Cooling a Quasi-Direct-Drive Joint Motor from Inside the Copper: A Simulation-Only Design Study

Quasi-direct-drive (QDD) robot joints trade gear ratio for transparency, and their continuous torque is set by how much winding heat the housing can shed. This preprint tests one hypothesis: that drive electronics are now efficient enough at high current that heat is the remaining limit, so a joint motor should be cooled directly, run hard, and geared less. The design studied is a Ø120 mm, 24-slot, 22-pole motor whose coils are hollow rectangular copper carrying dielectric oil in the bore, inside an outer jacket fed with the coldest coolant. It is evaluated with a first-order loss and hydraulic model, a nonlinear two-dimensional magnetostatic field solution, a lumped thermal model of the jacket-only alternative, and a parametric CAD assembly with interference checks. Nothing has been built or measured. In simulation the motor gives 16.6 N·m continuously for 850 W of heat with the copper at 126 °C, and 21.3 N·m at the limit of a 3 L/min pump, against about 5 N·m for the same iron cooled by air. The saturation limit first assumed was too pessimistic; trading copper for iron at fixed size does not raise torque at a given heat; and a simpler motor with solid conductors and only the jacket reaches about 70% of the hollow design's torque, so it is the one to build first. A 3:1 reduction matches a commercial 9:1 actuator's continuous torque with a ninth of the reflected inertia and about five times the holding power. The models and CAD were written by an AI coding agent under the author's direction; the paper describes that workflow and the modelling errors that cross-checks between the models exposed. No new cooling method is claimed.