CUSAT Previous Year Question Papers BE 5th Semester
Electrical and Electronics Engineering
Electrical Machines-II June 2011
EE 504 ELECTRICAL MACHINES II
(All questions carry equal marks)
I. (a) Derive the emf equation of the alternator.
(b) Discuss the main constructional features of cylindrical rotor and salient pole alternators.
II. (a) A 3 phase 12 pole star connected alternator has 180 slots with 10 conductors per slot and the conductors of each phase are connected in series. The coil span is 144° (electrical). Determine the phase and line values of emf if the machine runs at 600rpm and the flux per pole is 0.06 wb distributed over the pole.
(b) What are the cuases of harmonics in the voltage waveform of an alternator? How can these be minimized?
III. (a) Explain the effect of armature reaction on alternators terminal voltage at lagging, leading and unity power factor.
(b) Describe the emf method for determining the voltage regulation of an alternator.
IV. (a) The data obtained on 100KVA, 1100 V, 3 phase star connected alternator is DC resistance test, E between line = 6Vdc Current in lines = 10 A dc.
Open circuit test, Field current = 12.5 A dc Line voltage = 420 V Short circuit test, field current = 12.5 A Line current = rated value
Calculate the voltage regulation of alternator at 0.8 pf lagging to using emf method.
(b) What is short circuit ratio of an alternator? Explain the significance of SCR.
V. (a) What are the advantages of connecting the alternators in parallel? What are the conditions required to be fulfilled for the successful parallel operation of alternators.
(b) A 2000KVA, 3 phase, 8 pole alternator runs at 750 rpm in parallel with other machines on 6000V bus bars. Find the synchronising power on full load 0.8 pf lagging per mechanical degree of displacement and the corresponding synchronising torque. The synchronous reactance is 6 ohms per phase.
VI. (a) Derive the expression for power developed by a synchronous motor.
(b) A 75KW, 400V, 4Pole 3 Phase star connected synchronous motor has a resistance and synchronous reactance per phase of 0.04Q and 0.40 respectively. Determine for full load 0.8 pf lead, the open circuit emf per phase and gross mechanical power developed. Assume an efficiency of 92.5%.
VII. (a) Explain an experimental method for determining V – curves for a synchronous motor.
(b) What are synchronous condensers? Explain with phasor diagram.
VIII. (a) Explain the effect of varying the excitation of a synchronous machine connected to infinite bus bar.
(b) Derive the power Vs power angle relation of a salient pole synchronous machine. Also draw and explain the curve.
IX. (a) Discuss the phenomenon of sudden 3-phase short circuit of an alternator. Draw a typical wave shape of current and mark the different regions.
(b) What is meant by hunting in synchronous machine? How hunting can be avoided?
X. (a) Derive an expression for the natural time period of oscillation for a synchronous machine.
(b) A 1OMVA, 1OKV, 3 – Phase 50Hz, 1500 rpm alternator is paralleled with others of much greater capacity. The moment of inertia of the rotor is 2xl05 kg — m2 and the synchronous reactance of the machine is 40%. Calculate the frequency of oscillation of the rotor.