# Electrical Machines-II June 2011

EE 502 ELECTRICAL MACHINES II

(2006 Scheme)

PART-A

a)  Derive the relation between the number of poles, the frequency and speed of the alternator.

b) The coil span for the stator winding of an alternator is 120° (electrical). Find the chording factor for the fundamental and third harmonic.

c)  Why voltage regulation of an alternator is negative for leading power factor?

d) What are the advantages of connecting alternators in parallel?

e)  Draw the phasor diagrams of a synchronous motor for lagging, leading and unity power factor conditions.

f)  A 50KW, 400V 3-phase synchronous motor is operating at full load with an efficiency of 92%. If the field current is adjusted to make its power factor 0.8 leading calculate the armature current.

g)  Draw the power angle characteristics of a salient pole and cylindrical rotor machine.

h) What is hunting in synchronous machines?

PART-B

a) What are the different types of ac generators in use? Explain the essential differences in their construction,

b) What are the advantages of rotating field system in an alternate.

OR

III. a) What are the advantages of short pitching? Derive an expression for distribution  factor.

b) A3 phase, 8 pole, 750 rpm star connected alternator has 72 slots on the (10) armature. Each slot has 12 conductors and winding is short chorded by 2 slots. Find the induced emf between lines, given the flux per pole is 0.06wb.

IV. a) Define the terms synchronous reactance and voltage regulation of an alternator. Explain synchronous impedance method of determining regulation of alternator.

b) Why xj and x„ are different for salient pole alternators whereas they are same for smooth rotor machines?

OR

V.  3-phase star connected lOOOkVA, 2000V, 50Hz, alternator gave the  following open circuit and short circuit test readings.

Field current A:  10 20 25 30 40 50

Open circuit voltage V 800 1500 1760 2000 2350 2600 SC armature current A: 200 250 300

b) Draw the characteristic curves and calculate the full load regulation at 0.8pf lagging and leading using mmf method. The armature effective resistance/phase may be taken as 0.2Q.

VI. a. Derive the expression for mechanical power developed in a synchronous motor

b. A 2.3KV 3-(z) star connected synchronous motor has Zs = (0.2 + y’2.2)ohms per phase. The motor is operating at 0.5 pf leading with a line current of 200A. Determine the generated emf per phase.

OR

VII.   a. Explain the effects of varying the excitation on armature current and power factor of asynchronous motor.

b. A 500V, 50Hz, 3 phase circuit takes 20A at a lagging power factor of 0.8. A (6) synchronous motor is used to raise the power factor to unity. Calculate the KVA input to the motor and its power factor when driving a mechanical load of 7.5KW. The motor has an efficiency of 85%.

VIII.  a. Derive the expression for power developed in a salient pole alternator.

b. An alternator is connected to an infinite busbar and is running at no load. Explain briefly how to increase its active and reactive power outputs.

OR

IX. a. Explain transient and sub transient reactance of alternators.

b. A 10000KVA, 4 pole 6600V, 50Hz, 3 phase star connected alternator has a synchronous reactance of 25% and operates on constant voltage, constant frequency bus bars. If the natural period of oscillation while operating at full load and unity power factor is to be limited to 1.5 seconds. Calculate the moment of inertia of the rotating systems.