R09 - December, 2011 - Regular Examinations - Set - 4.

II B.Tech I Semester Examinations,December 2011
BASIC ELECTRICAL ENGINEERING
Common to Information Technology, Computer Science And Engineering
Time: 3 hours Max Marks: 75
Answer any FIVE Questions
All Questions carry equal marks
*****
1. (a) Explain how the rotating magnetic eld is developed in a 3- induction Motor.
(b) A 6 pole, 3- induction motor runs at 1140rpm on full load when supplied
power from a 60Hz supply. Determine the slip at full load. [7 8]
2. (a) A copper wire of diameter 1 cm had a resistance of 0.15ohm. It was drawn
under pressure so that its diameter was reduced to 50%. What is the new
resistance of the wire?
(b) Two wires of aluminum and copper have the same resistance and same length which
of these materials is lighter? Density of copper is  8.9 10 3 Kg/m3 and that
of aluminum is 2.7 103kg/m3. The resistivity of copper is 1.72 10??8 ohm-m
and that of aluminum is 2.6 10??8 ohm-m. [7 8]
3. (a) Using KVL determine total current drawn from the source in the circuit shown
below in gure 3 and also nd the potential drop across each resistor.
(b) Explain Norton's theorem. [9 6]
Figure 3:
4. (a) Derive the expression for
i. Back emf relation to the applied voltage in DC motor
ii. Torque equation of a DC motor.
(b) A 220V, shunt motor on no load runs at 750rpm and draws a 5A from supply.
The armature and eld resistance are 0.10ohms and 220ohms respectively.
Calculate the speed, when loaded and drawing full load current of 100A from
the supply if the armature reaction weakens the eld 
ux by 4%. [7 8]
5. (a) Draw the phasor diagram of Transformer under no load.
(b) A 1,000/200 V transformer takes 0.3 A at p.f of 0.2 on open circuit. Find the
magnetizing, and iron loss component of no-load primary current. [7 8]

6. (a) What are the classi cations of self excited DC generators and explain series
generator?
(b) A 4-pole lap wound D.C. shunt generator has a useful 
ux per pole of 0.06wb.
The armature winding consists of 240 turns each of 0.0035 
 resistance. Cal-
culate the terminal voltage when running at 920 rpm, if the armature current
is 60A. [7 8]
7. A moving coil voltmeter has a resistance of 200 ohms and a full scale is reached
when a potential di erence of 100 mV is applied across the terminals the moving
coil have e ective dimensions of 30mm 25mm and is wound with 100turns. The


flux density in air gap is 0.2 Wb/m2. Determine the control constant of the spring
if the nal de
ection is 1000 and a suitable diameter of copper wire for the coil
winding if 20% of total instrument resistance is due to the coil winding. Resistivity
of the copper is.1.7 10??8 ohm-meter. [15]
8. (a) The voltage applied to a circuit is V = 100 sin(wt 300), and the current
flowing in the circuit is i = 15(wt 600). Determine the impedance, resistance,
reactance, power and the power factor.
(b) A 230V, 50 Hz voltage is applied to a coil of L = 0.5 H and R = 200 
 in series
with a capacitor C. What value must C have in order that the total voltage
across the coil shall be 250V? [8 7]
*****