Sensorless control of a dual-airgap axial flux permanent magnet machine for flywheel energy storage system

This study presents the sensorless vector control of a dual-airgap axial flux permanent magnet (AFPM) machine optimised for use in flywheel energy storage system (FESS) applications. The proposed AFPM machine has two sets of three-phase stator windings but only requires a single power converter to control both electromagnetic torque and axial levitation force. The proper controllability of the latter is crucial as it can be utilised to minimise the vertical bearing stress to improve the efficiency of the FESS. The method for controlling the speed and axial displacement of the machine is discussed. An inherent speed sensorless observer is also proposed for speed estimation. The proposed observer eliminates the rotary encoder, which in turn reduces the overall weight and cost of the system while improving its reliability. The effectiveness of the proposed control scheme has been verified by simulations and experiments on a prototype machine.