Design of a direct-drive axle-mounted traction motor for boosting high-speed trains

Background High-speed railway traction systems must combine high efficiency and power density within constrained mechanical envelopes. This work presents the design and numerical assessment of a direct-drive permanent magnet synchronous motor (PMSM) for supplementary traction, designed to fit between the wheel and axlebox of an existing high-speed train while retaining its disc-brake hardware. Methods The design methodology combines multiobjective parametric optimisation with topology optimisation to limit magnetic flux leakage while preserving manufacturability. An oil-flooded, slot-through cooling system was integrated into the motor. Electromagnetic, thermal and structural simulations assessed nominal operation, fault conditions, overspeed, shock and vibration loads derived from railway standards. High-speed-boost and load-sharing strategies were benchmarked over a representative duty cycle against the reference geared induction-motor powertrain. Results This direct-drive architecture eliminates the gearbox and dedicated motor bearings. In the preliminary comparison based on estimated active-part masses, direct oil cooling enables approximately four times the power density of water-jacket cooling within the same envelope. Simulations predict 275 kW of continuous power and 95.54% nominal efficiency at 2275 rpm, corresponding to 360 km/h. The calculated assembly mass is 86.90 kg, including cooling oil, yielding 3.16 kW/kg. The motor achieves an envelope-based power density of 8.23 kW/L while retaining the existing disc-brake hardware. Duty-cycle simulations predict reductions of up to 5.82% in electricity-related operating costs and emissions with load sharing. Conclusions The results support the numerical feasibility of a compact, high-power-density direct-drive motor for supplementary railway traction. The proposed architecture integrates direct oil cooling and axle-mounted mechanical design while retaining the existing brakes, providing the basis for prototype manufacture and experimental validation in subsequent work.