AFPM
How it works
In an AFPM, permanent magnets on the rotor disc create a magnetic flux that crosses the air gap axially into a flat stator carrying windings. Current flowing in the stator windings, which sit in a field directed along the axis, generates a Lorentz force tangential to the disc, producing torque about the shaft. Control is electronic (brushless commutation, BLDC/PMSM) via an inverter that synchronises current with rotor position. In double-sided topologies two magnet sets or two stators balance the axial attraction forces and improve utilisation of active material. In the YASA variant the stator yoke is removed and segmented core-plus-winding modules shorten the flux path, cutting mass and iron losses. The large diameter combined with a short axial length yields high torque at relatively low rotational speeds, which is favourable for direct-drive and quasi-direct-drive applications, including robotic actuators.
Problem solved
Conventional radial-flux motors have limited torque and power density for a given mass and volume, and their elongated cylindrical shape is awkward in shallow installations (wheels, joints, propellers). The AFPM raises torque density and enables flat packaging, exploiting the fact that in axial geometry torque scales with the cube of the rotor diameter, whereas in radial geometry it scales only with the square.
Components
A disc (or discs) mounted on the shaft carrying permanent magnets (usually NdFeB) that generate the axial magnetic flux across the air gap.
A flat stator with coils; current flow in the magnetic field produces a tangential force and torque. It may have a yoke or be yokeless (YASA).
The axial gap between rotor and stator through which the flux passes; its uniformity and size are critical to efficiency and torque.
A ferromagnetic element that closes the magnetic circuit; in the YASA topology the stator yoke is eliminated and the core is split into segments.
The shaft transmits torque to the load; the bearings must carry the significant axial attraction forces between rotor and stator.
Implementation
Strong axial attraction forces between rotor and stator load the bearings and structure; they are especially large in single-sided topologies.
Assembly must ensure a small, uniform axial gap; deflection under load degrades efficiency and may cause rotor-stator contact.
A large-diameter rotor experiences high centrifugal stress, which limits the maximum rotational speed.
High current density in concentrated windings requires effective cooling despite the favourable heat-exchange surface area.
Evolution
The introduction of high-energy-density NdFeB magnets enabled compact permanent-magnet machines and the revival of axial-flux designs.
The Rolls-Royce electric aircraft set a speed record using three axial-flux motors, demonstrating the maturity of the technology for aviation propulsion.
A YASA prototype motor reached about 42 kW/kg power density, far exceeding typical radial-flux motors and underscoring the advantage of axial geometry.
Mercedes-Benz began large-scale production of axial-flux motors at its Berlin-Marienfelde plant, bringing AFPM technology into mass-market automotive use.
Hyperparameters (configurable axes)
The arrangement of rotor and stator counts and the presence of a yoke; determines axial force balance, torque density and manufacturing complexity.
The number of magnetic pole pairs; affects torque, electrical frequency and speed range.
The axial size of the gap; a smaller gap increases flux and torque but raises mechanical demands and attraction forces.
The type and grade of permanent magnets; higher energy product (NdFeB) increases torque density but lowers thermal tolerance.
The number of parallel rotor–stator sets stacked axially; increases torque and power at the cost of length and mass.