# (Supplementary) Electrical Machines-II

EE 502 ELECTRICAL MACHINES II

(2006 Scheme)

PART-A

(All questions carry EQUAL marks)

a)  Derive the e.m.f. equation of an alternator.

b) What are the causes of harmonics in the voltage waveform of an Alternator? How can these be minimized?

c) Explain the terms direct axis synchronous reactance and quadrature axis synchronous reactance of 3-phase synchronous machine.

d) Why does synchronous impedance method give a poorer voltage regulation?

e)  What is meant by synchronous condenser? Show the region of operation of the condenser on V-curves.

f)  Explain why synchronous motors are not self starting?

g)  Define transient and sub-transient reactance.

h) Describe the control of power factor in synchronous machines.

PART-B

II   a) State the advantages and disadvantages of using short pitched winding and distributed winding in alternator.

b) The stator of a 3-phase, 16 pole alternator has 144 slots and there are 4 conductors per slot connected in two layers and the conductors of each phase are connected in series. If the speed of the alternator is 375 r.p.m. calculate the e.m.f. induced per phase. Resultant flux in the air gap is .05 webers per pole sinusoidally distributed. Assume the coil span as 150° electrical.     (7 ‘A)

OR

III  a) Derive an expression for winding factor of an alternator.

b) A 4 pole, 50Hz, 3 phase star connected alternator has a single layer, full pitched winding with 21 slots per pole and two conductors per slot. The fundamental flux is 0.6wb and air gap flux contains a 3rd harmonics of 5% amplitude. Find the r.m.s. values of the phase e.m.f. due to the fundamental and the 3rd harmonic flux and total induced e.m.f.

IV a) What is armature reaction? Explain the effect of armature reaction on the terminal voltage of an alternator at (i) upf (ii) zero leading pf (iii) zero lagging pf. Draw relevant phasor Diagrams.

b) Determine the voltage regulation of a 2000 V, single phase alternator giving a current of 100A at 0.8 pf leading: Full load current of 100 A is produced on short circuit by a field excitation of 2.5 A. An ejn.f. of 500 V produced on open circuit by the same excitation. The armature resistance is 0.8 Q. Draw the vector diagram.

V a) Discuss the Potier Method of determination of voltage regulation.

b) Two 3 phase, star connected alternators 1 & 2 supply a total load of 18 MVA at 0.7 lagging pf at a line voltage of 6.6 KV. The two alternators are rated at 10 MVA 6.6 KV. The machine is operating on full load at 0.8 pf lagging. Find

i) the current supplied by machine 2

ii) Operating pf of machine 2

i)  Power delivered by each machine.

VI a) Justify the statement “synchronous motor either run at synchronous speed or do not run at all”.

b) Derive an expression for maximum power developed by a synchronous motor.

OR

VII a) Discuss the terms normal excitation, under excitation and over excitation in connection with synchronous motor operation.

b) A3 phase synchronous motor connected to 6600V mains has a star connected armature with an impedance of 2.5 + j\ 5 Q / phase. Then excitation of machine gives 7000V. The iron friction and excitation losses are 12KW. Find the maximum output of the motor.

VIII a) Explain the change in excitation on armature current and power factor,

b) The input to a 11000V 3 phase star connected synchronous motor is 60A. The effective resistance and synchronous reactance per phase are 1 fi and 30 Q respectively. Find the Power supplied to the motor and the induced e.m.f for a pf of 0.8 leading and 0.8 lagging.

OR

IX a) Describe the phenomenon of hunting in a synchronous machine. How is it remedied?

b) A 1000 KVA, 11000 V 3 phase star connected synchronous motor has an armature resistance and reactance per phase of 3,50 and 40£1 respectively. Determine the induced e.m.f and angular retardation of the motor when fully loaded at

 i) upf ii) 0.8 lag iii) 0.8 lead.