Controlled ac drives without speed sensors of the motor shaft are in increasing demand. Eliminating the speed sensor increases the reliability of the overall system and reduces its cost. The proposed method employs electrical quantities, the stator current and stator voltage space vectors, as input signals in a closed-loop observer to estimate the rotor speed. The sensorless observer is built around the fundamental model of the ac machine; the redundancy of different structures is exploited to endorse the system robustness. Issues of concern are drift in the signals of the input channels of the observer, deviation of machine parameters from their nominal values, and operation at very low stator frequency. The performance of the overall control system is evaluated at steady-state and dynamic operating conditions. Both simulation and experimental results are provided to support the reliability of the proposed sensorless observer.