High-Precision Sensorless Commutation Error Compensation for Downhole BLDC Motors With Nonideal Back EMF

Rotor position estimation in brushless dc (BLDC) motors often relies on the back electromotive force (back EMF), assuming ideal trapezoidal or sinusoidal waveforms. However, in real-world scenarios—especially under harsh operating conditions such as downhole drilling—the actual back EMF waveform deviates significantly due to the complex distribution of air-gap magnetic flux density. This leads to substantial errors in rotor position estimation and commutation. To address this, we propose a high-precision sensorless closed-loop control strategy that corrects commutation errors by utilizing flux linkage integration deviation. The approach begins with a detailed analysis of how different rotor structures affect the shape of the back EMF waveform. A synchronous sampling method is then introduced to reduce signal distortion caused by asynchronous acquisition, whereas an unbalance coefficient compensation algorithm further improves response speed and control accuracy. The proposed strategy is validated on a specialized downhole BLDC motor testing platform, demonstrating its effectiveness and feasibility in extreme environments. These results highlight the method’s potential for improving robustness and precision in sensorless control systems subject to nonideal conditions.