Gate Power Electronics Eee

SREENIVASA INSTITUTE OF TECHNOLOGY AND MANAGEMENT

STUDIES,CHITOOR

POWER ELECTRONICS OBJECTIVE TYPE QUESTIONS

1).The fully controlled thyristor converter in the figure is fed from a single-phase source. When the firing

angle is 00, the dc output voltage of the converter is 300V. What will be the output voltage for a firing

angle of 600, assuming continuous conduction?

A) 150V B) 210V C) 300V D) 100 V

Sol:

Given fully-controlled thyristor converter, when firing angle α = 0, dc output voltage Vdc0 = 300 V

If α = 600, then Vdc=?

we know for fully-controlled converter

Vdc0 = (2 √2Vdc1 / π) cos α

Since α = 0, Vdc0 = 300V

300 = (2 √2Vdc1 / π)cos 0

Vdc= 300π /2 √2

at α = 600, Vdc2 =?

Vdc0 = (2 √2 / π) * (300 π / 2 √2)cos 60

= 300 * 0.5 = 150 v.

Hence (A) is the correct option.

2).An SCR is considered to be a semi-controlled device because

(A) It can be turned OFF but not ON with a gate pulse.

(B) It conducts only during one half-cycle of an alternating current wave.

(C) It can be turned ON but not OFF with a gate pulse.

(D) It can be turned ON only during one half-cycle of an alternating voltage wave.

Sol:

SCR has the property that it can be turned ON but not OFF with a gate pulse, So SCR is being considered

to be a semi-controlled device.

Hence (C) is correct option.

3).The circuit shows an ideal diode connected to a pure inductor and is connected to a purely sinusoidal

50 Hz voltage source. Under ideal conditions the current waveform through the inductor will look like.

Sol:

Current wave form for iL

𝑉𝑙. = 𝐿𝑑𝑖𝑙

𝑑𝑡

𝑖𝑙 = 1

2∫ 𝑣𝑙𝑑𝑡

For 0 <𝑤𝑡<𝜋 𝑣𝑙 = 𝑣𝑖𝑛 = 10 sin 𝑤𝑡 =𝑑𝑖𝑙

𝑑𝑡

𝑖𝑙 =1

2∫ 𝑣𝑙𝑑𝑡 = − cos 100𝜋𝑡 + 𝑐

At 100 𝜋t=𝜋/2 il= 0, c= 0

𝑖𝑙 = − 100 cos 𝜋𝑡

𝑖𝑙 (Peak) = 1 AMP for π <ωtvL= vin= 0

10 = 𝐶1 ∗50

10−6

𝐶1 =50

10∗ 10−6 =0.2 micro farads.

Hence (A) is correct option.

4). Match the switch arrangements on the top row to the steady-state V -I characteristics on the lower

row. The steady state operating points are shown by large black dots.

(A) P-I, Q-II, R-III, S-IV (B) P-II, Q-IV, R-I, S-III

(C) P-IV, Q-III, R-I, S-II (D) P-IV, Q-III, R-II, S

Sol:

Characteristics are as


5)In the single phase voltage controller circuit shown in the figure, for what range of triggering angle (),

the input voltage (V0) is not controllable?

(A) 00<<450 (B) 450<<1350

(C) 900<<1800 (D) 1350<<1800

Sol:

R +jXL=50+50j

Since tanφ = ωL/R= 50/50 =1

φ = 450

So, firing angle ‘α’ must be higher the 450, Thus for 0<α<450, V0is uncontrollable.


6) A 3-phase voltage source inverter is operated in 1800conduction mode. Which one of the following

statements is true?

(A) Both pole-voltage and line-voltage will have 3rdharmonic components

(B) Pole-voltage will have 3rdharmonic component but line-voltage will be free from 3rd harmonic

(C) Line-voltage will have 3rd harmonic component but pole-voltage will be free from 3rd harmonic

(D) Both pole-voltage and line-voltage will be free from 3rdharmonic Components

Sol:

A 3-φ voltage source inverter is operated in 180c mode in that case third harmonics are absent in pole

voltage and line voltage due to the factor cos(nπ/6). So both are free from 3rd harmonic components.

Hence (D) is correct option.

7) A single phase source inverter is feeding a purely inductive load as shown in the figure the inverter is

operated at 50 Hz in 1800square wave mode. Assume that the load current does not have any dc

component. The peak value of the inductor current i0 will be

(A) 6.37 A (B) 10 A (C) 20 A (D) 40 A

Sol:

f=50 Hz t=20ms

Voltage source inverter feeds purely inductive load, then current is triangular

I peak =(Vs/L)*(t/4)=(200/0.1)*(20*10(-3)/4)

=10 amps

Answer is (B)

8) A single phase fully controlled converter bridge is used for electrical braking of a separately excited dc

motor. The dc motor load is represented by an equivalent circuit as shown in the figure.

Assume that the load inductance is sufficient to ensure continuous and ripple free load current. The

firing angle of the bridge for a load current t of I0 = 10 A will be

(A) 440 (B) 510 (C) 1290 (D) 1360

Sol:

Here for continuous conduction mode, by Kirchhoff’s voltage law,

Average load current

V −2Ia+150 = 0

Ia = (V+150)/2

I1 = 10 A,

So V =−130 V

2 (Vm/ π) cosα =−130

((2* √2*230)/ π)cos α =−130

α = 1290


9) A three phase fully controlled bridge converter is feeding a load drawing a constant and ripple free

load current of 10 A at a firing angle of 300. The approximate Total harmonic Distortion (%THD) and the

rms value of fundamental component of input current will respectively be

(A) 31% and 6.8 A (B) 31% and 7.8 A

(C) 66% and 6.8 A (D) 66% and 7.8 A

Sol: Total rms current 𝐼𝐴 = √2

3∗ 10 = 8.16 𝐴

Fundamental current𝐼𝐴1 = 0.78 ∗ 10 = 7.8 𝐴

Where THD = √1

𝐷𝐹2 − 1

𝐷𝐹 = 𝐼𝑎1

𝐼𝑎=

0.78∗10

0.816∗10= 0.955

THD = √1

0.9552 − 1 = 31%

Hence (B) is correct option.

10) In the circuit shown in the figure, the switch is operated at a duty cycle of 0.5. A large capacitor is

connected across the load. The inductor current is assumed to be continuous.

The average voltage across the load and the average current through the diode will respectively be

(A) 10 V, 2 A (B) 10 V, 8 A

(C) 40 V 2 A (D) 40 V, 8 A

Sol:

In the given diagram

When switch S is open en I0=IL=4A,Vs=20 V

When switch S is closed ID=0,V0=0 V

Duty cycle = 0.5 so average voltage is V/(1- δ)

Average current = (0+4)/2 = 2Amp

Average voltage = 20/ (1-0.5)= 40 V


11) A single-phase fully controlled thyristor bridge ac-dc converter is operating at a firing angle of

250and an overlap angle of 10c with constant dc output current of 20 A. The fundamental power factor

(displacement factor) at input ac mains is

(A) 0.78 (B) 0.827 (C) 0.866 (D) 0.9

Sol:

Firing angle α = 250

Overlap angle μ = 100

so,

I0 =Vm/ωLs[cos α − cos(α+μ )]

Since 20 =230√2

2𝜋∗50𝑙𝑠(cos 25 − cos(25 + 10))

Ls = 0.0045 H

𝑉𝑜 =2𝑉𝑚 cos 𝛼

𝜋−

𝑤𝐿𝑠𝐼𝑜

𝜋

= 187.73 − 9 = 178.740

Displacement factor = Vo Io/Vs. Is

= 0.78


12) A three-phase, fully controlled thyristor bridge converter is used as line commutated inverter to feed

50 kW power 420 V dc to a three-phase, 415V(line), 50 Hz ac mains. Consider dc link current to be

constant. The rms current of the thyristor is

(A) 119.05 A (B) 79.37 A (C) 68.73 A (D) 39.68 Al

Sol:

Given data P=50kw Vo=420v

V1=415 v

P=Vo*Io

Io=50000/420=119.047

RMS value =Io/3=68.73

Hence (C) is correct answer

13) A single phase full-wave half-controlled bridge converter feeds an inductive load. The two SCRs in

the converter are connected to acommon DC bus. The converter has to have a freewheeling diode.

(A) Because the converter inherently does not provide for freewheeling

(B) Because the converter does not provide for free-wheeling for high values of triggering angles

(C) Or else the free-wheeling action of the converter will cause shorting of the AC supply

(D) Or else if a gate pulse to one of the SCRs is missed, it will subsequently cause a high load current in

the other SCR.

Sol:

Answer is (A)

14) “Six MOSFETs connected in a bridge configuration (having no other power device) must be operated

as a Voltage Source Inverter (VSI)”. This statement is

(A) True, because being majority carrier devices MOSFETs are voltage driven.

(B) True, because MOSFETs hav inherently anti-parallel diodes

(C) False, because it can be operated both as Current Source Inverter (CSI) or a VSI

(D) False, because MOSFETs can be operated as excellent constant current sources in the saturation

region.

Answer is (B)

15) A single-phase voltages source inverter is controlled in a single pulse-width modulated mode with a

pulse width of 150c in each half cycle. Then find out the conduction time,

(A) 0μs<t <25μs (B) 25μs<t <50μs

(C) 50μs<t <75μs (D) 75μs<t <100μs

SOL:

Conduction time of auxiliarythyristor is LC=50μs .After this time ,mainThmwill turn off.

So answer is (C)

Common data

16) A 1:1 Pulse Transformer (PT) is used to trigger the SCR in the adjacent figure. The SCR is rated at 1.5

kV, 250 A with IL = 250 mA, IH = 150 mA, and IGmax = 150 mA, IGmin = 100 mA.The SCR is connected to

an inductive load, where L = 150 mH in series with a small resistance and the supply voltage is 200 V dc.

The forward drops of all transistors/diodes and gate-cathode junction during ON state is 1.0 V

1) The resistance R should be (A) 4.7 kΩ (B) 470 kΩ (C) 47 Ω (D) 4.7 Ω

Sol:

Here, Vm= maximum pulse voltage that can be appliedSo10 = 0 −1−1−1=7V

Here 1 V drop is in primary transistor side, so that we get 9V pulse on the secondary side. Again there is

1 V drop in diode and in gate Cathode junction each.

Ig max = 150 mA

So R = VmIIg max = 7/150 mA

= 46.67 Ω


2) The minimum approximate volt-second rating of pulse transformer suitable for triggering the SCR should be : (volt-second rating is the maximum of product of the voltage and the width of the pulse that may applied)

(A) 2000 μV-s (B) 200 μV-s (C) 20 μV-s (D) 2 μV-s

Sol:

We know that the pulse width required is equal to the time taken by iato rise uptoiL

So,

Vs= L (di /dt) +Ri(Vt approx. equal to 0)

Ia = (200/1) (1-𝑒−𝑇

0.5)

Here also t = T, ia=iL=0.25

0.25 =200[1−e−T/0.5]

T = 1.876 *10-4= 187.6 μs

Width of pulse = 187.6 μs

So,𝑉03

𝑉01𝑚𝑎𝑥=

4𝑉𝑠3𝜋

∗sin(3∗72)

4𝑉𝑠𝜋

sin 72= 19.61%


17) The speed of a 3-phase, 440 V, 50 Hz induction motor is to be controlled over a wide range from

zero speed to 1.5 time the rated speed using a 3-phase voltage source inverter. It is desired to keep the

flux in the machine constant in the constant torque region by controlling the terminal voltage as the

frequency changes. The inverter output voltage vs frequency characteristic should be

Sol:

In case of induction motorV/f

If is constantf (linear)

Hence answer is (c)

18) A voltage commutation circuit is shown in figure. If the turn-off time of the SCR is 50 μsec and a

safety margin of 2 is considered, then what will be the approximate minimum value of capacitor

required for proper commutation?

(A) 2.88 Μf (B) 1.44 μF (C) 0.91 μF (D) 0.72 μF

Sol:

We know that

T = RCln 2

So

C= T/(R* 0.693)

= 2.88 μF


19) A solar cell of 350 V is feeding power to an ac supply of 440 V, 50 Hz through a 3-phase fully

controlled bridge converter. A large inductance is connected in the dc circuit to maintain the dc current

at 20 A. If the solar cell resistance is 0.5 Ω then each thyristor will be reverse biased for a period of

(A) 1250 (B) 1200 (C) 600 (D) 550

Sol:

Let we have

Rsolar = 0.5 Ω, i0 = 20 A

SO,Vs =350−20*0.5=340 V

Hence 340=(3*440*√(2 cosα)/𝜋

cosα = 550

So each thyristor will reverse biased for 1800− 550 = 1250.


20) A single-phase bridge converter is used to charge a battery of 200 V having an internal resistance of

0.2 Ω as shown in figure. The SCRs are triggered by a constant dc signal. If SCR2 gets open circuited,

what will be the average charging current?

(A) 23.8 A (B) 15 A (C) 11.9 A (D) 3.54 A

Sol:

In this circuitry if SCR gets open circuited, than circuit behaves like a half wave rectifier.

So

Iavg = Average value of current

=1

2𝜋𝑅∫ (𝑉𝑚 sin 𝑤𝑡 − 𝐸)𝑑𝜃

𝜋−𝜃1

𝜃1

𝜃1 = sin−1𝐸

𝑉𝑚

=0.380=0.66 rad

Iavg= 11.9 A


21) An SCR having a turn ON times of 5 μsec, latching current of 50 A and holding current of 40 mA is

triggered by a short duration pulse and is used in the circuit shown in figure. The minimum pulse width

required to turn the SCR ON will be

(A) 251 μsec (B) 150 μsec (C) 100 μsec (D) 5 μsec

Sol:

Ton=5 μ sec IL=50 m amp

IH=40 m amp

From circuit i=(Vs/R)*(1-e(-R/L)*t)+Vs/R =IL=50 m amp

t=150μsec

Hence (B) is the correct

22) Common data

A voltage commutated chopper operating at 1 kHz is used to control the speed of dc as shown in figure.

The load current is assumed to be constant at 10 A

1)The minimum time in μsec for which the SCR M should be ON is

A) 280 b) 140 c) 70 d) 0

Sol:

Minimum time is required for change the polarity of capacitor from Vs to –Vs

i.e. t1=/w0=*LC

=140 μsec


2) The average output voltage of the chopper will be

a) 70V b) 47.5V c) 35V d)0V Sol:

V0=(Vs/T)*(Ton+2Tc)

Tc=CV/I0

V0=47.5v


23) The figure shows the voltage across a power semiconductor device and the current through the

device during switching transitions. If the transition a turn ON transition or a turn OFF transition? What

is the energy lost during the transition?

(A) Turn ON, VI/2(t1+ t2) (B) Turn OFF, VI(t1+ t2)

(C) Turn ON, VI(t1+ t2) (D) Turn OFF, VI/2(t1+ t2)

Sol:

Energy loss =V (0.5*I*t1) +I (0.5*V*t2)

=VI/2(t1+t2)

It is turn on process, voltage across device decrease &current increase

Answer is (A)

24) An electronics switch S is required to block voltage of either polarity during its OFF state as shown in

the figure (a). This switch is required to conduct in only one direction its ON state as shown in the figure

(b)

Which of the following are valid realizations of the switch S?

(A) Only P (B) P and Q (C) P and R (D) R and S

Sol:

Electronic switch described in the statement should have forward conduction state, forward blocking

state & reverse blocking state. SCR,NPN transistor with series diode exhibits the above states.

Hence answer is (c)

25) The given figure shows a step-down chopper switched at 1 kHz with a duty y ratio D = 0.5. The peak-

peak ripple in the load current is close to

(A) 10 A (B) 0.5 A (C) 0.125 A (D) 0.25 A

Sol:

∆𝐼𝑚𝑎𝑥 =𝑉𝑠

4𝑓𝐿= 0.125 A


26) An electric motor, developing a starting torque of 15 Nm, starts with a load torque of 7 Nm on its

shaft. If the acceleration at start is 2 rad/sec2, the moment of inertia of the system must be (neglecting

viscous and coulomb friction)

(A) 0.25 kg-m2 (B) 0.25 Nm2 (C) 4 kg-m2 (D) 4 Nm2

Sol:

Tst = 15 Nm

TL = 7 Nm

α = 2 rad/sec2

T = Iα

SO T=Tst−TL=8 Nm

I = 8/2 =4 kgm2


27) Consider a phase-controlled converter shown in the figure. The thyristor is fired at an angle α in

every positive half cycle of the input voltage. If the peak value of the instantaneous output voltage

equals 230 V, the firing angle α is close to

A) 600 B) 1350 C) 1250 D)1150 SOL:

We know that Vrms = 230 V

SO ,Vm= 230*√ 2 V

If whether α is< 900

Then Vpeak =Vmsinα=230

230*√ 2 sinα = 230

angleα = 1350


28) The triggering circuit of a thyristor is shown in figure. The thyristorrequires a gate current of 10 mA,

for guaranteed turn-on. The value of R required for the thyristorto turn on reliably under all conditions

of Vbvariation is

(A) 10000 Ω (B) 1600 Ω (C) 1200 Ω (D) 800 Ω

Sol:

Vb =12± 4 V

Vbmax = 16 V

Vbmin = 8 V

Required value of R=𝑉𝑏𝑚𝑖𝑛

𝐼𝑔=800Ω


29) The circuit in figure shows a 3-phase half-wave rectifier. The source is a symmetrical, 3-phase four-

wire system. The line-to-line voltage of the source is 100 V. The supply frequency is 400 Hz. The ripple

frequency at the output is

(A) 400 Hz (B) 800 Hz (C) 1200 Hz (D) 2400 Hz

SOL:

Ripple frequency =3f=3*400=1200 Hz

So from V0 ripple frequency = 1200 Hz


30) A MOSFET rated for 15 A, carries a periodic current as shown in figure. The ON state resistance of

the MOSFET is 0.15 Ω. The average ON state loss in the MOSFET is

(A) 33.8 W (B) 15.0 W (C) 7.5 W (D) 3.8 W

Sol:

31) The triac circuit shown in figure controls the ac output power to the resistive load. The peak power

dissipation in the load is

(A) 3968 W (B) 5290 W (C) 7935 W (D) 10580 W

Sol: Given circuit is AC voltage controller feeding resistive load

Vs=230*2 sin wt

R=10 ohm

Peak instantaneous voltage Vm peak=230*2

Peak power = (Vm peak)2/R

=10580 watts

Answer is(D)

32)Figure shows a chopper operating from a 100 V dc input. The duty ratio of the main switch S is 0.8.

The load is sufficiently inductive so that the load current is ripple free. The average current through

thediode D under steady state is

(A) 1.6 A (B) 6.4 A (C) 8.0 A (D) 10.0 A

Sol:

Given α=0.8 the avgout put in chopper ckt is

VO=α*Vs

=0.8*100=80

Avg current through diode Io=Vo/R=8 amp

During chopper operation switch s conducts during turn on time (Ton) & diode (D) conducts during turn

OFF time(Toff)

Avg current through

Diode (IDA)=Io*(Toff)/T

(Toff)=(1-α)T

(IDA)=1.6 amp

Answer is (A)

33) A variable speed drive rated for 1500 rpm, 40 Nm is reversing under no load. Figures show the

reversing torque & speed during the transient .The moment of inertia of drive is

(A) 0.048 kg-m2 (B) 0.064 km-m2 (C) 0.096 kg-m2 (D) 0.128 kg-m2

Sol:

Generated TorqueTe=j(dW/dt)

40 =j(2/60500-(-1500))/0.5

J=0.096 kg-m2

34) Figure shows a thyristor with the standard terminations of anode (A), cathode (K), gate (G) and the

different junctions named J1, J2 and J3. When the thyristor is turned on and conducting

(A) J1 and J2 are forward biased and J3 is reverse biased

(B) J1 and J3 are forward biased and J2 is reverse biased

(C) J1 is forward biased and J2 and J3 are reverse biased

(D) J1, J2 and J3 are all forward biased

Sol:

When thyristor turned on at that time J2 junction will break. So J1, J2, J3 all are in forward bias.


35)Figure shows a MOSFET with an integral body diode. It is employed as a power switching device in

the ON and OFF states through appropriate control. The ON and OFF states of the switch are given on

the VDS −IS plane by

Sol:

The ON-OFF state of switch h is given on VDS −IS plane as following

When VDS =+ve, diode conducts and IS = 0

VDS =−ve, diode opens, but IS = 0, D"−vepotential.


36) The speed/torque regimes in a dc motor and the control methods suitable for the same are given

respectively in List-II and List-I

List-I List-II

P. Field Control 1. Below base speed

Q. Armature Control 2. Above base speed

3. above base torque

4. below base torque

Codes:

(A) P-1, Q-3 (B) P-2, Q-1 (C) P-2, Q-3 (D) P-1, Q-4

Sol:

P. Field control-Above base speed

Q. Armature control-below base torque


37) A chopper is employed to charge a battery as shown in figure. The charging current is 5 A. The duty

ratio is 0.2. The chopper output voltage is also shown in the figure. The peak to peak ripple current in

the charging current is

(A) 0.48 A (B) 1.2 A (C) 2.4 A (D) 1 A

Sol:

Given α=0.2

Ton =200µ sec T=1msec

A constant DC voltage applied to an inductor then nature of current is linear.

i =((Vs-E)/L)*t

i=Ipeak t=Ton

Ipeak=0.48 amp

Answer is (A)

38)A 3 phase fully controlled converter is feeding power into a dc load at a constant Current of 150A

.The rms current through each thyristor of the converter is

(A)50A (B) 100A

(C)150 L2 /L3 (D)150/L3

Sol:

Average current of a thyristor IDT = Idc /3

RMS current of a thyristor IRT = Irms / L3

Hence (D) is answer

39) An inverter has a periodic output voltage with the output wave form as shown in figure

When the conduction angle α = 120c, the rms fundamental component of the output voltage is

(A) 0.78 V (B) 1.10 V (C) 0.90 V (D) 1.27 V

Sol:

Shape of output volt wave form is the output voltage of single pulse modulation.

Vo=0.78V

Answer is (A)

40) With reference to the output wave form given in above figure , the output of the converter will be

free from 5th harmonic when

(A) α= 720 (B) α = 360 (C) α = 1500 (D) α = 1200

Sol:

Sin nd=0

nd=

d=/n

Width of pulse (α)=2d=720

Answer is (A)

41) The MOSFET switch in its on-state may be considered equivalent to a

(A) resistor (B)inductor (C)capacitor (D)battery

SOL:

The MOSFET switch in on-state is equivalent to a capacitor.

Hence (C) is answer

42)when the firing angle α of a single phase fully controlled rectifier feeding constant dc current into a

load is 300 , the displacement power factor of the rectifier is

(A)1 (B) 0.5 (C)1/L3 (D)L3/2

SOL:

The displacement power factor DF =CosФ, where Ф is the angle between the fundamental component

of input current and voltage

Hence (B) is answer

43) The main reason for connecting a pulse transformer at the output stage of thyristor triggering circuit

is to

(A) Amplify the power of the triggering pulse

(B) Provide electrical isolation

(C) Reduce the turn on time of thyristor

(D) Avoid spurious triggering of the thyristor due to noise

Sol:

For providing electrical isolation it is necessary to connect a pulse transformer at the output stage of a

thyristor triggering circuit.


44) AC-to-DC circulating current dual converters are operated with the following relationship between

their triggering angles (α1 and α2)

(A) α1+α2=1800 (B) α1+α2=3600 (C) α1−α2=1800 (D) α1+α2=900

Sol:

In ac to dc circulating current dual converters if triggering angles are

α1and α2, than it is necessary that

α1+α2 = 1800


45)In a commutation circuit employed to turn off an SCR, satisfactory turn-off is

obtained when

A. circuit turn-off time < device turn-off time B. circuit turn-off time > device turn-off time C. circuit time constant < device turn-off time D. circuit time constant < device turn-off time

SOL:

Satisfactory turn-off is obtained in a commutation circuit when circuit turnoff time must be greater than

turn-off time of a thyristor.

Hence (B) is answer

46)The semiconductor switch S in the circuit of figure is operated at a frequency of 20 kHz and a duty

ratio D = 0.5. The circuit operates in the steady state. Calculate the power transferred from the dc

voltage source V2.

SOL:

Given f = 20 kHz

D = 0.5

Power transferred from source V1 to V2 = ?

Time period t 50 sec

47) The conduction loss versus device current characteristic of a power MOSFET is best approximated by

A) a parabola B) a straight line B) a rectangular hyperbola D)an exponentially decaying function

SOL:

The total conduction (on-state) power loss for a given MOSFET with forward current ID and on

resistance RDS(ON) is given by

Pon ,diss = ID2RDS(ON)

Hence A) is correct option.

48) The output voltage wave form of a three-phase square-wave inverter contains

A) only even harmonics B) both odd and even harmonics C) only odd harmonics D) only nipple harmonics

SOL:

Square wave having half wave symmetry and hence contain only odd harmonics.

Hence C) is correct option.

49) In case of an armature controlled separately excited DC motor drive with closed loop speed control

,an inner current loop is useful because it

A) Limits the speed of motor to a safe value B) Helps in improving the drive energy efficiency C) Limits the peak current of the motor to the permissible value D) Reduces the steady state speed error

SOL:

The closed-loop speed control schemes are provided with inner current control loop, in order to limit

the current within a safe limit and also decelerate the drive at maximum permissible current and torque

during transient operation.

Hence C) is correct option.

50) A six pulse thyristor rectifier bridge is connected to a balanced 50 Hz three phase ac source.

Assuming that the dc output current of the rectifier is constant, the lowest frequency harmonic

component in the ac source line current is

(A) 100 Hz (B) 150 Hz (C) 250 Hz (D) 300 Hz

SOL:

The ripple frequency of a six pulse thyristor rectifier bridge is increased to six times to the input

frequency. Given input frequency is 50HZ.

Hence ripple frequency will be 300HZ. Hence, lowest frequency component in the source line content

is 300HZ.

Hence d) is correct option

51). Match list-I(devices ) with list-II(switching time) and select correct answer using code

given below the lists:

List-I list-II

A. TRIAC 1. 5-10µs

B. SCR 2. 100-400 µs

C. MOSFET 3. 50-100 µs

D. IGBT 4.200-400 µs

Codes: A B C D

(a) 4 3 2 1

(b) 1 2 3 4

(c) 4 2 3 1

(d) 1 3 2 4

Sol: IGBT have high efficiency and fast switching. Thyristors & TRIACS are both bipolar

devices. They have very low on state voltage but ,because of minority carriers in the device must

be removed before they can block an applied voltage ,switching times are comparatively long.

Answer is (b)

52). The following is a uni-polar device:

A) BJT B) IGBT C) GTO D) MOSFET

SOL: answer is (D)

BJT, IGBT and GTO are bipolar devices. MOSFET is a unipolar device.

53) A thyristor has a PIV of 650 V. The voltage safety factor is 2. Then the voltage up to which

the device can be operated is given by

A) 1300V B) 650V C) 325V D) 230V

SOL: answer is (A)

PIV is the max voltage that thyristor can withstand in reverse direction without breaking down or

avalanching.

PIV=650V

Safety factor=2

Max voltage with standing=2*650=1300v

54) When a thyristor in the forward blocking state, then

A) All 3 junctions are reversed.

B) Anode and cathode junctions are forward biased but gate junction is reverse biased.

C) Anode junction is forward biased but other two are reverse biased.

D) Anode &gate junction are forward biased but cathode is reverse biased.

Sol: answer is (b)

Forward blocking state: anode & cathode junctions are forward biased but gate junction is

reverse biased.

Forward conduction state: all the three junctions are forward biased reverse blocking state.

Anode & cathode junctions are reversing biased but gate junction is forward biased.

55) AN SCR triggered by a current pulse applied to the gate cathode can be turned off

A) By applying a pulse to the cathode

B) By applying a pulse to the anode

C) By applying another pulse of opposite polarity to the gate cathode

D) By reversing the polarity of the anode and cathode voltage

Sol: answer is (D)

If current pulse is applied to gate cathode then the SCR can only turned off by

operating it in reverse blocking state or by reversing the polarity of the anode cathode voltage.

56) In forward bias portion of the thyristors i-v characteristic, the no of stable operating regions

is

A) One B) two C) three D) none

Sol: answer is (B)

57) A dc source of 100 v supplies a purely inductive load of 0.1 H. The controller is an SCR in

series with source and load. If the specified latching current is 100ma then the minimum width of

the gating pulse to ensure turn on of SCR would be

A) 10µs B) 50 µs c) 100 µs d) 1 µs

𝑉 = 𝐿𝑑𝑖

𝑑𝑡

∫ 𝐿𝑑𝑖100

0

= ∫ 𝑣𝑑𝑡𝑡

0

T=100 µs

Sol: answer is (C)

58) A voltage source inverter is normally employed when

A) Source inductance is large & load inductance is small

B) Source inductance is small & load inductance is large

C) Source inductance is small & load inductance is small

D) Source inductance is large & load inductance is large

Sol: answer is (b)

VSI, is one in which dc source has small impedance. In VSI using thyristors ,load

commutation is possible only if load is under damped .for RCL load the condition for load to be

under damped.

𝑅 < √4𝐿

𝑐

59) A current source inverter is obtained by inserting a large

A) Inductance in series with dc supply

B) Capacitance in parallel with dc supply

C) Inductance in parallel with dc supply

D) Capacitance in series with dc supply

Sol: answer is (A)

In current source inverter the input current to almost ripple free ,a inductance in series is used

with dc source.

60) A single phase ac regulator fed from 50HZ supply feeds a load having 4Ω resistance &

12.73mh inductance .the control range of firing angle will be

(a) 00 to 1800 B)450 to 1800 c)900to 1800 D)00to 450

Sol: answer is (b)

Min value of firing angle =load phase angle

= Tan−1 𝑤𝐿

𝑅

=450

Therefore 450to 1800

61) A three phase semi converter feeds the armature of a separately excited dc motor supplying a

nonzero load torque .for steady state operation the motor current is found to assume zero value at

certain instances of time . At such instances the armature voltage

A) is equal to the instantaneous value of ac voltage

B) is equal to the instantaneous value of motor back emf

C) Assumes an arbitrary value

D) Becomes zero

Sol: answer is (b)

62) A single phase two pulse converter feeds an RL load with insufficient smoothing but the

conduction is continuous. If the resistance of the circuit is increased, then

A) The ripple content of load current will remain the same

B) Ripple content of the load current will decrease

C) Ripple content of load current will increase

D) There is possibility of discontinuous conduction due to an increase in the ripple content.

Sol: answer is (d)

63) An SCR is in conducting state, a reserve voltage is applied between anode cathode ,but is

fails to turn off. What could be the reason?

A) Positive voltage is applied to the gate B) The reverse voltage is small

C) The anode current is more than the holding current

D) Turn off time of SCR is large

Sol: answer is (c)

Holding current is min value of anode current below which it must fall for turning off the

thyristor

64) A reverse conducting thyristor normally replace

A) A pair of anti parallel thyristor in a circuit

B) A combination of a thyristor and an anti parallel diode in a circuit

C) A thyristor in situation where it is not required to have reversed blocking capability at all

D) Conventional conversion grade thyristor having large turnoff time

Sol: answer is (b)

65) A structure obtained by lightly doped in drift region between the layers of a pn junction a

PIN diode is obtained . This structure is effective in

A) Making the diode support large reverse blocking voltages

B) Making reverse recovery process slow

C) Making the diode have high on state voltage drop

D) Reducing the voltage spike during turn off due to stray inductance

Sol: answer is (b)

A pin diode obeys the standard equation for low frequency signals. There is a lot

of stored charge in the intrinsic region. At low frequencies the charge can be removed and diode

turns off. At higher frequencies there is not enough time to turns off . The PIN diode has a poor

reverse recovery time

66) Which one of the following statements is not correct for a MOSFET?

A) Are easy to parallel for higher current

B) Leakage current is relatively high

C) Have more linear characteristics

D) Over load and peak current handling capacity is high

SOL: Answer is C

67) In a GTO anode current begins to fall when the gate current

A) Is negative peak at time t=0

B) is negative at t=storage period t

C) Just begins to become negative at t=0

D) Just begins to become positive at t=0

SOL: Answer is B

68) Consider the following statements

1) A thyristor requires turn off circuit while transistor does not

2) The voltage drop of a thyristor is less than that of a transistor

3) A thyristor require a continuous gate current

4) A transistor draws continuous base current

Which of these statements are correct?

A) 1, 2, 3 & 4 B) 1 & 2 C) 2 & 4 D) 1 & 4

SOL: Answer is D

Transistor can be turned on by applying gate pulse. Once thyristor gets turned on

the gate current can be removed. Thyristor remains turned on till anode current is higher than

holding current. So, thyristor does not require continuous gate current.

69) A field effect transistor with an anti-parallel body diode blocks:

A) Bidirectional voltage and passes Unidirectional current.

B) Bidirectional voltage and passes Bidirectional current.

C) Unidirectional voltage and passes Unidirectional current.

D) Unidirectional voltage and passes Bidirectional current.

SOL: Answer is D

70) For low-speed high-power reversible operation, the most suitable drives are

A) Voltage source inverter bed ac drives.

B) Current source inverter bed ac drives.

C) Dual converter bed dc drives.

D) Cyclo-converter bed ac drives.

SOL: Answer is D

Cyclo-converter control has the advantages of smooth low-speed operation with

regeneration braking and dynamic response. But it has low-speed range and because it uses large

number thyristors it becomes economically acceptable only when the drive rating is high.

71) The use of multiphase rectifier is in place of single phase rectifier results in

A) Increased output voltage and reduced harmonics.

B) Increased output voltage and increased harmonics.

C) Decreased output voltage and reduced harmonics.

D) Increased output voltage and no effect on harmonics.

SOL: Answer is A

Multiphase rectifiers are preferred because:

1) Higher dc voltage.

2) Better TUF.

3) Better input power factor.

4) Less ripple content in output current.

5) Lower size of filter circuit parameters because of higher ripple

frequency.

72) In a three phase semi converter, for firing angle less than are equal to 600, freewheeling

diode conducts for

A) 900 B) 600 C) 300 D) 00

SOL: Answer is D

For α ≤ 600 output voltage does not become negative. Therefore freewheeling

diode comes into picture.

73) What is the waveform of the current flowing through the diode in a buck-boost converter?

A) Square wave B) Triangular wave

C) Trapezoidal wave D) Sinusoidal wave

SOL: Answer is C

74) In order to simplify the design of a converter transformer, the two converters in a dual

converter should be connected using:

A) Series connection

B) Cross connection

C) Direct anti-parallel connection

D) Anti-parallel connection

SOL: Answer is C

75) An integral cycle a.c.voltage controller is feeding a purely resistive circuit from a single-

phase a.c. Voltage source. The current waveform consists alternately burst of N-complete cycle

of conduction followed by M-complete of extinction. The rms value of the load voltage equals

the rms value of supply voltage for

A) N=M B)N=0 C)N=M=0 D) M=0

SOL: Answer is A

In integral cycle control

𝑉𝑟 = 𝑉𝑠√𝑛

𝑛 + 𝑚

M=0

𝑉𝑟 = 𝑉𝑆

76) A constant current source inverter supplies 20 A to load resistance of 1Ω to a load resistance

change to 5Ω , then the load current

A) Remains same at 20A and the load voltage changes to 100V

B) Changes to 4A from 20A and the load voltage changes to 20A

C) Changes to 4A from 20A and the load voltage changes to 80V

D) And load voltage stay at 20A and 20V respectively

SOL: Answer is A

As it is CSI ,it supplies constant current what ever may be load. In according to load resistance

the load voltage changes.

77) Static VAR controllers are used to provide dynamic voltage regulation. These controllers are

primarily of

A) Thyristor switched inductors

B) Thyristor controlled capacitors

C) Thyristor switched resistor

D) Thyristor switched inductors and Thyristor controlled capacitors

SOL: Answer is D

For voltage control, reactive power compensation is used. Thyristor switched inductors &

thyristor switched capacitors are means of reactive power compensation.

78) Consider the following devices

1. Synchronous condenser

2. Saturable reactor

3. SCCR

4. FCCR

In which of these devices, the accuracy of compensation is very high and noise level is very low?

A)1,2,3 and 4 B)3 and 4 only C)3 only D)2 and

3 only

SOL: Answer is C

SCCR (switched capacitor & controlled reactor) will have very high accuracy & noise level is

low.

79) In an LC series circuit connected to a dc supply of E volts via thyristor when it turns off the

voltage that appear across the thyristor is

A) +E B)+2E C)-E D)-2E

SOL: Answer is C

When thyristor turn on the capacitor will be charged to 2E Volts at staring. So thyristor will be

reverse biased with –E volts. Even with the application gate pulse the thyristor will not conduct

after that.

80) Consider the following devices:

1. SCR

2. GTO

3. BJT

4. MOSFET

5. IGBT

Which of the following devices do not belong to family of transistors?

A) 1&2 only B)1,2& 3 only C)2,3&5 only D)1,2,3,4&5

SOL: Answer is A

SCR & GTO belong to the family of thyristors. While BJT, MOSFET&IGBT belong to the

family of transistors.

81) The average output voltage of fly-back converter is v0 =24 V at a resistance load of R=0.8 Ω.

The duty cycle ratio is α=50% and switching frequency is f=1 KHZ. The on state voltage drop of

transistors & diodes are Vt =1.2v & Vd =0.7v . The turns ratio of transformer is a NS/NP

=0.25.what is the efficiency of converter?

A) 86.5% B)96% C)75% D)90%

SOL: Answer is B

As α=50% the diode & transistors conducts same time. Efficiency of converter

=24 − (

1.2 + 0.72 )

24∗ 100%

=96%

82) The anode current through a conducting SCR is 10A. If it is made one fourth, then what will

be the anode current?

A) 0A B) 5A C)10A D)20A

SOL: Answer is C

When SCR is turned on. Gate current have no effect on the anode current.

83) In a power circuit of 3kv four thyristors each of rating 800V are connected in series. What is

the percentage series derating factor?

A) 50 B)25 C)12.5 D)6.25

SOL: Answer is D

Derating factor=1-η

== 1 −3∗100

4∗800= 6.25%

84) For elimination of 5th harmonics from the output of an inverter, what will be the position of

pulse in a PWM inverter?

A72 deg B) 36 deg C)60 deg D)90 deg

SOL: Answer is A

To eliminate 5th harmonic

Sin 5d=0

=0,2π,

Pulse width=2d=0,2π/5

2d=2π/5=72 deg

85) What is the effect of blanking time on output voltage in PWM inverter?

A) Distortion in instantaneous voltage at current zero crossing

B) Low order space harmonics in output voltage

C) Distortion in instantaneous voltage at voltage zero crossing

D) High order time harmonics in output voltage

SOL: Answer is D

86) AN SCR is rated for 650V PIV. What is the voltage for which the device can be operated if

the voltage safety factor is 2?

A) 325 V rms B)230 V rms C)459 V rms D)650 v rms

PIV=Vm *(voltage supply factor)

650=root(2)*Vrms*2

Vrms=230v

SOL: Answer is B

87) a single phase full converter feeds power to RLE load with R=10Ω ,L=10mh &E=50 V the

ac source voltage is 230V 50HZ.for continuous conduction ,what is the average value of load

current for firing angle delay of 600 ?

A) 4.63A B)6A C)6.5A D)5.35A

SOL: Answer is D

Average value of output voltage

𝑣0 =2𝑣𝑚

𝜋Cos 𝛼

V0=103.58 V

V0=E+IR

103.58=50+I*10

I=5.35A

88) A dc chopper is used in regenerative braking mode of a dc series motor. The dc supply is

600v, duty cycle is 70%. Average value of armature current is 100A. It is continuous & ripples

free. What is the value of power feedback to the supply?

A) 3 KW B) 9KW C)18KW D)35KW

SOL: Answer is C

Average armature terminal voltage

𝑉𝑡 = (1 − 𝛼)𝑉𝑠

=(1-0.7)*600=180V

Power returned to supply vtia=180*100=180KW

89) If a full wave fully controlled converter Is modified as a full wave half controlled converter,

what will be the maximum value of active power (P) & reactive power (Q)

P Q

(a) Double half

(B) unchanged unchanged

(c) Half double

(d) Unchanged half

SOL: Answer is B

90) For a single phase ac to dc controlled rectifier to operate in regenerative mode, which of the

following conditions should be satisfied?

A) Half controlled bridge, α<90 deg , source of emf in load

B) Half controlled bridge, α>90 deg , source of emf in load

C) Full controlled bridge, α>90 deg , source of emf in load

D) Full controlled bridge, α<90 deg , source of emf in load

SOL: Answer is C

In case half controlled output will not be negative. For α<90 deg converter voltage is positive

where as for inverter mode α>90 deg as 𝑉0 =2𝑉𝑚

𝜋Cos 𝛼

91) A single phase CSI is connected with capacitive load only the waveform of output voltage

across the capacitor for constant source current will be

A) Sine wave B) square wave C) triangular wave D) step function

SOL: Answer is C

𝐼𝑐 =𝑐𝑑𝑣

𝑑𝑡; 𝑉𝑐 =

1

𝑐 ∫ 𝐼𝑐𝑑𝑡

For constant IC

𝑉𝑐 =𝐼𝑐

𝑐𝑇

Vc can be triangular wave.

92) A modern power semiconductor device that combines the characteristics of BJT & MOSFET

is

A) GTO B)FCT C)IGBT D)MCT

SOL: Answer is C

IGBT possesses high input impedance like a PMOSFET &has low on state power loss as in a

BJT .further IGBT is free from second break down problem present in BJT

93) What are the advantages 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

A) 1&3 B) 2&3 C) 1 &2 D)1,2&3

SOL: Answer is B

Device operates as switch so not in active region only in saturation & cutoff.

94) 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

SOL: Answer is B

Cyclo converter is suitable where precise control required (means not for cement & compressor

drive)

95) A buck regulator has an input voltage of 12V & the required output voltage is 5V. What is

the duty cycle of the regulator?

A) 5/12 B)12/5 C)5/2 D)6

SOL: Answer is A

For buck regulator output voltage

𝑉0 = 𝛼𝑉𝑆

5=α *12

And α=5/12

96) Which one of the following is the correct statement?

In a two quadrant converter working in the 1st & 2nd quadrants

A) Load current & load voltage are positive

B) Load current is always negative

C) Load current can be positive or negative

D) Load current & load voltage are always negative

SOL: Answer is C

No option is correct. As load voltage can be positive & negative not current for dual converter

97) What is the maximum output voltage of a 3-phase bridge rectifier supplied with line voltage

of 440V?

A)528V B)396V C)594V D)616V

SOL: Answer is C

Maximum output voltage of a 3phase bridge is for α>30 deg

𝑉0 =3𝑉𝑚𝑙

𝜋=

3√2𝑉𝑙

𝜋=

3 ∗ 440√2

𝜋= 594𝑉

98) A single phase ac voltage controller is controlling current in a purely inductive load. If the

firing angle of SCR is α. What will be the conduction angle of the SCR?

A) Π B)(π-α) C)(2π-α) D)2π

SOL: Answer is B

Firing angle α>90 deg for purely inductive load.

99) The pulse width modulated inverter for the control of an ac motor is fed from which one of

the following?

A) Controlled rectifier B) uncontrolled rectifier C) ac regulator d) cycloconverter

SOL: Answer is B

Uncontrolled that is o/p voltage is obtained by adjusting on-off period of the inverter components

.this method is called P.W.M. control

100) Which one of the following is the main advantage of S M P S over linear power supply?

A) No transformer is required B) only one stage of conversion C)no filter is required

D) Low power dissipation

SOL: Answer is D

In SMPS filter required is easy to design there are multiple stage in SMPS transformer is also

used in SMPS small physical size & less weight is main reason to wide spread use of SMPS’s.

101) Triac cannot be used in

(a) AC voltage regulators (b) cyclo converters (c) solid state type of switch

(d) inverter

Sol:- a triac is a bidirectional thyristor with three terminals. It is used extensively for the

control of power in ac circuits.

Hence correct option is ©.

102) Snubber circuit is used in thyristor circuits for

(a) Triggering (b) dv/dt protection (c) di/dt protection (d) phase shift

Sol:- dv/dt protection to limit overheating.

103) A 4 quadrant chopper can’t be operated as

(a) 1 quadrant chopper (b) cyclo coverter

(c) inverter (d) bidirectional rectifier.

Sol:- a cycloconverter changes the frequency which is not possible with a chopper.

Hence (b) is correct option.

104) It is preferable to use a train of pulse of high frequency for gate triggering of SCR in

order to reduce

(a) Dv/dt protection (b) di/dt protection (c) the size of the pulse transformer

(d) the complexity of the firing circuit

Sol:- the size of the pulse transformer at high frequency is less to induce same emf at

same flux density. For same flux density

Area ×frequency = constant

Area α 1/frequency

Hence © is correct option

105) The quality of output ac voltage of a cyclo converter 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

Sol:- output voltage of cyclo converter can be improved by increasing output voltage as

well as frequency since ripple in output voltage decreases with increase in frequency.

Hence (b) is correct option

106) The most suitable device for high frequency inversion in SMPS is

(a) BJT (b) IGBT (c) MOSFET (d) GTO

Sol: - mosfets are widely used in low power high frequency converters, therefore best

suitable device for high frequency inversion in SMPS.

Hence, © is correct option

107) In case of voltage source inverter, freewheeling can be needed for the load of

(a) Inductive nature (b) capacitive nature (c) resistive nature (d) back EMF nature

Sol: - for inductive loads, load current will not be in phase with voltage across load and

freewheeling diodes connected in anti-parallel with thyristor allow the current to flow

when the main thyristor is turned off.

Hence (a)is correct option

108) 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

Sol: -

Note: relay are relatively slow to operate and have limited life.

Many relays designs do not tolerate high inrush current very well.

Hence (a) is correct option

109) A cyclo converter 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

Sol:- cyclo converter converts input power at one frequency to output power at different

frequency with one stage conversion.

Hence (d) is correct option

110) PWM switching Is preferred in voltage source inverter for the purpose of

(a) Controlling output voltage (b) output harmonics

© reducing filter size (d) controlling output voltage, harmonics and filter size

Sol:- using PWM switching, output voltage can be controlled by varying pulse width and

therefore harmonic content in the output waveform. PWM switching can reduce low

frequency harmonics hence reducing the size of filter.

Hence (d) is correct option.

111) . When cathode of a thyristor is made more positive than its anode .

A) all the junctions are reverse biased junction

B) outer the junctions are reverse biased junction and central one is forward biased

C) outer the junctions are forward biased junction and central one is forward biased

D) all the junctions are forward biased junction

Sol :- (B) when cathode is made more positive with respect to anode , thyristor operates in

reverse. Blocking mode , and the device behaves as it two diodes are in series with reverse

voltage applied across them .

112) sharing of voltage between thyristors operating in series is influenced by the

A) Di/dt capabilities B) dv/dt capabilities

C) junction temperature D) static v-I characteristics and leakage current

Sol :- (D) on account of inherent variation in there characteristics , the voltage share by SCR is

not same

113). R-C snubber is used in parallel with the thyristor

A) Reduce dv /dt across it B) reduce di/dt through it

C) limit current through it D) ensure its conduction after gate signal is removed

Sol:- (D) The capacitor Cs in with thyristor is sufficient to prevent unwanted dv/dt triggering of

thyristor .When switch is closed capacitor act as short ckt therefore voltage across thyristor is

zero with the passage of time voltage across capacitor builds up a slow rate such that dv/dt acoss

thyristor is less than specified max dv/dt rating of the device.

114). For a step up DC-DC chopper with an input dc voltage of 220 volts, if the output voltage

required is 330 volts and the non conducting time of thyristor – chopper is 100us, the on time of

thyristor - chopper would be

A) 66.6us B) 100us C) 150us D) 200us

Sol:- (D) non conducting time of thyristor means conducting time of thyristor chopper i.e.

Ton =100us

We know

Output voltage

Vo= Vs(1/1- ∝ )

330=220(1/1-∝)

1-∝=2/3

∝ =1/3 = Ton /Ton+Toff

1+Toff/Ton =3

Toff =2Ton

Toff =2*100us

I.e. On time of thyristor chopper

115). Thyristor controlled reactor is used to get

A) variable resistance B) variable capacitance

C) variable inductance D) improved reactor power factor

Sol:- (C) firing angle controls from ∝= 90 to 180 degrees ,the effective reactance of the reactor

,as seen bythe source , can be regulater from its actual value =2πfl when ∝= 90 to an infinite

value when ∝ = 180

Hence it can be used to get variable inductance

116). How many switches are used to construct a three – phase cycloconverter .

A) 3 B) 6 C)12 D) 18

Sol:- (D) 3 phase of 3-phase cyclo converter uses three sets of 3- phase half-wave

Circuit requires 6 thyristors, so totally 18 thyristor required

117). Compare 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

Sol :- (C) output voltage of single phase full bridge inverter is two times of output voltage of

single phase half bridge inverter and output power ∝(output voltage)^2.

118). In a self controlled synchronous motor fed from a variable frequency inverter

A) The rotor poles invariably have damper winding

B) There are stability problems

C) The speed of the rotor decides stator frequency

D) The frequency of the stator decides the rotor speed

Sol :- (D) Rotor rotates at synchronous speed

Ns = 120F/p

So the rotor speed is decided by the frequency of the stator.

119). The boost regulator has an input voltage of 5V and the average output voltage of 15V . The

duty cycle is

A) 3/2 B) 2/3 C) 5/2 D) 15/2

Sol :- B) boost regulator is a step up chopper . The output voltage is

Vo = Vs/1-∝

15 = 5/1-∝

∝ = 5/15= 1/3

∝=2/3

120). Trun on and Turn off time of transistor depends on

A) static characterstics B) junction capacitance

C) current gain D) none of the above

Sol :- (B)Trun on and Turn off time of transistor depends on , junction capacitance

Because of charging and discharging of junction capacitance a transistor does not turn on and

turn off instantly.

121). Which one of the following is correct ?

A) BJT B) GTO C)MOSFET D)THYRISTOR

Sol:- (B) GTO needs no commutation circuitand it can control output voltage . Thyristor needs

forced commutation

Therefore GTO is must suitable for DC-DC converter .

122). Which one of the following is correct ? In order to get best result per unit cost, the heat

sink on which the thyristors are mounted, are made of

A) aluminum B) copper C) nickel D) stainless steel

Sol:- (D)Heat sink are made from metal with high thermal conductivity copper , being a costly

metal , is seldom used as a heat sink material and aluminum is the most commonly used metal.

123). Which one of the following capicators is suitable for compensation of harmonic and

reactive power ?

A) Mica capacitor B) Glass capacatior C) polypylene capacatior D) Elcctrolytic capacatior

Sol:- Group of electrolytic capacitor are used to get higher overall operating voltage and make

voltage waveform smooth .

124). Which one of the following is correct ? The function of bleeder resistor in a power supply

is

A) To ensure a minimum current drain in the circuit

B) To increase the DC output voltage

C) To increase the DC output current

D) same that of load resistor

Sol:- (A) resistor not consume too much power while supply is ON and voltage decay quickly to

safe level when supply is OFF.

125). Which one of the following is mainly 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) TRAIC D) UJT

Sol : MOSFET has lowest switching losses and operates at high Frequencies (up to 100KHZ)

126). 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

Sol :- (C) A 3- phase semi Converter has the uniqe feratures of working asm a six – pulse

Converter for ∝<60 and has a three –pulse Converter for ∝ >60.

127). Power electronic device with poor turn –off gain is

A) A symmetrical thyristor B) A conventional thyristor

C) power bipolar junction transistor D) gate turn – off thyristor

Sol :- (D) The turn – off gin is the ratio of anode current Ia to gate current Ig needed to turn –off

the device .

If gate current is not able to turn on the GTO ; it behaves like a high voltage , low gain tranistor

with considerable anode current

128). In single – phase semi converter with discontinuous conduction and extention angle B<π ,

freewheeling action takes place for

A) ∝ B) ∝-B C) B- π D) zero degree

Sol:- (D) when B< π,load current becomes zero before π and freewheeling diode does not

conduct.

129). In a dual converter , the circulating current

A) allows smooth reversal of load current , but increase the response time

B) allows smooth reversal of load current , improves speed of response

C) does not allows smooth reversal of load current , but reduce the response time

D) flow if there is no interconnecting inductor

Sol:- (B) dual converter with circulating current mode is preferred if load is to be raversed quite

frequently and a fast response is desired

130). The pulse –width modulation inverted for the control of an motor is fed from which one of

the following ?

A) 00, 1800 degrees respectively B) 1800,00 degrees respectively

C) 00,00 respectively D) 1800,1800 respectively

Sol:- (A) output voltage of 1- φ full wave controlled rectifier

Vo= (2Vm/ π)cos∝

Minimum value of output voltage occur at firing angle ∝ is 1800

And maximum at 00 . The conduction angle is the difference between the excitation and firing

angle or the conduction angle is the total angle through which through which the SCR conducts

=00for maxmimum output voltage

=1800 for maxmimum output voltage

131). The pulse – width modulation inverter for the control of an ac motor is fed from which one

of the following ?

A) controlled rectifier B) un controlled rectifier

C) Ac regulator D) cyclo converter

Sol:- (B)

Uncontrolled i.e. O/p voltage is obtained by adjusting on-off period of the inverter

component . This method is called PWM control

132). A single phase ac voltage controller is controlling current in a purely inductive load . If the

firing angle of a SCR ∝, what will be the Conduction angle of that SCR ?

A) π B)π-∝ C) (2 π-∝) D) 2 π

Sol :- (B) firing angle ∝ = 900

For purely inductive load

133). A modern power semiconductor device that combine the characteristics of BJT and

MOSFET is

A) GTO B) FCT C) IGBT D) MCT

Sol (C) IGBT possesses high input impedance like PMOSFET and has low on state power loss

as in a BJT . Further, IGBT is free from second breakdown problem present in BJT.

134). A bulk regulator has an input voltage of 12V and the required output voltage is 5v . What

is the duty cycle of the regulator /

A) 5/12 B) 12/5 C) 5/2 D) 6

S0l:- (A)

For bulk regulator

Output voltage Vo=∝Vs

5= ∝*12 , ∝ =5/12

135). The anode current through a conducting SCR is 10A . If its gate current is made one-

fourth, then what will be the anode current ?

A) 0 A B) 5A C) 10A D) 20A

Sol:- (C) when SCR is turned ON . Gate current has no effect on the anode current.

136). In a power circuit of 3 kv , four tyristor s each of rating 800v are connected in series . What

is the percentage series derating factor ?

A) 50 B) 25 C) 12.5 D) 6.25

Sol:- (D) Derating factor =1-efficiency

= 1- (3*1000/4*800) = 6.25%

137). For elimination of 5th harmonics from the output of an inverter , what will be the position

of pulse in a PWM inverter

A) 720 B) 360 C) 600 D) 900

Sol:- to eliminate of 5th harmonic

Sin 5d =0

5d=0, π, 2 π , 3 π…………

Pulse width = 2d = 0, 2 π/5, 4 π/5………

2d = 2 π/5 =720

138). A cycloconverter – fed induction motor drive is most is suitable for which one of the

following ?

A) compressor drive B) machine tool drive

C)paper mill drive D) cement mill drive

Sol:- (B) cycloconverter is switch where precise control reqired . In paper mill constant speed/

smoothly variable speed is required . So it can be used to drive machine tool drive

139). A large d.c. Motor is reqired to control the speed of blower from a 3- phase a.c to d.c

converter

A) 3-phase fully controlled bridge converter

B) 3-phase fully controlled bridge converter with freewheeling diode

C) 3-phase half - controlled bridge converter

D) A pair 3-phase converters in sequence control

Sol:- (C) for only motoring half controlled bridge converter is most suitable as it will have

improved power factor operation

140). What are the advantages of switching power supplies over liner power supplies

1, The device operate in liner / active region

2, The device operates as switches.

3, power losses are less

Select the correct answer using answer using the code given below

A)1 and 3 B) 2 and 3 C) 1 and 2 D) 1,2 and 3

Sol;- Device operates as switch so not in active region in saturation &cutoff region

141) For the speed control of a 3 phase SCIM the most suitable solid state converter used is

A)load commutated converter B) cyclo converter

C)current source inverter D) voltage source inverter

Sol : Hence (B) is the correct option

142) In a self controlled permanent magnet synchronous motor drive

A) the stator supply tracks the rotor speed

B) the stator has permanent magnets

C) the stator frequency is independently controlled

D) the damper winding in rotor are used for damping rotor oscillations.

Sol : (A) is the correct option

143). For an SCR , the gate cathode characteristics has a straight line slope of 140 For triggering

source voltage of 20v and allowable gate power dissipation of 0.5 watts, what is the gate source

resistance ?

A) 200 OHMS B) 255OHMS C)195 OHMS D) 185 OHMS

Sol;- (C)

Vg Ig = 0.5

And Vg /Ig = 140

Therefore Ig=59.7ma

Vg=8.36ma

For the gate circuit Es =Igrs+Vg

20 = 0.0597R s+8.36

Rs = 195 ohms

144) Rotor resistance control in a three phase IM drive using a chopper in rotor circuit is used to

control the drive

A) above synchronous speed

B) below synchronous speed

C) both above and below synchronous speed

D) upto 0.2 times the base speed from stand still.

Sol :(B) is the correct option

145). An SCR is rated for 650 v PIV. What is the voltage for which the device can be operated if

the voltage safety factor is 2?

A) 325 V rms B) 230V rms C) 459 V rms D) 650 V rms

Sol:- (B)

PIV = VM *(voltage supply factor )

650 = √2

3* Vrms *2

Vrms = 230V

146). A d.c chopper is used in regenerative braking mode of a dc series motor . The dc supply is

600V , the duty cycle is 70%. The avg value of armature current is 104 A . It is comtinuous and

ripple free. What is the value of power feedback to supply /

A) 3KW B) 9KW C) 18KW D) 35KW

Sol:- (C) Avg Armature terminal voltage

Vt = (1-∝) Vs

= (1-0.7)*600

= 180 V

Power returned to supply

Vtia =180*100 =18KW

147) Which of the following inverter control schemes allows simultaneous control of output

voltage and output frequency in a 3 phase VSI

A) SPWM sceme B) 180 deg conduction mode

C) 120 deg conduction mod D) all of the above

Sol : (A) is the correct option

148). AC-to-DC circulating current dual converters are operated with the

Following relationship between their triggering angles(α1 and α2)

A) α1+ α2 = 180c B) α1+ α2 = 360c

C ) α1− α2 = 180c D) α1+ α2 = 90c

Sol:- (A)

In ac to dc circulating current dual converters if triggering angles are

Α1 and α2, than it is necessary that

Α1+ α2 = 180c

149). The main reason for connecting a pulse transformer at the output

Stage of thyristor triggering circuit is to

(A) amplify the power of the triggering pulse

(B) provide electrical isolation

(C) reduce the turn on time of thyristor

(D) avoid spurious triggering of the thyristor due to noise

Sol:- (B)

For providing electrical isolation it is necessary to connect a pulse Transformer at the output

stage of a thyristor triggering circuit.

150). A 3-phase voltage source inverter is operated in 180c conduction Mode. Which one of the

following statements is true ?

(A) Both pole-voltage and line-voltage will have 3rd harmonic

Components

(B) Pole-voltage will have 3rd harmonic component but line-voltage

Will be free from 3rd harmonic

(C) Line-voltage will have 3rd harmonic component but pole-voltage

Will be free from 3rd harmonic

(D) Both pole-voltage and line-voltage will be free from 3rd harmonic

Components

Sol:- (D)

A 3-φ voltage source inverter is operated in 180c mode in that case ,Third harmonics are absent

in pole voltage and line voltage due to

The factor cos (nπ/6). So both are free from 3rd harmonic components

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