Traditionally, most motors used in electric vehicles (EVs) and mobility platforms have been radial flux systems. In this approach to motor design the rotor and stator exist as two cylinders, with one sitting inside the other. This means the area of magnetic flux is perpendicular to the motor’s axis of rotation, hence “radial.”
In recent years, there have been major strides in alternative axial flux motor architectures. In these systems, a motor’s stator and rotor exist as discs, typically with either one rotor packaged between two stators or one stator packaged between two rotors. The result is that the area of magnetic flux runs through the axis of rotation, hence “axial.”
What exactly do these architectural differences mean for motor design and production?
Axial Flux’s Renaissance
Axial- and radial-flux motor designs have always existed. However, for the most part, radial flux systems have been the given option for mobility applications.
But as the EV ecosystem has continued to mature, a growing number of applications have explored axial flux systems. These have come in the form of extremely powerful, high-torque applications that do not have the room to package a cylindrical radial flux motor: good examples include drones and hypercars.
Along with reduced length, an axial flux motor also offers these applications an exciting opportunity: significant theoretical gains in power density. That’s because the sandwich-like structure of an axial flux motor allows its “air gap” — the gap that exists between the rotor and the stator — to have a larger outer radius, and surface area per unit of motor mass.
Since the amount of work a motor can do is a function of the air gap size, for short motors, this means axial flux motors can deliver gains in torque density. If the speed between the stator and rotor can be maintained (the ‘airgap velocity’), then in turn the power density could be increased.
Axial flux systems are commonly “yokeless,” whereby a second rotor completes the magnetic circuit. By contrast, traditional radial flux motors almost always use a stator iron “yoke” to complete the magnetic circuit.
Altogether, this means that axial flux systems can theoretically output significantly more power for less mass and volume, in some cases. For many engineers and technologists, this means the modern renaissance in axial flux motors will soon displace radial flux motors from many mobility niches where power and mass are at a premium.
Radial Flux’s Longevity
This does beg the question: If these theoretical benefits for axial flux systems have always been there, why haven’t they displaced radial flux motors already? The answer comes down to the realities of engineering and production.
Within a motor, the rotor is attracted toward the stator. For a motor to have balanced and consistent performance, it needs to find a way to mitigate this magnetic attraction. In a worst-case scenario, contact between the rotor and stator at thousands of RPM means catastrophic failure and the possibility of risk to drivers and bystanders.
Radial flux motors have a built-in solution to this challenge: The magnetic attraction between the cylindrical rotor and stator cancels itself out. At any given point where the stator and rotor are pulling on one-another, it’s nullified by attraction at the other side. Since this is happening across the entire surface area of the air gap, the rotor and stator are reliably fixed in place.
It’s this design difference that means an axial flux motor needs either two rotors or two stators — to cancel out the magnetic attraction and provide a so-called “balanced pull.” But this solution causes a new problem. There’s only ever a single air gap in a radial flux system, whereas there are two in an axial flux system.
While the surface area of the air gap increases the potential power of a magnetic circuit, the thickness of the air gap decreases it. By introducing a second air gap, the total ‘reluctance’ of the magnetic circuit, its resistance to the magnetic flux, is close to doubled. An axial flux motor must also find a way to keep these two air gaps near-identical in thickness. If it does not, imbalances can impact stability, performance, and potentially lead to catastrophic failure. This is a difficult engineering challenge, since the motor needs to find a way to now account for variance in temperature, vibration levels, and rotordynamic conditions.
This all means that an axial flux system lives — or dies — based on extremely fine production tolerances and precision engineering. And this, in turn, limits the scope for volume production and increases the costs of units.
In addition, measures that are key to the power density gains of axial flux designs come with significant costs. The “yokeless” double-rotor approach means trading out low-cost stator iron for some of the most expensive materials within a motor system, particularly the magnets themselves.
Ceilings on Volume Production
That is, axial flux motors can achieve a higher theoretical ceiling of power density in high aspect ratios — motors with short axial lengths and wide diameters. But this comes with a significant set of new engineering challenges that mean significant rises in costs, risk, and system reliability. These challenges are soluble but solving them is costly and puts a ceiling on production.
But there’s another ceiling on volume production also baked into axial flux motor systems: that’s the question of scalability.
It’s much easier to change design tooling and machinery to accommodate an axially longer system than one with greater diameter. For a motor, an application that requires double the torque is relatively easy to accommodate for a radial flux design: You double the surface area of the air gap by doubling the length of a stator and rotor. This is essentially a single parameter change in design and all the dependent tooling.
For an axial flux design, this becomes a lot harder. Increasing the diameter of the axial flux motor would require a more complete change in tooling. This is impractical for the realities of volume production where scalability is important, and certainly costly in low volumes. In practice, a comparable doubling of torque means adding a complete second motor, which means duplicating all the motor’s individual components and adding a second inverter to regulate its output.
We should not underplay the potential for advancements in radial flux designs, which means that the power densities they are able to deliver to the market continue to rise at a significant rate year-on-year. Along with improvements in thermal management (aided by the thermal management knowledge of engineers from the combustion world, as they pivot to e-mobility), gains in radial flux power density are continually arising through innovations in winding configurations, cooling or new motor geometries.
The Roles of Axial and Radial Flux
The axial flux renaissance has seen some truly impressive achievements that have raised the bar for motor engineering. And there is a clear market for these solutions: Ones where power is needed in a short axial length that is typically less well served by a radial flux solution.
But the challenges of the second air gap introduced by axial flux systems do introduce some significant challenges for volume production, and that’s before considering the question of tooling for and mass assembly. In practice, this means that the higher theoretical power density of axial flux systems comes with a tradeoff — getting there necessarily means greater expense and time.
By contrast, radial flux motors are continually making major gains in power density and efficiency — and in practice, they still dominate the leaderboards of power density. Beyond those few situations where there’s simply not enough axial length to accommodate one, a radial flux solution will continue to exist to meet a particular mobility need.
The right motor solution for any platform will reflect its operating conditions, form factor, and production constraints. In this regard, the axial flux renaissance has categorically been a good thing for maximizing optionality and innovation. At the same time, the advantages of radial flux systems will mean they will continue to be a foundation for e-mobility for years to come.
This article was written by Andrew Cross, Chief Innovation Officer, Helix (Milton Keynes, United Kingdom). For more information, visit here .
