Performance Benefits of Air Core Axial Flux Motors
Introduction
Axial Flux motor architecture is a form of rotary actuation that delivers torque in a non-traditional form factor. Unlike radial flux motors, where coils run in line, or parallel with the rotation axis, axial flux coils sit perpendicular to the axis of rotation, and when combined with a disk-shaped rotor, deliver a low-profile electromagnetic solution. When combined with an air core stator approach, these motors deliver cog-free, highly efficient torque in a high diameter-to-height aspect ratio, making them ideal solutions when axial space is limited.
Axial Length
The uniqueness of the axial flux motor is due to how the stator is orientated with respect to the rotation axis. The core principle is best described as taking a linear motor, and forming it into a flat washer, connecting each end of the stator forcer and magnet track to itself. In this architecture, there are sets of coils (and magnets) spaced evenly across the planar circumference (see Figure 1). The flat aspect ratio of both the stator and rotor combine to deliver an ultra-low-profile electromagnetic solution.
Inner Diameter
The inner diameter of axial flux motors is highly configurable. There are no set rules for an outer to inner diameter ratio. It is important to note, however, that torque generation scales with the square of the radius and therefore, there are diminishing returns for gaining torque by utilizing through-hole space.
Air Core
Figure 1: Allient SA Series Air Core Axial Flux Motor
The air core concept is not unique to axial flux motors. Air core is simply a stator design approach that eliminates iron in the center of each coil, thus eliminating cogging torque. The downside of this approach, however, is that the lack of iron generally lowers torque capacity of the motor. Higher continuous torque can be achieved by increasing the outer diameter of the motor. Due to the aforementioned relationship between torque and the motor radius, small increases in diameter make large gains in torque. This means that the fundamental axial flux architecture, that being a large OD to height ratio, is a perfect match for maximizing torque generation, while also providing smooth, cog-free motion. The air core approach also has an added benefit of reducing eddy currents that circulate in the iron within the magnetic path. Eddy current losses are speed dependent losses and can reduce the effective motor constant of any motor, making the motor less efficient. Lastly, the air core approach provides excellent linearity between current in the motor and torque generation. The linearity is captured in the torque constant and plays a major role in position and velocity loop stability, as well as output torque estimation based on drive current.
Summary
Ironless Axial Flux motors offer a unique electromagnetic solution, providing an excellent torque-to-height ratio, while also delivering smooth, cog-free torque for precise, low-speed operation and a highly efficient solution for high-speed applications. Allient offers a variety of Axial Flux designs. Contact us for more details.