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Department of Electrical EngineeringIndian Institute of
Technology, Kharagpur
Electrical Technology(EEllOOl) Date: FN /A NFirst Year sec-8 to
14 625 studentsTime: 2 hours. Spring Mid Semester Exam. 2010-2011
Full Marks: 100
Instructions: Answer any FIVE questions. Intermediate
calculation steps must be shown.Tick the question numbers you have
attempted, in the cover page of the answer script.
Q l. (a) In the circuit shown in Figure 1, calculate the value
of R such that its energy consumption rate becomesmaximum. Also
calculate the corresponding energy consumption rate. [5 + 10](b)
Calculate the power absorbed/delivered by the 2 V source for the
condition in (a). [5]
Q2. For the circuit shown in Figure 2, obtain the magnitude and
direction of the current flowing through R, when(i) R = 0 (zero)
and when (ii) R = 2 0. With R = 2 0, calculate th e reading of a
voltmeter, if connectedacross P and Q. [7 + 7 + 6]
R 2Q
SA 2V
Figure 1: Pertaining to Ql
p
Q'-------1+IJI-_--- - - - '
20V
Figure 2: Pertaining to Q2Q3. A balanced 3-phase delta connected
load of 30 kW at power factor 0.6 (lag) is connected to a
balanced
3-phase star connected load of 15 kW at power factor 0.8 (lead)
as shown in Figure 3.(a) Find the values of Rt:::,., Xt:::,. and
Ry, Xy.(b) Find the overall power factor of the combined load.(c)
Taking VAo as reference phasor ( i.e.,VAo = VAoL0), find the
currents lA, Is and lc .
Figure 3: Pertaining to Q3
Please Turn Over
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Q4. (a) Calculate the current through the 6ft resistor of the
circuit shown in Figure 4.(b) Calculate the RMS value of the
current waveform shown in Figure 5.
i inA
Figure 4: Pertaining to Q4(a) Figure 5: Pertaining to Q4(b)
[15][5]
Q5. Consider the parallel circuit shown in Figure 6, excited by
a single phase source of 10 V at 50 Hz withRL = 5n, Rc = 1n and L =
100 mH
'-----{ f'v f---- 'v
Figure 6: Pertaining to Q5(a) Draw a neat phasor diagram taking
the supply voltage as reference and showing the branch
currents,
total current and the voltage drops across each circuit elements
indicating all magnitudes and phaseangles. For this part of the
problem assume C = 100 !JF. [10](b) Show that the locus of the tip
of the current phasor in the R - C branch is a semi-circle of
radius V/ Rcas C is varied from zero to infinity. [6](c) Using
phasor diagram approach or otherwise, find the values of C that
result in unity power factor asseen from the source. [4]
Q6 . (a) The switch S shown in Figure 7 was in OF F position and
the capacitor voltage was 60 V with polarityas indicated. The
switch is now connected to position 1 for 10 sec and then moved to
position 2.(i) What will be the voltage across the capacitor,
immediately after S is moved to position 2 ? [2](ii) What will be
the current through the capacitor, immediately after S is moved to
position 2 ? [4]
(iii) Obtain expression for vc(t) after the switch is placed in
position 2. [4](b) The switch S of the circuit shown in Figure 8
was in open condition before t = 0 and the circuit reached
steady state condition. At t = 0, S is closed. Find an
expression for the current i(t) flowing throughthe inductor for t
0. [10]
IO!li(t)4H
Figure 7: Pertaining to Q6(a) Figure 8: Pertaining to Q6(b)
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