Effects of Eccentricity on Dual Rotor Single Stator Axial Flux PM Machines

Axial flux permanent magnet motors with a yokeless and segmented armature topology are gaining popularity due to their high power and torque density. However, these motors are more susceptible to eccentricity caused by manufacturing imperfections, improper assembly, bent shafts, or bearing faults. Eccentricity can occur in various forms, including rotor displacement within the plane, axial shift of the stator or rotor, and rotor tilt, with the unevenness of the two air gaps being either symmetric or asymmetric. The results of this study indicate that rotor displacement within the plane introduces in-plane forces and off-axis torque on the rotor. Additionally, the back electromotive force (BEMF) is affected, with some phases experiencing a phase shift, while others experience a change in magnitude. Stator axial displacement results in a nonzero axial force on the stator, which increases with the increase of the axial displacement. However, BEMF remains unaffected by this displacement. Axial displacement of one rotor, while keeping the other rotor fixed, increases the axial force on the stator. Moreover, torque and BEMF increase when one rotor moves closer to the stator but decrease when the rotor moves axially further from the stator.