What are the zero power factor characteristics (ZPFC) used for in the context of alternators?  

Explanation:

Zero Power Factor Characteristic (ZPFC):

• ZPFC of a generator is a curve of the armature terminal voltage and the field current.
• The machine is operated with constantly rated armature current at synchronous speed and zero lagging power factor.
• The Zero Power Factor Characteristic is also called as Potier Characteristic.
• For maintaining very low power factor, the alternator is loaded by means of reactors or by an under excited synchronous motor.
• The shape of ZPFC is very much like that of the O.C.C. The phasor diagram corresponding to zero power factor lagging is shown below.