Bangalore University 2006-2nd Sem B.E Electrical and Electronics Engineering First/ester , uary/ruary - Question Paper

NEW SCHEME
First/Second Semester B.E. Degree Examination, January/February 2006
Common to all branches.
Basic Electrical Engineering

Time: three Hrs.] [Max. Marks: 100

Note: ans any 5 full ques..

1. (a) Derive the expression for the energy stored in a magnetic field. (6 Marks)

(b) A flux of 0.5mwb is produced in a coil of 900 turns wound on a wooden ring by a current of 3A. compute.
i) Inductance of the coil.
ii) Average e.m.f induced in it when a current of 5A is switched off, assuming the current to fall to zero in 1ms.
iii) The mutual inductance ranging from the coils, if a 2nd coil of 600 turns was uniformly wound over the first. (6 Marks)

(c) In a circuit supplied from 50 Hz the voltage and current have maximum values of 500V and 10A respectively. At t= 0, their respective values are 400V and 4A both increasing positively.
i) Write expressions for their instantaneous values.
ii) Find the angle ranging from V and I and
iii) I at t=0.015 Sec (8 Marks)

2. (a). Show that the avg. power is a pure inductance with a sinusoidal voltage is zero. (5 Marks)

(b) The formula of al alternating current is provided by i=4242 sin628t. compute its
i) Maximum value ii) Frequency iii) RMS value
iv) avg. value V) Form Factor. (5 Marks)

(c) 2 circuit with impedances of Z1 = (10 + j15 )O and Z2 = (6-j8) O are connected in parallel. If the supply current is 20A, what is the power dissipated in every branch. (10 Marks)

3. (a) Show that in a three phase balanced circuit 2 wattmeters are sufficient to measure the total 3 phase power and power factor of the circuit. (10 Marks)

(b) A balanced begin connected load of (8+j6) O per phase is connected to a 3 phase, 230V supply. obtain the line current, power factor, power, reactive voltampere and total voltampere (10 Marks)

4. (a). With a neat sketch discuss the construction and working of a single phase, induction kind energymeter. (8 Marks)

(b) with a neat diagram and switching table discuss 2 point control of lamp. (6 Marks)

(c) With a neat sketch discuss any 1 method of earthing. (6 Marks)

5. (a) Draw the schematic representation of series, shunt and long-shunt compound generators. In every case, write the voltage balance and current balance equations. (6 Marks)

(b) provide reasons:
i) The armature winding is placed on the rotor in a dc machine.
ii) Shunt motors are used for constant speed application.
iii) Series motors should not be started without proper load. (6 Marks)

(c) A 6-pole dc shunt motor ahs a lap-connected armature with 492 conductors. The resistance of the armature is 0.2 O and the flux per pole is 50m Wb. The motor runs at 20 revolutions per 2nd when it is connected to a 500V supply for a particular load. What will be the speed of the motor when the load is decreased by 50%. Neglect contact drop and magnetic saturation. (8 Marks)

6. (a). elaborate the main parts of a transformer? What is the function and the main material of construction in every case? (6 Marks)

(b) With respect to a transformer, provide reasons for the subsequent.
i) There is small primary current even when the load is not connected.
ii) The core losses are constant for any load.
iii) There is an inrush of current in the primary when the secondary is loaded. (6 Marks)

(c) describe the power efficiency and regulation of a transformer. The maximum efficiency at full load and unit power factor of a single-phase 25kVA. 500/1000V. 50 Hz transformer is 98%. Determine its efficiency at
i) 75% load, 0.9pf; and ii) 50% load, 0.8pf. (8 Marks)

7. (a) elaborate the salient and nonsalient synchronous machines? provide any 2 characteristic features of every of them. (6 Marks)

(b) discuss 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-pitched coils with a distribution factor of 0.966. The flux per pole is 67.3mWb. Determine.
i) The frequency and the magnitude of the line voltage;
ii) The output kVA of the machine if the total current in every phase is 50A.
(8 Marks)


8. (a). describe synchronous speed, slip speed, and motor speed of a three-phase induction motor. discuss why the induction motor cannot run at synchronous speed. (6 Marks)

(b) Why is a starter needed for starting a 3-phase induction motor? discuss how a star-delta starter helps in safe starting of the motor. (6 Marks)

(c) The rotor-induced voltage of a three-phase, four pole squirrel cage induction motor fed by a salient pole alternator is observed to make 1.5 alterations per second. The begin connected alternator with 592, full-pitched armature conductors in series per phase with a distribution factor of 0.966 develops a line voltage of 6600 volts when the flux per pole is 60mWb. Determine the speed of the induction motor. (8 Marks)

Earning:   Approval pending.