The influence of the rotor time constant mismatch on the global stability of induction motors under indirect field oriented control (IFOC) is analyzed. A test for global stability is provided, based on necessary and sufficient conditions for the existence of quadratic Lyapunov functions for IFOC drives. This test yields robustness margins with respect to rotor time constant mismatches. A typical example is presented to show that the robustness margins obtained are significantly less conservative than the ones obtained with previously proposed stability criteria.