EE AND ECE IMPORTANT MCQ-Power Electronics 2

1)   Unipolar
modulation is generally used in
a. AC – AC converters
b. AC
– DC converters
c. DC
– AC converters
d. DC
– DC converters
ANSWER: DC – DC converter
2)   The
quadrant operation of BJT is represented by
a. 1
b. 2
c. 3
d. 4
ANSWER: 2
 3)   The
switching function of semiconductor devices can be characterized with
a. Duty ratio only
b. Frequency
only
c. Duty
ratio and frequency
d. Duty
ratio, frequency and time delay
ANSWER: Duty ratio, frequency and time delay
4)   AC
power in a load can be controlled by using
a. two SCR’s in parallel opposition
b. two
SCR’s in series
c. three
SCR’s in series
d. four
SCR’s in series
ANSWER: two SCR’s in parallel opposition
5)   An
SCR is made up of silicon because
a. silicon has large leakage current than germanium
b. silicon
has small leakage current than germanium
c. silicon
has small leakage voltage than germanium
d. silicon
has large leakage voltage than germanium
ANSWER: silicon has small leakage current
than germanium
6)   The
output power of the cascaded amplifier / attenuator system can be determined
using 
a. Actual gain of amplifier
b. Actual
gain of amplifier and attenuator
c. Gain
in dB of amplifier and attenuator
d. Actual
gain of attenuator
ANSWER: Actual gain of amplifier and
attenuator
7)   LISN is a device
used to measure conducted emissions. LISN stands for
a. Line integrated stabilization
network
b. Line
impedance stabilization network
c. Line
integrated stored network
d. Laser
integrated stabilization networking
ANSWER: Line impedance stabilization
network
8)   In current
commutated DC-DC choppers, the voltage spike appears across the load when
a. Voltage across the commutating
inductances collapses
b. The
capacitance voltage adds to the supply voltage
c. Both
(a) and (b)
d. None
of these
ANSWER: Both (a) and (b)
9)   In a load
commutated DC – DC chopper, the capacitor has a
a. Symmetric triangular voltage
across itself
b. Symmetric
rectangular voltage across itself
c. Symmetric
trapezoidal voltage across itself
d. Symmetric
sinusoidal voltage across itself
ANSWER: Symmetric trapezoidal voltage
across itself
10)   Voltage
commutation circuit can be converted into a current commutation by
interchanging the positions of
a. Diode and capacitor
b. Capacitor
and SCR
c. Inductor
and capacitor
d. Capacitor
and load
ANSWER: Diode and capacitor
11)   For swept
frequency measurements, the input impedance of the mismatched transmission
line would vary with frequency as the electrical length of the transmission
line would
a. Decrease with frequency
b. Remains
same with change in frequency
c. Increase
with frequency
d. Either
(a) or (b)
ANSWER: Increase with frequency
12)   In EMC signal,
the source delivers maximum power to the input of transmission line when
the transmission line input impedance
a. Is equal to the source
resistance
b. Greater
than the source resistance
c. Smaller
than the source resistance
d. None
of these
ANSWER: Is equal to the source resistance
13)   In radiative
coupling, the emitter radiation field
a. Decays as 1 / R, where R is the
separation distance between the emitter and the receptor
b. Decays
as R, where R is the separation distance between the emitter and the
receptor
c. Decays
as 1 / 2 R, where R is the separation distance between the emitter and the
receptor
d. Decays
as 2R, where R is the separation distance between the emitter and the
receptor
ANSWER: Decays as 1 / R, where R is the
separation distance between the emitter and the receptor
14)   The effects of
EMI can be reduced by 
a. Suppressing emissions
b. Reducing
the efficiency of the coupling path
c. Reducing
the susceptibility of the receptor
d. All
of these
ANSWER: All of these
15)   Between the
incoming and outgoing devices in voltage commutation
a. Large overlapping takes place
b. Small
overlapping operation
c. No
overlapping operation
d. None
of these
ANSWER: No overlapping operation
16)   During the
commutation period in 3 phase converter, overlap time is
a. Dependent on the load current
b. Dependent
on the voltage
c. Dependent
on both the load current and load voltage behind the short circuit current
d. Independent
on both the load current and load voltage
ANSWER: Dependent on both the load current
and load voltage behind the short circuit current
17)   In
a three phase converter, the number of notches per cycle is
a. One
b. Three
c. Six
d. Nine
ANSWER: Six
18)   In
a 3 phase CSI, if the required line output voltages are balanced and 120
degree out of phase, then the chopping angles are used to eliminate only the
harmonics at frequencies
a. 5
b. 7
c. 11
d. All
of these
ANSWER: All of these
19)   For
similar carrier and modulating signals, the line current used in CSI is
a. Identical to line voltage in a VSI
b. Identical
to line current in VSI
c. Identical
to phase voltage in VSI
d. Identical
to phase voltage in CSI
ANSWER: Identical to line voltage in a VSI
20)   Under
harmonic free load voltages, the 3 phase VSI 
a. Does not contains second harmonic
b. Does
not contains third harmonic
c. Does
not contains fifth harmonic
d. Does
not contains seventh harmonic
ANSWER: Does not contains second harmonic
21)   The
square wave operation of 3 phase VSI lines contains the harmonics. The
amplitudes are
a. Directly proportional to their harmonic order
b. Inversely
proportional to their harmonic order
c. Not
related to their harmonic order
d. None
of these
ANSWER: Inversely proportional to their
harmonic order
22)   In
bipolar modulation, the carrier is symmetric about zero with amplitude equal
to Cm, the PWM output 
a. Zero
b. Switches
between – 1 / 2 and + 1 / 2
c. Switches
between – 1 and + 1
d. Switches
between 0 and + 1
ANSWER: Switches between – 1 / 2 and + 1 / 2
23)   Double
fourier series analysis of PWM is 
a. Two dimensional functions
b. Three
dimensional functions
c. One
dimensional functions
d. All
of these
ANSWER: Two dimensional functions
24)   In
single phase VSI, the harmonic which is not present is
a. 2nd
b. 3rd
c. 5th
d. 7th
ANSWER: 2nd
25)   Very
large values of modulation index (greater than 3.24) lead to
a. Square AC output voltage
b. Sine
AC output voltage
c. Triangular
AC output voltage
d. Trapezoidal
AC output voltage
ANSWER: Square AC output voltage
26)   Single
phase VSI are mainly used in 
a. Power supplies
b. Ups
c. Multilevel
configuration
d. All
of these
ANSWER: All of these
27)   For
bidirectional operation of converters
a. A parallel combination of controllable switch and a diode
is used
b. A
parallel combination of controllable switch and capacitor is used
c. A
series combination of controllable switch and a diode is used
d. A
series combination of controllable switch and a capacitor is used
ANSWER: A parallel combination of
controllable switch and a diode is used
28)   For
a buck converter to reduce the conduction losses in diode
a. A high on – resistance switch can be added in parallel
b. A
low on – resistance switch can be added in parallel
c. A
high on – resistance switch can be added in series
d. A
low on – resistance switch can be added in series
ANSWER: A low on – resistance switch can be
added in parallel
29)   The
conduction losses in IGBT is
a. More than that of MOSFET
b. Lower
than that of MOSFET
c. Equal
to that of MOSFET
d. Equal
to that of BJT
ANSWER: Lower than that of MOSFET
30)   The
power MOSFET device is a
a. Current controlled unipolar device
b. Voltage
controlled unipolar device
c. Current
controlled bipolar device
d. Voltage
controlled bipolar device
ANSWER: Voltage controlled unipolar device
31)   With
increase in firing angle,
a. Both harmonic distortion and quality of input current
increases
b. Harmonic
distortion increases and quality of input current decreases
c. Harmonic
distortion decreases and quality of input current increases
d. Both
harmonic distortion and quality of input current decreases
ANSWER: Harmonic distortion increases and
quality of input current decreases
32)   The
most suited gate pulses given to the AC regulator with R – L load can be in
the form of
a. Continuous signal
b. Large
isolating pulse transformer
c. A
train of pulses
d. None
of these
ANSWER: A train of pulses
34)   Harmonics
in 3 phase inverters can be reduced by using
a. Passive filter
b. Active
filter
c. Both
passive and active filters
d. None
of these
ANSWER: Both passive and active filters
35)   The
advantage of using free – wheeling diode in half controlled bridge converter
is that
a. There is always a path for the dc current independent of
the ac line
b. There
is always a path for the ac current independent of the ac line
c. There
is always a path for the dc current dependent of the ac line
d. There
is always a path for the ac current independent of the ac line
ANSWER: There is always a path for the dc
current independent of the ac line
36)   The
input current waveform of a bridge controlled rectifier when the load is
perfectly filtered is
a. Sine wave
b. Square
wave
c. Saw
– tooth wave
d. Trapezoidal
wave
ANSWER: Square wave
37)   A
full wave rectifier with resistive load produces
a. Second harmonic
b. Third
harmonic
c. Fifth
harmonic
d. Do
not produce harmonics
ANSWER: Do not produce harmonics
38)   A crowbar is a
circuit which is used to protect a
a. voltage sensitive load from
excessive dc power supply output voltages
b. current
sensitive load from excessive dc power supply output voltages
c. voltage
sensitive load from excessive ac power supply output voltages
d. current
sensitive load from excessive ac power supply output voltages
ANSWER: voltage sensitive load from
excessive dc power supply output voltages
39)   An SCR can be
used 
a. as static conductor
b. for
power control
c. for
speed control of dc shunt motor
d. all
of these
ANSWER: all of these
40)   TRIAC is a
semiconductor power electronic device which contains 
a. Two SCR’s connected in reverse
parallel
b. Two
SCR’s connected in parallel
c. Two
SCR’s connected in series
d. Two
BJT’s connected in series
  
ANSWER: Two SCR’s connected in reverse
parallel
41)   Silicon
controlled rectifier can be turned on
a. By applying a gate pulse and
turned off only when current becomes zero
b. And
turned off by applying gate pulse
c. By
applying a gate pulse and turned off by removing the gate pulse
d. By
making current non zero and turned off by making current zero
ANSWER: By applying a gate pulse and
turned off only when current becomes zero
42)   In a square –
wave operation of 3 phase CSIs, the power values are on for
a. 60 degree
b. 90
degree
c. 120
degree
d. 150
degree
ANSWER: 120 degree
43)   A carrier based
PWM technique in CSI is composed of
a. A switching pulse generator and
a shorting pulse generator
b. A
shorting pulse distributor
c. A
switching and shorting pulse combination
d. All
of these
ANSWER: All of these
44)   In variable
frequency, PWM gain 
a. Phase lead helps to increase the
phase margin of the control loop
b. Phase
lag helps to increase the phase margin of the control loop
c. Phase
lead helps to decrease the phase margin of the control loop
d. Phase
lag helps to decrease the phase margin of the control loop
ANSWER: Phase lead helps to increase the
phase margin of the control loop
45)   In constant
frequency PWM, at perturbation the amplitude of the sinusoidal component is
a. Linear function
b. Non
linear function
c. Constant
function
d. None
of these
ANSWER: None of these
46)   The transfer
function of PWM is generally developed in
a. Time domain
b. Frequency
domain
c. Either
(a) or (b)
d. None
of these
ANSWER: Either (a) or (b)
47)   In the SPWM, the
modulating signal is 
a. Square
b. Sinusoidal
c. Triangular
d. Saw
– tooth
ANSWER: Sinusoidal
48)   In
a full bridge VSI, in order to avoid the short circuit across the DC bus and
the undefined AC output voltage condition, the modulating technique should
ensure that
a. Top switch of each leg is on at any instant
b. Bottom
switch of each leg is on at any instant
c. Either
(a) or (b)
d. None
of these
ANSWER: Either (a) or (b)
49)   In
a lossless inverter, the average power absorbed in one period by the load
must be
a. Equal to the average power supplied by the dc source
b. Greater
than the average power supplied by the dc source
c. Lesser
than the average power supplied by the dc source
d. Equal
to the average power supplied by the ac source
ANSWER: Equal to the average power supplied
by the dc source
50)   A
step – down choppers can be used in
a. Electric traction
b. Electric
vehicles
c. Machine
tools
d. All
of these
ANSWER: All of these

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EE AND ECE IMPORTANT MCQ-Power Electronics 3
ME ONLINE TEST-THERMODYNAMICS PART 4

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