Code No: 310205

III/IV B-Tech I-Semester Examinations

November - 2002
Set No: 1

ELECTROMECHANICS - III

(Electrical and Electronics Engineering)

[Time:3 hours]
Max. Marks: 80

Answer any FIVE questions

ALL questions carry equal marks

1. a) Define “Distribution factor” and find its value for a 3-phase with 600 phase spread when the winding is uniformly distributed over 9 slots per pole. Find the values for both fundamental and third harmonic.

b) Each winding of a 3-phase, 50Hz alternator has an e.m.f wave consisting of a fundamental with a maximum value of 1200volts, a 20% third harmonic and a 10% fifth harmonic. Calculate the R.M.S value of the line voltage of the cases, when the windings are connected in star and in delta.

2. a) Explain the A.S.A method of predetermining the regulation of an alternator.

NOTE:- You need not explain how to conduct the tests necessary for this method but take the test results. b) The open and short circuit test data on a 3-phase, 1MVA, 3.6KV, star connected synchronous generator is given below:-

Field current (Amps)   60     70     80     90      100   110

Voc line volts           2560  3000  3360  3600   3800  3960   

S.C. Test Amp.         180      -      -     -     -     -    -

Find unsaturated synchronous reactance, the adjusted synchronous reactance and the short circuit ratio.

3. a) Derive the power -angle characteristics of synchronous generator when it is connected to infinite bus bars.

b) Two similar 6.6 KV alternators supply a total load of 2000KW at a power factor of 0.8 lagging so that the power supplied by each of them is same. If the excitation of one of the alternators is adjusted so that its load current is 100 Amps, determine i) The load current of the second alternator and ii)the p.f at which each alternator is working.

4. a) With the help of phasor diagram, explain the working of a synchronous motor for constant power input and varying excitation.

b) A 400volts, 50Hz, 3-phase, star connected synchronous motor has an input of f10kw when it is working at full load and rated voltage. The synchronous reactance is 5ohms and the resistance is 1ohm per phase. Calculate the armature current and power factor when the excitation voltage is 480volts.

5. a) Explain the development of power circles for cylindrical rotor synchronous motor and show that zero (no load) power circle passes through origin.

b) A 433volts,3-phase, star connected synchronous motor has a synchronous reactance of 5 ohms. For a power output of 15Kw, find its minimum armature current excitation voltage and the power angle. Armature resistance is negligible.

6. a) Explain single-phase induction motors fail to start but continues to run once started by double revolving field theory.

b) Describe the construction, principle of working and application of shaded pole induction motor.

7. a) Explain the design modifications necessary for satisfactory operation of a d.c series motor from an a.c supply.

b) With a neat circuit diagram, explain the slip test. What do you determine from this test results?

8. Write short notes on the following:-

a) Armature reaction in synchronous machine. b) Two reaction theory. c) Cross field theory.