Power Electronics Comprehension

1. IGBT possessa) low input impedanceb) high input impedancec) high on-state resistanced) second breakdown problems

Answer: b

Explanation: Like MOSFET IGBT possess high input impedance.

2. IGBT & BJT both posses ___a) low on-state power lossesb) high on-state power lossesc) low switching lossesd) high input impedance

Answer: a

Explanation: Low on state power loss is one of the best parameters of both BJT & the IGBT.

3. The three terminals of the IGBT area) base, emitter & collectorb) gate, source & drainc) gate, emitter & collectord) base, source & drain

Answer: c

Explanation: IGBT is a three terminal device. It has a gate, a emitter & a collector.

4. In IGBT, the p+ layer connected to the collector terminal is called as thea) drift layerb) injection layerc) body layerd) collector Layer

Answer: b

Explanation: It is called as a injection layer, because it injects holes into the n- layer.

5. The controlling parameter in IGBT is thea) IG

b) VGE

c) IC

d) VCE

Answer: b

Explanation: The controlling parameter is the gate to emitter voltage, as the device is a voltage controlled

device.

6. In IGBT, the n- layer above the p+ layer is called as thea) drift layerb) injection layerc) body layerd) collector Layer

Answer: a

Explanation: It is called as the drift layer because its thickness determines the voltage blocking capabilities

of the device.

7. The voltage blocking capability of the IGBT is determined by thea) injection layerb) body layerc) metal used for the contactsd) drift layer

Answer: d

Explanation: The drift layer which is a n- layer determines the voltage blocking capabilities.

8. The controlled parameter in IGBT is thea) IG

b) VGE

c) IC

d) VCE

Answer: c

Explanation: The controlling parameter is the gate to collector current.

9. The structure of the IGBT is aa) P-N-P structure connected by a MOS gateb) N-N-P-P structure connected by a MOS gatec) P-N-P-N structure connected by a MOS gated) N-P-N-P structure connected by a MOS gate

Answer: c

Explanation: The IGBT is a semiconductor device with four alternating layers (P-N-P-N) that are controlled

by a metal-oxide-semiconductor (MOS) gate structure without regenerative action.

10. The major drawback of the first generation IGBTs was that, they hada) latch-up problemsb) noise & secondary breakdown problemsc) sluggish operationd) latch-up & secondary breakdown problems

Answer: d

Explanation: The earlier IGBT’s had latch-up problems (device cannot turn off even after the gate signal is

removed), and secondary breakdown problems (in which a localized hotspot in the device goes into thermal

runaway and burns the device out at high currents).

11. In the output characteristics of a MOSFET with low values of Vds, the value of the on-state resistance isa) Vds/Igb) Vds/Idc) 0d) ∞

Answer: b

Explanation: The o/p characteristics Is a plot of Id verses Vds, which for low values of Vds is almost

constant. Hence, the on-state resistance is constant & the slop is its constant value.

12. At turn-on the initial delay or turn on delay is the time required for thea) input inductance to charge to the threshold valueb) input capacitance to charge to the threshold valuec) input inductance to discharge to the threshold valued) input capacitance to discharge to the threshold value

Answer: b

Explanation:It is the time required for the input capacitance to charge to the thresholdvalue, which depends

on the device configuration. The device can start conducting only after this time.

13. Choose the correct statementa) MOSFET suffers from secondary breakdown problemsb) MOSFET has lower switching losses as compared to other devicesc) MOSFET has high value of on-state resistance as compared to other devicesd) All of the mentioned

Answer: b

Explanation: MOSFET has lower switching losses due to its unipolar nature & less turn off time. All of the

other statements are false.

14. Which among the following devices is the most suited for high frequency applications?a) BJTb) IGBTc) MOSFETd) SCR

Answer: c

Explanation: MOSFET has the least switching losses among the rest of the devices.

15. Choose the correct statementa) MOSFET has a positive temperature co-efficientb) MOSFET has a high gate circuit impedancec) MOSFET is a voltage controlled deviced) All of the above statements are correct

Answer: d

Explanation: MOSFETs are voltage controlled devices. They have high gate circuit impedance and are PTC

devices.

16. Consider an ideal MOSFET. If Vgs = 0V, then Id = ?a) Zerob) Maximumc) Id(on)d) Idd

Answer: a

Explanation: Gate current = 0 so device is off (ideally).

17. For a MOSFET Vgs=3V, Idss=5A, and Id=2A. Find the pinch of voltage Vpa) 4.08b) 8.16c) 16.32d) 0V

Answer: b

Explanation: Use Id = Idd x [1-Vgs/Vp]2

18. How does the MOSFET differ from the JFET?a) JFET has a p-n junctionb) They are both the samec) JFET is small in size

d) MOSFET has a base terminalView Answer

Answer: a

19. The basic advantage of the CMOS technology is thata) It is easily availableb) It has small sizec) It has lower power consumptiond) It has better switching capabilities

Answer: c

Explanation: Complementary MOS consumes very less power as compared to all the earlier devices.

20. The N-channel MOSFET is considered better than the P-channel MOSFET due to itsa) low noise levelsb) TTL compatibilityc) lower input impedanced) faster operation

Answer: d

Explanation: The N-channel are faster than the P-channel type.

21.For a power transistor, if the forward current gain α = 0.97, then β = ?a) 0.03b) 2.03c) 49.24d) 32.33

Answer: d

Explanation: Use the relation α = β/(β+1).

22. The power electronics devices have a very high efficiency becausea) cooling is very efficientb) the devices traverse active region at high speed & stays at the two states, on and offc) the devices never operate in active regiond) the devices always operate in the active region

Answer: b

Explanation: They are efficient due to their higher transition speeds.

23. For a power transistor, which of the following relations is true?a) Ie>Ic>Ib

b) Ib>Ic>Iec) Ic>Ie>Ibd) Ie=Ib

Answer: a

Explanation: Practically speaking Ie = Ib+Ic. Ie is the highest as it is the sum of thecollector and base

currents. The base current is the smallest.

24. High frequency operation of any device is limited by thea) forward voltage ratingb) switching lossesc) thermal conductivityd) heat Sink arrangementsView Answer

Answer: b

Explanation: Lower the switching losses higher the frequency of operation of the device.

25. The instantaneous power loss during the delay time of a transistor is given bya) Ic Vceb) Ib Vbec) Ic Vbed) Ib Vce

Answer: a

Explanation: During the delay time only the collector current flows & base to emittervoltage is zero.

26. For a power transistor, the average power loss during the delay time can be given by the equationa) Ic * Vcb) 1/T * ∫Td (Ic Vce) dtc) Ic * dVc/dt * Td) 1/T * ∫(Td *Tr) (Ic Vc) dt

Answer: b

Explanation: During the delay time only, the collector current flows & base to emittervoltage is zero. Hence

the average power can be found, simply by integrating it over the total delay time & dividing by the base time

period.

27. A 1mv of i/p gives an output of 1V, the voltage gain as such would bea) 0.001b) 0.0001

c) 1000d) 100

Answer: c

Explanation: 1V/1mv = 1000

28. Which of the following relations is true for a BJT?a) Ic ≈ Ieb) Ib ≈ Icc) Ie ≈ Ibd) Ib ≈ Ie ≈ Ic

Answer: a

Explanation: The collector & emitter current differ only by the base current, which is very very small.

29. Choose the correct statementa) A transistor will remain on as long the the base current is appliedb) A transistor remains on after a high to low pulse is applied at the basec) A transistor will remain on as long the the collector current is appliedd) A transistor remains on after a high to low pulse is applied at the collector

Answer: a

Explanation: Unlike the thyristor devices, all the transistor family devices remain in the conducting state as

long as the firing pulses are applied. This is a very important property of the transistor devices.

30. Let’s say that a transistor is operating at the middle of the load line, then a decrease in the current gain woulda) move the Q point upb) move the Q point downc) result in to & fro motion of the Q pointd) not change the Q point

Answer: b

Explanation:The current gain would decreases the collector current, shifting the Q point below.

31.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.

32).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.

33).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 li

Hence (A) is correct option.

34). 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, SSol:

Characteristics are as


35)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.


36) 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.

37) 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 ampsAnswer is (B)

38) 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


39) 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

Hence (B) is correct option.

40) 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


41)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


42) 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

43) 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)

44) “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)

45) 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)46) 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)

47) 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


48) 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)/

cosα = 550

So each thyristor will reverse biased for 1800− 550 = 1250.Hence (A) is correct option.

49) 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

50) 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

51)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


52) 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)

53) 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 SSol:

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)

53) 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 AHence (C) is correct option.

54) 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

Hence (C) is correct option.55) 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


56) 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 ΩSol:

Vb =12± 4 V

Vbmax = 16 V

Vbmin = 8 VRequired value of R= =800Ω


57) 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


58) 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:

59) 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*2Peak power = (Vm peak)2/R

=10580 watts

Answer is(D)

59)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)

60) 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

61) 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.


62)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.


63) 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


64) 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)

65)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

66) 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)

67) 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)

68) 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

69)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.


70) 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


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

obtained whenA. 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

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

A) only even harmonics B) both odd and even harmonicsC) 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.

73) 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.

74) 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

75). 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. 5-10µsB. SCR 2. 100-400 µsC. MOSFET 3. 50-100 µsD. 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)

76). 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.

77) 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

78) 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.

79) 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.

80) 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)

81) 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

Sol: answer is (C)

82) 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.

83) 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.

84) 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 1800D)00to 450

Sol: answer is (b)

85) 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)

Power Electronics Comprehension

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