Code No: NR-310205

III B-Tech I-Semester supplementary Examinations

May - 2004
Set No: 4

ELECTROMECHANICS - III

(Electrical and Electronic Engineering)

[Time:3 hours]
Max. Marks: 80

Answer any FIVE questions

ALL questions carry equal marks

1. a) Write the effect of short pitch on the induced emf of synchronous generator.

b) Calculate the distribution factor of a 3-phase winding with 120^ophase spread when the winding is (i) uniformly distributed (ii) occupies 6 slots per pole.

2. A 3-phase, 50Hz cylindrical- rotor synchronous machine has the following parameters. Self inductance per phase 3.15M.H
Armature leakage inductance = 0.35 m.h. For this machine, calculate the mutual inductance between armature phases and its synchronous reactance.

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

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

5. a) What are the salient features of a synchronous motor.

b) The input to a 11KV, 3Φ, Y connected synchronous motor is 60A. The effective resistance and synchronous reactance per phase are 1 Ω and 30 Ω respectively. Find (i) Power supplied to the motor and (ii) the induced emf for a power factor of 0.8 leading.

6. a) For a salient pole synchronous motor, working at lagging p.f. Show that

            Ia (Xq Cosθ - ra Sinθ)
tan δ = ------------------------
          Vt - Ia (Xq Sinθ + ra Cosθ).

b) Find an expression for power in terms of load angle δ for a salient pole synchronous motor working at a lagging p.f. Armature resistance may be neglected.

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. List out its industrial applications.

8. a) Compare various types of single phase induction motors in terms of construction and performance

b) The resistance and inductive reactance of each winding of a 50Hz single phase capacitor induction motor are 80 ohms and 237.5 ohms respectively. Additional resistance “R” and a capacitor “C” are in series with one winding inorder to achieve a phase difference of 90 degrees while both windings carry equal current. Calculate the value of R & C.