Biju Patnaik University of Technology 2008-2nd Sem B.Tech (B Tech) , basic electrical engineering . - Question Paper
BPUT(B Tech) , second semester basic electrical engineering ques. paper.
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BENG 1102
Second Semester Examination - 2008 BASIC ELECTRICAL ENGINEERING Full Marks -70
Time: 3 Hours
Answer Question No. 1 whtcf? is compulsory and afly five from the re$i.
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The figures in the nght*hand margin indicate marks.
1 Answer the following questions: 2x10
(a) A vottmelef V of 23 kilo-ohm resistance in series wilh a resistor R across a 230 V supply reads 92 volts. Calculate the value of resistor R.
(b) A resistance R is connected across a potential difference of 110 vofts and
. dissipates energy at the rate of 220 watts. Calculate the value of resistance R.
(c) Calculate the voltage at the terminals of the coil of resistance 5 ohms and inductance 10 henries at the instant when the current is 15 amperes and increasing at the rate of 5 ampere per second. What is the stored energy in the coil ?
(d) A 5-microfarad capacitor is connected to a constant DC voltage source of 230 volts through a resistance R. Across the capacitor a neon lamp is connected that strikes at 130 volts. Calculate R to make the lamp strike 4 seconds after switch has been closed.
(e) A circuit consists of a resistor of 15 ohms in series with an inductor of 0.2 H. The frequency is 50 Hz. Calculate the conductance and the susceptance of the circuit.
(f) A resistor of 115 ohms is connected in series with a 50 microfarad capacitor to a supply at 230 V, 50 Hz. Find the voltage across the capacitor.
(g) A 3-phase balanced star-connected 'load is connected to a symmetrical 3-phase 415 V balanced supply. The current in each phase is 45 amperes and lags 45 degrees behind the corresponding phase voltage. Find the phase voltage and the total power.
(h) An iron ring with a circular cross section of
4 cm diameter and a mean circumference of 80 cm is wound with a coil of 400 turns. For an exciting current of 2.5 A in the coil, the flux is found out to be 1.5 mWb. Calculate the relative permeability of iron. (H0 = 47i x 10_7H/m).
(i) What do you mean by the term 'slip' of a three-phase induction motor ? Calculate the siip of a 6-pole induction motor running at 1140 RPM while being connected to a 60 Hz three-phase source.
(j) Write down the expression for the emf per turn induced in the windings of a singlephase two winding transformer. Explain the various terms in it.
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and inductance 0.6 H in series with a capacitor. The resonant frequency of the circuit is 65 Hz. Jf the supply given to the above series combination is 300 V, 70 Hzt find
(i) the line current
(ii) the power factor and
(iii) the voltage across the capacitor. 7
5. (a) A magnetic circuit comprises two parts in series, each of uniform cross sectional area (c.s.a.). They are
(i) iron of length 90 mm and c.s.a.75 mm2
(ii) an airgap of length of 0,6 mm and c.s.a. 82 mm2.
A coil wound on the iron part takes a current of 100 A to produce a flux density of 0.56 Tesla in the airgap, The relative permeability of iron is 2500. Estimate the number of turns required in the coil assuming all the flux to pass through the given magnetic circuit. 7
(b) A 4-pole d.c. shunt generator supplies a load of 20 A at 220 V. The field current of the generator under this condition is 2 A. The armature resistance of the generator is 0,5 ohm. Calculate the generated emf under no-load. Neglect the brush-contact drops as well as armature reaction and field saturation. 3
6. (a) Derive the emf equation for the emfs
induced in the windings of a single-phase two winding transformer from first principles. 5
(b) How does a single-phase autotransformer differ from a single-phase two winding transformer ? Show the way a single-phase two winding transformer is connected to obtain a single-phase autotransformer,
5
7. (a) A moving coil instrument requires 25 mA
current for a full-scale deflection. It has a resistance of 5 ohms. Calculate the resistance required in parallel to enable the instrument to read up to 5 A. 3
(b) Derrve the expression lor the 'em! generated by a dc. generator. What are (he various methods of excitation adopted in case of a d.c. machine ? 4+3
8 (a) Draw neatly the lorquefp' characteristic of a 3*phase wound rotor induction motor and show therein the stable region of operation. How the curve wilt change tf an external resistance is added in series with each phase of the rotor circuit ?
5
(b) Explain with the help of a block diagram the principle of power generation in a nuclear power plant. 4
(c) What is the starting torque in case of a single-phase induction motor ? 1
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| Earning: Approval pending. |
