Improved Stability of Hybrid Flux Observer at Ultralow Speeds for PMSM Sensorless Control Considering DC Offsets

The stability of conventional hybrid flux observer (HFO) for sensorless control of permanent magnet synchronous machine deteriorates as speed decreases. In this paper, the capability of eliminating DC offsets and the stability of conventional HFO are analysed first. Although the conventional HFO with P controller has better stability at low speeds, the capability of eliminating DC offsets is poor, while HFO with PI controller are opposite. Considering the contradiction between the capability of eliminating DC offsets and the stability, an improved HFO with adaptive gain matrix is proposed to enhance stability. The capability of DC offset elimination is confirmed by frequency domain analyses and the stability of the whole system is proved according to the Popov's superstability theorem, and the guidelines for parameter design are also presented. The proposed HFO can operate at ultra-low speeds except for absolute zero speed while avoiding the influence of DC offsets. It is shown that the proposed observer has better performance on eliminating DC offsets than the conventional HFO with P controller and better stability than the conventional HFO with PI controller. The comparative experiments are carried out on a PMSM drive platform to validate the superiority of the proposed method.