Jawaharlal Nehru Technological University Kakinada 2008 B.Tech Electronics and Communications Engineering electrical technology - Question Paper

Code No: 07A3EC13 Set No. 1
II B.Tech I Semester Regular Examinations, November 2008
ELECTRICAL TECHNOLOGY
( Common to Electronics & Communication Engineering, Electronics &
Instrumentation Engineering, Bio-Medical Engineering, Electronics &
Control Engineering, Electronics & Telematics, Electronics & Computer
Engineering and Instrumentation & Control Engineering)
Time: three hours Max Marks: 80
ans any 5 ques.
All ques. carry equal marks
? ? ? ? ?
1. (a) Derive the formula for induced emf of a dc machine.
(b) explain the process of self excitation in a dc machine. What conditions must
be ful?lled for self excitation? [8+8]
2. (a) discuss the principle of operation of dc motor using suitable diagram.
(b) A 220V series motor runs at 800 rpm, when taking a current of 15 A. The
motor has Ra = 0.3O and Rf = 0.2 O. obtain the resistance to be connected in
series with armature if it has to take the identical current at the identical voltage at
600 rpm. presume ?ux is proportional to current. [8+8]
3. (a) What is an ideal transformer. Draw the no load phasor diagram of transformer.
(b) The maximum ?ux density in the core of 240/2400V, 50Hz, single phase trans-
former is 1.0Wb/sq.m. If the emf per turn is eight Volts, determine:
i. the primary and secondary turns and
ii. area of the core. [8+8]
4. (a) With neat circuit diagrams, discuss the procedure for conducting OC and SC
tests on a provided single - phase transformer to determine the regulation for
lagging and leading power factors.
(b) A 100 kVA transformer has iron losses of 1.2 kW and full load copper losses
of 1.5kW. obtain
i. the kVA for maximum e?ciency
ii. maximum e?ciency. [8+8]
5. A 440V, 4pole, 50 Hz, 3-phase stator connected slip ring induction motor has a
voltage of 80V ranging from the slip rings when full voltage is applied to the stator and
the slip rings are open circuited with the rotor stationary. The stator current is 2A
at a power factor of 0.2 lagging. The resistance and stand still reactance per phase
of the rotor winding are 0.05 and 0.25 ohms respectively. The rotor winding is star
connected. For the motor running with slip rings short circuited and a slip of 5%
determine:
(a) The mechanical power developed
(b) Rotor copper losses
1 of 2Code No: 07A3EC13 Set No. 1
(c) Stator current and power factor. (Neglect the stator resistance and reactance)
[4+4+8]
6. (a) discuss the synchronous impedance method of regulation with vector diagram.
(b) obtain the synchronous impedance and reactance of an alternator in which a
provided ?eld current produces an armature current of 200 A on short circuit and

Earning:   Approval pending.