Permanent Magnet Rotor Flux Linkage Control Through Direct Axis Field Amplification

Permanent magnets are popular for electric vehicle rotors due to their high energy density, making them excellent candidates for high-torque and high-speed machines. The downside of a permanent magnet machine is the inability to regulate the rotor field, creating nonideal behavior during high-speed or low-load operation, and potentially resulting in high currents and voltages in fault conditions. Several solutions to this have been explored, such as interior permanent magnet (combined reluctance and permanent magnet rotors), "hybrid" wound field and permanent magnet rotors, and variable flux machines with in-situ magnetization control or mechanical field weakening. This article proposes a novel method of regulating the air gap in axial flux machines, allowing for a low-cost mechanism allowing two degree of freedom operation without additional power electronics or modifications to the magnetics. The proposed method uses stacked linear springs to create a nonlinear bias against the attraction force, and then leverages direct axis current to control the air gap. The ideal constant current optimized field weakening spring curve is presented and the proposed concept is experimentally validated on a single-stator single-rotor axial flux machine.