Biju Patnaik University of Technology 2009 B.Tech Electronics and Communications Engineering Basic Electrical Engineering - Question Paper
5. (a) A soft iron ring of relative permeability of 2000 has a mean diameter of 75 cm and a cross sectional area of 20 cm2. A radial airgap of 2 mm width is cut In the ring that Is wound with a coll of 1500 turns. Calculate the current required to produce an airgap flux of 0.55 milli-weber. Neglect leakage and fringing. " Given = 4n x10"7 H/m. 5
(b) A sfngle-phase two winding transformer is designed to operate at 230/115 V,
50 Hz. Calculate the magnitude of the secondary no-foad voltage and its frequency if the high voltage side of the transformer is connected to 2+2+1
(i) 230 V, 40 Hz
(ii) 115 V, 25 Hz
(iii) 230 V dc
6. (a) A coil is connected in series with ai capacitor of 10 micro-farads and the series combination is connected across a 250 V single-phase ac mains. The resistance of the coil is 3.5 ohms. What
BENG 1102/BE 2102 6 Contd.
must be the inductance of the coil in order that maximum current occurs at a frequency of 5
(i) 30 Hz
(ii) 40 Hz
(iii) 110 Hz?
(b) A balanced three phase star conected load of 250 kW takes a lagging current of 200 A with a line voltage of 1000 V, 50 Hz. Find the circuit constants (resistance and inductance) of the load per phase. 5
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a) Derive the mathematical expression for the charge stored in the capacitor of a R-C series circuit connected across a d.c voltage source. 4
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b) The emf (electromotive force) per turn for a single phase, 2200/220 V, 50 Hz transformer is approximately 15 volts. Calculate
(i) the number of primary and secondary turns and
BENG 1102/BE 2102 7
(ii) the net cross sectional area of the core, for a maximum flux density of
1.25 Wb/m2 in the core. 3+3
8. (a) Explain with the help of a block diagram the principle of power generation in a
5
nuclear power plant.
(b) A moving coif instrument gives a full scale deflection of 25 mA when potential difference across its terminals is 250 mV.
Calculate
5
(i) the shunt resistance for measuring . currents up to 25 A and
(ii) series resistance for measuring voltage up to 500 V.
Total number of printed pages -8 B, Tech
BENG1102/BE 2102
Second Semester Examination - 2009 BASIC ELECTRICAL ENGINEERING
Full Marks-70
Time:3 Hours
Answer Question No. 1 which is compulsory and any five from the rest.
The figures in the right-hand margin indicate marks.
1. Answer the following questions : 2x10
(a) Three resistance of 12 ohms, 24 ohms and 30 ohms magnitude are connected in series and the series combination is connected across a 220 volts d.c. source. Compute the power dissipated in each resistor and the total power drawn from the source.
P.T.O.
(b) Four resistance of 15 ohms, 30 ohms, 35 ohms and 50 ohms magnitude are connected in parallel across a 230 volts d.c. source. Compute the power dissipated in each resistor and the total power drawn from the source. What is the total current ?
(c) An inductor of inductance 50 milli-henries is connected in series with a capacitance of 10 micro-farads. Find the impedance of the circuit when the frequency is
(i) 50 Hz and
(ii) 5 kHz
(d) A direct voltage of 200 V is applied to a coil of resistance 20 ohms and inductance of 2000 milli-henries. Find the time taken for the current through the coil to reach one-half of its final value.
(e) A resistor of 25 ohms in series with a
0.45 micro-farad capacitor is connected across a supply at 270 V, 70 Hz. Find the current through the capacitor.
(f) A Circuit consists of a resistor of 10 ohms in series with an ideal inductor of
4.5 henries. The frequency is 70 Hz. Calculate the conductance and the suscep-tance of the circuit.
(g) A 3-phase balanced star-connected load is connected to a symmetrical 3-phase 440 V balanced supply. The current in each phase is 60 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
7.5 cm diameter and a mean circumference of 150 cm is wound with a coil of 750 turns. Calculate the flux in the magnetic circuit for an exciting current of 2.5 A in the coil The relative permeability of iron is 1600 and Mo equals 4n x10"7 H/m.
(i) What do you mean by the term slip' of a three-phase induction motor ? Calculate the slip of a six-pole induction.motor
running at 960 RPM while drawing power from a 50 Hz three-phase balanced source.
(j) Two admittances 0.025 -50 and 0.015 Z45 are connected in parallel. Find out the resultant impedance in rectangular form.
2. (a) Using Superposition Theorem, find the current flowing in the 40-ohm resistor in Frg.1 below. * 6
(b) State and briefly explain Thevenins Theorem* with one example. 4
3. (a) Describe the voltage build up process in a d.c. shunt generator. Define 'critical field resistance. 5
(b) A single-phase a.c, supply voftage of 230 V at 50 Hz is applied lo a coil of Inductance 4.5 henries and resistance of
2.25 ohms in series with a capacitance C. Calculate the value of the capacitance C so as to obtain a potential difference of 255 V across the coil, 5
(a) A3-phaset3-wire,415volts, 50 Hz, RYB system of balance supply is connected to a delta connected load with Zfty=120 Z40 ohms, Zys=155 Z5Q" ohms and Z**=100 Z-30* ohms. Obtain the three line currents and draw the complete phasor diagram showing the line voltages, line currents and phase currents. The supply neutral is earthed. 7
(b) What are the expressions for
(i) the 'torque developed and
(ii) 'back emf induced' in a d.c. shunt motor ?
Explain all the terms in the expressions very briefly. 3
BENG 1102/BE 2102 5 p.m
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