This paper deals with rotor position and speed estimation of permanent magnet synchronous motors. A reduced order observers based on electromotive force estimation is proposed for estimating mechanical variables. Robustness against model uncertainties is analyzed. Under exact model assumption the estimation error converges to zero in exponential way, while a residual estimation error appears when uncertainties are presented. For this reason, estimation error bounds are calculated when both uncertainties in the mechanical and electrical submodels are taken into account. The observer performance is tested by simulation.