The chapter provides an experimental evaluation of a model reference adaptive system (MRAS) observer based on super twisting observer (STO) without a position sensor of surface permanent magnet synchronous motor (SPMSM). The observer is implemented and tested on experimental setup, with the aim to compare its speed and position tracking capability on high speeds and low speeds where particularly the motor state of SPMSM is unobservable. The first section of the chapter talks about SPMSM model and its observability analysis. The second section describes the MRAS observer for SPMSM. The third section illustrates experimental results of MRAS observer on SPMSM high- and low-speed benchmark. The final section draws some conclusions.