Analytical modeling and experimental validation of the churning torque based on partial oil immersion wet external rotor permanent magnet BLDC motor with an isolation sleeve

To solve the problem of rotary dynamic seals that are prone to failure at high speeds, oil-immersed wet motors are used in motor pumps. The resulting long-term erosive wear and deterioration of the winding insulation layer seriously affects the operational reliability of the motor. This paper presents a wet external rotor permanent magnet brushless DC (WER-PMBLDC) motor with an isolation sleeve separating the oil from the winding. The proposed design improves the reliability of the winding while removing the dynamic seal on the pump shaft. Its partial immersion working condition also offers the possibility of balancing flow resistance and heat dissipation. However, at high speeds, churning loss remains a major constraint on motor output torque. To quickly and accurately evaluate the churning loss of the WER-PMBLDC motor, an analytical model of the churning torque at different oil immersion ratios was established based on boundary layer theory, shearing and pushing flow distributions, and oil-air two-phase model. The flow field was verified by CFD simulation, the partial immersion coefficients of the analytical model were determined, and the influence of key structural parameters on the churning torque was analyzed. Finally, an external rotor churning prototype was manufactured and churning experiments were carried out. The study shows that the churning torque decreases with the decrease in rotation speed and the oil immersion ratio. The average error of the analytical model is only 5.34% at medium and high rotation speeds, which verifies the accuracy of the analytical model. This model can provide useful theoretical guidance for low flow resistance design of the wet motor.