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Code No: R5220401 R5II B.Tech II Semester(R05) Supplementary Examinations, December 2010

ELECTRICAL TECHNOLOGY(Electronics & Communications Engineering)

Time: 3 hours Max Marks: 80Answer any FIVE Questions

All Questions carry equal marks

1. (a) Why brushes and commutator are necessary for operation of a D.C. Machine.

(b) How D.C. Generators are classified?

(c) A 4-pole, lap wound DC shunt generator has a useful flux/pole 0.06Wb. The armature windingconsists of 200 turns, each turn having a resistance of 0.003. Calculate the terminal voltagewhen running at 1000rpm if armature current is 45A. [4+6+6]

2. (a) What is the power flow diagram of DC motor? And explain about losses involved in each stage?

(b) D.C. Shunt machine when running as a motor at no load takes 440W at 220V and runs at

1000r.p.m. the field current and armature resistance are 1A and 0.5 respectively. Calculate theefficiency of the machine when running

i. as a generator delivering 40A at 220V and

ii. As a motor taking 40A from a 220V supply. [8+8]

3. (a) What is an ideal transformer Draw its no load phasor diagram

(b) The primary winding of a 50 HZ single phase transformer has 480 turns and is fed from 6400V supply The secondary winding has 20 turns. Find the peak value of flux in the core and thesecondary Voltage. [8+8]

4. What is regulation of a transformer. How it can be obtained from equivalent circuit parameters. [16]

5. (a) Derive the expression for Torque-Slip characteristics of a 3-phase Induction Motor. Explain howthe torque of Induction Motor varies with speed with the help of a diagram.

(b) Deduce the condition for maximum torque of a 3-phase Induction Motor.

(c) A 12-pole, 3-phase Induction Motor is fed from 50Hz supply. If the frequency of rotor e.m.f atHalf full-load is 2 Hz, find the Half Full-load speed and %Slip

[8+4+4]

6. (a) Derive the expression for e.m.f induced in a 3-phase alternator in terms of pitch and distributionfactors and also discuss the factors affecting it.

(b) A 3-phase, 16-pole alternator has the following data: Number of slots = 192; Conductors / slot= 8 (conductors of each phase are connected in series); coil span = 1600(electrical); speed of the

alternator = 375 r.p.m.; flux / pole = 55 mwb. Calculate the phase and line voltages. [8+8]

7. Show that a single phase winding when excited by a single phase supply produce two equal and oppositerevolving fields. [16]

8. What are the requirements of indicating instruments. Explain them in detail.[16]

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