Monitoring of rotating electrical machines is still an essential part to increase reliability and operational safety of electrical systems and has been the subject of much research in recent decade. The methods conventionally used for the diagnosis of electrical machines are based on measurements of current, voltage, vibration and noise. This chapter presents an analytical modeling of an electrical machine with a stator inter‐turn short circuit fault, associated with a simplified decomposition of the external magnetic field. For modeling the faulty machine, a three‐phase stator winding is considered. The external magnetic field is an interesting variable to be exploited for the diagnosis of electrical machines. The chapter also presents a methodology that exploits amplitudes and amplitude variations of a sensitive harmonic measured in the voltage delivered by several sensors.