Thermal Analysis of Outer Rotor BLDC Motor for Drone Considering Airflow Ventilation Cooling

This paper presents a refined thermal analysis of a semi-closed outer rotor BLDC motor designed for drone applications, with a focus on ventilation cooling effects induced by motor-generated airflow. To reduce weight, small drone motors are typically designed without dedicated cooling systems, relying instead on airflow generated during flight. Considering these operating conditions, thermal analysis that accurately reflects the airflow characteristics around the motor is essential in the motor design process. Existing thermal analysis methods each have limitations. Computational Fluid Dynamics (CFD) can analyze flow and heat transfer phenomena with high resolution but involves high computational cost, making it impractical for iterative design processes. Conversely, Lumped-Parameter Thermal Network (LPTN) models offer fast computation but require pre-defined convective heat transfer coefficients and have difficulty capturing complex flow conditions. In this study, a hybrid thermal analysis approach combining CFD and LPTN methods is proposed. First, motor losses under various load conditions are calculated using 2D and 3D electromagnetic Finite Element Analysis (FEA), incorporating the effect of axial leakage flux. Next, CFD simulations are performed on a coaxial rotor drone system to evaluate airflow characteristics near the motor. Based on the CFD results, convective heat transfer coefficients are determined and applied to the LPTN model, achieving both high accuracy and computational efficiency in thermal analysis. The proposed method was validated through prototyping and propeller load tests simulating various operating conditions. The comparison between the measured and analyzed winding temperatures showed good agreement, confirming the accuracy and reliability of the proposed approach. Notably, the results confirmed that most of the airflow entering the motor is generated by the motor’s own rotation rather than by the propeller, for both single-rotor and coaxial-rotor configurations. This CFD–LPTN-based thermal analysis framework provides a practical and validated method to evaluate the thermal performance of air-cooled drone motors under various operating conditions.