Flexible Control for Wide Speed Range Operation of High Polarity Stator Coreless AFPM Machines with WBG Semiconductor Devices

This paper presents a dual-mode controller designed to enable a high-polarity coreless stator axial flux PM (AFPM) machine to operate over a wide range of speeds. The proposed approach includes a combination of sine-wave (field-oriented) and square-wave control schemes. Coreless machines offer the advantage of being effortlessly controlled using square-wave voltages, thanks to the absence of core losses and negligible torque ripple caused by the lack of a magnetic core. By adopting square-wave control, the need for high-precision encoders is eliminated, enabling the utilization of Hall effect sensors or sensorless drive schemes instead. This, in turn, facilitates the attainment of ultra-high speeds with enhanced efficiency. The square-wave control mode also extends the machine speed range while fully utilizing the DC-link voltage. The control system is implemented on full-bridge inverters based on wide bandgap (WBG) semiconductor devices operating at high switching frequencies. This is done to minimize current ripple due to the ultra-low inductance of this type of machines. The machine performance was simulated using finite element analysis (FEA) and MATLAB/Simulink, and it was also experimentally tested on a prototype two-phase PCB stator coreless AFPM machine connected to two single-phase full-bridge inverters.