Losses in axial-flux permanent-magnet coreless flywheel energy storage systems

This paper analyzes the losses of axial-flux permanent-magnet (AFPM) motor/generators integrated in flywheel energy storage applications with a coreless winding. This configuration has several advantages over traditional slotted ones for some applications since the losses in a toothless design are at a lower level than in an equivalent iron slotted devices. It has lower idle losses and potentially higher overall efficiency than conventional machines. 3-D finite element method (FEM) is used to calculate flux density in the air gap and to simulate the electromotive force (back EMF) and eddy current losses in the windings. A small-scale three-phase machine has been designed and constructed. Spin down tests have been performed in atmospheric pressure and at low pressure and eddy current, bearing and drag losses are presented.