The open-circuit saturation curve (also open-circuit characteristic, OCC) of a

synchronous generator An alternator (or synchronous generator) is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. For reasons of cost and simplicity, most alternators use a rotating magnetic field with ...

is a plot of the output

open circuit voltage Open or OPEN may refer to: Music * Open (band), Australian pop/rock band * The Open (band), English indie rock band * Open (Blues Image album), ''Open'' (Blues Image album), 1969 * Open (Gerd Dudek, Buschi Niebergall, and Edward Vesala album), ' ...

as a function of the excitation current or field. The curve is typically plotted alongside the

synchronous impedance curve The synchronous impedance curve (also short-circuit characteristic, SCC) of a synchronous generator is a plot of the output short circuit current as a function of the excitation current or Field (physics), field. The curve is typically plotted alon ...

. At the low field, the

permeable Permeability, permeable, and semipermeable may refer to: Chemistry *Drug permeability *Semipermeable membrane, a membrane which will allow certain molecules or ions to pass through it by diffusion *Vascular permeability, the movement of fluids a ...

iron in the

magnetic circuit A magnetic circuit is made up of one or more closed loop paths containing a magnetic flux. The flux is usually generated by permanent magnets or electromagnets and confined to the path by magnetic cores consisting of ferromagnetic materials lik ...

of the generator is not saturated, therefore the

reluctance Magnetic reluctance, or magnetic resistance, is a concept used in the analysis of magnetic circuits. It is defined as the ratio of magnetomotive force (mmf) to magnetic flux. It represents the opposition to magnetic flux, and depends on the ...

almost entirely depends on the fixed contribution of the air gap, so the part of the curve that starts at the point of origin is a linear "air-gap line" (output voltage is proportional to the excitation current). As the iron saturates with higher excitation and thus higher

magnetic flux In physics, specifically electromagnetism, the magnetic flux through a surface is the surface integral of the normal component of the magnetic field B over that surface. It is usually denoted or . The SI unit of magnetic flux is the we ...

, the reluctance increases, and the OCC deflects down from the air-gap line. The curve is obtained by rotating the generator at the rated

RPM Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or r⋅min−1) is a unit of rotational speed (or rotational frequency) for rotating machines. One revolution per minute is equivalent to hertz. Standards ISO 80000-3:2019 def ...

with the output terminals disconnected and the output voltage typically going to at least 120% of the rated for the device. The hydraulic units sometimes have to be tested at lower RPM with the resulting voltage scaled up to accommodate the differences in frequency. Since the test goes above the rated voltage, the step-up transformer is typically also disconnected to avoid damaging it. The open circuit saturation curve could be used together with the zero power factor curve in Potier Triangle Method.

References

Sources

* * Electrical generators {{electric-stub