The zero-power factor characteristic (ZPFC) of an alternator is a curve of the armature terminal voltage per phase plotted against the field current obtained by the operating the machine with constant rated armature current at synchronous speed and zero lagging power factor.The ZPFC is some times called potier characteristic after its originator.

For maintaining very low power factor,alternator is loaded by means of reactors of alternatively by an underexcited synchronous motor.The shape of ZPFC is very much like that of the O.C.C. displaced downwards and to the right.

The phasor diagram corresponding to zero-power factor lagging load is