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Foreword

Civil Services (Main) Examination

Dear Students,

It is observed that a very signifi cant number of the questions given in the engineering optional in Civil Services (Main) Examination during the period 1995-2018 have been regularly appearing in ESE main examination also. Good knowledge about these questions will be useful to Electrical and Communication Engineering students in the ESE Main Examination.

With a view to assisting candidates appearing for ESE examination, ACE Academy takes pride in bringing out a complete set of solutions of the Engineering related questions in the Civil Services (Main) Examination.

Most of these questions are conceptual and of high standard. Detailed solutions explaining how basic concepts are applied at various stages of solving the questions have been provided. This would enable the students to understand the importance of fundamentals and apply them to other similar problems.

The solutions have been prepared with utmost care. Inspite of this, if there are errors of any type, we will be grateful to be informed of the same to [email protected].

Best wishes to all the aspirants and also the engineering faculty who would like to go through the following pages.

Success is a result of effort and effort is a result of ambition.

With best wishes to all those who wish to go through the following pages.

Y.V. Gopala Krishna Murthy,M Tech. MIE,

Chairman & Managing Director,ACE Engineering Academy.

ACE Engineering Publications.

Civil Services Exam Syllabus (ELECTRICAL)

01. Circuit Theory: Circuit components; network graphs; KCL; KVL; circuit analysis methods: nodal analysis, mesh analysis; basic network theorems and applications; transient analysis: RL, RC and RLC circuits; sinusoidal steady state analysis; resonant circuits; coupled circuit; balanced 3-phase circuits; Two-port networks.

02. Signals & Systems: Representation of continuous-time and discrete-time signals & systems; LTI systems; convolution; impulse response; time-domain analysis of LTI systems based on convolution and differential/difference equations. Fourier transform, Laplace transform, Z-transform, Transfer function. Sampling and recovery of signals DFT, FFT Processing of analog signals through discrete-time system.

03. E.M. Theory: Maxwell’s equations, wave propagation in bounded media, Boundary conditions, re lection and refraction of plane waves. Transmission line: traveling and standing waves, impedance matching, Smith chart,

04. Analog Electronics: Characteristics and equivalent circuits (large and small-signal) of Diode, BJT, JFET and MOSFET. Diode circuits: clipping, clamping, recti ier. Biasing and bias stability. FET ampli iers. Current mirror; Ampli iers: single and multi-stage, differential, operational, feedback and power. Analysis of ampli iers; frequency-response of ampli iers. OPAMP circuits. Filters; sinusoidal oscillators: criterion for oscillation; single-transistor and OPAMP con igurations. Function generators and wave-shaping circuits. Linear and switching power supplies

05. Digital Electronics: Boolean algebra; minimization of Boolean functions; logic gates; digital IC families (DTL, TTL, ECL, MOS, CMOS). Combinational circuits: arithmetic circuits, code converters, multiplexers and decoders. Sequential circuits: latches and lip- lops, counters and shift-registers. Comparators, timers, multivibrators. Sample and hold circuits, ADCs and DACs. Semiconductor memories. Logic implementation using programmable devices (ROM, PLA, FPGA).

06. Energy Conversion: Principles of electromechanical energy conversion: Torque and emf in rotating machines. DC machines: characteristics and performance analysis; starting and speed control of motors, Transformers: principles of operation and analysis; regulation, ef iciency; 3-phase transformers. 3-phase induction machines and synchronous machines: characteristics and performance analysis, speed control.

07. Power Electronics and Electric Drives: Semiconductor power devices: diode, transistor, thyristor, triac, GTO and MOSFET-static characteristics and principles of operation; triggering circuits; hase control recti iers; bridge converters: fully-controlled and half controlled; principles of thyristor choppers and inverters; DC-DC converters; Switch mode inverter; basic concepts of speed control of DC and AC Motor drives applications of variable speed drives.

08. Analog Communication: Random variables: Continuous, discrete; probability, probability functions. Statistical averages; probability models; Random signals and noise: white noise, noise equivalent bandwidth; signal transmission with noise; signal to noise ratio. Linear CW modulation: Amplitude modulation: DSB, DSB-SC and SSB. Modulators and Demodulators: Phase and Frequency modulation: PM & FM signals; narrowband FM; generation & detection of FM and PM, Deemphasis, Preemphasis. CW modulation system: Superhetrodyne receivers, AM receivers, communication receivers, FM receivers, phase locked loop, SSB receiver Signal to noise ratio calculation for AM and FM receivers.

Electrical EngineeringCivil Serives (Main) Examination Paper-I Weightage

Weightage (2016 to 2018)

S.No. Name of the Subject 2016 2017 2018

01 Circuit Theory 40 50 50

02 Signals and Systems 40 50 30

03 EM Theory 60 50 50

04 Analog Electronics 60 50 50

05 Digital Electronics 20 50 50

06 Energy Conversion 80 50 60

07 Power Electronics & Electric Drives 60 50 70

08 Analog Communication 40 50 40

Total Marks 400 400 400

I. Circuit Theory

01. Basic Concepts ......................................................................................................... 02 - 14

02. Circuit Theorems .................................................................................................... 15 - 28

03. Transient Circuit Analysis.................................................................................... 29 - 39

04. AC Circuit Analysis ................................................................................................ 40 - 55

05. Magnetic Circuits .................................................................................................... 56 - 63

06. Two Port Networks ................................................................................................ 64 - 75

07. Graph Theory ............................................................................................................ 76 - 78

08. Three Phase Circuits .............................................................................................. 79 - 81

09. Network Functions ............................................................................................... 82 - 82

II. Signals and Systems

01. Introduction ......................................................................................................... 84 - 88

02. LTI Systems .......................................................................................................... 89 - 91

03. Fourier Series ..................................................................................................... 92 - 92

04. Fourier Transform ............................................................................................ 93 - 99

05. Laplace Transform ............................................................................................ 100 - 111

06. Z-Transform ......................................................................................................... 112 - 124

CONTENTSSNNOCONTENTS

III EM Theory

01. Static Fields ........................................................................................................ 126 - 134

02. Maxwell's Equations and EM Waves ........................................................ 135 - 161

03. Transmission Lines .......................................................................................... 162 - 170

04. Wave Guides ........................................................................................................ 171 - 173

05. Antenna ................................................................................................................ 174 - 176

06. Miscellaneous .................................................................................................... 177 - 178

IV Digital Electronics

01. Boolean Algebra ................................................................................................. 180 - 187

02. Combinational and Sequential Circuits..................................................... 188 - 210

03. Logic Gate Families .......................................................................................... 211 - 214

V Power Electronics and Electric Drives

01. Basics and Power Semiconductor Devices ............................................. 216 - 221

02. AC-DC Converters .............................................................................................. 222 - 243

03. DC-DC Converters ............................................................................................. 244 - 260

04. DC-AC Converters .............................................................................................. 261 - 272

05. Fundamentals of Drives .................................................................................. 273 - 282

VI Energy Conversion

01. Transformers ......................................................................................................... 284 - 318

02. Induction Machines ............................................................................................. 319 - 372

03. Synchronous Machines ...................................................................................... 373 - 434

04. DC Machines ........................................................................................................... 435 - 468

05. Traction .................................................................................................................... 469 - 480

VII Analog Electronics

01. Basics of Power Semiconductors and Diodes ............................................ 482 - 492

02. BJT Circuits ............................................................................................................... 493 - 511

03. FET Circuits ............................................................................................................. 512 - 517

04. Feedback Ampli iers and Oscillators ............................................................ 518 - 520

05. OP-Amp Circuits .................................................................................................... 521 - 557

06. Miscellaneous ........................................................................................................ 558 - 560

VIII Analog Communication

01. Amplitude Modulation ......................................................................................... 562 - 583

02. Angle Modulation ................................................................................................... 584 - 594

03. Receivers .................................................................................................................... 595 - 607

04. Random Variables and Noise ............................................................................ 608 - 620

Hyderabad • Delhi • Bhopal • Pune • Bhubaneswar • Lucknow • Patna • Bengaluru • Chennai • Vijayawada • Vizag • Tirupati • Kolkata • Ahmedabad ACE Engineering Publications

01. Using Thevenin’s theorem determine the

current in the 2 resistor in the circuit of Figure given below. (ICS-96-20M)

Sol: Vth is found from circuit shown in fig.1 Nodal analysis at node-A

210

36V20V

824V ththth

5Vth120+2Vth+4Vth–144 = 80

11Vth264 = 80

V11344Vth

Rth is found from circuit shown in Fig.2

Rth = 8//20//10

th

1 1 1 1R 8 20 10

1140R th

The Thevenin equivalent circuit shown in Fig.3

2R

VIth

th

2

1140

11344

= 5.54 A 02. Calculate the power fed to the 15 resistor by

using Thevenin’s equivalent circuit for the portion to the left of points A and B in the circuit of Figure given below. (ICS-97-20M)

8

10

20

36V

24V

2A

A

B

+ 2

8

10

20

36V

24V

2A A

B

+ Vth +

–

Fig.1

16

6.8

4 100V 0.147Vs

Vs

4.8 10

10

5

15

B

A

+ +

Fig.1

8

10 20

A

B Rth

Fig.2

+ 2

I A

B

Rth=40/11

V11

344thV

Fig.3

Chapter

2 CircuitTheorems

16

CircuitTheory ACE Engineering Publications


–10V 8A 1

1 1

5 +

Sol: The given circuit is shown in Fig. Vth is found from the circuit shown in Fig.2

)16//4(8.6

100I

2.38.6

100

I = 10 A Vs = 10 6.8 = 68 V 0.147 Vs = 0.147 68

= 9.996 Vth = 9.996 10 = 99.96 V 100 V Rth is found from the circuit shown in Fig.6 To find the Rth make all independent sources

= 0. hence Vs = 0

I = 0.147 0 = 0 (open circuit) Rth = 10+10+5 = 25 The Thevenin’s equivalent circuit with load

RL= 15 is shown in Fig.7

40

96.99I

RIP 215 = 15

4096.99 2

P15 = 93.675 W 03. Determine the current in the 5 ohm resistor

using Thevenin’s theorem. (ICS-99-20M) Sol: Vth is found from circuit shown in figure 1.

16

6.8

4 100V 0.147Vs

Vs

4.8 10

10

5 B

A

+ +

Fig.3 + 15

25

99.96V

A

B Fig.7

0.147Vs

4.8 10

10

5

Vth

B

A

Fig.5

16//4

6.8

100V

Vs + +

Fig.4

16

6.8

4 100V 0.147Vs

Vs

4.8 10

10

5

15

B

A

+ +

Fig.2

4.8 10

5

10

Fig.6

Rth


01. A filter has transfer function H(f) = rect

(f/2B). where

elsewhere0

2/1x1xrect

An input x(t) = 2W sinc (2Wt) is applied to this filter, where

x

xsinxcsin

(i) Find the output y(t) for W < B (ii) Find the output y(t) for W > B (iii) In which case does the output suffer

distortion? Justify your answer. [ICS-02-(7+7+6)M]

Sol: H(f) = rect

B2f

Input x(t) = 2W sinc (2Wt)

Fourier transform of Arect

t is

2

2

tj dtAe

2

2

tj

jeA

2

j2j

eej

A

2sinj2

jA

fsinf2

A2

Af

fsin

= A sinc (f) For input x(t) = 2W sinc (2Wt)

F.T of x(t) is rect

W2f

Filter function H(f) is Input function X(f) is

(i) y(t) for W < B = ? Y(f) = X(f) H(f) = X(f) [As W < B, whole X(f) is

passed] y(t) = x(t) y(t) = 2W sinc (2Wt)

Chapter

4 FourierTransform

A sinc (t) F.T

frectAfArect

sinc (t) F.T

frect

A rect

t

F.T A sinc(f)

2

2 t

A

Arect

t

–B

H(f)

B f

–W

X(f)

W f

1

94

Signals&Systems ACE Engineering Publications


(ii) for W > B Y(f) = X(f) H(f) = H(f) [As W > B only from –B

to B carrier will be passed] y(t) = h(t)

H(f) = rect

B2f

h(t) = 2B sinc(2Bt) y(t) = 2Bsinc(2Bt)

(iii)output suffers from distortion in case (ii) In case (i) whole input is passed through

the filter but in case (ii) some part of input is distorted, only from –B to B carrier is passed because of input, filter characteristics.

02. A rectangular pulse g(t) exists between

t = –(T/2) and t = + (T/2). Sketch the pulse obtained by convoluting g(t) with itself. The Fourier transform of g(t) is a sine function. Write down the Fourier transform of the pulse obtained by the above convolution.

(ICS-03-10M) Sol.

g(t) =

2Ttu

2Ttu

on convolution of g(t) with g(t), we get

2Ttu

2Ttu

2Ttu

2Ttu

= r(t + T) –r (t)–r(t) +r(t–T) F(t) = r(t+T)–2r(t) +r(t–T)

Fourier Transform of g(t) is

= dt1e2T

2T

tj

2T

2T

tj

je

2Tj

2Tj

eej1

2Tsinj2

j1

2Tsin

2

2T2Tsin

T

2TTsa

F(t) = g(t) g(t) By taking Fourier T/F on both sides, we get F() = G() G() = G2() Fourier T/F of convolved signal

= 2

2TsaT

=

2TsaT 22

=

2

2

2T2Tsin

T

= 2

2

2

2T

2Tsin

T

2T

2T t

g(t)

t

F(t)

–T T

T


t 01. Discuss the volt-ampere characteristic of a

Triac. Explain its usefulness in speed control applications. (ICS-97-20M)

Sol: Volt ampere characteristic of a triac:

A triac conducts in both directions.

A triac is a bidirectional thyristor with three terminals.

Blocking mode: With no gate signal, triac will block both half cycles of the ac applied voltage in case peak voltage is less then break device voltage of VB01, VB02 of the triac.

Taking MT1 as reference for measuring voltage and current at gate and MT2 terminal. 1. Conduction when MT2 is positive with

respect to MT1 In 1st quadrant: (i) With gate positive with respect to

MT1, MT2 positive with respect to MT1conduction in I quadrant

MT2 is positive with respect to MT1, P1N1, P2N2 are forward biased but P2N1 is reverse biased

When gate terminal is positive with respect to MT1, gate current flows through P2N2. When gate current has injected sufficient charge in P2 layer, reversed biased junction N1P2 breaks. As a result triac conducts through P1N1P2N2.

(ii) With gate negative with respect to MT1 and MT2 is positive with respect to MT1Conduction in I quadrant

Gate negative with respect to MT1, gate current flows through P2N3 junction and reverse biased N1P2 is

Chapter

1 BasicsandPowerSemiconductorDevices

–VB02 VB01

Ig2 Ig1 Ig0=0

–Ig2 0=Ig0 –Ig1

–V +V

MT2 is positive

MT2 is positive

MT2 is negative –Ia

Ia

MT2

MT1 G

MT2

MT1

T1 T2

G

N4

N2 N3

P1

N1

P2

MT1 G

MT2

217

Basics&PowerSemiconductorDevicesACE Engineering Publications


+Va(V)

– Ia(A)

Ia(A)

VAK is positive

–Va(V)

–VBO2

–Igo –Ig1 –Ig2

VBo1

Ig2 Ig1 Ig0 Ig2 > Ig1 > Ig0

VAK is negative

Static V-I characteristics of TRIAC

forward biased. Triac starts conducting through P1N1P2N2. Due to voltage drop across path. A current is established in P2 layer from left bright consisting of main structure P1N1P2N2 conducts.

In I quadrant with gate positive with respect to MT1sensitivity is greatest.

2. In III quadrant: MT2 negative with respect to MT1 (i) When gate is positive with respect to

MT1 Ig forward biases P2N2, layer N2

injects electrons into P2 layer, as a result N1P1 breaks down P2N1P1N4 is turned ON

(ii) When gate is negative with respect to MT1.

N3 acts as remote gate, Ig flows from P2 to N3

Reverse biased junction N1P1 broken.

P2N1P1N4 is turned ON. In III quadrant, with negative gate

with respect to MT1 sensitivity is highest.

02. (i) What is a TRIAC? Explain its

characteristics? Discuss its applications, advantages and disadvantages.

(ii) A voltage across SCR is 1 V when it is conducting. it has a holding current, of 2 mA when gate current (IG) = 0. If the SCR is triggered on by momentary pulse

of gate current, to what value must VA be reduced in order to turn the SCR off? The figure is shown below.

[ICS-05-(15+5)M]

Sol: (i) TRIAC is a bidirectional device which

conducts from both anode to cathode and cathode to Anode. Triac is derived by combining Triode and AC. Its operation is equivalent to two SCR’S connected in Antiparallel

Static V-I characteristics of TRIAC:

When VAK is positive, positive gate current is applied. After some break over voltage Triac current increases. If more

A

K G

A K

G

G Triac symbol

VA

VAK 45

218

PowerElectronics&Drives ACE Engineering Publications


gate current is applied, with less break over voltage only it conducts when VAK

is negative, negative gate current is applied, after break over voltage Triac conducts in Reverse direction. Some applications of triac are heat control (A.C voltage controller),for speed control of small single phase series and induction motors(Electric fans), lamp dimmers.

Advantages: It can switch both halves of an A.C

waveform. single gate controls conduction in both directions.

Disadvantages: Triac doesn’t fire symmetrically on both

sides of waveform, due to this leading to high level of harmonics. So more EMI problems. Care must be taken for firing the gate circuit when RL load is connected. If properly firing pulse is not applied, only one half of waveform only will get. Gate has no control over conduction once triac is on just like SCR. Triac has very high switching delay.

(ii) IH = 2 mA SCR will turn off when Ia Holding

current Ia 2mA

45

VVmA2 1A

Given voltage across SCR = 1V

45

1V102 A3

VA = (90 10–3)+1 = 0.09+1 VA = 1.09 V

03. The SCR shown in fig. has a dtdi limit of 10

A/s. It is to be operated from a 100 V d.c

supply with load resistance R = 50.

(i) What is the minimum value of load inductance L that will protect the SCR?

(ii) If an RsCs snubber is connected across the

SCR with Rs = 500, what will be the new

value of load inductance L to protect the SCR

against dtdi ? (ICS-16-10M)

Sol: Given that ;s/A10dtdi,50RL

Vs = 100V; Rs = 500

LtR

s e1RV)t(i

VA

VAK 45

V1

RL

L

VS

R = 50 L

VS = 100 V

+

–

SCR ig

284 Energy Conversion ACE

Hyderabad|Delhi|Bhopal|Pune|Bhubaneswar|Lucknow|Patna|Bengaluru|Chennai|Vijayawada |Vizag |Tirupathi|Kukatpally|Kolkatam | AhmedabadACE Engineering PublicationsACE Engineering PublicationsACE Engineering PublicationsACE Engineering PublicationsACE Engineering Publications

V

Chapter

1 Transformers

285TransformersACE


j

eL

e e

ZS SZ Z



LS

o

e

jSj Z

o

ejZ j

j

I KI

I

I KI

469TractionACE


Chapter

5 Traction



471 Traction ACE




473 Traction ACE


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