IES - Electrical Engineering - Power Electronics

7/27/2019 IES - Electrical Engineering - Power Electronics

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IES Electrical Engineering Topic wise Questions

Power Electronics

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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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IES EE Topic wise 2001-2008

Power Electronics

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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



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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


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


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


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


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


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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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


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


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 :


(B) Both A and R are true but R is NOT the correct explanation of

A



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 :


(B) Both A and R are true but R is NOT the correct explanation of

A



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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IES - Electrical Engineering - Power Electronics