Why the open circuit characteristics is nonlinear?

Because of saturation in iron parts of machine, the no load generated voltage Ef does not increase in the same proportion as the increase in field current and hence the OCC of Three Phase ALternator in Non-Linear.

Why do we use open circuit test on 3 phase ALternator?

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.

What is open circuit characteristics of an ALternator?

With the armature terminals open, Ia=0, so Eg = Vt It is thus possible to construct a plot of Eg or Vt vs If graph. This plot is called open-circuit characteristic (OCC) of a generator.

Why OCC curve is not linear?

The OCC will be nonlinear due to the saturation of the magnetic core at higher levels of field current. The SCC will be linear since the magnetic core does not saturate under short-circuit conditions.

Why SCC of an alternator is linear?

As the resultant air gap flux is only 0.15 of its rated value under normal voltage condition, such a low value of air gap flux does not saturate the iron parts of synchronous machine and hence the short circuit characteristics is a straight line.

What are the characteristics of open circuit?

An open circuit is the absence of a connection between two terminals. The open circuit maintains zero current and can allow any voltage drop, so its I-V characteristic is a horizontal line at I=0. Notice that an open circuit behaves identically to a zero current source with I_s=0\text{ A}.

Why does OCC look like BH curve?

However, as the flux density increases, the poles get saturated and the ɸ becomes practically constant. Thus, even we increase the If further, ɸ remains constant and hence, Eg also remains constant. Hence, the O.C.C. curve looks like the B-H characteristic.

Why SCC of synchronous generator is a straight line?

The OCC is a straight line till saturation point. As field current increases, Emf induced increases proportionally (according to emf eqn of synchronous machines). Thus we get a straight line.