A Simplified PLL-Like Observer for Sensorless PMSM Applications

Control of synchronous AC motors requires accurate knowledge of the orientation of the rotor magnetic field. This could most directly be obtained via measurement of the rotor mechanical angle, but the sensors required for this approach introduce cost and reliability issues to the system. Phase-locked loops (PLLs) are commonly utilized in conjunction with measured or observed back EMF signals to achieve a sensorless implementation. The stability of such systems, especially in the presence of unknown back EMFs, is difficult to prove. An observer design is proposed in this work which seeks to observe the rotor electrical angle without the need for intermediate observation of the back EMFs or a PLL. This holistic design is motivated by a Lyapunov stability analysis which proves boundedness of all observer signals and convergence of the observed rotor electrical angle. This analysis is further validated by experimental results.