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 0 0 , the dc output voltage of the converter is 300V. What will be the output voltage for a firing angle of 60 0 , assuming continuous conduction? A) 150V B) 210V C) 300V D) 100V Sol: Given fully-controlled thyristor converter, when firing angle α = 0, dc output voltage Vdc0 = 300 V If α = 60 0 , 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
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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.
(C) P-IV, Q-III, R-I, S-II (D) P-IV, Q-III, R-II, S
Sol:
Characteristics are as
Hence (C) is correct option.
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.
Hence (A) is correct option.
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
Hence (C) is correct option.
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
Hence (C) is correct option.
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
Hence (A) is correct option.
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
Hence (C) is correct option.
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%
Hence (B) is correct option.
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
Hence (A) is correct option.
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.
Hence (A) is correct option.
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
Hence (C) is correct option.
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
Hence (B) is correct option.
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
Hence (B) is correct option.
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 ¤t 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
Hence (C) is correct option.
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
Hence (C) is correct option.
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
Hence (B) is correct option.
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
Hence (D) is correct option.
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
Hence (C) is correct option.
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
In ac to dc circulating current dual converters if triggering angles are
α1and α2, than it is necessary that
α1+α2 = 1800
Hence (A) is correct option.
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