Calculation Method for Viscous Loss in Oil-Filled Motors Based on Corrected Friction Coefficient

Oil-filled motors (OFMs) are widely used in deep-sea, aerospace, and other high-reliability applications due to their superior insulation, lubrication, and pressure compensation capabilities. However, the additional viscous loss in the air gap significantly reduces motor efficiency, particularly during high-speed operation. As viscous loss is influenced by motor geometries, rotational speed, temperature, and rotor surface roughness, traditional analytical formulas have failed to account for these factors with sufficient accuracy. In this study, computational fluid dynamics (CFD) simulations investigate the effects of effective length, rotor outer radius, air gap, slot type, temperature, rotational speed, and rotor surface roughness on viscous loss. The CFD model is refined and validated through towing experiments, providing an accurate result of viscous loss. Based on the validated simulations and experimental data, the formula for viscous loss is corrected to incorporate the effects of rotor surface roughness on the friction coefficient and temperature-dependent viscosity. Compared with other methods, the corrected formula for viscous loss greatly improves calculation accuracy. The proposed method provides a reliable approach for calculating viscous loss, facilitating the design of high-efficiency OFMs suitable for demanding applications.