Bangalore University 2005 B.E -Basic Electrical Engineering - Question Paper

.2    ELE15/25

Explain the significance of back emf in dc motors.    (4 Marks)

A 4- pole 250V series motor has wave connected armature with 1254 conductors. The flux per pole is 22wb, when the motor is taking 50A. The armature and series field coil resistances are respectively 0.312 and 0.2S1. Calculate the speed and torque of the motor and also the power developed in Walts.

(10 Marks)

Draw the typical speed,' load characteristics of i) Shunt and ii] Series motors and draw the comments on their shapes and mention the applications of these motors.    (10 Marks)

Explain the need of starter for DC motors.    (5 Marks)

*A series motor should never be started without a load on it'. Justify this statement.    (5 Marks)

What are the losses in a transformer? Or. what factors do they depend? and how are they be minimised?    (3 Marks)

A single phase transformer has 400 primary and 1000 secondary turns. The net cross sectional area of the core is 60cm². The supply is 500V, 50Hz. Calculate:

i)    the peak value of flux density

ii)    voltage induced in the secondary and

iii)    the number of secondary turns to induce a voltage of 2500V. (s Marks)

Derive an expression for the terminal voltage on no load of a star connected tliree phase alternator.    (6 Marks)

Explain how' torque is produced in a three phase induction motor. (6 Marks)

A three phase 10 pole induction motor is supplied bv a 6-pole alternator running at 1200 rpm. Calculate the speed of the motor for a slip of 3 percent.

(6 Marks)

Find the number of armature conductors in series per phase of a three phase, 50Hz, 10 pole alternator having 90 slots. The winding is to be star connected to give a line voltage of 1 ikV. w hen the flux is 160 mwb. The winding factor is unity. Also'find the voltage regulation when the full load terminal voltage iS 10.80KV.    (8 Marks)

With a neat sketch explain the construction and working of a single phase energy meter.    <s Marks)

Explain the different types of wiring used in practice.    (s Marks)

Explain the brief:

i)    Fuses

ii)    Specification of wires

iiij Earthing and its necessity    (6 Marks)

H:

Page No... 1

irst/Second Semester B.E. Degree Examination, February/March 2005

Common to all branches

Basic Electrical Engineering

Time: 3 hrs.]    [Max.Marks : 100

Note: Answer any FIVE full questions.

1.    (a) Explain Fleming's rales as applied to induced emfs.

(b)    Define:

i)    Self inductance anti

ii)    Mutual inductance and obtain the relationship between these two.

(8 Marks)

(c)    Define RMS and average value of a sinusoidally varying current and find their values in terms of its peak value.    (7 Marks)

2.    {a} Show that the average power in a pure capacitive circuit with a sinusoidal

voltage is zero.    (6 Marks)

(b)    A coil of power factor 0.6 is in series with a 100 p,F capacitor. When connected to a 50Hz supply, the potential difference across the coil is equal to the potential difference across the capacitor. Find the resistance and inductance of the COil.    (9 Marks)

(c)    Two impedances (150 -f .7157)0 and (100 - jllO)fi are connected in parallel across a 220 V, 50Hz supply. Find the total current and power factor. (5 Marks)

3.    (a) Derive the relationship between the line and phase quantities of a balanced

star connected load.    (6 Marks)

(b)    Three 100S1 resistors are connected in i) Star & ii) Delta across a 415V. 50HZ, 3-phase supply. Calculate the line and phase currents and the power consumed in each case.    (8 Marks)

(c)    Estimate the power factor in each of the following cases of two Wattmeter method of measuring three phase power:

i)    Wattmeter readings are equal

ii)    Wattmeter readings are equal and opposite

iii)    Wattmeter readings are in the ratio 1:2 r^-)    One Wattmeter reads zero.

(5 Marks)

(6 Marks)

4. {a} Explain the following :

U Armature windings of a DC machine

ii)    Commutator of a DC machine

iii)    Brushes of a DC machine

Contd.... 2

Attachment: bangalore-university-2005-b-e-19348-1.pdf bangalore-university-2005-b-e-19348-2.pdf

Earning:   Approval pending.