Open Circuit Characteristics (OCC) of an Alternator: OCC is the curve that shows the relationship between the generated EMF at no-load (E0) and the field current (If) at constant speed.
To obtain the open circuit characteristics the machine is driven at rated speed without the load. Readings of the line-to-line voltage are taken for various values of field current. The voltage, except in very low voltage machines, is stepped down by the means of a potential transformer.
Experimental set up
OCC
If not for the magnetic saturation of the iron, the open circuit characteristics would be linear as represented by the air gap line.