power elex sec.1

7/23/2019 Power Elex Sec.1

http://slidepdf.com/reader/full/power-elex-sec1 1/26

Electronics and Communication Engineering: Power Electronics

Section 1

1.

A cycloconverter can be

A. step down

B. step up

C. step down or step up

D. none of the above

Answer: Option C

Explanation:

The output frequency can be more or less than input frequency.

2.

In the below figure the average load current is 15 A. The rms value of transformer secondary

current is

A. 15 A

B. 1.!1 A

C. ".5 A



D. 1#.1# A

Answer: Option B

Explanation:

$%& secondary current ' ' 15(1.#1# ' 1.!) A.

*.

In a * phase fully controlled converter the firing frequency is

A. * times the line frequency

B. ! times the line frequency

C. + times the line frequency

D. 12 times the line frequency

Answer: Option B

Explanation:

It is a ! pulse converter.

#.Two thyristor of same rating and same specifications

A. will have equal turn on and turn off periods

B. will have equal turn on but unequal turn off periods



C. may have equal or unequal turn on and turn off periods

D. will have unequal turn on and turn off periods

Answer: Option C

Explanation:

Turn on and turn off times of similar &,$s may not be same.

5.

The amount of dielectric heating is inversely proportional to frequency.

A. True

B. -alse

Answer: Option B

Explanation:

It is proportional to frequency.

!.

A semiconverter feeding an $/ load operates in

A. first quadrant only

B. first and second quadrants

C. first and fourth quadrants

D. first and third quadrants



Answer: Option A

Explanation:

&emiconvertor operates only in first quadrant.

".

The phenomenon of overlap in converter operation due to

A. source resistance

B. source inductance

C. both source resistance and source inductance

D. high value of firing angle

Answer: Option B

Explanation:

&ource inductance delays turn off and hence causes overlap.

).

A thyristor has a ma0imum allowable unction temperature of 12, and the ambient

temperature is #,. If thermal resistance is 1.! ,(34 the ma0imum allowable internal power dissipation is

A. 2 3

B. 5 3

C. +2 3

D. 12) 3



Answer: Option B

Explanation:

.

+.

-or a 12 pulse operation4 the two ! pulse units are fed by

A. 6 and 6 transformers

B. 7 6 7 and 7 6 7 transformers

C. 6 and 6 7 transformers

D. 6 7 and 6 7 transformers

Answer: Option C

Explanation:

* phase difference is necessary.

1.

In a single phase full wave converter 8% 6 2 connection9 feeding a highly inductive load4 the

firing angle for each thyristor is a in the respective half cycle. The period of conduction of

each thyristor is

A. 6

B.

C. :

D. 6 2



Answer: Option B

Explanation:

&ince conduction is from to : 4 period of conduction is in each half cycle.

11.

An &,$ is triggered at # in the positive half cycle only. The average anode current is 5 A.

If the firing angle is changed to )4 the average anode current is li;ely to be

A. 5 A

B. 25 A

C. less than 5 A but more than 25 A

D. less than 25 A

Answer: Option C

Explanation:

.

12.

The characteristics of a non6linear resistance is i ' kv

#

. If i becomes 1 times4 v becomes

A. about 1 times

B. about 1 times

C. about * times



D. about twice

Answer: Option C

Explanation:

v ∝ 8i91(# ∝ 8191(# < *.

1*.

In a step down chopper using pulse width modulation4 Ton ' * 0 16* and Toff ' 1 0 16*s. The

chopping frequency is

A. ***.** B. 25

C. 5 D. 1

Answer: Option B

Explanation:

.

1#.

Thyristors are suitable for dc circuit brea;ers but not for ac circuit brea;ers.

A. True B. -alse

Answer: Option B

Explanation:



They are suitable for both dc and ac circuit brea;ers.

15.Assertion (A): Inverters and choppers use fast switching thyristors

Reason (R): -ast switching &,$ has low turn off time.

A. =oth A and $ are correct and $ is correct e0planation of A

B. =oth A and $ correct but $ is not correct e0planation of A

C. A is correct but $ is wrong

D. A is wrong but $ is correct

Answer: Option A

Explanation:

ow turn off time enables fast switching.

1!.

The value of capacitor , for dynamic equalising circuit of series connected thyristors is

determined by

A. forward characteristics of thyristors

B. reverse recovery characteristics of thyristors

C. both forward and reverse recovery characteristics of thyristors


Answer: Option B



Explanation:

>alue of , is proportional to ma0imum difference between reverse charge of &,$s in series.

1".

A chopper

A. converts constant voltage dc into ac and then into variable voltage dc

B. converts constant voltage dc into variable voltage dc directly

C. converts ac of one frequency into ac of another frequency

D. converts ac to dc

Answer: Option B

Explanation:

A chopper is a dc6ac converter. It converts a fi0ed dc voltage into a variable dc voltage.

1).

Assertion (A): If the duty cycle of chopper is low the current would not become

discontinuous

Reason (R): If the time constant ($ of the load in chopper is low the current may becomediscontinuous.







Answer: Option C

Explanation:

If duty cycle is low4 current may become discontinuous.

1+.

In a single phase full wave regulator4 the firing angles in the positive and negative half cycles

are generally

A. equal

B. different

C. equal or different

D. different but sometimes equal

Answer: Option A

Explanation:

-iring angles are ;ept equal to get symmetrical output voltage.

2.

An electric heater is controlled by thyristors through on6off control. If ' .#4 the heating is

A. 1!? ma0imum

B. #? of ma0imum

C. !? of ma0imum

D. )#? of ma0imum



Answer: Option B

Explanation:

>l ' > . Therefore >

l

2

6 >2

8a9. @eating is proportional to .

21.

A two winding transformer is feeding a single phase half wave rectifier circuit. The load is

purely resistive. The rms value of transformer secondary current is I s and rms value of load

current in Irms. Then

A. I s ' Irms irrespective of the value of firing angle

B. I s ' .5 Irms irrespective of the value of firing angle

C. I s ' Irms if firing angle is less than *

D. I s ' Irms if firing angle is more than *

Answer: Option A

Explanation:

.

22.

The terminals of a power =T are called

A. emitter4 base4 collector

B. emitter4 base4 drain

C. source4 base4 drain

D. source4 base4 collector



Answer: Option A

Explanation:

In a =T terminals are emitter4 base and collector irrespective of rating.

2*.

In single phase half wave regulator4 the average current over one full cycle

A. is always positive

B. may be positive or negative

C. may be negative

D. is always negative

Answer: Option D

Explanation:

The input current during positive half cycle is less than that during negative half cycle.

2#.

The dynamic equalising circuit consists of a series combination of capacitor , and resistor

$ , across each thyristor. This resistance $ , 8along with parallel connected diode9

A. limits charging current

B. limits discharging current

C. limits both charging and discharging currents




Answer: Option B

Explanation:

Bischarge current flows through $ e .

25.

A single phase semiconverter is feeding a highly inductive load and has freewheeling diode

across the load. The waveshapes of output voltage and output current

A. are similar

B. are not similar

C. may be similar or dissimilar

D. are similar only if firing angle is Cero

Answer: Option B

Explanation:

3hen freewheeling diode conducts output current e0ists but output voltage is Cero.

2!.

,urrent derating may be necessary when thyristors are operating in

A. series

B. parallel

C. series or parallel




Answer: Option B

Explanation:

&ince current does not divide equally4 current derating is necessary.

2".

If duty cycle is and the load is a resistance $4 the rms value of output voltage is

A. >

B. >

C. 2>

D.

Answer: Option B

Explanation:

$%& output voltage

> ' >.

2).

-or a =T ' .+)4 then D '

A. .2 B. .5

C. #.+ D. #)

Answer: Option D



Explanation:

2+.

Assertion (A): Eower transistor has lower switching time than &,$

Reason (R): Eower transistor has greater switching time than &,$





Answer: Option D

Explanation:

Eower transistor does not have high rating.

*.

A * ;> circuit uses &,$ of ) > rating. If derating is 25?4 the number of &,$s in series is

A. # B. 5

C. ! D. )

Answer: Option B



Explanation:

.

*1.

In a circuit using a full wave converter 8% 6 2 connection9 the EI> of each thyristor is # >.-or the same output voltage and fully controlled bridge converter4 EI> will be

A. # >

B. ) >

C. 2 >

D. 22 >

Answer: Option C

Explanation:

In bridge connection EI> is half of that in %62 connection.

*2.

A dc separately e0cited motor has constant field current. The armature is fed from a single

phase supply through a full converter. 3hen ' 4 speed is 5 rpm. If ' #54 the speedis li;ely to be

A. about *5 rpm

B. about 25 rpm

C. about 1"5 rpm

D. about 125 rpm



Answer: Option D

Explanation:

Average output voltage∝

cos

and therefore reduced to ."" of the value for

' .&peed is nearly proportional to voltage.

**.

In a multiphase chopper4 all choppers operate together.

A. True B. -alse

Answer: Option B

Explanation:

They operate in a sequence.

*#.

The terminals of a power %O&-/T are called

A. emitter4 base4 collector

B. source4 gate4 drain

C. source4 base4 drain

D. emitter4 gate4 drain

Answer: Option B

Explanation:

In a %O&-/T terminals are source4 gate4 drain irrespective of rating.



*5.

Assertion (A): The surge current which an &,$ can withstand is much higher than rms onstate current.

Reason (R): The duration of surge current is very small.





Answer: Option A

Explanation:

&ince duration of surge current is small4 the associated energy is small.

*!. A single phase half wave controlled rectifier circuit has a free wheeling diode. The load is acombination of $ and . The firing angle is a. The period of conduction of &,$ and free

wheeling diode respectively are

A. a F Gt F p and p F Gt F 82p : a9

B. a F Gt F p and p F Gt F 2p

C. a F Gt F 8p : a9 and 8p : a9 F Gt F 2p

D. a F Gt F p and p F Gt F 82p 6 a9

Answer: Option A

Explanation:



&,$ conducts from a to p and free wheeling diode from p to 82p : a9.

*".Assertion (A): A transistor requires a continuous base signal for conduction but a thyristorrequires a gate pulse.

Reason (R): Transistor find widespread application in power electronic circuits.





Answer: Option C

Explanation:

Transistors are used but not very widely.

*).

In a thyristor the gate current is increased4 then

A. anode current will increase

B. anode current will decrease

C. anode current will remain constant

D. anode current may increase or decrease

Answer: Option C



Explanation:

Hate current does not affect the magnitude of anode current.

*+.

Thyristors are not suitable for logic circuits.

A. True B. -alse

Answer: Option B

Explanation:

Thyristors can be used for logic circuits.

#.

A single phase half wave rectifier is feeding a resistive load. Input voltage v ' >m sin Gt . The

output dc voltage is >dc and output rms voltage is >rms . If firing angle is 1)4 >dcand

>rms respectively are

A. and

B.

C. and 6 >m

D.

Answer: Option A

Explanation:



Eut a ' 1) in /qs

Otherwise also a ' 1) means no conduction.

@ence Cero output voltage.

#1.

The number of leads in an &,$ are

A. 2 B. *

C. # D. 5

Answer: Option B

Explanation:

Anode4 cathode and gate.

#2.

In a single phase full wave converter 8%62 connection9 feeding on $ 6 load4 the input

voltage is v ' >m sin Gt . The e0pression for dc output voltage is

A.

B.

C.



D.

Answer: Option C

Explanation:

Bue to inductive load the period of conduction is a to p : a in each half cycle.

#*.

A thyristor has a turn on time of ! Js. If the anode circuit is inductive4 the turn on time will be

A. ! Js

B. less than !Js

C. more than ! Js

D. either ! Js or less

Answer: Option C

Explanation:

Inductive load increases the turn on time.

##.The total number of leads in reverse conducting thyristor is

A. 2 B. *

C. 1 D. #



Answer: Option B

Explanation:

#5.

Assertion (A): A T$IA, is a bidirectional &,$.

Reason (R): A T$IA, is a four layers * terminal device and can conduct in both directions.





Answer: Option A

Explanation:

&ince a T$IA, can conduct in both directions it is bi directional &,$.

#!.

If >dc1 is the dc output voltage in half wave controlled rectifier circuit feeding resistive loadand >dc2 is the dc output voltage in full wave controlled rectifier circuit 8% 6 2 connection9

feeding resistive load4 then



A. >dc1 ' .5 >dc2 for all values of firing angle

B. >dc1 ' >dc2 for all values of firing angle

C. >dc1 K .5 >dc2 for all values of firing angle

D. >dc1 L .5 >dc2 for all values of firing angle

Answer: Option A

Explanation:

and >dc ' 81 : cos a9.

#".

A semi converter operates in two quadrants.

A. True B. -alse

Answer: Option B

Explanation:

It operates only in quadrant.

#).

-igure shows a chopper feeding $/ load4 The free wheeling diode conducts when



A. thyristor is on

B. thyristor is off

C. both when thyristor is on and thyristor is off

D. partly when thyristor is off and partly when thyristor is on

Answer: Option B

Explanation:

-B is forward biased only when thyristor is off.

#+.

Assertion (A): An ideal rectifier should have unity harmonic factor

Reason (R): An ideal rectifier should have unity displacement factor.







Answer: Option D

Explanation:

Ideal rectifier should have Cero harmonic factor and unity displacement factor.

5.

A single phase half wave rectifier is feeding a resistive load. 3hen a ' 4 dc output voltage is12" >. 3hen a ' *4 the dc output voltage will be about

A. 12" >

B. 12 >

C. 11 >

D. 1 >

Answer: Option B

Explanation:

Msing /q

' .+**.

@ence >dc for * firing angle ' .+** 0 12" ' < 12 >.

power elex sec.1

Documents