Basic Electrical Engg. - May 2016

First Year Engineering (P Cycle) (Semester 1)

TOTAL MARKS: 100
TOTAL TIME: 3 HOURS (1) Question 1 is compulsory.
(2) Attempt any four from the remaining questions.
(3) Assume data wherever required.
(4) Figures to the right indicate full marks.

Choose correct answers for the following;

1(a)(i) Fig.shows a part of a closed circuit. What is the potential difference between points A and B.

1(c) Find the unkown currents in the different sections of the circuit shows in Fig.Q1(c) using KCL. Also find the unknown voltage in each resistor source.

(5 marks)

Choose correct answers for the following;

2(a)(i) An alternating current is given by i=10 sin 314t.The time taken to genrate two cycle of genrate two cycle of current is,
A) 0.025
B) 0.01 S
C) 0.04 S
D) 0.05 S(1 marks) 2(a)(ii) The averge value of sin θ over a complete cycle is
A) Zero
B) +1
C) -1
D)½(1 marks) 2(a)(iii) An a.c. series circuit has R=6Ω,X_L=20Ω and X_C=12Ω.The circuit power factor will be
A) 0.8 lagging
B) 0.5 leading
C) 0.6 lagging
D) 0.6 leading(1 marks) 2(a)(iv) In the circuit shown in Fig.Q2 (a)-(iv), the current through the inductor is 0.7A, while that through the capacitor is 0.5 A. The current is
A) 0.1 A
B)12 A
C) 0.2A
D) 0.25 A

(1 marks) 2(b) Derive an equation from fundamental principle for R.M.S value of sinusoidal current.(5 marks) 2(c) An inductor coil is connected to supply of 250 V, 50Hz and takes a current of 5 A. The coil dissipates 750 W. Calculate (i) Power factor (ii) Resistance of coil (iii) Inductance of coil.(6 marks) 2(d) Two circuit with impedance of Z₁=(10+j15)Ω and Z₂=6-j8 Ω respectively are connected in parallell. If the current supplied is 15 A ,find each branch current and their phase angle with respect to the total current.What is voltage across combination?(5 marks)

Choose correct answers for the following;

3(a)(i) The advantage of star connected supply system is that
A) Line current is not equal to phase current.
B) Two voltage can be used.
C) Phase sequence can be easily changed.
D) None of these(1 marks) 3(a)(ii) The algebraic sum of instataneous phase voltages in a three phase circuit is equal to,
A) Zero
B) One
C) Line voltage
D) Phase voltage(1 marks) 3(a)(iii) Total instantaneous powerof three phase system is
A) Constant
B) Equal to the three times average power per phase
C) Zero
D) Both (A) and (B)(1 marks) 3(a)(iv) Three identical resistance connected in star consumes 4000 W. If same resistance are connected in Delta across same supply, the power cinsumed will be,
A) 4000 W
B) 6000 W
C) 8000 W
D) 12000 W(1 marks) 3(b) Derive equation, relates line current and phase current of Delta connected system. Draw phasor diagram.(5 marks) 3(c) A balanced star-connected load is supplied from symmetrical 3-phase, 400 V (L-L) supply. The current in each phase is 50 A and lags 30' behind the phase voltage. Find (i) Phase voltage (ii) Phase impedance (iii) Active and reactive power drawn.(5 marks) 3(d) The total power absorbed by certain 3-phase balanced load 68kw.The reactive power being 100 KVAR. Determine the power factor of the load readings on two wattmeters connected to read the power.(6 marks)

Choose correct answers for the following;

4(a)(i) Which of the following instrument is suitable for measuring both a.c and d.c.quantities.
A) Permanent magnet moving coil
B) induction type ammeter
C) Quadrant meter
D) Moving iron attraction type.(1 marks) 4(a)(ii) In the energy meter, constant speed of the disc is provided by
A) Shunt magnet
B) Series magnet
C) Breaking magnet
D) None of these(1 marks) 4(a)(iii) Good earthing should provide resistance in earth path.
A) Low
B) High
C) Medium
D) None of these(1 marks) 4(a)(iv) The material used for fuse wire should be,
A) Low resistivity and high melting point
B) High resistivity and high melting point
C) High resistivity and low melting
D) Low resistivity and low melting point(1 marks) 4(b) Explain necessity of earthing. With neat sketch explain pipe earthing.(7 marks) 4(c) With neat connection diagram and switching table. Explain the two way control of a lamp.(4 marks) 4(d) With neat diagram, explain the of operation of dyanomete type wattmeter.(5 marks)

Choose correct answers for the following;

5(a)(i) In dc machine, the field winding is placed on
A) Stator
B) Rotor
C) Both on stator and rotor
D) None of these(1 marks) 5(a)(ii) Torque in dc motor is porportional to,
A) Flux only
B) Armature current only
C) Both flux and armature current
D) None od these(1 marks) 5(a)(iii) In d.c shunt generator, field winding is connected
A) In series with armature
B) Across the armature
C) Combination of series and parallel
D) None of these(1 marks) 5(a)(iv) In dc generator terminal voltageis less than induced emf because
A) Drop in armature resistance only
B) Armature reaction
C) Brush drop
D) All these(1 marks) 5(b) Derive emf equation of DC genrator.(5 marks) 5(c) Explain working principle of DC motor.(5 marks) 5(d) A 220 V, 8.8 kW ,1000 rpm, dc shunt generator has armature resistance of 0.5Ω and field winding resistance of 110 ohm. The brush drop is 1 V per brush. Find the armature voltage at full load.(6 marks)

Choose correct answers for the following;

6(a)(i) Transformer operates on the basis of,
A) Faraday 's law
B) Ohm's law
C) Kirchoff 's
D) None of these(1 marks) 6(a)(ii) The winding connected to the energy source
A) Primary
B) Secondary
C) Bothe primary and secondary
D) None of these(1 marks) 6(a)(iii) In transformer , voltage are transformed in the
A) Direct ratio of turns
B) Direct ratio of currents
C) Current in inverse ration
D) Option (A) and (c)(1 marks) 6(a)(iv) Fixed loss in the transformer comprises of
A) Copper loss
B) Hysteresis loss
C) Eddy current loss
Both (A) and (c)(1 marks) 6(b) Derive emf equation of a 1-φ transformer .(5 marks) 6(c) Write a note on losses in the transformer.(6 marks) 6(d) A single phase 10 KVA , 11000/220 V transformer has core loss = 300 W at rated voltage and copper loss =400 W at full load . Find the efficiency of transformer feeding to a load of 8 KVA at 0.8 pf lagging.(5 marks)

Choose correct answers for the following;

7(a)(i) Sailent pole machines usually have
. A) More number of poles
B) Less number of poles.
C) Small diameter
D) Long cores.(1 marks) 7(a)(ii) A 4-pole , 1200rpm alternator will generate an emf at frequency
. A) 60Hz
B) 50Hz
C) 40Hz
D) 25Hz(1 marks) 7(a)(iii) For full pitch winding, coil span factor
. A) Less than one
B) One
C) Zero
None of these(1 marks) 7(a)(iv) The number of slip rings in synchronous generator is,
A) 5
B) 3
C) 2
D) Nil(1 marks) 7(b) With neat sketch explain salient role and non-salient pole construction of motor of an alternator .(6 marks) 7(c) Define coil span factor and distribution factor .(4 marks) 7(d) A these phase, 50 Hz, 20-pole, salient pole alternator with star connected stator winding has 180 slots on the stator These are 8 conductor per slot and coil are full pitch. The flux per pole is 25 mub. Assuming sinusoidaly distributed flux K_f=0.96. Calculate (i) Speed (ii) Generated emf (iii) the line e.m.f(6 marks)

Choose correct answers for the following;

8(a)(i) Magnitude of roating magnetic field in three phase induction moto
A) 1.5φ_max
B) 1.25φ_max
C) φ_max
None of these(1 marks) 8(a)(ii) Speed of the 3-φ induction motor
A) Equal to synchronous speed
B) Less than synchronous speed
C) More than synchronous speed
None of these(1 marks) 8(a)(iii) When an induction motor is stand still, its slip is
A) Zero
B) One
C) 0.5
D)∞(1 marks) 8(a)(iv) Speed of induction motor N equals to
A) N=(1-S)N_s
B) N=f 'N_s
C) N=_SNs
D) None of these(1 marks) 8(b) Explain concept of rotating magnetic field in the 3-φ induction motor(5 marks) 8(c) Explain construction of 3-φ squirrel cage induction motor.(6 marks) 8(d) A three phase induction motor is wound for four poles and is supplied from 50Hz supply. Calculate (i) Synchronous speed (ii) the speed of the rotor when slip is 4% (iii) the rotor frequency when the speed of the rotor is 1450 rpm.(5 marks)