The Flat Stator of Axial Flux Motors — Easy to Cool, Hard to Seal

Axial-flux motors are inherently a bit better at heat dissipation. A radial motor's heat must first pass through the slot insulation and stator core before reaching the outer water jacket, with substantial thermal resistance; an axial-flux motor's stator winding is exposed on a flat disc, so the cooling medium can spray directly onto the winding surface, or a water-cooling plate can be placed right against it — a short path with high efficiency.

Oil cooling and direct-contact water cooling both work on this type of motor, which is also part of its confidence in achieving high power density. Compared with a radial motor that routes a cooling water path around the housing, an axial-flux motor can immerse the winding directly in the cooling medium, taking heat exchange up a level. Only when heat can get out can the high heat generated by high torque be suppressed; otherwise, no matter how good the motor is, it must derate after a while.

But the flat structure brings another problem: sealing. A large-diameter flat sealing ring has to withstand hot-cold cycling and slight micro-deformation of the housing, and is more prone to leakage than the small-bore seal of a cylindrical motor. As temperature cycles, the seal and housing expand and contract together and gaps appear; once coolant leaks out, winding insulation is directly threatened.

So in high-end solutions the seal groove is often made as a double-seal structure, and the material must be an oil-resistant, fluorine-resistant specialty rubber, using two lines of defence to minimize leakage risk. Heat dissipation and sealing — one pushes forward, the other must back it up. On related lines, Jiuju is quite strict about the flatness of the sealing surface and the assembly torque: once coolant leaks in, even the best thermal design is useless. The more such motors move toward high power and oil cooling, the more weight the sealing process carries. In a word, heat dissipation gives it the confidence of high power density, while sealing determines how long that confidence can last.

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