Open Circuit Characteristics (OCC) of an Alternator

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.