Code No: NR-310205

III B-Tech I-Semester supplementary Examinations

May - 2004
Set No: 2

ELECTROMECHANICS - III

(Electrical and Electronic Engineering)

[Time:3 hours]
Max. Marks: 80

Answer any FIVE questions

ALL questions carry equal marks

1. Draw a factional pitch lap winding for 3-phases 4-pole, 2 layer with 2 slots per pole per phase. Discuss the merits and demerits between full pitch and fractional pitch windings.

2. a) Discuss the effect of armature reaction in an alternator.

b) A 3.3 KV, 3-phase star-connected alternator has a full-load current of 100A. under short circuit condition it takes 5A filed current . The emf on open circuit for the same excitation is 900V (line to line).The armature resistance is 0.9ohm per phase. Determine synchronous reactance per phase.

3. What are the precautions to be taken while conducting Slip test? Draw the Phasor diagram when the load connected is of Leading P.f.

4. a) Define the significance of transient & sub-transient reactances in an alternator.

b) Two 15KVA, 400V, 3-phase alternator in parallel supply a total load of 25KVA at 0.8 p.f. lagging. If one alternator shares half the power at unity power factor, determine the power factor and KVA shared by the other alternator.

5. 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 is now increased by 30% the power input being the same . Find the new current and power factor. Synchronous impedance equals to (0+2j) Ω/phase.

6. a) Draw the phasor diagram of a salient-pole synchronous motor working at leading p.f. And obtain therefrom an expression for power in terms of load angle δ. Neglect armature resistance.

b) A salient pole synchronous motor has the following per unit constants:

X_d = 1.25; X_q = 1.00. Find the excitation voltage when the motor takes rated current (leading) at rated voltage, delivering 0.5 p.u. mechanical power. Ignore all losses.

7. a) Draw the slip-torque characteristics of all types of single phase induction motor and compare their merits and demerits.

b) The full load slop of a single phase induction motor is higher than of corresponding 3 phase induction motor. Why ?

8. a) Show that in a shaded pole motor, the flux in the shaded part of pole segment lags behind the flux of the shaded pole segment pole segment both in space and time, resulting a rotational torque in the motor.

b) Explain the construction and operation of Universal motor. List out its merits and demerits.