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To detect the rotor angle of the permanent-magnet (PM) synchronous motor (PMSM), two linear Hall sensors are utilized to measure the fringe field of the PM from one end of the rotor. The measured signals look like sin θ and cos θ but contain large third-order harmonics. To remove the third-order harmonics, an adaptive notch filter (ANF) is utilized. Furthermore, the ANF is closely connected with an orthogonal phase-locked loop (PLL): The ANF eliminates the harmonics from the PLL input, and the PLL output is used for generating the harmonic reference signals that are required by the ANF. That is, the ANF utilizes the PLL outputs in removing the target harmonic components from the PLL inputs. The PLL angle estimates are used for implementing the field-oriented control of a PMSM. Experimental results in the low-speed region look better than the sensorless algorithms. This kind of edge-field detection method is advantageous since it is easy to install and less affected by the stator current, i.e., relatively robust to the armature reaction.