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no.27
37. A 25-kva 220-volt three-phase alternator delivers rated kilovolt-amperes at a power factor of 0.84. The effective a-c resistance, between armature-winding terminals is 0.18 ohm. The field takes 9.3 amp at 115 volts. If friction and windage loss is 460 watts and the core loss is 610 watts, calculate the per cent efficiency. 38. Each of two single-phase alternators has an armature winding whose resistance and synchronous reactance are, respectively, 0.025 and 0.06 ohm. If the machines are operating without load, calculate the circulating current in the windings if the alternators are paralleled: (a) when the emfs are equal at 230 volts, but are dis- placed 30° from a position of phase opposition; (b) when the voltages are in phase opposition, but one voltage is 230 and the other is 200 volts; (c) when the voltages are 230 and 200, and displaced 30° from a position of phase opposition. 29. Using the given data of Prob. 28, calculate the angle between two equal voltages of 230 if the circulating current is 1,000 amp when the alternators are paralleled.