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Though the hydraulic engineer is rarely required to select the electrical equipment for a hydroelectric project, he or she should have some knowledge of the basic requirements and limitations of this type of equipment. The alternating - current synchronous generator is most widely used in the United States. In its simplest form the generator is composed of two elements, a magnetic field consisting of an assembly of electromagnets of alternately opposite polarity and an armature or system of conductors in which an alternating electromotive force is induced when the armature is rotated. When a conductor has passed two adjacent poles of opposite polarity, a complete alternation of the electromotive force occurs, and this is referred to as a cycle. The number of cycles per second is termed the frequency. In the United States a frequency of 60 cycles per second is the accepted standard, though there are still a few systems operating at other frequencies. A turbine may be governed to operate a constant speed, and generators are designed with the proper number of poles to produce the desired frequency, at the selected speed.
Alternating - current generators are rated in kilovolt - amperes (kva) for a given temperature rise, usually the rise in temperature which the insulation can, safely stand. The apparent output (kva) of a generator differs from the actual, output according to the following expression:
Actual power (kW) = apparent power (kva) ×power factor
The power factor can never be greater than unity. Its value depends on the relationship between the inductance and resistance of the load. A load with very little inductance such as a lighting load will have a power factor close to unity. The usual system load has a power factor ranging from 0.8 to 0.9, but if the load contains a large number of induction motors, the power factor may be as low as 0.5.
Weights and dimensions of generators vary considerably depending on the kilovolt - ampere rating and the selected speed of operation. Alternating - current generators with a rating of 3000 kva vary in weight from about 18 tons (16,000 kg) for an operating speed of 900 rpm to 55 tons (50,000 kg) for a speed of 100 rpm. These generators would be about 10 ft (3 m) high and 8 or 16 ft (2.5 or 5 m) in diameter, respectively. Special structural problems often arise in providing proper support generators.
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