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, 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
As mentioned, the uniqueness of the Axial Flux motor comes in 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 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 falls off with the square as the radius gets smaller and smaller, meaning there are diminishing returns in gaining torque by using through hole space.
Air Core
The air core concept is not unique to Axial Flux motors. Air core is simply a motor stator design approach that eliminates iron in the centers of each coil, thus eliminating cogging torque. The downside of this approach, however, is that the lack iron generally lowers continuous torque. Gaining back continuous torque is done by designing the needed outer diameter. As mentioned previously, torque generation goes with the square of the radius, so small increments 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 (or eliminating) 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. Sierramotion offers a variety of Axial Flux designs. Contact us for more details.
