For the speed sensor-less control in the ultra lower speed range, the mechanical simulator method is applied to cope with the problems due to the extremely lower induced voltage. In the method, however, the mechanical parameters' error, such as the error of the inertia, affects the starting performance. To reveal the conditions for the stable starting, we carry out the phase plane analysis under the condition that the actual inertia J changes from the set inertia Js. Consequently, we find out Js ≥ J is the condition for the stable acceleration. The derived condition is useful as a design index of the mechanical simulator. Additionally, to cope with the large variation of the inertia, we carry out the phase plane analysis of the IM simulator method which compensates the estimation error of the rotor frequency. The analysis reveals that the method enables the stable acceleration while the estimation error of speed is remained in the case of inertia variation. Furthermore, we propose the modified IM simulator method which eliminates the deviation. By the phase plane analysis and experimental tests, we confirm the effectiveness of modified method.