Flow-Directed Oil Cooling of an Outer-Rotor PMSM for Electrohydraulic Applications

Abstract

The continuous torque of compact outer-rotor permanent-magnet synchronous machines (PMSMs) driving hydraulic pumps for hydraulic actuation is often limited by winding temperature. This paper proposes an improved flow-directed oil-cooling arrangement that supplies 24 axial stator channels and eight end-winding jets from a common pump. A nonlinear hydraulic-network model is used to select cooling-path dimensions and estimate flow distribution, while effective heat-transfer coefficients derived from analytical correlations and physically motivated assumptions are applied in a computationally efficient three-dimensional thermal finite-element model. The concept is implemented and validated in an outer-rotor PMSM prototype originally rated at 7 kW and 22.3 Nm. At 7 kW, 3000 rpm, and an oil flow rate of 1 L/min, the mean measured winding temperature decreased from approximately 155 °C with the original oil-circulation arrangement to 92 °C, while the model predicted 97 °C at the corresponding sensor-equivalent location. The improved cooling arrangement enabled continuous motor operation at 150% of the original rated torque at 3.7 L/min, with a mean measured winding temperature of approximately 91 °C and a corresponding model prediction of 96.5 °C. The validated model predicts that the winding temperature at the sensor-equivalent location reaches 155 °C at approximately 52 Nm, or 230% of the original rated torque. The combined experimental and sensitivity results support a broader design principle: moderate cooling distributed over several thermally critical surfaces can be more effective than maximizing the local HTC over a limited area.