Application of space phasors for the discrimination of short-circuits and high inertia startups

Short-circuit protection of electrical motors in explosive gas atmosphere, as in underground mines, is an important duty. Short-circuit currents of high magnitude may cause explosions and fire. But even the starting current of an induction motor is a multiple of nominal current. Consequently protection devices should have a high sensitivity to faults. This paper discusses the application of space phasors for the identification of high inertia startups. The complex motor impedance can be calculated from the voltage and current space phasor and runs along a circle during startup. This complex impedance can be used for monitoring and diagnosis purposes.