Design of Electric Machine Windings to Independently Control Multiple Airgap Harmonics

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Abstract
Multiphase electric machines offer benefits of fault tolerance and reduced power electronic switch ratings per phase. These machines are also capable of independently controlling multiple airgap fields with differing number of pole-pairs (spatial harmonics). This presents opportunity to improve torque density, magnetically levitate the rotor, and wirelessly transfer power to excite the rotor of wound-field synchronous machines. Although these performance improvement techniques have been studied separately in literature, there exists no general winding design approach that targets independent control of multiple airgap fields. This paper presents a generalized technique to design multiphase electric machine windings that can control the magnitude and angular location of multiple airgap magnetic fields, each with a different desired number of pole-pairs. Design examples are provided, and the control of different airgap harmonics is experimentally validated in two different prototype machines. The results show that the generalized approach presented in this paper is applicable across different motor topologies (radial and axial flux) and winding configurations (concentrated and distributed), demonstrating its utility for a wide range of application scenarios that benefit from the control of multiple airgap fields.

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