Question

A 400 v, 50 Hz, three phase alternator, rated at 75 kVA, is star connected and driven at 1000 r/min. The windings of the alternator have a resistance, per phase, of 0.03 Ω and synchronous reactance, at 50 Hz per phase, of j0.325 Ω. The stator of the alternator has 108 slots, with 8 conductors, per slots, in a double layer. The maximum pole flux is 8.772 x 10-3 Wb. To improve the wave form only 80% of the pole pitch is wound.

a. Determine the smallest lagging power factor at which the alternator can deliver its rated load [0.823]

b. According to available information about the alternator described above it may be driven 1200 r/min. When operated at 1200 r/min the rated voltage is 480 V and the maximum phase current is to be kept to the same value as the rated phase current when it is driven at 1000 r/min.

Determine for the alternator, when driven at 1200 r/min, the

(i) Rated output and,[305.95 Volts]

(ii) Smallest lagging power factor at which the alternator can deliver its rated load [0.815]