The sensorless control of permanent magnet synchronous motors (PMSMs) based on the model is usually affected by changes in motor parameters. For this reason, this paper proposes a novel method based on the neutral point third harmonic back electromotive force (EMF) and a second-order generalized integrator-based frequency-locked loop (SOGI-FLL). This method is not related to any motor parameters and only requires a neutral wire. Firstly, the relationship between neutral point voltage and third harmonic back EMF is analyzed, and a new low-cost third harmonic back EMF detection circuit and sampling method is presented. A cascaded structure consisting of a limiting filter and a sliding window median filter is designed to filter out switching spike pulses and sampling noise. Then, SOGI-FLL technology is applied to extract and track the rotor position and filter out non-third harmonic signals caused by the asymmetry of the three-phase back EMFs. Finally, simulation and experimental results demonstrate that the proposed method has high static and dynamic position estimation accuracy.