# WBUT Exam Papers Basic

## Electrical And Electronic Engineering-I B Tech 2nd Sem 2011

Full Marks : 35

The figures in the margin indicate full marks.

Candidates are required to give their answers in their own words

as far as practicable.

GROUP – A ( Multiple Choice Type Questions )

1. Choose the correct alternatives for any fwe of the following :                                                                                          5×1=5

i)              If a balanced load is connected across a balanced supply, the sum of three phase current is

a} zero                  ?                   b) IA

c) — Ij\ — ~ Iq                                  JC ~~ ^ B ‘

ii)           If the field of a d.c. shunt motor is opened,

a)            it will continue to run at its rated speed

b)            the speed of the motor will become very high

c)             the motor will stop

d)            the speed of the motor will decrease.

iii)         In a transformer connected to constant voltage source, increase in secondary load current will ultimately

a)           reduce the mutual flux in the core

b)           increase the mutual flux in the core

c)            not cause any change in the magnitude of mutual flux

d)           decrease the mutual flux when the secondary

current is below 0-8 pf lag.                              ,

iv)          The power drawn by 3-phase induction motor when first run is star connection & then delta is

a)  same b) V3 times that of star

c) 3 times that of star d)                      times that ol star.

v)            Frequency of the induced ernf in the rotor circuit is

a)           maximum at standstill

b)           maximum at synchronous speed

c)            zero at standstill

d)           none of these.

vl) In application of Gauss law. the current statement is/are

a)           Gauss law depends on symmetry. If symmetry does not exist, we cannot use Gauss’ law

b)           For closed surfaces D is either normal or tangential

c)            ^D.ds = Jv.D du

d)           all (a), (b) & (c).

Answer any two of the following.           2×5= 10

1. A positive charge Q is uniformly distributed over an
2. imaginary thin spherical shell of radius a in air. Using Gauss
3. law, calculate the held intensity at any point inside and

outside the shell. What shall be the field intensity at the surface of the shell ?

1. Derive speed-torque characteristics of a d.c. series motor. Discuss its applications.
2. Explain why the rotor of an induction motor rotates in the same direction as the stator magnetic field. Why the speed of the motor is less than the synchronous speed ?
3. Draw & explain the phasor diagram of a single phase transformer under loaded condition.

GROUP -C

Answer any two of the following. 2 x 10 = 20

1. a) Explain the conditions required for self excitation of a

d.c. generator.

b)          A shunt generator delivers 200 A at 250 V. The

armature resistance & shunt field resistance are 0 02 Q

& 50 £2 respectively. The iron & friction losses are equal to 950 W. Determine —

i)               the emf generated

ii)             copper losses

iii)           output of the prime mover

iv)            efficiency of the generator.                              2 + 8

1. a) A single phase 50 Hz core type transformer has core of

cross-section area 400 sq. cms. The permissible maximum flux density is 1 Wb/m2. Calculate the number of turns on the high & low voltage sides for a

3000/220 V ratio.

b)            Determine the full-load efficiency at unity power factor for the 4 kVA 200/400 V, 50 Hz single phase transformer of which the following are test figures.

G.C. test ; 200 V, 0-8 A, 70 W.

S.C. test : 17-5 V, 9 A, 50 W.                                     4 + 6

1. a} A 6-pole, 3~4>, 50 Hz induction motor develops a fu 11-load

torque of 150 nm when the rotor current makes 120 complete cycles per minute. Determine the shaft power output.

b)            A 3-phase 50 Hz, 4-pole, 400 volt induction motor has the following data referred to stator side :

jRj = 0 ■ 2 Q, R2 = 0 • 15 Q, Xj = J 0-5 Q, X2 = JO • 3 Q

The motor operates at 3% slip & the total losses including copper & constant losses are 2000 watt. The core effect may be neglected. Determine

i)                 thefstator current

ii)              p.f.

iii)          efficiency of the motor.

1. a) Explain the following terms :

Electronic field, Electric field, Intensity, Potential

difference.

b)            A balanced 3-phase star connected load is fed from a 208 volt, 3-phase supply. Each leg of the load has a resistance of 35 Q. Determine the power factor, the total power, the phase currents & line currents of the system.                          4 + 6

Full Marks : 35

The figures in the margin indicate full marks.

Candidates are required to give their answers in their own words

as far as practicable.

GROUP -A { Multiple Choice Type Questions )

1. Choose the correct alternatives for any five of the

following :

i) The action of JFET In its equivalent circuit can best be represented as a

a)           current controlled current source

b)           current controlled voltage source

c)            vflltage controlled voltage source

d)           voltage controlled current source.

iij The effective channel length of a MOSFET in saturation decreases with increase in

a)   gate voltage    b) drain voltage

c)  source voltage d) body voltage.

ili) A source follower using an FET usually has a voltage gain which is

a)           greater than +100

b)           slightly less than unity but positive

c)            exactly unity but negative

iv)         The ideal Op-Amp has which of the following characterises ?

a)            Rt = oo, A = oo, Ro = o b) Rt = 0, A – oo, Ro ■ o

c)            Rt = oo, A = oo, rq = oc d) Rt => 0, A = oo, Rq = oo.

v)            For self sustained oscillations the condition is a) Ap < 1     b) AP > 1

c)          Ap = 1       d) both (b) & (c).

vi)         Which one of the following feedback topologies offers high input impedance ?

a) Voltage series                   b) Voltage shunt

c)    Current series  d) Current shunt.

GROUP -B

Answer any two of the following.           2×5= 10

Identify the circuit and find out the output voltage VQ for the circuits given belo

U) (275.46)
10 -(?)2.
Perform the following number conversions : i) (11101.1011)2 =(?)2

Realize the Boolean expression using basic gates : y = (ABCE) + (aBCDE) + (ACD).

Deriving proper expressions explain the effect of positive feedback on gain, input impedance, output impedance, stability and bandwidth.   5×1

sinusoidal

Berkhausen criterion of

1. a) What is the relation between JFET parameters ?

b)             Draw and explain the drain characteristics of an n-channel enhancement type MOSFET.

c)             Calculate the drain current at gate voltage of – 2V for an n-channel JFET, which has saturation drain current at zero gate bias – 10 mA and pinch-off voltage = – 3-5 V.

3 + 4 + 3

1. a) What is an integrated circuit ? What is its advantage ?

b)             Draw and explain the principle of operation of an Op-Amp as an adder.                                              ( 2 + 2 ) + 6

1. Write short notes on any two of the following :                      2×5

a) Comparator

b} Universal gates

c)             Pinch-off condition of JFET

d)             CMRR.