When an axial flux motor is integrated into an EV wheel hub, the biggest challenge is often not whether it can deliver high torque. Instead, it is how to prevent rapid temperature rise—and the resulting power derating—during demanding operating conditions such as continuous hill climbing, high-speed cruising, or frequent stop-and-go driving.

 

 

In wheel-hub applications, there is no engine-bay airflow, no extra space for heat exchangers, and often no possibility of adding a dedicated cooling system. All thermal management must be achieved within an extremely thin and confined cavity.

 

Meanwhile, the axial flux motor’s disc-type architecture—while enabling exceptional power density—also eliminates most of the conventional cooling paths used in radial-flux machines.

 

Why Is Cooling an Axial Flux Motor So Difficult?

 

Cooling, at its core, is about creating an efficient thermal pathway from the heat source to the external environment.

 

However, the structural characteristics of an axial flux motor introduce multiple barriers along this path—making effective heat removal inherently challenging.

 

1) Deeply Embedded Heat Sources with Long and Tortuous Thermal Paths

 

In an axial flux motor, the windings are typically located within the stator layer sandwiched between two rotors. They are surrounded by multiple materials—insulation, air gaps, and permanent magnets—each introducing significant thermal resistance.

 

As a result, heat must pass through several high-impedance interfaces before it can reach the outer housing where cooling media are available.

 

Put simply, the longer the path and the more interfaces involved, the lower the overall thermal conductivity.

 

 

2) Lack of Natural Convection Mechanisms

 

Conventional radial-flux motors rely on rotor rotation to generate internal airflow channels, enabling a form of self-ventilation cooling.

 

In contrast, the disc-type architecture of an axial flux motor provides no such airflow mechanism. When installed inside a wheel hub, the motor is effectively sealed off from external air movement, leaving natural convection almost entirely ineffective.

 

3) Severe Space Constraints That Limit Cooling Circuit Integration

 

To achieve high power density, axial flux motors are designed to be extremely thin, leaving almost no spare volume for integrating cooling channels, oil circuits, or heat-dissipation fins.

 

Even when cooling passages are forced into the structure, they must often cross the interfaces between rotating and stationary components—introducing challenges related to sealing, wear, and potential leakage.

 

4) Wheel-Hub Operating Conditions Intensify Cooling Challenges

 

Frequent stop-and-go driving in urban traffic or continuous hill climbing on mountainous roads can generate substantial heat in a short period.

If the cooling system is insufficient, the temperature rises far faster than in conventional centralized drive motors, easily triggering protective derating and negating the motor’s performance advantages.

 

The cooling challenge of axial flux motors is not merely a single technical bottleneck—it reflects a fundamental conflict between structural advantages and thermal management requirements. Their compact design enables hub integration but simultaneously constrains cooling freedom.

Precisely because of these limitations, the industry has developed various cooling solutions for axial flux motors—but each comes with clear operational boundaries and trade-offs.

 

Oil-Immersion Cooling: Performance-First, High Cost

 

The entire motor is immersed in electrically insulated cooling oil, with a circulation system directly removing heat. This method offers high cooling efficiency and is well-suited for sustained high-load conditions.

 

However, it has notable limitations: the system is relatively complex, requiring additional pumps, piping, and sealing structures. Maintenance is challenging, costs are high, and widespread adoption in mainstream passenger vehicles is difficult.

 

Thermally Conductive Potting: Simplified Structure, Reduced Maintainability

 

High-thermal-conductivity epoxy or silicone is used to fill the internal gaps of the motor, enhancing overall heat transfer. This approach eliminates the need for external cooling circuits and allows for a compact design, making it suitable for the limited space of wheel-hub applications.

 

However, once potting is completed, the motor cannot be reworked. It places high demands on assembly cleanliness and first-pass success rate.

 

Embedded Microchannels: Precise and Efficient, Yet Process-Sensitive

 

Cooling channels are directly machined into the stator or rotor substrates, allowing the coolant to flow through the heat-generating regions. This approach offers rapid thermal response and high cooling efficiency.

 

However, because the system consists of multiple layers that must be precisely aligned, even slight misalignment can block the microchannels. The method is therefore highly sensitive to assembly precision and environmental cleanliness.

 

Passive Assisted Cooling: Suitable for Light-Load Scenarios

 

Methods such as high-thermal-conductivity housings or phase-change materials can only delay temperature rise. They are insufficient for supporting continuous high-load operation.

 

 

Although each of the cooling solutions described above offers specific advantages, all of them impose stringent requirements on manufacturing consistency and reliability.

 

For example, oil-immersion systems rely on high-precision sealing and press-fit assembly; potting processes demand flawless first-pass formation; and microchannel structures require precise alignment of multiple layers. Even minor deviations in any assembly step can lead to cooling failure, insulation breakdown, or complete motor scrapping.

 

In other words, the reliability of the cooling system begins to be established from the very first screw tightened.

 

This is precisely the focus of Honest Automation, as a provider of intelligent motor assembly solutions: cooling is not merely a thermal management challenge—it is a manufacturing implementation challenge as well.

 

Axial flux motors hold tremendous potential in wheel-hub drive applications, yet the essence of their cooling challenge lies in the systemic interplay of structure, materials, and manufacturing. No matter how sophisticated the cooling design, without matching assembly capabilities, it is difficult to translate into a stable, mass-producible product.

 

If you are evaluating the manufacturability of different cooling solutions or facing assembly challenges caused by cooling structures, we welcome you to get in touch.

 

Honest Automation, with years of accumulated expertise in motor assembly processes, is ready to provide manufacturing-side support for your axial flux motor projects.