Cochin University Previous Year Question Papers
BE EE Electrical Machines Nov 2008
EE 502 Electrical Machines II
(Answer ALL questions)
I. a) State the advantage of using a stationery armature and rotating field system in an alternator.
b) What are the various methods used for minimizing harmonics in an alternator.
c) Explain armature reaction in alternator.
d) State the conditions required for paralleling two synchronous generators.
e) An over excited synchronous motor is called a synchronous condenser. Explain?
f) Explain V-curves and inverted V-curves.
g) Explain the transient and sub transient reactance of alternators.
h) What is meant by hunting in a synchronous motor? How will you minimize it?
II. a) Discuss the main constructional features of cylindrical rotor and salient pole alternators.
b) A- 3 phase, 50Hz, 2 pole star-connected turbo alternator has 54 slots with 4 conductors per slot. The pitch of the coils is 2 slots less than the pole pitch. If the machine gives 3300V between lines on open circuit with sinusoidal flux distribution, determine the useful flux per pole?
III a) Derive the emf equation of an alternator.
b) A3 phase 16 pole synchronous generator has a resultant air — gap flux of 0.06 Wb per pole. The flux is distributed sinusoidally over the pole. The stator has 2 slots per pole per phase and 4 conductors per slot are accommodated in two layers The coil span is 150° electrical. Calculate the phase and line induced voltages when the machine runs at 375 rpm.
IV. a) Discuss the mmf method of determining voltage regulation
b) The following figures give the open circuit and full load zero p.f. saturation curves for a 15,000 KVA, 11,000 V, 3 ^, 50Hz, star connected turbo alternator.
Field AT in 103 : 10 18 24 30 40 45 50
O.C. line KV : 4.9, 8.4 10.1 11.5 12.8 13.3 13.65
Zero p.f. full
load line KV 0 10.2
Find the armature reaction, the armature reactants and the synchronous reactants.
Deduce the regulation for full load at 0.8 power lagging.
V. a) Describe the slip test method for the measurement of Xd and Xq of synchronous machines. lagging. The synchronous impedance per phase of machine A is (0.5 + j\ 0)Q and of machine B is (0.4 + j\ 2)Q .
(b) The excitation of machine A is adjusted so that it delivers 150 A at a lagging power factor and the governors are so set that load is shared equally between the machines. Determine the current, power factor, induced emf and load angle of each machine.
VI. a) Explain the operation of a synchronous motor under constant excitation and varying load.
b) A 3300 V, 1.5 MW, 3 <f>, star connected synchronous motor has Xd = 4fi /phase and Xq = 3 Q / phase. Neglecting all losses, calculate the excitation emf when motor supplies rated load at unity pf. Calculate the maximum mechanical power which the motor would develop for this field excitation.
VII. a) Derive an expression for the power developed in a 3 phase synchronous motor.
b) A 500V synchronous motor gives a net output mechanical power of 7.46 kw and operates at 0.9 pf lagging. Its effective resistance is 0.8 Q. If iron and friction losses are 500 W and excitation lossess are 800 W, estimate the armature current. Calculate the commercial efficiency?
VIII. a) Discuss the effect of short circuit on as alternators.
b). A 20 MVA 3 phase star connected 11 KV, 12 pole, 50 Hz salient pole synchronous motor has reactance of Xd = 5 Q, Xq = 3 Q . At full load, unity power factor and rated voltage, determine.
i) the excitation voltage.
ii) Active power
iii) Synchronizing power per electrical degree and corresponding torque.
iv) Maximum value of power angle and corresponding power
IX. (a) Derive the time period of oscillation of synchronous machine.
(b) Explain two important functions served by damper windings in a synchronous motor.