Code No: 310202

III/IV B-Tech I-Semester Examinations

November - 2002
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. a) Draw load characteristics of an alternator and explain how it is derived from phasor diagram.

b) Explain why alternators are rated in KVA and what is the necessity to mention pf on the name plates.

2. a) From the equivalent circuit of an alternator determine the expression for input power and output power.

b) A 3-phase Y connected alternator is operated at a constant voltage of 6.6KV and its excitation voltage is adjusted to 6.4KV. Find the maximum output power and pf at this power assuming X_s = 1 + j¹⁰ ohm per phase.

3. a) Explain how X_d and X_q are determined for a salient-pole machine.

b) In an alternator, a lagging current weakens the main field but in a synchronous machine it strengthens the main field. Explain why?

4. a) Draw the phasor diagram of an alternator corresponding to zero full-load regulation.

b) A 3-phase 50Hz, 10pole alternator has 90slots with a star-connected winding to give a no-load emf of 11KV.The coils are chorded by 1 slot. If the flux per pole is 0.11wb calculate the number of series turns required in each phase.

5. a) Explain Patier triangle method of finding regulation of an alternator.

b) Define synchronizing-power coefficient and synchronizing power. When does it come into action? How is it related to the stability limit of a synchronous machine?

6. a) Show that 2-phase currents in 2-phase winding will produce a synchronously rotating mmf wave of constant amplitude equal to the maximum mmf of any one phase.

b) Show how V and inverted V curve of a synchronous motor can be obtained experimentally.

7. a) A 6-pole 50HZ synchronous motor is supplied with a 4000V 3phase supply. Xd=10Ω, X_q = 7 Ω. Neglect losses. The excitation is adjusted to give an internal generated emf of 2000 V per phase at a power angle of 20⁰. Calculate developed torque, input current and pf (indicating lag or leed.) Draw complete phasor diagram showing J_dand J_q with magnitudes.

b) Prove that E_r = cosecδ - cosδ for a salient pole synchronous motor having r_a=0, X_d=1 p.u. and X_q=0.5 p.u. operating an infinite bus bar of 1 p.u. voltage at a synchronous power of 1 p.u.

8. Write short notes on:

a) Two axis theory of single phase induction motor.
b) AC series Motor.