Gearbox Lubricant Condition Monitoring Based on Motor Current Signals from Closed-Loop Control Drive

Gear failures are often linked to inadequate lubrication, inappropriate viscosity, and insufficient lubricant quantity, leading to significant gear damage. Insufficient lubrication alters the dynamic torque characteristics of gear systems, subtly changing motor current signal modulation. Industrial motors commonly employ closed-loop speed control systems, which inevitably introduce harmonic interference from electronic components and background noise. This interference results in a low signal-to-noise ratio (SNR), complicating the precise demodulation of minor current modulations and posing challenges to early fault diagnosis. This article explores fundamental frequency fluctuations in motor currents under speed-sensorless vector control systems and their impact on current signal demodulation. It introduces a novel technique, carrier-reconstructed modulation signal bispectrum (CR-MSB), which reconstructs the carrier to remove irrelevant components and utilizes phase alignment to reduce random noise, enabling precise extraction of nonlinear modulation features from current signals. To validate the proposed approach, a two-stage, helical industrial gearbox was tested under various operating conditions by altering the lubricant quantities and viscosities. The results revealed that under variable load conditions with subpar lubrication, the current sideband amplitudes exhibited the corresponding characteristics, allowing diagnosing the changes in lubrication condition. This verifies that CR-MSB based current signal analysis can be an effective tool for extracting monitoring information from motors with closed-loop control (CLC) drives.