Code No: 310205

III B-Tech I-Semester Supplementary Examinations

June - 2003
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. Describe the constructional features of cylindrical rotor synchronous generators and the purpose of each part.

2. A 3-phase, 50Hz cylindrical-rotor synchronous machine has the following parameters. Self inductance per phase =3.15mH

Armature leakage inductance = 0.35 mH. For this machine, calculate the mutual inductance between armature phase and its synchronous reactance.

3. a) Describe synchronous impedance method to determine regulation of an alternator for Lagging power factor.

b) A 600V, 60KVA, Single-phase alternator has an effective resistance of 0.2 Ω. A field current of 10A produces an armature current of 210A on short-circuit and an e.m.f of 480V on open –circuit. Calculate:

i) Synchronous impedance and reactance.
ii) Regulation with 0.8 P.f Lagging, and unity p.f Leading.

4. a) Explain the method of determining the voltage regulation of an alternator by ASA method

b) An alternator has a synchronous reactance of 20% and negligible resistance calculate its voltage regulation when working at full-load

a. 0.8 p.f Lag
b. Unity p.f and
c. 0.8 P.f. Lead

5. a) Mention the essential parts of a synchronous motor. Explain neatly with diagrams each one of them.

b)A 3Φ 220V, 50Hz,1500rpm,mesh connected synchronous motor has a synchronous impedance of 4Ω/phase. It receives an input line current of 30 Amps, at a leading p.f of 0.8. Find the line value of the inductance emf and the load angle in mechanical degrees. If the mechanical load is thrown off without change of excitation, determine the magnitudes of current under the new conditions. Neglect losses.

6. A 6000KVA, 5000V, 50Hz, 3-phase alternator with 4 poles and a synchronous reactance of 25% operates an constant voltage and constant frequency bus bars. The moment of inertia if the whole rotating system is 16800 kg-m². Calculate the time of one complete for full load and unity power factor.

7. a) Describe briefly the effect of varying excitation upon the armature current and p.f. of a synchronous motor when input power to a motor is maintained constant.

b) A 400V, 50Hz, 3Φ, 37.3KW, star connected synchronous motor has a full load efficiency of 88%.The synchronous impedance of the motor is (0.2+j1.6)Ω/phase .if the excitation of the motor is adjusted to give a leading p.f of 0.9,calculate for ful-load i) the induced emf ii) the total mechanical power developed.

8. a) "A pure single phase motor is not a self starting motor" justify the statement

b) Why cannot a shaded pole motor be made to rotate in the reverse direction?