This paper presents a methodology for the analysis of the natural self-excitation process under no load of the Induction Generator (IG). A spatial vector mathematical model of the Self-Excited Induction Generator (SEIG) is used as a starting point and the effect of the magnetic non-linearity on the magnetization inductance is considered, then the natural self-excitation region is determined and the self-excitation process of the Induction Generator is simulated. The methodology is applied using a three-phase induction motor with squirrel cage rotor of 3 HP, 230 V, 60 Hz, 4 poles, star connection and efficiency class IE1. Finally, simulations of the transient in the first instants are presented to verify the conditions that must be fulfilled to trigger the natural self-excitation process of the induction generator.