Tacholess Order Tracking Method Based on STFrFT for Fault Diagnosis of Wind Turbine Gearbox Under Variable Operating Conditions

Order tracking (OT) is an effective fault diagnosis method for rotating machinery under variable speed conditions. Accurate instantaneous frequency (IF) is the premise of this method. In recent years, the tacholess method has been widely used to estimate IF. However, the method’s accuracy requires further improvement. Thus, this article proposes an IF estimation method based on short-time fractional Fourier transform (STFrFT) for tacholess OT. First, the principle and main error sources of the OT are analyzed. On this basis, we present three quantitative evaluation indicators of IF accuracy that are applicable to different scenarios. Second, an efficient and accurate IF estimation method based on STFrFT that benefits from the first-order approximation of the linear frequency modulation (LFM) kernel function to the IF and a series of improvement measures to avoid redundant rotation angle calculation are proposed. Finally, order spectrum is employed after tacholess OT to achieve fault diagnosis. The effectiveness of the proposed method is proven by the wind turbine gearbox data collected in the field. Verification results show that the proposed method is superior to the comparison methods in terms of IF accuracy and efficiency.