In this paper, the low excitation process and
loss-of-excitition process of synchronous motor
is analyzed In three stages in detail, and the
corresponding power impedance circle, the
static-stable impedance and asynchronous
impedance are derived in detail and
demonstrated. When the synchronous motor loss
excitation, terminal locus of measured
impedance will be composed of second quadrant
into third quadrants, and the first to enter the
static-stable boundary, and then enter the
asynchronous boundary, in order to reduce the
harm, when the end locus of measured
impedance into the static-stable boundary, the
out of step protection will be activated. The
theory has broad application value and
versatility with practical value in daily life.