Biju Patnaik University of Technology 2008-1st Sem B.E Electronics Engineering Basic Electrical Engineering - Question Paper
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First Semester Examination - 2008 BASIC ELECTRICAL ENGINEERING Full Marks -70 Time: 3 Hours
Answer Question No. 1 which fs compulsory and any ftve from the rest
The figures in the right margin indicate fuff marks for the questions.
1. Answer the following questions: 2x10
(a) Three resistances of 10 ohms. 20 ohms and 25 ohms magnitude a connected m parallel across a 200 volts d.c. source. Compute the current and power drawn from the source.
P. TO.Four rewstences of 5 ohm*. 10 Ohms. 15 ohms and 20 ohms magnitude are connected n series and the series
(t
oombtfis>onuonnecisd across a200>*)*s dc.wurce Compute the voltage *oes eechrsMtor.
An inductor of Inductance 75 m*Mennes
(c)
camee a current of 10 A which reverses m
20 milliseconds. Whet is the average voltage induced the Inductor because of this current reversal ?
(d) A 15-mtciofarad capacitor in senes with a 15 k*H>hm resistance la oonneoted across a constant DC voltage source of 250 volts. The fuy charged capacitor Is disconnected from the supply and is discharged by connecting a 1000 ohm resistance across its terminals. Compute the initial value of the charging current and the initial value of the discharging current.
(0) A resistor of 20 ohms In series with a 05 H inductor Is connected across a supply at 250 V. 60 Hz. Fnd the current through the rtiuctor.
(f) A circuit consists of a resistor of 15 ohms m series with a capacitor of 50 microfarads. The frequency is 60 Hz. Calculate the conductance and the susceptance of the orcuft.
(g) A 3-phase balanced delta-connected load is connected to a symmetrical 3-phase 440 V balanced supply. The current in each
phase is 50 amperes and lags 30 degrees behind the corresponding phase voltage. Fnd the Kne client and the total power.
(h) An Iron ring with a circular cross section of 5 cm diameter and a mean circumference of 100 cm is wound with a coil of 500 turns. Calculate me exciting current in the coil to establish a flux of 1.75 mliH-weber in the magnetic circuit. The relative permeability of Iron is 1200 and m0 equals to4n xIO-7 HKn.
icNrt unsmF 2102 3 PXO.
0) What do you mean by the termsftpspeecf of e three-phase induction motor / Calculate the sfcp speed* of en eight-po* mAc&on mocor running at 730 RPW drawmg power from a 50 Hz three-phase balanced source.
09 Ttmvnpodnnoee2S/-5Pandi545>*e connectedmpemlef Find out fie rr..tant Impedance in rectangular form
2. (a) UsingThevennstheorem.ftndrecurrcrr
ow*ng in the 40-ohm resistor in Fg i below
Figure - i IING 1102/BE 2102 4
one example. 4
3 (* * pnncw. 01 operation oI a
0 C motor. Wnttt down and explain iho
8xPf8*0" f * %Kk amr mkm in themoior.
4<b) A single-phase a.c. supply voltage of 230 V at 50 Hz is connected to a 30-ohm resistor in series wttfi a cod The reeding of a voltmeter across the reststor150 volts and across the coil is 100 volts. Calculate ihe real power and reactive power of the co4 and overall power factor of the circuit.
6
4 (a) A3-phase.3*wire.415volt3,50 Hz, RYB
system of balanced supply is connected to a star connected load with 2 -110 40=
ohms = 105 z- 503 ohms and = 90 -''SO0 ohms, where tr is the neutral point of the star connection. Obtain the three line currents and draw the complete phasor diagram showing the line voltages, phase BCNQ 1102/BE 2102 5 pT0
voltages ano bnt currents. The supply neutral is earthed. 7
(b) Deea*>etf*venousm*hodsofexcrtatton of a d c. machine Show the connection digram match case. 3
5. (a) Anironnnghasamean<*amcterof45cm and a cross sectional area of 10 cm* It is wound with a col of 2500turns. An airgap of 1.5 mm width ts cut m the ring. If the current flowing in the coH it measured to be equal to 2.5 A. then determine the flux produced in the airgap. tf tho rotative permeabKtty of Iron under these conations is 1400. Neglect leakage and frinpng. &ven. M04* x lOHftn. 5
(b) Write the expressions for the emfs induced In both the windings of a single-phase two winding transformer. Calculate the magnitude of the induced emf in each winding for a frequency of 50 Hz and a peak fax of 1.2 webcr. The HV winding has
8ENG 1102/BE 2102 6 Contd.
2200 number of turns whe the LV winding has 440 number of turns. 5
6. (a) A coll of resistance 2.5 ohms and
inductance 0.02 H is connected in series ttflh a capacitor across230 V mains. What
must be the capacitance m order that
maximum current occurs at a frequency of
(i) 30 Hz (II) 60 Hz and (iii) 120 Hz ? Find also the voltage across the capacitor in each case 5
(b) A balanced three phasa star connected load
01 200 kW takes a leading current of 1 SO A with a line voltage of 1100 V, 50 Hz. Find the circuit constants (resistance and capacitance) of the toad per phase. 4
7. (a) Derive the mathematical expression of
growth and decay of current in a R-L circuit subjected to a d.c. voltage. 4 (b) A single phase transformer has 500 primary and 1500secondary turns. The net cross sectional area of the core Is 100 cm2. If the primary winding be connected to a
BENG 1102/BE 2102 7 P.T.O.
50 Hz supply at *'i!5w#s.cslcUate (0 the peak value of the density in the core and (*) iha voltage *uced m the
t#Condaryw'n<jng 3*3
8 (t) Artwvlngco<instrunertgivs*scale
deflection of 20 mA whan potential difference across its terminals is 200 mV Calculate (*) the shuni resistance for measuring currents up to 20 A and () senes resistance lor measuring vottage upto400 V 5
(b) t*piamwnntneherfaWockcegramihe principle of power generation m a thermal power pient 5
BENG 1102/BE 2102 8 - C
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