In this work, stability properties of a novel and easy-to-implement sensorless observer for induction motors are investigated. The considered solution is inspired by phase-locked-loop strategies and it takes advantage from the rotor flux derivatives to define a rotating vector to align with, even if such variables are not directly measured. Input-to-State Stability properties are proven for suitably defined electrical and magneto-mechanical observation-error subsystems. Time scale separation is, then, exploited to invoke Singular Perturbation arguments and guarantee boundedness of the observation errors trajectories. Finally, the Small Gain Theorem is adopted to prove semiglobal uniform asymptotic stability of the presented scheme. Some additional interesting properties are discussed, beside the "main stream" of the proposed analysis. Simulations indicate the observer can be profitably exploited for induction machines speed sensorless control applications.