The most commonly used machine for generation of electrical power for commercial purpose is the synchronous generator or alternator. An alternator works as a generator when its rotor carrying the field system is rotated by a prime-mover which in this case is DC shunt motor. The terminal voltage of an alternator changes with load.
Alternators are by far the most important source of electric energy. Alternators generate an AC voltage whose frequency depends entirely upon the speed of rotation. The generated voltage value depends upon the speed, the dc field excitation and the power factor of the load.
As the DC field excitation of an alternator is increased, its speed being held constant, the magnetic flux, and hence, the output voltage, will also increase in direct proportion to the current. However, with progressive increases in DC field current, the flux will eventually reach a high enough value to saturate the iron in the alternator. Saturation in the iron means that there will be a smaller increase in flux for a given increase in DC field current. Because the generated voltage is directly related to the magnetic flux intensity, it can be used as a measure of the degree of saturation.
When an alternator delivering full rated output voltage is suddenly subjected to a short-circuit, very large currents will initially flow. However, these large short-circuit currents drop off rapidly to safe values if the short-circuit is maintained. The output voltage of an alternator depends essentially upon the total flux in the air-gap. At no load this flux is established and determined exclusively by the DC field excitation.
Under load, however, the air-gap flux is determined by the ampere-turns of the rotor and the ampere-turns of the stator. The latter may aid or oppose the MMF (magnetomotive force) of the rotor depending upon the power factor of the load. Leading power factors assist the rotor, and lagging power factors oppose it.
The open-circuit test or the no-load test, is performed by driving the generator at its rated speed while the armature winding is left open. The field current is varied in suitable steps and the corresponding values of the open-circuit voltage varied in suitable steps and corresponding values of the open-circuit voltage between any two pair of terminals of the armature windings are recorded. The OCC follows a straight-line relation as long as the magnetic circuit of the synchronous generator does not saturate. In the linear region, most of the applied mmf is consumed by the air-gap; the straight line is appropriately called the air-gap line. As the saturation sets in, the OCC starts deviating from the air-gap line.