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Yokeless Axial Flux Topology as the Driver for Future Powertrains

The unique selling proposition of yokeless axial flux machines over their state-of-the-art radial flux cousins are:

  • Up to 3x the torque density 
  • Up to 2x power density 
  • Typically 1/3 of the axial length 
  • Around 45% lower CO2 footprint 

1. AXIAL LENGTH  

Thanks to their short axial length, yokeless axial flux machines are a perfect match for dual motor electric drive units (EDUs). This type of EDU is gaining traction (pun intended) in high-end electric vehicles thanks to their ability to perform torque vectoring.   

The main performance benefits of torque vectoring are increased traction and improved driving dynamics. These factors are of special importance to electric cars because of the huge weight penalty the battery pack imposes on the vehicle. As we all know, mass is the worst enemy of desirable driving dynamics.   

Figure 1: Two yokeless axial flux motors. Each motor drives a separate wheel at the rear axle, providing torque vectoring, which enhances safety, acceleration, and the ‘fun-to-drive’ factor. The whole dual-motor Electric Drive Unit (EDU) has a width (axial length) below 500 mm

For most passenger cars, the maximum allowable length of the EDU is quite limited due to packaging constraints. On both sides of the unit, sufficient space needs to be left for the suspension and steering components. This makes it problematic to fit two radial flux motors plus two gearboxes side-by-side.   

The high torque density of the yokeless axial flux machine allows using a gearbox with a lower reduction ratio (typically 1:6), which can be realized with a single stage geartrain. This makes the motors very well suited to be combined with a coaxial single stage planetary gearset.  

  • A first advantage is that single stage planetary gearsets are very compact, short and lightweight. Consequently, the coaxial combination with this type of gearbox further increases the power density on EDU level, and further decreases the length of the unit.  
  • A second advantage of having a single stage gearset is the higher efficiency of the gearbox. High speed radial flux motors require two or three stage gearing to get down to suitable output speeds. This can easily cost 2 or 3% of overall EDU efficiency, and results in a heavier, larger and more expensive gearbox.  

2. POWER AND TORQUE DENSITY  

The higher torque and power density of the yokeless axial flux machine translates to a lower EDU mass. Typically, around 20kg per motor can be saved when compared to state-of-the-art radial flux solutions, considering a higher segment passenger car.  

So for a tri-motor vehicle this adds up to around 60kg, for a quad motor vehicle it’s around 80kg that can be saved at vehicle level. These numbers are quite significant, particularly for a performance vehicle.  

The combination of lower mass plus the ability to perform torque vectoring will have a profound impact on the driving dynamics of an EV that’s equipped with yokeless axial flux technology, enabling it to set new performance benchmarks.  

3. SUSTAINABILITY  

Lower material usage does not only lead to a higher power and torque density and lower manufacturing costs. It also contributes to a better sustainable solution thanks to the significant reduction in overall carbon footprint.  

An evaluation of the Traxial yokeless axial flux machine compared to a state-of-the-art radial flux machine with comparable performance, concluded that the CO2 emissions on the account of the materials of the yokeless axial flux machine are about 45% lower than those of the radial flux machine.  

This result, mainly driven by the lower amount of electrical steel and copper, clearly indicates that YAF technology is a more future-proof option compared to radial flux technology, as the importance of life cycle emissions will undoubtedly continue to rise in the future. 

EDU VISION 

TORQUE VECTORING DUAL-MOTOR EDU

The short axial length of the yokeless axial flux motor, combined with its high power and torque density, makes the technology ideally suited for dual motor EDUs. A high-performance torque vectoring dual motor EDU can be made with similar or lower mass and smaller dimensions than a lower performing single motor radial flux EDU.  

As already stated above, for a dual, tri- or quad motor vehicle the lower motor and gearbox masses add up to a substantial weight saving on vehicle level.  

This also offers an interesting opportunity for OEMs who want to offer performance or premium variants of existing radial-flux equipped vehicles, as modifications to chassis and suspension will be minimized. 

The following post is the fourth in a 4-part series based on a paper presented at the 2023 CTI Symposium in Berlin by Traxial founder Peter Leijnen.

Part 1: Axial Flux Versus Radial Flux

Part 2: The Theory: Fundamental Advantages Delivering Substantial Performance Improvements in Axial Flux Motors

Part 3: The Practice: Resolving the Challenges

Get the technical data sheet of our latest axial flux motor, the AXF300-85s

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