Code No: 310205

III B-Tech I-Semester Regular Examinations

November - 2003
Set No: 2

ELECTROMECHANICS - III

(Electrical and Electronics Engineering)

[Time:3 hours]
Max. Marks: 80

Answer any FIVE questions

ALL questions carry equal marks

1. Discuss the difference in constructional features between cylindrical rotor and salient pole synchronous generators. Also explain the applications of each.

2. a) Discuss the factors affecting the terminal voltage if an alternator.

b) The following data was obtained of the OCC of a 10MVA, 13KV, 3-phase, 50Hz, Star-connected synchronous generator;

IF	50 7.5  100   125    150    162.5   200  250  300

VOC (Line)(KV)	     6.2 8.7  10.5  11.8  12.8    13.2    14.2  15.2  15.9

An excitation of 100A causes the full load current to flow during the short-circuit test. The excitation required giving the rated current at zero P.f. and rated voltage is 290A.

a) Calculate the synchronous reactance of the machine
b) Calculate the leakage reactance of the machine assuming the resistance to be negligible
c) Determine the excitation required when the machine supplies full load at 0.8p.f Lagging by using the leakage reactance and drawing the MMF phasor diagram. What is the voltage regulation of the machine?

4. A5000 KVA, 10KV, 1500rpm, 50Hz alternator runs in parallel with other machines. Its synchronous reactance is 20%. Find the synchronising power per unit mechanical angle of the phase displacement for

a) no load and
b) full load at 0.8 p.f. (lag)

Also calculate the synchronising torque if the mechanical displacement is 0.5^o

5. A 500V, 3-phase, mesh connected motor has an excitation emf of 600V. The motor synchronous impedance is (0.4+j5) Ω while the windage, friction and iron losses are 1200W. What maximum power output can it deliver? What is the corresponding line current, pf and motor efficiency.

6. a) What are the different methods of starting a synchronous motor?

b) Why it is necessary to increase the excitation to obtain minimum current with application of load?

c) What are the uses of damper winding in a synchronous motor?

7. a) Using double revolving field theory explain the torque-slip characteristics of a single phase induction motor and prove that it cannot produce staring torque.

b) Explain the constructional details and principle of operation of a split phase induction motor. Listout its industrial applications.

8. a) Explain the constructional features and principle lof operation of a capacitor start induction run motor. Draw the slip-torque characteristics and list out its merits over split phase motor.

b) In a direct load test on a single phase motor the following readings were obtained 
supply voltage : 230V   Line current: 12.0A   Watt meter reading 1.96kw
speed       :1410 rpm     Torque :1.01 kg-m
Determine (i) Slip (ii) efficiency iii) Power factor