Instructions.—(1) Attempt any five questions. All questions carry equal marks.
Answers must be written in English or Hindi. However all the
answers should be in one language only.
Parts of the same question, if any, must be answered together without
answers to other questions intervening.
Each question must be started on fresh page.
Marks will be deducted if handwriting is not easily legible.
Answers will be evaluated on the basis of logic, brevity and clarity
in exposition.
Use of ordinary calculators with six functions and twelve digits/log
tables is permitted.
1. An LC parallel circuit has an applied voltage 50 Cos (3000t+450) volts and
a total current iT 2 cos (3000t 450) amperes. It is also known that the
current in the L branch is five times greater than the current in the
C branch. Find L and C of this circuit.
The Circuit constants R and L of a coil are to be determined by connecting the
coil in series with a resistor of 25? and applying a 120 60 Hz source and
then reading the voltages across the resistor and the coil. Find R and L
if VR 70. 8 Volts and VCoil 86 Volts.
2. Explain the principle of transformer action and derive an expression for the emf
induced in a transformer winding. Show that the emf per turn in primary is
equal to emf per turn in the secondary.
Develop a phasor diagram of a single phase transformer under load for a lagging
power factor.
3. Find an expression for the magnetic force developed in a doubly excited
translational magnetic system.
Two air-cored coils have their magnetic axes coincident. Their parameters are
L1 =0.4 H L2 0.2H
M12 M21 0.1e- 4x
and where x is the distance between the two Coils.
For steady state current of I1 10A and I2 5A, calculate the magnetic force
developed. Calculate the magnitude of force for x 2.5 cm.
4. Discuss the effect of pitch and distribution factors is an alternator.
A 3-ph. Star connected alternator has a rating of 810 kVA at 2.5 kV and has
12 poles. Rated speed is 500 r.p.m. There are three slots per pole per phase
and the number of conductors per slot are 5. For a full pitched winding,
compute the flux per pole to give the rated terminal voltage at no load.
5. What is the effect of excitation, speed and load on the losses of a d.c. machine
A 230 V d.c. shunt motor is taking 5A when running at no load. The armature
resistance including brushes is 0.2? and field circuit resistance is 115? . For
and input current of 72A, calculate the shaft output and efficiency. Also
calculate the armature current at which the efficiency is maximum.
[Turn over
CON 1694 4
6. A 6 pole, 50 Hz, 3-phase induction motor has a rotor resistance of 0.2? per phase
and a maximum torque of 160 Nm at 875 r.p.m. Calculate the following
The torque for full load slip of 4%.
The resistance to be added to the rotor to obtain 80% of full load torque at
starting.
7. A synchronous motor is operating at 0.6 pf lag at half-full load. With Ef Vt, Xs
remaining constant, explain whether its power factor will be worsened or
improved when it is made to operate at full load.
A 433 3-phase, star-connected synchronous motor has a synchronous
reactance of per phase. For a power output of 15 kW, find its minimum
armature current, excitation voltage and the power-angle. Armature resistance
is negligible.
8. Derive expressions for the inductance per phase of a 3 phase line with equilateral
and unsymmetrical spacings, both, separately.
The 3 conductors of a 3-phase line are arranged in a horizontal plane and are
4 meters apart. The diameter of each conductor is 2 cm. Calculate the
capacitance between conductors.
9. Discuss the advantages and disadvantages of transmitting large amounts of
power over long distances using high voltage dc as compared to 3 phase
ac transmission.
What is the percentage saving in feeder conductor if the line voltage in a dc 2
wire system is raised from 220 volts to 500 volts for the same power to be
transmitted.
10. Derive an expression for the resistance to be provided across circuit breaker contacts
for critical damping when
Circuit breaker is located at the sub-station and followed by a transmission
line.
The circuit breaker is located at the end of a long transmission line and with
fault occuring on it.
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