V s / D
d. V 0 = V s / ( 1 – D )
 ANSWER: V 0 = D V s

53)   Choppers is a

a. AC – DC converters
b. AC – AC converters
c. DC – AC converters
d. DC – DC converters
 ANSWER: DC – DC converters

54)   In BJT, switching losses occurs

a. Only at turn – on
b. Only at turn – off
c. Both at turn on and off
d. None of these
 ANSWER: Both at turn on and off

55)   In BJT, the forward biased base emitter junction inject holes from base to emitter, the holes

a. Do not contribute to the collector current
b. Result in net current flow component into the base
c. Contribute to the collector current
d. Only (a) and (b)
e. Only (b) and (c)
 ANSWER: Only (a) and (b)

56)   As the breakdown voltage reached, the DIAC exhibits

a. Negative resistance characteristics
b. Goes into avalanche condition
c. Voltage drop snaps back
d. All of these
 ANSWER: All of these

57)   DIAC are specifically designed to trigger

a. TRIAC
b. SCR
c. GTO
d. Only (a) and (b)
 ANSWER: Only (a) and (b)
58)   In a 3 phase bridge rectifier the ripple frequency is

a. Equal to the input frequency
b. Twice the input frequency
c. Three times the input frequency
d. Six times the input frequency
 ANSWER: Six times the input frequency

59)   The sum of all phase current in a star connected primary winding with no neutral connection is equal to

a. Phase current
b. Three times the phase current
c. Three times the line current
d. Zero at all times
 ANSWER: Zero at all times

60)   For power output higher than 15 kW, the suitable rectifier is

a. Single phase
b. 3 phase
c. Poly phase
d. Only (b) and (c)
 ANSWER: Only (b) and (c)

61)   In a full wave rectifier, the rectification ratio is approximately equal to

a. 61%
b. 71%
c. 81%
d. 91%
 ANSWER: 81%

62)   In a single phase full wave rectifier, during blocking state the pea inverse voltage of diode is

a. V m
b. 2 V m
c. V m / 2
d. 4 V m
 ANSWER: 2 V m

63)   A single phase ac – dc converter is also known as

a. rectifier
b. inverter
c. chopper
d. regulator
 ANSWER: rectifier

64)   If the gate voltage of an SCR is removed, then the

a. anode current decreases
b. anode current does not decrease at all
c. anode current increases
d. cathode current increases
 ANSWER: anode current does not decrease at all

65)   Anode of an operational SCR is

a. Always positive w.r.t cathode
b. Always negative w.r.t anode
c. Always positive w.r.t anode
d. Always negative w.r.t cathode
 ANSWER: Always positive w.r.t cathode

66)   In a silicon controlled rectifier, the load is connected

a. Across anode
b. In series with anode
c. Across cathode
d. In series with cathode
 ANSWER: In series with anode

67)   Number of PN junction in an SCR is

a. Two
b. Three
c. Four
d. Five
 ANSWER: Three
68)   IGBT stands for

a. Insulated gate bipolar transistor
b. Insulated gate bidirectional transistor
c. Inductive gate bipolar transistor
d. Inductive gate bidirectional transistor
 ANSWER: Insulated gate bipolar transistor

69)   An ideal switch is

a. Lossless
b. Carry current in any direction when it is on
c. Does not carry any current in any direction when it is off
d. All of these
 ANSWER: All of these

70)   In ac – dc conversion, when the switch is closed then the sum of voltages around the loop is

a. Zero
b. Non zero
c. Equal to the sum of voltage when switch is open
d. Twice of the voltage when switch is open
 ANSWER: Non zero

71)   A rectifier with an external low pass filter is an example of

a. Indirect switch matrix circuits
b. Direct switch matrix circuits
c. Embedded converters
d. All of these
 ANSWER: Direct switch matrix circuits

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