Code No: 310205

III B-Tech I-Semester Supplementary Examinations

June - 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 why two types of machines are required.

2. The flux distribution in a salient pole machine is rectangular, the base being two thirds of pole-pitch .The peak value of flux density is 1 wb/m². Draw one complete cycle of the e.m.f induced in a single-turn coil which has the pitch of five-sixth of full pitch and moves with a uniform velocity of 30m per sec. The armature conductor length is 2m. Calculate the r.m.s. value of the voltage.

3. a) In brief explain the circuit model if synchronous machine (cylindrical rotor). What is synchronous reactance?

b) What is meant by regulation of an alternator ? Draw the Phasor diagram of a cylindrical rotor alternator supply Lagging P.f. load. Show the positions of induced e.m.f and terminal voltages

4. Explain the merits and demerits of EMF and MMF method. Explain what are the assumptions made in each case.

5. Two similar 6000V, 3-phase generators are running in parallel a constant voltage and frequency bus bars. Each has an equivalent resistance and reactance of 0.05 ohms and 0.5 ohms respectively and supplies one half of a total load of 10000KW at a lagging power factor of 0.8, the two machines being similarly excited, if the excitation of one machine is adjusted until the armature current is 438A and the steam supply to the turbine remains unchanged, find the armature current, the emf and the power factor of the other alternator.

6. a) Discuss the effect of armature current for change in excitation.

b) A 2,000V,3-Φ star-connected synchronous motor has and effective resistance and synchronous reactance of 0.2Ωand 2.2Ω respectively. The input is 800KW at normal voltage and the generated line emf is 2,500V. Calculate the line current and power factor.

7. a) Define back-emf. Draw the equivalent circuit diagram of synchronous motor. Also deduce the expression for i) excitation voltage ii) synchronous impedance and iii) armature current.

b) A 3-Φ,Y-connected synchronous motor takes 48KW at 693V (line),the p.f. Being 0.8 lag.The induced emf increased by 30%,the power input being the same. Find the new current and power factor. Synchronous impedance equals to (0+2j)Ω/phase.

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

b) What is the difference between conductively compensated series motor and an inductively compensated series motor.