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7/27/2019 IES - Electrical Engineering - Power Electronics
1/35
IES Electrical Engineering Topic wise Questions
Power Electronics
www.gatehelp.com
YEAR 2008
MCQ 1
For a single a.c. to d.c. controlled rectifier to operate in regenerative
mode, which of the following conditions should be satisfied ?
(A) Half-controlled bridge, 90< c , source of e.m.f. in load
(B) Half-controlled bridge, 90> c , source of e.m.f. in load
(C) Full-controlled bridge, 90> c , source of e.m.f. in load
(D) Full-controlled bridge, 90< c , source of e.m.f. in load
MCQ 2
A half-controlled bridge converter is operating from an r.m.s input
voltage of 120 V. Neglecting the voltage drops, what are the mean
load voltage at a firing delay angle of0c and 180c, respectively ?
(A) 120 2 2#
V and 0
(B) 0 and 120 2 2#
V
(C) 120 2
V and 0
(D) 0 and 120 2
V
MCQ 3
For a step-down d.c. chopper operating with discontinuous load
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current, what is the expression for the load voltage ? (K is duty ratio
of chopper)
(A) V V K0 d.c.#=
(B) /V V K0 d.c.=(C) /( )V V K10 d.c.=
(D) ( )V V K10 d.c.=
MCQ 4
An ideal chopper is operating at a frequency of 500 Hz from a 60 V
battery input. It is supplying a load having 3 resistance and 9 mH
inductance. Assuming the load is shunted by a perfect commutating
diode and assuming battery is low less, what is the mean load current
at an on/off ratio of 1/1 ?(A) 10 A
(B) 15 A
(C) 20 A
(D) None of the above
MCQ 5
Matching List I (Device) with List II (Monolithic construction of)
and select the correct answer using the code given below the lists :
List I List II
A. Triac 1. Two thyristors in anti-parallel
B. Reverse conducting thyristor 2. A thyristor and a diode in
anti-parallel
C. Diac 3. Two diodes in anti-parallel
Codes :
A B C
(A) 1 2 3
(B) 3 2 1
(C) 2 3 1
(D) 3 1 2
MCQ 6
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A two-quadrant d.c. to d.c. chopper can operate with which of the
following load conditions ?
1. ve+ voltage, ve+ current
2. ve voltage, ve+ current3. ve voltage, ve current
4. ve+ voltage, ve current
Select the correct answer using the code given below :
(A) 1 only
(B) 1 and 2
(C) 1 and 4
(D) 3 and 4
MCQ 7
A single-phase current source inverter is connected with capacitive
load only the waveform of the output voltage across the capacitor for
constant source current will be
(A) sine wave
(B) square wave
(C) triangular wave
(D) step function
MCQ 8
A modern power semiconductor device that combines the
characteristics of BJT and MOSFET is
(A) GTO
(B) FCT
(C) IGBT
(D) MCT
MCQ 9
What are the advantage of switching power supplies over linear power
supplies ?
1. The devices operate in linear/active region.
2. The devices operate as switches.
3. Power losses are less.
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Select the correct answer using the code given below :
(A) 1 and 3
(B) 2 and 3
(C) 1 and 2
(D) 1, 2 and 3
MCQ 10
Match List I (Controller) with List II (Type of load) and select the
correct answer using the code given below the lists :
List I List II
A. Chopper controlled resistance
in the rotor circuit of aninduction motor
1. Very low speed, high-power
reversible drive
B. Sub-synchronous converter-
cascade in the rotor circuit of
an induction motor
2. Centrifuges in sugar industry
C. 3-phase a.c. voltage controller 3. Blowers and compressors
D. Cycloconverter 4. Loads requiring good
starting performance
Codes :
A B C D
(A) 3 4 2 1
(B) 3 4 1 2
(C) 4 3 1 2
(D) 4 3 2 1
MCQ 11
A cycloconverter-fed induction motor drive is most suitable for which
one of the following ?
(A) Compressor drive
(B) Machine tool drive
(C) Paper mill drive
(D) Cement mill drive
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MCQ 12
A large d.c. motor is required to control the speed of blower from a
3-phase a.c. source. What is the most suitable a.c. to d.c. converter ?
(A) 3-phase fully controlled bridge converter(B) 3-phase fully controlled bridge converter with free wheeling diode
(C) 3-phase half-controlled bridge converter
(D) A pair of 3-phase converters in sequence control
MCQ 13
A single-phase full-bridge inverter is connected to a load of .2 4 . The
d.c. input voltage is 48 V. What is the r.m.s. output at fundamental
frequency ?
(A)2
4 48 V#
(B)2
2 48 V#
(C) 4 48 V#
(D) 2 48 V#
MCQ 14A buck regulator has an input voltage of 12 V and the required
output voltage is 5 V. What is the duty cycle of the regulator ?
(A) 5/12
(B) 12/5
(C) 5/2
(D) 6
YEAR 2007
MCQ 15
Which one of the following is the correct statement ?
In a two quadrant converter working in the 1st and 2nd quadrants
(A) load current and load voltage are always positive
(B) load current is always negative
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(C) load current can be positive or negative
(D) load current and load voltage are always negative
MCQ 16
The circuit shown in the below figure will work as which one of the
following ?
(A) Buck-Boost converter
(B) Buck converter
(C) Boost converter
(D) Dual converter
MCQ 17
What is the maximum output voltage of a 3-phase bridge rectifier
supplied with the line voltage of 440 V ?
(A) 528 V(B) 396 V
(C) 594 V
(D) 616 V
MCQ 18
A single phase ac voltage controller is controlling current in a purely
inductive load. If the firing angle of the SCR is , what will be the
conduction angle of the SCR ?
(A)
(B) ( )
(C) ( )2
(D) 2
MCQ 19
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In a single phase full wave controlled bridge rectifier, minimum
output voltage and maximum output voltage are obtained at which
conduction angles ?
(A) ,0 180c c respectively
(B) ,180 0c c respectively
(C) ,0 0c c respectively
(D) ,180 180c c respectively
MCQ 20
The pulse-width modulated inverter for the control of an a.c. motor
is fed from which one of the following ?
(A) Controlled rectifier
(B) Uncontrolled rectifier
(C) a.c. regulator
(D) Cycloconverter
MCQ 21
Consider the following statements :
1. Both voltage source inverter and current source inverter require
feedback diodes.
2. Only current source inverter requires feedback diodes.3. GTOs can not be used in a current source inverter.
4. Only voltage source inverter requires feedback diodes.
Which of the statements given above is/are correct ?
(A) 1 only
(B) 2 and 3
(C) 3 and 4
(D) 4 only
MCQ 22
Which one of the following is the main advantage of SMPS over
linear power supply ?
(A) No transformer is required
(B) Only one stage of conversion
(C) No filter is required
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(D) Low power-dissipation
MCQ 23
Assertion (A) : Power electronic converters are extensively used inadjustable speed drives.
Reason (R) : Power electronic converters do not produce harmonic
distortion.
(A) Both A and R are true and R is the correct explanation of A
(B) Both A and R are true but R is not the correct explanation of A
(C) A is true but R is false
(D) A is false but R is true
MCQ 24
MatchList I (Type of Device) with List II (Characteristic/Application)
and select the correct answer using the code given below the lists :
List I List II
A. MOSFET 1. Turn off by negative gate
B. GTO 2. Bi-directional switching
C. UJT 3. High speed switching
D. TRIAC 4. Triggering circuitCodes :
A B C D
(A) 3 1 4 2
(B) 3 1 2 4
(C) 1 2 3 4
(D) 1 2 4 3
MCQ 25
Snubber circuits are used to protect thyristor from which of the
following ?
(A)dt
di
dt
dvHigh and low
(B)dt
dv
dt
diHigh and low
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(C)dt
dv
dt
diLow and low
(D)dt
dv
dt
diHigh and high
MCQ 26
Consider the following statements in respect of IGBT :
1. It combines the attributes of MOSFET and BJT.
2. It has low forward voltage drop.
3. Its switching speed is very much lower than that of MOSFET.
4. It has high input impedance.
Which of the statements given above are correct ?
(A) 1, 2, 3 and 4
(B) 1, 2 and 4
(C) 1, 2 and 3
(D) 3 and 4
YEAR 2006
MCQ 27
A resistor connected across the gate and cathode of an SCR
(A) increases /dv dt rating of SCR
(B) increases holding current of SCR
(C) increases noise immunity of SCR
(D) increases turn-off time of SCR
MCQ 28
Turn-on of a thyristor takes place when
(A) anode to cathode voltage is positive(B) anode to cathode voltage is negative
(C) there is a positive current pulse at the gate
(D) the anode to cathode voltage is positive and there is a positive
current pulse at the gate.
MCQ 29
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Which is the most suitable power device for high frequency (>100
KHz) switching application ?
(A) Power MOSFET
(B) Bipolar junction transistor(C) Schotty diode
(D) Microwave transistor
MCQ 30
Match List I with List II and select the correct answer using the code
given below the lists :
List I List II
A. Voltage source inverter 1. Larger source inductanceB. Current source inverter 2. Poor power factor
C. Phase controlled a.c. to d.c.
converter
3. Inverter limit
D. Cycloconverter 4. Small source inductance
Codes :
A B C D
(A) 4 2 3 1
(B) 3 1 4 2
(C) 4 1 3 2
(D) 3 2 4 1
MCQ 31
In a dual converter, the circulating current
(A) allows smooth reversal of load current, but increases the response
time
(B) allows smooth reversal of load current with improved speed ofresponse
(C) does not allow smooth reversal of load current, but reduces the
response time
(D) flows if there is no interconnecting inductor
MCQ 32
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What is the power factor of a single phase a.c. regulator feeding a
resistive load ?
(A) ( )Per unit power 2
(B) ( )Per unit power/1 2
(C)( )
2
Per unit power 2
(D)( )
2Per unit power /1 2
MCQ 33
When the firing angle of a single phase fully controlled rectifier
feeding constant d.c. current into the load is 30c, what is the
displacement factor of the rectifier ?(A) 1
(B) 0.5
(C) 3
(D)23
MCQ 34
For the same voltage output,which one of the following has larger
peak inverse voltage of the thyristor ?(A) Single phase full wave centre tapped circuit
(B) Single phase full wave bridge circuit
(C) Three phase full wave bridge circuit
(D) Three phase full wave centre tapped circuit
MCQ 35
In a three phase voltage source inverter operating in square wave
mode, the output line voltage is free from
(A) 3rd harmonic
(B) 7th harmonic
(C) 11th harmonic
(D) 13th harmonic
MCQ 36
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A fully controlled line commutated converter functions as an inverter
when firing angle ( ) is in the range
(A) 0 90c c
(B) 90 180c c(C) 90 180c c only when there is a suitable d.c. source in the load
(D) 90 180c c only when it supplies a back e.m.f. load
MCQ 37
Which of the following devices should be used as a switch in a low
power switched mode power supply (SMPS) ?
(A) GTO
(B) MOSFET
(C) TRIAC
(D) THYRISTOR
YEAR 2005
MCQ 38
A converter which can operate both in 3-pulse and 6-pulse modes is a
(A) 1-phase full converter
(B) 3-phase half-wave converter
(C) 3-phase semi converter
(D) 3-phase full converter
MCQ 39
A gate turn off (GTO) thyristor has capacity to
(A) Amplify the gate-current
(B) Turn-off when positive current pulse is given at the gate
(C) Turn-off when a gate-pulse is given at the gate even though it isreverse biased
(D) Turn-off when a negative current pulse is given at the gate
MCQ 40
Power electronic device with poor turn-off gain is
(A) A symmetrical thyristor
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(B) A conventional thyristor
(C) Power bipolar junction transistor
(D) Gate turn-off thyristor
MCQ 41
Match List I (Limiting factor) with List II (Safe Operating Area
Portion) and select the correct answer using the code given below
the lists :
List I List II
A. The peak voltage limit 1. PQ
B. Secondary breakdown limit 2. QR
C. Power dissipation limit 3. RSD. Peak current limit 4. ST
Codes :
A B C D
(A) 2 1 4 3
(B) 4 3 2 1
(C) 2 3 4 1
(D) 4 1 2 3
MCQ 42
Which one of the following statements is not correct ?
(A) Power MOSFETs are so constructed as to avoid punch through
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(B) In a power MOSFET, the channel length is relatively large and
channel width is relatively small
(C) Power MOSFETs do not experience any minority carrier storage
(D) Power MOSFETs can be put in parallel to handle larger currents
MCQ 43
Carrier frequency gate drive is used for turn-on of a thyristor to
reduce
(A) /di dt
(B) Turn-on time
(C) /dv dt
(D) size of pulse transformer
MCQ 44
In a thyristor, /di dt protection is achieved by the use of
(A) An inductance L in series with the thyristor
(B) A resistor in series with the thyristor
(C) RC in series with the thyristor
(D) RL in series with the thyristor
MCQ 45
The circulating current inductor is required in a dual converter to
(A) Improve the p.f.
(B) Smoothen the waveform of circulating current
(C) Limit the circulating current
(D) Increase the circulating current
MCQ 46
In a single-phase semiconvector with discontinuous conduction and
extinction angle
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MCQ 47
A three pulse converter is feeding a purely resistive load. What is the
value of firing delay angle , which dictates the boundary between
continuous and discontinuous mode of current conduction ?(A) 0c =
(B) 30c =
(C) 60c =
(D) 150c =
MCQ 48
For a single phase a.c. voltage controller feeding a resistive load, what
is the power factor ?
(A) Unity for all values of firing angle
(B) ( ) sin121 2
/1 2
+; E& 0
(C) ( ) sin121 2
/1 2
+ +; E& 0
(D) ( ) sin121 2
/1 2
+ ; E& 0
Where id firing angle measured from voltage zero.
MCQ 49
Triacs cannot be used in AC voltage regulator for a
(A) Resistive load
(B) Back emf load
(C) Inductive load
(D) Resistive inductive load
MCQ 50
A motor armature supplies through phase controlled SCRs receives a
smoother voltage shape at
(A) High motor speed
(B) Low motor speed
(C) Rated motor speed
(D) None of the above
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MCQ 51
Which one of the following statements is correct ?
A voltage source inverter is normally employed
(A) when the source has low impedance and load has high reactance
(B) when the source has high impedance and load has low reactance
(C) when both the source and load have high values of impedance
and reactance respectively
(D) when both the source and load have low values of impedance and
reactance respectively
YEAR 2004
MCQ 52
A single-phase inverter has square wave output voltage. What is
the percentage of the fifth harmonic component in relation to the
fundamental component ?
(A) 40 %
(B) 30 %
(C) 20 %
(D) 10 %
MCQ 53
Which one of the following statements is correct ?
The turn off times of converter grade SCRs are normally in the range
of
(A) 1 to 2 microseconds
(B) 50 to 200 microseconds
(C) 500 to 1000 microseconds
(D) 1 to 2 milliseconds
MCQ 54
Which one of the following statements is correct ?
A triac is a
(A) 2 terminal switch
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(B) 2 terminal bilateral switch
(C) 3 terminal unilateral switch
(D) 3 terminal bidirectional switch
MCQ 55
Which one of the following is the most suitable device for a d.c.-d.c.
converter ?
(A) BJT
(B) GTO
(C) MOSFET
(D) Thyristor
MCQ 56
Which one of the following is correct ?
In a switched capacitor network for VAr compensation the SCRs are
commutated by
(A) forced commutation
(B) resonant commutation
(C) natural commutation
(D) delayed commutation
MCQ 57
Which one of the following shows current fold back characteristics
curve for an SCR controlled shunt regulated power supply ?
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MCQ 58
Which one of the following statements is correct ?
In a transistor, the reverse saturation current ICO
(A) doubles for every 10 Cc rise in temperature
(B) doubles for every 1 Cc rise in temperature
(C) increases linearly with temperature
(D) decreases linearly with temperature
MCQ 59
Which one of the following statements is correct ?
In a thyristor, the holding current IH is
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(A) more than the latching current IL
(B) less than IL
(C) equal to IL
(D) equal to zero
MCQ 60
Which one of the following statements is correct ?
For an SCR, /dv dt protection is achieved through the use of
(A) RL in series with SCR
(B) RC across SCR
(C) L in series with SCR
(D) RC in series with SCR
MCQ 61
A power diode is in the forward conduction mode and the forward
current is now decreased. The reverse recovery time of the diode is tr
and the rate of fall of the diode current is /di dt. What is the stored
charge ?
(A) .dt
ditrb l
(B) .dtdi t
21
r
2b l
(C) .dt
ditr
2b l
(D) .dt
dit
21
rb l
MCQ 62
In the buck-boost converter, what is the maximum value of the switch
utilization factor ?(A) 1.00
(B) 0.75
(C) 0.50
(D) 0.25
MCQ 63
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Which one of the following statements is correct ?
In order to get best results per unit cost, the heat sinks on which the
thyristors are mounted, are made of
(A) aluminium
(B) copper
(C) nickel
(D) stainless steel
MCQ 64
Which one of the following capacitor is suitable for compensation of
harmonic and reactive power ?
(A) Mica capacitor(B) Glass capacitor
(C) Polypropylene capacitor
(D) Electrolytic capacitor
MCQ 65
Which one of the following statements is correct ?
The following is bleeder resistor in a power supply is
(A) to ensure a minimum current drain in the current
(B) to increase the output d.c. voltage
(C) to increase the output current
(D) same as that of a load resistor
MCQ 66
Which one of the following is used as the main switching element in a
switched mode power supply operating in 20 kHz to 100 kHz range ?
(A) Thyristor
(B) MOSFET
(C) Triac
(D) UJT
YEAR 2003
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MCQ 67
Match List I (Converters) with List II (Type of conversion) and
select the correct answer :
List I List II
A. Controlled Rectifier 1. Fixed DC to variable voltage andvariable frequency AC
B. Chopper 2. Fixed DC to variable DC
C. Inverter 3. Fixed AC to variable DC
D. Cycloconverter 4. Fixed AC to variable frequency AC
Codes :
A B C D
(A) 2 3 1 4
(B) 3 2 4 1
(C) 2 3 4 1
(D) 3 2 1 4
MCQ 68
Turn-on and turn-off times of transistor depend on(A) static characteristic
(B) junction capacitances
(C) current gain
(D) None of the above
MCQ 69
Match List I (Characteristic/Action) with List II (Observations) and
select the correct answer using the codes given below the list :
List I List II
A. Turn-on time of thyristor 1. Depends on junction capacitances
B. Turn-on time of transistor 2. Is the sum of delay and rise times
C. Rate of rise of gate
current in thyristor
3. Thyristor switches back to Off-
state
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D. If the gate pulse is
removed, when the
thyristor is carrying
a current, less thanlatching current
4. Affect the delay time
Codes :
A B C D
(A) 2 1 4 3
(B) 1 2 3 4
(C) 2 1 3 4
(D) 1 2 4 3
MCQ 70
An ideal chopper operating at a frequency of 500 Hz, supplies a load
having resistance of 3 ohms and inductance of 9 mH from a 60 V
battery. The mean value of the load voltage for on/off ratio of 4/1,
(assuming that load is shunted by a perfect commutating diode and
battery is loss-less) is
(A) 240 V
(B) 48 V
(C) 15 V
(D) 4 V
MCQ 71
The characteristic features of discontinuous conduction compared to
continuous conduction in a four-pulse, single-phase bridge converter
are
(A) Larger average value of load voltage and larger ripple-content
(B) Larger average value of load voltage and smaller ripple-content(C) Smaller average value of load voltage and smaller ripple-content
(D) Smaller average value of load voltage and larger ripple-content
MCQ 72
If the r.m.s source voltage is V volts, the minimum and maximum
values of firing angles for a single-phase, half-wave controlled rectifier,
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supplying a load with a back e.m.f. of 40 volts are
(A) 0 180andc c
(B) (40/ ) 180sin 2 V and1 c =
(C) (40/ ) 40/sin sin2 2V and V1 1 = _ i9 C(D) 0 (40/ )sin 2and V1c 8 B
MCQ 73
An AC voltage-regulator using back-to-back connected SCRs is
feeding an RL load. The SCR firing angle
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For a single-phase, full-bridge inverter supplying power to a highly
inductive load as shown above, the correct sequence of operation of
switches and diodes is
(A) S S S S S S S S 1 4 3 2 1 4 3 2
(B) S S D D S S D D 1 2 1 2 3 4 3 4
(C) S D S S S D D D 1 3 1 4 4 2 2 3
(D) S D D D D D S S 2 4 4 1 1 3 3 2
MCQ 77
A single-phase, half-bridge inverter has input voltage of 48 V DC.
Inverter is feeding a load of .2 4 . The rms output voltage at
fundamental frequency is
(A) 2 48 V#
(B) 2 482 V#
(C) 482 V#
(D) 2 482 2
V
#
MCQ 78
A boost-regulator has an input voltage of 5 V and the average output
voltage of 15 V. The duty cycle is
(A) 3/2
(B) 2/3
(C) 5/2
(D) 15/2
MCQ 79
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An inter-group reactor is used in a single-phase cycloconverter circuit
to
(A) Reduce current-ripples
(B) Reduce voltage-ripples(C) Limit circulating current
(D) Limit /di dt in the semiconductor switch
Direction : The following item consist of two statements, one labelled
as Assertion A and the other labelled as Reason R. You are to
examine these two statements carefully and decide if the Assertion A
and the Reason R are individually true and if so, whether the Reason
is a correct explanation of the Assertion. Select your answers to these
items using the codes given below :
Codes :
(A) Both A and R are true and R is the correct explanation of A
(B) Both A and R are true but R is NOT the correct explanation of
A
(C) A is true but R is false
(D) A is false but R is true
MCQ 80
Assertion (A) : The critical rate of change of forward-voltage is thevalue of /dv dt, at which the device just goes into conduction without
a gate pulse.
Reason (R) : Thyristors go to a state of conduction with the
application of sharp rate of change of forward-voltage in the absence
of gate pulse, even before the break-forward voltage limit is reached.
YEAR 2002
MCQ 81
When cathode of a thyristor is made more positive than its anode
(A) all the junctions are reverse biased
(B) outer junctions are reverse biased and central one is forward
biased
(C) outer junctions are forward biased and central one is reverse
biased
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(D) all the junctions are forward biased
MCQ 82
The sharing of the voltages between thyristors operating in series isinfluenced by the
(A) /di dt capabilities
(B) /dv dt capabilities
(C) junction temperatures
(D) static v-i characteristics and leakage currents
MCQ 83
R-C snubber is used in parallel with the thyristor to
(A) reduce /dv dt across it
(B) reduce /di dt through it
(C) limit current through the thyristor
(D) ensure its conduction after gate signal is removed
MCQ 84
For a step up d.c.-d.c. chopper with an input d.c. voltage of 220 volts,
if the output voltage required is 330 volts and the non-conducting
time of thyristor-chopper is 100 s , the ON time of thyristor-chopperwould be
(A) 66.6 s
(B) 100 s
(C) 150 s
(D) 200 s
MCQ 85
A thyristor controlled reactor is used to get(A) variable resistance
(B) variable capacitance
(C) variable inductance
(D) improved reactor power factor
MCQ 86
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A single-phase full-bridge convertre with a free-wheeling diode feeds
an inductive load. The load resistance is .15 53 and it has a large
inductance providing constant and ripple free d.c. currnet. Input to
converter is from an ideal 230 V, 50 Hz single phase source. For a
firing delay angle of60c, the average value of diode current is
(A) 10 A
(B) 8.165 A
(B) 5.774 A
(D) 3.33 A
MCQ 87
The operation of an inverter fed induction motor can be shifted from
motoring to regenerative braking by
(A) reversing phase sequence
(B) reducing inverter voltage
(C) decreasing inverter frequency
(D) increasing inverter frequency
MCQ 88
In a three-phase full wave a.c. to d.c. converter, the ratio of output
ripple-frequency to the supply-voltage frequency is(A) 2
(B) 3
(C) 6
(D) 12
MCQ 89
A 6-phase bridge converter feeds a purely resistive load. The delay
angle is measured from the point of natural-commutation. Theeffective control of natural-commutation. The effective control of
voltage can be obtained when lies in the range
(A) 0 105c# #
(B) 0 120c# #
(C) 0 150c# #
(D) 0 180c# #
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MCQ 90
a.c. voltage regulators are widely used in
(A) traction drives
(B) fan drives(C) synchronous motor drives
(D) slip power recovery scheme of slip-ring induction motor
MCQ 91
When fed from a fully controlled rectifier, a d.c. motor, driving an
active load, can operate in
(A) forward motoring and reverse braking mode
(B) forward motoring and forward braking mode(C) reverse motoring and reverse braking mode
(D) reverse motoring and forward braking mode
MCQ 92
Compared to a single-phase half-bridge inverter, the output power of
a single-phase full-bridge inverter is higher by a factor of
(A) 12
(B) 8
(C) 4
(D) 2
MCQ 93
In a switched-mode power supply (SMPS), after conversion of
a.c. supply to a highly filtered d.c. voltage, a switching transistor
is switched ON and OFF at a very high speed by a pulse width
modulator (PWM) which generates very-high frequency square
pulses. The frequency of the pulses is typically in the range of(A) 100 Hz - 200 Hz
(B) 500 Hz - 1 kHz
(C) 2 kHz - 5 kHz
(D) 20 kHz - 50 kHz
MCQ 94
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How many switches are used to construct a three-phase cycloconverter
?
(A) 3
(B) 6(C) 12
(D) 18
MCQ 95
A 3-phase cycloconverter is used to obtain a variable-frequency
single-phase a.c. output. The single phase a.c. load is 220 V, 60 A
at a power factor of 0.6 lagging. The rms value of input voltage per
phase required is
(A) 376.2 V
(B) 311.12 V
(C) 266 V
(D) 220 V
YEAR 2001
MCQ 96
Match List I (1- Rectifier topology-feeding resistive load) with ListII (Average output voltage) and select the correct answer : ( is the
firing angle)
List I List II
A. Uncontrolled-half wave 1. / ( )cosV 1peak +
B. Controlled-half wave 2. / cosV2 peak
C. Controlled-full wave 3. /Vpeak
D. Semi-contorlled 4. / ( )cosV 2 1peak +
Codes :
A B C D
(A) 3 2 4 1
(B) 1 4 2 3
(C) 3 4 2 1
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(D) 1 2 4 3
Direction : The following item consist of two statements, one labelled
as Assertion A and the other labelled as Reason R. You are to
examine these two statements carefully and decide if the Assertion Aand the Reason R are individually true and if so, whether the Reason
is a correct explanation of the Assertion. Select your answers to these
items using the codes given below.
Codes :
(A) Both A and R are true and R is the correct explanation of A
(B) Both A and R are true but R is NOT the correct explanation of
A
(C) A is true but R is false
(D) A is false but R is true
MCQ 97
Assertion (A) : High frequency power supplies are lightweight.
Reason (R) : Transformer size get reduced at high frequency.
MCQ 98
The latching current in the above circuit is 4 mA. The minimum
width of the gate pulse required to turn on the thyristor is
(A) 6 s
(B) 4 s
(C) 2 s
(D) 1 s
MCQ 99
Triac cannot be used in
(A) ac voltage regulators
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(B) cycloconverters
(C) solid state type of switch
(D) inverter
MCQ 100
The snubber circuit is used in the thyristor circuits for
(A) triggering
(B) /dv dt protection
(C) /di dt protection
(D) phase shifting
MCQ 101
It is prefereable to use a train of pulse of high frequency for gate
triggering of SCR in order to reduce
(A) /dv dt problem
(B) /di dt problem
(C) the size of the pulse transformer
(D) the complexity of the firing circuit
MCQ 102
A four quadrant chopper cannot be operated as
(A) one quadrant chopper
(B) cycloconverter
(C) inverter
(D) bi-directional rectifier
MCQ 103
Match List I (Waveforms) with List II (Descriptions) and select the
correct answer :
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List II
1. Single phase fully controlled ac to dc converter
2. Voltage commutated dc to dc chopper with input dc voltage E.
3. Phase voltage of a three phase inverter with 180c conduction and
input dc voltage E.
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4. Line voltage of a three phase inverter with 120c conduction and
input dc voltage E.
5. Three-phase diode bridge rectifier.
Codes :
A B C D
(A) 3 4 1 5
(B) 5 1 4 2
(C) 3 1 4 5
(D) 5 4 1 2
MCQ 104
The total harmonic distortion (THD) of ac supply input current of
rectifiers is maximum for
(A) single-phase diode rectifier with dc inductive f filter
(B) 3-phase diode rectifier with dc inductive filter
(C) 3-phase thyristor with inductive filter
(D) Single-phase diode rectifier with capacitive filter
MCQ 105
A single phase ac voltage controller feeding a pure resistance load has
a load voltage of 200 V(rms) when fed from a source of 250 V(rms).
The input power factor of the controller is
(A) 0.64
(B) 0.8
(C) 0.894
(D) Difficult to estimate because of insufficiency of data
MCQ 106In a thyristor-controlled rectifier, the firing angle of thyristor is to be
controlled in the range of
(A) 0 90toc c
(B) 0 180toc c
(C) 90 180toc c
(D) 90 270toc c
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MCQ 107
A 3-phase wound rotor induction motor is controlled by a chopper-
controlled resistance in its rotor circuit. A resistance of 2 is
connected in the rotor circuit and a resistance of4 is additionallyconnected during OFF periods of the chopper. The average resistance
in the rotor circuit for the copper frequency of 200 Hz is
(A) 26/5
(B) /24 5
(C) /18 5
(D) /16 5
MCQ 108
The most suitable solid state converter for controlling the speed of
the three-phase cage motor at 25 Hz is
(A) cycloconverter
(B) current source inverter
(C) voltage source inverter
(D) load commutated inverter
MCQ 109
In case of voltage source inverter, free-wheeling can be needed for theload of
(A) inductive nature
(B) capacitive nature
(C) resistive nature
(D) back emf nature
MCQ 110
PWM switching is preferred in voltage source inverters for thepurpose of
(A) controlling output voltage
(B) output harmonics
(C) reducing filter size
(D) controlling output voltage, output harmonics and reducing filter
size
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MCQ 111
Which one of the following is not the advantage of solid state switching
of ac capacitors into ac supply over relay-based switching ?
(A) low transients(B) low losses
(C) fast response
(D) long life
MCQ 112
The most suitable device for high frequency inversion in SMPS is
(A) BJT
(B) IGBT(C) MOSFET
(D) GTO
MCQ 113
The quality of output ac voltage of a cycloconverter is improved with
(A) increase in output voltage at reduced frequency
(B) increase in output voltage at increased frequency
(C) decrease in output voltage at reduced frequency
(D) decrease in output voltage at increased frequency
MCQ 114
A cycloconverter is operating on a 50 Hz supply. The range of output
frequency that can be obtained with acceptable quality, is
(A) 0 - 16 Hz
(B) 0 - 32 Hz
(C) 0 - 64 Hz
(D) 0 - 128 Hz
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