Axial Force Negative Stiffness in Axial-Flux Electric Machines

This paper examines the axial force negative stiffness of induction and permanent magnet (PM) axial flux machines. First, simplified analytical models are used to identify the key normalised machine parameters which affect the variation of axial force with airgap length. For current-driven induction machines and PM machines, this is found to be the ratio of the effective magnetic length of the magnets and magnetic core divided by the nominal airgap. For voltage-driven (line-start) induction machines, it is the ratio of the stator leakage inductance to the nominal magnetising reactance. Next, the analytical results are validated against finite-element results and results from experimental axial force testing on induction and PM axial-flux machines.