Bangalore University 2006-2nd Sem B.E Electrical and Electronics Engineering First /ester , / - Question Paper
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First/Second Semester B.E. Degree Examination, January/February 2006
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the mutual inductance between the coils, if a second coil of 600 turns was uniformly wound over the first. (6 Marks)
(c) In a circuit supplied from 50Hz the voltage and current have maximum values of oOOV and 104 respectively. At t = 0, their respective values are 400and 4.4 both increasing positively.
Basic Electrical Engineering
Time: 3 tvs.)
Note: Answer any FIVE questions.
(a) Derive the expression for the energy stored in a magnetic field.
Inductance of the coii.
average e.m.f induced in it when a current of 5A is switched off, assuming the current to fall to zero in 1 ms.
(b) A flux of 0.5 mwb is produced in a coil of 900 turns wound on a wooden ring by current of 3A. Calculate
(6 Marks)
Write expressions for their instantaneous values. Find the angle between V and I and I at t 0.015sec
iii) 1 at t 0.015sec (8 Marks)
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2. (a) Show that the average power is a pure inductance with a sinusoidal voltage s
zero. (5 Marks)
(b) The equation of an alternating current is given by i = 42.42 sm628i. Calculate its
i) Maximum value ii) Frequency iii) RMS Value
iv) Average value v) Form Fcctor (5 Marks)
<c) Two circuits with impedances of Z\ = (10 + j 15)0 and z2 = (6 - j8)il ere connected in parallel. If the supply current is 20A, what is the power dissipated in each branch? (10 Marks)
3. (a) Show that in a 3 phase balanced circuit two wattmeters are sufficient to measure
the total three phase power and power factor of the circuit. (1 o Marks)
(b) A balanced star connected load of (8 + j6)fl per phase is connected to a three phase, 230V supply. Find the iine current, power factor, power, reactive voltampere and total voltampere. (1 o Marks)
4. (a) With a neat sketch explain the construction and working of a single phase, induction
type energymeter. (8 Marks)
(b) With a neat diagram and switching tabte explain two point control of lamp.
(6 Marks)
(c) With a neat sketch explain any one method of earthing. (6 Marks)
5. (a) Draw the schematic representation of series, shunt and long-shunt compound
generators. In each case, write the voyage balance and current balance equations. (6 Marks)
<b) Give reasons:
i) The armature winding is placed on the rotor in a dc machine
ii) Shunt motors are used tor-constant speed applications.
iii) Series motors should not be started without proper load, (6 Marks)
(c) A 6-pole, dc shunt motor has a lap-connected armature with 492 conductors. The resistance of the armature is 0.20 and the fiux per pole is 50mWb. The motor runs at 20 .revolutions per second when it is connected to a 5001 supply for a particular load. What will be the speed of the motor when the load is reduced by 60%. Neglect contact drop and magnetic saturation. (8 Marks)
6. (a) What are the main parts of a transformer? What is the function and the main
material of construction in each case? (6 Marks)
. (b) .With respect to a transformer, give reasons for the following:
i) there is small primary current even when the load is not connected.
0 the core losses are constant for any load. . t. . ,
Rb) there is an inrush of current in the primary when the secondary is loaded.
. _ - .. (6 Marks)
(c) Define the power efficiency and regulation of a transformer. The maximum efficiency at full load and unit power factor of a single-phase 25kVA,- 500/1000 V,
50 Hz. transformer is 98%. Determine its efficiency at
0 75% load. 0.9pf; and ii) 50% load, 0.8pf. (8 Marks)
7. (a) What are salient and nonsalient synchronous machines? Give any two characteristic
features of each of them. (6 Marks)
(b) Explain the terms: pitch factor, distribution factor, and voltage regulation of an alternator. (6 Marks)
(c) A 24-pole turbo alternator has a star connected armature winding with 144 slots and 10 conductors per slot. It is driven by a low speed Kaplan turbine at a speed of 250 revolutions per minute. The winding has full-pilched coils with a distribution factor of 0.966. The flux per pole is 67.3mW&. Determine
0 the frequency and the magnitude of the line voltage;
ii) the output kVA of the machine if the total current in each phase is 50A.
(8 Marks)
8. (a) Define synchronous speed, slip speed, and motor speed of a three-phase induction
motor. Explain why the induction motor cannot run at synchronous speed.(6 Marks)
(b) Why is a starter required for starting a 3-phase induction motor? Explain how a star-delta starter helps in safe starting of the motor. (6 Marks)
(c) The rotor-induced voltage of a three-phase. 4-pole squirrel cage induction motor fed by a salient pole alternator is observed to make 1.5 alternations per second. The star connected alternator with 592, fui! - pitched armature conductors in series per phase with a distribution factor of 0.966 develops a line voltage of 6600 voits when the flux per pole is 60m Wb. Determine the speed of the induction motor.
(8 Matks)
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| Earning: Approval pending. |
