# 270+ Power Electronics Solved MCQs

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1.

## In a flyback converter, the inductor of the buck-boost converter has been replaced by a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
2.

## 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. A parallel combination of controllable switch and a diode is used
3.

## 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» B. A low on - resistance switch can be added in parallel
4.

## A step - down choppers can be used in

A. Electric traction
B. Electric vehicles
C. Machine tools
D. All of these
5.

## 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» C. Either (a) or (b)
6.

## 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
7.

## 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» C. Cmin= ( 1 – 2 D ) V / 32 ( Vr L f2 )
8.

## 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
9.

## 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
10.

## 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» C. Both (a) and (b)
11.

## 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» C. Symmetric trapezoidal voltage across itself
12.

## 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
13.

## 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
14.

A. One
B. Three
C. Six
D. Nine
15.

## Unipolar modulation is generally used in

A. AC - AC converters
B. AC - DC converters
C. DC - AC converters
D. DC - DC converters
Answer» D. DC - DC converters
16.

## 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» A. Square AC output voltage
17.

## 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» B. Harmonic distortion increases and quality of input current decreases
18.

## 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» A. There is always a path for the dc current independent of the ac line
19.

## 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
20.

## A full wave rectifier with resistive load produces

A. Second harmonic
B. Third harmonic
C. Fifth harmonic
D. Do not produce harmonics
Answer» D. Do not produce harmonics
21.

## 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» B. 60 degree after the zero crossing of the corresponding line voltages
22.

## 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
23.

A. Positive
B. Negative
C. Zero
D. None of these
24.

## 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
25.

A. rectifier
B. inverter
C. chopper
D. regulator
26.

## 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
27.

A. True
B. False
C. none
D. all
28.

A. True
B. False
C. none
D. all
29.

A. 8D×Vin
B. 5D×Vin
C. 2D×Vin
D. D×Vin
30.

A. D×Vin
B. Vin ÷ (1-D)
C. D×Vin ÷ (1
31.

A. 8D×Vin
B. 5D×Vin
C. 2D×Vin
D. D×Vin
32.

A. .9
B. .7
C. .6
D. .4
33.

A. .8
B. .1
C. .2
D. .4
34.

## Inductor and Capacitor in Buck converter are used to ___________

A. Increase the cost
B. Decrease the cost
C. Filter out the harmonics
D. Increase the harmonics
Answer» C. Filter out the harmonics
35.

A. 711.1 V
B. 710.1 V
C. 722.2 V
D. 713.2 V
36.

A. 260 V
B. 264 V
C. 261 V
D. 268 V
37.

## 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» A. Does not contains second harmonic
38.

## A fully controlled converter uses

A. diodes only
B. thyristors only
C. both diodes and thyristors
D. none of the mentioned
39.

## In a B-2 type full controlled bridge converter

A. one SCR conducts at a time
B. two SCRs conduct at a time
C. three SCRs conduct at a time
D. four SCRs conduct at a time
Answer» B. two SCRs conduct at a time
40.

## In a step down chopper, if Vs = 100 V and the chopper is operated at a duty cycle of 75 %. Find the output voltage.

A. 100 V
B. 75 V
C. 25 V
D. none of the mentioned
41.

A. Vs
B. Vs/α
C. Vs/(1-α)
D. Vs/√2
42.

A. 240 V
B. 480 V
C. 560 V
D. 120 V
43.

## In the method of phase control, the phase relationship between ___ & ___ is controlled by varying the firing angle

A. supply current, supply voltage
44.

A. 2Vm/R
B. Vs/R
C. Vm/2
D. Vs/2
45.

A. Vm
B. 2Vm/π
C. Vm/π
D. Vm/α
46.

## The AC voltage controllers are used in __________ applications.

A. power generation
B. electric heating
C. conveyor belt motion
D. power transmission
47.

## A cycloconverter is a:

A. Frequency changer from higher to lower frequency with one-state conversion
B. Frequency changer from higher to lower frequency with two-stage conversion
C. Frequency changer from lower to higher frequency with one-stage conversion
D. Either a or c
Answer» D. Either a or c
48.

## IGT stands for

A. Insulated gate bipolar transistor
B. Insulated gate bidirectional transistor
C. Inductive gate bipolar transistor
D. Inductive gate bidirectional transistor
Answer» B. Insulated gate bidirectional transistor
49.

A. Two
B. Three
C. Four
D. Five
50.

## 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» B. In series with anode
51.

## 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» A. Always positive w.r.t cathode
52.

## 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» B. anode current does not decrease at all
53.

A. V m
B. 2 V m
C. V m / 2
D. 4 V m
54.

A. 61%
B. 71%
C. 81%
D. 91%
55.

## 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» D. Only (b) and (c)
56.

## 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» D. Zero at all times
57.

## 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» D. Six times the input frequency
58.

## DIAC are specifically designed to trigger

A. TRIAC
B. SCR
C. GTO
D. Only (a) and (b)
Answer» D. Only (a) and (b)
59.

## 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
60.

## 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)
Answer» D. Only (a) and (b)
61.

## 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» C. Both at turn on and off
62.

## Choppers is a

A. AC - DC converters
B. AC - AC converters
C. DC - AC converters
D. DC - DC converters
Answer» D. DC - DC converters
63.

## 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» B. V 0 = D V s
64.

A. 230 V
B. 250 V
C. 275 V
D. 325 V
65.

A. 2.5 A
B. 3 A
C. 3.25 A
D. 3.75 A.
66.

A. 6.3 A
B. 5.8 A
C. 4.1A
D. 3.8 A.
67.

A. 1.5
B. 1.1
C. 1.0
D. 0.942.
68.

## In an arc rectifier the drop in voltage at the cathode is approximately

A. 1 volt
B. 6 to 7 volts
C. 60 to 70 volts
D. 1.2 V7 volts.
Answer» B. 6 to 7 volts
69.

## In arc rectifiers mercury is chosen as the liquid for rectifier because

A. it has low specific heat
B. it has low ionization potential
C. it has high atomic weight
D. all of the above.
Answer» D. all of the above.
70.

A. 1 V
B. 2 V
C. 10.4 V
D. 104 V.
71.

A. 1-phase
B. 2 phase
C. 3 phase
D. 6 phase.
72.

## In a mercury arc rectifier the cathode voltage drop is due to

A. surface resistance
B. expenditure of energy in liberating electrons from the mercury
C. expenditure of energy in ionization
D. expenditure of energy in overcoming the electrostatic field.
Answer» B. expenditure of energy in liberating electrons from the mercury
73.

## In mercury arc rectifier, voltage drop at anode is due to

A. energy spent in overcoming the electrostatic field
B. high temperature inside the rectifier
C. self-restoring property of mercury
D. high ionization potential.
Answer» A. energy spent in overcoming the electrostatic field
74.

## The average life of the glass bulb rectifier is

A. 100 to 150 hours
B. 1000 to 1500 hours
C. 10,000 to 15,000 hours
D. 100,000 to 150,000 hours.
Answer» C. 10,000 to 15,000 hours
75.

## The vacuum inside the glass bulb of a mercury arc rectifier is of the order of

A. 5 x 10-3 cm of Hg
B. 5 x 10-4 cm of Hg
C. 5 x 10-5 cm of Hg
D. 5 x 10-8 cm of Hg.
Answer» C. 5 x 10-5 cm of Hg
76.

A. Aluminium
B. Copper
C. Tungsten
D. Graphite.
77.

A. 2
B. 3
C. 5
D. 7
78.

A. 180 degree
B. 190 degree
C. 200 degree
D. 210 degree
79.

## 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
80.

## 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» D. Voltage sensor in series with IGBT
81.

## 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» C. Constant voltage and frequency power supplies
82.

## An ideal diode is

A. Unidirectional
B. Bidirectional
C. Fixed voltage polarity
D. Only (a) and (c)
Answer» D. Only (a) and (c)
83.

A. Diode
B. BJT
C. SCR
D. TRIAC
84.

## 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
85.

## 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» C. former as forward characteristics and later as reverse characteristics
86.

## 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» C. Average value of current and voltage to its root mean square value
87.

## 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» B. Average value of the ac component of load voltage to the peak value of voltage
88.

## 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» C. DC output power to the transformer volt - amp rating required by secondary
89.

## Which of the following finds application in speed control of a dc motor?

A. FET
B. NPN transistor.
C. SCR.
D. None of the above.
90.

## In mercury arc rectifiers the voltage drop in arc is nearly

A. 0.1 volt per centimeter of arc length
B. 1 volt per centimeter of arc length
C. 10 volts per centimeter of arc length
D. 12 to 15 volts per centimeter of arc length.
Answer» C. 10 volts per centimeter of arc length
91.

## A cycloconverter is a _________

A. one stage power converter
B. one stage voltage converter
C. one stage frequency converter
D. none of the mentioned
Answer» C. one stage frequency converter
92.

## Applications of cycloconverters include

A. speed control of ac drives
B. induction heating
C. static VAr compensation
D. all of the mentioned
Answer» D. all of the mentioned
93.

## The single phase mid-point type cycloconverter uses __________ number of SCRs.

A. 4
B. 8
C. 6
D. none of the mentioned
94.

## The single phase bridge type cycloconverter uses __________ number of SCRs.

A. 4
B. 8
C. 6
D. none of the mentioned
95.

## The principle of three phase cycloconverter is to

A. add and remove number of SCRs
B. vary progressively the firing angle of the devices
C. keep the firing angle as 0° for all the devices
D. none of the mentioned
Answer» B. vary progressively the firing angle of the devices
96.

## In three phase cycloconverters, the reduction factor is given by

A. input frequency/output frequency
B. (input frequency/output frequency) -1
C. (input frequency/output frequency) -1/2
D. (input frequency/output frequency) 1/
Answer» B. (input frequency/output frequency) -1
97.

## In a three phase half-wave cycloconverter ___________

A. both inverting and converting action takes place
B. only inversion action takes place
C. only converting action takes place
D. none of the mentioned
Answer» A. both inverting and converting action takes place
98.

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

A. AC-DC
D. None of the mentioned
99.

A. AC to DC
B. DC to AC
C. DC to DC
D. AC to AC
100.

## A chopper may be thought as a

A. Inverter with DC input
B. DC equivalent of an AC transformer
C. Diode rectifier
D. DC equivalent of an induction motor
Answer» B. DC equivalent of an AC transformer

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