Problems

Author: E.E. Kimberly

	Problem	Answer
1	What is the full-load current per terminal of a 2300-volt, 3-phase, 2000-kv-a alternator?
2	A 2300-volt, Y-connected, 100-amp, 0.8-PF alternator has internal impedance of 1 +j5 ohms per phase. A full load of 100% PF is being supplied at the rated terminal voltage. If the load is removed, but the field-rheostat setting is kept the same, what will be the terminal voltage? If, then, a full load with 70% PF leading is applied, what will be the terminal voltage? Assume the internal impedance to remain constant.
3	A 40,000-kv-a, 13,200-volt, 3-phase, 60-cycle, Y-connected alternator has a stray power loss equal to 1.5% and a field loss equal to 0.4% of the rated output kv-a at 85% power factor lagging. The effective stator resistance is 0,10 ohm per phase. Calculate the efficiency at full load of 85% power factor, lagging.
4	A 50-kv-a, Y-connected, 3-phase, 220-volt, 1200-rpm, 0.8-power factor, 60-cycle alternator requires 2.5 hp to drive it at rated speed at no load with field excitation. The stator resistance is 0.05 ohm per phase. The field-circuit power input at full load is 2 kw. Calculate the efficiency at full load of 80% power factor lagging.
5	Two similar 3-phase, 2000-kv-a alternators in parallel supply a 3500-kw load of 80% power factor lagging. If the output of the first alternator is 1600 kw at 70% power factor, what are the kw and kv-a outputs of the other?
6	A 96-pole alternator is to be used to supply a 50-cycle power system. At what speed must it be run?
7	An alternator running at 375 rpm generates 25 cycles per second. How many poles does it have?
8	The stator of a 1000-kv-a alternator designed to be driven by a steam engine is much larger and shorter than one of the same electrical size designed for direct connection to a steam turbine. Explain.